Aggregation of [Au(CN)4]- anions: Examination by crystallography and 15N CP-MAS NMR and the structural factors influencing intermolecular Au⋯N interactions

Andrew R. Geisheimer, John E.C. Wren, Vladimir K. Michaelis, Masayuki Kobayashi, Ken Sakai, Scott Kroeker, Daniel B. Leznoff

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Abstract

To investigate the factors influencing the formation of intermolecular Au⋯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 15N{1H} CP-MAS NMR spectroscopy. Thus, [ nBu4N][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⋯NC interactions are present. trans-[Co(1,2-diaminoethane)2Cl2][Au(CN) 4] forms a supramolecular structure, where trans-[Co(en) 2Cl2]+ 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]3· (H 2O)4. In [Ni(1,2-diaminoethane)3][Au(CN) 4]2, 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]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⋯NC interactions, while the cationic charge, shape, size, and aromaticity have little effect. While the μCN 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), 15N{1H} 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 15N chemical shift also correlates with the intermolecular Au⋯N distances: the shortest Au-N distances also shift the 15N 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.

Original languageEnglish
Pages (from-to)1265-1274
Number of pages10
JournalInorganic Chemistry
Volume50
Issue number4
DOIs
Publication statusPublished - Feb 21 2011

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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