Formazanate Complexes of Bis-Cyclometalated Iridium

Abstract

<div>In this work we describe a series of bis-cyclometalated iridium(III) formazanate complexes, expanding the coordination chemistry of the redox-active formazanate class to iridium. A total of 18 new complexes are described, varying the substituent pattern on the formazanate and the identity of the cyclometalating ligand on iridium. Eight of the new compounds are structurally characterized by single-crystal X-ray diffraction, which along with NMR spectroscopy evinces two binding modes of the formazanate. Two of the compounds are isolated in a C2-symmetric geometry where the formazanate is bound in a six-member chelate “closed” conformation, involving the 1- and 5-positions of the 1,2,4,5-tetraazapentadienyl formazanate core. In most of the examples, the major isomer that forms and is exclusively isolated involves the formazanate bound in a five-member chelate “open” form, coordinating through the 1- and 4-positions of the formazanate core and resulting in C1 point-group symmetry. All complexes are characterized by UV-vis absorption spectroscopy and cyclic voltammetry, with these features depending primarily on the substitution pattern on the formazanate, and to a lesser extent on the identity of the cyclometalating ligand and formazanate binding mode.</div>

Trimetallic Iridium–Nickel–Iridium Bis(formazanate) Assemblies

“…In comparison, the absorption wavelength maximum of the free formazan Fz is 482 nm, and the large red shift observed upon coordination to nickel is consistently observed in many other nickel, platinum, and iridium formazanates. 11,12,35 In line with the CV data described above, which are consistent with frontier molecular orbitals primarily localized on the formazanate, we assign this low-energy transition as a formazanate-centered π → π* transition, perturbed by the chelation to nickel but minimally effected by the pyridine-coordinated iridium(III) center.…”

“…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.…”

“…7 A variety of formazanate complexes of main-group elements and firstand second-row transition metals have been extensively reported, 4,8−10 and our group's recent work has expanded the coordination chemistry of this ligand class to third-row 5d metals. 11,12 These studies devoted much effort to not only explore the coordination chemistry of formazanate ligands but also investigate the optical and redox properties of these compounds. For instance, boron formazanate-chelated complexes exhibit not only tunable redox properties but also can show near-IR photoluminescence, 13 leading to a wide range of applications such as optoelectronic materials and cellimaging agents.…”

“…22 Given the above-mentioned desirable redox and optical properties of formazanates, continued development of the chemistry of polynuclear formazanate compounds can bring new insights and discoveries to the above fields. Additionally, formazans are convenient substrates for constructing polynuclear complexes because of their coordination flexibility 12,23,24 and simple synthesis, making it easy to have additional coordination sites at different positions to template polynuclear assemblies.…”

“…Recognizing that formazanates are ideally suited as redoxactive and chromophoric bridging ligands for multimetallic assemblies, we have expanded the chemistry of formazanates in this context. 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).…”