Structural isomerism in crystals of redox-active secondary ortho-diamides:: The role of competing intra- and intermolecular hydrogen bonds in directing crystalline topologies

The solid-state chemistry of a series of seven ortho-bis(alkylamido)-ethylenedithiotetrathiafulvalene derivatives EDT-TTF-(CONHR)(2) (R = Me, 1; Et, 2; Pr, 3; Bu, 4; Pent, 5; Hex, 6; and Bz, 7), in their neutral and one-electron-oxidized, radical cation states, was investigated with an eye on the topology of intra- and intermolecular hydrogen-bond motifs. In the case of neutral, monomolecular solids, an intramolecular N-H...O hydrogen bond seals a constrained seven-membered ring for 1, 2, 3, and 7, which is disrupted in butyl derivative 4 in favor of an anti-parallel ladder at the expense of any intramolecular hydrogen bond. In the solid-state, the competition between intra- and intermolecular hydrogen bonding observed in solution depends on the packing of the molecules. Electrocrystallization of methyl derivative 1 with ReO4- or ClO4-, two anions of different volumes but otherwise identical charge and symmetry, revealed a fine sensitivity of the constrained seven-membered ring to the internal chemical pressure exerted by the anion. [1](2).+ReO4 (sigma(RT) = 8.5 S cm(-1), activation energy Delta = 600 K at high temperature) and beta"-[1](2).+ClO4 (sigma(RT) = 0.03 S cm(-1), Delta = 1600 K; under pressure at room temperature, the conductivity increases by three orders of magnitude up to 17 kbar with a linear variation of the activation energy with pressure, Delta = aP with a = 0.202 10(6) K kbar(-1)) have vastly different architectures, dimensionalities, electronic structures, and collective properties, a consequence of the presence of the closed or open structural isomers in one or the other. Ibis exemplifies the flexibility of functionalized TTF derivatives, even when the functional group is directly attached to the electro-active core. This allows an analogy to be drawn with tetrafunctionalized metallocenes, in which such flexibility has already been observed. The experimental data are supported by theoretical calculations of the energy profile of a model molecule on rotation of the amido groups.