New Ligands

To expand the scorpionate ligand denticity beyond the number of pz rings, Trofimenko and coworkers24 reported two ligands, hydrotris[3-(2,4-dimethoxyphenyl)pyrazol-l-yl]borate Xp(2,4(OMe)2Ph) anc J y-drobis [3-(2,4-dimethoxyphenyl)pyrazol-l-yl]borate Bp(2 4rt)Me)2Pllk able to coordinate through both pyrazolyl nitrogens and o-methoxy 

In a subsequent paper Trofimenko and others145 reported the synthesis of the ligands hydrotris(3-methyl-4,5-propylenepyrazol-l-yl)borate TpMe,mt3 and hydrotris(3-methyl-4,5-butylenepyrazol-l-yl)borate TpMe,mt4, and the structures of their representative cobalt and rhodium complexes, [Co(TpMe,3mt)(TpNp)] and [Rh(i 2- 

The new ligand hydrotris(3-(2/-furyl)-5-methylpyrazolyl)borate TpFl, Me reacts with zinc salts forming complexes [ZnX(TpFu,Me)] (X = Cl, Br, I, NCS, CH3COO, CF3COO). When zinc(II) perchlorate was employed, however, [Zn(TpFu,Me)2] afforded preferably. Careful control of the reaction conditions allows the formation of the enzyme model [Zn-OH(TpFu Me)], which mimics carbonic 

The structure of two thallium scorpionates derived from 3(5),4-trimethylene- and 3(5),4-hexamethylenepyrazole, Tp(3 5 (( I ll)3) and jp(3,4-(CH2)6) respectively, has been determined by multinuclear NMR, both in solution and in the solid state. As for the other 3-substituted pyrazolyl derivatives, these compounds can exist, in principle, in four isomeric forms (Fig. 1.15) depending on the substitution pattern. The isomeric distribution has been found the same in both phases suggesting a kinetic origin.151 

The synthesis of hydrotris(3,4,5-tribromopyrazol-l-yl)borate, (TpBr3), described also in the first edition, has been reported by Trofimenko and coworkers in detail in 20 02.152 [Mo(TpBr3)(t 3-methallyl)(CO)2], [Pd(TpBr3)(t 3-methallyl)], and [Rh(TpBr3)(CO)2] have been structurally characterized and compared with [Mo(Tp )- 

---

The change in colour when one ligand is replaced by another can be used to determine the coordination number thus if the colour change is measured in a colorimeter as the new ligand is added, the intensity of new colour reaches a maximum When the metal/ligand ratio is that in the new complex. 

The utility of complexation titrations improved following the introduction by Schwarzenbach, in 1945, of aminocarboxylic acids as multidentate ligands capable of forming stable 1 1 complexes with metal ions. The most widely used of these new ligands was ethylenediaminetetraacetic acid, EDTA, which forms strong 1 1 complexes with many metal ions. The first use of EDTA as a titrant occurred in... 

CAVEAT database generator for new ligands P. Bartiett at University of California, Berkeley... 

The two protons at C-1 are topologically nonequivalent, since substitution of one produces a product tiiat is stereochemically distinct fiom that produced by substitution of the other. Ligands of this type are termed heterotopic, and, because the products of substitution are enantiomers, the more precise term enantiotopic also applies. If a chiral assembly is generated when a particular ligand is replaced by a new ligand, the original assembly is prochiral. Both C-1 and C-3 of 1,3-propanediol are prochiral centers. 

The surprising stability of N-heterocyclic carbenes was of interest to organometallic chemists who started to explore the metal complexes of these new ligands. The first examples of this class had been synthesized as early as 1968 by Wanzlick [9] and Ofele [10], only 4 years after the first Fischer-type carbene complex was synthesized [2,3] and 6 years before the first report of a Schrock-type carbene complex [11]. Once the N-heterocyclic ligands are attached to a metal they show a completely different reaction pattern compared to the electrophilic Fischer- and nucleophilic Schrock-type carbene complexes. 

Reaction of Ph2PLi with Pr N = C = NPr in THF proceeds via insertion of the carbodiimide into the Li-P bond, affording the lithium phosphaguanidinate salt Li[Ph2PC(NPr )2] in 72% yield. The preparation and reactivity of this new ligand are summarized in Scheme 217. An X-ray crystal structure analysis of the product obtained after removal of the solvent from the reaction mixture revealed the presence of a mono-solvated, centrosymmetric dimer in the solid state (Figure... 

Oxidative additions occur of complexes when metals in low oxidation states (0, 1 +) are used to add AB compounds with rupture of the A—B bond. The addition of the new ligands A and B can take place to one or in some cases two metal centers, e.g. ... 

Davies [30] studied the PyBOx-induced conformational effects by testing several ligands sterically hindered on the oxazoUne moieties (Scheme 11, structures 18 and 19). However, these new ligands gave poorer results in terms of yields and enantioselectivities than ligand 16 for the Ru-catalyzed cyclopropanation reaction, indicating unfavorable steric interactions between styrene and the carbene complex. 

The SBDD approach affected computational chemists positively. The increased number of 3D structures of therapeutically relevant targets opened new opportunities for molecular modeling of the receptor sites. Computational chemists assisted the medicinal chemists in interpreting the fruits of crystallography for design of new ligands. 

C. (2001) l,l -Bis(2-pyridylthio)ferrocene a new ligand in gold and silver chemistry. 

Another class of new ligands was prepared in quantitative yields by Feringa et al., in 1997, by reaction between a-mercapto acids, aniline and 2-pyridine-carboxaldehyde." These pyridyl-substituted thiazolin-4-one ligands were further involved in the copper-catalysed conjugate addition of ZnEt2 to 2-cyclohexenone,... 

In 1999, Shi el al. showed that a diphenylthiophosphoramide derived from (li ,2i )-l,2-diaminocyclohexane could be used as a ligand in the catalytic asymmetric addition of ZnEt2 to aldehydes in the presence of Ti(Oi-Pr)4, providing the corresponding alcohols in enantioselectivities of 40-50% ee (Scheme 3.20). Another class of new ligands such as the phenylthio-phosphoramide of (7 )-1,1 -binaphthyl-2,2 -diamine was developed by the same group, and further tested as a ligand in the same conditions (Scheme 3.20). ... 

After finding the right combination for the diamine linkers, Yus et al. tried to determine whether it was compulsory to use two isoborneol-10-sulfonamide moieties. In this context, these authors have prepared the ligand depicted in Scheme 4.24 by reaction of the best amine linker, trani-cyclohexane-1,2-diamine, with camphorsulfonyl chloride and then with methanesulfonyl chloride, followed by reduction with AlH(i-Bu)2 and then hydrolysis.When this new ligand was involved in the enantioselective addition of ZnEt2 to acetophenone, the expected tertiary alcohol was obtained in excellent yield and enantioselectivity of 96% ee, as shown in Scheme 4.24. According to this result, the authors concluded that the second isoborneol unit seemed not to be necessary to obtain a high enantioselectivity. 

---