Number Of Coordinated Ligands Research Articles

Influences of coordination structures on the electronic properties of Zn ion and the adsorption of O-containing and S-containing molecules

Ligands in minerals have an important effect on the chemical properties of metal ions. The electronic properties of Zn ions formed by O and S ligands have been studied using density functional theory (DFT), and the interaction strength between O and S-containing molecules and Zn ions has been analyzed. The results show that the electronic properties of Zn ions may be influenced by the type of ligands, the number of ligands, and the distance between ligands and Zn ions. The adsorption capacity of zinc ions decreased with an the in increase ligand coordination number, but increased with an increase in the distance between the ligand and zinc ion. The adsorption of O- and S-containing molecules on sphalerite smithsonite and hemimorphite was then studied. It was indicated that O-containing molecules had a strong collecting ability for sphalerite, smithsonite and hemimorphite, but the adsorption capacity of S-containing molecules was weaker than that of O-containing molecules. The influence of water molecules on the adsorption behavior of O- and S-containing molecules was studied. The results of the calculations show that the relationship between H2O and O-containing sulfur-containing molecules is competitive adsorption on the surface of sphalerite smithsonite and hemimorphite. Adsorption of water molecules can reduce the adsorption energy of O-containing and S-containing molecules on mineral surfaces.

A hybrid forces Quantum Mechanical Charge Field Molecular Dynamics of Cs+ ion in aqueous ammonia: A solvation lability, “structure breaking” phenomenon and hydrogen bond properties

AbstractThe dynamics and structure of Cs+ solvation in an aqueous ammonia solution have been investigated using the Quantum Mechanical Charge Field Molecular Dynamics (QMCF‐MD) simulation method. The system contained 18.6% ammonia in aqueous solution at 298.15 K. The QM region was set to a radius of 7.0 Å to include the first and second solvation shells. The Hartree‐Fock (HF) level was applied to calculate the ion‐ligand and ligand–ligand interactions in the QM region using the LANL2DZ‐ECP basis set for the ion and DZP‐Dunning for the ligands. The radial distribution revealed only one solvation shell, indicating a labile solvation structure. The various coordination number of ligands suggests an instant exchange between them. The mean residence times of 1.29 and 0.9 ps were less than in the pure water and liquid ammonia system, indicating higher mobility ligands of the aqueous ammonia system. The preferential factor 0.55 supports the hypothesis that the Cs+ ion prefers to be dissolved in water. As the ligands moves further away from the ion, the observed number of hydrogen bonds increases, revealing the “structure‐breaking” property of the Cs+ ion. The minimum angle ligand‐ion‐ligand tends to be higher near the ion, confirming this phenomenon.

Role of Nafion Ionomer in Electrochemical Reduction of CO2 on Au Clusters

More than 80% of the global energy demand is produced by burning fossil fuels, which leads to the emission of environmentally unfriendly greenhouse gases that adversely affect climate and human health. Electrochemical reduction of CO2 into high-value chemicals or fuels can be a promising way to store renewable energy sources such as solar and wind energies into chemical form, along with alleviating the environmental concerns.1 Recently, atomically precise metal clusters have attracted increasing interest in the research field of catalysis due to their unique size-dependent electronic and chemical properties.2 For the robust application and reusability of catalysts, the metal clusters are generally deposited onto a support, and the electrode is prepared by mixing a binder with the active catalyst. The binder remains in the secondary/outer coordination sphere of the supported catalyst; thus, it can interact with the nanoparticles and modulate their chemical and electronic properties resulting in a change in the catalytic activity.3 In this work, we have studied the role of Nafion binder in the electrochemical reduction of CO2 on [Au9(PPh3)8](NO3)3 clusters supported onto a carbon black support. The atomically precise [Au9(PPh3)8](NO3)3 clusters were chemically synthesised, characterised, and deposited on a carbon black support via wet impregnation. The cathode material for the electrochemical CO2 reduction was prepared by spraying Au9/C catalyst along with Nafion ionomer onto a carbon felt. A potentiostatic electrochemical reduction of CO2 at -1.3 V vs. Ag|AgCl was studied in a three-electrode based electrochemical flow cell in 0.2 M KHCO3 electrolyte. The Au9/C electrocatalyst exhibited a gradual increase in the current density and CO Faradaic efficiency over time when studied for potentiostatic (-1.3 V vs. Ag|AgCl) electrochemical CO2RR suggesting activation of the catalyst. Besides, the process of activation was found to be accelerated when the electrode was treated at a higher potential (-1.7 V vs. Ag|AgCl).The physical mixing of Au9 clusters with the Nafion ionomer results in dilution of the solution colours, and a precipitate was obtained within seconds. A concentration-dependent investigation of the physical mixture of Au9 clusters and the Nafion ionomer using UV-Vis absorption and X-ray absorption spectroscopy indicated the formation of chemical bonds between the cluster metallic core and the Nafion ionomer. The X-ray absorption studies on the as-made Au9/C electrode, the electrode after 2 h of electrolysis and the electrode after electrochemical activation at higher potential (-1.7 V vs. Ag|AgCl) suggested that the interaction between the Nafion ionomer and Au9 clusters results in shielding of active sites in the as-made electrode (Figure). The EXAFS fitting of the Au9/C electrode after electrochemical activation show a lower metal ligand coordination number. The results demonstrate that the Nafion-cluster interface restructures itself upon applying an electrochemical potential for a long time or applying a higher potential. This restructuring of the interface allows the reactants to access the active sites resulting in an improvement in the catalytic activity during the electrolysis.An alternative approach for the preparation of cluster-based electrodes was developed to minimise the interactions between the Au9 clusters and the Nafion ionomer by first depositing a layer of carbon black then dropcasting Au9 clusters (Au9 DC electrodes). In addition to that, the effect of the amount of the Nafion ionomer in the catalytic layer and the role of solvent used in the fabrication of the catalytic layer on the electrocatalytic performance were studied.The work shows that the Nafion ionomer, one of the most commonly used binders, has a profound influence on the catalytic behaviour of ultra-small Au clusters. A detailed discussion on the experimental design, and the analysis of electrochemical and X-ray absorption studies will be given in the conference contribution.References Wang, G.; Chen, J.; Ding, Y.; Cai, P.; Yi, L.; Li, Y.; Tu, C.; Hou, Y.; Wen, Z.; Dai, L. Chemical Society Reviews 2021, 50, (8), 4993-5061.Tyo, E. C.; Vajda, S. Nature Nanotechnology 2015, 10, (7), 577-88.Andrews, E.; Katla, S.; Kumar, C.; Patterson, M.; Sprunger, P.; Flake, J. Journal of The Electrochemical Society 2015, 162, (12), F1373-F1378. Figure 1

All atom NMDA receptor transmembrane domain model development and simulations in lipid bilayers and water.

N-methyl-d-aspartate receptors (NMDARs) are members of the ionotropic glutamate receptor family that mediate excitatory synaptic transmission in the central nervous system. The channels of NMDARs are permeable to Ca2+ but blocked by Mg2+, distinctive properties that underlie essential brain processes such as induction of synaptic plasticity. However, due to limited structural information about the NMDAR transmembrane ion channel forming domain, the mechanism of divalent cation permeation and block is understood poorly. In this paper we developed an atomistic model of the transmembrane domain (TMD) of NMDARs composed of GluN1 and GluN2A subunits (GluN1/2A receptors). The model was generated using (a) a homology model based on the structure of the NaK channel and a partially resolved structure of an AMPA receptor (AMPAR), and (b) a partially resolved X-ray structure of GluN1/2B NMDARs. Refinement and extensive Molecular Dynamics (MD) simulations of the NMDAR TMD model were performed in explicit lipid bilayer membrane and water. Targeted MD with simulated annealing was introduced to promote structure refinement. Putative positions of the Mg2+ and Ca2+ ions in the ion channel divalent cation binding site are proposed. Differences in the structural and dynamic behavior of the channel protein in the presence of Mg2+ or Ca2+ are analyzed. NMDAR protein conformational flexibility was similar with no ion bound to the divalent cation binding site and with Ca2+ bound, whereas Mg2+ binding reduced protein fluctuations. While bound at the binding site both ions retained their preferred ligand coordination numbers: 6 for Mg2+, and 7–8 for Ca2+. Four asparagine side chain oxygens, a back-bone oxygen, and a water molecule participated in binding a Mg2+ ion. The Ca2+ ion first coordination shell ligands typically included four to five side-chain oxygen atoms of the binding site asparagine residues, two water molecules and zero to two backbone oxygens of the GluN2B subunits. These results demonstrate the importance of high-resolution channel structures for elucidation of mechanisms of NMDAR permeation and block.

Bio-orthogonal Coupling as a Means of Quantifying the Ligand Density on Hydrophilic Quantum Dots.

We describe the synthesis of two metal-coordinating ligands that present one or two lipoic acid (LA) anchors, a hydrophilic polyethylene glycol (PEG) segment and a terminal reactive group made of an azide or an aldehyde, two functionalities with great utility in bio-orthogonal coupling techniques. These ligands were introduced onto the QD surfaces using a combination of photochemical ligation and mixed cap exchange strategy, where control over the fraction of azide and aldehyde groups per nanocrystal can be easily achieved: LA-PEG-CHO, LA-PEG-N3, and bis(LA)-PEG-CHO. We then demonstrate the application of two novel bio-orthogonal coupling strategies directly on luminescent quantum dot (QD) surfaces that use click chemistry and hydrazone ligation under catalyst-free conditions. We applied the highly efficient hydrazone ligation to couple 2-hydrozinopyridine (2-HP) to aldehyde-functionalized QDs, which produces a stable hydrazone chromophore with a well-defined optical signature. This unique optical feature has enabled us to extract a measure for the ligand density on the QDs for a few distinct sizes and for different ligand architectures, namely mono-LA-PEG and bis(LA)-PEG. We found that the foot-print-area per ligand was unaffected by the nanocrystal size but strongly depended on the ligand coordination number. Additionally, we showed that when the two bio-orthogonal functionalities (aldehyde and azide) are combined on the same QD platform, the nanocrystal can be specifically reacted with two distinct targets and with great specificity. This design yields QD platforms with distinct chemoselectivities that are greatly promising for use as carriers for in vivo imaging and delivery.

Structural diversity and magnetic properties of six metal–organic coordination polymers based on semi-rigid V-shape tetracarboxylic acid ligand

Six Mn metal-organic frameworks have been synthesized under solvothermal conditions with V-shaped terphenyl tetracarboxylate ligands (H4ttac). Their structures were characterized by elemental analysis, infrared spectra, PXRD, thermogravimetric analysis, and single-crystal X-ray diffraction analysis. Crystal structures reveal that the coordination number of H4ttac ligand varies from 6 to 10, and each ligand links 4–8 Mn(II) ions. Coordination modes vary from η6μ4 to η10μ8. The existence of DMF solvent can increase coordination number of the ligand. The first coordination saturated phthalate is presented. The variable-temperature magnetic studies indicate that complexes exhibit dominant antiferromagnetic behaviors. Structural parameters and coordination modes were summarized. The porosity of these complexes is less than 15%, indicating that the V-shape ligand is not a good choice to construct porous coordination polymers.

Four Metal Complexes Based on Bulky Imidazole Ligands: Solvothermal Syntheses, Crystal Structures, and Fluorescence Properties

AbstractIn order to explore the influences of (de‐)protonation of the imidazole ring on the structural diversity of the resulting complexes, the imidazole‐based ligands 4, 5‐diphenylimidazole (Hdpi) and 1H‐phenanthro[9, 10‐d]imidazole (Hpi) were utilized as bulky building blocks to construct four complexes by solvothermal reactions, i.e. [Ag(Hdpi)2](NO3)·(H2O) (1), [Cu(dpi)]∞ (2), [Cu(Hpi)(NO3)]∞ (3), and [(H2pi)(NO3)]·H2O (4). In complex 1, two Hdpi ligands adopt a monodentate pattern and coordinate with one AgI ion to form a mononuclear unit, which is further connected by hydrogen bonds into a 1D supramolecular helix. The deprotonated dpi ligand of 2 acts in bidentate mode, and bridges CuI ions to afford a 1D chain. In 3, the NO3– ion, acts as a monodentate bridging ligand and joins CuI ions to generate a 1D chain. The Hpi ligand employs a monodentate mode to bond with CuI ions of the 1D chain. 4 is protonated and two H2pi nitrogen atoms are free of coordination. Interestingly, hydrogen bonds among the NO3– ion, the H2pi ligand, and the water molecule yield a macro ring R44(14). The resulting structural diversity reveals that the (de‐)protonation of imidazole ring directly steers the coordination number of ligand, and thus causes a significant effect on the structure, especially the dimensionality. Furthermore, the solid‐state fluorescence properties of the free ligands and compounds 1–4 were studied at room temperature.

Synthesis, structure, thermal stability and luminescence of five 2D coordination polymers based on 4-(4-oxypyridinium-1-yl) phthalic acid and transition metal ions

Five new coordination polymers, [Cd(η6-L)]n (1), [Cd(η4-L)(H2O)2]n (2), [Co(η4-L)(H2O)2]n (3), {[Cd(η4-L)(H2O)1.5]·H2O}n (4), and {[Cu2(η4-L)2(H2O)2]·2H2O}n (5) were synthesized by reactions of corresponding metal salts with 4-(4-oxypyridinium-1-yl)phthalic acid (H2L) hydrothermally. Ligands in complexes are deprotonated even in the absence of a base. Metal ions in these complexes are all in an octahedral environment. The coordination number of ligand varies from 4 to 6. Each ligand binds three or four metal ions. High temperature synthesis tends to give ligands with larger coordination numbers. 1–5 are all 2D double-layer coordination polymers. Complex 2 and 3 are isostructural. The ligands in 1–5 exist in 4(1H)-pyridinone form rather than commonly presented 4-oxypyridinium forms. Without water molecules, complex 1 is stacked through π–π interaction between 4(1H)-pyridinone moieties in adjacent layers. Compound 2 is connected by intrermolecular hydrogen bonds via coordinated water molecules to form a 3D structure. All capable protons in 2 and 3 form H-bonds. π–π interaction between pyridinone rings exists in 1–4 and is stronger than that between two benzene rings. 4 packed into 3D via H-bond and π–π interaction while 5 packs into 3D via H-bond only. The pyridinone oxygen either coordinates to metal ion or form very strong H-bond with water molecule at O⋯O distance of 2.58Å. Of all these complexes, anhydrous 1 with non-redox Cd(II) can stable up to 410°C and is the most thermally stable while redox active Cu(II) complex 5 is stable under 210°C. All the Cd(II) complexes have ligand centered emission at 430 and 480nm. According to bond lengths, ligand in complexes exists as pyridinone form rather than the commonly presented oxopyridinium form. Different from viologen (pyridinum) compound, the entire complex does not change color upon heating or light irradiation.

A multispectroscopic study of 3d orbitals in cobalt carboxylates: the high sensitivity of 2p3d resonant X-ray emission spectroscopy to the ligand field.

Determination of the ligand coordination number and symmetry of a transition metal ion is important to understand reaction mechanisms in inorganic chemistry. The problem can be addressed through diffraction techniques or by spectroscopy. Here we limit ourselves to the latter and note that, historically, the problem has been studied with UV/Vis, or optical absorption or electronic spectroscopy.1 Recently however the field of resonant X-ray emission spectroscopy (RXES) developed at a high pace.2 Here we show that metal 2p3d RXES is highly sensitive to the metal ion ligand field. We present a comparison of UV/Vis, 2p X-ray absorption spectroscopy (XAS), and 2p3d RXES on a series of cobalt(II) carboxylates. The X-ray data were acquired at the state-of-the-art ADRESS beamline.3 We show that 2p XAS and UV/Vis have a limited discriminative power compared to 2p3d RXES. Through ligand field multiplet (LFM) calculations we show that 2p3d RXES allows the most judicious analysis of the ligand field. While previous 2p3d RXES studies on metal oxides revealed its d–d sensitivity,4 this is the first such observation on inorganic complexes. More importantly, the notion that 2p3d RXES measures element-selective, more as well as more intense d–d excitations than UV/Vis, and that this allows a more reliable determination of the ligand field, is novel. 2p3d RXES will allow unraveling reaction mechanisms of important 3d-metal-mediated chemical processes.

Coordination and Spin States in Vanadium Carbonyl Complexes (V(CO)n+, n = 1–7) Revealed with IR Spectroscopy

The vibrational spectra of vanadium carbonyl cations of the form V(CO)(n)(+), where n = 1-7, were obtained via mass-selected infrared laser photodissociation spectroscopy in the carbonyl stretching region. The cations and their argon and neon "tagged" analogues were produced in a molecular beam via laser vaporization in a pulsed nozzle source. The relative intensities and frequency positions of the infrared bands observed provide distinctive patterns from which information on the coordination and spin states of these complexes can be obtained. Density functional theory is carried out in support of the experimental spectra. Infrared spectra obtained by experiment and predicted by theory provide evidence for a reduction in spin state as the ligand coordination number increases. The octahedral V(CO)(6)(+) complex is the fully coordinated experimental species. A single band at 2097 cm(-1) was observed for this complex red-shifted from the free CO vibration at 2143 cm(-1).

Photoluminescence of Mn2+ Centers in Chalcohalide Glasses

We report on photoluminescence from Mn2+-doped chalcohalide glasses of the GeS2–Ga2S3–CsCl system. Upon blue excitation at 447 nm, a broad emission band occurs in the green spectral range from 500 to 600 nm, indicating the presence of tetrahedrally coordinated Mn2+ species. When the Mn2+-concentration is increased up to 2 mol%, an increasingly intense secondary emission band evolves at about 610–660 nm. Meanwhile, the intensity of the green band decreases gradually and shifts toward the red, resulting a very flatten luminescence from 500 to 750 nm, which provides a promising white light emitting source. As for the origin of this unique luminescence behavior of Mn2+, it is proposed that the ligand coordination number of Mn2+ in the present chalcohalide glasses experiences a partial change from tetrahedral to octahedral coordination.

Density Functional Theory Studies of [Fe(O) 2 L] 2+ : What is the Role of the Spectator Ligand L with Different Coordination Numbers?

Density functional theory (DFT) studies were carried out on [Fe(O)2(L)]n+ [L = qpy (1), simple amines (2), and tpy (3); qpy = 2,2′:6′,2″:6″,2″′:6″′,2″″-quinquepyridine and tpy = terpyridine; n = 1 or 2] to study how the coordination number of the spectator ligand L affects the geometries and electronic structures of the complexes. It was found that qpy can act as both a tridentate and pentadentate ligand resulting in [Fe(O)2(qpy)]2+ (12+) having a trigonal bipyramidal (TBP) geometry in the former case, and a pentagonal bipyramidal (PBP) geometry in the latter case. The difference in coordination geometries has a significant impact on the electronic structures of 12+. With a TBP geometry, 12+ adopts a [FeV(O)2(qpy)+·]2+ formalism where a d3 quartet FeV ion ferromagnetically and antiferromagnetically couples to the qpy cation radical to give close-lying triplet and quintet states (within ca. 0.2 eV). With a PBP geometry, the FeV ion in 12+ also formally has three unpaired electrons (a d3 quartet) with the fourth unpaired electron localized on a single oxido ligand to give a quintet state. The unoccupied orbital of 12+ in PBP geometry is lower lying in energy and has higher oxido character than when the complex has TBP geometry. Thus, based on the MO energies and oxido character of the unoccupied orbital, 12+ with PBP geometry is proposed to be a more reactive oxidant than 12+ with TBP geometry. On the other hand, 12+ with TBP geometry has a similar electronic structure to heme Cpd I, and it is possible that these two compounds have similar oxygen atom transfer reaction mechanisms. By varying the ligand coordination number using different spectator ligands L, the dioxido-iron complex [Fe(O)2(L)]2+ can change from a high-spin triplet when L = tpy, to a low-spin singlet when L = simple amines, to a quasi-degenerate triplet and quintet state when L = qpy.

Affinity of small ligands to myoglobin studied by the 3D-RISM theory

The affinity of small ligands including O 2, Xe, NO, CO, H 2S, and H 2O to myoglobin is studied based on the Three-dimensional Reference Interaction Site Model (3D-RISM) theory, the statistical mechanics of molecular liquids. The affinity is evaluated in terms of the coordination number of ligands in cavities in the protein, or the “Xe site,” which can be obtained from the radial distribution function of ligand molecules inside the cavities. It is found that NO, CO, and H 2S show greater affinity to the Xe sites than O 2 does, while the affinity of Xe is lower than that of O 2. A relevance of the results to the physiological activity of the protein is speculated. The physical origin of the difference in affinity among the ligands is discussed.

Effects of Ligand Coordination Number and Surface Curvature on the Stability of Gold Nanoparticles in Aqueous Solutions

The colloidal stability of gold nanoparticles (AuNPs) cap-exchanged with either monothiol- or dithiolane-terminated PEG-OCH(3) ligands was investigated. Three distinct aspects were explored: (1) effects of excess salt concentration; (2) ligation competition by dithiothreitol (DTT); and (3) resistance to sodium cyanide digestion. We found that overall ligands presenting higher coordination numbers (dithiolane) exhibit much better stability to excess added salt and against competition from DTT compared to their monodentate counterparts. Resistance to NaCN digestion indicated that there is a balance between coordination number and density of ligand packing on the NP surface. For smaller NPs, where a larger surface curvature reduces the ligand packing density, a higher coordination number is clearly beneficial. In comparison, a higher ligand density allowed by the smaller curvature for larger nanocrystals makes monothiol-PEG-capped NPs more resistant to cyanide digestion. The present study indicates that balance between the coordination number and surface packing density is crucial to enhancing the colloidal stability of AuNPs.

Factors Governing Metal−Ligand Distances and Coordination Geometries of Metal Complexes

The metal-ligand (M-L) distances play a central role in determining the structure and reactivity of the metal complex and in metal discrimination in proteins. They reflect properties of the metal ion and the ligand during their coordination as well as the environment. However, the variation of the M-L distances as a function of the properties of the metal cation and its donor atoms had not been systematically analyzed, and no public website listing the M-L distances from metal complexes was available. Herein, the distances around 63 different types of metal ions in approximately 200,000 high-resolution crystal structures in the Cambridge Structural Database as well as their preferred coordination geometries have been determined. The dependence of these distances on (i) the donor atom's charge and charge-donating ability, (ii) the donor atom's size, (iii) the metal ion's oxidation state and charge-accepting ability, and (iv) the metal ion's size has also been determined. The concept of ligand coordination number (CN) was introduced to take into account the effect of the number of constituent ligand atoms on the M-L distances in addition to the number of metal-bound ligands. We propose grouping the M-L distances according to both the metal CN and the ligand CN. The mean M-L distance corresponding to a given metal CN and ligand CN was found to be linearly correlated with the metal's ionic radius in going down a main group or in going across the first three rows of the periodic table. The M-L distances as a function of the metal and ligand coordination numbers are available via http://bioit.ibms.sinica.edu.tw/CSD.htm

Surface oxidation of Co 2+ and its dependence on ligand coordination number in silica polyamine composites

Coordination of CoCl 2 solutions to the silica polyamine composite, WP-1, made with the branched polymer polyethylenimine ( PEI) shows irreversible binding resulting from surface oxidation of the Co 2+–Co 3+. This is not the case for the silica polyamine composite BP-1 made with the linear polymer polyallylamine where reversible binding and no oxidation is observed. These observations suggested that oxidation of the cobalt was related to the greater coordination number available with the branched polyamine relative to the linear polyamine. A study of the kinetics of cobalt binding to WP-1 indicated initial loading of Co 2+ at relatively low coordination number followed by desorption of Co 2+ leading to oxidation and irreversible binding. Exclusion of oxygen from the composite-cobalt solution mixtures resulted in irreversible binding at a level that was 14% of the initial experiments. These observations prompted us to undertake a study to elucidate the coordination number around cobalt in the case of the branched polymer PEI. Towards this end, we have synthesized the model complexes [(tren)Co(H 2O) 2] 3+3Cl − (tren = tris(2,2′,2″aminoethyl)amine) and [(dien)Co(H 2O) 3] 3+3Cl − (dien = diethylenetriamine). The UV–Vis spectra of these model complexes were compared with Co 3+ coordinated to PEI in solution and it was concluded that the UV–Vis spectrum of the tren complex was closer to that observed for the solution UV–Vis spectrum of Co 3+–PEI. These data indicated that coordination of four amines was needed to drive surface oxidation under ambient conditions. In order to further elucidate the coordination number of a metal coordinated to the surfaces of WP-1 and BP-1, we reacted these composites with the probe molecule [Ru(CO) 3(TFA) 3] −K + (TFA = trifluoroacetate) (1) where the carbonyl stretching frequencies could be used as a measure of coordination number and geometry of the adsorbed complex. The IR of this complex on WP-1 indicated a monocarbonyl species while the IR of this complex on BP-1 indicated the presence of dicarbonyl species on the surface. These data are consistent with a coordination number of four amines in the case of WP-1 and the coordination of two amines in the case of BP-1 based on our previous studies of the solution coordination chemistry of the 1. Subsequent 13C CPMAS solid-state NMR on 13CO enriched samples of the 1 adsorbed onto BP-1 and WP-1 were consistent with the IR data.

Cuaq+/Cuaq2+ Redox Reaction Exhibits Strong Nonlinear Solvent Response Due to Change in Coordination Number

We have carried out extensive Born-Oppenheimer molecular dynamics simulations to characterize the structure and energetics of the Cu(+)/Cu(2+) redox pair. Our simulations support recent experimental evidence suggesting that Cu(2+) adopts a 5-fold coordination in aqueous solution and not the traditionally assumed octahedral coordination. Cu(+) forms a linear dihydrate structure with a third water molecule occasionally binding in the equatorial plane. We show that the change in ligand coordination number from 2 for aqueous Cu(+) to 5 for aqueous Cu(2+) leads to marked deviations from the linear response assumption underlying Marcus theory of oxidation. The diabatic free energy curves deviate from parabolic behavior and the reorganization free energies for Cu(+) and Cu(2+) are asymmetric and differ by 1.0 eV. The calculated reorganization free energies can semiquantitatively explain the exceptionally small electron self-exchange rate between Cu(+) and Cu(2+) in aqueous solution.

Influence of coordination number and ligand size on the dissociation mechanisms of transition metal-monosaccharide complexes

Several features of metal-carbohydrate complexes, in the form of deprotonated metal/ N-glycoside ions, were varied in order to determine which types of complexes would best enable mass spectrometric differentiation of the stereochemical features of the coordinated monosaccharides. Metal complexes were generated by using transition metals such as nickel, copper, and zinc. Additionally, the size and coordination number of ligands in the complex, as well as the number of these ligands coordinated to the metal, were varied. By using a quadrupole ion trap mass spectrometer, multistage mass spectrometric experiments were performed on the electrospray-generated metal N-glycoside complexes. Several tricoordinate Ni N-glycoside systems were capable of differentiating the stereochemistry about the C-2 center in the monosaccharide ring, whereas the tricoordinate Cu/ en system allowed for the differentiation of both the C-2 and C-4 stereocenters. No stereochemical differentiation was possible from four- or five-coordinate species with the exception of the four-coordinate Zn/ dien complexes. Changes in the metal center or size of the N-glycoside ligand generated the greatest changes in the product ion spectra of the tricoordinate complexes. Such alterations in four- or five-coordinate complexes often did not result in greatly differing product ion spectra. Whereas the product ion spectra of most metal/ N-glycoside complexes could be easily categorized according to structural features of the precursor ion, exceptional species such as the four coordinate [Zn( dien/monosaccharide)–H] + precursor ion, also capable of differentiating the stereochemical features about C-2 and C-4, can give unexpected stereochemical information.

Selective cation binding with cis,cis-1,3,5-trioxycyclohexyl based ligands: application to ion transport and electrochemical detection and assessment of complexation by electrospray mass spectrometry

The synthesis is reported of two sets of oxa-amide ionophores based on 2-phenylglycerol and cis,cis-cyclohexane-1,3,5-triol, with ligand coordination numbers of four, five and six. Ionophores based on the hexadentate cyclohexyl triamide 9 show excellent Na+/K+ selectivity (-log KpotNa,K = 3.1), and the pentadentate analogue 10 shows good Li+/Na+ selectivity (-log KpotLi,Na = 2.2). Solution NMR and liquid–liquid extraction studies confirmed the formation of 1∶1 complexes and IR studies pinpointed the presence of amide binding. Ligands based on 2-phenylglycerol exhibited good Ca2+ selectivity which was highest for the hexadentate triamide, 6. Detailed ESMS studies revealed similar trends in ion-binding, performed under controlled conditions.

Differences in nature and stability of cadmium complexes with Group 15/Group 16 donor ligands as determined by multinuclear (phosphorus-31, selenium-77, cadmium-113) magnetic resonance and electrochemical techniques

Multinuclear ({sup 31}P, {sup 77}Se, {sup 113}Cd) magnetic resonance and electrochemical studies have been carried out to investigate the nature and stability of cadmium complexes with Ph{sub 2}PCH{sub 2}P(E)Ph{sub 2} (E = S (dpmS), Se (dpmSe)) and Ph{sub 2}P(E)CH{sub 2}P(E)Ph{sub 2} (E = S (dpmS{sub 2}), Se (dpmSe{sub 2})) in dichloromethane solution. The NMR studies indicate a cadmium preference for selenium or sulfur rather than phosphorus coordination in the dpmE complexes but are limited in scope with the NMR technique to examination rather than phosphorus coordination in the dpmE complexes but are limited in scope with the NMR technique to examination of the 1:2 cadmium to ligand stoichiometry due to the labile nature of the systems in the presence of excess ligand. Cadmium-selenium coupling is observed in some cases. The electrochemical reductions of both (Cd(dpmE){sub 2}){sup 2+} and (Cd(dpmE{sub 2}){sub 2}){sup 2+} at a mercury electrode are reversible two-electron processes. Ligand coordination numbers of 4 and 6 are attained with the dpmE complexes, but the dpmE{sub 2} complexes have a maximum ligand coordination number of 4 even at very high ligand to metal concentration ratios. The magnitudes of the calculated stability constants suggest that all of the dpmE and dpmE{submore » 2} complexes of cadmium(II) are essentially nondissociated in solution. The use of a Cd(Hg) amalgam electrode allows examination of the voltammetry of these cadmium systems in the presence of substoichiometric ligand concentrations and reemphasizes the high stability of the dpmE and dpmE{sub 2} complexes. The preference of cadmium for the group 16 donor atoms rather than phosphorus in the dpmE complexes is confirmed by an investigation of the interactions of cadmium perchlorate with PPh{sub 3} and PPh{sub 3}Se and by suitable mixing experiments. 25 refs., 4 figs., 5 tabs.« less