Structural characterization, infrared spectroscopy, and theoretical calculations for B(C6F5)3-stabilized benzene-ammonia and benzene-water complexes

Article abstract of DOI:10.1002/anie.201103904

O/N-H???π hydrogen bonds: Water-benzene and ammonia-benzene complexes are stabilized by the Lewis acid B(C₆F ₅)₃ and provide rare structural (X-ray) and infrared spectroscopic data for water-benzene and ammonia-benzene complexes in the solid state (see picture). The infrared spectra of the complexes showed that the O-H and N-H stretching frequencies decrease significantly on benzene complexation.

Full text of DOI:10.1002/anie.201103904

DOI: 10.1002/anie.201103904
Hydrogen Bonds
Structural Characterization, Infrared Spectroscopy, and Theoretical
Calculations for B(C6F5)3-Stabilized Benzene–Ammonia and Benzene–
Water Complexes**
Xinping Wang* and Philip P. Power*
À
Nonconventional hydrogen bonds between X H donors (X =
C, N, O) and the p-electron cloud of an aromatic moiety are a
well-documented phenomenon which is of great importance
been reported in the solid state. We now report isolation,
structures, IR spectra, and calculations for hydrogen-bonded
benzene complexes of water or ammonia stabilized by the
well-known Lewis acid B(C6F5)3,[10] which enhances the Lewis
acidic character of H2O and NH3. The new complexes
B(C6F5)3·H2O·C6H6 (1) and B(C6F5)3·NH3·C6H6 (2) feature
benzene–ammonia and benzene–water interactions with
located hydrogen atoms, and therefore permit a straightfor-
ward comparison of the spectroscopic and structural conse-
quences of benzene complexation.
in structural biology.[1] As prototypical models for C H···p,
À
À
À
O H···p, and N H···p interactions, benzene–CHX3 (X = hal-
ogen),[2] benzene–water,[3] and benzene–ammonia[4] com-
plexes have been the subject of numerous theoretical and
gas-phase experimental studies.[5] However, in these gas-
phase experiments, the hydrogen positions were not directly
determined. Experimental evidence for X H···p interactions
in the solid state is usually based on accurate X-ray crystallo-
graphic data, but surprisingly few structurally characterized
À
Single crystals of benzene complexes 1 and 2 were
obtained by recrystallization of B(C6F5)3·H2O[11] and B-
[12]
À
solid-state structures with C/N/O H···p interactions have
been reported. Examples include a calixarene ligand–water
complex featuring O H···p interactions, reported in 1991,
(C6F5)3·NH3
from benzene. The crystals are fragile and
readily decompose under reduced pressure or on exposure in
a dry box, which results in evaporative benzene loss. 1H NMR
experiments show that the chemical shifts of complexes in
CDCl3 are indistinguishable from those of B(C6F5)3·H2O or
B(C6F5)3·NH3 and free C6H6, which suggests that benzene in 1
and 2 is displaced by the polar solvent CDCl3. Nonetheless,
the benzene complexes proved sufficiently stable for IR
spectroscopic studies. Figure 1 shows IR spectra for 2 and
B(C6F5)3·NH3 (for 1 and B(C6F5)3·H2O, see Supporting
[6]
À
(mercuracarborand–water)2–benzene complex {(C2B10H10-
Hg)3·H2O}2·C6H6, in which the H atoms could not be located
accurately because of crystalline disorder,[7] and a hexaanthryl
octamino cryptand stabilized chloroform–benzene complex.[8]
Reed and co-workers showed that the hydronium ion H3O+
forms complexes to three benzene rings in the presence of
weakly coordinating anions.[9] These were obtained by the
reaction of H(carborane) acids with 1 equiv of water to give
well-characterized [H3O][carborane] salts that are soluble in
benzene, dichloromethane, and 1, 2-dichloroethane. The
benzene salts have the formula [H3O·3C6H6]+ [carborane]À
(carborane = CHB11R5X6; R = H, Me, Cl; X = Cl, Br, I) and
the X-ray crystal structure of [H3O·3C6H6][CHB11Cl11]·C6H6
was determined.[9a] At present, no benzene–ammonia com-
plex has been structurally characterized in the solid state and,
apart from the mercuracarborand complex mentioned above,
no structurally characterized benzene–water complex has
À
Information) in the N H stretching region. In comparison
to those of B(C6F5)3·H2O and B(C6F5)3·NH3,[12] the vibrational
À
À
bands of the O H and N H bonds in 1 and 2 display
considerable shifts to lower frequency of up to 87 cmÀ1 for 1
[*] Prof. X. Wang
State Key Laboratory of Coordination Chemistry, Nanjing National
Laboratory of Microstructures, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093 (China)
E-mail: xpwang@nju.edu.cn
Prof. P. P. Power
Department of Chemistry, University of California
Davis, CA 95616 (USA)
E-mail: pppower@ucdavis.edu
[**] The authors thank the US National Science Foundation (CHE-
0948417), the National Natural Science Foundation of China
(21021062), and the Fundamental Research Funds for the Central
Universities of China (020514330006) for financial support, and
Albemarle Corporation for donation of B(C6F5)3.
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103904.
Figure 1. The N H stretching region of the IR spectra of B(C6F5)3·NH3
and 2.
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10965

Communications
and 30 cmÀ1 for 2, which indicate significant interactions
between H2O/NH3 and C6H6.
but is comparable to the range observed in [H3O]
[CHB11Cl11]·3C6H6 (3.05–3.16 ꢀ).[9] We attribute the shorter
distances in the mercuracarborand complex to the fact that
the water molecule is confined within a cavity.[7] The
interactions of H3O+ with C6H6 in the [H3O·3C6H6]+ complex
differ from those in 1 in that the H atoms do not point directly
X-ray crystal studies reveal that 1 crystallizes as a
racemate of two chiral isomers 1-P and 1-M in space group
ꢀ
P1 (Figure 2) and 2 in space group P21/c (Figure 3). In 1 and 2,
water O and ammonia N atoms remain bound directly to
boron, and the hydrogen atoms of coordinated water or
À
at the center of the C6H6 rings but to C C bonds and afford
H···ring plane distances of 2.13, 2.16, and 2.26 ꢀ[9] (cf.
H···centroid distances of 2.58 ꢀ in 1). The O(N)···benzene
centroid distances in 1 and 2 are about 0.12 ꢀ (0.31 ꢀ) shorter
than those determined spectroscopically for a benzene–water
(benzene–ammonia) dimer in the vapor phase.[3,4] There are
also Hwater/ammonia···F contacts (Hwater···F 2.18(6), 2.42(5) ꢀ;
Hammonia···F 2.26(1) ꢀ; ꢁ van der Waals radii of H and F =
2.67 ꢀ) while HC H ···F contacts are very weak (HC H ···F ꢀ
6
6
6
6
2.67 ꢀ in 1 and ꢀ 2.61 ꢀ in 2). The angle between the H-O-H
plane and the C6H6 plane in 1 is about 59.3(1)8 (Figure 4),
whereas in the gas-phase water–benzene complex the two
planes are perpendicular to each other.[3] The difference in
water orientation in 1 from that in gas phase is probably
À
À
caused by intramolecular O H···F C hydrogen-bonding
Figure 2. Thermal ellipsoid (50%, 90 K) plot of 1. Selected bond
À
À
À
lengths [ꢀ] and angles [8]: O1 H1 0.78(5), O1 H2 0.83(7), O1 B1
À
À
1.630(3), H2 F5 2.18(6), H1 F11 2.42(5), O1···benzene centroid
3.231(3), H1···benzene centroid 2.58(5); H1-O1-H2 103(5).
Figure 3. Thermal ellipsoid (50%, 90 K) plot of 2. Selected bond
À
À
À
lengths [ꢀ] and angles [8]: N H1 0.93(1), N H2 0.88(1), N H3
À
0.90(1), N B 1.633(1), H2···F10 2.26(1), H1···F5 2.26(1), H1···benzene
centroid 2.64(1), N···benzene centroid 3.281(1); H1-N-H2 105(1), H1-
Figure 4. Angle between the H-O-H plane and the C6H6 plane in 1.
N-H3 107(1), H2-N-H3 107(1).
ammonia were located unambiguously from the difference
maps. The (C6F5)3B·H2O and (C6F5)3B·NH3 units interact with
the benzene rings such that the oxygen and nitrogen atoms
are located approximately over the centroids of the benzene
rings. One hydrogen atom of water or ammonia is oriented
toward the benzene ring. The O(N)···benzene centroid
distances, H···benzene centroid separations, and the H-
O(N)-H angles in 1 and 2 are listed in Table 1. It is noteworthy
that the O···benzene centroid distance of 1 (3.231(3) ꢀ) is
longer than that of {(C2B10H10Hg)3·H2O}2·C6H6 (2.99(1) ꢀ)[7]
interactions, as the geometry of B(C6F5)3·H2O with the lone
pair of oxygen approaching fluorine atoms is highest in energy
on the potential-energy surface (see Supporting Information).
To further study the strength of the hydrogen bonds in 1
and 2, and how they are affected by Lewis acid B(C6F5)3, we
performed ab initio calculations for model complexes
BH3·H2O·C6H6 (3), H2O·C6H6 (4), BH3·NH3·C6H6 (5), and
NH3·C6H6 (6).[13] They were found to have energy minima at
the HF/6-31G* level, and their geometries were further
optimized at the MP2/aug-cc-pVTZ level (Table 1 and
Table 1: Comparison of selected experimental and calculated structural parameters for 1, 2, and related complexes.
1
H2O·C6H6
BH3·H2O·C6H6 (3) H2O·C6H6 (4)
2
NH3·C6H6
BH3·NH3·C6H6 (5) NH3·C6H6 (6)
spectroscopic[a] calculated[b]
calculated[b]
spectroscopic[c] calculated[b]
calculated[b]
H···centroid [ꢀ]
O(N)···centroid [ꢀ] 3.231 (3) 3.347(5)
H-O(N)-H [8] 103(5)
2.58 (5)
–
2.15
3.028
105
2.71, 2.71
3.215
102
2.64(1)
3.281(1) 3.590(5)
106(1)
–
2.13
3.171
108
2.45
3.467
106
–
–
[a] Ref. [3]. [b] At the MP2/aug-cc-pVTZ level. [c] Ref. [4].
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Angew. Chem. Int. Ed. 2011, 50, 10965 –10968

Figure 5). BH3·H2O·C6H6 (3) has two chiral isomers 3-P and
3-M and a broadly similar structure to that of 1, except that in
3 the H-O-H plane is perpendicular to that of the benzene
ring. The experimentally determined O(N)···centroid distan-
Figure 6. Thermal ellipsoid (50%, 90 K) plot of 7. Selected bond
À
À
À
lengths [ꢀ] and angles [8]: N1 H1 0.83(6), N1 H2 0.76(7), N1 H3
À
0.90(7), N1 B1 1.633(6), H2···F15 2.22(6), H1···F6 2.19(5), H3···ben-
zene centroid 2.48(6), N1···benzene centroid 3.206(5); H1-N1-H2 111
(6), H1-N1-H3 113(5), H2-N1-H3 109(6).
acidities of water and ammonia on coordination to a Lewis
acid. The electron-donating substituent CH3 strengthens the
À
N H···p hydrogen bonding, which is reflected by a shorter
N···phenyl centroid distance in the complex B-
(C6F5)3·NH3·C7H8 (7). In addition, two chiral isomers are
observed as a racemate in 1 and supported by theoretical
calculations. A discrete chiral isomer of BH3·H2O·C6H6 (3) in
the gas phase is also possible and might be detected by
rotational spectroscopy.[14] Studies on complexes analogous to
1 and 2 involving other aryl rings are under way.
Figure 5. Calculated geometries of complexes 3–6.
ces in 1 and 2 are somewhat longer than those calculated for 3
and 5, but they are shorter than those spectroscopically
determined for H2O·C6H6 and H3N·C6H6, respectively
(Table 1).[3,4] This can be rationalized on the basis that
Lewis acids BH3 and B(C6F5)3 increase the acidities of the
hydrogen atoms of water and ammonia in complexes 1–3 and
Experimental Section
Compound 1: B(C6F5)3·H2O (0.305 g, 0.576 mmol) was dissolved in
benzene (ca. 2 mL). Overnight storage of the solution at about 58C
afforded colorless X-ray-quality crystals of 1. IR (KBr, Nujol, cmÀ1):
3537 (br, nas(OH)), 3412 (br, ns(OH)).
À
5, and thus increase the O(N) H···p hydrogen-bond affinities.
Correspondingly the binding energy of BH3·H2O (BH3·NH3)
to C6H6, corrected for basis set superposition error (BSSE), is
about 22.1 kJmolÀ1 (21.3 kJmolÀ1) larger than that of H2O
(NH3) to C6H6, calculated at the MP2/aug-cc-pVTZ level.[13]
Gas-phase and theoretical studies on ammonia–benzene
and ammonia–toluene complexes have shown that the
electron-donating substituent CH3 increases the p-electron
Compound 2: B(C6F5)3·NH3 (0.331 g, 0.626 mmol) was dissolved
in benzene (ca. 2 mL). Overnight storage of the solution at about 58C
~
afforded colorless X-ray-quality crystals of 2. IR (KBr, Nujol): n =
3363 (nas(NH)), 3330 (nas(NH)), 3265 cmÀ1 (nas(NH)).
Compound 7: B(C6F5)3·NH3 (0.284 g, 0.537 mmol) was dissolved
in toluene (ca. 1 mL). Overnight storage of the solution at about
À188C afforded colorless X-ray-quality crystals of 7.
À
cloud density of the phenyl ring and thus strengthens the N
H···p hydrogen bonding.[4b] To test this prediction in the solid
Crystal data for 1, 2, and 7 at 90(2) K with MoKa radiation (l =
[12]
state, recrystallization of B(C6F5)3·NH3 from toluene was
ꢀ
0.71073 ꢀ). 1: C24H8BF15O, M = 608.11, triclinic, space group P1, Z =
4, m = 0.197 mmÀ1, a = 10.630(1), b = 13.656(1), c = 15.388(1) ꢀ, a =
87.260(1), b = 86.322(1), g = 86.413(2)8, V= 2222.6(4) ꢀ3, R1 = 0.0468
for 8725 (I > 2s(I)) reflections, wR2 = 0.1421 (all data). 2:
C24H9BF15N, M = 607.13, monoclinic, space group P21/c, Z = 8, m =
carried out, and this afforded single crystals of B-
(C6F5)3·NH3·C7H8 (7). The structure of 7 (Figure 6) is similar
to that of 2 but the N···centroid distance is about 0.075 ꢀ
shorter. This is in agreement with spectroscopic and theoret-
ical predictions.[4b]
In summary, we have shown that water–benzene and
ammonia–benzene complexes are stabilized by the Lewis acid
B(C6F5)3 and provide rare examples of well-characterized
water–benzene and ammonia–benzene complexes in the solid
state. The H2O/NH3–C6H6 interactions in these complexes
0.194 mmÀ1
, a = 13.6490(7), b = 18.910(9), c = 17.9196(9) ꢀ, b =
90.474(1), V= 4449.1(4) ꢀ3, R1 = 0.0266 for 6998 (I > 2s(I)) reflec-
tions, wR2 = 0.0727 (all data). 7: C25H11BF15N, M = 621.16, triclinic,
space group P1, Z = 4, m = 0.189 mmÀ1, a = 10.536(2), b = 13.795(3),
ꢀ
c = 15.992(3) ꢀ, a = 86.281(3), b = 89.170(2), g = 87.176(2)8, V=
2316.5(8) ꢀ3, R1 = 0.0715 for 6635 (I > 2s(I)) reflections, wR2 =
0.2319 (all data). For 1, 2, and 7, all H atoms were found in difference
maps, and those attached to oxygen or nitrogen were refined freely.
À
À
lead to redshifts of the O H/N H stretching vibrations in
comparison to those of benzene-free H2O– or NH3–borane
complexes. In comparison to simple gas-phase water–benzene
The other
H atoms were refined by using a riding model.
CCDC 810110 (1), 810111 (2) and 810112 (7) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
and
ammonia–benzene
complexes,
the
shortened
O(N)···centroid distances are consistent with increased H
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Communications
Received: June 8, 2011
Revised: July 29, 2011
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Keywords: ammonia · hydrogen bonds · Lewis acids ·
p interactions · water chemistry
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