Coordinated Aqua vs Methanol Substitution Kinetics in fac-Re(I) Tricarbonyl Tropolonato Complexes

Water-soluble fac-[Re­(CO)3(L,L′-Bid)­(X)] (L,L′-Bid = tropolonato, X = H2O, methanol) complexes have been synthesized, and the aqua and methanol substitution reactions were investigated in water (pH range 6.3–10.0) and methanol, respectively, and compared. Thiocyanate ions were used as monodentate entering ligand. The complexes were characterized by UV–vis, IR, and NMR spectroscopy. The crystal structures of the complexes [NEt4] fac-[Re­(Trop)­(CO)3(H2O)].NO3.H2O (reactant) and fac-[Re­(CO)3(Trop)­(Py)], a substitution product, are reported. Overall it was found that the aqua substitution of fac-[Re­(CO)3(Trop)­(H2O)] is about 10 times faster than the methanol substitution reaction for fac-[Re­(CO)3(Trop)­(MeOH)], with forward and reverse rate and stability constants [k1 (M–1 s–1), k–1 (s–1), K1, (M–1)] for thiocyanate as monodentate entering ligand as follows: fac-[Re­(CO)3(Trop)­(H2O)] = 2.54 ± 0.03, 0.0077 ± 0.0005, 330 ± 22/207 ± 14 and fac-[Re­(CO)3(Trop)­(MeOH)] = 0.268 ± 0.002, 0.0044 ± 0.0002, (61 ± 3)/(52 ± 4). The activation parameters [ΔH⧧k1 (kJ mol–1), ΔS⧧k1 (J K–1 mol–1)] for the aqua and methanol complex respectively are 56.1 ± 0.7, −49 ± 2 and 64 ± 1, −43 ± 5.