Formation, stability, and structures of borenium and boronium cations derived from pentamethylazaferrocene-boranes by hydride or chloride abstraction reactions

Article abstract of DOI:10.1021/om500348u

Hydride abstraction reactions from 1′,2′,3′,4′, 5′-pentamethylazaferrocene-boranes (PMAF-boranes, where borane = BH ₃, 9-bora[3.3.1]bicyclononane) using trityl cation in CH ₂Cl₂ yielded the corresponding borenium cations, whose stability depends on the counteranion of the trityl salt. If the [BF ₄]^- salt was employed, then fluoride abstraction from the anion occurred immediately, yielding PMAF-BF₃ and other identifiable byproducts. If the [B(C₆F₅)₄]^- salt was employed, then the borenium ions were stable in solution at room temperature and could be characterized by ¹H, ¹³C, and ¹¹B NMR spectroscopy. Double hydroboration of [PMAF-BH₂]⁺ with 1,5-cycloooctadiene yields the borenium ion of PMAF-9-bora[3.3.1]- bicyclononane, [PMAF-9BBN]⁺. Reaction of [PMAF-BH₂] ⁺ with excess PMAF yields the boronium cation [(PMAF) ₂BH₂]⁺, which was isolated and crystallographically characterized as its [B(C₆F₅) ₄]^- salt. Reaction of [PMAF-9BBN]⁺ with 2 equiv of N-methylimidazole leads to the boronium cation [(N-methylimidazole) ₂(BBN)]⁺, which was also isolated and crystallographically characterized as its [B(C₆F₅)₄]^- salt. Chloride abstraction from PMAF-BCl₃ with AlCl₃ in CH₂Cl₂ yielded the borenium cation [PMAF-BCl ₂]⁺, which was characterized in solution by ¹H, ¹³C, and ¹¹B NMR spectroscopy. A single-crystal X-ray structure was also obtained of [PMAF-BCl₂]⁺[AlCl ₄]^-, revealing structural changes upon borenium ion formation, including rehybridization of the B center and a bending of the BCl₂ moiety toward the Fe center. The Lewis acidity of all three borenium cations was assessed by the Gutmann-Beckett method, and [PMAF-BH ₂]⁺ was found to be more acidic than B(C₆F ₅)₃. DFT calculations were performed on [PMAF-BH ₂]⁺ using the BP86 method with the LANL2DZ basis set for Fe and the TZVP basis set for C, H, N, and B. The optimized structure revealed substantial bending of the BH₂ moiety toward Fe with a dip angle of -23.7°. Inspection of the frontier MOs reveal that the electrophilic B center is stabilized through delocalized interactions that involve orbitals on Fe, the pyrrolyl ring, and the pentamethylcyclopentadienyl ring.

Full text of DOI:10.1021/om500348u

Article
pubs.acs.org/Organometallics
Formation, Stability, and Structures of Borenium and Boronium
Cations Derived from Pentamethylazaferrocene−Boranes by Hydride
or Chloride Abstraction Reactions
BriAnne Bentivegna,† Christine I. Mariani,† Jason R. Smith,† Shuhua Ma,† Arnold L. Rheingold,‡
and Tim J. Brunker*,†
†Department of Chemistry, Jess and Mildred Fisher College of Science and Mathematics, Towson University, 8000 York Road,
Towson, Maryland 21252, United States
‡Department of Chemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358, United States
S
* Supporting Information
ABSTRACT: Hydride abstraction reactions from
1′,2′,3′,4′,5′-pentamethylazaferrocene−boranes (PMAF-bor-
anes, where borane = BH3, 9-bora[3.3.1]bicyclononane)
using trityl cation in CH2Cl2 yielded the corresponding
borenium cations, whose stability depends on the counter-
anion of the trityl salt. If the [BF4]− salt was employed, then
fluoride abstraction from the anion occurred immediately, yielding PMAF-BF3 and other identifiable byproducts. If the
[B(C6F5)4]− salt was employed, then the borenium ions were stable in solution at room temperature and could be characterized
1
by H, 13C, and 11B NMR spectroscopy. Double hydroboration of [PMAF-BH2]+ with 1,5-cycloooctadiene yields the borenium
ion of PMAF-9-bora[3.3.1]-bicyclononane, [PMAF-9BBN]+. Reaction of [PMAF-BH2]+ with excess PMAF yields the boronium
cation [(PMAF)2BH2]+, which was isolated and crystallographically characterized as its [B(C6F5)4]− salt. Reaction of [PMAF-
9BBN]+ with 2 equiv of N-methylimidazole leads to the boronium cation [(N-methylimidazole)2(BBN)]+, which was also
isolated and crystallographically characterized as its [B(C6F5)4]− salt. Chloride abstraction from PMAF-BCl3 with AlCl3 in
CH2Cl2 yielded the borenium cation [PMAF-BCl2]+, which was characterized in solution by 1H, 13C, and 11B NMR
spectroscopy. A single-crystal X-ray structure was also obtained of [PMAF-BCl2]+[AlCl4]−, revealing structural changes upon
borenium ion formation, including rehybridization of the B center and a bending of the BCl2 moiety toward the Fe center. The
Lewis acidity of all three borenium cations was assessed by the Gutmann−Beckett method, and [PMAF-BH2]+ was found to be
more acidic than B(C6F5)3. DFT calculations were performed on [PMAF-BH2]+ using the BP86 method with the LANL2DZ
basis set for Fe and the TZVP basis set for C, H, N, and B. The optimized structure revealed substantial bending of the BH2
moiety toward Fe with a dip angle of −23.7°. Inspection of the frontier MOs reveal that the electrophilic B center is stabilized
through delocalized interactions that involve orbitals on Fe, the pyrrolyl ring, and the pentamethylcyclopentadienyl ring.
methods.2c To enable isolation of such species, electronic and/
or steric protection of the boron center is required to attenuate
the fierce reactivity of such species. Various borenium cations
INTRODUCTION
■
Neutral, tricoordinate boranes have wide applications in
synthetic chemistry, much of which derives from the electron
deficiency or Lewis acidity at boron.1 Considerable efforts have
have in this way been crystallographically characterized as
recently summarized2c and include DMAP11 and NHC adducts
been made to modulate the Lewis acidity by varying both the
12
13
+
+
of B(mesityl)2
and BCl2 , an acridine adduct of a 9-
electronic properties of the ligands and their steric effects.
Enhanced Lewis acidity should arise with cationic, tricoordinate
boron species or borenium ions, in which an anionic ligand is
replaced by a neutral donor such as an amine or N-heterocyclic
carbene (NHC).2 The reactivity of borenium ions has been
utilized for a variety of interesting and useful transformations,
including hydroboration,3 hydrosilylation,4 reductions,5 Diels−
Alder reactions,6 electrophilic arene borylation,7 and aliphatic
C−H borylation8 as well as frustrated Lewis acid−base pair
cleavage of hydrogen.9 In many of these cases borenium species
act catalytically.3−7,10 Oftentimes such borenium species (or
borenium ion equivalents in which solvent or another weakly
bound ligand is also present at B) are generated in situ via
protonation, halide, or hydride abstraction as well as other
borafluorenium cation,14 and BCl2 ,9b a borenium cation based
on the BODIPY structure,15 and a phosphine-supported
borenium cation derived from catechol−borane.16 Recently a
planar-chiral ferrocenylborenium cation with pyridyl and aryl
donors has also been reported.4b
+
In this study we approached the formation of borenium ions
from the standpoint of the isoelectronic relationship between
C−C- and N−B-bonded fragments and by extension between
methylferrocene and azaferrocene−borane (see Scheme 1).17
Formally, abstraction of hydride from the methyl or borane
Received: April 1, 2014
Published: May 23, 2014
© 2014 American Chemical Society
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prepared by the reaction of PMAF with 9-BBNH dimer or BCl3
in CH2Cl2 solution (Scheme 2). Both precipitated as pure
solids from the reaction mixture by addition of excess hexane in
almost quantitative yield. In their 11B NMR spectra, 2 displays a
doublet (1JBH = 91 Hz) at −5.5 ppm, whereas 8 displays a
singlet at 5.3 ppm: other pertinent data are also summarized in
Table 1. Single-crystal X-ray structures of both compounds
were determined, confirming their formulation as simple Lewis
acid−base adducts. Views of the structures are shown in Figures
1 and 2 along with significant structural parameters. The UV−
Scheme 1. Isoelectronic Relationships of Methylferrocenes,
Azaferrocene−Boranes, and Their Respective Cations
group would lead to the isoelectronic cations ferrocenylmethy-
lium and azaferrocene−borenium, both of which are also
isoelectronic with the neutral ferrocenylborane. Ferrocenylme-
thylium ions18 and ferrocenylboranes19 have been the subject of
extensive synthetic studies, and the extent of interaction of the
Fe center with the electron-deficient C or B atom has been
investigated theoretically and experimentally. In the ferrocenyl
cases it has been established that the vacant p orbital on the
exocyclic sp2 atom is partially stabilized through interactions
with Fe-based orbitals and those on the unsubstituted
cyclopentadienyl ring.18,20 We reasoned that similar stabilizing
interactions might allow observation and isolation of
azaferrocene−borenium ions, even in cases such as X = H,
where the anionic ligand confers minimal electronic or steric
stability. Previously we have synthesized 1′,2′,3′,4′,5′-pentam-
ethylazaferrocene−borane (PMAF-BH3) and herein report on
the hydride abstraction reaction from this borane, as well as for
the related dialkylborane adduct PMAF-9-borabicyclo[3.3.1]-
nonane, and on chloride abstraction from PMAF-BCl3 (Scheme
2). A portion of this work has previously been communicated.21
Figure 1. ORTEP plot of one of the independent molecules in the X-
ray crystal structure of 2 with thermal ellipsoids at 50% probability.
Selected bond lengths (averaged over both molecules) (Å): N−B,
1.619(4); Fe−N, 2.038(2); Fe−Pyr(Ct), 1.656; Fe−Cp*(Ct), 1.648.
Selected bond angles (averaged over both molecules) (deg): N1−B1−
C15, 109.7(2); N1−B1−C19, 112.3(2); C15−B1−C19, 106.1(2);
Fe−N−B 130.6(2), Cp*(Ct)−F−Pyr(Ct), 177.4; Pyr(Ct)−N−B,
173.4. Fe−N−B−H torsion angles (deg): 13.91 and −32.52.
vis spectra of 1, 2, and 8 are presented in Supporting
Information: all are characterized by a very broad, low-intensity
band around 450 nm and a shoulder at about 350 nm to a more
intense charge transfer band.
Scheme 2. Azaferrocenes and Boranes Utilized in This Study
Hydride Abstraction from PMAF-BH3 and PMAF-
9BBNH. Abstraction of hydride from PMAF-BH3 (1) and
PMAF-9BBNH (2) was investigated using trityl (triphenylcar-
benium, Tr+) salts in dry CH2Cl2 solvent, as shown in Scheme
3. The reaction of 1 with 1 equiv of Tr+BF4 under these
−
conditions yielded PMAF-BF3 and B2H6 as the only two
products observed by 11B{1H} NMR. PMAF-BF3, previously
synthesized and characterized by us,22 appears as a broad
quartet at −0.2 (1JBF = 11 Hz): its presence was also confirmed
by 19F NMR, which revealed a singlet at −148 ppm. Diborane
was identified from the singlet at 16.9 ppm in the 11B{1H}
NMR spectrum,23 which became a triplet of triplets in the H-
1
RESULTS
■
Formation of PMAF-9BBNH and PMAF-BCl3. The 9-
BBNH (2) and BCl3 (8) adducts of PMAF were conveniently
coupled 11B NMR spectrum (1JBH = 137 (terminal), 45
(bridging) Hz).24
1
Table 1. Comparison of Selected 11B, H, and 13C NMR Chemical Shift Data for Neutral Boranes (1, 2, and 8) and Their
Respective Borenium Ions (3, 4, and 9) in CD2Cl2 Solution
compound
11B
Pyr Hα
Pyr Hβ
Cp*-CH3
Cα
Cβ
Cp* quat C
Cp*-CH3
1
3
2
4
8
9
−18
+40
−8
4.88
5.46
4.96
5.68
5.64
6.03
3.52
5.29
4.47
5.34
4.70
5.70
1.66
1.88
1.90
1.88
1.93
1.87
89.13
89.32
86.75
83.7
74.48
87.10
77.44
83.0
83.09
91.02
84.95
88.3
10.41
10.92
10.37
10.9
+68
+5
85.78
86.13
78.6
85.29
90.1
10.9
+39
86.06
10.5
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1
attributed to 3 (see Table 1), the H NMR spectrum revealed
the expected byproduct Ph3CH. So far, attempts to isolate 3
through crystallization have been unsuccessful; however, it is
quite stable in dry CH2Cl2 solution at room temperature for
several hours.
The reaction of 2 with Tr+[B(C6F5)4]− gave a 11B{1H} NMR
spectrum with a broad peak at +68 ppm in addition to the
counteranion signal. Again we attribute this peak to the
1
borenium species [PMAF-9BBN]+ (4). 13C{1H} and H NMR
of the reaction mixture in CD2Cl2 are consistent with the
formation of 4 (see Table 1). Addition of hexane directly to this
reaction mixture and cooling to −35 °C gave crystalline
material, which was subjected to single-crystal X-ray diffraction
analysis; however, crystal twinning and poor crystal quality
precluded collection of a data set of sufficient quality for
solution and refinement. Attempts to grow crystals in which the
product was initially precipitated, redissolved in CH2Cl2, and
layered with hexanes gave good-quality crystals that were
subsequently shown to be of [PMAF-H]+[B(C6F5)4]− (5):
these presumably arose from hydrolysis of the borenium salt
from more prolonged contact with trace water in the solvents.
The X-ray structure and other characterization data for 5 are
given in the Supporting Information.
Figure 2. ORTEP plot of the X-ray crystal structure of 8 with thermal
ellipsoids at 50% probability. Selected bond lengths (Å): N1−B1,
1.5537(18); B1−Cl1, 1.8592(16); B1−Cl2, 1.8395(17); B1−Cl3,
1.8487(16); Fe−N, 2.0141(11); Fe−Cp*(Ct), 1.659; Fe−Pyr(Ct),
1.655. Selected bond angles (deg): N1−B1−Cl2, 108.92(10); N1−
B1−Cl3, 109.57(10); Cl2−B1−Cl3, 111.26(8); N1−B1−Cl1,
107.32(9); Cl2−B1−Cl1, 110.28(8); Cl3−B1−Cl1, 109.40(8); Fe−
N1−B1, 136.57(9); Cp*(Ct)−Fe−Pyr(Ct), 174.3; Pyr(Ct)−N−B,
168.1.
To further assess the reactivity of 3, its hydroboration
reactivity with 1,5-cyclooctadiene (COD) was studied. When 1
equiv of COD was added to a solution of 3 in CH2Cl2,
prepared as described above, complete clean conversion of 3 to
4 was observed within 24 h at room temperature, as monitored
by 11B NMR spectroscopy. We also note that addition of THF
to solutions of 4 caused slow polymerization of the THF.
The UV−vis spectra of 3 and 4 in CH2Cl2 are given in the
Supporting Information and are extremely similar. Two barely
resolved peaks at 410 and 435 nm are observed with molar
absorptivities consistent with charge transfer transitions
(2500−8000 M−1 cm−1).
Reaction of 2 with Tr+BF4 gave a mixture of products by
−
11B NMR, the major components being 9-fluoro-9-
borabicyclo[3.3.1]nonane and PMAF-BF3. The former was
identified as a doublet at 62 ppm (1JBF = 126 Hz), in agreement
with literature data.25 Other minor components were
(C8H14B)2O (58 ppm) and C8H15BF2 (29 ppm) both
being known hydrolysis products of 9-fluoro-9-
borabicyclo[3.3.1]nonane (9-BBNF) that we assume form
due to presence of trace water.25 In reactions in which 2 was
formed in situ from PMAF and 9-BBNH dimer but not
isolated, unreacted 9-BBNH (26 ppm) was observed along with
a minor, but broad, peak at 6 ppm, tentatively assigned as the
PMAF adduct of 9-fluoro-9-borabicyclo[3.3.1]nonane.
Boronium Ion Formation. To further confirm the
formation of 3 and 4, trapping experiments were performed
with Lewis bases to form more stable boronium ion species as
shown in Scheme 4. Reaction of 1 with Tr+[B(C6F5)4]− in the
presence of 1 equiv of PMAF gave a species with a broadened
singlet at −8 ppm in the 11B{1H} NMR spectrum. Again, no
Borenium Ion Formation and Reactivity. To attempt to
observe borenium species, a trityl salt with the less reactive
counterion [B(C6F5)4]− was investigated as shown in Scheme
4. The reaction of 1 with Tr+[B(C6F5)4]− in CH2Cl2 gave only
one species observable by 11B{1H} NMR, characterized by a
broad singlet at 40 ppm (w1/2h = 365 Hz) in addition to the
singlet resonance of the anion at −18 ppm. Although no
1
splitting was observed in the H-coupled 11B spectrum, but
broadening was observed. The reaction was performed on a
preparative scale, and a solid product was readily isolated from
the solution by precipitation with hexanes. Recrystallization of
the product from a CH2Cl2 solution layered with hexanes
1
splitting was resolved in the H-coupled 11B NMR spectrum,
the downfield peak was observed to broaden (w1/2h = 520 Hz),
suggesting unresolved splitting. When the spectrum was
recorded at −20 °C, conditions reported to minimize line
broadening in 11B NMR, no increase in resolution was
observed.26 We attribute the peak at 40 ppm to the borenium
ion species [PMAF-BH2]+ (3). The counterion [B(C6F5)4]−
was observed unchanged by both 11B and 19F NMR
1
yielded dark orange crystals. H and 13C NMR spectral data of
these crystals were consistent with a boronium species of
formula [(PMAF)2BH2]+[B(C6F5)4]− (6), with two equivalent
azaferrocene moieties. The BH2 group appeared as a very broad
1
feature centered at 2.5 ppm in the H NMR spectrum. Single
crystals suitable for X-ray diffraction were also isolated by the
same method, and solution of the structure confirmed the
proposed boronium ion formulation as shown in Figure 3.
1
spectroscopy. Performing the reaction in CD2Cl2 allowed H,
13C, and DEPT spectra to also be obtained. In addition to peaks
Scheme 3. Hydride Abstraction Reactions from 1 and 2 using Trityl Tetrafluoroborate
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Scheme 4. Summary of Borenium and Boronium Ion Chemistry of 1, 2, and 8
Figure 3. ORTEP plot of the X-ray crystal structure of 6 (cation only)
with thermal ellipsoids at 50% probability. Selected bond lengths (Å):
N1−B1, 1.570(3); N2−B1, 1.571(3); Fe1−N1, 2.0119(16); Fe2−N2,
2.0046(16); Fe1−Cp*(Ct), 1.654; Fe2--Cp*(Ct), 1.651; Fe1−Pyr-
(Ct), 1.655; Fe2−Pyr(Ct), 1.652. Selected bond angles (deg): N1−
B1−N2, 105.88(16); Fe1−N1−B1, 129.66(13); Fe2−N2−B1,
128.39(13); Cp*(Ct)−Fe1−Pyr(Ct), 176.07; Cp*(Ct)−Fe2−Pyr-
(Ct), 177.45; Pyr(Ct)−N1−B1, 175.04; Pyr(Ct)−N2−B1, 176.07.
Figure 4. ORTEP plot of one of the four independent molecules in
the X-ray crystal structure of 7 (cation only) with thermal ellipsoids at
50% probability. Selected bond lengths given as average (range from
all independent molecules) (Å): N−B, 1.600 (1.588(9)−1.617(9));
B−C, 1.603 (1.595(9)−1.616(9)). Selected bond lengths given as
average (range from all independent molecules) (deg): N−B−N,
102.7 (101.8(5)−103.4(4)); C−B−C, 107.2 (106.6(5)−107.6(5)).
The reaction of 2 with Tr+[B(C6F5)4]− in the presence of 1
equiv of PMAF gave a signal in the 11B NMR spectrum
attributable to (C8H14B)2O (58 ppm) and also a poorly
resolved, broad signal around 0 ppm. The latter peak is
consistent with a species of formulation [(PMAF)2(9-BBN)]+;
however, it appears extremely susceptible to hydrolysis by
adventitious water and could not be isolated or characterized
further, with only 5 being formed. We therefore turned to the
less sterically hindered Lewis base N-methylimidazole to trap
the borenium ion 4. 11B NMR experiments showed that
addition of 1 equiv of N-methylimidazole (N-MeIm) to a
solution of 4, prepared as described above, resulted in little
obvious change to the spectrum. However, addition of another
1 equiv of N-MeIm led to complete conversion to a new
species with an 11B chemical shift of −1 ppm. 1H and 13C NMR
data were consistent with the formulation [(N-MeIm)2(9-
BBN)]+[B(C6F5)4]− (7), and this was subsequently confirmed
by a single-crystal X-ray structure of crystals grown from the
reaction mixture. The structure of the cation is shown in Figure
4. It was not possible to independently synthesize 7 by the
reaction of 9-BBNH dimer with Tr+[B(C6F5)4]− in the
presence of excess N-MeIm. Although the presence of some
7 was indicated by 11B NMR spectroscopy, it was part of a
complex mixture and attempts to isolate the compound were
unsuccessful.
from reaction of a transient electrophilic borenium species with
the anions. Gratifyingly, reaction with AlCl3 in CH2Cl2 led to
the clean formation of a species with a 11B NMR shift of 39
ppm, consistent with the borenium salt [PMAF-BCl2]+ as
shown in Scheme 4. Addition of hexane and cooling gave red
crystals suitable for single-crystal X-ray analysis and confirmed
the structure to be [PMAF-BCl2]+[AlCl4]− (9). Two
independent ion pairs were found in the asymmetric unit, in
which one of the cations showed positional disorder of the
pyrrolyl-BCl2 moiety. Both cation/ion pairs also lie on a
crystallographic mirror plane. A view of the nondisordered
structure of the cation of 9 is given in Figure 5. Solid 9 may be
stored at room temperature under an inert atmosphere without
decomposition for several weeks. A UV−vis spectrum of 9 was
obtained in CH2Cl2, but at high dilution a white solid was seen
to precipitate.
Lewis Acidity Measurements. In an effort to further
quantify the Lewis acidity of 3, 4, and 9, we utilized the
Gutmann−Beckett method, which measures the downfield shift
(Δδ) of the 31P NMR resonance of triethylphosphine oxide
adducts of each ion in comparison to the resonance of free
Et3PO (51.2 ppm) in CD2Cl2 solution.27 Values of Δδ of 31.6,
28.2, and 29.0 ppm were observed for 3, 4, and 9, respectively.
To gauge the Lewis acidity of 3 relative to B(C6F5)3, we also
performed competition experiments in which 3-OPEt3 and
B(C6F5)3 were mixed in CD2Cl2 in a 1:1 mole ratio (see
Experimental Details). The ratio 3-OPEt3:B(C6F5)3-OPEt3 was
Chloride Abstraction from PMAF-BCl3. The reaction of
PMAF-BCl3 (8) with a variety of Ag+ salts (X− = triflate, BF4 ,
−
−
−
PF6 , SbF6 ) in CH2Cl2 led to complex mixtures of products
when monitored by 11B NMR spectroscopy, presumably arising
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Figure 6. BP86 computed structure of 3 with LANL2DZ basis set for
iron and TZVP basis set for C, H, N, and B.
Figure 5. ORTEP plot of the ordered independent molecule in the X-
ray crystal structure of 9 (cation only) with thermal ellipsoids at 50%
probability. Selected bond lengths (this molecule only) (Å): N2−B2,
1.445(8); B2−Cl2 1.723(4); Fe2−N2, 1.994(4); Fe2−Cp*(Ct),
1.666; Fe2−Pyr(Ct), 1.656. Selected bond angles (this molecule
only) (deg): B2−N2−Fe2, 114.3(3); N2−B2−Cl2, 119.3(2); Cl2−
B2−Cl2, 121.4(4); Cp*(Ct)-Fe2−Pyr(Ct), 173.9; Pyr(Ct)−B2−N2,
170.4.
interactions in this structure, the occupied molecular orbitals
(MOs) were calculated from the HOMO down to the HOMO-
12 and the lowest unoccupied MOs were calculated from
LUMO to LUMO+12 (Supporting Information); some of
these are depicted in Figure 7.
then determined by integration of the 31P NMR signals and
found to be between 3.2:1 and 7.3:1 in different samples. Slow
decomposition of the solutions was observed over time with the
formation of several new unidentified species by 31P NMR.
Calculations. To gain more insight into the structure of 3,
its structure was calculated using the BP86 method in the
Gaussian 09 package.28 The BP86 method has been found to
perform better than other DFT methods in predicting the
structures of borylated ferrocenes.20 In addition, we found that
the combination of the LANL2DZ basis set for Fe and the
TZVP basis set for C, H, N, and B performed well in
characterizing all the structures we tested (as discussed in the
Supporting Information). Here we focus our discussion on the
optimized structure of 3 using the BP86 method with the
LANL2DZ basis set for Fe and TZVP basis set for C, H, N, and
B. The optimized structure of 3 was compared to the
experimental and optimized structure of PMAF-BH3 (Table
2). A view of the calculated structure of 3 is given in Figure 6.
This structure shows Cs symmetry with the N−B bond
eclipsing a Cp* methyl group on the mirror plane and the z
axis perpendicular to that plane. To further explore the orbital
DISCUSSION
■
Characterization of Borenium Ions in Solution. Multi-
nuclear solution NMR data for 3, 4, and 9 are consistent with
their formulation as borenium ions, and all show similar
changes in comparison to the neutral parent borane adducts 1,
2, and 8 (see Table 1). All show substantial downfield shifts in
their 11B peaks upon cation formation, approximately 60 ppm
for both 3 and 4, consistent with a decrease in coordination
number at B.29 The 11B peak of 9 at 39 ppm is similar to that
observed for [pyridine-BCl2]+ (44 ppm):26 although the 11B
chemical shifts of 3 and 9 are very similar, the 1H and 13C data
readily distinguish the compounds. The H and 13C data in all
1
cases reveal a symmetrical PMAF derivative with unrestricted
rotation of the Cp* ring. DEPT spectra allowed the Cp*
quaternary signal to be distinguished from the Pyr C
resonances. There are several notable trends that are common
to each neutral adduct/cation pair. (1) In the 1H NMR spectra,
the deshielding of the pyrrolyl ring protons in the cations is
greater for the β-Hs (by 0.87−1.77 ppm) than for the α-Hs (by
0.39−0.72 ppm). (2) In the 13C spectra the accompanying
deshielding of the pyrrolyl β-carbons (by 5.6−12.6 ppm) is also
much greater than that of the α-Cs, which vary very little in
chemical shift from 1, 2, and 8. (3) The quaternary Cp*
carbons are considerably deshielded also in the cations (by
3.4−7.9 ppm). The similarity of these changes in each pair
points to a similar electronic structure in solution and hence a
similar stabilization of the boron center in each case.
Comparison of the UV−vis spectra of the borenium cations
with their neutral borane precursors show that in all cases a
low-intensity band around 450 nm in the boranes is replaced by
a much more intense pair of bands (3 and 4) or a broad band
(9) in the same region. Again the similarity of the spectra for 3
and 4 is strongly suggestive of very similar frontier orbital
characters for these cations.
Table 2. Comparison of Structural Parameters for 1 (from X-
ray Crystal Structure)22 and of 1 and [PMAF-BH2]+ (from
a
BP86 Calculations)
1 (exptl)
1 (computed)
[PMAF-BH2]+
B−N
B−H
Fe−N
Fe−C′ipso
Fe···B
1.564
0.980
2.017
2.066
3.269
1.642
1.657
6.6
1.592
1.464
1.193
2.003
2.127
2.677
1.682
1.671
−23.7
99.9
1.224
2.050
2.075
3.338
Fe−Cp*(Ct)
Fe−Pyr(Ct)
1.649
1.666
b
α
6.8
Borenium Ion Formation, Electrophilicity, and Reac-
tivity. The outcome of the hydride abstraction reactions of 1
and 2 with trityl BF4− indicate that the borenium cations 3 and
B−N−Fe
N−B−H
H−B−H
Cp*(Ct)−Fe−Pyr(Ct)
N−B−H−H
131.4
109.5
109.4
176.9
120.0
132.4
106.7 (av)
112.1 (av)
179.6
116.9
126.0
172.9
6.1
−
4 are sufficiently electrophilic to abstract fluoride from the BF4
anion, as has been previously observed.26 In the case of 2, the
BF3 formed displaces the more bulky acceptor 9BBN-F from
PMAF, forming PMAF-BF3 and free 9BBN-F. In the case of 1,
the initially formed PMAF-BH2F may similarly undergo Lewis
acid exchange with BF3it is known that BF3 will
116.9 (av)
a
b
Distances are given in Å and angles and torsion angles in deg. α =
180° − [Pyr(Ct)−N−B] angle, where a positive value indicates
bending away from Fe and a negative value bending toward Fe.
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Figure 7. Selected DFT calculated molecular orbitals of 3.
quantitatively displace BH3 from amine−BH3 adducts.30 We
presume that the resultant free BH2F undergoes rapid
symmetrization in solution to form BF3 and BH3, which then
dimerizes to the observed B2H6.25 Similarly, when 8 reacts with
silver salts, we propose that the initially formed [PMAF-BCl2]+
abstracts fluoride from the anions as the first step of a more
complex decomposition process. Noncoordinating anions such
as [B(C6F5)4]− are therefore necessary to stabilize 3 and 4. We
see no evidence of 3 reacting with 1 to form hydride-bridged
species of the type [L-BH2-H-BH2-L]+ as observed by de Vries
and Vedejs upon low-temperature reaction of pyridine-BH3
with Tr+[B(C6F5)4]−.26 The borenium ion 3 is also stable in
CH2Cl2 solution at room temperature for at least 1 day: the
reaction of pyridine-BH3 with Tr+[B(C6F5)4]− under those
conditions yields only [pyridine-BCl2]+, as the putative species
[pyridine-BH2]+ reacts with the solvent.26 The enhanced
stability of 3 vs “[pyridine-BH2]+” reflects both the increased
steric protection of the borenium ion by the FeCp* moiety and
enhanced electronic stabilization.
abstraction from 2 and comparison of the NMR spectra. No
evidence of the other possible isomer was found in this
hydroboration experiment, in contrast to the cyclic hydro-
boration of 1,5-cyclooctadiene with BH3-THF, which has been
reported to give an isomer mixture of 9-borabicyclo[3.3.1]- and
[4.2.1]nonanes.35 That reaction requires thermal equilibration
to form exclusively the more stable [3.3.1] isomer. Previous
related studies have shown that borenium ion equivalents (in
the form of hydride-bridged dimers, [L-BH2-H-BH2-L]+, and
iodoborane adducts, L-BH2I) can also undergo alkene hydro-
boration.2c N-heterocyclic carbene−borane adducts activated
with catalytic amounts of triflimide also affect hydroboration,
and the active hydroborating agent is proposed to be the
borenium triflimide or a hydride-bridged dimer also.3 Our
example may be the first in which a “naked” borenium ion
effects hydroboration. (It is possible that 3 and related species
exist as weakly bound solvent adducts in CH2Cl2 solution,
which would require prior dissocation of the solvent before
alkene binding.) Interestingly, the closely related neutral
compound pyrrolyl−borane (C5H4N-BH2) is an active hydro-
borating agent36 and reducing agent:36a,37 in this case the
zwitterionic canonical form can be considered as a borenium
ion.38 We are continuing to survey the reactivity of 3 and
related borenium ions with respect to hydroboration and other
related reactions.
X-ray Crystal Structure Comparisons. The structures of
two neutral PMAF-borane adducts (1 and PMAF-BF3) have
previously been reported by us, and here we disclose two new
adducts, 2 and 8. The N−B bond length of 8 (1.554(2) Å) is
slightly shorter than that for 1 (1.580(3) Å) and PMAF-BF3
(1.587(2) Å), whereas that of 2 is somewhat longer (average
1.619(4) Å). The Fe−N distances follow this same trend. The
variations in both of these parameters reflect the greater Lewis
acidity of BCl3 and the greater steric bulk of 9-BBNH in
comparison to the previous cases.
The structure of boronium cation 6 may also be compared to
that of 1.22 The average B−N bond length slightly decreases
from 1.580(3) Å in 1 to 1.571(2) in 6, which is consistent with
the cationic nature of 6. Little difference is otherwise seen in
the structural parameters within the PMAF moieties in each
structure. Despite the bulk of the PMAF moieties, the N−B−N
angle in 6 of 105.9° is close to the ideal tetrahedral angle. The
boronium cation 7 has slightly longer N−B bond lengths than 6
(average of 1.600(9) Å), and the N−B−N angles are slightly
smaller (average of 102.7°). These differences may arise from
the increased steric bulk at boron due to the bicyclic alkyl
substituents in this case.
The Gutmann−Beckett 31P NMR measurements given
greater Δδ values for 3, 4, and 9 than for B(C6F5)3, for
which Δδ values of 26.631 and 26.3 ppm32 were recently
disclosed. It has been shown that the sole use of Δδ values for
comparison of cationic and neutral Lewis acids can be
misleading and that competition experiments give more reliable
results.4b The result of the competition experiment between 3
and B(C6F5)3 for Et3PO show that 3 is more Lewis acidic
than B(C6F5)3. In comparison with other cationic Lewis acids, 3
is therefore more acidic than Jakle’s ferrocenylborenium ion4b
̈
but less acidic than the borinium ion reported by Del Grosso et
al.33 and the cyclic BH-containing borenium ion reported by
Vedejs.2c This might indicate a Lewis acidity similar to that of
AlCl3.34 The similarity of the Δδ values obtained for both 3 and
9 suggests that the stabilization of the boron center by the
azaferrocenyl moiety is significantly more important than that
by chloride donors, presumably as the Cl 3p(π)−B 2p(π)
overlap is inefficient. It is worth noting that the Lewis acidity
intrinsically depends on the Lewis base utilized; thus, 9 is a
weaker chlorophile than AlCl3, whereas 3 and 4 are more
potent fluorophiles than BF3.9a Steric hindrance of the boron
center is clearly a major contributor to the behavior of 4, as
shown by its inability to form isolable and stable boronium
species with PMAF or N-methylimidazole without decom-
position.
Cyclic hydroboration of 3 with 1,5-cyclooctadiene at room
temperature in CH2Cl2 yields the borenium cation 4, which we
have confirmed by the independent synthesis of 4 via hydride
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Figure 8. View of the extended structure of 9 down the c axis of the unit cell, showing the intermolecular short contacts within layers (dotted lines).
A comparison of the structure of 9 to that of 8 points toward
the origin of the electronic changes associated with borenium
ion formation in this system. On abstraction of chloride the B
center undergoes a change in hybridization from sp3 to sp2: the
N−B−Cl and Cl−B−Cl angles in 9 are essentially 120°,
whereas in 8 the B atom has 93.9% tetrahedral character, as
defined by Hopfl’s parameter (utilizing the six bond angles
̈
around B).39 The N−B bond distance decreases from 1.554(2)
to 1.445 (8) Å, and the average B−Cl distance decreases from
1.849(2) to 1.723(4) Å, consistent with an increase in Cl
3p(π)−B 2p(π) overlap at the sp2-hybridized B center. In
addition to the hybridization change, the BCl2 moiety bends
somewhat toward the Fe atom, whereas in 8 the BCl3 group
points away from the Fe. In 8, the B atom lies 0.325 Å above
the plane of the Pyr ring (away from the FeCp* moiety), as
indicated by a change of the Pyr(Ct)−N−B angle (α, dip
angle) of +11.9° from linear (where Ct = centroid). In contrast,
in 9, the B atom sits 0.202 Å below the plane of the Pyr ring
(toward the FeCp* moiety) as also shown by a dip angle of
−9.6° from linear. The Fe···B distance thus contracts from
3.319 to 2.905 Å. There is a slight decrease in the Fe−N
distance upon chloride abstraction from 2.0141(11) to
1.995(5) Å; however, the Fe−Pyr(centroid) distance is
essentially unchanged, whereas the Fe−Cp*(Ct) distance
increases slightly from 8 to 9 (1.658 to 1.666 Å). In a related
crystallographically characterized neutral species, Cp*Fe-
(C5H4BBr2), a dip angle of −12.3° was reported.40
In the structure of 9, an intricate network of short (less than
the sum of van der Waals radii) intermolecular contacts are
observed between the tetrachloroaluminate anions and the
borenium cations. Layers of cations and anions run parallel to
the crystallographic a axis with the Cp−Fe−Pyr vectors
pointing approximately parallel to the c axis (see Figure 8):
each independent cation and anion sits astride the crystallo-
graphic mirror plane which runs perpendicular to these layers.
One cation (containing Fe1) shows disorder of the BCl2 moiety
about this plane. Contacts from Cl atoms of the anions to
pyrrolyl β-Hs and Cp* methyl groups exist within these layers
(varying between 2.897 and 2.946 Å), whereas Cl···B contacts
bridge an anion in one layer to a cation in the other (see Figure
9). The B···Cl contacts (3.331 and 3.513 Å) clearly act to
stabilize the borenium cation pointing toward the formally
unoccupied B(2p) orbital in each case. Each independent anion
forms a different network of short contacts. Anion 1
Figure 9. View of the extended structure of 9 down the b axis of the
unit cell, showing the intermolecular short contacts within and
between layers (dotted lines).
(containing Al1) bridges three cations in the same layer
through interactions with the pyrrolyl β-Hs (of cation
containing Fe2) and a Cp* methyl group of the cation
(containing Fe1): the fourth Cl then contacts the B of the Fe2
cation in the adjacent layer. Anion 2 (containing Al2) bridges
two cations in the same layer. Two Cl atoms contact both
pyrrolyl β-Hs of the Fe2-containing cationthese same Cl
atoms also interact with the disordered BCl2 group of the Fe1-
containing cation in an adjacent layer. The third Cl contacts a
methyl group of a second Fe2-containing cation, whereas the
fourth Cl shows no close interactions. Solid-state IR spectra
show a strong, broad absorption between 3100 and 2900 cm−1,
which we assign to these C−H bonds: in neutral 8, the C−H
bands are extremely weak. It is likely that these B···Cl and Cl···
H contacts persist to a degree in solution, contributing to the
surprisingly low 11B NMR shift of the cation and perhaps also
explaining the strong deshielding of the pyrrolyl ring proton
resonances. The noninnocent nature of [AlCl4]− and related
anions in the solid state and solutions of NHC-stabilized
dichloroborenium cations has recently been reported.13
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Electronic Structure Calculations of [PMAF-BH2]+. In
order to better address the structural change due to hydride
abstraction from PMAF-BH3, the optimized geometries of 1
and 3 were compared. The structural parameters of 1 and 3 are
summarized in Table 2. The dip angle (α) of 1 was reproduced
very well from computations. Other computed geometric
parameters were also consistent with experimental values,
except for the B−H bond length, as the hydrogens were
modeled using a riding atom method with a B−H bond of
0.980 Å in the X-ray crystal structure: The calculated value is
1.224 Å, which is consistent with the experimental B−H
distance in H3N-BH3.41 In addition, the computed B−H bond
length in 3 also agrees with the B−H distance in H2N-BH2.42
There is excellent agreement between the calculated parameters
in 1 and the experimental parameters as shown in Table 2. The
calculated structure of [PMAF-BH2]+ (3) shows some
deviations in geometry similar to those of its neutral-borane
precursor, 1, as 9 does to 8. The hybridization change at B leads
to a shortening of the N−B bond distance by 0.128 Å
(comparing computed structures) and a bending of the B
toward the Fe center with a dip angle of −23.7°. For
comparison, the dip angle calculated for the theoretical
compound ferrocenylborane (FcBH2) is −25.6°20 and in two
different calculations of FcBMe2 dip angles of −13.4 and
−13.2° were found.20,43
localized on the Pyr ring, with the partial charge on N being
−0.565. The positive charge is distributed across Fe (0.645),
the BH2 group (0.311), and the Cp* ring (0.202). The partial
charge on the B atom is 0.349, indicating the electrophilic
nature of this site. Inspection of the LUMO reveals a primarily
antibonding interaction of the N−B π bond with Fe and Cp*
orbitals.
CONCLUSIONS
■
Azaferrocenes, and specifically PMAF, are effective at stabilizing
exocyclic borenium ions [PMAF-BX2]+, where X = H, alkyl, Cl.
DFT calculations reveal that the electronic stabilization in
[PMAF-BH2]+ is similar to that of the stabilization in
ferrocenylborane, in that two delocalized interactions involving
the Fe d orbitals, the Cp* and Pyr rings, and the N−B (π)
bond are primarily responsible. This stabilization is sufficient to
allow solution characterization of an [L-BH2]+ species, whose
pyridine analogue reacts with CH2Cl2 solvent under the same
conditions, further illustrating the electronic role of the FeCp*
moiety in these compounds. The Lewis acidity of these
borenium ions has been gauged by the Gutmann−Beckett
NMR method, allowing them to be benchmarked against other
common Lewis acids, and also by their reactivity. [PMAF-
BX2]+ species, where X = H, alkyl, are sufficiently electrophilic
to abstract fluoride from the tetrafluoroborate anion, and
[PMAF-BH2]+ undergoes dihydroboration. We are currently
investigating the potential of such species for further useful
reactivity and will report these results in due course.
An analysis of the frontier bonding MOs reveals six MOs
between the HOMO and HOMO-12 that show participation of
the exocylic BH2 fragment. Of these orbitals, HOMO-2,
HOMO-4, and HOMO-10 are symmetric (a′) with respect to
the mirror plane, with HOMO-10 approximating the N−B σ
bond. The two remaining a′ MO’s (HOMO-2 and HOMO-4)
show participation of the N−B π system: i.e., the involvement
of the formally unoccupied px orbital on B. (HOMO-5,
HOMO-7, and HOMO-9 are antisymmetric (a″ symmetry)
and represent B−H bonding orbitals). HOMO-2 a′ reveals a
delocalized interaction that principally involves a bonding
EXPERIMENTAL DETAILS
■
General Considerations. All reactions were performed under a
dry nitrogen atmosphere in oven-dried glassware using standard
Schlenk techniques or in a VAC Omni inert-atmosphere glovebox.
Tetrahydrofuran, toluene, and diethyl ether solvents were dried using a
VAC solvent purification system, whereas hexanes and dichloro-
methane were distilled from calcium hydride and stored under
nitrogen before use. Solvents for NMR spectroscopy were distilled
from calcium hydride (CD2Cl2) or potassium (C6D6) and stored
under nitrogen. NMR spectra were obtained using a JEOL ECS-400
MHz spectrometer at room temperature unless otherwise stated.
Chemical shifts are reported in ppm and referenced via residual
solvent resonances to Me4Si (1H and 13C) or externally referenced
using BF3-OEt2 (11B). IR spectra were collected on a Nicolet 360
FTIR as KBr disks. Elemental analyses were performed by QTI
Intertek, Whitehouse, NJ. 1′,2′,3′,4′,5′-Pentamethylazaferrocene
(PMAF) and PMAF-BH3 (1) were prepared by literature
procedures.22 Trityl salts (tetrakis(pentafluorophenylborate) and
tetrafluoroborate) were supplied by Strem Chemicals, and 1,5-
cyclooctadiene was supplied by Aldrich. AlCl3 (Aldrich) was purified
by sublimation and stored under nitrogen before use.
2
2
contribution of Fe(dx −y ), the α-C Pyr orbitals, and the N−B
(π) system. The HOMO-4 a′ orbital principally involves the
bonding interactions of Fe(dxy) with the N−B (π) system and
Cp* ring orbitals including methyl group contributions. Both of
these MOs reveal that stabilization of the B occurs through
quite delocalized interactions involving mainly orbitals on the
Pyr ring (HOMO-2) and the Cp* ring (HOMO-4) mediated
by Fe d orbitals of appropriate symmetry. The HOMO and
HOMO-3 of [PMAF-BH2]+ are a″ orbitals composed of
Fe(dyz) and d(xz) bonding interaction with both rings,
2
respectively. The HOMO-1 a′ orbital involves similar Fe(dz )
ring bonding interactions. The overall picture is quite similar to
that of FcBH2, particularly in the nature and ordering of the
MOs HOMO through HOMO-4.20 In that study it was
concluded that bending of the exocylic B toward Fe resulted in
significant stabilization of the a′ frontier MO’s, most
importantly HOMO-1 and the two MO’s involving the N−B
π system (HOMO-2 and HOMO-4), with only moderate
destabilization of the HOMO. We suggest that a similar
stabilization occurs in [PMAF-BH2]+, resulting in the
significant bending calculated.
Preparation of PMAF-9BBNH (2). A solution of 9-
borabicyclo[3.3.1]nonane dimer (200 mg, 0.819 mmol) in CH2Cl2
(2 mL) was added to a solution of PMAF (422 mg, 1.64 mmol) in
CH2Cl2 (2 mL) and the mixture stirred overnight. The solvent was
then removed in vacuo to leave a fine orange solid, which was washed
with cold hexane and dried in vacuo. This material was shown to be
pure by 1H NMR spectroscopy. Yield: 610 mg (1.61 mmol, 98%). The
solid was dissolved in toluene (5 mL), the solution was filtered, and
hexane (8 mL) was added. Storage of the solution at −35 °C for 1 day
yielded dark orange crystals suitable for single-crystal X-ray diffraction
and elemental analysis. A second crop was obtained from the
supernatant by addition of more hexane (4 mL) and recooling to −35
°C.
In order to further understand the Lewis acidity of [PMAF-
BH2]+, we performed natural bond orbital (NBO) analysis of
this molecule at the BP86/6-31G(d) level on the optimized
geometry obtained at the BP86/LANL2DZ(Fe) + TZVP(C, H,
N, and B) level. The full charge distribution is shown in the
Supporting Information. It indicates that the negative charge is
1H NMR (C6D6): δ 5.09 (s, 2H, Pyr), 3.58 (s, 2H, Pyr), 2.64 (m,
2H), 2.34 (m, 4H), 2.11 (m, 1H), 2.03 (m, 2H), 1.67 (overlapping m,
3H), 1.61 (s, 15H, Cp*), 1.18 (apparent s, 2H). 13C{1H} NMR
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(C6D6): δ 87.7 (Pyr), 82.8 (Cp* quat), 74.2 (Pyr), 36.2 (CH2), 30.6
(CH2), 26.6 (CH2), 25.8 (CH2), 25.7 (br, B-CH), 10.29 (Cp* CH3).
11B NMR (C6D6): δ −5.5 (d, J = 91). IR (ATR): ν 2910, 2880, 2840,
2315, 2280, 2245, 1455, 1383, 1380, 1200, 1190, 1025, 915, 910, 820,
690 cm−1. Anal. Calcd for C22H34NBFe: C, 69.64; H, 9.04; N, 3.69.
Found: C, 68.92; H, 9.22; N, 3.58.
Preparation of PMAF-BCl3 (8). BCl3 (0.97 mL, 1.0 M solution in
CH2Cl2) was added to a stirred solution of PMAF (250 mg, 0.97
mmol) in CH2Cl2 (3 mL). After 30 min, the crude product was
precipitated by the dropwise addition of a large excess of hexane to the
stirring solution, and then the solution was stored at −35 °C to ensure
complete precipitation. The supernatant was then decanted and the
orange product dried in vacuo, and this product was shown by NMR
to be pure enough for use. Yield: 360 mg (0.97 mmol, 99%).
Recrystallization of the product from a concentrated CH2Cl2 solution
layered with hexane at −35 °C yielded analytically pure material and
crystals suitable for single-crystal X-ray diffraction.
BCH). 11B{1H} NMR (CD2Cl2): δ −0.7 (br s, boronium), −17.7
(BArF anion).
General Protocol for Formation and Observation of
Borenium Ions 3 and 4. In a typical procedure, solutions of 1 or
2 (20−25 mg) and the Tr+ salt (1 equiv) were mixed in CH2Cl2 or
CD2Cl2 (total volume 1 mL) under nitrogen at −35 °C for 15 min
before transfer to an NMR tube, which was then sealed and monitored
by 11B NMR (and 19F NMR) at room temperature. On mixing the
solutions turned immediately from orange to dark red. Solutions for
UV−vis spectroscopy were prepared in a similar way.
Formation of 4 from 3. A solution of 1 (20.2 mg, 0.0745 mmol)
and [CPh3]+[B(C6F5)4]− (70 mg, 0.0758 mmol) were mixed in
CD2Cl2 (total volume 1.5 mL) under nitrogen at −35 °C for 15 min
before transfer to an NMR tube, which was then sealed with a rubber
septum. 11B NMR spectroscopy confirmed the formation of 3, as
indicated by a new peak at 40 ppm. 1,5-Cyclooctadiene (9.20 μL,
0.0750 mmol) was then added by syringe into the NMR tube and the
reaction monitored by 1H and 11B NMR spectroscopy. Complete
conversion of 3 to 4 was observed after 24 h by the formation of a new
11B peak at 68 ppm.
1H NMR (CD2Cl2): δ 5.64 (s, 2H, Pyr), 4.70 (s, 2H, Pyr), 1.93 (s,
15H, Cp*). 13C{1H} NMR (CD2Cl2): δ 85.78 (Pyr), 85.29 (Cp*
quat), 78.6 (Pyr), 10.9 (Cp* CH3). 11B NMR (CD2Cl2): 5.3 (s). IR
(ATR): ν 3130, 3100, 2990, 2905, 1490, 1460, 1425, 1390, 1202, 1060,
1150, 1145, 815, 795, 766, 715, 695 cm−1. Anal. Calcd for
C14H19NBCl3Fe: C, 44.88; H, 5.11; N, 3.74. Found: C, 45.05; H,
5.09; N, 3.61.
Preparation of [PMAF-BCl2]+[AlCl4]− (9). PMAF-BCl3 (60 mg,
0.16 mmol) and AlCl3 (21 mg, 0.16 mmol) were mixed in CH2Cl2 (3
mL) and stirred until all the AlCl3 dissolved to give a dark red solution.
Excess hexane (10 mL) was layered on top of the solution, which was
stored at −35 °C overnight. The supernatant was then decanted,
leaving dark red crystals (81 mg, 99% yield), which were dried in
vacuo. Single crystals suitable for X-ray diffraction were grown by the
same method.
Preparation of [(PMAF)2BH2]+[B(C6F5)4]− (6). 1 (51 mg, 0.19
mmol) and PMAF (50 mg, 0.19 mmol) were dissolved in CH2Cl2 (1
mL) and precooled to −35 °C. A solution of [CPh3]+[B(C6F5)4]−
(175 mg, 0.189 mmol) in CH2Cl2 (1 mL), also precooled to −35 °C,
was added and the solution stored at −35 °C for 20 min before stirring
at room temperature for 15 min. Hexane (10 mL) was added dropwise
until formation of a red precipitate was complete. The colorless
solution was then decanted, and the precipitate was dried in vacuo.
The red-orange solid was then redissolved in CH2Cl2 (1 mL), filtered,
and layered with excess hexane (10 mL). After 2 days at room
temperature, analytically pure red-orange crystals (146 mg, 62% yield)
suitable for single-crystal X-ray diffraction formed in a colorless
solution.
1H NMR (CD2Cl2): δ 6.03 (s, 2H, Pyr), 5.70 (s, 2H, Pyr), 1.87 (s,
15H, Cp*). 13C{1H} NMR (CD2Cl2): δ 90.1 (Cp* quat), 86.13 (Pyr),
86.06 (Pyr), 10.5 (Cp* CH3). 11B NMR (CD2Cl2): δ 39 (s). IR
(ATR): ν 3120−2900 (br), 1640, 1570, 1540, 1520, 1385, 1340, 1260,
1185, 1080, 1040, 1010, 980, 815, 795, 670 cm−1. Anal. Calcd for
C14H19NAlBCl6Fe: C, 33.11; H, 3.77; N, 2.76. Found: C, 32.13; H,
3.59; N, 2.17.
Lewis Acidity Measurements. Solutions of borenium cations 3,
4, and 9 were prepared as described above in CH2Cl2 or CD2Cl2 and
mixed with 1 equiv of Et3PO (in the case of cation 4 excess Et3PO (2−
3 equiv) was necessary to give a resolvable 31P NMR signal for the
adduct). The solution was placed in an NMR tube containing an insert
containing a solution of Et3PO in CD2Cl2, the tube was sealed, and
31P{1H} and 11B{1H} NMR spectra were obtained. Δδ values were
1H NMR (CD2Cl2): δ 4.96 (s, 2H, Pyr), 4.47 (s, 2H, Pyr), 2.55 (v
br, w1/2h = 235 Hz, −BH2−), 1.90 (s, 15H, Cp*). 13C{1H} NMR
(CD2Cl2): δ (cation only) 86.75 (Pyr), 84.95 (Cp* quat), 77.44 (Pyr),
10.37 (Cp* CH3). 11B{1H} NMR (CD2Cl2): −7.6 (br s, −BH2−),
−17.7 (s, BArF). IR (KBr disk): ν 2978, 2962, 2916, 2866, 2461, 2429,
1644, 1515, 1464, 1378, 1276, 1134, 1086, 1059, 979, 775, 756, 684,
661, 480 cm−1. Anal. Calcd for C52H40N2B2F20Fe2·CH2Cl2: C, 49.27;
H, 3.28; N, 2.17. Found: C, 49.12; H, 3.11; N, 2.13 (in agreement with
the X-ray crystal structure, which shows one molecule of CH2Cl2 per
ion pair).
obtained by comparing the 31P chemical shift of the adduct formed to
that of Et3PO in the insert solution. Attempts to perform competition
experiments in which in situ generated 3 and 9 were mixed with
equimolar B(C6F5)3 and Et3PO gave several additional unidentified
impurity peaks by 31P NMR. Therefore, a competition experiment
among 3, B(C6F5)3, and Et3PO was performed by mixing premade
[PMAF-BH2-OPEt3]+[B(C6F5)4]− (3-OPEt3, 1 equiv; for full NMR
characterization see the Supporting Information) and B(C6F5)3 (1
equiv) and monitoring the 31P{1H} and 11B{1H} NMR spectra. Under
these conditions the solutions were stable for 1−2 h and one minor
impurity peak (∼8% of total integration) was observed at 96 ppm in
the 31P{1H} NMR spectrum.
Details of X-ray Data Collection, Structure Determination,
and Refinement. In the crystal structure of 2, there are two
independent molecules in the asymmetric unit which differ primarily in
the Fe−N−B−H dihedral angle. The asymmetric unit of 5 contains
two independent molecules. The structure of 6 contains a molecule of
CH2Cl2 per ion pair (in agreement with analysis data), which lies on a
crystallographic inversion center. This molecule showed extreme
disorder and was therefore rendered using the SQUEEZE routine. The
asymmetric unit of 7 contains four independent ion pairs and one
molecule of CH2Cl2. The crystal displayed nonmerohedral twinning
with approximately 5% of the minor component, as the monoclinic β
angle is very close to 90°. In the structure of 9, two independent ion
pairs were found in the asymmetric unit. Both cation/ion pairs lie on a
crystallographic mirror plane and one of the cations showed positional
disorder of the pyrrolyl-BCl2 moiety between two orientations about
Preparation of [(N-methylimidazole)2B(C8H14)]+[B(C6F5)4]−
(7). 2 (20 mg, 0.053 mmol) was dissolved in CH2Cl2 (1 mL) and
the solution precooled to −35 °C. A solution of [CPh3]+[B(C6F5)4]−
(49 mg, 0.053 mmol) in CH2Cl2 (1 mL), also precooled to −35 °C,
was then added to the solution of 2, and the mixture was warmed to
room temperature before transfer to an NMR tube which was sealed
with a rubber septum. 11B NMR showed complete conversion to 4. N-
Methylimidazole (4.4 μL, 0.055 mmol) was added via syringe and the
solution again monitored by11B NMRin addition to peaks at 68 (4)
and −18 ppm (anion) a small peak at around 0 ppm was observed. An
additional portion of N-methylimidazole (4.2 μL, 0.053 mmol) was
then added, leading to complete loss of the peak at 68 ppm and
formation of a peak at −0.8 ppm in the 11B spectrum. Excess hexane
was added to the solution, causing a brown oil to form, which was
isolated by decantation and dried in vacuo. This oil was redissolved in
CH2Cl2 and cooled to −35 °C. Over a period of 2 weeks small
colorless crystals formed which were suitable for single-crystal X-ray
diffraction. Yield: 15 mg (0.016 mmol, 30%).
1H NMR (CD2Cl2): δ 7.86 (s, 2H, Im), 7.15 (s, 2H, Im), 7.03 (s,
2H, Im), 3.85(s, 6H, N-CH3), 1.90 (m, 2H), 1.79 (m, 4H), 1.50 (m,
4H), 1.36 (m, 4H). 13C{1H} NMR (CD2Cl2): δ 135.3 (Im), 122.95
(Im), 122.92 (Im), 35.6 (CH2), 30.2 (N-CH3), 23.4 (CH2), 21.0 (br,
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Organometallics
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this plane. Complete details of data collection and refinement of all
structures is given in the Supporting Information.
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DFT Calculations. DFT calculations were performed to character-
ize the structure of [PMAF-BH2]+ (3). Scheibitz et al.20 reported that
the combination of the BP86 method and TZVP basis set along with
the Ahlrich TZVP-J Coulomb fit basis worked well in predicting the
geometries of borylated ferrocenes, including ferrocene-BH2 (FcBH2).
In this work, we used their reported computed structure of FcBH2 and
our X-ray structure of [PMAF-BCl2]+ (9) as references and evaluated
several other basis sets (see the Supporting Information). Our
computations were carried out using the Gaussian 09 package,28 and
we only performed geometry optimizations with the BP86 method.
We found that the combination of the LANL2DZ basis set for Fe and
the TZVP basis set for C, H, N, and B performed well in reproducing
the structures of FcBH2 and [PMAF-BCl2]+ compared to the reference
structures. This combination of basis sets also performed well in
optimizing the geometry of PMAF-BH3 (1) and [PMAF-BH2]+ (3).
The X-ray crystal structure of PMAF-BH3 was used as the input
structure for geometry optimization of 1. The initial structures of 3
and 9 were generated from the X-ray crystal structures of PMAF-BH3
and PMAF-BCl3 with one hydride or chloride ion removed using
GaussView 5.0.44 The initial structure of FcBH2 was built with
GaussView 5.0 from scratch. The molecular orbitals of 3 were also
generated with GaussView 5.0.
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ASSOCIATED CONTENT
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■
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Figures, tables, and a CIF file giving complete details of X-ray
crystal structure data collection and refinements, character-
ization of 5, and complete details of DFT calculations including
comparisons of different basis sets for geometry optimizations
of borenium ions, complete pictures of frontier MO’s, and
Cartesian coordinates. This material is available free of charge
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AUTHOR INFORMATION
Corresponding Author
*T.J.B.: e-mail, tbrunker@towson.edu; tel, 410-704-3118.
Notes
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9364.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank both the American Chemical Society Petroleum
Research Fund (Undergraduate New Investigator grant) and
the NSF (MRI 0923051) for support of this research. T.J.B.
also thanks the Jess and Mildred Fisher College of Science and
Mathematics at Towson University for financial support of this
research.
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