Quadridentate ligand

Quadridentate Ligands. The effect of extent of chelation on aquation rates has been studied for the compounds raK5-[Co(N02)2(LLLL)]+, where LLLL = (NH3)4, (en 2, (pnja, or cyclam (5), and ci5-[Co(N02)2(LLLL)]+, where LLLL = (NHg)4, (en)a, or trien (6). Rates of aquation increase as the number and size of chelate rings decrease, which is consistent with a dissociative mechanism. Rates of aquation are also sensitive to steric crowding around the leaving group. This is well illustrated by the 10 -fold difference in aquation rates of [Co(C03)(LLLL)]+ when LLLL = (en)a or tet-b (7), and by rate variations in the series 

Varhelyi, and L. Banici, Studia Univ. Babes-Bolyai, Ser. Chim., 1968, 13, 

Varhelyi, and D. Dobocan, J. Inorg. Nuclear Chem., 1969, 31, 1459. 

Miscellaneous. Aquation of cw-[CoCl2phen2]+ also appears to proceed by a dissociative mechanism. The lower lability of the chloride in this complex than in ci5 -[CoCl2en2]+ is most readily explained by the different electron-releasing and -withdrawing properties of phen and en, which are obvious from their relative pJSTa values. Aquation of this and analogous bipy, and chromium(m) compounds has been briefly mentioned.  

The tra j -labilising effect of sulphite ligands is illustrated by the rates and activation parameters for aquation of the complexes [CoX(S03)(DMG)] and [CoX(S03H)(DMG)]. Variation of rates with composition of mixed solvents indicates an associative mechanism for these aquations.  

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Quadridentate ligands produce 3, and in some cases 4, rings on coordination, and so even greater restrictions on the stereochemistry of the complex may be imposed by an... 

One method of solving the kinetics dilemma is well known in coordination chemistry that is, start with a labile metal ion and render it inert during the course of the synthetic reaction. We have accomplished this in the case of zirconium(IV) by starting with tetrakis(salicylaldehydo)zirconium(IV), which is quite labile, and polymerization with 1,2,4,5-tetraaminobenzene in a Schiff-base condensation reaction in situ (6). The polymeric product contains a "double-headed" quadridentate ligand, which is much more inert to substitution. However, 1,2,4,5-tetraaminobenzene has become very expensive. Therefore, the synthesis of a zirconium polymer with 3,3, 4,4 -tetraaminobiphenyl (commercially 3,3 -diami nobenzidine) with zirconium salicylaldehyde, Zr(sal)4 (7) has been undertaken as shown below ... 

The quadridentate ligand (46), LH2, has been used to extract palladium from solutions containing platinum. Equilibrium studies indicated that a mono-deprotonated form of the ligand is... 

Dockal etal. [57] used slow-scan CV to determine the 21 values for 17 Cu(II/I) complexes in 80% methanol —20% water (w/w) - including nine complexes with macrocyclic terdentate, quadridentate, quinquedentate, and sexaden-tate thioethers and eight complexes with acyclic quadridentate ligands containing thioether sulfur and/or amine nitrogen donor atoms. (In naming the denticity of multidentate ligands, Dwyer, Lions, and coworkers have pointed out that dentate is a Latin root and proper nomenclature requires that Latin prefixes be used. 

Quadridentate ligands. One quadridentate derivative of en and dien is tren. It has a carboxylato analogue in the nitrilotriacetate ion (NTA3-) ... 

Figure 2.16 The redox-driven disassembling of a dicopper(I) double-strand helicate complex to give two mononuclear copper(II) complexes, in which each strand behaves as a quadridentate ligand. On subsequent reduction, the two mononuclear complexes reassemble to give the helicate. The illustrated process fits well the behavior of copper complexes of 16 in a MeCN solution.

The heteroatoms listed in Table 10 form complexes of stoichiometry [XWi0O36]8-. The structure, based on determinations of the CeIV and UIV anions, can be regarded as the attachment of two quadridentate ligands, derived from W6Oi9 through the loss of one W06 octahedron, to the heteroatom (Figure 18). The site symmetry of the latter is approximately D4d. The XW10 anions are of only moderate stability in aqueous solution (pH5.5-8.5). Electronic, vibrational and 170 NMR spectra have been recorded. The emission spectra and luminescence properties of several of the lanthanide anions have been discussed.84... 

Ni(NCS)2(aep)2 dissolves in acetone at room temperature after a period of weeks giving a blue product containing a quadridentate ligand formed by the condensation of two molecules of aep with two molecules of acetone (Scheme 4).938... 

All of the complexes structurally characterized have either a square planar or a square pyramidal structure with a planar arrangement of the four donor atoms. Complexes having general formulas [NiL]Y2 and [NiXL]Y (L = quadridentate ligand X = coordinated anion Y = uncoordinated anion) are invariably low-spin as one can easily predict on the basis of the high nucleophilicity of the donor atoms.1389... 

A minor change in the nature of the ligands sometimes produces a great change in the nature of a complex. Thus, if R" is —(CH2)2— in the branched quadridentate ligand (6), the nickel complex is a dimer in which each half is planar and diamagnetic, but if R" is —(CH2)3— the complex is polymeric and paramagnetic each nickel atom is at the center of a tetrahedron.23... 

Cyclization of coordinated primary amines on to coordinated aminoacetone has also been investigated in bis(l,2-diaminpethane)cobalt(ni) complexes, and the results show selectivity with respect to attack of the monoamine or 1,2-diaminoethane (equation 38).218 A similar complex with two coordinated aminoacetone molecules undergoes the same type of stereoselective kinetic template reactions and yields complexes primarily of a new quadridentate ligand (Scheme 48).219... 

A three-site system for peptide synthesis around a cobalt(III) complex has been studied. Instead of a quadridentate ligand as used in the above experiments, Wu and Busch chose the tridentate ligand diethylenetriamine. The formation of dipeptide and tetrapeptide complexes is shown in Scheme 92.360 The ester carbonyl group in the 0-bonded amide intermediate (127) cannot be activated by coordination because it cannot reach the metal ion. Isomerization to the jV-bonded amide complex (128) occurs with base and enables coordination and therefore activation of the ester carbonyl group. 

Using a quadridentate ligand derived from 1,3-propanediamine and 2,3-butanedione the cobalt-oxime complex (61) was prepared, and again in presence of the chiral base quinine (60) this was used to hydrogenate benzil according to equation (54) in an optical yield of 79%.277 These cobalt systems permitted only low turnover numbers to be achieved. 

Fig. 8.6. The porphin molecule. The NH hydrogen atoms are readily removed as protons and the resulting dianion acts as a quadridentate ligand.

Metal Containing Tetradentate Formazans. Formazans with two complex-forming groups in the 2- and 2 -positions of the N1- and N5-aryl substituents are potential quadridentate ligands. They form 1 1 coordination complexes with four- and six-coordinate metals ... 

A class of synthetic metalloporphyrins which are conveniently studied in aqueous solution derives from coordination compounds of the quadridentate ligand a,(S,7,5-tetra(4-pyridyl)-porphine. Although a method of synthesis for the zinc complex has been published,1 the yield is often poor and the method unreliable. If the same reaction is allowed to proceed under... 

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