Two terdentate pyrazine‐based amide ligands have been prepared from methyl pyrazine‐2‐carboxylate and 2‐(aminomethyl)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(L1M)2](BF4)·${1 \over 4}$H2O and [CoIII(L1E)2](BF4)·${1 \over 2}$H2O, have been prepared as building blocks and the silver(I) coordination of the ligands also probed, forming {[AgI(HL1M)]BF4}∞ and [AgI2(HL1LE)2](BF4)2. The [CoIII(L1E)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 [{CoIII(L1E)2}2AgI](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 [CoIII(L1Mox)(L1M)](BF4) and [CoIII(L1Mox)2](BF4). Fresh samples of [CoIII(L1M)2](BF4)·${1 \over 4}$H2O and [CoIII(L1E)2](BF4)·${1 \over 2}$H2O 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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Self-assembly of 1:1:2 M (BF ) (M=Zn or Fe), pyrazine-2,5-dicarbaldehyde (1) and 2-(2-aminoethyl)pyridine gave trimetallic triangle architectures rather than the anticipated tetrametallic [2×2] squares. Options for the nontrivial synthesis of 1 are considered, and synthetic details provided for both preferred routes. Rare cyclohelicate triangle architectures are observed for the pair of structurally characterized yellow-brown [Zn L ](BF ) and dark green [Fe L ](BF ) complexes of the neutral bis-terdentate Schiff base L. In order to form these pyrazine-edged triangles, the octahedral metal ions-with all 6 N-donors provided by the terdentate binding pockets of two L-are located 0.4-0.5 Å out of the plane of the bridging pyrazines, towards the center of the triangle. Density functional theory calculations confirm that simple particle counting entropic arguments, which predict triangles over squares, are correct here, with the triangles shown to be energetically favored over the corresponding squares. However, importantly, DFT analysis of these and related triangle versus square systems also show that vibrational contributions to entropy dominate and may significantly influence the preferred architecture, such that simple particle counting cannot in general be reliably employed to predict the observed architecture.

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