Acidity of dibasic carbon acids. Part 5.1-4 the second acidity constant of 9,10-dihydrodibenz[a,h]anthracene in tetrahydrofuran - Geometry, charge distribution of dianion, structure of dimetallic salts

Article abstract of DOI:10.1039/a604331c

The second equilibrium ion pair acidity constant (pK₂) of 9,10-dihydrodibenz[a,h]anthracene (DBDHA, 1H₂) with counter ions sodium (pK₂ 28.5) and potassium (pK₂ 30.4) has been determined in tetrahydrofuran (THF)

Full text of DOI:10.1039/a604331c

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Acidity of dibasic carbon acids. Part 5.1–4 The second acidity constant
of 9,10-dihydrodibenz[a,h]anthracene in tetrahydrofuran—geometry,
charge distribution of dianion, structure of dimetallic salts
Malka Nir, Israel O. Shapiro and Mordecai Rabinovitz*
Department of Organic Chemistry, The Hebrew University of Jerusalem, Givat Ram,
Jerusalem, Israel 91904
The second equilibrium ion pair acidity constant (pK2) of 9,10-dihydrodibenz[a,h]anthracene (D BD H A,
1H2) with counter ions sodium (pK2 28.5) and potassium (pK2 30.4) has been determined in
tetrahydrofuran (TH F ) at 298 K . This value is considerably lower than for other 9,10-dihydroanthracene
derivatives. D isodium, dipotassium and dirubidium salts of D BD H A, i.e. 12Ϫ, exist as contact ion triplets in
TH F at 298 K , although the dilithium salt behaves as a solvent-separated ion triplet under the same
conditions. In the dianion of D BD H A (12Ϫ) the negative charge is displaced from the deprotonated carbon
atoms towards the outer benzene rings.
12
This report deals with the dependence of the second acidity
11
1
13
14
constant (pK2) of 9,10-dihydroanthracene (DHA) derivatives
H
H
H
H
8
9
on the structure and size of the π-electron system of the dibasic
carbon acid as well as the counter ion. Previously, we measured
pK2 values of DHA, 9-phenyl- and 9,10-diphenyl-9,10-
dihydroanthracene (PDHA and DPDHA, respectively) in tetra-
hydrofuran (THF) with counter ions Na+, K+ and Rb+.1
8a
5a
1a
4a
7
6
2
3
18
17
5
10
4
15
16
1H2
12–
H
R1
and pK2 was found by measuring the equilibrium constant of
transmetallation reaction of dimetallic salts of DBDHA (1H2)
with several indicator carbon acids with known pK values.
H
R2
DHA: R1 = R2 = H
PDHA: R1 = H; R2 = Ph
DPDHA: R1 = R2 = Ph
Results
Preparation of salts and UV–VIS spectroscopic measurements
Dimetallic salts were obtained by consecutive two-electron
transfer to DBDHA (1) with alkali metals in THF. The first
step is the formation of an anion radical and the second is the
formation of the dimetallic salt [eqn. (1)], where M is an alkali
metal.
It was found that pK2 values of PDHA and DPDHA are
close to those of DHA and they are very strongly dependent on
the counter ion.
Analysis of the structure of PDHA and DPDHA dianions
shows that the angle between the planes of the phenyl substitu-
ent and the anthracene ring is 45Њ.2 It seems that the contribu-
tion of phenyl substituents to stabilization of the negative
charge of anions is small and is mainly connected with the
inductive effect. It is therefore interesting to estimate the con-
tribution of the conjugation effect of annelated phenyl groups
to the stabilization of the negative charge of the dianion of
DHA. In view of the possible effect of extending the π-electrons
array we studied dihydrodibenz[a,h]anthracene (DBDHA)
M, THF
M, THF
1
1
ϪM+
12Ϫ 2M+
(1)
᝽
The UV–VIS spectra of the anion radical and the dianion are
different. Therefore, it is possible to follow the reaction progress
and to follow the complete conversion of anion radical to dian-
ion. Dilithium and disodium salts of DBDHA (1H2) are stable
for several months. The dipotassium salt is less stable and
undergoes decomposition after several days; dirubidium salts
were decomposed after 5–6 h. The dicaesium salt is very
unstable; it decomposes after several seconds.
(1H2) and the corresponding dianion, i.e. 12Ϫ
.
According to our calculations (see below) the dianion of
DBDHA (12Ϫ) is planar and it follows that the two outer
annelated phenyl ring extend the conjugation and thus the sta-
bilization of the negative charge of the dianion of 9,10-
dihydroanthracene by conjugation.
Here we report results of the calculations of the dianion
geometry and the charge distribution, the structure of ion trip-
lets of dimetallic salts and the pK2 values of DBDHA (1H2)
(counter ions are Na+ and K+). Geometry and electron struc-
ture were calculated by the AM-1 method.5 Ion formation of
dimetallic salts of 1H2 is in the form of an ion triplet as it
consists of one anion and two cations. The structure of the ion
triplet (vida supra) was determined by UV–VIS spectroscopy
UV–VIS spectroscopic data (λmax and the molar extinction
coefficients, ε) of the dimetallic salts are given in Table 1. Each
extinction coefficient, ε, was determined in four to five concen-
trations, in the range 10Ϫ4 to 10Ϫ3 mol dmϪ3
.
In all cases Beer’s law is adhered to over the entire concen-
tration range used for pK2 determination. The ε values are
accurate to within ± 5%.
Equilibrium measurements
Ion triplet acidity (pK2) of the monometallic salt of DBDHA
(1H2), i.e. 1HϪM+, is determined by measuring the equi-
J. Chem. Soc., Perkin Trans. 2, 1997
329

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librium constant (Keq) of the transmetallation reaction [eqn. (2)]
pK2 values are given in Table 3. With each indicator two or three
independent determinations were made. With TPM (counter
ion K+) and BDPM (counter ion Na+) pK2 was determined only
once. All measurements were carried out at a constant temper-
ature of 25.0 ± 0.5 ЊC. It should be noted that the counter ion
did not affect the indicator absorption.
All our attempts to measure the equilibrium constant of the
reaction of the dilithium salt of DBDHA (1H2) with different
indicators were unsuccessful. The reaction is extremely slow.
The absorption of the dilithium salt in the reaction with 9-
phenylxanthene (9-Phx) did not change during 30 d.
Keq
12Ϫ 2M+ + InH
1HϪM+ + InϪ M+
(2)
of dimetallic salt (12Ϫ2M+) with an indicator (InH) that has a
known pKInH
.
The pK2 is determined by eqn. (3).
pK2 = log Keq + pKInH
(3)
The obtained ion triplet acidity constants1 are related to the
scale of ion pair acidities in caesium cyclohexylamide (Cs+).
We used indicator pK values measured in cyclohexylamine
(counter ion Cs+);6,7 the pK values of these indicators were also
determined in THF (counter ions Cs+) (Table 2).
Discussion
Geometry and electronic structure of the dianion
Geometry, electron structure and heat of formation of the dian-
ion were calculated by the AM-1 method. According to the
results obtained DBDHA (1H2)2Ϫ, i.e. 12Ϫ dianion, is planar.
The carbon–carbon bond lengths and the net charge on carbon
atoms of the dianions are given in Table 4. Dianion formation
leads to an alteration of the length of the carbon–carbon bonds
of the dibenzoanthracene system.1 Electron density is displaced
from the deprotonated carbon atoms to the carbon atoms of
the outer annelated benzene rings. The negative charge that is
concentrated on the deprotonated carbon atoms is considerably
less for the dianion of 1H2 than for the dianions of other di-
hydroanthracene derivatives studied.2 The heat of formation of
the dianion of 1H2 is also lower than that of the dianion of
phenyl-substituted derivatives of dihydroanthracene. (Table 5).
These results show that the dianion of 1H2 is more stable than
the dianion of dihydroanthracene and its phenyl substituted
derivatives.
The Keq is given by eqn. (4), where [1HϪ M+], [InϪ M+], [12Ϫ
Keq = ([1HϪ M+] [InϪ M+])/([12Ϫ 2M+] [InH])
(4)
2M+] and [InH] are equilibrium concentrations of monometal-
lic salt of DBDHA (1H2), metallic salt of the indicator, dimetal-
lic salt of DBDHA (1H2) and indicator, respectively.
The scale of the acidity of the indicators was measured in
several solvents (cyclohexylamine, dimethoxyethane, THF and
dimethyl sulfoxide) with different counter ions (Cs+, K+, Li+).8,9
It was found that the pKa values of the indicators—
alkylaromatic carbon acids—are practically independent of the
solvent and the counter ion. However, the scale of the indica-
tors in cyclohexylamine (counter ion Cs+) included more car-
bon acids than other indicator scales.8 This is the reason for
using this scale for the determination of the pK2 of DBDHA.
Equilibrium concentrations of [1HϪ M+], (InϪ M+] and [InH]
were found from the initial [12Ϫ 2M+]0 and equilibrium
[12Ϫ 2M+] concentration of dimetallic salt of DBDHA (1H2),
initial concentration of the indicator acid [InH]0 and the
equations of material balance [eqns. (5–7)], where A∞, A0 are the
Structure of ion triplet of dimetallic salts
The structure of the ion triplets of dimetallic salts of 1H2 was
studied by UV–VIS spectroscopy. UV–VIS spectra of di-
metallic salts of DBDHA (1H2) display absorptions in two
regions: ca. 435 and at ca. 673 nm (Table 1). The absorption
maxima of the dilithium salt is displaced towards longer wave-
lengths relative to the maxima of other salts. The absorption of
disodium, dipotassium and dirubidium salts is successively dis-
placed towards longer wavelengths as the cation radius is
increased. (Table 1).
[12Ϫ 2M+] = A∞/(l ε)
(5)
(6)
(7)
[InϪ M+] = [1HϪ M+]∞ = (A0 Ϫ A∞)/(l ε)
[InH] = [InH]0 Ϫ [(A0 Ϫ A∞)/(l ε])
These results show that disodium, dipotassium and dirubid-
ium salts of 1H2 exist as contact ion triplets (CIT) in THF at
289 K, however, the dilithium salt exists as a solvent separated
ion triplet (SSIT) under the same conditions.4,11 Previously, we
found that dialkali metal salts (including dilithium salt) of 9,10-
dihydroanthracene and its derivatives exist as CIT in THF at
298 K.4 Formation of SSIT for dilithium salts of 1H2 indicates
that there is a high degree of charge delocalization in its
dianion.
intensities of the equilibrium and initial absorptions of the
dimetallic salt of DBDHA, respectively, l is the length of the
light path, ε is the extinction coefficient of the dimetallic salt of
DBDHA, i.e. 12Ϫ
.
The pK2 values of DBDHA (1H2) were determined by meas-
uring Keq of the transmetallation reaction of the disodium and
dipotassium salts with three different indicators. The obtained
Table 1 UV–VIS spectra of dimetallic salts of DBDHA–(1H2) in THF
The maxima of absorption of the monometallic salts of 1H2
are displaced towards longer wavelengths relative to those of
the dimetallic salts of dihydroanthracene. These results show
that the mechanism of the negative charge stabilization of
dimetallic salts of 1H2 and phenyl substituted derivatives of
dihydroanthracene is different.
at 298 K
Cation
λmax/nm (ε/dmϪ3 molϪ1 cmϪ1
)
Li+
435 (—)
673 (—)
Na+
K+
406 (3.75 × 104)
409 (4.42 × 104)
412 (—)
656 (2.29 × 104)
665 (2.26 × 104)
669 (—)
Rb+
The contribution of p,π-conjugation to the charge stabiliz-
ation of dimetallic salts of 1H2 is more significant than that for
Table 2 Maxima of absorption (λmax) of caesium salt (in THF) and pK values of the indicator acids
Indicator
λmax/nm THF (Cs+)6,7
pK (CHA, Cs+)10
pK (THF, Cs+)6,7
9-Phenylxanthene (PhX)
Triphenylmethane (TPM)
Biphenyl-4-yldiphenylmethane (BDPM)
Bis(biphenyl-4-yl)methane (BBM)
491
488
568
557a
28.5
31.4
30.2
30.8
28.7
31.3
30.1
—
a
In cyclohexylamine, counter ion Cs+.
330
J. Chem. Soc., Perkin Trans. 2, 1997

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Table 3 Sodium, pK2 (Na+), and potassium, pK2 (K+), ion pair acidity
constants of 9,10-dihydrodibenz[a,h]anthracene 1 in THF at 298 K
moiety, they take part in the stabilization of the dianion by p,π-
conjugation. Therefore, the displacement of the negative charge
from the deprotonated carbon atoms into the π-array of 1H2 is
larger than the shift of charge delocalization into the π-array of
DPDHA or DHA. The negative charge which is placed on the
deprotonated carbon atoms is equal to 0.492 (electron charge)
for the dianion of 1H2 and 0.664 (electron charge) for the
dianion of DPDHA (Table 5).
Indicator
pK2 (Na+)
pK2 (K+)
9-PhX
TPM
BDPM
BBM
28.4 ± 0.1
28.6 ± 0.3
28.5
—
30.1
30.3 ± 0.2
30.5 ± 0.3
—
The increase of the contribution of the effect of p,π-
conjugation which stabilizes the dianion of 12Ϫ leads to a
decrease of the contribution of the cation effect. The difference
between pK2 (K+) and pK2 (Na+) is equal to 2.8 pK units and 1.6
pK units for DPDHA and 1H2, respectively (Table 3). Neverthe-
less, this result shows that the cation–anion interaction still
plays an important role in the stabilization of the dimetallic
salts of 1H2.
Average
28.5 ± 0.3
30.4 ± 0.3
Table 4 The C᎐C bond lengths and net atomic charge on the carbon
atoms of 12Ϫ a
C᎐C Bond
Length/Å
Carbon atom
Net charge
Ϫ0.121
C1᎐C2 (C5᎐C6)
C2᎐C3 (C6᎐C7)
C3᎐C4 (C7᎐C8)
1.4423
1.4107
1.4107
1.3747
1.4434
1.3678
1.4570
1.3946
1.3952
1.4009
1.3951
1.4296
1.4545
C1 (C5)
C2 (C6)
C3 (C7)
C4 (C8)
In conclusion, the high value of the second acidity con-
stant pK2 1H2 is rationalized by a considerable increase of the
contribution of p,π-conjugation effect and the cation–anion
+0.065
Ϫ0.354
Ϫ0.113
Ϫ0.089
+0.040
Ϫ0.069
Ϫ0.313
Ϫ0.099
Ϫ0.252
Ϫ0.241
C4᎐C4a (C8᎐C8a)
C4a᎐C10 (C8a᎐C9)
C10᎐C5a (C9᎐C1a)
C5a᎐C5 (C1a᎐C1)
C5᎐C15 (C1᎐C11)
C15᎐C16 (C11᎐C12)
C16᎐C17 (C12᎐C13)
C17᎐C18 (C13᎐C14)
C18᎐C6 (C14᎐C2)
C1a᎐C4a (C5a᎐C8a)
interaction on the dianion stabilization of 12Ϫ
.
C4a (C8a)
C1a (C5a)
C11 (C15)
C12 (C16)
C13 (C17)
C14 (C18)
C9 (C10)
Experimental
General
UV–VIS spectra were recorded on UVICON-860 spectrometer
fitted with a thermostatically controlled cell holder. All experi-
ments were carried out at constant temperature (25.0 ± 0.5 ЊC)
maintained by a thermostatted bath (Forma Scientific 2095).
Melting points were determined on a Thomas-Hoover capillary
melting point apparatus and are uncorrected.
a
Calculated by the AM-1 method.
Table 5 Negative charge on the deprotonated carbon atoms, heat of
dianion formation (∆Hf) and pK2 of DHA and its derivatives1,2
Materials
Net electron density
on the deprotonated
carbon atoms
∆Hf/kcal
Dibenz[a,h]anthracene 1. Commercially available dibenz-
[a,h]anthracene (Aldrich) was twice crystallized from benzene–
methanol, mp 266–268 ЊC (lit.,12 261–162 ЊC). Its UV–VIS
spectrum was identical with that in the literature.12
Triphenylmethane (TPM). Commercially available TPM
(Aldrich) was recrystallized from ethanol, mp 94 ЊC (lit.,13
94 ЊC).
9-Phenylxanthene (9PhX). 9PhX was prepared by reduction
of a commercially available sample of 9-phenylxanthene-9-ol
over 10% Pd/C under hydrogen pressure (2 bar) at room temp.,
and recrystallized from ethanol, mp 146–147 ЊC (lit.,14 147.8–
148.1 ЊC).
Bis(biphenyl-4-yl)methane (BBM). BBM was prepared by
hydrogenation of bis(biphenyl-4-yl)methanol over Pd/C. After
twice recrystallizing from methanol, the white crystals showed
mp 160–162 ЊC (lit.,14 162 ЊC). Bis(biphenyl-4-yl)methanol was
prepared by the procedure given in ref. 14.
Dianion
molϪ1 b
pK2 (Na+)
DHA=
Ϫ0.860
97.42
113.37
130.06
111.2a
34.1
33.6
9-PDHA=
DPDHA=
DBDHA=(12Ϫ
Ϫ0.701
Ϫ0.664
Ϫ0.492a
32.0
)
28.5 ± 0.3a
a
This paper. b 1 cal = 4.184 J.
the dimetallic salts of dihydroanthracene (long-wavelength dis-
placement of λmax). For dimetallic salts of the phenyl and
diphenyl derivative of dihydroanthracene, i.e. PDHA or
DPDHA, the contribution of p,π-conjugation to the negative
charge stabilization is reduced as compared with the dimetallic
salts of dihydroanthracene DHA (short-wavelength displace-
ment of λmax 1). It follows that the dimetallic salts of 9-phenyl-
and 9,10-diphenyl-dihydroanthracene, PDHA or DPDHA,
respectively are mainly stabilized by cation–anion interaction.
Biphenyl-4-yldiphenylmethane (BDPM).
BDPM
was
obtained by reduction of biphenyl-4-yldiphenylmethanol over
10% Pd/C under hydrogen pressure (2 bar). The product
obtained was recrystallized twice from methanol, mp 113–
114 ЊC (lit.,14 113.5–114.5 ЊC). Biphenyl-4-yldiphenylmethanol
was prepared from benzophenone and biphenyl-4-yllithium.14
Tetrahydrofuran (THF). The procedure for the purification of
the commercial solvent and its preparation for the transmetall-
ation reaction was given in ref. 1.
Second acidity constant
The values of second acidity constants of 1H2 are equal to
28.5 ± 0.3 (counter ion Na+) and 30.4 ± 0.3 (counter ion K+).
The values are related to ion pair acidity scale in cyclohexy-
lamine (counter ion Cs).8,10 The pK values of the indicators are
also measured in THF;6,7 therefore, the pK2 of 1H2 can be
related to the ion pair acidity scale in this solvent. In the ion
pair acidity scale in THF the pK2 of 1H2 is equal to 28.5 ± 0.3
(counter ion Na+) and 30.1 ± 0.2 (counter ion K+).
Equilibrium measurements. Apparatus and technical details
of equilibrium experiments were described in a previous paper.1
The value of pK2 of 1H2 is considerably lower than that of
other DHA derivatives and in particular the pK2 of DPDHA.1
The difference between the pK2 of DBDHA and DPDHA is
equal to 3.5 pK units (Na+ as counter ion) and 4.4 pK units (K+
as counter ion). Comparison of the pK2 of DHA (34.1; K+),
DPDHA (32.0; K+) and 1H2 (30.4; K+) as well as the structure
of their dianions shows that increase of the second acidity
constant of the latter compound is related to the extended
annelation with the outer benzene rings in the 12Ϫ dianion. As
these benzene rings are in the same plane of the anthracene
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Paper 6/04331C
Received 20th June 1996
Accepted 2nd October 1996
332
J. Chem. Soc., Perkin Trans. 2, 1997