(2,7-DIMETHYLOCTADIENEDIYL)RUTHENIUM(IV) COMPLEXES - ISOMERISM AND SOLUTION EQUILIBRIA FOR DICHLOROBIS(MU-CHLORO)BIS[(1-3-ETA-6-8-ETA)-2,7-DIMETHYLOCTADIENEDIYL]DIRUTHENIUM(IV) AND RELATED MONOMERIC SOLVATES

The1H NMR spectra of [RuC1(µ-C1)(?3:?3)-C10H16)]2(1) in noncoordinating solvents indicate that an equilibrium between two diastereoisomeric forms is rapidly established in solution [formula omitted] and provide evidence that the local C2symmetry of the [Ru(?3:?3-C10H116)] fragments is retained. Monomeric solvates of formula [RuC12(?3:?3-C10H16)S] form on dissolution of 1 in coordinating solvents and have been isolated for S = pyridine (2), DMSO (S-bound) (3), MeCN (4), and DMF (O-bound) (5). A trigonal-bipyramidal ruthenium coordination, with the octadienediyl ligand locked in a local C2symmetry configuration and occupying two of the equatorial sites, is proposed for all the solvates. In solution, 2 and 3 feature solvent coordination uniquely in the third equatorial site, and the chiral octadienediyl configuration results in diastereotopic methyl groups coordinated to sulfur in 3. Variable-temperature1H NMR measurements in acetone-d6show that these diastereotopic methyl groups are rendered equivalent as a result of intermolecular DMSO exchange between the enantiomers (ΔG*308= 66.5 ± 0.5 kJ mol-1). The formation of 4 and 5 from 1 proceeds via initial formation of equatorial solvates, which rapidly isomerize to an equilibrium mixture containing both axially and equatorially solvated isomers. Equilibrium constants and reaction enthalpies for the isomerizations have been determined. The kinetic activation parameters for the interconversion of the isomers of 4 have also been evaluated. A mechanism involving solvent exchange is proposed. Equilibria of the type [formula omitted] ;= [formula omitted] or [formula omitted]? 1 are set up on dissolution of 2-5 in certain coordinating or noncoordinating solvents, respectively. Evaluation of the equilibrium constants suggests that the affinities for coordination to the monomers of 1 fall in the sequence pyridine >DMSO >MeCN >DMF. © 1990, American Chemical Society. All rights reserved.