Diarylborenium cations: Synthesis, structure, and electrochemistry

Article abstract of DOI:10.1021/om701249n

Mes₂BF and Ar₂^NBF (Ar^N = 4-(Me₂N)-2,6-Me₂-C₆H₂) react with trimethylsilyltriflate and p-dimethylaminopyridine in chlorobenzene to afford the corresponding borenium salts [Mes₂B-DMAP]⁺ OTf ([1]⁺ OTf^-) and [Ar₂^NB-DMAP] ⁺ OTf^- ([2]⁺OTf^-), which have been fully characterized.

Full text of DOI:10.1021/om701249n

Organometallics 2008, 27, 1657–1659
1657
Diarylborenium Cations: Synthesis, Structure, and Electrochemistry
Ching-Wen Chiu and François P. Gabbaï*
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843
ReceiVed December 14, 2007
Summary: Mes2BF and ArN BF (ArN ) 4-(Me2N)-2,6-Me2-C6H2)
Scheme 1
2
react with trimethylsilyltriflate and p-dimethylaminopyridine in
chlorobenzene to afford the corresponding borenium salts
[Mes2B-DMAP]+ OTf- ([1]+OTf-) and [ArN B-DMAP]+ OTf-
2
([2]+OTf-), which haVe been fully characterized.
Introduction
Cationic boron derivatives continue to attract a great deal of
attention because of their instability and reactivity.1–6 Diphe-
nylborenium cations of general formula [Ph2B-L]+ (L ) neutral
ligand) provide a good example of such species, which had long
([1]+OTf-) (Scheme 1).14 In an effort to determine the effect
of remote amino-substituents on the stability of such borenium
ions, we also synthesized ArN2BF (ArN ) 4-(Me2N)-2,6-Me2-
C6H2) and converted it into [ArN2B-DMAP]+OTf- ([2]+OTf-)
by following the method used for [1]+OTf-. The formation of
been proposed but whose definitive characterization in solution
has only recently been achieved.7–11 Despite these recent
successes, diphenylborenium cations have not been structurally
characterized. Furthermore, the acridine-9-borafluorenium cation
is the only structurally characterized example of a diarylbore-
nium cation.12 The difficulties encountered in the isolation of
such cations can be correlated to the unsaturation of the boron
center and the weak π-donating ability of the aryl groups. In
an effort to overcome these limitations, we have probed the
use of more sterically protecting aryl substituents featuring
o-methyl groups. We have also probed the stabilizing effects
that can be provided by remote amino-substituents appended
to the aryl groups of diarylborenium cartions.13
1
[1]+OTf- and [2]+OTf- was confirmed by H NMR, which
showed the presence of one DMAP ligand bound to the boron
1
center. The H NMR spectra of [1]+OTf- and [2]+OTf- are
also different from those of [Mes2BF-DMAP] and [ArN2BF-
DMAP], respectively, which could be generated in situ by
simple mixing of the diarylboron fluoride with DMAP. Of
1
special note, the H NMR resonance of the Me2N group of
DMAP is shifted downfield by 0.36 ppm on going from
[Mes2BF-DMAP] to [1]+OTf- and 0.31 ppm on going from
[ArN2BF-DMAP] to [2]+OTf-. These observation are in agree-
ment with an ionic formulation for [1]+OTf- and [2]+OTf-.
This was confirmed by the detection of a 11B NMR signal at
64 ppm for [1]+OTf- and 62 ppm for [2]+OTf-. These chemical
shifts are comparable to the value reported for [(C6H5)2B(Py)]+
(58.2 ppm) which also feature a tricoordinate boron atom.11
While [1]+OTf- is isolated as colorless crystals, [2]+OTf- is
an orange solid.
Results and Discussion
The reaction of Mes2BF with trimethylsilyl triflate
(Me3SiOTf) and p-dimethylaminopyridine (DMAP) in refluxing
chlorobenzene affords after 24 h [Mes2B-DMAP]+OTf-
Bearing in mind that the only structurally characterized
diarylborenium salt features a fused 9-borafluorenium moiety,12
we undertook the characterization of [1]+OTf- and [2]+OTf-
* Corresponding author. E-mail: francois@tamu.edu. Fax: +1 979 845
4719. Phone: + 1 979 862 2070.
(1) Kölle, P.; Nöth, H. Chem. ReV. 1985, 85, 399.
(2) Piers, W. E.; Bourke, S. C.; Conroy, K. D. Angew. Chem., Int. Ed.
Engl. 2005, 44, 5016.
j
by X-ray analysis. Both salts crystallize in the P1 space group
with two molecules in the unit cell. Both structures are very
similar (Figure 1). The boron center of [1]+OTf- and [2]+OTf-
is trigonal planar as indicated by the sum of the bond angles,
which is equal to 360° in both cases. The B(1)-C(8) (1.560(3)Å
in [1]+, 1.550(4) Å in [2]+) and B(1)-C(18) bonds (1.570(3)Å
in [1]+, 1.532(4) Å in [2]+) connecting the aryl ligand to the
boron centers are significantly shorter than the CMes-B bonds
measured in Mes2BPh (1.579(2) Å),15 suggesting increased
π-donation to the electron-deficient boron center. The average
(3) (a) Vidovic, D.; Findlater, M.; Cowley, A. H. J. Am. Chem. Soc.
2007, 129, 8436. (b) Vidovic, D.; Findlater, M.; Cowley, A. H. J. Am. Chem.
Soc. 2007, 129, 11296.
(4) Braun, U.; Nöth, H. Eur. J. Inorg. Chem. 2007, 2296.
(5) Kato, T.; Tham, F. S.; Boyd, P. D. W.; Reed, C. A. Heteroatom
Chem. 2006, 17, 209.
(6) Chiu, C.-W.; Gabbai, F. P. Angew. Chem., Int. Ed. Engl. 2007, 46,
1723.
(7) Davidson, J. M.; French, C. M. J. Chem. Soc. 1962, 3364.
(8) Moodie, R. B.; Ellul, B.; Connor, T. M. Chem. Ind. 1966, 1966,
767.
(9) Kiyooka, S.-I.; Fujiyama, R.; Kawai, T.; Fujimoto, H.; Goh, K.
Tetrahedron Lett. 2001, 42, 4151.
(10) Uddin, M. K.; Fujiyama, R.; Kiyooka, S.-i.; Fujio, M.; Tsuno, Y.
Tetrahedron Lett. 2004, 45, 3913.
(14) These reaction conditions are those employed in the self-activated
silyl-assisted poly-onio-substitution protocol developed by Robert Weiss,
see for example: Weiss, R.; Puhlhofer, F. G. J. Am. Chem. Soc. 2007, 129,
547.
(11) Uddin, M. K.; Nagano, Y.; Fujiyama, R.; Kiyooka, S.-i.; Fujio,
M.; Tsuno, Y. Tetrahedron Lett. 2005, 46, 627.
(12) Narula, C. K.; Nöth, H. Inorg. Chem. 1985, 24, 2532.
(13) Schulz, A.; Kaim, W. Chem. Ber. 1989, 122, 1863.
(15) Fiedler, J.; Zalis, S.; Klein, A.; Hornung, F.; Kaim, W. Inorg. Chem.
1996, 35, 3039.
10.1021/om701249n CCC: $40.75
2008 American Chemical Society
Publication on Web 03/08/2008

1658 Organometallics, Vol. 27, No. 7, 2008
Notes
Figure 1. Crystal structure of [1]+ and [2]+ with thermal ellipsoids set at the 50% probability level. Hydrogen atoms are omitted for
clarity. Pertinent metrical parameters can be found in the text.
dihedral angles θ formed between the boron trigonal plane and
the plane containing the Mes ligand in [1]+ and the ArN ligand
in [2]+ are respectively 58.1° and 48.3°. The slight shortening
of the B(1)-C(8) and B(1)-C(18) bonds as well as the smaller
θ angle observed for [2]+ may be correlated to the stronger
π-donating ability of the ArN ligand, a conclusion supported
by the planarity of the N(3) and N(4) atoms (Σ(C-N-C) ) 359.8°
(N(3)) and 358.2° (N(4))). The B(1)-N(1) bonds connecting
the DMAP ligand to the boron centers (1.480(3)Å in [1]+,
1.501(4) Å in [2]+) in both structures are shorter that the value
computed for [(C6H5)2B(Py)]+ (1.552 Å).11 They are also shorter
than the B-NDMAP bond observed in Ar*PdB(DMAP)Tmp
(Ar* ) C6H3-2,6-(C6H2-2,4,6-iPr3)2; Tmp ) 2,2,6,6-tetra-
methylpiperidine) and are in fact comparable to traditional B-N
single bonds.16,17 The electron deficiency of the cationic boron
center as well as the strong σ-donor character of the DMAP
Figure 2. Cyclic voltammograms of [1]+ and [2]+ in CH2Cl2 with
a glassy-carbon working electrode (0.1 M nBu4NPF6) at scan rate
ligand are most certainly responsible for this feature.
In an attempt to better understand the properties of [1]+ and
[2]+, we have studied their electrochemistry. As indicated by
cyclic voltammetry, both [1]+ and [2]+ undergo an irreversible
reduction at Epeak -2.03 and -2.30 V (vs Fc/Fc+), respectively
(Figure 2). Because of their increased electron deficiency, the
reduction potentials of [1]+ and [2]+ are more positive than
that of neutral boranes, such as Mes3B, which is reduced at
-2.73 V (vs Fc/Fc+). It is also important to note that the
reduction potential of [1]+ is substantially more positive than
that of [2]+ once again indicating the greater donating ability
of the ArN ligand. The redox behavior of these derivatives is
different from that of neutral boranes which typically display a
reversible one-electron reduction wave corresponding to the
formation of stable radical anion.18 Hence, these results indicate
that the radical [Mes2B-DMAP]• and [ArN2B-DMAP]• are not
stable. It also suggests that radicals such as [Mes2B-pyridine]•
whose generation has been attempted19 might be too unstable
to actually observe in solution.20
of 300 mV s-1
.
observed experimentally. In both cases, the LUMO is localized
on the boron atom and DMAP ligand (Figure 3). An evaluation
of the orbital energies by using the PCM solvation model21,22
(solvent ) CH2Cl2) indicates that the LUMO of [1]+ is 0.37
eV lower than that of [2]+. This energy difference is close to
the results of our electrochemical measurements (∆Epeak(red)
) 0.27 V).
Experimental Section
General Considerations. Trimethylsilyl triflate (Me3SiOTf) was
purchased from TCI America and used without purification.
p-Dimethylaminopyridine (DMAP) was purchased from Lancaster
and used without purification. 4-Bromo-3,5-tetramethylaniline was
synthesized by following a published procedure.23 Chlorobenzene
was dried over P2O5 under N2 atmosphere and distilled prior to
A geometry optimization of [1]+ and [2]+ with DFT methods
(19) Leffler, J. E.; Dolan, E.; Tanigaki, T. J. Am. Chem. Soc. 1965, 87,
(B3LYP, 6-31G*) affords structures that closely match those
927.
(20) Weissman, S. I.; van Willigen, H. J. Am. Chem. Soc. 1965, 87,
(16) Rivard, E.; Merrill, W. A.; Fettinger, J. C.; Power, P. P. Chem.
Commun. 2006, 3800.
2285.
(21) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys. Lett.
(17) Rivard, E.; Merrill, W. A.; Fettinger, J. C.; Wolf, R.; Spikes, G. H.;
Power, P. P. Inorg. Chem. 2007, 46, 2971.
1996, 255, 327.
(22) Tomasi, J.; Cammi, R.; Mennucci, B. Int. J. Quantum Chem. 1999,
(18) Cummings, S. A.; Iimura, M.; Harlan, C. J.; Kwaan, R. J.; Trieu,
I. V.; Norton, J. R.; Bridgewater, B. M.; Jäkle, F.; Sundararaman, A.; Tilset,
M. Organometallics 2006, 25, 1565.
75, 783.
(23) Engman, L.; Stern, D.; Stenberg, B. J. Appl. Polym. Sci. 1996, 59,
1365.

Notes
Organometallics, Vol. 27, No. 7, 2008 1659
The sample could be further purified by an additional recrystalli-
zation from hot chlorobenzene. Anal. Calcd for C26H32BF3N2O3S:
C, 60.01; H, 6.20. Found: C, 60.16 H, 6.19.
Synthesis of [2]+OTf-. ArN2BF (0.28 g, 0.85 mmol), DMAP
(0.08 g, 0.65 mmol), and Me3SiOTf (0.11 mL, 0.61 mmol) were
dissolved in 5 mL of chlorobenzene and heated at reflux overnight
to give an orange solution. After cooling to room temperature,
addition of Et2O (20 mL) resulted in the precipitation of [2]+OTf-
as an orange solid that was isolated by filtration. It was washed
with Et2O (10 mL) and dried under reduced pressure (0.24 g, 69%
yield). Single crystals were obtained by vapor diffusion of Et2O
1
into a CHCl3 solution of the [2]+OTf-. H NMR (CDCl3, 399.59
MHz): δ 1.94 (s, 12H, o-Me), 2.98 (s, 12H, ArN-NMe2), 3.36 (s,
3
6H, DMAP-NMe2), 6.32 (s, 4H, ArN-CH), 6.91 (d, 2H, JH-H
)
3
8.0 Hz, DMAP-CH), 7.89 (d, 2H, JH-H ) 8.0 Hz, DMAP-CH).
13C NMR (CDCl3, 100.5 MHz): δ 23.4 (o-Me), 39.8 (ArN-NMe2),
40.9 (DMAP-NMe2), 108.5, 111.7, 144.3, 152.4, 157.9. 11B NMR
(CDCl3, 128.2 MHz): δ 62. Anal. Calcd for C28H38BF3N4O3S: C,
58.13; H, 6.62. Found: C, 57.60; H, 6.56.
Electrochemistry. Electrochemical experiments were performed
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 into 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 CH2Cl2 solution (5 mL) containing (n-Bu)4NPF6 (0.1 M) as a
support electrolyte and the analyte (7.7 mM for [1]+OTf-, 7.3 mM
for [2]+OTf-). The electrolyte was dried under vacuum prior to
use. In all cases, ferrocene was used as an internal standard, and
all reduction potentials are reported with respect to the E1/2 of the
Fc/Fc+ redox couple.
Figure 3. DFT optimized geometry of [1]+ with an overlay of the
LUMO (isovalue ) 0.03, H-atoms omitted).
use. Air-sensitive compounds were handled under N2 atmosphere,
using standard Schlenk and glovebox techniques. Elemental
analyses were performed at Atlantic Microlab (Norcross, GA).
NMR spectra were recorded on a Varian Unity Inova 400 FT NMR
spectrometer (399.59 MHz for 1H, 375.95 MHz for 19F, 128.2 MHz
for 11B, 100.5 MHz for 13C). Chemical shifts δ are given in ppm,
and are referenced against external Me4Si (H, 13C), BF3 · Et2O (11B),
and CFCl3 (19F).
Synthesis of ArN2BF (ArN ) 4-(Me2N)-2,6-Me2-C6H2). 4-Bromo-
3,5-tetramethylaniline (10 g, 0.04 mol) was allowed to react with
excess Mg powder in THF (30 mL) at reflux for 5 h. Once the
reaction mixture was cooled to room temperature, the THF solution
was filtered and then slowly added into an Et2O solution of
BF3 · OEt2 (2.7 mL, 0.02 mol) at -78 °C. The reaction mixture
was allowed to warm to room temperature and stirred overnight.
The solvents were then removed under reduced pressure and the
solid residue was extracted with hexane (3 × 50 mL). The combined
hexane fractions were dried under vacuum to afford a yellow solid.
This solid was washed with hexane (5 mL) to afford ArN2BF in
moderate yield (3.8 g, 53%). This compound was used without
Crystallography. The crystallographic measurements were
performed by using a Bruker APEX2 diffractometer, with graphite-
monochromated Mo KR radiation (λ ) 0.71069 Å). Specimens of
suitable size and quality were selected and mounted onto glass fiber
with apiezon grease. The structures were solved by direct methods,
which successfully located most of the non-hydrogen atoms.
Subsequent refinement on F2 with use of the SHELXTL/PC package
(version 5.1) allowed location of the remaining non-hydrogen atoms.
A summary of the pertinent crystallographic data follows:
Crystal data for [1]+OTf-: C26H32BF3N2O3S, Mr ) 520.41, T
1
further purification for the synthesis of [2]+OTf- (vide infra). H
j
) 140 K, space group P1, triclinic, a ) 8.2944(8) Å, b )
NMR (CDCl3, 399.59 MHz): δ 2.39 (d, 12H, o-Me), 3.06 (s, 12H,
ArN-NMe2), 6.46 (s, 4H, ArN-CH). 13C NMR (CDCl3, 100.5 MHz):
δ 23.2 (o-Me), 40.0 (ArN-NMe2), 111.3, 125.6, 144.4, 151.6. 19F
NMR (CDCl3, 375.95 MHz): δ -23.4. 11B NMR (CDCl3, 128.2
MHz): δ 52.
10.4347(10) Å , c ) 16.1597(15) Å, R ) 104.6880(10)°, ꢀ )
96.8560(10)°, γ ) 100.4320(10)°, V ) 1310.3(2) Å3, Z ) 2, Dc )
1.319 g cm-3, µ(Mo KR) ) 0.175 mm-1, 15106 reflections
measured, 6033 unique (Rint ) 0.0306), R1 ) 0.0575, wR2 ) 0.1185.
Crystal data for [2]+OTf- · (OEt2)0.5: C30H43BF3N4O3.5S, Mr )
Synthesis of [1]+OTf-. Dimesitylboron fluoride (0.60 g, 2.2
mmol), DMAP (0.23 g, 1.9 mmol), and Me3SiOTf (0.34 mL, 1.9
mmol) were dissolved in 5 mL of chlorobenzene and heated at
reflux overnight to give a colorless solution. Upon cooling,
compound [1]+OTf- precipitated and was isolated as a white solid
by filtration. It was washed with Et2O (10 mL) and dried under
reduced pressure (0.78 g, 80% yield). Single crystals were obtained
by slow cooling a hot chlorobenzene solution of [1]+OTf- to -40
j
615.55, T ) 140 K, space group P1, triclinic, a ) 8.110(3) Å, b )
13.043(4) Å, c ) 15.489(5) Å, R ) 85.454(3)°, ꢀ ) 78.149(3)°, γ
) 83.434(4)°, V ) 1590.4(9) Å3, Z ) 2, Dc ) 1.285 g cm-3, µ(Mo
KR) ) 0.158 mm-1, 11305 reflections measured, 5861 unique (Rint
) 0.0295), R1 ) 0.0841, wR2 ) 0.1276.
Acknowledgment. This work was supported by NSF
(CHE-0646916), the Welch Foundation (A-1423), and the
Petroleum Research Funds (Grant 44832-AC4).
1
°C. H NMR (CDCl3, 299.91 MHz): δ 2.01 (s, 12H, o-Me), 2.30
(s, 6H, p-Me), 3.43 (s, 6H, DMAP-NMe2), 6.87 (s, 4H, Mes-CH),
7.00 (d, 2H, 3JH-H ) 7.8 Hz, DMAP-CH), 7.87 (d, 2H, 3JH-H ) 7.8
Hz, DMAP-CH). 13C NMR (CDCl3, 100.5 MHz): δ 21.3 (o-Me),
22.5 (p-Me), 41.3 (DMAP-NMe2), 109.2, 122.3, 129.3, 133.2, 142.2,
143.9, 158.1. 11B NMR (CDCl3, 128.2 MHz): δ 64. Anal. Calcd
for C26H32BF3N2O3S: C, 60.01; H, 6.20. Found: C, 59.36; H, 6.16.
Supporting Information Available: Computational details and
crystallographic data in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM701249N