Coordination Chemistry Of Ligands Research Articles

Germylene-β-sulfoxide Hemilabile Ligand in Coordination Chemistry.

We describe herein the synthesis of a germylene-β-sulfoxide ligand, 1, and its abilities in coordination chemistry. Treatment of 1 with metal complexes [W(cod)(CO)4], [Mo(nbd)(CO)4] and [Ni(cod)2] afforded the corresponding (1)-chelated metal complexes (1)-W(CO)4 (2a), (1)-Mo(CO)4 (2b), and (1)-Ni(cod) (4a), clearly showing a bidentate ligation of the metal by the germanium(II) and sulfur centers. Coordination with [Ru(PPh3)3Cl2] afforded an unprecedented bridged bis(ruthenium) complex 3b. In the case of 4a, the hemilability of the bidentate ligand 1 was demonstrated by sulfoxide substitution by a CO ligand.

Iron(II) Complexes of an Anionic Bis(ylide)diphenylborate Ligand.

Double deprotonation of the salt [Ph2B(PMe3)2][OTf] (1) provides access to a bis(ylide)diphenylborate ligand that is readily transferred in situ to iron(II). Depending on the reaction stoichiometry, both the "ate" complex [Ph2B(Me2PCH2)2Fe(μ-Cl)2Li(THF)2] (2) and the homoleptic complex [Ph2B(Me2PCH2)2]2Fe(3) can be prepared from FeCl2(THF)1.5. Further reaction of 3 with FeCl2(THF)1.5 produces the chloride-bridged dimer [Ph2B(Me2PCH2)2Fe(μ-Cl)2Fe(CH2PMe2)2BPh2](4). Attempts to reduce or alkylate 4 provide 3 as the only isolable product, likely a consequence of the low steric hindrance of the bis(ylide)diphenylborate ligand. On the other hand, reaction of 4 with the strong field ligand CNtBu provides the six-coordinate, diamagnetic complex [Ph2B(Me2PCH2)2Fe(CNtBu)4][Cl](5). Electronic structure calculations for the bis(ylide)diphenylborate ligand and homoleptic complex 3 suggest that the C(ylide) atoms are strong σ-donors with little π-bonding character. These initial results suggest the potential for this bis(ylide)diphenylborate ligand in coordination chemistry.

The Metaphosphite (PO2 - ) Anion as a Ligand.

The utilization of monomeric, lower phosphorous oxides and oxoanions, such as metaphosphite (PO2 −), which is the heavier homologue of the common nitrite anion but previously only observed in the gas phase and by matrix isolation, requires new synthetic strategies. Herein, a series of rhenium(I–III) complexes with PO2 − as ligand is reported. Synthetic access was enabled by selective oxygenation of a terminal phosphide complex. Spectroscopic and computational examination revealed slightly stronger σ‐donor and comparable π‐acceptor properties of PO2 − compared to homologous NO2 −, which is one of the archetypal ligands in coordination chemistry.

The Metaphosphite (PO2−) Anion as a Ligand

AbstractThe utilization of monomeric, lower phosphorous oxides and oxoanions, such as metaphosphite (PO2−), which is the heavier homologue of the common nitrite anion but previously only observed in the gas phase and by matrix isolation, requires new synthetic strategies. Herein, a series of rhenium(I–III) complexes with PO2− as ligand is reported. Synthetic access was enabled by selective oxygenation of a terminal phosphide complex. Spectroscopic and computational examination revealed slightly stronger σ‐donor and comparable π‐acceptor properties of PO2− compared to homologous NO2−, which is one of the archetypal ligands in coordination chemistry.

Pyrone-derived Marine Natural Products: A Review on Isolation, Bio-activities and Synthesis

Marine natural products (MNPs) containing pyrone rings have been isolated from numerous marine organisms, and also produced by marine fungi and bacteria, particularly, actinomycetes. They constitute a versatile structure unit of bioactive natural products that exhibit various biological activities such as antibiotic, antifungal, cytotoxic, neurotoxic, phytotoxic and anti-tyrosinase. The two structure isomers of pyrone ring are γ- pyrone and α-pyrone. In terms of chemical motif, γ-pyrone is the vinologous form of α- pyrone which possesses a lactone ring. Actinomycete bacteria are responsible for the production of several α-pyrone compounds such as elijopyrones A-D, salinipyrones and violapyrones etc. to name a few. A class of pyrone metabolites, polypropionates which have fascinating carbon skeleton, is primarily produced by marine molluscs. Interestingly, some of the pyrone polytketides which are found in cone snails are actually synthesized by actinomycete bacteria. Several pyrone derivatives have been obtained from marine fungi such as Aspergillums flavus, Altenaria sp., etc. The γ-pyrone derivative namely, kojic acid obtained from Aspergillus fungus has high commercial demand and finds various applications. Kojic acid and its derivative displayed inhibition of tyrosinase activity and, it is also extensively used as a ligand in coordination chemistry. Owing to their commercial and biological significance, the synthesis of pyrone containing compounds has been given attention over the past years. Few reviews on the total synthesis of pyrone containing natural products namely, polypropionate metabolites have been reported. However, these reviews skipped other marine pyrone metabolites and also omitted discussion on isolation and detailed biological activities. This review presents a brief account of the isolation of marine metabolites containing a pyrone ring and their reported bio-activities. Further, the review covers the synthesis of marine pyrone metabolites such as cyercene-A, placidenes, onchitriol-I, onchitriol-II, crispatene, photodeoxytrichidione, (-) membrenone-C, lihualide-B, macrocyclic enol ethers and auripyrones-A & B.

Functionalised N-Heterocyclic Carbene Ligands in Bimetallic Architectures.

N‐Heterocyclic carbenes (NHCs) have become immensely successful ligands in coordination chemistry and homogeneous catalysis due to their strong terminal σ‐donor properties. However, by targeting NHC ligands with additional functionalisation, a new area of NHC coordination chemistry has developed that has enabled NHCs to be used to build up bimetallic and multimetallic architectures. This minireview covers the development of functionalised NHC ligands that incorporate additional donor sites in order to coordinate two or more metal atoms. This can be through the N‐atom of the NHC ring, through a donor group attached to the N‐atom or the carbon backbone, coordination of the π‐bond or an annulated π‐donor on the backbone, or through direct metalation of the backbone.

Direct Heterogenization of Salphen Coordination Complexes to Porous Organic Polymers: Catalysts for Ring-Expansion Carbonylation of Epoxides

Salen and salphens are important ligands in coordination chemistry due to their ability to form various metal complexes that can be used for a variety of organic transformations. However, salen/salphen complexes are difficult to separate from the reaction mixture, thereby limiting their application to homogeneous systems. Accordingly, considerable effort has been spent to heterogenize the metallosalen/salphen complexes; however, this has resulted in compromised activities and selectivities. Direct heterogenization of metallosalens to form porous organic polymers (POPs) shows promise for heterogeneous catalysis, because it would allow easy separation while retaining catalytic function. Thus, a facile synthetic strategy for preparing metallosalen/salphen-based porous organic polymers through direct molecular knitting using a Friedel-Crafts reaction is presented herein for the first time. As representative candidates, salphenM(III)Cl (M = Al3+ and Cr3+) complexes are knitted by covalent cross-linking using this facile, scalable, one-pot method to synthesize highly POPs in high yields. When incorporated with [Co(CO)4]- anions, the resulting heterogeneous Lewis acidic metal (Al3+ and Cr3+) POPs exhibit propylene oxide ring-expansion carbonylation activity on par with those of their homogeneous counterparts.

Nitrenium Salts in Lewis Acid Catalysis.

Molecular compounds featuring nitrogen atoms are typically regarded as Lewis bases and are extensively employed as donor ligands in coordination chemistry or as nucleophiles in organic chemistry. By contrast, electrophilic nitrogen-containing compounds are much rarer. Nitrenium cations are a new family of nitrogen-based Lewis acids, the reactivity of which remains largely unexplored. In this work, nitrenium ions are explored as catalysts in five organic transformations. These reactions are the first examples of Lewis acid catalysis employing nitrogen as the site of substrate activation. Moreover, these compounds are readily accessed from commercially available reagents and exhibit remarkable stability toward moisture, allowing for benchtop transformations without the need to pretreat solvents.

Morpholine-N-carboxylate as a ligand in coordination chemistry – Syntheses and structures of three heteroleptic copper(ii) and zinc complexes

The first three morpholine-N-carboxylate complexes, with copper(II) and zinc(II) have been prepared and studied by single crystal X-ray diffraction. All three compounds comprise of discrete mono- or dinuclear heteroleptic complex molecules. Three different ways of bonding of morphCOO– anions have been observed – as a monodentate, chelating, and as a bridging ligand. In all cases only the carboxyl group of the morpholinecarboxylate anion was found to bind to the metal cation, while the oxygen atom of the morpholine ring participates only in weak C–H‧‧‧O hydrogen bonds. Changes in the geometry of the coordinated morphCOO– anion relative to the free ligand (in its morpholinium salt) were observed in all three modes of binding to the metal center.

Periodic law: new formulation and equation description

Abstract The atomic weight, nuclear charge, electron configuration of an atom and the total number of i-electrons in an atom belonging to i-block (i = s, or p, or d, or f) are considered as the fundamental characteristics of an element (atom). Only in the latter case, the true periodicity is achieved: the repetitions occur at regular intervals. The total number of i-electrons in an atom belonging to i-block is used as the new basis for the description of the periodicity. This made possible to propose a new formulation of the Periodic law and to describe the Periodic law by an equation. The equation provides opportunities for large-scale prediction of the properties of elements and their compounds; it does not require special knowledge to work with it. Theoretical and applied aspects of the application of the new formulation and equation are outlined. Predictions are made for proton affinity and gas phase basicity of 20 elements, constants of inductive effects of 185 atoms and groups, electronic parameters of 222 neutral ligands. The suitability of the equation is exemplified by the description of the properties of atoms and elements, such as ionization energy, electron affinity, proton affinity, electronegativity, covalent atomic radii and the enthalpy of element formations in the gas phase. The equation makes possible to describe also the properties of compounds and their fragments: the acidic properties of the hydrogen compounds of the elements, the acidic properties of protonated atoms and molecules, gas phase basicity and proton affinity of compounds, inductive effects of ligands in coordination chemistry and substituents in organic chemistry, electronic parameters of neutral ligands, the electron effect constants of coordinating metals.

Metal coordination complexes in nonaqueous redox flow batteries

Abstract The ongoing search for new electroactive materials for next-generation redox flow batteries has within the last decade encompassed metal–ligand coordination chemistry. Here, we review the handful of metal coordination complexes proposed as redox flow battery electrolytes. We highlight examples with careful ligand design, driving research towards higher energy density storage materials. Metal coordination complexes designed to be highly soluble not only in the initial redox state but also in all charged states accessed during the battery cycle give particularly impressive performances. Optimisation of flow cell conditions for metal coordination complexes remains largely unexplored, with most complexes screened in nonideal H-cell experiments with little investigation into membrane and electrode materials.

Influence of Metal-Ligand Coordination on the Elemental Growth and Alloying Composition of Pt-Ni Octahedral Nanoparticles for Oxygen Reduction Electrocatalysis.

Understanding the role of surfactants or ligands on the growth mechanism of metal/alloy nanoparticles (NPs) is important for controlled synthesis of functional metallic NPs with tailored structures and properties. There have been a number of works showing the significant impact of surfactants/ligands on the shape-controlled synthesis of nanocrystals with well-defined surfaces. Beyond the morphological shape control, impact of the surfactants/ligands on the alloying structure of bimetallic nanocrystals, however, still remains largely unaddressed. We reveal here a significant effect of benzoic acid ligand on the elemental growth and alloying phase structure of octahedral Pt–Ni NPs, a class of highly active electrocatalyst for oxygen reduction reaction in fuel cells. Contrary to previous reports showing the critical role of benzoic acid in directing the growth of octahedral Pt–Ni NPs, we found that benzoic acid played a minor role in forming the octahedral shape; instead, it can strongly coordinate with Ni cation and significantly slows down its reduction rate, leading to a phase separation in the Pt–Ni NP products (a mixture of Pt-rich octahedral NPs and nearly pure Ni NPs). Such phase separation further resulted in a lower catalytic activity and stability. These results help us comprehensively understand the effect of metal–ligand coordination chemistry on the elemental growth mechanism and alloying phase structure of bimetallic NPs, complementing previous emphasis on the role of surfactants in purely morphological shape control.

Substituent effects in the zinc(II) coordination chemistry of isomeric pyridylpyrazole ligands

Pyrazoles with unsymmetric substitution are useful ligands in coordination chemistry, but are under-developed due to synthetic challenges in accessing the pure isomers. We have prepared four new structurally related N-(2-pyridyl)-3,5-dialkylpyrazole ligands, L1–L4, and probed their coordination chemistry in the crystalline phase and in solution to elucidate a relationship between steric influence of the alkyl substituents, the stability of the subsequent metal complexes, and their crystal packing influences. We find that L1 and L2, bearing linear or branched alkyl substituents, show similar stabilities and crystal packing motifs featuring π···π and C-H···Cl interactions in the crystalline complexes 1 and 2, respectively. The cyclohexyl-fused species L3 and L4 vary both in the solution stability of complexes 3 and 4, respectively, and in their crystal packing. Complex [ZnCl2(L3)] (3) is a mononuclear complex similar to 1 and 2, albeit with π···π interactions disallowed by the bulk of the cyclohexyl ring. Reaction of isomeric L4 with ZnCl2 gives two polymorphic complexes, 4α and 4β, of the form [Zn2Cl2(μ2-Cl)2(L4)2], varying only in their long-range packing modes. These results show the importance of understanding the steric influences in substituted pyridylpyrazoles, which determine both stability in solution and speciation in the crystalline phase.

The Triple-Decker Complex [Cp*Fe(µ,η⁵:η⁵-P₅)Mo(CO)₃] as a Building Block in Coordination Chemistry.

Although the triple-decker complex [Cp*Fe(µ,η5:η5-P5)Mo(CO)3] (2) was first reported 26 years ago, its reactivity has not yet been explored. Herein, we report a new high-yielding synthesis of 2 and the isolation of its new polymorph (2’). In addition, we study its reactivity towards AgI and CuI ions. The reaction of 2 with Ag[BF4] selectively produces the coordination compound [Ag{Cp*Fe(µ,η5:η5-P5)Mo(CO)3}2][BF4] (3). Its reaction with Ag[TEF] and Cu[TEF] ([TEF]− = [Al{OC(CF3)3}4]−) leads to the selective formation of the complexes [Ag{Cp*Fe(µ,η5:η5-P5)Mo(CO)3}2][TEF] (4) and [Cu{Cp*Fe(µ,η5:η5-P5)Mo(CO)3}2][TEF] (5), respectively. The X-ray structures of compounds 3–5 each show an MI ion (MI = AgI, CuI) bridged by two P atoms from two triple-decker complexes (2). Additionally, four short MI···CO distances (two to each triple-decker complex 2) participate in stabilizing the coordination sphere of the MI ion. Evidently, the X-ray structure for compound 3 shows a weak interaction of the AgI ion with one fluorine atom of the counterion [BF4]−. Such an Ag···F interaction does not exist for compound 4. These findings demonstrate the possibility of using triple-decker complex 2 as a ligand in coordination chemistry and opening a new perspective in the field of supramolecular chemistry of transition metal compounds with phosphorus-rich complexes.

Electrostatic polarization of nonpolar substrates: a study of interactions between simple cations and Mo-bound N2.

Although a great deal of catalytic studies have focused on covalent interactions between substrates and catalyst centers, recognition of the importance of noncovalent and ionic interactions is driving new approaches to catalyst design. Electrostatic interactions with simple cations (those with little covalency, such as alkali metals) play crucial roles in many catalytic processes, but these effects are challenging to study due to their complicated solvation and speciation behaviour. These effects are particularly difficult to study during cation-mediated reactions with weakly-polar or non-polar substrates. Dinitrogen is one of the most nonpolar substrates known to be affected by electrostatic interactions in both heterogeneous and homogeneous reactions but understanding the significance of these effects requires further exploration. To examine these effects systematically, a new multidentate ligand framework bearing pendent crown ethers has been developed and incorporated into a series of Mo(0)-based dinitrogen complexes. Prepared via both reduction and ligand substitution routes, the strength and impact of cation-N2 interactions have been studied experimentally (IR spectroscopy) and computationally. Although the smallest cation (Li+) has the largest impact on the ground-state heterobimetallic activation of N2, solvation interactions are highly competitive and result in low Li+-(N2)Mo binding affinities. Thus, although smaller cations can have the largest electronic impact on substrates, these interactions are also the least persistent.

Crowding out: ligand modifications and their structure directing effects on brucite-like {Mx(μ3-OH)y} (M = Co(ii), Ni(ii)) core growth within polymetallic cages.

Previous employment of the ligands 2-methoxy-6-[(methylimino)methyl]phenol (L1H) and 2-methoxy-6-[(phenylimino)methyl]phenol (L2H) has resulted in the self-assembly of pseudo metallocalix[6]arene complexes of general formulae: [M7(μ3-OH)6(Lx)6](NO3)y (M = Ni(ii), x = 1, y = 2 (1) and Co(ii/iii), x = 2, y = 3 (2)). Extrapolating upon this work, we report the coordination chemistry of ligands 2-methoxy-6-{[(2-methoxyphenyl)imino]methyl}phenol (L3H), 2-[(benzylimino)methyl]-6-methoxyphenol (L4H), 2-[(benzylamino)methyl]-6-methoxyphenol (L5H) and 2-[(benzylamino)methyl]-4-bromo-6-methoxyphenol (L6H), whose structures are modifications of ligands L1-2H. These ligands are employed in the synthesis and characterisation of the dimetallic complex [Ni(ii)2(L3)3(H2O)](NO3)·2H2O·3MeOH (3); the monometallic complexes [Ni(ii)(L4)2] (4) and [Co(iii)(L4)3]·H2O·MeOH (5a); and the tetranuclear pseudo metallocalix[4]arene complexes: [(NO3)⊂Co(ii)4(μ3-OH)2(L5)4(H2O)2](NO3)·H2O (6), [(NO3)⊂Ni(ii)4(μ3-OH)2(L5)4(H2O)2](NO3)·H2O (7) and [Ni(ii)4(μ3-OH)2(L6)4(NO3)2]·MeCN (8). The tetrametallic 'butterfly' core topologies in 6-8 are discussed with respect to their structural and topological relationship with their heptanuclear [M7] (M = Co(ii), Ni(ii)) pseudo metallocalix[6]arene ancestors (1 and 2).

Engineering Multifunctional DNA Hybrid Nanospheres through Coordination‐Driven Self‐Assembly

AbstractDeveloping simple and general approaches for the synthesis of nanometer‐sized DNA materials with specific morphologies and functionalities is important for various applications. Herein, a novel approach for the synthesis of a new set of DNA‐based nanoarchitectures through coordination‐driven self‐assembly of FeII ions and DNA molecules is reported. By fine‐tuning the assembly, Fe–DNA nanospheres of precise sizes and controlled compositions can be produced. The hybrid nanoparticles can be tailored for delivery of functional DNA to cells in vitro and in vivo with enhanced biological function. This highlights the potential of metal ion coordination as a tool for directing the assembly of DNA architectures, which conceptualizes a new pathway to expand the repertoire of DNA‐based nanomaterials. This methodology will advance both the fields of DNA nanobiotechnology and metal–ligand coordination chemistry.

Phosphines with N‐Heterocyclic Boryl‐Substituents: Ligands for Coordination Chemistry and Catalysis

Boryl‐substituted phosphines NHB–P(R)Ph (R = H, Ph, NHB = N‐heterocyclic boryl substituent) react with Fe2(CO)9 to give isolable Fe(CO)4 complexes, two of which were characterized by single‐crystal XRD studies. The electronic and steric properties for a series of the boryl phosphines were further assessed by evaluation of TEPs for in‐situ formed complexes [RhCl(NHB–PR1R2)(CO)2] (R1, R2 = H, Ph, Me, NMe2), and calculations of buried volumes for Fe(CO)4 complexes. The results imply that the NHB‐phosphines exhibit due to their conformational flexibility some variability in their steric bulk, and that some specimens may exhibit similar electron releasing power and steric demand as tBu3P. Studies of the amination of bromobenzene with 2,6‐diisopropylaniline confirmed that these properties can be exploited to promote Pd‐catalyzed C–N cross coupling reactions, and that formal replacement of a phenyl by a NHB substituent in the auxiliary phosphine has a beneficial effect on catalyst performance.

Surface Chemistry of Atomically Precise Coinage-Metal Nanoclusters: From Structural Control to Surface Reactivity and Catalysis.

A comprehensive understanding of chemical bonding and reactions at the surface of nanomaterials is of great importance in the rational design of their functional properties and applications. With the rapid development in cluster science, it has become clear that atomically precise metal clusters represent ideal models for resolving various important and/or unsolved issues related to surface science. This Account highlights our recent efforts on the fabrication of ligand-stabilized coinage nanoclusters with atomic precision from the viewpoint ofsurface coordination chemistry in particular. The successful synthesis of a large variety of metal clusters in our group has greatly benefitted from the development of an effective amine-assisted NaBH4 reduction method. First discussed in this Account is how the introduction of amines in the synthetic protocol enhances the long-term stability and high-yield production of Ag/Cu-based metals in air. Such a method allows the utilization of different organic ligands as surface stabilizing agents to manipulate both the core and surface structures of metal nanoclusters, helping to understand the role of surface ligands in determining the structures of metal nanoclusters. The coordination chemistry of ligands used in the synthesis of metal nanoclusters is crucial in determining their overall shape, metal arrangement, surface ligand binding structure, chirality and also metal exposure. Detailed discussions are given in the following four different systems: (1) The co-use of phosphines and thiolates with rich coordination structures (2 to 4-coordinated) helps to control the formation of a sequence of Ag nanoclusters with a near-perfectly cubic shape; (2) The metal arrangements and surface structures of AuCu clusters highly depend on metal precursors and counter cations used in the synthesis; (3) Metal clusters with intrinsic chirality are readily prepared by introducing chiral ligands or counterions, making it possible to obtain optically active enantiomers and understand the origin of chirality of metal nanoclusters; (4) The variation of metal exposure of the inner metal core of metal nanocluster can be controlled by the surface ligand coordination structure. Such capabilities to manipulate the surface structure of metal nanoclusters allow the creation of model systems for investigating the structure-reactivity relationship of metal nanomaterials. Several important examples are then discussed to highlight the importance of ligand coordination chemistry in tuning the surface reactivity and catalysis of metal nanoclusters. For example, bulky thiolates on Agare demonstrated to be more labile than small thiolates for making metal nanoclusters with both enhanced ligand exchange capability and catalysis. Alkynyl ligands can be thermally released from metal nanoclusters more easily than thiolates and halides while maintaining the overall structure, thereby serving as ideal systems for understanding the promoting effect of surface stabilizers on catalysis. Finally, we provide a perspective on the principles of surface coordination chemistry of metal nanoclusters and their potential applications with regards to catalysis of protected metal clusters.

4-(Ph3 B)-2,6-Mes2 Py]- : A Sterically Demanding Anionic Pyridine.

The introduction of a triphenylborate group at the 4-position of 2,6-dimesitylpyridine afforded a sterically demanding anionic pyridine. The charge introduced through the borate group drastically increases its basicity and measurement of its pKa value (18.46) revealed a significantly higher value than that of 4-dimethylaminopyridine (17.95). THF ring-opening was observed upon treating its lithium salt with TMSCl, which demonstrates its high nucleophilicity. The mesityl groups at the 2,6-positions are oriented orthogonal to the pyridine ring and do not block the nitrogen atom of the pyridine. The reaction of the protonated pyridine with Li[BH4 ] yielded the corresponding Lewis acid/base adduct, which shows that the title compound can be used as a monodentate ligand in coordination chemistry. The crystal structures of all the compounds presented in this work are reported.