CP/MAS NMR studies on binding environment of CH₃CN in Cu(I) complexes with disilane-bridged bis(methylpyridine) ligands

Abstract

CP/MAS NMR spectroscopies have developed as an important tool for studying the structure in the crystalline state. In this work, the structures of representative Cu(I) complexes 1–3 with disilane-bridged bis(methylpyridine) ligands in the crystalline state were investigated by CP/MAS NMR. ¹³C CP/MAS NMR confirmed the presence of CH₃CN in the crystals of 2 and 3, but the environment around CH₃CN could not be determined. Natural abundance (ca. 0.36%) ¹⁵N CP/MAS NMR measurements could confirm the difference of CH₃CN environment between coordinating solvent in 2 and crystal solvent in 3. The ¹⁵N CP/MAS of CH₃CN in 3 shows a singlet because it is not coordinated to Cu(I) and the crystal structure is stabilized by multiple intermolecular interactions. These data have provided valuable information on Cu(I) coordination environment, which was in good accordance with single-crystal X-ray analysis.

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Introduction

Flexible molecular structures are known to induce crystalline polymorphism. The transition between the stable and metastable polymorphs triggered through external stimuli can induce alternation of photophysical properties.^1,2 Among them, investigations on the relationship between structure and properties of Cu(I) complexes are of significant interest.^3–6 Copper(I) complexes have been shown to be excellent candidates for solid-state emitters due to their high earth-abundance and low cost, coupled with their ability of bright luminescence at rt, resulting in complexes with interesting photophysical characteristics. These compounds have been found to exhibit a variety of structures from mononuclear to polynuclear copper complexes. Recently, we reported the synthesis and photophysical properties of Cu(I) complexes with disilane-bridged dipyridine ligands. Some compounds showed responsiveness under external stimuli in the crystalline state.⁷

Solid-state NMR is a non-destructive measurement method, and various information can be obtained by the measurement.^8–12 Especially, CP/MAS NMR spectroscopy can provide information about the structure and bonding of metal complexes in the crystalline state. Recently, the techniques have been developed to improve resolution by rotating at high speeds.^{13 13}C and ¹⁵N CP/MAS NMR, along with single-crystal X-ray structure analysis, are powerful techniques for structural analysis.^14–17

The major difficulty in employing ¹⁵N NMR spectroscopy lies in its sensitivity, which is very low due to the low natural abundance (0.36%) of the ¹⁵N isotope. Although nitrogen-15 labelling compounds were used to employ ¹⁵N CP/MAS NMR, ¹⁵N NMR can be measured even for unlabelled ¹⁵N compounds by increasing accumulation numbers and using high-resolution NMR. Recently, we measured ¹⁵N CP/MAS NMR to track structural changes of organosilicon compounds upon mechanical stimulation.¹⁸

Cu complexes often decompose in solution, requiring solid-state NMR measurements. Previously, we reported the synthesis and optical properties of several Cu(I) complexes.¹⁹ The ¹³C CP/MAS NMR of 2 and 3 showed the peaks of CH₃CN as similar chemical shift, and it was impossible to distinguish existence state between the crystal solvent and coordinated ligand. ¹⁵N CP/MAS NMR of transition metal complexes were measured to distinguish binding mode of CH₃CN. As Cu(I) complex, several ¹⁵N CP/MAS NMR of Cu(I) complexes were reported in natural abundance.^20–23 However little clear evidence has been given on the effect of metal binding to the ¹⁵N nuclear shielding constants.^24–27 In this paper, we used natural abundance ¹⁵N CP/MAS NMR to study binding mode of CH₃CN in Cu(I) complexes 1–3 (Fig. 1).

Fig. 1 Chemical structures of copper complexes 1–3 studied in this work.

Experimental section

Materials

All chemicals and reagents were obtained from commercial sources and used without additional purification. 1,1,2,2-Tetramethyl-1,2-bis(4-methylpyridin-2-yl)disilane and 1,1,2,2-tetramethyl-1,2-bis(3-methylpyridin-2-yl)disilane were prepared according to our previous report.⁷

Synthesis of 1

CuI (380 mg, 2.0 mmol) and 1,1,2,2-tetramethyl-1,2-bis(4-methylpyridin-2-yl)disilane (300 mg, 1.0 mmol) were dissolved in CH₃CN (40 mL) at rt. The resulting yellow mixture was stirred at rt for 12 h. The solvent was evaporated under reduced pressure and the residue was washed with diethyl ether to give 1 as pale yellow crystalline solid. The solid was recrystallized from CH₃CN to obtain analytically pure complex 1 as colorless cubes. Yield: 42% (290 mg). Elemental analysis. Calcd for C₃₂H₄₈N₄Cu₄I₄Si₄: C, 28.20; H, 3.55; N, 4.11. Found: C, 28.47; H, 3.53; N, 4.24.

Synthesis of 2

CuI (380 mg, 2.0 mmol) and 1,1,2,2-tetramethyl-1,2-bis(3-methylpyridin-2-yl)disilane (300 mg, 1.0 mmol) were dissolved in CH₃CN (40 mL) at rt. The resulting yellow mixture was stirred at rt for 12 h. The solvent was evaporated under reduced pressure and the residue was washed with diethyl ether to give 2 as pale yellow crystalline solid. The solid was recrystallized from CH₃CN to obtain analytically pure complex 2 as yellow cubes. Yield: 44% (305 mg). Elemental analysis. Calcd for C₃₆H₅₄N₆Cu₄I₄Si₄: C, 29.92; H, 3.77; N, 5.82. Found: C, 29.76; H, 3.70; N, 5.78.

Synthesis of 3

CuI (190 mg, 1.0 mmol) in CH₃CN (15 mL) was added dropwise to a solution of 1,1,2,2-tetramethyl-1,2-bis(4-methylpyridin-2-yl)disilane (300 mg, 1.0 mmol) in CH₃CN (5 mL) over 5 min at rt. After stirring at rt for 10 min, 3 was obtained as a yellow crystalline solid (314 mg, 60%). The solid was recrystallized from CH₃CN to obtain analytically pure complex 3 as colorless cubes. The spectroscopic data were identical to our previous reported data.¹⁹

Single-crystal X-ray structural analyses

All single-crystal Xray diffraction measurements were conducted using a Rigaku Mercury CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and a rotating-anode generator. Each crystal was mounted on a loop using paraffin oil. Diffraction data were collected at around 100 K and processed using the Crystal Clear program. Structures were solved by direct methods using SIR-2011. Structural refinements were conducted by the full-matrix least-squares method using SHELXL-2013. All non-H atoms were refined anisotropically, and all H atoms were refined using the riding model. All calculations were performed using the Crystal Structure crystallographic software package. The crystallographic data were deposited in the Cambridge Crystallographic Data Centre.

Crystallographic data for 1

C₃₂H₄₈Cu₄I₄N₄Si₄, crystal dimensions: 0.15 mm × 0.15 mm × 0.05 mm. M = 1362.86, monoclinic, P12₁/n1, a = 9.8145(5) Å, b = 14.0010(8) Å, c = 16.7363(10) Å, α = 90°, β = 98.366(2)°, γ = 90°, V = 2275.3(2) ų, Z = 2, T = 109 K, D_calc = 1.989 g cm⁻³, F₀₀₀ = 1304.0, λ = 0.71073 Å (Mo Kα), μ = 4.690 mm⁻¹, R₁ = 0.0177 (I > 2σ(I)), wR₂ = 0.0462 (all data). GOF = 1.063. CCDC 2419410.

Crystallographic data for 2

C₃₆H₅₄Cu₄I₄N₆Si₄, crystal dimensions: 0.15 mm × 0.15 mm × 0.05 mm. M = 1444.97, triclinic, P1̄, a = 9.3981(5) Å, b = 10.1021(6) Å, c = 14.9219(8) Å, α = 81.667(2)°, β = 74.599(1)°, γ = 64.227(1)°, V = 1229.25(12) ų, Z = 1, T = 112 K, D_calc = 1.952 g cm⁻³, F₀₀₀ = 696.0, λ = 0.71073 Å (Mo Kα), μ = 4.348 mm⁻¹, R₁ = 0.0340 (I > 2σ(I)), wR₂ = 0.0706 (all data). GOF = 1.036. CCDC 2419411.

Crystallographic data for 3

C₁₈H₂₇CuIN₃Si₂, crystal dimensions: 0.15 mm × 0.13 mm × 0.05 mm. M = 532.04, orthorhombic, Pca2₁, a = 16.7137(6) Å, b = 10.0411(3) Å, c = 13.9102(5) Å, α = 90°, β = 90°, γ = 90°, V = 2334.46(14) ų, Z = 4, T = 100 K, D_calc = 1.514 g cm⁻³, F₀₀₀ = 1064.0, λ = 0.71073 Å (Mo Kα), μ = 2.367 mm⁻¹, R₁ = 0.0221 (I > 2σ(I)), wR₂ = 0.0431 (all data). GOF = 1.033. CCDC 2339248.

CP/MAS NMR spectroscopy

The sample was packed into a 4 mm zirconia rotor and measured with ¹³C and ¹⁵N cross-polarization/magic angle spinning (CP/MAS) NMR using a spectrometer (Bruker AVANCE III HD 600WB) at a Larmor frequency of 150.97 MHz (¹³C) and 60.86 MHz (¹⁵N). Bruker MAS probe head (MAS4DR) was used with a HR-MAS rotor with 4 mm diameter (HZ05538) and Teflon insert (50 μL), and sample spin rates were 10 kHz for ¹³C and 8 kHz for ¹⁵N, respectively. The chemical shifts refer to tetramethylsilane (¹³C) and nitromethane (¹⁵N) at 0.00 ppm. Glycine was used as a second reference material for ¹³C NMR, and its carbonyl signal was set at 176.46 ppm. NH₄Cl (10 atom% ¹⁵N) was used as a second reference material for ¹⁵N NMR, which was set at −341.15 ppm. The samples were measured at ambient probe temperature.

Results and discussion

The ¹³C CP/MAS NMR can be used to correlate solution NMR data and X-ray structure data.^{28 13}C CP/MAS NMR and X-ray analysis are shown in Fig. 2 and S1,† respectively. Chemical shifts are sensitive to the surrounding environment, and hence obvious changes can be found in different structures. In the solid-state spectra of copper complex 1, we observed a number of peaks than the number of chemically distinct carbon atoms in the molecules (Fig. 2(a)). The doublet-like signals with the intensity ratio of ca. 1 : 1 observed in some peaks of ¹³C CP/MAS NMR spectra arise due to structurally nonequivalent carbon of the complex in the unit cell.

Fig. 2 ¹³C CP/MAS NMR (151 MHz, 10 kHz MAS). (a) 1, (b) 2, and (c) 3. The parameters of CP/MAS are as follows: CP contact time: 2 ms, recycle delay: 8 s, and the number of scans: 2000. The signals with asterisks indicate spinning side band.

¹³C CP/MAS NMR reflects the unsymmetrized ligand molecule by crystallization and CH₃CN molecules. ¹³C CP/MAS of 2 shows signals of CH₃CN at 115 ppm (CH₃CN) and around 5 ppm (CH₃CN, overlapped with Si–CH₃) (Fig. 2(b)). The signal at 118 ppm and 5 ppm is attributed to the ¹³C peaks of CH₃CN in the crystal of 3 (Fig. 2(c)). However, ¹³C CP/MAS NMR did not show obvious information based on structural differences on CH₃CN between 2 and 3.

Nitrogen is a constituent of many ligands which are important in coordination chemistry. Natural abundance ¹⁵N CP/MAS NMR has been reported as a novel tool for investigating molecular information. Particularly, it is possible to consider the structure of metal complexes, and the ¹⁵N chemical shifts are sensitive to the coordinating or uncoordinating N atoms. Therefore, we measured ¹⁵N CP/MAS NMR of these copper complexes 1–3.

When the ¹⁵N NMR of copper complex 1 without CH₃CN was measured in the crystalline state, multiple peaks were observed at from −110 ppm to −126 ppm, which were assigned as ¹⁵N of pyridine moiety (Fig. 3(a)). The ¹⁵N chemical shift was consistent with the results in analogous systems in solution.^{29–33 15}N resonance of the free pyridine-based ligand showed at around −70 ppm while the coordination of the pyridine to Cu(I) in solution showed the resonance at around −100 ppm in the ¹⁵N NMR.³⁴ The complicated peaks in the solid state are due to not only the unsymmetrization observed in ¹³C spectra but also coupling between ¹⁵N and ⁶³Cu/⁶⁵Cu nuclei. In a similar system, ³¹P CP/MAS NMR of Cu(I) complexes having phosphine ligands show complicated peaks due to coupling between the ³¹P and ⁶³Cu/⁶⁵Cu.^35,36

Fig. 3 Natural abundance ¹⁵N CP/MAS NMR (61 MHz, 8 kHz MAS). (a) 1, (b) 2, and (c) 3. The parameters of CP/MAS are as follows: CP contact time: 5 ms, recycle delay: 10 s, and the number of scans: (a) 8000, (b) 12 000, and (c) 12 000.

Complexes 2 and 3 contain one CH₃CN, which exist in different states in the crystal. ¹⁵N NMR chemical shifts are sensitive to the environment of N moiety. Therefore, detectability of CH₃CN in crystals is discussed by CP/MAS ¹⁵N NMR for identifying specific chemical functional groups.

¹⁵N CP/MAS NMR of 2 was shown in Fig. 3(b). The multiple peaks from −87 ppm to −101 ppm were ¹⁵N chemical shifts of pyridine ligands in 2. The chemical shift moved to the downfield side in comparison with 1 are due to the difference in the substitution position of the methyl group on pyridine ring. The ¹⁵N resonance of CH₃CN in 2 was also observed as multiple peaks from −164 ppm to −172 ppm. The chemical shift in the characteristic ¹⁵N resonance was observed upon N-cyano coordination to Cu(I) via Cu⋯N≡C–CH₃ interactions.

CP/MAS ¹⁵N NMR of 3 was measured (Fig. 3(c)). The multiple peaks from −105 ppm to −122 ppm were assigned to ¹⁵N of pyridine ring coordinated to Cu(I). ¹⁵N peak of CH₃CN molecules as the crystal solvent was observed at −133 ppm as a sharp peak, which was lower field relative to 2. The CH₃CN peak of 3 is sharp because CH₃CN molecules have no Cu⋯N interaction in the crystalline state. Although these results on CH₃CN binding mode agree with X-ray structures of 2 and 3, single crystal X-ray determinations are not always available for analysing the structure. The structures can often not be determined by Rietveld analysis of microcrystals. There is a need for alternative spectroscopic methods to determine the ligand coordination mode. ¹⁵N CP/MAS NMR is a valuable tool for distinguishing the binding environments in the crystals. This study provides spectroscopic support for insight into the coordination mode of solvent in metal complexes.

CH₃CN molecules in 2 exist in the crystal through a coordination bond with Cu(I) (CH₃C≡N⋯Cu), and CH₃CN molecules in 3 exist through hydrogen bonds between N lone pair of CH₃CN and aromatic proton (CH₃C≡N⋯H–Ar) (Fig. S2†). The ¹⁵N NMR peak shapes of CH₃CN change depending on the coordination or uncoordination with Cu(I). ¹⁵N NMR peaks of CH₃CN in 2 are complicated by the coupling between ¹⁵N and ^63/65Cu nuclei due to the coordination of CH₃CN to Cu(I). On the other hand, there is no interaction between CH₃CN and Cu(I) in the crystal of 3, and ¹⁵N NMR peak of CH₃CN is observed as a sharp singlet.

Conclusion

This work describes detailed structural information on Cu(I) complexes in the crystalline state using natural abundance ¹⁵N CP/MAS NMR. ¹³C and ¹⁵N CP/MAS NMR spectra of 1–3 were in accord with the direct structural data on the complexes. CH₃CN in these complexes has restricted molecular motion in single crystals and could be observed by CP/MAS NMR.³⁷ Natural abundance ¹⁵N CP/MAS NMR spectra provided considerable information on the crystalline state, because the state of the solvents contained can be deduced from NMR spectroscopy in addition to single crystal X-ray structure analysis. ¹⁵N NMR peak shapes of these Cu(I) complexes are related to the environment of the nitrogen atoms in the crystal. The differences between coordinated and uncoordinated CH₃CN were able to be distinguished by ¹⁵N CP/MAS NMR. Molecular packing had an impact on the splitting patterns and peak shapes of the ¹⁵N peak of CH₃CN.

The ¹⁵N chemical shift and peak shape were sensitive to cyanide interactions observed in Cu(I) complexes. Sharp ¹⁵N signal was observed corresponding to CH₃CN of crystal solvent with the stabilization of multiple intermolecular interactions. On the contrary, multiple ¹⁵N signals of CH₃CN was observed corresponding to Cu⋯N≡C–CH₃ interactions. These observations suggest a detailed study of solvent environments in the crystal.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by Grant-in-Aids for Scientific Research (C) (no. JP22K05250 and JP25K08585) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Solid state NMR measurements were supported by AIST Nanocharacterization Facility (ANCF) in the Nanotechnology Platform Project (no. JPMXP1224AT5010) sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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Footnote

† Electronic supplementary information (ESI) available: ORTEP drawings of 1–3 and crystal packing of 3. CCDC 2339248, 2419410 and 2419411. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5ra02962g

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