Factors influencing mononuclear versus multinuclear coordination in a series of potentially hexadentate acyclic N6 ligands: 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 (Co(3+), Fe(2+), Ni(2+), Mn(2+)), and X-ray structures of [Mn(bmet)](ClO(4))(2), [Ni(bmet)](ClO(4))(2), [Fe(bmet)](ClO(4))(2), [Mn(bmpp)](ClO(4))(2)·2MeCN and [Co(bmpp)](ClO(4))(3)·H(2)O 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 Ni(2+) and bmpt, and Ni(2+), Fe(2+) and Mn(2+) 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.

Multifrequency cw-EPR and DFT Studies of an Apparent Compressed Octahedral Cu(II) Complex

“…The constraints associated with bmet contribute to particularly short M−N bonds involving the mutually trans pyridine rings along the tetragonal axis. A compressed tetragonal distortion is found also in the previously reported Co III , Mn II , Fe II , and Ni II analogues 26 and, most interestingly, in the parent d 10 [Zn(bmet)] 2+ complex. In this latter species, the compression must mainly originate from the steric constraints imposed by the ligand itself (additional packing effects can be also envisaged) because no electronic factors can affect this d 10 system.…”

“…The constraint of the rigid bmet ligand leads to short M−N bonds along the tetragonal axis that opposes the Jahn−Teller effect. However, the dissymmetry of the bmet ligand, 26 in which the nature of the trans ligands is different (a pyridine trans to an amine) breaks the symmetry of the complex inducing a pseudo Jahn− Teller effect. 47 The pseudo Jahn−Teller effect is a variant of the Jahn−Teller effect that occurs when the ground state interacts with excited states in nondegenerate systems.…”

“…The bmet ligand was prepared according to a previously reported procedure. 26 27 system using graphite-monochromated Mo Kα radiation with 0.5°frames. The data were corrected for Lorentz and polarization effects using SAINT 28 and scaled using SADABS.…”

“…The bmet ligand was prepared according to a previously reported procedure . All other reagents and solvents were used as received.…”

“…24 Recently, we reported a series of hexadentate nitrogen-based ligands in which two bipyridine units are linked by an aliphatic diamine chain, our objective being to study the effect of altering the length of the aliphatic chain on the nuclearities of the resulting metal complexes. 25,26 In the present study, we describe the syntheses of [Cu(bmet)](ClO 4 ) 2 and [Cu-(bmet)]Br 2 , two mononuclear octahedral Cu II complexes of the bmet ligand (Chart 2), as well as their characterization by single-crystal X-ray crystallography and multifrequency continuous-wave (cw)-EPR spectroscopy. The synthesis and characterization of the Zn II complex [Zn(bmet)](ClO 4 ) 2 , isostructural to [Cu(bmet)](ClO 4 ) 2 , is also described, to assist in rationalizing the structural properties of the Cu II complexes.…”