The complexation of lanthanides(III) by the tripodal ligands tpa (tris[(2-pyridyl)methyl]amine) and tpza (tris[(2-pyrazinyl)methyl]amine) has been investigated by solution NMR studies and by X-ray crystallography. The crystallographic studies show that both tpa and the new ligand tpza form complexes with a 1:1 metal:ligand ratio in which the tripodal amine acts as a tetradentate ligand. For the tpa complexes the remaining coordination sites are occupied by chloride counterions to give 7-coordination (Eu, Tb, Lu) or by chloride counterions and a methanol molecule to give 8-coordination (Nd). In [Nd(tpza)(H(2)O)(3)(CH(3)CN)(3)](ClO(4))(3).3H(2)O the remaining coordination sites are occupied by water and acetonitrile molecules to give 10-coordination while the perchlorate counterions remain non coordinating. Tpza complexes have been isolated from acetonitrile solution and dissociate completely in methanol, while the complexes of the more basic tpa can be isolated from methanol and exist in water in equilibrium with the free ligand. Solvent extraction studies of lanthanides(III) and actinides(III) from nitric acid solutions show that the new ligand tpza is, unlike tpa, a selective complexant of actinides(III). Considering their structural analogy, this difference could be explained in terms of the electronic differences between the two ligands resulting in a stronger affinity of actinides(III) for the softer donor tpza.

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.

Low-spin complexes of iron(III) chiroporphyrin, obtained from (1R)-cis-caronaldehyde acid methyl ester and pyrrole as the atropisomer, with R-imidazoles and cyanide have been studied by means of 1D and 2D (1)H NMR spectroscopy. A complete spectral assignment of resonances has been done on the basis of observed scalar, NOE, and EXSY correlations in 2D COSY and NOESY experiments. The chemical shift of beta-H pyrrole resonances have been used as a sensitive probe of electronic state of iron(III) metal ion. Cyanide anion coordination both in methanol and methylene dichloride results in formation of bis(cyanide) low-spin complexes with the rare (d(xz)(),d(yz)())(4) (d(xy)())(1) electronic ground state, revealed by pyrrole beta-H resonances at 11.12 and 10.56 ppm at 293 K, whereas imidazole and 1-methylimidazole produce the bis-ligated complexes with the (d(xy)())(2)(d(xz)(),d(yz)())(3) ground state. In case of sterically hindered imidazole derivatives, i.e., 2-methylimidazole and 1,2-dimethylimidazole, two rotational isomers have been observed at 293 K. Both electronic configurations contribute to the ground state of metal ion for the latter. The steric bulkiness of 2-methylimidazole (or 1,2-dimethylimidazole) is required to freeze a favorable configuration, even at room temperature, providing the perpendicular orientation of two imidazole planes which seems to be instrumental in the stabilization of the rare (d(xz)()d(yz)())(4)(d(xy)())(1) electronic state.

The chloroiron(III) complex of R R -tetramethylchiroporphyrin, FeCl(TMCP), was prepared, and its structure was determined by X-ray crystallography. Black crystals of FeCl(TMCP)‚0.72CH 2 Cl 2 form in the tetragonal space group P4 3 2 1 2 with a ) b ) 13.245(1) Å and c ) 26.355(5) Å at 130 K with Z ) 4. The structure shows an unusual five-coordinate high-spin iron(III) center in a strongly ruffled and domed porphyrin, with short equatorial bond distances (Fe-N(av) ) 2.034(9) Å), and the iron 0.64 Å out of the porphyrin mean plane toward the chlorine atom. The solid-state magnetic moment is 5.92 µ B at 20 K, slightly decreasing to 5.68 µ B at 300 K. In solution FeCl(TMCP) can be easily transformed to FeBr(TMCP) or FeOH(TMCP). The 1 H NMR spectra of the three species are consistent with their C 2 symmetry and S ) 5 / 2 spin state. The pyrrole proton resonances are shifted downfield to 80-100 ppm at 293 K, more than in the corresponding tetraaryl derivatives. The cyclopropyl protons on C 1 , R to the porphyrin meso position, appear at ca. 160-200 ppm, in keeping with the nearly perpendicular orientation of the C 1 -H bond with respect to the porphyrin mean plane. The temperature dependence of the 1 H NMR spectrum of FeCl(TMCP) suggests substantial zero-field splitting.

Cation-cation interactions are a key feature of actinide chemistry. These interactions can be used in two areas that currently attract great interest, namely the expansion of felement supramolecular chemistry and the enhancement of magnetic interactions in actinide compounds. [1][2][3][4][5][6][7][8][9][10][11][12] Moreover, oligomeric cation-cation species that present mutually coordinated actinyl ions are likely to play a crucial role in nuclear waste reprocessing and in the migration of radioactive actinides in the environment. [1] Cation-cation interactions are known to be important in neptunyl(V) structural chemistry, [13] but are more rarely found in uranyl(VI) compounds because of the lower basicity of the UO 2 2+ oxygen atoms. Dimeric compounds formed through the mutual binding of pentavalent uranyl(V) ions have been proposed as intermediates in the disproportionation of pentavalent uranyl ions to UO 2 2+ and U IV species. [14] As a result, bulky ligands have been used in the past few years to disfavor cation-cation interactions and allow the synthesis of rare UO 2 + complexes, [15][16][17][18][19][20][21] which have been the subject of two recent reviews. [21] In some of the reported UO 2 + systems, the ligand bulk does not prevent cation-cation interactions and results in decomposition. However, only two examples of UO 2 + ···UO 2 + intermediate complexes have been reported to date: the tetrameric [UO 2 (dbm) 2 ] 4 [K 4 (CH 3 CN) 4 ] (1) and the dimeric [{UO 2 (dbm) 2 K(18C6)} 2 ] (dbm À = dibenzoylmethanate, 18C6 = [18]crown-6) complexes. [8] The presence of antiferromagnetic coupling between the oxo-bridged uranium centers was unambiguously demonstrated for the dimetallic complex but is less evident for the tetrametallic complex. [8] The decomposition of these polymetallic complexes of pentavalent uranyl ions to UO 2 2+ and U IV species starts rapidly after dissolution in organic solvents and is accelerated by traces of water. From these reactivity studies, it occurred to us that the stability of these polymeric systems could possibly be modulated by fine-tuning the electronic and steric proper-ties of the ligand and coordinating cation. Herein, we report the first example of a UO 2 + ···UO 2 + complex that is highly stable in organic solvents and, significantly, is stable toward hydrolysis. We also describe the selective synthesis and the structure of the first mixed-valent UO 2 + ···UO 2 2+ molecular complex, which provides a rare example of functionalization of the U VI =O group. [20,22] By breaking away from the current trend of using steric bulk to prevent dimer formation and the associated disproportionation of UO 2 + complexes, we show that the non-bulky Schiff base ligand salen 2À (N,N'-bis(salicylidene)ethylenediamine) can stabilize pentavalent uranyl ions through the formation of a highly stable cation-cation complex. Moreover, we demonstrate that the resulting tetrameric uranyl(V) complex exhibits unambiguous antiferromagnetic coupling between the uranium centers.The reaction of the recentl...

The unique electronic properties of lanthanide ions (longlived luminescence and sharp emission spectra) make them particularly suitable for the development of diagnostic tools in medical analysis. [1,2] Lanthanide complexes are increasingly used for sensor development [3] and as luminescent probes in time-resolved high-throughput assays and fluorescence imaging because of their ability to discriminate between background fluorescence and the target signal.[ ).[ [15][16][17][18] As the Laporte-forbidden 4f-4f transition prevents direct excitation of lanthanide luminescence, lanthanide ions require sensitization by suitable organic chromophores. Furthermore, for practical applications under physiological conditions, the lanthanide ion should be incorporated into highly stable complexes. The efficiency of ligand-to-lanthanide energy transfer, which requires compatibility between the energy levels of the ligand excited states and the accepting levels of lanthanide ions, is crucial in the design of highperformance probes.[19] Moreover, high quantum yield cannot be obtained without the prevention of nonradiative deactivation of the lanthanide excited states by OÀH oscillators of coordinated or closely diffusing water molecules. The incorporation of suitable chromophores in carefully designed polydentate ligands leads to increased stability of the lanthanide chelate in solution and allows for the metal center to be well protected from water molecules. However, the tendency of lanthanide ions to adopt high coordination numbers and their lack of stereochemical preferences make the design of such ligands very challenging.[20] A successful strategy adopted by several investigators relies on tripodal architectures for the organization of three tridentate binding units in nine-coordinate Ln iii complexes. [21][22][23][24][25] The design of polydentate ligands that allow the arrangement of four bidentate moieties around a lanthanide ion has received less attention in spite of the excellent luminescent properties observed for tetrakis complexes of bidentate chomophores, such as quinolinates [26] or tropolonates. [27] Recently, octadentate ligands incorporating four bidentate chromophores have been shown to yield lanthanide complexes with very efficient emissions in the visible or NIR regions. [15,28] The structures of these complexes have not been elucidated, but the highly flexible structure of the backbone that connects the bidentate units suggests nonoptimal protection of the metal center in such systems.Herein, we describe a new and particularly efficient way of assembling four picolinate chromophores around a lanthanide center with a multidentate ligand that yields highly luminescent and water-stable lanthanide complexes. The decadentate ligand N,N,N'N'-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine (H 4 tpaen) is readily obtained in five steps from commercially available pyridine-2,6-dicarboxylic acid and ethylenediamine in a yield of 26 % (Scheme 1).The water-soluble complexes of tpaen were isolated in 50-60 % yie...

Low-valent uranium clusters provide good models to understand the formation and behavior of UO 2 nanoparticles. In their Communication on page 5745 ff., M. Mazzanti et al. present a reproducible method for the synthesis of such clusters. Benzoic acid is used to promote the formation of large oxo-hydroxo clusters, and cluster size can be tuned by the choice of solvent and base, thus leading to a U16 compound that is the largest U IV cluster reported to date.

Siloxide unterstützen die Reduktion kleiner Moleküle durch Uran‐Komplexe. Die Behandlung von [U{N(SiMe3)2}3] mit HOSi(OtBu)3 (3 Äquiv.) liefert den neuartigen homoleptischen Uran(III)‐Siloxid‐Komplex 1, der als Zwei‐Elektronen‐Reduktionsmittel für CS2 und CO2 (siehe Schema) wirkt. Komplex 1 reduziert außerdem Toluol und bildet einen invertierten Diuran‐Sandwichkomplex.