Chiral Synthesis of a Mononuclear Nickel(II) Complex Formed from an Achiral Tripodal Amine Ligand: Spontaneous Resolution A. Srinivasa Rao, † Abhijit Pal, ‡ Rajarshi Ghosh,* ,‡ and Samar K. Das* ,† School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 046, India, and Department of Chemistry, The UniVersity of Burdwan, Burdwan 713 104, India Received November 12, 2008 A neutral chiral mononuclear Ni(II) complex cis-[Ni(NCS) 2 (tren)] (1) [tren ) tris(2-aminoethyl)amine] has been isolated in a reaction of nickel(II) salt with an achiral ligand tren and NH 4 SCN in a MeOH-H 2 O (1:1) mixture. The crystal structure of 1 reveals the encapsulation of Ni(II) ions in a facial {NiN 6 } coordination environment that includes four nitrogen donors from the tetradentate chelate tren and two nitrogen donors from two cis-NCS (mono- dentate) ligands. The chirality of 1 arises from symmetry breaking of the cis-(NCS) 2 -Ni(II) octahedral complex by the tetradentate chelate tren. Compound 1 exhibits both its enantiomers in a DMF solution through a Pfeiffer effect when l- and d-arabinose are used as environment substances. The particular synthesis, described herein, offers total spontaneous resolution, as is evidenced from crystal structure and circular dichroism spectral studies. A standard “Inorganic Chemistry” text book describes that a pure inorganic octahedral complex can possess chirality and optical activity, when the relevant octahedral complex has at least one chelate ring, as originally discovered by Alfred Werner. 1 A tris-chelate complex [Co(en) 3 ] 3+ (D 3 symmetry), for example, can be resolved into its enantiomers. The chirality for this tris-chelate arises from the spiral configuration of the chelate ring providing the basis for right (Δ) and left (Λ)-handed notations. The octahedral complex cis-[CoCl 2 (en) 2 ]Cl is also optically active because it has no S n element of symmetry. An enantiopure form of such a chiral coordination complex is of the utmost importance because of its potential applications in enantioselective synthesis and asymmetric catalysis. 2 The optical resolution of this type of chiral chelate into its pure enantiomers is a difficult task because of their instant racemization due to the labile nature of the chelate ring. An enantiopure chiral coordination complex can be obtained either by enantiose- lective synthesis using an enantiopure ligand or by a spontaneous resolution upon crystallization of a recemic compound. Spontaneous resolution of a racemic compound, generally, results in a conglomerate, where the two enanti- omers (left-handed and right-handed forms) crystallize out separately. Jaques et al. 3 have demonstrated that only less than 10% of the racemates form conglomerates. When a particular synthesis leads to the isolation of only homochiral (one-handed) crystals (totally asymmetric state), that syn- thesis is called total spontaneous resolution. However, the synthesis, which brings about total spontaneous resolution, yielding only homochiral crystals, is extremely rare and unpredictable a priori since the factors determining the process are less understood. 4 We have chosen a tripodal ligand, namely, tris(2-aminoethyl)amine (tren), and a mono- dentate ligand (NCS - ) to obtain an octahedral coordination complex cis-[Ni(NCS) 2 (tren)] (1), which is analogous to the optically active coordination complex cis-[CoCl 2 (en) 2 ] 1+ . We wish to report here a unique synthesis that leads to total spontaneous resolution of the chiral coordination complex cis-[Ni(NCS) 2 (tren)] (1). We have also demonstrated the molecular-level chirality (solution optical resolution) by using two pure enantiomers of arabinose through the Pfeiffer effect. How the molecular-level chirality of complex 1 has trans- formed to a supramolecular-level chiralty, leading to total spontaneous resolution, is an important theme of this report. The title compound cis-[Ni(NCS) 2 (tren)] (1) is isolated by the slow evaporation of an aqueous methanolic solution of Ni(OAc) 2 · 4H 2 O, the ligand tren, and NH 4 SCN. 5 Compound * To whom correspondence should be addressed. E-mail: skdsc@ uohyd.ernet.in (S.K.D.), rajarshi_chem@yahoo.co.in (R.G.). † University of Hyderabad. ‡ The University of Burdwan. (1) (a) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.; John Wiley & Sons: New York, 1988. (b) Douglas, B. E.; McDaniel, D. H.; Alexander, J. J. Concepts & Models of Inorganic Chemistry, 3rd ed.; John Wiley & Sons: New York, 1994. (2) (a) Fu, H.; Kondo, H.; Ichikawa, Y.; Look, G. C.; Wong, C.-H. J. Org. Chem. 1992, 57, 7265. (b) Ojima, I. Catalytic Asymmetric Synthesis, 2nd ed.; Wiley-VCH: New York, 2000. (3) Collet, A.; Brienne, M. J.; Jacques, J. Chem. ReV. 1980, 80, 215. (4) (a) Kondepudi, D. K.; Kaufman, R. J.; Singh, N. Science 1990, 250, 975. (b) Shieh, W.-C.; Carlson, J. A. J. Org. Chem. 1994, 59, 5463. (5) Synthesis of cis-[Ni(NCS) 2 (tren)] (1): Light blue, block-shaped crystals of 1 were obtained from the slow evaporation of a MeOH-H 2 O (1:1) solution (50 mL) of Ni(OAc) 2 · 4H 2 O (0.25 g, 1 mmol), tren (0.29 g, 1.98 mmol), and NH 4 SCN (0.16 g, 2.10 mmol). Yield: 0.112 g (35% based on Ni(OAc) 2 · 4H 2 O). Anal. calcd for C 8 H 18 N 6 S 2 Ni (1): C, 29.92; H, 5.65; N, 26.17. Found: C, 30.09; H, 5.77; N, 26.23. IR (KBr, cm -1 ): 3305s, 3281s, 2087m, 2075s, 1579s, 1405s. UV-vis (DMF) [λ max /nm (ε/M -1 cm -1 )]: 560 (120), 360 (190), 280 (2320). Inorg. Chem. 2009, 48, 1802-1804 1802 Inorganic Chemistry, Vol. 48, No. 5, 2009 10.1021/ic802171b CCC: $40.75  2009 American Chemical Society Published on Web 02/03/2009