Elucidating the Electronic Nature of Rh-basedPaddlewheel Catalysts from ¹⁰³Rh NMRChemical Shifts: Insights from QuantumMechanical Calculations

The tremendous importance of dirhodium paddlewheel com-plexes for asymmetric catalysis is largely the result of anempirical optimization of the chiral ligand sphere aboutthe bimetallic core. Only recently, a H(C)Rhtriple resonance 103Rh NMR experiment provided the long-awaited opportunity to examine – with previously incon-ceivable accuracy – how variation of the ligands impacts onthe electronic structure of such catalysts. The recorded ef-fects are dramatic: formal replacement of one out ofeight O-atoms in a dirhodium tetracarboxylate by an N-atom results in a shielding of thecorresponding Rh-site of no less than 1000 ppm. The cur-rent paper provides the framework that allowsthis and related experimental observations made with a setof 19 representative rhodium complexes to be interpreted.In line with symmetry considerations, it is shown that theshielding tensor responds only to the donor ability of theequatorial ligands along the perpendicular principal axis.Axial ligands, in contrast, have no direct effect on shield-ing but exert an electronic cis-effect that onto the neighboring equatorial sites. Furthermore, charge redistribution within the core as well as the electronic trans-effect of ligands ofdifferent donor strengths is reflected in the 103 Rh NMR shifts