Synthesis, structure, and reactivity of an anionic Zr-oxo relevant to CO2 reduction by a Zr/Co heterobimetallic complex

Abstract

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(MesNP iPr2)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(MesNPiPr 2)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. © 2013 American Chemical Society.