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]_n (1) and new {[Cd(HL²)(H₂O)₂(DMF)]·H₂O}_n (2), [Cd(H₃L³)]_n (3) and {[Cd₂(μ-H₂O)₂(μ-H₂L⁴)₂(H₂L⁴)₂]·2H₂O}_n (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.

Noncovalent Interactions at Lanthanide Complexes

Lanthanide complexes have attracted a widespread attention due to their structural diversity, as well as multifunctional and tunable properties. The development of lanthanide based functional materials has often relied on the design of the secondary coordination sphere of the corresponding lanthanide complexes. For instance, usually simple lanthanide salts (solvento complexes) do not catalyze effectively organic reactions or provide low yield of the expected product, whereas the presence of a suitable organic ligand with a noncovalent bond donor or acceptor centre (secondary coordination sphere) modifies the symmetry around the metal centre in lanthanide complexes which then successfully can act as catalysts in both homogenous and heterogenous catalysis. In this minireview, we discuss several relevant examples, based on X-ray crystal structure analyses, in which the hydrogen, halogen, chalcogen, pnictogen, tetrel and rare-earth bonds, as well as cation-π, anion-π, lone pair-π, π-π and pancake interactions, are used as a synthon in the decoration of the secondary coordination sphere of lanthanide complexes.

Crystal engineering strategies towards halogen-bonded metal–organic multi-component solids: salts, cocrystals and salt cocrystals

This highlight presents an overview of the current advances in the preparation of halogen bonded metal–organic multi-component solids, including salts and cocrystals comprising neutral and ionic constituents.