Solvent and temperature induced switching between structural isomers of Rh(I) phosphinoalkyl thioether (PS) complexes

To develop functional systems based on the weak-link approach (WLA), it is important to understand how solvent and ligand binding strength alter the coordination geometry of complexes formed from this method. A series of phosphinoalkyl thioether (PS) hemilabile ligands with varying electron donating abilities were synthesized and incorporated into homoligated Rh(I)(PS)2Cl complexes to help understand the effects of solvent and ligand binding strength on the preferred coordination modes. The switching between closed and semiopen structural isomers of these Rh(I)(PS)2Cl complexes was studied by variable temperature 31P NMR spectroscopy in different solvent mixtures of CH2Cl2 and tetrahydrofuran (THF) to obtain thermodynamic parameters (DeltaG(o), DeltaH(o), TDeltaS(o), and K(eq)). The isomers differ in the position of the chloride counterion. In the closed isomer, the Cl- anion occupies the outer coordination sphere, while in the semiopen isomer, the Cl- has moved inner sphere and displaced one of the Rh-S bonds. The closed isomer is favored in CH2Cl2 and the semiopen isomer is favored in THF. The preference for either isomer at equilibrium depends on the solvent polarity, based upon the E(T)(N) solvent polarity scale, as was determined from 15 different solvents, with more polar solvents favoring the closed isomer. The isomer preference also depends on the electron donating ability of the group attached to the sulfur of the PS ligand, with electron donating groups favoring the closed isomers and electron withdrawing groups favoring the semiopen isomers. The formation of the semiopen isomer from the closed isomer is entropically favored but enthalpically disfavored under all conditions studied. Elucidation of the principles and environments that determine the equilibrium between the two isomers will aid in the design of functional complexes prepared by the WLA.