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·BF4 {2+ = [Cu­(Mepypm)­(dppp)]+, where Mepypm
= 4-methyl-2-(2′-pyridyl)­pyrimidine and dppp = 1,3-bis­(diphenylphosphino)­propane}
and previously reported 1·BF4, 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·BF4 in CDCl3 (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 CD2Cl2, acetone-d6, and CD3CN. 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(C6F5)4–. The findings are valuable
for the design of molecular mechanical units that can be readily tuned
via weak electrostatic interactions.