Oxo ligand
Encyclopedia
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.
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.
, 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.
, but metal-oxides are capable of a variety of reactions including catalytic process. Examples here demonstrate the range of reactions of molecular metal oxo species. The formation of O2 by the oxygen evolving center can be viewed as an O-atom transfer from a manganese oxo group with water.
(KMnO4) and osmium tetroxide (OsO4). Compounds such as these have been known since the 1700s and are widely used in organic synthesis
, e.g. for converting alkenes to vicinal
diol
s and alcohols to ketones or carboxylic acids. More selective or gentler oxidizing reagents include pyridinium chlorochromate
(PCC) and pyridinium dichromate (PDC), which have become widely used since the 1970s. Metal oxo species have been found capable of catalytic, including asymmetric oxidations of various types. Some metal-oxo complexes can activate
C-H bonds to give an aldehyde or alcohol. Some metal oxides have been used to catalyze reduction of organic compounds.
cofactor
and commonly oxidize an alkyl group to an alcohol
, a very difficult oxidation to do synthetically. 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.
have the molybdopterin
prosthetic group which generally cycles between Mo(IV) and Mo(VI) in one electron steps. Though there is some variation among these enzymes, members from all three families involve oxygen atom transfer between the Mo center and the substrate. Representative reactions from each of the three structural classes are:
The three different classes of molybdenum cofactors are shown in the Figure. The biological use of tungsten mirrors that of molybdenum.
(PSII) is a Mn4OxCa centre with several bridging oxo ligands that participate in the oxidation of water to molecular oxygen. The OEC is proposed to utilize a terminal oxo intermediate as a part of the water oxidation reaction. This complex is responsible for the production of nearly all of earth's molecular oxygen. This key link in the oxygen cycle
is necessary for much of the biodiversity
present on earth.
Terminal oxo ligands are also rather rare for the titanium triad, especially zirconium and hafnium and is unknown for group 3 metals (scandium, yttrium, and lanthanum).
s, but the stable oxidation states of late transition metals do not have low enough d counts to stabilize terminal oxo formation. The higher d counts of late transition metals lead to electrons in anti-bonding molecular orbitals and destabilization of the metal-oxygen multipl bond (see Fig. 2). Generally, very high oxidation states are destabilized by electrostatics
: a buildup of localized charge is always very destabilizing. Metals such as the group 6 metals (Cr, Mo, and W), which are very often found in the high +6 oxidation state, are stabilized by orbital considerations. The +6 oxidation state here yields a noble gas electron configuration
, which is very stable from an electron orbital point of view, thus offsetting destabilizing electrostatic effects.
The d orbitals for the very early transition are quite high energy and diffuse. Thus they do not overlap well in energy with the oxygen p orbitals, and in a metal-oxo interaction, the electron density tends to remain more localized on the oxygen in a more ionic type interaction. This makes the oxygen very nucleophilic and likely to attack other metal centers to yield a bridging configuration to delocalize the charge over multiple metals.
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
Molybdenum cofactor
Molybdenum cofactor is a cofactor required for the activity of enzymes such as sulfite oxidase, xanthine oxidoreductase, and aldehyde oxidase. It is a coordination complex formed between molybdopterin and an oxide of molybdenum.Molybdopterins, in turn, are synthesized from guanosine triphosphate...
and in many iron-containing enzymes. One of the earliest synthetic compounds to incorporate an oxo ligand is sodium ferrate
Ferrate
In chemistry, ferrate refers either to the anion , in which iron is in the +6 oxidation state, or to a salt containing this anion. The term ferrate is often used to mean ferrate, although according to IUPAC naming conventions, it may also refer to other iron-containing oxyanions, such as ferrate...
(Na2FeO4) circa 1702.
Olation and acid-base reactions
A common reactions effected by metal-oxo compounds is olationOlation
In inorganic chemistry, olation is the process by which metal ions form polymeric oxides in aqueous solution. The phenomenon is important to understand the relationship between metal ions in aqueous solution and metal oxides, which are represented by many minerals.At low pH, many metal ions exist...
, 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.
Oxo-atom transfer
Oxygen-atom transfer is another common reaction of particular interest in organic chemistryOrganic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, but metal-oxides are capable of a variety of reactions including catalytic process. Examples here demonstrate the range of reactions of molecular metal oxo species. The formation of O2 by the oxygen evolving center can be viewed as an O-atom transfer from a manganese oxo group with water.
Molecular oxides
Some of the longest known and most widely used oxo compounds are oxidizing agents such as potassium permanganatePotassium permanganate
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...
(KMnO4) and osmium tetroxide (OsO4). Compounds such as these have been known since the 1700s and are widely used in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
, e.g. for converting alkenes to vicinal
Vicinal
Vicinal may refer to:* Vicinal , stands for any two functional groups bonded to two adjacent atoms.* Vicinal , a word where all letters have alphabetic neighbors....
diol
Diol
A diol or glycol is a chemical compound containing two hydroxyl groups A geminal diol has two hydroxyl groups bonded to the same atom...
s and alcohols to ketones or carboxylic acids. More selective or gentler oxidizing reagents include pyridinium chlorochromate
Pyridinium chlorochromate
Pyridinium chlorochromate is a reddish orange solid reagent used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. Pyridinium chlorochromate, or PCC, will not fully oxidize a primary alcohol to the carboxylic acid as does the Jones reagent. A disadvantage to using PCC is...
(PCC) and pyridinium dichromate (PDC), which have become widely used since the 1970s. Metal oxo species have been found capable of catalytic, including asymmetric oxidations of various types. Some metal-oxo complexes can activate
C-H bond activation
Carbon–hydrogen bond activation or C−H activation may be defined as a reaction that cleaves a carbon–hydrogen bond. Often the term is restricted to reactions involving organometallic complexes and proceeding by coordination of a hydrocarbon to the inner-sphere of metal, either via an...
C-H bonds to give an aldehyde or alcohol. Some metal oxides have been used to catalyze reduction of organic compounds.
Iron(IV)-oxo species
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 hemeHeme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
cofactor
Cofactor
Cofactor may refer to any of the following:* Cofactor , the signed minor of a matrix* Minor , an alternative name for the determinant of a smaller matrix than that which it describes...
and commonly oxidize an alkyl group to an alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
, a very difficult oxidation to do synthetically. Similarly, methane monooxygenase
Methane monooxygenase
Methane monooxygenase, or MMO, is an enzyme capable of oxidizing the C-H bond in methane as well as other alkanes. Methane monooxygenase belongs to the class of oxidoreductase enzymes ....
(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
C-H bond activation
Carbon–hydrogen bond activation or C−H activation may be defined as a reaction that cleaves a carbon–hydrogen bond. Often the term is restricted to reactions involving organometallic complexes and proceeding by coordination of a hydrocarbon to the inner-sphere of metal, either via an...
). Second, harsh oxidizing agents will generally oxidize an alcohol to a carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
, but these enzymes are able to oxidize an alkyl group to an alcohol without further oxidation to a carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
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 species
The oxo ligand (or analagous 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 use. The biologically transported species and starting point for biosynthesis is generally accepted to be oxometallates MoO4−2 or WO4−2. All Mo/W enzymes except nitrogenaseNitrogenase
Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...
have the molybdopterin
Molybdopterin
Molybdopterins, when reacted with molybdenum or tungsten in the form of molybdate or tungstate, are a class of cofactors found in most molybdenum and all tungsten enzymes...
prosthetic group which generally cycles between Mo(IV) and Mo(VI) in one electron steps. Though there is some variation among these enzymes, members from all three families involve oxygen atom transfer between the Mo center and the substrate. Representative reactions from each of the three structural classes are:
- SO3−2 + H2O → SO4−2 + 2H+ + 2e- - (Sulfite oxidaseSulfite oxidaseSulfite oxidase is an enzyme in the mitochondria of all eukaryotes. It oxidizes sulfite to sulfate and, via cytochrome c, transfers the electrons produced to the electron transport chain, allowing generation of ATP in oxidative phosphorylation...
) - H3CS(O)CH3 (DMSODimethyl sulfoxideDimethyl sulfoxide is an organosulfur compound with the formula 2SO. This colorless liquid is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water...
) + 2H+ + 2e- → H3CSCH3 (DMSDimethyl sulfideDimethyl sulfide or methylthiomethane is an organosulfur compound with the formula 2S. Dimethyl sulfide is a water-insoluble flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from cooking of certain vegetables, notably maize,...
) + H2O - (DMSO reductaseDMSO reductaseDMSO reductase is a molybdenum-containing enzyme capable of reducing dimethyl sulfoxide to dimethyl sulfide . This enzyme serves as the terminal reductase under anaerobic conditions in some bacteria, with DMSO being the terminal electron acceptor...
)
The three different classes of molybdenum cofactors are shown in the Figure. The biological use of tungsten mirrors that of molybdenum.
Oxygen-evolving complex
The active site for the oxygen-evolving complex (OEC) of Photosystem IIPhotosystem II
Photosystem II is the first protein complex in the Light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme uses photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce...
(PSII) is a Mn4OxCa centre with several bridging oxo ligands that participate in the oxidation of water to molecular oxygen. The OEC is proposed to utilize a terminal oxo intermediate as a part of the water oxidation reaction. This complex is responsible for the production of nearly all of earth's molecular oxygen. This key link in the oxygen cycle
Oxygen cycle
The Oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within its three main reservoirs: the atmosphere , the total content of biological matter within the biosphere , and the lithosphere...
is necessary for much of the biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
present on earth.
The "oxo wall"
The term "oxo wall" is jargon used to describe the fact that no molecular oxo complexes is known for octahedral metal centers with d-electron counts beyond 4. A correlary is that the reactivity of complexes containing a terminal oxo ligand depends on the metal d-electron count. For octahedral d4 oxo complexes, the oxo ligand is nucleophilic. When the d-electron count is lower than 4, the oxo ligands become electrophilic. Oxo compounds for the vanadium through iron triads (groups 3-8) are well known, whereas terminal oxo compounds for metals in the cobalt through zinc triads (groups 9-12) are rare and invariably feature metals with coordination numbers lower than 6. This trend holds for other metal-ligand multiple bonds.Terminal oxo ligands are also rather rare for the titanium triad, especially zirconium and hafnium and is unknown for group 3 metals (scandium, yttrium, and lanthanum).
Bonding in terminal metal oxides
Terminal oxo ligation is stable for low d electron countD electron count
The d electron count is a chemistry formalism used to describe the electron configuration of the valence electrons of a transition metal center in a coordination complex. The d electron count is an effective way to understand the geometry and reactivity of transition metal complexes...
s, but the stable oxidation states of late transition metals do not have low enough d counts to stabilize terminal oxo formation. The higher d counts of late transition metals lead to electrons in anti-bonding molecular orbitals and destabilization of the metal-oxygen multipl bond (see Fig. 2). Generally, very high oxidation states are destabilized by electrostatics
Electrostatics
Electrostatics is the branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges....
: a buildup of localized charge is always very destabilizing. Metals such as the group 6 metals (Cr, Mo, and W), which are very often found in the high +6 oxidation state, are stabilized by orbital considerations. The +6 oxidation state here yields a noble gas electron configuration
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
, which is very stable from an electron orbital point of view, thus offsetting destabilizing electrostatic effects.
The d orbitals for the very early transition are quite high energy and diffuse. Thus they do not overlap well in energy with the oxygen p orbitals, and in a metal-oxo interaction, the electron density tends to remain more localized on the oxygen in a more ionic type interaction. This makes the oxygen very nucleophilic and likely to attack other metal centers to yield a bridging configuration to delocalize the charge over multiple metals.
M=O groups with coordination numbers <6
The iridium oxo complex Ir(O)(mesityl)3 poses an apparent exception to the oxo-wall, but in this case, iridium is tetrahedral. Similarly with platinum-, palladium-, and gold-oxo feature square planar metals. These complexes have M=O double bonds.See also
- metal-ligand multiple bond
- oxideOxideAn oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
- polyoxometalatePolyoxometalateIn chemistry, a polyoxometalate is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form a large, closed 3-dimensional framework....
- metallateMetallateMetallate is the name given to any complex anion containing a metal ligated to several atoms or small groups. The spelling metalate is almost as common....
- oxophilic
- dioxygen complexDioxygen complexDioxygen complexes are coordination compounds that contain O2 as a ligand. The study of these compounds is inspired by oxygen-carrying proteins such as myoglobin, hemoglobin, hemerythrin, and hemocyanin. Several transition metals form complexes with O2, and many of these complexes form reversibly...