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A series of mononuclear nickel­(II) thiolate complexes (Et₄N)­Ni­(X-pyS)₃ (Et₄N = tetraethylammonium; X = 5-H (<b>1a</b>), 5-Cl (<b>1b</b>), 5-CF₃ (<b>1c</b>), 6-CH₃ (<b>1d</b>); pyS = pyridine-2-thiolate), Ni­(pySH)₄(NO₃)₂ (<b>2</b>), (Et₄N)­Ni­(4,6-Y₂-pymS)₃ (Y = H (<b>3a</b>), CH₃ (<b>3b</b>); pymS = pyrimidine-2-thiolate), and Ni­(4,4′-Z-2,2′-bpy)­(pyS)₂ (Z = H (<b>4a</b>), CH₃ (<b>4b</b>), OCH₃ (<b>4c</b>); bpy = bipyridine) have been synthesized in high yield and characterized. X-ray diffraction studies show that <b>2</b> is square planar, while the other complexes possess tris-chelated distorted-octahedral geometries. All of the complexes are active catalysts for both the photocatalytic and electrocatalytic production of hydrogen in 1/1 EtOH/H₂O. When coupled with fluorescein (Fl) as the photosensitizer (PS) and triethylamine (TEA) as the sacrificial electron donor, these complexes exhibit activity for light-driven hydrogen generation that correlates with ligand electron donor ability. Complex <b>4c</b> achieves over 7300 turnovers of H₂ in 30 h, which is among the highest reported for a molecular noble metal-free system. The initial photochemical step is reductive quenching of Fl* by TEA because of the latter’s greater concentration. When system concentrations are modified so that oxidative quenching of Fl* by catalyst becomes more dominant, system durability increases, with a system lifetime of over 60 h. System variations and cyclic voltammetry experiments are consistent with a CECE mechanism that is common to electrocatalytic and photocatalytic hydrogen production. This mechanism involves initial protonation of the catalyst followed by reduction and then additional protonation and reduction steps to give a key Ni–H^–/N–H⁺ intermediate that forms the H–H bond in the turnover-limiting step of the catalytic cycle. A key to the activity of these catalysts is the reversible dechelation and protonation of the pyridine N atoms, which enable an internal heterocoupling of a metal hydride and an N-bound proton to produce H₂

Ligand Effects on Hydrogen Atom Transfer from Hydrocarbons to Three-Coordinate Iron Imides

A new β-diketiminate ligand with 2,4,6-tri­(phenyl)­phenyl <i>N-</i>substituents provides protective bulk around the metal without exposing any weak C–H bonds. This ligand improves the stability of reactive iron­(III) imido complexes with FeNAd and FeNMes functional groups (Ad = 1-adamantyl; Mes = mesityl). The new ligand gives iron­(III) imido complexes that are significantly more reactive toward 1,4-cyclohexadiene than the previously reported 2,6-diisopropylphenyl diketiminate variants. Analysis of X-ray crystal structures implicates FeN–C bending, a longer FeN bond, and greater access to the metal as potential reasons for the increase in C–H bond activation rates

Ligand Effects on Hydrogen Atom Transfer from Hydrocarbons to Three-Coordinate Iron Imides

A new β-diketiminate ligand with 2,4,6-tri­(phenyl)­phenyl <i>N-</i>substituents provides protective bulk around the metal without exposing any weak C–H bonds. This ligand improves the stability of reactive iron­(III) imido complexes with FeNAd and FeNMes functional groups (Ad = 1-adamantyl; Mes = mesityl). The new ligand gives iron­(III) imido complexes that are significantly more reactive toward 1,4-cyclohexadiene than the previously reported 2,6-diisopropylphenyl diketiminate variants. Analysis of X-ray crystal structures implicates FeN–C bending, a longer FeN bond, and greater access to the metal as potential reasons for the increase in C–H bond activation rates

Spin Crossover during β‑Hydride Elimination in High-Spin Iron(II)– and Cobalt(II)–Alkyl Complexes

It is surprising that rapid β-hydride elimination (βHE) can take place in some high-spin iron­(II)– and cobalt­(II)–alkyl complexes despite the absence of empty d orbitals. In this study, density functional theory (DFT) is used to analyze the pathways for βHE in alkyl complexes of iron­(II) and cobalt­(II) supported by β-diketiminate that undergo βHE, and in tris­(pyrazolyl)­borate (Tp) iron­(II)–alkyl complexes that are resistant to βHE. Each reaction pathway includes spin crossover to a transition state with a lower spin and a vacant d orbital; importantly, only the spin crossover accelerated pathway matches experimental rates. The lower spin transition state has a square-planar geometry that is ideal for depopulating one in-plane d orbital that can accept the electrons from the β-hydrogen. The energy of the square-planar transition state is increased by steric bulk around the metal center and by increases in the coordination number at iron, explaining the resistance to βHE in TpFeR. Migratory insertion, the microscopic reverse of βHE, is also accelerated by spin crossover, as shown through an analogous analysis of the insertion of N₂H₂ into the Fe–H bond of a β-diketiminate supported iron­(II)–hydride complex

A Multi-iron System Capable of Rapid N₂ Formation and N₂ Cleavage

The six-electron oxidation of two nitrides to N₂ is a key step of ammonia synthesis and decomposition reactions on surfaces. In molecular complexes, nitride coupling has been observed with terminal nitrides, but not with bridging nitride complexes that more closely resemble catalytically important surface species. Further, nitride coupling has not been reported in systems where the nitrides are derived from N₂. Here, we show that a molecular diiron­(II) diiron­(III) bis­(nitride) complex reacts with Lewis bases, leading to the rapid six-electron oxidation of two bridging nitrides to form N₂. Surprisingly, these mild reagents generate high yields of iron­(I) products from the iron­(II/III) starting material. This is the first molecular system that both breaks and forms the triple bond of N₂ at room temperature. These results highlight the ability of multi-iron species to decrease the energy barriers associated with the activation of strong bonds

Cooperativity Between Low-Valent Iron and Potassium Promoters in Dinitrogen Fixation

A density functional theory (DFT) study was performed to understand the role of cooperativity between iron-β-diketiminate fragments and potassium promoters in N₂ activation. Sequential addition of iron fragments to N₂ reveals that a minimum of three Fe centers interact with N₂ in order to break the triple bond. The potassium promoter stabilizes the N^3– ligand formed upon N₂ scission, thus making the activated iron nitride complex more energetically accessible. Reduction of the complex and stabilization of N^3– by K⁺ have similar impact on the energetics in the gas phase. However, upon inclusion of continuum THF solvent effects, coordination of K⁺ has a reduced influence upon the overall energetics of dinitrogen fixation; thus, reduction of the trimetallic Fe complex becomes more impactful than coordination of K⁺ vis-à-vis N₂ activation upon the inclusion of solvent effects

Quantitation of the THF Content in Fe[N(SiMe₃)₂]₂·<i>x</i>THF

The absence of residual solvent in metal precursors can be of key importance for the successful preparation of metal complexes or materials. Herein, we describe methods for the quantitation of residual coordinated tetrahydrofuran (THF) that binds to Fe­[N­(SiMe₃)₂]₂, a commonly used iron synthon, when prepared according to common literature procedures. A simple method for quantitation of the amount of residual coordinated THF using ¹H NMR spectroscopy is highlighted. Finally, a detailed synthetic procedure is described for the synthesis of THF-free Fe­[N­(SiMe₃)₂]₂

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

Low-Coordinate Cobalt Fluoride Complexes: Synthesis, Reactions, and Production from C–F Activation Reactions

A cobalt­(II) fluoride complex, [L^<i>t</i>BuCo­(μ-F)]₂ [L^<i>t</i>Bu = 2,2,6,6-tetramethyl-3,5-bis­(2,6-diisopropylphenylimido)­hept-4-yl], was synthesized from L^<i>t</i>BuCo using Me₃SnF via homolytic cleavage of the Sn–F bond. L^<i>t</i>BuCo also performed the overall binuclear oxidative addition of fluorinated arenes to give [L^<i>t</i>BuCo­(μ-F)]₂ and a cobalt­(II) aryl complex of the corresponding fluorobenzene substrate in a 1:2 molar ratio. The C–F activation reaction has a first-order rate dependence on both cobalt and fluorobenzene concentrations. The rate is increased by <i>meta</i>-fluoride substituents, and slowed by <i>ortho</i>-fluoride substituents, suggesting electronic and steric influences on the transition state, respectively. The data are most consistent with a mechanism beginning with rate-limiting oxidative addition of the aryl fluoride to cobalt­(I), followed by rapid reduction of the cobalt­(III) aryl fluoride intermediate by a second molecule of L^<i>t</i>BuCo. [L^<i>t</i>BuCo­(μ-F)]₂ also reacts with Et₃SiH to give the hydride complex [L^<i>t</i>BuCo­(μ-H)]₂. This hydride complex has low reactivity toward alkenes and N₂, in contrast to an earlier report