Synthetic, redox and coordination chemistry of bis(pentachlorophenyl)boryl ferrocene, FcB(C6Cl5)2

Article abstract of DOI:10.1016/j.jorganchem.2014.07.003

Two synthetic approaches to bis(pentachlorophenyl)boryl ferrocene have been explored. One mirrors that used in a novel approach to FcB(C₆F ₅)₂ from FcBBr₂, but is less selective than its perfluorinated counterpart on account of the greater steric bulk of LiC ₆Cl₅ over LiC₆F₅. This approach does, however, provide a viable route to unsymmetrical mono(pentachlorophenyl) derivatives of the type FcB(C₆Cl₅)Ar through the intermediacy of the mono-substituted species FcB(C₆Cl₅)Br. FcB(C₆Cl₅)₂ itself is best synthesized from ferrocenyllithum and ClB(C₆Cl₅)₂ and is a violet-blue species featuring an extremely electron deficient Fe(II) centre (E_1/2 = +550 mV with respect to ferrocene/ferrocenium). A combination of structural, spectroscopic and reactivity studies of these and related ferrocenylboranes allow some general comments to be made concerning the relative steric and electronic properties of the C₆Cl₅ group. Thus, in terms of their relative capabilities as electron-withdrawing groups the substituents examined can be ranked C₆Cl₅ > C ₆F₅ > Mes, while steric properties are ordered Mes > C₆Cl₅ > C₆F₅.

Full text of DOI:10.1016/j.jorganchem.2014.07.003

Journal of Organometallic Chemistry 769 (2014) 11e16
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthetic, redox and coordination chemistry of
bis(pentachlorophenyl)boryl ferrocene, FcB(C6Cl5)2
Michael J. Kelly, Remi Tirfoin, Jessica Gilbert, Simon Aldridge*
Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Two synthetic approaches to bis(pentachlorophenyl)boryl ferrocene have been explored. One mirrors
that used in a novel approach to FcB(C6F5)2 from FcBBr2, but is less selective than its perfluorinated
counterpart on account of the greater steric bulk of LiC6Cl5 over LiC6F5. This approach does, however,
provide a viable route to unsymmetrical mono(pentachlorophenyl) derivatives of the type FcB(C6Cl5)Ar
through the intermediacy of the mono-substituted species FcB(C6Cl5)Br. FcB(C6Cl5)2 itself is best syn-
thesized from ferrocenyllithum and ClB(C6Cl5)2 and is a violeteblue species featuring an extremely
electron deficient Fe(II) centre (E1/2 ¼ þ550 mV with respect to ferrocene/ferrocenium). A combination of
structural, spectroscopic and reactivity studies of these and related ferrocenylboranes allow some gen-
eral comments to be made concerning the relative steric and electronic properties of the C6Cl5 group.
Thus, in terms of their relative capabilities as electron-withdrawing groups the substituents examined
can be ranked C6Cl5 > C6F5 > Mes, while steric properties are ordered Mes > C6Cl5 > C6F5.
© 2014 Elsevier B.V. All rights reserved.
Received 28 May 2014
Received in revised form
7 July 2014
Accepted 8 July 2014
Available online 17 July 2014
Keywords:
Borane
Ferrocene
Lewis acid
Introduction
base strength of the two components employed [8]. With this in
mind, and given the recent syntheses of systems of the type
Functionalized ferrocenes play diverse roles in modern chem-
istry, with systems featuring pendant Lewis basic groups acting as
donor ligands in various complexes exploited in homogenous
catalysis, and those bearing Lewis acidic groups finding applica-
tions, for example, in polymer synthesis and in detection/sensing
[1e4]. The latter field exploits cathodic shifts in the redox potential
of the Fe(II) centre which can be induced, for example, on coordi-
nation of a Lewis base to a pendant boryl (eBX2) unit [5,6]. The
conversion of a mesomerically electron-withdrawing three-coor-
dinate boryl function to an inductively donating four coordinate
borate has been exploited in the design of systems used in the
detection of anions such as fluoride and cyanide. More recently,
bis(pentafluorophenyl)boryl ferrocene, FcB(C6F5)2, originally syn-
thesized by Piers and co-workers [7], has been exploited as one
component of a ‘frustrated Lewis pair’ (FLP) capable of detecting
nitrous oxide [8,9]. The formation of the ambiphilic N2O adduct
B(C6F5)n(C6Cl5)3ꢀn [10], we sought to expand the current (very
limited) pool of ferrocenylboranes possessing potent enough Lewis
acidity for FLP chemistry. Thus in the current study we targeted
bis(pentachlorophenyl)boryl ferrocene, FcB(C6Cl5)2, and related
systems [11].
Experimental
General procedures
All manipulations were carried out using standard Schlenk line
or dry-box techniques under an atmosphere of argon or dinitrogen,
respectively. Solvents were degassed by sparging with argon and
dried by passing through a column of the appropriate drying agent
using a commercially available Braun SPS. Fluorobenzene was dried
by refluxing over calcium hydride, distilled, sparged and stored
over activated molecular sieves. NMR spectra were recorded in
chloroform-d, benzene-d6 or THF-d8, which were dried over mo-
lecular sieves, potassium or calcium hydride respectively, and
stored under argon in Teflon valve ampoules. NMR samples were
prepared under argon in 5 mm Wilmad 507-PP tubes fitted with J.
Young Teflon valves. 1H and 13C NMR spectra were recorded on
Varian Mercury-VX-300 or Bruker AVII-500 spectrometers and
referenced internally to residual protio-solvent (1H) or solvent (13C)
t
tBu3P$NNO$BFc(C6F5)2 in the presence of Bu3P/FcB(C6F5)2 brings
about a selective colorimetric and electrochemical response [9].
The activation/trapping of small molecules using an FLP
approach is known to be highly dependent on the cumulative acid/
* Corresponding author. Tel.: þ44 (0)1865 285201; fax: þ44 (0)1865 272690.
E-mail address: Simon.Aldridge@chem.ox.ac.uk (S. Aldridge).
http://dx.doi.org/10.1016/j.jorganchem.2014.07.003
0022-328X/© 2014 Elsevier B.V. All rights reserved.

12
M.J. Kelly et al. / Journal of Organometallic Chemistry 769 (2014) 11e16
resonances and are reported relative to tetramethylsilane
isotopomer) 688.7081; meas. 688.7066. Elemental microanalysis:
calc. for C22H9BCl10Fe, C 38.05%, H 1.31%; meas. C 37.85, 37.92%, H
(d
¼ 0 ppm). 11B, 19F and 31P NMR spectra were referenced with
respect to Et2O$BF3, CFCl3 and 85% aqueous H3PO4, respectively.
1.20, 1.21%. UVevis (fluorobenzene) lmax
,
nm (ε): 546
Chemical shifts are quoted in
d
(ppm) and coupling constants in Hz.
(358 L molꢀ1 cmꢀ1). E1/2 (0.05 M [nBu4N][B(C6F5)4] electrolyte in
UVevis spectra were collected on a Scintio UV S-2100 UV/Vis
spectrometer. Elemental analyses were carried out at London
Metropolitan University. 1-bromo-2,3,4,5,6-pentafluorobenzene
and ferrocene were sourced commercially and used without
further purification. FcBBr2 [12], ClB(C6Cl5)2 [10], FcBMes2[6d] and
a
,
a
,
a
-trifluorotoluene) ¼ þ550 mV, relative to FcH/FcHþ. Crystal-
lographic data for 2$C6D6: C28H9BCl10D6Fe, Mr ¼ 778.65, mono-
clinic, P 21/c, a ¼ 9.2004(1), b ¼ 22.2839(2), c ¼ 14.7467(1) Å,
b
¼ 101.7501(4)ꢁ, V ¼ 2960.03(5) Å3, Z ¼ 4, rc ¼ 1.747 Mg mꢀ3
,
T ¼ 150 K,
l
¼ 0.71073 Å. 12,403 reflections collected, 6721 inde-
MesLi
[13]
were
h
prepared
by
literature
routes
pendent [R(int) ¼ 0.000] which were used in all calculations.
[Fc ¼ ferrocenyl ¼ (
5-C5H5)Fe(
h
5-C5H4); Mes ¼ mesityl ¼ 2,4,6-
R1 ¼0.0381, wR2 ¼ 0.0987 for observed unique reflections [I > 2
s
(I)]
Me3C6H2]. Ferrocenyllithium, FcLi, was prepared by a modified
and R1 ¼ 0.0506, wR2 ¼ 0.1067 for all unique reflections. Max. and
min. residual electron densities 0.52 and ꢀ0.56 e Å3. CSD reference:
1001876.
t
literature route [14]: BuLi (45 mL of a 1.6 M solution in hexanes,
72 mmol) was added over 15 min to a suspension of ferrocene
(16.0 g, 86 mmol) in THF (80 mL) at 0 ꢁC. Immediately after the
addition of t-BuLi, hexane (150 mL) was added and the suspension
cooled to ꢀ78 ꢁC. The resulting pyrophoric orange precipitate was
separated by filtration at ꢀ78 ꢁC, washed with pre-cooled hexane
(4 ꢂ 50 mL) at ꢀ78 ꢁC and volatiles removed in vacuo to afford an
orange powder (approximately 15 g). This powder was continu-
ously extracted with hexane (200 mL) using a Soxhlet apparatus
over a period of 12 h, until the extracts were colourless. The
remaining solid was dried in vacuo to afford the target material as a
highly pyrophoric orange powder. Yield 8.17 g, 59%.
FcB(C6Cl5)Br (3)
nBuLi (88 mL of a 1.6 M solution in hexanes,14 mmol) was added
dropwise to a suspension of hexacahlorobenzene (4.01 g, 14 mmol)
in a 1:1 hexane/diethyl ether mixture (100 mL of each) at ꢀ78 ꢁC.
The resulting suspension was allowed to warm to
approximately ꢀ40 ꢁC, until the white insoluble material was
consumed, yielding a yellow solution, which was quickly trans-
ferred to a suspension of FcBBr2 (5.01 g, 14 mmol) in hexane
(100 mL) at ꢀ78 ꢁC. The resulting red suspension was allowed to
warm to room temperature over
2 h, darkening noticeably
Novel syntheses
around ꢀ30 ꢁC, and was then stirred at room temperature for a
further 12 h. Volatiles were removed in vacuo to afford a dark
maroon tar. Condensing argon onto the tar (ca. 20 mL), subsequent
mechanical manipulation of the solid to give a fine suspension in
argon and then evacuation for a period of 1 h gave rise to a maroon
powder. Yield 6.92 g, 93%. 1H NMR (300 MHz, chloroform-d, 20 ꢁC):
dH 4.87 (t, 2H, JHeH ¼ 1.8 Hz, C5H4B), 4.47 (t, 2H, JHeH ¼ 1.8 Hz,
C5H4B), 4.32 (s, 5H, C5H5). 11B{1H} NMR (96 MHz, chloroform-d,
20 ꢁC): dB 61. 13C{1H} NMR (75 MHz, THF-d8, 20 ꢁC): dC 139.8 (C6Cl5),
134.1 (C6Cl5), 132.0 (C6Cl5), 76.6 (C5H4B), 76.4 (C5H4B), 73.0 (C5H4B),
70.8 (C5H5). MS (ESI positive) m/z (%): 185.9 (100) [FcH]þ, 249.8 (34)
[C6Cl5H]þ, 461.8 (59) [FcB(C6Cl5)(OH)]þ, 523.8 (16) [FcB(C6Cl5)Br]þ;
accurate mass: calc. (for Mþ, 10B, 35Cl, 54Fe, 79Br isotopomer)
518.7850; meas. 518.8750. Elemental microanalysis: calc. (for
Novel route to FcB(C6F5)2 (1) from FcBBr2
nBuLi (1.6 M, 17.5 mL in hexanes, 28 mmol) was added dropwise
to a solution of 1-bromo-2,3,4,5,6-pentafluorobenzene (3.47 mL,
28 mmol) in hexane (30 mL) at ꢀ78 ꢁC. The resulting mixture was
stirred at ꢀ78 ꢁC for 45 min; a solution of FcBBr2 (5.00 g, 14 mmol)
in hexane (150 mL) was then added dropwise at ꢀ78 ꢁC. The
resulting mixture was allowed to warm to room temperature over a
period of 12 h. Volatiles were removed in vacuo and the residue was
extracted with hexane (3 ꢂ 50 mL). The combined organic extracts
were concentrated in vacuo and the residue cooled to ꢀ20 ꢁC to
produce maroon crystals, which were separated by filtration
at ꢀ20 ꢁC and dried in vacuo. Yield: 4.80 g, 65%. 1H, 11B and 19F NMR
data were in agreement with the literature values [7].
3
3
C
16H9BBrCl5Fe) C 36.60%, H 1.73%; meas. C 36.13, 36.21%, H 1.32,
1.37%. E1/2 (0.05
M a,a,a-
[nBu4N][B(C6F5)4] electrolyte in
FcB(C6Cl5)2 (2)
trifluorotoluene) ¼ þ382 mV, relative to FcH/FcHþ.
FcLi (0.33 g, 1.7 mmol) prepared as described above was added
slowly at 6 ꢁC to a suspension of ClB(C6Cl5)2 (0.95 g, 1.7 mmol) in
benzene (100 mL). The resulting purple suspension was allowed to
warm to room temperature over 15 min, and then stirred for 15 h.
The precipitate was separated by filtration and then extracted with
benzene (2 ꢂ 50 mL). The filtrate and extracts were combined and
dried in vacuo to afford a purple solid (0.49 g), which was analysed
by 1H NMR and found to consist of a mixture ferrocene and the
target material. The ferrocene impurity was removed by sublima-
tion (at 70 ꢁC and 10ꢀ2 mbar) to afford FcB(C6Cl5)2 (2) as a violet
solid. Yield: 0.33 g, 27%. Crystals of 2$C6D6 suitable for X-ray
diffraction were obtained by cooling a saturated solution in ben-
zene. 1H NMR (300 MHz, chloroform-d, 20 ꢁC): dH 5.00 (t, 2H,
FcB(C6Cl5)(Mes) (4)
A suspension of FcB(C6Cl5)Br (0.858 g, 1.6 mmol) and MesLi
(0.205 g, 1.6 mmol) in benzene (50 mL) was heated under reflux
for 2 h. The reaction mixture was cooled to room temperature,
volatiles removed in vacuo and the residue transferred as a sus-
pension in a minimal amount of hexane onto a silica/hexane
column. Elution with hexane gave rise to three bands which were
isolated, the volatiles removed in vacuo from each and their
composition analysed by 1H NMR spectroscopy. The first, yellow
band (0.03 g) was found to be ferrocene, the second, maroon band
(0.39 g, 42%) was found to be the target material and the last, red
band (0.08 g) was found to be unreacted starting material. 4 was
isolated as a maroon solid in >95% purity (by multinuclear NMR)
after removal of the hexane solvent. 1H NMR (500 MHz, benzene-
3
3JHeH ¼ 1.8 Hz, C5H4B), 4.71 (t, 2H, JHeH ¼ 1.8 Hz, C5H4B), 4.32 (s,
5H, C5H5). 1H NMR (300 MHz, benzene-d6, 20 ꢁC): dH 4.52 (t, 2H,
3
3JHeH ¼ 1.8 Hz, C5H4B), 4.49 (t, 2H, JHeH ¼ 1.8 Hz, C5H4B), 3.95 (s,
5H, C5H5). 11B{1H} NMR (96 MHz, chloroform-d, 20 ꢁC): dB 62. 13
C
d6, 20 ꢁC): dH 6.79 (s, 2H, CH of Mes), 4.49 (t, 2H, JHeH ¼ 2.0 Hz,
3
{1H} NMR (75 MHz, THF-d8, 20 ꢁC): dC 139.9 (C6Cl5), 132.4 (C6Cl5),
129.6 (C6Cl5), 124.2 (C6Cl5), 77.9 (C5H4B), 74.2 (C5H4B), 67.8 (C5H5).
No signal was observed for the boron bound carbon of C5H4B. MS
(CI negative), m/z (%): 249.8 (43) [C6Cl5]ꢀ, 537.7 (39)
[C5H4B(C6Cl5)(C6Cl4)]ꢀ, 628.6 (100) [Fe(C5H4B(C6Cl5)2)]ꢀ, 693.6 (16)
[FcB(C6Cl5)2]ꢀ; accurate mass: calc. (for Mþ, 10B, 35Cl, 56Fe,
C5H4B), 4.42 (t, 2H, 3JHeH ¼ 2.0 Hz, C5H4B), 3.95 (s, 5H, C5H5), 2.52
(s, 6H, ortho-CH3 of Mes), 2.15 (s, 3H, para-CH3 of Mes). 11B{1H}
NMR (96 MHz, benzene-d6, 20 ꢁC): dB 72 (br s). 13C{1H} NMR
(125 MHz, benzene-d6, 20 ꢁC): dC 140.4 (para-C of Mes), 139.5
(AreC), 138.2 (ortho-C of Mes), 133.6 (AreC), 133.4 (AreC), 132.2
(AreC), 129.6 (AreC), 129.1 (meta-C of Mes), 80.2 (C5H4B), 75.1

M.J. Kelly et al. / Journal of Organometallic Chemistry 769 (2014) 11e16
13
Scheme 1. A novel synthetic route to FcB(C6F5)2 (1) and attempted route to FcB(C6Cl5)2 (2).
(C5H4B), 70.3 (C5H5), 25.3 (ortho-CH3 of Mes), 21.1 (para-CH3 of
Mes). The boron bound carbon of C5H4B was not observed. MS (ESI
positive) m/z (%): 185.9 (31) [FcH]þ, 249.8 (16) [C6Cl5H]þ, 563.9
(100) [FcB(C6Cl5)Mes]þ; accurate mass: calc. (for Mþ, 10B, 35Cl,
54Fe, 79Br isotopomer) 558.9529; meas. 558.9525. UVevis (fluo-
robenzene) lmax, nm (ε): 529 (364 L molꢀ1 cmꢀ1).
FcBBr2. Thus, initial attempts to synthesize the novel borane
FcB(C6Cl5)2 (2) sought to exploit the analogous route from FcBBr2
and two equivalents of LiC6Cl5. However, under all reaction con-
ditions screened, mixtures of the target compound FcB(C6Cl5)2 (2),
unreacted FcBBr2 and partially substituted FcB(C6Cl5)Br (3) were
obtained. Attempts to separate 2 from 3 proved unsuccessful: at-
tempts at fractional recrystallization resulted in co-precipitation,
while column chromatography could not adequately separate the
two compounds and led additionally to contamination with traces
of Fc(C6Cl5)B(m2-O)B(C6Cl5)Fc (see ESI).
That the reaction of FcBBr2 with two equivalents of LiC6F5 pro-
ceeds without issue, while the analogous reaction with LiC6Cl5 does
not, is thought to be a consequence of steric effects. Consistently,
when only one equivalent of LiC6Cl5 is employed, the reaction with
FcBBr2 allows for the facile, controlled introduction of a single
perchlorophenyl group. The compound so generated, FcB(C6Cl5)Br
(3), is a maroon solid, available in good yield (90e95%) and has
been characterized by 1H, 11B and 13C NMR spectroscopy, mass
spectrometry and elemental microanalysis. Although the thermal
frailty of LiC6Cl5 prevents the subsequent use of forcing conditions
to substitute the remaining boron-bound bromide (to generate 2)
[17], the reaction of 3 with one equivalent of the more robust
lithium aryl LiMes in refluxing benzene yields FcB(C6Cl5)(Mes) (4;
Scheme 2). After work-up, compound 4 was isolated in moderate
yield (35e45%) as a maroon solid, and characterized by 1H, 11B and
13C NMR spectroscopy and mass spectrometry (including accurate
mass determination). Thus, under more forcing conditions, a sec-
ond aryl group can indeed be assimilated by the [FcB(C6Cl5)]
fragment.
An alternative, more selective, pathway to the target compound
2 was therefore investigated as shown in Scheme 3. This route
utilizes the chloroborane, ClB(C6Cl5)2, reported by Ashley et al. [10]
and FcLi, itself synthesized by a modified procedure from that re-
ported in the literature [14]. A complication arises from the reaction
of lithioferrocene with a CeCl rather than with the BeCl bond. This
unwanted side reaction was encountered when the two reagents
were mixed as solutions at ꢀ78 ꢁC and slowly warmed to room
temperature; its extent could, however, be minimized by adding
FcLi as a solid powder to a benzene solution of ClB(C6Cl5)2 at 6 ꢁC.
After work-up, 2 could be isolated as a purple solid, albeit in rela-
tively low yield (25e30%). The measured 1H and 13C NMR spectra
are fully consistent with the proposed structure, with further
confirmation of identity and purity coming from mass spectrom-
etry and elemental analysis, respectively. The structure of 2 in the
solid state was subsequently determined by X-ray crystallography
(Scheme 3).
Crystallography
Data for 2$C6D6, Fc(C6Cl5)B(m2-O)B(C6Cl5)Fc$½C6H6 and
DMAP$B(C6F5)2Fc were collected on a Nonius KappaCCD diffrac-
tometer at 150 K. Data collection and reduction were carried out
using Collect and Denzo/Scalepack, respectively, structure solution
using either Sir92 or Superflip, and refinement using CRYSTALS
[15]. Complete details of all structures are contained within the
respective CIFs which have been deposited with the CCDC
(1001876e1001878).
Determination of Lewis acid acceptor number (AN)
A modified Gutmann method utilizing the 31P NMR signal of
Et3PO and a 1:3 ratio of Lewis base to Lewis acid was employed,
ꢀ
following the protocol of Adamczyk-Wozniak et al. [16].
Electrochemistry
Electrochemical measurements were performed on a PAR
AMETEK VersaSTAT 3 potentiostat under nitrogen within a Saffron
Omega Scientific glove-box. The cyclic voltammetry measurements
were carried out in a supporting electrolyte of 0.05 M [nBu4N]
[B(C6F5)4] in a,a,a-trifluorotoluene, using a silver quasi-reference
electrode, a platinum working electrode and a platinum wire
auxiliary electrode.
Results and discussion
Synthetic and structural studies
FcB(C6F5)2 (1) can be synthesised conveniently and in good yield
(65%) by the treatment of FcBBr2 with two equivalents LiC6F5, itself
generated in situ from either C6F6 or C6F5Br and nBuLi at ꢀ78 ꢁC
(Scheme 1). This synthetic route avoids the use of either ClB(C6F5)2
or [HB(C6F5)2]n (and associated tin-containing precursors) which
are inherent in either of the published routes to 1 [7]. In addition, it
points to a simple, versatile approach for variation in the boron-
bound aryl substituents starting from a common intermediate, i.e.
Scheme 2. Synthetic route to FcB(C6Cl5)Br (3) and FcB(C6Cl5)(Mes) (4).

14
M.J. Kelly et al. / Journal of Organometallic Chemistry 769 (2014) 11e16
Scheme 3. Synthesis of FcB(C6Cl5)2 (2) from FcLi. Molecular structure of 2$C6D6 as determined by X-ray crystallography. Hydrogen atoms and benzene-d6 solvate molecule omitted
for clarity and thermal ellipsoids drawn at the 50% probability level. Key geometric parameters are given in Table 1.
The 11B NMR spectrum of 2 in benzene-d6 features a broad peak
at dB ¼ 62 ppm, i.e. somewhat downfield of that reported for
FcB(C6F5)2 (1; dB ¼ 53 ppm) [7]. In the case of 1, the chemical shift is
thought to reflect a degree of pyramidalization at boron caused by
the presence in solution of a weak Fe / B donor/acceptor inter-
action [7]. In the case of 2, however, crystallographic evidence in
the solid state argues against the presence of an analogous inter-
action (vide infra) and the 11B shift in solution is more in line with
those reported for unperturbed three-coordinate triarylboranes [7].
In certain aspects the molecular structure of 2 in the solid state
is similar to that reported for its pentafluorophenyl counterpart, 1
(Table 1). Thus, for example, the BeC bond lengths involving both
the ferrocenyl and aryl substituents are not significantly different
for the two systems. The structure of 1, however exhibits a
noticeable bending of the borane unit toward the iron centre, such
that the Cp centroideCipsoeB angle deviates significantly from
linearity (by ca. 16ꢁ). Piers and co-workers ascribe this effect to a
degree of electron donation from iron-based orbitals to the highly
electron deficient boron centre [7,18]. In the case of 2, however, the
corresponding Cp centroideCipsoeB angle is essentially linear
(177.4ꢁ) and the Fe/B contact much longer [3.185(3) Å, cf. 2.924 Å
for 1] [7]. Moreover, the sum of the CeBeC angles for 2 (359.8ꢁ) is
reflective of a strictly planar borane fragment. Thus, there is no
evidence for an analogous Fe / B donor/acceptor formulation for
2. These differences we again ascribe primarily to steric factors,
with the bulkier pentachlorophenyl substituents preventing any
structurally significant interaction.
irreversible electrochemical processes) [7]. In line with the findings
reported for B(C6X5)3 (X ¼ F, Cl) [10], we find that the C6Cl5 sub-
stituent is more electron withdrawing than its perfluorophenyl
analogue. Accordingly, the oxidation potential of the pendant fer-
rocenyl group in 2 is þ550 mV (with respect to ferrocene/ferroce-
nium), while that 1 is þ450 mV. Thus, 2 is shown to be intrinsically
more electron deficient than 1. In addition, the Lewis acidities
determined for 1 and 2 by the Gutmann-Beckett method using
Et3PO reveal that 2 is also the stronger Lewis acid [19]. Thus, the
acceptor numbers determined for 1 and 2 using this approach are
71.9 and 81.0, respectively. The value determined for 1 can be put
into context by that previously reported for B(C6F5)3 (78.2) [16], and
presumably reflects not only the more electron rich nature of the
ferrocenyl group (compared to C6F5), but also the retention in so-
lution of the Fe / B interaction [7], which further biases binding
thermodynamics against Et3PO coordination. Consistently, Piers
and co-workers report a Lewis acidity for 1 on the Childs scale of
0.37 [cf. 0.77 for B(C6F5)3 and 1.00 for BBr3] The greater steric de-
mands of the C6Cl5 group (over C6F5) are presumably responsible
for the lack of any significant Fe / B interaction in 2, which
consequently renders it more Lewis acidic, provided the incoming
Lewis base is not itself sterically impeded.
While studies of the reduction potentials of B(C6F5)3 and
B(C6Cl5)3 are (in keeping with our CV studies of 1 and 2) indicative
of the greater electron-withdrawing capabilities of the C6Cl5 group,
it is also known (i) that B(C6F5)3, but not B(C6Cl5)3, will bind Et3PO
in dichloromethane-d2 solution [10]; and (ii) that B(C6F5)3 has a
greater fluoride ion affinity than B(C6Cl5)3 [20]. Both of these ob-
servations are consistent with the dominance of steric (rather than
electronic) factors in Lewis base coordination by tris(pentahalo-
phenyl)boranes, in that the bulkier but more electron deficient
system B(C6Cl5)3 binds more weakly. That FcB(C6Cl5)2 (2) not only
binds Et3PO [unlike B(C6Cl5)3], but is determined to be a stronger
Lewis acid than FcB(C6F5)2 (1) e at least with this particular Lewis
basic ‘probe’ e would imply that the incorporation of the ferrocenyl
group markedly reduces the steric congestion at the boron centre
compared to B(C6X5)3 systems.
Probes of the relative electron deficiency and Lewis acidity of
FcB(C6Cl5)2
Comparisons with FcB(C6F5)2 (1)
The electrochemical properties of 1 and 2 have been compared
as solutions in
a,a,a
-trifluorotoluene using a [nBu4N][B(C6F5)4]
electrolyte ([nBu4N][PF6] has been shown previously to react with
the cation formed by single electron oxidation of 1 leading to
Table 1
Comparison of key bond lengths (Å) and angles (ꢁ) of FcB(C6F5)2 (1) [7] and
FcB(C6Cl5)2 (2).
Comparisons within the series FcB(C6Cl5)(Mes)2ꢀx
While we have been unable to access the mixed Lewis acid
FcB(C6F5)(C6Cl5), due to the explosive [21] incompatibility of LiC6F5
with the forcing conditions required to substitute the remaining
bromide in FcB(C6Cl5)Br (3), the related series of boranes
FcB(C6Cl5)2 (2), FcB(C6Cl5)(Mes) (4) and FcBMes2 (5)[6d] has now
been synthesized. We therefore sought to explore the trends in
electron deficiency and Lewis acidity at boron as a function of the
formal C6Cl5/Mes substitution process.
Parameter
FcB(C6F5)2 (1) [7]
FcB(C6Cl5)2 (2)
d(BeCaryl)/Å
d(BeCFc)/Å
1.604(4), 1.584(4)
1.501(4)
1.609(3), 1.617(3)
1.514(4)
:(CaryleBeCaryl)/ꢁ
:(CaryleBeCFc)/ꢁ
S{:(CeBeC)}/ꢁ
d(Fe/B)/Å
118.4(2)
118.0(2), 122.5(2)
358.9
118.4(2)
120.3(2), 121.1(2)
359.8
2.924
3.185(3)

M.J. Kelly et al. / Journal of Organometallic Chemistry 769 (2014) 11e16
15
Fig. 1. UVeVis spectra (left to right) of FcBMes2 (5), FcB(C6Cl5)(Mes) (4) and FcB(C6Cl5)2 (2) in fluorobenzene solution (ca. 1.3 mM).
Cyclic voltammetry measurements readily testify to the ex-
pected increase in electron-withdrawing capabilities for the C6Cl5
group over mesityl (e.g. E1/2 for 5 ¼ þ180 mV with respect to
ferrocene/ferrocenium, cf. þ550 mV for 2),[6d] and such electronic
factors are also shown to have pronounced spectroscopic conse-
quences. Thus, on increasing the number of perchlorophenyl
groups, the wavelength corresponding to maximum absorption
intensity in the UVevis spectrum (lmax) undergoes successive red
shifts, with maxima at 507 nm, 529 nm and 546 nm being
measured for fluorobenzene solutions of 5, 4 and 2 respectively
(Fig. 1). Accordingly, while such solutions of 5 are red and those of 4
maroon, those of 2 are unusual for a neutral ferrocene-derived
species in being blueeviolet in colour (SI). Typically the LUMO of
a borane of this type is known to feature a large contribution from
the formally vacant p orbital at boron [4]. Thus, successive
replacement of mesityl substituents with more electron-
withdrawing C6Cl5 groups lowers the LUMO energy and shifts the
radiation absorbed to longer wavelength.
ranked C6Cl5 > C6F5 > Mes, while steric properties are ordered
Mes > C6Cl5 > C6F5.
Appendix. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2014.07.003.
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