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