Synthesis and first use of pyridine-2,6-diylbis(pyrazine-2-ylmethanone) in metal cluster chemistry: a {MnIII3Na2} complex with an ideal trigonal bipyramidal geometry

Abstract

The successful organic synthesis of a new dipyrazole/pyridine-dicarbonyl organic molecule, namely pyridine-2,6-diylbis(pyrazine-2-ylmethanone) [(pz)CO(py)CO(pz)], followed by its employment in Mn coordination chemistry has yielded the neutral cluster compound [Mn₃Na₂O(N₃)₃(L)₃] (1), where L²⁻ is the (pz)C(CH₂COCH₃)(O⁻)(py)C(CH₂COCH₃)(O⁻)(pz) dianion. The latter group was formed in situ, presumably by the nucleophilic attack of the carbanion ⁻CH₂COCH₃ to the carbonyl carbon atoms of (pz)CO(py)CO(pz), in the presence of Mnⁿ⁺ ions under basic conditions and in solvent Me₂CO. Complex 1 possesses an almost ideal trigonal bipyramidal topology, with the two Na^I ions occupying the apical positions and the three Mn^III ions residing in the equatorial trigonal plane. The bridging ligation about the metal ions is provided by a μ₃-O²⁻ ion and six μ-OR⁻ groups from the L²⁻ ligand, while peripheral ligation is completed by three terminal azido groups and the pyridine N and carbonyl O atoms of L²⁻. Magnetic susceptibility studies revealed the presence of predominant antiferromagnetic exchange interactions between the paramagnetic Mn^III centres; the use of an anisotropic, equilateral Mn^III₃ triangle model allowed us to fit the magnetic data and obtain the best-fit parameters: J = −10.8 cm⁻¹, D = −5.3 cm⁻¹, and g = 1.99. The combined results demonstrate the rich chemical reactivity of carbonyl groups and the ability of poly-ketone ligands to stabilize cluster compounds with unprecedented structural motifs and interesting architectures.