Axial Coordination Sites Research Articles

Substitution of the axial Type 1 Cu Ligand Afford Binding of a Water Molecule in Axial Position Affecting Kinetics, Spectral, and Structural Properties of the Small Laccase Ssl1.

Multicopper oxidases use Cu ions as cofactors to oxidize various substrates. High reduction potential at Type 1 Cu is considered as crucial for effective catalysis. Previous studies have shown that replacing the axial methionine ligand of the Type 1 Cu with leucine or phenylalanine leads to an increased reduction potential, but not always to higher enzyme activity. Here we present a study of six variants of the small laccase Ssl1 from Streptomyces sviceus, where the axial methionine ligand was substituted and the effect of the axial ligand on reduction potential, activity, spectral properties and structure was investigated. Absorption, electronic circular dichroism and EPR spectra revealed the presence of a stronger coordinating axial ligand like oxygen, which influences the electronic and catalytic properties more than the nature of the amino acid side chain. The crystal structures of the Ssl1 variants were solved, which show that none of the amino acids side chains coordinates to the Cu. Instead, a water molecule is found in the axial coordination site, which support the spectroscopic data. Our findings highlight the importance of combining structural and spectroscopic methods to investigate the effect of amino acid exchange on multicopper oxidases.

Reversible Reactions of Nitric Oxide with a Binuclear Iron(III) Nitrophorin Mimic.

Construction of functional synthetic systems that can reversibly bind and transport the most biologically important gaseous molecules, oxygen and nitric oxide (NO), remains a contemporary challenge. Myoglobin and nitrophorin perform these respective tasks employing a protein-embedded heme center where one axial iron site is occupied by a histidine residue and the other is available for small molecule ligation, structural features that are extremely difficult to mimic in protein-free environments. Indeed, the hitherto reported designs rely on sophisticated multistep syntheses for limiting access to one of the two axial coordination sites in small molecules. We have shown previously that binuclear Ga(III) and Al(III) corroles have available axial sites, and now report a redox-active binuclear Fe(III) corrole, (1-Fe)2 , in which each (corrolato)Fe(III) center is 5-coordinate, with one axial site occupied by an imidazole from the other corrole. The binuclear structure is further stabilized by attractive forces between the corrole π systems. Reaction of NO with (1-Fe)2 affords mononuclear iron nitrosyls, and of functional relevance, the reaction is reversible: nitric oxide is released upon purging the nitrosyls with inert gases, thereby restoring (1-Fe)2 in solutions or films.

A Different Perspective on Tuning the Photophysical and Photochemical Properties: The Influence of Constitutional Isomers in Group 6 Carbonyl Complexes with Pyridyl-Mesoionic Carbenes.

Exploring novel and existing design principles to tune the photochemical and photophysical properties of transition-metal complexes is an important goal in contemporary research. Here, we highlight the influence of constitutional isomers of pyridyl-1,2,3-triazolylidene mesoionic carbene (MIC) ligands on the photophysical and photochemical properties of the corresponding tetracarbonyl group 6 metal complexes (M = Cr, Mo, W). All new complexes [M(C-C)] presented herein incorporate a C-C linked pyridyl-MIC ligand and were fully characterized by X-ray diffraction analysis, elemental analysis, and 1H NMR and IR spectroscopy. Detailed photophysical investigations reveal a single emission in the VIS region, which extends into the NIR with lifetimes of up to 3.5 μs in the solid state at lower temperatures. The quantum yields were determined for all three complexes, and, in particular, the W0 complex shows an unusually high quantum yield of 29% compared to the values of 0.02% obtained for the [M(C-N)] isomers investigated in earlier works. Beyond this, the investigated W0 complex also exhibits an emission at 717 nm in a fluid solution. The combination of luminescence and FTIR-step scan spectroscopy with theoretical calculations reveals an emissive 3MLCT state. Irradiation of the presented complexes leads to a clean cleavage of one axial CO ligand. A metastable 16 VE species with a vacant axial coordination site was detected in the solid state at low temperatures. In solution, the respective solvato complexes are formed. A dark reverse reaction is observed, as previously described for the [M(C-N)] analogues. The increased electron density induced by the C-C linked pyridyl-MIC ligand leads to an increased kinetic rate constant for the reformation of the starting species and is also reflected in the lower photodissociation quantum yields.

Coordinatively Unsaturated Metallates of Cobalt(II), Nickel(II), and Zinc(II) Guarded by a Rigid and Narrow Void.

Both natural enzymatic systems and synthetic porous material catalysts utilize well-defined and uniform channels to dictate reaction selectivities on the basis of size or shape. Mimicry of this design element in homogeneous systems is generally difficult owing to the flexibility inherent in most small molecular species. Herein, we report the synthesis of a tripodal ligand scaffold that orients a narrow and rigid cavity atop accessible metal coordination space. The permanent void is formed through a macrocyclization reaction whereby the 3,5-dihydroxyphenyl arms are covalently linked through methylene bridges. Deprotonative metallation leads to anionic and coordinatively unsaturated complexes of divalent cobalt, nickel, and zinc. An analogous series of trigonal monopyramidal complexes bearing a nonmacrocyclized variant of the tripodal ligand are also reported. Physical characterization of the coordination complexes has been carried out using multiple spectroscopic techniques (NMR, EPR, and UV-vis), cyclic voltammetry, and X-ray diffraction. Complexes of the macrocyclized [LOCH2O]3- ligand retain a rigid cavity upon metallation, with this cavity guarding the entrance to the open axial coordination site. Through a combination of spectroscopic and computational studies, it is shown that acetonitrile entry into the void is sterically precluded, disrupting anticipated coordination at the intracavity site.

Spin-crossover in [Fe(Quinazoline)2][Fe(CN)5NO]. Evidence of its framework flexibility

The titled material shows a thermally induced spin-crossover (SCO) in the 75–150 K temperature region, with a narrow hysteresis of about 3 K. The recorded DSC data show a similar small hysteresis. Such behavior suggests that its framework has high flexibility, making possible the reversible spin transition on the sample cooling and warming, with slight kinetic effects. The crystal structure of this material was solved and refined from powder XRD patterns, complemented with information from IR, Raman, TG, and Mössbauer data. It crystallizes with a monoclinic unit cell in the P21 space group. Its framework is formed by stacked [Fe(Quinazoline)2][Fe(CN)5NO] layers. The organic molecule occupies the axial coordination sites for the iron atom linked to the N ends of equatorial CN ligands. In the interlayer region, organic ligands from adjacent layers remain coupled through weak dipolar and dispersive forces and a π⋅⋅⋅π interaction. Such relatively weak forces between adjacent layers explain the high framework flexibility inferred from the magnetic and DSC data. The crystal structure for the low temperature (low spin) phase was obtained by periodic DFT calculations using the VASP package. The results herein discussed contribute to understanding the role of the solid framework flexibility on the spin-crossover behavior in Hofmann-like 2D coordination polymers.

A Phosphine-ß-diketiminate Nickel(I)-Complex for Small Molecule Activation.

A bis(diphenyl)-phosphine functionalized ß-diketimine ligand (PNac-H) was applied for the synthesis of a subvalent Ni(I) complex [PNac-Ni]. Here, the Ni(I) center is stabilized by a tetradentate PNNP-type pocket, forming a square planar coordination sphere. Subsequently, the Ni(I) complex was investigated with regard to its reactivity and the activation of small molecules. The reductive potential of Ni(I) enabled an activation of different substrate classes, such as CH2 X2 (X=Br, I), I2 or Ph2 E2 (E=S, Se). The ligand's design allows a stabilization of the reactive Ni(I) species while at the same time enabling activation processes due to a hemilabile coordination behavior and accessible axial coordination sites. The activation products have been characterized by single crystal X-ray diffraction, NMR and IR spectroscopy as well as elemental analysis.

Electrochemical Studies on Manganese (III) Complex with meso-5, 10, 15, 20-Tetrakis (o-nitrophenyl) porphyrin by Cyclic Voltammetry and UV-Visible Spectrophotometry

Meso-5,10,15,20-Tetrakis(o-nitrophenyl)porphyrin, [T (o-NO2)PP] and manganese (III) meso-5,10,15,20- Tetrakis(2,5-dimethoxyphenyl)porphyrin, Mn[T(o- NO2)PP] have been prepared. The Mn[T(o-NO2)PP] complex is characterized by UV-Vis spectrophotometric and cyclic voltametric (CV) studies. The preparation of Mn(III) porphyrin complex may be attributed due to a significant metal-porphyrin π interaction. In Mn(III) porphyrin, Mn is in a high spin d4 configuration with octahedral geometry. Mn(III) porphyrin geometry has been altered to square pyramidal Mn(II) porphyrin after addition of ethylamine. In square pyramidal Mn(II) porphyrin, Mn atom is in a high-spin d5 configuration with the occupied d(x 2 - y 2 ) orbital whose orbital energy is low. This shows that Mn(III) will be out of the plane of porphyrin. Successive addition of diethylamine, Mn(III) porphyrin geometry has been changed to tetragonal complex due to a weak π back bonding. It appears that π -bonding is lowered when the tetragonal complexes are formed. This might arise due to the expansion of the in-plane metal to porphyrin in a direction to occupy two axial ligands of ethylamine. The geometry changes indicate that the two ethyl amines groups occupy 5th and 6th places as axial coordination site due to the availability of two ethylamine from ethyl, diethyl and triethyl amines. Cyclic voltammogram confirmed the oxidation state and reduction behavior of Mn porphyrin complex.

Fe[4-(3-Phenylpropyl)Pyridine]2[Fe(CN)5NO]: A 2D Coordination Polymer with Thermally-Induced Spin Transition and Nature of Its Asymmetric Hysteresis Loop

The titled material crystallizes with an orthorhombic unit cell, in the P21212 space group (Nr. 18). Its crystal structure was solved and refined from powder XRD patterns. The solid framework is formed by stacked undulated sheets of inorganic nature, Fe[Fe(CN)5NO], separated by bimolecular organic pillars, [4-(3-Phenylpropyl)pyridine]2, which remain coordinated to the axial coordination sites for the iron atom. The molecules forming these pillars remain coupled through C–H⋯π and dispersive interactions between neighboring molecules. When this solid is cooled and then warmed, a reversible spin transition, from high to low spin (HS → LS), and vice versa, is observed. This transition occurs in the temperature interval of 135–165 K, with a hysteresis between them of about 30 K. That hysteresis loop appears with a pronounced asymmetry when the slopes for the HS → LS and LS → HS transitions are compared. This effect is discussed in terms of the related structural changes in the solid structure during the spin transitions. The transition was also monitored from IR and Raman spectra recorded at 80 and 300 K. Relevant information on the electronic structure for both, the LS and HS phases of this material, was derived from the corresponding XPS spectra recorded at 114 and 270 K. This contribution emphasizes the role of the nitrosyl group (NO) as an electron buffer for tuning the bonding properties of the inorganic layer at the CN 5σ orbital to make possible the observed thermally-induced SCO behavior. The thermally-induced spin transition in this solid shows an asymmetric hysteresis loop, which was ascribed to the nature of the pillar molecule.

Thermally-induced spin transition in Fe(4,4′-Azopyridine)[Fe(CN)5NO

The 4,4′-Azopyridine molecule was incorporated as a pillar between adjacent layers of Fe[Fe(CN)5NO] through a precipitation reaction. According to the solved and refined crystal structure, from powder XRD data, the pillar molecule is found occupying the axial coordination sites for the iron atom linked at the N ends of the equatorial CNs of the nitroprusside complex ion. The NO and axial CN are found unlinked at their O and N ends and oriented toward the interlayer region. The structural study, from XRD data, was complemented with spectroscopic information (IR, Raman). The hybrid solid, Fe(4,4′-Azopyridine)[Fe(CN)5NO], shows a thermally-induced spin transition in the 75–160 ​K temperature region, with a hysteresis of 8 ​K for the magnetic data recorded at 0.25 ​K/min, with the magnetometer operated in the settle mode. The spin transition was also monitored from DSC curves, and FTIR and Raman spectra at 80 and 295 ​K. The structure for the LS phase was calculated from computational simulation relaxing the refined HS structure. This method was validated through its application to materials where for both, HS and LS phases, the refined crystal structures are known. The preparation and study of this solid have not been previously reported. Unlike previous studies on spin-crossover in pillared ferrous nitroprussides, this molecule provides a pillar of enough height (∼9 ​Å) to minimize potential interactions between unbridged CN and NO ligands in the interlayer region.

A Combined Experimental and Theoretical Investigation of Arene-Supported Actinide and Ytterbium Tetraphenolate Complexes.

Modular tetraphenolate ligands tethered with a protective arene platform (para-phenyl or para-terphenyl) are used to support mononuclear An(IV) (An = Th, U) complexes with an exceptionally large and open axial coordination site at the metal. The base-free complexes and a series of neutral donor adducts were synthesized and characterized by spectroscopies and single-crystal X-ray diffraction. Anionic Th(IV) -ate complexes with an additional axial aryloxide ligand were also synthesized and characterized. The para-phenyl-tethered mononuclear complexes exhibit rare An(IV)-arene interactions, and the An(IV)-arene distance broadly increases with axial donor strength. The para-terphenyl-tethered complexes have almost no interaction with the arene base, isolating the central metal cation. Computational analysis of the mononuclear complexes and their reduced analogues, and Yb(III) congeners, as well as the effect of additional donor ligand binding, seek to elucidate the electronic structure of the metal-arene interactions and establish whether they, or their reduced or oxidized counterparts, could function as molecular qubits.

Palladium‐Aminopyridine Catalyzed C−H Oxygenation: Probing the Nature of Metal Based Oxidant

AbstractA mechanistic study of direct selective oxidation of benzylic C(sp3)−H groups with peracetic acid, catalyzed by palladium complexes with tripodal amino‐tris(pyriylmethyl) ligands, is presented. The oxidation of arylalkanes having secondary and tertiary benzylic C−H groups, predominantly yields, depending on the substrate and conditions, either the corresponding ketones or alcohols. One of the three 2‐pyriylmethyl moieties, which is pending in the starting catalyst, apparently, facilitates the active species formation and takes part in stabilization of the high‐valent Pd center in the active species, occupying the axial coordination site of palladium. The catalytic, as well as isotopic labeling experiments, in combination with ESI‐MS data and DFT calculations, point out palladium oxyl species as possible catalytically active sites, operating essentially via C−H abstraction/oxygen rebound pathway. For the ketones formation, O−H abstraction/в‐scission mechanism has been proposed.

Exploring different equatorial donors in a series of five-coordinate Cu(II) complexes supported by rigid tetradentate ligands

A series of tetradentate mixed-donor ligands has been synthesized containing an amine, thioether, or phosphine donor at the 8-position of a 2-(1,1-di(pyridin-2-yl)ethyl)quinoline backbone. Upon metalation with CuCl2, the preorganized N4, N3P, and N3S donor ligands enforce distorted trigonal bipyramidal coordination environments around the copper metal center, in which a chlorido donor occupies the remaining axial coordination site. The complexes were characterized by elemental analysis, high-resolution mass spectrometry, and X-ray crystallography. The non-pyridyl equatorial donor was the sole difference from among the tetradentate ligands, and is responsible for the structural and electronic variations across the series. Indeed, more intermediate geometries between the ideal trigonal bipyramidal and square pyramidal extremes are observed with the larger phosphorus and sulfur donors. Relative to the N4 derivative, the N3P and N3S systems have greater chemical stability upon redox cycling as observed in spectroelectrochemical experiments and lower inner-sphere reorganization energies for the CuII/I redox couples based on density functional theory calculations.

A ruthenium phthalocyanine functionalized with a folic acid unit as a photosensitizer for photodynamic therapy: Synthesis, characterization andin vitroevaluation

A folate-targeted ruthenium(II) phthalocyanine (Ru(FA-Py)(DMSO)(PEG)[Formula: see text]Pc), endowed with a pyridyl ligand functionalized with one folic acid unit (FA-Py) at one of the two axial coordination sites, and a dimethylsulfoxide (DMSO) ligand coordinated to the other axial position, respectively, is described. In order to enhance its biocompatibility, the RuPc is donated with eight PEG chains attached at the peripheral positions. The observed singlet oxygen quantum yields of the PS measured in DMSO and in water are of 0.74 and 0.36, respectively, in line with those observed for other RuPcs bearing comparable axial and peripheral substitution. In vitro PDT activity of the compound has been evaluated in HT-1376 human bladder cancer cell line. Ru(FA-Py)(DMSO)(PEG)[Formula: see text]Pc revealed a slightly higher cellular uptake than those observed for the corresponding carbohydrate-substituted PSs and a better photodynamic activity compared to the glucose-functionalized RuPc.

Effect of Sulfonamide and Carboxamide Ligands on the Structural Diversity of Bimetallic RhII-RhII Cores: Exploring the Catalytic Activity of These Newly Synthesized Rh2 Complexes.

A new class of dirhodium(II) complexes with tethered sulfonamide and carboxamide ligands was synthesized and characterized. A new type of coordination mode was found for the quinoline moiety containing a sulfonamide ligand, which afforded the axially coordination-free bimetallic dirhodium complexes. Studies were conducted on the catalytic properties of these complexes for cyclopropanation reactions, and the findings indicate that a free axial coordination site is crucial for achieving a high degree of reactivity.

Chromic Platinum Complexes Containing Multidentate Ligands

AbstractPlatinum(II) complexes display versatile chromic behavior as a consequence of their square planar geometry, which enables intra‐ and intermolecular Pt⋅⋅⋅Pt interactions via open axial coordination sites. These metallophilic interactions are known to generate metal‐metal‐to‐ligand‐charge transfer (MMLCT) transitions, in addition to the ligand‐to‐ligand charge transfer (LLCT) and metal‐to‐ligand‐charge‐transfer (MLCT) transitions that are already present. The electronic properties of such complexes, and consequently the magnitude and intensity of these transitions, can be modulated by various functional groups as well as environmental factors, affording control over the color and luminescence. The responsive behavior of these complexes makes them attractive candidates for chromic devices with applications in memory, encryption, sensors, and optoelectronics. This Minireview summarizes the mechanisms and reversibility of optical chromism in platinum(II) complexes, with a focus on the recent developments in the literature.

Fast kinetic redox process in layered cobaltous terephthalate MOF-type for aqueous hybrid devices. Magnetic properties as sensor of Co–Co interactions

The understanding of the fast-redox reactions origin is crucial for developing highly stable hybrid devices for energy storage. In this work two layered structures were considered, labelled as Co–H2O and Co-Pyz, where water and pyrazine molecules are occupying the axial coordination sites (pillars), respectively. Electrochemical characterization clearly revealed that these materials exhibit two faradic processes. The first one is controlled by diffusion (slow process) while the second process is meanly of superficial nature (fast process). These results are in accordance with apparent diffusion coefficient of charge-compensating ions estimated by Electrochemical Impedance Spectroscopy. In-situ XANES and FT-IR characterization unravelled that Co3+/Co2+ redox process paired with hydroxyl ions insertion and desertion from the MOF are involved in the fast redox process. When pyrazine is present as pillar between cobaltous terephthalate lamellas, the solid exhibits larger currents and lower charge transfer resistance than in presence of water as pillar. The improved performance of the pyrazine containing solid, is mainly ascribed to increased exchange interactions and a weaker spin-orbit coupling for the cobalt ion, which contributes to delocalize its electrons (unpaired electrons redistribution during redox reaction). This favours an easier electron mobility when an external stimulus is applied, resulting in a lower resistance and higher current, in comparison to the hydrated analogue. Finally, a hybrid device was assembled with Co-Pyz and commercial activated carbon, exhibiting a paramount stability by retaining its initial capacitance after 5000 GCD cycles at 2.5 Ag−1.

Two dimensional coordination polymers based on 3,5-di(1H-imidazol-1-yl)pyridine and their fluorescence properties

The solvothermal synthesis of three new 2D MOFs is reported. From the reaction of 3,5-di(1H-imidazol-1-yl)pyridine (dip) and the metal salts Co(NO3)2·6H2O, Cd(NO3)2·4H2O or Zn(NO3)2·4H2O, the three MOFs [Co(dip)2(SCN)2] (1), {[Cd(dip)2(H2O)2]·(NO3)2} (2), {[Zn(dip)2(H2O)2]·(ClO4)2} (3) were obtained. For these 2D MOFs, all the metal ions adopt a six-coordinate octahedral geometry with axial coordination sites occupied by SCN− or water molecules. The SCN−/water ligands were all mono-coordinate and led to layered MOF structures. Furthermore, the fluorescence properties of 2 and 3 have been investigated.

Structures of 1:1 Mononuclear Adducts of Zinc(II) Halides, ZnX2 (X=Cl, Br, I) and of New Hydrates of 1:1 Adducts of Zinc(II) Nitrate and Sulfate with Aromatic N,N′′-Bidentate Base Derivatives of Pyridine (bpy, phen)

Single crystal X-ray structure determinations have been executed for several title compounds of the mononuclear form [LMX2], namely [(bpy)ZnX2], X=Cl, Br; [(phen)ZnX2], X=Cl, Br, I (bpy=2,2′-bipyridine; phen=1,10-phenanthroline). Also reported are the crystal structures of ZnSO4/bpy/H2O (1:1:5) (≡ [(bpy)Zn(OH2)4](SO4)·H2O) and Zn(NO3)2/bpy/H2O (1:1:3) (≡ [(bpy)Zn(OH2)3(ONO2)](NO3)). The sulfate complex completes the array [LM(OH2)4]2+, L=bpy, phen, M=Zn, Cd, while the nitrate is the first nitrate example for [LM(OH2)3(OX)](+) (OX=ONO2, OSO3). In all examples of the sulfate and nitrate forms here and previously defined, the metal atoms are six-coordinate, with any anionic donors lying in axial coordination sites (i.e. M–O (quasi-) normal to the LM plane). The structures of [LHg(SCN)2] (L=phen, tpy) (tpy=2,2′:6′,2′-terpyridine), [(tpy)(CdBr2], and [Zn(tpy)2](NO3)2·2H2O are also recorded.

Thermal induced spin transition in a series of iron(II) layered inorganic-organic solids. Role of the intermolecular interactions in the interlayer region

The understanding of the factors that determine the spin crossover phenomenon in transition metal-containing solids is a relevant subject in order to design new materials with tunable physical and functional properties based on such spin transition. This is a process that has been studied for decades but even not fully understood in terms of the electronic and structural features that could determine its control and tuning for a given potential application. In this contribution, we shed light in that sense, through an evaluation of the iron atom coordination environment from Raman, Mössbauer, XPS, and structural data for a series formed by the intercalation of the pyridine (Py) molecule (L) and its halogen-substituted derivatives (3X-py) between neighboring layers of iron(II) tetracyanonickellate, Fe(L)2[Ni(CN)4]. In their structure, the organic molecule (L) is found occupying the axial coordination sites for the iron atom. The halogen atom determines both, the relative orientation of neighboring molecules in the interlayer region and the dominant intermolecular forces, through dispersive interactions. When such interaction generates a local distortion for the iron atom coordination environment, the thermal induced spin transition is inhibited. This is properly supported by the herein discussed experimental data.

Highly Efficient Singlet Oxygen Generators Based on Ruthenium Phthalocyanines: Synthesis, Characterization and in vitro Evaluation for Photodynamic Therapy.

The synthesis of ruthenium(II) phthalocyanines (RuPcs) endowed with one carbohydrate unit-that is, glucose, galactose and mannose-and a dimethylsulfoxide (DMSO) ligand at the two axial coordination sites, respectively, is described. Two series of compounds, one unsubstituted at the periphery, and the other one bearing eight PEG chains at the isoindole meta-positions, have been prepared. The presence of the axial DMSO unit significantly increases the phthalocyanine singlet oxygen quantum yields, related to other comparable RuPcs. The compounds have been evaluated for PDT treatment in bladder cancer cells. In vitro studies have revealed high phototoxicity for RuPcs unsubstituted at their periphery. The phototoxicity of PEG-substituted RuPcs has been considerably improved by repeated light irradiation. The choice of the axial carbohydrate introduced little differences in the cellular uptake for both series of photosensitizers, but the phototoxic effects were considerably higher for compounds bearing mannose units.