Influence of the Bite Angle of Dianionic C,N,C-Pincer Ligands on the Chemical and Photophysical Properties of Iridium(III) and Osmium(IV) Hydride Complexes

Abstract

The study of the reactions of IrH 5 (P i Pr 3) 2 (1) with 2,6-diphenylpyridine and 2-phenoxy-6-phenylpyridine and the photophysical characterization of some of the resulting compounds, as well as their osmium-counterparts, reveal that the bite angle of dianionic C,N,C-pincer ligands has a marked influence on the chemical and photophysical properties of iridium(III)-and osmium(IV)-hydride complexes. Complex 1 promotes a double o-CH bond activation of both disubstituted pyridines. The reaction with 2,6-diphenylpyridine directly leads to IrH{κ 3-C,N,C-(C 6 H 4-py-C 6 H 4)}(P i Pr 3) 2 (2), whereas the activation of 2-phenoxy-6phenylpyridine is sequential and slower. Initially, the phenyl activation gives IrH 2 {κ 2-C,N-(C 6 H 4-py-OPh)}(P i Pr 3) 2 (3), which subsequently evolves to IrH{κ 3-C,N,C-(C 6 H 4-py-OC 6 H 4)}(P i Pr 3) 2 (4). Complexes 2 and 4 are Brønsted bases, which react with HBF 4 •OEt 2. The protonation of the hydride position is elusive; the proton of the acid is selectively added to one of the metalated substituents. Thus, the reaction with 2 affords [IrH{κ 3-C,N,(C-H)-(C 6 H 4-py-Ph)}(P i Pr 3) 2 ]BF 4 (5), in which the formed C-H bond remains coordinated. The protonation of 4 selectively occurs at the phenoxy substituent to give the five-coordinate cationic complex [IrH{κ 2-C,N-(C 6 H 4-py-OPh)}(P i Pr 3) 2 ]BF 4 (6) bearing a free OPh group. In contrast to 5, complex 6 inserts acetylene and phenylacetylene into the Ir-H bond to yield [Ir{(E)-CH=CHR}{κ 2-C,N-(C 6 H 4-py-OPh)}(P i Pr 3) 2 ]BF 4 (R = H (7), Ph (8)). Complexes 2 and 4 and their osmium(IV)-counterparts OsH 2 {κ 3-C,N,C-(C 6 H 4-py-C 6 H 4)}(P i Pr 3) 2 (A) and OsH 2 {κ 3-C,N,C-(C 6 H 4-py-OC 6 H 4)}(P i Pr 3) 2 (B) are phosphorescent emitters, which display emission wavelengths between 473 nm and 619 nm and quantum yields between 0.03 and 0.96 depending upon the metal center and the presence or absence of the oxygen atom in the pincer ligand. RESULTS AND DISCUSSION C-H Bond Activation Reactions. Pentahydride IrH 5 (P i Pr 3) 2 (1) promotes the o-CH bond activation of both substituents of 2,6-diphenylpyridine and 2-phenoxy-6-phenylpyridine. Interestingly, the rate of the C-H rupture depends upon the substituent. Thus, the phenyl activation is much faster than that of the phenoxide group. The reaction of 1 with 2,6-diphenylpyridine in toluene under reflux leads to the monohydride-iridium(III) derivative IrH{κ 3-C,N,C-(C 6 H 4-py-C 6 H 4)}(P i Pr 3) 2 (2), which was isolated as a yellow solid in 52%, after 14 h (Scheme 3), and characterized by X-ray diffraction analysis. Figure 1 shows a view of the structure, which proves the formation of the C,N,C-pincer

Repercussion of a 1,3-Hydrogen Shift in a Hydride-Osmium-Allenylidene Complex

“…The influence of polydentate ligands bite angles on the coordination polyhedron of the complexes, and therefore on the stability of the different oxidation states of the central ion, is well demonstrated. 33 The 1 H and 31 P{ 1 H} NMR spectra, in dichloromethane- d 2 , at room temperature are consistent with the structure shown in Figure 6 . Thus, the 1 H contains at a doublet of doublets ( 2 J H–P = 36.1 and 32.4 Hz) at −12.57 ppm, due to the hydride ligand, whereas the 31 P{ 1 H} NMR displays two doublets ( 2 J P–P = 11.9 Hz) at −12.6 and −23.0 ppm, corresponding to the inequivalent P i Pr 2 groups.…”

Deacylative Alkylation vs. Photoredox Catalysis in the Synthesis of 3,3'‐Bioxindoles

“…In this context, it should be mentioned that in addition to the lower energy of its emission, complex 23 shows two noticeable differences with regard to 21 and 22 . It displays longer lifetime in 2‐methyltetrahydrofuran, at room temperature (2.4 and 1.6 µs, respectively, vs. 7.7 µs) and the presence of an oxygen atom between the pyridyl ring and one of the phenyl group of the C,N,C‐pincer ligand, which stabilizes the octahedral geometry of the catalyst, due to the increase of a N‐Ir‐C bite angle . The 24 W blue LED source afforded better conversions than the visible light, 20 W fluorescent bulb, or 20 W white bulb (not included in Table ).…”

Recent Advances in Synthesis of Molecular Heteroleptic Osmium and Iridium Phosphorescent Emitters

“…In contrast to that observed for the osmium compounds 21 and 22, the incorporation of the oxygen atom causes a significant blue shift of the emission and a notable increase in the efficiency of the emitter. Therefore, complex 54 displays green-blue emission (538-473 nm), with quantum yields of 0.87 in PMMA film at 5 wt % and 0.96 in 2-MeTHF at 298 K. [34] The comparison of the iridium(III) derivatives 52 and 54 with the osmium(IV) counterparts 21 and 22 emphasizes the crucial role of the bite angle of pincer ligands on the photophysical features of these emitters. The emissions are attributed to T 1 excited states.…”

“…The presence of an oxygen atom between the pyridine ring and one of its phenyl substituents, which opens the CÀ OsÀ C' angle, disfavors the 4 + oxidation state of the osmium and ruins the efficiency of the emitter. [34] Hexahydride 1 is perhaps the transition metal-polyhydride with the maximum ability to activate σ-bonds. Accordingly, it has been used to carry out infrequent bond ruptures, including the B-type fragmentation of the four-membered ring of βlactams.…”