From classical adducts to frustrated lewis pairs: Steric effects in the interactions of pyridines and B(C6F5)3

Article abstract of DOI:10.1021/ic901726b

The pyridine adducts of B(C₆F₅)₃, (4-tBu)C₅H₄NB(C₆F₅)₃ 1, ((2-Me)C₅H₄N)B(C₆F₅)₃ 2, ((2-Et)C₅H₄

Full text of DOI:10.1021/ic901726b

10466 Inorg. Chem. 2009, 48, 10466–10474
DOI: 10.1021/ic901726b
From Classical Adducts to Frustrated Lewis Pairs: Steric Effects in the Interactions
of Pyridines and B(C6F5)3
Stephen J. Geier,† Austin L. Gille,‡ Thomas M. Gilbert,‡ and Douglas W. Stephan*,†
†Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada, and
‡Department of Chemistry & Biochemistry, Northern Illinois University, DeKalb, Illinois 60115
Received August 31, 2009
The pyridine adducts of B(C6F5)3, (4-tBu)C5H4NB(C6F5)3 1, ((2-Me)C5H4N)B(C6F5)3 2, ((2-Et)C5H4N)B(C6F5)3 3,
((2-Ph)C5H4N)B(C6F5)3 4, ((2-C5H4N)C5H4N)B(C6F5)3 5, (C9H7N)B(C6F5)3 6, and ((2-C5H4N)NH(2-C5H4N))B(C6F5)3
7, were prepared and characterized. The B-N bond lengths in 2-7 reflect the impact of ortho-substitution, increasing
significantly with sterically larger and electron-withdrawing substituents. In the case of 2-amino-6-picoline, reaction with
B(C6F5)3 affords the zwitterionic species (5-Me)C5H3NH(2-NH)B(C6F5)3 8. In contrast, lutidine/B(C6F5)3 yields an
equilibrium mixture containing both the free Lewis acid and base and the adduct (2,6-Me2C5H3N)B(C6F5)3 9. This
equilibrium has a ΔH of -42(1) kJ/mol and ΔS of -131(5) J/mol K. Addition of H2 shifts the equilibrium and yields [2,6-
3
Me2C5H3NH][HB(C6F5)3] 10. The corresponding reactions of 2,6-diphenylpyridine or 2-tert-butylpyridine with B(C6F5)3
showed no evidence of adduct formation and upon exposure to H2 afforded [(2,6-Ph2)C5H3NH][HB(C6F5)3] 11 and [(2-
tBu)C5H4NH][HB(C6F5)3] 12, respectively. The energetics of adduct formation and the reactions with H2 are probed
computationally. Crystallographic data for compounds 1-10 are reported.
Introduction
nitriles,4 enamines, and silylenol-ethers.5 In addition, fru-
strated Lewis pairs have been exploited to effect the
activation of tetrahydrofuran,6 catecholborane,7 olefins,8
dienes,9 terminal alkynes,10 disulfides,11 CO2,12 and
N2O.13 The initial frustrated Lewis pairs systems reported
were based on bulky phosphines in combination with
the Lewis acid B(C6F5)3.14 Since then, linked phosphinobo-
ranes15 pairs of bulky carbenes,4b,16 and amines17 with
Recent advances in main group chemistry have provided
opportunities for rather unique small molecule activa-
tions. Among these, compounds incorporating “frustrated
Lewis pairs” which are mixtures of Lewis acids and bases
that do not form adducts because of steric constraints
have Lewis acidity and basicity which result in unique
abilities to activate small molecules including H2.1 Simi-
larly, species with element-element multiple bonds2 and
carbenes3 have garnered attention as a result of their ability
to activate H2. In the case of frustrated Lewis pairs, this
reactivity has been extended to allow the metal-free hydro-
genation catalysis of imines, aziridines, borane-bound
(7) Dureen, M., A.; Lough, A.; Gilbert, T. M.; Stephan, D. W. Chem.
Commun. 2008, 4303.
(8) McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem., Int.
Ed. 2007, 46, 4968.
(9) Ullrich, M.; Seto, K. S.-H.; Lough, A. J.; Stephan, D. W. Chem.
Commun. 2009, 2335.
(10) Dureen, M. A.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 8396.
(11) Dureen, M. A.; Welch, G. C.; Gilbert, T. M.; Stephan, D. W. Inorg.
Chem. 2009, DOI: 10.1021/ic901590s.
(12) Momming, C. M.; Otten, E.; Kehr, G.; Frohlich, R.; Grimme, S.;
Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2009, 48, 6643.
(13) Otten, E.; Neu, R. C.; Stephan, D. W., J. Am. Chem. Soc. 2009, 131,
9918.
(14) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.
(15) Geier, S. J.; Gilbert, T. M.; Stephan, D. W. J. Am. Chem. Soc. 2008,
130, 12632.
*To whom correspondence should be addressed. E-mail: dstephan@
chem.utoronto.ca.
€
€
(1) (a) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W.
Science 2006, 314, 1124. (b) Stephan, D. W. Dalton Trans. 2009, 3129.
(c) Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535.
(2) Spikes, G. H.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc. 2005,
127, 12232.
(3) Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand,
G. Science 2007, 316, 439.
(16) (a) Chase, P.; Gille, A. L.; Gilbert, T.; Stephan, D. W. Dalton Trans.
2009, 7179. (b) Holschumacher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.;
Tamm, M. Angew. Chem., Int. Ed. 2008, 47, 7428. (c) Holschumacher, D.;
Taouss, C.; Bannenberg, T.; Hrib, C. G.; Daniliuc, C. G.; Jones, P. G.; Tamm, M.
Dalton Trans. 2009, 6927.
(4) (a) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008,
1701. (b) Chase, P. A.; Stephan, D. W. Angew. Chem., Int. Ed. 2008, 47,
7433.
€
(5) Spies, P.; Schewendemann, S.; Lange, S.; Kehr, G.; Frohlich, R.;
Erker, G. Angew. Chem., Int. Ed. 2008, 47, 7543.
(6) (a) Welch, G. C.; Cabrera, L.; Chase, P. A.; Hollink, E.; Masuda, J. D.;
(17) (a) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.;
€
Wei, P.; Stephan, D. W. Dalton Trans. 2007, 3407. (b) Welch, G. C.; Prieto, R.;
Leskela, M.; Repo, T.; Pyykko, P.; Rieger, B. J. Am. Chem. Soc. 2008, 130,
€
€
Dureen, M. A.; Lough, A. J.; Labeodan, O. A.; Holtrichter-Rossmann, T.;
14117. (b) Sumerin, V.; Schulz, F.; Nieger, M.; Leskela, M.; T., R.; Rieger, B.
Stephan, D. W., Dalton Trans. 2009, 1559.
Angew. Chem., Int. Ed. 2008, 47, 6001.
r
pubs.acs.org/IC
Published on Web 10/07/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10467
Scheme 1. Main Group Systems That Activate H2
B(C6F5)3 was generously donated by NOVA Chemicals Cor-
poration.
Synthesis of (4-tBu)C5H4NB(C6F5)3 1, (2-Me)C5H4NB-
(C6F5)3 2, (2-Et)C5H4NB(C6F5)3 3, (2-Ph)C5H4NB(C6F5)3 4, (2-
C5H4N)C5H4NB(C6F5)3 5, C9H7NB(C6F5)3 6, (2-C5H4N)NH(2-
C5H4N)B(C6F5)3 7. These compounds were prepared in a similar
fashion and thus only one preparation is detailed. B(C6F5)3
(100 mg, 0.20 mmol) was added to a solution of 4-(t-Bu)C5H4N
(26 mg, 0.20 mmol) in 2 mL of toluene. The solution was stirred
for 4 h, 2 mL of hexanes was added, and the solution was stored
at -35 °C overnight. The solution was decanted from the
resulting white precipitate. The precipitate was washed with
an additional 2 mL of hexanes and dried in vacuo. Crystals were
grown from the hexane wash layer at -35 °C.
1. Yield: 98 mg (78%). 1H NMR (CD2Cl2): 1.40 (s, 9H, CH3),
7.64 (d, 3JH-H = 7 Hz, 2H), 8.46 (d, 3JH-H = 7 Hz, 2H); 19
F
NMR (CD2Cl2): -132.2 (d, 3JF-F = 19 Hz, 6F, o-C6F5), -158.2
3
3
(t, JF-F = 20 Hz, 3F, p-C6F5), -164.6 (dd, JF-F = 19 Hz,
3JF-F = 20 Hz, 6F, m-C6F5); 11B NMR (CD2Cl2): -4.1 (br s);
13C NMR (CD2Cl2): 30.0 (CH3), 36.1 (C-CH3), 123.0, 137.4
B(C6F5)3 and alkyl-linked phosphine-boranes5,18 have been
shown to be effective frustrated Lewis pairs (Scheme 1).
In seeking to explore and broaden the range of bases that
can be employed in frustrated Lewis pair chemistry, we were
motivated by a 1942 report by Brown et al.19 In that work, it
was reported that 2,6-lutidine was incapable of forming a
classical Lewis adduct with BMe3, as a result of steric conflict
of the B-bound methyl groups and the substituents on
pyridine. Although Brown and co-workers observed what
we now refer to as a frustrated Lewis pairs, they did not
explore the resulting reactivity of such systems. In this full
account we explore bothexperimentally and computationally
the ability of substituted pyridines and B(C6F5)3 to act both
as classical Lewis pairs, forming donor-acceptor adducts,
and as frustrated Lewis pairs effecting the activation of H2. A
portion of this work has been previously communicated.20
1
1
(dm, JC-F =241 Hz, CF), 140.3 (dm, JC-F =260 Hz, CF),
146.3, 148.1 (dm, 1JC-F = 248 Hz, CF), 168.8. Anal. Calcd for
C27H13BF15N: C, 50.11%; H, 2.02%; N, 2.16%. Found: C,
50.27%; H, 2.08%; N, 2.16%.
2. Yield: 89%. Crystals were grown from a layered solution of
CDCl3/pentane at -35 °C. 1H NMR (CDCl3): 2.51 (s, 3H, CH3),
7.43 (m, 2H), 7.99 (td, 3JH-H = 7 Hz, 4JH-H = 2 Hz, 1H), 8.62
(pseudo-q, J = 6 Hz, 1H); 19F NMR (CDCl3): -126.3 (m, 3JF-F
=22Hz, 1F, o-C6F5), -128.9 (m, 1F, o-C6F5), -132.4 (d, 3JF-F
=
22 Hz, 1F, o-C6F5), -133.2 (m, 1F, o-C6F5), -133.4 (m, 1F, o-
C6F5), -137.7 (m, 1F, o-C6F5), -155.6 (t, 3JF-F = 22 Hz, 1F, p-
C6F5), -156.2 (t, 3JF-F = 22 Hz, 1F, p-C6F5), -157.7 (t, 3JF-F
=
22 Hz, 1F, p-C6F5), -161.9 (td, 3JF-F = 21 Hz, 4JF-F = 9 Hz, 1F,
m-C6F5), -162.9 (td, 3JF-F =22 Hz, 4JF-F = 10Hz, 1F, m-C6F5),
-163.8 (td, 3JF-F = 22 Hz, 4JF-F = 9 Hz, 1F, m-C6F5), -163.9
3
4
(td, JF-F = 21 Hz, JF-F = 9 Hz, 1F, m-C6F5), -164.2 (td,
3JF-F = 22 Hz, 4JF-F = 9 Hz, 1F, m-C6F5), -164.5 (td, 3JF-F
22 Hz, 4JF-F = 8 Hz, 1F, m-C6F5); 11B NMR (CDCl3): -3.6; 13
=
C
Experimental Section
NMR (CDCl3) (partial): 14.3, 122.6, 129.3, 142.3, 147.9, 159.8.
Calcd for C24H7BF15N: C, 47.64%; H, 1.17%; N, 2.31%. Found:
C, 48.05%; H, 1.38%; N, 2.26%.
General Considerations. All preparations were done under an
atmosphere of dry, O2-free N2 employing both Schlenk line
techniques and an Innovative Technologies or Vacuum Atmo-
spheres inert atmosphere glovebox. Solvents (pentane, hexanes,
toluene, and methylene chloride) were purified employing a
Grubbs’ type column systems manufactured by Innovative
Technology and stored over molecular sieves (4 A). Molecular
˚
sieves (4 A) were purchased from Aldrich Chemical Co. and
dried at 140 °C under vacuum for 24 h prior to use. Deuterated
solvents were dried over CaH2 (CD2Cl2, CDCl3) and vacuum
distilled prior to use. All common organic reagents were purified
3. Yield: 88%. Crystals were grown from the pentane wash
layer at room temperature. 1H NMR (CD2Cl2): 0.80 (t, 3JH-H = 8
Hz, 3H, CH2CH3), 2.99 (dq, 2JH-H = 23 Hz, 3JH-H = 8 Hz, 1H,
2
3
CH2CH3), 3.05 (dq, JH-H = 23 Hz, JH-H = 8 Hz, 1H,
CH2CH3), 7.51 (t, 3JH-H = 7 Hz, 1H), 7.63 (d, 3JH-H = 8 Hz,
1H), 8.15 (td, 3JH-H = 8 Hz, 4JH-H = 1 Hz, 1H), 8.67 (q, J = 6
˚
3
Hz, 1H); 19F NMR (CD2Cl2): -126.5 (m, JF-F = 22 Hz, 1F,
o-C6F5), -129.6 (m, 1F, o-C6F5), -132.4 (d, 3JF-F = 22 Hz, 1F,
o-C6F5), -133.7 (m, 1F, o-C6F5), -134.7 (m, 1F, o-C6F5), -137.3
by conventional methods unless otherwise noted. All liquid
pyridines were stored over 4 A molecular sieves. H, 13C, 11B,
˚
3
4
1
(td, JF-F = 24 Hz, JF-F = 9 Hz, 1F, o-C6F5), -157.0 (t,
3
3JF-F = 21 Hz, 1F, p-C6F5), -157.3 (t, JF-F = 20 Hz, 1F,
=
and 19F nuclear magnetic resonance (NMR) spectroscopy spec-
tra were recorded on a Bruker Avance-300 spectrometer at
p-C6F5),-159.2 (t, 3JF-F =20Hz,1F,p-C6F5),-163.2 (td, 3JF-F
22 Hz, 4JF-F = 9 Hz, 1F, m-C6F5), -164.0 (td, 3JF-F = 22 Hz,
1
300 K unless otherwise noted. H and 13C NMR spectra are
4JF-F = 10 Hz, 1F, m-C6F5), -164.8 (td, 3JF-F = 22 Hz, 4JF-F
=
referenced to SiMe4 using the residual solvent peak impurity
of the given solvent. 11B and 19F NMR experiments were
referenced to BF3(OEt2), and CFCl3, respectively. Chemi-
cal shifts are reported in parts per million (ppm) and coupling
constants in hertz (Hz) as absolute values. DEPT and 2-D
1H/13C correlation experiments were completed for assign-
ment of the carbon atoms. Combustion analyses were per-
formed in house employing a Perkin-Elmer CHN Analyzer.
9 Hz, 1F, m-C6F5), -165.1 (td, 3JF-F = 21 Hz, 3JF-F = 9 Hz, 1F,
m-C6F5), -165.4 (td, 3JF-F = 22 Hz, 4JF-F = 9 Hz, 1F, m-C6F5),
-165.6 (td, 3JF-F = 22 Hz, 3JF-F = 8 Hz, 1F, m-C6F5); 11B NMR
(CD2Cl2): -3.6; 13C NMR (CD2Cl2) (partial): 13.2, 27.4, 122.6,
127.6, 142.8, 165.5. Calcd for C25H9BF15N: C, 48.50%; H, 1.47%;
N, 2.26%. Found: C, 48.25%; H, 1.58%; N, 2.26%.
4. Yield: 85%. Crystals were grown from toluene at room
temperature. 1H NMR (CDCl3): 7.05 (br s, 2H), 7.25 (t, 3JH-H
=
3
8 Hz, 1H), 7.38 (d, JH-H = 8 Hz, 1H), 7.40 (br s, 1H), 7.65
(t, 3JH-H = 8 Hz, 1H), 7.85 (br s, 1H), 8.13 (t, 3JH-H = 8 Hz,
1H), 8.93 (s, 1H); 19F NMR (CDCl3): -125.4 (br s, 1F, o-C6F5),
-128.6 (br s, 1F, o-C6F5), -131.2 (br s, 1F, o-C6F5), -131.9 (d,
3JF-F=18 Hz, 1F, o-C6F5), -133.9 (br s, 2F, o-C6F5), -155.4 (t,
3JF-F =20 Hz, 1F, p-C6F5), -157.7 (br s, 2F, p-C6F5), -162.0
€
(18) (a) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Frohlich, R.;
Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072. (b) Wang, H.;
€
Frohlich, R.; Kehr, G.; Erker, G. Chem. Commun. 2008, 5966.
(19) Brown, H. C.; Schlesinger, H. I.; Cardon, S. Z. J. Am. Chem. Soc.
1942, 64, 325.
(20) Geier, S. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 3476.

10468 Inorganic Chemistry, Vol. 48, No. 21, 2009
Geier et al.
3
3
(t, 1F, JF-F=23 Hz, m-C6F5), -162.9 (t,1F, JF-F = 23 Hz,
m-C6F5), -164.5 (br s, 2F, m-C6F5), -165.1 (br m, 1F, m-C6F5),
-165.6 (br s, 1F, m-C6F5), 11B NMR (CDCl3): -2.9; 13C NMR
(CDCl3) (partial): 124.3, 128.0, 128.5, 129.8, 131.6, 142.3, 148.5.
Calcd for C29H9BF15N: C, 52.21%; H, 1.36%; N, 2.10%. Found:
C, 51.77%; H, 1.71%; N, 2.32%.
3JH-H = 7 Hz), 8.65 (br s, 1H, pyridinium N-H); 19F NMR
3
(CDCl3): -133.7 (d, JF-F = 20 Hz, 6F, o-C6F5), -157.0 (t,
3JF-F = 19 Hz, 3F, p-C6F5), -163.0 (br s, 6F, m-C6F5); 11B
NMR (CDCl3): -11.1; 13C NMR (CDCl3) (partial): 19.3,
1
109.5, 114.5, 137.0 (dm, JC-F = 256 Hz, CF), 141.8, 142.3
(dm, 1JC-F = 242 Hz, CF), 148.1 (dm, 1JC-F = 240 Hz, CF),
155.2. Anal. Calcd for C24H8BF15N2: C, 46.48%; H, 1.30%; N,
4.52%. Found: C, 46.30%; H, 1.18%; N, 5.02%.
5. Yield: 79%. Crystals were grown from toluene at -35 °C.
1H NMR (CD2Cl2): 6.68 (d, JH-H = 8 Hz, 1H), 7.16 (ddd,
3
3JH-H = 8 Hz, 3JH-H=5 Hz, 4JH-H = 1 Hz, 1H), 7.43-7.48 (ov
m, 2H), 7.72 (ddd, 3JH-H = 8 Hz, 3JH-H = 6 Hz, 4JH-H=2 Hz,
1H), 8.17 (ddd, 3JH-H = 5 Hz, 4JH-H = 2 Hz, 4JH-H=1 Hz,
1H), 8.23 (td, 3JH-H =8 Hz, 4JH-H = 2 Hz, 1H), 8.82 (br s, 1H) ;
19F NMR (CD2Cl2): -125.2 (m, 1F, o-C6F5), -130.9 (m, 1F,
o-C6F5), -131.5 (m, 1F, o-C6F5), -133.1 (m, 2F, o-C6F5), -135.6
(d, 3JF-F = 21 Hz, 1F, o-C6F5), -156.7 (t, 3JF-F = 19 Hz, 1F,
p-C6F5), -158.3 (m, 1F, p-C6F5), -160.0 (t, 3JF-F = 21 Hz, 1F,
p-C6F5), -160.5 (m, 1F, m-C6F5), -163.3 (t, 3JF-F = 21 Hz, 1F,
Synthesis of (2,6-Me2C5H3N)B(C6F5)3 9. B(C6F5)3 (100 mg,
0.20 mmol) was added to 2,6-lutidine (21 mg, 0.20 mmol) in
2 mL of toluene. The solution was allowed to stir for 4 h and
2 mL of pentane was added. The solution was stored at -35 °C.
X-ray quality crystals precipitated from solution and were washed
with pentane (2 ꢀ 2 mL) and again pumped to dryness. Yield:
1
60 mg (51%). NMR data were acquired at -10 °C. H NMR
3
(CD2Cl2): 2.58 (s, CH3), 7.36 (d, 2H, JH-H = 8 Hz, m-CH),
7.89 (t, 1H, 3JH-H = 8 Hz, p-CH); 19F NMR (CD2Cl2): -131.4
(br s, 2F, o-C6F5), -132.4 (br s, 2F, o-C6F5), -133.0 (d, 2F,
3JF-F = 18 Hz, o-C6F5), -157.6 (t, 1F, 3JF-F = 20 Hz, p-C6F5),
3
m-C6F5), -163.8 (t, JF-F = 21 Hz, 1F, m-C6F5), -166.0 (t,
3JF-F = 20 Hz, 1F, m-C6F5), -166.5 (m, 1F, m-C6F5), -167.6
(m, 1F, m-C6F5); 11B NMR (CD2Cl2): -2.7; 13C NMR (CDCl3)
(partial): 123.7, 124.0, 125.0, 130.8, 136.4, 142.8, 148.5, 149.3,
153.6, 158.9. Calcd for C28H8BF15N2: C, 50.33%; H, 1.21%; N,
4.19%. Found: C, 49.87%; H, 1.44%; N, 4.32%.
-158.7 (t, 2F, 3JF-F = 20 Hz, p-C6F5), -164.4 (t, 2F, 3JF-F
=
21 Hz, m-C6F5), -165.2 (m, 4F, m-C6F5); 11B NMR (CD2Cl2):
-3.9. Anal. Calcd for C31H17BF15NO4: C, 48.50%; H, 1.47%;
N, 2.26%. Found: C, 48.70%; H, 1.85%; N, 2.23%.
6. Yield: 96%. 1H NMR (CDCl3): 7.77 (m, 2H), 7.84 (m, 1H),
8.09 (dd, 3JH-H = 8 Hz, 4JH-H = 2 Hz, 1H), 8.51 (d, 3JH-H = 9
Hz, 1H), 8.72 (d, 3JH-H=8 Hz, 1H), 9.19 (q, 3JH-H = 5 Hz, 1H);
Synthesis of [2,6-Me2C5H3NH][HB(C6F5)3] 10, [(2,6-Ph2)-
C5H3NH][HB(C6F5)3] 11, [(2-tBu)C5H4NH][HB(C6F5)3] 12.
These compounds were prepared in a similar fashion and thus
only one preparation is detailed. B(C6F5)3 (100 mg, 0.20 mmol)
was added to of 2,6-lutidine (21 mg, 0.20 mmol) in 10 mL of
toluene. The solution was subjected to 3 freeze-pump-thaw
cycles and backfilled with H2 at 77 K (∼4 atm). The solution was
allowed to stir overnight at room temperature and then pumped
to dryness. The solid was washed with pentane (2 ꢀ 2 mL) and
again pumped to dryness.
3
19F NMR (CDCl3): -126.6 (m, JF-F = 27 Hz, 1F, o-C6F5),
-128.8 (br m, 1F, o-C6F5), -131.9 (br m, 1F, o-C6F5), -132.9
3
(m, JF-F = 36 Hz, 1F, o-C6F5), -133.3 (br m, 1F, o-C6F5),
3
4
-133.7 (m, 1F, o-C6F5), -155.1 (tt, JF-F = 20 Hz, JF-F
=
4 Hz, 1F, p-C6F5), -156.2 (tt, 3JF-F = 20 Hz, 4JF-F = 3 Hz, 1F,
p-C6F5), -157.2 (tt, 3JF-F =20 Hz, 4JF-F = 3 Hz, 1F, p-C6F5),
-161.2 (td, 3JF-F = 21 Hz, 4JF-F = 8 Hz, 1F, m-C6F5), -162.3
(td, 3JF-F = 23 Hz, 4JF-F = 10 Hz, 1F, m-C6F5), -163.2 (td,
10. Yield: 105 mg (87%). X-ray quality crystals were grown
by slow evaporation of a toluene solution. 1H NMR (CD2-
Cl2): 2.61 (s, 6H, CH3), 3.55 (q, 1JB-H = 88 Hz, B-H), 7.53 (d,
3JH-H = 8 Hz, 2H, m-CH), 8.22 (t, 1H, 3JH-H = 8 Hz, p-CH),
12.01 (br s, 1H, N-H); 19F NMR (CD2Cl2): -136.8 (br d, 6F,
3JF-F = 18 Hz, o-C6F5), -165.8 (t, 3F 3JF-F = 20 Hz, p-C6F5),
-169.3 (br t, 6F, 3JF-F = 20 Hz, m-C6F5); 11B NMR (CD2Cl2):
-24.7 (d, 1JB-H = 88 Hz); 13C NMR (CD2Cl2) (partial): 19.9,
125.5, 136.7 (dm, 1JC-F = 245 Hz CF), 138.4 (dm, 1JC-F = 249
Hz, CF), 147.2, 148.2, (dm, 1JC-F = 238 Hz, CF), 153.8. Anal.
Calcd for C26H11BF15N: C, 48.34%; H, 1.78%; N, 2.25%.
Found: C, 48.49%; H, 2.06%; N, 2.43%.
3JF-F = 22 Hz, 4JF-F=8 Hz, 1F, m-C6F5), -163.7 (td, 3JF-F
=
4
22 Hz, JF-F = 9 Hz, 1F, m-C6F5), -163.8 (m, peaks over-
lapping, m-C6F5), -163.9 (m, peaks overlapping, 1F, m-C6F5);
11B NMR (CDCl3): -3.2; 13C NMR (CDCl3) (partial): 120.2,
122.4, 128.6, 129.6, 130.1, 133.1, 142.6, 145.0, 150.4, C27H7-
BF15N: C, 50.58%; H, 1.10%; N, 2.18%. Found: C, 50.23%; H,
0.98%; N, 2.35%.
7. Yield: 86%. X-ray quality crystals were grown by
slow evaporation from CD2Cl2. H NMR (CD2Cl2): 6.44 (d,
1
3JH-H=8 Hz, 1H), 6.96 (dd, 3JH-H=7 Hz, 3JH-H=5 Hz, 2H),
3
4
3
7.02 (td, JH-H=7 Hz, JH-H =1 Hz, 1H), 7.55 (td, JH-H
8 Hz, 4JH-H = 2 Hz, 1H), 7.67 (br s, NH), 7.91 (ddd, 3JH-H
=
=
11. Yield: 82%. X-ray quality crystals were grown from
dichloromethane. 1H NMR (CD2Cl2): 3.35 (q, 1JB-H=92 Hz,
B-H), 7.57-7.62 (m, 4H), 7.64-7.68 (m, 2H), 7.74-7.77 (m,
4H), 8.05 (d, 3JH-H = 8 Hz, 2H, m-CH), 8.51 (t, 3JH-H = 8 Hz,
1H, p-CH), 11.27 (br s, 1H, N-H); 19F NMR (CD2Cl2): -134.3
(br d, 3JF-F = 21 Hz, 6F, o-C6F5), -164.8 (t, 3JF-F = 20 Hz,
9 Hz, 3JH-H = 7 Hz, 4JH-H = 2 Hz, 1H), 8.21 (m, 2H), 8.58 (d,
3JH-H = 9 Hz, 1H); 19F NMR (CD2Cl23): -127.0 (m, 1F,
o-C6F5), -128.1 (m, 1F, o-C6F5), -131.6 (d, JF-F=23 Hz, 1F,
o-C6F5), -133.0 (m, 1F, o-C6F5); -135.7 (m, 1F, o-C6F5), -137.1
(m, 1F, o-C6F5), -157.0 (t, 3JF-F=20 Hz, 3F, p-C6F5), -163.0
3
3F, p-C6F5), -167.7 (br t, JF-F = 20 Hz, 6F, m-C6F5); 11B
3
4
(td, JF-F = 22 Hz, JF-F = 8 Hz, 1F, m-C6F5), -163.9 (tt,
NMR (CD2Cl2): -24.6 (d, 1JB-H = 92 Hz); 13C NMR (CD2Cl2)
(partial): 122.6, 126.9, 127.5, 130.2, 132.4. Anal. Calcd for
C35H15BF15N: C, 56.40%; H, 2.03%; N, 1.88%. Found: C,
56.19%; H, 2.18%; N, 2.09%.
3JF-F = 22 Hz, JF-F = 9 Hz, 2F, m-C6F5), -164.0 (td,
4
3JF-F = 21 Hz, JF-F = 7 Hz, 1F, m-C6F5), -164.2 (td,
4
3JF-F=22 Hz, JF-F = 8 Hz, 1F, m-C6F5); 11B NMR (CD2-
4
Cl2): -5.1; 13C NMR (CD2Cl2) (partial): 139.0, 142.8, 144.1 (m),
148.2, 151.2, 152.4 (m). Anal. Calcd for C28H9BF15N3: C,
49.23%; H, 1.33%; N, 6.15%. Found: C, 49.59%; H, 1.69%;
N, 6.13%.
1
12. Yield: 105 mg (83%). H NMR (CD2Cl2): 1.51 (s, 9H,
1
3
C-CH3), 3.66 (q, JB-H =88 Hz, 1H, B-H), 7.80 (t, JH-H
=
7 Hz, 1H), 7.96 (d, 3JH-H = 8 Hz, 1H), 8.45 (dd, 3JH-H = 8 Hz,
4JH-H = 2 Hz, 1H), 8.48 (d, 3JH-H = 7 Hz, 1H), 12.13 (br s, 1H,
N-H); 19F NMR (CD2Cl2): -134.7 (br d, 3JF-F = 22 Hz, 6F,
o-C6F5), -163.6 (t, 3JF-F = 21 Hz, 3F, p-C6F5), -167.1 (m, 6F,
Synthesis of (5-Me)C5H3NH(2-NH)B(C6F5)3 8. 2-Amino-6-
picoline (4.5 mg, 0.04 mmol) was added to a solution of B(C6F5)3
(20 mg, 0.039 mmol) in 2 mL of CH2Cl2. The solution was
allowed to stand for 2 h, then all volatiles were removed and the
residue was washed with pentane (2 ꢀ 2 mL). The resulting
white solid was again pumped to dryness. Yield: 23 mg (96%).
X-ray quality crystals were grown from a layered solution of
m-C6F5); 11B NMR (CD2Cl2): -24.7 (d, JB-H =87 Hz); 13C
1
NMR (CD2Cl2) (partial): 28.8, 37.1, 125.2, 125.3, 136.8 (dm,
1JC-F = 254 Hz, CF), 140.7, 147.7, 148.2 (dm, CF, 1JC-F = 240
Hz). Anal. Calcd for C27H15BF15N: C, 49.95%; H, 2.33%; N,
2.16%. Found: C, 49.76%; H, 2.22%; N, 2.06%.
1
CDCl3/pentane at room temperature. H NMR (CDCl3): 2.13
(s, 3H, CH3), 6.07 (br s, 1H, amide N-H), 6.23 (dm, 3JH-H = 7 Hz,
1H), 6.55 (br d, JH-H = 9 Hz, 1H), 7.38 (dd, JH-H=9 Hz,
X-ray Data Collection and Reduction. Crystals were coated
in Paratone-N oil in the glovebox, mounted on a MiTegen
3
3

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10469
Micromount and placed under an N2 stream, thus maintaining a
dry, O2-free environment for each crystal. The data were
collected on a Bruker Apex II diffractometer. The data were
collected at 150((2) K for all crystals. The frames were inte-
grated with the Bruker SAINT software package using a
narrow-frame algorithm. Data were corrected for absorption
effects using the empirical multiscan method (SADABS).
Structure Solution and Refinement. Non-hydrogen atomic
scattering factors were taken from the literature tabulations.21
The heavy atom positions were determined using direct methods
employing the SHELXTL direct methods routine. The remain-
ing non-hydrogen atoms were located from successive difference
Fourier map calculations. The refinements were carried out by
formation. Frequency analysis using the ONIOM model demon-
strated the structure to be a minimum. For further confirmation,
optimization of the structure using the M06/6-311G(d,p) and M06-
2x/6-311G(d,p) DFT26 approaches as implemented in the GA-
MESS program27 gave stationary point structures similar to that
from the ONIOM calculation. Single point energies using the ONIOM-
(MPW1K) structures and the M06, M06-2x, and MP228 models
and triple-ζbasis sets (Table 2) were determined using the GAMESS
program. Relative energies in the Table were corrected using scaled29
zero point energies (ZPEs) from the frequency analyses.
For 9, attempts to optimize “weakly interacting” (B-N
distances >2.5 A) and “transition state” structures (located
˚
by scans of the potential energy surface for B-N interaction)
using both the HF/3-21G and the ONIOM(MPW1K) ap-
proaches were undertaken. While stationary points were located,
and frequency analyses characterized them appropriately,
their B-N bond lengths and relative energies suggested they
were metastable artifact points. The “weakly interacting”
using full-matrix least-squares techniques on F, minimizing the
2
function ω(Fo - Fc)2 where the weight ω is defined as 4Fo
/
2σ(Fo2) and Fo and Fc are the observed and calculated structure
factor amplitudes, respectively. In the final cycles of each refine-
ment, all non-hydrogen atoms were assigned anisotropic tempera-
ture factors in the absence of disorder or insufficient data. In the
latter cases atoms were treated isotropically. C-H atom positions
were calculated and allowed to ride on the carbon to which they are
˚
complex optimized to structures with B-N distances of 3.4 A
(HF) and >5.6 A (ONIOM(MPW1K)). No stationary transi-
˚
tion state point was located at the HF level, while the ONIOM-
(MPW1K)-optimized “transition state” structure exhibited a
˚
bonded assuming a C-H bond length of 0.95 A. H-atom temp-
˚
long B-N distance (2.74 A) and an energy value essentially
erature factors were fixed at 1.10 times the isotropic temperature
factor of the C-atom to which they are bonded. The H-atom
contributions were calculated, but not refined. The locations of the
largest peaks in the final difference Fourier map calculation as well
as the magnitude of residual electron densities in each case were of
no chemical significance. Additional details are provided in Table 1
and in the Supporting Information data.
equal to that of the sum of the reactants. The M06, M06-2x,
and MP2 single point energies determined using this structure were
less than the sum of the reactant energies (i.e., gave nonphysical
negative barriers). Efforts to optimize the transition states using the
M06/6-311G(d,p) and M06-2x/6-311G(d,p) models resulted in
structures that optimized toward ones similar to those seen using
the ONIOM approach, again with nonphysical barrier energies. It
was concluded that the potential energy surface for coordination
of 2,6-lutidine to B(C6F5)3 is essentially flat until the two are
close enough to bond, at which point coordination is barrierless
or nearly so. Viewed in reverse, the barrier for dissociation of
dimethylpyridine from borane equals the endothermicity of the
process, and once this is surmounted, further movement of the
moieties from each other is essentially thermoneutral.
Computational Methods. Initial optimizations were per-
formed with the GAUSSIAN (G98) suite.22 (C5H5N)(B-
(C6F5)3 and (2-MeC5H4N)B(C6F5)3 were first optimized with-
out constraints at the HF/3-21G level. Examination of the
optimized structures by analytical frequency analyses at this
level demonstrated them to be minima (no imaginary frequen-
cies). The structures were reoptimized using a two-layer ONIOM
approach (denoted ONIOM(MPW1K),23 using the MPW1K
density functional theory (DFT) model24 and the 6-31þG(d)
basis set for the high levels, and the 3-21G basis set for the low
levels.25 The partitioning of these levels for B(C6F5)3 moieties
was reported previously; all atoms of the pyridine moieties were
placed in the high levels.
Optimizations and single point energy calculations in CH2Cl2
solvent employed the PCM solvent continuum30 approach
implemented in GAMESS. Default values for solvent and
tesserae parameters were employed. Cartesian coordinates of
molecules studied at the ONIOM(MPW1K) or M06-2x/
6-311G(d,p) levels are available as Supporting Information.
As optimization of the structure of 9 at the HF/3-21G level
˚
gave a B-N distance of about 3.5 A, it was optimized instead
using the ONIOM(MPW1K) approach and a starting structure
Results and Discussion
˚
with a B-N distance of 1.6 A. This gave the more reasonable
optimized structure given in Table 2 and the Supporting In-
The synthesis of pyridine adducts of B(C6F5)3 is facile and
well precedented in the literature. Indeed the first report of
(C5H5N)B(C6F5)3 appeared in 1966 by Massey and Park.31
In a similar and straightforward fashion, combination of 4-t-
butylpyridine and B(C6F5)3 yielded the adduct (4-tBu)C5-
H4NB(C6F5)3 1. Compound 1 exhibits a 11B NMR resonance
(21) Cromer, D. T.; Waber, J. T. Int. Tables X-Ray Crystallogr. 1974, 4, 71.
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.;
Malick, A. D.; Rabuck, K. D.; Raghavachari, K.; Foresman, J. B.;
Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; A., P. J.
Gaussian 98, Revision A.11.4; Gaussian, Inc.: Pittsburgh, PA, 1998.
(26) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215.
(27) (a) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;
Gordon, M. S.; Jensen, J. J.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su,
S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993,
14, 1347. (b) Gordon, M. S.; Schmidt, M. W. In Theory and Applications of
Computational Chemistry: The First Forty Years; Dykstra, C. E., Frenking, G.,
Kim, K. S., Scuseria, G. E., Eds.; Elsevier: Amsterdam, 2005; p 1167.
(28) Moller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618.
(29) (a) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (b) No
scaling factor exists for the ONIOM(MPW1K method. However, work of the
Truhlar group suggests that all MPW1K methods exhibit scaling factors of ca.
0.95. Therefore we used 0.95 as the scaling factor for the ONIOM(MPW1K)
ZPEs. See: ttp://comp.chem.umn.edu/database/freq_scale.htm.
ꢀ
(23) (a) Dapprich, S.; Komaromi, I.; Byun, K. S.; Morokuma, K.; Frisch,
M. J. J. Mol. Struct.: THEOCHEM 1999, 462, 1. (b) Svensson, M.; Humbel,
S.; Morokuma, K. J. Chem. Phys. 1996, 105, 3654. (c) Svensson, M.; Humbel,
S.; Froese, R. D. J.; Matsubara, T.; Sieber, S.; Morokuma, K. J. Phys. Chem.
1996, 100, 19357. (d) Matsubara, T.; Sieber, S.; Morokuma, K. Int. J. Quantum
Chem. 1996, 60, 1101. (e) Maseras, F.; Morokuma, K. J. Comput. Chem. 1995,
16, 1170.
(24) (a) Lynch, B. J.; Truhlar, D. G. J. Phys. Chem. A 2001, 105, 2936.
(b) Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664.
(25) Gille, A. L.; Gilbert, T. M. J. Chem. Theory Comput. 2008, 3, 1681.
(30) (a) Fedorov, D. G.; Kitaura, K.; Li, H.; Jensen, J. H.; Gordon, M. S.
J. Comput. Chem. 2006, 27, 976. (b) Li, H.; Jensen, J. H. J. Comput. Chem.
2004, 25, 1449. (c) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105,
2999. (d) Cammi, R.; Tomasi, J. J. Comput. Chem. 1995, 16, 1449.
(31) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1966, 5, 218.

10470 Inorganic Chemistry, Vol. 48, No. 21, 2009
Geier et al.
Table 1. Crystallographic Dataa
1
2-0.5CHCl3
3
4-0.5C7H8
5
formula
C
27H13BF15
N
C24.5H7.5BCl1.5F15
664.80
monoclinic
P21/n
9.2940(8)
14.3404(14)
18.4081(18)
90.00
98.874(4)
90.00
2424.1(4)
4
1.822
0.347
5542
3159
401
0.0528
0.1270
1.007
N
C25H9BF15
619.14
triclinic
P1
9.7020(9)
11.5301(11)
12.1581(11)
105.149(5)
94.518(5)
113.003(4)
1183.10(19)
2
1.738
0.185
7401
5857
379
0.0427
0.1238
1.093
N
C32.5H13BF15
713.25
monoclinic
P21/n
9.0823(18)
15.684(3)
20.242(4)
90.00
101.05(3)
90.00
2830.0(10)
4
1.674
0.167
6440
4163
466
0.0522
0.1506
1.045
N
C28H8BF15N2
668.17
monoclinic
P21/c
10.706(2)
16.332(3)
14.368(3)
90.00
91.27(3)
90.00
2511.6(8)
4
1.767
0.182
4389
2499
415
0.0504
0.1409
1.006
fFormula weight
crystal system
space group
647.19
monoclinic
P21/n
11.5466(4)
13.3174(5)
16.5172(6)
90.00
92.0820(10)
90.00
2538.18(16)
4
1.694
0.176
6669
4924
397
0.0392
0.0938
1.020
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
d(calc) g cm-1
μ, cm-1
data: total (indep)
data Fo2 > 3σ(Fo
variables
Rb
2
)
c
Rw
goodness of fit
6
7
8
9
10
formula
C27H7BF15
641.15
Triclinic
P1
9.9848(9)
10.9162(10)
11.6538(11)
107.046(5)
94.178(5)
101.061(5)
1180.52(19)
2
1.804
0.189
5413
4093
397
0.0549
0.0903
1.017
N
C28H9BF15N3
683.19
Triclinic
P1
9.6501(10)
9.7264(8)
14.9925(11)
107.830(3)
102.077(4)
98.166(3)
1277.46(19)
2
1.776
0.182
6580
4246
424
0.0427
0.1017
1.020
C24H8BF15N2
620.13
Triclinic
P1
9.8535(13)
11.1355(16)
11.5291(16)
74.467(8)
73.964(7)
80.673(7)
1166.1(3)
2
1.697
0.189
12524
7552
388
0.0416
0.1383
1.007
C25H9BF15
619.14
Monoclinic
P21/n
12.8108(4)
13.4663(4)
13.3937(4)
90.00
100.156(2)
90.00
2274.40(12)
4
1.808
0.192
7942
6118
379
0.0494
0.1475
1.055
N
C25H11BF15
621.16
Monoclinic
P21/c
17.8525(12)
9.8407(7)
15.2683(10)
90.00
115.010(3)
90.00
2430.8(3)
4
1.697
0.180
8206
4499
383
0.0516
0.1680
1.005
N
formula weight
crystal system
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
d(calc) g cm-1
μ, cm-1
data: total (indep)
data Fo2 > 3σ(Fo
variables
Rb
2
)
c
Rw
Goodness of Fit
P
P
P
P
This data were collected at 150 K with Mo KR radiation (λ = 0.71069 A). b R = ||Fo| - |Fc||/ |Fo|. c Rw = ( w(Fo2 - Fc2)2/ w(Fo)2])1/2
.
a
˚
˚
Table 2. Computational Energies (kJ/mol) for Models of B(C6F5)3/Pyridine
Interactions
data for 1 (Figure 1) showed a B-N distance of 1.618(2) A.
32
˚
This compares to the B-N distance of 1.628(2) A in
(C5H5N)B(C6F5)3,33 consistent with the notion that elec-
tron-donating substitution remote to N results in a shortened
B-N bond relative to that of the pyridine adduct.
B-Na
ONIOMb M06c M06-2xc MP2d
(C5H5N)B(C6F5)3 1.616 (1.628) -120
(2-MeC5H4N)-
B(C6F5)3
(2,5-Me2C5H4N)- 1.653 (1.661) -61
B(C6F5)3
-108 -138
-100 -133
-177
-166
1.634 (1.639) -102
A series of adducts with substitution in the ortho-position
were examined. The species ((2-Me)C5H4N)B(C6F5)3 2, ((2-
Et)C5H4N)B(C6F5)3 3, ((2-Ph)C5H4N)B(C6F5)3 4, ((2-C5H4-
N)C5H4N)B(C6F5)3 5, (C9H7N)B(C6F5)3 6, ((2-C5H4N)NH-
(2-C5H4N))B(C6F5)3 7 were readily achieved (Scheme 2).
NMR data for 2-7 showed the expected 11B resonances in
the vicinity of -3 to -5 ppm. In contrast to 1, species 2-7
exhibit 19F NMR spectra that are composed of 15 signals
reflecting the molecular dissymmetry of these products
and restricted rotation about the B-N bonds (Figure 2).
Crystallographic data for 2-7 (Figure 3-5) reveals the
structural impact of ortho-substitution. The B-N distances
-69 -103
-153
a Calcd (exptl) B-N bond distance. b Basis set: ONIOM(MPW1K).
c Basis set: 6-311G(d,p), d Basis set: 6-311þþG(d,p).
at -4.1 ppm consistent with quaternization of B, while the
1H and 13C NMR data were also consistent with the
adduct formulation. The 19F NMR spectrum of 1 shows
resonances at -132.2, -158.2, and -164.6 ppm consistent
with 3-fold molecular symmetry in solution. Crystallographic
˚
˚
˚
in 2-7 were found to be 1.639(4) A, 1.638(2) A, 1.654(7) A,
˚
˚
˚
(32) (a) Piers, W. E. Adv. Organomet. Chem. 2005, 52, 1. (b) Focante, F.;
Mercandelli, P.; Sironi, A.; Resconi, L. Coord. Chem. Rev. 2006, 250, 170.
(33) Lesley, M. J. G.; Woodward, A.; Taylor, N. J.; Marder, T. B.;
Cazenobe, I.; Ledoux, I.; Zyss, J.; Thornton, A.; Bruce, D. W.; Kakkar, A.
K. Chem. Mater. 1998, 10, 1355.
1.649(5) A, 1.641(2) A, and 1.629(2) A, respectively. The
lengthening of the B-N bonds in 2-7 reflects the steric
impact of ortho-substitution. The B-N bond lengths for
2 and 3 are similar and yet significantly shorter than those

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10471
Figure 1. ORTEP drawing of 1, 50% thermal ellipsoids are shown,
˚
hydrogen atoms are omitted for clarity. Selected bond distances (A) and
angles (deg).
Figure 2. 19F NMR spectrum of 6 (a: ortho-C6F5, b: para-C6F5,
c: meta-C6F5).
Scheme 2. Synthesis of 1-7
Figure 3. ORTEP drawings of (a) 2, (b) 3, 50% thermal ellipsoids are
shown, hydrogen atoms are omitted for clarity. Selected bond distances
˚
(A) and angles (deg).
˚
of 1.5602(12) A is substantially shorter than previously
characterized amine-B(C6F5)3 adducts32,34 and significantly
longer than that seen in the anion [(C6H5NH)B(C6F5)3]-
in 4 and 5 consistent with the larger steric demands
and increased electron-withdrawing abilities of the phenyl
and pyridyl substituents. Similarly for 6, the increased
steric bulk of and reduced basicity of quinoline results
in a B-N distance significantly longer than that seen in
(C5H5N)B(C6F5)3.33
Introduction of a second ortho-substituent on pyridine
provides interesting and contrasting reactivity. The reaction
of B(C6F5)3 with 2-amino-6-picoline proceeds to give the
product 8. Quaternization of B is evident from the 11B NMR
signal as -11.1 ppm, suggesting anionic character at B. The
corresponding 19F NMR spectrum shows signals at -133.7,
-157.0, and -163.0 ppm. The absence of a more complex
spectrum suggests that B does not coordinate to the pyridyl
nitrogen. 1H NMR resonances at 6.07 and 8.65 ppm suggest
the presence of amide and pyridinium protons. Crystallo-
graphic data for 8 revealed the amine N of 2-amino-6-pico-
line coordinates to B and a proton is transferred to thepyridyl
N affording a zwitterionic species (5-Me)C5H3NH(2-NH)B-
(C6F5)3 8 (Figure 6, Scheme 3). As a result, the B-N distance
(1.532(8) A),4b reflecting the electron withdrawing nature of
˚
the pyridinium fragment in 8. This bond length in 8 is
˚
˚
comparable to B-N distances of 1.552 A and 1.576 A, seen
in the salts [(12-crown-4)Li][NH2B(C6F5)3]35 and [(Et2O)Li]-
[pyrrolyl B(C6F5)3],36 respectively. This B-N bond length in
8 is also significantly shorter than that reported for the
˚
adduct (4-Me2NC5H4N)B(C6F5)3 (1.602(6) A) where B bind-
ing occurs to the pyridyl N.33 The N(1)-C(19) bond length is
˚
1.3240(13) A consistent with theelectron-withdrawing nature
of pyridinium substituent on the amido-boron linkage. It is
interesting to note that although the pyridyl N of 2-amino-6-
picoline is more basic than the amine substituent, the amine is
(34) Mountford, A. J.; Lancaster, S. J.; Coles, S. J.; Horton, P. N.;
Hughes, D. L.; Hursthouse, M. B.; Light, M. E. Inorg. Chem. 2005, 44, 5921.
(35) Mountford, A. J.; Clegg, W.; Coles, S. J.; Harrington, R. W.;
Horton, P. N.; Humphrey, S. M.; Hursthouse, M. B.; Wright, J. A.;
Lancaster, S. J. Chem.;Eur. J. 2007, 13, 4535.
(36) Kehr, G.; Roesmann, R.; Frohlich, R.; Holst, C.; Erker, G. Eur. J.
Inorg. Chem. 2001, 535.

10472 Inorganic Chemistry, Vol. 48, No. 21, 2009
Geier et al.
Figure 5. ORTEP drawing of (a) 6, (b) 7, 50% thermal ellipsoids are
shown, hydrogen atoms are omitted for clarity. Selected bond distances
˚
(A) and angles (deg).
Figure 4. ORTEP drawings of (a) 4, (b) 5, 50% thermal ellipsoids are
shown, hydrogen atoms are omitted for clarity. Selected bond distances
˚
(A) and angles (deg).
more accessible for coordination to B. Thus, the formation of
the zwitterion 8 is a ramification of steric congestion.
Given the departure from conventional reactivity afford-
ing 8 the reaction of the disubstituted pyridine 2,6-lutidine
and B(C6F5)3 was examined. Monitoring the reaction by 1H
and 19F NMR spectra revealed only broad spectral lines,
suggesting the existence of equilibrium between free lutidine/
B(C6F5)3 mixture and the adduct (2,6-Me2C5H3N)B(C6F5)3
9 (Scheme 4). Variable temperature NMR spectra showed
eight 19F NMR resonances at -10 °C consistent with
inequivalent C6F5-ring environments. This, together with a
11B NMR signal at -3.9 ppm were consistent with the
formation of 9. X-ray crystallographic analysis confirmed
the formulation of 9 (Figure 7). The B-N bond length in 9 of
Figure 6. ORTEP drawing of 8, 50% thermal ellipsoids are shown,
hydrogen atoms except for the NH are omitted for clarity. Selected bond
˚
distances (A) and angles (deg).
Scheme 3. Synthesis of 8
Lewis acid and base could be exploited for frustrated Lewis
pair reactivity. Thus, H2 (1 atm, 2 h) was added to the mixture
˚
1.661(2) A is exceptionally longer in comparison to adducts
1
1-7, presumably a result of the dramatic steric demands
limiting the approach of N to B.
The activation parameters associated with the equilibrium
process were determined by analysis of the NMR data. This
revealed ΔH of -42(1) kJ/mol and ΔS of -131(5) J/mol K.
This equilibrium suggests that access to a mixture of free
at 25 °C. This afforded a new product 10 in 87% yield. H
NMR data showed resonances at 12.01 and 3.55 ppm,
attributable to NH and BH units respectively. These data,
together with the 19F and 11B NMR resonances were con-
sistent with the formulation of 10 as [2,6-Me2C5H3NH][HB-
(C6F5)3] (Scheme 4). This was confirmed by X-ray diffraction

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10473
Figure 8. ORTEP drawing of 10, 50% thermal ellipsoids are shown,
hydrogen atoms except for the NH and BH are omitted for clarity.
˚
Selected bond distances (A) and angles (deg).
unquenched donor and B(C6F5)3 are frustrated Lewis pairs,
these mixtures did not react with H2. This stands in contrast
to ferrocenyldiphosphines where reaction at one P center
provides the steric hindrance affording frustrated Lewis pair
activation of H2 at the other P center.37 The present result is
consistent with the diminished basicity at the free nitrogen
resulting from the electron withdrawing effect of the pendant
pyridyl-borane adduct. This supports the notion that there is
a threshold of cumulative Lewis acidity and basicity required
for a frustrated Lewis pair to effect heterolytic H2 cleavage.14
Computational Studies. To assess the nature of the
equilibrium between 2,6-lutidine with B(C6F5)3, and to
understand the kinetic and thermodynamic differences
between this frustrated Lewis pair and the less-substituted
analogues, the reactions between B(C6F5)3 and pyridine,
2-methylpyridine, and 2,6-lutidine were examined com-
putationally (Table 2). The optimized structures exhibit
B-N distances in fine agreement with the experimental
values. This is somewhat misleading, given that in general
one expects “gas phase” computational distances for
dative B-N bonds to be longer than those observed in
solid state crystallography studies.38 The B-N distance
increases regularly with the number of methyl groups on
the pyridine moiety. Given that complex 9 exists in
equilibrium with the frustrated Lewis pair, the predicted
Figure 7. ORTEP drawing of 9, 50% thermal ellipsoids are shown,
˚
hydrogen atoms are omitted for clarity. Selected bond distances (A) and
angles (deg).
Scheme 4. Reactions of Substituted-Pyridines with H2
(Figure 8). The metric parameters of this salt have been
previously discussed.20
Monitoring reactions mixtures of 2,6-diphenylpyridine
with B(C6F5)3 showed no evidence of reaction by multi-
nuclear NMR spectroscopy even at low temperature. Thus,
the increased steric demands of Ph versus Me preclude the
classic Lewis donor-acceptor interaction completely. The
resulting frustrated Lewis pair reacts upon exposure to H2
affording the salt [(2,6-Ph2)C5H3NH][HB(C6F5)3] 11 in 82%
yield (Scheme 4).
˚
B-N distance of 1.65 A might represent a lower limit for
that parameter for reactive frustrated Lewis pairs.
Different models give rather different energy predic-
tions for these complexes. Notably, the predicted exother-
micities for formation of 9 are invariably larger than that
observed experimentally, substantially so for the MP2
model. Nonetheless, these computational and experimen-
tal data illustrate the following: the coordination energy
decreases non-linearly as the steric bulk of the pyridine
increases, wherein the effect of a single methyl substitu-
tion on pyridine is small, while the effect of a second
methyl substitution is substantial. The “tipping point”
where the energy is sufficiently small to allow equilibrium
formation of a frustrated Lewis pair lies in the range
60-100 kJ mol-1. This sets an upper limit on coordi-
nation energy that is consistent with that calculated for
the very reactive FLP (F5C6)3B/Pt-Bu3 (80 kJ mol-1).25
The above systems suggest that disubstituted pyridines are
sufficiently bulky to behave as frustrated Lewis pairs, while
monosubstituted pyridines form classic adducts. However,
2-tert-butylpyridine, shows no evidence of adduct formation
with B(C6F5)3 at temperatures as low as-60 °C. This mixture
of unquenched Lewis acid and base reacts with 1 atm H2 in
2 h, affording the quantitative formation of the pyridinium
borate salt [(2-tBu)C5H4NH][HB(C6F5)3] 12 (Scheme 4).
NMR data showed the characteristic pyridinium N-H and
1
B-H signals in the H NMR spectrum, a broad singlet at
12.13 ppm, and a broad 1:1:1:1 quartet at 3.66 ppm, respec-
tively, as well as the expected 11B and 19F resonances.
It is also interesting to note that generation of a frustrated
Lewis pair is not a sufficient condition to effect the heterolytic
activation of H2. For example, the adducts of 2,20-bipyridine
and 2,20-dipyridylamine, 6 and 7, respectively, failed to react
with a second equivalent of B(C6F5)3, presumably a result of
a combination of electronic and steric effects. While for-
mally the resulting combinations of these adducts with an
(37) Ramos, A.; Lough, A. J.; Stephan, D. W. Chem. Commun. 2009,
1118.
(38) (a) Phillips, J. A.; Halfen, J. A.; Wrass, J. P.; Knutson, C. C.; Cramer,
C. J. Inorg. Chem. 2006, 45, 722. (b) Dillen, J.; Verhoeven, P. J. Phys. Chem. A
2003, 107, 2570. (c) Giesen, D. J.; Phillips, J. A. J. Phys. Chem. A 2003, 107,
4009.

10474 Inorganic Chemistry, Vol. 48, No. 21, 2009
Geier et al.
Coordination energies above this range lead to formation
of stable Lewis acid-base complexes.
with several reported calculations involving dissociation
of H2 from BN systems, although they are on thelowside.39
However, computationally, the reaction between complex 9
and H2 is endothermic by 11-37 kJ mol-1, indicating that
the B-N bond in 9 must be broken, or at least substan-
tially weakened through stretching, before hydrogena-
tion.
In summary, systematic variation of the steric bulk of
substituents on pyridine prompts elongation of the re-
sulting B-N bonds in classical Lewis acid adducts.
Increasing the steric conflict further, as in the case of
2,6-lutidine, 2,6-diphenylpyridine and 2-tert-butylpyri-
dine prompts formation of frustrated Lewis pairs, cap-
able of heterolytic activation of H2. Only 2,6-lutidine was
observed to form both a classical Lewis adduct and a
frustrated Lewis pair. We continue to explore the scope of
reactivity of frustrated Lewis pairs and their applications
to hydrogen storage and catalysis.
The poor agreement between predicted and experimen-
tal coordination energies prompted an examination of
solvation effects. Encapsulating the reaction components
in CH2Cl2 solvent modeled as a polar continuum (PCM
model) had little effect on the predicted energy, in keeping
with the modest dielectric constant of CH2Cl2. This also
held when an explicit CH2Cl2 molecule was included in
the PCM solvent cavity; in particular, binding of Cl lone
pairs to the Lewis acidic boron in free B(C6F5)3 proved
minimal. The B-Cl distance in (F5C6)3B Cl2CH2 opti-
3
˚
mized to 3.12 A (i.e., the distance expected on the basis of
van der Waals radii), while the energy stabilization of this
“complex” versus the free molecules was less than 5 kJ
mol-1. Apparently the models used here inherently over-
estimate the coordination energy of 9; future work will
address this problem.
Experimentally, complex 9 reacts with H2 to form the
corresponding pyridinium hydroborate, 10. The models
confirm that this reaction must take place with the
frustrated Lewis pair form of the equilibrium system,
rather than with the coordinated complex. Calculations
for the hydrogenation find that reaction between “free”
B(C6F5)3 and 2,6-lutidine and H2 is exothermic by 55-
63 kJ mol-1, depending on the model. These compare well
Acknowledgment. The authors thank NSERC of
Canada for financial support. D.W.S. is grateful for the
award of a Canada Research Chair and a Killam Re-
search Fellowship. S.J.G. is grateful for an NSERC
postgraduate scholarship.
Supporting Information Available: Crystallographic data
in CIF format. Ordering information is given on any current
masthead page. CIF data is available online. This mate-
rial is available free of charge via the Internet at http://pubs.
acs.org.
(39) (a) Matus, M. H.; Anderson, K. D.; Camaloni, D. M.; Autrey, S. T.;
Dixon, D. A. J. Phys. Chem. A 2007, 111, 4411. (b) Nirmala, V.; Kolandaivel, P.
J. Mol. Struc.: THEOCHEM 2006, 758, 9.