Factors influencing mononuclear versus multinuclear coordination in a series of potentially hexadentate acyclic N6ligands: the roles of flexibility and chelate ring size

Abstract

The synthesis of the new potentially hexadentate ligands N,N′-bis(2,2′-bipyridin-6-ylmethyl)butane-1,4-diamine (bmbu), N,N′-bis(2,2′-bipyridin-6-ylmethyl)pentane-1,5-diamine (bmpt) and N,N′-bis(2,2′-bipyridin-6-ylmethyl)octane-1,8-diamine (bmot) from the condensation of 2,2′-bipyridine-6-carbaldehyde with the appropriate diamine (butane-1,4-diamine, pentane-1,5-diamine and octane-1,8-diamine, respectively) and subsequent reduction, is reported. Bmet, bmpp and bmbu all form mononuclear complexes with first-row transition metal ions (Co³⁺, Fe²⁺, Ni²⁺, Mn²⁺), and X-ray structures of [Mn(bmet)](ClO₄)₂, [Ni(bmet)](ClO₄)₂, [Fe(bmet)](ClO₄)₂, [Mn(bmpp)](ClO₄)₂·2MeCN and [Co(bmpp)](ClO₄)₃·H₂O are reported. As the aliphatic methylene chain increases in length, formation of dinuclear, and in some cases trinuclear, complexes becomes more pronounced, as evidenced by mass spectral analysis of solutions containing Ni²⁺ and bmpt, and Ni²⁺, Fe²⁺ and Mn²⁺ with bmot. The increasing preference for multinuclear complexes with increasing chain length is ascribed to the difficulty of incorporating a medium-sized (8 to 13-membered) chelate ring in a mononuclear complex.