Dalton Transactions COMMUNICATION Cite this: Dalton Trans., 2019, 48, 3825 Received 14th December 2018, Accepted 14th February 2019 DOI: 10.1039/c8dt04928a rsc.li/dalton Bi-stable spin-crossover characteristics of a highly distorted [Fe(1-BPP-COOC 2 H 5 ) 2 ](ClO 4 ) 2 ·CH 3 CN complex† Kuppusamy Senthil Kumar, * a Benoît Heinrich, a Sergi Vela, b Eufemio Moreno-Pineda, c Corinne Bailly d and Mario Ruben * a,c A highly distorted high spin Fe(II)-complex, [Fe(1-BPP-COOC 2 H 5 ) 2 ] (ClO 4 ) 2 ·CH 3 CN, with a trans-N( pyridine)–Fe–N(pyridine) angle (ϕ) of 158.83(17)° showed lattice solvent dependent bi-stable spin- state switching characteristics with T 1/2 = ca. 233 K and a high thermal hysteresis width (ΔT ) of 101 K, for the first cooling and heating cycle, unprecedented for the [Fe(BPP) 2 ] 2+ series of complexes; the results presented in this study are fundamentally important and have implications towards the realization of device architectures based on bi-stable SCO complexes. Mononuclear Fe(II)-complexes capable of undergoing spin- crossover (SCO) from the diamagnetic low-spin (LS) to para- magnetic high-spin (HS) state are suitable candidates for the development of molecule-based memory and switching architectures. 1–6 To be optimal candidates for applications, SCO systems should show abrupt and hysteretic SCO. 1,3,5,7,8 The complexes showing such a cooperative and hysteretic spin state switching are termed bi-stable, due to the presence of a hysteresis loop conferring memory effect. 9 In this context, several mononuclear and polymeric SCO complexes are reported to exhibit spin-state switching with a technologically relevant thermal hysteresis width, ΔT ≥ 45 K. 10 Notably absent in the list of mononuclear Fe(II)-complexes showing ΔT ≥ 45 K are prototypical [Fe(1-bpp-R) 2 ](X) 2 , R = functional groups 11–15 and X = BF 4 - , ClO 4 - , etc., complexes comprised of the 2,6-di ( pyrazol-1-yl)pyridine (1-bpp) family of ligands. 16,17 This family of complexes is one of the most studied SCO systems due to their propensity to undergo facile spin-state switching owing to the moderate ligand field strength of 1-bpp ligands. The parent [Fe(1-bpp-R) 2 ](BF 4 ) 2 complex, R = H, is reported to show SCO around 259 K with a narrow hysteresis loop of 3 K. 18 Further derivatization of the parent [Fe(1-bpp-R) 2 ](X) 2 skeleton with various functional groups at the 4-position of the central pyridyl ring resulted in a range of [Fe(1-bpp-R) 2 ](X) 2 systems exhibiting different switching temperatures and narrow thermal hysteresis widths. Thermal hysteresis widths (ΔT ) of 18 K and 35 K reported for the [Fe(1-bpp-CH 3 ) 2 ](ClO 4 ) 2 (T 1/2 = 184 K) 19 and [Fe(1-bpp-CH 2 Br) 2 ](ClO 4 ) 2 (T 1/2 = ca. 340 K) 20 complexes, respectively, are the highest values reported so far. From the structural view point, the HS [Fe(1-bpp-R) 2 ](X) 2 complexes are prone to Jahn–Teller (J–T) distortion. 21 The dis- torted [Fe(1-bpp-R) 2 ](X) 2 complexes exhibit (i) a reduced trans- N{pyridyl}–Fe–N{pyridyl} angle (ϕ) below 180° (cf. Chart 1) and (ii) twisting of the tridentate 1-bpp ligands from the ideal per- pendicular arrangement leading to a reduction in the dihedral angle (θ) from its ideal value of 90°. 16,19,22 It is observed that [Fe(1-bpp-R) 2 ](X) 2 complexes exhibiting too pronounced distortions from the ideal octahedral geome- try are trapped in the HS state. This is due to the rigid lattice blocking the large structural reorganizations required by the distorted HS system to attain an undistorted/less-distorted LS state. A critical structure–property analysis of [Fe(1-bpp) 2 ](X) 2 complexes in 2009 led to the conclusion that the complexes featuring the values of ϕ < 172° and θ < 76° are prone to be trapped in the HS state. 16 The recently reported [Fe(1-bpp- CH 3 ) 2 ](ClO 4 ) 2 complex by Halcrow and co-workers is a notable exception of this conclusion, and is one of the rare examples of highly distorted [Fe(1-bpp-R) 2 ](X) 2 complex systems (ϕ = 163.7(2)° at 240 K) undergoing SCO. 19 In this contribution, we report abrupt and hysteretic SCO exhibited by a highly distorted [Fe(1-bpp-COOC 2 H 5 ) 2 ] † Electronic supplementary information (ESI) available: Synthesis and character- ization details of the complex, thermogravimetric and differential scanning calorimetric (DSC) analyses, first derivatives of the χT vs. T plots, and crystallo- graphic parameters of the complex. CCDC 1589139. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt04928a a Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS-Université de Strasbourg, 23, rue du Loess, BP 43, 67034 Strasbourg cedex 2, France. E-mail: senthil.kuppusamy@ipcms.unistra.fr b Laboratoire de Chimie Quantique, UMR 7111, CNRS-Université de Strasbourg, 4 rue Blaise Pascal, F-67000 Strasbourg, France c Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. E-mail: mario.ruben@kit.edu d Service de Radiocristallographie, Fédération de Chimie Le Bel FR2010 CNRS-Université de Strasbourg, 1 rue Blaise Pascal, BP 296/R8, 67008 Strasbourg cedex, France This journal is © The Royal Society of Chemistry 2019 Dalton Trans. , 2019, 48, 3825–3830 | 3825 Published on 14 February 2019. Downloaded by KIT Library on 7/29/2019 3:06:35 PM. View Article Online View Journal | View Issue