Polyamine Macrocycles Incorporating a Phenanthroline Unit: Their Synthesis, Basicity, and Cu(II) Coordination

Abstract

Synthesis and characterization of the two new polyamine macrocycles 2,5,8,11,14-pentaaza[15]-16,29-phenanthrolinophane (L1) and 8-methyl-8-aza-1(1,4),11(1,4)-dipiperazina[15]-16,29-phenanthrolinophane (L2) are reported. Both ligands incorporate a 2,9-phenanthroline unit as integral part of their cyclic structure. L1 contains a pentaamine chain linking the 2,9-phenanthroline positions. L2 contains two piperazine rings within the macrocyclic framework. The basicity constants and the formation constants of their complexes with Cu(II) have been determined by potentiometric measurements in aqueous solution (298.1 K, I = 0.1 mol dm-3). Both L1 and L2 form mono- and binuclear complexes in aqueous solution. In the [CuL1]2+ and [CuL2]2+ complexes, the metal ion is coordinated by the aromatic nitrogens and one or two nitrogens adjacent to the phenanthroline moiety. The remaining polyamine chain is not coordinated. These results are confirmed by the crystal structure of [H2L2Cu(NCS)2](ClO4)2·3H2O (space group P1̄, a = 9.770(10) Å, b = 12.027(5) Å, c = 18.006(7) Å, α = 106.87(3)°, β = 90.22(8)°, γ = 104.48(8)°, V = 1954(2) Å3, Z = 2, R = 0.0971, wR2 = 0.2553). The metal ion is five-coordinated by the phenanthroline nitrogens, by one adjacent amine nitrogen, and by two isothiocyanate ions. The noncoordinated polyamine chain can bind a second Cu(II) ion in aqueous solution, giving binuclear [Cu2L]4+ complexes (L = L1 or L2). In the case of L2, the formation of the binuclear complex leads to piperazine inversion from the chair to the boat conformation, as shown by the crystal structure of [Cu2L2(μ-OH)(H2O)(ClO4)](ClO4)2·2H2O (space group P21/a, a = 19.573(2) Å, b = 9.320(10) Å, c = 21.552(2) Å, β = 108.556(9)°, V = 3727(4) Å3, Z = 4, R = 0.0835, wR2 = 0.2759). This structure displays a piperazine ring unusually bridging two metal cations.