Silver compounds based on N,N,N-tridentate pyridylpyrazolate ligands. An opportunity to build cyclic trimetallic and oligomeric luminescent liquid crystals

Abstract

Silver pyrazolate complexes [Ag(μ-pzR(n)py)]3 [R(n)=C6H4OCnH2n+1; 3 (n=1); 4 (n=10); 5 (n=12); 6 (n=16) and 7 (n=18)] have been prepared by reaction of Ag2O with 3-(4-n-alkyloxyphenyl)-5-(pyridin-2-yl)pyrazole ligands [HpzR(n)py]. The presence of the pyridine as substituent group on the ligand seems to favour the formation of silver cyclic trimers which were isolated in the solid state, as it was supported by the X-ray single crystal structure of the derivative bearing the shortest alkyl chain [Ag(μ-pzR(1)py)]3 (3). By contrast, in CDCl3 or CD2Cl2 solution, the formation of one-dimensional coordination oligomers [Ag(μ-pzR(n)py)]x is proposed. The lability and interchange of AgN pyrazolic bonds of the trimeric structures in polar solvents appear to be responsible for the self-organization of Ag/μ-pzR(n)py motifs into open-chain oligomers. However, in C6D6 solution, the presence of only one set of signals led us to consider that apolar solvents may not affect the AgN pyrazolic bonds, so that the cyclic trimeric structure found in the solid state could be maintained in solution. Complexes bearing the longest chains [5 (n=12); 6 (n=16) and 7 (n=18)] exhibited liquid crystal behaviour showing smectic A (SmA) mesophases in agreement with the open-chain oligomeric structure proposed for the mesomorphic state. Photophysical studies performed on [Ag(μ-pzR(12)py)]3 (5), chosen as a representative example of the series, reveal the presence of luminescence in solution at room temperature for these materials.