Intra- and intermolecular N-H...F-C hydrogen-bonding interactions in amine adducts of tris(pentafluorophenyl)borane and -alane

Article abstract of DOI:10.1021/ic050663n

The reaction between B(C₆F₅)₃ and NH ₃(g) in light petroleum yielded the solvated adduct H ₃N·B(C₆F₅)₃·NH ₃. Treatment with a second equivalent of B(C₆

Full text of DOI:10.1021/ic050663n

Inorg. Chem. 2005, 44, 5921−5933
Intra- and Intermolecular N−H‚‚‚F−C Hydrogen-Bonding Interactions in
Amine Adducts of Tris(pentafluorophenyl)borane and -alane
Andrew J. Mountford,† Simon J. Lancaster,*,† Simon J. Coles,‡ Peter N. Horton,‡ David L. Hughes,†
Michael B. Hursthouse,‡ and Mark E. Light‡
Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy, UniVersity
of East Anglia, Norwich, NR4 7TJ U.K., and School of Chemistry, UniVersity of Southampton,
Highfield, Southampton, SO17 1BJ U.K.
Received April 29, 2005
The reaction between B(C6F5)3 and NH3(g) in light petroleum yielded the solvated adduct H3N
with a second equivalent of B(C6F5)3 afforded H3N B(C6F5)3. Attempts to prepare the analogous alane adduct were
unsuccessful and resulted in protolysis. Related compounds of the form R ′′N(H) M(C6F5)3 were synthesized
from M(C6F5)3 and the corresponding primary and secondary amines (M B, Al; R′ ) H, Me, CH2Ph; R′′ ) Me,
CH2Ph, CH(Me)(Ph); R ′′ ) cyclo-C5H10). The solid-state structures of 13 new compounds have been elucidated
by single-crystal X-ray diffraction and are discussed. Each of the borane adducts has a significant bifurcated
intramolecular hydrogen bond between an amino hydrogen and two o-fluorines, while N C interactions in
the alane adducts are weaker and more variable. 19F NMR studies demonstrate that the borane adducts retain the
bifurcated C C hydrogen bond in solution. Compounds of the type R ′′N(H) M(C6F5)3 conform to
Etter’s rules for the prediction of hydrogen-bonding interactions.
‚B(C6F5)3‚NH3. Treatment
‚
′
R
‚
)
′
R
−H‚‚‚F−
−F
‚‚‚
H‚‚‚
F
−
′
R
‚
Introduction
van der Waals radii (ca. 2.55 Å) and preferably no longer
than ca. 2.2-2.3 Å, with obtuse H-F-C angles.
The inorganic fluoride ion is one of the best hydrogen-
bond acceptors, with the hydrogen bond in the bifluoride
Intermolecular X-H‚‚‚F-C interactions are believed to
play a role in certain biological recognition processes and
have been investigated as synthons in organic crystal
engineering.6,7 Interest in X-H‚‚‚F-C interactions has
coincided with intense academic research into pentafluo-
rophenyl compounds of the group 13 elements, particularly
tris(pentafluorophenyl)borane, because of their importance
in catalysis both as activators for single-site transition-metal
polymerization catalysts8 and as Lewis acid catalysts for
organic transformations.9 Inevitably, studies of this nature
-
anion (HF2 ) approaching the strength of a covalent bond.1
In contrast, the C-F group is a poor hydrogen-bond acceptor,
and until the late 1990s, there were relatively few reported
examples of interactions in which organofluorine might be
regarded as accepting of a hydrogen bond.2 The controversy
surrounding close X-H‚‚‚F-C contacts has been the subject
of a number of detailed analyses.3-5 Dunitz concluded that
organofluorine rarely accepts hydrogen bonds and suggested
that in order to be regarded as a hydrogen bond the H‚‚‚F
distance should be significantly shorter than the sum of the
(5) An alternative description of the attractive interaction between N-H
and F-C dipoles has recently been proposed: (a) Snyder, J. P.;
Chandrakumar, N. S.; Sato, H.; Lankin, D. C. J. Am. Chem. Soc. 2000,
122, 544. (b) Lankin, D. C.; Grunewald, G. L.; Romero, F. A.; Oren,
I. Y.; Snyder, J. P. Org. Lett. 2002, 4, 3557.
(6) (a) O’Hagan, D.; Rzepa, H. S. Chem. Commun. 1997, 645. (b)
Hoffmann, M.; Rychlewski, J. J. Am. Chem. Soc. 2001, 123, 2308.
(7) (a) Borwick, S. J.; Howard, J. A. K.; Lehmann, C. W.; O’Hagan, D.
Acta Crystallogr. E 1997, 53, 124. (b) Weiss, H.-C.; Boese, R.; Smith,
H. L.; Haley, M. M. Chem. Commun. 1997, 2403. (c) Thalladi, V.
K.; Weiss, H.-C.; Bla¨ser, D.; Boese, R.; Nangia, A.; Desiraju, G. R.
J. Am. Chem. Soc. 1998, 120, 8702. (d) Barbarich, T. J.; Rithner, C.
D.; Miller, S. M.; Anderson, O. P.; Strauss, S. H. J. Am. Chem. Soc.
1999, 121, 4280. For a recent review of fluorine in crystal engineering,
see: (e) Reichenba¨cher, K.; Su¨ss, H. I.; Hulliger, J. Chem. Soc. ReV.
2005, 34, 22.
* To whom correspondence should be addressed. E-mail:
S.Lancaster@uea.ac.uk. Tel: +44 1603 592009. Fax: +44 1603 592003.
† University of East Anglia.
‡ University of Southampton.
(1) Harrell, S. A.; McDaniel, D. H. J. Am. Chem. Soc. 1964, 86, 4497.
(2) Shimoni, L.; Glusker, J. P. Struct. Chem. 1994, 5, 383.
(3) (a) Howard, J. A. K.; Hoy, V. J.; O’Hagan, D.; Smith, G. T.
Tetrahedron 1996, 52, 12613. (b) Plenio, H.; Diodone, R. Chem. Ber.
1997, 130, 633. (c) Takemura, H.; Kon, N.; Yasutake, M.; Nakashima,
S.; Shinmyozu, T.; Inazu, T. Chem. Eur. J. 2000, 6, 2334. (d)
Brammer, L.; Bruton, E. A.; Sherwood, P. Cryst. Growth Des. 2001,
1, 277. (e) Hyla-Kryspin, I.; Haufe, G.; Grimme, S. Chem. Eur. J.
2004, 10, 3411.
(4) Dunitz, J. D.; Taylor, R. Chem. Eur. J. 1997, 3, 89.
10.1021/ic050663n CCC: $30.25
Published on Web 07/14/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 16, 2005 5921

Mountford et al.
Chart 1a
a Intermolecular O-H‚‚‚F-C interactions in I and II have been omitted.
Scheme 1a
have led to the characterization of a great number of Lewis
base adducts.10 Protic Lewis bases experience an increase
in Brønsted acidity and hydrogen-bond donor strength upon
adduct formation. Perhaps the most obvious and best studied
example is H2O‚B(C6F5)3 (I, Chart 1),11 which has an
estimated pKa of 8.4 in acetonitrile.10c The aluminum
analogue H2O‚Al(C6F5)3 (II) has recently been reported and
is one of the relatively few examples of Lewis base adducts
of Al(C6F5)3.12
We recently described the synthesis and structure of the
amidodiborate anion [H2N{B(C6F5)3}2]- (III), in which there
is a complex intramolecular hydrogen-bonding arrange-
ment.13 Further studies directed toward extending this family
of anions have involved the isolation of examples of primary
and secondary amine adducts of B(C6F5)3 in which a NH
group participates in a bifurcated hydrogen-bonding arrange-
ment.14-16 The hydrogen-bonding patterns observed in these
preliminary studies of boron complexes prompted further
a (i) B(C6F5)3. (ii) Dissolution in toluene and removal of volatiles under
vacuum.
investigation of their structural chemistry. We became
interested in whether similar patterns would be observed with
amine adducts of Al(C6F5)3. Herein we report the synthesis
and solid-state structures of a number of novel protic amine
adducts of B(C6F5)3 and Al(C6F5)3 and contrast the intramo-
lecular hydrogen-bonding patterns.
(8) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391. Bochmann,
M.; Lancaster, S. J.; Hannant, M. D.; Rodriguez, A.; Schormann, M.;
Walker, D. A.; Woodman, T. J. Pure Appl. Chem. 2003, 75, 1183.
Bochmann, M. J. Organomet. Chem. 2004, 689, 3982.
Results
The ammonia adduct H3N‚B(C6F5)3 was among the first
complexes of B(C6F5)3 reported, but to date, the solid-state
structure has not been described.17 Using a procedure similar
to that employed by Stone, NH3(g) was bubbled through a
light petroleum solution of B(C6F5)3, precipitating a colorless
solid (Scheme 1). Characterization by multinuclear NMR
(benzene-d6) confirmed that this crude material was indeed
an adduct, in which the 11B NMR resonance was high-field-
shifted from δ 59 for free B(C6F5)3 to δ -7. However, the
1H NMR spectrum consisted of two broad resonances of
approximately equal intensity at δ 3.71 and δ -0.57. The
N-H stretching region of the FT-IR spectrum was complex,
and bands were observed at 3396, 3373, 3362, 3330, and
3296 cm-1. Recrystallization from a dichloromethane/light
petroleum mixture yielded colorless crystals that retained the
spectroscopic characteristics of the crude material. Elemental
analysis gave a C-N ratio of close to 18:2, suggesting a
composition with two nitrogens to each boron atom. We
therefore formulated the product as H3N‚B(C6F5)3‚NH3 (1a‚
NH3), in which a second ammonia molecule is hydrogen-
bonded to the adduct. The solid-state structure (which is
(9) Ishihara, K.; Yamamoto, H. Eur. J. Org. Chem. 1999, 527.
(10) For example, see: (a) 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. (b) Jacobsen,
H.; Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.; Fro¨hlich, R.; Meyer,
O. Organometallics 1999, 18, 1724. (c) Bergquist, C.; Bridgewater,
B. M.; Harlan, C. J.; Norton, J. R.; Friesner, R. A.; Parkin, G. J. Am.
Chem. Soc. 2000, 122, 10581. (d) Beckett, M. A.; Brassington, D. S.;
Light, M. E.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 2001,
1768. (e) Drewitt, M. J.; Niedermann, M.; Kumar, R.; Baird, M. C.
Inorg. Chim. Acta 2002, 335, 43. (f) Vagedes, D.; Erker, G.; Kehr,
G.; Bergander, K.; Kataeva, O.; Fro¨hlich, R.; Grimme, S.; Mu¨ck-
Lichtenfeld, C. Dalton Trans. 2003, 1337.
(11) Doerrer, L. H.; Green, M. L. H. J. Chem. Soc., Dalton Trans. 1999,
4325.
(12) Chakraborty, D.; Chen, E. Y.-X. Organometallics 2003, 22, 207.
(13) Lancaster, S. J.; Rodriguez, A.; Lara-Sanchez, A.; Hannant, M. D.;
Walker, D. A.; Hughes, D. L.; Bochmann, M. Organometallics 2002,
21, 451.
(14) Lancaster, S. J.; Mountford, A. J.; Hughes, D. L.; Schormann, M.;
Bochmann, M. J. Organomet. Chem. 2003, 680, 193.
(15) Mountford, A. J.; Hughes, D. L.; Lancaster, S. J. Chem. Commun.
2003, 2148.
(16) Resconi and co-workers recently reported the reaction of protic
aromatic N-heterocycles with B(C6F5)3. For comparison, they also
prepared (2,3-dihydro-1H-indole)tris(pentafluorophenyl)boron, which
exhibits a bifurcated intramolecular hydrogen-bonding motif closely
related to those in this study: Guidotti, S.; Camurati, I.; Focante, F.;
Angellini, L.; Moscardi, G.; Resconi, L.; Leardini, R.; Nanni, D.;
Mercandelli, P.; Sironi, A.; Beringhelli, T.; Maggioni, D. J. Org. Chem.
2003, 68, 5445.
(17) Massey, A. G.; Park, A. J.; Stone, F. G. A. Proc. Chem. Soc. 1963,
212. Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245.
5922 Inorganic Chemistry, Vol. 44, No. 16, 2005

Amine Adducts of Tris(pentafluorophenyl)borane and -alane
Scheme 2
presented below) was elucidated by X-ray crystallography
and confirmed the proposed composition. Retention of the
second NH3 molecule during recrystallization, despite the
very low solubility of NH3 in dichloromethane at room
temperature, indicates that the hydrogen-bonding interaction
is maintained in solution. Similar behavior has been reported
for the hydrate, H2O‚B(C6F5)3 (I), which forms intermolecu-
lar associations and cocrystallizes with solvent molecules that
provide hydrogen-bond acceptors.18 I has also been shown
to interact further with water molecules in a toluene
solution.19
Table 1. δ(N-H) for the Amines HNR′R′′ and Adducts
HR′R′′N‚M(C6F5)3 (Benzene-d6, 20 °C)
Treatment of 1a‚NH3 with a second equivalent of B(C6F5)3
(Scheme 1) leads to the formation of a new adduct with only
one H NMR resonance at room temperature and a C-N
δ(NHR′R′′)/ppm
1
amine
free amine borane adduct (1) alane adduct (2)
ratio of 18:1, indicating the composition H3N‚B(C6F5)3 (1a).
We find that 1a is more conveniently isolated by preparing
1a‚NH3 in toluene, removing the volatiles under reduced
pressure, and recrystallizing the resulting solid from dichlo-
romethane/light petroleum. Compound 1a was also charac-
terized by X-ray crystallography, and the significant structural
features are discussed below.
a, NH3
2.67
4.29
4.42
5.25, 4.02
6.23
5.13
5.91
6.36
b, H2NtBu
0.77
0.86
0.99
0.23
0.87
0.71
1.09
3.92
3.26
3.29
2.76
2.68
4.30
3.99
c, H2NCH2Ph
d, H2NC(Me)(H)Ph
e, HNMe2
f, cyclo-(H)NC5H10
g, HNMeCH2Ph
h, HN(CH2Ph)2
The reported chemistry of Al(C6F5)3 differs from B(C6F5)3
in that it forms adducts with arenes, is poorly soluble in
hydrocarbons, and decomposes in chlorocarbons,20-22 while
there are relatively few reports of Lewis base adducts.12,23-25
Our attempts to prepare an aluminum analogue of 1a were
ultimately unsuccessful. Treatment of a colorless light
petroleum suspension of Al(C6F5)3 with NH3(g) resulted in
the slow formation of a sticky yellow solid, which proved
to be insoluble in dichloromethane. When the reaction with
amines HNMe2 (e), cyclo-(H)NC5H10 (f), HNMeCH2Ph (g),
and HN(PhCH2)2 (h) in a dichloromethane or toluene solution
results in the formation of adducts 1b-h (Scheme 2). The
syntheses and solid-state structures of 1e and 1f have been
described elsewhere, and we have previously reported the
synthesis and spectroscopic characterization of 1b.13,14,26 The
new adducts 1c, 1d, 1g, and 1h have been characterized by
1H, 13C, 11B, and 19F NMR spectroscopy, elemental analyses,
and single-crystal X-ray diffraction. The 11B NMR reso-
nances at δ -4.2 (1c), -4.4 (1d), -1.2 (1g), and -0.3 (1h)
[cf. δ -5.2 (1b), -1.4 (1e), and -2.1 (1f)] are characteristic
of four-coordinate neutral adducts of B(C6F5)3.
1
NH3(g) was performed in benzene-d6 and monitored by H
NMR, the formation of the yellow precipitate was found to
be accompanied by generation of C6F5H. In light of the
successful isolation of H3N‚B(C6F5)3, H2O‚Al(C6F5)3,12 and
the primary and secondary adducts 2b-g described below,
the evident instability of H3N‚Al(C6F5)3 toward protolysis
is somewhat surprising.
The aluminum analogues 2b-h were prepared in toluene
solutions at room temperature (Scheme 2). The crude
products were subsequently recrystallized from dichlo-
romethane/light petroleum mixtures at -25 °C to yield, for
all but 2g, crystallographic quality colorless crystals. Because
Al(C6F5)3 decomposes in a dichloromethane solution, the
stability of the reaction product toward recrystallization from
dichloromethane is itself evidence for the formation of stable
adducts.22 1H, 13C, and 19F NMR and elemental analyses of
2b-h were also consistent with adduct formation. We note
that while H2O‚Al(C6F5)3 and H(Me)O‚Al(C6F5)3 have half-
lives of 33 and 193 h, respectively, in a toluene solution, no
evolution of C6F5H was observed during the characterization
of 2b-h, indicating that at 20 °C the adducts are indefinitely
stable with respect to intra- or intermolecular protolysis
reactions.
Treatment of B(C6F5)3 with the primary amines H2NtBu
(b), H2NCH2Ph (c), and H2NCH(Me)Ph (d) and secondary
(18) For compounds of the form [H2O‚B(C6F5)3]‚Lx, see the following.
(a) For Lx ) 2H2O: Danopoulos, A. A.; Galsworthy, J. R.; Green,
M. L. H.; Cafferkey, S.; Doerrer, L. H.; Hursthouse, M. B. Chem.
t
Commun. 1998, 2529. (b) For Lx ) BuOH: Reference 10c. (c) For
Lx ) Me2SO2‚H2O: Coles, S. J.; Hursthouse, M. B.; Beckett, M. A.;
Dutton, M. Acta Crystallogr. E 2003, 59, 1354. (d) For Lx ) Et2O,
H2O: Lancaster, S. J.; O’Hara, S. M.; Bochmann, M. In Metalorganic
Catalysts for Synthesis and Polymerization; Kaminsky, W., Ed.;
Springer Verlag: Berlin, 2000.
(19) Beringhelli, T.; Maggioni, D.; D’Alfonso, G. Organometallics 2001,
20, 4927.
(20) Free (C6F5)3Al presents a significant explosion hazard if subjected to
thermal or physical shock and was therefore isolated only on a small
scale by the reaction of (C6F5)3B with Me3Al in a light petroleum
solution followed by removal of the volatiles and resuspension in light
petroleum. See: Reference 21.
(21) Biagini, P.; Lugli, G.; Abis, L.; Andreussi, P. U.S. Patent 5,602,269,
1997. Lee, C. H.; Lee, S. J.; Park, J. W.; Kim, K. H.; Lee, B. Y.; Oh,
J. S. J. Mol. Catal., A 1998, 132, 231.
(22) Chakraborty, D.; Chen, E. Y.-X. Inorg. Chem. Commun. 2002, 5, 698.
(23) Belgardt, T.; Storre, J.; Roesky, H. W.; Noltemeyer, M.; Schmidt,
H.-G. Inorg. Chem. 1995, 34, 3821.
1
For both the borane and alane adducts, the H NMR
resonance of the NH group exhibits a dramatic change in
chemical shift upon adduct formation. Table 1 presents the
δ(NH) resonance for the free amine, the borane adducts, and
the alane adducts. The greatest values of ∆δ(NHadduct
-
NHfree amine) are found for the secondary amine borane
(24) Bolig, A. D.; Chen, E. Y.-X. J. Am. Chem. Soc. 2001, 123, 7943.
(25) LaPointe, R. E.; Roof, G. R.; Abboud, K. A.; Klosin, J. J. Am. Chem.
Soc. 2000, 122, 9560.
(26) Schatte, G.; Chivers, T.; Tuononen, H. M.; Suontamo, R.; Laitinen,
R.; Valkonen, J. Inorg. Chem. 2005, 44, 443.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5923

Mountford et al.
Table 2. ν(N-H) for Selected Amines HNR′R′′ and Adducts
HR′R′′N‚M(C6F5)3
Table 3. Selected Bond Lengths; Mean (or Unique) Values, in Å, with
Standard Deviations (or Estimated Standard Deviations) in Parentheses
ν(N-H)/cm-1
distance
free
amine
borane
alane
compd
no.
no. of independent
molecules
mean
B-C
mean
B-N
mean
N-C
amine
adduct (1)a
adduct (2)a
a, NH3
3434, 3334c
3350, 3277b
3367, 3294b
3347c
3373, 3362, 3296
3346
3337, 3282
3330
3312
3318
1a‚NH3
1a
1b
1c
1d
1e
1f
1g
1h
1
2
2
1
1
2
2
1
1
1.637(1)
1.636(2)
1.646(5)
1.642(3)
1.651(3)
1.654(2)
1.658(5)
1.648(7)
1.645(6)
1.606(3)a
1.624(1)
1.645(1)
1.625(2)a
1.638(6)a
1.653(1)
1.630(1)
1.635(8)a
1.651(2)a
b, H2NtBu
3299, 3254
3315, 3273
3302
3274
3278
c, H2NCH2Ph
e, HNMe2
f, cyclo-(H)NC5H10
g, HNMeCH2Ph
h, HN(CH2Ph)2
1.550(1)
1.505(2)a
1.535(6)a
1.506(1)
1.515(3)
1.506(6)
1.514(6)
3272b
3322b
3307b
3315
3259
a Nujol mull. b At high dilution in CCl4. c Gas phase.
distance
adducts, in which we know the NH is engaged in a bifurcated
hydrogen-bonding interaction (see below). ∆δ is somewhat
less for the primary amine borane adducts, in which only
one of the two NH’s is strongly hydrogen-bonded. The alane
adducts give lower values of ∆δ, which is presumably due,
at least in part, to weaker intramolecular hydrogen bonding.
Selected compounds have been characterized by IR
spectroscopy, and the ν(N-H) values are collated in Table
2. The secondary amine adducts give a single sharp N-H
stretch, while with the exception of 1b, the primary amine
adducts exhibit distinguishable symmetric and asymmetric
stretches. In general, the stretching frequencies are some 30-
50 cm-1 lower for the aluminum compounds when compared
to their boron analogues. However, aside from the borane
versus alane shift, there is no distinctive trend on coordina-
tion. The data for the free amine, run at high dilution to
eliminate intermolecular hydrogen bonding, are given in
Table 2. Hydrogen bonding would be expected to lead to
lower values of ν(N-H) than those found for the isolated
amine molecule, but we do not observe a distinctive
correlation between ν(N-H) and the extent of intramolecular
hydrogen bonding (as elucidated by the X-ray studies
described below) and an explanation for this awaits further
investigation.
compd
no.
no. of independent
molecules
mean
Al-C
mean
Al-N
mean
N-C
2b
2c
2d
2e
2f
1
2
1
4
2
2
2.005(4)
1.9952(13)
1.993(2)
1.998(2)
1.997(2)
1.991(2)
1.997(2)a
1.976(2)
1.987(2)a
1.971(4)
1.975(3)
1.967(2)
1.531(2)a
1.5105(5)
1.508(4)a
1.494(3)
1.504(2)
1.506(3)
2g
a A unique value, with its estimated standard deviation in parentheses;
all other values are mean values, with standard deviations.
the range 106-111° but are still significantly less than the
other C-Al-C angles. The mean Al-C distance 1.997(2)
Å is similar to those of the basic adducts of Al(C6F5)3
reported to date: THF‚Al(C6F5)3,23 MMA‚Al(C6F5)3,24 H2O‚
Al(C6F5)3, and H(Me)O‚Al(C6F5)3.12 At 1.979(5) Å, the mean
Al-N bond length is slightly longer than that in the anion
[(C6F5)3Al(imidazole)Al(C6F5)3]-, which is 1.911(2) Å.25
In all of the borane and alane adducts, with the exception
of 2d, the arrangement of bonds about the B-N or Al-N
bond is staggered, and there are few examples where the
trans torsion angle differs by more than 10° from 180°. The
exception 2d (Figure 9) shows an eclipsed arrangement, with
a C-Al-N-C torsion angle of -1.3(2)°, and we suggest
that this results from a combination of the favorable overlap
of the phenyl ring C(3-8) with one of the C6F5 rings C(21-
26) and the overlap of the other C6F5 rings with symmetry-
related rings in neighboring molecules.
Crystallography
The hitherto unreported solid-state structures of the borane
adducts 1a-d, 1a‚NH3, 1g, and 1h and the new aluminum
compounds 2b-g have been determined by crystallographic
methods. Views of the boron compounds, 1a‚NH3, 1a-d,
1g, and 1h, are shown in Figures 1-7, and those of the
aluminum compounds, 2b and 2d-g, in Figures 8-13 (see
also the Supporting Information). The structures of the borane
adducts are similar in as far as the geometries about all of
the boron and nitrogen atoms are essentially tetrahedral. We
note a characteristically small C-B-C angle, ca. 105°,
between the two C6F5 groups linked in a bifurcated hydrogen-
bond scheme (see below); the other C-B-C angles are ca.
114°. There is little variation in bond lengths, and these are
summarized in Table 3. The overall mean bond lengths are
1.646(2) Å for B-C and 1.635(4) Å for B-N and lie within
the range previously reported for related adducts.14-16 There
are also only minor variations from tetrahedral geometry in
the alane adducts; for the rings involved in bifurcated
hydrogen-bonding systems, the C-Al-C angles are now in
Where there are corresponding structures of the boron and
aluminum compounds, we find that although the Al/B-
(C6F5)3 units are very similar, there may be variation in the
arrangements of the amine groups, as is seen in the NH2-
CHMePh groups of 1d and 2d. In the compounds with
symmetrical amine groups, for example, the NHMe2 groups
of 1e and 2e and the cyclo-(H)NC5H10 groups in 1f and 2f,
the pseudo mirror symmetry in these ligands extends into
the B/Al-(C6F5)3 groups; in all of these compounds, there
is one C6 ring with a N-B/Al-C-C torsion angle of almost
90° and two others twisted about 30° in opposite directions.
There is disorder in one of the C6F5 groups in one molecule
of 2f, but the general scheme of pseudosymmetry is
maintained. In 1b/2b, 1c/2c, and 1g/2g, there are significant
differences between the conformations of the boron and
aluminum analogues, presumably resulting from the in-
creased N-Al vs N-B bond distances, the flexibility of the
5924 Inorganic Chemistry, Vol. 44, No. 16, 2005

Amine Adducts of Tris(pentafluorophenyl)borane and -alane
Table 4. Short Hydrogen Atom Contacts and Hydrogen Bond Dimensions, in Å and Degreesa
symmetry
operation
D-H‚‚‚A
d(D-H)
d(H‚‚‚A)
d(D‚‚‚A)
(DHA)
Compound 1a‚NH3
2.982(3)
2.876(3)
2.655(3)
2.998(3)
3.280(3)
2.767(3)
2.655(3)
2.998(3)
N(1)-H(1n)‚‚‚F(5)
N(1)-H(2n)‚‚‚N(2)
N(1)-H(2n)‚‚‚F(15)
N(1)-H(2n)‚‚‚F(15I)
N(1)-H(3n)‚‚‚F(2II)
N(1)-H(3n)‚‚‚F(10)
N(1)-H(3n)‚‚‚F(15)
N(1)-H(3n)‚‚‚F(15I)
0.87(3)
0.93(4)
0.93(4)
0.93(4)
0.87(4)
0.87(4)
0.87(4)
0.87(4)
0.87(5)
0.87(5)
0.82(5)
2.40(3)
1.96(4)
2.57(3)
2.73(3)
2.59(4)
2.16(4)
2.22(4)
2.49(4)
2.64(4)
2.51(5)
2.47(5)
124(3)
172(3)
85(2)
*
98(2)
*
*
I: 1 - x, 2 - y, -z
137(3)
127(3)
110(3)
117(3)
115(4)
172(4)
144(3)
II: x - 1/2, 11/2 - y, z - 1/2
*
*
*
*
I: 1 - x, 2 - y, -z
N(2)-H(5n)‚‚‚F(7III
)
3.114(4)
3.375(4)
3.170(3)
III: x - 1/2, 11/2 - y, 1/2 + z
I: 1 - x, 2 - y, -z
N(2)-H(5n)‚‚‚F(10I)
N(2)-H(6n)‚‚‚F(4IV
)
IV: 1 - x, 1 - y, -z
Compound 1a
2.630(2)
2.747(2)
2.870(2)
2.981(2)
3.117(2)
2.630(2)
3.270(2)
2.954(2)
3.011(2)
2.865(2)
2.636(2)
3.201(2)
2.742(2)
3.160(2)
2.636(2)
N(1)-H(1a)‚‚‚F(2)
N(1)-H(1a)‚‚‚F(8)
N(1)-H(1b)‚‚‚F(14)
N(1)-H(1b)‚‚‚F(29)
N(1)-H(1b)‚‚‚F(36I)
N(1)-H(1c)‚‚‚F(2)
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
2.44
2.18
2.27
2.52
2.34
2.28
2.39
2.38
2.43
2.24
2.39
2.38
2.13
2.31
2.32
91.6
119.8
122.9
111.6
142.9
102.6
163.0
121.2
122.1
125.3
95.1
*
*
I: x, y - 1, z
N(1)-H(1c)‚‚‚F(28)
N(21)-H(21a)‚‚‚F(9I)
N(21)-H(21a)‚‚‚F(17II)
N(21)-H(21a)‚‚‚F(32)
N(21)-H(21b)‚‚‚F(22)
*
*
*
II: x - 1, 1 + y, z
III: x - 1, y, z
N(21)-H(21b)‚‚‚F(23IV
N(21)-H(21b)‚‚‚F(38)
N(21)-H(21c)‚‚‚F(18V)
N(21)-H(21c)‚‚‚F(22)
)
150.8
123.8
154.3
99.8
*
*
IV: -x, 1 - y, 1 - z
V: x - 1, y, z
Compound 1b
2.770(2)
2.808(2)
3.017(2)
3.232(2)
3.128(2)
2.832(2)
2.756(2)
N(1)-H(1a)‚‚‚F(2)
0.92
0.92
0.92
0.92
0.92
0.92
0.92
2.15
2.15
2.44
2.55
2.58
2.20
2.11
123.4
127.3
121.0
131.2
118.3
124.9
126.5
N(1)-H(1a)‚‚‚F(8)
N(1)-H(1b)‚‚‚F(14)
N(1)-H(1b)‚‚‚F(44I)
N(31)-H(31a)‚‚‚F(48)
N(31)-H(31b)‚‚‚F(36)
N(31)-H(31b)‚‚‚F(42)
*
*
*
I: x, 1 + y, z
Compound 1c
3.0282(11)
2.723(2)
N(1)-H(1na)‚‚‚F(9I)
N(1)-H(1nb)‚‚‚F(6)
N(1)-H(1nb)‚‚‚F(11)
0.89(2)
0.86(2)
0.86(2)
2.26(2)
2.12(2)
2.23(2)
145(2)
127(2)
118.3(15)
I: 1/2 - x, 1/2 + y, 1/2 - z
2.747(2)
Compound 1d
2.988(5)
3.226(5)
2.770(5)
2.819(5)
N(4)-H(4a)‚‚‚F(16)
N(4)-H(4a)‚‚‚F(35I)
N(4)-H(4b)‚‚‚F(26)
N(4)-H(4b)‚‚‚F(32)
0.90
0.90
0.90
0.90
2.40
2.44
2.32
2.16
123
144
111
129
I: 11/2 - x, y - 1/2, 1/2 - z
Compound 1e
2.755(3)
2.751(4)
2.737(4)
2.731(3)
N(4)-H(4)‚‚‚F(22)
N(4)-H(4)‚‚‚F(32)
N(8)-H(8)‚‚‚F(62)
N(8)-H(8)‚‚‚F(72)
0.91
0.91
0.91
0.91
2.15
2.10
2.10
2.09
123
128
126
126
Compound 1f
2.747(2)
2.733(2)
2.758(2)
2.745(2)
N(4)-H(4)‚‚‚F(12)
N(4)-H(4)‚‚‚F(26)
N(8)-H(8)‚‚‚F(52)
N(8)-H(8)‚‚‚F(66)
0.91
0.91
0.91
0.91
2.10
2.10
2.11
2.14
127
126
128
123
Compound 1g
2.725(6)
2.763(6)
N(1)-H(1n)‚‚‚F(5)
N(1)-H(1n)‚‚‚F(11)
0.92(5)
0.92(5)
2.10(5)
2.15(5)
125(3)
123(4)
Compound 1h
2.790(2)
2.710(2)
N(1)-H(1)‚‚‚F(6)
N(1)-H(1)‚‚‚F(11)
0.93
0.93
2.22
2.02
118.9
129.5
Compound 2b
2.953(2)
2.891(2)
N(4)-H(4a)‚‚‚F(12)
N(4)-H(4b)‚‚‚F(36)
0.90
0.90
2.28
2.14
132
140
Compound 2c
2.943(2)
3.337(2)
2.879(2)
2.835(2)
2.897(2)
3.298(2)
3.006(2)
N(1)-H(1n)‚‚‚F(5)
N(1)-H(1n)‚‚‚F(5I)
N(1)-H(1n)‚‚‚F(15)
N(2)-H(3n)‚‚‚F(21)
N(2)-H(3n)‚‚‚F(30)
N(2)-H(3n)‚‚‚F(30II)
N(2)-H(4n)‚‚‚F(20)
0.92(3)
0.92(3)
0.92(3)
0.90(2)
0.90(2)
0.90(2)
0.98(3)
2.47(3)
2.43(3)
2.36(3)
2.48(2)
2.43(2)
2.40(2)
2.49(3)
112(2)
167(2)
116(2)
104(2)
112(2)
173(2)
113(2)
*
*
I: 1 - x, -y, 1 - z
II: 2 - x, 1 - y, 1 - z
Inorganic Chemistry, Vol. 44, No. 16, 2005 5925

Mountford et al.
Table 4 (Continued)
D-H‚‚‚A
symmetry
operation
d(D-H)
d(H‚‚‚A)
d(D‚‚‚A)
(DHA)
Compound 2d
3.086(3)
2.970(3)
3.126(3)
3.304(3)
N(1)-H(1d)‚‚‚F(18I)
N(1)-H(1d)‚‚‚F(20)
N(1)-H(1e)‚‚‚F(14)
N(1)-H(1e)‚‚‚F(16II)
0.92
0.92
0.92
0.92
2.43
2.47
2.37
2.55
128.5
114.5
139.4
139.8
*
*
I: -x, -y, 2 - z
II: 1 + x, y, z
Compound 2e
3.001(4)
2.916(4)
3.114(4)
3.283(4)
2.929(4)
2.906(4)
3.013(4)
3.107(4)
3.266(4)
2.925(4)
N(1)-H(1)‚‚‚F(1)
N(1)-H(1)‚‚‚F(6)
N(1)-H(1)‚‚‚F(26I)
N(2)-H(2)‚‚‚F(14II)
N(2)-H(2)‚‚‚F(16)
N(3)-H(3)‚‚‚F(31)
N(3)-H(3)‚‚‚F(45)
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
2.49
2.36
2.34
2.55
2.20
2.26
2.47
2.38
2.51
2.38
115.2
118.2
140.0
136.5
134.3
126.2
117.6
134.6
139.0
117.1
*
*
I: 1 + x, y, 1 + z
II: x, y, z - 1
N(3)-H(3)‚‚‚F(52III
)
*
*
III: 1 - x,1/2 + y, 1 - z
IV: -x, y - 1/2, 1 - z
N(4)-H(4)‚‚‚F(41IV
N(4)-H(4)‚‚‚F(46)
)
Compound 2f
2.944(2)
2.912(2)
2.926(2)
2.933(2)
N(41)-H(41)‚‚‚F(16)
N(41)-H(41)‚‚‚F(26)
N(81)-H(81)‚‚‚F(62)
N(81)-H(81)‚‚‚F(72)
0.91
0.91
0.91
0.91
2.39
2.29
2.34
2.39
120
125
122
119
Compound 2g
2.906(2)
3.021(3)
N(1)-H(1)‚‚‚F(6)
N(31)-H(31)‚‚‚F(32)
N(31)-H(31)‚‚‚F(48)
0.93
0.93
0.93
2.34
2.51
2.49
119.0
114.7
113.6
2.989(3)
a An asterisk indicates an intermolecular interaction. Estimated standard deviations are in parentheses.
Figure 2. Molecular structure of one of the two independent but very
similar molecules of 1a with displacement ellipsoids at the 50% probability
level.
Figure 1. Molecular structure of 1a‚NH3 with displacement ellipsoids at
the 50% probability level.
three C6F5 rings, the bulk of the amine group, the competition
from intermolecular interactions, and not least the relative
tendency toward the formation of intramolecular N-H‚‚‚F
interactions.
To some extent all of the complexes reported here exhibit
N-H‚‚‚F interactions. The intra- and intermolecular contacts
are listed in Table 4. We define short H‚‚‚F contacts as those
less than 2.2 Å, medium length between 2.2 and 2.35 Å,
and longer contacts where the distance is between 2.35 and
2.55 Å (the last being the sum of van der Waals’ distances).
The short H‚‚‚F interactions in particular fulfill the Dunitz
criteria for classification as hydrogen bonds.4
tures.18 Second-coordination-sphere association through hy-
drogen bonding between ammonia molecules is somewhat
less common than that for water but has been previously
described for complexes of the alkaline-earth metals.27 Of
the remaining hydrogens, the second engages in a bifurcated
F‚‚‚H‚‚‚F interaction and the third has only a long (2.45 Å)
contact. Significantly, this pattern is similar to that of the
primary amine adducts (1b-d) described below.
The H atoms of the NH3 groups in the unsolvated adduct
1a (Figure 2) are involved in a complex series of intra- and
intermolecular H‚‚‚F interactions.28 Each of the hydrogens
has a short- or medium-length intramolecular contact to a
single o-F. H(1b) and H(1c) form further medium-length
interactions with fluorines from neighboring molecules. The
The presence of a short [1.96(4) Å] nearly linear [172-
(3)°] H‚‚‚N hydrogen bond is responsible for the stability
of 1a‚NH3 (Figure 1). This second sphere adduct is remi-
niscent of a number of reported [H2O‚B(C6F5)3]‚Lx struc-
(27) Rossmeier, T.; Reil, M.; Korber, N. Inorg. Chem. 2004, 43, 2206.
5926 Inorganic Chemistry, Vol. 44, No. 16, 2005

Amine Adducts of Tris(pentafluorophenyl)borane and -alane
Figure 5. Molecular structure of 1d with displacement ellipsoids at the
50% probability level. C-bonded H atoms are omitted for clarity.
increases in line with the steric bulk of the alkyl group 1b
< 1c < 1d. In contrast, H‚‚‚F contacts to the second H atom
do not conform to a pattern; they are weaker and may be
intramolecular, as in 1b and 1d, or intermolecular, such as
in 1c.30 In the solid-state structure of compound 1d, an
intramolecular aryl-perfluoroaryl interaction is also apparent.
Within each molecule of 1d, the phenyl ring overlaps one
of the C6F5 rings (interplanar distance ca. 3.26 Å); on the
opposite side, there is a symmetry-related Ph ring at a
distance of 3.62 Å.31
Figure 3. Structure of one of the two molecules of 1b with displacement
ellipsoids at the 50% probability level. C-bonded H atoms are omitted for
clarity.
Similar bifurcated intramolecular hydrogen-bonding ar-
rangements, in which each H has two short H‚‚‚F contacts,
are found in each of the secondary amine adducts, 1e-g.
The molecular structures of the new examples 1g and 1h
are presented in Figures 6 and 7, and the hydrogen-bonding
data for all four are presented in Table 4. The agreement
between the H‚‚‚F distances in all four compounds (six
independent molecules) is striking; in the bifurcated interac-
tion, the average H‚‚‚F distance is 2.12, 2.11, 2.13, and 2.12
Å for 1e, 1f, 1g, and 1h, respectively. The largest individual
variations are found in 1h (2.22 and 2.02 Å), where the
asymmetry is presumably a consequence of accommodating
the sterically bulky benzyl groups.
Figure 4. Molecular structure of 1c with displacement ellipsoids at the
50% probability level. C-bonded H atoms are omitted for clarity.
solid-state structure of 1a therefore has a number of features
in common with the monohydrate H2O‚B(C6F5)3 (I, Chart
1),11 in which each of the H atoms forms one short
intramolecular and one longer intermolecular hydrogen bond
to fluorine atoms.
The molecular structures of the three primary amine
adducts of B(C6F5)3, 1b-d (Figures 3-5), exhibit very
similar principal intramolecular hydrogen-bonding interac-
tions. In each case, one H is engaged in a bifurcated
interaction with two short-medium contacts to o-F’s.29 The
average H‚‚‚F contact distance for the bifurcated hydrogen
In contrast to the borane complexes of the primary amine
adducts of Al(C6F5)3 we have structurally characterized, only
2b (Figure 8) exhibits a short H(4b)‚‚‚F(36) contact (2.14
Å). The structure differs significantly from that of 1b, in
that there is no bifurcation and it is the second hydrogen
that exhibits a medium-length contact, H(4a)‚‚‚F(12) (2.28
Å) to another o-F. Any intramolecular hydrogen bonding in
molecules of 2c and 2d (Figure 9) is very weak, and while
in each molecule one of the H atoms has a medium-length
contact to an o-F, the next closest contact is intermolecular
(28) H3P‚B(C6F5)3 crystallizes with two independent molecules, neither of
which exhibits close intramolecular H‚‚‚F contacts, but there is an
intermolecular interaction at 2.39 Å: Bradley, D. C.; Hursthouse, M.
B.; Motevalli, M.; Zheng, D. H. J. Chem. Soc., Chem. Commun. 1991,
7.
(29) Remarkably similar bifurcated intramolecular hydrogen-bonding ar-
rangements were observed in 2,6-bis(2,6-difluorophenyl)piperidines:
Pham, M.; Gdaniec, M.; Polon˜ski, T. J. Org. Chem. 1998, 63, 3731.
t
(30) Bu(H)2P‚B(C6F5)3 crystallizes with two independent molecules, each
of which has one medium-length intramolecular H‚‚‚F contact (2.337
and 2.357 Å): Bradley, D. C.; Harding, I. S.; Keefe, A. D.; Motevalli,
M.; Zheng, D. H. J. Chem. Soc., Dalton Trans. 1996, 3931.
(31) Favorable intra- and intermolecular interactions between the comple-
mentary quadrupoles of fluoroaryl and hydroaryl groups are well-
known. See: Reference 7e and references therein.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5927

Mountford et al.
Figure 6. Molecular structure of 1g with displacement ellipsoids at the
50% probability level. C-bonded H atoms are omitted for clarity.
Figure 8. Molecular structure of 2b with displacement ellipsoids at the
50% probability level. C-bonded H atoms are omitted for clarity.
Figure 7. Molecular structure of 1h with displacement ellipsoids at the
50% probability level. C-bonded H atoms are omitted for clarity.
Figure 9. Molecular structure of 2d with displacement ellipsoids at the
50% probability level. C-bonded H atoms are omitted for clarity.
(see the Supporting Information). Similarly to 1d, in 2d the
ring of the phenyl group C(3-8) is flanked on both sides
by the overlapping and approximately parallel rings of the
C6F5 groups C(21-26) and C(21′-26′), arranged in infinite
stacks parallel to the a axis in an ABAB... fashion.31
The solid-state structure of each of the secondary amine
alane adducts 2e and 2f consists of at least two crystal-
lographically independent molecules. In 2e, there are four
independent molecules [2e(1)-2e(4)]. Those of Al(1) (Figure
10) and Al(4) have very similar conformations, while those
of Al(2) and Al(3) also have very similar conformations.
The principal differences between the two pairs are in the
torsion angles about the Al-N bond; for example, the angles
corresponding to C(1)-Al(1)-N(1)-C(20) in the four
molecules are 177.62(2), -168.0(3), -177.5(2), and 174.6-
(2)°. Correspondingly, the intramolecular hydrogen-
bonding arrangements are different: in each of molecules 2
and 3, there is a medium H‚‚‚F contact, 2.20 and 2.26 Å,
whereas the shortest in 1 and 4 are 2.36 and 2.38 Å. In all
molecules, there are also rather longer H‚‚‚F contacts, both
intramolecular (from 2.47 Å in molecule 3) and intermo-
Figure 10. Structure of one of the four independent molecules of 2e with
displacement ellipsoids at the 50% probability level. C-bonded H atoms
are omitted for clarity.
lecular (2.34-2.58 Å in the four molecules). These inter-
molecular interactions form one-dimensional columns (Figure
11).
5928 Inorganic Chemistry, Vol. 44, No. 16, 2005

Amine Adducts of Tris(pentafluorophenyl)borane and -alane
Figure 12. Structure of one of the two molecules of 2f with displacement
ellipsoids at the 50% probability level. C-bonded H atoms are omitted for
clarity.
Figure 11. One-dimensional column formed by intermolecular H‚‚‚F
interactions between one of the pairs of molecules in 2e.
The solid-state structure of 2f (Figure 12) contains two
very similar molecules, each with two intramolecular H‚‚‚F
interactions, and no significant intermolecular H‚‚‚F interac-
tions. In this instance, the hydrogen-bonding arrangement
resembles that of 1f with a bifurcated F‚‚‚H‚‚‚F interaction.
However, while in 1f, the average H‚‚‚F distance was 2.10 Å,
in 2f, the average distance, over both molecules, is 2.35 Å.
The two molecules of 2g are quite different. While
molecule 1 (Figure 13) has a single medium-length contact,
molecule 2 has a very weak bifurcated interaction.
Solution Structures: Variable-Temperature 19F NMR
Spectroscopy
Figure 13. Structure of one of the two independent molecules of 2g with
displacement ellipsoids at the 50% probability level. C-bonded H atoms
are omitted for clarity.
Variable-temperature 19F NMR spectroscopy has been
demonstrated to be useful in determining the presence and
nature of N-H‚‚‚F-C interactions in solution. Earlier, we
employed variable-temperature 19F NMR to deduce a bifur-
cated hydrogen-bonding pattern in 1b, and this has now been
established as the solid-state structure.14 The room temper-
ature 19F NMR (toluene-d8) spectra of 1c and 1d (Figure
14a) exhibit a single o-F resonance at δ -134.8 and -134.0,
respectively. The characteristic splitting and indicative high-
field shift of a hydrogen-bond-accepting o-F is apparent only
at lower temperatures.14 For 1c, cooling to -60 °C gives
rise to broad o-F resonances [δ -132.4 (4F) and -139 (2F)],
while at -60 °C, 1d shows six unique o-F’s (δ -129.3,
-130.5, -131.7, -134.8, -135.6, and -142.6; Figure 14b),
a pattern that is consistent with the asymmetric bifurcated
arrangement seen in the solid.
1h has three o-F resonances in a 2:2:2 ratio (Figure 15), a
pattern similar to what we reported for 1e.14 We therefore
propose that the bifurcated hydrogen bonds present in the
solid state are maintained in solution. The hydrogen-bonding
interaction makes a significant contribution to the barrier to
free rotation, and the effect is not merely steric. This is
apparent from the observation that the 19F NMR spectrum
of the Et2MeN adduct does not exhibit decoalescence down
to -80 °C.14 In addition, the coalescence temperature for
the significantly more bulky (PhCH2)2NH adduct (80 °C),
1h, is similar to that of the Me2NH adduct (75 °C), 1e. The
pattern of chemical shifts observed for 1g is similar to that
for 1h, but in this case, the inherent asymmetry renders all
of the o-F resonances inequivalent and those interacting with
N-H are found at δ -140.0 and -142.8. In contrast to 1b-
For the secondary amine adducts 1e-h, hindered rotation
is apparent at room temperature. The 19F NMR spectrum of
Inorganic Chemistry, Vol. 44, No. 16, 2005 5929

Mountford et al.
Figure 14. 19F NMR spectra for compound 1d: (a) toluene-d8, 20 °C; (b) toluene-d8, -60 °C.
Figure 15. 19F NMR (benzene-d6, 20 °C) for complex 1h.
h, the room temperature 19F NMR spectra of both the
secondary and primary amine adducts of Al(C6F5)3 show only
one well-resolved o-F resonance, suggesting weaker hydro-
gen bonding and less hindrance to rotation about the M-N
bond. Only at temperatures below -65 °C does the o-F
resonance of 2e begin to broaden and subsequently decoa-
lesce, whereas 2b shows no sign of decoalescence down to
-80 °C.
In agreement with the supposition that a bifurcated
interaction is required to significantly inhibit rotation, even
on cooling to -80 °C, the o-F resonances of 1a broaden but
do not decoalesce.
occurrence might be rationalized using the same approach
as hydrogen bonds to more conventional acceptors.
Etter formulated a number of general rules that can be
applied to predict the occurrence of hydrogen-bonding
interactions: (i) all good proton donors and acceptors are
used in hydrogen bonding; (ii) if intramolecular hydrogen
bonds completing a six-membered ring are possible, they
will usually form in preference to intermolecular hydrogen
bonds; (iii) the best proton donors and acceptors remaining
after hydrogen-bond formation form intermolecular hydrogen
bonds to one another.33
The distinctive feature of our compounds is the presence
of very good proton donors but poor proton acceptors. In
the simplest case, that of the secondary amine adducts, there
is only one NH function to serve as a proton donor.
Application of rule ii allows us to predict the formation of
a six-membered ring and intramolecular hydrogen bonding
Discussion
Despite the well-documented poor hydrogen-bond-accept-
ing ability of fluorine, since our recognition of short
intramolecular N-H‚‚‚F-C contacts in the anion III, we
have encountered many further examples. If it is indeed the
case, as recent investigations of X-H‚‚‚F-C interactions
have concluded, that they have the characteristics of weak
hydrogen bonds,3d,32 it would seem reasonable that their
(32) For an estimation of the strength of a O-H‚‚‚F-C hydrogen bond,
see: Takemura, H.; Kotoku, M.; Yasutake, M.; Sinmyozu, T. Eur. J.
Org. Chem. 2004, 2019.
(33) Etter, M. J. Phys. Chem. 1991, 95, 4601.
5930 Inorganic Chemistry, Vol. 44, No. 16, 2005

Amine Adducts of Tris(pentafluorophenyl)borane and -alane
Chart 2
We believe these differences result principally from the
difference in size between the boron and aluminum atoms.
In our complexes, the average Al-N bond length (1.97 Å)
is 0.33 Å longer than the average B-N bond length (1.64
Å), and similarly for Al-C, it is 1.99 versus 1.64 Å in B-C,
a difference of 0.35 Å.
The prevailing picture in the secondary amine adducts of
Al(C6F5)3 is of a medium-length intramolecular H‚‚‚F contact
and a second longer H‚‚‚F contact, which may be intra- or
intermolecular. There is no discernible pattern in the primary
amine adducts of Al(C6F5)3. Compound 2b is interesting
because it forms two two-centered hydrogen bonds and
adopts structure C. In contrast, each molecule of 2c and 2d
has only medium-length H‚‚‚F contacts.
to an o-F. Amino hydrogens are known to participate in
three-center (bifurcated) hydrogen bonds,33 so we would
therefore predict the formation of either structure A or B
(Chart 2).
For the borane adducts 1e-h, the B-N bond is short
enough to allow a favorable bifurcated intramolecular
hydrogen-bonding interaction between the NH proton and
the two o-F’s of the form illustrated by A. However,
accommodating the bifurcated arrangement does cause a
noticeable distortion of the tetrahedral geometry at boron,
so that in the two independent molecules of 1e the C-B-C
angles between the two C6F5 rings participating in hydrogen
bonding are 105.7(3)° and 105.4(3)°, whereas the other
C-B-C angles between C6F5 rings are 113.4(3)° and 115.3-
(3)° in molecule 1 and 114.1(3)° and 115.2(3)° in molecule
2.
Conclusion
Rather than being rare, N-H‚‚‚F-C interactions short
enough and with sufficiently obtuse angles to merit clas-
sification as hydrogen bonds are common to all of the protic
amine adducts of B(C6F5)3 that we have structurally char-
acterized. Compounds of this type are favorably disposed
to form six-membered rings through intramolecular hydrogen
bonds. If one accepts that organofluorine can function as an
(albeit poor) hydrogen-bond acceptor, then the arrangements
observed are consistent with Etter’s rules. These intramo-
lecular hydrogen bonds to organofluorine are particularly
strong where two fluorines connect to a single NH func-
tionality, to give a C-F‚‚‚H‚‚‚F-C arrangement. In such
cases, they can have a formative influence on the molecular
geometry and dynamics, in some cases giving observable
effects in room temperature solution NMR spectra.
For primary amine adducts, where there are two potential
hydrogen-bond donors, one can envisage the possible in-
tramolecular hydrogen-bonding patterns represented by C-E
(Chart 3).
One might have expected that 1b would adopt structure
C, in which there are two two-centered intramolecular
hydrogen bonds. However, our spectroscopic studies had
indicated a bifurcated intramolecular hydrogen-bonding
arrangement (structure D) in solution. The similarities in the
solid-state structures of 1b-d serve to confirm that the
bifurcated interaction is indeed more favorable. It follows
from Etter’s rules that the second proton donor should also
participate in intramolecular hydrogen bonding, giving rise
to structure E. However, the spatial arrangement is much
less favorable, and the balance between intramolecular (1b
and 1d) and intermolecular (1c) interactions is finely poised;
because the resulting contact distances to this second
hydrogen exceed the Dunitz criteria, we are reluctant to label
them hydrogen bonds.
The potential hydrogen-bond donors and acceptors in the
aluminum adducts are likely to have properties very similar
to those of their boron analogues. However, while we have
found three-centered hydrogen bonds with two short-
medium N-H‚‚‚F-C contacts in all of the primary and
secondary borane adducts, the H‚‚‚F contacts are generally
rather longer in the aluminum compounds, and there is a
greater likelihood of finding intermolecular interactions. The
existence of at least two molecules in the solid-state structures
of each of the secondary amine adducts of Al(C6F5)3, in
which there is a significant variation in the intramolecular
H‚‚‚F contacts, suggests that these interactions are weak and
easily deformed by competing packing forces.
The corresponding aluminum compounds do exhibit
N-H‚‚‚F-C interactions, but there are fewer and weaker
intramolecular contacts, along with an increased tendency
for intermolecular over intramolecular interactions, which
we ascribe to the greater Al-N bond length, disfavoring the
formation of intramolecular hydrogen bonds.
Experimental Section
General Procedures. All syntheses and manipulations were
carried out using standard Schlenk techniques. Solvents were
distilled under a dinitrogen atmosphere over sodium (toluene), Na/K
alloy [light petroleum (bp 40-60 °C)], or CaH2 (dichloromethane).
All NMR experiments were conducted in benzene-d6 at 20 °C unless
otherwise stated. Benzene-d6, toluene-d8, dichloromethane-d2, and
chloroform-d1 were degassed and dried over activated 4 Å molecular
sieves. NMR spectra were recorded on a Bruker DPX300 spec-
1
trometer. Chemical shifts for H NMR spectra were referenced to
residual solvent resonances and reported as parts per million relative
to tetramethylsilane. 19F and 11B NMR spectra are reported relative
to CFCl3 and Et2O‚BF3, respectively. B(C6F5)3 and Al(C6F5)3‚C7H8
were prepared according to the literature procedures.22,34 The amines
1b-h were purchased from Aldrich and dried over activated 4 Å
molecular sieves. The syntheses of compounds 1b, 1e, and 1f have
been reported elsewhere.14,15
Crystal Structure Analyses. Intensity data for the samples
examined were measured either at UEA on a Rigaku/MSC R-Axis-
(34) Lancaster, S. J. http://www.syntheticpages.org/pages/215.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5931

Mountford et al.
Chart 3
Table 5. Selected Crystal and Structure Refinement Data for the Boron Compoundsa
compd no.
formula
fw
1a
1a‚NH3
C18H3BF15N‚H3N C22H11BF15N
546.1 585.1
1b
1c
1d
C26H11BF15N‚CH2Cl2 C26H11BF15N
718.1 633.2
1g
1h
C18H3BF15N
529.0
C25H9BF15N
619.1
C32H15BF15N
709.3
color, habit
cryst syst
space group
a, Å
b, Å
c, Å
colorless, slab colorless, prism
colorless, shard colorless, plate colorless, prism
colorless, blade colorless, block
triclinic
monoclinic
P21/n (No. 14)
12.1321(10)
12.6612(13)
13.5485(10)
90
114.066(6)
90
1900.2(3)
4
triclinic
monoclinic
C2/c (No. 15)
22.507(5)
8.9216(18)
22.773(5)
90
98.91(3)
90
4517.6(16)
8
monoclinic
P21/n (No. 14)
10.736(2)
8.7560(18)
29.330(6)
90
96.60(3)
90
2738.9(10)
4
monoclinic
P21/n (No. 14)
9.5496(12)
20.022(3)
12.4075(12)
90
monoclinic
C2/c (No. 15)
22.8044(4)
13.2282(3)
23.0749(5)
90
P1h (No. 2)
8.3329(5)
12.9478(8)
16.3878(7)
86.481(5)
85.386(4)
89.509(5)
1759.06(17)
4
P1h (No. 2)
10.92740(10)
12.0071(2)
17.2015(3)
83.0820(10)
79.1170(10)
77.6070(10)
2157.18(6)
4
R, deg
â, deg
94.894(7)
90
116.9070(10)
90
γ, deg
V (Å3)
2363.6(5)
4
6207.2(2)
8
Z
d, calcd (g cm-3
)
)
1.998
1.909
1.802
1.821
1.741
1.779
1.518
abs coeff (mm-1
0.229
0.217
0.196
0.193
0.361
0.187
0.152
no. of unique reflns, 8031, 0.030
4360, 0.044
9839, 0.077
5160, 0.043
4288, 0.079
5118, 0.251
7081, 0.049
Rint
no. of obsd reflns
(I > 2σI)
R1 (obsd reflns)
wR2 (all reflns)
6536
2162
7609
4468
3213
2270
5106
0.035
0.092
0.050
0.150
0.045
0.114
0.037
0.097
0.075
0.236
0.100
0.194
0.046
0.134
a Data were obtained at 120 K for all but 1d (140 K).
Table 6. Selected Crystal and Structure Refinement Data for the Aluminum Compoundsa
compd no.
formula
fw
2b
2c
2d
2e
2f
2g
C22H11AlF15N
601.3
C25H9AlF15N
635.3
C26H11AlF15N
649.3
C20H7AlF15N
573.3
C23H11AlF15N
613.3
C26H11AlF15N
649.3
color, habit
cryst syst
space group
a, Å
b, Å
c, Å
colorless, prism
monoclinic
C2/c (No. 15)
18.451(4)
12.842(3)
19.911(4)
90
102.43(3)
90
4607.3(16)
8
colorless, block
triclinic
P1h (No. 2)
13.724(3)
13.825(3)
15.012(3)
65.61(3)
85.10(3)
83.27(3)
2574.2(9)
4
colorless, slab
triclinic
P1h (No. 2)
7.9878(3)
11.9032(5)
12.8855(4)
87.946(2)
85.528(2)
87.136(2)
1219.27(8)
2
colorless, block
monoclinic
P21 (No. 4)
15.6465(5)
13.1990(4)
20.3427(7)
90
94.5110(10)
90
4188.1(2)
8
colorless, prism
monoclinic
P21/c (No. 14)
21.346(4)
10.968(2)
22.090(4)
90
108.95(3)
90
4891.6(17)
8
colorless, shard
monoclinic
P21/c (No. 14)
21.3404(8)
12.7062(5)
20.2308(7)
90
111.726(2)
90
5096.0(3)
8
R, deg
â, deg
γ, deg
V (Å3)
Z
d, calcd (g cm-3
)
)
1.734
0.222
1.639
0.204
1.769
0.218
1.818
0.240
1.666
0.211
1.693
0.208
abs coeff (mm-1
no. of unique reflns, Rint
no. of obsd reflns (I > 2σI)
R1 (obsd reflns)
4213, 0.069
3514
0.042
11757, 0.031
9259
0.045
5544, 0.055
3831
0.052
13572, 0.048
11341
0.044
8691, 0.057
7053
0.037
11624, 0.097
5655
0.052
wR2 (all reflns)
0.117
0.136
0.128
0.109
0.101
0.118
a Data were obtained at 120 K for all but 2b and 2f (140 K).
IIc image-plate diffractometer equipped with a rotating-anode X-ray
source or by the EPSRC Crystallography Service at the Uni-
versity of Southampton on a Nonius Kappa CCD diffractometer.
Both systems used monochromated Mo KR radiation. Crystal data
for the boron compounds are collated in Table 5 and those for the
aluminum compounds in Table 6. The procedures of the analyses
were very similar, and that for compound 1d is described here.
From a sample of clear, colorless prisms of 1d under oil, one,
ca. 0.8 × 0.15 × 0.10 mm, was mounted on a glass fiber and fixed
in the cold nitrogen stream on the Rigaku R-Axis-IIc image-plate
diffractometer. The total number of reflections recorded, to θmax
25.4°, was 12 120, of which 4288 were unique (Rint ) 0.079); 3213
were “observed” with I > 2σ(I).
Data were processed using the DENZO/SCALEPACK pro-
grams.35 The structure was determined by the direct method routines
in the SHELXS program and refined by full-matrix least-squares
)
(35) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
5932 Inorganic Chemistry, Vol. 44, No. 16, 2005

Amine Adducts of Tris(pentafluorophenyl)borane and -alane
methods, on F2’s, in SHELXL.36 The non-hydrogen atoms were
refined with anisotropic thermal parameters. Hydrogen atoms were
included in idealized positions, and their Uiso values were set to
ride on the Ueq values of the parent carbon atoms.
BF15N: C, 40.87 (40.83); H, 0.57 (0.52); N, 2.65 (3.05). 1H NMR:
δ 2.67 (br, 3H, NH3). 11B NMR: δ -6.9. 19F NMR: δ -135.3 (d,
6F, JFF ) 22.6 Hz, o-F), -155.5 (t, 3F, JFF ) 19.8 Hz, p-F), -162.9
(m, 6F, m-F). IR: ν(N-H) 3372.5, 3362.0, and 3295.8 cm-1
.
In the final difference map, the highest peaks (to ca. 0.67 e Å-3
were close to the solvent (CH2Cl2) molecule.
)
H2(CH2C6H5)N‚B(C6F5)3 (1c). To a solution of B(C6F5)3 (1.0
g, 2.0 mmol) in toluene (10 mL) was added H2NCH2Ph (0.21 g,
2.0 mmol). The solvent was then removed under reduced pressure,
affording a colorless solid that gave single crystals suitable for X-ray
crystallography by the slow diffusion of light petroleum through a
dichloromethane solution of the product (0.99 g, 1.6 mmol, 80%).
Anal. Calcd (found) for C25H9BF15N: C, 48.50 (48.11); H, 1.47
(1.47); N, 2.26 (2.05). 1H NMR: δ 7.00-6.60 (m, 5H, C6H5), 4.42
(br, 2H, NH2), 3.05 (m, 2H, CH2). 13C NMR: δ 133.8, 129.9 (C6H5),
Scattering factors for neutral atoms were taken from ref 37.
Computer programs used in this analysis have been noted above,
in Table 4 of ref 38, or in ref 39 and were run on a Silicon Graphics
Indy at the University of East Anglia or a DEC-AlphaStation 200
4/100 in the Biological Chemistry Department, John Innes Centre.
H3N‚B(C6F5)3‚NH3 (1a‚H3N). NH3(g) was bubbled through a
solution of B(C6F5)3 (0.63 g, 1.2 mmol) in light petroleum (40 mL)
for 5 min at room temperature. The resulting precipitate was
separated by filtration and subsequently recrystallized from a
dichloromethane/light petroleum mixture to give colorless needle-
shaped crystals, which were confirmed as 1a‚NH3 by elemental
analysis and X-ray crystallography (0.63 g, 1.2 mmol, 94%). Anal.
Calcd (found) for C18H6BF15N2: C, 39.59 (39.63); H, 1.11 (1.16);
48.9 (CH2). 11B NMR: δ -4.2. 19F NMR: δ -134.8 (d, 6F, JFF
)
22.6 Hz, o-F), -155.3 (t, 3F, JFF ) 20.8 Hz, p-F), -162.4 (m, 6F,
m-F). IR: ν(N-H) 3337.3 and 3281.8 cm-1
.
t
H2 BuN‚Al(C6F5)3 (2b). To a solution of Al(C6F5)3‚C7H8 (1.74
g, 2.8 mmol) in toluene (20 mL) was added H2NtBu (0.20 g, 2.8
mmol) at room temperature. After a few minutes, the solvent was
removed under vacuum and the resultant solid was recrystallized
from a light petroleum/dichloromethane mixture to give 2b as
X-ray-quality colorless crystals (1.30 g, 2.2 mmol, 77%). Anal.
Calcd (found) for C22H11AlF15N: C, 43.95 (43.73); H, 1.84 (1.81);
N, 2.33 (2.28). 1H NMR: δ 3.92 (s, 2H, NH2), 1.38 (s, 9H,
C(CH3)3). 13C NMR: δ 149.8, 141.5, 136.9 (C6F5), 55.4 (C(CH3)3),
30.1 (C(CH3)3). 19F NMR: δ -160.7 (m, 6F, o-F), -122.4 (m,
3F, p-F), -151.9 (m, 6F, m-F). IR: ν(N-H) 3299.1 and 3253.6
1
N, 5.13 (4.32). H NMR: δ 3.71 (br, 3H, NH3), -0.57 (s, 3H,
NH3). 11B NMR: δ -7.1. 19F NMR: δ -135.3 (d, 6F, JFF ) 22.6
Hz, o-F), -155.8 (t, 3F, JFF ) 19.8 Hz, p-F), -163.1 (m, 6F, m-F).
IR: ν(N-H) 3395.5, 3372.5, 3361.8, 3330.2, and 3295.6 cm-1
.
H3N‚B(C6F5)3 (1a). 1a‚NH3 (0.67 g, 1.2 mmol) was dissolved
in ca. 5 mL of toluene before the product was taken to dryness
under vacuum. The colorless solid was recrystallized from a light
petroleum/dichloromethane mixture cooled to -25°C, giving block-
shaped crystals from which the solid-state structure could be
determined (0.61 g, 1.2 mmol, 94%). Anal. Calcd (found) for C18H3-
cm-1
.
Acknowledgment. We are grateful to the Engineering
and Physical Sciences Research Council for a studentship
(A.J.M.) and access to the National Crystallography Service
at the University of Southampton.
(36) Sheldrick, G. M. SHELX-97sProgram for crystal structure determi-
nation (SHELXS) and refinement (SHELXL); University of Go¨ttingen,
Go¨ttingen, Germany, 1997. At the conclusion of the refinement, wR2
) 0.236 and R1 ) 0.092 for all 4288 reflections weighted w ) [σ2-
2
2
2
(Fo ) + (0.123P)2 + 6.82P]-1 with P ) (Fo + 2Fc )/3; for the
“observed” data only, R1 ) 0.075.
Supporting Information Available: CIF data and figures
illustrating the solid-state structures of 1e, 1f, and 2c as well as
synthetic procedures and spectroscopic data for compounds 1d, 1f-
h, and 2c-g. This material is available free of charge via the
Internet at http://pubs.acs.org.
(37) International Tables for X-ray Crystallography; Kluwer Academic
Publishers: Dordrecht, The Netherlands, 1992; Vol. C, pp 193, 219,
and 500.
(38) Anderson, S. N.; Richards, R. L.; Hughes, D. L. J. Chem. Soc., Dalton
Trans. 1986, 245.
(39) Sheldrick, G. M. SHELXTL package, including XS (for structure
determination), XL (structure refinement) and XP (molecular graph-
ics); Siemens Analytical Inc.: Madison, WI, 1995.
IC050663N
Inorganic Chemistry, Vol. 44, No. 16, 2005 5933