Preparation of half-sandwich diazoalkane complexes of osmium
Graphical abstract
The synthesis of diazoalkane complexes and κ1-pyridine-diazo derivatives of osmium is described.
Introduction
Diazoalkanes Ar1Ar2CN2 can coordinate to transition metals, giving stable and isolable complexes [1], [2], [3], [4], [5], [6]. The properties of these complexes are of current interest, not only thanks to the close relationship with dinitrogen fixation [7], [8] but also for their rich and various reactivity [1], [2], [3], [4], [5], [6]. Thus, extrusion of dinitrogen with formation of carbene M = CAr1Ar2 was observed in η2-CN coordinated species [3](f), [9], [10], whereas η1-bonded diazoalkane gave dinitrogen [M]–N2 complexes [3f], transfer of carbene to imine [9f], or cleavage of the NN bond of the Ar1Ar2CN2 group [3g]. Dipolar (3 + 2) cycloaddition of coordinate diazoalkane with alkene and alkyne affording 3H-pyrazole derivatives has recently been reported [5], as well as hydrolysis of [M]–N2CAr1Ar2 yielding η2-diazene derivatives [6].
A number of diazoalkane complexes have been reported [1], [2], [3], [4], [5], [6] for several metal centres, displaying a variety of coordination modes (Chart 1) and reactivities. However, in contrast with Fe and Ru, diazoalkane complexes of osmium are very rare and, apart from [OsH(N2CAr1Ar2)L4]BPh4, described 15 years ago [11], no other example of this metal has been reported.
We are interested in the chemistry of diazoalkane complexes and have recently reported the synthesis and reactivity of half-sandwich ruthenium derivatives with p-cymene [4], cyclopentadienyl [5](a), [5](b) and indenyl [5c] as supporting ligands. The interesting properties shown by these compounds prompted us to extend our study to osmium, to test whether diazoalkane complexes could be prepared and how their properties change. The results are given here.
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. All solvents were dried over appropriate drying agents, degassed on a vacuum line, and distilled into vacuum-tight storage flasks. OsO4 was a Pressure Chemical Co. (USA) product, used as received. Phosphites P(OMe)3, P(OEt)3 and triisopropylphosphine P(iPr)3 were Aldrich products, purified by distillation, whereas phenyldiethoxyphosphine PPh(OEt)2 and
Preparation of diazoalkane complexes
Diazoalkane complexes [OsCl(η6-p-cymene)(N2CAr1Ar2){PPh(OEt)2}]BPh4 (1) were prepared by reacting the chlorocompound [OsCl2(η6-p-cymene){PPh(OEt)2}] with an excess of diazoalkane in ethanol, as shown in Scheme 1.
The reaction proceeds with the substitution of one chloride by Ar1Ar2CN2 and the formation of cationic complexes 1 and 2. Crucial for successful synthesis is the presence of NaBPh4 salt which, labilising the Cl− ligand, favours the formation of diazoalkane complexes, separated as BPh4−
Conclusions
This paper reports that the half-sandwich fragment [OsCl(η6-p-cymene)L]+ with PPh(OEt)2 as a supporting ligand can stabilise diazoalkane complexes. κ1-Pyridine-diazoalkane derivatives [OsCl(η6-p-cymene){κ1-(4-C5H4N)(Ph)CN2}L]BPh4 were also prepared. Reaction of [OsCl(η6-p-cymene)(N2CAr1Ar2){PPh(OEt)2}]BPh4 with ethylene and phenylacetylene proceeds with substitution of the Ar1Ar2CN2 group, yielding new derivatives.
Acknowledgment
We thank Mrs. Daniela Baldan, from the Università Ca’ Foscari Venezia, for her technical assistance.
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