PNNL

Abstract

The isomerization dynamics of tris-catecholate complexes have been investigated by variable temperature NMR methods, demonstrating that the intramolecular racemization of ? and ? enantiomers of d0 TiIV is facile and faster than that of d10 GaIII and GeIV analogs. Activation parameters for the racemization of K2[Ti23] (H22 = 2,3-dihydroxy-N,N’-diisopropylterephthalamide) were determined from lineshape analysis of 1H NMR spectra {methanol-d4: ?H‡ = 47(1) kJ/mol; ?S‡ = -34(4) J/molK; ?G‡298 = 57(3) kJ/mol; DMF-d7: ?H‡ = 55(1) kJ/mol; ?S‡ = -16(4) J/molK; ?G‡298 = 59(3) kJ/mol; D2O (pD* = 8.6, 20% MeOD): ?H‡ = 48(3) kJ/mol; ?S‡ = -28(10) J/molK; ?G‡298 = 56(3) kJ/mol}. The study of K2[Ti43] (H24 = 2,3-dihydroxy-N-tert-butyl-N’-benzylterephthalamide) reveals two distinct isomerization processes: faster racemization of mer-[Ti43]2- by way of a Bailar twist mechanism (D3h transition state) {Tc ? 242 K, methanol-d4}, and a slower mer/fac-[Ti43]2- isomerization by way of a Rây-Dutt mechanism (C2v transition state) {Tc ? 281 K, methanol-d4}. The solution behavior of the TiIV complexes mirrors that reported previously for analogous GaIII complexes, while that of analogous GeIV 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 grounds states and Bailar and Rây-Dutt transition states, which correctly predict the relative kinetic barriers of the three metal ions, in addition to faithfully reproducing the ground state structures. Orbital calculations support the conclusion that participation of the TiIV d0 orbitals in ligand bonding contributes to the greater stabilization of the prismatic TiIV transition states. Battelle operates the Pacific Northwest National Laboratory for the U.S. Department of Energy.