Alkyl alkyne mono((trimethylsilyl)cyclopentadienyl) niobium complexes.: Synthesis and chemical behavior in insertion processes.: X-ray crystal structures of [NbCp′(CH2SiMe3)2(Me3SiCCSiMe3)] and [NbCp′(NAr){η4-CH(SiMe3)C(SiMe3)C(CH2SiMe3)=CH(SiMe3)}],(Cp′=η5-C5H4SiMe3, Ar=2,6-Me2C6H3).: DFT studies of the model complexes [Nb(η5-C5H5)R2(HCCH)] (R = Cl, Me)

The dichloro niobium complex [NbCp'Cl-2(R'CCR')] (Cp' = eta(5)-C5H4SiMe3; R' = SiMe3; 1) can be obtained by reduction of NbCp'Cl-4 with excess of aluminum in the presence of 1 equiv of bis(trimethylsilyl)acetylene. The dialkyl derivatives [NbCp'R-2(R'CCR')] (Cp' = eta(5)-C5H4SiMe3; R'= SiMe3; R = Me (2), CH2SiMe3 (3), CH2CMe3 (4), CH2C6H5 (5)) were obtained by treating 1 with the stoichiometric amounts of the appropriate alkylating reagents. Reactions of the dialkyl alkyne complexes [NbCp'R-2(R'CCR')] (2-5) with 1 equiv of 2,6-Me2C6H3NC in C6D6 resulted in migration of only one of the two alkyl groups to give the alkyl alkyne eta(2)-iminoacyl complexes [NbCp'R{eta(2)-C(R)=NAr}(R'CCR')] (Cp' = eta(5)-C5H4SiMe3; R' = SiMe3; Ar = 2,6-Me2C6H3; R = Me (6), CH2SiMe3 (7), CH2CMe3 (8)). However, when hexane solutions of the dialkylated complexes 2 and 3 were treated with 1 equiv of isocyanide at 40-50 degreesC (2) and 80 degreesC (3), respectively, the imido niobacyclopent-3-ene [NbCp'(NAr){eta(4)-CH(SiMe3)C(SiMe3)=C(Me)CH2}] (9) and the imido niobacyclopropane(vinyl) [NbCp'(NAr){eta(4)-CH(SiMe3)C(SiMe3)C(CH2SiMe3)=CH(SiMe3)}] (10) (Cp' = eta(5)-C6H4SiMe3; Ar = 2,6Me(2)C(6)H(3)) can be isolated, probably via azaniobacyclopropane intermediates. In the case of the dialkylated complex 4 the reaction with 1 equiv of isocyanide at room temperature gives initially the alkyne neopentyl eta(2)-iminoacyl complex 8, which is slowly transformed into the imido niobacyclohept-2-ene [NbCp'(NAr){eta(2)-CH2CMe2CH2CH(CH2CMe3)C(SiMe3)=C(SiMe3)}] (Cp' = eta(5)-C5H4SiMe3; Ar = 2,6-Me2C6H3; 11), but the intermediate species were not observed. All the new compounds have been characterized by IR spectrophotometry, H-1 and C-13{H-1} NMR spectroscopy, and elemental analyses. The molecular structures of the complexes 3 and 10 have been determined by X-ray diffraction analyses. DFT calculations have been carried out on the model complexes [Nb(eta(5)-C5H5)R-2(HCCH)] (R = Cl, Me). The theoretical studies rationalize the disposition of the alkyne ligand in complex 3.