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(MesNP^<i>i</i>Pr₂)₃Co (<b>1</b>) followed by one-electron reduction leads to formation of an unusual terminal Zr–oxo anion [<b>2]­[Na­(THF)</b>_<b>3</b><b>]</b> in low yield. To facilitate further study of this compound, an alternative high-yielding synthetic route has been devised. First, <b>1</b> is treated with CO to form (THF)­Zr­(MesNP^<i>i</i>Pr₂)₃Co­(CO) (<b>3</b>); then, addition of H₂O to <b>3</b> leads to the Zr–hydroxide complex (HO)­Zr­(MesNP^<i>i</i>Pr₂)₃Co­(CO) (<b>4</b>). Deprotonation of <b>4</b> with Li­(N­(SiMe₃)₂) leads to the anionic Zr–oxo species <b>[2]­[Li­(THF)</b>_<b>3</b><b>]</b> or <b>[2]­[Li­(12-c-4)]</b> 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 <b>[2]­[Li­(12-c-4)]</b> reacts with electrophiles such as MeOTf and Me₃SiOTf to generate (MeO)­Zr­(MesNP^<i>i</i>Pr₂)₃Co­(CO) (<b>5</b>) and (Me₃SiO)­Zr­(MesNP^<i>i</i>Pr₂)₃Co­(CO) (<b>6</b>), respectively, and addition of acetic anhydride generates (AcO)­Zr­(MesNP^<i>i</i>Pr₂)₃Co­(CO) (<b>7</b>). Complex <b>[2]­[Li­(12-c-4)]</b> is also shown to bind CO₂ to form a monoanionic Zr–carbonate, [(12-crown-4)­Li]­[(κ²-CO₃)­Zr­(MesNP^<i>i</i>Pr₂)₃Co­(CO)] (<b>[8]­[Li­(12-c-4)]</b>). A more stable version of this compound <b>[8]­[K­(18-c-6)]</b> is formed when a K⁺ counteranion and 18-crown-6 are used. Binding of CO₂ to <b>[2]­[Li­(12-c-4)]</b> 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 <b>5</b> can be cleaved using H⁺ and the CO ligand can be released from Co under photolytic conditions in the presence of I₂