Synthesis, characterization and coordination chemistry of some selenium-containing macrocyclic Schiff bases

Abstract

The condensation of bis(o-formylphenyl) selenide with 1,2-diaminoethane or 1,3-diaminopropane yielded the new macrocyclic ligands 1 and 2 in very good yields. Crystals of 1 are monoclinic, space group P2₁/c with a=10.0648(10) Å, b=37.390(4) Å, c=8.0073(11) Å, Z=4 and those of 2 are triclinic, space group P1̅ with a=7.200(3), b=9.137(3), c=12.780(6) Å, Z=1. The coordination chemistry of 1 and 2 has been studied with the "soft" metal ion Pd^II and "hard" metal ions Ni^II and Co^II. Reaction of [PdCl₂(COD)] with 1 mol-equiv. of 1 or 2 in refluxing methanol and addition of NH₄PF₆ afforded the PdII complexes 5 and 6, respectively. Compound 5 is an unexpected partially hydrolyzed complex. It crystallizes in the tetragonal space group P4₁2₁2 with a=10.7737(10) Å, b=10.7737(10) Å, c=28.190(5) Å, Z=4. The structure shows the PdII ion coordinated to a square-planar arrangement of two selenium and two nitrogen atoms. Reaction of NiCl₂·6H₂O with 1 or 2 and NH4PF6 yielded complexes 7 and 8, respectively. The assignment of octahedral geometries to [Ni(L)](PF₆)₂] (7: L=1; 8: L=2) follows from their paramagnetism [μ_eff=3.23 (7), 1.81 (8) B.M.] and their UV/Vis spectra. This was also confirmed by single-crystal X-ray studies. The crystal structures of 7 and 8 showed a distorted octahedral nickel center coordinated by two selenium and four nitrogen atoms. 7: Monoclinic, space group P2₁/c; a=9.979(2) Å, b=18.901(4) Å, c=19.333(4) Å; Z=4. 8:Orthorhombic, space group Pna2₁; a=20.277(5), b=20.865(5), c=9.416(2) Å; Z=4. The cyclic voltammograms of 7 and 8 in MeCN solution show well-behaved quasi-reversible Ni^III/Ni^II couples. The analogous CoII complex, [Co(1)](PF₆)₂] (9) also shows a quasi-reversible Co^III/Co^II couple.