The Bis(ferrocenyl)phosphenium Ion Revisited

The bis(ferrocenyl)phosphenium ion, [Fc2P]+, reported by Cowley et al. (J. Am. Chem. Soc. 1981, 103, 714–715), was the only claimed donor‐free divalent phosphenium ion. Our examination of the molecular and electronic structure reveals that [Fc2P]+ possesses significant intramolecular Fe⋅⋅⋅P contacts, which are predominantly electrostatic and moderate the Lewis acidity. Nonetheless, [Fc2P]+ undergoes complex formation with the Lewis bases PPh3 and IPr to give the donor–acceptor complexes [Fc2P(PPh3)]+ and [Fc2P(IPr)]+ (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazole‐2‐ylidene)

Synthesis and Characterisation of Phosphenium Ions with Aromatic Amido Substituents

σ2λ3-Phosphenium cations A are valence isoelectronic and isolobal with silenes B and hence, carbenes C. Notably, the chemistry of these divalent phosphorus cations mirrors that of their carbon-based analogues (e.g. cycloaddition hemistry, C-H insertions, etc.), making them versatile reagents in a variety of transformations and reactions. Just as for carbenes (e.g. amino carbenes D and E), a combination of kinetic and electronic stabilisation has been used to prepare and isolate stable phosphenium ions such as F, with amido substituents. This approach is also followed in this work. In particular, the synthesis, characterisation, and properties of a range of phosphenium ions (4) with alkyl or aryl amido substituents (R2N, R = alkyl or aryl) is described. The desired phosphenium ions 4 are synthesized in three steps by reacting the secondary amines 1 with nBuLi, which results in the production of the appropriate lithium amide salts (2) followed by subsequent reaction with PCl3, which yields the chlorophosphines 3. Reaction of 3 with a halogen abstracting reagent such as AlCl3 (or TMSOTf) results in the formation of phosphenium ions (4), which were studied by NMR spectroscopy and X-ray diffraction

Reversible cooperative dihydrogen binding and transfer with a bis-phosphenium complex of chromium

The reversible reaction of H(2)with a bis-phosphenium complex of chromium provides a rare example of 3d transition metal/phosphenium cooperativity. Photolysis induces the activation of H(2)and yields a spectroscopically detectable phosphenium-stabilized (sigma-H-2)-complex, readily showing exchange with gaseous H(2)and D-2. Further reaction of this complex affords a phosphine-functionalized metal hydride, representing a unique example of reversible H(2)cleavage across a 3d MP bond. The same species is also accessibleviastepwise H+/H(-)transfer to the bis-phosphenium complex, and releases H(2)upon heating or irradiation. Dihydrogen transfer from the H-2-complex to styrene is exploited to demonstrate the first example of promoting hydrogenation with a phosphenium complex.Peer reviewe

Synthesis and characterization of n-heterocyclic phosphenium and arsenium salts

Two ligand types were employed for the one-step preparations of heavy group 15 cations, namely, diazabutadiene (DAB) and bis(imino)acenaphthene (BIAN). By exploiting the redox properties of these ligands, in conjunction with two different synthetic strategies, it was possible to synthesize a variety of phosphenium and arsenium salts in relatively high yields. The first synthetic method took advantage of the known equilibrium between PI/PI3 in solution. The second synthetic route employed SnCl2 as the reducing agent for converting the reactive trihalides ECl3 into the corresponding “ECl” moieties (E = P, As).Chemistr

The Synthesis and Reactivity of New Pnictogen(III) Cations

Chelating nitrogen based ligands are well known for their use with transition metals while their chemistry with p-block elements has been relatively underdeveloped. This thesis examines the structure, bonding and reactivity of group 15 elements supported by a pyridyl tethered 1,2-bis(imino)acenaphthene (“clamshell”) and various homo- and heteroleptic guanidinate frameworks. The unique “clamshell” ligand contains a pendant Lewis base and has been used to support N-heterocyclic phosphenium and arsenium cations. Reactivity studies with the phosphorus analogue demonstrate the ability of the ligand to act as a Lewis base, while the phosphorus centre provides a Lewis acidic site, showing the amphoteric nature of such a molecule. Cobaltocene has been used as a new one-electron reductant in the facile, high yielding synthesis of a diaminochloroarsine supported by the “clamshell” ligand. Unfortunately this method was not suitable for all Group 15 elements and resulted in low yields for phosphorus and insoluble black material for antimony and bismuth. In the absence of a reductant the “clamshell” ligand can be used to form hypervalent donor-acceptor complexes with heavy main group elements (Sn, Sb and Bi). The addition of a salt metathesis reagent to the hypervalent Sb and Bi complexes results in base-stabilized SbCl2+ and BiCl2+ cations. A series of diaminochlorophosphines has been synthesized with various dianionic guanidinate ligands, and their reluctancy to release a chloride ion using halide abstracting reagents has been noted. A comprehensive study of their reactivity has been completed and the following were observed: (i) that the chloride ion could be removed with the addition of a chelating base; (ii) that carbodiimide can be eliminated chemically and thermally from the four-membered ring; and, (iii) that a rare ring expansion by the insertion of a chloro(imino)phosphine into a P–N bond of the P–N–C–N framework takes place. The analogous chemistry with heavier pnictogens to form diaminochloropnictines does not occur; rather the products are diaminodichloropnictines where the guanidinate ligand is monoanionic. Halide abstraction from these molecules is facile for As and Sb and the addition of a chelating base allows for the removal of the second chloride ion to give base-stabilized dicationic species

Phosphirenium ions as masked phosphenium catalysts : mechanistic evaluation and application in synthesis

The EPSRC is thanked for funding. S.E.N. thanks Heriot Watt University for the award of a James Watt scholarship.The utilization of phosphirenium ions is presented; optimized and broadened three-membered ring construction is described together with the use of these ions as efficient pre-catalysts for metal-free carbonyl reduction with silanes. Full characterization of the phosphirenium ions is presented, and initial experimental and computational mechanistic studies indicate that these act as a "masked phosphenium"source that is accessed via ring opening. Catalysis proceeds via associative transfer of {Ph2P+} to a carbonyl nucleophile, Hâ'SiR3 bond addition over the C=O group, and associative displacement of the product by a further equivalent of the carbonyl substrate, which completes the catalytic cycle. A competing off-cycle process leading to vinyl phosphine formation is detailed for the hydrosilylation of benzophenone for which an inverse order in [silane] is observed. Experimentally, the formation of side products, including off-cycle vinyl phosphine, is favored by electrondonating substituents on the phosphirenium cation, while catalytic hydrosilylation is promoted by electron-withdrawing substituents. These observations are rationalized in parallel computational studies.Peer reviewe

Comparing the Ligand Behavior of N-Heterocyclic Phosphenium and Nitrosyl Units in Iron and Chromium Complexes

N-Heterocyclic phosphenium (NHP) and nitro-sonium (NO+) ligands are often viewed as isolobal analogues that share the capability to switch between different charge states and thus display redox "noninnocent" behavior. We report here on mixed complexes [(NHP)M(CO)(n)(NO)] (M = Fe, Cr; n = 2, 3), which permit evaluating the donor/acceptor properties of both types of ligands and their interplay in a single complex. The crystalline target compounds were obtained from reactions of N-heterocyclic phosphenium triflates with PPN[Fe(CO)(3)(NO)] or PPN[Cr(CO)(4)-(NO)], respectively, and fully characterized (PPN = nitride-bistriphenylphosphonium cation). The structural and spectroscopic (IR, UV-vis) data support the presence of carbene-analogue NHP ligands with an overall positive charge state and pi-acceptor character. Even if the structural features of the M-NO unit were in all but one product blurred by crystallographic CO/NO disorder, spectroscopic studies and the structural data of the remaining compound suggest that the NO units exhibit nitroxide (NO-) character. This assignment was validated by computational studies, which reveal also that the electronic structure of iron NHP/ NO complexes is closely akin to that of the Hieber anion, [Fe(CO)(3)(NO)](-). The electrophilic character of the NHP units is further reflected in the chemical behavior of the mixed complexes. Cyclic voltammetry and IR-SEC studies revealed that complex [(NHP)Fe(CO)(2) (NO)] (4) undergoes chemically reversible one-electron reduction. Computational studies indicate that the NHP unit in the resulting product carries significant radical character, and the reduction may thus be classified as predominantly ligand-centered. Reaction of 4 with sodium azide proceeded likewise under nucleophilic attack at phosphorus and decomplexation, while super hydride and methyl lithium reacted with all chromium and iron complexes via transfer of a hydride or methyl anion to the NHP unit to afford anionic phosphine complexes. Some of these species were isolated after cation exchange or trapped with electrophiles (H+, SnPh3(+)) to afford neutral complexes representing the products of a formal hydrogenation or hydrostannylation of the original M=P double bond.Peer reviewe

N-Phosphino-pyridyl Imines: Flexible, Multi-functional Reagents

Abstract Chapter I includes a summary of the chemistry of phosphines and bidentate phosphorus-nitrogen ligands of relevance to this thesis, and highlights their application in transition metal-based catalysis. In addition, a review of the characteristic properties and reactivity of iminophosphoranes and phosphenium cations is presented in preparation for work outlined later in this manuscript. The synthesis and coordination studies of a range of N-phosphino-pyridyl imines with the general structure (C5H5N)(Ar)C=NPR2 (1) {R = alkyl or aryl and Ar = aryl} are reported in chapter II. The molecular structures of their palladium dichloride complexes are presented, highlighting the effect of differing substituents on framework 1 upon the geometry of the complexes. The chemistry surrounding a novel equilibrium between two valence tautomers prepared from reaction of N-lithio pyridyl imine with bis(diisopropylamino)chlorophosphine is presented in chapters III and IV. The tautomers 1,1-bis(diisopropylamino)-3-phenyl-1λ5-[1,3,2]diazaphospholo [1,5-a]pyridine (2c) and bis(diisopropylamino)phosphino(phenyl-pyridin-2-yl methylene)amine (2o) exist in a ratio of 95 : 5, respectively, at ambient temperature. Investigations of the thermodynamic properties, the rate of interconversion and the possible mechanistic pathways of interconversion for 2o/2c have been conducted. The synthesis and characterisation of another equilibrium mixture, that of the tautomers 1,1-bis(diisopropylamino)-3-anthracenyl-1λ5-[1,3,2]diazaphospholo [1,5-a]pyridine (3c) and bis(diisopropylamino)phosphino(anthracenyl-pyridin-2-yl methylene)amine (3o), which exist in a ratio of 30 : 70, respectively, at ambient temperature is reported. Chapter V describes the reaction of bis(diisopropylamino)phosphenium triflate with 1,1-bis(diisopropylamino)-3-phenyl-1λ5-[1,3,2]diazaphospholo[1,5-a]pyridine, which affords 1,1,1',1'- tetrakis(diisopropylamino)-3,3'-diphenyl-1H,1'H-[5,5'-bi-1λ5-[1,3,2]diazaphospholo[1,5-a]pyridinylidene]-1,1'-diium bis(triflate) (4) via a homo-bimolecular coupling process. This apparent ‘oxidative’ coupling by a phosphenium cation has no literature precedent and hence a detailed exploration of the possible mechanism of the reaction is outlined. Comparative reactions involving established oxidants, for example ferrocenium triflate, are examined and also give the diphosphonium salt 4. Chapter VI outlines an exploration of the behaviour of N-dialkyl- and diaryl-phosphino-pyridyl imines in the coordination sphere of low valent palladium species. The isolation and characterisation of a novel dimeric palladium(I) complex is reported, which is found to undergo reactions with chlorobenzene and triphenylphosphine, acting as a source of ‘masked’ palladium(0). These reactions are consistent with redox processes occurring to give palladium(0) or palladium(II) species

Coordination isomerism in N-heterocyclic phosphenium thiocyanates

Two N-heterocyclic phosphines with exocyclic SCN substituents were synthesised via metathesis of chlorophosphine precursors with KSCN and fully characterised. The crystallographic studies reveal that the products exhibit pronounced structural differences. The thiocyanato unit binds in one case via the nitrogen atom to yield a molecular structure with a slightly elongated P-N single bond and, in the other case, via the sulfur atom to form a structure that is best described as an ion pair and forms a one-dimensional coordination polymer in the crystal. DFT calculations suggest that the P-N and P center dot center dot center dot S interactions can be described as covalent and dative bonds, respectively, and that the structural differences correlate with the different cation stabilities of the individual phosphenium cation fragments.Peer reviewe

Phosphirenium Ions as Masked Phosphenium Catalysts:Mechanistic Evaluation and Application in Synthesis

The utilization of phosphirenium ions is presented; optimized and broadened three-membered ring construction is described together with the use of these ions as efficient pre-catalysts for metal-free carbonyl reduction with silanes. Full characterization of the phosphirenium ions is presented, and initial experimental and computational mechanistic studies indicate that these act as a "masked phosphenium"source that is accessed via ring opening. Catalysis proceeds via associative transfer of {Ph2P+} to a carbonyl nucleophile, Hâ'SiR3 bond addition over the C=O group, and associative displacement of the product by a further equivalent of the carbonyl substrate, which completes the catalytic cycle. A competing off-cycle process leading to vinyl phosphine formation is detailed for the hydrosilylation of benzophenone for which an inverse order in [silane] is observed. Experimentally, the formation of side products, including off-cycle vinyl phosphine, is favored by electrondonating substituents on the phosphirenium cation, while catalytic hydrosilylation is promoted by electron-withdrawing substituents. These observations are rationalized in parallel computational studies.</p