Rare Silver–Histidine Cluster Complex and Its Single-Crystal-to-Single-Crystal Phase-Transition Behavior

Silver complexes with proteinogenic amino acid ligands are of interest for biomedical and antimicrobial applications. In this work, we obtained {[Ag7(l-his)4](NO3)3·3H2O}0.2{[Ag8(l-his)4(H2O)2](NO3)4·3H2O}0.8 (1) and {[Ag7(d-his)4](NO3)3·3H2O}0.2{[Ag8(d-his)4(H2O)2](NO3)4·3H2O}0.8 (2), which represent the first example of any Ag-exclusive complex featuring a cluster-type core motif and with only proteinogenic amino acid ligands. Upon immersion into acetonitrile, an interesting single-crystal-to-single-crystal transformation occurred to produce a new cluster complex of the formula [Ag8(l-his)4(NO3)(H2O)](NO3)3 (3). Using a racemic mixture of histidine, the reaction under otherwise identical conditions led to the production of the second example of a three-dimensional (3D) network structured Ag-exclusive complex with only a proteinogenic amino acid ligand. Compared with other Ag–histidine complexes in the literature, the significance of reaction conditions, particularly the Ag/histidine ratio and pH of the reaction mixture, is revealed. Temperature-dependent emission of 1 and 2 at 440 nm characteristic of silver–philophilic interactions was also observed.