Linear Mn₃^II and cubane Mn₄^II carboxylate clusters derived from di-2-pyridyl ketone: Synthesis, characterization and magnetic properties

The employment of di-2-pyridyl ketone, (py)₂CO, in manganese(II) benzoate chemistry is reported. The syntheses, crystal structures and spectroscopic (IR, EPR) characterization are described for [Mn₃^II (O₂CPh)₆{(py)₂CO}₂] (1), [Mn₃^II (O₂CPh)₆{(py)₂CO}₂] · 2MeCN (2 · 2MeCN) and [Mn₄^II (O₂CPh)₄{(py)₂C(OH)O}₄] (3), where (py)₂C(OH)O^- is the monoanion of the gem-diol form of (py)₂CO. Variable-temperature, magnetic susceptibility studies on 3 have also been performed. Complexes 1 and 2 · 2MeCN are linkage isomers. The trinuclear molecules of both complexes have a linear structure, with one η¹:η²:μ₂ and two syn, syn- η¹:η¹:μ₂PhCO₂ ^- groups spanning each pair of Mn^II atoms. The terminal Mn^II atoms are each capped by one κ²N,N′ (py)₂CO ligand in orange 1 and one κ²N,Ο (py)₂CO molecule in the yellow isomer 2 · 2MeCN. The molecule of 3 has a cubane topology with the Mn^II centers and the deprotonated oxygen atoms from the η¹:η³:η¹:μ₃ ligands occupying alternate vertices of the cube. A terminal monodentate PhCO₂^- group completes a distorted octahedral coordination at each Mn^II atom. The IR data are discussed in terms of the coordination modes of the ligands that are present in the complexes. The X-band EPR spectra of powdered 1 and 2 at 4 K indicate that the populated spin states exhibit a zero-field splitting, while the spectrum of 3 at 18 K is consistent with the presence of magnetically interacting Mn^II atoms in the solid state. The magnetic properties of 3 in the 300-5 K range have been modelled with two J values, which reveal weak antiferromagnetic interactions within the molecule. Some suggestions have been made concerning the great stability of complex 3.