Cerium(IV) Imido Complexes: Structural, Computational, and Reactivity Studies.

A series of alkali metal capped cerium(IV) imido complexes, [M(solv)x][Ce═N(3,5-(CF3)2C6H3)(TriNOx)] (M = Li, K, Rb, Cs; solv = TMEDA, THF, Et2O, or DME), was isolated and fully characterized. An X-ray structural investigation of the cerium imido complexes demonstrated the impact of the alkali metal counterions on the geometry of the [Ce═N(3,5-(CF3)2C6H3)(TriNOx)]- moiety. Substantial shortening of the Ce═N bond was observed with increasing size of the alkali metal cation. The first complex featuring an unsupported, terminal multiple bond between a Ce(IV) ion and a ligand fragment was also isolated by encapsulation of a Cs+ counterion with 2.2.2-cryptand. This complex shows the shortest recorded Ce═N bond length of 2.077(3) Å. Computational investigation of the cerium imido complexes using DFT methods showed a relatively larger contribution of the cerium 5d orbital than the 4f orbital to the Ce═N bonds. The [K(DME)2][Ce═N(3,5-(CF3)2C6H3)(TriNOx)] complex cleaves the Si-O bond in (Me3Si)2O, yielding the [(Me3SiO)CeIV(TriNOx)] adduct. The reaction of the rubidium capped imido complex with benzophenone resulted in the formation of a rare Ce(IV)-oxo complex, that was stabilized by a supramolecular, tetrameric oligomerization of the Ce═O units with rubidium cations.

An Alkali Metal-Capped Cerium(IV) Imido Complex.

Structurally authenticated, terminal lanthanide-ligand multiple bonds are rare and expected to be highly reactive. Even capped with an alkali metal cation, poor orbital energy matching and overlap of metal and ligand valence orbitals should result in strong charge polarization within such bonds. We expand on a new strategy for isolating terminal lanthanide-ligand multiple bonds using cerium(IV) complexes. In the current case, our tailored tris(hydroxylaminato) ligand framework, TriNOx(3-), provides steric protection against ligand scrambling and metal complex oligomerization and electronic protection against reduction. This strategy culminates in isolation of the first formal Ce═N bonded moiety in the complex [K(DME)2][Ce═N(3,5-(CF3)2C6H3)(TriNOx)], whose Ce═N bond is the shortest known at 2.119(3) Å.

Density Functional Theory as a Predictive Tool for Cerium Redox Properties in Nonaqueous Solvents.

Two methods to correlate and predict experimental redox potentials for cerium complexes were evaluated. Seventeen previously reported cerium complexes were computed using DFT methods in both the CeIII and CeIV oxidation states with a dichloromethane solvent continuum. In the first computational approach, the ΔGo(CeIV/CeIII) was determined for each of the compounds and these values were correlated with the experimental E1/2 values measured in dichloromethane, referenced to the ferrocene/ferrocenium couple. The second method involved correlating the energies of the CeIV LUMOs (lowest unoccupied molecular orbitals) with the experimental redox potentials, E1/2. The predictive capabilities of these two correlative methods were tested using a new cerium hydroxylamine complex, Ce(ODiNOx)2 (ODiNOx = bis(2-tert-butylhydroxylaminatobenzyl) ether). All 18 complexes studied in this paper were combined with the 15 complexes determined in acetonitrile from a previously published correlation by our group. These sets of data allowed us to develop two methods for predicting the redox potential of cerium complexes regardless of the solvent for the experimental measurement.

Spectroscopic and Structural Elucidation of Uranium Dioxophenoxazine Complexes.

Uranium derivatives of a redox-active, dioxophenoxazine ligand, (DOPO(q))2UO2, (DOPO(sq))UI2(THF)2, (DOPO(cat))UI(THF)2, and Cp*U(DOPO(cat))(THF)2 (DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate), have been synthesized from U(VI) and U(III) starting materials. Full characterization of these species show uranium complexes bearing ligands in three different oxidation states. The electronic structures of these complexes have been explored using (1)H NMR and electronic absorption spectroscopies, and where possible, X-ray crystallography and SQUID magnetometry.

Why is uranyl formohydroxamate red?

The complexation of UO2(2+) by formohydroxamate (FHA(-)) creates solutions with dark red coloration. The inherent redox activity of formohydroxamate leads to the possibility that these solutions contain U(V) complexes, which are often red. We demonstrate that the reaction of U(VI) with formohydroxamate does not result in reduction, but rather in formation of the putative cis-aquo UO2(FHA)2(H2O)2, whose polymeric solid-state structure, UO2(FHA)2, contains an unusually bent UO2(2+) unit and a highly distorted coordination environment around a U(VI) cation in general. The bending of the uranyl cation results from unusually strong π donation from the FHA(-) ligands into the 6d and 5f orbitals of the U(VI) cation. The alteration of the bonding in the uranyl unit drastically changes its electronic and vibrational features.

Substituted quinoline quinones as surrogates for the PQQ cofactor: an electrochemical and computational study.

Pyrroloquinoline quinones (PQQ) are important cofactors that shuttle redox equivalents in diverse metalloproteins. Quinoline 7,8-quinones have been synthesized and characterized as surrogates for PQQ to elucidate redox energetics within metalloenzyme active sites. The quinoline 7,8-quinones were accessed using polymer-supported iodoxybenzoic acid and the compounds evaluated using solution electrochemistry. Together with a family of quinones, the products were evaluated computationally and used to generate a predictive correlation between a computed ΔG and the experimental reduction potentials.

Control of cerium oxidation state through metal complex secondary structures.

A series of alkali metal cerium diphenylhydrazido complexes, M x (py) y [Ce(PhNNPh)4], M = Li, Na, and K, x = 4 (Li and Na) or 5 (K), and y = 4 (Li), 8 (Na), or 7 (K), were synthesized to probe how a secondary coordination sphere would modulate electronic structures at a cerium cation. The resulting electronic structures of the heterobimetallic cerium diphenylhydrazido complexes were found to be strongly dependent on the identity of the alkali metal cations. When M = Li+ or Na+, the cerium(iii) starting material was oxidized with concomitant reduction of 1,2-diphenylhydrazine to aniline. Reduction of 1,2-diphenylhydrazine was not observed when M = K+, and the complex remained in the cerium(iii) oxidation state. Oxidation of the cerium(iii) diphenylhydrazido complex to the Ce(iv) diphenylhydrazido one was achieved through a simple cation exchange reaction of the alkali metals. UV-Vis spectroscopy, FTIR spectroscopy, electrochemistry, magnetic susceptibility, and DFT studies were used to probe the oxidation state and the electronic changes that occurred at the metal centre.

A homoleptic η² hydroxylaminato Ce(IV) complex with S4 symmetry.

An electron rich, air-stable hydroxylamine ligand and a strongly stabilized cerium(IV) hydroxylaminato complex are described. The synthesis of Ce[η(2)-ON((t)Bu)(2-OMe-5-(t)Bu-C6H3)]4 (1) proceeded through a one pot protonolysis and oxidation procedure. Complex 1 crystallized with the molecule in exact S4 symmetry. The hydroxylaminato ligands were bound in an η(2) mode that is of interest for potential application in the separations chemistry of rare earth metals.

Variation of electronic transitions and reduction potentials of cerium(IV) complexes.

The trivalent compound K[Ce[N(SiHMe2)2]4] was synthesized and oxidized, providing a convenient route to the reported cerium(IV) compound Ce[N(SiHMe2)2]4. Protonolysis reactions of Ce[N(SiHMe2)2]4 with tert-butanol, substituted benzyl alcohols, and 2,6-diphenylphenol yielded the neutral tetravalent compounds Ce(O(t)Bu)4(py)2, Ce2(OCH2C6R5)8(thf)2 (R = Me, F), and Ce(Odpp)4 (dpp = 2,6-(C6H5)2-C6H3). Spectroscopic and electrochemical characterization of the monometallic cerium(IV) silylamide, alkoxide, and aryloxide compounds revealed variable ligand-to-metal charge transfer transitions and metal-based reduction potentials. Computational bonding analyses were performed to complement the physical characterization of the complexes.