Reaction of bis(aryldiazenido) complexes of rhenium with bromine: Preparation of new diazo derivatives
Graphical abstract
The reaction of bis(aryldiazenido) [Re(ArN2)2P3]BPh4 complexes with Br2 is presented. The synthesis of aryldiazene–aryldiazenido ReBr2(ArN2)(ArNNH)P2 derivatives was achieved by reacting bis(aryldiazenido) with hydride-donor species. Protonation studies on aryldiazene complexes are also reported.
Introduction
The synthesis and reactivity of organodiazo complexes of transition metals continue to be of interest, both for their relevance in the field of nitrogen fixation and for preparing nitrogen-containing organic compounds [1], [2], [3]. A large number of studies have been reported in the past 30 years [1], [2], [3], [4], [5], [6] highlighting the interesting properties of the aryldiazenido ArN2 ligand and its coordination modes, as well as the reactivity of related diazene RNNH and hydrazine RNHNH2 ligands. The easy transformation of aryldiazene into aryldiazenido complexes and vice versa through deprotonation/protonation reactions is well-known [4], as is the possibility of synthesizing aryldiazene complexes, either through insertion of aryldiazonium cations into the metal–hydride bond M–H [5] or by oxidation of coordinated hydrazine [6]. However, despite numerous reports [1], [2], [3], [4], [5], [6], little is known about the reactivity of bis(aryldiazenido) complexes [7], mainly toward oxidation or reduction reactions.
We are interested in organometallic diazo chemistry [8] and have recently reported [9] the reactivity of rhenium complexes with hydrazine, which allows the synthesis of bis(aryldiazenido) [Re(ArN2)2P3]BPh4 derivatives. We have now extended our studies to the reactivity of these bis(aryldiazenido) complexes, and the results, which highlight new properties of diazo complexes of rhenium, are reported here.
Section snippets
General considerations and physical measurements
All synthetic work was carried out in appropriate atmospheres (Ar, H2) using standard Schlenk techniques or a Vacuum Atmosphere dry-box. Once isolated, the complexes were found to be relatively stable in air, but were stored in an inert atmosphere at −25 °C. All solvents were dried over appropriate drying agents, degassed on a vacuum line, and distilled into vacuum-tight storage flasks. Metallic rhenium was a Chempur (Germany) product, used as received. Phenylhydrazine (PhNHNH2) was an Aldrich
Results and discussion
Pentacoordinate bis(aryldiazenido) [Re(ArN2)2P3]BPh4 complexes react with equimolar amounts of Br2 to give first the tris(phosphine) [ReBr(ArN2)2P3]2+ (1) species and then the [ReBr2(ArN2)2P2]+ (2, 3) derivatives, as shown in Scheme 1.
The reaction was studied between −80 and +20 °C by 1H and 31P NMR. Results showed that the addition of an equimolar amount of Br2 to the [Re(ArN2)2(PPh2OEt)3]+ cation at −80 °C caused the disappearance of precursor signals and the appearance of new signals
Acknowledgements
The financial support of MIUR (Rome) – Programmi di Ricerca Scientifica di Rilevante Interesse Nazionale, PRIN 2004 – is gratefully acknowledged. We thank Daniela Baldan for technical assistance.
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