Hexadentate

Use of the tripodal ligand Saltren creates a tetranuclear complex [Mn402(Sal-tren)2][MnCl4] 2CH3CN (172) having a fused open cubane structure without a carboxylate group. At 300 K the value of the effective magnetic moment for (172) is 5.1 /Ub, which decreases to 2.0 /iB at 4.2 K. The redox properties of (172) in propylene carbonate have been studied by cyclic voltammetry. Two oxidation responses are observed at = —0.32V and 0.43 V vs. SCE. 

---

EthylenediaminetetraaceticAcid. Ethylenediaminetetraacetic acid (EDTAH has six potential donor groups two nitrogen atoms and four carboxylate groups. If EDTA 4— acts as a hexadentate ligand to a metal, the resulting complex contains five five-membered chelate rings and has a charge that is four less than that of the metal ion. 

Eig. 2. Stmctural representations of chelates where (8) corresponds to M(acac)2 (9) to ML2, L = S-sulfo-S-hydroxyquiaoline (10) to M-Trien (11) to a tetracoordinate metal bound to the hexadentate ligand EDTA and (12) to the Ee(III) chelate of EHPG. Stmcture (13), which emphasizes the spatial... 

The most complex P ligand so far characterized is the astonishing hexadentate Pio unit in [ Cr( j5-C5H5)(CO)2 5Pio] (see ref. 62 for details). 

Multidentate ligands contain more than two coordinating atoms per molecule, e.g. 1,2-diaminoethanetetra-acetic acid (ethylenediaminetetra-acetic acid, EDTA),f which has two donor nitrogen atoms and four donor oxygen atoms in the molecule, can be hexadentate. 

A dinuclear iron(ll/Ill) complex bearing a hexadentate phenol ligand displayed moderate activity toward aziridination of alkenes with PhlNTs a large excess of alkene (2,000 equiv. vs PhlNTs) was required for good product yields (Scheme 22) [76]. It is noteworthy that complex 4 is active in the aziridination of aliphatic alkenes, affording higher product yields than copper (11) catalysts with tetradentate macrocyclic ligands [77]. 

A chelating ligand contains two or more donor atoms in a structure that allows the ligand to wrap around the metal. Examples featured in Chapter 18 are the bidentate ligand ethylenediamine (en, chemical formula H2 NCH2 CH2 NH2) and the hexadentate ethylenediaminetetraacetate (EDTA). 

One study has been made with a chelate complex, tran5-l,2-diaminocyclohexane-tetraaceatomanganate(III), which is either pentadentate or hexadentate, a water molecule occupying the sixth or seventh coordination position respectively, and hence chelation of the oxalate is very unlikely. The reaction is first-order both in oxidant and oxalate and is retarded by increase of acidity. The HC204 ion is, therefore, attacked more slowly than the 204 " ion but both forms are oxidised as follows... 

Ligand abbreviations acacjtrien = hexadentate Schiff base from triethylenetetramine (trien) and acetylacetone (acac) Saljtrien = hexadentate Schiff base from triethylenetetramine (trien) and salicylaldehyde. 

Fe(acac2trien)]N03. For the iron(III) complex of the hexadentate Schiff-base ligand acac2trien, the barrier heights have been determined from ultrasonic relaxation [94] as AG[h = 6-28 kcalmoD = 2196 cm and AGJil = 5.85 kcalmol= 2046 cm The difference of zero-point energies has been obtained from equilibrium studies as AG° = 0.43 kcal mol =... 

Fe(6-Mepy)3tren](PF6)2- Within the series of iron(II) complexes of the type [Fe(6-Mepy)3 (py) tren](PFg)2 containing the hexadentate ligand tris[4-(6-R)-2-pyridyl]-3-aza-3-butenyl]amine, R = H or CH3, the compounds with X = 1, 2 show a similar HS <- LS transition in solution as well as in the solid state [170]. In solution, spin-state conversion rates have been obtained as shown in Table 3. The complex with x = 0 is pure HS in solution, whereas in the solid state a gradual transition is observed extending over the range... 

Gower, J.D., Ambrose, I.J., Manek, S., Bright, P.S., Dobbin, P.S., Hider, R.C., Goddard, J.G., Thomiley, M.S. and Green, C.J. (1993). The effect of a synthetic hexadentate iron chelator (CP130) and desferrioxamine on rabbit kidneys exposed to cold and warm ischaemia. Agents Actions 40, 96-105. 

Complexation is a phenomenon that involves a coordinate bond between a central atom (the metal) and a ligand (the anions). In a coordinate bond, the electron pair is shared between the metal and the ligand. A complex containing one coordinate bond is referred to as a monodentate complex. Multiple coordinate bonds are characteristic of polydentate complexes. Polydentate complexes are also referred to as chelates. An example of a monodentate-forming ligand is ammonia. Examples of chelates are oxylates (bidentates) and EDTA (hexadentates). 

The encapsulating hexadentate ligand (148) forms [Ni(148)]2+ with a distorted octahedral geometry. This can be oxidized to a Ni111 species in which a substitution reaction by sulfate in the NiN6 chromophore is observed.514... 

---