Metallacrown-based compounds: Applications in catalysis, luminescence, molecular magnetism, and adsorption

Characterization of an unanticipated indium-sulfur metallocycle complex

We have produced a novel indium-based metallocycle complex (In-MeSH), which we initially observed as an unanticipated side-product in metal-organic framework (MOF) syntheses. The serendipitously synthesized metallocycle forms via the acid-catalysed decomposition of dimethyl sulfoxide (DMSO) during solvothermal reactions in the presence of indium nitrate, dimethylformamide and nitric acid. A search through the Cambridge Structural Database revealed isostructural zinc, ruthenium and palladium metallocycle complexes formed by other routes. The ruthenium analogue is catalytically active and the In-MeSH structure similarly displays accessible open metal sites around the outside of the ring. Furthermore, this study also gives access to the relatively uncommon oxidation state of In(II), the targeted synthesis of which can be challenging. In(II) complexes have been reported as having potentially important applications in areas such as catalytic water splitting.

Praseodymium Metallacrown-Based NMR Probe for Enantioselective Discrimination of Mandelate Anions in Water

Recently, the enhanced interest in water-soluble aminohydroximate Ln(III)-Cu(II) metallacrowns (MC) is largely due to their fascinating structural chemistry, diverse properties and ease of synthesis. We examined the water-soluble praseodymium(III) alaninehydroximate complex Pr(H₂O)₄[15-MC_Cu(II)Alaha-5]·3Cl (1) as a highly effective chiral lanthanide shift reagent for NMR analysis of the biologically relevant (R/S)-mandelate (MA) anions in aqueous media. The R-MA and S-MA enantiomers can be easily discriminated in the presence of small (1.2-6.2 mol %) amounts of MC 1 by the ¹H NMR signals of multiple protons exhibiting an enantiomeric shift difference (ΔΔδ) of 0.06 ppm up to 0.31 ppm. Additionally, a possibility of coordination of MA to the metallacrown was investigated by the ESI-MS technique and a Density Functional Theory modeling of the molecular electrostatic potential and noncovalent interactions.

Lanthanide SMMs Based on Belt Macrocycles: Recent Advances and General Trends

Enhancement of axial magnetic anisotropy is the central objective to push forward the performance of Single-Molecule Magnet (SMM) complexes. In the case of mononuclear lanthanide complexes, the chemical environment around the paramagnetic ion must be tuned to place strongly interacting ligands along either the axial positions or the equatorial plane, depending on the oblate or prolate preference of the selected lanthanide. One classical strategy to achieve a precise chemical environment for a metal centre is using highly structured, chelating ligands. A natural approach for axial-equatorial control is the employment of macrocycles acting in a belt conformation, providing the equatorial coordination environment, and leaving room for axial ligands. In this review, we present a survey of SMMs based on the macrocycle belt motif. Literature systems are divided in three families (crown ether, Schiff-base and metallacrown) and their general properties in terms of structural stability and SMM performance are briefly discussed.

Diversity in the Interaction of Amino Acid- and Peptide-Based Hydroxamic Acids with Some Platinum Group Metals in Solution

Complexes that incorporate both ligand(s) and metal(s) exhibiting cytotoxic activity can especially be interesting to develop multifunctional drug molecules with desired activities. In this review, the limited number of solution results collected in our laboratory on the complexes of Pd(II) and two other platinum group metals—the half-sandwich type, [(η⁶-p-cym)Ru(H₂O)₃]²⁺, and [(η⁵-Cp*)Rh(H₂O)₃]²⁺—with hydroxamic acid derivatives of three amino acids, two imidazole analogues, and four small peptides are summarized and evaluated. Unlike the limited number of coordination sites of these metal ions (four and three for Pd(II) and the organometallic cations, respectively), the ligands discussed here offer a relatively high number of donor atoms as well as variation in their position within the ligands, resulting in a large versatility of the likely coordination modes. The review, besides presenting the solution equilibrium results, also discusses the main factors, such as (N,N) versus (O,O) chelate; size of chelate; amino-N versus imidazole-N; primary versus secondary hydroxamic function; differences between hydrolytic ability of the metal ions studied; and hydrolysis of the coordinated peptide hydroxamic acids in their Pd(II) complexes, which all determine the coordination modes present in the complexes formed in measurable concentrations in these systems. The options for the quantitative evaluation of metal binding effectivity and selectivity of the various ligands and the comparison with each other by using solution equilibrium data are also discussed.

Crystal structure of a TbIII-CuII glycine-hydroxamate 15-metallacrown-5 sulfate complex

The core of the title complex, bis-[hexa-aqua-hemi-aqua-penta-kis-(μ3-glycine-hydroxamato)sulfato-penta-copper(II)terbium(III)] sulfate hexa-hydrate, [TbCu5(SO4)(GlyHA)5(H2O)6.5]2(SO4)·6H2O (1), which belongs to the 15-metalla-crown-5 family, consists of five glycine-hydroxamate dianions (GlyHA2-; C2H4N2O2) and five copper(II) ions linked together forming a metallamacrocyclic moiety. The terbium(III) ion is connected to the centre of the metallamacrocycle through five hydroxamate oxygen atoms. The coordination environment of the Tb3+ ion is completed to an octa-coordination level by oxygen atoms of a bidentate sulfate and an apically coordinated water mol-ecule, while the copper(II) atoms are square-planar, penta- or hexa-coordinate due to the apical coordination of water mol-ecules. Continuous shape calculations indicate that the coordination polyhedron of the Tb3+ ion in 1 is best described as square anti-prismatic. The positive charge of each pair of [TbCu5(GlyHA)5(H2O)6.5(SO4)]2 2+ fragments is compensated by a non-coordinated sulfate anion, which is located on an inversion center with 1:1 disordered oxygen atoms. Complex 1 is isomorphous with the previously reported compounds [LnCu5(GlyHA)5(SO4)(H2O)6.5]2(SO4), where Ln III = Pr, Nd, Sm, Eu, Gd, Dy and Ho.

Synthesis and Magnetic Properties of a Dimerized Trinuclear Ni String Complex, [Ni6Cl2(dpa)8](I5)2·0.25I2 (dpa- = 2,2'-Dipyridylamide Anion)

Metal string complexes, linearly aligned transition metal arrays coordinated with the multidentate organic ligands, have gained much attention both in unique electronic/structural properties and in potential applications as conductive molecular nanowires. Here we report on a dimerized Ni^II trinuclear complex, [Ni₆Cl₂(dpa)₈](I₅)₂·0.25I₂ (dpa^- = 2,2'-dipyridylamide anion). X-ray structural analysis revealed that two trinuclear moieties are bridged by a Cl anion to form a dimerized string structure. This is the first example of two Ni string complexes that are connected. In the electronic absorption and Raman spectra, characteristic absorption bands and a vibration mode based on the dimer string structure were observed. In the solid state, dimer complexes align in one dimension in an MMMXMMMX (M = metal, X = halogen) manner, leading to the intra- and interdimer antiferromagnetic interactions.

Robotic Stepwise Synthesis of Hetero-Multinuclear Metal Oxo Clusters as Single-Molecule Magnets

An efficient stepwise synthesis method for discovering new heteromultinuclear metal clusters using a robotic workflow is developed where numerous reaction conditions for constructing heteromultinuclear metal oxo clusters in polyoxometalates (POMs) were explored using a custom-built automated platform. As a result, new nonanuclear tetrametallic oxo clusters {FeMn₄}Lu₂A₂ in TBA₅[(A-α-SiW₉O₃₄)₂FeMn₄O₂{Lu(acac)₂}₂A₂] (II^A; A = Ag, Na, K; TBA = tetra-n-butylammonium; acac = acetylacetonate) were discovered by the installation of diamagnetic metal cations A⁺ into a paramagnetic {FeMn₄}Lu₂ unit in TBA₇[(A-α-SiW₉O₃₄)₂FeMn₄O₂{Lu(acac)₂}₂] (I). POMs II^A exhibited single-molecule magnet properties with the higher energy barriers for magnetization reversal (II^Ag, 40.0 K; II^Na, 40.3 K; II^K, 26.7 K) compared with that of the parent I (19.7 K). Importantly, these clusters with unique properties were constructed as designed by a step of the predictable sequential multistep reactions with the time-efficient platform.

Filling the Gap in the Metallacrown Family: The 9-MC-3 Chromium Metallacrown

In this work, we report on a long‐sought missing complex in the metallacrown family. We synthesized and characterized the novel chromium metallacrown (MC) complex {Cr^III(μ₂‐piv)₃[9‐MC_{Cr(III)N(shi)}‐3](morph)₃}⋅MeOH (in which shi³⁻=salicyl hydroxamate, piv=pivalate, and morph=morpholine). The MC with a 9‐MC‐3 cavity of kinetically inert chromium(III) ions was synthesized by a solvothermal reaction. Magnetization measurements reveal a high spin ground state.

Water-Soluble Bismuth(III) Polynuclear Tyrosinehydroximate Metallamacrocyclic Complex: Structural Parallels to Lanthanide Metallacrowns

Recently there has been a great deal of interest and associated research into aspects of the coordination chemistry of lanthanides and bismuth-elements that show intriguing common features. This work focuses on the synthesis and characterization of a novel bismuth(III) polynuclear metallamacrocyclic complex derived from aminohydroxamic acid, in order to compare the coordination ability of Bi3+ with the similarly sized La3+ ions. A polynuclear tyrosinehydroximate Bi(OH)[15-MCCu(II)Tyrha-5](NO3)2 (1) was obtained according to the synthetic routes previously described for water-soluble Ln(III)-Cu(II) 15-MC-5 metallacrowns. Correlations between structural parameters of Bi(III) and Ln(III) complexes were analyzed. DFT calculations confirmed the similarity between molecular structures of the model bismuth(III) and lanthanum(III) tyrosinehydroximate 15-metallacrowns-5. Analysis of the electronic structures revealed, however, stronger donor-acceptor interactions between the central ion and the metallamacrocycle in the case of the lanthanum analogue. Thermochromic properties of 1 were studied.

Synthesis and crystal structure of a penta-copper(II) 12-metallacrown-4: cis-di-aqua-tetra-kis-(di-methyl-formamide-κO)manganese(II) tetra-kis-(μ3-N,2-dioxido-benzene-1-carboximidate)penta-copper(II) di-methyl-formamide monosolvate

The title compound, [Mn(C3H7NO)4(H2O)2][Cu5(C7H4NO3)4]·C3H7NO or cis-[Mn(H2O)2(DMF)4]{Cu[12-MCCu(II)N(shi)-4]}·DMF, where MC is metallacrown, shi3- is salicyl-hydroximate, and DMF is N,N-di-methyl-formamide, crystallizes in the monoclinic space group P21/n. Two crystallographically independent metallacrown anions are present in the structure, and both anions exhibit minor main mol-ecule disorder by an approximate (non-crystallographic) 180° rotation with occupancy ratios of 0.9010 (9) to 0.0990 (9) for one anion and 0.9497 (8) to 0.0503 (8) for the other. Each penta-copper(II) metallacrown contains four CuII ions in the MC ring and a CuII ion captured in the central cavity. Each CuII ion is four-coordinate with a square-planar geometry. The anionic {Cu[12-MCCu(II)N(shi)-4]}2- is charged-balanced by the presence of a cis-[Mn(H2O)2(DMF)4]2+ cation located in the lattice. In addition, the octa-hedral MnII counter-cation is hydrogen bonded to both MC anions via the coordinated water mol-ecules of the MnII ion. The water mol-ecules form hydrogen bonds with the phenolate and carbonyl oxygen atoms of the shi3- ligands of the MCs.

Investigating the effect of lanthanide radius and diamagnetic linkers on the framework of metallacrown complexes

By changing the stoichiometric ratios, the one-pot reaction of the glycinehydroxamic acid (H2glyha) ligand with copper(ii) and lanthanide(iii) salts in the presence of diamagnetic [Na2{Fe(CN)5(NO)}] led to two series of isostructural complexes, which can be designated as heterotrimetallic dimeric clusters [{LnCu5(glyha)5}{Fe(CN)5(NO)}(H2O)4]2·xNO3·yH2O (x = 2, y = 11 for La (1), x = 2, y = 11 for Pr (2), and x = 2, y = 11 for Nd (3)) and heterotetrametallic coordination polymers [Na{LnCu5(glyha)5}{Fe(CN)5(NO)}2(H2O)x·yH2O]n (x = 6, y = 4 for Sm (4), x = 6, y = 0 for Gd (5), x = 6, y = 4 for Tb (6), x = 5, y = 5 Dy (7), and x = 6, y = 4 for Ho (8)). Each molecular structure contains LnIII[15-metallacrown-5] nodes and diamagnetic [Fe(CN)5(NO)]2- linkers. The resulting products demonstrate diversified structural frameworks due to the radius effect of LnIII ions and different bridging fashions of diamagnetic [Fe(CN)5(NO)]2- linkers. An analysis of magnetic susceptibilities reveals that 7 exhibits ferromagnetic coupling between CuII and DyIII ions and field-induced SMM behavior.

Three new heterometallic ZnII–LnIII complexes with a windmill-like framework and field-induced SMM behavior

We have prepared three 3d–4f heterometallic complexes by the reaction of o-vanillin oxime (H₂L) with Zn(NO₃)₂·6H₂O and Ln(NO₃)₃·6H₂O salts in the presence of triethylamine.

3d–4f Metallacrown complexes with a new sandwich core: synthesis, structures and single molecule magnet behavior

Two heterometallic metallacrown complexes, [Er{Cu₄(butyrat)₄}₂]·Cl₃·MeOH·26H₂O (1) and [Yb{Cu₄(butyrat)₄}₂]·Cl₃·MeOH·26H₂O (2) (H₂butyrat = 3-aminobutyric hydroxamic acid), have been reported.

Conformational rearrangements of G-quadruplex topology promoted by Cu(II) 12-MCCu(II)PyrAcHA-4 metallacrown

Cu(II) 12-MC_{Cu(II)PyrAcHA}-4 metallacrown was studied by several spectroscopic techniques as an interacting ligand with G-quadruplex DNA structures. Investigations were performed on oligonucleotides bearing human telomeric and protooncogenic c-myc sequences in buffered solution mimicking ionic conditions in cellular environment. The planar square-based Cu(II) 12-MC-4 metallacrown interacts with GQ via an end-stacking mode with 1:1 stoichiometry. Circular dichroism (CD) titration revealed capability of this metallacrown to induce transformation of the GQ hybrid topology into the parallel form. Thermal melting experiment indicated higher thermal stability of both antiparallel (ΔT_m = +15 °C) and parallel (ΔT_m = ≥27 °C) G-quadruplexes in the presence of Cu (II) 12-MC-4. Indirect GQ FID assay let to determine high binding affinity of the Cu(II) 12-MC-4 to antiparallel 22Htel/Na⁺ GQ (K_MC = 3.9 (±0.4) x 10⁶ M^-1). Comparing with lower binding constants previously reported for Ln (III) 15-MC-5 and Sm (III) 12-MC-4, one can conclude that the square planar geometry and the positive charge of metallacrown play an important role in MC/GQ interactions.

Trends and Exceptions in the Interaction of Hydroxamic Acid Derivatives of Common Di- and Tripeptides with Some 3d and 4d Metal Ions in Aqueous Solution

By using various techniques (pH-potentiometry, UV-Visible spectrophotometry, ¹H and ¹⁷O-NMR, EPR, ESI-MS), first time in the literature, solution equilibrium study has been performed on complexes of dipeptide and tripeptide hydroxamic acids—AlaAlaNHOH, AlaAlaN(Me)OH, AlaGlyGlyNHOH, and AlaGlyGlyN(Me)OH—with 4d metals: the essential Mo(VI) and two half-sandwich type cations, [(η⁶-p-cym)Ru(H₂O)₃]²⁺ as well as [(η⁵-Cp*)Rh(H₂O)₃]²⁺, the latter two having potential importance in cancer therapy. The tripeptide derivatives have also been studied with some biologically important 3d metals, such as Fe(III), Ni(II), Cu(II), and Zn(II), in order to compare these new results with the corresponding previously obtained ones on dipeptide hydroxamic acids. Based on the outcomes, the effects of the type of metal ions, the coordination number, the number and types of donor atoms, and their relative positions to each other on the complexation have been evaluated in the present work. We hope that these collected results might be used when a new peptide-based hydroxamic acid molecule is planned with some purpose, e.g., to develop a potential metalloenzyme inhibitor.

The crystal structures of {LnCu5}3+ (Ln = Gd, Dy and Ho) 15-metallacrown-5 complexes and a reevaluation of the isotypic EuIII analogue

Three new isotypic heteropolynuclear complexes, namely penta-aqua-carbonato-penta-kis-(glycinehydroxamato)nitrato-penta-copper(II)lanthanide(III) x-hydrate, [LnCu5(GlyHA)5(CO3)(NO3)(H2O)5]·xH2O (GlyHA2- is glycine-hydrox-amate, N-hy-droxy-glycinamidate or amino-aceto-hydroxamate, C2H4N2O2 2-), with lanthanide(III) (Ln III) = gadolinium (Gd, 1, x = 3.5), dysprosium (Dy, 2, x = 3.28) and holmium (Ho, 3, x = 3.445), within a 15-metallacrown-5 class were obtained on reaction of lanthanide(III) nitrate, copper(II) acetate and sodium glycinehydroxamate. Complexes 1-3 contain five copper(II) ions and five bridging GlyHA2- anions, forming a [CuGlyHA]5 metallamacrocyclic core. The Ln III ions are coordinated to the metallamacrocycle through five O-donor hydroxamates. The electroneutrality of complexes 1-3 is achieved by a bidentate carbonate anion coordinated to the Ln III ion and a monodentate nitrate anion coordinated apically to one of the copper(II) ions of the metallamacrocycle. The lattice parameters of complexes 1-3 are similar to those previously reported for an EuIII-CuII 15-metallacrown-5 complex with glycine-hydroxamate of proposed composition [EuCu5(GlyHA)5(OH)(NO3)2(H2O)4]·3.5H2O [Stemmler et al. (1999 ▸). Inorg. Chem. 38, 2807-2817]. High-quality X-ray data obtained for 1-3 have allowed a re-evaluation of the X-ray data solution proposed earlier for the EuCu5 complex and suggest that the formula is actually [EuCu5(GlyHA)5(CO3)(NO3)(H2O)5]·3.5H2O.

The first water-soluble polynuclear metallamacrocyclic Sr(ii)-Cu(ii) complex based on simple glycinehydroximate ligands

Recently, aminohydroximate ligands have found wide applications in the fascinating class of polynuclear metallamacrocyclic compounds named 15-MC-5 metallacrowns. The enhanced interest in water-soluble aminohydroximate Ln(iii)-Cu(ii) complexes is largely due to their rich coordination chemistry, diverse properties and ease of synthesis. We examined glycinehydroxamic acid as a simple ligand for the preparation of the first water-soluble polynuclear metallamacrocyclic Sr(ii)-Cu(ii) compound. The complex Sr(H2O)3[15-MCCuGlyha-5](Cl)2 was synthesized and characterized structurally and spectroscopically. The single-crystal structure reveals the classic metallamacrocyclic 15-MC-5 configuration. The Sr(ii) ion is located at the center of the 15-MCCu(II)Glyha-5 ring and coordinated by five oxygen atoms of the cycle in the equatorial plane and an additional three oxygen atoms of the water molecules at apical positions. Detailed DFT and QT AIM studies were carried out for the hydrated isoelectronic Sr[15-MCCuGlyha-5]2+ and Y[15-MCCuGlyha-5]3+ systems. The ionic contribution to the metal-ligand interactions appears to be higher for the Sr(ii) derivative despite the smaller charge separation. DFT calculations suggest a thermodynamically favorable substitution of the Sr(ii) central ion with Y(iii). Indeed, it was shown experimentally that the strontium ion can be easily replaced by yttrium. Preliminary cytotoxic studies revealed the low toxicity of the strontium complex and its yttrium analogue, so the water-soluble Sr(ii) and Y(iii) metallacrowns can be further investigated as possible platforms for the development of new 90Sr and 90Y radiotherapy drugs.

Complex formation of copper(ii), nickel(ii) and zinc(ii) with ethylophosphonoacetohydroxamic acid: solution speciation, synthesis and structural characterization

We present herein the thermodynamic and X-ray characterisation of a novel ethyl phosphonohydroxamic acid-based Cu(ii) metallacrown, predominating in solution in a broad pH range.

Versatility and trends in the interaction between Pd(ii) and peptide hydroxamic acids

The interaction between di- and tripeptide hydroxamic acids and Pd(ii) (as a Pt(ii) model but with faster ligand exchange reactions) was studied in aqueous solution in the presence of chloride ions by pH-potentiometric and NMR methods revealing ligand-dependent competition between the coordinative and hydrolytic processes.

Impact of molecular packing rearrangement on solid-state fluorescence: polyhalogenated N-hetarylamines vs. their co-crystals with 18-crown-6

Relationship between the hetarylamine chemical structure, crystal packing in homo- and co-crystals, and fluorescence effects (quenching, bathochromic and hypsochromic shifts).

Crystal structure and Hirshfeld surface analysis of tetra-aqua-bis-(isonicotinamide-κN 1)nickel(II) fumarate

The reaction of NiCl2 with fumaric acid and isonicotinamide in a basic solution produces the title complex, [Ni(C6H6N2O)2(H2O)4](C4H2O4). The nickel(II) ion of the complex cation and the fumarate anion are each located on an inversion centre. The NiII ion is coordinated octa-hedrally by four water O atoms and two N atoms of isonicotinamide mol-ecules. The fumarate anion is linked to neighbouring complex cations via Owater-H⋯Ofumarate hydrogen bonds. In the crystal, the complex cations are further linked by O-H⋯O, N-H⋯O and C-H⋯O hydrogen bonds, forming a three-dimensional supra-molecular architecture. Hirshfeld surface analysis and two-dimensional fingerprint plots were used to analyse the inter-molecular inter-actions present in the crystal and indicate that the most important contributions for the crystal packing are from H⋯O/O⋯H (41.8%), H⋯H (35.3%) and H⋯C/C⋯H (10.2%) inter-actions.

Complexes in aqueous cobalt(II)-2-picolinehydroxamic acid system: Formation equilibria, DNA-binding ability, antimicrobial and cytotoxic properties

The coordination properties of 2-picolinehydroxamic acid towards cobalt(II) in aqueous solution were determined by a pH-metric method and confirmed by spectroscopic (UV-Vis and ESI-MS) studies. The results show the formation of mononuclear complexes, as well as of metallacrowns (MC). All methods indicate a high tendency of 2-picolinehydroxamic acid to form cobalt(II) metallacrown 12-MC-4. ESI-MS additionally confirms 15-MC-5 and 18-MC-6, stabilized by a sodium ion and methanol. The complexes observed in the speciation model at a pH about 7.2 were studied for their DNA-binding ability. The decrease of absorbance in the range of ca 310-400 nm indicates effective binding to calf thymus DNA by 2-picolinehydroxamic acid complexes, via intercalative mode. The antimicrobial properties of 2-picolinehydroxamic acid, cobalt(II) ions and of the complexes formed in the Co(II) - ligand system were determined against Gram-positive bacteria (Enterococcus faecalis, Enterococcus faecium, Staphylococcus aureus, Bacillus subtilis), Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli, Helicobacter pylori) and fungal strains (Candida, Aspergillus niger). The results indicate that the complexes demonstrate greater antibacterial and antifungal activity for most strains than the ligand. Both the complexes and the ligand induce a slight decrease in the metabolic activity of cells, while the complexes do not damage the cell nuclei. The 2-picolinehydroxamic acid complexes activate the human monocytic cells, suggesting they have immunomodulating properties, which are particularly important in combating infections caused by strains resistant to other drugs.