Spin Isomers and Ligand Isomerization in a Three-Coordinate Cobalt(I) Carbonyl Complex

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• 作者：Malik H. Al-Afyouni ; Elizaveta Suturina ; Shubhrodeep Pathak ; Mihail Atanasov ; Eckhard Bill ; Daniel E. DeRosha ; William W. Brennessel ; Frank Neese ; Patrick L. Holland
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：August 26, 2015
• 年：2015
• 卷：137
• 期：33
• 页码：10689-10699
• 全文大小：665K
• ISSN：1520-5126

文摘

Hemilabile ligands, which have one donor that can reversibly bind to a metal, are widely used in transition-metal catalysts to create open coordination sites. This change in coordination at the metal can also cause spin-state changes. Here, we explore a cobalt(I) system that is poised on the brink of hemilability and of a spin-state change and can rapidly interconvert between different spin states with different structures (鈥渟pin isomers鈥?. The new cobalt(I) monocarbonyl complex L^tBuCo(CO) (2) is a singlet (¹2) in the solid state, with an unprecedented diketiminate binding mode where one of the C鈺怌 double bonds of an aromatic ring completes a pseudo-square-planar coordination. Dissolving the compound gives a substantial population of the triplet (³2), which has exceptionally large uniaxial zero-field splitting due to strong spin鈥搊rbit coupling with a low-lying excited state. The interconversion of the two spin isomers is rapid, even at low temperature, and temperature-dependent NMR and electronic absorption spectroscopy studies show the energy differences quantitatively. Spectroscopically validated computations corroborate the presence of a low minimum-energy crossing point (MECP) between the two potential energy surfaces and elucidate the detailed pathway through which the 尾-diketiminate ligand 鈥渟lips鈥?between bidentate and arene-bound forms: rather than dissociation, the cobalt slides along the aromatic system in a pathway that balances strain energy and cobalt鈥搇igand bonding. These results show that multiple spin states are easily accessible in this hemilabile system and map the thermodynamics and mechanism of the transition.