Spin Crossover and High-Spin State in Fe(II) Anionic Polymorphs Based on Tripodal Ligands

Abstract

Two new mononuclear Fe(II) polymorphs, [(C2H5)4N]2[Fe(py3C-OEt)(NCS)3]2 (1) and [(C2H5)4N][Fe(py3C-OEt)(NCS)3] (2) (py3C-OEt = tris(pyridin-2yl)ethoxymethane) have been synthesized and characterized by single crystal X-ray diffraction, by magnetic and photomagnetic measurements, and by detailed variable temperature infrared spectroscopy. The molecular structure, in both complexes, is composed by the same anionic [Fe(py3C-OEt)(NCS)3] -complex (two units for 1, and one unit for 2) generated by a coordination, to the Fe(II) metal centre, of one tridentate donor molecules such tetrathiafulvalene (TTF) derivatives for the design of multifunctional systems, such as fluorescent or conducting switchable materials. 6,60 ■ ASSOCIATED CONTENT Supporting informationThe Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX/acs.inorg-chem.XXXXXXX X-ray crystallographic data in CIF format: CCDC numbers 2025933-2025936 (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. Syntheses details and figures S1-S19 (pdf)

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

“…The tris-(pyridin-2-yl)ethoxymethane (py3C-OEt) ligand was prepared as previously described [34][35][36][37]. Compound 3 was obtained as a red polycrystalline powder and as single crystals by mixing a solution of [N(C2H5)4](NCSe) with a solution of FeCl2 and tris(pyridin-2-yl)ethoxymethane at -32 °C (see details in experimental section).…”

“…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].…”