Easy Separation of Δ and Λ Isomers of Highly Luminescent [Ir<sup>III</sup>]‐Cyclometalated Complexes Based on Chiral Phenol‐Oxazoline Ancillary Ligands

Marchi, Enrico; Sinisi, Riccardo; Bergamini, Giacomo; Tragni, Michele; Monari, Magda; Bandini, Marco; Ceroni, Paola

doi:10.1002/chem.201200709

Chemistry A European J

2012

DOI: 10.1002/chem.201200709

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Easy Separation of Δ and Λ Isomers of Highly Luminescent [Ir^III]‐Cyclometalated Complexes Based on Chiral Phenol‐Oxazoline Ancillary Ligands

Enrico Marchi

Riccardo Sinisi

Giacomo Bergamini

et al.

Abstract: A new class of neutral cyclometalated iridium(III) complexes with enantiomerically pure C(1)-symmetric phenol-oxazolines (3a,b) have been synthetized in high yields and fully characterized. Resolution of the corresponding Δ(R) and Λ(R) or Δ(S) and Λ(S) isomers was easily achieved by conventional flash chromatography. The corresponding Δ and Λ helicities have been confirmed by CD spectroscopy and X-ray crystallography. Regarding the absorption and luminescence properties with unpolarized light, no significant d… Show more

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Cited by 64 publications

(62 citation statements)

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“…The lack of diastereoselectivity in the syntheses and the fact that solutions of single diastereomers show no epimerisation (by 1 H NMR spectroscopy) over several days suggests that chirality at the metal is stable at room temperature. To investigate this further a sample of DS-2a was heated in DMSO-d 6 and no epimerisation took place up to 120 1C. Similarly a mixture of DS-2b : LS-2b (1 : 1.4) showed no change in ratio by 1 H NMR spectroscopy when heated to 120 1C in DMSO-d 6 .…”

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Preparation of single enantiomers of chiral at metal bis-cyclometallated iridium complexes

Davies

Singh

et al. 2013

Chem. Commun.

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) with enantiopure X^Y ligands. However, these complexes have chirality at the ligand as well as the metal; hence, the aim of this work was to prepare, on a synthetically useful scale, homochiral cyclometallated Ir(III) complexes which are only chiral at the metal.Our strategy was to prepare diastereomeric complexes [Ir(C^N) 2 (X^Y*)], separate the diastereomers, then remove the chiral auxiliary by protonation and replace it with another bidentate ligand. A similar strategy has recently been applied by Meggers for the synthesis of homochiral trisbidentate ruthenium complexes, 10 and in 2012 was applied to iridium complexes for the first time. 5The dimer [Ir(ppz) 2 Cl] 2 (a, Hppz = phenylpyrazole) was reacted with 2.2 equiv. of (S)-Na(L1) 11,12 in a mixture of DCMmethanol (2 : 1) at room temperature for 2-4 hours, to give 1a as a (1 : 1) mixture of diastereomers, DS and LS, in good combined yields (>75%) (Scheme 1). The reaction was repeated with only 0.8 equiv. of (S)-Na(L1) per dimer and the 1 H NMR spectrum of the product showed a 1 : 1 ratio of the two diastereomers along with the unreacted excess dimer. This suggests that there is no diastereoselectivity in the synthesis and there is an equal probability for the formation of the two diastereomers. The diastereomers of 1a could be separated by crystallisation from different solvents and they do not interconvert in solution, suggesting the chirality at the metal is stable at room temperature. The absolute configuration of both diastereomers was determined by X-ray crystallography 13 and the structures of LS-and DS-1a are shown in Fig. 1. 14 The structures show that both isomers have cis carbon atoms and trans nitrogen atoms for the C^N ligands and S configuration at the chiral carbon atom of the oxazoline ligand; one isomer has a L configuration at the Ir centre (Fig. 1, left), whereas the other isomer shows a D configuration (Fig. 1, right). The change in chirality at the metal leads to different orientations of L1 with respect to the

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Preparation of single enantiomers of chiral at metal bis-cyclometallated iridium complexes

Davies

Singh

et al. 2013

Chem. Commun.

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“…However, unlike [Ru(bipy) 3 ] 2+ the resolution of Ir(III) complexes is still in its infancy and very few reports have been published so far. [2][3][4][5][6] Separation by HPLC of the enantiomers of [Ir(ppy) 3 ] (Hppy = phenylpyridine) was reported in 2007, 3 and of [Ir(ppy-R) 2 (acac)] (R = F, OMe, Ph) in 2008. 4 Single 20 diastereomers have been isolated e.g.…”

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confidence: 99%

“…4 Single 20 diastereomers have been isolated e.g. [Ir(C^N*)] 3 and [Ir(C^N*) 2 (acac)] with enantiopure cyclometallating ligands, 7 or [Ir(C^N) 2 (X^Y*)] (X^Y* = amino acidate 5,8,9 , phenol oxazoline 6 ) with enantiopure X^Y ligands. However, these complexes have chirality at the ligand as well as the metal; hence, 25 the aim of this work was to prepare, on a synthetically useful scale, homochiral cyclometallated Ir(III) complexes which are only chiral at the metal.…”

mentioning

confidence: 99%

“…The lack of diastereoselectivity in the syntheses and the fact that 45 solutions of single diastereomers show no epimerisation (by 1 H NMR spectroscopy) over several days suggests that chirality at the metal is stable at room temperature. To investigate this further a sample of ∆S2a was heated in DMSO-d 6 and no epimerisation took place up to 120 C. Similarly a mixture of ∆S-2b:ΛS-2b (1:1.4) showed no change in ratio by 1 H NMR spectroscopy when heated to 120 C in DMSO-d 6 .…”

mentioning

confidence: 99%

“…To investigate this further a sample of ∆S2a was heated in DMSO-d 6 and no epimerisation took place up to 120 C. Similarly a mixture of ∆S-2b:ΛS-2b (1:1.4) showed no change in ratio by 1 H NMR spectroscopy when heated to 120 C in DMSO-d 6 . Therefore we conclude that the chirality is stable up to 120 C.…”

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confidence: 99%

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Atropisomerism in a thermally switchable, cyclometallated iridium complex

et al. 2012

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5,8,9 , phenol oxazoline 6 ) with enantiopure X^Y ligands. However, these complexes have chirality at the ligand as well as the metal; hence, 25 the aim of this work was to prepare, on a synthetically useful scale, homochiral cyclometallated Ir(III) complexes which are only chiral at the metal.Our strategy was to prepare diastereomeric complexes [Ir(C^N) 2 (X^Y*)], separate the diastereomers, then remove the 30 chiral auxiliary by protonation and replace it with another bidentate ligand. A similar strategy has recently been applied by Meggers for the synthesis of homochiral trisbidentate ruthenium complexes, 10 and in 2012 was applied to iridium complexes for the first time. 5 35 The dimer [Ir(ppz) 2 Cl] 2 (a, Hppz = phenylpyrazole) was reacted with 2.2 equiv of (S)-Na(L1) 11,12 in a mixture of DCM/ methanol (2:1) at room temperature for 2-4 hrs, to give 1a as a (1:1) mixture of diastereomers, ∆S and ΛS, in good combined 45 yields (>75%) (Scheme 1). The reaction was repeated with only 0.8 equiv of (S)-Na(L1) per dimer and the 1 H NMR spectrum of the product showed a 1:1 ratio of the two diastereomers along with the unreacted excess dimer. This suggests that there is no diastereoselectivity in the synthesis and there is an equal 50 probability for the formation of the two diastereomers. The diastereomers of 1a could be separated by crystallisation from different solvents and they do not interconvert in solution, suggesting the chirality at the metal is stable at room temperature. The absolute configuration of both diastereomers was determined 55 by X-ray crystallography 13 and the structures of S-and S-1a are shown in Fig. 2. 14 The structures show that both isomers have cis carbon atoms and trans nitrogen atoms for the C^N ligands and S configuration at the chiral carbon atom of the oxazoline ligand; one isomer has a Λ configuration at the Ir centre (Fig. 2 60 left), whereas the other isomer shows a ∆ configuration (Fig. 2 right). The change in chirality at the metal leads to different orientations of L1 with respect to the [Ir(C^N) 2 ] fragment. In the ΛS isomer the pyrazole [N(4)-C(12)] pointing towards the oxazoline nitrogen is on the unsubstituted side of the oxazoline 65 and there is no steric conflict. In contrast, in the ∆S isomer, the pyrazole [N(2)-C(3)] pointing towards the oxazoline nitrogen is on the same side as the isopropyl which leads to steric congestion which is clearly evident in the planarity of L1 (Fig. 2 right). Thus, in the ΛS isomer the angle between the planes of the 70 phenol and the oxazoline is less than 3 whilst in the ∆S isomer the corresponding angle is > 20.The structures are retained in solution as determined by key NOEs between the X^Y ligand and the [Ir(C^N) 2 ] fragment (see ESI Fig S1). In ΛS-1a the isopropyl group lies above a ppz ligand

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Bimetallic Cyclometalated Iridium(III) Diastereomers with Non‐Innocent Bridging Ligands for High‐Efficiency Phosphorescent OLEDs

et al. 2014

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Easy Separation of Δ and Λ Isomers of Highly Luminescent [Ir^III]‐Cyclometalated Complexes Based on Chiral Phenol‐Oxazoline Ancillary Ligands

Cited by 64 publications

References 54 publications

Preparation of single enantiomers of chiral at metal bis-cyclometallated iridium complexes

Preparation of single enantiomers of chiral at metal bis-cyclometallated iridium complexes

Atropisomerism in a thermally switchable, cyclometallated iridium complex

Bimetallic Cyclometalated Iridium(III) Diastereomers with Non‐Innocent Bridging Ligands for High‐Efficiency Phosphorescent OLEDs

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Easy Separation of Δ and Λ Isomers of Highly Luminescent [IrIII]‐Cyclometalated Complexes Based on Chiral Phenol‐Oxazoline Ancillary Ligands

Cited by 64 publications

References 54 publications

Preparation of single enantiomers of chiral at metal bis-cyclometallated iridium complexes

Preparation of single enantiomers of chiral at metal bis-cyclometallated iridium complexes

Atropisomerism in a thermally switchable, cyclometallated iridium complex

Bimetallic Cyclometalated Iridium(III) Diastereomers with Non‐Innocent Bridging Ligands for High‐Efficiency Phosphorescent OLEDs

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Easy Separation of Δ and Λ Isomers of Highly Luminescent [Ir^III]‐Cyclometalated Complexes Based on Chiral Phenol‐Oxazoline Ancillary Ligands