Redox behaviour of bis(β-diketonato)copper(II) complexes

Cu(β-diketonato)2 bathochromic shifts from the ultraviolet towards the visible region

CONTEXT

The DFT-calculated ultraviolet/visible properties of 11 different Cu(β-diketonato)2 complexes are presented. The selected β-diketonato ligands on the Cu complex contain none, one or two aromatic rings. The experimentally measured absorbance maxima range of the ultraviolet/visible is observed at 295-390 nm, and the calculated range is 302-425 nm, for the 11 complexes in this study. More aromatic rings on the ligand lead to bathochromic shifts of the experimentally measured absorbance maxima from the ultraviolet towards the visible region. Absorbance maxima of the Cu(β-diketonato)2 complexes with no aromatic rings on the ligand are found to be predominantly ligand-to-metal charge transfer excitations, whereas introducing one or two aromatic rings shifts the excitations to predominantly ligand-to-ligand charge transfer.

METHODS

DFT calculations were conducted on the neutral molecules with multiplicity 2, using the PBEh1PBE functional and the aug-cc-pVDZ basis set as implemented in the Gaussian 16 package. The selected solvent was acetonitrile, the solvent in which most of the experimental UV/Vis are reported. The molecules were all optimized in the solvent phase, using the IEFPCM. The initial coordinates for the compounds were generated using Chemcraft.

HIGHLIGHTS

TDDFT of 11 different Cu(β-diketonato)2 complexes follow the experimental trend. Aromatic rings on the ligand lead to Bathochromic shifts of UV/Visible spectra. No aromatic rings on the ligand lead to ligand-to-metal charge transfer excitations. Aromatic rings on the ligand lead to ligand-to-ligand charge transfer excitations.

Boosting a practical Li-CO2 battery through dimerization reaction based on solid redox mediator

Li-CO2 batteries offer a promising avenue for converting greenhouse gases into electricity. However, the inherent challenge of direct electrocatalytic reduction of inert CO2 often results in the formation of Li2CO3, causing a dip in output voltage and energy efficiency. Our innovative approach involves solid redox mediators, affixed to the cathode via a Cu(II) coordination compound of benzene-1,3,5-tricarboxylic acid. This technique effectively circumvents the shuttle effect and sluggish kinetics associated with soluble redox mediators. Results show that the electrochemically reduced Cu(I) solid redox mediator efficiently captures CO2, facilitating Li2C2O4 formation through a dimerization reaction involving a dimeric oxalate intermediate. The Li-CO2 battery employing the Cu(II) solid redox mediator boasts a higher discharge voltage of 2.8 V, a lower charge potential of 3.7 V, and superior cycling performance over 400 cycles. Simultaneously, the successful development of a Li-CO2 pouch battery propels metal-CO2 batteries closer to practical application.

Bicyclopentylation of Alcohols with Thianthrenium Reagents

Herein we present the first method for the synthesis of bicyclo[1.1.1]pentyl (BCP) alkyl ethers from alcohols. The reaction uses BCP-thianthrenium reagents and is catalyzed by a dual copper/photoredox catalyst system. Unlike known alkylations of tertiary alcohols via carbocation intermediates, our Cu-mediated radical process circumvents the labile BCP carbocations. The approach demonstrates a broad tolerance for functional groups when applied to primary, secondary, and even tertiary alcohols. In addition, we highlight the utility of this method in late-stage functionalizations of both natural products and pharmaceuticals as well as in the rapid construction of BCP analogs of known pharmaceuticals that would otherwise be difficult to access.

DFT study of UV-vis-properties of thiophene-containing Cu(β-diketonato)2 - Application for DSSC

Experimental and theoretically calculated UV-vis properties of three Cu(β-diketonato)₂ complexes are presented. The Cu(β-diketonato)₂ contains β-diketones without (β-diketone = acetylacetone, (CH₃)COCH₂CO(CH₃), complex (1)), with one (β-diketone = thenoyltrifluoroacetone, (CF₃)COCH₂CO(C₄H₃S), complex (2)) and with two thiophene (β-diketone = (CF₃)COCH₂CO(C₄H₂S) (C₄H₃S), complex (3)) groups. More thiophenes on the β-diketonato ligand of Cu(β-diketonato)₂, lead to a red shift of the experimental absorbance maxima of the UV-vis of the complex, from 295 nm for complex (1), to 340 nm for complex (2) to 390 nm for complex (3). Theoretical time dependant density functional theory calculations indicate that both the two strongest absorbance peaks of the ultraviolet-visible spectrum of Cu(acetylacetonato)₂ are mainly ligand-to-metal charge-transfer excitations. However, the absorbance maxima of the UV-vis of thiophene-containing Cu(β-diketonato)₂ are mainly ligand-to-ligand charge-transfer excitations. Calculated properties such as light harvesting energy (LHE = 0.47, 0.94 and 0.99 for (1)-(3) respectively), driving force for electron injection (ΔG_inject = 1.43, 0.76 and 0.63 for (1)-(3) respectively), and driving force of dye regeneration (ΔG_regenerate = 1.85, 2.16 and 1.49 for (1)-(3) respectively), are favourable for (1)-(3) to be considered as dyes in DSSCs. However, some structural modifications are needed to prevent intramolecular charge recombination after excitation.

Reduction Data Obtained from Cyclic Voltammetry of Benzophenones and Copper-2-Hydroxyphenone Complexes

This article provides detailed redox data on nine differently substituted benzophenones and ten square planar copper(II) complexes containing 2-hydroxyphenones obtained by cyclic voltammetry (CV) experiments. The information provided is related to the published full research articles “An electrochemical and computational chemistry study of substituted benzophenones” (Electrochim. Acta2021, 373, 137894) and “Electrochemical behaviour of copper(II) complexes containing 2-hydroxyphenones” (Electrochim. Acta2022, 424, 140629), where the CVs and electrochemical data at mainly one scan rate, namely at 0.100 Vs−1, are reported. CVs and the related peak current and voltage values, not reported in the related research article, are provided in this article for nine differently substituted benzophenones and ten differently substituted copper-2-hydroxyphenone complexes at various scan rates over more than two orders of magnitude. The redox data presented are the first reported complete set of electrochemical data of nine 2-hydroxyphenones and ten copper(II) complexes containing 2-hydroxyphenone ligands.

β-Diketone Coordination Strategy for Highly Efficient and Stable Pb-Sn Mixed Perovskite Solar Cells

The narrow bandgap Pb-Sn hybrid perovskite materials with lower toxicities and adjustable optical bandgaps provide the opportunity to construct high-efficiency perovskite solar cells (PerSCs). To solve the issues of the uncontrollable crystallization rate of Pb-Sn perovskite and easy oxidation of Sn²⁺, a β-diketone-based additive, N,N,N',N'-tetraphenylmalondiamide (TPMA), is introduced to coordinate with Pb²⁺ and Sn²⁺. The introduction of TPMA can improve the morphology of perovskite films and decrease the density of defect states, resulting in an enhanced power conversion efficiency of >20% and improved stability. The PerSC without encapsulation retains 94% of its initial efficiency after being stored for 1000 h in a nitrogen-filled glovebox and shows a lifetime of only 8% degradation after being continuously heated for 100 h at 80 °C. This work represents a new strategy of introducing a β-diketone ligand as an additive in precursor engineering for achieving efficient and stable PerSCs.

DFT data to relate calculated LUMO energy with experimental reduction potentials of Cu(II)-β-diketonato complexes

We present data on the computed lowest unoccupied molecular orbital energy (E_LUMO) of two series of Cu(II)-β-diketonato complexes, calculated via density functional theory (DFT). These are correlated to experimental reduction potential data (E_pc), obtained by cyclic voltammetry under different experimental conditions (solvent, working and reference electrodes). All calculations were done with the B3LYP functional in the gas phase. Knowledge of the influence of different ligands on the redox potential of copper complexes, as measured by DFT calculated energy data, are very useful. These theoretical correlations are vital in the further design of similar compounds, to be customized for specific applications. The correlations can be used to predict and fine-tune redox potentials prior to synthesis, saving experimental chemists time and laboratory expenses. Redox potentials influence the catalytic property of bis(β-diketonato)copper(II) compounds. New catalysts can therefore be customized with a specific reduction potential and catalytic activity. Further, the Cu(II/I) redox couple is a potential alternative as electrolyte for dye-sensitized solar cells [1], [2], [3]. The redox potential of the electrolyte can drastically affect the photovoltage output and should therefore be optimized for efficiency and durability. By adjusting the reduction potential via different ligands on the complex, the properties of copper dyes can be fine-tuned at molecular level. For more insight into the reported data, see the related research article “Synthesis, Characterization, DFT and Biological Activity of Oligothiophene β-diketone and Cu-complexes” published in Polyhedron [4].

Bis(acetylacetonato)copper(II) - structural and electronic data of the neutral, oxidized and reduced forms

Bis(acetylacetonato)copper(II) can be synthesized economically and with ease by the reaction between acetylacetone and a copper salt (Cu(OAc)₂ or CuCl₂·2H₂O). When used as catalyst, bis(acetylacetonato)copper(II) is sometimes being oxidized to Cu(III) or reduced to Cu(I), although only the structure of the neutral form is known experimentally. The content of this paper provides computational chemistry calculated data of the geometry, electronic structure, spin state and frontier orbitals for the neutral, as well as the oxidized and reduced forms of the bis(acetylacetonato)copper(II) molecule. This data shows that both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the neutral molecule are copper based. The neutral molecule is a spin = ½ system. The data shows that the spin state of both the oxidized and reduced molecules is zero.

Sterically encumbered β-diketonates and base metal catalysis

Metal coordination complexes of the sterically hindered β-diketonate, 2,6-dimesitylbenzoyl pinacolone (esac), are reported for Co, Ni, Cu, and Zn. All four form ML2-type complexes with typical coordination behavior for late-metal β-diketonates, however the effects on established electrochemistry and reactivity vary somewhat per metal. For example, the striking chemical and electrochemical inertness of CoII(esac)2 to oxidation and disproportionation is atypical. Conversely, the behavior of CuII(esac)2 is rather typical relative to other CuII(β-diketonate)2 complexes. These data suggest a relative disfavoring of certain reaction pathways, and represent an important step in modulating the catalysis of the base metals via sterically hindered β-diketonates.