Polynuclear Iron(II) Pyridonates: Synthesis and Reactivity of Fe4 and Fe5 Clusters

The combination of pyridonate ligands with transition metal ions enables the synthesis of an especially richset of diverse coordination compounds involving various Kappa- and mu-bonding modes and higher nuclearities. With iron(II) ions, this chemical space is rather poorly explored beyond some biomimeticmodels of the pyridone iron-containing hydrogenase. Here, the topologically new Fe5and Fe4clusters, Fe5(LH)6[N(SiMe3)2]4(1)and Fe4(LMe)6[N(SiMe3)2]2(2), were synthesized (LH=2-pyridonate; LMe= 6-methyl-2-pyridonate). Complex1containedan unprecedented diamondoid Fe@Fe4tetrahedron with a central-to-peripheral Fe-Fe distance of similar to 3.1 A. The crystal structure of complex2displayed an Fe4O6butterfly motif containing a planarFe4arrangement. Mo??ssbauer spectroscopy confirmed the high-spin ferrous character of all iron ions. SQUID magnetometry revealsthat the Fe(II) ions are involved in weak magnetic exchange coupling across the pyridonate bridges that results in antiferromagneticinteractions. The Fe4cluster exhibits slow relaxation of magnetization under an applied magneticfield with an effective energy barrierof 38.5 K, rarely observed among the very rare examples of Fe(II) cluster-based single-molecule magnets. Studies of protolyticsubstitution of the amido ligands demonstrated the lability of the diamondoid Fe5core in1and the stability of the Fe4rhomboid in2.

Automated summary

The combination of pyridonate ligands with transition metal ions allows the synthesis of diverse coordination compounds. This study synthesized novel Fe5 and Fe4 clusters, with Fe5 cluster exhibiting a diamondoid structure and Fe4 cluster showing a butterfly motif. Magnetic tests revealed the rare single-molecule magnet behavior of Fe4 cluster.