First Isolated Active Titanium Peroxo Complex:  Characterization and Theoretical Study

Abstract

The protonated titanium peroxo complex [Bu(4)N](4)[HPTi(O(2))W(11)O(39)] (1) has been first prepared via interaction of the micro-oxo dimeric heteropolytungstate [Bu(4)N](8)[(PTiW(11)O(39))(2)O] (3) with an excess of 30% aqueous H(2)O(2) in MeCN. Peroxo complex 1 has been characterized by using elemental analysis, UV-vis, IR, resonance Raman (RR), (31)P and (183)W NMR spectroscopy, cyclic voltammetry, and potentiometric titration. The electronic and vibrational spectra of 1 are very similar to those of the well-known unprotonated titanium peroxo complex [Bu(4)N](5)[PTi(O(2))W(11)O(39)] (2), while (31)P and (183)W NMR spectra differ significantly. A compilation of the physicochemical techniques supports a monomeric Keggin type structure of 1 bearing one peroxo ligand attached to Ti(IV) in a eta(2)-coordination mode. The protonation of the titanium peroxo complex results in an increase of the redox potential of the peroxo group, E(1/2) = 1.25 and 0.88 V relative to Ag/AgCl reference electrode for 1 and 2, respectively. In contrast to 2, 1 readily reacts with 2,3,6-trimethylphenol (TMP) at 40 degrees C in MeCN to give 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol (BP) and 2,3,5-trimethyl-p-benzoquinone (TMBQ). The proportion between BP and TMBQ in the reaction products depends on the TMP/1 ratio. When a 2-fold excess of TMP is used, the main reaction product is BP (90%), while using a 2-fold excess of 1 leads to TMBQ (95%). On the basis of the product study, a homolytic oxidation mechanism that implicates the formation of phenoxyl radicals is suggested. The RR deuterium labeling experiments show that the activating proton is most likely localized at a Ti-O-W bridging oxygen rather than at the peroxo group. Theoretical calculations carried out at the DFT level on the protonated and unprotonated titanium peroxo derivatives also propose that the most stable complex is formed preferentially after protonation of the Ti-O-W site; however, both Ti-OH-W and TiOO-H protonated anions could coexist in solution.

Dititanium‐Containing 19‐Tungstodiarsenate(III) [Ti2(OH)2As2W19O67(H2O)]8−: Synthesis, Structure, Electrochemistry, and Oxidation Catalysis

“…However, we obtained 1 for the first time accidentally during a reaction of TiOSO 4 with Na 9 [a-AsW 9 O 33 ] in a molar ratio of 4:1 in aqueous medium at pH 4. Then we decided to reproduce 1 via a more rational synthetic procedure by using the dilacunary precursor […”

“…According to the literature, [9,24,25] this band can be attributed to the O 2 !Ti ligand-to-metal charge-transfer band. Importantly, the intensity of this band does not decrease while keeping the solution at 70 8C for several days.…”

“…[3j, 4,9] In sharp contrast, polyanion 1 affords epoxide, diol, and products of the diol overoxidation but none of the allylic oxidation products (Scheme 2).…”

Titanium-monosubstituted polyoxometalates: relation between homogeneous and heterogeneous Ti-single-site-based catalysis

“…The oxidation of alkylphenols with H 2 O 2 over Ti,Siand Ti-POM-catalysts has been studied using 2,3,6-trimethylphenol (TMP) as model substrate [73][74][75][76][77]135,136]. For both types of catalysts we found the same products, 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, vitamin E precursor) and 2,2´,3,3´,5,5´-hexamethyl-4,4´-biphenol (BP) (scheme 5).…”

“…In this review we would like to summarise our experience in the application of Ti-POMs for studying mechanisms of H 2 O 2 -based oxidations [130][131][132][133][134][135][136][137] and to show that these compounds can serve as a real bridge between homogeneous and heterogeneous Ti-catalysis and thus can provide mechanistic insight in surface chemistry processes. The following topics will be discussed.…”