Ion-pairing assemblies of anion-responsive helical Pt^II complexes

Abstract

Naphthylisoquinoline-appended dipyrrolyldiketone Pt^II complexes as helical π-electronic systems were synthesized. The Pt^II complexes, showing chiroptical properties in solution, exhibited anion-binding behaviour, resulting in the formation of anion complexes as pseudo π-electronic anions. In combination with the π-electronic cation, the receptor–anion complexes formed charge-by-charge ion-pairing assemblies, with the narcissistic self-sorting columnar structures comprising either of the enantiomers, in the crystal state.

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Introduction

Assemblies of π-electronic systems show fascinating electronic and electrooptical properties depending on the arrangements of constituting species. In particular, modulating the assembly modes of π-electronic systems with photoluminescence properties affords various electrooptical materials, such as light-emitting diodes, lasers and sensors.¹ Among various luminescent π-electronic molecules reported thus far, organoplatinum(II) complexes with square-planar geometries exhibit photoluminescence properties,^2,3 such as triplet energy transfer and phosphorescence anisotropy amplification.^3h,j Unique assembling behaviours, including stacking structures and Pt^II⋯Pt^II interactions, result in characteristic electronic and photoluminescence properties.

The electronic and photoluminescence properties of Pt^II complexes in solution and assembly states can be controlled by introduced π-electronic ligand molecules.³ As phosphorescent π-electronic Pt^II complexes, dipyrrolyldiketone Pt^II complexes (e.g., 1, Fig. 1a)⁴ showing anion-responsive abilities form anion complexes as pseudo π-electronic anions by binding guest anions and resulting ion pairs in combination with countercations. The π-electronic anions form ion-pairing assemblies through ⁱπ–ⁱπ interactions with π-electronic cations to form charge-by-charge assemblies.^5,6 The phosphorescence properties of the assemblies depend on countercations that modulate the arrangement of components. For example, the enhancement of the solid-state phosphorescence was observed by the formation of charge-by-charge assemblies with bulky countercations.^4b Thus, an appropriate combination with countercations and modification of π-electronic ligands would result in modulated electronic properties.

Fig. 1 (a) Anion complexation of dipyrrolyldiketone Pt^II complex 1 (X⁻: guest anion) and (b) the dipyrrolyldiketone Pt^II complex with a helical π-electronic unit (left) and the conceptual diagram of charge-by-charge ion-pairing assemblies (right).

Modification of the π-electronic ligands affects the chiroptical properties of Pt^II complexes. For example, chiral pendants attached to Pt^II complexes induce chiroptical properties.⁷ Furthermore, Pt^II complexes with helical π-electronic ligands, as seen in those with helicene-like structures, exhibit chiroptical properties.⁸ Such chiral Pt^II complexes with phosphorescence properties have been reported thus far for circularly polarized luminescence (CPL) properties. The chiroptical properties of Pt^II complexes can be modulated by binding guest species. In particular, ion-pairing assemblies of chiral Pt^II complexes would provide phosphorescent CPL materials. However, there are no reports on helical Pt^II complexes with anion-responsive properties (Fig. 1b). In light of this, it would be important to synthesize anion-responsive chiral Pt^II complexes that form ion-pairing assemblies in order to control the chiroptical characteristics of phosphorescence. This report describes the synthesis and ion-pairing assembly of helical Pt^II complexes to provide insights into future chiroptical ion-pairing materials.

Results and discussion

Synthesis and characterization of Pt^II complexes

As a preliminary examination for the reactivity in the Pt^II complexation of dipyrrolyldiketones⁹ and naphthylisoquinoline ligands,¹⁰ Pt^II complex 2a was synthesized in 32% yield by treating naphthylisoquinoline^10a with [(PtMe₂)₂(SMe₂)₂]¹¹ at r.t. in the presence of trifluoromethanesulfonic acid (TfOH) (1 equiv.), followed by the addition of dipyrrolyldiketone^10a and K₂CO₃ (1.5 equiv.) (Fig. 2). As a helical π-electronic ligand for Pt^II complexation, 8-phenyl-substituted naphthylisoquinoline^10b was selected for inducing chiral states in anion-complexing and ion-pairing forms. Under the same reaction conditions, Pt^II complexes 3a,b were prepared in 35% and 19% yields, respectively, as yellow solids from the corresponding dipyrrolyldiketones. The obtained Pt^II complexes were characterized by ¹H and ¹³C NMR and ESI-TOF-MS. Theoretical investigations for the most stable conformations of 2a and 3a,b at the B3LYP/6-31G(d,p) level with LanL2DZ for Pt suggested that doubly pyrrole-inverted geometries suitable for anion binding were less stable by 6.35, 6.60 and 6.60 kcal mol⁻¹, respectively (Fig. S19–21†),¹² as observed in the previously reported dipyrrolyldiketone Pt^II complexes including 1.^4a The naphthylisoquinoline unit in the optimized structures of 3a,b exhibited helical structures with a ligand-phenyl unit partially overlapped with the isoquinoline plane. The helical pitch of 3a (3.14 Å) was longer by 0.15 Å than that of 2a (2.99 Å), suggesting that the sterically hindered ligand-phenyl unit affords helical conformations without helicity inversion.

Fig. 2 Dipyrrolyldiketone Pt^II complexes with a naphthylisoquinoline ligand 2a and 3a,b.

As observed in the optimized structures of 3a,b, the X-ray analysis of 3a for the single crystal obtained from CH₂Cl₂/n-hexane revealed a helical geometry and packing structure (Fig. 3 and Fig. S12†). The diketone-naphthylisoquinoline-coordinated Pt^II unit showed angles of 80.4°–94.7° around Pt^II with a τ₄ value¹³ of 0.09, indicating an almost square-planar geometry. The ligand-phenyl unit was partially overlapped with the isoquinoline unit to induce the helical geometry with a helical pitch of 3.12 Å, being shorter by 0.02 Å than that of the optimized structure presumably due to the stacking effect. In the crystal structure, P- and M-helical enantiomers formed a stacked dimer with a stacking distance of 3.29 Å based on the mean plane of O, O, N, C and Pt and a Pt⋯Pt distance of 4.16 Å.

Fig. 3 Single-crystal X-ray structure of 3a: (a) packing structure and (b) top and side views as the P-helix. The colours brown, pink, blue, red and grey in this figure and Fig. 6 represent carbon, hydrogen, nitrogen, oxygen and platinum, respectively.

Solution-state properties and anion-binding behaviours

Pt^II complexes 2a and 3a exhibited UV/vis absorption spectra with λ_max at 380 and 386 nm and shoulders at 359/407 and 352/408 nm, respectively, in CH₂Cl₂, whereas phenyl-substituted 3b showed red-shifted absorption with λ_max at 452 nm and shoulders at 382 and 430 nm (Fig. S4†). The main absorption band of 3a at 386 nm was assigned to the HOMO−1-to-LUMO+1 transition, whereas that of 3b at 452 nm was attributed to the HOMO-to-LUMO+1 transition (Fig. S23, 24, 26 and 27†). Interestingly, in contrast to the phosphorescence emissive properties of dipyrrolyldiketone Pt^II complexes, such as 1, possessing an arylpyridine ligand,⁴2a and 3a,b showed luminescence with low quantum efficiencies at 509/676, 571/711 and 582/713 nm when excited at 380, 387 and 452 nm, respectively, in deoxygenated CH₂Cl₂ (Fig. S5†). The luminescence intensities at longer wavelengths for the Pt^II complexes decreased under atmospheric conditions, suggesting that the luminescence bands at the shorter and longer wavelengths were derived from fluorescence and phosphorescence, respectively.^8g Theoretical calculations of 2a and 3a at CAM-B3LYP/6-31+G(d,p) with LanL2DZ for Pt revealed that the electron density differences of T₁–S₀ were mainly observed at the naphthylisoquinoline unit, whereas those at the dipyrrolyldiketone unit were seen for 1. The S₀–T₁ excitation energies of 2a and 3a are 1.64 and 1.54 eV, respectively, which are lower than that of 1 (2.41 eV). The low phosphorescence intensities of the Pt^II complexes with a naphthylisoquinoline unit resulted from lower T₁ excited-state energy levels, inducing T₁–S₀ nonradiative transitions.

Pt^II complexes 3a,b, as racemic mixtures, were resolved via chiral HPLC separation to obtain the enantiopure P and M enantiomers (Fig. S6†). Circular dichroism (CD) spectra of the first fraction of 3a in CH₂Cl₂ (3 × 10⁻⁵ M) showed negative-to-positive signs in the 250–300 nm region (Fig. 4), whereas 3b exhibited the opposite CD signs for the first fraction (Fig. S7†). These CD spectra can be assigned as characteristic binaphthyl-type patterns.¹⁴ Theoretically estimated CD spectra of 3a,b at the B3LYP/6-31+G(d,p) level for the optimized structures suggest that the first fractions included the P- and M-helical enantiomers, respectively (Fig. S29†).

Fig. 4 CD spectra of 3a in CH₂Cl₂ (3 × 10⁻⁵ M) (inset: optimized structures of 3a as P- and M-helices at B3LYP/6-31G(d,p)). The solid and dotted lines correspond to the spectra of the first and second fractions, respectively, in chiral HPLC separation.

Dipyrrolyldiketone Pt^II complexes exhibit anion-binding behaviour through multiple hydrogen bonding⁴ as seen in boron complexes. The solution-state anion-binding behaviour was evaluated using ¹H NMR and UV/vis absorption spectral changes. Upon the addition of Cl⁻ as a tetrabutylammonium (TBA⁺) salt to 3a in CD₂Cl₂ (1.0 × 10⁻³ M) at −50 °C, the signals at 9.51 and 6.51 ppm ascribable to those of pyrrole NH and bridging CH, respectively, were shifted downfield to 12.56/12.51 and 7.98 ppm, respectively, suggesting the formation of a [1 + 1]-type Cl⁻ complex through hydrogen bonding (Fig. 5). Similarly, the [1 + 1]-type anion complexation of 2a and 3b was confirmed with ¹H NMR spectral changes upon Cl⁻ addition (Fig. S38 and 40†). In particular, the o-CH of the phenyl rings attached to the pyrrole units in 3b also supported weak hydrogen bonding for anion binding. The [1 + 1]-type Cl⁻ binding constants (K_a) for 2a and 3a were estimated to be 1 × 10³ and 2 × 10³ M⁻¹, respectively, based on the ¹H NMR measurements in CD₂Cl₂ at −50 °C (Fig. S38 and 39†). Conversely, the K_a values of 3b were evaluated by UV/vis absorption spectral changes upon the addition of anions as TBA salts. The K_a values of 3b at r.t. were estimated to be 2400, 300 and 5600 M⁻¹ for Cl⁻, Br⁻ and CH₃CO₂⁻, respectively, which are similar to those of Pt^II complexes with arylpyridines.^4a In contrast to 3b, α-unsubstituted 2a and 3a exhibited no drastic changes in the UV/vis absorption spectra upon the addition of anions (Fig. S34 and 35†). The conformation changes in the anion-free and anion-binding forms resulted in very small changes in the direction of the transition dipole moments. In addition, the chiral π-electronic ligand on Pt^II did not significantly affect the pyrrole-inverted structure and the resulting anion-binding properties. Furthermore, slightly decreased CD intensities at 300–400 nm were observed for 3a,b upon the addition of Cl⁻ (Fig. S37†), suggesting that changes in the conformations and electronic states due to anion binding only marginally affected the chirality of the naphthylisoquinoline ligand.

Fig. 5 (a) ¹H NMR of 3a (1 mM) upon the addition of Cl⁻ as a TBA salt in CD₂Cl₂ at −50 °C and (b) Cl⁻-binding behaviour of 3a shown by optimized structures at B3LYP/6-31G(d,p).

Ion-pairing assemblies of chiral Pt^II complexes

The exact structures of the anion complexes and ion-pairing assemblies were revealed using single-crystal X-ray analysis. The single crystal of 2a·Cl⁻-TATA⁺ (TATA⁺ = 4,8,12-tripropyl-4,8,12-triazatriangulenium cation)¹⁵ suitable for X-ray analysis was prepared by vapour diffusion of n-hexane into a CH₂Cl₂ solution of a 1 : 1 mixture of 2a and TATACl. In the solid state, 2a·Cl⁻ exhibited a [1 + 1]-type binding mode with the pyrrole N(–H)⋯Cl⁻ and bridging C(–H)⋯Cl⁻ distances of 3.19/3.22 and 3.58 Å, respectively (Fig. 6a(i) and Fig. S13†). 2a·Cl⁻-TATA⁺ formed a charge-by-charge assembly with alternately stacked 2a·Cl⁻ and TATA⁺ units with stacking distances of 3.42 and 3.48 Å (Fig. 6a(ii)). Single crystals of 3a·Cl⁻-TATA⁺ and 3b·Cl⁻-TATA⁺ were also obtained from vapour diffusion of n-hexane into a CH₂Cl₂ solution of a 1 : 1 mixture of 3a,b and TATACl (Fig. 6b,c(i) and Fig. S14, 15†). Similarly, 3a·Cl⁻-TATA⁺ and 3b·Cl⁻-TATA⁺ formed charge-by-charge assemblies of the Cl⁻ complexes and TATA⁺ with stacking distances of 3.39 and 3.45/3.47 Å, respectively (Fig. 6b,c(ii)). TATA⁺ was mainly stacked with the plane of the anion complexes to form columnar structures aligned along the a-, b- and c-axis directions for 2a·Cl⁻-TATA⁺, 3a·Cl⁻-TATA⁺ and 3b·Cl⁻-TATA⁺, respectively. In all the ion pairs based on the Pt^II complexes including 2a, the charge-by-charge columns comprise either of the enantiomers and the corresponding enantiomers, exhibiting narcissistic self-sorting of the enantiomers via ion pairing with the π-electronic cation. This observation contrasts with that of the anion-free 3a, which shows social self-sorting stacking columns in the crystal structure. Notably, the arrangement of helical structures can be modulated via anion binding and ion pairing. The charge-by-charge columns of the same enantiomer were aligned along the b-, a- and b-axis directions for 2a·Cl⁻-TATA⁺, 3a·Cl⁻-TATA⁺ and 3b·Cl⁻-TATA⁺, respectively, to form layer structures, which were alternately arranged with the layers of opposite chirality along the c-, c- and a-axis directions (Fig. 6a–c(iii)).

Fig. 6 Single-crystal X-ray structures of (a) 2a·Cl⁻-TATA⁺, (b) 3a·Cl⁻-TATA⁺ and (c) 3b·Cl⁻-TATA⁺ as (i) representative stacked ion pairs and (ii) side and (iii) top views of packing structures. Green spheres in (i) and (iii) represent chlorine. The colours magenta and cyan in (ii) represent anions and cations, respectively. The colours orange and green in (iii) represent P- and M-helical enantiomers, respectively.

Hirshfeld surface analysis¹⁶ for the charge-by-charge structures clearly showed red and blue triangles arranged in bow-tie shapes on the shape-index surface and a flat region on the curvedness, suggesting characteristic mapping patterns for π–π stacking structures of the planar charged species (Fig. S17 and 18†). The interaction energies for the stacking of TATA⁺ and the anion complexes were estimated by energy decomposition analysis (EDA)¹⁷ in the framework of the fragment molecular orbital (FMO) method at FMO2-MP2 using mixed basis sets including NOSeC-V-DZP with TZP for Pt with MCP (Tables S1–3 and Fig. S31, 32†).^18–20 The EDA calculations determine electrostatic, dispersion, charge transfer and exchange-repulsion interaction energies along with the total interaction energy. In all the ion-pairing assemblies, the main interaction energies were derived from electrostatic and dispersion forces. These energies and total energies for the representative pairing Cl⁻ complexes and TATA⁺ in 2a·Cl⁻-TATA⁺, 3a·Cl⁻-TATA⁺ and 3b·Cl⁻-TATA⁺ were calculated to be −89.1/–134.7/–202.4, −77.1/–143.8/–197.6 and −82.2/–155.7/–214.0 kcal mol⁻¹, respectively, suggesting ⁱπ–ⁱπ interactions.⁶ Narcissistic self-sorting charge-by-charge columns were formed by stacking of the chiral Pt^II complexes with the achiral π-electronic cation through ⁱπ–ⁱπ interactions.

Conclusions

Dipyrrolyldiketone Pt^II complexes with a helical π-electronic ligand were prepared as anion-responsive molecules. The synthesized chiral Pt^II complexes, showing weak phosphorescence emissions, exhibited anion-binding properties, and their electronic properties and assembly behaviours were modulated by anion binding. The crystal-state ion-pairing assemblies based on the anion complexes of the chiral Pt^II complexes include narcissistic self-sorting charge-by-charge columns comprising either of the enantiomers and those of the corresponding enantiomers. The chiral charge-by-charge columns are aligned in one direction to form layers that are alternately arranged with layers of opposite chirality. Further modifications of introduced π-electronic ligands would control the electronic states, enhancing the phosphorescence quantum efficiency. Charge-by-charge ion-pairing assemblies comprising single enantiomers of chiral Pt^II complexes are fascinating chiroptical materials.

Experimental

General information

Starting materials were purchased from FUJIFILM Wako Pure Chemical Corp., Nacalai Tesque Inc., Tokyo Chemical Industry Co., Ltd and Sigma-Aldrich Co. and were used without further purification unless otherwise stated. NMR spectra used in the characterization of products were recorded using a JEOL ECA-600 600 MHz spectrometer. All NMR spectra were referenced to the solvent. UV-visible absorption spectra were recorded using a Hitachi U-4100 spectrometer. Electrospray ionization mass spectrometry (ESI-MS) was performed with a BRUKER microTOF using the ESI-TOF method. TLC analyses were carried out on aluminum sheets coated with silica gel 60 (Merck 5554). Column chromatography was performed using Wakogel C-300.

(1,3-Dipyrrol-2-yl-1,3-propanedionato-κO¹,κO³)-[1-(naphthalen-1-yl-κC)isoquinoline-κN]platinum, 2a

According to the literature procedure,^4a in a dried Schlenk flask, [(PtMe₂)₂(SMe₂)₂]¹¹ (31.4 mg, 0.054 mmol) was dissolved in THF (1.0 mL) under a N₂ atmosphere. To the solution was added 1-(naphthalen-1-yl)isoquinoline^10a (25.7 mg, 0.100 mmol). The resulting mixture was stirred for 1 h at r.t., and TfOH (8.8 μL, 0.100 mmol) was added dropwise. The reaction mixture was stirred for 30 min, and then a solution of 1,3-dipyrrol-2-yl-1,3-propanedione^9a (21.2 mg, 0.104 mmol) and K₂CO₃ (19.8 mg, 0.143 mmol) in MeOH/THF (1 : 1, 2.0 mL) was added. The mixture was stirred for 4 h. After the removal of the solvent under vacuum, the residue was purified by column chromatography over silica gel (Wakogel C-300; eluent: CH₂Cl₂) to give 2a (11.3 mg, 0.017 mmol, 32%) as a yellow solid. R_f = 0.79 (CH₂Cl₂). ¹H NMR (600 MHz, DMSO-d₆, 20 °C): δ (ppm) 11.76 (br, 1H, NH), 11.55 (br, 1H, NH), 9.30 (d, J = 6.0 Hz, 1H, Ar–H), 8.18 (d, J = 7.8 Hz, 1H, Ar–H), 8.12 (m, 2H, Ar–H), 7.94 (d, J = 7.8 Hz, 1H, Ar–H), 7.89 (m, 2H, Ar–H), 7.85 (d, J = 7.8 Hz, 1H, Ar–H), 7.74 (d, J = 8.4 Hz, 1H, Ar–H), 7.60 (t, J = 7.2 Hz, 1H, Ar–H), 7.41 (t, J = 6.6 Hz, 1H, Ar–H), 7.34 (t, J = 8.4 Hz, 1H, Ar–H), 7.25 (m, 2H, pyrrole-H), 7.15 (m, 2H, pyrrole-H), 6.65 (s, 1H, CH), 6.29 (m, 2H, pyrrole-H). ¹³C NMR (151 MHz, DMSO-d₆, 20 °C): δ (ppm) 170.25, 169.43, 168.79, 145.60, 140.01, 138.95, 137.38, 131.19, 131.81, 131.44, 130.82, 130.46, 129.69, 128.90, 128.85, 128.43, 127.11, 126.79, 125.26, 124.73, 124.16, 123.77, 123.38, 123.34, 119.77, 113.09, 112.62, 110.24, 110.10, 93.01. UV/vis (CH₂Cl₂, λ_max[nm] (ε, 10⁴ M⁻¹ cm⁻¹)): 380 (3.3). ESI-TOF-MS (HR): 649.1210. Calcd for C₃₀H₂₀N₃O₂Pt ([M − H]⁻): 649.1209. This compound was further characterized as the ion-pairing form by single-crystal X-ray analysis.

(1,3-Dipyrrol-2-yl-1,3-propanedionato-κO¹,κO³)-[1-(8-phenylnaphthalen-1-yl-κC)isoquinoline-κN]platinum, 3a

According to the literature procedure,^4a in a dried Schlenk flask, [(PtMe₂)₂(SMe₂)₂]¹¹ (29.5 mg, 0.051 mmol) was dissolved in THF (1.0 mL) under a N₂ atmosphere. To the solution was added 1-(8-phenylnaphthalen-1-yl)isoquinoline^10b (33.4 mg, 0.101 mmol). The resulting mixture was stirred for 1 h at r.t., and TfOH (8.8 μL, 0.100 mmol) was added dropwise. The reaction mixture was stirred for 30 min, and then a solution of 1,3-dipyrrol-2-yl-1,3-propanedione^9a (22.1 mg, 0.109 mmol) and K₂CO₃ (28.6 mg, 0.207 mmol) in MeOH/THF (1 : 1, 2.0 mL) was added. The mixture was stirred for 3 h. After the removal of the solvent under vacuum, the residue was purified by column chromatography over silica gel (Wakogel C-300; eluent: CH₂Cl₂) to give 3a (12.8 mg, 0.018 mmol, 35%) as a yellow solid. R_f = 0.79 (CH₂Cl₂). ¹H NMR (600 MHz, DMSO-d₆, 20 °C): δ (ppm) 11.74 (br, 1H, NH), 11.53 (br, 1H, NH), 8.98 (d, J = 6.0 Hz, 1H, Ar–H), 8.08 (d, J = 7.8 Hz, 1H, Ar–H), 7.96 (d, J = 8.4 Hz, 1H, Ar–H), 7.89 (d, J = 8.4 Hz, 1H, Ar–H), 7.69 (d, J = 8.4 Hz, 1H, Ar–H), 7.60 (d, J = 8.4 Hz, 1H, Ar–H), 7.54 (m, 2H, Ar–H), 7.45 (d, J = 6.6 Hz, 1H, Ar–H), 7.34 (d, J = 7.2 Hz, 1H, Ar–H), 7.28 (t, J = 7.2 Hz, 1H, Ar–H), 7.24 (br, 2H, pyrrole-H), 7.18 (s, 1H, pyrrole-H), 7.15 (s, 1H, pyrrole-H), 6.69 (m, 5H, Ar–H), 6.65 (s, 1H, CH), 6.29 (m, 2H, pyrrole-H). ¹³C NMR (151 MHz, DMSO-d₆, 20 °C): δ (ppm) 170.61, 170.06, 168.75, 144.61, 142.43, 139.08, 138.91, 136.46, 135.73, 132.57, 131.43, 130.80, 130.47, 129.14, 128.98, 128.93, 128.36, 127.86, 126.78, 126.52, 126.31, 125.61, 125.08, 123.71, 123.66, 123.31, 118.51, 112.98, 112.55, 110.19, 110.06, 92.94 (four signals were overlapped). UV/vis (CH₂Cl₂, λ_max[nm] (ε, 10⁴ M⁻¹ cm⁻¹)): 387 (4.1). ESI-TOF-MS (HR): 725.1525. Calcd for C₃₆H₂₄N₃O₂Pt ([M − H]⁻): 725.1525. This compound was further characterized by single-crystal X-ray analysis.

(1,3-Di(5-phenylpyrrol-2-yl)-1,3-propanedionato-κO¹,κO³)-[1-(8-phenylnaphthalen-1-yl-κC)isoquinoline-κN]platinum, 3b

According to the literature procedure,^4a in a dried Schlenk flask, [(PtMe₂)₂(SMe₂)₂]¹¹ (30.5 mg, 0.053 mmol) was dissolved in THF (1.0 mL) under a N₂ atmosphere. To the solution was added 1-(8-phenylnaphthalen-1-yl)isoquinoline^10b (35.2 mg, 0.113 mmol). The resulting mixture was stirred for 1 h at r.t., and TfOH (8.8 μL, 0.100 mmol) was added dropwise. The reaction mixture was stirred for 30 min, and then a solution of 1,3-di(5-phenylpyrrol-2-yl)-1,3-propanedione^9b (38.9 mg, 0.110 mmol) and K₂CO₃ (20.7 mg, 0.150 mmol) in MeOH/THF (1 : 1, 2.0 mL) was added. The mixture was stirred for 3 h. After the removal of the solvent under vacuum, the residue was purified with column chromatography over silica gel (Wakogel C-300; eluent: CH₂Cl₂) to give 3b (9.11 mg, 0.010 mmol, 19%) as a yellow solid. R_f = 0.72 (CH₂Cl₂/n-hexane = 2/1 (v/v)). ¹H NMR (600 MHz, DMSO-d₆, 20 °C): δ (ppm) 11.80 (br, 1H, NH), 11.65 (br, 1H, NH), 9.09 (d, J = 6.6 Hz, 1H, Ar–H), 8.08 (d, J = 8.4 Hz, 1H, Ar–H), 7.96 (d, J = 8.4 Hz, 1H, Ar–H), 7.93 (m, 3H, Ar–H), 7.90 (d, J = 8.4 Hz, 2H, Ar–H), 7.68 (d, J = 8.4 Hz, 1H, Ar–H), 7.63 (d, J = 8.4 Hz, 1H, Ar–H), 7.55 (m, 3H, Ar–H), 7.48 (m, 4H, Ar–H), 7.41 (m, 1H, pyrrole-H), 7.35 (m, 3H, Ar–H), 7.29 (t, J = 8.4 Hz, 1H, Ar–H), 7.25 (m, 1H, pyrrole-H), 6.88 (s, 1H, CH), 6.80 (m, 2H, pyrrole-H), 6.70 (m, 5H, Ar–H). ¹³C NMR (151 MHz, DMSO-d₆, 20 °C): δ (ppm) 170.57, 170.00, 168.05, 144.64, 142.47, 139.25, 139.20, 136.95, 136.52, 136.25, 135.80, 133.30, 132.61, 132.37, 131.89, 131.82, 130.84, 129.17, 129.06, 129.01, 128.85, 128.81, 128.17, 127.94, 127.91, 127.26, 127.24, 127.21, 127.11, 126.81, 126.56, 126.33, 125.69, 125.66, 125.54, 125.35, 125.21, 125.11, 123.74, 118.71, 115.33, 114.75, 108.95, 108.80, 94.15 (three signals were overlapped). UV/vis (CH₂Cl₂, λ_max[nm] (ε, 10⁴ M⁻¹ cm⁻¹)): 452 (6.5). ESI-TOF-MS (HR): 877.2148. Calcd for C₄₈H₃₂N₃O₂Pt ([M − H]⁻): 877.2148. This compound was further characterized as the ion-pairing form by single-crystal X-ray analysis.

Method for single-crystal X-ray analysis

Crystallographic data are summarized in Table 1. A single crystal of 3a was obtained by vapour diffusion of n-hexane into a CH₂Cl₂ solution. The data crystal was an orange block of approximate dimensions 0.05 mm × 0.05 mm × 0.03 mm. A single crystal of 2a·Cl⁻-TATA⁺ was obtained by vapour diffusion of n-hexane into a CH₂Cl₂ solution of a mixture of 2a and TATA⁺ as a Cl⁻ salt (TATACl)^15c in a 1 : 1 ratio. The data crystal was a red prism of approximate dimensions 0.10 mm × 0.03 mm × 0.02 mm. A single crystal of 3a·Cl⁻-TATA⁺ was obtained by vapor diffusion of n-hexane into a CH₂Cl₂ solution of a mixture of 3a and TATACl in a 1 : 1 ratio. The data crystal was an orange prism of approximate dimensions 0.04 mm × 0.02 mm × 0.01 mm. A single crystal of 3b·Cl⁻-TATA⁺ was obtained by vapour diffusion of n-hexane into a CH₂Cl₂ solution of a mixture of 3b and TATACl in a 1 : 1 ratio. The data crystal was an orange prism of approximate dimensions 0.100 mm × 0.050 mm × 0.020 mm. The data of 3b·Cl⁻-TATA⁺ were collected at 100 K using a DECTRIS PILATUS3 CdTe 1M diffractometer with Si (311) monochromated synchrotron radiation (λ = 0.4136 Å) at BL02B1 (SPring-8),²¹ whereas those of 3a and 2a·Cl⁻-TATA⁺ were collected at 90 K on a DECTRIS EIGER X 1 M diffractometer with Si (111) monochromated synchrotron radiation (λ = 0.802 and 0.8105 Å, respectively) at BL40XU (SPring-8).²² The data of 3a·Cl⁻-TATA⁺ were collected at 90 K using a Bruker D8 Venture diffractometer with MoKα radiation (λ = 0.71073 Å) focused with a multilayer confocal mirror. All the structures were solved by the dual-space method. The structures were refined by a full-matrix least-squares method using SHELXL 2014²³ (Yadokari-XG).²⁴ In each structure, the non-hydrogen atoms were refined anisotropically. For 2a·Cl⁻-TATA⁺, the disordered solvents, presumably n-hexane, were removed using the SQUEEZE protocol included in PLATON.²⁵

Table 1 Crystallographic details

	3a	2a·Cl^−-TATA^+	3a·Cl^−-TATA^+	3b·Cl^−-TATA^+
a Synchrotron radiation. b MoKα radiation.
Formula	C36H25N3O2Pt·CH2Cl2	C30H21N3O2PtCl·C28H30N3·C6H14	C36H25N3O2PtCl·C28H30N3·2.772CH2Cl2	C48H33N3O2PtCl·C28H30N3·2CH2Cl2
Fw	811.60	1180.76	1406.13	1492.71
Crystal size, mm	0.05 × 0.05 × 0.03	0.10 × 0.03 × 0.02	0.04 × 0.02 × 0.01	0.100 × 0.050 × 0.020
Crystal system	Triclinic	Monoclinic	Monoclinic	Orthorhombic
Space group	P1̄ (no. 2)	P21/n (no. 14)	P21/c (no. 14)	Pna21 (no. 33)
a, Å	9.721(7)	7.925(8)	13.9514(15)	36.864(7)
b, Å	12.828(7)	19.815(13)	14.7192(14)	21.004(4)
c, Å	12.984(7)	32.173(18)	30.005(3)	8.479(17)
α, °	71.794(12)	90	90	90
β, °	87.54(2)	95.72(7)	102.175(4)	90
γ, °	81.166(11)	90	90	90
V, Å^3	1519.8(16)	5027(7)	6023.0(10)	6565(2)
ρ calcd, g cm^−3	1.774	1.560	1.551	1.510
Z	2	4	4	4
T, K	90(2)	90(2)	90(2)	100(2)
μ, mm^−1	6.539^a	4.050^a	2.671^b	0.592^a
No. of reflns	18 108	52 646	95 598	188 969
No. of unique reflns	6690	8882	10 676	14 998
Variables	406	616	742	832
λ, Å	0.802^a	0.8105^a	0.71073^b	0.4136^a
R 1 (I > 2σ(I))	0.0556	0.0342	0.0610	0.0255
wR2 (I > 2σ(I))	0.1379	0.0870	0.1450	0.0651
GOF	1.060	1.042	1.251	1.021

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Calculations for the geometrical optimizations and energy decomposition analysis

DFT calculations for the theoretical optimizations, electrostatic potentials (ESPs), molecular orbitals and UV/vis spectra were carried out using the Gaussian 16 program.¹² Energy decomposition analysis (EDA) calculations for the packing structures were performed using the GAMESS program.¹⁸

Data availability

The data that support the findings of this study are available in the ESI† of this article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Numbers JP22H02067 and JP23K23335 for Scientific Research (B), JP22K05253 and JP24K08389 for Scientific Research (C), JP23K17951 for Challenging Research (Exploratory) and JP20H05863 for Transformative Research Areas (A) “Condensed Conjugation” and Ritsumeikan Global Innovation Research Organization (R-GIRO) project (2022–27). Theoretical calculations were partially performed at the Research Center for Computational Science, Okazaki, Japan (22-IMS-C077, 23-IMS-C069 and 24-IMS-C067) and Information Initiative Center, Hokkaido University. We thank Dr Yuiga Nakamura, Dr Kouhei Ichiyanagi, Dr Toshiyuki Sasaki and Dr Nobuhiro Yasuda, JASRI/SPring-8 (2023A1240 and 2023B1755) for synchrotron radiation single-crystal X-ray analysis, Prof. Osamu Tsutsumi, Ritsumeikan University, for single-crystal X-ray analysis and Prof. Hitoshi Tamiaki, Ritsumeikan University, for various measurements including single-crystal X-ray analysis.

References

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, analytical data, computational details and cif files for the single-crystal X-ray analysis. CCDC 2381822–2381825. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qo01697a

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