FACILE PROPELLER ROTATION IN METALLACYCLOPROPANES - SYNTHESIS AND DYNAMIC BEHAVIOR OF NEW TETRAFLUOROETHYLENE-RUTHENIUM COMPLEXES - CRYSTAL AND MOLECULAR-STRUCTURE OF [RU(ETA-5-C5ME5)CL(ETA-2-C2F4)]2

Reaction of the tetramer [RuCp*Cl]4 (CP* = C5Me5) with tetrafluoroethylene gives the sparingly soluble dimer [RuCp*(C2F4)Cl]2 (3), the molecular structure of which has been determined by a single crystal X-ray diffraction study. Crystal data for 3: monoclinic, space group P2(1)/n; Z = 2; a = 8.059(2) angstrom, b = 20.504(6) angstrom, c = 8.830(1) angstrom, beta = 116.28(1)-degrees; V = 1308.3(5) angstrom3; T = 298 K; R = 0.0456; R(w) = 0.0530 based on 2461 reflections for F(o) greater-than-or-equal-to nsigma(F(o)) (n = 0). Reaction of this dimer with a neutral donor ligand L gives soluble monomeric complexes [RuCp*(C2F4)(L)Cl] (5a-d L = PMe3, P(OMe)3, pyridine, t-BuNC), while reaction with anionic chelating ligands LX- gives soluble monomers [RuCp* (C2F4) (LX)] (6a,b LX = 8-hydroxyquinolinato, 8-hydroxyquinaldinato; 4a-c LX = acetylacetonato, trifluoroacetylacetonato, 2,6-dimethylheptane-3,5-dionato). Despite having metallacyclopropane structures that are usually associated with high barriers to propeller rotation, variable temperature F-19-NMR spectra of the soluble tetrafluoroethylene complexes 4-6 reveal unprecedently low barriers to propeller rotation of the C2F4 ligand about the Ru-olefin axis. A series of free energies of activation (DELTAG(double dagger)) for propeller rotation have been measured. The observed low barriers are explained by considering the [RuCp*LX] fragment, to a first order approximation, as an ML5 fragment with filled, orthogonal, and almost degenerate orbitals available on the Ru fragment for pi back-bonding with the olefin.