Preparation and reactivity with azo-species of hydride and dihydrogen complexes of osmium stabilised by tris(pyrazolyl)borate and phosphite ligands

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Abstract

Mixed-ligand OsCl(Tp)L(PPh3) complexes 1 [Tp = hydridotris(pyrazolyl)borate; L = P(OMe)3, P(OEt)3 and PPh(OEt)2] were prepared by allowing OsCl(Tp)(PPh3)2 to react with an excess of phosphite. Treatment of chlorocomplexes 1 with NaBH4 in ethanol afforded hydride OsH(Tp)L(PPh3) derivatives 2. Stable dihydrogen [Os(η2-H2)(Tp)L(PPh3)]BPh4 derivatives 3 were prepared by protonation of hydrides 2 with HBF4 · Et2O at −80 °C. The presence of the η2-H2 ligand is supported by short T1 min values and JHD measurements on the partially deuterated derivatives. Treatment of the hydride OsH(Tp)[P(OEt)3](PPh3) complex with the aryldiazonium salt [4-CH3C6H4N2]BF4 afforded aryldiazene [Os(4-CH3C6H4Ndouble bondNH)(Tp){P(OEt)3}(PPh3)]BPh4 derivative 4. Instead, aryldiazenido [Os(4-CH3C6H4N2)(Tp)[P(OEt)3](PPh3)](BF4)2 derivative 5 was obtained by reacting the hydride OsH(Tp)[P(OEt)3](PPh3) first with methyltriflate and then with aryldiazonium [4-CH3C6H4N2]BF4 salt. Spectroscopic characterisation (IR, 15N NMR) by the 15N-labelled derivative strongly supports the presence of a near-linear Os–Ntriple bondN–Ar aryldiazenido group. Imine [Os{η1-NHdouble bondC(H)Ar}(Tp){P(OEt)3}(PPh3)]BPh4 complexes 6 and 7 (Ar = C6H5, 4-CH3C6H4) were also prepared by allowing the hydride OsH(Tp)[P(OEt)3](PPh3) to react first with methyltriflate and then with alkylazides.

Graphical abstract

The synthesis of hydride and dihydrogen complexes of osmium with phosphite and tris(pyrazolyl)borate as supporting ligands is described. Reactivity studies towards aryldiazonium cations and organic azide allow the first organometallic diazo complexes of osmium to be prepared.

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Introduction

Dihydrogen complexes of transition metals are an important class of compounds which have been extensively studied in the last twenty years, both from a fundamental point of view and in an attempt to provide insights into the stoichiometric and catalytic activation of H2 by coordination [1], [2], [3]. Numerous examples of stable η2-H2 complexes have been prepared for several transition metals with a wide range of ancillary ligands, including mono and polydentate tertiary phosphines, carbonyl, nitrosyl and cyclopentadienyl species [1], [2], [3].

Less attention has been paid to the tris(pyrazolyl)borate (Tp) ligand, whose dihydrogen chemistry has led to relatively few papers with respect to the other ligands [4], [5], and only two examples of stable η2-H2 complexes have been reported for osmium [6].

We are interested in the chemistry of classical and non-classical metal hydride complexes, and have reported the synthesis and reactivity of several η2-H2 derivatives of manganese and iron triads stabilised by phosphite ligands [7], [8]. We have also focused attention on the use of these hydrides as precursors in “organometallic” diazo chemistry, which allows the preparation of numerous examples of aryldiazene, aryldiazenido and hydrazine derivatives [9]. Recently, we have extended these studies to polypyridine [10] and tris(pyrazolyl)borate as supporting ligands, and have reported the synthesis and reactivity of classical and non-classical hydride complexes of ruthenium stabilised by the Ru(Tp)L(PPh3) fragment [11].

Now, as part of ongoing studies, we report the synthesis and reactivity towards azo-species of new hydride and dihydrogen complexes of osmium containing Tp and phosphite as ancillary ligands.

Section snippets

General considerations

All synthetic work was carried out in an appropriate atmosphere (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. (NH4)2OsCl6 salt was a Pressure Chem (USA) product, used as received. Potassium hydridotris(pyrazolyl)borate

Synthesis of hydride complexes

The triphenylphosphine OsCl(Tp)(PPh3)2 complex [6b] reacts with phosphites in toluene to give mixed-ligand OsCl(Tp)L(PPh3) (1) derivatives in good yields (Scheme 1).

The reaction proceeds with the substitution of only one PPh3 ligand, exclusively giving phosphine–phosphite complexes 1. A higher temperature of reaction with xylene as solvent or a longer reaction time only result in a lower yield for compounds 1, owing to some decomposition.

Treatment of chloro-complexes 1 with NaBH4 in refluxing

Conclusions

This report describes the synthesis of classical and non-classical hydride complexes of osmium, stabilised by phosphite, triphenylphosphine and tris(pyrazolyl)borate as supporting ligands. T1 measurements and JHD values give information on the H–H distance of the η2-H2 ligand, with values consistent with “elongated” dihydrogen complexes. The hydride OsH(Tp)L(PPh3) derivatives behave as precursors of the first tris(pyrazolyl)borate diazo complexes of osmium, allowing an easy route for the

Acknowledgement

We thank Daniela Baldan for technical assistance.

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