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Orientation and Restricted Rotation of Lopsided Aromatic Ligands. Octahedral Complexes Derived from cis-RuCl2(Me2SO)4

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journal contribution
posted on 24.04.1996, 00:00 by Enzo Alessio, Mario Calligaris, Marian Iwamoto, Luigi G. Marzilli
The solution and solid state structures of two octahedral Ru(II) complexes, cis,cis,cis-RuCl2(Me2SO)2(py)(Me3Bzm) (Me3Bzm = 1,5,6-trimethylbenzimidazole, py = pyridine) (1) and cis,cis,cis-RuCl2(Me2SO)2(Me3Bzm)2 (2), were compared. 2, the subject of a preliminary report, is described in more detail here. 1 has two possible geometric isomers with py trans to Cl in one (position “a”) and trans to Me2SO in the other (position “b”), Me3Bzm occupying the other position in each isomer. The X-ray structure of 1 revealed that py is at “a”. Since Me3Bzm is lopsided, each Me3Bzm has two possible orientations related by a rotation of ∼180° about the Ru−N3 bond; there are two possible atropisomers for each geometric isomer of 1 and four for 2. For 1, the solid state structure shows that Me3Bzm adopts the orientation with H2 (H on C between the two N's) pointing between the two cis Cl ligands, the same disposition as Me3Bzm “b” in 2 in the solid. For 1, the py signals (two broad py α and β signals, a sharp γ signal) in CDCl3 show that py “a” is rotating on the NMR time scale and that only one atropisomer is present. This interpretation was supported by ROESY and EXSY 1H NMR spectra. The 1H NMR shift pattern and the NOE data can be understood best if Me3Bzm “b” remains primarily in the orientation found in the solid. The solution data for 1, with the nonlopsided and sterically less demanding py ligand, provide insight into the more complicated properties of 2. For 2, there is a marked dispersion of 1H NMR signals of Me3Bzm “a” between the two atropisomers, which have nearly equal stability. One atropisomer is a head-to-head (HH) and the other a head-to-tail (HT) species. Me3Bzm “a” flips between the two species. Thus, ligand “a” is fluxional in both complexes. The dispersion of Me3Bzm “a” signals is due to the effect of Me3Bzm “b” anisotropy. For 1 and both atropisomers of 2, Me3Bzm “b” prefers one orientation, which appears to be the most hindered orientation. We postulate that the H2 of Me3Bzm “b” is electrostatically attracted to the two cis halides, accounting for this surprising result. Crystallographic details for 1 are as follows:  C19H29Cl2N3O2RuS2, P21/c, a = 10.947(1) Å, b = 9.046(1) Å, c = 24.221(2) Å, D(calcd) = 1.580 g cm-3, Z = 4, R = 0.026 for 4627 independent reflections.

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