Reduction–oxidation properties of organotransition-metal complexes. Part 26. The synthesis of paramagnetic triazenido- and aryl(1-aryliminoethyl)amido-bridged dirhodium carbonyl derivatives containing the [Rh2]3+ core

Abstract

Cyclic voltammetry shows that [Rh₂(CO)₄(µ-L′)₂][1, L′= RNNNR, R =p-tolyl; 2, L′= R′NC(Me)NR′, R′= Ph], [Rh₂(CO)₂L₂(µ-RNNNR)₂](3, L = PPh₃ or CNBu^t), [Rh₂(CO)₂(PPh₃)L{µ-R′NC(Me)NR′}₂](4,L = CO), [Rh₂(η⁴-diene)₂(µ-RNNNR)₂][5, diene = nbd (norborna-2,5-diene) or cod (cyclo-octa-1,5-diene)], and [Rh₂(CO)₃(CNBu^t)(µ-RNNNR)₂](6) undergo at least two diffusion-controlled one-electron oxidations at a platinum bead electrode in CH₂Cl₂. The first process is reversible for all of the complexes and corresponds to the formation of a monocation with a [Rh₂]³⁺ core. The second step is fully reversible only for (1), (3, L = PPh₃), and (5, diene = nbd), and for the first two of these complexes a third reversible oxidation wave is also observed. The paramagnetic monocations (3⁺, L = PPh₃ or CNBu^t), (4⁺, L = CO), and (5⁺, diene = nbd) have been isolated as stable hexafluorophosphate salts via the oxidation of the appropriate neutral complex with either [N₂C₆H₄F_-p][PF₆] or [Fe(η-C₅H₅)₂][PF₆] in CH₂Cl₂. The related complex [Rh₂(CO)₂(µ-dppm)(µ-RNNNR)₂][PF₆](7⁺, dppm = Ph₂PCH₂PPh₂) was also prepared, by the [Fe(η-C₅H₅)₂][PF₆] oxidation of a mixture of (1) and dppm. The tricarbonyl (4⁺, L = CO) undergoes substitution with Lewis bases, L, to give the cationic dicarbonyls [4⁺, L = PPh₃, AsPh₃, or P(OPh)₃] which are readily reduced by [NBuⁿ₄][BH₄] in CH₂Cl₂ to give the otherwise inaccessible neutral species [4, L = PPh₃, AsPh₃, or P(OPh)₃].