Anion binding with some 22- and 28-membered selenaaza macrocycles: Structural aspects and 77 Se NMR studies Snigdha Panda a , Sanjio S. Zade a , Arunashree Panda a , Harkesh B. Singh a, * , Ray J. Butcher b a Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India b Department of Chemistry, Howard University, Washington, DC 20059, USA Received 4 April 2005; received in revised form 11 November 2005; accepted 15 February 2006 Available online 6 March 2006 Abstract A series of macrocyclic adducts of the 22- and 28-membered selenaaza macrocycles (1 and 2, respectively) with different counter anions such as halides, sulfate, perchlorate, phosphate, trifluoroacetate and nitrate has been prepared. The adducts have been charac- terized by elemental analysis, IR, 1 H NMR, 77 Se NMR and ESI-MS analysis. The 77 Se NMR spectrum of the SO 2 4 adduct (7) shows an upfield shift compared to the parent macrocycle. The bromo (5), iodo (6), sulfate (7), trifluroacetate (10) adducts of the 22-membered selenaaza macrocycle and perchlorate (16), trifluroacetate (18) adducts of the 28-membered selenaaza macrocycle have been structurally characterized. The crystal structures show extensive hydrogen bonding networks. The molecular structures of all the compounds show the macrocycle to be fully protonated except the trifluroacetate adduct of the 22-membered macrocycle (10), which is only diprotonated. The binding constants of the neutral 22-membered selenaaza macrocycle towards, fluoride, bromide, iodide and sulfate ion have been determined by the NMR titration method. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Selenaaza macrocycles; 77 Se NMR; Anion binding; EQNMR; Hydrogen bonding 1. Introduction The molecular recognition of the anions with synthetic receptors is an expanding field of research [1]. Synthetic anion receptors, to date, have typically employed various combinations of Lewis acids [2], pyrroles [3], guanidiniums [4], amides [5] and polyammonium macrocycles [6]. The selectivity of these hosts comes from both the size and shape complementarity as well as the anion basicity. The polyamine macrocycles can act as receptors for cations, neutral molecules as well as anions. These polyamine mac- rocycles are capable of undergoing polyprotonation in solution forming positively charged polyammonium cat- ions, which can bind selectively a variety of inorganic, organic and biologically important anions by electrostatic forces and hydrogen bonding. The chalcogen (Se, Te) atoms exhibit weak intra- or inter-molecular chalcogen–X (X = nitrogen [7], chalcogen [8], phosphorous [9], hydrogen [10], fluorine [11], iodine [12]) interactions. The heavier chalcogens (Se,Te) are more prone towards secondary interactions than sulfur. In par- ticular, the chemistry of tellurium has numerous examples of ‘‘intramolecular coordination’’ in its derivatives such as diazenes, Schiff bases, pyridines, amines, and carbonylic compounds [10d]. We thought that the incorporation of selenium into the azamacrocycles should show selectivity in anion binding compared to the simple azamonocycles due to; (i) the geometrical arrangement required around Se is V-shape, which would lead to a puckered structure instead of a flat one for the monocycle. The puckered struc- ture is more effective towards anion binding. (ii) The strong propensity of selenium for secondary interactions with 0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2006.02.016 * Corresponding author. Tel.: +91 22 2576 7190; fax: +91 22 2572 3480. E-mail address: chhbsia@chem.iitb.ac.in (H.B. Singh). www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 691 (2006) 2793–2809