This report documents our attempts at synthesizing a terminal [W-VI equivalent to S] complex supported by a tetradentate, diamido/dithiolate ligand ([N2S2](4-)). The target compound was selected because it would serve as a synthetic model for the active sites of formate dehydrogenase (FDH) enzymes. Although the desired [N2S2]W-VI equivalent to S species was observed as an NEt3 adduct by mass spectrometry in one case, generally unwanted side reactions prevented isolation and definitive characterization of the target compound. Instead, isolated products characterized by X-ray crystallography included {[N2S2]H}W-VI(S-2)Cl from redox chemistry of the terminal sulfide, ([N2S2]W-VI)(2)(mu-[N2S2]) from dissociation of the terminal sulfide, ({[N2S2]H}W-V)(2)(mu-S)(2) from metal reduction and mu-sulfide bridge formation, and {[N2S2]H}(2) from disulfide bond formation via thiolate redox chemistry. A product formed from adventitious exposure to air/moisture, {[N2S2]H-2}W-VI(O)(2), was also characterized. The diverse range of products formed simply from attempted metalation of the [N2S2](4-) ligand with Cl4W(VI)equivalent to S highlights the synthetic challenges toward building active sites that are structurally faithful to FDH.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Formate dehydrogenase (FDH) enzymes catalyze redox interconversion of CO2 and HCO2-, with a key mechanistic step being the transfer of H- from HCO2- to an oxidized active site featuring a [MVI≡S] group in a sulfur-rich environment (M = Mo or W). Here, we report reactivity studies with HCO2- and other reducing agents of a synthetic [WVI≡S] model complex ligated by dithiocarbamate (dtc) ligands. Reactions of [WVIS(dtc)3][BF4] (1) conducted in MeOH solvent generated [WVIS(S2)(dtc)2] (2) and [WVS(μ-S)(dtc)]2 (3) products by a solvolysis pathway that was accelerated by the presence of [Me4N][HCO2] but did not require it. Under MeOH-free conditions, the reaction of 1 with [Et4N][HCO2] produced some [WIV(μ-S)(μ-dtc)(dtc)]2 (4), but predominantly [WV(dtc)4]+ (5), along with stoichiometric CO2 detected by headspace gas chromatography (GC) analysis. Stronger hydride sources such as K-selectride generated the more reduced analogue, 4, exclusively. The reaction of 1 with the electron donor, CoCp2, also produced 4 and 5 in varying amounts depending on reaction conditions. These results indicate that formates and borohydrides act as electron donors rather than hydride donors toward 1, an outcome that diverges from the behavior of FDHs. The difference is ascribed to the more oxidizing potential of [WVI≡S] complex 1 when supported by monoanionic dtc ligands that allows electron transfer to outcompete hydride transfer, as compared to the more reduced [MVI≡S] active sites supported by dianionic pyranopterindithiolate ligands in FDHs.

One of the more active areas in bioorganometallic chemistry is the preparation and reactivity studies of active site mimics of the [NiFe]-hydrogenases. One area of particular recent progress involves reactions that interconvert Ni(μ-X)Fe centers for X = OH, H, CO, as described by Song et al. Such reactions illustrate new ways to access intermediates related to the Ni-R and Ni-SI states of the enzyme. Most models are derivatives of the type (diphosphine)Ni(SR)2Fe(CO)3-n(PR'3)n. In recent work, the methodology has been generalized to include FeII(diphosphine) derivatives of Ni(N2S2), where N2S22- is the tetradentate diamine-dithiolate (CH2N(CH3)CH2CH2S-)2. Indeed, models based on Ni(N2S2) have proven valuable, but these studies also highlight challenges in working with heterobimetallic complexes, specifically the tendency of some such Ni-Fe complexes to convert to homometalliic Ni-Ni derivatives. This kind of problem is not readily detected by X-ray crystallography. With this caution in mind, we argue that one series of complexes recently described in this journal are almost certainly misassigned.

[FeFe]-hydrogenases use a unique organometallic complex, termed the H cluster, to reversibly convert H2 into protons and low-potential electrons. It can be best described as a [Fe4S4] cluster coupled to a unique [2Fe]H center where the reaction actually takes place. The latter corresponds to two iron atoms, each of which is bound by one CN- ligand and one CO ligand. The two iron atoms are connected by a unique azadithiolate molecule (-S-CH2-NH-CH2-S-) and an additional bridging CO. This [2Fe]H center is built stepwise thanks to the well-orchestrated action of maturating enzymes that belong to the Hyd machinery. Among them, HydG converts l-tyrosine into CO and CN- to produce a unique l-cysteine-Fe(CO)2CN species termed complex-B. Very recently, HydE was shown to perform radical-based chemistry using synthetic complex-B as a substrate. Here we report the high-resolution crystal structure that establishes the identity of the complex-B-bound HydE. By triggering the reaction prior to crystallization, we trapped a new five-coordinate Fe species, supporting the proposal that HydE performs complex modifications of complex-B to produce a monomeric "SFe(CO)2CN" precursor to the [2Fe]H center. Substrate access, product release, and intermediate transfer are also discussed.

Described are the syntheses of several Ni(μ-SR)2Fe complexes, including hydride derivatives, in a search for improved models for the active site of [NiFe]-hydrogenases. The nickel(II) precursors include (i) nickel with tripodal ligands: Ni(PS3)- and Ni(NS3)- (PS33- = tris(phenyl-2-thiolato)phosphine, NS33- = tris(benzyl-2-thiolato)amine), (ii) traditional diphosphine-dithiolates, including chiral diphosphine R,R-DIPAMP, (iii) cationic Ni(phosphine-imine/amine) complexes, and (iv) organonickel precursors Ni( o-tolyl)Cl(tmeda) and Ni(C6F5)2. The following new nickel precursor complexes were characterized: PPh4[Ni(NS3)] and the dimeric imino/amino-phosphine complexes [NiCl2(PCH═NAn)]2 and [NiCl2(PCH2NHAn)]2 (P = Ph2PC6H4-2-). The iron(II) reagents include [CpFe(CO)2(thf)]BF4, [Cp*Fe(CO)(MeCN)2]BF4, FeI2(CO)4, FeCl2(diphos)(CO)2, and Fe(pdt)(CO)2(diphos) (diphos = chelating diphosphines). Reactions of the nickel and iron complexes gave the following new Ni-Fe compounds: Cp*Fe(CO)Ni(NS3), [Cp(CO)Fe(μ-pdt)Ni(dppbz)]BF4, [( R,R-DIPAMP)Ni(μ-pdt)(H)Fe(CO)3]BArF4, [(PCH═NAn)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF4, [(PCH2NHAn)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF4, [(PCH═NAn)Ni(μ-pdt)(H)Fe(dppbz)(CO)]BF4, [(dppv)(CO)Fe(μ-pdt)]2Ni, {H[(dppv)(CO)Fe(μ-pdt)]2Ni]}BF4, and (C6F5)2Ni(μ-pdt)Fe(CO)2(dppv) (DIPAMP = (CH2P(C6H4-2-OMe)2)2; BArF4- = [B(C6H3-3,5-(CF3)2]4-)) Within the context of Ni-(SR)2-Fe complexes, these new complexes feature new microenvironments for the nickel center: tetrahedral Ni, chirality, imine, and amine coligands, and Ni-C bonds. In the case of {H[(dppv)(CO)Fe(μ-pdt)]2Ni}+, four low-energy isomers are separated by ≤3 kcal/mol, one of which features a biomimetic HNi(SR)4 site, as supported by density functional theory calculations.

We report on the synthesis, redox, electronic, and catalytic behavior of two new cobalt(iii) complexes, namely [Co-III(L-1)MeOH] (1) and [Co-III(L-2)MeOH] (2). These species contain nitro-rich, phenolate-based pentadentate ligands and present dramatically distinct properties associated with the position in which the -NO2 substituents are installed. Species 1 displays nitro-substituted phenolates, and exhibits irreversible redox response and negligible catalytic activity, whereas 2 has fuctionalized phenylene moieties, shows much improved redox reversibility and catalytic proton reduction activity at low overpotentials. A concerted experimental and theoretical approach sheds some light on these drastic differences.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

We report the solution and structural chemistry of nickel(ii) complexes of the phosphine-oxime Ph2PC6H4-2-CH?NOH (PCH?NOH). PCH?NOH invariably binds in a bidentate manner as illustrated by cis-Ni(PCH?NOH)(2)Cl-2 and cis-[Ni(PCH?NOH)(2)](2+) (as its BF4- salt). Treatment of PCH?NOH with Ni(OAc)(2)(H2O)(4) gave charge-neutral trans-[Ni(PCH?NO)(2)](0). Treatment of trans-[Ni(PCH?NO)(2)](0) with BF3 gave [Ni(PCH?NO)(2)BF2]BF4. The cation features a planar NiP2N2 center wherein the pair of oximate groups are linked by the difluoroboryl center. The 1:1 complexes of the oxime and the oximate are illustrated by [Ni(PCH?NOH)Cl-2](2) and [Ni(C6F5)(PCH?NO)](2), which feature five- and four-coordinate Ni(ii) centers, respectively. All complexes in this series hydrolyze to give the trinickel oxo-phosphine-oximate complex [Ni-3(PCH?NO)(3)O](+). One feature of the PCH?NOH ligand is its wide bite angle combined with its protic OH center. These aspects are manifested in the structures of Ni(PCH?NOH)(2)Cl-2 and [Ni(PCH?NOH)Cl-2](2), which show intramolecular hydrogen bonding to terminal chloride ligands.

Co and Fe dihalide complexes of a new rigidly planar PNN ligand platform are prepared and examined as precatalysts for hydrosilylation of alkenes. Lithiation of Thummel's 8-bromo-2-(pyrid-2'-yl)quinoline followed by treatment with (i-Pr)(2)PCl and (C6F5)(2)PCl afforded the phosphine-quinoline-pyridine ligands, abbreviated (R)PQpy for R = i-Pr and C6F5, respectively. These ligands form 1:1 adducts with the dichlorides and dibromides of iron and cobalt. Crystallographic characterization of FeBr2((ipr)FIPQpy), FeBr2((ArF)PQpy), CoCl2((iPr)PQpy)), CoBr2((iPr)PQpy), and CoCl2((ArF)PQpy) confirmed that the M-P-C-C-N-C-C-N portion of these complexes is planar within 0.078 angstrom unlike previous generations of PNN complexes where deviations from planarity were similar to 0.35 angstrom. Bond distances as well as magnetism indicate that the Fe complexes are high spin and the cobalt complexes are high spin or participate in spin equilibria. Also investigated were the NNN analogues of the RPQpy ligands, wherein the phosphine group was replaced by the mesityl ketimine. The complexes FeBr2((Mes)NQpy) and CoCl2((Mes)NQpy) were characterized crystallographically. Reduction of MX2(RPQpy) complexes with NaBHEt3 generates catalysts active for anti-Markovnikov silylation of simple and complex 1-alkenes with a variety of hydrosilanes. Catalysts derived from (Mes)NQpy exhibited low activity. Fe-(R)PQpy derived catalysts favor hydrosilylation, whereas Co-(R)PQpy based catalysts favor dehydrogenative silylation. Catalysts derived from CoX2((iPr)PQpy) convert hydrosilanes and ethylene to vinylsilanes. Related experiments were conducted on propylene to give propenylsilanes.

We report the solution and structural chemistry of nickel(II) complexes of the phosphine-oxime Ph2PC6H4-2-CHNOH (PCHNOH). PCHNOH invariably binds in a bidentate manner as illustrated by cis-Ni(PCHNOH)2Cl2 and cis-[Ni(PCHNOH)2]2+ (as its BF4− salt). Treatment of PCHNOH with Ni(OAc)2(H2O)4 gave charge-neutral trans-[Ni(PCHNO)2]0. Treatment of trans-[Ni(PCHNO)2]0 with BF3 gave [Ni(PCHNO)2BF2]BF4. The cation features a planar NiP2N2 center wherein the pair of oximate groups are linked by the difluoroboryl center. The 1 : 1 complexes of the oxime and the oximate are illustrated by [Ni(PCHNOH)Cl2]2 and [Ni(C6F5)(PCHNO)]2, which feature five- and four-coordinate Ni(II) centers, respectively. All complexes in this series hydrolyze to give the trinickel oxo-phosphine-oximate complex [Ni3(PCHNO)3O]+. One feature of the PCHNOH ligand is its wide bite angle combined with its protic OH center. These aspects are manifested in the structures of Ni(PCHNOH)2Cl2 and [Ni(PCHNOH)Cl2]2, which show intramolecular hydrogen bonding to terminal chloride ligands.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

A mechanistic investigation of the changes associated with distinct oxidation states of cobalt oximes and relevant for catalytic hydrogen generation.

A new pentadentate oxime has been designed to drive the preferential coordination favored by CoI in catalysts used for proton/water reduction. The ligand incorporates water upon metal coordination and is water soluble. This Co-III species is doubly reduced to CoI and exhibits H+ reduction activity in the presence of weak acids in MeCN and evolves H-2 upon protonation suggesting that the ligand design increases catalyst effectiveness. Superior catalysis is observed in water with a turnover number (TON) of 5700 over 18 h. However, the catalyst yields Co-based nanoparticles, indicating that the solvent media may dictate the nature of the catalyst.

We investigate the redox, spectroscopy and catalytic reactivity of new cobalt(iii) complexes based on phenolate-rich [N2O3] ligands. These complexes are described as [Co(III)(L(X))MeOH], where X indicates the presence of chloro (), bromo (), iodo (), or tert-butyl () substituents in the 3(rd) and 5(th) positions of each phenolate ring. These substituents modulate the Co(iii) ← PheO(-) LMCT bands of the parent complexes with (451) > (453) > (456) > (468 nm) and the redox potentials involved with the Co(iii)/Co(ii) and ligand reduction and with the phenolate/phenoxyl oxidation processes. The influence of the substituents on the phenolate pendant arms was also observed on the kinetic parameters; presented a rate constant of 1.0 × 10(-3) s(-1) whereas showed a considerably slower rate (5.3 × 10(-5) s(-1)). Species and are electrocatalysts towards proton reduction in the presence of weak acid in acetonitrile. A TON of 10.8 was observed for after 3 h of bulk electrolysis at -2.20 VFc/Fc(+) using a mercury pool as the working electrode.