Character Of Ligand Research Articles

Rationalizing the Effect of Triethylaluminum on the Cr/SiO2Phillips Catalysts

In contrast to most of the other olefin polymerization catalysts, the Cr/SiO2 Phillips catalyst does not need any activator to develop its activity. However, it is known that the addition of a small amount of metal-alkyls has drastic effects on the catalyst performance, and it is one of the strategies adopted in industrial practice to tailor the properties of the polyethylene produced. In this work, we conducted a systematic investigation on the effect of triethylaluminum on the Cr(VI)/SiO2 catalyst, with the ultimate goal to determine the properties of the Cr sites at a molecular level and to correlate them to the properties of the obtained polyethylene. To this aim, we coupled kinetic tests, spectroscopic methods (diffuse reflectance UV–vis–near-infrared, electron paramagnetic resonance, Fourier-transform infrared spectroscopy of probe molecules), and different polymer characterization techniques. We found that, at an Al/Cr ratio of 2:1, only ca. 50% of the original Cr(VI) sites are reduced to a variety of species comprising: (1) Cr(IV) bisalkyl sites, which are probed by carbon monoxide and are the major actors in the polymerization of ethylene, explaining the faster polymerization initiation rate; (2) 6-fold coordinated Cr(III) sites, which are not accessible by probes and whose amount does not correlate with the catalyst activity; and (3) two types of monografted Cr(II) sites, all deriving from the over-reduction of the Cr(IV) bisalkyl sites, having, respectively, weak and strong Lewis acid characters, the former responsible for in situ α-olefin generation (and hence contributing to the enchainment of short polymer branching) and the latter accounting for the formation of the high-molecular-weight polymer fraction. Overall, our results demonstrate that the catalytic performance of the Cr sites depends not only on the oxidation state but rather on a combination of molecular structure, acidic character, and nature of the ancillary ligands. Hence, by tuning the structure and acidity of the Cr sites, it is possible to move from a system that essentially affords oligomers from ethylene to one that affords high-molecular-weight polymers.

Ancillary ligand electro-activity effects towards phenyl acetylene homocoupling reaction by a nickel(ii) complex of a non-innocent O-amino phenol ligand: a mechanistic insight.

A new Ni(ii) complex, was synthesized from the reaction of a non-innocent o-aminophenol ligand, and Ni(OAc)2. The crystal structure of NiIIL2NIS (in which, IS stands for iminosemiquinone radical ligand with cyanide (shown by N in NIS) substituent on phenolate rings) exhibits the square planar environment of Ni(ii). The complex has been crystalized in the monoclinic system and Ni(ii) was surrounded by two oxygen and two nitrogen atoms of two ligands. Variable-temperature magnetic susceptibility measurement for crystalline samples of complex shows the effective magnetic moment per molecule (μeff) of near zero and the diamagnetic nature of the complex (S = 0) which emphasize that strong antiferromagnetic coupling prevailed between the two unpaired electrons of LNIS ligands and Ni(ii) high spin electrons. The complex is EPR silent which confirms the diamagnetic character of the Ni(ii) complex. Electrochemical measurement (CV) indicates the redox-active character of ligand and metal. NiIIL2NIS complex proved to be effective for free metal- or base counterpart homocoupling of phenyl acetylene at room temperature. To the best of our knowledge, this is the first example of using Ni(ii) complex without using any reducing agent due to the promotion ancillary effect of non-innocent o-aminophenol ligand which acts as an “electron reservoir” and can reversibly accept and donate electrons in the catalytic cycle. The theoretical calculation confirms the magnetostructure, electronic spectrum and confirmed the suggested mechanism of phenyl acetylene homocoupling with emphasis on the role of non-innocent ligand electro-activity and the effect of ligand substituent on the efficiency and stability of the complex.

A New Look on Octet‐Compliant Macrocyclic Organoaluminum Carboxylates as Dormant Poly‐Lewis Acids

A more in‐depth understanding of the multi‐faceted chemistry of organoaluminum complexes is continually vital for the comprehension and development of new reagents and catalytic systems as well as molecular building blocks for modern functional materials. In this report we utilize a macrocyclic tetranuclear organoaluminum derivative of 2,3‐naphthalenedicarboxylic acid (naphtha‐H2), [(Me2Al)2(naphtha)]2, as a model complex to probe both the dormant Lewis acidity and self‐assembly processes of octet‐compliant multinuclear macrocyclic organoaluminum compounds in the presence of mono‐ or bidentate pyridine ligands. The results nicely demonstrate that multinuclear octet‐compliant organoaluminum clusters may effectively act as so‐called poly‐Lewis acids, utilizing their dormant Lewis acidity with the preservation or cleavage of the parent macrocycle depending on the character of both donor ligands and aromatic subunits of the dicarboxylate ligands.

Revisiting the role of π acceptor character of polypyridinc ligands in the redox behavior of tris and bis Fe(II) complexes and in the electron transfer with glucose oxidase

In this work we revisit the role of π-acceptor character of polypyridinc ligands in the electrochemical behavior of bis and tris Fe(II) complexes [FeL3]2+ and [FeL′2]2+ and in the electron transfer with a representative redox enzyme, glucose oxidase GOD; (L = 1,10-phen, 5-chloro-1,10-phen, 5-nitro-1,10-phen, 5-methyl-1,10-phen, 4-methyl-1,10-phen, 5,6-dimethyl-1,10-phen, 4,7-dimethyl-1,10-phen, 2,2′-bipy, 5,5′-dimethyl-2,2′-bipy, 4,4′-dimethyl-2,2′-bipy, L′ = 2,2′6′,6″-terpy and 4-chloro-2,2′:6′,6″-terpy). The compounds presented a reversible electron transfer [Fe(II)Ln]2+ → [Fe(III)Ln]3+ 1e, (n = 1,2) followed by an unexpected coupled chemical reaction, ECi mechanism. It was established that the redox potential (E°) and the rate constant (kf) of couple reaction are tuned by the pKa of non-coordinated ligand (π acceptor character descriptor). Using UV–vis spectroelectrochemistry different products are proposed for the ECi mechanism. DFT calculations, allow us to state that changes in E°, and pKa are related to HOMO energies. Linear relationships among the electrochemical and kinetic values E°, kf, with the pKa of the ligands were obtained. Using Foot of the Wave Analysis (FOWA) the homogenous electron transfer rate constant (ks) for GOD mediation was calculated. From data obtained in this work and choosing similar complexes from literature, it was explored the role of redox potential in this redox mediation.

Lewis Acidic PSbP Pincer Ligand in Pt-Catalyzed 1,6-Enyne Cycloisomerization: A Theoretical Study.

The progressively improved heterobimetallic antimony transition metal complex PSbP-Pt (I1) provides superior activity in catalyzed 1,6-enyne cycloisomerization. Our DFT calculations demonstrate that the noninnocent character of the antimony ligand enhances the self-activation of the catalyst precursor through a substrate-aided intramolecular chloride migration, which triggers subsequent reaction. Designed alternative redox noninnocent active species with strong electron-withdrawing groups also show promising catalytic ability due to an electron-deficient antimony ligand, which lowers the typical reaction barrier for the cycloisomerization of 1,6-enyne.

Extended Assemblies of Ru(bpy)(CO)2X2 (X = Cl, Br, I) Molecules Linked by 1,4-Diiodotetrafluoro-Benzene (DITFB) Halogen Bond Donors

The ruthenium carbonyl compounds, Ru(bpy)(CO)2X2 (X = Cl, Br or I) act as neutral halogen bond (XB) acceptors when co-crystallized with 1,4-diiodotetrafluoro-benzene (DITFB). The halogen bonding strength of the Ru-X⋅⋅⋅I halogen bonds follow the nucleophilic character of the halido ligand. The strongest halogen bond occurs between the chlorido ligand and the iodide atoms of the DITFB. All three halogen bonded complexes form polymeric assemblies in the solid state. In Ru(bpy)(CO)2Cl2⋅DITFB (1) and in Ru(bpy)(CO)2Br2⋅DITFB (2) both halido ligands are halogen bonded to only one DITFB donor. In Ru(bpy)(CO)2I2⋅DITFB (3) only one of the halido ligands is involved in halogen bonding acting as ditopic center for two DITFB donors. The polymeric structures of 1 and 2 are isomorphic wave-like single chain systems, while the iodine complexes form pairs of linear chains attached together with weak F⋅⋅⋅O≡C interactions between the closest neighbors. The stronger polarization of the iodide ligand compared to the Cl or Br ligands favors nearly linear C-I⋅⋅⋅I angles between the XB donor and the metal complex supporting the linear arrangement of the halogen bonded chain.

Unexpected Role of Ru(II) Orbital and Spin Contribution on Photoinduced Ligand Exchange: New Mechanism To Access the Photodynamic Therapy Window

A series of 12 Ru(II) complexes of the type [Ru(tpy)(L)(CH3CN)]n+, 1–12, containing the tridentate tpy ligand (tpy = 2,2′:6′,2″-terpyridine) and various bidentate ancillary ligands, L, were synthesized and evaluated for their ability to photodissociate CH3CN, a model for nitrile-containing drugs. Although the bidentate ligands chosen display a similar degree of steric bulk around the metal center in the ground state, the photosubstitution efficiencies of 1–12 vary by approximately an order of magnitude. The complexes containing the most electron-donating bidentate ligands, 8–11, exhibit the larger quantum yield values for ligand exchange in water. Complexes 8–11 also possess the smallest energy gap between the ground state and the lowest energy triplet metal-to-ligand charge transfer (3MLCT) excited state. In addition, density functional theory calculations indicate that a large degree of ligand character is present in their highest occupied molecular orbitals (HOMOs) and in their 3MLCT excited states. These results show that ligand mixing affects the π-backbonding ability of the metal center in the excited state, increasing the quantum yields of nitrile ligand dissociation. Linear relationships were observed between the quantum yield of ligand exchange and both the Mulliken spin density on the metal center in the excited state and the percent of Ru(d) character in the ground state HOMO. These findings, driven by the presence of π-donating ancillary ligands that enhance excited state reactivity, can be used to design new complexes with higher quantum yields for the release of nitrile-containing drugs and biological probes without affecting their stability in the dark.

Efficiency enhancement of iridium-based organometallic light emitting diodes

The efficiency of the Organic Light Emitting Diode (OLED) is given either by the internal quantum efficiency of the organometallic compound or by the charge transport across OLED. IrQ(ppy)2 is a new organometallic compound which gives green and red electroluminescence. This dual emitter compound exhibits a lower internal quantum efficiency compared with classical Ir(ppy)3 green emitter because of a weak coupling between Ir3+ and oxygen ions which significantly reduces the charge transfer towards quinoline ligand. This lower internal quantum efficiency is compensated by the higher electron donor character of the quinoline ligand which induces better change transport in OLED structures. In the case of Ir(ppy)3 green emitter, the efficiency can be improved by adding magnetic or metallic nanoparticles which significantly change the charge transport for the Ir(ppy)3 based OLED structures. The metallic or magnetic nanoparticles embedded in the transparent and conductive polymer, reduce the electron injection, acting as filling traps, which directly increases the electroluminescence and the current at the same voltage.

Spin-State Variations of Iron(III) Complexes with Tetracarbene Macrocycles.

Starting from their six-coordinate iron(II) precursor complexes [L8R Fe(MeCN)]2+ , a series of iron(III) complexes of the known macrocyclic tetracarbene ligand L8H and its new octamethylated derivative L8Me , both providing four imidazol-2-yliden donors, were synthesized. Several five- and six-coordinate iron(III) complexes with different axial ligands (Cl- , OTf- , MeCN) were structurally characterized by X-ray diffraction and analyzed in detail with respect to their spin state variations, using a bouquet of spectroscopic methods (NMR, UV/Vis, EPR, and 57 Fe Mößbauer). Depending on the axial ligands, either low-spin (S=1/2) or intermediate-spin (S=3/2) states were observed, whereas high-spin (S=5/2) states were inaccessible because of the extremely strong in-plane σ-donor character of the macrocyclic tetracarbene ligands. These findings are reminiscent of the spin state patterns of topologically related ferric porphyrin complexes. The ring conformations and dynamics of the macrocyclic tetracarbene ligands in their iron(II), iron(III) and μ-oxo diiron(III) complexes were also studied.

Synthesis and Catalytic Activity of Iron Hydride Ligated with Bidentate N-Heterocyclic Silylenes for Hydroboration of Carbonyl Compounds

We report the synthesis of a novel bidentate N-heterocyclic silylene (NHSi) ligand, N-(LSi:)-N-methyl-2-pyridinamine (1) (L = PhC(NtBu)2), and the first bischelate disilylene iron hydride, [(Si,N)(Si,C)Fe(H)(PMe3)] (2), and monosilylene iron hydride, [(Si,C)Fe(H)(PMe3)3] (2′), through Csp2–H activation of the NHSi ligand. Compounds 1 and 2 were fully characterized by spectroscopic methods and single-crystal X-ray diffraction analysis. Density functional theory calculations indicated the multiple-bond character of the Fe–Si bonds and the π back-donation from Fe(II) to the Si(II) center. Moreover, the strong donor character of ligand 1 enables 2 to act as an efficient catalyst for the hydroboration reaction of carbonyl compounds at room temperature. Chemoselective hydroboration is attained under these conditions. This might be the first example of hydroboration of ketones and aldehydes catalyzed by a silylene hydrido iron complex. A catalytic mechanism was suggested and partially experimentally verified.

Electrochemical CO2 reduction by a cobalt bipyricorrole complex: decrease of an overpotential value derived from monoanionic ligand character of the porphyrinoid species.

A newly synthesized Co(ii) complex with a monoanionic bipyricorrole ligand is found to catalytically promote a selective CO2 electroreduction to CO with Faradaic efficiency of 75%. Catalytic Tafel plots show that the overpotential of Co(ii) bipyricorrole is 0.35 V lower than that of a Co(ii) complex with the dianionic tetraphenylporphyrin ligand.

Probing the Electron Accepting Orbitals of Ni-Centered Hydrogen Evolution Catalysts with Noninnocent Ligands by Ni L-Edge and S K-Edge X-ray Absorption.

The valence electronic structure of several square planar Ni-centered complexes, previously shown to catalyze the hydrogen evolution reaction, are characterized using S K-edge and Ni L-edge X-ray absorption spectroscopy and electronic structure calculations. Measurement of the atomic Ni 3d and S 3p contributions enables assessment of the metal-ligand covalency of the electron accepting valence orbitals and yields insight into the ligand-dependent reaction mechanisms proposed for the catalysts. The electron accepting orbital of the Ni(abt)2 (abt = 2-aminobenzenethiolate) catalyst is found to have large ligand character (80%), with only 9% S 3p (per S) character, indicating delocalization over the entire abt ligand. Upon two proton-coupled reductions to form the Ni(abt-H)2 intermediate, the catalyst stores 1.8 electrons on the abt ligand, and the ligand N atoms are protonated, thus supporting its role as an electron and proton reservoir. The electron accepting orbitals of the Ni(abt-H)2 intermediate and Ni(mpo)2 (mpo = 2-mercaptopyridyl- N-oxide) catalyst are found to have considerably larger Ni 3d (46-47%) and S 3p (17-18% per S) character, consistent with an orbital localized on the metal-ligand bonds. This finding supports the possibility of metal-based chemistry, resulting in Ni-H bond formation for the reduced Ni(abt-H)2 intermediate and Ni(mpo)2 catalyst, a critical reaction intermediate in H2 generation.

Backbone Dehydrogenation in Pyrrole-Based Pincer Ligands.

Treatment of both [CoCl( tBuPNP)] and [NiCl( tBuPNP)] ( tBuPNP = anion of 2,5-bis((di- tert-butylphosphino)methyl)pyrrole) with one equivalent of benzoquinone affords the corresponding chloride complexes containing a dehydrogenated PNP ligand, tBudPNP ( tBudPNP = anion of 2,5-bis((di- tert-butylphosphino)methylene)-2,5-dihydropyrrole). Dehydrogenation of PNP to dPNP results in minimal change to steric profile of the ligand but has important consequences for the resulting redox potentials of the metal complexes, resulting in the ability to isolate both [CoH( tBudPNP)] and [CoEt( tBudPNP)], which are more challenging (hydride) or not possible (ethyl) to prepare with the parent PNP ligand. Electrochemical measurements with both the Co and Ni dPNP species demonstrate a substantial shift in redox potentials for both the M(II/III) and M(II/I) couples. In the case of the former, oxidation to trivalent Co was found to be reversible, and subsequent reaction with AgSbF6 afforded a rare example of a square-planar Co(III) species. Corresponding reduction of [CoCl( tBudPNP)] with KC8 produced the diamagnetic Co(I) species, [Co(N2)( tBudPNP)]. Further reduction of the Co(I) complex was found to generate a pincer-based π-radical anion that demonstrated well-resolved EPR features to the four hydrogen atoms and lone nitrogen atom of the ligand with minor contributions from cobalt and coordinated N2. Changes in the electronic character of the PNP ligand upon dehydrogenation are proposed to result from loss of aromaticity in the pyrrole ligand, resulting in a more reducing central amido donor. DFT calculations on the Co(II) complexes were performed to shed further insight into the electronic structure of the pincer complexes.

Cover Feature: Bowl Shaped Monodentate Phosphines by Condensation of Imidazole Units with Boranes (Z. Anorg. Allg. Chem. 14/2018)

The cover picture shows a new class of chiral phosphine ligands bearing three imidazole units bridged by boranes. The bowl shaped, C3 symmetric molecules are obtained by a simple condensation procedure. The steric bulk of these monodentate ligands can easily be adjusted over a large range by the use of different chiral or achiral boranes. The ligand character is determined by the almost pure s-character of the phosphorous lone-pair, leading to very short metal-P distances. Details are discussed in the article by Markus Enders, Helene Schall, and Olaf Fritz, et al. on page 630 ff.

Gas-Phase Photoluminescence and Photodissociation of Silver-Capped Hexagold Clusters.

We report on the radiative and nonradiative deactivation pathways of selected charge states of the stoichiometric hexagold phosphine-stabilized ionic clusters, [(C)(AuDppy)6Ag2·(BF4) x](4- x)+ with x = 2 and 3 (Dppy = diphenylphosphino-2-pyridine), combining gas-phase photoluminescence and photodissociation with quantum chemical computations. These clusters possess an identical isostructural core made of a hyper-coordinated carbon at their center octahedrally surrounded by six gold ions, and two silver ions at their apexes. Their luminescence and fragmentation behavior upon photoexcitation was investigated under mass and charge control in an ion trap. The experimental and computational results shed light on the electronic states involved in the optical transitions as well as on their core, ligand, or charge transfer character. Gas-phase results are discussed in relation with condensed phase measurements, as well as previous observations in solution and on metal-organic frameworks. The monocationic species ( x = 3) is found to be less stable than the dicationic one ( x = 2). In the luminescence spectrum of the monocationic species, a shoulder at short wavelength can be observed and is assigned to fragment emission. This fragment formation appears to be favored for the monocation by the existence of a low lying singlet state energetically overlapping with the triplet state manifold, which is populated quickly after photoexcitation.

Computational Design of Quaterpyridine‐Based Fe/Mn–Complexes for the Direct Hydrogenation of CO2 to HCOOH: A Direction for Atom‐Economic Approach

AbstractOn the basis of first principle calculations, we propose a new class of earth‐abundant quaterpyridine complexes for the direct hydrogenation of CO2 to HCOOH. The mechanistic studies reveal that the complexes are advantageous in many aspects. First of all, they can perform both in added‐base and base‐free conditions, the latter being atom‐economic as well as a greener way. Most effectively, the energy barrier for the heterolytic hydrogen cleavage and hydride transfer processes could easily be monitored by changing the π‐acidic character of the axial ligand, solvent polarity of the medium and also by ligand modification through substitution.

How Changing the Bridgehead Can Affect the Properties of Tripodal Ligands

AbstractAlthough a multitude of studies have explored the coordination chemistry of classical tripodal ligands containing a range of main‐group bridgehead atoms or groups, it is not clear how periodic trends affect ligand character and reactivity within a single ligand family. A case in point is the extensive family of neutral tris‐2‐pyridyl ligands E(2‐py)3 (E=C−R, N, P), which are closely related to archetypal tris‐pyrazolyl borates. With the 6‐methyl substituted ligands E(6‐Me‐2‐py)3 (E=As, Sb, Bi) in hand, the effects of bridgehead modification alone on descending a single group in the periodic table were assessed. The primary influence on coordination behaviour is the increasing Lewis acidity (electropositivity) of the bridgehead atom as Group 15 is descended, which not only modulates the electron density on the pyridyl donor groups but also introduces the potential for anion selective coordination behaviour.

How Changing the Bridgehead Can Affect the Properties of Tripodal Ligands.

Although a multitude of studies have explored the coordination chemistry of classical tripodal ligands containing a range of main-group bridgehead atoms or groups, it is not clear how periodic trends affect ligand character and reactivity within a single ligand family. A case in point is the extensive family of neutral tris-2-pyridyl ligands E(2-py)3 (E=C-R, N, P), which are closely related to archetypal tris-pyrazolyl borates. With the 6-methyl substituted ligands E(6-Me-2-py)3 (E=As, Sb, Bi) in hand, the effects of bridgehead modification alone on descending a single group in the periodic table were assessed. The primary influence on coordination behaviour is the increasing Lewis acidity (electropositivity) of the bridgehead atom as Group 15 is descended, which not only modulates the electron density on the pyridyl donor groups but also introduces the potential for anion selective coordination behaviour.

Ground-State Charge-Density Distribution in a Crystal of the Luminescent ortho-Phenylenediboronic Acid Complex with 8-Hydroxyquinoline.

This contribution is devoted to the first electron density studies of a luminescent oxyquinolinato boron complex in the solid state. ortho-Phenylenediboronic acid mixed with 8-hydroxyquinoline in dioxane forms high-quality single crystals via slow solvent evaporation, which allows successful high resolution data collection (sinθ/λ = 1.2 Å-1) and charge density distribution modeling. Particular attention has been paid to the boron-oxygen fragment connecting the two parts of the complex, and to the solvent species exhibiting anharmonic thermal motion. The experiment and theory compared rather well in terms of atomic charges and volumes, except for the boron centers. Boron atoms, as expected, constitute the most electron-deficient species in the complex molecule, whereas the neighboring oxygen and carbon atoms are the most significantly negatively charged ones. This part of the molecule appears to be very much involved in the charge transfer occurring between the acid fragment and oxyquinoline moiety leading to the observed fluorescence, as supported by the time-dependent density functional theory (TDDFT) results and the generated transition density maps. TDDFT calculations indicated that p-type atomic orbitals contributing to the HOMO-1, HOMO, and LUMO play the major role in the lowest energy transitions, and enabled further comparison with the charge density features, which is discussed in details. Furthermore, the results confirmed the known fact the Q ligand character is most important for the spectroscopic properties of this class of complexes.

Unusually high stability of B12(BO)122− achieved by boronyl ligand manipulation: Theoretical investigation

Density functional theory calculations of dodecaboronyldodecaborate (B12(BO)122−) were performed to examine its electronic properties. Formation of this dianion with unusually high stability was accomplished by substituting boronyl (BO) for H in a typical dianion, B12H122−. The binding energy of the second electron of the B12(BO)122− dianion was 5.89 eV (PBE0/def2-TZVP) and 5.96 eV (MN12-SX/def2-TZVP); this represents its stability and is the largest value reported to date. We clarify that the high stability of B12(BO)122− originates from the weak π orbital interaction character of the BO ligand. All calculated results clearly indicate the deficient π orbital interaction ability of BO.