Cation-controlled self-assembly of a hexameric europium wheel

The simple asymmetric tetradentate ligand 2,2':6',2' '-terpyridine-6-carboxylic acid leads to the self-assembly of the first europium nanowheel containing europium ions in two different coordination environments. Moreover the self-assembly of bis(terpyridinecarboxylate) europium species to form a hexameric wheel capable of strongly binding LnIII cations is controlled by the ligand/cation ratio.     

A new class of C1-symmetric monosulfoximine ligands for enantioselective hetero Diels-Alder reactions

Quinoline-based C1-symmetric sulfoximines have been used as chiral ligands in copper-catalyzed asymmetric hetero Diels-Alder reactions leading to cycloadducts with up to 96% ee.     

A modular nanoparticle-based system for reagentless small molecule biosensing

Metalloprotein tethered CdSe nanoparticles have been generated to provide selective and reagentless maltose biosensing. As opposed to cell or protein detection by semiconducting nanoparticle bioconjugates, a modular method for small-molecule detection using semiconducting nanoparticle bioconjugates has been difficult. Here we report a method for reagentless protein-based semiconducting nanoparticle biosensors. This method uses Ru(II) complex-CdSe nanoparticle interactions and the maltose-induced conformation changes of maltose binding protein to alter the CdSe nanoparticle fluorescence emission intensity. In this proof-of-principle system, the maltose-induced protein conformation changes alter the Ru(II) complex-CdSe nanoparticle interaction, which increases the CdSe emission intensity. Altered CdSe emission intensity effects are best described as electron transfer from the Ru(II) complex to the CdSe excited state forming the nonfluorescent CdSe anion. Four surface-cysteine, Ru(II) complex-attached maltose-binding proteins have been studied for maltose dependent alteration of CdSe emission intensities. With 3.0-3.5 nm diameter CdSe nanoparticles, all ruthenated maltose-binding proteins display similar maltose-dependent increases (1.4-fold) in CdSe emission intensity and maltose binding affinities (KA = 3 x 106 M-1). For these four systems, the only difference was the sample-to-sample variation in maltose-dependent responses. Thus, very few surface cysteine mutations need to be examined to find a successful biosensor, as opposed to analogous systems using organic fluorophores. This strategy generates a unimolecular, or reagentless, semiconducting nanoparticle biosensor for maltose, which could be applied to other proteins with ligand-dependent conformation changes.     

The effect of pyridinecarboxylate chelating groups on the stability and electronic relaxation of gadolinium complexes

The ligand N,N'-bis[(6-carboxy-2-pyridylmethyl]ethylenediamine-N,N'-diacetic acid (H(4)bpeda) was synthesised using an improved procedure which requires a reduced number of steps and leads to a higher yield with respect to the published procedure. It was obtained in three steps from diethylpyridine-2,6-dicarboxylate and commercially available ethylenediamine-N,N[prime or minute]-diacetic acid with a total yield of approximately 20%. The crystal structure of the hexa-protonated form of the ligand which was determined by X-ray diffraction shows that the four carboxylates and the two amines are protonated. The crystal structure of the polynuclear complex [Gd(bpeda)(H(2)O)(2)](3)[Gd(H(2)O)(6)](2)Cl(3)(2), isolated by slow evaporation of a 1:1 mixture of GdCl(3) and H(4)bpeda at pH approximately 1, was determined by X-ray diffraction. In complex three [Gd(bpeda)(H(2)O)(2)] units, containing a Gd(III) ion ten-coordinated by the octadentate bpeda and two water molecules, are connected in a pentametallic structure by two hexa-aquo Gd(3+) cations through four carboxylato bridges. The protonation constants (pK(a1)= 2.9(1), pK(a2)= 3.5(1), pK(a3)= 5.2(2), and pK(a4)= 8.5(1)) and the stability constants of the complexes formed between Gd(III) and Ca(II) ions and H(4)bpeda (log beta(GdL)= 15.1(3); log beta(CaL)= 9.4(1)) were determined by potentiometric titration. The unexpected decrease in the stability of the gadolinium complex and of the calcium complex of the octadentate ligand bpeda(4-) with respect to the hexadentate ligand edta(4-) has been interpreted in terms of an overall lower contribution to stability of the metal-nitrogen interactions. The EPR spectra display very broad lines (apparent DeltaH(pp) approximately 800-1200 G at X-band and 90-110 G at Q-band depending on the temperature), indicating a rapid transverse electron spin relaxation. At X-band, Gd(bpeda) is among the fastest relaxing Gd(3+) complexes to date suggesting that the presence of pyridinecarboxylate chelating groups in itself does not lead to slow electron relaxation.     

Single metal four-electron reduction by U(ii) and masked “U(ii)” compounds

The redox chemistry of uranium is dominated by single electron transfer reactions while single metal four-electron transfers remain unknown in f-element chemistry. Here we show that the oxo bridged diuranium(iii) complex [K(2.2.2-cryptand)]₂[{((Me₃Si)₂N)₃U}₂(μ-O)], 1, effects the two-electron reduction of diphenylacetylene and the four-electron reduction of azobenzene through a masked U(ii) intermediate affording a stable metallacyclopropene complex of uranium(iv), [K(2.2.2-cryptand)][U(η²-C₂Ph₂){N(SiMe₃)₂}₃], 3, and a bis(imido)uranium(vi) complex [K(2.2.2-cryptand)][U(NPh)₂{N(SiMe₃)₂}₃], 4, respectively. The same reactivity is observed for the previously reported U(ii) complex [K(2.2.2-cryptand)][U{N(SiMe₃)₂}₃], 2. Computational studies indicate that the four-electron reduction of azobenzene occurs at a single U(ii) centre via two consecutive two-electron transfers and involves the formation of a U(iv) hydrazide intermediate. The isolation of the cis-hydrazide intermediate [K(2.2.2-cryptand)][U(N₂Ph₂){N(SiMe₃)₂}₃], 5, corroborated the mechanism proposed for the formation of the U(vi) bis(imido) complex. The reduction of azobenzene by U(ii) provided the first example of a “clear-cut” single metal four-electron transfer in f-element chemistry.

Both a masked and the actual complex [U(ii){N(SiMe₃)₂}₃]⁺ effect the reduction of azobenzene to yield a U(vi) bis-imido species providing the first example of a “clear-cut” metal centred four-electron reduction in f-element chemistry.     

Oxidation chemistry of uranium(III) complexes of Tpa: synthesis and structural studies of oxo,hydroxo, and alkoxo complexes of uranium(IV)

The crystal structure of the complex [U(tpa)(2)]I(3), 1 (tpa = tris[(2-pyridyl)methyl]amine), has been elucidated. The complex exists as only one enantiomer in the crystal leading to the chiral space group P2(1)2(1)2(1). The coordination geometry of the metal can be described as a distorted cube. Accidental oxidation of [U(tpa)(2)]I(3) led to the isolation of the unusual mononuclear bishydroxo complex of uranium(IV) [U(tpa)(2)(OH)(2)]I(2).3CH(3)CN, 2, which was structurally characterized. The controlled reaction of [U(tpa)(2)]I(3) with water resulted in the oxidation of the metal center and led to the formation of protonated tpa and of the trinuclear U(IV) oxo complex ([U(tpa)(mu-O)I](3)(mu(3)-I))I(2), 3. The solid state and solution structures of this trimer are reported. The pathway suggested for the formation of this complex is the oxidation of the [U(tpa)(2)]I(3) complex by H(2)O to form a U(IV) hydroxo complex which then decomposes, eliminating mono-protonated tpa. The comparison with the reported reaction with water of cyclopentadienyl derivatives points to a higher reactivity toward water reduction of the bis(tpa) complex with respect to the cyclopentadienyl derivatives. The reaction of U(III) with methanol in the presence of the supporting ligand tpa leads to formation of alkoxo complexes similarly to what is found for amide or cyclopentadienyl derivatives. The monomethoxide complex [U(tpa)I(3)(OMe)], 4, has been prepared in good yield by alcoholysis of the U(III) mono(tpa) complex. The crystal structure of this complex has been determined. The reaction of [U(tpa)(2)]I(3) with 2 equiv of methanol in acetonitrile allows the isolation of the bismethoxo complex of U(IV) [U(tpa)I(2)(OMe)(2)], 5, in 35-47% yield, which has been fully characterized. To account for the oxidation of U(III) to U(IV) the suggested mechanism assumes that hydrogen is evolved in both reactions.     

CS2 Reductive Coupling to Acetylenedithiolate by a Dinuclear Ytterbium(II) Complex

The activation of CS₂ is of interest in a broad range of fields and, more particularly, in the context of creating new C−C bonds. The reaction of the dinuclear ytterbium(II) complex [Yb₂L₄], 1 , [L=(OtBu)₃SiO⁻] with carbon disulfide led to the isolation of unprecedented reduction products. In particular, the crystallographic characterization of complex [Yb₂L₄(μ-C₂S₂)], 2 , provided the first example of an acetylenedithiolate ligand formed from metal reduction of CS₂. Computational studies indicated that this unprecedented reactivity can be ascribed to the unusual binding mode of CS₂²⁻ in the isolated “key intermediate” [Yb₂L₄(μ-CS₂)], 3 , which results from the dinuclear nature of 1 .     

A Practical Synthesis of 1,4,7,10-Tetraaza-Cyclododecane,A Pivotal Precursor for MRI Contrast Agents

A practical preparation of the versatile macrocycle 1,4,7,10-tetraazacyclododecane (cyclen) was developed starting from cheap and easily available starting materials as ethylenediamine and glyoxal.     

Tuning Lanthanide Reactivity Towards Small Molecules with Electron‐Rich Siloxide Ligands

The synthesis, structure, and reactivity of stable homoleptic heterometallic LnL₄K₂ complexes of divalent lanthanide ions with electron‐rich tris(tert‐butoxy)siloxide ligands are reported. The [Ln(OSi(OtBu)₃)₄K₂] complexes (Ln=Eu, Yb) are stable at room temperature, but they promote the reduction of azobenzene to yield the KPhNNPh radical anion as well as the reductive cleavage of CS₂ to yield CS₃^2? as the major product. The Eu^III complex of the radical anion PhNNPh is structurally characterized. Moreover, [Yb(OSi(OtBu)₃)₄K₂] can reduce CO₂ at room temperature. Release of the reduction products in D₂O shows the quantitative formation of both oxalate and carbonate in a 1:2.2 ratio. The bulky siloxide ligands enforce the labile binding of the reduction products providing the opportunity to establish a closed synthetic cycle for the Yb^II‐mediated CO₂ reduction. These studies show that the presence of four electron‐rich siloxide ligands renders their Eu^II and Yb^II complexes highly reactive.