Synthesis and anion binding properties of multi-phosphonium triarylboranes: Selective Sensing of cyanide ions in buffered water at pH 7

Article abstract of DOI:10.1021/om3010542

A series of mono-, di-, and triphosphonium-substituted triarylboranes, [Mes₂BAr^P]I ([2]I), [MesBAr^P₂]I ₂ ([3]I₂), and [BAr^P₃]I₃ ([4]I₃) (Ar^P = 4-(MePh₂P)-2,6-Me ₂-C₆H₂), were prepared from the corresponding neutral boranes 2a-4a. The crystal structure of [4]I₃ determined by X-ray diffraction study reveals peripheral decoration of aryl groups with phosphonium moieties. The anion affinity of the cationic boranes for fluoride and cyanide ions was investigated by UV-vis absorption titrations in aqueous solution. The triarylboranes, [2]I-[4]I₃ bind both fluoride and cyanide ions in a DMSO/H₂O (7:3 v/v) mixture with high binding constants (K). Comparison of the K values of triarylboranes for fluoride reveals that fluorophilicity increases with the increasing number of phosphonium moieties: [2]⁺ (K = 2.3 × 10¹ M^-1) < [3]²⁺ (3.6 × 10⁵ M^-1) < [4] ³⁺ (1.0 × 10⁷ M^-1). A similar trend is also observed in the cyanide binding, with K values that are greater by 2-4 orders of magnitude than those in the fluoride binding. These results indicate an apparent additive effect of multiple phosphonium substitutions on the Lewis acidity enhancement of triarylboranes. The triphosphonium borane [4]Cl ₃, a water-soluble form of [4]I₃, was further utilized in evaluating the anion affinity in water. While [4]³⁺ is shown to hardly bind fluoride in buffered water at pH 7, it binds cyanide with a high binding constant (1.7 × 10⁷ M^-1).

Full text of DOI:10.1021/om3010542

Article
pubs.acs.org/Organometallics
Synthesis and Anion Binding Properties of Multi-phosphonium
Triarylboranes: Selective Sensing of Cyanide Ions in Buffered Water
at pH 7
Ki Cheol Song,† Kang Mun Lee,† Nguyen Van Nghia,‡ Woo Young Sung,‡ Youngkyu Do,*,†
and Min Hyung Lee*,‡
†Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea
‡Department of Chemistry and EHSRC, University of Ulsan, Ulsan 680-749, Republic of Korea
S
* Supporting Information
ABSTRACT: A series of mono-, di-, and triphosphonium-
substituted triarylboranes, [Mes2BArP]I ([2]I), [MesBArP ]I2
2
([3]I2), and [BArP ]I3 ([4]I3) (ArP = 4-(MePh2P)-2,6-Me2-
3
C6H2), were prepared from the corresponding neutral boranes
2a−4a. The crystal structure of [4]I3 determined by X-ray
diffraction study reveals peripheral decoration of aryl groups
with phosphonium moieties. The anion affinity of the cationic
boranes for fluoride and cyanide ions was investigated by UV−vis absorption titrations in aqueous solution. The triarylboranes,
[2]I−[4]I3 bind both fluoride and cyanide ions in a DMSO/H2O (7:3 v/v) mixture with high binding constants (K).
Comparison of the K values of triarylboranes for fluoride reveals that fluorophilicity increases with the increasing number of
phosphonium moieties: [2]+ (K = 2.3 × 101 M−1) < [3]2+ (3.6 × 105 M−1) < [4]3+ (1.0 × 107 M−1). A similar trend is also
observed in the cyanide binding, with K values that are greater by 2−4 orders of magnitude than those in the fluoride binding.
These results indicate an apparent additive effect of multiple phosphonium substitutions on the Lewis acidity enhancement of
triarylboranes. The triphosphonium borane [4]Cl3, a water-soluble form of [4]I3, was further utilized in evaluating the anion
affinity in water. While [4]3+ is shown to hardly bind fluoride in buffered water at pH 7, it binds cyanide with a high binding
constant (1.7 × 107 M−1).
groups into the triarylboranes.6−15 Among such approaches,
in particular, the cationic triarylboranes have been shown to
INTRODUCTION
■
The detection and quantification of anions such as fluoride and
largely increase the anion affinity of the boron center enough to
cyanide ions are of great interest owing to the harmful effects of
operate in aqueous media, due to the Coulombic and inductive
such anions in physiological and environmental systems.
effects of cationic groups that assist boron−anion dative
Fluoride is added to drinking water and toothpaste for dental
interactions.6,8−14
health and is also used in treating osteoporosis,1 but taking
For example, as shown in Chart 1, various types of cationic
excessive fluorides may lead to fluorosis.2 Cyanide is a toxic
̈
triarylboranes were recently reported by Gabbai and co-
anion that inhibits cytochrome c oxidase; thus widespread
industrial uses of cyanide may raise physiological and
environmental concerns.3 For these reasons, the selective
detection of these anions has received considerable attention
during the past decade.
workers. The monoammonium borane [I]+ selectively binds
cyanide ions in DMSO/H2O (4:6 v/v, pH 7) with a high
binding constant (K = 3.9 × 108 M−1), while the ortho-
ammonium derivative [II]+ is selective for fluoride in the same
medium.14 The sulfonium borane [III]+ selectively captures
cyanide in water at pH 7 by favorable Coulombic effects and
the sulfonium−cyanide interaction, leading to fluorescence
quenching at the sub-ppm level of cyanide.8 Fluoride binding
studies using a series of monophosphonium boranes ([IV]+)
revealed that they can bind fluoride in H2O/MeOH (9:1 v/v),
and the binding constant increases with their hydrophobicity
(KMe < KEt < KnPr < KPh).10,13 The fluoride ion affinity can also
be drastically enhanced by the ortho-phosphonium borane [V]+,
which binds fluoride via chelation from the neighboring
Among the receptors for the recognition of fluoride and
cyanide ions developed to date, triarylborane compounds are
shown to be one of the most effective molecular platforms due
to their high affinity toward nucleophilic anions.4 While most
triarylborane receptors have successfully operated in organic
media and provided various detection methods,5 they none-
theless showed a limited binding ability in the presence of
water, which interferes with the anion binding, leaving them
hardly utilized in water or aqueous solution. In addition, the
competitive protonation of cyanide ions in neutral water may
complicate cyanide detection. To resolve this issue, many
efforts have been devoted to enhancing the Lewis acidity of
triarylborane by introducing strong electron-withdrawing
Received: November 5, 2012
Published: January 31, 2013
© 2013 American Chemical Society
817
dx.doi.org/10.1021/om3010542 | Organometallics 2013, 32, 817−823

Organometallics
Article
and a Bruker AM 300 spectrometer (96.29 MHz for 11B, 121.5 MHz
for 31P) at ambient temperature. Chemical shifts are given in ppm and
are referenced against external Me4Si (1H, 13C), BF3·OEt2 (11B), and
85% H3PO4 (31P). HR EI-MS measurement (JEOL JMS700) was
carried out at Korea Basic Science Institute. Elemental analyses were
performed on an EA1110 (Fisons Instruments) by the Environmental
Analysis Laboratory at KAIST. UV−vis absorption spectra were
recorded on a Jasco V-530 spectrophotometer.
Chart 1
Synthe sis of (4-Bromo-3, 5-dimethylphenyl)-
diphenylphosphine (1). A hexane solution of n-BuLi (3.6 mL,
9.07 mmol) was added at −78 °C to a solution of 2,5-dibromo-m-
xylene (2.18 g, 8.25 mmol) in Et2O (20 mL), and the mixture was
stirred for 45 min at this temperature. A solution of chlorodiphenyl-
phosphine (1.48 mL, 8.25 mmol) in THF (5 mL) was slowly added to
the mixture. The reaction mixture was stirred for 1 h at −78 °C and
allowed slowly to warm to room temperature. After stirring for 4 h, all
volatiles were removed under vacuum. The white residue was extracted
with n-hexane (30 mL) and filtered. After evaporating the solvent, the
remaining sticky residue was purified by column chromatography
(eluent: hexane/ethyl acetate, 10:1 v/v), which afforded 1 as a
colorless oil (2.57 g, 84%). 1H NMR (CDCl3): δ 2.34 (s, 6H, Me2Ph),
6.98 (d, J = 7.5 Hz, 2H, Me2Ph-CH), 7.31 (m, 10H, Ph-CH). 13C
NMR (CDCl3): δ 23.87 (Me2Ph), 128.52 (d, JC−P = 6.9 Hz), 128.77,
phosphonium and borane moieties.12 In addition to these
monocationic boranes, the Lewis acidity is shown to be
enhanced by the introduction of multiple cationic moieties.9,11
While the mono- and diammonium derivatives fail to bind
cyanide in water, the triammonium borane [VI]3+ selectively
binds cyanide in water at pH 7.9
131.51 (d, JC−P = 18 Hz), 133.28 (d, JC−P = 20 Hz), 133.65 (d, JC−P
=
19 Hz), 135.53 (d, JC−P = 11 Hz), 136.89 (d, JC−P = 11 Hz), 138.44 (d,
JC−P = 7.5 Hz). 31P NMR (CDCl3): δ −5.8. HR EI-MS: m/z calcd for
C20H18BrP, 368.0329; found, 368.0327.
Although the foregoing studies on the cationic boranes
successfully demonstrated fluoride or cyanide binding in
aqueous solution or water at a low level of concentration,
their low stability in such medium, partly due to the increased
Lewis acidity, often made it difficult to measure the exact
binding constant for the quantification of anions. In particular,
the effect of the multiple introductions of a cationic moiety on
the increase in the binding constant was not adequately
evaluated. For example, multiammonium boranes showed
sluggish binding and low affinity in water.9 To better
understand the additive effect of multiple substitutions of the
cationic moiety on the Lewis acidity enhancement of
triarylboranes and to quantify the resulting increase in the
binding constant, we turned our attention to the para-
phosphonium-substituted boranes, which showed high stability
in aqueous medium.10,13 In this report, we prepared a series of
mono-, di-, and triphosphonium-substituted triarylboranes and
compared their fluoride and cyanide ion affinities in aqueous
solution. The triphosphonium borane was further utilized in
evaluating the anion affinity in water. Details of the synthesis,
characterization, and anion binding studies of the multi-
phosphonium boranes are described.
Synthesis of Dimesityl(4-(diphenylphosphino)-2,6-
dimethylphenyl)borane (2a). A hexane solution of n-BuLi (0.44
mL, 1.1 mmol) was added at 0 °C to a solution of 1 (0.37 g, 1.0
mmol) in Et2O (15 mL), and the reaction mixture was stirred for 30
min. After stirring for 1 h at room temperature, a solution of Mes2BF
(0.30 g, 1.0 mmol) in Et2O (10 mL) was added to the mixture at −78
°C. The reaction mixture was allowed to slowly warm to room
temperature and stirred overnight. The reaction was quenched with
saturated aqueous NH4Cl (30 mL). The organic layer was separated,
and the aqueous layer was extracted with Et2O (20 mL). The
combined organic portions were dried over MgSO4 and concentrated
under reduced pressure. Purification by column chromatography
(eluent: CH2Cl2/hexane, 1:8 v/v) afforded 2a as a white powder (0.24
g, 45%). 1H NMR (CDCl3): δ 1.92 (s, 6H, Ar-CH3), 1.96 (s, 6H, Mes-
CH3), 1.97 (s, 6H, Mes-CH3), 2.24 (s, 6H, Mes-CH3), 6.72 (s, 4H,
Mes-CH), 6.79 (d, J = 8.2 Hz, 2H, Ar−CH), 7.29 (m, 10H, Ph-CH).
13C NMR (CDCl3): δ 21.22 (Mes-CH3), 22.79, 22.84, 22.88 (Ar-CH3
and Mes-CH3), 128.36 (d, JC−P = 7.1 Hz), 128.56, 128.63 (d, JC−P
=
3.0 Hz), 132.49 (d, JC−P = 18.2 Hz), 133.67 (d, JC−P = 3.0 Hz), 133.87
(d, JC−P = 3.8 Hz), 137.20 (d, JC−P = 8.3 Hz), 137.54 (d, JC−P = 9.7
Hz), 139.37, 140.19 (d, JC−P = 6.8 Hz), 140.45, 140.67. 31P NMR
(CDCl3): δ −5.7. 11B NMR (CDCl3): δ 79.8. Anal. Calcd for
C38H40BP: C, 84.75; H, 7.49. Found: C, 84.58; H, 7.59.
Synthesis of (4-(Dimesitylboryl)-3,5-dimethylphenyl)-
methyldiphenylphosphonium iodide ([2]I). To a solution of 2a
(0.11 g, 0.20 mmol) in Et2O (10 mL) was added excess MeI (1.02 mL,
2.0 mmol). After stirring overnight at room temperature, the pale
yellow precipitate was filtered and washed with Et2O (3 × 10 mL) and
pentane (3 × 10 mL), affording [2]I as a pale yellow solid (0.072 g,
52%). 1H NMR (CDCl3): δ 1.92 (s, 6H, Mes-CH), 1.98 (s, 6H, Mes-
CH), 2.06 (s, 6H, ArP-CH3), 2.24 (s, 6H, Mes-CH3), 3.15 (d, J = 13.0
Hz, 3H, P-CH3), 6.74 (s, 4H, Mes-CH), 7.14 (d, J = 13.8 Hz, 2H, ArP-
EXPERIMENTAL SECTION
■
General Considerations. All operations were performed under an
inert nitrogen atmosphere using standard Schlenk and glovebox
techniques. Anhydrous grade solvents (Aldrich) were dried by passing
them through an activated alumina column and stored over activated
molecular sieves (5 Å). Spectrophotometric grade DMSO (Aldrich)
was used as received. Commercial reagents were used without any
further purification after purchasing from Aldrich (chlorodiphenyl-
phosphine (Ph2PCl), dimesitylboron fluoride (Mes2BF), MeI,
Amberlite IRA-400 chloride form, BF3·OEt2, n-BuLi (2.5 M solution
in n-hexanes), N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid
(HEPES), tetra-n-butylammonium fluoride (TBAF), tetra-n-butylam-
monium cyanide (TBACN), KF, NaCN). Buffer solution (HEPES 10
mM in H2O, pH 7),16 dimethyl mesitylboronate (MesB(OMe)2),17
and 2,5-dibromo-m-xylene18 were analogously prepared according to
the reported procedures. Deuterated solvents from Cambridge Isotope
Laboratories were used. NMR spectra were recorded on a Bruker
CH), 7.69 (m, 10H, Ph-CH). 13C NMR (CDCl3): δ 11.62 (d, JC−P
=
56.9 Hz, P-CH3), 21.18 (Mes-CH3), 22.93 (ArP-CH3), 22.96 (Mes-
CH3), 118.03 (d, JC−P = 88.0 Hz), 119.09 (d, JC−P = 88.0 Hz), 128.99
(d, JC−P = 4.6 Hz), 130.37 (d, JC−P = 12.9 Hz), 131.03 (d, JC−P = 9.9
Hz), 133.18 (d, JC−P = 9.9 Hz), 135.01 (d, JC−P = 3.8 Hz), 140.07,
140.46, 141.09, 142.43, 142.55. 31P NMR (CDCl3): δ 21.2. 11B NMR
(CDCl3): δ 81.4. Anal. Calcd for C39H43BIP: C, 68.84; H, 6.37.
Found: C, 69.17; H, 6.18.
Synthes is of Bis(4-(diphenylphosphino)-2, 6-
dimethylphenyl)mesitylborane (3a). To a solution of 1 (0.52 g,
1.41 mmol) in Et2O (15 mL) was added n-BuLi (0.59 mL, 1.48 mmol)
1
Avance 400 spectrometer (400.13 MHz for H, 100.62 MHz for 13C)
818
dx.doi.org/10.1021/om3010542 | Organometallics 2013, 32, 817−823

Organometallics
Article
a
Scheme 1
a
(i) n-BuLi, Ph2PCl, THF/ether, −78 °C, 84%. (ii) n-BuLi, Mes2BF, ether, −78 °C, 45%. (iii) n-BuLi, 1/2 MesB(OMe)2, ether, −78 °C, 13%. (iv) n-
BuLi, 1/3 BF3·OEt2, ether, −78 °C, 50%.
at 0 °C, and the mixture was stirred for 30 min at this temperature.
After stirring for 1 h at room temperature, a solution of MesB(OMe)2
(0.11 g, 0.56 mmol) in Et2O (10 mL) was added to the mixture at −78
°C. The reaction mixture was allowed slowly to warm to room
temperature and stirred overnight. After quenching with saturated
aqueous NH4Cl (30 mL), the organic layer was separated and the
aqueous layer was extracted with Et2O (20 mL). The combined
organic portions were dried over MgSO4 and concentrated under
reduced pressure. Purification by column chromatography (eluent:
CH2Cl2/hexane, 1:5 v/v) afforded 3a as a white powder (0.052 g,
= 12.9 Hz), 131.81 (d, JC−P = 12.9 Hz), 132.48 (d, JC−P = 9.9 Hz),
133.79 (d, JC−P = 10.6 Hz), 134.44 (d, JC−P = 10.6 Hz), 134.49 (d, JC−P
= 10.6 Hz), 136.01 (d, JC−P = 3.0 Hz), 136.15 (d, JC−P = 3.8 Hz),
138.71 (d, JC−P = 11.4 Hz), 140.07, 142.79, 143.23, 143.40 (d, JC−P
=
12.9 Hz), 146.04 (d, JC−P = 10.4 Hz). 31P NMR (CDCl3): δ 19.3, 21.6.
11B signal was not observed. Anal. Calcd for C51H53BI2P2: C, 61.72; H,
5.38. Found: C, 61.94; H, 5.39.
Synthesis of Tris(4-(diphenylphosphino)-2,6-
dimethylphenyl)borane (4a). To a solution of 1 (1.93 g, 5.20
mmol) in Et2O (20 mL) was added n-BuLi (2.1 mL, 5.25 mmol) at 0
°C, and the mixture was stirred for 30 min at this temperature. After
stirring for 1 h at room temperature, the mixture was cooled to −78
°C. BF3·OEt2 (0.2 mL, 1.58 mmol) was added to the mixture and then
allowed to warm to room temperature. After stirring for 24 h, the
reaction was quenched with saturated aqueous NH4Cl (30 mL). The
organic layer was separated, and the aqueous layer was extracted with
Et2O (20 mL). The combined organic portions were dried over
MgSO4 and concentrated under reduced pressure. Recrystallization of
the residue from n-hexane at −20 °C afforded 2 as a white solid (2.28
g, 50%). Single crystals suitable for X-ray diffraction study were grown
1
13%). H NMR (CDCl3): δ 1.99 (s, 6H, Ar-CH3), 2.03 (s, 6H, Ar-
CH3), 2.14 (s, 6H, Mes-CH3), 2.30 (s, 3H, Mes-CH3), 6.82 (s, 2H,
Mes-CH), 6.89 (d, J = 12.0 Hz, 2H, Ar-CH), 7.17 (d, J = 3.8 Hz, 2H,
Ar-CH), 7.29 (m, 20H, Ph-CH). 13C NMR (CDCl3): δ 21.22 (Mes-
CH3), 23.50, 23.54, 23.64, 23.82 (Ar-CH3 and Mes-CH3), 127.68,
128.38 (d, JC−P = 7.6 Hz), 128.42 (d, JC−P = 5.3 Hz), 128.58, 131.63,
131.82, 132.03 (d, JC−P = 6.2 Hz), 132.18, 133.71, 133.91, 136.10 (d,
JC−P = 3.8 Hz), 136.22, 137.23, 137.34 139.06, 140.36 (d, JC−P = 6.8
Hz), 141.14, 144.45, 144.60, 144.88. 31P NMR (CDCl3): δ −14.1,
−5.4. 11B signal was not observed. Anal. Calcd for C49H47BP2: C,
83.05; H, 6.68. Found: C, 82.59; H, 6.87.
1
from cooling of a THF/MeOH solution of 4a. H NMR (CDCl3): δ
1.95 (s, 18H, Ar-CH3), 6.82 (d, J = 8.1 Hz, 6H, Ar-CH), 7.31 (m, 30H,
Ph-CH). 13C NMR (CDCl3): δ 22.95 (Ar-CH3), 128.45 (d, JC−P = 6.9
Hz), 128.67, 132.67 (d, JC−P = 18 Hz), 133.81 (d, JC−P = 19 Hz),
S yn t he sis o f 1 , 1′-Mesitylboranediylbis((3, 5-
dimethylphenyl)methyldiphenylphosphonium) Diiodide
([3]I2). To a solution of 3a (0.032 g, 0.045 mmol) in CHCl3 (3
mL) was added excess MeI (0.67 mL, 1.35 mmol). After stirring
overnight at room temperature, the resulting solution was treated with
Et2O (20 mL), which precipitated out a pale yellow solid. Filtration
followed by washing with Et2O (3 × 10 mL) and pentane (3 × 10 mL)
afforded [3]I2 as a pale yellow solid (0.037 g, 84%). 1H NMR
(CDCl3): δ 2.02 (s, 6H, Mes-CH3), 2.12 (s, 6H, ArP-CH3), 2.15 (s,
6H, ArP-CH3), 2.28 (s, 3H, Mes-CH3), 3.13 (d, J = 13.2 Hz, 3H, P-
CH3), 3.24 (d, J = 12.7 Hz, 3H, P-CH3), 6.84 (s, 2H, Mes-CH), 7.27
(d, J = 5.0 Hz, 2H, ArP-CH), 7.35 (d, J = 14.0 Hz, 2H, ArP-CH), 7.71
(m, 20H, Ph-CH). 13C NMR (CD3OD): δ 9.00 (d, JC−P = 57.9 Hz, P-
CH3), 14.44 (d, JC−P = 57.2 Hz, P-CH3), 21.35 (Mes-CH3), 23.78,
24.18, 24.85, 24.91 (ArP-CH3 and Mes-CH3), 121.17 (d, JC−P = 89.2
Hz), 121.51 (d, JC−P = 82.3 Hz), 123.72 (d, JC−P = 86.2 Hz), 130.13,
131.42 (d, JC−P = 12.9 Hz), 131.44 (d, JC−P = 12.9 Hz), 131.46 (d, JC−P
136.95 (d, JC−P = 11 Hz), 138.60 (d, JC−P = 11 Hz), 140.35 (d, JC−P
=
6.4 Hz), 146.89. 31P NMR (CDCl3): δ −5.5. 11B signal was not
observed. Anal. Calcd for C60H54BP3: C, 82.00; H, 6.19. Found: C,
81.76; H, 6.17.
Synthesis of 1,1′,1″-Boranetriyltris((3,5-dimethylphenyl)-
methyldiphenylphosphonium) Triiodide ([4]I3). To a solution
of 4a (70 mg, 0.080 mmol) in CH2Cl2 (15 mL) was added excess MeI
(0.05 mL, 0.8 mmol). The mixture was stirred overnight at room
temperature. After removal of all volatiles, the residue was suspended
in a CH3CN/Et2O mixed solvent. The insoluble part was collected by
filtration and washed with Et2O to afford [4]I3 as a pale yellow solid
(70 mg, 68%). Single crystals suitable for X-ray diffraction study were
grown from vapor diffusion of pentane into a CH3CN/THF solution
1
of [4]I3. H NMR (CD3OD): δ 2.16 (s, 18H, ArP-CH3), 3.02 (d, J =
819
dx.doi.org/10.1021/om3010542 | Organometallics 2013, 32, 817−823

Organometallics
Article
12 Hz, 9H, P-CH3), 7.42 (d, J = 12 Hz, 6H, ArP-CH), 7.75 (m, 24H,
Ph-CH), 7.87 (m, 6H, Ph-CH). 13C NMR (CD3OD): δ 8.93 (d, JC−P
= 57 Hz, P-CH3), δ 23.68 (ArP-CH3), 120.75 (d, JC−P = 88 Hz), 123.35
(d, JC−P = 87 Hz), 131.45 (d, JC−P = 13 Hz), 133.44 (d, JC−P = 10 Hz),
to the mesityl and 2,6-dimethylphenyl moieties, the C−H
protons of the Ar group appear as doublets due to the coupling
between 31P and 1H nuclei. The 31P signals detected at ca. δ −5
ppm are consistent with the presence of a neutral phosphine
group. In particular, while the three Ar groups in 4a are
chemically equivalent in solution, the two Ar groups in 3a are
shown to be different in chemical environment, judging from
the two sets of methyl and C−H proton signals of the Ar
groups, as well as two 31P resonances at δ −5 and −14 ppm.
Although the 11B NMR signals for di- and trisubstituted 3a and
4a were not observed despite a prolonged acquisition time, the
signal for 2a at δ 80 ppm confirms the presence of the trigonal
boron atom. Furthermore an X-ray diffraction study revealed
the molecular structure of 4a (Figure 1 and Table S1). The
134.50 (d, JC−P = 10 Hz), 136.18 (d, JC−P = 3.0 Hz), 143.91 (d, JC−P
=
13 Hz), 152.35. 31P NMR (CD3OD): δ 21.9. 11B signal was not
observed. Anal. Calcd for C63H63BI3P3: C, 58.00; H, 4.87. Found: C,
57.61; H, 5.16.
Synthesis of 1,1′,1″-Boranetriyltris((3,5-dimethylphenyl)-
methyldiphenylphosphonium) Trichloride ([4]Cl3). [4]I3 (70
mg, 0.054 mmol) and Amberite IRA-400(Cl) ion-exchange resin (1.34
g) were mixed in MeOH/water (1:1 v/v, 10 mL) and stirred for 12 h.
The resin was removed by filtration, and the procedure was repeated
three times. In the last cycle, 1 mL of HCl solution (2.0 M) was added.
Evaporation of the filtrate and drying under vacuum at 80 °C
1
overnight yielded [4]Cl3 as a white solid (40 mg, 72%). H NMR
(CD3OD): δ 2.16 (s, 18H, ArP-CH3), 3.02 (d, J = 16 Hz, 9H, P-CH3),
7.43 (d, J = 12 Hz, 6H, ArP-CH), 7.77 (m, 24H, Ph-CH), 7.88 (m, 6H,
Ph-CH). 13C NMR (CD3OD): δ 8.56 (d, JC−P = 57 Hz, P-CH3), δ
23.41 (ArP-CH3), 120.76 (d, JC−P = 88 Hz), 123.66 (d, JC−P = 87 Hz),
131.46 (d, JC−P = 13 Hz), 133.41 (d, JC−P = 10 Hz), 134.44 (d, JC−P
=
11 Hz), 136.28 (d, JC−P = 3.0 Hz), 143.93 (d, JC−P = 13 Hz), 152.29.
31P NMR (CD3OD): δ 22.1. 11B signal was not observed. Anal. Calcd
for C63H63BCl3P3·(H2O)3: C, 69.78; H, 6.41. Found: C, 69.45; H,
6.10.
UV−Vis Titration Experiments. UV−vis titrations of anions were
performed in DMSO/H2O (7:3 v/v) or in buffered water (10 mM
HEPES, pH 7) with a 1 cm quartz cuvette at ambient temperature.
Typically, a solution of cationic borane (3.0 mL, 5 × 10−5 M) was
titrated with incremental amounts of anions. The absorbance data
obtained were fitted to a 1:1 binding isotherm. The pKa of HCN in
DMSO/H2O (7:3 v/v, pH 7) was measured to be 9.5 (potentiometric
titration of NaCN with HCl) and used in the determination of the
cyanide binding constant.14 The detailed conditions are given in the
figure captions.
X-ray Crystallography. Single crystals of 4a and [4]I3 were coated
with Paratone oil and mounted onto a glass capillary. The
crystallographic measurement was performed on a Bruker SMART
Apex II CCD area detector diffractometer with a graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å). The structure
was solved by direct methods, and all non-hydrogen atoms were
subjected to anisotropic refinement by full-matrix least-squares on F2
by using the SHELXTL/PC package. Hydrogen atoms were placed at
their geometrically calculated positions and refined riding on the
corresponding carbon atoms with isotropic thermal parameters. For
[4]I3, disordered solvent molecules were removed by the squeeze
operation in PLATON. The detailed crystallographic data are given in
Table S1 (see Supporting Information).
Figure 1. Crystal structure of 4a (30% ellipsoid). H atoms and one
THF molecule were omitted for clarity. Selected bond lengths (Å) and
angles (deg): B−C(1) 1.558(12), B−C(21) 1.589(11), B−C(41)
1.605(11), P(1)−C(4) 1.815(13), P(2)−C(24), 1.842(9), P(3)−
C(44) 1.847(8); C(1)−B−C(21) 121.6(7), C(1)−B−C(41)
119.4(7), C(21)−B−C(41) 118.9(8), C(4)−P(1)−C(9) 99.4(8),
C(4)−P(1)−C(15) 101.7(6), C(24)−P(2)−C(29) 103.8(5),
C(24)−P(2)−C(35) 100.3(5), C(44)−P(3)−C(49) 102.5(4),
C(44)−P(3)−C(55) 102.9(5).
central boron atom adopts a trigonal planar geometry
(∑(C−B−C) = 359.9°), and the Ph2P moieties are attached at
the para-positions of the Ar groups. The three Ar groups form a
propeller-like conformation probably due to steric repulsion
between the six ortho-methyl groups around the boron atom.
The neutral boranes 2a−4a were converted into the
corresponding iodide salts of methylated phosphonium boranes
[2]I−[4]I3 by treating them with excess MeI in different
solvents (Scheme 1). The identity of the cationic borane salts
has been characterized by multinuclear NMR spectroscopy and
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthetic proce-
dures toward neutral mono-, di-, and triphosphinoboranes
Mes2BAr (2a), MesBAr2 (3a), and BAr3 (4a) (Ar = 4-(Ph2P)-
2,6-Me2-C6H2) and their cationic derivatives, [Mes2BArP]I
([2]I), [MesBArP ]I2 ([3]I2), and [BArP ]I3 ([4]I3) (ArP = 4-
2
3
(MePh2P)-2,6-Me2-C6H2), are shown in Scheme 1. Selective
lithiation at the C-5 position of 2,5-dibromo-m-xylene with n-
BuLi in ether followed by reaction with Ph2PCl in THF/ether
produced (4-bromo-3,5-dimethylphenyl)diphenylphosphine
(1) in high yield (84%). The neutral borane compounds 2a−
4a were obtained by reaction of a lithium salt derived from 1
with 1 equiv of Mes2BF, a half equiv of MesB(OMe)2, and one-
third equiv of BF3·OEt2 in ether at −78 °C, respectively, in
moderate to low yield (45% for 2a, 13% for 3a, and 50% for
4a). The formation of 2a−4a was characterized by multinuclear
1
elemental analysis. The H NMR spectra showed the methyl
proton resonances at ca. δ 3 ppm as a doublet, indicating
methylation on the phosphorus atom. The presence of a four-
coordinated phosphonium moiety was also confirmed by the
31P signals at ca. δ 21 ppm. As similarly observed in the neutral
boranes, the C−H protons of the ArP group in [2]+−[4]3+
appear as doublets and the two ArP groups in [3]2+ are in a
different chemical environment, giving rise to the two sets of
methyl and C−H proton signals of the ArP groups, as well as
NMR spectroscopy and elemental analysis. While H and 13C
1
NMR spectra showed the expected resonances corresponding
820
dx.doi.org/10.1021/om3010542 | Organometallics 2013, 32, 817−823

Organometallics
Article
a
two 31P resonances at δ 19 and 22 ppm (Figures S1−S3). It was
shown that theses signals do not coalesce at elevated
temperatures, up to 100 °C (Figure S2). Although the 11B
NMR signals for the multiphosphonium boranes [3]2+ and
[4]3+ were not observed, the monophosphonium borane [2]+
showed the 11B signal at δ 81 ppm, confirming the presence of
the trigonal boron atom. Among the cationic boranes, the
crystal structure of [4]I3 was unequivocally determined by the
X-ray diffraction method (Figure 2 and Table S1). As similarly
Scheme 2 .
a
(i) Amberite IRA-400(Cl) ion-exchange resin, MeOH/H2O, 25 °C,
72%.
1:1 binding isotherms are equal to K = 2.3 × 101 M−1 for [2]+,
3.6 × 105 M−1 for [3]2+, and 1.0 × 107 M−1 for [4]3+.
Comparison of these K values reveals a drastic increase of
fluorophilicity with the increasing number of phosphonium
moieties; the K value increases by 4 and 6 orders of magnitude
on going from mono- to dication and to trication, respectively
(insets in Figure 3). These results indicate that the introduction
of multiple phosphonium moieties into the triarylborane has an
apparent additive effect on the Lewis acidity enhancement of
the triarylborane probably due to the increased Coulombic and
inductive effects of the phosphonium groups.9,15
Next, cyanide ion affinities of the phosphonium boranes [2]I
and [3]I2 were investigated and compared with their fluoride
ion affinities. UV−vis titrations were carried out in buffered
DMSO/H2O (7:3 v/v, 10 mM HEPES, pH 7) solution. Due to
competitive protonation of cyanide in aqueous solution, the
pKa of HCN in DMSO/H2O (7:3 v/v) was first measured,
which afforded a pKa of 9.5, and was used in the determination
of the cyanide binding constant.14 The low-energy absorption
bands in the region 300−375 nm for [2]I and [3]I2 are
gradually quenched upon the addition of incremental amounts
of cyanide (Figure 4). On the basis of the 1:1 binding
isotherms, the cyanide binding constants (K) were calculated to
be K = 5.8 × 104 M−1 for [2]+ and 5.7 × 107 M−1 for [3]2+,
respectively (insets in Figure 4).
Figure 2. Crystal structure of [4]3+ in [4]I3 (50% ellipsoid). H atoms
and iodides were omitted for clarity. Selected bond lengths (Å) and
angles (deg): B−C(1) 1.584(5), P−C(4) 1.792(5), P−C(21)
1.775(6); C(1)−B−C(1a) 120.0, C(4)−P−C(9) 108.2(2), C(4)−
P−C(20) 107.4(2), C(4)−P−C(21) 111.0(3).
noted in neutral 4a, the three MePh2P moieties are appended at
the para-positions of the ArP groups, which form a propeller-
like conformation around the trigonal boron atom. Since [4]I3
crystallizes in the rhombohedral crystal system, the C3-axis
perpendicular to the trigonal boron center is clearly observed.
Although the iodide salts of cationic boranes [2]I−[4]I3 are
highly soluble in alcohol and partially in aqueous solution, they
are poorly soluble in water, limiting their use in the anion
binding in water. Thus, the chloride salt of triphosphonium
borane [4]Cl3 was prepared by repeated treatments of [4]I3
with a chloride-exchange resin (Scheme 2).19 It was found that
[4]Cl3 freely dissolves in water, and the spectroscopic features
are essentially identical to those for [4]I3.
Anion Binding Studies. To investigate anion binding
properties and an additive effect of multiple cationic moieties
on the Lewis acidity enhancement of triarylborane, UV−vis
titration experiments with fluoride were first carried out using
[2]I−[4]I3 in a DMSO/H2O (7:3 v/v) mixture. The choice of
DMSO/H2O (7:3 v/v) was to differentiate between the
binding constants of the cationic boranes within a measurable
range by absorption titration (vide infra). The phosphonium
boranes [2]I, [3]I2, and [4]I3 feature low-energy absorption
bands in the region 300−375 nm (Figure 3). Upon addition of
incremental amounts of fluoride, all absorption bands are
gradually quenched as a result of fluoride binding to the boron
center. The fluoride binding constants (K) estimated from the
These results indicate that both [2]+ and [3]2+ have a high
affinity for cyanide in the DMSO/H2O (7:3 v/v) mixture. The
K values of [2]+ and [3]2+ for cyanide are shown to be greater
by 4 and 2 orders of magnitude than those for fluoride,
respectively. An increase of cyanide ion affinity by 3 orders of
magnitude for dicationic [3]2+ compared with that for
monocationic [2]+ is also consistent with that observed in
the fluoride binding studies, indicating an additive effect of the
cationic moiety on the Lewis acidity enhancement of the
triarylborane. Because neutral triarylboranes such as Mes3B do
not form any detectable quantities of cyanoborate or
fluoroborate complexes in aqueous solution, these results
clearly demonstrate that the introduction of multiple
phosphonium moieties into the triarylborane significantly
enhances the Lewis acidity of the boron atom, the extent of
which also increases with the increasing number of
phosphonium moieties.
Encouraged by these results, we decided to attempt anion
binding studies in real physiological systems, such as in water
by use of triphosphonium borane. The water-soluble chloride
salt [4]Cl3 was used in the UV−vis titration experiments.
Compound [4]Cl3 features a low-energy absorption band
centered at 318 nm (log ε = 4.13) in buffered H2O (10 mM
HEPES, pH 7) (Figure 5). Addition of cyanide ions to a
821
dx.doi.org/10.1021/om3010542 | Organometallics 2013, 32, 817−823

Organometallics
Article
Figure 3. Spectral changes in the UV−vis absorption of a solution of [2]I (left), [3]I2 (middle), and [4]I3 (right) in DMSO/H2O (7:3 v/v, 5.0 ×
10−5 M) upon addition of TBAF ((0−1.0) × 10−2 M for [2]I, (0−9.8) × 10−5 M for [3]I2, (0−7.0) × 10−5 M for [4]I3). The inset shows the
absorbance at 317, 319, and 320 nm, respectively, as a function of [F−]. The line corresponds to the binding isotherm calculated with K = 2.3 × 101
M−1 for [2]I, 3.6 × 105 M−1 for [3]I2, and 1.0 × 107 M−1 for [4]I3.
contrast, the absorption spectrum of [4]Cl3 in water (10 mM
HEPES, pH 7) is very slightly affected in the presence of
fluoride ions, resulting in the absorption quenching of less than
4% upon the addition of up to 16 equiv of fluoride (Figure 6,
Figure 4. Spectral changes in the UV−vis absorption of a solution of
[2]I (left) and [3]I2 (right) in buffered DMSO/H2O (7:3 v/v, 5.0 ×
10−5 M, 10 mM HEPES, pH 7) upon addition of TBACN ((0−5.9) ×
10−3 M for [2]I, (0−9.8) × 10−5 M for [3]I2). The inset shows the
absorbance at 318 nm as a function of [CN−]. The line corresponds to
the binding isotherm calculated with K = 5.8 × 104 M−1 for [2]I and
5.7 × 107 M−1 for [3]I2.
Figure 6. Spectral changes in the UV−vis absorption of a solution of
[4]Cl3 in buffered H2O (3.9 × 10−5 M for F− and 3.4 × 10−5 M for
OH−, 10 mM HEPES, pH 7) upon addition of KF (left) and NaOH
(right).
left). Finally, because hydroxide ions may potentially bind to a
boron center of phosphonium borane in water at pH 7,10 we
monitored the absorbance of [4]Cl3 in the presence of
hydroxide ions (Figure 6, right). However, the absorption
band of [4]Cl3 remains unaffected upon the addition of up to
16 equiv of hydroxide in buffered H2O solution. The steric
protection provided by the six ortho-methyl groups to the
boron center is most likely responsible for the lack of hydroxide
binding. These results thus attest that the triphosphonium
Figure 5. Spectral changes in the UV−vis absorption of a solution of
borane [4]Cl3 can act as a selective receptor for cyanide ions in
[4]Cl3 in buffered H2O (3.9 × 10−5 M, 10 mM HEPES, pH 7) upon
water at neutral pH.
addition of NaCN ((0−1.93) × 10−4 M). The inset shows the
absorbance at 318 nm as a function of [CN−]. The line corresponds to
CONCLUSION
the binding isotherm calculated with K = 1.7 × 107 M−1.
■
We have synthesized and characterized a series of mono-, di-,
and triphosphonium-substituted triarylboranes, [2]+, [3]2+, and
buffered H2O solution of [4]Cl3 led to a gradual decrease in the
intensity of the absorption band, as similarly observed in the
fluoride binding of [4]I3 in DMSO/H2O (7:3 v/v). This
phenomenon reflects conversion of triphosphonium borane
into the corresponding triphosphonium cyanoborate [4CN]2+,
whose stability constant is equal to K = 1.7 × 107 M−1. In
[4]3+. It was demonstrated by absorption titrations with
fluoride and cyanide ions in a DMSO/H2O (7:3 v/v) mixture
that the introduction of multiple phosphonium moieties into
the triarylborane significantly enhances the Lewis acidity of a
boron center and has an additive effect on the Lewis acidity
enhancement probably due to the increased Coulombic and
822
dx.doi.org/10.1021/om3010542 | Organometallics 2013, 32, 817−823

Organometallics
Article
Mullen, K. J. Am. Chem. Soc. 2008, 130, 12477−12484. Huh, J. O.; Do,
̈
inductive effects of phosphonium groups. The results also
showed that the cyanide binding constants of the cationic
boranes are greater by 2−4 orders of magnitude than those in
the fluoride binding. More importantly, while fluoride hardly
binds the boron center of triphosphonium borane [4]3+ in
water at pH 7, [4]3+ complexes cyanide ions under the same
conditions with a high binding constant of 1.7 × 107 M−1.
These results indicate that multiphosphonium boranes such as
[4]3+ can be used as a selective sensor for cyanide in real
physiological systems.
Y.; Lee, M. H. Organometallics 2008, 27, 1022−1025. Day, J. K.;
Bresner, C.; Coombs, N. D.; Fallis, I. A.; Ooi, L.-L.; Aldridge, S. Inorg.
Chem. 2008, 47, 793−804. Broomsgrove, A. E. J.; Addy, D. A.;
Bresner, C.; Fallis, I. A.; Thompson, A. L.; Aldridge, S. Chem.Eur. J.
2008, 14, 7525−7529. Sakuda, E.; Funahashi, A.; Kitamura, N. Inorg.
Chem. 2006, 45, 10670−10677. Parab, K.; Venkatasubbaiah, K.; Jakle,
̈
́
F. J. Am. Chem. Soc. 2006, 128, 12879−12885. Melaïmi, M.; Sole, S.;
Chiu, C.-W.; Wang, H.; Gabbaï, F. P. Inorg. Chem. 2006, 45, 8136−
8143. Hudnall, T. W.; Melaimi, M.; Gabbaï, F. P. Org. Lett. 2006, 8,
2747−2749. Liu, X. Y.; Bai, D. R.; Wang, S. Angew. Chem., Int. Ed.
2006, 45, 5475−5478. Melaïmi, M.; Gabbaï, F. P. J. Am. Chem. Soc.
2005, 127, 9680−9681. Liu, Z. Q.; Shi, M.; Li, F. Y.; Fang, Q.; Chen,
Z. H.; Yi, T.; Huang, C. H. Org. Lett. 2005, 7, 5481−5484.
ASSOCIATED CONTENT
■
S
* Supporting Information
Sundararaman, A.; Victor, M.; Varughese, R.; Jakle, F. J. Am. Chem.
̈
Crystallographic data in cif format. This material is available
free of charge via the Internet at http://pubs.acs.org.
́
Soc. 2005, 127, 13748−13749. Sole, S.; Gabbaï, F. P. Chem. Commun.
2004, 1284−1285. Kubo, Y.; Yamamoto, M.; Ikeda, M.; Takeuchi, M.;
Shinkai, S.; Yamaguchi, S.; Tamao, K. Angew. Chem., Int. Ed. 2003, 42,
2036−2040. Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J.
Am. Chem. Soc. 2002, 124, 8816−8817. Yamaguchi, S.; Akiyama, S.;
Tamao, K. J. Am. Chem. Soc. 2001, 123, 11372−11375.
(6) Wade, C. R.; Ke, I.-S.; Gabbaï, F. P. Angew. Chem., Int. Ed. 2012,
51, 478−481. Zhao, H.; Gabbaï, F. P. Organometallics 2012, 31, 2327−
2335. Wade, C. R.; Gabbaï, F. P. Organometallics 2011, 30, 4479−
4481. Kim, Y.; Huh, H.-S.; Lee, M. H.; Lenov, I. L.; Zhao, H.; Gabbaï,
F. P. Chem.Eur. J. 2011, 17, 2057−2062. Zhao, H.; Gabbaï, F. P.
Nat. Chem. 2010, 2, 984−990. Wade, C. R.; Gabbaï, F. P. Inorg. Chem.
2010, 49, 714−720. Matsumoto, T.; Wade, C. R.; Gabbaï, F. P.
Organometallics 2010, 29, 5490−5495. Wade, C. R.; Gabbaï, F. P.
Dalton Trans. 2009, 9169−9175. Hudnall, T. W.; Gabbaï, F. P. Chem.
Commun. 2008, 4596−4597. Chiu, C.-W.; Gabbaï, F. P. Dalton Trans.
2008, 814−817. Lee, M. H.; Gabbaï, F. P. Inorg. Chem. 2007, 46,
8132−8138. Chiu, C.-W.; Gabbaï, F. P. J. Am. Chem. Soc. 2006, 128,
14248−14249.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ykdo@kaist.ac.kr; lmh74@ulsan.ac.kr.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Basic Science Research
Program (No. 2010-0008264 for Y.D. and No. 2012039773 for
M.H.L.) and Priority Research Centers program (No. 2009-
0093818) through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and
Technology.
(7) Sun, Y.; Hudson, Z. M.; Rao, Y.; Wang, S. Inorg. Chem. 2011, 50,
3373−3378. Xu, W.-J.; Liu, S.-J.; Zhao, X.-Y.; Sun, S.; Cheng, S.; Ma,
T.-C.; Sun, H.-B.; Zhao, Q.; Huang, W. Chem.Eur. J. 2010, 16,
7125−7133. Huh, J. O.; Kim, H.; Lee, K. M.; Lee, Y. S.; Do, Y.; Lee,
M. H. Chem. Commun. 2010, 46, 1138−1140. Sun, Y.; Wang, S. Inorg.
Chem. 2009, 48, 3755−3767. Sun, Y.; Ross, N.; Zhao, S.-B.; Huszarik,
K.; Jia, W.-L.; Wang, R.-Y.; Macartney, D.; Wang, S. J. Am. Chem. Soc.
2007, 129, 7510−7511.
REFERENCES
■
(1) Aaseth, J.; Shimshi, M.; Gabrilove, J. L.; Birketvedt, G. S. J. Trace
Elem. Exp. Med. 2004, 17, 83−92. Kleerekoper, M. Endocrinol. Metab.
Clin. North Am. 1998, 27, 441−452.
(2) Carton, R. J. Fluoride 2006, 39, 163−172.
(3) Young, C. A.; Twidwell, L. G.; Anderson, C. G. Cyanide: Social,
Industrial and Economic Aspects; Minerals, Metals, and Materials
Society: Warrendale, 2001. Baskin, S. I.; Brewer, T. G. Cyanide
Poisoning. In Medical Aspects of Chemical and Biological Warfare;
Sidell, F. R.; Takafuji, E. T.; Franz, D. R., Eds.; TMM Publications:
Washington, D.C., 1997; pp 271−286.
(8) Kim, Y.; Zhao, H.; Gabbaï, F. P. Angew. Chem., Int. Ed. 2009, 48,
4957−4960.
(9) Chiu, C.-W.; Kim, Y.; Gabbaï, F. P. J. Am. Chem. Soc. 2009, 131,
60−61.
(10) Kim, Y.; Gabbaï, F. P. J. Am. Chem. Soc. 2009, 131, 3363−3369.
(11) Agou, T.; Sekine, M.; Kobayashi, J.; Kawashima, T. Chem.Eur.
J. 2009, 15, 5056−5062.
(4) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge, S.; Gabbaï, F. P.
Chem. Rev. 2010, 110, 3958−3984. Jakle, F. Chem. Rev. 2010, 110,
3985−4022. Hudnall, T. W.; Chiu, C.-W.; Gabbaï, F. P. Acc. Chem. Res.
2009, 42, 388−397. Hudson, Z. M.; Wang, S. Acc. Chem. Res. 2009, 42,
1584−1596. Cametti, M.; Rissanen, K. Chem. Commun. 2009, 2809−
2829.
̈
(12) Hudnall, T. W.; Kim, Y.-M.; Bebbington, M. W. P.; Bourissou,
D.; Gabbaï, F. P. J. Am. Chem. Soc. 2008, 130, 10890−10891.
(13) Lee, M. H.; Agou, T.; Kobayashi, J.; Kawashima, T.; Gabbaï, F.
P. Chem. Commun. 2007, 1133−1135.
(5) Sung, W. Y.; Park, M. H.; Park, J. H.; Eo, M.; Yu, M.-S.; Do, Y.;
Lee, M. H. Polymer 2012, 53, 1857−1863. Vadavi, R. S.; Kim, H.; Lee,
K. M.; Kim, T.; Lee, J.; Lee, Y. S.; Lee, M. H. Organometallics 2012, 31,
31−34. Schmidt, H. C.; Reuter, L. G.; Hamacek, J.; Wenger, O. S. J.
Org. Chem. 2011, 76, 9081−9085. Park, M. H.; Kim, T.; Huh, J. O.;
Do, Y.; Lee, M. H. Polymer 2011, 52, 1510−1514. He, X.; Yam, V. W.-
W. Org. Lett. 2011, 13, 2172−2175. Hudson, Z. M.; Liu, X.-Y.; Wang,
S. Org. Lett. 2011, 13, 300−303. Siewert, I.; Fitzpatrick, P.;
Broomsgrove, A. E. J.; Kelly, M.; Vidovic, D.; Aldridge, S. Dalton
Trans. 2011, 40, 10345−10350. Fu, G.-L.; Pan, H.; Zhao, Y.-H.; Zhao,
C.-H. Org. Biomol. Chem. 2011, 9, 8141−8146. Broomsgrove, A. E. J.;
Addy, D. A.; Di Paolo, A.; Morgan, I. R.; Bresner, C.; Chislett, V.;
Fallis, I. A.; Thompson, A. L.; Vidovic, D.; Aldridge, S. Inorg. Chem.
2010, 49, 157−173. Sun, Y.; Wang, S. Inorg. Chem. 2010, 49, 4394−
4404. You, Y.; Park, S. Y. Adv. Mater. 2008, 20, 3820−3826. Kawachi,
A.; Tani, A.; Shimada, J.; Yamamoto, Y. J. Am. Chem. Soc. 2008, 130,
4222−4223. Zhao, Q.; Li, F.; Liu, S.; Yu, M.; Liu, Z.; Yi, T.; Huang, C.
Inorg. Chem. 2008, 47, 9256−9264. Zhou, G.; Baumgarten, M.;
(14) Hudnall, T. W.; Gabbaï, F. P. J. Am. Chem. Soc. 2007, 129,
11978−11986.
(15) Lee, K. M.; Huh, J. O.; Kim, T.; Do, Y.; Lee, M. H. Dalton
Trans. 2011, 40, 11758−11764.
(16) Good, N. E.; Winget, G. D.; Winter, W.; Connolly, T. N.; Izawa,
S.; Singh, R. M. M. Biochemistry 1966, 5, 467−477.
(17) Matsumi, N.; Chujo, Y. Polym. Bull. 1997, 38, 531−536.
(18) Grisdale, P. J.; Williams, J. L. R.; Glogowski, M. E.; Babb, B. E. J.
Org. Chem. 1971, 36, 544−549.
(19) Mohr, B.; Lynn, D. M.; Grubbs, R. H. Organometallics 1996, 15,
4317−4325.
823
dx.doi.org/10.1021/om3010542 | Organometallics 2013, 32, 817−823