摘要 : It is shown by spectrophotometric titration of nitromethane solutions of the macrocyclic complex Ni(DMC)(ClO4)(2) (DMC=C-meso-5,12-dimethyl-1,4,8,11-tetraazacyclotetradecane) with N,N-dimethylformamide (DMF) that the four-coordina... 展开 It is shown by spectrophotometric titration of nitromethane solutions of the macrocyclic complex Ni(DMC)(ClO4)(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+) (K-1=1.95+/-0.09 M(-1)) and Ni(DMC)(DMF)(2)(2+) (beta=K1K2=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)(ClO4)(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 (NaClO4) or 0.6 M ([(n-butyl)(4)N]PF6) 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+/-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 N-4-coordinate intermediate (k(2)(Ni)=2.35+/-0.19 S-l). The slow second phase of the overall reaction consists of two steps, leading to an intermediate (k(2)(1:1)=(8.40+/-0.13) x 10(-4) s(-1)) and to the product species trans-III Ni(DMC)(DMF)(2)(2+) (k(3)(1:1)=(1.80+/-0.02) x 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. [References: 18] 收起