School of Chemistry - Research Publications
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ItemGas-Phase Models for the Nickel- and Palladium-Catalyzed Deoxygenation of Fatty Acids(WILEY-V C H VERLAG GMBH, 2020-11-05)Using fatty acids as renewable sources of biofuels requires deoxygenation. While a number of promising catalysts have been developed to achieve this, their operating mechanisms are poorly understood. Here, model molecular systems are studied in the gas phase using mass spectrometry experiments and DFT calculations. The coordinated metal complexes [(phen)M(O2CR)]+ (where phen=1,10‐phenanthroline; M=Ni or Pd; R=CnH2n+1, n≥2) are formed via electrospray ionization. Their collision‐induced dissociation (CID) initiates deoxygenation via loss of CO2 and [C,H2,O2]. The CID spectrum of the stearate complexes (R=C17H35) also shows a series of cations [(phen)M(R’)]+ (where R’ < C17) separated by 14 Da (CH2) corresponding to losses of C2H4‐C16H32 (cracking products). Sequential CID of [(phen)M(R’)]+ ultimately leads to [(phen)M(H)]+ and [(phen)M(CH3)]+, both of which react with volatile carboxylic acids, RCO2H, (acetic, propionic, and butyric) to reform the coordinated carboxylate complexes [(phen)M(O2CR)]+. In contrast, cracking products with longer carbon chains, [(phen)M(R)]+ (R>C2), were unreactive towards these carboxylic acids. DFT calculations are consistent with these results and reveal that the approach of the carboxylic acid to the “free” coordination site is blocked by agostic interactions for R > CH3.
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ItemPhotoexcited Pd(ii) auxiliaries enable light-induced control in C(sp(3))-H bond functionalisation(ROYAL SOC CHEMISTRY, 2020-03-07)Herein we report the photophysical and photochemical properties of palladacycle complexes derived from 8-aminoquinoline ligands, commonly used auxiliaries in C-H activation. Spectroscopic, electrochemical and computational studies reveal that visible light irradiation induces a mixed LLCT/MLCT charge transfer providing access to synthetically relevant Pd(iii)/Pd(iv) redox couples. The Pd(ii) complex undergoes photoinduced electron transfer with alkyl halides generating C(sp3)-H halogenation products rather than C-C bond adducts. Online photochemical ESI-MS analysis implicates participation of a mononuclear Pd(iii) species which promotes C-X bond formation via a distinct Pd(iii)/Pd(iv) pathway. To demonstrate the synthetic utility, we developed a general method for inert C(sp3)-H bond bromination, chlorination and iodination with alkyl halides. This new strategy in auxiliary-directed C-H activation provides predictable and controllable access to distinct reactivity pathways proceeding via Pd(iii)/Pd(iv) redox couples induced by visible light irradiation.