Iron complexes polydentate

Iron polycations are not as well known as chromium or aluminum polycations because of the lability of ferric complexes. Only a few polycations (dimers, trimers) have been characterized in acidic solutions (pH < 1.5) [39]. [Fe2(OH)2] and [Fc20] dimers are present in organic complexes such as L3(H20)Fe(OH>2-Fc(OH2)L3 and LjFeOFeLs, where the L3 ligand is a tridentate picolinate and L5 a tridentate amine [16,40,41]. Other polydentate ligands, such as proteins, are able to stabilize many polynuclear iron complexes [42-45]. The existence of lire aquo complexes [(H20)4Fe2(0H)2(0H2)4] " and [(H20)5Fe20(0H2)s] is very probable in spite of the lack of structural data. 

The importance of connectivity in polydentate ligands has been demonstrated by Dahlenberg.193 The classic QP ligand of Venanzi, P(o-PPh2C6H4)3, containing four phosphorus donor atoms, forms a trigonal bipyramidal complex with iron(II), [FeX(QP)]X (X = Cl, Br, I).194 When flexibility is allowed between the phosphorus donor atoms in P(CH2CH2CH2PMe3)3 a distorted octahedral... 

Iron(II) forms345 stable complexes with a range of polyethyleneamine and related polydentate amines. There has been considerable research effort346 devoted to the measurement of the thermodynamics of formation of these complexes in aqueous solution. Thus, for example, ethyl-enediamine347 forms mono, bis and tris complexes in aqueous media with successive enthalpies of reaction being 21,1,43.5 and 66.3 kJ mol-1 respectively. Similar thermochemical measurements have been made on a number of other polydentate amines, for example diethylenetriamine,348 2,2, 2"-tria-minotriethylamine,349 triethylenetetramine,350 tetraethylenepentamine351 and N,N,N, N -tetra(2-aminoethyl)ethylenediamine.352... 

A large number of polydentate Schiff base ligands have been reported, and of particular interest are the iron(II) complexes of ligands such as (42) (R = H, Ph) because they are low-spin, even though they contain only two diimine units.578 The rate of acid aquation of [Fe(42)]2+ is slow, but this depends on acid concentration. Reaction with hydroxide ion is second order, as is that with cyanide ion. The product of the last reaction is not [Fe(CN)6]4-, but [Fe(CN)4(42)]2 that has only one diimine bound to the metal. Related ligands have been prepared and much of the early work has been reviewed.54615"5464... 

The epoxidation of alkenes by 02 is catalyzed by polydentate iron and by cobalt porphyrin complexes the latter also operate in a perfluoroalkane/MeCN biphasic system.166 Ruthenium complexes, such as (22-XL), catalyze alkene epoxidations with bis(acetoxy)iodobenzene.167... 

Polynuclear iron(II) and cobalt(III) oximehydrazonates have arisen from the template macrocyclization of the initial nonmacrocyclic tris-complexes with polydentate ligands resulting from the condensation of the corresponding diketones and their monooximes with hydrazine [193]. The tris-complexes formed have... 

High spin/low spin equilibria Pressure effects on the distribution of electronic states have also been investigated for d iron] 11) complexes of the type FeL, where L is a polydentate nitrogen donor Ugand. For certain L such as pyim (Fig. 6.8), the... 

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