N-heterocyclic carbene induced reductive coupling of phosphorus tribromide. Isolation of a bromine bridged P–P bond and its subsequent reactivity

Abstract

The room temperature reaction of a 1 : 1 mixture of phosphorus tribromide (PBr₃) and the N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) quantitatively affords the Lewis acid–base adduct (IPr)PBr₃ (1). Interestingly, when 1 is heated between 55 and 65 °C for a period of several days, dark red crystals slowly begin to form in the reaction vessel accompanied by the release of bromine. The resulting crystalline sample, [P₂(IPr)₂Br₃]Br ([2]Br), results from the reductive coupling of two equivalents of 1, and contains a cationic moiety with a P–P bond that is bridged by a bromine atom. Anion exchange reactions with Na[BAr^F₄] (BAr^F₄ = B(3,5-{CF₃}₂C₆H₃)₄) afford [2][BAr^F₄]. Abstraction of two equivalents of bromine allows for the isolation of the unprecedented dicationic species [P₂(IPr)₂Br₂]²⁺ (3) which was isolated and structurally authenticated as two different [BAr^F₄]⁻ salts. Reaction of 2 with mild reductants such as SnBr₂ or tetrakis(dimethylamino)ethylene (TDAE) affords [P₂(IPr)₂Br]⁺ (4) and the known radical cation [P₂(IPr)₂]˙⁺ (5), respectively. These studies show that relatively weak P–Br bonds present in compounds 1–4 can be cleaved in a straightforward manner to afford low oxidation state compounds in high yields.