[3 + 2] cycloaddition of azido-bridged molybdenum(ii) complex with nitriles and alkynes

Abstract

The azido-bridged molybdenum complex [N(CH₃)₄][(μ_1,1-N₃)₃{Mo(η³-C₃H₅)(CO)₂}₂], 1, was synthesized and its reactions with unsaturated nitriles and alkynes were investigated. The isolated [3 + 2] cycloaddition products were the N(2), N(3) bound tetrazolate complexes [N(CH₃)₄][(μ_1,1-N₃)₂(μ-N₄C{R}-κ²N²:N³){Mo(η³-C₃H₅)(CO)₂}₂] (R = C(CN)C(CN)₂ (2), C₆H₄NO₂, (3)) and [N(CH₃)₄][(μ-N₄C{R}-κ²N²:N³)₂(μ_1,1-N₃){Mo(η³-C₃H₅)(CO)₂}₂] (R = C(CN)C(CN)₂ (4), C₆H₄NO₂ (5)), and the N(1), N(2) bound triazolate complexes [N(CH₃)₄][(μ-N₃C₂{R}₂-κ²N¹:N²)(μ_1,1-N₃)₂{Mo(η³-C₃H₅)(CO)₂}₂] (R = CO₂CH₃ (6) and R = CO₂CH₂CH₃ (7). The reactivity of these cycloaddition reactions could be determined by the electronic properties of both metal azide and dipolarophile. In the reaction of 1 with nitriles, at most two bridging azido groups can participate in the cycloaddition reactions and elevated temperature is required for the preparations of 3 and 5. In the case of alkynes, only one azido group is active for the reaction. These complexes are fluxional in solution, and isomers were found in 3 and 5. The molecular structures of the above complexes were determined by single-crystal X-ray diffraction analysis, which reveals a distorted octahedral geometry around each molybdenum atom, and the two metal atoms are connected through three bridging ligands. The formation of these heterocycles demonstrated the [3 + 2] cycloaddition reaction could also be applied to the less electron-rich azido-bridged molybdenum complex.