Kinetic and mechanistic studies of substitution reactions of solvated cobalt(II), nickel(II), copper(II) and zinc(II) ions with 2,2′:6′,2"-terpyridine and several 2,2′:6′,2"-terpyridine derivatives.: Evidence for the formation of intermediates

Multiwavelength stopped-flow spectrophotometry has been used to investigate the rates and mechanisms of reactions of solvates of Co(II), Ni(II), Cu(II) and Zn(II) with 2,2':6',2"-terpyridine (terpy) and its 5-methyl-, 4-(4-tolyl)-, 4'-(4-tolyl)-, 4'-phenyl-, 4'-(4-t-butylphenyl)-, 4'-(4-nitrophenyl)-, 4'-(4-pyridyl)-, 4'-(I-naphthyl)-, and 4'-(2-naphthyl)-derivatives. The reactions of Co(U), Ni(H) and Zn(H) with 4'-(1-naphthyl)terpy were also investigated by stopped-flow fluorimetry. Using excess ligand, a single pseudo-first-order rate process is observed for reactions with either Co(II), Ni(II) or Zn(II) that corresponds to the rate of formation of the mono(ligand)metal(II) ion, followed by rapid conversion of the mono- to the bis-(terpy)metal(II) complexes. However, using excess Co(II), Ni(II) and Zn(H), intermediates are observed prior to the formation of the mono(terpy)metal(II) species, in a two-step consecutive first-order process. The results are consistent with the formation of an intermediate mono(ligand)metal(II) complex in which the terpy acts as a bidentate 2,2'-bipyridine (bipy) donor, followed by a measurable final chelate-ring-closure step. For the intermediates involved in the reactions of Zn2+ with terpy and 4'-phenylterpy, at 25 degreesC the kinetically determined equilibrium constants, estimated from the ratios of slopes/intercepts of plots of k(obs) versus [Zn2+], are log(K-1 / dm(3) mol(-1)) = 5.14 +/- 0.08 and 4.80 +/- 0.12 respectively. These values of K, are too small for the formation of mono(terpy)zinc(II) complexes, but are as expected for the formation of a mono(ligand)zinc(E) complex in which the terpy acts as a bidentate (bipy) donor. The kinetics of the reaction of [Cu(OH2)(6)](2+) with excess terpy in water buffered at pH 6.1 are more complex, with several kinetic steps observed. The first, very rapid stage involves the largest absorbance changes, and is attributed to the formation of mono(terpy)Cu(II) (at 25 degreesC, 10(-7)k(f) = 1.2 +/- 0.1 dm(3) mol(-1) s(-1)). Subsequent reactions are attributed to the rapid formation and slow rearrangement of a five-coordinate bis(terpy)copper(II) intermediates, with one terpy acting as a terdentate donor and the other terpy as a bidentate ligand. Reaction of pre-formed [Cu(terpy)(OH2)(2)](2+), with excess terpy also revealed the rapid formation and slow rearrangement of bis(terpy)copper(II) species. O-17 NMR and ESR line broadening were used to determine the rate of water exchange with [Cu(terpy)(OH2)(2)](2+); assuming [Cu(terpy)(OH2)(2)](2+) has a trigonal bipyramidal geometry with both solvent molecules in equivalent positions in the equatorial plane, and that they undergo solvent exchange at the same rate, for each coordinated solvent the following results were obtained: 10(-8)k(ex) = 6.6 +/- 0.9 s(-1): DeltaH(not equal) = 20.7 +/- 2 kJ mol(-1), DeltaS(not equal) = -6.6 +/- 6 J K-1 mol(-1), DeltaV(not equal) = 0.0 +/- 0.2 cm(3) mol(-1).