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.