Cyclometalated Ir(iii) complexes bridged by µ-OH and µ-3,5-bis(4-methoxyphenyl)-pyrazole ligands

Abstract

Recent advances of cyclometalated Ir(III) complexes are due to their varied structural features and rich photophysical properties. Having this in mind, in the present investigation neutral, cyclometalated heterobridged Ir(III) dimers [(ppy)₂Ir(µ-OH){µ-(PhOMe)₂Pz}Ir(ppy)₂] (1) and [(tpy)₂Ir(µ-OH){µ-(PhOMe)₂Pz}Ir(tpy)₂] (2) (ppyH = 2-phenylpyridine; tpyH = 2-(p-tolyl)pyridine; (PhOMe)₂PzH = 3,5-bis(4-methoxyphenyl)-pyrazole) are synthesized and characterized by various analytical techniques such as NMR, ESI-MS, UV-Vis, emission and cyclic voltammetric studies. Complex 1, has been also characterized by single crystal X-ray diffraction analysis. Both complexes are found to be emissive in aerated dichloromethane solution with emission maxima centered at around 550 nm for complex 1 and 546 nm for complex 2. Furthermore, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been carried out on both complexes 1 and 2 to provide insights into their photophysical characteristics. The observed photophysical properties align well with the DFT and TDDFT results. Our electrochemical investigations demonstrate the stability of the complexes and also the fact that the bridging ligands play an important role in facilitating metal–metal communication within the dimers. In our important findings, we observe a unique interplay of ligand roles where modifying the ancillary ligand does not alter the photophysical properties of the resultant Ir(III) complexes, but are essential for the structural integrity and stability of the complexes. The pyrazole ligand used in the study, 3,5-bis(4-methoxyphenyl)-pyrazole, facilitates the assembly of robust dimeric structures, and the –OH ligand helps shift the spectral properties towards the red end of the spectrum. This study demonstrates that steric modulation can be used to significantly improve the synthesis and isolation of dinuclear Ir(III) complexes without relying on changes in electronic structure. This study provides a clear contrast to prior electronically driven approaches and establishes steric design as a strategy for the rational construction of dimeric cyclometalated Ir(III) complexes. Our results also underscore the significant impact of the bridging ligand on the electronic structure and spectroscopic properties of the cyclometalated Ir(III) complexes, potentially augmenting our understanding of the structure–property variations of such compounds and paving the way for their potential applications.