Unprecedented large temperature dependence of silver(I)-silver(I) distances in some N-heterocyclic carbene silver(I) complex salts

摘要

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 [PF_6]~-, [SbF _6]~-, [ClO_4]~-, or [AgBr_2]~- anions, respectively, were prepared in high yields and characterized by elemental analysis, ~1H and ~(13)C 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 [PF_6]~- anions, a contraction from 3.498(8) to 3.180(2) ? 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.