Facile microwave synthesis, structural diversity and herbicidal activity of six novel alkaline-earth metal complexes (AECs) based on skeletal isomerization chlorophenoxyacetic acids

Six novel AECs of [Sr(o-CPA)(2)(H2O)(4)](o-CPA) 1, [Ba(o-CPA)(2)(H2O)]H2O 2, [Sr(m-CPA)(2)(H2O)(4)](m-CPA) 3, [Ba(m-CPA)(2)(H2O)(4)](2)2(m-CPA)H2O 4, [Sr(p-CPA)(2)(H2O)] 5 and [Ba(p-CPA)(2)(H2O)] 6 (o-CPA = 2-chlorophenoxyacetic acid, m-CPA = 3-chlorophenoxyacetic acid and p-CPA = 4-chlorophenoxyacetic acid) were synthesized by a facile microwave-assistant reaction. The solid-state structures were well established by X-ray crystallography and routine analyses of Fourier transform infrared, elemental analysis, field emission scanning electron microscope (FESEM) and thermogravimetric analysis. The structure data reveal that complexes of 1, 3 and 4 are one-dimensional chains with a zigzag arrangement, while 2, 5 and 6 are (4, 4) rhomboid two-dimensional grid structures. It is noticed that the carboxyl group in these complexes displays multiple coordination modes of mu(2)-(1):(1) (1, 2, 3, 4, 5 and 6), mu(3)-(1):(2) (1, 2, 3 and 4), mu(2)-(1):(2) (2), mu(3)-(2):(2) (5 and 6). Interestingly, the oxygen atoms of flexible -OCH2- in phenoxy groups take part in coordination behavior with metal centers through the rotation of the C-O and C-C bonds in complexes 2, 5 and 6, while only carboxyl group coordination can be found in complexes 1, 3 and 4. FESEM images indicate that the surface appearances of complexes are totally different from ligands after coordination. All ligands and complexes were evaluated for activity as plant-growth inhibitors against Amaranth (Amaranthus spp.) and barnyard grass. Compared with ligands, complexes 1, 3 and 4 exhibited better response index (RI) values of Shoot elongation against barnyard grass. Moreover, complexes 1, 3 and 4 demonstrated a higher inhibitory activity than 2, 5 and 6. It is significant to develop a new kind of environmentally-friendly herbicide based on these kinds of complexes for their low toxicity and high efficiency.