Non-Redox-Assisted Oxygen−Oxygen Bond Homolysis in Titanocene Alkylperoxide Complexes: [Cp₂Ti^IV(η¹-OO<i>^t</i>Bu)L]^+/0, L = Cl^-, OTf^-, Br^-, OEt₂, Et₃P

The titanium(IV) alkylperoxide complex Cp2Ti(OOtBu)Cl (1) is formed on treatment of Cp2TiCl2 with NaOOtBu in THF at −20 °C. Treatment of 1 with AgOTf at −20 °C gives the triflate complex Cp2Ti(OOtBu)OTf (2), which is rapidly converted to the bromide Cp2Ti(OOtBu)Br (3) on addition of nBu4NBr. The X-ray crystal structures of 1 and 3 both show η1-OOtBu ligands. Complex 2 is stable only below −20 °C; 1H, 13C, and 19F NMR spectra suggest that it also contains an η1-OOtBu ligand. Removal of the chloride from 1 with [Ag(Et2O)2]BAr‘4 (Ar‘ = 3,5-(CF3)2C6H3) yields the etherate complex [Cp2Ti(OOtBu)(OEt2)]BAr‘4 (4). Again, coordination of a fourth ligand to the Ti center indicates an η1-OOtBu ligand in 4. These peroxide complexes do not directly oxidize olefins or phosphines. For instance, the cationic etherate complex 4 reacts with excess Et3P simply by displacement of the ether to form [Cp2Ti(η1-OOtBu)(Et3P)]BAr‘4 (5). Compounds 1−5 all decompose by O−O bond homolysis, based on trapping and computational studies. The lack of direct oxygen atom transfer reactivity is likely due to the η1 coordination of the peroxide and the inability to adopt more reactive η2 geometry. DFT calculations indicate that the steric bulk of the tBu group inhibits formation of the hypothetical [Cp2Ti(η2-OOtBu)]+ species.四价钛烷基过氧配合物二(环戊二烯基)叔丁基过氧氯化钛（Cp₂Ti(OOtBu)Cl，记为化合物1），可通过在-20 ℃的四氢呋喃（THF）体系中，使二(环戊二烯基)二氯化钛（Cp₂TiCl₂）与叔丁基过氧钠（NaOOtBu）反应制得。将化合物1与三氟甲磺酰银（AgOTf）在-20 ℃下反应，得到三氟甲磺酸根配合物二(环戊二烯基)叔丁基过氧三氟甲磺酰钛（Cp₂Ti(OOtBu)OTf，记为化合物2）；向反应体系中加入四正丁基溴化铵（nBu₄NBr），化合物2可快速转化为溴代配合物二(环戊二烯基)叔丁基过氧溴化钛（Cp₂Ti(OOtBu)Br，记为化合物3）。化合物1和3的X射线单晶衍射结构均显示，其配体OOtBu以η¹模式配位。配合物2仅在-20 ℃以下稳定；通过氢谱（¹H NMR）、碳谱（¹³C NMR）及氟谱（¹⁹F NMR）的表征结果可推知，该配合物同样含有η¹配位的OOtBu配体。使用二乙醚合银四(3,5-双(三氟甲基)苯基)硼酸盐（[Ag(Et₂O)₂]BAr‘₄，其中Ar‘=3,5-(CF₃)₂C₆H₃）脱除化合物1中的氯离子，可得到醚合配合物[Cp₂Ti(OOtBu)(OEt₂)]BAr‘₄（记为化合物4）。同理，钛中心结合了第四位配体，证明化合物4中的OOtBu配体仍为η¹配位模式。该系列过氧配合物均无法直接氧化烯烃或膦类化合物。例如，阳离子型醚合配合物4与过量三乙基膦（Et₃P）反应时，仅发生二乙醚配体的置换，生成配合物[Cp₂Ti(η¹-OOtBu)(Et₃P)]BAr‘₄（记为化合物5）。基于捕获实验与计算化学研究，化合物1至5均通过O-O键的均裂发生分解。这类配合物缺乏直接氧原子转移反应活性的根源，可能在于过氧配体采取η¹配位模式，且无法转变为反应活性更高的η²配位构型。密度泛函理论（DFT）计算结果表明，叔丁基（tBu）的空间位阻效应会抑制假想中间体[Cp₂Ti(η²-OOtBu)]⁺的生成。