The interaction with Cu2+ and dopamine of three polyazacyclophanes containing pyrazole fragments
as spacers is described. Formation of mixed complexes Cu2+−macrocycle−dopamine has been studied by
potentiometric methods in aqueous solution. The crystal structures of the complexes [Cu2(L
1)(H2O)2](ClO4)4·2H2O (4) (L
1 = 13,26-dibenzyl-3,6,9,12,13,16,19,22,25,26-decaazatricyclo[22.2.1.111,14]octacosa-1(27),11,14(28),24-tetraene) and [Cu2(H-
1
L
3)](HClO4)(ClO4)2·2H2O (6) (L
3 = 1,4,7,8,11,14,17,20,21,24,29,32,33,36-tetradecaazapentacyclo[12.12.12.16,9.119,22.131,34]hentetraconta-6,9(41),19(40),21,31,34(39)-hexaene) are presented. In
the first one (4), each Cu2+ coordination site is made up by the three nitrogens of the polyamine bridge, a sp2
pyrazole nitrogen and one water molecule that occupies the axial position of a square pyramid. The distance
between the copper ions is 6.788(2) Å. In the crystal structure of 6, the coordination geometry around each
Cu2+ is square pyramidal with its base being formed by two secondary nitrogens of the bridge and two nitrogen
atoms of two different pyrazolate units which act as exobidentate ligands. The axial positions are occupied
by the bridgehead nitrogen atoms; the elongation is more pronounced in one of the two sites [Cu(1)−N(1),
2.29(2) Å; Cu(2)−N(6), 2.40(1) Å]. The Cu−N distances involving the deprotonated pyrazole moieties are
significantly shorter than those of the secondary nitrogens. The Cu(1)···Cu(2) distance is 3.960(3) Å. The
pyrazole in the noncoordinating bridge does not deprotonate and lies to one side of the macrocyclic cavity.
One of the aliphatic nitrogens of this bridge is protonated and hydrogen bonded to a water molecule, which
is further connected to the sp2 nitrogen of the pyrazole moiety through a hydrogen bond. The solution studies
reveal a ready deprotonation of the pyrazole units induced by coordination to Cu2+. In the case of L
2
(L
2 =
3,6,9,12,13,16,19,22,25,26-decaazatricyclo[22.2.1.111,14]octacosa-1(27),11,14(28),24-tetraene), deprotonation of
both pyrazole subunits is already observed at pH ca. 4 for 2:1 Cu2+:L
2 molar ratios. All three free receptors
interact with dopamine in aqueous solution. L
3 is a receptor particularly interesting with respect to the values
of the interaction constants over five logarithmic units at neutral pH, which might suggest an encapsulation of
dopamine in the macrocyclic cage. All three receptors form mixed complexes Cu2+−L−dopamine. The affinity
for the formation of ternary dopamine complexes is particularly high in the case of the binuclear Cu2+ complexes
of the 1-benzyl derivative L
1.