Synthesis, Structure, Redox Properties, and Catalytic Activity of New Ruthenium Complexes Containing Neutral or Anionic and Facial or Meridional Ligands:  An Evaluation of Electronic and Geometrical Effects

Abstract

The synthesis of a family of new Ru complexes containing meridional or facial tridentate ligands with the general formula [Ru(II)(T)(D)(X)](n+) [T = 2,2':6',2' '-terpyridine or tripyrazolylmethane; D = 4,4'-dibenzyl-4,4',5,5'-tetrahydro-2,2'-bioxazole (S,S-box-C) or 2-[((1'S)-1'-(hydroxymethyl)-2'-phenyl)ethylcarboxamide]-(4S)-4-benzyl-4,5-dihydrooxazole (S,S-box-O); X = Cl, H(2)O, MeCN or pyridine] has been described. All complexes have been spectroscopically characterized in solution through (1)H NMR and UV-vis techniques. Furthermore, all of the chloro complexes presented here have also been characterized in the solid state through monocrystal X-ray diffraction analysis. The oxazolinic S,S-box-C ligands undergo a Ru-assisted hydrolysis reaction generating the corresponding amidate anionic oxazolinic ligands S,S-box-O, which are also strongly attached to the metal center and produce a strong sigma-donation effect over the Ru metal center. The redox properties of all complexes have also been studied by means of cyclic voltammetry, strongly reflecting the nature of the ligands; both effects, geometrical (facial vs meridional) and electronic (neutral vs anionic), can be unveiled and rationalized. Finally, the reactivity of the Ru-OH(2) complexes has been tested with regard to the epoxidation of trans-stilbene, and it has been shown that, in this particular case, the reactivity is practically not dependent on the redox potentials of the catalyst but, in sharp contrast, it is strongly dependent on the geometry of the tridentate ligands.

Powerful Bis-facially Pyrazolate-Bridged Dinuclear Ruthenium Epoxidation Catalyst

“…First, as a general trend, the reported Ru mononuclear species in the literature show lower epoxide selectivities and substrate conversions. 50,51 And second, to our knowledge 3 2+ is more than 30 times faster than the best reported mononuclear Ru catalyst. Therefore, both figures suggest the existence of a potential cooperative effect between the two metal centers strategically situated in 3 3+ (see discussion below).…”

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

“…The results obtained are gathered in Table 4 together with other Ru-oxazoline complexes for purposes of comparison. 9 No epoxidation occurred in the absence of catalyst in any case.…”

“…9 The complexes exhibit ligand based πÀπ* bands below 350 nm and relatively intense bands above 350 nm assigned mainly to dπÀπ* MLCT transitions. For the RuÀCl complexes the MLCT bands are shifted to the red with regard to the analogous RuÀOH 2 species because of the relative destabilization of the dπ(Ru) levels provoked by the chloro ligand.…”

“…This ringopening is attributed to a Ru-assisted process once the oxazoline ligand is coordinated, because of an enhancement of the iminic C atom electrophilicity upon coordination as has previously been described by our group. 9 These RuÀCl complexes are easily separated by column chromatography since 2c is cationic whereas 2o is a neutral complex. The corresponding RuÀOH 2 complexes 3 are easily obtained from the corresponding RuÀCl complexes in the presence of stoichiometric amounts of Ag þ .…”

“…9 The pK a values for the Ru II ÀOH 2 and Ru III ÀOH 2 species, gathered in Table 3, can be calculated from the slope breaks of the Ru III / Ru II couple.…”