NICKEL(II) COMPLEXES OF AZACYCLAMS - OXIDATION AND REDUCTION BEHAVIOR AND CATALYTIC EFFECTS IN THE ELECTROREDUCTION OF CARBON-DIOXIDE

A wide range of functionalized azacyclam complexes of Ni(II) have been prepared through a template reaction which involves the open-chain tetramine complex [Ni(II)(2.3.2-tet)]2+, formaldehyde, and a locking fragment which is a primary carboxamide or sulfonamide, either aliphatic or aromatic (RNH2). The crystal and molecular structure has been determined for the low-spin [3-(4-tolylsulfonyl)-1,3,5,8,12-pentaazacyclotetradecane]nickel(II) complex. The crystal structure consists of three crystallographycally indipendent cationic complexes. Single-crystal X-ray diffraction data were collected with the use of Ni-filtered Cu Kalpha radiation: space group Pbca with a = 23.128(3) angstrom, b = 22.891(3) angstrom, c = 27.595(4) angstrom, alpha = 90-degrees, beta = 90-degrees, gamma = 90-degrees; V = 14609(3) angstrom3, and Z = 24 (R = 0.05 1, R(w) = 0.059). All the investigated azacyclam derivatives display the typical solution behavior of the Ni(II) macrocyclic complexes: inertness toward demetalation by strong acids and the blue-to-yellow interconversion in coordinating media. Electrochemical investigations in acetonitrile have shown that E1/2 values associated with Ni(III)/Ni(II) and Ni(II)/Ni(I) couples are influenced by the nature of the substituent appended to the azacyclam ring: such an effect is interpreted on the basis of a pi interaction between the metal center and the fifth nitrogen atom inserted in the azacyclam ring. Water soluble Ni(II)-azacyclam complexes catalyze the electroreduction Of CO2, as indicated by cyclic voltammetry investigations and controlled-potential coulometry studies, with an efficiency comparable to that of [Ni(II)(cyclam)]2+. Such a high efficiency is strictly related to the structural features of the cyclam and azacyclam framework: a 14-membered cycle forming a 5,6,5,6 sequence of chelate rings. Even small deviations from such a geometrical arrangement cause the electrocatalytic effect to be drastically reduced or completely lost.