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

Abstract

Several pincer ligated nickel mercapto complexes, [2,6-(R PCH ) C H ]NiSH (R=tBu, 1 a; iPr, 1 b), [2,6-(R PO) C H ]NiSH (R=tBu, 2 a; iPr, 2 b) and [4-MeOCO-2,6-(tBu PO) C H ]NiSH (3 a), were synthesized and fully characterized. The reactivity of the mercapto groups against boron hydrides and organic bases was investigated. It was found that the mercapto groups are difficult to be deprotonated by boron hydrides or organic bases. The treatment of complex 2 a or 2 b with an excess amount of catecholborane (HBcat) afforded the corresponding pincer ligated nickel borohydride complexes and the HBcat degradation product. The treatment of complex 1 a, 2 a or 2 b with an excess amount of BH ⋅THF produced the corresponding nickel borohydride species and the S-bridged triborane species THF⋅BH -μ -S(B H ) (5). No reactions between these complexes and organic bases were observed. DFT calculations were carried out to understand this reactivity and get mechanistic insights into the reactions.

Application of POCOP Pincer Nickel Complexes to the Catalytic Hydroboration of Carbon Dioxide

“…The 1 H, 13 C{ 1 H} and 31 P{ 1 H} NMR spectra of 1c were in good agreement with the structure depicted in Scheme 2. Complex 1c showed the expected spectra characteristics for a POCOP pincer ligated nickel complex [14][15][16]27,[33][34][35][36][37][38][39][40][41][42]. Similar to other reported transition metal mercapto complexes [37,38,[43][44][45], the 1 H NMR resonance of the -SH proton of 1c appeared in a relatively high-field (−0.76 ppm) as a triplet.…”

“…As shown in Figure 1, the square-planar geometry for the Ni center of 1c is distorted; this is quite common for POCOP pincer nickel complexes [14][15][16]27,[33][34][35][36][37][38][39][40][41][42]. Both the C ipso -Ni and Ni-S bond lengths in complex 1c are comparable to those of the related pincer nickel mercapto complexes with t Bu 2 P or i Pr 2 P arms [37,38]. This indicates that the C ipso -Ni and Ni-S bond strengths are not considerably affected by the substituents on the phosphine arms.…”

“…Complexes 1a [37], 1b [38], 2a-c [35] and 3a-c [35] were synthesized as described in the literatures. Complex 1c has never been reported before and was synthesized by the reaction between [2,6-(Ph 2 PO) 2 C 6 H 3 ]NiCl and NaSH in THF/methanol at room temperature (Scheme 2).…”

“…We previously reported that POCOP pincer ligated nickel mercapto complexes with t Bu 2 P or i Pr 2 P arms (1a and 1b) can interact with HBcat at room temperature to generate POCOP pincer nickel hydride species [38]. In order to obtain more information, we checked the interactions of the other complexes (1c, 2a-c and 3a-c) with HBcat.…”

“…We have been interested in the chemistry of transition metal complexes bearing bis(phosphinite) (POCOP) pincer ligands [14,15,26,27,[33][34][35][36][37][38]. Recently, we applied some POCOP pincer nickel thiolato complexes, [2,6-(R 2 PO) 2 C 6 H 3 ]NiY (R = t Bu, i Pr; Y = SC 6 H 4 -p-OCH 3 , SC 6 H 4 -p-CH 3 , SPh, SC 6 H 4 -p-CF 3 , SCH 2 Ph), to the catalytic reduction of CO 2 with catecholborane (HBcat) [27].…”