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

Abstract

Titanium-oxygen bonds derived from stable nitroxyl radicals are remarkably weak and can be homolyzed at 60 degrees 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, deltaH++ = 27(+/- 1) kcal/mol, deltaS++ = 6.9(+/- 2.3) eu for 3 and deltaH++ = 28(+/- 1) kcal/mol, deltaS++ = 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.

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

“…It is known that Cp 2 Ti III Cl and TEMPO form a bond of approximately 25 kcal mol −1 ; in light of this, we were unable to observe any conjugate amination at elevated temperatures using these two catalysts or the known complex Cp 2 Ti IV Cl-OTEMP [234][235][236]. Our DFT calculations estimate the strength of the Cp* 2 Ti IV Cl-OTEMP bond to be 23 kcal mol −1 weaker than that of the Cp 2 Ti IV Cl-OTEMP complex, putting an approximate value of this BDFE at 2 kcal mol −1 .…”

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

“…It is known that Cp 2 Ti III Cl and TEMPO form a bond of approximately 25 kcal mol −1 ; in light of this, we were unable to observe any conjugate amination at elevated temperatures using these two catalysts or the known complex Cp 2 Ti IV Cl-OTEMP [234-236]. Our DFT calculations estimate the strength of the Cp* 2 Ti IV Cl-OTEMP bond to be 23 kcal mol −1 weaker than that of the Cp 2 Ti IV Cl-OTEMP complex, putting an approximate value of this BDFE at 2 kcal mol −1 .…”

Probing ‘Spin-Forbidden’ Oxygen-Atom Transfer: Gas-Phase Reactions of Chromium−Porphyrin Complexes

“…The Cr–OP(OMe) 3 binding energy is estimated to be near 13 ± 5 kcal mol −1 based on the CID experiments at variable collision energy (Table 2). The Cr-O bond strength in 1 -TEMPO is taken as equal to the Ti–O BDE in Cp 2 TiCl(TEMPO), 25 kcal mol −1 43. This should be an upper limit because Cr is less oxophilic than Ti and because Cr IV is more oxidizing than Ti IV [these compounds are formally M 4+ –(TEMPO − )].…”