C^C* cyclometalated platinum( II ) N-heterocyclic carbene complexes with a sterically demanding β -diketonato ligand – synthesis, characterization and photophysical properties †

a Neutral cyclometalated platinum( II ) N-heterocyclic carbene complexes [Pt(C^C*)(O^O)] with C^C* ligands based on 1-phenyl-1,2,4-triazol-5-ylidene and 4-phenyl-1,2,4-triazol-5-ylidene, as well as acetylacetonato (O^O = acac) and 1,3-bis(2,4,6-trimethylphenyl)propan-1,3-dionato (O^O = mesacac) ancillary ligands were synthesized and characterized. All complexes are emissive at room temperature in a poly(methyl methacrylate) (PMMA) matrix with emission maxima in the blue region of the spectrum. High quantum e ﬃ ciencies and short decay times were observed for all complexes with mesacac ancillary ligands. The sterically demanding mesityl groups of the mesacac ligand e ﬀ ectively prevent molecular stacking. The emission behavior of these emitters is in general independent of the position of the nitrogen in the backbone of the N-heterocyclic carbene (NHC) unit and a variety of substituents in 4-position of the phenyl unit, meta to the cyclometalating bond.


Synthesis
The synthesis of the triazoles 1 and 7-9, and the methyl substituted triazolium salts 2 and 10-12 (Schemes 1 and 2) has been reported previously. 68The triazolium salts 3 and 13-15 were prepared by a quaternisation reaction using benzyl bromide (BnBr), 2 and 10-12 using iodomethane in tetrahydrofuran (THF).All complexes were synthesized in moderate to good yields by a multistep-reaction. 66,68The triazolium salts were reacted with silver(I) oxide in 1,4-dioxane to generate a silver(I) carbene complex in situ, which was then transmetalated with dichloro(1,5-cyclooctadiene)platinum(II) (Pt(COD)Cl 2 ) in a 1,4dioxane/butanone solvent mixture and cyclometalated at the C2 carbon atom of the phenyl ring at higher temperatures.The reported complexes were obtained after removal of the volatiles by reaction of the intermediate with the respective β-diketone and potassium tert-butoxide (KO t Bu) in dimethylformamide (DMF).
Although this method was successful with imidazoliumbased ligand precursors, the synthesis of platinum(II) triazol-5ylidene complexes with mesacac ancillary ligands suffered from unsatisfying yields.Optimization of the reaction conditions leads to improved yields by using DMF already for the generation of the silver(I)carbene as well as for the transmetalation step.In comparison to the previously used 1,4dioxane/butanone solvent mixture, we could more than double the yield of complex 17 (see exp. details, compound 17, method B).

Solid state structure determination
Single crystals suitable for X-ray diffraction analysis were grown from 4 by slow vapor diffusion of diethyl ether into a highly concentrated solution of the respective complex in dichloromethane and from 20 by slow evaporation of the solvent from the solution of the complex in dichloromethane.All details of the solid state structure determinations are summarized in the ESI.† The complexes 4 (Fig. 1 and 2) and 20 (Fig. 3 and 4) crystallize in the monoclinic space group P2 1 /c and are devoid of any solvent molecules.X-ray diffraction analysis confirms a quasi-square planar coordination environment of the platinum(II) center with bond lengths of the

Dalton Transactions Paper
coordinative bonds similar to those of the previously reported [Pt(C^C*)(O^O)] complexes. 68,71,72he Pt(1)-O(2) bond lengths of both complexes are slightly elongated when compared to the Pt(1)-O(1) bond lengths, due to the trans effect of the cyclometalated phenyl ring.
The planar structure with coplanar arranged ligands allows 4 to stack with short intermolecular distances.In the solid state two of the molecules form dimers with a Pt-Pt contact shorter than twice the van-der-Waals radius of the platinum atom.However, in the case of 20 the sterically demanding mesityl groups of the β-diketonato ancillary ligand are twisted out of the coordination plane.This efficiently prevents molecular stacking of these complexes so that they are orthogonally arranged to each other.

Photoluminescence properties
This class of complexes shows very weak emissions from a solution in dichloromethane (see ESI †), but are significantly stronger emissive in a poly(methyl methacrylate) (PMMA) matrix.Therefore, the photophysical properties of all complexes were investigated from polymer films with 2% of the complexes in a PMMA matrix at room temperature.Emission spectra of 5-6 and 16-21 from neat complex films were also recorded to get an insight into the emission behavior of agglomerated complexes.Neat emitter films of 4 and 22 could not be obtained due to rapid crystallization of the complexes during the film preparation.The absorption spectra of all complexes in a PMMA matrix (Fig. 5) display strong bands below 270 nm, which arise from spin allowed 1 π-π transitions.In this region the absorption maximum of the complex with a 1-benzyl-4-phenyl triazol-5-ylidene unit and mesacac ancillary ligand 5 is located at around 230 nm, whereas the absorption maxima of all other complexes are detected at around 220 nm.Further bands of moderate intensity up to 350 nm arise from transitions involving the metal center, such as a metal-toligand charge transfer 1 MLCT transition.Here complex 5 exhibits another absorption maximum at around 310 nm.
All complexes are strongly emissive in a PMMA matrix at room temperature.The emission spectra (Fig. 6) and the PL data (Table 1) of 4 and 16 correspond well to those of analogous platinum(II) imidazol-2-ylidene complexes containing an acetylacetonato ancillary ligand and a methyl group instead of a benzyl group bound to the imidazol-N3 atom. 68This clearly indicates that the emission behavior of these complexes is    independent of the benzyl group (R 1 ).Both emitters display a structured emission band with the maximum located in the blue region of the spectrum, which indicates strong contributions of a ligand centered state ( 3 LC) to the emission process.The well-structured emission band of 16 reveals a vibronical progression of about 1300 cm −1 which corresponds to vibrations of the NHC ligand as determined by the frequency analysis from the density functional theory (DFT) calculations.However, complexes 5-6 and 17-22 reveal an essentially different emission behavior, which is caused by the mesacac ancillary ligand.In contrast to the vibronic structure of the 3 LC emission profiles of the emitters containing an acac ancillary ligand (4 and 16), the complexes 5-6 and 17-22 exhibit unstructured emission bands with only one maximum located between 471 and 478 nm.Therefore, another transition contributes essentially to the emission process of the complexes with a mesacac ligand.This transition is supposed to be a charge transfer transition involving the metal center, as the complexes with a mesacac ligand also show significantly enhanced quantum efficiencies (70-83%) and reduced decay times below 8 μs (Table 1).Due to the change in the emission process to a transition with reduced 3 LC character, different substituents at the cyclometalated ligand do not significantly change the emission properties, like quantum yield and emission wavelength.All complexes, even the emitters based on 4-phenyl triazole 5 and 6 reveal high quantum yields, whereas very low efficiencies have been previously reported for several 4-phenyl triazole complexes with acac ancillary ligands in comparison to the 1-phenyl triazole complexes with acac ancillary ligands. 68While in the case of 1-phenyl and 4-phenyl triazole complexes with acac ligands, substitution in 4-position of the phenyl ring of the cyclometalating ligand had a significant effect on the emission wavelengths, this was not observed for the complexes with mesacac ligands.Although the emissions of the respective acac complexes are shifted slightly to lower energies by several substituents, this effect is relatively small for the mesacac complexes 18, 19, 21 and 22.
For the acac complexes 4 and 16 we could only get the neat film emission for 16 as 4 shows early crystallization.These are the only two complexes prone for stacking as the mesacac prevents short intermolecular contacts as shown in the solid state structure of 20.The PL spectrum of the neat film of 16 (Fig. 7) reveals a low energy emission in the green region of the spectrum, similar to other acac complexes [Pt(C^C*)(acac)]. 68This emission is probably caused by a multi-molecular species, such as a stack of emitter molecules.The planar geometry of the complexes with acac ligands supports aggregation in the solid state with a short intermolecular separation and Pt-Pt distances lower than twice the van-der-Waals radius of platinum.The quantum efficiency of the neat film of 16 (Table 2) is significantly higher than that of the complex in a 2% PMMA matrix.In contrast to 16, the emissions from the neat films of the mesacac complexes 5, 6 and 17-21 correspond well to the results of the measurements of 2% complex in a PMMA matrix, although the emission maxima are slightly shifted to higher wavelengths by 7-22 nm.With respect to the results of the molecular structure determination, which shows no stacking behaviour of 20 in the solid state, the emissions from the neat films of 5-6 and 17-21 are expected to be mono-molecular emissions.

DFT calculations
We recently published a methodology which allows us to predict the emission wavelengths of the platinum complexes. 73lso for the new complexes reported in this study we could successfully use this methodology, where we optimize the singlet and triplet state geometries of all complexes in the gas phase using DFT methods, e.g.BP86 together with a 6-31G(d) basis set and Hay-Wadt-ECP for platinum.The difference between the calculated and the measured wavelength (see Table S2, ESI †) turned out to be ≤10 nm for all complexes reported here with the exception of 4 (21 nm).The geometries of the optimized complexes 4 and 20 are also in good agreement with the data of the solid state structures obtained by X-ray diffraction analysis, which confirms that the DFT functionals used are capable of describing the bonding situation in these complexes.

Conclusion
We synthesized ten new platinum(II) complexes with C^C* cyclometalating ligands based on 1-phenyl-1,2,4-triazol-5ylidene and 4-phenyl-1,2,4-triazol-5-ylidene, as well as acetylacetonato (O^O = acac) and 1,3-bis(2,4,6-trimethylphenyl) propan-1,3-dionato) (O^O = mesacac) ancillary ligands.The complexes were thoroughly characterized and investigated in terms of their photophysical properties for a potential application as emitter molecules for OLEDs.The solid-state structure determination of two complexes reveals the steric effect of the O^O ancillary ligands on the stacking behavior of the complexes, which consequently has an impact on the emissions of the neat emitter films.The emission properties of the complexes strongly depend on the O^O ligand, but less on the substituents in 4-position of the cyclometalating phenyl ring.All complexes containing 1,3-bis(2,4,6-trimethylphenyl)propan-1,3-dionato as O^O ligand are strongly emissive at lower energies and reveal shorter decay times in comparison to the emitters with an acetylacetonato ancillary ligand.

General procedure for the synthesis of platinum(II) complexes
A flame-dried schlenk tube was charged with triazolium salt and silver(I) oxide.The reactants were dried in vacuo and the tube was refilled with argon.1,4-Dioxane was added and the mixture was stirred for 21 h at room temperature under a positive pressure of argon.Afterwards, dichloro(1,5-cyclooctadiene) platinum(II) and butanone were added, and the mixture was slowly heated and stirred for 21 h at 115 °C.The solvent was removed under reduced pressure and the remaining solid was dissolved in DMF.Potassium tert-butoxide and β-diketone were added, and the mixture was stirred for 21 h at room temperature and another 6 h at 100 °C.Afterwards the solvent was removed under reduced pressure and the residue was washed with water and purified by flash chromatography.

Fig. 2
Fig. 2 Intermolecular arrangement of 4 in the solid state.All hydrogen atoms are omitted for clarity.

Fig. 4
Fig. 4 Intermolecular arrangement of 20 in the solid state.All hydrogen atoms are omitted for clarity.
Potassium tetrachloroplatinate (II) was purchased from Pressure Chemicals Co. NMR spectra were recorded on a Bruker NMR spectrometer and referenced to the internal resonances of the solvents ( 1 H: 7.26, 13 C: 77.0 for CDCl 3 ; 1 H: 2.50, 13 C: 39.43 for DMSO-d 6 ).
are given in ppm, coupling constants J in Hz.