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.