Activation of Rhenium(I) Toward Substitution in fac-[Re(N,O′-Bid)(CO)3(HOCH3)] by Schiff-Base Bidentate Ligands (N,O′-Bid)

A series of fac-[Re(N,O'-Bid)(CO)(3)(L)] (N,O'-Bid = monoanionic bidentate Schiff-base ligands with N,O donor atoms; L = neutral monodentate ligand) has been synthesized, and the methanol substitution reactions have been investigated. The complexes were characterized by NMR, IR, and UV-vis spectroscopy. X-ray crystal structures of the compounds fac-[Re(Sal-mTol)(CO)(3)(HOCH3)], fac-[Re(Sal-pTol)(CO)(3)(HOCH3)], fac-[Re(Sal-Ph)(CO)(3)(HOCH3)], and fac-[Re(Sal-Ph)(CO)(3)(Py)] (Sal-mTol = 2-(m-tolyliminomethyl)phenolato; Sal-pTol = 2-(p-tolyliminomethyl)phenolato; Sal-Ph = 2-(phenyliminomethyl)phenolato; Py = pyridine) are reported. Significant activation for the methanol substitution is induced by the use of the N,O bidentate ligand as manifested by the second order rate constants, with limiting kinetics being observed for the first time. Rate constants (25 degrees C) (k(1) or k(3)) and activation parameters (Delta Hk(double dagger), kJ mol(-1); Delta S-k(double dagger), J K-1 mol(-1)) from Eyring plots for entering nucleophiles as indicated are as follows: fac-[Re(Sal-mTol)(CO)(3)(HOCH3)] 3-chloropyridine: (k(1)) 2.33 +/- 0.01 M-1 s(-1); 85.1 +/- 0.6, 48 +/- 2; fac-[Re(Sal-mTol)(CO)(3)(HOCH3)] pyridine: (k(1)) 1.29 +/- 0.02 M-1 s(-1); 92 +/- 2, 66 +/- 7; fac-[Re(Sal-mTol)(CO)(3)(HOCH3)] 4-picoline: (k(1)) 1.27 +/- 0.05 M-1 s(-1); 88 +/- 2, 53 +/- 6; (k(3)) 3.9 +/- 0.03 s(-1); 78 +/- 8, 30 +/- 27; (k(f)) 1.7 +/- 0.02 M-1 s(-1); 86 +/- 2, 49 +/- 6; fac-[Re(Sal-mTol)(CO)(3)(HOCH3)] DMAP (k(3)) 1.15 +/- 0.02 s(-1); 88 +/- 2, 52 +/- 7. An interchange dissociative mechanism is proposed.