7t-Acidic

As expected from the design of the experiment, the HPLC column packed with CSP 14 containing all 36 members of the library with tt-basic substituents separated 7t-acid substituted amino acid amides. Although encouraging since it suggested the presence of at least one useful selector, this result did not reveal which of the numerous selectors on CSP 14 was the most powerful one. Therefore, a deconvolution process involving the preparation of series of beads with smaller numbers of attached selectors was used. The approach is schematically outlined in Fig. 3-17. 

When the absorption spectra of 2, 26 and 27 in the region around 300 nm are compared 26 shows a band at 297 nm, whereas 2 has a less intense shoulder at 302 nm. 27 lacks an absorption band in this region, which suggests that the 302-nm band of 26 is associated with an electron donor-acceptor transition, i.e. a tz-tz transannular interaction in an electron-withdrawing 7t-acidic 39> tetrafluorophenylene ring. 

Fewer electrochemical studies have been performed on low-oxidation-state complexes of vanadium, compared to higher oxidation states. Complexes in oxidation states lower than V(I) are limited to those which contain ligands such as strong 7T acids (CO and RNC), arenes, or tropy-lium. For example, the [Cp2Co][V(CO)6] salt contains V(-I) which undergoes a reversible oxidation at —0.54 V in CH2CI2 to form the 17-electron V(0) species [3]. 

The first ionically bonded phase was presented by Pirkle it contained (/ )-3,5-dinitrobenzoyl phenylglycine [13]. The most commonly used 7t-acid moiety is the 3,5-dinitrophenyl group introduced by the reaction of 3,5-dinitrobenzoyl chloride (DNB-C1) on chiral selectors such as amino acids, amino alcohols, and amines. In addition, pentafluorobenzoyl derivatives have also been reported [14,15]. The 7r-basic phases are complimentary to the re-acidic phases. These CSPs include the presence of phenyl- or alkyl-substituted phenyl groups. Macaudiere et al. [9] designed a CSP containing both re-acidic and re-basic... 

Several questions remain. In particular, why is it that only one alkylidyne ligand appears to be susceptible to attack (addition/insertion). Perhaps the answer to this question lies in a combination of steric and electronic factors associated with the initial insertion/addition products. In some cases these lack any formal W—W bond, the tungsten atoms being in a formal +6 oxidation state, whereas in others the nature of the W—W bond and its accessibility toward entering 7t-acid ligands is at present not well understood. Further studies are clearly warranted. 

The coordination chemistry of iridium(I) involves 7t-acid ligands. Both four- and five-coordinate species form. Iridium(I) complexes are invariably prepared via some form of reduction, either from analogous iridium(III) complexes or from halide complexes in the presence of the complexing ligand, or via substitution. 

Isocyanides, in contrast, are much poorer 7t-acids and stronger o-donors than CO. Accordingly, they appear less able to stabilize... 

For coordinatively saturated carbene complexes ligated by strongly 7t-acidic co-ligands, with the metal in a comparatively low formal oxidation state, the carbene carbon typically shows electrophilic character. Accordingly, the less reactive examples of these typically bear one or two ju-dative carbene substituents (OR, NR0, SR). 

An alternative and widely employed approach involves the initial synthesis of a c-alkynyl complex, followed by treatment with a variety of electrophiles. As with the heterocarbonyls shown in Figure 3.19, the atom P to the metal is activated towards electrophilic attack (Figure 5.49). This strategy is most effective for alkynyls bound to highly rc-basic (commonly d6) metal centres for two reasons. Firstly, the more electron rich the metal centre, the more the P-carbon will be activated (via retrodonation) to electrophilic attack. Secondly, the vinylidene ligand is a potent 7t-acid, and will accordingly be most stabilized by coordination to a 7t-basic metal centre. 

The 7c-acidity of vinylidene ligands also points towards reactions with nucleophiles at Ca. This will be particularly true if the metal centre is positively charged or co-ligated by other strong 7t-acids, which competitively compromise M— Ca retrodonation. In the case of monobasic... 

The previous discussion focused on coordination of arenes to metal centres ligated by 7t-acids, the net result being that the role of the arene was primarily as a donor and thereby activated towards nucleophilic attack. A complementary situation arises when the metal centre has only pure o-donor co-ligands. The resulting retrodative capacity of the metal... 

C0)5M[S(CH2R )R ]> with BuLi and subsequent alkylation. Reaction of these thiometal complexes with BuLi in the presence of a 7t-acid ligand L (t-butyl isocyanide, phosphanes, phosphites) produces after alkylation complexes in which another CO ligand is replaced by L... 

There have been only a handful of studies on exchange coupled ruthenium dimers and yet the information available on metal-metal coupling is potentially as valuable as that obtained from mixed-valence complexes. The reason for this lack of activity has been the greater familiarity of researchers with the chemistry of Ru(II) bound to 7T-acid ligands. The synthetic pathways to complexes of this type are well explored (7) and possess the tremendous advantages of stability and ease of handling. Ru(II) complexes that incorporate anionic n donor... 

That exo multiple bonding to an orz/zo-carborane cage hypercarbon atom is possible (but with orientational constraints that prevent its occurrence in benzocarborane, and with fascinating imphcations for the use of ortho-carborane as a 7t-acidic probe by which to measure the n-donor capacity of substituents) has been demonstrated by a series of studies on derivatives of phenyl-orz/io-carborane PhCBioHioCX bearing assorted zi-donor C-attached... 

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