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 P (1) over bar with Z = 1, alpha = 9.120(3) Angstrom, b = 9.2563(18) Angstrom, c = 11.576(2) Angstrom, alpha = 76.256(15)degrees, beta = 76.405(18)degrees, gamma = 84.983(19)degrees, and V = 922.1(4) Angstrom(3) in contrast to 2 in the monoclinic space group C2/c with Z = 4, alpha = 21.851(3) Angstrom, b = 13.4113(15) Angstrom, c = 16.859(3) Angstrom, beta = 128.153(9)degrees, and V = 3885.0(9) Angstrom(3). 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 approximate to 0.2 Angstrom longer than that of the LS state. Complex 1 represents the first spin-crossover iron(II) complex with triaryltriazole and traps-thiocyanate ligands.