RESUMEN
With large surface area and versatile electronic behaviour, the composites of Fe4S4(SRS)4 nanoclusters and functionalized carbon nanotubes (f-CNTs), are expected to catalyze the conversion of protons to hydrogen gas at lower electro-potentials with higher output than a platinum electrode. In search of a non-noble metal based catalytic material, we report for the first time, the isolation of unimolecular iron-sulfur cubane cluster, [Fe4(µ-S)4(mnt)4], (1) (mnt = maleonitriledithiolate) as nanocubes (63×85×120 nm) in MeCN-EtOH (MeCN is acetonitrile, while EtOH is ethanol) solvents and bimolecular [NBu4]4[Fe4(µ-S)4(mnt)4] as nanocuboctahedra (120×121×125 nm) in pure EtOH. The cubic shape of the nanocrystal reminds its geometrical relationship with a molecular cube and one of sides of the nanocube and nanocuboctahedron matches at 120 nm. The nanocubes of Fe4S4(SRS)4 have been immobilized on f-CNTs and characterized by SEM and TEM methods which indicate that clusters of Fe4S4 of diameter (8-9 nm) interact with surfaces, sidewalls and tip of the f-CNTs. A ferrocene type of interaction could not be observed with f-CNTs, because nanoparticles are not found in CNT-inner cavity. The interaction could be either adsorption or hydrogen bonding interactions between -COOH/-OH groups of f-CNTs and N≡C terminals of iron-sulphur nanoclusters leading to immobilization of an iron-sulfur nanocluster on a single CNT molecule or the iron-sulfur nanoclusters can be entrapped between two CNT molecules.
RESUMEN
The development of alternate catalysts that utilize non-precious metal based electrode materials such as the first row transition metal complexes is an important goal for economic fuel cell design. In this direction, a new Fe4S4 cubane type cluster, [PPh4]2[Fe4S4(DMET)4] (1) (DMET = cis-1,2-dicarbomethoxyethylene dithiolate) and its composite with functionalized graphene, (1@graphene) have been synthesized and characterized. The presence of nanocrystalline structures on graphene matrix in TEM and SEM images of 1@graphene indicate that the cluster (1) has been immobilized. The composite, 1@graphene evolves H2 gas from p-toluene sulfonic acid (TsOH) in a mixture of H2O and CH3CN under ambient conditions with a significant turnover number of 3200. 1@graphene electro-catalyzes H2 evolution at Ep, -1.2 V with remarkable throughput, catalytic efficiency and stability in only H2O or in only CH3CN. The Fe4S4 cluster (1) alone electro-catalyzes hydrogen evolution at Ep, -0.75 V from TsOH in CH3CN. The X-ray crystal structure of the Fe4S4 cluster (1) (λmax, CH2Cl2, 823 nm; ε, 2200 mol-1 cm-1) shows that it is dianionic with a cumulative oxidation state of +2.5 for the iron centers and short C-S bond distances (ca., 1.712 Å & 1.727 Å) indicating the presence of sulfur based radicals.