Lucio Senatore

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.

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.

Nucleophilic substitution at four-co-ordinate sulphur. Mobility of the leaving group

The leaving group effect has been measured for the reactions of benzenesulphonyl halides with aniline, n-butylamine, and hydroxide ion. The specific rate constants of displacement at 25° with I, Br, Cl, and F, respectively, as leaving groups, are the following: with aniline, 3·55 × 10–2, 31·2 × 10–2, 4·27 × 10–2, and 2·6 × 10–7; with n-butylamine, 21·9, 103, 42·6, and 1·01 × 10–2; and with hydroxide ion, 43·6, 28·9, 15·5, and 3·40. The almost identical leaving group mobility of I, Br, and Cl for each nucleophile and the enormous change in relative group mobility of fluorine on changing the pKa of the nucleophile, point to a mechanism involving an intermediate complex with bond forming or bond breaking as the rate-limiting step according to the substrate. The activation parameters are also reported and agree with this interpretation.

Neutral and alkaline hydrolysis of 2,4,6-trimethylbenzenesulphonyl chloride

The kinetics of the alkaline hydrolysis of 2,4,6-trimethylbenzenesulphonyl chloride have been re-studied. Contrary to a previous report, the reaction does depend on the concentration of nucleophile. This eliminates the only hitherto undisputed example of an SN1 mechanism in nucleophilic substitution at sulphur.

Chemistry of sulphenates in acidic media

Methyl toluene-p-sulphenate is hydrolysed rapidly in moist solvents (dioxan, benzene, chloroform, or nitrobenzene) yielding methyl toluene-p-sulphinate, di-p-tolyl disulphide, and methanol. The reaction is acid catalysed. At higher concentration of water (>1%) the products are: p-tolyl toluene-p-thiolsulphonate, di-p-tolyl disulphide, and methanol. A mechanism is suggested where thiolsulphinate, which is usually suggested as the intermediate in the hydrolysis of sulphenyl derivatives, combines with unchanged sulphenate ester to yield an intermediate sulphonium salt. The sulphonium slat then rapidly reacts with either methanol or water to yield the products. Some implications for the chemistry of sulphenic acids are also discussed.

Nucleophilic substitution at sulphur. Reaction of p-nitrophenyl triphenylmethanesulphenate with n-butylamine and benzamidine

The reaction of n-butylamine with p-nitrophenyl triphenylmethanesulphenate is of the second order in nucleophile, but the reaction of benzamidine with the same substrate in the same solvent is of the first order in nucleophile. The same pattern is found when n-butylamine and benzamidine react with carboxylic esters and aromatic derivatives. It is suggested that this pattern has a common origin and that the first-order behaviour of benzamidine can be explained without resorting to bifunctional catalysis.

Reactions of thiolsulphonates with amines

The equilibrium constants for the reaction of morpholine with methyl or t-butyl toluene-p-thiolsulphonates have been measured in acetonitrile–water (7 : 1,v/v) at 25 °C. Contrary to previous data they are 2·5 × 10–3 and 3·4 for the methyl and t-butyl substrates respectively. In the same solvent medium have been measured the reaction rates and the activation parameters of the nucleophilic displacement reaction of morpholine with the same substrates. The overall mechanism is discussed.

Nucleophilic substitution at two-co-ordinate sulphur. Brønsted relationships with nitrogen nucleophiles

The rate of reaction of p-nitrophenyl triphenylmethanesulphenate with a variety of amines has been measured in 45% dioxan–water. The kinetic data, plotted in a Bronsted fashion against the pKa of the conjugate acid of the nucleophile, yield linear relationships characterized by β= 1·5 for anilines, 0·84 for pyridines, 0·75 for secondary heterocyclic amines, and 0·58 for primary aliphatic amines. The large effect of the basicity of the nucleophile on the rate of reaction is interpreted in terms of large bond formation in the transition state and the much larger slope found for anilines is discussed.