Development of the spin crossover model for compressible and nonuniform chains of exchange clusters

A model of a compressible chain of exchange clusters, which retains the cooperative behavior during thermal crossover, was developed. The model was generalized for periodical chains with two and more exchange clusters per unit cell. A general expression was obtained for the effective magnetic moment of the unit cell with several exchange clusters. A two-position approximation for the partition function of the Jahn-Teller paramagnetic center Cu^II (spin 1/2) was proposed for chains with the “head-head” motif, which makes it possible to derive the equation for the effective magnetic moment in quadratures. The model developed describes the cases of both smooth and sharp (cooperative) spin crossover for chains of three-spin exchange clusters.     

Metal–Organic Frameworks Constructed from Versatile [WS4Cux]x−2 Units: Micropores in Highly Interpenetrated Systems

Five metal–organic frameworks (MOFs) formed by [WS₄Cu_x]^x?2 secondary building units (SBUs) and multi‐pyridyl ligands are presented. The [WS₄Cu_x]^x?2 SBUs function as network vertexes showing various geometries and connectivities. Compound 1 contains one‐dimensional channels formed in fourfold interpenetrating diamondoid networks with a hexanuclear [WS₄Cu₅]³⁺ unit as SBU, which shows square‐pyramidal geometry and acts as a tetrahedral node. Compound 2 contains brick‐wall‐like layer also with a hexanuclear [WS₄Cu₅]³⁺ unit as SBU. The [WS₄Cu₅]³⁺ unit in 2 is a new type of [WS₄Cu_x]^x?2 cluster unit in which the five Cu⁺ ions are in one plane with the W atom, forming a planar unit. Compound 3 shows a nanotubular structure with a pentanuclear [WS₄Cu₄]²⁺ unit as SBU, which is saddle‐shaped and acts as a tetrahedral node. Compound 4 contains large cages formed between two interpenetrated (10,3)‐a networks also with a pentanuclear [WS₄Cu₄]²⁺ unit acting as a triangular node. The [WS₄Cu₄]²⁺ unit in 4 is isomeric to that in 3 and first observed in a MOF. Compound 5 contains zigzag chains with a tetrahedral [WS₄Cu₃]⁺ unit as SBU, which acts as a V‐shaped connector. The influence of synthesis conditions including temperature, ligand, anions of Cu^I salts, and the ratio of [NH₄]₂WS₄ to Cu^I salt on the formation of these [WS₄Cu_x]^x?2‐based MOFs were also studied. Porous MOF 3 is stable upon removal and exchange of the solvent guests, and when accommodating different solvent molecules, it exhibits specific colors depending on the polarity of incorporated solvent, that is, it shows a rare solvatochromic effect and has interesting prospects in sensing applications.     

Effect of pH on complexation in mixed dilute aqueous solutions of poly(acrylic acid), poly(ethylene glycol), and Cu2+ ions

The phase states of mixed dilute solutions of PAA, PEG, and Cu²⁺ ions largely determines the mechanism governing the growth of metal nanoparticles during the subsequent reduction of copper ions. Mixtures with PAA: PEG > 1 base-mol/base-mol and PAA: Cu²⁺ ≥ 5 base-mol/mol are studied. It is shown that the simultaneous complexation of PAA with PEG and Cu²⁺ ions in these mixtures at pH values below the intrinsic pH of a solution is accompanied by phase separation related to insolubility of PAA-PEG interpolymer complexes. A decrease in the pH of the ternary mixture is caused by the release of a strong low-molecular-mass acid due to complexation with Cu²⁺ ions. The minimum pH value, above which the PAA-PEG-Cu²⁺ system becomes single-phase (a transparent solution), depends on the concentration ratio between PAA and PEG chains (the mean degree of polymerization). This value is either 6.8–7.0 (if all macromolecules are incorporated in the insoluble interpolymer complex with PEG) or 4.0 (if chains occur in excess). Methods of preparing single-phase systems in the pH range 4.0–7.0 via exchange reactions of the PAA-Cu²⁺ complex with PEG or the nonstoichiometric soluble interpolymer complex PAA-PEG are developed. Viscometry, electron microscopy, and dynamic light scattering are used to investigate the compositions and structures of soluble complexes, in which either each chain (if the chain is long) may be linked with both PEG and Cu²⁺ ions or PAA chains are redistributed between two complexes (at comparable lengths of PAA and PEG chains).     

Copper Crystallization from Aqueous Solution: Initiation and Evolution of the Polynuclear Clusters

The initial steps of copper electrocrystallization process from aqueous solutions have been studied at DFT level of theory. It has been shown that Cu(H₂O) unit is the final product of Cu²⁺-ions electroreduction. From this particle clusters Cu_n·aq are formed and grow. Aggregation of copper atoms to the Cu_n·aq clusters consists of two steps. The first step includes condensation of Cu(H₂O) units to hydrated clusters Cu_n(H₂O)_n (n = 2–6). At the second step bonding of Cu(H₂O) particles is accompanied by dehydration of clusters yielding Cu_n(H₂O)_m structures (n > m). Cluster Cu₇·aq has been singled out as key structure based on calculated values of energies and Cu–Cu bond distances of Cu_n·aq clusters. This cluster is of D_5h symmetry which is typical for copper microcrystals formed from aqueous solutions in electrocrystallization processes on foreign surface. This key particle could be considered as a critical nucleus. Number of copper atoms therein matches average dimension of critical nucleus.     

Host-lattice effects on the electron spin resonance spectra of copper(II) complexes in diluted and undiluted forms

ESR spectra of the Cu²⁺ ion, doped in a di-[(2-aminoethyl)amine)]-di-isothiocyanato-cadmate(II) lattice in its single-crystal form, have been investigated. Cd(den) (SCN)₂ has unusual coordination, in that two separate CdN₆ and CdN₄S₂ entities are present in the unit cell. Cu²⁺ substitutes predominantly for the CdN₆ structural unit, and spectra recorded at both ambient and liquid-nitrogen temperatures are fitted to a spin-Hamiltonian of orthorhombic symmetry. In contrast, the weak spectral lines corresponding to the Cu²⁺ ions which substitute for the CdN₄S₂ structural unit are characteristic of axial symmetry. A single-crystal study was also made on di-[(2-aminoethyl)amine]-di-isothiocyanato- copper(II), in which the Cu₂₊ ion has a coordination number of five and “C_2v” point symmetry. A comparison of the spectral features shown in the two lattices was made and some interesting conclusions are drawn.     

MOFs Conferred with Transient Metal Centers for Enhanced Photocatalytic Activity

Highly effective photocatalysts for the hydrogen‐evolution reaction were developed by conferring the linkers of NH₂‐MIL‐125(Ti), a metal–organic framework (MOF) constructed from TiO_x clusters and 2‐aminoterephthalic acid (linkers), with active copper centers. This design enables effective transfer of electrons from the linkers to the transient Cu²⁺/Cu⁺ centers, leading to 7000‐fold and 27‐fold increase of carrier density and lifetime of photogenerated charges, respectively, as well as high‐rate production of H₂ under visible‐light irradiation. This work provides a novel design of a photocatalyst for hydrogen evolution using non‐noble Cu²⁺/Cu⁺ as co‐catalysts.     

Separation of catechin epimers by complexation using ion mobility mass spectrometry

Ion mobility coupled with mass spectrometry provides a fast and repeatable method to separate catechin epimers by previous complexation with selected chiral modifiers and transition metals. Several combinations with chiral ligands such as D‐ and L‐amino acids and/or additional metal cations, chiral crown ethers, tartaric acid and heptakis(2,6‐di‐O‐methyl)‐β‐cyclodextrin were screened for their ability to affect the separation efficiency. The clusters having the form of [2M + D‐amino acid + Cu²⁺ ? 3H]^? (M stands for (?)‐epicatechin or (+)‐catechin) showed improvement in stereodifferentiation between two epimeric catechins in comparison to the analysis of pure epimers, where no separation was observed or the separation was hampered by the formation of mixed dimer complexes. Among various examined D‐amino acids only those possessing hydrophobic side chains induced the improvement of separation efficiency. The best peak‐to‐peak resolution (R_p–p) was determined to be 0.71 for [2M + D‐Leucine + Cu²⁺ ? 3H]^? clusters. Copyright © 2015 John Wiley & Sons, Ltd.     

Multiple scattering method applied to the electronic structure and spectra of the impurity ions Cu2+, Ni2+, Co2+, Fe2+ and their tetrahedral environments in ZnO,ZnS and CdS

The multiple scattering method with local exchange and muffin-tin potentials is applied to tetrahedral clusters with Cu²⁺, Ni²⁺, Co²⁺ and Fe²⁺ as central ions and oxygen and sulphur as ligands. The calculated orbital energy differences ?(t₂^*) - ?(e^*) are compared to Δ_t values deduced from optical spectra. The agreement is fairly good.     

Reactive capture of gold nanoparticles by strongly physisorbed monolayers on graphite

Anthracene Diels Alder adducts (DAa) bearing two long side chains (H-(CH₂)₂₂O(CH₂)₆OCH₂-) at the 1- and 5-positions form self-assembled monolayers (SAMs) at the phenyloctane - highly oriented pyrolytic graphite (HOPG) interface. The long DAa side chains promote strong physisorption of the monolayer to HOPG and maintain the monolayer morphology upon rinsing or incubation in ethanol and air-drying of the substrate. Incorporating a carboxylic acid group on the DAa core enables capture of 1-4 nm diameter gold nanoparticles (AuNPs) provided (i) the monolayer containing DAa-carboxylic acids is treated with Cu²⁺ ions and (ii) the organic coating on the AuNP contains carboxylic acids (11-mercaptoundecanoic acid, MUA-AuNP). AuNP capture by the monolayer proceeds with formation of Cu²⁺ - carboxylate coordination complexes. The captured AuNP appear as mono- and multi-layered clusters at high coverage on HOPG. The surface density of the captured AuNPs can be adjusted from AuNP multi-layers to isolated AuNPs by varying incubation times, MUA-AuNP concentration, the number density of carboxylic acids in the monolayer, the number of MUA per AuNP, and post-incubation treatments.     

Harnessing the Flexibility of Peptidic Scaffolds to Control their Copper(II)‐Coordination Properties: A Potentiometric and Spectroscopic Study

Designing small peptides that are capable of binding Cu²⁺ ions mainly through the side‐chain functionalities is a hard task because the amide nitrogen atoms strongly compete for Cu²⁺ ion coordination. However, the design of such peptides is important for obtaining biomimetic small systems of metalloenyzmes as well as for the development of artificial systems. With this in mind, a cyclic decapeptide, C‐Asp, which contained three His residues and one Asp residue, and its linear derivative, O‐Asp, were synthesized. The C‐Asp peptide has two Pro? Gly β‐turn‐inducer units and, as a result of cyclization, and as shown by CD spectroscopy, its backbone is constrained into a more defined conformation than O‐Asp, which is linear and contains a single Pro? Gly unit. A detailed potentiometric, mass spectrometric, and spectroscopic study (UV/Vis, CD, and EPR spectroscopy) showed that at a 1:1 Cu²⁺/peptide ratio, both peptides formed a major [CuHL]²⁺ species in the pH range 5.0–7.5 (C‐Asp) and 5.5–7.0 (O‐Asp). The corrected stability constants of the protonated species (log K*_CuH(O?Asp)=9.28 and log K*_CuH(C?Asp)=10.79) indicate that the cyclic peptide binds Cu²⁺ ions with higher affinity. In addition, the calculated value of K_eff shows that this higher affinity for Cu²⁺ ions prevails at all pH values, not only for a 1:1 ratio but even for a 2:1 ratio. The spectroscopic data of both [CuHL]²⁺ species are consistent with the exclusive coordination of Cu²⁺ ions by the side‐chain functionalities of the three His residues and the Asp residue in a square‐planar or square‐pyramidal geometry. Nonetheless, although these data show that, upon metal coordination, both peptides adopt a similar fold, the larger conformational constraints that are present in the cyclic scaffold results in different behaviour for both [CuHL]²⁺ species. CD and NMR analysis revealed the formation of a more rigid structure and a slower Cu²⁺‐exchange rate for [CuH(C‐Asp)]²⁺ compared to [CuH(O‐Asp]²⁺. This detailed comparative study shows that cyclization has a remarkable effect on the Cu²⁺‐coordination properties of the C‐Asp peptide, which binds Cu²⁺ ions with higher affinity at all pH values, stabilizes the [CuHL]²⁺ species in a wider pH range, and has a slower Cu²⁺‐exchange rate compared to O‐Asp.     

Kinetics of the exchange between MnO or Mn3O4 and some divalent cations in aqueous solution

The kinetics of the exchange between MnO or Mn₃O₄ and Co²⁺, Cu²⁺, Ni²⁺, or Zn²⁺ ions in solution, was determined by measuring the γ-activity of⁵⁶Mn acquired by the solution after shaking with the neutron irradiated solid. The results indicated a fast exchange followed by a slower apparently diffusion-controlled exchange. The exchange capacity increased in the series: Co²⁺<Ni²⁺<Cu²⁺=Zn²⁺ for MnO and Ni²⁺<Co²⁺<Zn²⁺<Cu²⁺ for Mn₃O₄. The trends could not be satisfactorily explained by the ionic radii or crystal field stabilisation-energies. In the case of MnO, the results were discussed in terms of the estimated standard enthalpy change of the exchange reaction.     

Investigating the New Interaction Model between Cu(II) and PVA by the Proton Relaxation Method

An aqueous PVA-Cu²⁺ solution at pH = 3 is in the hydrated form and becomes green at pH ? 6 with a decrease in viscosity. The structure of the copper ion is suggested to be that of a polynuclear complex at pH > 6. For the green solution the polynuclear chains of the copper complex are believed to be surrounded by the PVA chains with the hydrophobic backbones facing toward the inside and the hydrophilic OH groups oriented toward the outside facing the bulk water. The proton spin-lattice relaxation rate 1/T_1p and the spin-spin relaxation rate 1/T₂ of CH and CH₂ in PVA and H₂O for aqueous PVA-Cu²⁺ solutions at pH = 3, can be explained by the two site exchange model in the region of the fast exchange limit. The dipolar correlation time τ_c is dominated by the reorientational process with a dipolar correlation time of 2.11 × 10^?11 s. When the pH rises from pH=3 to pH=12.5, the variation of 1/T₁p and 1/T₂p of CH and CH₂ in PVA with Cu²⁺ ion concentration in aqueous PVA-Cu²⁺ solution at pH=12.5 can be explained in terms of the relaxation by an inclusive model of the polynuclear copper complex and PVA. Furthermore, the frequency (or field) dependence of 1/T₁p, 1/T₂p of CH in PVA for aqueous PVA-Cu²⁺ solution at pH = 12.5 suggests that the dipolar relaxation is dominated by the electron-spin relaxation with the electron spin relaxation time T_1e = 1 ? 2 × 10^?10 s. The invariance of 1/T₁p and 1/T₂p of H₂O with the variation of the Cu²⁺ ion concentration in aqueous PVA-Cu²⁺ solution at pH = 12.5 supports the hypothesis that the water is not directly bound to the Cu²⁺ ion.     

Formation and Dissociation Mechanism of Amide Complexes,IV. Protonation and deprotonation of the Cu2+ and Ni2+ complexes of 3,7-diazanonanediamide and 3,7-diazanonane-N,N′-diethylamide

The protonation and deprotonation rates of the coordinated amide groups in the Ni²⁺ and Cu²⁺ complexes of 3,7-diazanonanediamide (DANA) and in the Cu²⁺ complex of 3,7-diazanonane-N,N′-diethylamide (DANEA) have been studied by stopped-flow techniques. For the interconversion M(H_?2L) ? ML, two consecutive reactions are observed in the case of Cu²⁺ with DANA or DANEA, whereas there is only one reaction for Ni²⁺ with DANA. Cu(H_?2DANEA) is unusually labile, indicating a strong interaction between the N-ethyl groups. The conversion of the O- into the N-coordinated amide groups in NiDANA²⁺ is 25 times slower than in CuDANA²⁺. In the case of Ni²⁺ this excludes a step with water substitution, which is involved in one of the reaction paths observed for the Cu²⁺ complexes, since the rates of water exchange differ by a factor of 10⁵ for the two metal ions.     

Double Rotors with Fluxional Axles: Domino Rotation and Azide–Alkyne Huisgen Cycloaddition Catalysis

The simple preparation of the multicomponent devices [Cu₄( A )₂]⁴⁺ and [Cu₂( A )( B )]²⁺, both rotors with fluxional axles undergoing domino rotation, highlights the potential of self‐sorting. The concept of domino rotation requires the interconversion of axle and rotator, allowing the spatiotemporal decoupling of two degenerate exchange processes in [Cu₄( A )₂]⁴⁺ occurring at 142 kHz. Addition of two equiv of B to rotor [Cu₄( A )₂]⁴⁺ afforded the heteromeric two‐axle rotor [Cu₂( A )( B )]²⁺ with two distinct exchange processes (64.0 kHz and 0.55 Hz). The motion requiring a pyridine→zinc porphyrin bond cleavage is 1.2×10⁵ times faster than that operating via a terpyridine→[Cu(phenAr₂)]⁺ rupture. Finally, both rotors are catalysts due to their copper(I) content. The fast domino rotor (142 kHz) was shown to suppress product inhibition in the catalysis of the azide–alkyne Huisgen cycloaddition.     

Water‐soluble hyperbranched polyelectrolytes with high fluorescence quantum yield: Facile synthesis and selective chemosensor for Hg2+ and Cu2+ ions

New water‐soluble hyperbranched polyfluorenes bearing carboxylate side chains have been synthesized by the simple “A2 + B2 + C3” protocol based on Suzuki coupling polymerization. The linear polyfluorene analogue LPFA was also synthesized for comparative investigation. The optical properties of the neutral precursory polymers in CHCl₃ and final carboxylic‐anionic conjugated polyelectrolytes in buffer solution were investigated. The obtained hyperbranched polyelectrolyte HPFA2 with lower content of branch unit (2%) showed excellent solubility and high fluorescence quantum yield (?_F = 89%) in aqueous solution. Fluorescence quenching of HPFA2 by different metal ions was also investigated, the polyelectrolyte showed high selectivity for Hg²⁺ and Cu²⁺ ions relative to other various metal ions in buffer solution. The Stern‐Volmer constant K_sv was determined to be 0.80 × 10⁶ M^?1 for Hg²⁺ and 3.11 × 10⁶ M^?1 for Cu²⁺, respectively, indicating the potential application of HPFA2 as a highly selective and sensitive chemosensor for Hg²⁺ and Cu²⁺ ions in aqueous solution. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3431–3439, 2010     

Hyperbranch-polyethyleniminated functional polymers II: effect of polyethyleniminated polyoxypropylenediamines on copper nanoparticle formation in aqueous solution

Colloidal aqueous solution of zerovalent copper (Cu(0)) nanoparticles were prepared from the Cu²⁺ ions coordinated with polyethyleniminated polyoxypropylenediamines (D400(EI) _x ) followed by chemical reduction of NaBH₄. Aqueous solution of copper clusters formed in the presence of D400(EI)₈ with a loading ratio of [EI]/[Cu²⁺] = 3 were stable without precipitation for standing more than 1 month. The protective effects of D400(EI) _x and the particle size of the resulted Cu nanoparticle are regulated by the attachments of ethylenimine (EI) groups per polymer backbone and the normality ratio of [EI]/[Cu²⁺] used. It is found that the more EI-content per polymer backbone results in the smaller particle size and the narrower size dispersity of the colloidal Cu(0) particles, and the average particle size of 5.07 nm with standard deviation of 0.86 nm was obtained in the presence of D400(EI)₈ with the ratio of [EI]/[Cu²⁺] = 3. As the polymer concentration of D400(EI)₈ increases (the increase of [EI]/[Cu²⁺]), the average particle size of the prepared Cu(0) nanoparticle slightly changes, but interestingly, the size dispersity gradually decreases, where the standard deviation for the concentration at [EI]/[Cu²⁺] = 5 is 0.82 nm approaching that for monodispersed nanoparticles (0.5 nm).     

Self-Assembled Anion-Binding Cryptand for the Selective Liquid–Liquid Extraction of Phosphate Anions

The ligands L¹ and L² form trinuclear self-assembled complexes with Cu²⁺ (i.e. [( L¹ )₂Cu₃]⁶⁺ or [( L² )₂Cu₃]⁶⁺) both of which act as a host to a variety of anions. Inclusion of long aliphatic chains on these ligands allows the assemblies to extract anions from aqueous media into organic solvents. Phosphate can be removed from water efficiently and highly selectively, even in the presence of other anions.     

Microwave-assisted template-free synthesis of butterfly-like CuO through Cu2Cl(OH)3 precursor and the electrochemical sensing property

An energy-efficient and environmentally friendly microwave-assisted method was adopted for synthesis of butterfly-like CuO assembled by nanosheets through a Cu₂Cl(OH)₃ precursor, using no template. Formation mechanism of the butterfly-like CuO was explored and discussed systematically for the first time on the basis of both experimental results and crystal structure transformations in atomic level. The electrochemical sensing properties of the butterfly-like CuO modified electrode to ascorbic acid (AA) were studied for the first time. The results reveal that Cu(OH)₂ nanowires were formed once the Cu²⁺ ions, located in between two CuO₄ parallelogram chains of a Cu₂Cl(OH)₃ precursor, dissolve into the solution as Cu(OH)₄²⁻ complex ions after ion exchange reactions and simultaneous assemble along a axis. Upon microwave irradiation, the adjacent CuO₄ parallelogram chains of the Cu(OH)₂ nanowires dehydrate and assemble along c axis, forming CuO nanosheets with (002) as the main exposed facet, which were further assembled to butterfly-like CuO under the action of microwave field, suggesting that microwave field functions like a ‘directing agent’. The butterfly-like CuO modified electrode shows good electrochemical sensing properties to AA with a low detecting limit, short response time and wide linear response range.     

Substituent Effect to Fine-Tune Energy Levels of Atom-Precise [MoOS3]2− Modified Copper(I) Thiolate Clusters Boosting Recyclable Photocatalysis

The propulsion of photocatalytic hydrogen (H₂) production is limited by the rational design and regulation of catalysts with precise structures and excellent activities. In this work, the [MoOS₃]²⁻ unit is introduced into the Cu^I clusters to form a series of atomically-precise Mo^VI−Cu^I bimetallic clusters of [Cu₆(MoOS₃)₂(C₆H₅(CH₂)S)₂(P(C₆H₄−R)₃)₄] ⋅ xCH₃CN (R=H, CH₃, or F), which show high photocatalytic H₂ evolution activities and excellent stability. By electron push-pull effects of the surface ligand, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of these Mo^VI−Cu^I clusters can be finely tuned, promoting the resultant visible-light-driven H₂ evolution performance. Furthermore, Mo^VI−Cu^I clusters loaded onto the surface of magnetic Fe₃O₄ carriers significantly reduced the loss of catalysts in the collection process, efficiently addressing the recycling issues of such small cluster-based catalyst. This work not only highlights a competitively universal approach on the design of high-efficiency cluster photocatalysts for energy conversion, but also makes it feasible to manipulate the catalytic performance of clusters through a rational substituent strategy.