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

This 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 ‐[PdCl 2 ( 1 ‐Np)(PPh 3 )], trans ‐[PdCl 2 ( 1 ‐Np) 2 ] and trans ‐[NiCl 2 ( 1 ‐Np)(PPh 3 )] have been prepared by the reaction of 1 ‐Np with trans ‐[PdCl 2 (PhCN) 2 ], [PdCl 2 (PPh 3 ) 2 ], cis ‐[PdCl 2 (COD)] (COD = cycloocta‐1,5‐diene) and [NiCl 2 (PPh 3 ) 2 ], respectively. In addition, the carbene–borane adduct 1 ‐Np–BF 3 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‐ i Pr) in the context of homogeneous catalysis, which utilises analogous palladium NHC complexes known to be particularly effective in the case of NHC = THP‐ i Pr. The catalytic performance of cis ‐[PdCl 2 ( 1 ‐Np)(PPh 3 )] in Suzuki–Miyaura cross‐coupling reactions of PhB(OH) 2 with aryl bromides Br– p ‐C 6 H 4 –R is similar to that reported for cis ‐[PdCl 2 (THP‐ i Pr)(PPh 3 )]. With aryl chlorides, however, the latter complex is superior to its 1 ‐Np analogue. The catalytic performance reported for trans ‐[PdCl 2 (THP‐ i Pr) 2 ] in Mizoroki–Heck reactions of n ‐butyl acrylate with aryl bromides Br– p ‐C 6 H 4 –R is equal to (R = H) or moderately better than that of trans ‐[PdCl 2 ( 1 ‐Np) 2 ] [R = C(O)Me, OMe].