Nonheme binuclear transition metal complexes with hydrosulfide and polychalcogenides.

The intriguing chemistry of chalcogen (S, Se)-containing ligands and their capability to bridge multiple metal centres have resulted in a plethora of reports on transition metal complexes featuring hydrosulfide (HS-) and polychalcogenides (En2-, E = S, Se). While a large number of such molecules are strictly organometallic complexes, examples of non-organometallic complexes featuring HS- and En2- with N-/O-donor ligands are relatively rare. The general synthetic procedure for the transition metal-hydrosulfido complexes involves the reaction of the corresponding metal salts with HS-/H2S and this is prone to generate sulfido bridged oligomers in the absence of sterically demanding ligands. On the other hand, the synthetic methods for the preparation of transition metal-polychalcogenido complexes include the reaction of the corresponding metal salts with En2- or the two electron oxidation of low-valent metals with elemental chalcogen, often at an elevated temperature and/or for a long time. Recently, we have developed new synthetic methods for the preparation of two new classes of binuclear transition metal complexes featuring either HS-, or Sn2- and Sen2- ligands. The new method for the synthesis of transition metal-hydrosulfido complexes involved transition metal-mediated hydrolysis of thiolates at room temperature (RT), while the method for the synthesis of transition metal-polychalcogenido complexes involved redox reaction of coordinated thiolates and exogenous elemental chalcogens at RT. An overview of the synthetic aspects, structural properties and intriguing reactivity of these two new classes of transition metal complexes is presented.

Transition Metal Mediated Hydrolysis of C-S Bonds: An Overview of a New Reaction Strategy.

Desulfurization of organosulfur substrates is highly important due to its relation with the industrial hydrodesulfurization (HDS) process of fossil fuels, which helps to eliminate the sulfur-containing impurities such as thiols, sulfide, thiophenes, etc. from crude oil for the production of easily processed and more cleanly combusted fuel with very low sulfur content. While the HDS process involves a hydrogenolysis reaction under a high pressure of hydrogen gas at high temperature, the hydrolysis of C-S bonds of organosulfur substrates at ambient conditions may very well be considered as a potential alternative for model desulfurization reactions. However, unlike the availability of an appreciable number of reports on base, acid, and metal ion mediated hydrolysis of thioesters in the literature, reports on the hydrolysis of more difficult substrates such as thiolates, sulfides, and other organosulfur substrates remained unavailable until 2017. The very recent discovery of a transition metal mediated hydrolysis reaction of C-S bonds at ambient conditions, however, has rapidly filled in this gap within the past few years. Development of this new stoichiometric reaction allowed the desulfurization of a large number of organosulfur substrates, including aliphatic and aromatic thiols, thiocarboxylic acids, sulfides, disulfides, thiophenes, and dibenzothiophene, at ambient conditions and was subsequently converted to a catalytic process for the hydrolysis of thiols. A brief overview of this new reaction strategy, a proposed reaction mechanism, a critical analysis of the efficiency, and future prospects are presented.

Hydrolysis and Transfer Reactivity of the Coordinated Thiolate, Thiocarboxylate, and Selenolate in Binuclear Zinc(II) Complexes.

A new binuclear Zn(II) complex, [Zn2(PhBIMP)(DMF)2]3+ (1) (where PhBIMP1 is the anion of 2,6-bis[bis[(N-1-methyl-4,5-diphenylimidazoylmethyl)amino]methyl]-4-methylphenol), has been shown for the first time to mediate the hydrolytic C-S bond cleavage of a series of aliphatic and aromatic thiolates to yield the corresponding alcohols/phenols along with the formation of a hydrosulfide-bridged complex, [Zn2(PhBIMP)(µ-SH)(DMF)]2+ (2), which has been thoroughly characterized in comparison with the corresponding chloride complex, [Zn2(PhBIMP)(Cl)(DMF)]2+ (3), as a control. The binuclear Zn(II)-thiolate complexes [Zn2(PhBIMP)(µ-SR)]2+ (R = Ph, 4a; 3-Br-C6H4, 4b) have also been synthesized by avoiding the C-S bond cleavage reaction. Based on the experimental results for the effects of H2O and Et3N on 1, 4a, and 4b, the complex [Zn2(PhBIMP)(µ-SR)(OH)]1+ has been proposed to be the active intermediate that precedes the C-S bond cleavage of thiolates. The complex [Zn2(PhBIMP)(µ-SCOPh)(DMF)]2+ (5) also demonstrates the hydrolysis of the coordinated thiobenzoate to produce [Zn2(PhBIMP)(µ-O2CPh)(MeCN)]2+ (6). However, unlike 4a and 5, the benzeneselenolate-bridged complex, [Zn2(PhBIMP)(µ-SePh)]2+ (7), does not generate the species, [Zn2(PhBIMP)(µ-SePh)(OH)]1+, in solution, and in line with that, the coordinated benzeneselenolate in 7 does not undergo hydrolysis to generate hydroselenide and phenol. Finally, a comparative study for the transfer reactivity of the bridging -SH, -SPh, -SC(O)Ph, and -SePh ligands in 2, 4a, 5, and 7, respectively, toward selected organic substrates has been performed to reveal the distinct differences in the reactivity of these bridging ligands.

Catalytic Hydrolysis of Thiolates to Alcohols.

A new and efficient catalytic hydrolysis of aliphatic and aromatic thiolates under ambient conditions is presented. Previously, we have demonstrated (Ganguly et al., Inorg. Chem. 2018, 57, 11306-11309) the Co(II) mediated stoichiometric hydrolysis of thiols to produce alcohols/phenols along with a binuclear dicobalt(II)-hydrosulfide complex, [Co2(PhBIMP)(µ2-SH)(DMF)]2+ (1) (PhBIMP is the anion of 2,6 bis[(bis((N-1-methyl-4,5- diphenylimidazoylmethyl) amino)methyl]- 4-methylphenol). In the present work, we have shown that the product of the stoichiometric reaction, 1, may act as an efficient catalyst for the catalytic hydrolysis of a broad range of aliphatic and aromatic thiolates in DMF at room temperature to produce alcohols/phenols. Complex 1 takes up a thiolate (RS-) and a water molecule to generate an active intermediate complex, [Co2(PhBIMP)(µ2-SH)(RS)(H2O)]1+ (2), which, in turn, releases the alcohol/phenol (ROH), hydrosulfide (HS-), and regenerates 1.