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

Bis(benzimidazole)amino thio- and selenoether Iron(II) complexes as proton reduction electrocatalysts

Two novel Iron (II) complexes featuring tetrapodal bis(benzimidazole)amino thio- and selenoether ligands (LS and LSe) were synthesized, characterized, and tested as electrocatalysts for the hydrogen evolution reaction. The bromide complexes [Fe(LS,LSe)Br₂] (1-2) are highly insoluble, but their DMSO solvates were characterized by single crystal X-ray diffraction, revealing an octahedral coordination environment that does not feature coordination of the chalcogen atoms. The corresponding triflate derivatives [Fe(LS,LSe)(MeCN)₃]OTf₂ (1c-2c) were employed for electrocatalytic proton reduction, with 1c exhibiting higher activity, thus suggesting that the thioether may participate as a more competent pendant ligand for proton transfer.

Thiolate Coordination vs C-S Bond Cleavage of Thiolates in Dinickel(II) Complexes

A detailed study for the synthesis of dinickel(II)-thiolate and dinickel(II)-hydrosulfide complexes and the complete characterization of the relevant intermediates involved in the C-S bond cleavage of thiolates are presented. Hydrated Ni(II) salts mediate the hydrolytic C-S bond cleavage of thiolates (NaSR/RSH; R = Me, Et, ⁿBu, ^tBu), albeit inefficiently, to yield a mixture of a dinickel(II)-hydrosulfide complex, [Ni₂(BPMP)(μ-SH)(DMF)₂]²⁺ (1), and the corresponding dinickel(II)-thiolate complexes, such as [Ni₂(BPMP)(μ-SEt)(ClO₄)]¹⁺ (2) (HBPMP is 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol). A systematic study for the reactivity of thiolates with Ni(II) was therefore pursued which finally yielded 1 as a pure product which has been characterized in comparison with the dinickel(II)-dichloride complex, [Ni₂(BPMP)(Cl)₂(MeOH)₂]¹⁺ (3). While the reaction of thiolates with anhydrous Ni(OTf)₂ in dry conditions could only yield [Ni₂(BPMP)(OTf)₂]¹⁺ (5) instead of the expected dinickel(II)-thiolate compound, the C-S bond cleavage could be suppressed by the use of a chelating thiol, such as PhCOSH, to yield [Ni₂(BPMP)(SCOPh)₂]¹⁺ (6). Finally, with the suitable choice of a monodentate thiol, a dinickel(II)-monothiolate complex, [Ni₂(BPMP)(SPh)(DMF)(MeOH)(H₂O)]²⁺ (7), was isolated as a pure product within 1 h of reaction, which after a longer time of reaction yielded 1 and PhOH. Complex 7 may thus be regarded as the intermediate that precedes the C-S bond cleavage and is generated by the reaction of a thiolate with an initially formed dinickel(II)-solvento complex, [Ni₂(BPMP)(MeOH)₂(H₂O)₂]³⁺(4). Selected dinickel(II) complexes were explored further for the scope of substitution reactions, and the results include the isolation of a dinickel(II)-bis(thiolate) complex, [Ni₂(BPMP)(μ-SPh)₂]¹⁺ (8).

A bioinspired thiolate-bridged dinickel complex with a pendant amine: synthesis, structure and electrocatalytic properties

By employing X(CH₂CH₂S^-)₂ (X = S, tpdt; X = O, opdt; X = NPh, npdt) as bridging ligands, four thiolate-bridged dinickel complexes supported by phosphine ligands, [(dppe)Ni(μ-1SSS':2SS-tpdt)Ni(dppe)][PF₆]₂ (1[PF₆]₂, dppe = Ph₂P(CH₂)₂PPh₂), [(dppe)Ni(μ-1SSN:2SS-npdt)Ni(dppe)][PF₆]₂ (2[PF₆]₂) and [(dppe)Ni(t-Cl)(μ-1SSX:2SS-C₄H₈S₂X)Ni(dppe)][PF₆] (3[PF₆], X = S; 4[PF₆], X = O) were facilely obtained by the salt metathesis reaction. These four thiolate-bridged dinickel complexes have all been fully characterized by spectroscopic methods and X-ray crystallography. In 2[PF₆]₂, elongation of the Ni-N bond distance, possibly caused by steric hindrance, indicates that the pendant nitrogen group shuttles between the two nickel centers in solution, which is evidenced by ³¹P{¹H} NMR spectroscopic results. Furthermore, these thiolate-bridged dinickel complexes have all been proved to be electrocatalysts for proton reduction to hydrogen. Notably, complex 2[PF₆]₂ featuring a pendant amine group in the secondary coordination sphere exhibits the best catalytic activity at a relatively low overpotential.