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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.

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.

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).

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.

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.

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.

First-row transition metal complexes of the tris(pyrrolyl)ethane (tpe) trianion have been prepared. The tpe ligand was found to coordinate in a uniform eta(1),eta(1),eta(1)-coordination mode to the divalent metal series as revealed by X-ray diffraction studies. Magnetic and structural characterization for complexes of the type [(tpe)M(II)(py)][Li(THF)(4)] (M: Mn, Fe, Co, Ni) reveal each divalent ion to be high-spin and have a distorted trigonal-monopyramidal geometry in the solid state. The pyridine ligand binds significantly canted from the molecular C(3) axis due to a stabilizing pi-stacking interaction with a ligand mesityl substituent. Cyclic voltammetry on the [(tpe)M(II)(py)](-) series reveals a common irreversible oxidation pathway that is entirely ligand-based, invariant to the divalent metal bound. This latter observation indicates that fully populated ligand-based orbitals from the tpe construct are energetically most accessible in the electrochemical experiments, akin to their dipyrromethane analogues. Chemical oxidation of [(tpe)Fe(II)(py)](-) yields a product in which the ligand has dissociated one pyrrole (following tpe oxidation and H-atom abstraction) and binds a second equivalent of pyridine to form the neutral, tetrahedral Fe(II) species (kappa(2)-tpe)Fe(py)(2). Similarly, chemical oxidation of the Zn(II) analogue shows evidence for tpe oxidation by electron paramagnetic resonance spectroscopy (77 K, toluene glass) with an isotropic signal for the organic radical at g = 2.002. Density functional theory analysis on this family of complexes reveals that the highest lying molecular orbitals are completely ligand-based, corroborating our proposed electronic structure assignment.

In this Communication, we report an intramolecular C-H bond amination reaction of a dipyrromethene ferrous complex with organic azides. Monitoring of the spectral changes (variable-temperature NMR and UV-vis) of the Fe(II) complex reveals no buildup of an intermediate during conversion of the starting material into the nitrene-inserted product. The rate-determining step appears to be azide addition to the 14-electron Fe(II) complex, hinting at the potential that these and related platforms may have to effect atom- and group-transfer processes.

This paper provides an entry point to the coordination chemistry of the group VIII chemistry of the bis(8-quinolinyl)amine (BQA) ligand. In this context, mono- and disubstituted BQA complexes of iron, ruthenium, and osmium are described. For example, the low-spin bis-ligated Fe(III) complex [Fe(BQA)(2)][BPh(4)] has been prepared via amine addition to FeCl(3) in the presence of a base and NaBPh(4). Complexes featuring a single BQA ligand are more readily prepared for Ru and Os. Auxiliary ligands featuring a single BQA ligand, along with two other L-type donor ligands, allow for a variety of ligand types to occupy a sixth coordination site. Representative examples include the halide and pseudohalide complexes trans-(BQA)MX(PPh(3))(2) (M = Ru, Os; X = Cl, Br, N(3), OTf), as well as the hydride and alkyl complexes trans-(BQA)RuH(PMe(3))(2) and trans-(BQA)RuMe(PMe(3))(2). Electrochemical studies are discussed that help to contextualize the BQA ligand with respect to its neutral counterpart 2,2',2''-terpyridine (terpy) in terms of electron-releasing character. Bidentate ligands have been explored in conjunction with the BQA ligand. Thus, the bidentate, monoanionic aryl(8-quinolinyl)amido ligand 3,5-(CF(3))(2)-(C(6)H(3))QA has been installed onto the (BQA)Ru platform to provide (BQA)Ru(3,5-(CF(3))(2)-(C(6)H(3))QA)(PPh(3)). A bis(phosphino)borate ligand stabilizes the five-coordinate complex [Ph(2)B(CH(2)PPh(2))(2)]Ru(BQA). Finally, access to dinitrogen complexes of the types [(BQA)Ru(N(2))(PPh(3))(2)][PF(6)], [(BQA)Ru(N(2))(PMe(3))(2)][PF(6)], and [(BQA)Os(N(2))(PPh(3))(2)][PF(6)] is provided by exposure of the sixth coordination site under a N(2) atmosphere.

Metal-oxos are critical intermediates for the management of oxygen and its activation. The reactivity of the metal-oxo is central to the formation of O-O bonds, which is the essential step for oxygen generation. Two basic strategies for the formation of O-O bonds at metal-oxo active sites are presented. The acid-base (AB) strategy involves the attack of a nucleophilic oxygen species (e.g., hydroxide) on an electrophilic metal-oxo. Here, active-site designs must incorporate the assembly of a hydroxide (or water) proximate to a high-valent metal-oxo of even d electron count. For the radical coupling (RC) strategy, two high-valent metal-oxos of an odd d electron count are needed to drive O-O coupling. This Forum Article focuses on the different electronic structures of terminal metal-oxos that support AB and RC strategies and the design of ligand scaffolds that engender these electronic structures.

X-ray absorption spectroscopy has been used to characterize the novel nitridoiron(IV) units in two [PhBPR3]Fe(N) complexes (R=iPr and CyCH2) and obtain direct spectroscopic evidence for a very short Fe-N distance. The distance of 1.51-1.55 A reflects the presence of an FeN triple bond in accord with the observed FeN vibration observed for one of these species (nuFeN=1034 cm(-1)). This highly covalent bonding interaction results in the appearance of an unusually intense pre-edge peak, whose estimated area of 100(20) units is much larger than those of the related tetrahedral complexes with FeI-N2-FeI, FeII-NPh2, and FeIIINAd motifs, and those of recently described six-coordinate FeVN and FeVIN complexes. The observation that the FeIV-N distances of two [PhBPR3]Fe(N) complexes are shorter than the FeIV-O bond lengths of oxoiron(IV) complexes may be rationalized on the basis of the greater pi basicity of the nitrido ligand than the oxo ligand and a lower metal coordination number for the Fe(N) complex.

A series of divalent, monovalent, and zerovalent nickel complexes supported by the electron-releasing, monoanionic tris(phosphino)borate ligands [PhBP3] and [PhBPiPr3] ([PhBP3] = [PhB(CH2PPh2)3]-, [PhBPiPr3] = [PhB(CH2PiPr2)3]-) have been synthesized to explore fundamental aspects of their coordination chemistry. The pseudotetrahedral, divalent halide complexes [PhBP3]NiCl (1), [PhBP3]NiI (2), and [PhBPiPr3]NiCl (3) were prepared by the metalation of [PhBP3]Tl or [PhBPiPr3]Tl with (Ph3P)2NiCl2, NiI2, and (DME)NiCl2 (DME = 1,2-dimethoxyethane), respectively. Complex 1 is a versatile precursor to a series of complexes accessible via substitution reactions including [PhBP3]Ni(N3) (4), [PhBP3]Ni(OSiPh3) (5), [PhBP3]Ni(O-p-tBu-Ph) (6), and [PhBP3]Ni(S-p-tBu-Ph) (7). Complexes 2-5 and 7 have been characterized by X-ray diffraction (XRD) and are pseudotetrahedral monomers in the solid state. Complex 1 reacts readily with oxygen to form the four-electron-oxidation product, [[PhB(CH2POPh2)2(CH2PPh2)]NiCl] (8A or 8B), which features a solid-state structure that is dependent on its method of crystallization. Chemical reduction of 1 using Na/Hg or other potential 1-electron reductants generates a product that arises from partial ligand degradation, [PhBP3]Ni(eta2-CH2PPh2) (9). The more sterically hindered chloride 3 reacts with Li(dbabh) (Hdbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) to provide the three-coordinate complex [kappa2-PhBPiPr3]Ni(dbabh) (11), also characterized by XRD. Chemical reduction of complex 1 in the presence of L-type donors produces the tetrahedral Ni(I) complexes [PhBP3]Ni(PPh3) (12) and [PhBP3]Ni(CNtBu) (13). Reduction of 3 following the addition of PMe3 or tert-butyl isocyanide affords the Ni(I) complexes [PhBPiPr3]Ni(PMe3) (14) and [PhBPiPr3]Ni(CNtBu) (15), respectively. The reactivity of these [PhBP3]NiIL and [PhBPiPr3]NiIL complexes with respect to oxidative group transfer reactions from organic azides and diazoalkanes is discussed. The zerovalent nitrosyl complex [PhBP3]Ni(NO) (16) is prepared by the reaction of 1 with excess NO or by treating 12 with stoichiometric NO. The anionic Ni(0) complexes [[kappa2-PhBP3]Ni(CO)2][nBu4N] (17) and [[kappa2-PhBPiPr3]Ni(CO)2][ASN] (18) (ASN = 5-azoniaspiro[4.4]nonane) have been prepared by reacting [PhBP3]Tl or [PhBPiPr3]Tl with (Ph3P)2Ni(CO)2 in the presence of R4NBr. The photolysis of 17 appears to generate a new species consistent with a zerovalent monocarbonyl complex which we tentatively assign as [[PhBP3]Ni(CO)][nBu4N], although complete characterization of this complex has been difficult. Finally, theoretical DFT calculations are presented for the hypothetical low spin complexes [PhBP3]Ni(NtBu), [PhBPiPr3]Ni(NtBu), [PhBPiPr3]Ni(NMe), and [PhBPiPr3]Ni(N) to consider what role electronic structure factors might play with respect to the relative stability of these species., (Copyright 2004 American Chemical Society)

This paper introduces a sterically encumbered, strong-field tris(diisopropylphosphino)borate ligand, [PhBP(iPr)(3)] ([PhBP(iPr)(3)] = [PhB(CH(2)P(i)Pr(2))(3)](-)), to probe aspects of its conformational and electronic characteristics within a host of complexes. To this end, the Tl(I) complex, [PhBP(iPr)(3)]Tl (1), was synthesized and characterized in the solid-state by X-ray diffraction analysis. This precursor proves to be an effective transmetallating agent, as evidenced by its reaction with the divalent halides FeCl(2) and CoX(2) (X = Cl, I) to produce the monomeric, 4-coordinate, high-spin derivatives [PhBP(iPr)(3)]FeCl (2) and [PhBP(iPr)(3)]CoX (X = Cl (3), I (4)) in good yield. Complexes 2-4 were each characterized by X-ray diffraction analysis and shown to be monomeric in the solid-state. For conformational and electronic comparison within a system exhibiting higher than 4-coordination, the 16-electron ruthenium complexes [[PhBP(iPr)(3)]Ru(mu-Cl)](2) (5) and [[PhBP(3)]Ru(mu-Cl)](2) (6) were prepared and characterized ([PhBP(3)] = [PhB(CH(2)PPh(2))(3)](-)). The chloride complexes 2 and 3 reacted with excess CO to afford the divalent, monocarbonyl adducts [PhBP(iPr)(3)]FeCl(CO) (7) and [PhBP(iPr)(3)]CoCl(CO) (8), respectively. Reaction of 4 with excess CO resulted in the monovalent, dicarbonyl product [PhBP(iPr)(3)]Co(I)(CO)(2) (9). Complexes 5 and 6 also bound CO readily, providing the octahedral, 18-electron complexes [PhBP(iPr)(3)]RuCl(CO)(2) (10) and [PhBP(3)]RuCl(CO)(2) (11), respectively. Dimers 5 and 6 were broken up by reaction with trimethylphosphine to produce the mono-PMe(3) adducts [PhBP(iPr)(3)]RuCl(PMe(3)) (12) and [PhBP(3)]RuCl(PMe(3)) (13). Stoichiometric oxidation of 3 with dioxygen provided the 4-electron oxidation product [PhB(CH(2)P(O)(i)Pr(2))(2)(CH(2)P(i)Pr(2))]CoCl (14), while exposure of 3 to excess oxygen results in the 6-electron oxidation product [PhB(CH(2)P(O)(i)Pr(2))(3)]CoCl (15). Complexes 2 and 4 were characterized via cyclic voltammetry to compare their redox behavior to their [PhBP(3)] analogues. Complex 4 was also studied by SQUID magnetization and EPR spectroscopy to confirm its high-spin assignment, providing an interesting contrast to its previously described low-spin relative, [PhBP(3)]CoI. The difference in spin states observed for these two systems reflects the conformational rigidity of the [PhBP(iPr)(3)] ligand by comparison to [PhBP(3)], leaving the former less able to accommodate a JT-distorted electronic ground state.

This paper reports the synthesis of the first bis(amino)borate ligand, [Ph(2)B(CH(2)NMe(2))(2)](-), an anionic equivalent of tertiary diamines. Anionic [Ph(2)B(CH(2)NMe(2))(2)] is an excellent bidentate ligand auxiliary and is used to prepare a series of N-chelated, zwitterionic rhodium(I) complexes.