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The room temperature reaction of a 1 : 1 mixture of phosphorus tribromide (PBr 3 ) and the N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) quantitatively affords the Lewis acid-base adduct (IPr)PBr 3 ( 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, [P 2 (IPr) 2 Br 3 ]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 F  4 ] (BAr F  4 = B(3,5-{CF 3 } 2 C 6 H 3 ) 4 ) afford [ 2 ][BAr F  4 ]. Abstraction of two equivalents of bromine allows for the isolation of the unprecedented dicationic species [P 2 (IPr) 2 Br 2 ] 2+ ( 3 ) which was isolated and structurally authenticated as two different [BAr F  4 ] - salts. Reaction of 2 with mild reductants such as SnBr 2 or tetrakis(dimethylamino)ethylene (TDAE) affords [P 2 (IPr) 2 Br] + ( 4 ) and the known radical cation [P 2 (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.

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