Selective colorimetric sensing of fluoride ion and its use for insitu cyclization of the sensor Soumen Ghosh a , Md. Akhtarul Alam b,⇑ , Aniruddha Ganguly a , Sasanka Dalapati a , Nikhil Guchhait a,⇑ a Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India b Department of Chemistry, Aliah University, DN-18, Sector-V, Saltlake, Kolkata 700091, India article info Article history: Received 21 October 2014 Received in revised form 11 December 2014 Accepted 17 December 2014 Available online 7 February 2015 Keywords: Schiff base Colorimetric detection Fluoride ion Cyclization Benzimidazole derivative abstract Sensor 1 (a Schiff base) obtained by the condensation of 4-nitro-o-phenyldiamine and 2-hydroxy-1-naph- thaldehyde can detect fluoride ion selectively through visual color change. Moreover, insitu cyclisation of sensor 1 to benzimidazole derivative 2 takes place by the help of fluoride ion as evidenced by X-ray crys- tal structure. The crystal structures of the sensor 1 and benzimidazole derivative 2 show that several types of weak C–H...X (X = O, N), C–H...p and p...p interactions play a crucial role along with intra/inter molecular H-bonding interactions for the stabilization of crystal lattice. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction In recent years, the design and recognition of receptors capable of selective sensing of anions is an important research area of supramolecular chemistry for their crucial role in biological, indus- trial and environmental science [1–4]. Specially, development of colorimetric anion sensors is an appreciated field for qualitative and quantitative detection of anions. There are some examples of highly selective, sensitive and easy to use colorimetric anion sensor [5,6]. Among various important anions fluoride ion detection is most important because it plays vital role in biological system [7–12].A number of compounds/sensors that are able to detect fluoride ion have been reported till now [13–16]. However, the challenge of selective detection and amplifying the fluoride ion sensing event to produce a measurable output still remains demanding. In this regard our interest is to synthesis colorimetric anion sensors that would allow the naked-eye detection of fluoride without using any spectroscopic instrumentation. Here, we report a remarkable chromogenic chemosensor (Schiff base 1) synthesized by the condensation between 2-hydroxy-1-naphthaldehyde and 5-nitro orthophenyl diamine, which detects fluoride ion selectively. This work not only focus on a new naked-eye selective F  ion sensor but also during this detection process fluoride ion helps in situ cyclization of the sensor to form a pharmaceutically important benzimidazole derivative 2 [17,18]. Furthermore, the formation of chemosensor 1 and benzimidazole derivative 2 have been sup- ported by X-ray crystal structures. Surprisingly, the crystal structure of the sensor (Schiff base) 1 shows that it exists in the keto form rather than enol form. More- over, it forms a 3D supramolecular structure through different types of H-bonding interactions and p...p stacking interactions. On the other hand, the crystal lattice of compound 2 and tetrabutyl ammonium ion is stabilized by several C–H...O, C–H...N and C– H...p interactions. 2. Experimental 2.1. Reagents All reagents and solvents were used as received from commer- cial sources without further purification. 4-nitro-o-phenyl dia- mine, 2-hydroxy 1-naphthaldehyde and 1-naphthaldehyde were purchased from Sigma Aldrich Chemicals. Spectroscopic grade sol- vents were purchased from Spectrochem and were used after prop- er distillation. The anions, tetrabutylammonium fluoride (Bu 4 N + F  ) hydrate (98%), tetrabutylammonium acetate (Bu 4 N + AcO  ) (97%), tetrabutylammonium dihydrogenphosphate (Bu 4 N + H 2 PO 4  ) (97%), http://dx.doi.org/10.1016/j.ica.2014.12.025 0020-1693/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding authors. E-mail addresses: alam_iitg@yahoo.com (Md.A. Alam), nguchhait@yahoo.com (N. Guchhait). Inorganica Chimica Acta 429 (2015) 39–45 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica