Correlations of structural, magnetic, and dielectric properties of undoped and doped CaCu3Ti4O12

The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and x-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T, showing a minor but significant dependence of the dielectric constant on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the Curie-Weiss constant.Comment: 10 pages, 11 figure

Influence of mixed thiolate/thioether versus dithiolate coordination on the accessibility of the uncommon +I and +III oxidation states for the nickel ion: an experimental and computational study

Sulfur-rich nickel metalloenzymes are capable of stabilizing NiI and NiIII oxidation states in catalytically relevant species. In an effort to better understand the structural and electronic features that allow the stabilization of such species, we have investigated the electrochemical properties of two mononuclear N2S2 NiII complexes that differ in their sulfur environment. Complex 1 features aliphatic dithiolate coordination ([NiL], 1), and complex 2I is characterized by mixed thiolate/thioether coordination ([NiLMe]I, 2I). The latter results from the methylation of a single sulfur of 1. The X-ray structure of 2I reveals a distorted square planar geometry around the NiII ion, similar to what was previously reported by us for 1. The electrochemical investigation of 1 and 2+ shows that the addition of a methyl group shifts the potentials of both redox NiII/NiI and NiIII/NiII redox couples by about 0.7 and 0.6 V to more positive values. Through bulk electrolyses, only the mononuclear dithiolate [NiIL]− (1-) and the mixed thiolate/thioether [NiIIILMe]2+ (22+) complexes were generated, and their electronic properties were investigated by UV−vis and EPR spectroscopy. For 1- (NiI, d9 configuration) the EPR data are consistent with a dx2-y2 based singly occupied molecular orbitals (SOMOs). However, DFT calculations suggest that there is also pronounced radical character. This is consistent with the small g-anisotropy observed in the EPR experiments. The spin population (Mulliken analysis) analysis of 1- reveals that the main contribution to the SOMO (64%) is due to the bipyridine unit. Time dependent density functional theory (TD-DFT) calculations attribute the most prominent features observed in the electronic absorption spectrum of 1- to metal to ligand charge transfer (MLCT) transitions. Concerning 22+, the EPR spectrum displays a rhombic signal with gx = 2.236, gy = 2.180, and gz = 2.039 in CH3CN. The giso value is larger than 2.0, which is consistent with metal based oxidation. The unpaired electron (NiIII, d7 configuration) occupies a Ni-dz2 based molecular orbital, consistent with DFT calculations. Nitrogen hyperfine structure is observed as a triplet in the gz component of the EPR spectrum with AN = 51 MHz. This result indicates the coordination of a CH3CN molecule in the axial position. DFT calculations confirm that the presence of a fifth ligand in the coordination sphere of the Ni ion is required for the metal-based oxidation process. Finally, we have shown that 1 exhibits catalytic reductive dehalogenation activity below potentials of −2.00 V versus Fc/Fc+ in CH2Cl2

Structural, spectroscopic and redox properties of a mononuclear Co^II thiolate complex - the reactivity toward S-alkylation: an experimental and theoretical study

The structural, spectroscopic, redox properties and also the reactivity toward S-alkylation of a new mononuclear N2S2 dithiolate CoII complex [CoL] (1), with H2L = 2,2′-(2,2′-bipyridine-6,6′-diyl)bis(1,1-diphenylethanethiol), have been investigated. The X-ray structure of 1 has revealed an unusual distorted square planar geometry for a CoII ion within a thiolate environment. The X-band EPR spectrum of 1 displays a rhombic S = 1/2 signal consistent with a low spin configuration for the d7 CoII ion with a large g-anisotropy (gx = 2.94, gy = 2.32 and gz = 2.01). By pulsed EPR experiments (HYSCORE), two weak hyperfine couplings (hfc) of 3.2 and 2.2 MHz have been measured and attributed respectively to protons and nitrogen nuclei of the bipyridine unit. In addition, another hyperfine coupling (hfc) of 7.5 MHz has been attributed to the cobalt ion. DFT calculations have successfully reproduced the Co and 14N hfc parameters. However, multiconfigurational ab initio calculations were required to predict the g-tensor of 1. The cyclic voltammogram (CV) displays two one-electron metal based processes: a quasi-reversible CoIII/CoII oxidation wave at E1/2 = −0.5 V vs. Fc+/Fc and a quasi-reversible CoII/CoI reduction wave at E1/2 = −1.7 V. 1 reacts with CH3I, generating the mono S-methylated complex, [CoLMeI] (1Me). The X-band EPR spectrum of 1Me displays a typical signal of a high spin (S= 3/2) CoII species. An optimized structure of 1Me, calculated by DFT, is consistent with its EPR and UV-visible spectra. Time dependent density functional theory (TD-DFT) calculations attribute the most prominent features observed in the electronic absorption spectra of 1 and 1Me. The singly occupied MO (SOMO) of 1 shows a notable delocalization of the unpaired electron over the metal (85%) and the ligand, especially over the sulphur atoms (10.5%), indicating a certain degree of covalency for the Co-S bonds. In 1Me, for two of the three SOMOs, the unpaired electron is notably delocalized over the metal (78.5 and 77.6%, respectively) and the ligand (12.5 and 7.8%, respectively over the sulphur of the thiolate function). For the third SOMO, the unpaired electron is mainly localized on the metal (92.2%). There is no electronic density spread on the sulphur atom of the thioether function in any of these SOMOs. The reactivity and the electronic properties of 1 are also compared with those of the analogous [ZnL] and [NiL] complexes

Cobalt(III) tetraaza-macrocyclic complexes as efficient catalyst for photoinduced hydrogen production in water: Theoretical investigation of the electronic structure of the reduced species and mechanistic insight

We recently reported a very efficient homogeneous system for visible-light driven hydrogen production in water based on the cobalt(III) tetraaza-macrocyclic complex [Co(CR)Cl2]+ (1) (CR = 2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) as a noble metal-free catalyst, with [RuII(bpy)3]2+ (Ru) as photosensitizer and ascorbate/ascorbic acid (HA-/H2A) as a sacrificial electron donor and buffer (PhysChemChemPhys 2013, 15, 17544). This catalyst presents the particularity to achieve very high turnover numbers (TONs) (up to 1000) at pH 4.0 at a relative high concentration (0.1 mM) generating a large amount of hydrogen and having a long term stability. A similar activity was observed for the aquo derivative [CoIII(CR)(H2O)2]3+ (2) due to substitution of chloro ligands by water molecule in water. In this work, the geometry and electronic structures of 2 and its analog [ZnII(CR)Cl]+ (3) derivative containing the redox innocent Zn(II) metal ion have been investigated by DFT calculations under various oxidation states. We also further studied the photocatalytic activity of this system and evaluated the influence of varying the relative concentration of the different components on the H2-evolving activity. Turnover numbers versus catalyst (TONCat) were found to be dependent on the catalyst concentration with the highest value of 1130 obtained at 0.05 mM. Interestingly, the analogous nickel derivative, [NiII(CR)Cl2] (4), when tested under the same experimental conditions was found to be fully inactive for H2 production. Nanosecond transient absorption spectroscopy measurements have revealed that the first electron-transfer steps of the photocatalytic H2-evolution mechanism with the Ru/cobalt tetraaza/HA-/H2A system involve a reductive quenching of the excited state of the photosensitizer by ascorbate (kq = 2.5 × 107 M-1 s-1) followed by an electron transfer from the reduced photosensitizer to the catalyst (ket = 1.4 × 109 M-1 s-1). The reduced catalyst can then enter into the cycle of hydrogen evolution

[Rh^III(dmbpy)₂Cl₂]⁺ as a highly efficient catalyst for visible-light-driven hydrogen production in pure water: comparison with other rhodium catalysts

We report a very efficient homogeneous system for the visible-light-driven hydrogen production in pure aqueous solution at room temperature. This comprises [RhIII(dmbpy)2Cl2]Cl (1) as catalyst, [Ru(bpy)3]Cl2 (PS1) as photosensitizer, and ascorbate as sacrificial electron donor. Comparative studies in aqueous solutions also performed with other known rhodium catalysts, or with an iridium photosensitizer, show that 1) the PS1/1/ascorbate/ascorbic acid system is by far the most active rhodium-based homogeneous photocatalytic system for hydrogen production in a purely aqueous medium when compared to the previously reported rhodium catalysts, Na3[RhI(dpm)3Cl] and [RhIII(bpy)Cp*(H2O)]SO4 and 2) the system is less efficient when [IrIII(ppy)2(bpy)]Cl (PS2) is used as photosensitizer. Because catalyst 1 is the most efficient rhodium-based H2-evolving catalyst in water, the performance limits of this complex were further investigated by varying the PS1/1 ratio at pH 4.0. Under optimal conditions, the system gives up to 1010 turnovers versus the catalyst with an initial turnover frequency as high as 857 TON h−1. Nanosecond transient absorption spectroscopy measurements show that the initial step of the photocatalytic H2-evolution mechanism is a reductive quenching of the PS1 excited state by ascorbate, leading to the reduced form of PS1, which is then able to reduce [RhIII(dmbpy)2Cl2]+ to [RhI(dmbpy)2]+. This reduced species can react with protons to yield the hydride [RhIII(H)(dmbpy)2(H2O)]2+, which is the key intermediate for the H2 production

Synthesis, characterization, and photocatalytic H₂-evolving activity of a family of [Co(N4Py)(X)]^<i>n</i>+ complexes in aqueous solution

International audienceA series of [CoIII(N4Py)(X)](ClO4)n (X = Cl-, Br-, OH-, N3 -, NCS--κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3]2+ as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [CoII(N4Py)(HA)]+ is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10-6 M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3]2+ by ascorbate followed by an electron transfer from [RuII(bpy)2(bpy•-)]+ to the [CoII(N4Py)(HA)]+ catalyst. The reduced catalyst then enters into the H2-evolution cycle