Controllable Preparation and Direct Observation of an Interconversion between Kinetically and Thermodynamically Stable Luminescent Multicuprous Coordination Complexes

We report the controlled preparation of kinetically and thermodynamically luminescent multinuclear cuprous coordination complexes. On reaction of the bidentate P/N-ligand (2-(diphenylphosphino)-pyridine, abbreviated as dppy) and copper-(I) with a similar stoichiometry but in different concentration conditions at ambient temperature, hexagonal crystals of [Cu-2(mu-dppy)(3)Cl]-PF6 center dot CH2Cl2 (1) or polyhedron crystals of [Cu4(mu-dppy)(6)Cl]-(PF6)(3) (2) were formed as kinetic or thermodynamic products, respectively. Investigation of the structures, morphologies, theoretical calculation, and photophysical properties of the multinuclear cuprous coordination complexes demonstrates the formation of and differences between kinetically and thermodynamically stable conformations. Single-crystal X-ray structural analyses indicated that 1 features the head-to-head orientation dppy bridged dinuclear trefoil-like Cu-2-core with one terminal Cl-, and tetranuclear 2 can be regarded as a Cl- bridged dimer of 1 with two-third flipping head-to-tail dppy ligands. The density functional theory (DFT) calculations demonstrated that 2 had a lower frontier molecular orbital energy than 1, with an energy difference of 320.76 kJ/mol. Intriguingly, the controllable reversible transformation between 1 and 2 can be achieved under suitable conditions. By soaking the crystal in the mother liquor or alcohol, the kinetically stable 1 can slowly transform to the thermodynamically stable 2, or conversely, 2 can also quickly reconstruct to 1 when it is soaked in a chloride solution. Steadily changing morphologies, photoluminescence, as well as powder X-ray diffraction patterns were observed during the gradual transformation process. Based on structural analyses, a feasible transforming mechanism was proposed. The energetic and structural analyses of and the interconversion process between kinetically and thermodynamically stable multicuprous complexes here provide insights into the relations among the structures, morphologies, and properties of the metal-organic coordination materials at the molecular level.