Phthalic acid, a versatile building block in organic-organometallic crystal engineering

Abstract

Phthalic acid [C₆H₄-1,2-(COOH)₂, H₂PA] and terephthalic acid [C₆H₄-1,4-(COOH)₂, H₂TPA] have been reacted with aqueous solutions of the hydroxides [(η⁵-C₅H₅)₂Co]⁺[OH]^- and [(η⁶-C₆H₆)₂Cr]⁺[OH]^- produced insitu by oxidation of the parent neutral molecules. The acid–base reaction leads to self-assembly of the deprotonated acid anions into honeycomb superstructures held together by hydrogen-bonding interactions of the O–H···O and charged O–H···O^- types. The superanions accommodate the [(η⁵-C₅H₅)₂Co]⁺ and the paramagnetic [(η⁶-C₆H₆)₂Cr]⁺ organometallic cations via charge-assisted C–H^δ+···O^δ- hydrogen bonds. Four novel organic–organometallic cocrystals, namely {[(η⁵-C₅H₅)₂Co]⁺}₄{[HPA]^-}₂[PA]^2-·4H₂O (1), [(η⁶-C₆H₆)₂Cr]⁺-[HPA]^-[H₂PA] (2), {[(η⁵-C₅H₅)₂Co]⁺}₂[TPA]^2-·6H₂O (3) and {[(η⁶-C₆H₆)₂Cr]⁺}₂[TPA]^2-·6H₂O (4) have been isolated and structurally characterized by low-temperature X-ray diffraction measurements. It is shown that phthalic acid is a very versatile building block in the formation of hydrogen-bonded networks and unprecedented superanionic architectures. The role played by water molecules in the stabilization of the crystal structures in the absence of all or almost all acidic protons is discussed.