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), N-(bis-2,2-(4-pyridyloxymethyl)-3-(4-pyridyloxy)propyl))pyridone-4 (L⁴), and 4-(N,N-diethylamino)phenyl-bis-2,6-(4-pyridyl)pyridine (L⁵) resulted in the formation of novel coordination polymers [Fe₂NiO(Piv)₆(L²)]_n (2), [Fe₂NiO(Piv)₆(L³)]_n (3), [Fe₂NiO(Piv)₆(L⁴)]_n•nHPiv (4), and [{Fe₂NiO(Piv)₆}₄{L⁵}₆]_n•3nDEF (5, where DEF is N,N-diethylformamide), which were crystallographically characterized. The topological analysis of 3, 4, and 5 disclosed the 3,3,4,4-connected 2D (3, 4) or 3,4,4-connected 1D (5) underlying networks which, upon further simplification, gave rise to the uninodal 3-connected nets with the respective fes (3, 4) or SP 1-periodic net (4,4)(0,2) (5) 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)]_n (1) and {Fe₂NiO(Piv)₆}₈{L⁶}₁₂ (6), where L¹ and L⁶ are tris(4-pyridyl)pyridine and 4-(N,N-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 2 and 3 possess a porous structure, as confirmed by the N₂ and H₂ sorption data at 78 K. Methanol and ethanol sorption by 2 was also studied indicating that the pores filled by these substrates did not induce any structural rearrangement of this sorbent. Additionally, porous coordination polymer 2 was also applied as a heterogeneous catalyst for the condensation of salicylaldehyde or 9-anthracenecarbaldehyde with malononitrile. The best activity of 2 was observed in the case of salicylaldehyde substrate, resulting in up to 88% conversion into 2-imino-2H-chromen-3-carbonitrile.