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The bicyclic cryptand 1,4,7,10,13,16,21,24-octaaza-bicyclo[8.8.8]hexacosan-5,6,14,15,22,23-hexone (2) bearing oxamide groups along the eight-atom bridges has been synthesized and its crystal structure solved by single-crystal X-ray analysis. 2 crystallizes in the monoclinic system, space group P21n, with a = 12.246(4) A, b = 14.421(2) A, c = 13.708(3)°, β = 112.21(3)° and Z = 4. The structure consists of discrete neutral macrobicyclic units. The two cycles share the two tertiary amine nitrogen atoms which exhibit an endo-endo conformation. The two tertiary amine groups are linked by three identical branches formed by N,N′-bis(ethyl)oxamide. The protonation reactions of 2 and its complex formation with copper(II) have been investigated by potentiometry in water and in a dmso-water (80:20 w/w) mixture as solvents. 2 acts as a bis-base through its two N-bridgehead and exhibits a strong cooperativity that favours the first protonation and makes difficult the second one (ΔpK = 5.2). An inward rotation of the oxamide groups to form hydrogen bonds accounts for this cooperativity. The interaction of 2 with copper(II) leads to several dinuclear complexes which different proton contents. The anionic ones are unstable and undergo hydrolytic decomposition to yield the novel dinuclear copper(II) complex {1, 10-bis(2-aminoethyl)-1,4,7,10,13,16-hexaazacyclooctadecane-5,6,14,15-tetronato(4-)} dicopper(II) (3). |
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