Mixed sandwich imido complexes of Uranium(V) and Uranium(IV): Synthesis, structure and redox behaviour

Abstract

The mixed sandwich U(III) complex {U[η 8-C8H6(1,4-Si(i Pr)3)2](Cp*)(THF)} reacts with the organic azides RN3 (R = SiMe3, 1-Ad, BMes2) to afford the corresponding, structurally characterised U(V) imido complexes {U[η 8-C8H6(1,4-Si(i Pr)3)2](Cp*)(NR)}. In the case of R=SiMe3, the reducing power of the U(III) complex leads to reductive coupling as a parallel minor reaction pathway, forming R-R and the U(IV) azide-bridged complex{[U]}2(µ-N3)2, along with the expected [U]=NR complex. All three [U] =NR complexes show a quasi-reversible one electron reduction between-1.6 to-1.75 V, and for R= SiMe3, chemical reduction using K/Hg affords the anionic U(IV) complex K + {U[η 8-C8H6(1,4-Si(i Pr)3)2](Cp*)=NSiMe3}-. The molecular structure of the latter shows an extended structure in the solid state in which the K counter cations are successively sandwiched between the Cp* ligand of one [U] anion and the COT tips2 ligand of the next.

How the Ancillary Ligand X Drives the Redox Properties of Biscyclopentadienyl Pentavalent Uranium Cp2U(═N–Ar)X Complexes

“…65 Interestingly, the computed shorter UN Ar bond lengths as well as the nearly lined up UN−C Ar atoms are reminiscent of multiple uranium−imido bonds, well in line with the corresponding geometrical parameters observed for the structurally characterized high-valent (U IV , U V , and U VI ) imido species. 7,27,[35][36][37][38][39][40][41]49,59 As expected, the computed Cp-(centroid)−U−Cp(centroid) angles in the U V /U IV redox couple are also well reproduced, with typical values of ca. 125°, agreeing with the familiar pseudo tetrahedral bent sandwich configuration of the Cp 2 U molecular fragment.…”

“…At the dawn of the 21st century, the redox chemistry of organoactinide complexes has experienced a remarkable revival and growth both experimentally and theoretically. − Indeed, in addition to the usual ligands such as chloride, carbocyclic ligands (C 5 R 5 , C 7 H 7 , C 8 H 8 ), and amides, NR 2 , the use of a wider range of functionalized groups have led to high oxidation states actinide compounds (>+3) exploiting the stabilization induced by metal–ligand multiple bonds. , Furthermore, contrarily to the 4f lanthanide electrons, which are essentially core electrons, the 5f actinide electrons are involved in the bonding. The nature of the ligands influences the electrochemical, magnetic, and optical properties of actinide systems. , Moreover, the investigation of new ligands that could provide thermodynamic stabilization to high-valent uranium species, is interesting, first on a fundamental point of view but also to elaborate new separation techniques and storage methods and for the processing of nuclear wastes from the nuclear plants. ,,,, Indeed, uranium complexes are able to access several oxidation states ranging from U II to U VI . , Their redox properties and the availability of valence 5f/6d-orbitals to interact make uranium complexes remarkably suitable for exploring new catalytic reactions especially for small-molecule activation chemistry. − …”

“…Mixed-ligand metallocene complexes of the type (C 5 Me 5 ) 2 U­(X)­(Y) (X = halogen; Y = triflate, alkyl, phenyl, amide or imide, ketimide, alkoxide/aryloxide, phosphide) have played a crucial role in the development of organometallic actinide chemistry, serving as potent starting materials for the preparation of various functionalized uranium complexes. ,,,− Several research groups were pioneers in the determination of the redox potentials for a wide range of actinide-containing complexes. ,,,,− …”

“…During the last two decades, the Kiplinger’s group provided a large number of experimental data (X-ray structures, E 1/2 half-wave redox potentials, spectroscopic and magnetic measurements) for various U IV /U III , U V /U IV , and U VI /U V redox couples of those mixed-ligand complexes. ,,− This availability of experimental data is highly valuable, in particular for pentavalent uranium­(V) species, ,,,,,,− because of their usual instability leading to their facile disproportionation into uranium­(IV) and uranium­(VI) species. Even though electron affinity (EA) plays a major role in many areas of pure chemistry, catalysis, materials science, and environmental chemistry, its direct measurement is difficult but a wide range of half-wave ( E 1/2 ) reduction potentials of uranium complexes have been measured. ,− …”

Breaking a Paradigm: Observation of Magnetic Order in the Purple U(IV) Phosphite: U(HPO3)2

“…Uranium can exhibit a wide range of oxidation states (+2 to +6), with all of the oxidation states except +6 showing paramagnetic behavior. All of the possible oxidation states between +2 and +6 have been observed, − although only the +4 and +6 states are prevalent in extended solid-state materials. The +5 state exists, but is rare, as is the +3 state, which is typically not observed in oxide structures.…”