Intermediates of Carbon Monoxide Oxidation on Praseodymium Monoxide Molecules: Insights from Matrix-Isolation IR Spectroscopy and Quantum-Chemical Calculations

Spin-orbit coupling of electrons on separate lanthanide atoms of Pr2O2 and its singly charged cation

Although it plays a critical role in the photophysics and catalysis of lanthanides, spin-orbit coupling of electrons on individual lanthanide atoms in small clusters is not well understood. The major objective of this work is to probe such coupling of the praseodymium (Pr) 4f and 6s electrons in Pr2O2 and Pr2O2+. The approach combines mass-analyzed threshold ionization spectroscopy and spin-orbit multiconfiguration second-order quasi-degenerate perturbation theory. The energies of six ionization transitions are precisely measured; the adiabatic ionization energy of the neutral cluster is 38 045 (5) cm-1. Most of the electronic states involved in these transitions are identified as spin-orbit coupled states consisting of two or more electron spins. The electron configurations of these states are 4f46s2 for the neutral cluster and 4f46s for the singly charged cation, both in planar rhombus-type structures. The spin-orbit splitting due to the coupling of the electrons on the separate Pr atoms is on the order of hundreds of wavenumbers.

C-O Bond Activation in Mononuclear Lanthanide Oxocarbonyl Complexes OLn(η2-CO) (Ln = La, Ce, Pr, and Nd)

Fundamental investigation of metal-CO interactions is of great importance for the development of high-performance catalysts to CO activation. Herein, a series of side-on bonded mononuclear lanthanide (Ln) oxocarbonyl complexes OLn(η²-CO) (Ln = La, Ce, Pr, and Nd) have been prepared and identified in solid argon matrices. The complexes exhibit uncommonly low C-O stretching bands near 1630 cm^-1, indicating remarkable C-O bond activation in these Ln analogues. The η²-CO ligand in OLn(η²-CO) can be claimed as an anion on the basis of the experimental observations and quantum chemistry investigations, although the CO anion is commonly considered to be unstable with electron auto-detachment. The CO activation in OLn(η²-CO) is attributed to the photoinduced intramolecular charge transfer from LnO to CO rather than the generally accepted metal → CO π back-donation, which conforms to the traditional Dewar-Chatt-Duncanson motif. Energy decomposition analysis combined with natural orbitals for chemical valence calculations demonstrates that the bonding between LnO and η²-CO arises from the combination of dominant ionic forces (>76%) and normal Lewis "acid-base" interactions. The fundamental findings provide guidelines for the catalyst design of CO activation.

Matrix-Isolation Infrared Spectra and Electronic Structure Calculations for Dinitrogen Complexes with Uranium Trioxide Molecules UO3(η1-NN)1-4

Investigations of the interactions of uranium trioxide (UO₃) with other species are expected to provide a new perspective on its reaction and bonding behaviors. Herein, we present a combined matrix-isolation infrared spectroscopy and theoretical study of the geometries, vibrational frequencies, electronic structures, and bonding patterns for a series of dinitrogen (N₂) complexes with UO₃ moieties UO₃(η¹-NN)_1-4. The complexes are prepared by reactions of laser-ablated uranium atoms with O₂/N₂ mixtures or laser-ablated UO₃ molecules with N₂ in solid argon. UO₃(η¹-NN)_1-4 are classified as "nonclassical" metal-N₂ complexes with increased Δν(N₂) values according to the experimental observations and the computed blue-shifts of N-N stretching frequencies and N-N bond length contractions. Electronic structure analysis suggests that UO₃(η¹-NN)_1-4 are σ-only complexes with a total lack of π-back-donation. The energy decomposition analysis combined with natural orbitals for chemical valence calculations reveal that the bonding between the UO₃ moiety and N₂ ligands in UO₃(η¹-NN)_1-4 arises from the roughly equal electrostatic attractions and orbital mixings. The inspection of orbital interactions from pairwise contributions indicates that the strongest orbital stabilization comes from the σ-donations of the 4σ*- and 5σ-based ligand molecular orbitals (MOs) into the hybrid 7s/6d_x²-y² MO of the U center. The electron polarization induced by electrostatic effects in the N_inner ← N_outer direction provides complementary contributions to the orbital stabilization in UO₃(η¹-NN)_1-4. In addition, the reactions of UO₃ with N₂ ligands and the origination of the nonclassical behavior in UO₃(η¹-NN)_1-4 are discussed.

Insights into the Metal-CO Bond in O2M(η1-CO) (M = Cr, Mo, W, Nd, and U) Complexes

Investigations on the structures and bonding properties of metal carbonyl compounds provide fundamental understandings on the origin of small-molecule activations. Herein, the geometry and bonding trends of a series of isovalent metal oxocarbonyl complexes O₂M(η¹-CO) (M = Cr, Mo, W, Nd, and U) were studied by combined matrix-isolation infrared spectroscopy and advanced quantum chemical calculations. The title complexes present red shift of C-O stretching bands in the range from 122 to 244 cm^-1, indicating the difference of CO activation ability for the series of isovalent metal dioxides. Density functional theory calculations predict T-shaped structures with a C_2v symmetry for all the title molecules. O₂Nd(η¹-CO) bears little resemblance to the other complexes in bonding characters because of the weak interactions between the NdO₂ and CO moiety. For the other complexes, natural localized molecular orbital analysis reveals a gradual increase of covalent character in M-CO bonds along the metal series Cr → Mo → W→ U. Energy decomposition analysis with natural orbitals for chemical valence calculations demonstrates that the M-CO bonding patterns conform to the conventional Dewar-Chatt-Duncanson motif. The contributions from orbital interactions in total attractions increase from Cr (41.7%) to U (52.7%). The breakdown of the orbital term into pairwise interactions shows that contributions of the M ← CO σ donation decrease from Cr (59.2%) to U (28.4%), while the M → CO π* backdonation increases significantly from Cr (23.8%) to U (67.3%). The more effective overlap and the better energy matching of U 5f and U 6d valence orbitals with CO π* orbitals result in much stronger U → CO π backdonation than the other metal elements.