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

Abstract

The previously unknown Zr(IV)-monosubstituted Keggin-type polyoxometalates (Zr-POMs), (n-Bu4N)7H[{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 H5PW11ZrO40.14H2O. 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 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. (83W 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 (31)P 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 (31)P 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 H(2)O and H(2)O(2) in MeCN has been studied by both (31)P and (183)W NMR. The stability of the [PW(11)O(39)ZrOH](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 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 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, 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. The Zr-monosubstituted polyoxometalates can be used as tractable homogeneous probes of Zr single-site heterogeneous catalysts in studying mechanisms of H2O2-based oxid...

Increased Lewis Acidity in Hafnium‐Substituted Polyoxotungstates

“…Yet, we cannot entirely rule out alternative indirect Brønsted catalysis for 1, the key feature of which to explain the differences between 2 and 3 would be the relative facility of accessing oxo ligands from hydroxo ones. [12] However, as we never observed double deprotonation (a bis-hydroxo species by deprotonation of a second water molecule on the hafnium ion, or formation of an oxo ligand on Hf), both of these compounds should rather act as direct Lewis acids (coordination of substrate), than as water activating agents. Our attempts to evidence complexation of benzaldehyde and diphenylimine to 1 or 2 either by MS or NMR spectroscopy has so far not been successful.…”

“…Gratifyingly, aldol products were obtained (entries 8-14). Good yields were reached with aromatic aldehydes, provided the aromatic groups were not too electron-rich (compare entries 8-10 to entries [11][12]. In those cases, the POM ligand reduces the Lewis acidity of Hf IV relative to HfCl 4 or HfA C H T U N G T R E N N U N G (OTf) 4 (see Table 2).…”

“…Besides, the complexation constants for lanthanides in the a 1 series are extremely high, [15b] and we can assume the one for Hf 4 + to be similar, or even stronger given the increased charge/ size ratio, which is a leading factor for the complexation equilibrium. Acid catalysis with POMs generally arises from proton counterions (as in H 3 A C H T U N G T R E N N U N G [PW 12 O 40 ]), but we could exclude such direct Brønsted catalysis, because 1 has no proton as counterion. Alternatively, protons can be generated through water activation by metal counterions.…”

Fe2 and Fe4 Clusters Encapsulated in Vacant Polyoxotungstates: Hydrothermal Synthesis, Magnetic and Electrochemical Properties, and DFT Calculations

“…[31] The dimerization of transition-metal mono-substituted POMs has also been studied for the titanium (in organic medium), [32] zirconium [33] and ruthenium [34] derivatives, but the structural characterization of a m-oxo bridged dimer has only very recently been performed in the case of [{SiW 11 O 39 Ru IV } 2 O] 10À .…”

Synthesis, Structural Investigation and Characterization of an α-Keggin-based Organic-Inorganic Hybrid, [Cu(BIIM)2]{[Cu(BIIM)2(H2O)][Cu(BIIM)2BW12O40]·3H2O}2 (BIIM = 2,2´-Biimidazole)

“…These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.com.ac.uk/data request/cif. 2 ] 2+ ion through one terminal oxygen O(1) atom. The Cu(1)-centered square pyramid is defined by four N atoms from two BIIM ligands in the basal plane and an apical oxygen atom bridging to a WO 6 octahedron with a Cu(1)−O distance of 2.349(7)Å and Cu(1)−N bond lengths in the range of 1.995(9) -2.020(9)Å.…”