Ligand-Induced Cleavage Of Dinitrogen By A Hafnium Metallocene Complex

Abstract

Side-on bound dinitrogen complexes of zirconium and hafnium, [([eta]5C5Me3H2)2M]2([eta]2, [eta]2-N2) (M = Zr, Hf), have been prepared by alkali metal reduction of the corresponding diiodide precursors. UV-Visible spectroscopy and X-ray diffraction studies established a higher degree of N2 activation in these complexes than those of the type [([eta]5-C5Me4H)2M]2([eta]2, [eta]2-N2) (M = Zr, Hf), likely due to increased metal-nitrogen orbital overlap resulting from contracted metal-metal distances. Addition of four atmospheres of dihydrogen afforded the expected hydrido metallocene azenido complexes [([eta]5-C5Me3H2)2MH]2N2H2 (M = Zr, Hf). The iodo hafnocene azenido compound [([eta]5-C5Me3H2)2HfI]2N2H2 was obtained by treatment of the hydrido hafnocene azenido precursor with methyl iodide. The first example of an bridging hafnium nitride compound, [([eta]5-C5Me3H2)2Hf(DMAP)]([MICRO SIGN]-N)[([eta]5-C5Me3H2)2Hf(NCO)], was isolated upon treatment of [([eta]5-C5Me3H2)2Hf]2([eta]2, [eta]2-N2) with carbon monoxide gas in the presence of 4-dimethylaminopyridine. The molecule proved to be a source of the hafnium-nitride fragment, which reacted with terminal alkynes via 1,2-addition at elevated temperatures and furnished the [MICRO SIGN]-imido acetylido complexes [([eta]5C5Me3H2)2Hf(CCR)]([MICRO SIGN]-NH)[([eta]5-C5Me3H2)2Hf(NCO)] (R = Ph, SiMe3). The hafnium nitride has also been shown to react with an additional molecule of carbon monoxide to yield the [MICRO SIGN]-isocyanate isocyanato compound [([eta]5-C5Me3H2)2Hf]([eta]3, [eta]1, [MICRO SIGN]-NCO)[([eta]5C5Me3H2)2Hf(NCO)]. The hafnocene-N2 complex also undergoes a unique series of reactions with primary silanes, leading to the production of monosilylated dinitrogen units. Complete N-N bond scission is achieved upon thermoylsis of the mono-silylated compounds, and N-C bond formation can be achieved by upon treatment of the silylated compound with CO. Trapping of the hafnium [MICRO SIGN]-nitride and N2 functionalization with silanes are reactions which are unique to the 1,2,4-trimethylcyclopentadienyl hafnocene-N2 compound. The origin of this unique reactivity has been attributed to the reduced steric profile of the trisubstituted metallocene, which allows shorter metalmetal contact distances and produces a highly activated, but sterically accessible dinitrogen fragment.