Mixed-valence Complexes Research Articles

Redox-Transmetalation Reactions: Easy Access to Homo- and Heterodimetallic d8,d10 Complexes.

The ability of the imine PyCH═N-CH2Py (Py = 2-pyridyl, bpi) to behave as a heteroditopic ligand, which is suitable for creating two separate compartments to host metals in different oxidation states, has been developed by studying the reactions of the mixed-valence complexes [(cod)M-Ι(μ-bpi)MΙ(cod)] (M = Rh, Ir) with [M'(Cl)2(PPh3)2] (M' = Pd, Ni). The results depend on the molar ratio of the reagents used (1:1 or 1:2) to give the heterometallic complexes {d10-M',d8-M}-[(PPh3)(Cl)M'0(μ-bpi)MΙ(cod)] (Pd,Rh, 4; Pd,Ir, 5; Ni,Rh, 8; Ni,Ir, 9) and the two-electron mixed-valent compounds [(PPh3)(Cl)M'0(μ-bpi)M'ΙΙ(Cl)] (M' = Ni, 10; Pd, 11), respectively. A redox process occurs in the replacement of the low-valent [(cod)M-I] fragment, whereas the exchange of the [(cod)MI] fragment is redox-neutral. The metal with a d8 configuration in the products exhibits a square-planar geometry coordinated to two (Rh/Ir) or three (Ni/Pd) nitrogen atoms of the bridging bpi ligand. Conversely, the metal with a d10 configuration adopts trigonal-planar geometries, π-bonded to the imine C═N bond. The isolated complexes 4/5 and 10/11, along with the hypothetical heterometallic Pd,Ni compound (12), were studied by DFT methods. Additionally, the T-shaped moiety 'M'ΙΙ(PPh3)(Cl)(η1-CH-N(bpi))', stabilized by a secondary γ-agostic interaction, and the 'M'II(Cl)(κ3N-bpi)' fragment was found to be accessible redoxomers of complexes 10 and 11 by DFT calculations.

A Personal Account on Inorganic Reaction Mechanisms.

The presented Review is focused on the latest research in the field of inorganic chemistry performed by the van Eldik group and his collaborators. The first part of the manuscript concentrates on the interaction of nitric oxide and its derivatives with biologically important compounds. We summarized mechanistic information on the interaction between model porphyrin systems (microperoxidase) and NO as well as the recent studies on the formation of nitrosylcobalamin (CblNO). The following sections cover the characterization of the Ru(II)/Ru(III) mixed-valence ion-pair complexes, including Ru(II)/Ru(III)(edta) complexes. The last part concerns the latest mechanistic information on the DFT techniques applications. Each section presents the most important results with the mechanistic interpretations.

Valence Delocalization and Metal-Metal Bonding in Carbon-Bridged Mixed-Valence Iron Complexes.

The carbide ligand in the iron-molybdenum cofactor (FeMoco) in nitrogenase bridges iron atoms in different oxidation states, yet it is difficult to discern its ability to mediate magnetic exchange interactions due to the structural complexity of the cofactor. Here, we describe two mixed-valent diiron complexes with C-based ketenylidene bridging ligands, and compare the carbon bridges with the more familiar sulfur bridges. The ground state of the [Fe2 (μ-CCO)2 ]+ complex with two carbon bridges (4) is S= , and it is valence delocalized on the Mössbauer timescale with a small thermal barrier for electron hopping that stems from the low Fe-C force constant. In contrast, one-electron reduction of the [Fe2 (μ-CCO)] complex with one carbon bridge (2) affords a mixed-valence species with a high-spin ground state (S= ), and the Fe-Fe distance contracts by 1 Å. Spectroscopic, magnetic, and computational studies of the latter reveal an Fe-Fe bonding interaction that leads to complete valence delocalization. Analysis of near-IR intervalence charge transfer transitions in 5 indicates a very large double exchange constant (B) in the range of 780-965 cm-1 . These results show that carbon bridges are extremely effective at stabilizing valence delocalized ground states in mixed-valent iron dimers.

The Atomic Arrangement of the Sherwoodite Structural Unit: Redefinition of the Sherwoodite Vanadoaluminate Heteropoly Complex as [AlV4+V5+12O39]11−

Abstract Sherwoodite, putatively Ca4.5(AlV5+12V4+2O40)·28H2O, was among the early naturally occurring polyoxometalates to be reported from the Uravan Mineral Belt; the ideal formula of sherwoodite is here redefined as Ca5.5[AlV4+V5+12O39]·28H2O. In an earlier preliminary structure report (R = 22%), researchers defined the composition of vanadoaluminate heteropoly complex in sherwoodite as (AlV4+,5+14O40)n−, the first natural occurrence of that polyoxometalate cluster. The cluster forms a framework around intermolecular channels that contain disordered arrays of H2O and charge-balancing Ca ions. A detailed structure study described herein [a = 27.9085(7), c = 13.8090(5), space group I41/amd, R1 < 6%] reveals new details of the vanadoaluminate heteropoly complex, indicating that it crystallizes as a structural variant of the putative (AlV4+,5+14O40)n− vanadoaluminate complex, [AlV4+V5+12O39]11−. That variant is derived through the random removal of one of the two apical vanadate octahedra, elucidating the cause of difficulties in the refinement of the preliminary structure report. The [AlV4+V5+12O39]11− complex is capped on one end by a Ca atom that bonds to four octahedral apices in a half-occupied site, and on the opposite end by the V1 and O1 atoms in half-occupied sites, similar to the compound potassium 13-vanadanomangate(IV). The capping octahedron is the locus of the V4+ ions in the mixed-valence complex. New electron microprobe analyses yielded an empirical formula, on the basis of (V4+V5+12), of (Ca4.26Mg1.09Na0.35K0.04Sr0.03)[Al0.87(V4+V5+12)O39]·28.15 H2O.

Terpyridine-containing 5-(2-pyridylmethylene)-2-thioimidazolones and their coordination compounds with copper(II) chloride: synthesis and cytotoxicity

Methods for the synthesis of new ditopic organic ligands containing coordinating fragments of two types, terpyridine and thiohydantoin, have been developed. The synthesized ligands were studied in complex formation reactions with copper(II) chloride. As a result, two types of coordination compounds were isolated and characterized by electron spectroscopy, elemental analysis, and cyclic voltammetry: bis-ligand tetranuclear mixed-valent complexes of copper(I, II) and a monoligand binuclear complex containing two copper(II) ions. The cytotoxic activity of the obtained coordination compounds with respect to the MCF7, A549, and HEK293 cell lines was studied.

Spin polarization effects in trigonal mixed-valence complexes exhibiting double exchange supported by external spin-cores.

The theory of the magnetic coupling between the localized spins, mediated by the mobile excess electron, is generalized to the case of a trigonal, six-center, four-electron molecule with partial valence delocalization. The combination of the electron transfer occurring within the valence-delocalized subsystem and the interatomic exchange producing coupling of the spin of the mobile electron of valence-delocalized fragment with the three localized spins forming the valence-localized subsystem leads to the appearance of a special kind of double exchange (DE), termed the "external core double exchange" (ECDE), in order to distinguish such DE from the conventional "internal core double exchange" for which the mobile electron is coupled with the spin-cores on the same center via the intra-atomic exchange. The effect of the ECDE on the ground spin state of the considered trigonal molecule is compared with earlier reported effect produced by DE in the four-electron, mixed-valence (MV) trimer. A high diversity of the ground spin states is revealed, depending on the relative magnitudes and signs of the electron transfer and interatomic exchange parameters, with part of these states not appearing to be the ground states in a trigonal trimer exhibiting DE. We briefly discuss some examples of trigonal MV systems from the point of view of the possibility to have different combinations of signs of the transfer and exchange parameters and, accordingly, different ground spin states. The tentative role of the considered systems in molecular electronics and spintronics is also noticed.

Syntheses, crystal structures and MMCT properties of cyanidometal-bridged trinuclear Ru-Ru-Ru complexes

Two pairs of cyanidometal-bridged trinuclear complexes trans-[Cp*(modppe)Ru(μ-NC)Ru(L)4(μ-CN)Ru(modppe)Cp*][PF6]n (n = 2, 3; L = pyridine (py), 1[PF6]n; 4-methoxypyridine (mopy), 2[PF6]n; Cp* = 1,2,3,4,5-pentamethylcyclopentadiene) in two distinct states have been synthesized and fully characterized. The investigated results suggest that 1[PF6]3 and 2[PF6]3 are Class II mixed valence complexes. The IR spectroscopy and UV–vis-NIR spectroscopy analyses showed that with the increase of the electron-donating ability of the auxiliary ligand of the cyanidometal bridge, the degree of delocalization increases and the energy of the metal-to-metal charge transfer (MMCT) of the mixed valence complexes decreases, supported by TDDFT calculations.

Iminophenol as a ligand or cation in cobalt(II and II,III) complexes: synthetic approaches, spectral and structural studies

New complexes [H2L]2CoCl4 (І) and [CoІІCоІІІCl2L3]H2O (ІІ), where HL is a condensation product of o-vanillin and methylamine, were synthesized and characterized by IR, UV/Vis spectroscopies, and X-ray structural analysis. For the synthesis of I, cobalt (II) chloride was used as a metal source, while for II, cobalt powder and its salt were used simultaneously. In the preparation of I, the use of methylamine hydrochloride contributed to the formation of tetrachlorocobaltate(II) with the protonated Schiff base H2L+as a cation. The monovalent cations are almost planar and show the presence of intramolecular O/N–HО hydrogen bonds. In the crystal, columns of cations connected by - stacking alternate with double-row columns of tetrahedral [CoCl4]2– anions and are additionally joined by a branched system of O/N–HCl hydrogen bonds. Crystal II is built of neutral molecules CoІІСоІІІCl2L3 and uncoordinated water molecules. In the molecule, the main CoІІІL3 fragment with a metal atom in the octahedral environment of three deprotonated ligands attaches a cobalt(II) atom at the CoIICoIII distance of 3.17 Å. The cobalt(II) atom has a highly distorted five-coordinate СоІІО3Cl2 environment formed by two bridging oxygen atoms of deprotonated phenoxy groups, the O atom of the methoxy group of the ligands, and two chlorine atoms. The obtained results allow us to propose the use of the metal salt+its powder combination for the purposeful synthesis of mixed-valent complexes of metals capable of exhibiting various oxidation states. Deliberate protonation of the Schiff base can be used to prepare organic-inorganic metal halide hybrids with functionalized cations.

A Highly Sensitive Dual-Signal Strategy via Inner Filter Effect between Tween 20-Gold Nanoparticles and CdSe/ZnS Quantum Dots for Detecting Cu2.

A highly sensitive and accurate dual-signal strategy is developed for trace Cu2+ detection based on the inner filter effect (IFE) between Tween 20-gold nanoparticles (AuNPs) and CdSe/ZnS quantum dots (QDs). Tween 20-AuNPs are utilized as colorimetric probes and excellent fluorescent absorbers. The fluorescence of CdSe/ZnS QDs can be quenched efficiently by Tween 20-AuNPs via IFE. In the presence of D-penicillamine, D-penicillamine induces the aggregation of Tween 20-AuNPs and the fluorescent recovery of CdSe/ZnS QDs at high ionic strength. Upon addition of Cu2+, D-penicillamine tends to selectively chelate with Cu2+ and then forms the mixed-valence complexes, which consequently inhibits the aggregation of Tween 20-AuNPs and the fluorescent recovery. The dual-signal method is used to quantitatively detect trace Cu2+, with low detection limits of 0.57 μg/L and 0.36 μg/L for colorimetry and fluorescence, respectively. In addition, the proposed method using a portable spectrometer is applied to the detection of Cu2+ in water. This sensitive, accurate and miniature sensing system has potential in environmental evaluations.

Understanding the Magnetic Relaxation Mechanism in Mixed-Valence Dilanthanide Complexes with Metal-Metal Bonding: A Theoretical Investigation.

Theoretical investigations on mixed-valence dilanthanide complexes (CpiPr5)2Ln2I3 (Ln = Tb, Dy, and Ho) indicate that the total spin of the 4f shell couples preferentially to the σ electron spin and then to the orbital angular momentum, improving the strength of spin-orbit coupling (SOC) for each magnetic center. On the other hand, the concentration of negative charges containing the delocalized σ electron in the axial direction leads to a large crystal-field (CF) splitting. Both strong SOC and large CF splitting lead to the largest energy barrier Ueff of such complexes up to now. In addition, our calculations show that the introduction of σ electron can better suppress the quantum tunneling of magnetization in the ground spin-orbit state, and the Ueff of (CpiPr5)2Ln2I3 is expected to originate from the contribution of both Ln ions under such strong Ln-σ exchange coupling.

Influence of Rare-Earth Ion Radius on Metal-Metal Charge Transfer in Trinuclear Mixed-Valent Complexes.

We report the synthesis and characterization of a highly conjugated bisferrocenyl pyrrolediimine ligand, Fc2PyrDIH (1), and its trinuclear complexes with rare earth ions─(Fc2PyrDI)M(N(TMS)2)2 (2-M, M = Sc, Y, Lu, La). Crystal structures, nuclear magnetic resonance (NMR) spectra, and ultraviolet/visible/near-infrared (UV/vis-NIR) data are presented. The latter are in good agreement with DFT calculations, illuminating the impact of the rare earth ionic radius on NIR charge transfer excitations. For [2-Sc]+, the charge transfer is at 11,500 cm-1, while for [2-Y]+, only a d-d transition at 8000 cm-1 is observed. Lu has an ionic radius in between Sc and Y, and the [2-Lu]+ complex exhibits both transitions. From time-dependent density functional theory (TDDFT) analysis, we assign the 11,500 cm-1 transition as a mixture of metal-to-ligand charge transfer (MLCT) and metal-to-metal charge transfer (MMCT), rather than pure metal-to-metal CT because it has significant ligand character. Typically, the ferrocenes moieties have high rotational freedom in bis-ferrocenyl mixed valent complexes. However, in the present (Fc2PyrDI)M(N(TMS)2)2 complexes, ligand-ligand repulsions lock the rotational freedom so that rare-earth ionic radius-dependent geometric differences increasingly influence orbital overlap as the ionic radius falls. The Marcus-Hush coupling constant HAB trends as [2-Sc]+ > [2-Lu]+ > [2-Y]+.

Symmetry‐Broken Intermolecular Charge Separation of Cationic Radicals

AbstractA quadrupolar compoundPyr‐BAwith two pyrrole‐type nitrogen atoms doped externally was prepared in this work. In high contrast with other π ionic radicals, its cationic radicalPyr‐BA⋅+undergoes unusual symmetry‐broken charge separation (SB‐CS), generating the mixed valence complex ofPyr‐BA+1−q⋅⋅⋅Pyr‐BA+1+q, whereqis the degree of charge transfer. Variable‐temperature (VT) single‐crystal analysis, absorption and EPR experiments all confirmed that aggregation and lower temperature would help to facilitate this SB‐CS process. Gibbs energy calculations and gauge‐including magnetically induced current simulation both validate that, forPyr‐BA⋅+, SB‐CS behavior is more favorable than the conventional dimerization mode. To the best of our knowledge, this is the first study that shows solid single‐crystal evidence for spontaneous SB‐CS between identical ionic radicals. Such a unique phenomenon is of great significance both in terms of fundamental aspects and uncharted material science.

Symmetry-Broken Intermolecular Charge Separation of Cationic Radicals.

A quadrupolar compound Pyr-BA with two pyrrole-type nitrogen atoms doped externally was prepared in this work. In high contrast with other π ionic radicals, its cationic radical Pyr-BA⋅+ undergoes unusual symmetry-broken charge separation (SB-CS), generating the mixed valence complex of Pyr-BA+1-q ⋅⋅⋅Pyr-BA+1+q , where q is the degree of charge transfer. Variable-temperature (VT) single-crystal analysis, absorption and EPR experiments all confirmed that aggregation and lower temperature would help to facilitate this SB-CS process. Gibbs energy calculations and gauge-including magnetically induced current simulation both validate that, for Pyr-BA⋅+ , SB-CS behavior is more favorable than the conventional dimerization mode. To the best of our knowledge, this is the first study that shows solid single-crystal evidence for spontaneous SB-CS between identical ionic radicals. Such a unique phenomenon is of great significance both in terms of fundamental aspects and uncharted material science.

Ancillary Ligand Coordination Directed Modes of Aggregation in Mixed-Valence Tetranuclear Cobalt Complexes: Synthesis, Structure, Field-Induced SIM Behavior, and Theoretical Insights

The contrasting metal–ion coordination reactivity behavior for the multisite ligand H3L ([2,6-bis-{(2-hydroxymethyl-phenylimino)methyl}-4-methylphenol]) with Co(ClO4)2·6H2O in the presence of ancillary ligands NaPiv (Piv = (CH3)3CCO2–) and NH4SCN afforded two different types of mixed-valence tetranuclear cobalt complexes, [CoIII3CoII(HL)(L)(μ4-O)(μ-OH)((CH3)3CCOO)2]ClO4·H2O (1) and [CoIII2CoII2(L)2(NCS)4(CH3CN)2]·CH3CN (2). In the first case, the presence of bridging pivalate anions triggered the oxido-hydroxido-bridged dual control for aggregation, whereas in the second case, terminal thiocyanate coordination forced the dicubane type of aggregation. The connectivity for different numbers of CoIII and CoII ions are achieved through contrasting bridges by ancillary water-molecule-derived HO– plus O2– linkers and ligand-anion-based double phenolato bridges, where terminal monodentate coordinations finally fulfill the octahedral geometries around the CoII ions. Alternating current/direct current (AC/DC) magnetic studies revealed field-induced slow magnetic relaxation for complex 1 that arises from the single octahedral CoII ion connected to three other diamagnetic CoIII ions. Whereas in complex 2, having a tetranuclear CoIII2CoII2 structure, the two CoII centers showed an antiferromagnetic interaction (J = −0.39 cm–1). The structural distortions in these two types of aggregates lead to different magnitudes of easy-axis magnetic anisotropy (D = −51.31 cm–1 for 1 and −31.9 cm–1 for 2) and a small but non-negligible transverse component (E/D = 0.263 for 1 and 0.255 for 2). The static and dynamic magnetic data were analyzed using DFT and CASSCF-based calculations.

Mixed-valence realizations of quantum dot cellular automata

This article reviews recent progress in the design and theoretical investigation of molecular implementations of quantum-dot cellular automata (QCA) for field-coupled nanocomputing applications. QCA is a classical computing paradigm based on Coulombic quantum interactions between nanoscale subunits consisting of two or more quantum dots. Shrinking these dots to the molecular scale maximizes device density and permits operation at ambient temperatures. The essential feature of molecular QCA systems is the presence of two or more coupled redox centers separated by spacers. In this work candidate systems ranging from simple organic molecules to self-assembled multi-center mixed-valence organometallic complexes are surveyed, and some of the challenges remaining to be faced both in theoretical understanding and practical implementation are discussed.

Excited State Mixed-Valence Complexes: From the Special Pair to the Creutz-Taube Ion and Beyond.

Compounds and complexes with mixed-valence electronic ground states, such as the Creutz-Taube ion, have proven to be excellent vehicles through which to study intramolecular electron-transfer processes. In a recent contribution by Cadranel and co-workers, time-resolved pump-probe spectroscopy reveals photo-induced metal-to-bridge charge transfer within the homovalent analogue of the Creutz-Taube ion, [{(NH3 )5 Ru}(μ-pz){Ru(NH3 )5 }]4+ , giving rise to two closely lying excited states with mixed-valence character, one with a shorter lifetime (τ=136 ps) and weakly-coupled (Robin-Day Class II) character, the other a longer-lived (τ=2.8 ns) configurational isomer with more delocalized electronic structure. Electron transfer reactions from the longer-lived species demonstrate analogies with the photo-induced reactions of the photosynthetic special pair, suggesting this state as a reference system for excited state mixed-valency, and a framework from which to explore photocatalytic reactions.

Two-Electron Mixed Valency in a Heterotrimetallic Nickel–Vanadium–Nickel Complex

Mixed-valence complexes represent an enticing class of coordination compounds to interrogate electron transfer confined within a molecular framework. The diamagnetic heterotrimetallic anion, [V(SNS)2{Ni(dppe)}2]-, was prepared by reducing (dppe)NiCl2 in the presence of the chelating metalloligand [V(SNS)2]- [dppe = bis(diphenylphosphino)ethane; (SNS)3- = bis(2-thiolato-4-methylphenyl)amide]. Vanadium-nickel bonds span the heterotrimetallic core in the structure of [V(SNS)2{Ni(dppe)}2]-, with V-Ni bond lengths of 2.78 and 2.79 Å. One-electron oxidation of monoanionic [V(SNS)2{Ni(dppe)}2]- yielded neutral, paramagnetic V(SNS)2{Ni(dppe)}2. The solid-state structure of V(SNS)2{Ni(dppe)}2 revealed that the two nickel ions occupy unique coordination environments: one nickel is in a square-planar S2P2 coordination environment (τ4 = 0.19), with a long Ni···V distance of 3.45 Å; the other nickel is in a tetrahedral S2P2 coordination environment (τ4 = 0.84) with a short Ni-V distance of 2.60 Å, consistent with a formal metal-metal bond. Continuous-wave X-band electron paramagnetic resonance spectroscopy, electrochemical investigations, and density functional theory computations indicated that the unpaired electron in the neutral V(SNS)2{Ni(dppe)}2 cluster is localized on the bridging [V(SNS)2] metalloligand, and as a result, V(SNS)2{Ni(dppe)}2 is best described as a two-electron mixed-valence complex. These results demonstrate the important role that metal-metal interactions and flexible coordination geometries play in enabling multiple, reversible electron transfer processes in small cluster complexes.

Tweaking the Optoelectronic Properties of S-Doped Polycyclic Aromatic Hydrocarbons by Chemical Oxidation.

Peri-thiaxanthenothiaxanthene, an S-doped analog of peri-xanthenoxanthene, is used as a polycyclic aromatic hydrocarbon (PAH) scaffold to tune the molecular semiconductor properties by editing the oxidation state of the S-atoms. Chemical oxidation of peri-thiaxanthenothiaxanthene with H2 O2 led to the relevant sulfoxide and sulfone congeners, whereas electrooxidation gave access to sulfonium-type derivatives forming crystalline mixed valence (MV) complexes. These complexes depicted peculiar molecular and solid-state arrangements with face-to-face π-π stacking organization. Photophysical studies showed a widening of the optical bandgap upon progressive oxidation of the S-atoms, with the bis-sulfone derivative displaying the largest value (E00 =2.99 eV). While peri-thiaxanthenothiaxanthene showed reversible oxidation properties, the sulfoxide and sulfone derivatives mainly showed reductive events, corroborating their n-type properties. Electric measurements of single crystals of the MV complexes exhibited a semiconducting behavior with a remarkably high conductivity at room temperature (10-1 -10-2 S cm-1 and 10-2 -10-3 S cm-1 for the O and S derivatives, respectively), one of the highest reported so far. Finally, the electroluminescence properties of the complexes were tested in light-emitting electrochemical cells (LECs), obtaining the first S-doped mid-emitting PAH-based LECs.

From sandwich complexes to dendrimers: journey toward applications to sensing, molecular electronics, materials science, and biomedicine.

This review links various areas of inorganic chemistry around the themes developed by our research group during the last four decades. It is firstly based on the electronic structure of iron sandwich complexes, showing how the metal electron count dictates their reactivities, with various applications (via C-H activation, C-C bond formation) as reducing and oxidizing agents, redox and electrocatalysts and precursors of dendrimers and catalyst templates through bursting reactions. Various electron-transfer processes and consequences are explored, including the influence of the redox state on the acidity of robust ligands and the possibility to iterate in situ C-H activation and C-C bond formation to build arene-cored dendrimers. Examples of how these dendrimers are functionalized are illustrated using the cross olefin metathesis reactions, with application to the synthesis of soft nanomaterials and biomaterials. Mixed and average valence complexes give rise to remarkable subsequent organometallic reactions, including the salt influence on these reactions. The stereo-electronic aspect of these mixed valencies is pointed out in star-shaped multi-ferrocenes with a frustration effect and other multi-organoiron systems, with the perspective of understanding electron-transfer processes among dendrimer redox sites involving electrostatic effects and application to redox sensing and polymer metallocene batteries. Dendritic redox sensing is summarized for biologically relevant anions such as ATP2- with supramolecular exoreceptor interactions at the dendrimer periphery in parallel with the seminal work on metallocene-derived endoreceptors by Beer's group. This aspect includes the design of the first metallodendrimers that have applications in both redox sensing and micellar catalysis with nanoparticles. These properties provide the opportunity to summarize the biomedical (mostly anticancer) applications of ferrocenes, dendrimers and dendritic ferrocenes in biomedicine (in particular the contribution from our group, but not only). Finally, the use of dendrimers as templates for catalysis is illustrated with numerous reactions including C-C bond formation, click reactions and H2 production reactions.

An efficient mixed-valence copper pyrazolate catalyst for the conversion of carbon dioxide and epoxides into cyclic carbonates.

In this paper, a cyclic (CuIpz)3·CH3CN (1) precursor and a mixed-valence pentanuclear complex CuI3CuII2(OH)pz6·CH3CN (2) have been synthesized and structurally characterized by single-crystal X-ray diffraction, where pzH = 4-chloro-3,5-diphenylpyrazole. The excellent catalytic activity of 2 has been demonstrated in the chemical fixation of CO2 into value-added cyclic carbonates, which can be carried out at ambient pressure and room temperature along with ultra-high yield and perfect steric hindrance tolerance. Based on density functional theory (DFT) calculations and comparison with the catalytic performance of 1, it is proposed that the coordinatively unsaturated CuII atoms of 2 are probably the active sites for this catalytic reaction.