Cyanide anion binding by a triarylborane at the outer rim of a cyclometalated ruthenium(II) cationic complex

Article abstract of DOI:10.1021/ic9020349

As part of our ongoing interest In the design of boron-based cyanide anion receptors, we have synthesized a triaryl borane decorated by a cationic Ru(II) complex and have investigated its anion binding properties. This new borane, [(2,2′-bpy)Ru(k-C,N-2-(dlmesltylborylphenyl)pyridinato)]OTf ([2]OTf), binds both fluoride and cyanide anions In organic solvents to afford 2-F and 2-CN whose crystal structures have been determined. UV-vis titrations in 9/1 CHCI₃/DMF (vol.) afforded K_(F-1) = 1.1(±0.1) × 10⁴M^-1and K_(CN-) = 3.0(±1.0) × 10⁶M^-1 indicating that [2]⁺has a higher affinity for cyanide than for fluoride In this solvent mixture. These elevated binding constants show that the cationic Ru(II) complex Increases the anion affinity of these complexes via Coulombic and Inductive effects. The UV-vis spectral changes which accompany either fluoride or cyanide binding to the boron center are similar and include a 30 nm bathochromic shift of the metal-to-ligand charge transfer band. This shift is attributed to an increase In the donor ability of the boron-substituted phenylpyrldine ligand upon anion binding to the boron center. Accordingly, cyclic voltammetry revealed that the Ru^II/III redox couple of [2]OTf (E_1/2 = +0.051 V vs Fc/Fc⁺) undergoes a cathodic shift upon F^- ΔE _1/2 = -0.242 V vs Fc/Fc⁺) or CN^- (ΔE _1/2 = -0.198 V vs Fc/Fc⁺) binding.

Full text of DOI:10.1021/ic9020349

714 Inorg. Chem. 2010, 49, 714–720
DOI: 10.1021/ic9020349
Cyanide Anion Binding by a Triarylborane at the Outer Rim of a Cyclometalated
Ruthenium(II) Cationic Complex
Casey R. Wade and Franc-ois P. Gabba¨ı*
Department of Chemistry, Texas A&M University, College Station, Texas 77843
Received October 14, 2009
As part of our ongoing interest in the design of boron-based cyanide anion receptors, we have synthesized a triaryl
borane decorated by a cationic Ru(II) complex and have investigated its anion binding properties. This new borane,
[(2,20-bpy)Ru(κ-C,N-2-(dimesitylborylphenyl)pyridinato)]OTf ([2]OTf), binds both fluoride and cyanide anions in
organic solvents to afford 2-F and 2-CN whose crystal structures have been determined. UV-vis titrations in 9/1
CHCl3/DMF (vol.) afforded K(F-) = 1.1((0.1) ꢀ 104 M-1 and K(CN-) = 3.0((1.0) ꢀ 106 M-1 indicating that [2]þ has a
higher affinity for cyanide than for fluoride in this solvent mixture. These elevated binding constants show that the
cationic Ru(II) complex increases the anion affinity of these complexes via Coulombic and inductive effects. The
UV-vis spectral changes which accompany either fluoride or cyanide binding to the boron center are similar and
include a 30 nm bathochromic shift of the metal-to-ligand charge transfer band. This shift is attributed to an increase in
the donor ability of the boron-substituted phenylpyridine ligand upon anion binding to the boron center. Accordingly,
cyclic voltammetry revealed that the RuII/III redox couple of [2]OTf (E1/2 = þ0.051 V vs Fc/Fcþ) undergoes a cathodic
shift upon F- (ΔE1/2 = -0.242 V vs Fc/Fcþ) or CN- (ΔE1/2 = -0.198 V vs Fc/Fcþ) binding.
Introduction
the electron deficiency of the boron center while providing a
Coulombic drive for the complexation of the anion.6,7
Triaryl boranes are receiving a great deal of interest as
receptors for small nucleophilic anions1 including the highly
toxic cyanide anion.2-5 The validity of this approach can be
illustrated by the ability of the cationic boranes [I]þ and [II]þ
to complex cyanide anions in neutral water at concentrations
aslow as 50 ppb.3 These unusualpropertiescan beassignedto
the presence of the cationic group which inductively enhances
In parallel to these developments, an increasing amount of
attention has been devoted to the synthesis and anion binding
properties of boron derivatives containing a transition
metal moiety.5,8-10 The study of such compounds has been
*To whom correspondence should be addressed. E-mail: francois@tamu.
edu.
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2009, 131, 60–61. Agou, T.; Sekine, M.; Kobayashi, J.; Kawashima, T. Chem.;
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r
pubs.acs.org/IC
Published on Web 12/11/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 715
motivated by the unusual physicochemical properties im-
parted by the presence of the transition metal and their
response to anion binding at the boron center. Examples of
such compounds include III whose oxidation potential un-
dergoes a cathodic shift of 560 mV upon binding of cyanide
to the boron center.5 Interestingly, relatively little effort has
been devoted to the study of cationic transition metal/boron
derivatives as anion receptors. Anticipating the occurrence of
favorable Coulombic and inductive effects, we have set out to
synthesize examples of such compounds and probe their
affinity toward cyanide among other anions. Prior efforts
in this area of chemistry have yielded derivatives such as
[IV]þ-[VII]þ.8 Although such derivatives have been well
characterized, relatively little is known about their affinity
for anions other than fluoride, and the structures of the
fluoride adducts remain unknown.
Synthesis of [(2,20-bpy)Ru(K-C,N-2-(dimesitylborylphenyl)-
pyridinato)]OTf ([2]OTf). AgOTf (0.102 g, 0.400 mmol) was
added as a solid to a solution of bpy2RuCl2 2H2O (0.100 g,
3
0.2 mmol) and 1 (0.400 g, 1.00 mmol) in 3 mL of CH2Cl2 and
10 mL of MeOH. The mixture was heated to reflux for 2 h and
the solvent removed in vacuo. The resulting purple solid was
extracted with 20 mL of CH2Cl2 and filtered over Celite. The
solvent was again removed, and the residue washed with 3 ꢀ
10 mL of Et2O and 5 mL of cold acetonitrile. Recrystallization
of the crude product by slow diffusion of Et2O into a CH2Cl2
solution gave the pure product in 52% yield as dark purple
needles. 1H NMR (399.9 MHz, pyridine-d5): δ 2.01 (s, 12H,
Mes-CH3), 2.31 (s, 6H, Mes-CH3), 6.72 (s, 1H, Ph-CH), 6.81 (s,
3
4H, Mes-CH), 6.94 (t, JH-H = 7.08 Hz, 1H, Pyr-CH), 7 (t,
3JH-H = 7.14 Hz, 1H, Pyr-CH), 7.05-7.09 (m, 2H, Pyr-CH),
3
3
7.34 (d, JH-H = 7.88 Hz, 1H, Pyr-CH), 7.40 (t, JH-H
=
6.41 Hz, 1H, Pyr-CH), 7.54 (t, 3JH-H = 8.06 Hz, 1H, Pyr-CH),
7.58(t, 3JH-H =7.33Hz, 1H, Pyr-CH), 7.65(t, 3JH-H = 8.06Hz,
1H, Pyr-CH), 7.68 (d, 3JH-H = 7.68 Hz, 1H, Pyr-CH), 7.72 (d,
3JH-H = 6.04 Hz 1H, Pyr-CH), 7.82 (d, 3JH-H = 5.67 Hz 1H,
Targeting the cyanide anion as an analyte, we decided to
focus on a robust complex whose transition metal center
would not react with the cyanide ligand. In this paper, we
report a series of results that we have obtained on the
synthesis and study of such a compound.
3
Pyr-CH), 7.89 (d, JH-H = 8.24 Hz, 1H, Pyr-CH), 8.01 (d,
3JH-H = 5.67 Hz, 1H, Pyr-CH), 8.05 (d, 3JH-H = 7.87 Hz, 1H,
Pyr-CH), 8.12 (d, JH-H = 5.31 Hz, 1H, Pyr-CH), 8.16 (d,
3
3JH-H = 8.24 Hz, 1H, Pyr-CH), 8.32-8.29 (m, 2H, Pyr-CH),
8.46 (d, 3JH-H = 8.42 Hz, 1H, Pyr-CH), 8.50 (d, 3JH-H = 7.69
3
Hz, 1H, Pyr-CH), 8.61 (d, JH-H = 8.24 Hz, 1H, Pyr-CH).
13C NMR (100.5 MHz, acetone-d6): δ 21.07 (Mes-p-CH3),
23.25 (Mes-o-CH3), 120.38, 123.50, 123.58, 123.70, 123.93,
124.04, 124.29, 126.59, 126.74, 126.88, 127.89, 128.54, 128.91,
133.94, 134.28, 135.65, 136.44, 137.07, 138.53, 140.70, 142.87,
144.01, 150.00, 150.02, 150.12, 150.84, 151.12, 154.87, 155.96,
157.25, 157.61, 158.63, 167.92, 183.91, 192.04 (Ph-C-Ru). 11B
NMR (128.2 MHz, CD2Cl2):
δ 74. Anal. Calcd for
C50H45BF3N5O3SRu: C, 62.24; H, 4.70; N, 7.26. Found: C,
62.15; H, 4.67; N, 7.24.
Generation of 2-F and 2-CN. 2-F and 2-CN were prepared and
characterized by multinuclear NMR in situ by addition of a
slight excess of TBAF or TEACN, respectively, to solutions of
[2]OTf in pyridine-d5. or acetone- d6. Data for 2-CN: 1H NMR
(399.9 MHz, pyridine-d5): δ 2.14 (s, 6H, Mes-CH3), 2.24 (s, 3H,
Mes-CH3), 2.29 (s, 3H, Mes-CH3), 2.45 (s, 6H, Mes-CH3), 6.53 (s,
1H, Ph-CH), 6.64 (s, 2H, Mes-CH), 6.68 (t, 3JH-H = 6.77 Hz,
1H, Pyr-CH), 6.75 (s, 2H, Mes-CH), 6.84-6.95 (m, 4H, Pyr-CH),
7.30 (d, 3JH-H = 7.14 Hz, 1H, Pyr-CH), 7.40 (d, 3JH-H = 7.14
Hz, 1H, Pyr-CH), 7.48-7.60 (m, 3H, Pyr-CH), 7.82-7.90 (m,
3H, Pyr-CH), 7.96 (m, 2H, Pyr-CH), 8.07 (m, 2H, Pyr-CH), 8.23
Experimental Section
General Considerations. 2-(40-Dimesitylborylphenyl)pyridine
was synthesized by a published procedure.11 Dimesitylboron
fluoride (Mes2BF) and tetraethylammonium cyanide (TEACN)
3
(d, 1H, Pyr-CH, JH-H = 8.06 Hz), 8.34 (d, 1H, Pyr-CH,
3
3JH-H = 7.69 Hz), 8.49 (d, 1H, Pyr-CH, JH-H = 78.24 Hz),
were purchased from Aldrich. n-Bu4NF 3H2O (TBAF) was
3
3
8.52 (d, 1H, Pyr-CH, JH-H = 5.5 Hz), 8.62 (d, 1H, Pyr-CH,
purchased from Alfa Aesar and used as received. Solvents were
dried by passing through an alumina column (n-hexane,
CH2Cl2) or refluxing under N2 over Na/K (Et2O and tetrahy-
drofuran (THF)). Air-sensitive compounds were handled under
a N2 atmosphere using standard Schlenk and glovebox techni-
ques. UV-vis spectra were recorded on either an HP8453
spectrophotometer or an Ocean Optics USB4000 spectrometer
with an Ocean Optics ISS light source. Elemental analyses were
performed at Atlantic Microlab (Norcross, GA). NMR
spectra were recorded on Varian Unity Inova 400 FT NMR
(399.59 MHz for 1H, 375.99 MHz for 19F, 128.19 MHz for 11B,
100.45 MHz for 13C) spectrometer at ambient temperature
unless otherwise stated. Chemical shifts δ are given in parts
per million, and are referenced against external Me4Si (1H, 13C)
3JH-H = 8.06 Hz). 13C NMR (100.5 MHz, pyridine-d5): δ 21.09
(Mes-p-CH3), 21.12 (Mes-p-CH3), 25.84 (Mes-o-CH3), 26.42
(Mes-o-CH3), 118.04, 120.64, 122.69, 122.75, 123.03, 123.21,
125.56, 125.77, 125.84, 127.17, 129.24, 129.38, 130.93, 131.24,
131.44, 132.42, 133.18, 134.12, 135.27, 140.31, 142.67, 142.76,
143.67, 149.01, 149.12, 155.08, 156.76, 157.13, 158.02, 165.89,
169.50, 189.48 (Ph-C-Ru). 11B NMR (128.2 MHz, pyridine-d5): δ
-12.7 Data for 2-F: 1H NMR (399.9 MHz, acetone- d6): δ 1.28 (s,
3H, Mes-CH3), 1.61 (s, 6H, Mes-CH3), 1.89 (s, 6H, Mes-CH3),
2.14 (s, 3H, Mes-CH3), 6.13 (bs, 1H, Ph-CH), 6.26 (bs, 2H, Mes-
CH), 6.33 (bs, 2H, Mes-CH), 6.75 (t, 1H, Pyr-CH, 3JH-H = 6.52
Hz), 6.94 (br, 1H, Pyr-CH), 7.15-7.21 (bm, 3H, Pyr-CH), 7.37
(br, 2H, Pyr-CH), 7.45 (d, 1H, Pyr-CH, 3JH-H = 5.50 Hz), 7.50
(d, 1H, Pyr-CH, 3JH-H = 6.4 Hz), 7.55 (t, 1H, Pyr-CH, 3JH-H
=
and BF3 Et2O (11B, 19F).
3
7.60Hz), 7.61 (t, 1H, Pyr-CH, 3JH-H =7.62Hz), 7.67(t, 1H, Pyr-
CH, 3JH-H = 6.8 Hz), 7.83-7.88 (m, 3H, Pyr-CH), 8.02-8.07
(m, 3H, Pyr-CH), 8.19 (br, 1H, Pyr-CH), 8.32 (br, 1H, Pyr-CH),
8.70 (d, 1H, Pyr-CH, 3JH-H = 8.26 Hz), 8.80 (d, 1H, Pyr-CH,
3JH-H = 8.26 Hz). 13C NMR (100.5 MHz, pyridine-d5): δ 21.27
(Mes-p-CH3), 21.30 (Mes-p-CH3) 25.82 (Mes-o-CH3), 26.22
(10) Zhou, G.; Ho, C.-L.; Wong, W.-Y.; Wang, Q.; Ma, D.; Wang, L.;
Lin, Z.; Marder, T. B.; Beeby, A. Adv. Funct. Mater. 2008, 18, 499–511. Rao,
Y.-L.; Wang, S. Inorg. Chem. 2009, 48, 7698–7713.
(11) Wade, C. R.; Gabbaı, F. P. Dalton Trans. 2009, 9169-9175.

716 Inorganic Chemistry, Vol. 49, No. 2, 2010
Wade and Gabbaı
(Mes-o-CH3), 117.76, 120.18, 122.69, 122.75, 123.03, 123.12,
124.06, 125.46, 125.68, 125.92, 127.10, 128.79, 128.86, 130.48,
130.78, 132.22, 133.07, 133.98, 135.11, 139.69, 142.35, 148.91,
149.25, 149.37, 155.28, 155.70, 156.89, 157.14, 158.01, 168.60,
169.91, 188.98 (Ph-C-Ru). 19F NMR (375.97 MHz, pyri-
dine-d5): δ -176.39 (90%), 176.19 (10%). Anal. Calcd for
C54H50BFN6Ru: C, 70.97; H, 5.51; N, 9.20. Found: C, 70.30;
H, 5.43; N, 9.25.
Scheme 1
Titration of [2]OTf with Fluoride and Cyanide in THF/DMF.
Solutions of [2]OTf (3.0 mL, 2.5 ꢀ 10-5 M, 9/1 DMF/THF)
were titrated with incremental (5 μL) amounts of fluoride or
cyanide anions by addition of a 3.5 ꢀ 10-3 M solution of TBAF
in DMF or a 3.0 ꢀ 10-3 M solution of TEACN in DMF.
Electrochemistry. Electrochemical experiments were per-
formed with an electrochemical analyzer from CH Instruments
(Model 610A) with a glassy carbon working electrode and a
platinum auxiliary electrode. The reference electrode was built
from a silver wire inserted in a small glass tube fitted with a
porous vycor frit at the tip and filled with a THF solution
containing (n-Bu)4NPF6 (0.1 M) and AgNO3 (0.005 M). All
three electrodes were immersed in a N,N-dimethylformamide
(DMF) solution (3 mL) containing 0.1 M of supporting electro-
lyte ((n-Bu)4NPF6) and 0.001 M of [2]OTf. 2-F and 2-CN were
prepared in situ in the electrochemical cell by addition of
0.1 mL of 0.03 M TEACN or TBAF solutions in DMF. In all
cases, ferrocene was used as an internal standard, and all
potentials are reported with respect to the E1/2 of the Fcþ/Fc
redox couple.
phenylpyridinato) have been previously studied and shown
to possess a relatively inert core.13-16 Moreover, such com-
plexes possess well-understood electrochemical and photo-
physical properties.13-16 Encouraged by these precedents, we
decided to investigate the synthesis of a borylated analogue of
such a complex. Consequently, the known 2-(40-dimesityl-
borylphenyl)pyridine (1),11 which has been incorporated in
iridium and platinum complexes,10 was allowed to react with
(2,20-bpy)2RuCl2 in refluxing CH2Cl2/MeOH in the presence
of AgOTf (Scheme 1) to afford [2]OTf as dark purple needles
in 52% yield after recrystallization from CH2Cl2/Et2O.16 Salt
[2]OTf has been fully characterized, and its crystal structure
determined. It displays moderate solubility in polar organic
solvents such as THF, CH2Cl2, DMSO, acetone, and pyr-
idine but is insoluble in H2O and less polar solvents such as
Et2O and pentane. The 1H NMR spectrum of [2]OTf
(pyridine-d5) displays distinct resonances for each of the aryl
CH groups of the 2,20-bpy and phenylpyridyl ligands. The
presence of single mesityl aromatic CH and ortho- and meta-
CH3 resonances indicates equivalency of boron mesityl
substituents at room temperature. The broad signal observed
at 74 ppm in the 11B NMR spectrum (CD2Cl2) is character-
istic of a triarylborane.
The UV-vis spectrum of [2]OTf (THF/DMF, 9/1 vol.)
displaystwo prominent features: a sharp band at 335 nm(ε =
33 460), assigned to absorption of the triarylborane-based
chromophore, and a broad metal-to-ligand-charge-transfer
(MLCT) band with λmax = 550 nm (ε = 11 650) spanning the
450-625 nm range. This MLCT band appears in the
same range as that reported for the parent cation [(bpy)2-
Ru(2-ppy)]þ.14 Cyclic voltammetry experiments performed
on 2[OTf] show that it undergoes a fully reversible RuII/III
redox couple at E1/2 = þ0.051 V versus Fc/Fcþ (1 mM, 0.1
M TBAPF6, DMF, glassy carbon electrode, 200 mV scan
rate). To understand the effects, if any, of the pendant
dimesitylboryl group on the RuII/III redox couple, the cyclic
voltammogram of the non-borylated complex, [(bpy)2Ru-
(2-ppy)]OTf, was recorded. Under the same conditions,
this complex undergoes oxidation at E1/2 = þ0.025 V versus
Fc/Fcþ, indicating that decoration of the phenylpyridyl
ligand with a dimesitylboryl moiety has only a limited
influence on the redox properties of the complex
(Supporting Information, Figure S1).
Crystallography. Single crystals of [2]OTf could be obtained by
slow diffusion of Et2O into a solution of the compound in CH2Cl2.
Single crystals of 2-F could be obtained by slow evaporation of a
mixture of [2]OTf and TBAF in pyridine-d5. Single crystals of 2-CN
were obtained by slow evaporation of a mixture of [2]OTf and
KCN in 1/1 acetone/MeOH. The crystallographic measurement of
[2]OTf, 2-F, and 2-CN were performed using a Bruker APEX-II
CCD area detector diffractometer, with a graphite-monochro-
˚
mated Mo KR radiation (λ = 0.71069 A). A specimen of suitable
size and quality was selected and mounted onto a nylon loop. The
structure was solved by direct methods, which successfully located
most of the non-hydrogen atoms. Subsequent refinement on F2
using the SHELXTL/PC package (version 6.10) allowed location
of the remaining non-hydrogen atoms.12
Theoretical Calculations. Density functional theory (DFT)
calculations (full geometry optimization) were carried out on
[2]þ, 2-CN, and 2-F starting from the crystal structure geome-
tries with Gaussian03 utilizing the gradient-corrected Becke
exchange functional (B3LYP) and the Lee-Yang-Parr corre-
lation functional. A 6-31þg(d0) basis set was used for C, H, N, B,
and F. A Stuttgart RSC 1997 ECP basis set was used for Ru.
Frequency calculations were also carried out on the optimized
geometry, showing no imaginary frequencies. Single point en-
ergy calculations were performed using the Polarizable Con-
tinuum Model (PCM) and THF as a solvent.
Results and Discussion
Coordinatively saturated cyclometaþlated ruthenium com-
plexes such as [(2,20-bpy)2Ru(2-ppy)] (2-ppy = κ-C,N-2-
(12) Sheldrick, G. M. SHELXTL/PC, Version 6.10; Bruker AXS Inc.:
Madison, WI, 1999.
(13) Reveco, P.; Schmehl, R. H.; Cherry, W. R.; Fronczek, F. R.; Selbin,
J. Inorg. Chem. 1985, 24, 4078–4082.
(14) Constable, E. C.; Holmes, J. M. J. Organomet. Chem. 1986, 301, 203–
208. Reveco, P.; Cherry, W. R.; Medley, J.; Garber, A.; Gale, R. J.; Selbin, J. Inorg.
Chem. 1986, 25, 1842–1845.
(15) Constable, E. C.; Leese, T. A. J. Organomet. Chem. 1987, 335, 293–
299. Constable, E. C.; Housecroft, C. E. Polyhedron 1990, 9, 1939–1947.
(16) Sasaki, I.; Vendier, L.; Sournia-Saquet, A.; Lacroix, P. G. Eur. J.
Inorg. Chem. 2006, 3294–3302.
The crystal structure of [2]OTf confirms the presence of
the cyclometalated ligand bearing a dimesitylboryl group
(Figure 1, Table 1). The Ru-N bond distances are all within
˚
the 2.035(4)-2.070(4) A range, with the exception of the
Ru(1)-N(5) bond trans to the Ru(1)-C(7) bond that is
˚
somewhat elongated (2.152(4) A) because of the strong trans
effect of the carbanionic ligand. The Ru(1)-C(7) bond was
˚
measured to be 2.038(5) A, in the range of Ru-C distances

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 717
2-CN
Table 1. Crystal Data, Data Collections, and Structure Refinements
crystal data
formula
Mr
2[OTf]
2-F
C
964.85
50H45BF3N5O3SRu
C49H45BFN5Ru-C6H5N
917.91
C50H45BN6Ru-CH3OH
873.84
crystal size/mm
crystal system
space group
0.17 ꢀ 0.12 ꢀ 0.08
monoclinic
P2(1)/c
14.3700(4)
21.1565(7)
14.8607(5)
103.511(2)
4392.9(2)
4
1.459
0.467
1984
110(2)
0.18 ꢀ 0.15 ꢀ 0.10
monoclinic
P2(1)/c
14.003(4)
23.236(7)
16.816(4)
128.800(15)
4381.2(19)
4
1.392
0.408
1912
110(2)
0.35 ꢀ 0.08 ꢀ 0.06
tetragonal
P4(3)
˚
a/A
11.2355(10)
11.2355(10)
32.768(3)
˚
b/A
˚
c/A
β/deg
3
˚
V/A
4136.5(6)
4
1.392
0.426
1812
110(2)
Z
F
calc/g cm-3
μ/mm-1
F(000)
T/K
scan mode
hkl range
ω, j
ω, j
ω, j
-16 f þ16
-24 f þ21
-16 f þ16
32052
6657 [0.0585]
6657
614
1.001
0.0809, 0.1281
0.921, -1.111
-16 f þ16
-26 f þ26
-19 f þ19
45426
6856 [0.0888]
6856
568
1.000
0.0908, 0.1715
2.051, -1.010
-13 f þ13
-14 f þ14
-37 f þ43
30058
9663 [0.0836]
9663
543
1.000
0.0775, 0.1073
0.528, -0.576
measd reflns
unique reflns [Rint
]
reflns used for refinement
refined parameters
GoF
R1,awR2b (all data)
-3
˚
fin (max., min.)/e A
F
P
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2; w = 1/[σ2(Fo2) þ (ap)2 þ bp]; p = (Fo þ 2Fc2)/3 with a = 0.0694 for [2]OTf,
2
0.0710 for 2-F, and 0.0362 for 2-CN; and b = 0 for [2]OTf, 21 for 2-F, and 0 for 2-CN.
observed for similar complexes.13,16,17 The boron atom dis-
˚
plays B-C distances in the range 1.556(8)-1.587(8) A,
comparable to those observed in similar triarylboranes.10
Addition of fluoride or cyanide anions to solutions of [2]þ
in acetone resulted in a color change from deep purple to
nearly black. To better understand the origin of this color
change, we decided to monitor these reactions using UV-vis
spectroscopy. The UV-vis spectrum of 2[OTf] (3.0 mL, 2.5ꢀ
10-5 M, THF/DMF, 9/1 vol.) was monitored upon incre-
mental addition of either tetraethylammonium cyanide
(TEACN, 3.0 ꢀ 10-3 M, DMF) or tetrabutylammonium
fluoride (TBAF, 3.5 ꢀ 10-3 M, DMF) (Figure 2). Similar
spectral changes were observed for both titrations suggesting
a similar mode of interaction of [2]þ with both fluoride and
cyanide (Figure 2). The absorption band at 335 nm, assigned
to the absorbance of the triarylborane chromophore, is
quenched upon addition of the first equivalent of fluoride
or cyanide ions. This phenomenon results from a loss of
conjugation in the boron-centered chromophore, indicating
formation of 2-F and 2-CN (Scheme 2).18,19 Formation of
these complexes is also accompanied by the appearance
of a broad band spanning the 360-450 nm range with
Figure 1. Structure of [2]þ. Displacement ellipsoids are scaled to the
˚
50% probability level. Selected bond lengths (A) and angles (deg) for
˚
˚
[2]OTf: Ru(1)-C(7) 2.038(5) A, Ru(1)-N(1) 2.068(4) A, Ru(1)-N(2)
˚
˚
˚
2.035(3) A, Ru(1)-N(3) 2.035(4) A, Ru(1)-N(4) 2.070(4) A, Ru(1)-N-
˚
(5) 2.152(4) A; C(7)-Ru(1)-N(1) 79.80(18)°, N(2)-Ru(1)-N(3)
˚
79.24(15)°, N(4)-Ru(1)-N(5) 77.38(15)°, B(1)-C(9) 1.587(8)A, B-
˚
˚
(1)-C(12) 1.556(8) A, B(1)-C(21) 1.564(8) A, C(12)-B(1)-C(21)
123.4(5)°, C(12)-B(1)-C(9) 115.4(5)°, C(21)-B(1)-C(9) 121.2(4)°.
shift of the Ru(II) dπ f 2,20-bpy MLCT band with a shift of
the lowest energy edge of the band from 550 to 580 nm. This
bathochromic shift can be rationalized by invoking the
increased energy and electron-releasing ability of the bory-
lated ligand in 2-F and 2-CN (vs [2]þ) leading to a more
electron rich Ru(II) center. This explanation is in agreement
with the observation that phosphine or 2,20-bpy ligands
decorated by borate units have increased electron-releasing
abilities.20
λmax = 390 nm (ε = 14 200) for both 2-F and 2-CN. This
behavior is reminiscent of that observed upon addition of
fluoride anions to the N-methylated pyridinium cation [1-
Me]þ and is tentatively assigned to a charge transfer transi-
tion involving the dimesitylfluoroborate as the donor and the
metalated pyridyl ring or 2,20-bpy ligands as the acceptor.11
Formation of 2-F and 2-CN also results in a bathochromic
The spectral changes induced by fluoride binding to [2]þ in
THF/DMF (9/1 vol.) can be fitted to a 1:1 binding isotherm
to provide a fluoride binding constant K(F-) of 8.0((2.0) ꢀ
106 M-1 (Figure 2). Encouraged by the magnitude of these
(17) Andres, R.; Brissard, M.; Gruselle, M.; Train, C.; Vaissermann, J.;
Malezieux, B.; Jamet, J.-P.; Verdaguer, M. Inorg. Chem. 2001, 40, 4633–
4640. Brissard, M.; Gruselle, M.; Malezieux, B.; Thouvenot, R.; Guyard-
Duhayon, C.; Convert, O. Eur. J. Inorg. Chem. 2001, 1745–1751.
(18) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2001,
123, 11372–11375.
(20) Thomas, C. M.; Peters, J. C. Organometallics 2005, 24, 5858–5867.
Thomas, C. M.; Peters, J. C. Inorg. Chem. 2004, 43, 8–10.
ꢀ
(19) Sole, S.; Gabbaı, F. P. Chem. Commun. 2004, 1284–1285.

718 Inorganic Chemistry, Vol. 49, No. 2, 2010
Wade and Gabbaı
Figure 3. Rendering of the frontier molecular orbitals of [2]þ (left), 2-F
(middle), and 2-CN (right) Isovalues are set at 0.02.
THF/DMF (9/1 vol.) (Figure 2) and K(CN-) = 3.0((1.0) ꢀ
106 M-1 in CHCl3/DMF 9/1 vol. (Supporting Information,
Figure S4) thus indicating that [2]þ has a higher affinity for
cyanide than fluoride. Reciprocally, these results show that,
in these solvent mixtures, the cyanide anion displays a higher
basicity toward the boron center of [2]þ than fluoride does.
Figure 2. Spectral changes in the UV-vis absorption spectra of [2]OTf
(2.5 ꢀ 10-5 M) in 9/1 DMF/THF upon incremental addition of a TBAF
solution (top, 3.5 ꢀ 10-3 M in DMF) and TEACN solution (bottom,
3.0 ꢀ 10-3 M in DMF). The isotherms are plotted based on the absorb-
ance measured at 338 nm.
Interestingly, this behavior is in agreement with the Bronsted
e
acidity displayed by HCN and HF (pKa(HCN) = 9.3,
pKa(HF) = 3.18) in water but contrasts with that measured
in non-aqueous solvents such as DMSO (pKa(HCN) = 12.9,
pKa(HF) = 15).23
Scheme 2
The binding constant measured in CHCl3/DMF 9/1 vol.
for [2]þ is also significantly higher than that measured for 1 in
the same solvent (K(CN-) = 4.0((2.0) ꢀ 105 M-1, Supporting
Information, Figure S5) once again pointing to the favorable
influence of the cationic Ru(II) moiety. Addition of other
anions, including Cl-, Br-, I-, or NO3 resulted in no
-
changes in the UV-vis spectrum of [2]OTf, indicating that
these anions do not bind to the boron center. In addition, the
reversible nature of fluoride binding to the boron center was
confirmed by reappearance of the absorption bands corre-
sponding to [2]þ upon addition of a small amount of the
fluoride ion scavenger Al(NO3)3 to solutions of 2-F
(Supporting Information, Figure S6).
binding constants, we decided to carry out a similar titration
in CHCl3/DMF 9/1 vol. The elevated acceptor number of
CHCl321 (AN = 23.1 for CHCl3 vs 8.9 for THF and 16.0 for
DMF) should make such a medium more competitive for
anion binding.22 Accordingly, this experiment afforded a
substantially lower K(F-) of 1.1((0.1) ꢀ 104 M-1 (Supp-
orting Information, Figure S2). A lower binding constant
K(F-) of 7.5((0.5) ꢀ 102 M-1 was obtained for the free ligand
1 in CHCl3/DMF 9/1 vol. (Supporting Information, Figure
S3) thus providing evidence for the beneficial influence of the
cationic Ru(II) moiety which increases the anion affinity of
the boron center through both inductive and Coulombic
effects. The observed fluoride binding constant is, however,
notably smaller than the value of 6.5((0.5) ꢀ 106 M-1
measured for the phosphonium borane [p-(Mes2B-C6H4-
PPh2Me)]þ in CHCl3.19 Presumably, the cationic charge of
the Ru(II) moiety is largely dissipated on the two 2,20-bpy
ligands leading to a decrease of its inductive influence on the
phenylpyridine ligand and boron center. The bulk of the
Ru(II) moiety may also hamper the anion-induced tetrahe-
dralization of the boron center.
The structures of [2]þ, 2-F, and 2-CN have been opti-
mized using DFT methods (B3LYP, Stuttgart RSC 1997
ECP basis set for Ru and 6-31þg(d0) for all other atoms)
and subjected to single point energy calculations using the
Polarizable Continuum Model (PCM) with THF as a
solvent. Inspection of the frontier orbitals shows that the
lowest unoccupied molecular orbital (LUMO) is localized
on the 2,20-bpy ligands and a Ru dπ orbital in each of the
molecules (Figure 3). It is worth noting that, in the case of
[2]þ, the boron empty p-orbital contributes to the
LUMOþ2 rather than the LUMO as in typical triarylbor-
anes (Supporting Information, Figure S7). The highest
occupied molecular orbital (HOMO) is also similar in each
of the three cases, with electron density residing primarily
on a Ru dπ orbital and the phenylpyridine ligand. The
effects of anion binding are reflected in the marked
decrease of the HOMO-LUMO gap going from [2]þ
(2.78 eV) to 2-F (2.52 eV) and 2-CN (2.56 eV) which
provides support for the observed redshift in the lowest
energy bands of the UV-vis spectrum.
Next, the affinity of [2]þ for cyanide ions was investigated
under conditions analogous to those used in the case of
fluoride. These experiments afforded K(CN-) > 107 M-1 in
To confirm the formation of 2-F and 2-CN, crystallization
experiments were undertaken (Figure 4, Table 1). Single
(21) Mayer, U.; Gutmann, V.; Gerger, W. Monatsh. Chem. 1975, 106,
1235–1257.
(22) Miyasaka, S.; Kobayashi, J.; Kawashima, T. Tetrahedron Lett. 2009,
50, 3467–3469.
(23) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456–463.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 719
crystals of 2-F were obtained by slow evaporation of a
pyridine solution of [2]OTf and TBAF at room temperature.
The structure of 2-F confirms the presence of a fluorine atom
coordinated to the boron center via a B(1)-F(1) length of
distances when compared to 2[OTf] (Ru(1)-N, 2.031(5)-
˚
˚
2.138(5) A; Ru(1)-C(7), 2.036(6) A). Crystals of 2-CN were
obtained by slow evaporation of a CH3OH/acetone solution
containing KCN and 2[OTf]. The structure clearly displays a
cyanide anion bound to the boron center with a B(1)-C(50)
˚
1.468(8) A, which is comparable to that found in other
7,18
˚
˚
triarylfluoroborate anions (1.47 A).
No significant
distance of 1.635(8) A. While the Ru(1)-N bond distances in
˚
changes were observed in the Ru(1)-N or Ru(1)-C(7) bond
2-CN (Ru(1)-N, 2.033(4)-2.141 (4) A) again experience no
significant changes versus 2[OTf] or 2-F, the Ru(1)-C(7)
˚
bond length is slightly elongated (2.062(5) A), possibly
resulting from increased ligand steric repulsion caused by
pyramidalization of the boron center.
1
The formation of 2-F and 2-CN was also studied by H
NMR. Addition of TBAF to a solution of 2[OTf] in acetone-
d6 resulted in the appearance of a new set of 1H NMR signals
assigned to 2-F. Significant broadening of many of the signals
in the 1H NMR spectrum of 2-F suggests restricted molecular
motion caused by pyramidalization of the boron atom.
Nonetheless, addition of Al(NO3)3 to the NMR sample leads
to a revival of the original purple color, as well as of the sharp
resonances assigned to [2]þ thus supporting formation of 2-F.
Despite extended acquisition times, the 11B NMR signal of 2-
F could not be detected. Its 19F NMR spectrum is dominated
by a resonance at -174 ppm appearing in the expected range
for a fluoroborate species. The 1H NMR spectrum of 2-CN
displays two distinct sets of Mes-CH signals at 6.64 and 6.75
ppm as well as two ortho-Mes-CH3 (2.14 and 2.45 ppm) and
para-Mes-CH3 (2.24 and 2.29 ppm) resonances, in agreement
with the diastereotopic relationship of the two mesityl
groups. The11B NMR spectrum of 2-CN displays a sharp
signal at -12.7 ppm, in the typical range for a triaryl
cyanoborate.3-5
Figure 4. Structure of 2-F (left) and 2-CN (right). Displacement ellip-
soids are scaled to the 50% probability level. Interstitial solvent molecules
˚
and hydrogen atoms omitted for clarity. Selected bond lengths (A) and
angles (deg) for: 2-F: Ru(1)-C(7) 2.036(6), Ru(1)-N(1) 2.068(5),
Ru(1)-N(2) 2.031(5), Ru(1)-N(3) 2.043(5), Ru(1)-N(4) 2.055(5),
Ru(1)-N(5) 2.138(5); C(7)-Ru(1)-N(1) 79.6(2), N(2)-Ru(1)-N(3)
78.65(19), N(4)-Ru(1)-N(5) 77.76(19), B(1)-F(1) 1.468(8), B(1)-C(9)
1.651(10), B(1)-C(12) 1.652(10), B(1)-C(21) 1.675(9), F(1)-B(1)-C(9)
103.6(5), F(1)-B(1)-C(12) 111.4(5), C(9)-B(1)-C(12) 109.4(5),
F(1)-B(1)-C(21) 102.9(5), C(9)-B(1)-C(21) 119.0(5), C(12)-B(1)-
C(21) 110.1(5). 2-CN: Ru(1)-C(7) 2.062(5), Ru(1)-N(1) 2.076(4),
Ru(1)-N(2) 2.141(4), Ru(1)-N(3) 2.063(4), Ru(1)-N(4) 2.053(4),
Ru(1)-N(5) 2.033(4); C(7)-Ru(1)-N(1) 80.08(17), N(2)-Ru(1)-N(3)
76.89(16), N(4)-Ru(1)-N(5) 78.33(17), B(1)-C(50) 1.635(8), B(1)-C(9)
1.648(7), B(1)-C(12) 1.659(7), B(1)-C(21) 1.675(7), C(9)-B(1)-C(12)
108.3(4), C(9)-B(1)-C(21) 118.9(4), C(12)-B(1)-C(21) 112.0(4),
C(50)-B(1)-C(9) 103.2(4), C(50)-B(1)-C(12) 114.6(4), C(50)-B(1)-
C(21) 99.4(4).
Encouraged by the high affinity that [2]þ displays for
cyanide and fluoride, we decided to determine if the readily
accessible RuII/III redox couple could be used to report anion
binding events occurring at the boron center. To this end, we
Figure 5. Cyclic voltammograms of [2]OTf and in situ generated 2-F (left), and 2-CN (right) in DMF ([[2]ΟΤf] = 1 mΜ, [n-Βu4PF6] = 0.1 M ν =
200 mV s-1).

720 Inorganic Chemistry, Vol. 49, No. 2, 2010
Wade and Gabbaı
first recorded the cyclic voltammogram of [2]þ (1 mM, 0.1 M
TBAPF6, DMF) which displays a reversible oxidation wave
for the RuII/III redox couple at E1/2 = 0.051 V versus Fc/Fcþ.
Addition of 1 equiv of TBAF to [2]þ in the electrochemical
Conclusion
In conclusion, we report a cationic cyclometalated
ruthenium(II) complex ([2]þ) decorated by a peripheral Lewis
acidic boryl moiety. This new complex reacts with fluoride and
cyanide anions to afford the corresponding zwitterionic fluor-
oborate and cyanoborate species. The elevated stability con-
stants of these new species indicate that the cationic transition
metal moiety increases the Lewis acidity of the boron center
via inductive and Coulombic effects. The cyanide binding
constant of [2]þ is elevated thus suggesting that such com-
pounds could be used to monitor very low concentrations of
this anion. Lastly, anion binding to the boron center affects the
photophysical and electrochemical properties of the ruthe-
nium center which can be used to monitor anion binding. Most
notably, the potential of the RuII/III couple experiences a
cathodic shift upon anion binding making such compounds
attractive for electrochemical sensing applications.
cell resulted in the appearance of a new wave at E1/2
=
-0.191 V versus Fc/Fcþ assigned to the presence of 2-F
(Figure 5). This cathodic shift of the RuII/III redox process is
in agreement with an increase in the donor strength of the
cyclometallating ligand induced by anion binding20 and a
decrease in the overall charge of the complex. Appearance of
the new wave at E1/2 = -0.191 V was accompanied by a
decrease in the intensity of the oxidation wave of [2]þ
(Figure 5). Increasing the fluoride ion concentration resulted
in a net decrease of both oxidation waves corresponding to
[2]þ and 2-F. This unexpected phenomenon is assigned to
further conversion of [2]þ to 2-F and subsequent preci-
pitation of the latter at higher fluoride concentration. In
agreement with this interpretation, addition of the fluoride
ion scavenger Al(NO3)3 to the electrochemical cell led to the
Acknowledgment. This work was supported by the
National Science Foundation (CHE-0646916 and CHE-
0541587), the Welch Foundation (Grant A-1423), and the
U.S. Army Medical Research Institute of Chemical De-
fense.
nearly full reappearance of the oxidation wave at E1/2
=
þ0.051 V versus Fc/Fcþ, indicating regeneration of the
cationic species [2]þ. Addition of cyanide ions to a solution
of [2]þ in the electrochemical cell triggered formation of 2-
CN as evidenced by the detection of an new oxidation wave at
E
1/2 = -0.147 V versus Fc/Fcþ. Conversion of [2]þ into 2-
Supporting Information Available: Optimized geometries of
[4]þ, 2-F, and 2-CN. UV-vis titration data in CHCl3, X-ray
crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
CN was not complicated by precipitation; moreover, the
conversion appeared quantitative, in agreement with the
elevated cyanide binding constant displayed by [2]þ.