| Abstract
| - Titanium−oxygen bonds derived from stable nitroxyl radicals are remarkably weak and can behomolyzed at 60 °C. The strength of these bonds depends sensitively on the ancillary ligation at titanium.Direct measurements of the rate of Ti−O bond homolysis in Ti−TEMPO complexes Cp2TiCl(TEMPO) (3)and Cp2TiCl(4-MeO-TEMPO) (4) (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl, 4-MeO-TEMPO = 2,2,6,6-tetramethyl-4-methoxypiperidine-N-oxyl) were conducted by nitroxyl radical exchange experiments. Eyringplots gave the activation parameters, ΔH⧧ = 27(±1) kcal/mol, ΔS⧧ = 6.9(±2.3) eu for 3 and ΔH⧧ = 28(±1)kcal/mol, ΔS⧧ = 9.0(±3.0) eu for 4, consistent with a process involving the homolysis of a weak Ti−Obond to generate the transient Cp2Ti(III)Cl and the nitroxyl radical. Thermolysis of the titanocene TEMPOcomplexes in the presence of epoxides leads to the Cp2Ti(III)Cl-mediated ring-opening of the epoxidefollowed by trapping by the nitroxyl radical. The X-ray crystal structure of the Ti−TEMPO derivative, Cp2TiCl(4-MeO-TEMPO) (4), is reported. DFT (B3LYP/6-31G*) calculations and experimental studies revealthat the strength of the Ti−O bond decreases dramatically with the number of cyclopentadienyl groups ontitanium. The calculated Ti−O bond strength of the monocyclopentadienyl complex 2 is 43 kcal/mol, whereasthat of the biscyclopentadienyl complex 3 is 17 kcal/mol, a difference of 26 kcal/mol. These studies revealthat the strength of these Ti−O bonds can be tuned over an interesting and experimentally accessibletemperature range by appropriate ligation on titanium.
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