Formazanate Complexes of Bis-Cyclometalated Iridium

Abstract

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 C 2-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 C 1 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.

Dinuclear Complexes of Flexidentate Pyridine‐Substituted Formazanate Ligands

“…Apart from intense peaks in the UV (l < 350 nm) attributed to localized p!p*t ransitions of the aryl substituents from both C^N and formazanate, an otable absorption between 460 nm and 700 nm stands out among all compounds,w hich is assigned as a p!p*t ransition within the formazanate framework with as mall amount of metal-to-ligand charge transfer (MLCT) character,o nt he basis of many other platinum and iridium formazanates previously studied experimentally and computationally. [29][30][31]33] The two free formazans have similar UV/Vis absorption wavelengths, with the low-energy transition occurring 482 nm in Fza and 466 nm in Fzb.C oordination through the 4-pyridyl substituent of Fza,a so bserved in complex 1,h as am inimal effect on the absorption wavelength, which occurs at nearly identicalw avelength (483 nm) in this complex. When the formazanate chelates the metal center, as observed in most other complexes,weo bserve as izeable red-shift of the p!p*t ransition, as we have previously noted in studying many other Pt and Ir formazanates.…”

“…[6] Av ariety of formazanate complexes of many main-group metals [7][8][9][10][11][12][13][14][15][16] andf irst-and second-row transition metals [17][18][19][20][21][22][23] have been described.T hese studies demonstrate the versatile coordination chemistry of formazanate ligandsa nd provides ignificant insight into the opticala nd redox properties of these compounds.S ome coppercomplexesc an also mediate oxygen activation, [24,25] certain cobalt and iron complexes exhibit unique magnetic characteristics, [17,26] and boronc omplexes in many cases feature not only the tunable redoxp roperties but also visible to nearinfrared photoluminescence, [9][10][11][12][13] finding applicationsa sc ellimaging agents [27,28] and electrochemiluminescence emitters. [10] Our group has expanded the coordination chemistry of formazanatest ot hird-row transition metals with as eries of hetero-leptic cyclometalated platinumc omplexes and bis-cyclometalated iridium complexes, [29][30][31] and accessed homoleptic azo-iminate platinum complexes and azo-1,2,3-triazolide iridium complexesv ia hydrogenative cleavage or [3+ +2] cyclization of formazanates, respectively. [32,33] Compared to the numerous formazanate complexes where as ingle metal atom is chelated within the NNCNN framework, polynuclear compounds involving formazanates remain rare.…”

“…Havinge xtensively studied the coordination chemistry of formazanates with Ir III [31,33] and Pt II , [29,30] our initial effort presented herein includes as eries of complexes containing different combinations of these two metals bridged by pyridyl-substitutedf ormazanates.W ee laborate the coordination chemistry of formazans with either 4-and 2-pyridyl substituents as the central aryl ring. The pyridyl substituents offer one additional coordination site, and we showi nt his work the diversity of coordination modes possible with these ligands.…”

Formazanate coordination compounds: synthesis, reactivity, and applications

“…38 Recent work from the Teets group has investigated a series of formazanate iridium(III) complexes with cyclometalated (C^N) ligands. 39 Both the substituent pattern on the formazanate as well as the nature of the C^N ligand were systematically varied to allow rational tuning of the electrochemical and optical properties of these octahedral Ir complexes. Stirring the dimeric bis-cyclometallated iridium chloride precursors with formazans at 80-85 1C in EtOH in the presence of NEt 3 afforded the formazanate complexes 33-36 in moderate to good yields (Scheme 12).…”

Dinuclear Complexes of Flexidentate Pyridine-Substituted Formazanate Ligands

“…36 Scheme 1 depicts the synthetic route of a mononuclear cyclometalated Ir(III) complex of Fza. Initially we used reaction conditions identical to those outlined in our previous report on cyclometalated iridium formazanate complexes, 31 so our hypothesis was that Fza would chelate the iridium center through the formazanate backbone. We treated the chloro-bridged dimer [Ir(F 2 ppy) 2 (μ-Cl)] 2 , where F 2 ppy is 2-(2,4-difluorophenyl)pyridine, with a stoichiometric amount of Fza in refluxing ethanol with excess triethylamine present.…”