Heterobi- and Trimetallic Cerium(IV) tert-Butoxides with Mono-, Di-, and Trivalent Metals (M = K(I), Ge(II), Sn(II), Pb(II), Al(III), Fe(III))

Neptunium Alkoxide Chemistry: Expanding Alkoxides to the Transuranium Elements

Two oxo-containing neptunium(IV) tert-butoxides, [Np₃O(O^tBu)₁₀] (1) and [K₄Np₂O(O^tBu)₁₀] (2), were synthesized using the ligand substitution between neptunium(IV) silylamides and HO^tBu, whereas the salt metathesis between [NpCl₄(DME)₂] (DME = dimethoxyethane) and various amounts of LiO^tBu resulted in the formation of oxo-free alkoxides [Np(O^tBu)₄(py)₂] (3; py = pyridine) and [Li(THF)]₂[Np(O^tBu)₆] (4; THF = tetrahydrofuran). These complexes are the first structurally characterized neptunium(IV) alkoxides using single-crystal X-ray diffraction and solid-state absorption spectroscopy, which provide data for the development of anhydrous metal-organic neptunium chemistry.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

Hydrostannylation of carbon dioxide by a hydridostannylene molybdenum complex

Reaction of the aryltin(II) hydrides {Ar^iPr₄Sn(μ-H)}₂ or {Ar^iPr₆Sn(μ-H)}₂ (Ar^iPr₄ = -C₆H₃-2,6-(C₆H₃-2,6-ⁱPr₂)₂, Ar^iPr₆ = -C₆H₃-2,6-(C₆H₂-2,4,6-ⁱPr₃)₂) with two equivalents of the molybdenum carbonyl [Mo(CO)₅(THF)] afforded the divalent tin hydride transition metal complexes, Mo(CO)₅{Sn(Ar^iPr₆)H}, (1), or Mo(CO)₅{Sn(Ar^iPr₄)(THF)H} (2), respectively. Complex 1 effects the facile hydrostannylation of carbon dioxide, to yield Mo(CO)₅{Sn(Ar^iPr₆)(κ²-O,O'-O₂CH)}, (3), which features a bidentate formate ligand coordinating the tin atom. Reaction of 3 with the pinacolborane, HBpin (pin = pinacolato) in benzene regenerated 1 in quantitative yield. All complexes were characterized by X-ray crystallography, as well as UV-visible, IR, and multinuclear NMR spectroscopies. The isolation of 1 and 2 is consistent with the existence of monomeric forms of {Ar^iPr₄Sn(μ-H)}₂ and {Ar^iPr₆Sn(μ-H)}₂ in solution. Regeneration of 1 from 3via reaction with pinacolborane as the hydrogen source shows the catalytic potential of 1 in the hydrogenation of CO₂.

Molecular Level Synthesis of InFeO3 and InFeO3/Fe2O3 Nanocomposites

New heterometallic In-Fe alkoxides [InFe(O^tBu)₄(PyTFP)₂] (1), [InFe₂(O^neoPen)₉(Py)] (2), and [InFe₃(O^neoPen)₁₂] (3) were synthesized and structurally characterized. The arrangement of metal centers in mixed-metal framework was governed by the In:Fe ratio and the coordination preferences of Fe(III) and In(III) centers to be in tetrahedral and octahedral environments, respectively. 3 displayed a star-shaped so-called "Mitsubishi" motif with the central In atom coordinated with three tetrahedral {Fe(O^neoPen)₄}^- anionic units. The deterministic structural influence of the larger In atom was evident in 1 and 2 which displayed the coordination of neutral coligands to achieve the desired coordination number. Thermal decomposition studies of compounds 1-3 under inert conditions with subsequent powder diffraction studies revealed the formation of Fe₂O₃ and In₂O₃ in the case of 3 and 2, whereas 1 intriguingly produced elemental In and Fe. In contrary, the thermal decomposition of 1-3 under ambient conditions produced a ternary oxide, InFeO₃, with additional Fe₂O₃ present as a secondary phase in a different stoichiometric ratio predetermined through the In:Fe ratio in 2 and 3. The intimate mixing of different phases in InFeO₃/Fe₂O₃ nanocomposites was confirmed by transmission electron microscopy of solid residues obtained after the decomposition of 1 and 2. The pure InFeO₃ particles demonstrated ferromagnetic anomalies around 170 K as determined by temperature-dependent field-cooled and zero-field-cooled magnetization experiments. A first-order magnetic transition with an increase in the ZFC measurements was explained by temperature-induced reduction of the Fe-Fe distance and the corresponding increase in superexchange.

Lanthanoid Complexes as Molecular Materials: The Redox Approach

The development of molecular materials with novel functionality offers promise for technological innovation. Switchable molecules that incorporate redox-active components are enticing candidate compounds due to their potential for electronic manipulation. Lanthanoid metals are most prevalent in their trivalent state and usually redox-activity in lanthanoid complexes is restricted to the ligand. The unique electronic and physical properties of lanthanoid ions have been exploited for various applications, including in magnetic and luminescent materials as well as in catalysis. Lanthanoid complexes are also promising for applications reliant on switchability, where the physical properties can be modulated by varying the oxidation state of a coordinated ligand. Lanthanoid-based redox activity is also possible, encompassing both divalent and tetravalent metal oxidation states. Thus, utilization of redox-active lanthanoid metals offers an attractive opportunity to further expand the capabilities of molecular materials. This review surveys both ligand and lanthanoid centered redox-activity in pre-existing molecular systems, including tuning of lanthanoid magnetic and photophysical properties by modulating the redox states of coordinated ligands. Ultimately the combination of redox-activity at both ligands and metal centers in the same molecule can afford novel electronic structures and physical properties, including multiconfigurational electronic states and valence tautomerism. Further targeted exploration of these features is clearly warranted, both to enhance understanding of the underlying fundamental chemistry, and for the generation of a potentially important new class of molecular material.

Synthesis of a terminal Ce(iv) oxo complex by photolysis of a Ce(iii) nitrate complex

Reaction of [Ce(NR2)3] (R = SiMe3) with LiNO3 in THF, in the presence of 2,2,2-cryptand, results in the formation of the Ce(iii) "ate" complex, [Li(2,2,2-cryptand)][Ce(κ2-O2NO)(NR2)3] (1) in 38% yield. Photolysis of 1 at 380 nm affords [Li(2,2,2-cryptand)][Ce(O)(NR2)3] (2), in 33% isolated yield after reaction work-up. Complex 2 is the first reported example of a Ce(iv) oxo complex where the oxo ligand is not supported by hydrogen bonding or alkali metal coordination. Also formed during photolysis are [Li(2,2,2-cryptand)]2[(μ3-O){Ce(μ-O)(NR2)2}3] (3) and [Li(2,2,2-cryptand)][Ce(OSiMe3)(NR2)3] (4). Their identities were confirmed by X-ray crystallography. Complex 4 can also be prepared via reaction of [Ce(NR2)3] with LiOSiMe3 in THF, in the presence of 2,2,2-cryptand. When synthesized in this fashion, 4 can be isolated in 47% yield. To rationalize the presence of 2, 3, and 4 in the reaction mixture, we propose that photolysis of 1 first generates 2 and NO2, via homolytic cleavage of the N-O bond in its nitrate co-ligand. Complex 2 then undergoes decomposition via two separate routes: (1) ligand scrambling and oligomerization to form 3; and, (2) abstraction of a trimethylsilyl cation to form a transient Ce(iv) silyloxide, [CeIV(OSiMe3)(NR2)3], followed by 1e- reduction to form 4. Alternatively, complex 4 could form directly via ·SiMe3 abstraction by 2.

Ceric Cyclopentadienides Bearing Alkoxy, Aryloxy, Chlorido, or Iodido Co-Ligands

A series of heteroleptic tris(cyclopentadienyl) Ce^IV complexes has been isolated and crystallographically characterized, revealing the broad accessibility of such organocerium(IV) compounds. The oxidation of complexes Cp^Me₃ Ce(thf) and Cp'₃ Ce(thf) (Cp^Me =C₅ H₄ Me, Cp'=C₅ H₄ SiMe₃ ), bearing monosubstituted electron-poor cyclopentadienyl ligands, with 0.5 equivalents of 1,4-benzoquinone or C₂ Cl₆ gave the cerium(IV) hydroquinolate complexes Cp^Me₃ Ce(OC₆ H₄ O)CeCp^Me₃ and Cp'₃ Ce(OC₆ H₄ O)CeCp'₃ , or the chloride complexes Cp^Me₃ CeCl and Cp'₃ CeCl, respectively; the iodide complex Cp'₃ CeI was obtained from the reaction of Cp'₃ Ce(thf) with elemental iodine. The behavior of Cp'₃ CeCl in salt metathesis protocols employing alkali metal amides or alkyl, alkoxide, and aryloxide reagents was investigated, which gave rise to the robust and isolable cerium(IV) alkoxide Cp'₃ Ce(OtBu). Trivalent [Cp'₂ CeCl]₂ was synthesized by AlMe₄ →Cl exchange utilizing Cp'₂ Ce(AlMe₄ ) and AlMe₂ Cl. The reactivity of [Cp'₂ CeCl]₂ towards the oxidants Ph₃ CCl, C₂ Cl₆ , 1,4-benzoquinone, and I₂ has been assessed, and has provided useful information on Ce^III /Ce^IV redox deactivation pathways. In addition to X-ray structure analysis, all the complexes were characterized by NMR, DRIFT (diffuse reflectance IR Fourier transform), and UV/Vis spectroscopy as well as elemental analysis. The tetravalent compounds were further analyzed for their magnetic susceptibility by using Evans' method.

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.

Cerium tetrakis(diisopropylamide)--a useful precursor for cerium(IV) chemistry

Homoleptic cerium(IV) diisopropylamide was synthesized via oxidation of ate complex Ce(NiPr2)4Li(thf) with trityl chloride or hexachloroethane. Due to its ready accessibility, and high pKa value of the proligand HNiPr2, Ce(NiPr2)4 features a promising candidate for protonolysis reactions, as shown for the synthesis of dimeric cerium(IV) tetrakis(1,1,3,3-N,N,N',N'-tetramethylguanidinate).

Bidentate nitroxide ligands stable toward oxidative redox cycling and their complexes with cerium and lanthanum

A redox non-innocent bidentate nitroxide ligand stabilizes a tetravalent cerium cation and subsequently an electron–hole upon the oxidation on the electrochemical time scale.

All-metallagermoxane with an adamantanoid cage structure: [(Cp*Ru(CO)2Ge)4(μ-O)6] (Cp* = η5-C5Me5)

The first tetrametallagermoxane [(Cp*Ru(CO)₂Ge)₄(μ-O)₆], with an adamantanoid cage structure, has been synthesized and structurally characterized with an exo-{Cp*Ru(CO)₂} fragment in each germanium atom.