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‐(aminomethyl)pyridine (H L 1M ) or 2‐(2‐aminoethyl)pyridine (H L 1E ) 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 ](BF 4 ) · ${1 \over 4}$ H 2 O and [Co III ( L 1E ) 2 ](BF 4 ) · ${1 \over 2}$ H 2 O, have been prepared as building blocks and the silver(I) coordination of the ligands also probed, forming {[Ag I (H L 1M )]BF 4 } ∞ and [Ag I 2 (H L 1LE ) 2 ](BF 4 ) 2 . The [Co III ( L 1E ) 2 ](BF 4 ) 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 ](BF 4 )(NO 3 ) 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 )](BF 4 ) and [Co III ( L 1Mox ) 2 ](BF 4 ). Fresh samples of [Co III ( L 1M ) 2 ](BF 4 ) · ${1 \over 4}$ H 2 O and [Co III ( L 1E ) 2 ](BF 4 ) · ${1 \over 2}$ H 2 O undergo a chemically reversible one‐electron reduction in dry acetonitrile, at –0.71 and –0.48 V vs. 0.01 M AgNO 3 /Ag, respectively, consistent with the methylene‐linked ligand being better able to stabilise the higher oxidation state of cobalt than the ethylene‐linked ligand.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)