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.

We report the catalytic reduction of a C–O bond and the borylation by a rhodium complex bearing an X-type PAlP pincer ligand. We have revealed the reaction mechanism based on the characterization of the reaction intermediate and deuterium-labeling experiments. Notably, this novel catalytic system shows steric-hindrance-dependent chemoselectivity that is distinct from conventional Ni-based catalysts and suggests a new strategy for selective C–O bond activation by heterobimetallic catalysis.

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.

No abstract

Transition-metal complexes that bear group-13 X-type metalloligands have attracted considerable interest on account of their strong σ-donicity, which can dramatically tune the reactivity of transition-metal catalysts. Among the group-13 X-type metalloligands, X-type aluminyl ligands are especially interesting due to the low electronegativity and strong Lewis acidity of the Al atom. However, transition-metal complexes that bear X-type aluminyl ligands had until recently remained elusive. In this perspective, we describe the reactivity of transition-metal complexes that carry a well-designed X-type PAlP pincer-ligand and their characteristic features, i.e., strong σ-donicity, trans-influence, reverse bond polarization, and Lewis acidity. These properties enable various catalytic transformations, including transfer dehydrogenation, highly efficient CO2 reduction, site-selective C–H-bond functionalization, and cooperative C–F- and C–O-bond functionalization, which are challenging to achieve using conventional catalyst systems.