Activation of organonitriles toward β-electrophilic attack.: Synthesis and characterization of methyleneamide (azavinylidene) complexes of rhenium

Treatment of the nitrile complexes cis-[ReCl(NCR)(dppe)(2)] (R = aryl; dppe = Ph2PCH2CH2PPh2) with [Et2OH][BF4] or SiMe3CF3SO3 in CH2Cl2 led to the formation of the methyleneamide (alkylideneamide or azavinylidene) compounds cis-[ReCl{NC(E)C6H4X-4}(dppe)(2)]Y [E = H; X = OMe (1b), Me (1c), H (1d), F (1e), or Cl (1f), Y = BF4; E = SiMe3, X = Me (1c'), Y = CF3SO3] and trans-[ReCl{NC(H)C6H4X-(4)}(dppe)(2)][BF4] [X = NEt2 (2a), OMe (2b), H (2d), F (2e), or Cl (2f)]. They were characterized by multinuclear NMR spectrometry and X-ray crystallography (2e) which shows that the methyleneamide ligand exhibits linear (three-electron donor) geometry and behaves as a strong pi-electron acceptor. The complex P-31{H-1} and H-1 [NC(H)C6H4X-4] NMR patterns for the cis isomers were analyzed as ABCD and ABCDX (X part) spin systems, respectively. Complexes 1 and 2 undergo deprotonation by base to form the trans isomers of the corresponding nitrile complexes, trans-[ReCl(NCC6H4X-4)(dppe)(2)] [X = NEt2 (3a), OMe (3b), H (3d), F (3e), or Cl (3f)] whose spectroscopic data are also presented and which on protonation give the corresponding trans isomers of the methyleneamide complexes (2). Reactions of cis-[ReCl(NCR)(dppe)(2)] with [Et3O][PF6] resulted in their oxidation and isomerization to afford trans-[ReCl(NCC6H4X-4) (dppe)(2)] [PF6] [X = Me (4c), H (4d), or Cl (4f)].