Synthesis of trifluoroborate functionalised imidazolium salts as precursors to weakly coordinating bidentate NHC ligands

Article abstract of DOI:10.1016/j.ica.2010.12.030

A new class of trifluoroborate functionalised N-heterocyclic carbene precursors have been synthesised, isolated and characterised structurally. The ligands were obtained via a serendipitous one-pot reaction in which deprotection, cyclisation and fluorination of boryl-functionalised diarylethylenediamine derivatives occur concurrently. Deprotonation of the imidazolium salts was found to yield the free carbene, though 18-crown-6 was found necessary to prevent further reactivity of the resulting aryl potassium trifluoroborate salts; in the absence of 18-crown-6, elimination of KF resulted in a cyclic carbene-BF₂ arene adduct. Complexation to rhodium was facile, and yielded four-coordinate complexes in which the Rh-BF₃ interaction was determined by ¹⁹F NMR spectroscopy to be weak.

Full text of DOI:10.1016/j.ica.2010.12.030

Inorganica Chimica Acta 369 (2011) 180–189
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis of trifluoroborate functionalised imidazolium salts as precursors
to weakly coordinating bidentate NHC ligands
b
a
Andrew L. Gott a, Warren E. Piers a, , Robert McDonald , Masood Parvez
⇑
a Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4
b Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta, Canada T6G 2G2
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 23 December 2010
A new class of trifluoroborate functionalised N-heterocyclic carbene precursors have been synthesised,
isolated and characterised structurally. The ligands were obtained via a serendipitous one-pot reaction
in which deprotection, cyclisation and fluorination of boryl-functionalised diarylethylenediamine deriv-
atives occur concurrently. Deprotonation of the imidazolium salts was found to yield the free carbene,
though 18-crown-6 was found necessary to prevent further reactivity of the resulting aryl potassium tri-
fluoroborate salts; in the absence of 18-crown-6, elimination of KF resulted in a cyclic carbene–BF2 arene
adduct. Complexation to rhodium was facile, and yielded four-coordinate complexes in which the Rh–BF3
interaction was determined by 19F NMR spectroscopy to be weak.
Dedicated to Prof. Robert G. Bergman for his
inspirational contributions to
organometallic chemistry
Keywords:
N-heterocyclic carbene
Bidentate
Ó 2010 Elsevier B.V. All rights reserved.
Trifluoroborate
Fluorination
Rhodium
Hemilability
1. Introduction
by Shaughnessy and co-workers [30]. Hoveyda and co-workers
[31–40] have elaborated upon this concept, yielding a variety of
Since the report of the first stable N-heterocyclic carbenes
(NHCs) [1,2], their ever-increasing use as ligand systems in organo-
metallic chemistry [3] is a demonstration of their improved donor
properties in comparison to the ubiquitous phosphine ligand fam-
ily. Highly modular synthetic routes allow for wide manipulation
of steric [4] and electronic [5–11] properties, and as such, these li-
gand systems are now widely utilised in catalytic transformations
of major significance. Olefin metathesis [12–17], and Pd-mediated
cross coupling [18–20] are two such examples, whilst other cata-
lytic systems have been reviewed extensively from both synthetic
[21–26] and theoretical [27] perspectives. In the drive to further
explore these systems, substituents with additional donor func-
tionalities have been incorporated onto the nitrogen atoms of the
imidazolium moiety [28]. One example of this strategy is to add
anionic tethering groups with a view to increasing the strength
of interaction of NHC ligands with early, electropositive, transition
elements [29]. As reviewed by Arnold et al. the anionic moieties
incorporated into NHCs thus far are largely alkoxide, amide, thio-
late and cyclopentadienyl derivatives; relatively little work has
been reported which includes anionic groups where the negative
charge is delocalised over a number of atoms. One such example
is the recent reports of sulphonated NHC complexes, first reported
chiral complexes which demonstrate a rich and diverse catalytic
chemistry. Similar NHC ligands have recently been reported by
Nozaki et al. [41] for use in olefin polymerisation chemistry.
It was to this end that we became interested in replacing the
sulphonate group with trifluoroborate moieties that could poten-
tially yield ligands that displayed unique coordination behaviour;
the close similarity to the weakly coordinating tetrafluoroborate li-
gand [42] offered the possibility of hemilabile coordination versus
the stronger/harder sulfoxide donor. Herein we report our synthe-
ses of such trifluoroborate functionalised NHC precursors and li-
gand systems and an initial investigation into their hemilabile
coordination behaviour.
2. Experimental details
2.1. Materials and instrumentation
Unless otherwise indicated, all manipulations were carried out
in either an MBraun inert atmosphere glovebox or on a greaseless
dual-manifold vacuum line using Teflon (Kontes) valves and swi-
vel-frit type glassware. Toluene, THF and hexanes were dried using
a Grubbs/Dow solvent purification system and stored in evacuated
bombs prior to use. Benzene-d6, toluene-d8 and THF-d8 were dried
over sodium/benzophenone, vacuum transferred into bombs
⇑
Corresponding author. Tel.: +1 403 220 5746; fax: +1 403 289 9488.
E-mail address: wpiers@ucalgary.ca (W.E. Piers).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.12.030

A.L. Gott et al. / Inorganica Chimica Acta 369 (2011) 180–189
181
sealed with Kontes valves, and stored in the glovebox. CDCl3 and
(CD3)2SO were purchased from Cambridge Isotopes and used as re-
ceived. Chloroacetyl chloride, DMAP, 18-crown-6, potassium io-
dide, KN(SiMe3)2, and triethylorthoformate were purchased from
Aldrich and used as received. 2,4,6-Trimethylaniline and 2,6-
diisopropylaniline were purchased from Aldrich and purified by
short-path distillation prior to use. 2-Aminophenylboronic acid
pinacol ester was purchased from Boron Molecular (US) and used
as received. [RhCl(COD)]2 was prepared according to published
procedures [43]. NMR spectra were obtained on Bruker AMX-
300, AMX-400 and DRX-400 spectrometers. 1H and 13C{1H} NMR
spectra were referenced to tetramethylsilane, 11B{1H} NMR spectra
to BF3.OEt2, and 19F NMR spectra to CFCl3. Full structural assign-
ment was confirmed by 2-D NMR spectra where necessary. Ele-
mental analyses and mass spectra were obtained by the
Instrumentation Facility of the Department of Chemistry, Univer-
sity of Calgary. Elemental analysis figures are reported as the aver-
age of two separate determinations.
(57.4 mmol, 93%). If necessary, the compound was purified by
recrystallisation from hot ethyl acetate. 1H NMR (300 MHz, CDCl3,
298 K): d 1.40 ppm (s, 12H, 4 ꢀ CH3 of pinacol ester), 3.83 (s, 2H,
3
4
ICH2CONH), 7.10 (t of d, 1H, aryl C–H, JHH = 8 Hz, JHH = 1 Hz),
3
4
7.47 (t of d, 1H, aryl C–H, JHH = 8 Hz, JHH = 1 Hz), 7.77 (d of d,
3
4
1H, aryl C–H, JHH = 8 Hz, JHH = 1 Hz), 8.42 (d, 1H, aryl C–H,
3JHH = 8 Hz), 9.79 (s, 1H, br, N–H). 13C{1H} NMR (75.5 MHz, CDCl3,
298 K): d ꢁ0.3 ppm (ICH2CONH), 24.9 (CH3 of pinacol ester), 84.6
(Cq of pinacol ester), 119.0, 123.5, 132.8, 136.2 (all aryl C-H),
144.1 (aryl Cq), 165.5 (ICH2CONH). 11B{1H} NMR (128.4 MHz,
CDCl3, 298 K): d +29.5 ppm (s, br, C6H4B(OR)2). Anal. Calc. for
C
14H19BINO3: C, 43.35; H, 4.95; N, 3.62. Found: C, 43.15; H, 4.89;
N, 3.53%. MS (EI +ve): m/z 387 [M+], 372 [M+ꢁCH3], 201
[M+ꢁICH2CONH], 179 [M+ꢁpinacol ester].
2.2.3. (2,4,6-Me3–C6H2NHCH2CONH(2-pinacolboryl-C6H4), 3a
A 100 mL round-bottomed flask was charged with N-iodoace-
tyl-2-aminophenylboronic acid pinacol ester 2 (6.75 g, 17.4 mmol).
Added was 2,4,6-trimethylaniline (40 mL, ca. 260 mmol, ꢂ15
equivalents), and DMAP (0.106 g, 0.87 mmol, 5 mol%). The viscous
yellow suspension was left to stir at ambient temperature for
4 days. The flask was then attached to a Y-shaped adaptor on a high
vacuum line and most of the excess aniline was removed by short-
path distillation (oil bath temperature 50–55 °C, full vacuum) to
yield a dark yellow/brown oily residue which solidified at room
temperature. The residue was slurried in dichloromethane
(100 mL) and washed with 1 M NaOH (2 ꢀ 100 mL), before drying
over Na2SO4. All volatiles were removed under vacuum to yield a
viscous, dark red oil which was dried under high vacuum before
dissolution in pentane (30 mL). Storage at ꢁ20 °C overnight yielded
the title compound as a crystalline off-white solid. Yield: 4.30 g
(10.9 mmol, 63%). Concentration of the supernatant and storage
at ꢁ20 °C for three to four days resulted in a further crop of mate-
rial. Total yield: 5.09 g (12.9 mmol, 74%). 1H NMR (300 MHz, CDCl3,
298 K): d 1.19 ppm (s, 12H, 4 ꢀ CH3 of pinacol ester), 2.22 (s, 3H,
CH3 of mesityl), 2.28 (s, 6H, 2 ꢀ CH3 of mesityl), 3.76 (s, 2H,
CH2NHMes), 6.82 (s, 2H, aryl C–H of mesityl), 7.08 (t of d, 1H, aryl
C–H, 3JHH = 8 Hz, 4JHH = 1 Hz), 7.47 (t of d, 1H, aryl C–H, 3JHH = 8 Hz,
4JHH = 1 Hz), 7.77 (d of d, 1H, aryl C-H, 3JHH = 8 Hz, 4JHH = 1 Hz), 8.62
2.2. Syntheses
2.2.1. N-chloroacetyl-2-aminophenylboronic acid pinacol ester, 1
A 500 mL round-bottomed flask was charged with a suspension
of anhydrous potassium carbonate (18.3 g, 132.4 mmol) in acetoni-
trile (250 mL). Added portionwise, with stirring, was 2-amin-
ophenylboronic acid pinacol ester (14.5 g, 66.2 mmol), resulting
in a yellow suspension. Added dropwise was a solution of chloro-
acetyl chloride (5.3 mL, 7.48 g, 66.2 mmol) in acetonitrile (20 mL)
with a concomitant colour change to orange/pink. The reaction
was left to stir at ambient temperature overnight, whereupon the
pink colouration dissipated and the colouration of the reaction
mixture became yellow. The suspension was filtered and the iso-
lated potassium salts were washed colourless with acetonitrile
(50 mL). The solvent was removed under reduced pressure to yield
a viscous yellow/orange oil which was triturated with pentane and
dried under high vacuum to yield the title compound as a spectro-
scopically pure, crystalline white solid which retained small
amounts of potassium chloride after recrystallisation. Yield:
18.54 g (62.7 mmol, 95%). 1H NMR (300 MHz, CDCl3, 298 K): d
1.35 ppm (s, 12H, 4 ꢀ CH3 of pinacol ester), 4.18 (s, 2H, CH2 of
3
(d, 1H, aryl C–H, JHH = 8 Hz), 10.87 (s, br, 1H, N–H). 13C{1H} NMR
3
4
ClCH2CONH), 7.10 (t of d, 1H, aryl C–H, JHH = 8 Hz, JHH = 1 Hz),
(75.5 MHz, CDCl3, 298 K): d 18.3, 20.6 ppm (both CH3 of mesityl),
24.7 (CH3 of pinacol ester), 52.9 (CH2 of NHCH2CONH), 84.1 (Cq of
pinacol ester), 119.1, 123.2, (both aryl C–H), 129.0 (aryl Cq), 129.7
(aryl C–H), 131.8 (aryl Cq), 132.7, 136.3 (both aryl C–H), 142.5,
144.1 (both aryl Cq), 170.2 (NHCH2CONH). 11B{1H} NMR
(128.4 MHz, CDCl3, 298 K): d +30.0 ppm (s, br, C6H4B(OR)2). Anal.
Calc. for C23H31BN2O3: C, 70.06; H, 7.92; N, 7.10. Found: C, 69.77;
H, 8.15; N, 7.12%. MS (EI +ve): m/z 394 [M+].
3
4
7.47 (t of d, 1H, aryl C–H, JHH = 8 Hz, JHH = 1 Hz), 7.77 (d of d,
3
4
1H, aryl C–H, JHH = 8 Hz, JHH = 1 Hz)), 8.50 (d, 1H, aryl C–H,
3JHH = 8 Hz), 10.35 (m, br, 1H, NH). 13C{1H} NMR (75.5 MHz, CDCl3,
298 K): d 24.8 ppm (CH3 of pinacol ester), 43.2 (ClCH2CONH), 84.6
(Cq of pinacol ester), 119.3, 123.7, 132.8, 136.2 (all aryl C–H), 142.7,
143.8 (both aryl Cq), 164.6 (ClCH2CONH). 11B{1H} NMR (128.4 MHz,
CDCl3, 298 K): d +30.3 ppm (s, br, C6H4B(OR)2). Anal. Calc. for
C
14H19BClNO3.KCl0.25: C, 53.52; H, 6.10; N, 4.46. Found: C, 53.66;
H, 6.50; N, 4.44%. MS (EI +ve): m/z 295 [M+], 280 [M+ꢁCH3], 265
[M+ꢁ2CH3], 237 [M+ꢁ4CH3], 179 [M+ꢁpinacol ester].
2.2.4. (2,6-iPr2–C6H2NHCH2CONH(2-pinacolboryl-C6H4), 3b
As above, using N-iodoacetyl-2-aminophenylboronic acid pina-
col ester 2 (8.00 g, 20.7 mmol), 2,6-diisopropylaniline (50 mL, ca.
300 mmol, ꢂ15 equivalents), and DMAP (0.126 g, 1.01 mmol,
5 mol%). Yield: 6.91 g (15.8 mmol, 76%). 1H NMR (300 MHz, CDCl3,
298 K): d 1.24 ppm (d, 12H, 4 ꢀ CH3 of iPr, 3JHH = 7 Hz), 1.27 (s, 12H,
2.2.2. N-iodoacetyl-2-aminophenylboronic acid pinacol ester, 2
A 500 mL round-bottomed flask was charged with a slurry of
potassium iodide (20.54 g, 123.7 mmol) in acetone (250 mL).
Added portionwise, with stirring, was N-chloroacetyl-2-amin-
ophenylboronic acid pinacol ester 1 (18.29 g, 61.9 mmol), and the
reaction mixture left to stir at ambient temperature overnight.
The resulting bright yellow suspension was concentrated to dry-
ness under reduced pressure. Dichloromethane (250 mL) was
added, and the resulting dark yellow suspension filtered by suction
to remove insoluble material; the inorganic solids were washed on
the filter with dichloromethane until colourless. The yellow
solution was dried over Na2SO4 and the solvent removed under
reduced pressure, yielding the title compound as a yellow, crystal-
line solid which was dried under high vacuum. Yield: 22.25 g
i
3
4 ꢀ CH3of pinacol ester), 3.28 (hept, 2H, 2 ꢀ CH of Pr, JHH = 7 Hz),
3.78 (s, 2H, NHCH2CONH), 7.12 (m, 4H, aryl C–H), 7.50 (t of d, 1H,
3
4
aryl C–H, JHH = 8 Hz, JHH = 1 Hz), 7.78 (d of d, 1H, aryl C–H,
3JHH = 8 Hz, JHH = 1 Hz), 8.52 (d, 1H, aryl C–H, JHH = 8 Hz), 10.28
4
3
(s, br, 1H, N–H). 13C{1H} NMR (75.5 MHz, CDCl3, 298 K):
d
i
i
24.3 ppm (CH3 of Pr), 24.9 (CH3 of pinacol ester) 27.8 (CH of Pr),
55.6 (CH2 of NHCH2CONH), 84.2 (Cq of pinacol ester), 119.5 (aryl
C–H), 122.7 (aryl Cq), 123.4, 123.8, 124.3, 132.7, 136.2 (all aryl C–
H), 142.1, 143.7 (both aryl Cq), 169.4 (NHCH2CONH). 11B{1H}
NMR (128.4 MHz, CDCl3, 298 K): d +30.3 ppm (s, br, C6H4B(OR)2).

182
A.L. Gott et al. / Inorganica Chimica Acta 369 (2011) 180–189
Anal. Calc. for C26H37BN2O3: C, 71.56; H, 8.55; N, 6.42. Found: C,
71.54; H, 8.63; N, 6.46%. MS (EI +ve): m/z 436 [M+].
diffraction were grown by the slow evaporation of a chloroform
solution. 1H NMR (300 MHz, CDCl3, 298 K): d 1.26 ppm (d, 6H,
i
3
i
3
2 ꢀ CH3 of Pr, JHH = 7 Hz), 1.29 (d, 6H, 2 ꢀ CH3 of Pr, JHH = 7 Hz),
i
3
2.2.5. 1-(2,4,6-Trimethylphenyl)-3-(2-trifluoroboratophenyl)-4,5-
dihydro-1H-imidazole 4a
3.13 (septet, 2H, 2 ꢀ CH of Pr, JHH = 7 Hz), 4.25 (m, 2H, CH2 of
NCH2CH2N0), 4.60 (m, 2H, CH2 of NCH2CH2N0), 7.13 (d, 1H, aryl C–
3
A 250 mL round-bottomed flask was charged with a solution of
3a (3.45 g, 8.75 mmol) in THF (50 mL). Added via syringe was a
solution of BH3.THF (1 M in THF, 60 mL, 60 mmol, ꢂ7 equiv) and
the reaction refluxed under argon overnight. The colour of the
reaction changed from yellow to almost colourless during the
course of the reaction. The reaction was allowed to cool to ambient
temperature and methanol was added slowly, with vigorous effer-
vescence, to hydrolyse excess borane. All volatiles were removed in
vacuo and the process repeated twice more to yield the diamine as
a viscous off-white or yellow oil in effectively quantitative yield
which contained some residual methanol [1H NMR (300 MHz,
CDCl3, 298 K): d 1.32 ppm (s, 12H, 4 ꢀ CH3 of pinacol ester), 2.22
(s, 3H, CH3 of mesityl), 2.26 (s, 6H, 2 ꢀ CH3 of mesityl), 3.19 (m,
3H, CH2 of NCH2CH2N0 and N–H, overlapping), 3.36 (q, br, 2H,
H, JHH = 7 Hz), 7.23–7.35 (m, 4H, aryl C–H), 7.44 (t, 1H, aryl C–H,
3JHH = 7 Hz), 7.87 (d of d, 1H, aryl C–H, JHH = 7 Hz, JHH = 2 Hz),
3
4
8.21 (s, 1H, imidazolium C–H). 13C{1H} NMR (75.5 MHz, CDCl3,
i
i
298 K): d 24.5, 24.8 ppm (both CH3 of Pr), 28.5 (CH of Pr), 52.5,
53.3 (both CH2 of NCH2CH2N0), 122.1 (aryl C–H), 125.0, 127.4,
128.7, 129.7, 131.1, (all aryl C–H), 134.5, 137.3, 147.0 (all aryl Cq),
158.0 (imidazolium C-H). 11B{1H} NMR (128.4 MHz, CDCl3, 298 K):
d +2.6 ppm (s, br, C6H4BF3-). 19F NMR (376.59 MHz, CDCl3, 298 K):
d ꢁ139.8 ppm (s, br, C6H4BF3ꢁ), Anal. Calc. for C21H26BF3N2.CH3CN:
C, 66.52; H, 7.04; N, 10.12. Found: C, 66.46; H, 7.13; N, 9.99%. MS
(EI +ve): m/z 355 [M+ꢁF], 305 [M+ꢁBF3ꢁH].
2.2.7. Lewis pair, 5b
In the glovebox, a 100 mL round-bottomed flask fitted with a
Y-shaped adapter and stir bar was charged with imidazolium salt
4b (0.300 g, 0.80 mmol) and KN(SiMe3)2 (0.160 g, 0.80 mmol). The
flask was attached to a high vacuum line and THF (10 mL) was con-
densed in at ꢁ78 °C, before the vessel was allowed to warm to
ambient temperature. The clear yellow solution was left to stir at
room temperature overnight before all volatiles were removed in
vacuo to yield the title compound as an off-white solid. Yield:
0.252 g (0.71 mmol, 88%). Recrystallisation of the crude product
(in air) from ethyl acetate yielded the title compound as colourless
needles. 1H NMR (400 MHz, CDCl3, 298 K): d 1.29 ppm (d, 6H, CH3
of iPr, 3JHH = 7 Hz), 1.33 (d, 6H, CH3 of iPr, 3JHH = 7 Hz), 3.04 (septet,
3
3
CH2 of NCH2CH2N0, JHH = 5 Hz), 6.03 (t, br, 1H, N–H, JHH = 5 Hz),
6.57–6.66 (m, 2H, aryl C–H), 6.82 (s, 2H, aryl C–H of mesityl),
3
4
7.29 (t of d, 1H, aryl C–H, JHH = 7 Hz, JHH = 2 Hz), 7.65 (d of d,
3
4
1H, aryl C–H, JHH = 7 Hz, JHH = 2 Hz)]. The oil thus obtained was
slurried in triethylorthoformate (25 mL), and ammonium tetrafluo-
roborate (4.97 g, 47.5 mmol, 5 equiv.) added. The pale yellow, het-
erogeneous suspension was heated to 125 °C under argon for 24 h
(turning orange at ca. 90 °C), whereupon a beige solid was
observed to have precipitated from the orange solution. The solids
were isolated by filtration and washed with pentane (50 mL), be-
fore addition, with vigorous shaking, to 200 mL of dichlorometh-
ane; the unreacted ammonium tetrafluoroborate was then
removed by filtration and the solids washed with dichloromethane
until colourless. Removal of volatiles in vacuo yielded the title com-
pound as a pale beige/yellow solid which was recrystallised from
hot acetonitrile where necessary. Yield: 1.94 g (5.84 mmol, 67%).
1H NMR (300 MHz, CDCl3, 298 K): d 2.29 ppm (s, 3H, CH3 of mesi-
tyl), 2.35 (s, 6H, 2 ꢀ CH3 of mesityl), 4.23 (m, 2H, CH2 of
NCH2CH2N0), 4.56 (m, 2H, CH2 of NCH2CH2N0), 6.95 (s, 2H aryl C–
i
3
2H, CH of Pr, JHH = 7 Hz), 4.15 (app. t, 2H, CH2 of NCH2CH2N0,
2
2JHH = 9 Hz), 4.35 (app. t, 2H, CH2 of NCH2CH2N0, JHH = 9 Hz), 6.72
3
3
(d, 1H, aryl C–H, JHH = 7 Hz), 7.13 (t, 1H, aryl C–H, JHH = 7 Hz),
7.23–7.30 (m, 3H, aryl C–H), 7.43 (m, 2H, aryl C–H). 13C{1H} NMR
i
(100.6 MHz, CDCl3, 298 K): d 23.7 ppm (CH of Pr), 25.2, 28.7 (CH3
i
of Pr), 43.0, 58.6 (both CH2 of NCH2CH2N0), 109.9, 124.6, 125.1,
128.1, 130.2, 130.4 (all aryl C–H), 131.4, 146.7 (both aryl Cq). Car-
bene carbon not located due to adjacent boron atom. 11B{1H}
NMR (128.4 MHz, CDCl3, 298 K): d +4.3 ppm (s, br, C6H4BF2CNHC).
3
H), 7.10 (d, 1H, aryl C–H, JHH = 7 Hz), 7.26–7.31 (m, 2H, aryl C–
3
4
H), 7.84 (d of d, 1H, aryl C–H, JHH = 7 Hz, JHH = 2 Hz), 8.14 (s, 1H,
imidazolium C–H). 13C{1H} NMR (100.6 MHz, (CD3)2SO, 298 K): d
17.5, 21.0 ppm (CH3 of mesityl), 50.8, 53.0 (both CH2 of
NCH2CH2N0), 124.5, 127.6, 128.2, 129.7, 131.7 (all aryl C–H),
134.2, 136.4, 139.3, 139.8 (all Cq), 159.0 (imidazolium C–H).
11B{1H} NMR (128.4 MHz, (CD3)2SO, 298 K): d +2.5 ppm (s, br,
C6H4BF3ꢁ). 19F NMR (376.59 MHz, CDCl3, 298 K): d ꢁ139.1 ppm
(s, br, C6H4BF3ꢁ). Anal. Calc. for C18H20BF3N2: C, 65.08; H, 6.07; N,
8.43. Found: C, 64.67; H, 6.05; N, 8.57%. MS (EI +ve): m/z 313
[M+ꢁF], 263 [M+ꢁBF3ꢁH].
19F NMR (376.59 MHz, CDCl3, 298 K):
d
ꢁ166.9 ppm (s, br,
C6H4BF2CNHC). Anal. Calc. for C21H25BF2N2.(C4H8O2)0.25: C, 70.23;
H, 7.23; N, 7.45. Found: C, 70.56; H, 7.01; N, 7.80%. MS (EI +ve):
m/z 354 [M+], 306 [M+ꢁBF2].
2.2.8. Free N-heterocyclic carbene, 6a
In the glovebox, a 100 mL round-bottomed flask equipped with
Y-adaptor and Kontes tap was charged with 4a (0.300 g, 0.90 mmol),
KN(SiMe3)2 (0.182 g, 0.90 mmol, 1 equiv.) and 18-crown-6 (0.238 g,
0.90 mmol). The flask was attached to a high vacuum line and THF
was condensed in at ꢁ78 °C. The heterogeneous suspension was al-
lowed to warm to room temperature, with stirring, to yield a clear,
dark yellow solution. The reaction was allowed to stir at room tem-
perature for one hour before all volatiles were removed in vacuo.
Hexanes (20 mL) were condensed in at ꢁ78 °C, and the resulting
brown suspension stirred for 30 min; removal of the solvent yielded
the title compound as a crystalline, pale brown solid. Yield: 0.540 g
(0.85 mmol, 94%). 1H NMR (300 MHz, C6D6, 298 K): d 2.09 ppm (s,
3H, CH3 of mesityl), 2.34 (s, 6H, 2 ꢀ CH3 of mesityl), 3.11 (s, 24H,
CH2 of 18-crown-6), 3.40 (app. t, 2H, CH2 of NCH2CH2N0,
2.2.6. 1-(2,6-Diisopropylphenyl)-3-(2-trifluoroboratophenyl)-4,5-
dihydro-1H-imidazole, 4b
Using the procedure described for 4a above, using 3b (6.85 g,
15.7 mmol) and BH3 THF (1 M in THF, 75 mL, 75 mmol, 5 equiv.)
yielded a viscous, colourless gum in effectively quantitative yield
which was used immediately in the next step [1H NMR (400 MHz,
i
3
CDCl3, 298 K): d 1.14 (d, 12H, 4 ꢀ CH3 of Pr, JHH = 7 Hz), 1.31 (s,
i
12H, 4 ꢀ CH3 of pinacol ester, 3.10–3.26 (m, 5H, CH of Pr, NCH2
and N–H, all overlapping, 3.49 (m, 2H, NCH2), 6.06 (s, v. br, N–H),
6.59–6.65 (m, 2H, aryl C–H, 7.06 (m, 3H, aryl C–H), 7.28 (t, 1H, aryl
2
2JHH = 10 Hz), 4.26 (app. t, 2H, CH2 of NCH2CH2N0, JHH = 10 Hz),
3
3
C–H, JHH = 7 Hz), 7.65 (d, 1H, aryl C–H, JHH = 7 Hz)]. The residue
thus isolated was reacted with triethylorthoformate (20 mL) and
ammonium tetrafluoroborate (8.23 g, 78.5 mmol, 5 equiv.) as de-
scribed above to yield the title compound 4b as a beige solid. Yield:
2.03 g (5.4 mmol, 35%). The compound was recrystallised from hot
acetonitrile where necessary. Single crystals suitable for X-ray
6.72 (s, 2H, aryl C–H), 7.20 (m, 2H, aryl C–H, overlapping), 7.44 (m,
3
4
1H, aryl C–H), 8.33 (d of d, 1H, aryl C–H, JHH = 7 Hz, JHH = 2 Hz).
13C{1H} NMR (75.5 MHz, C6D6, 298 K): d 18.0, 20.6 ppm (both CH3
of mesityl), 51.1, 53.5 (both CH2 of NCH2CH2N0), 69.6 (CH2 of 18-
crown-6), 124.8, 126.0, 126.8, 128.7 (all aryl C–H), 129.5, 135.5
(both aryl Cq), 137.0 (aryl C–H), 140.2, 148.0 (both aryl Cq), 235.3

A.L. Gott et al. / Inorganica Chimica Acta 369 (2011) 180–189
183
(CNHC). 11B{1H} NMR (128.4 MHz, C6D6, 298 K): d
+3.5 ppm
aryltrifluoroborate salts [46]. To incorporate such moieties into
the NHC framework required incorporation of substituents bearing
boronic acids or derivatives either directly, or by quenching of lith-
iated derivatives with suitable boron electrophiles [47]. The com-
mercially available 2-aminophenylboronic acid pinacol ester
shown in Scheme 1 was chosen as the source of the borate group.
In comparison to the synthesis of symmetrically substituted NHCs
[48], the synthesis of NHCs in which the two N-substituents are dif-
ferent is more involved, generally taking the form of addition of the
C2 backbone to an aniline, followed by the addition of the second
substituent, either by condensation [49,50], Pd-mediated cross-
coupling [40,51], or nucleophilic substitution [52]. Numerous routes
to unsymmetrically substituted NHCs now exist [53,54] and we be-
gan our studies utilising the methodology of Kotschy and co-
workers [54], as shown in the first step of Scheme 1.
Treatment of 2-aminophenylboronic acid pinacol ester with
chloroacetyl chloride in the presence of potassium carbonate
yielded the chloroalkyl derivative 1 in essentially quantitative
yield. This reaction was amenable to scale up to ca. 30 g scale.
Coupling of the two reagents was demonstrated by the 1H NMR
shift of the chloromethyl group from d 4.62 ppm in the starting
material to 4.18 ppm in 1. Incorporation of an aniline derivative
into synthons related to 1 has been shown previously within this
group, and others [54] as unsuccessful; aryl amines are insuffi-
ciently nucleophilic to effect displacement of the chloride group
of 1. Therefore, exchange of chloride for iodide was necessary,
and was accomplished by reaction with KI to give the iodoalkyl
species 2 in high yield (ca. 95%). Conversion of 1–2 was corrobo-
rated by the shift of the methylene carbon to higher field due to
the presence of the less electronegative iodide. Both 1 and 2 were
prone to slow decomposition over prolonged periods, and so were
generally used as soon as prepared. Following methodology
established in this laboratory [55], stirring 2 in an excess (>15
equivalents) of either neat 2,4,6-trimethylaniline or 2,6-diisopro-
pylaniline in the presence of a catalytic amount of 4-dimethylami-
nopyridine furnished the secondary amines 3 in good yields, after
removal of excess aniline by short-path distillation and crystallisa-
tion from pentane. Reduction of the amide functionality in 3 was
easily accomplished by refluxing with borane-tetrahydrofuran
complex overnight, yielding the corresponding diamines in essen-
tially quantitative yields. Complete reduction was demonstrated
by the disappearance of the amido proton resonances at ca.
10.9 ppm and the appearance of broad resonances at ca. 6 ppm
for the NHR2 functionalities.
Two related methods for the cyclisation of NHC precursors ex-
ist: quaternisation of the amines by reaction with concentrated
acid, followed by reflux in triethylorthoformate or derivatives
[48]; or reflux in triethylorthoformate in the presence of ammo-
nium tetrafluoroborate [56]. The former route yields chloride salts
and the latter tetrafluoroborate derivatives; to avoid acid cleavage
of the ester, we chose the latter route. Refluxing the diamines 3 in
the presence of one equivalent of ammonium tetrafluoroborate un-
der an inert atmosphere resulted, after a few hours, in the crystal-
lisation of a pale brown solid in moderate-to-poor yield. After
isolation by filtration, the compound was analysed by NMR spec-
troscopy. The compounds 4 were shown to be a single compound
by 1H NMR spectroscopy, and displayed resonances consistent
with cyclisation to a dihydroimidazole containing species. These
resonances (in the case of 4b) consist of two triplets in a typical
AA0BB0 pattern at d 4.25 and 4.60 ppm, both integrating to two
hydrogen atoms, consistent with the saturated backbone of the
carbene skeleton. In addition, a singlet resonance at 8.21 ppm (cor-
responding to a 13C{1H} resonance at 158.0 ppm) is typical of the
imidazolium proton of NHC rings. Also noteworthy were only
two resonances corresponding to the isopropyl methyls, instead
of the expected four. Significant by their absence were the reso-
(C6H4BF3K(18-crown-6)). 19F NMR (376.59 MHz, C6D6, 298 K): d
ꢁ138.7 ppm (C6H4BF3K(18-crown-6)). HRMS (ESI ꢁve): Calcd for
[Mꢁ] m/z 339.1598; Found 339.1592.
2.2.9. Free N-heterocyclic carbene, 6b
As for 4a above, using 4b (0.505 g, 1.35 mmol), KN(SiMe3)2
(0.268 g, 1.35 mmol) and 18-crown-6 (0.36 g, 1.35 mmol) in THF
(25 mL). Yield: 0.845 g (1.24 mmol, 93%). 1H NMR (300 MHz, C6D6,
i
298 K): d 1.22 ppm (d, 6H, CH3 of Pr, 3JHH = 7 Hz), 1.32 (d, 6H, CH3
of iPr, 3JHH = 7 Hz), 3.06 (s, 24H, CH2 of 18-crown-6), 3.45–3.61 (m,
4H, CH of iPr and CH2 of NCH2CH2N0, overlapping), 4.53 (app. t, 2H,
CH2 of NCH2CH2N0, 2JHH = 10 Hz), 7.13–7.25 (m, 5H, aromatic C–H),
3
7.88 (d, 1H, aryl C–H, JHH = 7 Hz), 8.27 (d of d, 1H, aryl C–H,
4
3JHH = 7 Hz, JHH = 2 Hz). 13C{1H} NMR (75.5 MHz, C6D6, 298 K): d
i
i
24.1, 24.8 ppm (both CH3 of Pr), 28.1 (CH of Pr), 52.5, 54.5 (both
CH2 of NCH2CH2N0), 69.8 (CH2 of 18-crown-6), 123.5, 123.9, 126.1,
135.0 (all aryl C–H) 140.5 (Cq), 147.5 (aryl C–H), 148.6 (Cq), 240.4
(CNHC). 11B{1H} NMR (128.4 MHz, C6D6, 298 K): d
+4.2 ppm
(C6H4BF3K(18-crown-6)). 19F NMR (376.56 MHz, C6D6, 298 K): d
ꢁ137.9 ppm (C6H4BF3K(18-crown-6)). HRMS (ESI ꢁve): Calcd for
[Mꢁ+H2O] m/z 391.2169; Found 391.2185.
2.2.10. Rh complex, 7
A J. Young NMR tube was charged with a solution of free car-
bene (41 mg, 60.8
lmol) in C6D6 (0.2 mL). Added was a solution
of [RhCl(COD)]2 (15 mg, 30.4
l
mol) in C6D6 (0.2 mL) at room tem-
perature, turning dark red, with the immediate precipitation of a
white crystalline solid. The sample was then characterised by
NMR spectroscopy. 1H NMR (400 MHz, C6D6, 298 K): d 0.99 ppm
i
3
i
3
(d, 6H, CH3 of Pr, JHH = 7 Hz), 1.42 (d, 6H, CH3 of Pr, JHH = 7 Hz),
1.84–1.99 (m, 2H, CH2 of COD), 2.13–2.20 (m, 6H, CH2 of COD),
2.97 (heptet, 2H, CH of iPr), 3.08 (m, 2H, CH2 of NCH2CH2N0), 3.25
2
(m, 2H, CH2 of NCH2CH2N0), 3.94 (d, 2H, CH of COD, JRhH = 2 Hz),
2
5.27 (d, 2H, CH of COD, JRhH = 2 Hz), 6.41 (d, 1H, aryl C–H,
3JHH = 7 Hz), 6.98–7.18 (m, 5H, aryl C–H), 7.43 (d, 1H, aryl C–H,
3JHH = 7 Hz). 13C{1H} NMR (75.5 MHz, toluene-d8, 298 K): d 25.3,
i
i
26.2 ppm (both CH3 of Pr), 28.6 (CH of Pr), 30.6, 31.8 (both CH2
1
of COD), 44.1, 55.0 (both NCH2), 83.7 (d, CH of COD, JRhC = 9 Hz),
1
90.5 (d, CH of COD, JRhC = 9 Hz), 108.4, 122.4, 134.9, 136.6, 146.8
(all aryl C–H), 152.4, 154.0, 154.5 (all aryl Cq), 205.7 (CNHC
,
1JRhC = 59 Hz). 11B{1H} NMR (128.4 MHz, toluene-d8, 298 K): d
+5.5 ppm (C6H4BF3ꢁ). 19F NMR (376.59 MHz, C6D6, 298 K):
d
ꢁ138.0 ppm (s, br, C6H4BF3ꢁ). Variable temperature NMR samples
were prepared similarly in toluene-d8.
2.3. X-ray crystallography
Single crystals of 4b were obtained from the slow evaporation
of a saturated chloroform solution at room temperature. Single
crystals of 5 were obtained similarly from a dichloromethane solu-
tion. X ray data for 4b were collected on a Bruker PLATFORM/
SMART 1000 CCD diffractometer using graphite monochromated
Mo K
a
radiation at ꢁ80 °C. Diffraction data for 5 were collected
on a Bruker P4/RA/SMART 1000 CCD diffractometer using Mo K
a
radiation at ꢁ100 °C. All data were corrected for absorption using
SADABS [44], and the structures solved in SHELXS97 [45] and refined
using full-matrix least squares on F2 using SHELXL97 [45].
3. Results and discussion
3.1. Ligand synthesis
The fluorination of boronic acids or esters with potassium dihy-
drogen fluoride has been shown to be a facile route to potassium

184
A.L. Gott et al. / Inorganica Chimica Acta 369 (2011) 180–189
Scheme 1. Synthesis of imidazolium salts 4.
nances corresponding to methyl groups of the pinacol ester in
either of the 1H or 13C{1H} NMR spectra, indicating cleavage of
the ester protecting group during the reaction. The fate of the boryl
ester was investigated by 11B NMR spectroscopy; this revealed a
single broad resonance at +2.6 ppm in the 11B{1H} NMR spectrum
indicative of four-coordinate boron and a marked change from
the chemical shift of the precursor which displayed a typical chem-
ical shift for three coordinate boron [57]. Furthermore, the 19F NMR
spectrum displayed a broad singlet at ꢁ139.1 ppm in agreement
with reported literature values for species which contain the
the imidazolium proton, as the distance of 3.00 Å lies well outside
the sum of the van der Waals radii of 2.67 Å. All other bond lengths
are otherwise unremarkable.
The low yields of the compounds 4 in the above described pro-
tocol likely stemmed from a deficiency of fluoride; accordingly, the
reaction was optimised when addition of a fivefold excess of
ammonium tetrafluoroborate resulted in an overall yield of 67%
for 4a and 35% for 4b, respectively. Characterisation of the contents
of the respective supernatants revealed multiple unidentified
products. The lower yield of 4b is likely due to the increased steric
bulk of the 2,6-diisopropylaniline moiety in the cyclisation site. In
terms of the mechanism of the ‘one pot’ reaction, we speculate that
residual methanol (present from the workup of the unsymmetric
diamines) and ethanol released as the cyclisation proceeds reacts
with triethylorthoformate, yielding formic acid. The formic acid
then performs the role of deprotecting the boryl ester whilst the
cyclisation reaction proceeds. Ammonium tetrafluoroborate then
acts as a latent fluorinating agent via the dissociation into ammo-
nium fluoride and BF3; ammonium fluoride is a known fluorinating
agent [58,59]. In light of there being three possible steps in this
reaction, an overall yield of 35% for the synthesis of 4b is relatively
respectable if it is considered as three separate steps.
-
PhBF3 moiety [46]. The above observations indicated that the
pinacol ester had been cleaved during the course of the reaction,
and an in situ fluorination had taken place, to yield a compound
with spectral properties consistent with the target.
This fortuitous turn of events was confirmed by X-ray crystallo-
graphic analysis on suitable crystals of 4b. An ORTEP representa-
tion is shown in Fig. 1, whilst relevant bond lengths and angles
are given in Table 1, with data collection and refinement parame-
ters in Table 3. The structural study established that the borate
ester had been cleaved during the course of the reaction, demon-
strating that the product of the cyclisation reaction is formally
zwitterionic in nature, with the negative charge of the aryl trifluo-
roborate moiety balanced by the positive charge which resides at
the nitrogen atoms of the imidazolium ring. A similar compound,
(1S,2S)-diphenyl-3-(2,6-diisopropylphenyl)-5-(2-sulfonylphenyl)-
dihydroimidazole was recently reported by Hoveyda and co-
workers [37,40], although not structurally characterised. As might
be expected, the C(1)–N(1) and C(1)–N(2) bond lengths are essen-
tially the same (1.303(3) and 1.315(3) Å, respectively), showing full
delocalisation of the dihydroimidazolium double bond. The
2,6-diisopropylphenyl group is oriented orthogonal to the five-
membered imidazolium ring. There appears to be little hydrogen
bonding between fluoride ions of the trifluoroborate moiety and
3.2. Deprotonation of imidazolium salts
A commonly applied deprotonation route is via reaction with
silver(I) oxide to yield silver(I) salts which can be used as a transfer
reagent in salt metathesis reactions [60]. Under a variety of differ-
ent reaction conditions, imidazolium salts 4 showed no reactivity
toward silver oxide, even in the presence of 4 Å molecular sieves.
This was surprising, given the report from Nozaki and co-workers
whilst this work was underway [41], and other reports from Hov-
eyda and co-workers of related sulphonated NHC ligands [37,40]

A.L. Gott et al. / Inorganica Chimica Acta 369 (2011) 180–189
185
Fig. 1. Molecular structure of 4b. Displacement ellipsoids are shown at the 50% probability level.
Table 1
Table 3
Bond lengths and angles for 4b.
Crystallographic data, acquisition parameters and processing parameters.
Bond
Length (Å)
Bond
Length (Å)
4b
5b
C(1)–N(1)
C(1)–N(2)
N(1)–C(2)
N(2)–C(3)
B(1)–F(1)
B(1)–F(3)
1.303(3)
1.315(3)
1.476(3)
1.477(3)
1.396(3)
1.377(3)
Angle (°)
113.5(2)
110.04(18)
109.97(10)
102.70(17)
C(2)–C(3)
N(1)–C(11)
N(2)–C(21)
B(1)–C(12)
B(2)–F(2)
1.531(3)
1.446(3)
1.445(3)
1.649(4)
1.391(3)
Formula
C
21H26BF3N2
C21H25BF2N2
354.24
0.24 ꢀ 0.18 ꢀ 0.14
colourless block
monoclinic
P21/c
12.800(3)
14.078(4)
11.964(4)
90
115.363(18)
90
1948.1(10)
4
0.083
1.208
752
Molecular weight
Crystal size (mm)
Morphology
Crystal system
Space group
a (Å)
374.25
0.18 ꢀ 0.16 ꢀ 0.11
colourless prism
monoclinic
P21/c
8.8297(13)
11.9506(18)
18.731(3)
90
99.946(3)
90
1946.8(5)
4
Angle (°)
N(2)–C(1)–N(1)
C(3)–N(2)–C(1)
C(2)–N(1)–C(1)
C(3)–C(2)–N(1)
C(2)–C(3)–C(2)
C(1)–N(1)–C(11)
C(1)–N(2)–C(21)
102.26(17)
126.15(19)
125.94(18)
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
l
(Mo K
a
, mmꢁ1
)
0.094
1.277
792
14 108
Table 2
Bond lengths and angles for 5b.
Dcalc
F(0 0 0)
Total reflections
Bond
Length (Å)
Bond
Length (Å)
8680
Independent reflections
R(int)
Data/restraints/parameters
R1 [I > 2r(I)]
wR2
3711
0.0681
3711/0/244
0.0537
0.1166
1.012
0.452, ꢁ0.234
4448
0.0363
4448/0/240
0.0476
0.1140
0.990
0.216, ꢁ0.167
C(1)–N(1)
N(1)–C(2)
C(2)–C(3)
B(1)–C(21)
B(1)–F(2)
N(1)–C(4)
1.321(2)
1.484(2)
1.538(3)
1.635(2)
1.399(2)
1.440(2)
Angle (°)
110.06(14)
113.47(14)
103.46(13)
109.76(14)
110.00(14)
110.67(14)
C(1)–N(2)
N(2)–C(3)
C(1)–B(1)
B(1)–F(1)
N(2)–C(16)
C(16)–C(21)
1.331(2)
1.468(2)
1.682(2)
1.397(2)
1.426(2)
Goodness-of-fit (GOF)
Largest peak/hole (e Å3)
Angle (°)
N(1)–C(1)–N(2)
C(1)–N(2)–C(3)
N(1)–C(2)–C(3)
F(1)–B(1)–F(2)
C(16)–C(21)–B(1)
N(2)–C(16)–C(21)
C(1)–N(1)–C(2)
C(2)–C(3)–N(2)
B(1)–C(1)–N(2)
F(2)–B(1)–C(21)
C(1)–N(2)–C(16)
C(1)–N(1)–C(4)
111.32(13)
101.04(13)
109.54(13)
114.74(13)
113.91(14)
125.08(13)
the carbene carbon resonance could not be located by 13C NMR
spectroscopy, even in concentrated samples. In addition, the prod-
uct was completely air stable and unreactive towards molecules
such as chloroform and phenylacetylene which are species known
to react with free carbenes [61]. Also, the 19F NMR spectrum
showed a significant high field shift, exhibiting a singlet resonance
at ꢁ166.9 ppm, inconsistent with the range of values reported for
the PhBF3K moiety which resonate between ꢁ130 and
ꢁ140 ppm. Taken as a whole, these observations implied an
unforeseen reactivity. Structural characterisation of the material
revealed the compound to be a Lewis acid/base pair, between the
deprotonated carbene and an aryldifluoroborate moiety with loss
of potassium fluoride from the postulated initial product. The
molecular structure of 5b can be seen in Fig. 2, with bond lengths
and angles in Table 2.
which react smoothly at elevated temperatures to yield
multinuclear silver complexes. The usual alternative to the
procedure described above is to react imidazolium salts with
bulky, non-nucleophilic bases such as KOtBu and KN(SiMe3)2. An
NMR-scale reaction in THF-d8 saw instant, clean reaction between
4b and KN(SiMe3)2, as demonstrated by a disappearance of the imi-
dazolium proton and concomitant formation of hexamethyldisilaz-
ane to form a new species 5b as shown in Scheme 2.
A preparative-scale reaction exhibited the same behaviour,
however it was apparent from spectroscopic examination that

186
A.L. Gott et al. / Inorganica Chimica Acta 369 (2011) 180–189
Scheme 2. Deprotonation of imidazolium salts 4.
Fig. 2. Molecular structure of 5b. Displacement ellipsoids are shown at the 50% probability level.
The structure shows that the compound is formally a Lewis
twisting of the C(2)–C(3) backbone relative to the plane of the ring;
the angle between mean least squares planes defined by C(1)–
N(1)–C(2) and C(1)–N(2)–C(3) is 5.2°. The boron atom lies
0.108 Å out of the plane defined by the five atoms of the imidazo-
lium ring. We suggest that the compound 5b is formed via the ini-
tial formation of a potassium carbene complex – for which
precedent exists in the literature [63–67] – followed by the extru-
sion of potassium fluoride and stabilisation of the three coordinate
boron centre by the free carbene [68–72]. The lack of appearance of
the carbene carbon in the 13C NMR spectrum is in hindsight obvi-
ous due to coupling to the adjacent quadrupolar boron nucleus of
the aryldifluoroborate group. There are relatively few compounds
in the literature with which to compare the spectroscopic proper-
ties of 5b; (C6F5)2BF2K resonates at ꢁ145.8 ppm in the 19F NMR
spectrum [73], and the related MBF2CN2 (M = Li, K) show reso-
nances at ca. ꢁ153 ppm [74]. The high field shift is demonstrative
of a more electron rich boron centre due to the strong donor prop-
erties of the carbene.
acid/base adduct, formed between the deprotonated N-heterocy-
clic carbene and a boron centre which has lost a fluoride ion, thus
rendering it three coordinate and so able to accept the free car-
bene’s electron pair. As with 4b, the 2,6-diisopropylphenyl group
is essentially orthogonal to the rest of the molecule. Compound
5b is related to the compound IMesꢃBF3 which has been reported
by Arduengo and co-workers [62]. The B(1)–C(1) bond distance
of 1.68 Å is slightly longer than the corresponding distance in
IMesꢃBF3 (1.635(5) Å), which is somewhat surprising given the
improved donor strength of carbenes obtained from dihydroimi-
dazolium salts versus imidazolium salts; in this instance, steric
abutment of the bulky 2,6-diisopropylphenyl substituent would
appear to limit the approach of the carbene to the boron centre.
The C(1)–N(1) and C(1)–N(2) bond distances of 1.321(2) and
1.331(2) Å are marginally shorter than the analogous distances in
IMesꢃBF3 (1.350(3) and 1.353(5) Å, respectively, due to the non
aromaticity of the former versus the latter, and thus free from
the geometric constraints imposed by the double bond in the
carbene backbone. In addition, in comparison to IMesꢃBF3, the
C(2)–N(1) and C(3)–N(2) bond distances (1.484(2) and
1.468(2) Å), and the ‘backbone’ C(2)–C(3) bond (1.538(3) Å) of 5b
are longer than the analogous distance in IMesꢃBF3. There is a slight
To yield a free carbene in which the trifluoroborate moiety
remained intact, we hoped that sequestration of the potassium cat-
ion by a crown ether would obviate the formation of potassium
fluoride, by preventing the potassium cation from closely
approaching the free carbenic carbon. To that end, deprotonation

A.L. Gott et al. / Inorganica Chimica Acta 369 (2011) 180–189
187
of the imidazolium salts 4 was cleanly accomplished by their
treatment with KN(SiMe3)2 in dry tetrahydrofuran in the presence
of 18-crown-6 to yield the free carbenes 6a/b as air- and moisture-
sensitive pale brown solids. The compounds are insoluble in pen-
tane/hexane and diethyl ether, though very highly soluble in
benzene and THF, so much so that it precludes crystallisation.
The free carbenes are slowly reactive toward chlorinated solvents,
matching similar behaviour as reported by Arduengo. The free
carbenes were distinguished from the imidazolium precursors by
an upfield shift of the multiplet CH2 resonances of the carbene
backbone (for example d 3.11 and 3.40 ppm in 6a), with concomi-
tant disappearance of the singlet imidazolium resonance at ca.
8 ppm in the 1H NMR spectrum. The trifluoroborate moiety was
confirmed as being intact via 19F NMR spectroscopy as illustrated
by a broad singlet at ca. ꢁ138 ppm, rather than a high field shift
as per 5b. Over a period of several months, 6a/b decomposes in
solution to both KBF4 and 5a/b (integration 4F–2F) as determined
by 19F NMR spectroscopy. The carbene carbon resonance was lo-
cated at +240.4 ppm in the 13C{1H} NMR spectrum for 4b, in excel-
lent agreement with the many known saturated N-heterocyclic
carbene ligands in the literature [75].
confirmed by diagnostic resonances at d 3.94 and 5.28 ppm,
displaying small (ca. 2 Hz) coupling to rhodium. The minor product
(ca. 40%) was determined by 1H NMR spectroscopy to be 5b. The
principal feature of the 13C{1H} NMR spectrum was a low field dou-
blet centred at 205.7 ppm, consistent with a ligated N-heterocyclic
carbene coupling to a 103Rh (I = ½) nucleus. The coupling constant
was 59 Hz which lies at the higher end of literature values [75].
Nolan has correlated NHC–Pt coupling constant as a measure of
carbene donor strength [86], perhaps implying slightly improved
donor strength versus ‘unfunctionalised’ SIMes derivatives. The
chemical shift of the carbene carbon (typically 180–220 ppm for
[(SIMes)RhCl(COD)] and derivatives) of d 205.7 ppm is consistent
with literature values [87–90]. The
g
4-1,5-cyclooctadiene ligand
was confirmed as present by the observation of two resonances
at 83.7 and 90.6 ppm, which also displayed a small coupling to
rhodium (1JRhC = 9 Hz). 11B{1H} NMR confirmed the presence of
the aryltrifluoroborate moiety, showing a broad resonance at
+5.5 ppm.
Three major products were observed by 19F NMR spectroscopy.
These consisted of a broad resonance at ꢁ138 ppm (ca. 25% of total
intensity by integration), which although comparable to free 6b by
19F NMR spectroscopy, reference spectra confirmed no free ligand
was present by 1H NMR and so was assigned as Rh complex 7 as
shown in Scheme 3.
3.3. Complexation
The second resonance (ca. 50% by total intensity) at d 151.4 ppm
corresponds to free tetrafluoroborate. The third resonance (ca. 25%
of total intensity) at d 166 ppm is consistent with the presence of 5.
The relative integrals compared to much smaller percentages of
identical impurities (roughly 10%) in ‘aged’ 6 suggests that either
a) the decomposition of the ligand is rapidly accelerated in the
presence of rhodium (the ratio of fluoroborate to 5 is consistent
with ligand decomposition as already shown); or b) the complex
formed is so inherently unstable it decomposes.
With the free ligands in hand, we sought to undertake an initial
exploration of their coordination chemistry and chose to explore
some Rh chemistry, in the hope of ascertaining the degree of inter-
action of the trifluoroborate moiety with transition metal centres
via observable Rh–F coupling in the NMR spectra. NHC complexes
of rhodium were first reported in the 1970s by Lappert and co-
workers [76–84] by the treatment of Wanzlick’s dimer with vari-
ous Rh-containing precursors. Rh-NHC chemistry has been widely
studied in recent years, and has been extensively reviewed by
Crudden [85]. Treatment of [RhCl(COD)]2 with 6b in C6D6 yielded
an immediate reaction to give a dark red solution containing an ex-
tremely air sensitive rhodium complex 7 along with the precipita-
tion of copious amounts of white solid; the chloride coligand
appears to have been lost in a salt metathesis reaction, resulting
in the precipitation of KCl(18-crown-6). Despite numerous at-
tempts, single crystals of 7 suitable for X-ray crystallography could
not be grown therefore the complex was characterised in situ by
NMR spectroscopy.
Variable temperature 19F NMR was employed to determine the
behaviour of the trifluoroborate moiety in 7; metal tetrafluorobo-
rate complexes have been shown to display low temperature 19F
NMR spectra containing separate resonances for bridging and ter-
minal fluorides, consistent with the formulation M–F–BF3 [91,92].
In an attempt to determine whether aryltrifluoroborates displayed
similar behaviour, a low temperature study was undertaken. Low
temperature 19F NMR spectroscopy of 7 revealed that the reso-
nance at ꢁ138 ppm decoalesced into a doublet at low temperature.
The coupling constant was determined to be 22 Hz. Given the sim-
ilarity of the room temperature chemical shift of the aryltrifluorob-
orate moiety with that of free ligand, it must be surmised that the
interaction with the Rh centre is extremely weak as demonstrated
by rapid rotation of the BF3 moiety, essentially yielding a three-
coordinate Rh(I) species, as shown in Scheme 3. Cooling of the
sample reduces the thermal motion of the complex such that rota-
tion about the carbene–Rh vector is somewhat slowed, yielding
some interaction between rhodium and trifluoroborate, but as such
is still weak, due to still only observing an average signal for the
The 1H NMR spectrum showed that complexation was facile
upon mixing; no free precursor and free ligand were present. The
major species present (ca. 60% by integration) exhibited two dou-
blets corresponding to the isopropyl methyl signals; these signals
were found to be much further separated in the complex (d 0.99
and 1.41 ppm) than in the free ligand (d 1.22 and 1.32 ppm). Con-
versely, the resonances associated with the NHC backbone (d 3.08
and 3.25 ppm) were much more closely spaced than in the free
ligand (d 3.61 and 4.51 ppm), in addition to a marked high field
shift. The retention of the coordinated cyclooctadiene ligand was
Scheme 3. Complexation of free carbene 6b.

188
A.L. Gott et al. / Inorganica Chimica Acta 369 (2011) 180–189
[22] O. Schuster, L.R. Yang, H.G. Raubenheimer, M. Albrecht, Chem. Rev. 109 (2009)
3445.
[23] D. McGuinness, Dalton Trans. (2009) 6915.
[24] D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 107 (2007) 5605.
[25] V. Cesar, S. Bellemin-Laponnaz, L.H. Gade, Chem. Soc. Rev. 33 (2004) 619.
[26] S.P. Nolan, N-Heterocyclic Carbenes In Synthesis, Wiley-VCH, 2006.
[27] H. Jacobsen, A. Correa, A. Poater, C. Costabile, L. Cavallo, Coord. Chem. Rev. 253
(2009) 2784.
fluorine atoms rather than three separate resonances due to
relatively unhindered rotation of the BF3 moiety. The observed
low temperature Rh–F coupling constant of 22 Hz is consistent
with a weak interaction; though fully covalently bonded rhodium
fluorides are extremely rare, a comparable example of the recently
reported RhF(PPh3)3 displays a coupling constant of 77 Hz [93].
[28] O. Kuhl, Chem. Soc. Rev. 36 (2007) 592.
[29] S.T. Liddle, I.S. Edworthy, P.L. Arnold, Chem. Soc. Rev. 36 (2007) 1732.
[30] L.R. Moore, S.M. Cooks, M.S. Anderson, H.J. Schanz, S.T. Griffin, R.D. Rogers, M.C.
Kirk, K.H. Shaughnessy, Organometallics 25 (2006) 5151.
[31] K.S. Lee, A.H. Hoveyda, J. Am. Chem. Soc. 132 (2010) 2898.
[32] K. Akiyama, F. Gao, A.H. Hoveyda, Angew. Chem., Int. Ed. 49 (2010) 419.
[33] Y.M. Lee, A.H. Hoveyda, J. Am. Chem. Soc. 131 (2009) 3160.
[34] Y. Lee, B. Li, A.H. Hoveyda, J. Am. Chem. Soc. 131 (2009) 11625.
[35] Y. Lee, H. Jang, A.H. Hoveyda, J. Am. Chem. Soc. 131 (2009) 18234.
[36] K.S. Lee, A.H. Hoveyda, J. Org. Chem. 74 (2009) 4455.
[37] T.L. May, M.K. Brown, A.H. Hoveyda, Angew. Chem., Int. Ed. 47 (2008)
7358.
[38] Y. Lee, K. Akiyama, D.G. Gillingham, M.K. Brown, A.H. Hoveyda, J. Am. Chem.
Soc. 130 (2008) 446.
[39] D.G. Gillingham, A.H. Hoveyda, Angew. Chem., Int. Ed. 46 (2007) 3860.
[40] M.K. Brown, T.L. May, C.A. Baxter, A.H. Hoveyda, Angew. Chem., Int. Ed. 46
(2007) 1097.
4. Conclusions
A novel family of trifluoroborate functionalised N-heterocyclic
carbenes (and precursors) were synthesised. The synthesis benefits
from an unforeseen one pot, three step process in which deprotec-
tion of the boronic ester, closure of the dihydroimidazolium ring
and fluorination of the boronic ester group occurs in the same reac-
tion. Manipulation of the reaction conditions gave the desired
dihydroimidazolium salts in moderate-to-good yield, one of which
has been structurally characterised. The zwitterionic imidazolium
salts formed are, surprisingly, unreactive towards silver(I) oxide
but can be deprotonated in facile manner by alkali metal amides.
In the presence of 18-crown-6, the desired free carbene is ac-
cessed; in the absence of 18-crown-6, elimination of potassium
fluoride occurs to yield a Lewis acid/base adduct. Complexation
to rhodium is facile and yields complexes that display weak coor-
dination of the trifluoroborate moiety to the metal centre, as
demonstrated by NMR spectroscopy, however significant decom-
position of the ligand appears to occur during complexation. The
potential elimination of 5 from 7 at ambient temperatures raises
doubts about the suitability of such ligand systems in catalytic pro-
cesses, which typically take place at higher temperatures. Alterna-
tive, more robust ligand systems, in which the carbene is replaced
with a different donor will be reported in due course.
[41] Y. Nagai, T. Kochi, K. Nozaki, Organometallics 28 (2009) 6131.
[42] W. Beck, K. Sunkel, Chem. Rev. 88 (1988) 1405.
[43] G. Giordano, R.H. Crabtree, Inorg. Synth. 28 (1990) 88.
[44] G.M. Sheldrick, SADABS, Program for Area Detector Absorption Correction,
Institute for Inorganic Chemistry, University of Gottingen, Germany, 1996.
[45] G.M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr. 64 (2008) 112.
[46] E. Vedejs, R.W. Chapman, S.C. Fields, S. Lin, M.R. Schrimpf, J. Org. Chem. 60
(1995) 3020.
[47] L.F. Groux, T. Weiss, D.N. Reddy, P.A. Chase, W.E. Piers, T. Ziegler, M. Parvez, J.
Benet-Buchholz, J. Am. Chem. Soc. 127 (2005) 1854.
[48] A.J. Arduengo, R. Krafczyk, R. Schmutzler, H.A. Craig, J.R. Goerlich, W.J.
Marshall, M. Unverzagt, Tetrahedron 55 (1999) 14523.
[49] A.G. Wenzel, R.H. Grubbs, J. Am. Chem. Soc. 128 (2006) 16048.
[50] G.C. Vougioukalakis, R.H. Grubbs, Organometallics 26 (2007) 2469.
[51] K. Vehlow, S. Maechling, S. Blechert, Organometallics 25 (2006) 25.
[52] N. Hadei, E.A.B. Kantchev, C.J. O’Brien, M.G. Organ, J. Org. Chem. 70 (2005)
8503.
[53] A. Furstner, M. Alcarazo, V. Cesar, C.W. Lehmann, Chem. Commun. (2006)
2176.
Acknowledgements
[54] A. Paczal, A.C. Benyei, A. Kotschy, J. Org. Chem. 71 (2006) 5969.
[55] E.M. Leitao, S.R. Dubberley, W.E. Piers, Q. Wu, R. McDonald, Chem. Eur. J. 14
(2008) 11565.
[56] S. Saba, A.M. Brescia, M.K. Kaloustian, Tetrahedron Lett. 32 (1991) 5031.
[57] S. Hermanek, Chem. Rev. 92 (1992) 325.
This work was generously supported by LANXESS GmbH. The
authors would like to thank Erin Leitao for useful discussions.
[58] M. Shimizu, Y. Nakahara, H. Yoshioka, Tetrahedron Lett. 26 (1985) 4207.
[59] P. Svec, P. Novak, M. Nadvornik, Z. Padelkova, I. Cisarova, L. Kolarova,
A. Ruzicka, J. Holecek, J. Fluorine Chem. 128 (2007) 1390.
[60] J.C. Garrison, W.J. Youngs, Chem. Rev. 105 (2005) 3978.
[61] A.J. Arduengo, J.C. Calabrese, F. Davidson, H.V.R. Dias, J.R. Goerlich, R. Krafczyk,
W.J. Marshall, M. Tamm, R. Schmutzler, Helv. Chim. Acta 82 (1999) 2348.
[62] A.J. Arduengo, F. Davidson, R. Krafczyk, W.J. Marshall, R. Schmutzler, Monatsh.
Chem. 131 (2000) 251.
[63] P.L. Arnold, M. Rodden, C. Wilson, Chem. Commun. (2005) 1743.
[64] P.L. Arnold, S.T. Liddle, Organometallics 25 (2006) 1485.
[65] S.P. Downing, A.A. Danopoulos, Organometallics 25 (2006) 1337.
[66] S.P. Downing, S.C. Guadano, D. Pugh, A.A. Danopoulos, R.M. Bellabarba,
M. Hanton, D. Smith, R.P. Tooze, Organometallics 26 (2007) 3762.
[67] K.E. Krahulic, H.M. Tuononen, M. Parvez, R. Roesler, J. Am. Chem. Soc. 131
(2009) 5858.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.12.030.
References
[1] A.J. Arduengo, R.L. Harlow, M. Kline, J. Am. Chem. Soc. 113 (1991) 361.
[2] A.J. Arduengo, Acc. Chem. Res. 32 (1999) 913.
[3] P. de Fremont, N. Marion, S.P. Nolan, Coord. Chem. Rev. 253 (2009) 862.
[4] H. Clavier, S.P. Nolan, Chem. Commun. 46 (2010) 841.
[5] D.G. Gusev, Organometallics 28 (2009) 6458.
[6] R. Tonner, G. Frenking, Organometallics 28 (2009) 3901.
[7] R.A. Kelly, H. Clavier, S. Giudice, N.M. Scott, E.D. Stevens, J. Bordner, I.
Samardjiev, C.D. Hoff, L. Cavallo, S.P. Nolan, Organometallics 27 (2008) 202.
[8] G. Song, Y. Zhang, X. Li, Organometallics 27 (2008) 1936.
[9] S. Leuthausser, D. Schwarz, H. Plenio, Chem. Eur. J. 13 (2007) 7195.
[10] W.A. Herrmann, J. Schutz, G.D. Frey, E. Herdtweck, Organometallics 25 (2006)
2437.
[68] T.K. Wood, W.E. Piers, B.A. Keay, M. Parvez, Angew. Chem., Int. Ed. 48 (2009)
4009.
[69] D. Holschumacher, T. Bannenberg, C.G. Hrib, P.G. Jones, M. Tamm, Angew.
Chem., Int. Ed. 47 (2008) 7428.
[70] A.D. Phillips, P.P. Power, Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 61
(2005) O291.
[71] N. Kuhn, R. Fawzi, H. Kotowski, M. Steimann, Z. Kristallogr. – New Cryst. Struct.
212 (1997) 259.
[72] N. Kuhn, G. Henkel, T. Kratz, J. Kreutzberg, R. Boese, A.H. Maulitz, Chem. Ber.
126 (1993) 2041.
[73] N.Y. Adonin, V.V. Bardin, U. Florke, H.J. Frohn, Z. Anorg. Allg. Chem. 631 (2005)
2638.
[74] E. Bernhardt, M. Berkei, H. Willner, M. Schurmann, Z. Anorg. Allg. Chem. 629
(2003) 677.
[11] R. Dorta, E.D. Stevens, N.M. Scott, C. Costabile, L. Cavallo, C.D. Hoff, S.P. Nolan, J.
Am. Chem. Soc. 127 (2005) 2485.
[12] C. Samojlowicz, M. Bieniek, K. Grela, Chem. Rev. 109 (2009) 3708.
[13] S. Monfette, D.E. Fogg, Chem. Rev. 109 (2009) 3783.
[14] M.S. Sanford, J.A. Love, R.H. Grubbs, Organometallics 20 (2001) 5314.
[15] S.B. Garber, J.S. Kingsbury, B.L. Gray, A.H. Hoveyda, J. Am. Chem. Soc. 122
(2000) 8168.
[16] J.S. Kingsbury, J.P.A. Harrity, P.J. Bonitatebus, A.H. Hoveyda, J. Am. Chem. Soc.
121 (1999) 791.
[75] D. Tapu, D.A. Dixon, C. Roe, Chem. Rev. 109 (2009) 3385.
[76] D.J. Cardin, M.J. Doyle, M.F. Lappert, J. Chem. Soc., Chem. Commun. (1972) 927.
[77] D.J. Cardin, M.J. Doyle, M.F. Lappert, J. Organomet. Chem. 65 (1974) C13.
[78] B. Cetinkaya, P. Dixneuf, M.F. Lappert, J. Chem. Soc., Dalton Trans. (1974) 1827.
[79] M.J. Doyle, M.F. Lappert, J. Chem. Soc., Chem. Commun. (1974) 679.
[80] M.J. Doyle, M.F. Lappert, G.M. McLaughlin, J. McMeeking, J. Chem. Soc., Dalton
Trans. (1974) 1494.
[17] M. Scholl, S. Ding, C.W. Lee, R.H. Grubbs, Org. Lett. 1 (1999) 953.
[18] N. Marion, S.P. Nolan, Acc. Chem. Res. 41 (2008) 1440.
[19] S. Wurtz, F. Glorius, Acc. Chem. Res. 41 (2008) 1523.
[20] E.A.B. Kantchev, C.J. O’Brien, M.G. Organ, Angew. Chem., Int. Ed. 46 (2007)
2768.
[21] S. Diez-Gonzalez, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612.
[81] P.B. Hitchcock, M.F. Lappert, P.L. Pye, J. Chem. Soc., Dalton Trans. (1977) 2160.

A.L. Gott et al. / Inorganica Chimica Acta 369 (2011) 180–189
189
[82] P.B. Hitchcock, M.F. Lappert, P. Terreros, K.P. Wainwright, J. Chem. Soc., Chem.
Commun. (1980) 1180.
[83] A.W. Coleman, P.B. Hitchcock, M.F. Lappert, R.K. Maskell, J.H. Muller,
J. Organomet. Chem. 250 (1983) C9.
[84] M.J. Doyle, M.F. Lappert, P.L. Pye, P. Terreros, J. Chem. Soc., Dalton Trans. (1984)
2355.
[87] S. Wolf, H. Plenio, J. Organomet. Chem. 694 (2009) 1487.
[88] L. Benhamou, V. Cesar, H. Gornitzka, N. Lugan, G. Lavigne, Chem. Commun.
(2009) 4720.
[89] A.P. Blum, T. Ritter, R.H. Grubbs, Organometallics 26 (2007) 2122.
[90] H. Turkmen, O. Sahin, O. Buyukgungor, B. Cetinkaya, Eur. J. Inorg. Chem. (2006)
4915.
[85] J.M. Praetorius, C.M. Crudden, Dalton Trans. (2008) 4079.
[86] S. Fantasia, J.L. Petersen, H. Jacobsen, L. Cavallo, S.P. Nolan, Organometallics 26
(2007) 5880.
[91] M. Appel, W. Beck, J. Organomet. Chem. 319 (1987) C1.
[92] K. Sunkel, G. Urban, W. Beck, J. Organomet. Chem. 252 (1983) 187.
[93] V.V. Grushin, W.J. Marshall, J. Am. Chem. Soc. 126 (2004) 3068.