Conformational Effects of [Ni2 (µ-ArS)2 ] Cores on Their Electrocatalytic Activity.

Two nickel complexes supported by tridentate NS2 ligands, [Ni2 (κ-N,S,S,S'-NPh {CH2 (MeC6 H2 R')S}2 )2 ] (1; R'=3,5-(CF3 )2 C6 H3 ) and [Ni2 (κ-N,S,S,S'-NiBu {CH2 C6 H4 S}2 )2 ] (2), were prepared as bioinspired models of the active site of [NiFe] hydrogenases. The solid-state structure of 1 reveals that the [Ni2 (µ-ArS)2 ] core is bent, with the planes of the nickel centers at a hinge angle of 81.3(5)°, whereas 2 shows a coplanar arrangement between both nickel(II) ions in the dimeric structure. Complex 1 electrocatalyzes proton reduction from CF3 COOH at -1.93 (overpotential of 1.04 V, with icat /ip ≈21.8) and -1.47 V (overpotential of 580 mV, with icat /ip ≈5.9) versus the ferrocene/ferrocenium redox couple. The electrochemical behavior of 1 relative to that of 2 may be related to the bent [Ni2 (µ-ArS)2 ] core, which allows proximity of the two Ni⋅⋅⋅Ni centers at 2.730(8) Å; thus possibly favoring H+ reduction. In contrast, the planar [Ni2 (µ-ArS)2 ] core of 2 results in a Ni⋅⋅⋅Ni distance of 3.364(4) Šand is unstable in the presence of acid.

Easily reduced bis-pincer (NS2)2molybdenum(iv) to (NHS2)2Mo(ii) by alcohols vs. redox-inert (NS2)(NHS2)iron(iii) complexes.

Iron and molybdenum complexes supported by a pincer-type dianionic [NS2]2- donor were prepared to compare their structural, spectroscopic, and electrochemical properties. The versatility of the [NS2]2Mo(iv) complex (2) to access different oxidation states was evidenced in the activation of methanol and isopropanol, oxidising them to formaldehyde or acetone with concomitant reduction and protonation to afford [NHS2]2Mo(ii), complex (3). This redox behaviour contrasts with the null reactivity observed for the analogous ferric complex [NS2][NHS2]Fe(iii) (1). Complex 2 presents a quasi-reversible process at E1/2 = -0.80 V relative to the ferrocenium/ferrocene couple (Fc+/Fc), which is attributed to the Mo(iv)/Mo(v) redox couple. Two irreversible cathodic processes were observed at Ecp = -1.59 and -2.20 V, which are attributed to the Mo(iv)/Mo(iii) and Mo(iii)/Mo(ii) redox couples. Cyclic voltammetry and solid-state structures obtained by X-ray crystallography support a 2H+ and 2e- process, whereby the Mo(iv) centre in 2 is reduced sequentially to Mo(iii), and finally to Mo(ii) in 3. These redox events were observed at Ecp = -1.22 and -2.15 V (vs. Fc+/Fc) in the anodic cyclic voltammograms of 2 in THF in the presence of acid. A new reduction peak was detected under these conditions at Ecp = -2.30 V, consistent with electrocatalytic proton reduction. This was corroborated for 2 as a catalyst precursor in the presence of increasing amounts of p-toluenesulfonic acid, with the addition of 2 to 14 equivs resulting in an increase of the current measured.