Roxana Haase

Phenolate Hydroxylation in a Bis(μ-oxo)dicopper(III) Complex : Lessons from the Guanidine/Amine Series

A new hybrid permethylated-amine-guanidine ligand based on a 1,3-propanediamine backbone (2L) and its Cu-O2 chemistry is reported. [(2L)CuI(MeCN)]1+ complex readily oxygenates at low temperatures in polar aprotic solvents to form a bis(mu-oxo)dicopper(III) (O) species (2b), similar to the parent bis-guanidine ligand complex (1b) and permethylated-diamine ligand complex (3b). UV-vis and X-ray absorption spectroscopy experiments confirm this assignment of 2b as an O species, and full formation of the 2:1 Cu-O2 complex is demonstrated by an optical titration with ferrocene-monocarboxylic acid (FcCOOH). The UV-vis spectra of 1b and 2b with guanidine ligation show low-intensity visible features assigned as guanidine pi --> Cu2O2 core transitions by time-dependent density functional theory (TD-DFT) calculations. Comparison of the reactivity among the three related complexes (1b-3b) with phenolate at 195 K is particularly insightful as only 2b hydroxylates 2,4-di-tert-butylphenolate to yield 3,5-di-tert-butylcatecholate (>95% yield) with the oxygen atom derived from O2, reminiscent of tyrosinase reactivity. 1b is unreactive, while 3b yields the C-C radical-coupled bis-phenol product. Attenuated outer-sphere oxidative strength of the O complexes and increased phenolate accessibility to the Cu2O2 core are attributes that correlate with phenolate hydroxylation reactivity observed in 2b. The comparative low-temperature reactivity of 1b-3b with FcCOOH (O-H BDE 71 kcal mol(-1)) to form the two-electron, two-proton reduced bis(mu-hydroxo)dicopper(II,II) complex is quantitative and presumably precedes through two sequential proton-coupled electron transfer (PCET) steps. Optical titrations along with DFT calculations support that the reduced complexes formed in the first step are more powerful oxidants than the parent O complexes. These mechanistic insights aid in understanding the phenol to bis-phenol reactivity exhibited by 2b and 3b.

Catching an Entatic State—A Pair of Copper Complexes

The structures of two types of guanidine-quinoline copper complexes have been investigated by single-crystal X-ray crystallography, K-edge X-ray absorption spectroscopy (XAS), resonance Raman and UV/Vis spectroscopy, cyclic voltammetry, and density functional theory (DFT). Independent of the oxidation state, the two structures, which are virtually identical for solids and complexes in solution, resemble each other strongly and are connected by a reversible electron transfer at 0.33 V. By resonant excitation of the two entatic copper complexes, the transition state of the electron transfer is accessible through vibrational modes, which are coupled to metal-ligand charge transfer (MLCT) and ligand-metal charge transfer (LMCT) states.

A Halide‐Induced Copper(I) Disulfide/Copper(II) Thiolate Interconversion

Shortly after their discovery as protein active sites, copper sulfur complexes entered the stage of modern synthetic coordination chemistry. In this respect, the combination of copper (II) and potentially reducing thiolate ligands appears especially attractive owing to its relevance to the CuA within cytochrome-c oxidase and N2O reductase. [2] Despite extensive research efforts, dinuclear copper(II) or mixed-valent copper(I/II) thiolate complexes with protein active site properties are rare. This situation can be traced back to the ligands under investigation, which are not capable of preventing Cu from being reduced to Cu along with the formation of organodisulfides. On the other hand, the selective and reversible oxidation of thiols/thiolates to organo-disulfides (e.g., cysteine to cystine) is one of the most important biological reactions resulting in the formation of disulfide bridges within peptides and proteins. In addition, the reaction system thiol– disulfide is an important electron source for a number of redox processes in biological systems, making it an indispensable component of basic regulatory processes during signal transduction and enzyme activity. Nevertheless, disulfide– thiolate redox processes are largely unexplored in an inorganic context, although they were investigated in terms of the participation of copper(II) ions in kinetic studies more than 50 years ago. Quite recently, further reports have been published on this subject, and in 2002 a unique model system was described which—under the influence of copper and controlled by halide ions—is able to shift the thiolate– disulfide equilibrium reversibly and completely from the one side to the other. This surprising discovery indicates an enormous but largely unrecognized potential for such reaction systems to act as novel electron sources and sinks, which has motivated us to explore this topic more deeply. We report herein a previously unknown chloride-induced disulfide–thiolate interconversion, leading from the copper(I) disulfide complex cation [Cu2{(NGuaS-)2}2] 2+ (1) to the electrically neutral copper(II) thiolate species [Cu2(NGuaS)2Cl2] (2 ; there is no longer an S S bond in the NGauS ligands, thus it is no longer written as (NGuaS )2). Both compounds (1 as 1[OTf]2) were characterized by X-ray crystallography. The proposed oxidation states of the Cu ions were confirmed by K-edge measurements. The reverse reaction can be initiated by removal of the chloride ligands from the corresponding thiolate complex (Scheme 1).

Stabilisation of a Highly Reactive Bis(μ-oxo)dicopper(III) Species at Room Temperature by Electronic and Steric Constraint of an Unconventional Nitrogen Donor Ligand

Herein we present an innovative combination of EXAFSspectroscopy and resonant Raman scattering for the charac-terisation of the ground state and structural dynamics of athermally stable binuclear bisACHTUNGRE(m-oxo) dicopperACHTUNGRE(III) species.Peralkylated bis(guanidine)-based ligands are used in thesynthesis of this compound.

Neue Bisguanidin-Kupfer-Komplexe und ihre Anwendung in der ATRP/ New Bisguanidine-Copper Complexes and their Application in ATRP

The ligands TMG2e [bis(N,N,N´,N´-tetramethylguanidino)ethane] and DMEG2e [N1,N2-bis(1,3-dimethylimidazolin-2-ylidene)ethane-1,2-diamine] were used in the complexation of copper cations to give the new complexes [Cu(TMG2e)2][Cu2I4], [Cu(TMG2e)Cl2] and [Cu(DMEG2e)2]-[CuCl2]. Single-crystal structure determination shows that the complexes [Cu(TMG2e)Cl2] and [Cu(DMEG2e)2][CuCl2] both crystallise in the monoclinic space group C2/c, the complex [Cu(TMG2e)2][Cu2I4] in the orthorhombic space group Pbca. The copper atoms in all complex cations reside in a coordination environment between tetrahedral and square-planar geometry. The application of copper complexes with TMG2e and DMEG2e as ligands in atom transfer radical polymerisation (ATRP) was investigated with styrene as monomer. The polymerisation process with both ligand systems shows even at low temperature unexpected high conversions and molecular weight distributions that are evidence of a well controlled ATRP. These first results in the application of guanidine ligands in ATRP show that these ligands have high potential, but that further process optimisations and ligand tuning are necessary to develop highly active catalysts for ATRP. Graphical Abstract Neue Bisguanidin-Kupfer-Komplexe und ihre Anwendung in der ATRP/ New Bisguanidine-Copper Complexes and their Application in ATRP

Colloidal Gels Formed by Dilute Aqueous Dispersions of Surfactant and Fatty Alcohol

Mixtures of surfactants, fatty alcohol as cosurfactant, and water often form gels, even at high dilution. We have investigated highly dilute samples of the system sodium dodecyl sulfate/cetyl alcohol/water (SDS/CA/D2O) at varying SDS/CA ratio. Gel-like samples are obtained only at low SDS/CA ratios. The phase structure and the dynamics of the molecules have been determined by a combination of proton and carbon-13 NMR spectroscopy, cryo-transmission electron microscopy, very-small-angle neutron and x-ray scattering, differential scanning calorimetry, rheology, and pulsed gradient spin echo NMR diffusometry. The gel-like character is found to be caused by jammed uni- and multilamellar vesicles.

Dissecting the role of guanidine copper complexes in atom transfer radical polymerization by density functional theory

Since the development of the living / controlled radical polymerization method ATRP (= atom transfer radical polymerization) in 1995 [1] new catalysts for this reaction have been intensively investigated. This method conquered rapidly numerous fields in chemistry ranging from organic and polymer synthesis to materials science and nanotechnology. Guanidine copper complexes display high activity in ATRP of styrene but the factors imposed on the activator/deactivator equilibrium are multifaceted [2]. Herein we report on new copper complexes with the guanidine ligand 1,3,3-tetramethyl-2-(quinolin-8-yl)guanidine which produce polystyrene with a narrow weight distribution and in high yields. Kinetic studies showed that the polymerization is of living character. Structural characterization leads us to a proposal for the activator and deactivator structures which control the ATRP (Figure ​(Figure1).1). By density functional theory, we were able to dissect the influences on the position of the equilibrium between the Cu(I) and Cu(II) complex (ligand donor strength, halogene bond strength, redox potential, coordinative space) operating the polymerization process. Figure 1 Activator/deactivator equilibrium in ATRP.

Characterization of the optically excited state of a bis (μ-oxo)-dicopper(III) species mimicking the hemocyanin and tyrosinase active sites

Optical excited molecules play an increasingly important role in research at light sources. Here we compare two approaches to structurally characterize such states, pumped-XAS and an innovative combination of EXAFS spectroscopy and resonant Raman scattering. The later combination allows to study efficiently charge-transfer complexes in their ground and excited state. The design of the experimental setups for pumped-XAS and resonant Raman scattering at different temperatures as well as results obtained are presented. We receive twofold information on the structural and electronic properties of both states elucidating the alterations upon induced charge transfer in the Cu2O2-core of a system mimicking the active site of tyrosinase and hemocyanin.