Synthesis and Redox Characterization of the Polyoxo Anion, γ*-[S₂W₁₈O₆₂]^4-:  A Unique Fast Oxidation Pathway Determines the Characteristic Reversible Electrochemical Behavior of Polyoxometalate Anions in Acidic Media

Abstract

The synthesis and characterization of (Bu4N)4[S2W18O62].1.23MeCN.0.27H2O are reported. It crystallizes in the monoclinic space group C2/c with a = 22.389(6) A, b = 22.104(3) A, c = 25.505(5) A, beta = 95.690(15) degrees, V = 12560(5) A3, and Z = 4. The anion exists as the gamma* isomer, the second example of this isomer type to be characterized structurally. The equivalent molybdenum salt occurs as the alpha isomer. gamma*-[S2W18O62]4- in MeCN solution displayed four electrochemically reversible one-electron redox processes at E1/2 values of -0.24, -0.62, -1.18, and -1.57 V versus the Fc+/Fc couple. Upon addition of acid in MeCN/H2O (95/5 v/v), the two most cathodic processes converted to an overall two-electron process at -0.71 V. The total data suggested that this process actually comprises two one-electron transfer processes, occurring at different potentials, with associated proton-transfer reactions. The interpretation is supported by simulation of the effect of acid titration upon the cyclic voltammetry. While multiple pathways for correlated reduction and protonation are present in both the molybdenum and tungsten systems, only a single fast oxidation pathway is available. As the reduced forms of [S2W18O62]4- are much weaker bases than those of [S2Mo18O62]4-, the individual oxidation pathways are not the same. However, their existence determines the highly reversible electrochemical behavior that is characteristic of these anions, and that of polyoxometalate systems in general.

Experimental and Theoretical Investigations of the Sulfite‐Based Polyoxometalate Cluster Redox Series: α‐ and β‐[Mo18O54(SO3)2]4−/5−/6−

“…Synthesis: Crystalline samples of the molybdenum sulfite polyoxometalates were produced by adopting a solventmixture reaction previously reported [19] as follows: A solution of Na 2 MoO 4 ·2 H 2 O (4.8 g, 20 mmol) and Na 2 SO 3 (0.45 g, 3.6 mmol) in H 2 O (30 mL) was added with HCl 37 % (9 mL), followed by the addition of 4À , [35] whereby the central anion templates are represented as yellow polyhedra. The bonds between the oxo positions of the heteroanions and the adjoined Mo positions are shown in green (S: yellow; O shell : red; O heteroanion : orange; capping Mo: dark blue; "belt" Mo: light blue).…”

“…[15,16] The conventional Dawson structure incorporates two tetrahedral anions such as PO 4 3À , [15,16] AsO 4 3À , [17] SO 4 2À , [18][19][20] or ClO 4 À [21,22] within the elongated polyoxometalate cluster shell. In contrast only a few examples of {M 18 } Dawson-like cluster hosts that contain non-tetrahedral anions are available.…”

Discovery of a Family of Isopolyoxotungstates [H4W19O62]6− Encapsulating a {WO6} Moiety within a {W18} Dawson‐like Cluster Cage

“…[20] Again, an additional W site is located at the center of the cluster and coordinates to six oxido ligands to form a central {WO 6 } centrosymmetric template of octahedral geometry. As in 2 a, two further m 3 -oxido ligands inside the cluster each bridge one of the capping {W 3 } triangles with an average W À O bond length of 2.28(1) .…”

“…), [15,20] that is, the same {W 18 O 54 } cage variations and interior bridging oxido group orientations are present; we therefore adopt the same isomer designations (a, a*, b, b*, g, g*).…”

β,β-Isomer of Open-Wells–Dawson Polyoxometalate Containing a Tetra-Iron(III) Hydroxide Cluster: [{Fe4(H2O)(OH)5}(β,β-Si2W18O66)]9−

“…Baker and Figgis predicted the existence of six possible structural isomers of the Wells-Dawson POM (α, β, γ, α*, β*, γ*) [31]. So far, only the α-, β-, γ-, and γ*-isomers have been experimentally confirmed [32][33][34][35][36][37]. The α-Wells-Dawson POM has D 3h symmetry ( Figure 1, upper left).…”