Indian Journal of Chemistry Vol. 48A, December 2009, pp. 1662-1666 First alkaline earth 2-carbamoyl-4- nitrobenzoate: Synthesis and supramolecular structure Bikshandarkoil R Srinivasan a, *, Santosh Y Shetgaonkar a & Pallepogu Raghavaiah b a Department of Chemistry, Goa University, Goa 403 206, India b School of Chemistry, University of Hyderabad, Hyderabad 500 046, India Email: srini@unigoa.ac.in Received 30 September 2009; accepted 17 November 2009 2-Carbamoyl-4-nitrobenzoic acid (2-ca-4nbaH) has been prepared by alkaline hydrolysis of 4-nitrophthalimide. Aqueous reaction of MgCO 3 with 2-ca-4nbaH results in the formation of [Mg(H 2 O) 6 ](2-ca-4nba) 2 , which crystallizes in the centrosymmetric monoclinic space group P2 1 /c with Mg(II) situated on an inversion center. The crystal structure consists of a distorted octahedral hexaaquamagnesium(II) cation and a free uncoordinated 2-ca-4nba anion. The cation and anion are involved in three varieties of H-bonding interactions, resulting in a supramolecular bilayer of 2-ca-4nba anions sandwiched between layers of [Mg(H 2 O) 6 ] 2+ cations. Keywords: Coordination chemistry, Magnesium, Crystal structure, Supramolecular bilayer IPC Code: Int. Cl. 9 C07F3/02 Chemistry of s-block elements is a topical area of research 1-6 . Non-toxic and water soluble nature, variable structural chemistry 6 , affinity for O-donor ligands 7 , relevance in biology 8 are some factors responsible for the growing interest in the chemistry of s-block elements. As part of our longstanding research interest 9,10 , we are investigating the coordination chemistry of the amino- (aba) and nitro- (nba) benzoates of s-block elements. In earlier work, we have reported the spectral characteristics, structural aspects and thermal properties and have unraveled a rich structural chemistry of alkaline earth nitro-(nba) benzoates 5,11-14 . A comparison of the structural features of alkaline earth nitrobenzoates with those of the aminobenzoates 15-17 reveals that in the case of Mg(II), the compounds contain the [Mg(H 2 O) 6 ] 2+ unit charge balanced by the corresponding carboxylate anion 14,15,17 while the aminobenzoates of the heavier alkaline earths exhibit M-N (M = Sr or Ba) bonding 16 . In the nitrobenzoates, no metal to N bonding is observed, excepting in [Ba(H 2 O) 3 (2-nba) 2 ] which shows coordination of Ba to the nitro oxygen 11 . In terms of secondary interactions, the amino group acts as H-donor while the oxygen atom of the nitro moiety functions as a hard H-bond acceptor leading to novel supramolecular assemblies. It is of interest to investigate the structural aspects of metal- carboxylates, containing both H-acceptor and H-donor groups in the same ligand, in addition to the carboxylate. Herein, we report the synthesis of 2-carbamoyl-4-nitrobenzoic acid (2-ca-4nbaH) (1) in which an amide (H-donor) and a nitro group (H-acceptor) are disposed ortho and trans respectively with respect to the carboxylate group. The presence of such substituents in (1) results in an interesting supramolecular architecture in [Mg(H 2 O) 6 ](2-ca-4nba) 2 (2), which constitutes the first example of a structurally characterized alkaline earth 2-carbmoyl-4-nitrobenzoate. Experimental Nitration of phthalimide by a literature method 18 afforded a mixture of the isomeric 3- and 4-nitrophthalimide. After separation from the soluble 3-nitro isomer (minor product), the insoluble 4-nitrophthalimide (10 g, 0.052 mol) was taken in water (50 mL) containing NaOH (2.08 g, 0.052 mol) at room temperature and stirred for 15 min to obtain a clear brown solution. To this, dilute HCl was added in drops (pH = 4-5) resulting in the separation of the crude product. The solid was filtered and washed well with water till free of chloride and recrystallized from alcohol to obtain (1) in 8 g yield. (m.pt.: 144-145 °C) UV-vis:  max = 293 nm ( = 5210 L mol -1 cm -1 ); IR (cm -1 ): 3396, 3268, 3219, 3116, 3093, 2872, 2778, 2596, 2480, 1725  as (-COO), 1667  CO (amide), 1611, 1572  as (-NO 2 ) 1502, 1485, 1441, 1380, 1354  s (-NO 2 ), 1306, 1267, 1246, 1120, 1068, 977, 910, 861, 807, 777, 737, 685, 643, 620, 547, 429; 1 H NMR (DMSO-d 6 )  (in ppm) 7.64 (d, J = 8.4 Hz, H3), 8.20 (dd, J = 2.4 Hz, J = 8.4 Hz, H5), 8.36 (d, J = 2.4 Hz, H6), 8.94 (br s, J = 2.4 Hz, -N-H). For assignment, the numbering for H-atom is same as the crystallographic labeling in Fig. 1.