Ligand Field Transitions Research Articles

Impurity incorporation and exchange interactions in Co2+-doped CdSe/CdS core/shell nanoplatelets.

The intentional incorporation of transition metal impurities into colloidal semiconductor nanocrystals allows an extension of the host material's functionality. While dopant incorporation has been extensively investigated in zero-dimensional quantum dots, the substitutional replacement of atoms in two-dimensional (2D) nanostructures by magnetic dopants has been reported only recently. Here, we demonstrate the successful incorporation of Co2+ ions into the shell of CdSe/CdS core/shell nanoplatelets, using these ions (i) as microscopic probes for gaining distinct structural insights and (ii) to enhance the magneto-optical functionality of the host material. Analyzing interatomic Co2+ ligand field transitions, we conclude that Co2+ is incorporated into lattice sites of the CdS shell, and effects such as diffusion of dopants into the CdSe core or diffusion of the dopants out of the heterostructure causing self-purification play a minor role. Taking advantage of the absorption-based technique of magnetic circular dichroism, we directly prove the presence of sp-d exchange interactions between the dopants and the band charge carriers in CdSe/Co2+:CdS heteronanoplatelets. Thus, our study not only demonstrates magneto-optical functionality in 2D nanocrystals by Co2+ doping but also shows that a careful choice of the dopant type paves the way for a more detailed understanding of the impurity incorporation process into these novel 2D colloidal materials.

Luminescent, Redox-Active Dithiolatobis(imine)platinum(II) Divergent Complexes with Exchangeable Imine Ligands: An Experimental/Computational Study Versus Their Diiminedithiolatoplatinum(II) Convergent Congeners

This article provides a third manifestation of a new tradition by which the editors of Comments on Inorganic Chemistry wish to lead by example, whereby we start publishing original research content that, nonetheless, preserves the journal’s identity as a niche for “critical discussion” of contemporary literature in inorganic chemistry. (For the first and second manifestations, see: (1) Otten, B. M.; Melancon, K. M.; Omary, M. A. “All that Glitters is Not Gold: A Computational Study of Covalent vs Metallophilic Bonding in Bimetallic Complexes of d10 Metal Centers: A Tribute to Al Cotton on the 10th Anniversary of His Passing,” Comments Inorg. Chem. 2018, 38, 1–35; (2) Yaseen, W. K.; Sanders, S. F.; Almotawa, R. M.; Otten, B. M.; Bhat, S.; Alamo, D. C.; Marpu, S. B.; Golden, T. D.; Omary, M. A. “Are Metal Complexes ‘Organic’, ‘Inorganic’, ‘Organometallic’, or ‘Metal-Organic’ Materials? A Case Study for the Use of Trinuclear Coinage Metal Complexes as ‘Metal-Organic Coatings’ for Corrosion Suppression on Aluminum Substrates,” Comments Inorg. Chem. 2019, 39, 1–26.) Herein, we contrast the electronic structure of two categories of Pt(II) complexes with mixed imine/thiolato ligands: a new class of [Pt(N^N)2(S^S)] bis(κ1-diimine)dithiolatoplatinum(II) divergent complexes—whereby only one N atom from the “back-to-back” diimine ligand actually coordinates to a Pt(II) center in a non-bridging monodentate manner—vis-à-vis the hitherto well-studied [Pt(N^N)(S^S)] (κ2-diimine)dithiolatoplatinum(II) convergent congeners—whereby both the diimine and dithiolate coordinate as κ2-chelating bidentate. Thus, in pursuit of using luminescent building blocks to generate light-harvesting supramolecular coordination compounds, we report the synthesis, characterization, and reactivity of Pt(pyz)2(mnt), 1, in high yield (pyz = pyrazine, mnt = maleonitriledithiolate). The complex exhibits solvent-dependent exchange of the pyrazine ligands, which can be utilized in the formation of larger complexes containing platinum dithiolate moieties. As proof of concept, 1 has been converted into the more inert Pt(4,4ʹ-bpy)2(mnt), 2, and Pt(4-ap)2(mnt), 3, (4-ap = 4-aminopyridine) complexes. These complexes exhibit redox properties, are strong absorbers of ultraviolet and visible light, and exhibit bright-luminescence at 77 K. Single-crystal X-ray diffraction analysis for 1 and 3 confirms the monotopic coordination of the ligands with intramolecular Pt-S and Pt-N bond lengths being comparable to other complexes of type (κ2-diimine)(dithiolate)Pt(II), whereas significant torsion is exhibited by the two heterocyclic rings due to the lack of tethering to one another. Solvent-dependent stability is uncovered for 1 crystals, whereas the crystal structure of 3 reveals an interesting supramolecular quadrangle resulting from hydrogen bonding between the amine groups of two complexes and two interstitial water molecules. A commentary is presented to contrast the electronic structure of the divergent complexes herein with that of a diimine-dithiolate analogue. Experimentally, the luminescence is believed to be sensitized in the new class of divergent complexes, given that their lack of conjugation of two aromatic rings renders them less-efficient π-acceptors compared to analogous diimines such as 2,2ʹ-bipyridine (2,2ʹ-bpy), hence blue-shifting the absorption toward the blue/near-UV and assisting in keeping the emission well within the visible region, whereby energy gap law considerations would suppress quenching effects in the red/near-infrared regions, especially under experimental conditions conducive for phosphorescence and aggregation effects in the solid state and higher-concentration fluid and/or frozen solutions. Computational studies for monomeric models of the three divergent complexes plus the diimine-dithiolate congener, Pt(2,2ʹ-bpy)(mnt), attain reasonable agreement with experimental structural, spectroscopic, and redox properties and provide excellent insights for the comparison between the two categories of complexes upon which we focus the commentary section. The computed photophysical properties reveal phosphorescence due to higher-lying triplet states, as the T1 is found to luminesce in the near-IR region and entail a ligand-field (dd) transition origin, whereas higher-lying states are shown to be in the visible region, close to experimental phosphorescence energies, and are consistent with the expected charge transfer transition to imine/diimine lowest-unoccupied molecular orbitals (LUMOs) from admixed dithiolate/platinum highest-occupied molecular orbitals (HOMOs). The T1 state is likely the culprit for the vanishing emission intensity at ambient temperature for both classes of complexes. We also find that both the T1 and D1 states for the neutral exciton and radical polaron (anion or cation) species, respectively, are severely distorted in 1–3 vs Pt(2,2ʹ-bpy)(mnt) models. Near-perfect orthogonality of the two heterocyclic rings can be attained for such excited and redox models of Pt(κ1-imine)2(dithiolate), whereas this distortion is hindered in Pt(κ2-diimine)(dithiolate) models due to the steric constraint in their CCNNPt metallacycle.

Tuning the Luminescence of Layered Halide Perovskites

Layered halide perovskites offer a versatile platform for manipulating light through synthetic design. Although most layered perovskites absorb strongly in the ultraviolet (UV) or near-UV region, their emission can range from the UV to the infrared region of the electromagnetic spectrum. This emission can be very narrow, displaying high color purity, or it can be extremely broad, spanning the entire visible spectrum and providing high color rendition (or accurately reproducing illuminated colors). The origin of the photoluminescence can vary enormously. Strongly correlated electron-hole pairs, permanent lattice defects, transient light-induced defects, and ligand-field transitions in the inorganic layers and molecular chromophores in the organic layers can be involved in the emission mechanism. In this review, we highlight the different types of photoluminescence that may be attained from layered halide perovskites, with an emphasis on how the emission may be systematically tuned through changes to the bulk crystalline lattice: changes in composition, structure, and dimensionality.

Spectroscopic and Electronic Structure Study of ETHE1: Elucidating the Factors Influencing Sulfur Oxidation and Oxygenation in Mononuclear Nonheme Iron Enzymes.

ETHE1 is a member of a growing subclass of nonheme Fe enzymes that catalyzes transformations of sulfur-containing substrates without a cofactor. ETHE1 dioxygenates glutathione persulfide (GSSH) to glutathione (GSH) and sulfite in a reaction which is similar to that of cysteine dioxygenase (CDO), but with monodentate (vs bidentate) substrate coordination and a 2-His/1-Asp (vs 3-His) ligand set. In this study, we demonstrate that GSS- binds directly to the iron active site, causing coordination unsaturation to prime the site for O2 activation. Nitrosyl complexes without and with GSSH were generated and spectroscopically characterized as unreactive analogues for the invoked ferric superoxide intermediate. New spectral features from persulfide binding to the FeIII include the appearance of a low-energy FeIII ligand field transition, an energy shift of a NO- to FeIII CT transition, and two new GSS- to FeIII CT transitions. Time-dependent density functional theory calculations were used to simulate the experimental spectra to determine the persulfide orientation. Correlation of these spectral features with those of monodentate cysteine binding in isopenicillin N synthase (IPNS) shows that the persulfide is a poorer donor but still results in an equivalent frontier molecular orbital for reactivity. The ETHE1 persulfide dioxygenation reaction coordinate was calculated, and while the initial steps are similar to the reaction coordinate of CDO, an additional hydrolysis step is required in ETHE1 to break the S-S bond. Unlike ETHE1 and CDO, which both oxygenate sulfur, IPNS oxidizes sulfur through an initial H atom abstraction. Thus, factors that determine oxygenase vs oxidase reactivity were evaluated. In general, sulfur oxygenation is thermodynamically favored and has a lower barrier for reactivity. However, in IPNS, second-sphere residues in the active site pocket constrain the substrate, raising the barrier for sulfur oxygenation relative to oxidation via H atom abstraction.

Co2+-Doping of Magic-Sized CdSe Clusters: Structural Insights via Ligand Field Transitions

Magic-sized clusters represent materials with unique properties at the border between molecules and solids and provide important insights into the nanocrystal formation process. However, synthesis, doping, and especially structural characterization become more and more challenging with decreasing cluster size. Herein, we report the successful introduction of Co2+ ions into extremely small-sized CdSe clusters with the intention of using internal ligand field transitions to obtain structural insights. Despite the huge mismatch between the radii of Cd2+ and Co2+ ions (>21%), CdSe clusters can be effectively synthesized with a high Co2+ doping concentration of ∼10%. Optical spectroscopy and mass spectrometry suggest that one or two Co2+ ions are substitutionally embedded into (CdSe)13 clusters, which is known as one of the smallest CdSe clusters. Using magnetic circular dichroism spectroscopy on the intrinsic ligand field transitions between the different 3d orbitals of the transition metal dopants, we demonstrate that the Co2+ dopants are embedded on pseudotetrahedral selenium coordinated sites despite the limited number of atoms in the clusters. A significant shortening of Co-Se bond lengths compared to bulk or nanocrystals is observed, which results in the metastability of Co2+ doping. Our results not only extend the doping chemistry of magic-sized semiconductor nanoclusters, but also suggest an effective method to characterize the local structure of these extremely small-sized clusters.

Spectroscopic Identification of the α-Fe/α-O Active Site in Fe-CHA Zeolite for the Low-Temperature Activation of the Methane C-H Bond.

The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown to be active in oxygen atom abstraction from N2O to form a highly reactive α-O, capable of methane activation at room temperature to form methanol. The methanol product can subsequently be desorbed by online steaming at 200 °C. For the intermediate steps of the reaction cycle, the evolution of the Fe active site is monitored by UV-vis-NIR and Mössbauer spectroscopy. A B3LYP-DFT model of the α-Fe site in CHA is constructed, and the ligand field transitions are calculated by CASPT2. The model is experimentally substantiated by the preferential formation of α-Fe over other Fe species, the requirement of paired framework aluminum and a MeOH/Fe ratio indicating a mononuclear active site. The simple CHA topology is shown to mitigate the heterogeneity of iron speciation found on other Fe-zeolites, with Fe2O3 being the only identifiable phase other than α-Fe formed in Fe-CHA. The α-Fe site is formed in the d6r composite building unit, which occurs frequently across synthetic and natural zeolites. Finally, through a comparison between α-Fe in Fe-CHA and Fe-*BEA, the topology's 6MR geometry is found to influence the structure, the ligand field, and consequently the spectroscopy of the α-Fe site in a predictable manner. Variations in zeolite topology can thus be used to rationally tune the active site properties.

Phase analysis, FTIR/Raman, and optical properties of Fe3BO6 nanocrystallites prepared by glass route at moderate temperature in ambient air

In this paper, a facile synthesis method is explored from a supercooled liquid Fe2O3B2O3 precursor using microwave furnace in order to obtain a single phase Fe3BO6 compound. Study includes X-ray diffraction (XRD), Field emission scanning electron microscope (FESEM), high resolution transmission electron microscopy (HRTEM) images, FTIR/Raman and optical property of sample. The crystal structure and size of the Fe3BO6 crystallites have been characterized in terms of XRD pattern in correlation to the FESEM/HRTEM images. A single phase compound Fe3BO6 of an orthorhombic crystal structure with Pnma space group and average crystallites size D = 49 nm is analyzed from the XRD pattern. IR and Raman bands in the oxygen polygons confer the results of forming Fe3BO6 with a bonded surface layer. UV–visible absorption spectrum over a spectral range 200–800 nm of wavelengths reveals two high-energy bands 222 and 277 nm possibly represent a ligand to metal charge transfer transition while one broad and relatively weak band appears in the visible region at 400 nm ascribed to a ligand field transition 6A1 → 4T1 of the 3 d5 electrons in the Fe3+ ions occupied. This compound also endures good optical properties in the visible and ultraviolet regions that can be combined to magnetic and other properties useful for developing multifunctional features for possible applications.

An insight into the origin of room-temperature ferromagnetism in SnO2 and Mn-doped SnO2 quantum dots: an experimental and DFT approach.

SnO2 and Mn-doped SnO2 single-phase tetragonal crystal structure quantum dots (QDs) of uniform size with control over dopant composition and microstructure were synthesized using the high pressure microwave synthesis technique. On a broader vision, we systematically investigated the influence of dilute Mn ions in SnO2 under the strong quantum confinement regime through various experimental techniques and density functional theoretical (DFT) calculations to disclose the physical mechanism governing the observed ferromagnetism. DFT calculations revealed that the formation of the stable (001) surface was much more energetically favorable than that of the (100) surface, and the formation energy of the oxygen vacancies in the stable (001) surface was comparatively higher in the undoped SnO2 QDs. X-ray photoelectron spectroscopy (XPS) and first-principles modeling of doped QDs revealed that the lower doping concentration of Mn favored the formation of MnO-like (Mn2+) structures in defect-rich areas and the higher doping concentration of Mn led to the formation of multiple configurations of Mn (Mn2+ and Mn3+) in the stable surfaces of SnO2 QDs. Electronic absorption spectra indicated the characteristic spin allowed ligand field transitions of Mn2+ and Mn3+ and the red shift in the band gap. DFT calculations clearly indicated that only the substitutional dopant antiferromagnetic configurations were more energetically favorable. The gradual increase of magnetization at a low level of Mn-doping could be explained by the prevalence of antiferromagnetic manganese-vacancy pairs. Higher concentrations of Mn led to the appearance of ferromagnetic interactions between manganese and oxygen vacancies. The increase in the concentration of metallic dopants caused not just an increase in the total magnetic moment of the system but also changed the magnetic interactions between the magnetic moments on the metal ions and oxygen. The present study provides new insight into the fundamental understanding of the origin of ferromagnetism in transition metal-doped QDs.

Ligand-field helical luminescence in a 2D ferromagnetic insulator

Bulk chromium triiodide (CrI$_3$) has long been known as a layered van der Waals ferromagnet. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet, providing a new platform for investigating light-matter interactions and magneto-optical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI$_3$ under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI$_3$ photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI$_3$ bilayers. Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors, our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI$_3$. We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr$^{3+}$ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.

Structural characterization of two solvates of a luminescent copper(II) bis-(pyridine)-substituted benzimidazole complex.

Copper(II) complexes of benzimidazole are known to exhibit biological activity that makes them of inter-est for chemotherapeutic and other pharmaceutical uses. The complex bis-(acetato-κO){5,6-dimethyl-2-(pyridin-2-yl)-1-[(pyridin-2-yl)meth-yl]-1H-benzimidazole-κ2N2,N3}copper(II), has been prepared. The absorption spectrum has features attributed to intra-ligand and ligand-field transitions and the complex exhibits ligand-centered room-temperature luminescence in solution. The aceto-nitrile monosolvate, [Cu(C2H3O2)2(C20H18N4)]·C2H3N (1), and the ethanol hemisolvate, [Cu(C2H3O2)2(C20H18N4)]·0.5C2H6O (2), have been structurally characterized. Compound 2 has two copper(II) complexes in the asymmetric unit. In both 1 and 2, distorted square-planar N2O2 coordination geometries are observed and the Cu-N(Im) bond distance is slightly shorter than the Cu-N(py) bond distance. Inter-molecular π-π inter-actions are found in 1 and 2. A weak C-H⋯π inter-action is observed in 1.

Photophysical properties of [Ru(2,2′-bipyridine)3]2+ encapsulated within the Uio-66 zirconium based metal organic framework

The ability to encapsulate photo-active guest molecules within the pores of metal organic frameworks (MOFs) affords the opportunity to develop robust photocatalysts as well as solar energy conversion systems. An important criteria for such systems is stability of the new materials towards moisture, high temperatures, etc which preclude the use of many MOF frameworks. Here, the ability to encapsulate [Ru(II)(2,2′-bipyridine)3]2+([Ru(bpy)3]2+) into the cavities of the zirconium based MOF Uio-66 as well as the photophysical properties of the complex are reported. The X-ray powder diffraction data of the orange Uio-66 powder are consistent with the formation of Uio-66 in the presence of [Ru(bpy)3]2+. The steady state emission exhibits a significant bathochromic shift from 603nm in ethanol to 610nm in Uio-66. The corresponding emission decay of the encapsulated [Ru(bpy)3]2+ complex is biexponential with a fast component of 128ns and a slower component of 1176ns (20degC). The slow component is consistent with encapsulation of [Ru(bpy)3]2+ into cavities with restricted volume that prevents the population of a triplet ligand field transition that is anti-bonding with respect to the Ru-N bonds. The origin of the fast component is unclear but may involve interactions of the [Ru(bpy)3]2+ encapsulated within large cavities formed through missing ligand defect sites within the Uio-66 materials. Co-encapsulated quenchers contained within these larger cavities gives rise to the reduced lifetimes of the [Ru(bpy)3]2+ complexes.

Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene-Oxoiron(IV) Complex.

In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O2-activating iron enzymes. This work details an electronic-structure investigation of [FeIV(O)(LNHC)(NCMe)]2+ (LNHC = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe-O stretching vibration at 832 ± 3 cm-1. By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm-1 in the E(dxy → dxz,yz) excited state. Both values are similar to those measured for [FeIV(O)(TMC)(NCMe)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm-1. We can unequivocally assign them to the ligand field transitions of dxy → dxz,yz, dxz,yz → dz2, and dxz,yz → dx2-y2, respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [FeIV(O) (SR-TPA)(NCMe)]2+ (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO)2+ core of complex 1 have similar transition energies, whereas the excitation energy for dxz,yz → dx2-y2 is significantly higher (∼12 000 cm-1 for [FeIV(O)(SR-TPA)(NCMe)]2+). Our results thus substantiate that the tetracarbene ligand (LNHC) of complex 1 does not significantly affect the bonding in the (FeO)2+ unit but strongly destabilizes the dx2-y2 orbital to eventually lift it above dz2. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed.

Fe3+ red phosphors based on lithium aluminates and an aluminum lithium oxyfluoride prepared from LiF as the Li Source

LiF was used as the Li source to prepare Fe3+ red phosphors based on lithium aluminates and an aluminum lithium oxyfluoride. LiF led to the three types of the host materials of α-LiAlO2, γ-LiAlO2 and Al4LiO6F with the different synthetic conditions. Metastable α-LiAlO2 crystallized up to 800°C under Li-rich conditions in the starting mixture (eg. LiF/Al(OH)3=3). α-LiAlO2 converted to γ-LiAlO2 above 800°C. The flux effect of the excess LiF resulted in the enhancement of the photoluminescence (PL) intensity at higher temperatures with the growth of the crystalline grains of γ-LiAlO2:Fe3+. The reaction between LiF and γ-Al2O3 at Li/Al=0.25 brought the formation of Al4LiO6F at 800°C and above. Red PL from Fe3+ was observed at ca. 740nm in γ-LiAlO2, 730nm in α-LiAlO2 and 690nm in Al4LiO6F under the irradiation of ultra-violet (UV) light. They had charge-transfer excitation bands in the UV region and weak excitation bands in the visible region due to the d-d (or ligand field) transition. The red PL in these phosphors originated in the tetrahedrally coordinated Fe3+.

Optical studies of cobalt implanted rutile TiO2 (110) surfaces

Present study investigates the photoabsorption properties of single crystal rutile TiO2 (110) surfaces after they have been implanted with low fluences of cobalt ions. The surfaces, after implantation, demonstrate fabrication of nanostructures and anisotropic nano-ripple patterns. Creation of oxygen vacancies (Ti3+ states), development of cobalt nano-clusters as well as band gap modifications have also been observed. Results presented here demonstrate that fabrication of self organized nanostructures, upon implantation, along with the development of oxygen vacancies and ligand field transitions of cobalt ion promote the enhancement of photo-absorbance in both UV (∼2 times) and visible (∼5 times) regimes. These investigations on nanostructured TiO2 surfaces can be important for photo-catalysis.

Spin-State Energetics of Fe(III) and Ru(III) Aqua Complexes: Accurate ab Initio Calculations and Evidence for Huge Solvation Effects.

Aqua complexes of transition metals are useful models for understanding the electronic structure of metal-oxide species relevant in photocatalytic water splitting. Moreover, spin-forbidden d-d transitions of aqua complexes provide valuable experimental data of spin-state energetics, which can be used for benchmarking of computational methods. Here, low-energy spin states of Fe(III) and Ru(III) aqua complexes are studied with an array of DFT and high-level wave function methods (CASPT2, RASPT2, NEVPT2, CCSD(T)-F12, and other coupled cluster methods up to full CCSDT). The results from single-reference and multireference methods are cross-checked, and the amount of multireference character for both considered spin states of [Fe(H2O)6](3+) is carefully analyzed. In addition to small [M(H2O)6](3+) clusters (M = Fe, Ru), we also employ larger models [M(H2O)6·(H2O)12](3+), with explicit water molecules in the second coordination sphere, to describe the situation in aqueous solution. By comparing the results for both types of models, our calculations evidence large and systematic solvation effects on the spin-state energetics. It is found that, due to the interaction with hydrogen-bonded water molecules in the second coordination sphere, the first coordination sphere undergoes a noticeable contraction and deformation. In consequence, the presence of solvation shell affects the relative energies of spin states by as much as 3-4 × 10(3) cm(-1) (∼10 kcal/mol). Once this solvation effect is accounted for, the spin-state energetics from CCSD(T) and NEVPT2 calculations turn out to be in an excellent agreement with the experimental estimates, which was not the case for isolated [M(H2O)6](3+) species is gas phase. We thus postulate that significant discrepancies between theory and experimental data for [Fe(H2O)6](3+) that were previously reported in the literature may be plausibly resolved and attributed to the neglect of explicit solvation effects and also, to some extent, to incompleteness of the active space and/or basis set used in the previous theoretical studies. The findings of this work contradict an anecdotal conjecture that energies of ligand-field (d-d) transitions are almost unaffected by solvation. On the contrary, it is highlighted that medium effects may contribute very significantly to spin-state energetics of transition metal complexes.

Electronic Structure of a Cu(II)-Alkoxide Complex Modeling Intermediates in Copper-Catalyzed Alcohol Oxidations.

In the copper-catalyzed oxidation of alcohols to aldehydes, a Cu(II)-alkoxide (Cu(II)-OR) intermediate is believed to modulate the αC-H bond strength of the deprotonated substrate to facilitate the oxidation. As a structural model for these intermediates, we characterized the electronic structure of the stable compound Tp(tBu)Cu(II)(OCH2CF3) (Tp(tBu) = hydro-tris(3-tert-butyl-pyrazolyl)borate) and investigated the influence of the trifluoroethoxide ligand on the electronic structure of the complex. The compound exhibits an electron paramagnetic resonance (EPR) spectrum with an unusually large gzz value of 2.44 and a small copper hyperfine coupling Azz of 40 × 10(-4) cm(-1) (120 MHz). Single-crystal electron nuclear double resonance (ENDOR) spectra show that the unpaired spin population is highly localized on the copper ion (≈68%), with no more than 15% on the ethoxide oxygen. Electronic absorption and magnetic circular dichroism (MCD) spectra show weak ligand-field transitions between 5000 and 12,000 cm(-1) and an intense ethoxide-to-copper charge transfer (LMCT) transition at 24,000 cm(-1), resulting in the red color of this complex. Resonance Raman (rR) spectroscopy reveals a Cu-O stretch mode at 592 cm(-1). Quantum chemical calculations support the interpretation and assignment of the experimental data. Compared to known Cu(II)-thiolate and Cu(II)-alkylperoxo complexes from the literature, we found an increased σ interaction in the Cu(II)-OR bond that results in the spectroscopic features. These insights lay the basis for further elucidating the mechanism of copper-catalyzed alcohol oxidations.

Structural and Magnetic Properties of Dilute Ca²⁺ Doped Iron Oxide Nanoparticles.

Undoped and calcium substituted hematite (α-Fe₂O₃) nanoparticles are synthesized by surfactant-directed co-precipitation and post annealing method. The annealed nanoparticles were found to be in single phase in nature and crystallize in the rhombohedral structure with space group R3c as confirmed by Rietveld refinement of the X-ray diffraction (XRD) data. Average crystallite sizes are calculated to be 20 to 30 nm and 50 to 60 nm for the nanoparticles annealed at 400 and 600 °C respectively. Mössbauer spectra for all the nanoparticles could be fitted with a sextet corresponding to the single magnetic state of the iron atoms in its Fe³⁺ state in the hematite matrix. The FTIR and Raman spectra of all the samples correspond to specific modes of α-Fe₂O₃. UV-Vis spectra of annealed samples showed broad peaks in the range of 525-630 nm resulting from spin-forbidden ligand field transition together with the spin-flip transition among the 2t₂g states. The estimated band gap energies were in the range of 1.6 to 1.9 eV which are much lower than the reported values for nano hematite. From the room temperature magnetic hysteresis loop measurements, weak ferromagnetic behavior is observed in all undoped and Ca²⁺ doped hematite samples. Morin temperature (T(M)) is calculated to be 257 and 237 K for 1.45% doped samples with particle size 54 and 27 nm respectively. The sample with Ca content of 1.45 wt% when annealed at 400 °C showed that the particles were of different shapes which included both quasi spherical and rod shaped. On annealing the same sample at 600 °C, the nanorods collapsed to form bigger spherical and ellipsoidal particles.

A new homoleptic coordination compound of ruthenium and norfloxacin and its interaction with human serum albumin

This work presents a new coordination compound of ruthenium and the norfloxacin antibiotics, as well as its interaction with human serum albumin (HSA). The ESI HRMS spectrum of [Ru(nor)3] (nor=norfloxacinate, C16H17FN3O3−) displayed the protonated molecular ion at m/z 1057.3, while the HRMS/MS spectra displayed peaks at m/z 958.4 and 639.3 assigned to the gaseous phase norfloxacin adducts (Hnor)3·H+ and (Hnor)2·H+. Coordination to Ru(III) remarkably enhances the water solubility of norfloxacin. The complex absorption spectrum in aqueous medium is dominated by the norfloxacin transitions in the UV region but also displays a broad shoulder around 370nm, which possibly overlaps with ligand-field transitions. Stern–Volmer analysis of human serum albumin (HSA) fluorescence quenching by [Ru(nor)3] showed that both dynamic and static mechanisms contribute to the quenching. Lifetime measurements demonstrated that the predominance of one pathway over the other depends on temperature variations. Finally, the analysis of thermodynamic parameters leads to the conclusions that the interaction of the novel complex with HSA is spontaneous and that this interaction occurs through the formation of hydrogen bonds and van der Waals forces (ΔH=−259.2kJ/mol and ΔS=−779.9kJ/mol·K).

Synthesis, optical, and photocatalytic properties of cobalt mixed-metal spinel oxides Co(Al1−xGax)2O4

Co(Al1−xGax)2O4 spinels synthesized from molecular precursors exhibit low energy (<2.5 eV) ligand–field transitions that contribute between 46 and 72% of the photocatalytic activity.

Synthesis, Structural and Spectroscopic Characterization of a New Coordination Polymer Based on a Tetraiminodiphenolate Macrocycle and Piperazine: [{Cu2(tidf)(µ-ppz)}(ClO4)2

Herein we report on the synthesis, single crystal X-ray structure and spectroscopic properties of [{Cu2(tidf)(µ-ppz)}(ClO4)2]n (tidf = a Robson-type macrocyclic ligand obtained by condensation of 2,6-diformyl-4-methylphenol and 1,3–diaminopropane). The two copper(II) centers in the dicopper(II,II) chromophore share identical coordination environment. The geometry around the copper(II) is square pyramidal and has [Cu2(tidf)]2+ units bridged by piperazine and forming a 1D–coordination polymer. The structure is also supported by non-classic hydrogen bonding such as C–H···Operchlorate. Electronic spectroscopy agrees with the C4v micro-symmetry around the metal center as ligand-field transitions are splitted when compared to the spectrum of the parent complex [Cu2(tidf)(ClO4)2(H2O)2]. The molecular structure and spectroscopic properties of [{Cu2(tidf)(µ-ppz)}(ClO4)2]n