Two Synthetic Tools to Deepen the Understanding of the Influence of Stereochemistry on the Properties of Iridium(III) Heteroleptic Emitters

Abstract

Two complementary procedures are presented to prepare cis-pyridyl-iridium(III) emitters of the class [3b+3b+3b'] with two orthometalated ligands of the 2-phenylpyridine type (3b) and a third ligand (3b'). They allowed to obtain four emitters of this class and to compare their properties with those of the trans-pyridyl isomers. The finding starts from IrH₅(PⁱPr₃)₂, which reacts with 2-(p-tolyl)pyridine to give fac-[Ir{κ²-C,N-[C₆MeH₃-py]}₃] with an almost quantitative yield. Stirring the latter in the appropriate amount of a saturated solution of HCl in toluene results in the cis-pyridyl adduct IrCl{κ²-C,N-[C₆MeH₃-py]}₂{κ¹-Cl-[Cl-H-py-C₆MeH₄]} stabilized with p-tolylpyridinium chloride, which can also be transformed into dimer cis-[Ir(μ-OH){κ²-C,N-[C₆MeH₃-py]}₂]₂. Adduct IrCl{κ²-C,N-[C₆MeH₃-py]}₂{κ¹-Cl-[Cl-H-py-C₆MeH₄]} directly generates cis-[Ir{κ²-C,N-[C₆MeH₃-py]}₂{κ²-C,N-[C₆H₄-Isoqui]}] and cis-[Ir{κ²-C,N-[C₆MeH₃-py]}₂{κ²-C,N-[C₆H₄-py]}] by transmetalation from Li[2-(isoquinolin-1-yl)-C₆H₄] and Li[py-2-C₆H₄]. Dimer cis-[Ir(μ-OH){κ²-C,N-[C₆MeH₃-py]}₂]₂ is also a useful starting complex when the precursor molecule of 3b' has a fairly acidic hydrogen atom, suitable for removal by hydroxide groups. Thus, its reactions with 2-picolinic acid and acetylacetone (Hacac) lead to cis-Ir{κ²-C,N-[C₆MeH₃-py]}₂{κ²-O,N-[OC(O)-py]} and cis-Ir{κ²-C,N-[C₆MeH₃-py]}₂{κ²-O,O-[acac]}. The stereochemistry of the emitter does not significantly influence the emission wavelengths. On the contrary, its efficiency is highly dependent on and associated with the stability of the isomer. The more stable isomer shows a higher quantum yield and color purity.

Scheme 1. Synthesis Methods of Homoleptic-Ir(III) Complexes

Scheme 2. Preparation of Heteroleptic-Ir(III) Emitters

Scheme 3. Synthesis of Iridaimidazo[1,2-a]pyridine and Iridaoxazole Complexes

Scheme 4. Reactions of IrH₅(PⁱPr₃)₂ with 2-(p-Tolyl)pyridine, 4,5-Dimethyl-2-phenylpyridine, and 1-Phenylisoquinoline

Figure 1

Molecular diagram of complex 3 (displacement ellipsoids shown at 50% probability). All hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Ir–C(1) = 1.987(3), Ir–C(14) = 1.972(3), Ir–C(27) = 2.053(4), Ir–N(1) = 2.175(3), Ir–N(2) = 2.089(3), Ir–N(3) = 2.118(3); C(1)–Ir–N(1) = 83.13(13), C(14)–Ir–N(2) = 86.15(13), C(27)–Ir–N(3) = 81.62(13), C(1)–Ir–N(2) = 172.55(13), C(14)–Ir–N(3) = 171.88(13), C(27)–Ir–N(1) = 171.60(11), N(1)–Ir–N(2) = 92.07(11), N(1)–Ir–N(3) = 92.61(12), N(2)–Ir–N(3) = 88.12(11).

Scheme 5. Preparation of Starting Materials to Generate Heteroleptic Emitters with cis-Pyridyl Arrangement

Figure 2

Molecular diagram of complex 5 (displacement ellipsoids shown at 50% probability). All hydrogen atoms (except that of pyridinium) are omitted for clarity. Selected bond distances (Å) and angles (deg): Ir–C(1) = 2.004(6), Ir–C(13) = 2.006(6), Ir–N(1) = 2.127(6), Ir–N(2) = 2.019(6), Ir–Cl(1) = 2.5151(16), Ir–Cl(2) = 2.3935(16), Cl(1)–H(3A) = 2.02(8), N(3A)–H(3A) = 1.13(8); Cl(1)–Ir–Cl(2) = 91.16(6), N(2)–Ir–Cl(2) = 177.04(16), Cl(1)–Ir–C(1) = 173.78(19), N(1)–Ir–N(2) = 96.0(2), C(1)–Ir–N(2) = 90.3(2), C(1)–Ir–N(1) = 80.0(2), C(13)–Ir–N(2) = 80.4(2), C(13)–Ir–N(1) = 173.8(2).

Figure 3

¹H NMR spectra (300 MHz, DMSO-d₆, 298 K) of cis-[IrCl{κ²-C,N-[C₆MeH₃-py]}₂{κ¹-S-[S(O)Me₂]}] (7) generated from 5 (a) or 6 (b) and its comparison with its trans isomer (c).

Scheme 6. Synthesis of [3b+3b+3b′] Emitters

Figure 4

Molecular diagram of complex 9a (displacement ellipsoids shown at 50% probability). All hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Ir–C(1) = 2.001(3), Ir–C(16) = 2.017(3), Ir–C(28) = 2.007(3), Ir–N(1) = 2.110(3), Ir–N(2) = 2.127(3), Ir–N(3) = 2.135(3); C(1)–Ir–N(1) = 78.53(13), C(16)–Ir–N(2) = 79.55(13), C(28)–Ir–N(3) = 79.22(14), C(28)–Ir–N(2) = 172.81(12), C(16)–Ir–N(1) = 172.04(12), C(1)–Ir–N(3) = 172.42(13).

Figure 5

Molecular diagram of one of the two chemically equivalent but crystallographically independent molecules of complex 9b (displacement ellipsoids shown at 50% probability). All hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Ir(1)–C(1) = 2.086(7), 2.091(7), Ir(1)–C(16) = 1.996(7), 2.002(7), Ir(1)–C(28) = 2.065(7), 2.054(9), Ir(1)–N(1) = 2.150(6), 2.139(7), Ir(1)–N(2) = 2.035(6), 2.041(7), Ir(1)–N(3) = 2.043(6), 2.046(6); N(2)–Ir(1)–N(3) = 174.2(2), 172.4(3), C(28)–Ir(1)–C(1) = 174.0(3), 172.5(3), C(16)–Ir(1)–N(1) = 174.9(3), 171.2(3), C(1)–Ir(1)–N(1) = 77.3(3), 77.0(3), C(16)–Ir(1)–N(2) = 80.3(3), 80.7(3), N(3)–Ir(1)–C(28) = 79.7(3), 79.8(4).

Scheme 7. Preparation of [3b+3b+3b′]-Picolinate Isomers

Figure 6

Molecular diagram of complex 11a (displacement ellipsoids shown at 50% probability). All hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Ir–C(1) = 2.008(4), Ir–C(13) = 2.010(4), Ir–N(1) = 2.124(3), Ir–N(2) = 2.028(4), Ir–N(3) = 2.137(4), Ir–O(1) = 2.068(3); C(1)–Ir–N(2) = 80.04(19), C(13)–Ir–N(3) = 79.6(2), O(1)–Ir–N(1) = 78.53(12), C(1)–Ir–N(3) = 176.10(16), C(13)–Ir–N(1) = 170.74(17), N(2)–Ir–O(1) = 174.13(15).

Scheme 8. Preparation of [3b+3b+3b′]-acac Isomers

Figure 7

DFT calculated spin density for the T₁ states of isomers a and b of complexes 9–12 at 0.002 au contour level.