Competitive insertion of isocyanide and carbon dioxide into niobium- and silicon-amido bonds

Reaction of the amido-niobium complex [Nb((eta)5-C5H4SiMe2((NHBu)-Bu-t))((NBu)-Bu-t)Cl((NHBu)-Bu-t)] (2) with 1 equiv of CNAr (Ar = 2,6-Me2C6H3) in hexane gave the iminocarbamoyl compound [Nb(eta (5)-C5H4SiMe2((NHBu)-Bu-t))((NBu)-Bu-t)Cl(eta (2)-C((NHBu)-Bu-t)=NAr)] (5) in quantitative yield at room temperature, whereas heating to 120 degreesC was required when similar reactions were carried out with the bridged silyl amido compounds [Nb(eta (5)-C5H4SiMe2(eta (1)-(NBu)-Bu-t))((NBu)-Bu-t)Cl] (3) and [Nb(eta (5)-C5H4SiMe2(eta (1)-(NBu)-Bu-t))((NBu)-Bu-t)((NHBu)-Bu-t)] (4) to give the bridging [Nb(eta (5)-C5H4SiMe2(N(t)Bt)eta (2)-(C=NAr))((NBu)-Bu-t)Cl] (6) and terminal iminocarbamoyl [Nb(eta (5)-C5H4SiMe2(eta (1)-NtBu))((NBu)-Bu-t)(eta (2)-C((NHBu)-Bu-t)=NAr)] (7) compounds, respectively. Benzylation of 6 with Mg(CH2Ph)(2).2THF provided the benzyl provided the benzyl eta (2)-iminocarbamoyl compound [Nb(eta (5)-C5H4SiMe2((NBu)-Bu-t)-eta (2)-(C=NAr))((NBu)-Bu-t)(CH2Ph)] (8), which rearranges to the silyll-eta (1)-amido eta (2)-iminoacyl derivative [Nb(eta (5)-C5H4SiMe2(eta (1)-(NBu)-Bu-t)]((NBu)-Bu-t)(eta (2)-C(CH2Ph)=NAr)] (9) on heating its C6D6 solution to 145 degreesC. Reaction of compound 2,with CO2 gave the dicarbamato complex [Nb(eta (5)-C5H4SiMe2(eta (1)-(OOCNHBu)-Bu-t)](172-OOCNHtBu)((NBu)-Bu-t)Cl] (10), whereas the same reaction with [Nb[eta (5)-C5H4SiMe2((NHBu)-Bu-t)](NtBu)Cl-2] (1) gave an unidentified mixture of products, which were transformed after heating to 130-140 degreesC into the oxo derivative [(Nb((NBu)-Bu-t)Cl-2)(2)(eta (5)-C5H4SiMe2-O-SiMe2-eta (5)-C5H4)] (11), which was further benzylated to give [(Nb((NBu)-Bu-t)(CH2Ph)(2))(2)(eta (5)-C5H4SiMe2-O-SiMe2-eta (5)-C5H4)] (12). Similar reaction of 3 with CO2 in toluene gave the oxo compounds [Nb((NBu)-Bu-t)Cl-mu-(eta (5)-C5H4SiMe2-eta -O)](2) (13) and [(Nb((NBu)-Bu-t)Cl)(2)(mu -O)(eta (5)-C5H4SiMe2-O-SiMe2-eta (5)-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(eta (5)-C5H4SiMe2Cl)((NBu)-Bu-t)Cl-2] in the presence of NEt3.