Altering the spectroscopy, electronic structure, and bonding of organometallic curium(III) upon coordination of 4,4'-bipyridine

Curium (Z = 96) is a heavy actinide most stable in the +3 oxidation state with a [Rn]5f7 electron configuration conferring enhanced stability and typically reduced f-electron participation in bonding. Separating Cm from Am and lanthanides is challenging due to similar ionic radii and trivalent chemistry, yet essential because Cm significantly contributes to nuclear waste radiotoxicity. Despite many Cm(III) compounds, no single-crystal structural characterization of a complex containing a Cm–C bond had been reported. Understanding Cm–C interactions could reveal how to engage actinide frontier orbitals and modulate covalency with soft donors. Prior work on related Am(III) Cp complexes suggested partially covalent metal–Cp interactions but largely ionic Am–N bonding to 4,4′-bipyridine. The present study aims to synthesize and structurally characterize the dinuclear organometallic complex (Cp′3Cm)2(µ-4,4′-bpy) (1-Cm), compare its structure and spectroscopy to Sm and Gd analogs, and use spectroscopy and multireference electronic structure theory with bonding analyses (NBO/NLMO and QTAIM) to elucidate Cm–C and Cm–N bonding and the impact of 4,4′-bpy coordination on emission.

The paper builds on extensive f-block organometallic literature demonstrating diverse oxidation states and spectroscopic ff-transition splitting in lanthanides and early actinides. Historically, cyclopentadienyl ligands enabled discovery of low-valent Ln/An complexes and insights into covalency. However, mid-actinide organometallics remain sparse due to isotope scarcity, facility requirements, and high air/moisture sensitivity, limiting structural studies of An–C bonds. Prior reports identified formal +2 oxidation states for Pu and Np in Cp′ systems, and related Am and Cf organometallics including an Am Cp complex exhibiting ionic Am–N interactions to 4,4′-bpy. Spectroscopy of Cm(III) is well-documented, typically showing red–orange emission (590–620 nm) in crystals and solution, with earlier observations of red luminescence in CmCp3. Yet a structurally authenticated Cm–C bond in a single crystal had not been captured. Comparisons with lanthanide congeners (Sm, Gd) and earlier U/Am analogs of 4,4′-bpy-bridged Cp′3M species provide benchmarks for evaluating bonding trends and covalency across f-elements.

Safety and handling: All work with 248Cm (t1/2 = 348,000 y; α-emitter with spontaneous fission and neutron emission) was conducted under Category II radiological controls in HEPA-equipped fume hoods and gloveboxes. Air- and moisture-free techniques employed Schlenk and glovebox methods under Ar. Solvents were rigorously dried and purified. Synthesis: Ln precursors (KCp′, Cp′3Sm, Cp′3Gd) were prepared per literature. For 1-Sm: 4,4′-bpy was added to Cp′3Sm in toluene, forming a precipitate; after workup, crystallization from hot toluene by controlled cooling yielded yellow crystals. For 1-Gd: Cp′3Gd was prepared by salt metathesis of GdCl3 with KCp′ in toluene at 70 °C, followed by addition of 4,4′-bpy and analogous crystallization to afford orange-yellow crystals. For 1-Cm: CmBr3·(DME)n was generated from Cm3+ aqueous stock via hydroxide precipitation, conversion to bromide, and drying with DME/TMS-Br. Treatment with KCp′ in toluene at 70 °C afforded putative Cp′3Cm (isolated as a tan oil; microcrystals used for emission). Addition of 4,4′-bpy in toluene followed by controlled cooling produced gold crystals of (Cp′3Cm)2(µ-4,4′-bpy). Crystallography: Single crystals of 1-Sm, 1-Gd, and 1-Cm were obtained from toluene by slow cooling from ~120 °C to room temperature and analyzed by single-crystal X-ray diffraction. All crystallized in P1̄, isomorphous, showing dinuclear units bridged by 4,4′-bpy with pseudo-tetrahedral geometry at metal centers (three Cp′ centroids + Nbpy). Spectroscopy: Photoluminescence of putative Cp′3Cm microcrystals and 1-Cm was measured upon 420 nm excitation at −180 °C (common excitation wavelengths 365 and 420 nm tested). Solid-state absorption spectra for 1-Sm, 1-Gd, 1-Cm recorded at room temperature from 350–1700 nm; 1-Cm also at −180 °C. Solution UV–vis–NIR spectra for 1-Sm, 1-Gd, 1-Cm, and putative Cp′3Cm were collected from 250–1700 nm (toluene). Computational methods: Spin–orbit CASSCF combined with MC-PDFT (SO-PDFT) was used to compute electronic states of model systems comprising Cp′3M coordinated to pyridine (M = Sm, Gd, Cm) to reduce computational cost. Transitions were assigned by vertical excitations from the SO ground state with J labels and predominant 2S+1L terms. Natural Bond Orbital (NBO) analysis with Natural Localized Molecular Orbitals (NLMOs) was used to quantify orbital mixing and metal hybrid contributions for M–Cp′ and M–Nbpy bonds. Quantum Theory of Atoms in Molecules (QTAIM) analyses provided topological metrics of the electron density at bond critical points (ρ(r), ∇2ρ(r), V(r), G(r), H(r), |V|/G, H/ρ, ellipticity ε, delocalization index δ(r)), and Wiberg bond indices (WBI). Ground-state-specific CASSCF densities were used for QTAIM. Optimized geometries for 1-Sm, 1-Gd, 1-Cm, and Cp′3Cm were provided in Supplementary Data.

- Synthesis and structure: The first single-crystal structural characterization of a Cm–C bonded organometallic complex, (Cp′3Cm)2(µ-4,4′-bpy) (1-Cm), was achieved, alongside isomorphous Sm and Gd analogs. All adopt dinuclear barbell structures with 4,4′-bpy bridging two metal centers. - Metal–N distances (Å): 1-Sm 2.626(3), 1-Gd 2.592(3), 1-Cm 2.5962(16). Cm–N is within error of Gd–N but shorter than Sm–N; across actinides, 1-Cm shows shorter M–N than U/Am analogs, indicating a non-linear trend and suggesting increased covalency for Cm–N relative to Am. - Metal–centroid distances (Å, mean ± std): 1-Sm 2.516 ± 0.017, 1-Gd 2.498 ± 0.019, 1-Cm 2.517 ± 0.016. Coordination by 4,4′-bpy increases coordination number and distorts Cp′ ring binding compared to Cp′3M precursors; ranges span ~0.04 Å. - Metal–C distances (Å, mean ± std): 1-Sm 2.788 ± 0.041, 1-Gd 2.771 ± 0.048, 1-Cm 2.789 ± 0.047, with broad intraring ranges (e.g., 1-Gd 2.714–2.889; 1-Cm 2.728–2.903), evidencing ring shifting upon 4,4′-bpy coordination. - Photoluminescence: Putative Cp′3Cm shows a broad emission band centered ~670 nm (≈14,925 cm⁻1) with FWHM ~52 nm (≈1225 cm⁻1), substantially red-shifted from typical Cm3+ emission (590–620 nm). 1-Cm emission is completely quenched upon 4,4′-bpy coordination at common excitation wavelengths (365, 420 nm), attributed to nonradiative deactivation via resonance between Cm emissive states (~15,000–16,000 cm⁻1) and the fifth harmonic of 4,4′-bpy C–H vibrations (~5×3000 cm⁻1). - Absorption spectroscopy: Charge-transfer bands obscure high-energy ff transitions in all three. Onset of CT bands: 1-Sm ~575 nm (17,391 cm⁻1), 1-Gd ~600 nm (16,667 cm⁻1), 1-Cm ~615 nm (16,260 cm⁻1). For 1-Cm, weak ff transitions appear at 587 nm (17,036 cm⁻1, J=7/2), 597 nm (16,750 cm⁻1, J=5/2), and 630–650 nm (15,873–15,385 cm⁻1, J=7/2), red-shifted by ~30 nm versus literature. In solution, a 1-Cm transition at 411 nm (24,331 cm⁻1) is bathochromically shifted by ~15 nm (~919 cm⁻1) from prior reports. - Electronic structure (SO-PDFT): 1-Sm SO-GS J=5/2 (4H); excited manifolds in 900–1700 nm region match experiment (errors 105–760 cm⁻1). 1-Gd SO-GS J=7/2 (8S), first excited manifold ~29,344 cm⁻1 (341 nm), bathochromically shifted vs literature (~32,467–31,645 cm⁻1). 1-Cm SO-GS J=7/2 (8S) with splitting ~385 cm⁻1; emissive manifolds J=7/2 (6D) ~15,216 cm⁻1 (657 nm) and J=5/2 (6D) ~16,650 cm⁻1 (601 nm). Calculations reproduce Cp′3Cm emission band 615–700 nm with peaks near 645, 660, 670 nm, consistent with a nephelauxetic effect from Cp′ ligation. - Bonding analyses: NLMO/NBO indicate for M–Cp′ bonds, 1-Sm exhibits the greatest f-orbital mixing/metal hybrid contributions, exceeding Cm and Gd; for M–Nbpy bonds, 1-Cm shows stronger orbital mixing and greater f participation than Sm/Gd. Coordination of 4,4′-bpy has little effect on Cm–Cp′ orbital mixing versus Cp′3Cm. - QTAIM/topology: Electron density at BCP for M–Cp′ is similar across congeners, but energetic analyses (H(r)/ρ(r)) indicate greater covalent stabilization for Sm–C relative to Cm and Gd by ~4 and ~17 kJ mol⁻1, respectively. For M–Nbpy, Cm–N shows higher ρ(r), δ(r), and WBI and retains low covalent character, whereas Sm–N and Gd–N show positive H(r) and negligible covalency. Coordination of 4,4′-bpy reduces BCP electron density and total energy density for Cm–Cp′ compared to Cp′3Cm (H(r) −21.9 vs −51.0 kJ mol⁻1 Å⁻3), indicating reduced covalency despite similar orbital mixing. - Comparative trends: 1-Cm shows M–N similarities to 1-Gd but M–C similarities to 1-Sm, deviating from simple ionic radius expectations and highlighting distinct An–C versus Ln–C bonding behaviors.

The work addresses the long-standing gap in structurally authenticated Cm–C bonding by synthesizing and crystallographically characterizing (Cp′3Cm)2(µ-4,4′-bpy). Structural metrics reveal that Cm–N distances are shorter than in related U and Am systems, consistent with enhanced covalency for Cm–N compared to Am, while Cm–C metrics align more closely with Sm analogs despite Cm’s actinide character. Spectroscopically, Cp′3Cm exhibits an unusually red-shifted emission band in the vis–NIR that is fully quenched upon 4,4′-bpy coordination; calculations show the emissive manifolds are essentially unchanged between Cp′3Cm and 1-Cm, supporting a ligand-centered vibrational quenching mechanism via high-order C–H overtones in 4,4′-bpy. Multireference calculations combined with NLMO and QTAIM analyses demonstrate that simple orbital mixing metrics are insufficient to quantify covalency: Sm–Cp′ bonds show the highest mixing yet QTAIM energetics differentiate the covalent stabilization across Sm, Cm, and Gd; for metal–Nbpy, only Cm–N presents low but detectable covalent character while Ln–N remains largely ionic. Overall, the results refine understanding of how soft-donor Cp′ ligation and neutral N-donor bridges modulate f-element covalency and emissive behavior, providing nuanced benchmarks for comparing Ln and An bonding with carbon and nitrogen donors.

This study reports the first single-crystal structure demonstrating Cm–C bonding in an organometallic complex and reveals unique photophysical behavior: Cp′3Cm shows markedly red-shifted emission that is completely quenched by 4,4′-bpy coordination. Comparative Sm and Gd analogs, together with multireference electronic structure calculations and bonding analyses, uncover atypical bonding trends: increased Sm–Cp′ orbital mixing and covalent stabilization relative to Cm/Gd, and a distinctive, slightly covalent Cm–Nbpy interaction absent in the lanthanides. Coordination of 4,4′-bpy reduces the electron density and energetic covalency at Cm–Cp′ bond critical points without significantly altering orbital mixing, underscoring the need to use multiple descriptors of covalency. These findings open avenues for rationally tuning actinide covalency and emissive properties via ligand design. Future work should explore alternative N-donor or deuterated ligands to mitigate vibrational quenching, pressure- or field-induced modulation of f-electron interactions, broader spectroscopic characterization (including time-resolved and vibrationally resolved studies), and expanded series across mid-actinides to map periodic trends in An–C and An–N covalency.

- Material constraints: Work with sub-5 mg quantities of 248Cm under stringent radiological and air-/moisture-free conditions limits the breadth of synthetic exploration and some characterization (e.g., no single-crystal XRD for Cp′3Cm precursor). - Spectroscopic masking: Strong charge-transfer bands obscure many high-energy ff transitions, restricting direct experimental assignment comparisons, especially for Gd(III). - Computational model simplification: Electronic structure and bonding analyses used pyridine in place of 4,4′-bpy for tractability, which may omit subtle electronic effects of the bridging ligand. - Quenching mechanism: The proposed vibrational quenching pathway via C–H overtones is inferential (resonance arguments) and not directly verified by, for example, isotopic substitution or time-resolved measurements. - Generalizability: Findings pertain to Cp′ and 4,4′-bpy ligand sets; extrapolation to other ligand classes or solution environments requires further validation.