Outer Coordination Sphere Research Articles

Designing electrochemically reversible H2 oxidation and production catalysts

The most energy-efficient electrocatalysts mediate forward and reverse reactions at high rates with minimal overpotential requirements. Such electrocatalytic reversibility is commonly observed for redox enzymes and is an attribute that we have sought to bestow on synthetic molecules to realize highly active and robust catalysts for applications in renewable energy. The recent development of the first synthetic molecular catalysts that reversibly mediate H2 ⇌ 2 H+ + 2e− exploits an enzyme-inspired outer coordination sphere that works in concert with both first and second coordination spheres. In this Perspective, we discuss a series of molecular Ni catalysts for H2 production and oxidation that exhibit electrochemical reversibility. Study of these catalysts allows us to identify important first, second and outer coordination sphere features necessary for efficient conversions of H2 and provides direction for the rational design of electrocatalysts that operate on other small molecules. This Perspective describes how reversible catalysis — a hallmark of enzymes — can be reproduced in synthetic catalysts by rationally designing first and second coordination spheres, as well as amino acid-based outer coordination spheres. We describe this in the context of Ni prototypes for efficient H2 oxidation and evolution.

Extraction of metal bromides from chloride brines with mixtures of molecular iodine and tributyl phosphate

The process of recovery by direct extraction of metal bromides from calcium chloride brines using mixtures of molecular iodine with tributyl phosphate has been studied. It was found that iodine significantly increases the recovery of the bromides of alkali and alkaline earth metals due to the formation of hydrophobic iodine complexes with the halide anions. Bromides of calcium and lithium are the most selectively extracted. The extracted complexes significantly dissociate into ions in the organic phase. The extraction of calcium halide over a wide range of its concentration in an aqueous solution results in the extraction of a mixed chloride-bromide salt. This is due to a significant difference in the separation factors for bromide and chloride ions in the interphase exchange reactions for the halide ions of the inner and outer coordination spheres of the extracted complex.

Going beyond Structure: Nickel-Substituted Rubredoxin as a Mechanistic Model for the [NiFe] Hydrogenases.

Well-defined molecular systems for catalytic hydrogen production that are robust, easily generated, and active under mild aqueous conditions remain underdeveloped. Nickel-substituted rubredoxin (NiRd) is one such system, featuring a tetrathiolate coordination environment around the nickel center that is identical to the native [NiFe] hydrogenases and demonstrating hydrogenase-like proton reduction activity. However, until now, the catalytic mechanism has remained elusive. In this work, we have combined quantitative protein film electrochemistry with optical and vibrational spectroscopy, density functional theory calculations, and molecular dynamics simulations to interrogate the mechanism of H2 evolution by NiRd. Proton-coupled electron transfer is found to be essential for catalysis. The coordinating thiolate ligands serve as the sites of protonation, a role that remains debated in the native [NiFe] hydrogenases, with reduction occurring at the nickel center following protonation. The rate-determining step is suggested to be intramolecular proton transfer via thiol inversion to generate a NiIII-hydride species. NiRd catalysis is found to be completely insensitive to the presence of oxygen, another advantage over the native [NiFe] hydrogenase enzymes, with potential implications for membrane-less fuel cells and aerobic hydrogen evolution. Targeted mutations around the metal center are seen to increase the activity and perturb the rate-determining process, highlighting the importance of the outer coordination sphere. Collectively, these results indicate that NiRd evolves H2 through a mechanism similar to that of the [NiFe] hydrogenases, suggesting a role for thiolate protonation in the native enzyme and guiding rational optimization of the NiRd system.

Disorder-Induced Breaking of the Local Inversion Symmetry in Rhombohedral Pyrochlores M2La3Sb3O14 (M = Mg or Ca): A Structural and Spectroscopic Investigation.

A detailed investigation of the overall crystal structure, and in particular of the local structure around the cations in M2La3Sb3O14 (M = Mg, Ca) was accomplished using X-ray diffraction, steady state luminescence spectroscopy and decay kinetics, and state of the art density functional calculations. The computational tool was also used to investigate the structure of Mn2La3Sb3O14. The Eu3+ dopant ion was employed as an optical probe of the local symmetry at the cationic sites. The use of these complementary techniques shows that the antimonates under investigation belong to the rhombohedral pyrochlore family with space group R3̅ m (No. 166), but while Mg2La3Sb3O14 and Mn2La3Sb3O14 show an ordered cationic configuration, the Ca2+ and La3+ of Ca2La3Sb3O14 are disordered because of their similar ionic radii. In both the Mg- and the Ca-based compounds, the Eu3+ ions formally occupy centrosymmetric sites, but in the case of Ca2La3Sb3O14 the presence of disorder in the outer coordination spheres removes the local inversion symmetry in these sites. This has a strong influence on the Eu3+ luminescence spectrum and on the radiative decay rate of the 5D0 emitting level.

Adsorption of Cs onto Biogenic Birnessite: Effects of Layer Structure, Ionic Strength, and Competition Cations

Although the adsorption of cesium (Cs) onto phyllosilicate minerals has been widely studied, the effect of Cs on redox-sensitive biogenic Mn oxide is not well understood. In this study, the structural transformation of biogenic birnessite stimulated by commonly occurring natural heavy metals, and the influence of those metals on the adsorption behavior of Cs over a wide range of concentrations (1 × 10–10 to 0.1 mol/L) was carefully examined via solution chemistry and X-ray absorption fine structure techniques. The Cs was reversibly adsorbed onto the mineral surface to form an outer-sphere coordination for all biogenic birnessite samples. The presence of heavy metals (e.g., Zn and Ni) during bio-oxidation of Mn(II), followed by acid treatment, increased the number of available layer vacancies, which consequently increased the adsorption capacity of Cs in the final product. The analysis of the dependence of sorption values on the ionic strength showed distinct results on biogenic birnessite and chemically s...

Synergistic separation of copper(II) in sulfate solutions through outer-sphere coordination of dinonylnaphthalene disulfonic acid

Abstract Synergistic extraction is a promising strategy to achieve the separation and concentration of metal ions from complex effluents. Herein, the selective extraction behaviors of Cu(II) with 2-ethylhexyl pyridine-4-carboxylate ester (LII) and dinonylnaphthalene disulfonic acid (H2DNNDS) have been investigated. The extraction of impurity ions including Al(III), Fe(III), Mn(II), Mg(II) and Ca(II) can be effectively depressed at the high molar ratio of the ligand LII and H2DNNDS, and the separation factors of Cu/Al, Cu/Ca, Cu/Fe, Cu/Mg, and Cu/Mn can reach 840, 1707, 2205, 2241, and 1269, respectively. Moreover, the relationship between the coordination chemistry and the extraction behavior has been demonstrated by X-ray single crystal diffraction, Fourier transform infrared spectroscopy (FT-IR) and electrospray ionization mass spectrometry (ESI-MS). The spectra analyses indicate that the extracted Cu(II) complex would have a similar distorted octahedral coordination environment with the Cu(II) model complex with the corresponding short-chain analogues of the ligand LII and H2DNNDS. The inner coordination sphere of Cu(II) complex is occupied with three N atoms from pyridine ring of the ligand LII and three O atoms from water molecules, and the deprotonated H2DNNDS acts as a non-coordinating ion-pairing agent to form an outer-sphere coordination structure. Hence, the extraction efficiency enhancement of Cu(II) with the ligand LII after mixed with H2DNNDS, can be attributed to the dual contribution of inner- and outer-sphere coordination of synergistic ligands.

Hydrogen Transfer Catalysis beyond the Primary Coordination Sphere

In this Perspective, we outline recent examples of homogeneous transition-metal hydrogen transfer catalysts for which functionality within the complex’s outer coordination sphere influences the outcome of a reaction. Secondary-sphere groups are often applied to hydrogen transfer reactions, but their specific role during catalysis is not always well-understood. New experimental and theoretical work details the complexity associated with predicting secondary-sphere interactions and therefore designing improved catalysts. This Perspective highlights examples of catalysts containing secondary-sphere groups that (1) accelerate a key turnover-limiting step such as H2 heterolysis or hydride transfer, (2) limit competing catalytic cycles, (3) prevent catalyst decomposition, and/or (4) provide access to new catalysts through postmetalation modifications. The examples described herein emphasize numerous roles of the secondary sphere in hydrogen transfer catalysis and illustrate how the optimal use of these interact...

Cyclopentadienyl molybdenum(II) compounds bearing carboxylic acid functional group

Abstract This work describes a procedure giving cyclopentadienyl molybdenum(II) compounds bearing carboxylic acid function group. It involves synthesis of carboxylic acid ester functionalized cyclopentadienides, their coordination to molybdenum(II) precursor and saponification of ester function groups. The method is not limited only to compounds with the function group directly attached in the cyclopentadienyl ring but also to those functionalized in the side chain. The attempts to synthesize the indenyl analogues were only partially successful due to low stability in the saponification step. All reported structure types were elucidated from spectroscopic measurements and verified by X-ray crystallography. The second part of the work describes an effect of the outer-coordination sphere on cytotoxicity of the cationic molybdenum(II) compounds bearing N,N-chelating ligands. The cytotoxicity of the modified species bearing phenanthroline ligand toward human leukemia cells MOLT-4 (IC50 = 10.5 ± 0.5 μmol l−1) is higher than reported for cisplatin (IC50 = 15.8 ± 1.9 μmol l−1).

Influence of copper(II) ions on the noncovalent interactions between cytidine-5′-diphosphate or cytidine-5′-triphosphate and biogenic amines putrescine or spermidine

Potentiometric and NMR spectroscopic studies of the nucleotide (NucP)/polyamine (PA) system (where NucP = CDP, CTP, PA = putrescine or spermidine) revealed the formation of molecular complexes (NucP)(Hx+y)(PA) (where Hx+y = number of protons; x - from NucP and y - from PA). Their thermodynamic parameters were determined and the modes of their interactions were proposed. The main reaction centers were found to be the protonated amine groups of polyamine (positive centers) and phosphate groups of nucleotide (negative centers). The pH ranges in which the complex occurs correspond to those of amine protonation and -PO3x- group deprotonation, which unambiguously confirms the dipole-dipole type of interaction. In the pH range of total deprotonation of NHx+ groups from the polyamine, the molecular complexes disappear. The equilibrium and spectroscopic studies of the ternary systems Cu(II)/NucP/PA evidenced the formation of Cu(NucP)Hx+y(PA) type coordination compounds and Cu(NucP)⋯(PA)(Hx) type molecular complexes with polyamine in the outer coordination sphere. The main sites of metal ion bonding in the latter species are the phosphate groups of the nucleotide, while in the coordination compounds – besides the phosphate groups – also the donor nitrogen atoms from the polyamines. In this paper we have also quantitatively calculated the effect of metal ions on the formation of the molecular complexes.

The Supramolecular and Coordination Chemistry of Cobalt(II) Extraction by Phosphinic Acids

A combination of mass spectrometry, DFT calculations and 31P{1H} NMR spectroscopy has been used to define the mode of action of the commercial cobalt extractant, bis(2,4,4‐trimethylpentyl)phosphinic acid (CYANEX®272, L1H) in Co recovery. The nature of the CoII complexes formed in the water‐immiscible phase is determined largely by the propensity of phosphinates to form strong interligand H‐bonds in the outer coordination sphere and also to form stable µ2‐Co–O–P–O–Co bridges. At low Co loading levels, the predominant species is the 4:1 complex, [Co(L1·L1H)2], in which coordinated neutral phosphinic acid ligands form strong H‐bonds to adjacent anionic phosphinates. At higher Co loading, oligomers such as [(L1·L1H)Co(L12Co)n(L1·L1H)] are formed with µ2‐phosphinate bridging, resulting in a substantial increase in the viscosity of the water‐immiscible phase. The presence of tris(2,4,4‐trimethylpentyl)phosphine oxide (L2) in the commercial formulation reduces the viscosity because its incorporation into oligomeric complexes such as [(L2)Co{L13CoL1CoL13Co}mL1] can terminate chains, resulting in a lower average molecular weight. The uptake of Zn by L1H shows a very similar dependence of viscosity on loading, and DOSY spectra and mass spectrometry demonstrate that higher molecular weight species are present at high Zn loading.

Dimorphism of Hexaaquanickel(Ii) Bis(p-Nitrobenzoate) Dihydrate Salt: A New Triclinic Crystal Form

Room temperature crystallization from a water/ethanol solution of NiSO4, p-nitrobenzoic acid (HPNBA), and thiourea yields single crystals of [Ni(H2O)6](PNBA)2·2H2O. Crystal data are: a = 5.9626(8), b = 7.3552(11), c = 12.3459(17) A, α = 97.885(12), β = 96.714(11), γ = 91.158(11)°, V = 532.28(13) A3, space group Pī, Z = 1. An alternative monoclinic crystal form of the same compound with a slightly lower packing efficiency was previously obtained. The Ni(II) cation is located on a crystallographic inversion center and octahedrally coordinated by six water molecules. The PNBA moieties occupy an outer coordination sphere acting as counter-anions. An intricate system of H bonds between the complex cation, the PNBA anions, and the co-crystallized water molecules results in a three-dimensional network.

Cu(II) complexes of cytosine and 1-methylcytosine with bromide: old motifs and new structures

The preparation from aqueous solutions and crystal structures of the compounds trans-[Cu(1-Mecyt)2Br2] (1), trans-[Cu(cyt)2Br2] (2), [Cu(cyt)4]Br2·2H2O (3), and [Cu(cyt)Br3][Hcyt] (4) (where 1-Mecyt = 1-Methylcytosine and cyt = cytosine) is reported. Compounds 1 and 2 are made of trans-CuL2Br2 molecules [L = 1-Mecyt (1) or cyt (2)]. In 1, the coordination geometry around the central Cu(II) is distorted square pyramidal, while in 2 no such distortion is observed. Two solid-state polymorphs of 1 have been isolated, 1a and 1b, both of which contain no crystallization solvent and thus only differ in the relative disposition of vicinal [Cu(1-Mecyt)2Br2] units through space. As far as we know, 1 is the first example of a 1:2 Cu(II) complex with 1-Mecyt showing a trans arrangement of the ligands. The obtaining of 2 is also worth of note, as this species was previously reported as elusive. Compound 3 comprises tetra-cytosine-Cu(II) cations and bromides, these latter building, together with water molecules, a H-bonded tetrameric [Ow2Br2] motif. Compound 4 is constituted by [Cu(cyt)Br3]− anions and Hcyt+ cations, the cytosinium cations being in the outer metal coordination sphere. Likely, a pH-mediated equilibrium between 2 and 4 exists, 4 being only observable in traces as the neutral 1:2 complex with two metal–cyt direct bonds (2) is the thermodynamically favored product.

Hybrid Macrocycles for Selective Binding and Sensing of Fluoride in Aqueous Solution.

Synthesis and anion binding properties of hybrid macrocycles containing ammonium and hydrogen bond donor groups are reported. Receptor properties were studied in a 10 mM MES buffer solution at pH 6.2, at which the receptors carry two positive charges at the secondary amine groups. Receptor 1 was found to bind fluoride with the highest affinity (105 M-1) and selectivity among the synthesized receptors. It was the only receptor that demonstrated fluorescence increase upon addition of fluoride. Other titration experiments with halides and oxyanions led to an anion-induced aggregation and fluorescence quenching. The mechanism of the particular turn-on fluorescence for fluoride was explained by the ability of receptor 1 to encapsulate several fluoride anions. Multiple anion coordination resulted in the protonation of the tertiary amine group and subsequent hindering of the PET process. 1H and 19F NMR titrations, single-crystal X-ray structure of chloride complex, and DFT calculation suggest that 1 can perfectly accommodate two fluoride anions in the inner cavity but only one chloride, keeping the second chloride in the outer coordination sphere. Thus, the importance of size selectivity, which is reflected in a collective behavior of molecules in an aqueous solution, represents a new strategy for the design of highly selective probes for fluoride functioning in an aqueous solution.

Homogeneously Catalyzed Electroreduction of Carbon Dioxide-Methods, Mechanisms, and Catalysts.

The utilization of CO2 via electrochemical reduction constitutes a promising approach toward production of value-added chemicals or fuels using intermittent renewable energy sources. For this purpose, molecular electrocatalysts are frequently studied and the recent progress both in tuning of the catalytic properties and in mechanistic understanding is truly remarkable. While in earlier years research efforts were focused on complexes with rare metal centers such as Re, Ru, and Pd, the focus has recently shifted toward earth-abundant transition metals such as Mn, Fe, Co, and Ni. By application of appropriate ligands, these metals have been rendered more than competitive for CO2 reduction compared to the heavier homologues. In addition, the important roles of the second and outer coordination spheres in the catalytic processes have become apparent, and metal-ligand cooperativity has recently become a well-established tool for further tuning of the catalytic behavior. Surprising advances have also been made with very simple organocatalysts, although the mechanisms behind their reactivity are not yet entirely understood. Herein, the developments of the last three decades in electrocatalytic CO2 reduction with homogeneous catalysts are reviewed. A discussion of the underlying mechanistic principles is included along with a treatment of the experimental and computational techniques for mechanistic studies and catalyst benchmarking. Important catalyst families are discussed in detail with regard to mechanistic aspects, and recent advances in the field are highlighted.

Dimorphism of hexaaquanickel(II) bis(p-nitrobenzoate) dihydrate salt: A new triclinic crystal form

Room temperature crystallization from a water/ethanol solution of NiSO4, p-nitrobenzoic acid (HPNBA), and thiourea yields single crystals of [Ni(H2O)6](PNBA)2·2H2O. Crystal data are: a = 5.9626(8), b = 7.3552(11), c = 12.3459(17) Å, α = 97.885(12), β = 96.714(11), γ = 91.158(11)°, V = 532.28(13) Å3, space group P1̅, Z = 1. An alternative monoclinic crystal form of the same compound with a slightly lower packing efficiency was previously obtained. The Ni(II) cation is located on a crystallographic inversion center and octahedrally coordinated by six water molecules. The PNBA moieties occupy an outer coordination sphere acting as counter-anions. An intricate system of H bonds between the complex cation, the PNBA anions, and the co-crystallized water molecules results in a three-dimensional network.

Computation of Energy, Intensity and Thermodynamic Parameters for the Interaction of Ln(III) with Nucleic Acid: Analysis of Structural Conformations, Chemical Kinetics and Thermodynamic Behaviour through 4f-4f Transition Spectra as Probe

Nucleosides and Nucleotides are polydentate ligands, offering potential binding sites for metal ions. Energy interaction parameters: Slater Condon Fk (cm-1), Spin Orbit interaction ξ4f (cm-1), Nephelauxetic Ratio (β), Bonding (b1/2) and Covalency (δ) parameter for the interaction of Pr(III) with Nucleoside and Nucleotide are evaluated to study the mode of binding of the Nucleic Acid Components (Guanosine and Guanosine Triphosphate) with Pr(III). Further Intensity Parameters like Oscillation Strength and Judd Ofelt Parameter (T2, T4, T6) have been evaluated to investigate degree of inner or outer sphere co-ordination of Pr(III) with Nucliec Acid ligands. Comperative Absorption Spectra in different solvents substantiate the informations, gathered from the evaluated values of both energy interaction and intensity parameters. Further evaluation of Thermodynamic parameters (ΔG, ΔH and ΔS) through Kinetic studies enable to provide the detailed information about the reaction pathways and thermodynamic behavour of the complexation process of neucleosides and nucleotides with Pr(III) and Ca2+.

Inner-Sphere Coordination of Divalent Metal Ion with Nucleobase in Catalytic RNA.

Identification of the function of metal ions and the RNA moieties, particularly nucleobases, that bind metal ions is important in RNA catalysis. Here we combine single-atom and abasic substitutions to probe functions of conserved nucleobases in ribonuclease P (RNase P). Structural and biophysical studies of bacterial RNase P propose direct coordination of metal ions by the nucleobases of conserved uridine and guanosine in helix P4 of the RNA subunit (P RNA). To biochemically probe the function of metal ion interactions, we substituted the universally conserved bulged uridine (U51) in the P4 helix of circularly permuted Bacillus subtilis P RNA with 4-thiouridine, 4-deoxyuridine, and abasic modifications and G378/379 with 2-aminopurine, N7-deazaguanosine, and 6-thioguanosine. The functional group modifications of U51 decrease RNase P-catalyzed phosphodiester bond cleavage 16- to 23-fold, as measured by the single-turnover cleavage rate constant. The activity of the 4-thiouridine RNase P is partially rescued by addition of Cd(II) or Mn(II) ions. This is the first time a metal-rescue experiment provides evidence for inner-sphere divalent metal ion coordination with a nucleobase. Modifications of G379 modestly decrease the cleavage activity of RNase P, suggesting outer-sphere coordination of O6 on G379 to a metal ion. These data provide biochemical evidence for catalytically important interactions of the P4 helix of P RNA with metal ions, demonstrating that the bulged uridine coordinates at least one catalytic metal ion through an inner-sphere interaction. The combination of single-atom and abasic nucleotide substitutions provides a powerful strategy to probe functions of conserved nucleobases in large RNAs.

Infrared Spectroscopic Analysis of the Adsorption of Pyridine Carboxylic Acids on Colloidal Ceria

Surface adsorption of a homologous series of pyridine carboxylic acids on a hydrated colloidal cerium dioxide (ceria) film is characterized using the combination of experimental and computationally determined infrared (IR) spectra. Experimental analyses employ attenuated total reflectance (ATR) IR spectroscopy of deposited colloidal ceria thin films equilibrated with three pyridine carboxylic acids at pH 3.0, 5.5, and 8.5. The corresponding computational IR spectra for the energy-minimized intermediate and base forms of the pyridine carboxylic acids use density functional theory calculations at the B3LYP/6-311++G** level of theory. Solvent effects are modeled using both the COSMO implicit solvation model and the inclusion of explicit water molecules. Experimental IR spectra show that the adsorptive interactions between the pyridine carboxylic acids and ceria surface are due to the outer-sphere coordination of cerium ions in the films. Vibrational assignments based on combined experimental and computational results indicate that both pyridyl ring nitrogen and carboxylate functional groups account for the interaction of pyridine carboxylic acids at ceria surfaces. Experimentally determined Langmuir constants point to the intermediate form of picolinic acid (pyridine-2-carboxylic acid) as having the strongest adsorption to ceria compared to the other pyridine carboxylic acids investigated. The enhanced adsorption of picolinic acid is attributed to the adjacency of the protonated pyridyl nitrogen and the carboxylate group relative to nicotinic acid (pyridine-3-carboxylic acid) and isonicotinic acid (pyridine-4-carboxylic acid).

Molybdenum and nickel atrane complexes with a rigid tripodal tetraphosphine ligand incorporating imidazole rings

Abstract Tripodal, tetradentate tetraphosphine ligand tris(1H-2-diisopropylphosphinoimidazol-1-yl)phosphine (1) was synthesized in three straightforward steps from widely available starting materials. The ligand crystallized in a C3-like conformation suitable for metal coordination. Nickel(II) complex 2 was prepared from 1 and Ni(dme)Br2 and adopted a classical atrane structure with trigonal pyramidal geometry at the metal, a bromide covalently bonded in the axial position, and a bromide ion in the outer coordination sphere. Molybdenum(III) complex 3 was prepared from 1 and MoCl3(thf)3 and could be structurally characterized using a co-crystal with Cp2Co and [Cp2Co]Cl. It displayed a face-capped octahedral geometry at the seven-coordinated metal. Complexes 2 and 3 proved prone to decomposition under reductive and oxidative conditions, hindering the investigation of their chemistry. Crystallization attempts from acetonitrile led to the isolation of dinuclear hydrolysis product 4, with the Mo(III) centres bridged by two chlorides and two 2-(diisopropylphosphino)imidazole moieties in a paddlewheel arrangement.

Synthesis and Characterization of an Isomorphous Lanthanide-Thiophenemonocarboxylate Series (Ln = La–Lu, except Pm) Amenable to Color Tuning

A series of 14 lanthanide compounds bearing 2-thiophenecarboxylate (TC) have been prepared under aqueous conditions and structurally characterized. Single crystal X-ray diffraction studies reveal a full isomorphous series across the 4f block of the general formula [(Ln(TC)3(H2O)2)·(HPy·TC)]n, (Ln = La (La-1) – Lu (Lu-1), excluding Pm), with observable contraction of the cell volume and average metal–oxygen bond distances occurring when moving from lanthanum to lutetium. The 8-coordinate Ln metal centers are bound to two water molecules as well as six oxygen atoms from one bidentate and four bridging TC units that link LnO8 structural units into 1D chains. One lattice stabilizing pyridinium and one outer coordination sphere TC anion are present per formula unit. Reaction optimization provided moderate to high yields of the title products; a lanthanum byproduct [La(TC)3(HTC)2]n (La-2) was formed by performing the reaction at reduced temperature, or alternatively, successive grinding of La-1 in ethanol resul...