The crystal structure of K3Cu(CN)4

Sodium bis-(ethyl-enedi-amine)-copper(II) tetra-cyanido-cuprate(I)

The title compound, Na[Cu(en)2][Cu(CN)4], where en represents ethyl-enedi-amine, NH2CH2CH2NH2, crystallizes as a salt with two distinct cations, Na(+) and [Cu(II)en2](2+), and discrete [Cu(I)(CN)4](3-) anions. The anion geometry is tetra-hedral, with angles at the copper atom ranging from 105.0 (1) to 115.4 (1)°. The Cu-C distances are in the range 1.976 (3) to 1.993 (3) Å. The divalent copper atom is coordinated by four N atoms of the two bidentate en ligands in a slightly distorted square-planar geometry. In the crystal, each sodium ion inter-acts with cyanide N atoms of four different anions, with Na-N distances lying in the narrow range of 2.344 (3) to 2.367 (3) Å, and an approximately tetra-hedral arrangement around the sodium ions. The inter-acting sodium ions and [Cu(I)(CN)4](3-) anions form a three-dimensional network with channels which contain the [Cu(en)2](2+) cations. One of the chelate rings in the cation shows partial disorder between two different conformations and the C atoms were refined with occupancies in the ratio 0.817 (15):0.183 (15).

Article

Direct NMR observation of copper-63/65 nuclei in solid K3Cu(CN)4 provides the first experimental example of anisotropic copper chemical shielding. Axially symmetric by virtue of the space group symmetry, the shielding tensor spans 42 ppm, with the greatest shielding when the unique axis is perpendicular to the applied magnetic field. The nuclear quadrupole coupling constant is also appreciable, CQ(63Cu) = -1.125 MHz, reflecting a deviation of the Cu(CN)43- anion from pure tetrahedral symmetry. Spin-spin coupling to 13C nuclei in an isotopically enriched sample is quantified by line-shape simulations of both 13C and 63/65Cu magic-angle spinning (MAS) NMR spectra to be 300 Hz. It is shown that this information is also directly available by 63/65Cu triple-quantum (3Q) MAS NMR. The relative merits of these three approaches to characterizing spin-spin couplings involving half-integer quadrupolar nuclei are discussed. Chemical shielding tensors for nitrogen-15 and carbon-13 are obtained from NMR spectra of non-spinning samples, and are compared to those of tetrahedral group 12 tetracyanometallates. Finally, 2J(63/65Cu,15N) detected in 15N MAS experiments are found to be 19 and 20 Hz for the two crystallographically distinct cyanide ligands.Key words: NMR, quadrupolar nucleus, chemical shielding tensor, multiple-quantum magic-angle spinning, metal cyanide, spin-spin coupling.