Kinetics and Mechanism of the Reaction between Chromium(III) and 2,3-Dihydroxybenzoic Acid in Weak Acidic Aqueous Solutions

The reaction between chromium(III) and 2,3-dihydroxybenzoic acid (2,3-DHBA) takes place in at least three stages, involving various intermediates. The ligand (2,3-DHBA)-to-chromium(III) ratio in the final product of the reaction is 1 : 1. The first stage is suggested to be the reaction of [Cr(H2O)5(OH)]2+ with the ligand in weak acidic aqueous solutions that follows an Id mechanism. The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters are ΔH≠2(obs) = 61.2 ± 3.1 kJmol−1, ΔS≠2(obs) = −91.1 ± 11.0 JK−1mol−1, ΔH≠3(obs) = 124.5 ± 8.7 kJmol−1, and ΔS≠3(obs) = 95.1 ± 29.0 JK−1mol−1. These two stages are proposed to proceed via associative mechanisms. The positive value of ΔS≠3(obs) can be explained by the opening of a four-membered ring (positive entropy change) and the breaking of a hydrogen bond (positive entropy change) at the associative step of the replacement of the carboxyl group by the hydroxyl group at the chromium(III) center (negative entropy change in associative mechanisms). The reactions are accompanied by proton release, as shown by the pH decrease

Kinetics and Mechanism of the Reaction between Chromium(III) and 3,4-Dihydroxy-Phenyl-Propenoic Acid (Caffeic Acid) in Weak Acidic Aqueous Solutions

Our study of the complexation of 3,4-dihydroxy-phenyl-propenoic acid by chromium(III) could give information on the way that this metal ion is available to plants. The reaction between chromium(III) and 3,4-dihydroxy-phenyl-propenoic acid in weak acidic aqueous solutions has been shown to take place by at least three stages. The first stage corresponds to substitution (Id mechanism) of water molecule from the Cr(H2O)5OH2+ coordination sphere by a ligand molecule. A very rapid protonation equilibrium, which follows, favors the aqua species. The second and the third stages are chromium(III) and ligand concentration independent and are attributed to isomerisation and chelation processes. The corresponding activation parameters are ΔH2(obs)≠ = 28.6 ± 2.9 kJ mol−1, ΔS2(obs)≠ = −220 ± 10 J K−1mol−1, ΔH3(obs)≠ = 62.9 ± 6.7 kJ mol−1 and ΔS3(obs)≠ = −121 ± 22 J K−1mol−1. The kinetic results suggest associative mechanisms for the two steps. The associatively activated substitution processes are accompanied by proton release causing pH decrease

Reaction of Chromium(III) with 3,4-Dihydroxybenzoic Acid: Kinetics and Mechanism in Weak Acidic Aqueous Solutions

The interactions between chromium(III) and 3,4-dihydroxybenzoic acid (3,4-DHBA) were studied resulting in the formation of oxygen-bonded complexes upon substitution of water molecules in the chromium(III) coordination sphere. The experimental results show that the reaction takes place in at least three stages, involving various intermediates. The first stage was found to be linearly dependent on ligand concentration k1(obs)′ = k0 + k1(obs)[3, 4-DHBA], and the corresponding activation parameters were calculated as follows: ΔH1(obs)≠ = 51.2 ± 11.5 kJ mol−1, ΔS1(obs)≠ = −97.3 ± 28.9 J mol−1 K−1 (composite activation parameters) . The second and third stages, which are kinetically indistinguishable, do not depend on the concentrations of ligand and chromium(III), accounting for isomerization and chelation processes, respectively. The corresponding activation parameters are ΔH2(obs)≠ = 44.5 ± 5.0 kJ mol−1, ΔS2(obs)≠ = −175.8 ± 70.3 J mol−1 K−1. The observed stages are proposed to proceed via interchange dissociative (Id, first stage) and associative (second and third stages) mechanisms. The reactions are accompanied by proton release, as is shown by the pH decrease

Study of the interactions of platinum group metals with Derivatives of dithiocarbazic acid

This work is aimed to the study of the complexation of S-benzyl dithiocarbazate and S-benzyl- b-N-isopropylidenedithiocarbazate with platinum, palladium and ruthenium, the comparison of the chemistry of these three metals and the effort to prepare platinum blue compounds. In the prepared complexes, ligands exist in their deprotonated anionic form. Complexes are chelates, with ligands coordinating by thiolato sulfur and b nitrogen atoms. Crystals of the complexes [PdL2], [PdL΄2] and [PtL΄2], where L and L΄ are respectively the deprotonated forms of S-benzyl dithiocarbazate and S-benzyl- b-N-isopropylidenedithiocarbazate, were studied by X-ray diffraction techniques. In [PdL2] the coordination geometry of the metal is trans square planar. In [PdL΄2] and [PtL΄2] the coordination geometry of the metal is cis distorted square planar, with the planes of the two phenyl rings being practically vertical to the coordination plane of the metal and forming a dihedral angle of ~ 130°. A blue platinum compound was prepared in inert atmosphere using K2[PtCl6] and S-benzyl- b- N-isopropylidenedithiocarbazate as starting materials. The dark green solid prepared from K2[PtCl4] and S-benzyl dithiocarbazate with ratio 1 χ 2 in the presence of equimolar to the ligand quantity of triethylamine is a mixture of two complexes, A and B, of which only B absorbs in the visible region. In DMF solution A is oxidized to B in a first- or pseudo-first-order reaction where the activated complex is formed with an associative mechanism.Η παρούσα εργασία αποσκοπεί στη μελέτη της συμπλεκτικής δράσης του διθειοκαρβαζικού S-βενζυλεστέρα και του β-Ν-ισοπροπυλιδενοδιθειοκαρβαζικού S-βενζυλεστέρα με λευκόχρυσο, παλλάδιο και ρουθήνιο, τη σύγκριση της χημικής συμπεριφοράς των τριών μετάλλων και την προσπάθεια να συντεθούν σύμπλοκα ανήκοντα στην κατηγορία των μπλε ενώσεων του λευκοχρύσου. Στα παρασκευασθέντα σύμπλοκα οι υποκαταστάτες βρίσκονται με την αποπρωτονιωμένη ανιοντική τους μορφή. Τα σύμπλοκα είναι χηλικά, με τους υποκαταστάτες να συμπλέκονται μέσω του θειολικού ατόμου θείου και του β ατόμου αζώτου. Κρύσταλλοι των συμπλόκων [PdL2], [PdL΄2] και [PtL΄2], όπου L και L΄ οι αποπρωτονιωμένες μορφές αντίστοιχα του διθειοκαρβαζικού S-βενζυλεστέρα και του β-Ν-ισοπροπυλιδενοδιθειοκαρβαζι- κού S-βενζυλεστέρα, μελετήθηκαν με τη μέθοδο της περίθλασης ακτίνων Χ. Στο [PdL2] η γεωμετρία του μετάλλου είναι trans επίπεδη τετραγωνική. Στα [PdL΄2] και [PtL΄2] η γεωμετρία του μετάλλου είναι cis παραμορφωμένη επίπεδη τετραγωνική, με τα επίπεδα των δύο φαινυλίων να είναι πρακτικά κάθετα στο επίπεδο σύμπλεξης του μετάλλου και να σχηματίζουν δίεδρη γωνία ~ 130°. Μπλε ένωση του λευκοχρύσου παρασκευάστηκε σε αδρανή ατμόσφαιρα χρησιμοποιώντας ως πρώτες ύλες K2[PtCl6] και β-Ν-ισοπροπυλιδενοδιθειοκαρβαζικό S-βενζυλεστέρα. Το σκούρο πράσινο στερεό που παρασκευάζεται από K2[PtCl4] και διθειοκαρβαζικό S-βενζυλεστέρα σε αναλογία 1 χ 2 παρουσία ισομοριακής ως προς τον υποκαταστάτη ποσότητας τριαιθυλαμίνης είναι μίγμα δύο συμπλόκων, Α και Β, από τα οποία μόνο το Β απορροφά στην περιοχή του ορατού. Σε διάλυμα σε DMF το Α οξειδώνεται στο Β μέσω μιας αντίδρασης πρώτης ή ψευδοπρώτης τάξης όπου το ενεργοποιημένο σύμπλοκο σχηματίζεται με προσεταιριστικό μηχανισμ