Plastic waste poses a major environmental challenge, yet it also represents a valuable feedstock to produce high-value chemicals. In this work, we report an ecofriendly and efficient mechanochemical strategy for upcycling of bio- and fossil-based polyesters under mild conditions into synthetically useful building blocks. Bio-based polyethylene furanoate (PEF), polybutylene furanoate (PBF), and polylactic acid (PLA), as well as fossil-derived polyethylene terephthalate (PET), were successfully transformed into their corresponding transesterification and amidation products in excellent yields using sodium methoxide as a catalyst. These reactions generate the corresponding diol, methanol, and sodium chloride as byproducts, which can be recovered and reused. Furthermore, using the same protocol, PEF was converted into bio-based plasticizers, including diethylhexyl furanate (DEHF) and diisoamyl furanoate (DIAF) in excellent yields. Importantly, the method is not limited to pure polymers but is also effective for commercially available PET- and PLA-based packaging materials. The products were isolated by simple aqueous workup and characterized using NMR, IR, HRMS, and XRD techniques. Overall, this mechanochemical route offers a sustainable, cost-effective, and versatile approach for polyester waste valorization, contributing significantly to a circular plastic economy.

Biradical(oid)s are key intermediates in the formation and cleavage of chemical bonds, and their application as molecular switches is of particular interest. In contrast, tetraradical(oid)s remain far less explored, although they may, for example, consist of two biradical(oid) units. Herein, we report the synthesis of phosphorus-centred bi- and tetraradicaloids coordinated to group 4 metallocene fragments of titanium and zirconium. The target compounds were obtained via ring-expansion reactions of metallocene-substituted isonitriles with one or two equivalents of the biradicaloid [·P(μ-NTer)]2, affording group 4 metallocene complexes bearing one or two bridged P-centred, five-membered heterocyclic biradicaloid units. These systems function as mono- and double-molecular switches, respectively, each based on a biradicaloid motif. Upon irradiation with light (638 nm), the biradicaloid heterocycles undergo transannular bond formation in one or both five-membered heterocyclic biradicaloid units, resulting in the generation of housane-type species. The photoinduced switching behaviour as well as CO2 activation were investigated in solution by NMR and UV/Vis spectroscopy. Quantum-chemical calculations were further performed to examine the design of the isonitrile ligands, the mechanism of their insertion reactions, and the electronic structure and photoswitching behaviour, supporting the experimental findings.

The heterodinuclear zirconocene/titanocene complexes, [Cp2Zr(μ-Me)(μ-C2R)(TiCp2)] where R = SiMe3 and R = Ph, were prepared using the previously reported comproportion reaction of the zirconocene alkynyl methyl complex with Rosenthal's zirconocene source. Examination of the molecular structure revealed alkynyl group migration from Zr to Ti, as confirmed by single-crystal x-ray analysis, nuclear magnetic resonance spectroscopy, and quantum chemical calculations. The homodinuclear (Zr/Zr) and heterodinuclear (Ti/Zr) complexes were activated with [Ph3C][B(C6F5)4], B(C6F5)3, and a mixture of B(C6F5)3 with excess Et3SiH (SiHB system) as aluminium-free activators and tested in ethylene polymerization. Depending on the type of activator, unique reactivity was observed, resulting in either electron or methyl abstraction and the formation of cationic species. The catalytically active species are proposed to have dinuclear character, forming linear polyethylenes with unsaturated groups predominantly on the Zr cationic center. In contrast, the Ti cationic center generated by a SiHB system only in dichloromethane produced silicon-terminated polyethylenes.

The understanding of the nature of catalyst–substrate complexes is key for the optimization of catalytic transformations such as asymmetric hydrogenation of prochiral olefins. We present a crystallographic and NMR spectroscopic study of coordination of the prochiral olefin α‐methyl‐β‐amidoitaconate to two [Rh(diphosphine)] + fragments under varying pH conditions. While under neutral conditions substrate coordination is observed, yielding the expected Rh(I)–substrate complexes, the coordination under basic conditions results in the deprotonation of the amide group, giving neutral Rh(I) complexes that possess covalent Rh–N linkages. At low pH, oxidative addition of the acid induces 1,2‐insertion of the olefin functional group into the Rh hydride, resulting in the formation of Rh(III) alkyl complexes that are commonly regarded as catalytically inactive species for hydrogenation. The NMR spectroscopic aspects of these rare transformations are investigated using a combination of low‐field flow and high‐field NMR spectroscopy. Finally, the relevance of these observations for asymmetric hydrogenation of α‐methyl‐β‐amidoitaconate is discussed.

Herein we disclose a novel route to redox-active vanadacyclobutadiene (VCBD) and vanadatetrahedrane (VTd) complexes, which circumvents a vanadium alkylidyne precursor ([VV][triple bond, length as m-dash]CR). [VV] VCBD salts and VTd's are prepared through an unusual, non-classical oxidative addition reaction via the addition of electrophiles to a low-spin [VIII] deprotiovanadacyclobutadiene (dVCBD). The microscopic reverse reaction, a reductive elimination, reverts the [VV] VCBD and VTd scaffolds back to the low-spin [VIII] dVCBD species via the addition of a Brønsted base. The interconversion of the [VV] VCBD salts and VTd's is mediated through anion exchange, which promotes a change in the spin-state and geometry of the organometallic species, highlighting the redox active nature of the allene ligand (C3) bound to vanadium (V). Single electron reduction of the [VV] VCBD or VTd with cobaltocene (CoCp2) led to the formation of neutral and radical-based [VIV] VCBDs. Oxidation of the [VIV] VCBDs with ferrocenium based salts ([FeCp2][X], X = BArF20 -, BArF24 -, OTf-) or chloride-delivering oxidants (i.e. triphenylmethyl chloride (Ph3CCl) or lead(ii) chloride (PbCl2)) regenerates the discrete [VV] VCBD salts or VTd's, respectively. Cyclic voltammetry studies reveal quasi-reversible one electron redox couples, while X-band electron paramagnetic resonance (EPR) spectroscopic studies confirms the presence of a paramagnetic [VIV], d1 system in the VCBD. Computational analysis of the dVCBD, VTd, and VCBD complexes affords detailed insight into the structure and bonding of this unusual class of molecules and delineates the role of the anion in their interconversion. Further examination of the reaction of dVCBD with electrophiles corroborates the nonclassical character of the oxidative addition sequence involving metal-ligand cooperation, with ligand-centered redox activity, rather than a formal two-electron oxidation at the metal center. The present work demonstrates how these rare dVCBD, VCBD, and VTd scaffolds can all be interconverted via reversible C-C bond formation and splitting pathways using the right combination of anion exchange, redox, and Brønsted acid-base chemistry.

The dinuclear title compound [(Cp2Zr)2(μ-Me)(μ-C2Ph)] 5 was prepared from a zirconocene alkynyl methyl complex and Rosenthal's zirconocene source [Cp2Zr(py)(η2-Me3SiC2SiMe3)] in a formal comproportionation reaction. This complex shows catalytic activity for the dehydrocoupling of amine boranes, with a dinuclear hydride-bridged alkynyl complex 6 being formed as a catalytically relevant species. The structure of this complex was confirmed for the first time by single-crystal X-ray analysis. The reaction of complex 5 with hydrogen results in hydrogenation of the alkynyl ligand, yielding a highly labile trinuclear hydride-bridged complex as a possible intermediate of zirconocene dihydride/ethylbenzene formation. This complex shows an unusual distorted planar tetracoordinate environment at the central carbon atom positioned between the three Zr centers. The reaction of complex 5 with 2-cyanopyridine and acetonitrile is characterized by a reduction of the substrates. The herein reported reactivity of complex 5 demonstrates the remarkable potential of well-established dinuclear zirconocenes to stabilize unusual bond situations, which were analyzed comprehensively using spectroscopic, structural, and computational methods.

The catalytic dehydrocoupling of amine boranes produces well-defined B-N compounds and polymers that are isovalent electronic to hydrocarbon analogs. In this study, the synthesis and characterization of a set of Rh, Ir, and Ru complexes with tridentate imidazolylphosphine PN(H)N ligands of the type (imidazolyl)CH2N(H)CH2CH2PR2 (R = iPr, tBu) are presented. These complexes potentially engage in metal-ligand cooperative dehydrogenation of amine boranes, followed by B-N coupling. All complexes show facial coordination of the PN(H)N ligands to the metal centers with pronounced hydrogen bonding to an outer-sphere chloride ligand. All Rh and Ir complexes are active catalysts for the dehydrogenation of H3B center dot NMeH2 and H3B center dot NMe2H, producing mainly linear and cyclic B-N oligomers. Only the Ru complex [Ru(PN(H)N)(PPh3)(CO)(H)]Cl (8) produces low-molecular-weight poly(aminoboranes) from H3B center dot NMeH2 with high activity after activation with KOtBu, supporting metal-ligand cooperativity during activation of the amine borane substrates.

Phosphaalkenes are an underrepresented class of phosphorus-based ligands that hold great potential for the stabilisation of low-valent transition metal and main group fragments. We present the synthesis of a monoanionic PNP-type bis-phosphaalkene ligand and its reactions with [Rh(cod)(Cl)]2, which furnish two unique dinuclear Rh(I) complexes. These complexes show C-H activation of the Mes* groups of the PNP ligand and the presence of three different phosphaalkene coordination modes, respectively. Computational analyses suggest the presence of a unique Rh-Rh donor-acceptor interaction in the C-H-activated complex. Stabilisation of the desired tricoordinate PNP-supported Rh(I) species is possible in the presence of the strongly donating CO ligand, yielding a mononuclear square-planar complex of the type [(PNP)Rh(CO)].

The challenge of producing new environmentally friendly and fossil-free polyesters has strongly encouraged the development of bio-based alternatives such as polyethylene furanoate (PEF) and polybutylene furanoate (PBF) as alternatives to commodity plastics such as polyethylene terephthalate (PET) for everyday applications. In this contribution, we report the mechanochemical depolymerisation of these polymers using NaOH in the presence of NaCl as an additive along with the synthesis of high-molecular weight PEF and PBF. Efficient depolymerisation, producing 2,5-furandicarboxylic acid (FDCA) and the corresponding diols in quantitative yields after aqueous acidic workup, is possible within 30 minutes milling time. Using slightly modified reaction conditions, transesterification with MeOH produces the 2,5-furandicarboxylic acid dimethyl ester (FuMe2), which can potentially be reused for polymer synthesis. Notably, the furan ring remains stable under the mechanochemical conditions used. The applicability of these straight-forward, environmentally friendly protocols on a large scale is demonstrated through multigram scale reactions.

Knowledge of the potential degradation products of active pharmaceutical ingredients (APIs) is of major interest for the development and approval of new drugs. Therefore, methodologies for the time-efficient and precise prediction of degradation products and pathways are of great importance. Traditional degradation assessments typically involve solution-based forced degradations under acidic, basic, thermal, or photolytic conditions. However, such conditions often fail to accurately replicate degradation pathways relevant to solid-state formulations. A promising addition to the established solvent-based approaches are forced degradation processes in the solid-state using mechanochemistry. The newly developed methodologies enable a time-efficient and accurate simulation of degradation pathways under mild reaction conditions in the solid-state. Herein, the general principles of forced mechanochemical degradations will be discussed on the basis of published case studies involving marketed drugs.

Starting from the alkyne complex Cp2Zr(py)(η2-Me3SiC2SiMe3) (Cp = η5-cyclopentadienyl, py = pyridine), the synthesis and complete characterisation of a zirconocene(IV) triazenido hydride complex and its use in the activation of small molecules is reported. The reaction with CO2 led to the formation of a zirconocene(IV) triazenido-formate complex, which was further investigated for its stability towards different bases with respect to the formation of formic acid. The experimentally observed reaction pathway was investigated computationally using DFT methods, revealing the favourable role of pyridine coordination in the hydrogen transfer from the triazene to the alkyne unit of the zirconocene reagent.

The dinuclear title compound [(Cp 2 Zr) 2 ( μ ‐Me)( μ ‐C 2 Ph)] 5 was prepared from a zirconocene alkynyl methyl complex and Rosenthal's zirconocene source [Cp 2 Zr(py)( η 2 ‐Me 3 SiC 2 SiMe 3 )] in a formal comproportionation reaction. This complex shows catalytic activity for the dehydrocoupling of amine boranes, with a dinuclear hydride‐bridged alkynyl complex 6 being formed as a catalytically relevant species. The structure of this complex was confirmed for the first time by single‐crystal X‐ray analysis. The reaction of complex 5 with hydrogen results in hydrogenation of the alkynyl ligand, yielding a highly labile trinuclear hydride‐bridged complex as a possible intermediate of zirconocene dihydride/ethylbenzene formation. This complex shows an unusual distorted planar tetracoordinate environment at the central carbon atom positioned between the three Zr centers. The reaction of complex 5 with 2‐cyanopyridine and acetonitrile is characterized by a reduction of the substrates. The herein reported reactivity of complex 5 demonstrates the remarkable potential of well‐established dinuclear zirconocenes to stabilize unusual bond situations, which were analyzed comprehensively using spectroscopic, structural, and computational methods.

Biradicals are important intermediates in the formation and breaking of a chemical bond. Their use as molecular switches is of particular interest. Much less is known about tetraradicals, which can, for example, consist of two biradical(oid) units. Here we report the synthesis of the first persistent phosphorus-centred tetraradical bound to a transition metal fragment. Starting from a zirconocene complex, rac-(ebthi)ZrCl2 (rac-(ebthi)=1,2-ethylene-1,10-bis(eta 5-tetrahydroindenyl), two cyclo-1,3-diphospha-pentane-1,3-diyls were successfully introduced, which finally led to the isolation of a deep green zirconcene-bridged bis(biradicaloid) complex (5) that can act as a double molecular switch. Under the influence of light (570 nm), this tetraradical forms a transannular bond in each of the two five-membered biradical units, leading to the formation of housane 5 h. Upon irradiation at 415 nm, the reverse reaction is observed, fully recovering tetraradical 5. Through single-crystal-to-single-crystal transformation, both stable species of the molecular switch could be structurally characterised using SCXRD. The switching under the influence of light and the activation of molecular hydrogen were analysed in solution using NMR and UV spectroscopy. It was found that the addition of one or two equivalents of molecular hydrogen can be switched on and off by light. The first synthesis of a phosphorus-centred tetraradical linked to a zirconocene fragment is presented, which can act as a molecular switch both in solution and in the solid state. This new hybrid switch was used to switch H2 addition on and off, which can be controlled photochemically. image

The introduction of heteroatoms into conjugated organic molecules is an important strategy to tune their reactivity and physical properties. In this realm triazabutadienes (TBDs) of the general from R2C=N-N=NR' are an interesting class of compounds, however, general synthetic protocols for their generation are limited. Based on the serendipitous finding that the sterically encumbered azide Mes*N3 (Mes*=2,4,6-tBu3C6H2) reacted with PMe3 in the presence of an aromatic aldehyde to form a TBD, we now report on the "Azide-Wittig" reaction. This azide-Wittig reaction is shown to be a versatile tool for the synthesis of a variety of TBDs, tolerating a wide range of aldehydes and organic azides as coupling partners. The preference for azide-Wittig, rather than aza-Wittig reactivity was rationalized using computational methods. This study shows how kinetic control can significantly alter the reaction pathway, thereby switching from an aza-Wittig to an azide-Wittig regime.

On-line reaction monitoring of hydrogenation reactions of oxygen-sensitive organometallic complexes is done via a 31P benchtop NMR spectrometer using the Multi-Resonance Sensitive Homogeneous And Resolved PEaks in Real time (MR-SHARPER) sequence.

Oxidative addition of PC(H)P pincer ligands at late transition metals typically occurs via C-H activation in the 2-position of the ligand, yielding [(PCP)MHX] type pincer complexes (X = monoanionic ligand). We present formation of the dinuclear Ir(III) complex [((PSCSPiPr)-P-iPr)Ir(H)(MeCN)(2)](2)[OTf](2) as a rare example of meta-C-H activation at a pincer ligand.

The reactions of two [rac-(ebthi)Ti]- and [rac-(ebi)Zr]-based 1-metallacyclobuta-2,3-diene complexes with diazenes are described. With azobenzene, cleavage of the N & boxH;N bond is observed in the case of Zr, leading to the formation of dinuclear Zr(IV) imido complex 1. In contrast, the insertion of azobenzene into the Ti-C bond of the titanacycle produces six-membered aza-metallacycle 6, which could only be characterized by nuclear magnetic resonance spectroscopy. With geometrically restricted benzo[c]cinnoline, activation and dearomatization of the phenyl group of the substrate occur, leading to the formation of unusual mononuclear [M = Zr (3)] and dinuclear complexes [M = Ti (7)]. Reaction mechanisms leading to the formation of these unusual complexes are proposed on the basis of control experiments.

Ammonia borane and amine boranes are main group analogues of alkanes, which are characterised by their large gravimetric hydrogen content. This hydrogen can be released in dehydrocoupling and dehydropolymerisation reactions to obtain B-N oligomers and polymers that are of importance as precursors for functional B-N materials. Furthermore, amine boranes are potent reagents for application in transfer hydrogenation reactions, representing a versatile, easy-to-handle alternative to the use of gaseous hydrogen for the reduction of organic compounds. Compared to late transition metals, complexes of readily available and comparatively inexpensive electropositive group 4 metals have been used to a much lesser extent. This review summarises the developments in the field of dehydrocoupling of amine boranes and transfer hydrogenation with these reagents, catalysed by complexes of group 4 metals. We analyse the background for these developments using examples and reaction mechanisms and provide an outlook for future developments in this field of research.

The synthesis of group 4 metal 1-metallacyclobuta-2,3-dienes as organometallic analogues of elusive 1,2-cyclobutadiene has so far been limited to SiMe3 substituted examples. We present the synthesis of two Ph substituted dilithiated ligand precursors for the preparation of four new 1-metallacyclobuta-2,3-dienes [rac-(ebthi)M] (M=Ti, Zr; ebthi=1,2-ethylene-1,10-bis(eta 5-tetrahydroindenyl)). The organolithium compounds [Li2(RC3Ph)] (1 b: R=Ph, 1 c: R=SiMe3) as well as the metallacycles of the general formula [rac-(ebthi)M(R1C3R2)] (2 b: M=Ti, R1=R2=Ph, 2 c: M=Ti, R1=Ph, R2=SiMe3; 3 b: M=Zr, R1=R2=Ph; 3 c: M=Zr, R1=Ph, R2=SiMe3) were fully characterised. Single crystal X-ray diffraction and quantum chemical bond analysis of the Ti and Zr complexes reveal ligand influence on the biradicaloid character of the titanocene complexes. X-band EPR spectroscopy of structurally similar Ti complexes [rac-(ebthi)Ti(Me3SiC3SiMe3)] (2 a), 2 b, and 2 c was carried out to evaluate the accessibility of an EPR active triplet state. Cyclic voltammetry shows that introduction of Ph groups renders the complexes easier to reduce. 13C CPMAS NMR analysis provides insights into the cause of the low field shift of the resonances of metal-bonded carbon atoms and provides evidence of the absence of the beta-C-Ti interaction. Ring-substituted 1-metallacyclobuta-2,3-dienes were prepared from suitable organolithium precursors. The effect of introduction of phenyl groups was studied using a combination of experimental and theoretical methods. For the first time cyclic voltammetry, EPR, and solid state NMR spectroscopy were used to support structural and DFT data. image

Direct coupling of benzotriazole to unsaturated substrates such as allenes represents an atom-efficient method for the construction of biologically and pharmaceutically interesting functional structures. In this work, the mechanism of the N-2-selective Rh complex-catalyzed coupling of benzotriazoles to allenes was investigated in depth using a combination of experimental and theoretical techniques. Substrate coordination, inhibition, and catalyst deactivation was probed in reactions of the neutral and cationic catalyst precursors [Rh(mu-Cl)(DPEPhos)](2) and [Rh(DPEPhos)(MeOH)(2)](+) with benzotriazole and allene, giving coordination, or coupling of the substrates. Formation of a rhodacycle, formed by unprecedented 1,2-coupling of allenes, is responsible for catalyst deactivation. Experimental and computational data suggest that cationic species, formed either by abstraction of the chloride ligand or used directly, are relevant for catalysis. Isomerization of benzotriazole and cleavage of its N-H bond are suggested to occur by counteranion-assisted proton shuttling. This contrasts with a previously proposed scenario in which oxidative N-H addition at Rh is one of the key steps. Based on the mechanistic analysis, the catalytic coupling reaction could be optimized, leading to lower reaction temperature and shorter reaction times compared to the literature.