Synthetic Designs and Structural Investigations of Biomimetic Ni–Fe Thiolates

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)]2and [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)4site, as supported by density functional theory calculations.