Alkene Complexes of Divalent and Trivalent Ruthenium Stabilized by Chelation. Dependence of Coordinated Alkene Orientation on Metal Oxidation State

Abstract

Zinc amalgam reduction of tris(acetylacetonato)ruthenium(III), [Ru(acac)3], in the presence of the chelating olefinic N- and O-donor ligands (LL‘) 2-vinyl-N,N-dimethylaniline, o-CH2CHC6H4NMe2 (1), 2-isopropenyl-N,N-dimethylaniline, o-CH2C(CH3)C6H4NMe2 (2), 3-butenyldimethylamine, CH2CHCH2CH2NMe2 (3), 2-allylpyridine, CH2CHCH2C5H4N (4), isomesityl oxide (4-methyl-4-penten-2-one), CH2C(CH3)CH2COCH3 (5), 2-methoxystyrene, o-CH2CHC6H4OMe (6), and 3-butenylmethyl ether, CH2CHCH2CH2OCH3 (7) gives the corresponding bis(acetylacetonato)ruthenium(II) complexes [Ru(acac)2(LL‘)] (8−14). These undergo one-electron oxidation by cyclic voltammetry to the corresponding cations [Ru(acac)2(LL‘)]+, the process being reversible at both room temperature and −60 °C. The cations were isolated as deep blue, paramagnetic PF6 or SbF6 salts from the oxidation of the ruthenium(II) precursors 8−12 and 14 with Ag+ or [FeCp2]+ salts; they are the first stable alkene complexes 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 potentials E1/2 (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 established only after several hours at ca. 100 °C, the ratio in the complexes immediately after isolation being ca. 2:3. Selective removal of the more easily oxidized diastereomer of the 2-vinyl-N,N-dimethylaniline complex 8a by treatment of a 2:3 mixture with ca. 0.5 equiv of Ag+ provides pure 8b, which undergoes reversible one-electron oxidation at −60 °C to 8b+. Above −10 °C, 8b+ isomerizes to an equilibrium mixture (ca. 85:15) of 8a+ and 8b+, as shown by UV−visible spectroelectrochemistry. Thus, both diastereomeric preference and rate of interconversion are strongly dependent on the oxidation state (number of metal d-electrons). The metrical parameters pertaining to alkene coordination in the diastereomers 8a and 8b do not differ significantly, the metal−carbon distances being 2.159(4), 2.144(4) Å (8a), 2.142(2), 2.153(3) Å (8b) and the CC distances being 1.383(5) Å (8a) and 1.382(4) Å (8b). The corresponding distances in the Ru(III) complex [8a]+[SbF6]- [Ru−C = 2.239(6), 2.236(7) Å; CC = 1.355(9) Å] indicate that the alkene is more weakly bound than in either of its diastereomeric Ru(II) precursors.