CRYSTALS CONTAINING CONFORMERS - A RARE CASE IN WHICH A SOLID CLOSELYREFLECTS A SOLUTION EQUILIBRIUM MIXTURE

The factors influencing the structural, energetic and dynamic chemistr y of complexes with coordinated aromatic N-donor ligands (L) are not w ell understood. Complexes of the type cis,cis,cis-RuCl2(Me2SO)(2)(L1)( L2) with L1 = L2 and L1 not equal L2 (L1 trans to Cl, L2 trans to Me2S O) have afforded highly informative results. Here we report studies on two such complexes. One complex provides a rare example of a compound in which the solid state closely reflects the solution state, both in the structure of species involved in a conformational equilibrium and in the position of that equilibrium. In effect,, the components of a fluxional equilibrium in solution are trapped, almost unchanged, in th e solid state. Specifically, the complex cis,cis,cis-RuCl2(Me2SO)(2)(3 ,5-lut)(1,2-Me(2)Im) (2) (L1 = 3,5-lut = 3,5-lutidine; L2 = 1,2-Me(2)I m = 1,2-dimethylimidazole) exists as mio conformers (R1 and R2) in sol ution and in the solid slate (ratio = 0.63(2):0.37(2)); these rotamers differ by a 180 degrees rotation of 1,2-Me(2)Im about the Ru-N bond. The orientations of 3,5-lut and 1,2-Me(2)Im found in the solid stale e xplain the solution data well, both in terms of H-1 NMR chemical shift dispersion and rotation rate of the N-donor ligands. However, there i s a displacement of the 1,2-Me(2)Im toward the 3,5-lut in the less abu ndant conformer R2 in the solid (N(1)-Ru-N(2) angle 78.1(3)degrees in R2 vs 93.5(2)degrees in R1), this clear feature in the solid is a type of result that cannot normally be derived from solution investigation s or from the study of one conformer in the solid state. Although cis, cis,cis-RuCl2(Me2SO)(2)(1,2-Me(2)Im)(Me(3)Bzm) (1) (Me(3)Bzm = 1,5,6-t rimethylbenzimidazole) exists as a mixture of dynamic rotamers in solu tion, the crystal structure is typical and reveals the presence of jus t the rotamer shown to be most abundant by NMR methods. 1 crystallizes in the triclinic space group <P(1)over bar>, Z = 2, with a = 8.863(4) Angstrom, b = 11.907(6) Angstrom, c = 12.406(6) Angstrom, alpha = 74. 32(4)degrees, beta = 83.27(4)degrees, and gamma = 88.89(3)degrees; 2 i n the orthorhombic space group Pbca, Z = 8, with a = 12.587(2) Angstro m, b = 12.329(3) Angstrom, and c = 28.382(5) Angstrom. With the added results from this study, it is now clear that the cis,cis-RuCl2(Me2SO) (2) moiety maintains a relatively fixed arrangement that appears also to dominate in solution. The stability of this arrangement is sufficie nt that the strain evident in the structures appears to be relieved by displacement of the L Ligands.