Carbonate-bridged dinuclear lanthanide(III) complexes of chiral macrocycle

Abstract

Mononuclear Eu(III) and Dy(III) complexes of the chiral hexaaza macrocycle L, 2(R),7(R),18(R),23(R)-1,8,15,17,24,31-hexaazatricyclo[25.3.1.1.0.0]-dotriaconta-10,12,14,26,28,30-hexaene have been obtained as chloride derivatives (1 and 2, respectively) and characterized by using spectroscopic methods. The X-ray crystal structure of 2 has been determined. The reaction of two equivalents of mononuclear 1 or 2 complexes with one equivalent of sodium carbonate results in the formation of dinuclear complexes [Eu2L2(µ2-CO3)(H2O)3]Cl43 and [Dy2L2(µ2-CO3)(H2O)3]Cl44, respectively. The X-ray crystal structure of complex 3 reveals two macrocyclic [EuL]3+ units bridged by the carbonate anion. The carbonate anion is bound to the Eu(III) ions using the η1:η2-µ2 bridging mode. The binding of carbonate anions to the [EuL]3+ or [DyL]3+ macrocyclic units has also been studied in solution using spectroscopic methods. The titration of the starting paramagnetic mononuclear chloride derivatives 1 and 2 monitored by 1H NMR spectroscopy revealed initial formation of dinuclear species, followed by the formation of complexes possessing additional carbonate ligands. The dinuclear carbonate complexes 3 and 4 are also formed in the reactions of the respective mononuclear chloride complexes with gaseous CO2. Lanthanide(III) complexes of the chiral hexaaza macrocycle L are effective for catalytic hydrolysis of bis(4-nitrophenyl) phosphate BNPP. Within the series of the studied mononuclear complexes: nitrate derivatives [LnL(NO3)(H2O)](NO3)2 (Ln = Ho, Er), [LnL(H2O)2](NO3)3 (Ln = Tm, Yb, Lu) and chloride derivatives [LnL(H2O)2]Cl3 (Ln = Tm, Yb, Lu, Y) the catalytic activity of the complexes is increased with increasing ionic size. Kinetic studies revealed that most active is the dinuclear europium(III) complex 3 which catalyzes the hydrolysis of BNPP with a rate kobs = 6.5 × 10−5 s−1 (at 0.3 mM concentration of the catalyst) at pH 8.5 and 37 °C.