Transition metals as Lewis bases: "Z-type" boron ligands and metal-to-boron dative bonding

While the vast majority of inorganic chemistry involves electron donation from main-group atoms to metals, an intriguing yet flip-side exists: where Lewis-basic metals donate electron density to Lewis-acidic main-group atoms (most often boron). These so-called "Z-type" ligands, along with other less clear-cut complexes, are examples of this metal-ligand role reversal. This perspective article offers an introduction to metal-to-boron dative bonding, and attempts to correlate spectroscopic and structural data from the complexes.     

Synthesis and characterisation of polymeric materials consisting of {Fe2(CO)5}-unit and their relevance to the diiron sub-unit of [FeFe]-hydrogenase

By using "click" chemistry between a diazide and a diiron model complex armed with two alkynyl groups, two polymeric diiron complexes (Poly-Py and Poly-Ph) were prepared. The two polymeric complexes were investigated using infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), M?ssbauer spectroscopy, and cyclic voltammetry (Poly-Py only, due to the insolubility of Poly-Ph). To probe the coordinating mode of the diiron units in the two polymeric complexes, two control complexes (3 and 4) were also synthesised using a monoazide. Complexes 3 and 4 were well characterised and the latter was further crystallographically analysed. It turns out that in both complexes (3 and 4) and the two polymeric diiron complexes, one of the two iron atoms in the diiron unit coordinates with one of the triazole N atoms. Our results revealed that both morphologies and properties of Poly-Py and Poly-Ph are significantly affected by the organic moiety of the diazide. Compared to the protonating behaviour of the complexes 3 and 4, Poly-Py exhibited proton resistance. In electrochemical reduction, potentials for the reduction of the diiron units in Poly-Py and hence its catalytic reduction of proton in acetic acid-DMF shifted by over 400 mV compared to those for complexes 3 and 4. It is likely that the polymeric nature of Poly-Py offers the diiron units a "protective" environment in an acidic medium and more positive reduction potential.     

Scope, limitations and mechanistic aspects of the photo-induced cross-linking of proteins by water-soluble metal complexes

BACKGROUND: Chemical cross-linking is a valuable tool with which to study protein-protein interactions. Recently, a new kind of cross-linking reaction was developed in which the photolysis of associated proteins with visible light in the presence of ammonium persulfate and tris(2,2'-bipyridyl)ruthenium(II) dication or palladium(II) porphyrins results in rapid and efficient covalent coupling (Fancy, D.A. & Kodadek, T. (1999). Proc. Natl. Acad. Sci. USA 96, 6020-6024 and Kim, K., Fancy, D.A. & Kodadek, T. (1999). J. Am. Chem. Soc. 121, 11896-11897). Here, mechanistic and practical aspects of the reaction of importance for its application to biochemical problems are examined. RESULTS: It is shown that the photo-initiated cross-linking chemistry can be optimized for the analysis of protein-protein interactions in crude cell extracts. A number of commonly used epitope or affinity tags survive the reaction in functional form, allowing the simple visualization of the cross-linked products, or their isolation. It is shown that very little light-independent oxidation of protein residues occurs and that significant perturbation of complexes of interest prior to the brief photolysis period does not occur. Finally, evidence is presented that is consistent with a mechanistic model in which ammonium persulfate functions simply as an electron acceptor, facilitating the generation of the key high valent metal complex from the photoexcited species by electron transfer. In the absence of an electron acceptor, a much lower efficiency reaction is observed that appears to involve products resulting from reaction of the excited state metal complex with molecular oxygen. CONCLUSIONS: These results provide useful practical information for chemists and biochemists who may wish to employ this new cross-linking chemistry for the analysis of protein complexes. They also shed new light on the mechanism of this interesting reaction.     

Organometallic manganese complexes as scaffolds for potential molecular wires

This article reviews recent work in the area of organomanganese chemistry designing organometallic based molecular wires for potential applications in molecular electronics utilising the bottom-up approach. The field of molecular electronics has recently received much attention in the pursuit of continued miniaturization of electronics. Molecular wires that can allow a through-bridge exchange of an electron/electron hole between its remote ends/terminal groups are the basic motifs of single electron devices. Our recent work in this field has been the design and development of transition-metal complexes with a special emphasis on the half sandwich dinuclear manganese complexes and the bis dmpe dinuclear Mn(II)/Mn(II). In this review, we would like to highlight the importance of the nature of the transition metal and their significant effect on the redox process, which is of paramount importance for the design of systems that could be ultimately wired into circuits for various applications.     

Electronic structure of linear thiophenolate-bridged heteronuclear complexes [LFeMFeL](n)(+) (M = Cr, Co, Fe; n = 1-3): a combination of kinetic exchange interaction and electron delocalization

The electronic properties of the isostructural series of heterotrinuclear thiophenolate-bridged complexes of the general formula [LFeMFeL](n)(+) with M = Cr, Co and Fe where L represents the trianionic form of the ligand 1,4,7-tris(4-tertbutyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, synthesized and investigated by a number of experimental techniques in the previous work(1), are subjected now to a theoretical analysis. The low-lying electronic excitations in these compounds are described within a minimal model supported by experiment and quantum chemistry calculations. It was found indeed that various experimental data concerning the magnetism and electron delocalization in the lowest states of all seven compounds are completely reproduced within a model which includes the electron transfer between magnetic orbitals at different metal centers and the electron repulsion in these orbitals (the Hubbard model). Moreover, due to the trigonal symmetry of the complexes, only the electron transfer between nondegenerate orbital, a(1), originating from the t(2g) shell of each metal ion in a pseudo-octahedral coordination, is relevant for the lowest states. An essential feature resulting from quantum chemistry calculations, allowing to explain the unusual magnetic properties of these compounds, is the surprisingly large value and, especially, the negative sign of the electron transfer between terminal iron ions, beta'. According to their electronic properties the series of complexes can be divided as follows: (1). The complexes [LFeFeFeL](3+) and [LFeCrFeL](3+) show localized valences in the ground electronic configuration. The strong antiferromagnetic exchange interaction and the resulting spin 1/2 of the ground-state arise from large values of the transfer parameters. (2). In the complex [LFeCrFeL](+), due to a higher energy of the magnetic orbital on the central Cr ion than on the terminal Fe ones, the spin 3/2 and the single unpaired a(1) electron are almost localized at the chromium center in the ground state. (3). The complex [LFeCoFeL](3+) has one ground electronic configuration in which two unpaired electrons are localized at terminal iron ions. The ground-state spin S = 1 arises from a kinetic mechanism involving the electron transfer between terminal iron ions as one of the steps. Such a mechanism, leading to a strong ferromagnetic interaction between distant spins, apparently has not been discussed before. (4). The complex [LFeFeFeL](2+) is characterized by both spin and charge degrees of freedom in the ground manifold. The stabilization of the total spin zero or one of the itinerant electrons depends on beta', i.e., corresponds to the observed S = 1 for its negative sign. This behavior does not fit into the double exchange model. (5). In [LFeCrFeL](2+) the delocalization of two itinerant holes in a(1) orbitals takes place over the magnetic core of chromium ion. Although the origin of the ground-state spin S = 2 is the spin dependent delocalization, the spectrum of the low-lying electronic states is again not of a double exchange type. (6). Finally, the complex [LFeCoFeL](2+) has the ground configuration corresponding to the electron delocalization between terminal iron atoms. The estimated magnitude of the corresponding electron transfer is smaller than the relaxation energy of the nuclear distortions induced by the electron localization at one of the centers, leading to vibronic valence trapping observed in this compound.     

Henry Taube: inorganic chemist extraordinaire

The numerous innovative contributions of Henry Taube to modern inorganic chemistry are briefly reviewed. Highlights include the determination of solvation numbers and lability, elucidation of substitution mechanisms, discovery and documentation of inner-sphere electron transfer, and discovery of the remarkable coordination chemistry of ruthenium and osmium ammine complexes with unsaturated ligands and mixed-valence complexes and their fundamental relationship to intramolecular electron transfer.     

The first complexes and cyclodimerisations of methylphosphaalkyne (P[triple bond]CMe)

The first complexes and cyclodimerisations of methylphosphaalkyne, P[triple bond]CMe, are reported to arise from its reactions with a range of platinum(0) complexes and [W(CO)5(THF)]. A number of differences between the chemistry of this phosphaalkyne and that of its bulkier analogues have been highlighted and explained on steric grounds.     

One‐Electron Bonds in Frustrated Lewis Pair TPB Ligands: Boron Behaving as a Lewis Base

Owing to its versatility in synthetic chemistry, TPB (tris[2‐diisopropylphospino)phenyl]borane) is a very important frustrated Lewis Pair. The unusual stability of the neutral radical (TPB)Cu has been related to the presence of a one‐electron B−Cu bond. Herein we show, through the use of different computational chemistry methods, that the existence and nature of this kind of A⋅⋅⋅M bond (A=donor atom, M=transition metal) depends on the surrounding chemical structure, and can be genuine one‐electron sigma bonds only if appropriate metal ligands (Y), able to trap the charge in the desired region, are chosen. This ability is modulated by the subtle balance between the electronegativity of the different atoms along the A⋅⋅⋅M⋅⋅⋅Y bond paths. Most importantly, contrary to many TPB complexes in which boron acts as a Lewis acid, in one‐electron‐bond‐containing structures boron behaves as a Lewis base.     

Dimeric Rare‐Earth BINOLate Complexes: Activation of 1,4‐Benzoquinone through Lewis Acid Promoted Potential Shifts

Reaction of p‐benzoquinone (BQ) with a series of rare‐earth metal/alkali metal/1,1′‐BINOLate (REMB) complexes (RE: La, Ce, Pr, Nd; M: Li) results in the largest recorded shift in reduction potential observed for BQ upon complexation. In the case of cerium, the formation of a 2:1 Ce/BQ complex shifts the two‐electron reduction of BQ by greater than or equal to 1.6 V to a more favorable potential. Reactivity investigations were extended to other RE^III (RE=La, Pr, Nd) complexes where the resulting highly electron‐deficient quinone ligands afforded isolation of the first lanthanide quinhydrone‐type charge‐transfer complexes. The large reduction‐potential shift associated with the formation of 2:1 Ce/BQ complexes illustrate the potential of Ce complexes to function both as a Lewis acid and an electron source in redox chemistry and organic‐substrate activation.     

Expanding the chemistry of cationic N-heterocyclic carbenes: alternative synthesis, reactivity, and coordination chemistry

A new synthetic route to complexes of the cationic N-heterocyclic carbene ligand 2 has been developed by the attachment of a cationic pentamethylcyclopentadienylruthenium ([RuCp*](+)) fragment to a metal-coordinated benzimidazol-2-ylidene ligand. The coordination chemistry and the steric and electronic properties of the cationic carbene were investigated in detail by experimental and theoretical methods. X-ray structures of three carbene-metal complexes were determined. The cationic ligand 2 is a poorer overall electron donor relative to the related neutral carbene, which is evident from cyclic voltammetry (CV) and IR measurements.     

The Formazanate Ligand as an Electron Reservoir: Bis(Formazanate) Zinc Complexes Isolated in Three Redox States

The synthesis of bis(formazanate) zinc complexes is described. These complexes have well‐behaved redox‐chemistry, with the ligands functioning as a reversible electron reservoir. This allows the synthesis of bis(formazanate) zinc compounds in three redox states in which the formazanate ligands are reduced to “metallaverdazyl” radicals. The stability of these ligand‐based radicals is a result of the delocalization of the unpaired electron over four nitrogen atoms in the ligand backbone. The neutral, anionic, and dianionic compounds (L₂Zn^0/?1/?2) were fully characterized by single‐crystal X‐ray crystallography, spectroscopic methods, and DFT calculations. In these complexes, the structural features of the formazanate ligands are very similar to well‐known β‐diketiminates, but the nitrogen‐rich (NNCNN) backbone of formazanates opens the door to redox‐chemistry that is otherwise not easily accessible.     

Silicon surfaces as electron acceptors: dative bonding of amines with Si(001) and Si(111) surfaces.

The bonding of the trimethylamine (TMA) and dimethylamine (DMA) with crystalline silicon surfaces has been investigated using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and density-functional computational methods. XPS spectra show that TMA forms stable dative-bonded adducts on both Si(001) and Si(111) surfaces that are characterized by very high N(1s) binding energies of 402.2 eV on Si(001) and 402.4 eV on Si(111). The highly ionic nature of these adducts is further evidenced by comparison with other charge-transfer complexes and through computational chemistry studies. The ability to form these highly ionic charge-transfer complexes between TMA and silicon surfaces stems from the ability to delocalize the donated electron density between different types of chemically distinct atoms within the surface unit cells. Corresponding studies of DMA on Si(001) show only dissociative adsorption via cleavage of the N-H bond. These results show that the unique geometric structures present on silicon surfaces permit silicon atoms to act as excellent electron acceptors.     

Polarity matching of radical trapping: high yielding 3-exo and 4-exo cyclizations

A catalyzed synthesis of cyclopropanes and cyclobutanes via radical chemistry is described. The method that generally proceeds in high yields uses epoxides as radical precursors and titanocene(III) complexes as the electron transfer catalysts (see scheme). The key to the success of the transformation is constituted by the chemoselectivity of radical reduction. Electrophilic enol radicals generated through cyclization are reduced rapidly whereas their precursors, nucleophilic alkyl radicals, remain unaffected.     

Boron(II) Cations: Interplay between Lewis‐Pair‐Acceptor and Electron‐Donor Properties

Boron(III) cations are widely used as highly Lewis acidic reagents in synthetic chemistry. In contrast, boron(II) cations are extremely rare and their chemistry almost completely unknown. They are both Lewis acids and electron donors, properties that are commonly associated with catalytically active late‐transition‐metal complexes. This double reactivity pattern ensures a rich and diverse chemistry. Herein we report the facile synthesis of several new boron(II) cations starting with a special diborane with two easily exchangeable triflate substituents. By increasing the π‐acceptor character of the neutral σ‐donor reaction partners, first reactions were developed in which the combined Lewis acidity and electron‐donor properties of boron(II) cations are applied for the reduction of organic molecules. The results of our study pave the way for applications of these unusual compounds in synthetic chemistry.     

Complexes and clusters of water relevant to atmospheric chemistry: H2O complexes with oxidants.

Experimental observations and data from quantum chemical calculations on complexes between water molecules and small, oxygen-containing inorganic species that play an important role as oxidants in the atmosphere (O(1D), O(3P), O2(X3sigmag), O2(b1sigmag+), O3, HO, HOO, HOOO, and H2O2) are reviewed, with emphasis on their structure, hydrogen bonding, interaction energies, thermodynamic parameters, and infrared spectra. In recent years, weakly bound complexes containing water have increasingly attracted scientific attention. Water in all its phases is a major player in the absorption of solar and terrestrial radiation. Thus, complexes between water and other atmospheric species may have a perceivable influence on the radiative balance and contribute to the greenhouse effect, even though their concentrations are low. In addition, they can play an important role in the chemistry of the Earth's atmosphere, particularly in the oxidation of trace gases. Apart from gas-phase complexes, the interactions of oxidants with ice surfaces have also received considerable advertency lately due to their importance in the chemistry of snow, ice clouds, and ice surfaces (e.g., ice shields in polar regions). In paleoclimate--respectively paleoenvironmental--studies, it is essential to understand the transfer processes from the atmosphere to the ice surface. Consequently, special attention is being paid here to the intercomparison of the properties of binary complexes and the complexes and clusters of more complicated compositions, including oxidants adsorbed on ice surfaces, where ice is considered a kind of large water cluster. Various facts concerning the chemistry of the Earth's atmosphere (concentration profiles and possible influence on radical reactions in the atmosphere) are discussed.     

A novel manganese(III)-peroxo complex bearing a proline-derived pentadentate aminobenzimidazole ligand

A novel nonheme manganese(III)-peroxo complex bearing a proline-derived pentadentate aminobenzimidazole ligand was synthesized and spectroscopically characterized, and its reactivity in aldehyde deformylation was investigated.     

The vitality of uranium molecular chemistry at the dawn of the XXIst century

The intent of this Dalton Perspective is to highlight the recent advances in uranium molecular chemistry, with the results reported during the 2000-2006 period. This discipline is currently witnessing an impressive development, together with the theoretical chemistry and solid-state chemistry of the f-elements, and its face has profoundly changed, revealing unsuspected structural and reactivity features. This progress required and was facilitated by the use of new precursors. Studies of low-valent compounds gave a better insight into lanthanide(III)/actinide(III) differentiation and led to the discovery of unusual reactions, including activation of small molecules. A number of tetravalent uranium complexes, in particular polynuclear compounds, have been synthesized, which exhibit exciting structures and physicochemical properties. The potential of uranium(III) and uranium(IV) complexes in catalysis has been confirmed. The uranyl complexes, from mononuclear species to supramolecular assemblies, reveal a variety of novel structures, changing the generally accepted ideas on the coordination geometry and the stability of the UO2(2+) ion.     

电子转移光氧化反应在有机合成中的应用

电子转移光氧化反应与光敏化的单重态氧反应是光氧化反应的两个最重要的组成部分。电子转移光氧化是随着光诱导电子转移反应研究的广泛开展而得以迅速发展的。近年来,与光诱导电子转移反应有密切关联的过渡金属配合物的可见光催化反应也已成为研究热点。一些过渡金属配合物催化的电子转移光氧化反应也已出现。本文根据电子转移光氧化反应的不同机理,对这些反应进行分类,介绍了不同类型的电子转移敏化剂（包括氰基芳烃类光敏剂、鎓盐类阳离子光敏剂、过渡金属配合物类光敏剂以及有机染料类光敏剂）引发的电子转移光氧化反应,并讨论了各类电子转移光氧化反应中底物的各种活性中间体、反应中氧的活性形式、可能的反应途径以及在有机合成中的应用。     

New Transition Metal Clusters with Ligands from Main Groups Five and Six

In both physics and chemistry, increased attention is being paid to metal clusters. One reason for this attitude is furnished by the surprising results that have been obtained from studies of the preparation, structural characterization and physical and chemical properties of the clusters. Whereas investigations of cluster reactivity are at present generally limited to three- or four-membered clusters, successful syntheses of clusters with many more metal atoms have recently been designed. These substances occupy an intermediate position between solid state chemistry and the chemistry of metal complexes. This review presents a versatile method for synthesizing metal clusters: the reaction of complexes of transition metal halides with silylated compounds such as E(SiMe₃)₂ (E = S, Se, Te) and E′R(SiMe₃)₂ (R = Ph, Me, Et; E′ = P, As, Sb). Although some of the compounds thus formed have already been prepared by other routes, the method affords ready access to both small and large transition metal clusters with unusual structures and valence electron concentrations; a variety of reactions in the ligand sphere are also possible.     

Theoretical characterization of the gas-phase O(3)HO hydrogen-bonded complex.

We report a theoretical study on two gas-phase hydrogen-bonded complexes formed between ozone and hydroxyl radical that have relevance to atmospheric chemistry. This study was carried out by using CASSCF, CASPT2, QCISD, and CCSD(T) theoretical approaches in conjunction with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. Both complexes have a planar structure and differ from each other in the orientation of the electronic density of the unpaired electron associated with the HO radical moiety. Our calculations predict their stabilities to be 0.87 and 0.67 kcal mol(-1), respectively, at 0 K and show the importance of anharmonic effects in computing the red shift of the HO stretch originating from the hydrogen-bonding interaction. We also report two transition states involving the movement of the HO moiety on the potential energy surfaces of these hydrogen-bonded complexes.