1,720,958 research outputs found
Elimination from Azide Diradical Ligands
No full textTransition metal nitrides/nitrenes are highly promising reagents for catalytic nitrogen atom transfer reactivity. They are typically prepared in situ upon optically induced N2‐elimination from azido precursors. A full exploitation of their catalytic potential, however, requires in‐depth knowledge of the primary photo‐induced processes and the structural/electronic factors mediating the N2‐loss with birth of the terminal metal‐nitrogen core. Using femtosecond infrared‐spectroscopy, we elucidate here the primary molecular‐level mechanisms responsible for the formation of a unique platinum(II) nitrene with a triplet ground state from a closed‐shell platinum(II) azide precursor. The spectroscopic data in combination with quantum‐chemical calculations provide compelling evidence that product formation requires the initial occupation of a singlet excited state with an anionic azide diradical ligand that is bound to a low‐spin d8‐configured PtII ion. Subsequent intersystem‐crossing generates the Pt‐bound triplet azide diradical, which smoothly evolves into the triplet nitrene via N2–loss in a near barrierless adiabatic dissociation. Our data highlight the importance of the productive, N2‐releasing state possessing azide ππ* character as a design principle for accessing efficient N‐atom‐transfer catalysts
Dioxygen Splitting by a Tantalum(V) Complex Ligated by a Rigid, Redox Non‐Innocent Pincer Ligand**
No full textThe reaction of TaMe3Cl2 with the rigid acridane-derived trisamine H3NNN yields the tantalum(V) complex [TaCl2(NNNcat)]. Subsequent reaction with dioxygen results in the full four-electron reduction of O2 yielding the oxido-bridged bimetallic complex [{TaCl2(NNNsq)}2O]. This dinuclear complex features an open-shell ground state due to partial ligand oxidation and was comprehensively characterized by single crystal X-ray diffraction, LIFDI mass spectrometry, NMR, EPR, IR and UV/VIS/NIR spectroscopy. The mechanism of O2 activation was investigated by DFT calculations revealing initial binding of O2 to the tantalum(V) center followed by complete O2 scission to produce a terminal oxido-complex
C═C Dissociative Imination of Styrenes by a Photogenerated Metallonitrene
Full text linkPhotolysis of a platinum(II) azide complex in the presence of styrenes enables C═C double bond cleavage upon dissociative olefin imination to aldimido (PtII–N═CHPh) and formimido (PtII–N═CH2) complexes as the main products. Spectroscopic and quantum chemical examinations support a mechanism that commences with the decay of the metallonitrene photoproduct (PtII–N) via bimolecular coupling and nitrogen loss as N2. The resulting platinum(I) complex initiates a radical chain mechanism via a dinuclear radical-bridged species (PtII–CH2CHPhN•–PtII) as a direct precursor to C–C scission. The preference for the PtI mediated route over styrene aziridination is attributed to the distinct nucleophilicity of the triplet metallonitrene.Photolysis of a platinum(II) azide complex in the presence of styrenes enables C═C double bond cleavage upon dissociative olefin imination to aldimido (PtII–N═CHPh) and formimido (PtII–N═CH2) complexes as the main products. Spectroscopic and quantum chemical examinations support a mechanism that commences with the decay of the metallonitrene photoproduct (PtII–N) via bimolecular coupling and nitrogen loss as N2. The resulting platinum(I) complex initiates a radical chain mechanism via a dinuclear radical-bridged species (PtII–CH2CHPhN•–PtII) as a direct precursor to C–C scission. The preference for the PtI mediated route over styrene aziridination is attributed to the distinct nucleophilicity of the triplet metallonitrene
Nitrogen Atom Transfer Catalysis by Metallonitrene C–H Insertion: Photocatalytic Amidation of Aldehydes
No full textC−H amination and amidation by catalytic nitrene transfer are well-established and typically proceed via electrophilic attack of nitrenoid intermediates. In contrast, the insertion of (formal) terminal nitride ligands into C−H bonds is much less developed and catalytic nitrogen atom transfer remains unknown. We here report the synthesis of a formal terminal nitride complex of palladium. Photocrystallographic, magnetic, and computational characterization support the assignment as an authentic metallonitrene (Pd−N) with a diradical nitrogen ligand that is singly bonded to PdII. Despite the subvalent nitrene character, selective C−H insertion with aldehydes follows nucleophilic selectivity. Transamidation of the benzamide product is enabled by reaction with N3SiMe3. Based on these results, a photocatalytic protocol for aldehyde C−H trimethylsilylamidation was developed that exhibits inverted, nucleophilic selectivity as compared to typical nitrene transfer catalysis. This first example of catalytic C−H nitrogen atom transfer offers facile access to primary amides after deprotection.European Research Council http://dx.doi.org/10.13039/100010663Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659U.S. Department of Energy http://dx.doi.org/10.13039/100000015Welch Foundation http://dx.doi.org/10.13039/100000928National Science Foundation http://dx.doi.org/10.13039/10000000
Electron Rich Metallo Nitrene Complexes For Nitrogen Transfer
Full text linkThe advent of C–N bond formation by catalytic C–H amination had a huge impact on the synthesis of nitrogen-containing compounds, in academia as well as industry. In contrast to nitrogen-group transfer, complementary nitrogen-atom transfer is heavily underdeveloped. This can be attributed to the high stability of most reported (early) transition metal nitrides, while the more reactive late transition metal nitrides are too reactive for isolation or even characterization. In this work, rare group 10 transition metal nitrides (metallo-nitrenes) are synthesized and their electronic nature is analyzed by spectroscopic and computational methods. The reactivity of Pd and Pt metallonitrene complexes towards C–H and C=C bonds is investigated, revealing a unique nucleophilic behavior of the transient nitrene. The Pd metallonitrene is capable of catalytic C-H bond amidation, being the first example for catalytic nitrogen-atom transfer. Reactivity of the Pt metallo-nitrene with styrene leads to a rare case of C=C bond cleavage, leading to C=N bond formation by imination. Mechanistic studies, most importantly EPR, suggest a radical mechanism, which is initiated by catalytic amounts of PtI.
Transition metal nitrides are an important class of compounds, especially with regard to fundamental structure and bonding. They can be synthesized readily by photoinduced N2 loss from the parent azide complex, however, the elementary primary events that facilitate N2 elimination after electronic excitation remain largely unknown. The here reported pincer complex of a Pt azide serves as an ideal probe for transient absorption IR spec-troscopy, revealing the individual steps from electronic excitation to N2 loss. A triplet azide, produced by triplet sensitization from the strongly absorbing pincer backbone, seems to be the key species leading to N2 loss. These findings might serve as a design princi-ple, which allows highly efficient photoinduced N2 loss of organic or inorganic azides on the triplet surface.2025-01-1
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Nitrogen Atom Transfer Catalysis by Metallonitrene C−H Insertion: Photocatalytic Amidation of Aldehydes
No full textH2020 European Research Council https://doi.org/10.13039/100010663Deutsche Forschungsgemeinschaft https://doi.org/10.13039/501100001659U.S. Department of Energy https://doi.org/10.13039/100000015Welch Foundation https://doi.org/10.13039/100000928National Science Foundation https://doi.org/10.13039/10000000
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