Auration of an Aryltris [(Triarylphosphine)Aurio(I)] Phosphonium(+) Cation to Give a Hypercoordinate Aryltetrakis [(Triarylphosphine) Aurio(I)] Phosphonium(2+) Dication

Treatment of (2-methylphenyl)phosphine with tris[(triphenylphosphine)aurio(I)]oxonium(+) tetrafluoroborate(-) affords colorless, air-stable (2-methylphenyl)tris[(triphenylphosphine)aurio(l)] phosphonium(+) tetrafluoroborate(-): (2-MeC₆H₄)[Ph₃PAu]₃P⁺BF₄^-(1); Me = methyl, Ph = phenyl. The crystal structure features tetrahedral cations with a central phosphorus atom surrounded by three gold atoms and one carbon atom. With the Au-P-Au angles showing only slight deviations from the tetrahedral value and the P-Au-P angles close to linear, the Au-Au distances are too long to allow bonding metal-metal interactions. An analogous arsenic compound (2-MeC₆H₄)[(Ph₃PAu]₃As+BF₄^-(3) is available from (2-methylphenyl)arsine and shows similar properties. Reaction of 1 with [Ph₃PAu]⁺BF₄^-gives high yields of the colorless, crystalline, air-stable species (2-MeC₆H₄)[Ph₃PAu]₄P₂⁺(BF₄^-)₂(2). The dications have a square-pyramidal structure with phosphorus at the center, the o-tolyl group at the apex, and the four gold atoms forming the base with short Au-Au contacts (ca. 3.0 Å). These short distances are held responsible for the ease of formation and stability of this novel hypercoordinate unit. An analogous arsenic compound could not be prepared. Crystal data for 1: a = 13.341 (3) Å, b = 15.363 (3) Å, c = 16.369 (3) Å, a = 107.88 (1)°, γ= 98.17 (1)°, γ-103.09 (1)°, triclinic, space group Pi, Z = 2. Crystal data for 2-4C₆H₆: a = 13.712 (2) Å, 6 = 17.873 (3) Å, c = 20.748 (2) Å, a = 95.04 (1)°, β= 104.58 (1)°, γ= 96.08 (1)°, triclinic, space group PI, Z = 2.