Spectroscopic and Electrochemical Properties of Electronically Modified Cycloplatinated Formazanate Complexes

Abstract

Formazanates are a class of monoanionic, chromophoric, redox-active chelating ligands, and until recently, the coordination chemistry of formazanates with third-row transition metals was unexplored. Described here is a series of heteroleptic cyclometalated platinum formazanate complexes of the type Pt(C^N)(Fz), where "C^N" is the cyclometalating ligand and "Fz" is the formazanate. This work includes a set of 14 complexes derived from four different cyclometalating ligands and five different formazans, greatly expanding the structural and electronic diversity of this class of complexes. The formazans fall into two major categories: triarylformazans with either electron-donating (Me, OMe) or electron-withdrawing (CN) para substituents, and even more electron-deficient diarylformazans where the center position of the backbone is a cyano group. Seven of the new compounds are characterized by single-crystal X-ray diffraction, and all are characterized by cyclic voltammetry and UV-vis absorption spectroscopy to provide insight into the frontier orbitals. The complexes can be electrochemically reduced by three electrons, with the first two electrons populating formazanate-centered frontier orbitals and the third being added to the Pt-C^N fragment. The formazanate-based reduction potentials are highly sensitive to the substitution pattern of the formazanate and can be tuned over a wide range of ca. 0.8 V by altering substituents. The formazanates also impart these complexes with very strong, low-energy visible absorption bands assigned to HOMO → LUMO transitions, which are greatly perturbed from the free ligand due to interactions of the formazanate frontier orbitals with platinum-centered orbitals. Unlike the redox properties, the formazanate-derived visible absorption band is relatively insensitive to substituent effects. Additional visible absorption bands, attributed to Pt(d) → C^N(π*) MLCT transitions, can be tuned over a wide range by changing the structure of the C^N ligand, giving rise to strong panchromatic absorption for some members of the series.

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.…”

“…First, we note that complex 2,w hichh as ac helated Ir III center, is significantly more difficult to reduce than any of the Pt II chelates, at rend we previouslyo bserved in many mononuclear formazanate complexes. [30,31] Other insights come from comparing mononuclear Pt II complex 4 to the two dinuclear complexes which contain this buildingb lock, 3 and 5.T he reductionp otentiali n4 is À1.55 V, which shift anodically to À1.46 Vw hen the pyridine is capped with another Pt II unit in 3,a nd similarly to À1.48 Vw hen Ir III binds to the pyridine in 5. Likewise, the one-electron oxidation couples are influenced to as mall extent,w ith 3 and 5 more difficult to oxidize by % 0.1-0.2 V. This observation suggests that when the pyridine coordinates to another Lewis-acidic metal center,t he formazanatecentered frontier orbitals are stabilized by as mall amount,i ndicatingt hat dincuclear assembly can influence formazanatebased orbital energies.…”

“…[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

“…The properties of platinum formazanate complexes 46-49 with cyclometallated ligands were studied by Teets and co-workers with a wide range of ligand substituents (Scheme 17a). 49,50 Formazanate coordination to the platinum center was shown by a combination of spectroscopic, electrochemical and computational studies to lead to substantial changes in comparison to the free ligands, with absorption maxima that are red-shifted to ca. 660 nm.…”