Complexes of the potentially hexadentate ligand bis{3-[6-(2,2′-bipyridyl)]pyrazol-1-yl}hydroborate with representative s-, p-, d- and f-block metal ions: factors promoting formation of mononuclear or double-helical dinuclear complexes

Abstract

Complexes of the new potentially hexadentate ligand bis{3-[6-(2,2′-bipyridyl)]pyrazol-1-yl}hydroborate (L^–), containing two terdentate chelating arms linked by a –BH₂– spacer, were prepared and crystallographically characterised with K⁺, Cu²⁺, Gd³⁺ and Tl⁺ as representatives of the s-, d-, f- and p-block metals respectively. The crystal structure of the K⁺ complex revealed it to be the double-helical dinuclear [K₂L₂], in which each metal ion is six-co-ordinated by a terdentate arm from each of the two ligands; the two ligands are therefore bridging, and folded at the flexible –BH₂– spacer group. The complex [Cu₂L₂][BF₄]₂ has a similar double-helical dinuclear cation with six-co-ordinate metal centres, but with a greater metal–metal separation because of the greater electrostatic repulsion between two dipositive metal ions compared to [K₂L₂]. The complex [GdL(NO₃)₂] in contrast is mononuclear with the ligand co-ordinated in a pseudo-equatorial manner, having a shallow helical twist to avoid steric interference between the terminal pyridyl groups. The two pseudo-axial bidentate nitrate ligands complete the ten-fold co-ordination. Formation of a (triple) helical complex between Gd³⁺ and L^–, known with other bis-terdentate compartmental ligands, is thought to be disfavoured in this case because of the electrostatic repulsion between the two +3 metal centres that would occur given the relatively short metal–metal separations imposed by the ligand. In [TlL] the Tl⁺ ion, which is comparable in size and identical in charge to K⁺, has a preference for lower co-ordination numbers, which is reflected in the fact that not all of the ligand binding sites are co-ordinated and there are three relatively short M–N interactions and two long, weak ones.