Abstract
Mixed-metal cyanides (Cu1/2Au1/2)CN, (Ag1/2Au1/2)CN, and (Cu1/3Ag1/3Au1/3)CN adopt an AuCN-type structure in which metal-cyanide chains pack on a hexagonal lattice with metal atoms arranged in sheets. The interactions between and within the metal-cyanide chains are investigated using density functional theory (DFT) calculations, 13C solid-state NMR (SSNMR), and X-ray pair distribution function (PDF) measurements. Long-range metal and cyanide order is found within the chains: (−Cu–NC–Au–CN−)∞, (−Ag–NC–Au–CN−)∞, and (−Cu–NC–Ag–NC–Au–CN−)∞. Although Bragg diffraction studies establish that there is no long-range order between chains, X-ray PDF results show that there is local order between chains. In (Cu1/2Au1/2)CN and (Ag1/2Au1/2)CN, there is a preference for unlike metal atoms occurring as nearest neighbors within the metal sheets. A general mathematical proof shows that the maximum average number of heterometallic nearest-neighbor interactions on a hexagonal lattice with two types of metal atoms is four. Calculated energies of periodic structural models show that those with four unlike nearest neighbors are most favorable. Of these, models in space group Immm give the best fits to the X-ray PDF data out to 8 Å, providing good descriptions of the short- and medium-range structures. This result shows that interactions beyond those of nearest neighbors must be considered when determining the structures of these materials. Such interactions are also important in (Cu1/3Ag1/3Au1/3)CN, leading to the adoption of a structure in Pmm2 containing mixed Cu–Au and Ag-only sheets arranged to maximize the numbers of Cu···Au nearest- and next-nearest-neighbor interactions.
Highlights
The group 11 cyanides CuCN,[1−4] AgCN,[4−7] and AuCN4,8−10 and the bimetallic cyanides (Cu1/2Au1/2)CN, (Ag1/2Au1/2)CN, and (CuxAg1−x)CN,[11,12] exhibit a surprising degree of structural complexity belied by their simple formula and apparently simple basic structures based on (−M−C N−)metal-cyanide chains
We show, using X-ray pair distribution function (PDF) studies for (Cu1/2Au1/2)CN and (Ag1/2Au1/2)CN, that is the maximum number of nearest-neighbor heterometallic interactions within the metallic layers of four achieved locally and next-nearest neighbor interactions are important in determining which of the possible structures is adopted
The absence of this splitting in the 13C NMR spectrum of (Ag1/2Au1/2)CN confirms that, in this compound, the C end of the CN ligand is again attached to Au, yielding chains of the form (−Ag−NC−Au−CN−)∞, as we proposed in our neutron PDF study.[11]
Summary
The group 11 cyanides CuCN,[1−4] AgCN,[4−7] and AuCN4,8−10 and the bimetallic cyanides (Cu1/2Au1/2)CN, (Ag1/2Au1/2)CN, and (CuxAg1−x)CN,[11,12] exhibit a surprising degree of structural complexity belied by their simple formula and apparently simple basic structures based on (−M−C N−). No superlattice peaks are seen in the powder X-ray diffraction (PXRD) patterns of the two cyanides (Figures S.1 and S.2), which can be indexed on simple hexagonal cells (Table 4) Examination of the X-ray PDF of (Cu1/3Ag1/3Au1/3)CN shows that the model built in P6mm (K) containing three homometallic sheets can immediately be rejected because it is a poor description of the local order This conclusion was reached in our previous paper[16] because, in this model, the large number of Au−Au nearest neighbors produces far too much intensity in the peak centered around ∼3.35 Å corresponding to the distance between nearestneighbor metals in the metal sheets. The DFT calculations show that nearest-neighbor Cu···Au interactions are more favorable than Ag···Au interactions, with structure M lying at lower energy than structure N, and this explains why structure M is the one adopted
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