Kinetics and mechanism of incorporation of nickel(II) into N 4 macrocycles of the cyclam type: how many steps and intermediates are involved?

Abstract

It is shown by spectrophotometric titration of nitromethane solutions of the macrocyclic complex Ni(DMC)(ClO 4) 2 (DMC = C- meso-5,12-dimethyl-1,4,8,11-tetraazacyclotetradecane) with N,N-dimethylformamide (DMF) that the four-coordinate species Ni(DMC) 2+ equilibrates with DMF to form the five-coordinate and six-coordinate adduct Ni(DMC)(DMF) 2 2+ (Kt = 1.95 t 0.09 M −1) and Ni(DMC)(DMF) 2 2+ (β = K 1 K 2 = 4.50 ± 1.25 M −2), respectively. Rapid-scan stopped-flow spectrophotometry at 280–620 nm was used to study the reaction of DMC with Ni(DMF) 6(ClO 4) 2 in DMF at 298 K to form the species Ni(DMC) solv 2+. The kinetic experiments were carried out at an ionic strength of 0.1 M (NaClO 4) or 0.6 M ([(n-butyl) 4N]PF 6) under stoichiometric conditions as well as with an excess of DMC over nickel(II) and vice versa. The overall reaction, leading finally to the species Ni(DMC)(DMF) 2 2+ has an initial fast stage and a consecutive slow stage. In the fast stage the nickel ions equilibrate with DMC (K(Ni) = 10.5 t 1.8 M −1) to form a complex with partially coordinated DMC, which is rapidly converted to a N 4-coordinate intermediate ( k 1(Ni) = 77.8 ± 11.3 s −1). This intermediate isomerizes quickly to form another N4-coordinate intermediate (k 2(Ni) = 2.35± 0.19 s −1). The slow second phase of the overall reaction consists of two steps, leading to an intermediate ( k 2(1:1) = (8.40 ± 0.13) × 10 −4 s −1) and to the product species trans-III Ni(DMC)(DMF) 2 2+ (k3(1:1) = (1.80 t 0.02) × 10 −5 s-1). The latter two steps are most probably stereochemical rearrangement steps, which are subject to base catalysis through DMC and, less efficiently, through triethylamine. A mechanistic interpretation is presented which refines and details earlier mechanistic concepts for the complex formation with cyclam-like N 4 macrocycles in aprotic solvents.