Towards Spontaneous Heterolysis of the Homonuclear PP Bond in Diphosphines: The Case of Diazaphospholeniumtriphospholides

Abstract

Computational studies on a series of polyphospholyl-substituted N-heterocyclic phosphines (CH)(2)(NR)(2) P-P(n)(CH)(5-n) (R=Me, n=1-5) disclosed that increasing formal replacement of CH units in the phosphole ring by phosphorus atoms is associated with an increase in P-P distances and charge separation, and a decrease in covalent bond orders. Altogether, these trends imply that the CH versus P substitution enhances ionic P-P bond polarization in these compounds. Experimental verification of this hypothesis was obtained for the triphospholyl diazaphospholenes (CR)(2)(NR')(2)P-P(3)(CtBu)(2) (8a: R=H, R'=tBu; 8b: R=Me, R'=Mesityl [Mes]), which were prepared through metathesis reactions from suitable precursors and identified by solution and solid-state NMR data and a single-crystal X-ray diffraction study of 8a. Analysis of J(PP) coupling patterns suggested that both species are characterized by the absence of a strong covalent P-P bond connecting both rings. This interpretation was confirmed by the finding of a unique P-P distance of 2.79 A for crystalline 8 a, and further supported by computational studies, which led to the conclusion that both species are better described as diazaphospholenium-triphospholide contact ion pairs rather than covalent molecules. Variable-temperature (VT) NMR spectra of 8b showed a collapse of J(PP) couplings between atoms in different rings, which indicates scrambling of the diazaphospholenium and triphospholide units between different molecules in solution, and further substantiates the proposed view on the molecular structure.

Unusual Reactivity of cyclo‐(P5Ph5): Oxidative Addition at a Group 6 Metal Carbonyl and Insertion of Acetonitrile into a P–P Bond

“…In all complexes, the C(37)–P(4) [1.859(6)–1.889(3) Å] bonds are slightly longer than P–C bonds of known P–C=N–P moieties (1.803–1.870 Å), while the P(5)–N(1) [1.719(3)–1.748(3) Å] bonds are within the reported range (P–N 1.697–1.802 Å) , , . However, the P(5)–N(1) and C(37)–P(4) bonds are longer than P–N bonds [1.665(2)–1.710(2) Å], , in diazaphospholene and P–C bonds [1.799(2)–1.835(5) Å] in phosphole rings. The C(37)–N(1) bonds [1.266(4)–1.280(6) Å] correspond to C=N double bonds and are similar to the C–N bond lengths [1.261(4)–1.276(4)], , in P–C=N–P moieties, but shorter than that [1.320(3) Å] reported by Hoffmann et al for a hexahydro‐1,4‐diaza‐2,3,5,6‐tetraphosphorine derivative (C 32 H 26 Cr 2 N 2 O 6 P 4 ).…”

An Objective Alternative to IUPAC's Approach To Assign Oxidation States

“…This is indeed the expected result for Lewis adducts where both the LA and LB moieties bear the same central atom, like for instance some unsymmetrically substituted diphosphines described in ref. [25]. In the simplest model system (55) on Figure 5, the EOS analysis clearly supports the Lewis pair picture originated from the heterolytic cleavage of the homonuclear dative bond (R=82%).…”

Diazaphospholenes:N-Heterocyclic Phosphines between Molecules and Lewis Pairs

“…Starting from the concepts that the strong polarization in P-phospholyl NHPs benefits from aromatic stabilization in both the cation and the anion fragment of a canonical structure IIIb (Chart 3) and that formal replacement of additional CH units in the phosphole ring by phosphorus atoms further enhances the aromatic stabilization in the anion fragment, 31 it was expected that polyphospholyl-substituted NHPs IV (Chart 4) should offer a still closer approach to true "phosphenium-phosphides". 26 The validity of this approach was endorsed by computational studies (trends in computed structural data, electron distributions, and ligand exchange reaction energies of model compounds of type IV indicated that the energy for charge separation declines and the P-P bond ionicity goes up with increasing number of phosphorus atoms in the phospholyl ring) and finally experimentally confirmed through the characterization of some triphospholyl-substituted NHPs 3 (Chart 4). Unique features of these derivatives include a further rise in P-P bond distances (g2.80 Å, Figure 5), the observation of small or even vanishing 1 J PP coupling constants, which exclude a strong covalent P-P interaction both in the solid state and in solution, and the observation of temperature-dependent NMR spectra, which reveal the occurrence of intermolecular exchange of cyclic phosphenium and triphospholide fragments in solution.…”

“…In light of the previous remarks, the bonding situation in P -phospholyl derivatives is without doubt a most interesting case due to the introduction of ionic polarization into a homonuclear P−P bond, which relates them to the systems with polarized C−C bonds studied by Arnett et al Further consideration of this aspect raised the question whether it was possible to design diphosphines with even more polarized P−P bonds that are no longer governed by covalent contributions but are predominantly ionic. Starting from the concepts that the strong polarization in P -phospholyl NHPs benefits from aromatic stabilization in both the cation and the anion fragment of a canonical structure IIIb (Chart ) and that formal replacement of additional CH units in the phosphole ring by phosphorus atoms further enhances the aromatic stabilization in the anion fragment, it was expected that polyphospholyl-substituted NHPs IV (Chart ) should offer a still closer approach to true “phosphenium-phosphides” . The validity of this approach was endorsed by computational studies (trends in computed structural data, electron distributions, and ligand exchange reaction energies of model compounds of type IV indicated that the energy for charge separation declines and the P−P bond ionicity goes up with increasing number of phosphorus atoms in the phospholyl ring) and finally experimentally confirmed through the characterization of some triphospholyl-substituted NHPs 3 (Chart ).…”