Transition metal dinitrogen complex

{{Short description|Coordination compounds with N2}} [[File:RuA5N2.png|thumb|right|175px|Structure of [Ru(NH3)5(N2)]2+.]] [[File:ReCl(dppe)2N2-3D-balls.png|thumb|right|175px|Ball-and-stick model of Re Cl (dppe)2N2]] thumb|150px|right|Fe(0)-N2 complex.<ref name=Peters>{{cite journal |doi=10.1021/acs.chemrev.9b00638|title=Catalytic N2-to-NH3 (or -N2H4) Conversion by Well-Defined Molecular Coordination Complexes|year=2020|last1=Chalkley|first1=Matthew J.|last2=Drover|first2=Marcus W.|last3=Peters|first3=Jonas C.|journal=Chemical Reviews|volume=120|issue=12|pages=5582–5636|pmc=7493999}}</ref> '''Transition metal dinitrogen complexes''' are coordination compounds that contain transition metals as ion centers the dinitrogen molecules (N2) as ligands.<ref name=":0">{{cite book|title=Transition Metal-Dinitrogen Complexes: Preparation and Reactivity|editor=Yoshiaki Nishibayashi|publisher=Wiley-VCH|year=2019|isbn=978-3-527-34425-3}}</ref>

== Historical background == Transition metal complexes of N2 have been studied since 1965 when the first complex was reported by Allen and Senoff.<ref>{{cite journal | last1 = Senoff | first1 = Caesar V. | year = 1990 | title = The discovery of [Ru(NH3)5N2]2+: A Case of Serendipity and the Scientific Method | journal = Journal of Chemical Education | volume = 67 | issue = 5| page = 368 | doi = 10.1021/ed067p368 | bibcode = 1990JChEd..67..368S }}</ref> This diamagnetic complex, [[Pentaamine(nitrogen)ruthenium(II) chloride|[Ru(NH3)5(N2)]2+]], was synthesized from hydrazine hydrate and ruthenium trichloride and consists of a [Ru(NH3)5]2+ centre attached to one end of N2.<ref name=Allen&Senoff>{{cite journal | title = Nitrogenopentammineruthenium(II) complexes |author1=A. D. Allen |author2=C. V. Senoff | journal = Journal of the Chemical Society, Chemical Communications | issue = 24 | pages = 621 | year = 1965 | doi = 10.1039/C19650000621}}</ref><ref>{{cite journal | last1 = Fryzuk | first1 = Michael D. | year = 2013 | title = N2 Coordination | journal = Chem. Commun. | volume = 49 | issue = 43| pages = 4866–4868 | doi = 10.1039/C3CC42001A | pmid = 23609888 }}</ref> The existence of N2 as a ligand in this compound was identified by IR spectrum with a strong band around 2170–2100 cm−1.<ref name=Allen&Senoff/> In 1966, the molecular structure of [Ru(NH3)5(N2)]Cl2 was determined by Bottomly and Nyburg by X-ray crystallography.<ref>{{Cite journal|last1=Bottomley|first1=F.|last2=Nyburg|first2=S. C.|date=1968-10-15|title=Molecular nitrogen as a ligand. The crystal structure of nitrogenpentaammineruthenium(II) dichloride and related salts|journal=Acta Crystallographica Section B |volume=24|issue=10|pages=1289–1293|doi=10.1107/S056774086800419X|issn=0567-7408|doi-access=free}}</ref>

The dinitrogen complex ''trans''-[IrCl(N2)(PPh3)2] is made by treating Vaska's complex with aromatic acyl azides. It has a planar geometry.<ref>{{Cite journal|last1=Collman|first1=James P.|last2=Kubota|first2=Mitsuru.|last3=Vastine|first3=Frederick D.|last4=Sun|first4=Jui Yuan.|last5=Kang|first5=Jung W.|date=September 1968|title=Iridium complexes of molecular nitrogen|journal=Journal of the American Chemical Society|volume=90|issue=20|pages=5430–5437|doi=10.1021/ja01022a018|issn=0002-7863}}</ref>

The first preparation of a metal-dinitrogen complex using dinitrogen was reported in 1967 by Yamamoto and coworkers. They obtained [Co(H)(N2)(PPh3)3] by reduction of Co(acac)3 with AlEt2OEt under an atmosphere of N2. Containing both hydrido and N2 ligands, the complex was of potential relevance to nitrogen fixation.<ref>{{Cite journal|last1=Yamamoto|first1=Akio|last2=Kitazume|first2=Shoji|last3=Pu|first3=Lyong Sun|last4=Ikeda|first4=Sakuji|date=January 1971|title=Synthesis and properties of hydridodinitrogentris(triphenylphosphine)cobalt(I) and the related phosphine-cobalt complexes|journal=Journal of the American Chemical Society|volume=93|issue=2|pages=371–380|doi=10.1021/ja00731a012|issn=0002-7863}}</ref>

From the late 1960s, a variety of transition metal-dinitrogen complexes were made including those with iron,<ref>{{Cite journal|last1=Aresta|first1=M.|last2=Giannoccaro|first2=P.|last3=Rossi|first3=M.|last4=Sacco|first4=A.|date=1971-03-01|title=Nitrogen fixation.: II. Dinitrogen-complexes of iron|journal=Inorganica Chimica Acta|volume=5|pages=203–206|doi=10.1016/S0020-1693(00)95914-0|issn=0020-1693}}</ref> molybdenum<ref>{{Cite journal|last1=Hidai|first1=M.|last2=Tominari|first2=K.|last3=Uchida|first3=Y.|last4=Misono|first4=A.|date=1969|title=A molybdenum complex containing molecular nitrogen|journal=Journal of the Chemical Society D: Chemical Communications|issue=14|pages=814|doi=10.1039/c29690000814|issn=0577-6171}}</ref> and vanadium<ref>{{Cite journal|last1=Song|first1=Jae-Inh|last2=Gambarotta|first2=Sandro|date=October 1996|title=Preparation, Characterization, and Reactivity of a Diamagnetic Vanadium Nitride|journal=Chemistry - A European Journal|volume=2|issue=10|pages=1258–1263|doi=10.1002/chem.19960021012|issn=0947-6539}}</ref> as metal centers. Interest in such complexes arises because N2 comprises the majority of the atmosphere and because many useful compounds contain nitrogen. Biological nitrogen fixation probably occurs via the binding of N2 to those metal centers in the enzyme nitrogenase, followed by a series of steps that involve electron transfer and protonation.<ref>{{Cite journal|last1=Li|first1=Jiapeng|last2=Yin|first2=Jianhao|last3=Yu|first3=Chao|last4=Zhang|first4=Wenxiong|last5=Xi|first5=Zhenfeng|date=2017|title=Direct Transformation of N2 to N-Containing Organic Compounds|journal=Acta Chimica Sinica|volume=75|issue=8|pages=733|doi=10.6023/a17040170|issn=0567-7351|doi-access=free|language=zh}}</ref>

==Bonding modes== In terms of its bonding to transition metals, N2 is related to CO and acetylene as all three species have triple bonds. A variety of bonding modes have been characterized. Based on whether the N2 molecules are shared by two more metal centers, the complexes can be classified into mononuclear and bridging. Based on the geometric relationship between the N2 molecule and the metal center, the complexes can be classified into end-on or side-on modes. In the end-on bonding modes of transition metal-dinitrogen complexes, the N-N vector can be considered in line with the metal ion center, whereas in the side-on modes, the metal-ligand bond is known to be perpendicular to the N-N vector.<ref name="61g"/>

===Mononuclear, end-on=== As a ligand, N2 usually binds to metals as an "end-on" ligand, as illustrated by [Ru(NH3)5N2]2+. Such complexes are usually analogous to related CO derivatives. This relationship is illustrated by the pair of complexes IrCl(CO)(PPh3)2 and IrCl(N2)(PPh3)2.<ref>{{cite book | title = ''trans''-Chloro(nitrogen)bis(triphenylphosphine)iridium (I) |author1=Collman, J. P. |author2=Hoffman, N. W. |author3=Hosking, J. W. | series = Inorganic Syntheses | volume = 12 | pages = 8–11 | year = 2000 | doi = 10.1002/9780470132432.ch2 | isbn = 978-0-470-13171-8}}</ref> In these mononuclear cases, N2 is both as a σ-donor and a π-acceptor. The M-N-N bond angles are close to 180°.<ref name=":0"/> N2 is a weaker pi-acceptor than CO, reflecting the nature of the π* orbitals on CO vs N2. For this reason, few examples exist of complexes containing ''both'' CO and N2 ligand.

Transition metal-dinitrogen complexes can contain more than one N2 as "end-on" ligands, such as ''mer''-[Mo(N2)3(PPr''n''2Ph)3], which has octahedral geometry.<ref>{{cite journal | title = A tris-dinitrogen complex. Preparation and crystal structure of ''mer''-[Mo(N2)3(PPr''n''2Ph)3] |author1=Anderson, S. N. |author2=Hughes, D. L. |author3=Richards, R. L. | journal = Journal of the Chemical Society, Chemical Communications | issue = 15 | pages = 958–959 | year = 1984 | doi = 10.1039/C39840000958}}</ref> In another example, the dinitrogen ligand in Mo(N2)2(Ph2PCH2CH2PPh2)2 can be reduced to produce ammonia.<ref>Modern Coordination Chemistry: The Legacy of Joseph Chatt" G. J. Leigh, N. W. Winterton Springer Verlag (2002). {{ISBN|0-85404-469-8}}</ref> Because many nitrogenases contain Mo, there has been particular interest in Mo-N2 complexes.

===Bridging, end-on=== N2 also serves as a bridging ligand with "end-on" bonding to two metal centers, as illustrated by {[Ru(NH3)5]2(μ-N2)}4+. These complexes are also called multinuclear dinitrogen complexes. In contrast to their mononuclear counterpart, they can be prepared for both early and late transition metals.<ref name=":0"/>

In 2006, a study of iron-dinitrogen complexes by Holland and coworkers showed that the N–N bond is significantly weakened upon complexation with iron atoms with a low coordination number. The complex involved bidentate chelating ligands attached to the iron atoms in the Fe–N–N–Fe core, in which N2 acts as a bridging ligand between two iron atoms. Increasing the coordination number of iron by modifying the chelating ligands and adding another ligand per iron atom showed an increase in the strength of the N–N bond in the resulting complex. It is thus suspected that Fe in a low-coordination environment is a key factor to the fixation of nitrogen by the nitrogenase enzyme, since its Fe–Mo cofactor also features Fe with low coordination numbers.<ref>{{Cite journal|last1=Smith|first1=Jeremy M.|last2=Sadique|first2=Azwana R.|last3=Cundari|first3=Thomas R.|last4=Rodgers|first4=Kenton R.|last5=Lukat-Rodgers|first5=Gudrun|last6=Lachicotte|first6=Rene J.|last7=Flaschenriem|first7=Christine J.|last8=Vela|first8=Javier|last9=Holland|first9=Patrick L.|date=2006-01-01|title=Studies of Low-Coordinate Iron Dinitrogen Complexes|journal=Journal of the American Chemical Society|volume=128|issue=3|pages=756–769|doi=10.1021/ja052707x|pmid=16417365|issn=0002-7863|url=https://digital.library.unt.edu/ark:/67531/metadc77137/}}</ref>

The average bond length of those bridging-end-on dinitrogen complexes is about 1.2 Å. In some cases, the bond length can be as long as 1.4 Å, which is similar to those of N-N single bonds.<ref>{{Cite journal|last1=Fryzuk|first1=Michael D.|last2=Haddad|first2=T. S.|last3=Mylvaganam|first3=Murugesapillai|last4=McConville|first4=David H.|last5=Rettig|first5=Steven J.|date=1993-04-01|title=End-on versus side-on bonding of dinitrogen to dinuclear early transition-metal complexes|journal=Journal of the American Chemical Society|volume=115|issue=7|pages=2782–2792|doi=10.1021/ja00060a028|issn=0002-7863}}</ref> Hasanayn and co-workers have shown that the Lewis structures of end-on bridging complexes can be assigned based on π-molecular-orbital occupancy, in analogy with simple tetratomic organic molecules. For example the cores of N2-bridged complexes with 8, 10, or 12 π-electrons can generally be formulated, respectively, as M≡N-N≡M, M=N=N=M, and M-N≡N-M, in analogy with the 8-, 10-, and 12-π-electron organic molecules HC≡C-C≡CH, O=C=C=O, and F-C≡C-F.<ref>{{Cite journal|last1=Hasanayn|first1=Faraj|last2=Holland|first2=Patrick L.|last3=Goldman|first3=Alan S.|last4=Miller|first4=Alexander J. M.|date=2023-02-16|title=Lewis Structures and the Bonding Classification of End-on Bridging Dinitrogen Transition Metal Complexes|journal=Journal of the American Chemical Society|doi=10.1021/jacs.2c12243|issn=0002-7863|pmc=9983020}}</ref>

=== Mononuclear, side-on === In comparison with their end-on counterpart, the mononuclear side-on dinitrogen complexes are usually higher in energy and the examples of them are rare. Dinitrogen act as a π-donor in these types of complexes. Fomitchev and Coppens has reported the first crystallographic evidence for side-on coordination of N2 to a single metal center in a photoinduced metastable state. When treated with UV light, the transition metal-dinitrogen complex, [Os(NH3)5(N2)]2+ in solid states can be converted into a metastable state of [Os(NH3)5(η2-N2)]2+, where the vibration of dinitrogen has shifted from 2025 to 1831 cm−1.

Some other examples are considered to exist in the transition states of intramolecular linkage isomerizations. Armor and Taube has reported these isomerizations using 15N-labelled dinitrogen as ligands.<ref>{{Cite journal|last1=Armor|first1=John N.|last2=Taube|first2=Henry.|date=April 1970|title=Linkage isomerization in nitrogen-labeled [Ru(NH3)5N2]Br2|journal=Journal of the American Chemical Society|volume=92|issue=8|pages=2560–2562|doi=10.1021/ja00711a066|issn=0002-7863}}</ref>

===Bridging, side-on=== In a second mode of bridging, bimetallic complexes are known wherein the N-N vector is perpendicular to the M-M vector, which can be considered as side-on fashion. One example is [(η5-C5Me4H)2Zr]2(μ2,η2,η2-N2).<ref>{{cite journal | title = Kinetics and Mechanism of N2 Hydrogenation in Bis(cyclopentadienyl) Zirconium Complexes and Dinitrogen Functionalization by 1,2-Addition of a Saturated C-H Bond |author1=Bernskoetter, W. H. |author2=Lobkovsky, E. |author3=Chirik, P. J. | journal = Journal of the American Chemical Society | volume = 127 | issue = 40 | pages = 14051–14061 | year = 2005 | doi = 10.1021/ja0538841 | pmid = 16201827}}</ref> The dimetallic complex can react with H2 to achieve the artificial nitrogen fixation by reducing N2.<ref>{{Cite journal|last1=Pool|first1=Jaime A.|last2=Lobkovsky|first2=Emil|last3=Chirik|first3=Paul J.|title=Hydrogenation and cleavage of dinitrogen to ammonia with a zirconium complex|journal=Nature|volume=427|issue=6974|pages=527–530|doi=10.1038/nature02274|pmid=14765191|bibcode=2004Natur.427..527P|year=2004|s2cid=4379465}}</ref> A related ditantalum tetrahydride complex could also reduce N2.<ref name="61g">{{Cite journal|last=Fryzuk|first=Michael D.|date=2008-09-20|title=Side-on End-on Bound Dinitrogen: An Activated Bonding Mode That Facilitates Functionalizing Molecular Nitrogen|journal=Accounts of Chemical Research|volume=42|issue=1|pages=127–133|doi=10.1021/ar800061g|pmid=18803409}}</ref>

==Reactivity== thumb|center|350px|Hypothesized cycle for M-catalysed nitrogen fixation according to Chatt et al.<ref name=":0"/>

===Cleavage to nitrides=== When metal nitrido complexes are produced from N2, the intermediacy of a dinitrogen complex is assumed. Some Mo(III) complexes also cleave N2:<ref>{{cite journal |doi=10.1021/ja960574x|title=Dinitrogen Cleavage by Three-Coordinate Molybdenum(III) Complexes: Mechanistic and Structural Data1|year=1996|last1=Laplaza|first1=Catalina E.|last2=Johnson|first2=Marc J. A.|last3=Peters|first3=Jonas C.|last4=Odom|first4=Aaron L.|last5=Kim|first5=Esther|last6=Cummins|first6=Christopher C.|last7=George|first7=Graham N.|last8=Pickering|first8=Ingrid J.|journal=Journal of the American Chemical Society|volume=118|issue=36|pages=8623–8638}}</ref><ref>{{cite journal |doi=10.1021/ja960574x|title=Dinitrogen Cleavage by Three-Coordinate Molybdenum(III) Complexes: Mechanistic and Structural Data1|year=1996|last1=Laplaza|first1=Catalina E.|last2=Johnson|first2=Marc J. A.|last3=Peters|first3=Jonas C.|last4=Odom|first4=Aaron L.|last5=Kim|first5=Esther|last6=Cummins|first6=Christopher C.|last7=George|first7=Graham N.|last8=Pickering|first8=Ingrid J.|journal=Journal of the American Chemical Society|volume=118|issue=36|pages=8623–8638|bibcode=1996JAChS.118.8623L |url=https://resolver.caltech.edu/CaltechAUTHORS:20141204-152547970 }}</ref>

:2{{nbsp}}Mo(NR2)3 + N2 → (R2N)3Mo-N2-Mo(NR2)3 :(R2N)3Mo-N2-Mo(NR2)3 → 2{{nbsp}}N≡Mo(NR2)3

===Attack by electrophiles=== Some electron-rich metal dinitrogen complexes are susceptible to attack by electrophiles on nitrogen. When the electrophile is a proton, the reaction is of interest in the context of abiological nitrogen fixation. Some metal-dintrogen complexes even catalyze the hydrogenation of N2 to ammonia in a cycle that involves N-protonation of a reduced M-N2 complex.<ref>{{Cite journal|last1=Yandulov|first1=Dmitry V.|last2=Schrock|first2=Richard R.|date=2003-07-04|title=Catalytic Reduction of Dinitrogen to Ammonia at a Single Molybdenum Center|journal=Science|volume=301|issue=5629|pages=76–78|doi=10.1126/science.1085326|issn=0036-8075|pmid=12843387|bibcode=2003Sci...301...76Y|s2cid=29046992}}</ref><ref name=":6">{{Cite journal|last1=Arashiba|first1=Kazuya|last2=Miyake|first2=Yoshihiro|last3=Nishibayashi|first3=Yoshiaki|title=A molybdenum complex bearing PNP-type pincer ligands leads to the catalytic reduction of dinitrogen into ammonia|journal=Nature Chemistry|volume=3|issue=2|pages=120–125|doi=10.1038/nchem.906|pmid=21258384|bibcode=2011NatCh...3..120A|year=2011}}</ref>

===Photochemistry=== Photolytic nitrogen splitting has been demonstrated.<ref>{{cite journal | last1 = Rebreyend | first1 = C. | last2 = de Bruin | first2 = B. | year = 2014 | title = Photolytic {{chem|N|2}} Splitting: A Road to Sustainable {{chem|NH|3}} Production? | journal = Angew. Chem. Int. Ed. | volume = 54| issue = 1| pages = 42–44| doi = 10.1002/anie.201409727 | pmid = 25382116}}</ref><ref>{{cite journal | last1 = Solari | first1 = E. | last2 = Da Silva | first2 = C. | last3 = Iacono | first3 = B. | last4 = Hesschenbrouck | first4 = J. | last5 = Rizzoli | first5 = C. | last6 = Scopelliti | first6 = R. | last7 = Floriani | first7 = C. | year = 2001 | title = Photochemical Activation of the N≡N Bond in a Dimolybdenum–Dinitrogen Complex: Formation of a Molybdenum Nitride | journal = Angew. Chem. Int. Ed. | volume = 40 | issue = 20| pages = 3907–3909 | doi = 10.1002/1521-3773(20011015)40:20<3907::AID-ANIE3907>3.0.CO;2-# | pmid = 29712125}}</ref><ref>{{cite journal | last1 = Huss | first1 = Adam S. | last2 = Curley | first2 = John J. | last3 = Cummins | first3 = Christopher C. | last4 = Blank | first4 = David A. | year = 2013 | title = Relaxation and Dissociation Following Photoexcitation of the (''μ''-{{chem|N|2}})[Mo(N[''t''-Bu]Ar)3]2 Dinitrogen Cleavage Intermediate | journal = J. Phys. Chem. B | volume = 117 | issue = 5| pages = 1429–1436 | pmid = 23249096 | doi = 10.1021/jp310122x }}</ref><ref>{{cite journal | last1 = Kunkely | first1 = H. | last2 = Vogler | first2 = A. | year = 2010 | title = Photolysis of Aqueous [({{chem|NH|3}})5Os(''μ''-{{chem|N|2}})Os({{chem|NH|3}})5]5+: Cleavage of Dinitrogen by an Intramolecular Photoredox Reaction | journal = Angew. Chem. Int. Ed. | volume = 49 | issue = 9| pages = 1591–1593 | doi = 10.1002/anie.200905026 | pmid = 20135653}}</ref><ref>{{cite journal | last1 = Miyazaki | first1 = T. | last2 = Tanaka | first2 = H. | last3 = Tanabe | first3 = Y. | last4 = Yuki | first4 = M. | last5 = Nakajima | first5 = K. | last6 = Yoshizawa | first6 = K. | last7 = Nishibayashi | first7 = Y. | year = 2014 | title = Cleavage and Formation of Molecular Dinitrogen in a Single System Assisted by Molybdenum Complexes Bearing Ferrocenyldiphosphine | journal = Angew. Chem. Int. Ed. | volume = 53 | issue = 43| pages = 11488–11492 | doi = 10.1002/anie.201405673 | pmid = 25214300 | bibcode = 2014ACIE...5311488M }}</ref>

==References== {{reflist|35em}}

Category:Dinitrogen complexes