Dopamine Interaction in the Absence and in the Presence of Cu2+ Ions with Macrocyclic and Macrobicyclic Polyamines Containing Pyrazole Units. Crystal Structures of [Cu2(L1)(H2O)2](ClO4)4 and [Cu2(H-1L3)](ClO4)3·2H2O
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Abstract
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(L1)(H2O)2](ClO4)4*2H2O (4) (L1 = 13,26-dibenzyl-3,6,9,12,13,16,19,22,25,26-decaazatricyclo[22.2.1.1(11,14)]octacosa-1(27),11,14(28),24-tetraene) and [Cu2(H-1L3)](HClO4)(ClO4)2*2H2O (6) (L3 = 1,4,7,8,11,14,17,20,21,24,29,32,33,36-tetradecaazapentacyclo[12.12.12.1(6,9).1(19,22).1(31,34)]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) A. 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) A; Cu(2)-N(6), 2.40(1) A]. 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) A. 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 L2 (L2 = 3,6,9,12,13,16,19,22,25,26-decaazatricyclo[22.2.1.1(11,14)]octacosa-1(27),11,14(28),24-tetraene), deprotonation of both pyrazole subunits is already observed at pH ca. 4 for 2:1 Cu2+:L2 molar ratios. All three free receptors interact with dopamine in aqueous solution. L3 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 L1.
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