Synthesis, Electronic Properties, and Lewis Acidity of Rhodium Complexes Bearing X-Type PBP, PAlP, and PGaP Pincer Ligands

We have synthesized rhodium complexes that bear X-type PBP and PGaP pincer ligands. X-ray diffraction analysis and IR spectroscopy of the PBP- and PGaP-Rh(CO) complexes showed that the sigma-donicity and trans-influence of the X-type boryl ligand are stronger than those of the X-type gallyl ligand. Moreover, the Lewis acidity of the PEP-Rh complexes (E = B, Al, Ga) was examined. The PBP-Rh complex did not interact with the Lewis base N,N-dimethyl-4-aminopyridine (DMAP), while the PAlP-Rh complex coordinated DMAP more strongly via the Al atom than the PGaP-Rh complex via the Ga atom. DFT calculations of the PEP-Rh complexes revealed the details of the electronic properties and Lewis acidity of the X-type PEP pincer ligands. Their s-donicity increases in the order Ga < B < Al, and the strength of their trans-influence follows the order Ga < Al < B. The polarity of the E delta+-Rhd(delta-) bond was found to be the greatest for Al and the smallest for B. The Lewis acidity based on the calculated binding energy increases in the order B (1) Ga < Al, which is consistent with the experimental results.