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-ylidene)­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, 1H and 13C 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) Å 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.