A reaction of [2,6-(^tBu₂PO)₂C₆H₃]NiSCH₂Ph with BH₃·THF: borane mediated C–S bond cleavage

Abstract

C-S bond activation of thiophenols and mercaptans is of great importance but has rarely been reported. In this paper we report the C-S bond cleavage of the thiolato ligand of [2,6-(tBu2PO)2C6H3]NiSCH2Ph (1) mediated by BH3·THF. The treatment of 1 with an excess amount of BH3·THF in THF at room temperature afforded the borohydride complex [2,6-(tBu2PO)2C6H3]Ni(η2-BH4) (2) as the only product. The reaction of 1 with 2 equiv. of BH3·THF in THF at room temperature for 48 h produced the hydride complex [2,6-(tBu2PO)2C6H3]NiH (3) and the mercapto complex [2,6-(tBu2PO)2C6H3]NiSH (5). As a new complex, 5 was also independently synthesized by a salt metathesis reaction of the corresponding chloride complex with NaSH and fully characterized by multinuclear NMR, FTIR, HRMS, X-ray crystallography and elemental analysis. A possible mechanism for the formation of 5 was proposed. It was supposed that 5 formed through BH3 mediated C-S bond cleavage of the thiolato ligand of 1. The result would initiate further studies on the transition metal catalyzed borane mediated C-S bond activation of mercaptans.

The Reactivity of Mercapto Groups against Boron Hydrides in Pincer Ligated Nickel Mercapto Complexes

“…We recently reported the synthesis of complex 2 a by a salt metathesis reaction of the corresponding nickel chloride complex with NaSH . Complexes 1 a , 1 b , 2 b and 3 a were synthesized similarly and isolated in moderate yields (Scheme ).…”

“…The 1 H, 13 C{ 1 H} and 31 P{ 1 H} NMR spectra of complexes 1 a , 1 b , 2 b and 3 a (SI Figures S1–S12) were consistent with the structures shown in Scheme , and displayed the expected spectra features for POCOP,,, or PCP (PCP=2,6‐(R 2 PCH 2 ) 2 C 6 H 3 ) pincer ligated nickel complexes. Like complex 2 a , the 1 H NMR resonances of the mercapto protons of 1 a , 1 b , 2 b and 3 a appeared in a relatively high‐field region (−0.92 to −1.60 ppm) as virtual triplets with H‐P coupling constants of 17–20 Hz. Similar 1 H NMR resonances and H‐P coupling constants were previously reported for other transition metal mercapto complexes ,.…”

“…A distorted square‐planar geometry for the nickel centers of complexes 1 a , 1 b , 2 b and 3 a was observed; this is similar to other reported POCOP,,, or PCP pincer ligated nickel complexes. The C ipso –Ni bond lengths in 1 a and 1 b (1.942(2) and 1.937(5) Å) are slightly longer than those of 2 a , 2 b and 3 a (1.9009(17), 1.894(7) and 1.889(3) Å). The Ni‐S bond lengths in 1 a and 1 b (2.2164(7) and 2.194(2) Å) are comparable to those of 2 a , 2 b and 3 a (2.2006(6), 2.183(5) and 2.1862(9) Å).…”

“…The C ipso –Ni bond lengths in 1 a and 1 b (1.942(2) and 1.937(5) Å) are slightly longer than those of 2 a , 2 b and 3 a (1.9009(17), 1.894(7) and 1.889(3) Å). The Ni‐S bond lengths in 1 a and 1 b (2.2164(7) and 2.194(2) Å) are comparable to those of 2 a , 2 b and 3 a (2.2006(6), 2.183(5) and 2.1862(9) Å). This indicates that the Ni‐S bond strength is not significantly influenced by the POCOP or PCP pincer backbones.…”

“…We are interested in the reactivity and catalytic application of transition metal complexes bearing bis(phosphinite) (also known as POCOP) pincer ligands . Recently, we obtained a POCOP pincer ligated nickel mercapto complex, [2,6‐( t Bu 2 PO) 2 C 6 H 3 ]NiSH ( 2 a ), by the borane mediated C–S bond cleavage of the corresponding nickel thiolate complex or the salt metathesis reaction of the corresponding nickel chloride complex with NaSH . In order to get information about the reactivity of the mercapto groups of this type of complexes, in this work, we synthesized more novel benzene‐based pincer ligated nickel mercapto complexes, [2,6‐(R 2 PCH 2 ) 2 C 6 H 3 ]NiSH (R= t Bu, 1 a ; i Pr, 1 b ), [2,6‐( i Pr 2 PO) 2 C 6 H 3 ]NiSH ( 2 b ), [4‐MeOCO‐2,6‐( t Bu 2 PO) 2 C 6 H 2 ]NiSH ( 3 a ), and reacted them with boron hydrides and organic bases.…”