New bimetallic complexes of late transition metals involving pyrazole-bridged bis N-heterocyclic carbene ligands

Bimetallic Ni, Rh, and Ir complexes of pyrazolate biscarbene containing bulky substituents have been synthesized and characterized by X-ray crystallography; the Ni complex dimerizes to a highly congested L(2)Ni(2) structure, whereas the corresponding Rh and Ir complexes form bimetallic LM(2) structures.     

Development of 7-membered N-heterocyclic carbene ligands for transition metals

We recently reported the first example of a seven-membered N-heterocyclic carbene (NHC) ligand for transition metals. These ligands are attractive because the heterocyclic framework, derived from 2,2′-diaminobiphenyl, exhibits a torsional twist that results in a chiral, C₂-symmetric structure. The present report outlines the synthetic efforts that led to the development of these ligands together with the synthesis and structural characterization of metal complexes bearing seven-membered NHCs as ancillary ligands. The identity of nitrogen substituent, neopentyl versus 2-adamantyl, influences the synthetic accessibility and stability of the seven-membered amidinium salts and the NHC–metal complexes obtained via in situ deprotonation/metallation. Computational analysis of the seven-membered ring structures reveals the Hückel antiaromatic 8π electron system achieves significant Möbius aromatic stabilization upon undergoing torsional distortion of the heterocyclic ring.     

"Click" 1,2,3-triazoles as tunable ligands for late transition metal complexes

The potential of "click" 1,2,3-triazoles to act as mono-dentate ligands in late transition metal complexes is explored relative to other commonly-used Lewis bases and it is shown that their coordination strength, determined by their 1- and 4-substituents, is easily tunable.     

A Schiff base expanded porphyrin macrocycle that acts as a versatile binucleating ligand for late first-row transition metals

The coordination chemistry of the Schiff base polypyrrolic octaaza macrocycle 1 toward late first-row transition metals was investigated. Binuclear complexes with the divalent cations Ni(II), Cu(II), and Zn(II) and with the monovalent cation Cu(I) were prepared and characterized. Air oxidation of the Cu(I) ions in the latter complex to their divalent oxidation state resulted in a change in the coordination mode relative to the macrocycle.     

Transmetalation of a nucleophilic carbene fragment: from early to late transition metals

Early transition metal nucleophilic carbene complexes have been used as stoichiometric carbene transfer agents in a transmetalation process.     

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.     

Transmetalation reactions from Fischer carbene complexes to late transition metals: a DFT study

Transmetalation reactions from chromium(0) Fischer carbene complexes to late-transition-metal complexes (palladium(0), copper(I), and rhodium(I)) have been studied computationally by density functional theory. The computational data were compared with the available experimental data. In this study, the different reaction pathways involving the different metal atoms have been compared with each other in terms of their activation barriers and reaction energies. Although the reaction profiles for the transmetalation reactions to palladium and copper are quite similar, the computed energy values indicate that the process involving palladium as catalyst is more favorable than that involving copper. In contrast to these transformations, which occur via triangular heterobimetallic species, the transmetalation reaction to rhodium leads to a new heterobimetallic species in which a carbonyl ligand is also transferred from the Fischer carbene to the rhodium catalyst. Moreover, the structure and bonding situation of the so far elusive heterobimetallic complexes are briefly discussed.     

Aryloxide-based multidentate ligands for early transition metals and f-element metals

This article presents an overview of the chemistry of early transition metal and f-element complexes stabilized by aryloxide-based multidentate ligands. Preparations and reactivity studies of these compounds are discussed. The presence of the bridging units in this ligand system imposes a strong geometry constraint to the aryloxide groups, which leads the way to novel patterns of structure and reactivity.     

Palladium complexes with abnormal N-heterocyclic carbene ligands derived from 1,2,3-triazolium ions and their application in Suzuki coupling

Chiral and achiral pincer type palladium complexes bearing abnormal N-heterocyclic carbene ligands derived from 1,2,3-triazole are reported. Both complexes are effective as catalysts for non-asymmetric Suzuki coupling reaction for the synthesis of biphenyl derivatives. However, both the complexes failed to catalyze the formation of binaphthyl derivatives, by the asymmetric Suzuki coupling in case of the chiral complex and the non-asymmetric version in case of the achiral pincer complex. Instead deboronation of arylboronic acid occurred very efficiently leaving the aryl bromide intact.     

Rhodium carbene complexes as versatile catalyst precursors for Si-H bond activation

Rhodium(III) complexes comprising monoanionic C,C,C-tridentate dicarbene ligands activate Si-H bonds and catalyse the hydrolysis of hydrosilanes to form silanols and siloxanes with concomitant release of H(2). In dry MeNO(2), selective formation of siloxanes takes place, while changing conditions to wet THF produces silanols exclusively. Silyl ethers are formed when ROH is used as substrate, thus providing a mild route towards the protection of alcohols with H(2) as the only by-product. With alkynes, comparably fast hydrosilylation takes place, while carbonyl groups are unaffected. Further expansion of the Si-H bond activation to dihydrosilanes afforded silicones and polysilyl ethers. Mechanistic investigations using deuterated silane revealed deuterium incorporation into the abnormal carbene ligand and thus suggests a ligand-assisted mechanism involving heterolytic Si-H bond cleavage.     

Synthesis of heterocyclic carbene ligands via 1,2,3-diheterocyclization of allenylidene complexes with dinucleophiles

Heterocyclic carbene complexes are accessible from π-donor-substituted allenylidene complexes, [(CO)₅CrCCC(NMe₂)Ph] (1) and [(CO)₅CrCCC(O-endo-Bornyl)OEt] (4), and various dinucleophiles by 1,2,3-diheterocyclization. The reaction of 1 with 1,2-dimethylhydrazine gives the 1,2-dimethylpyrazolylidene complex (2) in high yield in addition to small amounts of the α,β-unsaturated carbene complex [(CO)₅CrC(NMe₂)-C(H)C(NMe₂)Ph] (3). The analogous reaction of 4 with 1,2-dimethylhydrazine affords the 1,2-dimethylpyrazolylidene complex (5) and, via displacement of the C_γ-bound ethoxy substituent, the hydrazinoallenylidene complex [(CO)₅CrCCC(O-endo-Bornyl){NMe-N(H)Me}] (6). Treatment of 6 with catalytic amounts of acids induces cyclization to 5. On addition of 1,1-dimethylhydrazine to 1 the zwitterionic pyrazolium-5-ylidene complex (7) is formed. The reaction of 1 with 1,2-diaminocyclohexane affords a octahydro-benzo[1,4]diazepinylidene complex (10) and, via intermolecular substitution, a binuclear bisallenylidene complex (11). Thiazepinylidene complexes (12-14), containing 7-membered N/S-heterocyclic carbene ligands, are formed highly selectively in the reaction of 1 with 2-aminoethanethiol or related cysteine derivatives by a substitution/cyclization sequence. The analogous reaction of 1 with homocysteine methylester yields a thiazocanylidene complex (15). All new heterocyclic carbene ligands are strong donors exhibiting σ-donor/π-acceptor ratios similar to those of the known imidazolylidene complexes. On photolysis of 2 and 12 in the presence of triphenylphosphine, the corresponding cis-carbene tetracarbonyl triphenylphosphine complexes (16 and 17) are formed. The solid state structure of complexes 2, 7, 14, 15, and 16 is established by X-ray structural analysis.     

1,2,4-Triazolium-5-ylidene and 1,2,4-triazol-3,5-diylidene as new ligands for transition metals

Ab initio calculations show the 1,2,4-triazolium-5-ylidene (3a) and 1,2,4-triazol-3,5-diylidene (4a) are true minima on the potential surface. As expected, 4a is much higher in energy than its triazole isomers 5a, 6a and 8a and the 1,2,4-triazol-3-ylidene (7a). Sodium methoxyde adds to the diquaternary salt of 1,2,4-triazoles (9b,c) to give the corresponding monocationic heterocycles 10b,c in 70 and 50% yield, respectively. One equivalent of silver(I) acetate reacts with 9b leading to the bis(1,2,4-triazolium-5-ylidene)silver(I) complex (11b) in 80% yield. Under the same experimental conditions, but using two equivalents of silver(I) acetate, solid-state one-dimensional polymers 12′b,c featuring the coplanar 1,2,4-triazol-3,5-diylidene ligands coordinated to silver(I) are obtained in 90% yield. 12′c has been fully characterized including a single-crystal X-ray diffraction study.     

Olefin oligomerization,homopolymerization and copolymerization by late transition metals supported by (imino)pyridine ligands

In this review, the key advances achieved over more than 10 years on the design and development of (imino)pyridyl transition metal complexes as catalyst precursors for the transformation of ethylene, higher α-olefins and cyclic olefins into either linear/branched homopolymers or oligomers are highlighted. Particular attention has been paid to the relationships between the catalytic activity exhibited by the catalysts and their electronic and geometrical structure.     

Oxazolines as chiral building blocks for imidazolium salts and N-heterocyclic carbene ligands

Enantiomerically pure imidazolium triflates can be readily prepared from bioxazolines and oxazolineimines; deprotonation of imidazolium triflate 2 gives a chiral N-heterocyclic carbene that can act as a ligand in a catalytically active palladium complex.     

Designing catalysts for nitrene transfer using early transition metals and redox-active ligands

In this Forum Article, we discuss the use of redox-active pincer-type ligands to enable multielectron reactivity, specifically nitrene group transfer, at the electron-poor metals tantalum and zirconium. Two analogous ligand platforms, [ONO] and [NNN], are discussed with a detailed examination of their similarities and differences and the structural and electronic constraints they impose upon coordination to early transition metals. The two-electron redox capabilities of these ligands enable the transfer of organic nitrenes to tantalum(V) and zirconium(IV) metal centers despite formal d(0) electron counts. Under the correct conditions, the resulting metal imido complexes can participate in further multielectron reactions such as imide reduction, nitrene coupling, or formal nitrene transfer to an isocyanide.     

Polymer derived non-oxide ceramics modified with late transition metals

This tutorial review highlights the methods for the preparation of metal modified precursor derived ceramics (PDCs) and concentrates on the rare non-oxide systems enhanced with late transition metals. In addition to the main synthetic strategies for modified SiC and SiCN ceramics, an overview of the morphologies, structures and compositions of both, ceramic materials and metal (nano) particles, is presented. Potential magnetic and catalytic applications have been discussed for the so manufactured metal containing non-oxide ceramics.     

Benzene-o-dithiolate ligands as versatile building blocks in supramolecular chemistry

Polydentate ligands with benzene-o-dithiolato donor groups are useful building blocks in supramolecular coordination chemistry. The coordination chemistry of bis- and tris(benzene-o-dithiolato) ligands and mixed benzene-o-dithiolato/catecholato ligands is reviewed. These ligands exhibit a versatile coordination chemistry both in solution and in the solid state.     

Smooth C(alkyl)-H bond activation in rhodium complexes comprising abnormal carbene ligands

Rhodation of trimethylene-bridged diimidazolium salts induces the intramolecular activation of an alkane-type C-H bond and yields mono- and dimetallic complexes containing a formally monoanionic C,C,C-tridentate dicarbene ligand bound to each rhodium centre. Mechanistic investigation of the C(alkyl)-H bond activation revealed a significant rate enhancement when the carbene ligands are bound to the rhodium centre via C4 (instantaneous activation) as compared to C2-bound carbene homologues (activation incomplete after 2 days). The slow C-H activation in normal C2-bound carbene complexes allowed intermediates to be isolated and suggests a critical role of acetate in mediating the bond activation process. Computational modelling supported by spectroscopic analyses indicate that halide dissociation as well as formation of the agostic intermediate is substantially favoured with C4-bound carbenes. It is these processes that discriminate the C4- and C2-bound systems rather than the subsequent C-H bond activation, where the computed barriers are very similar in each case. The tridentate dicarbene ligand undergoes selective H/D exchange at the C5 position of the C4-bound carbene exclusively. A mechanism has been proposed for this process, which is based on the electronic separation of the abnormal carbene ligand into a cationic N-C-N amidinium unit and a metalla-allyl type M-C-C fragment.