We report rhodium complexes bearing PAlP pincer ligands with an X-type aluminyl moiety. IR spectroscopy and single-crystal X-ray diffraction analysis of a carbonyl complex exhibit the considerable σ-donating ability of the aluminyl ligand, whose Lewis acidity is confirmed through coordination of pyridine to the aluminum center. The X-type PAlP-Rh complexes catalyze C2-selective monoalkylation of pyridine with alkenes.

The unique Rh–Al bond in recently synthesized Rh­(PAlP) 1 {PAlP = pincer-type diphosphino-aluminyl ligand Al­[NCH2­(P i Pr2)]2­(C6H4)2­NMe} was investigated using the DFT method. Complex 1 has four doubly occupied nonbonding d orbitals on the Rh atom and one Rh d orbital largely participating in the Rh–Al bond which exhibits considerably large bonding overlap between Rh and Al atoms like in a covalent bond. Interestingly, Rhδ−−Alδ+ polarization is observed in the bonding MO of 1, which is reverse to Rhδ+−Eδ− (E = coordinating atom) polarization found in a usual coordinate bond. This unusual polarization arises from the presence of the Al valence orbital at significantly higher energy than the Rh valence orbital energy. Characteristic features of 1 are further unveiled by comparing 1 with similar Rh complexes RhL­(PMe3)2 (2 for L = AlMe2, 3 for L = Al­(NMe2)2, 4 for L = BMe2, 5 for L = SiMe3, 6 for L = SiH3, 7 for L = CH3, 8 for L = OMe, and 9 for L = Cl). As expected, 7, 8, and 9 exhibit usual Rhδ+−Eδ− polarization (E = coordinating atom) in the Rh–E bonding MO. On the other hand, the reverse Rhδ−−Eδ+ polarization is observed in the Rh–E bonding MOs of 2–5 like in 1, while the Rh–Si bond is polarized little in 6. These results are clearly understood in terms of the valence orbital energy of the ligand. Because the LUMO of 1 mainly consists of the Rh 4dσ, 5s, and 5p orbitals and the Al 3s and 3p orbitals, both Rh and Al atoms play the role of coordinating site for a substrate bearing a lone pair orbital. For instance, NH3 and pyridine coordinate to both Al and Rh atoms with considerably large binding energy. PAlP exhibits significantly strong trans influence, which is as strong as that of SiMe3 but moderately weaker than that of BMe2. The trans influence of these ligands is mainly determined by the valence orbital energy of the ligand and the covalent bond radius of the coordinating E atom.

DFT modelling suggests a metallacycle mechanism for the dimerization of ethene over a faujasite-supported Rh(i) complex, rationalizing the experimental selectivity.

Asymmetric transfer hydrogenation of various cyclic imines proceeded efficiently with H2O/MeOH (1 : 1, v/v) co-solvent media in 20 min with excellent yields and enantioselectivities by using Rh–TsDPEN catalyst and HCOONa as a hydrogen donor.