Electrospray identification of new polyoxochromate species

Electrospray ionization mass spectrometry (ESI MS) has been conducted on the ammonium and alkali metal (A=Li+, Na+ and K+) dichromate systems. A large number of previously unknown polyoxochromate species have been characterized. Major series that have been identified include [Ax+1HxCrVIxO4x]+ (Li+, x=1–5; Na+, x=1–7; K+, x=1–4) and [A2x−1CrVIxO4x−1]+ (Li+, x=2, 3; Na+, x=2–4; K+, x=2, 3) in the alkali metal dichromate systems, and [HCrVIxO3x+1]− (x=1–5) in the ammonium dichromate system. Several series also contain mixed oxidation state species, ranging from Cr(V) to Cr(II) in conjunction with Cr(VI), which is consistent with the ease of reduction of Cr(VI). Negative ion ESI MS spectra clearly demonstrate the existence of [HCrO4]− as the most abundant ion at −20 V, suggesting that its existence in solution is not just hypothetical, as was previously thought. The polymerization units for the series observed include {AHCrO4}, {A2CrO4} and {CrO3}, with the latter prominent in the alkali metal systems. This presumably arises from the fragmentation of dichromate, A2Cr2O7→{A2CrO4}+{CrO3}. Moreover, the ESI MS of the dichromate compounds have illustrated that the preservation of tetrahedral stereochemistry is of paramount importance for these systems, which leads to only limited polymerization compared to the related molybdate and tungstate systems.