PHOTOCATALYSIS - MECHANISTIC STUDIES OF HOMOGENEOUS PHOTOCHEMICAL WATER GAS SHIFT REACTION CATALYZED UNDER MILD CONDITIONS BY NOVEL CATIONIC IRIDIUM(III) COMPLEXES

The photochemical water gas shift reaction (WGSR) catalyzed, under mild conditions (25-degrees-C, 1 atm CO, visible light, pH = 7), by [(eta5-Me5C5)IrIII(bpy)X]+ (bpy = 2,2'-bipyridine, X = H, Cl), [(eta5-Me5C5)IrIII(phen)X]+ (phen = 1,10-phenanthroline, X = H, Cl), or [(eta5-Me5C5)IrIII(bpyRR')Cl]+ (R = R' = COOH, COOiPr, Br, NO2, NMe2 in the 4,4'-positions or R = R' = COOH, R = H and R' = SO3H in the 5,5-positions of the bpy ligand) has been investigated. A turnover frequency for H-2 formation of 32 h-1 was obtained in an aqueous phosphate buffer containing [(eta5-Me5C5)IrIII(bpy-4,4'-(COOH)2Cl]+ as catalyst, over a 7-b irradiation period at a constant CO pressure of 1 atm. An increase of 1 order of magnitude in catalytic activity was observed for the bpy ligand substituted with two carboxylate groups in the 4,4- or 5,5'-positions or with one sulfonate group in the 5-position (over the nonsubstituted bpy equivalent). Conversely, catalytic activity was lost when the bpy was substituted with two dimethylamino groups. The presence of an electron withdrawing group on the bpy-chelate was shown to decrease the activation energy of the process (E(a) = 14.6 kJ mol-1 for R = COOH, E(a) = 22.2 kJ mol-1 for R = COOiPr), cf. the unsubstituted ligand (E(a) = 29.6 kJ mol-1 for R = H). Decarboxylation of the intermediate [(eta5-C5Me5)IrIII-(bpyRR')COOH]+ (rate limiting step) seems therefore to be favored by the presence of an electron withdrawing group on the bpy-chelate. Three of the four intermediates involved in the WGS catalytic cycle have been characterized by NMR and Fr-IR spectroscopics: (i) the highly reactive [(eta5-Me5C5)IrIII(bpyRR')CO]2+ species formed by thermal displacement of the Cl- anion of thc starting complex; (ii) the iridium(I) complex [(eta5-Me5C5)IrI(bpyRR')], formed by decarboxylation of the hydroxycarbonyl complex; and (iii) the hydrido complex [(eta5-Me5C5)IrIII(bpyRR')H]+, formed by protonation of [(eta5-Me5C5)IrI(bpyRR')]. This latter complex (with R = COOH in the 4,4'-position of the bipyridine) has been characterized by a crystal structure determination. The photochemical step of the cycle was found to be the protonation of the hydride generating H-2 and the starting complex. The global catalytic system (for [(eta5-Me5C5)IrIII(bpy-4,4'-(COOH)2)Cl]+) has a quantum yield of 12.7% at 410 +/- 5 nm, which is independent of light intensity but strongly dependent on the pH of the solution.