Hans_Bauer96

A systematic study is presented of the coordination chemistry of imidazolylphosphine-based PN(H)N pincer ligands at Rh, Ir, and Ru centers. The corresponding cationic complexes are active catalysts for the dehydrocoupling of the amine boranes H₃B·NMe₂H and H₃B·NMeH₂. While Rh and Ir diolefin complexes mainly yield linear and cyclic BN oligomers, only a Ru carbonyl hydride produces low-molecular-weight poly(aminoboranes).

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·NMeH₂ and H₃B·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·NMeH₂ with high activity after activation with KOtBu, supporting metal-ligand cooperativity during activation of the amine borane substrates.

Sterically hindered potassium phosphinophosphido dithioformates K[TerP(CS₂)PR₂] (R = iPr, tBu) are synthesized from the conversion of the potassium phosphinophosphides K[TerP–PR₂] with CS₂. In solution, the dithioformate species rearrange to anionic phosphanylthioketones K[TerPC(=S)–P(S)R₂].

The synthesis of sterically demanding 2,6-bis(2,4,6-trimethylphenyl)phenyl (Ter)-stabilized potassium phosphinophosphido dithioformates 1a–b K[TerP(CS₂)PR₂] via conversion of the corresponding potassium phosphinophosphides K[TerP–PR₂] (R = iPr, tBu) with CS₂ is reported. In solution, the dithioformates 1a–b undergo a migration reaction, resulting in the formation of anionic phosphanylthioketones, K[TerPC(=S)–P(S)R₂] (2a-b), bearing phosphanylthiolate groups. The cocrystallization of compounds 1a and 2a allows simultaneous structural characterization of both, and the crystallization of 2a from THF or benzene affords specific solvates with solvent-dependent discrete dimers or polymeric chains, respectively.

The Front Cover shows how, like an octopus, the catalyst works with many components simultaneously to reach its target. New RuPOP pincer catalysts upgrade ethanol to butan-1-ol. Adding to the mechanistic understanding of ethanol upgrading, M. Nielsen and co-workers show in their Research Article (DOI: 10.1002/cctc.202500700) that catalysts void of the Noyori unit are fully capable of catalysing the reaction; this contrasts with the common understanding in the community of the role of the catalyst.

In their communication, Mindiola and coworkers demonstrate how a reduced Ti complex can reversibly bind CO₂ to produce organic and inorganic carbonates. They access this chemistry through a unique Ti^III oxo anion that can be generated both chemically and electrochemically. The subtle electronic and structure parameters of this species promote its reactivity and establish an exciting route to the capture, functionalization, and release of CO₂.

Abstract

Carbon dioxide capture and functionalization sequesters carbon dioxide in more robust products and offers a viable route to reducing greenhouse gas emissions. We present herein a unique molecular Ti^III oxo anion that reversibly binds CO₂ to allow both its sequestration and functionalization. The reduction of [(PN)₂Ti═O] (1) [PN = (2-Pⁱⁱ Pr₂-4-methylphenyl)(mesityl)amide] with KC₈ and 2.2.2-cryptand (crypt) resulted in formation of [K(crypt)][(PN)₂Ti═O] (2), which was fully characterized and shown to contain a Ti-centered radical. Complex 2 reacts with Al(CH₃)₃ to form [K(crypt)][(PN)₂Ti{O(Al(CH₃)₃}] (3), which can be independently prepared from [K(crypt)][(PN)₂Ti(OCP)] and Al(CH₃)₃. Whereas 1 does not react with CO₂, 2 rapidly captures the gas (1 atm, 25 °C) to produce a Ti^III carbonate [K(crypt)][(PN)₂Ti(κ²-O₂C═O)] (4). Chemical and electrochemical oxidation of 4 releases CO₂ to regenerate 1 while a soluble organic carbonate [Me₃SiOC(O)OSiMe₃, Me = CH₃] is obtained from reaction of 4 with ClSiMe₃.

Es wird ein neuer Syntheseweg zu anionischen N-heterocyclischen Olefinen (NHO) durch Ligandenaustausch in metallierten Yliden/Diazomethanen mit freien Carbenen beschrieben. Die anionischen NHOs weisen abgewinkelte Strukturen mit delokalisierter anionischer Ladung auf, was eine vielseitige Reaktivität ermöglicht, einschließlich der Darstellung hochfunktionalisierter NHOs, der thermischen Umlagerung zu N-heterocyclischen Iminen und der Bildung von Carbo(phosphino)carben-Liganden.

Kurzzusammenfassung

N-heterocyclische Olefine (NHOs) haben sich als effiziente Basen und Nukleophile mit breiten Anwendungsmöglichkeiten in der Synthese und Katalyse etabliert. Wir berichten nun über einen neuen synthetischen Ansatz zur Darstellung ihrer anionischen Derivate durch einen formalen Ligandenaustausch an metallierten Yliden oder Diazomethaniden mit freien Carbenen. Die Reaktion erwies sich als abhängig von der Art des Carbens, wobei elektrophilere Carbene leichter unter N₂- bzw. PPh₃-Austausch reagieren. Spektroskopische und kristallographische Analysen in Kombination mit quantenchemischen Rechnungen zeigten, dass die anionischen NHOs gewinkelte Strukturen aufweisen, wobei die anionische Ladung am zentralen Kohlenstoffatom teilweise in das Carben und den zweiten Substituenten (Z) delokalisiert ist. Diese Ladungsverteilung verleiht den anionischen NHOs eine vielseitige Reaktivität durch einen möglichen Angriff am zentralen Kohlenstoffatom, dem Stickstoff des N-heterocyclischen Carbens oder dem Z-Substituenten. Während die überwiegende Reaktivität am zentralen Kohlenstoffatom stattfindet – was die einfache Synthese funktionalisierter NHOs ermöglicht – unterliegen anionische NHOs darüber hinaus einer thermisch induzierten Umlagerung zu N-heterocyclischen Iminen über Ringöffnung des N-heterocyclischen Carbens.

A family of molecular anionic calcium alkyl complexes is described. Compared with their neutral analogues, these anionic systems show markedly higher basicity, exemplified by the templated twofold deprotonation of benzene to give an inverse-crown areneide complex.

Abstract

Anionic calcium alkyl complexes have been synthesised through the reduction of ethylene by an anionic calcium hydride. The alkyl complexes are demonstrated to be superbasic, rapidly deprotonating ethers below room temperature, leading to C─O bond cleavage. In reactions with benzene, selective 1,4-metalation of benzene is observed, forming an inverse-crown areneide complex. This transformation, previously inaccessible to calcium, proceeds via a stepwise mechanism, which can also be facilitated by co-complexation with organometallic reagents such as tert-butyllithium and phenylpotassium.

Die Synthese, Isolierung und vollständige Charakterisierung des bisher unbekannten Heterocumulens Ph₂SCCO wird vorgestellt. Ausgehend von einfachen Vorläufern ermöglicht Ph₂SCCO den direkten Zugang zu der wertvollen Klasse der (bicyclischen) α-Cyclopropylcarbonylverbindungen. Reaktivitätsstudien zeigten, dass es sich um ein ausgezeichnetes Reagenz für den CCO-Fragmenttransfer in der organischen Synthese handelt.

Kurzzusammenfassung

Ketenyliden (C₂O) ist ein elektronisch faszinierendes kleines Molekül mit signifikantem Potenzial als C₂-Synthon in der organischen Synthese. Seine hohe Reaktivität hat jedoch bislang praktische Anwendungen verhindert. Hier berichten wir über die Synthese eines neuen Reagenzes, des Heterocumulens Ph₂S═C═C═O (1), einem wohldefinierten C₂O-Äquivalent, das das chemische Reaktivitätsprofil von C₂O auf kontrollierte Weise nachbildet. 1 integriert auf einzigartige Weise ketenähnliche Reaktivität und klassisches Carbenverhalten. In Gegenwart von Brønsted-Säuren reagiert 1 in einer initialen 1,2-Addition, um ein Schwefelylid-Zwischenprodukt zu bilden, das für nachfolgende Reaktionen wie Cyclopropanierung, Epoxidierung oder X‒H-Addition genutzt werden kann. Die CCO-Fragmenttransferstrategie, die von einfachen Vorläufern ausgeht, ermöglicht einen effizienten Zugang zu α-Cyclopropyl- sowie α-Epoxycarbonyl-Derivaten und strukturell komplexen bicyclischen Cyclopropanen.

Strong nucleophiles play an important role in catalysis and CO₂ conversion. In this work the emerging versatile roles of 4-aminopyridines in CO₂ conversion, ranging from structural elements to catalytic components, are discussed with an eye to their interaction with CO₂ and mechanistic aspects.

Abstract

Organic superbases are a family of compounds endowed with high nucleophilicity and basicity. Several powerful nucleophiles such as DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene), or DMAP (4-dimethylaminopyridine) are involved in CO₂ conversion but their catalytic roles may differ from a mechanistic standpoint. In this work, we show the versatile application of 4-aminopyridines in CO₂ fixation leading to products of CO₂ reduction as well as cyclic carbonates and fine chemicals. In such cases, 4-aminopyridines serve not just as organocatalysts, but as recurring motifs performing as bases, structural components, ligands for electronic modulation of metals and full-fledged catalytic components. Such roles are highlighted herein with an eye to the understanding of mechanistic aspects and the interaction between 4-aminopyridines and CO₂ through the discussion of several catalytic studies.

Diese Arbeit zeigt, wie die projektionsbasierte Einbettungstheorie erfolgreich angewendet werden kann, um chemische Reaktionen auf Metalloberflächen zu untersuchen, indem sie Richtlinien für eine angemessene Systempartitionierung und eine geeignete Beschreibung der Wechselwirkungen zwischen den Subsystemen liefert. Dieser effiziente und systematisch verbesserbare Ansatz trägt wesentlich zum Werkzeugkasten der computergestützten Chemie für die heterogene Katalyse bei, da er den Weg für eine routinemäßige Anwendung ebnet.

Zusammenfassung

Das Screening von Katalysatoren stellt eine anspruchsvolle Aufgabe für die computergestützte Chemie dar, da die große strukturelle Vielfalt von Oberflächen unter Operando-Bedingungen mit hohen Anforderungen an die Genauigkeit der kinetischen Vorhersagen einhergeht. Einbettungsmethoden, die es ermöglichen den rechnerischen Aufwand auf die chemisch aktiven Bereiche zu konzentrieren, sind vielversprechende Werkzeuge, um ein Gleichgewicht zwischen Genauigkeit und Effizienz herzustellen. Für metallische Oberflächenkatalysatoren gestaltet sich die notwendige Trennung des Systems in einen aktiv behandelten Bereich und ein Umgebungssystem jedoch als technisch schwierig, da die Elektronen in der leitfähigen Oberfläche delokalisiert sind. Aus diesem Grund sind Studien, die das Potenzial von Einbettungsmethoden für das Screening heterogener (elektro-)katalytischer Systeme untersuchen, bislang selten. In dieser Arbeit zeigen wir, dass einfache Einbettungsansätze zur Untersuchung metallischer Katalysatoren durchaus realisierbar sind, wenn i) der aktive Orbitalraum entlang der Reaktionskoordinaten konstant gehalten wird und ii) das zur Berechnung des Einbettungspotenzials verwendete nicht-additive Austausch-Korrelationsfunktional einen Anteil exakter Austauschwechselwirkung enthält, um Delokalisierungsfehler zu minimieren. Wir verifizieren diesen Ansatz anhand einer Auswahl offenschaliger und geschlossenschaliger Zwischenprodukte der CO₂-Reduktionsreaktion, welche an verschiedenen Adsorptionsplätzen einer Cu(111)-Oberfläche adsorbiert sind. Die Oberflächen sind durch Clustermodelle repräsentiert und zeigen, dass das Screening von Katalysatoren mithilfe von Einbettungsmethoden möglich ist.

The anionic iminoborane [Ar─B≡N─Flu]^– features a B≡N─C: unit that enables cooperative reactivity. It functions as an acyclic boryl anion synthon via a stepwise C-to-B relay and undergoes concerted BNC-1,3-dipolar cycloadditions with unsaturated substrates.

Abstract

Classical iminoboranes, extensively investigated since the 1980s, are defined by 1,2-dipolar reactivity in which the boron center remains exclusively electrophilic. Here, we report an anionic iminoborane, [Ar─B≡N─Flu]⁻, in which substitution at nitrogen with a fluorenyl anion generates a conjugative B≡N─C: unit that enables cooperative reactivity. This species functions as a nucleophilic boryl anion synthon, engaging N-, P-, Se-, and Au-based electrophiles through a stepwise pathway initiated by carbanion attack followed by 1,3-migration to boron. In parallel, it acts as a BNC-1,3-dipole, undergoing [3 + n] cycloadditions with unsaturated substrates. These results demonstrate a rationally designed anionic iminoborane that provides a versatile platform for building diverse organoboron architectures beyond the classical chemistry of iminoboranes.

The isolation of elusive ketene radical cation and radical anion, achieved using a ketene derived from a singlet carbene, is reported. These species exhibit distinct structural, spectroscopic, and reactivity properties compared to the neutral ketene. The findings demonstrate that singlet carbenes not only mimic transition metals in small molecule activation, but also in subsequent redox chemistry.

Abstract

Ketenes are among the most versatile compounds in organic chemistry, participating in a wide range of transformations, including cycloadditions, nucleophilic and electrophilic additions, and polymerizations. However, their redox chemistry, particularly the preparation of stable radical anions and radical cations, remains largely unexplored. Here, we report the isolation and characterization of a cyclic (alkyl)(amino)carbene (CAAC)-derived ketene in three oxidation states, including its radical anion and cation, representing the first example of such species. These ketene radicals exhibit unique electronic properties: the radical anion adopts a bent geometry that places the radical out of conjugation with the CAAC substituent, while the radical cation results from oxidation of an electron-withdrawing carbonyl group. Both radicals were characterized in detail using infrared (IR) and UV–vis electronic absorption spectroscopy, EPR spectroscopy, structural analysis, and computations, revealing parallels with the redox chemistry of transition metal carbonyl complexes. Reactivity studies indicate that the radical anion and radical cation react via nucleophilic and radical pathways, respectively, contrasting with the behavior of neutral ketenes. This work demonstrates a novel redox-based strategy for investigating uncharted transformations of ketenes, expanding their synthetic utility and opening new avenues for redox-organocatalysis of carbenes with small molecules.

The hydroboration of carbonyl compounds by pinacolborane catalysed by {HC(MeCDippN)₂}Al(OSO₂CF₃)H represented a landmark in main group catalysis, advancing a now widely-accepted mechanistic paradigm. In contradiction of the original study, we show that: i) turnover via Al─O/B─H metathesis does not occur; ii) when using purified HBpin, the ‘catalysed’ reaction with acetophenone shows no conversion; and iii) the catalytically active species is a BH₃ adduct.

Abstract

The hydroboration of aldehydes and ketones by pinacolborane (HBpin) catalysed by (Nacnac)^DippAl(OTf)H ((Nacnac)^Dipp = HC(MeCDippN)₂; Dipp = C₆H₃ ⁱ Pr₂-2,6; Tf = SO₂CF₃) was first reported in 2015. This study represented a landmark in main group catalysis, and advanced a widely-accepted and oft-cited mechanistic paradigm. However, in contradiction of that study, we show here that: i) the mechanism proposed, involving turnover via Al─O/B─H metathesis at the intermediate (Nacnac)^DippAl(OTf)(OCH₂Ph), does not occur; ii) when using pre-purified HBpin, the hydroboration reaction with acetophenone ‘catalysed’ by (Nacnac)^DippAl(OTf)H (reportedly giving 51% conversion over 6 h at 2 mol% loading), actually shows no conversion; and iii) the active species in catalysis is a BH₃ adduct derived either from the use of impure HBpin, or from the degradation of HBpin by the action of aluminium species present in the reaction mixture. More broadly, our study i) calls into question the nature of the true catalyst species in reports of carbonyl hydroboration by aluminium complexes (since Al─O/B─H metathesis proceeds spontaneously in the reverse direction to that necessitated catalytically); and ii) presents further evidence that the hydroboration of benzaldehyde by HBpin is not a good catalytic probe, given the significant rate of the uncatalysed background reaction.

Iron-catalyzed alkene metathesis has long remained elusive. This study identifies β-hydride elimination from an iron metallacyclobutane as a key deactivation pathway in iron-catalyzed alkene metathesis, through combined computational and experimental efforts. By elucidating spin-state, ligand, and steric effects novel reactivity descriptors are identified, that aid in designing next-generation iron-based metathesis catalysts.

Abstract

Iron-catalyzed alkene metathesis holds great promise as a sustainable alternative to its precious metal congeners, yet its development has been hindered by poor mechanistic understanding and rapid catalyst deactivation. Here, we report the combined computational and experimental identification of β-hydride elimination as a key decomposition pathway from an iron metallacyclobutane, an essential intermediate in metathesis catalysis. Using our previously reported PC_NHCP-ligated iron(0) complex [(PC_NHCP)Fe(N₂)₂], we observe under metathesis conditions the formation of an iron(II) allyl hydride product, consistent with our computational predictions of a low-energy β-hydride elimination pathway. Detailed spin-state-resolved DFT analysis reveals that while metallacyclobutane formation is feasible across multiple spin surfaces, subsequent reactivity is strongly governed by the singlet state. Coordination of N₂ is shown to inhibit metathesis and promote decomposition by raising the transition-state barrier for cycloreversion while facilitating β-hydride elimination. Subsequent calculations show that upon suppressing this decomposition channel productive metathesis is restored. These findings offer mechanistically grounded design principles for next-generation iron-based metathesis catalysts and highlight the importance of spin-state control, ligand environment, and substrate selection in overcoming catalyst deactivation and provide a foray into productive iron catalyzed alkene metathesis.

The stepwise insertion of acetylene into the E═E bonds of digallenes and diindenes is reported. This leads to the selective formation of four- and six-membered rings with 2π E₂C₂ and 4π E₂C₄ cores. The six-membered cyclohexadiene analogues are Lewis-acidic and can form stable, isolable adducts with ammonia, which—upon heating—undergo ammonolysis releasing ethylene.

Abstract

Sequential [2 + 2] cycloaddition reactions between acetylene and the digallene and diindene compounds (ETer)₂ (E = Ga, In; Ter = 2,6-Dipp₂-C₆H₃; Dipp = 2,6-diisopropylphenyl) are described. Careful control of the reaction conditions leads to selective formation of four- and six-membered rings with 2π E₂C₂ and 4π E₂C₄ cores, respectively. A structural analysis of the heterocycles by single crystal X-ray diffraction suggests limited electronic delocalization within the rings, which is borne out in their reactivity. For example, the six-membered cyclohexadiene analogues exhibit Lewis-acidic behavior and can form stable, isolable adducts with ammonia. Upon heating, these adducts transform into the corresponding bimetallic triel amides with concomitant generation of ethene.