Synthesis and characterization of green-to-yellow emissive Ir(III) complexes of pyridylbenzothiadiazine ligand

Abstract Reaction of tetrakis(2-phenylpyridinato-C2,N′)(μ-dichloro)di-iridium with 3-(pyridin-2-yl)-4H-benzo[e][1,2,4]thiadiazine (L1) under neutral and basic media afforded the charged and neutral Ir-complexes, 1 and 2, respectively, in good yields (63–79%). Single-crystal XRD analysis confirms that the ancillary ligands in both 1 and 2 bind to the iridium via coordination of N2 of the benzo[e][1,2,4]thiadiazine. Upon isolation of the neutral complex, the sulfur rapidly oxidizes to form 2. The UV–vis absorption spectra of the complexes exhibit both ligand-centered and mixed metal-to-ligand and ligand-to-ligand charge transfer transitions that are typical of many heteroleptic iridium complexes. Complexes 1 and 2 were emissive at room temperature in the green and yellow region of the electromagnetic spectrum, respectively, albeit with poor photoluminescence quantum yield (ΦPL = 1.1–2.5%).


Introduction
Since the pivotal work by Watts and co-workers [1], interest in the photophysical and electrochemical properties of cationic heteroleptic iridium complexes of the form [Ir(ppy) 2 (bpy)] + , where ppyH is 2-phenylpyridine and bpy is 2,2′-bipyridine, has increased tremendously [2]. Cationic Ir(III) complexes, which have found wide use as emitters in electroluminescent devices [3], as photocatalysts [4] and in biolabeling [5], have been derivatized mostly by incorporation of electron donating or withdrawing groups onto either the cyclometallating or ancillary ligands [6,7]. Though several pyridylazole ancillary ligands have been explored as ancillary ligands, the overall structural diversity of heterocycles employed remains relatively poor.
The family of benzo[e] [1,2,4]thiadiazine with sulfur in various oxidation states has attracted attention for both materials and pharmaceutical applications [8]. While there are a number of synthetic methodologies to access specific derivatives of fused aryl-1,2,4-thiadiazines depending on the fused substituent [8][9][10], to the best of our knowledge, until now there is only one report of using 3-(pyridin-2-yl)-4H-benzo[e] [1,2,4]thiadiazine (L1) as a bidentate N,N′-chelating ligand, and this with respect to complexation with first row transition metals [11]. We thus became interested to attach L1 to the [Ir(ppy) 2 ] + core and study the structure-property relationship of the resulting complexes.
Herein, we report synthesis and characterization of a charged (1) and a neutral (2) Ir(III) complex, obtained under neutral and basic complexation conditions with L1, respectively. The optoelectronic properties of these complexes are discussed in the context of combined experimental and density functional theory (DfT) studies.

For purification, NMR spectroscopy, melting point determination and mass spectrometry
All reactions were performed using standard Schlenk techniques under an inert (N 2 ) atmosphere with reagent grade solvents. flash column chromatography was performed using silica gel (Silia-P from Silicycle, 60 Å, 40-63 mm). Analytical thin layer chromatography was performed with silica plates with aluminum backings (250 mm with indicator f-254). Compounds were visualized under UV light. 1 H and 13 C NMR spectra were recorded on a Bruker Avance spectrometer at 500 and 126 MHz, respectively. Deuterated chloroform (CDCl 3 ) and deuterated acetonitrile (CD 3 CN) were used as solvents of record and TMS was used as an internal standard. Melting points (Mps) were recorded using open-ended capillaries on an electrothermal melting point apparatus and are uncorrected. for high-resolution mass spectrometry the compounds were dissolved in DCM before being diluted in MeoH with NH 4 oAc for analysis on the Thermo Scientific LTQ orbitrap XL in positive nano-electrospray ionization mode. The instrument was calibrated with a mixture of caffeine, UltraMark and MRfA.

For cyclic voltammetry
Electrochemical measurements were carried out in N 2 -purged purified MeCN at 25 °C with a Potentiostat CH620E instrument. The working electrode was glassy carbon, the counter electrode was a Pt wire, and the pseudo-reference electrode was a Ag wire. The reference was an internal 1 mM solution of ferrocene/ferrocinium (380 mV versus SCE in MeCN) [13]. The concentration of the compounds analyzed was 1 mM. Tetra-n-butylammonium hexafluorophosphate was used as the supporting electrolyte (0.10 M). Cyclic voltammograms of 1 and 2 were obtained at a scan rate of 100 mV s −1 . The criteria for reversibility were (a) a separation of 60 mV between cathodic and anodic peaks, (b) a ratio of the intensities of the cathodic and anodic currents > 0.9, and (c) the lack of dependency of the peak potential on scan rate.

For UV-vis absorption spectroscopy
Electronic spectra were recorded using a Shimadzu UV-1800 double beam spectrophotometer with a 1 cm quartz cell from 200-1000 nm at room temperature. All samples were prepared in HPLC grade acetonitrile with varying concentrations in the order of mM. Molar absorptivity determination was verified by linear least-squares fit of values obtained from at least four independent solutions at varying concentrations with absorbance ranging from 8.08 × 10 −5 to 8.99 × 10 −6 M. Broad peaks are marked as shoulder, "sh".

For steady-state and time-resolved emission spectroscopy
Steady-state emission and excitation spectra and time-resolved emission spectra were recorded at 298 K using an Edinburgh Instruments f980. The sample solutions for the emission spectra were prepared in HPLC-grade acetonitrile and degassed via three freeze-pump-thaw cycles. Luminescence lifetimes were determined by time-correlated single-photon counting with an Edinburgh EPL-378 spectrometer (nanosecond-pulse diode laser at 378 nm). All samples for steady-state measurements were excited at 360 nm while samples for time-resolved measurements were excited at 378 nm.

For determination of photoluminescence quantum yield
Photoluminescence quantum yields (Φ PL ) were determined using the optically dilute method [14,15]. A stock solution with absorbance of ca. 0.5 was prepared and then four dilutions were prepared with dilution factors of 5, 6.6, 10, and 20 to obtain solutions with absorbances of ca. 0.1, 0.075, 0.05 and 0.025, respectively. The Beer-Lambert law was found to be linear at the concentrations of the solutions. The emission spectra were then measured after the solutions were rigorously degassed via three freezepump-thaw cycles prior to spectrum acquisition. for each sample, linearity between absorption and emission intensity was verified through linear regression analysis and additional measurements were acquired until the Pearson regression factor (R 2 ) for the linear fit of the data-set surpassed 0.9. Individual relative quantum yield values were calculated for each solution and the values reported represent the slope value. The equation Φ s = Φ r (A r /A s )(I s /I r )(n s /n r ) 2 was used to calculate the relative quantum yield of each of the samples, where Φ r is the absolute quantum yield of the reference, n is the refractive index of the solvent, A is the absorbance at the excitation wavelength, and I is the integrated area under the corrected emission curve. The subscripts s and r refer to the sample and reference, respectively. A solution of quinine sulfate in 0.5 M H 2 So 4 (Φ r = 54.6%) was used as the external reference [16].

X-ray crystallography
X-ray diffraction data for 1 and 2 were collected at 173 K using a Rigaku fR-X Ultrahigh brilliance Microfocus RA generator/confocal optics and Rigaku XtaLAB P200 system, with Mo Kα radiation (λ = 0.71075 Å). Intensity data were collected using ω steps accumulating area detector images spanning at least a hemisphere of reciprocal space. All data were corrected for Lorentz polarization effects. A multiscan absorption correction was applied by using CrystalClear [17]. Structures were solved by Patterson (PATTy [18]) methods and refined by full-matrix least-squares against F 2 (SHELXL-2013 [19]). Non-hydrogen atoms were refined anisotropically and hydrogens were refined using a riding model. All calculations were performed using the CrystalStructure [20] interface. Crystallographic data are summarized in table 1.

DFT studies
All calculations were performed with the Gaussian 03 [21] suite of programs employing the DfT method, the Becke three-parameter hybrid functional [22] and Lee-yang-Parr's gradient-corrected correlation functional (B3LyP) [23]. Singlet ground-state geometry optimizations for [1] + (1 without associated anion) and neutral [2] were carried out at the (R)B3LyP level in the gas phase, using the structures of 1 and 2 determined by X-ray crystallography as starting points. All elements except Ir were assigned the 6-31G(d,p) basis set [24]. The double-ζ quality LANL2DZ ECP basis set [25] with an effective core potential and one additional f-type polarization functional was employed for Ir. Vertical electronic excitations based on (R)B3LyP-optimized geometries were computed for [1] + , [2] and a model complex, [Ir(ppy) 2 (bpy)] + using the TD-DfT formalism [26,27] in MeCN using the conductor-like polarizable continuum model [28][29][30]. Vibrational frequency calculations were performed to ensure that the optimized geometries represent the local minima, and there are only positive eigenvalues. The electronic distribution and localization of the singlet excited states were visualized using the electron density difference maps. GaussSum 2.2 [31] and Chemissian [32] were employed to visualize the absorption spectra (simulated with Gaussian distribution with a full-width at half maximum (fwhm) set to 3000 cm −1 ) and to calculate the fractional contributions of various groups to each molecular orbital. All calculated structures and Kohn-Sham orbitals were visualized with ChemCraft [33].

Synthesis and characterization of the complexes
The Ir(III)-complexes 1 and 2 were synthesized involving a protocol that proceeds via isolation of the μ-dichloro-bridged Ir(III)-dimer described by Nonoyama 12 followed by its cleavage with 2.1 equiv. of ancillary L1 (scheme 1). The charged 1 was synthesized by reaction of [Ir(ppy) 2 Cl] 2 with L1 in a N 2degassed dichloromethane-methanol solution, whereas the synthesis of the neutral 2 requires a prior deprotonation of L1 with 10 equiv. of anhydrous K 2 Co 3 in N 2 -degassed methanol followed by its addition to a N 2 -degassed dichloromethane solution of the Ir-dimer. Despite maintaining the N 2 -atmosphere, the neutral complex was isolated as the S(IV) complex, 2. The facile oxidation of ligand L1 is indicative of the electron-rich benzothiadiazinate heterocycle, which has been previously observed by Rawson et al., where L1 undergoes facile Cu(II)-promoted aerial oxidation to give access to the benzothiadiazine-S-oxide heterocycle [34].

Solid-state structures
Crystals suitable for analysis by X-ray diffraction were grown by slow vapor diffusion of diisopropyl ether into an acetonitrile solution of 1, or of diethyl ether into a solution of 2 in dichloromethane (CCDC 1445661 and 1445662). Specific crystallographic parameters in comparison to those obtained from DfT calculations are summarized in table 2. Both structures reveal coordinatively saturated Ir(III) in a distorted octahedral geometry, where in both complexes L1 binds to Ir(III) in an N,N′-chelate fashion (figure 2). The Ir-N ppy and Ir-C ppy bond lengths and C ppy -Ir-N ppy and N L1 -Ir-N L1 bite angles were similar in both complexes and fall in close agreement with those calculated by DfT. Noticeable differences in bond lengths and angles are observed in the N32-C31-N40 fragments of the neutral and anionic versions of L1 in 1 and 2, respectively. In 1, the shorter C31-N32 bond length compared to that of C31-N40 indicates localized double-and single-bond characters for C31-N32 and C31-N40, respectively. In 2, due to the anionic nature of L1, there is a delocalized double bond character throughout the N32-C31-N40 core. Similarly, there is a more defined sp 2 character to C31 of the N32 = C31-N40 core in 1 compared to relatively less defined sp 2 character of the same core in 2, as judged by the deviation of N32-C31-N40 angle from the ideal sp 2 bond angle of 120°. The decrease in N32-S33 bond length in 2 compared to that in 1 is presumably due to extended delocalization of the N40-C31-N32-S33-o33 moiety in 2.

UV-vis absorption properties of 1 and 2
The UV-visible absorption spectra of L1, 1 and 2 were recorded in MeCN solutions at room temperature (figure 3) and data are summarized in table 3. overlays of experimental absorption spectra of 1 and 2 with their predicted singlet transitions calculated by time-dependent DfT (TD-DfT) are shown in figure  4. At higher energy (≤ 290 nm), ligand-centered (LC) π → π* transitions are observed for both complexes. In the case of 1, the transitions at 374 and 413 nm are predominantly due to singlet ligand-to-ligand charge transfer ( 1 LLCT), transitions, originating from ppy-to-L1 with a minor contribution of singlet metal-to-ligand charge transfer ( 1 MLCT) transition involving Ir(dπ) → L1(π*), as predicted by TD-DfT (table 4). In the case of 2, the contribution from the metal center comes into play at much higher energies as in the transition at 292 nm (table 5). The transitions at 368, 390 and 444 nm are all composed of a predominant 1 LLCT character along with a minor component of 1 MLCT transition. The tailing of the lowest energy 1 CT transition of 1 at 413 nm up to ~560 nm compared to that of 2 (λ max = 444 nm) and that of the prototype [Ir(ppy) 2 (bpy)] + (λ max = 465 nm) is in agreement with the trends revealed from the

Conclusion
Two new Ir(III) complexes, 1 and 2, bearing the unusual ancillary ligand, 3-(pyridin-2-yl)-4H-benzo[e] [1,2,4]thiadiazine L1, have been synthesized and characterized by various analytical techniques. L1 coordinates to Ir(III) as both neutral and anionic forms depending on the reaction conditions used. In the solid-state structures of these complexes, the fused phenylene ring of L1 was spatially away from the ppy moiety to overcome steric hindrance. In 2, L1 underwent oxidation to the S-oxide analog. Both complexes exhibit mixed ligand-centered and charge-transfer transitions at energies lower than 300 nm, with the absorption in 1 trailing further into the visible region. The complexes exhibit facile oxidation of both the metal and the cyclometallating ligand centers and reduction of the ancillary ligand. Both complexes were poorly emissive with the emission of 1 blue-shifted compared to that of 2.