P(CH)P Pincer Rhodium(I) Complexes: The Key Role of Electron-Poor Imidazoliophosphine Extremities

The coordination chemistry of a potentially pincer-type
dicationic meta-phenylene-bis­(imidazoliophosphine)
ligand 3 to neutral and cationic carbonylrhodium­(I) centers
has been investigated. Similarly to what was observed previously for
its ortho-phenylene counterpart, 3 was
found to bind to the RhCl­(CO) fragment in a trans-chelating manner
that makes possible a weak Rh–C­(H) interaction, inferred from
the nonbonding but relatively short Rh–C and Rh–H contacts
observed in the solid state structure of the dicationic adduct (3)­RhCl­(CO) (5). Formation of the target PCP-type
pincer complex could not be triggered despite multiple attempts to
deprotonate the central C–H moiety in the initial dicationic
adduct 5, or in the tricationic species [(3)­Rh­(CO)]+ (8) generated by abstraction of
the chloride ion from 5. Complex 8 was identified
on the basis of NMR and IR analyses as a Rh­(I)-stabilized P­(CH)­P-intermediate
en route to the anticipated classical PCP-type pincer complex. Analysis
of the electronic structure of this intermediate computed at the density functional level of theory (DFT level) revealed
a bonding overlap between a Rh d-orbital and π-orbitals of the m-phenylene ring. NBO analyses and calculated Wiberg indices
confirm that this interaction comprises an η1-C–Rh
bonding mode, with only secondary contributions from the geminal C
and H atoms. Although the target PCP-type pincer complex could not
be generated, treatment of the tricationic intermediate with methanol
induced a P–CN2 bond cleavage at both imidazoliophosphine
moieties, resulting in the formation of a dicationic “open
pincer” species, that is, a nonchelated bis­((MeO)­PPh2)-stabilized aryl-Rhodium complex that is the κC-only analogue of the putative κP,κC,κP-PCP complex sought initially. Theoretical
studies at the DFT level of experimental
or putative species relevant to the final C–H activation process
ruled out the oxidative addition pathway. Two alternative pathways
are proposed to explain the formation of the “open pincer”
complex, one based on an organometallic α-elimination step,
the other based on an organic aromatization-driven β-elimination
process.