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

Qiu

et al. 2022

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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 O 2 M(η 1 -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. O 2 Nd(η 1 -CO) bears little resemblance to the other complexes in bonding characters because of the weak interactions between the NdO 2 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.

Section: Experimental and Theoretical Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 91%

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

Qiu

et al. 2022

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“…The UO 2 disc target with a size of Φ 10 mm × 2 mm was prepared by powder sintering at 1700 K for 6.5 h . The experimental details have been described previously. , A Nd:YAG laser fundamental beam (1064 nm, 10 Hz repetition rate, and 10 ns pulse width) with a typical laser power of 10–20 mJ/pulse was focused on the rotating uranium metal or UO 2 ceramic target. The laser-ablated U atoms or UO 3 molecules were co-deposited with an O 2 /N 2 (>99.99%, Messer Inc.) mixture or N 2 , respectively, in excess argon onto a 9 K KBr window for 40–60 min.…”

Section: Experimental and Theoretical Methodsmentioning

confidence: 99%

Matrix-Isolation Infrared Spectra and Electronic Structure Calculations for Dinitrogen Complexes with Uranium Trioxide Molecules UO₃(η¹-NN)_1–4

Qiu

et al. 2022

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Investigations of the interactions of uranium trioxide (UO 3 ) 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 (N 2 ) complexes with UO 3 moieties UO 3 (η 1 -NN) 1−4 . The complexes are prepared by reactions of laser-ablated uranium atoms with O 2 /N 2 mixtures or laser-ablated UO 3 molecules with N 2 in solid argon. UO 3 (η 1 -NN) 1−4 are classified as "nonclassical" metal−N 2 complexes with increased Δν(N 2 ) 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 UO 3 (η 1 -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 UO 3 moiety and N 2 ligands in UO 3 (η 1 -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 x 2 −y 2 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 UO 3 (η 1 -NN) 1−4 . In addition, the reactions of UO 3 with N 2 ligands and the origination of the nonclassical behavior in UO 3 (η 1 -NN) 1−4 are discussed.

“…For mononuclear metal carbonyls, those side-on bonded species with C–O stretching frequency lower than 1700 cm –1 are rare. , Such complexes have been characterized in several early transition metal (M) carbonyl anions and neutral molecules, such as OM(η 2 -CO) (M = Sc, Ti, V, and Y) with the C–O stretching frequencies in the range of 1500–1650 cm –1 , , WCO – with that at 1640.8 cm –1 . Recently, we have reported a praseodymium oxocarbonyl complex OPr(η 2 -CO) as one intermediate in CO oxidation to CO 2 by praseodymium monoxide molecules . The C–O bond is found to be activated in OPr(η 2 -CO) with a very low C–O stretching band at 1624.5 cm –1 .…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2022

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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.