Self-Assembled Metallacycles with Pyrazine Edges:  A New Example in Which the <i>Unexpected</i> Molecular Triangle Prevails over the <i>Expected</i> Molecular Square

The combination of cis-protected metal fragments with linear linkers is expected to yield molecular squares. We found instead that treatment of the 90° angular precursor <i>trans</i>-[RuCl<sub>2</sub>(dmso−S)<sub>4</sub>] (<b>1</b>) with an equivalent amount of the linear and rigid pyrazine (pyz) linker unexpectedly yields, in a number of different experimental conditions, the molecular triangle [{<i>trans,cis</i>-RuCl<sub>2</sub>(dmso−S)<sub>2</sub>(μ-pyz)}<sub>3</sub>] (<b>3</b>), together with polymeric material. Very similar results were also obtained from the reaction between <b>1</b> and the preformed corner fragment <i>trans,cis,cis</i>-[RuCl<sub>2</sub>(dmso−S)<sub>2</sub>(pyz)<sub>2</sub>] (<b>6</b>). In both cases, the expected molecular square [{<i>trans,cis</i>-RuCl<sub>2</sub>(dmso−S)<sub>2</sub>(μ-pyz)}<sub>4</sub>] (<b>4</b>) was observed only as a transient species. These results suggest that <b>3</b>, which is the first example of a <i>neutral</i> molecular triangle with octahedral metal corners and pyrazine edges, is both the thermodynamic and the kinetic product of the reactions described above. The X-ray structure of <b>3</b> shows that the main distortions from ideal coordination geometry concern the N−Ru−N angles, which are narrower than 90°, and the coordination bonds of pyz. The pyrazine molecules, which are basically planar, are significantly tilted from linearity. Calculations performed on <b>6</b> indicated that the N−Ru−N angle is ca. six times more rigid than the tilt angle of pyrazine. The structural and theoretical findings on <b>3</b> and <b>6</b>, together with the previous examples of molecular triangles and squares with cis-protected metal corners and linear pyz edges, suggest that the entropically favored molecular triangles might be preferred over the expected molecular squares with metal corner fragments that spontaneously favor N<sub>pyz</sub>−M−N<sub>pyz</sub> angles narrower than 90° because of the presence of ancillary ligands with significant steric demand on the coordination plane. The rather-flexible coordination geometry of pyrazine can accommodate the moderate distortions from linearity required to close the small metallacycle with modest additional strain.