Förster energy transfer from poly(arylene–ethynylene)s to an erbium–porphyrin complex

We study the infrared emission at 1.54 μm of an organolanthanide complex, Er(III)-tetraphenylporphyrin [Er(TPP)acac], both as a result of direct optical excitation and via energy transfer from host π-conjugate polymers of type poly(arylene–ethynylene) [PAE]. In the first case, the emission of the neat complex is characterized in inert transparent materials and a value of the quantum yield at 1.54 μm φ_IR=4×10⁻⁴ is measured. Then, fluorescence resonance transfer is investigated in blends of Er(TPP)acac with PAEs by monitoring the quenching of the polymer fluorescence along with the enhancement of both the visible emission of the ligand and the near-infrared band of Er³⁺. These different procedures allow a detailed analysis of the transfer efficiency within a specific implementation of the Förster model for polymeric donors. The experimental values of the critical radius R₀, ranging from 1.3 to 2.5 nm for the different blends, are in good agreement with theory for a wide interval of the physical and spectroscopic parameters. This suggests that other mechanisms for excitation transfer do not play a significant role in these materials.     

Orbifold fibrations of Eschenburg spaces

Most of the few known examples of compact Riemannian manifolds with positive sectional curvature are the total space of a Riemannian submersion. In this article we show that this is true for all known examples, if we enlarge the category to orbifold fibrations. For this purpose we study all almost free isometric circle actions on positively curved Eschenburg spaces, which give rise to principle orbifold bundle structures, and we examine in detail their geometric properties. In particular, we obtain a new family of 6-dimensional orbifolds with positive sectional curvature whose singular locus consists of just two points.      

Methyl displacements from cyclopentadienyl ring planes in sterically crowded (C5Me5)3M complexes

The displacements of the methyl substituents away from the metal and out of the cyclopentadienyl ring plane are compared in sterically crowded (C(5)Me(5))(3)M complexes vs sterically normal f-element complexes in an attempt to evaluate the utility of this parameter in predicting unusual (C(5)Me(5))(1-) ring reactivity. The out-of-plane displacements of 16 sterically crowded tris(cyclopentadienyl) complexes of general formula (C(5)Me(5))(3)M, (C(5)Me(4)R)(3)M (R = Et, (i)Pr, (t)()Bu, SiMe(3)), (C(5)Me(5))(3)MX (X = anion), and (C(5)Me(5))(3)ML (L = neutral ligand) are compared with [(C(5)Me(5))(2)U](2)(C(6)H(6)), (C(5)Me(5))(2)Sm(PC(4)H(2)(t)Bu(2)), and 33 representative examples of f-element bis(cyclopentadienyl) complexes with normal cyclopentadienyl behavior and coordination numbers ranging from 6 to 10. In general, the methyl displacement values of sterically crowded complexes overlap with those in the other complexes, which demonstrates that the basis of the structural distortions is complex. However, if the most extreme out-of-plane displacement in each of the sterically crowded complexes is examined vs the analogous maximum out-of-plane displacement in less crowded systems, there appears to be a basis for predicting cyclopentadienyl reactivity.     

Dinitrogen reduction by TmII,DyII, and NdII with simple amide and aryloxide ligands

Dinitrogen can be reduced to the planar M2(mu-eta2:eta2-N2) structure without employing cyclopentadienyl or complicated polydentate ligands using the recently discovered divalent oxidation states of Tm(II), Dy(II), and Nd(II). Complexes of these ions with common monodentate amide and aryloxide ligands can effect N2 reduction. THF solutions of LnI2 (Ln = Tm, Dy) in the presence of 2 equiv of NaN(SiMe3)2 reduce dinitrogen to form {[(Me3Si)2N]2(THF)Ln}2(mu-eta2:eta2-N2) complexes that have planar Ln2N2 units and 1.264(7) and 1.305(6) A NN bonds consistent with (N2)2- moieties. With the stronger reductant Nd(II), aryloxides are sufficient ancillary ligands: the NdI2/2KOC6H3tBu2-2,6 (KOAr) system forms [(ArO)2(THF)2Nd]2(mu-eta2:eta2-N2), which has a 1.242(7) A NN bond.     

Reactivity of "Eu(OiPr)2" with phenols: formation of linear Eu3, square pyramidal Eu5, cubic Eu8, and capped cubic Eu9 polymetallic europium complexes

The direct reaction of europium with 2-propanol and phenols has been investigated under a variety of conditions. The reaction of europium metal with 2,6-dimethylphenol and 2,6-diisopropylphenol in 2-propanol at reflux revealed that polymetallic europium complexes could be generated by this method. Hx[Eu8O6(OC6H3Me2-2,6)12(OiPr)8], 1, and H5[Eu5O5(OC6H3iPr2-2,6)6(NCCH3)8], 2, were isolated by recrystallization in the presence of hexanes and acetonitrile, respectively, and characterized by X-ray crystallography. Complex 1 has a cubic arrangement of europium ions with face-bridging mu 4-O donor atoms, edge-bridging mu-O(phenoxide/phenol) ligands, and terminal O(isopropoxide/2-propanol) ligands. Complex 2 is mixed valent and has a square pyramidal europium core with four Eu(II) ions at the basal positions and one Eu(III) ion at the apex. Since these reactions gave complicated mixtures of products from which 1 and 2 could only be obtained in low yields, direct reactions under less forcing reaction conditions were investigated. Europium reacts slowly at room temperature to form arene-soluble divalent [Eu(OiPr)2(THF)x]n, 3. Complex 3 reacts with 2,6-dimethylphenol to form the arene-insoluble complex (H[Eu(OC6H3Me2)2(OiPr)])n, 4. Recrystallization of 4 in the presence of THF results in the crystallographically characterizable divalent trimetallic complex [Eu(OC6H3Me2-2,6)2(THF)2]3, 5, which has an unusual linear metal geometry. In the presence of HOiPr at ambient conditions in the glovebox, crystals of 5 slowly convert to the mixed valent H10[Eu8O8(OC6H3Me2-2,6)10(OiPr)2(THF)6], 6, which was found to have a cubic arrangement of europium atoms similar to 1 by X-ray crystallography. Complex 4, upon heating under vacuum, followed by reaction with THF, forms the arene-soluble divalent complex H18([Eu9O8(OC6H3Me2-2,6)10(THF)7][Eu9O9(OC6H3Me2-2,6)10(THF)6]), 7, which contains two types of capped cubic arrangements of europium ions in the solid state.     

Facile syntheses of unsolvated UI3 and tetramethylcyclopentadienyl uranium halides

In the course of comparing the reaction chemistry of (C5Me5)3U, 1, and its slightly less crowded analogue (C5Me4H)3U, 2, new syntheses of UI3, (C5Me4H)3U, (C5Me4H)3UCl, 3, and (C5Me5)3UCl, 4, have been developed. Additionally, (C5Me4H)3UI, 5, and (C5Me4H)2UCl2, 6, have been identified for the first time. A facile synthesis of unsolvated UI3 is reported that proceeds in high yield with inexpensive equipment from iodine and hot uranium turnings. Both UI3 and UI3(THF)4 react with KC5Me4H in toluene to make unsolvated (C5Me4H)3U in higher yield than in previous reports that involve reduction of tetravalent (C5Me4H)3UCl, 3. A more atom-efficient synthesis of complex 3 is also reported that proceeds from reduction of t-BuCl, PhCl, or HgCl2 by 2. Similarly, (C5Me4H)3U reacts with PhI or HgI2 to generate (C5Me4H)3UI. These studies also provided a basis to improve the synthesis of (C5Me5)3UCl from 1 by employing t-BuCl or HgCl2 as the halide source. Like (C5Me5)3UCl, the (C5Me4H)3UCl complex reacts with HgCl2 to form (C5Me4H)2 and (C5Me4H)2UCl2, 6, but unlike (C5Me5)3UX (X = Cl or I), the less substituted (C5Me4H)3UX complexes do not reduce t-BuCl or PhX. The synthesis of 6 from (C5Me4H)MgCl x THF and UCl4 is also included.     

Synthesis of arene-soluble mixed-metal Zr/Ce,Zr/Y,and related [[Zr2(OiPr)9]LnX2]n complexes using the dizirconium nonaisopropoxide ligand

The utility of polydentate monoanionic [Zr2(OiPr)9]- in generating arene-soluble, unsolvated, mixed-metal Zr/Ce and Zr/Y complexes is described. The synthesis of other mixed-metal zirconium lanthanide complexes was also studied to explore the relationship of metal size to structure. Lanthanide trihalides react in THF with KZr2(OiPr)9 to form unsolvated dimers, [[Zr2(OiPr)9]LnCl2]2, with the larger metals, Ln = Ce (1), Ho (2), Y (3), and unsolvated monomers, [Zr2(OiPr)9]LnCl2, with the smaller elements, Ln = Er (4), Yb (5). The synthesis of a monomeric iodide analogue, [Zr2(OiPr)9]TmI2, 6, by reduction of Zr2(OiPr)8(iPrOH)2 with TmI2(DME)3 is also reported. In all of these complexes, the [Zr2(OiPr)9]- subunit is tetradentate. 1-6 are compared with related cyclopentadienyl halide complexes to evaluate the special features of the dizirconium nonaisopropoxide ligand versus cyclopentadienide.