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

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.