Templating Schiff-Base Lateral Macrobicycles: An Experimental and Theoretical Structural Study of the Intermediates

In this paper, we report a structural study both in the solid state and in solution of barium complexes with the
diamine N,N‘-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L2), that allows us to rationalize the template effect of the
metal ion in the synthesis of Schiff-base lateral macrobicycles resulting from the condensation of L2 with different
dicarbonyl compounds. The X-ray crystal structures of [Ba(L2)(ClO4)](ClO4) (3) [triclinic space group P1̄ with Z =
2, a = 10.467(2) Å, b = 10.4755(2) Å, c = 16.9911(3) Å, α = 85.075(1)°, β = 80.907(1)°, and γ = 61.627(4)°]
and [Ba(L2)(NCS)(H2O)](SCN) (4) [monoclinic space group P21/n with Z = 4, a = 9.954(5) Å, b = 29.193(5) Å,
c = 11.313(5) Å, and β = 91.371(5)°] demonstrate that in the solid state the barium(II) ion induces an anti
conformation of the receptor in the complexes. Variable temperature 1H and 13C NMR data point out that in solution
compounds 3 and 4 exist as a mixture of syn and anti isomers. The presence of the syn isomer in solution,
independent of the counterion employed (perchlorate or thiocyanate), accounts for the effectiveness of the barium(II)
ion as a template agent in the synthesis of the lateral macrobicycles resulting from the condensation of L2 with
different dicarbonyl compounds. Density functional theory calculations (at the B3LYP/LanL2DZ level) for [Ba(L2)]2+
predict the syn conformation to be more stable both in vacuo and in solution (PCM model). In order to asses which
of the two isomers predominates in acetonitrile solution, the 13C NMR shielding tensors of the two isomers of
[Ba(L2)]2+ were calculated for the in vacuo optimized structures by using the GIAO method, and the results were
compared with the experimental ones. According to these analyses, a syn stereochemistry is assigned to the major
species in solution.