A Double-Functionalized Cyclen with Carbamoyl and Dansyl Groups (Cyclen = 1,4,7,10-Tetraazacyclododecane):  A Selective Fluorescent Probe for Y3+ and La3

A cyclen (=1,4,7,10-tetraazacyclododecane) doubly functionalized with three carbamoylmethyl groups and one dansylaminoethyl (dansyl = 2-(5-(dimethylamino)-1-naphthalenesulfonyl) group (L2 = 1-(2-(5-(dimethylamino)-1-naphthalenesulfonylamido)ethyl)-4,7,10-tris(carbamoylmethyl)-cyclen) was synthesized and characterized. Potentiometrtic pH titration and UV spectrophotometric titration of L2 served to determine deprotonation of the pendant dansylamide (L2 → H- 1L2) with a pK a value of 10.6, while the fluorometric titration disclosed a pK a value of 8.8 ± 0.2, which was assigned to the dansyl deprotonation in the excited state. The 1:1 M3+−H- 1L2 complexation constants (log K app = 6.0 for Y3+ and 5.2 for La3+, where K app(M−H- 1L2) = [M3+−H- 1L2]/[M3+]free[L2]free (M-1) at pH 7.4) were determined by potentiometric pH titration and UV and fluorescence spectrophotometric titrations (excitation at 335 nm and emission at 520 nm) in aqueous solution (with I = 0.1 (NaNO3)) and 25 °C. The X-ray structure analysis of the Y3+−H- 1L complex showed nine-coordinated Y3+ with four nitrogens of cyclen, three carbamoyl oxygens, and the deprotonated nitrogen and a sulfonyl oxygen of the dansylamide. The crystal data are as follow: formula C28H49N11O13.5SY (Y3+−H- 1L2·2(NO3 -)·2.5H2O), M r = 876.73, monoclinic, space group P21/n (No. 14), a = 18.912(3) Å, b = 17.042(3) Å, c = 24.318(4) Å, β = 95.99(1)°, V = 7794(2) Å3, Z = 8, R1 = 0.099. Upon M3+−H- 1L2 complexation, the dansyl fluorescence greatly increased (8.6 and 3.8 times for Y3+ and La3+, respectively) in aqueous solution at pH 7.4. Other lanthanide ions also yielded Ln3+−H- 1L2 complexes with similar K app values, although all the dansyl fluorescences were weakly quenched. On the other hand, zinc(II) formed only a 1:1 Zn2+−L2 complex at neutral pH with negligible fluorescence change. The X-ray crystal structure of the Zn2+−L2 complex confirmed the pendant dansylamide being noncoordinating. The crystal data are as follow: formula C28H51N11O14SZn (Zn2+−L2·2(NO3 -)·3H2O), M r = 863.22, monoclinic, space group C2/n (No. 15), a = 35.361(1) Å, b = 13.7298(5) Å, c = 18.5998(6) Å, β = 119.073(2)°, V = 7892.3(5) Å3, Z = 8, R1 = 0.084. Other divalent metal ions did not interact with L2 at all (e.g., Mg2+ and Ca2+) or interacted with L2 with the dansyl fluorescence quenched (e.g., Cu2+).