Cyclopentadienyl-Silyl-Amido versus Imido Niobium Complexes. The Role of Additional Amine Functionalities: A Combined Experimental and Theoretical Study

The reaction of the chlorosilyl-substituted cyclopentadienyl niobium compound [Nb(η5-C5H4SiMe2Cl)Cl4] (1) with 1 equiv of propylenediamine or N-methylpropylenediamine in toluene in the presence of 2 equiv of NEt3 affords the cyclopentadienyl-silyl-amido-amine derivatives [Nb{η5-C5H4SiMe2-η-N(CH2)3-η-NHR}Cl3] (R = H, 3a; Me, 3b). In contrast, a similar reaction using the N,N-dimethylpropylene diamine regiospecifically yields the imido compound [Nb(η5-C5H4SiMe2Cl){N(CH2)3NMe2}Cl2] (4a). However, treatment of 1 with 3 equiv of N-methylpropylene diamine in the absence of NEt3 gives the imido complex [Nb(η5-C5H4SiMe2Cl){N(CH2)3NHMe}Cl2] (4b). The reaction of 1 with 0.5 equiv of NH2(CH2)3NH2 in the presence of 2 equiv of NEt3 proceeds with the formation of the tethered dinuclear diimido compound [{Nb(η5-C5H4SiMe2Cl)Cl2}2{μ-N(CH2)3-η-N}] (5a). Analogous dinuclear diimido compounds [{Nb(η5-C5H4SiMe2Cl)Cl2}2{μ-N(CH2)n-η-N}] (n = 4, 5b; 5, 5c) are obtained when 1 reacts with longer unsubstituted diamines NH2(CH2)nNH2 (n = 4 and 5), regardless of the reaction proportions. The transient dinuclear diamine adducts of such a process, [{Nb(η5-C5H4SiMe2Cl)Cl4}2{μ-NH2(CH2)n-η-NH2}] (n = 3, 6a; 4, 6b; 5, 6c), have been prepared and isolated when these reactions were performed in the absence of a base. All compounds have been characterized by elemental analysis and NMR spectroscopy, and the crystal structures of the complexes [Nb{η5-C5H4SiMe2-η-N(CH2)2NMe2}Cl3] (2c) and [{Nb(η5-C5H4SiMe2Cl)Cl4}(μ-NH2(CH2)4NH2)] (6b) have been determined by X-ray diffraction methods. The experimental work has been assisted by DFT calculations to determine the role played by the pendant amine group in this kind of process and the cause for the highfield shifted resonances for the cyclopentadienyl ipso-carbon in constrained-geometry complexes.