Insertion reactions of phenyl isocyanate into hafnium nitrogen bonds: Synthesis and reactivity of hafnium complexes bearing substituted pyrrolyl ligands

A bis(diethylamido)hafnium compound [C₄(CH₂NMe ₂)-2]₂Hf(NEt₂)₂ (1) has been prepared in 79% yield by reacting Hf(NEt₂)₄ with 2 equiv. of [C₄H(CH₂NMe₂)-2] in heptane via deamination. Reacting compound 1 with 2 equiv. of phenyl isocyanate at room temperature in diethyl ether results in the PhNCO being inserted seletively into hafnium-NEt₂ bonds to generate [C₄(CH₂NMe₂)-2]₂Hf[PhNC (NEt₂)O]₂ (2) in 56% yield. Similarly, while reacting 1 with 2 equiv. of phenyl isocyanate for a week in toluene produces a mixture of 2 and [C₄(CH₂NMe ₂)-2]Hf[PhNC(NEt₂)O]₃ (3). For comparison, reacting Hf(NEt₂)₄ with 4 equiv. of PhNCO in a toluene solution at room temperature results in the PhNCO inserted into Hf-N bonds, and forms a tetrakis-ureato hafnium compound Hf[PhNC(NEt₂)O]₄ (4) in 88% yield. A theoretical calculation found that the unpaired electrons of the ureato fragments of 2 are resonance delocalized between the C-O, C-NPh, and C-NEt₂ bonds, which are all partially doubly bonded.