Solid-state Structural and Magnetic Investigations of M[HC(3,5-Me₂pz)₃]₂(BF₄)₂ (M = Fe, Co, Ni, Cu): Observation of a Thermally Induced Solid-state Phase Change Controlling an Iron(II) Spin-state Crossover


The reaction of M(BF4)2·xH2O (M = Co, Ni, and Cu) and HC(3,5-Me2pz)3 in a 1:2 ratio yields Co[HC(3,5-Me2- pz)3]2(BF4)2 (2), Ni[HC(3,5-Me2pz)3]2 (BF4)2 (3), and Cu[HC(3,5-Me2pz)3]2 (BF4)2 (4). Over the temperature range from 5 to 350, 345, or 320 K, Curie law behavior is observed for microcrystalline samples of all three compounds showing them to have three, two, and one unpaired electrons, respectively, with no spin-crossover observed for 2. Crystalline samples of these compounds torque in the applied magnetic field the first time the sample is cooled to 5 K. The solid-state structures of all three are isomorphous at 220 K, monoclinic in the space group C2/c. The metal is located on a unique crystallographic site and has a trigonally distorted octahedral structure, with 4 showing the expected Jahn-Teller distortions. Cooling crystals of all three to low temperatures leads to the observation of the same phase change to triclinic in the new space group P1 with nonmerohedral twinning. This change is reversible and yields two crystallographically unique metal sites at low temperature. The bond angles and distances for the two different metal sites for each compound in the low temperature structures are very similar to each other and to those in the 220 K structures. The same phase change, monoclinic to triclinic, has been observed previously for Fe[HC(3,5-Me2pz)3]2 (BF4)2 (1), except in this case, the phase change results in half of the cations changing over from the high-spin state to the low-spin state while the other half of the cations remain high-spin, with the low-spin form decreasing its Fe-N bond distances by 0.19 Å. The new results with 2-4 show that it is the phase transition, which occurs in complexes of the type M[HC(3,5-Me2pz)3]2 (BF4)2 with first row transition metals, that is driving the unusual spin-crossover behavior of Fe[HC(3,5-Me2pz)3]2 (BF4)2.



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