Disclosing Interfaces of ZnO Nanocrystals Using Dynamic Nuclear Polarization: Sol-Gel versus Organometallic Approach.

The unambiguous characterization of the coordination chemistry of nanocrystal surfaces produced by wet-chemical synthesis presently remains highly challenging. Here, zinc oxide nanocrystals (ZnO NCs) coated by monoanionic diphenyl phosphate (DPP) ligands were derived by a sol-gel process and a one-pot self-supporting organometallic (OSSOM) procedure. Atomic-scale characterization through dynamic nuclear polarization (DNP-)enhanced solid-state NMR (ssNMR) spectroscopy has notably enabled resolving their vastly different surface-ligand interfaces. For the OSSOM-derived NCs, DPP moieties form stable and strongly-anchored µ2 - and µ3 -bridging-ligand pairs that are resistant to competitive ligand exchange. The sol-gel-derived NCs contain a wide variety of coordination modes of DPP ligands and a ligand exchange process takes place between DPP and glycerol molecules. This highlights the power of DNP-enhanced ssNMR for detailed NC surface analysis and of the OSSOM approach for the preparation of ZnO NCs.

Safe-by-Design Ligand-Coated ZnO Nanocrystals Engineered by an Organometallic Approach: Unique Physicochemical Properties and Low Toxicity toward Lung Cells.

The unique physicochemical properties and biocompatibility of zinc oxide nanocrystals (ZnO NCs) are strongly dependent on the nanocrystal/ligand interface, which is largely determined by synthetic procedures. Stable ZnO NCs coated with a densely packed shell of 2-(2-methoxyethoxy)acetate ligands, which act as miniPEG prototypes, with average core size and hydrodynamic diameter of 4-5 and about 12 nm, respectively, were prepared by an organometallic self-supporting approach, fully characterized, and used as a model system for biological studies. The ZnO NCs from the one-pot, self-supporting organometallic procedure exhibit unique physicochemical properties such as relatively high quantum yield (up to 28 %), ultralong photoluminescence decay (up to 2.1 µs), and EPR silence under standard conditions. The cytotoxicity of the resulting ZnO NCs toward normal (MRC-5) and cancer (A549) human lung cell lines was tested by MTT assay, which demonstrated that these brightly luminescent, quantum-sized ZnO NCs have a low negative impact on mammalian cell lines. These results substantiate that the self-supporting organometallic approach is a highly promising method to obtain high-quality, nontoxic, ligand-coated ZnO NCs with prospective biomedical applications.

From Well-Defined Alkylzinc Phosphinates to Quantum-Sized ZnO Nanocrystals.

Despite various applications of alkylzinc carboxylates in chemistry and materials science, the corresponding organozinc derivatives of organophosphorus compounds still represent an insufficiently explored area. To fill this gap, we report on the synthesis of alkylzinc phosphinates and their use as efficient precursors of phosphinate-coated ZnO nanocrystals in the quantum size regime. Examples of a series of alkylzinc phosphinates with the general formula [RZn(O2 PR'2 )] (R=tBu or Et) have been prepared through equimolar reactions between ZnR2 and a selected phosphinic acid, namely dimethylphosphinic acid (dmpha-H), methylphenylphosphinic acid (mppha-H), diphenylphosphinic acid (dppha-H), or bis(4-methoxyphenyl)phosphinic acid (dmppha-H). The reactivities of alkylzinc phosphinate complexes toward H2 O and O2 have also been investigated, which resulted in the isolation of two oxo-zinc phosphinate clusters, that is, [Zn4 (µ4 -O)(dppha)6 ] and [Zn4 (µ4 -O)(dmppha)6 ], as well as the unique alkoxy(oxo)zinc cluster [Zn4 (µ4 -O)(µ2 -OtBu)(dppha)5 ]. Analysis of the crystal structures has revealed that organozinc complexes incorporating phosphinate ligands exhibit a unique capacity for shape-driven self-assembly to produce extended networks, including noncovalent quasi-porous materials. Finally, monodispersed and quantum-sized ZnO NCs coated with phosphinate ligands have been prepared using a non-external-surfactant-assisted wet-chemical organometallic approach based on well-defined [RZn(O2 PR'2 )]-type compounds. The resulting brightly luminescent ZnO NCs exhibit average core sizes and hydrodynamic diameters in the ranges 2-4.5 nm and 5-8 nm, respectively. The size of the inorganic core is slightly affected by the character of the incorporated phosphinate ligand, being smallest for ZnO NCs coated by asymmetrically substituted mppha ligands. Regardless of whether or not various phosphinate coating ligands could be controllably applied on the ZnO NC surface, no significant differences were found in the luminescence profiles of the analyzed nanosystems.

Alkylzinc diorganophosphates: synthesis, structural diversity and unique ability to incorporate zincoxane units.

Equimolar reactions between ZntBu2 and diphenyl phosphate (dpphe-H) or dimethyl phosphate (dmphe-H) result in the formation of [tBuZn(O2P(OR')2)]-type compounds which crystallize as tetranuclear aggregates, [tBuZn(dpphe)]4 (14) or [tBuZn(dmphe)]4 (24), with the phosphate ligands spanning 4-coordinate Zn centers. The utility of ZnMe2 instead of ZntBu2 dramatically changes the reaction outcome and leads to [MeZn(O2P(OR')2)]-type moieties incorporating zincoxane units, i.e., pentanuclear [{MeZn(dpphe)}3(Me2Zn2O)(THF)2] (3) and nonanuclear [{MeZn(dmphe)}6(Me2Zn3O2)] (4) aggregates. The resulting compounds were characterized by 1H and 31P NMR spectroscopy and single-crystal X-ray diffraction analysis.

'Clickable' ZnO nanocrystals: the superiority of a novel organometallic approach over the inorganic sol-gel procedure.

We demonstrate for the first time a highly efficient Cu(i)-catalyzed alkyne-azide cycloaddition reaction on the surface of ZnO nanocrystals with retention of their photoluminescence properties. Our comparative studies highlight the superiority of a novel self-supporting organometallic method for the preparation of brightly luminescent and well-passivated ZnO nanocrystals over the traditional sol-gel procedure.

Chemoselective alcoholysis of lactide mediated by a magnesium catalyst: an efficient route to alkyl lactyllactate.

Alkyl-(S,S)-O-lactyllactate was prepared by chemoselective alcoholysis of lactide LA mediated by a magnesium catalyst. When ROH reacted with LA it yielded the ring-opened product R-(S,S)-O-lactyllactate exclusively, which remained intact as long as LA was present in the reaction mixture. Consumption of LA caused the reaction to proceed further giving R-(S)-lactate.