Competitive Insertion of Isocyanide and Carbon Dioxide into Niobium− and Silicon−Amido Bonds

Reaction of the amido−niobium complex [Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)Cl(NHtBu)] (2)
with 1 equiv of CNAr (Ar = 2,6-Me2C6H3) in hexane gave the iminocarbamoyl compound
[Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)Cl{η2-C(NHtBu)NAr}] (5) in quantitative yield at room
temperature, whereas heating to 120 °C was required when similar reactions were carried
out with the bridged silyl amido compounds [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)Cl] (3) and
[Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu)(NHtBu)] (4) to give the bridging [Nb{η5-C5H4SiMe2(NtBu)-η2-(CNAr)}(NtBu)Cl] (6) and terminal iminocarbamoyl [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu){η2-C(NHtBu)NAr}] (7) compounds, respectively. Benzylation of 6 with Mg(CH2Ph)2·2THF
provided the benzyl η2-iminocarbamoyl compound [Nb{η5-C5H4SiMe2(NtBu)-η2-(CNAr)}(NtBu)(CH2Ph)] (8), which rearranges to the silyl-η1-amido η2-iminoacyl derivative [Nb{η5-C5H4SiMe2(η1-NtBu)}(NtBu){η2-C(CH2Ph)NAr}] (9) on heating its C6D6 solution to 145 °C.
Reaction of compound 2 with CO2 gave the dicarbamato complex [Nb{η5-C5H4SiMe2(η1-OOCNHtBu)}(η2-OOCNHtBu)(NtBu)Cl] (10), whereas the same reaction with [Nb{η5-C5H4SiMe2(NHtBu)}(NtBu)Cl2] (1) gave an unidentified mixture of products, which were
transformed after heating to 130−140 °C into the oxo derivative [{Nb(NtBu)Cl2}2{η5-C5H4SiMe2-O-SiMe2-η5-C5H4}] (11), which was further benzylated to give [{Nb(NtBu)(CH2Ph)2}2{η5-C5H4SiMe2-O-SiMe2-η5-C5H4}] (12). Similar reaction of 3 with CO2 in toluene gave the
oxo compounds [Nb(NtBu)Cl-μ-(η5-C5H4SiMe2-η-O)]2 (13) and [{Nb(NtBu)Cl}2(μ-O){η5-C5H4SiMe2-O-SiMe2-η5-C5H4}] (14). The molecular structure of 13 was determined by X-ray
diffraction methods. Formation of the oxo complexes 11, 13, and 14 also resulted from
hydrolysis of [Nb(η5-C5H4SiMe2Cl)(NtBu)Cl2] in the presence of NEt3.