Intra- and inter-molecular C-H...F-C and N-H...F-C hydrogen bonding in secondary amine adducts of B(C6F5) 3: Relevance to key interactions in alkene polymerisation catalysis

Article abstract of DOI:10.1039/b305613a

The reactions between the cyclic sec. amines pyrrolidine and piperidine with B(C₆F₅)₃ yield Lewis acid-base adducts with both intra- and inter-molecular hydrogen bonding interactions between C-H and N-H groups and aryl-flu

Full text of DOI:10.1039/b305613a

Intra- and inter-molecular C–H…F–C and N–H…F–C hydrogen
bonding in secondary amine adducts of B(C6F5)3: relevance to key
interactions in alkene polymerisation catalysis
Andrew J. Mountford, David L. Hughes and Simon J. Lancaster*
Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy, University of East
Anglia, Norwich, UK. E-mail: S.Lancaster@uea.ac.uk; Fax: 44 1603 592009; Tel: 44 1603 592009
Received (in Cambridge, UK) 21st May 2003, Accepted 3rd July 2003
First published as an Advance Article on the web 15th July 2003
The reactions between the cyclic sec. amines pyrrolidine and
piperidine with B(C6F5)3 yield Lewis acid-base adducts with
both intra- and inter-molecular hydrogen bonding inter-
actions between C–H and N–H groups and aryl-fluorines in
the solid state.
Perfluoroaryl boranes and borates have attracted considerable
interest in the field of 1-alkene polymerisation catalysis.1 In this
context we recently reported the synthesis of
[NH2{B(C6F5)3}2]2 from NH2
and 2 equivalents of
2
B(C6F5)3.2 The stability of this anion appears to be enhanced by
a number of N–H…F–C hydrogen bonding interactions. In the
solid state these hydrogen bonds vary in length between 1.90(2)
and 2.42(2) Å. Hydrogen bonding to covalently bonded organic
fluorine is regarded as rare but is of significance in crystal
engineering and enzyme substrate recognition and has been the
subject of detailed studies.3 O–H…F–C hydrogen bonding is
found
in
(C6F5)3B(OH2)
and
the
anion
[(C6F5)3B(OH)B(C6F5)3]2.4 Very recently intra- and inter-
molecular O–H…F–C hydrogen bonding interactions have
been identified in the H2O and MeOH adducts of Al(C6F5)3.5
C–H…F–C interactions and their designation as hydrogen
bonds have until recently been the subject of particular
controversy but are believed to play a role in determining the
crystal structure of fluorobenzenes and fluorine substituted
ribonucleic acids.3b,d,6 They have also been invoked to explain
the behaviour of fluorine substituted polymerisation catalysts.7
Dunitz and Taylor have laid down criteria for designation as
hydrogen bonds, i.e. the H…F–C interaction should be
significantly shorter than the sum of the van der Waals radii (ca.
2.55 Å) and preferably no longer than ca 2.2–2.3 Å, with obtuse
X–H–F angles.3a,8,9
In the course of studies designed to gauge the extent of
hydrogen bonding in B(C6F5)3 adducts we prepared the sec.
amine adducts (cyclo-C4H8NH)B(C6F5)3 (1) and (cyclo-
C5H10NH)B(C6F5)3 (2).† The molecular structures‡ of 1 (Fig.1)
and 2 have a number of similarities: in both cases the amino-
hydrogen is ca. 2.1 Å from two o-F atoms and is involved in a
bifurcated, intramolecular F…H…F hydrogen bonding system
(Table 1).
Evidence that the N–H…F–C interactions in 1 and 2 are
maintained in solution is provided by VT 19F NMR spectros-
copy. At room temperature the 19F NMR spectra of 1 and 2 are
complex, with highfield resonances for the hydrogen bonded
o-F atoms. The o-F resonances coalesce only on warming to
80 °C. This cannot be dismissed as hindered rotation about the
B–N and/or B–C bonds on steric grounds alone since (Et2-
MeN)B(C6F5)3, which is of greater steric bulk but cannot
participate in N–H…F–C hydrogen bonding, gives no indica-
tion in the 19F NMR spectrum for hindered rotation down to
260 °C.10
Fig. 1 Molecular structure of 1, showing intramolecular N–H…F–C and
C–H…F–C interactions.
Table 1 Hydrogen atom contacts (Å and °); esds are in parentheses
D–H…A
D…A
H…A
D–H…A
Structure 1:
Intramolecular contacts:
N(4)–H(4)…F(12)
N(4)–H(4)…F(22)
C(41)–H(41b)…F(32)
Intermolecular contacts:
C(41)–H(41a)…F(16A)
C(42)–H(42b)…F(13B)
2.736(2)
2.766(2)
3.085(2)
2.15
2.18
2.20
122
122
151
3.181(2)
3.351(2)
2.43
2.48
134
149
Structure 2:
Intramolecular contacts:
N(4)–H(4)…F(12)
N(4)–H(4)…F(26)
N(8)–H(8)…F(52)
N(8)–H(8)…F(66)
Intermolecular contact:
C(85)–H(85a)…F(25Ú)
2.747(2)
2.733(2)
2.758(2)
2.745(2)
2.10
2.10
2.11
2.14
127
126
128
123
3.175(3)
2.34
143
1
1
1
1
Symmetry ops.: A : x 2 2, 12 2 y, z 2 2; B : 1 2 x, 2 2 y, 2z; Ú : x 2 2, 1
1
2 y, 2 + z.
Unexpectedly, the crystal structure of 1 also shows a close
contact between an a-hydrogen of the cyclic amine and an aryl
o-F atom on the third C6F5 group. This intramolecular
interaction (Chart 1) appears to be more than a weak van der
Waals interaction brought about by the proximity of the C–H
and F–C groups after complex formation. The large C–H…F
angle (151°) and short H…F distance (2.20 Å) places it within
Chart 1
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CHEM. COMMUN., 2003, 2148–2149
This journal is © The Royal Society of Chemistry 2003

(Cyclo-C5H10NH)B(C6F5)3 2 was prepared from B(C6F5)3 (1.60 g, 3.1
mmol) and HNC5H10 (0.31 cm3, 3.1 mmol) following the same procedure
as 1 (1.55 g, 84%). Anal. found: C, 45.74; H, 1.99; N, 2.22%. Calcd. for
C23H11BF15N: C, 46.26; H, 1.86; N, 2.35 %. dH (300 MHz, C6D6, 20 °C)
5.24 (br. m, 1H, NH), 2.94 (d, 4H, J 12.5 Hz, CH2), 1.76 (q, 4H, J 11.7 Hz,
CH2), 1.07 (m, 2H, J 12.5 Hz, CH2); dC (75.47 MHz, C6D6, 20 °C) 50.2,
25.5, 22.5; dB (96.29 MHz, C6D6, 20 °C) 22.1; dF (282.40 MHz, C6D6, 20
°C) 2127.69 (s, 2F, o-F), 2127.76 (s, 2F, o-F), 2142.91 (s, 2F, o-F),
2154.22 (t, 1F, J 19.8 Hz, p-F), 2156.39 (t, 2F, J 22.6 Hz, p-F), 2161.08
(s, 2F, m-F), 2162.72 (m, 2F, m-F), 2163.02 (m, 2F, m-F).
‡
Crystal data for 1: C22H9BF15N, M = 583.1, monoclinic, space group
P21/n (equiv. to no. 14), a = 14.232(1), b = 11.510(1), c = 14.543(1) Å,
b = 117.22(1)°, V = 2118.5(3) Å3, Z = 4, Dc = 1.828 Mg m23, F(000) =
1152, T = 140(1) K, m(MoKa) = 0.200 mm21, l(MoKa) = 0.71073 Å,
12139 reflections measured, 3616 unique (Rint = 0.060), F2 refinement, R1
= 0.033 [I > 2s(I)], wR2 = 0.095 (all data).
Fig. 2 Structure of 2 indicating intramolecular N–H…F–C and inter-
molecular C–H…F–C interactions.
Crystal data for 2: C23H11BF15N, M = 597.1, monoclinic, space group
Pn (equiv. to no. 7), a = 10.186(2), b = 13.026(3), c = 17.501(4) Å, b =
96.63(3)°, V = 2306.5(8) Å3, Z = 4, Dc = 1.720 Mg m23, F(000) = 1184,
T = 140(1) K, m(MoKa) = 0.186 mm21, l(MoKa) = 0.71073 Å, 12844
the Dunitz criteria for X–H…F–C hydrogen bonds. It is
significantly shorter than any previously reported intra- or inter-
molecular C–H…F–C bond.6
There are also weak intermolecular C–H…F–C interactions
(Table 1) linking molecules of 1 in planar nets. By contrast,
2 does not adopt an extended supramolecular structure. There
are two independent molecules in the crystal, linked by a
C–H…F–C interaction (Fig. 2).
Intramolecular C–H…F–C interactions, similar to that ob-
served for 1, between the o-F atoms of fluorinated phenyl
groups and the b-H of the polymeryl chain (Structure I, Chart 1)
have recently been proposed as being responsible for decreasing
the likelihood of b-H transfer and causing the observed
differences in molecular weight between polyalkenes prepared
with phenyl and perfluorophenyl substituted phenoxy-imine
catalysts. The F–Hb distance predicted by the DFT calculations
for this interaction was 2.28 Å.7 However; there was little
structural precedence for bonding between a C–H group and the
o-F of a C6F5 ring. Chan et al. have very recently reported a
model complex with an intramolecular interaction between a
methylene hydrogen atom on a zirconium bonded benzyl group
and a (sp3)-CF3 ligand substituent (H…F 2.47 Å and C–H…F
114°).11 1 and 2 exhibit intra- and inter-molecular hydrogen
bonding to a (sp2)-C6F5 group with structural parameters
suggesting a significantly stronger interaction, comparable to
that found in the DFT studies.
reflections measured, 7190 unique (Rint = 0.042), F2 refinement, R1
=
0.031 [I > 2s(I)], wR2 = 0.075 (all data). CCDC 211286 and 211287. See
http://www.rsc.org/suppdata/cc/b3/b305613a/ for crystallographic data in
.cif or other electronic format.
1 E. Y-X. Chen and T. J. Marks, Chem. Rev., 2000, 100, 1391.
2 S. J. Lancaster, A. Rodriguez, A. Lara-Sanchez, M. D. Hannant, D. A.
Walker, D. L. Hughes and M. Bochmann, Organometallics, 2002, 21,
451.
3 (a) J. D. Dunitz and R. Taylor, Chem. Eur. J., 1997, 3, 89; (b) V. R.
Thalladi, H. C. Weiss, D. Bläser, R. Boese, A. Nangia and G. R.
Desiraju, J. Am. Chem. Soc., 1998, 120, 8702; (c) G. R. Desiraju, Acc.
Chem. Res., 2002, 35, 565; (d) T. J. Barbarich, C. D. Rithner, S. M.
Miller, O. P. Anderson and S. H. Strauss, J. Am. Chem. Soc., 1999, 121,
4280.
4 (a) A. A. Danopoulos, J. R. Galsworthy, M. L. H. Green, S. Cafferkey,
L. H. Doerrer and M. B. Hursthouse, Chem. Commun., 1998, 2529; (b)
L. H. Doerrer and M. L. H. Green, J. Chem. Soc., Dalton Trans., 1999,
4325; (c) M. J. Drewitt, M. Niedermann and M. C. Baird, Inorg. Chim.
Acta, 2002, 340, 207.
5 D. Chakraborty and E. Y-X. Chen, Organometallics, 2003, 22, 207.
6 The shortest C–H…F–C distance we are aware of was reported for an
intermolecular interaction in 4-fluoroethynylbenzene (2.26 Å): H-C.
Weiss, R. Boese, H. L. Smith and M. M. Haley, Chem. Commun., 1997,
2403; J. Parsch and J. W. Engels, J. Am. Chem. Soc., 2002, 124,
5664.
7 M. Mitani, J-i. Mohri, Y. Yoshida, J. Saito, S. Ishii, K. Tsuru, S. Matsui,
R. Furuyama, T. Nakano, H. Tanaka, S-i. Kojoh, T. Matsugi, N.
Kashiwa and T. Fujita, J. Am. Chem. Soc., 2002, 124, 3327; M. Mitani,
R. Furuyama, J-i. Mohri, J. Saito, S. Ishii, H. Terao, T. Nakano, H.
Tanaka and T. Fujita, J. Am. Chem. Soc., 2003, 125, 4293.
8 In addition to anion 1 a re-examination of the previously published
structures (Flu)B(C6F5)2·NH2CMe3 (R. Duchateau, S. J. Lancaster, M.
The electron-withdrawing properties of organofluorine sub-
stituents are employed widely, both in ligand design and in the
preparation of least coordinating anions.1,12 Our observations
and those of Fujita and Chan suggest that subtle interactions
between X–H groups and organofluorines may have a sig-
nificant influence on catalytic behaviour.
Thornton-Pett and M. Bochmann, Organometallics, 1997, 16, 4995)
5
and (CpB)CpTi(m-NHCMe3)Cl (CpB
=
h -C5H4B(C6F5)2) (S. J.
We are grateful to the Engineering and Physical Sciences
Research Council for a studentship (A. J. M).
Lancaster, S. Al-Benna, M. Thornton-Pett and M. Bochmann, Organo-
metallics, 2000, 19, 1599) identified hitherto unrecognised N–H…F–C
interactions. For further details see reference 9.
Notes and references
9 S. J. Lancaster, A. J. Mountford, D. L. Hughes, M. Schormann and M.
Bochmann, J. Organomet. Chem., 2003, DOI: 10.1016/S0022-
328X(03)00346-2.
10 This adduct must be prepared at low temperature. If warmed to room
temperature significant generation of an iminium salt of [HB(C6F5)3]2
is observed. Related chemistry has been observed in aniline adducts: N.
Millot, C. C. Santini, B. Fenet and J. M. Basset, Eur. J. Inorg. Chem.,
2002, 3328.
11 S. C. F. Kui, N. Zhu and M. C. W. Chan, Angew. Chem., Int. Ed., 2003,
42, 1628.
12 J. Zhou, S. J. Lancaster, D. A. Walker, S. Beck, M. Thornton-Pett and
M. Bochmann, J. Am. Chem. Soc., 2001 , 123, 223.
†
Synthesis: (cyclo-C4H8NH)B(C6F5)3 (1). To a solution of B(C6F5)3
(1.46 g, 2.9 mmol) in toluene (40 cm3) HNC4H8 (0.24 cm3, 2.9 mmol) was
added. After stirring for 1 h the toluene was removed under reduced
pressure. The product was isolated as colourless crystals after recystallisa-
tion from a light petroleum–dichloromethane mixture (1.21 g, 72%). Anal.
found: C, 45.27; H, 1.54; N, 2.40%. Calcd. for C22H9BF15N: C, 45.32; H,
1.56; N, 2.40%. dH (300 MHz, C6D6, 20 °C) 5.80 (br. s, 1H, NH), 2.31 (dt,
4H, J 158.1 and 8.3 Hz, CH2), 0.84 (t, 4H, J 4.0 Hz, CH2); dC (75.47 MHz;
C6D6, 20 °C) 49.56, 23.08; dB (96.29 MHz, C6D6, 20 °C) 23.1; dF (282.40
MHz, C6D6, 20 °C) 2127.90 (br, 4F, o-F), 2143.37 (br, 2F, o-F), 2155.95
(br, 3F, p-F), 2162.81 (br, 6F, m-F).
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