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
✍ Scribed by Tilo Poth; Helmut Paulus; Horst Elias; Rudi van Eldik; Andreas Grohmann
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 360 KB
- Volume
- 1999
- Category
- Article
- ISSN
- 1434-1948
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✦ Synopsis
Multi-wavelength stopped-flow spectrophotometry was used inertness of complex 2 towards base hydrolysis is discussed on the basis of the special structural features of the ligand L. to study the kinetics of base hydrolysis of the octahedral cobalt(III) complex CoLCl 2+ (2), in which the tetrapodal
The pK a of the coordinated water in the aqua species CoL(H 2 O) 3+ (3) was determined to be pK a = 6.0 ± 0.1 at I = pentadentate ligand L has an NN 4 donor set and forms a square-pyramidal coordination cap [L = 2,6-bis(1Ј,3Ј-0.1 M. The X-ray crystal structure analysis of the hydroxo complex [CoL(OH)](ClO 4 ) 2 (4) shows a mononuclear terminal diamino-2Ј-methylprop-2Ј-yl)pyridine, 1]. The kinetic investigation, carried out at different temperatures, pressures (hydroxo)Co III complex, in which the ligand provides a regular square-pyramidal NN 4 donor cap for the octahedrally 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 = six-coordinate Co III ion [4, monoclinic, space group C2/m, a = 16.841(6) A ˚, b = 8.541(3) A ˚, c = 15.112(5) A ˚, β = 109.13(1)°, 0.0570 ± 0.0004 M -1 s -1 (I = 1.0 M) at 298 K. The temperature and pressure dependence of k OH resulted in ∆H ‡ = 119 ± 3 Z = 4]. Coordination of L is accompanied by the formation of 6 six-membered chelate rings, all of which adopt a boat kJmol -1 (I = 0.1 M) and ∆H ‡ = 123 ± 3 kJmol -1 (I = 1.0 M), ∆S ‡ = + 130 ± 13 Jmol -1 K -1 (I = 0.1 M) and ∆S ‡ = + 151 ± 15 conformation. In the crystal lattice, pairs of cations are associated via four (hydroxo)O•••H-N(primary amine) Jmol -1 K -1 (I = 1.0 M) and ∆V ‡ = + 27.6 ± 0.6 cm 3 mol -1 (I = 1.0 M). The kinetic results support the operation of the conjugate hydrogen bonds and related through an inversion center. Full spectroscopic data for 4 ( 1 H-, 13 C-NMR, IR, MS) are base mechanism, D cb , with intermediate deprotonation occuring cis to the leaving chloride ion. The observed presented.