Oxygen binding and activation by the complexes of PY2- and TPA-appended diphenylglycoluril receptors with copper and other metals

Room-Temperature Cu(II) Radical-Triggered Alkyne C-H Activation

A dimeric Cu(II) complex [Cu(II)2L2(μ2-Cl)Cl] (1) built from an asymmetric tridentate ligand (2-(((2-aminocyclohexyl)imino)methyl)-4,6-di-tert-butylphenol) and weakly coordinating anions has been synthesized and structurally characterized. In dichloromethane solution, 1 exists in a monomeric [Cu(II)LCl] (1') (85%)-dimeric (1) (15%) equilibrium, and cyclic voltammetry (CV) and electron paramagnetic resonance (EPR) studies indicate structural stability and redox retention. Addition of phenylacetylene to the CH2Cl2 solution populates 1' and leads to the formation of a transient radical species. Theoretical studies support this notion and show that the radical initiates an alkyne C-H bond activation process via a four-membered ring (Cu(II)-O···H-Calkyne) intermediate. This unusual C-H activation method is applicable for the efficient synthesis of propargylamines, without additives, within 16 h, at low loadings and in noncoordinating solvents including late-stage functionalization of important bioactive molecules. Single-crystal X-ray diffraction studies, postcatalysis, confirmed the framework's stability and showed that the metal center preserves its oxidation state. The scope and limitations of this unconventional protocol are discussed.

Heteroleptic dirhodium(II,II) paddlewheel complexes as carbene transfer catalysts

This review highlights the applications of dirhodium(II,II) paddlewheel complexes with a heteroleptic scaffold. Dirhodium(II,II) paddlewheel complexes are well known as highly efficient and selective carbene transfer catalysts. While the majority of described complexes are homoleptic, comparatively fewer studies have concerned heteroleptic complexes. Here, we emphasise the use of heteroleptic complexes in order to highlight their benefits as carbene transfer catalysts and spur future research. Methods to synthesise heteroleptic dirhodium(II,II) paddlewheel complexes are discussed as well as a categorical review of their types of heteroleptic complexes and the carbene reactions in which they have been used.

Activation of dioxygen by copper metalloproteins and insights from model complexes

Nature uses dioxygen as a key oxidant in the transformation of biomolecules. Among the enzymes that are utilized for these reactions are copper-containing metalloenzymes, which are responsible for important biological functions such as the regulation of neurotransmitters, dioxygen transport, and cellular respiration. Enzymatic and model system studies work in tandem in order to gain an understanding of the fundamental reductive activation of dioxygen by copper complexes. This review covers the most recent advancements in the structures, spectroscopy, and reaction mechanisms for dioxygen-activating copper proteins and relevant synthetic models thereof. An emphasis has also been placed on cofactor biogenesis, a fundamentally important process whereby biomolecules are post-translationally modified by the pro-enzyme active site to generate cofactors which are essential for the catalytic enzymatic reaction. Significant questions remaining in copper-ion-mediated O2-activation in copper proteins are addressed.

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.

Early stages of catalyst aging in the iridium mediated water oxidation reaction

The structure of an iridium Cp* water oxidation catalyst was evaluated as a function of time and applied potential.

Supramolecular catalysis. Part 2: artificial enzyme mimics

The design of artificial catalysts able to compete with the catalytic proficiency of enzymes is an intense subject of research. Non-covalent interactions are thought to be involved in several properties of enzymatic catalysis, notably (i) the confinement of the substrates and the active site within a catalytic pocket, (ii) the creation of a hydrophobic pocket in water, (iii) self-replication properties and (iv) allosteric properties. The origins of the enhanced rates and high catalytic selectivities associated with these properties are still a matter of debate. Stabilisation of the transition state and favourable conformations of the active site and the product(s) are probably part of the answer. We present here artificial catalysts and biomacromolecule hybrid catalysts which constitute good models towards the development of truly competitive artificial enzymes.

Synthetic Models of the Active Site of Cytochromec Oxidase: Influence of Tridentate or Tetradentate Copper Chelates Bearing a HisTyr Linkage Mimic on Dioxygen Adduct Formation by Heme/Cu Complexes

Two synthetic models of the active site of cytochrome c oxidase--[(LN4-OH)CuI-FeII(TMP)]+ (1 a) and [(LN3-OH)CuI-FeII(TMP)]+ (2 a)-have been designed and synthesized. These models each contain a heme and a covalently attached copper moiety supported either by a tetradentate N4-copper chelate or by a tridentate N3-copper chelate including a moiety that acts as a mimic of the crosslinked His-Tyr component of cytochrome c oxidase. Low-temperature oxygenation reactions of these models have been investigated by spectroscopic methods including UV/Vis, resonance Raman, ESI-MS, and EPR spectroscopy. Oxygenation of the tetradentate model 1 a in MeCN and in other solvents produces a low-temperature-stable dioxygen-bridged peroxide [(LN4-OH)CuII-O2-FeIII(TMP)]+ {nuO--O=799 (16O2)/752 cm(-1) (18O2)}, while a heme superoxide species [(TMP)FeIII(O2-)CuIILN3-OH] {nuFe--O2: 576 (16O2)/551 cm(-1) (18O2)} is generated when the tridentate model 2 a is oxygenated in EtCN solution under similar experimental conditions. The coexistence of a heme superoxide species [(TMP)FeIII(O2-)CuIILN3-OH] and a bridged peroxide [(LN3-OH)CuII-O2-FeIII(TMP)]+ species in equal amounts is observed when the oxygenation reaction of 2 a is performed in CH2Cl(2)/7 % EtCN, while the percentage of the peroxide (approximately 70 %) in relation to superoxide (approximately 30 %) increases further when the crosslinked phenol moiety in 2 a is deprotonated to produce the bridged peroxide [(LN3-OH)CuII-O2-FeIII(TMP)]+ {nuO--O: 812 (16O2)/765 cm(-1) (18O2)} as the main dioxygen intermediate. The weak reducibility and decreased O2 reactivity of the tricoordinated CuI site in 2 a are responsible for the solvent-dependent formation of dioxygen adducts. The initial binding of dioxygen to the copper site en route to the formation of a bridged heme-O2-Cu intermediate by model 2 a is suggested and the deprotonated crosslinked His-Tyr moiety might contribute to enhancement of the O2 affinity of the CuI site at an early stage of the dioxygen-binding process.

Homo- and Hetero-Dinuclear Anion Complexes

The tripodal system 4, in which urea fragments are appended to the three terminal amine nitrogen atoms of a tris(2-aminoethyl)amine (tren) subunit, includes a Cu(II) ion and two anions X-, according to a cascade mechanism through three well defined stepwise equilibria in a DMSO solution. The first anion X- (halide, N3-, NCS-, NO2-, H2PO4-) seeks the Cu(II) centre coordinated by the tren moiety; the second anion X- interacts with the trisurea cavity, but this occurs only if the stronger H-bond acceptors, such as N3- and H2PO4-, are used. Binding of the second X- ion is favoured by the preorganising effect exerted by the metal and disfavoured by the steric and electrostatic repulsions between the anions. Under the appropriate conditions, heterodinuclear complexes of formula [Cu(II)(4)(Cl)(H2PO4)] can be obtained in solution, in which Cl- is bound to the metal centre and H2PO4- interacts with the trisurea compartment.