A Pyridyl-1,2-azaborine Ligand for Phosphorescent Neutral Iridium(III) Complexes

Abstract

A novel 1,2-azaborine (i.e., 4-methyl-2-(pyridin-2-yl)-2,1-borazaronaphthalene, 1a) has been synthesized and used for the first time as a B–N alternative to common cyclometalating ligands to obtain neutral phosphorescent iridium(III) complexes (i.e., 2a, 3, and 4) of general formula [Ir(C∧N)2(N∧NB)], where C∧N indicates three different cyclometalating ligands (Hppy = 2-phenylpyridine; Hdfppy = 2-(2,4-difluoro-phenyl)pyridine; Hpqu = 2-methyl-3-phenylquinoxaline). Moreover, the azaborine-based complex 2a was compared to the isoelectronic CC iridium(III) complex 2b, obtained using the corresponding 2-(naphthalen-2-yl)pyridine ligand 1b. Due to the dual cyclometalation mode of such CC ligand, the isomeric complex 2c was also obtained. All new compounds have been fully characterized by NMR spectroscopy and high-resolution mass spectrometry (MS), and the X-ray structure of 2a was determined. The electronic properties of both ligands and complexes were investigated by electrochemical, density functional theory (DFT), and photophysical methods showing that, compared to the naphthalene analogues, the azaborine ligand induces a larger band gap in the corresponding complexes, resulting in increased redox gap (basically because of the highest occupied molecular orbital (HOMO) stabilization) and blue-shifted emission bands (e.g., λmax = 523 vs 577 nm for 2a vs 2b, in acetonitrile solution at 298 K). On the other hand, the 3LC nature of the emitting state is the same in all complexes and remains centered on the pyridyl-borazaronaphthalene or its CC pyridyl-naphthalene analogue. As a consequence, the quantum yields of such azaborine-based complexes are comparable to those of the more classical CC counterparts (e.g., photoluminescence quantum yield (PLQY) = 16 vs 22% for 2a vs 2b, in acetonitrile solution at 298 K) but with enhanced excited-state energy. This proves that such type of azaborine ligands can be effectively used for the development of novel classes of photoactive transition-metal complexes for light-emitting devices or photocatalytic applications.

A dysprosium(iii)-based triple helical-like complex as a turn-on/off fluorescence sensor for Al(iii) and 4,5-dimethyl-2-nitroaniline

“…This reduces the vibration of the molecule, causing it to become the lowest excited singlet (π*, π) type and presenting bright ligand-characteristic luminescence. [34][35][36] In addition, the emission intensity of 1 was significantly enhanced compared to the ligand. This is probably because the ligand binding to the metal centre facilitates the reduction of non-radiative relaxation, resulting in the strong emission.…”

Synthesis and crystal structure of bis(9-mesityl-9,10-dihydro-10-aza-9-borabenzo[h]quinolinato-κ2 N 1,N 10)zinc(II)

“…Moreover, one study describes several complexes of a bidentate ligand with aluminum (Appiarius et al, 2023). However, there are only three reports of 1,2-azaborinines with N-transition-metal bonds, including zirconium (refcode JIZQEP; Pan et al, 2008), ruthenium (refcode DOXBEY; Pan et al, 2008) and iridium (refcode NEZXAV; Baschieri et al, 2023). Also, the structure of a 6-pyridyl-1,2azaborinine has been reported, which is structurally similar to HL and was used for the preparation of a dimesitylboron complex (refcode WUGMIW; Baggett et al, 2015).…”

Complexation of Boron and Aluminum with a Bidentate Hydroxy-BN-naphthalene Ligand

“…It has been demonstrated that these anionic monocycles may serve as η 1 -donating ligands for the complexation of electrophiles or metals (Be, B, Al, Ga, Si, Sn, Zr), establishing stable covalent bonds via the nitrogen atoms [14,15]. More recently, deprotonated pyridyl-substituted BN aromatics have been used for the chelation of iridium(III) ions [16] or a dimesitylboron unit [17] (Figure 1, bottom left).…”

“…Inorganics 2023, 11, x FOR PEER REVIEW 2 of 21 covalent bonds via the nitrogen atoms [14,15]. More recently, deprotonated pyridyl-substituted BN aromatics have been used for the chelation of iridium(III) ions [16] or a dimesitylboron unit [17] (Figure 1, bottom left). was expedient.…”