Square-planar Configuration Research Articles

Square-Planar Nickel Bis(phosphinopyridyl) Complexes for Long-Lived Photocatalytic Hydrogen Evolution.

Phosphinopyridyl ligands are used to synthesize a class of Ni(II) bis(chelate) complexes, which have been comprehensively characterized in both solid and solution phases. The structures display a square-planar configuration within the primary coordination sphere, with axially positioned labile binding sites. Their electrochemical data reveal two redox couples during the reduction process, suggesting the possibility of accessing two-electron reduction states. Significantly, these complexes serve as robust catalysts for homogeneous photocatalytic H2 evolution. In a system utilizing an organic photosensitizer and a sacrificial electron donor, an optimal turnover number of 27,100 is achieved in an alcohol-containing aqueous solution. A series of photophysical and electrochemical measurements were conducted to elucidate the reaction mechanism of photocatalytic hydrogen generation. Density function theory calculations propose a catalytic pathway involving two successive one-electron reduction steps, followed by two proton discharges. The sustained photocatalytic activity of these complexes stems from their distinct ligand system, which includes phosphine and pyridine donors that aid in stabilizing the low oxidation states of the Ni center.

Organoplatinum Complex Exhibiting Aggregation-Enhanced Emission (AEE) and Dual-Channel Ion-Sensing Properties by Terminating the Molecular Configuration Transformation (MCT) and Excited-State Intramolecular Proton Transfer (ESIPT).

Emitters produce weak emissions when they undergo structural changes such as molecular configuration transformation (MCT) or excited-state intramolecular proton transfer (ESIPT) but give out strong emissions after terminating these distortions. Herein, an organoplatinum complex, Pt-ppy-ABP, carrying a salicylaldehyde-based Schiff base unit is synthesized. It exhibits weak emission in dilute solutions but shows bright emission at the aggregated state or after interacting with F- and Zn2+. This suggests that it has an aggregation-enhanced emission (AEE) property and holds potential in ion detection. Supported by theoretical calculations and femtosecond transient absorption results, this complex suffers excited-state structural changes including MCT from a square-planar configuration to a tetrahedral one, as well as intramolecular rotation of a monodentate ligand and ESIPT, showing weak emission in its solutions. At the aggregated state, it releases strong yellow emissions because of the restraints of MCT and ligand rotation. Upon interacting with F- or Zn2+, it emits bright-red or -green emissions, achieving detection limits of 10-7 M. The sensing mechanism is concluded as deprotonation- and coordination-induced ESIPT terminations, respectively. Given its AEE property and ion-responsive emissions, its application in information encryption is also explored. Finally, these findings should provide valuable clues for the developments of chemosensors with dual-channel recognition abilities.

Atomically dispersed asymmetric cobalt electrocatalyst for efficient hydrogen peroxide production in neutral media

Electrochemical hydrogen peroxide (H2O2) production (EHPP) via a two-electron oxygen reduction reaction (2e- ORR) provides a promising alternative to replace the energy-intensive anthraquinone process. M-N-C electrocatalysts, which consist of atomically dispersed transition metals and nitrogen-doped carbon, have demonstrated considerable EHPP efficiency. However, their full potential, particularly regarding the correlation between structural configurations and performances in neutral media, remains underexplored. Herein, a series of ultralow metal-loading M-N-C electrocatalysts are synthesized and investigated for the EHPP process in the neutral electrolyte. CoNCB material with the asymmetric Co-C/N/O configuration exhibits the highest EHPP activity and selectivity among various as-prepared M-N-C electrocatalyst, with an outstanding mass activity (6.1 × 105 A gCo−1 at 0.5 V vs. RHE), and a high practical H2O2 production rate (4.72 mol gcatalyst−1 h−1 cm−2). Compared with the popularly recognized square-planar symmetric Co-N4 configuration, the superiority of asymmetric Co-C/N/O configurations is elucidated by X-ray absorption fine structure spectroscopy analysis and computational studies.

Crystal structure of bis{2-[5-(3,4,5-trimethoxyphenyl)-4H-1,2,4-triazol-3-yl]pyridine}palladium(II) bis(trifluoroacetate) trifluoroacetic acid disolvate

The new palladium(II) complex, [Pd(C16H16N4O3)2](CF3COO)2·2CF3COOH, crystallizes in the triclinic space group P\overline{1} with the asymmetric unit containing half the cation (PdII site symmetry Ci ), one trifluoroactetate anion and one co-crystallized trifluoroacetic acid molecule. Two neutral chelating 2-[5-(3,4,5-trimethoxyphenyl)-4H-1,2,4-triazol-3-yl]pyridine ligands coordinate to the PdII ion through the triazole-N and pyridine-N atoms in a distorted trans-PdN4 square-planar configuration [Pd—N 1.991 (2), 2.037 (2) Å; cis N—Pd—N 79.65 (8), 100.35 (8)°]. The complex cation is quite planar, except for the methoxo groups (δ = 0.117 Å for one of the C atoms). The planar configuration is supported by two intramolecular C—H...N hydrogen bonds. In the crystal, the π–π-stacked cations are arranged in sheets parallel to the ab plane that are flanked on both sides by the trifluoroacetic acid–trifluoroacetate anion pairs. Apart from classical N/O—H...O hydrogen-bonding interactions, weak C—H...F/N/O contacts consolidate the three-dimensional architecture. Both trifluoroacetic moieties were found to be disordered over two resolvable positions with a refined occupancy ratio of 0.587 (1):0.413 (17) and 0.530 (6):0.470 (6) for the protonated and deprotonated forms, respectively.

Iodonium cation stabilizes square-planar configuration of the silver(I) tetratriflate

In this work, we have experimentally and theoretically shown that halogen bonding can stabilize the rare example of square-planar configuration of the silver(I) center. The crystallographic analysis of tris-dibenziodolium silver(I) tetratriflate reveals two types of halogen bonding interactions between the triflate ligands and the dibenziodolium cation, which might stabilize the such square-planar coordination geometry. Theoretical calculations confirm the noncovalent nature of the I⋅⋅⋅O contacts and provide insights into their electronic properties.

(S)-(+)-1-(4-Bromophenyl)-N-[(4-methoxyphenyl)methylidene]ethylamine and bis{(S)-(+)-1-(4-bromophenyl)-N-[(4-methoxyphenyl)methylidene]ethylamine-κN}dichloridopalladium(II)

The (S)-(+)-1-(4-bromophenyl)-N-[(4-methoxyphenyl)methylidene]ethylamine ligand, C16H16BrNO, (I), was synthesized through the reaction of 4-methoxyanisaldehyde with (S)-(−)-1-(4-bromophenyl)ethylamine. It crystallizes in the orthorhombic space group P212121 belonging to the Sohncke group, featuring a single molecule in the asymmetric unit. The refinement converged successfully, achieving an R factor of 0.0508. The PdII complex bis{(S)-(+)-1-(4-bromophenyl)-N-[(4-methoxyphenyl)methylidene]ethylamine-κN}dichloridopalladium(II), [PdCl2(C16H16BrNO)2], (II), crystallizes in the monoclinic space group P21 belonging to the Sohncke group, with two molecules in the asymmetric unit. The central atom is tetracoordinated by two N atoms and two Cl atoms, resulting in a square-planar configuration. The imine moieties exhibit a trans configuration around the PdII centre, with average Cl—Pd—N angles of approximately 89.95 and 90°. The average distances within the palladium complex for the two molecules are ∼2.031 Å for Pd—N and ∼2.309 Å for Pd—Cl.

(NH4 )2 (I5 O12 )(IO3 ) and K1.03 (NH4 )0.97 (I5 O12 )(IO3 ): Mixed-Valent Polyiodates with Unprecedented I5 O12 - Unit Exhibiting Strong Second-Harmonic Generation Responses and Giant Birefringence.

Second-harmonic generation (SHG) response and birefringence are crucial properties for linear and nonlinear optical (NLO) materials, while it is difficult to further optimize these two key properties by using a single traditional functional building block (FBB) in one compound. Herein, a novel IO4 5- unit is identified, which possesses a square-planar configuration and two stereochemically active lone-pairs (SCALPs). By combining IO4 5- and IO3 - units, the first examples of mixed-valent polyiodates featuring an unprecedented bowl-shaped I5 O12 - polymerized unit, namely (NH4 )2 (I5 O12 )(IO3 ) and K1.03 (NH4 )0.97 (I5 O12 )(IO3 ), are successfully synthesized. Excitingly, both crystals exhibit strong SHG responses (16 × KDP and 19.5 × KDP @1064nm) as well as giant birefringence (∆nexp =0.431 and 0.405 @546nm). Detailed structure-property analyses reveal that the parallel aligned planar IO4 5- units induce the properly aligned high-density SCALPs, leading to strong SHG response and giant birefringence for both materials. This work not only provides two new potential NLO and birefringent crystals, but also discovers a novel promising FBB (IO4 5- ) for developing high-performance linear and nonlinear optical materials.

Cation-/Ligand-Induced Solvent-Assisted Transformations of Zn(II) and Cu(II) Complexes Featuring Single-Pocket Multidentate Chelating Members

A new family of single-pocket metal complexes bearing O,N,O-tridentate and O,N-bidentate chelating members {Cu, 1b (P21/n); Ni, 1c (C2/c); Mn, 1d (I2/a); Cu, 2b; and Ni, 2c (both P21/c)}, starting from synthesized and fully characterized Zn(II) (1a; I2/a) and Cu(II) (2a; C2) precursors, were conveniently prepared via cation-induced solvent-assisted and ligand-induced solvent-assisted transformations. Herein, we show multistep solvent-assisted transformations from cis-1a → trans-1b → cis-1c → cis-1d, as well as all-trans 2a → 2b → 2c. All processes are one-way irreversible, as substantiated by thermodynamic aspects (enthalpies based on Gibbs free energies) derived from density functional theory calculations. On the other hand, complex 2a′ (C2/c; a polymorphic form of 2a) was obtained through a routine synthetic procedure. The compounds have been established by various spectroscopic techniques (infrared, UV–vis, ESI-MS, 1H, and 13C NMR), elemental analysis, and X-ray crystallography. Single-crystal X-ray studies reveal that complexes 1a–d exhibit a pseudo-octahedral geometry around each metal center, with 2a displaying a four-coordinate seesaw geometry Cu(II) sphere (Addison parameter; τ = 0.42), while 2a′ (τ = 0.00), 2b (τ = 0.00), and 2c (τ = 0.00) possess a perfect square-planar configuration around each metal center. Furthermore, distortion is stabilized by the presence of peripheral O-donor atoms from the bulky −OMe group, and by virtue of its size, increased bond lengths and angles are accommodated. Ligand substitution induced coordination geometry transformation from quasi-square-planar 2a to perfect square-planar 2b. Assessment of the metric parameter shows that the distances between the two Cu–Omethoxy are all largely positive due to Jahn–Teller distortion, indicating an unprecedented tetragonal bipyramidal geometry in 1b.

Tuning Fe Spin Moment in Fe-N-C Catalysts to Climb the Activity Volcano via a Local Geometric Distortion Strategy.

As the most promising alternative to platinum-based catalysts for cathodic oxygen reduction reaction (ORR) in proton exchange membrane fuel cells, further performance enhancement of Fe-N-C catalysts is highly expected to promote their wide application. In Fe-N-C catalysts, the single Fe atom forms a square-planar configuration with four adjacent N atoms (D4h symmetry). Breaking the D4h symmetry of the FeN4 active center provides a new route to boost the activity of Fe-N-C catalysts. Herein, for the first time, the deformation of the square-planar coordination of FeN4 moiety achieved by introducing chalcogen oxygen groups (XO2 , X=S, Se, Te) as polar functional groups in the Fe-N-C catalyst is reported. The theoretical and experimental results demonstrate that breaking the D4h symmetry of FeN4 results in the rearrangement of Fe 3d electrons and increases spin moment of Fe centers. The efficient spin state manipulation optimizes the adsorption energetics of ORR intermediates, thereby significantly promoting the intrinsic ORR activity of Fe-N-C catalysts, among which the SeO2 modified catalyst lies around the peak of the ORR volcano plot. This work provides a new strategy to tune the local coordination and thus the electronic structure of single-atom catalysts.

Biological activity of bis-(morpholineacetato)palladium(II) complex: Preparation, structural elucidation, cytotoxicity, DNA-/serum albumin-interaction, density functional theory, in-silico prediction and molecular modeling

In an effort to discover a novel potential bioactive compound, a mono-nuclear Pd(II) complex with an amino acid derivative as ligand was synthesized and characterized through experimental and computational methodologies. A square-planar configuration was suggested for palladium(II) complex utilizing density functional theory. MEP map and Mulliken atomic charge were detected electrophilic and nucleophilic regions of the compound for reactions. The lipophilicity and cytotoxic activity of the complex was more effective than cisplatin. Also, OSIRIS DataWarrior revealed proper oral bioavailability and good drug-likeness for the compound. In-vitro binding behavior of the Pd(II) complex with DNA and serum albumin (BSA) were fully determined via variety of procedures including fluorescence, UV–Vis, CD, viscosity, gel electrophoresis experiments and molecular simulation. The negative signs of ΔH° and ΔS° for Pd(II) complex-CT-DNA/-BSA systems indicated the existence of hydrogen bonding/van der Waals interactions for both binding systems. Additionally, docking simulation illustrated the interaction of Pd(II) complex with the minor groove of DNA and the hydrophobic cavity of the BSA (drug binding site I).

NiII mol-ecular complex with a tetra-dentate amino-guanidine-derived Schiff base ligand: structural, spectroscopic and electrochemical studies and photoelectric response.

The new mol-ecular nickel(II) complex, namely, {4-bromo-2-[({N'-[(2-oxidobenzylidene)amino]carbamimidoyl}imino)methyl]phenolato}nickel(II) N,N-di-methyl-formamide solvate monohydrate, [Ni(C15H11BrN4O2)]·C3H7NO·H2O, (I), crystallizes in the triclinic space group P with one mol-ecule per asymmetric unit. The guanidine ligand is a product of Schiff base condensation between amino-guanidine, salicyl-aldehyde and 5-bromo-salicyl-aldehyde templated by Ni2+ ions. The chelating ligand mol-ecule is deprotonated at the phenol O atoms and coordinates the metal centre through the two azomethine N and two phenolate O atoms in a cis-NiN2O2 square-planar configuration [average(Ni-N/O) = 1.8489 Å, cis angles in the range 83.08 (5)-95.35 (5)°, trans angles of 177.80 (5) and 178.29 (5)°]. The complex mol-ecule adopts an almost planar conformation. In the crystal, a complicated hydrogen-bonded network is formed through N-H⋯N/O and O-H⋯O inter-molecular inter-actions. Complex (I) was also characterized by FT-IR and 1H NMR spectroscopy. It undergoes an NiII ↔ NiIII redox reaction at E 1/2 = +0.295 V (vs Ag/AgCl) in methanol solution. In a thin film with a free surface, complex (I) shows a fast photoelectric response upon exposure to visible light with a maximum photovoltage of ∼178 mV.

An isolable, crystalline complex of square-planar silicon(IV)

SummaryThe structure and reactivity of silicon(IV), the second most abundant element in our Earth's crust, is determined by its invariant tetrahedral coordination geometry. Silicon(IV) with a square-planar configuration (ptSiIV) represents a transition state. Quantum theory supported the feasibility of stabilizing ptSiIV by structural constraint, but its isolation has not been achieved yet. Here, we present the synthesis and full characterization of the first square-planar coordinated silicon(IV). The planarity provokes an extremely low-lying unoccupied molecular orbital that induces unusual silicon redox chemistry and CH-agostic interactions. The small separation of the frontier molecular orbitals enables visible-light ligand-element charge transfer and bond-activation reactivity. Previously, such characteristics have been reserved for d-block metals or low-valent p-block elements. Planarization transfers them, for the first time, to a p-block element in the normal valence state.

Reactivity of styrene with tert-butyl hydroperoxide over cu-based double hydroxide catalysts

Copper(II) ions are inserted into brucite-like sheets in the Zn-Cu-Al ternary hydroxides via the precipitation route. The amount of octahedral Cu(II) sites in the Zn-Al(OH) interlayers depends on the copper loadings. The square-planar configuration (CuO) was observed in the high-copper content samples. The Cu-inserted zinc aluminum hydroxides have lamellar structure, modest surface area, and hexagonally uniform plate-like particles. All synthesized hydroxides were tested for the liquid-phase oxidation of styrene. The overall conversion of styrene obtained about 60–80 % and styrene oxide selectivity is about 70 %. The catalytic activity is correlated with the electronic configurations of copper(II) ions and reaction variables. The copper(II) ions in the brucite-like sheets proceeded through the oxidation of styrene following the metal-superpoxo pathway while the extra-lattice copper(II) ions underwent reaction with free radical route. The experimental results also indicated Cu-intercalated layered double hydroxide catalysts are more active than copper(II) oxide in the liquid-phase oxidation of styrene under the same conditions.

Stereochemistry of Pt(η2-P2L)Cl2 derivatives

AbstractThe coordination chemistry of platinum (Pt) covers a huge field as shown by a recent survey covering the crystallographic and structural data of almost 200 monomeric examples of the Pt(η2-P2L)Cl2type. About 20% of these complexes exist as isomers and are discussed in this review. Included are distortion (64%),cis-trans(20%), and ligand (16%) isomers. These are discussed in terms of the metallocyclic rings, and coordinations are drawn between donor atoms, bond lengths, and interbond angles, with attention to anytrans-influence. Distortion isomers, differing only by the degree of distortion in Pt-L and L-Pt-L angles, are the most numerous. In this series of distortion and ligand isomers, the square-planar configuration about the Pt(II) atom has onlycis-configuration. The total mean values of Pt-L bond distances (cis- vs.trans-configuration) are 2.228 Å (P) and 2.353 Å (Cl) vs. 2.312 (P) and 2.302 Å (Cl).

Synthesis, characterization and antitumor activity of novel gold (III) compounds with cisplatin-like structure

Gold(III) centers are isoelectronic to Pt(II) compounds and adopt square-planar configurations similar to that of cisplatin, gold (III) compounds could display strong effects of tumor cell growth inhibition by a non-cisplatin-like mode of action. In our present work, we synthesized and characterized three gold(III) complexes which are coordinated by the N atom of amide group and investigated their in vitro anticancer activity, including their mechanism of action. In screening their in vitro activity, we found three gold complexes to exhibit selectivity of cytotoxicity, and complex 3 display better anticancer activity than another two gold(III) compounds and cisplatin. The three gold (III) complexes show preference to accumulation in mitochondria. The complex 3 could inhibit the growth of tumor cell by inducing the apoptosis mediated by ER-stress through loss of cellular homeostasis disrupts Ca2+ signaling. Based on our results, we believe complex 3 to be a promising anticancer agent or lead compound for further anticancer drug development.

On the Geometrical Stability of Square-Planar Platinum(0) Complexes That Bear a PNP-Pincer-Type Phosphaalkene Ligand (Eind2 -BPEP).

The four-coordinate Pt0 complex [Pt(PPh3 )(Eind2 -BPEP)] (Eind=1,1,3,3,5,5,7,7-octaethyl-1,2,3,5,6,7-hexahydro-s-indacen-4-yl; BPEP=2,6-bis(1-phenyl-2-phosphaethenyl)pyridine), which bears a PNP-pincer-type phosphaalkene ligand (Eind2 -BPEP; PNP=N,N-bis(diphenylphosphine)-2,6-diaminopyridine), were found to adopt a square-planar configuration around the Pt center (τ4 =0.11). This coordination geometry is very uncommon for formal d10 complexes. In this study, a series of ligands with different electronic properties (i.e., DMAP, 2,6-lutidine, PMe3 , tBuNC, and CO) were introduced in place of PPh3 , and their effects on the coordination geometry were examined. X-ray diffraction analysis revealed that all complexes adopted a square-planar configuration (τ4 =0.20-0.27). In contrast, DFT calculations indicated that the geometrical stability towards distortion around Pt varied with the ligand. The complexes with pyridine-based ligands had rigid planar structures, whereas those with π-accepting ligands, such as CO, were relatively flexible towards distortion. The electronic effects of the ligands were reflected in the spectroscopic properties of the complexes, which showed a large color change in the near-infrared region.

Role of lattice oxygen content and Ni geometry in the oxygen evolution activity of the Ba-Ni-O system

The activity of oxides towards the oxygen evolution reaction (OER) is usually tuned by changing the transition metal components and/or the surface facets. An important yet less studied feature is the repercussion of the lattice oxygen content of the oxide in the active site's coordination and catalytic performance. This is illustrated here for the Ba-Ni-O system. We synthesized two oxides with Ni in different coordination and oxidation states, namely BaNiO2 with Ni2+ in square-planar positions, and BaNiO2.78(2) with Ni3+ and Ni4+ in octahedral positions. We show that the square-planar configuration of Ni endows BaNiO2 with high intrinsic OER activity, comparable to the best catalysts in the literature. DFT indicates that progressively lowering the lattice oxygen content from BaNiO3 to BaNiO2 increases the Ni sites' affinity for the reaction intermediates, thereby lowering the OER overpotential. Thus, oxygen content is an important parameter in oxide catalysts, as it modulates the coordination, orbital splitting, oxidation number, and catalytic activity of the active sites.

Local Environment Sensitivity of the Cu K-Edge XANES Features in Cu-SSZ-13: Analysis from First-Principles

Cu K-edge X-ray absorption near-edge spectra (XANES) have been widely used to study the properties of Cu-SSZ-13. In this Letter, the sensitivity of the XANES features to the local environment for a Cu+ cation with a linear configuration and a Cu2+ cation with a square-linear configuration in Cu-SSZ-13 is reported. When a Cu+ cation is bonded to H2O or NH3 in a linear configuration, the XANES has a strong peak at around 8983 eV. The intensity of this peak decreases as the linear configuration is broken. As for the Cu2+ cations in a square-planar configuration with a coordination number of 4, two peaks at around 8986 and 8993 eV are found. An intensity decrease for both peaks at around 8986 and 8993 eV is found in an NH3_4_Z2Cu model as the N-Cu-N angle changes from 180 to 100°. We correlate these features to the variation of the 4p state by PDOS analysis. In addition, the feature peaks for both the Cu+ cation and Cu2+ cation do not show a dependence on the Cu-N bond length. We further show that the feature peaks also change when the coordination number of the Cu cation is varied, while these feature peaks are independent of the zeolite topology. These findings help elucidate the experimental XANES features at an atomic and an electronic level.

Ligand-Controlled Electron Structure of Catalytically Active Ni Complexes.

We have performed a systematic study of the electron structure of a series of Ni(I) and Ni(II) iodo and methyl complexes with a variety of di- and tridentate nitrogen ligands to study the influence of these ligands in the structure of catalytically active complexes in cross-coupling reactions. Ni(II) compounds show the expected square-planar configuration typical of complexes of d8 metals, regardless of the kind of coordinating nitrogen atom (sp2 or sp3) found in ligands derived from either trialkylamines or pyridines. In contrast, Ni(I) complexes show different structures. Thus, the absence of orbitals capable of delocalizing the unpaired electron (such as in TMEDA and PMDTA derivatives) leads to nonplanar iodo or methyl tetracoordinate complexes. In contrast, the presence of ligands derived from pyridine allows delocalization of the unpaired electron on the ligand. This delocalization is especially effective for terpyridine species.

Characterization, crystal structure determination, and luminescent properties of [Pd(4,4′-bit)Cl2

Complex [Pd(4,4′-bit)Cl2] (1) is synthesized by the reaction of PdCl2 and 4,4′-bithiazole (4,4′-bit) in acetonitrile and crystallized by vapor diffusion of methanol into a dimethyl sulfoxide solution of the complex. This complex is thoroughly characterized by elemental analysis, IR, 1H NMR, UV-Vis, and luminescence spectroscopy and its structure is studied by the single crystal X-ray diffraction method. The X-ray structure determination shows that in the structure of this complex, the Pd(II) atom is fourcoordinated in a slightly distorted square-planar configuration by two nitrogen atoms from a bidentate 4,4′-bithiazole ligand and two chloride ions.