X-Ray diffraction and molecular mechanics studies of 12-, 13-, and 14-membered tetraaza macrocycles containing pyridine: effect of the macrocyclic cavity size on the selectivity of the metal ion

The single crystal structures of complexes [(CuLBr)-Br-1]ClO4 1, [(CuLBr)-B r-2]PF6 2, and [NiL2][ClO4](2) 3 were determined (L-1 is 3,6,9,15-tetraazab icyclo[9.3.1]pentadeca-1(15),11,13-triene and L-2 is 3,6,10,16-tetraazabicy clo[10.3.1]hexadeca-1(16),12,14-triene). The asymmetric unit of 1 contains two [(CuLBr)-Br-1](+) cations having different five-co-ordinated environmen ts. One (A) exhibits a distorted square pyramidal arrangement, with the bas al plane defined by three nitrogen atoms of the macrocycle and the bromine, and the apical position occupied by the nitrogen opposite to the pyridine ring. In the other (B) the donor atoms are distorted from this geometry tow ards a trigonal bipyramid with the equatorial plane formed by two nitrogen atoms of the macrocycle and Br, and the axial positions occupied by the nit rogen atoms contiguous to the pyridine ring. The complex cation [(CuLBr)-Br -2](+) 2 exhibits a distorted square pyramidal environment with the basal p lane defined by the four nitrogen atoms of the macrocycle and the apical co -ordination by the bromine atom. In [NiL2](2+) 3 the four nitrogen atoms of the macrocycle form a distorted square planar environment around the nicke l centre. Molecular mechanics calculations are used to determine the best-f it sizes for metal ions accommodated into L-1 and L-2 by evaluation of all sterically allowed conformers for five-co-ordination geometry. The results obtained, together with those of L-3 (3,7,11,17-tetraazabicyclo[11.3.1]hept adeca-1(17),13,15-triene), published previously, clearly establish the effe ct of macrocyclic cavity size on metal ion selectivity. These macrocycles p refer a planar conformation to accommodate small metal ions but folded conf ormations are preferred for longer M-N distances. The increase of the macro cyclic cavity size leads to an increase of the M-N(sp(3)) distances at whic h the folded conformer(s) become the most stable form: 1.90, 2.14 and 2.18 Angstrom for 12-, 13- and 14-membered macrocycles, respectively.