Solvated-ion-pairing-sensitive molecular bistability based on copper(I)-coordinated pyrimidine ring rotation

We describe herein the effect of solvated ion pairing on the molecular motion of a pyrimidine ring coordinated on a copper center. We synthesized a series of heteroleptic copper(I) complex salts bearing an unsymmetrically substituted pyridylpyrimidine and a bulky diphosphine. Two rotational isomers of the complexes were found to coexist and interconvert in solution via intramolecular ligating atom exchange of the pyrimidine ring, where the notation of the inner (i-) and outer (o-) isomers describes the orientation of the pyrimidine ring relative to the copper center. The stability of the pyrimidine orientation was solvent- and counterion-sensitive in both 2·BF(4) {2(+) = [Cu(Mepypm)(dppp)](+), where Mepypm = 4-methyl-2-(2'-pyridyl)pyrimidine and dppp = 1,3-bis(diphenylphosphino)propane} and previously reported 1·BF(4), which possesses a bulky diphosphine ligand (1(+) = [Cu(Mepypm)(DPEphos)](+), where DPEphos = bis[2-(diphenylphosphino)phenyl] ether). Two rotational isomers of 2(+) were separately obtained as single crystals, and the structure of each isomer was examined in detail. Both the enthalpy and entropy values for the rotation of 2·BF(4) in CDCl(3) (ΔH = 6 kJ mol(-1); ΔS = 25 J K(-1) mol(-1)) were more positive than that tested under other conditions, such as in more polar solvents CD(2)Cl(2), acetone-d(6), and CD(3)CN. The reduced contact of the anion to the cation in a polar solvent seems to contribute to the enthalpy, entropy, and Gibbs free energy for rotational isomerization. This speculation based on solvated ion pairing was further confirmed by considering the rotational behavior of 2(+) with a bulky counterion, such as B(C(6)F(5))(4)(-). The findings are valuable for the design of molecular mechanical units that can be readily tuned via weak electrostatic interactions.