Metallacrowns are the inorganic analogues of the crown ethers, and they can be ideally obtained from the latter by replacing the carbon-carbon atoms with metal-heteroatom bonds. Metallacrowns with Cu2+ as the ring metal vacant metallacrowns (i.e. lacking the centralmetal ion) cannot be formed and isolated. For this reason, copper(II) metallcrowns are intended as structural and not functional analogues of crown ethers. Starting from preliminary observations that in copper(II) 15-MC-5 the Ca2+ ion can be displaced by Ln3+, we have addressed a series of investigations on these equilibria. The equilibrium constants for the substitution of Ca2+ by lanthanides(III) (La, Nd, Gd, Dy, Er, Yb) for the 15-MC-5 of (S)-α-tryptophanhydroxamic acid (Scheme 2, 3) showed that this process is favoured for heavier lanthanides, in very good agreement with several 15-crown-5 ethers. Vacant 15-metallacrowns-5 cannot be formed with Cu2+ as the ring metal, but our results demonstrate that they can be obtained with Ni2+ (Scheme 2, 4). For these complexes we have however observed that when K+ or Na+ salts are used as the ionic medium, the stability constant of the 15-MC-5 is higher than that obtained using the same ligand but with Bu4N+ as ionic medium cation. These results demonstrate that Ni2+ is capable to give rise to vacant 15-MC-5, although the enhancement of the stability observed using K+ and Na+ suggests that when these cations are employed, they are encapsulated into the metallacrown scaffold.
The functional behaviour of 15-metallacrowns-5: are they really different from parent crown ethers?
BACCO, Dimitri;REMELLI, Maurizio;
2009
Abstract
Metallacrowns are the inorganic analogues of the crown ethers, and they can be ideally obtained from the latter by replacing the carbon-carbon atoms with metal-heteroatom bonds. Metallacrowns with Cu2+ as the ring metal vacant metallacrowns (i.e. lacking the centralmetal ion) cannot be formed and isolated. For this reason, copper(II) metallcrowns are intended as structural and not functional analogues of crown ethers. Starting from preliminary observations that in copper(II) 15-MC-5 the Ca2+ ion can be displaced by Ln3+, we have addressed a series of investigations on these equilibria. The equilibrium constants for the substitution of Ca2+ by lanthanides(III) (La, Nd, Gd, Dy, Er, Yb) for the 15-MC-5 of (S)-α-tryptophanhydroxamic acid (Scheme 2, 3) showed that this process is favoured for heavier lanthanides, in very good agreement with several 15-crown-5 ethers. Vacant 15-metallacrowns-5 cannot be formed with Cu2+ as the ring metal, but our results demonstrate that they can be obtained with Ni2+ (Scheme 2, 4). For these complexes we have however observed that when K+ or Na+ salts are used as the ionic medium, the stability constant of the 15-MC-5 is higher than that obtained using the same ligand but with Bu4N+ as ionic medium cation. These results demonstrate that Ni2+ is capable to give rise to vacant 15-MC-5, although the enhancement of the stability observed using K+ and Na+ suggests that when these cations are employed, they are encapsulated into the metallacrown scaffold.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
