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(mu-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 carbeneborane adduct 1-NpBF3 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 SuzukiMiyaura cross-coupling reactions of PhB(OH)2 with aryl bromides Brp-C6H4R 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 MizorokiHeck reactions of n-butyl acrylate with aryl bromides Brp-C6H4R is equal to (R = H) or moderately better than that of trans-[PdCl2(1-Np)2] [R = C(O)Me, OMe].