Synthesis of Pyridylimido Complexes of Tantalum and Niobium by Reductive Cleavage of the N-N Bond of 2,2′-Azopyridine: Precursors for Early-Late Heterobimetallic Complexes

Kento Kawakita, Yuya Kakiuchi, Evan P. Beaumier, Ian A. Tonks, Hayato Tsurugi, Kazushi Mashima

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Abstract

We report the syntheses of 2-pyridylimido complexes of tantalum and niobium by N-N bond cleavage of 2,2′-azopyridine. Reaction of MCl5 (M = Ta and Nb) with 2,2′-azopyridine in the presence of 0.5 equiv of 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated Si-Me-CHD) afforded a dark red solution (for Ta) and a dark blue solution (for Nb) with some insoluble precipitates. After removing the solids, another 0.5 equiv of Si-Me-CHD was added to each solution, giving [M(-Npy)Cl3]n (1a: M = Ta; 1b: M = Nb) through reductive cleavage of the N-N bond of 2,2′-azopyridine. The initial products of the above reactions were determined to be 2,2′-azopyridine-bridged dinuclear complexes, [(MCl4)2(μ-pyNNpy)] (2a: M = Ta; 2b: M = Nb), which were isolated by treating MCl5 with 2,2′-azopyridine and Si-Me-CHD in a 2:1:1 molar ratio. In 2a and 2b, the N-N bond was reduced to a single bond via two-electron reduction. Further reduction of complexes 2a and 2b with 1 equiv of Si-Me-CHD afforded complexes 1a and 1b. An anionic doubly μ-imido-bridged ditantalum complex, [nBu4N][Ta2(μ-Npy)2Cl7] (3a), was generated upon addition of nBu4NCl to complex 1a, while addition of nBu4NCl to niobium complex 1b gave a polymeric terminal imido complex, [nBu4N]n/2[{Nb(-Npy)Cl3}2(μ-Cl)]n/2 (3b). Complexations of 1a and 1b with 1 equiv of 2,2′-bipyridine resulted in the formation of mononuclear 2-pyridylimido complexes, M(-Npy)Cl3(bipy) (4a: M = Ta; 4b: M = Nb), whose main structural feature is intramolecular hydrogen bonding between the ortho hydrogen atom of 2,2′-bipyridine and the nitrogen atom of the pyridyl group on the imido ligand. Isolated 2-pyridylimido complexes 4a and 4b reacted with [RhCl(cod)]2 to produce the corresponding early-late heterobimetallic complexes, (bipy)MCl3(μ-Npy)RhCl(cod) (5a: M = Ta; 5b: M = Nb).

Original languageEnglish (US)
Pages (from-to)15155-15165
Number of pages11
JournalInorganic chemistry
Volume58
Issue number22
DOIs
StatePublished - Nov 18 2019

Bibliographical note

Funding Information:
K.K. thanks the financial support by the JSPS Research Fellowships for Young Scientist. H.T. acknowledges the financial support by JSPS KAKENHI grant No. 15KK0186, a Fund for the Promotion of Joint International Research (Fostering Joint International Research), and Multidisciplinary Research Laboratory System of Graduate School of Engineering Science, Osaka University. K.M. acknowledges financial supports by JSPS KAKENHI Grant Nos. 15H05808 and 15K21707 in Precisely Designed Catalysts with Customized Scaffolding (No. 2702). Financial support was provided by the National Institutes of Health (1R35GM119457), and the Alfred P. Sloan Foundation (I.A.T. is a 2017 Sloan Fellow).

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