Preparation and reactivity with azo-species of hydride and dihydrogen complexes of osmium stabilised by tris(pyrazolyl)borate and phosphite ligands
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.
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.
References (32)
- et al.
Chem. Rev.
(1998)et al.Coord. Chem. Rev.
(1998)et al.Coord. Chem. Rev.
(1999) - et al.
Organometallics
(1997)et al.J. Chem. Soc., Dalton Trans.
(1998)et al.J. Organomet. Chem.
(2000) - et al.
Inorg. Chem.
(1990)et al.Inorg. Chem.
(1995)et al.J. Chem. Soc., Dalton Trans.
(2000) - et al.
Inorg. Chem.
(1996)et al.J. Am. Chem. Soc.
(1996)et al.J. Am. Chem. Soc.
(1996) - et al.
J. Chem. Soc., Dalton Trans.
(1973)et al.J. Organomet. Chem.
(1982) - et al.
J. Chem. Soc., Dalton Trans.
(1988) - et al.
Inorg. Chem.
(1984)et al.Inorg. Chem.
(1975)et al.Inorg. Chem.
(1975)et al.J. Chem. Soc., Dalton Trans.
(1989) Metal Dihydrogen and σ-Bond Complexes
(2001)et al.Recent Advances in Hydride Chemistry
(2001)- et al.
Coord. Chem. Rev.
(1992)et al.Chem. Rev.
(1993)Angew. Chem. Int. Ed.
(1993) - et al.
J. Am. Chem. Soc.
(1995)et al.Organometallics
(1997)et al.Inorg. Chim. Acta
(1997)