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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 [Cu2(μ-dppy)3Cl]PF6·CH2Cl2 (1) or polyhedron crystals of
[Cu4(μ-dppy)6Cl](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 Cu2-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