Axial Tri-tert-butylphosphane Coordination to Rh2(OAc)4: Synthesis, Structure, and Catalytic Studies

The introduction of strong σ-donor axial ligands to the Rh–Rh metal bond has been utilized as an effective way to provide new chemical reactivities to bimetallic dirhodium­(II) complexes. In this report, Rh2(OAc)4 complexes with axial bulky alkylphosphane ligands (PR3), in particular P­(t-Bu)3, were prepared and characterized. The net σ-donation from the PR3 to the Rh–Rh bond is the result of the competition between the electron-donating ability of the R group and the steric profile of the PR3 at the Rh2 core. Analysis of the crystal structure data showed that the strong σ-donor P­(t-Bu)3 coordinates to the rhodium with an amount of σ-donation to the rhodium similar to that of the aryl phosphane ligand PPh3, but has an unusually long Rh–P bond distance (2.663 Å). During catalytic trials to synthesize 3-aryl-3-hydroxy-2-oxindole by the addition of arylboronic acids to isatin derivatives, this longer Rh–P bond distance in Rh2(OAc)4(P­(t-Bu)3)2 (Cat-1) facilitates substitution of one of the axial phosphane ligands by the arylboronic acid. This σ-donating effect greatly accelerated the arylation reaction in comparison to alternative catalysts. Additionally, Rh2(OAc)4 was easily recovered after completion of the reaction.