Elsevier

Journal of Organometallic Chemistry

Volume 822, 1 November 2016, Pages 259-268
Journal of Organometallic Chemistry

Reactivity with alkene and alkyne of pentamethylcyclopentadienyl half-sandwich diazoalkane complexes of ruthenium

https://doi.org/10.1016/j.jorganchem.2016.07.016Get rights and content

Highlights

  • Synthesis of half-sandwich η2-alkene derivatives of ruthenium.

  • Cycloaddition of coordinated diazoalkane affording 3H-pyrazole derivatives.

  • Synthesis of vinylidene derivatives of ruthenium.

Abstract

Treatment of diazoalkane complexes [Ru(η5-C5Me5)(N2CAr1Ar2){P(OR)3}L]BPh4 (15) [Ar1double bondAr2double bondPh, Ar1double bondPh, Ar2double bondp-tolyl, Ar1Ar2double bondC12H8; Rdouble bondMe, Et; Ldouble bondP(OR)3, PPh3] with ethylene and maleic anhydride (ma) afforded η2-alkene derivatives [Ru(η5-C5Me5)(η2-CH2double bondCH2){P(OR)3}(PPh3)]BPh4 (7, 8) and [Ru(η5-C5Me5)(η2-ma){P(OR)3}L]BPh4 (6, 9), respectively. Acrylonitrile also reacted with diazoalkane complexes 15 to give dipolar (3 + 2) cycloaddition, affording 3H-pyrazole derivatives [Ru(η5-C5Me5)

Image 2
{P(OR)3}(PPh3)]BPh4 [A] and [Ru(η5-C5Me5)
Image 3
{P(OR)3}(PPh3)]BPh4 [B] (10, 11). Treatment of complexes 15 with acetylene HCtriple bondCH under mild conditions (1 atm, room temperature) led to dipolar cycloaddition, affording 3H-pyrazole complexes [Ru(η5-C5Me5){η1-
Image 4
}{P(OR)3}L]BPh4 (12, 15), whereas reaction with terminal alkynes HC≡CR gave vinylidene derivatives [Ru(η5-C5Me5){double bondCdouble bondC(H)R}{P(OR)3}L]BPh4 (13, 14, 16, 17). The complexes were characterised spectroscopically (IR and 1H, 31P, 13C NMR) and by X-ray crystal structure determination of [Ru(η5-C5Me5)
Image 5
{P(OEt)3}2]BPh4 (12). A DFT study on the reaction of diazoalkane complexes with CH2double bondCH2 is also reported.

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 (35), 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 CH2double bondC(H)CN and acetylene HCtriple bondCH 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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      More complex examples that involve Group 8 metallacumulenes are presented in subsequent paragraphs and schemes. Examples that illustrated the reaction pathways in Scheme 52 include: (1) formation of cationic parent iron vinylidene complex 769 from trimethylsilylacetylene and transformation to the Fischer carbene complexes (e.g. 770) in refluxing methanol (or direct formation of cyclic carbene complexes using alkynols) [896]; (2) formation of a cationic osmium carbene complex (e.g. 772) through reaction of an osmium diphenyldiazomethane complex (e.g. 771) with phenylacetylene in ethanol (the diazo compound dissociates) [897]; (3) formation of ruthenium vinylidene complexes from terminal alkynes and N-bound diazomethane complexes [898]; (4) generation of cationic ruthenium vinylidene complexes from ethynylferrocene and subsequent deprotonation to afford bimetallic alkynylmetal complexes (e.g. 773), addition of alkyl halides to the neutral alkynylruthenium complexes to afford substituted cation vinylidene complexes (e.g. 774, from addition of BrCH2CN to 773), and reaction with nitrogen and oxygen nucleophiles to afford either carbene complexes of alkyne-coupled products [899]; (5) formation of a dicationic bis(ruthenium-allenylidene) complex (775) from the propargylic alcohol and a ruthenium chloride [900]; and (6) formation of cationic carbon-rich (γ,γ-diaryl substituted) ruthenium allenylidene complexes (e.g. 776) from the corresponding propargylic alcohols and formation of π-complex adducts with polyaromatic molecules in solid state and solution [901]. Examples where the alkynylmetal complexes are produced through tandem terminal alkyne to vinylidene complex conversion followed by deprotonation, often without isolation of the metallacumulene, include: (1) formation of tris(alkynylruthenium) complexes linked through a tris(thiophene) group (e.g. 777) [902]; (2) formation bis(alkynyl)ruthenium complexes containing an azobenzene group (e.g. 778) [903]; (3) formation of alkynylruthenium complexes containing nickel-salen functionality and linked through an azobenzene group [904]; (4) mixed cationic alkynyl allenylidene ruthenium complexes for photo-electrochemical applications (e.g. 779) [905]; (5) fluorenylethynylruthenium complexes [906]; (6) dithiophene-bridged 2,5-dialkynyl diiron complexes (and analogs featuring more alkyne groups in the spacer) [907], and (7) bis(alkynyl)ruthenium (and platinum) complexes further conjugated to aromatic rings and other alkyne groups [908].

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