Diphosphenes

In 1877, Köhler and Michaelis claimed what would have been the first isolated diphosphene (PhP=PPh),{{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}} The structure of Köhler and Michaelis' product was later revised.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=,|url-access=subscription }}{{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}} and X-ray crystallographic analysis{{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}} proved that this "diphosphene" only had P-P single bonds and was in fact primarily a four-membered ring of the form (PPh)₄. The isolation of phosphorus ylide and phosphaalkenes suggested that compounds with P=P bonds could be made.{{wikicite|reference=Corbridge, D. E. C. (1995) "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. § 6.19. {{ISBN|0-444-89307-5}}.|ref={{harvid|Corbridge|1995}}}}

Yoshifuji et al's isolated a sterically-hindered diphosphene in 1981. That compound's P-P bond distance is 2.034 Å, which is much shorter than the average bond length in (C₆H₅P)₅ (2.217 Å) and (C₆H₅P)₆ (2.237 Å) and indicates double-bond character.{{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}}

Following Maasaka Yoshifuji and his coworkers' 1981 preparation of bis(2,4,6-tri-tert-butylphenyl)diphosphene, most disphosphene syntheses involve dehalogenation of bulkyl aryldichlorophosphine (ArPCl₂). Mg is a typical dehalogenation reagent:{{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}}

:{{chem2|2 ArPCl2 + 2 Mg -> ArP\dPAr + 2 MgCl2}}

Such a synthesis works also for trisalkylsilylphosphines, or N-heterocyclic boro-phosphines.{{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}}

File:B-P-1 (cropped).png

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.{{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}}

Cyclic voltammetry and UV/Vis spectra indicate that boryl-substituted diphosphenes have lower LUMO level and larger HOMO-LUMO gap than aryl-substituted diphosphenes.

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; $\backslash angle$P-P-C = 102.8 (1)^o; $\backslash angle$C-P-P-C = 172.2 (1)^o. Compared with the length of a P-P single bond in H₂PPH₂ (2.238 Å),{{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}} the P-P bond distance is much shorter, which reveals double bond character. The trans orientation is the thermodynamically preferred isomer.{{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}}

Diphosphene compounds usually exhibit a symmetry-allowed ($\backslash pi\; \backslash rightarrow\; \backslash pi^*$) (intense) and symmetry-forbidden ($n\backslash rightarrow\; \backslash pi^*$) (weak) electronic transitions.{{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}} In the Raman spectrum, the P=P vibration is enhanced by resonance with allowed the $\backslash pi\; \backslash rightarrow\; \backslash pi^*$ transition than with the forbidden $n\backslash rightarrow\; \backslash pi^*$ transition due to different geometries of excited states and enhancement mechanism.{{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 }} 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.{{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}}{{Failed verification|date=June 2024|reason=Ctrl-F for "Raman" has no hits}}

Lithium aluminium hydride reduces diphosphene to give diphosphanes.{{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}}

File:Color-filled map of electron density.png

Carbenes add across the double bond, to give diphosphiranes, which further rearrange to 1,3-diphospha-allenes in strong bases.{{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}}

: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.{{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}}{{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}} The reaction with ozone is much more rapid and indicates a 2:1 (ozone:diphosphene) stoichiometry.

File:Oxidation-2.png

When treated with strongly nucleophilic NHC's, the P=P bond cleaves giving phosphinidene compounds:{{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}}

:{{chem2|RP\dPR + L -> 2 RP\sL}}

File:Complex-2 (cropped).png

Diphosphines form a variety of coordination complexes. Diphosphenes can bind to transition metal either in a η¹ or in a η² mode.

{{chem2|[Fe(CO)4]2[P2(CH(SiMe3)]2}} is obtained by treating Na₂[Fe(CO)₄] with dichlorobis(trimethylsilyl)methylphosphine.{{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}} The related complex [ArP=PAr]Fe(CO)₄ (Ar=2,4,6-tri-tert-butylphenyl) arises by treating diphosephene with Fe₂(CO)₉.{{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}}

η²-coordination is illustrated by {{chem2|(M(PhP\dPPh)L2)}} (with M=Pt or Pd and L = (PPh₃)₂ or {{chem2|Ph2P[CH2]2PPh2}}).{{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}}

• Diazene
• Double bond rule

{{Reflist}}

Category:Organophosphorus compounds

Category:Functional groups