Dehydrosulfurization of Isothiocyanates by Gas‐Phase Fe(I) Cations

Abstract

Bare Fe(1) cations act in the gas phase as dehydrosulfurization catalysts for butyl isothiocyanate by specifically coupling the S-atom with remote H-atoms from the o and (o -1) position of the alkyl chain. Other losses induced by Fe(1) are due to 'remote functionalization', the formation of ion/dipole intermediates, and insertionlphydrogen shift processes.Introduction. -Catalytic hydrodesulfurization (HDS) is a very important process in the petroleum and petrochemical industry. In fact, the removal of sulfur from crude oils and coal liquids by conversion of the organic sulfur content to H,S is one of the largest-scale chemical processes [ 11. For this hydrogenolytic reaction, the catalysts used in industry are normally derived from oxides of group-VI elements (Mo, Wj, promoted with either Co or Ni, and supported on y-alumina, that are in situ persulfided with H,S/H, [2]. In view of the industrial importance of HDS, it is no wonder that a better understanding of the basic principles of this process and development of improved catalysts are an area of intense research [3].One approach to study the intrinsic properties of the catalytically active sites, i.e. the transition-metal atoms or ions, is to investigate the reactions of bare or ligated metal ions with organic model substrates in the gas phase. Gas-phase studies are not hampered with the problems of solvation, ion pairing, or ligand effects that strongly influence the condensed-phase studies on catalysis [4], and it is, thus, not surprising that organometallic chemistry in the gas phase has developed to an actively pursued topic (for reviews, see [5]). The rapidly growing data nowadays allows a much better understanding of the principles and mechanisms of C-H and/or C-C bond activation, and we, therefore, decided to investigate the influence that a S-atom exerts on the reactivity of monofunctional substrates in their reactions with bare metal ions, also in the hope to learn more about the mechanisms of HDS.Although it is well known that the presence of sulfur can have substantial effects on the performance of catalysts, by, for example, poisoning the isomerization or hydrogenation properties [6], it has so far received little attention in either condensed-phase catalysis [7] or gas-phase studies. Freiser and coworkers have studied the reactions of a few transition-metal cations with ethylene sulfide, which led to the formation of metal polysulfide ions [8], the chemistry of transition-metal anions with simple thiols, sulfides, and disulfides [9], and of MS' ions (M = Fe, Co, Ni) with alkanes [lo]. Finally, the reactions of Fe+, Co+, and Ni' with thiols and sulfides have been studied by Carlin [l 11 and Lombarski and Allison [12].

Gas‐Phase Chemistry of Ti+ to Zn+ with Butyl Isothiocyanate

“…Loss of propene for Fe+ (Eqn. 6 ) was also noted before [12], and the same labeling distribution is observed. We propose the mechanism given in Scheme 3 which is in accord with all experimental findings.…”

“…Thus, it can be safely concluded that the products arise from a common intermediate, such as M(C,H,)(HNCS)'. Although neither the structure of the MHNCS' ions could be determined [12], nor could the identity of the HNCS neutrals [15] be established, the H/D scrambling observed here strongly points to the operation of the ion/dipole mechanism. This is further supported by the observation that Cu' exclusively reacts via Eqns.…”

“…This intramolecular variant of the industrially important hydrodesulfurization (HDS) [ 191 was much more prominent in the MI spectra and also proceeded with a slightly different labeling distribution. The mechanism we proposed [12] is given in Scheme 2 and commences with S abstraction, which yields a complex of FeS' with butyl isocyanide (2). Sulfur abstraction in the gas phase has been noted for Fe+ before [20] [21] and is also well known in solution for isothiocyanates [22] as…”

“…The operation of the ion/dipole mechanism has also been assumed to account for the formation of FeHNCS+ ions and losses of HNCS in reactions of Fe'with isothiocyanates [12]. To obtain more evidence for this contention we decided to study all first d-row transition-metal ions from Ti+ through Zn' with BuNCS (1) and its isotopomers l a 4 using a Fourier-transform ion-cyclotron resonance (FTICR) spectrometer [ 131.…”

Formation and Reactivity of Gaseous Iron‐Sulfur Clusters

“…Note that this process corresponds to a formal reduction of the iron cluster-core. The second reaction channel (20%) results in hydro-desulfuration [22] [reaction (5)] and thus highlights the redox character of the reactions in an even more pronounced manner. It remains unclear whether the ionic product contains an intact benzene molecule or if further bond activation involving the remaining S atom has occurred.…”