Rhenium(VII) monoimido alkylidene complexes: Synthesis, structure, and Lewis-acid-cocatalyzed olefin metathesis

Article abstract of DOI:10.1021/om00052a034

Five-coordinate alkylidene complexes of the general type Re(CH-t-Bu)(NAr)(OAr′)₃ (NAr = N-2,6-C₆H₃-i-Pr₂; OAr′ = OC₆F₅ (1a), O-2,6-C₆H₃Cl₂ (1b), OC₆Cl₆ (1c)) can be prepared in high yield by treating [Re(C-t-Bu)(NHAr)Cl₃]₂ with 6 equiv of potassium phenoxide. Re(CH-t-Bu)(NAr)(O-2,6-C₆H₃Cl₂)₃ (P2₁/c, a = 11.461 (3) ?, b = 32.86 (1) ?, c = 11.680 (2) ?, β = 103.31 (2)°) is essentially a square pyramid in which the alkylidene ligand occupies the apical position. Re(CH-t-Bu)(NAr)(OC₆F₅)₃(THF) (2) is prepared in high yield from Re(C-t-Bu) (NHAr)Cl₃(THF) and 3 equiv of KOC₆F₅. Re(CH-t-Bu)(NAr)(OC₆F₅)₃(THF) (P1, a = 11.037 (7) ?, b = 17.66 (1) ?, c = 10.981 (6) ?, α = 90.28 (6)°, γ = 93.94 (6)°, γ = 94.15 (5)°) is pseudooctahedral with THF bound at the coordination site trans to the alkylidene ligand. Re(CH-t-Bu)(NAr)(OAr′)₃ complexes do not react with terminal, internal, or strained cyclic olefins, possibly because an olefin can bind only to the site trans to the alkylidene ligand. Re(CH-t-Bu)(NAr)(OAr′)₃ complexes react with pyridine to form alkylidyne complexes of the type Re(C-t-Bu)(NAr)(OAr′)₂(py) (OAr′ = O-2,6-C₆H₃Cl₂ (3a), OC₆Cl₅ (3b)). Alkylidene complexes of the type Re(CH-t-Bu)(NAr)(OAr′)Cl₂ (OAr′ = O-2,6-C₆H₃Cl₂ (4a), O-2,6-C₆H₃Me₂ (4b), OAr (O-2,6-C₆H₃-i-Pr₂), (4c), O-2,6-C₆H₃(OMe)₂ (4d)) can be prepared in high yield by treating Re(C-t-Bu)(NAr)Cl₂(DME) with 1 equiv of Ar′OH. NMR data suggest that Re(CH-t-Bu)(NAr)(OAr′)Cl₂ complexes are isostructural with Re(CH-t-Bu)(NAr)(OAr′)₃ complexes. Re(CH-t-Bu)(NAr)(OAr′)Cl₂ complexes also do not readily react with terminal, internal, or strained cyclic olefins. However, addition of GaBr₃ to Re(CH-t-Bu)(NAr)(OAr′)Cl₂ (OAr′ = O-2,6-C₆H₃Cl₂, O-2,6-C₆H₃Me₂, OAr) yields highly active catalysts that metathesize trans-3-heptene at a rate of ～1000 turnover min^-1. In some cases propagating alkylidene species can be observed by NMR spectroscopy. Re(CH-t-Bu)-(NAr)(OAr′)Cl₂ complexes react with pyridine to produce complexes of the type Re(C-t-Bu)(NAr)Cl₂(py)₂.