Ambient and high-pressure kinetic investigation of methanol substitution in fac-[Re(Trop)(CO)3(MeOH)] by different monodentate nucleophiles

Abstract

Methanol substitution in the fac-[Re(CO)₃(Trop)(MeOH)] complex (Trop⁻ = tropolonate) was studied with a range of seven nucleophiles, namely pyridine (Py), 4-dimethylaminopyridine (DMAP), imidazole (Im), thiourea (TU), 1-methyl-2-thiourea (MeTU), bromide (Br⁻) and iodide (I⁻) at variable temperature, and at ambient and high pressure. The substitution products were characterized by NMR, IR and UV/vis spectroscopy, and by chemical analysis, and the crystal structures of two of these, namely fac-[Re(Trop)(CO)₃(Im)] and fac-[Re(Trop)(CO)₃(DMAP)], are reported. High-pressure kinetic studies with four of these entering nucleophiles in methanol at 25 °C on fac-[Re(Trop)(CO)₃(MeOH)] yielded the following activation volumes, ΔV^≠_(kL), for the ligation by four nucleophiles as defined by k_L (cm³ mol⁻¹): Im: 9.0 ± 0.2; Py: 10.1 ± 0.2; TU: 10.0 ± 0.3 and MeTU: 14.5 ± 0.3. Since these experimental ΔV^≠_(kL) values were positive but smaller than expected, it was interpreted that these indicated a dissociative/dissociative interchange pathway for these substitution reactions. Kinetic studies at ambient pressure and variable temperature in methanol on fac-[Re(Trop)(CO)₃(MeOH)] with a range of eight entering nucleophiles pointed more clearly to a dissociative pathway and yielded the following results, wherein a clear linear free-energy relationship (LFER) was established for the entering nucleophiles Py, DMAP, Im, TU, MeTU, NCS⁻, Br⁻ and I⁻, within the following ranges: k_L (ligation; M⁻¹ s⁻¹), 0.263 ± 0.001 to 0.765 ± 0.002; k_−L (solvolysis; s⁻¹), (0.07 ± 0.01) × 10⁻³ to 0.674 ± 0.001; K_L (equilibrium; M⁻¹); 1.06 ± 0.01 to 2000 ± 500; ΔH^≠_(kL) (kJ mol⁻¹), 58.0 ± 0.7 to 76.1 ± 0.6, and ΔS^≠_(kL) (J K⁻¹ mol⁻¹); −55 ± 2 to 6 ± 3.