Laura Carbonaro

Activity and osmotic coefficients from the emf of liquid-membrane cells. VII : Co(ClO4)2, Ni(ClO4)2, K2SO4, CdSO4, CoSO4, and NiSO4

The activity coefficients of CdSO4 CoSO4, and NiSO4 are determined from the emf of liquid-membrane cells, like those described in the papers I–VI of this series. The activity coefficients of the auxiliary salts Co(ClO4)2, Ni(ClO4)2, and K2SO4, needed to eliminate the problem of extrapolation to infinite dilution of the 2–2 salts, are also measured. CdSO4 CoSO4 and NiSO4 in the dilute region deviate downward from the limiting law by a larger extent than believed in the past, thus creating the need for the activity coefficients to be recalculated and systematically lowered by 8–16%. The activity coefficients of Co(ClO4)2 and Ni(ClO4)2, too, need to be corrected by around –3%. For K2SO4, the original values, although scattered, were substantially correct. Pitzers theory best-fit parameter, able to provide the activity and osmotic coefficients of the salts considered, are reported.

Self‐Assembly of Nickel(II) Complexes of New Bis‐Bidentate Schiff Base Ligands

Nickel(II) complexes derived from three new bis-bidentate Schiff bases, H2L1−3, having two N-n-propyl-salicylaldiminate units linked by an O−(CH2)n−O spacer (n = 4, 8, 12, respectively) at the 3-position of the salicyl moieties, are described. The pyridine adduct of the complex with the shorter spacer forms a cyclic trinuclear structure, [Ni3(L1)3(py)6] (2), in the solid state, where each NiII ion is octahedrally coordinated by two N,O-bidentate arms of two different ligands and two pyridine nitrogens. The base with n = 8 leads to the double-helical dinuclear complex, [Ni2(L2)2]·THF (3), which is the first example of a structurally characterised helicate based on a square-planar N2O2 coordination geometry. In CHCl3 solution, 3 undergoes a disassembly process which results in an equilibrium mixture where the mononuclear species is largely predominant.

A NEW SYNTHETIC ROUTE FOR TRANSITION-METAL COMPLEXES WITH SCHIFF-BASES

Abstract A new preparation method of Schiff base metal complexes is proposed, based on the transimination reaction between ammonium or ammonium-like salts and the imino portion of some N-substituted salicylaldimine-metal chelates. The reaction was carried out through a general acid catalysis induced by the ammonium ions and by the metal centre. It was mainly applied to the preparation of Ni(II), Cu(II) and Co(II) complexes. The reaction took place rapidly on addition of a slight excess of the ammonium salt to a non-aqueous solution of the Schiff base complex, at room temperature. The method can also be applied to the preparation of N -salicylidene amino acid complexes.

Kinetics and Equilibria of Nickel(II)Schiff Base Adducts Formation

The interactions between NiII cations and bidentate Schiff base ligands, N-alkyl-5-X-salicylaldimine HL (X = H; R = Et, nPr, tBu; X = Cl, OMe, NO2; R = nPr), and bis(N-n-alkyl-5-X-salicylaldiminato)nickel(II) complexes [NiL2] (X = H; R = Et, nPr, nBu; X = Cl, OMe, Me; R = nPr) were investigated by UV/Vis spectrophotometry in acetonitrile. The kinetics for the formation of NiII/HL 1:1 adducts was studied by stopped-flow techniques, which showed that the process followed a two-consecutive mechanism. Experimental evidence and theoretical calculations indicated the formation of an unidentate intermediate in the first step, according to a dissociative interchange mechanism. Ring-closure occurs in the second step and its rate is slower for electron-withdrawing-substituted Schiff bases. Nickel(II) cations interact with [NiL2] affording [NiL]+ and [Ni2L3]+ species; the associated equilibrium constants were found to be related to the electronic effects of the 5-X ligand substituents. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Computational Screening of Weak Hydrogen Bond Networks: Predicting Stable Structures for Difluoromethane Oligomers

The stability and the structure of small difluoromethane oligomers are studied by combining classical Monte Carlo and quantum mechanical calculations. A hierarchical procedure was adopted to validate the accuracy of the whole protocol: the force field used in Monte Carlo simulations is parametrized on the basis of dimer intermolecular energies computed with density functional theory. The density functional is similarly chosen by comparing the interaction energies with reference values, purposely computed at a coupled cluster level, extrapolated at the complete basis set. The structures of dimers, trimers, and tetramers identified by the screening as local minima are first characterized by some geometrical parameters and by their dipole moment and eventually validated by comparison with results of microwave spectroscopy. The results are found in very good agreement with the experiment for all considered structures.

Mechanistic study of Schiff base release from bis(N-phenyl-salicylaldiminato)nickel(II) in acetonitrile

The reaction of bis(N-phenylsalicylaldiminato)nickel(II) and ammonium or ammonium-like ions has been studied kinetically in acetonitrile solution. The general features of the release of N-phenylsalicylaldimine were investigated. Attempts have been made to assign the observed reaction rates to different species existing in solution. The results show that the protonable groups of the Schiff base are involved in an acid–base equilibrium with ammonium ions, in a complexes stepwise reaction mechanism. The equilibria also involve stereoisomeric nickel complexes, the free metal ion and the free ligand. The elimination of the first Schiff base shows a rate increase with increasing acidity of the ammonium ions, which can be correlated with their pKa values. The reaction with NHEt3+ is approximately three-fold slower than with NH3Et+, presumably because of steric hindrance. The formation of diphenyl(N-phenylsalicylaldiminato)boron, from the nickel complex and anilinium tetraphenylborate is also briefly discussed.

Mechanistic study of the transimination of bis(N-alkylsalicylaldiminato)nickel(II) with ammonium ion in acetonitrile

Electronic spectrophotometry has been empolyed to study the mechanism of transimination in bis(N-alkylsalicylaldiminato)nickel(II) complexes (alkyl = Me, Et, Pri, Prn or Bun) by ammonium ion in acetonitrile. The reaction is clearly biphasic: fast addition of ammonium and slow elimination of alkylammonium ions. The addition reaction involving the two azomethine bonds of the complex occurs by two consecutive processes and a two-term rate law was found for R = Bun. In the successive elimination reaction only one rate constant was observed which depends on the pKa and steric strain of the leaving alkylammonium ions.

Kinetic study of ligand release promoted by ammonium ion on bis(N-alkylsalicylaldiminato)zinc(II) complexes in aprotic solvent

Abstract The reaction between ammonium ion and bis(N-alkylsalicylaldiminato)zinc(II) complexes [Zn(R-sal)2] (R-sal=anion of N-alkylsalicylaldimine; R=H, n-Bu=n-butyl, t-Bu=tert-butyl, i-Pr=iso-propyl, Et=ethyl) in dry acetonitrile, leads to the release of the ligand. The reaction was studied kinetically at different temperatures and an analysis by mean of a multi-wavelength stopped-flow technique evidences a first-order kinetic as a general rule, and a two exponential profile for the t-butyl substituted complex for the release of one of the bidentate ligands of the Zn(II) complexes. Kinetic and 1H NMR data indicate that the first step of the release reaction of the ligand is the OZn bond breaking.