Tridentate oxygen ligand

Even better results were achieved using the tridentate oxygen ligand - 1,1,1-tris (hydroxymethyl)ethane (Scheme 3.32). Nearly quantitative yields were found in the reaction involving various aryl iodides (including o-MeO-substituted) with aryl thiols and alkyl thiols [63]. 

Iron(III) complexes of 2-acetylpyridine Af-oxide iV-methyl- and 3-azabicyclo[3.2.2.]nonylthiosemicarbazone, 24 and 25, respectively, have been isolated from both iron(III) perchlorate and chloride [117], The perchlorate salt yields low spin, octahedral, monovalent, cationic complexes involving two deprotonated, tridentate thiosemicarbazone ligands coordinated via the N-oxide oxygen, azomethine nitrogen and thiol sulfur based on infrared spectral studies. Their powder ESR g-values are included in Table 1 and indicate that bonding is less covalent than for the analogous thiosemicarbazones prepared from 2-acetylpyridine, 3a and 4. Starting with iron (III) chloride, compounds with the same cations, but with tetrachloroferrate(III) anions, were isolated. 

Another type of ligand is the monoanionic, tridentate oxygen donor [(C5H4R)Co-(P(0)R R")3] (Lor), which has been used to prepare the complexes of technetium [37] and rhenium [38] [M03L] and [MOX2L] (X Cl, Br). These complexes are stable in organic solvents but hydrolyse slowly in water. In order to evaluate their usefulness in radioimmunotherapy, the corresponding compounds were also prepared with radioactive rhenium isotopes. 

The advances that have taken place over the past five years in catalytic, enantioselective aldol addition reactions is evident in a number of important respects. The types of transition metals and their complexes that function competently as catalysts have been expanded considerably. Thus, in addition to B(III), Ag(I), Au(I), Sn(II), and La(III), chiral catalysts prepared from Cu(II), Ti(IV), Ln(III), Si(IV), Pt(II) and Pd(II) have been introduced. The expansion in the use of transition metals has taken place hand-in-hand with the design and synthesis of new bidentate and tridentate organic ligands based on nitrogen, oxygen, and phosphorus donors. Additionally, whereas the older methods primarily relied on the use of Lewis-acids for the activation of... 

Tridentate nitrogen ligand is one of the useful ligands capping coordination sites of metal ions. Hydrotris(pyrazolyl)borate derivatives have been used for this purpose. The starting material, [LW02(0H)] (L = hydrotris(3,5-dimethylpyrazol-l-yl)borate) was prepared from [LWI(CO)3] via transfer of two oxygen atoms from dimethyl sulfoxide. This reaction required water as the source... 

Fig. 7. Schematic drawing showing oxalate, ATP, and the two divalent cations bound at the active site of pyruvate kinase. M is the divalent cation at site 1 and M2 is the divalent cation at site II. Four oxygen ligands for M have been identified by EPR experiments with O labeling. Ligands X and X are contributed from the protein. The a,/3,y-tridentate coordination scheme for M2 was determined for Mn(II) at this site in one of the hybrid complexes formed in the presence of Cd(ll). Ligands Z, Z, and Z" are likely water molecules given the facile binding of the aquo-Cr "ATP species at this site (Si).

---