View source for Diphosphenes

{{short description|Organophosphorus compound with a phosphorus–phosphorus double bond}}
{{Technical|date=June 2020}}
{{Distinguish|diphosphine|phosphaalkenes}}
'''Diphosphene''' is a type of [[organophosphorus compound]] that has a phosphorus–phosphorus [[double bond]], denoted by R-P=P-R'. These compounds are not common, but their properties have [[theoretical chemistry|theoretical]] importance. 

Normally, compounds with the empirical formula RP exist as rings.&nbsp; However, like other multiple bonds between heavy main-group elements, P=P double bonds can be stabilized by large [[steric effects|steric]] hindrance.<ref>{{cite journal |last=Power|first=Philip P.|date=2010-01-14|title=Main-group elements as transition metals|journal=Nature|language=en|volume=463|issue=7278|pages=171–177|doi=10.1038/nature08634|pmid=20075912|issn=1476-4687|bibcode=2010Natur.463..171P|s2cid=205219269}}</ref>  In general, diphosphenes react like [[Alkene|alkenes]].  

==History==
In 1877, Köhler and Michaelis claimed what would have been the first isolated diphosphene (PhP=PPh),<ref>{{cite journal |last1=Kohler|first1=H|last2=Michaelis|first2=A|date=1877|title=Ueber Phenylphosphin und Phosphobenzol (Diphosphenyl)|url=https://zenodo.org/record/1599236|journal=Ber. Dtsch. Chem. Ges.|volume=10|pages=807–814|via=|doi=10.1002/cber.187701001222}}</ref> The structure of Köhler and Michaelis' product was later revised.<ref>In {{Cite journal |last1=Horner |first1=Leopold |last2=Hoffmann |first2=Hellmut |last3=Beck |first3=Peter |date=August 1958 |title=Phosphororganische Verbindungen, XVI. Wege zur Darstellung primärer, sekundärer und tertiärer Phosphine |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cber.19580910803 |journal=Chemische Berichte |language=de |volume=91 |issue=8 |pages=1583–1588 |doi=10.1002/cber.19580910803 |issn=0009-2940 |postscript=,}}</ref><ref>{{cite journal |last1=Kuchen|first1=W|last2=Grilnewald|first2=W|date=1965|title=Zur Kenntnis der Organophosphorverbindungen, VIII. Über ein neues Verfahren zur Darstellung von Organooligophosphinen|url=|journal=Chem. Ber.|volume=98|issue=2|pages=480–486|doi=10.1002/cber.19650980220}}</ref> and X-ray crystallographic analysis<ref>{{cite journal |last1=Daly|first1=J|last2=Maier|first2=L|date=1964|title=Molecular Structure of Phosphobenzene|url=|journal=Nature|volume=203|issue=4950|pages=1167|bibcode=1964Natur.203.1167D|doi=10.1038/2031167b0|s2cid=4213722}}</ref> proved that this "diphosphene" only had P-P single bonds and was in fact primarily a four-membered ring of the form (PPh)<sub>4</sub>.  The isolation of phosphorus [[ylide]] and [[phosphaalkene]]s suggested that compounds with P=P bonds could be made.<ref name=":0">{{wikicite|Corbridge, D.&nbsp;E.&nbsp;C. (1995) "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. &sect;&nbsp;6.19.  {{ISBN|0-444-89307-5}}.|ref={{harvid|Corbridge|1995}}}}</ref>  

Yoshifuji ''et al''<nowiki/>'s isolated a sterically-hindered diphosphene in 1981.<ref name=":0" />  That compound's P-P bond distance is 2.034 Å, which is much shorter than the average bond length in (C<sub>6</sub>H<sub>5</sub>P)<sub>5</sub> (2.217 Å) and (C<sub>6</sub>H<sub>5</sub>P)<sub>6</sub> (2.237 Å) and indicates double-bond character.<ref name=":02">{{cite journal |last1=Yoshifuji |first1=M. |last2=Shima |first2=I. |last3=Inamoto |first3=N. |last4=Hirotsu |first4=K. |last5=Higuchi |first5=T. |date=1981-07-01 |title=Synthesis and structure of bis(2,4,6-tri-tert-butylphenyl)diphosphene: isolation of a true phosphobenzene |journal=Journal of the American Chemical Society |volume=103 |issue=15 |pages=4587–4589 |doi=10.1021/ja00405a054 |issn=0002-7863}}</ref>

== Synthesis ==
Following Maasaka Yoshifuji and his coworkers' 1981 preparation of bis(2,4,6-tri-tert-butylphenyl)diphosphene,<ref name=":02" /> most disphosphene syntheses involve dehalogenation of bulkyl aryldichlorophosphine (ArPCl<sub>2</sub>).  Mg is a typical dehalogenation reagent:<ref name=":1">{{cite journal |last1=Cowley |first1=A. H. |last2=Kilduff |first2=J. E. |last3=Newman |first3=T. H. |last4=Pakulski |first4=M. |date=1982-10-01 |title=Diphosphenes (RP:PR). Synthesis and NMR characterization |journal=Journal of the American Chemical Society |volume=104 |issue=21 |pages=5820–5821 |doi=10.1021/ja00385a060 |issn=0002-7863}}</ref>  
:{{chem2|2 ArPCl2  +  2 Mg  ->  ArP\dPAr  +  2 MgCl2}}
Such a synthesis works also for trisalkylsilylphosphines,<ref name=":1" /> or [[N-Heterocyclic carbene boryl anion|N-heterocyclic boro]]-phosphines.<ref name=":2">{{cite journal |last1=Asami |first1=Shun-suke |last2=Okamoto |first2=Masafumi |last3=Suzuki |first3=Katsunori |last4=Yamashita |first4=Makoto |date=2016-10-04 |title=A Boryl-Substituted Diphosphene: Synthesis, Structure, and Reaction with n-Butyllithium To Form a Stabilized Adduct by pπ-pπ Interaction |journal=Angewandte Chemie |language=en |volume=128 |issue=41 |pages=13019–13023 |bibcode=2016AngCh.12813019A |doi=10.1002/ange.201607995 |issn=1521-3757}}</ref>   
[[File:B-P-1 (cropped).png|Synthesis of diboryldiphosphene (dipp = 2,6-diisopropylphenyl)|center|thumb|upright=1.8]]

===Ylide-stabilized diphosphenes===
Examples of di-vinyl-substituted diphosphenes arise via a ring opening/dimerization process from kinetically unstable 2H-phosphirenes.  However, the conjugation caused the compounds to exhibit reactivity closer to a [[phosphinidene]].<ref>{{cite journal |last1=Liu|first1=Liu Leo|last2=Zhou|first2=Jiliang|last3=Cao|first3=Levy L.|last4=Stephan|first4=Douglas W.|date=2018-11-15|title=Facile Cleavage of the P=P Double Bond in Vinyl-Substituted Diphosphenes|journal=Angewandte Chemie International Edition|language=en|volume=58|issue=1|pages=273–277|doi=10.1002/anie.201812592|pmid=30444313|s2cid=53564701|issn=1521-3757}}</ref>

==Structure==
[[Cyclic voltammetry]] and [[UV/VIS spectroscopy|UV/Vis spectra]] indicate that boryl-substituted diphosphenes have lower LUMO level and larger HOMO-LUMO gap than aryl-substituted diphosphenes.<ref name=":2" />

===Geometry===
According to [[X-ray crystallography]], the following parameters describe bis(2,4,6-tri-tert-butylphenyl)diphosphene: P-P = 2.034 (2) Å; P-C = 1.826 (2) Å; <math>\angle</math>P-P-C = 102.8 (1)<sup>o</sup>; <math>\angle</math>C-P-P-C = 172.2 (1)<sup>o</sup>.<ref name=":02" /> Compared with the length of a P-P single bond in H<sub>2</sub>PPH<sub>2</sub> (2.238 Å),<ref>{{cite journal |last1=Matus|first1=Myrna H.|last2=Nguyen|first2=Minh Tho|last3=Dixon|first3=David A.|date=2007-03-01|title=Heats of Formation of Diphosphene, Phosphinophosphinidene, Diphosphine, and Their Methyl Derivatives, and Mechanism of the Borane-Assisted Hydrogen Release|journal=The Journal of Physical Chemistry A|volume=111|issue=9|pages=1726–1736|doi=10.1021/jp067892v|pmid=17298044|issn=1089-5639|bibcode=2007JPCA..111.1726M}}</ref> the P-P bond distance is much shorter, which reveals double bond character. The [[Cis–trans isomerism|''trans'' orientation]] is the thermodynamically preferred isomer.<ref name=":3">{{cite journal |last1=Caminade |first1=Anne-Marie |last2=Verrier |first2=Martine |last3=Ades |first3=Claude |last4=Paillous |first4=Nicole |last5=Koenig |first5=Max |date=1984-01-01 |title=Laser irradiation of a diphosphene: evidence for the first cis–trans isomerization |journal=J. Chem. Soc., Chem. Commun. |language=en |issue=13 |pages=875–877 |doi=10.1039/c39840000875 |issn=0022-4936}}</ref>  

===Spectroscopic properties===
Diphosphene compounds usually exhibit a symmetry-allowed (<math>\pi \rightarrow \pi^*</math>) (intense) and symmetry-forbidden (<math>n\rightarrow \pi^*</math>) (weak) electronic transitions.<ref>{{cite journal |last1=Sasamori|first1=Takahiro|last2=Tokitoh|first2=Norihiro|date=2008-03-05|title=Doubly bonded systems between heavier Group 15 elements|journal=Dalton Trans.|language=en|issue=11|pages=1395–1408|doi=10.1039/b715033d|pmid=18322615|s2cid=6269387 |issn=1477-9234}}</ref>  In the [[Raman spectrum]], the P=P vibration is enhanced by resonance with allowed the <math>\pi \rightarrow \pi^*</math> transition than with the forbidden <math>n\rightarrow \pi^*</math> transition due to different geometries of excited states and enhancement mechanism.<ref>{{cite journal |last1=Copeland|first1=Tiffany|last2=Shea|first2=Michael P.|last3=Milliken|first3=Matt C.|last4=Smith|first4=Rhett C.|last5=Protasiewicz|first5=John D.|last6=Simpson|first6=M.Cather|title=Raman excitation profile of a sterically protected diphosphene [ArP=PAr]|journal=Analytica Chimica Acta|language=en|volume=496|issue=1–2|pages=155–163|doi=10.1016/s0003-2670(03)00996-6|year=2003|bibcode=2003AcAC..496..155C }}</ref>  Also the observed strong Raman shifts for {{chem|(C|H|(|Si|Me|3|)|2|)|2|P|2}}and {{chem2|(CH(SiMe3)2P\dPC(SiMe3)2)}} suggest stronger dipnictenes feature{{Which|date=June 2024}} of diphosphene compared with P-P single bond.<ref>{{cite journal |last=Yoshifuji|first=Masaaki|date=2017-03-01|title=Sterically protected organophosphorus compounds of unusual structures|journal=Pure and Applied Chemistry|language=en|volume=89|issue=3|pages=281–286|doi=10.1515/pac-2016-1029|issn=1365-3075|doi-access=free}}</ref>{{Failed verification|date=June 2024|reason=Ctrl-F for "Raman" has no hits}}

== Reactivity ==
[[Lithium aluminium hydride]] reduces diphosphene to give diphosphanes.<ref>{{cite journal |last1=Yoshifuji |first1=Masaaki |last2=Shibayama |first2=Katsuhiro |last3=Inamoto |first3=Naoki |last4=Watanabe |first4=Tokuko |date=1983-04-05 |title=Reduction of Diphosphene: Formation of dl- and meso-Diphosphanes |journal=Chemistry Letters |volume=12 |issue=4 |pages=585–588 |doi=10.1246/cl.1983.585 |issn=0366-7022}}</ref> 
[[File:Color-filled map of electron density.png|thumb|Color-filled map of electron density of P<sub>2</sub>H<sub>2</sub>]]

Carbenes add across the double bond, to give [[Diphosphirane|diphosphiranes]], which further rearrange to [[Diphosphaallene|1,3-diphospha-allenes]] in strong bases.<ref name=":22">{{cite journal |last1=Yoshifuji |first1=Masaaki |last2=Sasaki |first2=Shigeru |last3=Niitsu |first3=Takashi |last4=Inamoto |first4=Naoki |year=1989 |title=A convenient new route from diphosphene to 1,3-diphospha-allene and dynamic NMR studies of the 2,4,6-tri-t-butylphenyl derivative |journal=Tetrahedron Letters |language=en |volume=30 |issue=2 |pages=187–188 |doi=10.1016/s0040-4039(00)95155-4}}</ref>  

:[[image:Carbene-1 (cropped).png|720px]]

Diphosphene is inert to oxygen but cycloadds to [[ozone]] to give highly unstable phosphorus-oxygen rings that tend to attack the phosphorus' organyl substituents.<ref>{{cite journal |last1=Caminade |first1=A. M. |author-link1=Anne-Marie Caminade |last2=Khatib |first2=F. E. |last3=Ades |first3=C. |last4=Verrier |first4=M. |last5=Paillous |first5=N. |last6=Koenig |first6=M. |date=1987-04-14 |title=ChemInform Abstract: Oxidation and Isomerization of Diphosphene. |journal=ChemInform |language=en |volume=18 |issue=15 |pages=no |doi=10.1002/chin.198715272 |issn=1522-2667}}</ref><ref name=":4">{{cite journal |last1=Caminade |first1=Anne-Marie |author-link1=Anne-Marie Caminade |last2=Couret |first2=Claude |last3=Escudie |first3=Jean |last4=Koenig |first4=Max |date=1984-01-01 |title=Ozonolysis of bis[tris(trimethylsilyl)methyl]diphosphene |journal=Journal of the Chemical Society, Chemical Communications |language=en |issue=24 |pages=1622 |doi=10.1039/c39840001622 |issn=0022-4936}}</ref> The reaction with ozone is much more rapid and indicates a 2:1 (ozone:diphosphene) stoichiometry.<ref name=":4" /> 

[[File:Oxidation-2.png|center|thumb|825x825px|Two oxidations of diphosphenes: a) Oxidation by triplet oxygen; b) Oxidation by ozone]]
When treated with strongly nucleophilic NHC's, the P=P bond cleaves giving [[phosphinidene]] compounds:<ref name=":32">{{cite journal |last1=Hayakawa |first1=Naoki |last2=Sadamori |first2=Kazuya |last3=Tsujimoto |first3=Shota |last4=Hatanaka |first4=Miho |last5=Wakabayashi |first5=Tomonari |last6=Matsuo |first6=Tsukasa |date=2017-05-15 |title=Cleavage of a P=P Double Bond Mediated by N-Heterocyclic Carbenes |journal=Angewandte Chemie International Edition |language=en |volume=56 |issue=21 |pages=5765–5769 |doi=10.1002/anie.201701201 |issn=1521-3773 |pmid=28429503}}</ref> 
:{{chem2|RP\dPR  + L  ->  2 RP\sL}}

===Coordination to transition metals===
[[File:Complex-2 (cropped).png|Coordination modes typical for diphosphenes.|120px|thumb|left]]
Diphosphines form a variety of [[coordination complexes..  Diphosphenes can bind to transition metal either in a η<sup>1</sup> or in a η<sup>2</sup> mode.

[{{chem2|Fe(CO)4[P2(CH(SiMe3)2}) is obtained by treating [[Collman's reagent|Na<sub>2</sub>[Fe(CO)<sub>4</sub>]]] with dichlorobis(trimethylsilyl)methylphosphine.<ref>{{cite journal |last1=Flynn|first1=Kathy M.|last2=Olmstead|first2=Marilyn M.|last3=Power|first3=Philip P.|date=1983-04-01|title=Simple one-step route to a transition-metal complex containing a phosphorus–phosphorus double bond. Synthesis and x-ray crystal structure of {trans-bis[bis(trimethylsilyl)methyl]diphosphene}bis[tetracarbonyliron(0)]|journal=Journal of the American Chemical Society|volume=105|issue=7|pages=2085–2086|doi=10.1021/ja00345a080|issn=0002-7863}}</ref>  The related complex [ArP=PAr]Fe(CO)<sub>4</sub> (Ar=2,4,6-tri-tert-butylphenyl) arises by treating diphosephene with Fe<sub>2</sub>(CO)<sub>9</sub>.<ref>{{cite journal |last1=Cowley|first1=A. H.|last2=Kilduff|first2=J. E.|last3=Lasch|first3=J. G.|last4=Norman|first4=N. C.|last5=Pakulski|first5=M.|last6=Ando|first6=F.|last7=Wright|first7=T. C.|date=1983-12-01|title=Reactivity of diphosphenes and phosphaarsenes toward metal carbonyls|journal=Journal of the American Chemical Society|volume=105|issue=26|pages=7751–7752|doi=10.1021/ja00364a051|issn=0002-7863}}</ref>

η<sup>2</sup>-coordination is illustrated by {{chem2|(M(PhP\dPPh)L2)}} (with M=Pt or Pd and L = (PPh<sub>3</sub>)<sub>2</sub> or {{chem2|Ph2P[CH2]2PPh2}}).<ref>{{cite journal |last1=Chatt|first1=Joseph|last2=Hitchcock|first2=Petter B.|last3=Pidcock|first3=Alan|last4=Warrens|first4=Christopher P.|last5=Dixon|first5=Keith R.|date=1982-01-01|title=Synthesis and31P n.m.r. spectroscopy of platinum and palladium complexes containing side-bonded diphenyldiphosphene. The X-ray crystal and molecular structure of [Pd(PhP=pph){bis(diphenyl-phosphino)ethane}]|journal=J. Chem. Soc., Chem. Commun.|language=en|issue=16|pages=932–933|doi=10.1039/c39820000932|issn=0022-4936}}</ref>

==See also==
* [[Diazene]]
* [[Double bond rule]]

==References==
{{Reflist}}

[[Category:Organophosphorus compounds]]
[[Category:Functional groups]]