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Heterolytic bond activation at gold: evidence for gold(iii) H-B, H-Si complexes, H-H and H-C cleavage.

ABSTRACT: The coordinatively unsaturated gold(iii) chelate complex [(C^N-CH)Au(C₆F₅)]⁺ (1 ⁺) reacts with main group hydrides H-BPin and H-SiEt₃ in dichloromethane solution at -70 °C to form the corresponding σ-complexes, which were spectroscopically characterized (C^N-CH = 2-(C₆H₃Bu ^t )-6-(C₆H₄Bu ^t )pyridine anion; Pin = OCMe₂CMe₂O). In the presence of an external base such as diethyl ether, heterolytic cleavage of the silane H-Si bond leads to the gold hydrides [{(C^N-CH)AuC₆F₅}₂(μ-H)]⁺ (2 ⁺) and (C^N-CH)AuH(C₆F₅) (5), together with spectroscopically detected [Et₃Si-OEt₂]⁺. The activation of dihydrogen also involves heterolytic H-H bond cleavage but requires a higher temperature (-20 °C). H₂ activation proceeds in two mechanistically distinct steps: the first leading to 2 plus [H(OEt₂)₂]⁺, the second to protonation of one of the C^N pyridine ligands and reductive elimination of C₆F₅H. By comparison, formation of gold hydrides by cleavage of suitably activated C-H bonds is very much more facile; e.g. the reaction of 1·OEt₂ with Hantzsch ester is essentially instantaneous and quantitative at -30 °C. This is the first experimental observation of species involved in the initial steps of gold catalyzed hydroboration, hydrosilylation and hydrogenation and the first demonstration of the ability of organic C-H bonds to act as hydride donors towards gold.

SUBMITTER: Rocchigiani L

PROVIDER: S-EPMC6425858 | biostudies-literature | 2019 Mar

REPOSITORIES: biostudies-literature

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Heterolytic bond activation at gold: evidence for gold(iii) H-B, H-Si complexes, H-H and H-C cleavage.

Rocchigiani Luca L Budzelaar Peter H M PHM Bochmann Manfred M

Chemical science 20190116 9

The coordinatively unsaturated gold(iii) chelate complex [(C^N-CH)Au(C6F5)]+ (1 +) reacts with main group hydrides H-BPin and H-SiEt3 in dichloromethane solution at -70 °C to form the corresponding σ-complexes, which were spectroscopically characterized (C^N-CH = 2-(C6H3Bu t )-6-(C6H4Bu t )pyridine anion; Pin = OCMe2CMe2O). In the pre ...[more]

PMID: 30996979

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Project description:Heme hydroperoxidases catalyze the oxidation of substrates by H2O2. The catalytic cycle involves the formation of a highly oxidizing species known as Compound I, resulting from the two-electron oxidation of the ferric heme in the active site of the resting enzyme. This high-valent intermediate is formed upon facile heterolysis of the O-O bond in the initial FeIII-OOH complex. Heterolysis is assisted by the histidine and arginine residues present in the heme distal cavity. This chemistry has not been successfully modeled in synthetic systems up to now. In this work, we have used a series of iron(iii) porphyrin complexes (FeIIIL2(Br), FeIIIL3(Br) and FeIIIMPh(Br)) with covalently attached pendent basic groups (pyridine and primary amine) mimicking the histidine and arginine residues in the distal-pocket of natural heme enzymes. The presence of pendent basic groups, capable of 2nd sphere hydrogen bonding interactions, leads to almost 1000-fold enhancement in the rate of Compound I formation from peracids relative to analogous complexes without these residues. The short-lived Compound I intermediate formed at cryogenic temperatures could be detected using UV-vis electronic absorption spectroscopy and also trapped to be unequivocally identified by 9 GHz EPR spectroscopy at 4 K. The broad (2000 G) and axial EPR spectrum of an exchange-coupled oxoferryl-porphyrin radical species, [FeIV[double bond, length as m-dash]O Por˙+] with g eff ⊥ = 3.80 and g eff ‖ = 1.99, was observed upon a reaction of the FeIIIL3(Br) porphyrin complex with m-CPBA. The characterization of the reactivity of the FeIII porphyrin complexes with a substrate in the presence of an oxidant like m-CPBA by UV-vis electronic absorption spectroscopy showed that they are capable of oxidizing two equivalents of inorganic and organic substrate(s) like ferrocene, 2,4,6-tritertiary butyl phenol and o-phenylenediamine. These oxidations are catalytic with a turnover number (TON) as high as 350. Density Functional Theory (DFT) calculations show that the mechanism of O-O bond activation by 2nd sphere hydrogen bonding interaction from these pendent basic groups, which are protonated by a peracid, involves polarization of the O-O σ-bond, leading to lowering of the O-O σ*-orbital allowing enhanced back bonding from the iron center. These results demonstrate how inclusion of 2nd sphere hydrogen bonding interaction can play a critical role in O-O bond heterolysis.

Dataset Information

Heterolytic bond activation at gold: evidence for gold(iii) H-B, H-Si complexes, H-H and H-C cleavage.

Publications

Heterolytic bond activation at gold: evidence for gold(iii) H-B, H-Si complexes, H-H and H-C cleavage.

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