Synthesis and Characterization of a Tetrapodal NO44– Ligand and Its Transition Metal Complexes

We present the synthesis and characterization of alkali metal salts of the new tetraanionic, tetrapodal ligand 2,2′-(pyridine-2,6-diyl)­bis­(2-methylmalonate) (A4[PY­(CO2)4], A = Li+, Na+, K+, and Cs+), via deprotection of the neutral tetrapodal ligand tetraethyl 2,2′-(pyridine-2,6-diyl)­bis­(2-methylmalonate) (PY­(CO2Et)4). The [PY­(CO2)4]4– ligand is composed of an axial pyridine and four equatorial carboxylate groups and must be kept at or below 0 °C to prevent decomposition. Exposing it to a number of divalent first-row transition metals cleanly forms complexes to give the series K2[(PY­(CO2)4)­M­(H2O)] (M = Mn2+, Fe2+, Co2+, Ni2+, Zn2+). The metal complexes were comprehensively characterized via single-crystal X-ray diffraction, 1H NMR and UV–vis absorption spectroscopy, and cyclic voltammetry. Crystal structures reveal that [PY­(CO2)4]4– coordinates in a pentadentate fashion to allow for a nearly ideal octahedral coordination geometry upon binding an exogenous water ligand. Additionally, depending on the nature of the charge-balancing countercation (Li+, Na+, or K+), the [(PY­(CO2)4)­M­(H2O)]2– complexes can assemble in the solid state to form one-dimensional channels filled with water molecules. Aqueous electrochemistry performed on [(PY­(CO2)4)­M­(H2O)]2– suggested accessible trivalent oxidation states for the Fe, Co, and Ni complexes, and the trivalent Co3+ species [(PY­(CO2)4)­Co­(OH)]2– could be isolated via chemical oxidation. The successful synthesis of the [PY­(CO2)4]4– ligand and its transition metal complexes illustrates the still-untapped versatility within the tetrapodal ligand family, which may yet hold promise for the isolation of more reactive and higher-valent metal complexes.