Anion-Controlled Assembly of Silver(I) Complexes of Multiring Heterocyclic Ligands: A Structural and Photophysical Study

Abstract

Four multiring heterocyclic ligands with benzimidazole (L1 and L3) and benzothiazole nuclei (L2 and L4) are reported. Their silver(I) complexes involving a variety of anions (both organic and inorganic) have been prepared by the process of self-assembly and structurally characterized by single-crystal X-ray diffraction analyses. Discrete metallocyclic complexes [Ag(L3)(X)]2 (X = NO3−, 3a; cis-HOOCCH═CHCOO−, 3b; 0.5SiF62-, 3c) and [Ag(L4)(Y)]2 (Y = NO3−, 4a; CF3SO3−, 4b) have been formed with the ligands L3 and L4, respectively, where the pyridine nitrogen atom N1 is in the 3-position as against the coordination polymers [Ag(L1)(H2O)(NO3)]n, 1a, [Ag(L1)(CF3COO)]n, 1b and {2[Ag(L2)2(ClO4)].0.5.C2H5OH}n 2a, and [Ag(L2)2(cis-HOOCCH═CHCOO)]n, 2b, with the ligands L1 and L2, respectively, in which the N1 atom occupies the 4-position in the pyridine ring. In addition to the primary ligands (L1−L4), the counteranions also have a dominant influence on the overall structures of these compounds. Secondary bonding interactions, namely, hydrogen bonding, π···π-stacking, and C−H···π interactions, are also proven effective in shaping the dimensionalities of the solid state structures. Thus, a zigzag chain structure of 1a mediated by a nitrate anion generates a more complicated double layer structure in 1b where trifluoroacetate has replaced nitrate as the counterion. Discrete 12-membered metallocycles in 3a−3c generate 2-D arrays of flat (3a) and undulating topologies (3b and 3c), depending upon the type of their associated anions. Metallocycles 4a and 4b have less complicated structures compared to those of 3a−3c because of the replacement of NH by S in the heterocyclic ring, thereby reducing the hydrogen-bonding potential in the primary ligand in going from L3 to L4. In the solid state, the complexes show enhanced phosphorescence at 77 K with triplet lifetime in the range of 0.5−0.8 s, much shorter than those for the free ligands (2.3 − 3.3 s) because of increased spin−orbit coupling introduced by the coordinated Ag+ ion. This heavy-atom effect also has a causative influence in shortening the fluorescence lifetimes of these compounds.