Synthesis, structures, DFT calculations, and catalytic application in the direct arylation of five-membered heteroarenes with aryl bromides of novel palladium-N-heterocyclic carbene PEPPSI-type complexes

A new series of Pd-catalysts was synthesized and fully characterized via spectroscopic methods and the Pd–NHC complexes were evaluated via the direct arylation process.

Simple NMR predictors of catalytic hydrogenation activity for [Rh(cod)Cl(NHC)] complexes featuring fluorinated NHC ligands

A series of imidazolium salts precursors for N-heterocyclic carbenes (NHCs) featuring fluoroaryl substituents have been prepared along with their selenides and rhodium complexes. Tests of the catalytic activity of the [Rh(cod)Cl(NHC)] complexes in the transfer hydrogenation of acetophenone with iPrOH shows that the rhodium complexes bearing fluorinated NHCs are better than their non-fluorinated counterparts. The order of activity being 4-F-C6H4 < 2,4-F2-C6H3 < 2,4,5-F3-C6H2 < 2,6-F2-C6H3 < 2,4,6-F3-C6H2. This order of reactivity is consistent with a number of simple NMR measures of the electronic properties of these systems, including 1JCH of the NHC·HBF4 salts, δ(77Se) of the NHC selenides and 1JRh-C and δ(13Ccarbene) of the [Rh(cod)Cl(NHC)] complexes.

Mono- and bimetallic manganese-carbonyl complexes and clusters bearing imidazol(in)ium-2-dithiocarboxylate ligands

Five complexes with the generic formula fac-[MnBr(CO)₃(S₂C·NHC)] were obtained by reacting [MnBr(CO)₅] with a set of representative imidazol(in)ium-2-dithiocarboxylate zwitterions. These ligands are the adducts of N-heterocyclic carbenes (NHCs) and carbon disulfide. The mononuclear Mn(i) derivatives were coupled with Na[Mn(CO)₅] to afford bimetallic [Mn₂(CO)₆(S₂C·NHC)] clusters. Yet, the most convenient strategy to access these dinuclear Mn(0) products implied a direct carbonyl substitution from the [Mn₂(CO)₁₀] dimer. The molecular structures of three monometallic and four bimetallic compounds were elucidated by single crystal X-ray diffraction analysis. In the monometallic complexes, the NHC·CS₂ ligands exhibited a bidentate κ²-S,S' coordination mode with an S-C-S bite angle of about 116°. In the dinuclear clusters, the CS₂^- unit acted as a chelate toward one manganese center and as a pseudoallylic ligand toward the other one. The S-C-S bite angle was reduced to ca. 104°. Thus, the zwitterions displayed a remarkable flexibility, which also permitted a staggered arrangement of the carbonyl groups in the bimetallic systems. Examination of the [small nu, Greek, macron]_CO absorption bands on IR spectroscopy helped identify the presence of fac-Mn(CO)₃ or Mn₂(CO)₆ motifs, while the ¹³C NMR chemical shift of the CS₂^- moiety was a reliable indicator for monitoring its hapticity. Whereas the dinuclear clusters were air- and moisture-stable crystalline solids, mononuclear halido derivatives displayed only a limited stability under aerobic conditions. Both types of compounds underwent rather unselective, extensive fragmentations in the gas phase, in sharp contrast with the analogous rhenium derivatives that led to clean sequential decarbonylation processes upon collision-induced dissociation.

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.

Facile synthesis of [(NHC)MX(cod)] and [(NHC)MCl(CO)2] (M = Rh, Ir; X = Cl, I) complexes

The reactions of [MCl(cod)]2 (M = Rh, Ir) with different NHC·HX (X = Cl, I), K2CO3 in technical grade acetone under air provide simple access to various [(NHC)MX(cod)] complexes; a facile one-pot synthesis of [(NHC)MCl(CO)2] (M = Rh, Ir) is also reported.

A comparison between oxazoline-imidazolinylidene, -imidazolylidine, -benzimidazolylidene hydrogenation catalysts

Imidazolinylidene, imidazolylidine, benzimidazolylidene complexes 1a-c were prepared and tested in asymmetric hydrogenations of a series of largely unfunctionalized alkenes. Similarities and differences in the catalytic performance of these complexes were rationalized in terms of the predicted mechanisms of these reactions, and their relative tendencies to generate protons under the hydrogenation conditions.

Dinuclear gold(I) complexes with propylene bridged N-heterocyclic dicarbene ligands: synthesis, structures, and trends in reactivities and properties

Four novel dinuclear N-heterocyclic dicarbene gold(I) complexes with a propylene linker between the carbene moieties have been synthesized and their luminescence and electrochemical properties, together with their reactivity towards bromine oxidative addition, have been screened. All the complexes emit in the solid state in the blue-green spectral range (400-500 nm) with appreciable intensities (Φ(em) up to ≈10%). In cyclic voltammetry, the Au(I)/Au(0) peak splits at low temperature into two separate peaks relative to the couples Au(I)-Au(I)/Au(I)-Au(0) and Au(I)-Au(0)/Au(0)-Au(0), thus indicating the presence of an Au···Au interaction in the dinuclear complex. Oxidative addition of bromine affords as a major or unique product Au(II)-Au(II) complexes most likely as a consequence of the interaction between the two gold centres favoured by the propylene linker.