Synthesis and Characterization of a Tetrapodal NO₄^4- 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.