Structural Characterization, Solution Dynamics, and Reactivity of Palladium Complexes with Benzimidazolin‐ 2‐ylidene N‐Heterocyclic Carbene Ligands

Abstract

Mononuclear and halide-bridged dinuclear palladium(II) complexes with symmetrically (1 and 3) and nonsymmetrically (2, 4-6) substituted benzimidazolin-2-ylidene N-heterocyclic carbene ligands were synthesized and characterized by 1 H and 13 C NMR spectroscopy, elemental analysis, and X-ray crystallography. The mononuclear complexes exist as a mixture of cis and trans isomers, and the identity of these was ascertained by NMR spectroscopy and structural characterization. The dinuclear complexes 4-6 with nonsymmetrically substituted benzimidazolin-2-ylidene ligands form rotamers (cis/anti, cis/syn, trans/anti, and trans/syn) at room temperature, which can be transformed to a single rotamer at higher temperatures as evidenced by 1 H NMR spectroscopy.[a] 2131Crystal structure analyses of representative examples show that the Pd II centers are bound in a distorted square-planar environment by halides and carbene C ligands, and the carbene ligands are perpendicular to the Pd-halide plane. All of the complexes were tested for their efficiency as (pre)catalysts in the Suzuki-Miyaura cross-coupling reaction, as well as in hydrodehalogenation reactions, and they show good activity. The Suzuki-Miyaura coupling reactions can be performed under air and in water as an environmentally benign solvent. The most active catalyst for the Suzuki-Miyaura coupling was also used in a sequence together with a "click reaction" to synthesize valuable sterically demanding tripodal triazole ligands.

NHC‐Stabilized Bis(trimethylsilyl)phosphido Complexes of PdII and NiII

“…The two carbene and bromido ligands are arranged trans to each other with an angle of 180° owing to the site symmetry. However, the plane Br1/Br1A/C1/C1A lies at an angle of 76.7° from the plane defined by the N atoms of the two bimy ligands, similar to what is observed in the related Pd II complex 8b. The Ni–C [1.903(3) Å] bond lengths are close to those reported for [NiBr 2 ( i Pr 2 ‐bimy) 2 ]9 [1.898(2) Å].…”

“…A similar trend was seen in the resonances of all diastereotopic CH 2 protons in 3 , which were split into two sets of multiplets with equal intensities. All protons in 3 were deshielded compared to those in [PdI 2 ( n Bu 2 ‐bimy) 2 ] 8b. The 13 C NMR spectra of 2 and 3 are in line with their 1 H NMR spectra.…”

“…The reaction conditions to form these complexes are easily changed based on the size, shape and reactivity of the azolium salts 8a. For instance, cis and trans isomers of [PdI 2 ( n Bu 2 ‐bimy) 2 ] were obtained by heating the corresponding benzimidazolium salt with Pd(OAc) 2 at 90 °C for 12 h, and after 10 months in DMF solution at ambient temperature, quantitative conversion of the cis to the trans isomer had occurred 8b. We observed that trans ‐[PdI 2 ( n Bu 2 ‐bimy) 2 ] can be prepared as the only detectable product by the in situ deprotonation of the N , N′ ‐di‐ n ‐butylbenzimidazolium iodide salt with Pd(OAc) 2 , in a 2:1 ratio, in DMSO at 120 °C for 12 h. After aqueous workup, a yellow crystalline solid was isolated from the reaction solution and washed with diethyl ether to obtain 70 % yield of pure trans ‐[PdI 2 ( n Bu 2 ‐bimy) 2 ] complex.…”

“…The Ni–C [1.903(3) Å] bond lengths are close to those reported for [NiBr 2 ( i Pr 2 ‐bimy) 2 ]9 [1.898(2) Å]. The percent buried volume (% V bur ) of the NHCs in [PdI 2 ( n Bu 2 ‐bimy) 2 ] (26.9 %) and 1 (29.4 %) can be readily calculated with SambVca,11 where the radius of the sphere is 3.5 Å and the distance from the sphere is 2.02 Å {in [PdI 2 ( n Bu 2 ‐bimy) 2 ]8b} and 1.90 Å (in 1 ) with a mesh step of 0.050 .…”

“…Due to the poor solubility of [MI 2 ( i Pr 2 ‐bimy) 2 ] (M = Ni,9 Pd8a) in common solvents, their reaction with Li[P(SiMe 3 ) 2 ] in thf required heating to 60 °C to obtain clear red (Ni) or yellow (Pd) solutions. The reaction of [PdI 2 ( n Bu 2 ‐bimy) 2 ]8b and [NiBr 2 ( n Bu 2 ‐bimy) 2 ] ( 1 ), however, was carried out at room temperature. In order to eliminate lithium halide salts, pentane was added to concentrated thf mixtures, and any insoluble material was removed by filtering through Celite.…”