Regulation of the Rate of Dinucleation of a Monocopper(I) Complex Containing Bipyrimidine Rotary Units by Restricted Double Pyrimidine Rotation

New copper­(I) complexes with coordinated
2-(4′-methyl)­pyrimidinyl moieties were fabricated, and the
isomerism of their pyrimidine ring linkage was investigated. The ligands
bis­[2-(diphenylphosphino)­phenyl] ether (DPEPhos) and 4,4′-dimethyl-2,2′-bipyrimidine
(dmbpm) were used to synthesize a heteroleptic copper­(I) complex,
[CuI(DPEPhos)­(dmbpm)]·BF4 (1·BF4), and a dinuclear copper­(I) complex, [(CuI)2(DPEPhos)2(μ-dmbmp)]­(BF4)2 [2·(BF4)2]. The X-ray crystallographic structures, UV–vis absorption
spectra, and luminescence properties of the complexes were analyzed.
The thermodynamic and kinetic aspects of the isomerism of 1·BF4 were examined by variable-temperature NMR. Double
pyrimidine ring rotation was found to be restricted sterically by
the bulky DPEPhos ligands. This limited the number of the possible
isomers: 1·BF4 showed only isomers with
either one (io isomer) or both (oo isomer) of the two methyl groups positioned away from the copper
center, while dinuclear 2·(BF4)2 was only found as a symmetric (io–io) isomer, with each of the two methyl groups positioned
toward different copper centers. The addition of [Cu­(MeCN)2(DPEPhos)] (3·BF4) allowed both isomers
of 1·BF4 to form 2·(BF4)2, although at different rates and via different
pathways, which were analyzed using time-dependent UV–vis spectroscopy.
The io isomer dinucleated more quickly than the oo isomer owing to it being able to form 2·(BF4)2 (i) without bond dissociation and (ii) without
a sterically congested ii configuration around the
copper center. In contrast, oo-1·BF4 required (i) recombination of the bipyrimidine coordination
bonds or (ii) formation of a product with higher thermodynamic energy,
unsymmetric (ii–oo) 2·(BF4)2. These findings are interpreted
as demonstrating a novel kinetic property: a conversion rate determined
by pyrimidine ring inversion.