Ag(I) NHC Complexes: General Synthetic Methods and Characterization

1.4.1.1 Synthesis

Generally, there are three main routes for the synthesis of silver NHCs.^{[9a, 123]} The first one includes preparation of a free NHC which is then reacted with a silver salt (route a, Scheme 1.4.1). This method was firstly applied by Arduengo in 1993, who treated ^HIMes with Ag(O3SCF3) to obtain the corresponding bis(NHC) complex.^[126] While this method can be utilized for a plethora of compounds the conditions required for the generation of free carbenes can facilitate unwanted deprotonation of other acidic protons in the ligand framework, therefore causing its decomposition.

An alternative method for the preparation of Ag(I) NHC compounds is the deprotonation of the respective azolium salts in the presence of silver salt using an external base (route b, Scheme 1.4.1). Lin and Wang described this method by using basic phase transfer catalyst in the presence of AgBr and a benzimidazolium salt.^[127] Unfortunately, this procedure did not lead to the successful synthesis in other cases.^[128]

In the same study Lin and Wang also introduced the third possible way for generation of Ag(I) NHCs by reporting the reaction of a benzimidazolium salt of the mild internal base Ag2O in

CH2Cl2 (route c, Scheme 1.4.1).^[127] Hereby they paved the way for the synthesis of numerous compounds obtained by this facile and nowadays the most widely applied synthetic protocol.

Scheme 1.4.1. Synthetic methods generally applied for the generation of Ag(I) NHC complexes.

The advantages of the Ag2O technique are enormous and can be summarize as follows^[123]:

─ Reactions can be carried out under aerobic conditions

─ No additional base is required

─ No dried or degassed solvents are necessary

─ Ag2O is very mild base, which deprotonates almost exclusively the C2 position of the imidazolium salts and ignores other acidic positions

Also other silver bases such as Ag(OAc) or Ag2CO3 can employed, however, to some extent they require longer reaction times.^[128-129] Generally, bis(imidazolium) salts react slower with Ag2O than mono(imidazolium) salts.^{[9a, 128]} Also the reactions with bulkier ligands proceed slower than with less sterically demanding substituents; in some cases with especially high steric hindrance of the ligand higher reaction temperatures are required instead of the “normal”

procedure at RT^[128]. Moreover, functional groups have been reported to affect the reactivity via this method as well.^{[33, 130]}

The fact that silver NHC complexes can be synthesized in water using Ag2O route suggests that the deprotonation and formation of carbene complexes proceed via a concerted mechanism.^[9a] More detailed DFT calculations in order to understand the mechanism of the reaction of Ag2O with N,N’-dimethyl imidazolium iodide in CH2Cl2 have been conducted by Peris, who has shown that the most favourable pathway for generation of Ag(I)-NHCs consists of the deprotonation and metalation of two imidazolium moieties in two steps (Scheme 1.4.2).^[131] The first deprotonation of the C2 position of one of the imidazolium cations is driven by the difference in pKa value between [NHC–H]⁺ and [Ag2–OH]⁺ acids. Additionally, the formation of a strong Ag–C bond contributes to overall stabilization of the first step. Notably, the second imidazolium salt assists the formation of the first silver NHC by C2–H∙∙∙OAg

hydrogen bond. In the second step the driving force is almost entirely the formation of another Ag–C bond, since the pKa of [AgOH2]⁺ is very close to that of the imidazolium cation. Therefore, the deprotonation of the second [NHC–H]⁺ by AgOH is an almost thermoneutral equilibrium that does not contribute to the overall thermodynamic stabilization of the system (>70 kcal∙mol

-1). Notably, all of the steps in this mechanism have also very low Gibbs activation energies.

Scheme 1.4.2. The most favourable mechanism of the formation of Ag(I) NHC compounds in CH2Cl2

obtained by DFT calculations.

The Peris’s results may explain some experimental observations described above, e. g. the slower kinetics of the deprotonation of poly(imidazolium) salts.^[131] Because the formation of the first Ag NHC is assisted by a second imidazolium salt, topological restrictions on the poly(imidazolium) compounds may slow down the reaction.

1.4.1.2 Structural Trends and Properties

The structures of Ag(I) NHC complexes in the solid state are very diverse and quite complex, especially when halide anions are involved.^[9a] The bonding motifs can be roughly characterised in seven categories, which are depicted in Figure 1.4.1.

Reliably predicting the solid state structure of silver NHCs containing halide anions is often very difficult, since the structure is affected by several parameters such as the nature of halide, sterics and flexibility of the ligand, solvent, silver‒halide and silver‒silver interactions.^[9a]

Noteworthy, argentophilic interactions often play an important role of many diverse architectures in the solid state. For the Ag(I) NHC complexes with non-coordinating anions the structural outcome is much easier to predict: in most cases a bis(NHC) complex with a non-coordinating anion for charge balance is obtained.

The silver‒carbene bond distances in characterised solid state structures range from roughly 2.06 till 2.52 Å, for bis(carbene) complexes the range is even more narrow (2.06-2.12 Å).^[9a]

The C–Ag–C bond angles range from a significant deviation (≈160°) till almost perfectly linear compounds (≈170°).

Figure 1.4.1. Bonding motifs of silver(I) NHC complexes (X = halide, Y = non-coordinating anion).

In the solution the most convenient way of monitoring the formation of silver NHC complexes is by recording NMR spectra, especially ¹³C NMR. The resonance values for CC in Ag(I) NHCs span quite a long range (213-163 ppm). For dinuclear bis(NHC) complexes the range for the chemical shift is much more narrow and the carbene resonance appears normally at around 180 ppm. The coupling constants for CC to two naturally abundant silver isotopes, ¹⁰⁷Ag (51.839 %) and ¹⁰⁹Ag (48.161 %), both spin ½, amount to 180-234 Hz and 204-270 Hz respectively. However, the most compounds show no coupling pattern for the carbene resonance. There is also a significant number of Ag(I) NHC with no observable NCN resonance at all. In some cases, the appearance of CC resonance was found to be dependent on the concentration of the sample; sharper singlets were observed in diluted samples.^[128] Lin and others speculated that a fast fluxional behaviour on the NMR time scale is the reason for the variance in the appearance of carbene resonances.[127, 132] Therefore, according to this assumption, on NMR time scale static complexes show very pronounced coupling pattern to both silver isotopes which often extends to the backbone of the NHC ring. With increased fluxionally the signals broaden, eventually coalesce and finally become a sharp singlet.

Moreover, such behaviour should be temperature dependent. Indeed, Bergbreiter studied the influence of the temperature on ¹³CC resonances by conducting VT NMR experiments with ¹³C labelled silver NHC compounds.^[132a] By analysing the data he postulated an equilibrium between neutral [(NHC)AgX] 110b and cationic [(NHC)2Ag]AgX2 110a species (Scheme 1.4.3).

Further evidence from DFT calculations and studies on ligand exchange rates with different halides supported the mechanism of the ligand exchange assisted by μ-halide and μ-NHC intermediates previously proposed by Lin (Scheme 1.4.3, a).^[127] Interestingly, Bergbreiter and

co-workers were also able to obtain a sample of analogue cationic bis(NHC) complex with iodide as counter anion (111, Scheme 1.4.3, b). For this compound they observed no dynamic ligand exchange processes in the solution in the whole temperature range from –85 °C till +20 °C.^[132a] Only the addition of 110b promoted the dynamic behaviour, which also correlates to the concentration of mono(NHC) silver halide 110b.

Scheme 1.4.3. Dynamic equilibrium on the ligand exchange for Ag NHC complexes containing halide anions.

The reason for the absence of carbene resonances for a significant number of compounds is however still unclear, but a fast dynamic behaviour combined with the poor relaxation of quaternary NCN carbon could account for this fact.^[9a]

Theoretical calculations on Group 11 metal NHC complexes support experimental observations that the bond strength of M‒C bond generally follows an order: Au>Cu>Ag.^[133]

Nevertheless, the bonds in Ag‒NHCs are quite strong. Frenking and co-workers analysed the bonding orbitals in Ag(I) and postulated a hybridization of filled dz2 and s orbitals caused by Coulombic repulsion form the lone pair of the NHC.^[133a] The same group also suggested that M‒C bonds for coinage metals are mostly ionic in nature with non-negligible covalent interactions.^[133b] Furthermore, by analysing both silver NHC halide and silver bis(NHC) complexes they followed that stabilization energy from the orbital interactions can comprise up to 30 % of π interactions suggesting quite a high degree of π-backbonding. Meyer and colleagues also confirmed these calculation by conducting theoretical studies on a series of complexes [(TIME^Me)2M3](PF6)3 (M = Ag, Cu, Au; TIME^Me = [1,1,1-tris(3-methylimidazolium-1-yl)methyl]ethane).^[133c]