Flexible, Bowl-Shaped N-Heterocyclic Carbene Ligands: Substrate Specificity in Iridium-Catalyzed Ketone Hydrosilylation

Synthesis of bacteriochlorins bearing diverse β-substituents

Eleven bacteriochlorins have been prepared for surface attachment, bioconjugation, water-solubilization, vibrational studies, and elaboration into multichromophore arrays.

Late-Stage Functionalization of the Periphery of Oligophenylene Dendrimers with Various Arene Units via Fourfold C-H Borylation

Late-stage functionalization of the periphery of oligophenylene dendrimers was efficiently achieved via site-selective C-H activation of a preconstructed, readily accessible dendron. By fourfold iridium-catalyzed C-H borylation followed by Suzuki-Miyaura cross-coupling, various arene units were introduced into the end points of the 1,3,5-phenylene-based hydrocarbon dendron. Coupling of the modified dendrons with a core unit, such as 2,6-dibromobenzoic acid derivatives, afforded the periphery-functionalized dendrimers that also have an endohedral functionality at the core position.

Solvent Induced Helix Folding of Defined Indolenine Squaraine Oligomers

A protecting group strategy was employed to synthesise a series of indolenine squaraine dye oligomers up to the nonamer. The longer oligomers show a distinct solvent dependence of the absorption spectra, that is, either a strong blue shift or a strong red shift of the lowest energy bands in the near infrared spectral region. This behaviour is explained by exciton coupling theory as being due to H- or J-type coupling of transition moments. The H-type coupling is a consequence of a helix folding in solvents with a small Hansen dispersity index. DOSY NMR, small angle neutron scattering (SANS), quantum chemical and force field calculations agree upon a helix structure with an unusually large pitch and open voids that are filled with solvent molecules, thereby forming a kind of clathrate. The thermodynamic parameters of the folding process were determined by temperature dependent optical absorption spectra.

N-Heterocyclic Carbenes in Materials Chemistry

N-Heterocyclic carbenes (NHCs) have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, the stabilization of reactive molecular fragments, and biochemistry. More recently, NHCs have found applications in materials chemistry and have allowed for the functionalization of surfaces, polymers, nanoparticles, and discrete, well-defined clusters. In this review, we provide an in-depth look at recent advances in the use of NHCs for the development of functional materials.

A facile synthetic route to benzimidazolium salts bearing bulky aromatic N-substituents

An atom-economic synthetic route to benzimidazolium salts is presented. The annulated polycyclic systems: 1,3-bis(2,4,6-trimethylphenyl)-1H-benzo[d]imidazol-3-ium chloride (1-Cl), 1,3-bis(2,6-diisopropylphenyl)-1H-benzo[d]imidazol-3-ium chloride (2-Cl), 1,3-diphenyl-1H-benzo[d]imidazol-3-ium chloride (3-Cl), and 1,3-di(pyridin-2-yl)-1H-benzo[d]imidazol-3-ium chloride (4-Cl) were prepared in a two-step synthesis avoiding chromatographic work-up. In the key step triethyl orthoformate is reacted with the corresponding N¹,N²-diarylbenzene-1,2-diamines and then further transformed in situ, by alkoxy abstraction using trimethylsilyl chloride (TMSCl), and concomitant imidazole ring closure.

Ruthenium catalysts bearing a benzimidazolylidene ligand for the metathetical ring-closure of tetrasubstituted cycloolefins

Deprotonation of 1,3-di(2-tolyl)benzimidazolium tetrafluoroborate with a strong base afforded 1,3-di(2-tolyl)benzimidazol-2-ylidene (BTol), which dimerized progressively into the corresponding dibenzotetraazafulvalene. The complexes [RhCl(COD)(BTol)] (COD is 1,5-cyclooctadiene) and cis-[RhCl(CO)2(BTol)] were synthesized to probe the steric and electronic parameters of BTol. Comparison of the percentage of buried volume (%VBur) and of the Tolman electronic parameter (TEP) of BTol with those determined previously for 1,3-dimesitylbenzimidazol-2-ylidene (BMes) revealed that the two N-heterocyclic carbenes displayed similar electron donicities, yet the 2-tolyl substituents took a slightly greater share of the rhodium coordination sphere than the mesityl groups, due to a more pronounced tilt. The anti,anti conformation adopted by BTol in the molecular structure of [RhCl(COD)(BTol)] ensured nonetheless a remarkably unhindered access to the metal center, as evidenced by steric maps. Second-generation ruthenium-benzylidene and isopropoxybenzylidene complexes featuring the BTol ligand were obtained via phosphine exchange from the first generation Grubbs and Hoveyda-Grubbs catalysts, respectively. The atropisomerism of the 2-tolyl substituents within [RuCl2(=CHPh)(PCy3)(BTol)] was investigated by using variable temperature NMR spectroscopy, and the molecular structures of all four possible rotamers of [RuCl2(=CH-o-O(i)PrC6H4)(BTol)] were determined by X-ray crystallography. Both complexes were highly active at promoting the ring-closing metathesis (RCM) of model α,ω-dienes. The replacement of BMes with BTol was particularly beneficial to achieve the ring-closure of tetrasubstituted cycloalkenes. More specifically, the stable isopropoxybenzylidene chelate enabled an almost quantitative RCM of two challenging substrates, viz., diethyl 2,2-bis(2-methylallyl)malonate and N,N-bis(2-methylallyl)tosylamide, within a few hours at 60 °C.

Pyridine versus acetonitrile coordination in rhodium–N-heterocyclic carbene square-planar complexes

Experimental and theoretical studies on the factors that control the coordination chemistry of N-donor ligands in square-planar complexes of the type RhCl(NHC)L¹L² (NHC = N-heterocyclic carbene) are presented.

Bimetallic N-heterocyclic carbene-iridium complexes: investigating metal-metal and metal-ligand communication via electrochemistry and phosphorescence spectroscopy

Bimetallic [Ir(COD)Cl] and [Ir(ppy)(2)] (COD = 1,5-cyclooctadiene; ppy = 2-phenylpyridyl) complexes bridged by 1,7-dimethyl-3,5-diphenylbenzobis(imidazolylidene) (1), in addition to their monometallic analogues supported by 1-methyl-3-phenylbenzimidazolylidene (2), were synthesized and studied. Electrochemical analyses indicated that 1 facilitated moderate electronic coupling between [Ir(COD)Cl] units (DeltaE = approximately 60 mV), but not [Ir(ppy)(2)]. The metal-based oxidation potentials for the bimetallic complexes were within 20 mV of those for their monometallic analogues. Furthermore, spectroscopic analyses of the [Ir(ppy)(2)] bimetallic and monometallic complexes revealed nearly identical phosphorescence profiles, indicating that carbene coordination does not affect the energy of the emissive states. Collectively, these results suggest that N-heterocyclic carbenes (NHCs) such as 1 could link together two emissive fragments without altering their fundamental phosphorescence profiles. Ultimately, employing multitopic NHCs as non-interfering molecular connectors could facilitate the rational design of new phosphorescent materials as well as second-generation phosphor dopants.

IPr* an easily accessible highly hindered N-heterocyclic carbene

Herein, we wish to describe the efficient three-step synthesis of a novel highly hindered, but flexible, N-heterocyclic carbene and its coordination chemistry to Ag(i) and Rh(i).

1,1'-Bis(N-benzimidazolylidene)ferrocene: synthesis and study of a novel ditopic ligand and its transition metal complexes

Diiridum complexes containing 1,1'-bis(N-benzimidazolylidene)ferrocene, a novel ditopic ligand comprised of two N-heterocyclic carbenes (NHCs) linked directly via their N-substituents to each cyclopentadienyl ring of a ferrocene moiety, were synthesized. Crystallographic analyses of these C(2)-symmetric bimetallic complexes revealed the benzimidazolylidene moieties were intramolecularly stacked in nearly opposing orientations, effectively forming Janus-type bis(NHC) structures in the solid state. Using a variety of electrochemical techniques, the oxidation potentials of the ferrocenyl groups in these complexes were found to depend on the auxillary ligands coordinated to the Ir centers (i.e., 1,5-cyclooctadiene vs. carbonyl). Similarly, the nu(CO) of carbonyls ligated to the Ir centers varied in accord with the oxidation state of the ferrocene contained with the bis(NHC) ligand. These results suggest that the Ir and Fe centers in these complexes are electronically coupled and that the electron donating ability of the bis(NHC) ligand reported herein can be tuned electrochemically.