Coordinatively Unsaturated Metallates of Cobalt(II), Nickel(II), and Zinc(II) Gated by a Rigid and Narrow Void

Both natural enzymatic systems and synthetic porous material catalysts utilize well-defined and uniform channels to dictate reaction selectivities on the basis of size or shape. Mimicry of this design element in homogeneous systems is generally difficult owing to the flexibility inherent to most small molecular species. Herein, we report the synthesis of a tripodal ligand scaffold which orients a narrow and rigid cavity atop accessible metal coordination space. The permanent void is formed through a macrocyclization reaction whereby the 3,5-dihydroxyphenyl arms are covalently linked through methylene bridges. Deprotonative metallation leads to anionic and coordinatively unsaturated complexes of divalent cobalt, nickel, and zinc. An analogous series of trigonal monopyramidal complexes bearing a non-macrocyclized variant of the tripodal ligand are also reported. Physical characterization of the coordination complexes has been carried out using multiple spectroscopic techniques (NMR, EPR, UV-Vis), cyclic voltammetry, and X-ray diffraction. Complexes of the macrocyclized [LOCH2O]3– ligand retain a rigid cavity upon metallation, with this cavity gating entrance to the open axial coordination site. Through a combination of spectroscopic and computational studies, it is shown that acetonitrile entry into the void is sterically precluded, disrupting anticipated coordination at the intracavity site.