Ralph Hübner

Spectroelectrochemical Evidence for the Nitrosyl Redox Siblings NO+, NO., and NO− Coordinated to a Strongly Electron‐Accepting FeII Porphyrin: DFT Calculations Suggest the Presence of High‐Spin States after Reduction of the FeII–NO− Complex

Experimental and computational results for the electron-deficient porphyrin complex [Fe(NO)(TFPPBr(8))] (1; TFPPBr(8)=2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(pentafluorophenyl)porphyrin) are reported with respect to its electron-transfer behavior. Complex 1 undergoes three one-electron processes: two reversible reductions and one irreversible oxidation. Spectroelectrochemical measurements (IR and UV/Vis/NIR spectroscopy) of (14)NO- and (15)NO-containing material indicate that the first reduction to 1(-) occurs largely on the NO ligand to produce nitroxyl anion (NO(-)) character, as evident from the considerable change in ν(NO) from 1715 to around 1550 cm(-1). The second reduction to 1(2)(-) does not result in a further shift of ν(NO) to lower frequencies, but to a surprising high-energy shift to 1590 cm(-1). This and the notable changes of the characteristic porphyrin vibrations as well as significant changes of the UV/Vis absorptions indicate a porphyrin-centered process; DFT calculations predict the shift of ν(NO) to higher frequencies for the intermediate- and high-spin states of 1(2-). The oxidation of 1 is irreversible on the voltammetry timescale, but chemically reversible in spectroelectrochemical experiments, suggesting that the cationic form dissociates to the corresponding ferric porphyrin and NO. DFT calculations support the interpretation of the experimental results.

Electronic, charge and magnetic interactions in three-centre systems

Three-centre systems [QCuQ]n containing electron transfer-active and donor-substituted o-iminoquinone ligands were synthesised and studied as models for electronic, charge and magnetic coupling within materials involving coordinative bonding. The neutral precursors (n = 0) involve spin-bearing semiquinone radical ions, Q˙−, bridged by paramagnetic copper(II). Crystal structures and electron paramagnetic resonance (EPR) measurements reveal a delicate sensitivity of structures and of spin–spin coupling in these three-centre configurations, depending on weak secondary interactions involving substituents at the coordinatively ambivalent (hemilabile) ligands Q×. Stepwise electron loss or uptake by [QCuQ]n is accompanied by considerable redox potential splitting and by spectroelectrochemically monitored change of UV-Vis-NIR absorption. Whereas the monoanions exhibit long-wavelength ligand-to-ligand intervalence charge transfer bands around 2000 nm for π(Q2−) → π*(Q˙−) transitions, the monocations are formed in a reorganisation step, achieving coordinative saturation at the metal. Weak secondary interactions are thus shown to be sufficient to cause significant structural changes as well as qualitative differences in spin–spin interaction and in excited state structures.

The metal-insulator transition in the organic conductor β″-(BEDT-TTF)2Hg(SCN)2Cl

We explore the nature of the metal-insulator transition in the two-dimensional organic compound β″-(BEDT-TTF)2Hg(SCN)2Cl by x-ray, electrical transport, ESR, Raman, and infrared investigations. Magnetic and vibrational spectroscopy concurrently reveal a gradual dimerization along the stacking direction (a-b), setting in already at the crossover temperature of 150 K from the metallic to the insulating state. A spin gap of Δσ=47 meV is extracted. From the activated resistivity behavior below T = 55 K, a charge gap of Δρ=60 meV is derived. At TCO = 72 K, the C=C vibrational modes reveal the development of a charge-ordered state with a charge disproportionation of 2δρ=0.34e. In addition to a slight structural dimerization, charge-order causes stripes most likely perpendicular to the stacks.