Cobalt−Dinitrogen Complexes with Weakened N−N Bonds

Ding, Keying; Pierpont, Aaron W.; Brennessel, William W.; Lukat-Rodgers, Gudrun S.; Rodgers, Kenton R.; Cundari, Thomas R.; Bill, Eckhard; Holland, Patrick L.

doi:10.1021/ja808783u

Cited by 120 publications

(145 citation statements)

References 23 publications

(23 reference statements)

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“…However, a broad band (including the 1,753-and 1,746-cm −1 band) was also observed with an absorption band at 1,687 cm −1 due to the residual 14 N 2 always being present in the DRIFTS cell. This isotopic band shift of ∼66 cm −1 from 1,753 cm −1 to 1,687 cm −1 is consistent with literature reports (19,(35)(36)(37). Our DRIFTS results demonstrate that N 2 can bind to the chalcogel under light illumination.…”

Section: Resultssupporting

confidence: 92%

“…The Comparing with the stretching frequencies of the free N 2 , azobenzene (PhN=NPh), and hydrazine (H 2 N-NH 2 ) (ν NN = 2,331, 1,442, and 1,111 cm −1 , respectively), the N≡N triple bond of N 2 weakens after binding to the chalcogel clusters, suggesting the potential for further functionalization, and ultimate N-N bond cleavage to form ammonia (38). The N-N stretching mode frequency is lower on the Fe 4 S 4 cluster than on Mo 2 Fe 6 S 8 (SPh) 3 cluster, implying that the photoactivated Fe 4 S 4 weakens the N≡N bond more significantly (35). This could partially explain why the FeS-SnS chalcogel outperforms the FeMoS-FeS-SnS chalcogel in terms of NH 3 production.…”

Section: Resultsmentioning

confidence: 98%

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Nitrogenase-mimic iron-containing chalcogels for photochemical reduction of dinitrogen to ammonia

Liu

Kelley

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

219

176

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A nitrogenase-inspired biomimetic chalcogel system comprising double-cubane [Mo 2 Fe 6 S 8 (SPh) 3 ] and single-cubane (Fe 4 S 4 ) biomimetic clusters demonstrates photocatalytic N 2 fixation and conversion to NH 3 in ambient temperature and pressure conditions. Replacing the Fe 4 S 4 clusters in this system with other inert ions such as Sb 3+ , Sn 4+ , Zn 2+ also gave chalcogels that were photocatalytically active. Finally, molybdenum-free chalcogels containing only Fe 4 S 4 clusters are also capable of accomplishing the N 2 fixation reaction with even higher efficiency than their Mo 2 Fe 6 S 8 (SPh) 3 -containing counterparts. Our results suggest that redox-active iron-sulfide-containing materials can activate the N 2 molecule upon visible light excitation, which can be reduced all of the way to NH 3 using protons and sacrificial electrons in aqueous solution. Evidently, whereas the Mo 2 Fe 6 S 8 (SPh) 3 is capable of N 2 fixation, Mo itself is not necessary to carry out this process. The initial binding of N 2 with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS). 15 N 2 isotope experiments confirm that the generated NH 3 derives from N 2 . Density functional theory (DFT) electronic structure calculations suggest that the N 2 binding is thermodynamically favorable only with the highly reduced active clusters. The results reported herein contribute to ongoing efforts of mimicking nitrogenase in fixing nitrogen and point to a promising path in developing catalysts for the reduction of N 2 under ambient conditions. nitrogenase mimics | chalcogel | N 2 fixation | ammonia synthesis | photocatalytic T he reduction of atmospheric nitrogen to ammonia is one of the most essential processes for sustaining life. Currently, roughly half of the fixed nitrogen is supplied biologically by nitrogenase, while nearly the other half is from the industrial Haber-Bosch process, which operates under high temperature (400-500°C) and high pressure (200-250 bar) in the presence of a metallic iron catalyst (1). Nitrogenase, a two-component protein system comprising a MoFe protein and an associated Fe protein, carries out this "fixation" in nature under ambient temperature and pressure (2-4). N 2 substrate binding and activation take place at the ironmolybdenum-sulfur cofactor (FeMoco), and in some cases, Mofree iron-sulfur cofactor FeFeco and iron-vanadium-sulfur cofactor FeVco cofactors. Electron transfer during this catalytic process is believed to proceed from a [4Fe:4S] cluster located in the Fe protein to another Fe/S cluster (the P cluster) buried in the MoFe protein and finally to the FeMoco (Fig. 1A) (2, 5, 6). Whereas the role of Mo in the reactivity of nitrogenase has been the subject of long debate, iron is now well recognized as the only transition metal essential to all nitrogenases, and recent biochemical and spectroscopic data point to iron as the site of N 2 binding in the FeMoco (7-9). Naturally, understanding and mimicking how the nitrogenas...

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Section: Resultssupporting

confidence: 92%

Section: Resultsmentioning

confidence: 98%

Nitrogenase-mimic iron-containing chalcogels for photochemical reduction of dinitrogen to ammonia

Liu

Kelley

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

219

176

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“…[123] Dinitrogen complexes of cobalt were synthesized by reduction of the iodide precursor with Mg 0 or Na/Hg amalgam in a N 2 atmosphere: the mononuclear [(PhBP CoL tBu ], (M = Na, K), analogous to the previously published iron complex 13. [126] In 2013 the group expanded their studies to cobaltmagnesium dinitrogen complexes. The reduction of a cobalt chloride precursor with activated Rieke magnesium (Mg*) in tetrahydrofuran in a N 2 atmosphere led to the formation of {L tBu Co(μ-N 2 )} 2 Mg(THF) 4 .…”

Section: Cobalt Rhodium and Iridiummentioning

confidence: 99%

Late Metal Scaffolds that Activate Both, Dinitrogen and Reduced Dinitrogen Species N_xH_y

Köthe

Limberg

2014

Zeitschrift anorg allge chemie

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Abstract. In the context of biological nitrogen fixation, the question whether molybdenum or iron is the site for substrate binding and reduction is still discussed controversially. Therefore, the development of relevant model compounds containing early or late transition metals is essential in understanding the nature and behavior of intermediates bound to the FeMo-cofactor. Ligated early transition metal atoms are known to strongly activate N 2 and mediate its cleavage, but the resulting complexes contain metal-nitrogen bonds, which often elude

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“…4 Binding of N 2 is relatively weak, as only 95% conversion is evident in cyclohexane-d 12 with 1 atm N 2 . The conversion to L tBu CoNNCoL tBu is >95% when the pressure of N 2 is increased to 8 atm.…”

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confidence: 99%

“…The addition of tetrahydrofuran (THF) to a solution of L tBu CoNNCoL tBu [L tBu = 2,2,6,6-tetramethyl-3,5-bis(2,4,6-triisopropylphenylimido)hept-4-yl] 4 in C 6 D 6 gives an immediate color change from brown to dark-green. This color change corresponds to the appearance of a set of signals in the 1 H NMR spectrum for a new species, L tBu Co(THF) (1), in quantitative yield.…”

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confidence: 99%

A Masked Two-Coordinate Cobalt(I) Complex That Activates C–F Bonds

et al. 2011

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We report the isolation, characterization, and reactions of the unsaturated complex L tBu Co (L tBu = bulky β-diketiminate ligand). The unusual slipped kN,η 6 -arene binding mode in L tBu Co interconverts rapidly and reversibly with the traditional k 2 N,N 0 ligation mode upon binding of Lewis bases, making it a "masked" two-coordinate complex. The mechanism of this isomerization is demonstrated using kinetic studies. L tBu Co is a stable yet reactive synthon for low-coordinate cobalt(I) complexes and is capable of cleaving the CÀF bond in fluorobenzene.H emilabile ligands, which contain a Lewis basic moiety that can reversibly dissociate from a metal, create transient coordinative unsaturation that can be used for bond activation and catalysis. 1 Notable examples include catalysts for olefin metathesis and cross-coupling reactions. 2 In this communication, we report that a bulky β-diketiminate ligand on cobalt(I) undergoes a novel isomerization that allows it to behave as a hemilabile ligand. Though β-diketiminates are represented in thousands of metal complexes, 3 this is the first example of this new binding mode. We show that the ligand isomerization is rapidly reversible and that it provides a "masked two-coordinate" cobalt(I) center for ligand binding and for activation of a strong CÀF bond.The addition of tetrahydrofuran (THF) to a solution of L tBu CoNNCoL tBu [L tBu = 2,2,6,6-tetramethyl-3,5-bis(2,4,6-triisopropylphenylimido)hept-4-yl] 4 in C 6 D 6 gives an immediate color change from brown to dark-green. This color change corresponds to the appearance of a set of signals in the 1 H NMR spectrum for a new species, L tBu Co(THF) (1), in quantitative yield. On a preparative scale, 1 can be produced from the reaction of L tBu CoCl with KC 8 in THF under Ar, which gives 1 as a dark-green crystalline solid in 64% yield. Complex 1 (Figure 1 left) has pseudo-C 2 symmetry in the solid state. The threecoordinate cobalt atom is planar, as the sum of the NÀCoÀN and NÀCoÀO bond angles is 359.7(2)°. The THF ligand is bent slightly toward one aryl arm of the β-diketiminate ligand, with NÀCoÀO angles of 135. Dissolution of 1 in C 6 D 6 under Ar gives a dark-orange solution whose 1 H NMR spectrum shows peaks from 1 plus a number of additional resonances. Evaporation of the volatile materials and redissolution in fresh C 6 D 6 leads to an 1 H NMR spectrum that contains very little 1. The displacement of THF with aromatic solvents can be used for the synthesis of the unknown species on a gram scale. Thus, 1 was dissolved in toluene, and two cycles of solvent removal and addition of more toluene removed all of the THF. Crystallization from a concentrated pentane solution at À45°C under Ar gave L tBu Co (2) in 72% yield as brown crystals.The molecular structure of 2 (Figure 1 right) shows a cobalt atom and a single β-diketiminate ligand with no additional donors. In contrast to the typical k 2 N,N 0 binding mode for L tBu , the β-diketiminate ligand in 2 is bound to cobalt in a kN,η 6 -arene mode in which the co...

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Cobalt−Dinitrogen Complexes with Weakened N−N Bonds

Cited by 120 publications

References 23 publications

Nitrogenase-mimic iron-containing chalcogels for photochemical reduction of dinitrogen to ammonia

Nitrogenase-mimic iron-containing chalcogels for photochemical reduction of dinitrogen to ammonia

Late Metal Scaffolds that Activate Both, Dinitrogen and Reduced Dinitrogen Species N_xH_y

A Masked Two-Coordinate Cobalt(I) Complex That Activates C–F Bonds

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Cobalt−Dinitrogen Complexes with Weakened N−N Bonds

Cited by 120 publications

References 23 publications

Nitrogenase-mimic iron-containing chalcogels for photochemical reduction of dinitrogen to ammonia

Nitrogenase-mimic iron-containing chalcogels for photochemical reduction of dinitrogen to ammonia

Late Metal Scaffolds that Activate Both, Dinitrogen and Reduced Dinitrogen Species NxHy

A Masked Two-Coordinate Cobalt(I) Complex That Activates C–F Bonds

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Late Metal Scaffolds that Activate Both, Dinitrogen and Reduced Dinitrogen Species N_xH_y