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