Reactivity with alkene and alkyne of pentamethylcyclopentadienyl half-sandwich diazoalkane complexes of ruthenium
Graphical abstract
Introduction
Transition metal complexes containing diazoalkanes as ligands have attracted interest for a long time [1], [2], [3], not only because of the different coordination mode and reactivity shown by coordinated N2CAr1Ar2 groups, but also due to their potential use in the synthesis of metal carbene derivatives [4], [5]. Diazoalkane is also of interest as a model for the dinitrogen fixation process [6].
Diazoalkane complexes have been reported for several transition metals [1], [2], [3] and reactivity studies have highlighted the different behaviour of the N2CAr1Ar2 groups, depending on their coordination mode and the nature of the supporting ligands. For example, extrusion of dinitrogen with the formation of carbene has been observed in η2-CN coordinated species [4], [5], whereas a η1-N bound diazoalkane can yield dinitrogen [M]–N2 complexes [2g], convert carbene to imine [5b] or cleave the N–N bond of the N2CAr1Ar2 group [2i]. Dipolar (3 + 2) cycloaddition of coordinated diazoalkane with alkene and alkyne has recently been reported [3](d), [3](e), [3](f).
Our ongoing interest in the chemistry of diazoalkane complexes has led us to the synthesis of half-sandwich compounds with cyclopentadienyl [Ru(η5-C5H5)(N2CAr1Ar2)(PPh3)L]BPh4 [3](d), [3](e), indenyl [Ru(η5-C9H7)(N2CAr1Ar2)(PPh3)L]BPh4 [3a] and tris(pyrazolyl)borate [Ru(Tp)(N2CAr1Ar2)(PPh3)L]BPh4 [3b], and to the study of (3 + 2) cycloaddition of the coordinated N2CAr1Ar2 with alkenes and alkynes, affording 3H-pyrazole derivatives. These interesting results prompted us to extend study of the reactivity of coordinated diazoalkanes with alkenes and alkynes to pentamethylcyclopentadienyl half-sandwich complexes [Ru(η5-C5Me5)(N2CAr1Ar2){P(OR)3}L]BPh4 [7], with the aim of testing whether (3 + 2) cyclisation occurs and how reactivity is influenced by the nature of the half-sandwich fragments. The results of this study are reported here.
Section snippets
General comments
All synthetic work was carried out in an appropriate atmosphere (Ar, N2) with standard Schlenk techniques or in an inert atmosphere dry-box. All solvents were dried over appropriate drying agents, degased on a vacuum line, and distilled into vacuum-tight storage flasks. RuCl3·3H2O was a Pressure Chemical Co. (USA) product, whereas pentamethylcyclopentadiene C5Me5H was a STREM product, used as received. Phosphites P(OMe)3 and P(OEt)3 were Aldrich products, used as received, whereas
Reactivity with alkenes
Reactivity studies of the half-sandwich pentamethylcyclopentadienyl diazoalkane complexes [Ru(η5-C5Me5)(N2CAr1Ar2){P(OR)3}L]BPh4 [7] were undertaken and some results are shown in Scheme 1.
First of all, different behaviour is shown towards ethylene (1 atm, RT) by the two types of complexes, containing either only phosphites (1, 2) or mixed ligands (3–5), with PPh3 and P(OR)3 or PPh(OEt)2. In the first case, the reactions do not proceed, and the starting diazoalkane compounds 1 and 2 can be
Conclusions
According to the nature of the phosphite ligands, diazoalkane complexes can give dipolar cycloaddition with acrylonitrile CH2C(H)CN and acetylene HCCH under mild conditions (1 atm, RT), yielding 3H-pyrazole derivatives. Unlike the closely related cyclopentadienyl analog, pentamethylcyclopentadienyl fragments [Ru(η5-C5Me5){P(OR)3}L]+ do not activate coordinated diazoalkane to cyclisation with ethylene. Displacement of the Ar1Ar2CN2 ligand by ethylene was observed in bis(phosphite) complexes. A
Acknowledgments
Thanks go to Mrs. Daniela Baldan for technical assistance.
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