A NEW CYCLAM WITH AN NH2-PENDANT DONOR, 6-AMINO-1,4,8,11-TETRAAZACYCLOTETRADECANE, AND ITS NICKEL(II) COMPLEXES

A new cyclam with a pendant primary amino donor, 6-amino-1,4,8,11-tetraazacyclotetradecane, 3, has been synthesized from diethyl aminomalonate and 1,9-diamino-3,7-diazanonane in refluxing MeOH, followed by BH3.THF reduction. Reaction of 3 with Ni(ClO4)2.6H2O in a pH 8.5 aqueous solution yielded a Ni(II) complex 14, whose X-ray crystal structure has shown that the pendant NH2 axially coordinates to Ni(II) and the cyclam moiety takes a trans I configuration. X-ray crystal data of 14: C10H25N5Ni(ClO4)2.2H2O, monoclinic, space group P2, with a = 8.943(5) angstrom, b = 13.552(7) angstrom, c = 8.729(5) angstrom, beta = 98.96(5)-degrees, rho(c) = 1.617 g cm-3 for Z = 2, V = 1045(2) angstrom3, R(w) = 0.075, and R = 0.075. The axial NH2 donor in an acidic solution dissociates with a protonation constant (log K) of 3.8 at 25-degrees-C, I = 0.10 (NaClO4), as measured by the spectroscopic titration, yielding a transient NH3+-pendant N4-coordinate complex 15 with the same trans I configuration. 15 slowly isomerizes with a first-order rate constant of 0.40 h-1 at 60-degrees-C and pH 3.0 to an N4-coordinate 16 with a more stable trans III configuration, as determined by X-ray crystal analysis. X-ray crystal data of 16: C10H26N5Ni(ClO4)3-1.5H2O, monoclinic, space group P2(1)/a, with a = 15.288(9) angstrom, b = 17.217(10) angstrom, c = 8.451(8) angstrom, beta = 93.77(5)-degrees, rho(c) = 1.797 g cm-3 for Z = 4, V = 2219(5) angstrom3, R(w) = 0.088, and R = 0.062. The NH3+ pendant in 16 reversibly deprotonates to 17 with log K of 6.0 at 25-degrees-C and I = 0.10 (NaClO4). At pH 7.6 and 25-degrees-C, the orange solution of 17 is converted back to the purple solution of 14 with a first-order rate constant of 2.3 x 10(-2) h-1. The NH3+-pendant, N4-coordinate, trans III cyclam complex 16 is more efficient than any other isomer mentioned above or cyclam complex 8 in catalyzing electrochemical reduction of CO2 to CO.