Valence‐to‐Core X‐ray Emission Spectroscopy as a Probe of O−O Bond Activation in Cu2O2 Complexes

Valence‐to‐Core (VtC) X‐ray emission spectroscopy (XES) was used to directly detect the presence of an O?O bond in a complex comprising the [Cu^II₂(μ‐η²:η²‐O₂)]²⁺ core relative to its isomer with a cleaved O?O bond having a [Cu^III₂(μ‐O)₂]²⁺ unit. The experimental studies are complemented by DFT calculations, which show that the unique VtC XES feature of the [Cu^II₂(μ‐η²:η²‐O₂)]²⁺ core corresponds to the copper stabilized in‐plane 2p π peroxo molecular orbital. These calculations illustrate the sensitivity of VtC XES for probing the extent of O?O bond activation in μ‐η²:η²‐O₂ species and highlight the potential of this method for time‐resolved studies of reaction mechanisms.     

Observation of a CuII2(μ‐1,2‐peroxo)/CuIII2(μ‐oxo)2 Equilibrium and its Implications for Copper–Dioxygen Reactivity

Synthesis of small‐molecule Cu₂O₂ adducts has provided insight into the related biological systems and their reactivity patterns including the interconversion of the Cu^II₂(μ‐η²:η²‐peroxo) and Cu^III₂(μ‐oxo)₂ isomers. In this study, absorption spectroscopy, kinetics, and resonance Raman data show that the oxygenated product of [(BQPA)Cu^I]⁺ initially yields an “end‐on peroxo” species, that subsequently converts to the thermodynamically more stable “bis‐μ‐oxo” isomer (K_eq=3.2 at ?90 °C). Calibration of density functional theory calculations to these experimental data suggest that the electrophilic reactivity previously ascribed to end‐on peroxo species is in fact a result of an accessible bis‐μ‐oxo isomer, an electrophilic Cu₂O₂ isomer in contrast to the nucleophilic reactivity of binuclear Cu^II end‐on peroxo species. This study is the first report of the interconversion of an end‐on peroxo to bis‐μ‐oxo species in transition metal‐dioxygen chemistry.     

Bis‐μ‐oxo and μ‐η2:η2‐peroxo dicopper complexes studied within (time‐dependent) density‐functional and many‐body perturbation theory

Based on the equilibrium geometries of [Cu₂(dbdmed)₂O₂]²⁺ and [Cu₂(en)₂O₂]²⁺ obtained within density‐functional theory, we investigate their molecular electronic structure and optical response. Thereby results from occupation‐constrained as well as time‐dependent DFT (ΔSCF and TDDFT) are compared with Green's function‐based approaches within many‐body perturbation theory such as the GW approximation (GWA) to the quasiparticle energies and the Bethe‐Salpeter equation (BSE) approach to the optical absorption. Concerning the ground‐state energies and geometries, no clear trend with respect to the amount of exact exchange in the DFT calculations is found, and a strong dependence on the basis sets is to be noted. They affect the energy difference between bis‐μ‐oxo and μ‐η²:η²‐peroxo complexes by as much as 0.8 eV (18 kcal/mol). Even stronger, up to 5 eV is the influence of the exchange‐correlation functional on the gap values obtained from the Kohn‐Sham eigenvalues. Not only the value itself but also the trends observed upon the bis‐μ‐oxo to μ‐η²:η²‐peroxo transition are affected. In contrast, excitation energies obtained from ΔSCF and TDDFT are comparatively robust with respect to the details of the calculations. Noteworthy, in particular, is the near quantitative agreement between TDDFT and GWA+BSE for the optical spectra of [Cu₂(en)₂O₂]²⁺. © 2013 Wiley Periodicals, Inc.     

[Cu6(NGuaS)6]2+ and its oxidized and reduced derivatives: Confining electrons on a torus

The hexanuclear thioguanidine mixed‐valent copper complex cation [Cu₆(NGuaS)₆]⁺² (NGuaS = o‐SC₆H₄NC(NMe₂)₂) and its oxidized/reduced states are theoretically analyzed by means of density functional theory (DFT) (TPSSh + D3BJ/def2‐TZV (p)). A detailed bonding analysis using overlap populations is performed. We find that a delocalized Cu‐based ring orbital serves as an acceptor for donated S p electrons. The formed fully delocalized orbitals give rise to a confined electron cloud within the Cu₆S₆ cage which becomes larger on reduction. The resulting strong electrostatic repulsion might prevent the fully reduced state. Experimental UV/Vis spectra are explained using time‐dependent density functional theory (TD‐DFT) and analyzed with a natural transition orbital analysis. The spectra are dominated by MLCTs within the Cu₆S₆ core over a wide range but LMCTs are also found. The experimental redshift of the reduced low energy absorption band can be explained by the clustering of the frontier orbitals. © 2017 Wiley Periodicals, Inc.     

Dioxygen Activation by a Macrocyclic Copper Complex Leads to a Cu2O2 Core with Unexpected Structure and Reactivity

We report the Cu^I/O₂ chemistry of complexes derived from the macrocylic ligands 14‐TMC (1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane) and 12‐TMC (1,4,7,10‐tetramethyl‐1,4,7,10‐tetraazacyclododecane). While [(14‐TMC)Cu^I]⁺ is unreactive towards dioxygen, the smaller analog [(12‐TMC)Cu^I(CH₃CN)]⁺ reacts with O₂ to give a side‐on bound peroxo‐dicopper(II) species (^SP), confirmed by spectroscopic and computational methods. Intriguingly, 12‐TMC as a N4 donor ligand generates ^SP species, thus in contrast with the previous observation that such species are generated by N2 and N3 ligands. In addition, the reactivity of this macrocyclic side‐on peroxo‐dicopper(II) differs from typical ^SP species, because it reacts only with acid to release H₂O₂, in contrast with the classic reactivity of Cu₂O₂ cores. Kinetics and computations are consistent with a protonation mechanism whereby the TMC acts as a hemilabile ligand and shuttles H⁺ to an isomerized peroxo core.     

Influence of the protein and DFT method on the broken‐symmetry and spin states in nitrogenase

The enzyme nitrogenase contains a complicated MoFe₇CS₉ cofactor with 35 possible broken‐symmetry (BS) states. We have studied how the energies of these states depend on the geometry, the surrounding protein, the DFT functional and the basis set, studying the resting state, a one‐electron reduced state and a protonated state. We find that the effect of the basis set is small, up to 11 kJ/mol. Likewise, the effect of the surrounding protein is restricted, up to 10 and 7 kJ/mol for the electrostatic and van der Waals energy terms. Single‐point energies calculated on a single geometry give a good correlation (R² = 0.92‐0.98) to energies calculated after geometry optimization, but some BS states may be disfavored by up to 37 kJ/mol. A change from the pure TPSS functional to the hybrid B3LYP functional may change the relative energies by up to 58 kJ/mol and the correlation between the two results is only 0.57‐0.72. Both functionals agree that BS7 is the most stable BS state and that the ground spin state is the quartet for the resting state and the quintet for the reduced state. With the TPSS functional, the BS6 state is the second most stable state, always at least 21 kJ/mol less stable than the BS7 state. However, with the B3LYP functional, BS10 is the second most stable state and for the protonated state it comes close in energy. Based on these results, we suggest a procedure how to consider the 35 BS states in future investigations of the nitrogenase reaction mechanism.     

Reactive adsorption of hydrogen sulfide by promoted sorbents Cu‐ZnO/SiO2: active sites by experiment and simulation

In the Cu_x‐Zn_(1‐x)O/SiO₂ sorbents for ultradeep adsorptive removal of H₂S from gaseous fuel reformates for fuel cells at room temperature, Cu promoter sites significantly increase sulfur uptake capacity of the sorbents. We report characterization of the family of Cu_x‐Zn_(1‐x)O/SiO₂ sorbents for reactive adsorption of H₂S using X‐ray diffraction (XRD), Brunauer‐Emmett‐Teller (BET) surface area analysis, electron spin resonance (ESR), ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy (DRS) and calculations by the density functional theory (DFT). Both the supported ZnO phase and Cu promoter sites in the Cu_x‐Zn_(1‐x)O/SiO₂ sorbents are nano‐dispersed, as shown by XRD. The Cu_x‐Zn_(1‐x)O/SiO₂ sorbents contain Cu promoter as the Cu²⁺ site of octahedral geometry, as found by the complementary ESR and UV–vis DRS. Mechanism of the promoter effect of the Cu²⁺ site in the Cu_x‐Zn_(1‐x)O/SiO₂ sorbents in reaction with H₂S is proposed based on DFT calculations. Copyright © 2012 John Wiley & Sons, Ltd.     

Computational Assessment of MLCT versus MC Stabilities in First-to-Third-Row d6 Pseudo-Octahedral Transition Metal Complexes

An accurate modeling of metal-to-ligand-charge-transfer (MLCT) and metal-centered (MC) excited state energies is key to predict the photoinduced response in transition metal complexes (TMCs). Herein, the importance of the ground state and excited state reference geometries is addressed for three-prototype d⁶ pseudo-octahedral TMCs, each displaying a different potential energy landscape of MLCT versus MC relative stabilities. Several functionals are used within the time-dependent density functional theory (TDDFT), as well as multireference wave-function theory (MS-CASPT2), applied to [Mn(im)(CO)₃(phen)]⁺, [Ru(im)₂(bpy)₂]²⁺, and [Re(im)(CO)₃(phen)]⁺, (im: imidazole, bpy: bypiridine, phen: phenantroline). The results revel that TDDFT is robust except when using B3LYP functional for first-row d⁶ TMCs. In contrast, MS-CASPT2 calculations are strongly biased in those cases with competitive MLCT/MC states. The results reinforce the reliability of B3LYP to describe the excited states in d⁶ TMCs, but question the validity of assessing the density functional theory (DFT)/TDDFT performance via direct comparison with MS-CASPT2 performed at the same DFT reference geometry as a standard strategy. © 2019 Wiley Periodicals, Inc.     

Mononuclear Manganese–Peroxo and Bis(μ‐oxo)dimanganese Complexes Bearing a Common N‐Methylated Macrocyclic Ligand

Mononuclear Mn^III–peroxo and dinuclear bis(μ‐oxo)Mn^III₂ complexes that bear a common macrocyclic ligand were synthesized by controlling the concentration of the starting Mn^II complex in the reaction of H₂O₂ (i.e., a Mn^III–peroxo complex at a low concentration (≤1 mM ) and a bis(μ‐oxo)Mn^III₂ complex at a high concentration (≥30 mM )). These intermediates were successfully characterized by various physicochemical methods such as UV–visible spectroscopy, ESI‐MS, resonance Raman, and X‐ray analysis. The structural and spectroscopic characterization combined with density functional theory (DFT) calculations demonstrated unambiguously that the peroxo ligand is bound in a side‐on fashion in the Mn^III–peroxo complex and the Mn₂O₂ diamond core is in the bis(μ‐oxo)Mn^III₂ complex. The reactivity of these intermediates was investigated in electrophilic and nucleophilic reactions, in which only the Mn^III–peroxo complex showed a nucleophilic reactivity in the deformylation of aldehydes.     

Quantum‐chemical study of the spin transition complex [Fe(bt)2(NCS)2] (bt=2,2′‐bithiazoline)

The spin crossover compound [Fe(bt)₂(NCS)₂] has been studied by several density functionals and basis sets. In the calculation, optimized geometries of the compound in the low‐, intermediate‐, and high‐spin states, the vibrational modes and IR spectra, spin splittings energies, excited states, and UV/vis absorption spectra were obtained. © 2012 Wiley Periodicals, Inc.     

Theoretical study of the magnetic exchange coupling behavior substituting Cr(III) with Mo(III) in cyano‐bridged transition metal complexes

Molecular magnetism in a series of cyano‐bridged first and second transition metal complexes has been investigated using density functional theory (DFT) combined with the broken‐symmetry (BS) approach. Several exchange‐correlation (XC) functionals in the ADF package were used to investigate complexes I [?(Me₃tacn)₂(cyclam)NiMo₂(CN)₆]²⁺, II [?(Me₃tacn)₂(cyclam)Ni‐Cr₂(CN)₆]²⁺, III [(Me₃tacn)₆MnMo₆(CN)₁₈]²⁺, and IV [(Me₃tacn)₆MnCr₆(CN)₁₈]²⁺ (Me₃tacn = N,N′,N?‐trimethyl‐1,4,7‐triazacyclononane). For models A (the molded structure of complex I) and B (the modeled structure of complex II), all the XCs given qualitatively reasonable results and predict ferromagnetic coupling character between M (M = Mo^III for A or Cr^III for B) and Ni^II in coincidence with the experimental results (see Tables I and II ). The calculated using Operdew, OPBE, O3LYP, and B3LYP functionals and experimental J values show that substituting Cr^III with Mo^III will enhance the ferromagnetic exchange coupling interactions. But VWN, PW91, PBE, VSXC, and tau‐HCTH functionals have no way to differentiate the relative strength of the intramolecular magnetic exchange coupling interactions of A and B correctly. For models C (the modeled structure of complex III) and D (the modeled structure of complex IV), all the XCs in ADF and B3LYP in Gaussian 03 with several basis sets show that substituting Cr^III with Mo^III will enhance the antiferromagnetic exchange coupling interactions. From the above calculations, the substitution of Cr^III by Mo^III will enhance the magnetic coupling interactions, whether the magnetic coupling interactions are ferro‐ or antiferromagnetic. Moreover, Kahn's model was applied to investigate the above facts. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

A Tetravalent Cerium Complex Containing a CeO Bond

Whereas terminal oxo complexes of transition and actinide elements are well documented, analogous lanthanide complexes have not been reported to date. Herein, we report the synthesis and structure of a cerium(IV) oxo complex, [Ce?O(L_OEt)₂(H₂O)]?MeC(O)NH₂ ( 1 ; L_OEt^?=[Co(η⁵‐C₅H₅){P(O)(OEt)₂}₃]^?), featuring a short Ce?O bond (1.857(3) Å). DFT calculations indicate that the hydrogen bond to cocrystallized acetamide plays a key role in stabilizing the Ce?O moiety of 1 in the solid state. Complex 1 exhibits oxidizing and nucleophilic reactivity.     

Theoretical Modelling of Photoswitching of Hyperpolarisabilities in Ruthenium Complexes

Static excited‐state polarisabilities and hyperpolarisabilities of three Ru^II ammine complexes are computed at the density functional theory (DFT) and several correlated ab initio levels. Most accurate modelling of the low energy electronic absorption spectrum is obtained with the hybrid functionals B3LYP, B3P86 or M06 for the complex [Ru^II(NH₃)₅(MeQ⁺)]³⁺ (MeQ⁺=N‐methyl‐4,4′‐bipyridinium, 3 ) in acetonitrile. The match with experimental data is less good for [Ru^II(NH₃)₅L]³⁺ (L=N‐methylpyrazinium, 2 ; N‐methyl‐4‐{E,E‐4‐(4‐pyridyl)buta‐1,3‐dienyl}pyridinium, 4 ). These calculations confirm that the first dipole‐ allowed excited state (FDAES) has metal‐to‐ligand charge‐transfer (MLCT) character. Both the solution and gas‐phase results obtained for 3 by using B3LYP, B3P86 or M06 are very similar to those from restricted active‐space SCF second‐order perturbation theory (RASPT2) with a very large basis set and large active space. However, the time‐dependent DFT λ_max predictions from the long‐range corrected functionals CAM‐B3LYP, LC‐ωPBE and wB97XB and also the fully ab initio resolution of identity approximate coupled‐cluster method (gas‐phase only) are less accurate for all three complexes. The ground state (GS) two‐state approximation first hyperpolarisability β_2SA for 3 from RASPT2 is very close to that derived experimentally via hyper‐Rayleigh scattering, whereas the corresponding DFT‐based values are considerably larger. The β responses calculated by using B3LYP, B3P86 or M06 increase markedly as the π‐conjugation extends on moving along the series 2 → 4 , for both the GS and FDAES species. All three functionals predict substantial FDAES β enhancements for each complex, increasing with the π‐conjugation, up to about sevenfold for 4 . Also, the computed second hyperpolarisabilities γ generally increase in the FDAES, but the results vary between the different functionals.     

Diselenophosphate‐Induced Conversion of an Achiral [Cu20H11{S2P(OiPr)2}9] into a Chiral [Cu20H11{Se2P(OiPr)2}9] Polyhydrido Nanocluster

A polyhydrido copper nanocluster, [Cu₂₀H₁₁{Se₂P(OiPr)₂}₉] ( 2_H ), which exhibits an intrinsically chiral inorganic core of C₃ symmetry, was synthesized from achiral [Cu₂₀H₁₁{S₂P(OiPr)₂}₉] ( 1_H ) of C_3h symmetry by a ligand‐exchange method. The structure has a distorted cuboctahedral Cu₁₃ core, two triangular faces of which are capped along the C₃ axis, one by a Cu₆ cupola and the other by a single Cu atom. The Cu₂₀ framework is further stabilized by 9 diselenophosphate and 11 hydride ligands. The number of hydride, phosphorus, and selenium resonances and their splitting patterns in multinuclear NMR spectra of 2_H indicate that the chiral Cu₂₀H₁₁ core retains its C₃ symmetry in solution. The 11 hydride ligands were located by neutron diffraction experiments and shown to be capping μ₃‐H and interstitial μ₅‐H ligands (in square‐pyramidal and trigonal‐bipyramidal cavities), as supported by DFT calculations on [Cu₂₀H₁₁(Se₂PH₂)₉] ( 2_H′ ) as a simplified model.     

Theoretical and Experimental Study of Complex Ions from Reactions of Al+ (Cu+) with Amine Molecules

The gas phase reactions of metal ions (Al⁺, Cu⁺) with amine molecules [CH₃NH₂=MA, (CH₃)₂NH=DMA] were investigated using a laser ablation‐molecular beam method. The directly associated product complex ions,Al⁺‐MA and Al⁺‐DMA, and the dehydrogenation product ions, Cu⁺(CH₂NH) and Cu⁺(C₂H₅N), as well as hydrated ion Cu⁺(NC₂H₅·H₂O), have been obtained and recorded from the reactions of the metal ions and organic amine molecules, and density functional theory (B3LYP) calculations have been performed to reveal the optimized geometry, energetics, and reaction mechanism of the title reactions with basis set 6‐311+G(d,p) adopted.     

Tetrathiafulvalene‐amido‐2‐pyridine‐N‐oxide as Efficient Charge‐Transfer Antenna Ligand for the Sensitization of YbIII Luminescence in a Series of Lanthanide Paramagnetic Coordination Complexes

The tetrathiafulvalene‐amido‐2‐pyridine‐N‐oxide ( L ) ligand has been employed to coordinate 4f elements. The architecture of the complexes mainly depends on the ionic radii of the lanthanides. Thus, the reaction of L in the same experimental protocol leads to three different molecular structure series. Binuclear [Ln₂(hfac)₅(O₂CPhCl)( L )₃] ? 2 H₂O (hfac^?=1,1,1,5,5,5‐hexafluoroacetylacetonate anion, O₂CPhCl^?=3‐chlorobenzoate anion) and mononuclear [Ln(hfac)₃( L )₂] complexes were obtained by using rare‐earth ions with either large (Ln^III=Pr, Gd) or small (Ln^III=Y, Yb) ionic radius, respectively, whereas the use of Tb^III that possesses an intermediate ionic radius led to the formation of a binuclear complex of formula [Tb₂(hfac)₄(O₂CPhCl)₂( L )₂]. Antiferromagnetic interactions have been observed in the three dinuclear compounds by using an extended empirical method. Photophysical properties of the coordination complexes have been studied by solid‐state absorption spectroscopy, whereas time‐dependent density functional theory (TD‐DFT) calculations have been carried out on the diamagnetic Y^III derivative to build a molecular orbital diagram and to reproduce the absorption spectrum. For the [Yb(hfac)₃( L )₂] complex, the excitation at 19 600 cm^?1 of the HOMO→LUMO+1/LUMO+2 charge‐transfer transition induces both line‐shape emissions in the near‐IR spectral range assigned to the ²F_5/2→²F_7/2 (9860 cm^?1) ytterbium‐centered transition and a residual charge‐transfer emission around 13 150 cm^?1. An efficient antenna effect that proceeds through energy transfer from the singlet excited state of the tetrathiafulvalene‐amido‐2‐pyridine‐N‐oxide chromophore is evidence of the Yb^III sensitization.     

Synthesis,Crystal Structure,and Visible Light Responses of Ti4Cu4-Oxo Clusters with Mixed Valence Copper Ions

Summaryof main observation and conclusion In order to extend the absorption spectrum of polyoxo-titanium clusters into the visible region,two new heterometal-oxo clusters Ti4CuII2CuI2(μ3-O)6(benzoate)10(MeCN)4(PTC-153)and Ti4CuII2CuI2(μ3-O)6(benzoate)8(CH3COO)2(MeCN)4(PTC-154)were success-fully synthesized.Single-crystal X-ray diffraction and X-ray photoelectron spectroscopy studies showed that these two heterometallic Ti4Cu4-oxo clusterspossessed Chinese knot-shape structure and mixed valence Cu^1+/2+ions.UV-visible spectroscopyanalysis demonstrated that the visible light region ab-sorption of PTC-153and PTC-154 could be significantly enhanced by doping copper.Furthermore,their visible-light driven photocurrent responses were studied by using samples of PTC-153and PTC-154as electrode precursors.