Oxorhenium(V) Complexes with Pyrazole Based Aryloxide Ligands and Application in Olefin Epoxidation

Abstract

We synthesized and characterized a set of new oxorhenium(V) complexes coordinated by various pyrazole containing phenol (L1-L3) and naphthol ligands (L4-L7). Depending on the starting material, we were able to selectively synthesize monosubstituded or disubstituted complexes of the type [ReOBr(2)L(PPh(3))] (1-7; L = L1-L7) and [ReOClL(2)] (L = L1 8; L2 9; L4 10; L6 11), respectively. All complexes are stable to air and moisture, both in solid state as well as in solution. Furthermore, the cationic oxorhenium(V) complex [ReO(L1)(2)(NCMe)](OTf) (8a) was obtained upon chloride abstraction with silver triflate from 8. All new complexes were able to catalyze the epoxidation of cis-cyclooctene in yields up to 64%. The ease of preparation and their tolerance to air and moisture, as well as the simple ligand modifications, make them an interesting class of novel catalysts. An attempted reduction of perchlorate ClO(4)(-) with complex 8 was unsuccessful. Molecular structures of complexes 1, 4, 6, 7, 8, 8a, 10, and 11 were determined by single crystal X-ray diffraction analyses.

Oxidorhenium(V) Complexes with Oxazolinylmethoxido Ligands – Structure and Catalytic Epoxidation

“…However, consistent with two isomers, the 31 P NMR spectrum of 3c in CDCl 3 revealed two resonances for a coordinated PPh 3 molecule at δ = -16.20 and -17.30 ppm, which is diagnostic of triphenylphosphane in the coordination sphere of Re V . [15,[34][35][36][37] The IR spectra of 3a-3d exhibit characteristic ν(Re=O) bands at 965.00 (3a), 974.62 (3b), 973.43 (3c) and 965.72 cm -1 (3d), which are in good agreement with those of similar oxidorhenium compounds. [8,15,34,36] The bands at 1670.05 (3a), 1644.63 (3b), 1644.59 (3c) and 1648.17 cm -1 (3d) are assigned to the ν(C=N) mode of the oxazoline ring, which represents a shift of 4-25 cm -1 to a lower frequency compared to the free ligands, indicating that the imine nitrogen atom of the oxazoline ring is strongly coordinated to the metal centre.…”

“…[4,8,9] Most interestingly, they were found to successfully catalyze the reduction of perchlorate (ClO 4 -) to chloride ions, [4,8] a reaction that is environmentally interesting (waste water cleaning) but kinetically hindered. [10] The success of oxazoline ligands as stabilizing agents in catalysis as well as our ongoing interest in rhenium-catalyzed epoxidations [11][12][13][14][15] prompted us to investigate a largely unexplored oxazoline ligand in oxidorhenium(V) chemistry. We were surprised to find that 2-(hydroxymethyl)-2-oxazoline and derivatives thereof have, in general, limited used incoordinationchemistry.Thus,with(4,5-dihydrooxazol-2-yl)-methanol only a few Cr III , [16] Ni II , [17,18] Zr IV [19] and Hf IV [20] [a] Institute of Chemistry, Karl-Franzens-University Graz, Schubertstrasse 1, 8010 Graz, Austria E-mail: nadia.…”

Oxorhenium(V) Complexes with Phenolate–Pyrazole Ligands for Olefin Epoxidation Using Hydrogen Peroxide

“…Therefore, we decided to activate the complexes by introducing electronwithdrawing and/or polar groups R in the ligand backbone (1a, R = Br; 1b, R = NO 2 ; 1c, R = OMe) in contrast to ligands bearing electron donating and apolar substituents. 21 ■ RESULTS AND DISCUSSION Ligand Synthesis. Ligands 1a−c were prepared according to Scheme 1.…”

“…To probe the electronic influence of ligands 1a−c on the metal center, we performed cyclic voltammetry experiments on complexes 3a−c and compared the obtained redox potentials to the redox potential of the previously published complex 4 (R = Me). 21 Whereas complexes 3a−b displayed the electrochemistry of quasireversible redox couples, the cyclic voltammogram of complex 3c was more complex and showed several irreversible faradaic processes (Supporting Information). These additional irreversible peaks are also observed in the cyclic voltammogram of ligand 1c and are therefore most likely caused by irreversible oxidations/reductions of the ligand backbone.…”

“…According to the electrochemical investigation, the redox potentials of 3c and 4 are largely similar; however, their catalytic behavior is not. 21 Using H 2 O 2 , where the media is biphasic (H 2 O and 1,2-C 2 H 2 Cl 2 ), the polar substituents might allow for reaction of the catalytic species with H 2 O 2 across the phase boundaries, permitting the use of H 2 O 2 as oxidant. In a previous investigation on the mechanism of TBHP catalyzed epoxidation with a Re(V) complex, we proposed a cationic trans-dioxorhenium species as a likely intermediate.…”