Citric Acid as Multidentate Flexible Ligand for Multinuclear Late‐3 d‐Metal Complexes and Single‐Molecule Magnets

Abstract

Citric acid (citH4) is a ubiquitous product in nature with an interesting chemistry owing to its three carboxy and one hydroxy groups, which can be deprotonated in a stepwise manner into six kinds of citrate anions involving two protonation isomers. Deprotonated citrates easily coordinate to late‐3d‐metal ions (Mn, Fe, Co, Ni, Cu, and Zn), affording complexes including mono‐, di‐, tri‐, tetra‐, hexa‐, hepta‐, octa‐, nona‐, and 21‐nuclear complexes with a variety of structures. Magnetic interactions between metal centers are mainly generated via superexchange through hydroxylate or carboxylate bridging, whereas weaker magnetic dipole interactions appear in the absence of superexchange interactions. In citrate complexes, both ferromagnetic and antiferromagnetic interactions have been found. Citrate has a great potential to serve as a bridge between spin quantum magnetism and green chemistry. In this review, the structures and magnetism of discrete citrate complexes of late‐3dmetals, as well as their single‐molecule magnet properties are discussed.