Synthesis and coordination chemistry of a new chiral tridentate PCPN-heterocyclic carbene ligand based on a ferrocene backbone

The new chiral imidazolium salt 1,3-bis[(R)-1-((S)-2-diphenylphosphinoferrocenyl)ethyl]-imidazolium iodide (4, (PCPH)I) was prepared in three steps from commercially available N,N-dimethyl-1-ferrocenylethylamine (1) in 55% overall yield. Salt 4 is the precursor for the in situ generation of a novel tridentate carbene ligand that could not be isolated upon deprotonation of 4 with (NaOBu)-Bu-t. Reaction of 4 with [Pd(OAC)(2)](3) afforded (SP-4)-1,3-bis[(R)1-((S)-2-diphenylphosphino-kappaP-ferrocenyl)ethyl]imidazol-2-ylideneiodol)alladium(II) acetate, [PdI(PCP)]OAc (6). Treatment of 4 with (NaOBu)-Bu-t and [RuCl2(PPh3)(3)] gave the mixed halide complex (SP-5)-1,3-bis[(R)-1-((S)-2-diphenylphosphino-kappaP-ferrocenyl)ethyl]imidazol-2-ylidene-chloroiodoruthenium(II) [RuClI(PCP)] (7a), as a mixture of two isomers in 60% yield. Additional complexes that could be obtained from 4 are [PdCl(PCP)]PF6 (5), [RuCl2(PCP)] (7b), and [RuI(PCP)(NCCH3)(2)]PF6 (8). The crystal and molecular structures of 5 and 7a were determined by X-ray diffraction. The Ru(II) center undergoes a weak agostic interaction with one of the two stereogenic methine units.