κ²N¹³,N^13′-Hydro-rac-2,2′-bis­[2-(1-propylbenzimidazol-2yl)]­bi­phenyl tri­fluoromethane­sulfonate, the salt of a novel proton sponge

The title salt, C₃₂H₃₁N₄⁺·CF₃SO₃⁻, contains chains of monopro­tonated rac-2,2′-bis[2-(1-propyl­benzimidazol-2yl)]­bi­phenyl cations separated by tri­fluoro­methane­sulfonate anions. The cations are linked by head-to-tail π-interactions of their benz­imidazole fragments. Protonation results in a strong hydrogen bond involving the imine N atoms, which gives the cations a tweezer or U shape.

Key indicators

• Single-crystal X-ray study
• T = 298 K
• Mean [sigma](C-C) = 0.003 Å
• Disorder in solvent or counterion
• R factor = 0.046
• wR factor = 0.146
• Data-to-parameter ratio = 12.4

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level C:



REFLT_03
         From the CIF: _diffrn_reflns_theta_max           24.97
         From the CIF: _reflns_number_total               2479
         TEST2: Reflns within _diffrn_reflns_theta_max
         Count of symmetry unique reflns         2732
         Completeness (_total/calc)             90.74%
          Alert C: < 95% complete

PLAT_302  Alert C Anion/Solvent Disorder .......................      50.00 Perc.


 0 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 2 Alert Level C = Please check

---

S1. Comment

Proton sponges, defined as species that exhibit high basicity and low nucleophilicity, have been used as auxiliary bases in organic synthesis (Llamas-Saiz et al., 1994). Compound (I) is considered to be the prototypical proton sponge, but others, such as (II), are known (Miyahara et al., 2001). Biphenyl-based, basic bidentate ligands, such as (III) and (IV), with three torsional degrees of freedom, have been used to enforce tetrahedral coordination geometries for transition metal complexes in the +1 and +2 oxidation states (Knapp et al., 1990; Xie et al., 1999; Stibrany, 2001). We have observed that benzimidazole biphenyl-based species of type (IV) can also serve as proton sponges.

The structure of the title salt (V) contains monoprotonated bis(benzimidazole) cations (Fig. 1) and trifluoromethanesulfonate anions arranged in layers along c (Fig. 2), offset appropriately to be consistent with the body-centered lattice. Within a given layer, the cations form chains along a in which individual centrosymmetrically related cations are linked head-to-tail by π-interactions of their benzimidazole fragments.

The cations exhibit 4 e site symmetry, with the twofold axes passing through the midpoints of the C21—C21' and N13—N13' vectors. Consistent with this symmetry, H13 is disordered on two sites about the twofold axis. Both the benzimidazole (bz) and phenyl (ph) rings are planar and are canted with respect to each other by 64.60 (6)° (bz/ph) and 88.60 (6)° (ph/ph) to give the cation a U or tweezers shape. Metric parameters of the benzimidazole fragments agree well with those reported for neutral structures containing 2-phenyl-benzimidazole groups (Bei et al., 2000; Elerman & Kabak, 1997). In particular, the C12—N(imine) distance is shorter than the C12—N(amine) distance and both are shorter than the N—C11,C13 linkages. In (V,) the near equality of C12—N distances found in imidazolium salts (Stibrany et al., 2002a) is less apparent, most likely because two benzimidazoles are monoprotonated.

The N13···N13' distance [2.652 (2) Å] is consistent with hydrogen bonding involving H13. In principle, this atom may be bonded either symmetrically or asymmetrically to the two imine nitrogen atoms. Both bonding modes were examined and better agreement with the data was obtained with H13 removed from the twofold axis to yield disordered, asymmetric hydrogen bonds. In IH⁺ with different anions, both symmetric (X-ray diffraction, Woźniak et al., 1990) and asymmetric (neutron diffraction, Woźniak et al., 1996; Fox et al., 2001) bonding has been reported. Recent structural (Pozharski et al., 2002; Fox et al., 2001) and NMR (Perrin & Ohta, 2001; Pietrzak et al., 2001) studies of proton sponges suggest that asymmetric bonding with a shallow double-minimum potential well is the norm. We conclude that, in the present system, H13 is disordered over two sites about the twofold axis and that placing it on the twofold axis, which would result in an unusually large N13···H13 distance (1.34 Å), would be an artifact of the refinement.

For H13, the shortest interionic contact is to H16 of an adjacent cation (3.82 Å), indicating that the hydrogen-bonded proton is encapsulated within the cation and that the center of positive charge is relatively far removed from other species, including the trifluoromethanesulfonate anions. To estimate the base strength of (IVa) and (IVb), competitive NMR experiments were performed. In deuterated acetonitrile, (IVb) deprotonates (I)H⁺ while (IVa) does not, suggesting that, in this solvent, (IV) and (I) have comparable pK_a values (Stibrany et al., 2002b).

S2. Experimental

The title salt (V) was obtained by the addition of an equimolar amount of a 2.57 M solution of triflic acid in methanol to an acetonitrile solution of rac-2,2'-bis[2-(1-propylbenzimidazol-2yl)]biphenyl. Crystals were obtained by slow vapor diffusion of diethyl ether. ¹H NMR (400 MHz, CDCl₃): δ=16.03(br s, 1H), 7.67(d, J=7.5 Hz, 2H), 7.48 (m, 12H),7.21(d, J=6.0 Hz, 2H), 4.40(m, 4H), 1.90(m, 4H), 0.97(t, J=7.2 Hz, 6H). ¹³C NMR (400 MHz, CDCl₃): δ= 150.6 140.3, 135.1, 133.1, 131.6, 130.4, 130.3, 129.3, 125.4, 125.3, 125.0, 116.5, 112.3, 46.9, 23.2, 11.3. ¹⁹F NMR (400 MHz, CDCl₃): δ=-78.6.

S3. Refinement

The structure was originally refined in space group C2/c with β = 131.81 (1)°; the space group was converted to I2/a with β = 92.85 (1)° to reduce correlation among x- and z-related parameters. The trifluoromethanesulfonate anions adopt the staggered-ethane conformation and are disordered centrosymmetrically about the 4c sites such that the CF₃– and SO₃– fragments, which have approximately the same size and shape, overlap to a large extent. Attempts to refine the atoms of these fragments individually resulted in large correlation coefficients (some greater than 0.95) and unrealistic parameters. Consequently, the anion was refined as a four-atom fragment (50% C and S in one position, 50% O and F in the remaining three positions); this refinement gave acceptable correlation coefficients and better agreement with the data than unconstrained or restrained refinements. Refinement in space group Ia was attempted, but did not remove the hydrogen or anion disorders. However, as expected, parameter s.u. values were substantially larger in Ia than in I2/a.

Computing details

Data collection: SMART (Bruker, 2000); cell refinement: SMART; data reduction: SAINT (Bruker, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPIII (Burnett & Johnson, 1996) and ORTEP-32 (Farrugia, 1997); software used to prepare material for publication: SHELXTL (Bruker, 2000).

Figures

	Fig. 1. Molecular structure of the cation in (V), showing 25% probability displacement ellipsiods. For clarity, only one of the disordered atoms H13 is shown.
	Fig. 2. Unit-cell (001) projection, showing cation chains along the a axis and the disordered protons H13. Other H atoms have been omitted for clarity.

κ²N¹³,N⁴³Hydro-rac-2,2'-bis[2-(1-propyl benzimidazol-2yl)]biphenyl trifluoromethanesulfonate

Crystal data

C32H31N4^+·CF3O3S^−	F(000) = 1296
Mr = 620.69	Dx = 1.323 Mg m^−3
Monoclinic, I2/a	Mo Kα radiation, λ = 0.71073 Å
a = 14.383 (3) Å	Cell parameters from 903 reflections
b = 14.217 (3) Å	θ = 4.0–23.6°
c = 15.252 (5) Å	µ = 0.16 mm^−1
β = 92.85 (1)°	T = 298 K
V = 3115.1 (14) Å^3	Plate, colorless
Z = 4	0.50 × 0.25 × 0.18 mm

Data collection

CCD area detector diffractometer	2479 independent reflections
Radiation source: fine-focus sealed tube	1925 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.017
ϕ and ω scans	θmax = 25.0°, θmin = 3.9°
Absorption correction: multi-scan (SADABS; Blessing, 1995)	h = −16→16
Tmin = 0.886, Tmax = 1.00	k = −16→15
6104 measured reflections	l = −17→14

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.046	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.146	H-atom parameters constrained
S = 1.00	w = 1/[σ^2(Fo^2) + (0.091P)^2 + 0.8886P] where P = (Fo^2 + 2Fc^2)/3
2479 reflections	(Δ/σ)max < 0.001
200 parameters	Δρmax = 0.16 e Å^−3
0 restraints	Δρmin = −0.31 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
N13	0.83945 (11)	0.11531 (11)	0.98105 (10)	0.0558 (4)
H13	0.7832	0.1003	0.9916	0.067*	0.50
N11	0.97272 (11)	0.19208 (12)	0.98308 (10)	0.0572 (4)
C11	0.98249 (14)	0.11396 (14)	0.92935 (12)	0.0574 (5)
C12	0.88542 (13)	0.18963 (13)	1.01185 (12)	0.0517 (5)
C21	0.77122 (12)	0.31520 (12)	1.04595 (12)	0.0514 (5)
C22	0.84688 (13)	0.25889 (12)	1.07207 (13)	0.0526 (5)
C23	0.88680 (16)	0.26674 (15)	1.15696 (14)	0.0668 (6)
H23	0.9374	0.2291	1.1741	0.080*
C13	0.89854 (15)	0.06590 (13)	0.92862 (13)	0.0561 (5)
C26	0.73839 (15)	0.37888 (14)	1.10652 (16)	0.0699 (6)
H26	0.6887	0.4179	1.0900	0.084*
C14	0.88513 (17)	−0.01570 (14)	0.87972 (14)	0.0707 (6)
H14	0.8296	−0.0492	0.8802	0.085*
C24	0.85221 (17)	0.32932 (17)	1.21519 (15)	0.0767 (7)
H24	0.8792	0.3338	1.2717	0.092*
C25	0.77820 (17)	0.38515 (17)	1.19049 (17)	0.0809 (7)
H25	0.7546	0.4274	1.2303	0.097*
C17	1.05576 (16)	0.08367 (19)	0.88043 (15)	0.0778 (7)
H17	1.1122	0.1156	0.8809	0.093*
C18	1.04190 (16)	0.26652 (18)	0.99780 (17)	0.0785 (7)
H18A	1.1037	0.2393	0.9972	0.094*
H18B	1.0348	0.2939	1.0553	0.094*
C15	0.9564 (2)	−0.04462 (17)	0.83111 (17)	0.0850 (8)
H15	0.9490	−0.0984	0.7967	0.102*
C16	1.0399 (2)	0.0041 (2)	0.83160 (16)	0.0896 (8)
H16	1.0869	−0.0182	0.7974	0.108*
C19	1.03293 (19)	0.34320 (18)	0.9292 (2)	0.0975 (9)
H19A	0.9697	0.3672	0.9265	0.117*
H19B	1.0450	0.3170	0.8721	0.117*
C110	1.1001 (2)	0.4233 (2)	0.9493 (3)	0.1507 (16)
H11A	1.0927	0.4451	1.0082	0.226*
H11B	1.0873	0.4739	0.9089	0.226*
H11C	1.1628	0.4017	0.9438	0.226*
S1	0.21709 (7)	0.27171 (9)	0.20321 (9)	0.0884 (4)	0.50
O1	0.23619 (16)	0.37411 (15)	0.21598 (16)	0.1313 (7)	0.50
O3	0.12409 (12)	0.26448 (18)	0.21989 (14)	0.1328 (8)	0.50
O2	0.24321 (16)	0.2450 (2)	0.12751 (14)	0.1479 (10)	0.50
C1	0.21709 (7)	0.27171 (9)	0.20321 (9)	0.0884 (4)	0.50
F1	0.23619 (16)	0.37411 (15)	0.21598 (16)	0.1313 (7)	0.50
F3	0.12409 (12)	0.26448 (18)	0.21989 (14)	0.1328 (8)	0.50
F2	0.24321 (16)	0.2450 (2)	0.12751 (14)	0.1479 (10)	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N13	0.0620 (10)	0.0485 (9)	0.0574 (10)	0.0074 (7)	0.0080 (8)	0.0017 (7)
N11	0.0491 (9)	0.0683 (10)	0.0537 (10)	0.0078 (7)	−0.0037 (7)	−0.0011 (8)
C11	0.0603 (12)	0.0657 (12)	0.0460 (10)	0.0195 (9)	−0.0001 (9)	0.0051 (9)
C12	0.0509 (10)	0.0541 (11)	0.0498 (11)	0.0114 (8)	−0.0010 (8)	0.0054 (8)
C21	0.0463 (10)	0.0414 (10)	0.0662 (12)	−0.0026 (7)	−0.0008 (8)	−0.0027 (8)
C22	0.0507 (10)	0.0488 (10)	0.0581 (12)	0.0020 (8)	0.0012 (9)	−0.0013 (8)
C23	0.0667 (13)	0.0704 (13)	0.0622 (14)	0.0139 (10)	−0.0085 (10)	−0.0063 (10)
C13	0.0698 (12)	0.0497 (10)	0.0491 (11)	0.0146 (9)	0.0059 (9)	0.0077 (8)
C26	0.0563 (12)	0.0567 (12)	0.0956 (17)	0.0090 (9)	−0.0075 (11)	−0.0204 (11)
C14	0.0975 (17)	0.0474 (11)	0.0682 (14)	0.0110 (11)	0.0130 (12)	0.0020 (10)
C24	0.0797 (15)	0.0823 (15)	0.0665 (14)	0.0062 (12)	−0.0111 (12)	−0.0249 (12)
C25	0.0746 (15)	0.0791 (15)	0.0884 (17)	0.0078 (12)	−0.0025 (13)	−0.0394 (13)
C17	0.0649 (14)	0.1004 (18)	0.0685 (14)	0.0229 (13)	0.0085 (11)	0.0008 (13)
C18	0.0538 (12)	0.1054 (19)	0.0758 (16)	−0.0090 (12)	−0.0024 (11)	−0.0189 (14)
C15	0.118 (2)	0.0635 (14)	0.0745 (16)	0.0270 (14)	0.0141 (15)	−0.0035 (11)
C16	0.099 (2)	0.101 (2)	0.0704 (16)	0.0456 (17)	0.0173 (14)	−0.0040 (14)
C19	0.0805 (17)	0.0738 (16)	0.138 (3)	−0.0122 (13)	0.0053 (16)	−0.0066 (16)
C110	0.110 (3)	0.097 (2)	0.248 (5)	−0.038 (2)	0.047 (3)	−0.047 (3)
S1	0.0616 (6)	0.1061 (9)	0.0963 (9)	−0.0108 (5)	−0.0066 (6)	0.0153 (6)
O1	0.1354 (17)	0.1142 (16)	0.1428 (17)	−0.0230 (12)	−0.0095 (13)	0.0137 (13)
O3	0.0600 (10)	0.215 (2)	0.1221 (16)	−0.0004 (12)	−0.0108 (10)	−0.0073 (14)
O2	0.1100 (16)	0.253 (3)	0.0797 (13)	−0.0256 (16)	−0.0030 (11)	0.0173 (15)
C1	0.0616 (6)	0.1061 (9)	0.0963 (9)	−0.0108 (5)	−0.0066 (6)	0.0153 (6)
F1	0.1354 (17)	0.1142 (16)	0.1428 (17)	−0.0230 (12)	−0.0095 (13)	0.0137 (13)
F3	0.0600 (10)	0.215 (2)	0.1221 (16)	−0.0004 (12)	−0.0108 (10)	−0.0073 (14)
F2	0.1100 (16)	0.253 (3)	0.0797 (13)	−0.0256 (16)	−0.0030 (11)	0.0173 (15)

Geometric parameters (Å, º)

N13—C12	1.320 (2)	C24—H24	0.9300
N13—C13	1.387 (2)	C25—H25	0.9300
N13—H13	0.8600	C17—C16	1.367 (4)
N11—C12	1.351 (2)	C17—H17	0.9300
N11—C11	1.392 (3)	C18—C19	1.513 (4)
N11—C18	1.462 (3)	C18—H18A	0.9700
C11—C13	1.387 (3)	C18—H18B	0.9700
C11—C17	1.390 (3)	C15—C16	1.386 (4)
C12—C22	1.473 (3)	C15—H15	0.9300
C21—C26	1.393 (3)	C16—H16	0.9300
C21—C22	1.393 (3)	C19—C110	1.515 (4)
C21—C21^i	1.501 (4)	C19—H19A	0.9700
C22—C23	1.394 (3)	C19—H19B	0.9700
C23—C24	1.368 (3)	C110—H11A	0.9600
C23—H23	0.9300	C110—H11B	0.9600
C13—C14	1.387 (3)	C110—H11C	0.9600
C26—C25	1.380 (3)	S1—O2	1.289 (3)
C26—H26	0.9300	S1—O3	1.378 (2)
C14—C15	1.359 (3)	S1—O1	1.493 (2)
C14—H14	0.9300	S1—S1^ii	1.783 (2)
C24—C25	1.366 (3)
C12—N13—C13	107.47 (16)	C26—C25—H25	120.0
C12—N13—H13	126.3	C16—C17—C11	116.0 (2)
C13—N13—H13	126.3	C16—C17—H17	122.0
C12—N11—C11	107.06 (16)	C11—C17—H17	122.0
C12—N11—C18	127.24 (17)	N11—C18—C19	112.42 (19)
C11—N11—C18	125.46 (17)	N11—C18—H18A	109.1
C13—C11—C17	121.7 (2)	C19—C18—H18A	109.1
C13—C11—N11	106.54 (16)	N11—C18—H18B	109.1
C17—C11—N11	131.7 (2)	C19—C18—H18B	109.1
N13—C12—N11	111.23 (16)	H18A—C18—H18B	107.9
N13—C12—C22	124.01 (17)	C14—C15—C16	121.5 (2)
N11—C12—C22	124.73 (17)	C14—C15—H15	119.2
C26—C21—C22	118.02 (18)	C16—C15—H15	119.2
C26—C21—C21^i	118.94 (16)	C17—C16—C15	122.5 (2)
C22—C21—C21^i	122.87 (17)	C17—C16—H16	118.7
C21—C22—C23	119.95 (17)	C15—C16—H16	118.7
C21—C22—C12	121.23 (17)	C18—C19—C110	111.7 (3)
C23—C22—C12	118.81 (16)	C18—C19—H19A	109.3
C24—C23—C22	120.6 (2)	C110—C19—H19A	109.3
C24—C23—H23	119.7	C18—C19—H19B	109.3
C22—C23—H23	119.7	C110—C19—H19B	109.3
N13—C13—C11	107.70 (17)	H19A—C19—H19B	107.9
N13—C13—C14	131.4 (2)	C19—C110—H11A	109.5
C11—C13—C14	120.93 (19)	C19—C110—H11B	109.5
C25—C26—C21	121.26 (19)	H11A—C110—H11B	109.5
C25—C26—H26	119.4	C19—C110—H11C	109.5
C21—C26—H26	119.4	H11A—C110—H11C	109.5
C15—C14—C13	117.2 (2)	H11B—C110—H11C	109.5
C15—C14—H14	121.4	O2—S1—O3	118.18 (16)
C13—C14—H14	121.4	O2—S1—O1	110.12 (17)
C25—C24—C23	120.2 (2)	O3—S1—O1	102.87 (16)
C25—C24—H24	119.9	O2—S1—S1^ii	116.63 (16)
C23—C24—H24	119.9	O3—S1—S1^ii	107.94 (14)
C24—C25—C26	120.0 (2)	O1—S1—S1^ii	98.41 (14)
C24—C25—H25	120.0
C12—N11—C11—C13	−0.5 (2)	C12—N13—C13—C14	−178.8 (2)
C18—N11—C11—C13	−175.29 (18)	C17—C11—C13—N13	−177.92 (18)
C12—N11—C11—C17	177.4 (2)	N11—C11—C13—N13	0.3 (2)
C18—N11—C11—C17	2.7 (3)	C17—C11—C13—C14	1.1 (3)
C13—N13—C12—N11	−0.4 (2)	N11—C11—C13—C14	179.28 (17)
C13—N13—C12—C22	−178.60 (16)	C22—C21—C26—C25	−1.2 (3)
C11—N11—C12—N13	0.6 (2)	C21^i—C21—C26—C25	−176.6 (2)
C18—N11—C12—N13	175.24 (18)	N13—C13—C14—C15	176.9 (2)
C11—N11—C12—C22	178.76 (16)	C11—C13—C14—C15	−1.8 (3)
C18—N11—C12—C22	−6.6 (3)	C22—C23—C24—C25	−0.3 (4)
C26—C21—C22—C23	0.6 (3)	C23—C24—C25—C26	−0.3 (4)
C21^i—C21—C22—C23	175.82 (18)	C21—C26—C25—C24	1.0 (4)
C26—C21—C22—C12	179.80 (18)	C13—C11—C17—C16	0.2 (3)
C21^i—C21—C22—C12	−5.0 (3)	N11—C11—C17—C16	−177.5 (2)
N13—C12—C22—C21	−64.8 (2)	C12—N11—C18—C19	−87.2 (3)
N11—C12—C22—C21	117.3 (2)	C11—N11—C18—C19	86.5 (3)
N13—C12—C22—C23	114.4 (2)	C13—C14—C15—C16	1.3 (4)
N11—C12—C22—C23	−63.5 (3)	C11—C17—C16—C15	−0.7 (4)
C21—C22—C23—C24	0.1 (3)	C14—C15—C16—C17	−0.1 (4)
C12—C22—C23—C24	−179.1 (2)	N11—C18—C19—C110	175.6 (2)
C12—N13—C13—C11	0.1 (2)

Symmetry codes: (i) −x+3/2, y, −z+2; (ii) −x+1/2, −y+1/2, −z+1/2.