Halogen bonding in cadmium(II) MOFs: its influence on the structure and on the nitroaldol reaction in aqueous medium

A solvothermal reaction of Cd(II) with the dicarboxyl-functionalized arylhydrazone pro-ligands, 5-(2-(2,4,6-trioxotetrahydro-pyrimidin-5(2H)-ylidene)hydrazineyl)isophthalic acid (H₅L¹) and 5-(2-(2,4-dioxopentan-3-ylidene)hydrazineyl)isophthalic acid (H₃L²), or their halogen bond donor centre(s) decorated analogs 2,4,6-triiodo-5-(2-(2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazineyl)isophthalic acid (H₅L³) and 5-(2-(2,4-dioxopentan-3-ylidene)hydrazineyl)-2,4,6-triiodoisophthalic acid (H₃L⁴), leads to the formation of known [Cd(H₃L¹)(H₂O)₂] (1) and new {[Cd(HL²)(H₂O)₂(DMF)]·H₂O} (2), [Cd(H₃L³)] (3) and {[Cd₂(μ-H₂O)₂(μ-H₂L⁴)₂(H₂L⁴)₂]·2H₂O} (4) coordination compounds, respectively. The aggregation of mononuclear units via Cd-OC and Cd-OH₂ coordination and C_Ar-I⋯I types of intramolecular halogen bonds lead to a dinuclear tecton 4. Both C_Ar-I⋯O and C_Ar-I⋯I types of intermolecular halogen bonds play a fundamental role in the supramolecular architectures of the obtained metal-organic frameworks 3 and 4. Theoretical (DFT) calculations confirmed the presence of the C_Ar-I⋯O and C_Ar-I⋯I halogen bonds in 3 and 4 and allowed their characterisation. The formation of intermolecular noncovalent interactions between the attached iodine substituents to the hydrazone ligands and polar solvent (water or methanol) molecules promoted, at least in part, the solubility of the corresponding complexes (3 and 4), which act as homogeneous catalyst precursors in the Henry reaction between aldehydes and nitroethane. The corresponding β-nitroalkanol products were obtained in good yields (66-79%) and with good diastereoselectivity (threo/erythro ca. 72 : 28) in water at room temperature.