σ-Acceptor, Z-type ligands for transition metals

The ability of Lewis acids to coordinate to transition metals as σ-acceptor ligands was recognized as early as in the 1970's, but so-called Z-type ligands remained curiosities until the early 2000's. Over the last decade, significant progress has been made in this area, especially via the incorporation of Lewis acid moieties into multidentate, ambiphilic ligands. Our understanding of the nature and influence of TM → Z interactions has considerably improved and the scope of Lewis acids susceptible to behave as σ-acceptor ligands has been significantly extended. This feature article summarizes these recent achievements.     

η-Phosphole tricarbonylmanganates: a new type of chelating ligands for transition metals

3,4-Dimethylphosphacymantrene (1) reacts with tert-butyllithium to give the corresponding (η⁴-3,4-dimethyl-1-tert-butylphosphole)tricarbonylmanganate (2) which can act as a chelating ligand (L) toward a Pd₂Cl₂ dimeric core. The X-ray crystal structure of L₂Pd₂Cl₂ (3) shows a bite angle of 60.5°.     

Bimetallic catalysis using transition and Group 11 metals: an emerging tool for C-C coupling and other reactions

Bimetallic catalysis refers to homogeneous processes in which either two transition metals (TM), or one TM and one Group?11 (G11) element (occasionally Hg also), cooperate in a synthetic process (often a C-C coupling) and their actions are connected by a transmetalation step. This is an emerging research area that differs from the isolated or tandem applications of the now classic processes (Stille, Negishi, Suzuki, Hiyama, Heck). Most of the reactions used so far combine Pd with a second metal, often Cu or Au, but syntheses involving very different TM couples (e.g., Cr/Ni in the catalyzed vinylation of aldehydes) have also been developed. Further development of the topic will soon demand a good knowledge of the mechanisms involved in bimetallic catalysis, but this knowledge is very limited for catalytic processes. However, there is much information available, dispersed in the literature, coming from basic research on exchange reactions occurring out of any catalytic cycle, in polynuclear complexes. These are essentially the same processes expected to operate in the heart of the catalytic process. This Review gathers together these two usually isolated topics in order to stimulate synergy between the bimetallic research coming from more basic organometallic studies and the more synthetic organic approaches to this chemistry.     

Bidentate ligands by supramolecular chemistry--the future for catalysis?

In this Frontier Article we give our view on recent developments in transition-metal catalyst development that evolve from a combination of supramolecular strategies and traditional ligand design and development.     

The quest for affinity chromatography ligands: are the molecular libraries the right source?

Affinity chromatography separations of proteins call for highly specific ligands. Antibodies are the most obvious approach; however, except for specific situations, technical and economic reasons are arguments against this choice especially for preparative purposes. With this in mind, the rationale is to select the most appropriate ligands from collections of pre‐established molecules. To reach the objective of having a large structural coverage, combinatorial libraries have been proposed. These are classified according to their nature and origin. This review presents and discusses the most common affinity ligand libraries along with the most appropriate screening methods for the identification of the right affinity chromatography selective structure according to the type of library; a side‐by‐side comparison is also presented.     

Are carbodiphosphoranes better ligands than N-heterocyclic carbenes for Grubb's catalysts?

Theoretical investigations suggest that substitution of an N-heterocyclic carbene by a carbodiphosphorane in the Grubb's catalyst for olefin metathesis might lead to enhanced reactivity.     

Can prenylcysteines be exploited as ligands for mammalian multidrug-resistance transporters?

The overexpression of specific transport proteins in the membrane of many cancer cells renders these cells resistant to many therapeutic drugs. Some lipid-modified cysteine compounds inhibit one drug-transporting protein, indicating the potential of developing such compounds as therapeutic agents.     

Halide-bridged polymers of divalent metals with donor ligands – structures and properties

Coordination polymers and organic–inorganic hybrid materials represent an area of very active research. Halide-bridged polymers of divalent transition or post-transition metals coordinated to donor ligands are reviewed. The article is organized according to structural features of the compounds: the main sections of chain polymers, crosslinked systems and networks of high metal-halide content are subdivided according to connectivity aspects. Structural trends are highlighted and the correlation between chemical composition and topology on the one hand and derived properties and dynamics on the other hand is discussed.     

Triallylborane and some its analogs as ligands in transition metal π-complexes: theoretical investigations

The structure, stability, and potential existence of chromium complexes with triallylborane B(CH₂—CH=CH₂)₃ and its analogs with coordination of the Cr atom to three double bonds are discussed. Complexes LCr(CO)₃ are studied where L = (C₃H₅)₃CH, (C₃H₅)₃B, (C₃H₅)₃BNMe₃, [(C₃H₅)₃BF]−, (C₂H₄)₃, C₆H₆, (C₃H₅)₃CPh, (C₃H₅)₃B(NC₅H₅). The calculations are carried out in DFT terms in approximations PBE/3z and BP86/TZ2P. The calculated dissociation energies of the studied complexes into the fragments Cr(CO)₃ and L range from 48.3 to 63.0 kcal mol⁻¹ (BP86/TZ2P) and from 54.2 to 66.9 kcal mol⁻¹ (PBE/3z). Ligands (C₃H₅)₃ER (ER = CPh, B(NC₅H₅)) can form two isomeric complexes A and B due to coordination of tricarbonylchromium to either three double bonds or benzene or pyridine ring. In the former case, isomer A is less stable as compared to isomer B, while in the latter case isomer Ais more stable. The possibility of selective synthesis of one of these isomers, namely, tris(η²-allyl)pyridineborane complex, is predicted.     

New synthetic routes to cationic arene π-complexes of transition metals

A review is given of the authors' studies on the development of new methods for the synthesis of cationic arene complexes of transition metals. The synthesis in strong protic acids facilitated obtaining -arene cations of Mn, Re, Ru, Os, Co, Rh and Ir. New nonacidic methods allowed the set of available cationic arene -complexes of Mn, Ru, Rh and Ir to be increased considerably. Based on the developed methods a number of previously unknown binuclear organometallic cations are obtained.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1186–1195, July, 1993.     

The band structures of some transition metals and their NaCl-type compounds

The band structures of Group IVB (Ti,Zr,Hf),VB (V,Nb,Ta) and VIB (Cr,Mo,W) transition metals and some of their carbides and nitrides (TiN,ZrN,HfN,VC,NbC,TaC,VN,NbN,TaN) with NaCl-type (Bl-type) structure have been calculated by using the tight-binding method within the Extended Hiickel approximation (EHT).The energy bands,densities of states and crystal orbital overlap populations are given.The relationship between the bonding properties and the superconducting transition temperatures (Tc) of them is discussed.The influences of various kinds of metallic atoms and changes of bond lengths on Tc are also discussed.     

Hindered amine light stabilisers—Interactions with transition and non-transition metals

Experiments have shown that hindered amine light stabilisers (HALS) are able to complex strongly with transition metal ions. During this reaction, in air, there is a marked tendency for oxidation of the metal ion species and the reaction is shown to be general across the first transition series. Using these results, in addition to evidence from other work, a scheme is proposed whereby these HALS, because of their complexing ability, will act as photo-oxidation stabilisers for polymers.Preliminary results show that HALS are also able to complex with non-transition metals.     

Can human prolidase enzyme use different metals for full catalytic activity?

The catalytic hydrolysis of the Gly-Pro substrate by the bimetallic prolidase active site model cluster has been investigated at the DF/B3LYP level of theory, in order to provide fundamental insights into the still poorly understood mechanism of prolidase catalysis. To date, the majority of prolidases exhibits metal-dependent activity, requiring two divalent cations such as Zn(2+), Mn(2+), or Co(2+) for maximal activity. In addition, it has been shown recently that two different metal ions in the active site of human prolidase (Zn and Mn) can coexist, with the protein remaining partially active. With the purpose of identifying which is the most efficient dimetallic center for the prolidase catalyzed reaction, Zn(II), Co(II), and Mn(II) have been examined as potential catalytic metals for this enzyme. Furthermore, to better elucidate the exact roles played by the metals occupying the site 1 and site 2 positions, the hetero-bimetallic active site having Zn and Mn cations has been also investigated, considering the two derivatives Mn1-Zn2 and Zn1-Mn2. The rate-determining step of the hydrolysis reaction is always found to be the nucleophilic addition of the hydroxide ion on the carbonyl carbon of the scissile peptide bond, followed by the less energetically demanding proline-peptide C-N bond scission. The analysis of the involved energy barriers does not indicate clearly a preference for a particular metal by the prolidase enzyme. Instead, we may point out a slightly better behavior of the cobalt-containing cluster as far as both tetrahedral formation and its decomposition are concerned, due to a greater degree of ligands-to-metals charge transfer. The mixed Mn-Zn hetero-dimetallic clusters appear to be also able to perform the hydrolysis of the Pro-Gly substrate, with a slight preference for the Mn1-Zn2 configuration.     

Acetyl-BINOL as mimic for chiral β-diketonates: a building block for new modular ligands

α-Acetyl-(S)-BINOL was prepared by ortho-lithiation and subsequent acetylation of acetal-protected (S)-BINOL. The β-hydroxyketone moiety of this compound is herein a structural mimic for a β-diketonate and forms six-membered chelates with transition metal ions. The second hydroxy-function was submitted to esterification with several carboxylic acids bearing another donor function, thus, new tridentate chiral ligands were obtained. Out of this library the l-proline-α-acetyl-(S)-BINOL-ester was identified to be most effective for the titanium-mediated addition of Et₂Zn to PhCHO yielding the respective secondary alcohol with up to 93% ee, which is better than with using (S)-BINOL itself. Besides a solvent dependency (use of MeCN is optimal), the proper choice of the counter-ion is crucial: anion exchange of bromide by trifluoroacetate gave a significant increase of enantioselectivity.     

Spectroscopic studies on a ‘turn-on’ fluorescent sensor for transition metals with selective ‘turn-off’ for mercury(II) ions

Spectroscopic studies on a dansyl-based fluorescent sensor N-(4-aminophenyl)-5-(dimethylamino)naphthalene-1-sulphonamide (1) and its interactions with transition metals are reported. The sensor's absorption and emission spectra exhibited different profiles at different polarities and pH values. Enhanced fluorescence was observed in the presence of chromium(III) ions in methanol, while the emission was quenched significantly in the presence of mercury(II) ions both in methanol and aqueous solution. The binding constants of 1 to chromium(III) in methanol and to mercury(II) ions in methanol and aqueous solution were calculated.     

Are metals made from molecules?

In the traditional view, covalently bound materials differ in a fundamental way from metallic substances. Though both are built from more basic units that are, in turn, constructed from a small number of atoms, for these two materials classes the nature of these units is thought to be quite different. For covalent solids and liquids, these units are considered to be molecular, meaning that they possess properties and bonding that are retained in the condensed phase and thus they continue to be identifiable within the larger system. For metallic materials, these basic units are considered to be mere constructs that are not observable against the delocalized bonding of metals or alloys. The perceived dissimilarity of metallic and covalently bound materials has fostered distinctly different approaches to their design and improvement. Here, the delocalized view of metallic bonding is examined. This examination suggests that much of the rationale used in the design of molecular materials my be applied to metals and alloys as well.     

Chiral furanoside phosphite–phosphoroamidites: new ligands for asymmetric catalytic hydroformylation

We have designed a new series of phosphite–phosphoroamidites ligands 1–4 based on a furanoside backbone. These ligands were screened in the Rh-catalyzed asymmetric hydroformylation of styrene, inducing high regioselectivities with 2-phenylpropanal and moderate enantioselectivities (up to 65% e.e.). The results showed that the configuration of the stereogenic carbon atom C(3) at the ligand backbone had remarkable effects on the activity and enantioselectivity. Replacing the tert-butyl substituents with methoxy substituents at the para positions of the biphenyl moieties improved the enantioselectivities. We have also studied the solution structures of HRh(PP)(CO)₂ complexes.     

Covalently bonded 1(infinity)[Pt]- chains in BaPt: extension of the Zintl-Klemm concept to anionic transition metals?

BaPt has been synthesized by reaction of a 1:1 mixture of Ba and Pt at 1223 K in argon and characterized by single-crystal X-ray structure determination and electrical resistivity and magnetic susceptibility measurements. The compound crystallizes in the NiAs structure type with an extremely low value for the c/a ratio (hexagonal space group P6(3)/mmc with a = 5.057(2) A, c = 5.420(3) A, c/a = 1.072, R1 = 0.0297, N(hkl)() = 93). This c/a ratio reflects structural features that are unusual for the NiAs type: Pt is coordinated linearly by two other Pt atoms at a distance as short as 2.71 A, thus forming a chain along [001] of homonuclearly bonded platinum. Band structure calculations and electron localization function analyses reveal a covalent character of Pt-Pt interactions along the c axis and two-dimensional metallic conductivity parallel to the ab plane. In terms of the Zintl-Klemm concept, in its most general sense, BaPt can be formulated as (Ba(2+)(1(infinity)[Pt]-) x e-), and it can be considered a new example of platinide anions in the solid state.     

1′-Phthalazine hydrazone of diacetyl monooxime and its complexes with transition metals: Physicochemical and theoretical study

1′-Phthalazine hydrazone of diacetyl monooxime and its Cu(II) and Ni(II) complexes were synthesized. The acid-base properties of the hydrazone were potentiometrically and spectrophotometrically studied, and quantum-chemical calculations of the ionization constants and the energies of possible tautomeric forms and conformations were performed. Complexes of copper(II) prepared by the reaction of the hydrazone with copper(II) acetate, chloride, and bromide were shown to have the dimeric structure with the bidentatebridging N-O-groups of the oxime fragment, whereas the copper(II) complex prepared from copper(II) nitrate, forms the dimer via the endocyclic nitrogen atoms of the phthalazine fragment.