Bis-meridional Fe2+ spincrossover complexes of phenyl and pyridyl substituted 2-(pyridin-2-yl)-1,10-phenanthrolines.

A series of Fe2+ SCO complexes of substituted 2-(pyridin-2-yl)-1,10-phenanthrolines 2 was prepared and the SCO (spincrossover) properties were characterized in the solid state (X-ray crystallography, SQUID magnetometry) and in solution (VT-1H NMR spectroscopy), augmented by theoretical modelling. Bis-meridional coordination of the tridendate 2a-c and tetradentate 2d ligands gives octahedral and distorted trigonal-dodecahedral complexes [Fe(2)2]2+, respectively, which were identified as SCO complexes with the transition temperature T1/2 below room temperature. SCO in the solid state is limited to bromo-substituted [Fe(2a)2]2+ (Dalton Trans., 2017, 46, 6218-6229) and [Fe(2b)2]2+ with a pyridine-appended phenyl group, whereas solution state NMR studies reveal SCO behaviour for all complexes, which is in agreement with DFT derived results. As anticipated from its N6(+2) coordination in the HS state, DFT structure modelling of [Fe(2d)2]2+ identified deviation from a structure-conserving SCO reaction coordinate; that is, Fe-N breathing is accompanied by a change in the coordination number. Accordingly, a remarkably slow SCO is observed in [Fe(2d)2]2+, owing to an extended coordinate. De-novo defined characteristic temperatures T(τHSLS) are introduced as structure-dependent parameters deemed to define the onset of phenomenological "slow" SCO. The rich phenomenology of the NMR spectra of [Fe(2)2]2+ is identified to be largely controlled by the dynamics of spin-state exchange and a qualitative illustration of the NMR-reporters of SCO is suggested.

Spin state variability in Fe2+ complexes of substituted (2-(pyridin-2-yl)-1,10-phenanthroline) ligands as versatile terpyridine analogues.

Fe2+ spin crossover complexes [Fe(L)2]2+ (L = 2-(6-R1-pyridin-2-yl)-1,10-phenanthroline with R1 = H, methoxy, bromo, -(1H-pyrazol-1-yl) or L = 2-(3-methoxy-pyridin-2-yl)-1,10-phenanthroline) were prepared. These air stable and durable complexes show SCO behaviour with very different transition temperatures T1/2 ranging from 130 K to 600 K depending on the substitution pattern. The use of 1H NMR spectroscopy to elucidate the thermodynamics and kinetics of SCO in a solution of this series is described in detail. By introduction of an additional pyrazole donor (R1) in the ortho-position to the pyridine, the N6 octahedral coordination sphere is expanded to N8 coordination with a trigonal dodecahedral structure. This leads to a strong stabilization of the high spin state and an increased longitudinal relaxation R1 of the proton spins. The larger R1 values were ascribed to different electronic structures with non-orbital degenerate quintet ground states and a larger energetic separation from the first excited state. These results are also supported by Mössbauer spectroscopy. The N8 coordination sphere stabilizes the complex in the high spin state and no indication for SCO was found. DFT calculations confirmed the experimentally obtained order of T1/2 and allowed the calculation of the complex structure in experimentally non-accessible spin states. Complexes of this series can be oxidized to the Fe3+ complexes in a chemically reversible fashion. Interestingly, the lowest oxidation potential was observed for the N8 coordinated complex.

(1)H NMR spectroscopic elucidation in solution of the kinetics and thermodynamics of spin crossover for an exceptionally robust Fe(2+) complex.

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/(kHL + kLH) = 33.7/35.8 ns for NMR/LFP). The solvent dependence of the transition temperature T1/2 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.

Efficiency and chemical recycling capability of magnetite-rich clay towards Eriochrome Black T remediation in the fixed-bed system.

The present study investigates a fixed-bed column by using magnetite-rich clay from Cameroon as an adsorbent for the removal of the Eriochrome Black T (EBT) from aqueous solution. The effect of various parameters such as bed depths and EBT concentrations was investigated. The Adams-Bohart, Thomas and Yoon-Nelson models were applied to adsorption under varying experimental conditions to predict the breakthrough curves and to evaluate the model parameters of the fixed-bed column that can be useful for the process design. The column regeneration studies were carried out for three different sorption-desorption cycles using HCl or NaOH solution at 1 M. The 1 M HCl solution was found to have the best bed regeneration capacity and the fixed-bed could be reused for several sorption-desorption cycles. The elution efficiency of EBT, 75 mg L(-1) was greater than 99.1% for all three cycles.