Coordination Flexibility of the Rh(PXP) Complex to NH3, CO, and C2H4 (PXP = Diphosphine-Based Pincer Ligand; X = B, Al, and Ga): Theoretical Insight

Abstract

The recently synthesized rhodium-aluminum bimetallic complex Rh(PAlP) 1 (PAlP = pincer-type diphosphino-aluminyl ligand Al{[N(C6H4)]2NMe}[CH2P(iPr)2]2) containing a unique Rh-Al direct bond exhibits coordination flexibility because Rh and Al can play the role of coordination site for the substrate. DFT calculations of NH3, CO, and C2H4 adducts with 1 show that the Rh atom is favorable for all these substrate but the Al atom is as favorable as the Rh atom for NH3 and unfavorable for CO and C2H4. NH3 and CO prefer the coordination at the Rh-axial (Ax) site to the Rh-equatorial (Eq) site, but C2H4 prefers coordination at the Rh-Eq site to the Rh-Ax site. Consequently, two CO and C2H4 molecules coordinate with 1 at the Rh-Ax and Rh-Eq sites to afford trigonal bipyramidal complexes Rh(PAlP)(CO)2 and Rh(PAlP)(C2H4)2, which is consistent with the experimental observation of Rh(PAlP)(CO)2. Energy decomposition analysis reveals that an electrostatic term plays an important role for NH3 coordination with the Al atom of 1, because Al has a significantly large positive charge and NH3 has a much negatively charged N atom and exhibits a considerably negative electrostatic potential at the Al position. In B and Ga analogues Rh(PBP) 2 and Rh(PGaP) 3, B and Ga atoms are not good for CO and C2H4 like the Al atom in 1. NH3 adducts with 2 and 3 at the B and Ga sites are less stable than those adducts at the Rh-Ax site unlike the NH3 adduct with 1 at the Al site. This difference in the NH3 adduct between Rh(PAlP) and others (Rh(PBP) and Rh(PGaP)) arises from much less positive charges of B and Ga and a smaller atomic size of B than that of Al. These results indicate that the significantly large electropositive nature and appropriate atomic size of Al are responsible for the characteristic coordination flexibility of Rh(PAlP).

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