关键词: 钳形配体, 钴配合物, 小分子活化, 低价金属配合物

Nonsymmetric pincer complexes have received much attention due to their hemilability and more easily modulated spatial and electronic effects. However, their synthesis and reactivity studies remain considerably limited in comparison to symmetric pincer complexes. Common strategies to enhance the reactivity of complexes, such as reducing coordination sites or lowering metal center valences, invariably complicate synthesis. In this work, employing the strategy of introducing reducing agents, the PNNCoCl (1) (PNN: [6-(^tBu₂PN)C₅H₃N-2-(3-Mes)C₃H₂N₂]) reacts with various small molecules to yield a variety of nonsymmetric pincer cobalt complexes, which were characterized by single-crystal X-ray diffractometry analysis and spectroscopy. The reaction of PNNCoCl, potassium graphite (or KHBEt₃) with 1 equiv. of CNXyl (2,6-dimethylphenyl isocyanide) or carbon monoxide successfully yielded the monovalent cobalt complexes 2 (PNNCoCNXyl) and 3 (PNNCoCO) in 64% and 41% yields, respectively. Moreover, the reaction of complex 1 with N-heterocyclic carbene (1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene), involving C—H bond cleavage and Co—C bond formation, afforded the corresponding divalent cobalt complex 4. Furthermore, by using different reducing agents, we achieved the modulation of the selective reaction of azobenzene C—H or N=N bonds. When KC₈ (potassium graphite) was used as the reducing agent, the reaction of PNNCoCl and azobenzene in tetrahydrofuran afforded the azobenzene C—H bond-breaking product 5 in 53% yield. The cobalt atom is pentacoordinate in a tetragonal pyramid geometry. The N—N lengths of 0.12835(17) nm in complex 5 are in the range of normal N=N double bonds (0.122~0.130 nm). In contrast, the reaction of 1 with azobenzene in the presence of KHBEt₃ gave the cobalt complex PNNCoNHPh (6), as confirmed by X-ray diffraction analysis. The structure suggests N=N double bond cleavage of azobenzene in the formation of the aniline complex. The N—H stretching bands of 6 were recorded at 3131 cm⁻¹, and the solution magnetic moment was µ_eff=2.2 µ_B. In addition, the treatment of complex 1 with 3,5-di-tert-butyl-o-benzoquinone in the presence of 1.2 equiv. of KC₈ gave rise to the cobalt complex 7. X-ray diffraction analysis of 7 revealed that the PNN ligand and o-benzoquinone parts are linked by forming a P—O single bond. The o-benzoquinone was reduced to the catechol ion. Complex 7 exhibits a high-spin electronic structure (S = 3/2), as evidenced by the solution magnetic moment of 4.0 µ_B.

Key words: pincer ligand, cobalt complex, small molecule activation, low-valent metal complex