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Self-Assembled Metallacycles with Pyrazine Edges: A New Example in Which the Unexpected Molecular Triangle Prevails over the Expected Molecular Square
journal contribution
posted on 2007-12-24, 00:00 authored by Sofia Derossi, Massimo Casanova, Elisabetta Iengo, Ennio Zangrando, Mauro Stener, Enzo AlessioThe 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.
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distortionNew Examplepyrazine moleculespyrazine edgestrancoordination planepyz edgesequivalent amountMolecular SquareThe combinationmetal corner fragmentssteric demandcoordination geometry concernoctahedral metal cornerslinkerlinearitycoordination bondstilt angleUnexpected Molecular Triangle Prevails
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