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The reaction chemistry of the bis-tetrazinyl pyridine ligand (btzp) towards nitrogen oxyanions coordinated to zinc is studied in order to explore the reduction of the NOx- substrates with a redox-active ligand in the absence of redox activity at the metal. Following syntheses and characterization of (btzp)ZnX2 for X=Cl, NO3 and NO2 , featuring O-Zn linkage of both nitrogen oxyanions, it is shown that a silylating agent selectively delivers silyl substituents to tetrazine nitrogens, without reductive deoxygenation of NOx-1 . A new synthesis of the highly hydrogenated H4 btzp, containing two dihydrotetrazine reductants is described as is the synthesis and characterization of (H4 btzp)ZnX2 for X=Cl and NO3 , both of which show considerable hydrogen bonding potential of the dihydrotetrazine ring NH groups. The (H4 btzp)ZnCl2 complex does not bind zinc in the pincer pocket, but instead H4 btzp becomes a bridge between neighboring atoms through tetrazine nitrogen atoms, forming a polymeric chain. The reaction of AgNO2 with (H4 btzp)ZnCl2 is shown to proceed with fast nitrite deoxygenation, yielding water and free NO. Half of the H4 btzp reducing equivalents form Ag0 and thus the chloride ligand remains coordinated to the zinc metal center to yield (btzp)ZnCl2 . To compare with AgNO2 , experiments of (H4 btzp)ZnCl2 with NaNO2 result in salt metathesis between chloride and nitrite, highlighting the importance of a redox-active cation in the reduction of nitrite to NO.

The catalytic transposition of double bonds holds promise as an ideal route to alkenes of value as fragrances, commodity chemicals, and pharmaceuticals; yet, selective access to specific isomers is a challenge, normally requiring independent development of different catalysts for different products. In this work, a single cation-responsive iridium catalyst selectively produces either of two different internal alkene isomers. In the absence of salts, a single positional isomerization of 1-butene derivatives furnishes 2-alkenes with exceptional regioselectivity and stereoselectivity. The same catalyst, in the presence of Na+, mediates two positional isomerizations to produce 3-alkenes. The synthesis of new iridium pincer-crown ether catalysts based on an aza-18-crown-6 ether proved instrumental in achieving cation-controlled selectivity. Experimental and computational studies guided the development of a mechanistic model that explains the observed selectivity for various functionalized 1-butenes, providing insight into strategies for catalyst development based on noncovalent modifications.

γ-Irradiation of crystalline hexaaquamagnesium trans-2-butenoate dihydrate affords rel-(3S,4R)-1-hexene-3,4-dicarboxylate by a single-crystal-to-single-crystal reaction. The reaction proceeds by a ...

Hemilabile ligands are found in many leading organometallic catalysts, but it can be challenging to tune the degree of hemilability in a particular catalyst. This work explores the impact of cation...

The N2 analogue phosphorus nitride (PN) was the first phosphorus-containing compound to be detected in the interstellar medium; however, this thermodynamically unstable compound has a fleeting existence on Earth. Here, we show that reductive coupling of iron(IV) nitride and molybdenum(VI) phosphide complexes assembles PN as a bridging ligand in a structurally characterized bimetallic complex. Reaction with C≡NtBu releases the mononuclear complex [(N3N)Mo—PN]−, N3N = [(Me3SiNCH2CH2)3N]3–), which undergoes light-induced linkage isomerization to provide [(N3N)Mo—NP]−, as revealed by photocrystallography. While structural and spectroscopic characterization, supported by electronic structure calculations, reveals the PN multiple bond character, coordination to molybdenum induces a nucleophilic character at the terminal atom of the PN/NP ligands. Indeed, the linkage isomers can be trapped in solution by reaction with a Rh(I) electrophile., The highly reactive interstellar gas, phosphorus nitride, is assembled and stabilized as a reactive ligand in transition metal complexes.

As the utility of electroanalytical methods to study reaction mechanisms continues to develop, a wider scope of chemical reactions will fall under the scrutiny of electrochemical investigation. We ...

A rare example of a dinuclear iron core with a non-linearly bridged dinitrogen ligand is reported in this work. One-electron reduction of [(tBupyrr2py)Fe(OEt2)] (1) (tBupyrr2py2- = 2,6-bis((3,5-di-tert-butyl)pyrrol-2-yl)pyridine) with KC8 yields the complex [K]2[(tBupyrr2py)Fe]2(μ2-η1:η1-N2) (2), where the unusual cis-divacant octahedral coordination geometry about each iron and the η5-cation-π coordination of two potassium ions with four pyrrolyl units of the ligand cause distortion of the bridging end-on μ-N2 about the FeN2Fe core. Attempts to generate a Et2O-free version of 1 resulted instead in a dinuclear helical dimer, [(tBupyrr2py)Fe]2 (3), via bridging of the pyridine moieties of the ligand. Reduction of 3 by two electrons under N2 does not break up the dimer, nor does it result in formation of 2 but instead formation of the ate-complex [K(OEt2)]2[(tBupyrr2py)Fe]2 (4). Reduction of 1 by two electrons and in the presence of crown-ether forms the tetraanionic N2 complex [K2][K(18-crown-6)]2(tBupyrr2py)Fe]2(μ2-η1:η1-N2) (5), also having a distorted FeN2Fe moiety akin to 2. Complex 2 is thermally unstable and loses N2, disproportionating to Fe nanoparticles among other products. A combination of single-crystal X-ray diffraction studies, solution and solid-state magnetic studies, and 57Fe Mossbauer spectroscopy has been applied to characterize complexes 2-5, whereas DFT studies have been used to help explain the bonding and electronic structure in these unique diiron-N2 complexes 2 and 5.

The nitride ligand in iron(IV) complex PhB(MesIm)3Fe≡N reacts with excess H3SiPh to afford PhB(MesIm)3Fe(μ-H)3(SiHPh) as the major product, which has been structurally and spectroscopically characterized. Bulkier silane HaSiPh2 provides iron(II) amido complex PhB(MesIm)3FeN(H)(SiHPh2) as the initial product of the reaction, with excess H2SiPh2 affording diamagnetic PhB(MesIm)3Fe(μ-H)3(SiPh2) as the major product. Unobserved iron(II) hydride PhB(MesIm)3Fe–H is implicated as an intermediate in this reaction, as suggested by the results of the reaction between iron(II) amido PhB(MesIm)3FeN(H)tBu and H3SiPh, which provides PhB(MesIm)3Fe(H)(μ-H)2(Si(NHtBu)Ph) as the sole product.

The direct scission of the triple bond of dinitrogen (N2) by a metal complex is an alluring entry point into the transformation of N2 to ammonia (NH3) in molecular catalysis. Reported herein is a pincer-ligated rhenium system that reduces N2 to NH3 via a well-defined reaction sequence involving reductive formation of a bridging N2 complex, photolytic N2 splitting, and proton-coupled electron transfer (PCET) reduction of the metal-nitride bond. The new complex (PONOP)ReCl3 (PONOP = 2,6- bis(diisopropylphosphinito)pyridine) is reduced under N2 to afford the trans,trans-isomer of the bimetallic complex [(PONOP)ReCl2]2(μ-N2) as an isolable kinetic product that isomerizes sequentially upon heating into the trans,cis and cis,cis isomers. All isomers are inert to thermal N2 scission, and thetrans,trans-isomer is also inert to photolytic N2 cleavage. In striking contrast, illumination of the trans,cisand cis,cis-isomers with blue light affords the octahedral nitride complex cis-(PONOP)Re(N)Cl2 in 47% spectroscopic yield and 11% quantum efficiency. The photon energy drives an N2 splitting reaction that is thermodynamically unfavorable under standard conditions, producing a nitrido complex that reacts with SmI2/H2O to produce a rhenium tetrahydride complex and furnish ammonia in 74% yield.

Outfitting an aza-crown ether with an organotransition-metal pendant provides a mechanism for tuning its supramolecular properties. The binding affinity can be tuned by more than 2 orders of magnit...

Reduction of the bis-(pyrazolyl)pyridine complex [LCr]2 with stoichiometric KC8 in THF produces a species that is reactive with CO2 to produce an aggregate composed of paramagnetic K2 L2 Cr2 (CO3 ) linked by KCl into a product of formula [K2 L2 Cr2 (CO3 )]4 ⋅2KCl. X-ray diffraction reveals a pincer hydrocarbon exterior and an inorganic interior composed of K+ , Cl- and carbonate oxygens. Every Cr is five coordinate and square pyramidal, with the axial N donor weakly bonded to Cr due to the Jahn-Teller effect of a high spin d4 configuration. Reaction with 13 CO2 confirms that carbonate here is derived from CO2 , that oxide is derived from CO2 , and that CO is indeed released, since it is not a competent ligand to CrII . Guiding principles for selectivity in CO2 reduction are deduced from the diverse successful molecular constructs to date.

The consequences of installing two versions of a potentially redox active pincer ligand on Cr(III) and Cr(II) are described, and reveal influence of the Jahn-Teller effect. The ligand involves two pyrazole rings attached by pyrazole carbons to the ortho positions of pyridine, thus creating two 1,4-diazabutadiene moieties. In one version, each pyrazole carries an NH proton (H2L), while a comparison version (LMe) involves methyl in place of the two weakly acidic protons, to identify the impact of hydrogen bonding. Complexes synthesized include (H2L)CrX3, where X Cl, triflate and nitrate, (LMe)CrCl3 and (LMe)Cr(OTf)3, together with the divalent metal complexes (H2L)CrCl2, (LMe)CrCl2 and (LMe)Cr(OTf)2. Cyclic voltammetry establishes the relationship between the tri- and divalent triflate complexes of LMe, and DFT calculations indicate further reduction of the latter occurs at the LMe ligand. Nitrate is shown to be a stronger ligand than triflate to Cr(III) here. NaBArF24 in THF removes only one chloride from (H2L)CrCl2, to give square pyramidal [(H2L)CrCl(THF)]+.

The planar pincer ligand bis(pyrazol-3-yl)pyridine, H2L, binds to copper(I) iodide to give a product of formula (H2L)2(CuI)3 which single crystal X-ray diffraction reveals to have two unprecedented features for this ligand: κ2 binding of one H2L and bridging of the second H2L across two copper centers. The broader implications of this finding are discussed.

A C–H bond activation strategy based on electrochemical activation of a metal hydride is introduced. Electrochemical oxidation of (tBu4PCP)IrH4 (tBu4PCP is [1,3-(tBu2PCH2)-C6H3]−) in the presence of pyridine derivatives generates cationic Ir hydride complexes of the type [(tBu4PCP)IrH(L)]+ (where L = pyridine, 2,6-lutidine, or 2-phenylpyridine). Facile deprotonation of [(tBu4PCP)IrH(2,6-lutidine)]+ with the phosphazene base tert-butylimino-tris(pyrrolidino)phosphorane, tBuP1(pyrr), results in selective C–H activation of 1,2-difluorobenzene (1,2-DFB) solvent to generate (tBu4PCP)Ir(H)(2,3-C6F2H3). The overall electrochemical C–H activation reaction proceeds at room temperature without need for chemical activation by a sacrificial alkene hydrogen acceptor. This rare example of undirected electrochemical C–H activation holds promise for the development of future catalytic processes.

Bis(carbene)borate ligand transfer to the nickel nitrosyl synthon Ni(NO)(PPh3)2Br provides the new nickel nitrosyl complex Ph2B(tBuIm)2Ni(NO)(PPh3). The solid state structure reveals a trigonal pyramidal nickel ion with a very long bond to the apical PPh3 ligand. The complex reversibly dissociates PPh3 in solution to afford three-coordinate Ph2B(tBuIm)2Ni(NO), with NMR data providing evidence for PPh3 binding at low temperatures. Ligand transfer to Ni(PMe3)2Cl2 provides the square planar complex, Ph2B(tBuIm)2Ni(PMe3)Cl, which shows no evidence for rearranging to the form a homoleptic complex with two bis(carbene)borate ligands. This complex is a suitable synthon for the [Ph2B(tBuIm)2Ni]+ fragment, as demonstrated by the synthesis of the π-allyl complex Ph2B(tBuIm)2Ni(η3-C7H7). The π-allyl complex reacts with O2 to provide benzaldehyde and an unstable complex that is tentatively identified as the nickel(II) hydroxide [Ph2B(tBuIm)2Ni(μ-OH)]2.

Two-electron, one-proton reactions of a family of [CoCp(dxpe)(NCCH3)]2+ complexes (Cp = cyclopentadienyl, dxpe = 1,2-bis(di(aryl/alkyl)phosphino)ethane) form the corresponding hydride species [HCoCp(dxpe)]+ (dxpe = dppe (1,2-bis(diphenylphosphino)ethane), depe (1,2-bis(diethylphosphino)ethane), and dcpe (1,2-bis(dicyclohexylphosphino)ethane)) through a stepwise proton-coupled electron transfer process. For three [CoCp(dxpe)(NCCH3)]2+ complexes, peak shift analysis was employed to quantify apparent proton transfer rate constants from cyclic voltammograms recorded with acids ranging 22 pKa units. The apparent proton transfer rate constants correlate with the strength of the proton source for weak acids, but these apparent proton transfer rate constants curiously plateau (kpl) as the reaction becomes increasingly exergonic. The absolute apparent proton transfer rate constants across both these regions correlate with the steric bulk of the chelating diphosphine ligand, with bulkier ligands leading to slower kinetics (kplateau,depe = 3.5 × 107 M-1 s-1, kplateau,dppe = 1.7 × 107 M-1 s-1, kplateau,dcpe = 7.1 × 104 M-1 s-1). Mechanistic studies were conducted to identify the cause of the aberrant kPTapp-ΔpKa trends. When deuterated acids are employed, deuterium incorporation in the Cp ring is observed, indicating protonation of the CoCp(dxpe) species to form the corresponding hydride proceeds via initial ligand protonation. Digital simulations of cyclic voltammograms show ligand loss accompanying initial reduction gates subsequent PCET activity at higher driving forces. Together, these experiments reveal the details of the reaction mechanism: reduction of the Co(III) species is followed by dissociation of the bound acetonitrile ligand, subsequent reduction of the unligated Co(II) species to form a Co(I) species is followed by protonation, which occurs at the Cp ring, followed by tautomerization to generate the stable Co(III)-hydride product [HCoCp(dxpe)]+. Analysis as a function of chelating disphosphine ligand, solvent, and acid strength reveals that the ligand dissociation equilibrium is directly influenced by the steric bulk of the phosphine ligands and gates protonation, giving rise to the plateau of the apparent proton transfer rate constant with strong acids. The complexity of the reaction mechanism underpinning hydride formation, encompassing dynamic behavior of the entire ligand set, highlights the critical need to understand elementary reaction steps in proton-coupled electron transfer reactions.

Persistent anion binding in a wide range of solution environments is a key challenge that continues to motivate and demand new strategies in synthetic receptor design. Though strong binding in low-polarity solvents has become routine, our ability to maintain high affinities in high-polarity solvents has not yet reached the standard set by nature. Anions are bound and transported regularly in aqueous environments by proteins that use secondary and tertiary structure to isolate anion binding sites from water. Inspired by this principle of solvent exclusion, we created a sequence-defined foldameric capsule whose global minimum conformation displays a helical folded state and is preorganized for 1:1 anion complexation. The high stability of the folded geometry and its ability to exclude solvent were supported by solid-state and solution phase studies. This capsule then withstood a 4-fold increase in solvent dielectric constant (er) from dichloromethane (9) to acetonitrile (36) while maintaining a high and solvent-independent affinity of 105 M-1; ΔG ∼ 28 kJ mol-1. This behavior is unusual. More typical of solvent-dependent behavior, Cl- affinities were seen to plummet in control compounds, such as aryl-triazole macrocycles and pentads, with their solvent-exposed binding cavities susceptible to dielectric screening. Finally, dimethyl sulfoxide denatures the foldamer by putative solvent binding, which then lowers the foldamer's Cl- affinity to normal levels. The design of this capsule demonstrates a new prototype for the development of potent receptors that can operate in polar solvents and has the potential to help manage hydrophilic anions present in the hydrosphere and biosphere.

The catalytic transposition of double bonds holds promise as an ideal route to alkenes with value as fragrances, commodity chemicals, and pharmaceuticals; yet, selective access to specific isomers is a challenge, requiring independent development of different catalysts for different products. In this work, a single cation-responsive iridium catalyst is developed for the selective production of either of two different internal alkene isomers. In the absence of salts, a single positional isomerization of 1-butene derivatives furnishes 2-alkenes with exceptional regioselectivity and stereoselectivity. The same catalyst, in the presence of Na+, mediates two positional isomerizations to produce 3-alkenes. The synthesis of new iridium pincer-crown ether catalysts based on an aza-18-crown-6 ether proved instrumental in achieving cation-controlled selectivity. Experimental and computational studies guided the development of a mechanistic model that explains the observed selectivity for various functionalized 1-butenes, providing insight into strategies for catalyst development based on non-covalent modifications.

A low-coordinate, high spin (S = 3/2) organometallic iron(I) complex is a catalyst for the isomerization of alkenes. A combination of experimental and computational mechanistic studies supports a m...

Summary Fluorescence is critical to applications in optical materials including OLEDs and photonics. While fluorescent dyes are potential key components of these materials, electronic coupling between them in the solid state quenches their emission, preventing their reliable translation to applications. We report a universal solution to this long-standing problem with the discovery of a class of materials called small-molecule ionic isolation lattices (SMILES). SMILES perfectly transfer the optical properties of dyes to solids, are simple to make by mixing cationic dyes with anion-binding cyanostar macrocycles, and work with major classes of commercial dyes, including xanthenes, oxazines, styryls, cyanines, and trianguleniums. Dyes are decoupled spatially and electronically in the lattice by using cyanostar with its wide band gap. Toward applications, SMILES crystals have the highest known brightness per volume and solve concentration quenching to impart fluorescence to commercial polymers. SMILES materials enable predictable fluorophore crystallization to fulfill the promise of optical materials by design.

The N2 analogue phosphorus nitride (PN) was the first phosphorus containing compound to be detected in the interstellar medium, however this thermodynamically unstable compound has a fleeting existence on Earth. Here, we show that reductive coupling of iron(IV) nitride and molybdenum(VI) phosphide complexes assembles PN as a bridging ligand in a structurally-characterized bimetallic complex. Reaction with C≡NtBu releases the mononuclear complex [(N3N)Mo-PN]−, N3N = [(Me3SiNCH2CH2)3N]3−), which undergoes light-induced linkage isomerization to provide [(N3N)Mo-NP]−, as revealed by photocrystallography. While structural and spectroscopic characterization, supported by electronic structure calculations reveal PN multiple bond character, coordination to molybdenum creates nucleophilic character at the terminal atom of the PN/NP ligands. Indeed, the linkage isomers can be trapped in solution by reaction with a Rh(I) electrophile.

Deoxygenation of nitrite oxygen with divalent cobalt was achieved using (PNNH)CoCl2, carrying a pyridyl pincer ligand with one P(t-Bu)2 arm and one pyrazole arm. Reaction of (PNNH)CoCl2 with NaNO2 at a 2 : 5 mole ratio promptly forms equimolar (PNNH)Co(NO2)3 and (PNN)Co(NO2)(NO), {CoNO}8 with the lost ligand proton combined with removed oxo as hydroxide. These two CoIII products are characterized, showing a bent CoNO unit as the fate of the reduced nitrogen. DFT calculations are consistent with two one-electron CoII reductants binding to one NO2− bridge, then proton transfer being needed for facile N/O bond scission. A species detected by low temperature execution of this reaction contains cobalt in two oxidation states with an N,O bridging nitro group and pincer ligands that have been deprotonated, showing the active participation of the proton responsive ligand.

The octahedral, S = 2, bis -pyrazolatopyridyl complex LFe(DMAP) 3 reacts with CO 2 in CH 3 CN to yield (LH)Fe(DMAP) 2 (O 3 C)Fe 2 (L)(HL), which is the product of capture of one CO 2 by one doubly deprotonated water molecule, then binding of one iron to each carbonate oxygen. In order to eliminate DMAP as a participant, [LiClFe 2 L 2 ] is shown to add one water per Fe 2 unit to give [LiClFe 2 (HL)(L)(OH)], with the Fe 2 L 2 unit capturing hydroxide, and this entire formula unit is linked into a dimer by Li/OH interactions, supplemented by hydrogen bonds to OH from nucleophilic pyrazolate nitrogens. This is suggested to be the process where carbonate is formed, promoted by the ability of the LFe unit to increase the nucleophilicity of H 2 O, and then attack CO 2 . Related to that conversion, we establish the structure of the conjugate base [Fe 2 L 2 (O)] 2- . Collectively, this shows the combined action of multiple Fe(II) reactants, together with pyrazolate nucleophile and Bronsted base in the hydration of CO 2 , the conversion effected by carbonic anhydrase.

The divalent cobalt complex of the diprotic pincer ligand bis-pyrazolylpyridine, (H2 L)CoCl2 , is dehydrohalogenated twice by LiN(SiMe3 )2 in the presence of PEt3 to give monomeric S=1/2 LCo(PEt3 )2 (1), fully characterized in the solid-state and solution as a square pyramidal monomer with a long axial Co-P bond. This 17-electron species reacts in time of mixing with N2 O to form L2 Co2 (μ-OPEt3 ) (2)+3 OPEt3 , the former the first example of phosphine oxide bridging two transition metals. The same products are formed from O2 , and divalent cobalt persists even in the presence of excess oxidant. Species (2) catalyzes oxygen atom transfer (OAT) for generation of O=PEt3 from PEt3 from either N2 O or O2 . Bridging and terminal cobalt oxo intermediates are suggested, and the electron donor power, and potential redox activity of the dianionic pincer ligand is emphasized.

The behavior of the complex (H2L)CoCl2, where H2L is a bis-(pyrazol-3-yl)pyridine, towards Bronsted bases is studied, to evaluate peripheral NH deprotonation as a route to a dianionic pincer ligand on a d7 center. Deprotonation is found to also remove chloride from cobalt, and the decreased metal coordination number is then satisfied by bimolecular reaction of the newly formed peripheral deprotonated pyrazolate nitrogen, leading to Co2 units bridged by some of the pyrazolates, in the analogous species [Co2(L)(LH)]2(L) and [Co2(L)(HL)]2[Co(L)2], but also occasionally by chloride retention, in LiCo2L2Cl. Reacting LiCo2L2Cl with tBuNC, yields monomeric LCo(tBuNC)2, shown to be a 17 valence electron species. Use of excess LiN(SiMe3)2 in deprotonation of (H2L)CoCl2 leads to a product containing a Co[N(SiMe3)2]2 substructure, which illustrates opening of the Co2L2 dimer in response to an attacking nucleophile.

The new iron(IV) nitride complex PhB(iPr2Im)3Fe≡N reacts with 2 equiv of bis(diisopropylamino)cyclopropenylidene (BAC) to provide PhB(iPr2Im)3Fe(CN)(N2)(BAC). This unusual example of a four-electron reaction involves carbon atom transfer from BAC to create a cyanide ligand along with the alkyne iPr2N-C≡C-NiPr2. The iron complex is in equilibrium with an N2-free species. Further reaction with CO leads to formation of a CO analogue, which can be independently prepared using NaCN as the cyanide source, while reaction with B(C6F5)3 provides the cyanoborane derivative.

The acid-base switching of complexes formed from anti-electrostatic anion-anion homodimers of organophosphates and cyanostar macrocycles was investigated for the first time. High-fidelity 2:2 complexes were selected by using suitably sized organo substituents. Reversible and direct switching occurs with triflic acid and hydroxide base. An unexpected acid⋅⋅⋅anion heterodimer was discovered with weaker picric acid, which helped reveal some of the elementary steps. Switching can also proceed in a cooperative (strong anion then weak acid) or stepwise manner (weak acid then strong anion).

The process of removal of H+ and of Cl-, dehydrohalogenation, from (H₂L)FeCl₂ is investigated systematically, to understand the reactivity of the implied transient LFe(II). Reaction of (H₂L)FeCl₂ with 2 equivalents of LiN(SiMe₃)₂ yields LiClFe₂L₂, as its dimer with every iron 5-coordinate in an FeN₄Cl environment. To avoid Li+ cation derived from base, reaction of Na₂L with FeCl₂ gives a product Na₂[NaFe(HL)(L)]₂(LFeO) from addition of water, which reveals Na/pyrazolate Nβ interactions and a five coordinate oxo group in the OFe₃Na₂ core. Dehydrohalogenation in the presence of Ph₂PC₂H₄PPh₂ gives diamagnetic [LFe(κ²-dppe)]₂(μ-dppe) and LFe(κ²-dppe)(κ¹-dppe). Dehydrohalogenation in the presence of tBuNC gives diamagnetic LFe(CNtBu)₃. This is selectively methylated at both pyrazolate β-N to give LMeFe(tBuNC)₃²+ which is reduced with KC₈ to diamagnetic LMeFe(tBuNC)². Structure determination of these and IR data on isonitrile bands, show L²- to be a stronger donor than LMe. First installing triflate (to avoid the more persistent Cl-) facilitates access to LFe(Lewis base)₃²+ complexes, but this cation shows relatively weak binding of CO to LFeII, which implicates weak π basicity of that d6 species. Production of bis-pincer complexes (H₂L)₂Fe²+ and (LMe)₂Fe²+ in the presence of abundant Lewis base in polar medium is demonstrated, which illustrates a pincer ligand redistribution challenge to be kept in mind when trying to maintain a 1:1 Fe:pincer ratio, for highest reactivity.

The complex (PNP)Ti═CHtBu(CH2tBu) (PNP = N[2-PiPr2-4-methylphenyl]2–) dehydrogenates cyclohexane to cyclohexene by forming a transient low-valent titanium-alkyl species, [(PNP)Ti(CH2tBu)], which reacts with 2 equiv of quinoline (Q) at room temperature to form H3CtBu and a Ti(IV) species where the less hindered C2═N1 bond of Q is ruptured and coupled to another equivalent of Q. The product isolated from this reaction is an imide with a tethered cycloamide group, (PNP)Ti═N[C18H13N] (1). Under photolytic conditions, intramolecular C—H bond activation across the imide moiety in 1 occurs to form 2, and thermolysis reverses this process. The reaction of 2 equiv of isoquinoline (Iq) with intermediate [(PNP)Ti(CH2tBu)] results in regioselective cleavage of the C1═N2 and C1—H bonds, which eventually couple to form complex 3, a constitutional isomer of 1. Akin to 1, the transient [(PNP)Ti(CH2tBu)] complex can ring-open and couple two pyridine molecules, to produce a close analogue of 1, complex (PNP)Ti═N[C10H9N] (4...

The synthesis of bis(N1-phenyl-5-hydroxypyrazol-3-yl)pyridines ("L") is described, and these are silylated to achieve analogues ("Si2L") without the variable of the hydroxyl proton mobility. One hydroxyl example is characterized in its bis-pincer iron(II) complex, which shows every OH proton involved in hydrogen bonding. The steric bulk of the silylated N-phenyl-substituted ligands allows the synthesis and characterization of paramagnetic (Si2L)FeCl2 complexes, and one of these is reduced, under CO, to give the diamagnetic (Si2L)Fe(CO)2 species. Structural comparison and density functional theory calculations of the dichloride and dicarbonyl species show that much, but not all, of the reduction occurs at both the ligand pyridine and pyrazole rings, and thus this ligand type is more resistant to reduction than the simpler bis(iminopyridines). The OSiR3 substituent offers a useful diagnostic of reduction at pyrazole via the degree of π-donation to pyrazole by the oxygen lone pairs, and the stereoelectronic features of the NPh moiety are analyzed. The X-ray photoelectron spectroscopy binding energies of both iron and nitrogen are analyzed to show details of the locus of reduction.

A pyridyl pincer ligand was developed to incorporate steric bulk, through a PtBu2 arm, and proton responsivity, through a pyrazole pincer ligand arm, together with reactivity at benzylic hydrogen and redox activity within a 1,4 diazabutadiene moiety. Binding it to CoCl2 yielded square-pyramidal [(PNNH)CoCl2 ], which was deprotonated by Li[N(SiMe3 )2 ] to form [{Li(THF)2 PNN}CoCl2 ]. Reduction of this LiCl adduct with KC8 under CO atmosphere led to formation of CoI mono- and dicarbonyl complexes, which can be protonated but also further deprotonated at the benzylic CH group to give a dearomatized pyridyl group. The ligand was characterized in its neutral, monoanionic, and dianionic forms, and the anions were shown to exist as intimate ion pairs with Li+ bound to pyrazolate N and chloride bound to Lewis acidic cobalt. X-ray photoelectron spectroscopy was used to assay both Li content and cobalt oxidation states. The general character of binding of LiCl to a metal complex acidic at metal and nucleophilic at ligand (pyrazolate Nβ) is discussed, as are potential catalytic applications of the concept.

Bulky tris(carbene)borate ligands provide access to high spin iron(II) phosphido complexes. The complex PhB(MesIm)3FeP(H)Ph is thermally unstable, and [PPh] group insertion into a C–H bond of the supporting ligand is observed. An arrested α-hydride migration mechanism suggests increased nucleophilicity of the phosphorus atom facilitates [PPh] group transfer reactivity.

Fluorescence is critical to many advanced materials including OLEDs and bioimaging. While molecular fluorophores that show bright emission in solution are potential sources of these materials, their emission is frequently lost in the solid state preventing their direct translation to optical applications. Unpredictable packing and coupling of dyes leads to uncontrolled spectral shifts and quenched emission. No universal solution has been found since Perkin made the first synthetic dye 150 years ago. We report the serendipitous discovery of a new type of material that we call small-molecule ionic isolation lattices(SMILES) tackling this long-standing problem. SMILES are easily prepared by adding two equivalents of the anion receptor cyanostar to cationic dyes binding the counter anions and inducing alternating packing of dyes and cyanostar-anion complexes. SMILES materials reinstate solution-like spectral properties and bright fluorescence to thin films and crystals. These positive outcomes derive from the cyanostar. Its wide 3.45-eV band gap allows the HOMO and LUMO levels of the dye to nest inside those of the complex as verified by electrochemistry. This feature simultaneously enables spatial and electronic isolation to decouple the fluorophores from each other and from the cyanostar-anion lattice. Representative dyes from major families of fluorophores, viz, xanthenes, oxazines, styryls, cyanines, and trianguleniums, all crystalize in the characteristic structure and regain their attractive fluorescence. SMILES crystals of rhodamine and cyanine display unsurpassed brightness per volume pointing to uses in demanding applications such as bioimaging. SMILES materials enable predictable fluorophore crystallization to fulfil the promise of optical materials by design.

The paramagnetic cyano-bridged complex PhB(tBuIm)3Fe-NC-Mo(NtBuAr)3 (Ar = 3,5-Me2C6H3) is readily assembled from a new four-coordinate, high-spin (S = 2) iron(II) monocyanide complex and the three-coordinate molybdenum(III) complex Mo(NtBuAr)3. X-ray diffraction and IR spectroscopy reveal that delocalization of unpaired electron density into the cyanide π* orbitals leads to a reduction of the C-N bond order. Direct current (dc) magnetic susceptibility measurements, supported by electronic structure calculations, demonstrate the presence of strong antiferromagnetic exchange between spin centers, with a coupling constant of J = -122(2) cm-1. To our knowledge, this value represents the strongest magnetic exchange coupling ever to be observed through cyanide. These results demonstrate the ability of low-coordinate metal fragments to engender extremely strong magnetic exchange coupling through cyanide by virtue of significant π-backbonding into the cyanide ligand.

This work reports a combined experimental and computational mechanistic investigation into the electrocatalytic reduction of nitrite to ammonia by a cobalt macrocycle in an aqueous solution. In the presence of a nitrite substrate, the Co(III) precatalyst, [Co(DIM)(NO2)2]+ (DIM = 2,3-dimethyl-1,4,8,11-tetraazacyclotetradeca-1,3-diene), is formed in situ. Cyclic voltammetry and density functional theory (DFT) calculations show that this complex is reduced by two electrons, the first of which is coupled with nitrite ligand loss, to provide the active catalyst. Experimental observations suggest that the key N-O bond cleavage step is facilitated by intramolecular proton transfer from an amine group of the macrocycle to a nitro ligand, as supported by modeling several potential reaction pathways with DFT. These results provide insights into how the combination of a redox active ligand and first-row transition metal can facilitate the multiproton/electron process of nitrite reduction.

A series of air-stable boron complexes 1-5 were prepared by using N-aryl iminopyrrolide ligands. Designed as minimalist structural mimics of the privileged BODIPY motif, these new BOIMPY (BOron complexes of IMinoPYrrolide ligands) fluorophores feature low molecular symmetry that promotes emission from CT-type excited states with large Stokes shifts and little self-quenching. Through comparative studies on the homologous set of compounds 1-4, we have confirmed that a delicate interplay between conformational twisting and donor-acceptor interaction dictates the mechanism of de-excitation, which responds sensitively to solvent polarity as well as protonation states. Over a wide visible spectral range, the structure-dependent light-emitting properties of BOIMPY molecules are well manifested, even in the solid-state. In order to exploit the environment-sensitive nature of CT-type emission, the BOIMPY motif was elaborated further into a bioprobe molecule 5. Live-cell fluorescence imaging studies have established that 5 is localized exclusively at lipid droplets to produce well-resolved staining patterns without affecting cell viability. These findings promise future elaboration of BOIMPY-based functional molecules for applications in biological imaging, chemical sensing, and molecular switching.

The Fe(II) coordination chemistry of bis(pyrazole-3-yl)pyridine ligands with both proton or methyl substituents on pyrazole nitrogen are investigated, including the willingness of the ligand to undergo redox change. Protons on the pyrazole nitrogen promote intermolecular hydrogen bonding and lead to redox irreversibility; N methylation of those nitrogens eliminates those intermolecular interactions and leads to reversible outer-sphere reducibility. The resulting anion radical of the N-methylated ligand has more spin in the pyridine moiety than in the pyrazolyl pincer ligand arms, but also detectably delocalized into the electron withdrawing pyrazolyl pincer ligand arms; EPR and density functional calculations assist in characterizing the ligand radical anion, as its potassium complex.

Reaction of 3,6-disubstituted-1,2,4,5-tetrazines with water and PEt3 forms the corresponding 1,4-dihydrotetrazine and OPEt3. Thus PEt3, as a stoichiometric reductant, reduces water, and the resulting two reducing equivalents serve to doubly hydrogenate the tetrazine. A variety of possible initial interactions between electron-deficient tetrazine and electron-rich PR3, including a charge transfer complex, were evaluated by density functional calculations which revealed that the energy of all these make them spectroscopically undetectable at equilibrium, but one of these is nevertheless suggested as the intermediate in the observed redox reaction. The relationship of this to the Mitsunobu reaction, which absorbs the components of water evolved in the conversion of alcohol and carboxylic acid to ester, with desirable inversion at the alcohol carbon, is discussed. This enables a modified Mitsunobu reaction, with tetrazine replacing EtO2CN=NCO2Et (DEAD), which has the advantage that dihydrotetrazine can be recycled to tetrazine by oxidation with O2, something impossible with the hydrogenated DEAD. For this tetrazine version, a betaine-like intermediate is undetectable, but its protonated form is characterized, including by X-ray structure and NMR spectroscopy.

The bulky bis(carbene)borate ligand Ph2B(tBuIm)2– stabilizes low-coordinate and low-valent iron complexes. The four-coordinate iron(II) complex Ph2B(tBuIm)2FeCl(THF) is a precursor to the low-spin iron(I) complex Ph2B(tBuIm)2Fe(CO)3. Despite a reversible reduction wave in the cyclic voltammogram, access to the iron(0) complex [Ph2B(tBuIm)2Fe(CO)3]− requires reduction under a CO atmosphere. Without excess CO, KC8 reduction leads to formation of the dimer K2[Ph2B(tBuIm)2Fe(CO)2] along with loss of CO. Each iron center in this complex adopts a distorted-square-planar geometry. Spectroscopic analysis of these carbonyl complexes reveals the very high strength of the bis(carbene)borate ligand.

The niobium methylidene [{(Ar'O)2 Nb}2 (μ2 -Cl)2 (μ2 -CH2 )] (2) can be cleanly prepared via thermolysis or photolysis of [(Ar'O)2 Nb(CH3 )2 Cl] (1) (OAr'=2,6-bis(diphenylmethyl)-4-tert-butylphenoxide). Reduction of 2 with two equivalents of KC8 results in formation of the first niobium methylidyne [K][{(Ar'O)2 Nb}2 (μ2 -CH)(μ2 -H)(μ2 -Cl)] (3) via a binuclear α-hydrogen elimination. Oxidation of 3 with two equiv of ClCPh3 reforms 2. In addition to solid state X-ray analysis, all these complexes were elucidated via multinuclear NMR experiments and isotopic labelling studies, including a crossover experiment, support the notion for a radical mechanism as well as a binuclear α-hydrogen abstraction pathway being operative in the formation of 2 from 1.

Reaction of the readily reduced pincer ligand bis-tetrazinylpyridine, btzp, with the zerovalent metal source M(CO)3(MeCN)3 yields M(btzp)2 for M = Cr, Mo. These diamagnetic molecules show intrapincer bond lengths consistent with major charge transfer from metal to ligand, a result which is further supported by X-ray photoelectron spectroscopy. These molecules show up to five reversible outer-sphere electron transfers by cyclic voltammetry. The electronic structure of neutral M(btzp)2 is analyzed by DFT and CASSCF calculations, which reveal the degree of back-donation from the metal into pincer π* orbitals and also subtle differences in metal-ligand interaction for Mo vs Cr. Near-IR absorptions exhibited by both M(btzp)2 species originate from charge transfer among differently reduced tetrazine rings, which thus further support pincer reduction in these species.

Double deprotonation of [(H2L)FeCl2]2·FeCl2 with LiN(SiMe3)2 under varying conditions occurs to give the acid [(THF)2LiClFe2L2]2[Fe(OH)2], 1, and the lithium salt of its doubly deprotonated conjugate base Li2([(THF)1.5LiClFe2L2]2[Fe(O)2]), 2. Single crystal X-ray determination of both structures shows that replacement of two protons by two Li+ causes both oxo and chloride ligands to adopt higher connectivity bridging roles. This illustrates the core event of moving towards more closed and higher dimensionality. The added lithium ions bind to various nucleophilic centers in the resulting intimate ion pair, all with general contraction of the metal framework, as an illustration of the general shrinkage towards a compact solid as hydroxide is converted to oxide.

1,4-Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene is an effective silyl transfer reagent towards the oxygen of nitrate coordinated to Cr(iii) in a pincer complex., 1,4-Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene is an effective silyl transfer reagent towards the oxygen of nitrate coordinated to Cr(iii) in a pincer complex. Two nitrate oxygens are removed to give the 17 valence electron octahedral complex (H2L)Cr(NO3)2(NO). This is shown by a variety of spectroscopic methods, together with DFT, to be a Cr(i) complex with a linear CrNO unit. This work also identifies future applications of this reductive silylation process.

Tetrazines react with OCP-1 through a reverse electron demand Diels-Alder process to produce 3,6-disubstituted-1,2,4-diazaphosphinin-5-olates. DFT calculations reveal that both Diels-Alder and subsequent aromatization barriers are low for both EWG and ED tetrazine substituents. The structure of the solid sodium salt shows the interaction of Na+ with aryloxide and also both nitrogens of a neighboring anion, leading to coordination polymer character. 1,2,4-Diazaphosphinin-5-olates react as nucleophiles towards MeI and R3SiCl, respectively, and were installed on the (Ph3P)2Ru(CO)H fragment to investigate their properties as ligands.

A low-coordinate iron(II) complex (CztBu(PztBu)2)Fe[N(SiMe3)2], 1 bearing an NNN-pincer ligand was prepared and fully characterized. Intramolecular C–H activation on the 5-position of a pyrazole at elevated temperatures was observed. Complex 1 was found to be an efficient and chemoselective pre-catalyst for the hydrosilylation of organo carbonyl substrates.