Titanium Complexes of Tridentate Aminebiphenolate Ligands Containing Distinct N‑Alkyls: Profound N‑Substituent Effect on Ring-Opening Polymerization Catalysis

The synthesis, structural characterization, and reactivity studies of titanium complexes supported by tridentate amine biphenolate ligands of the type [RN­(CH2-2-O-3,5-C6H2(tBu)2)2]2– {[R-ONO]2–; R = tBu (1a), iPr (1b), nPr (1c)} are described. Alcoholysis of Ti­(OiPr)4 with H2[1a–1c] in diethyl ether solutions at 25 °C generates quantitatively the corresponding [R-ONO]­Ti­(OiPr)2 (2a–2c) as a yellow crystalline solid. X-ray diffraction studies of 2b and 2c showed them to be five-coordinate, trigonal-bipyramidal species. Ring-opening polymerization of ε-caprolactone (ε-CL) catalyzed by 2b and 2c proved to be living, as evidenced by the narrow molecular weight distributions of the derived polymers and the linear dependence of number-averaged molecular weights on the monomer-to-catalyst ratios or polymerization time. Kinetic studies revealed that the polymerization rates are first-order in the concentration of ε-CL and first-order in that of 2b and 2c. The propagation rate of 2c is ca. 15 times faster than that of 2b, highlighting a profound substituent effect of primary versus secondary N-alkyls. In sharp contrast, reactions employing catalytic 2a produce either low-molecular-weight oligomers or polymers characteristic of somewhat wider molecular weight distributions, depending on the polymerization temperatures.