Diphosphenes

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 importance.

Normally, compounds with the empirical formula RP exist as rings.  However, like other multiple bonds between heavy main-group elements, P=P double bonds can be stabilized by large steric hindrance.[1] In general, diphosphenes react like alkenes.

History

In 1877, Köhler and Michaelis claimed what would have been the first isolated diphosphene (PhP=PPh),[2] The structure of Köhler and Michaelis' product was later revised.[3][4] and X-ray crystallographic analysis[5] 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.[6]

Yoshifuji et al's isolated a sterically-hindered diphosphene in 1981.[6] 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.[7]

Synthesis

Following Maasaka Yoshifuji and his coworkers' 1981 preparation of bis(2,4,6-tri-tert-butylphenyl)diphosphene,[7] most disphosphene syntheses involve dehalogenation of bulkyl aryldichlorophosphine (ArPCl2). Mg is a typical dehalogenation reagent:[8]

2 ArPCl2 + 2 Mg → ArP=PAr + 2 MgCl2

Such a synthesis works also for trisalkylsilylphosphines,[8] or N-heterocyclic boro-phosphines.[9]

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.[10]

Structure

Cyclic voltammetry and UV/Vis spectra indicate that boryl-substituted diphosphenes have lower LUMO level and larger HOMO-LUMO gap than aryl-substituted diphosphenes.[9]

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; P-P-C = 102.8 (1)o; C-P-P-C = 172.2 (1)o.[7] Compared with the length of a P-P single bond in H2PPH2 (2.238 Å),[11] the P-P bond distance is much shorter, which reveals double bond character. The trans orientation is the thermodynamically preferred isomer.[12]

Spectroscopic properties

Diphosphene compounds usually exhibit a symmetry-allowed () (intense) and symmetry-forbidden () (weak) electronic transitions.[13] In the Raman spectrum, the P=P vibration is enhanced by resonance with allowed the transition than with the forbidden transition due to different geometries of excited states and enhancement mechanism.[14] Also the observed strong Raman shifts for (CH(SiMe
3)
2)
2P
2and (CH(SiMe3)2P=PC(SiMe3)2) suggest stronger dipnictenes feature of diphosphene compared with P-P single bond.[15]

Reactivity

Lithium aluminium hydride reduces diphosphene to give diphosphanes.[16]

Carbenes add across the double bond, to give diphosphiranes, which further rearrange to 1,3-diphospha-allenes in strong bases.[17]

Diphosphene is inert to oxygen but cycloadds to ozone to give highly unstable phosphorus-oxygen rings that tend to attack the phosphorus' organyl substituents.[18][19] The reaction with ozone is much more rapid and indicates a 2:1 (ozone:diphosphene) stoichiometry.[19]

When treated with strongly nucleophilic NHC's, the P=P bond cleaves giving phosphinidene compounds:[20]

RP=PR + L → 2 RP−L

Coordination to transition metals

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[P2(CH(SiMe3)2}) is obtained by treating Na2[Fe(CO)4] with dichlorobis(trimethylsilyl)methylphosphine.[21] The related complex [ArP=PAr]Fe(CO)4 (Ar=2,4,6-tri-tert-butylphenyl) arises by treating diphosephene with Fe2(CO)9.[22]

η2-coordination is illustrated by (M(PhP=PPh)L2) (with M=Pt or Pd and L = (PPh3)2 or Ph2P[CH2]2PPh2).[23]

See also

References

  1. ↑ Power, Philip P. (2010-01-14). "Main-group elements as transition metals". Nature. 463 (7278): 171–177. Bibcode:2010Natur.463..171P. doi:10.1038/nature08634. ISSN 1476-4687. PMID 20075912. S2CID 205219269.
  2. ↑ Kohler, H; Michaelis, A (1877). "Ueber Phenylphosphin und Phosphobenzol (Diphosphenyl)". Ber. Dtsch. Chem. Ges. 10: 807–814. doi:10.1002/cber.187701001222.
  3. ↑ In Horner, Leopold; Hoffmann, Hellmut; Beck, Peter (August 1958). "Phosphororganische Verbindungen, XVI. Wege zur Darstellung primärer, sekundärer und tertiärer Phosphine". Chemische Berichte (in German). 91 (8): 1583–1588. doi:10.1002/cber.19580910803. ISSN 0009-2940,
  4. ↑ Kuchen, W; Grilnewald, W (1965). "Zur Kenntnis der Organophosphorverbindungen, VIII. Über ein neues Verfahren zur Darstellung von Organooligophosphinen". Chem. Ber. 98 (2): 480–486. doi:10.1002/cber.19650980220.
  5. ↑ Daly, J; Maier, L (1964). "Molecular Structure of Phosphobenzene". Nature. 203 (4950): 1167. Bibcode:1964Natur.203.1167D. doi:10.1038/2031167b0. S2CID 4213722.
  6. 1 2 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.
  7. 1 2 3 Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. (1981-07-01). "Synthesis and structure of bis(2,4,6-tri-tert-butylphenyl)diphosphene: isolation of a true phosphobenzene". Journal of the American Chemical Society. 103 (15): 4587–4589. doi:10.1021/ja00405a054. ISSN 0002-7863.
  8. 1 2 Cowley, A. H.; Kilduff, J. E.; Newman, T. H.; Pakulski, M. (1982-10-01). "Diphosphenes (RP:PR). Synthesis and NMR characterization". Journal of the American Chemical Society. 104 (21): 5820–5821. doi:10.1021/ja00385a060. ISSN 0002-7863.
  9. 1 2 Asami, Shun-suke; Okamoto, Masafumi; Suzuki, Katsunori; Yamashita, Makoto (2016-10-04). "A Boryl-Substituted Diphosphene: Synthesis, Structure, and Reaction with n-Butyllithium To Form a Stabilized Adduct by pπ-pπ Interaction". Angewandte Chemie. 128 (41): 13019–13023. Bibcode:2016AngCh.12813019A. doi:10.1002/ange.201607995. ISSN 1521-3757.
  10. ↑ Liu, Liu Leo; Zhou, Jiliang; Cao, Levy L.; Stephan, Douglas W. (2018-11-15). "Facile Cleavage of the P=P Double Bond in Vinyl-Substituted Diphosphenes". Angewandte Chemie International Edition. 58 (1): 273–277. doi:10.1002/anie.201812592. ISSN 1521-3757. PMID 30444313. S2CID 53564701.
  11. ↑ Matus, Myrna H.; Nguyen, Minh Tho; Dixon, David A. (2007-03-01). "Heats of Formation of Diphosphene, Phosphinophosphinidene, Diphosphine, and Their Methyl Derivatives, and Mechanism of the Borane-Assisted Hydrogen Release". The Journal of Physical Chemistry A. 111 (9): 1726–1736. Bibcode:2007JPCA..111.1726M. doi:10.1021/jp067892v. ISSN 1089-5639. PMID 17298044.
  12. ↑ Caminade, Anne-Marie; Verrier, Martine; Ades, Claude; Paillous, Nicole; Koenig, Max (1984-01-01). "Laser irradiation of a diphosphene: evidence for the first cis–trans isomerization". J. Chem. Soc., Chem. Commun. (13): 875–877. doi:10.1039/c39840000875. ISSN 0022-4936.
  13. ↑ Sasamori, Takahiro; Tokitoh, Norihiro (2008-03-05). "Doubly bonded systems between heavier Group 15 elements". Dalton Trans. (11): 1395–1408. doi:10.1039/b715033d. ISSN 1477-9234. PMID 18322615. S2CID 6269387.
  14. ↑ Copeland, Tiffany; Shea, Michael P.; Milliken, Matt C.; Smith, Rhett C.; Protasiewicz, John D.; Simpson, M.Cather (2003). "Raman excitation profile of a sterically protected diphosphene [ArP=PAr]". Analytica Chimica Acta. 496 (1–2): 155–163. Bibcode:2003AcAC..496..155C. doi:10.1016/s0003-2670(03)00996-6.
  15. ↑ Yoshifuji, Masaaki (2017-03-01). "Sterically protected organophosphorus compounds of unusual structures". Pure and Applied Chemistry. 89 (3): 281–286. doi:10.1515/pac-2016-1029. ISSN 1365-3075.
  16. ↑ Yoshifuji, Masaaki; Shibayama, Katsuhiro; Inamoto, Naoki; Watanabe, Tokuko (1983-04-05). "Reduction of Diphosphene: Formation of dl- and meso-Diphosphanes". Chemistry Letters. 12 (4): 585–588. doi:10.1246/cl.1983.585. ISSN 0366-7022.
  17. ↑ Yoshifuji, Masaaki; Sasaki, Shigeru; Niitsu, Takashi; Inamoto, Naoki (1989). "A convenient new route from diphosphene to 1,3-diphospha-allene and dynamic NMR studies of the 2,4,6-tri-t-butylphenyl derivative". Tetrahedron Letters. 30 (2): 187–188. doi:10.1016/s0040-4039(00)95155-4.
  18. ↑ Caminade, A. M.; Khatib, F. E.; Ades, C.; Verrier, M.; Paillous, N.; Koenig, M. (1987-04-14). "ChemInform Abstract: Oxidation and Isomerization of Diphosphene". ChemInform. 18 (15): no. doi:10.1002/chin.198715272. ISSN 1522-2667.
  19. 1 2 Caminade, Anne-Marie; Couret, Claude; Escudie, Jean; Koenig, Max (1984-01-01). "Ozonolysis of bis[tris(trimethylsilyl)methyl]diphosphene". Journal of the Chemical Society, Chemical Communications (24): 1622. doi:10.1039/c39840001622. ISSN 0022-4936.
  20. ↑ Hayakawa, Naoki; Sadamori, Kazuya; Tsujimoto, Shota; Hatanaka, Miho; Wakabayashi, Tomonari; Matsuo, Tsukasa (2017-05-15). "Cleavage of a P=P Double Bond Mediated by N-Heterocyclic Carbenes". Angewandte Chemie International Edition. 56 (21): 5765–5769. doi:10.1002/anie.201701201. ISSN 1521-3773. PMID 28429503.
  21. ↑ Flynn, Kathy M.; Olmstead, Marilyn M.; Power, Philip P. (1983-04-01). "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 of the American Chemical Society. 105 (7): 2085–2086. doi:10.1021/ja00345a080. ISSN 0002-7863.
  22. ↑ Cowley, A. H.; Kilduff, J. E.; Lasch, J. G.; Norman, N. C.; Pakulski, M.; Ando, F.; Wright, T. C. (1983-12-01). "Reactivity of diphosphenes and phosphaarsenes toward metal carbonyls". Journal of the American Chemical Society. 105 (26): 7751–7752. doi:10.1021/ja00364a051. ISSN 0002-7863.
  23. ↑ Chatt, Joseph; Hitchcock, Petter B.; Pidcock, Alan; Warrens, Christopher P.; Dixon, Keith R. (1982-01-01). "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}]". J. Chem. Soc., Chem. Commun. (16): 932–933. doi:10.1039/c39820000932. ISSN 0022-4936.
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