Transition metal oxo complex#Oxygen-atom transfer

Image:sodium decavanadate.png, one of many polyoxometallate salts. The structure illustrates terminal oxo, doubly-bridging oxo, triply bridging oxo, and six-fold bridging oxo ligands.]]

A common reaction exhibited by metal-oxo compounds is olation, the condensation process that converts low molecular weight oxides to polymers with M-O-M linkages. Olation often begins with the deprotonation of a metal-hydroxo complex. It is the basis for mineralization and the precipitation of metal oxides. For the oxides of d0 metals, V^V, Nb^V, Ta^V, Mo^VI, and W^VI, the olation process affords polyoxometallates, a large class of molecular metal oxides.

Metal oxo complexes are intermediates in many metal-catalyzed oxidation reactions. Oxygen-atom transfer is common reaction of particular interest in organic chemistry and biochemistry.{{cite journal | journal = Chem. Rev. | doi = 10.1021/cr00082a005 | title = Metal-centered oxygen atom transfer reactions | year = 1987 | last1 = Holm | first1 = R. H. | volume = 87 | issue = 6 | pages = 1401–1449}} Some metal-oxos are capable of transferring their oxo ligand to organic substrates. One such example of this type of reactivity is from the enzyme superfamily molybdenum oxotransferase.

In water oxidation catalysis, metal oxo complexes are intermediates in the conversion of water to O₂.

Transition 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.{{cite journal|last1=Meunier|first1=Bernard|last2=de Visser|first2=Samuël P.|last3=Shaik|first3=Sason|title=Mechanism of Oxidation Reactions Catalyzed by Cytochrome P450 Enzymes|journal=Chemical Reviews|volume=104|issue=9|year=2004|pages=3947–3980|issn=0009-2665|doi=10.1021/cr020443g|pmid=15352783}}

Some of the longest known and most widely used oxo compounds are oxidizing agents such as potassium permanganate (KMnO₄) and osmium tetroxide (OsO₄).{{cite journal |last1=Du|first1=G.|last2=Abu-Omar|first2=M. M. | title = Oxo and Imido Complexes of Rhenium and Molybdenum in Catalytic Reductions | journal = Current Organic Chemistry | year = 2008 | volume = 12 | pages = 1185–1198 | doi = 10.2174/138527208785740238 | issue = 14}} 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.

:File:MOvarietypackPlus.png (d⁰), a tungsten oxo carbonyl (d²), permanganate (d⁰), [ReO₂(pyridine)₄]⁺ (d²), simplified view of compound I (a state of cytochrome P450, d⁴), and Ir(O)(mesityl)₃ (d⁴).]]

File:CYP mechanism.png used by cytochrome P450 enzymes for oxidation of aliphatic groups to alcohols by the action of Compound I (adapted from {{cite journal|last1=Huang|first1=Xiongyi|last2=Groves|first2=John T.|title=Beyond Ferryl‑Mediated Hydroxylation: 40 Years of the rebound mechanism and C–H activation|journal=J Biol Inorg Chem|volume=22|pages=185–207|year=2017|issue=2–3|doi=10.1007/s00775-016-1414-3|pmid=27909920|pmc=5350257|doi-access=free}}]]

Iron(IV)-oxo compounds are intermediates in many biological oxidations:

• Alpha-ketoglutarate-dependent hydroxylases activate O₂ by oxidative decarboxylation of ketoglutarate, generating Fe(IV)=O centers, i.e. ferryl, that hydroxylate a variety of hydrocarbon substrates.{{cite journal |last1=Hausinger|first1=R. P.| title = Fe(II)/α-Ketoglutarate-Dependent Hydroxylases and Related Enzymes | journal = Crit. Rev. Biochem. Mol. Biol. | volume = 39 | issue = 1 | pages = 21–68 |date=January–February 2004 | pmid =15121720 | doi = 10.1080/10409230490440541 |s2cid=85784668}}
• Cytochrome P450 enzymes, use a heme cofactor, insert ferryl oxygen into saturated C–H bonds,{{cite journal|last1=Ortiz de Montellano|first1=Paul R.|title=Hydrocarbon Hydroxylation by Cytochrome P450 Enzymes|journal=Chemical Reviews|volume=110|issue=2|year=2010|pages=932–948|issn=0009-2665|doi=10.1021/cr9002193|pmid=19769330|pmc=2820140}} epoxidize olefins,{{Cite journal |title = Epoxidation of olefins by cytochrome P450: Evidence from site-specific mutagenesis for hydroperoxo-iron as an electrophilic oxidant |last = Coon |first = M. J. |date = 1998-01-20 |journal = Proceedings of the National Academy of Sciences |doi = 10.1073/pnas.95.7.3555|pmid = 9520404 |volume=95 |issue = 7 |pages=3555–60 |pmc=19874|bibcode=1998PNAS...95.3555V |doi-access = free}}{{cite journal|last1=Farinas|first1=Edgardo T|last2=Alcalde|first2=Miguel|last3=Arnold|first3=Frances|title=Alkene epoxidation catalyzed by cytochrome P450 BM-3 139-3|journal=Tetrahedron|volume=60|issue=3|year=2004|pages=525–528|issn=0040-4020|doi=10.1016/j.tet.2003.10.099}} and oxidize aromatic groups.{{cite journal|last1=Korzekwa|first1=Kenneth|last2=Trager|first2=William|last3=Gouterman|first3=Martin|author-link3=Martin Gouterman|last4=Spangler|first4=Dale|last5=Loew|first5=Gilda|year=1985|title=Cytochrome P450 mediated aromatic oxidation: a theoretical study|journal=Journal of the American Chemical Society|volume=107|issue=14|pages=4273–4279|doi=10.1021/ja00300a033|bibcode=1985JAChS.107.4273K |issn=0002-7863}}
• Methane monooxygenase (MMO) oxidizes methane to methanol via oxygen atom transfer from an iron-oxo intermediate at its non-heme diiron center.{{cite journal | author = Brunold, T.C. | title = Synthetic Iron-Oxo 'Diamond Core' Mimics Structure of Key IIntermediate in Methane Monooxygenase Catalytic Cycle | journal = Proc. Natl. Acad. Sci. U.S.A. | year = 2007 | volume = 104 | issue = 52 | pages = 20641–20642 | doi = 10.1073/pnas.0710734105| pmid = 18093936 |bibcode = 2007PNAS..10420641B | pmc = 2409203 | doi-access = free}} Much effort is aimed at reproducing reactions with synthetic catalysts.{{cite journal |last1=Gunay|first1=A. |last2=Theopold|first2=K. H. | title = C-H Bond Activations by Metal Oxo Compounds | journal = Chem. Rev. | year = 2010 | volume = 110 | pages = 1060–1081 | doi = 10.1021/cr900269x |pmid=20143877 | issue = 2}}

Image:Mo cofactors.png. The DMSO reductase features two molybdopterin ligands attached to molybdenum. They are omitted from the figure for simplicity. The rest of the heterocycle is similar to what is shown for the other two cofactors.]]

The 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 MoO₄²⁻ or WO₄²⁻. 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(VI)) and tetravalent (M(IV)) 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.{{cite journal | last1 = Schwarz|first1=G.|last2=Mendel|first2=R. R.|last3=Ribbe|first3=M. W.| title = Molybdenum Cofactors, Enzymes and Pathways | journal = Nature | year = 2009 | volume = 460 | pages = 839–847 | doi = 10.1038/nature08302 | pmid = 19675644 | issue = 7257|bibcode = 2009Natur.460..839S |s2cid=205217953}} Representative reactions from each of the three structural classes are:

{{clear|left}}

• Sulfite oxidase: SO₃²⁻ + H₂O → SO₄²⁻ + 2 H⁺ + 2 e⁻
• DMSO reductase: H₃C–S(O)–CH₃ (DMSO) + 2 H⁺ + 2 e⁻ → H₃C–S–CH₃ (DMS) + H₂O
• Aldehyde ferredoxin oxidoreductase: R–CHO + H₂O → R–CO₂H + 2 H⁺ + 2 e⁻

The three different classes of molybdenum cofactors are shown in the adjacent figure. The biological use of tungsten mirrors that of molybdenum.{{cite journal |last1=Mukund|first1=S.|last2=Adams|first2= M. W. W. | title = Molybdenum and Vanadium Do not Replace Tungsten in the Catalytically Active Forms of the Three Tungstoenzymes in the Hyperthermophilic Archaeon Pyrococcus furiosus | journal = J. Bacteriol. |volume=178 | year = 1996 |issue=1| pages = 163–167|doi=10.1128/jb.178.1.163-167.1996 |pmid=8550411|pmc=177634|doi-access=free}}

The active site for the oxygen-evolving complex (OEC) of photosystem II (PSII) is a Mn₄O₅Ca centre with several bridging oxo ligands that participate in the oxidation of water to molecular oxygen.{{cite journal|last1=Umena|first1=Yasufumi|last2=Kawakami|first2=Keisuke|last3=Shen|first3=Jian-Ren|last4=Kamiya|first4=Nobuo|title=Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å|journal=Nature|volume=473|issue=7345|year=2011|pages=55–60|issn=0028-0836|doi=10.1038/nature09913|pmid=21499260|bibcode=2011Natur.473...55U|s2cid=205224374|url=http://ousar.lib.okayama-u.ac.jp/files/public/4/47455/20160528084139320094/Nature_473_55–60.pdf}} 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.

File:Oxygen Evolving Complex Crystal structure to 1.9 Angstrom Resolution.png

Image:Metal-oxo MO (d0).png

The term "oxo wall" is a theory used to describe the fact that no terminal oxo complexes are known for metal centers with octahedral symmetry and d-electron counts beyond 5.{{cite book|last1=Winkler|first1=Jay R.|authorlink1=Jay R. Winkler|last2=Gray|first2=Harry B.|authorlink2=Harry B. Gray|chapter=Electronic Structures of Oxo-Metal Ions|editor1-first=David Michael P.|editor1-last=Mingos|editor1-link=David Michael P. Mingos|editor2-first=Peter|editor2-last=Day|editor2-link=Peter Day (chemist)|editor3-first=Jens Peder|editor3-last=Dahl|title=Molecular Electronic Structures of Transition Metal Complexes I |year=2012|volume=142|pages=17–28|publisher=Springer Nature|doi=10.1007/430_2011_55|series=Structure and Bonding|isbn=978-3-642-27369-8}}{{cite journal|doi=10.1038/s41570-020-0197-9|title=Iron and manganese oxo complexes, oxo wall and beyond|year=2020|last1=Larson|first1=Virginia A.|last2=Battistella|first2=Beatrice|last3=Ray|first3=Kallol|last4=Lehnert|first4=Nicolai|last5=Nam|first5=Wonwoo|journal=Nature Reviews Chemistry|volume=4|issue=8|pages=404–419|pmid=37127969 |s2cid=220295993}}

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. Claimed exceptions to this rule{{Cite journal|last1=Anderson|first1=Travis M.|last2=Neiwert|first2=Wade A.|last3=Kirk|first3=Martin L.|last4=Piccoli|first4=Paula M. B.|last5=Schultz|first5=Arthur J.|last6=Koetzle|first6=Thomas F.|last7=Musaev|first7=Djamaladdin G.|last8=Morokuma|first8=Keiji|last9=Cao|first9=Rui|last10=Hill|first10=Craig L.|date=2004-12-17|title=A Late-Transition Metal Oxo Complex: K7Na9[O=PtIV(H2O)L2], L = [PW9O34]9-|url=https://www.science.org/doi/10.1126/science.1104696|journal=Science|language=en|volume=306|issue=5704|pages=2074–2077|doi=10.1126/science.1104696|issn=0036-8075|pmid=15564312|s2cid=41123922}}{{Retracted|doi=10.1126/science.337.6092.290-a|pmid=22822129|http://retractionwatch.com/2012/07/19/closing-loop-science-retracts-hill-group-oxo-paper/ Retraction Watch|intentional=yes}}{{Cite journal|last1=Anderson|first1=Travis M.|last2=Cao|first2=Rui|last3=Slonkina|first3=Elena|last4=Hedman|first4=Britt|last5=Hodgson|first5=Keith O.|last6=Hardcastle|first6=Kenneth I.|last7=Neiwert|first7=Wade A.|last8=Wu|first8=Shaoxiong|last9=Kirk|first9=Martin L.|last10=Knottenbelt|first10=Sushilla|last11=Depperman|first11=Ezra C.|date=2005-08-01|title=A Palladium-Oxo Complex. Stabilization of This Proposed Catalytic Intermediate by an Encapsulating Polytungstate Ligand|url=https://doi.org/10.1021/ja054131h|journal=Journal of the American Chemical Society|volume=127|issue=34|pages=11948–11949|doi=10.1021/ja054131h|pmid=16117527|bibcode=2005JAChS.12711948A |issn=0002-7863|url-access=subscription}}{{Retracted|doi=10.1021/ja207910h|pmid=22873777|http://retractionwatch.com/2012/06/08/jacs-science-retracting-three-papers-from-leading-emory-chemist-craig-hill/ Retraction Watch|intentional=yes}}{{Cite journal|last1=Cao|first1=Rui|last2=Anderson|first2=Travis M.|last3=Piccoli|first3=Paula M. B.|last4=Schultz|first4=Arthur J.|last5=Koetzle|first5=Thomas F.|last6=Geletii|first6=Yurii V.|last7=Slonkina|first7=Elena|last8=Hedman|first8=Britt|last9=Hodgson|first9=Keith O.|last10=Hardcastle|first10=Kenneth I.|last11=Fang|first11=Xikui|date=2007-09-01|title=Terminal Gold-Oxo Complexes|url=https://doi.org/10.1021/ja072456n|journal=Journal of the American Chemical Society|volume=129|issue=36|pages=11118–11133|doi=10.1021/ja072456n|pmid=17711276|bibcode=2007JAChS.12911118C |issn=0002-7863}}{{Retracted|doi=10.1021/ja207909y|pmid=22873776|http://retractionwatch.com/2012/06/08/jacs-science-retracting-three-papers-from-leading-emory-chemist-craig-hill/ Retraction Watch|intentional=yes}} have been retracted.{{cite journal | doi = 10.1021/ic2008914 | title = Revisiting the Polyoxometalate-Based Late-Transition-Metal-Oxo Complexes: The "Oxo Wall" Stands | year = 2012 | last1 = O’Halloran | first1 = Kevin P. | last2 = Zhao | first2 = Chongchao | last3 = Ando | first3 = Nicole S. | last4 = Schultz | first4 = Arthur J. | last5 = Koetzle | first5 = Thomas F. | last6 = Piccoli | first6 = Paula M. B. | last7 = Hedman | first7 = Britt | last8 = Hodgson | first8 = Keith O. | authorlink8 = Keith Hodgson | last9 = Bobyr | first9 = Elena | last10 = Kirk | first10 = Martin L. | last11 = Knottenbelt | first11 = Sushilla | last12 = Depperman | first12 = Ezra C. | last13 = Stein | first13 = Benjamin | last14 = Anderson | first14 = Travis M. | last15 = Cao | first15 = Rui | last16 = Geletii | first16 = Yurii V. | last17 = Hardcastle | first17 = Kenneth I. | last18 = Musaev | first18 = Djamaladdin G. | last19 = Neiwert | first19 = Wade A. | last20 = Fang | first20 = Xikui | last21 = Morokuma | first21 = Keiji | last22 = Wu | first22 = Shaoxiong | last23 = Kögerler | first23 = Paul | last24 = Hill | first24 = Craig L. | journal = Inorganic Chemistry | volume = 51 | issue = 13 | pages = 7025–7031 | pmid = 22694272| display-authors = 8 }}{{Cite web|last=Ritter|first=Stephen K.|date=June 12, 2012|title=Metal-Oxo Papers Retracted|url=https://cen.acs.org/articles/90/web/2012/06/Metal-Oxo-Papers-Retracted.html|access-date=2021-05-15|website=cen.acs.org}}{{Cite web|last=Hadlington2012-06-14T00:00:00+01:00|first=Simon|title=Oxo wall still stands as inorganic papers retracted|url=https://www.chemistryworld.com/news/oxo-wall-still-stands-as-inorganic-papers-retracted/5108.article|access-date=2021-05-15|website=Chemistry World|language=en}}

The iridium oxo complex Ir(O)(mesityl)₃ may appear to be an exception to the oxo-wall rule, but it is not because the complex is non-octahedral.{{cite journal | last1=Hay-Motherwell | first1=Robyn S. | last2=Wilkinson | first2=Geoffrey | authorlink2=Geoffrey Wilkinson | last3=Hussain-Bates | first3=Bilquis | last4=Hursthouse | first4=Michael B. | title = Synthesis and X-ray Crystal Structure of Oxotrimesityl-Iridium(V) | journal = Polyhedron | year = 1993 | volume = 12 | pages = 2009–2012 | doi = 10.1016/S0277-5387(00)81474-6 | issue = 16}} The trigonal symmetry reorders the metal d-orbitals below the degenerate MO π* pair. In three-fold symmetric complexes, multiple MO bonding is allowed for as many as 7 d-electrons.

Terminal oxo ligands are also rather rare for the titanium triad, especially zirconium and hafnium and are unknown for group 3 metals (scandium, yttrium, and lanthanum).

• Metal-ligand multiple bond
• Oxide
• Polyoxometalate
• Metallate
• Oxophilic
• Dioxygen complex

{{Reflist|2}}

{{Coordination complexes}}

Category:Ligands

Category:Transition metal oxides