IRON(II) COMPLEXES OF POLYDENTATE AMINOPYRIDYL LIGANDS AND AN EXCHANGEABLE 6TH LIGAND REACTIONS WITH PEROXIDES - CRYSTAL-STRUCTURE OF [FEL(1)(H2O)][PF6](2)CENTER-DOT-H2O THYL)-N,N'-BIS(2-PYRIDYLMETHYL)ETHANE-1,2-DIAMINE]

Iron(II) complexes of the pentadentate ligand yl-N,N',N'-tris(2-pyridy lmethyl)ethane-1,2-diamine (L(2)) and the potentially hexadentate liga nd ethyl)-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine (L(1)) have been isolated. Both ligands behave as pentadentate ligands in their iron(I I) complexes with the general formulation [FeL(X)](n+) (L = L(1) or L( 2), n = 1 or 2) with the sixth co-ordination site (X) occupied by an a uxiliary ligand e.g. H2O, Cl, SCN or CN. The crystal structure of [FeL (1)(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 n ot co-ordinated to the iron atom. Instead a water molecule occupies th e sixth co-ordination position. Complex 1 crystallizes in the triclini c space group P (1) over bar, with a = 10.485(2), b = 13.161(5), c = 1 4.853(6) Angstrom, alpha = 69.31(4), beta = 74.42(3), gamma = 66.37(3) degrees and Z = 2. The structure refined to a final R value of 0.052 f or 2531 reflections. The complexes show peroxidase activity, the avail ability of a labile site on the relatively stable iron(II) complexes. along with no apparent tendency to form oxo-bridged diiron(III) specie s, which are likely to be crucial factors in the mechanism of the oxid ation reactions. Efforts to isolate a complex with a co-ordinated pero xide in the sixth position were unsuccessful. However. we have spectro scopic evidence that peroxide reacts with the iron(II) complexes to fo rm an unstable species, probably of the form [Fe(III)L(OOR)](2+) (L = L(1) or L(2), R = H or Bu(t)). The peroxide ligand in this iron(III) c omplex is likely to be forced into an 'end-on' co-ordination mode.