International Journal of Inorganic Chemistry

International Journal of Inorganic Chemistry / 2010 / Article

Research Article | Open Access

Volume 2010 |Article ID 628639 | https://doi.org/10.1155/2010/628639

Neil A. Williams, Mahboub Merzouk, Peter B. Hitchcock, "Synthesis of Silver (I) Complexes of Iminoalkyl Functionalised 𝑁 -Heterocyclic Carbenes", International Journal of Inorganic Chemistry, vol. 2010, Article ID 628639, 5 pages, 2010. https://doi.org/10.1155/2010/628639

Synthesis of Silver (I) Complexes of Iminoalkyl Functionalised 𝑁 -Heterocyclic Carbenes

Academic Editor: W. T. Wong
Received07 Jan 2010
Accepted27 Mar 2010
Published10 Jun 2010

Abstract

A range of silver iminoalkyl imidazol-2-ylidene complexes have been isolated in good yield (50%–85%) and characterised by and NMR spectroscopy. A single crystal X-ray diffraction structure determination of 1- -(benzylhydrylidene-amino)-ethyl -benzyl imidazol-2-ylidene silver bromide indicated monodentate coordination of the ligand.

1. Introduction

N-heterocyclic carbenes such as imidazol-2-ylidenes have been established as an important class of ligands for transition metals. They typically have strong -donor properties but poor π-acceptor character and have been widely employed as alternative to phosphine ligands to stabilise transition metal complexes. Imidazol-2-ylidene ligands are conveniently prepared from the deprotonation of imidazolium salts. The use of Ag2O to deprotonate imidazolium salts, developed by Lin and coworkers [1] has been widely employed as it gives silver imidazol-2-ylidene complexes that are effective carbene transfer agents and are much more stable than the free carbene [2]. The method is particularly useful when the imidazolium salt contains a base sensitive functional group. A diverse range of transition metal imidazol-2-ylidene complexes have been prepared from silver imidazol-2-ylidene complexes [3, 4].

Metal carbene complexes have been widely employed to catalyse Heck reactions, cross-coupling (such as Suzuki-Miyaura coupling) [5], and alkene metathesis reactions [6]. More recently, chiral mixed-donor ligands such as containing an N-heterocyclic carbene donor group have been reported to achieve high enantioselectivity in allylic alkylation [7], hydrogenation [8], and conjugate addition [9, 10]. In these cases the ligand is chelated to the metal via the imine and carbene donor groups. Previously, we have reported the synthesis of chiral iminoalkyl imidazolium salts and the generation of palladium imidazol-2-ylidene catalysts via deprotonation with silver oxide and carbene transfer to palladium [11]. In the latter work, the silver intermediates were not isolated or characterised. Here we report the isolation and characterisation of a range of silver iminoalkyl imidazol-2-ylidene complexes. These are of interest as they may be used as chiral catalysts for silver catalysed enantioselective reactions [12].

2. Experimental

The single crystal X-ray diffraction data were recorded on a Nonius CCD Kappa diffractometer (graphite-monochromated Mo K radiation,  Å). Data were processed using program package WinGX, absorption correction MULTISCAN. Refinement was by using SHELXL-97. Crystallographic data of 2a has been deposited with Cambridge Crystallographic Data centre, Deposition no. CCDC 726775. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

2.1. Preparation 1- 2-(benzylhydrylidene-amino)-ethyl -3-ben-zylimidazol-2-ylidene of Silver (I) Bromide (2a)

Silver oxide (I) (0.45 g, 1.93 mmol) was added to a solution of imidazolium salt (1a) (1.50 g, 3.36 mmol) in dichloromethane (100 mL) in the presence 4 Å activated molecular sieves (4 g). The reaction mixture was protected from light and was refluxed for 2 days. The mixture was filtered through a layer of celite. The solvent was then removed under reduced pressure to give 2a as a white solid (1.47 g, 79%); mp 157.5–162.1 ; : 1623 cm−1; 1H NMR (DMSO-d6, 400 MHz) 3.63 ( , 2H, J = 5.3 Hz), 4.39 ( , 2H, J = 5.3 Hz), 5.26 (s, 2H), 6.79–6.86 (m, 2H), 7.15–7.60 (m, 15H,); 13C NMR (DMSO-d6, 100 MHz) 52.1 53.9 (2C), 121.8, 122.4, 127.0, 127.3, 127.9, 128.5, 130.1, 135.6, 137.1, 138.6, 168.7 (1C, C=N), 179.8 (1C, C-Ag); Anal. found: C 54.26%, H 4.00%, N 7.64%; (C25H23N3AgBr requires C 54.27%, H 4.19%, N 7.60%).

Complexes 2b-2h and 3a-3b were prepared according to the same procedure.

2.2. Compound 2b

White solid; yield: 0.21 g, 50%; 1H NMR (DMSO-d6, 400 MHz) 2.89 (dd, 1H, J = 8.8 Hz, J = 13.0 Hz), 2.95 (dd, 1H, J = 4.4 Hz, J = 13.0 Hz), 3.60–3.75 (m, 1H,), 4.31 (dd, 1H, J = 2.6 Hz, J = 13.3 Hz), 4.49 (dd, 1H, J = 9.9 Hz, J = 13.3 Hz), 5.05–5.25 (m, 2H), 5.66 (s, 1H,), 5.68 (s, 1H,), 6.90–7.52 (m, 20H); 13C NMR (DMSO-d6, 100 MHz) 49.4, 54.2, 56.4, 64.8,121.7, 122.9, 126.2, 126.4, 127.4, 127.6, 127.8, 128.0, 128.09, 128.13, 128.6, 129.7 (2C), 130.2, 132.6, 136.9, 137.9, 138.4, 168.4 (C=N), 179.8 (C-Ag); Anal. found: C 59.98%, H 4.18%, N 6.31% (C32H29N3AgBr requires C 59.74%, H 4.54%, N 6.53%).

2.3. Compound 2c

White solid; yield: 0.47 g, 84%; mp 85.5– ; 1H NMR (DMSO-d6, 400 MHz, ) 1.12 (d, 3H, J = 6.6 Hz), 3.57–3.69 (m, 1H) 4.17 (dd, 1H, J = 2.9 Hz, J = 13.2 Hz), 4.34 (dd, J = 9.5 Hz, J = 13.2 Hz), 5.05–5.40 (m, 2H), 6.54 (s, 1H), 6.56 (s, 1H,), 7.50–7.60 (m, 15H,); 13C NMR (DMSO-d6, 100 MHz, ) 19.2, 54.1, 57.5, 58.4, 121.8, 122.7, 126.7, 127.4, 127.9, 128.0, 128.1 128.46, 128.53, 130.2, 135.6, 137.1, 138.4, 167.4 (C=N), 179.6 (C-Ag).

2.4. Compound 2d

White solid; yield: 0.25 g, 68%; 1H NMR (DMSO-d6, 400 MHz) 0.63 (d, 3H, J = 6.4 Hz), 0.76 (d, 3H, J = 6.4 Hz), 1.37–1.59 (m, 3H), 3.55–3.68 (m, 1H), 4.22–4.50 (m, 2H), 5.07–5.27 (m, 2H), 6.39 (s, 2H), 7.09–7.55 (m, 15H); 13C NMR (DMSO-d6, 100 MHz,) 22.6, 23.0, 23.9, 42.8, 54.1, 56.4, 60.8, 121.7, 122.9, 126.9 (2C), 130.0 (2C), 127.4, 127.9, 128.0, 128.1, 128.2, 128.5, 135.5, 136.8, 138.6, 167.7 (C=N), 179.9 (C-Ag).

2.5. Compound 2e

White solid; yield: 0.34 g, 78%; 1H NMR (DMSO-d6, 400 MHz) 3.67 ( , 4H, J = 5.4 Hz), 4.45 (t, 4H, J = 5.4 Hz), 6.29 (s, 2H), 7.10–7.53 (m, 15H); 13C NMR (DMSO-d6, 100 MHz) 52.5, 53.9, 121.8, 123.4, 123.5, 126.9, 127.9, 128.0, 128.39, 128.43, 128.6 129.4, 129.5, 130.1, 135.6 (2C), 138.6 (2C), 139.6 (2C), 170.1 (C=N), 168.9 (C-Ag).

2.6. Compound 2f

White solid; yield: 0.16 g, 66%; 1H NMR (DMSO-d6, 400 MHz) 3.20–3.35 (m, 2H), 3.60–3.80 (m, 1H), 4.10–4.25 (m, 1H), 4.30–4.45 (m, 1H), 6.69 (s, 2H), 7.10–7.90 (m, 20H); 13C NMR (DMSO-d6, 100 MHz) 57.6 (2C), 58.4, 121.6, 123.5 (3C), 126.6, 127.9, 128.0, 128.3, 128.5, 129.6, 129.9, 130.1, 135.7 (2C), 138.5, 139.7, 167.4 (C=N), not vis. (1C, C-Ag).

2.7. Compound 2g

White solid 0.11 g, 91%; mp 85.0– ; 1H NMR (DMSO-d6, 400 MHz) 1.20 (d, 3H, J = 5.9 Hz), 3.68–3.85 (m, 1H), 4.17–4.33 (m, 1H), 4.40 (dd, 1H, J = 9.3 Hz, J = 13.4 Hz), 6.75 (s, 2H), 7.25–7.62 (m, 14H), 7.83 (s, 1H); 13C NMR (DMSO-d6, 100 MHz) 19.3 57.7, 58.5, 121.9, 123.5, 123.6, 126.6, 128.0, 128.1, 128.6, 129.7, 130.3, 135.7, 138.5, 139.6, 168.5, not vis. (1C, C-Ag).

2.8. Compound 2h

White solid; yield: 0.071 g, 60%; mp 68.2– ; 1H NMR (CDCl3, 400 MHz) 0.70 (d, 3H, J = 6.4 Hz), 0.82 (d, 3H, J = 6.4 Hz), 1.40–1.63 (m, 3H), 3.73–3.87 (m, 1H), 4.22–4.45 (m, 2H,), 6.57 (s, 1H), 6.59 (s, 1H), 7.10–7.65 (m, 15H); 13C NMR (CDCl3, 100 MHz) 22.8, 23.2, 24.6, 43.1, 57.7, 61.2, 121.0, 123.2, 123.6 (2C), 126.9 (2C), 128.1 (2C), 128.38 (2C), 128.41 (2C,), 128.7, 128.9, 129.8 (2C), 130.3, 135.9, 138.9, 139.6, 168.8 (C=N), 180.7 (C-Ag).

2.9. Compound 3a

White solid (0.27 g, 85%); 1H NMR (CDCl3, 400 MHz) 0.88 (d, 3H, J = 6.6 Hz), 0.93 (d, 3H, J = 6.6 Hz), 1.35–1.73 (m, 3H,), 3.63–3.72 (m, 1H), 4.22 (dd, 1H, J = 9.2 Hz, J = 13.6 Hz), 4.34 (dd, 1H, J = 3.7 Hz, J = 13.6 Hz), 5.20 (d, 2H, J = 9.2 Hz), 7.07–7.44 (m, 9H), 7.54–7.62 (m, 2H), 8.04 (s, 1H,), 10.19 (s, 1H); 13C NMR (CDCl3, 100 MHz) 21.6, 23.5, 24.4, 42.1, 55.6, 57.2, 70.1, 120.4, 122.9, 128.2 (2C), 128.5 (2C), 128.6 (2C), 129.1, 129.1 (2C), 131.2, 135.5, 137.4, 161.6 (1C, N=CH-C6H5), 181.2 (1C, C-Ag).

2.10. Compound 3b

White solid (0.064 g, 77%); 1H NMR (CD2Cl2, 400 MHz) 0.65 (d, 3H, J = 6.3 Hz), 0.72 (d, 3H, J = 6.3 Hz), 1.37–1.57 (m, 3H), 1.96 (s, 3H), 2.12 (s, 3H), 2.20 (s, 3H), 3.54–3.77 (m, 6H), 3.84–3.94 (m, 1H), 6.81 (s, 1H), 6.87 (s, 1H), 7.08 (s, 1H), 7.10 (s, 1H), 7.20–7.36 (m, 3H), 7.37–7.47 (m, 3H), 7.55–7.65 (m, 2H); 13C NMR (CD2Cl2, 100 MHz) 17.7, 17.8, 20.8, 22.77, 22.83, 24.8, 43.2, 50.6, 51.2, 56.9, 58.9, 127.7 (2C), 128.2 (2C), 128.7 (4C), 129.5 (3C), 130.3 (1C), 135.7, 135.8, 135.9, 136.6, 138.6, 139.3, 167.9 (C=N), not vis. (1C, C-Ag).

3. Results and Discussion

The synthesis of iminoalkyl imidazolium salts 1a-1h has been previously reported. The imidazolium salt 1a was deprotonated by silver (I) oxide in dry CH2Cl2 under reflux, in the presence of activated 4 Å molecular sieves [13, 14]. The reaction was carried out in the dark to prevent photodecomposition of the silver complex. After 2 days, the achiral silver imidazol-2-ylidene complex 2a was obtained in good yield (79%). The method was subsequently extended to the preparation of chiral silver imidazol-2-ylidene complexes 2b-2h from chiral iminoalkyl N-benzyl, N-phenyl imidazolium bromide salts 1b-1h (Figure 1). The yields ranged from 45% to 91% (Table 1). The steric bulk of the R1 substituent appears to have a significant effect on the yield of the reaction, for R1 = H or Me yields are in the range 78%–91% whilst for R1 = Bn, they are in the range 50%–68%. This is consistent with the observation of Lin et al. that yields of silver complex formation are dependent on steric hindrance around the imidazolium salt [15].


R1R2Product yield

1aHBn2a 79%
1bBnBn2b 50%
1cMeBn2c 84%
1d Bn2d 68%
1eHPh2e 78%
1fBnPh2f 66%
1gMePh2g 91%
1h Ph2h 60%

The silver carbene transfer agent (3a) was prepared as the reduced steric bulk of a benzylideneamino-donor group has been linked to improved selectivity in palladium catalysed allylic alkylation [7, 16]. In addition, the imidazolin-2-ylidene silver complex (3b) is of interest as the unsaturated imidazolin-2-ylidene is considered a stronger -donor ligand and hence could enable greater electronic differentiation of a coordinated allyl group when 3b is employed as a carbene transfer agent in palladium catalysed allylic alkylation. This should result in greater enantioselectivity (Figure 2).

The deprotonation of imidazolium salts 1a-1h was evidenced by the disappearance of imidazolium C1 proton singlet in the 1H NMR spectra between 9 and 10 ppm. The coordination of the carbene carbon to the Ag(I) centre was characterised by 13C NMR. 13C NMR analysis confirmed conservation of the imine double bond, with the quaternary carbon signals observed between 168.5 and 170.1 ppm. The characteristic C1 carbon signals of 2a-2h were generally shifted up field from c.a. 136 ppm for imidazolium compounds, to c.a. 180 ppm for the carbene with no measurable coupling with silver. The carbene carbon signals were generally not detected for chiral silver iminoalkyl N-phenylimidazol-2-ylidene complexes 2e-2h. The lack of Ag(I)-13C coupling is indicative of labile nature in silver complexes [17], Danopoulos [13] and Douthwaite [16] reported similar structural and spectroscopic observations for chiral silver iminoalkyl N-substituted imidazol-2-ylidene complexes. Diastereotopic methyl groups in compounds derived from leucine (2d, 2h) gave a rise to doublet of doublets in the 1H NMR spectra.

The deprotonation of N-functionalised imidazolium halides and their coordination to Ag(I) can lead to a variety of structures, including: ion pairs, mononuclear neutral complexes, halide bridged complexes, bridged tetranuclear complexes, or bis carbene complexes. Single crystals of 2a were grown by slow diffusion of diethyl ether into a saturated equivolume solution of CH2Cl2/Et2O. The single crystal X-ray diffraction structure determination indicated compound 2a was a mononuclear neutral complex with monodentate coordination of the imino-alkyl)imidazol-2-ylidene ligand (Table 2, Figure 3). However, there is evidence of weak interactions between monomers across an inversion centre (Figure 4).


2a
Empirical formulaC25 H23 Ag Br N3
Formula weight553.24
Temperature173(2) K
Wavelength0.71073 Å
Crystal systemTriclinic
Space groupP (No. 2)
Unit cell dimensionsa = 8.1675(2) Å = 92.909(2)°
b = 8.6875(3) Å = 92.682(2)°
c = 16.5972(6) Å = 106.820(2)°
Volume1123.49(6) Å3
Z2
Density (calculated)1.64 Mg/m3
Absorption coefficient2.69 mm−1
F(000)552
Crystal size0.25 0.20 0.10 mm3
Theta range for data collection3.89 to 25.03°.
Index ranges−9 8, −10 10, −19 l 19
Reflections collected16098
Independent reflections3930 [R(int) = 0.043]
Reflections with I 2sigma(I)3495
Completeness to theta = 25.03°98.9%
Refinement methodFull-matrix least-squares on F2
Data/restraints/parameters3930/0/363
Goodness-of-fit on F21.071
Final R indices [I 2sigma(I)]R1 = 0.027, wR2 = 0.063
R indices (all data)R1 = 0.033, wR2 = 0.066
Largest diff. peak and hole0.42 and −0.53 


Angles )Distances (Å)

C(1)-Ag-Br154.76(8)Ag-C(1)2.015(3)
C(1)-Ag- 109.20(7)Ag-Br2.4969(3)
Br-Ag- 95.823(11) Ag- 3.0549(4)
Ag-Br- 84.177(11)N(1)-C(1)1.353(4)
C(1)-N(1)-C(2)111.1(2)N(1)-C(2)1.382(4)
C(1)-N(1)-C(4)124.7(2)N(1)-C(4)1.460(4)
C(2)-N(1)-C(4)124.0(3)N(2)-C(1)1.350(4)
C(1)-N(2)-C(3)111.0(2)N(2)-C(3)1.392(4)
C(1)-N(2)-C(11)125.1(2)N(2)-C(11)1.462(4)
N(2)-C(1)-Ag129.5(2)N(3)-C(13)1.278(4)
N(1)-C(1)-Ag126.19(19)

X-ray crystallographic data of 2a indicates that the distance between the imine nitrogen (N(3)) and the Ag atom (3.402 Å) is significantly longer than that between the carbene carbon (C(1)) and Ag (2.105 Å) (Table 3). Although the imine nitrogen is directed toward the Ag atom, the two atoms are too far away to significantly interact. The structure of 2a with no imine N(3)-Ag coordination, is in agreement with the general trend of silver to form linear and low coordinate complexes [9]. The orientation of the imine moiety away from the metal centre is assumed to be due to steric hindrance.

4. Conclusion

A range of silver iminoalkyl imidazole-2-ylidene complexes have been synthesised and characterised. The single crystal X-ray diffraction study of 2a reveals a neutral mononuclear structure in which the imine donor group is not coordinated to the silver. Further work will be aimed at synthesising palladium iminoalkyl imidazole-2-ylidene complexes using these new compounds as carbene transfer agents.

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Copyright © 2010 Neil A. Williams et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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