Systematic Electrochemical Synthesis of Reduced Forms of the α-[S₂Mo₁₈O₆₂]^4-Anion¹

Abstract

As proton concentration increases, the first two reversible (1 e-)-reduction processes of the anion α-[S2Mo18O62]4- in 95/5 MeCN/H2O convert to an overall (2 e-)-reduction process. Half-wave potentials for reversible one-electron reduction of [S2Mo18O62]4- itself and its two one-electron-reduced forms [S2Mo18O62]5- and [HS2Mo18O64]4- were estimated by voltammetry to be 0.12, −0.13, and 0.35 V, respectively, versus the ferrocenium/ferrocene couple. Simulation of cyclic voltammograms provided estimates of association constants of 1.4, 1.6 × 108, and 102 M-1 for protonation of the respective products of the reductions, [S2Mo18O62]5-, [S2Mo18O62]6-, and [HS2Mo18O62]5-. Equilibrium constants for disproportionation of the (1 e-)-reduced species were derived. Rates of the electron transfer and protonation processes are very fast relative to the voltammetric time scale. Consideration of the equilibrium constants, plus information obtained from acid titrations monitored by steady state voltammetry, helped define conditions for the isolation of salts of the (1 e-)-, (2 e-)-, (2 e-, 1 H+)-, (2 e-, 2 H+)-, (4 e-, 2 H+)- and (4 e-, 4 H+)-reduced derivatives of the [S2Mo18O62]4- anion. Anions at the (6 e-)- and (8 e-)-reduced levels undergo spontaneous oxidation in the acid solutions and could not be isolated experimentally. The present work shows that directed synthesis of reduced species in different protonation states is possible for these complex systems if adequate voltammetric data are available.

Electron Transfer to a Phosphomolybdate Monolayer on Glassy Carbon: Ambivalent Effect of Protonation

“…72,63 The simulations presented here show that the system is kinetically limited by the electron transfer step to protonated species HI. The value of the electron transfer rate constant for the reduction of HI allowing the CV simulation is 7.5 × 10 7 s −1 ; however, the E p −log v variations show increasing peak potential separation at scan rates as low as 0.1 V/s.…”

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

“…[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.…”

“…Furthermore, structurally characterized polyoxomolybdosulfites of this kind are rare. Examples include (NH 4 ) 4 3 ) 2 ]·4 H 2 O, [30] (NH 4 ) 8 3 ) 2 ]·2 H 2 O [31] and the framework structures (NH 4 ) 20 12 O 24 A C H T U N G T R E N N U N G (SO 3 ) 16 ]·4 H 2 O [31] and (NH 4 ) 15 4 ] 2 }·5 H 2 O. [31] These structures are of potential biological interest as one kind of sulfite oxidase is based on molybdenum metalloenzymes.…”

The crystallisation of polyoxo-molybdate and -tungstate anions with phenylated phosphonium and arsonium cations, in relation to the crystal packing and species in solution

“…There is a need to be able to control and facilitate the isolation of polyoxometallate anions by crystallisation. The redox properties of polyoxo-molybdates and -tungstates are crucial to many applications, and electrochemical investigations 4,5 have revealed series of increasingly reduced species-[S 2 Mo 18 O 62 ] 42 can be reversibly reduced by up to 18 electrons 4 -which present serious challenges in the selection and use of counter-cations for their crystallisation. Our interest in controlling the size of polyoxomolybdate anions was stimulated also by the observation in one of our laboratories that dimolybdenum species created by metal oxide vapour synthesis (MOVS) show enhanced hetereogeneous catalytic activities.…”