Vanadyl sulfates: molecular structure, magnetism and electrochemical activity

Abstract

Reaction of differing amounts of vanadyl sulfate with p-tert-butylthiacalix[4]areneH₄ and base allows access to the vanadyl-sulfate species [NEt₄]₄[(VO)₄(μ₃-OH)₄(SO₄)₄]·½H₂O (1), [HNEt₃]₅[(VO)₅(μ₃-O)₄(SO₄)₄]·4MeCN (2·4MeCN) and [NEt₄]₂[(VO)₆(O)₂(SO₄)₄(OMe)(OH₂)]·MeCN (3·MeCN). Similar use of p-tert-butylsulfonylcalix[4]areneH₄, p-tert-butylcalix[8]areneH₈ or p-tert-butylhexahomotrioxacalix[3]areneH₃ led to the isolation of [HNEt₃]₂[H₂NEt₂]₂{[VO(OMe)]₂p-tert-butylcalix[8-SO₂]areneH₂} (4), [HNEt₃]₂[V(O)₂p-tert-butylcalix[8]areneH₅] (5) and [HNEt₃]₂[V^IV₂V^V₄O₁₁(OMe)₈] (6), respectively. Dc magnetic susceptibility measurements were performed on powdered microcrystalline samples of 1–3 in the T = 300–2 K temperature range. Preliminary screening for electrochemical water oxidation revealed some activity for 2 with turnover frequency (TOF) and number (TON) of 2.2 × 10⁻⁴ s⁻¹ and 6.44 × 10⁻⁶ (mmol O₂/mmol cat.), respectively. The compound 3 showed an improved electrochemical activity in the presence of water. This is related to the increased number and the rate of electrons exchanged during oxidation of V⁴⁺ species, facilitated by protons generated in the water discharge process.