“On−Off” Au(I)···Cu(I) Interactions in a Au(NHC)2 Luminescent Vapochromic Sensor

摘要

Reaction of the Au(I) N-heterocyclic carbene (NHC) compound [Au(im(CH₂py)₂)₂]PF₆ with 2 equiv of [Cu(MeCN)₄]PF₆ affords the tricationic compound [Au(im(CH₂py)₂)₂(Cu(MeCN)₂)₂](PF₆)₃ (1), which exhibits blue luminescence (λ_max = 462 nm). Reaction of 1 with either liquid MeOH or MeOH vapor affords [Au(im(CH₂py)₂)₂(Cu(MeOH))₂](PF₆)₃ (2), which produces green luminescence (λ_max = 520 nm) under UV excitation. The molecular structures of 1·2MeCN and 2·2MeOH·2Et₂O were determined by single-crystal X-ray diffraction. Compound 1 contains a linearly coordinated [Au(NHC)₂]+ core in which each picolyl side arm bridges a [Cu(MeCN)₂]+ center. The Au···Cu separations are long at 4.596 Å. Compound 2 exhibits two short Au···Cu interactions of 2.7195(7) Å, with the Au(NHC)₂ core acting as an additional ligand toward each copper center to complete its tetrahedral coordination mode. Exposure of 2 to atmosphere produces a partial loss of MeOH accompanied by a luminescence color change to yellow (λ_max = 543 nm). The uptake and loss of MeOH vapor is rapid and reversible. Exposure of 2 to vacuum affords complete loss of MeOH, and the luminescence changes to yellow-orange (λ_max = 573 nm). Treatment of 2 with MeCN vapor regenerates 1. The interconversion of 1 and 2 was confirmed by powder X-ray diffraction. Compound 1 also reacts with acetone and H₂O vapors, leading to species that produce yellow-orange (λ_max = 591 nm) and green (λ_max = 519 nm) emission, respectively. Compounds 1 and 2 are examples of molecular vapochromic materials that exhibit large changes in the emission though ligand substitution reactions between the solid complex and solvent vapors. The dramatic color change likely results from the “on−off” Au···Cu interactions induced by the ligand exchange reaction.