Kinetics and mechanism of base hydrolysis in cobalt(III) complexes - The case of a complex CoLCl2+ where L has the novel topology of a square-pyramidal NN4 coordination cap

Multi-wavelength stopped-now spectrophotometry was used to study the kineti cs of base hydrolysis of the octahedral cobalt(III) complex CoLCl2+ (2), in which the tetrapodal pentadentate Ligand L has an NN4 donor sei and forms a square-pyramidal coordination cap [L = 2,6-bis(1',3'-diamino-2'-methylpro p-2'-yl)pyridine, 1]. The kinetic investigation, carried out at different t emperatures, pressures and ionic strengths I, led to second-order kinetics, rate = k(OH) [2][OH-], with k(OH) = 0.139 +/- 0.001 M(-1)s(-1) (I = 0.1 M) and k(OH) = 0.0570 +/- 0.0004 M(-1)s(-1) (I = 1.0 M) at 298 K. The tempera ture and pressure dependence of k(OH) resulted in Delta H double dagger = 1 19 +/- 3 kJmol(-1) (I = 0.1 M) and Delta H double dagger = 123 +/- 3 kJmol( -1) (I = 1.0 M), Delta S double dagger = +130 +/- 13 Jmol(-1)K(-1) (I = 0.1 M) and Delta S double dagger = +151 +/- 15 Jmol(-1)K(-1) (I = 1.0 M) and D elta V double dagger = +27.6 +/- 0.6 cm(3)mol(-1) (I = 1.0 M). The kinetic results support the operation of the conjugate base mechanism, D-cb, with i ntermediate deprotonation occuring cis to the leaving chloride ion. The obs erved inertness of complex 2 towards base hydrolysis is discussed on the ba sis of the special structural features of the ligand L. The pK(a) of the co ordinated water in the aqua species CoL(H2O)(3+)(3) was determined to be pK (a) = 6.0 +/- 0.1 at I = 0.1 M. The X-ray crystal structure analysis of the hydroxo complex [CoL(OH)](ClO4)(2) (4) shows a mononuclear terminal (hydro xo)Co-III complex, in which the ligand provides a regular square-pyramidal NN4 donor cap for the octahedrally six-coordinate Co-III ion [4, monoclinic , space group C2/m, a = 16.841(6) Angstrom, b = 8.541(3) Angstrom, c = 15.1 12(5) Angstrom, beta = 109.13(1)degrees, Z = 4]. Coordination of L is accom panied by the formation of 6 six-membered chelate rings, all of which adopt a boat conformation. In the crystal lattice, pairs of cations are associat ed via four (hydroxo)O ... H-N(primary amine) hydrogen bonds and related th rough an inversion center. Full spectroscopic data for 4 (H-1-, C-13-NMR, I R, MS) are presented.