Synthesis of rhodium(III) complexes with tris/tetrakis‐benzimidazoles and benzothiazoles—quick identification of cyclometallation by nuclear magnetic resonance spectroscopy
Abstract:] (X = Cl, Br, I). Elemental analysis, IR and 1 H nuclear magnetic resonance (NMR) chemical shifts supported the binuclear nature of the complexes. Cyclometallation was detected by conventional 13 C NMR spectra that showed a doublet around ∼190 ppm. Cyclometallation was also detected by gradient-enhanced heteronuclear multiple bond correlation (g-HMBC) experiment that showed cross-peaks between the cyclometallated carbon and the central benzene ring protons of 1-3. Cyclometallation was substantiated by two-dim… Show more
“…Transition metal complexes based on nitrogen heterocycles with benzimidazole rings have been studied extensively [1][2][3][4][5]. Some of the Cu(II) complexes show good catalytic activity, particularly toward the synthetically important oxidation of substituted phenols [6].…”
Chloromethylated polystyrene beads crosslinked with 6.5 % divinylbenzene were functionalized with 2-(2 0 -pyridyl) benzimidazole (PBIMH) and on subsequent treatment with Cu(OAc) 2 in methanol gave a polymersupported diacetatobis(2-pyridylbenzimidazole)copper(II) complex [PS-(PBIM) 2 Cu(II)], which was characterized by physicochemical techniques. The supported complex showed excellent catalytic activity toward the oxidation of industrially important organic compounds such as phenol, benzyl alcohol, cyclohexanol, styrene, and ethylbenzene. An effective catalytic protocol was developed by varying reaction parameters such as the catalyst and substrate concentrations, reaction time, temperature, and substrateto-oxidant ratio to obtain maximum selectivity with high yields of products. Possible reaction mechanisms were worked out. The catalyst could be recycled five times without any metal leaching or much loss in activity. This catalyst is truly heterogeneous and allows for easy work up, as well as recyclability and excellent product yields under mild conditions.
“…Transition metal complexes based on nitrogen heterocycles with benzimidazole rings have been studied extensively [1][2][3][4][5]. Some of the Cu(II) complexes show good catalytic activity, particularly toward the synthetically important oxidation of substituted phenols [6].…”
Chloromethylated polystyrene beads crosslinked with 6.5 % divinylbenzene were functionalized with 2-(2 0 -pyridyl) benzimidazole (PBIMH) and on subsequent treatment with Cu(OAc) 2 in methanol gave a polymersupported diacetatobis(2-pyridylbenzimidazole)copper(II) complex [PS-(PBIM) 2 Cu(II)], which was characterized by physicochemical techniques. The supported complex showed excellent catalytic activity toward the oxidation of industrially important organic compounds such as phenol, benzyl alcohol, cyclohexanol, styrene, and ethylbenzene. An effective catalytic protocol was developed by varying reaction parameters such as the catalyst and substrate concentrations, reaction time, temperature, and substrateto-oxidant ratio to obtain maximum selectivity with high yields of products. Possible reaction mechanisms were worked out. The catalyst could be recycled five times without any metal leaching or much loss in activity. This catalyst is truly heterogeneous and allows for easy work up, as well as recyclability and excellent product yields under mild conditions.
Allylic C-H bond oxidative addition reactions, mediated by tris(oxazolinyl)borato rhodium(I) and iridium(I) species, provide the first step in a hydrocarbon functionalization sequence. The bond activation products To M MH(h 3 -C 8 H 13 ) (M = Rh (1), Ir (2)), To M MH(h 3 -C 3 H 5 ) (M = Rh (3), Ir (4)), and To M RhH(h 3 -C 3 H 4 Ph) (5) (To M = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) are synthesized by reaction of Tl[To M ] and the corresponding metal olefin chloride dimers. Characterization of these group 9 allyl hydride complexes includes 1 H-15 N heteronuclear correlation NMR experiments that reveal through-metal magnetization transfer between metal hydride and the trans-coordinated oxazoline nitrogen. Furthermore, the oxazoline 15 N NMR chemical shifts are affected by the trans ligand, with the resonances for the group trans to hydride typically downfield of those trans to h 3 -allyl and tosylamide. These group 9 oxazolinylborate compounds have been studied to develop approaches for allylic functionalization. However, this possibility is generally limited by the tendency of the allyl hydride compounds to undergo olefin reductive elimination. Reductive elimination products are formed upon addition of ligands such as CO and CN t Bu.
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