Solvent Polarity Predictably Tunes Spin Crossover T1/2 in Isomeric Iron(II) Pyrimidine Triazoles
Posted on 2018-05-16 - 13:34
Two
isomeric pyrimidine-based Rdpt-type triazole ligands
were made: 4-(4-methylphenyl)-3-(2-pyrimidyl)-5-phenyl-4H-1,2,4-triazole (L2pyrimidine) and 4-(4-methylphenyl)-3-(4-pyrimidyl)-5-phenyl-4H-1,2,4-triazole (L4pyrimidine). When reacted with [FeII(pyridine)4(NCE)2], where E = S, Se, or BH3, two families of mononuclear
iron(II) complexes are obtained, including six solvatomorphs, giving
a total of 12 compounds: [FeII(L2pyrimidine)2(NCS)2] (1), [FeII(L2pyrimidine)2(NCSe)2] (2), 2·1.5H2O, [FeII(L2pyrimidine)2(NCBH3)2]·2CHCl3 (3·2CHCl3), 3 and 3·2H2O,
[FeII(L4pyrimidine)2(NCS)2] (4), 4·H2O, [FeII(L4pyrimidine)2(NCSe)2] (5), 5·2CH3OH, 5·1.5H2O, and [FeII(L4pyrimidine)2(NCBH3)2]·2.5H2O (6·2.5H2O). Single-crystal
X-ray diffraction reveals that the N6-coordinated iron(II)
centers in 1, 2, 3·2CHCl3, 4, 5, and 5·2CH3OH have two bidentate triazole ligands equatorially bound
and two axial NCE co-ligands trans-coordinated. All structures are
high spin (HS) at 100 K, except 3·2CHCl3, which is low spin (LS). Solid-state magnetic measurements show
that only 3·2CHCl3 (T1/2 above 400 K) and 5·1.5H2O (T1/2 = 110 K) undergo spin crossover
(SCO); the others remain HS at 300–50 K. When 3·2CHCl3 is heated at 400 K it desorbs CHCl3 becoming 3, which remains HS at 400–50 K. UV–Vis
studies in CH2Cl2, CHCl3, (CH3)2CO, CH3CN, and CH3NO2 solutions for the BH3 analogues 3 and 6 led to a 6:1 ratio of Lnpyrimidine/Fe(II) being employed for
the solution studies. These revealed SCO activity in all five solvents,
with T1/2 values for the 2-pyrimidine
complex (247–396 K) that were consistently higher than for
the 4-pyrimidine complex (216–367 K), regardless of solvent
choice, consistent with the 2-pyrimidine ring providing a stronger
ligand field than the 4-pyrimidine ring. Strong correlations of solvent
polarity index with the T1/2 values in
those solvents are observed for each complex, enabling predictable T1/2 tuning by up to 150 K. While this correlation
is tantalizing, here it may also be reflecting solvent-dependent speciationso
future tests of this concept should employ more stable complexes.
Differences between solid-state (ligand field; crystal packing; solvent
content) and solution (ligand field; solvation; speciation) effects
on SCO are highlighted.
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Rodríguez-Jiménez, Santiago; Barltrop, Alexis S.; White, Nicholas G.; L. C. Feltham, Humphrey; Brooker, Sally (2018). Solvent Polarity Predictably Tunes Spin Crossover T1/2 in Isomeric Iron(II) Pyrimidine Triazoles. ACS Publications. Collection. https://doi.org/10.1021/acs.inorgchem.8b00128
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AUTHORS (5)
SR
Santiago Rodríguez-Jiménez
AB
Alexis S. Barltrop
NW
Nicholas G. White
HL
Humphrey L. C. Feltham
SB
Sally Brooker
KEYWORDS
bidentate triazole ligands equatoriallyNCBHSingle-crystal X-ray diffraction400 KHSSCOCH 3L 2 pyrimidineOHUVisomeric pyrimidine-based Rdptdesorbs CHCl 3NCE co-ligands trans-coordinatedCH 3 CNligand fieldFe IICH 2 Cl 2CO4- pyrimidine ringtype triazole ligandsBH 3 analogues 32- pyrimidine ringLSL 4 pyrimidineNCS