Dinuclear Complexes of Flexidentate Pyridine‐Substituted Formazanate Ligands

Abstract

The utility of flexidentate pyridyl‐substituted formazanate ligands for assembling dinuclear coordination complexes with iridium(III) and/or platinum(II) building blocks is demonstrated herein. The dinuclear complexes are prepared either via a stepwise strategy, adding one metal unit at a time, or via one‐pot self‐assembly. Eight of the new complexes, including both mononuclear precursors and dinuclear products, are structurally characterized by single‐crystal X‐ray diffraction and NMR spectroscopy, revealing several distinct binding modes of the formazanates. All complexes are characterized by UV/Vis absorption spectroscopy and cyclic voltammetry. The frontier orbitals are primarily localized on the formazanate ligand, and a characteristic, intense formazanate‐centered π→π* absorption band is observed in the absorption spectra.

Trimetallic Iridium–Nickel–Iridium Bis(formazanate) Assemblies

“…Table 1 also summarizes previous iridium complexes that we reported that contain pyridylformazan Fz coordinated to iridium through the pyridine ring. 28 The first reduction potential in nickelcoordinated 2 is very similar to that of the previously reported complex Ir(F 2 ppy) 2 (FzH)(Cl), 28 in which the formazanate core is not coordinated to a metal, suggesting little contribution of the nickel center to the LUMO. In these complexes with a single formazanate bridging ligand, only a single reduction wave is observed, in contrast to the two waves observed in 1−5, further underscoring the conclusion that the two waves represent successive reduction of the two formazanates.…”

“…All of the trimetallic complexes 2−5 were synthesized in CH 2 Cl 2 by mixing complex 1 and the respective iridium dimer in a 1:1 ratio. Also shown in Scheme 1, the trimetallic complexes can alternatively be synthesized by first coordinating the free pyridylformazan to iridium, accessing a precursor described in our previous work on these ligands, 28 and then treating with nickel(II) acetate. Although the target compounds were formed by this route, we also observed the formation of insoluble side products, making it difficult to purify the desired compounds and isolate them in a satisfactory yield.…”

“…Another of our major goals in the study of formazanate complexes of third-row transition metals has been to elucidate triplet-state photophysical processes involving formazanates. Until now, we have established that formazanate ligands effectively quench the triplet excited states of cyclometalated iridium and platinum complexes, which is true in both monometallic formazanate complexes of platinum and iridium, 11,12,25 as well as dinuclear assemblies 28 that we had previously investigated. In all of these previous complexes, the formazanate was directly chelated to a photophysically active 5d metal, and in the bimetallic assemblies, the bridging ligand consisted of a single formazanate ligand.…”

“…We had previously developed the coordination chemistry and reactivity of formazanate ligands in third-row transition metals such as platinum 11,25,26 and iridium 12,27 and recently used analogous building blocks in combination with dinucleating formazanates to access several bimetallic complexes featuring cyclometalated platinum and iridium centers spanned by pyridyl-substituted formazanate ligands. 28 In this work, we use one of these same pyridyl formazanates to assemble a new class of trimetallic compounds, which include a 3d metal (nickel) partnered with two 5d metal centers (iridium). These new constructs further our understanding of a few concepts related to multimetallic, redox-active formazanate assemblies.…”

“…The iridium dimers [Ir(C^N) 2 (μ-Cl)] 2 and the formazan Fz were prepared by following literature procedures. 28,29 Tetrabutylammonium hexafluorophosphate (TBAPF 6 ) was recrystallized from hot ethanol, and ferrocene was purified by sublimation before use in cyclic voltammetry (CV) experiments.…”