Group 10 Metal Complexes of a Ferrocene‐Based N‐Heterocyclic Carbene: Syntheses, Structures and Catalytic Applications

Abstract

AbstractThis paper focuses on complexes of the N‐heterocyclic carbene (NHC) 1‐Np, which contains a 1,1′‐ferrocenediyl backbone and neopentyl substituents at the N atoms flanking the divalent C atom. 1‐Np is a ring‐expanded NHC with a ring size of six atoms. The square‐planar group 10 metal complexes [{PdCl(μ‐Cl)(1‐Np)}2], cis‐[PdCl2(1‐Np)(PPh3)], trans‐[PdCl2(1‐Np)2] and trans‐[NiCl2(1‐Np)(PPh3)] have been prepared by the reaction of 1‐Np with trans‐[PdCl2(PhCN)2], [PdCl2(PPh3)2], cis‐[PdCl2(COD)] (COD = cycloocta‐1,5‐diene) and [NiCl2(PPh3)2], respectively. In addition, the carbene–borane adduct 1‐Np–BF3 has been obtained by serendipity. All new compounds have been structurally characterised by single‐crystal X‐ray diffraction, which demonstrates the pronounced trans influence of 1‐Np. 1‐Np has been compared with the ring‐expanded NHC 1,3‐diisopropyltetrahydropyrimid‐2‐ylidene (THP‐iPr) in the context of homogeneous catalysis, which utilises analogous palladium NHC complexes known to be particularly effective in the case of NHC = THP‐iPr. The catalytic performance of cis‐[PdCl2(1‐Np)(PPh3)] in Suzuki–Miyaura cross‐coupling reactions of PhB(OH)2 with aryl bromides Br–p‐C6H4–R is similar to that reported for cis‐[PdCl2(THP‐iPr)(PPh3)]. With aryl chlorides, however, the latter complex is superior to its 1‐Np analogue. The catalytic performance reported for trans‐[PdCl2(THP‐iPr)2] in Mizoroki–Heck reactions of n‐butyl acrylate with aryl bromides Br–p‐C6H4–R is equal to (R = H) or moderately better than that of trans‐[PdCl2(1‐Np)2] [R = C(O)Me, OMe].