Zr-IV-monosubstituted Keggin-type dimeric polyoxometalates: Synthesis, characterization, catalysis of H2O2-based oxidations, and theoretical study

摘要

The previously unknown Zr-IV-monosubstituted Keggin-type polyoxometalates (Zr-POMs), (n-Bu4N)(7)H[{PW11O39Zr(mu-OH)}(2)] (1), (n-Bu4N)(8)[{PW11O39Zr(mu-OH)}(2)] (2), and (n-Bu4N)(9)[{PW11O39Zr}(2)(mu-OH)(mu-O)] (3) differing in their protonation state, have been prepared starting from heteropolyacid H5PW11ZrO40 center dot 14H(2)O. The compounds were characterized by elemental analysis, potentiometric titration, X-ray single-crystal structure, and IR, Raman, and P-31 and W-183 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 Zr-IV 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. W-183 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 P-31 NMR spectra of 2 and 3 differ insignificantly in dry MeCN, showing only signals at delta -12.46 and -12.44 ppm, respectively, while the spectrum of 1 displays two resonances at delta - 12.3 ( narrow) and - 13.2 ( broad) ppm, indicating slow proton exchange on the P-31 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 P-31 and W-183 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 P-31 NMR signal at delta - 13.2 ppm, while interaction with H2O2 leads to the formation of an unstable peroxo species (delta - 12.3 ppm), which reacts rapidly with cyclohexene, producing 2-cyclohexen-1-one and transcyclohexane-1,2-diol. Both 1 and 2 show a pronounced catalytic activity in H2O2 decomposition and H2O2-based oxidation of organic substrates, including cyclohexene, alpha-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.