phosphinidene

File:Phosphinidene singlet triplet.png

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.{{Cite journal|last1=Benkő|first1=Zoltán|last2=Streubel|first2=Rainer|last3=Nyulászi|first3=László|date=2006-09-11|title=Stability of phosphinidenes—Are they synthetically accessible?|url=https://pubs.rsc.org/en/content/articlelanding/2006/dt/b608276a|journal=Dalton Transactions|language=en|issue=36|pages=4321–4327|doi=10.1039/B608276A|pmid=16967115 |issn=1477-9234}} This singlet-triplet energy gap is considerably larger than that of the simplest carbene methylene (9 kcal/mol).{{Cite journal|last1=Gronert|first1=Scott|last2=Keeffe|first2=James R.|last3=More O’Ferrall|first3=Rory A.|date=2011-03-16|title=Stabilities of Carbenes: Independent Measures for Singlets and Triplets|url=https://doi.org/10.1021/ja1071493|journal=Journal of the American Chemical Society|volume=133|issue=10|pages=3381–3389|doi=10.1021/ja1071493|pmid=21341749 |bibcode=2011JAChS.133.3381G |issn=0002-7863}}

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. 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. 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.

One way to generate phosphinidines employs the decyclization of phosphaanthracene complexes.{{Cite journal|last1=Velian|first1=Alexandra|last2=Cummins|first2=Christopher C.|date=2012-08-20|title=Facile Synthesis of Dibenzo-7λ3-phosphanorbornadiene Derivatives Using Magnesium Anthracene|url=https://pubs.acs.org/doi/pdf/10.1021/ja306902j|journal=Journal of the American Chemical Society|volume=134|issue=34|pages=13978–13981|doi=10.1021/ja306902j|pmid=22894133 |bibcode=2012JAChS.13413978V |issn=0002-7863}}

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).

File:RPA reaction.png

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.

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. 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. File:Stable phosphinidene1.png

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).

File:Bertrand phosphinidene HOMO-LUMO.png 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{{Cite journal|last1=Hansmann|first1=Max M.|last2=Jazzar|first2=Rodolphe|last3=Bertrand|first3=Guy|date=2016-06-30|title=Singlet (Phosphino)phosphinidenes are Electrophilic|url=https://pubs.acs.org/doi/10.1021/jacs.6b04232|journal=Journal of the American Chemical Society|language=EN|volume=138|issue=27|pages=8356–8359|doi=10.1021/jacs.6b04232|pmid=27340902 |bibcode=2016JAChS.138.8356H |issn=0002-7863}} 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.

File:Reactivity of phosphinidene.png

File:Phosphawittig.png

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. File:Phosphawittig reactions.png

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.{{Cite journal|last1=Marinetti|first1=Angela|last2=Mathey|first2=François|last3=Fischer|first3=Jean|last4=Mitschler|first4=André|date=1982-01-01|title=Stabilization of 7-phosphanorbornadienes by complexation; X-ray crystal structure of 2,3-bis(methoxycarbonyl)-5,6-dimethyl-7-phenyl-7-phosphanorbornadiene(pentacarbonyl)-chromium|url=https://pubs.rsc.org/en/content/articlelanding/1982/c3/c39820000667|journal=Journal of the Chemical Society, Chemical Communications|language=en|issue=12|pages=667–668|doi=10.1039/C39820000667|issn=0022-4936}}{{Cite journal|last=Mathey|first=François|date=1987|title=The Development of a Carbene-like Chemistry with Terminal Phosphinidene Complexes|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.198702753|journal=Angewandte Chemie International Edition in English|language=en|volume=26|issue=4|pages=275–286|doi=10.1002/anie.198702753|issn=1521-3773}} 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.{{Cite journal|last1=Marinetti|first1=Angela|last2=Mathey|first2=Francois|last3=Fischer|first3=Jean|last4=Mitschler|first4=Andre|date=1982-08-01|title=Generation and trapping of terminal phosphinidene complexes. Synthesis and x-ray crystal structure of stable phosphirene complexes|url=https://doi.org/10.1021/ja00380a029|journal=Journal of the American Chemical Society|volume=104|issue=16|pages=4484–4485|doi=10.1021/ja00380a029|bibcode=1982JAChS.104.4484M |issn=0002-7863}}

File:7-phosphanorbornadiene synthesis and reaction.png

Donor-stabilized terminal phosphinidene complexes are also known,{{Cite journal |last1=Schmer |first1=Alexander |last2=Junker |first2=Philip |last3=Espinosa Ferao |first3=Arturo |last4=Streubel |first4=Rainer |date=2021-04-06 |title=M/X Phosphinidenoid Metal Complex Chemistry |url=https://pubs.acs.org/doi/10.1021/acs.accounts.1c00017 |journal=Accounts of Chemical Research |language=en |volume=54 |issue=7 |pages=1754–1765 |doi=10.1021/acs.accounts.1c00017 |pmid=33734678 |issn=0001-4842}} which could release free phosphinidene complexes L_nM=P-R at mild conditions by P-donor dissociation reactions.{{Cite journal |last1=Biskup |first1=David |last2=Schnakenburg |first2=Gregor |last3=Boeré |first3=René T. |last4=Espinosa Ferao |first4=Arturo |last5=Streubel |first5=Rainer K. |date=2023-10-13 |title=Challenging an old paradigm by demonstrating transition metal-like chemistry at a neutral nonmetal center |journal=Nature Communications |language=en |volume=14 |issue=1 |pages=6456 |doi=10.1038/s41467-023-42127-3 |issn=2041-1723 |pmc=10575908 |pmid=37833259|bibcode=2023NatCo..14.6456B }}{{Cite journal |last1=Biskup |first1=David |last2=Schnakenburg |first2=Gregor |last3=Boeré |first3=René T. |last4=Ferao |first4=Arturo Espinosa |last5=Streubel |first5=Rainer |date=2023-10-03 |title=A novel access to phosphanylidene–phosphorane complexes via P-donor substitution and a detailed bonding analysis |url=https://pubs.rsc.org/en/content/articlelanding/2023/dt/d3dt02304d |journal=Dalton Transactions |language=en |volume=52 |issue=38 |pages=13781–13786 |doi=10.1039/D3DT02304D |pmid=37721045 |issn=1477-9234}} The phosphinidene complexes decomposed to white phosphorus if no unsaturated substrates were provided.

File:Synthesis+reactivity Do-to-phosphinidene complex adducts.png

Terminal phosphinidene complexes of the type Cp₂M=P-R (M = Mo, W) can be obtained by combining aryl-dichlorophosphines RPCl₂ with [Cp₂MHLi]₄.{{Cite journal|last1=Hitchcock|first1=Peter B.|last2=Lappert|first2=Michael F.|last3=Leung|first3=Wing-Por|date=1987-01-01|title=The first stable transition metal (molybdenum or tungsten) complexes having a metal–phosphorus(III) double bond: the phosphorus analogues of metal aryl- and alkyl-imides; X-ray structure of [Mo(η-C5H5)2(PAr)](Ar = C6H2But3-2,4,6)|url=https://pubs.rsc.org/en/content/articlelanding/1987/c3/c39870001282|journal=Journal of the Chemical Society, Chemical Communications|language=en|issue=17|pages=1282–1283|doi=10.1039/C39870001282|issn=0022-4936}}

File:Lappert metallocene phosphinidene.png

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 {{chem2|Fe3(PC6H5)2(CO)9}}, which forms from iron pentacarbonyl and phenylphosphine according to the following idealized equation:{{cite journal |doi=10.1021/ic50113a030 |title=Reactions of mono- and bis(organo)phosphines with iron carbonyls |date=1972 |last1=Treichel |first1=P. M. |last2=Dean |first2=W. K. |last3=Douglas |first3=W. M. |journal=Inorganic Chemistry |volume=11 |issue=7 |pages=1609–1615 }}

:{{chem2|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.{{cite journal|doi=10.1002/anie.198707431|title=RP-Bridged Metal Carbonyl Clusters: Synthesis, Properties, and Reactions|journal=Angewandte Chemie International Edition in English|volume=26|issue=8|pages=743–760|year=1987|last1=Huttner|first1=Gottfried|last2=Knoll|first2=Konrad}}

• Carbene analog
• Phosphorus compounds

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Category:Reactive intermediates

Category:Organophosphorus compounds

Category:Octet-deficient functional groups