THERMODYNAMICS OF COPPER(II) COMPLEXATION BY MACROCYCLIC TETRATHIOETHERS AT 25-DEGREES-C IN WATER-METHANOL MIXTURES AND ACETONITRILE - CORRELATIONS OF COMPLEXATION ENTHALPY AND LIGAND CONFORMATIONAL PREFERENCES

The stability constants for the interaction of copper(II) ions with three 14-membered tetrathioethers (i.e., [14]aneS4, Me2[14]aneS4, and Me4[14]aneS4) have been determined spectrophotometrically in water-methanol mixtures with various concentrations of ClO4- ion at 25-degrees-C. The stability constant values, K1, for Cu2+-L interaction increase from 2.48 for Cu([14]ane [S]4)2+ to 3.04 for Cu(Me2[14]aneS4)2+ and 3.56 for Cu(Me4[14]aneS4)2+ in 80% methanol. The enthalpies of complex formation (DELTA-H-1) measured calorimetrically at 25-degrees-C for Cu2+-L interaction in 0.1 M HClO4 in 90% methanol were-0.87 kcal.mol-1 for [14]aneS4,-2.45 kcal.mol-1 for Me2[14]aneS4 and -3.43 kcal-mol-1 for Me4[14]aneS4. Corresponding DELTA-H-1 values(kcal.mol-1)for acetonitrile (mu = 0.005) were -9.02 for [14]aneS4, -9,84 for Me2[14]aneS4, and -10.73 for Me4[14]aneS4. The perchlorate anion forms comparatively strong ternary complexes with Cu(II)-tetrathiomacrocycle cations. The activity-corrected values for perchlorate association with these cations in 80% MeOH are as follows: log K3 (25-degrees-C) = 2.81 for Cu*[14]aneS4)2+, 3.03 for Cu(Me2[14]aneS4)2+, and 3.25 for Cu(Me4[14]aneS4)2+. The thermodynamic data, in conjunction with prior structural results, support our earlier hypothesis (Desper, J. M.; Gellman, S. H. J. Am. Chem. Soc. 1991, 113, 704) that the gem-dimethyl pairs reduce the conformational enthalpy cost required to organize the ligand for chelation.