Pentavalent Lanthanide Compounds: Formation and Characterization of Praseodymium(V) Oxides

Abstract

The chemistry of lanthanides (Ln = La-Lu) is dominated by the low-valent + 3or+ 2oxidation state because of the chemical inertness of the valence 4f electrons.T he highest knownoxidation state of the whole lanthanide series is + 4f or Ce,Pr, Nd, Tb,and Dy.W ereport the formation of the lanthanide oxide species PrO 4 and PrO 2 + complexes in the gas phase and in as olid noble-gas matrix. Combined infrared spectroscopic and advanced quantum chemistry studies show that these species have the unprecedented Pr V oxidation state, thus demonstrating that the pentavalent state is viable for lanthanide elements in as uitable coordination environment.Oxidation state (OS) has become ac entral concept of chemistry and astaple of chemistry textbooks.[1] Thequestion of just how high an OS of an element can be reached has long piqued chemists interest.[2] Thed iscovery of novel species with high OS helps to expand chemical understanding of the behavior of elements and their compounds.U ntil fairly recently,t he highest observed OS had been + 8, which occurs only in afew compounds including RuO 4 ,OsO 4 ,IrO 4 , and XeO 4 . [2][3][4][5] Recently,t he IrO 4 + cation was successfully formed in the gas phase,a nd was identified to contain the highest reported Ir IX oxidation state. [6,7] Forthe light elements in each row of the main group,t ransition metals,a nd actinides,the highest OS often equals the number of available valence electrons (e.g. + 3to+ 6f or Al-S,Sc-Cr,and Ac-U).However,i nl anthanides (Ln) the 4f orbitals are usually extremely contracted in radial distribution and considerably lower in energy because they penetrate the [Xe] core,t hus hardly participating in bonding.The chemistry of lanthanides is thus generally dominated by the low-valent + 3o r+ 2 oxidation state despite the fact that most lanthanides have available valence 4f electrons. [8][9][10] Besides the omnipresent oxidation state + 3, the higher oxidation state Ln IV is rather common for Ce,and is encountered in Pr, Nd, Tb,and Dy as well, [10] as these lanthanides have the lowest fourth ionization energies and also the lowest + 4/ + 3r eduction potentials. [11] Thec hemistry of pentavalent lanthanides has not been explored to date.[10] Aprevious study presumed the presence of pentavalent praseodymium in PrO 3 À without bonding analysis.[12] However,recent density functional theory (DFT) and ab initio multiconfigurational wavefunction theory (WFT) calculations show that the Pr center in the PrO 3 À anion is in oxidation state + 4r ather than the anticipated + 5.[13] Herein, we report ac ombined experimental and theoretical study on the lanthanide oxide species PrO 4 in solid noble-gas matrix as well as the PrO 2 + ion in the gas phase and in solid noble-gas matrix. Combined gas-phase infrared photodissociation spectroscopy,m atrix-isolation infrared absorption spectroscopy,a nd high-level quantum chemistry studies reveal that these species have the Pr V oxidation state. Thepraseodymium oxide cation species were generated in the gas phase by usi...

Heptavalent Actinide Tetroxides NpO4– and PuO4–: Oxidation of Pu(V) to Pu(VII) by Adding an Electron to PuO4

“…Recent examples of the ability to identify the intrinsic stabilities of metal oxidation states in elementary gas-phase species include the discovery of the nonavalent oxidation state in Ir(IX)O 4 + , 19 and the first pentavalent lanthanides in Pr(V)O 2 + and NPrO. 20,21 Neither of the extreme IX oxidation state in the periodic table, nor the V oxidation state in the lanthanide series have yet been achieved in condensed phase.…”

The High‐Pressure Oxide Tb3O5 and its Non‐Centrosymmetric Low‐Temperature Polymorph–A Comprehensive Study

“…[45] Investigations of the electronic structure of the isostructuralc ompounds U 3 S 5 and U 3 Se 5 based on the ionic radiia nd XPS measurements showedt he same mixed valency for the uranium ions. The formula of thesec ompounds can therefore be written as U 3 + U 3 + U 4 + (S 2À ) 5 and U 3 + U 3 + U 4 + (Se 2À ) 5 ,r espectively. [8a, 9b,c, 20a] Furthermore,acomparison to ordered substitution variants of the U 3 S 5 structure type like Y 2 HfS 5 [23] is very interesting.…”

“…Higher oxides in the compositional range RE O x (1.5≤ x ≤2) are found for Ce, Pr, and Tb, due to their possible tetravalent oxidation state. Recently, even an oxidation state of +5 has been reported for praseodymium in PrO 4 and the PrO 2 + ion in a solid noble‐gas matrix and the gas phase, respectively . The dioxides RE O 2 ( RE =Ce, Pr, Tb) form under increased temperature and oxygen pressure, namely 573 K and around 1000 atm for TbO 2 , or through disproportionation from the oxides obtained by decomposition of precursors with O/ RE ratios between 1.5 and 2.…”

“…Recently,e ven an oxidation state of + 5h as been reported for praseodymium in PrO 4 and the PrO 2 + ion in as olid noble-gas matrix and the gas phase, respectively. [5] The dioxides REO 2 (RE = Ce, Pr,T b) form under increased temperature and oxygen pressure, namely 573 Ka nd around1 000 atm for TbO 2 ,o rt hrough disproportionation from the oxideso btained by decomposition of precursors with O/RE ratios between 1.5 and 2. All knownr are earth dioxides crystallize in the cubic fluorite structure (Fm3 m)w ith, for example, a = 522 pm for TbO 2 .…”

The duality of electron localization and covalency in lanthanide and actinide metallocenes

“…[21][22][23][24] Taken together, these studies have raised more basic questions regarding whether the concept of a multicongurational ground state should apply to all f-element compounds, how it equates to more established models of metal-ligand covalency, 25 and how it is manifested by changes in chemical reactivity or magnetic behavior. Although Ce(C 8 H 8 ) 2 is prototypical, such fundamental questions pertain to all formal Ce 4+ compounds [26][27][28][29][30][31][32][33][34][35] and high valent lanthanide compounds, [36][37][38][39][40][41] ytterbium complexes, [42][43][44][45][46][47][48] and the transuranic actinocenes 15,17,49 and have implications throughout the periodic table for the electronic structure models used to describe bonding near the limits of chemically accessible oxidation states. 50,51 Improved theoretical models of bonding in these systems are needed to develop new ligands and innovative concepts in lanthanide/actinide separations for nuclear energy.…”