Synthesis, spectral characterization and crystal structure of Ni(II) pyridoxal thiosemicarbazone complexes and their recyclable catalytic application in the nitroaldol (Henry) reaction in ionic liquid media Rajendran Manikandan, Panneerselvam Anitha, Govindan Prakash, Paranthaman Vijayan, Periasamy Viswanathamurthi ⇑ Department of Chemistry, Periyar University, Salem 636 011, India article info Article history: Received 5 April 2014 Accepted 4 July 2014 Available online 17 July 2014 Keywords: ONS donor ligand Nickel(II) complexes Crystal structure Henry reaction Recyclable abstract The new square planar nickel(II) complexes 1, 2 and 3 have been synthesized from the reaction of [NiCl 2 (PPh 3 ) 2 ] with the tridentate Schiff base ligand, pyridoxal thiosemicarbazone (L 1 ), pyridoxal N-methyl thiosemicarbazone (L 2 ) and pyridoxal N-phenyl thiosemicarbazone (L 3 ) respectively in ethanol. These complexes have been characterized by elemental analyses, IR, UV–Vis, 1 H NMR, 31 P NMR and ESI-MS spectroscopy. The molecular structure of the complex [Ni(L 2 )PPh 3 ](2) was determined by single-crystal X-ray diffraction, which reveals a distorted square planar geometry around the nickel(II) ion. The nitroaldol reaction was studied in detail using the nickel(II) complexes as catalysts in a homo- geneous solution formed by an ionic liquid and methanol. The effect of solvent, ionic liquid, time, temperature, catalyst loading and substituent of the ligand moiety on the reaction was also studied. The b-nitroalcohol products were obtained in good yields of up to 97%. A two step substrate addition mechanism was tentatively proposed based on ESI-MS spectral monitoring of the reaction mass. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Research based on new Schiff bases and their metal complexes represents one of the most promising areas of material science, catalysis and chemical research [1–3]. The Schiff bases are easily synthesized and form complexes with almost all metal ions. Stud- ies on such complexes of transition metals, including nickel, have received overwhelming attention in recent times due to their important catalytic properties [4–14]. The synthesis of nickel(II) complexes with mixed O, N and S donor sets is the research inter- est of many groups [15–17]. Furthermore, nickel complexes con- taining triphenylphosphine ligands play an important role in small molecular catalysts used for the synthesis of fine chemicals and drug molecules [18–22]. The nitroaldol (Henry) reaction is an attractive synthetic tool in organic synthesis. The resulting products, b-nitroalkanols, are important and versatile intermediates in the synthesis of nitroal- kenes, 2-amino alcohols and a-nitroketones [23]. 2-Amino alcohols are of particular significance in the synthesis of biologically impor- tant compounds, such as epinephrine [24], anthracycline antibiot- ics [25], while a-nitroketones are valuable intermediates in the synthesis of several natural products [26]. b-Nitroalkanols are gaining importance due to their fungicidal [27] and antibiotic [28,29] properties, in addition to their synthetic utility, like acting as intermediates in the synthesis of amino sugars [30]. The nitro group can be conveniently converted into several other functional- ities by reduction, the Nef reaction, nucleophilic displacement or other means, thus generating a-hydroxy ketones, aldehydes, car- boxylic acids, azides, sulfides and other important bifunctional compounds. Moreover, the reaction also turns out to be a key step in the preparation of some drugs [31,32]. In spite of many applications, the catalytic nitroaldol reaction is relatively less explored as compared to its aldol counterpart, due to the non-availability of catalysts. Shibasaki reported the first asym- metric version of the nitroaldol reaction in 1992 [33]. Since then various transition metals such as Cu [34], Zn [35], Co [36], Cr [37], Pd [38], Ru [39] and rare metals [40] have been applied to the catalytic version of the nitroaldol reaction. In contrast, nickel, an attractive surrogate for precious metals in terms of its abun- dance and low cost, has not been investigated so far in the nitroal- dol reaction. From a catalyst recycling point of view, very few reports are available [34e–g,41] to date which suggest a recyclable catalyst for the nitroaldol reaction. To overcome this problem ionic liquids can be used to facilitate the recovery and reuse of catalyst. http://dx.doi.org/10.1016/j.poly.2014.07.018 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Fax: +91 427 2345124. E-mail address: viswanathamurthi72@gmail.com (P. Viswanathamurthi). Polyhedron 81 (2014) 619–627 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly