The one-electron-reduced (OER) species of rhenium-based catalysts in the homogeneous photocatalytic reduction of CO₂ represents the starting point of light-induced deactivation processes, which lead to low catalyst activity and productivity. Herein, we report the suppression of these processes using pulsed light. Experimental parameters to avoid the irradiation of the OER species were estimated, leading us to conclude that pulse lengths shorter than 1 ns and repetition rates lower than 33 Hz should be employed. [Re(bpy)(CO)₃X] (bpy=2,2′-bipyridine; X=Cl (1), Br (3)) catalysts were employed in pulsed irradiation experiments using different light sources, pulse lengths and repetition rates. Pulsed irradiation experiments using LEDs revealed that a minimum average photon flux is necessary to enable CO₂ conversion. Furthermore, pulsed laser light with a 10 ns pulse length partially prevented lightinduced deactivation processes, whereas efficient suppression was achieved using a 30 ps pulse length.

Silicon nanocrystals (Si-NCs) are emerging as an attractive class of quantum dots owing to the natural abundance of silicon in the Earth's crust, their low toxicity compared to many Group II–VI and III–V based quantum dots, compatibility with the existing semiconductor industry infrastructure, and their unique optoelectronic properties. Despite these favorable qualities, Si-NCs have not received the same attention as Group II–VI and III–V quantum dots, because of their lower emission quantum yields, difficulties associated with synthesizing monodisperse particles, and oxidative instability. Recent advancements indicate the surface chemistry of Si-NCs plays a key role in determining many of their properties. This Review summarizes new reports related to engineering Si-NC surfaces, synthesis of Si-NC/polymer hybrids, and their applications in sensing, diodes, catalysis, and batteries.

Herein we present the functionalization of freestanding silicon nanosheets (SiNSs) by radical-induced hydrosilylation reactions. An efficient hydrosilylation of Si−H terminated SiNSs can be achieved by thermal initiation or the addition of diazonium salts with a variety of alkene or alkyne derivatives. The radical-induced hydrosilylation is applicable for a wide variety of substrates with different functionalities, improving the stability and dispersibility of the functional SiNSs in organic solvents and potentially opening up new fields of application for these hybrid materials.

Aufgrund der hohen Verfügbarkeit in der Erdkruste und seiner geringen Toxizität im Vergleich zu einigen anderen Halbleitern ist Silicium der Vorreiter in der Elektroindustrie. Daher finden Silicium-Nanokristalle (SiNKs) insbesondere aufgrund ihrer einzigartigen optoelektronischen Eigenschaften ein hohes Interesse in der Halbleiterindustrie und sie haben das Potenzial, die toxischen Quantenpunkte (Elemente der Gruppen II–VI und III–V) zu substituieren. Dennoch erhielten SiNKs wegen ihrer geringeren Photolumineszenz(PL)-Quantenausbeuten, einer schwierig zu erzielenden monodispersen Partikelverteilung und ihrer Oxidationsanfälligkeit nicht dieselbe Aufmerksamkeit wie ihre schwermetallhaltigen Analoga. Daher wurde vermehrt an der Funktionalisierung von SiNK-Oberflächen geforscht und die genannten Faktoren im Wesentlichen verbessert. Vor diesem Hintergrund fassen wir in diesem Aufsatz die neuesten Entwicklungen in der Funktionalisierung von SiNK-Oberflächen, beschriebene SiNK/Polymer-Hybridmaterialien und deren Anwendungen in den Bereichen Sensorentwicklung, Leuchtdioden, Katalyse und Akkumulatoren zusammen.

New monometallic amine-bis(phenolato)cobalt(II) [(ONNO)^RCo^II] (R = CMe₂Ph; Cl; Br) complexes have been synthesized and fully characterized including X-ray crystallographic analysis. These Co^II complexes show good activity for the formation of cyclic propylene carbonate in combination with tetrabutylammonium bromide (TBAB) as a co-catalyst. The reaction parameters such as carbon dioxide pressure, co-catalyst loadings and temperature were varied to determine the ideal reaction conditions. These catalysts were also employed in copolymerization reactions of cyclohexene oxide/CO₂ and propylene oxide/CO₂. [(ONNO)^ClCo^II]*(MeOH) was found to effectively copolymerize cyclohexene oxide (CHO) and CO₂. This is the first reported amine-bis(phenolato) cobalt(II) complex to be active in the copolymerization of CO₂ and CHO. In-depth stability studies were conducted (Evan's method) to validate Co^II as the active species required for copolymerization. End-group analysis via NMR, ESI-MS and MALDI-TOF revealed the presence of 4-(dimethylamino)pyridine (DMAP) and methoxy terminated chains.

Hydride-terminated photoluminescent silicon nanocrystals (SiNCs) were functionalized with organolithium compounds. The reaction is proposed to proceed through cleavage of SiSi bonds and formation of a SiLi surface species. The method yields colloidally stabilized SiNCs at room temperature with short reaction times. SiNCs with mixed surface functionalities can be prepared in an easy two-step reaction by this method by quenching of the SiLi group with electrophiles or by addressing free SiH groups on the surface with a hydrosilylation reaction.

The dinuclear zinc complex reported by us is to date the most active zinc catalyst for the co-polymerization of cyclohexene oxide (CHO) and carbon dioxide. However, co-polymerization experiments with propylene oxide (PO) and CO₂ revealed surprisingly low conversions. Within this work, we focused on clarification of this behavior through experimental results and quantum chemical studies. The combination of both results indicated the formation of an energetically highly stable intermediate in the presence of propylene oxide and carbon dioxide. A similar species in the case of cyclohexene oxide/CO₂ co-polymerization was not stable enough to deactivate the catalyst due to steric repulsion.

Poly(3-hydroxybutyrate) (PHB) is produced by numerous bacteria as carbon and energy reserve storage material. Whereas nature only produces PHB in its strictly isotactic (R) form, homogeneous catalysis, when starting from racemic (rac) β-butyrolactone (BL) as monomer, can in fact produce a wide variety of tacticities. The variation of the metal center and the surrounding ligand structure enable activity as well as tacticity tuning. However, no homogeneous catalyst exists to date that is easy to modify, highly active, and able to produce PHB with high isotacticities from rac-β-BL. Therefore, in this work, the reaction kinetics of various 2-methoxyethylamino-bis(phenolate) lanthanide (Ln=Sm, Tb, Y, Lu) catalysts are examined in detail. The order in monomer and catalyst are determined to elucidate the reaction mechanism and the results are correlated with DFT calculations of the catalytic cycle. Furthermore, the enthalpies and entropies of the rate-determining steps are determined through temperature-dependent in situ IR measurements. Experimental and computational results converge in one specific mechanism for the ring-opening polymerization of BL and even allow us to rationalize the preference for syndiotactic PHB.

Functioning as active catalysts for propylene oxide (PO) and carbon dioxide copolymerization, cobalt(III)-based salen and porphyrin complexes have drawn great attention owing to their readily modifiable nature and promising catalytic behavior, such as high selectivity for the copolymer formation and good regioselectivity with respect to the polymer microstructure. Both cobalt(III)–salen and porphyrin catalysts have been found to undergo reduction reactions to their corresponding catalytically inactive cobalt(II) species in the presence of propylene oxide, as evidenced by UV/Vis and NMR spectroscopies and X-ray crystallography (for cobalt(II)–salen). Further investigations on a TPPCoCl (TPP=tetraphenylporphyrin) and NaOMe system reveal that such a catalyst reduction is attributed to the presence of alkoxide anions. Kinetic studies of the redox reaction of TPPCoCl with NaOMe suggests a pseudo-first order in cobalt(III)–porphyrin. The addition of a co-catalyst, namely bis(triphenylphosphine)iminium chloride (PPNCl), into the reaction system of cobalt(III)–salen/porphyrin and PO shows no direct stabilizing effect. However, the results of PO/CO₂ copolymerization by cobalt(III)–salen/porphyrin with PPNCl suggest a suppressed catalyst reduction. This phenomenon is explained by a rapid transformation of the alkoxide into the carbonate chain end in the course of the polymer formation, greatly shortening the lifetime of the autoreducible PO-ring-opening intermediates, cobalt(III)–salen/porphyrin alkoxides.

The heterogeneous nature of β-butyrolactone (BL) polymerization towards tactic poly(3-hydroxybutyrate) (PHB) in the presence of chromium(III) salphen (salphen=N,N′-disalicylidene-o-phenylenediamine) complexes is supported by a number of experimental observations. Depending on the substitution pattern, initially soluble chromium(III) salphen chloride complexes can generate microcrystalline agglomerates under the polymerization conditions, driven by formation of μ-OH bridges between metal centers. Coordinated water molecules are suggested to be the source of such bridging ligands. The formation of these/this heterogeneous species is a prerequisite for the stereocontrolled ring-opening polymerization (ROP) of BL, whereas both iso- and syndioselective enchainment occurs simultaneously. According to the analysis of the ¹³C NMR spectra of the polymers, the ratio of the corresponding triads depends on a number of parameters in a not yet understood manner. Besides dual stereoselectivity, the heterogeneous chromium(III) salphen species feature catalytic sites with different activities, which is reflected in the very broad molecular mass distribution of the produced PHB. Highly active catalytic sites cause the formation of polymer chains with a high molecular mass at the beginning of polymerization. The described behavior is not inherent to truly homogeneous chromium salphen complexes and is more in line with a bimetallic ROP mechanism proposed earlier, which requires a particular mutual spatial orientation of two salphen complexes for the efficient catalysis of BL polymerization.

Photocatalytic reduction of CO₂ with rhenium(I) bipyridine complexes has been studied for several decades. Nonetheless, important parameters affecting the catalytic performance remain elusive to date. By using the standard catalyst [Re(dmb)(CO)₃Cl] (dmb=4,4′-dimethyl-2,2′-bipyridine), the effect of catalyst concentration and irradiation intensity is studied in detail and important correlations are revealed. The decomposition of the catalyst is investigated, and two main deactivation pathways are proposed, both of which involve the one-electron-reduced species and are likely to be valid for other homogeneous photocatalysts as well. The rate of deactivation is linked to the relative concentration of 1) the catalyst in its electronic ground state, 2) the catalyst in its excited state, 3) the one-electron-reduced species, and 4) quencher radicals. Adequate tuning of catalyst concentration and irradiation intensity leads to the highest quantum yield (Φ=0.53) reported to date for a single-molecule system.

We present a method to combine the functional features of poly(diethyl vinylphosphonate) (PDEVP) and photoluminescent silicon nanocrystals. The polymer–particle hybrids were synthesized in three steps through surface-initiated group transfer polymerization using Cp₂YCH₂TMS(thf) as a catalyst. This pathway of particle modification renders the nanoparticle surface stable against oxidation. Although SiNC properties are known to be sensitive toward transition metals, the hybrid particles exhibit red photoluminescence in water. The temperature-dependent coiling of PDEVP results in a change of the hydrodynamic radius of the hybrid particles in water. To the best of our knowledge, this is the first example of controlled catalytic polymerization reactions on a silicon nanocrystal surface.

Cobalt(III) tetraphenylporphyrin chloride (TPPCoCl) was experimentally proved to be an active catalyst for poly(propylene carbonate) production. It was chosen as a model catalyst in the present work to investigate the initiation step of propylene oxide (PO)/CO₂ copolymerization, which is supposed to be the ring opening of the epoxide. Ring-opening intermediates (1–7) were detected by using ¹H NMR spectroscopy. A first-order reaction in TPPCoCl was determined. A combination of monometallic and bimetallic ring-opening pathways is proposed according to kinetics experiments. Addition of onium salts (e.g., bis(triphenylphosphine)iminium chloride, PPNCl) efficiently promoted the PO ring-opening rate. The existence of axial ligand exchange in the cobalt porphyrin complex in the presence of onium salts was suggested by analyzing collected ¹H NMR spectra.

The reactivity of diazonium salts towards freestanding, photoluminescent silicon nanocrystals (SiNCs) is reported. It was found that SiNCs can be functionalized with aryl groups by direct reductive grafting of the diazonium salts. Furthermore, diazonium salts are efficient radical initiators for SiNC hydrosilylation. For this purpose, novel electron-deficient diazonium salts, highly soluble in nonpolar solvents were synthesized. The SiNCs were functionalized with a variety of alkenes and alkynes at room temperature with short reaction times.

We demonstrate a facile, yet efficient method for the functionalization of crosslinked polystyrene (PS) microspheres with biocompatible poly(vinylphosphonate)s via the combination of a UV grafting polymerization and a surface-initiated group transfer polymerization. Self-initiated photografting and photopolymerization of ethylene glycol dimethacrylate results in direct photografting of poly(ethylene glycol dimethacrylate) on the PS microspheres with dangling methacrylate functionalities, which are used to immobilize ytterbocene complexes to form the surface-bound rare-earth metal catalyst system. The surface-initiated GTP of dialkyl vinylphosphonates from the initiator system leads to the functionalization of PS microspheres with poly(vinylphosphonate) brushes. Polymerization kinetic investigation indicates that surface-initiated GTP leads to a constant and remarkably rapid weight gain of the microsphere (a microsphere weight increase of 600% within 3 min), owing to the highly living and efficient character of GTP. The surface-initiated GTP occurring inside the microsphere causes an accumulation of the tension between the polymer chains in the microsphere, which eventually induces fracture of the microsphere for longer polymerization time. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2919–2925

In this work, new, crosslinkable copolymers from propylene and di-tert-butoxy(methyl)(oct-7-enyl)silane are presented. The silane-functionalized monomer is obtained by hydrosilylation of 1,7-octadiene with dichloromethylsilane, followed by the substitution of the chloro atoms by tert-butoxy groups. Homopolymerization and copolymerization with propylene are performed using rac-[ethylenebis(indenyl)]zirconium dichloride. The tert-butoxysilane groups are easily cleaved by acid-catalyzed processes. The resulting copolymer can be completely crosslinked via the tert-butoxysilane functionality to obtain insoluble polymeric material and the gel content of the polymers with different silane content is determined. This method allows control of the copolymer composition and thus of the subsequent extent of crosslinking.

(Acetoxymethyl)silanes 2, 7 a–c, and 10 a–c with at least one alkoxy group, of the general formula (AcOCH₂)Si(OR)_3−n(CH₃)_n (R: Me, Et, iPr; n=0, 1, 2), were synthesized from the corresponding (chloromethyl)silanes 1, 6 a–c, and 9 a–c by treatment with potassium acetate under phase-transfer-catalysis conditions. These compounds were found to provide 2,2,5,5-organo-substituted 1,4-dioxa-2,5-disilacyclohexanes 3, 8 a–c, and 11 a–c if treated with organotin(IV) catalysts such as dioctyltin oxide. The reaction proceeds through transesterification of the acetoxy and alkoxy units followed by ring-closure to form a dimeric six-membered ring. The corresponding alkyl acetates are formed as the reaction by-products. With these mild conditions, the method overcomes the drawbacks of previously reported synthetic routes to furnish 2,2,5,5-tetramethyl-1,4-dioxa-2,5-disilacyclohexane (3) and even allows the synthesis of 1,4-dioxa-2,5-disilacyclohexanes bearing hydrolytically labile alkoxy substituents at the silicon atom in good yields and high purity. These new materials were fully characterized by NMR spectroscopy, elemental analysis, mass spectrometry, and X-ray analysis (trans-8 a).

For the first time, the adaptability of the CC double bond as a versatile precursor for the postsynthetic modification (PSM) of microporous materials was extensively investigated and evaluated. Therefore, an olefin-tagged 4,4′-bipyridine linker was synthesized and successfully introduced as pillar linker within a 9,10-triptycenedicarboxylate (TDC) zinc paddle-wheel metal–organic framework (MOF) through microwave-assisted synthesis. Different reactions, predominately used in organic chemistry, were tested, leading to the development of new postsynthetic reactions for the functionalization of solid materials. The postsynthetic oxidation of the olefin side groups applying osmium tetroxide (OsO₄) as a catalyst led to the formation of a microporous material with free vicinal diol functionalities. The epoxidation with dimethyldioxirane (DMDO) enabled the synthesis of epoxy-functionalized MOFs. In addition to that, reaction procedures for a postsynthetic hydroboration with borane dimethyl sulfide as well as a photoinduced thiol–ene click reaction with ethyl mercaptan were developed. For all of these PSMs, yields of more than 90 % were obtained, entirely maintaining the crystallinity of the MOFs. Since the direct introduction of the corresponding groups by means of pre-synthetic approaches is hardly possible, these new PSMs are useful tools for the functionalization of porous solids towards applications such as selective adsorption, separation, and catalysis.

The strong organoborane Lewis acid B(C₆F₅)₃ catalyzes the polymerization of phenylsilane at elevated temperatures forming benzene and SiH₄ as side-products. The resulting polymer is a branched polysilane with an irregular substitution pattern, as revealed by 2D NMR spectroscopy. Having explored the mechanism of this novel metal-free polymerization by computational chemistry methods at the DFT level, we have suggested that unusual cationic active species, namely monomer-stabilized silyl cations, propagate the polymerization. Hydride abstraction of SiH₃ moiety by the catalyst in the initiation step was found to be kinetically preferred by around 9 kcal mol⁻¹ over activation by coordination of the monomer at the aromatic ring. The formation of linear SiSi bonds during propagation was calculated to be less favorable than branching and ligand scrambling, which accounts for the branched and highly substituted form of the polymer that was obtained. This novel type of polymerization bears the potential for further optimization with respect to degree of polymerization and structure control for both primary as well as secondary silanes, which can be polymerized by sterically less hindered boranes.

Two dinuclear cobalt porphyrins comprising different structural tethering motifs at the porphyrin periphery were synthesised, along with a representative mononuclear cobalt porphyrin, and their catalytic activities tested towards carbon dioxide–propylene oxide copolymerisation in the presence of bis(triphenylphosphoranyl)ammonium chloride cocatalyst. The catalytic activities of the mononuclear and the bis-para-tethered dinuclear cobalt porphyrin with selective formation of poly(propylene carbonate) are largely comparable, showing no benefit of dinuclearity in contrast to the case of cobalt salen complexes and suggesting that polymer growth proceeds exclusively from one metal centre. The alternative bis-ortho-tethered porphyrin demonstrated considerably reduced activity, with dominant formation of cyclic propylene carbonate, as a result of hindered substrate approach at the metal centre. Time-resolved UV/Vis spectroscopic studies suggested a general intolerance of the cobalt(III) porphyrin catalysts towards the copolymerisation conditions in the absence of carbon dioxide pressure, leading to catalytically inactive cobalt(II) species. In the presence of carbon dioxide, the bis-ortho-tethered catalyst showed the fastest deactivation, which is related to an unfavourable steric arrangement of the linker fragment, as was also confirmed by NMR spectroscopic measurements.

Recent studies have shown that poly(vinylphosphonate)s are readily accessible by rare earth metal-mediated group transfer polymerization (GTP). This article highlights the progress in this new field and advantages of GTP in comparison to classical anionic and radical polymerization approaches. Late lanthanide metallocenes proved to be efficient initiators and highly active catalysts for vinylphosphonate polymerization yielding polymers of precise molecular weight and low polydispersity. Using this method, our group has developed a surface-initiated GTP to prepare poly(vinylphosphonate) brushes. In combination with different ester cleavage strategies, rare earth metal-mediated GTP is an efficient way to create well-defined high-molecular-weight poly(vinylphosphonic acid).

The development of a new tetraamine–iron complex as a catalyst for the cyclization of propylene oxide with carbon dioxide to form propylene carbonate is reported. The structure of the complex was confirmed by X-ray crystallography. The molecule exhibited an exceptionally long iron–chlorine bond and a high catalytic activity even without the addition of an activator. However, kinetic studies showed a second-order dependence on catalyst concentration. By using iron, we provide an ecologically as well as economically favourable alternative to the preferentially used toxic metals cobalt and chromium. On the basis of the kinetics and other experimental data, the catalytic cycle deduced as well as an understanding of the high activities.

In der Vergangenheit wurde eine Vielzahl von Methoden zur Verwendung von Kohlendioxid in der organischen Synthese entwickelt. Trotz der breiten Gesamtverfügbarkeit von CO₂ auf der Erde ist sein Einsatz als Reaktant, insbesondere in der Synthesechemie, recht selten. In den letzten 35 Jahren wurde intensiv daran geforscht, kosten- und energieeffiziente katalytische Prozesse zu finden, um CO₂ zu Carbonsäuren, Estern, Lactonen und Polymeren umzusetzen. Dieser Aufsatz fasst die bis heute vorhandenen homogenkatalytischen Routen zur Verwendung von CO₂ als C₁-Kohlenstoffquelle für die Synthese industrieller Produkte sowie Feinchemikalien zusammen.

Silicon-containing materials which possess the ability to form mesophases are promising systems for applications in the fields of electro-optical devices, nonlinear optics, and information storage media. In this work, the formation of supramolecular assemblies of a series of low molecular weight siloxane-containing mesogens is presented. Besides a novel synthesis route via Ru^II-catalyzed hydrosilylation of phenyl acetylene derivatives, mesophase characterization by modern analysis techniques is performed. As linker groups, leading to bi- and tetramesogens, linear disiloxane and cyclic tetrasiloxane are utilized. In the resulting class of materials, high thermal stability, induced by the formation of layered smectic-type structures, is predominant. The smectic-type phases were found to be monotropic. Layer distances in the assemblies, as well as the phase transition temperatures, can be controlled by the substitution motif on the mesogens (number and length of alkyl chains). In spin-cast thin films, the layered domains are visualized by atomic force microscopy; furthermore, domain dimensions and electron densities are determined by grazing-incidence small-angle X-ray scattering.

The homogeneous dinuclear zinc catalyst going back to the work of Williams et al. is to date the most active catalyst for the copolymerisation of cyclohexene oxide and CO₂ at one atmosphere of carbon dioxide. However, this catalyst shows no copolymer formation in the copolymerisation reaction of propylene oxide and carbon dioxide, instead only cyclic carbonate is found. This behaviour is known for many zinc-based catalysts, although the reasons are still unidentified. Within our studies, we focus on the parameters that are responsible for this typical behaviour. A deactivation of the catalyst due to a reaction with propylene oxide turns out to be negligible. Furthermore, the catalyst still shows poly(cyclohexene carbonate) formation in the presence of cyclic propylene carbonate, but the catalyst activity is dramatically reduced. In terpolymerisation reactions of CO₂ with different ratios of cyclohexene oxide to propylene oxide, no incorporation of propylene oxide can be detected, which can only be explained by a very fast back-biting reaction. Kinetic investigations indicate a complex reaction network, which can be manifested by theoretical investigations. DFT calculations show that the ring strains of both epoxides are comparable and the kinetic barriers for the chain propagation even favour the poly(propylene carbonate) over the poly(cyclohexene carbonate) formation. Therefore, the crucial step in the copolymerisation of propylene oxide and carbon dioxide is the back-biting reaction in the case of the studied zinc catalyst. The depolymerisation is several orders of magnitude faster for poly(propylene carbonate) than for poly(cyclohexene carbonate).

A plethora of methods have been developed over the years so that carbon dioxide can be used as a reactant in organic synthesis. Given the abundance of this compound, its utilization in synthetic chemistry, particularly on an industrial scale, is still at a rather low level. In the last 35 years, considerable research has been performed to find catalytic routes to transform CO₂ into carboxylic acids, esters, lactones, and polymers in an economic way. This Review presents an overview of the available homogeneous catalytic routes that use carbon dioxide as a C₁ carbon source for the synthesis of industrial products as well as fine chemicals.

Two novel 2D and 3D coordination polymers of 9,10-triptycenedicarboxylic acid and zinc nitrate, which form under solvothermal reaction conditions, are described. As determined by single-crystal X-ray structure analysis, their frameworks are assembled from dinuclear zinc coordination units which are interlinked by triptycenedicarboxylato (TDC) struts. In the 2D-MOF reported here, the extended network of van der Waals interactions between triptycene units as well as coordinated solvent molecules is responsible for the dense assembly of layers with a 4⁴ net topology into the final crystal structure. The framework of the 3D-MOF is composed from [Zn₂(TDC)_1.5]_∞ layers with a 6³ topology, the layers being interlinked by triptycenedicarboxylato pillars (bnn-net). This MOF possesses hexagonal channels which are filled with guest molecules.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)