Tris-{Hydridotris(1-pyrazolyl)borato}lanthanide Complexes: Synthesis, Spectroscopy, Crystal Structure and Bonding Properties

Activation and functionalisation of carbon dioxide by bis-tris(pyrazolyl)borate-supported divalent samarium and trivalent lanthanide silylamide complexes

Synthesis and reactivity with carbon dioxide (CO2) of divalent samarium in the bis-tris(pyrazolyl)borate ligand environment has been reported. In addition, CO2 activation and functionalisation by lanthanide silylamides in the bis-tris(pyrazolyl)borate ligand environment was demonstrated. Reduction of the Sm(III) precursor [Sm(Tp)2(OTf)] (Tp = hydrotris(1-pyrazolyl)borate; OTf = triflate) with KC8 yielded the insoluble Sm(II) multi-metallic coordination polymer [{Sm(Tp)2}n] 1-Sm. Addition of 1,2-dimethoxyethane (DME) to 1-Sm enabled isolation of the monomeric complex [Sm(Tp)2(DME)] 1-Sm(DME). Complex 1-Sm(DME) reduced CO2 to yield the oxalate-bridged dimeric Sm(III) complex [{Sm(Tp)2}2(μ-η2:η2-O2CCO2)] 2-Sm. The reactions of heteroleptic Ln(III) silylamide complexes [Ln(Tp)2(N'')] (Ln = Y, Sm; N'' = N(SiMe3)2) with CO2 yielded monomeric Ln(III) silyloxides [Ln(Tp)2(OSiMe3)] 3-Ln and trimethylsilyl isocyanate (OCNSiMe3). Complexes 3-Ln are the first crystallographically characterised examples of Ln(III)-OSiMe3 bonds accessed via CO2 activation and functionalisation. Full characterisation data are presented for all complexes, including solid-state molecular structure determination by single-crystal X-ray diffraction.

Tuning the optical and magnetic properties of lanthanide single-ion magnets using nitro-functionalized trispyrazolylborate ligands

We report the synthesis, crystal structures, photophysical and magnetic properties of 11 novel lanthanide complexes with the asymmetrically functionalized trispyrazolylborate ligand 4-nitrotrispyrazolylborate, 4-NO₂Tp^-: [Ln(4-NO₂Tp)₃] (Ln = La-Dy, except Pm). In-depth photophysical characterization of the ligands via luminescence, reflectance and absorption spectroscopic techniques, decay lifetimes, quantum yields supported by time-dependent density functional theory (TD-DFT) and natural bond order (NBO) analysis reveal that n-NO₂Tp^- ligands are dominated by intra-ligand charge transfer (ILCT) transitions and that second-sphere interactions are critical to the stabilization of the T₁ state of n-NO₂Tp^- ligands and hence their ability to sensitize Ln³⁺ emission. The luminescence properties of the complexes indicate that 4-NO₂Tp^- is a poor sensitizer of Ln³⁺ emission, unlike 3-NO₂Tp^-. Moreover, [Nd(4-NO₂Tp)₃] (crystallized as a hexane solvate) displays single-molecule magnet (SMM) properties, with longer relaxation times and larger barrier than the non-functionalized [NdTp₃], attributed to the addition of the NO₂-group and subsequent rigidification of the molecular structure.

Tris(carbene)borates; alternatives to cyclopentadienyls in organolanthanide chemistry

The chemistry of the tris-carbene anion phenyltris(3-alkyl-imidazoline-2-yliden-1-yl)borate, [C3Me]- ligand, is initiated for f-block metal cations. Neutral, molecular complexes of the form Ln(C3)2I are formed for cerium(III), while a separated ion pair [Ln(C3)2]I forms for ytterbium(III). DFT/QTAIM computational analyses of the complexes and related tridentate tris(pyrazolyl)borate (Tp) - supported analogs demonstrates the anticipated strength of the σ donation and confirms greater covalency in the metal-carbon bonds of the [C3Me]- complexes in comparison with those in the TpMe,Me complexes. The DFT calculations demonstrate the crucial role of THF solvent in accurately reproducing the contrasting molecular and ion-pair geometries observed experimentally for the Ce and Yb complexes.

Competing Metal-Ligand Interactions in Tris(cyclopentadienyl)-cyclohexylisonitrile Complexes of Trivalent Actinides and Lanthanides

The structure and bonding properties of 16 complexes formed by trivalent f elements (M=U, Np, Pu and lanthanides except for Pm and Pr) with cyclopentadienyl (Cp) and cyclohexylisonitrile (C≡NCy) ligands, (Cp)₃M(C≡NCy), were studied by a joint experimental (XRD, NMR) and theoretical (DFT) analysis. For the large La(III) ion, the bis-adduct (Cp)₃La(C≡NCy)₂ could also be synthesized and characterized. The metal–ligand interactions, focusing on the comparison of the actinides and lanthanides as well as on the competition of the two different ligands for M, were elucidated using the Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) models. The results point to interactions of comparable strengths with the anionic Cp and neutral C≡NCy ligands in the complexes. The structural and bonding properties of the actinide complexes reflect small but characteristic differences with respect to the lanthanide analogues. They include larger ligand-to-metal charge transfers as well as metal–ligand electron-sharing interactions. The most significant experimental marker of these covalent interactions is the C≡N stretching frequency.

HERFD-XANES and RIXS Study on the Electronic Structure of Trivalent Lanthanides across a Series of Isostructural Compounds

We performed a systematic study of the complexes of trivalent lanthanide cations with the hydridotris(1-pyrazolyl)borato (Tp) ligand (LnTp₃; Ln = La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu) using both high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) and resonant inelastic X-ray scattering (RIXS) at the lanthanide L₃ absorption edge. Here, we report the results obtained and we discuss them against calculations performed using density functional theory (DFT) and atomic multiplet theory. The spectral shape and the elemental trends observed in the experimental HERFD-XANES spectra are well reproduced by DFT calculations, while the pre-edge energy interval is better described by atomic multiplet theory. The RIXS data show a generally rather complex pattern that originates from the intra-atomic electron–electron interactions in the intermediate and final states, as demonstrated by the good agreement obtained with calculations using an atomic-only model of the absorber. Guided by theoretical predictions, we discuss the possible origins of the observed spectral features and the trends in energy splitting across the series. The insight into the electronic structure of trivalent lanthanide compounds demonstrated here and obtained with advanced X-ray spectroscopies coupled with theoretical calculations can be applied to any lanthanide-bearing compound and be of great interest for all research fields involving lanthanides.

This work represents a systematic investigation of the complexes of trivalent lanthanide cations with the hydridotris(1-pyrazolyl)borato (Tp) ligand (LnTp₃; Ln = La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu) using both high-energy-resolution fluorescence-detected X-ray absorption near-edge structure and resonant inelastic X-ray scattering at the lanthanide L₃ absorption edge. The experimental results are discussed against the calculations performed with the density functional theory and atomic multiplet theory approaches.