Design, Synthesis, and X-ray Structural Characterization of New Dinucleating Macrocyclic Ligands and a Novel Phenolate-Bridged Dilanthanum(III) Complex

Abstract

The [2+2] template condensation of 6,6‘-bis(aminomethyl)-2,2‘-bipyridyl with 2,6-diformyl-p-cresol in the presence of lanthanum acetate yields a homodinuclear complex of a Schiff base macrocycle (babp)2(dfc)2, 1. The crystal of [La21(OAc)4]·4CH3CN·3H2O is triclinic, space group P1̄, with cell constants a = 10.986(6) Å, b = 12.231(8) Å, c = 23.818(12) Å, α = 86.63(5)°, β = 85.99(5)°, γ = 79.45(4)°, V = 3135(3) Å3, and Z = 2. The decacoordination geometry around each identical La3+ ion is a bicapped dodecahedron made up of two bidentate acetate anions, two phenolate oxygens, and four imine nitrogens. The 1H, 13C, and 2D-COSY NMR and UV−visible spectra of the complex are consistent with the solid state structural information obtained by IR and X-ray diffraction studies. Treatment of a solution of La21(OAc)4 in methanol with sodium cyanotrihydroborate yields the hydrogenated octaaza macrocycle R(babp)2(dfc)2, 2. The [2+2] metal-free condensation of 2,6-bis(aminomethyl)-p-cresol with 2,6-diformylpyridine yields a Schiff base macrocycle (bac)2(dfp)2, 3. Reduction of 3 with sodium borohydride yields the hydrogenated hexaaza macrocycle R(bac)2(dfp)2, 4. Under strongly acidic conditions (48% HBr), the free ligand 4 crystallizes as a hydrobromide salt 4·H2O·7HBr in the monoclinic system, space group C2/m, with cell constants a = 26.737(11) Å, b = 16.885(7) Å, c = 10.325(4) Å, β = 101.94(3)°, V = 4560(3) Å3, and Z = 4. The [2+2] nature of the macrocycles 3 and 4 is identified by 1H and 13C NMR, FAB mass spectra and by X-ray crystallography.