Palladium(II) and Gold(I) Complexes of a New O-Functionalized N-Heterocyclic Carbene Ligand:  Synthesis, Structures, and Catalytic Application

Abstract

Synthetic and structural studies of Pd(II) and Au(I) complexes of a new O-functionalized N-heterocyclic carbene ligand, namely, 1-(o-methoxybenzyl)-3-tert-butylimidazol-2-ylidene, are reported. Specifically, the N-heterocyclic carbene precursor 1-(o-methoxybenzyl)-3-tert-butylimidazolium bromide (1a) was synthesized by the reaction of 2-methoxybenzyl bromide and tert-butylimidazole in 44% yield. The Au-(I) and Pd(IIwere prepared in 77% and 89% yields, respectively, by the commonly used silver carbene transfer route from the silver complex {[1-(omethoxybenzyl)-3-tert-butylimidazol-2-ylidene] 2 Ag} + Br -(1b). The silver complex 1b was in turn synthesized from the reaction of 1a with Ag 2 O. The molecular structures of the complexes 1c and 1d have been determined by X-ray diffraction studies, which revealed that the gold complex 1c possessed a linear geometry while the palladium complex 1d had a trans-square-planar geometry at their respective metal centers. The Pd(II) complex 1d was found to be an efficient catalyst for Suzuki-Miyaura type cross-coupling reactions of phenylboronic acid and aryl halides (ArX, X ) Br, I) in high yields and turnover numbers (up to 109 600).

N-heterocyclic carbene tethered amido complexes of palladium and platinum

“…In 10, the bond lengths and angles around the square planar Pd are extremely similar for the two independent molecules found in the unit cell, despite the two molecules having very different angles between the mean plane of the NHC and pyridine ring (14.9 and 40.2 °). The Pd-C bond lengths (1.957 (7) and 1.981 (7) ), alcohol and ether groups 51,53 have also been reported previously, but, to the best of our knowledge, 10 is the first amine functionalised complex of this type. Furthermore, the nucleophilic substitution reactions of alkyl or benzyl halides used to prepare the functionalised NHC ligands in these previously reported complexes contrasts with the cross-coupling strategy used to synthesise the pro-ligand 6, which enables efficient access to aryl substituted NHC ligands with the functional group (amine) also bound to the aromatic ring.…”

Synthesis, characterization and catalytic properties of an N-heterocyclic carbene palladium-based complex

“…These bond lengths are comparable to those observed in the literature [29,30]. In addition, the Pd-Cl bond in the complex is somewhat shorter than the average of 2.33(4) Å calculated for 2151 Pd-Cl distances in 1306 complexes reported to the Cambridge Structural Database (CSD; version 5.26, updated May 2005; [31]) and slightly larger than the sum of the individual covalent radii (2.273 Å) [32][33][34]. The Pd-Cl distances in these 1252 complexes span the range 2.222-2.516 Å; therefore, the Pd-Cl separation in the investigated complex is within the expected range [34].…”

Fluoride‐Free Hiyama and Copper‐ and Amine‐Free Sonogashira Coupling in Air in a Mixed Aqueous Medium by a Series of PEPPSI‐Themed Precatalysts

“…Our research revolves around exploring the utility of N/O-functionalized N-heterocyclic carbenes [9] in biomedical applications [10] as well as in chemical catalysis, [11] with special emphasis on developing robust, user-friendly, and highly efficient precatalysts for the Pd-mediated C-C crosscoupling reactions [8,12] and hence, here in this contribution, we report on the use of a series of new PEPPSI-themed (NHC)PdX 2 (pyridine) (X = halide) complexes of the C4-C5 saturated imidazole-(1-4) and triazole-based (5 and 6) N-heterocyclic carbenes for the two highly important but relatively less explored C-C cross-coupling reactions namely the Hiyama and Sonogashira couplings (Figure 1). …”