A ratiometric substrate for rapid evaluation of transfer hydrogenation efficiency in solution.

A cyclometalated iridium(III) complex bearing a self-immolative quinolinium moiety was developed as a ratiometric substrate for transfer hydrogenation studies. This photoluminescent probe allowed the rapid screening of a variety of Ir catalysts using a microplate reader, offering a convenient method to assess activity using a minimum amount of catalyst sample.

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.

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

A detailed study for the synthesis of dinickel(II)-thiolate and dinickel(II)-hydrosulfide complexes and the complete characterization of the relevant intermediates involved in the C-S bond cleavage of thiolates are presented. Hydrated Ni(II) salts mediate the hydrolytic C-S bond cleavage of thiolates (NaSR/RSH; R = Me, Et, nBu, tBu), albeit inefficiently, to yield a mixture of a dinickel(II)-hydrosulfide complex, [Ni2(BPMP)(µ-SH)(DMF)2]2+ (1), and the corresponding dinickel(II)-thiolate complexes, such as [Ni2(BPMP)(µ-SEt)(ClO4)]1+ (2) (HBPMP is 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol). A systematic study for the reactivity of thiolates with Ni(II) was therefore pursued which finally yielded 1 as a pure product which has been characterized in comparison with the dinickel(II)-dichloride complex, [Ni2(BPMP)(Cl)2(MeOH)2]1+ (3). While the reaction of thiolates with anhydrous Ni(OTf)2 in dry conditions could only yield [Ni2(BPMP)(OTf)2]1+ (5) instead of the expected dinickel(II)-thiolate compound, the C-S bond cleavage could be suppressed by the use of a chelating thiol, such as PhCOSH, to yield [Ni2(BPMP)(SCOPh)2]1+ (6). Finally, with the suitable choice of a monodentate thiol, a dinickel(II)-monothiolate complex, [Ni2(BPMP)(SPh)(DMF)(MeOH)(H2O)]2+ (7), was isolated as a pure product within 1 h of reaction, which after a longer time of reaction yielded 1 and PhOH. Complex 7 may thus be regarded as the intermediate that precedes the C-S bond cleavage and is generated by the reaction of a thiolate with an initially formed dinickel(II)-solvento complex, [Ni2(BPMP)(MeOH)2(H2O)2]3+(4). Selected dinickel(II) complexes were explored further for the scope of substitution reactions, and the results include the isolation of a dinickel(II)-bis(thiolate) complex, [Ni2(BPMP)(µ-SPh)2]1+ (8).

Comparative Study for the Cobalt(II)- and Iron(II)-Mediated Desulfurization of Disulfides Demonstrating That the C-S Bond Cleavage Step Precedes the S-S Bond Cleavage Step.

A unique Co(II)- and Fe(II)-mediated complete desulfurization of disulfides of the type RS-SR and RC(O)S-SC(O)R to yield the corresponding alcohols (ROH) and carboxylic acids (RCOOH), respectively, along with the formation of a dicobalt(II)/diiron(II)-hydrosulfide complex, [M2(PhBIMP)(µ2-SH)(DMF)]2+ (M = Co, Fe), has been demonstrated. This new desulfurization reaction involves cleavage of both C-S and S-S bonds, where the cleavage of the S-S bond (presumably two-electron reduction of the S-S bond) may generate two-electron-oxidized dicobalt(III)/diiron(III) species, [MIII2(PhBIMP)(H2O)2(DMF)2]5+ (M = Co, Fe), in solution. While the generation of such a solvent- and/or H2O-coordinated dicobalt(III) species in the reaction solution could not be established beyond a doubt, formation of the diiron(III) species [FeIII2(PhBIMP)(H2O)2(DMF)2]5+ according to the proposed reaction mechanism has been confirmed by a combination of mass spectrometry and UV-vis spectroscopy in comparison with an authentic sample, synthesized directly by an independent procedure using Fe(ClO4)3·xH2O. Interestingly, a comparative study using different types of disulfides and the molecular structure determination of a key reaction intermediate, [Fe2(PhBIMP)(MeCOSS)]2+, generated via the cleavage of only one C-S bond of MeC(O)S-SC(O)Me, demonstrates that the C-S bond cleavage step precedes the S-S bond cleavage step during the Fe(II)-mediated desulfurization of disulfides.

Iron(II) Mediated Desulfurization of Organosulfur Substrates Produces Nonheme Diiron(II)-hydrosulfides.

A reaction system involving Fe(BF4)2·6H2O and two dinucleating ligands, HBPMP and HPhBIMP, mediates the desulfurization of aliphatic and aromatic thiols at room temperature. This rare C-S bond cleavage reaction produces two nonheme diiron(II) complexes, [Fe2(BPMP)(SH)2(MeOH)2]1+ (1a) and [Fe2(PhBIMP)(µ-SH)(DMF)]2+ (2a), possibly via an active species similar to [Fe2(PhBIMP)(H2O)2(DMF)2]3+ (2c), while the thiols are converted to the corresponding alcohols/phenols. In the case of thioacetic acid, a bidentate chelating organosulfur substrate, the use of HBPMP produces the corresponding bis-thiocarboxylato bridged complex, [Fe2(BPMP)(CH3COS)2]1+ (1b), instead of 1a. However, the use of HPhBIMP allows the Fe(II) mediated desulfurization of thioacetic acid as well to yield 2a, along with the formation of [Fe2(PhBIMP)(CH3COS)(MeCN)]2+ (2b). This convenient desulfurization reaction has been demonstrated for different substrates in different solvents along with the structural and spectroscopic characterizations of the diiron(II)-hydrosulfide complexes in comparison with two isostructural chloride complexes, [Fe2(BPMP)(Cl)2(MeOH)2]1+ (1c) and [Fe2(PhBIMP)(µ-Cl)(DMF)]2+ (2d). The role of the individual reactants in the desulfurization process has been thoroughly investigated using control reactions, and on the basis of these results and the identification of intermediate species, such as [Fe2(PhBIMP)(StBu)(DMF)3]2+ and [Fe2(PhBIMP)(StBu)(H2O)(DMF)2]2+, in solution by mass spectrometry, a possible mechanism has been proposed.

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

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(BF4)2·6H2O 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 [Co2(BPMP)(µ2-SH)(MeCN)](BF4)2 (1a), [Co2(BPMP)(SH)2](BF4) (1b), and [Co2(PhBIMP)(µ2-SH)(X)](BF4)2 [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.