Electronic tuning of β-diketiminate ligands with fluorinated substituents: effects on the O2-reactivity of mononuclear Cu(i) complexes

Steric and electronic influence on Cu-Cu short contacts in β-thioketiminato tricopper(I) clusters

A series of β-thioketiminate copper(I) complex trimers [LCuI]3 were synthesized by modifying the ligand framework with electron-withdrawing groups (F and Cl) or electron-donating groups (iPr and Me) at the N-aryl ring as well as with CF3 groups on the chelating backbone. This ligand modification significantly impacts the enhancement of Cu⋯Cu short contacts, which can be rationalized by using steric and electronic factors of the chelated ligand. We observed that this intramolecular cuprophilicity among [LCuI]3 complexes is primarily governed by the size of N-aryl ortho-substituents. These findings were well supported by X-ray crystallography, Raman spectroscopy, and Mayer bond order analysis. The electronic effects induced by the ligand modification on the LCuI fragment were investigated using CO and 2,4,6-CNC6H2Me3 as probe molecules. Corroborated by the FTIR and CV measurements, our results reveal that the β-thioketiminate SN chelators induce more pronounced changes in the electronic character of the LCuI fragment due to the presence of CF3 groups on the chelating backbone in comparison with the F or Cl substituents on the N-aryl ring.

A π-extended β-diketiminate ligand via a templated Scholl approach

We report a templated Scholl oxidation strategy for the preparation of the first β-diketiminate (BDI) ligands embedded within a 24-electron π-system backbone. The resulting benzo[f,g]tetracene BDI ligand was coordinated to a zinc centre and electrochemical studies showed the redox active nature of the ligand.

Ligand Degradation Study of Unsymmetrical β-Diketiminato Copper Dioxygen Adducts: The Length Chelating Arm Effect

An investigation on the reactivity of O₂ binding to unsymmetrical β-diketiminato copper(I) complexes by spectroscopic and titration analysis was performed. The length of chelating pyridyl arms (pyridylmethyl arm vs pyridylethyl arm) leads to the formation of mono- or di-nuclear copper-dioxygen species at -80 °C. The pyridylmethyl arm adduct (L₁CuO₂) forms mononuclear copper-oxygen species and shows ligand degradation, resulting in the formation of (2E,3Z)-N-(2,6-diisopropylphenyl)-4-(((E)-pyridin-2-ylmethylene)amino)pent-3-en-2-imine, which slowly converts to its cyclization isomer 1-(2,6-diisopropylphenyl)-4,6-dimethyl-2-(pyridin-2-yl)-1,2-dihydropyrimidine after addition of NH₄OH at room temperature. On the other hand, the pyridylethyl arm adduct [(L₂Cu)₂(μ-O)₂] forms dinuclear species at -80 °C and does not show any ligand degradation product. Instead, free ligand formation was observed after the addition of NH₄OH. These experimental observations and product analysis results indicate that the chelating length of pyridyl arms governs the Cu/O₂ binding ratio and the ligand degradation behavior.

Mimicking the Cu Active Site of Lytic Polysaccharide Monooxygenase Using Monoanionic Tridentate N-Donor Ligands

In an effort to prepare small molecule mimics of the active site of lytic polysaccharide monooxygenase (LPMO), three monoanionic tridentate N donor ligands comprising a central deprotonated amide group flanked by two neutral donors were prepared, and their coordination chemistry with Cu(I) and Cu(II) was evaluated. With Cu(I), a dimer formed, which was characterized by X-ray crystallography and NMR spectroscopy. A variety of mononuclear and dinuclear Cu(II) species with a range of auxiliary ligands (MeCN, Cl^-, OH^-, OAc^-, OBz^-, CO₃ ^2-) were prepared and characterized by X-ray diffraction and various spectroscopies (UV-vis, EPR). The complexes exhibit structural similarities to the LPMO active site.

Electronic effect of a perfluorinated β-diketiminate ligand on the bonding nature of copper carbonyl complexes

Two copper complexes 17Fnac2Cu(C6H6) (3) and 17Fnac2CuCO (4) containing the monoanionic, perfluorinated β-diketiminate 17Fnac2- ligand (1) (17Fnac2 = FC[C(CF3)N(C6F5)]2) were synthesized and characterized by IR and NMR spectroscopy (1H, 13C, 19F), cyclovaltammometry (CV), elemental analysis and single crystal X-ray diffraction. The perfluorinated 17Fnac2- ligand marginally reduces the π-back-bonding capacity of the copper centre to the carbonyl group in 4 when compared with the corresponding 16Fnac2- substituted complexes but substantially when compared with the fluorine free substituted derivatives. Quantum chemical calculations gave deeper insight into the bonding situation of this carbonyl complex, while CV studies were performed to determine the oxidation potential of 3 in solution. Based on these data, the influence of the degree of fluorination in different β-diketimine ligands on the electronic nature of the corresponding copper complexes is discussed.

Carbonyl complexes of copper(i) stabilized by bridging fluorinated pyrazolates and halide ions

Halide ions provide a promising tool to stabilize – through bridging interactions – copper carbonyl clusters of fluorinated pyrazolates.

Exclusive imidazole ligation to CuIII2O2 and CuIIICuII2O2 cores

Direct oxygenation of imidazole-ligated Cu(i) generates dinuclear and trinuclear Cu(iii) species with exclusive imidazole ligation.

Reactivity Study of Unsymmetrical β-Diketiminato Copper(I) Complexes: Effect of the Chelating Ring

β-Diketiminato copper(I) complexes play important roles in bioinspired catalytic chemistry and in applications to the materials industry. However, it has been observed that these complexes are very susceptible to disproportionation. Coordinating solvents or Lewis bases are typically used to prevent disproportionation and to block the coordination sites of the copper(I) center from further decomposition. Here, we incorporate this coordination protection directly into the molecule in order to increase the stability and reactivity of these complexes and to discover new copper(I) binding motifs. Here we describe the synthesis, structural characterization, and reactivity of a series of unsymmetrical N-aryl-N'-alkylpyridyl β-diketiminato copper(I) complexes and discuss the structures and reactivity of these complexes with respect to the length of the pyridyl arm. All of the aforementioned unsymmetrical ß-diketiminato copper(I) complexes bind CO reversibly and are stable to disproportionation. The binding ability of CO and the rate of pyridyl ligand decoordination of these copper(I) complexes are directly related to the competition between the degree of puckering of the chelate system and the steric demands of the N-aryl substituent.

Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity

A longstanding research goal has been to understand the nature and role of copper-oxygen intermediates within copper-containing enzymes and abiological catalysts. Synthetic chemistry has played a pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper-oxygen cores. In addition to the number of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004, 104, 1013-1046 and Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047-1076), the field has seen significant expansion in the (1) range of cores synthesized and characterized, (2) amount of mechanistic work performed, particularly in the area of organic substrate oxidation, and (3) use of computational methods for both the corroboration and prediction of proposed intermediates. The scope of this review has been limited to well-characterized examples of copper-oxygen species but seeks to provide a thorough picture of the spectroscopic characteristics and reactivity trends of the copper-oxygen cores discussed.

Binuclear β-diketiminate complexes of copper(i)

The reaction of a series of dinucleating bis(β-diketiminate) pro-ligands with mesitylcopper in the presence and absence of mono and diphosphines has allowed the isolation of a new series of dicopper(i) complexes.

Effects of Ligand Halogenation on the Electron Localization, Geometry and Spin State of Low-Coordinate (β-Diketiminato)iron Complexes

This contribution explores the influences of incorporating electron-withdrawing CF3 and halide groups into β-diketiminate iron complexes of tetrazene and isocyanide. The synthesis of a new halogenated β-diketimine (LCF3,ClH) was accomplished via two different methods, including a novel microwave-assisted synthesis that improves the yield of the difficult condensation. Treatment of an iron(II) complex of this ligand with reductant and azide gives two diiron complexes with novel tetrazenes as bridging ligands. Structural and Mössbauer data show that the bridging tetrazene is a radical anion. The halogenation of the supporting ligand also influences iron(I) complexes of the type LFe(CNtBu)2, which are low-spin and square-planar with alkyl substituents but high-spin and pseudotetrahedral with halogen substituents. DFT calculations suggest that the changes from halogenation come from a combination of steric and electronic effects, and that the electronic influence of ligand halogenation is minor.

On the non-innocence of “Nacnacs”: ligand-based reactivity in β-diketiminate supported coordination compounds

While β-diketiminate (BDI or ‘nacnac’) ligands have been widely adopted to stabilize a wide range of metal ions in multiple oxidation states and coordination numbers, in several occurrences these ligands do not behave as spectators and participate in reactivity.

Tuning steric and electronic effects in transition-metal β-diketiminate complexes

β-Diketiminates are widely used supporting ligands for building a range of metal complexes with different oxidation states, structures, and reactivities. This Perspective summarizes the steric and electronic influences of ligand substituents on these complexes, with an eye toward informing the design of new complexes with optimized properties. The backbone and N-aryl substituents can give significant steric effects on structure, reactivity and selectivity of reactions. The electron density on the metal can be tuned by installation of electron withdrawing or donating groups on the β-diketiminate ligand as well. Examples are shown from throughout the transition metal series to demonstrate different types of effects attributable to systematic variation of β-diketiminate ligands.

Re-evaluating the Cu K pre-edge XAS transition in complexes with covalent metal-ligand interactions

Three [Me2NN]Cu(η2-L2) complexes (Me2NN = HC[C(Me)NAr]2; L2 = PhNO (2), (3), PhCH[double bond, length as m-dash]CH2 (4); Ar = 2,6-Me2-C6H3; ArF = 3,5-(CF3)2-C6H3) have been studied by Cu K-edge X-ray absorption spectroscopy, as well as single- and multi-reference computational methods (DFT, TD-DFT, CASSCF, MRCI, and OVB). The study was extended to a range of both known and theoretical compounds bearing 2p-element donors as a means of deriving a consistent view of how the pre-edge transition energy responds in systems with significant ground state covalency. The ground state electronic structures of many of the compounds under investigation were found to be strongly influenced by correlation effects, resulting in ground state descriptions with majority contributions from a configuration comprised of a Cu(ii) metal center anti-ferromagentically coupled to radical anion O2, PhNO, and ligands. In contrast, the styrene complex 4, which displays a Cu K pre-edge transition despite its formal d10 electron configuration, exhibits what can best be described as a Cu(i):(styrene)0 ground state with strong π-backbonding. The Cu K pre-edge features for these complexes increase in energy from 1 to 4, a trend that was tracked to the percent Cu(ii)-character in the ground state. The unexpected shift to higher pre-edge transition energies with decreasing charge on copper (Q Cu) contributed to an assignment of the pre-edge features for these species as arising from metal-to-ligand charge transfer instead of the traditional Cu1s → Cu3d designation.

Computational electrochemistry: prediction of liquid-phase reduction potentials

This article reviews recent developments and applications in the area of computational electrochemistry. Our focus is on predicting the reduction potentials of electron transfer and other electrochemical reactions and half-reactions in both aqueous and nonaqueous solutions. Topics covered include various computational protocols that combine quantum mechanical electronic structure methods (such as density functional theory) with implicit-solvent models, explicit-solvent protocols that employ Monte Carlo or molecular dynamics simulations (for example, Car-Parrinello molecular dynamics using the grand canonical ensemble formalism), and the Marcus theory of electronic charge transfer. We also review computational approaches based on empirical relationships between molecular and electronic structure and electron transfer reactivity. The scope of the implicit-solvent protocols is emphasized, and the present status of the theory and future directions are outlined.

Cu(I)/O2 chemistry using a β-diketiminate supporting ligand derived from N,N-dimethylhydrazine: a [Cu3O2]3+ complex with novel reactivity

A Cu(I) complex, LCu(CH(3)CN), was prepared and characterized, where L(-) is a sterically unencumbered β-diketiminate ligand, the deprotonated version of 4-(2,2-dimethylhydrazino)dimethylhydrazone-3-penten-2-one (LH). Analysis of FTIR spectra of the products of the reaction of LCu(CH(3)CN) with CO indicate that L(-) is strongly electron donating, and support an equilibrium in solution between monomeric and dimeric forms with terminal and bridging CO ligands, respectively. Low temperature oxygenation of LCu(CH(3)CN) generated a bis(μ-oxo)tricopper complex with a S = 1 [Cu(3)O(2)](3+) core that was identified on the basis of UV-vis (λ(max) (ε, M(-1) cm(-1) per Cu) = 328 (10700), 420 (1500), 590 (835) nm) and X-band electron paramagnetic resonance (EPR) spectroscopy (Δm(s) = 2 transition at 1500 G), electrospray ionization (ESI) mass spectrometry, and spectrophotometric titration (0.35(2) equiv of O(2) per copper atom), magnetic susceptibility (μ(eff) = 2.8(1) BM), and H(2)O(2) detection experiments (no H(2)O(2) evolved upon acidification). Unlike other reported variants supported by neutral N-donor ligands, L(3)Cu(3)O(2) is not reduced by ferrocene, does not abstract H-atoms from phenols or 1,2-dihydroanthracene, oxidizes PPh(3) to Ph(3)P═O, and generates carbonate species upon exposure to CO(2). This unique reactivity for a [Cu(3)O(2)](3+) complex may be traced to the anionic charge and strong electron donating characteristics of L(-).

Metal-dioxygen and metal-dinitrogen complexes: where are the electrons?

Transition-metal complexes of O(2) and N(2) play an important role in the environment, chemical industry, and metalloenzymes. This Perspective compares and contrasts the binding modes, reduction levels, and electronic influences on the nature of the bound O(2) or N(2) group in these complexes. The charge distribution between the metal and the diatomic ligand is variable, and different models for describing the adducts have evolved. In some cases, single resonance structures (e.g. M-superoxide = M-O(2)(-)) are accurate descriptions of the adducts. Recent studies have shown that the magnetic coupling in certain N(2)(2-) complexes differs between resonance forms, and can be used to distinguish experimentally between resonance structures. On the other hand, many O(2) and N(2) complexes cannot be described well with a simple valence-bond model. Defining the situations where ambiguities occur is a fertile area for continued study.

Density functional theory for transition metals and transition metal chemistry

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

Effects of electron-deficient beta-diketiminate and formazan supporting ligands on copper(I)-mediated dioxygen activation

Copper(I) complexes of a diketiminate featuring CF(3) groups on the backbone and dimethylphenyl substituents (4) and a nitroformazan (5) were synthesized and shown by spectroscopy, X-ray crystallography, cyclic voltammetry, and theory to contain copper(I) sites electron-deficient relative to those supported by previously studied diketiminate complexes comprising alkyl or aryl backbone substituents. Despite their electron-poor nature, oxygenation of LCu(CH(3)CN) (L = 4 or 5) at room temperature yielded bis(hydroxo)dicopper(II) compounds and at -80 degrees C yielded bis(mu-oxo)dicopper complexes that were identified on the basis of UV-vis and resonance Raman spectroscopy, spectrophotometric titration results (2:1 Cu/O(2) ratio), electron paramagnetic resonance spectroscopy (silent), and density functional theory calculations. The bis(mu-oxo)dicopper complex supported by 5 exhibited unusual spectroscopic properties and decayed via a novel intermediate proposed to be a metallaverdazyl radical complex, findings that highlight the potential for the formazan ligand to exhibit "noninnocent" behavior.

Mononuclear Cu-O2 complexes: geometries, spectroscopic properties, electronic structures, and reactivity

Using interwoven experimental and theoretical methods, detailed studies of several structurally defined 1:1 Cu-O 2 complexes have provided important fundamental chemical information useful for understanding the nature of intermediates involved in aerobic oxidations in synthetic and enzymatic copper-mediated catalysis. In particular, these studies have shed new light on the factors that influence the mode of O 2 coordination (end-on vs side-on) and the electronic structure, which can vary between Cu(II)-superoxo and Cu(III)-peroxo extremes.