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.

Not to be confused with diphosphine or phosphaalkenes.

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]

Synthesis of diboryldiphosphene (dipp = 2,6-diisopropylphenyl)

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; ${\displaystyle \angle }$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]

Spectroscopic properties

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^[which?] of diphosphene compared with P-P single bond.^{[15][failed verification]}

Reactivity

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

Color-filled map of electron density of P₂H₂

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]

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

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

Coordination modes typical for diphosphenes.

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

• Diazene
• Double bond rule