Diphosphenes

Not to be confused with diphosphine or 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 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. In general, diphosphenes react like alkenes.

History

In 1877, Köhler and Michaelis claimed what would have been the first isolated diphosphene (PhP=PPh), a publication that inspired little. However, the heavier pnictogens were known to form oligomers in oxidation state I, and by 1958, chemists had begun to reconsider the structure of Köhler and Michaelis' product. During the subsequent decade (the 1960s), molecular weight determination 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. Nevertheless, the contemporary discoveries of the first diphosphorus ylide and first phosphaalkene suggested that compounds with multiply-bonded phosphorus could be made.

The modern diphosphene field properly begins with field Yoshifuji et al's isolation of a more 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 isolation of bis(2,4,6-tri-tert-butylphenyl)diphosphene, most disphosphene syntheses dehalogenate a bulkyl alkyldichlorophosphine with an active metal. Such a synthesis works for arylphosphenes, trisalkylsilylphosphines, or N-heterocyclic boro-phosphines.

Synthesis of Bis(2,4,6-tri-tert-butylphenyl)diphosphene

Synthesis of diboryldiphosphene

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 illustrate that boryl-substituted diphosphenes have lower LUMO level and larger HOMO-LUMO gap than aryl-substituted diphosphenes.

Geometry

X-ray analysis indicates certain important bond lengths and angles of the first diphosphene, bis(2,4,6-tri-tert-butylphenyl)diphosphene: P-P = 2.034 (2) Å; P-C = 1.826 (2) Å; $\angle$P-P-C = 102.8 (1)^o; $\angle$C-P-P-C = 172.2 (1)^o. Compared with the bond 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 electronic transitions ($n\rightarrow \pi ^{*}$) (weak). Raman spectroscopy observes significant enhancement of P=P stretch in the resonance with allowed electron $\pi \rightarrow \pi ^{*}$ 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 (CH(SiMe
₃)
₂)
₂P
₂and (CH(SiMe₃)₂P=PC(SiMe₃)₂) suggest stronger dipnictenes feature of diphosphene compared with P-P single bond.

Reactivity

Lithium aluminium hydride reduces diphosphene to give diphosphanes:

Color-filled map of electron density of P₂H₂

Visible radiation induces cis-trans isomerization, although further irradiation can excite the molecule to a triplet diradical state. In triplet trans-HPPH, the P-P bond length is predicted to be 2.291 Å. It is not only longer than the P-P double bond in ground state trans-bis(2,4,6-tri-tert-butylphenyl)diphosphene, but also longer than that of P-P single bond in H₂PPH₂. Calculation of the dihedral angle of trans-HPPH suggests that it is almost 90 degree, which means the formation of $\pi$ and $\pi ^{*}$ P-P bonds is forbidden and σ bond is enhanced.

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.

Two oxidations of diphosphenes: a) Oxidation by triplet oxygen; b) Oxidation by ozone

Carbenes add across the double bond, to give diphosphiranes, which further rearrange to 1,3-diphospha-allenes in strong bases. Unlike with olefins, the ylides traditionally called persistent carbenes instead tend to cleave the central bond, forming two phosphaalkene/phosphinidene compounds.

a) carbene addition; b) "persistent carbene" mediated P=P double bond cleavage

Coordination to transition metals

The compounds form a wide variety of transition metal alkene complexes (see § Coordination to transition metals), as well as the traditional complexation to the phosphorus lone pair, or to any aryl moieties present.

Coordination modes typical for diphosphenes.

Diphosphenes can bind to transition metal either in a η¹ mode by donating a lone pair on phosphorus, or in a η² behavior via a $\pi$ interaction. If the bulky groups are aryl- groups, arene-coordinated products of η⁶-type coordination are also possible.

[Fe(CO)
₄][PCH(SiMe
₃)
₂]2}) is obtained by treating Na₂[Fe(CO)₄] and dichlorobis(trimethylsilyl)methylphosphine. ArP=PArFe(CO)₄ (Ar=2,4,6-tri-tert-butylphenyl) arises simply by treating diphosephene with Fe₂(CO)₉.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.</ref>

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

See also

• Diazene
• Double bond rule