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Anomalous X-ray diffraction (AXD) and neutron diffraction can be used to crystallographically distinguish between metals of similar electron density. Despite the use of AXD for structural characterization in mixed metal clusters, there are no benchmark studies evaluating the accuracy of AXD toward assessing elemental occupancy in molecules with comparisons with what is determined via neutron diffraction. We collected resonant diffraction data on several homo and heterometallic clusters and refined their anomalous scattering components to determine metal site occupancies. Theoretical resonant scattering terms for Fe 0 , Co 0 , and Zn 0 were compared against experimental values, revealing theoretical values are ill-suited to serve as references for occupancy determination. The cluster featuring distinct cation and anion metal compositions [CoCp 2 *][( tbs L)Fe 3 (μ 3 -NAr)] was used to assess the accuracy of different f ' references for occupancy determination ( f ' theoretical ± 15-17%; f ' experimental ± 10%). This methodology was applied toward calculating the occupancy of three different clusters: ( tbs L)Fe 2 Zn(py) ( 6 ), ( tbs L)Fe 2 Zn(μ 3 -NAr)(py) ( 7 ), and [CoCp* 2 ][( tbs L)Fe 2 Zn(μ 3 -NAr)] ( 8 ). The first two clusters maintain 100% Fe/Zn site isolation, whereas 8 showed metal mixing within the sites. The large crystal size of 8 enabled collection of neutron diffraction data which was compared against the results found with AXD. The ability of AXD to replicate the metal occupancies as determined by neutron diffraction supports the AXD occupancy methodology developed herein. Furthermore, the advantages innate to AXD (e.g., smaller crystal sizes, shorter collection times, and greater availability of synchrotron resources) versus neutron diffraction further support the need for its development as a standard technique.

Despite the utility of copper catalysts for the insertion of carbene moieties into C-H bonds, the copper carbene intermediate often invoked in these transformations has not been isolated. Herein, we describe the synthesis and structural characterization of a series of copper benzylidenes utilizing the sterically encumbered dipyrrin ligand ( Em L)H. These isolated copper carbenes demonstrate intramolecular insertion into the primary C(sp 3 )-H bond of the ligand ( Em L)H and intermolecular insertion into ethereal and allylic C-H bonds. The copper carbenes isolated are best described as Cu(I) carbene adducts akin to canonical Fischer carbenes, given their diamagnetic ground state and electrophilic carbene reactivity. Furthermore, the insertion chemistry can be rendered catalytic utilizing a more sterically exposed dipyrrin ligand ( ArF L)H. The ability to isolate and observe stoichiometric C-H insertion and olefin cyclopropanation from well-characterized copper benzylidenes illuminates their viability as catalytic intermediates and their participation in potential catalyst deactivation pathways.

Using a new hexanucleating anildophosphine ligand t Bu LH 3 (1,3,5-C 6 H 9 (NHC 6 H 3 -5-F-2-P( t Bu) 2 ) 3 ), the all-monovalent [Fe I 3 ] compound ( t Bu L)Fe 3 ( 1 ) was isolated and characterized by X-ray diffraction analysis, SQUID magnetometry, 57 Fe Mössbauer spectroscopy, and cyclic voltammetry. The molecular structure of 1 reveals very close Fe-Fe distances of 2.3825(7), 2.4146(8), and 2.3913(8) Å which results in significant Fe-Fe interactions and a maximum high-spin S = 9 / 2 spin state as determined by SQUID magnetometry and further supported by quantum chemical calculations. Compound 1 mediates the multielectron, oxidative atom transfer from inorganic azide ([Bu 4 N][N 3 ]), cyanate (Na[NCO]), and phosphonate (Na(dioxane) 2.5 [PCO]) to afford the [Fe 3 ]-nitrido (N 3- ) and [Fe 3 ]-phosphido (P 3- ) pnictides, ( t Bu L)Fe 3 (μ 3 -N) ( 2 ) and [( t Bu L)Fe 3 (μ 3 -P)(CO)] - ( 3 ), respectively. Compounds 1 - 3 exhibit rich electrochemical behavior with three (for 1 ), four (for 2 ) and five (for 3 ) distinct redox events being observed in the cyclic voltammograms of these compounds. Finally, the all-monovalent 1 and the formally Fe II /Fe II /Fe I compound 3 , were investigated by alternating current (ac) SQUID magnetometry, revealing slow magnetic relaxation in both compounds, with 3 being found to be a unique example of a [Fe 3 ]-phosphido single-molecule magnet having an energy barrier relaxation reversal of U = 30.7(6) cm -1 in the absence of an external magnetic field. This study demonstrates the utility of an all low-valent polynuclear cluster to perform multielectron redox chemistry and exemplifies the redox flexibility and unique physical properties that are present in the corresponding midvalent oxidation products.

Stepwise metalation of the hexadentate ligand tbs LH 6 ( tbs LH 6 = 1,3,5-C 6 H 9 (NHC 6 H 4 - o -NHSiMe 2 t Bu) 3 ) affords bimetallic trinuclear clusters ( tbs L)Fe 2 Zn(thf) and ( tbs L)Fe 2 Zn(py). Reactivity studies were pursued to understand metal atom lability as the clusters undergo ligand substitution, redox chemistry, and group transfer processes. Chloride addition to ( tbs L)Fe 2 Zn(thf) resulted in a mixture of species including both all-zinc and all-iron products. Addition of ArN 3 (Ar = Ph, 3,5-(CF 3 ) 2 C 6 H 3 ) to ( tbs L)Fe 2 Zn(py) yielded a mixture of two trinuclear products: ( tbs L)Fe 3 (μ 3 -NAr) and ( tbs L)Fe 2 Zn(μ 3 -NAr)(py). The two imido species were separated via crystallization, and outer sphere reduction of ( tbs L)Fe 2 Zn(μ 3 -NAr)(py) resulted in the formation of a single product, [2,2,2-crypt(K)][( tbs L)Fe 2 Zn(μ 3 -NAr)]. These results provide insight into the relationship between heterometallic cluster structure and substitutional lability and could help inform both future catalyst design and our understanding of metal atom lability in bioinorganic systems., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Metalation of the polynucleating ligand F,tbs LH 6 (1,3,5-C 6 H 9 (NC 6 H 3 -4-F-2-NSiMe 2 t Bu) 3 ) with two equivalents of Zn(N(SiMe 3 ) 2 ) 2 affords the dinuclear product ( F,tbs LH 2 )Zn 2 (1), which can be further deprotonated to yield ( F,tbs L)Zn 2 Li 2 (OEt 2 ) 4 (2). Transmetalation of 2 with NiCl 2 (py) 2 yields the heterometallic, trinuclear cluster ( F,tbs L)Zn 2 Ni(py) (3). Reduction of 3 with KC 8 affords [KC 222 ][( F,tbs L)Zn 2 Ni] (4) which features a monovalent Ni centre. Addition of 1-adamantyl azide to 4 generates the bridging μ 3 -nitrenoid adduct [K(THF) 3 ][( F,tbs L)Zn 2 Ni(μ 3 -NAd)] (5). EPR spectroscopy reveals that the anionic cluster possesses a doublet ground state (S = 1 / 2 ${{ 1/2 }}$ ). Cyclic voltammetry of 5 reveals two fully reversible redox events. The dianionic nitrenoid [K 2 (THF) 9 ][( F,tbs L)Zn 2 Ni(μ 3 -NAd)] (6) was isolated and characterized while the neutral redox isomer was observed to undergo both intra- and intermolecular H-atom abstraction processes. Ni K-edge XAS studies suggest a divalent oxidation state for the Ni centres in both the monoanionic and dianionic [Zn 2 Ni] nitrenoid complexes. However, DFT analysis suggests Ni-borne oxidation for 5., (© 2023 Wiley-VCH GmbH.)

Despite the myriad Cu-catalyzed nitrene transfer methodologies to form new C-N bonds ( e.g. , amination, aziridination), the critical reaction intermediates have largely eluded direct characterization due to their inherent reactivity. Herein, we report the synthesis of dipyrrin-supported Cu nitrenoid adducts, investigate their spectroscopic features, and probe their nitrene transfer chemistry through detailed mechanistic analyses. Treatment of the dipyrrin Cu I complexes with substituted organoazides affords terminally ligated organoazide adducts with minimal activation of the azide unit as evidenced by vibrational spectroscopy and single crystal X-ray diffraction. The Cu nitrenoid, with an electronic structure most consistent with a triplet nitrene adduct of Cu I , is accessed following geometric rearrangement of the azide adduct from κ 1 -N terminal ligation to κ 1 -N internal ligation with subsequent expulsion of N 2 . For perfluorinated arylazides, stoichiometric and catalytic C-H amination and aziridination was observed. Mechanistic analysis employing substrate competition reveals an enthalpically-controlled, electrophilic nitrene transfer for primary and secondary C-H bonds. Kinetic analyses for catalytic amination using tetrahydrofuran as a model substrate reveal pseudo-first order kinetics under relevant amination conditions with a first-order dependence on both Cu and organoazide. Activation parameters determined from Eyring analysis (Δ H ‡ = 9.2(2) kcal mol -1 , Δ S ‡ = -42(2) cal mol -1 K -1 , Δ G ‡ 298K = 21.7(2) kcal mol -1 ) and parallel kinetic isotope effect measurements (1.10(2)) are consistent with rate-limiting Cu nitrenoid formation, followed by a proposed stepwise hydrogen-atom abstraction and rapid radical recombination to furnish the resulting C-N bond. The proposed mechanism and experimental analysis are further corroborated by density functional theory calculations. Multiconfigurational calculations provide insight into the electronic structure of the catalytically relevant Cu nitrene intermediates. The findings presented herein will assist in the development of future methodology for Cu-mediated C-N bond forming catalysis., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The reactivity of ( Tr L)Co ( Tr L=5-mesityl-1,9-(trityl)dipyrrin) toward various aryl azides was examined to elucidate the electronic structure and reactivity of dipyrrinato cobalt aryl nitrenoid complexes. Herein, we demonstrate the synthesis of a Co II diketimide complex [( Tr L)Co(NC 6 F 5 )] 2 and its reversible C-C bond cleavage to yield a monomeric Co nitrenoid complex ( Tr L)Co(NC 6 F 5 ). Exposure of [( Tr L)Co(NC 6 F 5 )] 2 to an excess amount of an H-atom donor cleanly affords the Co II anilide complex ( Tr L)Co(NHC 6 F 5 ). The half-order decay of [( Tr L)Co(NC 6 F 5 )] 2 via H-atom abstraction (HAA) reveals saturation kinetic behavior indicating a pre-equilibrium between [( Tr L)Co(NC 6 F 5 )] 2 and ( Tr L)Co(NC 6 F 5 ) prior to HAA. Furthermore, ( Tr L)Co(NC 6 F 5 ) undergoes reductive coupling with another equivalent of azide to furnish the four-coordinate tetrazido complex ( Tr L)Co(κ 2 -N 4 (C 6 F 5 ) 2 ), expulsion of a fluorine atom to afford ( Tr L)CoF, and N-group transfer reactivity to PPh 3 ., (© 2022 Wiley-VCH GmbH.)

Quantum confinement in small symmetric clusters leads to the bunching of electronic states into closely packed shells, enabling the classification of clusters with well-defined valences as superatoms. Like atoms, superatomic clusters with filled shells exhibit enhanced electronic stability. Here, we show that octahedral transition-metal chalcogenide clusters can achieve filled shell electronic configurations when they have 100 valence electrons in 50 orbitals or 114 valence electrons in 57 orbitals. While these stable clusters are intrinsically diamagnetic, we use our understanding of their electronic structures to theoretically predict that a cluster with 107 valence electrons would uniquely combine high stability and high-spin magnetic moment, attained by filling a majority subshell of 57 electrons and a minority subshell of 50 electrons. We experimentally demonstrate this predicted stability, high-spin magnetic moment ( S = 7/2), and fully delocalized electronic structure in a new cluster, [NEt 4 ] 5 [Fe 6 S 8 (CN) 6 ]. This work presents the first computational and experimental demonstration of the importance of dual subshell filling in transition-metal chalcogenide clusters.

We report that exposing the dipyrrin complex ( EMind L)Cu(N 2 ) to air affords rapid, quantitative uptake of O 2 in either solution or the solid-state to yield ( EMind L)Cu(O 2 ). The air and thermal stability of ( EMind L)Cu(O 2 ) is unparalleled in molecular copper-dioxygen coordination chemistry, attributable to the ligand flanking groups which preclude the [Cu(O 2 )] 1+ core from degradation. Despite the apparent stability of ( EMind L)Cu(O 2 ), dioxygen binding is reversible over multiple cycles with competitive solvent exchange, thermal cycling, and redox manipulations. Additionally, rapid, catalytic oxidation of 1,2-diphenylhydrazine to azoarene with the generation of hydrogen peroxide is observed, through the intermittency of an observable ( EMind L)Cu(H 2 O 2 ) adduct. The design principles gleaned from this study can provide insight for the formation of new materials capable of reversible scavenging of O 2 from air under ambient conditions with low-coordinate Cu I sorbents.

In polynuclear biological active sites, multiple electrons are needed for turnover, and the distribution of these electrons among the metal sites is affected by the structure of the active site. However, the study of the interplay between structure and redox distribution is difficult not only in biological systems but also in synthetic polynuclear clusters since most redox changes produce only one thermodynamically stable product. Here, the unusual chemistry of a sterically hindered trichromium complex allowed us to probe the relationship between structural and redox isomerism. Two structurally isomeric trichromium imides were isolated: asymmetric terminal imide ( tbs L)Cr 3 (NDipp) and symmetric, μ 3 -bridging imide ( tbs L)Cr 3 (μ 3 -NBn) (( tbs L) 6- = (1,3,5-C 6 H 9 (NC 6 H 4 - o -NSi t BuMe 2 ) 3 ) 6- ). Along with the homovalent isocyanide adduct ( tbs L)Cr 3 (CNBn) and the bisimide ( tbs L)Cr 3 (μ 3 -NPh)(NPh), both imide isomers were examined by multiple-wavelength anomalous diffraction (MAD) to determine the redox load distribution by the free refinement of atomic scattering factors. Despite their compositional similarities, the bridging imide shows uniform oxidation of all three Cr sites while the terminal imide shows oxidation at only two Cr sites. Further oxidation from the bridging imide to the bisimide is only borne at the Cr site bound to the second, terminal imido fragment. Thus, depending on the structural motifs present in each [Cr 3 ] complex, MAD revealed complete localization of oxidation, partial localization, and complete delocalization, all supported by the same hexadentate ligand scaffold., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Intramolecular alkoxylation of C-H bonds can rapidly introduce structural and functional group complexities into seemingly simple or inert precursors. The transformation is particularly important due to the ubiquitous presence of tetrahydrofuran (THF) motifs as fundamental building blocks in a wide range of pharmaceuticals, agrochemicals, and natural products. Despite the various synthetic methodologies known for generating functionalized THFs, most show limited functional group tolerance and lack demonstration for the preparation of spiro or fused bi- and tricyclic ether units prevalent in molecules for pharmacological purposes. Herein we report an intramolecular C-H alkoxylation to furnish oxacycles from easily prepared α-diazo-β-ketoesters using commercially available iron acetylacetonate (Fe(acac) 2 ) as a catalyst. The reaction is proposed to proceed through the formation of a vinylic carboradical arising from N 2 extrusion, which mediates a proximal H-atom abstraction followed by a rapid C-O bond forming radical recombination step. The radical mechanism is probed using an isotopic labeling study (vinyl C-D incorporation), ring opening of a radical clock substrate, and Hammett analysis and is further corroborated by density functional theory (DFT) calculations. Heightened reactivity is observed for electron-rich C-H bonds (tertiary, ethereal), while greater catalyst loadings or elevated reaction temperatures are required to fully convert substrates with benzylic, secondary, and primary C-H bonds. The transformation is highly functional group tolerant and operates under mild reaction conditions to provide rapid access to complex structures such as spiro and fused bi-/tricyclic O-heterocycles from readily available precursors.

Several first-row transition metal complexes of the 1,9-bis(2',4',6'-triphenylphenyl)-5-mesityl dipyrrinato ligand and its tetrahalogenated analogues have been synthesized and their luminescence spectra obtained. The protonated ligands, as well as the Li(i), Mn(ii), Cu(i), Cu(ii), and Zn(ii) chelates show appreciable luminescence, despite the paramagnetism of the Mn(ii) and Cu(ii) ions. Fluorescence quantum yields (ΦF) as high as 0.67 were observed for the zinc complex. Luminescence was partially quenched by the introduction of heavy halogens to the backbone of the ligand, as well as by the introduction of paramagnetic metal ions. Room-temperature, solution state phosphorescence was observed from the halogenated dipyrrinato lithium salts, as well as from the non-halogenated Mn(ii) complex.

The metal-metal-bonded molecule [Bu 4 N][( H L) 2 Fe 6 (dmf) 2 ] (Fe 6 ) was previously shown to possess a thermally isolated spin S = 19 / 2 ground state and found to exhibit slow magnetization relaxation below a blocking temperature of ∼5 K [ J. Am. Chem. Soc. 2015 , 137 , 13949-13956]. Here, we present a comprehensive spectroscopic investigation of this unique single-molecule magnet (SMM), combining ultrawideband field-swept high-field electron paramagnetic resonance (EPR) with frequency-domain Fourier-transform terahertz EPR to accurately quantify the spin Hamiltonian parameters of Fe 6 . Of particular importance is the near absence of a 4th-order axial zero-field splitting term, which is known to arise because of quantum mechanical mixing of spin states on account of the relatively weak spin-spin (superexchange) interactions in traditional polynuclear SMMs such as the celebrated Mn 12 -acetate. The combined high-resolution measurements on both powder samples and an oriented single crystal provide a quantitative measure of the isolated nature of the spin ground state in the Fe 6 molecule, as well as additional microscopic insights into factors that govern the quantum tunneling of its magnetization. This work suggests strategies for improving the performance of polynuclear SMMs featuring direct metal-metal bonds and strong ferromagnetic spin-spin (exchange) interactions.

The dichromium Pacman complex ( tBu dmx)Cr 2 Cl 2 ·C 4 H 10 O (1) [( tBu dmx)H 2 is a dimethylxanthene-bridged cofacial (bis)dipyrrin, C 49 H 58 N 4 O] was synthesized by salt metathesis using anhydrous CrCl 2 and previously reported ( tBu dmx)K 2 . Treatment of 1 with two equivalents of the reductant potassium graphite afforded K 2 ( tBu dmx)Cr 2 Cl 2 (thf) 3 ·0.5C 4 H 10 O·0.5C 4 H 8 O (thf is tetrahydrofuran, C 4 H 8 O) (2), with both potassium ions intercalated between the pyrrolic subunits. Comparison of the solid-state structures for 1 and 2 reveals minimal changes in the primary coordination sphere of each Cr ion, with notable elongation of the dipyrrin C-C and C-N bonds upon reduction, consistent with computational support for a ligand-based reduction.

The trityl-substituted bisoxazoline ( TrH BOX) was prepared as a chiral analogue to a previously reported nickel dipyrrin system capable of ring-closing amination catalysis. Ligand metalation with divalent NiI 2 (py) 4 followed by potassium graphite reduction afforded the monovalent ( TrH BOX)Ni(py) ( 4 ). Slow addition of 1.4 equiv of a benzene solution of 1-adamantylazide to 4 generated the tetrazido ( TrH BOX)Ni(κ 2 -N 4 Ad 2 ) ( 5 ) and terminal iminyl adduct ( TrH BOX)Ni(NAd) ( 6 ). Investigation of 6 via single-crystal X-ray crystallography, NMR and EPR spectroscopies, and computations revealed a Ni(II)-iminyl radical formulation, similar to its dipyrrinato congener. Complex 4 exhibits enantioselective intramolecular C-H bond amination to afford N -heterocyclic products from 4-aryl-2-methyl-2-azidopentanes. Catalytic C-H amination occurs under mild conditions (5 mol % catalyst, 60 °C) and provides pyrrolidine products in decent yield (29%-87%) with moderate ee (up to 73%). Substrates with a 3,5-dialkyl substitution on the 4-aryl position maximized the observed enantioselectivity. Kinetic studies to probe the reaction mechanism were conducted using 1 H and 19 F NMR spectroscopies. A small, intermolecular kinetic isotope effect (1.35 ± 0.03) suggests an H-atom abstraction step with an asymmetric transition state while the reaction rate is measured to be first order in catalyst and zeroth order in substrate concentrations. Enantiospecific deuterium labeling studies show that the enantioselectivity is dictated by both the H-atom abstraction and radical recombination steps due to the comparable rate between radical rotation and C-N bond formation. Furthermore, the competing elements of the two-step reaction where H-removal from the pro -R configuration is preferred while the preferential radical capture occurs with the Si face of the carboradical likely lead to the diminished ee observed, as corroborated by theoretical calculations. Based on these enantio-determining steps, catalytic enantioselective synthesis of 2,5-bis-tertiary pyrrolidines is demonstrated with good yield (50-78%) and moderate ee (up to 79%).

Treatment of ( Ar L)CoBr ( Ar L = 5-mesityl-1,9-(2,4,6-Ph 3 C 6 H 2 )dipyrrin) with a stoichiometric amount of 1-azido-4-( tert -butyl)benzene N 3 (C 6 H 4 - p - t Bu) furnished the corresponding four-coordinate organoazide-bound complex ( Ar L)CoBr(N 3 (C 6 H 4 - p - t Bu)). Spectroscopic and structural characterization of the complex indicated redox innocent ligation of the organoazide. Slow expulsion of dinitrogen (N 2 ) was observed at room temperature to afford a ligand functionalized product via a [3 + 2] annulation, which can be mediated by a high-valent nitrene intermediate such as a Co III iminyl ( Ar L)CoBr( • N(C 6 H 4 - p - t Bu)) or Co IV imido ( Ar L)CoBr(N(C 6 H 4 - p - t Bu)) complex. The presence of the proposed intermediate and its viability as a nitrene group transfer reagent are supported by intermolecular C-H amination and aziridination reactivities. Unlike ( Ar L)CoBr(N 3 (C 6 H 4 - p - t Bu)), a series of alkyl azide-bound Co II analogues expel N 2 only above 60 °C, affording paramagnetic intermediates that convert to the corresponding Co-imine complexes via α-H-atom abstraction. The corresponding N 2 -released structures were observed via single-crystal-to-crystal transformation, suggesting formation of a Co-nitrenoid intermediate in solid-state. Alternatively, the alkyl azide-bound congeners supported by a more sterically accessible dipyrrinato scaffold t Bu L ( t Bu L = 5-mesityl-(1,9-di- tert -butyl)dipyrrin) facilitate intramolecular 1,3-dipolar cycloaddition as well as C-H amination to furnish 1,2,3-dihydrotriazole and substituted pyrrolidine products, respectively. For the C-H amination, we observe that the temperature required for azide activation varies depending on the presence of weak C-H bonds, suggesting that the alkyl azide adducts serve as viable species for C-H amination when the C-H bonds are (1) proximal to the azide moiety and (2) sufficiently weak to be activated.

A dipyrrin-supported nickel catalyst ( AdF L)Ni(py) ( AdF L: 1,9-di(1-adamantyl)-5-perfluorophenyldipyrrin; py: pyridine) displays productive intramolecular C-H bond amination to afford N-heterocyclic products using aliphatic azide substrates. The catalytic amination conditions are mild, requiring 0.1-2 mol% catalyst loading and operational at room temperature. The scope of C-H bond substrates was explored and benzylic, tertiary, secondary, and primary C-H bonds are successfully aminated. The amination chemoselectivity was examined using substrates featuring multiple activatable C-H bonds. Uniformly, the catalyst showcases high chemoselectivity favoring C-H bonds with lower bond dissociation energy as well as a wide range of functional group tolerance (e.g., ethers, halides, thioetheres, esters, etc.). Sequential cyclization of substrates with ester groups could be achieved, providing facile preparation of an indolizidine framework commonly found in a variety of alkaloids. The amination cyclization reaction mechanism was examined employing nuclear magnetic resonance (NMR) spectroscopy to determine the reaction kinetic profile. A large, primary intermolecular kinetic isotope effect (KIE = 31.9 ± 1.0) suggests H-atom abstraction (HAA) is the rate-determining step, indicative of H-atom tunneling being operative. The reaction rate has first order dependence in the catalyst and zeroth order in substrate, consistent with the resting state of the catalyst as the corresponding nickel iminyl radical. The presence of the nickel iminyl was determined by multinuclear NMR spectroscopy observed during catalysis. The activation parameters (Δ H‡ = 13.4 ± 0.5 kcal/mol; Δ S‡ = -24.3 ± 1.7 cal/mol·K) were measured using Eyring analysis, implying a highly ordered transition state during the HAA step. The proposed mechanism of rapid iminyl formation, rate-determining HAA, and subsequent radical recombination was corroborated by intramolecular isotope labeling experiments and theoretical calculations.

The effect of metal identity, d-electron count, and coordination geometry on the electronic structure of a metal-ligand multiple bond (MLMB) is an area of active exploration. Although high oxidation state Cr imidos have been extensively studied, very few reports on low-valent Cr imidos or the interconversion of redox isomers exist. Herein, we report the synthesis and characterization of a family of dipyrrinato Cr imido complexes in oxidation states ranging from CrIII to CrV, showcasing the influence of the weak-field dipyrromethene scaffold on the electronic structure and coordination geometries of these Cr imides.

Dicopper complexes templated by dinucleating, pacman dipyrrin ligand scaffolds ( Mes dmx, t Bu dmx: dimethylxanthine-bridged, cofacial bis-dipyrrin) were synthesized by deprotonation/metalation with mesitylcopper (CuMes; Mes: mesityl) or by transmetalation with cuprous precursors from the corresponding deprotonated ligand. Neutral imide complexes ( R dmx)Cu 2 (μ 2 -NAr) (R: Mes, t Bu; Ar: 4-MeOC 6 H 4 , 3,5-(F 3 C) 2 C 6 H 3 ) were synthesized by treatment of the corresponding dicuprous complexes with aryl azides. While one-electron reduction of ( Mes dmx)Cu 2 (μ 2 -N(C 6 H 4 OMe)) with potassium graphite initiates an intramolecular, benzylic C-H amination at room temperature, chemical reduction of ( t Bu dmx)Cu 2 (μ 2 -NAr) leads to isolable [( t Bu dmx)Cu 2 (μ 2 -NAr)] - product salts. The electronic structures of the thermally robust [( t Bu dmx)Cu 2 (μ 2 -NAr)] 0/- complexes were assessed by variable-temperature electron paramagnetic resonance spectroscopy, X-ray absorption spectroscopy (Cu L 2,3 /K-edge, N K-edge), optical spectroscopy, and DFT/CASSCF calculations. These data indicate that the formally Class IIIA mixed valence complexes of the type [( R dmx)Cu 2 (μ 2 -NAr)] -  feature significant NAr-localized spin following reduction from electronic population of the [Cu 2 (μ 2 -NAr)] π* manifold, contrasting previous methods for engendering iminyl character through chemical oxidation. The reactivity of the isolable imido and iminyl complexes are examined for prototypical radical-promoted reactivity (e.g., nitrene transfer and H-atom abstraction), where the divergent reactivity is rationalized by the relative degree of N-radical character afforded from different aryl substituents.

Metalation of the deprotonated dipyrrin ( AdF L)Li with NiCl 2 (py) 2 afforded the divalent Ni product ( AdF L)NiCl(py) 2 ( 1 ) ( AdF L: 1,9-di(1-adamantyl)-5-perfluorophenyldipyrrin; py: pyridine). To generate a reactive synthon on which to explore oxidative group transfer, we used potassium graphite to reduce 1 , affording the monovalent Ni synthon ( AdF L)Ni(py) ( 2 ) and concomitant production of a stoichiometric equivalent of KCl and pyridine. Slow addition of mesityl- or 1-adamantylazide in benzene to 2 afforded the oxidized Ni complexes ( AdF L)Ni(NMes) ( 3 ) and ( AdF L)Ni(NAd) ( 4 ), respectively. Both 3 and 4 were characterized by multinuclear NMR, EPR, magnetometry, single-crystal X-ray crystallography, theoretical calculations, and X-ray absorption spectroscopies to provide a detailed electronic structure picture of the nitrenoid adducts. X-ray absorption near edge spectroscopy (XANES) on the Ni reveals higher energy Ni 1s → 3d transitions ( 3 : 8333.2 eV; 4 : 8333.4 eV) than Ni I or unambiguous Ni II analogues. N K-edge X-ray absorption spectroscopy performed on 3 and 4 reveals a common low-energy absorption present only for 3 and 4 (395.4 eV) that was assigned via TDDFT as an N 1s promotion into a predominantly N-localized, singly occupied orbital, akin to metal-supported iminyl complexes reported for iron. On the continuum of imido ( i.e. , NR 2- ) to iminyl ( i.e. , 2 NR - ) formulations, the complexes are best described as Ni II -bound iminyl species given the N K-edge and TDDFT results. Given the open-shell configuration ( S = 1/2) of the iminyl adducts, we then examined their propensity to undergo nitrenoid-group transfer to organic substrates. The adamantyl complex 4 readily consumes 1,4-cyclohexadiene (CHD) via H-atom abstraction to afford the amide ( AdF L)Ni(NHAd) ( 5 ), whereas no reaction was observed upon treatment of the mesityl variant 3 with excess amount of CHD over 3 hours. Toluene can be functionalized by 4 at room temperature, exclusively affording the N -1-adamantyl-benzylidene ( 6 ). Slow addition of the organoazide substrate (4-azidobutyl)benzene ( 7 ) with 2 exclusively forms 4-phenylbutanenitrile ( 8 ) as opposed to an intramolecular cyclized pyrrolidine, resulting from facile β-H elimination outcompeting H-atom abstraction from the benzylic position, followed by rapid H 2 -elimination from the intermediate Ni hydride ketimide intermediate., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Seventeen Cu complexes with formal oxidation states ranging from Cu I to Cu III are investigated through the use of multiedge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations. Analysis reveals that the metal-ligand bonding in high-valent, formally Cu III species is extremely covalent, resulting in Cu K-edge and L 2,3 -edge spectra whose features have energies that complicate physical oxidation state assignment. Covalency analysis of the Cu L 2,3 -edge data reveals that all formally Cu III species have significantly diminished Cu d-character in their lowest unoccupied molecular orbitals (LUMOs). DFT calculations provide further validation of the orbital composition analysis, and excellent agreement is found between the calculated and experimental results. The finding that Cu has limited capacity to be oxidized necessitates localization of electron hole character on the supporting ligands; consequently, the physical d 8 description for these formally Cu III species is inaccurate. This study provides an alternative explanation for the competence of formally Cu III species in transformations that are traditionally described as metal-centered, 2-electron Cu I /Cu III redox processes.

We report the catalytic C-H amination mediated by an isolable Co III imido complex ( Tr L)Co(NR) supported by a sterically demanding dipyrromethene ligand ( Tr L = 5-mesityl-1,9-(trityl)dipyrrin). Metalation of ( Tr L)Li with CoCl 2 in THF afforded a high-spin ( S = 3/2) three-coordinate complex ( Tr L)CoCl. Chemical reduction of ( Tr L)CoCl with potassium graphite yielded the high-spin ( S = 1) Co I synthon ( Tr L)Co which is stabilized through an intramolecular η 6 -arene interaction. Treatment of ( Tr L)Co with a stoichiometric amount of 1-azidoadamantane (AdN 3 ) furnished a three-coordinate, diamagnetic Co III imide ( Tr L)Co(NAd) as confirmed by single-crystal X-ray diffraction, revealing a rare trigonal pyramidal geometry with an acute Co-N imido -C angle 145.0(3)°. Exposure of 1-10 mol % of ( Tr L)Co to linear alkyl azides (RN 3 ) resulted in catalytic formation of substituted N -heterocycles via intramolecular C-H amination of a range of C-H bonds, including primary C-H bonds. The mechanism of the C-N bond formation was probed via initial rate kinetic analysis and kinetic isotope effect experiments [ k H / k D = 38.4(1)], suggesting a stepwise H-atom abstraction followed by radical recombination. In contrast to the previously reported C-H amination mediated by ( Ar L)Co(NR) ( Ar L = 5-mesityl-1,9-(2,4,6-Ph 3 C 6 H 2 )dipyrrin), ( Tr L)Co(NR) displays enhanced yields and rates of C-H amination without the aid of a cocatalyst, and no catalyst degradation to a tetrazene species was observed, as further supported by the pyridine inhibition effect on the rate of C-H amination. Furthermore, ( Tr L)Co(NAd) exhibits an extremely low one-electron reduction potential ( E ° red = -1.98 V vs [Cp 2 Fe] +/0 ) indicating that the highly basic terminal imido unit contributes to the driving force for H-atom abstraction.

Terminal copper-nitrenoid complexes have inspired interest in their fundamental bonding structures as well as their putative intermediacy in catalytic nitrene-transfer reactions. Here, we report that aryl azides react with a copper(I) dinitrogen complex bearing a sterically encumbered dipyrrin ligand to produce terminal copper nitrene complexes with near-linear, short copper-nitrenoid bonds [1.745(2) to 1.759(2) angstroms]. X-ray absorption spectroscopy and quantum chemistry calculations reveal a predominantly triplet nitrene adduct bound to copper(I), as opposed to copper(II) or copper(III) assignments, indicating the absence of a copper-nitrogen multiple-bond character. Employing electron-deficient aryl azides renders the copper nitrene species competent for alkane amination and alkene aziridination, lending further credence to the intermediacy of this species in proposed nitrene-transfer mechanisms., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

In this report we examine a family of trinuclear iron complexes by multiple-wavelength, anomalous diffraction (MAD) to explore the redox load distribution within cluster materials by the free refinement of atomic scattering factors. Several effects were explored that can impact atomic scattering factors within clusters, including 1) metal atom primary coordination sphere, 2) M-M bonding, and 3) redox delocalization in formally mixed-valent species. Complexes were investigated which vary from highly symmetric to fully asymmetric by 57 Fe Mössbauer and X-ray diffraction to explore the relationship between MAD-derived data and the data available from these widely used characterization techniques. The compounds examined include the all-ferrous clusters [ n Bu 4 N][( tbs L)Fe 3 (μ 3 -Cl)] (1) ([ tbs L] 6- = [1,3,5-C 6 H 9 (NC 6 H 4 - o -NSi t BuMe 2 ) 3 ] 6- ]), ( tbs L)Fe 3 (py) (2), [K(C 222 )] 2 [( tbs L)Fe 3 (μ 3 -NPh)] (4) (C 222 = 2,2,2-cryptand), and the mixed-valent ( tbs L)Fe 3 (μ 3 -NPh) (3). Redox delocalization in mixed-valent 3 was explored with cyclic voltammetry (CV), zero-field 57 Fe Mössbauer, near-infrared (NIR) spectroscopy, and X-ray crystallography techniques. We find that the MAD results show an excellent correspondence to 57 Fe Mössbauer data; yet also can distinguish between subtle changes in local coordination geometries where Mössbauer cannot. Differences within aggregate oxidation levels are evident by systematic shifts of scattering factor envelopes to increasingly higher energies. However, distinguishing local oxidation levels in iso- or mixed-valent materials can be dramatically obscured by the degree of covalent intracore bonding. MAD-derived atomic scattering factor data emphasize in-edge features that are often difficult to analyze by X-ray absorption near edge spectroscopy (XANES). Thus, relative oxidation levels within the cluster were most reliably ascertained from comparing the entire envelope of the atomic scattering factor data., Competing Interests: The authors declare no conflict of interest.

Reduction of ( Ar L)Co II Br ( Ar L = 5-mesityl-1,9-(2,4,6-Ph 3 C 6 H 2 )dipyrrin) with potassium graphite afforded the novel Co I synthon ( Ar L)Co I . Treatment of ( Ar L)Co I with a stoichiometric amount of various alkyl azides (N 3 R) furnished three-coordinate Co III alkyl imidos ( Ar L)Co(NR), as confirmed by single-crystal X-ray diffraction (R: CMe 2 Bu, CMe 2 (CH 2 ) 2 CHMe 2 ). The exclusive formation of four-coordinate cobalt tetrazido complexes ( Ar L)Co(κ 2 -N 4 R 2 ) was observed upon addition of excess azide, inhibiting any subsequent C-H amination. However, when a weak C-H bond is appended to the imido moiety, as in the case of (4-azido-4-methylpentyl)benzene, intramolecular C-H amination kinetically outcompetes formation of the corresponding tetrazene species to generate 2,2-dimethyl-5-phenylpyrrolidine in a catalytic fashion without requiring product sequestration. The imido ( Ar L)Co(NAd) exists in equilibrium in the presence of pyridine with a four-coordinate cobalt imido ( Ar L)Co(NAd)(py) ( K a = 8.04 M -1 ), as determined by 1 H NMR titration experiments. Kinetic studies revealed that pyridine binding slows down the formation of the tetrazido complex by blocking azide coordination to the Co III imido. Further, ( Ar L)Co(NR)(py) displays enhanced C-H amination reactivity compared to that of the pyridine-free complex, enabling higher catalytic turnover numbers under milder conditions. The mechanism of C-H amination was probed via kinetic isotope effect experiments [ k H / k D = 10.2(9)] and initial rate analysis with para-substituted azides, suggesting a two-step radical pathway. Lastly, the enhanced reactivity of ( Ar L)Co(NR)(py) can be correlated to a higher spin-state population, resulting in a decreased crystal field due to a geometry change upon pyridine coordination.

Concomitant deprotonation and metalation of a dinucleating cofacial Pacman dipyrrin ligand platform t Bu dmxH 2 with Fe 2 (Mes) 4 results in formation of a diiron complex ( t Bu dmx)Fe 2 (Mes) 2 . Treatment of ( t Bu dmx)Fe 2 (Mes) 2 with one equivalent of water yields the diiron μ-oxo complex ( t Bu dmx)Fe 2 (μ-O) and free mesitylene. A two-electron oxidation of ( t Bu dmx)Fe 2 (μ-O) gives rise to the diferric complex ( t Bu dmx)Fe 2 (μ-O)Cl 2 , and one-electron reduction from this Fe III Fe III state allows for isolation of a mixed-valent species [Cp 2 Co][( t Bu dmx)Fe 2 (μ-O)Cl 2 ]. Both ( t Bu dmx)Fe 2 (μ-O) and [Cp 2 Co][( t Bu dmx)Fe 2 (μ-O)Cl 2 ] exhibit basic character at the bridging oxygen atom and can be protonated using weak acids to form bridging diferrous hydroxide species. The basicity of the diferrous oxo ( t Bu dmx)Fe 2 (μ-O) is quantified through studies of the p K a of its conjugate acid, [( t Bu dmx)Fe 2 (μ-OH)] + , which is determined to be 15.3(6); interestingly, upon coordination of neutral solvent ligands to yield ( t Bu dmx)Fe 2 (μ-O)(thf) 2 , the basicity is increased as observed through an increase in the p K a of the conjugate acid [( t Bu dmx)Fe 2 (μ-OH)(thf) 2 ] + to 26.8(6). In contrast, attempts to synthesize a diferric bridging hydroxide by two-electron oxidation of [( t Bu dmx)Fe 2 (μ-OH)(thf) 2 ] + resulted in isolation of ( t Bu dmx)Fe 2 (μ-O)Cl 2 with concomitant loss of a proton, consistent with the p K a of the conjugate acid [( t Bu dmx)Fe 2 (μ-OH)Cl 2 ] + determined computationally to be -1.8(6). The foregoing results highlight the intricate interplay between oxidation state and reactivity in diiron μ-oxo units.

The trichromium cluster ( tbs L)Cr 3 (thf) ([ tbs L] 6- =[1,3,5-C 6 H 9 (NC 6 H 4 -o-NSi t BuMe 2 ) 3 ] 6- ) exhibits steric- and solvation-controlled reactivity with organic azides to form three distinct products: reaction of ( tbs L)Cr 3 (thf) with benzyl azide forms a symmetrized bridging imido complex ( tbs L)Cr 3 (μ 3 -NBn); reaction with mesityl azide in benzene affords a terminally bound imido complex ( tbs L)Cr 3 (μ 1 -NMes); whereas the reaction with mesityl azide in THF leads to terminal N-atom excision from the azide to yield the nitride complex ( tbs L)Cr 3 (μ 3 -N). The reactivity of this complex demonstrates the ability of the cluster-templating ligand to produce a well-defined polynuclear transition metal cluster that can access distinct single-site and cooperative reactivity controlled by either substrate steric demands or reaction media., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Chemical oxidation and reduction of the all-ferrous ( H L) 2 Fe 6 in THF affords isostructural, coordinatively unsaturated clusters of the type [( H L) 2 Fe 6 ] n : [( H L) 2 Fe 6 ][BArF 24 ] (1, n = +1; where [BArF 24 ] - = tetrakis[(3,5-trifluoromethyl)phenyl]borate), [Bu 4 N][( H L) 2 Fe 6 ] (2a, n = -1), [P][( H L) 2 Fe 6 ] (2b, n = -1; where [P] + = tributyl(1,3-dioxolan-2-ylmethyl)phosphonium), and [Bu 4 N] 2 [( H L) 2 Fe 6 ] (3, n = -2). Each member of the redox-transfer series was characterized by zero-field 57 Fe Mössbauer spectroscopy, near-infrared spectroscopy, single-crystal X-ray crystallography, and magnetometry. Redox-directed trends are observed when comparing the structural metrics within the [Fe 6 ] core. The metal octahedron [Fe 6 ] decreases marginally in volume as the molecular reduction state increases as gauged by the Fe-Fe avg distance varying from 2.608(11) Å ( n = +1) to 2.573(3) ( n = -2). In contrast, the mean Fe-N distances and ∠Fe-N-Fe angles correlate linearly with the [Fe 6 ] oxidation level, or alternatively, the changes observed within the local Fe-N 4 coordination planes vary linearly with the aggregate spin ground state. In general, as the spin ground state ( S) increases, the Fe-N(H) avg distances also increase. The structural metric perturbations within the [Fe 6 ] core and measured spin ground states were rationalized extending the previously proposed molecular orbital diagram derived for ( H L) 2 Fe 6 . Chemical reduction of the ( H L) 2 Fe 6 cluster results in an abrupt increase in spin ground state from S = 6 for the all-ferrous cluster, to S = 19 / 2 in the monoanionic 2b and S = 11 for the dianionic 3. The observation of asymmetric intervalence charge transfer bands in 3 provides further evidence of the fully delocalized ground state observed by 57 Fe Mössbauer spectroscopy for all species examined (1-3). For each of the clusters examined within the electron-transfer series, the observed spin ground states thermally persist to 300 K. In particular, the S = 11 in dianionic 3 and S = 19 / 2 in the monoanionic 2b represent the highest spin ground states isolated up to room temperature known to date. The increase in spin ground state results from population of the antibonding orbital band comprised of the Fe-N σ* interactions. As such, the thermally persistent ground states arise from population of the resultant single spin manifolds in accordance with Hund's rules. The large spin ground states, indicative of strong ferromagnetic electronic alignment of the valence electrons, result from strong direct exchange electronic coupling mediated by Fe-Fe orbital overlap within the [Fe 6 ] cores, equivalent to a strong double exchange magnetic coupling B for 3 that was calculated to be 309 cm -1 .

We report herein the improved diastereoselective synthesis of 2,5-disubstituted pyrrolidines from aliphatic azides. Experimental and theoretical studies of the C-H amination reaction mediated by the iron dipyrrinato complex ( Ad L)FeCl(OEt 2 ) provided a model for diastereoinduction and allowed for systematic variation of the catalyst to enhance selectivity. Among the iron alkoxide and aryloxide catalysts evaluated, the iron phenoxide complex exhibited superior performance towards the generation of syn 2,5-disubstituted pyrrolidines with high diastereoselectivity., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Reduction of previously reported iminyl radical ( Ar L)FeCl( • N(C 6 H 4 -p- t Bu)) (2) with potassium graphite furnished the corresponding high-spin (S = 5 / 2 ) imido ( Ar L)Fe(N(C 6 H 4 -p- t Bu)) (3) ( Ar L = 5-mesityl-1,9-(2,4,6-Ph 3 C 6 H 2 )dipyrrin). Oxidation of the three-coordinate imido ( Ar L)Fe(NAd) (5) with chlorotriphenylmethane afforded ( Ar L)FeCl( • NAd) (6) with concomitant expulsion of Ph 3 C(C 6 H 5 )CPh 2 . The respective aryl/alkyl imido/iminyl pairs (3, 2; 5, 6) have been characterized by EPR, zero-field 57 Fe Mössbauer, magnetometry, single crystal X-ray diffraction, XAS, and EXAFS for 6. The high-spin (S = 5 / 2 ) imidos exhibit characteristically short Fe-N bonds (3: 1.708(4) Å; 5: 1.674(11) Å), whereas the corresponding iminyls exhibit elongated Fe-N bonds (2: 1.768(2) Å; 6: 1.761(6) Å). Comparison of the pre-edge absorption feature (1s → 3d) in the X-ray absorption spectra reveals that the four imido/iminyl complexes share a common iron oxidation level consistent with a ferric formulation (3: 7111.5 eV, 2: 7111.5 eV; 5: 7112.2 eV, 6: 7112.4 eV) as compared with a ferrous amine adduct ( Ar L)FeCl(NH 2 Ad) (7: 7110.3 eV). N K-edge X-ray absorption spectra reveal a common low-energy absorption present only for the iminyl species 2 (394.5 eV) and 6 (394.8 eV) that was assigned as a N 1s promotion into a N-localized, singly occupied iminyl orbital. Kinetic analysis of the reaction between the respective iron imido and iminyl complexes with toluene yielded the following activation parameters: E a (kcal/mol) 3: 12.1, 2: 9.2; 5: 11.5, 6: 7.1. The attenuation of the Fe-N bond interaction on oxidation from an imido to an iminyl complex leads to a reduced enthalpic barrier [Δ(ΔH ‡ ) ≈ 5 kcal/mol]; the alkyl iminyl 6 has a reduced enthalpic barrier (1.84 kcal/mol) as compared with the aryl iminyl 2 (3.84 kcal/mol), consistent with iminyl radical delocalization into the aryl substituent in 2 as compared with 6.

The reaction of nitroxyl radicals TEMPO (2,2',6,6'-tetramethylpiperidinyloxyl) and AZADO (2-azaadamantane-N-oxyl) with an iron(I) synthon affords iron(II)-nitroxido complexes ( Ar L)Fe(κ 1 -TEMPO) and ( Ar L)Fe(κ 2 -N,O-AZADO) ( Ar L=1,9-(2,4,6-Ph 3 C 6 H 2 ) 2 -5-mesityldipyrromethene). Both high-spin iron(II)-nitroxido species are stable in the absence of weak C-H bonds, but decay via N-O bond homolysis to ferrous or ferric iron hydroxides in the presence of 1,4-cyclohexadiene. Whereas ( Ar L)Fe(κ 1 -TEMPO) reacts to give a diferrous hydroxide [( Ar L)Fe] 2 (μ-OH) 2 , the reaction of four-coordinate ( Ar L)Fe(κ 2 -N,O-AZADO) with hydrogen atom donors yields ferric hydroxide ( Ar L)Fe(OH)(AZAD). Mechanistic experiments reveal saturation behavior in C-H substrate and are consistent with rate-determining hydrogen atom transfer., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We report the isolation of a room temperature stable dipyrromethene Cu(O 2 ) complex featuring a side-on O 2 coordination. Reactivity studies highlight the unique ability of the dioxygen adduct for both hydrogen-atom abstraction and acid/base chemistry towards phenols, demonstrating that side-on superoxide species can be reactive entities.

Reduction of previously reported ( Ar L)FeCl with potassium graphite furnished a low-spin (S = 1/2) iron complex ( Ar L)Fe which features an intramolecular η 6 -arene interaction and can be utilized as an Fe I synthon ( Ar L = 5-mesityl-1,9-(2,4,6-Ph 3 C 6 H 2 )dipyrrin). Treatment of ( Ar L)Fe with adamantyl azide or mesityl azide led to the formation of the high-spin (S = 5/2), three-coordinate imidos ( Ar L)Fe(NAd) and ( Ar L)Fe(NMes), respectively, as determined by EPR, zero-field 57 Fe Mössbauer, magnetometry, and single crystal X-ray diffraction. The high-spin iron imidos are reactive with a variety of substrates: ( Ar L)Fe(NAd) reacts with azide yielding a ferrous tetrazido ( Ar L)Fe(κ 2 -N 4 Ad 2 ), undergoes intermolecular nitrene transfer to phosphine, abstracts H atoms from weak C-H bonds (1,4-cyclohexadiene, 2,4,6- t Bu 3 C 6 H 2 OH) to afford ferrous amido product ( Ar L)Fe(NHAd), and can mediate intermolecular C-H amination of toluene [PhCH 3 /PhCD 3 k H /k D : 15.5(3); PhCH 2 D k H /k D : 11(1)]. The C-H bond functionalization reactivity is rationalized from a two-step mechanism wherein each step occurs via maximal energy and orbital overlap between the imido fragment and the C-H bond containing substrate.

Three ferric dipyrromethene complexes featuring different ancillary ligands were synthesized by one electron oxidation of ferrous precursors. Four-coordinate iron complexes of the type ( Ar L)FeX 2 [ Ar L = 1,9-(2,4,6-Ph 3 C 6 H 2 ) 2 -5-mesityldipyrromethene] with X = Cl or t BuO were prepared and found to be high-spin (S = 5 / 2 ), as determined by superconducting quantum interference device magnetometry, electron paramagnetic resonance, and 57 Fe Mössbauer spectroscopy. The ancillary ligand substitution was found to affect both ground state and excited properties of the ferric complexes examined. While each ferric complex displays reversible reduction and oxidation events, each alkoxide for chloride substitution results in a nearly 600 mV cathodic shift of the Fe III/II couple. The oxidation event remains largely unaffected by the ancillary ligand substitution and is likely dipyrrin-centered. While the alkoxide substituted ferric species largely retain the color of their ferrous precursors, characteristic of dipyrrin-based ligand-to-ligand charge transfer (LLCT), the dichloride ferric complex loses the prominent dipyrrin chromophore, taking on a deep green color. Time-dependent density functional theory analyses indicate the weaker-field chloride ligands allow substantial configuration mixing of ligand-to-metal charge transfer into the LLCT bands, giving rise to the color changes observed. Furthermore, the higher degree of covalency between the alkoxide ferric centers is manifest in the observed reactivity. Delocalization of spin density onto the tert-butoxide ligand in ( Ar L)FeCl(O t Bu) is evidenced by hydrogen atom abstraction to yield ( Ar L)FeCl and HO t Bu in the presence of substrates containing weak C-H bonds, whereas the chloride ( Ar L)FeCl 2 analogue does not react under these conditions.

The one-electron reduction of ((tbs)L)Fe₃(thf)¹ furnishes [M][((tbs)L)Fe₃] ([M]⁺ = [(18-C-6)K(thf)₂]⁺ (1, 76%) or [(crypt-222)K]⁺ (2, 54%)). Upon reduction, the ligand (tbs)L⁶⁻ rearranges around the triiron core to adopt an almost ideal C₃-symmetry. Accompanying the ((tbs)L) ligand rearrangement, the THF bound to the neutral starting material is expelled, and the Fe-Fe distances within the trinuclear cluster contract by ∼0.13 Å in 1. Variable-temperature magnetic susceptibility data indicates a well-isolated S = 11/2 spin ground state that persists to room temperature. Slow magnetic relaxation is observed at low temperature as evidenced by the out-of-phase (χ(M)″) component of the alternating current (ac) magnetic susceptibility data and by the appearance of hyperfine splitting in the zero-field ⁵⁷Fe Mössbauer spectra at 4.2 K. Analysis of the ac magnetic susceptibility yields an effective spin reversal barrier (U(eff)) of 22.6(2) cm⁻¹, nearly matching the theoretical barrier of 38.7 cm⁻¹ calculated from the axial zero-field splitting parameter (D = -1.29 cm⁻¹) extracted from the reduced magnetization data. A polycrystalline sample of 1 displays three sextets in the Mössbauer spectrum at 4.2 K (H(ext) = 0) which converge to a single six-line pattern in a frozen 2-MeTHF glass sample, indicating a unique iron environment and thus strong electron delocalization. The spin ground state and ligand rearrangement are discussed within the framework of a fully delocalized cluster exhibiting strong double and direct exchange interactions.

A sterically accessible tert-butyl-substituted dipyrrinato di-iron(II) complex [((tBu)L)FeCl]2 possessing two bridging chloride atoms was synthesized from the previously reported solvento adduct. Upon treatment with aryl azides, the formation of high-spin Fe(III) species was confirmed by (57)Fe Mössbauer spectroscopy. Crystallographic characterization revealed two possible oxidation products: (1) a terminal iron iminyl from aryl azides bearing ortho isopropyl substituents, ((tBu)L)FeCl((•)NC6H3-2,6-(i)Pr2); or (2) a bridging di-iron imido arising from reaction with 3,5-bis(trifluoromethyl)aryl azide, [((tBu)L)FeCl]2(μ-NC6H3-3,5-(CF3)2). Similar to the previously reported ((Ar)L)FeCl((•)NC6H4-4-(t)Bu), the monomeric iron imido is best described as a high-spin Fe(III) antiferromagnetically coupled to an iminyl radical, affording an S = 2 spin state as confirmed by SQUID magnetometry. The di-iron imido possesses an S = 0 ground state, arising from two high-spin Fe(III) centers weakly antiferromagnetically coupled through the bridging imido ligand. The terminal iron iminyl complex undergoes facile decomposition via intra- or intermolecular hydrogen-atom abstraction (HAA) from an imido aryl ortho isopropyl group, or from 1,4-cyclohexadiene, respectively. The bridging di-iron imido is a competent N-group transfer reagent to cyclic internal olefins as well as styrene. Although solid-state magnetometry indicates an antiferromagnetic interaction between the two iron centers (J = -108.7 cm(-1)) in [((tBu)L)FeCl]2(μ-NC6H3-3,5-(CF3)2), we demonstrate that in solution the bridging imido can facilitate HAA as well as dissociate into a terminal iminyl species, which then can promote HAA. In situ monitoring reveals the di-iron bridging imido is a catalytically competent intermediate, one of several iron complexes observed in the amination of C-H bond substrates or styrene aziridination.

The field of single molecule magnetism remains predicated on super- and double exchange mechanisms to engender large spin ground states. An alternative approach to achieving high-spin architectures involves synthesizing weak-field clusters featuring close M-M interactions to produce a single valence orbital manifold. Population of this orbital manifold in accordance with Hund's rules could potentially yield thermally persistent high-spin ground states under which the valence electrons remain coupled. We now demonstrate this effect with a reduced hexanuclear iron cluster that achieves an S = 19/2 (χ(M)T ≈ 53 cm(3) K/mol) ground state that persists to 300 K, representing the largest spin ground state persistent to room temperature reported to date. The reduced cluster displays single molecule magnet behavior manifest in both variable-temperature zero-field (57)Fe Mössbauer and magnetometry with a spin reversal barrier of 42.5(8) cm(-1) and a magnetic blocking temperature of 2.9 K (0.059 K/min).

The synthesis of polynuclear clusters with control over size and cluster geometry remains an unsolved challenge. Herein, we report the synthesis and characterization of open-shell octairon clusters supported by two heptaamine ligands [o-H2 NC6 H4 NH(CH2 )2 ]3 N ((tren) LH9 ). The crystal structure of the all-ferrous species ([(tren) L)2 Fe8 (PMe2 Ph)2 ] (1) displays a bicapped octahedral geometry with FeFe distances ranging from 2.4071(6) to 2.8236(5) Å, where the ligand amine units are formally in amine, amide, and imide oxidation states. Several redox states of the octairon cluster are accessible, as ascertained using cyclic voltammetry. The one-electron-reduced clusters [M](+) [((tren) L)2 Fe8 (PMe2 Ph)2 ](-) (M=Bu4 N (2 a); (15-crown-5)Na(thf) (2 b)) were isolated and characterized. Variable-temperature magnetic susceptibility data indicates that the exchange coupling within the [Fe8 ] core is antiferromagnetic which is attenuated upon reduction to the mixed valent anion., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

To assess the impact of terminal ligand binding on a variety of cluster properties (redox delocalization, ground-state stabilization, and breadth of redox state accessibility), we prepared three electron-transfer series based on the hexanuclear iron cluster [((H)L)2Fe6(L')m](n+) in which the terminal ligand field strength was modulated from weak to strong (L' = DMF, MeCN, CN). The extent of intracore M-M interactions is gauged by M-M distances, spin ground state persistence, and preference for mixed-valence states as determined by electrochemical comproportionation constants. Coordination of DMF to the [((H)L)2Fe6] core leads to weaker Fe-Fe interactions, as manifested by the observation of ground states populated only at lower temperatures (<100 K) and by the greater evidence of valence trapping within the mixed-valence states. Comproportionation constants determined electrochemically (Kc = 10(4)-10(8)) indicate that the redox series exhibits electronic delocalization (class II-III), yet no intervalence charge transfer (IVCT) bands are observable in the near-IR spectra. Ligation of the stronger σ donor acetonitrile results in stabilization of spin ground states to higher temperatures (∼300 K) and a high degree of valence delocalization (Kc = 10(2)-10(8)) with observable IVCT bands. Finally, the anionic cyanide-bound series reveals the highest degree of valence delocalization with the most intense IVCT bands (Kc = 10(12)-10(20)) and spin ground state population beyond room temperature. Across the series, at a given formal oxidation level, the capping ligand on the hexairon cluster dictates the overall properties of the aggregate, modulating the redox delocalization and the persistence of the intracore coupling of the metal sites.

Iron(III) complexes of the tris(pyrrolide)ethane trianion have been synthesized by reaction of one- and two-electron oxidants with [(tpe)Fe(THF)][Li(THF)4] (tpe = tris(5-mesitylpyrrolyl)ethane). X-ray crystallography, (57)Fe Mössbauer, (1)H NMR and EPR spectroscopy, SQUID magnetometry, and density functional theory calculations were employed to rigorously establish the iron 3+ oxidation state. All oxidants employed are proposed to operate via an inner-sphere electron transfer mechanism. Dialkyl peroxides and dibenzyldisulfide served to oxidize iron by one electron, and group transfer of an aryl nitrene unit to the Fe(2+) starting material resulted in formation of Fe(3+) amido species following H-atom abstraction by a presumed nitrenoid intermediate. Single electron transfer to and from diphenyldiazoalkane was also observed to yield a diphenyldiazomethanyl radical anion antiferromagnetically coupled to the S = 5/2 Fe(3+). Isolation of Fe(3+) complexes of tpe, in comparison with previous results wherein the tpe ligand was the redox active moiety, presents an unusual juxtaposition of two noncommunicating redox reservoirs, each accessible via different reaction pathways (namely, inner- and outer-sphere electron transfer).

Concomitant deprotonation and metalation of hexadentate ligand platform (tbs)LH6 ((tbs)LH6 = 1,3,5-C6H9(NHC6H4-o-NHSiMe2(t)Bu)3) with divalent transition metal starting materials Fe2(Mes)4 (Mes = mesityl) or Mn3(Mes)6 in the presence of tetrahydrofuran (THF) resulted in isolation of homotrinuclear complexes ((tbs)L)Fe3(THF) and ((tbs)L)Mn3(THF), respectively. In the absence of coordinating solvent (THF), the deprotonation and metalation exclusively afforded dinuclear complexes of the type ((tbs)LH2)M2 (M = Fe or Mn). The resulting dinuclear species were utilized as synthons to prepare bimetallic trinuclear clusters. Treatment of ((tbs)LH2)Fe2 complex with divalent Mn source (Mn2(N(SiMe3)2)4) afforded the bimetallic complex ((tbs)L)Fe2Mn(THF), which established the ability of hexamine ligand (tbs)LH6 to support mixed metal clusters. The substitutional homogeneity of ((tbs)L)Fe2Mn(THF) was determined by (1)H NMR, (57)Fe Mössbauer, and X-ray fluorescence. Anomalous scattering measurements were critical for the unambiguous assignment of the trinuclear core composition. Heating a solution of ((tbs)LH2)Mn2 with a stoichiometric amount of Fe2(Mes)4 (0.5 mol equiv) affords a mixture of both ((tbs)L)Mn2Fe(THF) and ((tbs)L)Fe2Mn(THF) as a result of the thermodynamic preference for heavier metal substitution within the hexa-anilido ligand framework. These results demonstrate for the first time the assembly of mixed metal cluster synthesis in an unbiased ligand platform.

High-spin trinuclear iron complex ((tbs)L)Fe3(thf) ([(tbs)L](6-) = [1,3,5-C6H9(NC6H4-o-NSi(t)BuMe2)3](6-)) (S = 6) facilitates 2 and 4e(-) reduction of NxHy type substrates to yield imido and nitrido products. Reaction of hydrazine or phenylhydrazine with ((tbs)L)Fe3(thf) yields triiron μ(3)-imido cluster ((tbs)L)Fe3(μ(3)-NH) and ammonia or aniline, respectively. ((tbs)L)Fe3(μ(3)-NH) has a similar zero-field (57)Fe Mössbauer spectrum compared to previously reported [((tbs)L)Fe3(μ(3)-N)]NBu4, and can be directly synthesized by protonation of the anionic triiron nitrido with lutidinium tetraphenylborate. Deprotonation of the triiron parent imido ((tbs)L)Fe3(μ(3)-NH) with lithium bis(trimethylsilyl)amide results in regeneration of the triiron nitrido complex capped with a thf-solvated Li cation [((tbs)L)Fe3(μ(3)-N)]Li(thf)3. The lithium capped nitrido, structurally similar to the pseudo C3-symmetric triiron nitride with a tetrabutylammonium countercation, is rigorously C3-symmetric featuring intracore distances of Fe-Fe 2.4802(5) Å, Fe-N(nitride) 1.877(2) Å, and N(nitride)-Li 1.990(6) Å. A similar 2e(-) reduction of 1,2-diphenylhydrazine by ((tbs)L)Fe3(thf) affords ((tbs)L)Fe3(μ(3)-NPh) and aniline. The solid state structure of ((tbs)L)Fe3(μ(3)-NPh) is similar to the series of μ(3)-nitrido and -imido triiron complexes synthesized in this work with average Fe-Nimido and Fe-Fe bond lengths of 1.941(6) and 2.530(1) Å, respectively. Reductive N═N bond cleavage of azobenzene is also achieved in the presence of ((tbs)L)Fe3(thf) to yield triiron bis-imido complex ((tbs)L)Fe3(μ(3)-NPh)(μ(2)-NPh), which has been structurally characterized. Ligand redox participation has been ruled out, and therefore, charge balance indicates that the bis-imido cluster has undergone a 4e(-) metal based oxidation resulting in an (Fe(IV))(Fe(III))2 formulation. Cyclic voltammograms of the series of triiron clusters presented herein demonstrate that oxidation states up to (Fe(IV))(Fe(III))2 (in the case of [((tbs)L)Fe3(μ(3)-N)]NBu4) are electrochemically accessible. These results highlight the efficacy of high-spin, polynuclear reaction sites to cooperatively mediate small molecule activation.

The asymmetric oxidation product [((Ph)L)Fe3(μ-Cl)]2 [(Ph)LH6 = MeC(CH2NHPh-o-NHPh)3], where each trinuclear core is comprised of an oxidized diiron unit [Fe2](5+) and an isolated trigonal pyramidal ferrous site, reacts with MCl2 salts to afford heptanuclear bridged structures of the type ((Ph)L)2Fe6M(μ-Cl)4(thf)2, where M = Fe or Co. Zero-field, (57)Fe Mössbauer analysis revealed the Co resides within the trinuclear core subunits, not at the octahedral, halide-bridged MCl4(thf)2 position indicating Co migration into the trinuclear subunits has occurred. Reaction of [((Ph)L)Fe3(μ-Cl)]2 with CoCl2 (2 or 5 equivalents) followed by precipitation via addition of acetonitrile afforded trinuclear products where one or two irons, respectively, can be substituted within the trinuclear core. Metal atom substitution was verified by (1)H NMR, (57)Fe Mossbauer, single crystal X-ray diffraction, X-ray fluorescence, and magnetometry analysis. Spectroscopic analysis revealed that the Co atom(s) substitute(s) into the oxidized dimetal unit ([M2](5+)), while the M(2+) site remains iron-substituted. Magnetic data acquired for the series are consistent with this analysis revealing the oxidized dimetal unit comprises a strongly coupled S = 1 unit ([FeCo](5+)) or S = 1/2 ([Co2](5+)) that is weakly antiferromagnetically coupled to the high spin (S = 2) ferrous site. The kinetic pathway for metal substitution was probed via reaction of [((Ph)L)Fe3(μ-Cl)]2 with isotopically enriched (57)FeCl2(thf)2, the results of which suggest rapid equilibration of (57)Fe into both the M(2+) site and oxidized diiron site, achieving a 1:1 mixture.

The manipulation of traditionally unreactive functional groups is of paramount importance in modern chemical synthesis. We have developed an iron-dipyrrinato catalyst that leverages the reactivity of iron-borne metal-ligand multiple bonds to promote the direct amination of aliphatic C-H bonds. Exposure of organic azides to the iron dipyrrinato catalyst furnishes saturated, cyclic amine products (N-heterocycles) bearing complex core-substitution patterns. This study highlights the development of C-H bond functionalization chemistry for the formation of saturated, cyclic amine products and should find broad application in the context of both pharmaceuticals and natural product synthesis.

The reaction of ((Ar)L)Co(py) with (t)BuN(3) afforded the isolable three-coordinate Co-imido complex ((Ar)L)Co(N(t)Bu), which is paramagnetic at room temperature. Variable-temperature (VT) (1)H NMR spectroscopy, VT crystallography, and magnetic susceptibility measurements revealed that ((Ar)L)Co(N(t)Bu) undergoes a thermally induced spin crossover from an S = 0 ground state to a quintet (S = 2) state. The reaction of ((Ar)L)Co(py) with mesityl azide yielded an isolable S = 1 terminal imido complex that was converted into the metallacycloindoline ((Ar)L)Co(κ(2)-NHC(6)H(2)-2,4-Me(2)-6-CH(2)) via benzylic C-H activation.

Transamination of divalent transition metal starting materials (M(2)(N(SiMe(3))(2))(4), M = Mn, Co) with hexadentate ligand platforms (R)LH(6) ((R)LH(6) = MeC(CH(2)NPh-o-NR)(3) where R = H, Ph, Mes (Mes = Mesityl)) or (H,Cy)LH(6) = 1,3,5-C(6)H(9)(NHPh-o-NH(2))(3) with added pyridine or tertiary phosphine coligands afforded trinuclear complexes of the type ((R)L)Mn(3)(py)(3) and ((R)L)Co(3)(PMe(2)R')(3) (R' = Me, Ph). While the sterically less encumbered ligand varieties, (H)L or (Ph)L, give rise to local square-pyramidal geometries at each of the bound metal atoms, with four anilides forming an equatorial plane and an exogenous pyridine or phosphine in the apical site, the mesityl-substituted ligand ((Mes)L) engenders local tetrahedral coordination. Both the neutral Mn(3) and Co(3) clusters feature S = (1)/(2) ground states, as determined by direct current (dc) magnetometry, (1)H NMR spectroscopy, and low-temperature electron paramagnetic resonance (EPR) spectroscopy. Within the Mn(3) clusters, the long internuclear Mn-Mn separations suggest minimal direct metal-metal orbital overlap. Accordingly, fits to variable-temperature magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange interactions through the bridging anilide ligands with exchange couplings ranging from J = -16.8 to -42 cm(-1). Conversely, the short Co-Co interatomic distances suggest a significant degree of direct metal-metal orbital overlap, akin to the related Fe(3) clusters. With the Co(3) series, the S = (1)/(2) ground state can be attributed to population of a single molecular orbital manifold that arises from mixing of the metal- and o-phenylenediamide (OPDA) ligand-based frontier orbitals. Chemical oxidation of the neutral Co(3) clusters affords diamagnetic cationic clusters of the type [((R)L)Co(3)(PMe(2)R)(3)](+). Density functional theory (DFT) calculations on the neutral (S = (1)/(2)) and cationic (S = 0) Co(3) clusters reveal that oxidation occurs at an orbital with contributions from both the Co3 core and OPDA subunits. The predicted bond elongations within the ligand OPDA units are corroborated by the ligand bond perturbations observed by X-ray crystallography.

The symmetric, high-spin triiron complex ((Ph)L)Fe(3)(THF)(3) reacts with mild chemical oxidants (e.g., Ph(3)C-X, I(2)) to afford an asymmetric core, where one iron bears the halide ligand ((Ph)L)Fe(3)X(L) and the hexadentate ((Ph)L = MeC(CH(2)NPh-o-NPh)(3)) ligand has undergone significant rearrangement. In the absence of a suitable trapping ligand, the chlorine and bromine complexes form (μ-X)(2)-bridged structures of the type [((Ph)L)Fe(3)(μ-X)](2). In the trinuclear complexes, the halide-bearing iron site sits in approximate trigonal-bipyramidal (tbp) geometry, formed by two ((Ph)L) anilides and an exogenous solvent molecule. The two distal iron atoms reside in distorted square-planar sites featuring a short Fe-Fe separation at 2.301 Å, whereas the distance to the tbp site is substantially elongated (2.6-2.7 Å). Zero-field, (57)Fe Mössbauer analysis reveals the diiron unit as the locus of oxidation, while the tbp site bearing the halide ligand remains divalent. Magnetic data acquired for the series reveal that the oxidized diiron unit comprises a strongly coupled S = (3)/(2) unit that is weakly ferromagnetically coupled to the high-spin (S = 2) ferrous site, giving an overall S = (7)/(2) ground state for the trinuclear units.

Utilizing a hexadentate ligand platform, a trinuclear manganese complex of the type ((H)L)Mn(3)(thf)(3) was synthesized and characterized ([(H)L](6-) = [MeC(CH(2)N(C(6)H(4)-o-NH))(3)](6-)). The pale-orange, formally divalent trimanganese complex rapidly reacts with O-atom transfer reagents to afford the μ(6)-oxo complex ((H)L)(2)Mn(6)(μ(6)-O)(NCMe)(4), where two trinuclear subunits bind the central O-atom and the ((H)L) ligands cooperatively bind both trinuclear subunits. The trimanganese complex ((H)L)Mn(3)(thf)(3) rapidly consumes inorganic azide ([N(3)]NBu(4)) to afford a dianionic hexanuclear nitride complex [((H)L)(2)Mn(6)(μ(6)-N)](NBu(4))(2), which subsequently can be oxidized with elemental iodine to ((H)L)(2)Mn(6)(μ(6)-N)(NCMe)(4). EPR and alkylation of the interstitial light atom substituent were used to distinguish the nitride from the oxo complex. The oxo and oxidized nitride complexes give rise to well-defined Mn(II) and Mn(III) sites, determined by bond valence summation, while the dianionic nitride shows a more symmetric complex, giving rise to indistinguishable ion oxidation states based on crystal structure bond metrics.

Reaction of NO(2)(-) with the octahedral cluster ((H)L)(2)Fe(6) in the presence of a proton source affords the hexanitrosyl cluster ((H)L)(2)Fe(6)(NO)(6). This species forms via a proton-induced reduction of six nitrite molecules per cluster, utilizing each site available on the polynuclear core. Formation of the hexanitrosyl cluster is accompanied by a near 2-fold expansion of the ((H)L)(2)Fe(6) core volume, where intracore Fe-Fe interactions are overcome by strong π-bonding between Fe centers and NO ligands. A core volume of this magnitude is rare in octahedral metal clusters not supported by interstitial atoms. Moreover, the structural flexibility afforded by the ((H)L)(2)Fe(6) platform highlights the potential for other reaction chemistry involving species with metal-ligand multiple bonds. Carrying out the reaction of the cluster [((H)L)(2)Fe(6)(NCMe)(6)](4+) with nitrite in the absence of a proton source serves to forestall the nitrite reduction and enables clean isolation of the intermediate hexanitro cluster [((H)L)(2)Fe(6)(NO(2))(6)](2-).

Systematic electronic variations were introduced into the monoanionic dipyrrinato ligand scaffold via halogenation of the pyrrolic β-positions and/or via the use of fluorinated aryl substituents in the ligand bridgehead position in order to synthesize proligands of the type 1,9-dimesityl-β-R(4)-5-Ar-dipyrrin [R = H, Cl, Br, I; Ar = mesityl, 3,5-(F(3)C)(2)C(6)H(3), C(6)F(5) in ligand 5-position; β = 2,3,7,8 ligand substitution; abbreviated ((β,Ar)L)H]. The electronic perturbations were probed using standard electronic absorption and electrochemical techniques on the different ligand variations and their divalent iron complexes. The free-ligand variations cause modest shifts in the electronic absorption maxima (λ(max): 464-499 nm) and more pronounced shifts in the electrochemical redox potentials for one-electron proligand reductions (E(1/2): -1.25 to -1.99 V) and oxidations (E(1/2): +0.52 to +1.14 V vs [Cp(2)Fe](+/0)). Installation of iron into the dipyrrinato scaffolds was effected via deprotonation of the proligands followed by treatment with FeCl(2) and excess pyridine in tetrahydrofuran to afford complexes of the type ((β,Ar)L)FeCl(py) (py = pyridine). The electrochemical and spectroscopic behavior of these complexes varies significantly across the series: the redox potential of the fully reversible Fe(III/II) couple spans more than 400 mV (E(1/2): -0.34 to +0.50 V vs [Cp(2)Fe](+/0)); λ(max) spans more than 40 nm (506-548 nm); and the (57)Fe Mössbauer quadrupole splitting (|ΔE(Q)|) spans nearly 2.0 mm/s while the isomer shift (δ) remains essentially constant (0.86-0.89 mm/s) across the series. These effects demonstrate how peripheral variation of the dipyrrinato ligand scaffold can allow systematic variation of the chemical and physical properties of iron dipyrrinato complexes.