Deeply reduced empty Keggin clusters [Mo^IV_xM^VI_12−xO_40−xpy_x] (x = 3, 6; M = Mo, W; py = pyridine): synthesis, structures, and Lewis field catalysis

In this research, we present the first examples of Mo^IV₃–W^VI hybrid polyoxometalates (Mo^IV₃-POMs) with empty Keggin structures: α-[Mo^IV₃Mo^VIW^VI₈O₃₁(OH)₆py₃]·2Hpy·4H₂O (1·2Hpy·4H₂O), γ-[Mo^IV₆W^VI₆O₂₈(OH)₆py₆]·2[H₂N(CH₃)₂]·6.5H₂O (2·2[H₂N(CH₃)₂]·6.5H₂O), γ-[Mo^IV₆W^VI₆O₂₉(OH)₅py₆Na(H₂O)₅]·2[NH₂(CH₃)₂]·2H₂O (3·2[NH₂(CH₃)₂]·2H₂O) and γ-[{Mo^IV₆Mo^VIW^VI₅O₂₈(OH)₆py₆}₂Znpy₄]·2Hpy·8H₂O (4·2Hpy·8H₂O). They were prepared through the solvothermal partial oxidation of a mixture of triangular bioxo-capped hexacarboxylate clusters [Mo^IV_xW^IV_3−xO₂(O₂CCH₃)₆L₃]^q (x = 0–3, L = H₂O, q = 2+; L = CH₃CO₂⁻, q = −1), providing a feasible route to the designed synthesis of mixed Mo^IV₃–W POMs. The much lower yield and poor repeatability of 1 compared to those of 2, 3, and 4 are ascribed to the fact that the triangularly Mo^IV–Mo^IV bonded Mo^IV₃ Lewis acid cluster unit of the former results from the breakage of M^IV–W^IV bonds (M = Mo, W) and recombination of Mo^IV in the Na[Mo^IVW^IV₂O₂(O₂CCH₃)₉] precursor because of the extremely easy oxidation of W^IV to W^VI as compared to Mo^IV. 2, 3, and 4 exhibit highly LCF-dependent (LCF = Lewis catalysis field) hydrogen transfer catalysis performance in the hydrazine reduction of nitrobenzene to aniline.