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

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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 CNBut), [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(CNBut)(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 CNBut), (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].