Insertion of V^IV Ions into the Polyoxotungstate Archetype {As₄W₄₀}

A new polyoxometalate compound consisting of the 39-tungsto-4-arsenate(III) unit with four incorporated V^IV ions, isolated as (NH₄)₂₂[(V^IVO)₂(V^IVO(H₂O))(As^IIIW^VI₉O₃₃)₂(As^IIIW^VI_8.5V^IV_0.5(OH)O₃₂)₂(W^VIO₂)₄]·48H₂O (NH-AsW(V)), was synthesized and fully characterized. SQUID magnetometry shows three weakly coupled V^IV centers with an antiferromagnetic exchange interaction and one isolated V^IV ion as a spin-¹/₂ Curie paramagnet. UV-vis spectroscopy indicates that the AsW(V) structure remains intact in aqueous solution for at least 24 h. To enable the deposition of AsW(V) from solution on gold surfaces, its trihexyltetradecylphosphonium salt, THTDP-AsW(V), was prepared. The IR spectra of both congeners reveal the structural identity of AsW(V) independent of the countercations. The X-ray absorption near-edge structure data confirm the presence of V^IV centers in a distorted square-pyramidal coordination geometry in NH-AsW(V) and THTDP-AsW(V). X-ray photoelectron spectroscopy of the latter, deposited on Au(111), shows that the 4 V and 35 W centers preserve their IV+ and VI+ oxidation states, while the remaining 4 W ions are reduced to IV+.