Thermochromism of CuI Tetrakisguanidine Complexes: Reversible Activation of Metal‐to‐Ligand Charge‐Transfer Bands

Tetranuclear, intensely blue‐coloured Cu^I complexes were synthesised in which two Cu₂X₃^? units (X=Br or I) are bridged by a dicationic GFA (guanidino‐functionalised aromatic) ligand. The UV/Vis spectra show a large metal‐to‐ligand charge‐transfer (MLCT) band around 638 nm. The tetranuclear “low‐temperature” complexes are in a temperature‐dependent equilibrium with dinuclear Cu^I “high‐temperature” complexes, which result from the reversible elimination of two CuX groups. A massive thermochromism effect results from the extinction of the strong MLCT band upon CuX elimination with increasing temperature. For all complexes, quantum chemical calculations predict a small and method‐dependent energy difference between the possible electronic structures, namely Cu^I and dicationic GFA ligand (closed‐shell singlet) versus Cu^II and neutral GFA ligand (triplet or broken‐symmetry state). The closed‐shell singlet state is disfavoured by hybrid‐DFT functionals, which mix in exact Hartree–Fock exchange, and is favoured by larger basis sets and consideration of a polar medium.     

A twofold interwoven two‐dimensional→two‐dimensional (2‐D→2‐D) cluster–organic network based on the [Cu2I2] cluster and the 4,4′‐(diazenediyl)dipyridine ligand: poly[[μ2‐4,4′‐(diazenediyl)dipyridine]‐μ2‐iodido‐copper(I)]

In the polymeric title compound, [CuI(C₁₀H₈N₄)]_n, the Cu^I atom is in a four‐coordinated tetrahedral geometry, formed by two I atoms and two pyridine N atoms from two different 4,4′‐(diazenediyl)dipyridine (4,4′‐azpy) ligands. Two μ₂‐I atoms link two Cu^I atoms to form a planar rhomboid [Cu₂I₂] cluster located on an inversion centre, where the distance between two Cu^I atoms is 2.7781 (15) Å and the Cu—I bond lengths are 2.6290 (13) and 2.7495 (15) Å. The bridging 4,4′‐azpy ligands connect the [Cu₂I₂] clusters into a two‐dimensional (2‐D) double‐layered grid‐like network [parallel to the (10) plane], with a (4,4)‐connected topology. Two 2‐D grid‐like networks interweave each other by long 4,4′‐azpy bridging ligands to form a dense 2‐D double‐layered network. To the best of our knowledge, this interwoven 2‐D→2‐D network is observed for the first time in [Cu₂I₂]–organic compounds.     

An Adaptable N-Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

A neutral hybrid macrocycle with two trans-positioned N-heterocyclic carbenes (NHCs) and two pyridine donors hosts copper in three oxidation states (+I–+III) in a series of structurally characterized complexes ( 1 – 3 ). Redox interconversion of [LCu]^+/2+/3+ is electrochemically (quasi)reversible and occurs at moderate potentials (E_1/2=−0.45 V and +0.82 V (vs. Fc/Fc⁺)). A linear C^NHC-Cu-C^NHC arrangement and hemilability of the two pyridine donors allows the ligand to adapt to the different stereoelectronic and coordination requirements of Cu^I versus Cu^II/Cu^III. Analytical methods such as NMR, UV/Vis, IR, electron paramagnetic resonance, and Cu Kβ high-energy-resolution fluorescence detection X-ray absorption spectroscopies, as well as DFT calculations, give insight into the geometric and electronic structures of the complexes. The XAS signatures of 1 – 3 are textbook examples for Cu^I, Cu^II, and Cu^III species. Facile 2-electron interconversion combined with the exposure of two basic pyridine N sites in the reduced Cu^I form suggest that [LCu]^+/2+/3+ may operate in catalysis via coupled 2 e⁻/2 H⁺ transfer.     

Luminescent Copper(I)-Complexes with an Anionic NHC obtained via a Coordination Polymer as Versatile Precursor**

The anionic diamido N-heterocyclic carbene 1 is used to prepare a series of linear as well as trigonal, heteroleptic Cu^I complexes with small molecular ligands such as pyridine derivatives or triphenylphosphine. A key role lies in the versatile precursor for these complexes, a moisture- and air-stable 1D coordination polymer [1 ⋅ Cu] _n composed of only the NHC ligand and Cu^I, such that the copper is linearly coordinated by the carbene carbon atom and one oxygen atom of the backbone of the carbene. This polymer can easily be cleaved into monomeric complexes by addition of the desired ligand to dispersions of the polymer in dichloromethane. In solution, the complexes are in equilibrium with this highly insoluble polymer and free ligand. Thus, analytical and spectroscopical experiments with the compounds are limited to their crystalline state, characterized by single crystal X-ray diffraction experiments. Some of the complexes exhibit visible luminescence in the solid state upon irradiation with ultraviolet light. The spectral features (emission wavelength, Stokes shift, width of the emission band, vibrational fine structure) significantly differ among the complexes. Quantum mechanical computations reveal a subtle interplay of several factors such as coordination number and charge transfer character of the emissive state.     

Luminescent properties of europium(III) and terbium(III) complexes with para- and ortho-ethoxybenzoic acids

The luminescent properties of europium(III) and terbium(III) complexes with para- and ortho-ethoxybenzoic acids are studied. The excitation energies of the triplet states of ligands are determined, a hypothesis is made about the efficient luminescence of europium(III) and terbium(III) complexes, the geometry of the coordination polyhedron of a europium complex is established, and the luminescence quantum yields of the complexes in solution are determined.     

Highly Luminescent Linear Complex Arrays of up to Eight Cuprous Centers

Linearly arranged metal atoms that are embedded in discrete molecules have fascinated scientists across various disciplines for decades; this is attributed to their potential use in microelectronic devices on a submicroscopic scale. Luminescent oligonuclear Group 11 metal complexes are of particular interest for applications in molecular light‐emitting devices. Herein, we describe the synthesis and characterization of a rare, homoleptic, and neutral linearly arranged tetranuclear Cu^I complex that is helically bent, thus representing a molecular coil in the solid state. This tetracuprous arrangement dimerizes into a unique octanuclear assembly bearing a linear array of six Cu^I centers with two additional bridging cuprous ions that constitute a central pseudo‐rhombic Cu^I₄ cluster. The crystal structure determinations of both complexes reveal close d¹⁰???d¹⁰ contacts between all cuprous ions that are adjacent to each other. The dynamic behavior in solution, DFT calculations, and the luminescence properties of these remarkable complexes are also discussed.     

An Adaptable N‐Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

A neutral hybrid macrocycle with two trans‐positioned N‐heterocyclic carbenes (NHCs) and two pyridine donors hosts copper in three oxidation states (+I–+III) in a series of structurally characterized complexes ( 1 – 3 ). Redox interconversion of [LCu]^+/2+/3+ is electrochemically (quasi)reversible and occurs at moderate potentials (E_1/2=?0.45 V and +0.82 V (vs. Fc/Fc⁺)). A linear C^NHC‐Cu‐C^NHC arrangement and hemilability of the two pyridine donors allows the ligand to adapt to the different stereoelectronic and coordination requirements of Cu^I versus Cu^II/Cu^III. Analytical methods such as NMR, UV/Vis, IR, electron paramagnetic resonance, and Cu Kβ high‐energy‐resolution fluorescence detection X‐ray absorption spectroscopies, as well as DFT calculations, give insight into the geometric and electronic structures of the complexes. The XAS signatures of 1 – 3 are textbook examples for Cu^I, Cu^II, and Cu^III species. Facile 2‐electron interconversion combined with the exposure of two basic pyridine N sites in the reduced Cu^I form suggest that [LCu]^+/2+/3+ may operate in catalysis via coupled 2 e^?/2 H⁺ transfer.     

Poly[μ2‐chloro‐μ2‐1,4‐oxathiane‐κ2S:S‐copper(I)]

The title complex, [CuCl(C₄H₈OS)]_n, contains infinite spiral (CuS)_n chains linked by bridging Cl atoms into layers. The Cl atoms do not form polymeric fragments with Cu^I, but combine into isolated centrosymmetric Cu₂Cl₂ units. The compound is non‐isomorphous with the Br‐containing analogue, which contains Cu₈S₈ rings linked by Br atoms into chains. The O atom of the 1,4‐oxathiane mol­ecule does not realize its coordination abilities in the known copper(I)–halide complexes, while in copper(II)–halide complexes, oxathiane is coordinated via the S and O atoms. This falls into a pattern of the preferred inter­actions, viz. weak acid (Cu^I atom) with weak base (S atom) and harder acid (Cu^II atom) with harder base (O atom).     

Binuclear Copper(I) Complexes for Near-Infrared Light-Emitting Electrochemical Cells

Two binuclear heteroleptic Cu^I complexes, namely Cu−NIR1 and Cu−NIR2, bearing rigid chelating diphosphines and π-conjugated 2,5-di(pyridin-2-yl)thiazolo[5,4-d]thiazole as the bis-bidentate ligand are presented. The proposed dinuclearization strategy yields a large bathochromic shift of the emission when compared to the mononuclear counterparts (M1–M2) and enables shifting luminescence into the near-infrared (NIR) region in both solution and solid state, showing emission maximum at ca. 750 and 712 nm, respectively. The radiative process is assigned to an excited state with triplet metal-to-ligand charge transfer (³MLCT) character as demonstrated by in-depth photophysical and computational investigation. Noteworthy, X-ray analysis of the binuclear complexes unravels two interligand π–π-stacking interactions yielding a doubly locked structure that disfavours flattening of the tetrahedral coordination around the Cu^I centre in the excited state and maintain enhanced NIR luminescence. No such interaction is present in M1–M2. These findings prompt the successful use of Cu−NIR1 and Cu−NIR2 in NIR light-emitting electrochemical cells (LECs), which display electroluminescence maximum up to 756 nm and peak external quantum efficiency (EQE) of 0.43 %. Their suitability for the fabrication of white-emitting LECs is also demonstrated. To the best of our knowledge, these are the first examples of NIR electroluminescent devices based on earth-abundant Cu^I emitters.     

Strong Visible‐Light‐Absorbing Cuprous Sensitizers for Dramatically Boosting Photocatalysis

Developing strong visible‐light‐absorbing (SVLA) earth‐abundant photosensitizers (PSs) for significantly improving the utilization of solar energy is highly desirable, yet it remains a great challenge. Herein, we adopt a through‐bond energy transfer (TBET) strategy by bridging boron dipyrromethene (Bodipy) and a Cu^I complex with an electronically conjugated bridge, resulting in the first SVLA Cu^I PSs ( Cu‐2 and Cu‐3 ). Cu‐3 has an extremely high molar extinction coefficient of 162 260 m ^?1 cm^?1 at 518 nm, over 62 times higher than that of traditional Cu^I PS ( Cu‐1 ). The photooxidation activity of Cu‐3 is much greater than that of Cu‐1 and noble‐metal PSs (Ru(bpy)₃²⁺ and Ir(ppy)₃⁺) for both energy‐ and electron‐transfer reactions. Femto‐ and nanosecond transient absorption and theoretical investigations demonstrate that a “ping‐pong” energy‐transfer process in Cu‐3 involving a forward singlet TBET from Bodipy to the Cu^I complex and a backward triplet‐triplet energy transfer greatly contribute to the long‐lived and Bodipy‐localized triplet excited state.     

Synthesis,X-ray Structures,and Photoluminescence of the Octahedral Cu4I4 Cluster with Bulky Bidentate Bis(phosphanyl)amine Ligand

The reaction of C₁₀H₇-1-N(PPh₂)₂ ( 1 ) with two equivalents of CuI in acetonitrile resulted in the formation of octahedron Cu₄I₄[ 1 ]₂ complex ( 2 ). The crystal structure of 2 showed it adopted a rare octahedral arrangement. The rectangular Cu₄ plane is μ⁴-capped by two of the iodides and is placed in axial positions above and below the Cu₄-plane form an octahedron, whereas the other two iodides are bonded to two copper atoms in a μ²-fashion. The luminescence of complex 2 arises from a triplet halide-to-ligand charge transfer (³XLCT) excited state and ³CC (Cu₄I₄ cluster-centered) excited state are not involved in the luminescence by the rigid bidentate ligand 1 in spite of the short Cu^I–Cu^I bond length. Complex 2 was identified and characterized by multinuclear NMR (¹H, ¹³C, ³¹P NMR) and IR spectroscopy. Crystal structure determinations of 1 and 2 were carried out.     

Structure and spectroscopic properties of the dimeric copper(I) N‐heterocyclic carbene complex [Cu2(CNCt‐Bu)2](PF6)2

In recent years, the use of copper N‐heterocyclic carbene (NHC) complexes has expanded to fields besides catalysis, namely medicinal chemistry and luminescence applications. In the latter case, multinuclear copper NHC compounds have attracted interest, however, the number of these complexes in the literature is still quite limited. Bis[μ‐1,3‐bis(3‐tert‐butylimidazolin‐2‐yliden‐1‐yl)pyridine]‐1κ⁴C²,N:N,C^2′;2κ⁴C²,N:N,C^2′‐dicopper(I) bis(hexafluoridophosphate), [Cu₂(C₁₉H₂₅N₅)₂](PF₆)₂, is a dimeric copper(I) complex bridged by two CNC, i.e. bis(N‐heterocyclic carbene)pyridine, ligands. Each Cu^I atom is almost linearly coordinated by two NHC ligands and interactions are observed between the pyridine N atoms and the metal centres, while no cuprophilic interactions were observed. Very strong absorption bands are evident in the UV–Vis spectrum at 236 and 274 nm, and an emission band is observed at 450 nm. The reported complex is a new example of a multinuclear copper NHC complex and a member of a compound class which has only rarely been reported.     

Transformations in Chemically Responsive Copper‐Calixarene Architectures

Self‐assembly of bis‐picolyl‐appended calix[4]arene ( L ) with Cu^I or Cu^II salts resulted in a collection of multinuclear architectures capable of expressing structural reconfigurations in response to various chemical stimuli: addition of copper salt, solvents, or oxidation. Coordination of L to CuX (X=Br, I) selectively yielded dinuclear macrocycles Cu^I ₂ L₂Br₂ ( 1 ) and Cu^I ₂ L₂I₂ ( 3 ) that were transformed into tetranuclear assemblies Cu^I ₄ L₂Br₄ ( 2 ) and Cu^I ₄ L₂I₄ ( 4 ) upon further addition of CuX. These supramolecules persist as robust and discrete entities in solution that display red emission; notably, 4 exhibits luminescence thermochromism. Assembly of L with CuCl₂ produced macrocycle Cu^II ₂ L₂Cl₄ ( 5 ), which crystallised as cage [Cu^II ₂ L₄(μ‐Cl)]³⁺ ( 6 ) in the presence of MeOH. Two chemical signals—introduction of CuCl₂ and addition of CH₃CN—regenerated macrocycle 5 . Coordination of L to Cu(OTf) yielded macrocycle Cu^I ₂ L₂(OTf)₂ ( 7 ) that also crystallised as cage 6 upon oxidation in CHCl₃.     

Biaryl and Aryl Ketone Synthesis via Pd‐Catalyzed Decarboxylative Coupling of Carboxylate Salts with Aryl Triflates

A bimetallic catalyst system has been developed that for the first time allows the decarboxylative cross‐coupling of aryl and acyl carboxylates with aryl triflates. In contrast to aryl halides, these electrophiles give rise to non‐coordinating anions as byproducts, which do not interfere with the decarboxylation step that leads to the generation of the carbon nucleophilic cross‐coupling partner. As a result, the scope of carboxylate substrates usable in this transformation was extended from ortho‐substituted or otherwise activated derivatives to a broad range of ortho‐, meta‐, and para‐substituted aromatic carboxylates. Two alternative protocols have been optimized, one involving heating the substrates in the presence of Cu^I/1,10‐phenanthroline (10–15 mol %) and PdI₂/phosphine (2–3 mol %) in NMP for 1–24 h, the other involving Cu^I/1,10‐phenanthroline (6–15 mol %) and PdBr₂/Tol‐BINAP (2 mol %) in NMP using microwave heating for 5–10 min. While most products are accessible using standard heating, the use of microwave irradiation was found to be beneficial especially for the conversion of non‐activated carboxylates with functionalized aryl triflates. The synthetic utility of the transformation is demonstrated with 48 examples showing the scope and limitations of both protocols. In mechanistic studies, the special role of microwave irradiation is elucidated, and further perspectives of decarboxylative cross‐couplings are discussed.     

Structural and Electronic Control of the Bidentate 1-(2-pyridyl)benzotriazole Ligand in Copper Chemistry with Application to Catalysis in the A3 Coupling Reaction

The hybrid bidentate 1-(2-pyridyl)benzotriazole (pyb) ligand was introduced into 3d transition metal catalysis. Specifically, [Cu^II(OTf)₂(pyb)₂] ⋅ 2 CH₃CN ( 1 ) enables the synthesis of a wide range of propargylamines by the A³ coupling reaction at room temperature in the absence of additives. Experimental and high-level theoretical calculations suggest that the bridging N atom of the ligand imposes exclusive trans coordination at Cu and allows ligand rotation, while the N atom of the pyridine group modulates charge distribution and flux, and thus orchestrates structural and electronic precatalyst control permitting alkyne binding with simultaneous activation of the C−H bond via a transient Cu^I species.     

17O NMR studies of ortho‐substituent effects in substituted phenyl tosylates

¹⁷O NMR spectra for 35 ortho‐, para‐, and meta‐substituted phenyl tosylates (phenyl 4‐methylbenzenesulfonates), 4‐CH₃‐C₆H₄SO₂OC₆H₄‐X, at natural abundance in acetonitrile at 50 °C were recorded. The ¹⁷O NMR chemical shifts, δ(¹⁷O), of the sulfonyl (SO₂) and the single‐bonded phenoxy (OPh) oxygens for para and meta derivatives correlated well with dual substituent parameter treatment using the Taft inductive, σ_I, and resonance, σº_R, constants. The influence of ortho substituents on the sulfonyl oxygen and the single‐bonded phenoxy oxygen chemical shifts, δ(¹⁷O), was found to be nicely described by the Charton equation: δ(¹⁷O)_ortho = δ(¹⁷O)_H + ρ_Iσ_I + ρ_Rσ°_R + δE_s^B when the data treatment was performed separately for electron‐donating +R substituents and electron‐attracting ?R substituents. Electron‐attracting meta and para substituents in the phenyl moiety caused deshielding while the electron‐donating meta, para and ortho +R substituents produce shielding effects on the sulfonyl (SO₂) and single‐bonded phenoxy (OPh) oxygens. The influence of ortho inductive and resonance effects in the case of +R substituents was found to be approximately twice higher than the corresponding influence from the para position. Due to the steric effect of ortho substituents a decrease in shielding of the oxygens at the sulfonyl group (δE_s^B > 0, E_s^B < 0) was detected. Copyright © 2012 John Wiley & Sons, Ltd.     

Structural and Spectroscopic Studies of Halocuprate(I) and Haloargentate(I) Complexes [M2XnX′4−n]2−

The complexes [Cu₂Br₄]^2?, [Cu₂I₄]^2?, [Cu₂I₂Br₂]^2?, [Cu₂I₃Cl]^2?, [Ag₂Cl₄]^2? have been characterized as their isomorphous bis(triphenylphosphoranylidene)ammonium ([Ph₃PNPPh₃]⁺ = PNP⁺) salts by single crystal structural determinations. All anions show the centrosymmetric doubly halogen‐bridged forms [XM(μ‐X)₂MX]^2? with three‐coordinate metal atoms that have been observed in [M₂X₄]^2? complexes with other large organic cations. In [Cu₂I₂Br₂]^2? the iodide ligands occupy the bridging positions and the bromide the terminal positions, while in [Cu₂I₃Cl]^2?, obtained in an attempt to prepare [Cu₂I₂Cl₂]^2?, two of the iodide ligands occupy the bridging positions with the third iodide and the chloride ligand occupying two statistically disordered terminal positions. In [Ag₂Cl₄]^2? the distortion from ideal trigonal coordination of the metal atom is greater than in the copper complexes, but less than in other previously reported [Ag₂Cl₄]^2? complexes with organic cations. The ν(MX) bands have been assigned in the far‐IR spectra, and confirm previous observations regarding the unexpectedly simple IR spectra of [Cu₂X₄]^2? complexes.     

Photoresponsive Propeller-like Chiral AIE Copper(I) Clusters

A pair of propeller-like chiral trinuclear Cu^I clusters ( R/S-Cu3 ) with unique photoinduced fluorescence enhancement were prepared. R/S-Cu3 showed intense variable luminescence after UV light irradiation, which was attributed to the stepwise oxidation of ligand in the clusters. It exhibited typical aggregation-induced emission (AIE) (α_AIE=17.3). Mechanism studies showed that metal cluster-centered (MCC) and triplet metal-to-ligand charge-transfer (³MLCT) processes are the origin of the luminescence; the processes are regulated by a restriction of intramolecular motions mechanism in a different state. The chiral structure and AIE feature endow R/S-Cu3 with remarkable circularly polarized luminescence (g_lum=2×10⁻²) in the aggregated state. It shows good capability for producing reactive oxygen species. This work enriches the kinds of atomically precise AIE clusters, gains insight into their luminescence mechanism, and offers the prospect of application in multifunctional materials.     

First‐row transition metal–pyridine (py)–sulfate [(py)xM](SO4) complexes (M = Ni,Cu and Zn): crystal field theory in action

The crystal structures of three first‐row transition metal–pyridine–sulfate complexes, namely catena‐poly[[tetrakis(pyridine‐κN)nickel(II)]‐μ‐sulfato‐κ²O:O′], [Ni(SO₄)(C₅H₅N)₄]_n, (1), di‐μ‐sulfato‐κ⁴O:O‐bis[tris(pyridine‐κN)copper(II)], [Cu₂(SO₄)₂(C₅H₅N)₆], (2), and catena‐poly[[tetrakis(pyridine‐κN)zinc(II)]‐μ‐sulfato‐κ²O:O′‐[bis(pyridine‐κN)zinc(II)]‐μ‐sulfato‐κ²O:O′], [Zn₂(SO₄)₂(C₅H₅N)₆]_n, (3), are reported. Ni compound (1) displays a polymeric crystal structure, with infinite chains of Ni^II atoms adopting an octahedral N₄O₂ coordination environment that involves four pyridine ligands and two bridging sulfate ligands. Cu compound (2) features a dimeric molecular structure, with the Cu^II atoms possessing square‐pyramidal N₃O₂ coordination environments that contain three pyridine ligands and two bridging sulfate ligands. Zn compound (3) exhibits a polymeric crystal structure of infinite chains, with two alternating zinc coordination environments, i.e. octahedral N₄O₂ coordination involving four pyridine ligands and two bridging sulfate ligands, and tetrahedral N₂O₂ coordination containing two pyridine ligands and two bridging sulfate ligands. The observed coordination environments are consistent with those predicted by crystal field theory.