Heterometallic Coordination Polymers Assembled from Trigonal Trinuclear Fe₂Ni-Pivalate Blocks and Polypyridine Spacers: Topological Diversity, Sorption, and Catalytic Properties

Linkage of the trigonal complex [Fe₂NiO­(Piv)₆] (where Piv^– = pivalate) by a series of polypyridine ligands, namely, tris­(4-pyridyl)­triazine (L²), 2,6-bis­(3-pyridyl)-4-(4-pyridyl)­pyridine (L³), <i>N</i>-(bis-2,2-(4-pyridyloxymethyl)-3-(4-pyridyloxy)­propyl))­pyridone-4 (L⁴), and 4-(<i>N</i>,<i>N</i>-diethylamino)­phenyl-bis-2,6-(4-pyridyl)­pyridine (L⁵) resulted in the formation of novel coordination polymers [Fe₂NiO­(Piv)₆(L²)]_<i>n</i> (<b>2</b>), [Fe₂NiO­(Piv)₆(L³)]_<i>n</i> (<b>3</b>), [Fe₂NiO­(Piv)₆(L⁴)]_<i>n</i>·<i>n</i>HPiv (<b>4</b>), and [{Fe₂NiO­(Piv)₆}₄{L⁵}₆]<i>_n</i>·3<i>n</i>DEF (<b>5</b>, where DEF is <i>N</i>,<i>N</i>-diethylformamide), which were crystallographically characterized. The topological analysis of <b>3</b>, <b>4</b>, and <b>5</b> disclosed the 3,3,4,4-connected 2D (<b>3</b>, <b>4</b>) or 3,4,4-connected 1D (<b>5</b>) underlying networks which, upon further simplification, gave rise to the uninodal 3-connected nets with the respective fes (<b>3</b>, <b>4</b>) or SP 1-periodic net (4,4)­(0,2) (<b>5</b>) topologies, driven by the cluster [Fe₂Ni­(μ₃-O)­(μ-Piv)₆] nodes and the polypyridine μ₃-L^3,4 or μ₂-L⁵ blocks. The obtained topologies were compared with those identified in other closely related derivatives [Fe₂NiO­(Piv)₆(L¹)]_<i>n</i> (<b>1</b>) and {Fe₂NiO­(Piv)₆}₈{L⁶}₁₂ (<b>6</b>), where L¹ and L⁶ are tris­(4-pyridyl)­pyridine and 4-(<i>N</i>,<i>N</i>-dimethylamino)­phenyl-bis-2,6-(4-pyridyl)­pyridine, respectively. It was shown that a key structure-driven role in defining the dimensionality and topology of the resulting coordination network is played by the type of polypyridine spacer. Compounds <b>2</b> and <b>3</b> possess a porous structure, as confirmed by the N₂ and H₂ sorption data at 78 K. Methanol and ethanol sorption by <b>2</b> was also studied indicating that the pores filled by these substrates did not induce any structural rearrangement of this sorbent. Additionally, porous coordination polymer <b>2</b> was also applied as a heterogeneous catalyst for the condensation of salicylaldehyde or 9-anthracenecarbaldehyde with malononitrile. The best activity of <b>2</b> was observed in the case of salicylaldehyde substrate, resulting in up to 88% conversion into 2-imino-2<i>H</i>-chromen-3-carbonitrile