%0 Journal Article %A Derossi, Sofia %A Casanova, Massimo %A Iengo, Elisabetta %A Zangrando, Ennio %A Stener, Mauro %A Alessio, Enzo %D 2007 %T Self-Assembled Metallacycles with Pyrazine Edges:  A New Example in Which the Unexpected Molecular Triangle Prevails over the Expected Molecular Square %U https://acs.figshare.com/articles/journal_contribution/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/2967199 %R 10.1021/ic7019507.s002 %2 https://ndownloader.figshare.com/files/4666375 %K distortion %K New Example %K pyrazine molecules %K pyrazine edges %K tran %K coordination plane %K pyz edges %K equivalent amount %K Molecular SquareThe combination %K metal corner fragments %K steric demand %K coordination geometry concern %K octahedral metal corners %K linker %K linearity %K coordination bonds %K tilt angle %K Unexpected Molecular Triangle Prevails %X 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 trans-[RuCl2(dmso−S)4] (1) with an equivalent amount of the linear and rigid pyrazine (pyz) linker unexpectedly yields, in a number of different experimental conditions, the molecular triangle [{trans,cis-RuCl2(dmso−S)2(μ-pyz)}3] (3), together with polymeric material. Very similar results were also obtained from the reaction between 1 and the preformed corner fragment trans,cis,cis-[RuCl2(dmso−S)2(pyz)2] (6). In both cases, the expected molecular square [{trans,cis-RuCl2(dmso−S)2(μ-pyz)}4] (4) was observed only as a transient species. These results suggest that 3, which is the first example of a neutral 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 3 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 6 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 3 and 6, 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 Npyz−M−Npyz 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. %I ACS Publications