Synthesis, Structure, and Reactivity of Bridging Cyanide Complexes of the Formula [(η⁵-C₅R₅)Re(NO)(PPh₃)CN(Ph₃P)(ON)Re(η⁵-C₅R‘₅)]⁺TfO^-(R, R‘ = H, Me)

Abstract

Reactions of (η5-C5R‘5)Re(NO)(PPh3)(OTf) (1a/b, R‘ = H/Me) and (η5-C5R5)Re(NO)(PPh3)(CN) (2a/b, R = H/Me) in toluene or CH2Cl2 give the title complexes (R/R‘ = H/H (4a), Me/H (4b), H/Me (4c), Me/Me (4d)) in 71−91% yields as >99:<1, 99−88:1−12, 95−66:5−34, and 71−60:29−40 mixtures of SR,RS/SS,RR diastereomers (pseudo meso/dl). Reaction of (S)-1a and (R)-2a gives (RR)-4a (retention)a diastereomer not formed from the racemates. IR and NMR spectra of 4a−d are analyzed, especially with reference to the cyanide ligands and charge distribution. Cyclic voltammetry shows pseudoreversible oxidations that become thermodynamically more favorable with increased numbers of pentamethylcyclopentadienyl ligands. A crystal structure of (SR,RS)-4a shows a slightly bent ReCNRe linkage, bond lengths close to those of related ReC⋮ and RC⋮NRe compounds, and van der Waals contacts across the bridge. The diastereomers of 4a−d and linkage isomers 4b,c do not equilibrate in CDCl3 (55 °C) or CHCl2CHCl2 (98 °C). The dissociation of 4a,d to 1a/2a or 1b/2b is excluded by unsuccessful trapping experiments involving 1a,b, 2a,b, and diallyl sulfide (25−120 °C). However, the diallyl sulfide complex [(η5-C5H5)Re(NO)(PPh3)(S(CH2CHCH2)2)]+ TfO- and 2a react (CHCl2CHCl2, 96 °C) to give 4a (75−63%; 50:50 SR,RS/SS,RR). These data show the cyanide bridges to have exceptional kinetic and thermodynamic stabilities.