C2-Symmetric Ferrocene-Bis(ureido)peptides: Synthesis, Conformation and Solid-State Structure

The extension of peptide derivatives of ferrocene-1,1'-dicarboxylic acid by formal insertion of NH units between ferrocene and peptide strands results in ferrocene-bis(ureido)peptides. Experimentally, alanine and dialanine methyl esters were attached to the 1- and 1'-position of 1,1'-diisocyanoferrocene to give the corresponding bis(ureido)peptide derivatives 3 and 4. The conformation of 3 has been determined in the solid state by X-ray crystallography. In solution the preferred conformation of 3 and 4 has been elucidated by NMR, IR and CD spectroscopy in concert with DFT calculations. The secondary structure of ferrocene-bis(ureido)peptides 3 and 4 is determined by double bifurcated intramolecular hydrogen bonds (IHBs). The different stability of the secondary structures of 3 and 4 is due to different types of IHBs.