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

Reactions of [MoO2 (acetylacetonate)(2)] with the proligands (N-hydroxyimino)diacetic acid (H(3)hidpa), R, R-2,2'-(N-hydroxyimino)dipropionic acid (R,R-H(3)hidpa) or R,S-2,2'-(N-hydroxyimino) dibutyric acid (R,S-H(3)hidba) yielded the compounds [PPh4][Delta,Lambda -Mo(hida)(2)]. CH2Cl2 1, [H5O2][Delta -Mo(R,R-hidpa)(2)] 2, [PPh4][Mo(R,S-hidba)(2)]. 2H(2)O 3a and Na[Delta,Lambda -Mo(R,S-hidba)(2)]. 1/4 Pr-i(2) O 3b, respectively. Reactions of H(3)hida 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.