Structure of Metiram, a Crystalline Zinc Coordination Complex with Amorphous Side Phase

Ethylenebis(dithiocarbamates) (EBDTCs) have been extensively used as fungicides in agriculture for nearly 80 years. Modern fungicides based on EBDTCs contain metal ions, such as zinc in Zineb and Metiram, manganese in Maneb, or combinations of both, such as in Mancozeb. Despite being commercially available since the 1940s, the molecular structure of the metal complexes of EBDTCs was not described in detail until the crystal structure of Zineb was published in 2020. Zineb (C4H6N2S4Zn) is a single-phase crystalline material. In this study, we present a comprehensive multimethod structural characterization of Metiram (C12H27N9S12Zn3), which is the active ingredient of Polyram WG, a highly effective and plant-compatible organic contact fungicide. Our findings reveal that Metiram comprises two distinct phases. The primary phase, phase I, of Metiram is ethylenebis(dithiocarbamate) zinc(II) ammine, which constitutes 81 wt % of the material, or three-quarters of EBDTC. It is a zinc-coordinating crystalline phase. The crystal structure of this phase was determined by powder X-ray diffraction, revealing 1D S-shaped chains of EBDTC connected by strongly distorted Zn[NH3][S4] tetragonal pyramids. These pyramids share their sulfur atoms, and ammonia molecules occupy the apex of the pyramids, pointing alternately up and down. The secondary phase, phase II, constitutes 19 wt % of the material or one-quarter of EBDTC, and is amorphous. Using a combination of different techniques, including microscopy, diffraction, and spectroscopy, we have concluded that phase II consists of Zn-free EBDTC. We infer that the primary structure of this secondary constituent aligns with previous assumptions, notably the absence of a significant amount of Zn and the presence of disulfide bonds.