Synthesis, characterization and catalytic properties of cationic N-heterocyclic carbene silver complexes

Three new dibenzimidazolium salts bridged by 2-methylenepropane-1,3-diyl group were synthesized. Their dinuclear N-heterocyclic carbene Ag(I) complexes were prepared by the reactions of these salts with Ag2O. The structures of the synthesized compounds were defined by nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FT-IR), elemental analysis, and LC-MSMS (for complexes) techniques. Stability of the silver complexes was confirmed by 1H NMR spectroscopy. Catalytic activities of Ag(I) compounds were tested for three-component coupling reaction of some aldehydes, amines, and phenylacetylene.


General procedure for A 3 -coupling reaction
NHC silver complex (3 mol%), aldehyde (1 mmol), amine (1.2 mmol) and phenylacetylene (168 mL, 1.5 mmol) were placed in a test tube with screw cap. The mixture was stirred at 80 °C for 18 h in dark medium. After cooling to room temperature, Et 2 O and MgSO 4 were added to the mixture. Filtration was done and Et 2 O was removed from the filtrate. Related propargylamine was obtained in pure form by column chromatography.

Results and discussion
Synthesis methods of dibenzimidazolium salts and dinuclear NHC-Ag(I) complexes are in Figure 1. Dibenzimidazole compound 1 was prepared by the reaction of two equivalents of benzimidazole and one equivalent of 1,1-bis(chloromethyl) ethylene by using NaH base in THF [15]. Quaternization of 1 with two equivalents of substituted benzyl bromides afforded the dibenzimidazolium dibromide salts 2a-2c. Transition metal complexes of N-benzylic benzimidazol-2-ylidene are of importance in organometallic chemistry, and there have been various studies on these compounds [50,51]. Dinuclear cationic NHC silver hexafluorophosphate complexes 3a-3c were procured by the reactions of 2a-2c with two equivalents of Ag 2 O and then salt metathesis reactions of bromide complexes with NH 4 PF 6 in methanol medium.
The results of elemental analysis confirm the expected formulations. While the signals of the acidic C2 protons of 2a-2c appear at 9.78-10.17 ppm in the 1 H NMR spectra, these signals resonating in a low field do not exist in those of the Ag(I) complexes. This observation points out formation of a NHC metal complex as previously reported [52]. The absence of carbene carbon signals in the 13 C NMR spectra of the silver complexes may be attributed to the fluxional behaviour of the NHC silver complexes [15,53]. IR peaks concerning the stretching vibrations of -C=N-groups for the salts are present at 1554-1562 cm -1 . Whereas, these values decrease to 1400 cm -1 for the metal complexes. These data are compatible with the literature [54]. The stretching frequencies related to P-F bond for the complexes appear in the range of 834-840 cm -1 . The sharp band observed in 3390 cm -1 for 2c is assigned to the n(O-H) of hydrated water. TGA/DTA analysis supports that this compound is a hydrate molecule. Unfortunately, single crystals required for XRD analysis were not obtained despite all efforts. The molecular weights of 2a-2c and 3a-3c were proved by LC-MSMS spectroscopic analysis. [M-Br] + and [M-2Br] + peaks are observed for 2a-2c. There are [M-PF 6 ] + signals at 1409.35, 1537.40, and 1746.60, respectively in the mass spectra of 3a-3c. Mass data affirm dinuclear [Ag 2 (L) 2 ](PF 6 ) 2 formulation. It is believed that the cationic silver complexes 3a-3c isolated as hexafluorophosphate salts do not form polymers.
Stabilities of the silver complexes in solution were studied by 1 H NMR spectroscopy for a period of ten days. 1 H NMR spectra were recorded on the day their dmso-d 6 solutions were prepared and after one, four, seven, and ten days. The spectra for stability testing are shown in Figures S7-S9 in supporting information. The results evidently point out that the complexes are stable in solution even after ten days.
While A 3 -coupling reactions have been catalyzed by many transition metal ions, the number of studies on using NHC silver complexes is limited. In this work, catalytic activities of Ag(I) compounds were studied for three-component coupling reaction of some aldehydes, amines and phenylacetylene. The reaction of p-formaldehyde, diethylamine, and phenylacetylene was carried out using different solvents and different amount of catalyst 3a (Table 1, entries 1-8). The results showed that solvent free medium and increased amount of catalyst raised the activity. N,N-diethyl-3-phenylprop-2-yn-1-amine was obtained in 78% yield with 3 mol% catalyst ( Table 1, entry 3). When the same reaction was performed by using piperidine instead of diethylamine, 51%-52% yields were obtained (Table 1, entries [11][12][13]. These data are comparable with the literature [25,30]. It was understood that each of the three complexes exhibited similar activities in both reactions examined. In our previous work, 59% yield was obtained for this reaction with a similar complex containing 3-methoxybenzyl group on NHC ligand [15]. The presence of a larger number of alkyl groups on the benzyl substituent causes a decrease in the catalytic activity. 3,5-dimethylbenzyl, 3,5-dimethoxybenzyl and 3,5-di-tert-butylbenzyl substituents on NHC ligands did not affect the catalytic behaviours of the catalysts. This consequence is consistent with the literature [23,55]. In the case of using aliphatic aldehydes and the amines, such as diethylamine and piperidine, the propargylamine compounds were gained in moderate yields (Table 1, entries 14,15). Using morpholine caused low yields (Table 1, entries [16][17][18]. When the results are compared, it is seen that the prepared complexes show less activity than monomeric NHC silver complexes possibly because of steric hindrance. This result is consistent with the literature [31]. Based on the literature [56][57][58], a mechanism can be proposed ( Figure 2). Firstly, C-H activation of phenylacetylene forms a silver-acetylide complex and acidic proton. The formation of this complex may proceed through a p-complex. Then, in situ formed silver acetylide reacts with iminium cation to give propargylamine and the catalyst. • 1 H NMR spectra of the silver complexes for the stability tests S8 • FT-IR spectra of the synthesized compounds S11 • LC-MSMS spectra of the synthesized compounds S14 • TGA/DTA graph of 2c S17 • Characterization data for the catalytic products S18 • 1 H NMR and 13 C NMR spectra of the catalytic products S22 • 1 H NMR spectra of the silver complexes for the stability tests      ppm. 13