Bimetallic Iridium(III) Complexes Consisting of Ir(ppy)₂ Units (ppy = 2-Phenylpyridine) and Two Laterally Connected N^∧N Chelates as Bridge:  Synthesis, Separation, and Photophysical Properties

Abstract

Reaction of the precursor complex Ir2(ppy)4Cl2 (ppy = 2-phenylpyridine) with the bridging ligand 3,8-dipyridyl-4,7-phenanthroline (L) affords, in 94% yield, the cyclometalated iridium dinuclear complex [(ppy)2Ir(mu-L)Ir(ppy)2]2+ (12+) as a mixture of three stereoisomers. This mixture consists of a meso form Delta,Lambda and a racemic form (enantiomeric pair Delta,Delta and Lambda,Lambda) in the ratio 1:1.5. Single-crystal X-ray characterization of the perchlorate salt of the meso form reveals (i) the distortion of the bridging ligand from the planarity and (ii) the location of the two iridium subunits above and below the medium plane of the bridging ligand. Ion-pair chromatography with Delta-TRISPHAT anion (TRISPHAT = tris(tetrachlorobenzenediolato)phosphate(V)) as resolving anion permits the separation of the three stereoisomers. The 1H NMR spectroscopic analysis of each fraction indicates high diastereomeric purity. Electronic circular dichroism properties and comparison with literature data establish their absolute configuration. The absorption and emission properties of the three stereoisomers show only very small variations. The anisotropic properties can be interpreted as distinct interactions of the isomers with the chiral resolving Delta-TRISPHAT anion.

Atropisomerism in a thermally switchable, cyclometallated iridium complex

“…18 Hence a milder acid treatment was used; ∆S-2a was reacted with TFA in a biphasic mixture of DCM:H 2 O (1:1) at room temperature for 48 hrs. Monitoring the reaction by 1 H NMR spectroscopy showed a spectrum very similar to that of 70 [Ir(ppz) 2 Cl] 2 . The FAB mass spectrum showed an ion at m/z 1071 corresponding to [Ir 2 (ppz) 4 (CF 3 CO 2 )] + hence the product is proposed to be ΛΛ-[Ir(ppz) 2 (CF 3 CO 2 )] 2 3a.…”

“…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.…”

“…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.…”

“…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 the chirality imposed by the metal has a bigger effect on the specific rotation than the chirality at the carbon of the ligand. 10 A similar reaction of (S)-Na(L1) with [Ir(ppy) 2 Cl] 2 gave 1b as a 1:1 mixture of diastereomers. In this case both diastereomers crystallised from the same solvent mixture but the crystals could be separated by hand picking due to a significant variation in colour and shape.…”