From Imidazole toward Imidazolium Salts and N-Heterocyclic Carbene Ligands: Electronic and Geometrical Redistribution.

Investigations dealing with N-heterocyclic carbenes and their derivatives are usually centered on the influence that they exert by acting as catalysts, ionic liquids, or metallodrugs and consequently on their capabilities to tune the properties and reactivity of these systems. In this context, we aimed to focus on the internal molecular changes undergone by imidazole derivatives, from electronic and geometrical points of view. This work represents an empirical evidence of the molecular modifications that an imidazole skeleton undergoes upon protonation, alkylation, and metalation.

Introduction

N-Heterocyclic carbenes (NHCs) have emerged along the last quarter century as versatile tools with numerous applications that have made a considerable impact in chemistry, physics, and material science.[1−6] There is a wide variety of symmetric and asymmetric NHC ligands bearing protons, alkyl, or aryl substituents at the nitrogen atoms. Among other uses, NHCs are excellent ligands capable of stabilizing alkali, alkaline earth, transition, and inner transition metal complexes.[7] Their strong electron donor ability modifies the electronic properties of the metals, including metallophilicity.[8,9] This enhancement of electron density is systematically exploited in homogeneous and heterogeneous catalysis[10,11] and in the design of potential antitumor metallodrugs.[12] NHCs also exhibit an interesting main-group reaction chemistry that involves, for example, the stabilization of E–E multiple bonds, activation of E–E bonds, ring expansion reactions, and so forth.[13] In addition, NHCs can be deposited on surfaces as anchoring or mobility units;[14] they have also been applied directly as organocatalysts, facilitating the synthesis of sophisticated organic synthons, molecules, and metal-free polymers.[15,16]

Given this background, it is evident that the importance of imidazoles and their cationic imidazolium derivatives results to a large extent from the fact that these heterocyclic rings are reactants in the synthesis of NHCs. However, imidazoles and imidazolium salts are important in their own right. Thus, imidazole rings are part of many natural products, and this building block is present in significant biological bases and in drugs of physiological activity. Likewise, imidazolium derivatives form ionic liquids with a variety of substituents and anions, which are commonly considered as green solvents and utilized for a wide range of applications.[17]

It is clear that the many chemical facets of imidazole and imidazolium salts make them important and ubiquitous compounds. It is, therefore, quite surprising that there is a lack of studies dealing with the structural and spectroscopic characteristics of imidazole/imidazolium systems, in a systematic manner, emphasizing, for example, the changes that the aromatic N-heterocyclic ring may undergo upon protonation or alkylation to form the imidazolium precursors. These kinds of comparative studies are usually biased because imidazole/imidazolium systems are often regarded as mere intermediates in the pathway toward the synthesis of NHCs.

In this work, we have focused on exploring this last topic, as detailed studies of this kind are scarce. Herein, we, therefore, report a thorough study of the structural, spectroscopic, and electronic properties of a stepwise synthesized imidazole and of two imidazolium derivatives (Scheme ). In particular, our synthetic approach includes: (i) a neutral imidazole derivative, 1-(2-phenoxyethyl)imidazole (1a, PhOEtIm); (ii) its protonated R,H-NHC form, 1-(2-phenoxyethyl)imidazolium tetrafluoroborate (1b, [HPhOEtIm]BF4); (iii) the corresponding bisNHC salt, methylenebis(N-2-phenoxyethyl)imidazolium diiodide (2, [H2bisPhOEtIm]I2); and (iv) the palladium complex, [Pd(bisPhOEtIm)I2] (3), which incorporates a methylenebis(N-2-phenoxyethyl)imidazol-2-ylidene unit (bisPhOEtIm) as the chelating ligand.

Scheme 1

Formation of Compounds 1a, 1b, 2, and 3

Results and Discussion

PhOEtIm (1a) and [HPhOEtIm]BF4 (1b)

Synthesis of 1-(2-phenoxyethyl)imidazole (PhOEtIm) (1a) was carried out starting from imidazole, which was deprotonated in refluxing acetonitrile with excess NaOH. After treatment with 2-phenoxyethylbromide, followed by separation and washing routines, PhOEtIm was obtained as a white precipitate and recrystallized. The 1H NMR spectrum of 1a (r.t., (CD3)2CO) shows the H2 proton signal as a singlet (7.63 ppm) and the H5 and H4 protons as pseudotriplets centered at 7.28 and 7.19 ppm, respectively. The phenyl ring protons are observed as multiplets (centered at 7.28 and 6.94 ppm), and the 1H signals assigned to the N—CH2 and CH2—O resonances appear as multiplets at 4.44 and 4.29 ppm, respectively. A complete NMR characterization (1H NMR, 13C{1H}-APT NMR, 1H—13C heteronuclear single quantum correlation (HSQC)/heteronuclear multiple bond correlation (HMBC)) is provided in the Supporting Information.

A view of compound 1a is given in Figure , showing the imidazole ring N-connected to the 2-phenoxyethyl unit. Table lists bond lengths and angles, determined by X-ray crystallography, for compound 1a, together with compounds 1b and 2, and the density functional theory (DFT)-calculated data for the optimized structures, Ia, Ib, and II. It should be noted that the hydrogen atoms in 1a were observed in the difference Fourier map. Both imidazole and phenyl rings form a dihedral angle of 56.82(9)°, whereas the N3—C6—C7—O8 torsion angle was found to be 63.8(3)°. In the crystal, there are no strong intermolecular interactions. The C2(H)···N1′ distance at 3.364(4) Å is the shortest intramolecular contact within the crystal lattice; but this appears to be too long to be regarded as even a weak hydrogen bonding interaction between two neighboring 1a molecules.

Figure 1

ORTEP-type representation of PhOEtIm (1a).

Table 1

Selected Interatomic Distances [Å] and Angles [deg] for Compounds 1a, 1b, and 2 and for Their DFT-Optimized Models Ia, Ib, and II

	1a/Ia	1b/Ib	2/II
N1—C2	1.316(4)/1.306	1.326(3)/1.315	1.348(2)/1.323
C2—N3	1.356(3)/1.340	1.328(2)/1.317	1.327(2)/1.314
N3—C4	1.376(3)/1.357	1.377(2)/1.361	1.378(2)/1.362
C4—C5	1.355(4)/1.361	1.340(3)/1.351	1.348(3)/1.348
C5—N1	1.383(3)/1.358	1.373(3)/1.355	1.379(3)/1.362
C2—N1—C5	104.6(2)/104.7	109.15(18)/109.87	109.13(16)/109.31
N1—C2—N3	112.3(2)/112.4	107.93(17)/107.77	107.27(17)/107.92
C2—N3—C4	106.7(2)/107.1	108.82(17)/109.18	109.56(16)/109.33
N3—C4—C5	105.9(2)/105.3	107.10(18)/106.81	107.21(17)/106.47
N1—C5—C4	110.6(2)/110.6	106.99(18)/106.36	106.82(17)/106.97

Protonation of PhOEtIm was achieved by stepwise addition of HNO3 to an aqueous solution of 1a. 1H NMR monitoring experiments in deuterated water revealed a pD-dependent shift of the proton signals upon the stepwise protonation (Supporting Information). The pKa value of the N1 site in the PhOEtIm/HPhOEtIm+ equilibrium was found to be 6.56 ± 0.03 (value converted to H2O).[18] For comparison, we also measured the acid dissociation constant of the related compound, 1-methylimidazole, obtaining a pKa value of 7.18 ± 0.02 in H2O. This value is similar to previously reported approximations.[19] Thus, as expected, the substituents have an evident influence on the acidity constant of imidazoles, which is a reflection of their electronic distribution and geometry.

After addition of LiBF4 to a solution of 1a, acidified with HNO3, colorless crystals of [HPhOEtIm]BF4 (1b) were isolated. We recorded 1H NMR spectra of 1a and 1b from crystalline samples in (CD3)2CO (Figure ). There is a large downfield shift in the proton resonances upon protonation of the N-heterocyclic ring, 1a, demonstrating an important electronic change in the molecule. The most affected protons are H2 (Δδ = 1.55 ppm), H5 (Δδ = 0.62 ppm), and H4 (Δδ = 0.56 ppm), although aliphatic (Δδ = 0.43 ppm, N—CH2 and 0.22 ppm, CH2—O) and, to a lesser extent, phenyl protons (Δδ = 0.01–0.03 ppm) are also altered (Supporting Information).

Figure 2

1H NMR spectra in (CD3)2CO for compounds 1a, 1b, and 2.

1H NMR spectra in (CD3)2n class="Chemical">CO for compounds 1a, 1b, and 2.

The crystal structure analysis of HPhOEtIm+ (cation 1b) revealed notable structural modifications when compared to that of 1a (Table ). A view of cation 1b is depicted in Figure . An interesting aspect concerns the symmetrization undergone by the imidazolium ring upon protonation, involving its bond distances and angles. These structural changes can be rationalized invoking the resonant forms of an imidazole ring versus an imidazolium ring (Scheme ). Both C2—N bond distances are almost identical in 1b: the C2—N3 distance becomes significantly shorter (from 1.356(3) Å in 1a to 1.328(2) Å in 1b), whereas the N1(H)—C2 length is 1.326(3) Å in 1b (1.316(3) Å in 1a). These structural attributes reflect the electronic changes that the imidazole ring suffers upon protonation at the N1 atom. The proton changes the aromatic character of the N-heterocyclic ring. Thus, the structural and NMR spectroscopic variation of the neutral versus alkylated or protonated systems may be rationalized as a change in the aromaticity of the ring, with the protonated and alkylated cationic species being more aromatic than the neutral imidazole precursors (vide infra).

Figure 3

View of short contacts between the imidazolium cation and three neighboring BF4– anions in the crystal of 1b. Symmetry operations: (a) x, 1 + y, z; (b) 2 – x, 1 – y, −z.

Scheme 2

Protonation of an Imidazole Derivative: Resonant Changes in the Ring

View of short contacts between the n class="Chemical">imidazolium cation and three neighboring BF4– anions in the crystal of 1b. Symmetry operations: (a) x, 1 + y, z; (b) 2 – x, 1 – y, −z.

In addition, it is worth mentioning here that, although the C4—C5 connection is usually depicted by a double line (Scheme , right), this bond is slightly elongated with respect to the C=C bond distances typically found in alkenes (ca. 1.34 Å). On the basis of the discussion above, this elongation is the result of the aromatic character of the N-heterocyclic pentagonal rings in imidazoles, imidazolium, and carbenes. Likewise, N1—C2 and C2—N3 bond distances (typically represented by a combination of solid and dashed lines) are between the typical values found for C—N single (1.47 Å) and C=N double (1.27 Å) bonds,[20] being closer from the single-bond value, reflecting again the effects of the electronic delocalization in the pentagonal rings. On the basis of the structural characteristics of the imidazole and imidazolium rings and their aromatic character (see below), a circle within the ring appears to be appropriate to illustrate the bonding within these N-heterocyclic rings. It is not surprising, therefore, that some authors prefer this latter representation for imidazole, imidazolium, and carbene molecules.

As commented above, endocyclic ring angles are also influenced by protonation. The presence of a proton at N1 widens by +4.6(2)° the corresponding C2—N1—C5 angle: 1a, 104.6(2)°; 1b, 109.15(18)°. This structural change could be rationalized from the point of view of the valence shell electron pair repulsion (VSEPR) theory that assigns a higher repulsion power to lone electron pairs than to sigma bonds; therefore, the protonation at the N1 atom reduces the steric repulsion, leading to the observed increase in the C2—N1—C5 angle. This structural phenomenon has been typically observed in imidazole and pyrimidine rings of nucleobases and actually serves as diagnostic for the presence of protonation in these systems.[21,22] In contrast, both adjacent N1—C2—N3 and N1—C5—C4 angles decrease, displaying differences of −4.4(2)° (1a: 112.3(2)°; 1b: 107.93(17)°) and −3.6(2)° (1a: 110.6(2)°; 1b: 106.99(18)°), respectively.

Cation 1b exhibits an amphiphilic nature: a polar central imidazolium core connected to the nonpolar hydrophobic phenyl ring. Figure outlines contacts involving the imidazolium cation of 1b that is surrounded by three BF4– anions. The acidic proton at the N1 site forms hydrogen bonds with two symmetry-related counter BF4– anions, namely, N1(H)···F11 (2.994(2) Å) and N1(H)···F11′ (2.903(2) Å). Additional intermolecular C—F distances are C4(H)···F14 (3.207(2) Å), C5(H)···F12 (3.171(2) Å), and C2(H)···F14 (3.169(2) Å). These latter contacts are too long to be regarded as C—H···F hydrogen bonds, although they could provide some weak lattice stabilization. It should be noted that the crystal structure of 1b does not exhibit substantial intermolecular interactions between neighboring cations.

[H2bisPhOEtIm]I2 (2)

The bisimidazolium salt, [H2bisPhOEtIm]I2 (2), was prepared from the reaction between CH2I2 and 1a. Upon recrystallization in acetonitrile, colorless crystals were isolated and studied. A complete NMR characterization of salt 2 (1H NMR, 13C{1H}-APT NMR, 1H—13C HSQC/HMBC) is provided in the Supporting Information. The 1H NMR spectrum of 2 [r.t., (CD3)2CO] conforms to the two-fold symmetry of the bisimidazolium dication. Figure illustrates that the imidazolium protons in 2 suffer a significant downfield shift with respect to the resonances of 1b: H2 (10.51 ppm), H5 (8.56 ppm), and H4 (7.96 ppm). These data suggest that the μ-CH2 bridge (singlet at δH 7.46 ppm) in 2 exerts a higher deshielding effect on the protons of the imidazolium ring than on the proton in compound 1b. Alternatively, the observed proton chemical shifts could be ascribed to different secondary interactions between the cation and the anion as revealed in the crystal structures of both salts. It is noteworthy that no significant 1H chemical shift variation between 1b and 2 is observed for the aliphatic and phenyl protons, indicating that, in this region, the effect of protonation at N1 seems to be similar to the bridging methylenation.

Iodide salt 2 of the H2bisPhOEtIm2+ cation crystallizes in the centrosymmetric 2/c space group (Figure ), with the bridging methylene group lying at the inversion center (C21 in Figure ). Selected crystallographically determined distances and angles are listed in Table together, for comparison, with those calculated by DFT. The imidazolium dication in salt 2 is transoid disposed, with a dihedral angle between both imidazolium rings that approaches 90° (86.91(6)°). The N1—C21—N1′ angle between them is 107.8(2)°, close to the expected tetrahedral angle supported by the bridging carbon atom. In addition, the aromatic tails of the phenoxyethyl substituents display a dihedral angle of 52.29(9)° with the imidazolium rings. The N3—C6—C7—O8 torsion angle is 68.8(2)°. These angle values resemble those found in the crystal structures of 1a and 1b.

Figure 4

View of the H2bisPhOEtIm2+ dication in iodide salt 2.

View of the H2bisPhOEtIm2+ dication in n class="Chemical">iodide salt 2.

The geometry variations found in the imidazolium rings of compound 2 follow the same trend discussed above for salt 1b. Thus, the N1—C2 bond distance is longer (1.348(2) Å) in 2 than in 1a and 1b and (by chance) identical to that of C4—C5. Similarly, both N3—C4 and C5—N1 bond lengths in 2 are identical (Table ). Besides, the C2—N3 bond distance does not vary with respect to that in both salts 1b and 2 and is shorter than the neighboring N1—C2 bond distance. Regarding the ring angles, no significant variations are observed between 1b and 2. Such changes are reflected in the aromaticity of the imidazole ring (see DFT Calculations section).

As in the case of 1b, dication 2 displays an amphiphilic nature. The crystal packing of 2 is dominated by anion/cation interactions. Interestingly, these interactions involve mainly the imidazolium rings and the μ-CH2 bridge, demonstrating that, as expected, the positive charge is localized around the imidazolium head of the molecule. Figure shows the C(H)···I interactions involving the cationic imidazolium rings. Although these interactions are relatively long (C5, 3.777(2) Å; C2, 3.783(2) Å; C4, 3.813(2) Å; C21, 3.870(1) Å; C6, 3.903(2) Å), and the protons involved are not very acidic, they seem to exhibit sufficient hydrogen-bond-donor capabilities to interact with the I1 anions in the crystal lattice. In any case, these short contacts are driven by packing and electrostatic effects (Supporting Information). In this regard, the large proton chemical shifts observed for H2 and H5 could be perhaps rationalized as the result of the secondary cation/anion interactions in solution.

Figure 5

Representation of the C(H)···I interactions of dication 2 with symmetry-related iodide anions. Symmetry operations: (a) +x, −y, 0.5 + z; (b) +x, 1 + y, z; (c) −x, 1 + y, 0.5 – z; (d) −x, +y, 0.5 – z; (e) −x, −y, −z.

[Pd(bisPhOEtIm)I2] (3)

The synthesis of 3 was carried out by the addition of 2 to a solution of palladium acetate in acetone. Immediately, the solution changed color from orange to dark brown-orange, and after 24 h stirring at room temperature, the solution turned progressively deep orange. The product was isolated by precipitation with ether, followed by filtration. Recrystallization from acetone/ether afforded compound 3 in 80% yield. Suitable crystals for X-ray crystallography were grown by slow diffusion of ether into a dichloromethane solution of complex 3 at room temperature.

The 1H NMR spectrum of 3 in deuterated acetone (r.t., Supporting Information) shows variations when compared to that of 1a, 1b, and 2. Metal coordination at the C2 site, with the subsequent substitution of the C2(H) proton, yields a high field shift of the proton signals, in particular, of those placed in the immediate vicinity of the PdII ion. Thus, the H5 (7.60 ppm) and H4 (7.38 ppm) protons appear in the spectra of 3 as doublets, displaying a Δδ compared to those of imidazolium salt 2 of −0.96 and −0.59 ppm, respectively. Phenyl proton multiplets show minor variations and are centered at 7.23 (meta), 6.90 (para), and 6.84 (ortho) ppm, respectively. The CH2 bridging unit exhibits an AB system,[8] centered at 6.45 ppm (Δδ = −1.01 ppm, in comparison with 2), whereas the N—CH2 and CH2—O are shown in the 4.2–5.2 ppm range. Regarding the 13C{1H} NMR spectra, metalation of the C2 sites produces an expected substantial shift of this resonance, from the range of 137.5–138.8 ppm (C2 sites in 1a–c) to 169.1 ppm (C2 site in 3). A complete NMR characterization is available in the Supporting Information.

The crystal structure of compound 3 shows a rather distorted square-planar coordination of the palladium ion, which incorporates a chelating bisPhOEtIm unit and two iodido ligands (Figure ). Salient intramolecular distances and angles are presented in Table . The angles C12—Pd1—I2 and C22—Pd1—I1 are 174.46(9) and 164.62(9)°, respectively. The latter exhibits a clear distortion from linearity that is found in less than 11% of PdL4 (L = any atom) square-planar complexes.a

Figure 6

View of complex [Pd(bisPhOEtIm)I2] (3).

Table 2

Selected Interatomic Distances [Å] and Angles [deg] in Compound 3

Pd1—C12	1.988(3)	Pd1—C22	1.983(3)
Pd1—I1	2.6373(3)	Pd1—I2	2.6465(4)
N11—C12	1.342(4)	N21—C22	1.358(4)
C12—N13	1.346(4)	C22—N23	1.342(4)
N13—C14	1.379(4)	N23—C24	1.404(4)
C14—C15	1.349(5)	C24—C25	1.340(5)
C15—N11	1.391(4)	C25—N21	1.385(4)
N11—C31	1.457(4)	N21—C31	1.461(4)
C12—Pd1—C22	82.45(13)	C12—Pd1—I2	174.46(9)
C12—Pd1—I1	90.93(9)	C22—Pd1—I1	164.62(9)
C22—Pd1—I2	92.01(9)	I1—Pd1—I2	94.430(11)
C12—N11—C15	111.4(3)	C22—N21—C25	111.3(3)
N11—C12—N13	105.4(3)	N21—C22—N23	105.0(3)
C12—N13—C14	110.1(3)	C22—N23—C24	110.3(3)
N13—C14—C15	108.0(3)	N23—C24—C25	107.1(3)
N11—C15—C14	105.2(3)	N21—C25—C24	106.3(3)

View of complex n class="Chemical">[Pd(bisPhOEtIm)I2] (3).

An interesting question concerns the structural modification undergone by the bisPhOEtIm ligand upon C2 cis-coordination to the palladium ion, which formally represents a substitution of the C2(H) protons by a PdII center. First, both imidazole rings change into a mutual cisoid orientation and display a tilt angle of 72.5(1)°, in contrast to the transoid orientation and almost right dihedral angle exhibited in cation 2. This rotation does not affect the geometry of the bridging sp3 hybridized CH2 unit, with angle and distances involving both pentanuclear rings in compound 3 of 107.3(3)° (N11—C31—N21), 1.457(4) Å (N11—C31), and 1.461(4) Å (N21—C31), respectively. Corresponding values in 2 were 107.8(2)° (N1—C21—N1′) and 1.455(2) Å (N1—C21). In reference to the imidazole geometry, it should be noted that the higher standard deviations exhibited by crystal 3, probably due to the presence of both iodido ligands, hinder a thorough comparison of records. However, some tendencies can be analyzed. Substantial variations include the following: (i) the C2—N3 distances in 3 (C12—N13, 1.346(4) Å; C22—N23, 1.342(4) Å) are longer than those in 2 (1.327(2) Å), namely, they possess a lower double-bond character as a result of the electron density donation to the metal center. (ii) Interestingly, the N1—C2—N3 angles are significantly smaller in 3 (N11—C12—N13, 105.4(3)°; N21—C22—N23, 105.0(3)°), when compared to those in 1a (112.3(2)°), 1b (107.93(17)°), or 2 (107.27(17)°). The decrease of this angle has usually been observed in NHC compounds with transition metals at the C2 site (Supporting Information). It should be recalled here that a (unprotonated) carbene has a mean N1—C2—N3 value of 101°,b even smaller than that observed in 3. (iii) A side consequence of this is that in 3 the C2—N1—C5 angles (C12—N11—C15, 111.4(3)°; C22—N21—C25, 111.3(3)°) are even wider than those in 1b (109.13(16)°).

Following the dictates of the VSEPR, as commented above, the reduction of the N1—C2—N3 angle upon deprotonation of the imidazolium salts can be rationalized as a consequence of the higher repulsion power of the electron lone pair at the ylidenic carbon atom versus the C2—H bond. Likewise, upon coordination of the carbene ligands to a metal center, the lone electron pair loses its inherent repulsion capabilities and the angle increases but not as much as that when the carbon is protonated, indicating that the steric effects of the metal center play, as expected, a role in the structural changes of the ligand.

DFT Calculations: Aromaticity

We carried out DFT optimizations (M11L, 311++G(d,p), SMD water) to study the calculated geometries of compounds 1a, 1b, 2, and 3. The resulting optimized intramolecular bond lengths and angles of the corresponding compounds (namely, Ia, Ib, II, and III) are listed in Table . The salient coordinates are summarized in the Supporting Information. Overall, calculated arrangements anticipate those obtained experimentally and follow the described trends for the crystal structures: bond angles fit well, although some of the bond length values display minor differences. Thus, the arguments and trends exposed for the crystal structures are also valid here.

An interesting feature concerns a calculated tautomer of 1a, namely Ia*, in which the proton at the C2 position in Ia has been shifted to the N1 site in Ia*, allowing for the study of the C2-carbene atom. As expected, in terms of relative energy, the Ia* tautomer is less stable than Ia (ΔE = 25.7 kcal mol–1). The geometry of Ia* shows a higher degree of symmetry in the imidazole tautomer ring, when compared to that of Ia (N1—C2, 1.347 Å; C2—N3, 1.350 Å; N3—C4, 1.368 Å; C4—C5, 1.347 Å; C5—N1, 1.361 Å). Furthermore, the N1—C2—N3 angle shows a value of 101.4°, which is in agreement with previously reported examples.a

As described in this work, protonation or alkylation at the N1 atom of the neutral imidazole ring leads to an increase in the aromatic character of the resulting cationic system. The higher degree of aromaticity is clearly reflected by the observed equilibration in the crystallographic bond distances of the N-heterocyclic rings, and it may be related to the large downfield chemical shift found in the 1H NMR spectra upon protonation and alkylation.

The aromaticity of the optimized models has been estimated by the nucleus-independent chemical shift (NICS). The NICS(0) value was calculated at the geometrical center of the aromatic ring, whereas NICS(1) are the values obtained at a perpendicular distance of 1.0 Å above the ring center. In our case, due to the low symmetry of the models, the value of NICS(1) depends on which side of the ring is considered, and therefore NICS(0) values have been considered. Most negative NICS values represent a higher aromaticity degree. Table lists salient NICS(0) values for Ia, Ia*, Ib, II, and III. We have calculated the aromaticity at both rings, namely, imidazole and phenoxy. The NICS(0) value for the imidazole ring of Ia is −12.5, whereas this value for the Ia* tautomer is −12.9 (Table ). The protonated form, Ib, exhibits a lower NICS(0) value, that is, −14.3, which corresponds, as expected from the experimental data, to a higher level of aromaticity. Alkylation of the imidazole ring in II results in an NICS(0) value of −13.8, which is between the neutral forms, Ia/Ia*, and the protonated cation, Ib. Complex III, which incorporates a chelating PdII ion at the C2 sites, displays the lowest aromaticity of the series (NICS(0), −10.4). Thus, symmetrization observed upon protonation and alkylation in the crystal structures results in a higher degree of aromaticity at the imidazole rings. For comparison, the H,H-imidazolium cation and the methyl derivative, [Ia—CH3]+, were optimized, giving NICS(0) values of −15.9 and −13.7. Thus, protonation has a higher effect than alkylation on the aromaticity of the imidazole derivatives, whereas the C-substituents at the endocyclic N atoms do not substantially affect the aromaticity of the imidazolium rings. In contrast, variations in the aromaticity of phenoxy rings are not remarkable.

Table 3

NICS Values for Ia, Ia*, Ib, II, and III

	Ia	Ia*	Ib	II	III
imidazole
NICS(0)	–12.5	–12.9	–14.3	–13.8	–10.4
phenoxy
NICS(0)	–8.9	–8.8	–8.9	–9.1	–8.6

Conclusions

In summary, we carried out a stepwise synthesis of an imidazole derivative (1a), its protonated imidazolium salt (1b), the related bisimidazolium salt (2), and a PdII-metal complex (3). A comparison between the structure of 1a and its protonated form 1b entails a geometrical transformation that can be envisaged as a symmetrization (aromatization) of the imidazole ring. These geometrical variations and the corresponding electronic redistribution are observed by NMR spectroscopy. This trend is further reinforced in the bisimidazolium salt, 2. Substitution of the C2(H) protons of 2 by a metal center, or, namely, coordination of a PdI2 unit to form 3, results in slight modifications of the geometry of the aromatic rings. The DFT-calculated bond distances and angles in the optimized models are consistent with the X-ray crystallographic data, and NICS criteria reveal that the imidazolium cations exhibit a higher aromaticity than the unprotonated forms and the metal complexes, and, in particular, protonation confers a higher aromaticity level than alkylation.

Experimental Section

Preparations

Reagents and solvents were obtained from commercial sources and used without further purification.

Synthesis of PhOEtIm (1a)

Compound 1a was synthesized following reported methods.[23]

Synthesis of [HPhOEtIm]BF4 (1b)

To a saturated solution of 1a (402.8 mg, 2.14 mmol) in water, n class="Chemical">concentrated HNO3 and LiBF4 (800 mg, 8.54 mmol) were added. After 4 days at 4 °C, crystals of 1b were picked up (116 mg, 0.42 mmol) from the beaker. Yield: 20%. 1H NMR ((CD3)2CO, δ ppm, 300 MHz): 9.18 (s, 1H; H2Im), 7.90 (m, 1H; H5Im), 7.75 (m, 1H; H4Im), 7.29 (m, 2H; Hm Ph), 6.96 (m, 3H; Hp + Ho Ph), 4.87 (m, 2H; N—CH2), 4.51 (m, 2H; CH2—O). 13C{1H}-APT NMR ((CD3)2CO, δ ppm, 75 MHz): 159.0 (s; C Ph), 137.5 (s; C2Im), 130.4 (s; Cm Ph), 123.6 (s; C4Im), 122.2 (s; Cp Ph), 120.7 (s; C5Im), 115.4 (s; Co Ph), 66.6 (s; CH2—O), 50.0 (s; N—CH2).

Synthesis of [H2bisPhOEtIm]I2 (2)

Compound 2 was obtained in an analogous manner to that described previously,[24] by adding diiodomethane (0.5 eq) to a solution of 1a in acetonitrile. After refluxing, and the addition of diethyl ether as a precipitating agent, compound 2 was isolated as a white solid. Recrystallization in acetonitrile and subsequent cooling (4 °C) resulted in colorless crystals. Yield: 46%. 1H NMR (CD3CN, δ ppm, 300 MHz): 9.90 (s, 2H; H2Im), 8.11 (m, 2H; H5Im), 7.60 (m, 2H; H4Im), 7.30 (m, 4H; Hm Ph), 6.99 (m, 8H; Hp + Ho Ph + CH2-bridge), 4.62 (m, 4H; N—CH2), 4.33 (m, 4H; CH2—O). 13C{1H}-APT NMR ((CD3)2CO, δ ppm, 75 MHz): 158.8 (s; C Ph), 138.8 (s; C2Im), 130.6 (s; Cm Ph), 124.9 (s; C4Im), 123.2 (s; C5Im), 122.5 (s; Cp Ph), 115.6 (s; Co Ph), 66.3 (s; CH2—O), 58.5 (s; CH2-bridge), 50.8 (s; N—CH2).

Synthesis of [Pd(bisPhOEtIm)I2] (3)

A mixture of [Pd(CH3COO)2] (300 mg, 1.34 mmol) and 2 (862 mg, 1.34 mmol) in acetone (80 mL) was stirred at room temperature for 24 h. Afterward, the solvent was removed under vacuum, and product 3 was isolated, after the addition of diethyl ether, as a yellow solid (800 mg, 1.07 mmol). Yield: 80%. Recrystallization was performed from a mixture of acetone–ether. 1H NMR (DMSO-d6, δ ppm, 300 MHz): 7.81 (d, 2H, JH—H = 1.8; H5Im), 7.63 (d, 2H, JH—H = 1.8; H4Im), 7.20 (m, 4H; Hm Ph), 6.92 (m, 2H; Hp Ph), 6.74 (m, 4H; Ho Ph), 6.60 (d, 1H, JH—H = 13.3; CH2-bridge), 6.32 (d, 1H, JH—H = 13.3; CH2-bridge), 4.20 (m, 4H; N—CH2/CH2—O), 4.07 (m, 2H; CH2—O), 3.72 (m, 2H; N—CH2). 13C{1H}-APT NMR (DMSO-d6, δ ppm, 75 MHz): 169.1 (s; C2Im), 157.6 (s; C Ph), 129.5 (s; Cm Ph), 123.0 (s; C5Im), 122.6 (s; C4Im), 121.2 (s; Cp Ph), 114.5 (s; Co Ph), 66.7 (s; CH2—O), 63.0 (s; CH2-bridge), 49.4 (s; N—CH2). Anal. Calcd for C23H24I2N4O2Pd: C, 36.80; H, 3.49; N: 7.47. Found: C, 36.62; H, 3.14; N, 7.39.

Instrumentation

1H NMR and 13C{1H}-APT NMR spectra were recorded on a Bruker ARX 300 MHz spectrometer. Chemical shifts are given as dimensionless δ values. The spectra were referenced to the corresponding residual solvent peaks or to sodium 3-(trimethylsilyl)propanesulfonate (TSP), used as an internal reference for D2O measurements. Coupling constants, J, are given in hertz. Identification of signals was achieved by 1H—13C HSQC/HMBC experiments (Supporting Information). The pD values of the NMR samples were measured by a glass electrode and adjusted by the addition of NaOD or DNO3.

Determination of pKa Values

Acidity constants were determined by pH-dependent 1H NMR spectroscopy. Chemical shift variations were registered at different pD values. These were obtained by adding 0.4 units to the pH meter reading (uncorrected pH*). pD was evaluated versus the chemical shift values by a nonlinear least-squares fit according to the Newton–Gauss method.[18] The obtained pKa values (for D2O) were converted to values valid for H2O.[25]

X-ray Crystallography

Crystal data and refinement parameters for 1a, 1b, 2, and 3 were collected on an APEX-II diffractometer with graphite monochromated Mo Kα radiation (0.71073 Å). Data reduction and cell refinements were carried out with the APEX2 software package.[26] The structures were solved by direct methods and refined by full-matrix least-squares on the basis of F2 using the SHELXL-97 and WinGX software.[27,28] All nonhydrogen atoms were located from difference Fourier maps and were refined anisotropically. Protons were either observed in the difference Fourier map (1a, 1b) or positioned geometrically (2, 3), and they were refined with isotropic displacement parameters following a riding model. Calculations were performed using the SHELXL-97 and WinGX programs.

Crystal Data for PhOEtIm (1a)

[C11H12N2O], orthorhombic, P2(1)2(1)2(1), a = 5.5786(8) Å, b = 7.5398(11) Å, c = 23.223(4) Å, Z = 4, Mr = 188.23, V = 976.8(2) Å3, Dcalcd = 1.280 g cm–3, λ(Mo Kα) = 0.71073 Å, T = 100 K, μ = 0.084 mm–1, 12 922 reflections n class="Chemical">collected, 2590 unique (Rint = 0.0627), 2136 observed, R1(Fo) = 0.0451 [I > 2σ(I)], w2 (Fo2) = 0.1209 (all data), GOF = 1.068. CCDC 1530558.

Crystal Data for [HPhOEtIm]BF4 (1b)

[C11H13n class="Chemical">BF4N2O], monoclinic, 21/, a = 5.6463(3) Å, b = 8.2933(4) Å, c = 26.5684(14) Å, β = 93.6800(10)°, Z = 4, Mr = 276.04, V = 1241.54(11) Å3, Dcalcd = 1.477 g cm–3, λ(Mo Kα) = 0.71073 Å, T = 100 K, μ = 0.135 mm–1, 15 747 reflections collected, 2449 unique (Rint = 0.0311), 1989 observed, R1(Fo) = 0.0417 [I > 2σ(I)], wR2 (Fo2) = 0.1071 (all data), GOF = 1.069. CCDC 1530559.

Crystal Data for [H2bisPhOEtIm]I2 (2)

[C23n class="Chemical">H26I2N4O2], monoclinic, 2/c, a = 36.0917(15) Å, b = 5.1773(2) Å, c = 13.8210(6) Å, β = 91.5830(10)°, Z = 4, Mr = 644.28, V = 2581.57(18) Å3, Dcalcd = 1.658 g cm–3, λ(Mo Kα) = 0.71073 Å, T = 100 K, μ = 2.461 mm–1, 15 708 reflections collected, 3108 unique (Rint = 0.0229), 2797 observed, R1(Fo) = 0.0209 [I > 2σ(I)], wR2 (Fo2) = 0.0698 (all data), GOF = 0.991. CCDC 1530560.

Crystal Data for [Pd(bisPhOEtIm)I2] (3)

[C23H24I2N4O2Pd], monoclinic, 21/, a = 9.9925(6) Å, b = 17.2375(10) Å, c = 14.6398(8) Å, β = 94.8950(10)°, Z = 4, Mr = 748.66, V = 2512.4(3) Å3, Dcalcd = 1.979 g cm–3, λ(Mo Kα) = 0.71073 Å, T = 100 K, μ = 3.223 mm–1, 59 437 reflections n class="Chemical">collected, 6184 unique (Rint = 0.0429), 5285 observed, R1(Fo) = 0.0311 [I > 2σ(I)], w2 (Fo2) = 0.1001 (all data), GOF = 1.041. CCDC 1530561.

Quantum Chemical Calculations

DFT calculations were performed to calculate the energy minima of the systems and the resulting molecular geometries, providing, therefore, good models for comparison with the crystallographically determined data. The M11L[29] functional was utilized, through the polarizable continuum method (SMD).[30] The basis sets used for C, N, O, and H were 6-311++G(d,p). Pd and I were represented with the use of the relativistic effective core potential from the Stuttgart group.[31,32] All calculations were performed with the Gaussian09 software.[33] NICS[34] values were computed using the Gauge-including atomic orbital method at the same level of theory,[35,36] either at the geometrical center of the rings (NICS(0)) or above the perpendicular plane of the rings (at 1.0 Å, NICS(1)). Coordinates are given in the Supporting Information.