NEW SERIES OF MULTIFUNCTIONALIZED NICKEL(II) - CYCLAM (CYCLAM = 1,4,8,11-TETRAAZACYCLOTETRADECANE) COMPLEXES - APPLICATION TO THE PHOTOREDUCTION OF CARBON-DIOXIDE

Two types of Ni(II)-cyclam complex (cyclam = 1,4,8,11-tetraazacyclotet radecane) have been synthesized for CO2 photoreduction catalysis. The first type (4a, -4-yl)methyl)-1,4,8,11-tetraazacyclotetradecane)]; 4b, yl)pyridin-4-yl)methyl)-1,4,8,11-tetraazacyclotetr adecane)];4c, -4-y l)methyl)-1,4,8,11-tetraazacyclotetradecane)]; 4d, pyridin-4-yl)methyl )-1,4,8,11-tetraazacyclotetrade cane)]) bear a quaternary pyridinium p endant moiety as a potential electron acceptor. The reduction potentia l of each cationic pyridinium moiety was found to be more positive tha n the respective Ni(II) center. Their catalytic behavior toward the ph otoreduction of CO2 with Ru(bpy)32+ as a photosensitizer was examined in ascorbate buffer (pH 5.1, 0.1 M) and the evolved amounts of CO by 4 a-d were respectively, 3, 5.3,5.8, and 5.1 times greater than that fro m underivatized Ni(II)-cyclam (1). A close correlation was found betwe en the reduction potential of each pyridinium cation and the quantity of CO evolved. As a second type of Ni(II) complex, a new bifunctional supermolecule Ru(bpy)2(bpy-py-cyclam-Ni(II)) (5) (bpy-py-cyclam = idin -4-yl)methyl)pyridin-4-yl)methyl)-1,4,8,11-tet raazacyclotetradecane) has been synthesized, and its spectroscopic, redox, and CO2 catalytic properties have been investigated. The lifetime of the excited state o f 5 is long enough to permit reductive quenching in the presence of as corbate. Although this process induced reductive photocleavage of 5, c atalytic studies show that twice the amount of CO is produced compared with a multimolecular system composite of Ru(bpy)3(2+), pyridinium sa lt, and Ni(II)-cyclam (1).