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Titolo:
Unifying the mechanisms for alkane dehydrogenation and alkene H/D exchangewith [IrH2(O2CCF3)(PAr3)(2)]: the key role of CF3CO2 in the "sticky" alkane route
Autore:
Gerard, H; Eisenstein, O; Lee, DH; Chen, JY; Crabtree, RH;
Indirizzi:
Yale Univ, Dept Chem, New Haven, CT 06511 USA Yale Univ New Haven CT USA 06511 Univ, Dept Chem, New Haven, CT 06511 USA Univ Montpellier 2, Lab Struct & Dynam Syst Mol & Solides, CNRS, UMR 5636,F-34095 Montpellier 05, France Univ Montpellier 2 Montpellier France 05 ,F-34095 Montpellier 05, France
Titolo Testata:
NEW JOURNAL OF CHEMISTRY
fascicolo: 9, volume: 25, anno: 2001,
pagine: 1121 - 1131
SICI:
1144-0546(200109)25:9<1121:UTMFAD>2.0.ZU;2-Z
Fonte:
ISI
Lingua:
ENG
Soggetto:
C-H BONDS; ORGANOMETALLIC REACTION-MECHANISMS; TRANSITION-METAL COMPLEXES; LOW-TEMPERATURE MATRICES; D6 ML5 COMPLEXES; OXIDATIVE ADDITION; REDUCTIVE ELIMINATION; HYDROGEN-EXCHANGE; CATALYTIC DEHYDROGENATION; IRIDIUM(III) COMPLEXES;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
75
Recensione:
Indirizzi per estratti:
Indirizzo: Eisenstein, O Yale Univ, Dept Chem, 225 Prospect St, New Haven, CT 06511 USA Yale Univ 225 Prospect St New Haven CT USA 06511 T 06511 USA
Citazione:
H. Gerard et al., "Unifying the mechanisms for alkane dehydrogenation and alkene H/D exchangewith [IrH2(O2CCF3)(PAr3)(2)]: the key role of CF3CO2 in the "sticky" alkane route", NEW J CHEM, 25(9), 2001, pp. 1121-1131

Abstract

To understand photochemical and thermal alkane activation with IrH2(O2CCF3)(PAr3)(2) (Ar = p-FC6H4), H/D isotope scrambling between alkenes and IrD2(O2CCF3)(PAr3)(2) was studied. No unique interpretation of the experimental data was possible, so DFT(B3PW91) calculations on the exchange process in Ir(H)(2)(O2CCF3)(PH3)(2)(C2H4) were carried out to distinguish between the possibilities allowed by experiment. Of several possible mechanisms for H/D scrambling, one was strongly preferred and is therefore proposed here. It involves the insertion of the olefin to give an alkyl hydride that reductively eliminates to lead to a transition state that contains an eta (3)-bound alkane. This transition state, which achieves a 1,1' geminal H/D exchange, is significantly lower in energy than a dihydrido carbene, located as a secondary minimum, eliminating the alternative carbene mechanism, The unexpectedly large binding energy (BDE) of the alkane ('sticky alkane") to the Ir(O2CCF3)(PH3)(2) fragment (BDE = 11.9 kcal mol(-1)) in this transition state is ascribed in part to the presence of a weakly sigma- and pi -donating (CF3CO2) group trans to the alkane binding site. The H/D exchange selectivity observed requires that 1,1'-shifts (i.e., M moving to a geminal C-H bond), but not 1,3-shifts, be allowed in the alkane complex. In a key finding, a 1,3-shift in which the metal moves down the alkane chain is indeed found to have a much higher activation energy than the 1,1'-process and is thereforeslow in our system. A 1,2-shift has not been considered since it would involve a strong steric hindrance at a tertiary carbon in this system. The mechanism via an alkane path provides an insight into the closely related photochemical and catalytic thermal alkane dehydrogenation processes mediated by IrH2(O2CCF3)(PAr3)(2); the thermal route requires (BuCH)-Bu-t=CH2 as the hydrogen acceptor. These two alkane reactions are intimately related mechanistically to the isotope exchange because they are proposed to have the same intermediates, in particular the sticky alkane complex. Remarkably, the rate determining step of the thermal (150 degreesC) alkane dehydrogenation process is predicted to be substitution of the hydrogen acceptor-derived alkane by the alkane substrate.

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Documento generato il 21/09/20 alle ore 12:39:39