Experimentally Quantifying Small-Molecule Bond Activation Using Valence-to-Core X‑ray Emission Spectroscopy

This work establishes the ability of valence-to-core X-ray emission spectroscopy (XES) to serve as a direct probe of N₂ bond activation. A systematic series of iron-N₂ complexes has been experimentally investigated and the energy of a valence-to-core XES peak was correlated with N–N bond length and stretching frequency. Computations demonstrate that, in a simple one-electron picture, this peak arises from the N₂ 2s2s σ* orbital, which becomes less antibonding as the N–N bond is weakened and broken. Changes as small as 0.02 Å in the N–N bond length may be distinguished using this approach. The results thus establish valence-to-core XES as an effective probe of small molecule activation, which should have broad applicability in transition-metal mediated catalysis

Alkali Metal Control over N–N Cleavage in Iron Complexes

Though N₂ cleavage on K-promoted Fe surfaces is important in the large-scale Haber–Bosch process, there is still ambiguity about the number of Fe atoms involved during the N–N cleaving step and the interactions responsible for the promoting ability of K. This work explores a molecular Fe system for N₂ reduction, particularly focusing on the differences in the results obtained using different alkali metals as reductants (Na, K, Rb, Cs). The products of these reactions feature new types of Fe–N₂ and Fe-nitride cores. Surprisingly, adding more equivalents of reductant to the system gives a product in which the N–N bond is not cleaved, indicating that the reducing power is not the most important factor that determines the extent of N₂ activation. On the other hand, the results suggest that the size of the alkali metal cation can control the number of Fe atoms that can approach N₂, which in turn controls the ability to achieve N₂ cleavage. The accumulated results indicate that cleaving the triple N–N bond to nitrides is facilitated by simultaneous approach of least three low-valent Fe atoms to a single molecule of N₂

Hydrogenation of CO₂ at Room Temperature and Low Pressure with a Cobalt Tetraphosphine Catalyst

Large-scale implementation of carbon neutral energy sources such as solar and wind will require the development of energy storage mechanisms. The hydrogenation of CO₂ into formic acid or methanol could function as a means to store energy in a chemical bond. The catalyst reported here operates under low pressure, at room temperature, and in the presence of a base much milder (7 p<i>K</i>_a units lower) than the previously reported CO₂ hydrogenation catalyst, Co­(dmpe)₂H. The Co­(I) tetraphosphine complex, [Co­(L3)­(CH₃CN)]­BF₄, where L3 = 1,5-diphenyl-3,7-bis­(diphenylphosphino)­propyl-1,5-diaza-3,7-diphosphacyclooctane (0.31 mM), catalyzes CO₂ hydrogenation with an initial turnover frequency of 150(20) h^–1 at 25 °C, 1.7 atm of a 1:1 mixture of H₂ and CO₂, and 0.6 M 2-<i>tert</i>-butyl-1,1,3,3-tetramethylguanidine

Regioselective Aliphatic Carbon–Carbon Bond Cleavage by a Model System of Relevance to Iron-Containing Acireductone Dioxygenase

Mononuclear Fe­(II) complexes ([(6-Ph₂TPA)­Fe­(PhC­(O)­C­(R)­C­(O)­Ph)]­X (<b>3-X</b>: R = OH, X = ClO₄ or OTf; <b>4</b>: R = H, X = ClO₄)) supported by the 6-Ph₂TPA chelate ligand (6-Ph₂TPA = <i>N</i>,<i>N</i>-bis­((6-phenyl-2-pyridyl)­methyl)-<i>N</i>-(2-pyridylmethyl)­amine) and containing a β-diketonate ligand bound via a six-membered chelate ring have been synthesized. The complexes have all been characterized by ¹H NMR, UV–vis, and infrared spectroscopy and variably by elemental analysis, mass spectrometry, and X-ray crystallography. Treatment of dry CH₃CN solutions of <b>3-OTf</b> with O₂ leads to oxidative cleavage of the C(1)–C(2) and C(2)–C(3) bonds of the acireductone via a dioxygenase reaction, leading to formation of carbon monoxide and 2 equiv of benzoic acid as well as two other products not derived from dioxygenase reactivity: 2-oxo-2-phenylethylbenzoate and benzil. Treatment of CH₃CN/H₂O solutions of <b>3-X</b> with O₂ leads to the formation of an additional product, benzoylformic acid, indicative of the operation of a new reaction pathway in which only the C(1)–C(2) bond is cleaved. Mechanistic studies show that the change in regioselectivity is due to the hydration of a vicinal triketone intermediate in the presence of both an iron center and water. This is the first structural and functional model of relevance to iron-containing acireductone dioxygenase (Fe-ARD′), an enzyme in the methionine salvage pathway that catalyzes the regiospecific oxidation of 1,2-dihydroxy-3-oxo-(<i>S</i>)-methylthiopentene to form 2-oxo-4-methylthiobutyrate. Importantly, this model system is found to control the regioselectivity of aliphatic carbon–carbon bond cleavage by changes involving an intermediate in the reaction pathway, rather than by the binding mode of the substrate, as had been proposed in studies of acireductone enzymes

Diazoalkanes in Low-Coordinate Iron Chemistry: Bimetallic Diazoalkyl and Alkylidene Complexes of Iron(II)

The addition of (trimethylsilyl)­diazomethane and its conjugate base to iron β-diketiminate precursors gives novel dinuclear complexes in which the bridges are either diazomethane derivatives or an alkylidene. One product is an unusual bridging alkylidene complex containing two three-coordinate iron­(II) centers. On the other hand, syntheses using the deprotonated diazomethane give two bridging diazomethyl species with binding modes that have not been observed in iron complexes previously. In the presence of a coordinating tetrahydrofuran solvent, a diiron­(II) compound with μ-N bridges rearranges to a more stable isomer with μ-N,C bridges, a process that is accompanied by a 1,3-shift of a silyl group

Influence of supporting ligand microenvironment on the aqueous stability and visible light-induced CO-release reactivity of zinc flavonolato species

<div><p>The visible light-induced CO-release reactivity of the zinc flavonolato complex [(6-Ph₂TPA)Zn(3-Hfl)]ClO₄ (<b>1</b>) has been investigated in 1 : 1 H₂O : DMSO. Additionally, the effect of ligand secondary microenvironment on the aqueous stability and visible light-induced CO-release reactivity of zinc flavonolato species has been evaluated through the preparation, characterization, and examination of the photochemistry of compounds supported by chelate ligands with differing secondary appendages, [(TPA)Zn(3-Hfl)]ClO₄ (<b>3</b>; TPA = tris-2-(pyridylmethyl)amine) and [(bnpapa)Zn(3-Hfl)]ClO₄ (<b>4</b>; bnpapa = <i>N</i>,<i>N</i>-bis((6-neopentylamino-2-pyridyl)methyl)-<i>N</i>-((2-pyridyl)methyl)amine)). Compound <b>3</b> undergoes reaction in 1 : 1 H₂O : DMSO resulting in the release of the free neutral flavonol. Irradiation of acetonitrile solutions of <b>3</b> and <b>4</b> at 419 nm under aerobic conditions results in quantitative, photoinduced CO-release. However, the reaction quantum yields under these conditions are lower than that exhibited by <b>1</b>, with <b>4</b> exhibiting an especially low quantum yield. Overall, the results of this study indicate that positioning a zinc flavonolato moiety within a hydrophobic microenvironment is an important design strategy toward further developing such compounds as CO-release agents for use in biological systems.</p></div

Diazoalkanes in Low-Coordinate Iron Chemistry: Bimetallic Diazoalkyl and Alkylidene Complexes of Iron(II)

The addition of (trimethylsilyl)­diazomethane and its conjugate base to iron β-diketiminate precursors gives novel dinuclear complexes in which the bridges are either diazomethane derivatives or an alkylidene. One product is an unusual bridging alkylidene complex containing two three-coordinate iron­(II) centers. On the other hand, syntheses using the deprotonated diazomethane give two bridging diazomethyl species with binding modes that have not been observed in iron complexes previously. In the presence of a coordinating tetrahydrofuran solvent, a diiron­(II) compound with μ-N bridges rearranges to a more stable isomer with μ-N,C bridges, a process that is accompanied by a 1,3-shift of a silyl group

Diazoalkanes in Low-Coordinate Iron Chemistry: Bimetallic Diazoalkyl and Alkylidene Complexes of Iron(II)

The addition of (trimethylsilyl)­diazomethane and its conjugate base to iron β-diketiminate precursors gives novel dinuclear complexes in which the bridges are either diazomethane derivatives or an alkylidene. One product is an unusual bridging alkylidene complex containing two three-coordinate iron­(II) centers. On the other hand, syntheses using the deprotonated diazomethane give two bridging diazomethyl species with binding modes that have not been observed in iron complexes previously. In the presence of a coordinating tetrahydrofuran solvent, a diiron­(II) compound with μ-N bridges rearranges to a more stable isomer with μ-N,C bridges, a process that is accompanied by a 1,3-shift of a silyl group

Alkali Metal Variation and Twisting of the FeNNFe Core in Bridging Diiron Dinitrogen Complexes

Alkali metal cations can interact with Fe–N₂ complexes, potentially enhancing back-bonding or influencing the geometry of the iron atom. These influences are relevant to large-scale N₂ reduction by iron, such as in the FeMoco of nitrogenase and the alkali-promoted Haber–Bosch process. However, to our knowledge there have been no systematic studies of a large range of alkali metals regarding their influence on transition metal–dinitrogen complexes. In this work, we varied the alkali metal in [alkali cation]₂[LFeNNFeL] complexes (L = bulky β-diketiminate ligand) through the size range from Na⁺ to K⁺, Rb⁺, and Cs⁺. The FeNNFe cores have similar Fe–N and N–N distances and N–N stretching frequencies despite the drastic change in alkali metal cation size. The two diketiminates twist relative to one another, with larger dihedral angles accommodating the larger cations. In order to explain why the twisting has so little influence on the core, we performed density functional theory calculations on a simplified LFeNNFeL model, which show that the two metals surprisingly do not compete for back-bonding to the same π* orbital of N₂, even when the ligand planes are parallel. This diiron system can tolerate distortion of the ligand planes through compensating orbital energy changes, and thus, a range of ligand orientations can give very similar energies

Structural and Spectroscopic Characterization of Iron(II), Cobalt(II), and Nickel(II) <i>ortho</i>-Dihalophenolate Complexes: Insights into Metal–Halogen Secondary Bonding

Metal complexes incorporating the tris­(3,5-diphenylpyrazolyl)­borate ligand (Tp^Ph2) and <i>ortho</i>-dihalophenolates were synthesized and characterized in order to explore metal–halogen secondary bonding in biorelevant model complexes. The complexes Tp^Ph2ML were synthesized and structurally characterized, where M was Fe­(II), Co­(II), or Ni­(II) and L was either 2,6-dichloro- or 2,6-dibromophenolate. All six complexes exhibited metal–halogen secondary bonds in the solid state, with distances ranging from 2.56 Å for the Tp^Ph2Ni­(2,6-dichlorophenolate) complex to 2.88 Å for the Tp^Ph2Fe­(2,6-dibromophenolate) complex. Variable temperature NMR spectra of the Tp^Ph2Co­(2,6-dichlorophenolate) and Tp^Ph2Ni­(2,6-dichlorophenolate) complexes showed that rotation of the phenolate, which requires loss of the secondary bond, has an activation barrier of ∼30 and ∼37 kJ/mol, respectively. Density functional theory calculations support the presence of a barrier for disruption of the metal–halogen interaction during rotation of the phenolate. On the other hand, calculations using the spectroscopically calibrated angular overlap method suggest essentially no contribution of the halogen to the ligand-field splitting. Overall, these results provide the first quantitative measure of the strength of a metal–halogen secondary bond and demonstrate that it is a weak noncovalent interaction comparable in strength to a hydrogen bond. These results provide insight into the origin of the specificity of the enzyme 2,6-dichlorohydroquinone 1,2-dioxygenase (PcpA), which is specific for <i>ortho</i>-dihalohydroquinone substrates and phenol inhibitors