Dicopper(ii) Complex of the Large Polyazacycloalkane 1,4,7,10,13,16,19,22-octaazacyclotetracosane (bistrien) - Synthesis, Crystal-structure, Electrochemistry, and Thermodynamics of Formation

The large macrocycle 1,4,7,10,13,16,19,22-octaazacyclotetracosan(eb istrien) was synthesized. The eight basicity constants and the stability constants of the dinuclear complexes [CuZ(bistrien)l4+[, C~~(bistrien)H~a]n~d' , [C~~(bistrien)OHw]~er+e determined by potentiometry at 25 OC in 0.5 mol dm-3 NaC10,. The formation enthalpy of [CuZ(bistrien)l4' was determined by batch microcalorimetry. The most important species, [Cu2(bistrien)14', is extremely stable (log k = 36.63) and exothermic (-AHo = 39.0 kcal mol-'), indicating that all nitrogen atoms are involved in the coordination to the metal ions. The stability of the hydroxo species [C~~(bistrien)OH]i~s 'h igh, and its magnetic susceptibility in solution shows metal-metal interaction. Crystals of the com und [Cuz(bistrien)C12](C104a)rez monoclinic, space group P2 , /m, with a = 11.811 (3) A, b = 11.352 ( 3 ) A, c = 11.385 (3) r@ = 99.1 1 (4)O, and 2 = 2. Refinement of the atomic parameters by least squares gave a final R factor of 0.080 (R, = 0.070) for 1340 unique reflections with F,, > 20(F,). The structure consists of centrosymmetric binuclear Cu2(bistrien) units held together by chlorine bridges forming pairs of polymeric ..-Cu-Cl-Cu-CI.-Cu zigzag chains parallel to the b axis. The copper atoms are coordinated by four nitrogen atoms of the macrocyclic ligand (average Cu-N = 2.04 (1) A) and one chloride ion (CuCl = 2.48 (2) A), forming a distorted square pyramid. One further chloride ion at 3.31 (2) A completes a distorted elongated octahedron. The electrochemical behavior of the dinuclear complex [CuZ(bistrien)l4' was investigated in aqueous solution by cyclic voltammetry. The complex undergoes a two-electron reversible reduction into the dinuclear Cu(1) species, by two independent monoelectronic steps. The reduced species is unstable with respect to the demetalation and reduction to metal.