Showing papers on "Spectrochemical series published in 2017"

Probing the Effects of Ligand Field and Coordination Geometry on Magnetic Anisotropy of Pentacoordinate Cobalt(II) Single-Ion Magnets.

Abstract: In this work, the effects of ligand field strength as well as the metal coordination geometry on magnetic anisotropy of pentacoordinated CoII complexes have been investigated using a combined experimental and theoretical approach. For that, a strategic design and synthesis of three pentacoordinate CoII complexes [Co(bbp)Cl2]·(MeOH) (1), [Co(bbp)Br2]·(MeOH) (2), and [Co(bbp)(NCS)2] (3) has been achieved by using the tridentate coordination environment of the ligand in conjunction with the accommodating terminal ligands (i.e., chloride, bromide, and thiocyanate). Detailed magnetic studies disclose the occurrence of slow magnetic relaxation behavior of CoII centers with an easy-plane magnetic anisotropy. A quantitative estimation of ZFS parameters has been successfully performed by density functional theory (DFT) calculations. Both the sign and magnitude of ZFS parameters are prophesied well by this DFT method. The theoretical results also reveal that the α → β (SOMO–SOMO) excitation contributes almost entir...

The effect of K+ cations on the phase transitions, and structural, dielectric and luminescence properties of [cat][K0.5Cr0.5(HCOO)3], where cat is protonated dimethylamine or ethylamine

Abstract: We report the synthesis, crystal structure, and dielectric, vibrational and emission spectra of two novel heterometallic perovskite-type metal–organic frameworks (MOFs) of the following formula: [(CH3)2NH2][K0.5Cr0.5(HCOO)3] (DMAKCr) and [C2H5NH3][K0.5Cr0.5(HCOO)3] (EtAKCr). DMAKCr crystallizes in a trigonal structure (R space group) and undergoes an order–disorder phase transition to the monoclinic system (P space group) at about 190 K. The dielectric studies confirm the presence of first-order relaxor-like structural transformation. In the high-temperature phase, the dimethylammonium cations are dynamically disordered over three equal positions and upon cooling the dynamical disorder evolves into a two-fold one. This partial ordering is accompanied by a small distortion of the metal–formate framework. EtAKCr crystallizes in a monoclinic structure (P21/n space group) with ordered EtA+ cations and does not experience any phase transition. The differences in the thermal behavior caused by the substitution of Na+ ions by larger K+ ions in the [cat]MIMIII (cat = DMA+, EtA+, MI = Na+, K+ and MIII = Cr3+ and Fe3+) heterometallic MOF family are discussed taking into account the impact of the hydrogen bond (HB) pattern and other factors affecting the stability of metal–formate frameworks. The optical studies show that DMANaCr and EtAKCr exhibit Cr3+-based emission characteristics for intermediate ligand field strength.

Heteroleptic iron(III) Schiff base spin crossover complexes: halogen substitution, solvent loss and crystallite size effects

Abstract: The influence of the halogen substituent on the qsal moiety of iron(III) heteroleptic compounds with the formulae [Fe(qsal-X)(thsa)]·nMeCN, where qsal-X− = X-substituted quinolylsalicylaldimine; thsa2− = thiosemicarbazone-salicylaldiminate; X = F; n = 2.5, 1·2.5MeCN and X = Cl 2, Br 3 and I 4, n = 1 (labelled 2·MeCN, 3·MeCN and 4·MeCN, respectively) has been systematically investigated. Magnetic studies on solid samples show incomplete spin crossover in 1–3 which can be related to MeCN solvent loss. Complex 4·MeCN remains fully LS up to 360 K. Single crystals have been examined at variable temperatures for samples possessing different degrees of solvation. Intermolecular C–X⋯H interactions are present for X = F, Cl and Br while a C–I⋯π interaction is uniquely observed in 4·MeCN. These preferential interactions result in different supramolecular packings of the various halogen substituted compounds. However, as the LS stability increases from F to I, the ligand field strength is then also suggested to increase from F to I. Consequently, in this family, the electronic structure resulting from halogen variation is believed to influence the magnetic properties more than crystal packing effects. Mossbauer spectra, at variable temperatures, confirm the presence of Fe(III) and the magnetic properties in these compounds. The effect of different drying methods as well as the crystal/powder effect on the magnetic properties are discussed in the case of 2·MeCN.

Synthesis and Characterization of Iron(II) Complexes with a BPMEN‐Type Ligand Bearing π‐Accepting Nitro Groups

Abstract: In this paper, we describe the synthesis, characterization and stability studies of new FeII complexes prepared with ligand N,N'-dimethyl-N,N'-bis-(4-nitro-pyridin-2-ylmethyl)-ethane-1,2-diamine (L₄²NO₂). The properties of these complexes, probed by magnetic, spectroscopic and electrochemical techniques, are compared to those of the parent complexes prepared with the unsubstituted ligand N,N'-dimethyl-N,N'-bis-(pyridin-2-ylmethyl)-ethane-1,2-diamine (BPMEN or L₄²). The para-functionalization of the pyridine moieties with a π-accepting nitro group increases the ligand field strength and the Lewis acidity of the metal center at the same time, without altering the complex stability.

Partial density of states ligand field theory (PDOS-LFT): Recovering a LFT-like picture and application to photoproperties of ruthenium(II) polypyridine complexes

Abstract: Gas phase density-functional theory (DFT) and time-dependent DFT (TD-DFT) calculations are reported for a data base of 98 ruthenium(II) polypyridine complexes. Comparison with X-ray crystal geometries and with experimental absorption spectra measured in solution show an excellent linear correlation with the results of the gas phase calculations. Comparing this with the usual chemical understanding based upon ligand field theory (LFT) is complicated by the large number of molecular orbitals present and especially by the heavy mixing of the antibonding metal e g * orbitals with ligand orbitals. Nevertheless, we show that a deeper understanding can be obtained by a partial density-of-states (PDOS) analysis which allows us to extract approximate metal t2g and e g * and ligand π* orbital energies in a well-defined way, thus providing a PDOS analogue of LFT (PDOS-LFT). Not only do PDOS-LFT energies generate a spectrochemical series for the ligands, but orbital energy differences provide good estimates of TD-DFT absorption energies. Encouraged by this success, we use frontier-molecular-orbital-theory-like reasoning to construct a model which allows us in most, but not all, of the cases studied to use PDOS-LFT energies to provide a semiquantitative relationship between luminescence lifetimes at room temperature and liquid nitrogen temperature.