Enhancing the stereoselectivity of Me2GaOR(NHC) species in the ring-opening polymerization of rac-lactide, with the help of the chelation effect.

Using dialkylgallium alkoxides with N-hetrocyclic carbenes (Me2GaOR(NHC)) for the ring-opening polymerization of rac-lactide, we have demonstrated the effect of the chelate interaction between the growing PLA chain and gallium on the stereoselectivity of dialkylgallium alkoxide propagating species - Me2Ga(OPLA)(NHC). In order to do so, we have conducted the structure-activity studies of both Me2Ga(OCH2CH2OMe)(NHC) (NHC = SIMes (1) and IMes (2)) and Me2Ga(OCH(Me)CO2Me)(NHC) (NHC = SIMes (3) and IMes (4)), the latter mimicking active species in the ROP of lactide with growing PLA chain. Based on VT NMR and FTIR spectroscopy, the effect of toluene, CH2Cl2 and THF on the structure of 3 and 4 have been demonstrated, especially with regard to the interaction of methyl lactate ligand with gallium. In a combination with the latter, the studies on the activity of 1 and 2 in the ROP of rac-LA, in different solvents, and at temperatures between -40 °C and 40 °C, have shown the extent of the chelation effect on the isoselectivity of Me2Ga(OPLA)(NHC) in the ROP of rac-LA, which varied between P m of 0.75 and 0.89 depending on the polymerization conditions. Both the latter, and the contribution resulting from the structure of Me2Ga(OPLA)(NHC) (P m = 0.75) have been decisive for the total isoselectivity observed under specific conditions. Our finding represents the first evidence demonstrating that the chelation effect, resulting from the weak interaction between the growing PLA chain and the metal centre, can be responsible for the enhancement of stereoselectivity in the ROP of rac-LA with metal alkoxide propagating species. It should remain of interest, especially in the case of metal based catalysts, which are able to carry out the stereoselective polymerization of rac-LA at mild conditions, under which the chelation effect can manifest itself.

Polylactide conjugates of camptothecin with different drug release abilities.

Camptothecin-polylactide conjugates (CMPT-PLA) were synthesized by covalent incorporation of CMPT into PLA of different microstructure, i.e., atactic PLA and atactic-block-isotactically enriched PLA (Pm = 0.79) via urethane bonds. The kinetic release of CPMT from CMPT-PLA conjugates, tested in vitro under different conditions, is possible in both cases and notably, strongly dependent on PLA microstructure. It shows that release properties of drug-PLA conjugates can be tailored by controlled design of the PLA microstructure, and allow in the case of CMPT-PLA conjugates for the development of highly controlled biodegradable CMPT systems-important delivery systems for anti-cancer agents.

The first facile stereoselectivity switch in the polymerization of rac-lactide--from heteroselective to isoselective dialkylgallium alkoxides with the help of N-heterocyclic carbenes.

The reaction of N-heterocyclic carbene (NHC) with dimeric dialkylgallium alkoxides, acting as nonselective or heteroselective catalysts in the polymerization of rac-LA, leads to highly active and isoselective monomeric Me(2)Ga(NHC)OR catalysts, resulting for the first time in the facile switch of stereoselectivity.

Multielectron redox reactions involving C-C coupling and cleavage in uranium Schiff base complexes.

The reaction of U(III) with Schiff base ligands and the reduction of U(IV) Schiff base complexes both promote C-C bond formation to afford dinuclear or mononuclear U(IV) amido complexes, which can release up to four electrons to substrates through the oxidative cleavage of the C-C bond.

Cation-cation complexes of pentavalent uranyl: from disproportionation intermediates to stable clusters.

Three new cation-cation complexes of pentavalent uranyl, stable with respect to the disproportionation reaction, have been prepared from the reaction of the precursor [(UO(2)py(5))(KI(2)py(2))](n) (1) with the Schiff base ligands salen(2-), acacen(2-), and salophen(2-) (H(2)salen = N,N'-ethylene-bis(salicylideneimine), H(2)acacen = N,N'-ethylenebis(acetylacetoneimine), H(2)salophen = N,N'-phenylene-bis(salicylideneimine)). The preparation of stable complexes requires a careful choice of counter ions and reaction conditions. Notably the reaction of 1 with salophen(2-) in pyridine leads to immediate disproportionation, but in the presence of [18]crown-6 ([18]C-6) a stable complex forms. The solid-state structure of the four tetranuclear complexes, {[UO(2)(acacen)](4)[µ(8)-](2)[K([18]C-6)(py)](2)} (3) and {[UO(2)(acacen)](4)[µ(8)-]}⋅2 [K([222])(py)] (4), {[UO(2)(salophen)](4)[µ(8)-K](2)[µ(5)-KI](2)[(K([18]C-6)]}⋅2 [K([18]C-6)(thf)(2)]⋅2 I (5), and {[UO(2)(salen)(4)][µ(8)-Rb](2)[Rb([18]C-6)](2)} (9) ([222] = [222]cryptand, py = pyridine), presenting a T-shaped cation-cation interaction has been determined by X-ray crystallographic studies. NMR spectroscopic and UV/Vis studies show that the tetranuclear structure is maintained in pyridine solution for the salen and acacen complexes. Stable mononuclear complexes of pentavalent uranyl are also obtained by reduction of the hexavalent uranyl Schiff base complexes with cobaltocene in pyridine in the absence of coordinating cations. The reactivity of the complex [U(V)O(2)(salen)(py)][Cp*(2)Co] with different alkali ions demonstrates the crucial effect of coordinating cations on the stability of cation-cation complexes. The nature of the cation plays a key role in the preparation of stable cation-cation complexes. Stable tetranuclear complexes form in the presence of K(+) and Rb(+), whereas Li(+) leads to disproportionation. A new uranyl-oxo cluster was isolated from this reaction. The reaction of [U(V)O(2)(salen)(py)][Cp*(2)Co] (Cp* = pentamethylcyclopentadienyl) with its U(VI) analogue yields the oxo-functionalized dimer [UO(2)(salen)(py)](2)[Cp*(2)Co] (8). The reaction of the {[UO(2)(salen)(4)][µ(8)-K](2)[K([18]C-6)](2)} tetramer with protons leads to disproportionation to U(IV) and U(VI) species and H(2)O confirming the crucial role of the proton in the U(V) disproportionation.

Synthesis, structure, and bonding of stable complexes of pentavalent uranyl.

Stable complexes of pentavalent uranyl [UO(2)(salan-(t)Bu(2))(py)K](n) (3), [UO(2)(salan-(t)Bu(2))(py)K(18C6)] (4), and [UO(2)(salophen-(t)Bu(2))(thf)]K(thf)(2)}(n) (8) have been synthesized from the reaction of the complex {[UO(2)py(5)][KI(2)py(2)]}(n) (1) with the bulky amine-phenolate ligand potassium salt K(2)(salan-(t)Bu(2)) or the Schiff base ligand potassium salt K(2)(salophen-(t)Bu(2)) in pyridine. They were characterized by NMR, IR, elemental analysis, single crystal X-ray diffraction, UV-vis spectroscopy, cyclic voltammetry, low-temperature EPR, and variable-temperature magnetic susceptibility. X-ray diffraction shows that 3 and 8 are polymeric and 4 is monomeric. Crystals of the monomeric complex [U(V)O(2)(salan-(t)Bu(2))(py)][Cp*(2)Co], 6, were also isolated from the reduction of [U(VI)O(2)(salan-(t)Bu(2))(py)], 5, with Cp*(2)Co. Addition of crown ether to 1 afforded the highly soluble pyridine stable species [UO(2)py(5)]I.py (2). The measured redox potentials E(1/2) (U(VI)/U(V)) are significantly different for 2 (-0.91 and -0.46 V) in comparison with 3, 4, 5, 7 and 9 (in the range -1.65 to -1.82 V). Temperature-dependent magnetic susceptibility data are reported for 4 and 7 and give mu(eff) of 2.20 and 2.23 mu(B) at 300 K respectively, which is compared with a mu(eff) of 2.6(1) mu(B) (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (g(x) = 1.98; g(y) = 1.25; g(z) = 0.74 (at 4 K) was measured for 4. The magnetic and the EPR data can be qualitatively analyzed with a simple crystal field model where the f electron has a nonbonding character. However, the temperature dependence of the magnetic susceptibility data suggests that one or more excited states are relatively low-lying. DFT studies show unambiguously the presence of a significant covalent contribution to the metal-ligand interaction in these complexes leading to a significant lowering of the pi(u)*. The presence of a back-bonding interaction is likely to play a role in the observed solution stability of the [UO(2)(salan-(t)Bu(2))(py)K] and [UO(2)(salophen-(t)Bu(2))(py)K] complexes with respect to disproportionation and hydrolysis.

Pentavalent uranyl stabilized by a dianionic bulky tetradentate ligand.

A pentavalent uranyl complex supported by a bulky dianionic tetradentate (ONNO)-type salan ligand has been prepared by direct synthesis from the iodide precursor {[UO(2)Py(5)][KI(2)Py(2)]}(n), and displays high stability towards disproportionation and ligand dissociation but reactivity towards oxidizing substrates.

Polynuclear cation-cation complexes of pentavalent uranyl: relating stability and magnetic properties to structure.

Reaction of {[UO2Pys][mu-KI2Py2]}n (1) with 2 equiv of potassium dibenzoylmethanate (Kdbm) in pyridine or acetonitrile affords, respectively, the corresponding tetranuclear complexes of pentavalent uranyl ([UO2(dbm),2]2[mu-K(Py)2]2[mu8-K(Py)]}2I2 x Py2 (2) (in 70% yield) and {[UO2(dbm)2]2[mu-K(MeCN)2][mu8-K]}2 (3) (in 40% yield) in which four UO2+ are mutually coordinated (T-shaped "cation-cation" interaction). The X-ray structures of 2 and 3 show also the presence of, respectively, six and four potassium cations involved in UO2+...K+ interactions. Reaction of 2 with an excess of 18-crown-6 (18C6) affords the dimeric complex [UO2(dbm)2K(18C6)]2 (4) presenting a diamond-shaped interaction between two UO2+ groups, in 45% yield. 1H and PFGSTE diffusion NMR spectroscopy of 2 and 3 in pyridine show unambiguously the presence of UO2+...UO2+ and UO2+...K+ interactions (tetrametallic species) in solution, which leads to a rapid (7 days) disproportionation of pentavalent uranyl to afford [U(dbm)4] and [UO2(dbm)2] species. The UO2+...K+ interaction plays an important synergistic role in the stabilization of the UO2+...UO2+ interactions. Accordingly, the lower affinity of (K(18C6))+ for the uranyl(V) oxygen in complex 4 results in a lower number of coordinated K+ and therefore in a weakened UP2+...UO2+ interaction. The UO24+...UO2+ interactions is completely disrupted in dmso or in the presence of Kdbm, preventing disproportionation of pentavalent uranyl. Solid-state variable-temperature magnetic susceptibility studies showed the unambiguous presence of antiferromagnetic coupling between the two oxo-bridged uranium centers of complex 4, with the appearance of a maximum in chi versus T at approximately 5 K. The different behavior of the tetrameric complex 3, which probably involves a magnetic coupling occurring at lower temperature, can be ascribed to the different geometric arrangement of the interacting uranyl(V) groups.

Simple generation of cationic aluminum alkyls and alkoxides based on the pendant arm tridentate schiff base.

The prepared in situ methyl(chloro)aluminum complex (2) from Me2AlCl and the pendant arm tridentate Schiff base (H-SchNMe2) was used to generate the methylaluminum cationic species [(SchNMe2)AlMe]+ in further reaction with 1 equiv of AlCl3 or NaBPh4 as the chloride abstracting reagents. The exposure of the resulting methylaluminum cationic species to an excess of dry dioxygen at 0 degrees C afforded the alkoxyaluminum cationic species, [(SchNMe2)AlOMe]+ or [(SchNMe2)AlOPh]+. The alkoxylaluminum cations proved to be a very efficient catalyst in the polymerization of epsilon-caprolactone.