Synthesis, characterisation and molecular structure of Re(iii) 2-oxacyclocarbenes stabilised by a benzoyldiazenido ligand

Abstract

A family of new Fischer-type rhenium(III) benzoyldiazenido-2-oxacyclocarbenes of formula [(ReCl₂{η¹-N₂C(O)Ph}{C(CH₂)_nCH(R)O}(PPh₃)₂] [n = 2, R = H (2), R = Me (3); n = 3, R = H (4), R = Me (5)] have been prepared by reaction of [ReCl₂{η²-N₂C(Ph)O}(PPh₃)₂] (1) with ω-alkynols, such as 3-butyn-1-ol, 4-pentyn-1-ol, 4-pentyn-2-ol, 5-hexyn-2-ol in refluxing THF. The correct formulation of the carbene derivatives 2-5 has been unambiguously determined in solution by NMR analysis and confirmed for compounds 2–4 by X-ray diffraction methods in the solid state. All complexes are octahedral with the benzoyldiazenido ligand, Re{N₂C(O)Ph}, adopting a “single bent” conformation. The coordination basal plane is completed by an oxacyclocarbene ligand and two chlorine atoms. Two triphenylphosphines in trans positions with respect to each other complete the octahedral geometry around rhenium. The reactivity of 1 towards different alkynes and alkenes including propargyl- and allylamine has been also studied. With propargyl amine, monosubstituted or bisubstituted complexes, [(ReCl₂{η¹-N₂C(O)Ph}{η¹-NH₂CH₂CCH}_n(PPh₃)_3−n] [n = 1 (6); n = 2 (7)], have been isolated depending on the reaction conditions. In contrast, the reaction with allylamine gave only the disubstituted complex [(ReCl₂{η¹-N₂C(O)Ph}{η¹-NH₂CH₂CHCH₂}₂(PPh₃)] (8). The molecular structure of the monosubstituted adduct 6 has been confirmed by X-ray analysis in the solid state.