Deprotonation of ferraborane clusters and their relative acidities

Article abstract of DOI:10.1021/ic000818l

The relative Bronsted acidities for a series of ferraborane clusters were determined. Relative acidities were studied by proton competition reactions between neutral and anionic species followed by IR spectroscopy. From these studies the relative trend in acidities was determined to be B₂H₆Fe₂(CO)₉ < HFe₃(CO)₉BH₄ = HFe₄(CO)₁₂BH₂ < HFe₃(CO)₁₀BH₂.

Full text of DOI:10.1021/ic000818l

1972
Inorg. Chem. 2001, 40, 1972-1974
Notes
Deprotonation of Ferraborane Clusters and Their
Relative Acidities
Deborah A. McCarthy,* Heather E. Durie,
Jennifer J. Embry, Lesley A. Imbur, and
Katherine A. Marhoefer
Department of Chemistry and Physics, Saint Mary’s College,
Notre Dame, Indiana 46556-5001
ReceiVed July 21, 2000
Introduction
The Brønsted acidities of transition metal hydrides and metal
carbonyl hydrides is of general interest because of their
importance in a variety of preparative reactions and theoretical
studies.1 The relative acidities of metal carbonyl cluster
Figure 1. Molecular structures of (a) HFe
BH , (c) HFe (CO)10BH , and (d) B Fe
4
(CO)12BH
2 3 9
, (b) HFe (CO) -
4
3
2
H
2 6
2
(CO)
6
.
,2
were distilled before use. Solvents for chromatographic separations were
stirred over molecular sieves and deoxygenated with N . The reactants
Fe(CO) , Fe (CO) , and BH S(CH [10 M] were purchased from
Aldrich and used as received. CF COOH and N(C were alsopur-
3
4,5
hydrides and those of borane clusters have been related to
the structural arrangements and the relative sizes of the cluster
cores. The only reported comparisons of Brønsted acidities in
2
5
2
9
3
3 2
)
3
2 5 3
H )
6
metallaborane clusters found that the Brønsted acidity of B2H5-
chased from Aldrich and deoxygenated with nitrogen before use.
Chromatography was performed under nitrogen at reduced temperature
on 60-200 mesh dried silica gel (Baker). Infrared spectra were recorded
on a Nicolet 205 FT-IR spectrometer. 11B NMR spectra were obtained
FeCo(CO)9 is less than that of B2H6Fe2(CO)9, contrary to
prediction based on the relative acidities of isoelectronic
monometallic analogues. The authors concluded that the number
of endo-bridging hydrogens in B2H6Fe2(CO)9 must have a
greater effect on the protic nature of a hydrogen atom than the
more subtle change in metal identities. We continue this work
using proton competition reactions between neutral and anionic
cluster pairs to determine the relative Brønsted acidities of four
on a Bruker 300 MHz spectrometer [external reference: NEt
4
B
3
H
8
(δ
(a),7 HFe
c
(CO)
BH
(b), HFe
7b
)
-29.7 ppm)]. HFe
4
(CO)12BH
Fe (CO)
2
3
9
4
3
(CO)10-
(c),7d and B
H
6
(d) were prepared using published
7a
BH
2
2
2
6
methods and standard glovebag and Schlenk line techniques.
Deprotonation of B Fe (CO) , HFe (CO) BH , HFe (CO)12BH
and HFe (CO)10BH . [HNEt ][B Fe (CO) ] (d′) was prepared in a
50 mL Schlenk flask from ca. 10 mL of concentrated hexane solution
of B Fe (CO) (d) by adding NEt slowly by drops from a syringe
H
2 6
2
6
3
9
4
4
2
,
3
2
3
H
2 5
2
6
7c
closely related metalloboranes, HFe4(CO)12BH2 (a), HFe3(CO)9-
2
BH4 (b),7b HFe3(CO)10BH2 (c), and B2H6Fe2(CO)6 (d),
7d
7a
H
2 6
2
6
3
(Figure 1).
with stirring. The bright-yellow color diminished immediately, forming
a brown precipitate that was washed with fresh hexanes and was
evaporated to dryness, leaving solid d′ in the flask:
Experimental Section
Under a nitrogen atmosphere, the reagent solvents tetrahydrofuran,
acetonitrile, diethyl ether, toluene, and hexane were stirred over
benzophenone ketyl, methanol was stirred over calcium chloride, and
dichloromethane was stirred over molecular sieves. All these solvents
B H Fe (CO) (d) + NEt f [HNEt ][B H Fe (CO) ] (d′) (1)
2
6
2
6
3
3
2
5
2
6
In similar reactions, deprotonation of HFe
BH (b), and HFe (CO)10BH (c) produced [HNEt
a′), [HNEt ][HFe (CO) BH ] (b′), and [HNEt ][Fe
respectively.
Proton Competition Reactions: Reaction of HFe
[HNEt ][B Fe (CO) ] and of B Fe (CO) with [HNEt
CO) BH ]. A dilute hexane solution (ca.10 mL) of HFe (CO)
b) was added to a flask containing a large excess of solid [HNEt ][B
Fe (CO) ] (d′). After an initial infrared spectrum of the hexane solution
4
(CO)12BH
2
3 9
(a), HFe (CO) -
4
3
2
3
][HFe
4
(CO)12BH]
] (c′),
(
3
3
9
3
3
3
(CO)10BH
2
(
1) (a) Moore, E. S.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc.
986, 108, 2257. (b) Pearson, R. G. Chem. ReV. 1985, 85, 41. (c)
Walker, H. W.; Pearson, R. G.; Ford, P. C. J. Am. Chem. Soc. 1983,
05, 1179.
1
3
(CO)
9
BH
][HFe
BH
4
with
1
2
H
H
-
3
5
2
6
2
6
2
6
3
3
(
2) (a) Schunn, R. A. Transition Metal Hydrides; Marcel Dekker: New
York, 1971. (b) Kristj a` nsd o` ttir, S. S.; Norton, J. R. Transition Metal
Hydrides; VCH Publishers: New York, 1992.
(
(
9
3
3
9
4
3
2 5
H -
(3) Kristj a` nsd o` ttir, S. S.; Moody, A. E.; Weberg, R. T.; Norton, J. R.
Organometallics 1988, 7, 1983.
2
6
was obtained, the mixture was stirred. Infrared spectra were taken of
aliquots (0.1 mL) at 10 min intervals until the spectral characteristics
Table 1) indicated that (1) proton transfer had occurred, (2) reactions
(
(
4) Parry, R. W.; Edwards, L. J. J. Am. Chem. Soc. 1959, 81, 3554.
5) (a) Johnson, H. D.; Shore, S. G.; Mock, N. L.; Carter, J. C. J. Am.
Chem. Soc. 1969, 91, 2131. (b) Remmel, R. J.; Johnson, H. D.;
Jaworiwsky, I. S.; Shore, S. G. 1975, 97, 5395.
(
other than proton transfer had occurred, or (3) no reaction had occurred
after 2-3 h. The hexane phase was removed by cannula, and the anion
phase was washed with hexane, evaporated, dissolved in diethyl ether,
(6) (a) Jun, C. S.; Fehlner, T. P. Organometallics 1994, 13, 2145. (b)
Jun, C. S.; Bandyopadhyay, A. K.; Fehlner, T. P Inorg. Chem. 1996,
11
3
5, 2189.
and analyzed by B NMR spectroscopy (Table 2). The proton
competition reaction of the complementary pair d and b′ was done in
a similar manner. By use of similar reaction conditions, proton
competition reactions of pairs of neutral and anionic clusters of d with
a and b with a were studied.
(
7) (a) Jacobsen, G. B.; Andersen, E. L.; Housecroft, C. E.; Hong, F.-E.;
Buhl, M. L.; Long, G. J.; Fehlner, T. P. Inorg. Chem. 1987, 26, 4040.
(
1
b) Vites, J.; Eigenbrot, C.; Fehlner, T. P. J. Am. Chem. Soc. 1984,
06, 4633. (c) Vites, J. C.; Housecroft, C. E.; Eigenbrot, C.; Buhl, M.
L.; Long, G. J.; Fehlner, T. P. J. Am. Chem. Soc. 1986, 108, 3304. (d)
Wong, K. S.; Scheidt, W. R.; Fehlner, T. P. J. Am. Chem. Soc. 1982,
Reactions between HFe
4 2 3 9 4
(CO)12BH (a) and HFe (CO) BH (b)
104, 1111.
Studied by 11B NMR Spectroscopy. A concentrated diethyl ether
1
0.1021/ic000818l CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/14/2001

Notes
Inorganic Chemistry, Vol. 40, No. 8, 2001 1973
solution (ca. 4 mL) of [HFe
4
(CO)12BH]- (a′) was cannula-transferred
Table 1. Neutral Ferraborane Infrared Spectral Bands (νCO, cm-1
)
to an NMR tube attached to a Schlenk flask and evaporated to dryness.
A concentrated hexane solution (ca. 4 mL) of HFe (CO) BH (b) was
transferred via cannula into the NMR tube, which was then sealed.
NMR spectra were obtained on the mixture over ca. 4 h, collecting
000 scans for each spectrum. In a similar manner b′ was reacted with
HFe
4
(CO)12BH
a)
2
HFe
3
(CO)
(b)
9
BH
4
HFe
3
(CO)10BH
(c)
2
2 6 2 6
B H Fe (CO)
3
9
4
(
(d)
11
B
a
a
2
095 m
2106 w
2054 vs
2041 sh
2031 m
2090 s
2
059 s
2058 vs
3
a
1
1
2047 vs
2
2046 vs
a, and the reaction was followed by B NMR spectroscopy.
a
034 s
2038 vs
2
2
026 vs
017 m sh
2026 s
2022 s
Results
2
022 m
When a was mixed with b′,
2
013 m
2010 m
1995 m
1990 s
1
-
2008 m sh
1984 m
HFe (CO) BH (a) + [HFe (CO) BH ] (b′) f
4
12
2
3
9
3
1996 s
-
1988 m
[
HFe (CO) BH] (a′) + other products (2)
4 12
a
868 m
a
the characteristic infrared signals for a disappeared over time.
After 2 h only very low-intensity bands attributed to Fe3(CO)12
and no bands corresponding to the presence of b were observed
in the hexane-phase infrared spectrum. The presence of a′ in
the products was indicated by observation of its characteristic
signal in the B NMR spectrum of the solid phase. When b
was mixed with a′, the infrared spectrum did not change over
a 3 h period. The 11B NMR spectrum of the solid phase indicated
the presence of a′ and c′.8
Bands used for compound identification.
Table 2. Ferraborane 11B NMR Spectral Data (ppm)
-
HFe4(CO)12BH2 (a) 106 (t)
HFe3(CO)9BH4 (b) 1.5 (m)
HFe3(CO)10BH2 (c) 56.5 (d) (20 °C) Fe3(CO)10BH2 (c′) 57.4 (br, d)
HFe4(CO)12BH (a′) 149 (br)
HFe3(CO)9BH3 (b′) 6.2 (br, q)
-
-
1
1
-
B2H6Fe2(CO)6 (d)
-24 (q)
B2H5Fe2(CO)6 (d′) -17.4 (br)
clusters were mixed (d with a′), no changes were observed in
the infrared spectra.
We found HFe3(CO)10BH2 difficult to prepare and keep in
quantities sufficient for this study. We never were able to
perform direct proton competition reactions under the conditions
we desired.
When b was reacted with a′ in an NMR tube,
-
HFe (CO) BH (b) + [HFe (CO) BH] (a′) f
3
9
4
4
12
-
HFe (CO) BH (a) + [HFe (CO) BH ] (b′) (3)
4
12
2
3
9
3
Discussion
a very small signal characteristic of a was observed growing in
over a 3 h time period. An 11B NMR spectrum of a diethyl
ether solution of the solid phase indicated the presence of both
the original a′ and small amounts of the anion b′. In attempts
to react the opposing pair of clusters, when a was mixed with
b′ in an NMR tube, only the signal attributed to a was observed
in the B NMR spectra obtained after 3 h. The B NMR
spectrum of a diethyl ether solution of the solid phase contained
only the signal attributed to the original reactant b′.
From the results of the reaction shown in eq 4, it is clear that
-
HFe (CO) BH (b) transferred a proton to [B H Fe (CO) ] (d′).
3
9
4
2
5
2
6
No proton transfer occurred when the opposing pair of clusters,
-
[HFe (CO) BH ] (b′) and B H Fe (CO) (d), were mixed. We
3
9
3
2
6
2
6
conclude that b is more acidic than d under these reaction
conditions. Likewise, proton transfer from reaction of HFe (CO) -
1
1
11
4
12
-
BH (a) with [B H Fe (CO) ] (d′) [eq 5] and no proton transfer
2
2
5
2
6
-
from mixing B H Fe (CO) (d) with [HFe (CO) BH] (a′)
2
6
2
6
4
12
Infrared spectra taken of samples of b mixed with d′,
indicate that a is a stronger Brønsted acid than d under these
conditions.
-
HFe (CO) BH (b) + [B H Fe (CO) ] (d′) f
The reactions involving HFe4(CO)12BH2 (a) with [HFe3-
3
9
4
2
5
2
6
-
-
-
(CO)9BH3] (b′) and HFe3(CO)9BH4 (b) with [HFe4(CO)12BH]
a′) are less straightforward and appear to be very dependent
[
HFe (CO) BH ] (b′) + B H Fe (CO) (d) (4)
3 9 3 2 6 2 6
(
on the relative physical states and the concentrations of the two
reacting species. The reactions were studied under different
conditions of relative concentration and were monitored by two
different methods. In the reactions monitored by infrared
spectroscopy, the anion was present in large excess. When the
-
1
afforded a new band at 2092 cm due to d while the band at
2
-
1
097 cm due to b was diminished. When d was reacted with
b′, the infrared spectrum remained unchanged over a 2 h time
period.
Mixtures of a with d′,
11
reactions were monitored by B NMR spectroscopy, the clusters
were present in approximately equimolar amounts. Under
conditions of excess anion, b′ deprotonated a but without
concomitant protonation of b′ to form b (eq 2). Under equimolar
reaction conditions b′ did not deprotonate a. Under conditions
of excess anion, a′ did not react with b. But when present in
approximately equimolar amounts, a′ did deprotonate b to a
small extent, producing very small amounts of both a and b′
(eq 3). It is clear that each anionic cluster is capable of
deprotonating the neutral cluster in certain circumstances. It
appears that the basicity of the anions in the solid state drives
these reactions. We never observe that a is capable of proto-
nating b′, but we do observe b protonating a′. From these results
it appears that a may be slightly less acidic than b. Without
further studies we can only conclude that the relative Brønsted
acidities of the clusters HFe4(CO)12BH2 (a) and HFe3(CO)9-
BH4 (b) are nearly identical under these conditions.
-
HFe (CO) BH (a) + [B H Fe (CO) ] (d′) f
4
12
2
2
5
2
6
-
[
HFe (CO) BH] (a′) + B H Fe (CO) (d) (5)
4 12 2 6 2 6
showed the presence of d by the growth of a new band in the
infrared spectrum at 2092 cm as the band due to a at 2059
cm simultaneously diminished. The final infrared spectrum
contained only peaks due to d. When the opposite pair of
-
1
-
1
(
8) Clusters a, c, and d are formed in the same synthesis and separated
using column chromatography on silica. While most of compound c
produced in these reactions is deprotonated during the chromatographic
separation, residual amounts are often detected by NMR spectroscopy
in solutions of a as a result of incomplete separation. We have noted
the presence of c in very small quantities in solutions of a due to
incomplete separation under the conditions employed. Thus, it is likely
that the c′ we find in products of reactions involving a′ is the result
of its presence in the initial solution when a was deprotonated.

1974 Inorganic Chemistry, Vol. 40, No. 8, 2001
Notes
As was noted in the Experimental Section, the placement of
The monoanion of HFe3(CO)10BH2 is formed by loss of the
M-H-M bridging hydrogen. Monoanions of every other
reported metallaborane cluster including those in this study,
HFe4(CO)12BH2, HFe3(CO)9BH4, and B2H6Fe2(CO)6, occur
through loss of the B-H-M bridging hydrogen. The competi-
tion for electron density between the M-H-M bridging
hydrogen and the bridging CO group in the cluster core may
have the strongest influence on the relative acidity of HFe3(CO)10-
BH2 compared to either HFe4(CO)12BH2 or HFe3(CO)9BH4.
It is not clear what most influences the relative acidities of
clusters HFe4(CO)12BH2 and HFe3(CO)9BH4. The earlier com-
parison of the dinuclear clusters attributed the greater acidity
of B2H6Fe2(CO)9 compared to B2H5FeCo(CO)9 to be the result
HFe3(CO)10BH2 (c) in the series could not be determined using
proton competition reactions. We place it as the most acidic of
all the clusters as the result of two observations. First, in the
-
attempted reaction of HFe3(CO)9BH4 (b) with [HFe4(CO)12BH]
-
(
a′) we find [Fe3(CO)10BH2] (c′) is also present in the products,
8
indicating that c′ is not protonated by b. Second, since c is
largely deprotonated to c′ under chromatographic separation
conditions on basic silica where a and d remain protonated, it
is reasonable to conclude that c is the most acidic of these three
clusters.
On the basis of the all these results, we propose that the
relative order of acidity for the four ferraboranes studied is
6
of the larger number of endo-bridging hydrogens. In our studies
the greater number of endo-bridging hydrogens in trinuclear
HFe3(CO)9BH4 (b) compared to HFe4(CO)12BH2 (a) may be
mitigating the greater acidity of the tetranuclear cluster under
certain conditions. As always, reaction conditions dictate which
compounds are found as products in these proton competition
reactions. We were interested in studying these compounds
under the same conditions used for the previously reported study
of the isoelectronic metalloboranes B2H5FeCo(CO)9 and B2H6-
Fe2(CO)9.
B H Fe (CO) (d) < HFe (CO) BH (a) )
2
6
2
6
4
12
2
HFe (CO) BH (b) < HFe (CO) BH (c)
3
9
4
3
10
2
3
Quantitative studies of metal carbonyl cluster hydrides and
6
qualitative studies of boranes have demonstrated that their
acidities can be related to the size and structure of the metal or
boron core, respectively. For example H2Os3(CO)12 was found
to be 3 orders of magnitude less acidic than H4Os4(CO)12. For
3
borane clusters the relative Brønsted acidity also increases with
size. This relationship does not strictly hold with these four
Acknowledgment. This work was done in collaboration with
Professor Thomas P. Fehlner at University of Notre Dame, Notre
Dame, IN. We thank him greatly for his financial support, the
use of his equipment and chemicals, and his wise counsel
throughout these studies.
5
ferraboranes. The tetranuclear cluster, HFe4(CO)12BH2, is
definitely less acidic than at least one of the trinuclear clusters,
HFe3(CO)10BH2, and it may be equally or slightly less acidic
than the other trinuclear cluster, HFe3(CO)9BH4. The smallest
cluster, B2H6Fe2(CO)6, is the least acidic.
IC000818L