Cobalt PCP Pincer Complexes via an Unexpected Sequence
of Ortho Metalations
- Publication date
- Publisher
Abstract
The cobalt PCP pincer complexes [Co{2,6-(CH<sub>2</sub>PPh<sub>2</sub>-κ<i>P</i>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>-κ<i>C</i><sup>1</sup>}(L)<sub>2</sub>], where L = PMe<sub>3</sub> (<b>1</b>), CO (<b>2</b>), have been prepared. Complex <b>1</b> is obtained by a transmetalation
reaction between 1-lithio-2,6-bis((diphenylphosphino)methyl)benzene
and [CoCl(PMe<sub>3</sub>)<sub>3</sub>]. Subsequent exposure of <b>1</b> to CO gave complex <b>2</b>. Complexes <b>1</b> and <b>2</b> can also be obtained from 1,3-bis((diphenylphosphino)methyl)benzene
and [CoMe(PMe<sub>3</sub>)<sub>4</sub>]. 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-(CH<sub>2</sub>PPh<sub>2</sub>-κ<i>P</i>)-4-(CH<sub>2</sub>PPh<sub>2</sub>)-C<sub>6</sub>H<sub>3</sub>-κ<i>C</i><sup>1</sup>}(PMe<sub>3</sub>)<sub>3</sub>] (<b>4</b>). After
reflux of the reaction mixture for 24 h, a rearrangement of <b>4</b> occurs to give pincer complex <b>1</b> with loss of
PMe<sub>3</sub> in ca. 50% yield; this rearrangement was accompanied
by some decomposition. The mechanism for the conversion of <b>4</b> to <b>1</b> 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 PPh<sub>2</sub> group in the product complex <b>1</b>, and the proposed mechanism for this rearrangement involves a four-membered cobaltacyclic
intermediate
