Effect of methylene versus ethylene linkers on structural properties of tert-butyl and mesityl bis(imidazolium) bromide salts

The crystal structures of ligand precursor bis(imidaozolium) salts 1,1′-methylenebis(3-tert-butylimidazolium) dibromide monohydrate, C15H26N4 +·2Br−·H2O or [ tBuNHC2Me][Br]2·H2O, 1,1′-(ethane-1,2-diyl)bis(3-tert-butylimidazolium) dibromide dihydrate, C16H28N4 +·2Br−·2H2O or [ tBuNHC2Et][Br]2·2H2O, 1,1′-methylenebis[3-(2,4,6-trimethylphenyl)imidazolium] dibromide dihydrate, C25H30N4 2+·2Br−·2H2O or [MesNHC2Me][Br]2·2H2O, and 1,1′-1,1′-(ethane-1,2-diyl)bis[3-(2,4,6-trimethylphenyl)imidazolium] dibromide tetrahydrate, C26H32N4 2+·2Br−·4H2O or [MesNHC2Et][Br]2·4H2O, are reported. At 293 K, [ tBuNHC2Me][Br]2·H2O crystallizes in the P21/c space group, while [ tBuNHC2Et][Br]2·2H2O crystallizes in the P21/n space group at 100 K. At 112 K, [MesNHC2Me][Br]2·2H2O crystallizes in the orthorhombic space group Pccn while [MesNHC2Et][Br]2·4H2O crystallizes in the P21/c space group at 100 K.

The crystal structures of ligand precursor bis(imidazolium) salts 1,1 0 -methylenebis(3-tert-butylimidazolium) dibromide monohydrate, C 15  O crystallizes in the P2 1 /c space group at 100 K. Bond distances and angles within the imidazolium rings are generally comparable among the four structures. All four bis(imidazolium) salts cocrystallize with one to four molecules of water.

Chemical context
Bis(imidazolium) salts are common precursors for the synthesis of bidentate N-heterocyclic carbene (NHC 2 ) ligands, which can be used to stabilize a variety of metal complexes and catalysts. Bis(imidazolium) salts, [ R NHC 2 R 1 ][X] 2 are relatively modular in that modifications can be relatively easily made to exterior groups attached to each NHC (R), the moiety linking the two NHC groups (R 1 ), and the counter-ion (X). One general synthetic approach for synthesizing bis-(imidazolium) salts is where two equivalents of an alkyl or aryl imidazole are combined with one equivalent of an organic dihalide reagent and refluxed to afford the final product (Gardiner et al., 1999). A simplified procedure for a variety of ligand salts using pressure tubes resulting in yields that were generally over 80% was also reported . Some reports have gone even further to minimize solvent in the synthesis of these ligand precursors, including a solventfree synthesis (Cao et al., 2011. This implies that the exterior R groups can easily be modified by changing the alkyl or aryl group on the starting imidazole. The linking group R 1 and counter-ion X can be modified by changing the organic dihalide reagent. In this fashion, a library of bis(imidazolium) salts can be relatively easily synthesized from alkyl or aryl imidazoles, and some are also commercially available.
Some of the most widely reported bis(imidazolium) salts are those with tert-butyl ( t Bu) and mesityl (Mes) exterior R groups and methylene (Me) or ethylene (Et) linking R 1 groups. [ Mes NHC 2 Et][Br] 2 was even reported to be a stand-alone catalyst for the conversion of arylaldehydes to carboxylic acids in combination with water and K 2 CO 3 in DMSO (Yang et al., 2013). Methylene linkers are quite commonly used for complexing to metals, and although examples with ethylene linkers are fewer, comparative studies report that changing the linker affects catalysis. For example, shorter methylene linkers (R 1 ) were reported to be more effective for hydrosilylation reactions with Rh I complexes than ethylene linkers (Riederer et al., 2010).

Supramolecular features
The supramolecular structure of [ tBu NHC 2 Me][Br] 2 ÁH 2 O is stabilized by hydrogen bonding (Fig. 5, Table 1). Distances between centroids of neighboring imidazoles are greater than 5 Å , suggesting no -stacking interactions (Janiak, 2000). Hydrogen bonding between one bromide atom and one water molecule is found with Br1Á Á ÁH1D having a distance of 2.575 (4) Å . One tert-butyl group has positional disorder.

Database survey
A survey of the Cambridge Structural Database (Web accessed March 24, 2022; Groom et al., 2016) andSciFinder (SciFinder, 2022) (Rheingold, 2019) with a slightly higher R1 of 3.94% and data collection at a higher temperature of 150 K, as compared to R1 of 3.18% and temperature of 112 K in the current report. As discussed in the introduction, the syntheses of all of the reported structures are reported based on the SciFinder search; however, no additional structural data were found.  View of four molecules of [ tBu NHC 2 Et][Br] 2 Á2H 2 O with 50% probability ellipsoids, highlighting intermolecular distances.

Figure 7
View of eight molecules of [ Mes NHC 2 Me][Br] 2 Á2H 2 O with 50% probability ellipsoids, highlighting intermolecular distances. NMR data were collected on a Varian 400 MHz spectrometer and referenced to residual CHCl 3 . Synthesis of 1-tert-butyl-1H-imidazole, ( tBu Im). The procedure was adapted from a literature procedure (Liu et al., 2003). A round-bottom flask was charged with 10.0 mL (95 mmol, 1 eq.) of tert-butylamine, 11.0 mL of 40% glyoxal (95 mmol, 1 eq.), approximately 100 mL of methanol, and approximately 25 mL of deionized water and a stir bar, then heated to 343 K under reflux. 7.81 mL of 37% formaldehyde (95 mmol, 1 eq.) were added, followed by 3.70 mL of ammonium hydroxide (95 mmol, 1 eq.) added dropwise over 5 minutes while stirring. The solution was refluxed at 343 K for 5 h, resulting in a light red-orange solution. Excess solvent was removed in vacuo, and the resulting product was diluted with approximately 150 mL of dichloromethane and washed twice with 50 mL of deionized H 2 O until the aqueous layers ran clear. The product was vacuum distilled at $373 K, yielding a clear liquid, which was weighed in a tared vial, resulting in 7.95 g (34% yield) of tBu Im, and characterized by 1 H NMR spectroscopy in CDCl 3 .
Synthesis of 1-(2,4,6-trimethylphenyl)-1H-imidazole, ( Mes Im). The procedure was adapted from a literature procedure (Liu et al., 2003;Gardiner et al., 1999). A 250 mL threeneck round-bottom flask was charged with 15.000 g (110.9 mmol, 1 eq.) of 2,4,6-trimethylaniline, 16.090 g (110.9 mmol, 1 eq) of 40% glyoxal, and $75 mL of methanol and stirred for 24 h after which the solution turned orange with a yellow precipitate. 11.86 g (221.8 mmol, 2 eq.) of ammonium chloride, 18.00 g (221.8 mmol, 2 eq.) of 37% formaldehyde, and 300 mL of methanol were added, and the solution was refluxed for 24 h at 373 K, at which point the solution was deep brown. After being cooled to room temperature, 25.57 g (221.8 mmol, 2 eq.) of 85% phosphoric acid were added dropwise over ten minutes and the solution was refluxed for 16 h at 368 K. Excess solvent was removed in vacuo at 313 K, and the viscous brown residue was poured over $300 g of ice and neutralized to pH 10 with a saturated solution of potassium hydroxide, resulting in a clear solution with a chunky brown precipitate. The product was taken into diethyl ether by washing the solution three times with $100 mL of diethyl ether. The diethyl ether solution was washed thrice with $100 mL of water, thrice with $100 mL of brine, and dried overnight over sodium sulfate. Sodium sulfate solids were gravity filtered from the solution and the solvent was removed in vacuo resulting in a brown solid. The product was recrystallized from hot ethyl acetate, resulting in 9.49 g (46% yield) of tan crystals, which were characterized by 1 H NMR spectroscopy and identified as Mes Im.
Synthesis of 1,1 0 0 0 -di(tert-butyl)-3,3 0 0 0 -methylene-diimidazolium dibromide, [ tBu NHC 2 Me][Br] 2. 1.850 g (14.9 mmol, 2.5 eq.) of tbu Im and 0.4194 mL (5.9 mmol, 1 eq.) of dibromomethane, a stir bar, and $20 mL of toluene were stirred in a 50 mL round-bottomed flask. The solution was then heated to 423 K and refluxed for 46 h, resulting in the formation of a dark orange-brown solution. The solution was cooled in an ice bath, resulting in a fine white precipitate which was collected via vacuum filtration, washed twice with $5 mL of cold toluene, filtered and dried. 1.120 g (78.02% yield) of a fine white solid identified as [ tBu NHC 2 Me][Br] 2 were isolated. Crystals suitable for X-ray diffraction were obtained by recrystallization from hot methanol. The product was characterized by 1 H NMR spectroscopy. The 1 H NMR data were consistent with those previously reported .
A 250 mL round-bottomed flask was charged with 2.017 g (16.2 mmol, 2.5 eq.) of tbu Im, 0.562 mL (6.45 mmol, 1 eq.) of dibromoethane, a stir bar, and $20 mL of toluene. The mixture was refluxed at 423 K and stirred for 46 h, at which point the solution was a rusty brown color. The flask was then placed in an ice bath, and the resulting precipitate was collected via vacuum filtration and washed twice with $5 mL of cold toluene. The resulting solids were dried and weighed, yielding 1.727 g (61% yield) of [ tBu NHC 2 Et][Br] 2 and single crystals suitable for X-ray diffraction were obtained via recrystallization from hot methanol. 1 H NMR data were consistent with those previously reported .
The procedure was adapted from a literature procedure (Gardiner et al., 1999). 5.00 g (26.8 mmol, 2.5 eq.) of Mes Im we added to a 50 mL roundbottomed flask with a stir bar and $20 mL of toluene. 0.754 mL (10.72 mmol, 1 eq.) of dibromomethane were added and the solution was refluxed at 423 K for 20 h. The solution was cooled in an ice bath, resulting in a white precipitate. The white solid was recrystallized from $12 mL of hot methanol. The product was obtained in 17% yield (1.10 g) as tan crystals identified as [ Mes NHC 2 Me][Br] 2 suitable for X-ray diffraction and characterized by 1 H NMR.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Most hydrogen atoms were placed in calculated positions using the AFIX commands of SHELXL and included as riding contributions with distances of 0.95 Å for C-H, 0.99 Å for CH 2 and 0.98 Å for CH 3 . Methyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. U iso values of riding H atoms were set to 1.2 times U eq (C) for CH and CH 2 , and 1.5 times U eq (C) for CH 3 and H 2 O. For [ tBu NHC 2 Me][Br] 2 , the SADI command of SHELX was used to model disorder in one of the tert-butyl moieties for N4-C0AA and N4-C12, C0AA-C00N and C14-C12, and C1AA-C0AA and C13-C12 to restrain distances within a sigma of 0.02 Å . The population parameters for the disordered tert-butyl groups are 0.54019 for C12-C14, and 0.45981 for C00N, C0AA, and C1AA. The highest peak and deepest hole are both near a heavy atom Br1 with a distance of 0.88 Å from the highest peak of 1.49 e Å À3 and a distance of 0.73 Å from the deepest hole of À1.10 e Å À3 . For all structures, data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016). Program(s) used to solve structure: SHELXT (Sheldrick, 2015a) for sces01006_0m, est01043_0m, est01041d_0ma; olex2.solve (Bourhis et al., 2015) for at01019_0ma. For all structures, program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

1,1′-Methylenebis(3-tert-butylimidazolium) dibromide monohydrate (sces01006_0m)
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) (2) 0.03323 (9)   Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. ( where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.33 e Å −3 Δρ min = −0.29 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.