Transition metal oxo complex
In coordination chemistry, an oxo ligand is an oxygen atom bound only to one or more metal centers. These ligands can exist as terminal or (most commonly) as bridging atom (Fig. 1). Oxo ligands stabilize high oxidation states of a metal.Nugent, W. A., Mayer, J. M. "Metal-Ligand Multiple Bonds." John Wiley & Sons, New York, 1988. Oxo ligands are pervasive, comprising the great majority of the Earth's crust. This article concerns a subset of oxides, molecular derivatives. They are also found in several metalloenzymes, e.g. in the molybdenum cofactor and in many iron-containing enzymes. One of the earliest synthetic compounds to incorporate an oxo ligand is sodium ferrate (Na2FeO4) circa 1702.
Olation and acid-base reactionsCommon reactions affected by metal-oxo compounds is olation, the condensation process that converts low molecular weight oxides to polymeric materials, including minerals. Olation often begins with the deprotonation of a metal-hydroxo complex.
Oxygen-atom transferOxygen-atom transfer is common reaction of particular interest in organic chemistry and biochemistry. Some metal-oxos are capable of transferring their oxo ligand to organic substrates. One such example of this type of reactivity is from and enzyme super-family Molybdenum oxotransferase.
Hydrogen Atom AbstractionTransition metal-oxo's are also capable of abstracting strong C–H, N–H, and O–H bonds. Cytochrome P450 contains a high-valent iron-oxo which is capable of abstracting hydrogen atoms from strong C–H bonds.
Molecular oxidesSome of the longest known and most widely used oxo compounds are oxidizing agents such as potassium permanganate (KMnO4) and osmium tetroxide (OsO4). Compounds such as these are widely used for converting alkenes to vicinal diols and alcohols to ketones or carboxylic acids. More selective or gentler oxidizing reagents include pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC). Metal oxo species are capable of catalytic, including asymmetric oxidations of various types. Some metal-oxo complexes promote C-H bond activation, converting hydrocarbons to alcohols. (d0), a tungsten oxo carbonyl (d2), permanganate (d0), ReO2(pyridine)4+ (d2), simplified view of compound I (a state of cytochrome P450, d4), and trismesityliridium oxide (d4).]]
Iron(IV)-oxo speciesfor conversion of hydrocarbons to alcohols via the action of "compound I", an iron(IV) oxide bound to a radical heme.|286x286px]]Iron(IV)-oxo compounds are intermediates in many oxidations catalysed by heme-containing enzymes. One of the most widely studied examples is cytochrome p450 enzymes, which use a heme cofactor that is capable of hydroxylation of saturated C–H bonds, epoxidation of olefins, and oxidation of aromatic groups. Similarly, methane monooxygenase (MMO) oxidizes methane to methanol via oxygen atom transfer from an iron-oxo intermediate at its non-heme di-iron center. First, C-H bonds are quite resistant to oxidation and are generally unreactive at moderate temperatures (see C-H bond activation). Second, harsh oxidizing agents will generally oxidize an alcohol to a carboxylic acid, but these enzymes are able to oxidize an alkyl group to an alcohol without further oxidation to a carbonyl or carboxylic acid. The oxidant used in these enzymatic reactions is molecular oxygen in contrast with the harsh, toxic chemicals often found in conventional synthetic organic oxidations. As is generally the case with enzymatic reactions, these oxidations are chemically selective and take place at fast rates in aqueous solvent. Much of the effort in producing synthetic C-H bond activation catalysts has been inspired by these well designed natural catalysts.
Molybdenum/tungsten oxo speciesThe oxo ligand (or analogous sulfido ligand) is nearly ubiquitous in molybdenum and tungsten chemistry, appearing in the ores containing these elements, throughout their synthetic chemistry, and also in their biological role (aside from nitrogenase). The biologically transported species and starting point for biosynthesis is generally accepted to be oxometallates MoO4−2 or WO4−2. All Mo/W enzymes, again except nitrogenase, are bound to one or more molybdopterin prosthetic group. The Mo/W centers generally cycle between hexavalent (M(IV)) and tetravalent (M(VI)) states. Although there is some variation among these enzymes, members from all three families involve oxygen atom transfer between the Mo/W center and the substrate. Representative reactions from each of the three structural classes are:
- Sulfite oxidase: SO3−2 + H2O → SO4−2 + 2 H+ + 2 e−
- DMSO reductase: H3CS(O)CH3 ( DMSO) + 2 H+ + 2 e− → H3CSCH3 ( DMS) + H2O
- Aldehyde ferredoxin oxidoreductase: RCHO + H2O → RCO2H + 2 H+ + 2 e−