Understanding the Multiconfigurational Ground and Excited States in Lanthanide Tetrakis Bipyridine Complexes from Experimental and CASSCF Computational Studies

Abstract

The 4f-block metal compounds with 2,2'-bipyridine of empirical composition M( 2 -bipy) 4 , M = La, Ce, and Nd were prepared fifty years ago by Herzog. [1][2] These compounds were prepared as part of a systematic investigation of the reaction of the metal salts with Li 2 (bipy) and bipy in thf. 3 The compounds with M = Eu, Yb were prepared shortly thereafter from the metals dissolved in liquid ammonia and bipyridine. 4 The synthesis of the remainder of the 4fblock metals, except Pm, was published in 2000, adapting the synthetic methodology used by Herzog, i.e. using metal iodides or bis-trimethylsilyl amides as starting materials. 5 The compounds are deeply colored, often described as black, air and moisture sensitive solids that are not volatile. They are insoluble in hydrocarbon and ethereal solvents. The solid-state effective magnetic moments ( eff ) at room temperature for M = La, Ce, and Nd were 1.82  B , 2.85  B and 3.62  B , respectively [1][2] show that the compounds are not based on the metals in their zero oxidation states. The room temperature solution (MeCN) magnetic moments for Eu and Yb of 5.8  B and 2.8  B , respectively, were ascribed to antiferromagnetic coupling between the metal-based spins and the ligand-based spins, the latter being formulated as bipy radical anions. 4 The UV-Vis spectra of M = Tb and Yb clearly show signatures of bipy radical anions but their complexity rendered a detailed interpretation impossible. 6 The solidstate effective moments ( eff ) at room temperature were reported for the entire 4f-block metals (except Pm) along with crystals structures of Sm, Eu, Yb, and Lu. 5,7 The  eff values agree with those reported by Herzog but disagree with those by Feistel 4 (see discussion) and were interpreted according to the Lewis structure [M(III)(bipy) 2-(bipy) •-(bipy) 2 ], except for Eu, which was based on Eu(II). The bipyridine ligands are clearly non-innocent or perhaps more descriptively referred to as redox active 8 but a deeper level of understanding of the electronic structure was not developed.

Catalytic Deoxygenation of Nitroarenes Mediated by High-Valent Molybdenum(VI)–NHC Complexes

“…In both cases, the ground state is found to be an open-shell singlet formed from two Mo­(V) d 1 centers. This open-shell singlet is therefore responsible for a TIP behavior at low temperature, that would account for the low magnetic moment found for the two complexes, as demonstrated by Booth. − Moreover, the first excited state is in both cases, the associated triplet and is very close in energy (0.09 eV for complex 4 and 0.07 eV for complex 5 ). Thus, the triplet state can be populated at room temperature which increases the magnetic moment of the two complexes.…”

Cerium(iv) complexes with guanidinate ligands: intense colors and anomalous electronic structures

“…Given that Ce is in a very similar crystallographic site for each compound, these data indicate that all complexes are formal Ce(IV) compounds with signicant multicongurational character, possibly due to some form of covalence. [51][52][53] The overall f occupancy (n f ) was determined from the relative area of the lower energy XANES peak to the total weight of the doublet peak. Fitting details are included in the SI.…”

Intermediate Valence States in Lanthanide Compounds

“…Recently, the electronic structures of lanthanide tetrakisbipyridine complexes were investigated [158] . Both magnetic susceptibility measurements and L 3 ‐edge XANES data on the Ce and Yb species were consistent with the metals being trivalent.…”