Ligands For Transition Metals Research Articles

Synthesis and Electronic Spectra of Fluorinated Aryloxide and Alkoxide [NiX4]2− Anions

Five new homoleptic [NiX(4)](2-) compounds have been prepared with the fluorinated ligands OC(6)F(5) (OAr(F)), OC(6)H(3)(CF(3))(2) (OAr'), and OC(4)F(9) (OR(F)) and characterized with X-ray crystallography, magnetic susceptibility, and elemental analysis. Electronic spectral studies show that these ligands engender a ligand-field environment similar to that of fluoride and thus act electronically like fluoride, but with none of the drawbacks of F(-) as a transition-metal ligand.

Abnormal Carbenes as Ligands in Transition Metal Chemistry: Curiosities with Exciting Perspectives

This review compiles the advances achieved in our laboratories using abnormal and less heteroatom-stabilized carbenes as ligands for transition metal chemistry. Fundamental studies allowed the evaluation of the impact of this new class of ligands both electronically and sterically. Based on these results, initial catalytic applications have been devised in the area of H–H and C–H bond activation, demonstrating the potential of abnormal carbenes as unique ligands for transition metals.

ChemInform Abstract: Fullerenes as a New Type of Ligands for Transition Metals

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Compounds with low-valent group 13 metals as ligands for electron poor main group and transition metals

The application of low-valent group 13 metal compounds as ligands in coordination chemistry results in the formation of new metal-to-metal bonds. Traditionally, the coordination chemistry of these compounds was focused on late transition metals. This article now reviews recent progress of the coordination chemistry of compounds with low-valent group 13 metals in the chemistry of electron poor main group and transition metal as well as in f-block elements.

P,N-Containing cyclophanes with large helical hydrophobic cavities: prospective precursors for the design of a molecular reactor

The highly effective self-assembly process between 2,4,6-tris(isopropyl)phenylphosphine or mesitylphosphine, formaldehyde and 1,4-bis(alpha-(4'-aminophenyl)isopropyl)benzene results in the formation of a novel type of heterocyclophane which is able to encapsulate organic molecules as well as being a potential ligand for transition metals inside the intramolecular cavity. Compounds of this type can be regarded as potential precursors for a new kind of molecular reactors.

Antimicrobial and toxicological studies of some metal complexes of 4-methylpiperazine-1-carbodithioate and phenanthroline mixed ligands

A few mixed ligand transition metal carbodithioate complexes of the general formula [M(4-MPipzcdt)x(phen)y]Y (M = Mn(II), Co(II), Zn(II); 4-MPipzcdt = 4-methylpiperazine-1-carbodithioate; phen = 1,10-phenanthroline; x = 1 and y = 2 when Y = Cl; x = 2 and y = 1 when Y = nil) were synthesized and screened for their antimicrobial activity against Candida albicans, Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Enterococcus faecalis by disk diffusion method. All the complexes exhibited prominent antimicrobial activity against tested pathogenic strains with the MIC values in the range <8-512 gmL-1. The complexes [Mn(4-MPipzcdt)2(phen)] and [Co(4-MPipzcdt)(phen)2]Cl inhibited the growth of Candida albicans at a concentration as low as 8 µgmL-1. The complexes were also evaluated for their toxicity towards human transformed rhabdomyosarcoma cells (RD cells). Moderate cell viability of the RD cells was exhibited against the metal complexes.

Chirality in the Absence of Rigid Stereogenic Elements: The Design of Configurationally Stable C3‐Symmetric Propellers

Residual stereoisomerism is a form of stereoisomerism scarcely considered so far for applicative purposes, though extremely interesting, since the production of stereoisomers does not involve classical rigid stereogenic elements. In three-bladed propeller-shaped molecules, a preferred stereomerization mechanism, related to the correlated rotation of the rings, allows the free interconversion of stereoisomers inside separated sets (the residual stereoisomers) that can interconvert through higher energy pathways. In light of possible future applications as chiral ligands for transition metals in stereoselective processes, some C(3)-symmetric phosphorus-centered propellers, which could exist as residual enantiomers, are synthesized and the possibility of resolving their racemates into residual antipodes is explored. While the tris(aryl)methanes are configurationally stable at room temperature, only selected tris(aryl)phosphane oxides display a configurational stability high enough to allow resolution by HPLC on a chiral stationary phase (CSP HPLC) at a semipreparative level at room temperature. Stability was evaluated through different techniques (circular dichroism (CD) signal decay, dynamic CSP HPLC (CSP DHPLC), dynamic NMR analysis (DNMR)) and the results compared and discussed. Phosphanes were found much less stable than the corresponding phosphane oxides, for which preliminary calculations suggest that the three-ring-flip enantiomerization mechanism (M(0)) would be easier than phosphorus pyramidal inversion. The parameters affecting the configurational stability of the residual enantiomers of C(3)-symmetric propellers are discussed.

Calculating interaction energies in transition metal complexes with local electron correlation methods

The results of density fitting and local approximations applied to the calculation of transition metal-ligand binding energies using second order Møller-Plesset perturbation theory are reported. This procedure accurately reproduces counterpoise corrected binding energies from the canonical method for a range of test complexes. While counterpoise corrections for basis set superposition error are generally small, this procedure can be time consuming, and in some cases gives rise to unphysical dissociation of complexes. In circumventing this correction, a local treatment of electron correlation offers major efficiency savings with little loss of accuracy. The use of density fitting for the underlying Hartree-Fock calculations is also tested for sample Ru complexes, leading to further efficiency gains but essentially no loss in accuracy.

1,2,3-Triazolylidenes as Versatile Abnormal Carbene Ligands for Late Transition Metals

The [3 + 2] cycloaddition of azides and acetylenes followed by nitrogen quaternization was applied for the generation of novel and highly modular triazolium salts. The selective substitution of the 1,3,4-substitution pattern presets such salts as precursors for a new class of abnormal carbene ligands, thus expanding the family of these high-impact ligands. Metalation of the triazolium salts is highly versatile and is illustrated by direct C-H bond activation as well as by applying a transmetalation protocol, thus providing access to Pd(II), Ru(II), Rh(I), and Ir(I) abnormal carbene complexes. The donor properties of these carbenes were analyzed by using Tolman electronic parameters and were found to be slightly stronger than those the most basic normal carbenes.

Hydrogen Absorption in Transition Metal Silicides: La3Pd5Si-Hydrogen System

Pressure-composition isotherm measurements show that the ternary lanthanum palladium silicide phase La3Pd5Si absorbs reversibly up to 5 hydrogen atoms per formula unit at 550 K and 14 bar hydrogen pressure. In-situ synchrotron and neutron powder diffraction reveals three phases, an alpha-phase having the limiting composition La3Pd5SiD approximately 1.6 at low deuterium pressure (at up to 9.5 bar D2 and 550 K), a beta-phase La3Pd5SiD approximately 2.30-4 at intermediate deuterium pressure (<9.5 bar D2 and 550 K), and a relatively unstable gamma-phase La3Pd5SiD approximately 5 at high deuterium pressure (obtained at 75 bar D2 and 293 K). While the alpha and beta phases retain the symmetry of the H-free La3Pd5Si (space group Imma), the gamma-phase undergoes a symmetry lowering (a(gamma) approximately a(beta), b(gamma) approximately 3b(beta) and c(gamma) approximately c(beta), V(gamma) approximately 3V(beta), space group Pmnb). The structure of the alpha-phase contains isolated [Pd-D-Pd] fragments, which are joined into polymeric (-Pd-D-Pd-)n zig-zag chains in the beta-phase. In the gamma-phase some D sites depopulate, while new D sites are occupied, thus leading to a partial interruption of the zig-zag chains and the formation of isolated [D-Pd-D-Pd] and [D-Pd-D-Pd-D] fragments. This unexpected behavior can be attributed to the onset of repulsive Si-D and D-D interactions (Si-D > 3.0 A, D-D > 2.1 A) that divide the structure into Si-poor slabs that absorb hydrogen and Si-rich slabs that do not. The competition between silicon and deuterium which act as a transition metal ligand is further underlined by the fact that Pd atoms having one Si ligand are capable of forming Pd-D bonds, whereas Pd atoms having two Si ligands are not.

Synthesis of Enantiopure 1‐r‐Alkyl‐2‐c,5‐t‐Diphenylphospholanes and Phospholanium Salts through Direct Alkylation of Phospholane

AbstractChiral enantiopure 1‐alkyl‐2,5‐diphenylphospholanium salts were obtained in one step through alkylation of phospholane with alkyl triflates. The resulting air‐stable phosphonium salts are electron‐rich trialkylphosphane precursor ligands for transition metals, and they offer a convenient route toward chiral quaternary phosphonium salts as phase‐transfer agents.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

Synthesis of an Extremely Bent Acyclic Allene (A “Carbodicarbene”): A Strong Donor Ligand

Pushed to the limit: Pushing CC π bonds to the breaking point by using a push–push substitution pattern forces allenes to bend (see structure; C light blue, N dark blue). An acyclic allene featuring a CCC bond angle of 134.8° has been isolated in which the typically sp-hybridized central carbon atom approaches a configuration that has two lone pairs of electrons, and acts as a very strong η1-donor ligand for transition metals.

Electronic structures of organometallic polymers containing μ 2-bridging pentalene and indacene ligands

Band structures of organometallic polymers containing μ 2-bridging pentalene or indacene as ligands for transition metals (Fe, Co, Ni) have been calculated by using the tight-binding crystal orbital method. These organometallic polymers are an infinite model of bimetallic complexes of pentalene or s-indacene with pentamethylcyclopentadiene. Polymers containing pentalene and s-indacene with iron atoms possess small band gaps with broad bands near the Fermi level. On the other hand, polymers containing cobalt or nickel atoms exhibit a metallic or semi-metallic character, which suggests the possibility of changes in their structures. We have analysed the crystal orbital phases to examine the electronic structures.

C4-bound imidazolylidenes: from curiosities to high-impact carbene ligands

This feature article summarizes the progress achieved thus far in using C4-bound imidazolylidenes as a new class of ligands for transition metals. Since the discovery of this unusual carbene bonding mode in 2001, various rational routes towards complexes containing C4-bound carbenes have evolved. These advances allowed for studying the impact of this new type of ligand on the transition metal center, both from a fundamental point of view as well as from a more applied perspective, in particular for catalytic applications. The promising results accomplished in this relatively short period of time demonstrate the potential of C4-bound imidazolylidenes as unique carbene ligands for inducing catalytic activity and for mediating unprecedented transformations.

Iptycene-Derived Pyridazines and Phthalazines

The synthesis of new heterocyclic oligo(phenylene) analogues based on soluble, nonaggregating 1,2-diazines is reported. Improved palladium-catalyzed reductive coupling methods were developed to allow for the preparation of large quantities of iptycene-derived bipyridazines and biphthalazines, and the controlled synthesis of well-defined oligomers up to sexipyridazine. Crystallographic, spectroscopic, and computational evidence indicate that in these analogues, hindrance at the ortho position is relaxed relative to poly(phenylenes). The resulting building blocks are promising for incorporation in conjugated electronics materials, and as new iptycene-derived ligands for transition metals.

Fullerenes as a new type of ligands for transition metals

Fullerenes are considered as ligands in transition metal π-complexes. The following aspects are discussed: metals able to form complexes with fullerenes; haptic numbers; homo-and heteroligand complexes; ligand compatibility with fullerenes for different metals, including fullerenes with a disturbed structure of conjugation.

Applications of bis(1-R-imidazol-2-yl)disulfides and diselenides as ligands for main-group and transition metals: kappa2-(N,N) coordination, S-S bond cleavage, and S-S/E-E (E = S, Se) bond metathesis reactions.

Bis(1-R-imidazol-2-yl)disulfides, (mim(R))2 (R = Ph, Bu(t)), and diselenides, (seim(Mes))2, serve as bidentate N,N-donor ligands for main-group and transition metals. For example, [kappa2-(mim(Bu)(t))2]MCl2 (M = Fe, Co, Ni, Zn), [kappa2-(mim(Ph))2]MCl2 (M = Co, Zn), [kappa2-(mim(Bu)(t))2]CuX (X = Cl, I), and [kappa2-(seim(Mes))2]MCl2 (M = Fe, Co, Ni) are obtained by treatment of (mim(Bu)(t))2 or (seim(Mes))2 with the respective metal halide and have been structurally characterized by X-ray diffraction. On the other hand, the zerovalent nickel complex Ni(PMe3)4 effects cleavage of the disulfide bond of (mim(Bu)(t))2 to give square-planar trans-Ni(PMe3)2(mim(Bu)(t))2 in which the (mim(Bu)(t)) ligands coordinate via nitrogen rather than sulfur, a most uncommon coordination mode for this class of ligands. Although [kappa2-(mim(R))2]MCl2 (M = Fe, Co, Ni, Zn) are not subject to homolytic cleavage of the S-S bond because the tetravalent state is not readily accessible, the observation that [kappa2-(mimPh)2]CoCl2 and [kappa2-(mim(Bu)(t))2]CoCl2 form an equilibrium mixture with the asymmetric disulfide [kappa2-(mim(Ph))(mim(Bu)(t))]CoCl2 indicates that S-S bond cleavage via another mechanism is possible. Likewise, metathesis between disulfide and diselenide ligands is observed in the formation of [kappa2-(mim(Bu)(t))(seim(Mes))]CoCl2 upon treatment of [kappa2-(mim(Bu)(t))2]CoCl2 with [kappa2-(seim(Mes))2]CoCl2.

N-Heterocyclic Carbene-Copper(I) Complexes in Homogeneous Catalysis

N-Heterocyclic carbenes (NHCs) have become increasingly popular as ligands for transition metals, thanks to their remarkable successes in metathesis and cross-coupling areas. Important advances have also been achieved in organocopper chemistry. To date, the reported neutral N-heterocyclic carbene-copper(I) complexes have shown outstanding catalytic activity. In this account, we describe the application of such complexes in a variety of organic transformations. 1 Introduction 2 Copper-Hydride-Mediated Reactions 2.1 Reduction of Carbonyl Compounds 2.1.1 Activity and Scope 2.1.2 Mechanistic Considerations 2.2 Related Transformations 3 Carbene Transfer Reactions 3.1 Cyclopropanation and Insertion Reactions 3.2 Olefination Reactions 4 [3+2] Cycloaddition of Azides and Alkynes 4.1 'Click Chemistry' 4.2 Use of Internal Alkynes: Mechanistic Implications 5 Miscellaneous Reactions 6 Concluding Remarks.

Main‐Chain Organometallic Polymers: Synthetic Strategies, Applications, and Perspectives

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Ge4Br4[Mn(CO)5]4 and Ge6Br2[Mn(CO)5]6: first germanium cluster compounds containing Mn(CO)5 ligands

Cluster compounds of germanium exhibiting germanium-germanium bonds, where the germanium atoms are additionally bound to transition metal ligands, are rare. Here a synthetic pathway to such cluster compounds is described, starting from metastable Ge I halide solutions leading to the two cluster compounds Ge4Br4[Mn(CO)5]4 and Ge6Br2[Mn(CO)5]6, being the first examples of germanium cluster compounds bearing Mn(CO)5 ligands. The Ge6 compound exhibits a novel arrangement of germanium atoms that has not been previously observed in ligand stabilized cluster compounds of germanium, neither with organic nor with transition metal ligands. The bonding situation inside the cluster compounds is discussed, together with a possible reaction pathway that opens the way to metalloid cluster compounds of germanium exhibiting Mn(CO)5 ligands.