Synthesis, structure, and reactivity of bridging cyanide complexes of the formula [(5-C5R5)Re(NO)(PPh3)CN(Ph 3P)(ON)Re(5-C5R5)] +TfO- (R, R = H, Me)
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Reactions of (5-C5R5)Re(NO)(PPh 3)(OTf) (1a/b, R = H/Me) and (5-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 ReCand RCNRe 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 [(5-C5H5)Re(NO)(PPh3)(S(CH 2CH=CH2)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.
