Novel carbon-centered heteroscorpionate ligands: Cu(I) complexes and luminescence properties

Herein, we describe the synthesis of N,N′,S donor ligands 2-(1-(3,5-diisopropyl-1H-pyrazol-1-yl)-3-(methythio)propyl)-4-methoxy-3,5-dimethylpyridine (L1) and 2-(1-(3,5-diisopropyl-1H-pyrazol-1-yl)-2-(methythio)ethyl)-4-methoxy-3,5-dimethylpyridine (L2). Cu(I) complexes were prepared by reacting L1 or L2 with [Cu(CH3CN)4]BF4 or CuCl. The coordination behavior of the thioether arm of the ligands was found to determine the nuclearity of the resulting complexes, in which [Cu(L1)PPh3]BF4 (1) is polynuclear, [Cu(L2)PPh3]BF4 (2) is mononuclear, while [Cu(L1)]2(BF4)2 (3), [Cu(L2)CH3CN]2(BF4)2 (4), and [Cu(L1)Cl]2 (5) are dinuclear. In the dimeric complex [Cu(L2)Cl]2 (6), the sulfur atoms are not metal-bound. Rather, the two bridging chloride ions link the two copper centers. Compounds 4–6 are luminescent in the solid state, and exhibit emission bands centered at 490 nm (4), 544 nm (5), and 562 nm (6), respectively. Their excitation spectra display bands at 280 nm and 380 nm. According to DFT calculations, the HOMO is distributed partially over the metal centers and partially over the chloride anions (5 and 6) or the sulfur atoms (4) of the ligands, while the LUMO is a π∗ antibonding pyridine orbital. This suggests that the emission properties are derived from metal-to-ligand charge-transfer (MLCT), halide-to-ligand charge-transfer (XLCT), and ligand-to-ligand charge-transfer (LLCT) excited states.