Temperature-dependent NMR Spectroscopy Research Articles

Revealing the Origin of Fast Delayed Fluorescence in a Donor Functionalized Bisterpyridine.

A new carbazole-substituted bisterpyridine with pronounced delayed fluorescence is presented. While the molecular donor-acceptor-donor design suggests the origin of this to be thermally activated delayed fluorescence (TADF), results from various photophysical characterizations, OLED characteristics, temperature-dependent NMR spectroscopy, and DFT calculations all point against the involvement of triplet states. The molecule exhibits blue emission at about 440 nm with two or more fast decay channels in the lower nanosecond range in both solution and thin films. The delayed emission is proposed to be caused by rotational vibrational modes. We suggest that these results are generally applicable, especially for more complex molecules, and should be considered as alternative or competitive emissive relaxation pathways in the field of organic light emitting materials.

Feinabstimmung der Redoxeigenschaften von heteroleptischen Molybdän‐Komplexen mittels Ligand‐Ligand‐Kooperativität

AbstractHeteroleptische Molybdänkomplexe mit 1,5‐Diaza‐3,7‐Diphosphacyclooctan (P2N2) und non‐innocent (nicht‐unschuldigen) Dithiolenliganden wurden synthetisiert und elektrochemisch charakterisiert. Dabei wurde festgestellt, dass die Reduktionspotentiale der Komplexe durch einen synergistischen Effekt moduliert werden, welcher durch DFT‐Berechnungen als Ligand‐Ligand‐Kooperativität über nicht‐kovalente Wechselwirkungen identifiziert werden konnte. Dieses Ergebnis konnte durch elektrochemische Studien in Kombination mit UV/Vis‐Spektroskopie und temperaturabhängiger NMR‐Spektroskopie bestätigt werden. Das beobachtete Verhalten erinnert an eine enzymatische Redoxmodulation durch Effekte basierend auf Interaktionen in der zweiten Ligandensphäre.

Fine-Tuning Redox Properties of Heteroleptic Molybdenum Complexes through Ligand-Ligand-Cooperativity.

Heteroleptic molybdenum complexes bearing 1,5-diaza-3,7-diphosphacyclooctane (P2 N2 ) and non-innocent dithiolene ligands were synthesized and electrochemically characterized. The reduction potentials of the complexes were found to be fine-tuned by a synergistic effect identified by DFT calculations as ligand-ligand cooperativity via non-covalent interactions. This finding is supported by electrochemical studies combined with UV/Vis spectroscopy and temperature-dependent NMR spectroscopy. The observed behavior is reminiscent of enzymatic redox modulation using second ligand sphere effects.

Dynamic Multicomponent Reactions-Directed Self-Assembled G-quadruplex Inherent Antibacterial Hydrogel.

Nowadays, inherent antibacterial hydrogels have gained significant attention due to their utilization against infectious bacteria. Herein, we focus on the development of an injectable, self-healable, dynamic, and G-quadruplex hydrogel with inherent antibacterial activity. The dynamic self-assembled hydrogel is constructed upon multicomponent reactions (MCR) among guanosine, 2-formylphenylboronic acid, and amino acid/peptides in the presence of potassium ions. The role of amino acid/peptides in the formation of the G-quadruplex hydrogel is studied in detail. The G-quadruplex structure is formed via the π-π stacking of G-quartets. The formation of G-quadruplex is investigated by thioflavin T binding assay, CD spectroscopy, and PXRD. The formation of the dynamic imino-boronate bond in the hydrogels is well characterized by temperature-dependent 11B NMR (VT-NMR) and FT-IR spectroscopy. Furthermore, HR-TEM images and rheological experiments reveal the fibrillar networks and viscoelastic property of the hydrogels. The presence of the dynamic imino-boronate ester bonds makes the hydrogel injectable and self-healable in nature. These dynamic G-quadruplex hydrogels show potential antibacterial activity against a series of Gram-positive and Gram-negative bacteria. The hydrogels have been used for the entrapment and sustained release of an anticancer drug doxorubicin over 48 h at different pHs (4.8, 7.4, and 8.5) and temperature without the influence of any external stimuli. Such injectable and self-healable hydrogels could be used in various applications in the field of biomedical science.

Superatom Pruning by Diphosphine Ligands as a Chemical Scissor.

A two-electron silver superatom, [Ag6{S2P(OiPr)2}4(dppm)2] (1), was synthesized by adding dppm (bis(diphenylphosphino)methane) into [Ag20{S2P(OiPr)2}12] (8e). It was characterized by single-crystal crystallography, multinuclear NMR spectroscopy, electrospray ionization-mass spectrometry, density functional theory (DFT), and time-dependent DFT calculations. The added dppm ligands, which carry out the nanocluster-to-nanocluster transformation, act as a chemical scissor to prune the nanocluster geometrically from an icosahedron-based Ag20 nanocluster (NC) to an octahedral Ag6 NC and electronically from eight-electron to two-electron. Eventually, dppm was involved in the protective shell to form a new heteroleptic NC. The temperature-dependent NMR spectroscopy confirms its fluxional behavior, showing the fast atomic movement at ambient temperature. Compound 1 exhibits a bright yellow emission under UV irradiation at ambient temperature with a quantum yield of 16.3%. This work demonstrates a new methodology to achieve nanocluster-to-nanocluster transformation via stepwise synthesis.

Sustainable freshwater recovery from radioactive wastewater by gas hydrate formation

Freshwater production from radioactive wastewater is a key issue not only to minimize environmental contamination by nuclear power plants but also to develop new types of nuclear technology such as small modular reactors. Here, we present a sustainable hydrate-based desalination (HBD) technology for freshwater recovery from radioactive wastewater, specifically from Cs, Sr, and I-containing wastewater. CHClF2 and SF6 were selected as a hydrate former for HBD processes, which can be operated under relatively mild conditions. X-ray diffraction and Raman spectroscopy measurements indicated that the crystal structure of the CHClF2 and SF6 hydrates formed with radioactive chemicals was identical to that of pure CHClF2 and SF6 hydrates, respectively. Formation kinetics experiments were performed to investigate the inhibition effect of Cs+, Sr2+, and I− on CHClF2 and SF6 hydrates, indicating that the apparent rate constant, initial reaction rate, and pressure drop caused by the formation of gas hydrate significantly depend on the sub-cooling temperature and concentrations of the radioactive chemicals. A close examination using scanning electron microscopy with energy dispersive X-ray spectroscopy and in situ temperature-dependent solid-state NMR spectroscopy found evidence of the exclusion of radioactive chemicals from solid gas hydrates, signaling the potential of HBD technology for use in freshwater production from radioactive wastewater. More importantly, the proposed HBD process is applicable to wastewater in the very large concentration ranges of Cs+, Sr2+, and I− covering from the percent to hundreds of parts per million (ppm) and even the sub-ppm ranges, revealing equivalent recovery and separation efficiency rates for all wastewater solutions. We conclusively present a clear-cut picture of the dual-functional HBD process of freshwater recovery and radioactive chemical separation from wastewater.

Ligand properties of boryl ligands in bis-boryl rhodium(III) complexes: a case study.

The oxidative addition of five diborane(4) derivatives, symmetrical and unsymmetrical, to [Rh(PMe3)3Cl] was studied. Only for the more electron poor diboron derivatives, B2cat2, B2pin2 and catB-Bpin the resulting octahedral bis-boryl complexes [(PMe3)3Rh(boryl)2Cl] were obtained, while for the more electron rich congeners only the equilibrium oxidative addition (catB-Bdmab) or no significant reaction (pinB-Bdmab) was observed (pin = (OCMe2)2, cat = 1,2-O2C6H4, dmab = 1,2-(NMe)2C6H4). By abstraction of the chlorido ligand with NaBArF (BArF = tetrakis-[3,5-bis-(trifluormethyl)-phenyl]-borat) in the presence of a neutral ligand (L = PMe3, MeCN, MeNC) the corresponding cationic octahedral complexes [(PMe3)3Rh(boryl)2L]+ were obtained. All isolated complexes were fully characterised including single crystal X-ray diffraction and heteronuclear, temperature dependent NMR spectroscopy. Whilst the complexes [(PMe3)3Rh(boryl)2Cl] and [(PMe3)3Rh(boryl)2L]+ show many similarities, their detailed structural and spectroscopic properties depend crucially on the properties of both boryl ligands.

Curved Perylene Diimides Fused with Seven-Membered Rings.

Curved perylene diimides fused with seven-membered rings have been synthesized using a regioselective bay-functionalization method and Pd-catalyzed intramolecular C-H/C-Br coupling reaction. X-Ray analysis and temperature-dependent NMR spectroscopy revealed the curved molecular structure with a certain degree of conformational flexibility. The curved and expanded π-conjugation altered the electronic properties while retaining the intrinsic properties of the parent perylene diimide. Despite the absence of solubilizing N-substituents, the curved perylene diimides showed sufficient solubility for application in solution-processed organic photovoltaic devices. The devices showed superior performance with a power conversion efficiency of up to 2.76% due to suppressed charge recombination. Our detailed investigations suggest that the introduction of a curved structure enables the removal of the bulky N-substituents, which is an effective way to achieve a thin-film morphology suitable for photoelectric conversion.

A New Class of Superionic Solid-State Lithium-Ion Conductors: Lithium-Phosphido Silicates, Germanates, and Aluminates

Superionic conductors as a solid-state electrolyte materials are essential for use for all-solid-state lithium ion batteries. Whereas intensive research on oxide and sulfide based lithium ion conductors resulted in optimized materials which reach ionic conductivities up to 25 mScm-1 other compound classes are disregarded. A design principle for fabricating new superionic conductors follows the ideas of increasing the carrier densities, changing the diffusion pathways of the mobile species, creating vacancies or increasing structural defects, and including anions that are more polarizable. For example to increase the carrier density, an effective way is especially the aliovalent substitution of cations: for example, in Li3PS4 making a formal substitution from “P5+” to “Ge4+” arises in Li3.25Ge0.25P0.75S4 with a four times higher ionic conductivity.We propose here phosphide-based materials as ionic conductors that result in principle from the aliovalent substitution of [TtS4]4- tetrahedra, which are an important key building block in sulfide-based electrolytes, by [TtP4]8- leading to the analogous complex anions based on phosphorus (Tt = tetrel element = Group-14 element). The resulting higher charge of the [TtP4]8- tetrahedra allows for twice the number of lithium ions per formula unit for charge compensation if compared to [TtS4]4- and in addition, the more negatively charged P3- is more polarizable than S2-. In this context we discovered for example Li8SiP4,[1] β-Li8GeP4,[2] and Li14SiP6 [3] with conductivities up to 1.1 mScm-1 . In addition we recently succeded to synthesize the lithium richest representative, obtained by formal aliovalent substitution of “Tt 4+” by “Al3+” and formation of in Li9AlP4.[4] The insertion of Al3+ leads to formally nine-fold negatively charged [AlP4]9- tetrahedra, resulting in an even higher lithium density per formula unit and a change in spatial extent of the diffusion pathways. This first representative of a novel compound class of lithium phosphidoaluminates has as an undoped material a remarkable fast ionic conductivity of ~ 3 mS cm-1 and a low activation energy of ~ 29 kJ mol−1 as determined by impedance spectroscopy. Temperature-dependent 7Li NMR spectroscopy supports the fast lithium motion. In addition, Li9AlP4 combines a very high lithium content with a very low theoretical density of 1.703 g·cm-3.In this paper we give an overview on the recent development in this material class. All crystalline materials are readily obtained as single phase materials by ball mill synthesis from the elements followed by moderate thermal treatment of the mixtures. In all cases the atomic structures are characterized by X-ray diffraction methods. The lithium-ion mobility was investigated by temperature-dependent nuclear magnetic resonance and electrochemical impedance spectroscopy revealing low activation energies for lithium hopping. In some cases lithium diffusion pathways via both, tetrahedral and octahedral voids are analyzed by temperature-dependent powder neutron diffraction measurements in combination with maximum entropy method (MEM) and DFT calculations. The role of lithium ion occupation in the various tetrahedral and octahedral sites that occur in those compounds is highlighted.

Na+ ion mobility in Na3+xSc2(SiO4)x(PO4)3−x (0.1 < x < 0.8) observed by 23Na NMR spectroscopy

The mobility of Na+ ions in Na3+xSc2(SiO4)x(PO4)3−x (0.1 < x < 0.8) was investigated by temperature-dependent 23Na NMR spectroscopy and relaxometry. These measurements are probing the local motion of the Na+ ions and give information about the average hopping rate of the ions and the activation barriers these ions have to overcome for single jumps. The results are compared with those obtained by impedance spectroscopy that is probing the long-range transport of the Na+ ions. A consistent picture is obtained with the fastest motion of the Na+ ions observed for the sample with x = 0.4.

NHC-phosphane rhodium complexes and their reaction with oxygen

The new homoleptic square planar Rh+I complexes [Rh(NHC-PPh2)2][X] (cis-2[X] and trans-2[X], X = Cl, PF6) with chelating phosphane N-heterocyclic carbene ligands were synthesized by reaction of [Rh(μ-Cl)(COD)]2 with the free ligand {1-[2-(diphenylphosphino)ethyl)]-3-ethylimidazol-2-ylidene)}. The ligand precursor was obtained by deprotonation of the imidazolium salts 1 {1-[2-(diphenylphosphino)ethyl)]-3-ethyl-imidazoliumiodide} 1[I] or hexafluorophosphate 1[PF6], respectively, with KN(SiMe3)2 in THF. Cis-2[Cl] and trans-2[Cl] coexist in a dynamic equilibrium which has been studied by temperature-dependent 31P NMR spectroscopy. cis-2[Cl] and trans-2[Cl] react with O2 at ambient temperature to afford the new cis and trans peroxo Rh+III complexes [Rh(η2-O2)(NHC-PPh2)2]Cl (cis-3[Cl] and trans-3b[Cl]). 31P NMR spectra prove that the bonding of O2 to cis-2[Cl] is reversible. In solution, cis-3[Cl] shows a fluxional behaviour as evidenced by 31P NMR spectra. The fluxionality is attributed to the flexibility of the chelating rings formed by the ligand and the metal centre. Cis-2[PF6] reacts with S8 to give the novel Rh+III complex [Rh(η2-S2)(NHC-PPh2)2][PF6] (cis-4[PF6]). Contrary to the analogous peroxo complex cis-3[Cl], no fluxionality could be observed in this case. All complexes have been characterised by 31P, 1H, 13C NMR spectroscopy and single-crystal X-ray analysis.

Gear Slippage in Molecular Bevel Gears Bridged with a Group 14 Element.

Ditriptycilmethanes are known as molecular bevel gears because the two triptycil groups show correlated rotation. In this report, molecular bevel gears bridged with a group 14 element, bis(methyltriptycil)X (X = SiH2, GeH2, GeF2), were synthesized, and their gearing properties were investigated. Gear slippage, that is an error in gear rotation, is observed in high-temperature solutions of molecular bevel gears. Heavy atom derivatives undergo gear slippage more easily due to the long bond lengths and wide angles between the two triptycil units and the bridging group 14 element. Activation energies of gear slippages were estimated by temperature-dependent NMR spectroscopy and DFT calculations, and theoretical thermodynamic parameters for gear slippage were found to be in excellent agreement with experimental values. The results indicate that theoretical calculations for gear rotation in molecular bevel gears can accurately reproduce experimental phenomena.

Fast Ionic Conductivity in the Most Lithium-Rich Phosphidosilicate Li14SiP6

Solid electrolytes with superionic conductivity are required as a main component for all-solid-state batteries. Here we present a novel solid electrolyte with three-dimensional conducting pathways based on "lithium-rich" phosphidosilicates with ionic conductivity of σ > 10-3 S cm-1 at room temperature and activation energy of 30-32 kJ mol-1 expanding the recently introduced family of lithium phosphidotetrelates. Aiming toward higher lithium ion conductivities, systematic investigations of lithium phosphidosilicates gave access to the so far lithium-richest compound within this class of materials. The crystalline material (space group Fm3m), which shows reversible thermal phase transitions, can be readily obtained by ball mill synthesis from the elements followed by moderate thermal treatment of the mixture. Lithium diffusion pathways via both tetrahedral and octahedral voids are analyzed by temperature-dependent powder neutron diffraction measurements in combination with maximum entropy method and DFT calculations. Moreover, the lithium ion mobility structurally indicated by a disordered Li/Si occupancy in the tetrahedral voids plus partially filled octahedral voids is studied by temperature-dependent impedance and 7Li NMR spectroscopy.

Cyclic Distannene or Bis(stannylene) with a Ferrocenyl Backbone: Synthesis, Structure, and Coordination Chemistry.

1,1'-Dilithioferrocene was reacted with 2 equiv of isopropyl (Ar*) or methyl (Ar') substituted terphenyl tin(II) chloride. Reaction product 1, carrying the bulkier terphenyl substituent Ar*, displays a bis(stannylene) structure in the solid state without formation of a tin-tin bond. Temperature-dependent solution 119Sn NMR spectroscopy, however, revealed a dynamic interplay between bis(stannylene) (100 °C) and cyclic distannene (-80 °C). In contrast to 1, the less bulky Ar' substituent results in a cyclic distannene 2. On the basis of temperature-dependent 119Sn NMR spectroscopy the Sn-Sn bond of compound 2 was preserved up to 100 °C. Both compounds were further characterized by solid-state 119Sn NMR spectroscopy as well as 119Sn and 57Fe Mössbauer spectroscopy. 1 reacted as a chelating ligand with nickel and palladium complexes [Ni(cod)2] and [Pd(nbe)3] (nbe = norbornene). In the resulting coordination compounds the nonstabilized stannylene acts as a donor as well as an acceptor ligand and shows a dynamic switch from donor to acceptor behavior in the monopalladium complex.

Tris(Trimethylsilyl)Germane (Me&lt;sub&gt;3&lt;/sub&gt;Si)&lt;sub&gt;3&lt;/sub&gt;GeH: A Molecular Model for Sulfur Passivation of Ge(111) Surfaces

Tristrimethylsilylgermane, (Me3Si)3GeH, was employed as a molecular model compound for hydrogen terminated Ge(111) surfaces. Time and temperature dependent NMR spectroscopy yielded rate constants for the reaction between (Me3Si)3GeH and elemental sulfur and allowed for the determination of the activation energy for this molecular model reaction to mimic germanium surface passivation.

Template Catalysis by Metal-Ligand Cooperation. C-C Bond Formation via Conjugate Addition of Non-activated Nitriles under Mild, Base-free Conditions Catalyzed by a Manganese Pincer Complex.

The first example of a catalytic Michael addition reaction of non-activated aliphatic nitriles to α,β-unsaturated carbonyl compounds under mild, neutral conditions is reported. A new de-aromatized pyridine-based PNP pincer complex of the Earth-abundant, first-row transition metal manganese serves as the catalyst. The reaction tolerates a variety of nitriles and Michael acceptors with different steric features and acceptor strengths. Mechanistic investigations including temperature-dependent NMR spectroscopy and DFT calculations reveal that the cooperative activation of alkyl nitriles, which leads to the generation of metalated nitrile nucleophile species (α-cyano carbanion analogues), is a key step of the mechanism. The metal center is not directly involved in the catalytic bond formation but rather serves, cooperatively with the ligand, as a template for the substrate activation. This approach of "template catalysis" expands the scope of potential donors for conjugate addition reactions.

Interstitial lithium diffusion pathways in γ-LiAlO2: a computational study.

Although the Li diffusion in single crystalline γ-LiAlO2 was studied with temperature-dependent Li-7 NMR spectroscopy and conductivity measurements recently, the exact diffusion pathways are not yet clearly identified. Therefore, the present study aims at elucidating the diffusion pathways in γ-LiAlO2 theoretically from first principles. Competing pathways for Li diffusion are investigated using the climbing-image nudged-elastic-band approach with periodic quantum-chemical density functional theory (DFT) method. Li can migrate between two regular LiO4 tetrahedral sites via Li point defect (VLi) and via a Li Frenkel defect (VLi + Lii). On the basis of calculated activation energies for Li diffusion pathways, it is concluded that Li conductivity is strongly dependent on the distribution of Li vacancies and interstitial Li in the lattice. For Frenkel defects where Lii is far away from the migrating Li atom, the calculated activation energies for jumps to nearest-neighbor vacant sites agree with experimental values.

η1-silolyl-FeCp(CO)2 complexes. Is there a way to sila-ferrocene?

The reaction of 1-chlorosilols (12, 13) and K[Fe(CO)2Cp] in THF yielded two new η1-silolyl-FeCp(CO)2 complexes (15, 16) in good yield. These complexes are the first covalent Fe–Si bonded silolyl complexes reported in the literature, which were structurally proven by single crystal X-ray diffraction and multinuclear NMR spectroscopy. By temperature dependent NMR spectroscopy a rotational barrier of 14.4 kcal/mol was revealed, in agreement with the calculated 14.9 kcal/mol barrier of the phenyl rotation. The attempted transformation of these complexes to the corresponding silaferrocene by CO elimination failed. DFT calculations revealed that the rather high stability of the Si–Fe comparing to the C–Fe bond might be responsible for the stability of the complexes.

Spin Isomers and Ligand Isomerization in a Three-Coordinate Cobalt(I) Carbonyl Complex.

Hemilabile ligands, which have one donor that can reversibly bind to a metal, are widely used in transition-metal catalysts to create open coordination sites. This change in coordination at the metal can also cause spin-state changes. Here, we explore a cobalt(I) system that is poised on the brink of hemilability and of a spin-state change and can rapidly interconvert between different spin states with different structures ("spin isomers"). The new cobalt(I) monocarbonyl complex L(tBu)Co(CO) (2) is a singlet ((1)2) in the solid state, with an unprecedented diketiminate binding mode where one of the C═C double bonds of an aromatic ring completes a pseudo-square-planar coordination. Dissolving the compound gives a substantial population of the triplet ((3)2), which has exceptionally large uniaxial zero-field splitting due to strong spin-orbit coupling with a low-lying excited state. The interconversion of the two spin isomers is rapid, even at low temperature, and temperature-dependent NMR and electronic absorption spectroscopy studies show the energy differences quantitatively. Spectroscopically validated computations corroborate the presence of a low minimum-energy crossing point (MECP) between the two potential energy surfaces and elucidate the detailed pathway through which the β-diketiminate ligand "slips" between bidentate and arene-bound forms: rather than dissociation, the cobalt slides along the aromatic system in a pathway that balances strain energy and cobalt-ligand bonding. These results show that multiple spin states are easily accessible in this hemilabile system and map the thermodynamics and mechanism of the transition.

Reactions of Titanocene and Zirconocene Complexes with ­Bis(tert‐butoxy)acetylene – Coordination and C–O Bond Cleavage

Abstract The reactivity of the bis‐oxygen substituted alkyne bis(tert‐butoxy)acetylene (3) towards titanocene‐ and zirconocene complexes of btmsa [btmsa = bis(trimethylsilyl)acetylene] was investigated. Those metallocenes with unsubstituted Cp ligands (Cp = η5‐cyclopentadienyl) afforded metallacyclopropenes by alkyne replacement, but only in low yields. The metallocenes bearing the sterically more demanding Cp* ligands (Cp* = η5‐pentamethylcyclopentadienyl) do not engage in replacement reactions with the alkyne 3. If titanocene monochloride is present in the reaction mixture of Cp2Ti(η2‐btmsa) with the alkyne 3, a rather unusual Calkyne–O bond cleavage occurs. The so‐formed acetylide moiety bridges two titanocene centres with an end‐on σ coordination to one of them and a π‐side‐on one to the other. This complex was investigated by X‐ray crystallography and temperature dependent NMR spectroscopy, revealing an unusual behavior.