Effect of Tethered Sulfur Ligands on the Association Constants of Dirhodium(II,II) Paddlewheel Complexes

Abstract

AbstractHerein we report on the association constants (Ka) of dirhodium(II,II) complexes bearing tethered thioether or sulfoxide ligands in the presence of binding analytes. The effect of substituent and tether length modifications on Ka and the stability of the complexes were analyzed by UV‐vis using pyridine and cysteine as binding analytes of interest. When pyridine was the incoming exogenous species, dirhodium(II,II) complexes with tethered sulfur ligands reduced the axial coordination of pyridine by up to four‐fold when compared to dirhodium(II,II) complexes without tethered axial coordination. Tuning the tethered sulfur coordination through changes in tether length, substituent, and electronics resulted in Ka values that spanned 2.6×103 M−1 to 8.1×103 M−1. The differences in Ka were correlated with structural features of crystals obtained from single crystal X‐ray diffraction and results pointed to release of strain as a factor in the association constants. Interestingly, cysteine interactions were greater for complexes with tethered, axial ligands than those without, alluding to an alternate mode of coordination with cysteine. These results demonstrate the tunability of tethered, axial coordination on dirhodium(II,II) paddlewheel complexes and provide access to controlled binding of exogenous species.