%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