1H NMR spectroscopic elucidation in solution of the kinetics and thermodynamics of spin crossover for an exceptionally robust Fe2+ complex

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) ¹H 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 ¹H NMR spectroscopy. An alternative scheme using a linear correction term C¹ 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 ¹H 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·(CH₃CN) composed of a 1 : 1 mixture of the Zn and Fe complexes with inverse chirality.