Reaction of NHOs with Bisphosphanes – Designing Diradicaloids, Zwitterions and Radicals

TL;DR

Scientists created a new class of phosphorus-based heterocycles that can act as both zwitterions and diradicaloids, with properties that can be adjusted by molecular rotation.

Contribution

A new class of P-heterocycles with tunable diradical character and zwitterionic properties was synthesized and characterized.

Findings

The diradical character and charge separation can be controlled by rotating the imidazole ring.

Imidazolium-diphospha-indenylides represent a new class of zwitterionic diradicals.

A highly reactive monoradical anion was isolated and fully characterized.

Abstract

The linkage of an imidazole‐based N‐heterocyclic olefin (NHO), containing a terminal CH2 donor group, with a phosphorus‐centered diradical molecular fragment leads to an open‐shell singlet diphospha‐indenylide system, a new class of P‐heterocycles, which can be interpreted both as a phosphorus‐centered diradicaloid and as a zwitterion with a permanent, overall charge separation between the N‐ and P‐heterocyclic ring systems. The rotation of the imidazole ring, which is thermally possible due to a central C−C bond with a weakened π‐component, changes both the charge separation and diradical character depending on the dihedral angle, as quantum mechanical calculations indicate. By varying the bulkiness of substituents at the imidazole‐based NHO, it was possible to obtain different diphospha‐indenylide species with different rotation angles in the solid state and hence varying diradical character. Imidazolium‐diphospha‐indenylides represent a new class of NHO‐based zwitterions with diradical character. Their synthesis, structure, and activation chemistry are described, as well as the quantum mechanical description of the electronic structure in these unusual heterocycles. In addition, along the synthesis route to diphospha‐indenylide, we also succeeded in isolating a highly reactive monoradical anion, which was also fully characterized.

By linking N‐heterocyclic olefins with a P‐substituted indenyl heterocycle, it was possible to synthesize a very exciting new heterocyclic substance class that is both zwitterionic and diradical. The diradical character of these heterocycles can be adjusted by rotation within the molecule, which was achieved by rational design. The thermochromism of these heterocycles was studied by experimental and theoretical methods. Furthermore, we have shown that these heterocycles can activate small molecules. The mechanism of such an activation chemistry was studied and shown to be reversible in some case. Finally, we studied the electronic structure using multireference methods.

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