Models for Deep Hydrodesulfurization of Alkylated Benzothiophenes. Reductive Cleavage of C−S Bonds Mediated by Precoordination of Manganese Tricarbonyl to the Carbocyclic Ring

Abstract

The reduction of (η6-benzothiophene)Mn(CO)3 + with cobaltocene under CO leads to insertion of the Mn(CO)4 - fragment into the C(aryl)−S bond to afford a neutral bimetallic complex. When there are methyl substituents at the 2,3-, 2,7-, or 3,7-positions, the regioselectivity and product distribution in the reductive insertion reactions are significantly affected, and several new types of complexes are formed. Low-temperature chemical reduction of (η6-benzothiophene)Mn(CO)3 + complexes affords unstable η4-species, (η4-BT)Mn(CO)3 -, which do not react with the η6-cation to give insertion products. Similarly, electrochemical reduction of (η6-benzothiophene)Mn(CO)3 + changes from a one-electron chemically irreversible process at room temperature to a two-electron chemically reversible but electrochemically irreversible process at −90 °C. It is concluded from these results that the room-temperature reductive cleavage of a C−S bond in (η6-benzothiophene)Mn(CO)3 + occurs by a radical mechanism. Crystallographic data and density function theory (DFT) calculations indicate that insertion into the C(aryl)−S and not the C(vinyl)−S bond of (η6-benzothiophene)Mn(CO)3 + is favored. A redetermination of the crystal structure of the η1-S complex (η1-3-MeBT)Re(Cp*)(CO)2 revealed that the C(aryl)−S and C(vinyl)−S bonds are of similar lengths, suggesting that an η1-S intermediate is not predisposed to insert into the latter bond, as was previously thought. DFT calculations of (η6-BT)Mn(CO)3 + bonded in an η1-S fashion to Mn(CO)4 - indicated that an intermediate of this sort is viable in the C−S insertion reactions.

The Remote Activation of Chemical Bonds via Metal Coordination

“…Interestingly, the initial product 15, which is stable if R H, derives from insertion into the C(aryl) ± S bond in BT. Our studies are suggestive [14,17,18] of a radical mechanism, which may be relevant to electron transfer processes thought to occur in HDS. The isomerization 15 3 16 occurs over several hours for R Me via reductive elimination to h 1 -S bonded Mn(CO) 4 followed by oxidative addition.…”

“…The results nicely explained the observed regioselectivity in the C À S cleavage reactions and confirmed that an h 1 -S species is a viable intermediate in HDS chemistry. [18] In analogy with the activation of BTs and DBTs described above, the C À S bonds in thiophene itself (T) can be activated by the direct h . [19] In this case the metal is directly bonded to the C À S linkage that is subsequently cleaved, so the activation can not be classified as ™remote∫.…”

Transition-metal mediated carbon–sulfur bond activation and transformations

“…In this section, only a concise overview of C-S bond cleavage by stoichiometric amounts of transition metal compounds is presented although a variety of reports have been documented in this area. [1][2][3][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Coordination models of thiophene rings to transition metals have been reported, demonstrating diverse C-S bond activation in the complexes (Scheme 1). 31 In particular, the insertion reactions of metal atoms to C-S bonds in thiophenes and other organosulfur compounds have been extensively studied in order to get better understanding of the heterogeneous HDS mechanisms.…”

Carbon−Phosphorus Bond Activation of Tri(2-thienyl)phosphine at Dirhenium and Dimanganese Centers

“…23 Likewise, the rhenium−sulfur bond [Re(2)-S(1) 2.5101(11) Å] is similar to that in B and related complexes. [16][17][18][19][20][21]23 It was not possible to unambiguously assign a structure to 7 in the absence of an X-ray crystallographic study, the result of which is summarized in Figure 5. The molecule contains a very unusual μ-κ 1 :κ 2 -Th2P(C4H2S)PTh ligand, which bridges the rhenium centers through one phosphorus atom, P was confirmed by a crystallographic study, and the molecular structure is shown in Figure 6.…”

“…These have been the subject of reviews, although the interaction of thiophenes with the catalyst surface during HDS is not yet properly understood. 1,4,[8][9][10][11][12][13][14][15][16][17][18][19][20][21] On the basis of these studies, it has been proposed that substrate binding to active metal sites through the sulfur atom is an initial step followed by carbon−sulfur bond cleavage. 17a,21 Numerous examples of thiophene coordination and activation have been reported from which it is clear that thiophene is a relatively poor ligand for low-valent metal complexes.…”