Late transition metal Fischer carbene complexes of group 9 and 11: synthesis and application

Abstract

Carbene ligand transfer from tungsten(0) pentacarbonyl (hetero)aryl-ethoxy Fischer carbene complexes (FCCs) to rhodium(I) and gold(I) metal precursors, results in new examples of rare class of mono-heteroatom stabilized rhodium(I) and gold(I) FCCs, respectively. Analogously, a series of novel iridium(I) FCCs were accessed via transmetallation reactions of the relevant tungsten(0) FCCs with the selected iridium(I) precursor. The (hetero)aryl substituents included 2-thienyl (Th), 2-furyl (Fu), para N,N-dimethylaniline (p-DMA), cyclopentadienylmetalcarbonyls CpˈFe(CO)2Me, CpˈMn(CO)3, and CpˈRe(CO)3, as well as ruthenocenyl (Rc), and ferrocenyl (Fc) entities. The isolated RhI , IrI , and AuI ethoxy-FCCs were modified by either aminolysis- or carbonylation reactions, for direct- or remote fine tuning of the electron- donating ability of the carbene ligand. The electronic properties were monitored by spectroscopic and structural characterization methods. Tolman electronic parameters (TEPs) of RhI and IrI complexes were calculated, and the relative electron donating strength of the FC ligands were determined as increasing in the order :C(OEt)(CpˈRe(CO)3) < :C(OEt)Fc ≈ :C(OEt)Rc < :C(OEt)p-DMA ≤ :C(NHnPr)Fc < :C(NHnPr)p-DMA < :C(NH(CH2)2NMe2)Fc, with the unexpected implication that p-DMA-substituted FC ligands are more electron donating than Fc and Rc analogues. The isolated RhI FCCs [Rh(LLˈ){C(OEt)Ar}Cl] (LLˈ = cod, (CO)2; Ar = p-DMA, Rc) were evaluated as precatalysts in the hydroformylation of 1-octene. The best catalytic activity was observed for metallocenyl-substituted complexes, where the possibility of bimetallic-activity at the central rhodium- and metallocenyl- substituent (iron- or, to a larger extent, ruthenium-) metal atom, cannot be ruled out. Chemical oxidation of the previously reported RhI Fc-FCC was also achieved, resulting in a significantly more electrophilic ferroceniumyl-substituted rhodium(I) FCC, which indicated improved regioselectivity in catalytic hydroformylation, at the expense of both catalytic activity and chemoselectivity. Iridium(I) FCCs [Ir(LLˈ){(NR)Ar}Cl] (LLˈ = cod, (CO)2; XR = OEt, NHnPr, NH(CH2)2NMe2; Ar = CpˈRe(CO)3, p-DMA, Fc), were used as precatalysts in the base-assisted transfer hydrogenation of acetophenone with the solvent iso-propanol acting as the sacrificial hydrogen atom donor. This study details the first report of the use of classical acyclic mono-heteroatom stabilized FCCs in molecular catalysis, results of which indicated an increase in catalytic activity with increasing electron-donating strength of the corresponding FC ligand. Excellent catalytic activities were observed for the p-DMA- mono-amino (- NHnPr) and Fc-substituted diamino (-NH(CH2)2NMe2) carbene complexes, with high turnover frequencies (TOFs) of 273 and 445 hr -1 , respectively. v Finally, AuI aminocarbene complexes [Au{(NR)Ar}Cl] (R = nPr, (CH2)2NMe2; Ar = Th, p-DMA, Fc) were evaluated in redox-switchable catalytic reactions mediated by the oxidant Magic Blue (MB). Significantly improved catalytic activity of the Fc- aryl-carbene complexes as compared to the Fu- and p-DMA analogues were observed. The use of diamino-substituted carbene ligands (-NH(CH2)2NMe2) instead of the monoamino analogues (-NHnPr), also yielded enhanced catalytic activity. The catalytic mechanism was investigated by means of experimental electron paramagnetic resonance (EPR) spectroscopy and computational density functional theory (DFT) studies. In all cases, the formation of catalytically active AuII species upon the addition of MB is implicated. For the organic- (Fu-, p-DMA-) aryl-substituted carbene complexes, MB directly (and irreversibly) oxidizes the AuI metal atom, resulting in the catalytically active AuII intermediate, which instantaneously decomposes to carbene ligand oxidation products and elemental gold. For the Fc-substituted precatalysts however, reversible oxidation of the FeII metal atom with subsequent in situ inter- and intramolecular transformations upon addition of MB, results in ferrocenophane-substituted active AuII species, and the AuI precatalysts could be regenerated by addition of a suitable reductant to the respective AuI -FCC/MB mixtures