Preparation and reactivity of half-sandwich hydrazine complexes of ruthenium and osmium

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Abstract

Hydrazine complexes [MCl(η6-p-cymene)(RNHNH2)L]BPh4 (16) [M = Ru, Os; R = H, Me, Ph; L = P(OEt)3, PPh(OEt)2, PPh2OEt] were prepared by allowing dichloro complexes MCl26-p-cymene)L to react with hydrazines RNHNH2 in the presence of NaBPh4. Treatment of ruthenium complexes [RuCl(η6-p-cymene)(RNHNH2)L]BPh4 with Pb(OAc)4 led to acetate complex [Ru(κ2–O2CCH3)(η6-p-cymene)L]BPh4 (7). Instead, the reaction of osmium derivatives [OsCl(η6-p-cymene)(CH3NHNH2)L]BPh4 with Pb(OAc)4 afforded the methyldiazenido complex [Os(CH3N2)(η6-p-cymene)L}]BPh4 (8). Treatment with HCl of this diazenido complex 8 led to the methyldiazene cation [OsCl(CH3Ndouble bondNH)(η6-p-cymene)L}]+ (9+). The complexes were characterised spectroscopically and by X-ray crystal structure determination of [OsCl(η6-p-cymene)(PhNHNH2){PPh(OEt)2}]BPh4 (6b) and [Ru(κ2–O2CCH3)(η6-p-cymene){PPh(OEt)2}]BPh4 (7b).

Graphical abstract

The preparation of hydrazine complexes of ruthenium and osmium stabilised by the p-cymene fragment [MCl(η6-p-cymene)L]+ is described. Oxidation with Pb(OAc)4 at −30 °C of [OsCl(η6-p-cymene)(CH3NHNH2)L]BPh4, leading to the methyldiazenido derivative [Os(CH3N2)(η6-p-cymene)L]BPh4, is also reported.

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Highlights

► Facile synthesis of half-sandwich hydrazine complexes of Ru and Os. ► Oxidation with Pb(OAc)4 leads to both Ru-acetate and Os-methyldiazenido complexes. ► Reaction of Os-methyldiazenido complexes with HCl affords methyldiazene cations.

Introduction

The chemistry of transition metal complexes containing hydrazine NH2NH2 or substituted hydrazine RNHNH2 as ligands continues to be studied, not only due to interest in the differing coordination modes and interesting reactivity shown by these ligands [1](a), [1](b), [1], [2](a), [2](b), [2], [3], [4](a), [4](b), [4](c), [4](d), [4], but also due to the relationship of hydrazine with the nitrogen fixation process [5](a), [5](b), [5](c), [5](d), [5](e), [5](f), [5], [6](a), [6](b), [6](c), [6](d), [6](e), [6], [7].

Hydrazine is reported to coordinate both η1- and η2- to a metal centre, and may also behave as a bridging μ–η2 ligand [1](a), [1](b), [1], [2](a), [2](b), [2], [3], [4](a), [4](b), [4](c), [4](d), [4]. Coordinated NH2NH2 can either give stable 1,2-diazene complexes [M]–NHdouble bondNH, by both oxidation [5](a), [5](b), [5](c), [5](d), [5](e), [5](f), [5] and deprotonation with a strong base [4](a), [4](b), [4](c), [4](d), [4], or undergo reduction to ammonia [5](b), [6](a), [6](b), [6](c), [6](d), [6](e), [6], [7](a), [8](a), [8](b), [8]. Hydrazine has also been shown to be a substrate of nitrogenase and has been trapped as an intermediate during enzyme turnover [9], [9](a), [9](b).

A number of hydrazine RNHNH2 complexes of several transition metals have been reported in the past 30 years, mainly with π-acceptors such as carbonyl, phosphine and cyclopentadienyl as ancillary ligands [1](a), [1](b), [1], [2](a), [2](b), [2], [3], [4](a), [4](b), [4](c), [4](d), [4]. Less attention has been devoted to arene ligands and, for the iron triad, only a few examples of hydrazine complexes containing arene as supporting ligand have been reported [10], [10](a), [10](b), [10](c).

We are interested in the chemistry of diazo complexes and have reported the synthesis and reactivity of hydrazine complexes of the iron triad stabilised by phosphite, carbonyl or tris(pyrazolyl)borate ligands, of the type [MH(RNHNH2)L4]+, [M(RNHNH2)2L4]2+, [M(CO)(RNHNH2)L4]2+, [M(Tp)(RNHNH2)L(PPh3)]+ [M = Fe, Ru, Os; Tp = tris(pyrazolyl)borate; L = P(OEt)3, PPh(OEt)2] [11](a), [11](b), [11](c), [11](d), [11](e), [11](f), [11](g), [11](h), [11].

We have now extended these studies with the aim of introducing the arene ligands into the diazo chemistry of the iron triad. In this paper, we report the synthesis and reactivity of new hydrazine complexes of ruthenium and osmium stabilised by the p-cymene ligand.

Section snippets

General comments

All synthetic work was carried out in an appropriate atmosphere (Ar, N2) 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. RuCl3·3H2O and OsO4 were Pressure Chemical Co. (USA) products, used as received. Phosphites PPh(OEt)2 and

Results and discussion

Half-sandwich dichloro complexes [15] of ruthenium and osmium MCl26-p-cymene)L react with hydrazine NH2NH2 or monosubstituted hydrazines RNHNH2 in the presence of NaBPh4 to give hydrazine derivatives [MCl(η6-p-cymene)(RNHNH2)L]BPh4 (16), which were isolated in good yields and characterised (Scheme 1).

Crucial for the separation of compounds 16 as solids was the use of equimolar amounts of reagents and starting the reaction at a low temperature. The addition of an excess of hydrazine to the

Conclusions

A facile method for the synthesis of hydrazine complexes of ruthenium and osmium, stabilised by the p-cymene ligand, is reported. Oxidation with Pb(OAc)4 allowed acetate [Ru(κ2–O2CCH3)(η6-p-cymene)L]BPh4 and methyldiazenido complexes [Os(CH3N2)(η6-p-cymene)L}]BPh4 to be prepared. Half-sandwich methyldiazene cations [OsCl(CH3Ndouble bondNH)(η6-p-cymene)L}]+ were also obtained from the reaction of methyldiazenido complexes with HCl.

Acknowledgement

The financial support of MIUR (Rome)-PRIN 2009 is gratefully acknowledge. We thank Mrs. Daniela Baldan, from the Università Ca’ Foscari Venezia, for her technical assistance.

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