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Hydride oxidation from a titanium-aluminum bimetallic complex: insertion, thermal and electrochemical reactivity.

ABSTRACT: We report the synthesis and reactivity of paramagnetic heterometallic complexes containing a Ti(iii)-?-H-Al(iii) moiety. Combining different stoichiometries amounts of Cp₂TiCl and KH₃AlC(TMS)₃ (Cp = cyclopentadienyl, TMS = trimethylsilyl) resulted in the formation of either bimetallic Cp₂Ti(?-H)₂(H)AlC(TMS)₃ (2) or trimetallic (Cp₂Ti)₂(?-H)₃(H)AlC(TMS)₃ (3) via salt metathesis pathways. While these complexes were indefinitely stable at room temperature, the bridging hydrides were readily activated upon exposure to heteroallenes, heating, or electrochemical oxidation. In each case, formal hydride oxidation occurred, but the isolated product maintained the +3 oxidation state at both metal centers. The nature of this reactivity was explored using deuterium labelling experiments and Density Functional Theory (DFT) calculations. It was found that while C-H activation from the Ti(iii) bimetallic may occur through a ?-bond metathesis pathway, chemical oxidation to Ti(iv) promotes bimolecular reductive elimination of dihydrogen to form a Ti(iii) product.

SUBMITTER: Brown AC

PROVIDER: S-EPMC6102661 | biostudies-literature | 2017 Jul

REPOSITORIES: biostudies-literature

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Hydride oxidation from a titanium-aluminum bimetallic complex: insertion, thermal and electrochemical reactivity.

Brown Alexandra C AC Altman Alison B AB Lohrey Trevor D TD Hohloch Stephan S Arnold John J

Chemical science 20170531 7

We report the synthesis and reactivity of paramagnetic heterometallic complexes containing a Ti(iii)-μ-H-Al(iii) moiety. Combining different stoichiometries amounts of Cp2TiCl and KH3AlC(TMS)3 (Cp = cyclopentadienyl, TMS = trimethylsilyl) resulted in the formation of either bimetallic Cp2Ti(μ-H)2(H)AlC(TMS)3 (2) or trimetallic (Cp2Ti)2(μ-H)3(H)AlC(TMS)3 (3) via salt ...[more]

PMID: 30155225

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Project description:The dimeric ?-diketiminato magnesium hydride, [(BDI)MgH]2, reacts at 80 °C with the terminal alkenes, 1-hexene, 1-octene, 3-phenyl-1-propene and 3,3-dimethyl-butene to provide the respective n-hexyl, n-octyl, 3-phenylpropyl and 3,3-dimethyl-butyl magnesium organometallics. The facility for and the regiodiscrimination of these reactions are profoundly affected by the steric demands of the alkene reagent. Reactions with the phenyl-substituted alkenes, styrene and 1,1-diphenylethene, require a more elevated temperature of 100 °C with styrene providing a mixture of the 2-phenylethyl and 1-phenylethyl products over 7 days. Although the reaction with 1,1-diphenylethene yields the magnesium 1,1-diphenylethyl derivative as the sole reaction product, only 64% conversion was achieved over a 21 day timeframe. Reactions with the ?,?-dienes, 1,5-hexadiene and 1,7-octadiene, provided divergent results. The initial 5-alkenyl magnesium reaction product of the shorter chain diene undergoes 5-exo-trig cyclisation via intramolecular carbomagnesiation to provide a cyclopentylmethyl derivative, which was shown by X-ray diffraction analysis to exist as a three-coordinate monomer. In contrast, 1,7-octadiene provided a mixture of two compounds, a magnesium oct-7-en-1-yl derivative and a dimagnesium-octane-1,4-diide, as a result of single or two-fold activation of the terminal C[double bond, length as m-dash]C double bonds. The magnesium hydride was unreactive towards internal alkenes apart from the strained bicycle, norbornene, allowing the characterisation of the resultant three-coordinate magnesium norbornyl derivative by X-ray diffraction analysis. Computational analysis of the reaction between [(BDI)MgH]2 and 1-hexene using density functional theory (DFT) indicated that the initial Mg-H/C[double bond, length as m-dash]C insertion process is rate determining and takes place at the intact magnesium hydride dimer. This exothermic reaction (?H = -14.1 kcal mol-1) traverses a barrier of 18.9 kcal mol-1 and results in the rupture of the dinuclear structure into magnesium alkyl and hydride species. Although the latter three-coordinate hydride derivative may be prone to redimerisation, it can also provide a further pathway to magnesium alkyl species through its direct reaction with a further equivalent of 1-hexene, which occurs via a lower barrier of 15.1 kcal mol-1. This Mg-H/C[double bond, length as m-dash]C insertion reactivity provides the basis for the catalytic hydrosilylation of terminal alkenes with PhSiH3, which proceeds with a preference for the formation of the anti-Markovnikov organosilane product. Further DFT calculations reveal that the catalytic reaction is predicated on a sequence of Mg-H/C[double bond, length as m-dash]C insertion and classical Si-H/Mg-C ?-bond metathesis reactions, the latter of which, with a barrier height of 24.9 kcal mol-1, is found to be rate determining.

Dataset Information

Hydride oxidation from a titanium-aluminum bimetallic complex: insertion, thermal and electrochemical reactivity.

Publications

Hydride oxidation from a titanium-aluminum bimetallic complex: insertion, thermal and electrochemical reactivity.

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