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Main Title: 1H NMR spectroscopic elucidation in solution of the kinetics and thermodynamics of spin crossover for an exceptionally robust Fe2+ complex
Author(s): Petzold, Holm
Djomgoue, Paul
Hörner, Gerald
Speck, J. Matthäus
Rüffer, Tobias
Schaarschmidt, Dieter
Type: Article
Language Code: en
Abstract: A series of Fe2+ 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 Fe2+ and Zn2+ complexes conserving their structure even in coordinating solvents like dmso at high temperatures. Their solution behavior was investigated using variable temperature (VT) H-1 NMR spectroscopy and VT Vis spectroscopy. SCO behavior was found for all Fe2+ 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 H-1 NMR spectroscopy. An alternative scheme using a linear correction term C-1 to model chemical shifts for Fe2+ SCO complexes is presented. The rate constant for the SCO of [Fe(rac-trans-5)](2+) obtained by VT H-1 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)](ClO4)(4)center dot(CH3CN) composed of a 1 : 1 mixture of the Zn and Fe complexes with inverse chirality.
Issue Date: 2016
Date Available: 24-Oct-2017
DDC Class: 540 Chemie und zugeordnete Wissenschaften
Sponsor/Funder: DFG, SFB 658, Elementarprozesse in molekularen Schaltern auf Oberflächen
Journal Title: Dalton transactions
Publisher: Royal Society of Chemistry
Publisher Place: Cambridge
Volume: 45
Issue: 35
Publisher DOI: 10.1039/c6dt01895e
Page Start: 13798
Page End: 13809
EISSN: 1477-9234
ISSN: 1477-9226
Appears in Collections:FG Bioanorganische Chemie » Publications

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