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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)62+ and Ni(DMF)5Cl+

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journal contribution
posted on 28.03.2000, 00:00 by Horst Elias, Rüdiger Schumacher, Jörg Schwamberger, Thorsten Wittekopf, Lothar Helm, André E. Merbach, Stefan Ulrich
13C NMR was used to study the rate of DMF exchange in the nickel(II) cation Ni(DMF)62+ and in the monochloro species Ni(DMF)5Cl+ with 13C-labeled DMF in the temperature range of 193−395 K in DMF (DMF = N,N-dimethylformamide). The kinetic parameters for solvent exchange are kex = (3.7 ± 0.4) × 103 s-1, ΔH = 59.3 ± 5 kJ mol-1, and ΔS = +22.3 ± 14 J mol-1 K-1 for Ni(DMF)62+ and kex = (5.3 ± 1) × 105 s-1, ΔH = 42.4 ± 4 kJ mol-1, and ΔS = +6.7 ± 15 J mol-1 K-1 for Ni(DMF)5Cl+. Multiwavelength stopped-flow spectrophotometry was used to study the kinetics of complex formation of the cation Ni(DMF)62+ and of the 100-fold more labile cation Ni(DMF)5Cl+ with TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and TEC (1,4,8,11-tetraethyl-1,4,8,11-tetraazacyclotetradecane) 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 spectrophotometric 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-order nickel incorporation with rate constants k1(TMC) = 99 ± 5 M-1 s-1 and k1(TEC) = 235 ± 12 M-1 s-1 leads to the intermediates Ni(TMC)int2+ and Ni(TEC)int2+, which have N4-coordinated nickel. In the second stage, these intermediates rearrange slowly to form the stereochemically most stable configuration. First-order rate constants for the one-step rearrangement of Ni(TMC)int2+ and the two-step rearrangment of Ni(TEC)int2+ are presented. Because of the rapid formation of Ni(DMF)5Cl+, the reactions of Ni(DMF)62+ with TMC and TEC are accelerated upon the addition of tetra-n-butylammoniumchloride (TBACl) and lead to the complexes Ni(TMC)Cl+ and Ni(TEC)Cl+, respectively. For initial concentrations such that [TBACl]o/[nickel]o ≥ 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.