Vapochromism Induced by Volatile Organic Compounds with Low Coordinating Ability in a Benzimidazole-Derived <i>N</i> -Heterocyclic Carbene–Ni(II) Complex

Transition-metal complexes that exhibit vapochromism are effective for detecting harmful volatile organic compounds (VOCs). Vapochromism of 3d-transition-metal complexes is generally induced by the coordination of VOC molecules to the metal center; however, this mechanism is applicable only to the detection of VOCs with relatively high coordinating ability. Herein, we demonstrate that the dicationic Ni(II) complex <b>1</b>, which contains a benzimidazole-derived <i>N</i>-heterocyclic-carbene (NHC) ligand, exhibits vapochromism via an unprecedented mechanism. The crystal structure of <b>1</b> consists of alternately stacked layers of dicationic <b>1</b> and H₂O molecules containing Cl^- anions. The color of the crystalline powder of <b>1</b> changes reversibly from yellow to green within 5 min upon exposure to the vapors of VOCs with low coordinating ability, such as acetone and dichloromethane. XANES and EXAFS measurements, as well as DFT calculations, indicated that this color change is induced by a hemilabile-ligand substitution reaction between the benzimidazole moiety and the Cl^- anion. Based on this mechanism, 3d-transition-metal complexes may find applications in the detection of VOC molecules with low coordinating ability.