Unprecedented Large Temperature Dependence of Silver(I)-Silver(I) Distances in Some N-Heterocyclic Carbene Silver(I) Complex Salts

Six examples from a series of complex salts containing bis(1,3-dialkylimidazol-2-yliderie)silver(I) cations (with dialkyl = dimethyl, diethyl, methyl, ethyl, diisopropyl) with [PF6](-), [SbF6](-), [ClO4](-), or [AgBr2]- anions, respectively, were prepared in high yields and characterized by elemental analysis, H-1 and C-13 NMR spectroscopy, and mass spectrometry. Single-crystal X-ray diffraction experiments reveal unprecedentedly large contractions of the metallophilic Ag(I)-Ag(I) distances in the solid-state structures upon cooling. In the salt containing diisopropyl-substituted ligands and [PF6](-) anions, a contraction from 3.498(8) to 3.180(2) angstrom was observed on cooling from 293(1) to 100(1) K. Photochemical measurements show strong hypsochromic shifts of the emission maxima upon cooling, underlining the metallophilic-based nature of the emission bands. Ab initio calculations show that the strong temperature dependence of the observed Ag-Ag distances can be attributed to some extent to both the shallowness and the anharmonicity of the intermetallic interaction potential. The Ag-Ag interaction potentials are found to be attractive only when relativity is accounted for in the calculations.