Diphosphenes

Template:Short description Template:Technical Template:Distinguish 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]

Template:Chem2

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 ( $π \to π^{*}$ ) (intense) and symmetry-forbidden ( $n \to π^{*}$ ) (weak) electronic transitions.^[13] In the Raman spectrum, the P=P vibration is enhanced by resonance with allowed the $π \to π^{*}$ transition than with the forbidden $n \to π^{*}$ transition due to different geometries of excited states and enhancement mechanism.^[14] Also the observed strong Raman shifts for Template:Chemand Template:Chem2 suggest stronger dipnictenes featureTemplate:Which of diphosphene compared with P-P single bond.^[15]Template:Failed verification

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]

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]

Template:Chem2

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 Template:Chem2 (with M=Pt or Pd and L = (PPh₃)₂ or Template:Chem2).^[23]

References

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Diphosphenes

Contents

History