Changes in redox potential of a nickel-pincer complex bearing reactive secondary thioamide units: Changes caused by deprotonation/protonation reactions on addition of NEt3 and DBU

Abstract

a b s t r a c tShifts of Ni(II)/Ni(III) redox potentials of a nickel-pincer complex, [Ni(SCS)Br] (SCS = 2,6-bis(benzylaminothiocarbonyl)phenyl), on addition of bases have been investigated. The complex showed two-step shifts of the Ni(II)/Ni(III) redox potential, and the shifts are associated to two-step deprotonation of the SCS ligand on addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). DBU led to a negative shift of the Ni(II)/Ni(III) redox potential by approximately 500 mV.In recent years, there have been significant interests in metalthioamide complexes [1]. In particular, transition metal complexes bearing secondary thioamides have been the subject of recent interest [2]. Several transition metal pincer complexes having reactive secondary thioamide unit(s) (-C(=S)-NH-) in the pincer ligand have been reported as exemplified by the complex shown in Scheme 1 [3].Deprotonation of the thioamide unit gives an anionic thioamidate unit, − C(= S) − NH − → − C(= S) − N − − + H + , which results in an increase in the electron-donating ability of the pincer ligand to the metal center; the oxidation potential of the complex is thought to be shifted to a lower potential by the deprotonation. Actually such an interesting shift in the oxidation potential by addition of base to pincer complexes have been reported for several transition metal complexes [4], however, there has not been such a report for Ni complexes to our knowledge. In this paper, we report the control of the oxidation potential of the secondary thioamide-containing nickel pincer complex, [Ni(SCS)Br] (cf. Scheme 1), by addition of bases.[Ni(SCS)Br] was prepared as reported previously [3a]. Fig. 1 displays the cyclic voltammogram (CV) of [Ni(SCS)Br] in DMF. As shown in Fig. 1, [Ni(SCS)Br] shows a reversible redox wave at E 1/2 =+0.156 V (vs. Fc + / Fc), which is assigned to an Ni(II)/Ni(III) redox process. The E 1/2(Ni(II)/Ni(III)) is comparable to that of a previously reported Ni-pincer complex, [Ni(NCN)Br] (NCN = C 6 H 3 (CH 2 NMe 2 ) 2 -o,o′; E 1/2(Ni(II)/Ni(III)) = + 0.15 V (vs. SCE)) [5]. The strong electron-donating ability of the pincer ligand seems to give such a lower Ni(II)/Ni(III) potential than that of usual Ni(II) complexes; for instance, E 1/2(Ni(II)/Ni(III)) of [Ni(bpy) 3 ] 2+ (bpy = 2,2′-bipyridyl) is observed at 1.66 V vs. SCE (or 1.13 V vs. Fc + /Fc) [6]. The second oxidation peak (shoulder) at E pa = +0.854 V may involve oxidation of the SCS ligand.Effects of the added base on the electrochemical processes were investigated using NEt 3 and DBU as a base. Fig. 2 shows changes in the CV curve of [Ni(SCS)Br] on addition of NEt 3 in DMF. As shown in Fig. 2, the most part of the original Ni(II)/Ni(III) redox wave of [Ni(SCS)Br] disappears on addition of NEt 3 , and a new redox wave appears at E 1/2 = − 0.08 V vs. Fc + /Fc. When methanesulfonic acid (MSA) is added to the [Ni(SCS)Br]-NEt 3 solution, the redox wave of original [Ni(SCS)Br] is recovered completely. These results suggest that the redox potential of [Ni(SCS)Br] can be controlled by ...

Synthesis, characterization, photophysical properties, and catalytic activity of an SCS bis(N-heterocyclic thione) (SCS-NHT) Pd pincer complex

“…The first SCS pincer complex, a thioether-based palladium(II) derivative, was reported by Shaw in 1980. 13 Sulfur-containing pincer ligands have been shown to exhibit a great deal of architectural diversity and variations in their design including lateral donors such as thioethers, [14][15][16][17][18][19][20][21] thioamides [22][23][24][25][26][27][28] or phosphine sulfides. [29][30][31] Although the chemistry of SCS pincer complexes is not as developed as that of the related PCP and NCN derivatives, they have been found to exhibit interesting photophysical properties [22][23][24]26,30,32 and catalytic applications ranging from the borylation of allylic alcohols 33 to a variety of cross-coupling reactions.…”

One‐pot synthesis of methyl n‐phenyl carbamate from aniline, carbon dioxide and methanol

“…Only DPU was detected when using DBU catalyst alone and without solvent. When acetonitrile was used as solvent, the catalytic activity of the organic base catalysts decreased in the following order: DBU > TMG > triethylamine ≈ NMI ≈ pyridine, roughly in accordance with their alkali strength . Therefore DBU was determined as the most suitable catalyst.…”

“…When acetonitrile was used as solvent, the catalytic activity of the organic base catalysts decreased in the following order: DBU > TMG > triethylamine ≈ NMI ≈ pyridine, roughly in accordance with their alkali strength. 14 excellent solvent performance of acetonitrile is attributed not only to its good solubility to CO 2 but also to shifting the reaction equilibrium forward since its hydration can consume the byproduct water. 17,18 In the system of DBU-acetonitrile combination, the united selectivity of both DPU and MPC was only 65.2%, suggesting that part of aniline has converted to side-products.…”

Iron Complexes of a Proton-Responsive SCS Pincer Ligand with a Sensitive Electronic Structure

“…In the parent system, the Fe 3+/2+ redox couple was found at E 1/2 = −0.84 V, and the p K a was determined to be at least 19.5, giving a BDFE of at least 62 kcal/mol; bracketing suggested a BDFE near 68 kcal/mol. Studies of nickel, platinum, and ruthenium SCS compounds also corroborate decreased N–H bond acidity and more negative reduction potential upon metalation, though these examples do not report bond energies. ,− …”

“…Studies of nickel, platinum, and ruthenium SCS compounds also corroborate decreased N-H bond acidity and more negative reduction potential upon metalation, though these examples do not report bond energies. 39,[76][77][78] There are interesting implications of our determination that deprotonating a distant thioamide site gives a highly-reducing iron(II) species, because this could be a mechanism for generating electron-rich active sites in metalloenzymes. This fits into a growing body of evidence that reduction potentials can may be modulated by the protonation state of sites outside the vicinity of metals in metallocofactors like the nitrogenase FeMoco.…”