Synthesis and Reactivity of Ruthenium Azido Complexes Containing a Hydridotris(pyrazolyl)borate Ligand and Dimerization of Terminal Alkynes in Organic and Aqueous Media

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Abstract

Facile ligand substitution is observed when the ruthenium azido complex (PPh3)[Ru]-N3 (1; [Ru] = Tp(PPh3)Ru) is treated with CH3CN, yielding the nitrile-substituted ruthenium azido complex (CH3CN)[Ru]-N3 (2). Alkylation, [3 + 2] cycloaddition, and catalytic reactions of complex 2 have been investigated. In the case of [3 + 2] cycloaddition reactions, the metal-bound heterocyclic complexes (CH3CN)[Ru]-N3C2(CO2Me)2 (3a), (CH3CN)[Ru]-N3C2HCO2Me (5), and (CH3CN)[Ru]-N3C2HCN (6) are obtained from dimethyl acetylenedicarboxylate, methyl propiolate, and fumaronitrile, respectively. The tetrazolato complex [(CN)2C═C(CN)2][Ru]-N4C[C(CN)═C(CN)2] (8) is prepared from 2 and TCNE. Alkylation of 3a with organic bromides affords N-alkylated five-membered-ring organic triazoles. The reaction of CS2 with 2 produces the thermally unstable thiothiatriazolate (CH3CN)[Ru]-N3CS(S) (10), which decomposes to the isothiocyanate (CH3CN)[Ru]-NCS (11). On the other hand, several cationic imine complexes {(CH3CN)[Ru]-(NH═CHR)}+ (12a, R = H; 12b, R = CH3; 12c, R = HC═CH2) are formed by the reaction of RCH2X with the negatively charged nitrogen atom of the azido ligand on RuII. The reaction proceeds via formation of an alkylimido intermediate followed by N2 evolution and proton transfer from the alkyl group to the imido nitrogen atom. Preliminary results on the catalytic activity of 2 are also presented. Interestingly, complex 2 catalyzes the dimerization of some terminal alkynes HC≡CR in organic and aqueous medium. Whereas with R = Ph, SiMe3, t-Bu isomeric mixtures of head-to-head and head-to-tail coupling products are obtained, in the case of R = COOMe, E-selective head-to-head dimerization takes place exclusively. The structures of 1, 8, and 12c have been determined by X-ray diffraction analysis.

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