Linear Copper Complex Arrays as Versatile Molecular Strings: Syntheses, Structures, Luminescence, and Magnetism

Twisted and Disconnected Chains: Flexible Linear Tetracuprous Arrays and a Decanuclear CuI Cluster as Blue- and Green/Yellow-Light Emitters

Defined arrays of transition metal ions embedded in tailored polydentate ligand scaffolds allow for a systematic design of their physical properties. Such molecular strings of closed-shell transition metal centers are particularly interesting for Group 11 metal ions in the oxidation state +1 if they undergo metallophilic d10···d10 contact interactions since these clusters are oftentimes efficient photoluminescence (PL) emitters. Copper is particularly attractive as a sustainable earth-abundant coinage metal source and because of the ability of several CuI complexes to serve as powerful thermally activated delayed fluorescence (TADF) emitters in molecular/organic light-emitting devices (OLEDs). Our combined synthetic, crystallographic, photophysical, and computational study describes a straight tetracuprous array possessing a centrally disconnected CuI2···CuI2 chain and a continuous helically bent CuI4 complex. This molecular helix undergoes a facile rearrangement in diethyl ether solution, yielding an unprecedented nanosized CuI10 cluster (2.9 × 2.0 nm) upon crystallization. All three clusters show either bright blue phosphorescence, TADF, or green/yellow multiband phosphorescence with quantum yields between 6.5 and 67%, which is persistent under hydrostatic pressure up to 30 kbar. Temperature-dependent PL investigations in combination with time-dependent density-functional theory (TD-DFT) calculations and void space analyses of the crystal packings complement a comprehensive correlation between the molecular structures and photoluminescence properties.

Supramolecular compounds assembled from the heteroleptic tetrahedral complex [{CpMo(CO)2}2(μ,η2-AsSb)] and metal salts

The reaction of the tetrahedral complex [{CpMo(CO)2}2(μ,η2-AsSb)] with CuI and AgI salts is presented which gives unprecedented neutral and cationic supramolecular aggregates featuring mixed As/Sb-donor molecules as ligands/linkers between metal ions.

Novel synthetic route towards heteroleptic pnictogen-rich organometallic-inorganic coordination compounds

The reaction of the Ag(I) dimer [Ag2(η2-A)2(μ,η1:η1-A)2][TEF]2 (A = [{CpMo(CO)2}2(μ,η2-P2)]) possessing labile η2-coordinated P2 ligands with the organometallic dipnictogen compounds [{CpMo(CO)2}2(μ,η2-EE')] (E = E' = As, Sb; E = P, E' = As, Sb) represents a facile synthetic route towards unprecedented heteroleptic pnictogen-rich supramolecular complexes. This method can also be extended to the analogous Cu(I) dimer and is studied by DFT computations.

Synthesis, characterization, DNA binding interactions, DFT calculations, and Covid-19 molecular docking of novel bioactive copper(I) complexes developed via unexpected reduction of azo-hydrazo ligands

In this work, we focused on the 3rd goal of the sustainable development plan: achieving good health and supporting well-being. Two redox-active hydrazo ligands namely, phenylcarbonohydrazonoyldicyanide (PCHD) and pyridin-4-ylcarbonohydrazonoyl-dicyanide (PyCHD), and their copper(I) complexes have been synthesized and characterized. The analytical data indicates the formation of copper(I) complexes despite starting with copper(II) perchlorate salt. The 1H-NMR and UV-visible spectral studies in DMSO revealed that PyCHD mainly exists in its azo-form, while PCHD exists in azo ↔ hydrazo equilibrium form, and confirmed the copper(I) oxidation state. XPS, spectral and electrochemistry data indicated the existence of copper(I) valence of both complexes. Cyclic voltammetry of PCHD and its copper(I) complex supported the reduction power of the ligand. The antimicrobial activity, cytotoxicity against the mammalian breast carcinoma cell line (MCF7), and DNA interaction of the compounds are investigated. All compounds showed high antimicrobial, and cytotoxic activities, relative to the standard drugs. Upon studying the wheat DNA binding, PCHD and PyCHD were found to bind through external contacts, while both [Cu(PCHD)2]ClO4.H2O and [Cu(PyCHD)2]ClO4.H2O were intercalated binding. In-silico molecular docking simulations against Estrogen Receptor Alpha Ligand Binding Domain (ID: 6CBZ) were performed on all produced compounds and confirmed the invitro experimentally best anticancer activity of [Cu(PyCHD)2]ClO4.H2O. The molecular docking tests against SARS-CoV-2 main protease (ID: 6 WTT) showed promising activity in the order of total binding energy values: [Cu(PCHD)2]ClO4.H2O > [Cu(PyCHD)2]ClO4.H2O > PCHD > PyCHD.

Mixed-Valence Tetrametallic Iridium Chains

Neutral [X-{Ir2 }-{Ir2 }-X] (X=Cl, Br, SCN, I) and dicationic [L-{Ir2 }-{Ir2 }-L]2+ (L=MeCN, Me2 CO) tetrametallic iridium chains made by connecting two dinuclear {Ir2 } units ({Ir2 }=[Ir2 (μ-OPy)2 (CO)4 ], OPy=2-pyridonate) by an iridium-iridium bond are described. The complexes exhibit fractional averaged oxidation states of +1.5 and electronic delocalization along the metallic chain. While the axial ligands do not significantly affect the metal-metal bond lengths, the metallic chain has a significant impact on the iridium-L/X bond distances. The complexes show free rotation around the unsupported iridium-iridium bond in solution, with a low-energy transition state for the chloride chain. The absorption spectra of these complexes show characteristic bands at 438-504 nm, which can be fine-tuned by varying the terminal capping ligands.

Selective coordination of coinage metals using orthogonal ligand scaffolds

Group 11 metal complexes with their ability to form metallophilic interations are widely pursued to develop multifunctional luminescent materials. Heteronuclear coinage metal complexes are promising candidates to tune electronic and optical properties which are not readily accessed by their homometallic congeners. In this review, we present the concept of orthogonal ligands which are rationally designed to access heteronuclear coinage metal complexes and studied in terms of their photophysical properties. Bifunctional ligands containing soft and hard donor atoms have the potential of providing different coordination modes to selectively synthesise heterobimetallic complexes in a predictable manner. This review deals with ligand sets composed of pyridine, bipyridine- or iminopyridine-substituted NHCs featuring C-N coordination modes, phosphine-based N-heterocycles and amidinate ligand scaffolds comprising of P-N functionalities and mixed phosphine-phosphine oxide with P-O donor sites. Therefore, the scope of this perspective is the discussion of heteronuclear coinage metal complexes supported by recently developed bifunctional ligands in terms of their synthesis, coordination geometries and tunability of optical properties when compared to their homometallic analogues.

Self-Assembly of Double-Helical Metallopolymers

Metal-containing polymers, or metallopolymers, have diverse applications in the fields of sensors, catalysis, information storage, optoelectronics, and neuromorphic computing, among other areas. The approach of metal-templated subcomponent self-assembly using dynamic covalent linkages allows complex architectures to be formed with relative synthetic ease. The dynamic nature of the linkages between subunits in these systems facilitates error checking during the assembly process and also provides a route to disassemble the structure, rendering these materials recyclable. This Account summarizes a class of double-helical metallopolymers. These metallopolymers are formed via subcomponent self-assembly and consist of two conjugated helical strands wrapping a linear array of Cu^I centers. Starting from discrete model helicates, we discuss how, through the judicious design of subcomponents, long helical metallopolymers can be obtained and detail their subsequent assembly into nanometer-scale aggregates. Two approaches to generate these helical metallopolymers are compared. We describe methods to govern (i) the length of the metallopolymers, (ii) the relative orientations (head-to-head vs head-to-tail) of the two organic strands, and (iii) the screw-sense of the double helix. Achieving structural control allowed the growth behavior of these systems to be probed. The structure influenced properties in ways that are relevant to specific applications; for example, the length of the metallopolymer determines the color of the light it emits in solution. In the solid state, the ionic nature of these helices renders them useful as both emitters and ionic additives in light-emitting electrochemical cells. Moreover, recent experimental work has clarified the role of the linear array of Cu ions in the transport of charge through these materials. The conductivity displayed by a film of metallopolymer depends upon its history of applied voltage and current, behavior characteristic of a memristor. In addition to the prospective applications already identified, others may be on the horizon, potentially combing stimuli-responsive electronic behavior with the chirality of the helical twist.

Non-conventional hydrogen bonding and dispersion forces that support embedding mesitylgold into a tailored bis(amidine) framework

A bis(amidine) ligand operates as a molecular lock for two AuMes fragments. The resulting complex retains a flexible double macrocycle with two non-conventional N-H⋯C_ipso hydrogen bonds and distinct intramolecular dispersion forces. Instead of unfolding of the double-ring structure through bond rupture in solution, a conformational ring inversion is observed.

Hydrogen bonds and dispersion forces serving as molecular locks for tailored Group 11 bis(amidine) complexes

A flexible polydentate bis(amidine) ligand LH2 operates as a molecular lock for various coinage metal fragments and forms the dinuclear complexes [LH2(MCl)2], M = Cu (1), Au (2), the coordination...

Tetra- and hexanuclear string complexes of the coinage metals

Reaction of the PNNP ligand system N,N'-bis[(2-diphenylphosphino)phenyl]formamidinate (dpfam) featuring different coordination compartments with [AuCl(tht)], [CuMes]₅, [AgMes]₄, or [AuC₆F₅(tht)] (tht = tetrahydrothiophene) resulted in tetranuclear homo- and heterometallic coinage metal complexes, as well as a hexanuclear gold complex. All of them feature a metal string conformation. Photophysical investigation revealed a significant dependence of the photoluminescence properties on the metal composition. Below 100 K, the PL efficiency of three compounds approaches nearly 100%.

Synthesis and Magnetic Properties of a Dimerized Trinuclear Ni String Complex, [Ni6Cl2(dpa)8](I5)2·0.25I2 (dpa- = 2,2'-Dipyridylamide Anion)

Metal string complexes, linearly aligned transition metal arrays coordinated with the multidentate organic ligands, have gained much attention both in unique electronic/structural properties and in potential applications as conductive molecular nanowires. Here we report on a dimerized Ni^II trinuclear complex, [Ni₆Cl₂(dpa)₈](I₅)₂·0.25I₂ (dpa^- = 2,2'-dipyridylamide anion). X-ray structural analysis revealed that two trinuclear moieties are bridged by a Cl anion to form a dimerized string structure. This is the first example of two Ni string complexes that are connected. In the electronic absorption and Raman spectra, characteristic absorption bands and a vibration mode based on the dimer string structure were observed. In the solid state, dimer complexes align in one dimension in an MMMXMMMX (M = metal, X = halogen) manner, leading to the intra- and interdimer antiferromagnetic interactions.

Bright Luminescence in Three Phases-A Combined Synthetic, Spectroscopic and Theoretical Approach

Combining phase-dependent photoluminescence (PL) measurements and quantum chemical calculations is a powerful approach to help understand the influence of the molecular surroundings on the PL properties. Herein, a phosphine functionalized amidinate was used to synthesize a recently presented bimetallic gold complex, featuring an unusual charge separation. The latter was subsequently used as metalloligand to yield heterotetrametallic complexes with an Au-M-M-Au "molecular wire" arrangement (M=Cu, Ag, Au) featuring metallophilic interactions. All compounds show bright phosphorescence in the solid state, also at ambient temperature. The effect of the molecular environment on the PL was studied in detail for these tetrametallic complexes by comparative measurements in solution, in the solid state and in the gas phase and contrasted to time-dependent density functional theory computations.

Bi- and trinuclear coinage metal complexes of a PNNP ligand featuring metallophilic interactions and an unusual charge separation

A selective synthesis of bi- and trinuclear complexes featuring a tetradentate monoanionic PNNP ligand is presented. The binuclear coinage metal complexes show a typical fourfold coordination for Cu and Ag, which changes to a bifold coordination for Au. The latter is accompanied by an unusual charge separation. Optical properties are investigated using photoluminescence spectroscopy and complemented by time-dependent density-functional-theory calculations. All compounds demonstrate clearly distinguished features dependent on the metals chosen and differences in the complex scaffold.

Improving the Solubility of Hexanuclear Heterometallic Extended Metal Atom Chain Compounds in Nonpolar Solvents by Introducing Alkyl Amine Moieties

The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) interaction at the d _{z ²} orbital between two kinds of metal complex is useful for obtaining heterometallic one-dimensional (1D) chains as well as heterometallic metal string compounds (HMSCs). Platinum dinuclear complexes, [Pt₂(piam)₂(NH₂R)₄]X₂ (piam = pivalamidate, R = CH₃, C₂H₅, C₃H₇, or C₄H₉, X = anion), comprising σ* as HOMO were mixed with [Rh₂(O₂CCH₃)₄] comprising σ* as LUMO in solvents to afford single crystals of [{Rh₂(O₂CCH₃)₄}{Pt₂(piam)₂(NH₂R)₄}₂]X₄ (2-5). Single-crystal X-ray analyses revealed that 2-5 are hexanuclear complexes that are one-dimensionally aligned as Pt-Pt-Rh-Rh-Pt-Pt with metal-metal bonds, where the alkyl moieties at end Pt atoms obstruct further 1D extension. Complexes 2-5 appear as if they are cut off from an infinite chain [{Rh₂(O₂CCH₃)₄}{Pt₂(piam)₂(NH₃)₄}₂] _n (PF₆)_4n ·6nH₂O (1) aligned as -{Pt-Pt-Rh-Rh-Pt-Pt} _n -. The diffuse reflectance spectrum of 1 depicts broad shoulder bands, which are not present in the spectra of 2-5, proving that the infinite chain 1 forms a band structure. Compounds 4 and 5 with propyl or butyl moieties at amine ligands, respectively, are soluble in nonpolar solvents, such as CH₂Cl₂, without the dissociation of their hexanuclear structures. Taking advantage of their solubility, measurement of cyclic voltammetry in CH₂Cl₂ become possible, which shows the quasi-reversible oxidation and reduction waves at 4: E _ox = 0.86 V and E _red = 0.69 V and 5: E _ox = 0.87 V and E _red = 0.53 V.

Transition Metal Chain Complexes Supported by Soft Donor Assembling Ligands

The chemistry of discrete molecular chains constituted by metals in low oxidation states, displaying metal-metal proximity and stabilized by suitable metal-bridging, assembling ligands comprising at least one soft donor atom is comprehensively reviewed; complexes with a single (hard or soft) bridging atom (e.g., μ-halide, μ-sulfide, or μ-PR₂ etc.) as well as "closed" metal arrays (that fall in the realm of cluster chemistry) are excluded. The focus is on transition metal-based systems, with few excursions to cases combining transition and post-transition elements. Most relevant supporting ligands have neutral C, P, O, or S donor (mainly, N-heterocyclic carbene, phosphine, ether, thioether) or anionic donor (mainly phenyl, ylide, silyl, phosphide, thiolate) groups. A supporting-ligand-based classification of the metal chains is introduced, using as the classifying parameter the number of "bites" (i.e., ligand bridges) subtending each intermetallic separation. The ligands are further grouped according to the number of donor atoms interacting with the metal chain (called denticity in the following) and the column of the Periodic Table to which the set of donor atoms belongs (in ascending order). A complementary metal-based compilation of the complexes discussed is also provided in a concise tabular form.

Electrically Induced Mixed Valence Increases the Conductivity of Copper Helical Metallopolymers

Controlling the flow of electrical current at the nanoscale typically requires complex top-down approaches. Here, a bottom-up approach is employed to demonstrate resistive switching within molecular wires that consist of double-helical metallopolymers and are constructed by self-assembly. When the material is exposed to an electric field, it is determined that ≈25% of the copper atoms oxidize from Cu^I to Cu^II , without rupture of the polymer chain. The ability to sustain such a high level of oxidation is unprecedented in a copper-based molecule: it is made possible here by the double helix compressing in order to satisfy the new coordination geometry required by Cu^II . This mixed-valence structure exhibits a 10⁴ -fold increase in conductivity, which is projected to last on the order of years. The increase in conductivity is explained as being promoted by the creation, upon oxidation, of partly filled d z 2 orbitals aligned along the mixed-valence copper array; the long-lasting nature of the change in conductivity is due to the structural rearrangement of the double-helix, which poses an energetic barrier to re-reduction. This work establishes helical metallopolymers as a new platform for controlling currents at the nanoscale.

Molecular gold strings: aurophilicity, luminescence and structure-property correlations

This review covers the compound class of one-dimensional gold strings. These compounds feature a formally infinite repetition of gold complexes as monomers/repeating units that are held together by aurophilic interactions, i.e. direct gold-gold contacts. Their molecular structures are primarily determined in the solid state using single crystal X-ray diffraction. The chemical composition of the employed gold complexes is diverse and furthermore plays a key role in terms of structure characteristics and the resulting properties. One of the most common features of gold strings is their photoluminescence upon UV excitation. The emission energy is often dependent on the distance of adjacent gold ions and the electronic structure of the whole string. In terms of gold strings, these parameters can be fine-tuned by external stimuli such as solvent, pH value, pressure or mechanical stress. This leads to direct structure-property correlations, not only with regard to the photophysical properties, but also electric conductivity for potential application in nanoelectronics. Concerning these correlations, gold strings, consisting of self-assembled individual complexes as building blocks, are the ideal compound class to look at, as perturbations by an inhomogeneity in the ligand sphere (such as the end of a molecule) can be neglected. Therefore, the aim of this review is to shed light on the past achievements and current developments in this area.

2D chain layer versus 1D chain: rigid aromatic benzoate disassembling flexible alicyclic dicarboxylate-based lanthanide coordination polymers with enhanced photoluminescence and characteristic single-molecule magnet behavior

Used as photosensitizer and structural separator, aromatic benzoate activator was grafted on cyclopropane dicarboxylate-based lanthanide coordination polymers with efficient photoluminescence and characteristic behavior of single molecule magnet.

Ethylene-Bridged Hexadentate Bis(amidines) and Bis(amidinates) with Variable Binding Sites

Hexadentate bis(amidines) form versatile networks of hydrogen bonds both in solid state and solution, as revealed by X-ray crystallography, IR, and NMR spectroscopy. Moreover, the corresponding bis(amidinates) produce blue and green emissions in THF solution. Tethered tetradentate bis(amidines) have emerged in coordination chemistry, enantioselective catalysis, as building blocks for polyfunctional heterocycles, and in photoluminescent materials. The next generation of flexible bis(amidine)/bis(amidinate) platforms with up to six N-donor sites has now been established.

The Potential of Molybdenum Complexes Bearing Unsubstituted Heterodiatomic Group 15 Elements as Linkers in Supramolecular Chemistry

The reactions of tetrahedral molybdenum complexes bearing unsubstituted heterodiatomic Group 15 elements, [Cp₂ Mo₂ (CO)₄ (μ,η² :η² -PE)] (Cp=C₅ H₅ ; E=As (1), Sb (2)), with Cu^I halides afforded seven unprecedented neutral supramolecular assemblies. Depending on the Mo₂ PE units and the Cu^I halide, the oligomers [⟨{Cp₂ Mo₂ (CO)₄ }{μ₄ ,η² :η² :η² :η¹ -PE}⟩₄ ⟨{CuX}{Cu(μ-X)}⟩₂ ] (E=As (X=Cl (3), Br (4)); E=Sb (X=Cl (6), Br (7))) or the 1D coordination polymers [{Cp₂ Mo₂ (CO)₄ }{μ₄ ,η² :η² :η¹ :η¹ -PAs}{Cu(μ-I)}]_n (5) and [{Cp₂ Mo₂ (CO)₄ }{μ₄ ,η² :η² :η² :η¹ -PSb}₂ {Cu(μ-X)}₃ ]_n (X=I (8), Br (9)) are accessible. These solid-state aggregates are the first and only examples featuring the organometallic heterodiatomic Mo₂ PE complexes 1 and 2 as linking moieties. DFT calculations demonstrate that complexes 1 and 2 present a unique class of mixed-donor ligands coordinating to Cu^I centers via the P lone pair and the P-E σ-bond, revealing an unprecedented coordination mode.