NXS, Morpholine, and HFIP: The Ideal Combination for Biomimetic Haliranium-Induced Polyene Cyclizations

In contrast to Nature that accomplishes polyene cyclizations seemingly with ease, such transformations are difficult to conduct in the lab. In our program dealing with the development of selective halogenations of alkenes, we now asserted that standard X⁺ reagents are perfectly suited for the biomimetic cation-π cyclization of both electron rich and poor linear polyenes in the presence of the Lewis base morpholine and the Lewis acid HFIP. The method stands out due to its broad substrate scope and practicability together with high chemical yields and excellent selectivities, even for highly challenging chloriranium-induced polyene cyclizations

Fluoridonitrosyl Complexes of Technetium(I) and Technetium(II). Synthesis, Characterization, Reactions, and DFT Calculations

A mixture of [Tc­(NO)­F₅]^2– and [Tc­(NO)­(NH₃)₄F]⁺ is formed during the reaction of pertechnetate with acetohydroxamic acid (Haha) in aqueous HF. The blue pentafluoridonitrosyltechnetate­(II) has been isolated in crystalline form as potassium and rubidium salts, while the orange-red ammine complex crystallizes as bifluoride or PF₆^– salts. Reactions of [Tc­(NO)­F₅]^2– salts with HCl give the corresponding [Tc­(NO)­Cl_4/5]^−/2– complexes, while reflux in neat pyridine (py) results in the formation of the technetium­(I) cation [Tc­(NO)­(py)₄F]⁺, which can be crystallized as hexafluoridophosphate. The same compound can be synthesized directly from pertechnetate, Haha, HF, and py or by a ligand-exchange procedure starting from [Tc­(NO)­(NH₃)₄F]­(HF₂). The technetium­(I) cation [Tc­(NO)­(NH₃)₄F]⁺ can be oxidized electrochemically or by the reaction with Ce­(SO₄)₂ to give the corresponding Tc­(II) compound [Tc­(NO)­(NH₃)₄F]²⁺. The fluorido ligand in [Tc­(NO)­(NH₃)₄F]⁺ can be replaced by CF₃COO^–, leaving the “[Tc­(NO)­(NH₃)₄]²⁺ core” untouched. The experimental results are confirmed by density functional theory calculations on [Tc­(NO)­F₅]^2–, [Tc­(NO)­(py)₄F]⁺, [Tc­(NO)­(NH₃)₄F]⁺, and [Tc­(NO)­(NH₃)₄F]²⁺

High-Melting, Elastic Polypropylene: A One-Pot, One-Catalyst Strategy toward Propylene-Based Thermoplastic Elastomers

This contribution provides the simple one-pot, one-catalyst synthesis of high-melting (<i>T</i>_m ∼ 140 °C), high-molecular-weight, elastic polypropylene (^<i>e</i>PP) offering an excellent reversible deformation behavior. The produced propylene-based thermoplastic elastomers contain of ^<i>i</i>PP–^<i>a</i>PP block structures embedded in an amorphous polypropylene matrix which is enabled by the variable stereoselective behavior of ethylene-bridged fluorenylindenyl (EBFI) <i>ansa</i>-metallocene complexes. For the tailored synthesis of these high-melting ^<i>e</i>PPs the intricate interplay of various mechanisms, which collectively define the stereoregularity of the produced polypropylenes, was examined, and a decisive impact of different chelate ring conformers was elucidated. In this connection, the accurate adjustment of conformational interconversion with respect to the chain propagation and termination rate facilitated a directed switching between iso- and unselective polypropylene sequences in the catalytic production of highly temperature-stable, elastic polypropylene

Pyrazolato-Bridged Dinuclear Complexes of Ruthenium(II) and Rhodium(III) with N‑Heterocyclic Carbene Ligands: Synthesis, Characterization, and Electrochemical Properties

Pyrazolato-bridged dinuclear complexes of ruthenium and rhodium were synthesized from N-heterocyclic carbene (NHC) precursors, 3,5-bis­[(methylimidazolium-1-yl)­methyl]-1<i>H</i>-pyrazole bis­(hexafluorophosphate), and the metal precursors [Ru­(<i>p</i>-cymene)­Cl₂]₂ and [Rh­(η⁵-C₅Me₅)­Cl₂]₂. Depending on the reaction conditions, dinuclear bis­(imidazolium) complexes or the corresponding bis­(NHC) complexes were formed. These complexes were characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The metal–metal distances are in the range 3.85–3.92 Å. Accordingly, a metal–metal bond can be excluded in all cases. The electronic properties were examined by cyclic voltammetry (CV) to detect possible electronic coupling between the metal centers. In the case of the imidazolium complexes irreversible processes are observed in CV, indicating decomposition. The Ru–bis­(NHC) complexcoordinatively saturated with six acetonitrile molecules instead of <i>p</i>-cymene ligandsshows three reversible redox processes. Density functional theory (DFT) calculations were used to verify the processes during CV. The Rh–bis­(NHC) complex decomposes through irreversible reductions

Pyrazolato-Bridged Dinuclear Complexes of Ruthenium(II) and Rhodium(III) with N‑Heterocyclic Carbene Ligands: Synthesis, Characterization, and Electrochemical Properties

Pyrazolato-bridged dinuclear complexes of ruthenium and rhodium were synthesized from N-heterocyclic carbene (NHC) precursors, 3,5-bis­[(methylimidazolium-1-yl)­methyl]-1<i>H</i>-pyrazole bis­(hexafluorophosphate), and the metal precursors [Ru­(<i>p</i>-cymene)­Cl₂]₂ and [Rh­(η⁵-C₅Me₅)­Cl₂]₂. Depending on the reaction conditions, dinuclear bis­(imidazolium) complexes or the corresponding bis­(NHC) complexes were formed. These complexes were characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The metal–metal distances are in the range 3.85–3.92 Å. Accordingly, a metal–metal bond can be excluded in all cases. The electronic properties were examined by cyclic voltammetry (CV) to detect possible electronic coupling between the metal centers. In the case of the imidazolium complexes irreversible processes are observed in CV, indicating decomposition. The Ru–bis­(NHC) complexcoordinatively saturated with six acetonitrile molecules instead of <i>p</i>-cymene ligandsshows three reversible redox processes. Density functional theory (DFT) calculations were used to verify the processes during CV. The Rh–bis­(NHC) complex decomposes through irreversible reductions

The Mechanism of Borane–Amine Dehydrocoupling with Bifunctional Ruthenium Catalysts

Borane–amine adducts have received considerable attention, both as vectors for chemical hydrogen storage and as precursors for the synthesis of inorganic materials. Transition metal-catalyzed ammonia–borane (H₃N–BH₃, AB) dehydrocoupling offers, in principle, the possibility of large gravimetric hydrogen release at high rates and the formation of B–N polymers with well-defined microstructure. Several different homogeneous catalysts were reported in the literature. The current mechanistic picture implies that the release of aminoborane (e.g., Ni carbenes and Shvo’s catalyst) results in formation of borazine and 2 equiv of H₂, while 1 equiv of H₂ and polyaminoborane are obtained with catalysts that also couple the dehydroproducts (e.g., Ir and Rh diphosphine and pincer catalysts). However, in comparison with the rapidly growing number of catalysts, the amount of experimental studies that deal with mechanistic details is still limited. Here, we present a comprehensive experimental and theoretical study about the mechanism of AB dehydrocoupling to polyaminoborane with ruthenium amine/amido catalysts, which exhibit particularly high activity. On the basis of kinetics, trapping experiments, polymer characterization by ¹¹B MQMAS solid-state NMR, spectroscopic experiments with model substrates, and density functional theory (DFT) calculations, we propose for the amine catalyst [Ru­(H)₂PMe₃{HN­(CH₂CH₂P<i>t</i>Bu₂)₂}] two mechanistically connected catalytic cycles that account for both metal-mediated substrate dehydrogenation to aminoborane and catalyzed polymer enchainment by formal aminoborane insertion into a H–NH₂BH₃ bond. Kinetic results and polymer characterization also indicate that amido catalyst [Ru­(H)­PMe₃{N­(CH₂CH₂P<i>t</i>Bu₂)₂}] does not undergo the same mechanism as was previously proposed in a theoretical study