Study on the isomerism in meso-amavadin and an amavadin analogue

Amavadin and Homologues as Mediators of Water Oxidation

The vanadium(IV) N-hydroxyiminodicarboxylate complexes [V(HIDPA)2](2-) and [V(HIDA)2](2-), close models of the amavadin (a natural product from Amanita fungi lacking the V=O group but exhibiting a rare NO-bound oxyiminate moiety), are shown to be the first recognized complexes of the early transition metals (up to periodic Group 7) that mediate the oxidation of water. The reactions were analyzed by visible spectrophotometry, mass spectrometry, and measurement of evolved dioxygen using Ce(4+) as sacrificial oxidant. A mechanism proposed on the basis of DFT calculations involves the reversible oxidation to the mononuclear V(V)-{OṄ<} center, where the redox active oxyimino group plays a key role and metal oxidation state variation is only one unit. The more similar model of the metallobiomolecule, [V(HIDPA)2](2-), displays a lower oxidation rate than [V(HIDA)2](2-) but does not undergo appreciable degradation, in contrast to the latter.

Molecular and supramolecular ionic aggregates HxOyzin organic and organometallic crystalline hydrates

Ionic aggregates of the form HxOyz(z≠ 0) have been characterized during an analysis of 245 crystal structures extracted from the Cambridge Structural Database [Allen (2002).Acta Cryst.B58, 380–388]. A systematic nomenclature is proposed for these species. Three modes of hydrogen bonding are described, characterized in part by the distance between contiguous O atoms: normal (NHB; O...O = 2.6–3.0 Å), charge assisted (CAHB; O...O = 2.5 Å) and molecular (MHB; O...O = 2.4 Å). The three modes are consistent with previous reports, our experimental results, and quantum-chemical-optimized geometries and energetics. No evidence is presented concerning the possible existence or stability of these aggregates in solution. Rather, emphasis is placed on the necessity in crystal structure analysis to develop thoroughly existing hydrogen-bonded networks, ignorance of which can lead to erroneous crystal structure models and other physico-chemical data associated with composition and charge balance.