Effects of sequential replacement of -NH2 by -OH in the tripodal tetraamine tren on its acidity and metal ion coordinating properties

The preparation is described of two modified derivatives of the tripodal te traamine tren, 2-hydroxy-N,N-bis(2-aminoethyl)ethylamine, NNO(2)222, and 2- amino-N,N-bis(2-hydroxyethyl)ethylamine NNO(2)222, in which one and two pri mary amines, respectively, have been replaced with hydroxyl groups. The aqu eous acid-base and metal ion (Ni2+, Cu2+: Zn2+ coordination properties of t hese two compounds were studied by potentiometric, spectrophotometric, and NMR titrations. Two and three acidity constants, respectively, were determi ned for NNO(2)222 and NN(2)O222 by potentiometry. NMR titrations proved tha t deprotonation of the two OH residues in NNO(2)222, and of the one in NN(2 )O222, corresponded to pK(a) > 14. Acidity constants related to deprotonati on-of the terminal primary amine functions were similar in both NNO2222 and NN(2)O222 land to those in the parent compound tren), whereas deprotonatio n of the tertiary ammonium N atom had a very different acidity constant in each of these three compounds. Charge repulsion, polar effects, and intramo lecular hydrogen bond formation an responsible for the discrepancy. Chelate d diamine metal complexes for each ligand studied depended only on the basi city of the corresponding two amines, suggesting that the hydroxyl group in teracted with the metal ion very weakly in acidic or neutral solutions. The ML2+ species further deprotonated to form M(L - H)(+) and M(L - 2H) comple xes, in which the protons are released from the coordinated OH group. A pM vs pH correlation showed that replacing an NH2 group with a OH group in tre n or NN(2)O222 makes the resulting metal complex less stable. Electronic sp ectra showed that the Cu(II) complexes of both NNO(2)222 and NN(2)O222 adop ted a square pyramidal geometry rather than a trigonal bipyramidal geometry . The X-ray crystal structure analysis of the zinc complex [Zn(OH)(mu -NNO( 2)222 - H)Zn(NNO(2)222)](2+), as its [BF4](-) salt, shows a dinuclear molec ule containing two zinc ions, each coordinated in a distorted trigonal bipy ramid. The coordination environment at one zinc atom is composed of the fou r donor groups of a mono-O-deprotonated ligand NNO(2)222 and a hydroxyl ion with the central nitrogen atom of the ligand and the hydroxyl ion in equat orial positions. The oxygen atom of the deprotonated alkoxo group bridges t o the second zinc atom, which is coordinated by this atom and one undeproto nated ligand NNO2222.