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 I2.二氧化碳（CO₂）对还原态锆/钴配合物（四氢呋喃，THF）Zr(MesNPiPr₂)₃Co（1）进行氧化加成，随后经单电子还原，以低产率得到一种罕见的端基Zr-氧阴离子[2][Na(THF)₃]。为便于对该化合物开展后续研究，我们开发了一条高产率的替代合成路线。首先，将1与一氧化碳（CO）反应，得到（四氢呋喃，THF）Zr(MesNPiPr₂)₃Co(CO)（3）；随后向3中加入水（H₂O），生成Zr-氢氧化物配合物(HO)Zr(MesNPiPr₂)₃Co(CO)（4）。使用双(三甲基硅基)氨基锂（Li(N(SiMe₃)₂)）对4进行去质子化，在无冠醚或存在12-冠-4（12-c-4）的条件下，分别得到阴离子Zr-氧物种[2][Li(THF)₃]或[2][Li(12-c-4)]。研究表明，锂离子抗衡阳离子的配位环境会导致这两种物种产生显著的结构差异。配合物[2][Li(12-c-4)]中的阴离子氧片段可与亲电试剂（如三氟甲磺酸甲酯（MeOTf）和三氟甲磺酸三甲基硅酯（Me₃SiOTf））反应，分别生成(MeO)Zr(MesNPiPr₂)₃Co(CO)（5）与(Me₃SiO)Zr(MesNPiPr₂)₃Co(CO)（6）；加入乙酸酐则可得到(AcO)Zr(MesNPiPr₂)₃Co(CO)（7）。此外，配合物[2][Li(12-c-4)]还可结合CO₂，生成单阴离子Zr-碳酸根配合物[(12-crown-4)Li][(κ²-CO₃)Zr(MesNPiPr₂)₃Co(CO)]（[8][Li(12-c-4)]）。当使用钾离子抗衡阳离子与18-冠-6（18-c-6）时，可得到该化合物更稳定的版本[8][K(18-c-6)]。通过同位素标记实验证实，[2][Li(12-c-4)]结合CO₂的过程是可逆的。为探索可将这些CO₂衍生产物纳入催化循环的方法，研究发现化合物5中的Zr-OMe键可被质子（H+）断裂，且在碘单质（I₂）存在的光解条件下，可从钴中心释放出CO配体。