Skye Fortier

The isolobal relationship between imidazolin-2-iminato and cyclopentadienyl ligands makes the iron bis(imidazolin-2-iminato) complex (Im ^R N) ₂ Fe an attractive target as a potential ferrocene analog, yet reported efforts toward its isolation have been unsuccessful. Herein, we report the synthesis of (Im ^Dipp N) ₂ Fe ( 1 ) by protonolysis of Fe ₂ Mes ₄ with Im ^Dipp NH, affording a linear, two-coordinate, high-spin Fe(II) complex (S = 2). Its coordinative unsaturation permits binding of THF, an aryl isocyanide, and TEMPO, affording complexes with varied geometries. Unlike ferrocene, the electrochemical properties of 1 depend strongly on its ancillary ligands. Oxidation with AgPF ₆ affords a Ag-bound Fe(II) intermediate 1-Ag that undergoes photochemical conversion to the Fe(III) complex [(Im ^Dipp N) ₂ Fe(Solv) ₂ ][PF ₆ ] ( 2 ). These results establish 1 as a distinct low-coordinate iron complex whose coordination and redox chemistry extend beyond its proposed ferrocene analogy.

One-electron reduction of the N,N′-tethered U(III) complex κ2:η6-LArUI(THF) (A), featuring a ligand imposed η6-arene-metal interaction, generates the reduced species [K(2.2.2-cryptand)][(κ2:η6-LAr)UI] (1) in good yield. Complex 1 exhibits severe distortion of the anchored arene, indicating ligand-based rather than metal-centered reduction. In line with this, close inspection of the structural parameters of 1 reveals bond localization consistent with two-electron reduction of the arene concomitant with reduction-induced oxidation of the metal center, giving a formally U(IV) complex. Nonetheless, 1 is a potent U(II) synthon capable of four-electron transfer chemistry, activating the small molecules PhN3, PhNO, and O2 to yield the multiply bonded, hexavalent products trans-(κ2:η6-LAr)U(NPh)2 (3) and trans-(κ2-LAr)U(E)(E’)(THF) (E = NPh, E′ = O (4); and E = E′ = O (5)). Formation of mononuclear uranyl complex 5 is especially significant, as it constitutes the first reported example of O2 activation via four-electron transfer from a single actinide center.

Metal-arene sandwich complexes are foundational to organometallic chemistry, yet analogous compounds of the f-elements remain rare, especially for the actinides. This scarcity has limited efforts to understand how this iconic metal-arene bonding motif changes when extended to actinide elements, where metal-carbon bonding, charge transfer, and ligand redox behaviour may depart from established d-block patterns. Here we report the first homoleptic tris(arenide) arenocenes of the felements, together with a related heteroleptic uranium arenocene, establishing a new class of actinide sandwich compounds supported by formally dianionic arene ligands. Crystallographic, spectroscopic, magnetic, X-ray absorption, and multireference computational analyses support their formulation as tetravalent uranium centres engaged in appreciable arene-to-uranium π-bonding interactions across varying coordination modes. The overall metal contribution to bonding decreases as the arene πsystem expands, while 5f-orbital participation becomes more pronounced in complexes bearing larger arene ligands. Direct comparison with the structurally related d-block complex [Hf(η4-C10H8)3]2− reveals greater orbital participation in the uranium system, providing comparative measure for 5f engagement in metal-arene interactions and establishing a new platform for investigating actinide valence orbital contributions to bonding.

Supporting Information Placeholder One-electron reduction of the N,N’-tethered U(III) complex κ2:η6-LArUI(THF) (A), featuring a ligand imposed η6arene-metal interaction, generates the reduced species [K(2.2.2-cryptand)][(κ2:η6-LAr)UI] (1) in good yield. Complex 1 exhibits severe structural distortion of the anchored arene moiety, indicating ligand-based rather than metal-centered reduction. In line with this, close inspection of the structural parameters of 1 reveals bond localization consistent with two-electron reduction of the arene concomitant with reduction-induced oxidation of the metal center, giving a formally U(IV) complex. Nonetheless, 1 is a potent U(II) synthon capable of four-electron transfer chemistry, activating the small molecules PhN3, PhNO, and O2 to yield the multiply bonded, hexavalent products trans-(κ2:η6-LAr)U(NPh)2 (3) and trans-(κ2-LAr)U(E)(E’)(THF) (E = NPh, E’ = O (4); and E = E’ = O (5)). The formation of mononuclear uranyl complex 5 is especially significant, as it constitutes the first reported example of O2 activation via four-electron transfer from a single f-element center.

A simple method is presented for encapsulating highly air-sensitive samples in paraffin wax, providing an airtight seal that preserves sample integrity during XAS analysis.

We report the use of the bulky phosphaylide reagent H₂C=P(Ar^tt)₃ (Ar^tt = 3,5-^tBu₂C₆H₃) to kinetically trap the alkyl-carbene molecule Cp*₂U(Me)[=C(H)P(Ar^tt)₃] (Cp* = C₅Me₅) (1-mono). However, despite the steric bulk of the Ar^tt groups, a significant amount of highly encumbered bis(carbene) species Cp*₂U[=C(H)P(Ar^tt)₃]₂ (2-bis) is produced. To circumvent this issue, an alternative route was devised in which the iodide compound Cp*₂U(I)[=C(H)P(Ar^tt)₃] (4-I) was synthesized. The subsequent addition of MeLi afforded 1 in good yields. The identity of the methylating reagent plays a key role in the success of the reaction, as addition of the Grignard reagent MeMgBr generates a mixture of products including Cp*₂UMe₂ as a major species. Finally, we demonstrate that access to other mixed carbene complexes such as Cp*₂U(O^tBu)[=C(H)P(Ar^tt)₃] (6-O^tBu) is possible.

We report the use of the bulky phosphaylide reagent H2C=P(tBuAr)3 (tBuAr = 3,5-tBu2C6H3) to kinetically trap the alkyl–carbene molecule Cp*2U(Me)[=C(H)P(tBuAr)3] (1-mono). However, despite the steric bulk of the tBuAr groups, a significant amount of the highly encumbered bis(carbene) species Cp*2U[=C(H)P(tBuAr)3]2 (2-bis) is produced. To circumvent this issue, an alternative route was devised in which the iodide compound Cp*2U(I)[=C(H)P(tBuAr)3] (4-I) was synthesized. The subsequent addition of MeLi affords 1 in good yields. The identity of the methylating reagent plays a key role in the success of the reaction as addition of the Grignard reagent MeMgBr generates a mixture of products including Cp*2UMe2 as a major species. Finally, we demon-strate that access to other mixed carbene complexes such as Cp*2U(OtBu)[=C(H)P(tBuAr)3] (6-OtBu) is possible.

To facilitate X-ray absorption spectroscopy (XAS) measurements of air-sensitive samples, we present a simple method in which materials are encased in common paraffin wax to protect them from air and moisture. We demonstrate the efficacy of this approach using a highly reducing, air- and moisture-sensitive uranium(III) complex, the tris(amide) U[N(SiMe3)2]3 (1). When finely dispersed in a boron nitride matrix and subsequently encased in inert paraffin wax, samples of 1 remain stable with no visible or spectroscopic degradation after several days under ambient conditions. The viability of this method for XAS measurements was further evaluated across a series of uranium compounds, ranging from uranyl species to highly air- and moisture-sensitive molecular complexes, at the uranium L₃-edge. Edge energy determinations were highly reproducible (±0.1 eV between replicates) and, where available, showed excellent agreement with literature values. This low-cost, effective, and versatile method offers a viable solution for XAS studies of air-sensitive compounds and materials.