Metal-Organic Frameworks and Their Biodegradable Composites for Controlled Delivery of Antimicrobial Drugs.

Antimicrobial resistance (AMR) is a growing global crisis with an increasing number of untreatable or exceedingly difficult-to-treat bacterial infections, due to their growing resistance to existing drugs. It is predicted that AMR will be the leading cause of death by 2050. In addition to ongoing efforts on preventive strategies and infection control, there is ongoing research towards the development of novel vaccines, antimicrobial agents, and optimised diagnostic practices to address AMR. However, developing new therapeutic agents and medicines can be a lengthy process. Therefore, there is a parallel ongoing worldwide effort to develop materials for optimised drug delivery to improve efficacy and minimise AMR. Examples of such materials include functionalisation of surfaces so that they can become self-disinfecting or non-fouling, and the development of nanoparticles with promising antimicrobial properties attributed to their ability to damage numerous essential components of pathogens. A relatively new class of materials, metal-organic frameworks (MOFs), is also being investigated for their ability to act as carriers of antimicrobial agents, because of their ultrahigh porosity and modular structures, which can be engineered to control the delivery mechanism of loaded drugs. Biodegradable polymers have also been found to show promising applications as antimicrobial carriers; and, recently, several studies have been reported on delivery of antimicrobial drugs using composites of MOF and biodegradable polymers. This review article reflects on MOFs and polymer-MOF composites, as carriers and delivery agents of antimicrobial drugs, that have been studied recently, and provides an overview of the state of the art in this highly topical area of research.

Metal-Organic Frameworks as Potential Agents for Extraction and Delivery of Pesticides and Agrochemicals.

Pesticide contamination is a global issue, affecting nearly 44% of the global farming population, and disproportionately affecting farmers and agricultural workers in developing countries. Despite this, global pesticide usage is on the rise, with the growing demand of global food production with increasing population. Different types of porous materials, such as carbon and zeolites, have been explored for the remediation of pesticides from the environment. However, there are some limitations with these materials, especially due to lack of functional groups and relatively modest surface areas. In this regard, metal-organic frameworks (MOFs) provide us with a better alternative to conventionally used porous materials due to their versatile and highly porous structure. Recently, a number of MOFs have been studied for the extraction of pesticides from the environment as well as for targeted and controlled release of agrochemicals. Different types of pesticides and conditions have been investigated, and MOFs have proved their potential in agricultural applications. In this review, the latest studies on delivery and extraction of pesticides using MOFs are systematically reviewed, along with some recent studies on greener ways of pest control through the slow release of chemical compounds from MOF composites. Finally, we present our insights into the key issues concerning the development and translational applications of using MOFs for targeted delivery and pesticide control.

Zirconium-Based MOFs and Their Biodegradable Polymer Composites for Controlled and Sustainable Delivery of Herbicides.

Adsorption and controlled release of agrochemicals has been studied widely using different nanomaterials and a variety of formulations. However, the potential for application of high surface-area metal-organic frameworks (MOFs) for the controlled release of agrochemicals has not been thoroughly explored. Herein, we report controlled and sustainable release of a widely used herbicide (2-methyl-4-chlorophenoxyacetic acid, MCPA) via incorporation in a range of zirconium-based MOFs and their biodegradable polymer composites. Three Zr-based MOFs, viz., UiO-66, UiO-66-NH2, and UiO-67 were loaded with MCPA either postsynthetically or in situ during synthesis of the MOFs. The MCPA-loaded MOFs were then incorporated into a biodegradable polycaprolactone (PCL) composite membrane. All three MOFs and their PCL composites were thoroughly characterized using FT-IR, TGA, SEM, PXRD, BET, and mass spectrometry. Release of MCPA from each of these MOFs and their PCL composites was then studied in both distilled water and in ethanol for up to 72 h using HPLC. The best performance for MCPA release was observed for the postsynthetically loaded MOFs, with PS-MCPA@UiO-66-NH2 showing the highest MCPA concentrations in ethanol and water of 0.056 and 0.037 mg/mL, respectively. Enhanced release of MCPA was observed in distilled water when the MOFs were incorporated in PCL. The concentrations of herbicides in the release studies provide us with a range of inhibitory concentrations that can be utilized depending on the crop, making this class of composite materials a promising new route for future agricultural applications.

Polynuclear Complexes as Precursor Templates for Hierarchical Microporous Graphitic Carbon: An Unusual Approach.

A highly porous carbon was synthesized using a coordination complex as an unusual precursor. During controlled pyrolysis, a trinuclear copper complex, [CuII3Cl4(H2L)2]·CH3OH, undergoes phase changes with melt and expulsion of different gases to produce a unique morphology of copper-doped carbon which, upon acid treatment, produces highly porous graphitic carbon with a surface area of 857 m2 g-1 and a gravimetric hydrogen uptake of 1.1 wt % at 0.5 bar pressure at 77 K.

Transition metal complexes of a versatile polyalkoxy oxazolidine-based ligand derived from in situ cyclization.

One-pot reaction between 8-hydroxyquinoline-2-carboxaldehyde (HQC) and tris(hydroxymethyl)aminomethane (TRIS) followed by in situ cyclization yielded an oxazolidine based ligand which produced four mononuclear complexes of MnII(1), CoII(2), NiII(3), ZnII(4), a tetranuclear iron (FeIII4) complex (5) and a trinuclear cobalt (CoIICoIII2) complex (6). Magnetic studies show dominant antiferromagnetic interaction in tetranuclear iron (FeIII4) complex 5 and presence of the slow relaxation of magnetisation in 6. The compounds were also studied for their antibacterial properties. The oxazolidine ligand (H3L2) of this study showed good antimicrobial activity not only against Gram-positive bacteria but against Gram-negative bacteria too. The antimicrobial efficacy of the metal complexes (1-6) is also reported.

Mixed-linker approach in designing porous zirconium-based metal-organic frameworks with high hydrogen storage capacity.

Three highly porous Zr(iv)-based metal-organic frameworks, UBMOF-8, UBMOF-9, and UBMOF-31, were synthesized by using 2,2'-diamino-4,4'-stilbenedicarboxylic acid, 4,4'-stilbenedicarboxylic acid, and combination of both linkers, respectively. The mixed-linker UBMOF-31 showed excellent hydrogen uptake of 4.9 wt% and high selectivity for adsorption of CO2 over N2 with high thermal stability and moderate water stability with permanent porosity and surface area of 2552 m(2) g(-1).

Chemical and structural stability of zirconium-based metal-organic frameworks with large three-dimensional pores by linker engineering.

The synthesis of metal-organic frameworks with large three-dimensional channels that are permanently porous and chemically stable offers new opportunities in areas such as catalysis and separation. Two linkers (L1=4,4',4'',4'''-([1,1'-biphenyl]-3,3',5,5'-tetrayltetrakis(ethyne-2,1-diyl)) tetrabenzoic acid, L2=4,4',4'',4'''-(pyrene-1,3,6,8-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid) were used that have equivalent connectivity and dimensions but quite distinct torsional flexibility. With these, a solid solution material, [Zr6 O4 (OH)4 (L1)2.6 (L2)0.4 ]⋅(solvent)x , was formed that has three-dimensional crystalline permanent porosity with a surface area of over 4000 m(2) g(-1) that persists after immersion in water. These properties are not accessible for the isostructural phases made from the separate single linkers.

New three-dimensional metal-organic framework with heterometallic [Fe-Ag] building units: synthesis, crystal structure, and functional studies.

A new three-dimensional metal-organic framework (MOF) containing Fe(II) and Ag(I) ions and 2-pyrazinecarboxylic acid (pcaH) was synthesized, and its magnetic and catalytic properties were studied. The MOF has a big window size of 12.1 × 12.0 Å, with 23.8% solvent-accessible space, and shows size selectivity toward Lewis acid catalysis.

Trigonal propeller-shaped [Mn(III)3M(II)Na] complexes (M = Mn, Ca): structural and functional models for the dioxygen evolving centre of PSII.

Two new polynuclear heterometallic cluster complexes with [Mn(III)(3)M(II)Na] (M = Mn, Ca) core were synthesized using two in situ formed Schiff bases. The compounds were structurally characterized by single crystal X-ray analysis. The compound with [Mn(III)Ca(II)Na] appeared to catalyse water oxidation which was followed by using Clark electrode and online mass spectrometry.

Magnetothermal studies of a series of coordination clusters built from ferromagnetically coupled {Mn(II) (4)Mn(III) (6)} supertetrahedral units.

Three high-nuclearity mixed valence manganese(II/III) coordination clusters, have been synthesised, that is, [Mn(III) (6)Mn(II) (4)(µ(3)-O)(4)(HL(1))(6)(µ(3)-N(3))(3)(µ(3)-Br)(Br)](N(3))(0.7)/(Br)(0.3)⋅3 MeCN⋅2 MeOH (1) (H(3)L(1)=3-methylpentan-1,3,5-triol), [Mn(III) (11)Mn(II) (6)(µ(4)-O)(8)(µ(3)-Cl)(4)(µ,µ(3)-O(2)CMe)(2)(µ,µ-L(2))(10)Cl(2.34)(O(2)CMe)(0.66)(py)(3)(MeCN)(2)]⋅7 MeCN (2) (H(2)L(2)=2,2-dimethyl-1,3-propanediol and py is pyridine), and [Mn(III) (12)Mn(II) (7)(µ(4)-O)(8)(µ(3)-η(1)N(3))(8)(HL(3))(12)(MeCN)(6)]Cl(2)⋅10 MeOH ⋅MeCN (3) (H(3)L(3)=2,6-bis(hydroxymethyl)-4-methylphenol) with high ground-spin states, S=22, 28±1, and 83/2, respectively; their magnetothermal properties have been studied. The three compounds are based on a common supertetrahedral building block as seen in the Mn(10) cluster. This fundamental magnetic unit is made up of a tetrahedron of Mn(II) ions with six Mn(III) ions placed midway along each edge giving an inscribed octahedron. Thus, the fundamental building unit as represented by compound 1 can be described as a Mn(10) supertetrahedron. Compounds 2 and 3 correspond to two such units joined by a common edge or vertex, respectively, resulting in Mn(17) and Mn(19) coordination clusters. Magnetothermal studies reveal that all three compounds show interesting long-range magnetic ordering at low temperature, originating from negligible magnetic anisotropy of the compounds; compound 2 shows the largest magnetocaloric effect among the three compounds. This is as expected and can be attributed to the presence of a small magnetic anisotropy, and low-lying excited states in compound 2.

Discrete tetrairon(III) cluster exhibiting a square-planar Fe4(mu4-O) core: structural and magnetic properties.

The aerobic reaction of the Schiff-base ligand N-(benzimidazol-2-yl)salicylaldimine (Hbisi) with iron(II) perchlorate in methanol leads to the formation of the remarkable coordination compound [Fe(4)(mu(4)-O)(mu-MeO)(4)(bisi)(4)](ClO(4))(2) x 4 MeOH (1), whose single-crystal X-ray structure reveals the presence of a discrete Fe(III)(4)(mu(4)-O) core. Magnetic and Mossbauer studies both show that the exchange interaction within the square tetranuclear iron(III) unit is dominated by the central bridging mu(4)-oxido ligand, the involvement of the mu-methoxido bridges being negligible.

Manganese(III)-mediated cyclodimerization of a hydrazinyl derivative generating an unprecedented 1,2,3,5,6-substituted leuco-verdazyl ring.

The one-pot reaction of Mn(OAc)(3) with two equivalents of 6-(pyridine-2-ylhydrazonomethyl)phenol (Hphp) in methanol leads to a unique leuco-verdazyl-type ring. The remarkable cyclodimerization reaction is proposed to involve a number of one-electron oxidative steps mediated by Mn(3+) ions, as well as a very uncommon 1,2-radical aryl migration and a nucleophilic attack of the solvent, i.e. methanol.

Hydrogen bond assisted co-crystallization of a bimetallic Mn(III)2Ni(II)2 cluster and a Ni(II)2 cluster unit: synthesis, structure, spectroscopy and magnetism.

A new bimetallic Schiff-base composite complex [Ni2(LH2)2(H2O)2Cl2][Mn2Ni2(LH)4]2Cl4(CH3OH) (1) has been synthesized by a simple one-pot reaction. The compound was structurally characterized by single-crystal X-ray diffraction. In the crystal structure the dinuclear nickel units are connected to the tetranuclear Mn2Ni2 units by means of strong hydrogen-bonding interactions. The compound was further characterized by ESI-MS, ligand-field and infrared spectroscopy. The magnetic properties of the compound have been studied in combination with preliminary DFT calculations, and have resulted in the successful determination of the nature of the magnetic exchange interactions between the metal ions, and hence the coupling constants.

An S-shaped [Fe(4)Dy(2)] complex exhibiting slow relaxation of magnetization: synthesis, magnetism, and crystal structures of a family of [Fe(4)Ln(2)][Ln] coordination compounds (Ln = Nd, Gd, Tb, Dy, and Ho).

A series of heterobimetallic 3d-4f cluster coordination compounds has been synthesized using a Schiff-base ligand (H(3)L), a trinuclear iron(III) precursor complex and lanthanide nitrates as reactants. Five new isostructural complexes were prepared with the ligand in 4 different forms, i.e., neutral and 3 different levels of deprotonation, and with the general formula [Fe(III)(4)Ln(III)(2)(H(2)L)(2)(HL)(2)L(2)(CH(3)OH)(2)((CH(3))(3)CCOO)(2)(NO(3))(2)][Ln(III)(NO(3))(4)(H(3)L)(CH(3)OH)].NO(3).H(2)O, where Ln is Nd, Gd, Tb, Dy, and Ho for compounds 1, 2, 3, 4, and 5, respectively. The single-crystal structures of two complexes were determined by X-ray diffraction, consisting of an original [Fe(III)(4)Ln(III)(2)](2+) moiety with a linear S-shaped [Ln-Fe(4)-Ln] core and an isolated Ln(III) ion coordinated by nitrate anions and the neutral Schiff-base ligand. The isostructural nature of all five coordination compounds is further illustrated both by XRPD and IR analysis. Magnetic properties of all five compounds have been studied and are discussed in light of magnetostructural considerations. Among these five compounds, the Dy (4) cluster shows frequency-dependent ac-susceptibility indicative of probable single-molecule magnet behavior.

Two edge-sharing MnII4MnIII6 supertetrahedra give an anisotropic S = 28 +/- 1 MnII6MnIII11 complex.

A complex with a [Mn(II)(6)Mn(III)(11)] core results via formal fusion of two isotropic Mn(II)(4)Mn(III)(6) supertetrahedra along a common edge leading to a high spin ground state anisotropic system showing Mvs. H hysteresis loops below 1 K.

Concentric Archimedean polyhedra: Mn(III)12Mn(II)9 aggregates linked into a cubic network.

A Mn(III)(12)Mn(II)(9) aggregate has a structure built up of concentric polyhedra with these units linked into a cubic network to give a remarkably pleasing structure isotypic with iron pyrites.