A sandwich complex of trimesitylborane Mes3B: Synthesis, characterization and anion binding properties of Mes2B[(η 6-Mes)FeCp]+

Article abstract of DOI:10.1039/c1nj20290a

As part of our continuing interest in the chemistry of Lewis acidic organoboron compounds as anion receptors, we have synthesized the hexafluorophosphate salt of the cationic borane Mes₂B[(η ⁶-Mes)FeCp]⁺ ([1]⁺) by reaction of [(η⁶-naphthalene)FeCp][PF₆] with Mes₃B. This compound, which undergoes two quasi-reversible reductions at E _1/2 = -1.56 V and -2.40 V, reacts with both fluoride and cyanide anions in THF or MeOH to afford the corresponding zwitterionic fluoro- (1-F) and cyano-borate (1-CN) species, respectively. Spectrophotometric titrations carried out in THF indicate that the fluoride and cyanide binding constants of [1]⁺ are much larger than those of Mes₃B. This difference points to the favorable inductive and coulombic effects imparted by the cationic [CpFe]⁺ moiety on the Lewis acidity of the boron center.

Full text of DOI:10.1039/c1nj20290a

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PAPER
A sandwich complex of trimesitylborane Mes3B: synthesis,
characterization and anion binding properties
of Mes2B[(g6-Mes)FeCp]+wz
a
Duxia Cao,yab Haiyan Zhaoya and Franc
¸ ois P. Gabbaı*
¨
Received (in Gainesville, FL, USA) 1st April 2011, Accepted 13th June 2011
DOI: 10.1039/c1nj20290a
As part of our continuing interest in the chemistry of Lewis acidic organoboron compounds as
anion receptors, we have synthesized the hexafluorophosphate salt of the cationic borane
Mes2B[(Z6-Mes)FeCp]+ ([1]+) by reaction of [(Z6-naphthalene)FeCp][PF6] with Mes3B. This
compound, which undergoes two quasi-reversible reductions at E1/2 = ꢀ1.56 V and ꢀ2.40 V, reacts
with both fluoride and cyanide anions in THF or MeOH to afford the corresponding zwitterionic
fluoro- (1-F) and cyano-borate (1-CN) species, respectively. Spectrophotometric titrations carried
out in THF indicate that the fluoride and cyanide binding constants of [1]+ are much larger than
those of Mes3B. This difference points to the favorable inductive and coulombic effects imparted by
the cationic [CpFe]+ moiety on the Lewis acidity of the boron center.
overcome in smaller complexes that more efficiently focus their
Introduction
positive charge close to the boron atom. This possibility is
illustrated by the seminal work of Shinkai et al. who showed
that oxidation of ferrocenyl boronic acid into ferrocenium
boronic acid ([III]+) leads to an increase in the fluoride
binding by 3.5 orders of magnitude.44 This phenomenon has
also been shown to impact the anion affinity of a series of
other ferrocene boron species27,38,45,46 including ferrocenyl-
dimesitylborane (IV), a complex recently synthesized by the
Aldridge group.47 Inspired by this body of work, we have now
decided to determine if similar effects could be observed in
borylated [CpFe(Z6-arene)]+ cationic sandwich complexes. In
this paper, we now report on the synthesis and anion binding
properties of Mes2B[(Z6-Mes)FeCp]+ ([1]+, Mes = 2,4,6-
Me3C6H2), a cationic analog of IV.
Owing to their electronic unsaturation and inherent Lewis
acidity, organoboranes have found widespread applications for
the recognition of nucleophilic anions including the toxic fluoride
and cyanide ions.1–6 These small anions are sufficiently basic to
bind to the boron atom of these derivatives leading to the
formation of the corresponding borate anions (Chart 1).7 In an
effort to enhance the strength of the anion–boron interaction, a
great deal of effort has been devoted to the peripheral decoration
of the organoborane by main group onium ions8–16 such
as ammonium,17–27 phosphonium,19,22,28–33 sulfonium19,34–36
and telluronium ions.36 The presence of these cationic
functionalities leads to favorable coulombic effects which
stabilize the fluoroborate moiety against dissociation
(Chart 1). The success of this approach is illustrated by the
ability of such cationic boron derivatives to bind cyanide and
fluoride anions in aqueous solutions.23,28,29
Cationic organoboranes can also be obtained by positioning
the boron moiety in the vicinity of a cationic transition metal
moiety37–41 as in [I]+ 42 and [II]+.43 In such cases however,
dissipation of the cationic charge of the transition metal
complex over several ligands may limit its impact on the anion
affinity of the boron center. Such limitations may be partly
a Texas A&M University, Department of Chemistry, College Station,
TX 77843, USA. E-mail: francois@tamu.edu
b School of Material Science and Engineering, University of Jinan,
Jinan 250022, Shandong, China
w Dedicated to our friend Didier Astruc, on the occasion of his 65th
birthday.
z CCDC reference numbers 817396–817398. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1nj20290a
y DC and HZ contributed equally to this work.
c
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Chart 1 Anion binding by neutral (left) and cationic (right) organoboranes; XQF, CN; R = organic substituent.
Table 1 Crystal data, data collections, and structure refinement for
[1][PF6], 1-F and 1-CN
Crystal data
[1][PF6]
1-F
1-CN
Empirical formula
C32H38BF6FeP C32H38BFFe C33H38BNFeꢁ
CHCl3
634.67
Formula mass
Crystal system
Space group
634.25 508.28
Orthorhombic Orthorhombic Monoclinic
Scheme 1 Synthesis of [1][PF6].
Pca21
Pna21
P21/c
a/A
b/A
c/A
b/1
20.827(2)
8.7404(10)
16.2270(18)
90.00
2953.9(5)
4
1.426
17.1967(10)
17.6328(10)
8.5350(5)
90.00
2588.0(3)
4
1.305
14.5594(7)
13.9523(6)
15.6438(7)
96.5310(10)
3157.2(2)
4
1.335
0.756
1328
Results and discussion
Synthesis and characterization of [1][PF6]
V/A3
Z
Sandwich complexes of trimesitylborane (Mes3B) have been
previously synthesized by the group of Elschenbroich who
reported the sequential decoration of the mesityl groups of
this derivative by Cr(CO)3 moieties.48 Encouraged by these
results, we allowed [(Z6-naphthalene)FeCp][PF6]49 to react
with trimesitylborane Mes3B in 1,3-dichloropropane (Scheme 1).
This reaction proceeded via smooth arene exchange to afford
Mes2B[(Z6-Mes)FeCp]+ as an hexafluorophosphate salt
([1][PF6]). This salt, which can be stored in air for extended
periods of time, is soluble in solvents of moderate polarity
such as CHCl3, THF and acetone. It is however poorly soluble
in MeOH and CH3CN. The 1H NMR spectrum of the
compound in CDCl3 exhibits one singlet resonance at 4.81 ppm
corresponding to the CH groups of the cyclopentadienyl ring.
The rest of the spectrum is more challenging to assign. Indeed,
since coordination of the [CpFe]+ moiety to one of the mesityl
r
calc/g cmꢀ3
m/mmꢀ1
F(000)
Crystal size/mm
0.624
1320
0.609
1080
0.20 ꢂ 0.12 ꢂ 0.35 ꢂ 0.09 ꢂ 0.38 ꢂ 0.12 ꢂ
0.05 0.07 0.05
ymin–ymax/1
2.33–22.19
110(2)
o, j
2.31–24.30
110(2)
o, j
2.33–27.73
110(2)
o, j
T/K
Scan mode
Index range
ꢀ23 r h r 23 ꢀ22 r h r 22 ꢀ19 r h r 19
ꢀ10 r k r 10 ꢀ23 r k r 23 ꢀ18 r k r 18
ꢀ18 r l r 18 ꢀ11 r l r 11 ꢀ20 r l r 20
Collected reflections 4636
Unique reflections 4011
Refined parameters 379
R1, wR2 (all data)
6449
4877
326
7834
6249
409
0.0554, 0.1107 0.0508, 0.0618 0.0544, 0.1077
r
Flack parameter
fin (max., min.)/eꢁAꢀ3 0.716, ꢀ0.417 0.309, ꢀ0.355 0.401, ꢀ0.517
0.02(2)
0.676(11)
—
group will differentiate the two faces of the boron-centered
Mes
propeller, the six CH3
as well as the four aromatic CHMes
groups are magnetically unequivalent. Because of accidental
overlaps, four resonances are observed in the methyl region.
Similarly, four resonances are observed in the 6.2 to 7.0 ppm
range for the aromatic CHMes groups. The 19F NMR spectrum
ꢀ
exhibits a doublet at ꢀ76 ppm which corresponds to the PF6
anion. The crystal structure of [1][PF6] has been determined
(Table 1). The boron atom adopts a trigonal planar geometry
as indicated by the sum of the Caryl–B–Caryl angles (S(C–B–C)
=
359.81). One of the mesityl substituents is coordinated to the
iron center in an Z6-fashion with a distance of 1.55 A between
the centroid of the coordinated mesityl ring and iron atom.
Examination of the Fe–CMes bonds which are within B0.1 A
from one another indicates that the iron atom is essentially
centrally located over the mesityl substituent. The increase in
steric encumbrance caused by attachment of the [CpFe]+
group leads to a 0.44 A displacement of the boron atom above
the plane containing the central six-membered ring of the
coordinated mesityl substituent (Fig. 1).
Fig. 1 Crystal structure of [1][PF6] with thermal ellipsoid plots (50%
probability). For clarity, the hydrogen, phosphorus and fluorine atoms
are omitted and the mesityl substituents are represented by a thin line.
Selected bond lengths (A) and angles (1): Fe(1)–C(6) 2.107(5),
Fe(1)–C(7) 2.086(4), Fe(1)–C(8) 2.116(4), Fe(1)–C(9) 2.163(4),
Fe(1)–C(10) 2.077(4), Fe(1)–C(11) 2.072(4), B(1)–C(9) 1.593(7),
B(1)–C(15) 1.575(6), B(1)–C(24) 1.584(7); C(15)–B(1)–C(24) 122.0(4),
C(15)–B(1)–C(9) 113.7(4), C(24)–B(1)–C(9) 124.1(4).
The UV-vis absorption spectrum of [1][PF6] in THF
displays two distinct low-energy bands centered at 335 nm
and 290 nm (Fig. 2). To elucidate the origin of these bands,
the structure of [1]+ was optimized using density functional
c
2300 New J. Chem., 2011, 35, 2299–2305
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Fig. 2 Top: experimental UV-vis spectrum of [1][PF6] (6.24
ꢂ
10ꢀ5 mol Lꢀ1) in THF overlaid with the calculated TD-DFT electronic
excitations. The accepting orbitals involved in these electronic
excitations are as follows: Ea–d: LUMO; Ee, LUMO and LUMO + 1,
Ef: LUMO + 1. Bottom: view of the LUMO and LUMO + 1
(isovalue = 0.02) of cation [1]+
.
theory (DFT) methods (functional: BP86; mixed basis set: Fe:
aug-cc-pvTz-pp; B: 6-31+g(d0); C, H: 6-31g). The optimized
structure, which is close to that determined experimentally,
was subjected to a time-dependent DFT (TD-DFT) calcula-
tion (functional: MPW1PW91; mixed basis set: Fe: aug-cc-
pvTz-pp; B: 6-31+g(d0); C, H: 6-31g) using the polarizable
continuum model with THF as a solvent. The TD-DFT
calculation indicates that the low-energy bands are dominated
by transitions from filled orbitals into the LUMO (Ea, Eb, Ec,
Ed) for the band observed at 335 nm and the LUMO + 1 (Ef,
Eg) for that observed at 290 nm (Fig. 2). Inspection of the
frontier molecular orbitals shows that the LUMO carries an
important contribution from the boron pp orbital, which is
typical for triarylboranes, and the LUMO + 1 bears an
increased contribution from the iron sandwich moiety with
both Fe and Cp/Mes ligand character.
Fig. 3 Cyclic voltammograms of [1][PF6] (top) and in situ generated
1-F (bottom) in THF with [[1][PF6]] = 1.9 mM, [TBAPF6] = 0.1 M
(TBA = tetra-n-butylammonium) and n = 300 mV sꢀ1
.
properties are neutralized by coordination of a fluoride anion.
The latter implies formation of a stable fluoride adduct,
namely 1-F, whose formation has indeed been confirmed
(vide infra). The similarity of the potential observed between
the second reduction wave of [1]+ (ꢀ2.40 V) and that of 1-F
(ꢀ2.37 V) suggests that these reduction waves involve addition
of a 19th valence electron to the iron sandwich moiety. The
formation of 19-electron iron sandwich complexes is a
well established phenomenon extensively documented by the
contribution of Astruc.50 The electronic independence of the
boron and iron moiety implied in the above discussion is
clearly an oversimplification. Indeed, comparison of the first
reduction potential of [1]+ (E1/2 = ꢀ1.56 V) with that of
[(Z6-mesitylene)FeCp]+ (E1/2 = ꢀ2.01 vs. Fc/Fc+)50 and
The cyclic voltammogram of [1][PF6] in THF shows two
quasi-reversible reduction waves at E1/2 = ꢀ1.56 V and
ꢀ2.40 V (vs. Fc/Fc+) as well as smaller intermediate feature
(Ered = ꢀ1.98 V) which may reflect the appearance of a new
species by partial decomposition of [1]+ under the condition
of the electrochemical measurement (Fig. 3). Interestingly,
addition of one equivalent of fluoride anion to the electro-
chemical cell afforded a species that only show one major
reduction wave at ꢀ2.37 V (vs. Fc/Fc+). In a simplistic
scenario (Fig. 3), these observations suggest that the wave
observed at E1/2 = ꢀ1.56 V for [1]+ corresponds to the
reduction of the boron center whose electron accepting
[4-(Me3N)-2,6-Me2-C6H2-BMes2]+ (E1/2
=
ꢀ2.55
V vs.
Fc/Fc+)21 indicates that the two electroactive boron and iron
moieties effectively communicate.
Encouraged by this result, we decided to investigate the
Lewis acidity of [1]+ by examination of its reaction with
fluoride and cyanide in organic solvents. Reactions of
[1][PF6] with excess KF or KCN in MeOH resulted in the
rapid precipitation of 1-F and 1-CN, which have been isolated
c
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by filtration. When the same reaction was attempted with
Mes3B, anion coordination to the boron center was not
observed thus attesting to the favorable coulombic effects
resulting from the decoration of a mesityl ring by a cationic
[CpFe]+ fragment. Compounds 1-F and 1-CN have been
isolated and characterized by multinuclear NMR spectro-
scopy, IR spectroscopy and single crystal X-ray diffraction.
The 11B NMR signals at 5.9 ppm for 1-F and ꢀ14.1 ppm for
1-CN are in the expected range for four coordinate boron
species.23,28,29 The 19F NMR signal of 1-F appearing at
ꢀ168.2 ppm indicates the presence of a triarylfluoroborate
moiety.23,29 An absorption band at 2170 cmꢀ1 in the IR
spectrum of 1-CN is typical for triarylcyanoborate species.23,28
Anion coordination to the boron atom leads to some notable
changes in the 1H NMR spectrum of the complex. While
signal overlaps make a clear assignment of the methyl
resonances difficult, the number of aromatic CHMes resonances
decreases from four in [1]+ to three resonances of 2 : 2 : 2
intensity in both 1-F and 1-CN. One of these three resonances,
which appears up-field at 5.56 ppm for 1-F and 5.68 ppm for
1-CN, can be assigned to the aromatic CHMes resonances of
the mesityl group coordinated to the iron center.51 The other
two resonances correspond to the aromatic CHMes groups
pointing, respectively, above and below the plane of the
coordinated mesityl substituent. Altogether, these NMR data
suggest that the molecular motion-averaged solution structure
of 1-F and 1-CN is that of a complex of Cs symmetry as
depicted in Scheme 2. The crystal structures of 1-F and 1-CN
have been determined (Fig. 4, Table 1). The B–F bond length
in 1-F (1.463(3) A) and B–CCN bond length in 1-CN (1.628(3) A)
are comparable to those of other triarylfluoroborate and
triarylcyanoborate species, respectively.18 The boron center
of these two compounds is distinctly pyramidalized as indicated
by the sum of the Caryl–B–Caryl angles in both structures
(S(C–B–C) = 340.71 for 1-F, and 340.11 for 1-CN). Interestingly,
pyramidalization of the boron center leads to a relief of
the steric repulsion occurring between the Mes2B and
[(Z6-Mes)FeCp]+ moieties of [1]+. The decreased steric
encumbrance in 1-F and 1-CN when compared to [1]+ is
reflected by the relatively small displacement of the boron
atom (0.10 A in 1-F, 0.19 A in 1-CN vs. 0.44 A in [1]+) above
the plane containing the central six-membered ring of the
coordinated mesityl substituent. The separation between the
iron atom and the centroid of the coordinated mesityl ring in
1-F (1.54 A) and 1-CN (1.55 A) remain virtually identical to
that observed in [1]+ (1.55 A).
To measure the effect induced by coordination of the
[CpFe]+ moiety to a mesityl ring of Mes3B, the anion affinity
of [1]+ has been measured in organic solvents. Addition of
TBAF or TBACN (TBA = tetra-n-butylammonium) to a
THF solution of [1][PF6] resulted in a progressive quenching
of the low energy band observed in the UV-vis spectrum of
[1]+ (Fig. 5). These changes, which can be rationalized by
involvement of the boron pp orbital in the formation of a B–X
bond (XQF or CN), indicate that both fluoride and cyanide
bind to the boron atom of [1]+. Fitting of the resulting data to
a 1 : 1 binding isotherm indicates that the fluoride (K(Fꢀ)) and
cyanide binding constants (K(CNꢀ)) exceed the measurable
range and are at least equal to 107 Mꢀ1. Although no accurate
number could be obtained, these experiments show that the
fluoride and cyanide binding constants of [1]+ exceed those of
Scheme 2 Left: reactions of [1][PF6] with potassium fluoride and
cyanide ions in methanol. Right: molecular motion-averaged structure
1
of 1-F and 1-CN observed by H NMR in CDCl3.
Fig. 4 ORTEP drawings of 1-F (left) and 1-CN (right) with thermal ellipsoid plots (50% probability). For clarity, the hydrogen atoms are
omitted and the mesityl substituents are represented by a thin line. Selected bond lengths (A) and angles (1) for 1-F: Fe(1)–C(6) 2.095(2),
Fe(1)–C(7) 2.059(2), Fe(1)–C(8) 2.086(2), Fe(1)–C(9) 2.185(2), Fe(1)–C(10) 2.084(2), Fe(1)–C(11) 2.067(2), B(1)–C(9) 1.684(3), B(1)–C(15)
1.678(3), B(1)–C(24) 1.663(3), F(1)–B(1) 1.463(3); C(24)–B(1)–C(15) 109.9(2), C(24)–B(1)–C(9) 118.6(2), C(15)–B(1)–C(9) 112.2(2),
F(1)–B(1)–C(9) 102.1(2). Selected bond lengths (A) and angles (1) for 1-CN: Fe(1)–C(6) 2.076(2), Fe(1)–C(7) 2.102(2), Fe(1)–C(8) 2.057(2),
Fe(1)–C(9) 2.090(2), Fe(1)–C(10) 2.199(2), Fe(1)–C(11) 2.106(2), C(10)–B(1) 1.682(3), C(15)–B(1) 1.684(3), C(24)–B(1) 1.628(3), C(25)–B(1)
1.668(3); C(24)–B(1)-C(10) 104.05(14);C(25)–B(1)–C(10) 118.68(14); C(25)–B(1)–C(15) 107.34(14); C(10)–B(1)–C(15) 114.09(14).
c
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Fig. 5 Top: changes in the UV-vis absorption spectra of [1][PF6] (6.24 ꢂ 10ꢀ5 mol Lꢀ1) in THF upon addition of TBAF (left); experimental and
calculated 1 : 1 fluoride binding isotherms (right). Bottom: changes in the UV-vis absorption spectra of [1][PF6] (7.70 ꢂ 10ꢀ5 mol Lꢀ1) in THF upon
addition of TBACN (left); experimental and calculated 1 : 1 cyanide binding isotherms (right).
Mes3B by at least two orders of magnitude (K(Fꢀ) = 3.3 ꢂ
105 Mꢀ1, K(CNꢀ) = 4.3 ꢂ 104 Mꢀ1 for Mes3B in THF). To
better discriminate between the anion affinity of [1]+ and
Mes3B, we also attempted to carry out these titrations in a
more competing solvent such as MeOH. In this solvent, NMR
experiments showed that Mes3B has no measurable affinity for
fluoride and cyanide anions (K o 0.01 Mꢀ1). By contrast, we
observed the rapid reaction of [1]+ with both fluoride and
cyanide anions, even at 0.05 mM concentrations. Although
precise titrations could not be completed because of the slow
decomposition of [1]+ in this solvent, the contrasting behavior
observed between Mes3B and [1]+ attests to the favorable
influence of the [CpFe]+ moiety on the acidity of the boron
center. Finally, [1]+ quickly decomposes in the presence of
water and could not be studied in aqueous solutions.
transition metal moieties to aromatic anion receptors has been
previously described,52,53 the results presented in this paper
show that this strategy could become applicable to a broad
range of Lewis acidic anion receptors that feature aromatic
substituents.
Experimental section
General consideration
[(Z6-naphthalene)FeCp][PF6] was synthesized according to a
published procedure.49 1,3-Dichloropropane was purchased
from TCI America and dried with CaH2 and distilled under a
nitrogen atmosphere. The fluoride and cyanide salts n-Bu4NFꢁ
3H2O (TBAF) and n-Bu4NCN (TBACN) were purchased
from Alfa Aesar and Aldrich, respectively. NMR spectra were
recorded on a Varian Unity Inova 400 FT NMR (399.6 MHz
for 1H, 375.9 MHz for 19F, 128.2 MHz for 11B, 100.5 MHz for
13C) spectrometer at ambient temperature. Chemical shifts
d are given in parts per million, and are referenced against
external Me4Si (1H, 13C) and BF3ꢁEt2O (11B, 19F). Elemental
analyses were performed at Atlantic Microlab (Norcross, GA).
Conclusion
In conclusion, we report that Mes3B is readily converted into a
cationic complex ([1]+) by reaction with [(Z6-naphthalene)-
FeCp][PF6]. This new cationic organoboron complex undergoes
two quasi-reversible reductions in agreement with the presence
of an electroactive boryl and [CpFe]+ moiety. This complex
reacts with fluoride and cyanide anions in MeOH to afford
the corresponding zwitterionic fluoroborate and cyanoborate
species. Titration experiments carried out in THF indicate that
the stability constants of the resulting zwitterionic fluoro-
and cyano-borate species are much larger than those of
Mes3B thus reflecting the favorable inductive and coulombic
effects imparted by the [CpFe]+ moiety on the Lewis acidity
of the boron center. While the coordination of cationic
Theoretical calculations
Density functional theory (DFT) calculations (full geometry
optimization) were carried out with the Gaussian 03 program
using the BP86 functional. Geometry optimization was carried
out with the following mixed basis set: Fe: aug-cc-pvTz-pp;
B: 6-31 + g(d0); C, H: 6-31g. Frequency calculations, which
were carried out on the optimized structure of the compound,
confirmed the absence of any imaginary frequencies.
TD-DFT calculations were carried out with the Gaussian 03
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program using the MPW1PW91 functional and the afore-
mentioned mixed basis set.
or KCN solution in MeOH (2 mL). Both compounds could be
isolated by filtration as light yellow solids. Data for 1-F:
1H NMR (399.6 MHz, CDCl3): d 1.53 (s, 9H, Mes-CH3), 2.18
(s, 12H, Mes-CH3), 2.33 (s, 3H, Mes-CH3), 2.52 (s, 3H,
Mes-CH3), 4.61 (s, 5H, Cp-CH), 5.56 (s, 2H, Mes-CH), 6.45 (s,
2H, Mes-CH), 6.71 (s, 2H, Mes-CH). 13C NMR (100.5 MHz,
CDCl3): 20.84 (Mes-CH3), 21.07 (Mes-CH3), 24.32 (Mes-CH3),
24.37 (Mes-CH3), 76.02 (Cp-CH), 88.12, 99.25, 128.57, 128.71,
128.85, 129.72, 133.08. 19F NMR (375.9 MHz, CDCl3): ꢀ168.24.
11B NMR (128.2 MHz, CDCl3): d 5.86. Anal. Calcd for
C32H38BFFe: C, 75.61, H, 7.54%. Found: C,73.99, H, 7.53%
(The deviation between the calculated and observed EA results
show that this compound could not be isolated in a pure form.
The EA results suggest the presence of inorganic impurities that
Electrochemistry. Electrochemical experiments were performed
with an electrochemical analyzer from CH Instruments
(Model 610A) with a gold working electrode, a platinum
auxiliary electrode and a reference silver electrode. The reference
electrode solution was built by immersing a silver wire in a
vicor-capped glass tube containing a THF solution of TBAPF6
(0.1 M) and AgNO3 (0.005 M). All the three electrodes were
placed in a THF solution (3 mL) containing TBAPF6 (0.1 M)
and [1][PF6] (0.002 M). Ferrocene was used as an internal
standard and the potentials are reported relative to the E1/2 of
the Fc+/Fc redox couple.
1
Titration of [1]+ with fluoride and cyanide in THF. A
solution of [1][PF6] in THF (3 mL) was titrated by addition
of incremental amounts of a THF solution of TBAF or
TBACN. Concentrations used for the fluoride titrations:
[[1][PF6]] = 6.24 ꢂ 10ꢀ5 M and [TBAF] = 9.50 ꢂ 10ꢀ3 M.
Concentrations used for the cyanide titrations: [[1][PF6]] =
7.70 ꢂ 10ꢀ5 M and [TBACN] = 7.30 ꢂ 10ꢀ3 M.
could not be removed). Data for 1-CN: H NMR (399.6 MHz,
CDCl3): d 1.51 (s, 6H, Mes-CH3), 2.17 (s, 12H, Mes-CH3), 2.36
(s, 3H, Mes-CH3), 2.52 (s, 3H, Mes-CH3), 3.06 (s, 3H,
Mes-CH3), 4.77 (s, 5H, Cp-CH), 5.68 (s, 2H, Mes-CH), 6.45 (s,
2H, Mes-CH), 6.75 (s, 2H, Mes-CH). 13C NMR (100.5 MHz,
CDCl3): 20.78 (Mes-CH3), 20.97 (Mes-CH3), 25.21 (Mes-CH3),
76.82 (Cp-CH), 88.88, 99.44, 128.85, 129.95, 133.66. 11B NMR
(128.2 MHz, CDCl3): ꢀ15.02. Anal. Calcd for C33H38BFeN-3/4
(CHCl3): C, 67.02, H, 6.46%. Found: C, 67.15, H, 6.47% (The
sample used for EA was obtained by recrystallization from
CHCl3; the EA results indicate partial loss of the interstitial
CHCl3 molecule found in the crystal structure).
Crystallography. Single crystals of compound [1][PF6] were
obtained by slow diffusion of hexanes into a solution of the
compound in CHCl3. Single crystals of 1-F and 1-CN were
obtained by slow diffusion of MeOH into CHCl3 solutions
of the compounds. The crystallographic measurement of
compounds [1][PF6], 1-F and 1-CN were performed using a
Bruker APEX-II CCD area detector diffractometer, with a
graphite-monochromated Mo Ka radiation (l = 0.71069 A).
The CIF files have been deposited with the Cambridge
structure database and assigned the following numbers:
CCDC 817396–817398.
Acknowledgements
This work was supported by the National Science Foundation
(CHE-0646916), the Welch Foundation (A-1423). DC thanks
the National Natural Science Foundation of China (20802026).
Synthesis of Mes2B[(g6-Mes)FeCp][PF6] ([1][PF6]). [(Z6-
Naphthalene)FeCp][PF6] (0.17 g, 0.43 mmol) was added to a
solution of trimesitylborane (0.9 g, 2.4 mmol) in 1,3-dichloro-
propane (2 mL) under nitrogen. The mixture was heated to
reflux for 1.5 h. Following cooling to room temperature,
CHCl3 (2 mL) was added to the reaction mixture which was
then filtered. Addition of hexanes (10 mL) to the filtrate
resulted in the precipitation of the product ([1][PF6]) as a red
solid. The product was recrystallized by slow diffusion of
hexanes into the solution of the compound in CHCl3, affording
[1][PF6] (0.09 g, yield: 33%) in a red microcrystalline form.
1H NMR (399.6 MHz, CDCl3): d 1.21 (s, 3H, Mes-CH3), 2.11
(s, 9H, Mes-CH3), 2.20 (s, 3H, Mes-CH3), 2.36 (s, 9H,
Mes-CH3), 2.51 (s, 3H, Mes-CH3), 4.81 (s, 5H, Cp-CH), 6.23
(s, 2H, Mes-CH), 6.60 (s, 1H, Mes-CH), 6.72 (s, 1H, Mes-CH),
6.94 (s, 2H, Mes-CH). 13C NMR (100.5 MHz, CDCl3): d 20.71
(Mes-CH3), 21.46 (Mes-CH3), 21.57 (Mes-CH3), 21.99
(Mes-CH3), 23.15 (Mes-CH3), 24.06 (Mes-CH3), 24.83
(Mes-CH3), 78.31 (Cp-CH), 87.61, 104.81, 111.07, 129.35,
129.73, 130.22, 138.95, 140.66, 140.87, 142.15, 142.24,
142.33, 142.49. 19F NMR (375.9 MHz, CDCl3): ꢀ76.06
(2JF–F = 712.7 Hz). Anal. Calcd for C32H38BF6FeP: C,
60.60, H, 6.04%. Found: C, 60.54, H, 6.02%.
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