Generation of rhodium(II) and rhodium(I) from the one-electron reduction of tris(2,2'-bipyridine)rhodium(III) ion in aqueous solution

The reaction of Rh(bpy)33+ with radiation-generated reducing radicals (e/sub aq/-,. CO2-, and (CH3)2COH) in aqueous solution quantitatively and rapidly (k = 109-1010 M-1 s -1) yields Rh(bpy)32+ (lambda/sub max/ 485 nm, epsilon/sub max/ 1.0 x 103 M-1 cm-1) which undergoes slow (k = 0.45 +- 0.05 s-1 at pH 3-10) loss of bpy at room temperature. Rh(bpy)32+ reacts with O2 (k = 4.9 x 108 M-1 s-1) via electron transfer. In alkaline solution, Rh(bpy)32+ undergoes disproportionation with ligand-labilized Rh(II) to form Rh(bpy)33+ and red-violet Rh(bpy)2+. At pH > 10, ligand-labilized Rh(II) reduces Rh(bpy)33+ resulting in a redox-catalyzed ligand-labilization chain reaction; at pH 14, G(bpy) approx. = G(-Rh(bpy)33+) > 300. The nature of O2-sensitive Rh(bpy)2+, its spectrum, and state of aggregation is highly dependent upon the pH of the solution, [Rh(I)], and the nature and concentration of the counteranion. At least four forms of Rh(bpy)2+ are clearly identified: (a) a red-violet soluble form (lambda/sub max/ 518 nm, epsilon/sub max/ 9500 M-1 cm-1) which is formulated as Rh(bpy)2(OH)/sub n//sup (1-n)/ and may be dimeric via hydroxide bridging (in neutral solution, very small changes in pH have a large effect on the spectrum which shows a main band at approx. 415 nm and a well-defined shoulder in the 470-nm region); (b) a violet insoluble form represented as Rh(bpy)2X where X = Cl-, ClO4-, etc.; (c) a transient green form observed when the red-violet form is acidified which is formulated as Rh(bpy)2(OH2)/sub n/+; (d) a colorless form in acidic solution which is assigned as a hydride in which the metal center is formally Rh(III), e.g., RhH(bpy)22+. At natural pH, H2 is produced with an efficiency of approx. 25% in the absence of any catalyst. The relevance of these results to solar energy conversion schemes is examined. 11 figures