Phosphinidene

Phosphinidenes (IUPAC: phosphanylidenes, formerly phosphinediyls) are low-valent phosphorus compounds analogous to carbenes and nitrenes, having the general structure RP.

[10] 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.

[11] Under thermal conditions, the RPA compound (R = NiPr2) decomposes to yield anthracene; kinetic experiments found this decomposition to be first-order.

[11] Molecular beam mass spectrometry has enabled the detection of the evolution of amino-phosphinidene fragments from a number of alkylamide derivatives (e.g. Me2NP+ and Me2NPH+ from Me2NPA) in the gas-phase at elevated temperatures.

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.

DFT calculations at the M06-2X/Def2-SVP level of theory on the phospino-phosphinidene with bulky 2,6-bis[4-tert-butylphenyl)methyl]-4-methylphenyl groups suggest that the tri-coordinated phosphorus atom exists in a planar environment.

[3] 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.

to form phosphinidene-nucleophile adducts[3][12] 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.

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.

A partucularly well-studied case is Fe3(PC6H5)2(CO)9, which forms from iron pentacarbonyl and phenylphosphine according to the following idealized equation:[21] A related example is the tert-butylphosphinidene complex (t-BuP)Fe3(CO)10.

General structure of a phosphinidene
Singlet and Triplet Phosphinidenes
Synthesis of RPA (R = NiPr 2 ) and an example phosphinidene transfer reaction with 1,3-cyclohexadiene [ 11 ]
Synthesis of a stable singlet phospino-phosphinidene with bulky 2,6-bis[4-tert-butylphenyl)methyl]-4-methylphenyl substituents as reported by Bertrand and coworkers. [ 3 ]
Frontier molecular orbitals of a model phosphino-phosphinidene with "Dipp" groups. Calculations were performed at the M06-2X/def2-TZVPP//M06-2X/def2-SVP level of theory. Reproduced from Bertrand and coworkers [ 3 ] with NBO 6.0 in ORCA. 4.2.0 and visualized in IBOview.
Reactivity of phosphino-phosphinidene with various nucleophiles [ 3 ] [ 12 ]
Dominant resonance structures of the phospha-Wittig reagent from Fritz et al. [ 13 ]
Reactivity of the phopha-Wittig reagent as described in Fritz et al. [ 13 ]
Synthesis and reactivity of several 7-phosphanorbornadiene complexes [ 14 ] [ 15 ] [ 16 ]
Synthesis and reactivity of donor-to-phosphinidene complex adducts. [ 18 ] [ 19 ]
Lappert and coworkers' synthesis of first stable terminal phosphinidene complex [ 20 ]