Cobalt and silver complexes of terdentate pyrazine-based amide ligands and assembly of monocobalt building blocks through a silver connector

Abstract

Two terdentate pyrazine-based amide ligands have been prepared from methyl pyrazine-2-carboxylate and 2-(amino-methyl)pyridine (HL1M) or 2- 2-aminoethyl)pyridine (HL1E) in order to probe the potential of the "spare" nitrogen atom "out: the back" of the pyrazine ring to coordinate to a different metal ion and thereby act as a linker between complexes. Two inert cobalt(III) complexes, [CoIII(L 1M)2](BF4) 1/4H2O and Co III(L1E)2](BF4)-1/2H20, have been prepared as building blocks and the silver(I) coordination of the ligands also probed, forming {[AgI(HL1M)]BF 4}∞, and [AgI2(HL1LE) 2]- (BF4)2. The [CoIII(L 1E)2](BF4) building block has been successfully connected to a second such complex by coordination of silver(I) to a "spare" pyrazine nitrogen atom on each complex, resulting in [{Co III(L1E)2}2AgI](BF 4)(N03)2, All five complexes have been structurally characterised. Mass spectra and cyclic voltammetry studies on "aged" (kept in solution in air for 2 d) samples clearly showed that the cobalt complex of the methylene-linked ligand was prone to slow ligand oxidation, forming [CoIII(L1Mox)(L1M)](BF 4) and [CoIII(L1Mox)2](BF 4). Fresh samples of [CoIII(L1M) 2](BF4)-1/4H20 and oIII(L 1M)2](BF4)-1/2H20 undergo a chemically reversible one- electron reduction in dry acetonitrile, at -0.71 and -0.48 V vs. 0.01 M AgN03/Ag, respectively, consistent with the methylene-linked ligand being better able to stabilise the higher oxidation state of cobalt than the ethylene-linked ligand. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.