Four-Coordinate Cobalt Pincer Complexes: Electronic Structure Studies and Ligand Modification by Homolytic and Heterolytic Pathways

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• 作者：Scott P. Semproni ; Carsten Milsmann ; Paul J. Chirik
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：June 25, 2014
• 年：2014
• 卷：136
• 期：25
• 页码：9211-9224
• 全文大小：688K
• ISSN：1520-5126

文摘

A family of cobalt chloride, methyl, acetylide and hydride complexes bearing both intact and modified tert-butyl substituted bis(phosphino)pyridine pincer ligands has been synthesized and structurally characterized and their electronic structures evaluated. Treatment of the unmodified compounds with the stable nitroxyl radical, TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxidanyl) resulted in immediate H- atom abstraction from the benzylic position of the chelate yielding the corresponding modified pincer complexes, (^tBumPNP)CoX (X = H, CH₃, Cl, CCPh). Thermolysis of the methyl and hydride derivatives, (^tBuPNP)CoCH₃ and (^tBuPNP)CoH, at 110 掳C also resulted in pincer modification by H atom loss while the chloride and acetylide derivatives proved inert. The relative ordering of benzylic C鈥揌 bond strengths was corroborated by H atom exchange experiments between appropriate intact and modified pincer complexes. The electronic structures of the modified compounds, (^tBumPNP)CoX were established by EPR spectroscopy and DFT computations and are best described as low spin Co(II) complexes with no evidence for ligand centered radicals. The electronic structures of the intact complexes, (^tBuPNP)CoX were studied computationally and bond dissociation free energies of the benzylic C鈥揌 bonds were correlated to the identity of the X-type ligand on cobalt where pure 蟽 donors such as hydride and methyl produce the weakest C鈥揌 bonds. Comparison to a rhodium congener highlights the impact of the energetically accessible one-electron redox couple of the first row metal ion in generating weak C鈥揌 bonds in remote positions of the supporting pincer ligand.