Photoelectron Spectra and Structures of Proazaphosphatranes

Abstract

The He I and He II photoelectron spectra of two proazaphosphatranes were recorded and interpreted by correlated ab initio quantum chemical calculations. The first ionization energy of the compounds investigated is among the lowest reported hitherto for phosphorus compounds. Geometry optimization of the ionic states of the parent proazaphosphatrane (at the HF and MP2 levels) revealed that there are two minima on the potential energy surface at different P−Nax distances representing two bond-stretch isomers. The structure with the long P−Nax distance (3.3 Å) is similar to that of the neutral molecule and thus has a favorable Frank−Condon factor for the ionization process. The other structure, with a short P−Nax distance (2.07 Å), is as much as 1 eV lower in energy than the other. Due to the unfavorable Frank−Condon factor, however, the corresponding adiabatic ionization energy cannot be seen in the spectrum. The short P−Nax distance in the more stable bond-stretch isomer is characteristic for azaphosphatranes. Its stability can thus be related to the formation of the dative P ← N bond. The unobserved adiabatic ionization energy should be about 1 eV lower than that of the observed one (ca. 5.6 eV), in full agreement with the strong basicity of proazaphosphatranes.

Stretching the P–C Bond. Variations on Carbenes and Phosphanes

“…In case of carbenes, which are not “too much” stabilized (Δ E ISO = 70–90 kcal/mol) and are also rather nucleophilic, two or three different types of adducts could be optimized with X: Cl and F. In case of 3 , 5–6 , and 11 , these three structures are F , E , and E′ , and in case of 4 and PF 3 , all the structures F , E , and E′ could be optimized. This is an interesting example of the rare and somewhat debated bond-stretch isomerism. − To our best knowledge, this is the first case where three distinct minima could be located from two reactants, without breaking the other bonds of the interacting units. We were also able to locate the transition states between the minima for the PF 3 adduct with carbene 4 (Figure ).…”

On Transannulation in Azaphosphatranes: Synthesis and Theoretical Analysis

“…In multiple computational studies, transannulation has been attributed to the donation of the lone pair of N ax into the antibonding orbital of the P–electrophile bond . An argument could also be made that transannulation is enhanced by an electrostatic interaction between a positively charged phosphorus and the axial nitrogen lone pair.…”

“…This phenomenon has been invoked to explain the high p K a of the conjugate acids of proazaphosphatranes (p K a = 32–34 in acetonitrile) as well as the superior performance of proazaphosphatranes as ligands in palladium catalysis compared to structurally similar aminophosphines that are unable to transannulate . Theoretical , and combined spectroscopic-computational , studies have provided insight into the bonding observed in azaphosphatranes, but experimental corroboration of these findings was not available at the time of some of these studies, and new questions concerning the nature of transannulation have emerged over time. …”

Tweaking the Charge Transfer: Bonding Analysis of Bismuth(III) Complexes with a Flexidentate Phosphane Ligand

“…The goal of the present study is to gain more insight into the bonding situation in bismuth­(III) complexes with the PS 3 ligand. Although other multidentate ligand systems are known in the literature, − due to their accordion-like structural flexibility the molecular skeleton of type PS 3 BiX 3 seems to be suitable for experimental investigations. First, we aim to predict computationally how the pnictogen interaction between the bridgehead atoms P and Bi observed in the neutral PS 3 BiX 3 compounds can be shifted into a dative (covalent) regime, and subsequently, we provide experimental evidence based on X-ray crystallography and NMR studies.…”