Ion-pairing of phosphonium salts in solution: C-H...halogen and C-H...π hydrogen bonds

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 0000, 00, 0 – 0

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Ion-Pairing of Phosphonium Salts in Solution

FULL PAPER

1

+

Table 2. Concentration-dependent

H NMR

chemical

shifts

d

H

Ph CHꢀPPh

ions (2a). The determination of the K_D

2

3

ꢀ

(

600 MHz) for C(a)ꢀH and dissociation constants K

D

[m] of 2a X with

values listed in Table 2 will be discussed below.

ꢀ

different counterions X in CD

2

Cl

2

.

ꢀ

[a]

[b]

Salt

A

C

H

T

U

N

G

T

R

E

N

U

N

G

[2aX ]/m

d

H

[ppm]

x

paired,exptl

K

D

[m]

x

paired,calcd

ꢀ

Benzyl triphenylphosphonium salts (1a,bX ): It was already

₄ꢀ

ꢀ5

ꢀ

2

aBPh

1.75ꢄ10

5.80

5.75

5.76

5.81

5.80

5.76

5.75

5.78

5.77

5.80

5.86

5.91

5.92

5.94

5.96

5.81

5.85

5.96

5.99

6.06

6.13

6.15

6.19

6.22

6.25

6.07

6.28

6.77

7.18

7.46

–

noted by Schiemenz and co-workers, that the “BPh₄effect”

For that reason, we also

tested the “BPh₄effect” in sterically less hindered systems

and investigated the concentration-dependent effects of the

counteranions on the H NMR chemical shifts d (600 MHz,

ꢀ4

ꢀ3

ꢀ2

1

4

1

8

2

5

.03ꢄ10

.07ꢄ10

.76ꢄ10

.25ꢄ10

.08ꢄ10

.47ꢄ10

[9,10]

decreases with steric shielding.

ꢀ

1

H

CD Cl ) of the C(a)ꢀH protons of the benzyl triphenylphos-

2

average d

2.13ꢄ10

H

+

phonium ions PhCH ꢀPPh (1a) and the 4-(trifluorome-

₆ꢀ

ꢀ5

2

3

2

aSbF

0.00

0.16

0.47

0.74

0.79

0.89

1.00

0.08

0.17

0.40

0.46

0.60

0.75

0.79

0.88

0.94

1.00

0.13

0.22

0.43

0.61

0.73

0.80

0.95

0.98

1.00

0.23

0.57

0.81

0.86

0.97

0.98

1.00

0.03

0.13

ꢀ4

ꢀ3

ꢀ2

thyl)benzyl triphenylphosphonium ions (p-CF -C H )CH ꢀ

1

5

1

3

5

.07ꢄ10

.02ꢄ10

.44ꢄ10

.23ꢄ10

.68ꢄ10

3

6

4

2

ꢀ4

+

6ꢀ10

A

H

U

G

R

N

U

G

PPh (1b) (Tables S1 and S2 in the Supporting Informa-

3

0.72

0.80

0.88

0.90

0.08

0.14

0.26

tion). Figure 2a and b illustrate the data.

4^ꢀ

ꢀ5

aBF

2.12ꢄ10

ꢀ5

ꢀ4

ꢀ3

ꢀ2

4

1

3

8

2

4

1

2

6

.22ꢄ10

.03ꢄ10

.36ꢄ10

.91ꢄ10

.01ꢄ10

.26ꢄ10

.13ꢄ10

.87ꢄ10

.06ꢄ10

ꢀ4

2.2ꢀ10

ACHTUNGTRNENUNG( 0.46)

0.61

0.72

0.80

0.87

0.92

0.94

0.17

0.26

ꢀ

ꢀ5

2

aBr

1.81ꢄ10

ꢀ5

ꢀ4

ꢀ3

ꢀ2

3

1

3

6

1

2

6

.58ꢄ10

.03ꢄ10

.04ꢄ10

.09ꢄ10

.83ꢄ10

.12ꢄ10

.51ꢄ10

.62ꢄ10

ꢀ5

7.6ꢀ10

ACHTUNGTRNENUNG( 0.43)

0.61

0.70

0.82

0.92

0.95

0.97

0.32

[c]

AHCTUNGTERNNUNG( 7.6)

7.95

8.02

8.07

6.38

7.24

7.87

8.01

8.29

8.28

8.32

8.37

ꢀ

ꢀ5

2

aCl

2.31ꢄ10

ꢀ4

ꢀ3

ꢀ2

ꢀ5

1

7

1

4

1

2

5

.00ꢄ10

.13ꢄ10

.76ꢄ10

.45ꢄ10

.14ꢄ10

.61ꢄ10

.83ꢄ10

3.4ꢀ10

ACHTUNGTRNENUNG( 0.57)

0.81

0.87

0.92

0.95

0.96

0.98

1

[

x

a] K

paired,exptl ꢁ 0.5. [b] Calculated using K

with signals of aryl protons.

D

derived from the data for phosphonium salt concentrations where

Figure 2. Concentration-dependent H NMR (600 MHz, 278C) chemical

ꢀ

shifts d

ferent counteranions X =Cl (*), Br (&), BF

CD Cl

H

of the benzylic C(a) H protons of a) 1aX or b) 1bX with dif-

ꢀ

D

from this table. [c] Superimposed

ꢀ

4

(N), or BPh

(^~) in

4

2

.

these concentrations we observe ion pairs almost exclusively.

Literature NMR spectra of phosphonium salts in CD Cl or

ꢀ

The phosphonium halides 1aCl and 1bBr , as well as

2

ꢀ

CDCl solution, which were recorded under typical condi-

the tetrafluoroborates 1a,bBF , show similar behavior as

3

4

tions of NMR measurements, can thus be expected to char-

acterize the ion pairs.

the corresponding benzhydryl triphenylphosphonium salts

ꢀ

2aX . In the concentration range [1a,bX ] ꢃ 0.02m, the d

H

ꢀ

3

At lower concentrations (<5ꢄ10 m), the chemical shifts

values of the C(a)ꢀH protons are virtually constant and we

can conclude that the phosphonium halides and tetrafluoro-

borates predominantly exist as ion pairs under these condi-

ꢀ

of the C(a)ꢀH protons of 2aX with all investigated anions

ꢀ

except BPh₄decrease markedly and approach d of the tet-

H

ꢀ

3

raphenylborate salt (Figure 1). Finally, at a concentration of

tions (Figure 2). At lower concentrations (<5ꢄ10 m), the

ꢀ

5

ꢀ5

2

.13ꢄ10 m, the chemical shift of the C(a)ꢀH proton of

d_Hvalues decrease: For 2ꢄ10 m solutions of the tetrafluor-

ꢀ

aSbF₆reaches a value of d =5.77 ppm (Table 2), which

oborate salts, we determined chemical shifts of d =4.39

H

ꢀ

is identical to d_Hobserved for 2aBPh . We can therefore

(1aBF ) and 4.49 ppm (1bBF ) for the C(a)ꢀH protons

4

assume that this d_Hvalue corresponds to the unpaired

(Tables S1 and S2 in the Supporting Information).

Chem. Eur. J. 2013, 00, 0 – 0

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

3

&

ÞÞ

These are not the final page numbers!

J. Ammer, K. Karaghiosoff et al.

The C(a)ꢀH protons of the tetraphenylborate salts

ꢀ

1

a,bBPh , however, show the opposite effect: Their chemi-

4

ꢀ

cal shifts are also virtually constant (1aBPh : 3.95 ppm;

4

ꢀ

1

bBPh₄: 3.68 ppm) at concentrations ꢃ 0.02m, but increase

with decreasing concentration until they reach the value of

ꢀ

4

d =4.32 ppm (1aBPh ) or 4.31 ppm (1bBPh ) at concen-

H

4

ꢀ

5

trations of 2ꢄ10 m (Figure 2).

ꢀ

Thus, the d_Hvalues of the C(a)ꢀH protons of 1aX with

ꢀ

different counteranions X approach a common value of

ꢀ

4

.32 < d_H< 4.39 ppm at low concentrations of 1a X , and

we can estimate d_H_,_u_n_p_a_i_r_e_dꢁ 4.37 ppm for the C(a)ꢀH pro-

tons of the free benzyl triphenylphosphonium ion 1a. Anal-

ogously, 4.31 < d_H< 4.49 ppm at low concentrations of

ꢀ

1

bX leads to an estimate of d_H_,_u_n_p_a_i_r_e_dꢁ 4.44 ppm for the

C(a)ꢀH protons of free 1b.

1

Figure 3. Plot of the H NMR chemical shifts d

H

for the C(a)ꢀH in aryl-

The knowledge of d for the unpaired phosphonium ions

H

methyl triarylphosphonium salts (ion pairs) with different counter-anions

(2a, 3t) and CDCl

energies of transfer DG

1

a,b allows us to directly compare the magnitude of the

ꢀ

X

in CD

2

Cl

2

3

(other salts) against the single free ion

ꢀ

“

^B^P^h4 effect” with the influence of C(a)ꢀH···halide hydro-

0

ꢀ

t

(H

2

O!CH

3

CN) of the anions X . The points

ꢀ

4^ꢀ

gen bonding. The large difference between the C(a)ꢀH pro-

for 1aBPh

and 2aBPh deviate from the correlation (not shown). See

4

ꢀ

Table S3 in the Supporting Information for numeric values and referen-

ces.

tons of the ion pairs 1aCl and 1aBPh

(Dd = +

4

H

ꢀ

1.54 ppm) is mostly due to the deshielding effect of Cl ,

ꢀ

whereas the shielding effect of BPh₄contributes less than

ꢀ

3

0% to the observed Dd . The smaller deshielding effects

C(a)ꢀH protons of 2aX and other arylmethyl phosphoni-

H

ꢀ

0

t

of the “normal” anions X =Cl and BF₄on the C(a)ꢀH

um salts in CD Cl or CDCl correlate linearly with DG

(H O ! CH CN) of the anions.

2

3

ꢀ

protons of 1a (e.g., Dd ꢁ +1.0 ppm for 1aCl relative to

H

2

3

unpaired 1a) compared to those for the analogous benz-

ꢀ

hydryl derivatives 2aX (e.g., Dd_Hꢁ +2.5 ppm for 2aCl

Other NMR signals of the phosphonium ions in CD Cl sol-

2 2

relative to unpaired 2a) are explained by the statistical

factor of two C(a)ꢀH protons in 1a (vs one in 2a) and the

ution

[6]

lower C(a)ꢀH acidity of 1a (pK 17.6 in DMSO) com-

Besides the large change in d_Hfor the C(a)ꢀH protons,

a

ꢀ

pared to that of 2a (pK ꢁ 9 in DMSO estimated from the

Tables 1 and 3 also show the effect of the counter-anion X

a

3

1

13

correlation equation published in ref. [5] and pK 30.6 for

on other P NMR, H NMR, and C NMR signals of 2a

and 1a in CD Cl .

a

[29]

Ph CH₂).

2

The p-CF₃substituent decreases the pK_avalue of the

phosphonium salt 1b in DMSO to 14.6, compared to

ꢀ

Benzhydryl triphenylphosphonium salts (2aX ): The excel-

[6]

1

pK 17.6 for the parent compound 1a, which results in

lent agreement between the H NMR chemical shifts of

2.13ꢄ10 m 2aSbF₆and the concentration-independent

H NMR chemical shifts of 2aBPh (Table 1) corroborates

the assumption that 2a SbF₆mostly exists in the form of

the free ions at this concentration. Pairing with the Cl

anion polarizes the bond between a-C and a-H (Dd = +

a

ꢀ

ꢀ5

ꢀ

stronger CꢀH···X hydrogen bonds in the 1bX ion pairs.

1

ꢀ

Accordingly, Figure 2b shows comparably large downfield

4

ꢀ

shifts for the C(a)ꢀH protons in the 1bBr and 1bBF ion

4

ꢀ

pairs. The “BPh₄effect” also increases with the C(a)ꢀH

acidity: The upfield shift of Dd ꢁ ꢀ0.78 ppm for the C(a)ꢀ

H

ꢀ

H protons of 1b that results from ion pairing with the BPh

2.53 ppm, Dd =ꢀ6.0 ppm), while the effect on the phospho-

4

C

ion is almost twice as large as the upfield shift of Dd_H

ꢁ

rus atom is rather small (Dd = +0.6 ppm). The coupling

P

ꢀ

1

ꢀ

4

ꢀ

0.44 for 1aBPh₄(Figure 2). The relative magnitudes of

constant J =128.7 Hz for C(a)ꢀH of 2a BPh is typical

H,C

ꢀ

3 [31]

the “ordinary anion” effects and the “BPh effect” in 1b

for sp carbons, and increases slightly in the presence of

4

ꢀ

1

X

are therefore similar to those in the parent compounds

the hydrogen-bond acceptor Cl (D J = +2.6 Hz). A

slight increase of J by a few Hz was previously observed

H,C

ꢀ

1

a X .

H,C

for other CꢀH···X hydrogen bonds and may result from the

0

ꢀ

Correlation with DG (H O!CH CN) of the anions X :

additional electric field component along the CꢀH bond in

t

2

3

[32]

1

2

Figures 1 and 2 show that the deshielding of the C(a)ꢀH

the presence of the hydrogen-bond acceptor. The J and J

coupling constants between C(a)ꢀH and P change by

ꢀ

protons in the 2aX and 1a,bX ion pairs increases in the

ꢀ

1

2

order SbF₆< BF₄< Br < Cl . An anionꢃs ability to act

as hydrogen bond acceptor is related to its single free ion

D J =ꢀ2.5 and D J = +1.2 Hz, respectively.

C,P

H,P

The ortho-protons of the phenyl groups are also deshield-

ed substantially (Dd_Hꢁ +0.50 ppm), especially if one con-

0

t

[30]

energy of transfer DG (H O ! CH CN), since a large

2

3

ꢀ

contribution to the transfer energy is the loss of the HOꢀ

siders that the effect of the Cl anion is averaged over six o-

ꢀ

H···X hydrogen bonds to the good hydrogen-bond donor

PPh or four o-CHPh protons. Other effects are small: The

3

2

H O. Figure 3 illustrates that the chemical shifts d_Hfor the

2

iso- and ortho-carbons are also deshielded slightly (Dd ꢁ

C

&

4

&

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 0000, 00, 0 – 0

ÝÝ

These are not the final page numbers!

Ion-Pairing of Phosphonium Salts in Solution

FULL PAPER

3

1

13

ꢀ

Table 3. P NMR (162 MHz), H NMR (400 MHz), and C NMR (100 MHz) data for the phosphonium ion 1a in CD

2

Cl

2

. Data for 1a X were deter-

mined at concentrations where the phosphonium salts exist as ion pairs.

+

P

d

CH

[ppm] ( JH,P

Hz])

2

P

o-CH

2

Ph

m-CH

2

Ph

p-CH

2

Ph

o-PPh

[ppm]

3

m-PPh

[ppm]

3

p-PPh

H

d [ppm]

3

2

1

[a]

Salt

P

[ppm]

d

H

J

H,C [Hz]

d

H

[ppm]

d

H

[ppm]

d

H

[ppm]

d

H

d

H

[

ꢀ

1

“

1

D

aCl

23.1

22.2

5.42 (14.7)

4.56 (14.1)

4.39 (14.0)

4.32 (14.0)

3.94 (13.8)

+1.03 (+0.7)

ꢀ0.38 (ꢀ0.2)

134.6

134.2

7.07–7.10

6.91–6.94

6.87–6.89

6.85–6.89

6.73–6.76

~ +0.20

7.14–7.18

7.20–7.25

7.25–7.28

7.26–7.28

7.21–7.28

~ꢀ0.10

7.24–7.29

7.31–7.36

7.38–7.41

7.38–7.44

7.34–7.39

~ꢀ0.13

7.70–7.76

7.48–7.54

7.43–7.46

7.38–7.44

7.21–7.28

~ +0.29

7.60–7.66

7.65–7.71

7.68–7.72

7.68–7.71

7.57–7.62

~ꢀ0.07

7.77–7.82

7.84–7.88

7.89–7.92

7.79–7.84

~ꢀ0.10

4^ꢀ

aBF

[

b]

d]

[c]

free” 1a

–

[c]

[e]

[f]

ꢀ

[f]

aBPh

4

21.6

133.9

ꢀ

[g]

[c]

A

C

H

T

U

N

G

T

R

E

N

U

N

G

(Cl )

–

4^ꢀ

[h]

[c]

D

A

C

H

T

U

N

G

T

R

E

N

U

N

G

(BPh

)

~ꢀ0.12

ꢄꢀ0.05

~ꢀ0.05

~ꢀ0.16

~ꢀ0.10

~ꢀ0.09

+

CH

[ppm] ( JC,P

Hz])

2

P

i-CH

2

Ph

o-CH

2

Ph

m-CH

2

Ph

p-CH

2

Ph

i-PPh

3

o-PPh

3

m-PPh

3

p-PPh

3

1

2

3

4

5

1

2

3

Salt

d

C

d

C

[ppm] ( JC,P d [ppm] ( JC,P d [ppm] ( JC,P d [ppm] ( JC,P d [ppm] ( JC,P d [ppm] ( JC,P d [ppm] ( JC,P d [ppm]

C C C C C C C

4

[

[Hz])

( JC,P [Hz])

135.5 (3.0)

136.1 (3.1)

136.2 (3.0)

ꢀ0.7 (ꢂ0)

ꢀ5

ꢀ

1

aCl

31.2 (46.9)

31.5 (49.0)

31.7 (49.0)

ꢀ0.5

128.1 (8.5)

127.0 (8.5)

126.5 (8.4)

+1.6 (ꢀ0.1)

131.9 (5.6)

131.5 (5.5)

131.4 (5.4)

129.4 (3.3)

129.8 (3.2)

129.0 (3.9)

129.6 (3.8)

129.7 (3.8)

ꢀ0.7 (+0.1)

118.5 (85.8)

117.6 (86.1)

117.1 (86.4)

+1.4 (ꢀ0.6)

1

135.0 (9.8)

134.6 (9.7)

134.4 (9.7)

+0.6 (+0.1) ꢀ0.4 (ꢂ0)

130.6 (12.6)

130.9 (12.6)

131.0 (12.6)

4^ꢀ

aBF

4^ꢀ

aBPh

(total)

[

i]

D

A

C

H

T

U

N

G

T

R

E

N

U

N

G

A

C

H

T

U

N

G

T

R

E

N

U

N

G

(ꢀ2.1)

+0.5 (+0.2) ꢀ0.4 (ꢀ0.1)

1

13

1

[

a] JH,C determined from C satellites in the H NMR (600 MHz) spectra. [b] Determined from H NMR (600 MHz) spectrum of a 2.08ꢄ10 m solution

of 1a BF

imposed with p-protons of BPh

m 1aBF

aBPh

ꢀ

1

ꢀ5

ꢀ

4

in CD

2

Cl

2

. [c] Not available. [d] Determined from H NMR (600 MHz) spectrum of a 1.75ꢄ10 m solution of 1a BPh

4

in CD

ꢀ

4

. [f] Two signals superimposed. [g] “Ordinary anion effect”: Difference between 1aCl ion pairs and free ions (2.08ꢄ

2 2

Cl . [e] Super-

ꢀ

5

ꢀ

ꢀ5

ꢀ

1

0

4

). [h] “BPh

4

effect”: Difference between 1a BPh

4

ion pairs and free ions (1.75ꢄ10 m 1aBPh ). [i] Difference between 1aCl and

4

ꢀ

1

4

.

ꢀ

+

0.5 to +2 ppm) in the presence of Cl , while the meta-

NMR Signals of the anions X in CD Cl solution

2 2

and para-positions of the aromatic rings are slightly shielded

1

9

(

Dd_Hꢁ 0 to ꢀ0.18 ppm, Dd ꢁ ꢀ0.7 to ꢀ1.3 ppm). The ef-

Figure 4a shows the F NMR (376 MHz) spectrum of

C

ꢀ

fects of Br , BF , and SbF on the NMR signals of 2a are

2aBF₄in CD Cl at a concentration where the salt exists as

ion pairs (6ꢄ10 m). We observed a 1:1:1:1 quartet ( J

4

6

2

ꢀ

1

similar but less pronounced.

ꢁ

F,B

1

ꢀ

1

.2 Hz) at d =ꢀ152.0 ppm for the main isotopomer, BF₄

F

ꢀ

3

11

Benzyl triphenylphosphonium salts (1aX ): Table 3 includes

(I= = for the B nucleus), together with the unresolved

2

1

+

10

two sets of H NMR data for the free PhCH ꢀPPh ion

signal for the B (I=3) isotopomer about 0.05 ppm further

downfield. The corresponding signal of the boron atom in

2

3

ꢀ

5

(

1a), one determined from a 2.08ꢄ10 m solution of

aBF , the other from a 1.75ꢄ10 m solution of 1a BPh .

ꢀ

ꢀ5

ꢀ

11

1

the B NMR (128 MHz) spectrum is at d =ꢀ2.0 ppm.

4

B

1

9

Again, the good agreement between the two data sets con-

The F NMR is sensitive enough so that we could also de-

ꢀ

firms the assignment to the unpaired phosphonium ion 1a.

termine the fluorine chemical shift of the BF anion at a

4

1

ꢀ

ꢀ5

ꢀ

Ion-pairing affects the H NMR signals of 1aCl in a simi-

concentration of 2ꢄ10 m, where 2aBF₄exists as free ions

lar way as those of the corresponding benzhydryl triphenyl-

in CD Cl (Figure 4b). Under these conditions, the signal

2

phosphonium ions (2a): The C(a)ꢀH protons experience

for the main isotopomer is found at d ꢁ ꢀ153.4 ppm, which

F

2

the largest deshielding (Dd

ꢁ

+1.03 ppm, D J

ꢁ

is upfield by Dd ꢁ ꢀ1.4 ppm compared to the paired

H

H,P

F

ꢀ

+

0.7 ppm); a smaller but still significant deshielding effect

2aBF₄salt.

is observed for the ortho-protons of the PPh group (Dd ꢁ

Figure 4c and d show the heteronuclear NMR spectra of

3

H

ꢀ

2

ꢀ

+

0.29 ppm) and the o-protons of the benzyl group (Dd ꢁ

a ca. 6ꢄ10 m solution of 2aSbF in CD Cl . The antimony

2 2

H

6

1

21

5

123

7

0.20 ppm). The meta- and para-protons of PPh and benzyl

isotopes Sb (I= = ) and Sb (I= = ) have similar natural

2 2

3

are slightly shielded (Dd ꢁ ꢀ0.07 to ꢀ0.13 ppm).

abundances and comparable magnetogyric ratios (g=

H

7

ꢀ1 ꢀ1

[33]

The same protons which experience a deshielding by pair-

6.4435ꢄ10 and 3.4892ꢄ10 radT s , respectively). The

ꢀ

19

ꢀ

ing with Cl are shielded in the 1aBPh ion pairs (Table 3):

F NMR spectrum of 2aSbF₆in CD Cl (Figure 4c) thus

2 2

4

Dd_Hꢁ ꢀ0.38 ppm for C(a)ꢀH, Dd ꢁ ꢀ0.16 and ꢁ

features two superimposed signals at d =ꢀ123.6 ppm: a

H

F

1

121

ꢀ

0.12 ppm for the ortho-protons of the PPh and benzyl

sextet with J

ꢁ 1950 Hz for SbF and an octet with

3

F,Sb

6

1

123

ꢀ

1

groups, respectively. The changes in the chemical shifts of

the other protons are in the same direction and of the same

J_F_,_S_bꢁ 1020 Hz for SbF . The ratio of the J coupling

6

F,Sb

constants of the two isotopomers corresponds to the ratio of

the magnetogyric ratios of the antimony isotopes (i.e., the

reduced coupling constants, which are proportional to J_F_,_S_b/

ꢀ

magnitude (Dd ꢁ ꢀ0.08 to ꢀ0.11 ppm) as in 1aCl .

H

31

1

The P NMR signals (Dd ꢁ +1.5 ppm when going from

P

ꢀ

13

1

aBPh₄to 1aCl ) and the C NMR signals of 1a, includ-

g , are the same for both isotopomers). The coupling con-

Sb

ꢀ

1

121

ing those of C(a), vary only little with the counteranion X

Table 3).

stant of J

ꢁ 1960 Hz is also found in the Sb NMR

Sb,F

(

(65 MHz) spectrum, in which five lines of the septet at d

Sb

ꢁ

86.2 ppm are observed (Figure 4d). Due to the broadness

of the signal, we could not detect the F NMR signal of

1

9

Chem. Eur. J. 2013, 00, 0 – 0

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

5

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J. Ammer, K. Karaghiosoff et al.

allow us to derive the mole fractions of the paired phospho-

nium ions, x_p_a_i_r_e_d_,_e_x_p_t_l[Eq. (1)].

^dH ^ꢀ^d

H,unpaired

x_p_a_i_r_e_d_,_e_x_p_t_l¼ _d

ð1Þ

H,paired

^ꢀ^dH,unpaired

At phosphonium salt concentrations where x_p_a_i_r_e_d_,_e_x_p_t_l

ꢁ

0

1

.5, we estimated the dissociation constants K (m) for

D

ꢀ

a,bX and 2aX in CD Cl as defined by Equation (2) in

2 2

+

which [R P X ] is the total salt concentration. The result-

ꢀ

0

4

ing values are listed in Table 4.

þ

ꢀ

2

½

R P ꢅ

ꢆ ½X ꢅ

ð1 ꢀ xpaired;exptl^Þ

4

unpaired

þ

ꢀ

0

K ¼

D

¼

ꢆ ½R P X ꢅ

þ

ꢀ

4

½R P X ꢅ

x

paired;exptl

4

paired

ð2Þ

ꢀ

Table 4. Dissociation constants K

and 2aX with different counter-anions X in CD

D

[m] for phosphonium salts 1a,bX

ꢀ

2

Cl

2

.

[

a]

0[b]

[c]

DGacid

DG

[kJmol

t

K

D

[m]

ꢀ

ꢀ1

ꢀ

X

[kcalmol

]

A

H

U

G

R

N

U

G

]

1aX

2.5ꢄ10

1bX

2aX

₄ꢀ

6^ꢀ

ꢀ

[d]

ꢀ4

[e]

BPh

SbF

ꢀ

ꢀ32.8

1.1ꢄ10

[d]

[f]

ꢀ4

256

288

315

324

–

6ꢄ10

[g]

ꢀ4

ꢀ5

ꢀ4

ꢀ5

1

9

₄ꢀ

BF

(~0)

A

H

U

G

R

N

U

G

)

A

H

U

G

R

N

U

G

)

2.2ꢄ10

7.6ꢄ10

3.4ꢄ10

Figure 4. Left: F NMR spectra (376 MHz) of 2a BF in CD

2

Cl

2

at con-

4

ꢀ

[f]

ꢀ5

ꢀ

2

ꢀ5

19

Br

31.3

42.1

centrations of a) 6ꢄ10 m, or b) 2ꢄ10 m. Right: c) F NMR spectrum

376 MHz) and d) Sb NMR spectrum (65 MHz) of 2a SbF

ca. 0.06m).

ꢀ

ꢀ5

[f]

1

21

ꢀ

6 2 2

in CD Cl

Cl

pK

6.8ꢄ10

(

[

h]

[h]

[i]

for C(a)ꢀH in DMSO:

17.6

14.6

(~9)

(

a

[

a] Calculated DGacid (298 K) for deprotonation of the conjugate acids

[

34]

0

t 2

HX in the gas phase. [b] Single free ion energies of transfer DG (H O

ꢀ

ꢀ [30]

2

aSbF₆at lower concentrations, where the salt is mostly

! CH

3

CN, 258C) for the anions X .

[c] Dissociation constants in

CD Cl

2

based on d

H

ꢀ

of the C(a)ꢀH protons; this work. [d] Not available.

unpaired.

ꢀ

[e] No effect of X on d

[

from the correlation equation published in ref. [5] and pK 30.6 for

a

Ph CH .

H

of the C(a)ꢀH protons. [f] Not determined.

^T^h^e^N^M^R^d^a^t^a^f^o^r^t^h^e^B^F4 ^aⁿ^d^S^b^F6 anions shown in

Figure 4a,c,d indicate a very high symmetry of the anions

despite the fact that these spectra were recorded under con-

ditions where the phosphonium salts predominantly exist as

ion pairs. This indicates that all fluorine atoms are equiva-

lent on the NMR time scale. The interaction between the

ꢀ

0

t

[35]

g] For BF

4

, DG

ꢁ 0 was estimated. [h] From ref. [6]. [i] Estimated

[

29]

2

With this method, more reliable values of K_Dcan be ob-

ꢀ

BF₄anion and the phosphonium ion 2a can only be noticed

tained than by evaluating K from all investigated solutions,

D

by the slight downfield shift of the signal by Dd_F

ꢁ

which also include those in which paired or non-paired spe-

cies are highly dominating. The mole fractions of unpaired

ions x_p_a_i_r_e_d_,_c_a_l_c_dcalculated from these K_Dvalues are in fair

ꢀ

1.4 ppm, which indicates the averaged effect over four flu-

orine atoms.

ꢀ

We also had a closer look at the NMR data of the BPh

agreement with the experimental values x

(Tables 2,

paired,exptl

4

ꢀ

anions in concentrated solutions of 1a,bBPh in CD Cl .

S1, S2). The K_Dvalues for 1a,bBF₄are only rough esti-

mates, as the effect of ion pairing on d_His small for these

salts (Dd_Hꢁ 0.17 to 0.23 ppm for C(a)ꢀH protons) and we

4

2

11

1

13

ꢀ

However, the B, H, and C NMR signals of the BPh

anion in the phosphonium salts 1a,bBPh₄in CD Cl do not

differ significantly from those of the free BPh ions or

those of 2aBPh₄which does not show any “BPh₄effect”

Table S4 in the Supporting Information).

4

ꢀ

2

ꢀ

cannot determine d

for the unpaired ions 1a,b very

4

H,unpaired

ꢀ

accurately.

(

The dissociation constants K determined in this manner

D

ꢀ

decrease in the order SbF₆> BF₄> Br > Cl (Table 4).

Thus, the degree of association of the phosphonium salts in-

creases with the deshielding of the C(a)ꢀH protons in the

Dissociation constants of the phosphonium salts in CD Cl

2

ꢀ

The large differences between the C(a)ꢀH proton chemical

shifts of the unpaired phosphonium ions (e.g., d

respective ion pairs (SbF <BF <Br <Cl ). In agree-

6 4

=

ment with the higher C(a)ꢀH acidity of 1b compared with

H,unpaired

[

6]

ꢀ

5

.77 ppm for 2a) and the paired phosphonium ions

d_H_,_p_a_i_r_e_d=maximum d_Hfor the a proton measured at the

highest concentration of the salts; see Tables 2, S1, and S2)

1a, the dissociation constants K for all 1bX salts are

D

ꢀ

(

smaller than those for the corresponding 1aX salts (e.g.,

K of 1bBr is already smaller than that of 1aCl although

ꢀ

D

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6

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ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Ion-Pairing of Phosphonium Salts in Solution

FULL PAPER

ꢀ

Cl is more basic than Br ) (Table 4). Since 2a has an even

higher C(a)ꢀH acidity than 1b, one might also expect lower

Effect of C(a)ꢀH acidity: Substituent effects on the NMR

spectra of phosphonium salts

ꢀ

dissociation constants K for 2aX than for 1bX . On the

contrary, the K_Dvalues for 2aX are higher than for 1bX

Table 4), that is, the salts 2aX dissociate more readily.

D

ꢀ

In a series of benzhydryl triphenylphosphonium salts

ꢀ

+

ꢀ

(

Ar CHꢀPPh X (2X ) with different Ar groups, the acidi-

2

3

The reason for this may be a combination of steric hin-

drance and a statistical effect due to the fact that there is

only one C(a)ꢀH proton in the benzhydryl derviatives

ties of the C(a)ꢀH groups increase with the electron-with-

drawing character of the substituents on the benzhydryl

moiety. The series of substituted benzhydryl triphenylphos-

phonium tetrafluoroborates 2a–tBF₄(Table 5), which we

required for our laser flash photolysis experiments,

ꢀ

2

aX but two in the benzyl derivatives 1bX .

[19]

It should be noted, however, that the differences in K_D

can

are rather small, and all K_Dvalues are in the same order of

thus be employed to study the interaction between C(a)ꢀH

ꢀ

magnitude as previously reported for benzhydrylium, trityli-

and BF as a function of C(a)ꢀH acidity.

4

[36]

ꢀ

um, pyrylium, and tetraalkylammonium salts in CH Cl .

The NMR data for the 2a–tBF ion pairs in CD Cl solu-

2

4

2

ꢀ

tion are collected in Table 5, where the salts 2a–tBF₄are

arranged according to the sums of their substituentsꢃ s pa-

1

ꢀ

Effect of the solvent: H NMR Signals for a protons of 2a

[37]

ꢀ

in CD CN solution

rameters. We use the s parameters as a measure for the

C(a)ꢀH acidities of the phosphonium ions here, because the

3

ꢀ

In CD CN, variation of the counteranion X has a much

pK values of 2a–tBF are not available and the pK values

3

a

4

a

1

smaller effect on the H NMR chemical shifts of C(a)ꢀH of

of the closely related benzyl triphenylphosphonium salts in

ꢀ

2

a X than in CD Cl . As in CD Cl , the d values for

DMSO have been shown to correlate with the s parame-

2

H

ꢀ

[6]

1

aBPh₄do not vary with the concentration (d =6.27 ppm,

Figure 5). The very similar d for 2aBF and 2aSbF₆sug-

ters of the benzyl substituents. The H NMR chemical

H

ꢀ

shifts of the C(a)ꢀH protons increase from d ꢁ 6.04 to ꢁ

H

4

H

gest that these compounds are also mostly unpaired at con-

6.80 ppm when the substituents of the benzhydryl group are

varied from electron-donating (2b) to strongly electron-

withdrawing (2s or 2t). Figure S1 in the Supporting Infor-

centrations of ~1ꢄ10^ꢀm in CD CN.

2

3

mation displays a moderate correlation of d for the C(a)ꢀ

H

ꢀ

H protons of 2BF₄with the sums of the s parameters.

If the observed increase of d of the C(a)ꢀH protons and

H

the less pronounced concomitant variations in d of the

C

C(a) atom and d_Pof the phosphorus atom (Table 5) are

ꢀ

linked with stronger interactions with the BF anion, one

4

1

9

should also observe the effect in the F NMR spectra of the

ꢀ

19

11

ꢀ

BF₄ions. Indeed, the F NMR chemical shifts for BF₄

increase from d_Fꢁ ꢀ152.2 to ꢁꢀ150.4 ppm when going

ꢀ

from 2b BF₄to 2tBF (Figure 6 and Table 5). Thus, the

4

greater the C(a)ꢀH acidity of the phosphonium ion 2, the

ꢀ

larger the upfield shift Dd for the BF anion due to the in-

F

4

1

Figure 5. Concentration-dependent H NMR (600 MHz, 278C) chemical

creasing strength of ion pairing with the phosphonium ion.

ꢀ

shifts d

anions X =Cl (*), Br (&), BF

CD CN.

H

of the benzylic C(a)ꢀH protons of 2aX with different counter-

ꢀ

The increasing C(a)ꢀH acidity from 2aBF ion pairs to

ꢀ

4

(N), SbF

6

(*), or BPh

4

(~) in

4

ꢀ

2

tBF ion pairs causes approximately the same shifts in the

4

3

1

19

H and F NMR signals (Dd_Hꢁ +0.45 ppm and Dd ꢁ

F

ꢀ

+

1.6 ppm) as going from the free ions 2a and BF to the

4

ꢀ

The chemical shifts of d_Hꢄ6.29 ppm determined for the

2aBF₄ion pairs (Dd ꢁ +0.46 ppm and Dd ꢁ +1.4 ppm)

H

F

ꢀ

C(a)ꢀH protons of 2a Br in CD CN at concentrations

(Figure 6). In analogous series of neutral benzhydryl deriva-

tives such as benzhydryl halides, substituent variations

induce considerably smaller changes of d_Hfor the C(a)ꢀH

3

ꢀ

4

ꢄ1ꢄ10 m indicate that ion pairing is negligible in this con-

centration range (Table S5 in the Supporting Information).

At larger concentrations, the phosphonium halides do form

ion pairs to some extent. However, the d_Hvalues of the

protons in the other direction (see Figure S1 in the Support-

ing Information).

phosphonium halides in CD CN do not reach a plateau in

Moreover, the a protons of the unpaired benzyl triphenyl-

3

ꢀ

+

the concentration range where 2a Br is soluble in CD CN

phosphonium ions PhCH -PPh

(1a) and p-(CF )-C H ꢀ

3

2

3

6

4

ꢀ

2

+

(

<1ꢄ10 m). Therefore, we cannot estimate the degree of

CH ꢀPPh (1b) have very similar d values despite the dif-

2

3

H

ion pairing or the dissociation constants K in CD CN from

fering substitution patterns (see Tables S1 and S2 in the

Supporting Information). All these observations suggest that

the variations in d_Hof the benzhydryl methine protons ob-

served for the ion pairs of the differently substituted benz-

D

3

the NMR data.

ꢀ

hydryl triphenylphosphonium tetrafluoroborates 2BF₄

Chem. Eur. J. 2013, 00, 0 – 0

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

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J. Ammer, K. Karaghiosoff et al.

4^ꢀ

2

solution under conditions where the phosphonium salts

Table 5. Selected NMR data for triphenylphosphonium tetrafluoroborates 2a–tBF in CD

Cl

exist as ion pairs.

ꢀ

[a]

+

11

4^ꢀ^[^b^]

Salt

Ss

P ꢀC(a)ꢀH

BF

1

2

1

R

d

H

[ppm] ( JH,P [Hz])

d

C

[ppm] ( JC,P [Hz])

d

P

[ppm]

d

F

[ppm] ( JF,B [Hz])

ion pairs

4^ꢀ

[c]

[d]

2

bBF

–

6.04 (17.2)

49.1 (42.8)

48.9 (43.1)

20.5

20.6

ꢀ152.2 (–

)

4^ꢀ

[c]

[d]

cBF

–

ꢀ152.3 (–

ꢀ152.1 (–

)

4^ꢀ

2

“

2

dBF

p-OMe

p-Me

p-OMe

p-Me

p-OPh

p-F

H

m-F

p-Cl

p-OMe

p-Me

p-OPh

p-Me

H

p-F

H

p-Cl

H

m-F

ꢀ0.52

ꢀ0.43

ꢀ0.36

ꢀ0.34

ꢀ0.26

ꢀ0.17

ꢀ0.10

ꢀ0.06

ꢀ0.03

0.00

0.34

0.38

0.65

0.68

1.02

1.30

1.36

6.15 (17.3)

6.08 (17.2)

6.18 (17.4)

6.04 (17.2)

6.20 (17.2)

6.20 (17.4)

6.31 (17.4)

6.49 (17.7)

6.40 (17.5)

6.23 (17.4)

6.39 (17.5)

6.48 (17.7)

6.53 (17.7)

6.51 (17.5)

6.52 (17.6)

6.61 (17.6)

6.80 (17.8)

6.68 (17.6)

48.2 (43.2)

48.9 (43.5)

48.4 (43.5)

49.4 (43.6)

48.9 (43.5)

49.1 (43.8)

48.6 (43.8)

47.3 (44.7)

48.1 (44.3)

49.6 (43.9)

48.5 (44.5)

47.4 (44.6)

48.4 (44.7)

47.9 (45.1)

47.7 (45.0)

47.1 (45.6)

47.5 (45.3)

46.6 (46.2)

20.8

21.0

21.1

21.3

21.5

21.9

21.8

22.0

21.8

22.1–22.2

22.2

22.3

22.5

22.4–22.5

22.6

4^ꢀ

eBF

fBF

ꢀ152.4 (1.0)

ꢀ152.1 (1.1)

ꢀ152.3 (1.1)

ꢀ152.1 (1.1)

ꢀ152.2 (1.1)

gBF

hBF

4^ꢀ

iBF

jBF

kBF

4^ꢀ

ꢀ151.8 (1.1)

4^ꢀ

[d]

ꢀ151.3 (-

)

4^ꢀ

lBF

ꢀ151.5 (1.2)

ꢀ152.0 (1.2)

ꢀ151.6 (1.1)

ꢀ151.2 (1.2)

ꢀ151.4 (1.2)

ꢀ151.1 (1.2)

4^ꢀ

aBF

4^ꢀ

m BF

ꢀ

nBF

oBF

pBF

qBF

4

4^ꢀ

3

p-CF

m,m’-F

m-F

2

4^ꢀ

2

rBF

sBF

tBF

m,m’-F

p-CF

ꢀ150.8 (1.3)

[d]

3

p-CF

3

ꢀ150.7 (–

)

m,m’-F

2

m,m’-F

2

ꢀ150.4 (1.3)

free” ions

[

e,g]

[c]

a

H

–

H

–

0.00

–

5.77 (17.0)

–

4^ꢀ

[f,g]

[d]

BF

–

ꢀ153.4 (–

)

1

0

4^ꢀ

1

[

(

BF

a] From ref. [37]. [b] Isotopomer signal for BF downfield by Dd

F

< +0.1 ppm. [c] Not available. [d] Not resolved. [e] Determined from H NMR

ꢀ

5

ꢀ

19

ꢀ5

600 MHz) spectrum of a 2.13ꢄ10 m solution of 2a SbF

6

in CD

2

Cl

2

. [f] Determined from F NMR (376 MHz) spectrum of a ~2ꢄ10 m solution of 2a

ꢀ

4

in CD

2

Cl

2

. [g] At the employed concentrations, the salts predominantly exist in the form of the free (unpaired) ions.

The C(a)ꢀH acidities of the benzhydryl triarylphosphoni-

+

ꢀ

um ions Ar CHꢀPAr

X are also affected by variation of

2

3

the PAr moiety. Table S6 in the Supporting Information il-

3

lustrates that the d_Hvalues for the C(a)ꢀH protons for the

tris(4-chlorophenyl)phosphonium

salts

Ar CHꢀP(p-Cl-

2

+

ꢀ

4

C H ) BF₄(3BF ) are 0.15 to 0.48 ppm higher and the

6

4

3

ꢀ

d_Fvalues for the BF₄anion are 0.7 to 2.1 ppm higher than

for the corresponding triphenylphosphonium salts Ar CHꢀ

2

+

ꢀ

4

PPh BF (2BF ). Thus, electron-withdrawing substitu-

3

4

ents in the triphenylphosphonium group have analogous ef-

fects on d_Hand d as substituents in the benzhydryl group.

F

1

9

Figure 6. Correlation of F NMR (376 MHz) chemical shifts d

F

of the

ꢀ

1

BF

4

anions versus the H NMR (400 MHz) chemical shifts d

H

ꢀ

Calculations and X-ray Analyses

C(a)ꢀH protons of the phosphonium ions 2 in the ion pairs 2a–tBF

in

4

2

CD

2

Cl

2

(d

F

=2.51d

H

ꢀ167.54; R =0.9299). The dashed lines show the

ꢀ

chemical shifts of the free ions 2a and BF

4

.

To learn more about the structure of the phosphonium salt

ion pairs in solution, we will now compare the NMR data

with the results of quantum chemical calculations as well as

with the CꢀH···X interactions in the crystals. The structural

mainly result from the different interactions of the methine

protons with the BF₄anions.

ꢀ

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Ion-Pairing of Phosphonium Salts in Solution

FULL PAPER

features in solution and in the crystals resemble each other

and will therefore be discussed together for each compound.

These interactions seem to be so favorable that they are

also found in the crystal (Figure 7e), which shows two short

ꢀ

Strong hydrogen bonds are characterized by short H···X

contacts between Cl and the C(a)ꢀH as well as one o-CPh

2

ꢀ

distances and CꢀH···X angles close to 1808, but there are

proton (H19···Cl distance 2.49 ꢅ and C19ꢀH19···Cl angle

1668; H31···Cl distance 2.82 ꢅ, C31ꢀH31···Cl angle 1478),

ꢀ

no clear cut-off criteria to decide whether a CꢀH···X con-

ꢀ

tact should be considered as a hydrogen bond. According to

but a significantly longer distance between the Cl anion

[8]

the latest IUPAC definition of the hydrogen bond, weak

hydrogen bonds may also be longer than the sum of the van

der Waals radii, and the angle of a hydrogen bond “should

preferably be above 1108”. In the calculated solution struc-

tures, as well as in the crystal structures, we have considered

and the o-PPh proton (H2···Cl distance 3.00 ꢅ and C2ꢀ

3

H2···Cl angle 1618) (Table 7). The packing of the molecules

ꢀ

is controlled by additional CꢀH···Cl hydrogen bonds in-

volving some of the meta- and para-protons of the PPh₃

group, resulting in a different orientation of the phenyl

groups compared to the solution structure. Particularly

ꢀ

all H···X distances up to 2.90 ꢅ, and also added some nota-

ꢀ

ble longer contacts, the most important of which are shown

as dashed lines in the Figures. Particularly short (shorter

strong is the CꢀH···Cl interaction for one of the p-PPh₃

protons (H10···Cl distance 2.58 ꢅ and C10ꢀH10···Cl angle

1718). Thus, the distances and angles for the two shortest Cꢀ

[38]

than the sum of the van der Waals radii) or close to linear

ꢀ

contacts (CꢀH···X angle ꢃ1608) are indicated by bold type

H···Cl interactions in crystals of 2aCl come very close to

ꢀ

[3]

in the Tables listing the lengths and angles of the CꢀH···X

the typical values of OꢀH···Cl hydrogen bonds.

contacts.

ꢀ

Benzhydryl triphenylphosphonium bromide (2aBr ): The

ꢀ

Benzhydryl triphenylphosphonium salts (2aX )

calculated structure of the 2aBr ion pair in CH Cl (Fig-

2

ure 7b) closely resembles that of the chloride (Figure 7a);

only the distances between the hydrogen (or carbon) atoms

and the halide ion are longer by 0.1 to 0.2 ꢅ (Table 6).

Calculated structures in CH Cl solution: The solution-phase

structures of the salts 2aX in dichloromethane (Figure 7a–

d) were determined by DFT calculations on the M06-2X 6-

2

ꢀ

2

ꢀ

Again, a similar motif is found in the 2aBr crystal (Fig-

3

1+G

A

C

H

T

U

N

G

T

R

E

N

U

N

G

(d,p) level of theory with a polarizable continuum

ure 7 f and Table 7): The strongest interactions between

cation and anion are the hydrogen bonds between the Br

[39–43]

ꢀ

model to describe the effect of the solvent.

structure of the BPh₄salt was not calculated due to the

large size of the ions. Table 6 lists the distances and angles

of the CꢀH···X contacts in the ion pairs.

The solution

ꢀ

anion and the C(a)ꢀH and o-PPh protons (H7···Br distance

3

2.90 ꢅ and C7ꢀH7···Br angle 1728; H6···Br distance 2.85 ꢅ,

C6ꢀH6···Br angle 1678), as well as the interaction of one m-

ꢀ

PPh proton with a second bromide anion (H16···Br distance

3

Crystal structures: Crystal structures of salts of the Ph CHꢀ

2.81 ꢅ and C16ꢀH16···Br angle 1518). Weaker interactions

2

+

ꢀ

PPh₃cation (2a) have not been described previously. In

are observed between Br and the o-CHPh protons as well

2

this work, we have therefore determined the crystal struc-

as another o-PPh proton, but these are already in the same

range as the interactions between Br and various phenyl

protons of further surrounding phosphonium ions (ꢃ3.0 ꢅ).

3

ꢀ

tures of the same salts 2aX which we have investigated in

ꢀ

solution. Only 2a BPh₄crystallizes as very long thin nee-

dles, and we could not obtain single crystals of sufficient size

in all three dimensions to determine the structure of this

salt by X-ray diffraction. In each of the crystal structures,

Benzhydryl triphenylphosphonium tetrafluoroborate (2a

ꢀ

BF ): In the calculated structure of 2aBF in CH Cl solu-

4

2

ꢀ

the phosphonium ions have many CꢀH···X interactions

tion (Figure 7c), the C(a)ꢀH (H19) as well as one o-PPh

3

with one particular anion and fewer contacts with other

anions (Figure 7e–h). The solid state structures thus resem-

ble the 1:1 ion pairs which are present in solution (Fig-

ure 7a–d). Table 7 lists the distances and angles of the clos-

(H12) and one o-CPh proton (H31) show bifurcated hydro-

2

gen bonds with two of the fluorine atoms (F2 and F3)

(Table 6). Additionally, the o-PPh proton (H12) has a third

3

ꢀ

CꢀH···FꢀBF interaction with a third fluorine atom (F1),

3

ꢀ

est CꢀH···X contacts in the crystals.

and the second phenyl group of the benzhydryl moiety also

ꢀ

shows one contact between o-CPh (H25) and FꢀBF (F3).

2

3

ꢀ

Benzhydryl triphenylphosphonium chloride (2a Cl ): The

In the 2aBF crystal, all fluorine atoms of the BF₄

4

ꢀ

calculated structure of 2aCl in CH Cl (Figure 7a) shows

anion exhibit multifurcated hydrogen bonds to several sur-

rounding phosphonium ions. The usual pattern of close in-

teractions between the anion and the C(a)ꢀH proton (H19),

2

ꢀ

two strong hydrogen bonds between the Cl anion and the

C(a)ꢀH and one o-PPh proton (H19···Cl distance 2.44 ꢅ

3

and C19ꢀH19···Cl angle 1768; H2···Cl distance 2.52 ꢅ and

one o-PPh (H12), and one o-CPh proton (H31) is also

3

2

C2ꢀH2···Cl angle 1768) (Table 6). The positioning of the

found (Figure 7g and Table 7), but it differs a bit from the

chloride anion near the C(a)ꢀH and o-PPh protons is fur-

calculated structure in solution. Again, the shortest contact

3

ꢀ

ther stabilized by two weaker hydrogen bonds to o-CPh

is the C(a)ꢀH···FꢀBF

interaction (H19···F2 distance

2

3

protons of both phenyl rings of the benzhydryl group

2.23 ꢅ and C19ꢀH19···F2 angle 1588), but in the crystal,

(

H31···Cl distance 2.81 ꢅ and C31ꢀH31···Cl angle 1478;

only the o-PPh proton (H12) shows bifurcated hydrogen

3

H25···Cl distance 3.13 ꢅ and C25ꢀH25···Cl angle 1408).

bonds, while the other interactions are directed towards

only one of the fluorine atoms. The fourth close CꢀH···Fꢀ

Chem. Eur. J. 2013, 00, 0 – 0

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

9

&

ÞÞ

These are not the final page numbers!

J. Ammer, K. Karaghiosoff et al.

+

ꢀ

4^ꢀ

ꢀ

6^ꢀ

ꢀ

Figure 7. a)–d) Calculated structures of Ph

2

CHꢀPPh

3

X

(2aX ) ion pairs in CH

2

Cl

2

solution: a) 2aCl , b) 2aBr , c) 2aBF , d) 2aSbF . The number-

+

ꢀ

ing corresponds to the atom numbers in the crystal structures. e)–h) X-ray structures of the salts Ph

2

CHꢀPPh

3

X (2aX ): e) 2aCl , f) 2aBr , g)

ꢀ

2

aBF

4

, h) 2aSbF

6

. All CꢀH···X contacts with d

ACHTUNGTRENNUNG

(H···X ) ꢄ 2.90 ꢅ are shown as orange lines, selected longer contacts are indicated by dashed lines.

ꢀ

For CꢀH···X bond lengths and angles, see Tables 6 and 7.

&

10

&

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 0000, 00, 0 – 0

ÝÝ

These are not the final page numbers!

Ion-Pairing of Phosphonium Salts in Solution

FULL PAPER

Table 6. Calculated distances and angles of CꢀH···X^ꢀcontacts in

Table 7. Distances and angles of CꢀH···X^ꢀcontacts in crystals of

+

ꢀ

+

ꢀ

Ph

2

CHꢀPPh

Donor

3

X

(2aX ) ion pairs in CH

2

Cl

2

solution.

Ph

2

CH-PPh

3

X (2a X ).

[

a]

ꢀ

[a]

ꢀ

Salt

Accep-

d

A

H

U

G

R

N

U

G

d

[ꢅ]

A

T

N

T

E

N

(H···X )

d

[ꢅ]

A

H

U

G

E

U

G

a

A

H

U

G

R

N

U

G

Salt

Donor

Accep- Code

d

[ꢅ]

A

H

U

G

R

N

U

G

d

[ꢅ]

A

H

U

T

E

N

a

A

C

H

T

U

N

G

T

R

E

N

U

N

G

(Cꢀ

[

a]

ꢀ

[a]

ꢀ

At or

C

H

T

U

N

G

T

R

E

N

U

N

G

H···X )

8]

A

H

U

G

R

N

H···X )

[

[8]

ꢀ

2

aCl

H19 (a-H) Cl1

1.1022

1.0907

1.0881

1.0869

1.1021

1.0901

1.0877

1.0869

1.0968

2.44

2.52

2.81

3.13

2.64

2.68

2.93

3.24

2.30

2.20

2.25

2.38

2.66

2.57

2.71

2.57

2.28

2.44

2.74

2.30

2.42

2.66

2.59

2.44

2.54

3.54

3.61

3.77

4.02

3.74

3.77

3.92

4.14

3.24

3.23

3.29

3.56

3.32

3.63

3.32

3.28

3.37

3.45

3.36

3.19

3.30

3.34

3.50

3.22

176

147

140

177

178

152

141

142

156

158

140

126

142

125

151

141

122

165

126

117

126

165

120

2aCl

–

e

c

g

–

e

l

2.49(3)

2.82

3.00

3.20

2.58

2.76

2.81

2.90

2.85

3.19

3.14

3.68

2.81

3.02

3.05

3.22

3.38

2.23

2.59

2.53

2.80

2.73

2.44

2.84

2.54

2.67

2.70

2.56

2.59

2.62

2.60

2.29

2.62

2.48

2.67

2.84

2.42

2.72

2.60

3.16

2.85

2.58

2.74

2.59

2.80

2.66

2.95

2.50

2.53

2.54

2.76

3.444(3)

3.654(3)

3.913 (3) 161

166(2)

147

H2

H31

H21

Cl1

H31

H2

H21

H10

H5

H11

H7 (a-H) Br1

H6

Cl1

3.900(3)

3.526(3)

3.573(3)

3.526(3)

3.894(5)

3.781(5)

4.000(5)

4.024(5)

4.491(5)

3.672(6)

3.915(6)

3.734(5)

3.828(5)

4.267(5)

3.218(4)

3.340(4)

3.329(4)

3.450(4)

3.423(4)

3.259(4)

3.643(5)

3.467(3)

3.236(4)

3.247(4)

3.482(5)

3.527(4)

3.192(4)

3.179(5)

3.221(3)

3.357(4)

3.407(4)

3.505(5)

3.683(5)

3.372(6)

3.293(5)

3.234(5)

4.096(5)

3.561(6)

3.074(5)

3.208(5)

3.265(5)

3.365(6)

3.373(6)

3.511(6)

3.097(6)

3.114(6)

3.374(6)

3.571(6)

132

171

144

133

172

167

144

156

146

151

158

131

123

156

158

136

141

127

130

144

143

164

119

117

163

170

119

120

167

135

164

151

153

178

120

125

169

133

113

111

129

119

132

119

121

120

147

144

aBr

H7 (a-H) Br1

H6

H25

H13

Br1

ꢀ

2aBr

4^ꢀ

aBF

H19 (a-H) F2

H19 (a-H) F3

Br1

H13

H25

H24

H16

H20

H22

H23

H28

H12

H31

H25

F2

F1

F3

F2

F3

1.0861

1.0863

[

b]

i

1.0857

1.0977

6^ꢀ

2

aSbF H13 (a-H) F4

H13 (a-H) F5

h

e

a

b

g

h

–

a

e

–

g

c

j

4^ꢀ

2aBF

H19 (a-H) F2

H2

H3

H21

H15

F4

F5

F2

F5

F3

F4

1.0860

H18

H12

H31

H23

H24

H3

H4

H8

H14

H15

H16

H17

H28

F2

F1

F2

F1

F2

F4

F3

F4

F1

F3

F4

1.0848

1.0865

1.0852

[

a] See Figure 7 for numbering of atoms.

ꢀ

BF₃contact is now to a second o-PPh proton (H18) in-

3

stead of the second o-CPh proton. This subtle variation be-

tween the solution and crystal structures is caused by the ad-

ditional interactions between BF₄and the other surround-

ing phosphonium ions in the crystal (Figure 7g and Table 7).

2

ꢀ

6^ꢀ

2

aSbF H13 (a-H) F4

H13 (a-H) F5

H2

Benzhydryl triphenylphosphonium hexafluoroantimonate

2aSbF ): According to the calculations, the fluorine atoms

of the SbF₆anions in the 2aSbF₆ion pairs in CH Cl solu-

tion also form multifurcated hydrogen bonds (Figure 7d and

F5

F2

F3

F1

F3

F2

F5

F3

F4

F6

ꢀ

(

6

ꢀ

H3

2

[

b]

H21

H25

H19

H29

H31

H30

H10

H11

H8

Table 6). The C(a)ꢀH proton forms a short bifurcated hy-

drogen bond with two of the fluorine atoms (H13···F4 dis-

tance 2.28 ꢅ and C13ꢀH13···F4 angle 1518; H13···F5 dis-

tance 2.44 ꢅ and C13ꢀH13···F5 angle 1418). The same two

fluorine atoms are also involved in a bifurcated hydrogen

[b]

bond with one of the o-PPh₃protons (H2···F5 distance

2

.30 ꢅ and C2ꢀH2···F5 angle 1658; H2···F4 distance 2.74 ꢅ

H9

H4

H5

and C2ꢀH2···F4 angle 1228), and each of them also has a

j

weaker interaction with an o-CPh proton (H15 or H21)

2

[

a] See Figure 7 for numbering of atoms and symmetry codes. [b] These

contacts are not shown in Figure 7.

(

Table 6). A further strong hydrogen bond is found between

one of the o-CPh₂protons and a third fluorine atom

H21···F3 distance 2.44 ꢅ and C21ꢀH21···F3 angle 1658).

Two weaker interactions are observed between the o-PPh

(

contact is between an o-PPh proton and one of the fluorine

3

(

H2) and m-PPh (H3) protons and a fourth fluorine atom

atoms (H2···F5 distance 2.42 ꢅ and C2ꢀH2···F5 angle 1788).

This proton also has a second weaker interaction with an-

other fluorine atom (F2), which also forms a hydrogen bond

3

F2) (Table 6).

The fluorine atoms of the SbF₆anions in the 2aSbF₆

ꢀ

crystal also form multifurcated hydrogen bonds (Figure 7h

to the adjacent m-PPh proton (H3). The bifurcated hydro-

3

ꢀ

and Table 7), but the CꢀH···F interactions differ somewhat

gen bonds between the C(a)ꢀH proton and the SbF₆anion

ꢀ

from those in CH Cl solution. The closest CꢀH···FꢀSbF

are significantly longer than in solution or in the crystals of

2

5

Chem. Eur. J. 2013, 00, 0 – 0

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

11

&

ÞÞ

These are not the final page numbers!

J. Ammer, K. Karaghiosoff et al.

ꢀ

the other salts (H13···F4 distance 2.67 ꢅ and C13-H13···F4

angle 1518; H13···F5 distance 2.84 ꢅ and C13-H13···F5 angle

compare the structures of 1aBF

with co-crystallized

4

[45]

CH Cl

and co-crystallized CHCl , respectively. Both

structures resemble that of 1aCl (Figure 8), except that

2

3

ꢀ

1538), and the typical interaction with one or more o-CPh₂

protons is not found (Figure 7 h). Instead, the o-CPh pro-

tons (H19 and H25) form hydrogen bonds to a second SbF₆

some of the hydrogen bonds are bifurcated towards two of

the BF₄anionsꢃ fluorine atoms (Figure 9 and Table 8). On

2

ꢀ

anion which is located on the far side of the C(a)ꢀH proton.

each side of the benzyl moiety, there are strong interactions

The packing of the ions is also influenced by several other

between the anion and the C(a)ꢀH (H1/H2 in Figure 9a;

contacts between the protons of the PPh groups and neigh-

H19 A/H19B in Figure 9b) and one p-PPh proton (H10/

3

ꢀ

boring SbF₆anions (Figure 7h and Table 7).

H19 in Figure 9a; H18/H8 in Figure 9b). The larger size of

ꢀ

the BF₄anion and a slight rotation of the benzyl groupꢃs

ꢀ

Benzyl triphenylphosphonium salts (1X )

phenyl ring allow for an additional contact between one o-

ꢀ

CPh proton and one of the BF₄anions, as shown on the

Halide and tetrafluoroborate salts: The calculated structures

left side in Figure 9a (H5) and 9b (H21).

The second BF₄anion (shown on the right side) cannot

ꢀ

of 1aCl and 1aBF₄in CH Cl solution closely resemble

2

ꢀ

those of the benzhydryl derivatives 2aX and are shown in

Figure S2 in the Supporting Information. In each case, the

anions form hydrogen bonds with three donors: one of the

undergo such an interaction, because the phenyl ring of the

benzyl moiety is already twisted in the wrong direction. In-

stead, this anion forms strong hydrogen bonds to a solvent

ꢀ

C(a)ꢀH, one o-PPh and one o-CPh proton. The crystal

molecule. In the case of 1aBF ·CH Cl there is also a con-

3

4

2

structures of these salts are worth discussing briefly because

of the additional possibility of an interaction of the anion

with the second C(a)ꢀH proton of the benzyl group belong-

tact between an o-PPh₃proton and a chlorine atom of

CH Cl (Figure 9a), but this interaction is rather weak

2

(H20···Cl1 distance 2.84 ꢅ, C22ꢀH20···Cl1 angle 1358).

ing to a second phosphonium ion.

ꢀ

4

The crystal structure of 1aCl has been reported previ-

The tetraphenylborate salts: Like 2aBPh , 1aBPh crys-

4

tallizes as very long fine needles and we could not obtain

[44]

ously (Figure 8). The shortest contact between cation and

anion is the interaction of one C(a)ꢀH proton with the

chloride anion (H1···Cl distance 2.52 ꢅ and C1ꢀH1···Cl

angle 1708), which forms a second hydrogen bond to one of

the o-PPh protons (H8···Cl distance 2.83 ꢅ and C9ꢀH8···Cl

3

angle 1768) (Table 8). The second C(a)ꢀH proton shows a

weaker hydrogen bond to a second chloride anion (H2···Cl

distance 2.66 ꢅ, C1ꢀH2···Cl angle 1638), which also has a

very short contact to another o-PPh proton (H13···Cl dis-

3

tance 2.57 ꢅ, C15ꢀH13···Cl angle 1768). These strong bidir-

ectional interactions result in the formation of one-dimen-

sional chains of alternating cations and anions in the crystal,

which interact by weaker contacts between the chloride

anions and some of the p- and m-PPh protons.

3

ꢀ

The crystal structure of 1aBF₄with co-crystallized

[45]

CH Cl₂has previously been reported.

Figure 9a and b

2

4^ꢀ

ꢀ

2

Cl

2

Figure 9. Crystal structures of a) 1aBF ·CH

(ref. [45]) and b)

ꢀ

[44b]

ꢀ

Figure 8. Crystal structure of 1aCl .

All contacts with

d

A

T

N

T

E

N

(H···X )

1aBF

4

·CHCl

3

(this work). All CꢀH···X contacts with

d

A

C

H

T

U

N

G

T

R

E

N

U

N

G

(H···X )

ꢀ

ꢄ2.90 ꢅ are shown as orange lines. For CꢀH···X bond lengths and

angles, see Table 8.

&

12

&

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 0000, 00, 0 – 0

ÝÝ

These are not the final page numbers!

Ion-Pairing of Phosphonium Salts in Solution

FULL PAPER

ꢀ

+

ꢀ

Table 8. Distances and angles of CꢀH···X contacts in crystals of ArCH

-PPh

X .

2

3

the phenyl ring is relatively large (3.40 ꢅ). The

[

a]

[a]

ꢀ

(Cꢀ

second C(a)ꢀH proton (H1A) forms a much

Salt

Donor

Accept- Code

d

[ꢅ]

A

H

U

G

R

N

U

G

d AHCTUNGERTNNUNG( C···X ) a ACHTNUGTRENNNUG

[ꢅ]

[a]

ꢀ

or

H···X )

8]

closer contact to a phenyl ring of another BPh₄

[

anion (distance: 2.80 ꢅ, sum of C and H van der

ꢀ

[44b]

[38]

1

aCl

H1 (a-H)

H8

H2 (a-H)

H13

H15

H16

H11

H20

H1 (a-H)

H10

Cl1

F3

F4

F1

F4

F1

F4

F1

F3

F1

F3

F1

F2

a

–

d

c

b

–

b

a

c

d

e

n

b

–

j

2.52

2.83

2.66

2.57

2.76

2.84

2.87

2.81

2.52

2.81

2.49

2.76

2.58

2.52

2.51

2.54

2.57

2.64

2.68

2.56

2.88

2.52

2.65

2.75

2.78

2.80

2.84

2.47

2.71

2.82

2.64

2.24(4)

2.50(3)

2.53

2.83

2.49

2.51

2.60

2.68

2.54

2.87

2.41

2.87

2.80

3.40

3.490(6)

3.803(5)

3.602(5)

3.550(6)

3.424(6)

3.472(7)

3.771(7)

3.532(7)

3.437(3)

3.643(3)

3.298(2)

3.594(3)

3.367(4)

3.478(5)

3.422(3)

3.403(3)

3.448(2)

3.490(3)

3.391(3)

3.435(3)

3.552(3)

3.283(3)

3.350(3)

3.376(3)

3.398(4)

3.396(3)

3.563(3)

3.390(3)

3.570(3)

3.696(3)

3.412(3)

3.192(3)

3.250(4)

3.443(3)

3.741(4)

3.253(3)

3.390(3)

3.479(4)

3.226(4)

3.436(3)

3.730(4)

3.209(3)

3.738(3)

3.633(2)

4.388(4)

170

176

163

176

123

153

132

158

145

146

150

139

169

157

148

157

152

134

158

130

140

133

125

123

135

159

149

154

139

169(3)

134(3)

157

162

138

154

153

117

157

151

141

152

142

180

Waals radii: 2.79 ꢅ ), but in this case the projec-

tion of the C(a)ꢀH bond does not point exactly

at the phenyl ring (angle 1428).

Comparison of calculated and experimental

NMR spectra

4^ꢀ

2

[45]

2

1

aBF ·CH

Cl

1

We also calculated the H NMR chemical shifts

for the ion pairs in CD Cl solution with the

2

H10

gauge-independent atomic orbital method

H4 (CH

2

Cl

2

)

[46]

[47]

(

GIAO) and the functional WP04 (Table 9).

2

This method has been developed especially for

H2 (a-H)

H19

H5

H6

H17

H13

H22

H23

H20

H19 A (a-H) F4

H19 A (a-H) F3

H18

H26 (CHCl

H19B (a-H) F4

H8

H21

H24

H23

H4

H10

H17

H16

H1A (a-H)

H1B (a-H)

1

[47a]

the calculation of H NMR data.

For the calcu-

lation of the chemical shifts we additionally used

pseudo potentials for all atoms from the third

[43]

period on.

For a comparison with the experi-

^m^eⁿ^t^a^l^d^a^t^a^,^t^h^e^dH values were averaged for

both a-H of the benzyl systems, all o-CPh pro-

tons, or all six o-PPh₃protons, respectively

F2

F3

F2

F4

F2

Cl1

(

Table 9). As the solvent certainly takes part in

the hydrogen bond network (compare Figure 9a),

the implicit solvent continuum used for the calcu-

lations is a simplification. Further deviations may

be caused by the fact that the calculations refer

to the most stable conformation, while the experi-

mental data reflect a statistical distribution of dif-

ferent conformations. Still, the experimentally ob-

served trends are fairly well reproduced by the

4^ꢀ

3

1

aBF ·CHCl

F3

F1

F3

F2

3

)

3

F4

F3

F4

F3

F1

F2

F1

1

calculated H NMR shifts of the optimized solu-

tion structures (Table 9).

k

m

f

–

d

Infrared Spectra

Hydrogen bonding is usually associated with red-

shifts of the IR stretching frequencies for the in-

F2

phenyl

4^ꢀ

[b]

1

bBPh

[

c]

[8]

volved bonds in the donor group. Figure 11

[

a] See Figures 8–10 for atom numbering and symmetry codes. [b] Center of six atoms

shows the CꢀH stretching regions of the IR spec-

C33, C34, C35, C36, C37, C38. [c] Center of six atoms C27, C28, C29, C30, C31, C32.

tra of different phosphonium salts in CD Cl solu-

2

tion, which were acquired under conditions

ꢀ

2

suitable material for X-ray structure analyses. We

where the phosphonium salts are mostly paired (3ꢄ10

solutions).

The red-shifted C(a)ꢀH stretch vibrations ( ꢀn ꢁ 2831 and

2791 cm ) in the benzhydryl triphenylphosphonium halides

m

+

ꢀ

could, however, crystallize (p-CF -C H )CH ꢀPPh BPh₄

3

6

4

2

3

ꢀ

(

1bBPh ) as platelets from CH Cl /Et O. Its crystal struc-

4 2 2 2

ꢀ

1

ture is shown in Figure 10; the CF group is disordered.

3

ꢀ

Both C(a)ꢀH bonds (C1ꢀH1A and C1ꢀH1B) point to-

2aCl and 2aBr are clearly visible (Figure 11a). The oc-

currence of two bands might be explained by couplings with

lower-frequency modes, which is a common phenomenon in

hydrogen bonding, or by the existence of two conforma-

tions, which are discernible on the IR time scale but not on

the NMR time scale. Further investigations are needed to

resolve this issue. Unfortunately, direct calculations of the

IR bands of ion pairs consisting of such large ions are

beyond the scope of this work, since reliable calculations

ꢀ

wards phenyl rings of the BPh anions. The nature of these

4

CꢀH···p contacts can be characterized by the distances be-

[48]

tween the H (or C) atoms and the centers of the phenyl

rings, as well as the angles between the CꢀH bonds and the

lines connecting the H atom and the center of the phenyl

ring (Table 8). One of the C(a)ꢀH bonds (C1ꢀH1B) points

ꢀ

exactly at the center of the phenyl ring of one BPh₄anion

(

angle: 1808) (Figure 10), but the distance to the center of

Chem. Eur. J. 2013, 00, 0 – 0

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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J. Ammer, K. Karaghiosoff et al.

4^ꢀ

Figure 10. Crystal structure of 1bBPh . For clarity, the carbon atoms of

ꢀ

the BPh

4

anions are shown in brown color. The orange dashed lines in-

dicate the distances between the a-protons and the centers of the phenyl

groups. For CꢀH···X^ꢀbond lengths and angles, see Table 8.

1

Table 9. Comparison of calculated and experimental H NMR chemical

ꢀ

shifts d

H 2 2

for 2aX and 1a,bX in CD Cl solution under conditions

where the salts exist as ion pairs.

+

[a]

salt

d

H

A

C

H

T

U

N

G

T

R

E

N

U

N

G

(CHP )

d

H

A

H

U

G

R

N

U

G

d

H

A

T

N

T

E

N

G

3

)

A

C

H

T

U

N

G

T

R

E

N

U

N

G

[ppm]

A

H

U

T

E

N

ACHTUNGTRENNUNG

[

b]

[b]

calcd

exptl

calcd

exptl

calcd

exptl

ꢀ

2

“

1

“

1

“

aCl

aBr

8.01

7.71

6.57

6.22

5.84

6.33

5.04

4.09

5.07

5.00

4.10

8.25

8.10

6.23

5.98

5.77

5.42

4.56

~4.37 6.39

5.78

4.72

~4.44 6.39

7.43

7.61

7.33

6.98

6.74

7.39

6.58

7.55–7.60 7.54

7.53–7.61 7.34

7.19–7.33 6.99

7.15–7.17 6.95

7.09–7.11 6.71

7.07–7.10 7.76

6.91–6.94 7.16

6.87–6.89 6.91

7.79–7.84

7.74–7.79

7.43–7.49

7.38–7.44

7.33–7.37

7.70–7.76

7.48–7.54

7.43–7.46

7.76–7.82

7.54–7.60

7.47–7.51

4^ꢀ

aBF

6^ꢀ

aSbF

[

c]

free” 2a

ꢀ

aCl

4^ꢀ

aBF

free” 1a

ꢀ

bBr

7.19

6.78

7.36

7.11

7.04

7.33

7.16

6.95

4^ꢀ

b BF

[

c]

free” 1b

[

H 3

a] Averaged d of both a protons of the benzyl systems, all six o-PPh

protons, or all o-CPh protons, respectively. [b] From quantum chemical

calculations (see text). [c] Experimental values determined from

1

ꢀ5

ꢀ

H NMR (600 MHz) spectra of ca. 2ꢄ10 m solutions of the SbF

6

, BF

4

ꢀ

and/or BPh

4

2 2

salts in CD Cl . At these concentrations, the phosphonium

salts predominantly exist in the form of the free (unpaired) ions.

would require molecular dynamics simulations with the ex-

plicit inclusion of at least two solvent shells.

The intensities of the aliphatic CꢀH stretching bands in

ꢀ

the other benzhydryl triphenylphosphonium salts 2aX with

ꢀ

4

X =BF , SbF , and BPh are very weak (Figure 11a).

ꢀ2

ꢀ

4

6

Figure 11. IR spectra of 3ꢄ10 m solutions of a) 2aX (vertical offset for

These bands can be better discerned in the IR spectra of the

benzyl triphenylphosphonium salts 1aX (Figure 11b) and

bX (Figure 11c). For the halides, we again observe pro-

ꢀ

6

visibility), b) 1aX , or c) 1bX with X =Cl , Br , BF

4

, SbF

, or

ꢀ

BPh

4

in CD

2

Cl

2

.

ꢀ

1

ꢀ

nounced red-shifts of the C(a)ꢀH stretching bands ( ꢀn ꢁ

with the BPh₄anions (Table 4). This may indicate the exis-

tence of so-called blue-shifting hydrogen bonds between the

phosphonium ions and the BF₄anions (see below).

ꢀ

1

2

850 and 2777 cm ).

ꢀ

Interestingly, the C(a)ꢀH bands of the tetrafluoroborates

ꢀ

ꢀ1

1

a,bBF₄( ꢀn ꢁ 2962 and 2919 cm ) in Figure 11b and 11c

As both the NMR and X-ray data indicated hydrogen

bonding between the counter-anions and aryl protons (see

above), it is also interesting to compare the aromatic CꢀH

are located at higher wave numbers (i.e., blue-shifted) and

have lower intensities than those of the corresponding tetra-

ꢀ

ꢀ1

phenylborates 1a,bBPh₄( ꢀn ꢁ 2935 and 2899 cm ), al-

stretching vibrations. Indeed, the C

A

H

U

G

R

N

U

G

ꢀ

though the dissociation constants K indicate stronger inter-

bands of the BF₄salts are found above 3000 cm , while

those of the halides extend further into the red to well

D

ꢀ

actions of the phosphonium ions with the BF₄anions than

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Ion-Pairing of Phosphonium Salts in Solution

FULL PAPER

ꢀ

1

below 3000 cm (Figure 11a–c). The IR spectra thus pro-

these pieces of evidence individually, one might not have

recognized the hydrogen bonds clearly in some cases.

ꢀ

vide further evidence for C

CD Cl solutions of the phosphonium halides.

A

H

U

G

R

N

U

G

ꢀ

The nature of the “BPh₄Effect” in benzyl triphenylphos-

phonium salts

ꢀ

CꢀH···X Hydrogen Bonds: What Is Typical?

ꢀ

While we did not find such interactions in 2aBPh , the

4

Birdꢁs eye view of the whole data set

crystallographic data in Figure 10 reveal CꢀH···p interac-

ꢀ

tions between 1b and BPh , where the protons reside

4

The NMR data of the phosphonium salts in CD Cl solution

show that ion pairing with the counteranion X mainly af-

above the centers of the phenyl rings. Unlike the typical

2

ꢀ

2

CH–p interaction, which is mainly based on dispersion inter-

[49]

ꢀ

fects the proton resonances of the C(a)ꢀH, o-PPh , and o-

actions, the interaction between 1b and BPh₄can be ex-

3

CPh protons of 2a (Table 1) or 1a (Table 3), respectively. A

comparison with the crystal structures of these salts reveals

that these protons are also involved in the shortest and least

pected to have a strong electrostatic component due to the

high acidity of the CꢀH bond and the negative charge on

ꢀ

the phenyl rings of the BPh anion. This notion is support-

4

ꢀ

bent CꢀH···X contacts in the crystals (Figure 7–9; Tables 7

ed by the strong directionality of the C1ꢀH1B···Ph interac-

ꢀ

and 8). The presence of CꢀH···X hydrogen bonds in solu-

tion, since the electrostatic interaction is the main source of

ꢀ

[49]

tions of 2X and 1X is consistent with the strong deshield-

ing of the respective protons, as well as with the fact that

the deshielding increases with increasing CꢀH acidity and

directionality in CH–p interactions.

Hence, the C(a)ꢀ

ꢀ

H···Ph interactions in 1b BPh₄can be viewed as hydrogen

bonds in which a phenyl ring of the tetraphenylborate anion

ꢀ

4

[2,50]

with increasing basicity of the anions X (SbF < BF

<

acts as the hydrogen bond acceptor.

Similar C(a)ꢀH···Ph

6

ꢀ

[34]

Br < Cl ). Moreover, the IR spectra clearly show that

the CꢀH stretch frequencies of the phosphonium salts in

hydrogen bonds have also been reported in the crystal struc-

+

ture of choline tetraphenylborate, Me N ꢀC(a)H ꢀCH OH

3

2

ꢀ

CD Cl solution depend on the counter-anion (Figure 11).

BPh (H···Ph distances 2.42 ꢅ and 2.38 ꢅ, CꢀH···Ph angles

2

4

[50b]

The results of our quantum chemical calculations also con-

168 and 1598).

ꢀ

firm CꢀH···X hydrogen bonds for the C(a)ꢀH, o-PPh , and

A similar interaction between cation and anion in CD Cl₂

3

2

ꢀ

o-CPh protons of 2aX and 1a,bX ion pairs in CH Cl sol-

solution can explain the upfield shift of the C(a)ꢀH reso-

2

ꢀ

1

ution (Figure 7a–d; Tables 6 and 9). The relevant data for

nances of 1a,bBPh in the H NMR spectra (Figure 2): The

4

ꢀ

[51]

the C(a)ꢀH···X hydrogen bonds in dichloromethane solu-

resulting ring current effect

downfield shift in the H NMR spectrum that may be ex-

pected due to the formation of a weak hydrogen bond.

over-compensates any small

1

tion are summarized again in Tables 10 and 11.

The data presented in the three previous Sections as a

ꢀ

whole thus clearly support the existence of CꢀH···X hydro-

gen bonds between the phosphonium ions and the anions

Blue-shifting hydrogen bonds with weak hydrogen bond ac-

ceptors

ꢀ

(

Tables 10 and 11). Some of these CꢀH···X interactions,

however, show spectroscopic characteristics which are worth

discussing in greater detail in the following. Looking at

Blue-shifting hydrogen bonds show stretching vibrations at

higher wave numbers, often accompanied by reduced inten-

Table 10. Summary of experimental and calculated data for the C(a)ꢀH···X^ꢀhydrogen bonds of the benzhydryl triphenylphosphonium ion pairs 2aX^ꢀ

in dichloromethane solution.

ꢀ

4^ꢀ

6^ꢀ

4^ꢀ

X

Cl

Br

BF

SbF

BPh

free

ion

(ꢁ free ion)

d

H

for C(a)ꢀH [ppm]

C,H for C(a)ꢀH [Hz]

8.25

131.3

8.10

131.1

6.23

130.2

5.98

129.3

5.72

128.7

5.77

n/a

1

J

ꢀ

1

n˜ CH [cm

]

2832 (s)

2829 (s)

2791 (s)

2903 (w)

2918 (w)

2889 (w)

2888 (w)

n/a

2791 (s)

calcd d

A

T

N

T

E

N

(CꢀH) [ꢅ]

1.1022

2.44

3.54

1.1021

2.64

3.74

1.0968

1.0977

n/a

1.0977

ꢀ

A

H( H···X ) [ꢅ]

U

G

R

N

U

G

2.30/2.20

3.24/3.23

142/156

2.28/2.44

3.28/3.37

151/141

–

A

H

U

G

R

N

(C···X ) [ꢅ]

ꢀ

calcd a

A

T

N

T

N

U

G

176

177

further H-bonds to…

also see Figure 7a–d)

H2 (o-PPh

H31 (o-CPh)

H21 (o’-CPh

3

)

H6 (o-PPh

H25 (o-CPh

H13 (o’-CPh

3

)

H12 (o-PPh

H31 (o-CPh

H25 (o’-CPh

3

)

3

H2 (o-PPh )

(

2

)

2

)

H21 (o-CPh

H15 (o’-CPh

H3 (m-PPh

2

)

n/a

–

2

)

2

)

2

)

2

)

3

)

Chem. Eur. J. 2013, 00, 0 – 0

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

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J. Ammer, K. Karaghiosoff et al.

Table 11. Summary of experimental and calculated data for the C(a)ꢀH···X^ꢀhydrogen bonds of the benzyl triphenylphosphonium ion pairs 1a,bX in

ꢀ

dichloromethane solution.

ꢀ

4^ꢀ

X

Cl

Br

BF

BPh

free ions

1

a

1b

1a

1b

1a

1b

1a

1b

d

H

for C(a)ꢀH [ppm]

C,H for C(a)ꢀH [Hz]

5.42

134.6

5.78

134.6

4.56

134.2

4.72

134.9

3.94

133.9

3.66

134.6

~4.37

n/a

~4.44

n/a

1

J

2

852 (s) 2848 (s) 2960 (m) 2963 (m)

776 (s) 2778 (s) 2916 (m) 2921 (m)

2935 (w)

2899 (m)

ꢀ

1

n˜ CH [cm

]

2899 (m)

n/a

[

a]

[b]

calcd d

A

H

U

G

R

N

(CꢀH) [ꢅ]

1.0983

2.52

3.57

1.0980

2.72

1.0929

1.0931

n/a

1.0941

–

1.0940

–

ꢀ

A

H

U

G

R

N

U

G

2.18/2.34 2.15/2.34 n/a

3.26/3.12 3.22/3.14 n/a

ꢀ

A

H

U

G

R

N

(C···X ) [ꢅ]

3.74

ꢀ

calcd a

A

H

U

G

R

N

U

G

154

172/126

168/128

n/a

–

3

o-PPh

o-CPh

3

o-PPh

o-CPh

3

o-PPh

o-CPh

3

o-CPh

[

0

a] Length of C(a)ꢀH bond acting as hydrogen bond donor. [b] Average of two C(a)ꢀH bonds of the free phosphonium ions 1a (ꢂ0.0007 ꢅ) or 1b (ꢂ

.0008 ꢅ), respectively.

[

52,53]

sities of the IR bands,

which is the opposite behavior of

This interpretation of the IR spectra is supported by the

quantum chemical calculations, which also show C(a)ꢀH

[8]

normal hydrogen bonds. The nature of blue-shifting hydro-

gen bonds has been discussed controversially,

there seems to be a general agreement that there is no fun-

damental difference between blue-shifting and normal red-

shifting hydrogen bonds.

[52,53]

but now

bond contractions (ca. ꢀ1 mꢅ) relative to the unpaired

phosphonium ions for the tetrafluoroborate salts and bond

elongations (ca. +4 mꢅ) for the halide salts (Tables 10 and

11). An experimental confirmation of the polarization of the

C(a)ꢀH bond, which is expected for both red- and blue-

[54]

[53]

According to Joseph and Jemmis, there are two oppos-

ing effects, when the hydrogen bond acceptor X ap-

ꢀ

shifting hydrogen bonds (see above), is provided by the

ꢀ

proaches the CꢀH proton. On the one hand, there is an at-

NMR data of 2a X (Table 1) and 1a X (Table 3): Com-

pared to the free ions, the H atoms are deshielded (i.e.,

lower electron density around H) and the C atoms are

shielded (i. e., higher electron density around C) in the hy-

tractive interaction between the positive H and the negative

ꢀ

X , which lengthens the CꢀH bond and reduces the force

ꢀ

constant. On the other hand, the presence of X induces a

greater polarization of the CꢀH bond, because it compen-

sates the resulting positive charge at H. As a result, the Cꢀ

H bond is contracted and the force constant increases. If the

former effect dominates, a classical red-shifting hydrogen

bond is the result. If the latter dominates, a blue-shift of the

drogen-bonded systems. While d may reflect changes in the

H

electron density caused by polarization of the CꢀH bond as

ꢀ

well as by interaction with the acceptor X , changes in d

C

primarily reflect polarization effects. This polarization is ob-

served in all studied systems and becomes more pronounced

with increasing interaction between cation and anion

(Tables 1 and 3).

[53]

frequency of the CꢀH stretch mode is observed.

Compared to classical hydrogen bond donors such as Oꢀ

H or NꢀH, the CꢀH bond is longer and less polar. An ap-

Cooperativity or anti-cooperativity phenomena between

the different types of hydrogen bonds may also play a role

ꢀ

proach of the hydrogen bond acceptor X will thus lead to a

ꢀ

considerable polarization of the CꢀH bond. Whether the in-

creased polarization causes a contraction of the CꢀH bond

and a blue shift of its IR stretching band depends on the rel-

ative importance of the compensating attractive interaction

in controlling the strengths of the CꢀH···X hydrogen

[55]

bonds. A point which may be relevant here is the fact that

ꢀ

the hydrogen bonds involving the BF₄and SbF₆acceptors,

which are characterized as blue-shifting in this work, are bi-

furcated hydrogen bonds (Figures 7 and 9). A theoretical

study of linear and bifurcated hydrogen bonds between the

proton donors H CZ (Z=O, S, Se) or H CZ (Z=F, Cl, Br)

ꢀ

between H and X . Increasing interaction energy between

the hydrogen bond donor and acceptor causes a blue shift at

ꢀ

relatively long equilibrium distances between H and X ,

2

ꢀ

which then decreases again and changes into a red shift as

and the halide ions Cl and Br found that all linear hydro-

gen bonds in the investigated systems were red-shifting,

while all bifurcated hydrogen bonds were blue-shifting.

ꢀ

the equilibrium distance between H and X becomes short-

[

53]

ꢀ

[56]

er. For strong acceptors such as Cl or Br , the attraction

ꢀ

between H and X clearly dominates and we observe the

The NMR data for the phosphonium ion pairs 2aX

ꢀ

classical red-shifting hydrogen bonds (Figure 11, Tables 10

(Table 1) or 1aX (Table 3) do not show any qualitative dif-

ꢀ

and 11). The hydrogen bond acceptor BF₄seems to be of

ferences between the blue-shifting and red-shifting hydrogen

ꢀ

an intermediate strength, where we observe a blue shift with

bonds. We note that in both the blue-shifting (2aBPh₄

ꢁ

ꢀ

the CꢀH hydrogen bond donors 1a and 1b in CD Cl solu-

“free” 2a ! 2aBF ) as well as the red-shifting series

2

4

ꢀ

tion (Figure 11b and c, Table 11).

(2aBr ! 2aCl ) of the benzhydryl triphenylphosphonium

salts, the chemical shifts d for the a-carbon atom of 2a de-

C

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Ion-Pairing of Phosphonium Salts in Solution

FULL PAPER

1

crease (Table 1) and the coupling constants J_H_,_Cfor the

C(a)ꢀH bond increase (Table 10) with increasing interaction

anion has also been observed in the crystal structure of

ꢀ

1bBPh , and the NMR spectra indicate that a similar inter-

4

energy. These parameters had previously been suggested as

indicators for the transition between blue- and red-shifting

hydrogen bonds, based on data for a series of single hydro-

gen bonds between fluoroform and various hydrogen bond

action is also relevant in dichloromethane solution. Ion pair-

ing thus plays an important role in solutions of phosphoni-

um salts even when weakly coordinating anions such as

ꢀ

BF , SbF , or BPh are employed in solvents such as

4

6

4

[57]

acceptors.

CH Cl or CHCl . In more polar solvents such as CH CN,

2 2 3 3

ꢀ

only stronger hydrogen-bond acceptors such as Cl or Br

Implications for the identification of CꢀH···X^ꢀhydrogen

form ion pairs with the phosphonium ions.

ꢀ

bonds

Similar CꢀH···X hydrogen bonds probably also play a

major role in solutions of other onium salts, as demonstrated

ꢀ

In the course of this work, we have encountered many of

the concepts that have been debated controversially in the

field of hydrogen bonding during the last decades: hydrogen

bonds involving CꢀH donors, bi- and multi-furcated hydro-

by the large number of examples for the “BPh effect” in

4

[

9,10]

[14]

[11]

[13]

phosphonium,

ammonium, anilinium, pyridinium,

[15]

[12]

sulfonium, arsonium, and stibonium salts, which were

collected by Schiemenz and co-workers. Indeed, the crystal

structures reported for tetraarylborate salts of other onium

show similar cation–anion interactions as described

for 1bBPh₄in this work (Figure 10). However, if the con-

gen bonds, “aromatic” hydrogen bonds with phenyl groups

as acceptors, and “improper” blue-shifting hydrogen bonds.

It seems that these are widespread phenomena which should

be considered when dealing with solutions of onium salts.

As hydrogen bonding can lead to both downfield or upfield

[50b,c]

ions

ꢀ

clusions drawn for the benzyl triphenylphosphonium salts

ꢀ

1a,bX in this work are also applicable to these other

1

shifts in the H NMR spectra, and both red- or blue-shifts in

the IR stretching bands, none of these techniques alone is

onium salts, the strongest anion effect has to be expected

ꢀ

for the “ordinary” halide salts and not the BPh₄salts. The

ꢀ

sufficient to unambiguously detect weak CꢀH···X interac-

fact that red-shifts of the C(a)ꢀH IR stretching bands in

tions in solution. The combination of these methods with

CHCl solution were reported even for alkyl triphenylphos-

3

ꢀ

[7]

variations of concentrations, CꢀH acidities, or X basicities

phonium halides illustrates that hydrogen bonding also

provides more detailed insights.

plays a role for substrates of lower CꢀH acidity.

This work once again demonstrates that the formation of

weak hydrogen bonds does not always induce red-shifts of

the CꢀH stretch vibrations. This may be a reason why the

Conclusion

decisive role of hydrogen bonding for the structure of

onium ion pairs has often been underappreciated in the

past, although the structural features of the ion pairs control

the spectroscopic characteristics and reactivities of the

The remarkably large counterion-induced shifts in the

1

+

H NMR spectra of the phosphonium ion Ph CHꢀPPh

2

3

ꢀ

(

2a) (e.g., C(a)ꢀH signals of 2aCl : 8.25 ppm; 2aBPh

:

4

[59]

5

.72 ppm in CD Cl ) have previously been attributed mainly

onium salts in solution.

2

ꢀ

to the shielding by the ring current effect of the BPh

anions. In contrast, we have now demonstrated that the

H NMR spectrum of the phosphonium ion 2a is not affect-

ed by BPh₄anions at all, and that the formation of ion

4

[9]

1

ꢀ

Experimental Section

ꢀ

pairs of 2a with Cl anions or other hydrogen bond accept-

+

ꢀ

Syntheses of the phosphonium salts ArCH

2

ꢀPPh

3

X

A

H

U

G

E

N

(1X ), Ar

2

CHꢀ

(3X ): The benzyl tri-

phenylphosphonium salts 1aCl and 1aBF are commercially available;

ors is responsible for the large downfield shifts of the C(a)ꢀ

+

ꢀ

+

ꢀ

PPh

3

X

A

H

U

G

R

N

(2X ), and Ar

2

CHꢀP(p-Cl-C

H ) X ACHTNUTGRENNUG

6 4 3

H signals of 2a in CD Cl relative to that of the unpaired

ꢀ

4

2

ꢀ

[58]

ꢀ

cation (Figure 1, Table 10). Even weakly coordinating

anions such as SbF₆or BF₄induce a noticeable downfield

1bBr was prepared by a literature method; 1bBF₄and 1a,bBPh₄

were prepared by anion exchange from 1aCl or 1bBr . The benzhydryl

ꢀ

+

ꢀ

4

triphenylphosphonium tetrafluoroborates Ar

2

CHꢀPPh

3

BF

4

(2BF )

shift of +0.2 to +0.4 ppm. In sterically less congested sys-

ꢀ

and 2aCl were synthesized by heating the benzhydrols Ar

2

CHꢀOH (4)

+

ꢀ

tems such as PhCH ꢀPPh

(1a), the BPh₄anion does

2

3

+

ꢀ

with Ph

3

PH

CH PAr

benzhydryl bromides Ar

X

(X =BF

4

or Cl ). The phosphonium bromides

Br (2Br and 3Br ) were obtained by reaction of the

induce a noticeable upfield shift, but its magnitude remains

second to the deshielding effect of Cl or Br anions

ꢀ

+

ꢀ

Ar

2

3

ꢀ

CHꢀBr (5) with PAr

3

. Subsequent anion meta-

2

ꢀ

4

thesis provided the phosphonium salts 2SbF

6

, 2BPh

4

, 3BF

, and

(

Figure 2, Table 11).

ꢀ

3

SbF

6

. Details of the synthetic procedures are described in the Support-

The counterion-induced NMR shifts in quaternary phos-

phonium salts are caused by the formation of charge-assist-

ing Information.

Calculated structures of phosphonium salts in solution: The solution

ꢀ

ed CꢀH···X hydrogen bonds between the anion and the

ꢀ

structures of the salts 1a,bX and 2aX were obtained by DFT calcula-

C(a)ꢀH protons of the cation. The o-PPh and o-CPh pro-

[39]

3

tions using the program package Gaussian09. The solution structures

ꢀ

tons are likewise involved in such CꢀH···X hydrogen

of the BPh

4

salts were not calculated due to the large size of the ions.

ꢀ

The geometries of the different ion pairs 1a,bX and 2aX were opti-

mized and confirmed by frequency analysis showing no imaginary fre-

quency. We used the recently developed hybrid meta-GGA functional

M06-2X for all geometry optimizations, which has been shown to de-

scribe intermolecular interactions adequately. We included implicit sol-

bonds. The strengths of the hydrogen bonds increase in the

ꢀ

order BPh₄< SbF₆< BF₄< Br < Cl and also increase

with increasing CꢀH-acidities of the phosphonium ions. A

[40]

ꢀ

[40]

CꢀH···p hydrogen bond between C(a)ꢀH and the BPh₄

Chem. Eur. J. 2013, 00, 0 – 0

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Magn. Reson. 1974 , 6 , 276–278.

&

17

&

ÞÞ

These are not the final page numbers!

J. Ammer, K. Karaghiosoff et al.

ꢀ

vent effects of dichloromethane using the integral equation formalism

when the anion X was varied from chloride to picrate. The fact

that the same anion variation caused much larger upfield shifts in ar-

omatic solvents (e.g., Dd_Hꢁ ꢀ1.6 ppm in 1-bromonaphthalene) was

explained by the ability of the smaller anions to form contact pairs

with the onium ions, thus preventing the interaction of the aromatic

solvent with the a protons of the cation.

[

41]

variant of the polarizable continuum model (IEFPCM). The optimiza-

[

42]

tions were calculated with the double z basis 6-31+G

A

H

U

G

E

N

(d,p).

For all

atoms in the fourth period and higher we used effective core poten-

[

43]

tials. Details of the calculations and the calculated structures are given

in Section S4 of the Supporting Information.

₄ꢀ

[18] a) G. P. Schiemenz, Angew. Chem. 1971, 83, 929–930; Angew. Chem.

Int. Ed. Engl. 1971, 10, 855; b) G. P. Schiemenz, H. Rast, Tetrahe-

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134 , 11481–11494.

Crystal structure determinations: CCDC-916846 (1aBF ·CHCl

3

4^ꢀ

),

)

ꢀ

-

916530 (1bBPh

4

), -916849 (2aCl ), -916848 (2aBr ), -916847 (2aBF

ꢀ

and -916850 (2aSbF

6

) contain the supplementary crystallographic data

for this paper. These data can be obtained free of charge from The Cam-

bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-

quest/cif.

[

20] J. Ammer, C. Nolte, H. Mayr, J. Am. Chem. Soc. 2012 , 134 , 13902–

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813–4823. In this study, an upfield shift of Dd

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ꢁ ꢀ0.4 ppm was

in CDCl solution,

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PPh

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X

3

&

18

&

Chem. 2011 , 83 , 1619–1636.

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 0000, 00, 0 – 0

ÝÝ

These are not the final page numbers!

Ion-Pairing of Phosphonium Salts in Solution

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Received: December 21, 2012

Revised: June 6, 2013

Published online: && &&, 0000

(

2

Chem. Eur. J. 2013, 00, 0 – 0

ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

&

19

&

ÞÞ

These are not the final page numbers!

J. Ammer, K. Karaghiosoff et al.

Hydrogen Bonds

J. Ammer,* C. Nolte, K. Karaghiosoff,*

S. Thallmair, P. Mayer,

R. de Vivie-Riedle, H. Mayr &&&& — &&&&

Ion-Pairing of Phosphonium Salts in

1

ꢀ

Hydrogen bonds: The H NMR chemi-

bonds with the anions X , which is

Solution: CꢀH···Halogen and CꢀH···p

cal shifts of the C(a)ꢀH protons of

benzhydryl and benzyl triphenylphos-

phonium salts in CD Cl solution

strongly depend on the counter-anions

see graphic). The large downfield

confirmed by quantum chemical calcu-

lations and by the crystal structures.

Hydrogen Bonds

ꢀ

The BPh₄anion forms CꢀH···Ph

2

hydrogen bonds causing upfield shifts

of the benzyl protons. IR spectra show

(

ꢀ

shifts result from CꢀH···X hydrogen

red- or blue-shifts depending on X .

&

20

&