Outer Coordination Sphere Research Articles

Versatile photocatalytic systems for H2 generation in water based on an efficient DuBois-type nickel catalyst.

The generation of renewable H2 through an efficient photochemical route requires photoinduced electron transfer (ET) from a light harvester to an efficient electrocatalyst in water. Here, we report on a molecular H2 evolution catalyst (NiP) with a DuBois-type [Ni(P2R′N2R″)2]2+ core (P2R′N2R″ = bis(1,5-R′-diphospha-3,7-R″-diazacyclooctane), which contains an outer coordination sphere with phosphonic acid groups. The latter functionality allows for good solubility in water and immobilization on metal oxide semiconductors. Electrochemical studies confirm that NiP is a highly active electrocatalyst in aqueous electrolyte solution (overpotential of approximately 200 mV at pH 4.5 with a Faradaic yield of 85 ± 4%). Photocatalytic experiments and investigations on the ET kinetics were carried out in combination with a phosphonated Ru(II) tris(bipyridine) dye (RuP) in homogeneous and heterogeneous environments. Time-resolved luminescence and transient absorption spectroscopy studies confirmed that directed ET from RuP to NiP occurs efficiently in all systems on the nano- to microsecond time scale, through three distinct routes: reductive quenching of RuP in solution or on the surface of ZrO2 (“on particle” system) or oxidative quenching of RuP when the compounds were immobilized on TiO2 (“through particle” system). Our studies show that NiP can be used in a purely aqueous solution and on a semiconductor surface with a high degree of versatility. A high TOF of 460 ± 60 h–1 with a TON of 723 ± 171 for photocatalytic H2 generation with a molecular Ni catalyst in water and a photon-to-H2 quantum yield of approximately 10% were achieved for the homogeneous system.

Minimal Proton Channel Enables H2 Oxidation and Production with a Water-Soluble Nickel-Based Catalyst

Hydrogenase enzymes use first-row transition metals to interconvert H2 with protons and electrons, reactions that are important for the storage and recovery of energy from intermittent sources such as solar, hydroelectric, and wind. Here we present Ni(P(Cy)2N(Gly)2)2, a water-soluble molecular electrocatalyst with the amino acid glycine built into the diphosphine ligand framework. Proton transfer between the outer coordination sphere carboxylates and the second coordination sphere pendant amines is rapid, as observed by cyclic voltammetry and FTIR spectroscopy, indicating that the carboxylate groups may participate in proton transfer during catalysis. This complex oxidizes H2 (1-33 s(-1)) at low overpotentials (150-365 mV) over a range of pH values (0.1-9.0) and produces H2 under identical solution conditions (>2400 s(-1) at pH 0.5). Enzymes employ proton channels for the controlled movement of protons over long distances-the results presented here demonstrate the effects of a simple two-component proton channel in a synthetic molecular electrocatalyst.

Synthesis and characterization of mixed-ligand copper(II) saccharinate complexes containing tridentate NNS Schiff bases. X-ray crystallographic analysis of the free ligands and one complex

Four new Schiff bases containing N and S heteroatoms (HL1–HL4) have been prepared and characterized, including determination of the X-ray crystal structures of HL1 and HL3. Spectroscopic evidence indicates that these Schiff bases behave as uninegatively charged tridentate NNS ligands in complexes of general formula [Cu(Ln)sac] (Ln is the anionic form of NNS, and sac represents saccharinate anion). Crystals of both HL1 and [Cu(L4)sac)(H2O)]·Hsac crystallized in triclinic system with P\( \bar{1} \) space group, while HL3 crystallized in monoclinic system with P 21/c space group. [Cu(L4)sac)(H2O)]·Hsac has a distorted square-pyramidal structure with a non-bonded saccharin molecule present in the outer coordination sphere. The tridentate NNS ligands are coordinated to Cu through pyridine nitrogen, azomethine nitrogen and thiolate sulfur atoms, while the fourth and fifth coordination positions are occupied by the N-bonded saccharinate anion and a water ligand, respectively.

Platinum(IV) complexes with N-alkylphenothiazines: synthesis, characterization, and antibacterial activity

Two new platinum(IV) complexes (1, trifluoperazinehydrochloride-aquapentachloridoplatinate(IV) and 2, chlorpromazine-chlorpromazinehydrochloridepentachloridoplatinate(IV)) were synthesized in the reaction of K2[PtCl6] with trifluoperazine dihydrochloride (TF·2HCl) or chlorpromazine hydrochloride (CP·HCl). The complexes were characterized by elemental analysis, molar conductivity measurement, and spectral (IR, 1H, 13C, 2D 1H–13C heteronuclear correlation spectra, 195Pt NMR, and MS) methods. Outer-coordination sphere was proposed for 1; while in 2, the ligand was coordinated to the metal. The complexes exhibit antibacterial effect on strains of Bacillus subtilis, Bacillus cereus, Bacillus pumilus, and methicillin-resistant Staphylococci as Gram-positive bacteria and an Escherichia coli as Gram-negative bacteria, as well as the reference strains.

Kinetics and mechanism of ligands substitution in the chromium(III) complex of tetraphenylporphin

The reaction of axial coordination of imidazole derivatives by (X)CrTPP in amphiprotic media was studied. The reaction includes two stages: consecutive substitution of the two alcohols molecules, first in the outer coordination sphere, and then in the inner sphere. For the reaction of the intrasphere substitution the solvolytic associative-dissociative mechanism has been found. The molecule of (chloro)chromium(III) porphyrin and its derivatives have been studied by density functional theory (DFT) computations utilizing the B3LYP hybrid method.

Electrocatalytic Oxidation of Formate with Nickel Diphosphine Dipeptide Complexes: Effect of Ligands Modified with Amino Acids

AbstractA series of nickel bis(diphosphine) complexes with dipeptides appended to the ligands were investigated for the catalytic oxidation of formate to carbon dioxide, a proton, and two electrons. Typical rates of approximately 7 s–1 were found, similar to that for the parent complex (ca. 8 s–1), with amino acid size and positioning contributing very little to the rate or operating potential. Hydroxy functionalities did result in lower rates, which were recovered by protecting the hydroxy group. The results suggest that the overall dielectric properties introduced by the dipeptides do not play an important role in catalysis, but free hydroxy groups do influence activity, implying contributions from intra‐ or intermolecular interactions. These observations are important in developing a fundamental understanding of the effect that an enzyme‐like outer coordination sphere can have upon molecular catalysts.

Sulfate Binding with a Tripodal Tris(4‐pyridylurea) Receptor

AbstractThe tris(2‐aminoethyl)amine (tren)‐based tris(4‐pyridylurea) receptor L has been synthesized and its anion binding properties were studied. The ligand forms a 2:1 (host/guest) complex with MgSO4, [SO42−⊂L2], in which a sulfate ion is encapsulated by six urea groups from the two ligands through multiple hydrogen bonds. The metal ions do not coordinate to the pyridyl groups but exist as the hydrate [Mg(H2O)6]2+ and interact with the [SO4⊂L2] capsules in the outer coordination sphere to form a three dimensional extended structure. The anion binding behavior of ligand L in solution was studied.

Investigation of the crystal and molecular structure of a novel coordination polymer formed by manganese(II) nitrate and 4,4,10,10-tetramethyl-1,3,7,9-tetraazospiro[5.5]undecan-2,8-dione

A coordination polymer {[Mn(H2O)2(C11H20N4O2)2]2+·2(NO3−)}n is synthesized and its structure is determined. The crystals are monoclinic: space group P21/c, a = 12.3771(3) A, b = 14.8775(3) A, c = 18.1388(4) A, β = 106.611(2)°, V = 3200.70(12) A3, dx = 1.44 g/cm3, Z = 4. Manganese ions are coordinated with four oxygen atoms of four organic ligands (two unique) and two water molecules. The coordination polyhedron is a distorted octahedron; the O-Mn-O angles between the adjacent oxygen atoms fall within 83.96(5)–98.11(5)°. The nitrate anions are in the outer coordination sphere. The Mn...Mn distances in the polymer are 8.56 A. In the crystal, polymeric coordination chains are joined in layers perpendicular to the b axis by hydrogen bonds involving the organic ligands, water molecules, and nitrate anions.

Cover Picture: The Role of a Dipeptide Outer-Coordination Sphere on H2 -Production Catalysts: Influence on Catalytic Rates and Electron Transfer (Chem. Eur. J. 6/2013)

Cover Picture: The Role of a Dipeptide Outer-Coordination Sphere on H₂ -Production Catalysts: Influence on Catalytic Rates and Electron Transfer (Chem. Eur. J. 6/2013)

Exploiting outer-sphere interactions to enhance metal recovery by solvent extraction

Interactions, particularly hydrogen bonds, between ligands in the outer coordination spheres of metal complexes have a major effect on their stabilities in the hydrocarbon solvents used in commercial solvent extraction and it is now possible to use these interactions to tune the strength and selectivity of extractants.

Studies of new phosphothreonine complexes formed in binary and ternary systems including biogenic amines and copper(II)

Binary and ternary systems of copper(II) with O-phospho-L-threonine (Thr-P) and biogenic amines (putrescine or spermidine or spermine) have been investigated. The studies were performed in aqueous solution. Overall stability constants of the complexes were determined by pH-metric study and the coordination sites were identified by 31P NMR, visible and electron paramagnetic resonance spectroscopies. Results of the equilibrium and spectral studies have shown that in the phosphothreonine binary system Cu(HThr-P), Cu(Thr-P) and Cu(Thr-P)(OH) are formed. In the binary system at low pH, only the phosphate group is involved in coordination but at higher pH, the efficiency of −PO4 2− is low and the amine and carboxylate are the main sites of coordination. Formation of complexes was established in ternary systems with all bioamines studied. In these systems the reaction centers of phosphothreonine are the same as in the binary systems: phosphate, carboxylate and amine. Additionally, amine groups from the polyamines (PAs) are involved in the complex formation. Besides the heteroligand complexes with intermolecular interactions, also molecular complexes with the protonated bioamine in the outer coordination sphere appeared. In these molecular species, PA was involved in non-covalent interactions with the anchoring phospho-threonine complex.

The Role of a Dipeptide Outer-Coordination Sphere on H2-Production Catalysts: Influence on Catalytic Rates and Electron Transfer

The outer-coordination sphere of enzymes acts to fine-tune the active site reactivity and control catalytic rates, suggesting that incorporation of analogous structural elements into molecular catalysts may be necessary to achieve rates comparable to those observed in enzyme systems at low overpotentials. In this work, we evaluate the effect of an amino acid and dipeptide outer-coordination sphere on [Ni(P(Ph)(2)N(Ph-R)(2))(2)](2+) hydrogen production catalysts. A series of 12 new complexes containing non-natural amino acids or dipeptides was prepared to test the effects of positioning, size, polarity and aromaticity on catalytic activity. The non-natural amino acid was either 3-(meta- or para-aminophenyl)propionic acid terminated as an acid, an ester or an amide. Dipeptides consisted of one of the non-natural amino acids coupled to one of four amino acid esters: alanine, serine, phenylalanine or tyrosine. All of the catalysts are active for hydrogen production, with rates averaging ∼1000 s(-1), 40 % faster than the unmodified catalyst. Structure and polarity of the aliphatic or aromatic side chains of the C-terminal peptide do not strongly influence rates. However, the presence of an amide bond increases rates, suggesting a role for the amide in assisting catalysis. Overpotentials were lower with substituents at the N-phenyl meta position. This is consistent with slower electron transfer in the less compact, para-substituted complexes, as shown in digital simulations of catalyst cyclic voltammograms and computational modeling of the complexes. Combining the current results with insights from previous results, we propose a mechanism for the role of the amino acid and dipeptide based outer-coordination sphere in molecular hydrogen production catalysts.

On the governing of alkaline earth metal nitrate coordination spheres by hexamethylenetetramine

Alkaline earth metal nitrate coordination compounds with hexamethylenetetramine, []], have been synthesized and characterized by elemental and thermal analysis, IR spectroscopy, and X-ray crystallography. Introduction of hmta to the outer coordination sphere allows changing of the inner coordination sphere giving the dinuclear coordination compounds of calcium and strontium nitrates. The compounds are air stable at room temperature and soluble in water. The ions and molecules are assembled via , and hydrogen bonds. Thermal analyses show crucial differences in decomposition processes (especially in the case of 3 that finally decomposes to the mixture of SrO and Sr(NO3)2). The IR spectrum of 3 shows lack of bending vibration (present in the spectra of the respective pure salts at 741–759 cm−1), while in 2 this band is weakened and shifted to lower frequencies. Only the IR spectrum of 3 clearly exhibits bands originating from overtones and combination bands (in the spectral range 2073–2474 cm−1).

Outer-Sphere Coordination Chemistry: Amido-Ammonium Ligands as Highly Selective Tetrachloridozinc(II)ate Extractants

Eight new amido functionalized reagents, L(1)-L(8), have been synthesized containing the sequence of atoms R(2)N-CH(2)-NR'-CO-R″, which upon protonation forms a six-membered chelate with a hydrogen bond between the tertiary ammonium N-H(+) group and the amido oxygen atom. The monocationic ligands, LH(+), extract tetrachloridometal(II)ates from acidic solutions containing high concentrations of chloride ions via a mechanism in which two ligands address the "outer sphere" of the [MCl(4)](2-) unit using both N-H and C-H hydrogen bond donors to form the neutral complex as in 2L + 2HCl + MCl(2) ⇌ [(LH)(2)MCl(4)]. The strengths of L(1)-L(8) as zinc extractants in these pH-dependent equilibria have been shown to be very dependent on the number of amide groups in the R(3-n)N(CH(2)NR'COR″)(n) molecules, anti-intuitively decreasing with the number of strong hydrogen bond donors present and following the order monoamides > diamides > triamides. Studies of the effects of chloride concentration on extraction have demonstrated that the monoamides in particular show an unusually high selectivity for [ZnCl(4)](2-) over [FeCl(4)](-) and Cl(-). Hybrid-DFT calculations on the tri-, di-, and monoamides, L(2), L(3), and L(4), help to rationalize these orders of strength and selectivity. The monoamide L(4) has the most favorable protonation energy because formation of the LH(+) cation generates a "chelated proton" structure as described above without having to sacrifice an existing intramolecular amide-amide hydrogen bond. The selectivity of extraction of [ZnCl(4)](2-) over Cl(-), represented by the process 2[(LH)Cl] + ZnCl(4)(2-) ⇌ [(LH)(2)ZnCl(4)] + 2Cl(-), is most favorable for L(4) because it is less effective at binding chloride as it has fewer highly polar N-H hydrogen bond donor groups to interact with this "hard" anion.

Evaluation of intensity and energy interaction parameters for the complexation of Pr(III) with selected nucleoside and nucleotide through absorption spectral studies

The interactions of Pr(III) with nucleosides and nucleotides have been studied in different organic solvents employing absorption difference and comparative absorption spectrophotometry. The magnitudes of the variations in both energy and intensity interaction parameters were used to explore the degree of outer and inner sphere co-ordination, incidence of covalency and the extent of metal 4f-orbital involvement in chemical bonding. Various electronic spectral parameters like Slater–Condon (Fk), Racah (Ek), Lande parameter (ξ4f), Nephelauxatic ratio (β), bonding (b1/2), percentage covalency (δ) and intensity parameters like oscillator strength (P) and Judd Ofelt electronic dipole intensity parameter (Tλ, λ=2,4,6) have been evaluated. The variation of these evaluated parameters were employed to interpret the nature of binding of Pr(III) with different ligands i.e. Adenosine/ATP in presence and absence of Ca2+.

A Biomimetic Pathway for Vanadium‐Catalyzed Aerobic Oxidation of Alcohols: Evidence for a Base‐Assisted Dehydrogenation Mechanism

The first step in the catalytic oxidation of alcohols by molecular O(2), mediated by homogeneous vanadium(V) complexes [LV(V)(O)(OR)], is ligand exchange. The unusual mechanism of the subsequent intramolecular oxidation of benzyl alcoholate ligands in the 8-hydroxyquinolinato (HQ) complexes [(HQ)(2)V(V)(O)(OCH(2)C(6)H(4)-p-X)] involves intermolecular deprotonation. In the presence of triethylamine, complex 3 (X = H) reacts within an hour at room temperature to generate, quantitatively, [(HQ)(2)V(IV)(O)], benzaldehyde (0.5 equivalents), and benzyl alcohol (0.5 equivalents). The base plays a key role in the reaction: in its absence, less than 12% conversion was observed after 72 hours. The reaction is first order in both 3 and NEt(3), with activation parameters ΔH(≠)=(28±4) kJ mol(-1) and ΔS(≠)=(-169±4) J K(-1) mol(-1). A large kinetic isotope effect, 10.2±0.6, was observed when the benzylic hydrogen atoms were replaced by deuterium atoms. The effect of the para substituent of the benzyl alcoholate ligand on the reaction rate was investigated using a Hammett plot, which was constructed using σ(p). From the slope of the Hammett plot, ρ=+(1.34±0.18), a significant buildup of negative charge on the benzylic carbon atom in the transition state is inferred. These experimental findings, in combination with computational studies, support an unusual bimolecular pathway for the intramolecular redox reaction, in which the rate-limiting step is deprotonation at the benzylic position. This mechanism, that is, base-assisted dehydrogenation (BAD), represents a biomimetic pathway for transition-metal-mediated alcohol oxidations, differing from the previously identified hydride-transfer and radical pathways. It suggests a new way to enhance the activity and selectivity of vanadium catalysts in a wide range of redox reactions, through control of the outer coordination sphere.

Adsorption mechanism of selenium(VI) onto maghemite

In this study, the sorption properties of maghemite (γ-Fe2O3) towards selenium(VI) were studied for the first time both on the macroscopic and the molecular level. Using batch experiments, we found that the retention reaction was very fast. Both increase of pH and ionic strength led to a decrease of selenium(VI) sorption. Electrophoretic mobility measurements showed that selenium(VI) sorption had no significant effect on the isoelectric point of maghemite. These macroscopic results strongly suggested the formation of outer-sphere complexes across the investigated pH range (3.5–8.0). At the molecular level, the structure of the sorbed surface species was elucidated in situ using Attenuated Total Reflection Fourier-Transform Infrared (ATR FT-IR) spectroscopy and Extended X-ray Absorption Fine-Structure (EXAFS) spectroscopy. The ATR FT-IR results suggested the formation of outer-sphere complexes showing an unexpected bidentate symmetry, which possibly revealed the lack of accuracy of the actual widely-used classification of inner- and outer-sphere coordination for anionic species. EXAFS results revealed, that in addition to outer-sphere complexes, there is also a small (15%) contribution of an inner-sphere complex in binuclear corner-sharing geometry present.

The Outer-Coordination Sphere: Incorporating Amino Acids and Peptides as Ligands for Homogeneous Catalysts to Mimic Enzyme Function

Great progress has been achieved in the field of homogeneous transition metal-based catalysis; however, as a general rule these solution-based catalysts are still easily outperformed by their analogous enzyme counterparts, in terms of both rates and selectivity. This includes structural mimics of enzymatic active sites. This observation suggests that the features of the enzyme beyond the active site, i.e., the outer-coordination sphere, are important for enzymes' exceptional function. Directly mimicking the outer-coordination sphere requires the incorporation of amino acids and peptides as ligands for homogeneous catalysts. This effort has been attempted for many homogeneous catalysts which span the manifold of catalytic reactions including hydrogenation, hydroformylation, hydrogen production, oxygen activation, dioxygen transport, electron transfer, disproportionation of the superoxide anion radical, acylation, phosphorylation, esterification, ester hydrolysis, and hydrogen peroxide disproportionation. How much of the outer-coordination sphere to include in molecular catalysts and the preferential structural conformation of the appended section are long-standing questions. Synthetically incorporating an amino acid- or peptide-based outer-coordination sphere requires care to avoid unwanted side reactions with the large number of functional groups. Catalysis with amino acid or peptide containing catalysts requires careful consideration of solvent and pH to promote solubility, stabilize structure, optimize catalysis, and avoid catalyst degradation. This article reviews the current capability of synthesizing and characterizing this often challenging but very promising category of metal-based catalysts.

Structure and catalytic activity of a new iron(II) complex with a tetradentate carboxamide ligand: The effect of the outer-sphere donor on the chemoselectivity of the metal complex catalyst

The interaction between Fe(ClO4)2 or Fe(OTf)2(MeCN)2 and 2 equiv of the potentially tetradentate ligand bis(2-pyridyl)methyl-2-pyridinecarboxamide (Py2CHNHCOPy, tpcaH) yields the iron(II) complex [FeII(tpcaH)2]X2 (X = ClO4, OTf), in which, according to X-ray crystallography data, tpcaH is facially coordinated to the iron atom as a tridentate ligand through the carbonyl group and two pyridyl donors of the bis(2-pyridyl)methylcarboxamide moiety and the third pyridyl group is uninvolved in coordination. The oxidation of saturated and unsaturated hydrocarbons with hydrogen peroxide involving this complex has been investigated. The presence of the uncoordinated nitrogen donor in the outer coordination sphere of the complex exerts a crucial effect on it catalytic properties.

Incorporating Amino Acid Esters into Catalysts for Hydrogen Oxidation: Steric and Electronic Effects and the Role of Water as a Base

Four derivatives of a hydrogen oxidation catalyst, [Ni(PCy2NBn-R2)2]2+ (Cy = cyclohexyl, Bn = benzyl, R = OMe, COOMe, CO-alanine-methyl ester, CO-phenylalanine-methyl ester), have been prepared to investigate steric and electronic effects on catalysis. Each complex was characterized spectroscopically and electrochemically, and thermodynamic data were determined. Crystal structures are also reported for the −OMe and −COOMe derivatives. All four catalysts were found to be active for H2 oxidation. The methyl ester (R = COOMe) and amino acid ester containing complexes (R = CO-alanine-methyl ester or CO-phenylalanine-methyl ester) had rates slower (4 s–1) than that of the parent complex (10 s–1), in which R = H, which is consistent with the lower amine pKa's and less favorable ΔGH2's found for these electron-withdrawing substituents. Dynamic processes for the amino acid ester containing complexes were also investigated and found not to hinder catalysis. The electron-donating methyl ether derivative (R = OMe) was prepared to compare electronic effects and has a catalytic rate similar to that of the parent complex. In the course of these studies, it was found that water could act as a weak base for H2 oxidation, although catalytic turnover requires a higher potential and utilizes a different sequence of catalytic steps than when using a base with a higher pKa. Importantly, these catalysts provide a foundation upon which larger peptides can be attached to [Ni(PCy2NBn2)2]2+ hydrogen oxidation catalysts in order to more fully investigate and implement the effects of the outer coordination sphere.