Rigid Ligand Research Articles

Denitrogenative Ring-Contraction of Pyridines to a Cyclopentadienyl Skeleton at a Dititanium Hydride Framework.

Selective removal of the nitrogen atom from an aromatic N-heterocycle, such as pyridine, is of significant interest and importance, yet it remains highly challenging. Here, we report an unprecedented denitrogenative ring-contraction reaction of pyridines at a dititanium hydride framework, yielding cyclopentadienyl and nitride species under mild conditions. The reaction of pyridine with a dititanium tetrahydride complex (1) bearing rigid acridane-based PNP-pincer ligands at room temperature produced a cyclopentadienyl/nitride complex (2), in which the two Ti atoms are bridged by a nitride atom and one Ti atom is bonded to a cyclopentadienyl group formed by pyridine denitrogenation and ring-contraction. The reactions of 2-, 3-, and 4-methylpyridines with 1 under similar conditions yielded the same product (3), a methylcyclopentadienyl-ligated analog of 2. When 2,4- or 3,5-dimethylpyridine reacted with 1 at 60 °C, the 1,3-dimethylcyclopentadienyl-ligated analog (5) formed almost quantitatively. The mechanistic details have been elucidated by isolation of key intermediates and density functional theory calculations. It was revealed that the reaction proceeded via coordination of the N atom of a pyridine unit to a Ti atom in 1 followed by H2 release, C═N reduction, two C-N bond cleavage (ring-opening and denitrogenation), and C-C coupling (ring closing). The involvement of C-H activation in an isopropyl group of a PNP ligand at the later stages of the reaction significantly contributed to the stabilization of the denitrogenative ring-contraction product. This work not only provides unprecedented mechanistic insights into denitrogenation of aromatic N-heterocycles but also represents a novel example of skeletal editing of aromatic N-heterocycles.

Rigid Ligand Confined Synthesis of Carbon Supported Dimeric Fe Sites with High-Performance Oxygen Reduction Reaction Activity for Quasi-Solid-State Rechargeable Zn-Air Batteries.

Dimeric metal sites (DiMSs) in carbon-based single atom catalysts (SACs) offer distinct advantages in optimizing the adsorption energies of the catalytic intermediates and reaction pathways over single atom sites, which inspires the investigations on the rational design of DiMSs-based SACs and the accurate discernment of catalytic mechanisms. Here, dimeric Fe sites on carbon blacks (DiFe-N/CBs) are prepared using the precursor of metal-organic complex with a controlled structure, and the rigid ligand confinement secures the preservation of dimeric Fe sites during the thermal treatment. DiFe-N/CBs shows excellent electrocatalytic performance for oxygen reduction reaction (ORR) with a high half-wave potential of 0.917 V, and excellent durability with negligible activity decay. Theoretical studies reveal that the dimeric Fe sites have an optimal adsorption of OOH* with the Yeager-type binding, illustrating the advantages of DiMSs over SAs in catalyzing ORR. The rechargeable aqueous and quasi-solid-state Zn-air batteries assembled using DiFe-N/CBs-based air cathodes achieve small voltage gaps after long term charge/discharge test, showing great promises for practical applications. This synthetic strategy serves a novel platform to produce a scope of catalysts incorporating multimeric metal sites, and studies on the catalytic mechanism lay the foundation for establishing cooperative effect for multidentate adsorption reactions.

Rigid Ligand Confined Synthesis of Carbon Supported Dimeric Fe Sites with High‐Performance Oxygen Reduction Reaction Activity for Quasi‐Solid‐State Rechargeable Zn‐Air Batteries

AbstractDimeric metal sites (DiMSs) in carbon‐based single atom catalysts (SACs) offer distinct advantages in optimizing the adsorption energies of the catalytic intermediates and reaction pathways over single atom sites, which inspires the investigations on the rational design of DiMSs‐based SACs and the accurate discernment of catalytic mechanisms. Here, dimeric Fe sites on carbon blacks (DiFe‐N/CBs) are prepared using the precursor of metal‐organic complex with a controlled structure, and the rigid ligand confinement secures the preservation of dimeric Fe sites during the thermal treatment. DiFe‐N/CBs shows excellent electrocatalytic performance for oxygen reduction reaction (ORR) with a high half‐wave potential of 0.917 V, and excellent durability with negligible activity decay. Theoretical studies reveal that the dimeric Fe sites have an optimal adsorption of OOH* with the Yeager‐type binding, illustrating the advantages of DiMSs over SAs in catalyzing ORR. The rechargeable aqueous and quasi‐solid‐state Zn‐air batteries assembled using DiFe‐N/CBs‐based air cathodes achieve small voltage gaps after long term charge/discharge test, showing great promises for practical applications. This synthetic strategy serves a novel platform to produce a scope of catalysts incorporating multimeric metal sites, and studies on the catalytic mechanism lay the foundation for establishing cooperative effect for multidentate adsorption reactions.

A Combined Experimental and Computational Exploration of Heteroleptic cis-Pd2L2L'2 Coordination Cages through Geometric Complementarity.

Heteroleptic (mixed-ligand) coordination cages are of interest as host systems with more structurally and functionally complex cavities than homoleptic architectures. The design of heteroleptic cages, however, is far from trivial. In this work, we experimentally probed the self-assembly of Pd(II) ions with binary ligand combinations in a combinatorial fashion to search for new cis-Pd2L2L'2 heteroleptic cages. A hierarchy of computational analyses was then applied to these systems with the aim of elucidating key factors for rationalising self-assembly outcomes. Simple and inexpensive geometric analyses were shown to be effective in identifying complementary ligand pairs. Preliminary results demonstrated the viability of relatively rapid semi-empirical calculations for predicting the topology of thermodynamically favoured assemblies with rigid ligands, whilst more flexible systems proved challenging. Stemming from this, key challenges were identified for future work developing effective computational forecasting tools for self-assembled metallo-supramolecular systems.

Synthesis and electrochemical properties of two cobalt(II) and nickel(II) metal-organic frameworks for excellent performance supercapacitors

Two novel metal-organic frameworks, [Co(HNTB)(bif)]·3H2O·DMA (1) and [(Ni2(μ2O)(HNTB)2(bif)2]·2H2O·2H3O·2DMA (2), were obtained with a tripod-type carboxylic ligand (4,4′,4″-Nitrilotrisbenzoic acid, H3NTB) and a rigid N-donor ligand (2,7-bis(1-imidazoly)fluorene, bif) by solvothermal method. Complex 1 exhibits a 2D layered structure and 2 is a 3D microporous structure with an anionic skeleton. Significantly, their electrochemical tests show that both of them own excellent capacitive performance. Complex 1 has a relatively high specific capacitance (301.2 F g−1 at 1 A g−1) with favorable cycling durability (retained 73.2 % over 3000 cycles). Complex 2 presents a high specific capacitance (329.6 F g−1 at 1 A g−1) with a relatively poor cycling durability (retained 58.9 % over 3000 cycles). Additionally, complex 1 // activates carbon (AC) as an asymmetrical ultra-capacitor exhibits a maximum energy density of 20.9 Wh kg−1 at 800.0 W kg−1 and a high cycle stability (retained 89.2 % over 8000 cycles). Complex 2 // AC owns an energy density of 19.3 Wh kg−1 at 750.0 W kg−1 and a satisfactory cycle stability (retained 88.6 % over 8000 cycles).

Ratiometric fluorescence sensor based on dye-encapsulated Zn-MOF for highly sensitive detection of diquat in tap water and apple samples

The development of convenient and intelligent visualization of rapid pesticide detection methods is key to ensuring food quality and safety. This paper presents a smartphone-based ratiometric fluorescence sensor for smart field visualization and detection of diquat (DQ). Three-dimensional Zn-MOFs are prepared by the solvothermal method using the rigid ligand 4,4′,4″-s-triazine-2,4,6-triyltribenzoic acid (H3TATB), the flexible ligand 1,4-bis((1H-imidazol-1-yl)methyl)benzene (bimb), and the transition metal (Zn2+), which shows good structural and thermal stability. Zn-MOF is a three-dimensional framework crystallizing in a triclinic crystal system in the Pī space group. Then, RhB@Zn-MOF is obtained by introducing rhodamine B (RhB) into the Zn-MOF framework using the same method. Internal Filter Effect (IFE) quenches red fluorescence and Fluorescence Resonance Energy Transfer (FRET) enhances blue fluorescence, enabling high sensitivity and visual detection of DQ. The sensor has a low detection limit of 10.50 nM and is linear over the 0–70.00 μM concentration range. Compared to Zn-MOF (detection limit of 16.90 nM), the introduction of RhB significantly improved the precision and sensitivity of DQ detection. The combination of a smartphone and RGB analysis enables fast and accurate quantitative analysis of DQ in tap water and apple samples. The sensor platform has the advantages of convenience, intelligent real-time detection, etc. It has a wide prospect of practical application and consolidates a solid foundation for food safety control.

Mono and binuclear Pt(II) complexes incorporated with NHC ligands: syntheses, photophysical properties, and acid vapor responses

Three mononuclear complexes (PtL1H, PtL2H, and PtL3H) and one binuclear complex (Pt2L1) have here been prepared using the bis-carbene ligand of 1,3-propylene-bis(1,3-dihydro-3-butyl-2H-imidazol-2-ylidene) and one of three rigid planar ligands of dibenzo[f,h]quinoxaline (L1H2), dibenzo[a,c]phenazine (L2H2), and tribenzo[a,c,i]phenazine (L3H2). These four complexes were fully characterized by nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS), and elemental analyses. Moreover, the two binuclear complexes Pt2L2 and Pt2L3 could not be isolated, but Pt2L3 was isolated and characterized by single-crystal X-ray diffraction (XRD). Complexes PtL1H, PtL2H, and Pt2L1 exhibited phosphorescence in N2-degassed dichloromethane (DCM) solution at 563 nm, 656 nm, and 624 nm, respectively. Also, Pt2L1 exhibited triplet emission at 553 nm in a 2 wt% PMMA film, with the highest Ф of 63 % and τ of 23.8 μs. Moreover, complexes PtL1H and Pt2L1 were deposited on quartz plates to monitor formic acid vapor and acetic acid vapor. The plates that were covered with PtL1H exhibited phosphorescent quenching in both formic acid and acetic acid. However, the plates that were covered with Pt2L1 did only show a prompt response to formic acid, but were not sensitive to acetic acid.

Enhancing Photoreduction of Cr(VI) through a Multivalent Manganese(II)-Organic Framework Incorporating Anthracene Moieties.

Exploiting a photocatalyst with high stability and excellent activity for Cr(VI) reduction under mild conditions is crucial yet challenging. Herein, the rigid aromatic multicarboxylate ligand with chromophore anthracene was selected to coordinate with multivalent metal ion manganese and to obtain a stable two-dimensional (2D) Mn-based metal-organic framework (MOF), LCUH-120, which can efficiently and quickly convert Cr(VI) into Cr(III) under light without the need for any additional photosensitizer. The efficient photosensitive anthracene group serves as a photosensitizer center and multivalent Mn(II) ion as a photocatalyst center in LCUH-120, and the conversion of Cr(VI) to Cr(III) can be realized completely in just 40 min. Specifically, the rate constant (k) and reduction rate of the Cr(VI) photocatalytic reaction can be high up to 0.134 min-1 and 2.50 mgCr(VI) g-1cata min-1 in an acidic environment (pH = 2), respectively. Compared to our previously reported three-dimensional (3D) Sm-MOF, LCUH-120 exhibits a significantly higher catalytic reaction rate, which might be ascribed to the fact that the photocatalyst center Mn node can improve the rate of electron transfer and promote the separation of holes and photogenerated electrons. In an acidic environment, the reaction mechanism can be verified through various contrast experiments and theoretical simulations.

A terbium coordination polymer based on mixed rigid and flexible organic ligands as a luminescent sensor for the convenient visual detection of ascorbic acid

Ascorbic acid (AA) is an essential antioxidant in the human body, whose detection is of significance in many fields. In this study, a two-dimensional terbium coordination polymer [Tb (dlba) (phen)]n (Tb-CP) based on a rigid 1,10-phenanthroline (phen) and a flexible 2-(3,5-dicarboxybenzyloxy) benzoic acid unit (H3dlba) was prepared under the hydrothermal conditions and characterized by Powder X-ray diffraction, elemental analysis, thermogravimetric analysis, infrared spectroscopy and single crystal X-ray diffraction. The Tb-CP synthesized via the mixed ligand strategy exhibits excellent water stability and acid-alkali resistance. As a luminescent sensor, the Tb-CP showed bright green luminescence characteristics of Tb3+, which was quenched after the addition of AA, with a quenching efficiency of 95.3 %. Additionally, the sensor shows high sensitivity and a rapid response time of 30 s. The detection limit is as low as 55.4 nM. In the actual sample test, the recovery rate of AA was 95–105 % (RSD< 1 %), indicating that the sensor has the potential for practical application. Furthermore, an integrated system based on Tb-CP and a smartphone can be employed for the visual detection of AA.

(Invited) Two-Dimensional Organic-Perovskite Hybrid Materials and Heterostructures

Epitaxial heterostructures based on oxide perovskites and III–V, II–VI and transition metal dichalcogenide semiconductors form the foundation of modern electronics and optoelectronics. Halide perovskites—an emerging family of tunable semiconductors with desirable properties—are attractive for applications such as solution-processed solar cells, light-emitting diodes, detectors and lasers. Their inherently soft crystal lattice allows greater tolerance to lattice mismatch, making them promising for heterostructure formation and semiconductor integration. Atomically sharp epitaxial interfaces are necessary to improve performance and for device miniaturization. However, epitaxial growth of atomically sharp heterostructures of halide perovskites has not yet been achieved, owing to their high intrinsic ion mobility and their poor chemical stability. Therefore, understanding the origins of this instability and identifying effective approaches to suppress ion diffusion are of great importance. In this talk I will present an effective strategy to substantially inhibit in-plane ion diffusion in two-dimensional halide perovskites by incorporating rigid π-conjugated organic ligands. Highly stable and tunable lateral and vertical epitaxial heterostructures, multiheterostructures and superlattices will be demonstrated. Furthermore, using these 2D heterostructures as a new platform, I will present our recent efforts in 1) quantitatively understanding the anion inter-diffusions and migrations, and 2) controlling and manipulating exciton transport and light-emission in halide perovskites.

Water-Stable Ln-MOF as a multi-emitting luminescent sensor for the detection of metal ions and pharmaceuticals

The development of innovative multi-emission sensors for the rapid and accurate detection of contaminants is both vital and challenging. In this study, utilizing two rigid ligands (H3ICA and H4BTEC), a series of water-stable bimetallic organic frameworks (EuTb-MOFs) were synthesized. Luminescent investigations have revealed that EuTb-MOF-1 exhibits prominent multiple emission peaks, attributed to the distinctive fluorescence characteristics of Eu(III) and Tb(III) ions. Therefore, EuTb-MOF-1 efficiently recognized various metal ions and pharmaceutical compounds through 2D decoded maps. Fe3+ and Pb2+ exhibited significant quenching effects on the luminescence of EuTb-MOF-1, which were attributed to the internal filtering effect and the interaction between Lewis basic sites within EuTb-MOF-1 and Pb2+ ions, respectively. Furthermore, EuTb-MOF-1 demonstrated high sensitivity to sulfonamide antibiotics, with detection limits of 0.037 μM for SMZ and 0.041 μM for SDZ, respectively. In addition, EuTb-MOF-1 was immobilized to prepare MOF-based test strips, enabling direct visual detection of sulfonamides as a portable sensor. With excellent water stability, multi-responsive recognition capabilities, and high sensitivity to specific analytes, EuTb-MOF-1 is a promising candidate for environmental contaminant detection in aquatic systems.

Unraveling the Geometry-Driven C═C Epoxidation and C-H Hydroxylation Reactivity of Tetra-Coordinated Nonheme Iron(IV)-Oxo Complexes.

The electronic structure and reactivity of tetra-coordinated nonheme iron(IV)-oxo complexes have remained unexplored for years. The recent synthesis of a closed-shell iron(IV)-oxo complex [(quinisox)FeIV(O)]+ (1) has set up a platform to understand how such complexes compare with the celebrated open-shell iron-oxo chemistry. Herein, using density functional theory and ab initio calculations, we present an in-depth electronic structure investigation of the C═C epoxidation [oxygen atom transfer (OAT)] and C-H hydroxylation [hydrogen atom transfer (HAT)] reactivity of 1. Using a solvent-coordinated geometry of 1 (1') and other potential tetra-coordinated iron(IV)-oxo complexes bearing rigid ligands (2 and 3), we established the geometric origin of spin-state energetics and reactivity of 1. Complex 1 featuring a strong Fe-O bond exhibits OAT and HAT reactivity in its quintet state. The lowest quintet OAT pathway has a lower barrier by ∼4 kcal/mol than the quintet HAT pathway, corroborating the experimentally observed gas-phase OAT reactivity preference. A conventional HAT reactivity preference for 2 and a comparable OAT and HAT reactivity for 3 are observed. This further supports the geometry-driven reactivity preference for 1. Noncovalent interaction analyses reveal a pronounced π-π interaction between the substrate and ligand in the OAT transition state, rationalizing the origin of the observed reactivity preference for 1.

Palladium-Catalyzed Enantioselective Migratory Hydroamidocarbonylation of Amide-Linked Alkenes to Access Chiral α-Alkyl Succinimides.

A Pd-catalyzed asymmetric isomerization-hydroamidocarbonylation of amide-containing alkenes was developed, affording a variety of chiral α-alkyl succinimides in moderate to good yields with high enantioselectivities. The key to success was introducing bulky 1-adamentyl P-substitution and 2,3,5,6-tetramethoxyphenyl group into the rigid P-chirogenic bisphosphine ligand to create stronger steric hinderance and deeper catalytic pocket. By this approach, regio- or stereo-convergent synthesis of enantiomeric succinimides from the mixture of olefin isomers was achieved.

Synthesis, structure, and magnetic properties of lanthanide(III) complexes incorporating a tris-chelate cobalt(III) complex as a metalloligand

A series of CoIII–LnIII–CoIII trinuclear complexes [Ln{Co(Hmtimn)3}2](NO3)3 were prepared by following a simple one-pot synthesis method, mixing an unsymmetrical bidentate ligand 2-(2-imidazolinyl)-6-methoxypheol (H2mtimn) with Co(NO3)2·6H2O and LnX3· 6H2O (where Ln = La, Ce, Pr, Nd, Gd or Dy and X = NO3 − or Cl−) in methanolic solution in the presence of triethylamine in air. In the reaction mixture, this leads to the in situ formation of a tris-chelate CoIII metalloligand unit, which can coordinate LnIII ions through the interactions of phenolato- and methoxy-O donors. X-ray structural analysis revealed that the CoIII metalloligand functions as a rigid tripodal ligand to coordinate to a LnIII ion by six oxygen donors. The two metalloligands in the complex exhibit homochiral aggregation (ΔΔ or ΛΛ) upon coordination to the LnIII ion. The coordination of the central LnIII ion was found to be a distorted icosahedral geometry. Magnetic susceptibility measurements reveal that both cobalt and central lanthanide ions exist in trivalent states.

Fac-[M(CO)3] (M = Mn or Re)-based capped metallacalix[3]arenes

A flexible tritopic ligand, 1,3,5-tri(5,6-di-(methoxycarbonyl)-benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene (Tebim) was designed, synthesized and characterized. Six neutral heteroleptic fac-[M(CO)3]-based capped metallacalix[3]arenes with/without six or twelve ester decorated motifs were synthesized via one-pot approach using M2(CO)10 (M = Mn or Re), rigid angular bidentate ligand (benzimidazole (H-bim) or 5,6-di-(methoxycarbonyl)-benzimidazole (H-ebim)), and tritopic ligand (1,3,5-tri(benzimidazol-1-ylmethyl)-benzene (Tbim) or Tebim). All capped metallacalix[3]arenes are characterized using FT-IR, 1H NMR spectroscopy and mass spectrometry. The 1H NMR spectra for all the complexes displayed a simple and symmetrical pattern with characteristic upfield and downfield shifts for the protons of both the coordinated ligands, which signifies that synthesized capped metallacalix[3]arenes retains the structural framework in the solution. Further, the molecular structures of 1 and 3b were confirmed by single crystal X-ray diffraction analysis.

Insensitive energetic azide-based metal-organic frameworks: Highly stable frameworks constructed by nitrogen-rich and rigid ligands

The design and development of azide-based energetic metal–organic frameworks (EMOFs) with high energy and reliable stabilities for energetic materials still remains significant challenges. Herein, we utilized two rigid triazole-based polydentate ligands to design and synthesize two stable azide-based EMOFs [Cd(N3)(TRZ)]n1 (HTRZ = 1,2,4-triazole) and [Cd(N3)(ATRT)(H2O)]n2 (HATRT = 5-amino-3-(4H-1,2,4-triazol-4-yl)-1H-1,2,4-triazole). Structural analysis reveals that 1 displays a unique 3D inserted 3D framework, and 2 presents a rigid 2D network with strong π-π packing interactions. Compounds 1 and 2 feature rarely insensitive to mechanical stimulation and exhibit excellent thermal stabilities reaching up to 371 and 366 °C, respectively. The calculated heats of detonation (ΔHdet) of 2 is 5.315 kJ·g−1, which is much higher than that of the traditional explosive 2,4,6-trinitrotoluene (TNT, ΔHdet = 3.72 kJ·g−1). The experimental results and theoretical calculations demonstrate that the specific 3D inserted 3D structure of 1 and the rigid 2D network of 2 produce high mechanical strength. This study not only provides two azide-based EMOFs with reliable stabilities as potential replacements for traditional energetic materials, but also illustrates a new insight into building stable structural models of azide-based EMOFs.

Cationic manganese(II)-based metal-organic framework constructed from viologen derivative with multifunctional properties

Utilizing the rigid and linear viologen carboxylic ligand as an organic linker, we have successfully synthesized and structurally characterized a multifunctional Mn-based cationic metal-organic framework (1). Compound 1 features a three-dimensional framework structure with a positively charged skeleton, wherein triangular pores accommodate [MnCl4]2− anions to preserve charge neutrality. Notably, compound 1 demonstrates a chemical chromism phenomenon, transitioning from yellow-brown to deep green upon exposure to ammonia. This color change is attributed to the nucleophilic attack of electron-rich ammonia molecules on the electron-deficient viologen moiety of viologen carboxylic ligand, leading to the formation of viologen radicals and charge transfer complexes. Moreover, compound 1 exhibits rapid and selective detection of antibiotics like metronidazole (MDZ) and dimetridazole (DTZ) via a competitive absorption-induced luminescence quenching process, featuring high quenching coefficients and low detection limits. Additionally, magnetic property investigations reveal predominant antiferromagnetic coupling between Mn(II) ions within compound 1.

Rational Designed Metal–Organic Framework with Nanocavity Traps for Selectively Recognizing and Separating of Radioactive Thorium in Rare Earth Wastewater

AbstractFacing the complex coexistence of various metal ions in wastewater and mineral resources, metal–organic frameworks (MOFs) with diverse functionalities and tunable pores are poised to serve as distinctive solutions for efficiently recognizing and selectively separating radioactive thorium (Th(IV)). Herein, a fluorescent MOF (DBT‐DHTA‐Cd) with unique thorium nanocavity traps is rationally designed by using two rigid ligands, including azole and hydroxyl groups, respectively. Notably, the synergistic effect of the appropriate pore size and dense hydroxyl···N nanocavity traps in DBT‐DHTA‐Cd results in selective capture of Th(IV), arising from the high charge density of nano‐trap and extremely low binding energy (Eads = −602.7 KJ mol−1). The fast adsorption kinetics (30 min) and rapid fluorescence response (1 min) of DBT‐DHTA‐Cd toward Th(IV) are attributed to the stronger electron holding capacity of the d and f orbitals belonged to Th(IV), making it more prone to accepting electrons transferred from the N/O active sites in the nano‐trap. As a verification test, tailing breakthrough experiments confirm that DBT‐DHTA‐Cd can efficiently purify Th(IV) from monazite mining solid waste in one step. This work provides profound insights into relationship between structure and property of MOFs and has valuable guidance for designing materials in the radioactive purification field.

Filling the gaps in icosahedral superatomic metal clusters.

Chemically modified superatoms have emerged as promising candidates in the new periodic table, in which Au13 and its doped M n Au13- n have been widely studied. However, their important counterpart, Ag13 artificial element, has not yet been synthesized. In this work, we report the synthesis of Ag13 nanoclusters using strong chelating ability and rigid ligands, that fills the gaps in the icosahedral superatomic metal clusters. After further doping Ag13 template with different degrees of Au atoms, we gained insight into the evolution of their optical properties. Theoretical calculations show that the kernel metal doping can modulate the transition of the excited-state electronic structure, and the electron transfer process changes from local excitation (LE) to charge transfer (CT) to LE. This study not only enriches the families of artificial superatoms, but also contributes to the understanding of the electronic states of superatomic clusters.

Characterization of Deformational Isomerization Potential and Interconversion Dynamics with Ultrafast X-ray Solution Scattering.

Dimeric complexes composed of d8 square planar metal centers and rigid bridging ligands provide model systems to understand the interplay between attractive dispersion forces and steric strain in order to assist the development of reliable methods to model metal dimer complexes more broadly. [Ir2 (dimen)4]2+ (dimen = para-diisocyanomenthane) presents a unique case study for such phenomena, as distortions of the optimal structure of a ligand with limited conformational flexibility counteract the attractive dispersive forces from the metal and ligand to yield a complex with two ground state deformational isomers. Here, we use ultrafast X-ray solution scattering (XSS) and optical transient absorption spectroscopy (OTAS) to reveal the nature of the equilibrium distribution and the exchange rate between the deformational isomers. The two ground state isomers have spectrally distinct electronic excitations that enable the selective excitation of one isomer or the other using a femtosecond duration pulse of visible light. We then track the dynamics of the nonequilibrium depletion of the electronic ground state population─often termed the ground state hole─with ultrafast XSS and OTAS, revealing a restoration of the ground state equilibrium in 2.3 ps. This combined experimental and theoretical study provides a critical test of various density functional approximations in the description of bridged d8-d8 metal complexes. The results show that density functional theory calculations can reproduce the primary experimental observations if dispersion interactions are added, and a hybrid functional, which includes exact exchange, is used.