NDLI: $^{1}H$ NMR spectroscopic elucidation in solution of the kinetics and thermodynamics of spin crossover for an exceptionally robust $Fe^{2+}$ complex

Content Provider	World Health Organization (WHO)-Global Index Medicus
Author	Schaarschmidt, Dieter Hörner, Gerald Rüffer, Tobias Petzold, Holm Speck, J. Matthäus Djomgoue, Paul
Description	Author Affiliation: Petzold H ( TU Chemnitz, Institut für Chemie, Anorganische Chemie, Straße der Nationen 62, 09111 Chemnitz, Germany. holm.petzold@chemie.tu-chemnitz.de.)
Abstract	A series of $Fe^{2+}$ spin crossover (SCO) complexes $[Fe(5/6)]^{2+}$ employing hexadentate ligands (5/6) with cis/trans-1,2-diamino cyclohexanes (4) as central building blocks were synthesised. The ligands were obtained by reductive amination of 4 with 2,2′-bipyridyl-6-carbaldehyde or 1,10-phenanthroline-2-carbaldehyde 3. The chelating effect and the rigid structure of the ligands 5/6 lead to exceptionally robust $Fe^{2+}$ and $Zn^{2+}$ complexes conserving their structure even in coordinating solvents like dmso at high temperatures. Their solution behavior was investigated using variable temperature (VT) $^{1}H$ NMR spectroscopy and VT Vis spectroscopy. SCO behavior was found for all $Fe^{2+}$ complexes in this series centred around and far above room temperature. For the first time we have demonstrated that the thermodynamics as well as kinetics for SCO can be deduced by using VT $^{1}H$ NMR spectroscopy. An alternative scheme using a linear correction term $C^{1}$ to model chemical shifts for $Fe^{2+}$ SCO complexes is presented. The rate constant for the SCO of $[Fe(rac-trans-5)]^{2+}$ obtained by VT $^{1}H$ NMR was validated by Laser Flash Photolysis (LFP), with excellent agreement $(1/(k_{HL}$ + $k_{LH})$ = 33.7/35.8 ns for NMR/LFP). The solvent dependence of the transition temperature $T_{1/2}$ and the solvatochromism of complex $[Fe(rac-trans-5)]^{2+}$ were ascribed to hydrogen bond formation of the secondary amine to the solvent. Enantiomerically pure complexes can be prepared starting with R,R- or S,S-1,2-diaminocyclohexane (R,R-trans-4 or S,S-trans-4). The high robustness of the complexes reduces a possible ligand scrambling and allows preparation of quasiracemic crystals of $[Zn(R,R-5)][Fe(S,S-5)](ClO_{4})_{4}·(CH_{3}CN)$ composed of a 1:1 mixture of the Zn and Fe complexes with inverse chirality.
ISSN	14779226
Issue Number	35
Journal	Dalton Trans.
Volume Number	45
e-ISSN	13645447
Language	English
Publisher	Royal Society of Chemistry
Publisher Date	2016-09-21
Publisher Place	Great Britain (UK)
Access Restriction	Subscribed
Subject Keyword	Chemistry
Content Type	Text
Resource Type	Article
Subject	Inorganic Chemistry

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