Aromatic Phosphenium Cations

Abstract

The aromatic stabilization of cyclic phosphenium cations (general type C2N2P+) was studied by experimental methods (synthesis, multinuclear NMR, single crystal X‐ray crystallography) and quantum chemical calculations (ab initio methods). The structures of the 1,3,2‐diazaphosphole derivatives [(tBuN–CH=CH–NtBu)P+]Cl− (1), (tBuN–CH2–CH2–NtBu)P–Cl (2), [(tBuN–CH=CH–NtBu)P]+PF6− (3) and [(tBuN–CH2–CH2‐NtBu)]P+PF6− (4) were examined by single crystal X‐ray diffraction. The chloro phosphane [(tBuN–CH=CH–NtBu)P]+Cl− (1) has an ionic P–Cl bond and contains an aromatically stabilized phosphenium cation [shortest distance P···Cl = 275.9(2) pm], while the CC‐saturated compound (tBuN–CH2–CH2–NtBu)P–Cl (2) is covalent. The two chloro‐phosphanes 1 and 2 differ sharply in their volatility and solubility in organic solvents. Compound 2 is soluble in hydrocarbons and sublimes readily at 90 °C/0.1 Torr but 1 is insoluble in hexanes and not volatile below 180 °C/0.1 Torr. The degree of aromatic stabilization in the phosphenium cation 1 was investigated by ab initio methods. For the model cations [RN–CH2–CH2–NR]P+ and [(RN–CH=CH–NR)P]+, thermochemical stabilization energies of 25.8 kcal · mol−1 (R = H) and 28.1 kcal · mol−1 (R = Me) were obtained from isodesmic hydrogenation reactions at the RHF/MP2/6–31G*//RHF/6–31G* level.

Diazaphospholenes:N-Heterocyclic Phosphines between Molecules and Lewis Pairs

“…The influence of solvent effects on phosphorus−halogen distances was further illustrated in case of the analogous P -chloro-NHP where the calculated P−Cl bond distance for a single molecule in the gas phase was found to increase continuously upon immersion in solvents of increasing polarity . It has been pointed out that this solvent-induced bond polarization explains both the unique solvent dependence of NMR chemical shifts of P -chloro-NHPs in solution , and the solvate-induced modification of P−Cl bonds in the solid state . A substantial solvent influence on P−P distances was likewise inferred for P -phospholyl-NHPs where it was established that the large deviations of 1 J PP coupling constants measured in solid-state and solution NMR spectra and the marked temperature dependence of the coupling in solution are attributable to the combined effects of a temperature-dependent equilibrium between rotamers and a relaxation of P−P bond distances in solution (Figure ) …”

“…These compounds had come into play as synthetic precursors for the cations, but it had been noted from the beginning that they displayed by themselves unique structural features like unusually long P-Cl bonds; 5 in fact, one compound was even addressed as ionic species, which had supposedly formed via spontaneous P-Cl bond heterolysis. 6 Although this hypothesis became questionable in the light of more detailed studies, the original idea, that the high stability of the cyclic π-electron system in cations I (E ) P) can act as driving force to enhance P-Cl bond ionicity in chlorophosphines II (X ) Cl), turned up again as a key motif in the representation of the bonding situation in terms of resonance between covalent (IIa) and ionic canonical structures (IIb). 7 In exploring the chemistry of these compounds, we found that a similar situation applies also to type II molecules with phosphino (X ) PR 2 ), 8 hydrogen (X ) H), 9 or cyclopentadienyl substituents (X ) C 5 R 5 ) 10 where the P-X bonds connect now two atoms of similar electronegativity.…”

“…In the context of the exploration of phosphenium cations I (E = P), also neutral N -heterocyclic P -chlorophosphines II (X = Cl) caught attention. These compounds had come into play as synthetic precursors for the cations, but it had been noted from the beginning that they displayed by themselves unique structural features like unusually long P−Cl bonds; in fact, one compound was even addressed as ionic species, which had supposedly formed via spontaneous P−Cl bond heterolysis . Although this hypothesis became questionable in the light of more detailed studies, the original idea, that the high stability of the cyclic π-electron system in cations I (E = P) can act as driving force to enhance P−Cl bond ionicity in chlorophosphines II (X = Cl), turned up again as a key motif in the representation of the bonding situation in terms of resonance between covalent ( IIa ) and ionic canonical structures ( IIb ) .…”

Bending Ferrocenes with Low Coordinated Bridging Units: The Investigation of Carbenes and Their Analogues with a Ferrocenophane Backbone

“…Not only carbene I turned out to be stable, but also heavier analogues [silylenes ( IV ), germylenes ( V ), and cationic 1,3,2-diazaphospholenes ( VI ) − ] were synthesized and thoroughly investigated . Furthermore, transient diaminophosphanyls VII (which exist in equilibrium with their PP-bonded dimers) are also known, − showing that this particular ring system stabilizes even incorporated radical centers.…”

A New Class of Two‐Deck Dicationic Heteropentalenes Existing by the Balance between Interdeck Aromaticity and Covalent P−P Bonding

“…When considering this reaction, it is important to note the high mobility of chlorine atoms in dichlorides (DDPCl 2 ) [31] due to aromatic 6π stabilization (Schemes S1, S2 in the Supporting Information), which is similar to the formation of 1,3,2-and 1,2,3-diazaphosphenium cations from the corresponding Phalo-substituted diazaphosphols. [2,32] It is also worth noting the high activity of DDP in the dehalogenation reactions of some chlorophosphines. In particular, cHexDDP is capable of converting Ph 2 PCl to Ph 2 P-PPh 2 and PCl 3 to red phosphorus in hexane at 20 °C).…”