Phosphinidenes (IUPAC: phosphanylidenes, formerly phosphinediyls) are low-valent phosphorus compounds analogous to and , having the general structure RP.
A variety of strategies have been employed to stabilize phosphinidenes (e.g. π-donation, Steric effects, transition metal complexation),
Electronic structure
Like carbenes, phosphinidenes can exist in either a
singlet state or
triplet state, with the triplet state typically being more stable.
The stability of these states and their relative energy difference (the singlet-triplet energy gap) depends on the substituents.
The ground state in the parent phosphinidene (PH) is a triplet that is 22 kcal/mol more stable than the lowest singlet state.
This singlet-triplet energy gap is considerably larger than that of the simplest carbene methylene (9 kcal/mol).
Ab initio calculations from Nguyen et al. found that alkyl- and silyl-substituted phosphinidenes have triplet ground states, possibly in-part due to a negative hyperconjugation.
Case studies
Dibenzo-7-phosphanorbornadiene derivatives
One way to generate phosphinidines employs the decyclization of phosphaanthracene complexes.
Treatment of a bulky phosphine chloride (RPCl2) with magnesium anthracene affords a dibenzo-7-phosphanorbornadiene compound (RPA).
Phosphino-phosphinidene
The first singlet phosphino-phosphinidene has been prepared using extremely bulky substituents.
The authors prepared a chlorodiazaphospholidine with bulky (2,6-bis(4-tert-butylphenyl)methyl-4-methylphenyl) groups, and then synthesized the corresponding phosphaketene. Subsequent photolytic decarbonylation of the phosphaketene produced the phosphino-phosphinidene product as a yellow-orange solid that is stable at room temperature but decomposes immediately in the presence of air and moisture.
P NMR spectroscopy shows assigned product peaks at 80.2 and -200.4 ppm, with a
J-coupling constant of J
PP = 883.7 Hz. The very high P-P coupling constant is indicative of P-P multiple bond character.
The air/water sensitivity and high solubility of this compound prevented characterization by X-ray crystallography.
[File:Stable-4-methylphenyl substituents as reported by Bertrand and coworkers.
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Density functional theory and Natural bond orbital (NBO) calculations were used to gain insight into the structure and bonding of these phosphino-phosphinidenes. DFT calculations at the M06-2X/Def2-SVP level of theory on the phospino-phosphinidene with bulky 2,6-bis4-tert-butylphenyl)methyl-4-methylphenyl groups suggest that the tri-coordinated phosphorus atom exists in a planar environment.
Calculations at the
M06-2X/def2-TZVPP//M06-2X/def2-SVP level of theory were applied to a simplified model compound with diisopropylphenyl (Dipp) groups so as to reduce the computational cost for detailed NBO analysis.
Inspection of the outputted wavefunctions shows that the HOMO and HOMO-1 are P-P π-bonding orbitals and the LUMO is a P-P π*-antibonding orbital.
Further evidence of multiple bond character between the phosphorus atoms was provided by natural resonance theory and a large Wiberg bond index (P
1-P
2: 2.34).
Natural population analysis assigned a negative partial charge to the terminal phosphorus atom (-0.34 q) and a positive charge to the tri-coordinated phosphorus atom (1.16 q).
Despite the negative charge on the terminal phosphorus atom, subsequent studies have shown that this particular phosphinidene is electrophilic at the phosphinidene center. This phosphino-phosphinidene reacts with a number of nucleophiles (CO, isocyanides, carbenes, phosphines, etc.) to form phosphinidene-nucleophile adducts
Upon nucleophilic addition, the tri-coordinated phosphorus atom becomes non-planar, and it is postulated that the driving force of the reaction is provided by the instability of the phosphinidene's planar geometry.
Phospha-Wittig fragmentation
In 1989, Fritz et al. synthesized the phospha-Wittig species shown to the right.
Phospha-Wittig compounds can be viewed as a phosphinidene stabilized by a phosphine. These compounds have been given the label of "phospha-Wittig" as they have two dominant resonance structures (a neutral form and a
Zwitterion form) that are analogous to those of the
Ylide that are used in the
Wittig reaction.
Fritz et al. found that this particular phospha-Wittig reagent thermally decomposes at 20 °C to give
tBu
2PBr, LiBr, and cyclophosphanes.
The authors proposed that the singlet phosphino-phosphinidene
tBu
2PP was formed as an intermediate in this reaction. Further evidence for this was provided by trapping experiments, where the thermal decomposition of the phospha-Wittig reagent in the presence of 3,4,-dimethyl-1,3-butadiene and cyclohexene gave rise to the products shown in the figure below.
Metal complexes
Terminal phosphinidine complexes
Terminal transition-metal-complexed phosphinidenes L
nM=P-R are phosphorus analogs of transition metal carbene complexes. The first "metal-phosphinidine" was reported by Marinetti et al. They generated the transient species (OC)
5M=P-Ph by fragmentation of 7-phosphanorbornadiene
molybdenum and
tungsten complexes inside a mass spectrometer.
Soon after, they discovered that these 7-phosphanorbornadiene complexes could be used to transfer the phosphinidene complex (OC)
5M=P-R to various unsaturated substrates.
Donor-stabilized terminal phosphinidene complexes are also known,
which could release free phosphinidene complexes L
nM=P-R at mild conditions by P-donor dissociation reactions.
The phosphinidene complexes decomposed to white phosphorus if no unsaturated substrates were provided.
Terminal phosphinidene complexes of the type Cp
2M=P-R (M = Mo, W) can be obtained by combining aryl-dichlorophosphines RPCl
2 with Cp
2MHLi
4.
Phosphinidine-based clusters
containing RP substituents are numerous. They typically arise by the reaction of metal carbonyls with primary phosphines (compounds with the formula RPH
2). A partucularly well-studied case is , which forms from iron pentacarbonyl and
phenylphosphine according to the following idealized equation:
A related example is the tert-butylphosphinidene complex (t-BuP)Fe3(CO)10.
See also
