Synthesis, structural, spectroscopic and DFT study on a palladium(II)-N-heterocyclic carbene complex

Accepted Manuscript Synthesis, structural, spectroscopic and DFT study on a palladium(II)-N-heterocyclic carbene complex Rukiye Fırıncı, M. Emin Günay, Namık Özdemir, Muharrem Dinçer PII:

S0022-2860(17)30788-3

DOI:

10.1016/j.molstruc.2017.06.012

Reference:

MOLSTR 23896

To appear in:

Journal of Molecular Structure

Received Date: 7 April 2017 Revised Date:

2 June 2017

Accepted Date: 4 June 2017

Please cite this article as: R. Fırıncı, M.E. Günay, Namı. Özdemir, M. Dinçer, Synthesis, structural, spectroscopic and DFT study on a palladium(II)-N-heterocyclic carbene complex, Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.06.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Synthesis, structural, spectroscopic and DFT study on a palladium(II)-N-heterocyclic carbene complex

Muharrem Dinçerc

Department of Chemistry, Faculty of Arts and Sciences, Adnan Menderes University, 09010 Aydın, Turkey b

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Rukiye Fırıncıa, M. Emin Günaya, Namık Özdemirb,*,

Department of Mathematics and Science Education, Faculty of Education,

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Ondokuz Mayıs University, 55139 Samsun, Turkey c

Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, 55139 Samsun, Turkey

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ABSTRACT

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* Corresponding author. E-mail address: [email protected]

A new palladium complex with N-heterocyclic carbene (NHC) and phosphine ligands was 13

C NMR and

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P NMR spectroscopies, IR

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prepared and fully characterized by 1H NMR,

spectroscopy, and X-ray crystallography. The solid-state structure of the complex shows that the metal center was surrounded by an N-heterocyclic carbene ligand, a phosphorus atom and two bromide ions in a cis-arrangement. Density-functional theory (DFT) calculations at the B3LYP/SDD level were also executed for the further explorations of the spectroscopic and structural properties. The obtained theoretical parameters adequately support the experimental findings in general.

Keywords: N-heterocyclic carbene; Crystal structure; Spectroscopy; Density functional theory

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1. Introduction Over the last two decades, the popularity of N-heterocyclic carbene ligands (NHC) has dramatically increased mainly due to their use in the development of very active and versatile catalysts. Complexes with N-heterocyclic carbene (NHC) ligands are extremely versatile ligands in homogeneous catalysis and coordination chemistry [1−4]. The steric and electronic properties of

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such ligands are easily modified through variation of the substituents at the nitrogen and carbon atoms, and they have been used as ancillary ligands for the preparation of various catalytically active complexes [5]. During the last decade, a major advance has been the development of catalysts enabling the cross-coupling reactions. A large number of complexes used in homogeneous catalysis contain phosphine or N-heterocyclic (NHC) ligands. While many complexes bearing N-

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heterocyclic ligands are known, but NHC ligands containing both NHC and phosphine donor groups are less common [6−9]. The use of such compounds in combination with phosphorus-based

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ligands within metal complexes has enabled the design of very active yet robust catalytic systems. In this work, a new palladium(II) complex containing imidazol-2-ylidene and phosphine ligands was obtained and characterized by IR, NMR, UV-vis spectroscopic methods and singlecrystal X-ray diffraction. Theoretical characterization of the structural and spectroscopic properties of the complex was done using the density functional theory (B3LYP) method with the SDD basis

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set, and the results were compared with the experimental data.

2. Materials and methods 2.1. General remarks

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The infrared spectra were recorded on an ATR unit in the range of 4000-450 cm−1 with a Perkin-Elmer Spectrum FTIR system. 1H, 13C and 31P NMR measurements were performed using a Varian AS 400 spectrometer operating at 400 and 100 MHz, respectively. The UV-Vis. spectra

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were measured on a Shimadzu UV-1601 UV-vis. spectrometer in 0.0001 M CH2Cl2. The synthesis scheme was drawn using MarvinSketch [10].

2.2. Synthesis

2.2.1. Synthesis of Imidazolium Salt (1) Unsymmetric dialkylimidazolium salt (1) was prepared according to known methods (Scheme 1) [11−13].



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ACCEPTED MANUSCRIPT 2.2.2. Synthesis of Pd-NHC Complex (2) Imidazolium bromide salt (1) (1.0 mmol), PdCl2 (1.1 mmol), PPh3 (1.0 mmol), excess KBr and K2CO3 (5.0 mmol) were mixed in toluene (5.0 mL). After the mixture was refluxed for 18 h, it was cooled down to room temperature. The solvent was removed in vacuo, the remaining precipitate was then dissolved in dichloromethane and recrystallization from CH2Cl2/Et2O afforded the complex. Yield: 30% (0.200 g). m.p.: 155 oC. Anal. Calc. for C37H45Br2N2OPPd: C, 53.48; H,

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5.46; N, 3.37. Found: C, 53.79; H, 5.35; N, 3.32%. FT-IR (cm-1): 3165, 3124, 3053, 2960, 2926, 2871, 1573, 1410, 745. 1H NMR (δ, 400 MHz, CDCl3): 0.86 [t, J = 7.2 Hz, 3H, CH3(CH2)3N]; 1.181.31 [m, 2H, CH3CH2(CH2)2N]; 1.82-1.92 [m, 2H, CH3CH2CH2CH2N]; 2.17 [s, 6H, C6-(CH3)5-oCH3]; 2.17 [s, 6H, C6-(CH3)5-m-CH3]; 2.23 [s, 3H, C6-(CH3)5-p-CH3]; 4.10-4.20 [m, 2H,

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CH3(CH2)2CH2N]; 4.65 [d, J = 14.1 Hz, 1H, NCH2-C6-(CH3)5]; 6.01 [d, J = 14.1 Hz, 1H, NCH2-C6(CH3)5]; 6.13 [d, J = 2.0 Hz, 1H, NCHCHN]; 6.54 [d, J = 2.0 Hz, 1H, NCHCHN]; 7.32-7.36 [m,

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5H, PPh3-CH]; 7.43-7.47 [m, 5H, PPh3-CH]; 7.53-7.55 [m, 5H, PPh3-CH]. 13C NMR (δ, 100 MHz, CDCl3): 13.6 [CH3(CH2)3N]; 16.7 [CH3CH2(CH2)2N]; 16.8 [C6-(CH3)5-o-CH3]; 17.1 [C6-(CH3)5-mCH3]; 20.2 [C6-(CH3)5-p-CH3]; 31.8 [CH3CH2CH2CH2N]; 50.7 [CH3(CH2)2CH2N]; 51.1 [NCH2-C6(CH3)5]; 110.0 [NCHCHN]; 126.9 [NCHCHN]; 128.5 [C6-C-(CH3)5]; 128.6 [C6-C-(CH3)5]; 133.1 [PPh3-C]; 133.2 [PPh3-C]; 133.9 [PPh3-C]; 134.2 [PPh3-CH]; 134.3 [PPh3-CH]; 136.3 [PPh3-CH];

2.3. X-ray crystallography

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159.9 [Pd-Ccarbene]. 31P NMR (δ, 162 MHz, CDCl3): 21.6 ppm. UV-Vis (CH2Cl2, nm) 230, 279.

Intensity data of the compound were collected on a STOE diffractometer with an IPDS II image plate detector. The diffraction measurements were performed at room temperature using

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graphite monochromated Mo Kα radiation by applying the ω-scan method. Data collection and cell refinement were carried out using X-AREA [14] while data reduction was applied using X-RED32

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[14]. The structure was solved by a dual-space algorithm using SHELXT-2014 [15] and refined with full-matrix least-squares calculations on F2 using SHELXL-2016 [16] implemented in WinGX [17] program suit. All H atoms were positioned geometrically and treated using a riding model, fixing the bond lengths at 0.93, 0.97 and 0.96 Å for CH, CH2 and CH3 atoms, respectively. The displacement parameters of the H atoms were fixed at Uiso(H) = 1.2Ueq (1.5Ueq for CH3) of their parent atoms. In the complex, there is a disordered solvent water molecule with very large displacement parameters which was difficult to model. Therefore, the SQUEEZE command of PLATON [18] was used to model the electron density in the void region. There is a cavity of volume 106 Å3 per unit cell centered at (0, 0, 0.5). The cavity contains approximately 31 electrons which were assigned to two solvent water molecules. With Z = 2, each Pd complex has one solvent water equivalent. In the final refinement, these contributions were removed from the intensity data

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ACCEPTED MANUSCRIPT that produced better refinement results. Furthermore, the butane and pentamethylbenzyl moieties were disordered over two positions, and the refined site-occupancy factors of the disordered parts are 0.61(3)/0.39(3)% for C5-C7 and 0.558(13)/0.442(13)% for C9-C19. Crystal data, data collection and structure refinement details are summarized in Table 1. Molecular graphic was generated by



2.4. Computational procedure

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using ORTEP-3 [17].

The structure of 2 was optimized using the three-parameter hybrid density functional

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(B3LYP) [19,20] and SDD [21] basis set. The vibrational frequencies were obtained at the same level to characterize the nature of the local minima with no imaginary frequency. A scale factor of 13

C

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0.961 has been used to correct the calculated harmonic vibrational frequencies. The 1H and

NMR chemical shifts were calculated within the gauge-independent atomic orbital (GIAO) approach [22,23] at the same level. The effect of solvent on the theoretical NMR parameters was included using the default solvation model [24]. Chloroform was used as a solvent. The electronic absorption spectra were obtained using the time dependent density functional theory (TD-DFT) [25−27] and dichloromethane (DCM) as solvent within the default solvation model. All the

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calculations in this paper were performed via the GaussView molecular visualization program [28] and Gaussian 03W package [29].

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3. Results and discussion

3.1. Experimental and theoretical structures The molecular structure of 2 is shown in Fig. 1(a), while the important bond lengths and

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angles are given in Table 2.

The complex crystallized as cis configuration, in which the palladium centre is surrounded by an NHC carbon atom, a triphenylphosphine phosphorus atom and two cis bromide ions in a distorted square planar fashion. The Pd─Br bond trans to the P atom is longer than that trans to the carbene ligand by ca. 0.05 Å, although the trans influence of the NHC ligand is slightly stronger than phosphine [30]. The coordination bond distances are comparable to those found in mononuclear phosphine/N-heterocyclic carbene-palladium complexes [31−34]. The cis [83.6(2)-

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ACCEPTED MANUSCRIPT 94.40(5)°] and trans angles [176.40(6) and 177.9(2)°] around the palladium ion deviate from the expected values (90 and 180°). The four-coordinate geometry index for the complex, τ4 [35], is 0.04 (where 0 would be perfectly square-planar, and 1 is perfectly tetrahedral). The value of τ4 shows that the coordination polyhedron of the palladium is a distorted tetrahedral. The carbene ring plane is oriented almost perpendicularly to the square-planar Pd coordination plane (PdCPBr2) with a

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dihedral angle of 88.67(3)°, which is typical for NHC complexes to relieve steric congestion [36].

The calculated structural parameters are also tabulated in Table 2. From the table, the

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biggest deviation in the bond lengths is 0.15 Å at Pd1─P1 and the biggest deviation in the bond angles is 3.72° at Br2─Pd1─C1. In theoretical structure, the τ4 value is 0.06, and the carbene ring

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plane makes a dihedral angle of 83.18° with coordination plane. When the x-ray and theoretical structures are superimposed by PLATON as illustrated in Fig. 1(b), the obtained root mean square (RMS) bond fit and angle fit values of 0.052 Å and 1.613° confirm an acceptable correlation between them.

3.2. Spectroscopic characterization

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3.2.1. Vibrational spectroscopy

The recorded and scaled theoretical spectra are given as superimposed in Fig. 2. The vibrational bands assignments have been done by using Gauss View molecular visualization

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program.

The formation of the imidazolium salt was evident through their IR spectrum, which showed a peak at 1608 cm−1 for the ν(C=N) bond of 1. The FT-IR data clearly indicated the presence of ν(C─N) at 1410 cm−1 for the Pd-NHC complex, which has been calculated at 1408 cm−1. The formation of a C─N module in the imidazole ring correlated with a shift of IR (C=N) band. The C─H aromatic stretching modes were observed at 3165, 3124, 3053 cm−1 experimentally and were calculated at 3128, 3109 and 3077 cm−1. The peaks recorded at 2960 and 2926 cm-1 are the aliphatic asymmetric and symmetric stretching vibration belonging to methyl groups. These bands have been calculated at 2999 and 2913 cm−1, respectively. We assigned the absorption band at 2871 cm−1 due to methylene stretching vibration that has been computed at 2925 cm−1. The experimental band at 1573 cm−1 corresponds to C=C stretching, which has been calculated at 1571 cm−1. The stretching

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of the P─C bonds was observed at 745 cm−1 while this band appeared at 671 cm−1 in the theoretical spectrum.

3.2.2. NMR spectroscopy The NMR spectra of 2 are given in Fig. 3. The expected composition of the complex was clearly verified by 1H and 13C NMR spectroscopies. The imidazolium salt includes an acidic NCHN

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proton, which can be deprotonated easily to form an NHC, at the C2 position of the imidazolium ring. The sharp salt peak indicating the synthesis of an imidazolium salt came quite downfield at 10.14 ppm in the 1H NMR spectrum. The NCHN peak of the carbene precursor was observed at 137.2 ppm in the

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C NMR spectrum. Complex 2 with an imidazolium moiety displays 13

C NMR spectra. For the 1H NMR

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characteristic signals for the phospine ligand in the 1H and

spectrum of the metal complex, a sharp peak in the lower field belonging to the imidazolium salt

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(NCHN) was not observed between 10 and 12 ppm.

The proton signals for the n-butyl bearing to the imidazole-N1 were appeared at 0.86, 1.181.31, 1.82-1.92 and 4.10-4.20 ppm. These signals were calculated at 1.13, 1.58, 2.28, and 3.51 ppm,

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respectively. The proton signals for the methyl substituted benzylic group bearing to the imidazoleN3 were monitored at 2.17 and 2.23 ppm, which were assigned at 2.22 and 2.31 ppm in the theoretical spectrum, respectively. In addition, two doublets observed at 4.65 and 6.01 ppm (calculated at 5.65 ppm) is due to the diastereotopic methylene group of the benzylic substituent.

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The H atoms on the imidazole backbone carbon atoms were resonated at 6.13 and 6.54 ppm as a doublet, while the proton signals for the phospine ligands were detected at 7.32-7.36, 7.43-7.47 and

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7.53-7.55 ppm. These signals were theoretically found at 6.57, 8.95, 6.90-7.90, 7.60-9.10 and 7.639.90 ppm, respectively.

The carbon signals for the n-butyl bearing to the imidazole-N1 were observed at 13.6, 16.7, 31.8 and 50.7 ppm that have been calculated at 17.45, 25.61, 39.17 and 59.28 ppm, respectively. The carbon signals for the methyl substituted benzylic group bearing to the imidazole-N3 were recorded at 16.8, 17.1 and 20.2 ppm, while the carbon signals for the benzylic group were monitored at 128.5-128.6 ppm. These signals were observed computationally at 21.24, 21.99, 22.48 and 132.61-142.10 ppm, respectively. The peak at 51.1 ppm was attributed to the methylene carbon of the benzylic substituent, while the shifts corresponding the carbon signals of the phosphine ligands were appeared in the region of 133.1-136.3 ppm. The imidazole backbone carbon atoms were observed at 110.0 and 126.9 ppm experimentally, and theoretically computed at 127.94 and

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3.2.3. UV-vis spectroscopy

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UV-vis spectrum of 2, measured in dichloromethane solution, is presented in Fig. 4. The higher energy band around at 230 nm might results from π → π* transition states of the imidazolyl

transfer (MLCT).

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moiety. The transitions in the region of 279 nm can be ascribed to the metal-to-ligand charge-

Electronic absorption spectrum of 2 was computed by the TD-DFT method at the same level. To designate the major contributions of the transitions, GaussSum program [39] was used. According to the TD-DFT calculation, a single absorption was predicted at 291 nm with oscillator strength of 0.1398 [major contributions: H−8→L (63%) and H−17→L (11%); H : HOMO and L: LUMO]. Furthermore, the energy separation from the H to L is 3.58 eV, emphasizing the stable

Supplementary materials.

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4. Conclusions

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character of the complex. Contour plots of selected molecular orbitals of 2 are shown in the

In the present work, we describe the synthesis and characterization of a novel palladium(II) complex containing imidazol-2-ylidene and phosphine ligands. FT-IR, NMR and UV-vis. were

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used for the spectroscopic characterization of the complex, while its structure was determined by single-crystal X-ray diffraction technique. Further characterization of the complex was achieved by using quantum chemical methods at the B3LYP/SDD level. X-Ray study reveals that the coordination geometry around PdII is distorted square-planar geometry. For the Pd-NHC complex, the absence of any ν(C=N) vibration around 1600 cm−1 in the FT-IR spectrum, and the disappearing of the NCHN peak in the 1H NMR spectrum confirm the formation of the complex. Despite the differences commonly arising from dissimilar phases, it is found an acceptable correlation between the theoretical and experimental results.

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ACCEPTED MANUSCRIPT Appendix A. Supplementary data CCDC 1520121 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44

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1223 336033).

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ACCEPTED MANUSCRIPT Table 1. Crystal data and structure refinement parameters for 2.

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1520121 Green/prism [PdBr2(C19H28N2)(C18H15P)]·H2O 830.94 296(2) 0.71073 Mo Kα Triclinic P−1 (No. 2)

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11.0793(5), 12.6207(6), 13.7263(6) 98.907(3), 95.826(3), 100.272(4) 1849.26(15) 2 1.492 2.736 Integration 0.2191, 0.6689 804 0.60 × 0.30 × 0.15 STOE IPDS II/rotation (ω scan) −13 ≤ h ≤ 13, −15 ≤ k ≤ 13, −16 ≤ l ≤ 16 1.666 ≤ θ ≤ 25.048 21074 6531/5246 0.049 Full-matrix least-squares on F2 6531/549/516 1.054 R1 = 0.0750, wR2 = 0.2339 R1 = 0.0903, wR2 = 0.2472 1.52, −2.54

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CCDC deposition no. Color/shape Chemical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell parameters a, b, c (Å) α, β, γ (°) Volume (Å3) Z Dcalc (g/cm3) µ (mm−1) Absorption correction Tmin, Tmax F000 Crystal size (mm3) Diffractometer/measurement method Index ranges θ range for data collection (°) Reflections collected Independent/observed reflections Rint Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2σ(I)] R indices (all data) ∆ρmax, ∆ρmin (e/Å3)

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2.4783(12) 2.4324(17) 2.253(2) 1.979(8) 1.332(11) 1.365(13) 1.488(13) 1.337(11) 1.379(13) 1.492(12)

2.532 2.545 2.403 2.011 1.373 1.404 1.476 1.376 1.405 1.497

94.40(5) 176.40(6) 83.6(2) 88.26(7) 177.9(2) 93.8(2) 106.2(8)

91.81 177.92 84.26 89.49 174.18 94.57 105.23

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Parameters Bond lengths (Å) Pd1─Br1 Pd1─Br2 Pd1─P1 Pd1─C1 N1─C1 N1─C3 N1─C4 N2─C1 N2─C2 N2─C8 Bond angles (°) Br1─Pd1─Br2 Br1─Pd1─P1 Br1─Pd1─C1 Br2─Pd1─P1 Br2─Pd1─C1 P1─Pd1─C1 N1─C1─N2

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Table 2. Selected experimental and calculated geometric parameters for 2.

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Figure Captions Scheme 1. Synthesis of imidazolium salt and palladium/N-heterocyclic carbene complex.

Figure 1. (a) Molecular structure of 2, showing atom-numbering scheme and 30% displacement

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ellipsoids. H atoms have been omitted for clarity and only the major parts of the disordered fragments are shown. (b) A molecular fit of the experimental and calculated structures shown in black and red, respectively. H atoms have been omitted for clarity

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Figure 2. Experimental and theoretical IR spectrum of 2.

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Figure 3. 1H and 13C-NMR spectrum of 2.

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Figure 4. UV-vis spectrum of 2 in dichloromethane.

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Scheme 1

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Figure 2

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Highlights

A new palladium(II)-N-heterocyclic carbene complex was synthesized.

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The structure of the compound was determined by X-ray crystallography.

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IR, NMR and UV spectra were interpreted and assigned using DFT calculations.

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