Insulated π-conjugated 2,2'-bipyridine transition-metal complexes: enhanced photoproperties in luminescence and catalysis

Abstract

2,2'-Bipyridine has been identified as a privileged ligand scaffold for photofunctional transition metal complexes. We herein report on the synthesis and photoproperties of an insulated π-conjugated 2,2'-bipyridine with a linked rotaxane structure consisting of permethylated α-cyclodextrin (PM α-CD) and oligo(p-phenylene ethynylene). The insulated π-conjugated 2,2'-bipyridine exhibited enhanced ligand performance in the solid-state emitting biscyclometalated Ir complexes and visible-light-driven Ni catalysts owing to π-extension and remote steric effects based on the linked rotaxane structure.

Fig. 1. (a) Conceptual diagrams illustrating issues encountered in π-extended 2,2′-bipyridine ligands. Red dotted lines show π–π and metal–metal interactions, and additional ligation to the metal center. (b) Structure of insulated π-conjugated 2,2′-bipyridine L1 with the linked rotaxane structure end-capped by pivaloyl groups. (c) Optimized molecular structure of L1 calculated using the B3LYP/6-31G(d,p) level.

Scheme 1. Synthesis and structures of L1–L3.

Fig. 2. (a) Molecular structure of [Ir(ppy)₂(L1)]PF₆. (b) Absorption spectra of [Ir(ppy)₂(L1)]PF₆, [Ir(ppy)₂(L3)]PF₆, and [Ir(ppy)₂(bpy)]PF₆ (in CH₂Cl₂, 1 × 10⁻⁵ M). (c) Emission spectra of [Ir(ppy)₂(L1)]PF₆, [Ir(ppy)₂(L3)]PF₆, and [Ir(ppy)₂(bpy)]PF₆ (in CH₂Cl₂, 1 × 10⁻⁶ M, under a N₂ atmosphere, excited at 405, 405, 365 nm, respectively). The asterisk indicates the second order of excitation wavelength. (d) Solution (1 × 10⁻⁶ M in CH₂Cl₂) and solid-state luminescence quantum yields (QYs) of [Ir(ppy)₂(L1)]PF₆ and [Ir(ppy)₂(L3)]PF₆ (excited at 405 nm under a N₂ atmosphere). (e) Schematic of the aggregation suppression of [Ir(ppy)₂(L1)]PF₆. (f) Natural transition orbitals (NTOs) of [Ir(ppy)₂(L3)]⁺ calculated using the CAM-B3LYP/6-31+G(d,p) (for C, H, N, O) and LANL2DZ (for Ir) levels with the PCM (CH₂Cl₂) solvation model.

Fig. 3. (a) Experimental (red solid line) and calculated (black dashed line) UV/Visible spectra of NiCl₂·DME/L1 (1 : 1, in DMF) and NiCl₂(L3), respectively. A vertical line is assigned to an intraligand charge transfer (ILCT) transition. (b) NTOs of NiCl₂(L3) calculated using the CAM-UB3LYP/6-31+G(d,p) (for C, H, N, O, Cl) and LANL2DZ (for Ni) levels with the PCM (DMF) solvation model.

Scheme 2. Substrate scope in the visible-light-driven Ni-catalyzed C–O coupling with L1. Conditions: aryl bromide (6, 0.1 mmol), HOR′ (4 mmol), NiCl₂·DME (1 mol%), L1 (1 mol%), iPr₂EtN (0.2 mmol), DMF (0.5 mL), 1,3,5-trimethoxybenzene as an internal standard (0.1 mmol), 427 nm LEDs, 20 h with cooling fans. Determined by ¹H NMR analysis using an internal standard. ^a Isolated yields. ^b Hydration product 7f was also obtained in 28% NMR yield. ^c Some unreacted 4-iodobenzonitrile remained in the crude product (83% conversion). Dehaloprotonation product, benzonitrile, was also obtained in 20% NMR yield. ^d6a (10 mmol), H₂O (40 mmol), NiCl₂·DME (0.02 mol%), L1 (0.02 mol%), iPr₂EtN (20 mmol), DMF (7.5 mL), 427 nm LEDs, 17 h with cooling fans. Complete conversion of 6a was observed. The dehaloprotonation product, benzonitrile, was also formed. ^e The homocoupling product, [1,1′-biphenyl]-2,2′-dicarbonitrile, was also obtained in 24% NMR yield.

Scheme 3. Visible-light-driven Ni-catalyzed C–N coupling. Conditions: 4-bromobenzonitrile (6a, 0.1 mmol), H₂N-nC₈H₁₇ (0.5 mmol), NiCl₂·DME (1 mol%), L1 (1 mol%), DABCO (0.2 mmol), DMA (0.5 mL), 1,3,5-trimethoxybenzene as an internal standard (0.1 mmol), 427 nm LEDs, 48 h with cooling fans. Determined by ¹H NMR analysis using an internal standard. Isolated yields are shown in parentheses. ^a Hydration product 7f was also obtained in 13% NMR yield.