SYNTHESIS AND CHARACTERIZATION OF NEUTRAL AND CATIONIC 2,7-DIMETHYLOCTADIENEDIYLRUTHENIUM(IV) COMPLEXES - X-RAY CRYSTAL-STRUCTURE OF CIS-BIS(ACETONITRILE)CHLORO[(1,2,3,6,7,8-ETA)-2,7-DIMETHYLOCTA-2,6-DIENE-1,8-DIYL]RUTHENIUM(IV) TETRAFLUOROBORATE

Reactions of [{Ru(eta-3:eta-3-C10H16)Cl(mu-Cl)}2] 1 with the ligands (L) P(OMe)3, Bu(t)NC or Pr(i)NC in chloroform solution give the complexes [Ru(eta-3:eta-3-C10H16)Cl2L] 2-4. Treatment of these complexes with Ag[BF4] and additional free ligand in acetone solution gives the salts [Ru(eta-3:eta-3-C10H16)ClL2][BF4] 5-7. The reaction of 1 with Ag[BF4] in acetonitrile solution yields [Ru(eta-3:eta-3-C10H16)Cl(NCMe)2][BF4] 8 which slowly converts in solution into an alternative stereoisomer 9. The structures proposed for 2-9 all feature an approximate trigonal-bipyramidal co-ordination at ruthenium, occupation of two equatorial sites by the eta-3:eta-3-2,7-dimethyloctadienediyl ligand, and a local C2-symmetry configuration for the octadienediyl chain. In 2-4 it is the two-electron ligand that occupies the third equatorial site with both chlorides axial. In 5-7 the chloride occupies the third equatorial site and both the two-electron ligands are axial. For the two acetonitrile-containing stereoisomers, the chloride is axial in 8 and equatorial in 9. An equilibrium 8 reversible 9 exists in acetonitrile solution with K298 = 30 +/- 5, k298(forward) = (7.6 +/- 0.6) x 10(-6) s-1, DELTA-H double-ended dagger = 123 +/- 3 kJ mol-1, DELTA-S double-ended dagger = 68 +/- 9 J K-1 mol-1, as determined by H-1 NMR spectroscopy and sampling techniques. The structure of 8 has been confirmed by X-ray diffraction: space group P1BAR with a = 7.80(1), b = 15.69(1), c = 7.63(1) angstrom, alpha = 96.5(1), beta = 87.1(1), gamma = 96.9(1)-degrees and Z = 2. The structure was refined to R = 0.082 for 1766 unique reflections.