Dimensionality Switching Through a Thermally Induced Reversible Single-Crystal-to-Single-Crystal Phase Transition in a Cyanide Complex

The heterometallic hexanuclear cyanide-bridged complex {[Mn(bpym)(H2O)](2)[Fe(HB(pz)(3))(CN)(3)](4)) (1), its (CN)-N-15 and D2O enriched forms {[Mn(bpym)(H2O)](2)[Fe(HB(Pz)(3))((CN)-N-15)(3)](4)} (2) and {[Mn(bpym)(D2O)](2)[Fe(HB(pz)(3))(CN)(3)](4)} (3), and the hexanuclear derivative complex {[Mn(bpym)(H2O)](2)[Fe(B(Pz)(4))(CN)(3)](4)} center dot 4H(2)O (4) [bpym = 2,2'-bipyrimidine, HB(pz)(3)(-) = hydrotris(1-pyrazolyl)borate, B(pz)(4)(-) = tetra(1-pyrazolyl)borate] have been synthesized. Their structures have been determined through single-crystal X-ray crystallography at different temperatures. Whereas 3 and 4 maintain a discrete hexanuclear motif during the entire temperature range investigated (down to 95 K), 1 and 2 exhibit a thermally induced reversible single-crystal to single-crystal phase transition driven by a remarkable concerted rearrangement of hydrogen and cyanide coordination bonds. While hexanuclear complexes are observed in the high temperature phases (noted la and 2a) above 200 K, the low temperature phases are composed of one-dimensional coordination polymers noted 1b and 2b. The magnetic properties of the four compounds have been investigated in the 2-300 K range, and they reveal the occurrence of an overall antiferromagnetic behavior. The thermal dependence of the optical reflectivity and the FT-IR absorbance have been studied for 1 in the range 10-300 K and 130-300K, respectively. A comparative analysis of the structural and electronic properties for 1-4 clearly underlines the major role of the intermolecular interactions in the topological and dimensional rearrangement observed during the structural phase transition. This result opens new perspectives in the design of cyanide-based switchable magnetic materials using coordination bonds rearrangements.