Aggregation of [Au(CN)4]- Anions: Examination by Crystallography and 15N CP-MAS NMR and the Structural Factors Influencing Intermolecular Au•••N Interactions

To investigate the factors influencing the formation of intermolecular Au center dot center dot center dot NC interactions between [Au(CN)(4)](-) units, a series of [cation](n+)[Au(CN)(4)](n) double salts was synthesized, structurally characterized and probed by IR and N-15{H-1} CP-MAS NMR spectroscopy. Thus, [(Bu4N)-Bu-n][Au(CN)(4)], [AsPh4][Au(CN)(4)], [N(PPh3)(2)][Au(CN)(4)], [Co(1,10-phenanthroline)(3)][Au(CN)(4)](2), and [Mn(2,2';6',2 ''-terpyridine)(2)][Au(CN)(4)](2) show [Au(CN)(4)](-) anions that are well-separated from one another; no Au-Au or Au center dot center dot center dot NC interactions are present. trans-[Co(1,2-diaminoethane)(2)Cl-2][Au(CN)(4)] forms a supramolecular structure, where trans-[Co(en)(2)Cl-2](+) and [Au(CN)(4)](-) ions are found in separate layers connected by Au-CN center dot center dot center dot H-N hydrogen-bonding; weak Au center dot center dot center dot NC coordinate bonds complete octahedral Au(III) centers, and support a 2-D (4,4) network motif of [Au(CN)(4)](-)-units. A similar structure-type is formed by [Co(NH3)(6)][Au(CN)(4)](3)center dot(H2O)(4). In [Ni(1,2-diaminoethane)(3)][Au(CN)(4)](2), intermolecular Au center dot center dot center dot NC interactions facilitate formation of 1-D chains of [Au(CN)(4)](-) anions in the supramolecular structure, which are separated from one another by [Ni(en)(3)](2+) cations. In [1,4-diazabicyclo[2.2.2]octane-H][Au(CN)(4)], the monoprotonated amine cation forms a hydrogen-bond to the [Au(CN)(4)](-) unit on one side, while coordinating to the axial sites of the gold(III) center through the unprotonated amine on the other, thereby generating a 2-D (4,4) net of cations and anions; an additional, uncoordinated [Au(CN)(4)](-)-unit lies in the central space of each grid. This body of structural data indicates that cations with hydrogen-bonding groups can induce intermolecular Au center dot center dot center dot NC interactions, while the cationic charge, shape, size, and aromaticity have little effect. While the nu(CN) values are poor indicators of the presence or absence of N-cyano bridging between [Au(CN)(4)](-)-units (partly because of the very low intensity of the observed bands), N-15{H-1} CP-MAS NMR reveals well-defined, ordered cyanide groups in the six diamagnetic compounds with chemical shifts between 250 and 275 ppm; the resonances between 260 and 275 ppm can be assigned to C-bound terminal ligands, while those subject to CN center dot center dot center dot H-N bonding resonate lower, around 250-257 ppm. The N-15 chemical shift also correlates with the intermolecular Au center dot center dot center dot N distances: the shortest Au-N distances also shift the N-15 peak to lower frequency. This provides a real, spectroscopically measurable electronic effect associated with the crystallographic observation of intermolecular Au center dot center dot center dot NC interactions, thereby lending support for their viability.