Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ‐donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus‐based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon‐based ligands (which are not NHCs), and which feature even stronger σ‐donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.     

pi-Conjugated chelating polymers with charged iridium complexes in the backbones: synthesis, characterization, energy transfer, and electrochemical properties

A series of pi-conjugated chelating polymers with charged iridium (Ir) complexes in the backbones were synthesized by a Suzuki polycondensation reaction, leading to homogeneous polymeric materials that phosphoresce red light. The fluorene and bipyridine (bpy) segments were used as polymer backbones. 5,5'-Dibromobipyridine served as a ligand to form a charged iridium complex monomer with 1-(9'9-dioctylfluorene-2-yl)isoquinoline (Fiq) as the cyclometalated ligand. Chemical and photophysical characterization confirmed that Ir complexes were incorporated into the backbones as one of the repeat units by means of the 5,5'-dibromobipyridine ligand. Chelating polymers showed almost complete energy transfer from the host fluorene segments to the guest Ir complexes in the solid state when the feed ratio was 2 mol %. In the films of the corresponding blend system, however, energy transfer was not complete even when the content of Ir complexes was as high as 16 mol %. Both intra- and intermolecular energy-transfer processes existed in this host-guest system, and the intramolecular energy transfer was a more efficient process. All chelating polymers displayed good thermal stability, redox reversibility, and film formation. These chelating polymers also showed more efficient energy transfer than the corresponding blended system and the mechanism of incorporation of the charged Ir complexes into the pi-conjugated polymer backbones efficiently avoided the intrinsic problems associated with the blend system, thus offering promise in optoelectronic applications.     

Synthesis of Carbene-Metal-Amido (CMA) Complexes and Their Use as Precatalysts for the Activator-Free,Gold-Catalyzed Addition of Carboxylic Acids to Alkynes

A straightforward synthetic protocol leading to carbene–metal–amido (CMA) complexes (metal=Au, Cu) using a mild base and an environmentally desirable solvent (EtOH) has been explored, with a focus on complexes bearing backbone-substituted N-heterocyclic carbene (NHC) ligands, including BIAN-NHCs (BIAN=bis(imino)acenaphthene). The novel CMAs were structurally characterized, and gold-based CMAs bearing diverse NHCs were screened as simple, Brønsted-basic precatalysts. The readily accessible complexes display high catalytic activity in the intermolecular and intramolecular hydrocarboxylation of internal alkynes and alkynoic acids respectively, while the screening reveals the ancillary ligand effect of NHCs in these catalytic systems.     

Isoquinolin-1-ylidenes as electronically tuneable ligands

The novel isoquinolin-1-ylidene ligands, introduced into Rh(I) complexes by exploiting the carbene-like reactivity of adducts , exhibit ligand properties similar to those of classic NHCs, and their electronic properties can be tuned by the introduction of electron-withdrawing or donating groups in the benzene ring.     

Synthesis of monodentate bis(N-heterocyclic carbene) complexes of iridium: Mixed complexes of abnormal NHCs, normal NHCs, and triazole NHCs

A stepwise synthesis of mixed monodentate bis-NHC complexes of Ir(I), employing Ag(I)NHC complexes as transfer agents, yields complexes with two monodentate NHCs having different steric and electronic characteristics. The crystal structure of the mixed complex (5) with both a triazole-derived NHC ligand and an imidazole-derived NHC ligand is reported and both the NHC ring geometry and the M-NHC bond lengths are similar to related complexes. The complexes maintain their integrity over time and do not disproportionate, consistent with the NHC ligands not being labile.     

Geometric and Electronic Structures of Boron(III)‐Cored Dyes Tailored by Incorporation of Heteroatoms into Ligands

Complexation of a boron atom with a series of bidentate heterocyclic ligands successfully gives rise to corresponding BF₂‐chelated heteroarenes, which could be considered as novel boron(III)‐cored dyes. These dye molecules exhibit planar structures and expanded π‐conjugated backbones due to the locked conformation with a boron center. The geometric and electronic structures of these BF₂ complexes can be tailored by embedding heteroatoms in the unique modes to form positional isomer and isoelectronic structures. The structure–property relationship is further elucidated by studying the photophysical properties, electrochemical behavior and quantum‐chemical calculations.     

A simple 1H NMR method for determining the σ-donor properties of N-heterocyclic carbenes

The σ-donor properties of NHC ligands (NHC?=?N-heterocyclic carbene) are crucial in controlling their interaction with transition metals, and as a consequence, to determine the selectivity and reactivity of NHCs in transition-metal-catalysis. Herein, we report a simple NMR method for estimating the σ-donor properties of NHC ligands based on a straightforward ¹H NMR measurement of ligand precursors. We present evaluation of σ-donating properties for a range of NHC ligands varied by structure and electronics that are relevant to transition-metal-catalysis. We expect that the simple measurement of σ-donating properties of NHCs, together with the known methods for evaluating sterics and π-backbonding, will enhance the understanding of the properties of NHCs in transition-metal-catalysis.     

Frontispiece: BIAN-NHC Ligands in Transition-Metal-Catalysis: A Perfect Union of Sterically Encumbered,Electronically Tunable N-Heterocyclic Carbenes?

The discovery of NHCs (NHC = N-heterocyclic carbenes) as ancillary ligands in transition-metal-catalysis ranks as one of the most important developments in synthesis and catalysis. It is now well-recognized that the strong σ-donating properties of NHCs along with the ease of scaffold modification and a steric shielding of the N-wingtip substituents around the metal center enable dramatic improvements in catalytic processes, including the discovery of reactions that are not possible using other ancillary ligands. In this context, although the classical NHCs based on imidazolylidene and imidazolinylidene ring systems are now well-established, recently tremendous progress has been made in the development and catalytic applications of BIAN-NHC (BIAN = bis(imino)acenaphthene) class of ligands. The enhanced reactivity of BIAN-NHCs is a direct result of the combination of electronic and steric properties that collectively allow for a major expansion of the scope of catalytic processes that can be accomplished using NHCs. BIAN-NHC ligands take advantage of (1) the stronger σ-donation, (2) lower lying LUMO orbitals, (3) the presence of an extended π-system, (4) the rigid backbone that pushes the N-wingtip substituents closer to the metal center by buttressing effect, thus resulting in a significantly improved control of the catalytic center and enhanced air-stability of BIAN-NHC-metal complexes at low oxidation state. Acenaphthoquinone as a precursor enables facile scaffold modification, including for the first time the high yielding synthesis of unsymmetrical NHCs with unique catalytic properties. Overall, this results in a highly attractive, easily accessible class of ligands that bring major advances and emerge as a leading practical alternative to classical NHCs in various aspects of catalysis, cross-coupling and C−H activation endeavors.     

Hybrid ligands with N-heterocyclic carbene and chiral phosphaferrocene components

N-Heterocyclic carbenes (NHCs) possessing one or two 3,4-dimethylphosphaferrocenyl substituents and either methylene or ethylene alkyl bridges have been prepared. These carbenes turned out to be remarkably stable and were characterized by NMR methods and partly by mass spectrometry. Their molybdenum and ruthenium complexes were examined in order to determine the electronic properties and the coordination behaviour of these chiral PC- and PCP-chelate ligands, which combine a NHC unit as a strong sigma-donor with pi-accepting phosphaferrocene moieties. Crystal structures of one ligand precursor and of three complexes have been determined.     

Ligand topology variations and the importance of ligand field strength in non-heme iron catalyzed oxidations of alkanes

A series of iron(II)-bis(triflate) complexes [Fe(L)(OTf)2] containing linear tetradentate bis(quinolyl)-diamine and bis(quinolylmethyl)-diamine ligands with a range of ligand backbones has been prepared. The coordination geometries of these complexes have been investigated in the solid state by X-ray crystallography and in solution by 1H and 19F NMR spectroscopy. Because of the labile nature of high-spin iron(II) complexes in solution, dynamic equilibria of complexes with different coordination geometries (cis-alpha, cis-beta, and trans) are observed with certain ligand systems. In these cases, the geometry observed in the solid-state does not necessarily represent the only or even the major geometry present in solution. The ligand field strength in the various complexes has been investigated by variable-temperature (VT) magnetic moment measurements and by UV-vis spectroscopy. The strongest ligand field is observed with the most rigid ligand that generates [Fe(L)(OTf)2] complexes with a cis-alpha coordination geometry, and the corresponding [Fe(L)(CH3CN)2]2+ complex displays spin crossover behavior. The catalytic properties of the complexes for the oxidation of cyclohexane have been investigated using hydrogen peroxide as the oxidant. An increased flexibility in the ligand results in a weaker ligand field, which increases the lability of the complexes. The activity and selectivity of the catalysts appear to be related to the strength of the ligand field and the stability of the catalyst.     

Synthesis and antioxidant, antimicrobial evaluation, DFT studies of novel metal complexes derivate from Schiff base

In Iraq, like most developing countries, attempts are being made to synthesize new compounds with several pharmacological properties. (E)-2-(3-(2-imino-1-methylimidazolidin-4-ylidene)-1-methylguanidino)acetic acid (L) has been synthesized and used as a ligand for the formation of Cr(III), Co(II), Ni(II), and Cu(II) complexes. The chemical structures of synthesized compounds were characterized using different spectroscopic methods. All chelates except Ni(II) chelate are found to be octahedral structures, Ni(II) chelate was square planar. The stability for the prepared complexes was studied theoretically using density function theory. The total energy for the complexes was calculated and it was shown that the copper complex is the most stable one. Ligand and complexes were tested against selected types of microbial organisms and showed significant activities. The free-radical scavenging activity of ligand and metal complexes have been determined by their interaction with the stable free-radical DPPH and all the compounds have shown encouraging antioxidant activities.     

Cationic Complexes of Boron and Aluminum: An Early 21st Century Viewpoint

Boron and aluminum are lighter Group 13 elements, found in daily life commodities, and considered environmentally benign. Nevertheless, they markedly differ in their elemental properties (e.g., metal character, atomic radius). The use of Lewis acidic complexes of boron and aluminum for methods of bond activation and catalysis (e.g., hydrogenation of unsaturated substrates, polymerization of olefins and epoxides) is quickly expanding. The introduction of cationic charge may boost the metalloid-centered Lewis acidity and allow for its fine-tuning particularly with regard to preference for “hard” or “soft” Lewis bases (i.e., substrates). Especially the isolation of low-coordinate cations (number of ligand atoms smaller than four) demands elaborate techniques of thermodynamic and kinetic stabilization (i.e., electronic saturation and steric shielding) by a ligand system. Furthermore, the properties of the solvent and the counteranion must be considered with care. Here, selected examples of boron and aluminum cations are described.     

Ferrocenyl-substituted Schiff base complexes of boron: synthesis, structural, physico-chemical and biochemical aspects

Biological important complexes of boron(III) derived from 1-acetylferrocenehydrazinecarboxamide (L1H), 1-acetylferrocenehydrazinecarbothioamide (L2H) and 1-acetylferrocene carbodithioic acid (L3H) have been prepared and investigated using a combination of microanalytical analysis, melting point, electronic, IR, 1H NMR and 13C NMR spectral studies, cyclic voltammetry and X-ray powder diffraction studies. Boron isopropoxide interacts with the ligands in 1:1, 1:2 and 1:3 molar ratios (boron:ligand) resulting in the formation of coloured products. On the basis of conductance and spectral evidences, tetrahedral structures for boron(III) complexes have been assigned. The ligands are coordinated to the boron(III) via the azomethine nitrogen atom and the thiolic sulfur atom/enolic oxygen atom. On the basis of X-ray powder diffraction study one of the representative boron complex was found to have orthorhombic lattice, having lattice parameters: a=9.9700, b=15.0000 and c=7.0000. Both the ligands and their complexes have been screened for their biological activity on several pathogenic fungi and bacteria and were found to possess appreciable fungicidal and bactericidal properties. Plant growth regulating activity of one of the ligand and its complexes has also been recorded on gram plant, and results have been discussed.     

Some chelating C-donor ligands in hydrogen transfer and related catalysis

Recent work is discussed that throws light on synthetic, steric, and electronic aspects of NHC complexes as well as on outer sphere effects in their reactivity. The chemistry of the NHC ligand is much more complex than the more traditional phosphines and provides much greater possibilities for altering steric and electronic properties for tuning reactivity. In synthetic work the Lin Ag₂O method is shown to be inapplicable to the synthesis of abnormal NHCs bound via C-4(5) where the C2 position is blocked with CH₃, because Ag(I) oxidizes the CH₃ group to formate with formation of the normal C-2 bound Ag-NHC. Linker effects on the behavior of chelating NHCs depend on the linker locking the azole rings into a conformation that depends on linker length. This gives rise to different complexes being formed when different linker lengths are employed. The failure of M-NHC bonds to reversibly dissociate can prevent potentially chelating bis and tris NHC precursors from forming the desired products but instead being trapped in a kinetic nonchelate form. Imidazolium carboxylates prove to be synthetically useful in that they can act as excellent NHC transfer agents to a variety of transition metals. The Tolman electronic parameter of NHCs can be determined by a variety of experimental and computational methods. Anion dependent chemistry can give rise to a switching of the product of imidazolium salt metallation from normal (C-2) to abnormal (C-4(5)) forms.     

An N‐Heterocyclic Carbene with a Saturated Backbone and Spatially‐Defined Steric Impact

The synthesis and coordination chemistry of a saturated analogue of a “bulky‐yet‐flexible” N‐heterocyclic carbene (NHC) ligand are described. “SIPaul” is a 4,5‐dihydroimidazol‐2‐ylidene ligand with unsymmetrical aryl N‐substituents, and is one of the growing class of “bulky‐yet‐flexible” NHCs that are sufficiently bulky to stabilize catalytic intermediates, but sufficiently flexible that they do not inhibit productive chemistry at the central metal atom. Here, the synthesis of SIPaul.HCl and its complexes with copper, silver, iridium, palladium, and nickel, and its selenourea, are reported. The steric impact of the ligand is quantified using percent buried volume (% V_bur), whereas the electronic properties are probed and quantified using the Tolman Electronic Parameter (TEP) and δ_Se of the corresponding selenourea. This work shows that despite the often very different performance of saturated versus unsaturated carbenes in catalysis, the effect of backbone saturation on measurable properties is very small.     

DFT survey of monoboron and diboron corroles: regio- and stereochemical preferences for a constrained, low-symmetry macrocycle

The structures of a number of mono- and diboron corrole complexes have been optimized using DFT methods in order to establish regio- and stereochemical preferences for bonding of one or two boron atoms to the corrole macrocycle. The formulations of the complexes were suggested either from preliminary experimental results (to be reported elsewhere) or by analogy with related diboron porphyrin compounds. The computational results suggest for the monoboron corroles BF(2)(H(2)corrole) and BPhH(H(2)corrole) that the regioisomer in which the boron is bound to a dipyrromethene site adjacent to the bipyrrole is preferred over the other possible regioisomers in which boron coordinates either in the bipyrrole or in the dipyrromethene site opposite the bipyrrole. In the N-substituted corrole complexes there are only two possiblities and, for each complex, the regioisomer with boron in the dipyrromethene site adjacent to the bipyrrole is lower in energy. For all four monoboron complexes the stereoisomers in which boron and both its substituents are displaced out of the mean N(4) plane are more stable than the boron in-plane stereoisomers. These regio- and stereochemical preferences are rationalised by an analysis of the deformations to the corrole macrocycle and the geometry at the boron atoms. The lowest energy structures in all cases correspond to the least strained configurations. In addition, all four complexes show significant BFHN hydrogen bonding and BHHN dihydrogen bonding interactions, which are maximised in the lowest energy configurations for each structure, indicating that these are important additional stabilising interactions. Three different regioisomers, each with cisoid or transoid stereochemistry were optimised for the diboron complex PhBOB(corrole) which contains a bridging BOB group. The dipyrromethene/dipyrromethene isomer is more stable than either of the dipyrromethene/bipyrrole isomers and cisoid stereochemistry is preferred over transoid. This contrasts with porphyrin complexes containing BOB groups for which both stereochemical possibilities are observed, and reflects the contracted size of the corrole macrocycle. Three further diboron corroles were investigated, the diboranyl cation [B(2)(corrole)](+) and its one- and two-electron reduced derivatives B(2)(corrole) and [B(2)(corrole)](-). These calculations were undertaken to determine whether the site of reduction of [B(2)(corrole)](+) is likely to be the diboron moiety or the macrocycle. The B-B bond lengths do not shorten upon reduction and an analysis of the molecular orbitals of each species indicates that reduction will be most likely to occur at the macrocycle, offering a potential route to an example of the two-electron reduced corrole ligand, an analogue of the 20-electron isophlorin ligand observed in the corresponding reduced porphyrin complex B(2)(porphine).     

The Measure of All Rings—N‐Heterocyclic Carbenes

Quantification and variation of characteristic properties of different ligand classes is an exciting and rewarding research field. N‐Heterocyclic carbenes (NHCs) are of special interest since their electron richness and structure provide a unique class of ligands and organocatalysts. Consequently, they have found widespread application as ligands in transition‐metal catalysis and organometallic chemistry, and as organocatalysts in their own right. Herein we provide an overview on physicochemical data (electronics, sterics, bond strength) of NHCs that are essential for the design, application, and mechanistic understanding of NHCs in catalysis.     

Palladacycles bearing COOH‐/ester‐functionalized N‐heterocyclic carbenes: Divergent syntheses and catalytic applications

Two new [C^N]‐type palladacyclic dinuclear complexes bearing carboxylate‐containing N‐heterocyclic carbenes (NHCs) were synthesized, and in both cases the carboxylato‐NHC ligand adopts a bridging mode. Both complexes proved to be suitable precursors, which can be used to divergently access palladacycles bearing ester‐ or COOH‐functionalized NHCs upon esterification or acidolysis. In the esterification reactions, alkyl halides are found to selectively react with the carboxylato moieties, and the palladacycle scaffold is retained even when excess haloalkane is employed. In the acidolysis reactions, the desired COOH‐tethered complexes can only be obtained when stoichiometric acid (with respect to Pd) is used, while excess acid destroys the metallacycle scaffold. Finally, a preliminary catalytic study reveals the good performances of all newly synthesized complexes in direct aromatic C─H functionalization reactions with alkynes. Poisoning experiments indicate that these hydroarylation reactions are likely to be homogeneously catalyzed.     

Theoretical studies on effective metal-to-ligand charge transfer characteristics of novel ruthenium dyes for dye sensitized solar cells

The development of ruthenium dye-sensitizers with highly effective metal-to-ligand charge transfer (MLCT) characteristics and narrowed transition energy gaps are essential for the new generation of dye-sensitized solar cells. Here, we designed a novel anchoring ligand by inserting the cyanovinyl-branches inside the anchoring ligands of selected highly efficient dye-sensitizers and studied their intrinsic optical properties using theoretical methods. Our calculated results show that the designed ruthenium dyes provide good performances as sensitizers compared to the selected efficient dyes, because of their red-shift in the UV–visible absorption spectra with an increase in the absorption intensity, smaller energy gaps and thereby enhancing MLCT transitions. We found that, the designed anchoring ligand acts as an efficient “electron-acceptor” which boosts electron-transfer from a –NCS ligand to this ligand via a Ru-bridge, thus providing a way to lower the transition energy gap and enhance the MLCT transitions.     

N-heterocyclic carbene ligands in copper-catalyzed addition of diethylzinc to N-sulfonylimines

N-Heterocyclic carbenes (NHCs) were generated in-situ from imidazolium and imidazolinium salts by deprotonation of C-2 hydrogen and were used as ligands in the copper-catalyzed addition of diethylzinc to N-sulfonylimines. The copper-NHC complexes were shown to possess an efficient ligand acceleration effect (LAE).