Cobalt and Silver Complexes of Terdentate Pyrazine-Based Amide Ligands and Assembly of Monocobalt Building Blocks through a Silver Connector

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, [Co-III(L-1M)(2)](BF4)center dot 1/4H(2)O and [Co-III(L-1E)(2)](BF4)center dot 1/2H(2)O, have been prepared as building blocks and the silver(I) coordination of the ligands also probed, forming {[Ag-I(HL1M)]BF4}(x) and [Ag-2(I)(HL1LE)(2)](BF4)(2). The [Co-III(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(L-1E)(2)}(2)Ag-I](BF4)(NO3)(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 [Co-III(L-1Mox)(L-1M)](BF4) and [Co-III(L-1Mox)(2)](BF4). Fresh samples of [Co-III(L-1M)(2)](BF4)center dot 1/4H(2)O and [Co-III(L-1E)(2)](BF4)center dot 1/2H(2)O undergo a chemically reversible one-electron reduction in dry acetonitrile, at -0.71 and -0.48 V vs. 0.01 M AgNO3/Ag, respectively, consistent with the methylene-linked ligand being better able to stabilise the higher oxidation state of cobalt than the ethylene-linked ligand. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany. 2009)