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DFT Calculations on a New Class of C3-Symmetric Organic Bases Highly Basic Proton Sponges and Ligands for Very Small MetalCations.

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Angewandte
Chemie
Proton Sponges
DFT Calculations on a New Class of
C3-Symmetric Organic Bases: Highly Basic
Proton Sponges and Ligands for Very Small
Metal Cations
Gtz Bucher*
The design and synthesis of strong organic bases has long
been an active field of research.[1–4] Extremely high basicity
can be achieved by different strategies. Spatial overlap of lone
[*] Dr. G. Bucher
Lehrstuhl fr Organische Chemie II
Ruhr-Universitt Bochum
Universittsstrasse 150, 44780 Bochum (Germany)
Fax: (+ 49) 234-3214-353
E-mail: goetz.bucher@rub.de
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 4039 –4042
DOI: 10.1002/anie.200351648
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4039
Communications
pairs leads to destabilization and thus enhanced basicity. This
concept has repeatedly been employed with nitrogen bases. It
led to the synthesis of proton sponges, such as 1,8-bis(dimethylamino)naphthalene (1)[5] and derivatives or quino[7,8h]quinoline (2).[6] The basicity of these compounds does not
only derive from the overlap of two nitrogen lone pairs in the
free bases, but is also due to the formation of a strong
intramolecular hydrogen bridge in the monoprotonated
compounds.
The conjugate acids of certain organic bases, such as
guanidines, are more strongly stabilized by resonance than the
free bases which also leads to compounds of significantly
increased basicity.[7a,b] The systematic exploitation of this
strategy has led to the synthesis of exceedingly basic species,
such as vinamidines[7c] or polyphosphazenes.[8] The two
concepts can also be combined, as has recently been
demonstrated by the synthesis of the highly basic 1,8bis(tetramethylguanidino)naphthalene (3).[9]
To date, all known examples of proton sponges involve
two nitrogen lone pairs. Herein the possibility of achieving
even more basic systems through the interaction of three or
four nitrogen lone pairs, and of possibly obtaining a C3symmetric hydrogen bridge is addressed. Molecules with
threefold symmetry can be obtained by joining three identical
ligands with a nitrogen or boron atom.
The bases introduced herein build on the framework of
syn-tris-8-quinolylborane (4). Additional bridging of 4 by a
C3-symmetric (CH2)3N bridge either in the 7-position (!5) or
in 2-position (!6) reduces the conformational flexibility of
4040
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
tris-8-quinolylborane unit and forces the nitrogen lone pairs
into close proximity. The use of a nitrogen and a boron atom
as molecular axis of threefold symmetry offers the advantage
of a stabilizing intramolecular NB donor–acceptor bond.[10]
The benzoquinolines 7–9 were calculated to investigate the
influence of a more rigid molecular framework. The choice of
B and N in different sequence (7 or 8) or of two C atoms (9) as
central atoms makes it possible to vary the space between the
N atoms. To investigate the influence of the third lone pair,
the bridged bis(quinolyl)phenylborane 11 was calculated.
While having a similar structure of the basic center, in
compound 11 an interaction of only two lone pairs takes
place.
The geometries and vibrational spectra of 4–9 as well as of
quinoline 10 and quino[7,8-h]quinoline 2 as reference compounds, each in different stages of protonation, were calculated by standard density functional theory (B3LYP/631G(d)). For the calculation of gas-phase proton affinities
single-point calculations employing a larger basis set were
performed (see Methods Section). Proton affinities in acetonitrile (PA(CH3CN)) were calculated according to the
isodensity surface polarized continuum model (ipcm).[11]
The pKa values of the conjugate acids in acetonitrile were
obtained from PA(CH3CN) by Equation (1).[12]
pKa ðtheorÞ ¼ 0:72888 PAðCH3 CNÞ189:5
ð1Þ
The results are displayed in Table 1. Relevant geometric
parameters chosen are the N–H and N–N separations which
are important for characterization of the intramolecular
hydrogen bridges, as well as the aN-H-N’ bond angles. If
applicable, the B–N (or C–C in the case of 9) bond lengths and
the dihedral around the central N–B unit (dCNBC) are also
tabulated. The latter indicates the degree of propeller-shape
assumed by the molecule.
The results show that in 5–9 the forced proximity of three
nitrogen atoms leads to a basicity that is far greater than that
of 2, although the N–N separation (which is a determining
factor for the destabilizing interaction of the lone pairs) is
shorter in 2 (274 pm) than in 5–9 (between 289 pm in 6 and
343 pm in 5). While the pKa value of the conjugate acid of 2
(pKa(MeCN) = 19.2) exceeds the pKa value of the conjugate
acid of the reference compound 10 (12.2) by 7 units, the C3symmetric proton sponges have pKa values, that are higher by
6.8 (5, 26.0) to 11 units (6, 30.2). In particular 6 is predicted to
show a basicity approaching that of the most basic known
neutral nitrogen bases.[12, 7] Taking the syn conformer of 4
(pKa(MeCN) = 14.5) as reference compound reveals that the
additional bridging in 5 or 6 results in a gain in basicity of 11.5
(5) to 15.7 (6) units. This, however, only applies to the free
bases. The differences are significantly less pronounced for
the monoprotonated compounds, while in the case of the
diprotonated compounds the more flexible system 4 shows a
higher basicity than 5 or even 6 (6/2H+: pKa = 15.2).
The bases 4–9 are propeller-shaped and thus have C3 and
not D3d symmetry. For 5 and 6, the deviation from D3d
symmetry is defined over the dihedral angle dCNBC, which is
given by C(H2)-N-B-C8.[13] In 7 it is defined by C9, N, B, and
C10. In general, dCNBC ranges between 15.28 (7) and 34.08 (6).
www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 4039 –4042
Angewandte
Chemie
Table 1: Calculated geometric parameters, proton affinities (PA), and pKa values of bases 2 and 4–11 and their conjugate acids.[a]
Compound
dCNBC
RB-N
RN-H
RN’-H
RN’-N
AN-H-N’
PA(Gas)[b]
PA(MeCN)[c]
–
–
–
–
–
–
–
–
–
–
173
–
179
197
195
394
–
189
191
193
309
–
–
–
199
206
186
212
–
194
–
171
–
176
–
181
–
–
274
263
–
–
–
140.4
–
163.8
161.8
176.3
155.6
–
154.8
164.0
174.0
162.5
120.0[g]
119.9[i]
–
136.2
119.8
169.0
170.0
–
148.2
–
159.6
–
156.7
–
165.4
228.0
–
253.7
–
251.8
175.3
112.8
276.7
–
286.3
n.c.[e]
279.9
273.0
269.1
12.2
–
19.2
n.c.[e]
14.5
9.5
6.6
–
265.6
185.8
102.4
–
295.6
273.7
263.3
–
26.0
10.0
2.4
–
–
–
268.1
176.7
–
–
–
301.4
270.5
–
–
–
30.2
7.7
79.9
–
264.5
n.c.[e]
255.4
n.c.[e]
262.3
n.c.[e]
257.4
–
239.2
–
292.5
n.c.[e]
290.3
n.c.[e]
290.2
n.c.[e]
290.3
–
10
10/H+
2
2/H+
4
4/H+
4/2H+
–
–
–
–
4/3H+
5
5/H+
5/2H+
–
28.1
21.6
19.6
–
172
170
169
5/3H+
5/Be++
5/Mg++
6
6/H+
31.9
17.3
25.7
34.0
18.6
20.2
1.2
27.0
15.2
4.4
22.1
17.1
19.3
11.0
25[l]
20[l]
165
166
164
457
456
–
102
–
105
–
105
104
104
102
–
105
104
104
102
167[f ]
204[h]
–
105
460
449
183
179
180
188
162[j]
164[j]
173
170
103
101
–
105
–
106
–
105
–
105
6/2H+
6/3H+ [k]
7
7/H+
8
8/H+
9
9/H+
11
11/H+
–
–
282
297
298
488
343
288
294
294
408
289
353
289
284
285
288
312
337
288
303
272
313
275
335
284
pKa(theor)[d]
15.2
–
23.7
n.c.[e]
22.1
n.c.[e]
22.0
n.c.[e]
22.1
–
[a] All bond lengths [pm] and bond angles [8] calculated with B3LYP/6-31G(d). [b] Calculated with B3LYP/6-311 + G(d,p)//B3LYP/6-31G(d), in
kcal mol1. [c] Calculated with ipcm B3LYP/6-31G(d), in kcal mol1. [d] pKa value of conjugate acid. [e] n.c. = not calculated. [f ] RN-Be. [g] aN-Be-N’.
[h] RN-Mg. [i] aN-Mg-N’. [j] CC bond length of the central bond of the triquinacene framework. [k] Protonation of the quinoline nitrogen atoms [l] Average
of all three dCNBC.
The values are smaller for the benzoquinolines 7–9 than for
the bridged methylquinolines 5 and 6, which is due to the
lower flexibility of the benzoquinolines. The benzoquinolines
can only avoid D3d symmetry by giving up planarity of the
benzoquinoline unit. Some of the mono- and diprotonated
compounds are predicted to show significantly reduced values
of d. In particular the dication of 6 (6/2H+) is almost CS
symmetric (d = 1.28). The intramolecular N-H-N hydrogen
bridges of the monoprotonated compounds of 5–9 are
generally unsymmetrical. The NH bond lengths are only
slightly elongated compared to the reference compound 10/
H+. The N–N’ separations of the monoprotonated compounds
(ranging from 275 pm (9/H+) to 288 pm (5/H+)) are slightly
longer than the N–N’ separations of the reference proton
sponge 2/H+ (263 pm). Also, the N-H-N bond angles of the
monoprotonated compounds deviate from the ideal value of
1808 (A ranges from 136.28 (6/H+) to 159.68 (8/H+)).
A comparison with the model compound 11 is of
particular interest. In the case of 11, the interaction of two
nitrogen lone pairs leads to a high basicity (pKa(MeCN) =
22.1). This value lies four units below the pKa value of the
analogous tris(quinoline) base 5. A comparison of the N-H-N’
bond angle of the intramolecular hydrogen bridge in 5/H+
(a = 154.88) and 11/H+ (a = 165.48) shows that the presence of
a third nitrogen lone pair in 5/H+ results in a stronger
Angew. Chem. Int. Ed. 2003, 42, 4039 –4042
deviation from the ideal value of a = 1808. Also, N’–H
separation is larger in 5/H+ than in 11/H+, while the N–H
separation is the same in both systems. The fact that 5 is
predicted to be a stronger base than 11 thus shows that the
destabilizing interaction of three lone pairs is more pronounced for the free bases than for the monoprotonated
species.
In the case of proton sponge 6, four nitrogen lone pairs are
in close proximity. In this system, the tertiary amine should
have basic character too, as an intramolecular donor–acceptor
bond to boron cannot be formed. Protonation of 6 should thus
also be possible at the tertiary amine moiety, in both endo and
exo positions. The pKa(MeCN) value for exo-protonation is
calculated as 6.8 which corresponds to a PA(CH3CN) =
269.3 kcal mol1. This extremely low basicity is because the
nitrogen atom of the tris(quinolylmethyl)amine moiety in 6
has almost trigonal-planar coordination (calculated dihedral
angle C-N-C-C = 1748). The lone pair thus has almost pure
p character.[14] In exo-monoprotonated 6/H+(exo) the dihedral angle C-N-C-C is calculated to be 1408, thus deviating
strongly from the ideal value of 1208. Endo-monoprotonation
of the tertiary nitrogen atom is not possible according to the
calculations. An attempted geometry optimization instead
gave the geometry of 6/H+ (monoprotonation in the tris(quinoline) system).
www.angewandte.org
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4041
Communications
The proton is bound by 5–9 through an unsymmetrical NH···N hydrogen bridge. C3-symmetric coordination of the
three quinoline nitrogen atoms should be possible in the case
of metal cation complexation in the cavity of 5–9. Owing to
the small size of the cavity (N–N = 343 pm for 5) only the
smallest metal cations have to be considered. Selected
geometric parameters for 5/Be2+ and 5/Mg2+ are given in
Table 1. The data show that complexation of a Be2+ ion should
lead to similar changes of the host geometry as protonation,
whereas complexation of the larger Mg2+ ion should result in
larger N–N separations and in a more pronounced twisting of
the molecule. Nevertheless, a perfectly shielded beryllium ion
is only expected to be found in the gas phase or in aprotic
solvents. Semiempirical calculations (PM3)[15] for the system
5 + Be2+ + 6 H2O indicate, that the presence of water should
lead to the formation of additional O-H-N hydrogen bridges,
to a more pronounced twisting of the molecule, and to larger
N–N separations.
In conclusion, interaction of three nitrogen lone pairs
should lead to a dramatic increase of the basicity of quinoline
derivatives. Furthermore, the tris(quinolyl)boranes 5 and 6
should be interesting three-dimensional chelating ligands for
very small metal cations such as Be2+ or Mg2+. The synthesis
of 5 and 6 is in preparation.
[6]
[7]
[8]
[9]
[10]
[11]
[12]
Methods Section
All calculations were performed with the Gaussian 98 suite of
programs.[16] All stationary points were unambiguously characterized
as minima by calculating their vibrational spectra. The calculated
energies include a zero-point vibrational-energy correction. The
B3LYP hybrid functional[17] in connection with a 6-31G(d) basis was
employed for geometry optimizations and calculations of IR spectra.
In addition, for all molecules investigated a single-point-calculation
employing a larger basis set (B3LYP/6-311 + G(d,p)//B3LYP/631G(d)) was performed. Proton affinities in acetonitrile were
calculated according to the isodensity surface polarized continuum
model (method: B3LYP/6-31G(d)).[11]
[15]
Received: April 14, 2003 [Z51648]
.
[16]
Keywords: density functional calculations · organic bases ·
proton sponges
[1] F. Hibbert, Acc. Chem. Res. 1984, 17, 115 – 120.
[2] H. A. Staab, T. Saupe, Angew. Chem. 1988, 100, 895 – 1040;
Angew. Chem. Int. Ed. Engl. 1988, 27, 865 – 879.
[3] R. W. Alder, Chem. Rev. 1989, 89, 1215 – 1223.
[4] R. W. Alder, Tetrahedron 1990, 46, 683 – 713.
[5] a) W. G. Brown, N. J. Netang, J. Am. Chem. Soc. 1941, 63, 358 –
361; b) R. W. Alder, P. S. Bowman, W. R. S. Steele, D. R.
Winterman, Chem. Commun. 1968, 723 – 724; c) R. W. Alder,
N. C. Goode, N. Miller, F. Hibbert, K. P. Hunte, H. J. Robbins, J.
Chem. Soc. Chem. Commun. 1978, 89 – 90; d) R. W. Alder, M. R.
4042
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[13]
[14]
[17]
Bryce, N. C. Goode, N. Miller, J. Owen, J. Chem. Soc. Perkin
Trans. 1 1981, 2840 – 2847; e) R. W. Alder, M. R. Bryce, N. C.
Goode, J. Chem. Soc. Perkin Trans. 2 1982, 477 – 483; f) F.
Hibbert, K. P. Hunte, J. Chem. Soc. Perkin Trans. 2, 1983, 1895 –
1899.
M. A. Zirnstein, H. A. Staab, Angew. Chem. 1987, 99, 460 – 461;
Angew. Chem. Int. Ed. Engl. 1987, 26, 460 – 461.
a) Z. B. Maksic, B. Kovacevic, J. Org. Chem. 2000, 65, 3303 –
3309; b) B. Kovacevic, T. Glasovac, Z. B. Maksic, J. Phys. Org.
Chem. 2002, 15, 765 – 774; c) R. Schwesinger, M. Mißfeldt, K.
Peters, H. G. von Schnering, Angew. Chem. 1987, 99, 1210 –
1212; Angew. Chem. Int. Ed. Engl. 1987, 26, 1165 – 1166.
a) R. Schwesinger, H. Schlemper, Angew. Chem. 1987, 99, 1212 –
1214; Angew. Chem. Int. Ed. Engl. 1987, 26, 1167 – 1169; b) R.
Schwesinger, H. Schlemper, C. Hasenfratz, J. Willaredt, T.
Dambacher, T. Breuer, C. Ottaway, M. Fletschinger, J. Boele, H.
Fritz, D. Putzas, H. W. Rotter, F. G. Bordwell, A. V. Satish, G.-J.
Ji, E.-M. Peters, K. Peters, H. G. von Schnering, L. Walz, Liebigs
Ann. 1996, 1055 – 1081.
a) V. Raab, J. Kipke, R. M. Gschwind, J. Sundermeyer, Chem.
Eur. J. 2002, 8, 1682 – 1693; b) B. Kovacevic, Z. B. Maksic, Chem.
Eur. J. 2002, 8, 1694 – 1702.
This does not apply to 6. In this molecule the N and B atoms are
too far apart to form a NB donor/acceptor bond.
J. B. Foresman, T. A. Keith, K. B. Wiberg, J. Snoonian, M. J.
Frisch, J. Phys. Chem. 1996, 100, 16 098 – 16 104.
a) B. Kovacevic, Z. B. Maksic, Org. Lett. 2001, 3, 1523 – 1526;
b) Equation (1) was derived analogous to in ref. [12a] by
correlating the calculated (ipcm B3LYP/6-31G(d)) proton
affinities (in acetonitrile) of a series of 13 bases with the
experimentally determined pKa values of the conjugate acids
(in acetonitrile). The basis set of the ipcm calculations presented
here is smaller than the basis set used in ref. [12a]. For that
reason, the correlation is not as good (r2 = 0.960) as in ref. [12a].
The calculated pKa values differ from the experimentally
determined pKa values by a maximum of 1.5 units. A scheme
showing the bases calculated for deriving Equation (1), as well as
a plot of PA(MeCN) vs. pKa(MeCN), is given in the Supporting
Information.
C8 belongs to the same methylenequinoline subunit as C(H2).
By planarizing the tertiary nitrogen atom, the distance between
the quinoline lone pairs is maximized.
a) J. J. P. Stewart, J. Comput. Chem. 1989, 10, 209 – 220; b) J. J. P.
Stewart, J. Comput. Chem. 1989, 10, 221 – 264.
Gaussian 98 (Revision A.7), M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G.
Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S.
Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain,
O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B.
Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A.
Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J.
V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M.
Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W.
Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A.
Pople, Gaussian, Inc., Pittsburgh, PA, 1998.
A. D. Becke, J. Chem. Phys. 1993, 98, 5648 – 5652.
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