Preparation and structure of nickel(II) complexes of N-substituted tetra-aza macrocycles: molecular structure of [(1RS,4SR,5RS,8SR,11RS,12SR)-1,5,8,12- tetramethyl-1,4,8,11-tetra-azacyclotetradecane]nickel(II) diperchloratet

Abstract

Reaction of N-methyl-1,2-diaminoethane monoperchlorate and but-3-en-2-one gives the N-substituted diaminodi-imine macrocycle 1,5,8,12-tetramethyl-1,4,8,11 -tetra-azacyclotetradeca-4,ll -diene, L⁶. The complexation of Ni^II to L⁶ followed by reduction of the CN double bonds with NaBH₄ leads to the exclusive formation of (C-meso-1,5,8,12-tetramethyl-1,4,8,11 -tetra-azacyclotetra-decane) nickel(II) ion (3). An X-ray crystal structure determination of the diperchlorate of (3) showed that it adopts an elongated octahedral geometry with C_i symmetry, where the perchlorates co-ordinate weakly to the apical sites. The N-methyl groups are located at the opposite side of the NiN₄ basal plane, and the complex has the N-meso configuration with both C-methyl groups being orientated equatorially. On the other hand, the free base of the C-meso macrocycle L⁷ was obtained highly selectively by direct reduction of L⁶ with NaBH₄ and the incorporation of Ni^II into L⁷ gave a complex (4), instead of (3). Proton n.m.r. analysis revealed that complex (4) has a structure which differs from that of (3) with regard to the orientation of two C-methyl groups, which are equatorial in (3) but axial in (4).