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Thermal C−H Bond Activation of Benzene, Toluene, and Methane with Cationic [M(X)(bipy)]+ (M = Ni, Pd, Pt; X = CH3, Cl; bipy = 2,2′-bipyridine): A Mechanistic Study

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journal contribution
posted on 28.03.2011, 00:00 by Burkhard Butschke, Helmut Schwarz
In the thermal gas-phase ion/molecule reactions of the mass-selected cationic 2,2′-bipyridine (bipy) complexes [M(CH3)(bipy)]+ (M = Ni, Pd, Pt) with benzene and toluene, hydrogen-atom abstraction occurs that results in the loss of neutral CH4. While the Ni and Pd complexes attack exclusively the arene C−H bonds of toluene (however, without any pronounced preference for the ortho-, meta-, or para-positions), in the reaction with [Pt(CH3)(bipy)]+ 10% of the hydrogen originates from the methyl group of toluene. Further, for the Pt complex extensive hydrogen/deuterium exchange between the Pt-bound methyl group and the incoming substrates C6D6 and C6D5CH3 takes place. In addition, it is only the platinum complex that brings about a degenerate H/D exchange with CD4, as demonstrated by the eliminations of CH4−nDn (n = 1−3) from the adduct complex. The experimental findings are in keeping with DFT calculations using the BP86 functional, which is superior as compared with several other functionals for this particular system. According to the computational studies, for the reactions of [Ni(CH3)(bipy)]+ and [Pd(CH3)(bipy)]+ with benzene and toluene a σ-bond metathesis scenario is favored energetically. However, at a [Pt(CH3)(bipy)]+ center, for C(sp2)−H bond activation a competition exists between a σ-bond metathesis and an oxidative addition/reductive elimination pathway (OA/RE), while in the activation of C(sp3)−H bonds, i.e., those of methane or of the benzylic position in toluene, the OA/RE mode prevails. The chloro analogues [M(Cl)(bipy)]+ (M = Ni, Pd, Pt) are not able to abstract hydrogen atoms from benzene and methane and to form HCl; according to the DFT calculations, these reactions are endothermic. Benzylic C−H bond activation of toluene, however, is predicted to be exothermic for all three chloro complexes. Nevertheless, only in the reaction of [Pt(Cl)(bipy)]+ with toluene is HCl loss from the adduct complex observed, but the corresponding signal is only slightly above the detection limit.