Reactions of the Terminal NiII-OH Group in Substitution and Electrophilic Reactions with Carbon Dioxide and Other Substrates: Structural Definition of Binding Modes in an Intramolecular NiII•••FeII Bridged Site

A singular feature of the catalytic C-cluster of carbon monoxide dehydrogenase is a sulfide-bridged Ni center dot center dot center dot Fe locus where substrate is bound and transformed in the reversible reaction CO + H2O = CO2 + 2H(+) + 2e(-). A similar structure has been sought in this work. Mononuclear planar Ni-II complexes [Ni(pyN(2)(Me2))L](1-) (pyN(2)(Me2) = bis(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate(2-)) derived from a NNN pincer ligand have been prepared including L = OH- (1) and CN- (7). Complex 1 reacts with ethyl formate and CO2 to form unidentate L = HCO2- (5) and HCO3- (6) products. A binucleating macrocycle was prepared which specifically binds Ni-II at a NNN pincer site and five-coordinate Fe-II at a triamine site. The Ni-II macrocyle forms hydroxo (14) and cyanide complexes (15) analogous to 1 and 7. Reaction of 14 with FeCl2 alone and with ethyl formate and 15 with FeCl2 affords molecules with the Ni-II-L-Fe-II bridge unit in which L = mu(2):eta(1)-OH- (17) and mu(2):eta(2)-HCO2- (18) and -CN- (19). All bridges are nonlinear (17, 140.0 degrees; 18, M-O-C 135.9 degrees (Ni), 120.2 degrees (Fe); 19, Ni-C-N 170.3 degrees, Fe-N-C 141.8 degrees) with Ni center dot center dot center dot Fe separations of 3.7-4.8 angstrom. The (NiFeII)-Fe-II complexes, lacking appropriate Ni-Fe-S cluster structures, are not site analogues, but their synthesis and reactivity provide the first demonstration that molecular Ni-II center dot center dot center dot Fe-II sites and bridges can be attained, a necessity in the biomimetic chemistry of C-clusters.