A Neutral Heteroleptic Cu(I) Complex with Diimine and Diphosphine Ligands

Abstract

Developing organic luminescent materials with the advantages of low cost, high thermal stability, and strong emission performance is incredibly desirable. In this work, we synthesized a new neutral heteroleptic Cu(I) complex characterized by single-crystal X-ray diffraction, FT-IR, NMR, and MALDI-TOF-MS. The neutral heteroleptic Cu(I) complex has a typical distorted tetrahedral configuration, and the complex molecules are connected into 1D chains via C-H···π interactions in crystal.

1. Introduction

Organic light-emitting diodes (OLEDs) have attracted extensive attention due to their high efficiency, low driving voltage, excellent flexibility, wide viewing angle, thinness, and other properties [1,2,3]. Organic luminescent materials based on transition metal complexes emitting phosphorescence/thermally activated delayed fluorescence (TADF) can make use of both singlet and triplet excitons resulting in high electroluminescence (EL) efficiencies [4,5]. However, most central ions in these transition metal complexes are noble metals, such as Ir(III) and Pt(II), which are expensive and scarce. Over the past two decades, luminescent Cu(I) complexes have long been considered economically viable alternatives for OLEDs due to their low cost and the abundance of copper resources [6,7,8,9,10,11,12,13,14,15].

It was reported that heteroleptic copper(I) complexes consisting of diimine and diphosphine ligands exhibit excellent luminescence properties [16,17,18,19]. In addition, neutral Cu(I) complexes without counterions are suitable for preparing OLEDs using the conventional vacuum deposition method. Based on the above considerations and our extensive experience in luminescent Cu(I) complexes [20,21,22,23], we present here the synthesis and structure of a neutral heteroleptic Cu(I) complex comprising a diphosphine ligand and deprotonated imidazole ligand.

2. Results and Discussion

2.1. Synthesis and Single-Crystal Structure

A neutral heteroleptic Cu(I) complex [(immp)Cu(POP)] was synthesized through the following steps (Scheme 1). One equivalent of [Cu(CH₃CN)₄]BF₄ precursor successively reacted with one equivalent of the bis[2-(diphenylphosphino)phenyl]ether (POP) ligand and one equivalent of the 2-(1H-imidazol-2-yl)-6-methylpyridine (Himmp) ligand precursor in dichloromethane solution at room temperature. After the solvent was evaporated, the solid residue was dissolved in methanol and treated with two equivalents of KOH to give the crude product. By slowly evaporating the dichloromethane/methanol solution, a pure neutral heteroleptic Cu(I) complex [(immp)Cu(POP)] was obtained in 63% yield as pale-green crystals. It should be noted that the Cu(I) coordination reaction occurred at room temperature and did not need heating to high temperature, making the procedure more energy efficient, green, and sustainable.

The complex [(immp)Cu(POP)] is stable enough to be exposed to air in its solid state. The single-crystal X-ray diffraction (XRD) analysis results show that this Cu(I) complex crystallizes in the triclinic space group P-1. The refinement metrics (R1 = 0.0537, wR2 = 0.1504) indicate a well-resolved structure with minimal residual electron density. A low X-ray absorption coefficient (µ = 0.638 mm⁻¹) and moderate density (ρ_calc = 1.214 g/cm³) were observed compared to those of pure copper metal (µ = 4.47 mm⁻¹, ρ_Cu = 8.940 g/cm³); the significantly lower µ value indicates that the coordination of Cu(I) with organic ligands in the [(immp)Cu(POP)] complex essentially reduced the electron density around the Cu(I) centre. The molecular structure of [(immp)Cu(POP)] determined by single-crystal XRD is shown in Figure 1, whereas the crystal data of the complex are given in Table S1.

The single-crystal structure of [(immp)Cu(POP)] exhibits a typical distorted tetrahedral geometry. The important bond lengths and angles are listed in

Table 1. Important bond lengths and angles of [(immp)Cu(POP)].

Bond lengths (Å)
Cu1-N1	2.017(3)	Cu1-N3	2.141(3)
Cu1-P1	2.2477(9)	Cu1-P2	2.2948(8)
Bond angles (°)
N1-Cu1-N3	81.00(12)	P1-Cu1-P2	113.87(3)
N1-Cu1-P1	123.60(8)	N3-Cu1-P1	117.10(7)
N1-Cu1-P2	107.08(8)	N3-Cu1-P2	109.67(7)

, whereas the complete bond lengths and angles are given in Tables S2 and S3, respectively. The bond length of Cu1-P1 (2.2477(9) Å) is similar to that of Cu1-P2 (2.2948(8) Å). In contrast, the bond length of Cu1-N1 (2.017(3) Å) is distinctly shorter than that of Cu1-N3 (2.141(3) Å), which indicates that the N atom in the imidazole unit is a stronger electron donor than the N atom in the pyridine unit. The coordination of Cu(I) with N atoms of immp and pyridine could result in C=N/C=C infrared vibrations at 1580–1630 cm⁻¹ (Figure S1). Additionally, the imidazole and pyridine rings in this complex have a small dihedral angle of about 5.98°, indicating the immp ligand has near coplanarity, which is beneficial for stabilizing the π-conjugation system of the immp ligand. The bond angles (81.00–123.60°) suggested the existence of tetrahedral distortion around the metal ion (Cu⁺) centre, which is widely observed in four-coordinated Cu(I) complexes with various organic ligands. Jahn-Teller distortion, along with its solvent-induced exciplex, may influence their luminescent properties [24,25]. Moreover, there are no intermolecular hydrogen bonds or π-π interactions among the complex molecules. The adjacent molecules are connected into 1D chains via C-H···π interactions along the a-axis (Figure 2), C18-H···π1, and C37-H···π2, where π1 is the C16-C21 ring and π2 is the C22-C27 ring.

2.2. NMR and MALDI-TOF-MS Spectra Analysis

The structure of the [(immp)Cu(POP)] complex was further characterized by a ³¹P NMR spectrum and MALDI-TOF-MS spectrum. Unfortunately, it is difficult to obtain clear ¹H and ¹³C NMR signals (there are broadened and abnormal ¹H signals at 7.1–7.5 ppm and ¹³C signals at 132–135 ppm; see Figures S2 and S3). In contrast, the ³¹P NMR spectrum exhibited a clear singlet peak at −15.1 ppm (Figure S4), which belongs to the P atoms on the Cu(I)-coordinated POP ligand. Moreover, MALDI-TOF-MS analysis of the repurified complex revealed a molecular ion peak at m/z 760.17 (Figure S5), which was very consistent with the calculated m/z, 760.28. In addition, the ³¹P NMR spectrum and MALDI-TOF-MS spectrum results further supported the above-mentioned single-crystal X-ray structure of the [(immp)Cu(POP)] complex. Similar to our work, several Cu(I) complexes with POP and heterocyclic coligands were synthesized and characterized in recent years [26,27].

3. Materials and Methods

3.1. Synthesis Procedure for [(Immp)Cu(POP)]

Himmp (2 mmol, 0.32 g) in dichloromethane (10 mL) was slowly added to a solution of [Cu(CH₃CN)₄]BF₄ (2 mmol, 0.63 g) in dichloromethane (20 mL), and the mixture was stirred for one hour at room temperature. Then, POP (2 mmol, 1.08 g) in dichloromethane (10 mL) was slowly added to the mixture and reacted for two hours at room temperature. After the solution was filtered and evaporated to dryness, the residue was dissolved in methanol (25 mL). Solid-state KOH (4 mmol, 0.23 g) was added to the solution, and the mixture was stirred for 3 h at room temperature. The precipitate was collected and washed with water three times. After drying, the precipitate was dissolved in a 30 mL ethanol/dichloromethane (3/1) mixed solution. The pure complex was obtained as a pale-green crystal by slowly evaporating the solvents. Yield: 63% (0.96 g).

3.2. X-Ray Crystal Structure Analysis of [(Immp)Cu(POP)]

Crystal data were collected on a Rigaku SuperNova CCD diffractometer with confocal monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods, i.e., using the program Olex2 as an interface together with the programs SHELXT and SHELXL to solve and refine the structure, respectively [28,29,30]. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were placed in the calculated positions. Crystal data for C₄₅H₃₆CuN₃OP₂ (M = 760.25 g/mol): triclinic crystal system, P-1 space group, a = 9.8461(3) Å, b = 12.7566(5) Å, c = 17.7329(7) Å, α = 75.775(3)°, β = 77.118(3)°, γ = 78.417(3)°, V = 2079.38(14) ų, Z = 2, T = 293(2) K, µ(Mo-Kα) = 0.638 mm⁻¹, ρcalc = 1.214 g/cm³, 17,360 reflections collected (4.506° ≤ θ ≤ 52.74°), and 8480 unique (Rint = 0.0541, Rsigma = 0.0794), which were used in all calculations. The final R1 was 0.0537 (I > 2.0 σ(I)), and the wR2 was 0.1504 (all data). CCDC 1511746 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures (accessed on 1 April 2025).

3.3. FT-IR, NMR, and MALDI-TOF-MS Analysis of [(Immp)Cu(POP)]

For the characterization of the [(immp)Cu(POP)] complex, a Fourier-transform infrared (FT-IR) spectrum was recorded on a Nicolet 6700 spectrometer (Thermo Scientific, USA)equipped with a DTGS detector (KBr pellets, 4000–400 cm⁻¹ range). NMR characterization was performed on a Bruker Avance III HD 400 MHz spectrometer (¹H nuclei: 400 MHz, ¹³C nuclei: 100 MHz, ³¹P nuclei: 162 MHz, with chemical shift in ppm) in CDCl₃ at 298 K, ¹H and ¹³C NMR spectra were recorded with the internal standard tetramethylsilane (TMS, δ = 0 ppm), and ³¹P NMR was referenced externally to 85% phosphoric acid (δ = 0 ppm). An MALDI-TOF-MS spectrum was recorded on a Bruker APEX II FT-ICR MALDI-TOF/TOF mass spectrometer (positive-ion mode, 20 kV acceleration voltage).

4. Conclusions

In summary, our research led to the successful synthesis of a new neutral Cu(I) complex [(immp)Cu(POP)]. This complex was formed by reacting [Cu(CH₃CN)₄]BF₄ with diimine and diphosphine ligands at room temperature, and its structure was confirmed through single-crystal XRD analysis, NMR, and MALDI-TOF-MS. The molecular geometry of [(immp)Cu(POP)] exhibits a typical distorted tetrahedral configuration. There are no intermolecular hydrogen bonds or π-π interactions among the [(immp)Cu(POP)] complex molecules. The adjacent molecules are self-organized into 1D chains via C-H···π interactions.