Reactions of Lanthanide Atoms with Oxygen Difluoride and the Role of the Ln Oxidation State

Spectroscopic and electrochemical characterization of a Pr4+ imidophosphorane complex and the redox chemistry of Nd3+ and Dy3+ complexes

The molecular tetravalent oxidation state for praseodymium is observed in solution via oxidation of the anionic trivalent precursor [K][Pr³⁺(NP(1,2-bis-^tBu-diamidoethane)(NEt₂))₄] (1-Pr(NP*)) with AgI at -35 °C. The Pr⁴⁺ complex is characterized in solution via cyclic voltammetry, UV-vis-NIR electronic absorption spectroscopy, and EPR spectroscopy. Electrochemical analyses of [K][Ln³⁺(NP(1,2-bis-^tBu-diamidoethane)(NEt₂))₄] (Ln = Nd and Dy) by cyclic voltammetry are reported and, in conjunction with theoretical modeling of electronic structure and oxidation potential, are indicative of principal ligand oxidations in contrast to the metal-centered oxidation observed for 1-Pr(NP*). The identification of a tetravalent praseodymium complex in in situ UV-vis and EPR experiments is further validated by theoretical modeling of the redox chemistry and the UV-vis spectrum. The latter study was performed by extended multistate pair-density functional theory (XMS-PDFT) and implicates a multiconfigurational ground state for the tetravalent praseodymium complex.

Bond Dissociation Energies Reveal the Participation of d Electrons in f-Element Halide Bonding

Bond dissociation energies (BDEs) reported in the literature for lanthanide monofluorides and lanthanide monochlorides LnX, where X = F or Cl, exhibit substantial irregular variations across the Ln series. It is demonstrated here that correlations of these variations with reported experimentally based atomic energies to prepare the Ln constituent for bonding reveal the nature of the bonding. Whereas some molecular characteristics are well understood in the context of highly ionic bonding, with LnX considered to be (Ln⁺)(X^-), some significant variations in BDEs are not well rationalized simply by ionization to convert Ln to Ln⁺ for bonding. Focusing here on lanthanide monofluorides LnF, a consideration of alternative Ln preparation schemes shows that a particularly good rationalization of BDEs is obtained by invoking the participation of a lanthanide 5d electron in bonding. This 5d participation could be in ionic (Ln⁺)(F^-) via π-donation from F^- 2p to empty Ln⁺ 5d orbitals or in covalent π-bonded Ln:F via polarization from Ln 5d to F 2p, with these ionic and polar covalent perspectives ultimately being equivalent. The inference of lanthanide 5d involvement suggests that the valence 4f and 6s electrons do not effectively participate in some key aspects of the bonding, presumably due to poor spatial overlap with F 2p orbitals. An extension to actinide monofluorides, AnF, assumes analogous ionic or polar covalent bonding involving a valence 6d electron and results in predictions for BDEs that include a general decrease from left to right across the series, except for a distinctive local minimum at AmF. Determining the BDE for AmF would serve to evaluate the predictions and the underlying assumption of 6d bonding. The BDE assessments/predictions for neutral monofluorides, LnF and AnF, are also applied to cationic LnF⁺ and AnF⁺, and it is noted that the approach can be directly extended to f-element monochlorides, monobromides, and monoiodides.

Metal Oxo-Fluoride Molecules OnMF2 (M = Mn and Fe; n = 1-4) and O2MnF: Matrix Infrared Spectra and Quantum Chemistry

On reacting laser-ablated manganese or iron difluorides with O₂ or O₃ during codeposition in solid neon or argon, infrared absorptions of several new metal oxo-fluoride molecules, including OMF₂, (η¹-O₂)MF₂, (η²-O₃)MF₂, (η¹-O₂)₂MF₂ (M = Mn and Fe), and O₂MnF, have been observed. Quantum chemical density functional and multiconfiguration wavefunction calculations have been applied to characterize these new products by their geometric and electronic structures, vibrations, charges, and bonding. The assignment of the main vibrational absorptions as dominant symmetric or antisymmetric M-F or M-O stretching modes is confirmed by oxygen isotopic shifts and quantum chemical calculations of frequencies and thermal stabilities. The tendency of Fe to form polyoxygen complexes in lower oxidation states than the preceding element Mn is affirmed experimentally and supported theoretically. The M-F stretching frequencies of the isolated metal oxo-fluorides may provide a scale for the local charge on the MF₂ sites in active energy conversion systems. The study of these species provides insights for understanding the trend of oxidation state changes across the transition-metal series.

Spin-orbit coupling and vibronic transitions of Ce(C3H4) and Ce(C3H6) formed by the Ce reaction with propene: Mass-analyzed threshold ionization and relativistic quantum computation

A Ce atom reaction with propene is carried out in a pulsed laser vaporization molecule beam source. Several Ce-hydrocarbon species formed by the C-H and C-C bond activation of propene are observed by time-of-flight mass spectrometry, and Ce(C₃H_n) (n = 4 and 6) are characterized by mass-analyzed threshold ionization (MATI) spectroscopy and density functional theory, multiconfiguration, and relativistic quantum chemical calculations. The MATI spectrum of each species consists of two vibronic band systems, each with several vibronic bands. Ce(C₃H₆) is identified as an inserted species with Ce inserting into an allylic C-H bond of propene and Ce(C₃H₄) as a metallocycle through 1,2-vinylic dehydrogenation. Both species have a C_s structure with the Ce 4f¹6s¹ ground valence electron configuration in the neutral molecule and the Ce 4f¹ configuration in the singly charged ion. The two vibronic band systems observed for each species are attributed to the ionization of two pairs of the lowest spin-orbit coupled states with each pair being nearly degenerate.

The chemical and physical properties of tetravalent lanthanides: Pr, Nd, Tb, and Dy

The thermochemistry, descriptive chemistry, spectroscopy, and physical properties of the tetravalent lanthanides (Pr, Nd, Tb and Dy) in extended phases, gas phase, solution, and as isolable molecular complexes are presented.

The decisive role of 4f-covalency in the structural direction and oxidation state of XPrO compounds (X: group 13 to 17 elements)

Lanthanide oxo compounds are of vital importance in lanthanide chemistry, as well as in environmental and materials sciences. Praseodymium, as an exceptional element in lanthanides which can form a +V formal oxidation state (OSf) besides the dominant +III among the 4f-block element, displays the significant participation of the Pr 4f orbitals in bonding interactions which is commonly crucial in stabilizing the high oxidation state of Pr in PrO2+ and NPrO species. Here, we report a systematic theoretical study on the structures and stabilities of a series of XPrO (X: B, Al, C, Si, N, P, As, O, S, F, Cl) compounds along with [XPrO]+ cation (X: O, S) and [X3PrO] complexes (X: F and Cl). This work reveals that Pr is able to achieve the lowest and highest OSf and the OSf exhibits periodic variation from +I in BOPr and AlOPr to +II in SiOPr to +III in CPrO, FPrO, ClPrO and AsPrO to +IV in OPrO and SPrO and even to +V in NPrO, [OPrO]+, [SPrO]+, F3PrO and Cl3PrO. We found that the molecular structures are correlated to the Pr oxidation state due to the highly important 4f orbital in the chemical bonding of the high oxidation state compounds. Thus, not only the electronegativity of the ligand but also the quasi-degenerate Pr valence 4f orbitals, namely energetic covalency, control the oxidation state and play a fundamental role in affecting the electronic structural stability of Pr(v) compounds as well. This work demonstrates the structurally directing role of the f-orbital in the formation of the linear structure and is constructive for achieving the higher oxidation state of a given element by tuning the ligand.

Fluoroborylene Complexes FBMF2 (M = Sc, Y, La, Ce): Matrix Infrared Spectra and Quantum Chemical Calculations

Laser-ablated group 3 transition metal and cerium atom reactions with boron trifluoride were explored in excess solid neon at 4 K through matrix isolation infrared spectroscopy and quantum chemical calculations. The fluoroborylene complexes FBMF₂ (M = Sc, Y, La, Ce) were trapped in inert gas and identified by the isotopic substitutions. The observed frequencies of FBMF₂ were reproduced by DFT, NEVPT2, and CASSCF calculations. From Sc to La, the observed F-¹¹B stretching mode has been observed at 1391.9 cm^-1 (Sc), 1370.8 cm^-1 (Y), and 1337.1 cm^-1(La); however, for Ce this mode shifts up to 1340.8 cm^-1, which is due to relativistic effects. The electron localization function (ELF) analysis and the theory of atoms in molecules (AIM) were applied to investigate the character of the B-M bond in FBMF₂ molecules, which favors bond order 1.5.

Assigning the Cerium Oxidation State for CH2CeF2 and OCeF2 Based on Multireference Wave Function Analysis

The geometric and electronic structure of the recently experimentally studied molecules ZCeF2 (Z = CH2, O) was investigated by density functional theory (DFT) and wave function-based ab initio methods. Special attention was paid to the Ce-Z metal-ligand bonding, especially to the nature of the interaction between the Ce 4f and the Z 2p orbitals and the possible multiconfigurational character arising from it, as well as to the assignment of an oxidation state of Ce reflecting the electronic structure. Complete active space self-consistent field (CASSCF) calculations were performed, followed by orbital rotations in the active orbital space. The methylene compound CH2CeF2 has an open-shell singlet ground state, which is characterized by a two-configurational wave function in the basis of the strongly mixed natural CASSCF orbitals. The system can also be described in a very compact way by the dominant Ce 4f(1) C 2p(1) configuration, if nearly pure Ce 4f and C 2p orbitals are used. In the basis of these localized orbitals, the molecule is almost monoconfigurational and should be best described as a Ce(III) system. The singlet ground state of the oxygen OCeF2 complex is of closed-shell character when a monoconfigurational wave function with very strongly mixed Ce 4f and O 2p CASSCF natural orbitals is used for the description. The transformation to orbitals localized on the cerium and oxygen atoms leads to a multiconfigurational wave function and reveals characteristics of a mixed valent Ce(IV)/Ce(III) compound. Additionally, the interactions of the localized active orbitals were analyzed by evaluating the expectation values of the charge fluctuation operator and the local spin operator. The Ce 4f and C 2p orbital interaction of the CH2CeF2 compound is weakly covalent and resembles the interaction of the H 1s orbitals in a stretched hydrogen dimer. In contrast, the interaction of the localized active orbitals for OCeF2 shows ionic character. Calculated vibrational Ce-C and Ce-O stretching frequencies at the DFT, CASSCF, second-order Rayleigh-Schrödinger perturbation theory (RS2C), multireference configuration interaction (MRCI), as well as single, doubles, and perturbative triples coupled cluster (CCSD(T)) level are reported and compared to experimental infrared absorption data in a Ne and Ar matrix.

Structures and Properties of the Products of the Reaction of Lanthanide Atoms with H2O: Dominance of the +II Oxidation State

The reactions of lanthanides with H2O have been studied using density functional theory with the B3LYP functional. H2O forms an initial Lewis acid-base complex with the lanthanides exothermically with interaction energies from -2 to -20 kcal/mol. For most of the Ln, formation of HLnOH is more exothermic than formation of H2LnO, HLnO + H, and LnOH + H. The reactions to produce HLnOH are exothermic from -25 to -75 kcal/mol. The formation of LnO + H2 for La and Ce is slightly more exothermic than formation of HLnOH and is less or equally exothermic for the rest of the lanthanides. The Ln in HLnOH and LnOH are in the formal +II and +I oxidation states, respectively. The Ln in H2LnO is mostly in the +III formal oxidation state with either Ln-O(-)/Ln-H(-) or Ln-(H2)(-)/Ln=O(2-) bonding interactions. A few of the H2LnO have the Ln in the +IV or mixed +III/+IV formal oxidation states with Ln=O(2-)/Ln-H(-) bonding interactions. The Ln in HLnO are generally in the +III oxidation state with the exception of Yb in the +II state. The orbital populations calculated within the natural bond orbital (NBO) analysis are consistent with the oxidation states and reaction energies. The more exothermic reactions to produce HLnOH are always associated with more backbonding from the O(H) and H characterized by more population in the 6s and 5d in Ln and the formation of a stronger Ln-O(H) bond. Overall, the calculations are consistent with the experiments in terms of reaction energies and vibrational frequencies.

Properties of Cerium Hydroxides from Matrix Infrared Spectra and Electronic Structure Calculations

Reactions of laser ablated cerium atoms with hydrogen peroxide or hydrogen and oxygen mixtures diluted in argon and condensed at 4 K produced the Ce(OH)3 and Ce(OH)2 molecules and Ce(OH)2(+) cation as major products. Additional minor products were identified as the Ce(OH)4, HCeO, and OCeOH molecules. These new species were identified from their matrix infrared spectra with D2O2, D2, and (18)O2 isotopic substitution and correlating observed frequencies with values calculated by density functional theory. We find that the amounts of Ce(OH)3 and of the Ce(OH)2(+) cation increase on UV (λ > 220 nm) photolysis, while Ce(OH)2, Ce(OH)4, and HCeO are photosensitive. The observed major species for Ce are in the +III or +II oxidation state, and the minor product, Ce(OH)4, is in the +IV oxidation state. The calculations for the vibrational frequencies with the B3LYP functional agree well with the experiment. The NBO analysis shows significant backbonding to the metal 4f and 5d orbitals for the closed shell species. Most open shell species have the excess spin in the 4f with paired spin in the 5d due to backbonding. The heats of formation of the observed species were derived from the available data from experiment and the calculated reaction energies. The major products in this study are different from similar reactions for Th where the tetrahydroxide was the major species.

Extending the Row of Lanthanide Tetrafluorides: A Combined Matrix-Isolation and Quantum-Chemical Study

Only the neutral tetrafluorides of Ce, Pr, and Tb as well as the [LnF7 ](3-) anions of Dy and Nd, with the metal in the +IV oxidation state, have been previously reported. We report our attempts to extend the row of neutral lanthanide tetrafluorides through the reaction of laser-ablated metal atoms with fluorine and their stabilization and characterization by matrix-isolation IR spectroscopy. In addition to the above three tetrafluorides, we found two new tetrafluorides, (3) NdF4 and (7) DyF4 , both of which are in the +IV oxidation state, which extends this lanthanide oxidation state to two new metals. Our experimental results are supported by quantum-chemical calculations and the role of the lanthanide oxidation state is discussed for both the LnF4 and [LnF4 ](-) species. Most of the LnF4 species are predicted to be in the +IV oxidation state and all of the [LnF4 ](-) anions are predicted to be in the +III oxidation state. The LnF4 species are predicted to be strong oxidizing agents and the LnF3 species are predicted to be moderate to strong Lewis acids.

Investigation of Praseodymium Fluorides: A Combined Matrix-Isolation and Quantum-Chemical Study

The chemistry of the lanthanides is mostly dominated by compounds in the oxidation state +III. Only few compounds of Ce, Pr, and Tb are known with the metal in the +IV oxidation state. Removal of the last f-electron on praseodymium +IV would lead to a closed-shell system with formal oxidation state V. In this work we investigated the stability of the PrF5 molecule by theory and matrix-isolation techniques through the reaction of laser-ablated praseodymium atoms with fluorine in excess of neon, argon, krypton, or neat fluorine. Besides the known PrF3 molecule, unreported IR bands for PrF4 could be observed, and there is evidence for the formation of PrF and PrF2 but not for the formation of PrF5.