Organometallics as potential protein labels: Pyrylium and pyridinium salts bearing (C6H5)Cr(CO)3, (C5H4)Mn(CO)3, and ferrocenyl substituents

Article abstract of DOI:10.1021/om00011a051

A series of 2,6-dimethyl- or 2,6-diphenylpyrylium cations, [R′-C₅H₂R″₂O]⁺, which bear organometallic substituents at the 4-position, have been synthesized. The molecules in which R′ = (C₆H₅)Cr(CO)₃, R″ = Me, 1, R′ = (C₅H₄)Mn(CO)₃, R″ = Ph, 8, R′ = (C₅H₄)Fe(C₅H₅), R″ = Me, 9, and R′ = (C₅H₄)Re(CO)₃, R″ = Ph, 10, have been identified spectroscopically, and the complexes 1 and 8 have also been characterized by X-ray crystallography as CrC¹⁶H₁₃O₄⁺ PF₆^- and MnC₂₅H₁₆O₄⁺ BF₄^-, respectively. The π-complexed ring bearing either a Cr(CO)₃ or a Mn(CO)₃ moiety is almost coplanar with the pyrylium ring, as are the phenyl rings in 8. The structural and spectroscopic data indicate that the delocalization of the positive charge on the pyrylium ring onto the metal is rather minimal. These pyrylium salts react readily with benzylamine, or with β-alanine ethyl ester, to form the corresponding pyridinium salts. In particular, the ferrocenyl derivative, 9, yields a crystalline pyridinium salt, 15, with the ethyl ester of β-alanine.