Palladium Complexes with Carbene and Phosphine Ligands: Synthesis, Structural Characterization, and Direct Arylation Reactions between Aryl Halides and Alkynes

Abstract

The new palladium complexes with NHC and phosphine ligands, cis-PdCl 2 (L 2 )(PPh 3 ) (2), cis-PdBr 2 (L 2 )(PPh 3 ) (3), and cis-PdCl 2 (L 3 )(PPh 3 ) (4) were prepared following a general protocol of a one pot reaction between PdCl 2 (COD), PPh 3 , and the ligand precursors LH 3 Y (Y=Cl, Br, BF 4 ) (L 2 = 1,3-dibenzylimidazolin-2-ylidene; L 3 =1,3-dibenzylimidazol-2-ylidene). The cis-PdCl 2 (L 3 )(PCy 3 ) complex (5) was prepared by the ligand substitution reaction between 4 and PCy 3 . The palladium complexes with NHC and pyridine complexes, trans-PdCl 2 (L)(py) (6: L = L 2 ; 7: L = L 3 ) were obtained by heating a mixture of PdCl 2 (COD) and LH 3 BF 4 in pyridine. A similar reaction condition using CH 3 CN as solvent with KO t Bu as base afforded cis-PdCl 2 (L 3 ) 2 (8). Complexes 2-8 were successfully characterized by X-ray crystallographic studies, among which, intriguingly, two polymorphs of 8 were obtained. Thermogravimetric analysis showed that the cis-PdX 2 (NHC)(PR 3 ) complexes are more thermally stable than the trans-PdCl 2 (NHC)(py) complexes. Together with the known cis-PdCl 2 (L 1 )(PCy 3 ) (1) (L 1 =1-benzyl-3-(N-phenylacetamido)imidazol-2-ylidene), they were screened for the direct arylation reaction between aryl halides and alkynes. The result indicate that the carbene/phosphine complexes 1-5 are superior precatalysts than 6-8 with higher activities than the commonly-used system of Pd(OAc) 2 /2PPh 3 .

A hybrid carbocyclic/N-heterocyclic carbene ligand

“…This is in agreement with the very few examples of NHC complexes for which also an exact analogue bearing a CHT has been reported suggesting that this observation can in fact be generalised. 13,33,34 Another interesting feature is the orientation of the carbenemetal bonds in relation to the CHT and NHC rings, because the formation of the five-membered metallacycle in 7 and 8 requires some deviation from the ideal binding geometries. To describe such distortions the NHC tilting angle Y NHC defined by Chaplin et al as the angle between the metal, carbene-C and the centroid of the NHC can be used, where 1801 corresponds to the ideal coordination geometry.…”

Crystal structure of trans-dichlorido[1,3-bis(9-methyl-9H-fluoren-9-yl) benzimidazol-2-ylidene](pyridine)palladium(II) – a compound with anagostic CH–Pd interactions, C40H31Cl2N3Pd

“…Note that, as shown by NMR solution studies, the methyl uorenyl groups do not rotate about the corresponding N1-C4 bonds [1]. The PdCl (2.323(1) and 2.331(1) Å), Pd-C18 (1.966(5) Å), and Pd-N2 (2.144(4) Å) bond lengths are comparable to those found in related trans-[PdCl 2 (NHC)(pyridine)] complexes [1,3].…”

2‐Hydroxyethyl‐Substituted Pd‐PEPPSI Complexes: Synthesis, Characterization and the Catalytic Activity in the Suzuki‐Miyaura Reaction for Aryl Chlorides in Aqueous Media

“…Pd–C(carbene) bond distance of 1.944(11) Å is slightly shorter than the similar trans ‐PdCl 2 (NHC)(py) complexes in literature. On the other hand, Pd–N(pyridine) bond length 2.120(9) Å is longer than the sum of the individual covalent radii of Pd and N (Pd–N=1.983 Å) and comparable to those found in related complexes . The longer Pd–N(pyridine) distance is attributed to the greater trans effect of the NHC ligand residing diagonally opposite to the pyridine …”