Nine non-symmetric pyrazine-pyridine imide-based complexes: reversible redox and isolation of [MII/III(pypzca)2]0/+ when M = Co, Fe

Abstract

The non-symmetric imide ligand Hpypzca (N-(2-pyrazylcarbonyl)-2-pyridinecarboxamide) has been deliberately synthesised and used to produce nine first row transition metal complexes: [M^II(pypzca)₂], M = Zn, Cu, Ni, Co, Fe; [M^III(pypzca)₂]Y, M = Co and Y = BF₄, M = Fe and Y = ClO₄; [Cu^II(pypzca)(H₂O)₂]BF₄, [Mn^II(pypzca)(Cl)₂]HNEt₃. These are the first deliberately prepared complexes of a non-symmetric imide ligand. X-ray crystal structures of [Cu^II(pypzca)₂]·H₂O, [Co^II(pypzca)₂], [Co^III(pypzca)₂]BF₄, [Cu^II(pypzca)(H₂O)₂]BF₄·H₂O and [Mn^II(pypzca)Cl₂]HNEt₃ show that each of the (pypzca)⁻ ligands binds in a meridional fashion via the N₃ donors. In the first three complexes, two such ligands are bound such that the ‘spare’ pyrazine nitrogen atoms are positioned approximately orthogonally to one another and also to the imide oxygen atoms. In MeCN the [M^II/III(pypzca)₂]^0/+ complexes, where M = Ni, Co or Fe, exhibit one reversible metal based M^II/III process and two distinct, quasi-reversible ligand based reduction processes, the latter also observed for M^II = Zn. [Mn^II(pypzca)Cl₂]HNEt₃ displays a quasi-reversible oxidation process in MeCN, along with several irreversible processes. Both copper(II) complexes show only irreversible processes. Variable temperature magnetic measurements show that [Fe^III(pypzca)₂]ClO₄ undergoes a gradual spin crossover from partially high spin at 298 K (3.00 BM) to fully low spin at 2 K (1.96 BM), and that [Co^II(pypzca)₂] remains high spin from 298 to 4 K. All of the complexes are weakly coloured, other than [Fe^II(pypzca)₂] which is dark purple and absorbs strongly in the visible region.