Copper co-ordination compounds of a chelating imidazole–thioether ligand. The molecular structures of [1,3-bis(5-methyl-4-imidazolyl)-2-thiapropane]-bis(nitrato)copper(II) and bis[1,3-bis(5-methyl-4-imidazolyl)-2-thiapropane]-copper(II) bis(tetrafluoroborate)–ethanol(1/2)

Abstract

The synthesis and spectroscopic properties of copper compounds with the tridentate ligand 1,3-bis(5-methyl-4-imidazolyl)-2-thiapropane (L) are described. Two series of compounds have been obtained, one green and the other purple. The green series has general formula [Cu(L)X2], in which X = Cl–, Br–, NCS–, or NO3– and [Cu(L)Cl(BF4)]. In these compounds both the ligand and the anions are co-ordinated to the copper(II) ion. The purple series has general formula [Cu(L)2]Y2·nH2O, with Y = Cl–, Br–, NO3–, or BF4– and n= 0–3, and [Cu(L)2][NCS][NO3]·H2O, with only the ligands L co-ordinated to the copper(II) ion. In all compounds L acts as a tridentate ligand with the thioether sulphur and both imidazole nitrogens co-ordinating. The compound [Cu(L)(NO3)2](A) crystallizes in the monoclinic space group P21/n with a= 18.060(5)A, b= 7.063(4)A, c= 13.229(3)A, β= 104.13(3)°, for Z= 4. The R′ value is 0.032 for 1971 significant reflections. The copper(II) ion has a distorted square-pyramidal geometry with an equatorial N2S donor set, an equatorial oxygen, and an axial oxygen from the nitrate ions. The Cu–N distances are 1.94 A, Cu–S 2.37 A, and the distance of the copper ion to the equatorial oxygen is 1.99 A and to the axial oxygen 2.41 A. The compound [Cu(L)2][BF4]2·2C2H5OH (B) crystallizes in the monoclinic space group P21/c with a= 8.262(2), b= 9.609(3), c= 22.582(5)A, β= 100.04(2)°, for Z= 2. The R′ value is 0.09 for 2 149 independent reflections (including low-intensity reflections). The copper(II) ion has a distorted-octahedral geometry with four equatorial N-donors and two long-distance axial sulphur atoms. The Cu–N distances are 2.00 A, the Cu–S distances 2.79 A. Electrochemistry of the two series (1 : 1 and 1 : 2) of copper(II) compounds points to two distinct one-electron reductions in both cases. Only the first copper(II)–copper(I) redox change, which occurs at rather high positive potentials, is chemically reversible. The stability of the copper(II)–copper(I) couple both in the five-co-ordinate [Cu(L)X2]0/– and six-co-ordinate [Cu(L)2]2+/+ assemblies gives evidence for the relative flexibility and versatility of the tridentate ligand.