Synthesis, structure and coordination of the ambiphilic ligand (2-picolyl)BCy2

Article abstract of DOI:10.1039/b704993p

The new pyridine-borane compound (2-picolyl)BCy₂, readily prepared from 2-picolyllithium and ClBCy₂, adopts a head-to-tail dimeric structure in the solid state as indicated by X-ray diffraction analysis and according to NMR and DFT studies, the dimeric form equilibrates in solution with a strained monomeric structure; the ambiphilic behavior of the new compound is illustrated by its bridging coordination to the (p-cymene)RuCl₂ unit. The Royal Society of Chemistry.

Full text of DOI:10.1039/b704993p

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Synthesis, structure and coordination of the ambiphilic ligand
(2-picolyl)BCy2†‡
Je´roˆme Vergnaud,a,b Tahra Ayed,c Khansaa Hussein,c,d Laure Vendier,a Mary Grellier,a Ghenwa Bouhadir,b
Jean-Claude Barthelat,c Sylviane Sabo-Etienne*a and Didier Bourissou*b
Received 2nd April 2007, Accepted 23rd April 2007
First published as an Advance Article on the web 9th May 2007
DOI: 10.1039/b704993p
The new pyridine–borane compound (2-picolyl)BCy2, readily
prepared from 2-picolyllithium and ClBCy2, adopts a head-
to-tail dimeric structure in the solid state as indicated by X-ray
diffraction analysis and according to NMR and DFT studies,
the dimeric form equilibrates in solution with a strained
monomeric structure; the ambiphilic behavior of the new
compound is illustrated by its bridging coordination to the
(p-cymene)RuCl2 unit.
hexane (eqn (1)).‡ After workup, a colorless solid was isolated
in 60% yield. Single crystals of 1 were obtained from a saturated
CH2Cl2 solution kept for several days at 4◦ C. The X-ray crystal
structure analysis§ shows a dimeric structure resulting from the
head-to-tail association of two (2-picolyl)BCy2 units (Fig. 1).‡
The eight-membered ring core adopts a bent shape conformation
˚
˚
and the two B–N bond distances [1.664(3) A and 1.659(3) A] fall
within the range reported previously for dimeric compounds.10c,12
(2-picolyl)Li + CIBCy2 → 1/2[(2-picolyl)BCy2]2
(1)
Polyfunctional derivatives featuring both donor and acceptor moi-
eties, so-called ambiphilic compounds, have attracted increasing
interest over the last few years. In particular, combinations of
amine and borane groups (NB systems) have been successfully
used as multi-center organocatalysts,1,2 non-linear (electr)optical
materials,3 and molecular sensors.4,5 Very recently, promising
new studies have also been initiated with related PAl and PB
systems6–8 and further developments will undoubtedly emerge in
the near future taking advantage of the almost infinite, as yet
underexplored, structural modularity of ambiphilic compounds.
In this context, most of the NB systems reported so far9 are
tertiary amines and are therefore not really suitable as ambiphilic
ligands for transition metals. This prompted us to investigate a
new NB system derived from 2-methylpyridine (2-picoline) and
here we report our preliminary results in this area. An X-ray
diffraction study of the new compound [(NC5H4)CH2BCy2]2 (1)
reveals a head-to-tail dimeric structure in the solid state while
NMR investigations combined with DFT calculations provide
evidence for a monomer/dimer equilibrium in solution.10 The
bifunctional behavior of 1 is illustrated by reaction with [(p-
cymene)RuCl2]2 affording a rare example of donor → M–Cl →
acceptor bridging interaction.6b,11
Fig. 1 ORTEP drawings for compound 1. Displacement ellipsoids are
drawn at 50% probability and hydrogen atoms are omitted for clarity.
Right side: cyclohexyl groups are omitted for clarity.
Surprisingly, multinuclear 1D and 2D NMR analyses indicated
the presence of two species when the crystals were dissolved in
CD2Cl2. At room temperature, the major species is assigned to
Compound 1 was obtained by mixing a pentane solution of (2-
picolyl)lithium with an equivalent of chlorodicyclohexylborane in
1
the dimeric structure 1 characterized by a 11B{ H} NMR signal
at 3.6 ppm consistent with a tetracoordinate boron atom. The
1H NMR resonance for the methylene groups consists of an AB
aLaboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne,
31077 Toulouse Cedex 04, France. E-mail: sabo@lcc-toulouse.fr; Fax: +33
5 6155 3003; Tel: +33 5 6133 3177
pattern (3.62 and 2.00 ppm, 2JHH = 13 Hz) and the 13C{ H} NMR
1
bLaboratoire He´te´rochimie Fondamentale et Applique´e du CNRS, (UMR
5069), Universite´ Paul Sabatier, 118, route de Narbonne, 31062 Toulouse
Cedex 09, France. E-mail: dbouriss@chimie.ups-tlse.fr; Fax: +33 5 6155
8204; Tel: +33 5 6155 7737
signal appears as a broad singlet at 36.5 ppm due to the proximity
of the boron atom. In contrast, the minor species displays a singlet
1H NMR resonance at 1.76 ppm for the CH2B group, associated
with a broad 13C signal at 18.5 ppm. A tentative formulation as a
monomeric form of the picolylborane 1 was supported by a DOSY
NMR experiment. Indeed, the hydrodynamic radius of the major
species 1 was found to be 1.5-fold larger than that of the minor
one. The 11B NMR chemical shift for the minor species (14.3 ppm)
was quite surprising and suggested a closed monomer structure as
depicted in Scheme 1.13
cLaboratoire de Chimie et Physique Quantique, IRSAMC/UMR 5626,
Universite´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex
04, France
dDepartment of Chemistry, Faculty of Sciences, Damascus University,
Damascus, Syria
† The HTML version of this article has been enhanced with colour images.
‡ Electronic supplementary information (ESI) available: Experimental,
spectroscopic and computational details. See DOI: 10.1039/b704993p
2370 | Dalton Trans., 2007, 2370–2372
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Scheme 1
To gain more insight into the structure of this minor species,
theoretical calculations were performed at both DFT (B3LYP,
B3PW91) and MP2 levels of theory.14 The optimised structure for
the dimer 1 closely matches that measured in the solid state by X-
ray diffraction. The calculated (GIAO) 11B NMR shift (3.1 ppm) is
in perfect agreement with the experimental value. Two monomeric
structures were found as local minima. The most stable one 2c
adopts a closed form as the result of an intramolecular N → B
Fig. 3 1H NMR spectra (Tol-d8) of 1/2 at various temperatures (aromatic
region).
the ruthenium complex [(p-cymene)RuCl2]2 was then investigated
(Scheme 2). After stirring both compounds in CH2Cl2 at room
temperature for 50 min, the new complex [(p-cymene)RuCl2-
{(NC5H4)CH2BCy2}] 3 was isolated in 46% yield.
˚
interaction characterized by a short NB distance (1.703 A) and a
noticeable pyramidalization of the boron (RBa 349.4◦). The other
minimum corresponds to the related open monomer 2o featuring a
˚
long B · · · N distance (2.672 A) and a trigonal planar environment
around the boron (RBa 359.4◦) (Fig. 2). Despite the ring strain
associated with the four-membered ring, the closed form 2c is
predicted to be significantly more stable than the open one 2o
(by 31.3 kJ mol−1 at the MP2 level). This strongly supports the
assignment of the minor species spectroscopically characterized
in solution as the closed monomeric structure, which is further
corroborated by comparing the experimental 11B NMR chemical
shift of 2 (14.3 ppm) with those calculated for 2c (9.2 ppm)
and 2o (83.1 ppm). Such a strained four-membered ring does
find precedent in the two structurally characterized aminopyridyl
Scheme 2
Single crystals of 3 were obtained from a CH2Cl2/pentane
mixture at 4 ◦C. Despite the modest quality of the data [R1
(I > 2r(I)) = 0.0665], the X-ray analysis indicated the bridging
coordination of the ambiphilic ligand to the (p-cymene)RuCl2
moiety via N → Ru–Cl → B interactions (Fig. 4). The Cl →
15a
compounds [6-(Cl2BN)-2-Me-C5H3N]BCl2 and [6-(Me3SiN)-2-
Me–C5H3N]BBr215b featuring more electrophilic and less sterically
hindered borane groups.
˚
B interaction observed in the solid state [B–Cl 2.103(9) A] is also
present in solution, as deduced from the 11B{ H} NMR signal
1
at 22.2 ppm in CD2Cl2. Notably, the p-cymene ring of 3 appears
desymmetrized in both the 1H and 13C NMR spectra (four signals
are observed for the CHarom groups), suggesting that the boron
atom does not shift rapidly from one chlorine to the other, at
least at the NMR timescale. The ambiphilic character of the new
NB ligand was further substantiated by DFT calculations carried
out on the real complex. Notably, the RuCl bond distance is only
Fig. 2 MP2-optimized structures of the closed and open monomeric
forms 2c and 2o.
Variable temperature (277–353 K, Tol-d8) 1H NMR studies were
then carried out to determine the thermodynamic parameters
for the monomer/dimer equilibrium (Fig. 3). As expected, the
proportion of the monomeric form 2 increases with temperature,
from ca. 8% at 277 K to ca. 90% at 353 K.16 The van’t Hoff plot
gives an endothermic DH = 79.2 2.3 kJ mol−1, and the DG =
f (T) plot leads to an entropy value of DS = 218 137 J K−1 mol−1
(with a 99% confidence).
In order to assess whether or not the presence of N → B
interactions in both the monomeric and dimeric forms of the
picolylborane would prevent ambiphilic behavior, reaction with
Fig. 4 ORTEP drawing for compound 3 at the 50% probability level.
Hydrogen atoms are omitted for clarity.
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˚
6 For versatile coordination properties towards transition metals in-
cluding unusual M → B interactions, see: (a) F.-G. Fontaine and D.
Zargarian, J. Am. Chem. Soc., 2004, 126, 8786; (b) S. Bontemps, H.
Gornitzka, G. Bouhadir, K. Miqueu and D. Bourissou, Angew. Chem.,
Int. Ed., 2006, 45, 1611; (c) S. Bontemps, G. Bouhadir, K. Miqueu and
D. Bourissou, J. Am. Chem. Soc., 2006, 128, 12056; (d) D. J. H. Emslie,
J. M. Blackwell, J. F. Britten and L. E. Harrington, Organometallics,
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Vargas-Baca, C. M. Robertson, L. E. Harrington and J. F. Britten,
Organometallics, 2006, 25, 5835.
7 For reversible hydrogen activation, see: G. C. Welch, R. R. San Juan,
J. D. Masuda and D. W. Stephan, Science, 2006, 314, 1124.
8 For photoisomerization heterodienes, see: M. W. P. Bebbington, S.
Bontemps, G. Bouhadir and D. Bourissou, Angew. Chem., Int. Ed.,
2007, 46, 3333.
9 For recently investigated amine boranes featuring phenyl, benzyl or
naphthyl spacers, see: (a) R. Roesler, W. E. Piers and M. Parvez,
J. Organomet. Chem., 2003, 680, 218; (b) R. L. Giles, J. A. K.
Howard, L. G. F. Patrick, M. R. Probert, G. E. Smith and A. Whiting,
J. Organomet. Chem., 2003, 680, 257; (c) S. W. Coghlan, R. L. Giles,
J. A. K. Howard, L. G. F. Patrick, M. R. Probert, G. E. Smith and A.
Whiting, J. Organomet. Chem., 2005, 690, 4784.
10 2-Pyridylboranes were reported to adopt dimeric structures both in the
solid state and in solution, see: (a) T. G. Hodgkins, Inorg. Chem., 1993,
32, 6115; (b) T. G. Hodgkins and D. R. Powell, Inorg. Chem., 1996,
35, 2140; (c) T. Murafuji, R. Mouri, Y. Sugihara, K. Takakura, Y.
Mikata and S. Yano, Tetrahedron, 1996, 52, 13933; (d) F. Garc´ıa, A. D.
Hopkins, R. A. Kowenicki, M. McPartlin, J. S. Silvia, J. M. Rawson,
M. C. Rogers and D. S. Wright, Chem. Commun., 2007, 586.
11 (a) S. J. Lancaster, S. Al-Benna, M. Thornton-Pett and M. Bochmann,
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Liebigs Ann. Chem., 1993, 189.
marginally elongated (2.446 vs. 2.396 A) by the Cl → B interaction
17
˚
(2.123 A).
In conclusion, both the monomeric and dimeric forms of (2-
picolyl)BCy2 have been evidenced. Despite the presence of N →
B interactions in both forms, the ability of this new NB system to
behave as an ambiphilic ligand was demonstrated. Its coordination
to the (p-cymene)RuCl2 fragment indeed afforded a rare example
of donor → M–Cl → acceptor bridging interaction.6c Structural
variations of the pyridine–borane framework are currently under
investigation and future research will seek particularly to deter-
mine the significance of such an ambiphilic character in transition-
metal catalysis.
Acknowledgements
We are grateful to the CNRS and UPS for financial support of this
work. The CINES (Montpellier, France) is warmly acknowledged
for a generous allocation of computer time. J. V. thanks the
French Ministry “Education Nationale, Enseignement Supe´rieur,
Recherche” for his PhD grant.
Notes and references
§ X-Ray crystal data for 1: Mw = 538.45, monoclinic space group P-21n,
˚
˚
colourless blocks, Mo-Kak = 0.71073 A, a = 14.2973(8) A, b = 10.1773(5)
◦
3
˚
˚
˚
A, c = 21.9682(11) A, b = 97.2812(44) , V = 3170.8(3) A , Z = 4, T =
100 K. X-Ray crystal data for 3: Mw = 575.41, triclinic space group P-1,
˚
yellow platelets, Mo-Ka k = 0.71073 A, a = 12.45◦40(11), b = 13.3645(16)
◦
◦
˚
A, c = 17.766(2), a = 86.727(10) , b = 71.861(9) , c = 86.425(9) , V =
3
˚
2802.4(5) A , Z = 4, T = 180 K. CCDC reference numbers 635560 (3) &
635561 (1). For crystallographic data in CIF or other electronic format see
13 An open monomer species should display a d 11B value higher than
50 ppm as reported for trivalent boron alkyl compounds. See: Nuclear
Magnetic Resonance Spectroscopy of boron compounds, ed. H. No¨th
and B. Wrackmeyer, Springer-Verlag, 1978.
14 DFT functionals were observed to strongly influence the geometric and
energetic predictions associated with N → B interactions, see: T. M.
Gilbert, J. Phys. Chem. A, 2004, 108, 2550.
15 (a) L. M. Engelhardt, G. E. Jacobsen, P. C. Junk, C. L. Raston and
A. H. White, J. Chem. Soc., Chem. Commun., 1990, 89; (b) S. Aldridge,
R. J. Calder, D. L. Coombs, C. Jones, J. W. Steed, S. Coles and M. B.
Hursthouse, New J. Chem., 2002, 26, 677.
DOI: 10.1039/b704993p
1 A cooperative mechanism was proposed early on for the reduction of
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and C. J. Helal, Angew. Chem., Int. Ed., 1998, 37, 1986.
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(b) H. Gro¨ger, Chem.–Eur. J., 2001, 7, 5247; (c) M. Shibasaki, M. Kanai
and K. Funabashi, Chem. Commun., 2002, 1989; (d) J.-A. Ma and D.
Cahard, Angew. Chem., Int. Ed., 2004, 43, 4566.
3 C. D. Entwistle and T. B. Marder, Chem. Mater., 2004, 16, 4574 and
references therein.
4 For chemosensing of saccharides, a-hydroxycarboxylates and vicinal
diols with boronic acids featuring a pendant amino group, see: (a) T. D.
James, K. R. A. S. Sandanayake and S. Shinkai, Angew. Chem., Int. Ed.
Engl., 1996, 35, 1910; (b) L. Zhu, Z. Zhong and E. V. Anslyn, J. Am.
Chem. Soc., 2005, 127, 4260.
5 For ferrocenyl-based amine-boranes as electrochemical probes for
hydrogen fluoride, see: C. Bresner, S. Aldridge, I. A. Fallis, C. Jones
and L.-L. Ooi, Angew. Chem., Int. Ed., 2005, 44, 3606.
16 The ratio between 1 and 2 is 82/18 at 296.4 K in C7D8 for a 0.053
0.001 mol L−1 overall concentration of the monomeric compound 2.
This ratio is determined by the relative integrations of the pyridinic
proton signals at 8.25 ppm for 1, and 7.41 ppm for 2. The equilibrium
constant for the reaction 1 to 2 is K = 2.44 × 10−3
.
17 A similar situation has been encountered in the complex (Tp)Os-
(NPhBPh2)Cl2,11b while a significant M–Cl elongation has been re-
ported for (Ind)[CpB(C6F5)2]TiCl2.11a
.
2372 | Dalton Trans., 2007, 2370–2372
This journal is
The Royal Society of Chemistry 2007
©