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-tetraazacyclotetradecane) have been synthesized for CO2 photoreduction catalysis. The first type (4a, [Ni(II)(6-((N-methylpyridin-4-yl)methyl)-1,4,8,11-tetraazacyclotetradecane)]; 4b, [Ni(II)(6-((N-(p-methoxylbenzyl)pyridin-4-yl)methyl)-1,4,8,11-tetraazacyclotetr adecane)];4c, [Ni(II)-(6-((N-benzylpyridin-4-yl)methyl)-1,4,8,11-tetraazacyclotetradecane)]; 4d, [Ni(II)(6-((N-(p-nitrobenzyl)pyridin-4-yl)methyl)-1,4,8,11-tetraazacyclotetrade cane)]) bear a quaternary pyridinium pendant moiety as a potential electron acceptor. The reduction potential of each cationic pyridinium moiety was found to be more positive than the respective Ni(II) center. Their catalytic behavior toward the photoreduction 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 4a-d were respectively, 3, 5.3,5.8, and 5.1 times greater than that from underivatized Ni(II)-cyclam (1). A close correlation was found between 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 = (6-((N-((4'-methyl-2,2'-bipyridin-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 of 5 is long enough to permit reductive quenching in the presence of ascorbate. Although this process induced reductive photocleavage of 5, catalytic studies show that twice the amount of CO is produced compared with a multimolecular system composite of Ru(bpy)3(2+), pyridinium salt, and Ni(II)-cyclam (1).