Structural, Electrochemical, and Magnetic Studies of Bulky Uranium(III) and Uranium(IV) Metallocenes

Abstract

Addition of the potassium salt of the bulky tetra-(isopropyl)cyclopentadienyl (Cp iPr4 ) ligand to UI 3 (1,4-dioxane) 1.5 results in the formation of the bent metallocene uranium(III) complex (Cp iPr4 ) 2 UI (1), which is then used to obtain the uranium(IV) and uraniumas mononuclear, donor-free complexes, for X − = F − , Cl − , Br − , and I − . Interestingly, reaction of 1 with chloride and cyanide salts of alkali metal ions leads to isolation of the chloride-and cyanide-bridged coordination solids [(Cp iPr4 ) 2 U(μ-Cl) 2 Cs] n (4-Cl) and [(Cp iPr4 ) 2 U(μ-CN) 2 Na(OEt 2 ) 2 ] n (4-CN). Abstraction of the iodide ligand from 1 further enables isolation of the "base-free" metallocenium cation salt [(Cp iPr4 ) 2 U][B(C 6 F 5 ) 4 ] (5) and its DME adduct [(Cp iPr4 ) 2 U(DME)][B(C 6 F 5 ) 4 ] (5-DME). Solid-state structures of all of the compounds, determined by X-ray crystallography, facilitate a detailed analysis of the effect of changing oxidation state or halide ligand on the molecular structure. NMR spectroscopy, X-ray crystallography, cyclic voltammetry, and UV−visible spectroscopy studies of 2-X and 3-X further reveal that the difluoride species in both series exhibit properties that differ significantly from trends observed among the other dihalides, such as a substantial negative shift in the potential of the [(Cp iPr4 ) 2 UX 2 ] uranium(III/IV) redox couple. Magnetic characterization of 1 and 5 reveals that both compounds exhibit slow magnetic relaxation of molecular origin under applied magnetic fields; this process is dominated by a Raman relaxation mechanism.

Isocyanide adducts of tri- and tetravalent uranium metallocenes supported by tetra(isopropyl)cyclopentadienyl ligands

“…The 1 H NMR spectrum of C 6 D 6 solutions of 3 exhibited a sharp (line width = 16 Hz) signal at −12.84 ppm assigned as the tert-butyl group of the isocyanide ligand, while the resonances for the Cp iPr4 ligand were very broad due to dynamic behavior we have discussed previously in related species. 29,31 Oxidation of in situ generated 3 with [Cp 2 Fe][B(C 6 F 5 ) 4 ] in the presence of excess t BuNC enabled isolation of 2 in higher yield (49%, compared to 23% via crude [(Cp iPr4 ) 2 U][B(C 6 F 5 ) 4 ]).…”

“…15,[17][18][19][20][21][22][23][24][25][26][27] Recently, the first base-free uranium(III) metallocenium cations [(Cp iPr5 ) 2 U][B(C 6 F 5 ) 4 ] and [(Cp iPr4 ) 2 U][B(C 6 F 5 ) 4 ] (Cp iPr5 = C 5 ( i Pr) 5 , Cp iPr4 = C 5 ( i Pr) 4 H) were reported. 28,29 To-date, the reactivity of these speices is minimal, amounting to coordination of 1,2-dimethoxyethane (DME) to [(Cp iPr4 ) 2 U][B(C 6 F 5 ) 4 ] to form the Lewis adduct [(Cp iPr4 ) 2 U(DME)][B(C 6 F 5 ) 4 ]. 29 As a part of our continuing studies using the (Cp iPr4 ) 2 U fragment with both uranium(III) and uranium(IV), [29][30][31] we sought to expand the reactivity of the [(Cp iPr4 ) 2 U] + cation.…”

“…28,29 To-date, the reactivity of these speices is minimal, amounting to coordination of 1,2-dimethoxyethane (DME) to [(Cp iPr4 ) 2 U][B(C 6 F 5 ) 4 ] to form the Lewis adduct [(Cp iPr4 ) 2 U(DME)][B(C 6 F 5 ) 4 ]. 29 As a part of our continuing studies using the (Cp iPr4 ) 2 U fragment with both uranium(III) and uranium(IV), [29][30][31] we sought to expand the reactivity of the [(Cp iPr4 ) 2 U] + cation.…”

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

“…At the dawn of the 21st century, the redox chemistry of organoactinide complexes has experienced a remarkable revival and growth both experimentally and theoretically. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] 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. 17,18 Furthermore, contrarily to the 4f lanthanide electrons which are essentially core electrons, 19 the 5f actinide electrons are involved in the bonding.…”

“…[26][27][28][29][30][31] 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. 7,12,16,[32][33][34][35][36][37][38][39][40][41][42][43] Several research groups were pioneers in the determination of the redox potentials for a wide range of actinide-containing complexes. 3,4,34,35,[44][45][46][47][48][49][50][51] 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 couple of those mixed-ligand complexes.…”