Significant Control on Zero-Field Quantum Tunneling of Magnetization in Dysprosium Based Single-Molecule Magnets via Orientation of the Anilato Ligand.

Tuning the bridging fashion of anilato ligand in dinuclear DyIII complexes, reveals a sizable effect on the slow relaxation of magnetization. Combined experimental and theoretical studies divulge that the geometry with high order axial symmetry (pseudo square antiprism) reduces the transverse crystal fields corresponding to QTM (quantum tunneling of magnetization) resulting in a significant increase in energy barrier (Ueff =518 cm-1 ) through the Orbach relaxation process whereas the geometry with lower symmetry (triangular dodecahedron, pseudo D2d ) enhances the transverse crystal fields that accelerate the ground state QTM process. Notably, the value 518 cm-1 represents the highest energy barrier among anilato ligand based SMMs.

Pyrimidoquinazolinophenanthroline Opens Next Chapter in Design of Bridging Ligands for Artificial Photosynthesis.

The synthesis and detailed characterization of a new Ru polypyridine complex containing a heteroditopic bridging ligand with previously unexplored metal-metal distances is presented. Due to the twisted geometry of the novel ligand, the resultant division of the ligand in two distinct subunits leads to steady state as well as excited state properties of the corresponding mononuclear Ru(II) polypyridine complex resembling those of prototype [Ru(bpy)3 ]2+ (bpy=2,2'-bipyridine). The localization of the initially optically excited and the nature of the long-lived excited states on the Ru-facing ligand spheres is evaluated by resonance Raman and fs-TA spectroscopy, respectively, and supported by DFT and TDDFT calculations. Coordination of a second metal (Zn or Rh) to the available bis-pyrimidyl-like coordination sphere strongly influences the frontier orbitals, apparent by, for example, luminescence quenching. Thus, the new bridging ligand motif offers electronic properties, which can be adjusted by the nature of the second metal center. Using the heterodinuclear Ru-Rh complex, visible light-driven reduction of NAD+ to NADH was achieved, highlighting the potential of this system for photocatalytic applications.

Structural Diversity of Nickel and Manganese Chloride Complexes with Pyridin-2-One.

Reactions of NiCl2·6H2O and pyridin-2-one (C5H5NO = Hhp) afforded novel molecular complexes, i.e., mononuclear [NiCl2(Hhp)4] (1), dinuclear [NiCl2(Hhp)(H2O)2]2.2Hhp (3) and [Ni2Cl4(Hhp)5]·2MeCN (4), and an ionic complex [Ni(Hhp)6]Cl2 (2). Single-crystal X-ray analyses revealed two modes of Hhp ligation in these complexes: a monodentate coordination of carbonyl oxygen in all of them and an additional µ2-oxygen bridging coordination in the dinuclear complex 4. Three bridging molecules of Hhp span two nickel(II) ions in 4 with a 2.9802 (5) Å separation of the metal ions. Complex 3 is a chlorido-bridged nickel dimer with a planar Ni2(µ-Cl)2 framework. Hydrogen bonds and parallel stacking arrangements of the Hhp molecules govern the connectivity patterns in the crystals, resulting in 1D structures in 1 and 5 or 2D in 3. A single manganese compound [MnCl2(Hhp)4] (5), isostructural to 1, was isolated under the similar conditions. This is in contrast to four nickel(II) chloride complexes with Hhp. Thermal analyses proved the stability of complexes 1 and 3 in argon up to 145 °C and 100 °C, respectively. The decomposition of 1 and 3 yielded nickel in argon and nickel(II) oxide in air at 800 °C.

Controlled Interconversion of a Dinuclear Au Species Supported by a Redox-Active Bridging PNP Ligand Facilitates Ligand-to-Gold Electron Transfer.

Redox non-innocent ligands have recently emerged as interesting tools to obtain new reactivity with a wide variety of metals. However, gold has almost been neglected in this respect. Here, we report mechanistic investigations related to a rare example of ligand-based redox chemistry in the coordination sphere of gold. The dinuclear metal-centered mixed-valent AuI -AuIII complex 1, supported by monoanionic diarylamido-diphosphine ligand PNPPr and with three chlorido ligands overall, undergoes a complex series of reactions upon halide abstraction by silver salt or Lewis acids such as gallium trichloride. Formation of the ultimate AuI -AuI complex 2 requires the intermediacy of AuI -AuI dimers 5 and 7 as well as the unique AuIII -AuIII complex 6, both of which are interconverted in a feedback loop. Finally, unprecedented ortho-selective C-H activation of the redox-active PNP ligand results in the carbazolyldiphosphine derivative PN*PPr via ligand-to-metal two-electron transfer. This work demonstrates that the redox-chemistry of gold may be significantly expanded and modified when using a reactive ligand scaffold.

Determination of the Bridging Ligand in the Active Site of Tyrosinase.

Tyrosinase is a type-3 copper enzyme that is widely distributed in plants, fungi, insects, and mammals. Developing high potent inhibitors against tyrosinase is of great interest in diverse fields including tobacco curing, food processing, bio-insecticides development, cosmetic development, and human healthcare-related research. In the crystal structure of Agaricus bisporus mushroom tyrosinase, there is an oxygen atom bridging the two copper ions in the active site. It is unclear whether the identity of this bridging oxygen is a water molecule or a hydroxide anion. In the present study, we theoretically determine the identity of this critical bridging oxygen by performing first-principles hybrid quantum mechanics/molecular mechanics/Poisson-Boltzmann-surface area (QM/MM-PBSA) calculations along with a thermodynamic cycle that aim to improve the accuracy. Our results show that the binding with water molecule is energy favored and the QM/MM-optimized structure is very close to the crystal structure, whereas the binding with hydroxide anions causes the increase of energy and significant structural changes of the active site, indicating that the identity of the bridging oxygen must be a water molecule rather than a hydroxide anion. The different binding behavior between water and hydroxide anions may explain why molecules with a carboxyl group or too many negative charges have lower inhibitory activity. In light of this, the design of high potent active inhibitors against tyrosinase should satisfy both the affinity to the copper ions and the charge neutrality of the entire molecule.

Protonated paramagnetic redox forms of di-o-quinone bridged with p-phenylene-extended TTF: A EPR spectroscopy study.

The chemical oxidation and reduction processes of deprotonated, direduced o-quinone-exTTF-o-quinone in protic solvents were studied by EPR spectroscopy. The formation of relatively stable paramagnetic protonated redox forms of the parent triad was very surprising. The character of spin-density distribution in the semiquinone-quinone and semiquinone-catechol redox forms indicates that the p-phenylene-extended tetrathiafulvalene connector provides a quite effective electronic communication channel between dioxolene coordination sites. It was found that the deprotonated, direduced o-quinone-exTTF-o-quinone is capable to reduction of the metal copper in solution. The radical anion species formed in this reaction exists in solution as a solvent-separated ion pair with a copper cation. A character of spin-density distribution in a radical anion species leads to the conclusion that the ligand corresponds to type III of the Robin-Day classification.

Crystal structure of di-µ-hydroxido-bis{[N,N'-bis-(2,6-di-methyl-phen-yl)pentane-2,4-diiminato(1-)]zinc}.

The title compound, [Zn2(C21H25N2)2(OH)2], is a binuclear zinc complex formed by two bidentate ß-diketiminate (nacnac) ligands and two µ-hydroxide O atoms, bridging two mononuclear units into a centrosymmetric dimeric unit. Each Zn(2+) cation is coordinated by two N-donor atoms from the nacnac ligand and two O-donor atoms of hydroxide anions to give a distorted tetra-hedral coordination environment. The Zn-O bond lengths are 1.9643 (13) and 2.0022 (14) Å, and the two Zn-N bond lengths are 1.9696 (14) and 1.9823 (14) Å. The distance between the two Zn(2+) cations in the dimer is 2.9420 (4) Å. Although hydroxide groups are present in the complex, no classical hydrogen-bonding inter-ations are observed because of the bulky ß-diketiminate ligands.

Crystal structure of tetra-kis-(µ-n-butyrato-κ(2) O:O')bis-[chlorido-rhenium(III)](Re-Re).

With an inversion center at the mid-point of the two Re(III) atoms, the title compound, [Re2Cl2{O2C(CH2)2CH3}4], exhibits a paddle-wheel or lantern-type structure with four n-butyrate groups bridging two Re(III) atoms in a syn-syn fashion. The axial chloride ligands together with the Re-Re quadruple bond [2.2330 (3) Å] complete an essentially octa-hedral geometry around each Re(III) atom. There is little distortion, with an Re-Re-Cl bond angle of 176.18 (3)° and typical cis-O-Re-O bond angles ranging from 89.39 (11) to 90.68 (11)°. There are two mol-ecules in the unit cell, and no significant inter-molecular inter-actions were noticed between mol-ecules in the crystal.

Introducing non-bridging ligand in metal-organic framework-based electrocatalyst enabling reinforced oxygen evolution in seawater.

Using seawater as the replacement of freshwater for electrolysis, with the integration of renewable energy, is deemed as an attractive manner to harvest green hydrogen. However, the complexity of seawater puts forward stricter requirement to the electrocatalyst to alleviate the chlorine electrochemistry and corrosion. Herein, a nanosheet array of NiFe-MOF@Ni2P/Ni(OH)2 is devised by partially substituting terephthalic acid (H2BDC) ligand by ferrocenecarboxylic acid (FcCA). Tailoring the active site into an under-coordinated fashion affords NiFe-MOF@Ni2P/Ni(OH)2 excellent performance towards oxygen evolution reaction (OER), only requiring the overpotentials of 302 mV and 394 mV in alkaline seawater to drive the current densities of 100 and 1000 mA cm-2, respectively. Moreover, the as-obtained electrocatalyst showed robust durability for operating more than 120 h at 500 mA cm-2 under harsh condition (6 M KOH + 1.5 M NaCl, 60 ℃). Density functional theory (DFT) calculations confirmed that tuning the coordination environment of Ni in NiFe-MOF by incorporating the non-bridging FcCA ligands could boost the formation of more active catalytic sites, which can simultaneously enhance the electronic conductivity and accelerate OER kinetics. This work provides beneficial enlightenment of combining MOF-based electrocatalyst with direct electrolysis of seawater.

Protonation of the oxo-bridged heme/copper assemblies: Modeling the oxidized state of the cytochrome c oxidase active site.

In this study on model compounds for the resting oxidized state of the iron­copper binuclear center in cytochrome c oxidase (CcO), we describe the synthesis of a new µ-oxo-heme/Cu complex, [(TPP)FeIII-O-CuII(tmpa)][B(C6F5)4] (2) {TPP: tetraphenyl porphyrinate(2-); TMPA: tris(2-pyridylmethylamine)}, as well as two protonation events for three µ-oxo-heme/Cu complexes with varying peripheral substituents on the heme site. The addition of increasing amounts of strong acid to these µ-oxo-heme/Cu systems successively led to the generation of the corresponding µ-hydroxo, µ-aquo, and the dissociated complexes. The heme/Cu assemblies bridged through a water ligand are reported here for the first time and the 1H NMR and 19F NMR spectral properties are consistent with antiferromagnetically coupled high-spin iron(III) and copper(II) centers. By titration using a series of protonated amines, the pKa values for the corresponding µ-hydroxo-heme/Cu species (i.e., the first protonation event) have been reported and compared with the pKa ranges previously estimated for related systems. These synthetic systems may represent structural models for the oxidized FeIII-X-CuII resting state, or turnover intermediates and can be employed to clarify the nature of proton/electron transfer events in CcO. SYNOPSIS: The resting oxidized state of the cytochrome c oxidase active site contains an Fea3-OHx-CuB moiety. Here, we investigated two successive protonation events, for a series of µ-oxo-heme/Cu assemblies and reported the pKa values for the first protonation event. The µ-aquo-heme/Cu complexes described here are the first examples of such systems.

An Effective Approach to Obtain Near-Infrared Emission from Binuclear Platinum(II) Complexes Involving Thiophenpyridine-Isoquinoline Bridging Ligand in Solution-Processed OLEDs.

Bimetallic complexes have become an emerging hot topic in field of luminous applications in recent years. Unlike the traditional modification on a cyclometalated ligand, grafting an additional metal ion provides a novel approach to tune molecular conjugation as well as the spin orbital coupling (SOC). Herein, we demonstrate a new kind of binuclear platinum(II) complex Pt-3 that possesses an asymmetric thiophenpyridine-isoquinoline bridging ligand. Compared to its mononuclear analogues of Pt-1 and Pt-2, an extremely large redshift emission from 576 and 618 nm to 721 nm was observed in solution. Binding of two metal ions helps to enhance molecular planarity, extend conjugation and suppress excited state distortion. However, their quantum yields tend to remarkably decrease with increasing red-shift emission as following the "energy gap law". The relatively larger HOMO/LUMO separation that induced by the second platinum ion also decreases the oscillator strength at the lowest singlet state, and goes against the fast radiative decay process. Solution-processed organic light-emitting diodes (OLEDs) based on Pt-1, Pt-2 and Pt-3 achieved external quantum efficiencies (EQEs) and luminance/radiant emittance of 13.6% and 13640 cd/m2 , 3.5% and 3754 cd/m2 , 0.9% and 7981 mW/Sr/m2 with the corresponding electroluminescent (EL) emission peaked at 580 nm, 625 nm and 708 nm, respectively. This work emphasizes the complement argument of the commonly largely reported symmetric binuclear configurations, and provides a new view to photophysical mechanism and design strategies for bimetallic species.

Crystal structure of 7,7'-[(pyridin-2-yl)methyl-ene]bis-(5-chloro-quinolin-8-ol).

In the title compound, C24H15Cl2N3O2, one quinoline ring system is essentially planar and the other is slightly bent. An intra-molecular O-H⋯N hydrogen bond involving the hy-droxy group and a pyridine N atom forms an S(5) ring motif. In the crystal, two mol-ecules are associated into an inversion dimer with two R 2 2(7) ring motifs through inter-molecular O-H⋯N and O-H⋯O hydrogen bonds. The dimers are further linked by an inter-molecular C-H⋯O hydrogen bond and four C-H⋯π inter-actions, forming a two-dimensional network parallel to (001).

A novel three-dimensional copper(I) cyanide coordination polymer constructed from various bridging ligands: synthesis, crystal structure and characterization.

The cyanide ligand can act as a strong σ-donor and an effective π-electron acceptor that exhibits versatile bridging abilities, such as terminal, µ2-C:N, µ3-C:C:N and µ4-C:C:N:N modes. These ligands play a key role in the formation of various copper(I) cyanide systems, including one-dimensional (1D) chains, two-dimensional (2D) layers and three-dimensional (3D) frameworks. According to the literature, numerous coordination polymers based on terminal, µ2-C:N and µ3-C,C,N bridging modes have been documented so far. However, systems based on the µ4-C:C:N:N bridging mode are relatively rare. In this work, a novel cyanide-bridged 3D CuI coordination framework, namely poly[(µ2-2,2'-biimidazole-κ2N3:N3')(µ4-cyanido-κ4C:C:N:N)(µ2-cyanido-κ2C:N)dicopper(I)], [Cu2(CN)2(C6H6N4)]n, (I), was synthesized hydrothermally by reaction of environmentally friendly K3[Fe(CN)6], CuCl2·2H2O and 2,2'-biimidazole (H2biim). It should be noted that cyanide ligands may act as reducing agents to reduce CuII to CuI under hydrothermal conditions. Compound (I) contains diverse types of bridging ligands, such as µ4-C:C:N:N-cyanide, µ2-C:N-cyanide and µ2-biimidazole. Interestingly, the [Cu2] dimers are bridged by rare µ4-C:C:N:N-mode cyanide ligands giving rise to the first example of a 1D dimeric {[Cu2(µ4-C:C:N:N)]n+}n infinite chain. Furthermore, adjacent dimer-based chains are linked by µ2-C:N bridging cyanide ligands, generating a neutral 2D wave-like (4,4) layer structure. Finally, the 2D layers are joined together via bidentate bridging H2biim to create a 3D cuprous cyanide network. This arrangement leads to a systematic variation in dimensionality from 1D chain→2D sheet→3D framework by different types of bridging ligands. Compound (I) was further characterized by thermal analysis, solid-state UV-Vis diffuse-reflectance and photoluminescence studies. The solid-state UV-Vis diffuse-reflectance spectra show that compound (I) is a wide-gap semiconductor with band gaps of 3.18 eV. The photoluminescence study shows a strong blue-green photoluminescence at room temperature, which may be associated with metal-to-ligand charge transfer.

First crystal structures of metal complexes with a 4-nitropyrazole-3-carboxylic acid ligand and the third crystal form of the ligand.

Pyrazole (pz)-derived ligands can, besides exhibiting a strong coordination ability toward different metal ions, exhibit a great diversity in their coordination geometry and nuclearity, which can be achieved by varying the type and position of the pz substituents. The present study reports the synthesis and crystal structure of two binuclear complexes, namely bis(µ-4-nitro-1H-imidazol-1-ide-5-carboxylato)-κ3N1,O:N2;κ3N2:N1,O-bis[aqua(dimethylformamide-κO)copper(II)], [Cu2(C4HN3O4)2(C3H7NO)2(H2O)2], (II), and bis(µ-4-nitro-1H-imidazol-1-ide-5-carboxylato)-κ2N1,O:N2;κ2N2:N1,O-bis[triaquacobalt(II)] dihydrate, [Co2(C4HN3O4)2(H2O)6]·2H2O, (III). These compounds represent rare examples of metal complexes comprising 3,4-substituted pz derivatives as a bridging ligand and also the first crystal structures of transition-metal complexes with ligands derived from 4-nitropyrazole-3-carboxylic acid. Recently, the crystal structures of the same ligand in the neutral and mixed neutral/anionic forms have been reported. We present here the third form of this ligand, where it is present in a fully deprotonated anionic form within a salt, i.e. ammonium 4-nitropyrazole-3-carboxylate, NH4+·C4H2N3O4-, (I). Single-crystal X-ray diffraction revealed that in the present complexes, the CuII and CoII centres adopt distorted square-pyramidal and octahedral geometries, respectively. In both cases, the N,N',O-coordinated pz ligand shows simultaneously chelating and bridging coordination modes, leading to the formation of a nearly planar six-membered M2N4 metallocycle. In all three crystal structures, the supramolecular arrangement is controlled by strong hydrogen bonds which primarily engage the carboxylate O atoms as acceptors, while the nitro group adopts the role of an acceptor only in structures with an increased number of donors, as is the case with CoII complex (III). The electrostatic potential, as a descriptor of reactivity, was also calculated in order to examine the changes in ligand electrostatic preferences upon coordination to metal ions.

One-dimensional copper(II) coordination polymers based on 1,3-bis(pyridin-4-yl)propane and diimine ligands.

Two one-dimensional (1D) coordination polymers (CPs), namely catena-poly[[[aqua(2,2'-bipyridine-κ2N,N')(nitrato-κO)copper(II)]-µ-1,3-bis(pyridin-4-yl)propane-κ2N:N'] nitrate], {[Cu(NO3)(C10H8N2)(C13H14N2)(H2O)]·NO3}n (1), and catena-poly[[[aqua(nitrato-κO)(1,10-phenanthroline-κ2N,N')copper(II)]-µ-1,3-bis(pyridin-4-yl)propane-κ2N:N'] nitrate], {[Cu(NO3)(C12H8N2)(C13H14N2)(H2O)]·NO3}n (2), have been synthesized using [Cu(NO3)(NN)(H2O)2]NO3, where NN = 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen), as a linker in a 1:1 molar ratio. The CPs were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis and single-crystal X-ray structure determination. The 1,3-bis(pyridin-4-yl)propane (dpp) ligand acts as a bridging ligand, leading to the formation of a 1D polymer. The octahedral coordination sphere around copper consists of two N atoms from bpy for 1 or phen for 2, two N atoms from dpp, one O atom from water and one O atom from a coordinated nitrate anion. Each structure contains two crystallographically independent chains in the asymmetric unit and the chains are linked via hydrogen bonds into a three-dimensional network.

Crystal structure of µ-oxalato-κ2O1:O2-bis-[(dimethyl sulfoxide-κO)tri-phenyl-tin(IV)].

In the previously reported [C2O4(SnPh3)2] complex [Diop et al. (2003 ▸). Appl. Organomet. Chem.17, 881-882.], the SnIV atoms are able to formally complete their coordination by addition of dimethyl sulfoxide (DMSO) mol-ecules provided by the reaction medium, affording the title complex, [Sn2(C6H5)6(C2O4)(C2H6OS)2]. The SnIV atoms are then penta-coordinated, with a common trans trigonal-bipyramidal arrangement. The asymmetric unit contains one half-mol-ecule, which is completed by inversion symmetry in space group type C2/c. The inversion centre is placed at the mid-point of the central bis-monodentate oxalate dianion, C2O42-, which bridges the [(SnPh3)(DMSO)] moieties. The mol-ecule crystallizes as a disordered system, with two phenyl rings disordered by rotation about their Sn-C bonds, while the DMSO mol-ecule is split over two positions due to a tetra-hedral inversion at the S atom. All disordered parts were refined with occupancies fixed of 0.5.

A pillared framework coordination polymer based on the Cd3(µ3-OH) unit: poly[[(µ4-5-aminotetrazolato-κ(4)N(1):N(2):N(3):N(4))chlorido-µ3-hydroxido-(µ3-isonicotinato-κ(3)N:O:O')dicadmium(II)] 0.14-hydrate].

The title coordination polymer, {[Cd2(CH2N5)(C6H4NO2)Cl(OH)]·0.14H2O}n, (I), was synthesized by the reaction of cadmium acetate and N-(1H-tetrazol-5-yl)isonicotinamide in aqueous ammonia, using hydrochloric acid to adjust the pH. Under hydrothermal conditions, N-(1H-tetrazol-5-yl)isonicotinamide slowly hydrolyzes to form isonicotinic acid (Hisonic) and 5-aminotetrazole (Hatz). The deprotonated form of isonicotinic acid (denoted isonic) acts as a bridging ligand in the structure. The polymer crystallizes in the monoclinic space group C2/m. In the structure, there is one Cd3(µ3-OH) unit of Cs symmetry, with one of the Cd(II) atoms and the O and H atoms located on a mirror plane. The other crystallographically independent Cd(II) cation is located on an inversion centre. Each edge of the Cd3(µ3-OH) isosceles triangle is bridged by an atz ligand in a µ1,2 or µ2,3/µ3,4 mode. The Cd3(µ3-OH) units are laced around with a belt of chloride ligands. The belts are further connected into undulating layers via weak inter-belt Cd-Cl bonds. The two organic ligands reside across mirror planes. The construction of a three-dimensional framework is completed by the pillaring isonic ligand. Water molecules partially occupy the voids of the framework.