New Ru(II) Complexes Containing Oxazoline Ligands As Epoxidation Catalysts. Influence of the Substituents on the Catalytic Performance

Abstract

The synthesis of a family of new Ru complexes containing the facial tridentate ligand with general formula [Ru(II)(T)(D)(X)](n+) (T = trispyrazolylmethane (tpm); D = ((4S,4'S)-(-)-4,4',5,5'-tetrahydro-4,4'-bis(1-methylethyl)-2,2'-bioxazole) (iPr-box-C) or N-(1-hydroxy-3-methylbutan-(2S)-(-)-2-yl)-(4S)-(-)-4-isopropyl-4,5-dihydrooxazole-2-carbimidate (iPr-box-O); X = Cl, H(2)O) has been described. All complexes have been spectroscopically characterized in solution through (1)H NMR and UV-vis techniques, and the redox properties of complexes have also been studied by means of cyclic voltammetry (CV). Furthermore, the chloro complexes presented here have been characterized in the solid state through monocrystal X-ray diffraction analysis. The oxazolinic iPr-box-C ligand undergoes a Ru-assisted hydrolysis reaction generating the corresponding amidate anionic ligand iPr-box-O, that keeps coordinated to the Ru metal center and that produces a strong σ-donation effect over it. The reactivity of the Ru-OH(2) complexes described in this paper together with other similar ones, previously synthesized by us, has been tested with regard to the epoxidation of different olefins. Complexes [Ru(II)(R-box-C)(tpm)OH(2)](BF(4))(2), R = Bz, 3'c/iPr, 3c, show high stereoselectivity in the epoxidation of cis-β-methylstyrene, with the exclusive formation of the cis-epoxide. However, there is a significant difference in regioselectivity between the two catalysts in the epoxidation of 4-vinylcyclohexene; complex 3'c leads to the regioselective oxidation at the ring alkene position, whereas complex 3c leads to the oxidation at the terminal position. Computational calculations indicate only small energy differences between the two possible products of 4-vinylcyclohexene epoxidation, but the energy barriers for the interaction of the catalytic systems with the alkene groups of 4-vinylcyclohexene agree with the reactivity differences found for the two catalysts having isopropyl or benzyl as substituent of the oxazole ligand. Computed local Fukui functions help to explain the observed reactivity trends.

Synergistic oxygen atom transfer by ruthenium complexes with non-redox metal ions

“…75 For the Ru-Cl complexes, the MLCT bands are red shifted with regard to the analogous Ru(bpy) 3 2+ species (λ max at 450 nm) because of the relative stabilization of the dπ(Ru) levels provoked by the chloro ligand. 43,75 Upon addition of 5 equiv. of PhI(OAc) 2 as oxidant, the deep purple color of Ru(bpy) 2 Cl 2 gradually changed to yellow, and we named this unknown species compound 1 (Fig.…”

“…Although the ratios of cis and trans-epoxide products reported herein are difficult to compare directly with other complexes from the literature because the catalysts and oxidants used are different, the results of catalytic epoxidation in this paper are consistent with a general trend that Ru(IV)vO based species provide a certain steric selectivity. 3,5,43 In the case of trans-stilbene, Ru(bpy) 2 Cl 2 combined with 2 equiv. of Al(III) as catalyst provides 36.2% yield of trans-epoxide with 17.6% yield of benzaldehyde, while Ru(bpy) 2 Cl 2 alone gave only 2.0% yield of trans-epoxide with 8.2% yield of benzaldehyde.…”

“…Attempts have been made to optimize the performance of ruthenium catalysts through chemical modification of their attached organic ligands. 6,18,29,[41][42][43][44] However, chemical modification of organic ligands still has major drawbacks, such as high cost, unpredictable stability, and time-consuming procedures; examples also exist where unexpected steric hindrance in the first coordination sphere strongly influences the reactivity of a RuvO system. 24,43 Alternatively, non-redox metal ions serving as Lewis acids offer another efficient strategy for regulating the reactivity of redox metal ions through bridging or ligation with the metaloxo or metal-hydroxo moiety in a variety of homogeneous chemical transformations.…”

Tuning the Electronic Properties by Width and Length Modifications of Narrow- Diameter Carbon Nanotubes for Nanomedicine

“…Conceptual DFT has allowed mathematical definitions to be given to a series of chemical concepts such as electronegativity and hardness. Thus it is a useful tool to obtain quantitative information about the chemical reactivity [19,[23][24][25].…”

Ruthenium(II) complexes bearing (NNN) ligand: catalytic evaluation of different solvent-mediated coordination modes

“…[8][9][10] On the other hand, there is a great interest in transition metal complexes with tridentate amine ligands, especially those containing a pyridine due to its unique electronic and steric properties. 8,[11][12][13][14][15][16][17] Some of them have good efficiency in the catalytic epoxidation reaction [18][19] and in transfer hydrogenation of aryl ketones. 20 Starting from this consideration here is reported the synthesis of a new tridentate ligand 1 (Figure 1) and the study of its different coordination modes to a ruthenium metal center.…”