Kinetics and mechanism of complex formation of nickel(II) with tetra-N-alkylated cyclam in N,N-dimethylformamide (DMF): Comparative study on the reactivity and solvent exchange of the species Ni(DMF)(6)(2+) and Ni(DMF)(5)Cl+

C-13 NMR was used to study the rate of DMF exchange in the nickel(II) catio n Ni(DMF)(6)(2+) and in the monochloro species Ni(DMF)(5)Cl+ with C-13-labe led DMF in the temperature range of 193-395 K in DMF (DMF = N,N-dimethylfor mamide). The kinetic parameters for solvent exchange are k(ex) = (3.7 +/- 0 .4) x 10(3) s(-1), Delta H-double dagger = 59.3 +/- 5 kJ mol(-1), and Delta S-double dagger = +22.3 +/- 14 J mol(-1) K-1 for Ni(DMF)(6)(2+) and k(ex) = (5.3 +/- 1) x 10(5) s(-1), Delta H-double dagger = 42.4 +/- 4 kJ mol(-1), and Delta S-double dagger = +6.7 +/- 15 J mol(-1) K-1 for Ni(DMF)(5)Cl+. M ultiwavelength stopped-flow spectrophotometry was used to study the kinetic s of complex formation of the cation Ni(DMF)(6)(2+) and of the 100-fold mor e labile cation Ni(DMF)(5)Cl+ with TMC (1,4,8,11-tetramethyl-1,4,8,11-tetra azacyclotetradecane) and TEC (1,4,8,11-tetraethyl-1,4,8,11-tetraazacyclotet radecane) in DMF at 298 K and I = 0.6 M (tetra-n-butylammoniumperchlorate). Equilibrium constants K for the addition of the nucleophiles DMF, Cl-, and Br- to the complexes Ni(TMC)(2+) and Ni(TEC)(2+) were determined by spectr ophotometric titration. Formation of the complexes Ni(TMC)(2+) and Ni(TEC)( 2+) was found to occur in two stages. In the initial stage, fast, second-or der nickel incorporation with rate constants k(1)(TMC) = 99 +/- 5 M-1 s(-1) and k(1)(TEC) = 235 +/- 12 M-1 s(-1) leads to the intermediates Ni(TMC)(in t)(2+) and Ni(TEC)(int)(2+), which have N4-coordinated nickel. In the secon d stage, these intermediates rearrange slowly to form the stereochemically most stable configuration. First-order rate constants for the one-step rear rangement of Ni(TMC)(int)(2+) and the two-step rearrangement of Ni(TEC)(int )(2+) are presented. Because of the rapid formation of Ni(DMF)(5)Cl+, the r eactions of Ni(DMF)(6)(2+) with TMC and TEC are accelerated upon the additi on of tetra-n-butylammoniumchloride (TBACl) and lead to the complexes Ni(TM C)Cl+ and Ni(TEC)Cl+, respectively. For initial concentrations such that [T BACl](0)/[nickel](0) greater than or equal to 20, intermediate formation is 230 times (TMC) and 47 times (TEC) faster than in the absence of chloride. The mechanism of complex formation is discussed.