Cobalt PCP Pincer Complexes via an Unexpected Sequence of Ortho Metalations

The cobalt PCP pincer complexes [Co­{2,6-(CH2PPh2-κP)2C6H3-κC1}­(L)2], where L = PMe3 (1), CO (2), have been prepared. Complex 1 is obtained by a transmetalation reaction between 1-lithio-2,6-bis­((diphenylphosphino)­methyl)­benzene and [CoCl­(PMe3)3]. Subsequent exposure of 1 to CO gave complex 2. Complexes 1 and 2 can also be obtained from 1,3-bis­((diphenylphosphino)­methyl)­benzene and [CoMe­(PMe3)4]. Instead of ortho metalation occurring directly at the C2 (pincer) position of the diphosphine, ortho metalation first occurs at the C4 position to form [Co­{2-(CH2PPh2-κP)-4-(CH2PPh2)-C6H3-κC1}­(PMe3)3] (4). After reflux of the reaction mixture for 24 h, a rearrangement of 4 occurs to give pincer complex 1 with loss of PMe3 in ca. 50% yield; this rearrangement was accompanied by some decomposition. The mechanism for the conversion of 4 to 1 has been probed using 1-deuterio-2,6-bis­((diphenylphosphino)­methyl)­benzene. Unexpectedly, the labeled ligand led to 15% deuterium enrichment of an ortho CH of the terminal PPh2 group in the product complex 1, and the proposed mechanism for this rearrangement involves a four-membered cobaltacyclic intermediate.本研究成功合成了钴PCP钳形配合物（cobalt PCP pincer complexes）[Co­{2,6-(CH₂PPh₂-κP)₂C₆H₃-κC₁}­(L)₂]，其中配体L分别为三甲基膦（PMe₃，对应配合物1）与一氧化碳（CO，对应配合物2）。配合物1可通过1-锂代-2,6-双((二苯基膦基)甲基)苯与[CoCl(PMe₃)₃]之间的金属转移反应（transmetalation）制备得到。将配合物1置于一氧化碳氛围中反应，即可得到配合物2。配合物1与2也可由1,3-双((二苯基膦基)甲基)苯与[CoMe(PMe₃)₄]反应制得。该反应并未直接在二膦配体的C2（钳形）位点发生邻位金属化（ortho metalation），而是首先在C4位点发生邻位金属化，生成中间体[Co­{2-(CH₂PPh₂-κP)-4-(CH₂PPh₂)-C₆H₃-κC₁}­(PMe₃)₃]（配合物4）。将反应混合物回流24小时后，配合物4发生重排反应，失去三甲基膦并生成钳形配合物1，产率约为50%，该重排过程伴随一定程度的分解。本研究利用1-氘代-2,6-双((二苯基膦基)甲基)苯对配合物4转化为1的反应机理进行了探究。出乎意料的是，使用氘标记配体后，产物配合物1末端PPh₂基团的邻位CH位点获得了15%的氘富集（deuterium enrichment），据此提出的重排反应机理涉及四元钴杂环中间体（cobaltacyclic intermediate）。