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-CH2CHC6H4NMe2 (1), 2-isopropenyl-N,N-dimethylaniline, o-CH2C(CH3)C6H4NMe2 (2), 3-butenyldimethylamine, CH2CHCH2CH2NMe2 (3), 2-allylpyridine, CH2CHCH2C5H4N (4), isomesityl oxide (4-methyl-4-penten-2-one), CH2C(CH3)CH2COCH3 (5), 2-methoxystyrene, o-CH2CHC6H4OMe (6), and 3-butenylmethyl ether, CH2CHCH2CH2OCH3 (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 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 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 CC 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; CC = 1.355(9) Å] indicate that the alkene is more weakly bound than in either of its diastereomeric Ru(II) precursors.

Effects of Ancillary Ligands on Redox and Chemical Properties of Ruthenium Coordinated Azoaromatic Pincer

“…The two d N–N bond lengths in the complex 4 are 1.309(3) and 1.300(2) Å. The Ru–O bond lengths in complex 4 are 2.0428(17) and 2.0821(15) Å, which are commonly observed − in the other Ru­(II)–acac complexes. The ligand DTBCat coordinates via deprotonation and can exists in different oxidation states (catecholate, semiquinone, and quinonoid), C–O bond length of the coordinated ligand is an indicator − of its oxidation state.…”

Redox routes to arenechromium complexes of two-, three- and four-electron alkynes; structure and bonding in paramagnetic [Cr(CO)L(η-RCCR)(η-arene)]+

“…[Note that although this orbital is bonding ] z (z = 0 and 1). 9 The Dewar-Chatt-Duncanson model 10 for metal-alkene bonding accounts for the difference in that oxidation will result in depopulation of a metal-alkene π-bonding orbital (either directly, or indirectly by depopulation of a metal orbital). Oxidation therefore inhibits metal-to-alkene π-back donation and the metal-alkene bond is weakened and lengthened.…”

Fluxional Processes in Diamagnetic and Paramagnetic Allyl Dicarbonyl and 2-Methylallyl Dicarbonyl Molybdenum Histidinato Complexes as Revealed by Spectroscopic Data and Density Functional Calculations

“…A 17-18-19-electron triad of stable isostructural organometallic FeCp complexes has been presented by Astruc and co-workers, [10] and Bennett et al have reported a series of air-stable ruthenium alkene complexes with 17 and 18 electrons. [17] Although significant progress has been made recently, it is probably fair to say that tools for the comprehensive study of paramagnetic organometallic species are by far not as well established as for coordination compounds. This includes spectroscopic techniques in solution as well as computational work.…”