Electronic Structures and Reduction Potentials of Cu(II) Complexes of [N,N′-Alkyl-bis(ethyl-2-amino-1-cyclopentenecarbothioate)] (Alkyl=Ethyl, Propyl, and Butyl)

Copper(II) complexes of N,N'-alkyl-bis(ethyl-2-amino-1-cyclopentenecarbodithioate) [alkyl=ethyl (L2), propyl (L3), and butyl(L4)] ligands have been synthesized and characterized. Analytical data for all three complexes show 1:1 copper-ligand stoichiometry. Well-resolved EPR spectra were recorded in toluene, benzene, and methylene chloride solutions at room temperature and in glassy toluene or toluene-methylene chloride mixtures in the range 20–150 K. The superhyperfine pattern unambiguously demonstrates coordination of two nitrogen atoms to copper; and the spin-Hamiltonian parameters [CuL2, g_∥=2.115, A_∥=187×10^(–4) cm^(–1); CuL3, g_∥=2.128, A_∥=165×10^(–4) cm^(–1); CuL4, g_∥=2.138, A_∥=147×10^(–4) cm^(–1)] are as expected for a CuN_2S_2 coordination core. Quasi-reversible electrochemical behavior was observed in methylene chloride: the Cu(II)/Cu(I) reduction potentials increase from –1.17 V (E° vs Ag/AgNO_3) for CuL2 to –0.74 V for CuL4, indicating greater stabilization of Cu(I) in CuL4. Taken together, these data demonstrate that lengthening the N,N′-alkyl chain distorts the planar CuN_2S_2 core (CuL2) toward a flattened tetrahedral geometry (CuL4).