Halogen Complexes of Anionic N‐Heterocyclic Carbenes

Abstract The lithium complexes [(WCA‐NHC)Li(toluene)] of anionic N‐heterocyclic carbenes with a weakly coordinating anionic borate moiety (WCA‐NHC) reacted with iodine, bromine, or CCl4 to afford the zwitterionic 2‐halogenoimidazolium borates (WCA‐NHC)X (X=I, Br, Cl; WCA=B(C6F5)3, B{3,5‐C6H3(CF3)2}3; NHC=IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene, or NHC=IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene). The iodine derivative (WCA‐IDipp)I (WCA=B(C6F5)3) formed several complexes of the type (WCA‐IDipp)I⋅L (L=C6H5Cl, C6H5Me, CH3CN, THF, ONMe3), revealing its ability to act as an efficient halogen bond donor, which was also exploited for the preparation of hypervalent bis(carbene)iodine(I) complexes of the type [(WCA‐IDipp)I(NHC)] and [PPh4][(WCA‐IDipp)I(WCA‐NHC)] (NHC=IDipp, IMes). The corresponding bromine complex [PPh4][(WCA‐IDipp)2Br] was isolated as a rare example of a hypervalent (10‐Br‐2) system. DFT calculations reveal that London dispersion contributes significantly to the stability of the bis(carbene)halogen(I) complexes, and the bonding was further analyzed by quantum theory of atoms in molecules (QTAIM) analysis.

The molecular structures of the 2-iodoimidazolium borates 2 b and 2 c were also determined by X-ray diffraction analysis; 2 b crystallized from a toluene/n-hexane solution as the toluene solvate 2 b·C 6 H 5 Me, whereas 2 c crystallized without the inclusion of solvent molecules from the THF/n-hexane solution. In contrast to 2 a·C 6 H 5 Me, the toluene solvate molecule in 2 b·C 6 H 5 Me does not interact with the iodine atom, which instead displays short intermolecular C arene ···I contacts of Figure 1. ORTEP diagram of 2 a·C 6 H 5 Cl and 2 a·ONMe 3 with thermal displacement parameters drawn at the 50 % probability level; hydrogen atoms are omitted for clarity. Pertinent structural data of all compounds 2 are assembled in Table 1. . ORTEP diagrams of 5 a·C 6 H 5 Cl and 5 b·C 6 H 5 Cl·n-hexane with thermal displacement parameters drawn at the 50 % probability level; hydrogen atoms and solvent molecules are omitted for clarity. 5 a must be regarded with care, owing to the usage of SQUEZZE. Pertinent structural data of all compounds 5 are assembled in Table 2. 3.1742(13), 3.4860 (15), and 3.5277(1) to the 2,4,6-trimethylphenyl (Mes) group of another molecule of 2 b, resulting in a head-to-head linkage of two molecules in the crystal structure. Likewise, 2 c forms dimeric units through two intermolecular C arene ···I contacts of 3.338(2) and 3.390(2) involving one of the 2,6-diisopropylphenyl (Dipp) substituents. Packing diagrams can be found in the Supporting Information (Figures S7 and  S9). The covalent carbon-iodine bond lengths are 2.0536 (13) in 2 b·C 6 H 5 Me and 2.0544 (16) in 2 c and identical with 2.0568(8) established for 2 a·C 6 H 5 Cl (Table 1).
The 2-bromoimidazolium borates 3 a and 3 b were prepared in a similar fashion by reaction of 1 a and 1 b with elemental bromine in chlorobenzene solution, which afforded both compounds as colorless solids in high yield (3 a: 74%, 3 b: 78%). For the preparation of the corresponding 2-chloroimidazolium borate 4, CCl 4 was used as the chlorination reagent, and its reaction with 1 a in chlorobenzene solution furnished 4 as a colorless solid in moderate yield (47 %, Scheme 2). It should be noted that the reaction of IDipp and IMes with CCl 4 , however, gives stable 4,5-dichloroimidazolin-2-ylidenes. [42,43] All complexes 3 and 4 were characterized by 1 H, 11 B{ 1 H}, 13 C{ 1 H}, and 19 F{ 1 H} NMR spectroscopy in [D 8 ]THF solution (see the Supporting Information for the presentation of all spectra). The NMR spectra are like those recorded for the iodine derivatives 2 a and 2 b, except for the 13 C NMR signals of the carbene carbon atoms, which are found at lower field. Single crystals of 3 a, 3 b, and 4·THF were subjected to X-ray diffraction analysis; the molecular structures are presented in the Supporting Information (Figures S10, S12, and S14), whereas pertinent structural data are assembled in Table 1. The bromine atom in 3 a shows only conventional Br···F and Br···H van der Waals contacts in the solid state, 3 b exhibits intermolecular C arene ···Br contacts of 3.3925(17) and 3.4470(17) , which are just below the sum of the van der Waals radii (3.53 ). [37] The chlorine atom in 4·THF does not interact with the THF solvate molecule, but shows a Cl···Cl contact of 3.2665 (8) , which is slightly shorter than twice the van der Waals radius (3.50 ). [37] The CÀBr bond lengths of 1.845(2) (3 a) and 1.8396(15) (3 b) as well as the CÀCl bond length of 1.6825 (13) in 4·THF fall in the expected ranges and are in good agreement with the values reported for other 2-bromo-and 2-chloroimidazolium salts. [44,45] Synthesis and characterization of bis(carbene)halogen(I) complexes The strong tendency of the 2-iodoimidazolium borate 2 a to act as a halogen bond donor by formation of the adducts 2 a·L (L= C 6 H 5 Cl, C 6 H 5 Me, CH 3 CN, THF, ONMe 3 ) prompted us to react 2 a·C 6 H 5 Cl with the neutral carbenes IDipp and IMes to generate hypervalent bis(carbene)iodine(I) complexes (Scheme 3). Layering a chlorobenzene solution containing a 1:1 mixture of 2 a·C 6 H 5 Cl and the respective NHC ligand with nhexane gave suitable single crystals of 2 a·IDipp (5 a) and 2 a·IMes (5 b). The structural parameters of 5 a have to be handled with care owing to the usage of SQUEZZE. [46] The molecular structures are shown in Figure 2, and pertinent structural data are listed in Table 2. The central C1-I-C46 unit is in both cases almost linear (5 a: 177.02(7)8, 5 b: 177.44(8)8) with slightly different carbon-iodine bond lengths of 2.260(2)/2.441(2) in 5 a and 2.288(2)/2.361(2) in 5 b. The shorter CÀI bonds are formed with the WCA-NHC ligand, implying that this ligand is a slightly stronger donor in comparison with the neutral NHC. However, they are significantly longer than the CÀI bond length of 2.0568(8) in 2 a·C 6 H 5 Cl, and this elongation can be attributed to the strong and partly covalent interaction with the neutral NHC ligands, in accordance with a hypervalent 10-I-2 bonding situation. The N-C-N angles lie between the values of the 2 a adducts (Table 1) and the neutral carbenes IDipp and IMes (101.48). [12,43] Overall, the structural parameters agree perfectly with those determined for the cationic congeners [(IMes) 2 I][BPh 4 ] (III a) and [(IDipp) 2 I]I (III b). [24,27] In the 13 C NMR spectra ( [D 8 ]THF solution), two signals at 170.6/156.5 ppm (5 a) and 162.9/150.8 ppm (5 b) can be assigned to the carbene carbon atoms of the NHC and WCA-NHC ligands, respectively, revealing a weaker donor and therefore more pronounced carbene character for the neutral NHC ligand as also observed in the solid state ( Table 2).
Attempts to combine 2 a with the sterically more demanding NHC 1,4-di-tert-butylimidazolin-2-ylidene (ItBu) failed, and  Figure S26). The formation of this compound indicates a high stability of the bis(carbene)iodine(I) anion in analogy with the bis(pentafluorophenyl)iodate(I) I (Scheme 1). Therefore, we aimed at the targeted synthesis of this anion in the following. In principle, the combination of 1 a and 2 a would lead to a corresponding lithium salt; however, to avoid interaction with the lithium ion, the phosphonium salt [PPh 4 ] [WCA-IDipp] (6) was prepared by reacting the lithiocarbene 1 a with tetraphenylphosphonium chloride in chlorobenzene (Scheme 4). Compound 6 was isolated as a pale-orange solid in satisfactory yield (70 %) after filtration through Celite and washing with toluene and n-hexane. Naturally, the NMR spectroscopic data of 6 in [D 8 ]THF largely correspond to those of the lithiocarbene precursor 1 a; the 13 C NMR resonance of the carbene carbon atom is found at 218.8 ppm, which is in the range of 1 a (217.4 ppm). [33] In addition, the expected signals for the [PPh 4 ] + counterion are observed, for example, a singlet at 23.8 ppm in the 31 P{ 1 H} NMR spectrum. Single crystals of 6 suitable for X-ray diffraction analysis were obtained by layering a concentrated THF solution with nhexane, and the molecular structure of the anion is shown in Figure 3. The carbene carbon atom does not show any significant intermolecular contacts and only displays a weak C···H contact with one phenyl group of the phosphonium ion. The structural parameters are very similar to those reported for IDipp [43] and also for the lithiocarbene 1 a, [34] and the small N1-C1-N2 angle of 101.93(11)8 is in line with the presence of a "free" carbene. A related example is the potassium salt of an amido-functionalized anionic NHC, in which the carbene and the K + ion are separated by complexation with 18-crown-6. [47] The phosphonium salt 6 was treated with 2 a·C 6 H 5 Cl or 2 b in chlorobenzene solution, furnishing the anionic bis(carbene)iodine(I) complexes 7 a and 7 b in moderate yield by crystallization from chlorobenzene or THF/n-hexane solution (Scheme 4). Both compounds were fully characterized by 1 H, 11 B{ 1 H}, 13 C{ 1 H}, 19 F{ 1 H}, and 31 P{ 1 H} NMR spectroscopy in [D 8 ]THF solution (see the Supporting Information for the presentation of all spectra). The symmetric complex 7 a exhibits only one set of signals for the two equivalent carbene ligands, for example, one singlet for the backbone CH hydrogen atoms at 6.25 ppm, which is intermediate between the chemical shifts of 2 a·C 6 H 5 Cl (7.33 ppm) and 6 (6.11 ppm). In contrast, two singlets at 6.67 and 6.22 ppm are observed for 7 b. Consequently, one 13 C NMR resonance is found at 156.2 ppm for 7 a, whereas two resonances at 164.3 and 150.6 ppm can be assigned to the carbene carbon atoms in 7 b. The 31 P{ 1 H} NMR 102.50 (16) C1-X-C46 177.02 (7) 177.44 (8) 178.48 (4) 178.71(6)/ 179.32 (6) 177.40 (6) Interplanar angle 49.70 (9) 92.78 (10) 42.58 (5) 53.13(7)/ 48.58 (7) 35.65 (8) Scheme 4. Synthesis of anionic bis(carbene)iodine(I) complexes. Dipp = 2,6diisopropylphenyl, Mes = 2,4,6-trimethylphenyl. spectra of both compounds exhibit one signal at 23.9 ppm for the PPh 4 + ion. The crystal structures of 7 a·2 THF and 7 b were determined by X-ray diffraction analysis, and Figure 4 shows the ORTEP diagrams of the bis(carbene)iodine(I) anions. Important bond lengths and angles are summarized in Table 2 Attempts to isolate similar bis(carbene)bromine(I) and bis-(carbene)chlorine(I) complexes proved difficult, and, for instance, mixing the 2-bromo-and 2-chloroimidazolium borates 3 a und 4 with IDipp or IMes led to decomposition and formation of imidazolium salts. However, layering a chlorobenzene solution of 3 a and 6 with n-hexane furnished colorless single crystals of [PPh 4 ][(WCA-IDipp) 2 Br]·C 6 H 5 Cl (8) suitable for X-ray diffraction analysis. The molecular structure of the anion in 8 is shown in Figure 5, revealing a linear C-Br-C unit with an angle of 177.40(6)8, which unlike the C-I-C units in 5 and 7 displays considerably different CÀBr bond lengths of 1.9010(18) and 2.822 (2) . The shorter bond (C1ÀBr) is noticeably longer compared with 1.845 (2) in 3 a, whereas the longer one is still significantly shorter than the sum of the van der Waals radii (3.53 ). [37] This asymmetry is also expressed in the clearly different N-C-N angles of 107.48 (15)8 and 102.50(16)8, revealing more imidazolium or more carbene character, respectively. Accordingly, the bonding in 8 is better conceived as halogen bonding between 3 a and the anionic carbene 6, but this interaction is certainly stronger compared with the Br···Br interaction in 2-bromoimidazolium bromides, which consistently feature shorter CÀBr bonds. [26,27,44,48] With the exception of bis(pyridine)bromine(I) cations, [49] we are unaware of other bromine systems with a linear L-Br-L arrangement. NMR spectroscopic characterization of 8 was hampered by its gradual decomposition in solution. Nevertheless, the NMR spectra in [D 8 ]THF reveal the signals for two equivalent WCA-IDipp ligands, whereas broadening indicates slow bromine exchange between the two carbene ligands on the NMR timescale. Unfortunately, the corresponding chlorine complex could not be isolated by the combination of 4 and 6, which can be ascribed to its lower thermodynamic stability, resulting in stronger dissociation and faster decomposition in solution. In particular, the zwitterionic imidazolium borates (WCA-IDipp)H (9 a) and (WCA-IMes)H (9 b) crystallized repeatedly as side products and were characterized by X-ray diffraction analysis (see the Supporting information, Figures S23 and S24).

Computational Studies
The isolation and crystallization of the solvate complexes 2 a·L (L = C 6 H 5 Cl, C 6 H 5 Me, CH 3 CN, THF) and of the trimethylamine Noxide complex 2 a·ONMe 3 prove the ability of the 2-iodoimidazolium borate 2 a to serve as a halogen bond donor. To assess the stability of these complexes, their structures were optimized by applying the density functional theory (DFT) method B97-D. [50] The calculated structural parameters agree with those found experimentally in the solid state by X-ray diffraction analysis, although the calculated gas-phase geometries   4 + counterion, the hydrogen atoms, and the chlorobenzene molecule are omitted for clarity. Pertinent structural data of all compounds 8·C 6 F 5 Cl are assembled in Table 2. consistently afford slightly longer bond lengths (see the Supporting Information). The resulting association enthalpies DH 298K for the formation of complexes 2 a·L (L= C 6 H 5 Cl, C 6 H 5 Me, CH 3 CN, THF, ONMe 3 ) from 2 a and L are also included in Table 1. For the solvate complexes, the enthalpies range from À5.7 kcal mol À1 for L = CH 3 CN to À10.8 kcal mol À1 for L = THF, which is typical, but at the stronger end for noncovalent halogen bonding. [15,16,51,52] A significantly stronger interaction is calculated for the trimethylamine N-oxide adduct with DH 298K = À17.5 kcal mol À1 , which is on the same order of magnitude as that calculated for N-iodosaccharin pyridine N-oxide complexes. [53] Complexation of 2 a with the carbenes IDipp and IMes turned out to be more exothermic (Table 3), revealing a significantly higher stability of 2 a·IDipp (5 a, À34.5 kcal mol À1 ) and 2 a·IMes (5 b, À33.2 kcal mol À1 ). In comparison, isocyanide adducts such as [(C 6 F 5 )I(CNMes)] are significantly more labile with dissociation energies of approximately 7 kcal mol À1 , [13] which can be ascribed to the higher s-donor ability of NHC ligands.
The corresponding anionic dicarbene complexes 7 can be assigned even higher stabilities of À50.0 kcal mol À1 (7 a) and À51.2 kcal mol À1 (7 b), which were calculated with respect to 2 a and the respective WCA-NHC ligand. Likewise, the formation of the cationic complex [(IDipp) 2 I] + (as in III a) from [(IDipp)I] + and IDipp was calculated to be equally exothermic (À47.7 kcal mol À1 ). The same trend was derived for the analogous neutral, anionic, and cationic bis(carbene)bromine(I) and -chlorine(I) complexes, with the overall stability decreasing together with the polarizability in the order I > Br > Cl (Table 3). A similar trend has been derived for the energies of heterolytic dissociation of bis(pyridine)halogen(I) cations; it was also found that solvent effects were significant as expected for charged species, with the stability decreasing on moving from the gas to the solution phase. [54] Therefore, our gas-phase calculations should be considered as the upper limit for the true stabilities, and despite the favorable thermodynamics calculated also for the formation of the corresponding bromine and chlorine species, only the complex [(WCA-IDipp) 2 Br] À has been obtained as the phosphonium salt 8 so far. We suspect that competing side reactions, especially with the free carbenes in solution, prevent the clean formation of the desired products and afford side products such as the imidazolium borates 9.
It has been shown that London dispersion decisively contributes to the thermodynamic stability of main-group element NHC adducts, [55] which has also been demonstrated for pnictogen complexes of anionic N-heterocyclic carbenes. [34] As similar factors might contribute to the stability of the halogen complexes reported here, the association enthalpies (DH 298K ) were calculated again for the bis(carbene)halogen(I) complexes employing the B3LYP-D3 [56] and B3LYP [57] levels of theory with and without dispersion correction (Table 3). The corrected DFT results (B3LYP-D3) are in excellent agreement with those previously discussed for the B97-D method (see above), whereas significantly lower stabilities were obtained with the uncorrected DFT method B3LYP. The dispersion stabilization (D3) ranges from À21 to À28 kcal mol À1 , with the highest contribution to the overall stability found for the homoleptic [(WCA-IDipp)X(W-CA-NHC)] À (X = I, Br, Cl) systems. These results indicate that the WCA-borate moieties clearly enhance the dispersion forces upon adduct formation and therefore enable the isolation of the analogous bromine complex [PPh 4 ][(WCA-IDipp) 2 Br] (8). Nevertheless, the calculations suggest that even bis(carbene)chlorine(I) complexes should become isolable if undesired side reactions could be avoided by optimization of the reaction conditions.
It should be noted that the calculations generally afforded structures with almost identical or very similar carbonÀiodine bond lengths for the homoleptic [(WCA-IDipp) 2 I] À and [(IDipp) 2 I] + systems. Therefore, these complexes should be described as static complexes with a symmetric three-center, four-electron (3c4e) bond, in agreement with the NMR spectroscopic results and despite the slight differences found in the crystal structures (cf. Tables 2 and 3). As expected, slightly different carbonÀiodine bond lengths were calculated for the heteroleptic congeners [(WCA-IDipp)I(WCA-IMes)] À , [(WCA-IDipp)I(IDipp)], and [(WCA-IDipp)I(IMes)] as also found by X-ray diffraction analysis, suggesting a similar, but more polarized 3c4e bonding situation. [7,58] Accordingly, the bonding situation in two examples, asymmetric [(WCA-IDipp)I(IDipp)] (5 a) and symmetric [(WCA-IDipp) 2 I] À (as in 7 a), was further examined by quantum theory of atoms in molecules (QTAIM) analysis, [59] which has previously been used to study the nature of halogen bonding. [13,51,60,61] Figure 6 shows the contour map of the Laplacian of the electron density, r 2 1(r), along the C-I-C axis in both bis(carbene)iodine(I) complexes, and topological values derived from the QTAIM analysis are assembled in Table 4. For both systems, the two expected (3, À1) bond critical points (bcp) are found along the CÀIÀC bond paths, which present typical properties of closed-shell interactions: The value of electron density, 1(r bcp ), is small, and the Laplacian of the electron density, r 2 1(r bcp ), is positive. [51,62] The asymmetric complex , which would suggest a stronger, but still a closed-shell carbon-iodine interaction.
It should be noted that the results of the above topological analysis must be treated with caution [63] and that the naive interpretation as a noncovalent closed-shell interaction is probably not valid in view of the similar topological values reported for the triiodide anion (I 3 À ), namely 1(r bcp ) = 0.064 e a 0 À3 and r 2 1(r bcp ) = 0.0305 e a 0 À5 for both IÀI bond critical points. [61] This anion is a classical textbook example for describing the bonding in a hypervalent (10-I-2) system through a 3c4e interaction. Thus, the bonding in the bis(carbene)iodine(I) complexes is probably best conceived as a Class II (L 2 Z) 3c4e interaction, [8] in which the two carbene ligands (L) each provide a pair of electrons and the iodine atom (Z) provides none. This is also in line with the description derived for the bonding in bis(pyridine)iodine(I) systems. [4,7] Alternatively, the formation of bis(carbene)iodine(I) complexes can be described by covalent interaction between the s*-orbital of the 2-iodoimidazolium species with the carbene lone pair, and a molecular orbital (MO) diagram of 7 a in line with this n!s* bonding model can be found in the Supporting Information ( Figure S27). [9] Natural bond orbital (NBO) analysis affords Wiberg bond indices (WBI) of 0.54 for both carbon-iodine bonds in symmetric 7 a, revealing the expected decrease of the bond order from 1.04 in 2 a, albeit in agreement with covalent 3c4e bonding. The transition from hypervalency to secondary, noncovalent bonding certainly takes place for the significantly less stable complexes 2 a·L (L= C 6 H 5 Cl, C 6 H 5 Me, CH 3 CN, THF, ONMe 3 ), [9] in which the 2-iodoimidazolium borate 2 a acts as a halogen bond donor. As noncovalent halogen bonding is often described with the so-called s-hole model, [15,18,20,22] we also calculated the electrostatic potential (ESP) surface of 2 a (Figure 7). Examination of the ESP reveals an area of significant positive potential (in blue) associated with the iodine atom that is available for the directed, linear interaction with the nucleophiles L (C 6 H 5 Cl, C 6 H 5 Me, CH 3 CN, THF, ONMe 3 , IDipp, IMes, WCA-IDipp, WCA-IMes) studied in this contribution. For comparison, the ESP of the bromine and chlorine congeners 3 and 4 were also calculated (Figure 7), which qualitatively show less pronounced areas of positive potential, that is, "smaller s-holes", which is in line with these systems acting as weaker halogen bond donors towards nucleophiles such as N-heterocyclic carbenes, affording lower stabilities for the corresponding bis(carbene)bromine(I) and -chlorine(I) complexes (Table 3).    Figure S28 in the Supporting Information for further details).

Conclusion
Zwitterionic 2-halogenoimidazolium borates of the type (WCA-NHC)X (X = I, Br, Cl; WCA = weakly coordinating anionic fluoroborate moiety) were obtained from the reactions of the lithium salts [(WCA-NHC)Li(toluene)] (1) with elemental iodine, bromine, or CCl 4 , respectively. Crystallization of the 2-iodoimidazolium borate (WCA-IDipp)I (2 a, WCA= B(C 6 F 5 ) 3 ) from different solvents or in the presence of nucleophiles affords several adducts of the type 2 a·L (L= C 6 H 5 Cl, C 6 H 5 Me, CH 3 CN, THF, ONMe 3 ), which demonstrates the ability of 2 a to act as an efficient halogen bond donor, and the calculated association enthalpies suggest weak noncovalent charge-transfer interactions. In contrast, significantly higher stabilities were calculated for bis(carbene)iodine(I) complexes, and accordingly, the neutral and anionic complexes [(WCA-IDipp)I(NHC)] (5) and [PPh 4 ] [(WCA-IDipp)I(WCA-NHC)] (7) could be isolated (NHC = IDipp, IMes), which display short carbon-iodine bond lengths in the solid state in agreement with the presence of more covalent three-center, four-electron (3c4e) bonding. The calculations also reveal that London dispersion contributes significantly to the stability of these complexes, which could be exploited for the isolation and structural characterization of the bis(carbene)bromine(I) complex [PPh 4 ][(WCA-IDipp) 2 Br] (8) as a rare example of a hypervalent 10-Br-2 system. The experimental and theoretical results also reveal that the anionic carbenes WCA-NHC act as stronger donors than their neutral NHC congeners, and with the phosphonium salt [PPh 4 ] [WCA-IDipp], we have introduced a "free" anionic N-heterocyclic carbene, which can be employed as a useful synthon in transition metal and main group element chemistry. In addition, we feel that the neutral 2-iodoimidazolium borates 2, and potentially also the corresponding bromine derivatives such as 3, represent useful additions to the family of strong halogen bond donors such as ubiquitous fluoroiodobenzenes, and their application in organic synthesis and catalysis will be further studied by our group.