84848-15-7Relevant articles and documents
Nuclear Magnetic Resonance Studies of Inversion and Diketonate R-Group Exchange in Dialkoxybis(β-diketonato)titanium(IV) Complexes. Evidence for a Twist Mechanism
Fay, Robert C.,Lindmark, Alan F.
, p. 2118 - 2127 (2007/10/02)
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; Rk = kinv/kex 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 Rk 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 Rk 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 Rk 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.
