Zinc Thiolate Complexes [ZnLn(SR)]+ with Azamacrocyclic Ligands, Part II: Mechanism of the Reaction with CS2

The thiolate complexes [Zn([15]aneN(4))(S-CH2-C6H5)]ClO4 (1) ([15]aneN(4) = 1,4,8,12-tetraazacyclopentadecane) and [Zn(i-[14]aneN(4))(S-CH2-C6H5)ICIO4 (2) (i-[14]aneN(4) = 1, 4,7, 11 -tetraazacyclotetradecane) have been reacted with carbon disulfide. The trithiocarbonate complexes [Zn([15]aneN(4)){S-C(S)-S-CH2-C6H5}]CIO4 {1a, monoclinic, space group P2(1)/n, Z = 8, a = 13,2338(1) angstrom, b = 12.9251(2) angstrom, c = 30.1669(4) angstrom, beta = 101.463(1)degrees, V = 5057.1(1) angstrom(3)} and [Zn(i[14]aneN(4)){S-C(S)-S-CH2-C6H5}]CIO4 {2a, orthorhombic, space group P2(1)2(1)2(1), Z = 8, a = 9.9936(1) angstrom, b 22.1261(4) angstrom, c = 22.3192(4) angstrom, V = 4935.2(l) angstrom(3)} were obtained. The reaction of 1 with CS2 is second order with a rate constant of k = (57.6 +/- 2.4) x 10(-3) m(-1.)s(-1) at 25 degrees C. The experimentally determined Eyring activation barrier is Delta H-exp(double dagger) = 65.3 +/- 0.7 kJ(.)mol(-1) (Delta S-exp(double dagger) = -49.9 +/- 2.5 J(.)mol(-1.)K(-1)) and a free energy of activation of Delta G* = 80.2 +/- 1.5 kJ(.)mol(-1) at 25 degrees C. To discriminate between an associative and a dissociative mechanism the barriers for both processes were calculated using density functional theory at the C-PCM(B98/G3MP2Large)//B3LYP/ 6-311+G(d) level. The associative mechanism is clearly favored with a difference in free energies of activation of delta Delta G(double dagger) approximate to 80kJ(.)mol(-1). Its calculated barrier Delta G(theor)(double dagger) = 114.3 kJ(.)mol(-1) is in reasonable agreement with the experimental value. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).