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Molecular Analogs of Surface Species. 3. The Mechanism of the Regioselective Homogeneous Hydrogenation of Benzothiophene by Use of [Rh(COD)(PPh<inf>3</inf>)<inf>2</inf>]PF<inf>6</inf> as the Catalyst Precursor. Kinetic and Theoretical Study

Abstract:

The mechanism of the regioselective hydrogenation of benzo[b]thiophene (BT) to 2,3- dihydrobenzo[b]thiophene (DHBT) in 2-methoxyethanol solution at 125 °C and ambient or subambient pressure of H2, using [Rh(COD)(PPh3)2]PF6 (1) (COD = 1,3-cyclooctadiene) as the catalyst precursor has been investigated by a combination of kinetic, chemical, and theoretical methods. The kinetic study led to a rate law ri= k7K6 [Rh]0[H2]/(1 + K6[H2]), where (kcat)exp = k7K6 = 0.726 M-1 s-1, k7 = 3.70 × 10-2 s-1, and K6 = 19.61 M-1. The calculated activation parameters are ΔH‡ = 20.1 ± 0.9 kcal/mol, ΔS‡ = −11.1 ± 0.8 eu, and ΔG‡ = 23 ± 3 kcal/mol. 1 probably reacts with hydrogen to produce an unstable intermediate [RhH2(η1-S-BT)2(PPh3)2]+ which evolves into [Rh(π-BT) (PPh3)2]+, where BT is likely to be η5-bonded through the thiophene ring. Semiempirical (CNDO) theoretical calculations on the interactions of the fragment [Rh-(PH3)2]+ with BT revealed that both η5-bonding through the thiophene ring and η6-bonding through the benzene ring are possible; in the former case, the bonding is dominated by donation from a filled 5a′ BT orbital localized mainly on the S lone pair to empty 3a1 and 2b1 metal orbitals, whereas in the latter the main interaction is donation from a delocalized 4a′ BT orbital to an empty b2 metal orbital. The dihydride [Rh(BT)(H)2(PH3)2]+ which is formed by reaction of [(η5-BT)Rh(PH3)2]+ with H2 may contain BT coordinated η1-S, η2-C2=C3 or η4; [Rh(η4-BT)(H)2(PH3)2]+ was found to be more stable than [Rh(η1-BT)(H)2(PH3)2]+ and [Rh(η2-BT)- (H)2(PH3)2]+. The largest activation of the C=C bond of the thiophene moiety is produced by the η2 mode of bonding. All this information leads to a hydrogenation mechanism in which the active species is most probably the 18 electron π-bonded species [Rh(η5-BT)(PPh3)2]+, which is in equilibrium with an olefin-like complex [Rh(H)2(η2-BT) (PPh3)2]+. The rate determining step of the catalytic reaction is the transfer of hydrides from Rh to η2-BT, to yield [Rh(η1-SDHBT)(PPh3)2]+; subsequent rapid reaction with BT liberates the product and restarts the catalytic cycle. This mechanism provides a good model for understanding the initial hydrogenation step in the heterogeneous hydrodesulfurization of BT on solid catalysts. © 1994, American Chemical Society. All rights reserved.