ALKENE COMPLEXES OF DIVALENT AND TRIVALENT RUTHENIUM STABILIZED BY CHELATION - DEPENDENCE OF COORDINATED ALKENE ORIENTATION ON METAL OXIDATION-STATE

Zinc amalgam reduction of tris (acetylacetonato)ruthenium(III), [Ru(ac ac)(3)], in the presence of the chelating olefinic N- and O-donor liga nds (LL') 2-vinyl-N,N-dimethylaniline, o-CH2=CHC6H4NMe2 (1), 2-isoprop enyl-N,N-dimethylaniline, o-CH2=C(CH3)C6H4NMe2 (2), 3-butenyldimethyla mine, CH2=CHCH2CH2NMe2 (3), 2-allylpyridine, CH2=CHCH2C5H4N (4), isome sityl oxide (4-methyl-4-penten-2-one), CH2=C(CH3)CH2COCH3 (5), 2-metho xystyrene, o-CH2=CHC6H4OMe (6), and 3-butenylmethyl ether, CH2=CHCH2CH 2OCH3 (7) gives the corresponding bis(acetylacetonato)ruthenium(II) co mplexes [Ru(acac)(2)(LL')] (8-14). These undergo one-electron oxidatio n by cyclic voltammetry to the corresponding cations [Ru(acac)(2)(LL') ](+), the process being reversible at both room temperature and -60 de grees C. The cations were isolated as deep blue, paramagnetic PF6 or S bF6 salts from the oxidation of the ruthenium(II) precursors 8-12 and 14 with Ag+ or [FeCp2](+) salts; they are the first stable alkene comp lexes of ruthenium(III). At both oxidation levels, coordination of the prochiral alkene gives rise to a pair of diastereomers, labeled a, b at the Ru(II) level, a(+), b(+) at the Ru(III) level, whose redox pote ntials E-1/2 (Ru3+/2+) differ by ca. 100 mV. The equilibrium a/b ratio at the Ru(II) level is ca. 1:9, although for 8, 10, and 11 this is es tablished only after several hours at ca. 100 degrees C, the ratio in the complexes immediately after isolation being ca. 2:3. Selective rem oval of the more easily oxidized diastereomer of the 2-vinyl-N,N-dimet hylaniline complex 8a by treatment of a 2:3 mixture with ca. 0.5 equiv of Ag+ provides pure 8b, which undergoes reversible one-electron oxid ation at -60 degrees C to 8b(+). Above -10 degrees C, 8b+ isomerizes t o an equilibrium mixture (ca. 85:15) of 8a(+) and 8b(+), as shown by U V-visible spectroelectrochemistry. Thus, both diastereomeric preferenc e and rate of interconversion are strongly dependent on the oxidation state (number of metal d-electrons). The metrical parameters pertainin g to alkene coordination in the diastereomers 8a and 8b do not differ significantly, the metal-carbon distances being 2.159(4), 2.144(4) Ang strom (8a), 2.142(2), 2.153(3) Angstrom (8b) and the C=C distances bei ng 1.383(5) Angstrom (8a) and 1.382(4) Angstrom (8b). The correspondin g distances in the Ru(III) complex [8a](+)[SbF6](-) [Ru-C=2.239(6), 2. 236(7) Angstrom; C=C = 1.355(9) A] indicate that the alkene is more we akly bound than in either of its diastereomeric Ru(II) precursors.