Polynuclear Iron(II) Pyridonates: Synthesis and Reactivity of Fe 4 and Fe 5 Clusters.

The combination of pyridonate ligands with transition metal ions enables the synthesis of an especially rich set of diverse coordination compounds involving various κ- and μ-bonding modes and higher nuclearities. With iron(II) ions, this chemical space is rather poorly explored beyond some biomimetic models of the pyridone iron-containing hydrogenase. Here, the topologically new Fe 5 and Fe 4 clusters, Fe 5 (L H ) 6 [N(SiMe 3 ) 2 ] 4 ( 1 ) and Fe 4 (L Me ) 6 [N(SiMe 3 ) 2 ] 2 ( 2 ), were synthesized (L H = 2-pyridonate; L Me = 6-methyl-2-pyridonate). Complex 1 contained an unprecedented diamondoid Fe@Fe 4 tetrahedron with a central-to-peripheral Fe-Fe distance of ∼3.1 Å. The crystal structure of complex 2 displayed an Fe 4 O 6 butterfly motif containing a planar Fe 4 arrangement. Mössbauer spectroscopy confirmed the high-spin ferrous character of all iron ions. SQUID magnetometry reveals that the Fe(II) ions are involved in weak magnetic exchange coupling across the pyridonate bridges that results in antiferromagnetic interactions. The Fe 4 cluster exhibits slow relaxation of magnetization under an applied magnetic field with an effective energy barrier of 38.5 K, rarely observed among the very rare examples of Fe(II) cluster-based single-molecule magnets. Studies of protolytic substitution of the amido ligands demonstrated the lability of the diamondoid Fe 5 core in 1 and the stability of the Fe 4 rhomboid in 2 .