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Copper bromide complexes based on a series of substituted guanidine-quinolinyl and -pyridinyl ligands are reported. The ligand systems were chosen based on the large variation with regard to their flexibility in the backbone, different guanidine moieties and influence by electron density donating groups. Relationships between the molecular structures and spectroscopic and electronic properties are described. Beside the expected increase in activity by substituting the 4-position (NMe 2 vs . H), we showed that a higher flexibility, such as TMG vs . DMEG moiety, leads to a better stabilsiation of the copper(II) complex. Due to the correlation of the potentials and K ATRP values, the catalyst based on the ligand TMGm4NMe 2 py is the most active copper complex for ATRP with a bidentate ligand system. The combination of the strong donating abilities of dimethylamine pyridinyl, the donor properties of the TMG substituent, and the improved flexibility due to the methylene bridging unit leads to high activity. With all NMe 2 -substituted systems standard ATRP experiments were conducted and with more active NMe 2 -substituted pyridinyl systems, ICAR ATRP experiments of styrene were conducted. Low dispersities and ideal molar masses have been achieved.

The synthesis of zinc complexes of guanidine-pyridine hybrid ligands [Zn(DMEGpy)Cl(2)] (C1), [Zn(TMGpy)Cl(2)] (C2), [Zn(DMEGqu)Cl(2)] (C3), [Zn(TMGqu)Cl(2)] (C4), [Zn(DMEGpy)(CH(3)COO)(2)] (C5), [Zn(TMGpy)(CH(3)COO)(2)] (C6), [Zn(DMEGqu)(CH(3)COO)(2)] (C7), [Zn(TMGqu)(CH(3)COO)(2)] (C8), [Zn(DMEGqu)(2)(CF(3)SO(3))][CF(3)SO(3)] (C9) and [Zn(TMGqu)(2)(CF(3)SO(3))][CF(3)SO(3)] (C10) is reported. These zinc complexes were completely characterised and screened regarding their activity in the ring-opening polymerisation of D,L-lactide. They proved to be active initiators in lactide bulk polymerisation, and polylactides with molecular weights (M(w)) up to 176,000 g mol(-1) could be obtained. They combine high activity with robustness towards moisture and air. The influence of reaction temperature and of the anionic component of the zinc salt on the activity of the catalyst, as well as the occurrence of undesired side reactions, was investigated. By correlating these findings with the structural study on the zinc complexes we could deduce a structure-reactivity relationship for the zinc catalysts. This study was accompanied by DFT calculations. The bis-chelate triflate complexes C9 and C10, supported by quinoline-guanidine ligands L3 and L4, exhibit by far the highest reactivity. Systematic comparison of these complexes with their mono-chelate counterparts and their bis-guanidine analogues allows the attributes that promote polymerisation by neutral guanidine ligand systems to be elucidated: accessibility to the zinc centre and Lewis acidity.

The catalytically active center of the enzyme tyrosinase, standing out for its unusual substrate diversity, consists of a side‐on μ‐η2:η2‐peroxido complex (SP). Several ligand systems stabilizing a SP and able to mimic the catalytic activity of the enzyme towards simple phenolic substrates are known. Only a few catalytically active systems based on the isoelectronic isomer structure of SP, a bis(μ‐oxido)dicopper(III) complex (O), were investigated until now. Two years ago, we presented with the TMGbenza a hybrid guanidine based tyrosinase model system stabilizing an O species with an exceptional substrate diversity. Herein we studied the catalytic activity of another O species stabilized by the bisguanidine ligand TMG2tol. The reaction conditions for the catalytic oxygenation were optimized and a broad spectrum of phenolic substrates like naphthol, quinolinols and indolols was tested. Naturally occurring phenazine derivatives like phenazine‐1‐carboxylamide (PCN) and phenazine‐1‐carboxylic acid (PCA) show antifungal as well as antibacterial activity, functioning as biological control agents against crop disease like take‐all. Hence, the synthesized phenazines were evaluated for their potential antimicrobial activities against representative gram‐positive and gram‐negative bacteria. [ABSTRACT FROM AUTHOR]

A previously reported non‐toxic guanidine‐iron catalyst active in the ring opening polymerization (ROP) of polylactide (PLA) under industrially relevant conditions was evaluated for its activity in the alcoholysis and aminolysis of PLA under mild conditions. Kinetic and thermodynamic parameters were determined for the methanolysis of PLA with [FeCl2(TMG5NMe2asme)] (C1) using 1H NMR spectroscopy. A comparison with the Zn analog of C1 showed that the metal center has a large impact on the activity for the alcoholysis. Further, the influence of different nucleophiles was tested broadening the scope of products from PLA waste. C1 is the first discrete metal catalyst reported to be active in the selective aminolysis of PLA. Catalyst recycling, scale‐up experiments and solvent‐free alcoholysis were conducted successfully strengthening the industrial relevance and highlighting aspects of green chemistry. Moreover, the selective depolymerization of PLA in polymer blends was successful. C1 is a promising catalyst for a circular (bio)plastics economy. [ABSTRACT FROM AUTHOR]

Copper guanidine‐quinoline complexes are an important class of bioinorganic complexes that find utilization in electron and atom transfer processes. By substitution of functional groups on the quinoline moiety the electron transfer abilities of these complexes can be tuned. In order to explore the full substitution space by simulations, the accurate theoretical description of the effect of functional groups is essential. In this study, we compare three different methods for the theoretical description of the structures. We use the semi‐empirical tight‐binding method GFN2‐xTB, the density functional TPSSh and the double‐hybrid functional B2PLYP. We evaluate the methods on five different complex pairs (Cu(I) and Cu(II) complexes), and compare how well calculated energies can predict the redox potentials. We find even though B2PLYP and TPSSh yield better accordance with the experimental structures. GFN2‐xTB performs surprisingly well in the geometry optimization at a fraction of the computational cost. TPSSh offers a good compromise between computational cost and accuracy of the redox potential for real‐life complexes. [ABSTRACT FROM AUTHOR]

Inspired by the efficiency of natural enzymes in organic transformation reactions, the development of synthetic catalysts for oxygenation and oxidation reactions under mild conditions still remains challenging. Tyrosinases serve as archetype when it comes to hydroxylation reactions involving molecular oxygen. We herein present new copper(I) guanidine halide complexes, capable of the activation of molecular oxygen at room temperature. The formation of the reactive bis(µ-oxido) dicopper(III) species and the influence of the anion are investigated by UV/Vis spectroscopy, mass spectrometry, and density functional theory. We highlight the catalytic hydroxylation activity towards diverse polycyclic aromatic alcohols under mild reaction conditions. The selective formation of reactive quinones provides a promising tool to design phenazine derivatives for medical applications. [ABSTRACT FROM AUTHOR]

Copper bromide complexes with the ligands TMG6NO2qu, TMG6Brqu, TMG6Methoxyqu, TMG6NMe2qu, TMG6EHOqu and TMG6dBAqu were examined regarding their activity in atom transfer radical polymerization (ATRP). The ligands were inspired by 1,1,3,3‐tetramethyl‐2‐(quinolin‐8‐yl)guanidine (TMGqu) and the substituents have been chosen with a large range between electron withdrawing and donating abilities. The donor properties of the ligands can be strongly influenced and further highly active catalysts based on these systems can be obtained. The ligands with strong donating moieties were in addition modified by alkyl groups to increase the solubility in apolar monomers like styrene. CuI and CuII bromide complexes were crystallised and the structural data correlated to the different substituents and the catalyst activity. The electrochemical potentials E1/2, the equilibrium constants KATRP and rate constants kact and kdeact were determined. Polymerizations of styrene were conducted in solution whereas the catalyst based on TMG6EHOqu shows a good solubility and performance in bulk. [ABSTRACT FROM AUTHOR]

The enzyme tyrosinase contains a reactive side‐on peroxo dicopper(II) center as catalytically active species in C−H oxygenation reactions. The tyrosinase activity of the isomeric bis(μ‐oxo) dicopper(III) form has been discussed controversially. The synthesis of bis(μ‐oxo) dicopper(III) species [Cu2(μ‐O)2(L1)2](X)2 ([O1](X)2, X=PF6−, BF4−, OTf−, ClO4−), stabilized by the new hybrid guanidine ligand 2‐{2‐((dimethylamino)methyl)phenyl}‐1,1,3,3‐tetramethylguanidine (L1), and its characterization by UV/Vis, Raman, and XAS spectroscopy, as well as cryo‐UHR‐ESI mass spectrometry, is described. We highlight selective oxygenation of a plethora of phenolic substrates mediated by [O1](PF6)2, which results in mono‐ and bicyclic quinones and provides an attractive strategy for designing new phenazines. The selectivity is predicted by using the Fukui function, which is hereby introduced into tyrosinase model chemistry. Our bioinspired catalysis harnesses molecular dioxygen for organic transformations and achieves a substrate diversity reaching far beyond the scope of the enzyme. [ABSTRACT FROM AUTHOR]

The increasing awareness of sustainability has led to enormous growth of the demand for bio‐based and biodegradable polymers such as poly(lactide) (PLA). In industry, polymerization of lactide is currently carried out using tin catalysts (e. g. tin(II) ethyl hexanoate, Sn(Oct)2). Since the catalyst remains in the polymer, it can accumulate in the soil or in the human body after degradation and cause damage due to its toxicity. Therefore, a search for a suitable substitute for this catalyst has been going on for decades. Guanidine metal complexes prove to be excellent catalysts in the polymerization of lactide. They are not only convincing because of their activity and the synthesis of high molar mass polymers, but also show a high robustness against high temperatures, oxidation as well as residual protic impurities in the monomer. Herein, key zinc and iron guanidine complexes are discussed with respect to their apparent rate constant (kapp) and rate constant of propagation (kp), produced molar masses and the mechanism involved. [ABSTRACT FROM AUTHOR]

Polylactide is a biodegradable versatile material based on annually renewable resources and thus CO2‐neutral in its lifecycle. Until now, tin(II)octanoate [Sn(Oct2)] was used as catalyst for the industrial ring‐opening polymerization of lactide in spite of its cytotoxicity. On the way towards a sustainable catalyst, three iron(II) hybrid guanidine complexes were investigated concerning their molecular structure and applied to the ring‐opening polymerization of lactide. The complexes could polymerize unpurified technical‐grade rac‐lactide as well as recrystallized l‐lactide to long‐chain polylactide in bulk with monomer/initiator ratios of more than 5000:1 in a controlled manner following the coordination–insertion mechanism. For the first time, a biocompatible complex has surpassed Sn(Oct)2 in its polymerization activity under industrially relevant conditions. [ABSTRACT FROM AUTHOR]

Non-toxic, highly active and robust complexes are the holy grail as ideal green catalysts for the polymerisation of biorenewable and biodegradable polylactide. Four new zinc guanidine complexes [ZnCl2(TMG4NMe2asme)], [ZnCl2(TMG5Clasme)], [ZnCl2(TMG5Measme)] and [ZnCl2(TMG5NMe2asme)] with different electron-donating and electron-withdrawing groups on the ligand's aromatic backbone have been synthesised. Ligands are derived from low-cost commercially available compounds and have been converted by a three- or four-step synthesis process into the desired ligand in good yields. The compounds have been fully characterised and tested in the ROP of rac-LA under industrially relevant conditions. The complexes are based on the recently published structure [ZnCl2(TMGasme)] which has shown high activity in the polymerisation of lactide at 150 °C. Different substituents in the para-position of the guanidine moiety significantly increase the polymerisation rate whereas positioning substituents in the meta-position causes no change in the reaction rate. With molecular weights over 71 000 g mol−1 being achievable, the best system produces polymers for multiple industrial applications and its polymerisation rate approaches that of Sn(Oct)2. The robust systems are able to polymerise non-purified lactide. The initiation of the polymerisation is suggested to occur due to impurities in the monomer. [ABSTRACT FROM AUTHOR]

In previous studies, copper halide complexes of the guanidinoquinoline (GUAqu) ligands 1,3‐dimethyl‐N‐(quinolin‐8‐yl)‐imidazolidin‐2‐imine (DMEGqu) and 1,1,3,3‐tetramethyl‐2‐(quinolin‐8‐yl)‐guanidine (TMGqu) were successfully implemented in atom transfer radical polymerization (ATRP) and could be further enhanced by introduction of alkyl substituents at C6 position of the quinoline backbone. Herein, the ligand DMEG6phqu is presented. The quinoline backbone of this ligand is equipped with a phenyl substituent at C6 position. This study deals with the influence of the phenyl substituent on solubility and molecular structural properties of DMEG6phqu CuI and CuII bromide complexes. In contrast to previously reported systems, the CuIBr complex of DMEG6phqu crystallizes as a trigonal coordinated monochelate complex. However, NMR and UV/Vis spectroscopic experiments indicate that DMEG6phqu forms a bischelate species in solution. The influence of the substituent on the complex redox potential and ATRP equilibrium constant KATRP is discussed. In contrast to expectations, it turned out that copper halide complexes of DMEG6phqu are completely insoluble in the apolar monomer styrene. However, ATRP kinetics were performed in solution and the results are compared to previous studies. [ABSTRACT FROM AUTHOR]

Ligands DMEG6etqu, TMG6etqu, DMEG6buqu, and TMG6buqu were developed on the basis of guanidine quinoline (GUAqu) ligands 1,3‐dimethyl‐N‐(quinolin‐8‐yl)imidazolidin‐2‐imine (DMEGqu) and 1,1,3,3‐tetramethyl‐2‐(quinolin‐8‐yl)guanidine (TMGqu). These ligands feature an alkyl substituent at the C6 of the quinoline backbone. The synthetic strategy developed here enables inexpensive syntheses of any kind of C6‐substituted GUAqu ligands. On one hand, the alkylation increases the solubility of corresponding copper complexes in apolar atom transfer radical polymerization (ATRP) monomers like styrene. On the other hand, it has a significant electronic influence and thus an effect on the donor properties of the new ligands. Seven CuI and CuII complexes of DMEG6etqu and TMG6etqu have been crystallized and were studied with regard to their structural and electrochemical properties. CuI and CuII complexes of DMEG6buqu and TMG6buqu turned out to be perfectly soluble in pure styrene even at room temperature, which makes them excellent catalysts in the ATRP of apolar monomers. The key characteristics of the ATRP equilibrium, KATRP and kact, were determined for the new complexes. In addition, we used our recently developed DFT methodology, NBO analysis, and isodesmic reactions to predict the influence of the introduced alkyl substituents. It turned out that high conformational freedom in the complex structures leads to a significant uncertainty in prediction of the thermodynamic properties. [ABSTRACT FROM AUTHOR]

Invited for this month's cover is the group of Professor Sonja Herres‐Pawlis at the RWTH Aachen (Germany). The cover image illustrates the complex yet flexible circular economy of (bio)plastics and the role of a Zn‐based catalyst therein. The Research Article itself is available at 10.1002/cssc.202300192. [ABSTRACT FROM AUTHOR]

Combining the new aromatic bis(guanidine) ligand bis(tetramethylguanidino)toluene (TMG²tol) and different copper salts led to four new complexes. The molecular structures were analysed by X-ray diffraction. Three of the complexes are active in biomimetic activation of oxygen. It is the first time a bis-µ-oxido species of an aromatic guanidine copper complex is analysed by Raman and UV/Vis spectroscopy. The formation and decay of the bis(µ-oxido)dicopper species were investigated at low temperatures up to 273 K by UV/Vis spectroscopy with a stopped-flow setup. The results showed that theCu2O2 intermediate of an aromatic guanidine-copper complex decays much faster than most known bis(guanidine) complexes. Moreover, natural bond orbital (NBO) analyses revealed that the aromatic guanidine is the stronger donor compared to the aliphatic one. [ABSTRACT FROM AUTHOR]

Front Cover: Open Loop Recycling - Guanidine Iron(II) Polymerization Catalyst for the Depolymerization of Polylactide (Chem. Alcoholysis, aminolysis, circular economy, iron catalyst, polylactide, recycling Keywords: alcoholysis; aminolysis; circular economy; iron catalyst; polylactide; recycling EN alcoholysis aminolysis circular economy iron catalyst polylactide recycling 1 1 1 03/03/23 20230301 NES 230301 B Besides closed loop recycling b approaches-monomer to product to monomer-open loop recycling is a promising strategy for a circular plastics economy, preventing waste and the use of fossil resources. [Extracted from the article]

The synthesis of the new hybrid guanidine ligands DMEGdmap, DMEGdeae, TMGdmab, DMEGdmab, TMGdeab and DMEGdeab is reported. These ligands were combined with zinc chloride, and the six obtained new complexes were structurally characterised by X-ray crystallography and NMR spectroscopy. Further six new zinc chloride complexes were obtained from the hybrid guanidine ligands TMGdmae, DMEGdmae, TMGdeae, TMGdmap, TMGdeap and TEGdeap. Each of the twelve complexes possesses a tetrahedral coordination geometry. The donor situation between guanidine and amine donors was evaluated by use of density functional theory. These complexes show robust activity in the melt polymerisation of technical unsublimed lactide. For selected complexes kinetic polymerisation experiments were performed; these showed first-order behavior. The end-group was established by NMR spectroscopy. [ABSTRACT FROM AUTHOR]

The new N-heterocyclic carbene 1-(2, 6-diisopropylphenyl)-3, 4,5-trimethylimidazolin-2-ylidene (IPrMe) ( 3) was prepared in three steps from methyl isothiocyanate and 2, 6-diisopropylaniline, affording N-methyl- N′-2, 6-diisopropylthiourea ( 1). Reaction of 1 with 3-hydroxybutanone gave the imidazolin-2-thione (IPrMe)S ( 2), which was reduced with elemental potassium to the carbene IPrMe ( 3). Treatment of 3 with trimethylsilyl azide (Me3SiN3) in boiling toluene, followed by desilylation in methanol, gave the imidazolin-2-imine (IPrMe)NH ( 4). The ethylene-bridged diimine ligand N, N′-bis[1-(2, 6-diisopropylphenyl)-3, 4,5-trimethylimidazolin-2-ylidene]-1, 2-ethanediamine, (IPrMe)NCH2CH2N(IPrMe) ( 5) was prepared by reaction of 4 with 1, 2-ethyleneditosylate and subsequent deprotonation with KO tBu. The reaction of 5 with metal dichlorides furnished the tetrahedral complexes [( 5) MCl2] [ M = Fe ( 6), M = Ni ( 7), M = Zn ( 8)] and the square-planar complex [( 5)PdCl2] ( 9), as revealed by X-ray diffraction analyses. The X-ray crystal structures of 2, 3, and 4 were also established. Complex 8 and related zinc(II) bis(imidazolin-2-imine) complexes were tested in the polymerization of d, l-lactide. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations of the equilibrium geometry, vibrational modes, ionization energies, electron affinities, and optical response of [Cu2(btmgp)2(μ-O)2]2+ (oxo) and [Cu2(btmgp)2(μ-η2:η2-O2)]2+ (peroxo) are presented. Comprehensive benchmarking shows that the description of the oxo-peroxo energetics is still a torture track for DFT, but finds the molecular geometry to be comparatively robust with respect to changes in the exchange-correlation functionals and basis sets. Pure functionals favor the oxo core found experimentally, whereas hybrid functionals shift the bias toward the peroxo core. Further stabilization of peroxo core results from relaxing the spin degrees of freedom using the broken-symmetry (BS) approach. Dispersion effects, conversely, tend to favor the oxo configuration. Triple-zeta basis sets are found to represent a sensible compromise between numerical accuracy and computational effort. Particular attention is paid to the modification of the electronic structure, optical transitions, and excited-state energies along the transition path between the oxo and peroxo species. The excited-state potential energy surface calculations indicate that two triplet states are involved in the transition that stabilize the BS solution. Charge decomposition and natural transition orbital analyses are used for obtaining microscopic insight into the molecular orbital interactions. Here, the crucial role of guanidine π-interactions is highlighted for the stabilization of the Cu2O2 core. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Ground- and excited-state properties of copper(II) charge-transfer systems have been investigated starting from density-functional calculations with particular emphasis on the role of (i) the exchange and correlation functional, (ii) the basis set, (iii) solvent effects, and (iv) the treatment of dispersive interactions. Furthermore (v), the applicability of TD-DFT to excitations of copper(II) bis(chelate) charge-transfer systems is explored by performing many-body perturbation theory (GW + BSE), independent-particle approximation and ΔSCF calculations for a small model system that contains simple guanidine and imine groups. These results show that DFT and TD-DFT in particular in combination with hybrid functionals are well suited for the description of the structural and optical properties, respectively, of copper(II) bis(chelate) complexes. Furthermore, it is found an accurate theoretical geometrical description requires the use of dispersion correction with Becke-Johnson damping and triple-zeta basis sets while solvent effects are small. The hybrid functionals B3LYP and TPSSh yielded best performance. The optical description is best with B3LYP, whereby heavily mixed molecular transitions of MLCT and LLCT character are obtained which can be more easily understood using natural transition orbitals. An natural bond orbital analysis sheds light on the donor properties of the different donor functions and the intraguanidine stabilization during coordination to copper(I) and (II). © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, guanidine copper compounds are reviewed with an emphasis on structural characteristics and their application in bioinorganic chemistry and catalysis. The literature survey includes the copper coordination chemistry of biological guanidine derivatives, peralkylated guanidines including bicyclic ones and of further nitrogen-rich guanidine-type systems such as azoimidazoles, triazolopyrimidines and triaminoguanidines. From a sporadic interest dating back to the 1960s, research on this ligand class and its use in copper coordination chemistry has gained new impetus since 2000. With the synthesis of examples with sophisticated substitution at the characteristic CN3 framework, complex problems can be addressed in several fields of chemistry. This paper analyses the different types of guanidines for their special donor properties and highlights the specific advantages of guanidines as neutral donor ligands in copper coordination chemistry where a great variety of coordination modes was found. These compounds offer the ability to distribute the formal positive charge of the metal throughout the guanidine unit and represent more than simple σ-donating ligands. [ABSTRACT FROM AUTHOR]

Keywords: copper catalysis; dioxygen activation; guanidines; phenazines; tyrosinase EN copper catalysis dioxygen activation guanidines phenazines tyrosinase 1 1 1 06/22/20 20200618 NES 200618 B The tyrosinase-like activity b of a new bioinspired bis( -oxo) dicopper(III) catalyst was investigated herein. The selective C-H activation of mono- and bicyclic substrates provides a new benchmark to future developments on tyrosinase model systems on the route to atom-economic oxygen-transfer reactions. Copper catalysis, dioxygen activation, guanidines, phenazines, tyrosinase. [Extracted from the article]

The structure of the title compound, catena-poly­[[μ-cyano-1:2 C: N-[1,3-bis­(tetra­methyl­guanidino)­propane-1κ2 N, N′]­di­copper(I)]-μ-cyano-2:1′ C: N], [Cu2(CN)2(C13H30N4)] n, shows one-dimensional zigzag {Cu(CN)}∞ chains with copper centres trigonally coordinated either by a chelating guanidine and a cyano ligand or by three cyano ligands. [ABSTRACT FROM AUTHOR]

The cover picture shows a guanidine–quinoline–copper complex as a new entatic‐state model. Small changes in the ligand strongly affect the molecular structure, which demonstrates the subtle balance of steric and electronic effects at a metal center. The unique combination of solid‐state and solution structural investigations by classical single‐crystal X‐ray diffraction and X‐ray absorption spectroscopy together with density functional theory reveals deep insights into this entatic‐state model. Details are discussed in the article by S. Herres‐Pawlis et al. on p. 4731 ff. For more on the story behind the cover research, see the Cover Profile. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations of the equilibrium geometry, vibrational modes, ionization energies, electron affinities, and optical response of [Cu2(btmgp)2(μ-O)2]2+ (oxo) and [Cu2(btmgp)2(μ-η2:η2-O2)]2+ (peroxo) are presented. Comprehensive benchmarking shows that the description of the oxo-peroxo energetics is still a torture track for DFT, but finds the molecular geometry to be comparatively robust with respect to changes in the exchange-correlation functionals and basis sets. Pure functionals favor the oxo core found experimentally, whereas hybrid functionals shift the bias toward the peroxo core. Further stabilization of peroxo core results from relaxing the spin degrees of freedom using the broken-symmetry (BS) approach. Dispersion effects, conversely, tend to favor the oxo configuration. Triple-zeta basis sets are found to represent a sensible compromise between numerical accuracy and computational effort. Particular attention is paid to the modification of the electronic structure, optical transitions, and excited-state energies along the transition path between the oxo and peroxo species. The excited-state potential energy surface calculations indicate that two triplet states are involved in the transition that stabilize the BS solution. Charge decomposition and natural transition orbital analyses are used for obtaining microscopic insight into the molecular orbital interactions. Here, the crucial role of guanidine π-interactions is highlighted for the stabilization of the Cu2O2 core. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: The syntheses of the aliphatic bidentate guanidine–amine-hybrid ligands DMEGdmae (L1), TMGdmae (L2), TMGdeae (L3) and DPipGdmae (L4) as well as the reaction of their Cu(I) complexes with molecular oxygen (monitored by UV–Vis spectroscopy) are reported. The molecular structures of 10 bis(μ-hydroxo) dicopper complexes based on these ligands are described. The solid state structures of [Cu2(μ-OH)2(DMEGdmae)2]X2 (X− =I− (1), CF3SO3 − (2), SbF6 − (3), PF6 − (4)), [Cu2(μ-OH)2(TMGdmae)2]X2 (X− =I− (5), CF3SO3 − (6)), [Cu2(μ-OH)2(TMGdeae)2]Cu2I4 (7) and [Cu2(μ-OH)2(DPipGdmae)2]X2 (X− =CF3SO3 − (8), SbF6 − (9), PF6 − (10)) show a square-planar distorted coordination of the copper(II) ion. The bis(μ-hydroxo) dicopper complex 1 exhibits a Cu···Cu distance of 2.860(1)Å, which is one of the smallest observed for hydroxo-bridged copper compounds so far. The influence of the anion on the structure of the bis(μ-hydroxo) dicopper(II) unit is analyzed for the reported complexes and a literature overview with emphasis on the structural characteristics of the Cu2O2 moiety of bis(μ-hydroxo) dicopper(II) and bis(μ-oxo) dicopper(III) is given. [Copyright &y& Elsevier]