Reversible Ligand Protonation in Noninnocent Constrained-Geometry-Like Group 4 Complexes

Abstract

The proligands C5H5CMe2CHPhOxR (R = Me2, CHMe2, and CMe3) react with M­(NMe2)4 (M = Ti, Zr, and Hf) to give monodeprotonated (OxRCHPhCMe2C5H4)­M­(NMe2)3, doubly deprotonated (C5H4CMe2CPhOxR)­M­(NMe2)2, or a mixture of both. The observed products depend on reaction conditions, the oxazoline substituent, and the metal center, with 4-t-butyl-oxazoline or Ti giving only the monodeprotonated (OxRCHPhCMe2C5H4)­M­(NMe2)3. Amine elimination from (OxRCHPhCMe2C5H4)­M­(NMe2)3 to give (C5H4CMe2CPhOxR)­M­(NMe2)2 is reversible for 4,4-dimethyl- and 4-isopropyl-oxazoline-based ligands in Zr or Hf complexes. Temperature-dependent kinetic studies of the equilibration of (C5H4CMe2CPhOxMe2)­Zr­(NMe2)2, HNMe2 and (OxMe2CHPhCMe2C5H4)­Zr­(NMe2)3 provide the experimental thermodynamic parameters ΔS° = −17.4 ± 2.6 cal·mol–1K–1 and ΔH° = −6.8 ± 0.8 kcal·mol–1. An Eyring plot of the rate constants, determined from the system as it approaches equilibrium, gives the activation entropy and activation enthalpy for the addition of (C5H4CMe2CPhOxMe2)­Zr­(NMe2)2 and HNMe2 as −36 ± 4 cal·mol–1 K–1 and 8.9 ± 1.3 kcal·mol–1, respectively; the elimination of HNMe2 from (OxMe2CHPhCMe2C5H4)­Zr­(NMe2)3 is characterized by ΔS ⧧ = −19 ± 5 cal·mol–1 K–1 and ΔH ⧧ = 15.7 ± 1.5 kcal·mol–1. DFT computational models indicate a single-step, nonlinear transfer of the H between the benzylic position of the noninnocent, oxazoline-coordinated ligand and NMe2. Computations also confirm the negative activation entropy and the trends in the barriers support the experimental results. Together, these studies indicate the importance of steric effects from the oxazoline ligand, metal center, ancillary ligands, and leaving group on the shuttling of the proton between HNMe2 and the noninnocent ligand. Overall, these effects suggest that coordination of oxazoline to the metal center is a key part of the benzylic deprotonation and noninnocent behavior of the cyclopentadienyl-oxazoline ligand.