N-heterocyclic carbene induced reductive coupling of phosphorus tribromide. Isolation of a bromine bridged P–P bond and its subsequent reactivity. Issue 12 (2nd August 2016)

• Record Type: Journal Article
• Authors: Waters, Jordan B.; Everitt, Thomas A.; Myers, William K.; Goicoechea, Jose M.
• Is Part Of: Chemical science. Volume 7:Issue 12(2016:Dec.)
• Page Start: 6981
• Page End: 6987
• Publication Date: 2016-08-02
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c6sc02343f
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : Thermal treatment of a 1:1 adduct of phosphorus tribromide and 1, 3-bis(2, 6-diisopropylphenyl)-imidazol-2-ylidene (IPr) results in the formation of a reduced phosphorus-containing dimer, which can be employed to access a range of low oxidation state compounds. 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, when1 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)2 Br3 ]Br ([2 ]Br), results from the reductive coupling of two equivalents of1, 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-{CF3 }2 C6 H3 )4 ) afford [2 ][BAr F 4 ]. Abstraction of two equivalents of bromine allows for the isolation of the unprecedented dicationic species [P2 (IPr)2 Br2 ] 2+ (3 ) which was isolated and structurally authenticated as two different [BAr F 4 ] − salts. Reaction of2 with mild reductants such as SnBr2 or tetrakis(dimethylamino)ethylene (TDAE) affords [P2 (IPr)2 Br] + (4 ) and the known radical cation [P2 (IPr)2 ]˙ + (5 ), respectively. These studies show that relatively weak P–Br bonds present in compounds1–4 can be cleaved in a straightforward manner to afford low oxidation state compounds in high yields.