Transition Metal Centers Research Articles

Supramolecular interaction controlled and calix[4]arene ligand assisted Pd-catalyzed C(sp3)−H arylation of aliphatic aldehydes

A calix[4]arene ligand assisted direct β-C−H arylation of tertiary aliphatic aldehydes has been developed via a Pd-catalyzed C(sp3)−H functionalization process. This strategy exhibited good functional group compatibility and C−H bond site-selectivity. Mechanism studies have shown that both synergistic effect and cationic-π supramolecular interaction between calixarene cavity and transition-metal catalytic center may play an important role in this catalytic cycle. This complementary method would be used in organic and medical chemistry due to the importance of tertiary aliphatic aldehydes.

Magnetic Interplay between π-Electrons of Open-Shell Porphyrins and d-Electrons of Their Central Transition Metal Ions.

Magnetism is typically associated with d‐ or f‐block elements, but can also appear in organic molecules with unpaired π‐electrons. This has considerably boosted the interest in such organic materials with large potential for spintronics and quantum applications. While several materials showing either d/f or π‐electron magnetism have been synthesized, the combination of both features within the same structure has only scarcely been reported. Open‐shell porphyrins (Pors) incorporating d‐block transition metal ions represent an ideal platform for the realization of such architectures. Herein, the preparation of a series of open‐shell, π‐extended Pors that contain magnetically active metal ions (i.e., CuII, CoII, and FeII) through a combination of in‐solution and on‐surface synthesis is reported. A detailed study of the magnetic interplay between π‐ and d‐electrons in these metalloPors has been performed by scanning probe methods and density functional theory calculations. For the Cu and FePors, ferromagnetically coupled π‐electrons are determined to be delocalized over the Por edges. For the CoPor, the authors find a Kondo resonance resulting from the singly occupied CoII d z 2 orbital to dominate the magnetic fingerprint. The Fe derivative exhibits the highest magnetization of 3.67 μB (S≈2) and an exchange coupling of 16 meV between the π‐electrons and the Fe d‐states.

EDA-NOCV analysis of carbene-borylene bonded dinitrogen complexes for deeper bonding insight: A fair comparison with a metal-dinitrogen system.

Binding of dinitrogen (N2 ) to a transition metal center (M) and followed by its activation under milder conditions is no longer impossible; rather, it is routinely studied in laboratories by transition metal complexes. In contrast, binding of N2 by main group elements has been a challenge for decades, until very recently, an exotic cAAC-borylene (cAAC=cyclic alkyl(amino) carbene) species showed similar binding affinity to kinetically inert and non-polar dinitrogen (N2 ) gas under ambient conditions. Since then, N2 binding by short lived borylene species has made a captivating news in different journals for its unusual features and future prospects. Herein, we carried out different types of DFT calculations, including EDA-NOCV analysis of the relevant cAAC-boron-dinitrogen complexes and their precursors, to shed light on the deeper insight of the bonding secret (EDA-NOCV=energy decomposition analysis coupled with natural orbital for chemical valence). The hidden bonding aspects have been uncovered and are presented in details. Additionally, similar calculations have been carried out in comparison with a selected stable dinitrogen bridged-diiron(I) complex. Singlet cAAC ligand is known to be an exotic stable species which, combined with the BAr group, produces an intermediate singlet electron-deficient (cAAC)(BAr) species possessing a high lying HOMO suitable for overlapping with the high lying π*-orbital of N2 via effective π-backdonation. The BN2 interaction energy has been compared with that of the FeN2 bond. Our thorough bonding analysis might answer the unasked questions of experimental chemists about how boron compounds could mimic the transition metal of dinitrogen binding and activation, uncovering hidden bonding aspects. Importantly, Pauling repulsion energy also plays a crucial role and decides the binding efficiency in terms of intrinsic interaction energy between the boron-center and the N2 ligand.

John-Teller Distortion in Octahedral hexaaquacopper (II) Transition for S=1/2, d9 Metal Complex

Jahn–Teller effect (JTE) describe how specific electrons distributions induce geometric deformation in molecules or ions. The distortion of Jahn–Teller (JT) takes place because of the uneven occupation of orbitals with the identical energies is disapproving. To prevent such inappropriate electronic arrangements, molecules distort (lose regularity), causing these orbitals to become non-degenerate. This distortion is most frequently seen in octahedral compounds, at which two axial bonds could be squeezed (compressed) or extend (elongated), leading to bond lengths that the different from equator bonds. The degeneracy of electronic states really isn’t accurately measured, but it would be measured precisely by the disparity in a concentration of electron among metal and the ligands upon on axil direction. The density functional theory has been used to calculate bond length in both cases geometry for , octahedral metal-complexes, the majority ordinary mechanism of distortion is elongation (z-out). In perfectly octahedral complexes deformation, the orbitals energy is changed more than the orbitals energy, causing elongation z-out distortion. The d-levels energy would be splitting in a ligand field throughout transition metal centers can frequently result in degenerate electron configurations that are subject to Jahn–Teller effects. In six-coordinate complexes of copper II ions. It is also significant in fewer regular oxidation levels of other transition ions with that count.

Altering Ligand Fields in Single-Atom Sites through Second-Shell Anion Modulation Boosts the Oxygen Reduction Reaction.

Single-atom catalysts based on metal-N4 moieties and anchored on carbon supports (defined as M-N-C) are promising for oxygen reduction reaction (ORR). Among those, M-N-C catalysts with 4d and 5d transition metal (TM4d,5d) centers are much more durable and not susceptible to the undesirable Fenton reaction, especially compared with 3d transition metal based ones. However, the ORR activity of these TM4d,5d-N-C catalysts is still far from satisfactory; thus far, there are few discussions about how to accurately tune the ligand fields of single-atom TM4d,5d sites in order to improve their catalytic properties. Herein, we leverage single-atom Ru-N-C as a model system and report an S-anion coordination strategy to modulate the catalyst's structure and ORR performance. The S anions are identified to bond with N atoms in the second coordination shell of Ru centers, which allows us to manipulate the electronic configuration of central Ru sites. The S-anion-coordinated Ru-N-C catalyst delivers not only promising ORR activity but also outstanding long-term durability, superior to those of commercial Pt/C and most of the near-term single-atom catalysts. DFT calculations reveal that the high ORR activity is attributed to the lower adsorption energy of ORR intermediates at Ru sites. Metal-air batteries using this catalyst in the cathode side also exhibit fast kinetics and excellent stability.

Ferrocenyl‐2‐pyridylimine derived d10‐configuration complexes as prospective co‐sensitizers in dye sensitized solar cells

Three complexes comprising ferrocenyl‐2‐pyridylimine and three d10 transition metal centers viz. Zn (II), Cd (II), and Hg (II) having formula [M (FcCH=NCH2(2‐py)] (M = Zn (II) (Zn‐Fc), Cd (II) (Cd‐Fc), and Hg (II) (Hg‐Fc); Fc = Ferrocene) are synthesized and their characterization have been done using microanalytical technique, FT‐IR and NMR spectroscopic methods. The optimized molecular geometries for all the three ferrocene‐based complexes revealed that the coordination geometries around the d10‐metal centers are distorted tetrahedral. The application of these complexes as sensitizers and co‐sensitizers/co‐adsorbents have been assessed in dye sensitized solar cells (DSSCs). Amid the three co‐sensitizers/co‐adsorbent based the DSSC set‐ups, the assembly fabricated using Zn‐Fc/N719 displayed superior performance with 5.71 ± 0.04% efficiency (η) indicating enhancement in the DSSC performance than to the set‐up constructed using solely N719 (η = 4.20 ± 0.05%).

Metallosurfactants Consisting of Amphiphilic Ligands and Transition Metals: Structure, Bonding, Reactivity, and Self-assembling Property

Metallosurfactants are emerging as a relatively new class of surfactants whose ligand moieties bind to various transition metals. Because transition metal centers are incorporated into the surfactant frameworks, they can form various self-assembled structures with metallic interfaces such as micelles, vesicles, and lyotropic liquid crystals. To reduce the lability of transition metal complexes under aqueous conditions, various amphiphilic ligands have been developed as surfactant frameworks. This review discusses some aspects of the design and chemical structures of amphiphilic ligands, as well as focus on various functions and types of chemical bonds present in metallosurfactants.

High performance Li-, Na-, and K-ion storage in electrically conducting coordination polymers.

Coordination polymers (CPs) made of redox-active organic moieties and metal ions emerge as an important class of electroactive materials for battery applications. However, the design and synthesis of high voltage alkali-cation reservoir anionic CPs remains challenging, hindering their practical applications. Herein, we report a family of electrically conducting alkali-cation reservoir CPs with the general formula of A2-TM-PTtSA (wherein A = Li+, Na+, or K+; TM = Fe2+, Co2+, or Mn2+; and PTtSA = benzene-1,2,4,5-tetra-methylsulfonamide). The incorporation of transition metal centers not only enables intrinsic high electrical conductivity, but also shows an impressive redox potential increase of as high as 1 V as compared to A4-PTtSA analogues, resulting in a class of organometallic cathode materials with a high average redox potential of 2.95–3.25 V for Li-, Na- and K-ion batteries. A detailed structure – composition – physicochemical properties – performance correlation study is provided relying on experimental and computational analysis. The best performing candidate shows excellent rate capability (86% of the nominal capacity retained at 10C rate), remarkable cycling stability (96.5% after 1000 cycles), outstanding tolerance to low carbon content (5 wt%), high mass loading (50 mg cm−2), and extreme utilisation conditions of low earth orbit space environment tests. The significance of the disclosed alkali-ion reservoir cathodes is further emphasized by utilizing conventional Li-host graphite anode for full cell assembly, attaining a record voltage of 3 V in an organic cathode Li-ion proof-of-concept cell.

Unveiling ultrafast dynamics in bridged bimetallic complexes using optical and X-ray transient absorption spectroscopies

In photosynthetic systems employing multiple transition metal centers, the properties of charge-transfer states are tuned by the coupling between metal centers. Here, we use ultrafast optical and X-ray spectroscopies to elucidate the effects of metal–metal interactions in a bimetallic tetrapyridophenazine-bridged Os(ii)/Cu(i) complex. Despite having an appropriate driving force for Os-to-Cu hole transfer in the Os(ii) moiety excited state, no such charge transfer was observed. However, excited-state coupling between the metal centers is present, evidenced by variations in the Os MLCT lifetime depending on the identity of the opposite metal center. This coupling results in concerted coherent vibrations appearing in the relaxation kinetics of the MLCT states for both Cu and Os centers. These vibrations are dominated by metal–ligand contraction at the Cu/Os centers, which are in-phase and linked through the conjugated bridging ligand. This study shows how vibronic coupling between transition metal centers affects the ultrafast dynamics in bridged, multi-metallic systems from the earliest times after photoexcitation to excited-state decay, presenting avenues for tuning charge-transfer states through judicious choice of metal/ligand groups.

Interaction of Trivalent Lanthanide (III) Cations with Bidentate Schiff Base (1E,2E) Diphenylethane-1,2- Hydrazone Oxime

The novel lanthanide (III) complexes [Ln(BMOH)3] (NO3) (Ln=Pr, Nd and La), where (BMOH)-= (1E,2E)-1,2- diphenylethane – 1,2 – diene hydrazone oxime, have been obtained by direct condensation reaction of the Schiff base HBMOH ligand and corresponding hydrated lanthanide (III) nitrates in ethanol. All prepared complexes were characterized by traditional physico-chemical and spectral techniques. In the molecular structure of synthesized complexes, central inner transition metal ions are surrounded by all donor atoms of the HBMOH ligand (two nitrogen donor atoms) and nitrogen atom belonging to monodentatae chelating nitrito ligand.

Quasi-Phthalocyanine Conjugated Covalent Organic Frameworks with Nitrogen-Coordinated Transition Metal Centers for High-Efficiency Electrocatalytic Ammonia Synthesis.

Developing high-performance nitrogen reduction reaction (NRR) electrocatalysts is an ongoing challenge. Herein, we report a pyrolysis-free synthetic method for introducing ordered quasi-phthalocyanine N-coordinated transition metal (Ti, Cu, or Co) centers into a conjugated two-dimensional (2D) covalent organic framework (COF) for enhanced NRR performance. Detailed experiments and characterizations revealed that the NRR activity of Ti-COF was clearly better than that of Cu-COF and Co-COF, because of the superior abilities of Ti metal centers in activating inert N2 molecules and suppressing the hydrogen evolution reaction (HER). The resulting Ti-COF exhibits a high NH3 yield of 26.89 μg h-1 mg-1cat. and a Faradaic efficiency of 34.62% for NRR. Density functional theory (DFT) calculations verify that Ti-COF can effectively adsorb and activate N2 molecules and inhibit HER compared with Cu-COF, Co-COF, and pristine COF catalysts. This work opens a new avenue for developing 2D-COF materials that contain abundant coordinated transition metal centers toward electrocatalytic NRR.

Understanding the Binding Properties of N‐heterocyclic Carbenes through BDE Matrix App

AbstractThe interaction of N‐heterocyclic carbene (NHC) ligands with transition metal centers is analyzed with a set of descriptors derived from the statistical treatment of density functional theory (DFT) computational results. These descriptors, labeled as hidden descriptors (HD), had been previously defined in our group and here are applied with the help of a user‐friendly web application developed for that purpose: BDE Matrix App. Five HDs are computed with little computational effort for each NHC under consideration. Expectations are confirmed in that the binding to the metal center is largely ruled by the first two descriptors, suitably associated to σ donation and π backdonation, and the approach leads to a straightforward comparison of their quantitative values. The study is extended beyond NHCs, and other neutral ligands, such as cyclopropenylidenes, phosphines, and amines are considered. The procedure is shown to be well suited to provide a unified framework for the comparison of these diverse ligands.

A Free Aluminylene with Diverse σ-Donating and Doubly σ/π-Accepting Ligand Features for Transition Metals*.

We report herein the synthesis, characterization, and coordination chemistry of a free N-aluminylene, namely a carbazolylaluminylene 2 b. This species is prepared via a reduction reaction of the corresponding carbazolyl aluminium diiodide. The coordination behavior of 2 b towards transition metal centers (W, Cr) is shown to afford a series of novel aluminylene complexes 3-6 with diverse coordination modes. We demonstrate that the tri-active ambiphilic Al center in 2 b can behave as: 1. a σ-donating and doubly π-accepting ligand; 2. a σ-donating, σ-accepting and π-accepting ligand; and 3. a σ-donating and doubly σ-accepting ligand. Additionally, we show ligand exchange at the aluminylene center providing access to the modulation of electronic properties of transition metals without changing the coordinated atoms. Investigations of 2 b with IDippCuCl (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) show an unprecedented aluminylene-alumanyl transformation leading to a rare terminal Cu-alumanyl complex 8. The electronic structures of such complexes and the mechanism of the aluminylene-alumanyl transformation are investigated through density functional theory (DFT) calculations.

A Free Aluminylene with Diverse σ‐Donating and Doubly σ/π‐Accepting Ligand Features for Transition Metals**

AbstractWe report herein the synthesis, characterization, and coordination chemistry of a free N‐aluminylene, namely a carbazolylaluminylene 2 b. This species is prepared via a reduction reaction of the corresponding carbazolyl aluminium diiodide. The coordination behavior of 2 b towards transition metal centers (W, Cr) is shown to afford a series of novel aluminylene complexes 3–6 with diverse coordination modes. We demonstrate that the tri‐active ambiphilic Al center in 2 b can behave as: 1. a σ‐donating and doubly π‐accepting ligand; 2. a σ‐donating, σ‐accepting and π‐accepting ligand; and 3. a σ‐donating and doubly σ‐accepting ligand. Additionally, we show ligand exchange at the aluminylene center providing access to the modulation of electronic properties of transition metals without changing the coordinated atoms. Investigations of 2 b with IDippCuCl (IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) show an unprecedented aluminylene‐alumanyl transformation leading to a rare terminal Cu‐alumanyl complex 8. The electronic structures of such complexes and the mechanism of the aluminylene‐alumanyl transformation are investigated through density functional theory (DFT) calculations.

Metal Centers as Nucleophiles: Oxymoron of Halogen Bond-Involving Crystal Engineering.

This review highlights recent studies discovering unconventional halogen bonding (HaB) that involves positively charged metal centers. These centers provide their filled d‐orbitals for HaB, and thus behave as nucleophilic components toward the noncovalent interaction. This role of some electron‐rich transition metal centers can be considered an oxymoron in the sense that the metal is, in most cases, formally cationic; consequently, its electron donor function is unexpected. The importance of Ha⋅⋅⋅d‐[M] (Ha=halogen; M is Group 9 (Rh, Ir), 10 (Ni, Pd, Pt), or 11 (Cu, Au)) interactions in crystal engineering is emphasized by showing remarkable examples (reported and uncovered by our processing of the Cambridge Structural Database), where this Ha⋅⋅⋅d‐[M] directional interaction guides the formation of solid supramolecular assemblies of different dimensionalities.

Reexamining the Formation of Environmentally Persistent Free Radicals from Aromatic Molecules on Metal Oxides

The subject is a debate on the formation mechanism of the environmentally persistent free radicals (EPFR), by interaction of aromatics with transition metal oxides. Literature reports contrasting opinions concerning the stabilization of the phenoxyradicals mainly as concerns the suggested reduction of interacting transition metal centers. The present paper, in accordance with the mechanisms assessed by a number of spectroscopic and computational results, excludes the necessity that the catalytic mechanism involves the reduction of the metal centers by interaction with the aromatics.Graphic

Determination of peracetic acid by an Ag nanoparticle decorated Cu-organic framework modified electrode

Peracetic acid (PAA) is commonly used as the strong oxidant in many areas. However, high concentration of PAA can be harmful to health of people. The quantitative determination of PAA is of great significance. Here, a metal–organic framework (MOF) is introduced into the electrochemical sensor for PAA detection. The influence of common transition metal centers on the performance of electrochemical detection of PAA was investigated and the results showed the Cu-1,3,5-trimesic acid (Cu-BTC) was the best original MOF. Ag nanoparticles were decorated in the Cu-BTC to further improve the electrocatalytical activity. The concentration of the added Ag salt and the quality of the reducing agent tannic acid (TA) had a great impact on the performance. The final sample optimized for the synthesis conditions was named as Ag NPs/Cu-BTC. The electrochemical sensor based on Ag NPs/Cu-BTC/GCE was fabricated for sensing PAA with a wide linear range (0.1–5000 μM), low detection limit (0.046 μM, S/N = 3) and high sensitivity (1013.4 μA mM−1 cm−2). The water stability of Ag NPs/Cu-BTC and the anti-interference ability of the sensor were good. The electrochemical sensor could be applied in detecting the concentration of PAA in the commercially available peracetic acid, suggesting the practical application value.

Tuning the Electrical and Thermoelectric Properties of Phthalocyanine and Metallo-Phthalocyanine Molecular Junction

In this work, a comparative study is presented that analyses the electrical and thermoelectrical properties of free base-Phthalocyanine (Bare-Pc) and Metallo-Phthalocyanine (MPc) molecule sandwiched between gold electrodes. The study investigates the transmission and conductance characteristics of Bare-Pc without central metal ion and with different transition metal ions at the centre of (Pc) such as (Mn, Co, Ni, and Zn) in two different molecular configurations, cis and trans. The results reveal that the conductance changed by varying the transition metal-centre in the order of Co > Ni > Zn in trans, and closely similar pattern Co > Ni > Mn in cis with higher values in trans configuration. This reflects the correlation between the spin-dependent transport properties with the transition metal-centre type and the molecular structure. Further, thermoelectrical properties such as thermal conductance (k), Seebeck coefficient (S) and figure of merit (ZT) are investigated. It was found that (k), (S) and (ZT) have higher values in case of trans configuration for Co-Pc, Ni-Pc and Zn-Pc structures. Thus, by varying the transition metal-centre type and the molecular configuration of (Pc) molecule, transport properties can be tuned to produce a good value of the transmission and electrical conductance, thermal conductance, Seebeck coefficient, and figure of merit of the (Pc) molecular junction for potential high conductance and efficient thermoelectric applications.

On the Accuracy of Mean-Field Spin-Orbit Operators for 3d Transition-Metal Systems.

We present an extensive study of the performance of mean-field approximations to the spin-orbit operators on realistic molecular systems, as widely used in applications like single-molecule magnets, molecular quantum bits, and molecular spintronic devices. The test systems feature a 3d transition-metal center ion (V, Cr, Mn, Fe, Co, and Ni) in various coordinations and a multitude of energetically close-lying open-shell configurations that can couple via the spin-orbit operator. We performed complete active space spin-orbit configuration interaction calculations and compared the full two-electron Breit-Pauli spin-orbit operator to different approximations: the one-center approximation, the spin-orbit mean-field approach with electron densities from different state-averaging procedures, and the atomic mean-field integral approximation. We show that the mean-field approaches can lead to significant errors in the spin-orbital coupling matrix elements, which becomes particularly visible for the computed zero-field splittings. The one-center approximation, keeping all relevant two-electron terms, seems to be a significantly more accurate choice for the examples from our test set.

Metal-Free Bond Activation by Carboranyl Diphosphines.

We report metal-free bond activation by the carboranyl diphosphine 1-PtBu2-2-PiPr2-C2B10H10. This main group element system contains basic binding sites and possesses the ability to cycle through two-electron redox states. The reported reactions with selected main group hydrides and alcohols occur via the formal oxidation of the phosphine groups and concomitant reduction of the boron cage. These transformations, which are driven by the cooperation between the electron-donating exohedral substituents and the electron-accepting cluster, differ from those of "regular" phosphines and are reminiscent of oxidative addition to transition metal centers, thus representing a new approach to metal-free bond activation.