Solvent Polarity Predictably Tunes Spin Crossover T_1/2 in Isomeric Iron(II) Pyrimidine Triazoles

Abstract

Two isomeric pyrimidine-based Rdpt-type triazole ligands were made: 4-(4-methylphenyl)-3-(2-pyrimidyl)-5-phenyl-4 H-1,2,4-triazole (L) and 4-(4-methylphenyl)-3-(4-pyrimidyl)-5-phenyl-4 H-1,2,4-triazole (L). When reacted with [Fe(pyridine)(NCE)], where E = S, Se, or BH, two families of mononuclear iron(II) complexes are obtained, including six solvatomorphs, giving a total of 12 compounds: [Fe(L)(NCS)] (1), [Fe(L)(NCSe)] (2), 2·1.5HO, [Fe(L)(NCBH)]·2CHCl (3·2CHCl), 3 and 3·2HO, [Fe(L)(NCS)] (4), 4·HO, [Fe(L)(NCSe)] (5), 5·2CHOH, 5·1.5HO, and [Fe(L)(NCBH)]·2.5HO (6·2.5HO). Single-crystal X-ray diffraction reveals that the N-coordinated iron(II) centers in 1, 2, 3·2CHCl, 4, 5, and 5·2CHOH 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 3·2CHCl, which is low spin (LS). Solid-state magnetic measurements show that only 3·2CHCl ( T above 400 K) and 5·1.5HO ( T = 110 K) undergo spin crossover (SCO); the others remain HS at 300-50 K. When 3·2CHCl is heated at 400 K it desorbs CHCl becoming 3, which remains HS at 400-50 K. UV-Vis studies in CHCl, CHCl, (CH)CO, CHCN, and CHNO solutions for the BH analogues 3 and 6 led to a 6:1 ratio of L /Fe(II) being employed for the solution studies. These revealed SCO activity in all five solvents, with T 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 T values in those solvents are observed for each complex, enabling predictable T 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.

Predictable Electronic Tuning By Choice of Azine Substituent in Five Iron(II) Triazoles: Redox Properties and DFT Calculations

“…In the solid state, both the ligand field imposed by the ligand and the crystal packing interactions, are critical. In the solution phase crystal packing interactions are absent, hence the ligand field is the predominant influence modulating the spin state, but this can be attenuated by solvent polarity …”

“… The symmetrical Rdpt 1,2,4‐triazole ligand (box), and the family of non‐symmetrically azine ‐substituted Rat 1,2,4‐triazole ligands ( L azine ) with five different azine rings ( azine =pyridazine, pyrazine, pyridine, 2‐pyrimidine and 4‐pyrimidine) used in this study and which have previously produced SCO‐active iron(II) complexes.…”

“…We have recently employed five monotopic azine ‐substituted 1,2,4‐ triazole ligands, L azine (Figure , Rat ligands) to generate a large family of mononuclear iron(II) complexes, [Fe II ( L azine ) 2 (NCE) 2 ] (E=S, Se and BH 3 ), that are SCO‐active in both the solid‐ and solution‐phase . The solution studies on these complexes have enabled us to (i) investigate the effects of solvent polarity on the spin state; (ii) place five azines in order of increasing ligand field strength in [Fe II ( L azine ) 2 (NCBH 3 ) 2 ] complexes (4‐pyrimidine <2‐pyrimidine < pyridine < pyrazine < pyridazine), and (iii) develop a simple and general method that enables us to predict, before synthesis, the spin state of metal complexes in solution, once a trend line is established for the family of interest [T 1/2 from Evans method on the complex vs. 15 N chemical shift for the ligand, either measured or calculated by density functional theory (DFT)] . Predictable tuning of spin state in advance of synthesis is a theme that is gaining increasing attention due to its importance in SCO‐materials and in other fields such as catalysis …”

Abrupt Spin Crossover Behavior in a Linear N1,N2-Triazole Bridged Trinuclear Fe(II) Complex

“…Currently, special attention is given to SCO compounds exhibiting pronounced cooperative behavior. Here, the mononuclear system [Fe(bpp) 2 ] 2+ (bpp = 2,6-di{pyrazol-1-yl}pyridine) [1][2][3][4], as well as the triazole-based mono-/bi-nuclear [Fe 2 (PMAT) 2 ] 4+ system [5][6][7][8], are among the most frequently reported compounds. Recently, we could enlarge the family of binuclear systems showing multiple SCO steps with reports on thiadiazole and oxadiazole complexes for which the cooperativity was studied in detail [9,10].…”

“…This family of compounds shows the attempt and endeavor to explore the famous 1D coordination polymers of general formula [Fe(Rtrz) 3 ](X) 2 (Rtrz = 4-subsitituted-1,2,4-triazole, X − = standard monoanions): some of which displayed wide thermal hysteresis loops around room temperature [11][12][13][14][15][16]. Compared to the typical molecular system mentioned above [1][2][3][4][5][6][7][8][9][10], the triazole-based compounds bear benefits of expanding the discrete nuclearities to multinuclear or 1D polymeric complexes, which is of vital importance…”