Issue 26, 2024

Lattice solvent- and substituent-dependent spin-crossover in isomeric iron(ii) complexes

Abstract

Spin-state switching in iron(II) complexes composed of ligands featuring moderate ligand-field strength—for example, 2,6-bi(1H-pyrazol-1-yl)pyridine (BPP)—is dependent on many factors. Herein, we show that spin-state switching in isomeric iron(II) complexes composed of BPP-based ligands—ethyl 2,6-bis(1H-pyrazol-1-yl)isonicotinate (BPP-COOEt, L1) and (2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)methylacetate (BPP-CH2OCOMe, L2)—is dependent on the nature of the substituent at the BPP skeleton. Bi-stable spin-state switching—with a thermal hysteresis width (ΔT1/2) of 44 K and switching temperature (T1/2) = 298 K in the first cycle—is observed for complex 1·CH3CN composed of L1 and BF4 counter anions. Conversely, the solvent-free isomeric counterpart of 1·CH3CN—complex 2a, composed of L2 and BF4 counter anions—was trapped in the high-spin (HS) state. For one of the polymorphs of complex 2b·CH3CN—2b·CH3CN-Y, Y denotes yellow colour of the crystals—composed of L2 and ClO4 counter anions, a gradual and non-hysteretic SCO is observed with T1/2 = 234 K. Complexes 1·CH3CN and 2b·CH3CN-Y also underwent light-induced spin-state switching at 5 K due to the light-induced excited spin-state trapping (LIESST) effect. Structures of the low-spin (LS) and HS forms of complex 1·CH3CN revealed that spin-state switching goes hand-in-hand with pronounced distortion of the trans-N{pyridyl}-Fe–N{pyridyl} angle (ϕ), whereas such distortion is not observed for 2b·CH3CN-Y. This observation points that distortion is one of the factors making the spin-state switching of 1·CH3CN hysteretic in the solid state. The observation of bi-stable spin-state switching with T1/2 centred at room temperature for 1·CH3CN indicates that technologically relevant spin-state switching profiles based on mononuclear iron(II) complexes can be obtained.

Graphical abstract: Lattice solvent- and substituent-dependent spin-crossover in isomeric iron(ii) complexes

Supplementary files

Article information

Article type
Paper
Submitted
14 Feb 2024
Accepted
15 May 2024
First published
22 May 2024
This article is Open Access
Creative Commons BY license

Dalton Trans., 2024,53, 10851-10865

Lattice solvent- and substituent-dependent spin-crossover in isomeric iron(II) complexes

S. K. Kuppusamy, A. Mizuno, L. Kämmerer, S. Salamon, B. Heinrich, C. Bailly, I. Šalitroš, H. Wende and M. Ruben, Dalton Trans., 2024, 53, 10851 DOI: 10.1039/D4DT00429A

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