Rhenium(VII) monoimido alkylidyne complexes. The importance of face selectivity in the metathesis of acetylenes via rhenacyclobutadiene intermediates

Article abstract of DOI:10.1021/ja00001a021

Re(NAr)₂(CH-t-Bu)Cl (Ar = 2,6-C₆H₃-i-Pr₂) reacts with 3 equiv of HCl in diethyl ether to give [ArNH₃][Re-(C-t-Bu)(NHAr)Cl₄] (1a) as an insoluble orange powder. [NEt₄][Re(C-t-Bu)(NHAr)Cl₄] (1b) can be prepared from 1a by cation exchange. Addition of ZnCl₂ to 1b yields [Re(C-t-Bu)(NHAr)Cl₃]₂ (2a), an X-ray study of which shows it to be a dimer in which each Re is a square pyramid with the neopentylidyne α-carbon atom in the apical position and a weakly bound bridging chloride ligand trans to it (space group P2₁/c, a = 9.953 (4) A?), b = 12.398 (9) A?, c = 19.720 (6) A?, β= 93.08 (3)°, V = 2430 (2) A?³, Z = 4, ρ= 1.473 g/cm³, R = 0.050, R_w = 0.084). Adducts of the type Re(C-t-Bu)(NHAr)Cl₃(L) (L = THF, py) can be prepared from 2a. Addition of DBU to Re(C-t-Bu)(NHAr)Cl₃(py) in the presence of pyridine gives Re(C-t-Bu)(NAr)Cl₂(py)₂ (4a). A more useful analogue, Re(C-t-Bu)(NAr)Cl₂(DME) (4b), can be prepared by adding ZnCl₂ to 1b followed by NEt₃ in DME. Complexes of the type Re(C-t-Bu)(NAr)(OR)₂ (OR = O-t-Bu (5a), OCMe₂(CF₃) (5b), O-2,6-C₆H₃-i-Pr₂ (5c), OC(CF₃)₂Me (5d), or OC(CF₃2H (as a diethyl ether adduct, 5e)) can be prepared from 1b or 4b. Reactions of 5d and 5e with symmetric internal acetylenes give fluxional rhenacyclobutadiene complexes, Re-(NAr)(C₃R₃)[OC(CF₃)₂Me] ₂ and Re(NAr)(C₃R₃)[OC(CF₃)₂H]₂ for R = Et, n-Pr, i-Bu, and i-Pr. Crystals of Re-(NAr)(C₃Et₃)[OC(CF₃)₂Me] ₂ belong to the space group P2₁/a with a = 19.553 (8) A?, b = 9.225 (4) A?, c = 20.246 (8) A?, β = 117.36 (3)°, V = 3243 (2) A?³, Z = 4, ρ_calcd = 1.73 g/cm³, μ= 40.70 cm^-1. The core structure is roughly a trigonal bipyramid containing axial carbon and oxygen atoms. The rhenacyclobutadiene ring is bent and asymmetric with Re-C_α bond lengths of 1.88 (1) and 2.09 (1) A? and C_α-C_β bond lengths of 1.33 (2) and 1.46 (1) A? All but one of the rhenacycles have analogous structures (type 1) according to NMR studies and are stable toward loss of an acetylene or further reactions with an internal acetylene for hours at 25 °C. The exception is Re(NAr)(C₃-i-Pr₃)[OC(CFj)2Me]2, a significantly different rhenacycle (type 2) that readily loses i-PrC≡C-i-Pr to give Re(C-i-Pr)(NAr)[OC(CF₃)₂Me]₂. It is proposed that rhenacycles of type 2 are the ones that are active for metathesis of internal acetylenes, that rhenacycles of type 2 form when an acetylene attacks a C/O/O face in Re(CR′)(NAr)(OR)₂, and that inactive rhenacycles of type 1 form when an acetylene attacks a C/N/O face in Re(CR′)(NAr)(OR)₂. Only acetylenes containing bulky groups are metathesized and only complexes that contain OC(CF₃)₂Me ligands are active. The ultimate fate of Re(VII) metallacyclobutadiene complexes over a period of hours to days is reduction to yield Re(V) complexes.