Generation of Rhodium(II) and Rhodium(I) from the One-Electron Reduction of Tris(2,2'-bipyridine)rhodium(III) Ion in Aqueous Solution

Article abstract of DOI:10.1021/ja00402a021

The reaction of Rh(bpy)3³⁺ with radiation-generated reducing radicals (e_aq^-, .CO2^-, and (CH3)2COH) in aqueous solution quantitatively and rapidly (k = 10⁹-10¹⁰ M^-1 s^-1) yields Rh(bpy)3²⁺ (λ_max 485 nm, e_max 1.0+10³ M^-1 s^-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*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) ca.G(-Rh(bpy)3³⁺) > 300.The nature of O2-sensitive Rh(bpy)2⁺, its spectrum, and the state of aggregation is highly depenednt upon the pH of the solution, , 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 (ι_max 518 nm, ε_max9500 M^-1 s^-1) which is formulated as Rh(bpy)2(OH)n^(1-n) and may be dimeric via hydroxide bridging (in neutral solution, very small changes of pH have a large effect on the spectrum which shows a main band at ca. 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)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 ca. 25percent in the absence of any catalyst.The relevance of these results to solar energy conversion schemes is examined.