N-heterocyclic Carbene Induced Reductive Coupling of Phosphorus Tribromide. Isolation of a Bromine Bridged P-P Bond and Its Subsequent Reactivity

The room temperature reaction of a 1 : 1 mixture of phosphorus tribromide (PBr) and the N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) quantitatively affords the Lewis acid-base adduct (IPr)PBr (). Interestingly, when 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, [P(IPr)Br]Br ([]Br), results from the reductive coupling of two equivalents of , and contains a cationic moiety with a P-P bond that is bridged by a bromine atom. Anion exchange reactions with Na[BAr] (BAr = B(3,5-{CF}CH)) afford [][BAr]. Abstraction of two equivalents of bromine allows for the isolation of the unprecedented dicationic species [P(IPr)Br] () which was isolated and structurally authenticated as two different [BAr] salts. Reaction of with mild reductants such as SnBr or tetrakis(dimethylamino)ethylene (TDAE) affords [P(IPr)Br] () and the known radical cation [P(IPr)]˙ (), respectively. These studies show that relatively weak P-Br bonds present in compounds can be cleaved in a straightforward manner to afford low oxidation state compounds in high yields.