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

A new hybrid pyrazine-terpyridine ligand L (C(34)H(22)N(8)) 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 [M(2)L(2)F(2)(H(2)O)](BF(4))(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 FeN(4)F(2)O coordination sphere, the other low-spin and octahedral with a FeN(6) 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 C(2) 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 [M(2)L(H(2)O)(n)(NO(3))(4-n)](NO(3))(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.