A trivalent 4f complex with two bis-silylamide ligands displaying slow magnetic relaxation

The best-performing single-molecule magnets (SMMs) have historically relied on pseudoaxial ligands delocalized across several coordinated atoms. This coordination environment has been found to elicit strong magnetic anisotropy, but lanthanide-based SMMs with low coordination numbers have remained synthetically elusive species. Here we report a cationic 4f complex bearing only two bis-silylamide ligands, Yb(III)[{N(SiMePh₂)₂}₂][Al{OC(CF₃)₃}₄], which exhibits slow relaxation of its magnetization. The combination of the bulky silylamide ligands and weakly coordinating [Al{OC(CF₃)₃}₄]^- anion provides a sterically hindered environment that suitably stabilizes the pseudotrigonal geometry necessary to elicit strong ground-state magnetic anisotropy. The resolution of the m_J states by luminescence spectroscopy is supported by ab initio calculations, which show a large ground-state splitting of approximately 1,850 cm^-1. These results provide a facile route to access a bis-silylamido Yb(III) complex, and further underline the desirability of axially coordinated ligands with well-localized charges for high-performing SMMs.

Isocyanate deoxygenation by a molecular magnesium silanide

The stoichiometric reactivity of a β-diketiminato (BDI) magnesium silanide towards a variety of organic isocyanates has been assessed. While the primary outcome of reactions of t-BuNCO, DippNCO and CyNCO was the production of β-diketiminato magnesium siloxide adducts of the isonitrile resulting from isocyanate deoxygenation, analogous treatment with i-PrNCO led to multiple products, four of which have been positively identified. Although the specificity of this latter reaction was hampered by competitive isocycanate addition at the γ-methine carbon of the BDI supporting ligand, the identification of [{i-PrNCO}CH{(Me)CNDipp}Mg{Me₂PhSi}C(O)Ni-Pr]₆ provided corroborative evidence for the likely generation of sila-amidate intermediates in all four reactions under study. The formation of [{Me₂PhSi}C(O)NR]^- anions as the most likely initial species formed en route to isonitrile and siloxide formation was, therefore, validated by a computational density functional theory (DFT) study.

Iron-Catalyzed Vinylsilane Dimerization and Cross-Cycloadditions with 1,3-Dienes: Probing the Origins of Chemo- and Regioselectivity

The selective, intermolecular, homodimerization and cross-cycloaddition of vinylsilanes with unbiased 1,3-dienes, catalyzed by a pyridine-2,6-diimine (PDI) iron complex is described. In the absence of a diene coupling partner, vinylsilane hydroalkenylation products were obtained chemoselectively with unusual head-to-head regioselectivity (up to >98% purity, 98:2 E/Z). In the presence of a 4- or 2-substituted diene coupling partner, under otherwise identical reaction conditions, formation of value-added [2+2]- and [4+2]-cycloadducts, respectively, was observed. The chemoselectivity profile was distinct from that observed for analogous α-olefin dimerization and cross-reactions with 1,3-dienes. Mechanistic studies conducted with well-defined, single-component precatalysts (MePDI)Fe(L2) (where MePDI = 2,6-(2,6-Me2-C6H3N═CMe)2C5H3N; L2 = butadiene or 2(N2)) provided insights into the kinetic and thermodynamic factors contributing to the substrate-controlled regioselectivity for both the homodimerization and cross cycloadditions. Diamagnetic iron diene and paramagnetic iron olefin complexes were identified as catalyst resting states, were characterized by in situ NMR and Mössbauer spectroscopic studies, and were corroborated with DFT calculations. Stoichiometric reactions and computational models provided evidence for a common mechanistic regime where competing steric and orbital-symmetry requirements dictate the regioselectivity of oxidative cyclization. Although distinct chemoselectivity profiles were observed in cross-cycloadditions with the vinylsilane congeners of α-olefins, these products arose from metallacycles with the same connectivity. The silyl substituents ultimately governed the relative rates of β-H elimination and C-C reductive elimination to dictate final product formation.

Efficient alkene hydrosilation with bis(8-quinolyl)phosphine (NPN) nickel catalysts. The dominant role of silyl-over hydrido-nickel catalytic intermediates

A cationic nickel complex of the bis(8-quinolyl)(3,5-di-tert-butylphenoxy)phosphine (NPN) ligand, [(NPN)NiCl]⁺, is a precursor to efficient catalysts for the hydrosilation of alkenes with a variety of hydrosilanes under mild conditions and low catalyst loadings. DFT studies reveal the presence of two coupled catalytic cycles based on [(NPN)NiH]⁺ and [(NPN)NiSiR₃]⁺ active species, with the latter being more efficient for producing the product. The preferred silyl-based catalysis is not due to a more facile insertion of alkene into the Ni–Si (vs. Ni–H) bond, but by consistent and efficient conversions of the hydride to the silyl complex.

A cationic nickel complex of the bis(8-quinolyl)(3,5-di-tert-butylphenoxy)phosphine (NPN) ligand, [(NPN)NiCl]⁺, is a precursor to efficient catalysts for the hydrosilation of alkenes with hydrosilanes under mild conditions and low catalyst loadings.

Electronic structures of bent lanthanide(III) complexes with two N-donor ligands

Low coordinate metal complexes can exhibit superlative physicochemical properties, but this chemistry is challenging for the lanthanides (Ln) due to their tendency to maximize electrostatic contacts in predominantly ionic bonding regimes. Although a handful of Ln2+ complexes with only two monodentate ligands have been isolated, examples in the most common +3 oxidation state have remained elusive due to the greater electrostatic forces of Ln3+ ions. Here, we report bent Ln3+ complexes with two bis(silyl)amide ligands; in the solid state the Yb3+ analogue exhibits a crystal field similar to its three coordinate precursor rather than that expected for an axial system. This unanticipated finding is in opposition to the predicted electronic structure for two-coordinate systems, indicating that geometries can be more important than the Ln ion identity for dictating the magnetic ground states of low coordinate complexes; this is crucial transferable information for the construction of systems with enhanced magnetic properties.

Reversible temperature-induced polymorphic phase transitions of [Y(OAr)3] and [Ce(OAr)3] (Ar = 2,6-tBu2-4-MeC6H2): interconversions between pyramidal and planar geometries

Exploring the balance of energetics between planar and pyramidal forms of [Y(O-2,6-^tBu₂-4-MePh)₃] and related complexes.

Homoleptic Trivalent Tris(alkyl) Rare Earth Compounds

Homoleptic tris(alkyl) rare earth complexes Ln{C(SiHMe₂)₃}₃ (Ln = La, 1a; Ce, 1b; Pr, 1c; Nd, 1d) are synthesized in high yield from LnI₃THF_n and 3 equiv of KC(SiHMe₂)₃. X-ray diffraction studies reveal 1a-d are isostructural, pseudo-C₃-symmetric molecules that contain two secondary Ln↼HSi interactions per alkyl ligand (six total). Spectroscopic assignments are supported by comparison with Ln{C(SiDMe₂)₃}₃ and DFT calculations. The Ln↼HSi and terminal SiH exchange rapidly on the NMR time scale at room temperature, but the two motifs are resolved at low temperature. Variable-temperature NMR studies provide activation parameters for the exchange process in 1a (ΔH^⧧ = 8.2(4) kcal·mol^-1; ΔS^⧧ = -1(2) cal·mol^-1K^-1) and 1a-d₉ (ΔH^⧧ = 7.7(3) kcal·mol^-1; ΔS^⧧ = -4(2) cal·mol^-1K^-1). Comparisons of lineshapes, rate constants (k_H/k_D), and slopes of ln(k/T) vs 1/T plots for 1a and 1a-d₉ reveal that an inverse isotope effect dominates at low temperature. DFT calculations identify four low-energy intermediates containing five β-Si-H⇀Ln and one γ-C-H⇀Ln. The calculations also suggest the pathway for Ln↼HSi/SiH exchange involves rotation of a single C(SiHMe₂)₃ ligand that is coordinated to the Ln center through the Ln-C bond and one secondary interaction. These robust organometallic compounds persist in solution and in the solid state up to 80 °C, providing potential for their use in a range of synthetic applications. For example, reactions of Ln{C(SiHMe₂)₃}₃ and ancillary proligands, such as bis-1,1-(4,4-dimethyl-2-oxazolinyl)ethane (HMeC(Ox^Me2)₂) give {MeC(Ox^Me2)₂}Ln{C(SiHMe₂)₃}₂, and reactions with disilazanes provide solvent-free lanthanoid tris(disilazides).

Formation of a Ce(IV) Oxo Complex via Inner Sphere Nitrate Reduction

Reaction of Ce(NO₃)₃(THF)₄ with Li₃(THF)₃(NN'₃) (NN'₃ = N(CH₂CH₂NR)₃, R = Si^tBuMe₂) in Et₂O, in the presence of 12-crown-4, results in the formation of [Li(12-crown-4)][(NN'₃)Ce(O)] (1) in 36% yield. This transformation proceeds via formation of a Ce(III) nitrate intermediate, [Li(12-crown-4)][(NN'₃)Ce(κ²-O₂NO)] (2), which undergoes inner sphere nitrate reduction. In addition, reaction of 1 with ^tBuMe₂SiCl results in the formation of (NN'₃)Ce(OSi^tBuMe₂) (3), confirming the nucleophilic character of its oxo ligand. Natural bond orbital and quantum theory of atoms-in-molecules data reveal the Ce-O interaction in 1 to be significantly covalent, and strikingly similar to analogous U-O bonding.

The Nature of Secondary Interactions at Electrophilic Metal Sites of Molecular and Silica-Supported Organolutetium Complexes from Solid-State NMR Spectroscopy

Lu[CH(SiMe3)2]3 reacts with [SiO2-700] to give [(≡SiO)Lu[CH(SiMe3)2]2] and CH2(SiMe3)2. [(≡SiO)Lu[CH(SiMe3)2]2] is characterized by solid-state NMR and EXAFS spectroscopy, which show that secondary Lu···C and Lu···O interactions, involving a γ-CH3 and a siloxane bridge, are present. From X-ray crystallographic analysis, the molecular analogues Lu[CH(SiMe3)2]3-x[O-2,6-tBu-C6H3]x (x = 0-2) also have secondary Lu···C interactions. The (1)H NMR spectrum of Lu[CH(SiMe3)2]3 shows that the -SiMe3 groups are equivalent to -125 °C and inequivalent below that temperature, ΔG(⧧)(Tc = 148 K) = 7.1 kcal mol(-1). Both -SiMe3 groups in Lu[CH(SiMe3)2]3 have (1)JCH = 117 ± 1 Hz at -140 °C. The solid-state (13)C CPMAS NMR spectrum at 20 °C shows three chemically inequivalent resonances in the area ratio of 4:1:1 (12:3:3); the J-resolved spectra for each resonance give (1)JCH = 117 ± 2 Hz. The (29)Si CPMAS NMR spectrum shows two chemically inequivalent resonances with different values of chemical shift anisotropy. Similar observations are obtained for Lu[CH(SiMe3)2]3-x[O-2,6-tBu-C6H3]x (x = 1 and 2). The spectroscopic data point to short Lu···Cγ contacts corresponding to 3c-2e Lu···Cγ-Siβ interactions, which are supported by DFT calculations. Calculated natural bond orbital (NBO) charges show that Cγ carries a negative charge, while Lu, Hγ, and Siβ carry positive charges; as the number of O-based ligands increases so does the positive charge at Lu, which in turns shortens the Lu···Cγ distance. The change in NBO charges and the resulting changes in the spectroscopic and crystallographic properties show how ligands and surface-support sites rearrange to accommodate these changes, consistent with Pauling's electroneutrality concept.

New perspectives in organolanthanide chemistry from redox to bond metathesis: insights from theory

A fifteen year contribution of computational studies carried out in close synergy with experiments is summarized.

Neutral "Cp-Free" Silyl-Lanthanide(II) Complexes: Synthesis, Structure, and Bonding Analysis

Complexes featuring lanthanide silicon bonds represent a research area still in its infancy. Herein, we report a series of Cp-free lanthanide (+II) complexes bearing σ-bonded silyl ligands. By reactions of LnI₂ (Ln = Yb, Eu, Sm) either with a 1,4-oligosilanyl dianion [K-Si(SiMe₃)₂SiMe₂SiMe₂Si(SiMe₃)₂-K)] (1) or with 2 (Me₃Si)₃SiK (3) the corresponding neutral metallacyclopentasilanes ({Me₂Si(Me₃Si)₂Si}₂)Ln·(THF)₄ (Ln = Yb (2a), Eu (2b), Sm (2c)), or the disilylated complexes ({Me₃Si}₃Si)₂Ln·(THF)₃ (Ln = Yb (4a), Eu (4b), Sm (4c)), were selectively obtained. Complexes 2b, 2c, 4b, and 4c represent the first examples of structurally characterized Cp-free Eu and Sm complexes with silyl ligands. In both series, a linear correlation was observed between the Ln–Si bond lengths and the covalent radii of the corresponding lanthanide metals. Density functional theory calculations were also carried out for complexes 2a–c and 4a–c to elucidate the bonding situation between the Ln(+II) centers and Si.

By reactions of LnI₂ (Ln = Yb, Eu, Sm) with oligosilanyl anions or dianions, either neutral disilylated complexes or metallacyclopentasilanes were obtained.

Heterolytic cleavage of H2 by bifunctional manganese(i) complexes: impact of ligand dynamics, electrophilicity, and base positioning

A balance of metal electrophilicity and ligand steric influences is required for facile, reversible H–H heterolytic cleavage in Mn complexes with pendant amines.

Synthesis and stability of homoleptic metal(III) tetramethylaluminates

Whereas a number of homoleptic metal(III) tetramethylaluminates M(AlMe(4))(3) of the rare earth metals have proven accessible, the stability of these compounds varies strongly among the metals, with some even escaping preparation altogether. The differences in stability may seem puzzling given that this class of metals usually is considered to be relatively uniform with respect to properties. On the basis of quantum chemically obtained relative energies and atomic and molecular descriptors of homoleptic tris(tetramethylaluminate) and related compounds of rare earth metals, transition metals, p-block metals, and actinides, multivariate modeling has identified the importance of ionic metal-methylaluminate bonding and small steric repulsion between the methylaluminate ligands for obtaining stable homoleptic compounds. Low electronegativity and a sufficiently large ionic radius are thus essential properties for the central metal atom. Whereas scandium and many transition metals are too small and too electronegative for this task, all lanthanides and actinides covered in this study are predicted to give homoleptic compounds stable toward loss of trimethylaluminum, the expected main decomposition reaction. Three of the predicted lanthanide-based compounds Ln(AlMe(4))(3) (Ln = Ce, Tm, Yb) have been prepared and fully characterized in the present work, in addition to Ln(OCH(2)tBu)(3)(AlMe(3))(3) (Ln = Sc, Nd) and [Eu(AlEt(4))(2)](n). At ambient temperature, donor-free hexane solutions of Ln(AlMe(4))(3) of the Ln(3+)/Ln(2+) redox-active metal centers display enhanced reduction to [Ln(AlMe(4))(2)](n) with decreasing negative redox potential, in the order Eu ≫ Yb ≫ Sm. Whereas Eu(AlMe(4))(3) could not be identified, Yb(AlMe(4))(3) turned out to be isolable in low yield. All attempts to prepare the putative Sc(AlMe(4))(3), featuring the smallest rare earth metal center, failed.