Bidentate bonding mode of tetrahydroborate and nitrite towards copper(II) in open-faced macrocyclic complexes

Novel seven-coordinate complexes formulated as CuL(BH₄)₂], CuL(BH₄)(NO₂)] and CuL(NO₂)₂] (L = 1,4,7-triazacyclononane) have been prepared and structurally characterized by elemental analyses, spectroscopic data (u.v., i.r. and e.p.r.), magnetic susceptibility and conductivity measurements. The results reveal that the complexes are non-electrolytic. The coordination geometry around the copper(II) ion is a seven coordinated square pyramidal structure with three nitrogen atoms of the 1,4,7-triazacyclononane and either four hydrogen atoms of two bidentate tetrahydroborate groups or two hydrogen atoms of the bidentate tetrahydroborate group and two oxygen atoms of the bidentate nitrite group or four oxygen atoms of two bidentate nitrito groups. A cyclic voltammetric study on the complexes indicates an irreversible redox couple (Cu^II/Cu^I) in DMF, giving a voltage of ca. −0.37 V versus s.c.e.