Quinquedentate co-ordination of amino-substituted tetraazacycloalkanes to cobalt(III). Part 2. Crystal structures of trans isomers, molecular mechanics calculations and base-hydrolysis kinetics

Abstract

Chlorocobalt(III) complexes of the pendant-arm macrocycles 12-methyl-1,4,7,10-tetraazacyclotridecan-12-amine (L¹³), 6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine (L¹⁴), 10-methyl-1,4,8,12-tetraazacyclopentadecan-10-amine (L¹⁵) and 3-methyl-1,5,9,13-tetraazacyclohexadecan-3-amine (L¹⁶) were isolated as trans isomers only for L¹⁶ and the minor isomer of L¹⁴. Of the two trans complexes, [Co(L¹⁴)Cl][ClO₄]₂ crystallized in the monoclinic space group P2₁/c, a= 9.107(3), b= 16.448(4), c= 13.898(5)Å and β= 99.16(3)°, and [Co(L¹⁶)Cl]Cl[ClO₄] crystallized in the same space group, a= 16.868(9), b= 7.531 (5), c= 20.51 (2)Å and β= 126.61 (5)°. Single-crystal X-ray structure determinations were refined to residuals of 0.056 and 0.071 for 2711 and 2314 ‘observed’ reflections respectively. In both cases the pendant primary amine and two adjacent secondary amines necessarily occupy an octahedral face, with the chloro ligand trans to the primary amine. Average macrocycle Co–N distances vary with ring size (1.96₁, 2.01₀Å for L¹⁴ and L¹⁶ respectively), as does the Co–Cl distance [2.244(2), 2.222(3)Å respectively], yet the Co–N (pendant) distance is constant within experimental error for both structures [1.961(4), 1.956(8)Å respectively]. Molecular mechanics calculations have been employed to predict the isomer preferences in all cases, and define the cis isomer of the L¹⁴ complex as more stable than the trans isomer by 4.4 kJ mol^–1(corresponding to a predicted cis:trans ratio of 86:14), reasonably consistent with the experimental ratio of ca. 98:2. For the trans isomers, base hydrolyses are rapid (k_OH 9100 and 11 100 dm³ mol^–1 s^–1 for L¹⁴ and L¹⁶ respectively), and not particularly sensitive to clear differences in Co–Cl distance. For the cis isomers the Co–Cl distances are minimized for L¹⁴(2.24₅, 2.23₆, 2.27₃Å for L¹³, L¹⁴ and L¹⁵ respectively), and this trend is reflected well in the comparative rate constants for base hydrolysis (k_OH 4300, 76 and 6700 dm³ mol^–1 s^–1 respectively). Variations in rate constants are tied to variations in the activation enthalpy, but not the activation entropy. Factors influencing base-hydrolysis rate constants for the series are discussed, and the significance of ground-state effects on hydrolysis rate is examined.