Synthesis of polyoxometalates for detailed solution reactivity studies

Abstract

Non-aqueous methodologies provide an opportunity to access a range of polyoxometalates that may not be stable in H2O and enable mechanistic studies into hydrolytic and protonation behaviours, which are fundamental to polyoxometalate chemistry. 17O-enriched (TBA)6[NaPW11O39] was prepared via an efficient non-aqueous route and shown to be a suitable precursor to [(L)MPW11O39]n- (M = Sn2+, Pb2+, Bi3+, Sb3+, Sn4+, Ti4+) for detailed systematic studies. Reactions were monitored by 31P NMR while products were characterised by FT-IR, multinuclear NMR (1H, 13C, 17O, 31P, 119Sn, 183W and 207Pb), solid state NMR, ESI-MS, CHN microanalysis, UV-Vis and/or single crystal XRD. Using this approach, the readily-hydrolysable tin derivatives, (TBA)4[(CH3O)SnPW11O39] and (TBA)8[(μ-O)(SnPW11O39)2] were prepared for the first time and the previously reported (TBA)4[(HO)TiPW11O39] was shown to be stable in DMSO for up to 3 months possibly due to interaction between HO- and DMSO. As a result of the more ionic character of Sn—OCH3 bond compared with Ti—OCH3, (TBA)4[(CH3O)SnPW11O39] was observed to hydrolyse faster than (TBA)4[(CH3O)TiPW11O39] whereas (TBA)4[ClTiPW11O39] with a large excess of H2O hydrolysed more readily than (TBA)4[ClSnPW11O39]. Although (TBA)4[(HO)TiPW11O39] underwent condensation to (TBA)8[(μ-O)(TiPW11O39)2] easily in acetonitrile at room temperature, this reaction only occured for (TBA)4[(HO)SnPW11O39] at elevated temperature (~120oC) in the presence of a water-scavenging agent such as N, N’-dicyclohexylcarbodiimide (DCC). These experimental observations were consistent with DFT calculations on the energetics of the hydrolysis and condensation of (TBA)4[(CH3O)SnPW11O39] and (TBA)4[(CH3O)TiPW11O39]. Protonation studies on the 17O-enriched POMs provided insights into protonation of the MOW sites in (TBA)4[(CH3O)TiPW11O39], (TBA)4[ClMIVPW11O39] (M = Sn, Ti), (TBA)5[MIIPW11O39] (M = Sn, Pb) and (TBA)4[MIIIPW11O39] (M = Sb, Bi) and protonation at both TiOW and TiOTi sites in (TBA)8[(μ-O)(TiPW11O39)2] whilst reactions between (TBA)8[(μ-O)(TiPW11O39)2] and electrophiles indicated possible formation of adducts. Treatment of (TBA)4[(L)SnPW11O39] (L = Cl, HO) with NaBH4 resulted in reduction of the tin heteroatom only whereas reaction between (TBA)5[SnIIPW11O39] and halogens (Br2 and I2) or the molybdate (TBA)3[PMo12O40] showed oxidation of tin (II). Electrochemical studies in acetonitrile revealed no redox processes associated with the heterometals in (TBA)4[(L)SnIVPW11O39] and (TBA)5[PbIIPW11O39] while redox waves assigned to Sn2+/Sn4+ were observed for Abstract ii (TBA)5[SnIIPW11O39] within the potential range studied. Finally, attempts to prepare Lindqvist-type derivatives, [(L)MW5O18]n- (M = Co2+, Mo2+, Sn2+, Pb2+, Fe2+, Cu2+, Cr3+, Sb3+, Bi3+) from a tungstate precursor prepared by hydrolysis of a 3:2 mixture of (TBA)2WO4 and WO(OMe)4 provided evidence that only in certain cases were the required heterometalates formed. Acetonitrile hydrolysis was observed under reaction conditions and the acetamide adduct (TBA)3[{CH3C(O)NH2}CoW5O17(OMe)] was characterised crystallographically. An attempt to prepare [(L)MoIIW5O18]4- produced the crystallographically characterised, one-electron reduced (TBA)3[W6O19].