d-Orbital effects on stereochemical non-rigidity:: Twisted TiIV intramolecular dynamics

The isomerization dynamics of tris-catecholate complexes have been investigated by variable-temperature NMR methods, demonstrating that the intramolecular racemization of Delta and Lambda enantiomers of d(0) Ti-IV is facile and faster than that of d(10) Ga-III and Ge-IV analogues. Activation parameters for the racemization of K-2[Ti2(3)] (H(2)2 = 2,3-dihydroxy-N,N'-diisopropylterephthalamide) were determined from line shape analysis of H-1 NMR spectra {methanol-d(4): Delta H-double dagger = 47(1) kJ/mol; Delta S-double dagger = -34(4) J/molK; Delta G(298)(double dagger) = 57(3) kJ/mol; DMF-d(7): Delta H-double dagger = 55(1) kJ/mol; Delta S-double dagger = -16(4) J/molK; Delta G(298)(double dagger) = 59(3) kJ/mol; D2O (pD(star) = 8.6, 20% MeOD): Delta H-double dagger = 48(3) kJ/mol; Delta S-double dagger = -28(10) J/molK; Delta G(298)(double dagger) = 56(3) kJ/mol}. The study of K-2[Ti4(3)] (H(2)4 = 2,3-dihydroxy-N-tert-butyl-N'-benzylterephthalamide) reveals two distinct isomerization processes: faster racemization of mer-[Ti4(3)](2-) by way of a Bailar twist mechanism (D-3h transition state) {T-c approximate to 242 K, methanol-d(4)}, and a slower mer reversible arrow fac[Ti4(3)](2-) isomerization by way of a Ray-Dutt mechanism (C-2v transition state) {T-c approximate to 281 K, methanol-d(4)}. The solution behavior of the Ti-IV complexes mirrors that reported previously for analogous Ga-III complexes, while that of analogous Ge-IV complexes was too inert to be detected by 1H NMR up to 400 K. These experimental findings are augmented by DFT calculations of the ML3 ground states and Bailar and Ray-Dutt transition states, which correctly predict the relative kinetic barriers of complexes of the three metal ions, in addition to faithfully reproducing the ground-state structures. Orbital calculations support the conclusion that participation of the Ti-IV d orbitals in ligand bonding contributes to the greater stabilization of the prismatic Ti-IV transition states.