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Probing the mer- to fac-isomerization of tris-cyclometallated homo- and heteroleptic (C,N)3 Iridium(III) complexes

By A.R. McDonald, M. Lutz, L.S. von Chrzanowski, G.P.M. van Klink, A.L. Spek and G. van Koten


We have developed techniques which allow for covalent tethering, via a “hetero” cyclometallating ligand, of heteroleptic tris-cyclometallated iridium(III) complexes to polymeric supports (for application in light-emitting diode technologies). This involved the selective synthesis and thorough characterization of heteroleptic [Ir(C,N)2(C′,N′)] tris-cyclometallated iridium(III) complexes. Furthermore, the synthesis and characterization of heteroleptic [Ir(C,N)2OR] complexes is presented. Under standard thermal conditions for the synthesis of the facial (fac) isomer of tris-cyclometallated complexes, it was not possible to synthesize pure heteroleptic complexes of the form [Ir(C,N)2(C′,N′)]. Instead, a mixture of homo- and heteroleptic complexes was acquired. It was found that a stepwise procedure involving the synthesis of a pure meridonial (mer) isomer followed by photochemical isomerization of this mer to the fac isomer was necessary to synthesize pure fac-[Ir(C,N)2(C′,N′)] complexes. Under thermal isomerization conditions, the conversion of mer-[Ir(C,N)2(C′,N′)] to fac-[Ir(C,N)2(C′,N′)] was also not a clean reaction, with again a mixture of homo- and heteroleptic complexes acquired. An investigation into the thermal mer to fac isomerization of both homo- and heteroleptic tris-cyclometallated complexes is presented. It was found that the process is an alcohol-catalyzed reaction with the formation of an iridium alkoxide [Ir(C,N)2OR] intermediate in the isomerization process. This catalyzed reaction can be carried out between 50 and 100 °C, the first such example of low-temperature mer−fac thermal isomerization. We have synthesized analogous complexes and have shown that they do indeed react so as to give fac-tris-cyclometallated products. A detailed explanation of the intermediates (and all of their stereoisomers, in particular when systems of the generic formula [M(a,b)2(a′,b′)] are synthesized) formed in the mer to fac isomerization process is presented, including how the formed intermediates react further, and the stereoisomeric products they yield

Year: 2008
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