ORIENTATION AND RESTRICTED ROTATION OF LOPSIDED AROMATIC LIGANDS - OCTAHEDRAL COMPLEXES DERIVED FROM CIS-RUCL2(ME(2)SO)(4)

The solution and solid state structures of two octahedral Ru(II) compl exes, cis,cis,cis-RuCl2(Me(2)SO)(2)(py)(Me(3-)Bzm) (Me(3)Bzm = 1,5,6-t rimethylbenzimidazole, py = pyridine) (1) and cis,cis,cis-RuCl2(Me(2)S O)(2)(Me(3)Bzm)(2) (2), were compared. 2, the subject of a preliminary report, is described in more detail here. 1 has two possible geometri c isomers with py trans to Cl in one (position ''a'') and trans to Me( 2)SO in the other (position ''b''), Me(3)Bzm occupying the other posit ion in each isomer. The X-ray structure of 1 revealed that py is at '' a''). Since Me(3)Bzm is lopsided, each Me(3)Bzm has two possible orien tations related by a rotation of similar to 180 degrees about the Ru-N 3 bond; there are two possible atropisomers for each geometric isomer of 1 and four for 2. For 1, the solid state structure shows that Me(3) Bzm adopts the orientation with H2 (H on C between the two N's) pointi ng between the two cis Cl ligands, the same disposition as Me(3)Bzm '' b'' in 2 in the solid, For 1, the py signals (two broad py alpha and b eta signals, a sharp gamma signal) in CDCl3 show that py ''a'' is rota ting on the NMR time scale and that only one atropisomer is present. T his interpretation was supported by ROESY and EXSY H-1 NMR spectra. Th e H-1 NMR shift pattern and the NOE data can be understood best if Me( 3)Bzm ''b'' remains primarily in the orientation found in the solid. T he solution data for 1, with the nonlopsided and sterically less deman ding py ligand, provide insight into the more complicated properties o f 2. For 2, there is a marked dispersion of H-1 NMR signals of Me(3)Bz m ''a'' between the two atropisomers, which have nearly equal stabilit y. One atropisomer is a head-to-head (HH) and the other a head-to-tail (HT) species. Me(3)Bzm ''a'' flips between the two species. Thus, lig and ''a'' is fluxional in both complexes. The dispersion of Me(3)Bzm ' 'a'' signals is due to the effect of Me(3)Bzm ''b'' anisotropy. For 1 and both atropisomers of 2, Me(3)Bzm ''b'' prefers one orientation, wh ich appears to be the most hindered orientation. We postulate that the H2 of Me(3)Bzm ''b'' is electrostatically attracted to the two cis ha lides, accounting for this surprising result. Crystallographic details for 1 are as follows: C19H29Cl2N3O2RuS2, P2(1)/c, a = 10.947(1) Angst rom, b = 9.046(1) Angstrom, c = 24.221(2) Angstrom, D(calcd) = 1.580 g cm(-3), Z = 4, R = 0.026 for 4627 independent reflections.