Preparation of rhenium hydride complexes with pyrazole and pyrazolato ligands
Abstract
Hydride-pyrazolato ReH(Rpz)(HRpz)(NO)(PPh3)2 complexes (R = H (1), 5-Me (2)) were prepared by allowing the ReH2(NO)(PPh3)3 derivative to react with pyrazole (HRpz) in refluxing toluene. The complexes were characterised spectroscopically (IR and NMR data) and by the X-ray crystal structure determination of the ReH(5-Mepz)(H-5-Mepz)(NO)(PPh3)2 derivative. An intramolecular hydrogen bond N–H⋯N between the pyrazolato and the pyrazole ligands was established both in the solid state and in solution. Protonation reaction with Brønsted acid of compound 1 was studied and led to the unstable bis(pyrazole) [ReH(Hpz)2(NO)(PPh3)2]+ cation. Acetylide-pyrazolato Re(C
CAr)(Rpz)(HRpz)(NO)(PPh3)2 (3, 4) complexes (Ar = Ph, p-tolyl; R = H, 5-Me) were also prepared by reacting hydride ReH(Rpz)(HRpz)(NO)(PPh3)2 species with terminal alkynes HCCAr in refluxing 1,2-dichloroethane.
Graphical abstract
The synthesis of rhenium hydride ReH(Rpz)(HRpz)(NO)(PPh3)2 complexes containing both pyrazole and pyrazolato as supporting ligands is described. An intramolecular hydrogen bond N–H⋯N between the pyrazolato and the pyrazole ligands is present both in the solid state and in solution. Acetylide pyrazolato Re(CCAr)(Rpz)(HRpz) (NO)(PPh3)2 complexes were also prepared by reacting the ReH(Rpz)(HRpz)(NO)(PPh3)2 hydrides with terminal alkynes.
Introduction
Pyrazole (Hpz) and pyrazolato (pz) have been extensively used as ligands in transition metal chemistry and a number of complexes have been reported in the past 25 years [1], [2], [3]. These studies have been developed rapidly due to the potentialities of Hpz and pz as ligands. In fact, although being a poor base [4], pyrazole is a good σ-donor by acting as a monodentate ligand through its pyridinic nitrogen atom, or as either a monodentate or a bidentate bridging ligand in a pyrazolato anion [1], [2], [3]. Furthermore, the possibility of using substituents on the heterocyclic ring may modify the steric and electronic properties of the pyrazole ligand giving rise to versatile and varied chemistry.
The introduction of these ligands in the chemistry of transition metal hydrides [5], however, is somewhat restricted and in the case of rhenium, for example, only the hydride clusters [Re3(μ-H)3 (μ-η2-pz)(CO)10]− and [Re3(μ-H)3 (μ-η2-pz)(CO)9Hpz]− with pyrazole and pyrazolato have been recently described [3].
We are interested in the chemistry of classical and non-classical transition metal hydrides [6] and recently have devoted our studies to the synthesis and reactivity of hydride complexes of the iron triad containing N-donor ligands of the [MH(N–N)P3]BPh4, [MH(N–N)2P]BPh4 (M = Fe, Ru, Os; N–N = 2,2′-bipyridine, 1,10-phenanthroline) and RuH(Tp)P2 [Tp = hydridotris(pyrazolyl)borate] (P = phosphite) types [7], [8]. We have now extended these studies with the aim of introducing pyrazole in the chemistry of hydride complexes and in this paper we report some results on rhenium as a central metal dealing with the synthesis and reactivity of the first mononuclear hydride complex containing pyrazole and pyrazolato as supporting ligands.
Section snippets
Results and discussion
Hydride ReH2(NO)(PPh3)3 complex reacts with pyrazole (Hpz) in refluxing toluene to give the pyrazole-pyrazolato ReH(pz)(Hpz)(NO)(PPh3)2 (1) complex which was isolated in good yield and characterised (Scheme 1).
The reaction proceeds also with the substituted methylpyrazole H-3-Mepz to give the related ReH(5-Mepz)(H-5-Mepz)(NO)(PPh3)2 (2) complex, in which the nitrogen sites of the pyrazole ligand have been interchanged (see below) giving the final 5-methylpyrazole derivatives (Scheme 2).
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.
Pyrazole (Hpz), 3-methylpyrazole (H-3-Mepz) and other reagents were purchased from commercial sources
Supplementary data
Crystallographic data in CIF format have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 266858.
Acknowledgements
The financial support of MIUR (Rome) – PRIN 2004 – is gratefully acknowledged. We thank Daniela Baldan for technical assistance.
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