Structural characterization of ((9-fluorenylidene) (ferrocenyl)methyl)palladium iodide as the catalytic intermediate in the synthesis of 9-(ferrocenyl (ferrocenylethynyl)methylene)-9H-fluorene

The reaction of 2,2 '-diiodo-1,1 '-biphenyl (1a) with ethynyl ferrocene (2) gave trace amounts of the expected 2,2 '-bis(ferrocenylethynyl)-1,1 '-biphenyl (3) together with 9-(ferrocenyl(ferrocenylethynyl)methylene)-9H-fluorene (4), formed via a cyclization process during the Sonogashira cross-coupling reaction, as the major product. In contrast, a similar reaction of the 2,2 '-diiodo-1,1 '-diphenyl ether analogue with 2 gave the anticipated 2,2 '-bis(ferrocenylethynyl) substituted product (11). Reaction of 11 with tetracyanoethylene gave its bis-[2 + 2] cycloaddition product in high yield (12), while the alkynyl motif in fluorenylidene 4 proved to be inert towards such reactions. Studies of the mechanism leading to formation of 4 using a Pd/dppf (dppf = 1,1 '-bis(diphenylphosphino)ferrocene) catalyst combination allowed for isolation of a {C = C(Fc)-PdI(dppf)} intermediate (13), which was structurally characterized by single crystal X-ray diffraction. Electrochemical investigations of 4, 11, 12 and 13 revealed a series of essentially ferrocene-based oxidation processes, which in the case of 4 and 11 could be better resolved using a non-coordinating anion based electrolyte. Electronic structure studies at the DFT level (B3LYP/def2-TZVP/CPCM(CH2Cl2) suggest that the first oxidation event in 4 can be assigned to the vinyl-bonded ferrocenyl moiety.