Spin Cross-Over (SCO) Anionic Fe(II) Complexes Based on the Tripodal Ligand Tris(2-pyridyl)ethoxymethane

Abstract

Reactions of Fe(II) with the tripodal chelating ligand 1,1,1-tris(2-pyridyl)ethoxymethane (py3C-OEt) and (NCE)− co-ligands (E = S, Se, BH3) give a series of mononuclear complexes formulated as [Fe(py3C-OEt)2][Fe(py3C-OEt)(NCE)3]2·2CH3CN, with E = S (1) and BH3 (2). These compounds are the first Fe(II) spin cross-over (SCO) complexes based on the tripodal ligand tris(2-pyridyl)ethoxymethane and on the versatile co-ligands (NCS)− and (NCBH3)−. The crystal structure reveals discrete monomeric isomorph structures formed by a cationic [Fe(py3C-OEt)2]2+ complex and by two equivalent anionic [Fe(py3C-OEt)(NCE)3]− complexes. In the cations the Fe(II) is facially coordinated by two py3C-OEt tripodal ligands whereas in the anion the three nitrogen atoms of the tripodal ligand are facially coordinated and the N-donor atoms of the three (NCE)− co-ligands occupy the remaining three positions to complete the distorted octahedral environment of the Fe(II) centre. The magnetic studies show the presence of gradual SCO for both complexes: A one-step transition around 205 K for 1 and a two-step transition for compound 2, centered around 245 K and 380 K.

Solvent-Induced Hysteresis Loop in Anionic Spin Crossover (SCO) Isomorph Complexes

“…The spin crossover (SCO) complexes are by far the most studied molecular systems among switchable systems during the last decade thanks to their many potential applications, in particular for the development of new generations of electronic devices such as displays [1][2][3][4], memory devices [4][5][6][7][8], sensors [9][10][11][12][13][14], and Organic Light-Emitting Diodes (OLED) [15]. Although the SCO phenomenon can be essentially observed in octahedral complexes based on metal ions allowing spin state changes between the high spin (HS) and low spin (LS) states under external stimulus such as temperature, pressure, light irradiation or magnetic field, those based on Fe(II) ion exhibiting d 6 electronic configuration remain the most studied systems [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. Nevertheless, such complexes are mostly either cationic or neutral, and the Fe(II) anionic complexes exhibiting SCO behavior are relatively very scarcely reported [27][28][29][30][31][32]…”

“…Although the SCO phenomenon can be essentially observed in octahedral complexes based on metal ions allowing spin state changes between the high spin (HS) and low spin (LS) states under external stimulus such as temperature, pressure, light irradiation or magnetic field, those based on Fe(II) ion exhibiting d 6 electronic configuration remain the most studied systems [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. Nevertheless, such complexes are mostly either cationic or neutral, and the Fe(II) anionic complexes exhibiting SCO behavior are relatively very scarcely reported [27][28][29][30][31][32][33][34][35]. Furthermore, the few anionic SCO examples are restricted to only three different systems.…”

“…The second one consists of the trinuclear [Fe II 3(μ-L)6(H2O)6] 6− complex involving the 4-(1,2,4-triazol-4-yl)ethanedisulfonate anion (L 2− ) [28], that displays a HS-HS-HS to HS-LS-HS transition above room temperature and a relatively large hysteresis loop (> 85 K). The last system concerns the series of mononuclear complexes based on the tripodal functionalized tris(2-pyridyl)methane (py3C-R, R = CnH2n+1, aryl group, O-CnH2n+1, O-aryl, O-CO-CnH2n+1) ligands (Scheme 1a) [29][30][31][32][33][34][35]. Such complexes, of general formula {A[Fe((py3C-R)(NCE)3)]m} (A = [(CnH2n+1)4N] + , [Fe(py3C-R)2] 2+ , E = S, Se, BH3), involve the [Fe((py3C-R)(NCE)3)] -anion generated by an Fe(II) metal centre, one py3C-R chelating tripodal ligand and three terminal N-SCE terminal coligands Scheme 1b).…”

“…Such complexes, of general formula {A[Fe((py3C-R)(NCE)3)]m} (A = [(CnH2n+1)4N] + , [Fe(py3C-R)2] 2+ , E = S, Se, BH3), involve the [Fe((py3C-R)(NCE)3)] -anion generated by an Fe(II) metal centre, one py3C-R chelating tripodal ligand and three terminal N-SCE terminal coligands Scheme 1b). The different studies, reported essentially by Ishida et al and some of us [29][30][31][32][33][34][35], concern essentially the study of the effect of the cationic counter-ion or the functional R group linked to the tripodal py3C motif, on the SCO characteristics such as the transition temperatures and the cooperative effects. More recently, some of us extended such effects to the effect of the crystal packing through the design of a series of polymorph complexes [35].…”

“…More recently, some of us extended such effects to the effect of the crystal packing through the design of a series of polymorph complexes [35]. , as counter-ion [34]. At the same time the NCSe − analogue complex (E = Se), that completes such isomorphic series, has been also prepared.…”