Modelling fundamental arene-borane contacts: Spontaneous formation of a dibromoborenium cation driven by interaction between a borane Lewis acid and an arene π system

Article abstract of DOI:10.1039/c1cc15259a

Reaction of 2,6-dimesityl pyridine (L^py) with BBr₃ leads to the spontaneous formation of the trigonal dibromoborenium cation [L^py·BBr₂]⁺via bromide ejection. Systematic structural and computational st

Full text of DOI:10.1039/c1cc15259a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 12295–12297
www.rsc.org/chemcomm
COMMUNICATION
Modelling fundamental arene–borane contacts: spontaneous formation of
a dibromoborenium cation driven by interaction between a borane Lewis
acid and an arene p systemw
Hassanatu B. Mansaray, Alexander D. L. Rowe, Nicholas Phillips, Jochen Niemeyer,
Michael Kelly, David A. Addy, Joshua I. Bates and Simon Aldridge*
Received 24th August 2011, Accepted 29th September 2011
DOI: 10.1039/c1cc15259a
Reaction of 2,6-dimesityl pyridine (Lpy) with BBr3 leads to the
spontaneous formation of the trigonal dibromoborenium cation
[LpyꢀBBr2]+ via bromide ejection. Systematic structural and
computational studies, and the reactivity displayed by a closely
related N-heterocyclic carbene (NHC) donor, reveal the role
played by arene–borane interactions in this chemistry. [LpyꢀBBr2]+
features a structurally characterized (albeit weak) electrostatic
interaction between the borane Lewis acid and flanking arene p
systems.
substrates than their neutral counterparts (based not least on
electrostatic considerations),11 and although less common,12
have been exploited in their own right in the borylation of
arenes,13 and as potent Lewis acid catalysts in organic synthesis.14
Among such systems, dihaloborenium species, [LꢀBhal2]+, offer
particularly potent Lewis acid behaviour, but their potential for
substrate binding has yet to be probed structurally; overwhelming
electron deficiency and the lability of the B–hal bonds mean that
reactivity studies have typically been carried out in situ.15 By
contrast, in the current study we report a straightforward one-
step procedure for the spontaneous formation of [LꢀBBr2]+ from
BBr3 using a sterically loaded pyridine donor, together with the
first crystallographic investigation of such a system. Moreover,
systematic structural studies, and the reactivity displayed by a
closely related NHC donor, reveal the role played by electrostatic
arene–borane interactions in driving this transformation.
The formation of arene p-complexes with potent electrophiles
represents a key step in the mechanistic pathway ultimately
leading to electrophilic aromatic substitution.1 Despite this,
and in contrast to the large number of complexes known with
transition metals,2 isolated arene adducts of p-block electro-
philes are largely confined to those featuring low-valent post-
transition elements.3 While Z6-binding tends to predominate
in such systems, a number of examples have been reported
{featuring cations such as Cl+, and [SiR3]+, or charge-neutral
heavier Group 13 Lewis acids, ER3 (E = Al, Ga or In)},4–7
which involve interaction with a single arene carbon, in a
fashion reminiscent of classical Wheland intermediates.1
The interactions of arenes with borane Lewis acids have
obvious relevance to aromatic borylation chemistry (e.g. by
BBr3),8 and at a more general level to Lewis acid catalysis of
electrophilic aromatic substitution.9 Despite this, unambiguous
structural verification of such interactions remains elusive,10 with
quantum chemical studies indicating that such binding events
are likely to be relatively weak (e.g. 18.8 ꢁ 11.2 kJ molꢂ1 for
BCl3/benzene) and to involve interaction of the boron centre
with a single arene carbon (at ca. 3.25 A for BCl3/benzene).9a,b
Cationic tri-coordinate boranes (e.g. borenium ions, [LꢀBX2]+)
offer the potential for stronger binding of Lewis basic
In contrast to its reactivity towards ECl3 (E = Al, Ga),
which generates the straightforward adducts LpyꢀECl3 (see
Scheme 1 and ESIw), 2,6-Mes2py (Lpy) reacts with BBr3 to
generate [LpyꢀBBr2][BBr4] in essentially quantitative yield
(73% after recrystallization).16–18 The cationic component of this
compound is characterized by downfield shifted 11B NMR and
pyridine 1H NMR resonances (dB = 42, dH = 7.86, 8.70 ppm,
cf. dB = 38 ppm for BBr3, dH = 7.24, 7.83 ppm for Lpy),16
and the formation of the tri-coordinate borenium species
[LpyꢀBBr2]+ was also confirmed crystallographically (Fig. 1).
The related [AlBr4]ꢂ salt has also been synthesized (see ESIw)
and characterized by closely related spectroscopic and struc-
tural data. While the spontaneous loss of bromide to generate
[LpyꢀBBr2]+ (and [BBr4]ꢂ) in the presence of BBr3 is unusual, it
is noteworthy that an analogous step to generate the active
borylation agent [pyꢀBBr2]+ has been proposed by Murakami
in the N-directed functionalization of arenes.8
Structurally, [LpyꢀBBr2]+ features a planar boron centre
{S angles at B(1) = 360.0(4), 360.0(5)1 for the [BBr4]ꢂ and
[AlBr4]ꢂ salts, respectively} with the BBr2 unit aligned essentially
orthogonal to the plane of the pyridine donor [inter-plane
torsion angle = 74.4, 87.21, respectively]. While the B–N and
B–Br distances are well precedented,19 the most unusual struc-
tural features of the [LpyꢀBBr2]+ cation are the close contacts
between the boron centre and the ipso carbons of both flanking
Inorganic Chemistry Laboratory, Department of Chemistry,
University of Oxford, South Parks Rd, Oxford, UK.
E-mail: simon.aldridge@chem.ox.ac.uk
w Electronic supplementary information (ESI) available: Data for all
compounds; details of DFT calculations (including ‘run’ files). CCDC
839365–839373. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1cc15259a
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12295–12297 12295

View Article Online
retains a planar geometry reflects the presence of two such
interactions involving opposite faces of the NBBr2 unit. Inter-
estingly, [LpyꢀBBr2]+ is unreactive towards the borylation of
external arenes with the steric/electronic obstacle presented by
the flanking rings presumably restricting access to the borenium
centre.5b
Related synthetic studies show that the corresponding Bcat
derivative [LpyꢀBcat]+ (cat = O2C6H4-1,2) can be generated
from BrBcat and Lpy by the use of stronger aluminium-
containing Lewis acids (see ESIw and Fig. 1). Structurally,
this cation also features a trigonal boron centre [S angles at
B(14) = 359.9(2)1] and short Bꢀ ꢀ ꢀCipso contacts [2.734(4) and
2.785(4) A]. A related feature common to these borenium
cations is a marked bending of the flanking arene rings towards
the boron centre. Thus, in [LpyꢀBBr2]+ and [LpyꢀBcat]+, the
‘bite’ angles defined by the centroid of the central pyridine ring
and those of the flanking mesityl substituents (Fig. 2) are
109.8/112.9 and 114.41, respectively, compared to 126.1 and
126.91 for the simple AlCl3/GaCl3 adducts (which feature no
short intramolecular contacts).
Scheme 1 Syntheses of borenium and related derivatives of
2,6-dimesitylpyridine (counter-ions omitted for clarity). Key reagents and
conditions; (i) BBr3 (2.0 equiv.), hexanes, room temperature, 2 h, yield:
73% (of [LPyꢀBBr2][BBr4]), or BBr3 (1.0 equiv.), fluorobenzene, then AlBr3
(1.0 equiv.), room temperature 12 h, yield: 57% (of [LPyꢀBBr2][AlBr4]);
(ii) BrBcat (1.0 equiv.), dichloromethane, then AlCl3 (2.0 equiv.), room
temperature, 12 h, yield: 68% (of [LpyꢀBcat][AlCl4]); (iii) Li2cat or H2cat,
fluorobenzene; (iv) ECl3 (1.0 equiv.), fluorobenzene, room temperature,
24 h, yield: 79% (of LpyꢀAlCl3), 51% (of LpyꢀGaCl3).
Reasoning that the stronger s-donor capabilities of the
N-heterocyclic carbene (NHC) class of ligand (coupled with
the similar steric profiles of the Lpy and LNHC systems)21
should lead to enhanced stabilization of the cationic borenium
system [LNHCꢀBBr2]+, the analogous reactivity of LNHC with
BBr3 was also examined. Somewhat counter-intuitively, the
1 : 2 reaction in fluorobenzene leads to the formation of the
simple adduct LNHCꢀBBr3; addition of the stronger Lewis acid
AlBr3, however, generates the borenium ion [LNHCꢀBBr2]+
(as the [AlBr4]ꢂ salt). Structural studies indicate that the
inter-arene ‘bite’ angle is somewhat wider in charge neutral
adducts of this NHC than in their Lpy counterparts [e.g. 137.5,
133.4, 133.01 for adducts of BBr3, AlCl3 and GaCl3, respec-
tively (see ESIw)]. However, consistent with the general
notion of arene stabilization of dibromoborenium systems,
[LNHCꢀBBr2]+ features a markedly narrowed ‘bite’ angle of
120.21 [cf. 137.51 for LNHCꢀBBr3] and Bꢀ ꢀ ꢀCipso contacts of
2.786(7) and 2.798(8) A.
Fig. 1 Molecular structures of the cationic components of
(a) [LpyꢀBBr2][BBr4], (b) [LpyꢀBcat][AlCl4] and (c) [LNHCꢀBBr2][AlBr4],
and of (d) the adduct LNHCꢀBBr3. H atoms and counter-ions omitted
for clarity; thermal ellipsoids: 40% probability level. Closest contacts
between boron and carbon atoms of the flanking rings shown by red
dashed lines. Key distances [A] and angles [1]: (for [LpyꢀBBr2]+) B(1)–
Br(2) 1.879(3), B(1)–N(4) 1.530(7), B(1)ꢀ ꢀ ꢀC(8) 2.766(5), Br(2)–
In order to differentiate between steric and electronic factors
underpinning these structural changes, and to quantify the
energy changes associated with the observed bending of the
Lpy ligand on borenium ion formation, Density Functional
Theory (DFT) calculations were carried out at the BLYP(tzp)
level using the ADF2010 software package and taking the
crystallographically determined geometries of the Lpy fragment
in ‘free’ Lpy or [LpyꢀBBr2]+ as the starting geometry (see ESIw).
These reveal that the energetic penalties associated with distorting
Lpy or LNHC from its minimum energy geometry to that found
in [LꢀBBr2]+ are 23.3 and 21.7 kJ molꢂ1, respectively;z such
data presumably give an indirect estimate of the ‘binding
energy’ associated with the arene borane interaction in these
B(1)–Br(2A) 120.0(3), Br(2)–B(1)–N(4) 120.0(4); (for [LpyꢀBcat]+
)
O(6)–B(14) 1.363(2), O(13)–B(14) 1.367(3), B(14)–N(15) 1.505(3),
B(14)ꢀ ꢀ ꢀC(17) 2.785(4), B(14)ꢀ ꢀ ꢀC(30) 2.734(4); O(6)–B(14)–O(13)
114.7(2), O(6)–B(14)–N(15) 122.2(2), O(13)–B(14)–N(15) 123.0(2);
(for [LNHCꢀBBr2]+) Br(1)–B(2) 1.889(7), B(2)–Br(3) 1.874(6), B(2)–
C(4) 1.604(7), B(2)ꢀ ꢀ ꢀC(6) 2.798(8), B(2)ꢀ ꢀ ꢀC(19) 2.786(7), Br(1)–
B(2)–Br(3) 119.7(3), Br(1)–B(2)–C(4) 118.1(4), Br(3)–B(2)–C(4)
122.1(4); (for LNHCꢀBBr3) Br(1)–B(2) 2.042(3), B(2)–Br(3) 2.016(4),
B(2)–Br(4) 2.015(4), B(2)–C(5) 1.668(5).
mesityl rings {2.766(5) and 2.754(5)/2.786(5) A for the [BBr4]ꢂ
and [AlBr4]ꢂ salts}, which are well within the sum of the
respective van der Waals radii (3.93 A). While these distances
are much longer than those found in Al(C6F5)3ꢀ(toluene)
[2.342(6), 2.366(2) A],6a a complex described as being intermediate
between an arene p-complex and a s-complex (or Wheland
intermediate), they are significantly shorter than the contacts
calculated for benzene p-complexes of neutral boranes [e.g. 3.25 A
for BCl3ꢀ(C6H6)].9a,b,20 That the boron centre in [LpyꢀBBr2]+
Fig. 2 Definitions of the inter-arene ‘bite’ angle for Lpy and LNHC
ligands.
c
12296 Chem. Commun., 2011, 47, 12295–12297
This journal is The Royal Society of Chemistry 2011

View Article Online
9 (a) P. Tarakeshwar, J. Y. Lee and K. S. Kim, J. Phys. Chem. A,
1998, 102, 2253–2255; (b) P. Tarakeshwar, S. J. Lee, J. Y. Lee and
K. S. Kim, J. Phys. Chem. A, 1999, 103, 184–191; (c) P. Tarakeshwar
and K. S. Kim, J. Phys. Chem. A, 1999, 103, 9116–9124;
(d) S. Kawahara, S. Tsuzuki and T. Uchimaru, Chem.–Eur. J.,
2005, 11, 4458–4464.
10 For a system featuring an aryl group bridging between otherwise
two-coordinate boron centres see: (a) M. Pilz, J. Allwohn,
W. Massa and A. Berndt, Angew. Chem., Int. Ed. Engl., 1990,
29, 399–400. For a recent paper examining alkene-borane p
coordination see: (b) X. Zhao and D. W. Stephan, J. Am. Chem.
Soc., 2011, 133, 12448–12450.
Fig. 3 HOMO of the Lpy ligand.
systems. Moreover, analyses of the electronic structures of
both Lpy and LNHC show that for both ligands, orbitals
featuring a significant contribution from the p system of the
flanking mesityl rings are found at energies close to the
11 For reviews of boron cation chemistry, see: (a) P. Kolle and
¨
¨
H. Noth, Chem. Rev., 1985, 85, 399–418; (b) W. E. Piers,
S. C. Bourke and K. D. Conroy, Angew. Chem., Int. Ed., 2005,
44, 5016–5036.
HOMO–LUMO gap (ꢂ5.23 and ꢂ5.22 eV for Lpy and LNHC
,
12 For recent borenium references see: (a) P. Jutzi, B. Krato,
M. Hursthouse and A. J. Howes, Chem. Ber., 1987, 120,
respectively). Somewhat surprisingly, for Lpy, this orbital is
the HOMO itself (Fig. 3) with the deeper lying HOMO-4
(at ꢂ5.48 eV) being of N-based lone pair character. As such,
the availability of energetically high-lying regions of electron
density at the flanking rings is demonstrated. That said,
the relatively long Bꢀ ꢀ ꢀC contacts measured for all of the
borenium systems reported here and the lack of significant
pyramidalization at the ipso carbon imply that the interaction
of the arene p system with the cationic boron centre is a
predominantly electrostatic one.18 Consistent with this notion,
the [(Lpy)H]+ cation also features a ‘bite’ angle (ca. 1111)
1091–1098; (b) W. F. Schneider, C. K. Narula, H. Noth and
¨
B. E. Bursten, Inorg. Chem., 1991, 30, 3919–3927;
(c) M. G. Davidson, M. A. Fox, T. G. Hibbert, J. A. K. Howard,
A. Mackinnon, I. S. Neretin and K. Wade, Chem. Commun., 1999,
1649–1650; (d) I. Ghesner, W. E. Piers, M. Parveza and
R. McDonald, Chem. Commun., 2005, 2480–2482; (e) T. S. De
Vries and E. Vedejs, Organometallics, 2007, 26, 3079–3081;
(f) C. W. Chiu and F. P. Gabbaı, Organometallics, 2008, 27,
¨
1657–1659; (g) M. A. Dureen, A. Lough, T. M. Gilbert and
D. W. Stephan, Chem. Commun., 2008, 4303–4305;
(h) C. Bonnier, W. E. Piers, M. Parvez and T. S. Sorensen, Chem.
Commun., 2008, 4593–4595; (i) T. Matsumoto and F. P. Gabbaı
¨
,
Organometallics, 2009, 28, 4252–4253; (j) C. I. Someya, S. Inoue,
C. Prasang, E. Irran and M. Driess, Chem. Commun., 2011, 47,
¨
somewhat narrower than that found in the free ligand Lpy 18
.
6599–6601; (k) D. McArthur, C. P. Butts and D. M. Lindsay,
Chem. Commun., 2011, 47, 6650–6652; (l) E. Tsurumaki,
S. Hayashi, F. S. Tham, C. A. Reed and A. Osuka, J. Am. Chem.
Soc., 2011, 133, 11956–11959.
We thank Dr Michael Ingleson (University of Manchester)
for preliminary reactivity studies on [LpyꢀBBr2]+ and the
EPSRC National Mass Spectrometry Service Centre; JN and
JIB thank the DFG and NSERC, respectively, for post-
doctoral fellowships.
13 Borylation using borenium species: (a) E. L. Muetterties, J. Am.
Chem. Soc., 1960, 82, 4163–4166; (b) E. L. Muetterties and
F. N. Tebbe, Inorg. Chem., 1968, 7, 2663–2664; (c) T. S. De Vries,
A. Prokofjevs, J. N. Harvey and E. Vedejs, J. Am. Chem. Soc.,
2009, 131, 14679–14687; (d) A. Del Grosso, R. G. Pritchard,
C. A. Muryn and M. J. Ingleson, Organometallics, 2010, 29,
241–249; (e) A. Del Grosso, P. J. Singleton, C. A. Muryn and
M. J. Ingleson, Angew. Chem., Int. Ed., 2011, 50, 2102–2106.
14 See, for example: E. J. Corey, Angew. Chem., Int. Ed., 2009, 48,
2100–2117.
Notes and references
z The related figures calculated using BP(tzp) are 22.1/21.9 kJ molꢂ1
1 P. Sykes, Guidebook to Mechanism in Organic Chemistry, Longman,
Harlow, 1986.
.
2 See for example: P. E. Kundig, Transition Metal Arene p-Complexes
¨
15 For an early report of [(4-Me-py)ꢀBCl2]+, see: G. E. Ryschkewitsch
and J. W. Wiggins, J. Am. Chem. Soc., 1970, 92, 1790–1791.
16 For a study of related lutidine/Group 13 systems, see: C. K. Narula
and H. Noth, Inorg. Chem., 1985, 24, 2532–2539.
¨
17 (a) E. Bosch and C. L. Barnes, Inorg. Chem., 2001, 40, 3234;
in Organic Synthesis and Catalysis, Springer, Heidelberg, 2004.
3 The coordination of arenes to low-valent post-transition element
cations has been extensively investigated: (a) H. Schmidbaur and
A. Schier, Organometallics, 2008, 27, 2361–2395. For additional
references to arene coordination by Hg2+ see: (b) M. R. Haneline,
(b) S. Hermanek, Chem. Rev., 1992, 92, 325–362.
´
¨
J. B. King and F. P. Gabbaı, Dalton Trans., 2003, 2686–2690.
18 In the presence of adventitious moisture, the reaction of Lpy with
GaCl3 generates [(Lpy)H][GaCl4]; see also T. Pell, D. P. Mills,
A. J. Blake, W. Lewis and S. T. Liddle, Polyhedron, 2010, 29,
120–125.
4 (a) R. Rathore, J. Hecht and J. K. Kochi, J. Am. Chem. Soc., 1998,
120, 13278–13279. See also: (b) G. A. Olah, H. C. Lin and
Y. K. Mo, J. Am. Chem. Soc., 1972, 94, 3667–3669.
5 (a) J. B. Lambert, S. Zhang, C. L. Stern and J. C. Huffman,
Science, 1993, 260, 1917–1918. For a very recent study of terphenyl
silylium cations valence isoelectronic with the current systems, see:
(b) P. Romanato, S. Duttwyler, A. Linden, K. K. Baldridge and
J. S. Siegel, J. Am. Chem. Soc., 2011, 133, 11184–11186.
6 (a) G. S. Hair, A. H. Cowley, R. A. Jones, B. G. McBurnett and
A. Voigt, J. Am. Chem. Soc., 1999, 121, 4922–4923. See also:
(b) J. F. Malone and W. S. McDonald, J. Chem. Soc., Dalton
Trans., 1972, 2646–2648.
19 For [LpyꢀBBr2][BBr4]: B(1)–Br(2) 1.879(3), B(1)–N(4) 1.530(7) A.
Compare with d(B–Br) = 1.902(5) and 1.897(7) A for 2,6-
Mes2C6H3BBr2 and 2,6-Trip2C6H3BBr2, and d(B–N) = 1.530(6) A
for [(phenanthridine)ꢀB{N(Me)CH}2]+
: (a) W. J. Grigsby and
P. P. Power, J. Am. Chem. Soc., 1996, 118, 7981–7988; (b) C. K.
Narula and H. Noth, J. Chem. Soc., Chem. Commun., 1984,
¨
1023–1024.
20 B-arene contacts as short as 2.730(3) A have been reported in
´
1,8-naphthalenediyl systems: J. D. Hoefelmeyer, S. Sole and
¨
7 (a) M. Schulte and F. P. Gabbaı, Chem.–Eur. J., 2002, 8,
F. P. Gabbaı, Dalton Trans., 2004, 1254–1258.
¨
3802–3807. See also: (b) J. F. Malone and W. S. McDonald,
J. Chem. Soc. (A), 1970, 3362–3367; (c) S. Ulvenlund,
A. Wheatley and L. A. Bengtsson, J. Chem. Soc., Dalton Trans.,
1995, 255–263; (d) M. Gorlov and L. Kloo, Coord. Chem. Rev.,
2008, 252, 1564–1576.
21 The respective % buried volumes calculated for the Lpy and LNHC
ligands using the Sambvca program (http://www.molnac.unisa.it/
OMtools/sambvca.php) are 35.3, 37.0% (based on the respective
AlCl3 complexes) or 35.9, 37.6% (based on LꢀGaCl3): A. Poater,
B. Cosenza, A. Correa, S. Giudice, F. Ragone, V. Scarano and
L. Cavallo, Eur. J. Inorg. Chem., 2009, 1759–1766.
8 For a pertinent recent example, see: N. Ishida, T. Moriya, T. Goya
and M. Murakami, J. Org. Chem., 2010, 75, 8709–8712.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12295–12297 12297