Phosphinidene

Like carbenes, phosphinidenes can exist in either a singlet state or triplet state, with the triplet state typically being more stable.[2][4] The stability of these states and their relative energy difference (the singlet-triplet energy gap) is dependent 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.[2][9] This singlet-triplet energy gap is considerably larger than that of the simplest carbene methylene (9 kcal/mol).[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 effect that stabilizes the triplet more than the singlet.[4] Substituents containing lone pairs (e.g. -NX₂, -OX, -PX₂ ,-SX) were found to stabilize the singlet state, presumably by π-donation into an empty phosphorus 3p orbital; in most of these cases, the energies of the lowest singlet and triplet states were close to degenerate.[4] A singlet ground state could be induced in amino- and phosphino-phosphinidenes by introducing bulky β-substituents, which are thought to destabilize the triplet state by distorting the pyramidal geometry through increased nuclear repulsion.[4]

Bertrand and coworkers synthesized a stable singlet phosphino-phosphinidene compound using extremely bulky substituents.[3] Hitherto, there had been no free singlet phosphinidenes that were characterized by spectroscopy.[3] 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.[3] ³¹P 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.[3] The air/water sensitivity and high solubility of this compound prevented characterization by X-ray crystallography.[3]

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-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.[3] 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.[3] Further evidence of multiple bond character between the phosphorus atoms was provided by natural resonance theory and a large Wiberg bond index (P₁-P₂: 2.34).[3] 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).[3]

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[3][11] 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.[11]

In 1989, Fritz et al. synthesized the phospha-Wittig species shown to the right.[12] 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 zwitterionic form) that are analogous to those of the phosphonium ylides that are used in the Wittig reaction.

Fritz et al. found that this particular phospha-Wittig reagent thermally decomposes at 20 °C to give ^tBu₂PBr, LiBr, and cyclophosphanes.[12] The authors proposed that the singlet phosphino-phosphinidene ^tBu₂PP 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.[12]

Terminal transition-metal-complexed phosphinidenes L_nM=P-R are phosphorus analogs of transition metal carbene complexes where L is a spectator ligand. The first terminal phosphinidene complex was reported by Marinetti et al., who observed the formation of the transient species [(OC)₅M=P-Ph] during the fragmentation of 7-phosphanorbornadiene molybdenum and tungsten complexes inside a mass spectrometer.[13][14] 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.[14][15]

Biskup et al. have reported the synthesis of donor-stabilized terminal electrophilic phosphinidene transition metal complexes via Li/Cl phosphinidenoid complexes[16] which could release free phosphinidene complexes L_nM=P-R at mild conditions by P-donor dissociation reactions.[17][18] The phosphinidene complexes decomposed to white phosphorus if no unsaturated substrates were provided.[17]

Lappert and coworkers reported the first synthesis of a stable terminal phosphinidene complex: lithium metallocene hydrides [Cp₂MHLi]₄ of Mo and W were reacted with aryl-dichlorophosphines RPCl₂ to yield Cp₂M=P-R, which were able to be characterized by single crystal X-ray diffraction.[19]

More common than complexes of terminal phosphinidene ligands are cluster compounds wherein the phosphinidene is a triply and less commonly doubly bridging ligand. One example is the ter-butylphosphinidene complex (t-BuP)Fe₃(CO)₁₀.[20]

A class of RPA (A = anthracene) compounds were developed and explored by Cummins and coworkers.[21]

Treatment of a bulky phosphine chloride (RPCl₂) with magnesium anthracene affords a dibenzo-7-phosphanorbornadiene compound (RPA).[21] Under thermal conditions, the RPA compound (R = NiPr₂) decomposes to yield anthracene; kinetic experiments found this decomposition to be first-order.[21] It was hypothesized that the amino-phosphinidene iPr₂NP is formed as a transient intermediate species, and this was corroborated by an experiment where 1,3-cyclohexadiene was used as a trapping agent, forming anti-iPr₂NP(C₆H₈).[21]

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.[5]

• Carbene analog
• Phosphorus compounds