Quinquedentate co-ordination of amino-substituted tetraazacycloalkanes to cobalt(III). Part 1. Complexes of macrocycles of differing ring size, and crystal structures of cis isomers
Abstract
The pendant-arm macrocycles 12-methyl-1,4,7,10-tetraazacyclotridecan-12-amine (L13), 6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine (L14) and 10-methyl-1,4,8,12-tetraazacyclopentadecan-10-amine (L15) react readily in water with cobalt(II) ion and air, followed by aqueous hydrochloric acid addition and equilibration with activated charcoal, to yield almost exclusively cis-[Co(Ln)Cl]2+ complexes (n= 13, 14 or 15). A trans isomer is detected in addition to the cis only in the case of L14, and then to only ca. 2% of total complexes. With 3-methyl-1,5,9,13-tetraazacyclohexadecan-3-amine (L16) a trans isomer is the sole product, however. The three cis complexes were crystallized readily as perchlorate or tetrachlorocobaltate salts: [Co(L13)Cl][CoCl4]·0.25MeOH, triclinic, space group P, a= 15.682(8), b= 14.864(2), c= 8.938(4)Å, α= 96.32(4), β= 97.34(6), γ= 102.59(2)°; [Co(L14)Cl][ClO4]2, monoclinic, space group P21/c, a= 9.856(4), b= 17.36(2), c= 12.08(1)Å, β= 98.95(5)°; and [Co(L15)Cl][ClO4]2·H2O, orthorhombic, space group Pn21a, a= 16.708(4), b= 13.343(3), c= 9.886(1)Å. Single crystal X-ray structure determinations were refined to residuals of 0.050, 0.051 and 0.037 for 4848, 3035 and 1549 ‘observed’ reflections respectively. In all cases the pendant primary amine and two adjacent secondary amines necessarily occupy an octahedral face, with the chloro ligand cis to the primary amine, and secondary amines adopt RRSS stereochemistries. There is an increase in average Co–N distances with macrocycle size (1.948, 1.954, 1.981Å for L13, L14 and L15 respectively), although the Co–N (pendant primary amine) distance is relatively invariant [range 1.955(4)–1.962(5)Å]. The Co–Cl distance, however, is minimized for L14[2.245(2), 2.236(2), 2.273(2)Å for L13, L14 and L15 respectively]. This latter trend is reflected in the comparative rate constants for stereoretentive base hydrolysis (kOH 4300, 76, 6700 dm3 mol–1 s–1 respectively), suggesting a ground-state influence on the reaction rate.