Iridium salts sensitizers

Iridium salts are of current interest as sensitizers for internal latent image formation (189,190). The iridium ions are incorporated in the grains during their preparation, probably as a complex associated with silver ion vacancies. 

Complexes of other metals are used to sensitize emulsions, and some are used to sensitize even further (supersensitize) gold-sensitized emulsions. " Among these are (NH4)2[PtCl4], (NH4)2[PdCl4], (NH4)2[PdCy, K2[IrCl6] and similar compounds with ruthenium and the other halides. Sensitization by iridium salts and complex ions has been reviewed recently. Mechanisms of action of palladium(II) salts and complex ions on gold-sensitized emulsions have been studied. Several phosphine complexes of palladium and platinum, for example (2), are reported to be effective sensitizers, as are many macrocyclic polyamine compounds and their metal complexes,for example the Cu", Ni", Fe " and Rh " chelates of the cyclen ligand, 1,4,7,10-tetraazacyclododecane (3). 

After extensive experimentation, a simple solution for avoiding catalyst deactivation was discovered, when testing an Ir-PHOX catalyst with tetrakis[3,5-bis (trifluoromethyl)phenyl]borate (BArp ) as counterion [5]. Iridium complexes with this bulky, apolar, and extremely weakly coordinating anion [18] did not suffer from deactivation, and full conversion could be routinely obtained with catalyst loadings as low as 0.02 mol% [19]. In addition, the BArp salts proved to be much less sensitive to moisture than the corresponding hexafluorophosphates. Tetrakis (pentafluorophenyl)borate and tetrakis(perfluoro-tert-butoxy)aluminate were equally effective with very high turnover frequency, whereas catalysts with hexafluorophosphate and tetrafluoroborate gave only low conversion while reactions with triflate were completely ineffective (Fig. 1). 

Few complexes of iridium(V) are known. Before 1965, literature references to TrFq actually refer to IrFs. Reduction of IrFe by H2 at 60 °C gives IrFs at 100°C, further reduction leads to Ir metal. IrFs is a yellow crystalline solid (/u-eff = 1.32 /xb at 296 K) with a tetrameric structure (1). It is moisture sensitive (equation 5). Other iridium(V) compounds include salts of the type (M)[IrF6] (M+ = NO+, K+, Na+, Rb+, Cs+, Ag+, XeF+, Xe2Fn+, CIO2+, CI4+). KIrFe and CsIrFe exhibit temperature-independent paramagnetism. 

Using a modified Epson ink-jet printer, they printed metal salt solutions of platinum, rhodium, osmium, ruthenium, and iridium to form 220 compositions in the phase space of ternary and quaternary alloys of these metals. They were converted to metallic form after printing the metal salt solutions and screened for performance using a pH-sensitive fluorescence indicator. This led to identification of new fuel cell catalysts Pt 2l h250s,3 and Pt44Ru4jOjoIr5, which provided 40% higher current densities. 

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