Cationic Sulfonium-based Tripodal Ligand and its Rh(I) Complexes

We report the first example of a cationic tripodal ligand consisting of a sulfonium donor embedded within a tripodal scaffold. Coordination of this ligand to Rh(I) gives rise to a series of mono- and bis-cationic Rh(I) complexes, all featuring a trigonal bipyramidal geometry. To elucidate the distinctive characteristics of the new sulfonium ligand, its Rh(I) carbonyl complex was subjected to a comprehensive structural (XRD) and spectroscopic (NMR, IR) analysis, in direct comparison with isostructural analogs featuring neutral P-based and anionic Si-based ligands. This study revealed systematic trends along this series, including a progressive shortening of the central E-Rh bond (E = Si, P, S) and an increasing blue shift of the CO stretching frequencies. These results were fully corroborated by density functional theory calculations, which also indicated a steady depletion of the local negative natural charge on the Rh atom along the series. Electrochemical studies (CV) further highlighted the profound effect imposed by the cationic sulfonium donor on the coordinated Rh(I) center, revealing an anodic shift of ~1.0 V in its reduction potential, relative to the P-based analog. The Lewis acidity of coordinatively unsaturated Rh(I) complexes was quantified using the Guttman-Becket method, yielding acceptor numbers approaching those of the highly Lewis acidic polyfluoroaryl boranes for the sulfonium complex. Finally, we demonstrated that the enhanced Lewis acidity of the Rh(I) center imparted by coordination to the sulfonium donor can be exploited in Lewis acid catalysis, as illustrated by a series of representative 2:1 double condensations of ketones with indole or pyrrole.