Synthesis, Structure, and Reactivity of an Anionic Zr–Oxo Relevant to CO2 Reduction by a Zr/Co Heterobimetallic Complex

Oxidative addition of CO2 to the reduced Zr/Co complex (THF)­Zr­(MesNPiPr2)3Co (1) followed by one-electron reduction leads to formation of an unusual terminal Zr–oxo anion [2]­[Na­(THF)3] in low yield. To facilitate further study of this compound, an alternative high-yielding synthetic route has been devised. First, 1 is treated with CO to form (THF)­Zr­(MesNPiPr2)3Co­(CO) (3); then, addition of H2O to 3 leads to the Zr–hydroxide complex (HO)­Zr­(MesNPiPr2)3Co­(CO) (4). Deprotonation of 4 with Li­(N­(SiMe3)2) leads to the anionic Zr–oxo species [2]­[Li­(THF)3] or [2]­[Li­(12-c-4)] in the absence or presence of 12-crown-4, respectively. The coordination sphere of the Li+ countercation is shown to lead to interesting structural differences between these two species. The anionic oxo fragment in complex [2]­[Li­(12-c-4)] reacts with electrophiles such as MeOTf and Me3SiOTf to generate (MeO)­Zr­(MesNPiPr2)3Co­(CO) (5) and (Me3SiO)­Zr­(MesNPiPr2)3Co­(CO) (6), respectively, and addition of acetic anhydride generates (AcO)­Zr­(MesNPiPr2)3Co­(CO) (7). Complex [2]­[Li­(12-c-4)] is also shown to bind CO2 to form a monoanionic Zr–carbonate, [(12-crown-4)­Li]­[(κ2-CO3)­Zr­(MesNPiPr2)3Co­(CO)] ([8]­[Li­(12-c-4)]). A more stable version of this compound [8]­[K­(18-c-6)] is formed when a K+ counteranion and 18-crown-6 are used. Binding of CO2 to [2]­[Li­(12-c-4)] is shown to be reversible using isotopic labeling studies. In an effort to address methods by which these CO2-derived products could be turned over in a catalytic cycle, it is shown that the Zr–OMe bond in 5 can be cleaved using H+ and the CO ligand can be released from Co under photolytic conditions in the presence of I2.