Cobalt(II)-Mediated Desulfurization of Thiophenes, Sulfides, and Thiols

Abstract

Desulfurization of organosulfur compounds is a highly important reaction because of its relevance to the hydrodesulfurization (HDS) process of fossil fuels. A reaction system involving Co(BF)·6HO and the dinucleating ligands HBPMP or HPhBIMP has been developed that could desulfurize a large number of thiophenes, sulfides, and thiols to generate the complexes [Co(BPMP)(μ-SH)(MeCN)](BF) (1a), [Co(BPMP)(SH)](BF) (1b), and [Co(PhBIMP)(μ-SH)(X)](BF) [X = DMF (2a), MeCN (2c)], while the substrates are mostly converted to the corresponding alcohols/phenols. This convenient desulfurization process has been demonstrated for 25 substrates in 6 different solvents at room temperature.

Thiolate Coordination vs C–S Bond Cleavage of Thiolates in Dinickel(II) Complexes

“…These results demonstrate that, unlike the clean and facile C−S bond cleavage reaction mediated by Co(II) 89,91 and Fe(II), 90,92 the reaction of a series of thiolates with hydrated Ni(II) salt leads to the formation of both the dinickel(II)− thiolate and dinickel(II)−hydrosulfide complexes, of which the latter complex is generated via the hydrolytic C−S bond cleavage of coordinated thiolates in the former complex. In our previous studies [89][90][91][92]108 It may, however, be noted that a coordinated −SH cannot be distinguished from a coordinated −Cl by only a molecular structure determination and that additional spectroscopic characterizations are absolutely necessary to confirm the presence of hydrosulfide (−SH) in 1. This argument has been demonstrated in our previous works involving Co(II) 91 and Fe(II) 90,92 mediated desulfurization reactions.…”

“…88,89 Interestingly, it was observed during these studies that the C−S bonds of the coordinated thiolates may be cleaved hydrolytically to yield diiron(II)− 90 and dicobalt(II)−hydrosulfide complexes. 89,91,92 A plausible mechanism for the reaction was initially proposed, 89 which was supported by detailed control experiments, isotope labeling studies using H 2 O 18 , and isolation/identification of the proposed dinuclear complexes using Co(II) and Fe(II) with different dinucleating ligand systems (Scheme 1; see also Chart 1). 91,92 The following points were perceived: order aliphatic thiols < aromatic thiols < heteroaromatic thiols < sulfides < thiophenes.…”

“…89,91,92 A plausible mechanism for the reaction was initially proposed, 89 which was supported by detailed control experiments, isotope labeling studies using H 2 O 18 , and isolation/identification of the proposed dinuclear complexes using Co(II) and Fe(II) with different dinucleating ligand systems (Scheme 1; see also Chart 1). 91,92 The following points were perceived: order aliphatic thiols < aromatic thiols < heteroaromatic thiols < sulfides < thiophenes. (vi) Substitutions on the benzene ring of PhSH, in general, lower the yield of the hydrolyzed products.…”

“…In our previous studies [89][90][91][92]108 It may, however, be noted that a coordinated −SH cannot be distinguished from a coordinated −Cl by only a molecular structure determination and that additional spectroscopic characterizations are absolutely necessary to confirm the presence of hydrosulfide (−SH) in 1. This argument has been demonstrated in our previous works involving Co(II) 91 and Fe(II) 90,92 mediated desulfurization reactions. Therefore, besides characterization of 1(ClO 4 ) 2 by molecular structure determination, elemental analysis, and mass spectrometry, 1 was characterized further by IR spectroscopy and X-ray photoelectron spectroscopy (XPS).…”

“…On the other hand, only a handful of reports on discrete mononuclear − and dinuclear − nickel–-hydrosulfide, and dinuclear nickel–sulfide , complexes are available in the literature. We have been exploring the reactivity of externally added thiolates with first row transition metal complexes in N,O-donor dinucleating ligand platforms for some time, which resulted into the isolation and characterization of several thiolate bridged diiron­(II) and dicobalt­(II) complexes. , Interestingly, it was observed during these studies that the C–S bonds of the coordinated thiolates may be cleaved hydrolytically to yield diiron­(II)– and dicobalt­(II)–hydrosulfide complexes. ,, A plausible mechanism for the reaction was initially proposed, which was supported by detailed control experiments, isotope labeling studies using H 2 O 18 , and isolation/identification of the proposed dinuclear complexes using Co­(II) and Fe­(II) with different dinucleating ligand systems (Scheme ; see also Chart ). , The following points were perceived: (i) The C–S bonds of thiolates are cleaved hydrolytically, and water is an essential component of the reaction.…”