supplementary materials

Recently, N-heterocyclic carbene compounds have become universal ligands in inorganic and organometallic chemistry (Hahn, 2006; Kirmse, 2004). The role of N-heterocyclic carbenes as alternative compounds to tertiary phosphines in homogeneous catalysis is now well established (Scott & Nolan, 2005; Herrmann, 2002). At the same time, it has become increasingly apparent that carbenes (and/or their imidazolium precursors) are quite susceptible to metal induced bond activation reactions, which may be crucial to both catalytic activity and possible deactivation pathways. The chemistry of N-heterocyclic carbenes has become a major area of research as these stable carbenes (Nair et al., 2004) have proven to be outstanding ligands for transition metals (Cavell & Guinness, 2004), as well as potent nucleophilic organic catalysts (Rangits & Kollar, 2006). In this field, the conjugate acids play a very important role. Indeed, they are by far the most frequently used precursors of N-heterocyclic carbenes, and oxidative addition of the CH bond to transition metal centers can even directly generate the carbine complexes (Herrmann, 2002). Moreover, the conjugate acids of N-heterocyclic carbenes (H⁺) have found numerous applications as ionic liquids (Richmond, 2000), which are important components of green chemistry.

We have previously reported the use of in situ formed imidazolidin-2-ylidene, tetrahydropyrimidin-2-ylidene and tetrahydrodiazepin-2-ylidene palladium (II) systems that exhibit high activity for various coupling reactions of aryl bromides and aryl chlorides (Özdemir et al., 2005a, 2005b). In order to obtain more stable, efficient and active systems, we have also investigated benzo-annelated derivatives (Özdemir et al., 2004). Recently our group reported that novel complexes of rhodium(I) 1,3-dialkyimidazolidin-2-ylidenes gave secondary alcohols in good yields by the addition of phenylboronic acid to aldehydes (Özdemir et al., 2005c). In recent years, we also investigated the crystal structures of new N-heterocyclic carbene derivatives (Arslan et al., 2004a, 2004b, 2005a, 2005b, 2007a, 2007b, 2007c).

Due to our interest in preparing stable ligands for selected catalytic processes, we prepared 1-(2,4,6-trimethylbenzyl)-3-(2-phenylbenzyl)-imidazolidinium bromide, (I) as a carbene precursor. The title compound was purified by re-crystallization from an ethanol:diethylether mixture (2:1) and characterized by elemental analysis, ¹H and ¹³C-NMR and IR spectroscopy (Özdemir et al. 2005d). The analytical and spectroscopic data are consistent with the proposed structure given in Scheme1. The structure consists of a 1-(2,4,6-trimethylbenzyl)-3-(2-phenylbenzyl)-imidazolidinium cation and a Br^-anion. The structure of (I) is shown in Fig. 1.

The crystallographic data for (I) will provide valuable information for assessing its electronic conjugation properties and a possible intra- and inter-molecular hydrogen bond, which may be correlated with the reactivity of the carben carbonatom. N—C_carben—N bond angles and C_carben—N bond lengths are related with the proximity of the proton in the carben compounds (Arduengo et al., 1995b). Though N—C_carben—N bond angles are observed as 101.4 (2) ° and 108.7 (4) ° for 1,3-dimesityl-2,3-dihydro-1H-imidazole-2-ylidene and 1,3-dimesityl-1H-imidazol-3-ium, respectively, (Arduengo et al., 1995b), N—C_carben—N bond angle is 113.0 (4) ° in the title compound. According to this result, the larger N—C_carben—N bond angle is more relaxed than the other carben derivatives. In addition, C3—H3···Br1 intermolecular hydrogen bond and C_carben—N bond lengths have confirmed this result (Arduengo et al., 1995a, 1995b). The same geometric behaviors are observed for 1,3-dimesitylimidazolidinium tetrachloridogold(III) and 1,3-dimesityl-4,5-dihydro-1H-imidazol-3-ium chloride where the N—C_carben—N bond angles are 113.9 (6) ° and 113.1 (4) °, respectively (Hagos et al., 2008; Arduengo et al., 1995b).

The C_carben—N bond distances in the imidazolidine ring are not the expected single and double bond lengths as reported in other imidazolidine derivatives (Arduengo et al., 1995a, 1995b). Although the C_carben—N bond distances (N1—C3 = 1.313 (6) Å and N2—C3 = 1.305 (6) Å) in the title compound are shorter than the average C_carben—N bond length for free carbene derivatives which have imidazolidine rings is 1.360 Å. The C—N bond lengths have intermediate values between the single and double C—N bond lengths (Allen et al., 1987) and agree with C_carben—N bond lengths values of 1,3-dimesitylimidazolidinium tetrachloridogold(III) and 1,3-dimesityl-4,5-dihydro-1H-imidazol-3-ium chloride (1.294 (7) and 1.320 (8) Å) (Hagos et al., 2008), (1.327 (5) and 1.310 (5) Å) (Arduengo et al., 1995b), respectively. Also, the sum of the bond angles around N1 (360.0 °) and N2 (360.0 °) indicate sp² hybridization. These results can be explained by existence of resonance in this part of the molecule. All the other bond lengths are in normal ranges (Allen et al., 1987).

The deviations from planarity of imidazolidine ring are C1 0.056 (5), C2 0.051 (5), N2 0.027 (4), C3 0.008 (5) and N1 0.039 (4) Å. The puckering parameters (Cremer & Pople, 1975) and the smallest displacement asymmetry parameters (Nardelli, 1983) for the imidazolidine ringare q₂ = 0.090 (5) Å, φ₂= 46 (3)° and ΔC₂(C3) = 13.0 (5), ΔC_s(C3) = 5.1 (5). According to these results, the imidazolidine ring adopts a twisted conformation with a pseudo-twofold axis passing through C3 and the C1—C2 bond.

The crystal packing is shown in Fig. 2. The crystal packing is dominated by intermolecular C3—H3···Br1 (x,1/2 - y,1/2 + z) hydrogen bonds, with H···Br = 2.52 Å and C—H···Br 170 ^o (Table 1).