Phosphenium

The first cyclic phosphenium compounds were reported in 1972 by Suzanne Fleming and coworkers.{{Cite journal|last1=Fleming|first1=Suzanne.|last2=Lupton|first2=Mary Kathryn.|last3=Jekot|first3=Kathleen.|date=1972-10-01|title=Synthesis of a cyclic fluorodialkylaminophosphine and its coordination with boron acids. Formation of a unique dialkylaminophosphine cation|journal=Inorganic Chemistry|language=EN|volume=11|issue=10|pages=2534–2540|doi=10.1021/ic50116a050|issn=0020-1669}} Acyclic phosphenium compounds were synthesized by Fleming's thesis advisor Robert Parry in 1976.

File:The first cyclic and acyclic phosphenium ions prepared.png

Several methods exist for the preparation of two-coordinate phosphorus ions. A common method involves halide abstraction from halophosphines:{{Cite journal|last1=Cowley|first1=A. H.|last2=Kemp|first2=R. A.|date=1985-10-01|title=Synthesis and reaction chemistry of stable two-coordinate phosphorus cations (phosphenium ions)|journal=Chemical Reviews|language=EN|volume=85|issue=5|pages=367–382|doi=10.1021/cr00069a002|issn=0009-2665}}

:R₂PCl + AlCl₃ → [R₂P⁺][{{chem|AlCl|4|−}}]

Protonolysis of tris(dimethylamino)phosphine affords the phosphenium salt:{{Cite journal|last=Dahl|first=Otto|date=1982-01-01|title=Reactions of aminophosphines with trifluormethanesulfonic acid: phosphenium ion (two-coordinate phosphorus ion) or tricovalent phosphorus products?|journal=Tetrahedron Letters|volume=23|issue=14|pages=1493–1496|doi=10.1016/s0040-4039(00)87141-5|issn=0040-4039}}

:P(NMe₂)₃ + 2 HOTf → [P(NMe₂)₂]OTf + [HNMe₂]OTf

Weakly coordinating anions are desirable. Triflic acid is often used.

N-heterocyclic phosphenium (NHP) have also been reported.{{Cite journal|last1=Reeske|first1=Gregor|last2=Cowley|first2=Alan H.|date=2007-02-01|title=One-Step Redox Route to N-Heterocyclic Phosphenium Ions|journal=Inorganic Chemistry|language=en|volume=46|issue=4|pages=1426–1430|doi=10.1021/ic061956z|pmid=17291126|issn=0020-1669}} Reaction of PI₃ with the α-diimine yields the NHP cation by reduction of the diimine and oxidation of iodine.

File:CowleyPhosphenium.svg

According to X-ray crystallography, [(i-Pr₂N)₂P]⁺ is nearly planar consistent with sp²-hybridized phosphorus center.{{Cite journal|last1=Cowley|first1=Alan H.|last2=Cushner|first2=Mike C.|last3=Szobota|first3=John S.|date=1978-11-01|title=Static and dynamic stereochemistry of dicoordinate phosphorus cations|journal=Journal of the American Chemical Society|language=EN|volume=100|issue=24|pages=7784–7786|doi=10.1021/ja00492a087|issn=0002-7863}} The planarity of the nitrogen center is consistent with the resonance of the lone pair of the nitrogen atom as a pi bond to the empty phosphorus 3p orbital perpendicular to the N−P−N plane. An idealized sp² phosphorus center would expect an N−P−N angle of 120°. The tighter N−P−N angle observed in the crystal structure can be interpreted as the result of repulsion between the phosphorus lone pair with the bulky i-Pr₂N ligands, as the {{chem|P(NH|2|)|2|+}} and {{chem|PH|2|+}} molecules have bond angles closer to 110° and 90°, respectively.

File:Electronic and bond structure of model phosphenium.png

Calculations also show that the analogy to carbenes is lessened by strongly π-donating substituents. With NH₂ substituents, the phosphenium cation assumes allyl character.{{Cite journal|last=Gudat|first=Dietrich|date=1998-08-01|title=Cation Stabilities, Electrophilicities, and "Carbene Analogue" Character of Low Coordinate Phosphorus Cations|journal=European Journal of Inorganic Chemistry|language=en|volume=1998|issue=8|pages=1087–1094|doi=10.1002/(sici)1099-0682(199808)1998:8<1087::aid-ejic1087>3.0.co;2-3|issn=1099-0682}} Generalized Valence Bond (GVB) calculations of the phosphenium ions as having a singlet ground state, singlet-triplet separation increases with increasing electronegativity of the ligands.{{Cite journal|last1=Harrison|first1=James F.|last2=Liedtke|first2=Richard C.|last3=Liebman|first3=Joel F.|date=1979-11-01|title=The multiplicity of substituted acyclic carbenes and related molecules|journal=Journal of the American Chemical Society|volume=101|issue=24|pages=7162–7168|doi=10.1021/ja00518a006|issn=0002-7863}} The singlet-triplet separation for {{chem|PH|2|+}} and {{chem|PF|2|+}} were calculated to be 20.38 and 84.00 kcal/mol, respectively. Additionally, the triplet state of the phosphenium ion displays a greater bond angle at the phosphorus. For example, the calculated bond angle of the singlet state of {{chem|PH|2|+}} is approximately 94° compared to 121.5° in the triplet state. Calculated bond lengths between the two states are not significantly impacted.{{Cite journal|last=Harrison|first=James F.|date=1981-12-01|title=Electronic structure of the phosphenium ions {{chem|PH|2|+}}, HPF⁺, and {{chem|PF|2|+}}|journal=Journal of the American Chemical Society|language=EN|volume=103|issue=25|pages=7406–7413|doi=10.1021/ja00415a002|issn=0002-7863}}

Phosphenium is isoelectronic with singlet (Fisher) carbenes and are therefore expected to be Lewis acidic. Adducts are produced by combining [P(NMe₂)₂]⁺ and P(NMe₂)₃:{{Cite journal|last1=Schultz|first1=C. W.|last2=Parry|first2=R. W.|date=1976-12-01|title=Structure of [2((CH₃)₂N)₂PCl]·AlCl₃, ((CH₃)₂N)₃P·((CH₃)₂N)₂PCl·AlCl₃, and related species-diphosphorus cations|journal=Inorganic Chemistry|language=EN|volume=15|pages=3046–3050|doi=10.1021/ic50166a022|issn=0020-1669}}

:P(NMe₂)₂]⁺ + P(NMe₂)₃ → [(Me₂N)₃P−P(NMe₂)₂]⁺

Being electrophilic, they undergo C−H insertion reactions.{{Cite journal|last1=Nakazawa|first1=Hiroshi|last2=Buhro|first2=William E.|last3=Bertrand|first3=Guy|last4=Gladysz|first4=J. A.|date=1984-10-01|title=Reactions of phosphorus electrophiles with [(η⁵-C⁵Me⁵)Fe(CO)²]⁻; spectroscopic evidence for a phosphinidene complex|journal=Inorganic Chemistry|language=EN|volume=23|issue=22|pages=3431–3433|doi=10.1021/ic00190a001|issn=0020-1669}}

Phosphenium intermediates are invoked as intermediates in the McCormack reaction, a method for the synthesis of organophosphorus heterocycles. An illustrative reaction involves phenyldichlorophosphine and isoprene:{{OrgSynth | author = W. B. McCormack | title = 3-Methyl-1-Phenylphospholene oxide| volume= 43 | pages = 73| year = 1963 | doi= 10.15227/orgsyn.043.0073}}

:320 px

Isolated phosphenium salts undergo this reaction readily.{{Cite journal|last1=Cowley|first1=A. H.|last2=Kemp|first2=R. A.|last3=Lasch|first3=J. G.|last4=Norman|first4=N. C.|last5=Stewart|first5=C. A.|date=1983-11-01|title=Reaction of phosphenium ions with 1,3-dienes: a rapid synthesis of phosphorus-containing five-membered rings|journal=Journal of the American Chemical Society|volume=105|issue=25|pages=7444–7445|doi=10.1021/ja00363a040|issn=0002-7863}}

There are few examples of reactions catalyzed by phosphenium. In 2018, Rei Kinjo and coworkers reported the hydroboration of pyridines by the NHP salt, 1,3,2-diazaphosphenium triflate. The NHP is proposed to act as a hydride transfer reagent in this reaction.{{Cite journal|last1=Rao|first1=Bin|last2=Chong|first2=Che Chang|last3=Kinjo|first3=Rei|date=2018-01-05|title=Metal-Free Regio- and Chemoselective Hydroboration of Pyridines Catalyzed by 1,3,2-Diazaphosphenium Triflate|journal=Journal of the American Chemical Society|language=EN|volume=140|issue=2|pages=652–656|doi=10.1021/jacs.7b09754|pmid=29303259|issn=0002-7863}}

File:Hydroboration of pyridine catalyzed by NHP.png

File:CINLAM.svg

Phosphenium ions serve as ligands in coordination chemistry. [(R₂N)₂PFe(CO)₄]⁺ was prepared by two methods: the first being the abstraction of a fluoride ion from (R₂N)₂(F)PFe(CO)₄ by PF₅. The second method is the direct substitution reaction of Fe(CO)₅ by the phosphenium ion [P(NR₂)]⁺.{{Cite journal|last1=Montemayor|first1=R. G.|last2=Sauer|first2=Dennis T.|last3=Fleming|first3=Suzanne|last4=Bennett|first4=Dennis W.|last5=Thomas|first5=Michael G.|last6=Parry|first6=Robert W.|date=1978-03-01|title=Iron carbonyl complexes containing positively charged phosphorus ligands|journal=Journal of the American Chemical Society|language=EN|volume=100|issue=7|pages=2231–2233|doi=10.1021/ja00475a044|issn=0002-7863}} Related complexes exist of the type Fe(CO)₄L, where L = [(Me₂N)₂P]⁺, [(Et₂N)₂P]⁺, [(Me₂N)(Cl)P]⁺, and [(en)P]⁺ (en = C₂H₄(NH₂)₂).

File:Phosphenium coordination to Mo complexes.pngN-heterocyclic phosphenium-transition metal complexes are anticipated due to their isoelectronicity to N-heterocyclic carbenes. In 2004, Martin Nieger and coworkers synthesized two Cobalt-NHP complexes. Experimental and computation analysis of the complexes confirmed the expected L→M σ donation and the M→L π backbonding, though the phosphenium was observed to have reduced σ donor ability. It was suggested that this is due to the greater s orbital-character of the phosphorus lone pair compared to the lone pair of the analogous carbene.{{Cite journal|last1=Burck|first1=Sebastian|last2=Daniels|first2=Jörg|last3=Gans-Eichler|first3=Timo|last4=Gudat|first4=Dietrich|last5=Nättinen|first5=Kalle|last6=Nieger|first6=Martin|date=2005-06-01|title=N-Heterocyclic Phosphenium, Arsenium, and Stibenium Ions as Ligands in Transition Metal Complexes: A Comparative Experimental and Computational Study|journal=Zeitschrift für Anorganische und Allgemeine Chemie|language=en|volume=631|issue=8|pages=1403–1412|doi=10.1002/zaac.200400538|issn=0044-2313}} Additional studies of NHP ligands by Christine Thomas and coworkers in 2012, likened the phosphenium to nitrosyl.{{Cite journal|last1=Pan|first1=Baofei|last2=Xu|first2=Zhequan|last3=Bezpalko|first3=Mark W.|last4=Foxman|first4=Bruce M.|last5=Thomas|first5=Christine M.|date=2012-03-14|title=N-Heterocyclic Phosphenium Ligands as Sterically and Electronically-Tunable Isolobal Analogues of Nitrosyls|journal=Inorganic Chemistry|language=EN|volume=51|issue=7|pages=4170–4179|doi=10.1021/ic202581v|pmid=22416761|issn=0020-1669}} Nitrosyl is well known for its redox non-innocence, coordinating in either a bent or linear geometry that possess different L–M bonding modes. It was observed that NHPs in complex with a transition metal may have either a planar or pyramidal geometry about the phosphorus, reminiscent of the linear versus bent geometries of nitrosyl. Highly electron-rich metal complexes were observed to have pyramidal phosphorus, while less electron-rich metals showed greater phosphenium character at the phosphorus. Pyramidal phosphorus indicates significant lone pair character at phosphorus, suggesting that the L→M σ donation and the M→L π backbonding interactions have been replaced with M→L σ donation, formally oxidizing the metal center by two electrons. File:NHP-Cobalt complex.png

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Category:Cations

Category:Phosphorus compounds