Multimetallic assembly of concave-shaped rectangular Mn4 clusters as efficient hydrogen evolution electrocatalysts

Abstract

Although great efforts have been dedicated to the investigation of active, stable and cost-effective electrocatalysts, only a few noteworthy breakthroughs have been accomplished until now. In this perspective, cluster-based molecular systems in which composition can be compared with electrocatalytic activity remain under-explored primarily because of the synthetic challenges. Thus, exploring novel electrocatalysts for water splitting with enhanced catalytic activity and stability is still a leading task. Manganese-grafted molecular clusters have garnered interest as models to design more precise replicas, which, in turn, provide a better perception of the probable pathways used in electrocatalysis. In this research, the design and synthesis of a novel class of fascinating concave-shaped Mn₄ cluster materials of molecular formula Na[Mn₄(L)₂(μ-O)(μ-OH)(μ-p-O₂CC₆H₄(Cl))₂](ClO₄)₂(CH₃OH)(H₂O) (1), Na[Mn₄(L)₂(μ-O)(μ-OH)(μ-p-O₂CC₆H₄(CH₃))₂](ClO₄)₂(CH₃OH)(H₂O)₂ (2) and Na[Mn₄(L)₂(μ-O)(μ-OH)(μ-p-O₂CC₆H₄(NO₂))₂](ClO₄)₂(H₂O) (3), and their evaluation of magnetic exchange interactions and electrocatalytic efficiency for water splitting to generate molecular hydrogen (H₃L = N,N′-bis[2-carboxybenzomethyl]-N,N′-bis[2-pyridylmethyl]-1,3-diaminopropan-2-ol; p-C₆H₄(Cl)(CO₂H) = para-chlorobenzoic acid; p-C₆H₄(CH₃)(CO₂H) = para-methylbenzoic acid; p-C₆H₄(NO₂)(CO₂H) = para-nitrobenzoic acid) have been described. Clusters 1–3 have been characterized by a combined approach of microanalysis, FTIR, UV-Vis, PXRD, FESEM and single crystal X-ray crystallography. The X-ray crystallographic analyses of 1–3 disclose that their molecular structures consist of two crystallographically equivalent dimeric [Mn₂L]³⁺ units, interconnected by one bridging oxide and hydroxide, and two bridging para-substituted benzoate linkers, forming concave-shaped rectangular motifs. Clusters 1–3 show the coexistence of ferromagnetic and antiferromagnetic exchange interactions as disclosed by low-temperature magnetic susceptibility studies, and the spin ground state (S = 2) of the clusters is correlated with the individuality of ancillary bridging ligands and structural arrangements of the [Mn₄(L)₂(μ-O)(μ-OH)(μ-p-O₂CC₆H₄(R))₂] cores [R = Cl (1), CH₃ (2) and NO₂ (3)]. All three Mn₄ clusters exhibit excellent electrocatalytic activity for the hydrogen evolution reaction (HER) with overpotential values of 284, 377 and 323 mV, and Tafel slopes of 87.33, 201.33 and 182.24 mV dec⁻¹ for 1, 2 and 3, respectively, to achieve a current density of 10 mA cm⁻². As revealed by chronoamperometric investigation, 1–3 show outstanding stability with insignificant degradation of current density. Density functional theory (DFT) computations were employed to propose the mechanistic aspects of HER studies, suggesting that the Mn(III)–O_oxide–Mn(III) connectivity is the active site for the HER. To our knowledge, 1–3 represent the first examples of Mn^III₄-based molecular clusters showing excellent electrocatalytic HER performance for the generation of molecular hydrogen.