Bis(ZnII–cyclen) Complex as a Novel Receptor of Barbiturates in Aqueous Solution

✍ Scribed by Dr. Tohru Koike; Megumi Takashige; Professor Eiichi Kimura; Dr. Haruto Fujioka; Dr. Motoo Shiro

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 819 KB
• Volume: 2
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.19960020604

✦ Synopsis

A new bis-zinc@) receptor (Zn,L), which has two macrocyclic 12membered tetraamine (cyclen) Zn" complexes connected through a p-xylene bridge, has been synthesized as a novel host molecule to recognize barbiturates (such as barbital (bar)) in aqueous solu- tion. Each of the zincfii) ions in the biszinc(I1) receptor was originally intended to match the dianionic barbital anion (bar' -) with supplementary hydrogen bonds between the cyclen NH's and the three carbonyl oxygens in complementary positions to yield a 1 : l complex, Zn2L-bar2-. From an aqueous solution of equimolar Zn,L and barbital at pH 8, however, a cyclic 2:2 complex, (Zn,L-bar2 -) 2 , was isolated and characterized by X-ray crystal analysis. The NMR study in 10% (v/v) D,O/H,O has revealed dissociation of (Zn2L-bar2-), solely into the original target 1 : I complex Zn,L-bar2-and established the dimerization constant for 2Zn,L-bar2-(Zn2L-bar2-),, Kd (= [(Zn,L-bar2-),]/ [Zn,L-bar2-I2) to be 1 0 3 . 4 ~-' . The thermodynamic parameters were evaluated from the NMR measurements at 25, 35, 45, and 55°C: AG = -1 . 9 ~1 0 ~ Jmol-', -49 Jmol-'K-' at 25 "C. Potentiometric pH titration of Zn,L (1 mM) and barbi-AH = -3.3 x lo4 Jmol-',