Structural and spectroscopic properties of a copper(I)-bis(N-heterocyclic)carbene complex

The structural and spectroscopic properties of a Cu(I) complex bearing a methylene-linked bis(N-heterocyclic carbene) ligand, [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) were investigated. X-ray single crystal structure analysis revealed that the complex is binuclear similar to the corresponding silver(I) complex. In [Cu(2)(mu-Me-mbim)(2)](PF(6))(2), cation-pi interaction between copper and the adjacent carbene carbon is observed. On the other hand, the copper-copper interaction is very weak in the crystal and almost negligible in solution. The absorption spectrum of [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) in methanol shows a strong absorption band (epsilon = 23 000 dm(3) mol(-1) cm(-1)) and a weaker shoulder (epsilon = 6200 dm(3) mol(-1) cm(-1)) at 261 nm and 300 nm, respectively. From molecular orbital calculations using TD-DFT, these absorption bands are assigned to the metal-centered transitions with some contribution from the NHC orbitals. The powdered sample of [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) shows bright blue-green phosphorescence with a high quantum yield (43%). The phosphorescence is of dual-emission character at room temperature with peak maxima at 374 nm and 482 nm whereas it changes to a single emission band centered around 500 nm at 77 K. Molecular orbital calculations indicate that the luminescence derives from the triplet MC and MLCT mixed excited states. A methanolic solution of [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) shows yellow-green phosphorescence with a peak maximum at 542 nm. Unlike in the solid state, no dual-emission was observed. These results suggest that the dual emission is caused by differences in the contribution of metal-metal interactions at room temperature in the solid state. The differences in the absorption and emission properties between [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) and the related Cu(I)-diphosphine complex, [Cu(2)(mu-dcpm)(2)](BF(4))(2) are discussed.