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

Abstract

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, [M 2 (PhBIMP)(μ 2 -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, [M III 2 (PhBIMP)(H 2 O) 2 (DMF) 2 ] 5+ (M = Co, Fe), in solution. While the generation of such a solvent-and/or H 2 O-coordinated dicobalt(III) species in the reaction solution could not be established beyond a doubt, formation of the diiron(III) species [Fe III 2 (PhBIMP)(H 2 O) 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(ClO 4 ) 3 •xH 2 O. Interestingly, a comparative study using different types of disulfides and the molecular structure determination of a key reaction intermediate, [Fe 2 (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.

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) , and Fe­(II), , 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 − , involving Fe­(II) and Co­(II), we have shown that the C–S bonds of thiolates are cleaved hydrolytically, and such C–S bond cleavage reactions are mediated by solvent coordinated dinuclear Fe­(II)/Co­(II) complexes ( A in Scheme ) in the presence of a base, such as Et 3 N. The proposed working mechanism considered the initial formation of a dinuclear Fe­(II)/Co­(II)–solvento complex (solvent = DMF/H 2 O) followed by its conversion to a dinuclear Fe­(II)/Co­(II) complex featuring a coordinated thiolate and water molecule/molecules ( B in Scheme ) which finally leads to the hydrolyzed organic product (alcohol) and diiron­(II)/dicobalt­(II)–hydrosulfide complexes ( C in Scheme ). The generation of the dinuclear Fe­(II)/Co­(II)–solvento complexes (initial complexes generated in the reaction solution) and the dinuclear Fe­(II)/Co­(II)–hydrosulfide complexes (final products) in the proposed working mechanism (Scheme ) was confirmed by molecular structure determinations and mass spectrometric study, ,, while the dinuclear intermediate featuring a coordinated thiolate and water molecule ( B in Scheme ) has been supported so far by only a mass spectrometric study .…”

“…However, synthesis of a dinickel­(II)–thiolate complex using a simple, monodentate thiolate remained to be achieved. Considering these results and the known difficulty experienced in the case of C–S bond cleavage of aromatic thiolates (such as NaSPh) as compared with the facile C–S bond cleavage reactions for the aliphatic thiolates (such as NaSR; R = Me, Et, t Bu), ,, the reaction of NaSPh with Ni­(ClO 4 ) 2 ·6H 2 O was therefore performed with the expectation of preparing the benzenethiolate coordinated dinickel­(II) compound as the sole product. Indeed, reaction of Ni­(ClO 4 ) 2 ·6H 2 O and HBPMP with NaSPh in the presence of Et 3 N in DMF for 12 h yielded the desired compound, [Ni 2 (BPMP)­(SPh)­(DMF)­(MeOH)­(H 2 O)]­(ClO 4 ) 2 ( 7 (ClO 4 ) 2 ) (Figure b).…”

“…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.…”

Improving Excited-State Potential Energy Surfaces via Optimal Orbital Shapes

“…Let us begin with the relatively simple example of C−S bond photodissociation, which can be important in astrochemistry, 32 biomedicine, 33 and catalysis. 34 For C−S single bond dissociation, nonradiative internal conversion between the first excited state 1 ππ* and the dissociative 1 πσ* state is key. 35 Here, we instead investigate the C−S double bond dissociation of thioacetone, (CH 3 ) 2 CS, using both SA-CASSCF and SS-CASSCF.…”

“…Let us begin with the relatively simple example of C–S bond photodissociation, which can be important in astrochemistry, biomedicine, and catalysis . For C–S single bond dissociation, nonradiative internal conversion between the first excited state 1 ππ* and the dissociative 1 πσ* state is key .…”