CpMetal N-heterocyclic carbene complexes : synthesis and applications

Abstract

The synthesis and catalytic application of new half-sandwich Ni(II), Cr(III), and Ru(II) complexes, all stabilised by either symmetric or asymmetric N-heterocyclic carbene (NHC) ligands, were investigated. The study entailed the synthesis of novel imidazolium halide salts as ligand precursors to the metal-NHC complexes. A range of nine (three symmetric, six asymmetric) imidazolium bromide salts were synthesised and characterized, with N-substituents including either electron-donating or electron-withdrawing groups. The ligands were reacted with nickelocene under inert conditions to form nine [CpNiBr(NHC)] complexes. These complexes were evaluated as catalysts in the Suzuki-Miyaura cross-coupling reaction for which moderate to good activity (53-79% conversion, maximum TOF = 320 h-1) was observed. The catalytic activity of these complexes in the anaerobic oxidation of secondary alcohols was also assessed. Moderate activity (52-84% conversion, TOF = 14-86 h-1) was observed using conventional heating, and excellent activity (90-98% conversion) using microwave heating. DFT studies provided insight into the better performance of the complexes featuring electron-donating NHCs as opposed to those with electron-withdrawing NHC ligands. In addition, DFT-supported energy profile studies explained the lack of formation of α-arylation byproducts, found for similar studies in the literature. By-product formation of the type [NiBr2(NHC)2] during synthesis of the above mentioned nickel NHC complexes led to the subsequent investigation of the reactivity and properties of the biscarbene complexes. Reaction of thiophenol with [CpNiBr(NHC)] or the biscarbene complexes, yielded complexes of the type [CpNi(SPh)(NHC)] and [Ni(SPh)2(NHC)2]. A detailed DFT study revealed structural preferences that were correlated with the experimentally observed structures. The electronic differences among the [CpNiBr(NHC)] complexes were further evaluated through a DFT-supported electrochemistry study, which showed some electrochemical variances due to the different N-alkyl substituents present in the nickel complexes. Reaction of chromocene with the novel non-NO2-containing imidazolium bromide salts, followed by chromium oxidation using CCl4, allowed for the formation, purification and characterisation of five [CpCrBrCl(NHC)] complexes. Exposure of these heterohalo complexes to excess CHCl3 or CCl4 leads to the formation of complexes [CpCrCl2(NHC)]. Upon reaction of chromocene with the novel NO2-containing imidazolium salts, a ligand decomposition reaction occurs. C-N bond cleavage and imidazole protonation result in the formation of [C4H7N2][CpCrBrCl2]. The catalytic activity of the six Cr(III)-NHC complexes was evaluated in the base-free glucose dehydration reaction for which moderate activity (52-81% conversion) was observed. Supplementary DFT studies added insight into the electrochemical properties of the Cr(III) complexes. A range of six new bidentate (benz)imidazolium chloride salts (one symmetric, five asymmetric), each bearing an N-2-methylallyl substituent were synthesised and complexed to ruthenium(II) (three different Ru(II) precursors) via silver transmetallation. Eight new cationic complexes [(C5H4R)Ru(EPh3)(NHC)]PF6 (R = H, Me; E = P, As; NHC = Im(R')(2-methylallyl), BIm(Bn)(2-methylallyl); R' = alkyl) were isolated, purified and characterised. DFT studies illustrated the electronic properties of the complexes, while a catalytic study (tandem transfer hydrogenation-epoxidation of ketone substrates) showed moderate activity (39-62% conversion, TOF = 4-12 h-1) for all of the complexes. Finally, a range of four new C(2)-protected bidentate imidazolium chloride salts (one symmetric, three asymmetric), each containing either an N-2-methylallyl or N-picolyl substituent, were synthesised and characterised. These ligands were found suitable as ligand precursors to abnormal NHCs, and hence were reacted with [(p-cym)RuCl2]2 or [CpRuCl(PPh3)2] to form ten new Ru(II)-NHC complexes (eight Ru-aNHC and two Ru-NHC complexes). The Ru-NHC complexes formed as by-products due to Ag-mediated C(2)-dealkylation in selected cases. The strength of the Ru-aNHC bond was probed in an 1H-NMR titration study with DCl, which showed its superior bond strength when compared to analogous normally bound Ru-NHC complexes. The catalytic activity of the latter ten complexes was screened in both the transfer hydrogenation and alcohol oxidation reactions. The complexes are therefore more suitable as transfer hydrogenation catalysts (50-93% conversion, TOF = 22-60 h-1) than for application in alcohol oxidation catalysis (56-77% conversion, TOF = 2-11 h-1).