Dinuclear Co(II)Co(III) Mixed-Valence and Co(III)Co(III) Complexes with N- and O-Donor Ligands: Characterization and Water Oxidation Studies

The tridentate ligand N-methyl-N,N-bis(2-pyridylmethyl)amine (L) has been employed to synthesize a dinuclear Co(II)Co(III) mixed-valence complex containing μ-methoxo and μ-carboxylato bridging ligands, [LCoII(μ-carboxylato)bis(μ-methoxo)CoIIIL](ClO4)2. In this complex, the two pseudo-octahedral Co centers have an identical ligand environment, yet the average Co–N and Co–O bond distances at the two Co ions differ significantly. Electrochemical, spectroscopic, and magnetic susceptibility measurements confirm that it belongs to a localized Class II mixed-valence system, despite the presence of a short Co···Co distance of 3.021 Å. Oxidation of this Co(II)Co(III) complex leads to formation of the corresponding Co(III)Co(III) complex that was characterized structurally and spectroscopically. In addition, dinuclear and trinuclear μ-hydroxo Co(III) complexes have been obtained in the presence of phosphate anions and absence of methanol, respectively, suggesting that an additional bridging ligand is needed to stabilize the CoIIIbis(μ-hydroxo)CoIII fragment. Moreover, the ability of the mixed-valence Co(II)Co(III) complex and the three related Co(III) complexes to electrocatalytically oxidize water was also investigated. The observed limited water oxidation catalytic ability for these systems suggests that a multinuclear Co cluster and/or presence of O-rich ligands may be needed for the generation of efficient molecular Co-based water oxidation catalysts.

本研究采用三齿配体N-甲基-N,N-双(2-吡啶甲基)胺（L），合成了一种含μ-甲氧基桥联配体与μ-羧酸根桥联配体的双核Co(II)Co(III)混合价配合物[LCoII(μ-羧酸根)bis(μ-甲氧基)CoIIIL](ClO4)2。该配合物中，两个准八面体配位的Co中心具有完全一致的配位环境，但两个Co离子的平均Co-N与Co-O键长存在显著差异。电化学、光谱及磁化率测试证实，尽管该配合物的Co…Co间距仅为3.021 Å，但其属于定域II类混合价体系。对该Co(II)Co(III)配合物进行氧化反应，可得到对应的Co(III)Co(III)配合物，该产物已通过结构与光谱手段完成表征。此外，在磷酸根阴离子存在的条件下与无甲醇的条件下，我们分别得到了双核与三核μ-羟基Co(III)配合物，这表明稳定CoIII双(μ-羟基)CoIII片段需要额外的桥联配体。此外，本研究还考察了该Co(II)Co(III)混合价配合物及三种相关Co(III)配合物的电催化水氧化性能。观测到的有限水氧化催化活性提示，构建高效的分子基钴基水氧化催化剂，可能需要多核钴团簇及/或富氧配体的参与。