Pillaring Role of 4,4′-Azobis(pyridine) in Substituted Malonate-Containing Manganese(II) Complexes: Syntheses, Crystal Structures, and Magnetic Properties

Abstract

Six new manganese(II) complexes of formulas [Mn 2 (Rmal) 2 (H 2 O) 2 (azpy)] n ( 1 – 3 ), [Mn(Phmal)(H 2 O)(azpy)] n ( 4 ), [Mn 2 (Et 2 mal) 2 (H 2 O) 4 (azpy) 2 ] n ( 5 ), and [Mn(Bzmal)(H 2 O) 3 (azpy)] ( 6 ) [azpy = 4,4′-azobispyridine ( 1 – 6 ), Rmal = methylmalonate (Memal) ( 1 ), dimethylmalonate (Me 2 mal) ( 2 ), and butylmalonate (Butmal) ( 3 ), Phmal = phenylmalonate ( 4 ), Et 2 mal = diethylmalonate ( 5 ), and Bzmal = benzylmalonate ( 6 )] were synthesized and structurally characterized by single-crystal X-ray diffraction. Complexes 1 – 3 are three-dimensional compounds whose structure consists of corrugated layers of manganese(II) linked through syn – anti carboxylate (Rmal) bridges, which are pillared through the bis-monodentate azpy molecule. Complex 4 has a layered structure of manganese(II) ions connected by carboxylate (Phmal) bridges in the syn – anti coordination mode as in 1 – 3, the azpy group acting here as a terminally bound monodentate ligand. The structure of 5 consists of Et 2 mal–Mn(II) neutral chains linked through the azpy ligand, giving rise to a complex three-dimensional network. Complex 6 is constituted by neutral [Mn(Bzmal)(H 2 O) 3 (azpy)] mononuclear units, which are interlinked through O–H···O, O–H···N, and π–π type intermolecular interactions to afford a three-dimensional supramolecular structure. The topological analysis of these crystallographic structures shows the occurrence of four different nets: a (3,4)-connected InS-type ( 1 – 3 ), a binodal layered hcb ( 4 ), a uninodal CdS-type ( 5 ), and a (4,5)-connected tcs topology ( 6 ). The magnetic properties of 1 – 5 were investigated in the 2.0–300 K temperature range. Overall antiferromagnetic behavior occurs in 1 – 4 with susceptibility maxima in the range 2.8–5.5 K, the exchange pathway being provided by the syn – anti carboxylate (substituted malonate) bridge [manganese–manganese separation in the range 5.4365(3)–5.5274(1) Å]. Very weak antiferromagnetic interactions are observed in 5 through the trans -bis-monodentate Et 2 mal ligand, the intrachain manganese–manganese separation being 7.328(3) Å. The much larger manganese–manganese separation through the bis-monodentate azpy ligand in 1 – 5 (values greater than 13.5 Å) accounts for the lack of any significant magnetic interaction though this extended bridge.