Hans_Bauer96

Titanocene phosphinidenes react with terminal alkynes in formal [2 + 2] cycloadditions to give unsaturated [C₂PTi] metallcycles. Diphosphene complexes in combination with aryl-substituted alkynes react similarly to provide metallacyclopentenes.

Titanocene-phosphinidene and diphosphanediide (or diphosphene) complexes Cp₂Ti(PMe₃)P^MesTer and Cp₂Ti(P₂Ar₂) (Ar=Tipp, Dipp) undergo formal [2 + 2] or [3 + 2] cycloaddition reactions with terminal alkynes, respectively, to give the corresponding 4- and 5-membered phosphatitanacycles. Cp₂Ti(PMe₃)P^MesTer is shown to react selectively with aryl-substituted and aliphatic terminal alkynes to give stable phosphatitanacyclobutene species, while the reactivity of Cp₂Ti(P₂Ar₂) is restricted to aryl-substituted terminal alkynes featuring strong electron-withdrawing substituents in para-position. These metallacycles were investigated spectroscopically and crystallographically.

We present the first synthesis and characterization of several stable cyclic Peterson olefination intermediates with four carbon bonded substituents at the silicon. The cyclic 1,2-oxasiletanide intermediates were synthesized via the reaction of an α-silyl carbanion with various ketones.

ABSTRACT

Carbonyl olefination reactions have become essential to organic chemistry since Wittig's report on the first reaction of this kind using a phosphorus ylide. While the reaction mechanism of the Wittig olefination is well understood the same cannot be said about the related silicon analogue, the Peterson olefination. Both an open chain, betaine like intermediate and a cyclic 1,2-oxasiletanide intermediate have been discussed since Peterson's original publication, with little evidence for the cyclic intermediates. Herein we present the synthesis and characterization of several stable cyclic Peterson olefination intermediates synthesized via the reaction of an α-silyl carbanion with various ketones. The α-silyl carbanion is stabilized by three pentafluoroethyl groups at the silicon atom. Furthermore, it bears a carbanion stabilizing phenyl group α to the carbanion.

Ir pincer complexes bearing a triazole architecture in the pincer backbone and O-ancillary chelating ligands are presented. These were employed in the catalytic transfer dehydrogenation of alkanes as well as of saturated heterocycles. Reactivity of these complexes with silver salts and Lewis acids produced various cationic iridium complexes.

ABSTRACT

An asymmetric ^{t Bu}POCN_Triaz type pincer ligand containing a triazole arm was developed and used to generate iridium pincer complexes with different oxygen atom binding ancillary ligands. The structure and reactivity of the complexes [( ^{t Bu}POCN_Triaz)Ir(H)(O₂CCH₃)], [( ^{t Bu}POCN_Triaz)Ir(H)(acac)], and [( ^{t Bu}POC)Ir(H)(O₂CCF₃)] and its cationic derivatives were studied. The complexes could be successfully employed in the catalytic transfer dehydrogenation of alkanes as well as of unsaturated heterocycles.

While late 3d metals dominate efforts to replace precious metals in homogeneous hydrogenation, molybdenum has remained largely ignored. This work reveals that a nine-coordinate pentahydrido molybdenum complex can hydrogenate a broad range of alkenes efficiently, demonstrating that early-metal polyhydrides can serve as genuine catalysts rather than mere curiosities.

ABSTRACT

The cationic nine-coordinate pentahydrido complex [MoH₅(depe)₂]BPh₄ (1·BPh₄, depe  =  1,2-(diethylphosphino)ethane) is, to the best of our knowledge, the first early transition metal polyhydride shown to efficiently catalyze the hydrogenation of a diverse set of unactivated alkenes. Preliminary mechanistic investigation by Density Functional Theory (DFT) calculations indicates that turnover relies on the ability of this 18-electron, d° complex to undergo thermally facile H₂ reductive elimination, which opens coordination sites and supplies the metal center the necessary electrons to engage in the reduction of the substrate. According to the computations, ethylene hydrogenation catalysis by 1·BPh₄ follows a sequence of migratory insertion prior to H₂ oxidative addition. These findings demonstrate that high-coordinate early transition metal polyhydrides are not mere structural curiosities but can act as genuine hydrogenation catalysts. The unique reactivity of 1·BPh₄ provides a foundation for new design concepts in base-metal homogeneous catalysis, beyond the current focus on late 3d metals.

The electronic structure of a variety of singlet biradicaloids may be described in the Lewis picture using single antibonds as the central bonding motif. In conjunction with a consistent set of biradical indicators, we can resolve some long-standing disputes over the biradical character of molecules such as O₃ and S₂N₂.

ABSTRACT

While the mathematical description of the electronic structure of biradical(oid)s is nowadays well understood, we feel that the chemical-conceptual understanding of the structure and bonding, especially of singlet biradicaloids, remains underdeveloped. In particular, the degree of “biradical character” of singlet biradicaloids is discussed in the literature using a heterogeneous selection of indices, which are often not directly comparable and therefore hamper the development of a systematic understanding as well as comparison of different biradicaloid species. This paper aims to ameliorate this situation, by taking a holistic approach considering a wide variety of biradical indicators (including e.g. the LUNO occupancy, singlet-triplet (ST) gap, bond order (BO) between the radical centers) and applying established computational methods (DFT, CCSD(T), CASSCF, NEVPT2, MRCI) to attain a unified description across the whole range of biradical character, from closed-shell molecules to “perfect” (open-shell) biradicals. We mainly focus on model systems of inorganic, four-membered ring systems which are formally isolobal to S₂N₂ to discuss our results, but the general conclusions should hold for any structure with biradical character. Most notably, we suggest describing the interaction between the radical electrons in biradicaloids with low to moderate biradical character using bonds and antibonds for an intuitive chemical understanding of these species.

Fluorinated diphosphine ligands such as 1,2-bis[bis(pentafluorophenyl)phosphino]ethane (dFppe) offer powerful electronic tunability and unusual reactivity. Despite known analogues of the heavier congeners Pd and Pt, Ni complexes of dFppe remain conspicuously absent. Here, we show that reaction of the highly labile Ni source, Ni(CDT) (CDT = trans,trans,trans-1,5,9-cyclododecatriene) with dFppe enables access to [Ni(dFppe)2]. Stability studies and preliminary reactivity of this fluorinated Ni(0) complex are described.

ABSTRACT

While 1,2-bis[bis(pentafluorophenyl)phosphino]ethane (dFppe) has been shown to impart unusual electronic properties and reactivity in Pd and Pt complexes, analogous Ni systems have remained unexplored. Leveraging the high lability of Ni(CDT) (CDT = trans,trans,trans-1,5,9-cyclododecatriene), we have synthesized a new Ni(0) complex, [Ni(dFppe)₂]. Herein, we describe its structural and spectroscopic characterization and evaluate air stability, ligand substitution behavior, and reactivity in C─C cross-coupling chemistry.

Wir berichten über die Synthese und umfassende Charakterisierung eines quadratisch-planaren Si(+IV)-Hydrids, das von einem unsymmetrischen, trianionischen und dearomatisierten N,N,N-Pincerliganden stabilisiert wird. Dieses System ermöglicht eine Element–Liganden-kooperative Reaktivität als Alternative zur siliciumzentrierten Redoxchemie und erweitert damit ein weitgehend unerforschtes Gebiet in der Silicium-Chemie.

ZUSAMMENFASSUNG

Die quadratisch-planare Koordination an vierwertigem Silicium ist stark benachteiligt, sodass strukturell eindeutig charakterisierte Si(+IV)-Komplexe dieses Typs außerordentlich selten sind. Im Folgenden berichten wir über die Synthese und Isolation eines quadratisch-planaren Silicium(+IV)-Hydrids, welches durch einen unsymmetrischen, trianionischen N,N,N-Pincer-Liganden mit dearomatisiertem Rückgrat stabilisiert wird. Die Einkristallröntgenstrukturanalyse dieser Verbindung belegt ein strikt planares, vierfach koordiniertes Siliciumzentrum, wobei spektroskopische Daten und quantenchemische Berechnungen dieses Bindungsmotiv zusätzlich untermauern. Reaktivitätsstudien zeigen eine Element–Liganden-kooperative Substrataktivierung, die durch die Rearomatisierung des Liganden angetrieben wird und damit Parallelen zu Reaktionen in konstanten Oxidationsstufen bei späten Übergangsmetallkomplexen aufweist. Dies stellt die bislang dominierende Rolle niedervalenter p-Block-Spezies für Bindungsaktivierungen auf den Prüfstand.

An operationally simple protocol using a strongly basic sodium amide in combination with sterically emcumbered silicon electrophiles enables the deprotonative C–H (multi)silylation of a myriad of (hetero)arenes. Mechanistic investigations highlight the pivotal role of steric and coordination effects in controlling the regioselectivity and efficiency of these reactions.

ABSTRACT

The importance of organosilicon compounds in synthetic and materials chemistry has prompted the search for efficient and broadly applicable routes to these invaluable scaffolds, which often depend on scarce transition metals. In contrast, main group-mediated strategies remain underdeveloped, with those reported generally limited to activated substrates and harsh reaction conditions. Here, a new sodium-mediated protocol for deprotonative C(sp ³)/C(sp ²)–H silylation of (hetero)arenes is presented, which relies on the power of the strongly basic sodium amide NaTMP (TMP = 2,2,6,6-tetramethylpiperidide) in combination with bulky chlorosilanes. This approach provides direct access to a myriad of silylated aromatic products, including toluene derivatives, non-activated arenes such as benzene and naphthalene, pyridines and electron-rich heterocycles. Mechanistic investigations, combining the isolation of key organometallic intermediates with theoretical calculations, underline the complementarity of NaTMP and the electrophilic chlorosilane, and reveal the steric and coordination effects that govern sodiation and silylation steps. Most notably, this protocol extends beyond conventional monosilylation, including orthogonal multisilylation of distinct C(sp ³) and C(sp ²)–H bonds, thereby broadening the scope of main group-mediated arene functionalization.

Cross-coupling/homologation sequence between a stable three-membered-ring carbene and isocyanides enables facile and rapid access to novel methyleneketenimines and the first bis(methyleneketenimine). Reactivity studies gave rise to the first methyleneketenimine-transition metal complexes, unique multimodal reactivity ranging from CO₂, isocyanate splitting, to formal alkyne C≡C bond insertion triggered via dipolar cycloaddition.

ABSTRACT

Methyleneketenimines, due to their highly unsaturated and CN-cumulenic nature, represent a rare class of heterocumulenes with potential as building blocks for pharmaceuticals and molecular electronics. However, their systematic chemistry has been limited. Herein, we report a concise C═C═N homologation reaction, driven by the strain release of the three-membered-ring carbene, bis(diisopropylamino)cyclopropenylidene (BAC), which unlocks rapid and scalable access to a series of isolable methyleneketenimines, including the first example of a bis(methyleneketenimine), featuring terminal azetidine moieties (azetidinylideneketenimines). Mechanistic studies, supported by control experiments and computational analysis, elucidate the important role of a methyleneketeniminyl carbene intermediate in forming the target heterocumulene. Spectroscopic and electrochemical investigations show that the visible-light absorption and redox potentials correlate with the degree of N-substituent π-conjugation. Most importantly, the highly polarized nature of the C═C═C═N framework enables a multimodal reactivity platform, spanning from unprecedented coordination to transition metals (Au(I), Rh(I)), CO₂ splitting, alkyne C≡C bond insertion, to divergent skeletal rearrangements via (2 + 2) or (3 + 2) cycloaddition. This work establishes methyleneketenimines as a versatile toolkit for accessing complex heterocycles and novel organometallic complexes.

The syntheses and the structures of N-heterocyclic allenes are reported. Allenes bearing fluorenylidene capping groups show markedly bent C═C═C units, a carbene-like reactivity, and they can act as potent C-donor ligands for metal complexes.

ABSTRACT

Allenes typically display a linear geometry. A notable exception occurs in allenes bearing two electron-donating N-heterocyclic capping groups, which can display a strongly bent C═C═C unit. These push-push allenes, known as carbodicarbenes, are powerful carbon-donor ligands with broad utility in chemistry. Computational studies have suggested that push-pull allenes, featuring both an electron-donating and withdrawing capping group, can also adopt a bent geometry. However, experimental confirmation has so far been lacking. Herein, we report allenes featuring an N-heterocyclic capping group on one side, and either diarylmethylidene or fluorenylidene capping groups on the other. The synthesis of these allenes was accomplished via a titanium vinylidene complex. The latter could be isolated and analyzed by X-ray diffraction, and it represents the first structurally characterized Ti vinylidene complex. The push-pull allenes with fluorenylidene capping groups revealed markedly bent C═C═C units (α_c-c-c < 140°), whereas the less polarized diarylmethylidene analogues were found to display a more linear geometry. The geometric differences correlate with a divergent reactivity upon thermal activation. Both types of N-heterocyclic allenes can be used as carbon-donor ligands for transition metal complexes.

Bindungsbruch: Die Reaktion zwischen NO[Al(OR^F)₄] (R^F = C(CF₃)₃) und dem Diphosphan PNP ^{t Bu} (2,6-Bis(di-tert-butylphosphino-methyl)pyridin) führt zur direkten und vollständigen Spaltung der NO⁺-Dreifachbindung, einer der stärksten bekannten chemischen Bindungen. Der Mechanismus dieser Reaktion wurde mithilfe quantenchemischer Rechnungen und NMR-spektroskopischer kinetischer Untersuchungen ermittelt.

ZUSAMMENFASSUNG

Wir stellen die einfache und direkte unimolekulare Spaltung der N≡O⁺-Dreifachbindung vor, welche mit einer Bindungsdissoziationsenergie von 1049 kJ mol⁻¹ eine der stärksten bekannten chemischen Bindungen ist. Die Reaktion von NO[Al(OR^F)₄] (R^F = C(CF₃)₃) und PNP ^tBu (2,6-Bis(di-tert-butylphosphinomethyl)pyridin) in CH₂Cl₂ bei Raumtemperatur ergibt das 1,2,3-Diazaphospholo[1,5-a]pyridinium-Derivat [DAPP ^tBu]⁺[Al(OR^F)₄]^– (1), das durch die vollständige Spaltung der N≡O⁺-Bindung entsteht. Bei −30°C kann das Reaktionszwischenprodukt [PNOP ^tBu]⁺[Al(OR^F)₄]^– (2) isoliert werden, welches ein neuartiges verbrückendes Strukturmotiv P = N−O−P⁺ enthält. Durch quantenchemische Rechnungen wurden mechanistische Einblicke gewonnen, die die Bildungswege von 1 und 2 aufklärten, während die Umwandlungsrate von 2 zu 1 durch NMR-spektroskopische Untersuchungen der Reaktionskinetik bestimmt wurde.

CO₂ hydrogenation to methanol using a Mn–MACHO catalyst was achieved without Lewis acid promoters, delivering TONs up to 45.2. Diamines, by driving a highly exergonic double amidation reaction, divert formate resting states toward the active catalyst. The correlation between ΔG _amidation and methanol productivity enables the rational design of the amine for novel Ru and Mn-MACHO catalysis.

Abstract

The conversion of CO₂ into energy-dense liquid fuels, such as methanol, represents a cornerstone of sustainable chemistry; however, most homogeneous catalytic systems still rely on noble metals or Lewis acid additives. Here, we report the first protocol for amine-assisted CO₂ hydrogenation to methanol using a Mn–MACHO catalyst without any Lewis acid co-catalyst, achieving turnover numbers up to 45.2, the highest reported for Mn systems. A combined computational, microkinetic, and experimental study reveals that diamines dramatically enhance activity compared to monoamines by promoting a highly exergonic double amidation step. This thermodynamic driving force shifts the equilibrium away from formate resting states toward the active catalyst, thereby accelerating methanol formation. The correlation established between amidation free energies (ΔG _amidation) and methanol productivity provides a rational design principle for tailoring amine promoters across Ru- and Mn-based MACHO catalysts. These insights advance the development of sustainable, base-metal-catalyzed CO₂ conversion strategies and open opportunities for integrated carbon capture and utilization.