Diphosphenes

History

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

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.

Synthesis

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

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.

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.

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

Spectroscopic properties

Diphosphene compounds usually exhibit a symmetry-allowed (\pi \rightarrow \pi^) (intense) and symmetry-forbidden (n\rightarrow \pi^) (weak) electronic transitions. In the Raman spectrum, the P=P vibration is enhanced by resonance with allowed the \pi \rightarrow \pi^* transition than with the forbidden n\rightarrow \pi^* transition due to different geometries of excited states and enhancement mechanism. Also the observed strong Raman shifts for and suggest stronger dipnictenes feature of diphosphene compared with P-P single bond.

Reactivity

Lithium aluminium hydride reduces diphosphene to give diphosphanes.

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

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

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

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.

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

η2-coordination is illustrated by (with M=Pt or Pd and L = (PPh3)2 or ).

References

References

1. Power, Philip P.. (2010-01-14). "Main-group elements as transition metals". Nature.
2. (1877). "Ueber Phenylphosphin und Phosphobenzol (Diphosphenyl)". Ber. Dtsch. Chem. Ges..
3. (August 1958). "Phosphororganische Verbindungen, XVI. Wege zur Darstellung primärer, sekundärer und tertiärer Phosphine". Chemische Berichte.
4. (1965). "Zur Kenntnis der Organophosphorverbindungen, VIII. Über ein neues Verfahren zur Darstellung von Organooligophosphinen". Chem. Ber..
5. (1964). "Molecular Structure of Phosphobenzene". Nature.
6. {{wikicite. 0-444-89307-5.. Corbridge. 1995
7. (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.
8. (1982-10-01). "Diphosphenes (RP:PR). Synthesis and NMR characterization". Journal of the American Chemical Society.
9. (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.
10. (2018-11-15). "Facile Cleavage of the P=P Double Bond in Vinyl-Substituted Diphosphenes". Angewandte Chemie International Edition.
11. (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.
12. (1984-01-01). "Laser irradiation of a diphosphene: evidence for the first cis–trans isomerization". J. Chem. Soc., Chem. Commun..
13. (2008-03-05). "Doubly bonded systems between heavier Group 15 elements". Dalton Trans..
14. (2003). "Raman excitation profile of a sterically protected diphosphene [ArP=PAr]". Analytica Chimica Acta.
15. Yoshifuji, Masaaki. (2017-03-01). "Sterically protected organophosphorus compounds of unusual structures". Pure and Applied Chemistry.
16. (1983-04-05). "Reduction of Diphosphene: Formation of dl- and meso-Diphosphanes". Chemistry Letters.
17. (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.
18. (1987-04-14). "ChemInform Abstract: Oxidation and Isomerization of Diphosphene.". ChemInform.
19. (1984-01-01). "Ozonolysis of bis[tris(trimethylsilyl)methyl]diphosphene". Journal of the Chemical Society, Chemical Communications.
20. (2017-05-15). "Cleavage of a P=P Double Bond Mediated by N-Heterocyclic Carbenes". Angewandte Chemie International Edition.
21. (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.
22. (1983-12-01). "Reactivity of diphosphenes and phosphaarsenes toward metal carbonyls". Journal of the American Chemical Society.
23. (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..