Marcel Kühling

One ligand fits all: lanthanide and actinide sandwich complexes comprising the 1,4-bis(trimethylsilyl)cyclooctatetraenyl (=COT′′) ligand

The series of anionic lanthanide(III) sandwich complexes of the type [Ln(COT′′)2]− (COT′′ = 1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion) has been largely extended by the synthesis of eight new derivatives ranging from lanthanum to lutetium. The new compounds [Li(DME)3][Ln(COT′′)2] (Ln = Y (1), La (2), Pr (3), Gd (4), Tm (6), Lu (8)) and [Li(THF)4][Ln(COT′′)2] (Ln = Ho (5), Tm (7)) were prepared in good yields following a straightforward synthetic protocol which involves the treatment of LnCl3 with 2 equiv. of in situ-prepared Li2COT′′ in either DME (=1,2-dimethoxyethane) or THF. The neutral actinide sandwich complexes An(COT′′)2 (An = Th (9), U (10)) and An(COT′′′)2 (COT′′′ = 1,3,6-tris(trimethylsilyl)cyclooctatetraenyl dianion; An = Th (11), U (12)) were synthesized in a similar manner, starting from ThCl4 or UCl4, respectively. The COT′′ ligand imparts excellent solubility even in low-polar solvents as well as excellent crystallinity to all new compounds studied. All twelve new f-element sandwich complexes have been structurally authenticated by single-crystal X-ray diffraction. All are nearly perfect sandwich complexes with little deviation from the coplanar arrangement of the substituted COT′′ rings. Surprisingly, all six [Li(DME)3][Ln(COT′′)2] complexes covering the entire range of Ln3+ ionic radii from La3+ to Lu3+ are isostructural (space group P). Compound 10 is the first uranocene derivative for which 13C NMR data are reported.

Investigation of the “bent sandwich-like” divalent lanthanide hydro-tris(pyrazolyl)borates Ln(TpiPr2)2 (Ln = Sm, Eu, Tm, Yb)

The series of homoleptic lanthanide(II) “bent sandwich-like” hydro-tris(pyrazolyl)borate complexes Ln(TpiPr2)2 (Ln = Sm (1), Eu (2), Tm (3), Yb (4); TpiPr2 = hydro-tris(3,5-diisopropylpyrazolyl)borate) has been completed by the synthesis of the hitherto unknown europium and ytterbium derivatives 2 and 4. Both compounds were prepared in high yields by treatment of LnI2(THF)2 (Ln = Eu, Yb) with 2 equiv. of KTpiPr2 in a THF solution. Although the molecules are sterically highly congested, an X-ray diffraction study of bright red 4 revealed a similar bent B–Yb–B arrangement (151.1° and 153.9°, two independent molecules) as in the previously investigated Sm(II) and Tm(II) complexes 1 and 3. An initial reactivity study showed a very different behavior with acetonitrile. While 2 and 4 proved to be unreactive toward acetonitrile, the more strongly reducing Sm(II) complex 1 yielded two new products. The major product was the dark green-black acetonitrile solvate SmII(TpiPr2)2·CH3CN (5), while the second product, the colorless (TpiPr2)SmIII(3,5-iPr2pz)2(NCCH3) (6) with two 3,5-diisopropyl-pyrazolate ligands, resulted from oxidation of samarium to the trivalent state and degradation of a TpiPr2 ligand. Disappointingly, from the most reducing Tm(II) complex 3 only the ligand fragmentation product pyrazabole, [HB(3,5-iPr2pz)2]2 (7), could be isolated and the fate of the Tm containing by-product(s) remains unknown. The new compounds 4–6 were structurally authenticated through single-crystal X-ray diffraction. The europium compound 2 shows an extremely bright yellow emission in solution, which can be observed also at daylight excitation, as well as in the solid state. The high intensity is even remarkable when compared to other Eu(II) containing materials. The photoluminescence was investigated with the conclusion that the rigidity of this complex is responsible for these outstanding luminescence properties.

The First Aziridinylguanidinates: New Precursors for Potentially Volatile Metal Guanidinates

The first lithium-aziridinylamidinates, Li[(C2H4N)C(NR)2]·THF (R = iPr (3), Cy (4); iPr = isopropyl, Cy = cyclohexyl), have been prepared by addition of N-aziridinyllithium, C2H4NLi (2), to either N,N′-diisopropylcarbodiimide or N,N′-dicyclohexylcarbodiimide. The cyclohexyl derivative 4 was crystallised from both diethyl ether (Et2O) and THF to afford the crystalline solvent adducts {Li[(C2H4N)C(NR)2]·S}2 (4: S = THF; 4a: S = Et2O) which were structurally characterised by X-ray diffraction. In the solid state, these lithium-aziridinylamidinates comprise ladder-type dimeric molecular structures.

Bright Photoluminescence of [{(CptBu2)2Ce(μ-Cl)}2]: A Valuable Technique for the Determination of the Oxidation State of Cerium

The synthesis and photoluminescence properties of the bright-yellow organocerium complex [{(CptBu2 )2 Ce(μ-Cl)}2 ] (CptBu2 =1,3-di(tert-butyl)cyclopentadienyl) are presented. This coordination compound exhibits highly efficient photoluminescence within the yellow-light wavelength range, with a high internal quantum yield of 61(±2) % at room temperature. The large red shift is attributed to the delocalizing ability of the aromatic ligands, whilst its quantum yield even makes this compound competitive with Ce3+ -activated LED phosphors in terms of its photoluminescence efficiency (disregarding its thermal stability). A bridging connection between two crystallographically independent Ce3+ ions is anticipated to be the reason for the highly efficient photoluminescence, even up to room temperature. The emission spectrum is characterized by two bands in the orange-light range at both 10 K and room temperature, which are attributed to the parity-allowed transitions 5d1 (2 D3/2 )→4f1 (2 F7/2 ) and 5d1 (2 D3/2 )→4f1 (2 F5/2 ) of Ce3+ , respectively. The photoluminescence spectra were interpreted in relation to the structure and vibrational modes of the coordination compound. The spectra and optical properties indicate that trivalent cerium ions are the dominant species in the ground state, which also resolves an often-encountered ambiguity in organocerium compounds. This result shows that photoluminescence spectroscopy is a versatile tool that can help elucidate the oxidation state of Ce in such compounds.

Formation and structural characterization of a europium(II) mono(scorpionate) complex and a sterically crowded pyraza­bole

Reaction of EuI2(THF)2 with K[HB(3,5-iPr2pz)] (= KTpiPr2, pz = pyrazolyl) afforded the new europium(II) scorpionate complex (KTpiPr2)(3,5-iPr2pzH)2EuIII (1) in addition to the sterically crowded pyrazabole derivative trans-{(3,5-iPr2pz)HB(μ-3,5-iPr2pz)}2 (2) which were both structurally characterized through X-ray diffraction.