Change in the Spin State of an Fe III Center upon One N‐to‐O Switch in the Coordination of a 2,6‐Pyridinedicarboxamido Unit: The Effect of Methyl Thioether and Methyl Ether Appendages at the Ligand Periphery

Two ligands bearing the 2,6‐pyridinedicarboxamide unit, namely PyPSMeH 2 and PyPOMeH 2 , have been used to prepare complexes of Fe III which have been structurally characterised. The potentially pentadentate thioether ligand PyPSMeH 2 affords only the bis complex (Et 4 N)[Fe(PyPSMe) 2 ] ( 1 ), irrespective of the metal to ligand ratio, due to the weak affinity of the thioether groups for the Fe III center. In [Fe(PyPSMe) 2 ] − , the two PyPSMe 2− ligands bind to the iron center in a mer fashion and steric crowding among the pendant thioether groups forces one of the four deprotonated carboxamido moieties to bind to the Fe III center through the carbonyl oxygen, giving rise to an FeN 5 O chromophore. The tridentate ligand PyPOMeH 2 , in its deprotonated form, also gives rise to a bis complex, (Et 4 N)[Fe(PyPOMe) 2 ] ( 2 ), in which the two PyPOMe 2− ligands bind to the iron center in a mer fashion. In this case, however, less steric crowding among the smaller pendant ether groups allows binding of all four carboxamido nitrogens to the Fe III center in [Fe(PyPOMe) 2 ] − giving rise to an FeN 6 chromophore. This one N‐to‐O switch in donor atom makes the two complexes 1 and 2 very different. While complex 2 is low spin ( g = 2.18, 1.94) much like the other reported Fe III bis complexes with two ligated 2,6‐pyridinedicarboxamido units, complex 1 is high spin ( g = 4.3). The spin flip due to a switch in only one N or O donor atom in these two complexes is unprecedented. The two complexes also exhibit different stability towards water. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)