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···NC interactions between Au(CN)(4) units, a series of cationAu(CN)(4) double salts was synthesized, structurally characterized and probed by IR and (15)N{(1)H} CP-MAS NMR spectroscopy. Thus, [(n)Bu(4)N][Au(CN)(4)], [AsPh(4)][Au(CN)(4)], [N(PPh(3))(2)][Au(CN)(4)], [Co(1,10-phenanthroline)(3)]Au(CN)(4), and [Mn(2,2';6',2''-terpyridine)(2)]Au(CN)(4) show Au(CN)(4) anions that are well-separated from one another; no Au-Au or Au···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···H-N hydrogen-bonding; weak Au···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(NH(3))(6)]Au(CN)(4)·(H(2)O)(4). In [Ni(1,2-diaminoethane)(3)]Au(CN)(4), intermolecular Au···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) 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···NC interactions, while the cationic charge, shape, size, and aromaticity have little effect. While the ν(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), (15)N{(1)H} 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···H-N bonding resonate lower, around 250-257 ppm. The (15)N chemical shift also correlates with the intermolecular Au···N distances: the shortest Au-N distances also shift the (15)N peak to lower frequency. This provides a real, spectroscopically measurable electronic effect associated with the crystallographic observation of intermolecular Au···NC interactions, thereby lending support for their viability.