Ditopic Bis Research Articles

Ditopic ligand effects on solution structure and redox chemistry in discrete [Cu12S6] clusters with labile Cu-S bonds.

Copper chalcogenide nanoclusters (Cu-S/Se/Te NCs) are a broad and diverse class of atomically precise nanomaterials that have historically been studied for potential applications in luminescent devices and sensors, and for their beautiful, mineral-like crystal structures. By the "cluster-surface" analogy, Cu-S/Se NCs are prime candidates for the development of nanoscale multimetallic catalysts with atomic precision. However, the majority of studies conducted to date have focused exclusively on their solid-state structures and physical properties, leaving open questions as to their solution stability, dynamics, and reactivity. Herein, we report the first detailed interrogation of solution structure, dynamics, electrochemistry, and decomposition of Cu-S NCs. Specifically, we report the detailed NMR spectroscopy, diffusion-ordered spectroscopy, MALDI mass spectrometry, electrochemical and stoichiometric redox reactivity studies, and DFT studies of a series of [Cu12S6] clusters with labile Cu-S bonds supported by monodentate phosphines and ditopic bis(diphenylphosphino)alkane ligands PPh2R (R = Et, -(CH2)5-, -(CH2)8-). We find that the ligand binding topology dictates the extent of speciation in solution, with complete stability being afforded by the longer octane chelate in dppo (1,8-bis(diphenylphosphino)octane) according to 1H and DOSY NMR and MALDI-MS studies. Furthermore, a combined electrochemical and computational investigation of [Cu12S6(dppo)4] reveals that the intact [Cu12S6] core undergoes a quasireversible one-electron oxidation at mild applied potentials ([Cu12S6]0/+: -0.50 V vs. Fc0/+). In contrast, prolonged air exposure or treatment with chemical oxidants results in cluster degradation with S atom extrusion as phosphine sulfide byproducts. This work adds critical new dimensions to the stabilization and study of atomically precise metal chalcogenide NCs with labile M-S/Se bonds, and demonstrates both progress and challenges in controlling the solution behaviour and redox chemistry of phosphine-supported copper chalcogenide nanoclusters.

Photoswitching of Co(ii)-based coordination cages containing azobenzene backbones

Incorporation of a photoswitchable azobenzene unit into a ditopic bis(pyrazolyl-pyridine) ligand allows a range of reversible light-induced transformations between different cage assemblies.

Light-Controlled Aggregation and Gelation of Viologen-Based Coordination Polymers.

Ditopic bis-(triazole/pyridine)viologens are bidentate ligands that self-assemble into coordination polymers. In such photo-responsive materials, light irradiation initiates photo-induced electron transfer to generate π-radicals that can self-associate to form π-dimers. This leads to a cascade of events: processes at the supramolecular scale associated with mechanical and structural transition at the macroscopic scale. By tuning the irradiation power and duration, we evidence the formation of aggregates and gels. Using microscopy, we show that the aggregates are dense, polydisperse, micron-sized, spindle-shaped particles which grow in time. Using microscopy and time-resolved micro-rheology, we follow the gelation kinetics which leads to a gel characterized by a correlation length of a few microns and a weak elastic modulus. The analysis of the aggregates and the gel states vouch for an arrested phase separation process, a new scenario to supramolecular systems.

Photophysical and Electrochemical Properties of a Zinc(II) [2×2] Metallogrid and its Bis(hydrazone) Ditopic Ligand

AbstractHerein we describe the synthesis and structural characterization of a [2x2] metallogrid of Zinc(II) and its ditopic bis(hydrazone) ligand, as well as the evaluation of their photo‐physical and electrochemical properties. The synthesis of the bis(hydrazone) involves an aromatic nucleophilic substitution followed by a condensation. Then, through complexation with Zinc(II) ions the [2x2] metallogrid was obtained. The bis(hydrazone) was subjected to UV light‐induced E/Z isomerization determining its kinetic constants through Nuclear Magnetic Resonance (NMR). A clear effect of the metal and the structure was observed on the redox potentials of the metallogrid and its ligand when studied by Cyclic Voltammetry (CV) and Osteryoung Square Wave Voltammetry (OSWV). All compounds were also studied through UV‐Vis and Fluorescence spectroscopy, observing, in some cases, a pronounced pH‐effect. Our results indicate that the photophysical properties of metallogrid can be tuned through the elimination of the acidic protons from the bis(hydrazone) ligands and, together with their electrochemical properties can be used as molecular switches.

A triangular palladium(II) supramolecular coordination complex based on 1,4-bis(1H-imidazol-1-yl)benzene and (2,2'-bipyridyl)palladium(II) nitrate: synthesis and crystal structure.

The self-assembly of ditopic bis(1H-imidazol-1-yl)benzene ligands (LH) and the complex (2,2'-bipyridyl-κ2N,N')bis(nitrato-κO)palladium(II) affords the supramolecular coordination complex tris[μ-bis(1H-imidazol-1-yl)benzene-κ2N3:N3']-triangulo-tris[(2,2'-bipyridyl-κ2N,N')palladium(II)] hexakis(hexafluoridophosphate) acetonitrile heptasolvate, [Pd3(C10H8N2)3(C12H10N4)3](PF6)6·7CH3CN, 2. The structure of 2 was characterized in acetonitrile-d3 by 1H/13C NMR spectroscopy and a DOSY experiment. The trimeric nature of supramolecular coordination complex 2 in solution was ascertained by cold spray ionization mass spectrometry (CSI-MS) and confirmed in the solid state by X-ray structure analysis. The asymmetric unit of 2 comprises the trimetallic Pd complex, six PF6- counter-ions and seven acetonitrile solvent molecules. Moreover, there is one cavity within the unit cell which could contain diethyl ether solvent molecules, as suggested by the crystallization process. The packing is stabilized by weak inter- and intramolecular C-H...N and C-H...F interactions. Interestingly, the crystal structure displays two distinct conformations for the LH ligand (i.e. syn and anti), with an all-syn-[Pd] coordination mode. This result is in contrast to the solution behaviour, where multiple structures with syn/anti-LH and syn/anti-[Pd] are a priori possible and expected to be in rapid equilibrium.

Self-assembled 1D infinite inorganic [2]catenane and 2D sheet framework with calix[8]phenylazoimidazole and [4+4]metallomacrocyclic motifs based on silver and ditopic bis(imidazolyl)methane ligands

Three novel supramolecular complexes, [Ag4(2-mBIM)4](ClO4)4(H2O) (1), [Ag2(2-mBIM)2](PF6)2 (2) and [Ag2(PA-BIM)2](ClO4)2(CH2Cl2) (3) (2-mBIM = bis(2-methyl- imidazol-1-yl)methane; PA-BIM = 1,1-bis[(2-phenylazo)imidazol-1-yl]methane), have been prepared and structurally characterized. The reported complexes bear [4+4]metallomacrocyclic motifs comprising four silver atoms and four ditopic bis(imidazolyl)methane ligands. Complex 1 exhibits a rare 1D infinite inorganic [2]catenane structure, which was self-assembled by the interlocking action of [4+4]metallomacrocyclic units. Complex 2 is a 2D layered supramolecular motif containing [4+4]macrometallacycle units with π-π interaction between imidazole rings. Complex 3 has a 2D sheet supramolecular framework through Ag–Ag interactions in [4+4]macrometallacyclic calix [8]phenylazoimidazole with a nanocavity. The results suggest that the bisimidazolium ligands and anions play crucial roles in the formation of the different host structures. The thermal stability and photoluminescence spectra of the synthesized complexes have also been discussed.

Dinuclear Metallacycles with Single M–X–M Bridges (X = Cl–, Br–; M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II)): Strong Antiferromagnetic Superexchange Interactions

A series of monochloride-bridged, dinuclear metallacycles of the general formula [M2(μ-Cl)(μ-L)2](ClO4)3 have been prepared using the third-generation, ditopic bis(pyrazolyl)methane ligands L = m-bis[bis(1-pyrazolyl)methyl]benzene (Lm), M = Cu(II), Zn(II), and L = m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (Lm*), M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II). These complexes were synthesized from the direct reactions of M(ClO4)2·6H2O, MCl2, and the ligand, Lm or Lm*, in the appropriate stoichiometric amounts. Three analogous complexes of the formula [M2(μ-Cl)(μ-L)2](BF4)3, L = Lm, M = Cu(II), and L = Lm*, M = Co(II), Cu(II), were prepared from the reaction of [M2(μ-F)(μ-L)2](BF4)3 and (CH3)3SiCl. The bromide-bridged complex [Cu2(μ-Br)(μ-Lm*)2](ClO4)3 was prepared by the first method. Three acyclic complexes, [Co2(μ-Lm)μ-Cl4], [Co2(μ-Lm*)Cl4], and [Co2(μ-Lm*)Br4], were also prepared. The structures of all [M2(μ-X)(μ-L)2]3+ (X = Cl-, Br-) complexes have two ditopic bis(pyrazolyl)methane ligands bridging two metals in a metallacyclic arrangement. The fifth coordination site of the distorted trigonal bipyramidal metal centers is filled by a bridging halide ligand that has an unusual linear or nearly linear M-X-M angle. The NMR spectra of [Zn2(μ-Cl)(μ-Lm*)2](ClO4)3 and especially [Cd2(μ-Cl)(μ-Lm*)2](ClO4)3 demonstrate that the metallacycle structure is maintained in solution. Solid state magnetic susceptibility data for the copper(II) compounds show very strong antiferromagnetic exchange interactions, with -J values of 536 cm-1 for [Cu2(μ-Cl)(μ-Lm)2](ClO4)3·xCH3CN, 720 cm-1 for [Cu2(μ-Cl)(μ-Lm*)2](ClO4)3, and 945 cm-1 for [Cu2(μ-Br)(μ-Lm*)2](ClO4)3·2CH3CN. Smaller but still substantial antiferromagnetic interactions are observed with other first row transition metals, with -J values of 98 cm-1 for [Ni2(μ-Cl)(μ-Lm*)2](ClO4)3, 55 cm-1 for [Co2(μ-Cl)(μ-Lm*)2](ClO4)3, and 34 cm-1 for [Fe2(μ-Cl)(μ-Lm*)2](ClO4)3. EPR spectra of [Cu2(μ-Cl)(μ-Lm*)2](BF4)3 confirm the dz2 ground state of copper(II). In addition, the sign of the zero-field splitting parameter D was determined to be positive for [Cu2(μ-F)(μ-Lm*)2](BF4)3. Electronic spectra of the copper(II) complexes as well as Mössbauer spectra of the iron(II) complexes were also studied in relation with the EPR spectra and magnetic properties, respectively. Density functional theory calculations were performed using ORCA, and exchange integral values were obtained that parallel but are slightly higher than the experimental values by about 30%.

Correction: 2D → 2D parallel interpenetration of (4,4) sheets constructed from a ditopic bis(4,2′:6′,4′′-terpyridine)

Correction for ‘2D → 2D parallel interpenetration of (4,4) sheets constructed from a ditopic bis(4,2′:6′,4′′-terpyridine)’ by Edwin C. Constable et al., CrystEngComm, 2014, 16, 3494–3497.

A Dinuclear Gold(I) Bis(Carbene) Complex Based on a Ditopic Cyclic (Aryl)(Amino)Carbene Framework

A dinuclear gold(I) bis(carbene) complex based on a 1,5‐dihydropyrazolo[3,4‐f]indazoline‐3,7‐diylidene ligand framework is reported. The bridging bis(carbene) ligand is the first example of a ditopic cyclic (aryl)(amino)carbene (CArAC). The ligand precursor 2,6‐diethyl‐1,5‐diphenyl‐1,5‐dihydropyrazolo[3,4‐f]indazole‐2,6‐diium (2H2) is readily available from the literature‐known starting material 1,5‐diphenyl‐1,5‐dihydropyrazolo[3,4‐f]indazole (1) and two equivalents of the Meerwein salt [Et3O][BF4]. The NHC gold(I) hydroxide complex [IPrAuOH] proved to be a valuable precursor for the synthesis of the dinuclear gold complex [(IPr)2Au2(μ‐2)][BF4]2 (3) [IPr = 1,3‐bis(2,6‐diisopropylphenyl)imidazole‐2‐ylidene]. The compounds under study were fully characterized by various methods, also including X‐ray crystallographic studies on single crystals of 2H2 and 3. The ligand was developed in order to explore a possibly enhanced mediation of electronic metal‐to‐metal communication via the π‐system. Density functional theory calculations revealed, however, that the electronic communication in the parent system, or in other ring‐substituted derivatives, is only small and predominantly mediated by the σ‐bond framework. The calculations also predict that the introduction of an additional donor functionality to the central arene ring of the ditopic bis(carbene), which is able to act as bridging entity and forms an additional metal‐ligand bond, increases the communication between the two central metal atoms. The amount of electronic communication in the bis(carbene) complexes under study appears to be a direct function of the number of metal‐ligand contacts.

Crystal Engineering of Polymeric Structures with Dative Boron–Nitrogen Bonds: Design Criteria and Limitations

A database analysis of known adducts between pyridyl donor ligands and Lewis acidic boronate esters revealed a preference for a particular conformation at the boron centers. On the basis of this analysis, we designed two tritopic pyridyl ligands, which were expected to form polymeric structures upon combination with ditopic bis(benzenedioxaborole) Lewis acids. Instead of two-dimensional networks, we observed the formation of chains of macrocycles for different ester/N-donor combinations. This kind of structural motif is unprecedented for polymeric B–N adducts. The predictability of the chemistry was compromised by the low strength of the dative B–N bond, which resulted in incomplete adduct formation and/or cocrystallization of starting materials.

One- and Two-Step Self-Assembly Processes in Zn(II) Supramolecular Frameworks with Ditopic Bis(pyrazolyl)methane Ligands. Chiral Recognition and Formation of Cyclic Helicates

The new ligands bis(pyrazol-1-yl)(pyridine-3-yl)methane (bpzm3py, 1) and bis(3,5-dimethylpyrazol-1-yl)(pyridine-3-yl)methane (bpz*m3py, 2) have been synthesized. Box-like cyclic dimers have been obtained by reaction of the ligands with Zn(NO3)2, namely, [Zn(NO3)2L]2 (L = 1, complex 3; L = 2, complex 4), after the self-assembly of two metal centers and two ligands in a head-to-tail fashion. In 3 and 4 the pyridine rings exhibit planar chirality and the two pR,pR and pS,pS enantiomers coexist in the crystal. A two-step self-assembly process using 3 as an SBU and sodium 1,4-benzenedicarboxylate (Na2BDC) as a linker leads to the formation of hexanuclear cyclic helicates with a distorted trigonal prism form [Zn2(BDC)(1)2(H2O)4]3(BDC)3 (5) constituted by cyclic dimers and μ2,κ2-BDC connectors that are oriented diagonally. The chiral recognition that is observed means that the three dimers of the helicate have the same chirality. In the 3-D structure, chains of helicates of the same chirality are formed and these interdigitate with chains of opposite chirality connected by hydrogen bonds and through a column of π–π stacking interactions involving alternate pyridine and benzene rings.

2D → 2D Parallel interpenetration of (4,4) sheets constructed from a ditopic bis(4,2′:6′,4′′-terpyridine)

1,4-Bis(n-octyloxy)-2,5-bis(4,2′:6′,4′′-terpyridin-4′-yl)benzene reacts with ZnCl2 to give corrugated 2D (4,4) nets which interpenetrate in a 2D → 2D parallel manner.

Reaction of a Bis(benzoylhydrazone) with Copper(II): Complex Formation, Hydroxylation, and DNA Cleavage Activity

AbstractThe reaction of the ditopic bis(benzoylhydrazone) of isophthalaldehyde (H2L) with copper(II) acetate in methanol under aerobic conditions afforded a green solid that contains the tetranuclear complex [Cu4L*2(OCH3)2] (1). The ligand L*, a bis(benzoylhydrazone) of 2,6‐diformylphenol, resulted from hydroxylation of the corresponding isophthalaldehyde derivative. H2L and 1 were characterized by different spectroscopic techniques, and their molecular structures were confirmed by single‐crystal X‐ray diffraction. The reaction of H2L with copper(II) under argon, on the other hand, resulted in a brown diamagnetic solid [Cu2L]·H2O (2) containing copper(I), which was readily transformed into 1 when exposed to air. In contrast, the reaction with [Cu(CH3CN)4][BF4] afforded a mononuclear complex [Cu(H2L)][BF4]·H2O (3), and single crystals of [Cu(H2L)2][BF4] (4) were isolated from its dichloromethane solution. H2L cleaved plasmid DNA in the presence of copper(II) independently of any reducing agent, but more efficiently when ascorbate was present. The cleavage process is likely oxidative because the enzyme catalase, which decomposes hydrogen peroxide, quenched the reaction. In the absence of ascorbate, the ligand H2L presumably acts as the reductant in the redox reaction, as exploited also in the synthesis of 1. As a consequence, a variety of reactive oxygen species were generated, eventually also leading to the cleavage of DNA (“self‐activating” chemical nuclease).

Ditopic bis(oxazolines): Synthesis and structural studies of zinc(II), copper(II) and nickel(II) complexes

The synthesis, crystal structures, magnetic and spectroscopic properties of zinc(II), nickel(II) and copper(II) dinuclear complexes 2–4 of a novel dinucleating polyoxazoline ligand 1 are reported. X-ray analysis revealed that the three complexes are centrosymmetric dinuclear species with an overall S shape, the bisoxazoline moieties pointing toward the aromatic core of the molecule. Magnetic susceptibility measurements suggest that there is a very weak exchange interaction between the copper or nickel ions in complexes 3 and 4.

Cobalt, nickel, copper and cadmium coordination polymers containing the bis(1,2,4-triazolyl)methane ligand

Co(II), Ni(II), Cu(II) and Cd(II) coordination polymers containing the flexible ditopic bis(1,2,4-triazol-1-yl)methane ligand (Btm) have been prepared by reaction of equimolar quantities of the corresponding cobalt, nickel, copper and cadmium salts in EtOH solution. Structure solution and refinement of polycrystalline materials were performed by powder diffraction technique (XRPD), using conventional laboratory data. The results show that architecturally different coordination polymers were obtained depending on the counter-ion employed. The XRPD results show also that non-covalent interactions are driving forces for the occurrence of different structures.

When is a metallopolymer not a metallopolymer? When it is a metallomacrocycle

We report reactions of cobalt(II) acetate with a series of ditopic bis(tpy) ligands (tpy = 2,2':6',2''-terpyridine) containing flexible polyethyleneoxy spacers (tpy-4'-O{(CH(2))(2)O}(n)-4'-tpy, n = 2, 3, 4 or 6 ligands 1-4, respectively) which result in the formation of complicated mixtures of species, presumed to be both open chain and cyclic species. Well resolved paramagnetically shifted (1)H NMR spectra are a powerful tool for the analysis of these solution systems. Upon equilibration, [n + n] metallomacrocycles are isolated as the dominant (thermodynamic) species in some cases, and the single crystal X-ray structures of [Co(2)(3)(2)][PF(6)](4)·6MeCN and [Co(2)(4)(2)][PF(6)](5)·2MeCN (a mixed cobalt(ii)/cobalt(iii) species) are presented. Oxidation of the equilibrated cobalt(II) mixtures to kinetically inert cobalt(III) species provides additional evidence for the formation of metallomacrocycles as the thermodynamic products. Single crystal structural data for [Co(3)(2)(3)][PF(6)](9)·2MeCN·3.5H(2)O, [Co(2)(3)(2)][PF(6)](6)·10MeCN and [Co(2)(4)(2)][PF(6)](6)·6MeCN confirm the assembly of [2 + 2] and [3 + 3] metallomacrocycles. PGSE NMR spectroscopy has been used to determine the hydrodynamic radii of the solution species.

Conformationally-locked metallomacrocycles—prototypes for a novel type of axial chirality

A series of ditopic ligands incorporating two 2,2′:6′,2″-terpyridine (tpy) metal-binding domains linked to a central naphthalenediyl core by ethyleneoxy chains of various lengths have been prepared and their iron(II) complexes prepared. The major thermodynamic products of complexation are ferracycles of [1 + 1], [2 + 2] or [3 + 3] stoichiometry. In the case of the [1 + 1] complexes, the linker between the two tpy domains is spatially restricted and the cations exhibit chirality associated with the conformation about the central N–Fe–N bonds. The dynamic processes by which the enantiomeric forms interconvert are investigated in the presence of chiral anions by NMR and circular dichroism spectroscopy making use of the Pfeiffer effect. We have shown that ditopic bis(2,2′:6′,2″-terpyridine) ligands give rise to conformationally restricted complexes with iron(II) centres. The absolute configuration of the cations in the diastereomeric ion pairs obtained through the Pfeiffer effect has been calculated using TDDFT methods.

Self-assembly and layer-by-layer deposition of metallosupramolecular perylene bisimide polymers

2,2′:6′,2″-Terpyridine (tpy)-functionalized red and green perylene bisimide (PBI) building blocks were synthesized, and their metal-ion-directed self-assembly was studied in detail by 1H NMR, DOSY NMR, and UV-vis spectroscopy. These studies revealed that the newly synthesized ditopic bis(tpy)-PBI ligands, in which the tpy units are directly connected to PBI moieties at the imide positions, form supramolecular coordination polymers upon addition of one equivalent of Zn(II) ions. Atomic force microscopy (AFM) investigations confirmed the formation of extended, rigid polymers with average chain lengths of up to 30–35 repeat units. These coordination polymers create closely packed films with linear arrangement on mica and HOPG surfaces. The Zn(II) centers were mapped by applying current imaging tunnelling spectroscopy (CITS) to the red-coloured polymer deposited on graphite surfaces. An enhanced local density of unoccupied states close to the Fermi energy was observed in positions with metal ions. Alternate multilayer films of two different coordination polymers were prepared by the layer-by-layer deposition technique and surface densities Γ of perylene bisimide chromophores in these films were determined.

Rh I-complexes of ditopic bis(pyrazol-1-yl)borate ligands: Assessment of their catalytic activity towards phenylacetylene polymerization

Two dinuclear Rh I-cyclooctadiene complexes [1,4-(cod)Rh(B(R’)pz 2)-C 6H 4-(B(R’)pz 2)Rh(cod)], linked by a ditopic scorpionate ligand, have been prepared and fully characterized (R′ = Ph ( 2), C 6F 5 ( 2 F ); pz = pyrazolide). Both compounds were tested as catalysts for phenylacetylene polymerization but showed no catalytic activity. Attempts at the synthesis of corresponding complexes of the sterically more demanding ligands [ 1 , 4 - ( B ( R ′ ) pz 2 Ph ) - C 6 H 4 - ( B ( R ′ ) pz 2 Ph ) ] 2 - (R′ = Ph ( 4), C 6F 5 ( 4 F ); pz Ph = 3-phenylpyrazolide) resulted in B–N bond cleavage and formation of the dinuclear complex [(cod)Rh(μ-pz Ph) 2Rh(cod)] ( 5). Complex 5 proved to be an efficient catalyst for the preparation of highly stereoregular head-to-tail cis- transoidal poly(phenylacetylene).

A Manganese(II) Coordination Polymer with Ditopic Bis(pyrazol‐1‐yl)borate Bridges

AbstractThe synthesis and characterization of the ditopic bis(pyrazol‐1‐yl)borate ligand Li2[p‐C6H4(B(C6F5)pz2)2] is reported (pz = pyrazol‐1‐yl). Compared to the corresponding t‐butyl derivative Li2[p‐C6H4(B(t‐Bu)pz2)2], the C6F5‐substituted scorpionate is significantly more stable towards hydrolysis. Reaction of Li2[p‐C6H4(B(C6F5)pz2)2] with two equivalents of MnCl2 leads to the formation of coordination polymers {(MnCl2)2(Li(THF)3)2[p‐C6H4(B(C6F5)pz2)2]}∞ featuring penta‐coordinate MnII ions chelated by one bis(pyrazol‐1‐yl)borate fragment and further bonded to three chloride ions. Two of the three chloride ions are also coordinated to a neighbouring MnII ion; the third chloro ligand is shared between the MnII centre and a Li(THF)3 moiety.