Inorganic Chemistry, Vol.39, No.21, 4769-4775, 2000
Kinetic origin of the chelate effect. Base hydrolysis, H-exchange reactivity, and structures of syn,anti-[Co(cyclen)(NH3)(2)](3+) and syn,anti-[Co(cyclen)(diamine)](3+) ions (diamine = H2N(CH2)(2)NH2, H2N(CH2)(3)NH2)
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) Angstrom, b = 12.123(3) Angstrom, c = 9.493(2) Angstrom, alpha = beta = gamma = 90 degrees, beta = 4, R1 = 0.030. 1(ClO4)(3): monoclinic, P2(1)/n, a = 8.892(2) Angstrom, b = 15.285(3) Angstrom, c = 15.466(3) Angstrom, alpha = 90 degrees, beta = 1.05(3)degrees, gamma = 90 degrees, Z = 4, R1 = 0.0657. 2Br(3): orthorhombic, Pca2(1), a = 14.170(4) Angstrom, b = 10.623(3) Angstrom, c = 12.362(4) Angstrom, alpha = beta = gamma = 90 degrees, Z = 4, R1 = 0.0289. Rate constants for H/D exchange (D2O, I = 1.0 M, NaClO4, 25 degreesC) of the syn and anti NH protons (rate law: k(obs) = k(o) + k(H)[OD-]) and the apical NH, and the NH3 and NH2 protons (rate law: k(obs) = k(H)[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)(OR)(2)](+) via [Co(cyclen)(amine)OH](2+) monodentates. Hydrolysis of 3 is two step: k(obs(1)) = k(OH(1))[OH-], k(obs(2)) = k(o) + k(OH(2))[OH-] (k(OH(1)) = (2.2 +/- 0.4) x 10(4) M-1 s(-1), k(o) = (5.1 +/- 1.2) x 10(-4) s(-1), k(OH(2)) = 1.0 +/- 0.1 M-1 s(-1)). Hydrolysis of 2 is biphasic: k(obs(1)) = k(1)K[OH-]/(1 + K[OH-]) (k(1) = 5.0 +/-0.2 s(-1), K = 28 M-1), kobs(2)= k(2)K(2)[OH-]/(1 + K-2[OH-]) (k(2) = 3.5 +/- 1.2 s(-1), K-2 = 1.2 +/- 0.8 M-1). Hydrolysis of 1 is monophasic: k(obs) = k(1)k(2)KK(2)[OH-](2)/(1 + K[OH-])(k(-1) + k(2)K(2)[OH-]) (k(1) = 0.035 +/- 0.004 s(-1), k(-1) = 2.9 +/- 0.6 s(-1), K = 5.5 M-1, k(2)K(2) = 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 interms of a reduced ability of the former system to allow the bond angle expansion required to produce the S(N)1CB trigonal bipyramidal intermediate.