Cluster self-assembly of di[gold(I)]halonium cations

Treatment of gold(l) halide complexes of the type L-Au-X [where L = PPh3, PEt3 with X = Cl, Br, I, or L = 2,6-(MeO)(2)C6H3PPh2 with X = Cl] with AgSbF6 in the molar ratio 2:1 in dichloromethane/tetrahydrofuran at -78degreesC affords high yields of di[gold(l)]halonium salts of the formula {X[Au(PR3)](2)}(+) SbF6- (2-8). A determination of the crystal structures of the four triarylphosphine complexes (2-4, 8) revealed the presence of novel tetranuclear dications with a highly symmetrical structure (point group S-4) that arises from self-assembly of the dinuclear monocations through a set of four equivalent aurophilic Au-Au interactions. A comparison with two reference structures of corresponding chloronium perchlorate and bromonium tetrafluoroborate salts with monomeric, dinuclear cations shows that the geometry of the latter is greatly altered on dimerization to optimize the interactions between the closed-shelf metal centers (Au: 5d(10)). Weak metallophilic bonding clearly becomes significant only in crystal lattices where anions with a larger ionic radius (SbF6- vs. BF4-, ClO4-) reduce the otherwise dominant role of strong interionic Coulomb forces, The results indicate that aurophilic bonding is indeed an ubiquitous, quite dependable mode of intermetallic interactions provided that the right environment is chosen to allow the weak forces to become operative.