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.

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.³⁴ and X-ray crystallographic analysis⁵ 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

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

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

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

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; ${\displaystyle \angle }$P-P-C = 102.8 (1)^o; ${\displaystyle \angle }$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

Diphosphene compounds usually exhibit a symmetry-allowed (${\displaystyle \pi \rightarrow \pi ^{*}}$) (intense) and symmetry-forbidden (${\displaystyle n\rightarrow \pi ^{*}}$) (weak) electronic transitions.¹³ In the Raman spectrum, the P=P vibration is enhanced by resonance with allowed the ${\displaystyle \pi \rightarrow \pi ^{*}}$ transition than with the forbidden ${\displaystyle n\rightarrow \pi ^{*}}$ 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

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.¹⁸¹⁹ 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

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

[Fe(CO)₄]₂[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