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

Two novel complexes trans-[Fe(MBPT)₂(NCS)₂] (1) and cis-[Fe(mMBPT)₂(NCS)₂] (2) have been synthesized and their structure characterized at 293 K by single-crystal X-ray crystallography [MBPT (or mMBPT) = 4-p(or m)-methylphenyl-3,5-bis(pyridin-2-yl)-1,2,4-triazole]. Complexes 1 and 2 are isomers, and 1 crystallizes in the triclinic space group P1̄ with Z = 1, a = 9.120(3) Å, b = 9.2563(18) Å, c = 11.576(2) Å, α = 76.256(15)°, β = 76.405(18)°, γ = 84.983(19)°, and V = 922.1(4) ų in contrast to 2 in the monoclinic space group C2/c with Z = 4, a = 21.851(3) Å, b = 13.4113(15) Å, c = 16.859(3) Å, β = 128.153(9)°, and V = 3885.0(9) ų. Both complexes have a similar pseudo-octahedral [FeN₆] core with the NCS^- groups in the trans arrangement in 1 but cis in 2. Variable-temperature magnetic susceptibility and IR spectral measurements reveal that 1 shows an abrupt high-spin ↔ low-spin (HS ↔ LS) transition centered at T_1/2 around 231 K, in contrast to 2 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 1 is ≈0.2 Å longer than that of the LS state. Complex 1 represents the first spin-crossover iron(II) complex with triaryltriazole and trans-thiocyanate ligands.