Two molecules are better than one: cooperative effects of two sulfur dioxide and sulfur dioxide/water in bond forming and hydrolysis reactions of polyvanadate dianions in the gas phase

Vanadium oxides are mostly used as catalysts in many oxidation reactions such as of SO2 to SO3, whereas hydrated vanadium oxides have potential application in Li-ion batteries [1, 2]. Following our interest in the chemistry of SO2, we recently reported an unprecedented bond-forming and hydrolysis reactions of doublycharged vanadium oxide and hydroxide cluster anions promoted by SO2, leading to singly-charged ions, as a result of a charge separation process [3, 4]. The reactions have been studied in the gas phase at room temperature by an in-house modified linear quadrupole ion trap mass spectrometer. NaV3O9 2- and H2NaV3O10 2- dianions, prepared by electrospray ionization (ESI) of NaVO3 aqueous solutions, were reacted in the gas-phase with SO2. The rate constant, branching ratio and the efficiency of the reactions have been measured. NaV3O9 2- reacts with sulfur dioxide giving a first stable association product [NaV3O9SO2] 2-, that further reacts with a second molecule of SO2 forming two singly charged products, NaV2O6SO 2- and VO3SO 2-. The reaction involves formation of new V-O and S-O bonds, by breaking a V-O bond containing a bridging oxygen atom. By contrast, H2NaV3O10 2-, the hydrated form of NaV3O9 2-, reacts with only one SO2 also giving the hydrolysis products HOSO2 - and HNaV3O9 - in addition to those arising from the bond-forming process, H2VO4 - and NaV2O6SO2 -. Both reactions proceed through the formation of long-lived intermediates, where the bonding to the neutral SO2 is favoured with respect to the charge separation, that would arise from an electron transfer to SO2. Instead, the charge separation is the consequence of a chemical reaction leading to the formation of two singly charged products. A common feature is the crucial role played by sulfur dioxide in promoting charge separation processes. The sequential addition of two molecules of sulfur dioxide to thermal doubly-charged vanadium oxide anions, NaV3O9 2-, results in the breaking of the stable V3O9 bone, whereas in the case of the hydroxide H2NaV3O10 2- anions one SO2 molecule is sufficient to promote also hydrolysis. These fast and efficient reactions are associated with charge separation processes that result in two singly charged ions by formation of new V-O and S-O bonds. Cooperativity of two SO2 or SO2/H2O molecules in the bonding to polyvanadate dianions has proved fundamental in addressing bond-forming or hydrolysis reactions.