TOPO ligands

Preparation of TpEncapsulated CdSe/ZnS Nanocrystals via TOPO Ligand Displacement. 

PBE dendrons coordinate to the surface of II-VI semiconductor nanocrystals (e.g., CdSe [33] and CdSe/ZnS core/shell structure [34, 35]) to modulate the photoluminescence of the nanocrystals [32]. Trioctylphosphine oxide (TOPO)-capped II-VI semiconductor nanocrystals of several-nanometers diameter have been synthesized by a pyrolysis reaction of organometallics in TOPO [33-35]. The capping ligand (TOPO) can be replaced by stronger ligands such as thiol compounds [36], suggesting that dendrons bearing sulfur atom(s) at the focal point replace TOPO as well. 

Figure 2. Co-nanocrystals synthesized using high-temperature thermal decomposition technique using thermal OA and TOPO as ligands.

Another study of the surface structure of CdSe NCs of 3.7 nm size used 31P MAS-NMR and 31P/77Se rotational-echo double-resonance (REDOR) to identify overlapping broad peaks from two surface species trioctylphosphine oxide (TOPO) at 29.3 ppm and trioctylphosphine selenide (TOPSe) at 22.2 ppm [343]. Both the isotropic chemical shift and CSA of the surface-bound TOPO were substantially different from those of the free ligand. Spin-echo experiments on 31P were stated to indicate an average P-P distance of 8-10 A at the surface, consistent with capping at alternate atomic sites (all Cd but not Se). 

While pH plays an important role in the extraction of metal ions by the acidic chelating extractants, counteranions such as N03, CE, etc., significantly influence the extraction of metal ions by solvating extractants (L) like TBP, TOPO, etc. The extracted species thus forms solvating species such as MX4 n. or M02X2 nL for tetravalent and hexavalent actinide ions, respectively, where X is a representative counteranion and n is the number of ligand molecules in the extracted species. In... 

Though TBP and DOSO adducts of Pu(IV) were observed when HTTA was used as the primary extractant, no such adducts were reported with the Pu(IV)-HPMBP system (110, 111). On the other hand, synergism was observed for Pu(IV) extraction with HTTA, HPMBP, and HPBI (with stringent stereochemical requirements) when TOPO was used as the auxiliary ligand (27, 33). Other tetravalent actinide ions such as Th(IV) and Np(IV) have shown similar extraction behavior (29, 30, 34). Some adduct formation constants (I<0 for U(VI) and tetravalent actinide ions are listed in Table 2.4. It is necessary to consider both electronic and steric factors of the ligands to explain the observed trends. 

Subramanian and coworkers developed polymeric sorbents using different support materials (such as Merrifield chloromethylated resin, Amberlite XAD 16) and complexing ligands (amides, phosphonic acids, TTA), and evaluated their binding affinity for U(VI) over other diverse ions, even under high acidities. The practical utility of these sorbents was demonstrated using simulated waste solutions (220-222). Shamsipur et al. reported the solid-phase extraction of ultra trace U(VI) in natural waters using octadecyl silica membrane disks modified by TOPO (223). The method was found satisfactory for the extraction and determination of uranium from different water samples. 

Extraction of thorium and europium by these same compounds shows an increase from 051 to Os5 (Table 4.26). Thorium is equally extracted by the linear tetramer and pentamer, whereas europium is even better extracted by linear tetramer than by the linear pentamer. Most of the CMPO calixarenes extract europium better than TOPO and OOCMPO. All these extractants are stronger extractants of thorium than europium, because similar efficiencies require ligand concentrations of 10-3 M for... 

Recently, the primary processes were investigated using pulse radiolysis with two extractant-alkane systems (182, 292). Transient optical absorption spectra proved that in the presence of ligands like TODGA, the excited species of -dodecane (singlet excited state and radical cation) disappeared immediately. Results showed that an energy transfer occurred from the excited alkane to the extractant molecule (TBP, TOPO, or amide), which constituted an additional decomposition route, as described in the following set of reactions ... 

Often (e.g. in asymmetric synthesis) one is interested in the fact that in certain molecules, such as propionic acid (2, Fig. 1), an achiral center (here C ) can be transformed into a chiral center by replacement of one or other of two apparently identical1 ligands2 by a different one. Thus the replacement of HA at C in propionic add (Fig. 1) by OH generates the chiral center of (lactic acid. C in propionic acid is therefore called a prochiral center 4) HA and HB are called heterotopic ligands 5 7) (from Greek heteros = different and topos" = place — see also below). Prochiral axes and planes may similarly be defined in relation to chiral axes and planes (see below)... 

The term equivalent is overly general and therefore bland and of equivocal meaning. Thus the methylene hydrogen atoms in propionic acid (Fig. 1) are equivalent when detached (i.e. they are homomorphic), but, as already explained, they are not equivalent in the CH3CH2C02H molecules because of their placement — i.e. they are heterotopic. Ligands that are equivalent by the criteria to be described in the sequel are called homotopic from Greek homos = same and topos = place 6>, those that are not are called heterotopic . 

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