Phosphinidenes (IUPAC: phosphanylidenes, formerly phosphinediyls) are low-valent phosphorus compounds analogous to carbenes and nitrenes, having the general structure RP.[1][2] The parent phosphinidine has the formula PH. More common are the organic analogues where R = alkyl or aryl. In these compounds phosphorus has only 6 electrons in its valence level.[2] Most phosphinidenes are highly reactive and short-lived, thereby complicating empirical studies on their chemical properties.[3][4]
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A variety of strategies have been employed to stabilize phosphinidenes (e.g. π-donation.steric protection, transition metal complexation),[2][3] Furthermore reagents and systems have been developed that can generate and transfer phosphinidenes as intermediates in the synthesis of various organophosphorus compounds.[5][6][7][8]
Electronic structure
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) 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.[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.[4] Substituents containing lone pairs (e.g. -NX2, -OX, -PX2 ,-SX) 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]
Case studies
Dibenzo-7-phosphanorbornadiene derivatives
One way to generate phosphinidines employs the decyclization of phosphaanthracene complexes.[11]
Treatment of a bulky phosphine chloride (RPCl2) with magnesium anthracene affords a dibenzo-7-phosphanorbornadiene compound (RPA).[11] Under thermal conditions, the RPA compound (R = NiPr2) decomposes to yield anthracene; kinetic experiments found this decomposition to be first-order.[11] 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).[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.[5]
Phosphino-phosphinidene
The first singlet phosphino-phosphinidene has been prepared using extremely bulky substituents.[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 JPP = 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][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.[12]
Phospha-Wittig fragmentation
In 1989, Fritz et al. synthesized the phospha-Wittig species shown to the right.[13] 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.[13] 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.[13]
Metal complexes
Terminal phosphinidine complexes
Terminal transition-metal-complexed phosphinidenes LnM=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.[14][15] 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.[15][16]
Donor-stabilized terminal phosphinidene complexes are also known,[17] which could release free phosphinidene complexes LnM=P-R at mild conditions by P-donor dissociation reactions.[18][19] The phosphinidene complexes decomposed to white phosphorus if no unsaturated substrates were provided.[18]
Terminal phosphinidene complexes of the type Cp2M=P-R (M = Mo, W) can be obtained by combining aryl-dichlorophosphines RPCl2 with [Cp2MHLi]4.[20]
Phosphinidine-based clusters
Metal clusters containing RP substituents are numerous. They typically arise by the reaction of metal carbonyls with primary phosphines (compounds with the formula RPH2). 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]
- 3 Fe(CO)5 + 2 C6H5PH2 → Fe3(PC6H5)2(CO)9 + 2 H2 + 6 CO
A related example is the tert-butylphosphinidene complex (t-BuP)Fe3(CO)10.[22]
See also
References
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