Oxidative dissolution of synthetic vivianites as a method for the crystallization of molecular structural motifs

Abstract

Isolation of structural motifs from minerals can be used to improve the understanding of fundamental processes such as catalytic mechanisms and spectroscopic vibrations. In this study, we designed an oxidative dissolution reaction using 2,2′-bipyridine in combination with minerals from the vivianite family. Vivianites are structurally similar to amorphous cobalt oxide water oxidation catalysts (CoCat). Specifically, we used pakhomovskyite (Pak: Co3(PO4)2•8H2O) and erythrite (Ery: Co3(AsO4)2•8H2O)). Through the designed synthetic route, we crystallized two mononuclear isostructural complexes containing phosphate or arsenate and a cobalt-arsenate trinuclear cluster. We observed three main binding modes from the anions: (i) both arsenates or phosphates bind to cobalt forming an O-μ2-bridge, (ii) arsenate forms a bridge between two cobalt atoms, and (iii) arsenate binds between three cobalt atoms expanding its coordination sphere to five. This structural information allows us to propose an updated structural model for amorphous cobalt oxide catalysts containing arsenate.