Kinetic Origin of the Chelate Effect. Base Hydrolysis, H-Exchange Reactivity, and Structures of <i>syn,anti</i>-[Co(cyclen)(NH₃)₂]³⁺ and <i>syn,anti</i>-[Co(cyclen)(diamine)]³⁺ Ions (diamine = H₂N(CH₂)₂NH₂, H₂N(CH₂)₃NH₂)

The synthesis of syn,anti-[Co(cyclen)en](ClO4)3 (1(ClO4)3) and syn,anti-[Co(cyclen)tn](ClO4)3 (2(ClO4)3) is reported, as are single-crystal X-ray structures for syn,anti-[Co(cyclen)(NH3)2](ClO4)3 (3(ClO4)3). 3(ClO4)3: orthorhombic, Pnma, a = 17.805(4) Å, b = 12.123(3) Å, c = 9.493(2) Å, α = β = γ = 90°, Z = 4, R1 = 0.030. 1(ClO4)3: monoclinic, P2(1)/n, a = 8.892(2) Å, b = 15.285(3) Å, c = 15.466(3) Å, α = 90°, β = 91.05(3)°, γ = 90°, Z = 4, R1 = 0.0657. 2Br3: orthorhombic, Pca2(1), a = 14.170(4) Å, b = 10.623(3) Å, c = 12.362(4) Å, α = β = γ = 90°, Z = 4, R1 = 0.0289. Rate constants for H/D exchange (D2O, I = 1.0 M, NaClO4, 25 °C) of the syn and anti NH protons (rate law: kobs = ko + kH[OD-]) and the apical NH, and the NH3 and NH2 protons (rate law: kobs = kH[OD-]) in the 1, 2, and 3 cations are reported. Deprotonation constants (K = [Co(cyclen-H)(diamine)2+]/[Co(cyclen)(diamine)3+][OH-]) were determined for 1 (5.5 ± 0.5 M-1) and 2 (28 ± 3 M-1). In alkaline solution 1, 2, and 3 hydrolyze to [Co(cyclen)(OH)2]+ via [Co(cyclen)(amine)OH)]2+ monodentates. Hydrolysis of 3 is two step: kobs(1) = kOH(1)[OH-], kobs(2) = ko + kOH(2)[OH-] (kOH(1) = (2.2 ± 0.4) × 104 M-1 s-1, ko = (5.1 ± 1.2) × 10-4 s-1, kOH(2) = 1.0 ± 0.1 M-1 s-1). Hydrolysis of 2 is biphasic: kobs(1) = k1K[OH-]/(1 + K[OH-]) (k1 = 5.0 ± 0.2 s-1, K = 28 M-1), kobs(2) = k2K2[OH-]/(1 + K2[OH-]) (k2 = 3.5 ± 1.2 s-1, K2 = 1.2 ± 0.8 M-1). Hydrolysis of 1 is monophasic: kobs = k1k2KK2[OH-]2/(1 + K[OH-])(k-1 + k2K2[OH-]) (k1 = 0.035 ± 0.004 s-1, k-1 = 2.9 ± 0.6 s-1, K = 5.5 M-1, k2K2 = 4.0 M-1 s-1). The much slower rate of chelate ring-opening in 1, compared to loss of NH3 from 3, is rationalized in terms of a reduced ability of the former system to allow the bond angle expansion required to produce the SN1CB trigonal bipyramidal intermediate.