RESUMEN
The unsymmetrical coordination of gold(I) by 2,2'-bipyridine (bipy) in some planar, three-coordinate cations has been examined by crystallographic and computational studies. The salts [(Ph3P)Au(bipy)]XF6 (X = P, As, Sb) form an isomorphic series in which the differences in Au-N distances range from 0.241(2) to 0.146(2) Å. A second polymorph of [(Ph3P)Au(bipy)]AsF6 has also been found. Both polymorphs exhibit similar structures. The salts [(Et3P)Au(bipy)]XF6 (X = P, As, Sb) form a second isostructural series. In this series the unsymmetrical coordination of the bipy ligand is maintained, but the gold ions are disordered over two unequally populated positions that produce very similar overall structures for the cations. Although many planar, three-coordinate gold(I) complexes are strongly luminescent, the salts [(R3P)Au(bipy)]XF6 (R = Ph or Et; X = P, As, Sb) are not luminescent as solids or in solution. Computational studies revealed that a fully symmetrical structure for [(Et3P)Au(bipy)]+ is 7 kJ/mol higher in energy than the observed unsymmetrical structure and is best described as a transition state between the two limiting unsymmetrical geometries. The Au-N bonding has been examined by natural resonance theory (NRT) calculations using the "12 electron rule". The dominant Lewis structure is one with five lone pairs on Au and one bond to the P atom, which results in a saturated (12 electron) gold center and thereby inhibits the formation of any classical, 2 e- bonds between the gold and either of the bipy nitrogen atoms. The nitrogen atoms may instead donate a lone pair into an empty Au-P antibonding orbital, resulting in a three-center, four-electron (3c/4e) P-Au-N bond. The binuclear complex, [µ2-bipy(AuPPh3)2](PF6)2, has also been prepared and shown to have an aurophillic interaction between the two gold ions, which are separated by 3.0747(3) Å. Despite the aurophillic interaction, this binuclear complex is not luminescent.
RESUMEN
Non-luminescent, isostructural crystals of [(C6H11NC)2Au](EF6)·C6H6 (E = As, Sb) lose benzene upon standing in air to produce green luminescent (E = As) or blue luminescent (E = Sb) powders. Previous studies have shown that the two-coordinate cation, [(C6H11NC)2Au]+, self-associates to form luminescent crystals that contain linear or nearly linear chains of cations and display unusual polymorphic, vapochromic, and/or thermochromic properties. Here, we report the formation of non-luminescent crystalline salts in which individual [(C6H11NC)2Au]+ ions are isolated from one another. In [(C6H11NC)2Au](BArF24) ((BArF24)- is tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) each cation is surrounded by two anions that prohibit any close approach of the gold ions. Crystallization of [(C6H11NC)2Au](EF6) (E = As or Sb, but not P) from benzene solution produces colorless, non-emissive crystals of the solvates [(C6H11NC)2Au](EF6)·C6H6. These two solvates are isostructural and contain columns in which cations and benzene molecules alternate. With the benzene molecules separating the cations, the shortest distances between gold ions are 6.936(2) Å for E = As and 6.9717(19) Å for E = Sb. Upon removal from the mother liquor, these crystals crack due to the loss of benzene from the crystal and form luminescent powders. Crystals of [(C6H11NC)2Au](SbF6)·C6H6 that powder out form a pale yellow powder with a blue luminescence with emission spectra and powder X-ray diffraction data that show that the previously characterized [(C6H11NC)2Au](SbF6) is formed. In the process, the distances between the gold(i) ions decrease to â¼3 Å and half of the cyclohexyl groups move from an axial orientation to an equatorial one. Remarkably, when crystals of [(C6H11NC)2Au](AsF6)·C6H6 stand in air, they lose benzene and are converted into the yellow, green-luminescent polymorph of [(C6H11NC)2Au](AsF6) rather than the colorless, blue-luminescent polymorph. Paradoxically, the yellow, green-luminescent powder that forms as well as authentic crystals of the yellow, green-luminescent polymorph of [(C6H11NC)2Au](AsF6) are sensitive to benzene vapor and are converted by exposure to benzene vapor into the colorless, blue-luminescent polymorph.