Spin Crossover\nQuenching by “Racemization”\nin a Family of <i>trans</i>-1,2-Di(tetrazol-1-yl)cyclopentane-Based\nFe(II) 1D Coordination Polymers

Optically pure (<i>RR</i>)- and racemic (<i>RR</i>/<i>SS</i>)-<i>trans</i>-1,2-di­(tetrazol-1-yl)­cyclopentane\nwere synthesized and used to prepare homo- and heterochiral Fe­(II)\ncoordination compounds. [Fe­((<i>RR</i>/<i>SS</i>)-C<sub>7</sub>H<sub>10</sub>N<sub>8</sub>)<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub>]­(BF<sub>4</sub>)<sub>2</sub> (<b>1A</b>), [Fe­((<i>RR</i>/<i>SS</i>)-C<sub>7</sub>H<sub>10</sub>N<sub>8</sub>)<sub>2</sub>(C<sub>2</sub>H<sub>5</sub>CN)<sub>2</sub>]­(BF<sub>4</sub>)<sub>2</sub> (<b>2A</b>), [Fe­((<i>RR</i>)-C<sub>7</sub>H<sub>10</sub>N<sub>8</sub>)<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub>]­(BF<sub>4</sub>)<sub>2</sub>·2CH<sub>3</sub>CN (<b>1B·solv</b>), and [Fe­((<i>RR</i>)-C<sub>7</sub>H<sub>10</sub>N<sub>8</sub>)<sub>2</sub>(C<sub>2</sub>H<sub>5</sub>CN)<sub>2</sub>]­(BF<sub>4</sub>)<sub>2</sub> (<b>2B</b>) form\na family of one-dimensional coordination polymers. Fe­(II) cations\nin these complexes are characterized by a heteroleptic coordination\nenvironment: the neighboring metal centers are bridged by two 1,2-di­(tetrazol-1-yl)­cyclopentane\nmolecules, while the nitrile molecules (acetonitrile or propionitrile,\nrespectively) occupy the axial positions. Independently of the kind\nof nitrile coligands, an ability to thermally induce spin crossover\n(SCO) is governed by chirality. <b>1B·solv</b> and <b>2B</b> exhibit abrupt and complete SCO occurring at <i>T</i><sub>1/2</sub> = 144 K and <i>T</i><sub>1/2</sub> = 228\nK, respectively. Desolvated form, <b>1B</b> (of the same stoichiometry\nas <b>2B</b>), also exhibits SCO (<i>T</i><sub>1/2</sub> = 215 K). In contrast, an exchange within the polymeric chain of\nhalf of the <i>RR</i> molecules with the <i>SS</i> enantiomeric form results in formation of <b>1A</b> and <b>2A</b>, which remain in stable high-spin (HS) form down to 10\nK. It has been shown that moving from a homochiral to a heterochiral\nsystem changes the structure of the polymeric unit (while maintaining\nthe same polymer dimensionality and bridging fashion) that leads to\nthe deep reorganization of the further coordination spheres, including\nthe anion network.