Many solid-state physical properties, such as pyroelectricity, piezoelectricity, ferroelectricity, second harmonic generation (SHG), and triboluminescence, are only found in noncentrosymmetric bulk materials. [1] In spite of the practical importance of these properties in many technological applications, including telecommunications, optical storage, and information processing, the generation of acentric solids from achiral building blocks still depends upon ™Edisonian∫ approaches. The rational construction of structurally ordered noncentrosymmetric and chiral metal±organic solids remains a great challenge. Recently, attempts to generate acentric metal±organic coordination polymers (MOCPs) and organic solids by employing crystal engineering strategies have met with some success. [2] However, crystal engineering has not developed to the stage where a desired structure or crystal symmetry can be ensured. [3] Although it is not always possible to exert synthetic control, a fascinating variety of acentric and novel MOCPs have been obtained from hydrothermal reactions. [4] These reactions are often quite complicated and may involve in situ hydrolysis, oxidation, and ligand synthesis. The products generated under hydrothermal conditions cannot normally be obtained by direct mixing of solutions of metal ions and ligands. [4] Recently, Sharpless et al. reported a safe, convenient, and environmentally friendly procedure for the synthesis of a variety of 5-substituted 1H-tetrazoles in water. The 1Htetrazoles are prepared by addition of azide to nitriles in water with Zn salts as Lewis acid catalysts (Scheme 1). [5] The role of Zn in this reaction is unclear, but it was suggested that the solid intermediate in the reaction of PhCN with ZnBr 2 and NaN 3 is (PhCN 4 ) 2 Zn. The characterization of such an intermediate may provide important clues to the role of Zn in this reaction, and this in turn may allow synthetic chemists to further optimize this synthetic approach.
We recently combined metal salts with potentially bridging organic ligands under hydrothermal conditions to produce a range of new materials. [4b±f, 6] Our experience with such systems prompted us to conduct structural studies of the products or ™intermediates∫ in the hydrothermal reaction of ZnCl 2 (or CdCl 2 ) with 3-cyanopyridine (or 4-cyanopyridine) and NaN 3 in water (Scheme 2). To our surprise, the intermediates trapped or synthesized at high temperature all crystallize in acentric space groups and display SHG responses in the solid state which are unprecedented in tetrazole± metal coordination chemistry, as far as we are aware. [7] Here we report their solid-state structures and preliminary SHG properties.