CuI Complexes with N,N′,S,S′ Scorpionate Ligands: Evidence for Dimer−Monomer Equilibria
文摘
The heteroscorpionate N,N′,S,S′ donor ligands 4-methoxy-3,5-dimethyl-2-(3-(methylthio)-1-(3-(2-(methylthio)phenyl)-1H-pyrazol-1-yl)propyl)pyridine (La) and 4-methoxy-3,5-dimethyl-2-(2-(methylthio)-1-(3-(2-(methylthio)phenyl)-1H-pyrazol-1-yl)ethyl)pyridine (Lb) were prepared. The CuI complexes [Cu(La)]2(BF4)2 (a2(BF4)2) and [Cu(Lb)]2(BF4)2 (b2(BF4)2) were synthesized and characterized by X-ray crystallography. Both compounds exhibit a dinuclear structure, presenting each CuI center in a distorted N,N′,S,S′ tetrahedral environment. On the basis of nuclear magnetic resonance (NMR) and ESI−mass data, the presence of a mononuclear complex in equilibrium with the dimer was hypothesized for both complexes. The dimerization constants of the processes, 2a+ = a22+ and 2b+ = b22+, were obtained by 1H NMR dilution experiments (fast-exchange regime) in CD3CN: log K(a22+) = 3.55(6) and log K(b22+) = 3.23(5) at 300 K. Thermodynamic parameters were determined by a van’t Hoff analysis (280−310 K temperature range): ΔH0(a22+) = −12(1) kJ mol−1, ΔH0(b22+) = −10(1) kJ mol−1, ΔS0(a22+) = +27(4) kJ mol−1, and ΔS0(b22+) = +28(4) kJ mol−1. Pulsed gradient spin–echo (PGSE) NMR experiments provided the weighted-average hydrodynamic volume (VH) of the species present in CD3CN solution at different copper concentrations (CCu). Nonlinear interpolation of VH as a function of CCu for a dimer−monomer equilibrium led to the hydrodynamic volumes of both monomers (VH0(M)) and dimers (VH0(D)): VH0(a+) = 620(40) Å3, VH0(b+) = 550(10) Å3, VH0(a22+) = 950(20) Å3, and VH0(b22+) = 900(10) Å3. Cyclic voltammetry experiments performed in CH3CN and CH2Cl2 showed a quasi-reversible to irreversible behavior of the CuI/CuII redox couple for both complexes.