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Receptor versus Counterion: Capability of N , N′ ‐Bis(2‐aminobenzyl)‐diazacrowns for Giving Endo‐ and/or Exocyclic Coordination of Zn II
Author(s) -
Vaiana Lea,
PlatasIglesias Carlos,
EstebanGómez David,
Avecilla Fernando,
de Blas Andrés,
RodríguezBlas Teresa
Publication year - 2007
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200601012
Subject(s) - chemistry , perchlorate , acetonitrile , counterion , crystallography , metal , coordination sphere , ionic radius , stereochemistry , coordination complex , inorganic chemistry , medicinal chemistry , ion , crystal structure , organic chemistry
The structure of Zn II complexes with receptors L 1 and L 2 [L 1 = N , N′ ‐bis(2‐aminobenzyl)‐1,10‐diaza‐15‐crown‐5 and L 2 = N , N′ ‐bis(2‐aminobenzyl)‐4,13‐diaza‐18‐crown‐6] was studied both in the solid state and in acetonitrile solution. Both receptors form mononuclear Zn II complexes in this solvent, while no evidence for the formation of dinuclear complexes was obtained. This is in contrast with previous investigations that demonstrated the formation of dinuclear complexes of L 2 with first‐row transition metals such as Ni II , Co II and Cu II . Compounds of formula [Zn(L 1 )](ClO 4 ) 2 ( 1 ), [Zn(L 1 )](NO 3 ) 2 · 2CH 3 CN ( 2 ), [Zn(L 2 )](ClO 4 ) 2 ( 3 ) and [Zn(L 2 )(NO 3 ) 2 ] ( 4 ) were isolated and structurally characterised by X‐ray diffraction analyses. L 1 forms seven‐coordinate Zn II complexes in the presence of both nitrate and perchlorate anions, as a consequence of the good fit between the macrocyclic cavity and the ionic radius of the metal ion. The Zn II ion is deeply buried into the receptor cavity and the anions are forced to remain out of the metal coordination sphere. The cation [Zn(L 1 )] 2+ present in 1 and 2 is one of the very few examples of seven‐coordinate Zn complexes. Receptor L 2 provides a very rare example of a macrocyclic receptor allowing endocyclic and exocyclic coordination on the same guest cation, depending on the nature of the anion present. Thus, in 3 the Zn II ion is endocyclically coordinated, placed inside the crown hole coordinated to four donor atoms of the ligand in a distorted tetrahedral environment, whereas in 4 , the presence of a strongly coordinating anion such as nitrate results in an exocyclic coordination of Zn II , which is directly bound only to the two primarily amine groups of L 2 and two nitrate ligands. Spectrophotometric titrations of [Zn(L 2 )] 2+ with tetrabutylammonium nitrate in acetonitrile solution demonstrate the stepwise formation of 1:1 and 1:2 adducts with this anion in acetonitrile solution. The [Zn(L 1 )] 2+ , [Zn(L 2 )] 2+ and [Zn(L 2 )(NO 3 ) 2 ] systems were characterised by means of DFT calculations (B3LYP model). The calculated geometries show an excellent agreement with the experimental structures obtained from X‐ray diffraction analyses. Calculated binding energies of the macrocyclic ligands to Zn II are also consistent with the experimental data.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)