Extension of Azine-Triazole Synthesis to Azole-Triazoles Reduces Ligand Field, Leading to Spin Crossover in Tris-L Fe(II)

Abstract

The first examples of azole-triazole Rat ligands, bidentate L 4NMeIm (3-(1-methyl-1H-imidazol-4-yl)-5-phenyl-4-(p-tolyl)-4H-1,2,4-triazole) and L 4SIm (4-(5-phenyl-4-(ptolyl)-4H-1,2,4-triazol-3-yl)thiazole), have been prepared, by extension of the general synthesis used to access many examples of azine-triazoles. The tris-L Fe II complexes of the azine-triazoles are consistently low spin (LS). As intended, these new azole-triazole ligands provide lower field strengths, resulting in high-spin (HS) [Fe II (L 4NMeIm ) 3 ](BF 4 ) 2 (1•4H 2 O) and spin crossover (SCO) active [Fe II ( L 4SIm ) 3 ](BF 4 ) 2 (2•0.5H 2 O). Single-crystal structure determinations revealed that at 100 K 1• solvents is HS whereas 2•solvents is LS. Solid-state variable temperature magnetic studies of air-dried crystals showed that the methylimidazole-triazole complex 1•4H 2 O remains HS while the thiazole-triazole complex 2•0.5H 2 O undergoes a two-step gradual SCO (T 1/2 approximately 275 and 350 K). Variable-temperature Evans method NMR studies of 2, in five different solvents (CD 3 NO 2 , CD 3 CN, CD 3 COCD 3 , CD 2 Cl 2 , and CDCl 3 ) gave T 1/2 values in a relatively narrow range, 214−259 K. These T 1/2 values did not correlate with the solvent polarity index P′ (R 2 = 0.25) but did correlate with the solvent basicity parameter SB (R 2 = 0.90). Variable-temperature UV−vis studies on a golden yellow CH 3 CN solution of 2, with monitoring of the d−d transition at 530 nm (ε = 39 L mol −1 cm −1 at 293 K) while the solution was heated from 253 to 303 K, showed that the high-spin fraction increased from 0.51 to 0.77. Cyclic voltammetry studies in CH 3 CN revealed a Fe(III)/Fe(II) redox process that was reversible for 1 and irreversible for 2, with significant tuning of the E pa value: the methylimidazole-triazole complex 1 is significantly easier to oxidize (0.46 V) than the thiazole-triazole complex 2 (0.68 V; both vs 0.01 M Ag/AgNO 3 ).

Tuning spin-crossover transition temperatures in non-symmetrical homoleptic meridional/facial [Fe(didentate)3]2+complexes: what for and who cares about it?

“…The non-linear least-square fits of the experimental paramagnetic data depicted in Fig. 6 16,61,[97][98][99][100][101][102] One notes that C HS > 3.00 cm 3 K mol −1 (spin only value) can be taken as the signature of some partially unquenched orbital momentum in this pseudo-octahedral environment. The thermodynamic parameters 19.5 ≤ ΔH SCO ≤ 23 kJ mol −1 and 71 ≤ ΔS SCO ≤ 83 J mol −1 K −1 measured for [Fe(L6) 3 ] 2+ and [Fe(L7) 3 ] 2+ also match the usual range found for similar complexes in solution.…”

Correlations between ligand field Δo, spin crossover T1/2 and redox potential Epa in a family of five dinuclear helicates

“…Recently, we reported that our general synthetic strategy for accessing Rdpt ligands 38,56 could be extended to access the first examples of ditopic azine-triazole analogues 57 and of azole-triazole monotopic Rat ligands. 58 The first four ditopic azine-triazole ligands prepared, L npym-meta and L npym-para (Figure 2), 57 featured n-pyrimidine-triazole binding sites linked by either meta-(for helicates) or para-(for tetrahedral cages) substituted phenylene linkers. All four of the structurally characterised iron(II) complexes, both helicates and cages, of these first ditopic Rat ligand systems were found to be LS.…”

“…With this goal in mind, very recently, we reported the first examples of monotopic azoletriazole L 4NMe/SIm ligands (Figure 2), which, as expected, imposed a weaker ligand field: [Fe(L 4NMeIm ) 3 ](BF 4 ) 2 is HS whereas [Fe(L 4SIm ) 3 ](BF 4 ) 2 is SCO-active in both the solid state and in MeCN solution. 58 The focus herein is on the development of a new family of robust yet easily modified ligands for the assembly of SCOactive triply bridged iron(II) dinuclear helicates. Hence the ditopic versions of these weaker fields azole-triazole ligands (L 4NMe/SIm ) are prepared, using a meta-phenylene linker, to enable access to a new class of SCO-active [Fe II 2 L 3 ] 4+ dinuclear helicates.…”

“…2 ), which, as expected, imposed a weaker ligand field: [Fe( L4NMeIm ) 3 ](BF 4 ) 2 is HS whereas [Fe( L4SIm ) 3 ](BF 4 ) 2 is SCO-active in both the solid state and in MeCN solution. 58 …”

Enhancing Magnetic Cooperativity in Fe(II) Triazole‐based Spin‐crossover Nanoparticles by Pluronic Matrix Confinement

“…Among the known SCO molecules reported in literature, polymeric one‐dimensional (1D) triazole‐based Fe II materials offer synthetic flexibility in design, such as the easy engineering of robust chains by virtue of the N1,N2‐triazole bridges (Figure a) linking the metal cations. Furthermore, it has been well validated in literature the possibility to add various functionalities on the N4‐triazole site (e. g. with −NH 2 , ‐alkyl, ‐ethylpropionate, ‐benzamido, ‐metoxyethyl, ‐hydroxyalkyl groups) as well as on the C3,C5 positions of the triazole ring, and to obtain 1D polymeric complexes in which the triazole rings can be present in the protonated (HTrz) or deprotonated, negatively charged forms (Trz) . Triazole‐based SCO materials have been obtained in diverse morphologies, such as nanoparticles, nanotubes and nanorods, they have been dispersed in composite resins, in organic and inorganic matrixes, such as mesoporous silica, and isotactic polystyrene matrix .…”