Curium (Cm), discovered in 1944, is one of the heaviest elements available in sufficient quantities for traditional synthetic chemistry. It's most stable in the +3 oxidation state, exhibiting a [Rn]5f¹ electron configuration. The half-filled 5f shell contributes to its stability and resistance to oxidation state changes, posing challenges in separating it from americium and lanthanides during nuclear fuel recycling. Despite the extensive study of Cm³⁺ compounds, single-crystal structural characterization of a complex with a Cm–C bond has been lacking. Examining this interaction could provide crucial insights into methods for engaging the frontier orbitals of curium and other actinides in forming partially covalent bonds. This control over electronic structure is essential for understanding and manipulating these complex elements. Quantum mechanical evaluations of americium(III) complexes have shown a mixture of covalent and ionic interactions, but such complexes remain rare due to the low availability of isotopes, specialized research facilities, and air/moisture sensitivity. Recent advancements have made the structural characterization of An–C bonds (An = Pu, Am, Cf) more accessible, paving the way for similar studies with curium. Lanthanide analogs are crucial for optimizing synthetic chemistry and providing benchmarks for comparisons with the 5f series, allowing for insights into oxidation states, electron configurations, and f-f transition splitting.

Previous research on organometallic actinide chemistry, particularly with lanthanides and early actinides, has led to the characterization of new oxidation states, electron configurations, and spectral features, contributing to our understanding of these elements. Utilizing lanthanide analogs with similar ionic radii and valence electron configurations serves as an excellent basis for comparison with mid-actinides. Studies on americium(III) cyclopentadienyl complexes have revealed a mixture of covalent and ionic bonding characteristics, but these are rare. The synthesis of An-C bonds has recently become accessible; however, it is still difficult. There is a lack of single-crystal structural characterization of a complex containing a Cm-C bond before this study. Understanding Cm-C bonding is necessary to gain insight into the methods for forming partially covalent bonds and controlling the electronic structure of curium.

The synthesis of (Cp′₃M)₂(µ-4,4′-bpy) (M = Sm, Gd, Cm) involved the reaction of 4,4′-bipyridine with two equivalents of the respective tris(trimethylsilylcyclopentadienyl) metal complex (Cp′₃M). Single crystals were obtained by cooling hot toluene solutions to room temperature. All reactions involving curium were conducted in a Category II radiological facility with stringent safety protocols due to the radioactive nature of ²⁴⁸Cm. Anhydrous lanthanide chlorides (SmCl₃ and GdCl₃), 4,4′-bipyridine, bromotrimethylsilane, hydrobromic acid, and solvents were used. Solvents were rigorously purified and dried before use. Schlenk line and glovebox techniques were employed to maintain an inert atmosphere. The synthesis of Cp′₃Cm involved multiple steps, including the preparation of CmBr₃(DME)n from Cm(OH)₃, followed by the reaction with KCp′. Photoluminescence spectroscopy was employed to study the emission properties of Cp′₃Cm and (Cp′₃Cm)₂(µ-4,4′-bipy). Absorption spectroscopy (solid-state and solution) was conducted to analyze the electronic transitions. Electronic structure calculations (spin-orbit CASSCF and MC-PDFT methods) were performed to study the electronic states of the complexes, using a simplified model of one Cp′₃M unit coordinated to pyridine. Natural Bond Orbital (NBO) analysis and Quantum Theory of Atoms in Molecules (QTAIM) calculations were used to investigate the nature of the metal-ligand bonds. Single crystal x-ray diffraction data was collected and analyzed to determine the molecular structures of all three complexes (1-Sm, 1-Gd, and 1-Cm).

The synthesis and single-crystal X-ray diffraction analysis revealed the isomorphous structures of (Cp′₃M)₂(µ-4,4′-bpy) for M = Sm, Gd, and Cm, exhibiting a dinuclear barbell shape. Metal-nitrogen (M-N) distances showed that 1-Gd has a slightly shorter distance than 1-Sm, as expected from the lanthanide contraction. However, 1-Cm's M-N distance was similar to 1-Gd but shorter than 1-Sm, deviating from expected trends based on ionic radii. Metal-centroid (M-Cent) distances were similar for 1-Sm and 1-Cm, but 1-Gd showed a slightly shorter distance. M-C bond distances were similar between 1-Sm and 1-Cm. Significant variation in M-C distances was observed due to ring shifting upon 4,4′-bipyridine coordination. Cp′₃Cm showed low energy red emission, significantly redshifted compared to typical curium complexes. Upon coordination with 4,4′-bipyridine, photoluminescence was completely quenched. Solid-state absorption spectroscopy revealed significant splitting of f-f transitions, particularly in 1-Sm. Charge transfer bands masked high-energy f-f transitions in 1-Gd and 1-Cm. Solution absorption spectra showed weak f-f transitions for 1-Cm. SO-PDFT calculations accurately reproduced the observed bathochromic shift in the photoluminescence of Cp′₃Cm, assigning it to transitions from the J=7/2 (⁶D) manifold to the ground state. The calculations suggested that the quenching of emission in 1-Cm is likely due to the resonance between C-H vibrational modes of 4,4′-bipyridine and the emissive state of the curium ion. NBO analysis indicated that 1-Sm showed the most significant f-orbital involvement in the metal-Cp′ bonds, contrasting with the metal-N bonds where 1-Cm showed greater f-orbital mixing. QTAIM analysis indicated that Sm-C bonds displayed greater energetic stabilization from covalent interactions compared to Cm-C and Gd-C bonds. Cm-N bonds showed some covalent character, while Sm-N and Gd-N bonds did not. The 4,4′-bipyridine coordination reduced electron density at the bond critical point in Cm-Cp′ bonds, leading to reduced covalency.

The findings highlight the unique bonding and electronic properties of organometallic curium(III) complexes. The unexpected low-energy emission and complete quenching upon 4,4′-bipyridine coordination challenge existing understanding of curium's photophysical behavior. The structural and bonding analyses reveal deviations from typical trends observed in lanthanide analogs, emphasizing the importance of considering relativistic and ligand-field effects in actinide chemistry. The observed differences in orbital mixing and covalency between Cm-Cp′ and Cm-N bonds suggest a complex interplay between the metal center and the ligand environment. The study reveals that the increased covalency in Cm-N bonds compared to its Am analog and the substantial variations in M-C distances upon ligand coordination contribute significantly to the unique spectroscopy of this element. These observations necessitate further investigation into the interaction between actinide 5f orbitals and soft-donor ligands under varying conditions.

This research presents the first single-crystal structural characterization of a curium–carbon bond and reveals unexpected spectroscopic behavior in a curium complex. The complete quenching of emission upon 4,4′-bipyridine coordination highlights the sensitivity of curium's electronic structure to ligand effects. The bonding analysis provides valuable insights into the nature of f-block metal-carbon and metal-nitrogen interactions, indicating significant differences between the behavior of curium and its lanthanide analogs. Future studies should explore other soft-donor ligands and investigate the influence of pressure and other external factors on the electronic properties of curium complexes. The unique findings in this work contribute substantially to the fundamental understanding of curium chemistry and organometallic actinide chemistry.

The study utilizes a simplified model for theoretical calculations due to computational constraints. The limited availability of curium isotopes and the specialized experimental conditions needed for handling radioactive materials restrict the scope of synthetic and experimental investigations. The small scale of the reactions also limits the quantity of the products available for further studies. Furthermore, the interpretation of bonding characteristics relies on theoretical models, which inherently include approximations and assumptions. While every effort has been made to minimize errors and account for systematic uncertainties, the results are subject to these limitations.