Iron complexes with nitrogen ligands

Martin et al. have developed a unique series of capped tris(l,2-di-aminoethane) cages which can encapsulate divalent transition metal ions in a near octahedral geometry (28). The iron(II) complex with the ligand (NH2)2sar turns out to be a crossover system in solution [34], but the solid triflate salt is low-spin [43]. This is the only Fe(II) crossover system having 6 identical aliphatic nitrogen donors. 

At the same time the bond angles (C—N—C) at the N7 position vary from 112 (-fp3 as expected for an amine ligand) in the manganese complex (where repulsion is least) up to a maximum value of 120 in the iron complex with maximum repulsions. The tertiary amine nitrogen atom (N7) corresponds to a three-ribbed umbrella that has been inverted by the wind (the handle is the lone pair directed at the metal). As the a, and If levels fill, the repulsions increase, the metal-nitrogen distance increases, and the umbrella begins to flatten ... 

Table 1 Mossbauer Parameters for Some Iron(III) Complexes with Nitrogen Donor Ligands...

Table 6.13 Mossbauer parameters for iron(lll) complexes with nitrogen as ligand... 

Chemical differences between imide and sulfide ligand types, however, are substantive and dictate synthetic tactics. In ionic form, N-anions are considerably more basic than sulfur anions [e.g. in DMSO PhNH2, = 30.6 PhSH, 10.3) and, when coordinated to weak-field iron, the former remain more reactive than the latter. Furthermore, redox transformations coupled to weak-field iron are much more accessible with sulfur than nitrogen. As a result, imide ligation is introduced in Scheme 5.9 by protolysis rather than the salt-metathesis or redox routes typical in Fe-S chemistry. Protolysis requires iron precursors with reactive ligands as latent bases the relative instability of these complexes forces the incorporation of imide (or equivalent N-anions) early in the synthetic sequence. 

The reduction of nitrogen co-ordinated to Mo and W in complexes with phosphine ligands most probably takes place in a series of six successive protonation and reduction steps. This suggests a mechanism for nitrogenase in which dinitrogen is bound in an intermediate oxidation state and in which electrons are passed through molybdenum from iron-sulphur clusters in the enzyme as protonation proceeds. 

Although [2-f2] cycloaddition reactions such as (7.5.1.2) are rare, an iron complex with a neutral tridentate nitrogen ligand has been shown to catalyze it. Here, as shown by reaction 7.5.1.3, structure 7.67 is involved as an intermediate. 

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