An overview on the recent progress in bimetallic MOFs regarding their design strategies, syntheses and multifarious applications.

The increasing use of antibiotics in our society is now another source of aquatic pollution as these antibiotics now exist in wastewater discharge. Therefore, it is necessary to identify this class of harmful components present in wastewater discharge. Metal–organic frameworks (MOFs) are a peculiar class of multidimensional materials that can be employed as sensors to selectively and sensitively detect the antibiotics that are present in wastewater discharge. Herein, a stable Cd(II)-based MOF, with the composition {[Cd₂L(H₂O)_1.5]}_n (1) (H₄L = 4,5-di(4′-carboxylphenyl)phthalic acid), has been synthesized and characterized. Single-crystal X-ray analysis reveals that the MOF possesses a 3,8-connected topology with 1D channels occupied by lattice water molecules with the Schläfli symbol (3·⁵¹4⁷⁸·5²⁴). The MOF has been employed as a luminescent sensor to detect antibiotics in aqueous media. The results reveal that 1 sensitively detects trace amounts of the antibiotic sulfasalazine (SLA) up to 2.66 × 10⁻⁶ M via the decline in its emission intensity. Hence, 1 is an appropriate luminescent sensor for the SLA antibiotic in aqueous media. Further, attempts have been made to explain the sensing mechanism both experimentally and using theoretical calculations, which suggest that photoinduced electron transfer and competitive absorption are responsible for the decline in the emission intensity of 1 in the presence of antibiotic analytes.

In this study, two new heteroleptic complexes with compositions [Ni(mtsq)_1.85Cl_0.15(dppe)](CH₂Cl₂)_0.06(CH₃OH)_0.09 (Ni-mtsq) and [Cu(mtsq)(PPh₃)₂] (Cu-mtsq) (dppe = 1,2-bis-(diphenylphosphino)ethane; PPh₃ = triphenylphosphane and mtsq = 3-ethoxycyclobutenedione-4-thiolate) have been synthesized and characterized by spectroscopy analyses as well as by single crystal X-ray diffraction technique. X-ray analyses reveal square planar geometry around Ni(II) in Ni-mtsq, where Ni(II) coordinates with two sulfur centres of two mtsq ligands with monodentate coordination and two phosphorus centres of the dppe ligand in chelating mode. Unlike Ni-mtsq, Cu(I) in Cu-mtsq adopts a tetrahedral geometry, which is satisfied by sulfur and oxygen centres of one mtsq ligand with a bidentate coordination mode and two phosphorus centres of two PPh₃ ligands. The supramolecular architecture of both Ni-mtsq and Cu-mtsq is sustained by intermolecular C–H⋯π and O⋯H interactions. Also, Ni-mtsq exhibits intriguing intramolecular semicoordination Ni⋯O and π⋯π non-covalent interactions. The nature of these interactions has been addressed with the aid of Hirshfeld surface analysis, density functional theory and quantum theory of atoms in molecules (QTAIM) analyses and has been visualized with the help of non-covalent interactions reduced density gradient (NCI-RDG). Further, the emissive nature of Cu-mtsq in a solution and solid phase suggests aggregation-induced emission with a bathochromic shift in the solid phase as compared to the solution phase. Both complexes have been used as electrocatalysts for oxygen and hydrogen evolution reactions, which suggests that Cu-mtsq offers better catalysis due to the presence of a redox active, less hindered Cu(I) centre with an η¹⁰ of 600 mV and a Tafel slope of 228 mV dec⁻¹ in the OER, while in HER too, Cu-mtsq exhibited the best onset potential of −0.36 V with an η¹⁰ of 647 mV. Also, homogeneous electrocatalysis for the HER for both complexes was investigated using trifluoroacetic acid as the hydrogen source, which suggests a better electrocatalytic performance of Cu-mtsq.

Two Schiff base complexes of copper(II) and cobalt(III) having the formulae [CuL₂] (Cu-Sal) and [CoL₃] (Co-Sal) (HL = 2-(((2-hydroxyethyl)imino)methyl)phenol) have been synthesized and characterized microanalytically, spectroscopically and in the case of Cu-Sal using single crystal X-ray diffraction technique. The single crystal X-ray analysis reveals a square planar geometry around Cu(II) satisfied by phenoxide oxygen and imine nitrogen of the L⁻ ligand to generate a six membered chelate ring. The solid state structure of Cu-Sal is satisfied by varied intermolecular non-covalent interactions. The nature of these interactions has been addressed with the aid of Hirshfeld surface analysis. Both compounds have been used as sensitizers in TiO₂ based dye sensitized solar cells (DSSCs) and the DSSC experiments revealed that Co-Sal offers better photovoltaic performance in comparison to Cu-Sal. The Co-Sal exhibited a J_sc of 9.75 mA cm⁻² with a V_oc of −0.648 V, incident photon to current conversion efficiency (IPCE) of 57% and η of 3.84%. The relatively better photovoltaic performance of Co-Sal could be attributed to better light absorption and dye loading than that of Cu-Sal.

Two new heteroleptic cobalt(III)dialkyldithiophosphate complex cations with a 1,2-bis(diphenylphosphino)ethane (dppe) ancillary ligand having formula [Co{S₂P(OR)₂}₂(dppe)](B(C₆H₅)₄) (R = –C₂H₅ (Co–Et); –CH(CH₃)₂ (Co–Pr)) have been synthesized and characterized spectroscopically as well as via single crystal X-ray diffraction analysis. The single crystal X-ray diffraction analyses reveal that the immediate geometries around Co(III) in both complexes are distorted octahedral, which are satisfied by four sulfur atoms of two dialkyldithiophosphate ligands and two phosphorus centers of the dppe ligand. These complex cations are neutralized by tetraphenyl borate B(C₆H₅)₄ anions. The solid-state framework of both complexes are stabilized by C–H⋯π and C–H⋯C interactions. The nature of these interactions have been addressed with the help of Hirshfeld surface analysis, and percent contributions of pertinent interactions in the crystal structure of these complexes have been addressed using fingerprint plots. Both complexes have been used as heterogeneous electrocatalysts for the oxygen evolution reaction (OER). Results suggest that Co–Et is a better electrocatalyst for the OER, displaying an onset potential of 1.68 V and Tafel slope of 114 mV dec⁻¹. Furthermore, in the homogeneous electrocatalysis of the hydrogen evolution reaction (HER) using trifluoroacetic acid, the overpotential for Co–Et and Co–Pr have been 1.05 V and 0.92 V, respectively, with turnover frequencies of 1518 s⁻¹ and 287.5 s⁻¹, respectively. Overall, heterogeneous OER and homogenous HER results suggest that Co–Et can be used as an efficient molecular electrocatalyst for oxygen/hydrogen evolution reactions.