Solvent Polarity Predictably Tunes Spin Crossover <i>T</i><sub>1/2</sub> in Isomeric Iron(II) Pyrimidine Triazoles

Published on 2018-05-16T13:34:45Z (GMT) by
Two isomeric pyrimidine-based <b>Rdpt</b>-type triazole ligands were made: 4-(4-methylphenyl)-3-(2-pyrimidyl)-5-phenyl-4<i>H</i>-1,2,4-triazole (<b>L</b><sup><b>2pyrimidine</b></sup>) and 4-(4-methylphenyl)-3-(4-pyrimidyl)-5-phenyl-4<i>H</i>-1,2,4-triazole (<b>L</b><sup><b>4pyrimidine</b></sup>). When reacted with [Fe<sup>II</sup>(pyridine)<sub>4</sub>­(NCE)<sub>2</sub>], where E = S, Se, or BH<sub>3</sub>, two families of mononuclear iron­(II) complexes are obtained, including six solvatomorphs, giving a total of 12 compounds: [Fe<sup>II</sup>(<b>L</b><sup><b>2pyrimidine</b></sup>)<sub>2</sub>­(NCS)<sub>2</sub>] (<b>1</b>), [Fe<sup>II</sup>(<b>L</b><sup><b>2pyrimidine</b></sup>)<sub>2</sub>­(NCSe)<sub>2</sub>] (<b>2</b>), <b>2</b>·1.5H<sub>2</sub>O, [Fe<sup>II</sup>(<b>L</b><sup><b>2pyrimidine</b></sup>)<sub>2</sub>­(NCBH<sub>3</sub>)<sub>2</sub>]·2CHCl<sub>3</sub> (<b>3</b>·2CHCl<sub>3</sub>), <b>3</b> and <b>3</b>·2H<sub>2</sub>O, [Fe<sup>II</sup>(<b>L</b><sup><b>4pyrimidine</b></sup>)<sub>2</sub>­(NCS)<sub>2</sub>] (<b>4</b>), <b>4</b>·H<sub>2</sub>O, [Fe<sup>II</sup>(<b>L</b><sup><b>4pyrimidine</b></sup>)<sub>2</sub>­(NCSe)<sub>2</sub>] (<b>5</b>), <b>5</b>·2CH<sub>3</sub>OH, <b>5</b>·1.5H<sub>2</sub>O, and [Fe<sup>II</sup>(<b>L</b><sup><b>4pyrimidine</b></sup>)<sub>2</sub>­(NCBH<sub>3</sub>)<sub>2</sub>]·2.5H<sub>2</sub>O (<b>6</b>·2.5H<sub>2</sub>O). Single-crystal X-ray diffraction reveals that the N<sub>6</sub>-coordinated iron­(II) centers in <b>1</b>, <b>2</b>, <b>3</b>·2CHCl<sub>3</sub>, <b>4</b>, <b>5</b>, and <b>5</b>·2CH<sub>3</sub>OH 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 <b>3</b>·2CHCl<sub>3</sub>, which is low spin (LS). Solid-state magnetic measurements show that only <b>3</b>·2CHCl<sub>3</sub> (<i>T</i><sub>1/2</sub> above 400 K) and <b>5</b>·1.5H<sub>2</sub>O (<i>T</i><sub>1/2</sub> = 110 K) undergo spin crossover (SCO); the others remain HS at 300–50 K. When <b>3</b>·2CHCl<sub>3</sub> is heated at 400 K it desorbs CHCl<sub>3</sub> becoming <b>3</b>, which remains HS at 400–50 K. UV–Vis studies in CH<sub>2</sub>Cl<sub>2</sub>, CHCl<sub>3</sub>, (CH<sub>3</sub>)<sub>2</sub>CO, CH<sub>3</sub>CN, and CH<sub>3</sub>NO<sub>2</sub> solutions for the BH<sub>3</sub> analogues <b>3</b> and <b>6</b> led to a 6:1 ratio of <b>L</b><sup><b><i>n</i>pyrimidine</b></sup>/Fe­(II) being employed for the solution studies. These revealed SCO activity in all five solvents, with <i>T</i><sub>1/2</sub> 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 <i>T</i><sub>1/2</sub> values in those solvents are observed for each complex, enabling predictable <i>T</i><sub>1/2</sub> tuning by up to 150 K. While this correlation is tantalizing, here it may also be reflecting solvent-dependent speciationso 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.