Proton Conductivity in Photomagnetic Cu^II₂-[M^IV(CN)₈] Frameworks (M = Mo^IV and W^IV) 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 (<b>1</b>) and W^IV (<b>2</b>)), 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 <i>Fdd</i>2 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 (<b>1</b>) and 8.6(3) × 10^-7 S cm^-1 (<b>2</b>) at 298 K and 98% relative humidity. A strong humidity dependence of conductivity and activation energy values of 0.34 eV (<b>1</b>) and 0.36 eV (<b>2</b>) indicate the Grotthuss mechanism of proton transport that is mediated by crystallization water molecules. In the ground state, <b>1</b> and <b>2</b> are paramagnets due to Cu^II (<i>S</i> = 1/2) centers separated by diamagnetic [M^IV(CN)₈]^4- anions (<i>S</i> = 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 <b>1</b> and <b>2</b> are the first examples of cyanido-bridged assemblies combining proton conductivity and the photomagnetic effect, advancing the reticular chemistry of cyanido-bridged frameworks.