Organorhenium Imido Complexes: Syntheses, Structure, and Reactivity

Imido complexes of heptavalent rhenium have been synthesized by three different routes: (i) aminolysis of the half-sandwich complex (η5-C₅Me₅)ReCl₄ (1) (Me = CH₃) with primary amines gives the imido derivatives (η5-C₅Me₅)Re(N^tBu)Cl₂ (2a,^tBu = tert-butyl) and (η⁵-C₅Me₅)Re(NCH₃)Cl₂ (2b). The rhenium- nitrogen distance in compound 2a (1.709 (3) Å) corresponds to a triple bond (single-crystal X-ray diffraction), while the analogous oxo congener (η⁵-C₅Me₅)ReOCl₂ has relatively little rhenium-oxygen triple-bond character, (ii) Sequential deoxygenation of methyltrioxorhenium(VII) (6) via decarboxylation upon treatment with 2,6-diisopropylphenyl isocyanate yields mono- and dinuclear methylrhenium(VII) imido complexes 7-9 in good yields; imido bridging occurs when the mononuclear compounds of types CH₃ReO(NR)₂ (8a) and CH₃Re(NR)₃ (9a) are dissolved in polar solvents such as typically 1,2-dimethoxyethane. (iii) Nucleophilic substitution of Me₃SiO-Re(N_tBu)₃ (10) by organolithium or Grignard reagents was used to synthesize the new methyl-, allyl-, ethyl-, and phenylalkynyl derivatives R’-Re(N^tBu)₃ (R’ = CH₃,11a; C₃H₅, 11b; C₂H₅, 11c; C₆H₅C≡C, 11d). These compounds are thermally very robust even though 11b,c exhibit alkyl groups with β-hydrogen atoms, α- instead of π-coordination of the allyl ligand of 11b underlies the strong electron-releasing character of the three imido ligands; this suggests that even π-ligands R’ in compounds of type R’-Re(NR)₃ will be only α-coordinated, in contrast to the oxo congeners R’-Reo₃. [2 + 2] Cycloaddition intermediates can be trapped in the reaction of the oxorhenium(III) alkyne complexes of type (η⁵-C₅Me₅)ReO(η²-RC≡CR) (3a, R = CH₃; 3b, C⁶H₅) with phenyl isocyanate to give phenyl carbamidato-N,O complexes 4a,b that undergo consecutive decarboxylations and isocyanate addition to yield finally ureato-NN’ compounds 5a,b.