Proton Conductivity in Photomagnetic CuII2-[MIV(CN)8] Frameworks (M = MoIV and WIV) Facilitated by the Tetracarboxy-Derivative of Cyclam

Two 3D bimetallic cyanido-bridged coordination polymers, [Cu^II(H₄tetac)]₂[M^IV(CN)₈]·4H₂O (M = Mo^IV (1) and W^IV (2)), based on copper(II) complex of tetracarboxylic-derivative of cyclam, H₄tetac (=1,4,8,11-tetrazacyclotetradecane-6,6,13,13-tetracarboxylic acid), have been synthesized and characterized in terms of structure, proton conductivity, and the photomagnetic effect. The isostructural compounds crystallize in the polar Fdd2 space group and present a diamond-like topology. The carboxylic groups in the structure yield proton conductivities of 5.4(3) × 10^–7 S cm^–1 (1) and 8.6(3) × 10^–7 S cm^–1 (2) at 298 K and 98% relative humidity. A strong humidity dependence of conductivity and activation energy values of 0.34 eV (1) and 0.36 eV (2) indicate the Grotthuss mechanism of proton transport that is mediated by crystallization water molecules. In the ground state, 1 and 2 are paramagnets due to Cu^II (S = 1/2) centers separated by diamagnetic [M^IV(CN)₈]^4– anions (S = 0). Upon 450 nm light irradiation at 10 K, both compounds show a photomagnetic response. The Mo^IV system shows higher photoconversion rates, while the W^IV analogue exhibits full reversibility. Compounds 1 and 2 are the first examples of cyanido-bridged assemblies combining proton conductivity and the photomagnetic effect, advancing the reticular chemistry of cyanido-bridged frameworks.