Unsolvated Al(C6F5)3: Structural features and electronic interaction with ferrocene

Article abstract of DOI:10.1039/c5dt03895b

Alkyl/aryl ligand exchange between AlEt₃ and B(C₆F₅)₃ in hexanes enables the formation and isolation of the unsolvated Al(C₆F₅)₃ as a crystalline solid, the structure of which ha

Full text of DOI:10.1039/c5dt03895b

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Unsolvated Al(C6F5)3: structural features and
electronic interaction with ferrocene†
Cite this: DOI: 10.1039/c5dt03895b
Jiawei Chen and Eugene Y.-X. Chen*
Alkyl/aryl ligand exchange between AlEt3 and B(C6F5)3 in hexanes enables the formation and isolation of
the unsolvated Al(C6F5)3 as a crystalline solid, the structure of which has been determined by single-
crystal X-ray diffraction analysis. Instead of forming the anticipated Al⋯F contacts with the seemingly
more accessible meta- and para-F’s of –C6F5 groups, two Al(C6F5)3 molecules form a dimeric structure
with double Al⋯F interactions between the Al center of one molecule and the ortho-F atom of the –C6F5
group on the other molecule. This mode of interactions is apparently linked to the thermal and shock
sensitivity of the unsolvated Al(C6F5)3 in the solid state. To compare with the B(C6F5)3/ferrocene frustrated
Lewis pair system, the complexation between Al(C6F5)3 and ferrocene has also been studied, which
affords a stable adduct formed through the η1-coordination of Al to one of the CCp atoms, similar to the
alane–toluene or benzene complex.
Received 5th October 2015,
Accepted 29th October 2015
DOI: 10.1039/c5dt03895b
www.rsc.org/dalton
Recently we demonstrated that Al(C6F5)3, which is known to
form an isolable adduct with toluene or benzene,29 can also
Introduction
Installation of pentafluorophenyl (–C6F5) groups onto group 13 form an isolable complex with triethylsilane,25 a weaker donor
elements, typically B and Al,1–5 provides a powerful approach (base) than toluene; in sharp contrast, B(C6F5)3 is not capable
to construct highly Lewis acidic catalysts6–9 with considerable of forming a stable adduct with either weak basic donor for
stability. Such an enhancement in both acidity and stability is SC-XRD measurement.30 Interestingly, in this alane–silane
largely due to the attached strongly electron-withdrawing –C6F5 complex, a secondary Al⋯F contact was also present, which
groups that render a more electron deficient B or Al center and indicates that, even upon complexation with a donor such as
also offer steric protection and less polar element–carbon Et3SiH, Al(C6F5)3 tends to seek for additional stabilization
bonds around B or Al. For instance, the strongly Lewis acidic through Al⋯F interactions. On the other hand, although the
and chemically robust B(C6F5)3 serves as the most widely unsolvated Al(C6F5)3 has been synthesized by transmetalation
studied Lewis acid in the emerging frustrated Lewis pair (FLP) or alkyl/aryl ligand exchange between AlEt3 and B(C6F5)3 in
chemistry.10–12 Its heavier and less explored congener Al(C6F5)3 non-coordinating solvents such as hexanes,1–3 its solid state
was recognized to possess higher Lewis acidity13,14 and hence structure has never been obtained and analysed. Instead, the
also considered as a desired candidate in the FLP type chem- structural characterization of Al(C6F5)3 was performed through
istry in recent years.15–23 Although B(C6F5)3 was reported in the SC-XRD analysis of its adduct with toluene or benzene.29
2,24
literature to be a stronger Lewis acid than Al(C6F5)3
a large Notably, the bonding interaction in the alane·toluene adduct
number of observations from both experimental and compu- features η1-coordination at the para-position of toluene
tational studies indicate that Al(C6F5)3 is a stronger Lewis (Chart 1), which resembles the coordination behavior of the
acid.1,13,14,25–28 One possible reason for the Lewis acidity Et3Si+·toluene adduct.31,32 Other observations, such as silane
discrepancy could be the unique ability of Al(C6F5)3 to complexation and the related catalysis, also suggest that the
form stable adducts with even very weak donor molecules properties of Al(C6F5)3 are more comparable to those of Et3Si+,
(vide infra), while the structure of the uncomplexed, unsolvated
Al(C6F5)3 is currently unknown.
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-
1872, USA. E-mail: Eugene.Chen@Colostate.edu; Fax: (+1) 970-491-1801
†Electronic supplementary information (ESI) available: NMR spectra and X-ray
structural data. CCDC 1429349 and 1429350. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c5dt03895b
Chart 1 Known alane complexes with weak donors.
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and less comparable to those of B(C6F5)3.25 Given the high
level of interest and difficulty in pursuing the “free” Et3Si+
cation, owing to the high tendency of such silylium ions to
interact with its environments,33–35 such as their pendent
groups, coordinating solvents, and counterions, it would be of
great interest to investigate the structural features of its neutral
analogue, the unsolvated Al(C6F5)3.
Results and discussion
Isolation and characterization of unsolvated Al(C6F5)3
To isolate the unsolvated crystalline Al(C6F5)3, B(C6F5)3 was
suspended in hexanes and one equivalent of AlEt3 was added.
The gradual dissolution of B(C6F5)3 indicates the B/Al aryl/
alkyl ligand exchange36 between B and Al. After full dissolution
of B(C6F5)3, the clear solution mixture was capped tight and
left undisturbed for 2 days at room temperature in a glovebox.
Colourless crystals were developed, decanted, and washed with
hexanes. The resulting crystals had limited solubility in non-
coordinating solvents such as hexanes and cyclohexane. Never-
1
theless, the H and 19F NMR spectra of a saturated solution of
Al(C6F5)3 in C6D12 were recorded, indicating that no other com-
ponent was present except the alane (Fig. S1–3†). The 19F NMR
signals at δ −124.9, −146.7 and −157.6 ppm were downfield
shifted (for meta- and para-F’s), compared to those of the
silane– and toluene–alane complexes. Notably, these signals
are relatively broad, presumably due to the exchange between
the tricoordinate and tetracoordinate environments at the Al
center with the fluorine atoms of the –C6F5 group (vide infra).
To investigate the possible Al⋯F contacts in the solid state,
we performed SC-XRD analysis of the unsolvated alane. As
expected, there are strong Al⋯F interactions. But to our sur-
prise and delight, instead of forming Al⋯F contacts with the
Fig. 1 X-ray structure (50% thermal displacement) of [Al(C6F5)3]2.
Selected interatomic distances (Å) and angles (°): Al1⋯F16 1.962(1),
Al2⋯F1 1.983(5), F16–C20 1.423(2), F1–C2 1.447(5), Al1–C1 1.984(2),
Al2–C19 1.982(2), C1–C2 1.366(3), C19–C20 1.371(2). (A) The highlighted
double Al⋯F bridging; (B) the overall structure; (C) the highlighted
8-membered Al2C4F2 heterocycle.
meta- or para-fluorine atoms, which are considered to be less the Al center features intramolecular Al⋯F interactions with
crowded and more accessible, two of the alane molecules fluorine atoms on its pendent groups. The X-ray structure of
adopt a dimeric structure via the ortho-F bridge (Fig. 1). In PhF → Al[OC(CF3)3]3 revealed the Al⋯F distance to be 1.864(2) Å,
such geometry, the Al center of one molecule receives dative which is about 0.1 Å shorter than that observed for the un-
fluorine interaction from the other alane molecule, and in the solvated alane, while the CPh–F distance of 1.447(3) Å is nearly
meantime it also serves as the fluorine donor to the latter. identical for both Al complexes.
Such double Al⋯F interactions within the dimer afford a dis-
Previous reports on the unsolvated Al(C6F5)3 always empha-
torted boat shape 8-membered Al2C4F2 heterocycle consisting sized its thermal and shock sensitivity,1,14 as an explosion was
of Al, F, and two adjacent C atoms on the –C6F5 ring. This reported in an attempt to sublime the crude etherate, prepared
specific arrangement also brings two free –C6F5 groups (one from the reaction of AlCl3 and 3 equivalents of C6F5MgBr in
on each alane molecule) in close proximity to form a pi-stack- ether, or in an attempt to prepare the unsolvated Al(C6F5)3 by
ing interaction with the shortest C17 to –C6F5 plane distance heating a mixture of B(C6F5)3 and AlEt3 to 70 °C.38 This obser-
of 3.316(2) Å. In addition, the Al⋯F interaction is rather vation actually resembles the explosive nature of LiC6F5 at
strong, as reflected by short Al⋯F distances of 1.962(1) and above −20 °C, which was due to the exothermic elimination of
1.983(5) Å and the elongation of the involving C–F bonds LiF to form the tetrafluorobenzyne species.39 We speculate
(1.423(2) and 1.447(5) Å), when compared to an average of that the thermal and shock sensitivity of the unsolvated
other C–F bond lengths (ca. 1.340 Å). Overall, our observation Al(C6F5)3 might be due to a similar decomposition pathway,
is consistent with what Krossing et al. reported on a similar which is indeed indicative of the dimeric structure. A closer
Lewis acidic alane Al[OC(CF3)3]3.37 From their computational look at the X-ray structure reveals that the endocyclic (with
results, the fluoride ion affinities (FIA) of Al[OC(CF3)3]3 and respect to the Al2C4F2 ring) CvC bond lengths (1.366(3) and
Al(C6F5)3 are almost identical (537 and 530 KJ mol−1, respect- 1.371(2) Å) are shorter than those of the exocyclic ones, which
ively). According to the calculated geometry of Al[OC(CF3)3]3, gave us hints on the possible pathway for the rearrangement
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action came from the distorted tetrahedral geometry at the Al
center with a sum of the C–Al–C angles ΣC–Al–C = 338.63(6)° for
the Al(C6F5)3 unit. Interestingly, the attachment of Al also
renders the corresponding C(Cp) atom to be pyrimidalized,
which perturbs the aromaticity of the Cp ring. A large alterna-
tion of the bond distance of the complexed Cp was observed.
For the C1–C2–C3–C4–C5 ring, the C–C bond distances are
1.464(2), 1.414(2), 1.426(2), 1.418(2) and 1.465(2) Å. Last but
not least, the proton attached to the same C1 was refined
without constraint or restraint, which yields a short C1–H1 dis-
tance of 0.90(2) Å (the regular Ccp–H distance for other protons
using the riding mode is set at 1.000 Å). This proton carries a
partial positive charge, as it bends out of the Cp plane towards
the iron center, forming a Fe⋯Hδ+ distance of 2.56(2) Å. In
comparison, the alane–toluene complex features an Al–C
(toluene) distance of 2.366(2) Å and an angle of 96.1°, which
results in the elongation of the related C–H bond length, but
with only slight perturbation of the phenyl ring. In the alane–
ferrocene complex, we observed a shorter Al1–C1 contact of
2.214(2) Å and a larger angle of 99.44(6)°. As a result, this
interaction leads to quite considerable alternation of the Cp
ring and contraction of the C1–H1 distance due to the Fe⋯Hδ+
interaction. It is also worth noting that the stabilization and
shuffling of protons via the iron center have been proposed in
the metathesis involving 1,2-disubstituted stannylated ferro-
cene derivatives and boron halides, which usually yields a
mixture of 1,2-, 1,3- and 1,1′-products.41–45
Scheme 1 Possible thermal decomposition pathway of [Al(C6F5)3]2.
of the dimer towards elimination of [Al]F, in the form
of (C6F5)2AlF (Fig. S9†), and tetrafluorobenzyne shown in
Scheme 1.
Complexation of Al(C6F5)3 with ferrocene
Next, we examined the complexation between Al(C6F5)3 and
ferrocene (Cp2Fe). In their recent communication Agapie
et al.40 utilized B(C6F5)3 and ferrocene, which showed no inter-
action between them, as a FLP system for dioxygen reduction.
We envisioned that, since ferrocene is more electron rich than
benzene or toluene, it should form a stable and/or isolable
adduct with the alane. Indeed, upon mixing equimolar
Al(C6F5)3 and ferrocene, we observed an immediate change of
yellow colour of ferrocene to red. The downfield shift of the Cp
1H NMR signal from 4.05 to 4.24 ppm also indicates the inter-
action between Al(C6F5)3 and ferrocene, as well as the partial
removal of the electron density from the ferrocene as a result
of this interaction. Deep red crystals of the resulting complex
were obtained by laying hexanes onto a 1,2-dichlorobenzene
solution of this mixture. The X-ray structure reveals the 1 : 1
adduct formation, via the η1-coordination of the alane onto C1
of the Cp ring (Fig. 2A). The C1–Al1 distance is 2.214(2),
which is shorter than the value (2.366(2) Å) reported for the
Al(C6F5)3·toluene adduct.29 Further evidence of the strong inter-
To further confirm the conclusion that ferrocene is a stron-
ger donor than toluene toward the alane drawn from the
SC-XRD analysis, a separate NMR scale experiment was per-
formed. After dissolving an equimolar amount of Al(C6F5)3·
(toluene)0.5 adduct and ferrocene in C6D5Br, a similar red colour
1
solution was obtained. H NMR analysis indicates the replace-
ment of toluene by ferrocene according to the signal at
4.23 ppm. This solution was then heated at 100 °C for 3 days
1
to give a dark green solution (Fig. 2C) and the H NMR signal
Fig. 2 (A) X-ray structure (50% thermal displacement) of Cp2Fe·Al(C6F5)3.
Selected interatomic distances (Å) and angles (°): C1–H1 0.90(2),
Fe1⋯H1 2.56(2), Al1–C1 2.214(2), Al1–C11 1.985(2), Al1–C17 1.998(2),
Al1–C23 1.994(2), ΣC–Al–C 338.63(6), C1–C2 1.464(2), C2–C3 1.414(2),
C3–C4 1.426(2), C4–C5 1.418(2), C1–C5 1.465(2). Centcp–C1–H1
11.8(12), Centcp–C1–Al 99.44(6). (B) Color of ferrocene and Cp2Fe·Al(C6F5)3;
(C) after heating at 100 °C for 3 days.
Fig. 3 Overlay of UV-Vis spectra of Cp2Fe (yellow), Cp2Fe·Al(C6F5)3
(red) and a mixture of Cp2Fe and Al(C6F5)3 after being heated at 100 °C
for 3 days (green).
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Et2O, THF, and CH2Cl2) followed by passage through Q-5 sup-
ported copper catalyst (for toluene and hexanes) stainless steel
columns. Benzene-d6, bromobenzene-d5, and cyclohexane-d12
were dried over a sodium/potassium alloy and vacuum-distilled
or filtered, whereas CD2Cl2 and CDCl3 were distilled over CaH2
and then stored over activated Davison 4 Å molecular sieves.
NMR spectra were recorded on Varian Inova 400 MHz (FT
400 MHz, 1H; 100 MHz, 13C; 376 MHz, 19F) and 500 MHz spec-
trometers. Chemical shifts for all spectra were referenced to
internal solvent resonances and were reported in parts per
million relative to SiMe4, whereas 19F NMR spectra were refer-
enced to external CFCl3. EPR spectra were recorded on a
Bruker ESP-300 spectrometer. UV-Vis spectra were recorded on
a Varian Cary 500 UV-Vis-NIR spectrometer. Elemental ana-
lyses were performed by Robertson Microlit Laboratories,
Madison, NJ.
Triethylaluminum and ferrocene were purchased from
Sigma-Aldrich Chemical Co. and used as received. Tris(penta-
fluorophenyl)borane, B(C6F5)3, was obtained as a research gift
from Boulder Scientific Co. and further purified by double
sublimation under vacuum. Other commercial reagents were
purchased from Sigma-Aldrich and used as received. Literature
procedures were employed or modified for the preparation
Scheme 2 Proposed pathway for the oxidation of ferrocene through
electronic interaction with the alane.
of Cp became broader and also shifted to a downfield region
of 11.4 ppm (Fig. S7†), all of which are a typical indication of
gradual oxidation of ferrocene. The presence of the Cp2Fe+
cation was also evidenced by the emergence of a LMCT band
at around 630 nm from the UV-Vis absorption measurement
(Fig. 3). The 19F NMR spectrum (Fig. S8†) indicates the gene-
ration of a complex mixture of aluminum-based species, pre-
sumably due to the decomposition of the transient aluminum
based radical anion (Scheme 2). Indeed, the attempts on iso-
lation of the boron based radical anion also proved futile.46–48
of the following compounds or adducts: Al(C6F5)3 and
Conclusions
1–3
Al(C6F5)3(toluene)0.5
.
(Extra caution should be exercised when
In summary, we report the crystal structure of the unsolvated
Al(C6F5)3, which features the dimeric structure with the double
Al⋯F interactions between the Al and the ortho-F of the adja-
cent molecules. This finding indicates that, due to the high
FIA of the monomeric Al(C6F5)3, even in non-coordinating sol-
vents such as hexanes, it seeks for additional stabilization
through its pendent –C6F5 group. In practice, when comparing
the Lewis acidity of the alane, the dimeric structure should be
considered. In addition, the thermal and shock sensitivity of
the unsolvated Al(C6F5)3 might have originated from its
dimeric structure, which favors the exothermic elimination of
[Al]F and formation of tetrafluorobenzyne, in a fashion similar
to the LiC6F5 decomposition. The complexation between
Al(C6F5)3 and ferrocene results in the formation of the stable
adduct Cp2Fe·Al(C6F5)3, which is in contrast to B(C6F5)3. Elec-
tronic interaction between the alane and ferrocene rendering
oxidation of ferrocene also occurred in such a system, albeit
with the decomposition of the aluminum based radical anion.
handling these materials, especially the unsolvated alane, due to
its thermal and shock sensitivity!)
X-ray diffraction intensities were collected on a Bruker
SMART APEX CCD Diffractometer using CuKα (1.54178 Å) or
MoKα (0.71073 Å) radiation at 100 or 120 K. The structures
were solved by direct methods and refined using the Bruker
SHELXTL program library by full-matrix least squares on F2 for
all reflections.49 All non-hydrogen atoms were refined with ani-
sotropic displacement parameters, whereas hydrogen atoms
were included in the structure factor calculations at idealized
positions expect the H1 (at distorted position due to the attack
of Al) in adduct [Cp2Fe·Al(C6F5)3]. Hydrogen H1 was found on
the difference density map and refined without restraint
or constraint. Crystallographic data for the structures of
[Al(C6F5)3]2 (CCDC 1429349) and [Cp2Fe·Al(C6F5)3] (CCDC
1429350) have been deposited with the Cambridge Crystallo-
graphic Data Center as supplementary publications.
Isolation of the unsolvated Al(C6F5)3
In a glovebox, to a suspension of B(C6F5)3 (512 mg, 1.00 mmol)
in 10 mL of hexane was added neat AlEt3 (0.137 mL,
1.00 mmol, 1.00 equiv.) in a 20 mL vial. The mixture gradually
became clear after stirring for 10 min. The vial was capped
Experimental
Materials, reagents, and methods
All syntheses and manipulations of air- and moisture-sensitive tight and left undisturbed at room temperature for 2 days,
materials were carried out in flamed Schlenk-type glassware after which X-ray quality crystals were developed. The crystals
on a dual-manifold Schlenk line, on a high-vacuum line, or in were collected by filtration, washed with hexane (2 mL) twice,
an inert gas (Ar or N2)-filled glovebox. NMR-scale reactions and dried under vacuum, affording 400 mg of the product.
were conducted in Teflon-valve-sealed J. Young-type NMR Additional crop of 80 mg was obtained by reducing the mother
tubes. HPLC-grade organic solvents were first sparged exten- liquor to 2 mL followed by crystallization at −30 °C (yield:
sively with nitrogen during the filling of 20 L solvent reservoirs 480 mg, 91%). 19F NMR (C6D12, 25 °C): δ −124.9 (br, 6F, o-F),
and then dried by passage through activated alumina (for −146.7 (br, 3F, p-F), −157.6 (br, 6F, m-F) ppm. 19F NMR (C6D6,
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25 °C): δ −122.7 (d, 3J = 17.7 Hz, 6F, o-F), −150.6 (pst, 3J = 19.9
Hz, 3F, p-F), −160.5 (m, 6F, m-F) ppm.
8 H. Yamamoto, Lewis acids in organic synthesis, Wiley-VCH,
Weinheim, New York, Chichester, Brisbane, Singapore,
Toronto, 2000.
Isolation of Cp2Fe·Al(C6F5)3
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11 D. W. Stephan and G. Erker, Frustrated Lewis Pairs I & II,
Top. Curr. Chem., Springer-Verlag, Berlin, Germany, 2013.
12 D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010,
49, 46–76.
13 E. Y.-X. Chen, in Encyclopedia of Reagents for Organic
Synthesis, John Wiley & Sons, Ltd, 2012, DOI: 10.1002/
047084289X.rn01382.
14 E. Y.-X. Chen, W. J. Kruper, G. Roof and D. R. Wilson,
J. Am. Chem. Soc., 2001, 123, 745–746.
In a glovebox, ferrocene (186 mg, 1.00 mmol) and Al(C6F5)3
(528 mg, 1.00 mmol, 1.00 equiv.) were dissolved in 2 mL of
1,2-dichlorobenzene. The mixture was layered with 2 mL of
hexanes and placed in a freezer at −30 °C. X-ray quality crystals
were developed overnight. The crystals were collected by fil-
tration, washed with hexanes (2 mL) twice, dried under
vacuum, affording 600 mg of the product (yield: 84%).
1H NMR (C6D5Br, 25 °C): δ 4.24 (br, 10H, Cp). 13C NMR (C6D5Br,
25 °C): δ 149.8, 142.3, 136.9, 111.4 (C6F5), 70.1 (Cp). 19F NMR
(C6D5Br, 25 °C): δ −120.4 (d, 3J = 16.9 Hz, 6F, o-F), −149.3 (pst,
3J = 19.9 Hz, 3F, p-F), −159.4 (m, 6F, m-F) ppm. Elemental ana-
lysis for C28H10AlF15Fe: calcd C 47.09, H 1.41; found C 46.95,
H 1.49%.
15 G. Menard and D. W. Stephan, Dalton Trans., 2013, 42,
5447–5453.
16 G. Menard, T. M. Gilbert, J. A. Hatnean, A. Kraft,
I. Krossing and D. W. Stephan, Organometallics, 2013, 32,
4416–4422.
17 E. Y.-X. Chen, Top. Curr. Chem., 2013, 334, 239–260.
18 Y. Zhang, G. M. Miyake, M. G. John, L. Falivene,
L. Caporaso, L. Cavallo and E. Y.-X. Chen, Dalton Trans.,
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19 R. C. Neu, G. Menard and D. W. Stephan, Dalton Trans.,
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20 G. Menard and D. W. Stephan, Angew. Chem., Int. Ed., 2012,
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NMR scale reaction of Cp2Fe and Al(C6F5)3·(toluene)0.5
A Teflon-valve-sealed J. Young-type NMR tube was charged
with ferrocene (10.0 mg, 0.025 mmol), Al(C6F5)3·(toluene)0.5
(30.9 mg 0.025 mmol, 1.00 equiv.) and 0.7 mL of C6D5Br at
ambient temperature. The mixture was allowed to react for
10 min before NMR analysis, which indicates the adduct for-
mation between ferrocene and Al(C6F5)3. The NMR tube was
heated at 100 °C for 3 days. 1H and 19F NMR spectra were
recorded at different time intervals. The gradual color change
from red to dark green also indicates the oxidation of ferro-
cene, which was also confirmed by UV-Vis absorption study.
21 G. Menard and D. W. Stephan, Angew. Chem., Int. Ed., 2012,
51, 8272–8275.
22 Y. Zhang, G. M. Miyake and E. Y.-X. Chen, Angew. Chem.,
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25 J. Chen and E. Y.-X. Chen, Angew. Chem., Int. Ed., 2015, 54,
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Acknowledgements
This work was supported by the United States National Science
Foundation (CHE-1507702). We thank Dr Roger A. Lalancette
for the generous access to the SC-XRD facility at Rutgers-
Newark for determining the structure of the alane–ferrocene
adduct and Boulder Scientific Co. for the research gift of
B(C6F5)3.
26 J. Chen and E. Y.-X. Chen, Molecules, 2015, 20, 9575–9590.
27 A. Y. Timoshkin and G. Frenking, Organometallics, 2008,
27, 371–380.
28 A. Rodriguez-Delgado and E. Y.-X. Chen, J. Am. Chem. Soc.,
2005, 127, 961–974.
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