Isomerization and Oxygen Atom Transfer Reactivity in Oxo−Mo Complexes of Relevance to Molybdoenzymes

Abstract

Both dioxo Mo(VI) and mono-oxo Mo(V) complexes of a sterically restrictive N2O heteroscorpionate ligand are found to exist as cis and trans isomers. The thermodynamically stable isomer differs for the two oxidation states, but in each case, we have isolated the kinetically labile isomer and followed its isomerization to the thermodynamically stable form. The Mo(VI) complex is more stable in the cis geometry and isomerizes more than 6 times faster than the Mo(V) complex, which prefers the trans geometry. In OAT reactions with PPh3, the trans isomer of the dioxo-Mo(VI) reacts approximately 20 times faster than the cis isomer. Thus, there are both oxidation state and donor atom dependent differences in isomeric stability and reactivity that could have significant functional implications for molybdoenzymes such as DMSO reductase.

The difference one ligand atom makes – An altered oxygen transfer reaction mechanism caused by an exchange of selenium for sulfur

“…In this particular case the selenium complex was the better oxo transfer catalyst compared to the sulfur complex although both performed less efficiently than other known molybdenum based oxo transfer catalysts [13][14][15][16][17][18] . The slowness of the reactions, however, provided the opportunity to investigate them in detail.…”

Dioxomolybdenum(VI) and -Tungsten(VI) Amino Bisphenolates as Epoxidation Catalysts

“…Dioxomolybdenum(VI) and tungsten(VI) complexes are known to catalyse oxotransfer reactions from DMSO to organic substrates or phosphines. [23,24,[27][28][29]38]. Generally, molybdenum-based catalysts are more active in these reactions than their tungsten counterparts.…”

“…On the grounds of structurally comparable molybdenum-and tungsten-based catalyst systems, we can assume that the dioxometal(VI) complexes react with benzoin to yield oxometal(IV) complexes and benzil. The central metal ions are then rapidly oxidized back to the initial dioxocompounds with DMSO to complete the catalytic cycle [23,24]. Although reduced molybdenum and tungsten species are expected to be strongly coloured, only rather weak colour changes were observed during these catalytic reactions indicating that the re-oxidation process of metal is very rapid and/or only small parts of the complexes are actually reacting.…”

“…For example, W can replace Mo in sulphite oxidase or nitrate reductase at the same coordination site but with remarkable differences in their behaviour, as these enzymes are less active or completely inactive upon the substitution of Mo by W [16][17][18]. Dioxomolybdenum(VI) complexes with amine bisphenol ligands have generally been prepared from stable and easily available MoO2(acac)2 or MoO2Cl2 and relevant ligand precursors in high yields using simple ligand displacement reactions [19][20][21][22][23][24][25][26], whereas occasional examples employ the use of an acidic solution of (NH4)6Mo7O24•4H2O [27][28][29]. Corresponding W compounds are made under dry conditions using [WO2Cl2(dme)] and ligands as free bases or sodium salts [30,31] or mixing free ligands and [W(eg)3] (eg = ethylene glycol) in alcohol solutions in the presence of ambient moisture [19,32].…”

Synthesis and structural characterization of mono- and dinuclear Mo(V)-oxo-complexes containing bis(3,5-dimethylpyrazol-1-yl)acetate anion as ligand

“…The exchange of tris(pyrazolyl) borate (Tp) ligands with tripodal N,N,X species rises the number of biologically appropriate atoms bound to Mo center but preserves the isostructurality and isoelectronicity with Tp ligands. The similarity between such compounds offers the possibility of investigation of effects of donor atom identity on the properties of the Mo center in a series of analogous complexes [12].In this report we describe the preparation [13] and crystallographic characterization of new mono-and dinuclear Mo(V) complexes with tripodal NNO ligand [14]. In Scheme 1 (a) the exchange of Cl and H 2 O ligands in [MoOCl 4 (H 2 O)] − by tridentate bdmpza molecule as well as (b) dimerisation of the resulted bdmpza compound are shown.…”