Iron(II) complexes of polydentate aminopyridyl ligands and an exchangeable sixth ligand; reactions with peroxides. Crystal structure of [FeL1(H2O)][PF6]2·H2O [L1=N,N′-bis-(6-methyl-2-pyridylmethyl)-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine]

Abstract

Iron(II) complexes of the pentadentate ligand N-methyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine (L²) and the potentially hexadentate ligand N,N′-bis(6-methyl-2-pyridylmethyl)-N,N′-bis(2-pyridylmethyl)-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine (L¹) have been isolated. Both ligands behave as pentadentate ligands in their iron(II) complexes with the general formulation [FeL(X)]ⁿ⁺(L = L¹ or L², n= 1 or 2) with the sixth co-ordination site (X) occupied by an auxiliary ligand e.g. H₂O, Cl, SCN or CN. The crystal structure of [FeL¹(H₂O)][PF₆]₂·H₂O 1 shows that the nitrogen atom of one of the 6-methyl-2-pyridylmethyl groups of the formally hexadentate ligand is not co-ordinated to the iron atom. Instead a water molecule occupies the sixth co-ordination position. Complex 1 crystallizes in the triclinic space group P, with a= 10.485(2), b= 13.161(5), c= 14.853(6)Å, α= 69.31(4), β= 74.42(3), γ= 66.37(3)° and Z= 2. The structure refined to a final R value of 0.052 for 2531 reflections. The complexes show peroxidase activity, the availability of a labile site on the relatively stable iron(II) complexes, along with no apparent tendency to form oxo-bridged diiron(III) species, which are likely to be crucial factors in the mechanism of the oxidation reactions. Efforts to isolate a complex with a co-ordinated peroxide in the sixth position were unsuccessful. However, we have spectroscopic evidence that peroxide reacts with the iron(II) complexes to form an unstable species, probably of the form [Fe^IIIL(OOR)]²⁺(L = L¹ or L², R = H or Bu^t). The peroxide ligand in this iron(III) complex is likely to be forced into an ‘end-on’ co-ordination mode.