A Trinuclear Copper(II) Cryptate and Its μ₃‐CO₃ Cascade Complex: Thermodynamics, Structural and Magnetic Properties

Abstract

The 2,4,6-triethylbenzene-capped hexaamine macrobicycle with pyridyl spacers (pyr) was able to coordinate three copper(II) ions within its cavity. Potentiometric studies performed at 298.2 K in MeOH/H(2)O (50:50 v/v) and at ionic strength 0.10 mol dm(-3) in KNO(3) revealed that trinuclear species predominate in solution from pH 5.0, the hydroxo complexes being the main species, which start forming at unusual very low pH values. The single-crystal X-ray determination of the trinuclear complex showed that the three copper centres have square-planar geometry, arranged in an almost equilateral triangle, and have carbonate bridging the three metal centres. The presence of carbonate resulted from fixation of atmospheric CO(2). The present study represents the first μ(3)-CO(3)-bridged trinuclear copper(II) complex located in the interior of a macrobicyclic cavity. The magnetic data of [Cu(3)(pyr)(μ(3)-CO(3))]·(ClO(4))(4) showed ferromagnetic intramolecular interactions [J=3.80 cm(-1), based on the Hamiltonian H=-J(S(1)S(2)+S(2)S(3)+S(1)S(3))] yielding a spin quartet, S=3/2, ground state. Density functional calculations on the experimental geometry of the trinuclear complex showed that the ferromagnetic nature of the magnetic coupling can be attributed to the syn-anti conformation of the carbonato bridge, and a magneto-structural correlation, based on the different conformations (syn-anti, syn-syn and anti-anti), is presented. The interesting properties observed, namely the lowering of the pK(a) of coordinated water molecules to unusual values and the good fit of the carbonate anion between the copper centres, derive from the special architecture of pyr.

Synthesis, structure, magnetism, and hydrolase and catecholase activity of a new trinuclear copper(II) complex

“…For comparison, the weak antiferromagnetism (J = À1.0 cm À1 ) between Cu(1) and Cu(2) falls in the range of J values observed for many Cu(l-alkoxo)(l-Acetate)Cu complexes reported in the literature[22,23]. On the other hand, the weak ferromagnetic exchange (J 1,3 = J 2,3 ffi + 4.0 cm À1 ) between the Cu(3) and Cu(1)/Cu(2) centers are in agreement with the ferromagnetic coupling observed for open type dinuclear Cu(II) com-…”

“…The hydrolase and catecholase-like activities of 1 were determined by measuring the hydrolysis reaction of the model substrate bis (2,4-dinitrophenyl)phosphate (2, and the oxidation of the substrate 3,5-di-tert-butylcatechol (3,5-DTBC), respectively, in a Varian Cary 50 BIO UV-Vis spectrophotometer fitted with a thermostated water-jacketed cell holder. The reactions were accompanied by monitoring the increase in the 2,4-dinitrophenolate characteristic absorption band at 400 nm (pH/e L mol À1 cm À1 = 3.5/2125; 4.0/3408; 4.5/7182; 5.0/10 078; 5.5/11 405; 6.0/ 12 004; 6.5-10.0/12 100) at 50°C for the hydrolase-like activity [18] and the formation of 3,5-di-tert-butylquinone (3,5-DTBQ) at 400 nm (e = 1900 M À1 cm À1 ) for the catecholase-like activity at 25°C.…”

“…The study of model complexes of these systems should not only provide a better understanding of the biological molecules but also assist in the development of new lowmolecular-weight catalysts [2]. In fact, trinuclear copper(II) complexes with trigonal symmetry have attracted attention as magnetic materials and systems mimicking trinuclear metalloenzyme sites present in copper oxidases, such as ascorbate oxidase, ceruplasmin and laccase [3].…”

“…The structure of 1 was determined by X-ray crystallography and variable-temperature magnetic data in the temperature range of 4-300 K indicate weak antiferromagnetism (J 1,2 ffi À1.0 cm À1 ) between the Cu(1) and Cu (2) centers bridged by the l-alkoxo and l-acetate groups and weak ferromagnetic coupling (J 1,3 = J 2,3 ffi + 4.0 cm À1 ) involving the {Cu(1)(l-phenoxo)Cu(3)} and {Cu(2)(l-phenoxo)Cu(3)} structural units. Complex 1 shows significant catalytic activity towards the hydrolysis of the model substrate bis (2,4-dinitrophenyl)phosphate (2, as well as catechol oxidase activity since it efficiently catalyzes the oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butyl-o-benzoquinone by dioxygen. A comparison of the catalytic efficiency of 1 with its dinuclear parent complex [Cu 2 (H 2 bbppnol)(l-OAc)] 2+ (H 3 bbppnol = N,N 0 ,N,N 0 -bis[(2-hydroxybenzyl)(2-methylpyridyl)]-1,3-propanediamine-2-ol) reveals that the hydrolase efficiency (k cat /K M ) is 12 times higher for 1, while the catecholase efficiencies of these two compounds are similar.…”

Trinuclear zinc complexes for biologically relevant μ3-oxoanion binding and carbon dioxide fixation

“…In addition to the abovementioned aspects, the chemical fixation and activation of CO 2 via transition metal complexes have drawn extensive interest due to the intriguing coordination chemistry of CO 2 (refs 32-34) and its hydration products 28,[35][36][37][38][39] . In the zinc area, triply bridging carbonate units are often found in the solid structures of multinuclear zinc complexes and coordination polymers.…”