Ferrocene-based Lewis acids and Lewis pairs: Synthesis and structural characterization

Article abstract of DOI:10.1007/s12039-012-0356-8

Optically active Lewis acids and Lewis pairs were synthesized and characterized by multinuclear NMR, UV/Vis spectroscopy and elemental analysis. Optical rotation measurements were carried out and the absolute configuration of the new chiral molecules conf

Full text of DOI:10.1007/s12039-012-0356-8

c
J. Chem. Sci. Vol. 125, No. 1, January 2013, pp. 41–49. ꢀ Indian Academy of Sciences.
Ferrocene-based Lewis acids and Lewis pairs: Synthesis and structural
characterization
PAGIDI SUDHAKAR and PAKKIRISAMY THILAGAR∗
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
e-mail: thilagar@ipc.iisc.ernet.in
MS received 7 September 2012; revised 25 October 2012; accepted 2 November 2012
Abstract. Optically active Lewis acids and Lewis pairs were synthesized and characterized by multinuclear
NMR, UV/Vis spectroscopy and elemental analysis. Optical rotation measurements were carried out and the
absolute configuration of the new chiral molecules confirmed by single crystal X-ray diffraction.
Keywords. Ferrocene/Lewis acid; optical rotation; frustrated Lewis pairs; organoborane.
1. Introduction
in chiral synthesis. It is noteworthy to mention that
Aldridge and co-workers recently have devised a sim-
ple route for the synthesis of planar chiral frustrated
Lewis pairs (PCFLP’s) from 1,1ꢁ-dibromoferrocene, but
the final products were in racemic form.14a Instead,
use of chiral ferrocenyl sulphoxide can be visua-
lized as a precursor for optically pure isomers both
1-phosphino-2-borylferrocenes (SP) and 2-phosphino-
1-borylferrocenes (RP). We anticipate that the prepa-
ration of PCFLP’s could open up a new entry into
enantioselective catalysts and also that the reversible
redox chemistry at the metal centre can be used to fine
tune the activity of the PCFLP’s. While the work was
under progress in our lab14b–d Siewert and coworkers14e
have independently reported the syntheses of homochi-
ral Sp-1,2-fc(PPh2)(BMes2) using ferrocene sulphox-
ide as a precursor, but they have not explored the
possibility of synthesis of Rp-1,2-fc)(BMes2)(PPh2)
from the same precursor. Prior to the report of Siewert
et al., the preliminary accounts of our work reported
in this manuscript have been presented in one interna-
tional and two national conferences.14b–d In this com-
munication, we report our independent results on the
synthesis and characterization of both 1-phosphino-2-
borylferrocene (SP) and 2-phosphino-1-borylferrocene
(RP) from single precursor chiral ferrocenylsulphoxide.
The design and synthesis of molecules containing
non-interacting Lewis base and Lewis acid groups
[Frustrated Lewis pairs (FLP’s)] have received intense
attention due to their potential applications in
the area of molecular catalysis.1–3 For example,
Stephen’s and co-workers have demonstrated that
the unquenched Lewis acidity and Lewis basicity of
(C6H2Me)2PH(C6F4)BF(C6F5)2 reversibly activate
molecular hydrogen, in the absence of transition me-
tals.2 FLP’s can also be used to activate C-H, B-H and
N-H bonds. Erker and co-workers have demonstrated
the reversible activation of H2 by metallocene based
FLPs.3 Surprisingly, there is no report on the synthesis
of enantiomerically pure FLP’s, which would be very
important for chiral organic transformations.1–3
Lewis acidic organoboranes play key role both as
reagents and catalysts in asymmetric organic syn-
thesis.4–9 The rigid three-dimensional structure and
inherent planar chirality of 1,2-disubstituted ferrocenes
bearing non-identical atoms provide excellent chiral
environment for enantioselective synthesis.9,10 Several
planar chiral ferrocene based phosphines and amines
are known and their catalytic activity in the pre-
sence of transition metals is well documented.10 Piers,8
Wagner11 and Aldridge12 have independently prepared
various ferrocenylboranes and studied their applica-
tions in catalysis and as anion sensors. Jakle et al.9,13
have prepared verious ferrocenylboranes and studi-
ed their applications in anion binding as well as
2. Experimental
2.1 General procedure
n-Butyl lithium, t-butyl lithium (1.7M in hexanes),
bis-mesitylfluroborane and PPh2Cl & were purchased
∗For correspondence
41

42
Pagidi Sudhakar and Pakkirisamy Thilagar
from Aldrich. Caution! Lithium reagents and Ph2PCl 2.24 (s, 6H), 2.32 (s, 12H), 2.419 (s, 3H), 4.12 (s, 5H),
are toxic and highly corrosive and should be handled 4.43 (m, 1H), 4.62 (m, 2H), 6.74 (s, 4H), 7.24 (s, 2H),
appropriately with great care. All reactions and manipu- 7.56–7.54 (d, 2H). 13C NMR (100 MHz, CDCl3, 25◦C, δ
lations were carried out under an atmosphere of pre- (ppm)), 21.54, 21.95, 25.25, 71.59, 72.33, 73.34, 81.01,
purified nitrogen using Schlenk techniques. Due to 101.10, 125.65, 129.4, 129.6, 137.9, 140.3, 140.6,
the unpleasant odour of Ph2PCl, most of the manipu- 141.4, 143.2: ESI Mass Spectrometry: Mcalc = 572.2 :
lations were carried out in a well-ventilated fume found: 595.0 [M + Na]+ ; 611.0 [M + K]+, (UV-Vis)
hood. Thin-layer chromatography (TLC) analyses were (CH2Cl2, 1.001 × 10–5 M): λ max = 496 nm (ε =
carried out on pre-coated silica gel plates (Merck), 2.6 × 103). Elemental analysis for C37H52BFeOS: C,
and spots were visualized by UV irradiation. Col- 72.67; H, 8.57, found C, 72.12; H, 8.25
umn chromatography was performed on glass columns
loaded with silica gel. THF and hexane were distilled
from sodium/benzophenone. Chlorinated solvents were
2.3 Preparation of (3(SP, SS))
stirred for 24 h over anhydrous CaH2, then degassed
To a solution of 2(SP, SS) in THF was added distilled
via several freeze pump thaw cycles and stored over
water (0.1 mL). The reaction mixture was stirred for
3 Å molecular sieves. 400 MHz 1H NMR, 100.613 MHz
6 h at room temperature. The product was extracted
13C NMR, 128.378 MHz, 11B NMR and 161.976 MHz
with diethyl ether and volatiles were removed in vacuo
31P NMR spectra were recorded on a Bruker 400 MHz
to obtain desired product. Yield: 60%. [α]2D4 = 463,
1
NMR spectrometer. Solution H and 13C NMR spec-
1H NMR (400 MHz, CDCl3, 25◦C, δppm): 2.01 (s,
1H), 2.20 (s, 3H), 2.26 (s, 8H), 4.09 (m, 1H), 4.45
tra were referenced internally to the solvent signals.
11B NMR spectra to BF3.OEt2 (δ = 0) in C6D6.
(s, 5H), 4.52 (d, 1H), 4.99 (m, 1H), 6.77 (d, 2H),
Mass spectral studies were carried out using a Q-
7.17 (m, 2H), 7.41 (d, 2H). 10.74 (s, 1H) 13C NMR
TOF micro mass spectrometer or Bruker Daltonics
Esquire 6000 plus mass spectrometer with ESI-MS
mode analysis. The melting point was determined in
open capillary using an ANALAB melting-point appa-
ratus. UV-visible absorption data were acquired on
a UV-vis/NIR perkin Elmer Lambda 750 spectropho-
(100 MHz, CDCl3, 25◦C) δ 21.67, 21.76, 71.27, 73.29,
74.76, 79.59, 100.25, 124.64, 127.60, 130.13, 137.76,
141.34, 142.17. 11B NMR (160 MHz, CDCl3, 25◦C)
δ : 46.21, ESI Mass Spectrometry: Mcalc: 470.1;
found: 507.4 [M+ OMe+H];. Elemental analysis for
C26H27BFeO2S; C, 66.41; H, 5.79; found C, 66.10;
tometer. Solutions were prepared using a microbalance
H, 5.35. UV-Vis (CH2Cl2, 1.001 × 10–5 M): λmax =
( 0.1 mg) and volumetric glassware and then charged
into quartz cuvettes with sealing screw caps. Opti-
cal rotation analysis was performed on JASCO p-1020
438 nm (ε = 1.0 × 102).
III polarimeter, using a tungsten-halogen light source
2.4 Preparation of (4(SP))
operating at λ = 589 nm. CCDC 823721–823724 con-
tain the supplementary crystallographic data for this
paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
To a solution of 2(SP, SS) (100 μg, 0.17 mmol) in
THF (10 mL) at −78◦C was added t-BuLi (77 μL,
0.18 mmol) and the reaction mixture stirred for
1 h. Chlorodiphenylphosphine (33 μL, 0.64 mmol) was
added and the reaction mixture was allowed to warm
up to room temperature. The reaction mixture was
kept stirring overnight. Volatiles were removed in
vacuo to yield crude product, which was purified
2.2 Preparation of (2(SP, SS))
To a solution of Ferrocenyl-p-tolylsulphoxide (1.00 g, by column chromatography (EtOAc-hexane) to give
3.05 mmol) in THF (30 mL) at −78◦C was added red solid. Yield: 10 mg, 10%. [α]2D1 = −694.92.
drop-wise LDA (2 mL, 0.34 mmol) and the reaction 1H NMR (400 MHz, CDCl3, 25◦C, δppm) 2.21 (s,
mixture was stirred for 1 h. A solution of dimesityl fluo- 12H), 2.27 (s, 3H), 2.38 (s, 3H), 4.07 (s, 1H),
roborane (1 g, 4.06 mmol) in THF (4 mL) was added. 4.18 (s, 5H), 4.26 (s, 1H), 4.66 (m, 1H), 6.59 (s,
The mixture was allowed to warm up to room tempe- 3H), 6.64 (d, 1H), 6.81 (m, 3H ), 7.09 (t, 2H),
rature and kept stirring for an additional 6 h. After stan- 7.19 (m, 1H), 7.32 (m, 4H), 13C NMR(100 MHz,
dard aqueous work-up, the crude product was purified CDCl3, 25◦C: 21.36, 25.15, 30.42, 70.16, 73.45, 79.13,
by column chromatography (EtOAc-hexane 1:3 ratio) 83.79, 88.86, 89.04, 133.67, 133.89, 134.72, 134.93,
to obtain orange crystal. Yield: 0.6 g, 80% [α]D24
=
137.47, 139.86, 140.08, 143.81, 31P NMR (160 MHz,
−676. 1H NMR (400 MHz, CDCl3, 25◦C, δ (ppm)): CDCl3, 25◦C δ ppm): −21.5, ESI Mass Spectrometry:

Optically active chiral Lewis pairs
43
Mcalc = 618.3, found: 619 [M + H]+ ; 641[M + Na]+ ; (Diphenylphosphino)-1-(p-tolylsulfinyl)ferrocene as a
656.9 [M + K]+, UV-Vis (CH2Cl2, 1.001 × 10−5 M): yellow solid in 27% yield. 1H-NMR (400 MHz, CDCl3,
λmax = 506 nm (ε = 1.2 × 103). Elemental analysis for 25◦C) δ 2.35 (s, 3H), 4.05 (m, 5H), 4.30 (s, 1H), 4.50
C41H52BFeP calcd C, 76.65; H, 8.16; found C, 76.22; (d, 1H), 4.51 (d, 1H), 7.17 (s, 2H), 7.37 (m, 6H),
H, 7.98.
7.74–7.57 (m, 6H). 31P NMR (160 MHz, CDCl3, 25◦C):
δ −24.
2.5 Preparation of (5(SP, SS))
2.6 Preparation of (4(RP))
To a solution of Ferrocenyl-p-tolylsulphoxide (1.0 g,
3.05 mmol) in freshly distilled THF (30 ml), was added
LDA (1.7 ml, 3.36 mmol) at −78◦C. The reaction
mixture was stirred at −78◦C for 1 h and PPh2Cl
(600 μl, 3.1 mmol) was added. The resulting solu-
tion was warmed to room temperature, stirred for 6 h
and quenched with water (5 ml). The organic layer
was separated and the aqueous layer was extracted
with diethyl ether (2 × 20 ml). The combined organic
extracts were washed with brine (10 ml) and dried over
anhydrous Na2SO4. The solvent was evaporated under
reduced pressure and the residue was purified by col-
umn chromatography on silica gel (60–120 mesh) using
petether/diethyl ether (1:1) as eluent to obtain (S,S)-2-
To a solution of 5(SP, SS) (0.09 g, 0.17 mmol) in THF
(10 mL) at −78◦C was added t-BuLi (77, 0.18 mmol)
and the reaction mixture was stirred for 2 h. Dime-
sityl fluoroborane (33 μL, 0.64 mmol) was then added
and the reaction mixture was warmed to room temper-
ature and stirred for overnight. Volatiles were removed
in vacuo to yield crude product which was purified by
column chromatography (EtOAc-hexane) to give red
solid. Yield: 20%. [α]2D1 = 674.9. H NMR (400 MHz,
1
CDCl3, 25◦C, δppm) 2.22 (s, 12H), 2.26 (s, 3H), 2.39
(s, 3H), 4.10 (s, 1H), 4.20 (s, 5H), 4.25 (s, 1H), 4.67
(m, 1H), 6.59 (s, 3H), 6.65 (d, 1H), 6.80 (m, 3H),
7.10 (t, 2H), 7.20 (m, 1H), 7.31 (m, 4H). 13C NMR
Table 1. Details of X-ray crystal structure analyses of complexes 2(SP, SS), 3(SP, SS), 4(SP) and 5(SP, SS).
Compound
2(SP, SS)
3(SP, SS)
4(SP)
5(SP, SS)
Empirical formula
C35H37BFeOS
572.39
C26H27BFeO2S
470.21
C40H40BFeP
618.37
C29H25FeOPS
508.39
MW
T , K
273(2)
273(2)
363(2)
293(2)
Wavelength, Å
Crystal system
Space group
0.71073
Orthorhombic
P212121
0.71073
Monoclinic
P21
0.71073
Orthorhombic
P212121
0.71073
Monoclinic
P21
a, Å
b, Å
c, Å
9.360(2)
11.884(3)
25.331(6)
2817.8(11)
4
1.349
0.637
7.433(3)
9.196(3)
17.092
1166.8(7)
2
1.338
0.756
13.8055(17)
14.9786(18)
15.4878(19)
3202.7(7)
3
7.727(5)
14.233(8)
10.969(7)
1196.5(12)
2
2.386
0.861
V, Å3
Z
ρcalc, g cm−3
1.282
0.548
0.20 × 0.20 × 0.20
μ (Mo/Cu Kα), mm−1
Crystal size, mm
0.30 × 0.20 × 0.20
0.25 × 0.22 × 0.20
0.15 × 0.15 × 0.15
θ range, deg
Limiting indices
1.89 to 28.11
−12<=h<=12
−15<=k<=15
−33<=l<=33
2.39 to 28.07
−9<=h<=9
−12<=k<=11
−22<=l<=22
1.89 to 28.01
−18<=h<=18
−19<=k<=19
−20<=l<=20
1.87 to 26.37 deg
−9<=h<=9,
−17<=k<=17,
−13<=l<=13
Reflns collected
Independent reflns
32711
6740 [R(int) = 0.0579]
SADABS
13432
5416 [R(int) = 0.0237]
SADABS
37168
7646 [R(int) = 0.0566]
SADABS
12595
4879 [R(int) = 0.0853]
SADABS
Absorption correction
data/restraints/parameters
6740/0/359
5416/1/285
7646/0/394
4879/1/300
Goodness-of-fit on F2
0.981
1.059
1.016
0.959
Final R indices [I >2 σ(I)][a]
R1 = 0.0339
R1 = 0.0322,
R1 = 0.0379
R1 = 0.0696,
wR2 = 0.0700
wR2 = 0.0832
wR2 = 0.0739
wR2 = 0.1372
R indices (all data)[a]
R1 = 0.0453
R1 = 0.0338,
wR2 = 0.0844
R1 = 0.0536
R1 = 0.1409,
wR2 = 0.1625
wR2 = 0.0719
wR2 = 0.0778
Peakmax/holemin(e Å−3
Absolute structure parameter
)
0.497 and −0.250
0.232 and −0.325
0.469 and −0.251
0.667 and −0.315
0.012(11)
0.075(11)
0.038(11)
0.07(3)
ꢀ
ꢁ
ꢄ
ꢁ
ꢄꢅ
1/2
ꢂ
ꢃ
ꢂ
ꢃ
2
2
[a] R1 = ||Fo| − |Fc|| / |Fo| ; wR2 =
w F2o − F2c
/
w F2o

44
Pagidi Sudhakar and Pakkirisamy Thilagar
(100 MHz, CDCl3, 25◦C) δ 21.35, 25.16, 30.41, 70.14, 3. Results and discussion
73.42, 79.11, 83.77, 88.87, 89.06, 133.66, 133.90,
134.71, 134.92, 137.41, 139.87, 140.10, 143.80. 31P
NMR (160 MHz, CDCl3, 25◦C δppm) −20.5. ESI Mass
Spectrometry: Mcalc = 618.37, found: 619 [M + H]+,
656.9 [M + K]+, UV-Vis, (CH2Cl2, 1.001 × 10−5 M):
λ max = 506 nm (ε = 1.2 × 103). Elemental analy-
sis for C41H52BFeP calcd C, 76.65; H, 8.16, found C,
76.20; H, 8.0.
The synthetic strategy followed in the syntheses of
compounds 2(SP, SS)-5 is described in scheme 1.
The synthetic access to the chiral ferrocenylsulphox-
ide (1) was made possible by the principal studi-
es of Kagan who converted stanylferrocene to 1 by
the action of n-BuLi followed by (S,S)-menthyl-p-
tolylsulphinate.10e,10d,15 The second step of the process
is the diastereo-selective ortholithiation of 1 by LDA at
−78◦C in THF and followed by quenching with Mes2BF
gave 2(SP, SS) in 80% yield after silica gel column
1
purification (scheme 1). The H and 13C NMR spec-
2.7 Structure determination of compounds 2(SP, SS),
3(SP, SS), 4(SP) and 5(SP, SS)
tra of 2(SP, SS) are consistent with a 1,2-disubstituted
ferrocene derivatives, and a resonance at δ = 49 ppm
in the 11B NMR spectrum confirms the attachment of
the BMes2 group. The 11B NMR signal is considerably
upfield shifted compared to other triorganyl boranes (in
general they resonate at 60–70 ppm).8–13 This may be due
to the interaction between the boron in -BMes2 unit and
the oxygen of tolylsulphinate moiety. The absolute con-
figuration of 2(SP, SS) was assigned from the single-
crystal X-ray structure, which confirms diastereoselec-
tive ortho lithiation of 1 (figure 1a).
The molecular structure of 2(SP, SS) also gives evi-
dence for B—O (3.293 Å) interaction (figure 1a). Such
kind of interaction was first noted by Aldridge and
co-workers (B—O, 3.304 Å).14 In contrast to the obser-
vation noted by Siewert et al., compound 2(SP, SS)
is not stable at atmospheric conditions and prone to
Single-crystal X-ray diffraction studies were carried out
with a Bruker SMART APEX diffractometer equipped
with 3-axis goniometer. The crystals were kept under
a steady flow of cold dinitrogen during the data col-
lection. The details regarding the data collection and
refinement for compounds 2(SP, SS), 3(SP, SS), 4(SP)
and 5(SP, SS) are given in table 1. The data were inte-
grated using SAINT, and an empirical absorption cor-
rection was applied with SADABS. The structures were
solved by direct methods and refined by full matrix
least-squares on F2 using SHELXTL software. All the
non-hydrogen atoms were refined with anisotropic dis-
placement parameters, while the hydrogen atoms were
refined isotropically on the positions calculated using a
riding model.
B(OH)Mes
O
S
Fe
BMes2
Atmospheric
Moisture
O
O
a
S
3(Sp, SS)
S
Fe
b
Fe
BMes2
PPh2
c
2(Sp, SS)
1
d
Fe
a
d
PPh2
4(SP)
PPh2
O
BMes2
c
S
Fe
b
Fe
4(RP)
5(Sp, SS)
Scheme 1. Synthesis of compounds 2(SP, SS), 3(SP, SS), 4(SP), 4(RP) and 5(SP, SS)
(a) LDA, (b) FBMes2, (c) t-BuLi and (d) ClPPh2.

Optically active chiral Lewis pairs
45
(a)
(b)
(c)
(d)
Figure 1. Molecular structures of (a) 2(SP, SS), (b) 3(SP, SS), (c) 4(SP) and (d)
5(SP, SS). All the hydrogen atoms are omitted for clarity. (a) Molecular structure
of 2(SP, SS). Selected interatomic distances [Å] and angles [◦]: C(1)-B(1) 1.573(3),
C(20)-B(1) 1.584(3), C(11)-B(1) 1.589(3), S(1)-O(1) 1.5034(16), S(1)-C(2) 1.776(2),
S(1)-C(29)1.804(2), O(1)-S(1)-C(2)107.67(9), O(1)-S(1)-C(29) 105.45(1), C(2)-S(1)-
C(29) 99.16(9), C(1)-B(1)-C(20)119.60(2), C(1)-B(1)-C(11)116.83(2), C(20)-B(1)-
C(11)122.01(2). (b) Molecular structure of 3(SP, SS). Selected interatomic distances
[Å] and angles [◦]: S(1)-O(2) 1.5058(17), S(1)-C(1) 1.770(2), S(1)-C(11) 1.799(2),
O(1)-B(1) 1.351(3), B(1)-C(18) 1.575(3), B(1)-C(2) 1.577(3), O(2)-S(1)-C(1)
109.39(10), O(2)-S(1)-C(11) 106.10(1), C(1)-S(1)-C(11) 96.93(9), O(1)-B(1)-C(2)
118.68(2), O(1)-B(1)-C(2) 121.04(2), C(18)-B(1)-C(2) 120.28(2). (c) Molecular struc-
ture of 4(SP), selected interatomic distances [Å] and angles [◦]. P(1)-C(1)1.824(2),
P(1)-C(11) 1.836(2), P(1)-C(17) 1.842(2), B(1)-C(2)1.557(3), B(1)-C(29)1.599(3),
B(1)-C(23)1.599(3), C(1)-P(1)-C(11) 99.24(9), C(1)-P(1)-C(17) 99.26(1), C(11)-
P(1)-C(17)100.48(1), C(2)-B(1)-(29)114.14(2), C(2)-B(1)-C(23)126.31(2), C(29)-
B(1)-C(23)119.43(2). (d) Molecular structure of 5. Selected interatomic distances [Å]
and angles [◦]: S(1)-O(1) 1.470(5), S(1)-C(2) 1.780(8), S(1)-C(23) 1.781(6), P(1)-C(1)
1.784(6), P(1)-C(11) 1.819(7), P(1)-C(18) 1.836(6), O(1)-S(1)-C(2) 109.6(3), O(1)-
S(1)-C(23) 106.8(3), C(2)-S(1)-C(23) 98.0(3), C(1)-P(1)-C(11) 100.4(3), C(1)-P(1)-
C(18) 101.9(3), C(11)-P(1)-C(18) 100.9(3).
hydrolysis. Over a period of a week it slowly underwent 3(SP, SS). Later, compound 3(SP, SS) was prepared by a
selective hydrolysis of one of the two B-Mes bonds different route (scheme 2).
by reacting with atmospheric moisture. The hydroly-
When compound 2(SP, SS) was allowed to react with
sed product was separated by using silica gel column one equivalent of water in THF 3(SP, SS), the quan-
1
chromatography technique and found to be compound titative yield was obtained. The H NMR spectrum

46
Pagidi Sudhakar and Pakkirisamy Thilagar
Mes
1,2-disubstituted Cp ring, and a singlet at δ = 4.18 ppm
OH
O
BMes2
for the free Cp ring. A signal at 77.6 ppm in the 11B
NMR spectrum confirms that BMes2 is intact and a
resonance at δ = −20.5 ppm in the 31P NMR spec-
trum confirms the attachment of PPh2. The appearance
of protonated molecular ion [M + H]+ peak at 619
in the ESI mass spectrum confirms the formation of
4(SP). The 11B and 31P resonances clearly indicate the
presence of unquenched tricoordinated phosphine and
borane centres in 4(SP) in solution.1–3,14 The optical
purity of 4(SP) was confirmed by single crystal X-ray
analysis and optical rotation studies. Compound 5(SP,
SS) was prepared by adopting known literature15d proce-
dure (scheme 1). Compound 4(RP) was prepared from
B
O
S
H2O/THF
S
Fe
Fe
3(SP, SS)
2(SP, SS)
Mes
H2O/THF
BMes2
Fe
B
OH
X
Fe
Scheme 2. Synthesis of 3(SP, SS) from 2(SP, SS).
of 3(SP, SS) shows three different resonances at 4.09, 5(SP, SS) following a procedure similar to that used for
4.52 and 4.99 ppm for substituted Cp and a single res- 4(SP). Compound 4(RP) was characterized by multin-
onance for free Cp at 4.45 ppm. The hydrolysis might uclear NMR (1H, 13C, 11B and 31P), ESI mass, opti-
have occurred because of the intramolecular Tolyl-S- cal rotation, and elemental analysis and UV-Vis spec-
1
O—B interaction. The upfield shifted 11B resonance troscopy. H NMR integration and molecular ion peak
of 3(SP, SS) (46.2 ppm) (see Supporting Information in ESI Mass spectrum confirms the formation of 4(RP)
Figure S9) relative to the parent compound 2(SP, SS) and are consistent with 4(SP). The 11B (77.2 ppm) and
(49 ppm) support the intramolecular interaction dis- 31P (−20.5 ppm) resonances are also in the range of free
cussed vide-supra. In order to demonstrate the role of tricoordinated phosphine and borane, respectively.1–3
the B—O interaction in the hydrolysis reaction, con-
Molecular structure of compounds 2(SP, SS), 3(SP,
trol experiment was designed in which FcBMes2, which SS), 4(SP) and 5(SP, SS) are confirmed by single crys-
lacks the Tolyl-S-O functionality, was tested for hydrol- tal X-ray diffraction studies. The molecular structures
ysis. FcBMes2 was treated with 10 equiv of water for are shown in figure 1 with important geometric para-
two days using THF as solvent (scheme 2). No B–C meters. Recently, Aldrige and co-workers14 reported
bond cleavage was observed and FcBMes2 was com- the crystal structure of 2(SP, SS) and 3(SP, SS), but the
pletely recovered. The molecular structure of 3(SP, inter and intramolecular bonding parameters vary con-
SS) is shown in figure 1. Although free rotation is siderably in the present report. In addition, the synthetic
possible at the boron centre (due to the absence of procedure for these compounds reported in the present
one bulky mesityl group), we observed only one iso- study is different from the literature. The dihedral angle
mer. This might be due to the strong intramolecular between BC2/BCO plane and plane of substituted Cp
OH—O (1.956 Å) interaction between sulphinate oxy- ring is considerably smaller for 4(SP) with 11.7◦ in
gen and B–OH moiety. The more downfield shifted res- comparison to the highly tilted 2(SP, SS), (59.2◦), while
onance of B–OH (10.74 ppm) in solution state 1H NMR the angle found for 3(SP, SS) lies in between at 27.7◦
clearly indicates that the intramolecular interaction also (table 2). This might be due to the steric bulk of mesityl
persists in solution state (see Supporting Information substituents in 2(SP, SS), and 4(SP). Steric effects are
Figure S2).
also evident from a comparison of the Cp//Cp tiltangles
Reaction of 2(SP, SS) with t-BuLi in THF at −78◦C of 2(SP, SS)-5(SP, SS), which for 4(SP) is 8.6◦, whereas
generates chiral lithioferrocene, which was trapped for 2(SP, SS), 3(SP, SS) and 5(SP, SS) they are 4.32,
with PPh2Cl to give compound 4(SP). The H NMR 1.18 and 3.19◦, respectively. 3(SP, SS) shows more pro-
1
spectrum shows three signals at δ = 4.66 (dd), 4.26 nounced Fe—B interaction (3.214 Å) compared to 2(SP,
(pseudo triplet), and 4.07 ppm (dd), as expected for a SS) (3.44 Å) and 4(SP) (3.36 Å). This can be rationa-
Table 2. Selected intramolecular interactions (distance (Å) and angles (◦)) involved in 2(SP, SS), 3(SP, SS), 4(SP), and 5(SP,
SS).
Compound
2(SP, SS)
3(SP, SS)
4(SP)
5(SP, SS)
Cp//Cp
BC2/BCO//Cp
Fe—B
4.32
59.23
3.442
1.18
27.74
3.214
8.61
11.73
3.369
3.19
—–
—–

Optically active chiral Lewis pairs
47
other electron deficient groups like pentafluorophenyl
and 1,3,5-trifluoromethylphenyl to fine tune the Lewis
acidity of boron centre and to set-up a general route
to enantiomerically pure Planar Chiral Frustrated Lewis
Pairs (PCFLP’s).
Supporting information
1H NMR and 13C NMR, 11B and 31P spectra and
HRMS of compounds 2(SP, SS), 3(SP, SS), 4(SP),
4(RP) and 5(SP, SS). CCDC 823721 - 823724 con-
tain the supplementary crystallographic data for this
paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Supplementary figures S1–S13 are given as supple-
mentary material (see www.ias.ac.in/chemsci).
Figure 2. Uv-Vis spectra of 2(SP, SS), 3(SP, SS), 4(SP),
4(RP) and 5(SP, SS).
Acknowledgements
This work was supported by the Department of Science
and Technology (DST), New Delhi, India and Indian
Institute of Science (IISc), Bangalore. PS thanks IISc,
Bangalore for research fellowship.
lized by the electronic factor. The boron centre in 3(SP,
SS), (connected with OH and one mesityl group, respec-
tively) is more electron deficient than in 2(SP, SS) and
4(SP) (connected with two mesityl groups).
The boron centre in 3(SP, SS), and 4(SP) is planar
with the sum of angle around boron is 360◦, in the case
of 2(SP, SS) little pyramidalization occurred.14 The P-
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