Zr^IV-Monosubstituted Keggin-Type Dimeric Polyoxometalates: Synthesis, Characterization, Catalysis of H₂O₂-Based Oxidations, and Theoretical Study

The previously unknown ZrIV-monosubstituted Keggin-type polyoxometalates (Zr-POMs), (n-Bu4N)7H[{PW11O39Zr(μ-OH)}2] (1), (n-Bu4N)8[{PW11O39Zr(μ-OH)}2] (2), and (n-Bu4N)9[{PW11O39Zr}2(μ-OH)(μ-O)] (3) differing in their protonation state, have been prepared starting from heteropolyacid H5PW11ZrO40·14H2O. The compounds were characterized by elemental analysis, potentiometric titration, X-ray single-crystal structure, and IR, Raman, and 31P and 183W NMR spectroscopy. The single-crystal X-ray analysis of 2 reveals that two Keggin structural units [PW11O39Zr]3- are linked through two hydroxo bridges Zr−(OH)−Zr with ZrIV in 7-fold coordination. The IR spectra of 1 and 2 show a characteristic band at 772 cm-1, which moves to 767 cm-1 for 3, reflecting deprotonation of the Zr−(OH)−Zr bond. Potentiometric titration with methanolic Bu4NOH indicates that 1−3 contain 2, 1, and 0 acid protons, respectively. 183W NMR reveals Cs symmetry of 2 and 3 in dry MeCN, while for 1, it discovers nonequivalence of its two subunits and their distortion resulting from localization of the acidic proton on one of the Zr−O−W bridging O atoms. The 31P NMR spectra of 2 and 3 differ insignificantly in dry MeCN, showing only signals at δ −12.46 and −12.44 ppm, respectively, while the spectrum of 1 displays two resonances at δ −12.3 (narrow) and −13.2 (broad) ppm, indicating slow proton exchange on the 31P NMR time scale. The theoretical calculations carried out at the density functional theory level on the dimeric species 1−3 propose that protonation at the Zr−O−Zr bridging site is more favorable than protonation at Zr−O−W sites. Calculations also revealed that the doubly bridged hydroxo structure is thermodynamically more stable than the singly bridged oxo structure, in marked contrast with analogous Ti- and Nb-monosubstituted polyoxometalates. The interaction of 1−3 with H2O and H2O2 in MeCN has been studied by both 31P and 183W NMR. The stability of the [PW11O39ZrOH]4- structural unit toward at least 100-fold excess of H2O2 in MeCN was confirmed by both NMR and Raman spectroscopy. The interaction of 1 and 2 with H2O in MeCN produces most likely monomeric species (n-Bu4N)3+n[PW11O39Zr(OH)n(H2O)3-n] (n = 0 and 1) showing a broad 31P NMR signal at δ −13.2 ppm, while interaction with H2O2 leads to the formation of an unstable peroxo species (δ −12.3 ppm), which reacts rapidly with cyclohexene, producing 2-cyclohexen-1-one and trans-cyclohexane-1,2-diol. Both 1 and 2 show a pronounced catalytic activity in H2O2 decomposition and H2O2-based oxidation of organic substrates, including cyclohexene, α-pinene, and 2,3,6-trimethylphenol. The oxidation products are consistent with those of a homolytic oxidation mechanism. On the contrary, 3 containing no acid protons reacts with neither H2O nor H2O2 and shows negligible catalytic activity. The Zr-monosubstituted polyoxometalates can be used as tractable homogeneous probes of Zr single-site heterogeneous catalysts in studying mechanisms of H2O2-based oxidations.