Ligand Substitution Research Articles

Mechanochemical synthesis of iron aluminyl complexes.

A series of iron aluminyl complexes have been synthesised in good crystalline yields from reactions between bulky diamido aluminium iodide complexes and K[Fe(CO)2Cp] in the solid state. The series of metal-metal bonded complexes have been characterised by X-ray crystallography and were investigated using density functional theory to probe the effects of ligand substitution on the Al-Fe bond.

Exploring efficient and air-stable d2 Re(v) alkylidyne catalysts: toward room temperature alkyne metathesis.

Transition metal-catalyzed alkyne metathesis has become a useful tool in synthetic chemistry. Well-defined alkyne metathesis catalysts comprise alkylidyne complexes of tungsten, molybdenum and rhenium. Non-d0 Re(v) alkylidyne catalysts exhibit advantages such as remarkable tolerance to air and moisture as well as excellent functional group compatibility. However, the known Re(v) alkylidynes with a pyridine leaving ligand require harsh conditions for activation, resulting in lower catalytic efficiency compared to d0 Mo(vi) and W(vi) alkylidynes. Herein, we report the first non-d0 alkylidyne complex capable of mediating alkyne metathesis at room temperature, namely, the Re(v) aqua alkylidyne complex Re([triple bond, length as m-dash]CCH2Ph)( Ph PO)2(H2O) (14). The aqua complex readily dissociates a water ligand in solution, confirmed by ligand substitution reactions with other σ-donor ligands. The aqua complex can be readily prepared on a large scale, and is stable to air and moisture in the solid state and compatible with a variety of functional groups. The versatile ability of the catalyst has been demonstrated through examples of alkyne cross-metathesis (ACM), ring-closing alkyne metathesis (RCAM), and acyclic diyne metathesis macrocyclization (ADIMAC) reactions. All in all, this work presents a solution for an efficient and air-stable alkyne metathesis catalytic system based on d2 Re(v)-alkylidynes.

Lewis base adducts of NpCl4.

Np(IV) Lewis base adducts were prepared by ligand substitution of NpCl4(DME)2. Using acetonitrile and pyridine, NpCl4(MeCN)4 (1) and NpCl4(pyr)4 (2) were isolated, respectively. Addition of t-butylbipyridine and triphenylphosphine oxide generated the respective Lewis base adducts, NpCl4(tBuBipy)2 (3) and NpCl4(OPPh3)2 (4). All species were fully characterized using spectroscopic and structural analyses.

A bowl-shaped phosphangulene-protected cubic Cu58 nanocluster.

A bowl-shaped phosphangulene-protected cubic Cu58 nanocluster has been synthesized. The structure was determined by X-ray crystallography and further analyzed by multiple techniques. The phosphangulenes not only enable ligand substitutions with triphenylphosphines in a cluster-to-cluster transformation way, but also facilitate inter-cluster interactions with fullerenes. These interactions further influence the entirety's photocurrent response and ability to liberate hydrogen when stimulated by light.

Phase-Pure Zirconium Metal–Organic Polyhedra Enabled by a Ligand Substitution Strategy

Phase-Pure Zirconium Metal–Organic Polyhedra Enabled by a Ligand Substitution Strategy

Tunable Crystallinity and Electron Conduction in Wavy 2D Conjugated Metal-Organic Frameworks via Halogen Substitution.

Currently, most reported 2D conjugated metal-organic frameworks (2D c-MOFs) are based on planar polycyclic aromatic hydrocarbons (PAHs) with symmetrical functional groups, limiting the possibility of introducing additional substituents to fine-tune the crystallinity and electrical properties. Herein, a novel class of wavy 2D c-MOFs with highly substituted, core-twisted hexahydroxy-hexa-cata-benzocoronenes (HH-cHBCs) as ligands is reported. By tailoring the substitution of the c-HBC ligands with electron-withdrawing groups (EWGs), such as fluorine, chlorine, and bromine, it is demonstrated that the crystallinity and electrical conductivity at the molecular level can be tuned. The theoretical calculations demonstrate that F-substitution leads to a more reversible coordination bonding between HH-cHBCs and copper metal center, due to smaller atomic size and stronger electron-withdrawing effect. As a result, the achieved F-substituted 2D c-MOF exhibits superior crystallinity, comprising ribbon-like single crystals up to tens of micrometers in length. Moreover, the F-substituted 2D c-MOF displays higher electrical conductivity (two orders of magnitude) and higher charge carrier mobility (almost three times) than the Cl-substituted one. This work provides a new molecular design strategy for the development of wavy 2D c-MOFs and opens a new route for tailoring the coordination reversibility by ligand substitution toward increased crystallinity and superior electric conductivity.

Pd(II) and Pt(II) terpyridyl complexes: topoisomerase I inhibition and cytotoxicity

Abstract Pd(II) and Pt(II) terpyridyl complexes [MCl(terpy-COOH)]Cl where M = Pd(1) or Pt(2); terpy-COOH = 2,2′:6′,2″-terpyridine-4′-carboxylic acid] were synthesized. The Pd(II) complex was rapidly hydrolyzed because a Pd(II) ion was active in ligand substitution, while hydrolysis of the Pt(II) complex was slow because of its inactivity. Their topoisomerase inhibitory activity was examined: the Pd complex showed higher activity than the Pt complex. The complexes also bound with calf thymus DNA (ct-DNA): the binding constant of 1 was about twice as large as that of 2. The model studies suggested that the Pd complex coordinated faster with the His residue, to which topoisomerase bound phosphate ester, than the Pt complex. The cytotoxicity against HeLa cells was evaluated by adding bovine serum albumin (BSA): the Pd complex 1 was more cytotoxic than cisplatin, while no cytotoxicity was observed for the Pt complex 2. The binding constants of 2 with BSA were confirmed to be similar to those of 1. It is fascinating that the Pd complex, which has been conventionally considered to have low anticancer activity, shows higher cytotoxicity than the Pt complex with the same structure.

Electron and Spin Delocalization in [Co6 Se8 (PEt3 )6 ]0/+1 Superatoms.

Molecular clusters can function as nanoscale atoms/superatoms, assembling into superatomic solids, a new class of solid-state materials with designable properties through modifications on superatoms. To explore possibilities on diversifying building blocks, here we thoroughly studied one representative superatom, Co6 Se8 (PEt3 )6 . We probed its structural, electronic, and magnetic properties and revealed its detailed electronic structure as valence electrons delocalize over inorganic [Co6 Se8 ] core while ligands function as an insulated shell. 59 Co SSNMR measurements on the core and 31 P, 13 C on the ligands show that the neutral Co6 Se8 (PEt3 )6 is diamagnetic and symmetric, with all ligands magnetically equivalent. Quantum computations cross-validate NMR results and reveal degenerate delocalized HOMO orbitals, indicating aromaticity. Ligand substitution keeps the inorganic core nearly intact. After losing one electron, the unpaired electron in [Co6 Se8 (PEt3 )6 ]+1 is delocalized, causing paramagnetism and a delocalized electron spin. Notably, this feature of electron/spin delocalization over a large cluster is attractive for special single-electron devices.

Mono-β-diketiminate supported cerium(III) complex: Synthesis, characterization and reactivity

Mono-β-diketiminate cerium(III) complex 1 was easily accessible from the anhydrous metal tribromide and LNa (L = N,N-2,6-diisopropylphenyl-2,4-pentanediimine anion) in THF at room temperature. Interaction of 1 with 1,2-dimethoxyethane, pyridine and 4-dimethylaminopyridine gave the ligand substitution products 2, 3 and 4 in good yield. All of those complexes exhibited molecular structure similar to that of 1, and the coordination environment around cerium center was the distorted octahedral geometry. The reaction of 1 with 2,6-dimethylphenyl isocyanide gave dinuclear complex 5 with an unusual triple bromine bridge in the molecule. It was worth noting that the reaction of 1 with one equivalent of cyclopentadienylsodium (CpNa) did not afford the mixed-ligand complex but the undesirable ligand redistributed product Cp2CeL (6). Treatment of 1 with arylamido lithium reagent whereas obtained the dissociated product 7. All the complexes were well characterized and further broadened the utility of β-diketiminate ligands.

Steric effect on the photoreaction of chromium(III) porphyrin complexes with imidazole ligands

The steric effect in the photoreaction and ligand substitution reaction of axially coordinated imidazole ligands in chromium(III) porphyrin complexes was investigated in toluene. Imidazole ligands that have various alkyl substituents at the carbon atom [Formula: see text] to the coordinating nitrogen atom were used. The quantum yield of the photodissociation reaction of the axial imidazole ligand shows a remarkable steric effect. Reflecting the steric bulkiness, the magnitude of the quantum yield is isopropyl > ethyl > methyl > (H). The steric effect also appears in the thermal ligand substitution reactions. The mechanism of the steric effect on reactivity will be discussed in terms of the steric hindrance between the porphyrin ligand and the alkyl substituents of imidazole.

Reductive decomplexation of Cu(II)-complexes from copper wastewater by zero-valent aluminum material

In order to effectively remove the extremely stable Cu(II)-complexes, decomplexation is typically required, with previous efforts focusing on the destruction or substitution of organic ligands. Now, we have a new concept that if starting with Cu(II) group and reducing it directly from the complex state, the reductive decomplexation of Cu(II)-complexes may be attained along with the recovery of Cu(II) to Cu0. In this study, for the first time, we introduced a carbon-modified zero-valent aluminum material (C@mZVAl) for one-step reductive decomplexation of Cu(II)-complexes. It was found that 96.84% Cu-EDTA removal and 91.99% Cu0 recovery could be reached. Furthermore, this efficient reductive decomplexation approach was also applicable to other complexes such as copper pyrophosphate, copper tartrate and copper citrate. SEM-EDS, XPS and XRD showed that Cu0 appeared on the material's surface, providing evidences of successful reduction of complexed Cu(II). Cu-EDTA was firstly adsorbed because of the carbon’s adsorption, and then accepted the electrons released from the powerful reducer zero-valent aluminum to accomplish reductive decomplexation. Furthermore, without any pretreatment, two real industrial copper-plating wastewater containing copper pyrophosphate and Cu-EDTA were treated and the total Cu recovery rates were both achieved up to 98.00%. In conclusion, our innovative approach offers a novel method of decomplexation for the efficient recovery of reusable metal products from complexed metal wastewater through a direct reduction reaction.

Valence Tautomerism in Chromium Half-Sandwich Triarylmethylium Dyads

Valence tautomerism (VT) may occur if a molecule contains two chemically different redox-active units, which differ only slightly in their intrinsic redox potential. Herein, we present three new half-sandwich complexes [(η6-arene)Cr(CO)2L]+ with a triarylmethylium substituent appended to the π-coordinated arene and different coligands L (L = CO, P(OPh)3, PPh3, 1+–3+) at the chromium atom. Ligand substitution purposefully lowers the half-wave potential for chromium oxidation and thereby the redox potential difference towards tritylium reduction. For the PPh3-substituted complex 3+, cyclic voltammetry measurements indicate that chromium oxidation and tritylium reduction occur at (almost) the same potential. This renders the diamagnetic Cr(0)-C6H4-CAr2+ form 3+, and its paramagnetic diradical Cr(I)+•-C6H4-CAr2• valence tautomer 3+•• energetically nearly degenerate. Temperature-dependent IR spectroscopy indeed shows two pairs of carbonyl bands that are assignable to a Cr(0) and a Cr(I) species, coexisting in a T-dependent equilibrium with almost equal quantities for both at −70 °C. The diradical form with one unpaired spin at the trityl unit engages in a monomer ⇌ dimer equilibrium, which was investigated by means of quantitative EPR spectroscopy. The diradical species 1+••–3+•• were found to be highly reactive, leading to several identified reaction products, which presumably result from hydrogen atom abstraction via the trityl C atom, e.g., from the solvent.

Halogen-Dependent Diversity and Weak Interactions in the Heterometallic Ni/Cd Complex Solids: Structural and Theoretical Investigation.

Three novel heterometallic Ni/Cd coordination compounds [Ni(en)3][CdCl4]∙3dmso (1), [Ni(en)2(dmf)2][CdBr4] (2), and [Ni(en)3]2[CdI4](I)2 (3) have been synthesized through the self-assembly process in a one-pot reaction of cadmium oxide, nickel salt (or nickel powder), NH4X (X = Cl, Br, I), and ethylenediamine in non-aqueous solvents dmso (for 1) or dmf (for 2 and 3). Formation of the one- (1) or three-dimensional (2 and 3) hydrogen-bonded frameworks has been observed depending on the nature of the [CdX4]2- counter-anion, as well as on the nature of the solvent. The electronic structures of [Ni(en)3]2+ and [Ni(en)2(dmf)2]2+ cations were studied at the DFT and CASSCF levels, including the ab initio ligand field theory (AILFT) calculations. The non-covalent intermolecular contacts between the cationic nickel and anionic cadmium blocks in the solid state were investigated by the QTAIM analysis. The mechanism of ligand substitution at the nickel center in [Ni(en)2(dmf)2]2+ was theoretically investigated at the ωB97X-D4/ma-def2-TZVP//DLPNO-CCSD(T)/ma-def2-TZVPP level. The results demonstrate that thermodynamic factors are structure-determining ones due to low energy barriers of the rotation of dmf ligands in [Ni(en)2(dmf)2]2+ (below 3 kcal mol-1) and the reversible transformation of [Ni(en)2(dmf)2]2+ into [Ni(en)3]2+ (below 20 kcal mol-1).

Substitution of apical iodide ligands in octahedral molybdenum cluster complexes by N-donor molecules: Synthesis and investigation of [{Mo6I8}(L)I5]– and cis-[{Mo6I8}(L)2I4] (L = CH3CN or 1,2,4-triazole)

In this work, reactivity of the [{Mo6I8}I6]2– cluster anion toward some N-donor ligands was investigated. Heating of the solution of Cs2[{Mo6I8}I6] salt in acetonitrile leads to the partial substitution of apical iodide ligands by CH3CN molecules forming crystalline solid Cs0.1[{Mo6I8}(CH3CN)I5]0.1·cis-[{Mo6I8}(CH3CN)2I4]0.9·0.9CH3CN (1). The solid includes anionic [{Mo6I8}(CH3CN)I5]– and neutral cis-[{Mo6I8}(CH3CN)2I4] cluster units at a ratio of 1:9. In the crystal structure of 1, these clusters occupy common crystallographic sites, which allows us to consider this compound as a solid solution. When 1,2,4-triazole (tz) was added to the reaction mixture, a solid solution of mono- and disubstituted products was also formed and crystallized in a ratio of approximately 1:3, yielding the compound Cs0.27[{Mo6I8}(tz)I5]0.27·cis-[{Mo6I8}(tz)2I4]0.73 (2). It was assumed that compound 2 was formed as a product of direct substitution of the apical iodide ligands by tz molecules. Structural features and luminescence properties of the new compounds 1 and 2 were studied.

Recent Advancements in Mechanistic Studies of Palladium- and Nickel-Catalyzed Ethylene Copolymerization with Polar Monomers.

The introduction of polar functional groups into polyolefin chain structures creates opportunities to enhance specific properties, such as adhesion, dyeability, printability, compatibility, thermal stability, and electrical conductivity, which widen the range of potential applications for these modified materials. Transition metal catalysts, especially late transition metals, have proven to be highly effective in copolymerization processes due to their reduced Lewis acidity and electrophilicity. However, when compared to the significant progress and summary of synthetic methods, there is a distinct lack of a comprehensive summary of mechanistic studies pertaining to the catalytic systems involved in ethylene copolymerization catalyzed by palladium and nickel catalysts. In this review, we have provided a comprehensive summary of the latest developments in mechanistic studies of ethylene copolymerization with polar monomers catalyzed by late-transition-metal complexes. Experimental and computational methods were employed to conduct a detailed investigation of these organic and organometallic systems. It is mainly focused on ligand substitution, changes in binding modes, ethylene/polar monomer insertion, chelate opening, and β-H elimination. Factors that control the catalytic activity, molecular weight, comonomer incorporation ratios, and branch content are analyzed, these include steric repulsions between ligands and monomers, electronic effects arising from both ligands and monomers, and so on.

Π-Character of Chromium Germylyne Complex in the Reactions with Enone, Butadiene, and Alkynes: Formation of Germacycles through [2+4] Cycloaddition with Conjugated Molecules.

Germylyne complex [Cp*(OC)2Cr≡Ge{C(SiMe3)3}] (1) reacted with methyl vinyl ketone to give an η3-allyl complex 2 with an oxagermacyclopentenyl ring. An analogous η3-allyl complex 3 with a germacyclopentenyl ring was obtained by the reaction with butadiene, a non-polar conjugated molecule, under photoirradiation. These reactions are accompanied by cleavage of the Cr≡Ge triple bond. On the other hand, the reactions of complex 1 with alkynes under photoirradiation resulted in clean substitution of a CO ligand of 1 to afford (η2-alkyne)germylyne complexes, where the Cr≡Ge triple bond is intact.

Mechanistic Study on Steric Activity Interplay of Olefin/Polar Monomers for Industrially Selective Late Transition Metal Catalytic Reactions.

A significant issue in developing metal-catalyzed plastic polymer materials is obtaining distinctive catalytic characteristics to compete with current plastics in industrial commodities. We performed first-principle DFT calculations on the key insertion steps for industrially important monomers, vinyl fluoride (VF) and 3,3,3-trifluoropropene (TFP), to explain how the ligand substitution patterns affect the complex's polymerization behaviors. Our results indicate that the favorable 2,1-insertion of TFP is caused by less deformation in the catalyst moiety of the complexes in contrast to the 1,2-insertion mode. In contrast to the VF monomer, the additional interaction between the fluorine atoms of 3,3,3-trifluoropropene and the carbons of the catalyst ligands also contributed to favor the 2,1-insertion. It was found that the regioselectivity of the monomer was predominated by the progressive alteration of the catalytic geometry caused by small dihedral angles that were developed after the ligand-monomer interaction. Based on the distribution of the 1,2- and 2,1-insertion products, the activity and selectivity were influenced by the steric environment surrounding the palladium center; thus, an increased steric bulk visibly improved the selectivity of the bulkier polar monomer (TFP) during the copolymerization mechanism. In contrast, better activity was maintained through a sterically less hindered Pd metal center; the calculated moderate energy barriers showed that a catalyst with less steric hindrance might provide an opportunity for a wide range of prospective industrial applications.

Shape-Memory Effect Enabled by Ligand Substitution and CO2 Affinity in a Flexible SIFSIX Coordination Network.

We report that linker ligand substitution involving just one atom induces a shape-memory effect in a flexible coordination network. Specifically, whereas SIFSIX-23-Cu,[Cu(SiF6)(L)2]n, (L = 1,4-bis(1-imidazolyl)benzene, SiF62- = SIFSIX) has been previously reported to exhibit reversible switching between closed and open phases, the activated phase of SIFSIX-23-CuN, [Cu(SiF6)(LN)2]n (LN = 2,5-bis(1-imidazolyl)pyridine), transformed to a kinetically stable porous phase with strong affinity for CO2. As-synthesized SIFSIX-23-CuN, α, transformed to less open, γ, and closed, β, phases during activation. β did not adsorb N2 (77 K), rather it reverted to α induced by CO2 at 195, 273 and 298 K. CO2 desorption resulted in α', a shape-memory phase which subsequently exhibited type-I isotherms for N2 (77 K) and CO2 as well as strong performance for separation of CO2/N2 (15/85) at 298 K and 1 bar driven by strong binding (Qst = 45-51 kJ/mol), high CO2 capacity (209 cm3/g) and excellent CO2/N2 selectivity (up to 700). Interestingly, α' reverted to β after resolvation/desolvation. Density functional theory (DFT) calculations provide insight into the properties of SIFSIX-23-CuN.

Shape‐Memory Effect Enabled by Ligand Substitution and CO2 Affinity in a Flexible SIFSIX Coordination Network

AbstractWe report that linker ligand substitution involving just one atom induces a shape‐memory effect in a flexible coordination network. Specifically, whereas SIFSIX‐23‐Cu, [Cu(SiF6)(L)2]n, (L=1,4‐bis(1‐imidazolyl)benzene, SiF62−=SIFSIX) has been previously reported to exhibit reversible switching between closed and open phases, the activated phase of SIFSIX‐23‐CuN, [Cu(SiF6)(LN)2]n (LN=2,5‐bis(1‐imidazolyl)pyridine), transformed to a kinetically stable porous phase with strong affinity for CO2. As‐synthesized SIFSIX‐23‐CuN, α, transformed to less open, γ, and closed, β, phases during activation. β did not adsorb N2 (77 K), rather it reverted to α induced by CO2 at 195, 273 and 298 K. CO2 desorption resulted in α′, a shape‐memory phase which subsequently exhibited type‐I isotherms for N2 (77 K) and CO2 as well as strong performance for separation of CO2/N2 (15/85) at 298 K and 1 bar driven by strong binding (Qst=45–51 kJ/mol) and excellent CO2/N2 selectivity (up to 700). Interestingly, α′ reverted to β after re‐solvation/desolvation. Molecular simulations and density functional theory (DFT) calculations provide insight into the properties of SIFSIX‐23‐CuN.

The influence of electronic effects on the substitution reactions of N,N′-donor cis-palladium(II) complexes with bidentate (pyridin-2-yl)methyl-aniline ligands

The substitution behaviour of bidentate N,N′-donor (pyridin-2-yl)methyl-aniline chelates with different substituents on the mononuclear Pd(II) complexes was investigated. The complexes were synthesized and characterized using 1H NMR, 13C NMR, FTIR, LC–MS, and elemental analysis. The study explored the kinetics and mechanistic behaviour of N,N′-pyridyl Pd(II) complexes, viz. dichloro-(N-((pyridin-2-yl)methyl)aniline)palladium(II) (PdL1), dichloro-(4-fluoro-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL2), dichloro-(4-methoxy-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL3) and dichloro-(4-ethyl-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL4). The effect of concentration and temperature on substitution behaviour of coordinated chloride ligands by three bio-relevant thiourea nucleophiles, viz. thiourea (TU), N,N′-dimethylthiourea (DMTU) and N,N,N′,N′-tetramethylthiourea (TMTU), of different steric demands was studied in a 0.1 M (LiCl) solution of ultra-pure water under pseudo-first order conditions using standard Stopped-Flow and UV–Visible spectrophotometric techniques. The substitution of the chloride ligands from the Pd(II) metal by thiourea nucleophiles was a two-step reaction, where the chloride trans to the pyridine ligand was substituted first due to the strong trans effect on the pyridine ring compared to the amine group. The rate of substitution of the chloride by thiourea nucleophiles increased with the presence of an electron-withdrawing substituent and decreased when an electron-donating group was attached to the para position of the phenyl moiety. The negative activation entropies and positive activation enthalpy for all the substitution reactions support an associative mode of substitution mechanism.