View source for Biology:Transition metal dioxygen complex

'''Dioxygen complexes''' are coordination compounds that contain O<sub>2</sub> as a [[Chemistry:Ligand|ligand]].<ref>{{cite book|first1=Gereon M.|last1=Yee |first2=William B. |last2=Tolman |editor1-first=Peter M. H. |editor1-last=Kroneck |editor2-first=Martha E. |editor2-last=Sosa Torres|title=Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases|series=Metal Ions in Life Sciences|volume=15|year=2015|publisher=Springer|chapter=Chapter 5: Transition Metal Complexes and the Activation of Dioxygen|pages=131–204|doi=10.1007/978-3-319-12415-5_5|pmid=25707468}}</ref><ref>{{cite book|last1=Holleman|first1= A. F. |last2=Wiberg |first2=E. |title=Inorganic Chemistry |publisher=Academic Press |location=San Diego, CA|date=2001 |isbn=0-12-352651-5}}</ref> The study of these compounds is inspired by oxygen-carrying proteins such as [[Biology:Myoglobin|myoglobin]], [[Biology:Hemoglobin|hemoglobin]], [[Biology:Hemerythrin|hemerythrin]], and [[Biology:Hemocyanin|hemocyanin]].<ref>{{cite book|first1=S. J. |last1=Lippard |first2=J. M. |last2=Berg |title=Principles of Bioinorganic Chemistry |publisher=University Science Books|location= Mill Valley, CA|date= 1994| isbn=0-935702-73-3}}</ref>  Several [[Chemistry:Transition metal|transition metal]]s form complexes with O<sub>2</sub>, and many of these complexes form reversibly.<ref>{{cite encyclopedia|last=Berry|first= R. E. |encyclopedia= Comprehensive Coordination Chemistry II|date= 2004|volume= 1|pages= 625–629|isbn=978-0-08-043748-4|doi=10.1016/B0-08-043748-6/01161-0|chapter= Reactivity and Structure of Complexes of Small Molecules: Dioxygen}}</ref>  The binding of O<sub>2</sub> is the first step in many important phenomena, such as [[Biology:Cellular respiration|cellular respiration]], [[Chemistry:Corrosion|corrosion]], and industrial chemistry.  The first synthetic oxygen complex was demonstrated in 1938 with cobalt(II) complex reversibly bound O<sub>2</sub>.<ref>{{cite journal | first= Tokuichi |last=Tsumaki | title = Nebenvalenzringverbindungen. IV. Über einige innerkomplexe Kobaltsalze der Oxyaldimine|trans-title=Secondary valence ring compounds. IV. On some inner-complex cobalt salts of [[Chemistry:Salen ligand|oxyaldimine]] | journal = [[Organization:Bulletin of the Chemical Society of Japan|Bulletin of the Chemical Society of Japan]] | year = 1938 | volume = 13 |issue=2 | pages = 252–260 | doi = 10.1246/bcsj.13.252| doi-access = free}}</ref>
==Mononuclear complexes of O<sub>2</sub>==
O<sub>2</sub> binds to a single metal center either "end-on" ([[Chemistry:Hapticity|''η''<sup>1</sup>-]]) or "side-on" (''η''<sup>2</sup>-). The bonding and structures of these compounds are usually evaluated by single-crystal [[Physics:X-ray crystallography|X-ray crystallography]], focusing both on the overall geometry as well as the O–O distances, which reveals the [[Chemistry:Bond order|bond order]] of the O<sub>2</sub> ligand.
:[[Image:TMdioxygenCmpx.png|220px]]

===Complexes of ''η''<sup>1</sup>-O<sub>2</sub> ligands===
thumb|right|220px|A picket-fence porphyrin complex of Fe, with axial coordination sites occupied by methylimidazole (green) and [[dioxygen (R = amide groups).<ref>S. J. Lippard, J. M. Berg "Principles of Bioinorganic Chemistry" University Science Books: Mill Valley, CA; 1994.  {{ISBN|0-935702-73-3}}.</ref>]]
O<sub>2</sub> [[Chemistry:Adduct|adduct]]s derived from [[Chemistry:Cobalt|cobalt]](II) and [[Chemistry:Iron|iron]](II) complexes of [[Biology:Porphyrin|porphyrin]] (and related anionic macrocyclic ligands) exhibit this bonding mode. Myoglobin and hemoglobin are famous examples, and many synthetic analogues have been described that behave similarly.  Binding of O<sub>2</sub> is usually described as proceeding by [[Physics:Electron transfer|electron transfer]] from the metal(II) center to give [[Chemistry:Superoxide|superoxide]] ({{chem|O|2|−}}) complexes of metal(III) centers. As shown by the mechanisms of [[Biology:Cytochrome P450|cytochrome P450]] and alpha-ketoglutarate-dependent hydroxylase, Fe-''η''<sup>1</sup>-O<sub>2</sub> bonding is conducive to formation of Fe(IV) oxo centers. O<sub>2</sub> can bind to one metal of a bimetallic unit via the same modes discussed above for mononuclear complexes. A well-known example is the active site of the protein [[Biology:Hemerythrin|hemerythrin]], which features a diiron [[Chemistry:Carboxylate|carboxylate]] that binds O<sub>2</sub> at one Fe center. Dinuclear complexes can also cooperate in the binding, although the initial attack of O<sub>2</sub> probably occurs at a single metal.

===Complexes of ''η''<sup>2</sup>-O<sub>2</sub> ligands===
''η''<sup>2</sup>-bonding is the most common motif seen in coordination chemistry of dioxygen.  Such complexes can be generated by treating low-valent metal complexes with oxygen.  For example, [[Chemistry:Vaska's complex|Vaska's complex]] reversibly binds O<sub>2</sub> (Ph = C<sub>6</sub>H<sub>5</sub>):
:IrCl(CO)(PPh<sub>3</sub>)<sub>2</sub> + O<sub>2</sub> {{eqm}} IrCl(CO)(PPh<sub>3</sub>)<sub>2</sub>O<sub>2</sub>
The conversion is described as a 2&nbsp;e<sup>−</sup> [[Chemistry:Redox|redox]] process: Ir(I) converts to Ir(III) as dioxygen converts to [[Chemistry:Metal peroxide|peroxide]]. Since O<sub>2</sub> has a triplet ground state and Vaska's complex is a singlet, the reaction is slower than when [[Chemistry:Singlet oxygen|singlet oxygen]] is used.<ref>{{cite journal|last1=Selke|first1= M. |last2=Foote|first2= C. S.|title=Reactions of Organometallic Complexes with Singlet Oxygen. Photooxidation of Vaska's Complex|journal= J. Am. Chem. Soc.|date= 1993| volume=115 |issue= 3 |pages=1166–1167|doi=10.1021/ja00056a061}}</ref> The magnetic properties of some  ''η''<sup>2</sup>-O<sub>2</sub> complexes show that the ligand, in fact, is superoxide, not peroxide.<ref>{{cite journal |doi=10.1021/ja00162a069|title=Crystal structure of a side-on superoxo complex of cobalt and hydrogen abstraction by a reactive terminal oxo ligand|year=1990|last1=Egan|first1=James W.|last2=Haggerty|first2=Brian S.|last3=Rheingold|first3=Arnold L.|last4=Sendlinger|first4=Shawn C.|last5=Theopold|first5=Klaus H.|journal=Journal of the American Chemical Society|volume=112|issue=6|pages=2445–2446}}</ref>

Most complexes of ''η''<sup>2</sup>-O<sub>2</sub> are generated using [[Chemistry:Hydrogen peroxide|hydrogen peroxide]], not from O<sub>2</sub>.  [[Chemistry:Chromate ion|Chromate]] ([CrO<sub>4</sub>)]<sup>2−</sup>) can for example be converted to the tetraperoxide [Cr(O<sub>2</sub>)<sub>4</sub>]<sup>2−</sup>.  The reaction of hydrogen peroxide with aqueous titanium(IV) gives a brightly colored peroxy complex that is a useful test for titanium as well as hydrogen peroxide.<ref>{{cite book|last1=Greenwood|first1= N. N.|last2= Earnshaw|first2= A. |date=1997 |title=Chemistry of the Elements |edition=2nd |location=Oxford|publisher=Butterworth-Heinemann |isbn=0-7506-3365-4}}</ref>

==Binuclear complexes of O<sub>2</sub>==
[[Image:oxyhemocyanin full.png|thumb|right|O<sub>2</sub>-bound form of hemocyanin, the O<sub>2</sub> carrier for certain molluscs.]]
These binding modes include ''μ''<sub>2</sub>-''η''<sup>2</sup>,''η''<sup>2</sup>-, ''μ''<sub>2</sub>-''η''<sup>1</sup>,''η''<sup>1</sup>-, and ''μ''<sub>2</sub>-''η''<sup>1</sup>,''η''<sup>2</sup>-.  Depending on the degree of electron-transfer from the dimetal unit, these O<sub>2</sub> ligands can again be described as peroxo or superoxo. [[Biology:Hemocyanin|Hemocyanin]] is an O<sub>2</sub>-carrier that utilizes a bridging O2 binding motif. It features a pair of [[Chemistry:Copper|copper]] centers.<ref name=Tolman>{{cite journal|title=Copper–Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity|first1=Courtney E. |last1=Elwell|first2=Nicole L. |last2=Gagnon|first3=Benjamin D. |last3=Neisen|first4=Debanjan |last4=Dhar|first5=Andrew D. |last5=Spaeth|first6=Gereon M. |last6=Yee|first7=William B.|last7=Tolman|journal=Chemical Reviews|year=2017|volume=117|issue=3 |pages=2059–2107 |doi=10.1021/acs.chemrev.6b00636|pmid=28103018 |pmc=5963733}}</ref>
:[[Image:Dimetal dioxygen complexes (molecular diagrams).png|420px]]
[[File:DOESCF10.svg|thumb|right|Structure of [Co(salen)([[Chemistry:Dimethylformamide|dmf]])]<sub>2</sub>O<sub>2</sub>.<ref>{{cite journal|journal=J. Chem. Soc. A|author=M. Calligaris |author2=G. Nardin |author3=L. Randaccio |author4=A. Ripamonti |year=1970|page=1069|title=Structural Aspects of the Synthetic Oxygen-Carrier NN′-Ethylenebis(Salicylideneiminato)cobalt(II): Structure of the Addition Compound with Oxygen Containing Dimethylformamide|doi=10.1039/j19700001069}}</ref>]].
[[Chemistry:Salcomine|Salcomine]], the cobalt(II) complex of [[Chemistry:Salen ligand|salen ligand]] is the first synthetic O<sub>2</sub> carrier.<ref>{{cite journal | author = Tokuichi Tsumaki | title = Nebenvalenzringverbindungen. IV. Über einige innerkomplexe Kobaltsalze der Oxyaldimine | journal = [[Organization:Bulletin of the Chemical Society of Japan|Bulletin of the Chemical Society of Japan]] | year = 1938 | volume = 13 | issue = 2 | pages = 252–260 | doi = 10.1246/bcsj.13.252| doi-access = free }}</ref> Solvated derivatives of the solid complex bind 0.5 equivalent of O<sub>2</sub>:
:2 Co(salen) + O<sub>2</sub> → [Co(salen)]<sub>2</sub>O<sub>2</sub>

Reversible electron transfer reactions are observed in some dinuclear O<sub>2</sub> complexes.<ref>{{cite journal |doi=10.1021/ic50062a022|title=Structure of Decaammine-μ-Peroxo-Dicobalt Disulfate Tetrahydrate|year=1968|last1= Schaefer|first1=William Palzer|journal= Inorganic Chemistry|volume=7|issue=4 |pages=725–731}}</ref>
[[File:Co2O2noparam.svg|thumb|center|398px|Oxidation of the dicobalt peroxy complex gives the complex of superoxide (O<sub>2</sub><sup>−</sup>). The Co-O-O-Co core flattens in the process and the O-O distance contracts by 10%.]]

==Relationship to other oxygenic ligands and applications==
Dioxygen complexes are the precursors to other families of oxygenic ligands.  Metal oxo compounds arise from the cleavage of the O–O bond after complexation. [[Chemistry:Hydroperoxide|Hydroperoxo]] complexes are generated in the course of the reduction of dioxygen by metals. The reduction of O<sub>2</sub> by metal catalysts is a key [[Chemistry:Half-reaction|half-reaction]] in [[Physics:Fuel cell|fuel cell]]s.

Metal-catalyzed oxidations with O<sub>2</sub> proceed via the intermediacy of dioxygen complexes, although the actual oxidants are often oxo derivatives. The reversible binding of O<sub>2</sub> to metal complexes has been used as a means to purify oxygen from air, but cryogenic distillation of [[Physics:Liquid air|liquid air]] remains the dominant technology.

==References==
<references />
{{Coordination complexes}}

[[Category:Coordination complexes]]
[[Category:Biochemistry]]
[[Category:Inorganic chemistry]]

{{Sourceattribution|Transition metal dioxygen complex}}