Phosphenium ions, not to be confused with phosphonium or phosphirenium, are divalent cations of phosphorus of the form [PR2+. Phosphenium ions have long been proposed as reaction intermediates.
The first cyclic phosphenium compounds were reported in 1972 by Suzanne Fleming and coworkers. [1] Acyclic phosphenium compounds were synthesized by Fleming's thesis advisor Robert Parry in 1976. [2]
Several methods exist for the preparation of two-coordinate phosphorus ions. A common method involves halide abstraction from halophosphines: [3]
Protonolysis of tris(dimethylamino)phosphine affords the phosphenium salt: [4]
Weakly coordinating anions are desirable. Triflic acid is often used. [3]
N-heterocyclic phosphenium (NHP) have also been reported. [5] Reaction of PI3 with the α-diimine yields the NHP cation by reduction of the diimine and oxidation of iodine.
According to
X-ray crystallography, [(i-Pr2N)2P]+ is nearly planar consistent with sp2-hybridized phosphorus center.
[6] 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 P(NH
2)+
2 and PH+
2 molecules have bond angles closer to 110° and 90°, respectively.
[3]
[6]
Calculations also show that the analogy to carbenes is lessened by strongly π-donating substituents. With NH2 substituents, the phosphenium cation assumes allyl character.
[7]
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.
[3]
[8]
[9] The singlet-triplet separation for PH+
2 and 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 PH+
2 is approximately 94° compared to 121.5° in the triplet state. Calculated bond lengths between the two states are not significantly impacted.
[9]
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: [2]
Being electrophilic, they undergo C−H insertion reactions. [10]
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: [11]
Isolated phosphenium salts undergo this reaction readily. [12]
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. [13]
Phosphenium ions serve as ligands in coordination chemistry. [2] [(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)]+. [14] 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). [3]
N-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. [15] Additional studies of NHP ligands by Christine Thomas and coworkers in 2012, likened the phosphenium to nitrosyl. [16] 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. [16]
Phosphenium ions, not to be confused with phosphonium or phosphirenium, are divalent cations of phosphorus of the form [PR2+. Phosphenium ions have long been proposed as reaction intermediates.
The first cyclic phosphenium compounds were reported in 1972 by Suzanne Fleming and coworkers. [1] Acyclic phosphenium compounds were synthesized by Fleming's thesis advisor Robert Parry in 1976. [2]
Several methods exist for the preparation of two-coordinate phosphorus ions. A common method involves halide abstraction from halophosphines: [3]
Protonolysis of tris(dimethylamino)phosphine affords the phosphenium salt: [4]
Weakly coordinating anions are desirable. Triflic acid is often used. [3]
N-heterocyclic phosphenium (NHP) have also been reported. [5] Reaction of PI3 with the α-diimine yields the NHP cation by reduction of the diimine and oxidation of iodine.
According to
X-ray crystallography, [(i-Pr2N)2P]+ is nearly planar consistent with sp2-hybridized phosphorus center.
[6] 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 P(NH
2)+
2 and PH+
2 molecules have bond angles closer to 110° and 90°, respectively.
[3]
[6]
Calculations also show that the analogy to carbenes is lessened by strongly π-donating substituents. With NH2 substituents, the phosphenium cation assumes allyl character.
[7]
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.
[3]
[8]
[9] The singlet-triplet separation for PH+
2 and 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 PH+
2 is approximately 94° compared to 121.5° in the triplet state. Calculated bond lengths between the two states are not significantly impacted.
[9]
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: [2]
Being electrophilic, they undergo C−H insertion reactions. [10]
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: [11]
Isolated phosphenium salts undergo this reaction readily. [12]
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. [13]
Phosphenium ions serve as ligands in coordination chemistry. [2] [(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)]+. [14] 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). [3]
N-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. [15] Additional studies of NHP ligands by Christine Thomas and coworkers in 2012, likened the phosphenium to nitrosyl. [16] 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. [16]