resumo
The 2,4,6-triethylbenzenecapped 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/H2O (50: 50 v/v) and at ionic strength 0.10 moldm(-3) in KNO3 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 CO2. The present study represents the first mu(3)-CO3-bridged trinuclear copper(II) complex located in the interior of a macrobicyclic cavity. The magnetic data of [Cu-3(pyr)(mu(3)-CO3)]center dot(ClO4)(4) showed ferromagnetic intramolecular interactions [J = 3.80 cm(-1), based on the Hamiltonian H = -J(S1S2+S2S3+S1S3)] 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 magne-to-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.
palavras-chave
TRANSITION-METAL-COMPLEXES; ATMOSPHERIC CARBON-DIOXIDE; EXCHANGE COUPLING-CONSTANTS; GAUSSIAN-BASIS SETS; X-RAY-STRUCTURE; CRYSTAL-STRUCTURE; CO2 FIXATION; MACROCYCLIC COMPLEXES; COORDINATION MODE; ZN(II) COMPLEXES
categoria
Chemistry
autores
Mateus, P; Delgado, R; Lloret, F; Cano, J; Brandao, P; Felix, V
nossos autores
Paula Cristina Ferreira da Silva Brandão
Investigador Auxiliar
Vítor Félix
Professor Associado com Agregaçãoagradecimentos
The authors acknowledge FCT, with co-participation of the European Community funds FEDER, POCI, QREN and COMPETE for the financial support under project PTDC/QUI/67175/2006. The authors also thank the MICINN (Spain) (Projects CTQ2010-15364 and the Consolider Ingenio in Molecular Nanoscience CSD2007-00010) and the Generalitat Valenciana (Spain) (Project PROMETEO/2009/108). The NMR spectrometers are part of The National NMR Network (REDE/1517/RMN/2005), supported by