1H NMR spectroscopic elucidation in solution of the kinetics and thermodynamics of spin crossover for an exceptionally robust Fe2+ complex. Issue 35 (10th August 2016)

• Record Type: Journal Article
• Authors: Petzold, Holm; Djomgoue, Paul; Hörner, Gerald; Speck, J. Matthäus; Rüffer, Tobias; Schaarschmidt, Dieter
• Is Part Of: Dalton transactions. Volume 45:Issue 35(2016)
• Page Start: 13798
• Page End: 13809
• Publication Date: 2016-08-10
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/c6dt01895e
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : A series of exceptionally robust Fe 2+ spin crossover complexes has been synthesized and investigated by 1 H NMR spectroscopy in solution. 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 of4 with 2, 2′-bipyridyl-6-carbaldehyde or 1, 10-phenanthroline-2-carbaldehyde3 . The chelating effect and the rigid structure of the ligands5 /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 )](ClO4 )4 ·(CH3 CN) composed of a 1 : 1 mixture of the Zn and Fe complexes with inverse chirality.