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Inorganic Chemistrytriazole)copper(II) ( J / k = -17.9 to -19.O"K) than in ligand atoms of dichloro(l,2,4-triazole)copper(II).13 copper(I1) benzoate trihydrate ( J / k = -12.7'K), deThus, although normal magnetic moments have been spite the fact that the Cu-Cu distance of 3.40 A in the observed for these compounds a t room temperature, former is longer than 3.15 A in the latter. Therefore, the spins are not free from one another, but antiferrothe superexchange interaction rather than the intermagnetic interaction exists between neighboring spins action of direct nature is predominant. This conchin a chain. sion is supported by the existence of spin densities in the (16) M . Inoue and &I, x7torg. them., 5, 70 (1066).The repulsion between the lone electron pairs of the nitrogen atoms in the staggered hydrazine molecule influences the length of the nitrogen-nitrogen bond; diminution of the repulsion causes shortening of the bond in those derivatives of hydrazine where the lone pair is attracted to bonds. the shifts of v(h--N) are in agreement with this point of view. The band shifts from 880 toward 1000 cm-' when the repulsion between the lone pairs diminishes. I n metal complexes of hydrazine, shifts of the band are also observed, depending on the field effect of the metal cations on the lone pairs, even if the nitrogen-nitrogen bond distance remains unaltered.The infrared spectra of several hydrazine derivatives have been examined:The hydrazine molecule H2N-NH2 is composed of two tetrahedra in staggered configuration, 1, one corner in each being occupied by a lone pair. The nitrogennitrogen boiid distance is N-N = 1.46 A3 in solid hydrazine but it becomes shorter in the cation +H3N-

A complete thermodynamic study of the protonation and Cu(II) complex formation equilibria of a series of alpha- and beta-aminohydroxamic acids in aqueous solution was performed. The thermodynamic parameters obtained for the protonation of glycine-, (S)-alpha-alanine-, (R,S)-valine-, (S)-leucine-, beta-alanine- and (R)-aspartic-beta-hydroxamic acids were compared with those previously reported for gamma-amino- and (S)-glutamic-gamma-hydroxamic acids. The enthalpy/entropy parameters calculated for the protonation microequilibria of these three types of ligands are in very good agreement with the literature values for simple amines and hydroxamic acids. The pentanuclear complexes [Cu5L4H(-4)]2+ contain the ligands acting as (NH2,N-)-(O,O-) bridging bis-chelating and correspond to 12-metallacrown-4 (12-MC-4) which are formed by self-assembly between pH 4 and 6 with alpha-aminohydroxamates (HL), while those with beta- and gamma-derivatives exist in a wider pH range (4-11). The stability order of these metallomacrocycles is beta- >> alpha- > gamma-aminohydroxamates. The formation of 12-MC-4 with alpha-aminohydroxamates is entropy-driven, and that with beta-derivatives is enthalpy-driven, while with gamma-GABAhydroxamate both effects occur. These results are interpreted on the basis of specific enthalpies or entropy contributions related to chelate ring dimensions, charge neutralization and solvation-desolvation effects. The enthalpy/entropy parameters of 12-MC-4 with alpha-aminohydroxamic acids considered are also dependent on the optical purity of the ligands. Actually, that with (R,S)-valinehydroxamic acid presents an higher entropy and a lower enthalpy value than those of enantiopure ligands, although the corresponding stabilities are almost equivalent. Moreover, DFT calculations are in agreement with a more exothermic enthalpy found for metallacrowns with enantiomerically pure ligands.

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Growing attention has been devoted in the recent years to a class of metallamacrocycles known as metallacrowns (MCs). They are structural analogues of crown ethers where the methylene bridges have been substituted by coordinative bonds formed by a transition metal ion ("ring" metal) and a nitrogen atom. The cavity of the metallacrown can accommodate an additional metal ion ("core" metal) either identical or different from the ring metal, thus forming a homo- or hetero-metallic MC. The most studied ring metal ion is certainly Cu(2+) and the aminohydroxamic acids have proved to be very suitable ligands to form MCs. The behavioural analogies between Cu(2+) and Ni(2+) in forming complexes, along with recent literature data in the solid state, prompted us to investigate the possible MC formation between Ni(2+) and both (S)-α-alaninehydroxamic acid and (S)-valinehydroxamic acid, in aqueous solution. Two metallacrowns, a 12-MC-4 and an unexpected 15-MC-5 have been detected by potentiometry and confirmed by ESI-MS results. Their structures are discussed on the basis of potentiometric, calorimetric, spectroscopic data and DFT calculations. The existence of a vacant 15-MC-5 species in solution can be put forward for the first time, making the present metal/ligand systems very interesting for their potential applications in cation recognition and separation. Finally, the crystal structure of the binary complex K[NiL(2)H(-1)]·5/3 H(2)O of (S)-α-alaninehydroxamic acid (LH) is also reported.

The solution equilibria of gamma-aminobutanehydroxamic acid (GABAha) with H+ and Cu2+ were investigated by potentiometry, titration calorimetry, spectrophotometry, NMR spectroscopy, and ESI-MS. The thermodynamic parameters of the CuII [12]metallacrown-4 obtained for GABAha were compared with those of the corresponding complexes of (S)-alpha-Alaha and beta-Alaha. The stability (-DeltaG0) sequence was beta-Alaha>>alpha-Alaha>GABAha, whereas the order of formation enthalpies (-DeltaH0) was beta-Alaha>>GABAha>alpha-Alaha. These data were interpreted on the basis of the dimensions of the chelate rings and the planarity of the metallamacrocycles. The CuII [12]metallacrown-4 ([12]MC-4) complex of GABAha was isolated and its crystal structure, which is the first reported for a [12]MC-4 of a gamma-aminohydroxamic acid, fully supports the structural features interpreted from the thermodynamic data.

Formation constants of ternary complexes of Cu" with (S)-amino-acid amides (phenylalaninamide, proiinamide, and tryptophanamide) and (R)-or (S)-amino acids (valine, phenylalanine, proline, and tryptophan) were determined potentiometrically at 25" and I = 0 . 1~ (KC1). Significant stereoselectivity was found for the systems (S)-tryptophanamide/proline, (S)-prolinamide/tryptophan, and (S)-phenylalaninamide/proline, the diastereoisomeric complexes with 'homochiral' ( S S ) being more stable than with 'heterochiral' ( S R ) ligands. The stereoselectivity observed may be explained on the basis of repulsive interactions between the ligand side-chain residues. The present data on the stability of mixed complexes in solution allow to draw some conclusions on the mechanism of chiral discrimination of amino acids in HPLC (reversed-phase) using Cu" complexes of (S)-amino-acid amides as selectors for ligand-exchange chromatography (LEC).1. Introduction. -Several investigations on mixed complexes of transition-metal ions with amino acids or dipeptides and different ligands are reported in the literature both in solution and in the solid state because of their significance as models for metal-ionassisted biological processes [ 13 [2]. The stabilities of these complexes were often explained on the basis of noncovalent interactions such as hydrophobic effects, aromatic ring stacking, H-bonding, or electrostatic ligand-ligand interactions [3].Stereoselectivity in mixed complexes of Cu" with a -amino acids has received little attention [4], and it appears to be essentially restricted to systems in which phenylalanine [5] [6], proline [7] [8], tryptophan [5-71, tyrosine [8] [9], and histidine [5] [6] are mutually involved, the effect being negligible, whenever one or both amino acids are aliphatic. Significant enantioselective effects were reported for Cu" complexes of N-alkylated valine and proline [lo], or histidine [5] with amino acids, although some data are not reliable (low precision) [lo]. N-carboxymethyl derivatives of (S)-Val, Ser, Ileu, Ala, Asp, Glu are enantioselective towards (R)/(S)-Val, Thr, and Leu [ 1 I].