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At least 1,315 records · Page 73
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Redox-Active vs Redox-Innocent: A Comparison of Uranium Complexes Containing Diamine Ligands

Here uranium complexes ( Mes DAE) 2 U(THF) ( 1-DAE ) and Cp 2 U( Mes DAE) ( 2-DAE ) ( Mes DAE = [ArN-CH 2 CH 2 -NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), bearing redox-innocent diamide ligands, have been synthesized and characterized for a full comparison with previously published, redox-active diimine complexes, ( Mes DAB Me ) 2 U(THF) ( 1-DAB ) and Cp 2 U( Mes DAB Me ) ( 2-DAB ) ( Mes DAB Me = [ArN=C(Me)C(Me)=NAr]; Ar = Mes). These redox-innocent analogues maintain an analogous steric environment to their redox-active ligand counterparts to facilitate a study aimed at determining the differing electronic behavior around the uranium center. Structural analysis by X-ray crystallography showed 1-DAE and 2-DAE have a structural environment very similar to 1-DAB and 2-DAB , respectively. The main difference occurs with coordination of the ene-backbone to the uranium center in the latter species. Electronic absorption spectroscopy reveals these new DAE complexes are nearly identical to each other. X-ray absorption spectroscopy suggests all four species contain +4 uranium ions. The data also indicates that there is an electronic difference between the bis(diamide)-THF uranium complexes as opposed to those that only contain one diamide and two cyclopentadienyl rings. Finally, magnetic measurements reveal that all complexes display temperature-dependent behavior consistent with uranium(IV) ions that do not include ligand radicals. Overall, this study determines that there is no significant bonding difference between the redox-innocent and redox-active ligand frameworks on uranium. Furthermore, there are no data to suggest covalent bonding character using the latter ligand framework on uranium, despite what is known for transition metals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Experimental and Theoretical Comparison of Transition-Metal and Actinide Tetravalent Schiff Base Coordination Complexes

A series of homoleptic tetravalent transition-metal and actinide Schiff-base coordination complexes, ML 2 {M = Zr, Hf, Th, U; L = N,N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized that feature a rigid phenyl backbone. These complexes create the opportunity for comparing a series of complexes containing metal cations in the formal IV+ oxidation state by structural, spectroscopic, and theoretical analysis that also incorporate the previously reported Ce(IV) and Pu(IV) analogues. X-ray crystallographic analysis reveals that all complexes are isomorphous and feature a co-facial ligand geometry. TD-DFT and other quantum mechanical methods were used to explore bonding differences across between the complexes, and resulting calculated absorbance spectra for ML 2 are in good agreement with the experimental data. The computational results also suggest that U(IV) and Pu(IV) analogs have more covalent character in their bonding than found with the other metal cations reported here.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Intriguing Effects of Halogen Substitution on the Photophysical Properties of 2,9-(Bis)halo-Substituted Phenanthrolinecopper(I) Complexes

Three new copper(I) complexes [Cu-(L-x)(2)](+)(PF6-) (where L-x stands for 2,9-dihalo-1,10-phenanthroline and X = Cl, Br, and I) have been synthesized in order to study the impact of halogen substituents tethered in the alpha position of the chelating nitrogen atoms on their physical properties. The photophysical properties of these new complexes (hereafter named Cu-X) were characterized in both their ground and excited states. Femtosecond ultrafast spectroscopy revealed that early photoinduced processes are faster for Cu-I than for Cu-Cl or Cu-Br, both showing similar behaviors. Their electronic absorption and electrochemical properties are comparable to benchmark [Cu(dmp)(2)](+) (where dmp stands for 2,9-dimethyl-1,10-phenanthroline); furthermore, their optical features were fully reproduced by time-dependent density functional theory and ab initio molecular dynamics calculations. All three complexes are luminescent at room temperature, showing that halogen atoms bound to positions 2 and 9 of phenanthroline are sufficiently bulky to prevent strong interactions between the excited Cu complexes and solvent molecules in the coordination sphere. Their behavior in the excited state, more specifically the extent of the photoluminescence efficiency and its dependence on the temperature, is, however, strongly dependent on the nature of the halogen. A combination of ultrafast transient absorption spectroscopy, temperature-dependent steady-state fluorescence spectroscopy, and computational chemistry allows one to gain a deeper understanding of the behavior of all three complexes in their excited state.

Brown-Xu, Samantha↗
DOE OSTI2019

Bonding Interactions in Uranyl α-Diimine Complexes: A Spectroscopic and Electrochemical Study of the Impacts of Ligand Electronics and Extended Conjugation

Uranyl complexes of aryl-substituted α-diimine ligands gbha (UO 2 -1a-f and phen-BIAN (UO 2 -2a-f) (gbha (1) = glyoxal bis(2-hydroxyanil); phen-BIAN (2) = N,N' -bis-(iminophenol)acenaphthene; R = OMe (a), t-bu (b), H (c), Me (d), F (e), and naphthyl (f)) were designed, prepared, and characterized by X-ray diffraction, FT-IR, NMR, UV-Vis, and electrochemical methods. Furthermore these ligand frameworks contain a salen-type O-N-N-O binding pocket, but are redox-non-innocent, leading to unusual metal complex behaviors. Here, we describe three solid-state structures of uranyl complexes UO 2 -1b, UO 2 -1c, and UO 2 -1f, and observe manifestations of ligand non-innocence for the U(VI) complexes UO 2 -1b and UO 2 -1c. The impacts of accessible π-systems and ligand substitution on the axial uranium-oxo interactions can be evaluated spectroscopically via the intraligand charge-transfer (ILCT) processes that dominate the absorption spectra of these complexes and through changes to the asymmetric (ν 3 ) O=U=O stretching frequency. This, in combination with electrochemical data, reveals the effects of the inclusion of the conjugated acenaphthene backbone and the importance of ligand electronic structure on uranyl’s bonding interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Trifluoromethylated Phenanthroline Ligands Reduce Excited-State Distortion in Homoleptic Copper(I) Complexes

Herein, we report the synthesis and excited-state dynamics for a series of homoleptic copper(I) trifluoromethylated phenanthroline complexes with two, three, and four trifluoromethyl functional groups. Our analysis of the steady-state absorbance and emission, transient-absorption spectroscopy, and electronic-structure-theory calculations results enable in-depth analysis of the pseudo-Jahn–Teller distortion inhibition from increased steric hindrance of the trifluoromethyl functional group relative to the prototypical dimethyl phenanthroline complex. Surprisingly, our results demonstrate that the greatest degree of pseudo-Jahn–Teller distortion inhibition is achieved with trifluoromethylation of only the 2 and 9 positions by an unusual combination of steric hindrance and stabilization of a nondistorted 1 MLCT manifold observed by transient kinetic lifetimes and optimized excited-state structures. The intersystem-crossing (ISC) lifetime for the 2,9-bis(trifluoromethyl)-1,10-phenanthroline Cu(I) complex is 69 ps, while the triplet excited-state lifetime and emission quantum yield are 106 ns and 4 × 10 –3 , respectively. Further trifluoromethylation of the phenanthroline yields a greater σ bond inductive withdrawing force on the phenanthroline nitrogens, ultimately resulting in weaker coordination to the copper. Last, the surprising success of the 2,9-bis(trifluoromethyl)-1,10-phenanthroline Cu(I) complex by adjusting both ligand sterics and electronic properties outlines a new strategy for developing long-lived Cu(I) charge-transfer complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Machine Learning Prediction of the Experimental Transition Temperature of Fe(II) Spin-Crossover Complexes

Spin-crossover (SCO) complexes are materials that exhibit changes in the spin state in response to external stimuli, with potential applications in molecular electronics. It is challenging to know a priori how to design ligands to achieve the delicate balance of entropic and enthalpic contributions needed to tailor a transition temperature close to room temperature. Here, we leverage the SCO complexes from the previously curated SCO-95 data set [Vennelakanti et al. J. Chem. Phys. 159, 024120 (2023)] to train three machine learning (ML) models for transition temperature (T 1/2 ) prediction using graph-based revised autocorrelations as features. We perform feature selection using random forest-ranked recursive feature addition (RF-RFA) to identify the features essential to model transferability. Of the ML models considered, the full feature set RF and recursive feature addition RF models perform best, achieving moderate correlation to experimental T 1/2 values. We then compare ML T 1/2 predictions to those from three previously identified best-performing density functional approximations (DFAs) which accurately predict SCO behavior across SCO-95, finding that the ML models predict T 1/2 more accurately than the best-performing DFAs. In addition, we study ML model predictions for a set of 18 SCO complexes for which only estimated T 1/2 values are available. Upon excluding outliers from this set, the RF-RFA RF model shows a strong correlation to estimated T 1/2 values with a Pearson’s r of 0.82. In contrast, DFA-predicted T 1/2 values have large errors and show no correlation to estimated T 1/2 values over the same set of complexes. Overall, our study demonstrates slightly superior performance of ML models in comparison with some of the best-performing DFAs, and we expect ML models to improve further as larger data sets of SCO complexes are curated and become available for model training.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Electronic Structure and Dynamics of Higher-Lying Excited States in Light Harvesting Complex 1 from Rhodobacter sphaeroides

Light harvesting in photosynthetic organisms involves efficient transfer of energy from peripheral antenna complexes to core antenna complexes, and ultimately to the reaction center where charge separation drives downstream photosynthetic processes. Antenna complexes contain many strongly coupled chromophores, which complicates analysis of their electronic structure. Two-dimensional electronic spectroscopy (2DES) provides information on energetic coupling and ultrafast energy transfer dynamics, making the technique well suited for the study of photosynthetic antennae. Here in this paper, we present 2DES results on excited state properties and dynamics of a core antenna complex, light harvesting complex 1 (LH1), embedded in the photosynthetic membrane of Rhodobacter sphaeroides . The experiment reveals weakly allowed higher-lying excited states in LH1 at 770 nm, which transfer energy to the strongly allowed states at 875 nm with a lifetime of 40 fs. The presence of higher-lying excited states is in agreement with effective Hamiltonians constructed using parameters from crystal structures and atomic force microscopy (AFM) studies. The energy transfer dynamics between the higher- and lower-lying excited states agree with Redfield theory calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Infrared Spectroscopy of the Entrance Channel Complex Formed Between the Hydroxyl Radical and Methane in Helium Nanodroplets

Here, the entrance channel complex in the exothermic OH + CH 4 → H 2 O + CH 3 reaction has been isolated in helium nanodroplets following the sequential pick-up of the hydroxyl radical and methane. The a -type OH stretching band was probed with infrared depletion spectroscopy, revealing a spectrum qualitatively similar to that previously reported in the gas phase, but with additional substructure that is due to the different internal rotation states of methane (j CH 4 = 0, 1, or 2) in the complex. We fit the spectra by assuming the rotational constants of the complex are the same for all internal rotation states; however, subband origins are found to decrease with increasing j CH 4 . Measurements of deuterated complexes have also been made (OD–CH 4 , OH–CD 4 , and OD–CD 4 ), the relative linewidths of which provide information about the flow of vibrational energy in the complexes; vibrational lifetime broadening is prominent for OH–CH 4 and OD–CD 4 , for which the excited OX stretching state has a nearby CY 4 stretching fundamental (X, Y = H or D).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Role of Vibrational Dynamics on Excited-State Electronic Coherence in a Binuclear Platinum Complex

Long-lived quantum coherence between excited electronic states can enable highly efficient energy and charge transport processes in chemical systems. Recent pump-probe experiments on binuclear platinum complexes identified persistent, periodic beating of transient absorption anisotropy signals, indicating long excited-state coherence lifetimes. Our previous simulations of the electronic dynamics of these complexes indicate that coherence lifetimes are sensitive to the balance between competitive electronic couplings. The complexes with shorter Pt-Pt distances underwent no appreciable dephasing in the limit of static nuclei, motivating the inclusion of nuclear motion into our simulations. The tert-butyl-substituted complex is studied in this work using the Ehrenfest method for mixed quantum classical dynamics to investigate the role of vibrational dynamics on a complex shown to support coherence lifetimes. Results indicate that the inclusion of excited-state vibrations drives a rapid collapse of the two-state coherence prior to the experimentally determined intersystem crossing. This further suggests singlet excited-state coherences may not be prerequisites for long-lived triplet coherences.

Radler, Joseph J.↗
DOE OSTI2021

Complex Formation and Dissociation Dynamics on Amorphous Silica Surfaces

Benzene complex formation and dissociation dynamics with silanols on the amorphous silica surfaces of nanoporous SiO 2 , from a benzene/carbon tetrachloride solution, were measured by the growth of off-diagonal peaks in the two dimensional infrared (2D IR) chemical exchange spectrum of the isolated Si-OD stretch. The presence of two types of isolated silanols, referred to as type I and II, was revealed, with dissociation time constants of 82 ps and 4.0 ps for type I and II complexes, respectively. The type I silanols are associated with the main IR absorption feature in the Si-OD stretching region, while the type II silanols give rise to a broader shoulder to lower frequency. Polarization selective pump-probe (PSPP) measurements provided the vibrational lifetimes and orientational relaxation rates of the two silanols in the CCl 4 (free) and benzene (complex) environments. The type II silanols constitute roughly 30% of the isolated silanol population and exhibit a substantially faster rate of vibrational relaxation, making the type I dynamics the dominant contribution to the PSPP and 2D IR signals. From the measured dissociation times, the enthalpies of formation for the two surface complexes were obtained; the formation of the type I complex being significantly more exothermic. Furthermore, as the type II site is preferentially removed from the amorphous silica surface with increasing activation temperature, the results provide a reasonable explanation for the increased exothermicity of benzene adsorption on silica with increasing activation temperature in previous calorimetry experiments.

14 SOLAR ENERGY↗
DOE OSTI2025

The Impact of Carotenoid Energy Levels on the Exciton Dynamics and Singlet–Triplet Annihilation in Isolated Bacterial Light-Harvesting 2 Complexes

The light-harvesting 2 (LH2) complex of purple phototrophic bacteria plays a critical role in absorbing solar energy and distributing the excitation energy. Exciton dynamics within LH2 complexes are controlled by the structural arrangement and energy levels of the bacteriochlorophyll (BChl) and carotenoid (Car) pigments. However, there is still debate over the competing light-harvesting versus energy-dissipation pathways. In this work, we compared five variants of the LH2 complex from genetically modified strains of Rhodobacter sphaeroides, all containing the same BChls but different Cars with increasing conjugation: zeta-carotene (N = 7; LH2 Zeta ), neurosporene (N = 9; LH2 Neu ), spheroidene (N = 10; LH2 Spher ), lycopene (N = 11; LH2 Lyco ), and spirilloxanthin (N = 13; LH2 Spir ). Absorption measurements confirmed that the Car excited-state energy decreased with increasing conjugation. Similarly, fluorescence spectra showed that the B850 BChl emission peak had an increasing red shift from LH2 Zeta →(LH2 Neu /LH2 Spher )→LH2 Lyco →LH2 Spir . In contrast, time-resolved fluorescence and ultrafast transient absorption (fs-TA) revealed similar excited-state lifetimes (∼1 ns) for all complexes except LH2 Spir (∼0.7 ns). From fs-TA analysis, an additional ∼7 ps nonradiative dissipation step from B850 BChl was observed for LH2 Zeta . Further, singlet− singlet and singlet−triplet annihilation studies showed a ∼50% average fluorescence lifetime reduction in LH2Zeta at high laser power and high repetition rate, compared to ∼10−15% reductions in LH2 Neu /LH2 Spher /LH2 Lyco and minimal lifetime change in LH2 Spir . In LH2 Zeta , the fastest decay component (<50 ps) became prominent at high repetition rates, consistent with strong singlet−triplet annihilation. Nanosecond TA measurements revealed long-lived (>40 μs) BChl triplet states in LH2 Zeta and signs of damage caused by singlet oxygen, whereas other LH2s showed faster triplet quenching (∼18 ns) by Cars. These findings highlight a key design principle of LH2 complexes: the Car triplet energy must be significantly lower than the BChl triplet energy to efficiently quench BChl triplets that otherwise act as potent “trap states,” causing exciton annihilation in laser-based experiments or photodamage in native membranes.

Absorption↗
DOE OSTI2018

Structure-Based Exciton Hamiltonian and Dynamics for the Reconstituted Wild-type CP29 Protein Antenna Complex of the Photosystem II

Here we provide an analysis of the pigment composition of reconstituted wild type CP29 complexes. The obtained stoichiometry of 9 ± 0.6 Chls a and 3 ± 0.6 Chls b per complex, with some possible heterogeneity in the carotenoid binding, is in agreement with 9 Chls a and 3.5 Chls b revealed by the modeling of low-temperature optical spectra. We find that ~50% of Chl b614 is lost during the reconstitution/purification procedure, while Chls a are almost fully retained. The excitonic structure and the nature of the low-energy (low-E) state(s) are addressed via simulations (using Redfield theory) of 5 K absorption and fluorescence/nonresonant hole-burned (NRHB) spectra obtained at different excitation/burning conditions. We show that, depending on laser excitation frequency, reconstituted complexes display two (independent) low-E states (i.e., the A and B traps) with different NRHB and emission spectra. The red-shifted state A near 682.4 nm is assigned to a minor (~10%) subpopulation (Sub. II) that most likely originates from an imperfect local folding occurring during protein reconstitution. Its lowest energy state A (localized on Chl a604) is easily burned with λ B = 488.0 nm and has a red-shifted florescence origin band near 683.7 nm that is not observed in native (isolated) complexes. Prolonged burning by 488.0 nm light reveals a second low-E trap at 680.2 nm (state B) with a fluorescence origin-band ~681 nm which is also observed when using a direct lowfluence excitation near 650 nm. The latter state is mostly delocalized over the a611, a612, a615 Chl trimer, and corresponds to the lowest energy state of the major (~90%) subpopulation (Sub. I) which exhibits a lower hole-burning quantum yield. Thus, we suggest that major Sub. I correspond to the native folding of CP29, whereas the red-shift of the Chl a604 site energy observed in the minor Sub. II occurs only in reconstituted complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Synthesis of Pyridine– and Pyrazine–BF 3 Complexes and Their Characterization in Solution and Solid State

Following the discovery of the redox-active 1,4- bis-BF 3 -quinoxaline complex, we undertook a structure- activity study with the objective to understand the active nature of the quinoxaline complex. Through systematic synthesis and characterization, we have compared complexes prepared from pyridine and pyrazine derivatives, as heterocyclic core analogues. This paper reports the structural requirements that give rise to the electrochemical features of the 1,4-bis-BF 3 -quinoxaline adduct. Using solution and solidstate NMR spectroscopy, the role of aromatic ring fusion and nitrogen incorporation in bonding and electronics was elucidated. We establish the boron atom location and its interaction with its environment from 1D and 2D solution NMR, X-ray diffraction analysis, and 11B solid-state NMR experiments. Crystallographic analysis of single crystals helped to correlate the boron geometry with 11B quadrupolar coupling constant (CQ) and asymmetry parameter (ηQ), extracted from 11B solid-state NMR spectra. Additionally, computations based on density functional theory were performed to predict electrochemical behavior of the BF 3 -heteroaromatic complexes. We then experimentally measured electrochemical potential using cyclic voltammetry and found that the redox potentials and CQ values are similarly affected by electronic changes in the complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Cation-π Bonding in Actinides: UO x + (Benzene) ( x = 0, 1, 2) Complexes Studied with Threshold Photodissociation Spectroscopy and Theory

Cation-π complexes of the form UO x + (benzene) (x = 0, 1, 2) are produced by laser vaporization and cooled in a supersonic molecular beam. These ions are mass selected and studied with UV–visible laser photodissociation spectroscopy. Each of these complexes photodissociates by elimination of the benzene ligand. Above an energetic threshold, the absorption and photodissociation are continuous, indicating a high density of strongly coupled electronic states. The thresholds for the dissociation of each of these three complexes are measured and assigned as their respective bond dissociation energies. The bond energies determined [U + –(benzene): 42.5 ± 0.3 kcal/mol; UO + –(benzene): 41.0 ± 0.3 kcal/mol; UO 2 + –(benzene): 39.7 ± 0.3 kcal/mol] are comparable to those of transition metal ion-benzene complexes. Computational studies at the DFT/B3LYP level complement the experiments, predicting dissociation energies in reasonably good agreement with the experiments. Experiments and theory agree that the U+(benzene) complex is more strongly bound than its corresponding oxide ions. This new thermochemistry on actinide cation-π bonding should stimulate higher-level computational studies on these systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Observation of Electronic Excitation Transfer Through Light Harvesting Complex II Using Two-Dimensional Electronic-Vibrational Spectroscopy

Light-harvesting complex II (LHCII) serves a central role in light harvesting for oxygenic photosynthesis and is arguably the most important photosynthetic antenna complex. In this article, we present two-dimensional electronic–vibrational (2DEV) spectra of LHCII isolated from spinach, demonstrating the possibility of using this technique to track the transfer of electronic excitation energy between specific pigments within the complex. We assign the spectral bands via comparison with the 2DEV spectra of the isolated chromophores, chlorophyll a and b, and present evidence that excitation energy between the pigments of the complex are observed in these spectra. Lastly, we analyze the essential components of the 2DEV spectra using singular value decomposition, which makes it possible to reveal the relaxation pathways within this complex.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Energy Transfer in Aqueous Light Harvesting Antennae Based on Brush-like Inter-Conjugated Polyelectrolyte Complexes

Conjugated polyelectrolytes (CPEs) have the potential to serve as building blocks of artificial light-harvesting systems. This is primarily due to their delocalized electronic states and potential for hierarchical self-assembly. We showed previously that inter-CPE complexes composed of oppositely charged exciton-donor and exciton-acceptor CPEs displayed efficient electronic energy transfer. However, near ionic charge equivalence, complexed CPE chains become net-neutral and thus experience a precipitous drop in aqueous solubility. To increase the stability and to rationally manipulate the phase behavior of inter-CPE complexes, we synthesized a series of highly water-soluble exciton-donor CPEs composed of alternating ionic and polar nonionic fluorene monomers. The nonionic monomer contained oligo(ethyleneglycol) sidechains of variable length. We then formed exciton donor-acceptor complexes and investigated their relative energy transfer efficiencies in the presence of a fixed exciton-acceptor CPE. We find that, even when the polar nonionic sidechains become quite long (nine ethyleneglycol units), the energy transfer efficiency is hardly affected so long as the inter-CPE network retains a net polyelectrolyte charge. However, near the onset of spontaneous phase separation, we observe a clear influence of the length of the oligo(ethyleneglycol) sidechains on the photophysics of the complex. Our results have implications for the use of polyelectrolyte phase separation to produce aqueous light-harvesting soft materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Coordination-Induced N–H Bond Weakening in a Molybdenum Pyrrolidine Complex: Isotopic Labeling Provides Insight into the Pathway for H 2 Evolution

The synthesis and characterization of a cationic molybdenum pyrrolidine complex is described that exhibits significant coordination-induced N–H bond weakening. Here, the N–H bond dissociation free energy (BDFE) of the coordinated pyrrolidine in [( Ph Tpy)(PPh 2 Me) 2 Mo(NH(pyrr))][BArF 24 ] ([1-NH(pyrr)] + ; PhTpy = 4'-Ph-2,2',6',2''-terpyridine, NH(pyrr) = pyrrolidine, ArF 24 = [C 6 H 3 -3,5-(CF 3 ) 2 ] 4 ) was determined to be between 41–51 kcal mol -1 by thermochemical analysis and supported by a density functional theory (DFT) computed value of 48 kcal mol -1 . The complex [1-NH(pyrr)] + underwent proton-coupled electron transfer (PCET) to 2,4,6-tri-tert-butylphenoxyl radical, as well as spontaneous H 2 evolution upon gentle heating to furnish the corresponding molybdenum pyrrolidide complex, [( Ph Tpy)(PPh 2 Me) 2 Mo(N(pyrr))][BArF 24 ] ([1-N(pyrr)] + ). Thermolysis of the deuterated isotopolog, [1-ND(pyrr)] + still produced H 2 with concomitant incorporation of the isotopic label into the pyrrolidide ligand in the product [(1-N(pyrr-d n )] + (n = 0–2), consistent with an H 2 evolution pathway involving intramolecular H–H bond formation followed by an intermolecular product-forming PCET step. These observations provide context for understanding H 2 evolution in the nonclassical ammine complex [( Ph Tpy)(PPh 2 Me) 2 Mo(NH 3 )][BArF 24 ] ([1-NH 3 ] + ) and are supported by DFT-computed reaction thermochemistry. Overall, these studies offer rare insight into the H 2 formation pathway in nonclassical amine complexes with N–H BDFEs below the thermodynamic threshold for H 2 evolution and inform the development of well-defined, thermody-namically potent PCET reagents.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Si–Cl Bond Activations at Ni(0) to Give Bimetallic Ni(I) μ 1,2 -Cl–SiR 1 R 2 Complexes that Undergo Selective Hydrogenolyses to R 1 R 2 SiH 2 Dihydrosilanes

Chlorosilanes are cheap and abundant raw materials as crucial building blocks in silicon chemistry, yet the metal-mediated activation and functionalization of Si–Cl bonds typically require precious metal sources due to their thermodynamic inertness. Herein, we report the stoichiometric, facile activation, and hydrogenolysis of chlorosilanes mediated by a series of low-valent NHC–Ni (NHC = N-heterocyclic carbene) complexes. Treatment of a Ni(0) complex (IPr)Ni(η 6 -toluene) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene) with chlorosilanes (R 1 R 2 SiCl 2 , R 1 = Cl, R 2 = Cl, Me, Ph, or R 1 = R 2 = Me, Et, Ph, 4-MePh) rapidly afforded di-Ni(I) complexes with a bridging silyl ligand ([(IPr)Ni] 2 (μ-SiR 1 R 2 Cl)(μ-Cl), 1 R1,R2 ) in high yields. Use of a bulkier chlorosilane, Ph 2 SiCl 2 , allowed the isolation of the mono-Ni(II) silyl complex (IPr)Ni(SiPh 2 Cl)Cl (2 Ph ) as an intermediate generated via Si–Cl oxidative addition, which underwent comproportionation with (IPr)Ni(η 6 -toluene) to form 1 Ph,Ph in nearly quantitative yield. Interestingly, 1 R1,R2 was found to react with H 2 at room temperature to form mono- or di-hydrosilanes in moderate to high yields, and the product selectivity was found to be highly dependent on the identity of substituents on Si. In conclusion, these results demonstrate a novel example of facile Si–Cl activation and hydrogenolysis mediated by low-valent mono- and dinuclear NHC–Ni complexes under mild conditions.

Liu, Tianchang [University of California, Berkeley↗