MOLECULAR ANALOGS OF SURFACE SPECIES .3. THE MECHANISM OF THE REGIOSELECTIVE HOMOGENEOUS HYDROGENATION OF BENZOTHIOPHENE BY USE OF [RH(COD)(PPH3)2]PF6 AS THE CATALYST PRECURSOR - KINETIC AND THEORETICAL-STUDY

The mechanism of the regioselective hydrogenation of benzo[b]thiophene (BT) to 2,3-dihydrobenzo[b]thiophene (DHBT) in 2-methoxyethanol solut ion at 125-degrees-C and ambient or subambient pressure of H-2, using [Rh(COD) (PPh3)2] PF6 (1) (COD = 1,3-cyclooctadiene) as the catalyst p rocursor has been investigated by a combination of kinetic, chemical, and theoretical methods. The kinetic study led to a rate law r(i) = k7 K6 [Rh]0[H2]/(l + K6[H2]), where (k(cat))exp = k7K6 = 0.726 M-1 s-1, k 7 = 3.70 x 10(-2) s-1, and K6 = 19.61 M-1. The calculated activation p arameters are DELTAH(double dagger) = 20.1 +/- 0.9 kcal/mol, DELTAS(do uble dagger) = -11.1 +/- 0.8 eu, and DELTAG(double dagger) = 23 +/- 3 kcal/mol. 1 probably reacts with hydrogen to produce an unstable inter mediate [RhH2(eta1-S-BT)2(PPh3)2]+ which evolves into [Rh(pi-BT)(PPh3) 2]+, where BT is likely to be eta5-bonded through the thiophene ring. Semiempirical (CNDO) theoretical calculations on the interactions of t he fragment [Rh-(PH3)2]+ with BT revealed that both eta5-bonding throu gh the thiophene ring and eta6-bonding through the benzene ring are po ssible; 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 3 a1 and 2b1 metal orbitals, whereas in the latter the main interaction is donation from a delocalized 4a' BT orbital to an empty b2 metal orb ital. The dihydride [Rh(BT)(H)2(PH3)2]+ which is formed by reaction of [(eta5-BT)Rh(PH3)2]+ with H-2 may contain BT coordinated eta1-S, eta2 -C2=C3, or eta4; [Rh(eta4-BT)-(H)2(PH3)2]+ was found to be more stable than [Rh(H)2(eta2-BT)(PPH3)2]+ and [Rh(eta2-BT)-(H)2(PH3)2]+. The lar gest activation of the C=C bond of the thiophene moiety is produced by the eta2 mode of bonding. All this information leads to a hydrogenati on mechanism in which the active species is most probably the 18 elect ron pi-bonded species [Rh(eta5-BT)(PPh3)2]+, which is in equilibrium w ith an olefin-like complex [Rh(H)2(eta2-BT)(PPh3)2]+. The rate determi ning step of the catalytic reaction is the transfer of hydrides from R h to eta2-BT, to yield [Rh(eta1-S-DHBT) (PPh3)2]+; subsequent rapid re action with BT liberates the product and restarts the catalytic cycle. This mechanism provides a good model for understanding the initial hy drogenation step in the heterogeneous hydrodesulfurization of BT on so lid catalysts.