Homolysis of Weak Ti−O Bonds: Experimental and Theoretical Studies of Titanium Oxygen Bonds Derived from Stable Nitroxyl Radicals

Titanium−oxygen bonds derived from stable nitroxyl radicals are remarkably weak and can be
homolyzed 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. Eyring
plots 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−O
bond to generate the transient Cp2Ti(III)Cl and the nitroxyl radical. Thermolysis of the titanocene TEMPO
complexes in the presence of epoxides leads to the Cp2Ti(III)Cl-mediated ring-opening of the epoxide
followed 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 reveal
that the strength of the Ti−O bond decreases dramatically with the number of cyclopentadienyl groups on
titanium. The calculated Ti−O bond strength of the monocyclopentadienyl complex 2 is 43 kcal/mol, whereas
that of the biscyclopentadienyl complex 3 is 17 kcal/mol, a difference of 26 kcal/mol. These studies reveal
that the strength of these Ti−O bonds can be tuned over an interesting and experimentally accessible
temperature range by appropriate ligation on titanium.