Rh Complex with Unique Rh-Al Direct Bond: Theoretical Insight into its Characteristic Features and Application to Catalytic Reaction via σ-Bond Activation

A new type of catalyst for reactions via sigma-bond activation is Rh(PAlP): Rh-Al heterobimetallic complex of diphosphine-based tridentate pincer-type PAlP ligand bearing an aluminyl moiety at the center coordination site. This work reports results of theoretical and computational studies of the unique Rh-Al direct bond, flexible behavior of Rh(PAlP) for substrate coordination, and new catalytic functions of Rh(PAlP) via the C-H and C-F sigma-bond activations. It is particularly interesting that the Rh-Al bond exhibits an unusual Rh delta--Al delta+ polarization of the Rh-Al sigma-bonding MO, opposite to M delta+-L delta- polarization of the usual coordinate bond. The other characteristic feature of PAlP is its strong trans-influence, which is as strong as that of SiMe3 ligand but much stronger than that of CH3 ligand. Because the Al atom of PAlP plays a role of a Lewis acid in Rh(PAlP), Rh(PAlP) has three coordination sites: Al site, Rh axial site, and Rh equatorial site. There, NH3 coordinates to the Al site and the Rh axial site with similar stability, whereas CO coordinates to the Rh axial site and C2H4 coordinates to the Rh equatorial site. Electrostatic interaction plays a crucially important role in the NH3 coordination to the Al site. Rh(PAlP) catalyses conversion of fluorobenzene to benzoic acid via C-F sigma-bond cleavage and C(2)-regioselective alkylation of pyridine with alkene via C-H sigma-bond activation. Theoretical studies show clearly that the Rh and Al sites participate cooperatively in C-F and C-H sigma-bond activations. The unusual Rh delta--Al delta+ polarization and coordination flexibility play important roles in these catalytic reactions.

Automated summary

Rh(PAlP) is a new type of catalyst for reactions via sigma-bond activation, showing unique Rh-Al direct bond, flexible substrate coordination, and new catalytic functions for C-H and C-F sigma-bond activations. The unusual Rh delta--Al delta+ polarization and coordination flexibility play important roles in these catalytic reactions.