Diamagnetic Transition Metal Complexes Research Articles

Characterization of Reactive Organometallic Species via MicroED.

Here we apply microcrystal electron diffraction (MicroED) to the structural determination of transition-metal complexes. We find that the simultaneous use of 300 keV electrons, very low electron doses, and an ultrasensitive camera allows for the collection of data without cryogenic cooling of the stage. This technique reveals the first crystal structures of the classic zirconocene hydride, colloquially known as “Schwartz’s reagent”, a novel Pd(II) complex not amenable to solution-state NMR or X-ray crystallography, and five other paramagnetic and diamagnetic transition-metal complexes.

H2 Binding, Splitting, and Net Hydrogen Atom Transfer at a Paramagnetic Iron Complex.

While diamagnetic transition metal complexes that bind and split H2 have been extensively studied, paramagnetic complexes that exhibit this behavior remain rare. The square planar S = 1/2 FeI(P4N2)+ cation (FeI+) reversibly binds H2/D2 in solution, exhibiting an inverse equilibrium isotope effect of KH2/ KD2 = 0.58(4) at -5.0 °C. In the presence of excess H2, the dihydrogen complex FeI(H2)+ cleaves H2 at 25 °C in a net hydrogen atom transfer reaction, producing the dihydrogen-hydride trans-FeII(H)(H2)+. The proposed mechanism of H2 splitting involves both intra- and intermolecular steps, resulting in a mixed first- and second-order rate law with respect to initial [FeI+]. The key intermediate is a paramagnetic dihydride complex, trans-FeIII(H)2+, whose weak FeIII-H bond dissociation free energy (calculated BDFE = 44 kcal/mol) leads to bimetallic H-H homolysis, generating trans-FeII(H)(H2)+. Reaction kinetics, thermodynamics, electrochemistry, EPR spectroscopy, and DFT calculations support the proposed mechanism.

An EPR Study of Two-Electron Sensitization by Fragmentable Electron Donors

Fragmentable electron donors (FEDs) are molecules designed to undergo bond fragmentation after capturing the hole created by photoexposure of silver halide. By design, the radical remaining after fragmentation is a potent reductant expected to be capable of injecting an electron into the conduction band of the silver halide. Thus, the addition of a FED to the AgX surface should allow the creation of two electrons for each photon absorbed by the substrate. Photographic studies have confirmed that the addition of FEDs can increase the photosensitivity of AgX materials. In this work, EPR spectroscopy was employed to study the processes of hole capture, donor fragmentation, and secondary electron injection by FEDs in AgBr dispersions. To do so, we used AgBr microcrystals doped with diamagnetic transition metal complexes that act as deep electron traps. For samples exposed to actinic light at 15 K, secondary electron injection was detected as an increase in the EPR signal from electrons trapped at the dopant upon annealing the samples above 50 K. Organic radical intermediates and self-trapped hole centers were the other paramagnetic species monitored in this study. The results presented here confirm that the FED sensitization mechanisms originally proposed by Gould et al. take place at silver halide surfaces and result in additional electrons in the silver halide conduction band.

On the absence of strong ESCA satellites in diamagnetic transition metal complexes. a study of some octahedral cyanides

Abstract The electronic structure of four octahedral transition metal cyanides is studied by the multiple scattering molecular orbital method. It is found that the charge relaxations at ionization from the inner shells of the transition metal ions are small and do not give rise to any strong satellites in the ESCA spectrum. It is suggested that the absence of satellites for a number of other diamagnetic complexes can be similarly explained.

Mass spectrometric study of paramagnetic and diamagnetic transition metal complexes

The mass spectra and the ionization potentials of several paramagnetic and diamagnetic transition metal complexes of the type M(NO) 2HaIPR 3 [M = Co, Fe; Hal = Br, I; PR 3 = P( nC 4H 9) 3, P( iOC 3H 7) 3, P(C 6H 5) 3] have been measured. Both the mass spectra and the ionization potentials of these two classes of complexes are very similar to each other. The mass spectral results are interpreted in terms of the driving forces of the ionization-fragmentation processes. From the ionization potential values, information on the nature of the last occupied orbitals of these complexes are obtained.