Isolation of a (Dinitrogen)Tricopper(I) Complex

Insights into small molecule activation by multinuclear first-row transition metal cyclophanates

The rational design of trimetallic transition metal clusters supported by a trinucleating cyclophane ligand, L³⁻, and the reactivities of these complexes with dinitrogen and carbon dioxide are discussed.

Reactivity of Hydride Bridges in High-Spin [3M-3(μ-H)] Clusters (M = FeII, CoII)

The designed [3M-3(μ-H)] clusters (M = Fe(II), Co(II)) Fe3H3L (1-H) and Co3H3L (2-H) [where L(3-) is a tris(β-diketiminate) cyclophane] were synthesized by treating the corresponding M3Br3L complexes with KBEt3H. From single-crystal X-ray analysis, the hydride ligands are sterically protected by the cyclophane ligand, and these complexes selectively react with CO2 over other unsaturated substrates (e.g., CS2, Me3SiCCH, C2H2, and CH3CN). The reaction of 1-H or 2-H with CO2 at room temperature yielded Fe3(OCHO)(H)2L (1-CO2) or Co3(OCHO)(H)2L (2-CO2), respectively, which evidence the differential reactivity of the hydride ligands within these complexes. The analogous reactions at elevated temperatures revealed a distinct difference in the reactivity pattern for 2-H as compared to 1-H; Fe3(OCHO)3L (1-3CO2) was generated from 1-H, while 2-H afforded only 2-CO2.

Dinitrogen binding and cleavage by multinuclear iron complexes

The iron–molybdenum cofactor of nitrogenase has unprecedented coordination chemistry, including a high-spin iron cluster called the iron-molybdenum cofactor (FeMoco). Thus, understanding the mechanism of nitrogenase challenges coordination chemists to understand the fundamental N₂ chemistry of high-spin iron sites. This Account summarizes a series of studies in which we have synthesized a number of new compounds with multiple iron atoms, characterized them using crystallography and spectroscopy, and studied their reactions in detail. These studies show that formally iron(I) and iron(0) complexes with three- and four-coordinate metal atoms have the ability to weaken and break the triple bond of N₂. These reactions occur at or below room temperature, indicating that they are kinetically facile. This in turn implies that iron sites in the FeMoco are chemically reasonable locations for N₂ binding and reduction.

The careful evaluation of these compounds and their reaction pathways has taught important lessons about what characteristics make iron more effective for N₂ activation. Cooperation of two iron atoms can lengthen and weaken the N–N bond, while three working together enables iron atoms to completely cleave the N–N bond to nitrides. Alkali metals (typically introduced into the reaction as part of the reducing agent) are thermodynamically useful because the alkali metal cations stabilize highly reduced complexes, pull electron density into the N₂ unit, and make reduced nitride products more stable. Alkali metals can also play a kinetic role, because cation−π interactions with the supporting ligands can hold iron atoms near enough to one another to facilitate the cooperation of multiple iron atoms. Many of these principles may also be relevant to the iron-catalyzed Haber–Bosch process, at which collections of iron atoms (often promoted by the addition of alkali metals) break the N–N bond of N₂.

The results of these studies teach more general lessons as well. They have demonstrated that N₂ can be a redox-active ligand, accepting spin and electron density in complexes of N₂^2–. They have shown the power of cooperation between multiple transition metals, and also between alkali metals and transition metals. Finally, alkali metal based cation−π interactions have the potential to be broadly useful for bringing metals close together with sufficient flexibility to allow multistep, multielectron reactions. At the same time, the positive charge on the alkali metal cation stabilizes charge buildup in intermediates.

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.