New molybdenum(v) analogues of Amavadin and their redox properties

Abstract

Reactions of [MoO₂(acetylacetonate)₂] with the proligands (N-hydroxyimino)diacetic acid (H₃hidpa), R,R-2,2′-(N-hydroxyimino)dipropionic acid (R,R-H₃hidpa) or R,S-2,2′-(N-hydroxyimino)dibutyric acid (R,S-H₃hidba) yielded the compounds [PPh₄][Δ,Λ-Mo(hida)₂]·CH₂Cl₂1, [H₅O₂][Δ-Mo(R,R-hidpa)₂] 2, [PPh₄][Mo(R,S-hidba)₂]·2H₂O 3a and Na[Δ,Λ-Mo(R,S-hidba)₂]·¼ⁱPr₂O 3b, respectively. Reactions of H₃hida with a methanolic solution of [PPh₄][MoOCl₄(H₂O)] in the presence of NaOH (ca. pH 8) provided an alternative synthesis for 1. The complex of 1 when transferred into CH₂Cl₂ using [PPh₄]Br yielded brown block-like crystals from a CH₂Cl₂–EtOH solution, however, 2 and 3b were crystallised from H₂O and MeCN solutions with [H₅O₂]⁺ and [Na]⁺ counter cations, respectively. X-Ray crystallography confirmed the same distinctive eight-co-ordinate geometry of the complex anions of 1, 2 and 3b as identified for Amavadin, the form in which vanadium(IV) is bound in Amanita muscaria mushrooms. EPR and UV/vis spectra recorded for 1, 2 and 3a are consistent with the presence of molybdenum(V). Cyclic voltammetric studies using a glassy carbon working electrode in CH₂Cl₂ for 1 exhibited a reversible Mo^VI/Mo^V and a quasi-reversible Mo^V/Mo^IV redox couple at E_1/2 = +0.96 and −0.99 V (vs. a saturated calomel electrode), respectively. Complex 3a also displayed a reversible Mo^VI/Mo^V redox couple at E_1/2 = +0.77 V, whereas the Mo^V/Mo^IV couple was irreversible (E_pc = −1.28 V). Additional electrochemical studies with 2 recorded a reversible Mo^VI/Mo^V redox couple in Me₂SO (E_1/2 = +0.77 V), however in H₂O this one-electron oxidation process is irreversible.