Nuclear Magnetic Resonance Studies of Inversion and Diketonate R-Group Exchange in Dialkoxybis(β-diketonato)titanium(IV) Complexes. Evidence for a Twist Mechanism

Article abstract of DOI:10.1021/ja00346a004

Dialkoxybis(β-diketonato)titanium(IV) complexes, Ti(dik)2(OR)2, where dik = CH3COCHCOCH3 (acac) or t-C4H9COCHCO-t-C4H9 (dpm) and R = CH2C6H5, CH2CH(CH3)2, CH(CH3)2, or C(CH3)2C6H5, have been synthesized and characterized by chemical analysis, NMR and IR spectroscopy, and conductance measurements.Rates and activation parameters for inversion and diketonate R-group exchange in these complexes have been determined by ¹H NMR spectroscopy (total line-shape analysis).Inversion, probed by the diastereotopic groups in the alkoxide ligands, and exchange of diketonate methyl (or tert-butyl) groups occurs at comparable rates by a common, intramolecular mechanism; R_k = k_inv/k_ex decreases from ca. 2.0 to ca. 1.0 with increasing steric bulk of the alkoxide ligand.Activation parameters for the two processes are identical within experimental uncertainty.Permutational analysis indicates that the R_k values are consistent with a linear combination of NMR averaging sets A₅ and A₆.Certain intramolecular bond-rupture mechanisms can be ruled out as the sole rearrangement pathway on the basis of the R_k values.A dramatic decrease in rearrangement rate and increase in ΔH<*> with increasing bulk of the OR group strongly suggest that these complexes rearrange by a twist mechanism.Supporting evidence includes (i) lack of solvent effects, (ii) negative values of ΔS<*>, and (iii) lack of a correlation between rates and metal-diketonate bond strength in Ti(dik)2(OR)2 and related complexes.The variation in R_k is attributed to a mixture of twists about the various octahedral C₃ axes, with the relative importance of the various twist transition states depending on steric effects.