Metal Coordination Chemistry Research Articles

Fifteen Years of Scientific Investigation into Main Groups and Transition Metal Coordination Chemistry with Allan White

The outcomes of the investigations on the structures and reactivity of a massive number of main group and transition metal complexes containing different families of ligands are reviewed. All the data result from the scientific collaboration between the research groups of Claudio Pettinari and Allan White which lasted fifteen years.

Conformational Stability Effect of Polymeric Iron Chelators.

SummaryThe design and synthesis of metal chelators with extraordinary metal affinities is a basic and challenging scientific problem of both fundamental and practical importance. Here, we demonstrate a “conformational stability effect” that can significantly enhance the metal affinity of ligands after conjugation to polymer chains with the ability to spontaneously adopt a specific conformation as an optimal “soft” scaffold to ensure maximum thermodynamic stability of the metal complexes. Using iron chelators as models, we show that simple conjugation of small molecule catechol ligands to a polyallylamine chain resulted in more than 8–9 orders of magnitude enhancement of the iron-binding affinity, which is comparable to that of enterobactin, the strongest iron chelator ever known. This study demonstrates that flexible polymer chelators may realize the highest possible metal affinities of the conjugated ligands owing to their ability to achieve an optimal conformation, which could advance the identification of strong metal chelators.

The Chemistry of Lanthanides in Biology: Recent Discoveries, Emerging Principles, and Technological Applications.

The essential biological role of rare earth elements lay hidden until the discovery in 2011 that lanthanides are specifically incorporated into a bacterial methanol dehydrogenase. Only recently has this observation gone from a curiosity to a major research area, with the appreciation for the widespread nature of lanthanide-utilizing organisms in the environment and the discovery of other lanthanide-binding proteins and systems for selective uptake. While seemingly exotic at first glance, biological utilization of lanthanides is very logical from a chemical perspective. The early lanthanides (La, Ce, Pr, Nd) primarily used by biology are abundant in the environment, perform similar chemistry to other biologically useful metals and do so more efficiently due to higher Lewis acidity, and possess sufficiently distinct coordination chemistry to allow for selective uptake, trafficking, and incorporation into enzymes. Indeed, recent advances in the field illustrate clear analogies with the biological coordination chemistry of other metals, particularly CaII and FeIII, but with unique twists—including cooperative metal binding to magnify the effects of small ionic radius differences—enabling selectivity. This Outlook summarizes the recent developments in this young but rapidly expanding field and looks forward to potential future discoveries, emphasizing continuity with principles of bioinorganic chemistry established by studies of other metals. We also highlight how a more thorough understanding of the central chemical question—selective lanthanide recognition in biology—may impact the challenging problems of sensing, capture, recycling, and separations of rare earths.

Porphyrinoid f-Element Complexes.

Porphyrin and related pyrrole-containing macrocycles, collectively porphyrinoids, are versatile ligands that allow access to a multitude of coordination modes. Judicious modification of the porphyrin core as well as the pendant substituents has extended the coordination chemistry of porphyrinoids to include systems that are able to stabilize f-block element complexes with possible utility. This review focuses on our group's efforts to prepare expanded porphyrin and porphyrinogen ligands that can serve as tools to study and apply f-element metal coordination chemistry: it covers the background of the topic, selected syntheses, and application of these species in the chemical and medical sciences.

Energy transfer in liquid and solid nanoobjects: application in luminescent analysis

Abstract Radiationless resonance electronic excitation energy transfer (ET) is a fundamental physical phenomenon in luminescence spectroscopy playing an important role in natural processes, especially in photosynthesis and biochemistry. Besides, it is widely used in photooptics, optoelectronics, and protein chemistry, coordination chemistry of transition metals and lanthanides as well as in luminescent analysis. ET involves the transfer of electronic energy from a donor (D) (molecules or particles) which is initially excited, to an acceptor (A) at the ground state to emit it later. Fluorescence or phosphorescence of the acceptor that occurs during ET is known as sensitized. There do many kinds of ET exist but in all cases along with other factors the rate and efficiency of ET in common solvents depends to a large extent on the distance between the donor and the acceptor. This dependency greatly limits the efficiency of ET and, correspondingly, does not allow the determination of analytes in highly diluted (10–9–10–15 M) solutions. To solve the problem of distance-effect, the effects of concentrating and bring close together the donor and acceptor in surfactant micelles (liquid nanosystems) or sorption on solid nanoparticles are used. Various approaches to promote the efficiency of ET for improvement determination selectivity and sensitivity using liquid and solid nanoobjects is reviewed and analyzed.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt.

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Subwavelength Spatially Resolved Coordination Chemistry of Metal-Organic Framework Glass Blends.

Microstructured metal-organic framework (MOF) glasses have been produced by combining two amorphous MOFs. However, the electronic structure of these materials has not been interrogated at the length scales of the chemical domains formed in these glasses. Here, we report a subwavelength spatially resolved physicochemical analysis of the electronic states at visible and UV energies in a blend of two zeolitic imidazolate frameworks (ZIFs), ZIF-4-Co and ZIF-62-Zn. By combining spectroscopy at visible and UV energies as well as at core ionization energies in electron energy loss spectroscopy in the scanning transmission electron microscope with density functional theory calculations, we show that domains less than 200 nm in size retain the electronic structure of the precursor crystalline ZIF phases. Prototypical signatures of coordination chemistry including d- d transitions in ZIF-4-Co are assigned and mapped with nanoscale precision.

(2-Pyridyloxy)silanes as Ligands in Transition Metal Coordination Chemistry

Proceeding our initial studies of compounds with formally dative TM→Si bonds (TM = Ni, Pd, Pt), which feature a paddlewheel arrangement of four (N,S) or (N,N) bridging ligands around the TM–Si axis, the current study shows that the (N,O)-bidentate ligand 2-pyridyloxy (pyO) is also capable of bridging systems with TM→Si bonds (shown for TM = Pd, Cu). Reactions of MeSi(pyO)3 with [PdCl2(NCMe)2] and CuCl afforded the compounds MeSi(µ-pyO)4PdCl (1) and MeSi(µ-pyO)3CuCl (2), respectively. In the latter case, some crystals of the Cu(II) compound MeSi(µ-pyO)4CuCl (3) were obtained as a byproduct. Analogous reactions of Si(pyO)4, in the presence of HpyO, with [PdCl2(NCMe)2] and CuCl2, afforded the compounds [(HpyO)Si(µ-pyO)4PdCl]Cl (4), (HpyO)2Si[(µ-pyO)2PdCl2]2 (5), and (HpyO)2Si[(µ-pyO)2CuCl2]2 (6), respectively. Compounds 1–6 and the starting silanes MeSi(pyO)3 and Si(pyO)4 were characterized by single-crystal X-ray diffraction analyses and, with exception of the paramagnetic compounds 3 and 6, with NMR spectroscopy. Compound 2 features a pentacoordinate Si atom, the Si atoms of the other complexes are hexacoordinate. Whereas compounds 1–4 feature a TM→Si bond each, the Si atoms of compounds 5 and 6 are situated in an O6 coordination sphere, while the TMCl2 groups are coordinated to pyridine moieties in the periphery of the molecule. The TM–Si interatomic distances in compounds 1–4 are close to the sum of the covalent radii (1 and 4) or at least significantly shorter than the sum of the van-der-Waals radii (2 and 3). The latter indicates a noticeably weaker interaction for TM = Cu. For the series 1, 2, and 3, all of which feature the Me–Si motif trans-disposed to the TM→Si bond, the dependence of the TM→Si interaction on the nature of TM (Pd(II), Cu(I), and Cu(II)) was analyzed using quantum chemical calculations, that is, the natural localized molecular orbitals (NLMO) analyses, the non-covalent interaction (NCI) descriptor, Wiberg bond order (WBO), and topological characteristics of the bond critical points using the atoms in molecules (AIM) approach.

Rational Design of Dodecahedral MnCo2O4.5 Hollowed‐Out Nanocages as Efficient Bifunctional Electrocatalysts for Oxygen Reduction and Evolution

AbstractEfficient bifunctional electrocatalysts are in demand to increase kinetics of oxygen reduction and evolution reactions (ORR and OER) for rechargeable zinc–air batteries. In this work, dodecahedral MnCo2O4.5 hollowed‐out nanocages are synthesized using a seed‐mediated coordination chemistry of metal–organic frameworks. The hollowed‐out MnCo2O4.5 nanocages used as bifunctional oxygen electrocatalysts exhibit high catalytic activity and excellent stability for both ORR and OER in alkaline media, which can be attributed to both the hierarchical porosity and stable hollowed‐out structure that not only facilitate fast charge transfer reaction but also prevent the agglomeration of MnCo2O4.5 nanoparticles. The dodecahedral hollowed‐out MnCo2O4.5 nanocages as the air electrode catalyst can deliver good stability in a rechargeable zinc–air battery. Scanning electron microscopy and transmission electron microscopy images, X‐ray diffraction and X‐ray photoelectron spectroscopy of the electrocatalyst were measured again after durability tests. There is no significant influence on the morphologies, compositions, and structures before and after durability tests.

Mechanically strong and tough hydrogels with excellent anti-fatigue, self-healing and reprocessing performance enabled by dynamic metal-coordination chemistry

Strong and tough hydrogels with good self-healing property and processability are of great interest in the recent decades, however, how to simultaneously realize high mechanical properties and self-healing performance still remains a challenge. Here, we report a fully physical dual-network (DN) hydrogel consisting of poly(acrylic acid) (PAA) network crosslinked by ferric ions and chitosan (CS) network crosslinked with calcium ions and chain entanglement. The resulting hydrogel displays a tensile strength of 12.2 MPa and a tensile strain of 1290%. Under the strain of 1000%, 35.7 MJ m−3 of energy can be dissipated by the hydrogel network. Due to the reversibility of the physical crosslinks, particularly, the hydrogel presents high self-recovery and self-healing performances, that is, its tensile strength is recovered to 2.3 MPa after healing. Additionally, even if fragmented, the hydrogel can still acquire a tensile stress of 4.3 MPa after re-processing. This finding indicates the effective strategy to construct high-performance hydrogels can bring benefits to more soft materials.

Biomimetic strategies to design metallic proteins for detoxification of hazardous heavy metal

Discharge of hazardous heavy metals in to the environment poses a serious threat to the ecosystem owing to its non-degradability and indestructability. Physical and chemical techniques for the removal of heavy metals from industrial effluent is expensive and causes secondary pollution. On the other hand, biological processes using microorganisms play a vital role due to their large surface area to volume ratio, which increases the interactions with metal ions present in the environment. Here, we developed a third generation biological tool for the removal of heavy metal (copper) from the effluent through the biosynthesis of intracellular and surface displayed metallic proteins with novel metal co-ordination chemistry. We evaluated the cell viability for maximum heavy metal adsorption and metal tolerance of synthesized congener metallic proteins. Finally, to eliminate the cost associated with incorporation of metal binding aminoacid, we have introduced a genetic circuit in order to evolve a novel magnetotactic bacterium. The bioreactor studies of the consortia of metallic protein expressing cells immobilized on functionalized granular activated carbon revealed that 97% of copper was adsorbed from the industrial effluent. It is evident that the use of congener metallic proteins will be a futuristic approach for the treatment of wastewater facilitating environmental detoxification.

Metal-Coordination-Induced Fusion Creates Hollow Crystalline Molecular Superstructures.

In this work, we report the formation of superstructures assembled from organic tubular crystals mediated by metal-coordination chemistry. This template-free process involves the crystallization of molecules into crystals having a rectangular and uniform morphology, which then go on to fuse together into multibranched superstructures. The initially hollow and organic crystals are obtained under solvothermal conditions in the presence of a copper salt, whereas the superstructures are subsequently formed by aging the reaction mixture at room temperature. The mild thermodynamic conditions and the favorable kinetics of this unique self-assembly process allowed us to ex-situ monitor the superstructure formation by electron microscopy, highlighting a pivotal and unusual role for copper ions in their formation and stabilization.

Functional Calixphyrins: Synthetic Strategies and Applications

Calixphyrins are hybrid macrocycles that contain both sp2- and sp3-hybridized carbon atoms and hence bear analogy to both porphyrins and calixpyrroles. Due to the presence of sp3-hybridized carbon atoms, π-conjugation is disrupted in calixphyrins, which leads to conformational flexibility. Hence, these molecules find a use in anion binding, host-guest chemistry and metal-coordination chemistry. During the last decade, studies on calixphyrins have been a topic of wide interest to researchers. Various functionalities have been introduced to the structure of calixphyrins, leading to the development of expanded calixphyrins, core-modified calixphyrins and N-confused calixphyrins with diverse applications. This review outlines the detailed historical origin of the synthesis of calixphyrins with emphasis on current research and development in this area. The modular syntheses of normal calixphyrins, expanded calixphyrins, core-modified calixphyrins and N-confused calixphyrins are also discussed, along with their applicative aspects.

Diasteroselective multi-component assemblies from dynamic covalent imine condensation and metal-coordination chemistry: mechanism and narcissistic stereochemistry self-sorting.

Self-assembly of a modified tris(2-pyridylmethyl)amine TPMA ligand, zinc(ii) or cobalt(ii) ions, and amino acids have been used effectively as stereo dynamic optical probes for the determination of the enantiomeric excess of free amino acids either using Electronic or Vibrational Circular Dichroism (CD and VCD). Herein, we report the mechanistic and stereochemical study of the self-assembly process which reveals a complex equilibrium in solution where even small variations in the experimental conditions can profoundly affect the final products of the reaction. In particular, variation on the metal stoichiometry switch give rises to an entirely enantio narcissistic self-assembly of the structure.

Coordination chemistry of iron triad metals with organic N-donor ligands and boron cluster anions [B10H10]2−, [B12H12]2−, and [B10Cl10]2−: Complexation and accompanying processes

Synthesis and structures of coordination compounds of the iron triad metals with N-donor ligands (Bipy, Phen, and BPA) and boron cluster anions [B10H10]2−, [B12H12]2−, and [B10Cl10]2− are described. The processes of complexation and accompanying redox reactions involving the metal atoms and boron clusters and substitution of exo-polyhedral hydrogen atoms in the decahydro-closo-decaborate anion are considered. The conditions under which the accompanying reactions become the main processes and predominate over complexation are determined.

X‐ray elemental mapping techniques for elucidating the ecophysiology of hyperaccumulator plants

Contents Summary 432 I. Introduction 433 II. Preparation of plant samples for X-ray micro-analysis 433 III. X-ray elemental mapping techniques 438 IV. X-ray data analysis 442 V. Case studies 443 VI. Conclusions 446 Acknowledgements 449 Author contributions 449 References 449 SUMMARY: Hyperaccumulators are attractive models for studying metal(loid) homeostasis, and probing the spatial distribution and coordination chemistry of metal(loid)s in their tissues is important for advancing our understanding of their ecophysiology. X-ray elemental mapping techniques are unique in providing insitu information, and with appropriate sample preparation offer results true to biological conditions of the living plant. The common platform of these techniques is a reliance on characteristic X-rays of elements present in a sample, excited either by electrons (scanning/transmission electron microscopy), protons (proton-induced X-ray emission) or X-rays (X-ray fluorescence microscopy). Elucidating the cellular and tissue-level distribution of metal(loid)s is inherently challenging and accurate X-ray analysis places strict demands on sample collection, preparation and analytical conditions, to avoid elemental redistribution, chemical modification or ultrastructural alterations. We compare the merits and limitations of the individual techniques, and focus on the optimal field of applications for inferring ecophysiological processes in hyperaccumulator plants. X-ray elemental mapping techniques can play a key role in answering questions at every level of metal(loid) homeostasis in plants, from the rhizosphere interface, to uptake pathways in the roots and shoots. Further improvements in technological capabilities offer exciting perspectives for the study of hyperaccumulator plants into the future.

Microwave alkylation of lithium tetrazolate

N1-substituted tetrazoles are interesting ligands in transition metal coordination chemistry, especially in the field of spin crossover. Their synthesis is performed in most cases according to the Franke-synthesis, using a primary amine as reagent introducing the substitution pattern. To enhance flexibility in means of substrate scope, we developed a new protocol based on alkylation of lithium tetrazolate with alkyl bromides. The N1–N2 isomerism of the tetrazole during the alkylation was successfully suppressed by use of highly pure lithium tetrazolate and 30 vol.% aqueous ethanol as solvent, leading to pure N1-substituted products. The feasibility of this reaction was demonstrated by a selection of different substrates.Graphical abstract

Copper(II) Complexes of Phenanthroline and Histidine Containing Ligands: Synthesis, Characterization and Evaluation of their DNA Cleavage and Cytotoxic Activity.

Copper(II) complexes have been intensely investigated in a variety of diseases and pathological conditions due to their therapeutic potential. The development of these complexes requires a good knowledge of metal coordination chemistry and ligand design to control species distribution in solution and tailor the copper(II) centers in the right environment for the desired biological activity. Herein we present the synthesis and characterization of two ligands HL1 and H2L2 containing a phenanthroline unit (phen) attached to the amino group of histidine (His). Their copper(II) coordination properties were studied using potentiometry, spectroscopy techniques (UV-vis and EPR), mass spectrometry (ESI-MS) and DFT calculations. The data showed the formation of single copper complexes, [CuL1]+ and [CuL2], with high stability within a large pH range (from 3.0 to 9.0 for [CuL1]+ and from 4.5 to 10.0 for [CuL2]). In both complexes the Cu2+ ion is bound to the phen unit, the imidazole ring and the deprotonated amide group, and displays a distorted square pyramidal geometry as confirmed by single crystal X-ray crystallography. Interestingly, despite having similar structures, these copper complexes show different redox potentials, DNA cleavage properties and cytotoxic activity against different cancer cell lines (human ovarian (A2780), its cisplatin-resistant variant (A2780cisR) and human breast (MCF7) cancer cell lines). The [CuL2] complex has lower reduction potential (Epc= -0.722 V vs -0.452 V for [CuL1]+) but higher biological activity. These results highlight the effect of different pendant functional groups (carboxylate vs amide), placed out of the coordination sphere, in the properties of these copper complexes.

The silver(I) complex [HB{3-(CF3),5-(CH3)Pz}3]AgNCCH3 supported by a partially fluorinated scorpionate ligand.

Tris(pyrazolyl)borates are used extensively in metal coordination chemistry and belong to a class of ligands generally referred to as scorpionates. The steric and electronic properties of these ligands can be modified quite easily by varying the substituents on the 3-, 4-, and 5-positions of the pyrazolyl moieties on the B atom. Fluorinated tris(pyrazolyl)borates are useful in the stabilization of rare silver(I) complexes. The silver(I) adduct (acetonitrile-κN){tris[5-methyl-3-(trifluoromethyl)pyrazol-1-yl-κN2]hydroborato}silver(I), [Ag(C15H13BF9N6)(CH3CN)] or [HB{3-(CF3),5-(CH3)Pz}3]AgNCCH3, was obtained by treating [HB{3-(CF3),5-(CH3)Pz}3]Na with CF3SO3Ag in the presence of acetonitrile, and was isolated in 85% yield. Single-crystal X-ray diffraction analysis reveals that the AgI center has a pseudo-tetrahedral all-nitrogen coordination sphere, and is supported by a tris(pyrazolyl)borate ligand that binds to the AgI center in a κ3-fashion.

An unusual bis-heteroscorpionate complex with anomalous ligands: [tris(3,4-dibromo-5-phenylpyrazolyl)hydroborato][hydrotris(3-neopentylpyrazolyl)borato]nickel(II).

Tridentate trispyrazolylborate (Tp) ligands, introduced by Trofimenko in 1966, have been widely utilized in metal coordination chemistry because of the relatively facile synthetic modification of their electronic and steric factors. The title heteroscorpionate, [Ni(C27H16BBr6N6)(C24H40BN6)], features one ligand, namely hydrotris(3-neopentylpyrazolyl)borate, that has previously displayed variable steric effects, and a brominated ligand, namely tris(3,4-dibromo-5-phenylpyrazolyl)hydroborate, that, atypical in trispyrazolylborate chemistry, coordinates such that the less bulky pyrazole substituent is oriented facing toward the metal ion. The potential molecular threefold symmetry in scorpionates can allow axial chirality. Although crystallized in the centrosymmetric P-1 space group, a closer inspection of the structure of the title compound reveals axial diastereomers.