Biradicaloid and Zwitterion Reactivity of Dicarbondiphosphide Stabilized with N‐Heterocyclic Carbenes

Abstract

Organic biradicals are usually very short lived species under standard laboratory conditions, which makes their experimental studies difficult. In contrast, heteroatom-substituted analogues of these biradicals show enhanced stability due to π-electron delocalization, which is why main group biradicals (or biradicaloids) of archetypical heterocyclobutanediyls have been thoroughly investigated. Herein, N-heterocyclic carbene (NHC)-stabilized dicarbondiphosphide compounds of the type (L )C P (L=NHC) were utilized to activate both single and multiple bonds in small molecules with a feasible concerted mechanism. This reactivity is explicable by considerable biradicaloid character of the dicarbondiphosphide compounds, which is further corroborated by theoretical studies. Furthermore, the dicarbondiphosphide compounds also exhibited Lewis base properties towards Lewis acids, such as borane and metal halides.

The Chemistry of the 2‐Phosphaethynolate Anion

“…Subsequently,these compounds rearrange in an intramolecular Wittig-type reaction in which the oxygen centre of the carbonyl group is transferred to the terminal phosphorus centre.This step is rate determining according to DFT calculations with model compounds.S ubsequently, ap hosphorotropic tautomersim, R 2 PÀP(=O)R 2 !R 2 PÀOÀ PR 2 , [61] takes places and leads to the final products.Remarkably,inthis process the sequence OCP has been converted to CPO and the final products can be viewed as NHC-stabilized isocyaphides, DCP À R. This rearrangement reaction also occurs when Ph 3 Si À P = C = Oi sr eacted with nucleophilic N-heterocyclic carbenes,w hich leads to NHC=C=PÀOÀSiPh 3 . [64] These compounds (NHC) 2 C 2 P 2 react with av ariety of reagents including H 2 which add cis-specifically across the P···P vector and are also easily oxidized to give the stable deep blue or green radical cations with ad elocalized 5p-electron configuration (Scheme 17). [63] These are best described as non-KekulØ molecules with aromatic 6pelectron systems with as mall but significant contribution of as inglet biradical character.…”

“…[63] These are best described as non-KekulØ molecules with aromatic 6pelectron systems with as mall but significant contribution of as inglet biradical character. [64] These compounds (NHC) 2 C 2 P 2 react with av ariety of reagents including H 2 which add cis-specifically across the P···P vector and are also easily oxidized to give the stable deep blue or green radical cations with ad elocalized 5p-electron configuration (Scheme 17).…”

N‐Heterocyclic Carbene Stabilized Dicarbondiphosphides: Strong Neutral Four‐Membered Heterocyclic 6π‐Electron Donors

“…These molecules also have a substantial biradical character as shown in I and as such are related to S 2 N 2 and related phosphorus heterocycles such as (ClC) 2 (PMes) 2 , (TerN) 2 P 2 , and other diradicaloid heterocycles . Also zwitterionic structures such as III and IV in Scheme contribute significantly to the electronic ground state …”

Die Chemie des 2‐Phosphaethinolat‐Anions

“…Die OP(NR) 2 (CR 1 ) 2 ‐Einheiten sind potenzielle Austrittsgruppen und werden tatsächlich bei der Reaktion mit KC 8 glatt als K + [OP(NR) 2 (CR 1 ) 2 ] − abgespalten, sodass die NHC‐stabilisierten C 2 P 2 ‐Ringe erhalten werden . Diese lassen sich am besten als Nicht‐Kekulé‐Moleküle mit aromatischen 6π‐Elektronensystemen und einem kleinen, aber signifikanten Beitrag einer Singulett‐Diradikalspezies beschreiben . Die Verbindungen (NHC) 2 C 2 P 2 reagieren mit zahlreichen Reagentien, z.…”

“…[63] Diese lassen sich am besten als Nicht-KekulØ-Moleküle mit aromatischen 6p-Elektronensystemen und einem kleinen, aber signifikanten Beitrag einer Singulett-Diradikalspezies beschreiben. [64] Die Verbindungen (NHC) 2 C 2 P 2 reagieren mit zahlreichen Reagentien, z. B. addiert H 2 cis-spezifisch über den P•••P-Vektor,u nd Oxidationen verlaufen glatt zu den stabilen tiefblauen oder grünen Radikalkationen mit delokalisierter 5p-Elektronenkonfiguration (Schema 17).…”