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

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.