First-row Transition Metal Ions Research Articles

Chelate Complexes of 3d Transition Metal Ions─A Challenge for Electronic-Structure Methods?

Different electronic-structure methods were assessed for their ability to predict two important properties of the industrially relevant chelating agent nitrilotriacetic acid (NTA): its selectivity with respect to six different first-row transition metal ions and the spin-state energetics of its complex with Fe(III). The investigated methods encompassed density functional theory (DFT), the random phase approximation (RPA), coupled cluster (CC) theory, and the auxiliary-field quantum Monte Carlo (AFQMC) method, as well as the complete active space self-consistent field (CASSCF) method and the respective on-top methods: second-order N-electron valence state perturbation theory (NEVPT2) and multiconfiguration pair-density functional theory (MC-PDFT). Different strategies for selecting active spaces were explored, and the density matrix renormalization group (DMRG) approach was used to solve the largest active spaces. Despite somewhat ambiguous multi-reference diagnostics, most methods gave relatively good agreement with experimental data for the chemical reactions connected to the selectivity, which only involved transition-metal complexes in their high-spin state. CC methods yielded the highest accuracy followed by range-separated DFT and AFQMC. We discussed in detail that even higher accuracies can be obtained with NEVPT2, under the prerequisite that consistent active spaces along the entire chemical reaction can be selected, which was not the case for reactions involving Fe(III). A bigger challenge for electronic-structure methods was the prediction of the spin-state energetics, which additionally involved lower spin states that exhibited larger multi-reference diagnostics. Conceptually different, typically accurate methods ranging from CC theory via DMRG-NEVPT2 in combination with large active spaces to AFQMC agreed well that the high-spin state is energetically significantly favored over the other spin states. This was in contrast to most DFT functionals and RPA which yielded a smaller stabilization and some common DFT functionals and MC-PDFT even predicting the low-spin state to be energetically most favorable.

Catalytic Hydrogenolysis by Atomically Dispersed Iron Sites Embedded in Chemically and Redox Non-innocent N-Doped Carbon.

Atomically dispersed first-row transition metals embedded in nitrogen-doped carbon materials (M-N-C) show promising performance in catalytic hydrogenation but are less well-studied for reactions with more complex mechanisms, such as hydrogenolysis. Their ability to catalyze selective C-O bond cleavage of oxygenated hydrocarbons such as aryl alcohols and ethers is enhanced with the participation of ligands directly bound to the metal ion as well as longer-range contributions from the support. In this article, we describe how Fe-N-C catalysts with well-defined local structures for the Fe sites catalyze C-O bond hydrogenolysis. The reaction is facilitated by the N-C support. According to spectroscopic analyses, the as-synthesized catalysts contain mostly pentacoordinated FeIII sites, with four in-plane nitrogen donor ligands and one axial hydroxyl ligand. In the presence of 20 bar of H2 at 170-230 °C, the hydroxyl ligand is lost when N4FeIIIOH is reduced to N4FeII, assisted by the H2 chemisorbed on the support. When an alcohol binds to the tetracoordinated FeII sites, homolytic cleavage of the O-H bond is accompanied by reoxidation to FeIII and H atom transfer to the support. The role of the N-C support in catalytic hydrogenolysis is analogous to the behavior of chemically and redox-non-innocent ligands in molecular catalysts based on first-row transition metal ions and enhances the ability of M-N-Cs to achieve the types of multistep activations of strong bonds needed to upgrade renewable and recycled feedstocks.

Structural, photophysical, and electrochemical redox properties of meso-tetrakis(pentafluorophenyl)porphyrins

This study delineates the synthesis and comprehensive characterization of metal complexes of meso-tetrakis(pentafluorophenyl)porphyrin (H2TPF[Formula: see text]P, 1) with first-row transition metal ions (M = Mn(III) 2, Fe(III) 3, Co(II) 4, Ni(II) 5, Cu(II) 6, and Zn(II) 7) to elucidate their structural, electronic spectral, and electrochemical redox properties. Co, Ni, Cu and Zn Complexes exhibit planar structure ([Formula: see text]24 = 0.003-0.021 Å and [Formula: see text]C[Formula: see text] = 0.004−0.041 Å) which is also confirmed by the single crystal data of complex ZnTPF[Formula: see text]P, 7 shows the planar geometry, while Fe and Mn complexes show the slightly nonplanar structure ([Formula: see text]24 = 0.101–0.143 Å and [Formula: see text]C[Formula: see text] = 0.080–0.112 Å), which are distorted due to their occupied axial sites by −Cl ligand as revealed by density functional theory (DFT) calculations. Furthermore, electronic spectroscopy revealed a gradual bathochromic shift following the order of NiTPF[Formula: see text]P < CoTPF[Formula: see text]P < CuTPF[Formula: see text]PCl < H2TPF[Formula: see text]P = FeTPF[Formula: see text]PCl < ZnTPF[Formula: see text]P < MnTPF[Formula: see text]PCl by appending different metal ions in the porphyrin core. MnTPF[Formula: see text]PCl exhibited the most significant bathochromic shift ([Formula: see text] = 63 nm) in the Soret band compared to H2TPF[Formula: see text]P. Also, MnTPF[Formula: see text]PCl and FeTPF[Formula: see text]PCl show a unique pattern of split Soret band at around 364 and 345 nm due to the interaction between the metal [Formula: see text]-orbitals and the surrounding ligands. The electrochemical redox potentials of the filled 3d orbital of Zn endow ZnTPF[Formula: see text]P (ring centered) with the lowest oxidation potential.

Preparation and Characterization of Aceclofenac Complexes of First Row Transition Metals Mn(II), Co((II), Ni(II), Cu(II), and Zn(II)

A number of complex compounds of aceclofenac with different first-row transition metal ions like Co(II), Mn(II), Ni(II), Cu(II), and Zn (II) were synthesized in an aqueous medium and characterized on the basis of their UV-Visible, FTIR spectra, elemental (C, H, N) analyses, and metal estimation data. Thermogravimetric analysis (TGA) was also done. Conductivity measurement and magnetic susceptibility measurement were also done to have information about the electrolytic behavior of the compounds and oxidation states of the central metal atoms respectively.
 Dhaka Univ. J. Sci. 71(2): 153-159, 2023 (July)

Location of Carbonate Ions in Metal-Doped Carbonated Hydroxylapatites

The environment model for the description of the location of carbonate ions in apatites predicts that approximately half of the carbonate occupies the apatite channel. This model relies on the influence of entities surrounding the carbonate on its IR spectrum and can be used to determine how various substituents affect the location and structure of that ion. Careful deconvolution (peak-fitting) of the asymmetric carbonate IR region was used to determine the percentage of A-type (channel) ions, A′-type (channel with either a Ca2+ vacancy or substitution of Na+ for Ca2+) ions, and B-type (substitution for phosphate) ions. In our previous applications of this model, we have looked at the effect of alkali metal ions, such as sodium, lithium, and potassium, the ammonium ion, and the rare earth europium ion. In the present work, we explore the incorporation of the first-row transition metal ions and find that they have little effect on the location of the carbonate ion. Like the un-substituted carbonated apatite, these apatites contain about half of the carbonate in the channel, at least in derivatives that contain up to half a mole of the metal ion per mole of apatite. Attempts to incorporate greater amounts of metal ions by aqueous ion-combination reactions generally lead to lower-resolution XRD patterns and IR spectra that produce greater uncertainties in the peak-fitting modeling.

Four - Electron Oxidative Addition of an N=N Double Bond at a Chromium Metallocyclopropene.

This report describes the synthesis of a pseudo-tetrahedral chromium alkyne complex supported by a bidentate phosphinimide ligand and its reactivity with azobenzene. Characterization of the former by structural and computational methods reveals an unprecedented extent of alkyne activation by a formal chromium(II) center, suggesting that this complex is best described as a chromium(IV)-metallocyclopropene. Exposure of this compound to an azobenzene derivative results in the formation of a chromium(VI) diimido complex, which constitutes a rare 4-electron oxidative addition of an N=N double bond. The isolation of a chromium(IV)-hydrazido intermediate enabled mechanistic investigations of this challenging bond cleavage process. This work substantiates the notion that terminal phosphinimide ligands can engender first-row transition metal ions with exceptional reactivity.

Ligand-Copper(I) Primary O2-Adducts: Design, Characterization, and Biological Significance of Cupric-Superoxides.

In this Account, we overview and highlight synthetic bioinorganic chemistry focused on initial adducts formed from the reaction of reduced ligand-copper(I) coordination complexes with molecular oxygen, reactions that produce ligand-CuII(O2•-) complexes (O2•- ≡ superoxide anion). We provide mostly a historical perspective, starting in the Karlin research group in the 1980s, emphasizing the ligand design and ligand effects, structure, and spectroscopy of these O2 adducts and subsequent further reactivity with substrates, including the interaction with a second ligand-CuI complex to form binuclear species. The Account emphasizes the approach, evolution, and results obtained in the Karlin group, a synthetic bioinorganic research program inspired by the state of knowledge and insights obtained on enzymes possessing copper ion active sites which process molecular oxygen. These constitute an important biochemistry for all levels/types of organisms, bacteria, fungi, insects, and mammals, including humans.Copper is earth abundant, and its redox properties in complexes allow for facile CuII/CuI interconversions. Simple salts or coordination complexes have been well known to serve as oxidants for the stoichiometric or catalytic oxidation or oxygenation (i.e., O-atom insertion) of organic substrates. Thus, copper dioxygen- or peroxide-centered synthetic bioinorganic studies provide strong relevance and potential application to synthesis or even the development of cathodic catalysts for dioxygen reduction to hydrogen peroxide or water, as in fuel cells. The Karlin group's focus however was primarily oriented toward bioinorganic chemistry with the goal to provide fundamental insights into the nature of copper-dioxygen adducts and further reduced and/or protonated derivatives, species likely occurring in enzyme turnover or related in one or more aspects of formation, structure, spectroscopic properties, and scope of reactivity toward organic/biochemical substrates.Prior to this time, the 1980s, O2 adducts of redox-active first-row transition-metal ions focused on iron, such as the porphyrinate-Fe centers occurring in the oxygen carrier proteins myoglobin and hemoglobin and that determined to occur in cytochrome P-450 monooxygenase turnover. Deoxy (i.e., reduced Fe(II)) heme proteins react with O2, giving FeIII-superoxo complexes (preferably referred to by traditional biochemists as ferrous-oxy species). And, it was in the 1970s that great strides were made by synthetic chemists in generating hemes capable of forming O2 adducts, their physiochemical characterization providing critical insights to enzyme (bio)chemistry and providing ideas and important goals leading to countless person years of future research.

The case for an oxidopyrylium intermediate in the mechanism of quercetin dioxygenases

The quercetin dioxygenases (QDOs) are unusual metalloenzymes in that they display ring-opening dioxygenase activity with several different first-row transition metal ions which do not undergo redox changes during turnover. The QDOs are also unique in that the substrate binds as an η1-flavonolate rather than the η2 -bidentate mode seen in all reported model complexes. The flavonol substrates were early examples of excited state intramolecular proton transfer (ESIPT) phenomena, in which photoexcitation causes an H-atom exchange between the adjacent hydroxyl and ketone, generating an oxidopyrylium emissive state. These oxidopyryliums undergo ring-opening dioxygenations analogous to the enzymatic reactions. Our hypothesis is that lability of the divalent metal ion may allow access to a reactive oxidopyrylium intermediate via coordination switching from the oxy to ketone position, which allows reaction with O2. In this report, we use a straight-forward methylation strategy to generate a panel of flavonol and thioflavonol derivatives modeling several η1- and η2−coordination modes. Methylation of 3-hydroxythioflavone generates an air stable η1 hydroxopyrylium salt, which undergoes rapid ring-opening dioxygenation by deprotonation or photoexcitation. By comparison, the η1-methoxyflavonol does not react with O2 under any condition. We find that any of the studied flavonol derivatives, η1 or η2, which demonstrates ESIPT-like oxidopyrylium emissions undergo QDO-like ring-opening reactions with dioxygen. The implications of these results concerning the mechanism of QDOs and related dioxygenases is discussed.

A Combined Wave Function and Density Functional Approach for K-Edge X-ray Absorption Near-Edge Spectroscopy: A Case Study of Hydrated First-Row Transition Metal Ions.

The prediction of X-ray absorption spectra (XAS) of transition metal complexes has important and broad application areas in chemistry and biology. In this letter, we have investigated the predictive ability of multiconfiguration pair-density functional theory (MC-PDFT) for X-ray absorption spectra by calculating the metal K pre-edge features of aquated 3d transition metal ions in common oxidation states. MC-PDFT results were compared with experimentally measured spectra as well as analyzed against results from restricted active-space second-order perturbation theory (RASPT2) and time-dependent density functional theory (TDDFT). As expected, TDDFT performs well for excited states that can be accurately represented by singly excited configurations but fails for excited states where higher order excitations become important. On the other hand, both RASPT2 and MC-PDFT provide quantitatively accurate results for all excited states irrespective of their character. While core-level spectroscopy with RASPT2 is accurate, it is computationally expensive. Our results show that MC-PDFT performs equally well with significantly lower computational cost and is an encouraging alternate approach for X-ray spectroscopies.

17O hyperfine spectroscopy in surface chemistry and catalysis

Oxide-based materials are of key technological importance in different areas including advanced functional materials, solid state chemistry and catalysis. Many of the key questions concerning these areas involve understanding the chemical bond between the metal and the oxygen ions in the first or subsequent coordinating shells. The spectroscopic study of oxygen is therefore of fundamental importance to elucidate the complex interfacial coordination chemistry that underlies the development of metal-oxide supported catalysts and other advanced materials. Oxygen atoms at solid surfaces or lining the pores of zeolite frameworks play a vital role in stabilizing and defining the electronic and geometric structure of single metal atoms or clusters that act as catalytically active sites. In the case of paramagnetic species, EPR and its related hyperfine techniques offer a unique opportunity to explore and understand the nature of the chemical bonding in metal-oxide systems through the detection of the 17O hyperfine interaction. In this perspective we offer an overview of experimental considerations and relevant examples specific to 17O hyperfine spectroscopy of transition metal ions in zeolites relevant to catalysis. 17O hyperfine coupling values are obtained, which allow discriminating σ- and π-bonding channels in metal-oxygen bonds involving first-row transition metal ions. An exhaustive collection of 17O hyperfine and nuclear quadrupole couplings in different systems including molecular and biomolecular chemistry is provided, emphasizing the connection between interfacial and molecular inorganic coordination chemistry.

Underlying Role of Hydrophobic Environments in Tuning Metal Elements for Efficient Enzyme Catalysis.

The catalytic functions of metalloenzymes are often strongly correlated with metal elements in the active sites. However, dioxygen-activating nonheme quercetin dioxygenases (QueD) are found with various first-row transition-metal ions when metal swapping inactivates their innate catalytic activity. To unveil the molecular basis of this seemingly promiscuous yet metal-specific enzyme, we transformed manganese-dependent QueD into a nickel-dependent enzyme by sequence- and structure-based directed evolution. Although the net effect of acquired mutations was primarily to rearrange hydrophobic residues in the active site pocket, biochemical, kinetic, X-ray crystallographic, spectroscopic, and computational studies suggest that these modifications in the secondary coordination spheres can adjust the electronic structure of the enzyme-substrate complex to counteract the effects induced by the metal substitution. These results explicitly demonstrate that such noncovalent interactions encrypt metal specificity in a finely modulated manner, revealing the underestimated chemical power of the hydrophobic sequence network in enzyme catalysis.

The role of CopA in Streptococcus pyogenes copper homeostasis and virulence

Maintenance of intracellular metal homeostasis during interaction with host niches is critical to the success of bacterial pathogens. To prevent infection, the mammalian innate immune response employs metal-withholding and metal-intoxication mechanisms to limit bacterial propagation. The first-row transition metal ion copper serves critical roles at the host-pathogen interface and has been associated with antimicrobial activity since antiquity. Despite lacking any known copper-utilizing proteins, streptococci have been reported to accumulate significant levels of copper. Here, we report that loss of CopA, a copper-specific exporter, confers increased sensitivity to copper in Streptococcus pyogenes strain HSC5, with prolonged exposure to physiological levels of copper resulting in reduced viability during stationary phase cultivation. This defect in stationary phase survival was rescued by supplementation with exogeneous amino acids, indicating the pathogen had altered nutritional requirements during exposure to copper stress. Furthermore, S. pyogenes HSC5 ΔcopA was substantially attenuated during murine soft-tissue infection, demonstrating the importance of copper efflux at the host-pathogen interface. Collectively, these data indicate that copper can severely reduce the viability of stationary phase S. pyogenes and that active efflux mechanisms are required to survive copper stress in vitro and during infection.

Slow magnetic relaxation of a S = 1/2 copper(II)-substituted Keggin-type silicotungstate.

This is the first report on slow magnetic relaxation in an S = 1/2 system based on a first-row transition metal ion with the polyoxometalate skeleton [(n-C4H9)4N]4H2[SiW11O39Cu] (1). The X-band electron-spin-resonance spectrum of 1 measured at room temperature indicates that the copper ion experiences significantly reduced intermolecular interactions compared to the potassium salt and that it adopts a five-coordinated square-pyramidal coordination geometry. The AC magnetic-susceptibility measurements revealed that 1 undergoes slow magnetic relaxation in an applied static magnetic field (Hdc). The extracted spin-lattice relaxation time (92 ms at 1.8 K and Hdc = 5000 Oe) for 5% magnetically diluted 1, [(n-C4H9)4N]4H2[SiW11O39Cu0.05Zn0.95] (dil.1), is comparable to those of other potential S = 1/2 spin qubits. A relaxation-time analysis indicated that Raman spin-lattice relaxation dominates even at low temperatures in an optimized field. The extracted Raman exponent (n = 2.30) is smaller than those of other S = 1/2 complexes that carry organic ligands, which implies that the decrease in relaxation time at higher temperatures is likely to be moderate.

A Theoretical Study of Tris-(<i>o</i>-benzoquinonediimine)-First-Row Divalent Transition Metal Complexes

The ligand o-phenylenediamine (opda) and its oxidized form, o-benzoquinonediimine (bqdi), act as a fascinating candidate coordinating toward transition metal ions leading to the photochemical hydrogen production in absence of photosensitizers. Herein, we report the systematic study of the interaction between the oxidized form bqdi ligand, tris-(o-benzoquinonediimine) with divalent first-row transition metal series using DFT calculations. The lowest energy structures, bond length, binding energies, frontier molecular orbital analysis, natural bond orbitals, and global reactivity descriptor were calculated using B3LYP/6-311G(d,P) level of theory. The time dependent-DFT at the CAM-B3LYP/6-311+G(d,p) level of theory was applied to determine the electronic structures and the optical spectra. The theoretical binding trend of the divalent first-row transition metal series is decreasing as follows: Cu >Ti > V > Co > Ni > Fe > Cr > Zn >Mn. Among them, the binding potency of iron (II) by the bqdi ligand was not predominantly sturdy as compared to other first-row divalent transition metal ions. The origin of strong coordination with Fe(II) is attributed to its extra capability to induce covalent coordination of bqdi ligands. The complex exhibited two strong peaks at 370 nm and 452 nm, due to the HOMO-3 to LUMO+1 and HOMO-1 to LUMO transitions, respectively. Natural bond orbital analysis showed that the major interaction happens between the N lone pair electrons of the ligand with an anti-bonding orbital of metal ions, in which Ti showed the highest interaction energy than other metal ions. The present systemic DFT study of bqdi ligands with the first-row transition metals strongly encourages the future establishment of photochemical hydrogen production in absence of photosensitizers.

Research on three complex compounds of divalent first-row transition metal ions having the Schiff base derived from sulfanilamide and N-acetylisatin as bidentate ligand

The present paper aims to describe the research performed on three complex compounds formed by divalent first-row transition metal ions – namely M = Co(II), Ni(II) and Zn(II) (representing the complex kernels) with the Schiff base derived from the condensation of sulfanilamide with N-acetylisatin (which proved itself to be able to act as a bidentate ligand in all these cases). This Schiff base is N-(1-acetyl-2-oxoindolin-3-ylidene)-4-aminobenzenesulfonamide, which appears twice in each complex, but is not the only ligand in the structure, as two water molecules are also part of the coordination sphere; there also is an ionization sphere, represented by two chloride ions whose role is to equilibrate the electrical charge of the complex kernel (all the ligands being charge-free) – the general molecular formula of the complexes being thus: [ML2(H2O)2]Cl2. From the electrical point of view, we dealt with three complex compounds that all showed themselves to be electrolytes. As far as magnetic properties are concerned, two of them proved themselves to be paramagnetic, whereas the last one was found to be diamagnetic. After adding different spectral data, corroborating them with the prior information and with the elemental analysis – we have drawn the conclusion that the complexes are six-coordinated, so they all will tend to adopt an octahedral geometry.

A genetically encoded fluorescent sensor for manganese(II), engineered from lanmodulin

The design of selective metal-binding sites is a challenge in both small-molecule and macromolecular chemistry. Selective recognition of manganese (II)-the first-row transition metal ion that tends to bind with the lowest affinity to ligands, as described by the Irving-Williams series-is particularly difficult. As a result, there is a dearth of chemical biology tools with which to study manganese physiology in live cells, which would advance understanding of photosynthesis, host-pathogen interactions, and neurobiology. Here we report the rational re-engineering of the lanthanide-binding protein, lanmodulin, into genetically encoded fluorescent sensors for MnII, MnLaMP1 and MnLaMP2. These sensors with effective Kd(MnII) of 29 and 7 µM, respectively, defy the Irving-Williams series to selectively detect MnII in vitro and in vivo. We apply both sensors to visualize kinetics of bacterial labile manganese pools. Biophysical studies indicate the importance of coordinated solvent and hydrophobic interactions in the sensors' selectivity. Our results establish lanmodulin as a versatile scaffold for design of selective protein-based biosensors and chelators for metals beyond the f-block.

Tunable Construction of Sandwich-Type Double-[1 + 1] and Half-Folded [2 + 2] Schiff-Base Complexes Controlled by the Combination of Primary and Secondary Template Effects.

The first-row transition-metal ions Mn2+-Cu2+ could serve as effective templates to construct three types of double-[1 + 1], [2 + 2], and [1 + 1] Schiff-base dinuclear macrocyclic complexes in the presence of dialdehydes with different pendant arms and a common 1,8-diamine. The extremely flexible nature of macrocyclic ligands allows for the multiple template-directed syntheses, but the final products could be finely tuned by the subtle variations of Mn2+-Cu2+ ions in a 3d-electronic configuration, radius, and coordination number/geometry as well as the auxiliary (pendant-armed and anionic) template effect at the same time. Two borderlines are observed at the Co2+ ion for forming double-[1 + 1] and [2 + 2] metallacycles involving the H2pdd precursor and the [1 + 1] Cu2+ complex for double-[1 + 1] and [2 + 2] macrocycles containing the H2hpdd unit, respectively. The structural diversity is originated from the non-perfect match between [1 + 1]/[2 + 2] Schiff-base macrocycles and dinuclear metal centers; hence, a compromise between the metal coordination modes and alterations of the ligand conformation takes place.

Design of a Flexible, Zn-Selective Protein Scaffold that Displays Anti-Irving-Williams Behavior.

Selective metal binding is a key requirement not only for the functions of natural metalloproteins but also for the potential applications of artificial metalloproteins in heterogeneous environments such as cells and environmental samples. The selection of transition-metal ions through protein design can, in principle, be achieved through the appropriate choice and the precise positioning of amino acids that comprise the primary metal coordination sphere. However, this task is made difficult by the intrinsic flexibility of proteins and the fact that protein design approaches generally lack the sub-Å precision required for the steric selection of metal ions. We recently introduced a flexible/probabilistic protein design strategy (MASCoT) that allows metal ions to search for optimal coordination geometry within a flexible, yet covalently constrained dimer interface. In an earlier proof-of-principle study, we used MASCoT to generate an artificial metalloprotein dimer, (AB)2, which selectively bound CoII and NiII over CuII (as well as other first-row transition-metal ions) through the imposition of a rigid octahedral coordination geometry, thus countering the Irving-Williams trend. In this study, we set out to redesign (AB)2 to examine the applicability of MASCoT to the selective binding of other metal ions. We report here the design and characterization of a new flexible protein dimer, B2, which displays ZnII selectivity over all other tested metal ions including CuII both in vitro and in cellulo. Selective, anti-Irving-Williams ZnII binding by B2 is achieved through the formation of a unique trinuclear Zn coordination motif in which His and Glu residues are rigidly placed in a tetrahedral geometry. These results highlight the utility of protein flexibility in the design and discovery of selective binding motifs.

Near-Quantitative Predictions of the First-Shell Coordination Structure of Hydrated First-Row Transition Metal Ions Using K-Edge X-ray Absorption Near-Edge Spectroscopy.

The solvation structure of transition metal ions is important for applications in geochemistry, biochemistry, energy storage, and environmental chemistry. We study the X-ray absorption pre-edge and near-edge spectra at the K-edge of a nearly complete series of hydrated first-row transition metal ions with d orbital occupancy from d2 to d10. We optimize all of the structures at the density functional theory (DFT) level with explicit solvation and then compute the pre-edge X-ray absorption spectra with time-dependent DFT (TDDFT) and restricted active space second-order perturbation theory (RASPT2). TDDFT provides accurate results for spectra that are dominated by single excitations, while RASPT2 correctly distinguishes between singly and doubly excited states with quantitative accuracy compared with experiment. We analyze the pre-edge features for each metal ion to reveal the impact of the variations in d orbital occupancy on the first-shell coordination environment. We also report the lowest-energy ligand field d-d transitions using complete active space second-order perturbation theory.

In Situ X-ray Fluorescence Evaluation of Metal Ion Adsorption on Ferric Oxyhydroxide in Frozen Solutions

Cryosphere plays an important role in the global circulation of various chemical substances through the chemical processes that occur therein. Metal ions are adsorbed on adsorbents, such as ferric (oxy)hydroxides (FeOxH) and iron manganese oxides, and can be accumulated in the frozen phases. The metal ions can be supplied from the cryosphere to the hydrosphere in the bioavailable form by the desorption of metal ions during ice melting. Therefore, quantitative evaluation of the adsorption of metal ions in the frozen state is important to understand their exchange between the frozen and solution phases. Here, we propose in situ synchrotron X-ray fluorescence (XRF) as an appropriate method to quantitatively measure the adsorption of metal ions on FeOxH in a frozen solution. When NaCl solution freezes, a freeze-concentrated solution (FCS) is separated from ice. XRF imaging confirms that the first-row transition metal ions, that is, Mn2+, Co2+, Cu2+, and Zn2+, are enriched with FeOxH in the FCS. While in the presence of tris(hydroxymethyl)aminomethane (tris), Cu2+ forms a stable tris-complex and does not adsorb on FeOxH, other metal ions are partially adsorbed on FeOxH. Therefore, Cu2+ is always dissolved in the FCS, but other metal ions are distributed in both FCS and FeOxH. The linear correlations between the normalized XRF intensity of Cu2+ and those of other metal ions dissolved in the FCS are confirmed. Also, the normalized XRF intensity of Fe in FeOxH is proportional to those of the adsorbed metal ions. Thus, the XRF signals of Cu and Fe are considered to represent the other metal ions contained in the FCS and those adsorbed on FeOxH, respectively. The multivariate regression analysis provides the relative contributions of these two different states of the metal ion in frozen samples. From the regression coefficients, the adsorption ratio of the metal ion in the frozen phase can be estimated. The time change in the adsorption ratio is also successfully evaluated using this method.