A Novel Bis(<i>trans</i>-thiocyanate)iron(II) Spin-Transition Molecular Material with Bidentate Triaryltriazole Ligands and Its Bis(<i>cis-</i>thiocyanate)iron(II) High-Spin Isomer

Two novel complexes <i>trans</i>-[Fe(MBPT)<sub>2</sub>(NCS)<sub>2</sub>] (<b>1</b>) and <i>cis</i>-[Fe(mMBPT)<sub>2</sub>(NCS)<sub>2</sub>] (<b>2</b>) have been synthesized and their structure characterized at 293 K by single-crystal X-ray crystallography [MBPT (or mMBPT) = 4-<i>p(</i>or <i>m)</i>-methylphenyl-3,5-bis(pyridin-2-yl)-1,2,4-triazole]. Complexes <b>1</b> and <b>2</b> are isomers, and <b>1 </b>crystallizes in the triclinic space group <i>P</i>1̄ with<i> Z</i> = 1, <i>a</i> = 9.120(3) Å, <i>b</i> = 9.2563(18) Å, <i>c</i> = 11.576(2) Å, <i>α</i> = 76.256(15)°, <i>β</i> = 76.405(18)°, <i>γ</i> = 84.983(19)°, and <i>V</i> = 922.1(4) Å<sup>3</sup> in contrast to <b>2</b> in the monoclinic space group <i>C</i>2/<i>c</i> with<i> Z</i> = 4, <i>a</i> = 21.851(3) Å, <i>b</i> = 13.4113(15) Å, <i>c</i> = 16.859(3) Å, <i>β</i> = 128.153(9)°, and <i>V</i> = 3885.0(9) Å<sup>3</sup>. Both complexes have a similar pseudo-octahedral [FeN<sub>6</sub>] core with the NCS<sup>-</sup> groups in the trans arrangement in <b>1</b> but cis in <b>2</b>. Variable-temperature magnetic susceptibility and IR spectral measurements reveal that <b>1</b> shows an abrupt high-spin ↔ low-spin (HS ↔ LS) transition centered at <i>T</i><sub>1/2</sub> around 231 K, in contrast to <b>2 </b>staying in a high-spin state in the observed temperature range of 75−300 K. The extended X-ray absorption fine structures (EXAFS) spectra confirm that the average Fe−N distance for the HS state in <b>1</b> is ≈0.2 Å longer than that of the LS state. Complex <b>1</b> represents the first spin-crossover iron(II) complex with triaryltriazole and <i>trans</i>-thiocyanate ligands.