Synthesis, Characterization, and Studies of Coordination-Polymeres With Isomeric Pyridylcyanoximes: Route to Metal Ribbons With Very Short Tl···Tl Separations

Two isomeric pyridylcyanoximes, H­(3PCO) and H­(4PCO),
containing
nitrogen atoms of the heterocycle at 3- and 4-positions, respectively,
were synthesized and thoroughly characterized using IR (including 15N labeling), UV–visible, 1H,13C one- and two-dimensional NMR spectroscopy, and X-ray analysis.
The H­(3PCO) exists as a mixture of syn- and anti-isomers in solutions, contrary to H­(4PCO) which is
present only as an anti-isomer. Both compounds have
planar geometry in the solid state and form elegant zigzag chains
via H-bonding between the oxime group and the N-atom of the heterocycle.
The H­(3PCO) and H­(4PCO) upon addition of a base form colored conjugated
and solvatochromic anions in solutions. The reaction of the hot aqueous
solution of Tl2CO3 with solid isomeric pyridylcyanoximes
leads to TlL, which were characterized by elemental analyses, IR spectroscopy,
and X-ray analysis. Both complexes represent three-dimensional coordination
polymers of different complexities where cyanoxime anions act as bridging
ligands. Thallium­(I) atoms in Tl­(3PCO) and Tl­(4PCO) form infinite,
planar “metal ribbons” with unusually short intermetallic
distances: 3.705 Å in Tl­(3PCO) and 3.635 Å in Tl­(4PCO) respectively,
which are close to that in metallic thallium (3.465 Å). The latter
compound has the second shortest Tl···Tl thallophillic
interaction reported in the literature thus far. Both yellow Tl­(3PCO)
and orange Tl­(4PCO) are insulators at ambient conditions: R ∼ 1 × 1011 Ohm.