Phosphinidene

General structure of a phosphinidene

Phosphinidenes (IUPAC: phosphanylidenes, formerly phosphinediyls) are low-valent phosphorus compounds analogous to carbenes and nitrenes, having the general structure RP. The "free" form of these compounds is conventionally described as having a singly-coordinated phosphorus atom containing only 6 electrons in its valence level. Most phosphinidenes are highly reactive and short-lived, thereby complicating empirical studies on their chemical properties. In the last few decades, several strategies have been employed to stabilize phosphinidenes (e.g. π-donation, steric protection, transition metal complexation), and researchers have developed a number of reagents and systems that can generate and transfer phosphinidenes as reactive intermediates in the synthesis of various organophosphorus compounds.

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) is dependent on the substituents.

Singlet and Triplet Phosphinidenes

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 effect that stabilizes the triplet more than the singlet. 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. 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.

Stable Monomeric Phosphino-Phosphinidene

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

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.

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. 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 (P₁-P₂: 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).

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 with NBO 6.0 in ORCA. 4.2.0 and visualized in IBOview.

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.

Reactivity of phosphino-phosphinidene with various nucleophiles

Phospha-Wittig Fragmentation

Dominant resonance structures of the phospha-Wittig reagent from Fritz et al.

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 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. 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.

Reactivity of the phopha-Wittig reagent as described in Fritz et al.

Phosphinidene complexes

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. 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.

Synthesis and reactivity of several 7-phosphanorbornadiene complexes

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.

Lappert and coworkers' synthesis of first stable terminal phosphinidene complex

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.

Dibenzo-7-phosphanorbornadiene derivatives

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

Treatment of a bulky phosphine chloride (RPCl₂) with magnesium anthracene affords a dibenzo-7-phosphanorbornadiene compound (RPA). Under thermal conditions, the RPA compound (R = NiPr₂) decomposes to yield anthracene; kinetic experiments found this decomposition to be first-order. 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).

Synthesis of RPA (R = NiPr₂) and an example phosphinidene transfer reaction with 1,3-cyclohexadiene

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.

See also

• Carbene analog
• Phosphorus compounds