Reactivity of the Bifunctional Ambiphilic Molecule 8-(dimesitylboryl) quinoline: Hydrolysis and Coordination to CuI, AgI and PdII

Article abstract of DOI:10.1039/c0dt00798f

The ambiphilic molecule 8-(dimesitylboryl)quinoline (1) was synthesized by treatment of 8-bromoquinoline or 8-iodoquinoline with n-BuLi followed by dimesitylboronfluoride. Hydrolysis of 1 is unusually rapid compared to bulky triorganoboranes with the sequential loss of mesitylene and formation of mesityl(quinolin-8-yl)borinic acid (2) and 8-quinoline boronic acid dimer (3). Cooperativity within the bifunctional ambiphilic site leads to water activation and protodeboronation of the B-C(Mes) bonds. Blocking of the ambiphilic site of 1 by methylation of the quinoline nitrogen atom leads to an air-stable N-methyl-quinolinium salt. Coordination complexes were formed by reaction of 1 with CuCl, Ag(OTf), and PdCl₂(PhCN)₂ with coordination of the quinolinyl nitrogen to the metal ion. The Cu^I and Ag^I centers are stabilized by η³-BC_ipsoC_ortho π-interaction. The isolated Pd^II complex is a product of cyclometalation, resulting from elimination of HCl upon deprotonation of the ortho-methyl group of nearby mesityl. The bonding in 7 could be understood as a 16-electron Pd complex that features an anionic η³-C _ipso-C_ortho-C_benzyl allylic ligand fragment plus a Pd→B bond, or an η⁴-BC_ipso-C _ortho-C_benzyl boratabutadiene anion fragment. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0dt00798f

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www.rsc.org/dalton | Dalton Transactions
Reactivity of the Bifunctional Ambiphilic Molecule
8-(dimesitylboryl)quinoline: Hydrolysis and Coordination to CuI,
AgI and PdII†
Jung-Ho Son, Michael A. Pudenz and James D. Hoefelmeyer*
Received 6th July 2010, Accepted 4th September 2010
DOI: 10.1039/c0dt00798f
The ambiphilic molecule 8-(dimesitylboryl)quinoline (1) was synthesized by treatment of
8-bromoquinoline or 8-iodoquinoline with n-BuLi followed by dimesitylboronfluoride. Hydrolysis of 1
is unusually rapid compared to bulky triorganoboranes with the sequential loss of mesitylene and
formation of mesityl(quinolin-8-yl)borinic acid (2) and 8-quinoline boronic acid dimer (3).
Cooperativity within the bifunctional ambiphilic site leads to water activation and protodeboronation
of the B–C(Mes) bonds. Blocking of the ambiphilic site of 1 by methylation of the quinoline nitrogen
atom leads to an air-stable N-methyl-quinolinium salt. Coordination complexes were formed by
reaction of 1 with CuCl, Ag(OTf), and PdCl2(PhCN)2 with coordination of the quinolinyl nitrogen to
3
the metal ion. The CuI and AgI centers are stabilized by h -BCipsoCortho p-interaction. The isolated PdII
complex is a product of cyclometalation, resulting from elimination of HCl upon deprotonation of the
ortho-methyl group of nearby mesityl. The bonding in 7 could be understood as a 16-electron Pd
3
complex that features an anionic h -Cipso-Cortho-Cbenzyl allylic ligand fragment plus a Pd→B bond, or an
4
h -BCipso-Cortho-Cbenzyl boratabutadiene anion fragment.
unique due to cooperative engagement of the Lewis centers of an
Introduction
ambiphilic molecule, the centers must be constrained to avoid
stable intra-24–28 or intermolecular dative bonds, giving rise to
Frustrated Lewis Pairs (FLPs). Such geometry could be useful
for host–guest interactions in which cooperative interaction of
the Lewis centers with the guest species (especially ambiphilic
types) lead to its sequestration or activation. Additionally, co-
operative interaction of Lewis pairs with guest species occurs
with bimolecular FLPs. Recently the thermodynamics associated
with the reactivity of FLPs was considered wherein the entropic
component differs significantly between linked and non-linked
systems.29
FLPs have been reported to activate small molecules such as H2,
alkene, CO2, RSSR, carbonyls, etc.30–44 Polyfunctional ambiphilic
molecules have been shown to stabilize transition metals.45–53 The
electrophilic center of the ambiphilic ligand can accept electron
density from the transition metal. For example, metallaboratranes
feature M→B bonding wherein the low oxidation number late
transition metal is the electron donor in the dative bond.54–61
Alternatively, the electrophilic domain of the ambiphilic ligand
can accept electron density from an adjacent ligand bound to the
central metal, such as halide.47,50–52,62
Cooperative interaction of molecular functional groups is funda-
mentally important inthe reactivityofnumerous chemicalsystems.
The most sophisticated examples can be found in nature, for
instance the active sites of proteins, in which amino acid residues
are positioned in a way that enables high reaction activity and
selectivity in catalysis. The concept of polyfunctional catalysts
was defined as those that involve concerted action by two or
more functional groups.1,2 Synthetic designs that incorporate
multiple functional groups include modified porous materials,3–6
polyfunctional Lewis acids7–10 and Lewis bases,11,12 or molecules
with pendant arms or tethered functional groups.13
Ambiphilic molecules, identified by the presence of electrophilic
and nucleophilic sites, are of fundamental interest. The molecules
can exhibit high hyperpolarizability, and have found use as non-
linear optical materials.14–17 Ambiphilic molecules can conceivably
form a variety of supramolecular structures such as rings, cate-
nanes, and coordination polymers.18–20 They have been found as
linkers in supramolecular assembly.21,22 Ambiphilic molecules with
suitable geometry can behave as receptors for small molecules or
as catalysts.23
The reactivity of ambiphilic molecules depends on the relative
placement of the electrophilic (Lewis acid) and nucleophilic
(Lewis base) centers, and the steric and electronic properties of
the Lewis centers. In order to observe reaction modes that are
Attachment of a Lewis acid group at the 8-position of a
quinoline ring is a route to a bifunctional ambiphilic molecule.
Letsinger et al. reported the synthesis of 8-quinoline boronic acid,
and its ability to catalyze the hydrolysis of chloro-alcohols.63–65 The
reactivity was attributed to the cooperative interaction of the Lewis
centers. An important feature of the molecule is the separation of
the Lewis centers by two carbon atoms within a rigid ring system.
Within this geometric constraint, an intramolecular dative bond
would result in an energetically unfavorable four-member ring.
Thus, non-coordinated Lewis centers are retained, giving rise to
highly active ambiphilic sites. Additionally, large steric groups
Department of Chemistry, University of South Dakota, 414 E. Clark St.,
Vermillion, SD 57069, USA. E-mail: jhoefelm@usd.edu
† Electronic supplementary information (ESI) available: CIF files for
crystals 1, 1a, 2–7, synthesis of 8-iodoquinoline, ORTEP drawing of 1a,
1
1
VT H NMR data and corresponding Eyring plots for 5 and 6, and H
and 13C NMR data for 1–7. CCDC reference numbers 758512–758515,
759392, 759393 and 761001. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0dt00798f
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Table 1 Comparison of C9–C8–B1 and dihedral angle between the borane mean plane and quinoline plane in compounds 1, 1a, 4–7
1
1a
4
5
6
7
C9–C8–B1 (deg)
125.24(17)
49.45(6)
115.2(4)
57.05(11)
133.05(21)
55.12(7)
125.3(2)
41.80(8)
128.6(2)
52.07(7)
123.4(4)
45.45(10)
Dihedral angle (deg)a
a Angle between the borane mean plane and the quinoline plane.
about the Lewis centers should prevent intermolecular dative
bonding that would lead to dimerization. The synthesis of the
bifunctional ambiphilic molecule 8-(dimesitylboryl)quinoline, its
unusually rapid hydrolysis, use as an ambiphilic ligand toward the
coordination of group 11 metal salts (CuCl and Ag(OTf)), and
coordination to PdII via cyclometalation are reported herein.
was found to be occupationally disordered on a special position
and was refined isotropically as C3. Crystallographic parameters
are summarized in Table 2. The CIF files are deposited in CCDC
crystallographic database and can be retrieved according to the
following database number and in the ESI:† 1 (CCDC 758512), 1a
(CCDC 759392), 2 (CCDC 758513), 4 (CCDC 758514), 5 (CCDC
758515), 6 (CCDC 759393), 7 (CCDC 761001).
Experimental section
8-(dimesitylboryl)quinoline (1). The synthesis can be per-
formed using 8-bromoquinoline or 8-iodoquinoline with similar
results. In a 250 mL round-bottom flask, 2 g (9.6 mmol) of 8-
bromoquinoline was dissolved in 80 mL of THF using magnetic
stirrer and cooled to -78 ◦C. 6 mL of 1.6 M n-butyl lithium
solution in hexane (9.6 mmol) was slowly added to the solution
through syringe. The solution was kept at -78 ◦C for 2 h. After that,
2.86 g (9.6 mmol) of dimesitylboron fluoride (90%) dissolved in
40 mL THF was added via cannula and the flask was brought
to room temperature overnight. The crude product obtained
after solvent removal in vacuo was washed with cold acetonitrile
(3 ¥ 5 mL) and cold pentane (3 ¥ 5 mL), leaving a pale yellow
powder. Single crystals (triclinic system, 1) were obtained by
prolonged storage of CH2Cl2–pentane solution at -30 ◦C. The
crystals bearing disorder (orthorhombic system, 1a) were grown by
room temperature evaporation of a hexane, toluene, or acetonitrile
solution. Yield: 2.6 g (72%). Mp: 165–167 ◦C. dH (200 MHz;
CDCl3; CH2Cl2): 2.00 (12 H, s, Mes o-CH3), 2.30 (6 H, s, Mes
p-CH3), 6.77 (4 H, s, Mes m-CH), 7.29 (1 H, dd, J 8.4 and 4.2,
C(3)H), 7.48 (1 H, dd, J 8.0 and 6.8, C(6)H), 7.61 (1 H, dd, J 7.0
and 1.6, C(7)H), 7.85 (1 H, dd, J 8.0 and 1.8, C(5)H), 8.10 (1 H,
dd, J 8.2 and 2.0, C(4)H), 8.69 (1 H, dd, J 4.1 and 1.7, C(2)H).
dC (50 MHz; CDCl3; CHCl3): 21.2 (Mes p-CH3), 23.1 (Mes o-
CH3), 120.8 (C3), 126.0 (C6), 127.3 (C10), 128.0 (Mes m-CH),
130.0 (C5), 134.1 (C7), 135.5 (C4), 138.4 (Mes p-CCH3), 140.3
(Mes o-CCH3), 149.6 (C2), 150.5 (C9). dB (128 MHz; CDCl3;
BF3OEt2): 74.9 (br s, n1/2 ª 968 Hz). Found: C, 85.70; H, 7.47; N,
3.87. C27H28BN requires C, 85.94; H, 7.48; N, 3.71%.
General considerations
8-Bromoquinoline was purchased from Frontier Scientific. 8-
Iodoquinoline was synthesized according to a modified procedure
(see ESI†).66–68 Dimesitylboron fluoride (90%), 1.6 M n-butyl
lithium in hexane, methyl triflate, and CaH2 were purchased
from Sigma-Aldrich. CuCl was purchased from Janssen Chemical.
Silver triflate was obtained from Strem Chemical. PdCl2(PhCN)2
was synthesized according to the reported procedure.69 Deuterated
solvents were purchased from Cambridge Isotope Laboratories.
CDCl3 was dried and distilled from CaH2. Acetone-d6 was dried
˚
and distilled from activated 4 A molecular sieves. Dry solvents
were obtained from Pure-solv solvent dryer and kept over 4
˚
A molecular sieves. Procedures were carried out using standard
Schlenk line and glove box techniques under nitrogen atmosphere
unless noted otherwise. Elemental analysis was carried out using
an Exeter CE-440 Elemental Analyzer or at Atlantic Microlab
1
(Norcross, GA). H and 13C NMR spectra were collected using
Varian 200 MHz, Bruker Avance 400 MHz and JEOL ECS
400 MHz NMR spectrometers. J values are given in Hz. Variable
temperature (VT) 1H NMR data were acquired with Bruker
Avance 400 MHz NMR spectrometer. Line shape analysis was
performed with WinDNMR program.70 11B, 1D NOE and 2D
NMR experiments (COSY and HMQC) were carried out with
JEOL ECS 400 MHz NMR spectrometer.
Crystallography
The crystallographic measurements were performed using a
Bruker SMART Apex II diffractometer with CCD area detector
Mesityl(quinolin-8-yl)borinic acid (2). 100 mg (0.26 mmol) of 1
was suspended in 10 mL of anhydrous ethanol (Pharmco Inc.). The
cloudy solution became clear within a few minutes after addition of
180 mg of deionized water. The solvent was immediately removed
in vacuo, and a light tan precipitate remained. The precipitate
was washed with hexane and extracted with dichloromethane.
A tan crystalline powder was recovered after evaporation of
dichloromethane. Recrystallization in acetone produced crystals
suitable for X-ray diffraction study. Yield: 45 mg (63%). Mp: 150–
152 ◦C. dH (200 MHz; CDCl3; CHCl3): 2.25 (6 H, s, Mes o-CH3),
2.33 (3 H, s, Mes p-CH3), 6.87 (2 H, s, Mes m-CH), 7.47–7.54 (2
H, m, C(3)H/C(6)H), 7.80 (1 H, dd, J 6.8 and 1.8, C(7)H), 7.93
(1 H, dd, J 8.2 and 1.6, C(5)H), 8.28 (1 H, dd, J 8.4 and 1.6,
C(4)H), 8.94 (1 H, dd, J 4.4 and 2.0, C(2)H), 13.29 (1 H, s, OH).
˚
using Mo-Ka radiation (l = 0.71069 A). Single crystals were
mounted onto glass fibers with epoxy resin. The structures were
solved by direct methods, which successfully located most of the
non-hydrogen atoms. Subsequent refinements on F2 using the
SHELXTL/PC package (version 5.1) allowed location of the re-
maining non-hydrogen atoms.71 Multiscan absorption correction
was applied.72 The structure of 1a is disordered, arising from two
partially superimposed positions on a 2-fold rotation axis that
passes through B(1) and midpoint of C(9) and C(9)i (Ortep draw-
ing included in ESI†). Positionally disordered dichloromethane
solvent in 6 was refined with partial occupancy. In the structure
of 7, 0.25 equivalent acetonitrile solvent per one molecular unit
11082 | Dalton Trans., 2010, 39, 11081–11090
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This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 11081–11090 | 11083
©

View Online
dC (50 MHz; CDCl3; CHCl3) 21.2(Mes p-CH3), 22.1(Mes o-CH3),
120.9(C3), 126.9 (Mes m-CH), 127.1 (C6), 127.8 (C10), 131.3 (C5),
137.3 (Mes p-CCH3), 137.5 (C4), 139.3 (Mes o-CCH3), 141.9 (C7),
148.9 (C2), 152.6 (C9). dB (128 MHz; CDCl3; BF3OEt2) 48.5 (br
s, n1/2 ª 748 Hz). Found: C, 77.40; H, 6.38; N, 4.68. C18H18BNO
requires C, 78.57; H, 6.59; N, 5.09%.
of AgI, and stirred overnight. The solvent was removed in vacuo,
which produced a mixture of colorless crystalline material and
brown powder. The product was dissolved in CH2Cl2 and filtered
to remove the brown powdery impurity. Colorless crystals were
obtained upon evaporation of the solvent. Yield: 112 mg (88%).
Mp: 210–212 ◦C. dH (400 MHz; CDCl3; CHCl3): 1.55 (3 H, br s,
Mes o-CH3), 1.81 (3 H, br s, Mes o-CH3), 1.99 (3 H, br s, Mes o-
CH3), 2.13 (3 H, br s, Mes o-CH3), 2.29 (6 H, s, Mes p-CH3), 6.81
(3 H, br s, Mes m-CH), 7.12 (1 H, br s, Mes m-CH), 7.56–7.60 (2 H,
m, C(3)H and C(6)H), 7.67 (1 H, dd, J 7.0 and 1.4, C(7)H), 7.99 (1
H, dd, J 8.0 and 1.2, C(5)H), 8.39 (1 H, dd, J 8.3 and 1.6, C(4)H),
8.93 (1 H, dd, J 4.6 and 1.7, C(2)H). dC (50 MHz; CDCl3; CHCl3):
21.4 (Mes p-CH3), 23.4 (br, Mes o-CH3), 122.2 (C3), 127.8 (C6),
128.5 (C10), 132.1 (C5), 138.7 (C7), 140.0 (C4), 148.6 (C9), 154.0
(C2). dB (128 MHz; CDCl3; BF3OEt2): 72.4 (br s, n1/2 ª 1753 Hz).
Found: C, 52.85; H, 4.37; N, 2.51. C56H56Ag2B2F6N2O6S2 requires
C, 53.02; H, 4.45; N, 2.21%.
8-quinolyl boronic acid dimer (3). 113 mg (0.3 mmol) of 1 was
dissolved in 10 mL of acetone and 3 mL of deionized water. After
slow evaporation of solvent over several days, pale yellow cubic
crystals formed. Yield: 44 mg (85%). Mp: > 300 ◦C. dH (200 MHz;
CD3OD; CH3OH): 7.55 (1 H, dd, J 8.2 and 6.6, C(6)H), 7.67
(1 H, dd, J 8.4 and 5.2, C(3)H), 7.73 (1 H, dd, J 8.2 and 1.6,
C(7)H), 8.08 (1 H, dd, J 6.6 and 1.6, C(5)H), 8.45 (1 H, dd, J
8.4 and 1.8, C(4)H), 9.28 (1 H, dd, J 5.0 and 1.6, C(2)H). dC
(100 MHz; CD3OD; CH3OH): 122.5 (C3), 128.8 (C7), 130.2 (C6),
131.3 (C10), 135.7 (C5), 144.1 (C4), 145.4 (C2), 147.1 (C9). dB
(128 MHz; CD3OD; BF3OEt2): 5.2 (s, n1/2 ª 230 Hz). Found: C,
62.28; H, 4.55; N, 7.99. C18H16B2N2O4 requires C, 62.49; H, 4.66;
N, 8.10%.
Chloro({2 - [mesityl(quinolin - 8 - yl - jN)boryl] - 3,5 - dimethyl-
phenyl}methyl-jC)palladium(II) (7)73. A flask was charged with
95 mg (0.25 mmol) of 1 and 100 mg (0.26 mmol)◦of PdCl2(PhCN)2
dissolved in 15 mL toluene and heated to 100 C for 2 h. After
cooling, the solvent was removed in vacuo, leaving a yellow-
brown powder and black powder. The product was extracted
with dichloromethane and recrystallized from acetonitrile by slow
evaporation. Yield: 118 mg (91%). Mp: 304–306 ◦C. dH (400 MHz;
CDCl3; CHCl3): 1.49 (3 H, s, Mes C(19)H3), 1.72 (3 H, s, Mes
C(28)H3), 2.22 (3 H, s, Mes C(27)H3), 2.35 (3 H, s, Mes C(18)H3),
2.57 (3 H, s, Mes C(26)H3), 3.07 (1 H, d, J 2.6, Mes H17a), 3.90
(1 H, d, J 2.8, Mes H17b), 6.69 (1 H, s, Mes m-C(24)H), 6.78 (1
H, s, Mes m-C(22)H), 6.85 (1 H, s, Mes m-C(13)H), 7.02 (1 H, s,
Mes m-C(15)H), 7.44 (1 H, dd, J 8.2 and 5.0, C(3)H), 7.51 (1 H,
t, J 7.8, C(6)H), 7.58 (1 H, d, J 6.4, C(7)H), 7.90 (1 H, d, J 7.8,
C(5)H), 8.26 (1 H, d, J 8.2, C(4)H), 8.64 (1 H, d, J 4.1, C(2)H). dC
(50 MHz; CDCl3; CHCl3): 21.2 (C27), 21.6 (C18), 22.9 (C28), 24.8
(C26), 25.2 (C19), 51.8 (C17), 99.8, 121.7 (C13), 122.8 (C3), 127.6,
127.8 (C24), 128.2 (C6), 128.3, 128.8 (C22), 129.8 (C5), 130.4,
132.9 (C15), 134.9 (C7), 138.4 (C4), 138.6, 139.8, 143.1, 146.7,
148.2, 150.1, 151.5 (C2), 152.2. dB (128 MHz; CDCl3; BF3OEt2):
49.3 (br s, n1/2 ª 1720 Hz). Found: C, 62.16; H, 5.29; N, 3.04.
C27H27BClNPd requires C, 62.58; H, 5.25; N, 2.70%.
8-(dimesitylboryl)-1-methylquinolinium triflate (4). 110 mg
(0.29 mmol) of 1 was dissolved in 5 mL of CH2Cl2. 48 mg
(0.29 mmol) of methyl triflate was added to the solution and gently
swirled to ensure homogeneous mixing. The solvent was allowed
to evaporate, leaving a pale y◦ellow crystalline compound. Yield:
120 mg (76%). Mp: 206–208 C. dH (200 MHz; CDCl3; CHCl3):
2.03 (12 H, br s, Mes o-CH3), 2.26 (6 H, s, Mes p-CH3), 4.27 (3
H, s, CH3N), 6.83 (4 H, br s, Mes m-CH), 7.80 (1 H, t, J 7.6,
C(6)H), 8.07-8.19 (2 H, m, C(7)H/C(3)H), 8.38 (1 H, dd, J 8.4
and 1.4, C(5)H), 9.12 (1 H, d, J 8.2, C(4)H), 9.36 (1 H, d, J 6.0,
C(2)H). dC (50 MHz; CDCl3; CHCl3): 21.2 (Mes p-CH3), 23.0 (br,
Mes o-CH3), 49.9 (CH3N), 123.1 (C3), 129.7 (br, Mes m-CH),
130.9 (C6), 134.3 (C5), 141.5 (br, Mes o-CH3), 143.0 (Mes p-
CH3), 143.2 (C7), 148.4 (C4), 151.5 (C2). dB (128 MHz; CDCl3;
BF3OEt2): 75.8 (br s, n1/2 ª 3778 Hz). Found: C, 63.68; H, 5.66; N,
2.74. C29H31BF3NO3S requires C, 64.33; H, 5.77; N, 2.59%.
Chloro[8-(dimesitylboryl)quinoline-jN]copper(I) (5). A solu-
tion of 50 mg (0.50 mmol) CuCl in 7 mL of CH3CN was prepared.
To this, 180 mg (0.50 mmol) of 1 and 7 mL of CH2Cl2 were added.
The mixture turned clear yellow and was stirred overnight. The
solvent was removed in vacuo, which afforded red crystalline prod-
uct. The product was washed with methanol and recrystallized
from CH2Cl2–pentane. Yield: 159 mg (67%). Mp: 222–224 ◦C. dH
(400 MHz; CDCl3; CHCl3): 1.49 (3 H, s, br, Mes o-CH3), 1.92 (6
H, br s, Mes o-CH3), 2.03 (3 H, br s, Mes o-CH3), 2.25 (6 H, s, Mes
p-CH3), 6.77 (3 H, br s, Mes m-CH), 7.05 (1 H, br s, Mes m-CH),
7.53 (1 H, dd, J 8.3 and 4.8, C(3)H), 7.57 (1 H, t, J 7.5, C(6)H),
7.65 (1 H, dd, J 6.9 and 1.5, C(7)H), 7.99 (1 H, dd, J 8.1 and 1.5,
C(5)H), 8.36 (1 H, dd, J 8.3 and 1.7, C(4)H), 9.01 (1 H, dd, J 4.7
and 1.7, C(2)H). dC (50 MHz; CDCl3; CHCl3): 21.4 (Mes p-CH3),
23.9 (br, Mes o-CH3), 122.2 (C3), 128.0 (C6), 131.4 (C5), 137.4
(C7), 139.4 (C4), 148.1 (C9), 152.3 (C2). dB (128 MHz; CDCl3;
BF3OEt2): 69.2 (br s, n1/2 ª 1521 Hz). Found: C, 67.11; H, 5.79; N,
2.98. C54H56B2Cl2Cu2N2 requires C, 68.08; H, 5.92; N, 2.94%.
Results and Discussion
The synthesis and structure of 8-(dimesitylboryl)quinoline (1)
The bifunctional ambiphilic molecule 8-(dimesitylboryl)-
quinoline, was obtained by treatment of 8-bromoquinoline
or 8-iodoquinoline with n-BuLi, followed by reaction with
dimesitylboron fluoride (Scheme 1). Single crystals of 8-
(dimesitylboryl)quinoline could be obtained as a triclinic (1) or
disordered orthorhombic (1a) polymorph (Fig. S1, see ESI†),
Triflato[8-(dimesitylboryl)quinoline-jN]silver(I) (6). A solu-
tion of 50 mg (0.20 mmol) Ag(OTf) in 5 mL of CH3CN was
prepared. To this, 76 mg (0.20 mmol) of 1 and 5 mL of toluene were
added. The vial was covered with foil to minimize photoreduction
Scheme 1 The synthesis of 8-(dimesitylboryl)quinoline (1).
11084 | Dalton Trans., 2010, 39, 11081–11090
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Scheme 2 Hydrolysis of 1 proceeds stepwise to the formation of a borinic acid (2) followed by the formation of a bis(quinolyl)boroxide (3) that likely
proceeds through a boronic acid intermediate followed by condensation reaction.
depending on the crystallization solvent. In both polymorphs,
8-(dimesitylboryl)quinoline is monomeric; the structure of 1 is
storage of an NMR tube containing 1 in CDCl3 under ambient
condition produced crystals of 8-quinoline boronic acid dimer (3).
These observations indicate that the reaction of 1 with water leads
to sequential hydrolysis of the mesityl groups, followed by a ring-
closing condensation reaction (Scheme 2). Initially, 2 forms from
hydrolysis of 1. The structure of 2 in the crystal is shown in Fig. 2.
The mean plane defined by boron and its adjacent atoms is almost
parallel to the quinoline plane with an angle of 2.68(10)◦. The
borinic acid function is engaged in an intramolecular N ◊ ◊ ◊ HO
shown in Fig. 1. The boron atom is three-coordinate in an
ꢀ
approximately trigonal planar geometry (
C–B–C angles =
359.43◦). The C(9)–C(8)–B(1) angle is 125.24(17)◦. The angle
between the mean plane defined by boron and its neighboring
carbon atoms and the quinoline mean plane is approximately 49◦
˚
(Table 1). The distance between B(1) and N(1) is 3.003(3) A. The
structural evidence does not indicate significant intramolecular
dative bonding between boron and nitrogen. With the observed
geometry, the unoccupied boron 2p orbital is not directly pointed
toward the filled sp2 orbital of nitrogen. The broad 11B NMR
resonance at 75 ppm is within the typical chemical shift range
for triorganoboranes also supporting the absence of N→B dative
˚
hydrogen bond (1.85(3) A), completing a planar six-member
ring. The mesityl ring is oriented almost perpendicular to the
quinoline plane (85.46(5)◦). A 11B NMR peak appears at 49 ppm,
upfield relative to 1 due to the p electron donation from oxygen.
Further hydrolysis of 2 occurs much slower upon addition of
excess water. After evaporation of solvent, tiny colorless cubic
crystals of 8-quinoline boronic acid dimer (3) formed. The crystal
structure coincided with the previously reported structure of 8-
quinoline boronic acid dimer with one water of crystallization,
which has been shown to form upon spontaneous dimerization
of 8-quinoline boronic acid.74 Thus we conclude that the mesityl
group in 2 was further hydrolysed to form 8-quinoline boronic acid
that dimerized and underwent a condensation reaction to form the
boroxide linkage.
1
bond. The H NMR spectrum of 1 shows two sharp resonances
in a 1 : 2 ratio for the p-CH3 and o-CH3 mesityl groups, indicating
free rotation about the B–C bonds.
Fig. 1 ORTEP view of dimesityl-8-quinolylborane (1). Selected bond
˚
lengths (A) and angles (deg): C(8)–B(1) = 1.578(3), C(11)–B(1) = 1.588(3),
C(20)–B(1) = 1.575(3), C(9)–C(8)–B(1) = 125.24(17), C(7)–C(8)–B(1) =
117.21(17), C(11)–B(1)–C(8) = 113.27(16), C(20)–B(1)–C(8) = 121.85(17),
C(20)–B(1)–C(11) = 124.31(17). Thermal ellipsoids are shown at the 50%
probability level.
Fig. 2 ORTEP view of mesityl-8-quinolylborinic acid (2). Selected bond
˚
lengths (A) and angles (deg): C(8)–B(1) = 1.584(3), C(11)–B(1) = 1.590(3),
Hydrolysis of 1
B(1)–O(1) = 1.354(2), C(9)–C(8)–B(1) = 123.13(15), C(7)–C(8)–B(1) =
120.25(16), O(1)–B(1)–C(8) = 120.25(16), O(1)–B(1)–C(11) = 118.15(16),
C(8)–B(1)–C(11) = 121.59(15). Thermal ellipsoids are shown in 50%
probability level.
The unexpected discovery of crystals of mesityl(quinolin-8-
yl)borinic acid (2) by evaporation of the acetone solution of 1
in air initiated a hydrolysis study of 1. Furthermore, prolonged
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Hydrolysis of organoboranes proceeds through separate path-
ways depending on whether oxygen is present. In the absence
of oxygen, hydrolysis is sluggish; however, in acidic or basic
conditions, it is accelerated via protodeboronation of R3B or
R3BOH-, respectively. In both cases, R–H and B–OH groups are
formed with cleavage of a B–C bond.75 In the presence of oxygen,
borinic ester, boronic ester, and triorganoborate are formed
stepwise due to sequential oxygen atom insertion into the B–C
bonds. The B–OR group is readily hydrolyzed, yielding B–OH and
alcohol.76–78 Therefore, oxidative hydrolysis of triorganoboranes is
distinguishable from anaerobic hydrolysis due to the difference
in the reaction products. The steric groups surrounding boron
influence the rate of oxygen atom insertion (into B–C bonds)
and hydrolysis. Triphenylborane (TPB) hydrolyzes rapidly in cold
water and reacts with oxygen; whereas trinaphthylborane (TNB)
and trimesitylborane (TMB) are inert to reaction with water or
oxygen.79
of 4 is shown in Fig. 3. The molecule is crowded at the boron
atom, as evidenced by an outward bending of the dimesitylboryl
group away from◦the N-methylquinolinium group (C(9)–C(8)–
1
B(1) = 133.05(21) ). The H NMR spectrum shows broadening
of proton resonances of the mesityl groups, due to the hindered
rotation that arises from the increased bulk after N-methylation.
The 11B NMR chemical shift of 4 is almost unchanged from 1 but
very broad.
Hydrolysis of 1 under aerobic and anaerobic (N2 atmosphere,
deoxygenated water) conditions was investigated. In both condi-
1
tions, mesitylene was identified as a by-product upon H NMR
spectroscopic analysis of the reaction solution. Borinic or boronic
esters were not observed from 1, and it may be difficult for oxygen
to penetrate the boron atom through the bulky mesityl groups.
Indeed, recrystallization of 1 from Lewis base solvents (CH3CN,
THF, or acetone) did not lead to an adduct. If the reaction involved
oxidation, then 2,4,6-trimethylphenol should be a by-product;
1
however, it was not observed in the H NMR spectrum. Upon
Fig. 3 The structure of 8-dimesitylboryl-N-methyl-quinolinium tri-
flate (4) in the crystal. Selected bond lengths (A) and angles (deg):
hydrolysis reaction of 1 with D2O the 1H NMR signal of the borinic
acid (13.30 ppm) was suppressed and the aryl C–H resonance
intensity ratio of the mesityl group in 2 and byproduct mesitylene
was almost 1 : 1. Given the steric properties of 1, its apparent
inert reactivity toward oxygen is consistent with the reported
bulky triorganoboranes. These observations lead to the following
proposed mechanism: a molecule of water forms a hydrogen bond
with the nitrogen atom in 1; there is enhanced polarization of
the water molecule by the closely located bifunctional boron and
nitrogen sites; protodeboronation leads to cleavage of a B–C(Mes)
bond. The observed reactivity is remarkable given that TNB and
TMB are inert toward water.79 In addition, we note the close
proximity of a dimesitylboryl Lewis acid center to an S-bound
DMSO ligand was invoked in the slow hydrolysis of B–C(Mes)
bonds.80 The hydrolysis rate from 2 to 3 was much slower (several
days) than that from 1 to 2 (less than one hour with various
solvents). The slow rate in the conversion from 2 to 3 suggests that
intramolecular hydrogen bond stabilization in 2 could have limited
the access of additional water to the quinolinyl nitrogen, which is
necessary for the activation of water for further protodeboronation
from 2 to 3. Also, the p-donating nature of the oxygen present in 2
should have limited the ability of boron to accept electron density
from additional water.
˚
C(8)–B(1) = 1.592(3), C(11)–B(1) = 1.577(4), C(20)–B(1) = 1.569(3),
C(29)–N(1) = 1.474(3), C(9)–C(8)–B(1) = 133.0(2), C(7)–C(8)–B(1) =
110.30(19), C(20)–B(1)–C(11) = 122.4(2), C(20)–B(1)–C(8) = 116.9(2),
C(11)–B(1)–C(8) = 120.0(2), C(9)–N(1)–C(29) = 121.72(18). Thermal
ellipsoids are shown at the 50% probability level.
Coordination of 1 with Ag(OTf), CuCl, and PdCl2(PhCN)2
Stoichiometric reactions of 1 with group 11 metal salts CuCl
or Ag(OTf) yield dimeric compounds 5 and 6 (Scheme 3). The
structures of the dimeric units in 5 and 6 are shown in Fig. 4 and
Fig. 5, respectively. A Cu2Cl2 or Ag2(OTf)2 ring is found at the
core of the dimeric structure. The quinolinyl nitrogen coordinates
I
˚
˚
M with N–Cu = 2.031(2) A and N–Ag = 2.266(3) A. Structural
data suggests h -BCipsoCortho p-interaction with MI as evidenced
3
by Cortho ◊ ◊ ◊ M, Cipso ◊ ◊ ◊ M, B ◊ ◊ ◊ M = 2.422(2), 2.126(2), 2.660(3)
˚
˚
A and 2.596(3), 2.359(3), 2.902(3) A for 5 and 6, respectively.
The B ◊ ◊ ◊ M distances in 5 and 6 are less than the sum of
VDW radii, indicative of the weak M→B interaction; however,
the distances are slightly longer than B ◊ ◊ ◊ M distances within
similar bonding motifs reported in the literature. Bourissou et al.
I
I
˚
reported the first observation of Cu →B (2.508(2) A) and Ag →B
˚
For comparison, 1-dimesitylboryl naphthalene was synthesized
according to the literature.81 The compound was stable in air,
and could be kept for years without decomposition. Additionally,
N-methylation of 1 gave compound 4, which was also air stable
for years. In compound 4 the FLP motif is partially blocked,
preventing hydrolysis by the bifunctional reaction site. In light
of the moisture sensitive nature of 1, these observations imply
that the quinoline nitrogen in 1 is responsible for the unusually
fast hydrolysis of the B–C(Mes) bond. The X-ray crystal structure
(2.540(2) A) interaction with an ambiphilic triphosphine borane
ligand.46 Mono- and diphosphinoborane complexes with CuCl
3
2
were synthesized, which feature h -BCC and h -BC stabilization
I
I
˚
˚
of the Cu center with Cu →B = 2.555(2) A and 2.396(5) A,
respectively.53 Similar h -BCipsoCortho stabilization of Rh0, Ni0, or
3
Pd0 by a phenyl borane group has been reported by Emslie and
coworkers upon coordination of a phosphine/thioether/borane
ambiphilic ligand.50,51 They concluded that the interaction includes
2
contribution from two resonance forms, an h -alkene-M plus
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Scheme 3 Synthesis of 5, 6, and 7 from 1.
˚
Fig. 4 Structure of 5 in the crystal. Thermal ellipsoids are shown at the 50% probability level. Selected bond lengths (A) and angles (deg): N(1)–Cu(1) =
2.031(2), C(21)–Cu(1) = 2.422(2), C(20)–Cu(1) = 2.126(2), Cl(1)–Cu(1) = 2.3474(6), Cl(1)–Cu(1)i = 2.3979(6), B(1)–Cu(1) = 2.660(3), C(9)–C(8)–B(1) =
125.3(2). i = -x + 1, -y, -z.
˚
Fig. 5 Structure of 6 in the crystal. Thermal ellipsoids are shown at the 50% probability level. Selected bond lengths (A) and angles (deg): N(1)–Ag(1) =
2.266(2), C(20)–Ag(1) = 2.359(3), C(21)–Ag(1) = 2.596(3), B(1)–Ag(1) = 2.902(3), O(1)–Ag(1) = 2.339(2), Ag(1)–O(3)i = 2.426(3), C(9)–C(8)–B(1) =
128.6(2). i = -x + 1, -y, -z + 1.
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M→B structure and borataallyl-M structure. Although their
interactions. Examination of the structural data can reveal the
nature of stabilization of the PdII ion. Some structural parameters
in the mesityl ring suggest h -BCipsoCorthoCbenzyl boratabutadiene
structures have negligible pyramidalization of boron, the 11B NMR
4
chemical shifts suggest tetracoordinate boron. Similarly, trigonal
ꢀ
planar boron were observed in 5 and 6 ( C–B–C angles = 359.4
and 359.6, respectively). A 11B NMR resonance appears slightly
upfield from 1 at 69 ppm (5) and 72 ppm (6) suggesting the presence
anion contribution in the crystal structure of 7 (Scheme 4).
˚
The C(13)–C(14) and C(15)–C(16) bond distances (1.356(7) A
˚
and 1.366(7) A) are contracted in comparison to the other C–C
1
of a weak M→B interaction. The H NMR spectra of 5 and 6
bond distances within the ring. The C(12)–C(17) bond distance
˚
show broadening of resonances assigned to o-CH3 and m-H of
the mesityl group. This spectroscopic feature can be attributed
to restricted rotation about the B–C(Mes) bonds that arise from
(1.434(6) A) is shorter than unperturbed methyl groups on the
˚
same mesityl group (i.e. C(14)–C(18) = 1.510(6) A and C(16)–
˚
C(19) = 1.508(6) A), implying partial double bond character.
3
the reversible h -BCC–M bond and steric bulk in the molecules.
These structural features are consistent with localized p-bonding
Variable temperature (VT) 1H NMR was carried out to investigate
the rotational energy barrier (see ESI†). Line shape analysis of the
o-CH3 peaks and Eyring plot gave DG‡ = 15.1(1) kcal mol-1 (5)
and DG‡ = 15.2(3) kcal mol-1 (6). Activation energy was calculated
from the slope of the Arrhenius plot, which gave EA = 15.1(4) kcal
mol-1 (5) and 10.8(4) kcal mol-1 (6).
in the boratabutadiene resonance form. However, the B(1)–C(11)
˚
distance (1.582(6) A) is not shorter than the B(1)–C(20) distance
˚
(1.581(6) A) in the free mesityl group, which is inconsistent with
the boratabutadiene description. On the other hand, the C(11)–
˚
C(12) bond length (1.465(6) A) is comparable to C(12)–C(17) bond
3
˚
length (1.434(6) A), suggestive of a delocalized h -Cipso-Cortho-Cbenzyl
A
PdII complex (7) was obtained by heating
1
with
allylic anion fragment. The 11B NMR spectrum shows a resonance
PdCl2(PhCN)2 in toluene (Scheme 3). Unlike the group 11
complexes 5 and 6 which decompose in air, 7 is air stable. In the
crystal structure of 7, two crystallographically unique molecular
units are found that are almost identical (one of the molecules
is shown in Fig. 6). The benzonitrile ligands were displaced and
coordination of PdII by the quinolinyl nitrogen resulted. Cyclomet-
alation occurs concomitantly with elimination of HCl, and leads
to a borapalladacycle. Cyclometalation upon coordination of a
transition metal and activation of a nearby C–H bond has been
well established, especially for Pd.82 Compound 7 contains PdII
with nitrogen and chloride in the coordination sphere and four
close contacts to boron and mesityl. The bonding in 7 warrants
discussion. If one neglects the presence of the [Cl–PdII]+ fragment,
the anionic ligand has several resonance forms. Each resonance
form potentially presents a unique set of metal–ligand orbital
at 49 ppm that is significantly upfield from that of 1, and indicates
the presence of Pd→B electron backdonation. Therefore, 7 could
3
be understood as a 16-electron complex that features an h -Cipso
-
Cortho-Cbenzyl allylic anion ligand fragment and Pd→B bond. The
˚
Pd–B distances in 7 (B(1)–Pd(1) = 2.379(5) A and B(2)–Pd(2) =
˚
2.363(5) A) are less than the sum of the Van der Waals radii
ꢀ
83
˚
(
= 3.63 A). Previous examples of Pd–B bond distances,
Pd –B = 2.650(3) A, Pd –B = 2.200(5) A, Pd –B = 2.073(4) A
and 2.050(8) A, are comparable to those in 7.
vdw
II
48
II
84
0
˚
˚
˚
85
˚
3
Scheme 4 Resonance structures of 7. Left: h -Cipso-Cortho-Cbenzyl allylic
4
anion ligand fragment and Pd→B bond; right: h -BCipsoCorthoCbenzyl bo-
ratabutadienyl anion-PdII p-complex.
Conclusion
The
bifunctional
ambiphilic
molecule,
8-(dimesityl-
boryl)quinoline (1), was synthesized. The molecule features
bulky groups around the electrophilic boron center and a rigid
two-carbon spacer that prevent inter- and intramolecular dative
bonding. The steric bulk around boron is sufficient to prevent
the approach of oxygen to the boron atom; however, small
molecules can approach the nitrogen donor site. The molecule
exhibits unusually rapid hydrolysis compared to similarly bulky
triarylboranes, which can be attributed to the unique active site.
N-Methylation of 1 suppresses hydrolysis completely. 1 forms
3
coordination complexes with CuI and AgI, wherein h -BCipsoCortho
stabilization from boron and mesityl is a common structural
feature. Reaction with PdII led to simultaneous coordination via
quinolinyl nitrogen and deprotonation of an o-CH3 group of
mesityl. The complex 7 could be understood as a 16-electron
Fig. 6 Structure of 7 in the crystal. Thermal ellipsoids are shown at
the 50% probability level. Selected bond lengths (A) and angles (deg):
Pd(1)–C(17) = 2.074(4), Pd(1)–N(1) = 2.118(3), Pd(1)–C(11) = 2.163(4),
Pd(1)–C(12) = 2.248(4), Pd(1)–Cl(1) = 2.349(1), Pd(1)–B(1) = 2.379(5),
C(9)–C(8)–B(1) = 123.4(4).
˚
3
complex that features an h -Cipso-Cortho-Cbenzyl allylic anion
4
ligand fragment plus a Pd→B bond or an h -BCipsoCorthoCbenzyl
11088 | Dalton Trans., 2010, 39, 11081–11090
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boratabutadiene anion. Further studies will probe deeper into
the reactivity of the bifunctional ambiphilic molecule, and the
effects of tuning electronic, steric, and geometric properties. While
addition of ambiphilic substrates to the bifunctional ambiphilic
molecule is conceptually straightforward, future efforts should
examine heterolytic cleavage of non-polar bonds.
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Acknowledgements
We thank Dr Jay Shore in the Department of Chemistry, South
Dakota State University for generous help with VT NMR
experiments. We thank Mr Brandon Gustafson in the Department
of Chemistry, Augustana College (Sioux Falls, SD) for 11B, 1D
NOE and 2D NMR experiments. This work was supported
by the National Science Foundation (CHE-0552687), the U.S.
Department of Energy (Director, Office of Science, Office of
Biological & Environmental Research, Biological Systems Science
Division, of the U.S. Department of Energy under Contract
No. DE-FG02-08ER64624 and DE-FG02-08ER64624), South
Dakota 2010 Initiative ‘Center for Research and Development of
Light-Activated Materials’, and startup funds from the University
of South Dakota. Purchase of the single crystal X-ray diffrac-
tometer and glovebox was made possible by the National Science
Foundation (EPS-0554609).
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11090 | Dalton Trans., 2010, 39, 11081–11090
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The Royal Society of Chemistry 2010
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