English

• 0.   Introduction

  Cyclometalated cationic Pt(Ⅱ)-isocyanide complexes have received great interest due to their planar coordination geometry, which can self-organize into various nanostructures and chromonic mesophases through intermolecular Pt…Pt and/or ligand-ligand interactions, demonstrating versatile potential in waveguiding, semiconducting, electroluminescent devices, and DNA-binding agents^[1-10]. Due to the large ionic polarizability of chloride and sulfate anions in the Hofmeister anion series, most of the cationic Pt(Ⅱ)-isocyanide complexes with chloride and sulfate anions could dissolve well in water up to 20%^[11]. Therefore, an aqueous/organic phase-transfer reaction was employed to prepare the cationic Pt(Ⅱ)-isocyanide complexes, and the targeted cationic products were enriched in the aqueous phase with yields exceeding 80%^[11-12]. In addition, other anions, such as perchlorate and hexafluorophosphate, were used to replace chloride, and pure products as precipitates in methanol could be obtained^[13-14].

  Previously, Che et al. attempted to prepare κ³-[(N^C^N)Pt(CNXyl)]Cl complexes by a simple ligand-substitution reaction in homogeneous solution (N^C^N=1,3-bis(2′-pyridyl)benzene, CNXyl=2,6-dimethylphenyl isocyanide)^[15]. However, they had not obtained the targeted cationic product. And they figured out that the reactant [(N^C^N)PtCl] and the desired product κ³-[(N^C^N)Pt(CNXyl)]Cl were coexisting due to reversible ligand exchange reactions. Similarly, we found that pure cyclometalated κ³-[(N^C^N′)Pt(CNXyl)]Cl complexes with the functionalized-pinene ligand were also difficult to separate, and the reaction showed a fairly low yield^[16]. Pinenes are accessible natural products with rigid lipophilic groups, and can be easily modified on pyridine planes through Kröhnke-type reaction^[17-18]. In addition, the introduction of aliphatic pinene groups would improve the solubility of square-planar Pt(Ⅱ) complexes in nonpolar solvents^{[12, 14, 16]}. Recently, we prepared two different coordination configurations of difluoro-substituted, pinene-appended Pt(Ⅱ)-isocyanide complexes, namely, the κ³-N^C^N′-coordinated [1A]OTf ([1A]OTf is referred to as [1-isomer A]OTf in the reference^[19].) and the κ²-N^C^N′-coordinated 1B (1B is referred to as 1-isomer B in the reference^[19].) by controlling the chloride source (Fig.1). Accordingly, our tentative hypothesis is whether these two coordination configurations are concurrent in the direct reaction of the [Pt(N^C^N′)Cl] precursor with CNXyl.

  Figure 1

  

  Figure 1.  Chemical structures of reported difluoro-substituted N^C^N′-coordinated Pt(Ⅱ)-isocyanide complexes^[19]

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  1.   Materials and methods

  1.1   General methods

  All reagents were purchased from commercial suppliers and used as received. High-resolution ESI (HR-ESI) mass spectrometry spectra were acquired on a Thermo Fisher Scientific Q Exactive Focus Mass Spectrometer. The ¹H, ¹³C, and ¹⁹⁵Pt NMR spectra were obtained on a Bruker DRX-400 spectrometer. Photoluminescence (PL) spectra were measured by a Hitachi F-4600 PL spectrophotometer (λ_ex=420 nm).

  1.2   Synthesis

  The non‑substituted/substituted unsymmetric pinene ligands (N^C^N′) and [Pt(N^C^N′)Cl] precursors with different substituents were prepared according to the reported literature^{[14, 19]}. The mixture of [Pt(N^C^N′)Cl] precursor and CNXyl was stirred in CH₂Cl₂ for 30 min at RT. Then the solvent was evaporated in a vacuum, and the obtained solids were used in NMR and photoluminescence tests.

  [1A]OTf. ¹H NMR (400 MHz, CD₂Cl₂, RT): δ 8.66 (dd, J₁=5.2 Hz, J₂=1.2 Hz, 1H), 8.15 (s, 1H), 7.88 (d, J₁=9.6 Hz, J₂=1.2 Hz, 1H), 7.64 (dd, J₁=8.8 Hz, J₂=3.6 Hz, 1H), 7.61 (s, 1H), 7.58-7.50 (m, 1H), 7.46 (t, J=7.6 Hz, 1H), 7.32 (d, J=7.6 Hz, 2H), 7.02 (dd, J₁=12.4 Hz, J₂=8.4 Hz, 1H), 3.22 (s, 2H), 2.81 (s, 1H), 2.79 (s, 1H), 2.60 (s, 6H), 2.43-2.37 (m, 1H), 1.43 (s, 3H), 1.27 (s, 1H), 0.72 (s, 3H). ¹³C NMR (100 MHz, CD₂Cl₂, RT): δ 169.8, 166.1, 165.3, 158.4 (d, J₁=268 Hz, C—F), 156.5 (d, J₁=250 Hz, C—F), 155.1, 152.8, 152.3, 149.5, 145.6, 140.6, 140.5, 136.4, 131.5, 130.3 (d, J₂=22 Hz, C—C—F), 129.1, 128.1, 128.0, 121.1, 119.8, 115.8 (d, J₂=27 Hz, C—C—F), 45.0, 39.9, 39.6, 33.9, 31.7, 25.7, 21.6, 19.4 (Fig.S1-S7, Supporting information).

  1B. ¹H NMR (400 MHz, CD₂Cl₂, RT): δ 9.08 (s, 1H), 8.26-8.29 (m, 1H), 7.70 (dd, J₁=8.4 Hz, J₂ =6.0 Hz, 1H), 7.58 (s, 1H), 7.20 (t, J=7.6 Hz, 1H), 7.09-7.14 (m, 1H), 7.07 (d, J=8.0 Hz, 2H), 7.00 (t, J=8.8 Hz, 1H), 6.63-6.68 (m, 1H), 3.14 (s, 2H), 2.96 (t, J=5.2 Hz, 1H), 2.72-2.78 (m, 1H), 2.37-2.31 (m, 1H), 2.27 (s, 6H), 1.43 (s, 3H), 1.25 (dd, J₁=9.6 Hz, J₂ =3.2 Hz, 1H), 0.69 (s, 3H). ¹³C NMR (100 MHz, CD₂Cl₂, RT): δ 164.0, 161.3 (d, J₁=251 Hz, C—F), 158.2 (d, J₁=254 Hz, C—F), 151.09, 151.05, 148.0, 146.1 (d, J₃=10 Hz, C—C—C—F), 145.4 (d, J₄=4 Hz, C—C—C—C—F), 144.2, 142.7, 142.6, 142.4, 134.8, 129.5, 128.2, 127.9, 126.0 (d, J₃=10 Hz, C—C—C—F), 124.7, 122.9 (d, J₂=20 Hz, C—C—F), 118.8, 111.8 (d, J₂=24 Hz, C—C—F), 45.0, 40.2, 39.6, 33.9, 31.9, 25.9, 21.6, 18.7 (Fig.S8-S14).

  CN-substituted N^C^N′ ligand (substituted at 3-position of the lateral pyridine ring). ¹H NMR (400 MHz, CDCl₃): δ 8.91 (dd, J₁=5.2 Hz, J₂=1.6 Hz, 1H), 8.50 (t, J=1.6 Hz, 1H), 8.28 (s, 1H), 8.22 (d, J=8.0 Hz, 1H), 8.11 (dd, J₁=8.0 Hz, J₂=1.6 Hz, 1H), 7.99 (d, J=7.6 Hz, 1H), 7.66 (t, J=7.6 Hz, 2H), 7.41 (dd, J₁=7.6 Hz, J₂=4.8 Hz, 1H), 3.08 (d, J=2.4 Hz, 2H), 2.90 (t, J=5.6 Hz, 1H), 2.71-2.77 (m, 1H), 2.35 (m, 1H), 1.44 (s, 3H), 1.26 (d, J=9.6 Hz, 1H), 0.68 (s, 3H) (Fig.S15). HR ESI‑MS (m/z): [M+H]⁺ Calcd. for C₂₄H₂₂N₃⁺, 352.180 82; Found, 352.361 77.

  CH₃O-substituted N^C^N′ ligand (substituted at 3-position of the lateral pyridine ring). ¹H NMR (400 MHz, CDCl₃): δ 8.45 (t, J=1.6 Hz, 1H), 8.33 (dd, J₁=4.8 Hz, J₂=1.6 Hz, 1H), 8.22 (s, 1H), 8.04 (dt, J₁=8.0 Hz, J₂=1.6 Hz, 1H), 7.93 (dt, J₁=8.0 Hz, J₂=1.6 Hz, 1H), 7.60 (s, 1H), 7.53 (t, J=7.6 Hz, 1H), 7.30 (dd, J₁=8.4 Hz, J₂=1.2 Hz, 1H), 7.22-7.25 (m, 1H), 3.86 (s, 1H), 3.02 (d, J=2.8 Hz, 2H), 2.85 (t, J=5.6 Hz, 1H), 2.68-2.73 (m, 1H), 2.31 (m, 1H), 1.42 (s, 3H), 1.25 (d, J=9.6 Hz, 1H), 0.66 (s, 3H) (Fig.S16). HR ESI-MS (m/z): [M+H]⁺ Calcd. for C₂₄H₂₅N₂O⁺, 357.196 14; Found, 357.302 05.

  CF₃-substituted N^C^N′ ligand (substituted at 3-position of the lateral pyridine ring). ¹H NMR (400 MHz, CDCl₃): δ 8.86 (d, J=4.0 Hz, 1H), 8.22 (s, 1H), 8.08-8.12 (m, 3H), 7.53-7.57 (m, 3H), 7.44 (dd, J₁=8.0 Hz, J₂=4.8 Hz, 1H), 3.01 (d, J=2.4 Hz, 2H), 2.85 (t, J=5.6 Hz, 1H), 2.68-2.73 (m, 1H), 2.31 (m, 1H), 1.42 (s, 3H), 1.24 (d, J=9.6 Hz, 1H), 0.66 (s, 3H) (Fig.S17). HR ESI-MS (m/z): [M+H]⁺ Calcd. for C₂₄H₂₂F₃N₂⁺, 395.172 96; Found, 395.286 75.

  Cl-substituted N^C^N′ ligand (substituted at 3-position of the lateral pyridine ring). ¹H NMR (400 MHz, CDCl₃): δ 8.61 (dd, J₁=4.8 Hz, J₂=1.6 Hz, 1H), 8.29 (t, J=1.6 Hz, 1H), 8.23 (s, 1H), 8.07 (d, J=8.0 Hz, 1H), 7.82 (dd, J₁=8.0 Hz, J₂=1.2 Hz, 1H), 7.75 (d, J=7.6 Hz, 1H), 7.58 (s, 1H), 7.55 (d, J=7.6 Hz, 1H), 7.24 (t, J=5.2 Hz, 1H), 3.02 (d, J=2.4 Hz, 2H), 2.85 (t, J=5.6 Hz, 1H), 2.68-2.73 (m, 1H), 2.32 (m, 1H), 1.42 (s, 3H), 1.25 (d, J=9.6 Hz, 1H), 0.66 (s, 3H) (Fig.S18). HR ESI-MS (m/z): [M+H]⁺ Calcd. for C₂₃H₂₂ClN₂⁺, 361.146 60; Found, 361.221 58.

  CH₃-substituted N^C^N′ ligand (substituted at 3-position of the lateral pyridine ring). ¹H NMR (400 MHz, CDCl₃): δ 8.45 (d, J=4.8 Hz, 1H), 8.21 (s, 1H), 8.17 (s, 1H), 8.09 (m, 1H), 7.99-8.02 (m, 1H), 7.60 (d, J=8.0 Hz, 1H), 7.49 (t, J=2.0 Hz, 1H), 7.35 (d, J=8.0 Hz, 1H), 7.20 (t, J=4.8 Hz, 1H), 2.99 (d, J=2.8 Hz, 2H), 2.84 (t, J=5.6 Hz, 1H), 2.67-2.72 (m, 1H), 2.38 (s, 3H), 2.30 (m, 1H), 1.41 (s, 3H), 1.24 (d, J=9.6 Hz, 1H), 0.65 (s, 3H) (Fig.S19). HR ESI-MS (m/z): [M+H]⁺ Calcd. for C₂₄H₂₅N₂⁺, 341.201 23; Found, 341.363 31.

  CN-substituted [Pt(N^C^N′)Cl] precursor (substituted at the 3-position of the lateral pyridine ring). ¹H NMR (400 MHz, CD₂Cl₂): δ 9.53 (dd, J₁=6.0 Hz, J₂=1.6 Hz, 1H), 8.62 (s, 1H), 8.11-8.15 (m, 2H), 7.46 (s, 1H), 7.42 (d, J=7.2 Hz, 1H), 7.29 (dd, J₁=7.6 Hz, J₂=6.0 Hz, 1H), 7.16 (t, J=7.6 Hz, 1H), 3.07 (d, J=2.4 Hz, 2H), 2.88 (t, J=5.6 Hz, 1H), 2.66-2.71 (m, 1H), 2.28 (m, 1H), 1.36 (s, 3H), 1.19 (d, J=10.0 Hz, 1H), 0.64 (s, 3H) (Fig.S20). HR ESI-MS (m/z): [M]⁺ Calcd. for C₂₄H₂₀ClN₃Pt⁺, 580.099 35; Found, 580.201 56.

  CH₃O-substituted [Pt(N^C^N′)Cl] precursor (substituted at 3-position of the lateral pyridine ring). ¹H NMR (400 MHz, CD₂Cl₂): δ 8.92 (dd, J₁=5.6 Hz, J₂=1.2 Hz, 1H), 8.70 (s, 1H), 7.93 (dd, J₁=7.6 Hz, J₂=0.8 Hz, 1H), 7.43-7.46 (m, 2H), 7.33 (dd, J₁=7.6 Hz, J₂=0.8 Hz, 1H), 7.18 (dd, J₁=8.8 Hz, J₂=5.6 Hz, 1H), 7.11 (t, J=7.6 Hz, 1H), 3.97 (s, 3H), 3.03 (d, J=2.4 Hz, 2H), 2.86 (t, J=5.6 Hz, 1H), 2.63-2.69 (m, 1H), 2.25 (m, 1H), 1.34 (s, 3H), 1.16 (d, J=9.6 Hz, 1H), 0.62 (s, 3H) (Fig.S21). HR ESI-MS (m/z): [M]⁺ Calcd. for C₂₄H₂₃ClN₂OPt⁺, 585.114 67; Found, 585.265 63.

  CF₃-substituted [Pt(N^C^N′)Cl] precursor (substituted at 3‑position of the lateral pyridine ring). ¹H NMR (400 MHz, CD₂Cl₂): δ 8.80 (dd, J₁=5.6 Hz, J₂=1.2 Hz, 1H), 8.79 (s, 1H), 8.31 (d, J=7.6 Hz, 1H), 7.88 (d, J=6.8 Hz, 1H), 7.56 (s, 1H), 7.52 (d, J=7.6 Hz, 1H), 7.42 (dd, J₁=8.0 Hz, J₂=5.6 Hz, 1H), 7.27 (t, J=7.6 Hz, 1H), 3.14 (d, J=2.4 Hz, 2H), 2.96 (t, J=5.6 Hz, 1H), 2.72-2.78 (m, 1H), 2.35 (m, 1H), 1.43 (s, 3H), 1.25 (d, J=10.0 Hz, 1H), 0.71 (s, 3H) (Fig.S22). HR ESI-MS (m/z): [M]⁺ Calcd. for C₂₄H₂₀ClF₃N₂Pt⁺, 623.091 48; Found, 623.233 71.

  Cl-substituted [Pt(N^C^N′)Cl] precursor (substituted at the 3-position of the lateral pyridine ring). ¹H NMR (400 MHz, CD₂Cl₂): δ 9.56 (dd, J₁=5.6 Hz, J₂=1.6 Hz, 1H), 8.85 (s, 1H), 7.30 (d, J=7.6 Hz, 1H), 7.95 (dd, J₁=8.0 Hz, J₂=1.6 Hz, 1H), 7.48 (s, 1H), 7.45 (d, J=7.2 Hz, 1H), 7.19-7.24 (m, 2H), 3.11 (d, J=2.0 Hz, 2H), 2.95 (t, J=5.6 Hz, 1H), 2.70-2.76 (m, 1H), 2.34 (m, 1H), 1.42 (s, 3H), 1.23 (d, J=10.0 Hz, 1H), 0.69 (s, 3H) (Fig.S23). HR ESI-MS (m/z): [M]⁺ Calcd. for C₂₃H₂₀Cl₂N₂Pt⁺, 589.065 13; Found, 589.232 87.

  CH₃-substituted [Pt(N^C^N′)Cl] precursor (substituted at 3-position of the lateral pyridine ring). ¹H NMR (400 MHz, CD₂Cl₂): δ 9.43 (dd, J₁=5.6 Hz, J₂=1.2 Hz, 1H), 8.89 (s, 1H), 7.71 (d, J=6.8 Hz, 1H), 7.67 (d, J=8.0 Hz, 1H), 7.47 (s, 1H), 7.40 (d, J=7.2 Hz, 1H), 7.21 (t, J=7.6 Hz, 1H), 7.16 (dd, J₁=7.6 Hz, J₂=5.6 Hz, 1H), 3.10 (d, J=2.4 Hz, 2H), 2.95 (t, J=5.6 Hz, 1H), 2.76 (s, 3H), 2.70-2.74 (m, 1H), 2.34 (m, 1H), 1.42 (s, 3H), 1.23 (d, J=10.0 Hz, 1H), 0.69 (s, 3H) (Fig.S24). HR ESI-MS (m/z): [M]⁺ Calcd. for C₂₄H₂₃ClN₂Pt⁺, 569.119 75; Found, 569.275 16.

  1.3   Crystal structure determination

  The crystals of 5B and 7B, suitable for X-ray diffraction, have been obtained in mixed CH₂Cl₂/methanol (1∶2, V/V) solutions. Single-crystal X-ray diffraction measurements were performed on a Xcalibur, Atlas, and Gemini Ultra. Intensities were collected with graphite monochromatized Mo Kα radiation (λ= 0.071 073 nm) operating at 50 kV and 30 mA, using ω/2θ scan mode. The data reduction was made with the Bruker SAINT package. Absorption corrections were performed using the SADABS program. The structures were solved by direct methods and refined on F² by full-matrix least-squares using SHELXL‑2018/3 with anisotropic displacement parameters for all non‑hydrogen atoms in the two structures. Hydrogen atoms bonded to carbon atoms were placed in calculated positions and refined as riding mode, with C—H length of 0.093 nm (methane) or 0.096 nm (methyl) and U_iso(H)=1.2U_eq(C_methane) or U_iso(H)=1.5U_eq(C_methyl). All computations were carried out using the SHELXL-2018/3 program package^[20].

  1.4   Calculation methods

  The calculations of structure optimization and transition state searching were carried out using the Gaussian program suite (Gaussian 16 Revision B.01). The PBE0 hybrid functional with the assistance of GD3BJ dispersion-correction scheme was introduced, and the def2-TZVP basis set and the corresponding ECP for Pt were used^[21-24]. The transition state structures were followed by the intrinsic reaction coordinate calculations^[25].

  2.   Results and discussion

  2.1   Analysis of reaction products

  Here, we continue to utilize the non-substituted/substituted unsymmetric pinene-derived [Pt(N^C^N′)Cl] precursors and CNXyl as starting materials (Scheme 1). For all [Pt(N^C^N′)Cl] precursors, after reacting with CNXyl in CH₂Cl₂ for 30 min at RT, the reactants [Pt(N^C^N′)Cl] reacted well and could not be detected through TLC analysis. Taking non-substituted unsymmetric pinene-derived [Pt(N^C^N′)Cl] as the reactant, we examined the ESI-MS spectrum of the reacted solution. As revealed in the ESI-MS spectrum (Fig.2), positive m/z peaks regarding [Pt(N^C^N′)(CNXyl)]⁺ and [Pt(N^C^N′)(CNXyl)Cl+H]⁺ were found at 651.207 86 and 687.184 13, respectively. Therefore, we assumed that the two isomers ([2A]Cl and 2B) should coexist in the reaction equilibrium (Scheme 1). Furthermore, concerning our previously obtained pure [1A]OTf and 1B^[19], the ¹⁹⁵Pt NMR spectrum of the above-reacted solution was also consistent with the presence of two isomers in solution (Fig.3). Two chemical shifts of ¹⁹⁵Pt were distinguished in the reaction product^[26-28]. The ¹⁹⁵Pt signal of positive [Pt(κ³-N^C^N′)(CNXyl)]⁺ ions ([2A]⁺) resided at δ=-3 636.70. In contrast, the ¹⁹⁵Pt signal of neutral [Pt(κ²-N^C-N′)(CNXyl)Cl] complex (2B) was located at δ=-3 789.04. Therefore, we are convinced by the presence of a coordination equilibrium between the κ³‑N^C^N′ coordinated [Pt(N^C^N′)(CNXyl)]⁺ and the κ²-N^C^N′ coordinated [Pt(N^C-N′)(CNXyl)Cl] in the reaction of [Pt(N^C^N′)Cl] and CNXyl.

  Scheme 1

  

  Scheme 1.  Reaction between the non-substituted/substituted unsymmetric pinene-derived [Pt(N^C^N′)Cl] precursor and CNXyl

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  Figure 2

  

  Figure 2.  ESI-MS spectrum of the reacted solution between the non-substituted unsymmetric pinene-derived [Pt(N^C^N′)Cl] and CNXyl

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  Figure 3

  

  Figure 3.  ¹⁹⁵Pt NMR spectra of the reacted solution between the non-substituted unsymmetric pinene-derived [Pt(N^C^N′)Cl] and CNXyl

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  In addition, we still used the previously obtained pure [1A]⁺ and 1B as references^[19], and their ¹H signals were assigned by measuring 2D NMR spectra (Fig.S1-S14). The complex [1A]⁺ in CD₂Cl₂ showed a characteristic doublet peak (δ=8.66) in the low field of ¹H NMR spectrum, which came from the H15 signal next to the N atom of the pyridine ring without pinene groups (Fig.4). In contrast, the ¹H NMR spectrum featured a singlet peak (δ=9.08) in the low field for 1B. The peak was caused by a signal from the H12 atom next to the N atom of the pyridine ring with pinene groups (Fig.4). Not controlling the chloride source, we just performed the direct reaction of difluoro-substituted [Pt(N^C^N′)Cl] and CNXyl in CH₂Cl₂ for 30 min at RT. It was found that the ¹H NMR spectrum of the reaction product showed only a singlet at δ 9.08 with no signals detected at 8.66. So, we believe that only 1B was obtained completely, and 1B surpassed [1A]⁺ overwhelmingly in the coordination equilibrium.

  Figure 4

  

  Figure 4.  Characteristic ¹H NMR signals of [1A]⁺ and 1B in the lowest field

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  2.2   Influencing factors of coordination equilibrium

  Whether isomer B always prevails over isomer [A]Cl in the coordination equilibrium, the reactions between CNXyl and [Pt(N^C^N′)Cl] with different substituents at the 3-position of the lateral pyridine ring have been inspected (Scheme 1). In principle, the molar ratio between the isomers would vary with the introduction of electron-withdrawing and electron-donating groups. However, isomer B dominated the product composition (Table 1), and only an insignificant amount of [A]⁺ was present, which could be neglected according to the NMR spectra (Fig.S25-S31). For all the substituents at the 3-position of the lateral pyridine ring, a prominent singlet at ca. 9.0 was observed in ¹H NMR spectra (Fig.S25-S31). Accordingly, the variance of substituents may have a small impact on the equilibrium between two isomers.

  Table 1

  

  Table 1.  Molar ratios of [A]⁺ and B with different substituents at the 3-position of the lateral pyridine ring

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  Substituents at the 3-position of the lateral pyridine ring	$ {n}_{{\left[{\rm{A}}\right]}^{+}} $∶nB
  —H	42∶58
  —CH3	8∶92
  —OCH3	9∶91
  —CF3	0∶100
  —F	0∶100
  —Cl	3∶97
  —CN	2∶98

  Apart from the influence of substituent types^[29], the repulsion between the substituents of the pyridine ring and the H atom of the benzene ring may be an essential factor in affecting coordination equilibrium and generating a specific product^[30-32]. As shown in Table 1, the isomer A/B molar ratios in solutions of different substituted precursors reacting with CNXyl have been obtained (Table 1). To eliminate repulsion as much as possible, we examined the reaction between the non-substituted unsymmetric pinene-derived [Pt(N^C^N′)Cl] precursor and CNXyl. Through monitoring ¹H signals in the low field (Fig.S25), it can be found that the A/B molar ratio is 42∶58. According to our previous studies, cationic [1A]⁺ exhibited stronger emission than neutral 1B in CH₂Cl₂ solution^[19]. As revealed in Fig.S32, the reacted solution between non‑substituted [Pt(N^C^N′)Cl] and CNXyl displayed a more intense emission than those containing substituted mixtures, further confirming a distinct component of [2A]⁺ in the reacted mixture between non-substituted [Pt(N^C^N′)Cl] and CNXyl. Therefore, the proportion of [A]⁺ increased significantly as any substituent was removed to reduce the repulsion. The repulsion may play a more substantial role in tuning the reaction equilibrium. Eliminating steric hindrance should favor the formation of tridentate N^C^N-coordinated [A]⁺. In addition, the variation of solvent and temperature would exert an influence on the product^[33], and increasing the polarity of the reaction solution could increase the molar ratio of [A]⁺. However, the change of solvent and temperature would induce the shift or broadening of ¹H NMR signals. Solvent and temperature effects on reaction equilibrium are complex and are being investigated.

  2.3   Structural analysis of isomers A and B

  The structures of CF₃- and Cl-substituted isomer B (5B and 7B) have been confirmed by single-crystal X-ray diffraction (Table 2 and Table S1-S2), revealing a slightly distorted square planar geometry with trans-N_N^C^N′, CNR configuration (Fig.5)^[34-37]. Inspecting the structural characteristics of [A]⁺ and B, we also found that B is superior to [A]⁺ in the coordination equilibrium, revealing the difference in structural stability. According to reported crystal structural data^{[19, 34-37]}, for cationic tridentate [A]⁺, the trans N—Pt—N angles (158°-163°) deviate severely from linearity, and this configuration suffers from a significantly chelating ring strain. In addition, strong Pt—C(aryl) and Pt—C(isocyanide) bonds occupy the trans-position, and in this case, the central Pt atom is pulled tightly by two strong Pt—C bonds, possibly resulting in the instability of tridentate N^C^N-coordinated [A]⁺. However, neutral bidentate isomer B keeps a trans-N_N^C^N, CNR configuration of Pt—N(pyridine) and Pt—C(isocyanide) (Fig.5). The chloride ion resides at the trans position relative to the C atom of the cyclometalated ligand. The trans C—Pt—Cl and C—Pt—N angles (174°-179°) are close to linearity. The central Pt atom is pulled by a strong Pt—C bond and a weak Pt—Cl(N) bond with one tension and one relaxation in this trans position. The coordination configuration of isomer B seems much stable than that of [A]Cl.

  Table 2

  

  Table 2.  Crystallographic data of 5B and 7B

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  Parameter	5B	7B
  Formula	C33H29ClF3N3Pt	C32H29Cl2N3Pt
  Formula weight	755.13	721.57
  Crystal system	Monoclinic	Orthorhombic
  Space group	P21	P212121
  a / nm	1.334 82(8)	1.315 94(8)
  b / nm	1.565 91(9)	1.444 34(8)
  c / nm	1.433 93(7)	1.510 39(11)
  β / (°)	90.544(5)
  V / nm3	2.997 1(3)	2.870 7(3)
  Z	4	4
  T / K	293(2)	293(2)
  Dc / (g·cm-3)	1.674	1.670
  μ / mm-1	4.817	5.100
  F(000)	1 480	1 416
  θ range / (°)	3.318-26.000	3.402-25.998
  Reflection measured	17 929	11 355
  Unique reflection	9 535	5 145
  Rint	0.075 8	0.046 1
  Reflection with I > 2σ(I)	6 308	3 658
  Number of parameters	747	335
  Goodness-of-fit on F2	1.024	1.076
  R1 [I > 2σ(I)]	0.069 1	0.059 6
  wR2 (all data)	0.185 2	0.180 3
  (Δρ)max, (Δρ)min / (e·nm-3)	2 842, -2 178	3 728, -1 539
  Flack parameter	-0.041(13)	-0.025(9)

  Figure 5

  

  Figure 5.  Crystal structures of CF₃- and Cl-substituted isomers B with ellipsoid probability levels of 30%

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  2.4   Theoretical calculation

  The structural superiority of isomer B to isomer [A]Cl in the reaction equilibrium has also been verified by the theoretical calculation^[38]. As shown in Fig.6 and 7, relative to the starting reactant, the F-substitution stabilizes the neutral product type but destabilizes the cationic type. Both for R=H and R=F, the reaction route for the cationic product has lower forward and reverse barriers than the reaction route for the neutral product, so the neutral products are thermodynamically more stable. Before the Cl^- ion is precipitated from the solution phase, the reverse reaction of route B is more difficult to occur for R=F, due to the much higher reverse barrier (126.8 kJ·mol^-1 for R=F vs 107.3 kJ·mol^-1 for R=H). And, when the Cl^- ion is to be precipitated, the reverse reaction of route A is more unlikely to occur for R=H, thanks to the slightly lower reverse barrier (53.8 kJ·mol^-1 for R=H vs 50.8 kJ·mol^-1 for R=F).

  Figure 6

  

  Figure 6.  Free energy profiles of two reaction routes, respectively for R=H and R=F, calculated using PBE0(D3BJ)/def2-TZVP with the solvation effect of CH₂Cl₂

  Routes A and B are for the reactions towards the ionic and neutral type of products, respectively.

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  Figure 7

  

  Figure 7.  Starting reactants, transition state structures, and product structures involved in routes A and B for R=H and R=F

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  3.   Conclusions

  In summary, the non‑substituted/substituted unsymmetric pinene-derived [Pt(N^C^N′)Cl] precursors reacted with CNXyl by simple substitution to give two isomeric products, cationic [Pt(κ³-N^C^N′)(CNXyl)]Cl complex ([A]Cl) and neutral [Pt(κ²-N^C-N′)(CNXyl)Cl] complex (B). The reactions produce predominantly neutral isomer B regardless of substituents on the lateral pyridine. A theoretical study indicates that neutral products are thermodynamically more stable. In addition, F-substitution could stabilize the neutral product type, but destabilize the cationic type. The study guides separating cyclometalated Pt(Ⅱ) complexes with specific configurations.

  
  Acknowledgments: This work is supported by the National Natural Science Foundation of China (Grant No.22461019) and the Natural Science Foundation of Hainan Province (Grant No.ZDYF2023SHFZ106). We gratefully acknowledge Prof. Yang Xiaoliang (Nanjing University) for ¹⁹⁵Pt NMR measurements. Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Figure 1  Chemical structures of reported difluoro-substituted N^C^N′-coordinated Pt(Ⅱ)-isocyanide complexes^[19]

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  Scheme 1  Reaction between the non-substituted/substituted unsymmetric pinene-derived [Pt(N^C^N′)Cl] precursor and CNXyl

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  Figure 2  ESI-MS spectrum of the reacted solution between the non-substituted unsymmetric pinene-derived [Pt(N^C^N′)Cl] and CNXyl

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  Figure 3  ¹⁹⁵Pt NMR spectra of the reacted solution between the non-substituted unsymmetric pinene-derived [Pt(N^C^N′)Cl] and CNXyl

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  Figure 4  Characteristic ¹H NMR signals of [1A]⁺ and 1B in the lowest field

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  Figure 5  Crystal structures of CF₃- and Cl-substituted isomers B with ellipsoid probability levels of 30%

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Free energy profiles of two reaction routes, respectively for R=H and R=F, calculated using PBE0(D3BJ)/def2-TZVP with the solvation effect of CH₂Cl₂

  Routes A and B are for the reactions towards the ionic and neutral type of products, respectively.

  下载: 全尺寸图片 幻灯片
  

  Figure 7  Starting reactants, transition state structures, and product structures involved in routes A and B for R=H and R=F

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  Table 1.  Molar ratios of [A]⁺ and B with different substituents at the 3-position of the lateral pyridine ring

  Substituents at the 3-position of the lateral pyridine ring	$ {n}_{{\left[{\rm{A}}\right]}^{+}} $∶nB
  —H	42∶58
  —CH3	8∶92
  —OCH3	9∶91
  —CF3	0∶100
  —F	0∶100
  —Cl	3∶97
  —CN	2∶98

  下载: 导出CSV

  Table 2.  Crystallographic data of 5B and 7B

  Parameter	5B	7B
  Formula	C33H29ClF3N3Pt	C32H29Cl2N3Pt
  Formula weight	755.13	721.57
  Crystal system	Monoclinic	Orthorhombic
  Space group	P21	P212121
  a / nm	1.334 82(8)	1.315 94(8)
  b / nm	1.565 91(9)	1.444 34(8)
  c / nm	1.433 93(7)	1.510 39(11)
  β / (°)	90.544(5)
  V / nm3	2.997 1(3)	2.870 7(3)
  Z	4	4
  T / K	293(2)	293(2)
  Dc / (g·cm-3)	1.674	1.670
  μ / mm-1	4.817	5.100
  F(000)	1 480	1 416
  θ range / (°)	3.318-26.000	3.402-25.998
  Reflection measured	17 929	11 355
  Unique reflection	9 535	5 145
  Rint	0.075 8	0.046 1
  Reflection with I > 2σ(I)	6 308	3 658
  Number of parameters	747	335
  Goodness-of-fit on F2	1.024	1.076
  R1 [I > 2σ(I)]	0.069 1	0.059 6
  wR2 (all data)	0.185 2	0.180 3
  (Δρ)max, (Δρ)min / (e·nm-3)	2 842, -2 178	3 728, -1 539
  Flack parameter	-0.041(13)	-0.025(9)

  下载: 导出CSV
•