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···FeIIBridged Site

Abstract

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 Ni^II complexes [Ni(pyN₂^Me2)L]¹⁻ (pyN₂^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 = 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 Ni^IIFe^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.