Reactions of the Terminal Ni^II−OH Group in Substitution and Electrophilic Reactions with Carbon Dioxide and Other Substrates: Structural Definition of Binding Modes in an Intramolecular Ni^II···Fe^II Bridged Site

A singular feature of the catalytic C-cluster of carbon monoxide dehydrogenase is a sulfide-bridged Ni···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 NiII complexes [Ni(pyN2Me2)L]1− (pyN2Me2 = 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 NiII at a NNN pincer site and five-coordinate FeII at a triamine site. The NiII 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 NiII−L−FeII bridge unit in which L = μ2:η1-OH− (17) and μ2:η2-HCO2− (18) and −CN− (19). All bridges are nonlinear (17, 140.0°; 18, M−O−C 135.9° (Ni), 120.2° (Fe); 19, Ni−C−N 170.3°, Fe−N−C 141.8°) with Ni···Fe separations of 3.7−4.8 Å. The NiIIFeII complexes, lacking appropriate Ni−Fe−S cluster structures, are not site analogues, but their synthesis and reactivity provide the first demonstration that molecular NiII···FeII sites and bridges can be attained, a necessity in the biomimetic chemistry of C-clusters.