Self-assembly of transition metal ion complexes of a hybrid pyrazine–terpyridine ligand

Abstract

A new hybrid pyrazine–terpyridine ligand L (C₃₄H₂₂N₈) and its complexes with different transition metal ions, M (M = Mn(II) 1, Zn(II) 2, Fe(II) 3, Co(II) 4, Cu(II) 5 and Cd(II) 6), have been synthesised. In the presence of a nitrate counter-anion, both Cu(II) and Cd(II) give complexes in which the ratio M : L is 2 : 1, whereas with perchlorate, trifluoromethanesulfonate or tetrafluoroborate, the other metal ions provide solids in which this ratio is 1 : 1. From mass spectral measurements and a single crystal, X-ray structure determination for the Fe(II) complex 3, however, all the latter species are concluded to be 2 : 2 complexes. Both the Fe(II) complex 3 and the Co(II) complex 4, generated from tetrafluoroborate reactant salts, have the composition [M₂L₂F₂(H₂O)](BF₄)₂, the presence of fluoride ligands being presumed to reflect the abstraction of fluoride ions from tetrafluoroborate by the metal ions under the preparative conditions. The crystal structure of complex 3 shows the Fe(II) centres to be inequivalent, one being high-spin and heptacoordinate with a FeN₄F₂O coordination sphere, the other low-spin and octahedral with a FeN₆ sphere. The two ligand molecules differ markedly, one being heptadentate, the other clearly “hypodentate”, with only three N-donor atoms of a terpyridine-like arm coordinated, although their conformations are similar, showing significant differences from that of C₂ symmetry found for the free ligand by a crystal structure determination. Mass spectra are consistent with the Cu(II) and Cd(II) complexes having the composition [M₂L(H₂O)_n(NO₃)_4−n](NO₃)_4−n, and the weak antiferromagnetic coupling observed for the Cu(II) complex is consistent with a preliminary crystal structure determination which indicates that the two Cu(II) centres are not bridged by a pyrazine unit.