1,4-Addition of Alkyl Halides to a Side-on Bound Hafnocene Dinitrogen Complex

Attempts to functionalize the side-on coordinated dinitrogen ligand in [(η5-C5Me4H)2Hf]2(μ2, η2, η2-N2) by addition of methyl halides such as CH3X (X = Cl, Br, I) or alkyl halides (R-X = CH3CH2Br, CH3CH2I, and CH3CH2CH2Cl) resulted in 1,4-addition to yield metastable, weakly activated hafnocene end-on dinitrogen complexes, [(η5-C5Me4H)2HfX][(η5-C5Me4H)2HfR](μ2, η1, η1-N2). The product of methyl iodide addition, [(η5-C5Me4H)2HfI][(η5-C5Me4H)2HfCH3](μ2, η1, η1-N2) was structurally characterized and confirmed the hapticity of the N2 ligand. Addition of the radical clocks 6-chloro-1-hexene and chloromethylcyclopropane also yielded 1,4-addition products with accompanying cyclization and ring-opening, respectively. In contrast, treatment of [(η5-C5Me4H)2Hf]2(μ2, η2, η2-N2) with benzyl chloride, benzyl bromide, or allyl bromide yielded only small quantities of the 1,4-addition product, with the majority of the mixture being the hafnocene dihalide (η5-C5Me4H)2HfX2 and organic products of radical coupling. In benzene-d6 solution at 23 °C, the 1,4-addition products undergo disproportionation with loss of N2 to yield the hafnocene alkyl halide complexes (η5-C5Me4H)2Hf(R)X along with concomitant regeneration of the strongly activated side-on dinitrogen complex [(η5-C5Me4H)2Hf]2(μ2, η2, η2-N2). This rearrangement is sensitive to both the halide and the alkyl substituents, where smaller groups undergo the most rapid rates of reaction, likely due to the accessibility of side-on bound N2 intermediates.