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Preparation and characterization of terdentate [C,N,N] acetophenone and acetylpyridine hydrazone platinacycles. A DFT insight into the reaction mechanism

Abstract

Reaction of N-orto-chlorophenyl substituted acetylpyridine hydrazones (a and d) with K2[PtCl4] (n-butanol/water, 100°C) gave mononuclear complexes 1a and 1d with the ligands as [N,N] bidentate. Contrastingly, reaction of N-phenyl or N-meta-chlorophenyl hydrazones (b and c, respectively) under analogous reaction conditions gave the cycloplatinated species 2b and 2c with the ligand as [C,N,N] terdentate. Treatment of the mononuclear complexes 1a and 1d with NaOAc (n-butanol, 100° C) gave the corresponding cycloplatinated complexes 2a and 2d. Acetophenone hydrazone platinacycle 2e was prepared in a similar fashion and its reaction with tertiary mono and triphosphines gave mono or trinuclear species depending on the reaction conditions. The X-ray crystal structures of some of these complexes showed interesting π-π slipped stacking interactions between metallacyclic rings which, according to NCI analyses, showed aromatic character. Aimed to the rationalization of the different reactivity showed by acetylpyridine hydrazones and the precise role of acetate anion, the energy profiles for the three main steps for the cycloplatination (iminoplatinum complex formation, chelation and cyclometallation) have been determined by DFT (M06) methods. Calculations indicate that cycloplatination of 1b proceeds via electrophilic substitution, involving the direct replacement of chloride anion at the Pt(II) centre by the N-phenyl moiety as the rate-determining step, to give an agostic intermediate 5b+ that, subsequently, undergoes elimination of a proton as hydrogen chloride. When present as an “external” base, acetate enters the coordination sphere around the Pt(II) centre and facilitates hydrazone N-H deprotonation and electrophilic C-H activation through a dissociative route, leading to a Wheland-type σ-complex intermediate 9ac.

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Publication details

The article was received on 13 Sep 2017, accepted on 10 Nov 2017 and first published on 13 Nov 2017


Article type: Paper
DOI: 10.1039/C7DT03418K
Citation: Dalton Trans., 2017, Accepted Manuscript
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    Preparation and characterization of terdentate [C,N,N] acetophenone and acetylpyridine hydrazone platinacycles. A DFT insight into the reaction mechanism

    I. Marcos, V. Ojea, D. Vazquez-Garcia, J. J. Fernández Sánchez, A. Fernández, M. Lopez-Torres, J. Lado and J. M. VILA, Dalton Trans., 2017, Accepted Manuscript , DOI: 10.1039/C7DT03418K

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