Influence of Ring-Expanded <i>N</i>-Heterocyclic Carbenes on the Structures of Half-Sandwich Ni(I) Complexes: An X-ray, Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) Study

Pelties, Stefan; Carter, Emma; Folli, Andrea; Mahon, Mary F.; Murphy, D. M.; Whittlesey, Michael K.; Wolf, Robert

doi:10.1021/acs.inorgchem.6b01540

Cited by 28 publications

(14 citation statements)

References 128 publications

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“…Only a relatively small number of Ni(0)–NHC complexes have been isolated, and their stoichiometric and/or catalytic reactivity investigated. ,, In all cases, five-membered ring NHCs, are present with the majority of examples also containing bulky N-substituents in order to generate highly reactive low-coordinate Ni centers. During the course of our work with 6- and 7-membered ring carbenes (so-called ring-expanded carbenes or RE-NHCs) for the synthesis of two- and three-coordinate Ni(I) carbene complexes, we chanced upon the formation of the Ni(0) 6-membered ring carbene complex, Ni(6-Mes)(PPh 3 ) 2 ( 1 ; 6-Mes =1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene). We now report our initial studies of both its stoichiometric and catalytic chemistry.…”

Section: Introductionmentioning

confidence: 99%

Stoichiometric and Catalytic Reactivity of Ni(6-Mes)(PPh₃)₂

et al. 2017

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The three-coordinate Ni(0) N-heterocyclic carbene complex Ni(6-Mes)(PPh 3 ) 2 (1; 6-Mes = 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) is formed in the reaction of Ni(cod) 2 with a 1:2 mixture of 6-Mes and PPh 3 or upon reduction of Ni(6-Mes)(PPh 3 )Br ( 2) with KO t Bu. Facile substitution of PPh 3 in 1 gave a range of Ni(6-Mes)(PPh 3 )(L) products (L = PhCCMe (3), PhCHCH 2 (4), Ph 2 CO (5), PhCHO ( 6)). Oxidative addition of C 6 F 6 gave Ni(6-Mes)(PPh 3 )(C 6 F 5 )F ( 7), while 1 was also oxidized by 4-BrC 6 H 4 F to afford a mixture of 2 and Ni(6-Mes)(PPh 3 )(C 6 H 4 F)Br (8). Surprisingly, 1 was also oxidized upon reaction with the small 5-membered ring NHC IMe 4 to give the terminal Ni(II) phosphido complex Ni(IMe 4 ) 2 (PPh 2 )Ph ( 9). Compounds 1 and 5 proved to be active as a precursors for the catalytic transfer hydrogenation of ketones.

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Section: Introductionmentioning

confidence: 99%

Stoichiometric and Catalytic Reactivity of Ni(6-Mes)(PPh₃)₂

et al. 2017

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“…224 The driving force in this field, in recent years, has focused on the choice of ligands employed to stabilise the active oxidation states, such as N-heterocyclic carbenes (NHCs) and bisphosphines, and these ligands have also been found to improve the overall selectivity in cross-coupling reactions. [225][226][227][228] For example, Whittlesey et al, 224,228,229 designed a series of air-sensitive [Ni I (PPh 3 )(NHC)X] complexes (X ¼ Br, Cl) using bulky NHC ligands. They investigated the effect of sterically demanding substitution at the central heteroaromatic ring which is required to stabilise the low-coordinate and low-valent Ni 11 complex and to avoid any intermolecular reactions.…”

Section: C-c Cross-couplingmentioning

confidence: 99%

“…They investigated the effect of sterically demanding substitution at the central heteroaromatic ring which is required to stabilise the low-coordinate and low-valent Ni 11 complex and to avoid any intermolecular reactions. 225,229,230 The authors used a combination of CW and pulsed EPR spectroscopy, with complimentary DFT calculations, to extract information about the fundamental properties (structure and bonding) and catalytic efficiency of the complex in the Kumada coupling of aryl-flourides and aryl-chlorides. The EPR spectra revealed a rhombic symmetry for the low coordinated Ni 11 systems, with a large super-hyperfine coupling to the 31 P and 79,81 Br nuclei.…”

Section: C-c Cross-couplingmentioning

confidence: 99%

Paramagnetic species in catalysis research: a unified approach towards (the role of EPR in) heterogeneous, homogeneous and enzyme catalysis

Bracci¹,

Bruzzese²,

Famulari³

et al. 2020

Electron Paramagnetic Resonance

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Paramagnetic (open-shell) systems, including transition metal ions, radical intermediates and defect centres, are often involved in catalytic transformations. Despite the prevalence of such species in catalysis, there are relatively few studies devoted to their characterisation, compared to their diamagnetic counterparts. Electron Paramagnetic Resonance (EPR) is an ideal technique perfectly suited to characterise such reaction centres, providing valuable insights into the molecular and supramolecular structure, the electronic structure, the dynamics and even the concentration of the paramagnetic systems under investigation. Furthermore, as EPR is such a versatile technique, samples can be measured as liquids, solids (frozen solutions and powders) and single crystals, making it ideal for studies in heterogeneous, homogeneous and enzyme catalysis. Coupled with the higher resolving power of the pulsed, higher frequency and hyperfine techniques, unsurpassed detail on the structure of these catalytic centres can be obtained. In this Chapter, we provide an overview to demonstrate how advanced EPR methods can be successfully exploited in the study of open-shell paramagnetic reaction centres in heterogeneous, homogeneous and enzymatic catalysts, including heme-based enzymes for use in biocatalysts, polymerisation based catalysts, supported microporous heterogeneous catalytic centres to homogeneous metal complexes for small molecule actions.

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“…As reference groups we have somewhat arbitrarily chosen those of Marina Bennati (Göttingen) [209][210][211], Robert Bittl (Berlin) [212][213][214], Dave Britt (UC Davis) [215][216][217], Jack Freed (Cornell) [218][219][220], Daniella Goldfarb (Weizmann) [221][222][223], Brian Hoffman (Northwestern) [224][225][226], Gunnar Jeschke (Zurich) [227][228][229], Chris Kay (London) [230,231], Yasuhiro Kobori (Kobe) [232][233][234], Wolfgang Lubitz (Mülheim/Ruhr) [235][236][237], Damian Murphy (Cardiff) [238][239][240], Thomas Prisner (Frankfurt/Main) [241][242][243], Christiane Timmel (Oxford) [244][245][246], Sabine van Doorslaer (Antwerp) [247][248][249], and Stefan Weber (Freiburg) [250][251][252].…”

Section: Pulse Endormentioning

confidence: 99%

Biomolecular EPR Meets NMR at High Magnetic Fields

et al. 2018

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In this review on advanced biomolecular EPR spectroscopy, which addresses both the EPR and NMR communities, considerable emphasis is put on delineating the complementarity of NMR and EPR regarding the measurement of interactions and dynamics of large molecules embedded in fluid-solution or solid-state environments. Our focus is on the characterization of protein structure, dynamics and interactions, using sophisticated EPR spectroscopy methods. New developments in pulsed microwave and sweepable cryomagnet technology as well as ultrafast electronics for signal data handling and processing have pushed the limits of EPR spectroscopy to new horizons reaching millimeter and sub-millimeter wavelengths and 15 T Zeeman fields. Expanding traditional applications to paramagnetic systems, spin-labeling of biomolecules has become a mainstream multifrequency approach in EPR spectroscopy. In the high-frequency/high-field EPR region, sub-micromolar concentrations of nitroxide spin-labeled molecules are now sufficient to characterize reaction intermediates of complex biomolecular processes. This offers promising analytical applications in biochemistry and molecular biology where sample material is often difficult to prepare in sufficient concentration for NMR characterization. For multifrequency EPR experiments on frozen solutions typical sample volumes are of the order of 250 μL (S-band), 150 μL (X-band), 10 μL (Q-band) and 1 μL (W-band). These are orders of magnitude smaller than the sample volumes required for modern liquid- or solid-state NMR spectroscopy. An important additional advantage of EPR over NMR is the ability to detect and characterize even short-lived paramagnetic reaction intermediates (down to a lifetime of a few ns). Electron–nuclear and electron–electron double-resonance techniques such as electron–nuclear double resonance (ENDOR), ELDOR-detected NMR, PELDOR (DEER) further improve the spectroscopic selectivity for the various magnetic interactions and their evolution in the frequency and time domains. PELDOR techniques applied to frozen-solution samples of doubly spin-labeled proteins allow for molecular distance measurements ranging up to about 100 Å. For disordered frozen-solution samples high-field EPR spectroscopy allows greatly improved orientational selection of the molecules within the laboratory axes reference system by means of the anisotropic electron Zeeman interaction. Single-crystal resolution is approached at the canonical g-tensor orientations—even for molecules with very small g-anisotropies. Unique structural, functional, and dynamic information about molecular systems is thus revealed that can hardly be obtained by other analytical techniques. On the other hand, the limitation to systems with unpaired electrons means that EPR is less widely used than NMR. However, this limitation also means that EPR offers greater specificity, since ordinary chemical solvents and matrices do not give rise to EPR in contrast to NMR spectra. Thus, multifrequency EPR spectroscopy plays an important role in better understanding paramagnetic species such as organic and inorganic radicals, transition metal complexes as found in many catalysts or metalloenzymes, transient species such as light-generated spin-correlated radical pairs and triplets occurring in protein complexes of photosynthetic reaction centers, electron-transfer relays, etc. Special attention is drawn to high-field EPR experiments on photosynthetic reaction centers embedded in specific sugar matrices that enable organisms to survive extreme dryness and heat stress by adopting an anhydrobiotic state. After a more general overview on methods and applications of advanced multifrequency EPR spectroscopy, a few representative examples are reviewed to some detail in two Case Studies: (I) High-field ELDOR-detected NMR (EDNMR) as a general method for electron–nuclear hyperfine spectroscopy of nitroxide radical and transition metal containing systems; (II) High-field ENDOR and EDNMR studies of the Oxygen Evolving Complex (OEC) in Photosystem II, which performs water oxidation in photosynthesis, i.e., the light-driven splitting of water into its elemental constituents, which is one of the most important chemical reactions on Earth.

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Influence of Ring-Expanded N-Heterocyclic Carbenes on the Structures of Half-Sandwich Ni(I) Complexes: An X-ray, Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) Study

Cited by 28 publications

References 128 publications

Stoichiometric and Catalytic Reactivity of Ni(6-Mes)(PPh₃)₂

Stoichiometric and Catalytic Reactivity of Ni(6-Mes)(PPh₃)₂

Paramagnetic species in catalysis research: a unified approach towards (the role of EPR in) heterogeneous, homogeneous and enzyme catalysis

Biomolecular EPR Meets NMR at High Magnetic Fields

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Influence of Ring-Expanded N-Heterocyclic Carbenes on the Structures of Half-Sandwich Ni(I) Complexes: An X-ray, Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) Study

Cited by 28 publications

References 128 publications

Stoichiometric and Catalytic Reactivity of Ni(6-Mes)(PPh3)2

Stoichiometric and Catalytic Reactivity of Ni(6-Mes)(PPh3)2

Paramagnetic species in catalysis research: a unified approach towards (the role of EPR in) heterogeneous, homogeneous and enzyme catalysis

Biomolecular EPR Meets NMR at High Magnetic Fields

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Product

Resources

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Stoichiometric and Catalytic Reactivity of Ni(6-Mes)(PPh₃)₂

Stoichiometric and Catalytic Reactivity of Ni(6-Mes)(PPh₃)₂