A Well-Defined Terminal Vanadium(III) Oxo Complex
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The ubiquity of vanadium oxo complexes in the V+ and IV+ oxidation states has contributed to a comprehensive understanding of their electronic structure and reactivity. However, despite being predicted to be stable by ligand-field theory, the isolation and characterization of a well-defined terminal mononuclear vanadium(III) oxo complex has remained elusive. We present the synthesis and characterization of a unique terminal mononuclear vanadium(III) oxo species supported by the pentadentate polypyridyl ligand 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine (PY5Me2). Exposure of [VII(NCCH3)(PY5Me2)]2+ (1) to either dioxygen or selected O-atom-transfer reagents yields [VIV(O)(PY5Me2)]2+ (2). The metal-centered one-electron reduction of this vanadium(IV) oxo complex furnishes a stable, diamagnetic [VIII(O)(PY5Me2)]+ (3) species. The vanadium(III) oxo species is unreactive toward H- and O-atom transfer but readily reacts with protons to form a putative vanadium hydroxo complex. Computational results predict that further one-electron reduction of the vanadium(III) oxo species will result in ligand-based reduction, even though pyridine is generally considered to be a poor π-accepting ligand. These results have implications for future efforts toward low-valent vanadyl chemistry, particularly with regard to the isolation and study of formal vanadium(II) oxo species.
钒氧配合物在V+与IV+氧化态下的广泛存在，推动了学界对其电子结构与反应活性的全面认知。然而，尽管配体场理论（ligand-field theory）预测结构明确的末端单核钒(III)氧配合物具备稳定性，但精准分离并表征这类物种始终极具挑战性。本研究报道了一种由五齿聚吡啶配体（pentadentate polypyridyl ligand）2,6-双[1,1-双(2-吡啶基)乙基]吡啶（PY5Me2）稳定的独特末端单核钒(III)氧物种的合成与表征。将[VII(NCCH3)(PY5Me2)]2+（物种1）暴露于氧气或选定的氧原子转移试剂中，可得到[VIV(O)(PY5Me2)]2+（物种2）。对该钒(IV)氧配合物进行金属中心单电子还原，可得到稳定的反磁性[VIII(O)(PY5Me2)]+（物种3）。该钒(III)氧物种对氢原子转移与氧原子转移反应均无活性，但可与质子快速反应，生成推定的钒羟基配合物。计算结果表明，尽管吡啶通常被认为是弱π接受配体（π-accepting ligand），但对该钒(III)氧物种进行进一步单电子还原时，还原过程将发生在配体层面。上述研究结果对未来开展低价钒氧化学（low-valent vanadyl chemistry）相关研究具有指导意义，尤其是在分离并研究形式上的钒(II)氧物种方面。
