(EDT-TTF-CONH2)6[Re6Se8(CN)6], a Metallic Kagome-Type Organic−Inorganic Hybrid Compound: Electronic Instability, Molecular Motion, and Charge Localization

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Abstract
(EDT-TTF-CONH2)6[Re6Se8(CN)6], space group R3̄, was prepared by electrocrystallization from the primary amide-functionalized ethylenedithiotetrathiafulvalene, EDT-TTF-CONH2 (E1/21 = 0.49 V vs SCE in CH3CN), and the molecular cluster tetraanion, [Re6Se8(CN)6]4- (E1/2 = 0.33 V vs SCE in CH3CN), equipped with hydrogen bond donor and hydrogen bond acceptor functionalities, respectively. Its Kagome topology is unprecedented for any TTF-based materials. The metallic state observed at room temperature has a strong two-dimensional character, in coherence with the Kagome lattice symmetry, and the presence of minute amounts of [Re6Se8(CN)6](3-)• identified by electron spin spectroscopy. A structural instability toward a distorted form of the Kagome topology of lesser symmetry is observed at ca. 180 K. The low-temperature structure is associated with a localized, electrically insulating electronic ground state and its magnetic susceptibility accounted for by a model of uniform chains of localized S = 1/2 spins in agreement with the 100 K triclinic crystal structure and band structure calculations. A sliding motion, within one out of the three (EDT-TTF-CONH2)2 dimers coupled to the [Re6Se8(CN6)(3-)•]/[Re6Se8(CN6)4-] proportion at any temperature, and the electronic ground state of the organic−inorganic hybrid material are analyzed on the basis of ESR, dc conductivity, 1H spin−lattice relaxation, and static susceptibility data which qualify a Mott localization in [EDT-TTF-CONH2]6[Re6Se8(CN)6]. The coupling between the metal−insulator transition and a structural transition allows for the lifting of a degeneracy due to the ternary axis in the high temperature, strongly correlated metallic phase which, in turn, leads to Heisenberg chains at low temperature.