Quinquedentate co-ordination of amino-substituted tetraazacycloalkanes to chromium(III)

Abstract

The pendant-arm macrocycles 12-methyl-1,4,7,10-tetraazacyclotridecane-12-amine (L³), 6-methyl-1,4,8,11-tetraazacyclotetradecane-6-amine (L⁴) and 10-methyl-1,4,8,12-tetraazacyclopentadecane-10-amine (L⁵) react readily in methanol with chromium(II) ion, followed by dilution with aqueous hydrochloric acid and chromatography, to yield cis- and or trans-[Cr(Lⁿ)Cl]²⁺ complexes (n= 3, 4 or 5). The two trans complexes were crystallized readily as perchlorate salts: [Cr(L⁴)Cl][ClO₄]₂·H₂O, monoclinic, space group P2₁/c, a= 11.015(4), b= 13.549(4), c= 17.238(3)Å, β= 125.20(3)°; [Cr(L⁵)Cl][ClO₄]₂, triclinic, space group P, a= 13.270(3), b= 9.445(3), c= 9.396(3)Å, α= 100.04(2), β= 96.14(2), γ= 107.15(2)°. Single-crystal X-ray structure determinations were refined to residuals of 0.030 and 0.052 for 2946 and 3211 ‘observed’ reflections respectively. In each case the pendant primary amine and two adjacent secondary amines necessarily occupy an octahedral face, with the chloro ligand trans to the primary amine, and secondary amines adopting RRSS stereochemistries. There is an increase in average Cr–N distances with macrocycle size (2.06₉, 2.07₉Å for the L⁴ and L⁵ complexes respectively), although the Cr–N (pendant primary amine) distance is invariant [2.077(3)Å in each case]. The Cr–Cl distance is smaller for L⁴[2.297(1)Å] than for L⁵[2.306(1)Å]. The second-order rate constants for base hydrolysis are k_OH 1.4 × 10⁵, 2.6 × 10⁷, 0.7, and 8.2 dm³ mol^–1 s^–1 for cis-L³, cis- and trans-L⁴, and trans-L⁵ respectively, with markedly greater values for the cis isomers arising largely from lower activation enthalpies, and a greater value for the longer Cr–Cl bond length in the two trans isomers.