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

摘要

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 + H_2O (?) CO_2 + 2H~+ + 2e~-. A similar structure has been sought in this work. Mononuclear planar Ni" 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 CO_2 to form unidentate L = HCO_2~- (5) and HCO_3~- (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 FeCI_2 alone and with ethyl formate and 15 with FeCI_2 affords molecules with the Ni~(II)-L-Fe~(II) bridge unit in which L = μ_2:η~1-OH~- (17) and μ_2:η~2-HCO_2~- (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 A. The Ni~(II)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)...Fe~(II) sites and bridges can be attained, a necessity in the biomimetic chemistry of C-clusters.