Dimerization of a Metal Complex through Thermally Induced Single‐Crystal‐to‐Single‐Crystal Transformation or Mechanochemical Reaction

Abstract

Single-crystal-to-single-crystal (SCSC) transformations, which involve cooperative movement of atoms in the solid state, have received much attention from chemists. Through SCSC transformation, new complexes, which may not be obtained under conventional conditions, can be obtained in high yield. The SCSC transformation was first reported as a photoreaction in organic complexes, which could be considered as a new organic synthetic method. Solid-state photochemical [2+2] cycloadditions of organic molecules have been well studied. Recently, SCSC transformation has been extended to other complexes, such as metal complexes and coordination polymers (or metal–organic frameworks). In the past decades, many metal–organic frameworks (MOFs) that undergo SCSC transformation have been reported. Most of these reports describe the sliding of layers or breathing of 3D porous MOFs through solvent exchange. SCSC transformations involving breaking and forming bonds in the solid state remain quite rare. The MacGillivray and Foxman groups attained considerable success in solid-state photodimerization of organic groups mediated by metals. Converting a finite metal complex into an infinite coordination network was also reported by the groups of MacGillivray and Vittal. The reported topochemical dimerizations of metal complexes involve the organization of two double or triple bonds in the solid state: the geometric criteria are very important for such SCSC reactions. However, the coordination geometry of the metal ion remains unchanged in these topochemical dimerizations. More recently, zur Loye and co-workers described a 3D cobalt–organic framework exhibiting reversible shrinkage and expansion involving a change in the cobalt coordination geometry through SCSC transformation induced by the removal and re-absorption of guest molecules. Studies on the change in the metal coordination geometry caused by the removal and addition of ligands from the framework itself through SCSC transformation remain less common. Herein, we describe a new type of SCSC dimerization of a mononuclear copper complex, specifically, thermally induced head-to-tail dimerization involving the formation of a new Cu Oligand bond and a change in the metal coordination geometry. The transformation induced a color change from blue to green, which was caused by the change in the copper coordination geometry (Scheme 1). Interestingly, the trans-