Zinc thiolate complexes [ZnL _n(SR)] ⁺ with azamacrocyclic ligands, part II: Mechanism of the reaction with CS ₂

The thiolate complexes [Zn([15]aneN ₄)(S-CH ₂-C ₆H ₅)]ClO ₄ (1) ([15]aneN ₄ = 1,4,8,12-tetraazacyclopentadecane) and [Zn(i-[14]aneN ₄)(S-CH ₂-C ₆H ₅)]ClO ₄ (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- CH ₂-C ₆H ₅}]ClO ₄ {1a, monoclinic, space group P2 ₁/n, Z = 8, a = 13.2338(1) Å, b = 12.9251(2) Å, c = 30.1669(4) Å, β = 101.463(1)°, V = 5057.1(1) Å ³} and [Zn(i-[14]aneN ₄){S-C(S)-S-CH ₂-C ₆H ₅}]ClO ₄ {2a, orthorhombic, space group P2 ₁2 ₁2 ₁, Z = 8, a = 9.9936(1) Å, b = 22.1261(4) Å, c = 22.3192(4) Å, V = 4935.2(1) Å ³} were obtained. The reaction of 1 with CS ₂ is second order with a rate constant of k = (57.6 ± 2.4) × 10 ^-3 M ^-1·s ^-1 at 25°C. The experimentally determined Eyring activation barrier is ΔH _exp ^‡ = 65.3 ± 0.7 kJ·mol ^-1 (ΔS _exp ^‡ = -49.9 ± 2.5 J·mol ^-1·K ^-1) and a free energy of activation of ΔG ^Dagger; = 80.2 ± 1.5 kJ·mol ^-1 at 25°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 δΔG ^‡ ≈ 80 kJ·mol ^-1. Its calculated barrier ΔG _theor ^‡ = 114.3 kJ·mol ^-1 is in reasonable agreement with the experimental value.