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 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 Fe5 and Fe4 clusters, 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). Complex 1 contained an unprecedented diamondoid Fe@Fe4 tetrahedron with a central-to-peripheral Fe-Fe distance of ∼3.1 Å. The crystal structure of complex 2 displayed an Fe4O6 butterfly motif containing a planar Fe4 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 Fe4 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 Fe5 core in 1 and the stability of the Fe4 rhomboid in 2.

Cobalt-catalyzed double hydroboration of pyridines.

Cobalt(ii) complexes were prepared from a modular phosphinopyridonate platform and applied to the hydroboration of pyridines. The synthetically useful, yet challenging, double hydroboration toward tetrahydropyridine derivatives was successfully performed with high activity and regiocontrol. This new method enabled the direct synthesis of N-heterocyclic allylic boronates from commercial pyridines and pinacolborane (HBpin). One-pot acetylation afforded the bench-stable borylated N-acetyl tetrahydropyridines in good yields. The synthetic utility of this procedure was demonstrated by a gram-scale double hydroboration-acetylation sequence followed by chemical diversification. Mechanistic experiments indicated metal-ligand cooperativity involving ligand-centered C-H activation and the intermediacy of a cobalt(iii) hydride species.

Synthesis and Catalysis of Anionic Amido Iron(II) Complexes.

Low-coordinate, open-shell 3d metal complexes have attracted great attention due to their critical role in several catalytic transformations but have been notoriously difficult to prepare and study due to their high lability. Here, we report the synthesis of a heteroleptic tri-coordinate amidoferrate that displays high catalytic activity in the regioselective hydrosilylation of alkenes.