Redox-Active Ligands and Organic Radical Chemistry

Zhu, Di; Thapa, Indira; Korobkov, Ilia; Gambarotta, Sandro; Budzelaar, Peter H. M.

doi:10.1021/ic2002145

Cited by 124 publications

(75 citation statements)

References 73 publications

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“…The most likely explanation for the observed NMR behavior is thermal population of a low-lying paramagnetic excited state. This phenomenon was first identified in bis(imino)pyridine chemistry by Budzelaar and coworkers for the ( R PDI)CoR class of compounds 24,41 and later verified by our laboratory with the spin crossover behavior of alkyl-substituted bis(imino)pyridine cobalt halide complexes. 42 The temperature dependence of the chemical shifts of the ( iPr RPDI)FeN 2 compounds are more dramatic than the cobalt alkyls suggesting a more energetically accessible triplet state.…”

Section: Resultssupporting

confidence: 65%

Electronic Effects in 4-Substituted Bis(imino)pyridines and the Corresponding Reduced Iron Compounds

et al. 2012

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The electronic structures of the four- and five-coordinate aryl-substituted bis(imino)pyridine iron dinitrogen complexes, (iPrPDI)FeN2 and (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-(2,6-iPr2-C6H3-N=CMe)2C5H3N), have been investigated by a combination of spectroscopic techniques (NMR, Mössbauer, X-ray Absorption and X-ray Emission) and DFT calculations. Homologation of the imine methyl backbone to ethyl or isopropyl groups resulted in the preparation of the new bis(imino)pyridine iron dinitrogen complexes, (iPrRPDI)FeN2 (iPrRPDI = 2,6-(2,6-iPr2-C6H3-N=CR)2C5H3N; R = Et, iPr), that are exclusively four coordinate both in the solid state and in solution. The spectroscopic and computational data establish that the (iPrRPDI)FeN2 compounds are intermediate spin ferrous derivatives (SFe = 1) antiferromagnetically coupled to bis(imino)pyridine triplet diradical dianions (SPDI = 1). While this ground state description is identical to that previously reported for (iPrPDI)Fe(DMAP) (DMAP = 4-N,N-dimethylaminopyridine) and other four-coordinate iron compounds with principally σ-donating ligands, the d-orbital energetics determine the degree of coupling of the metal-chelate magnetic orbitals resulting in different NMR spectroscopic behavior. For (iPrRPDI)Fe(DMAP) and related compounds, this coupling is strong and results in temperature independent paramagnetism where a triplet excited state mixes with the singlet ground state via spin orbit coupling. In the (iPrRPDI)FeN2 family, one of the iron SOMOs is essentially dz2 in character resulting in poor overlap with the magnetic orbitals of the chelate, leading to thermal population of the triplet state and hence temperature dependent NMR behavior. The electronic structures of (iPrRPDI)FeN2 and (iPrPDI)Fe(DMAP) differ from (iPrPDI)Fe(N2)2, a highly covalent molecule with a redox non-innocent chelate that is best described as a resonance hybrid between iron(0) and iron(II) canonical forms as originally proposed in 2004.

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Section: Resultssupporting

confidence: 65%

Electronic Effects in 4-Substituted Bis(imino)pyridines and the Corresponding Reduced Iron Compounds

et al. 2012

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“…Budzelaar and coworkers 52,74 and subsequently our laboratory 60,70,75 have demonstrated the utility of 1 H NMR chemical shifts of the in-plane hydrogens of bis(imino)pyridine chelates to be diagnostic of radical chemistry and participation in the overall electronic structure. Deviations of the in-plane chelate hydrogens from their diamagnetic reference values have been attributed to either thermal population of a triplet excited state 52 or from temperature independent paramagnetism.…”

Section: Resultsmentioning

confidence: 99%

Catalytic Hydrogenation Activity and Electronic Structure Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes

et al. 2013

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The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, (iPrCNC)CoCH3, was evaluated for the catalytic hydrogenation of alkenes. At 22 °C and 4 atm of H2 pressure, (iPrCNC)CoCH3 is an effective pre-catalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-methylstilbene, 1-methyl-1-cyclohexene and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, (iPrCNC)CoH was accomplished by hydrogenation of (iPrCNC)CoCH3. Over the course of 3 hours at 22 °C, migration of the metal-hydride to the 4-position of the pyridine ring yielded (4-H2-iPrCNC)CoN2. Similar alkyl migration was observed upon treatment of (iPrCNC)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redoxactive, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2-ylidene)pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity and suggest a wide family of pyridine-based pincers may also be redox active.

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“…Dinitrogen adducts of Ni are uncommon; (N 2 )NiInL is one of only seven structurally characterized examples of an end-on Ni-N 2 adduct to our knowledge [39,47,48,60,61]. Though N 2 binding in (N 2 )NiInL is irreversible under vacuum, the high ν(N 2 ) of 2144 cm −1 and the short N−N bond length suggest that π-back-bonding from Ni to N 2 is minimal [40].…”

Section: 3mentioning

confidence: 99%

Leveraging molecular metal–support interactions for H2 and N2 activation

Cammarota

Clouston

2017

Coordination Chemistry Reviews

164

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Many challenging chemical reactions require precious metal catalysts to proceed. Bio-inspired catalysts featuring multiple earth-abundant metals are an attractive alternative, as they offer boundless possibilities for facilitating processes that the constituent metals cannot mediate on their own. Our work utilizes a supporting metal as an electronic lever for tuning a base metal (Co, Ni) active site via a metal-metal bond. This approach has allowed for the development of metal-support catalysts for reductive N 2 silylation and olefin hydrogenation. The bimetallic catalysts display markedly enhanced activity compared to the analogous single metal centers. In this review, we investigate the role of the supporting metal in substrate binding, activation, and catalysis, to inform future efforts in the optimization and development of molecular metalsupport catalysts. Stoichiometric N−Si Bond Formation by an Iron Bimetallic Catalytic N−Si Bond Formation by Cobalt Bimetallics Mechanistic Investigation of N 2 Silylation Conclusions Highlights• The catalyst activity of a single metal was enhanced by a supporting metal.• Supporting metals were varied across the first-row period and down Group 13.• Metal-support bonding affects the metal's electronic state and ligand lability.• Group 13 supports induce H 2 binding at Ni and act cooperatively to cleave H 2 .• Transition metal supports promote N 2 binding and catalytic N 2 silylation at Co.

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Redox-Active Ligands and Organic Radical Chemistry

Cited by 124 publications

References 73 publications

Electronic Effects in 4-Substituted Bis(imino)pyridines and the Corresponding Reduced Iron Compounds

Electronic Effects in 4-Substituted Bis(imino)pyridines and the Corresponding Reduced Iron Compounds

Catalytic Hydrogenation Activity and Electronic Structure Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes

Leveraging molecular metal–support interactions for H2 and N2 activation

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