Intermetal Distance Research Articles

Inter-Metal Interaction with a Threshold Effect in NiCu Dual-Atom Catalysts for CO2 Electroreduction.

Dual-atom catalysts (DACs) have become an emerging platform to provide more flexible active sites for electrocatalytic reactions with multi-electron/proton transfer, such as the CO2 reduction reaction (CRR). However, the introduction of asymmetric dual-atom sites causes complexity in structure, leaving an incomprehensive understanding of the inter-metal interaction and catalytic mechanism. Taking NiCu DACs as an example, herein, a more rational structural model is proposed, and the distance-dependent inter-metal interaction is investigated by combining theoretical simulations and experiments, including density functional theory computation, aberration-corrected transmission electron microscopy, synchrotron-based X-ray absorption fine structure, and Monte Carlo experiments. A distance threshold around 5.3Å between adjacent NiN4 and CuN4 moieties is revealed to trigger effective electronic regulation and boost CRR performance on both selectivity and activity. A universal macro-descriptor rigorously correlating the inter-metal distance and intrinsic material features (e.g., metal loading and thickness) is established to guide the rational design and synthesis of advanced DACs. This study highlights the significance of identifying the inter-metal interaction in DACs, and helps bridge the gap between theoretical study and experimental synthesis of atomically dispersed catalysts with highly correlated active sites.

Effects of intermetal distance on the electrochemistry-induced surface coverage of M-N-C dual-atom catalysts.

The often-overlooked electrocatalytic bridge-site poisoning of the emerging dual-atom catalysts (DACs) has aroused broad concerns very recently. Herein, based on surface Pourbaix analysis, we identified a significant change in the electrochemistry-induced surface coverages of DACs upon changing the intermetal distance. We found a pronounced effect of the intermetal distance on the electrochemical potential window and the type of pre-covered adsorbate, suggesting an interesting avenue to tune the electrocatalytic function of DACs.

Aggregation tendency of guest Fe in NaCo1\u2212xFexO2 (x\u2009

In transition metal (M) compounds, the partial substitution of the host transition metal (Mh) to guest one (Mg) is effective to improve the functionality. To microscopically comprehend the substitution effect, degree of distribution of Mg is crucial. Here, we propose that a systematic EXAFS analysis against the Mg concentration can reveal the spatial distribution of Mg. We chose NaCo1−xFexO2 as a prototypical M compound and investigated the local intermetal distance around the guest Fe [dFe–M(x)] against Fe concentration (x). dFe–M(x) steeply increased with x, reflecting the larger ionic radius of high-spin Fe3+. The x-dependence of dFe–M(x) was analyzed by an empirical equation, {d}_{mathrm{F}mathrm{e}-M}(x)=sxd_{mathrm{F}mathrm{e}-mathrm{F}mathrm{e}}+(1-sx)d_{mathrm{F}mathrm{e}-mathrm{C}mathrm{o}}, where dFe–Fe and dFe–Co are the Fe–Fe and Co–Fe distances, respectively. The parameter s represents degree of distribution of Fe; s = 1, > 1, < 1 are for random, attractive, and repulsive distribution, respectively. The obtained s value (= 4.8) indicates aggregation tendency of guest Fe.

Retraction of "Pushing Single-Oxygen-Atom-Bridged Bimetallic Systems to the Right: A Cryptand-Encapsulated Co-O-Co Unit".

A dicobalt(II) complex, [Co2(mBDCA-5t)](2-) (1), demonstrates a cofacial arrangement of trigonal monopyramidal Co(II) ions with an inter-metal separation of 6.2710(6) A. Reaction of 1 with potassium superoxide generates an encapsulated Co-O-Co core in the dianionic complex, [Co2O(mBDCA-5t)](2-) (2); to form the linear Co-O-Co core, the inter-metal distance has diminished to 3.994(3) A. Co K-edge X-ray absorption spectroscopy data are consistent with a +2 oxidation state assignment for Co in both 1 and 2. Multireference complete active space calculations followed by second-order perturbation theory support this assignment, with hole equivalents residing on the bridging O-atom and on the cryptand ligand for the case of 2. Complex 2 acts as a 2-e(-) oxidant toward substrates including CO and H2, in both cases efficiently regenerating 1 in what represent net oxygen-atom-transfer reactions. This dicobalt system also functions as a catalase upon treatment with H2O2.

Palladium bis-pincer complexes with controlled rigidity and inter-metal distance

We report a series of redox-active bis(pincer) Pd(ii) complexes in which the redox active units are based on either a diarylamido or a carbazolide framework.

Pushing Single-Oxygen-Atom-Bridged Bimetallic Systems to the Right: A Cryptand-Encapsulated Co-O-Co Unit.

A dicobalt(II) complex, [Co2(mBDCA-5t)](2-) (1), demonstrates a cofacial arrangement of trigonal monopyramidal Co(II) ions with an inter-metal separation of 6.2710(6) Å. Reaction of 1 with potassium superoxide generates an encapsulated Co-O-Co core in the dianionic complex, [Co2O(mBDCA-5t)](2-) (2); to form the linear Co-O-Co core, the inter-metal distance has diminished to 3.994(3) Å. Co K-edge X-ray absorption spectroscopy data are consistent with a +2 oxidation state assignment for Co in both 1 and 2. Multireference complete active space calculations followed by second-order perturbation theory support this assignment, with hole equivalents residing on the bridging O-atom and on the cryptand ligand for the case of 2. Complex 2 acts as a 2-e(-) oxidant toward substrates including CO and H2, in both cases efficiently regenerating 1 in what represent net oxygen-atom-transfer reactions. This dicobalt system also functions as a catalase upon treatment with H2O2.

Towards the Synthesis of Guanidinate- and Amidinate-Bridged Dimers of Mn and Ni

Reactions of Cp2M (Cp = cyclopentadienyl, M = Mn, Ni) with lithium amidinates and guanidinates are reported. The highly oxophilic nature of Mn leads to the isolation of the interstitial oxide Mn4O(MeN···CH···NMe)6 (4) in preference to the intended paddle-wheel homodimer Mn2(MeN···CH···NMe)4 when employing the sterically uncongested amidinate [MeN···CH···NMe]– ligand. In contrast, an analogous reaction using Cp2Ni yielded Ni2(MeN···CH···NMe)4 (5). The use of monoprotic guanidinate ligands also gave contrasting results for Mn and Ni. In the first case, the highly unusual spirocycle Mn{μ-NC(NMe2)2}4Li2·3THF (6) was produced in low yield. For M = Ni, use of the [hpp]– (1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) ligand gives results comparable with the synthesis of 5, with Ni2(hpp)4 (7) isolated. In contrast to recent data obtained using Cp2Cr, the guanidinate ligands do not sequester coformed CpLi. Density functional theory analysis corroborates the view that the intermetal distance in each of the reported dinickel paddle-wheel complexes (2.4846(8) and 2.3753(5) Å in 5 and 7 respectively) is defined by the geometric parameters of the bidentate ligands and that intermetal bonding is not present.

Structural insights into the ferroxidase site of ferritins from higher eukaryotes.

The first step of iron biomineralization mediated by ferritin is the oxidation at the ferroxidase active site of two ferrous ions to a diferric oxo/hydroxo species. Metal-loaded ferritin crystals obtained by soaking crystals of frog ferritin in FeSO(4) and CuSO(4) solutions followed by flash freezing provided X-ray crystal structures of the tripositive iron and bipositive copper adducts at 2.7 and 2.8 Å resolution, respectively. At variance with the already available structures, the crystal form used in this study contains 24 independent subunits in the asymmetric unit permitting comparison between them. For the first time, the diferric species at the ferroxidase site is identified in ferritins from higher eukaryotes. Anomalous difference Fourier maps for crystals (iron crystal 1) obtained after long soaking times in FeSO(4) solution invariantly showed diferric species with a Fe-Fe average distance of 3.1 ± 0.1 Å, strongly indicative of the presence of a μ-oxo/hydroxo bridge between the irons; protein ligands for each iron ion (Fe1 and Fe2) were also unequivocally identified and found to be the same in all subunits. For copper bound ferritin, dicopper(II) centers are also observed. While copper at site 1 is essentially in the same position and has the same coordination environment as Fe1, copper at site 2 is displaced toward His54, now acting as a ligand; this results in an increased intermetal distance (4.3 ± 0.4 Å). His54 coordination and longer metal-metal distances might represent peculiar features of divalent cations at the ferroxidase site. This oxidation-dependent structural information may provide key features for the mechanistic pathway in ferritins from higher eukaryotes that drive uptake of bivalent cation and release of ferric products at the catalytic site. This mechanism is supported by the X-ray picture obtained after only 1 min of soaking in FeSO(4) solutions (iron crystal 2) which reasonably contain the metal at different oxidation states. Here two different di-iron species are trapped in the active site, with intermetal distances corresponding to those of the ferric dimer in crystal 1 and of the dicopper centers and corresponding rearrangement of the His54 side chain.

Structure and Bonding of the Manganese(II) Phosphide Complex (t-BuPH2)(η5-Cp)Mn{μ-(t-BuPH)}2Mn(Cp)(t-BuPH2)

Rather than achieving bis-deprotonation of the phosphine, reaction of Cp2Mn (Cp = cyclopentadienyl) with t-BuPH2 at room temperature yields monodeprotonation of half of the available phosphine in the product (t-BuPH2)(η5-Cp)Mn{μ-(t-BuPH)}2Mn(Cp)(t-BuPH2) (1). This complex comprises a Mn(II) phosphide and is a dimer in the solid state, containing a Mn2P2 diamond core. Consistent with the observation of a relatively short intermetal distance of 2.8717(4) A in 1, DFT analysis of the full structure points to a singlet ground state stabilized by a direct Mn–Mn single bond. This is in line with the diamagnetic character of 1 and an 18-electron count at Mn.

Calculation of electron transfer rates in proteins

Electron transfer rates in Zn/Ru modified myoglobins are calculated using a molecular orbital method to obtain the electronic factor and empirical values of the reorganization energy and driving force. The protein is partitioned into three subsystems, A, B, and C, where A and C consist of the metal ions and their ligands and B is the protein bridge between A and C. Molecular orbitals are calculated in each subsystem. Interaction matrix elements between the subsystems are calculated and the total eigenvalue problem solved using a partitioning technique. The distance dependence of the rate is only roughly exponential as a function of intermetal distance. In particular, the rate of the His-12 derivative is larger than the average exponential decrease, in agreement with the experiments. If the indole group in the pathway between the metal ions is replaced by an equally large (CH{sub 2})n-chain, the calculated rate is close to the average exponential decrease. The electronic spectra of zinc porphin (ZnP) and Zn-substituted myoglobin (ZnMb) are calculated using a CNDO/S approach, and the results compare favorably with the corresponding experimental spectra.

Response of a Designed Metalloprotein to Changes in Metal Ion Coordination, Exogenous Ligands, and Active Site Volume Determined by X-ray Crystallography

The de novo protein DF1 is a minimal model for diiron and dimanganese metalloproteins, such as soluble methane monooxygenase. DF1 is a homodimeric four-helix bundle whose dinuclear center is formed by two bridging Glu side chains, two chelating Glu side chains, and two monodentate His ligands. Here, we report the di-Mn(II) and di-Co(II) derivatives of variants of this protein. Together with previously solved structures, 23 crystallographically independent four-helix bundle structures of DF1 variants have been determined, which differ in the bound metal ions and size of the active site cavity. For the di-Mn(II) derivatives, as the size of the cavity increases, the number and polarity of exogenous ligands increases. This collection of structures was analyzed to determine the relationship between protein conformation and the geometry of the active site. The primary mode of backbone movement involves a coordinated tilting and sliding of the first helix in the helix-loop-helix motif. Sliding depends on crystal-packing forces, the steric bulk of a critical residue that determines the dimensions of the active site access cavity, and the intermetal distance. Additionally, a torsional motion of the bridging carboxylates modulates the intermetal distance. This analysis provides a critical evaluation of how conformation, flexibility, and active site accessibility affect the geometry and ligand-binding properties of a metal center. The geometric parameters defining the DF structures were compared to natural diiron proteins; DF proteins have a restricted active site cavity, which may have implications for substrate recognition and chemical stability.

Calix[4]arene-Based Zn2+ Complexes as Shape- and Size-Selective Catalysts of Ester Cleavage

[structures: see text] The kinetics of methanolysis of a number of esters endowed with a carboxylate anchoring group have been investigated in the presence of di- and trinuclear Zn2+ complexes of calix[4]arenes functionalized at the upper rim with nitrogen ligands. The results (i) emphasize the importance of a good match between ester size and intermetal distance, (ii) reveal a substrate independent superiority of the 1,2-vicinal dinuclear catalyst 1-Zn2 to its 1,3-distal regioisomer 2-Zn2, and (iii) provide further evidence for the concurrence of the three metal ions of 3-Zn3 in the catalytic mechanism.

Dinuclear barium(II) complexes based on a calix[4]arene scaffold as catalysts of acyl transfer.

Two novel regioisomeric calix[4]arene derivatives (2 and 3), decorated with two aza[18]crown-6 units at vicinal (1,2) or diagonal (1,3) positions of the upper rim, were synthesized. The catalytic activities of their dinuclear Ba2+ complexes were investigated in the ethanolysis of esters 8-11, endowed with a carboxylate anchoring group. Major results are as follows: 1) the two metal ions in the dinuclear catalysts work together in a cooperative fashion; 2) the vicinal calix[4]arene catalyst 2 is far superior to its diagonal regioisomer 3 in the reactions of all of the investigated esters; and 3) the distance between the carboxylate and ester carbonyl, which increases regularly from 8 to 11, influences reactivity of catalytic ester cleavage in a way that is decidedly suggestive of the importance of a good match between ester size and metal-to-metal distance. However, the superiority of the vicinal catalyst 2 relative to 3 cannot be explained on the basis of the putative match of ester size to intermetal distance, thus providing an indication that additional, still poorly understood effects may contribute significantly to catalytic efficiency.

Variational Approach to Electron Transfer in Bridged Diruthenium Complexes

Abstract In this work, we address the energetics of electron transfer in bridged diruthenium complexes from a theoretical and numerical perspective. The electronic structure of the bridged complexes is described by a tight-binding Hamiltonian, which is extended by an on-site spin pairing energy. The coupling of the system to a polarizable solvent is taken into account by a nonretarded reaction field. The resulting nonlinear Hamiltonian is solved within the Hartree–Fock mean-field approximation, and the emerging potential energy surface is analysed using Marcus´ theory of electron transfer reactions. For the systems studied here, the charge transfer rate depends on the chemical nature of the bridge rather than the intermetal distance. We discuss the special nature of the Creutz–Taube cation, which is on the border of charge delocalization.

Long-range magnetic coupling through extended pi-conjugated aromatic bridges in dinuclear copper(II) metallacyclophanes.

Self-assembly of 1,4-phenylenebis(oxamate) and 4,4'-biphenylenebis(oxamate) ligands and Cu2+ ions gives two new dinuclear copper(II) metallacyclophanes where the two metal centers are connected by double para-substituted aromatic diamide bridges. Despite the relatively large intramolecular Cu-Cu distance of ca. 8 and 12 A for each complex, magnetic susceptibility measurements evidence strong to moderately strong intramolecular antiferromagnetic couplings (-J values of the order of 100 and 10 cm-1, respectively). Density functional theory calculations on these compounds and their homologues with linear oligo-p-phenylenediamide bridges predict a rather slow exponential decay of magnetic coupling with increasing intermetal distance (r values up to 25 A) along this novel series of dicopper metalla-amidocyclophanes.

Coordination polymers based on cadmium(ii) pyridine complexes: synthesis, range of existence, and structureDedicated to Prof. Dr. G. E. Herberich on the occasion of his 65th birthday.

A series of nine new cadmium compounds of formal composition [Cd(μ-X)2py2] (X = Cl, Br, I; py = 3,5-disubstituted pyridine ligands with methyl, chloro, and bromo substituents) were synthesized and characterized by single crystal X-ray diffraction. Only for the combination of the sterically most demanding ligands, namely I and 3,5-dimethylpyridine, is a mononuclear tetrahedral complex obtained; in all other eight cases one-dimensional coordination polymers are found. In these chains cadmium(II) cations are linked in an edge-sharing fashion by halogen bridges in their pseudoequatorial plane, with terminal pyridine ligands completing the pseudo-octahedral coordination sphere. In all polymers the distance between neighboring Cd atoms corresponds to the shortest lattice constant: metal⋯metal separations systematically depend on both bridging and terminal ligands and range between 3.77 and 4.14 Å. This intermetal distance and the coordination mode of the donor ligands are found to be correlated. Five of the Cd chain polymers adopt the structure type found for [Zn(μ-Cl)2py2]∞. In addition to previous work the zinc derivatives [ZnX2(3,5-Me2py)2] (X = Cl, Br) were investigated for comparison and shown to be mononuclear.

Conformational changes in yeast pyruvate kinase studied by 205Tl+ NMR.

The interaction of the monovalent cation with yeast pyruvate kinase (yPK) has been investigated by 205Tl+ NMR. TlNO3 activates yPK to 80-90% activity compared to KCl with an apparent Ka of 1.00 +/- 0.03 mM in the presence of 4 mM Mn(NO3)2 as the activating divalent cation. At higher concentrations of Tl+, enzyme inhibition is observed with an apparent KI of 180 +/- 10 mM. The extent of inhibition is dependent on the nature and concentration of the divalent cation. The effect of Mn2+ on the 1/T1 and 1/T2 values of 205Tl+ in the presence of yPK was determined at 173.02 MHz (300 MHz, 1H) and 346.03 MHz (600 MHz, 1H). The temperature dependence of the relaxation rates indicates that fast exchange conditions prevail for 205Tl+ longitudinal relaxation rates. The correlation time, tauc, for the Mn2+-205Tl+ interaction was estimated by a frequency dependence of 1/T1m for several enzyme complexes, and an average value of tauc was determined to be 0.91 ns. The distance between Tl+ and Mn2+ at the active site of yPK was calculated from the paramagnetic contribution of Mn2+ to the longitudinal (1/T1m) relaxation rates of Tl+ bound to yPK. For the apo yPK complex, the Tl+ to Mn2+ distance is 6.7 +/- 0.2 A. Upon addition of phosphoenolpyruvate (PEP) to form the yPK-Tl-Mn-PEP complex, the inter-cation distance decreases to 6.1 +/- 0.3 A. The addition of the allosteric activator fructose 1,6-bisphosphate (FBP) to form the yPK-Tl+-Mn2+-PEP-FBP complex gives an intermetal distance of 6.2 +/- 0.2 A. In the yPK-Tl-Mn-FBP complex, a Tl+-Mn2+ distance of 6.0 +/- 0.1 A is observed, indicating that FBP causes a conformational change at the active site in the absence of PEP. Analogous 205Tl NMR experiments with competitive inhibitors of PEP (oxalate, BrPEP) indicate that these ligands do not induce the same conformational changes as do the physiological substrates and activators. Similar experiments with the nonallosteric rabbit muscle PK were also performed and analyzed.

Investigating the effects of posttranslational adenylylation on the metal binding sites of Escherichia coli glutamine synthetase using lanthanide luminescence spectroscopy.

Lanthanide luminescence was used to examine the effects of posttranslational adenylylation on the metal binding sites of Escherichia coli glutamine synthetase (GS). These studies revealed the presence of two lanthanide ion binding sites of GS of either adenylylation extrema. Individual emission decay lifetimes were obtained in both H2O and D2O solvent systems, allowing for the determination of the number of water molecules coordinated to each bound Eu3+. The results indicate that there are 4.3 +/- 0.5 and 4.6 +/- 0.5 water molecules coordinated to Eu3+ bound to the n1 site of unadenylylated enzyme, GS0, and fully adenylylated enzyme, GS12, respectively, and that there are 2.6 +/- 0.5 water molecules coordinated to Eu3+ at site n2 for both GS0 and GS12. Energy transfer measurements between the lanthanide donor-acceptor pair Eu3+ and Nd3+, obtained an intermetal distance measurement of 12.1 +/- 1.5 A. Distances between a Tb3+ ion at site n2 and tryptophan residues were also performed with the use of single-tryptophan mutant forms of E. coli GS. The dissociation constant for lanthanide ion binding to site n1 was observed to decrease from Kd = 0.35 +/- 0.09 microM for GS0 to Kd = 0.06 +/- 0.02 microM for GS12. The dissociation constant for lanthanide ion binding to site n2 remained unchanged as a function of adenylylation state; Kd = 3.8 +/- 0.9 microM and Kd = 2.6 +/- 0.7 microM for GS0 and GS12, respectively. Competition experiments indicate that Mn2+ affinity at site n1 decreases as a function of increasing adenylylation state, from Kd = 0.05 +/- 0.02 microM for GS0 to Kd = 0.35 +/- 0.09 microM for GS12. Mn2+ affinity at site n2 remains unchanged (Kd = 5.3 +/- 1.3 microM for GS0 and Kd = 4.0 +/- 1.0 microM for GS12). The observed divalent metal ion affinities, which are affected by the adenylylation state, agrees with other steady-state substrate experiments (Abell LM, Villafranca JJ, 1991, Biochemistry 30:1413-1418), supporting the hypothesis that adenylylation regulates GS by altering substrate and metal ion affinities.

Binuclear copper(II) complexes bridged by oxamidate and dithio-oxamidate groups. Synthesis, structure, and magnetism

Binuclear copper(II) complexes bridged by oxamidate (oxd) or dithio-oxamidate (dto) groups [Cu2L2(oxd)]X2 and [Cu2L2(dto)]X2[L = 2,2′-bipyridyl (bipy) or 1,10-phenanthroline, X = NO3 or ClO4], have been synthesized. Single-crystal X-ray analysis for [Cu2(bipy)2(oxd)][ClO4]2·2dmf (dmf = dimethylformamide) reveals that the oxd2– ion in trans conformation bridges two copper(II) ions with an intermetal distance of 5.192(2)A. The configuration around each copper is square pyramidal with two nitrogens of bipy and one nitrogen and one oxygen of oxd in the basal plane and a perchlorate oxygen at the apex [2.433(9)A]. The dto-bridged complexes are characterized by two intense i.r. bands at 1 540 and 1 440 cm–1 and a charge-transfer band at (21.6–21.9)× 103 cm–1. Magnetic analyses in the temperature range 80–300 K on the basis of the Bleaney–Bowers equation gave –2J values (singlet–triplet energy separation) of 488–566 cm–1 for the oxd-bridged complexes and 726–982 cm–1 for the dto-bridged complexes, indicating that the dithio-oxamidate bridge mediates a stronger antiferromagnetic spin exchange between copper(II) ions than does the oxamidate bridge. Cyclic voltammograms for the oxd complexes show an irreversible couple at +0.1 V vs. the saturated calomel electrode which is assigned to the process CuII2→ CuICuII.

Chromium (III)-nucleotide complexes as paramagnetic probes for catalytic sites of phosphoryl transfer enzymes

Cr/sup 3 +/-nucleotides, as exchange-inert, paramagnetic analogs of Mg/sup 2 +/-nucleotides, are useful paramagnetic probes for nuclear relaxation studies to determine intersubstrate distances between phosphoryl donor and acceptor substrates on phosphoryl transfer enzymes and, for enzymes with an additional metal ion site to estimate the intermetal distance between the enzyme- and nucleotide-bound metal ions. A basic assumption of this approach, that Cr/sup 3 +/-nucleotides bind at the Mg/sup 2 +/-nucleotide sites in a structurally similar manner, can be tested by observing kinetic inhibition patterns in the presence of Cr/sup 3 +/-nucleotides and, where possible, by their ability to activate partial reactions and to participate as substrates in phosphoryl transfer reactions.