Kinetic hydricity of silane hydrides in the gas phase

Article abstract of DOI:10.1039/c9sc02118c

Herein, gas phase studies of the kinetic hydricity of a series of silane hydrides are described. An understanding of hydricity is important as hydride reactions figure largely in many processes, including organic synthesis, photoelectrocatalysis, and hydrogen activation. We find that hydricity trends in the gas phase differ from those in solution, revealing the effect of solvent. Calculations and further experiments, including H/D studies, were used to delve into the reactivity and structure of the reactants. These studies also represent a first step toward systematically understanding nucleophilicity and electrophilicity in the absence of solvent.

Full text of DOI:10.1039/c9sc02118c

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Kinetic hydricity of silane hydrides in the gas phase†
*
Jiahui Xu,
Allison E. Krajewski, Yijie Niu, G. S. M. Kiruba and Jeehiun K. Lee
Cite this: DOI: 10.1039/c9sc02118c
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Herein, gas phase studies of the kinetic hydricity of a series of silane hydrides are described. An
understanding of hydricity is important as hydride reactions figure largely in many processes, including
organic synthesis, photoelectrocatalysis, and hydrogen activation. We find that hydricity trends in the gas
phase differ from those in solution, revealing the effect of solvent. Calculations and further experiments,
including H/D studies, were used to delve into the reactivity and structure of the reactants. These studies
also represent a first step toward systematically understanding nucleophilicity and electrophilicity in the
absence of solvent.
Received 30th April 2019
Accepted 9th July 2019
DOI: 10.1039/c9sc02118c
rsc.li/chemical-science
“sN” is a nucleophile-specic slope parameter, “N” is
a nucleophilicity parameter and “E” is an electrophilicity
Introduction
parameter. k is a normalized rate constant for the nucleophile–
electrophile reactions. Mayr's studies of hydride donor ability
are one type of reaction examined, toward his developing
a general nucleophilicity–electrophilicity scale.12 As Mayr's work
has been done in the solution phase, solvent is a potentially
complicating factor.
To ll these major gaps in knowledge of general intrinsic
nucleophilicity and electrophilicity, as well as specically,
hydricity, we have studied, in the gas phase, the reactions of
a series of silane hydrides with substituted benzhydryl cations.
While there have been some prior studies of nucleophilicity and
electrophilicity in the gas phase, including a Mayr-like study on
the association of benzhydryl cations with amines, the system-
atic application to hydricity has not heretofore been conduct-
ed.13–26 Knowledge of the intrinsic kinetic hydricities of metal-
free hydrides will be useful for understanding the role of
solvent in reactivity. These results are of import both for further
developing the utility of hydrides for organic and catalytic
applications, as well as for generally understanding nucleo-
philicity and electrophilicity in the absence of solvent.
Hydride transfers play an important role in many areas of
chemistry.1 In organic chemistry, hydride transfer is a key step
in many invaluable reactions, including the Cannizzaro, the
Meerwein–Ponndorf–Verley, and the Tischenko reactions.
Furthermore, metal hydrides such as lithium aluminum
hydride and sodium borohydride are indispensable in synthetic
chemistry for reducing ketones and aldehydes to alcohols.2
Nicotinamide adenine dinucleotide phosphate (NADPH) and
avin adenine dinucleotide (FADH2) are important enzymatic
cofactors that are hydride donors for many biological
processes.3 Recent interest in metal-free hydride donors as
catalysts for hydrogen activation further solidies the impor-
tance of hydride transfer reactions.4–8
Thermodynamic hydricity is the standard Gibbs free energy
change for the dissociation of a hydride donor RH to R+ and Hꢀ.
These values are always positive and therefore, lower values
indicate a better hydride donor.6 Kinetic hydricity is usually
expressed in terms of a nucleophilicity factor N, which is an
empirical value obtained from rate constants for the reaction of
hydride donors with reference acceptors.6,9,10 Stronger hydride
donors have a higher N value.
The development of the nucleophilicity parameter N has
been accomplished largely by Mayr and coworkers, who have
demonstrated that the rates of reactions of electrophiles and
nucleophiles can be described by a linear-free energy relation-
ship (eqn (1)).9–11
Results
Experimental results: hydride abstraction from silanes
We have examined hydride abstraction from silanes by benz-
hydryl cations; the general reaction is shown in Fig. 1.
For this model to succeed in the gas phase, the reaction must
be exergonic. This poses some challenges in design since the
parent benzhydryl cation (R1 ¼ R2 ¼ Ph) is quite stable. At
log k ¼ sN(N + E) [at 20 ꢁC]
(1)
Department of Chemistry and Chemical Biology, Rutgers The State University of New
Jersey, New Brunswick, NJ 08901, USA. E-mail: jee.lee@rutgers.edu
† Electronic supplementary information (ESI) available: Cartesian coordinates for
all calculated species, full citations for references with greater than 16 authors,
and representative mass spectra are available. See DOI: 10.1039/c9sc02118c
Fig. 1 General hydride transfer reaction studied herein.
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Table 1 DG for reactions shown in Fig. 1 (R1, R2 ¼ Ph)a
Calculated DG for
reactions are also known to proceed in solution, which would
allow for direct comparison between gas and solution phase
reactivity.10
Silane substitution (Fig. 1)
reaction in Fig. 1 (kcal molꢀ1
)
Unfortunately, we found that although these reactions are
calculated to be slightly exergonic in the gas phase, we do not
observe reaction under our experimental conditions. We
hypothesize that the reactions may in reality be slightly ender-
gonic. The reactions may also be favored by solvation of the silyl
cation, which is not possible in vacuo.
R3
Me
iPr
R4
Me
iPr
R5
Ph
iPr
ꢀ1.6
ꢀ1.3
a
Calculations at B3LYP/6-31+G(d).
To drive these reactions further, we added electron with-
drawing groups to the benzhydryl cation. The reactions between
a series of uoride-substituted benzhydryl cations and silanes
shown in Fig. 2 were examined.
Briey, to conduct this experiment, we vaporize the alcohol
1-OH into our Fourier transform mass spectrometer (Fig. 3).
Reaction with hydronium ions should generate the desired
benzhydryl cation. We then allow the cation to react with the
silane hydrides. The rate constants for the reactions between 1
and the various silane hydrides are reported in Table 2.
Further interpretation of these data will follow, in the
“Discussion” section. We next describe additional studies
delving into reaction mechanisms.
Reaction details: reaction paths
Two concerns that arose in the course of our studies were
whether (i) the desired hydride transfer, and not a proton
transfer type reaction was taking place; and (ii) the structure of
the reactant cation was in fact a benzhydryl cation, and not
a tropylium.
Fig. 2 Benzhydryl cations and silanes studied herein.
i. Hydride transfer versus proton abstraction. One concern
of ours was that the silane hydride might abstract a proton from
the benzhydryl cation to form a carbene (Path B, Fig. 4), instead
of the desired hydride transfer (Path A, Fig. 4). Calculations on
various benzhydryl cations and silanes indicate that while
hydride transfer (Path A) is very exergonic, proton abstraction
(Path B) is extremely (>35 kcal molꢀ1) endergonic, so should not
be a competing pathway.
To lend further insight into which pathway we see, we
compared the rate constant for reaction of the deuterated per-
uorobenzhydryl cation 1a-D with that of the undeuterated
compound 1a. As shown in Fig. 4, if hydride transfer occurs
(Path A), the carbon with the H(D) should undergo a hybridiza-
tion change from sp2 to sp3. This should yield a secondary KIE
of less than 1. If the H(D) is abstracted (Path B), then the KIE
should be normal and primary, a value greater than 1.
Comparison of the rate constants for the reaction of cation
1a versus that of the deuterated 1a-D (Table 2) indicate an
Fig. 3 Generation of benzhydryl cations and subsequent reaction with
silane hydrides.
B3LYP/6-31+G(d), the benzhydryl cation has a calculated hydride
affinity (DG) of 245 kcal molꢀ1, while a silyl cation with methyl
groups (R3 ¼ R4 ¼ R5 ¼ methyl) has a computed hydride affinity
of 252 kcal molꢀ1. The reaction between these two species would
therefore be endergonic. Although our experiments measure
kinetic, not thermodynamic, hydricity, our gas phase conditions
are such that we do not generally observe endergonic reactions
(more details in Experimental section).13,27–31
We rst allowed Me2PhSiH and iPr3SiH to react with the
parent benzhydryl cation, as calculations (Table 1) indicated
that these reactions should be exergonic in the gas phase. These
Table 2 Gas phase rate constants k (ꢂ10ꢀ10 cm3 per moleculeꢀ1 per sꢀ1) for the reaction of 1 with silanes 2–5
1a-D
1a
1b
1c
1d
1e
2
3
4
5
5.62 ꢃ 0.17
3.31 ꢃ 0.11
3.30 ꢃ 0.27
0.49 ꢃ 0.07
4.13 ꢃ 0.26
2.82 ꢃ 0.12
2.60 ꢃ 0.19
0.42 ꢃ 0.11
3.47 ꢃ 0.22
2.09 ꢃ 0.04
1.84 ꢃ 0.09
0.14 ꢃ 0.08
3.16 ꢃ 0.27
1.90 ꢃ 0.11
1.16 ꢃ 0.20
0.13 ꢃ 0.01
2.76 ꢃ 0.17
1.78 ꢃ 0.05
1.27 ꢃ 0.11
0.08 ꢃ 0.02
1.69 ꢃ 0.45
0.81 ꢃ 0.06
0.57 ꢃ 0.09
Too slow
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Fig. 4 Hydride abstraction versus proton abstraction.
Fig. 5 Benzhydryl cation versus tropylium structures.
inverse secondary KIE of ꢄ0.8; we therefore believe hydride
abstraction is indeed taking place.
ii. Benzhydryl cation versus tropylium. The second concern
is whether, in the mass spectrometer, we are generating the
benzhydryl or the tropylium cation (Fig. 5).
We rst calculated the free energy change associated with
the isomerization of the benzhydryl cation to the tropylium
(Table 3). In all cases, the isomerization is endergonic (albeit by
just 0.5 kcal molꢀ1 for 1b).
Fig. 6 Characteristic benzyl cation reaction with toluene in the gas
phase.
We also sought to determine structure experimentally,
through reactions with toluene. It is known that the benzyl
cation reacts with toluene in the gas phase, while the tropylium
remains unreactive (Fig. 6).32–36
toluene is not the same as that of benzyl cations with toluene.
We conducted calculations on the parent benzhydryl cation
plus toluene reaction, to ascertain the barriers to reaction.
Calculations indicate that the barrier to nucleophilic attack of
the benzhydryl cation by toluene is relatively higher, by roughly
20 kcal molꢀ1, than that of the benzyl cation with toluene.
Unlike benzyl cation, the benzhydryl cation has a quite sterically
hindered center, which might slow down any reaction. Thus,
the lack of reaction between toluene and the cations derived
from benzhydrol would seem inconclusive; we cannot say that
we do not have the benzhydryl cation structure as the reaction
with toluene may simply be unfavored.
While benzyl cation and tropylium display the divergent
reactivity shown in Fig. 6, to our knowledge, such reactions have
not been studied with benzhydryl cations. As a control, we
examined the reaction of benzyl cation with toluene under our
+
conditions, and we do produce the expected CH3C6H4CH2
cation. However, when we allow the cations derived from the
parent benzhydrol to react with toluene, we observe no reaction.
We also tried the cations derived from the peruorobenzhydrol;
these also show no reaction with toluene.
Overall, we do have reason to believe that our cations are
benzhydryl in structure. First, the generation method should
favor the benzhydryl cation, not the tropylium. We produce the
cation from the corresponding benzhydryl alcohol, via proton-
ation. Sen Sharma and Kebarle established that abstraction of
chloride from benzyl chloride in the gas phase yields the benzyl
cation, not the tropylium.33 This reaction is similar to our
generation method, which should favor the benzhydryl cation
structure. Tropylium is usually produced by direct electron
impact, which we are not using.34–41 Furthermore, both
computational and experimental evidence indicates that under
thermoneutral conditions, rearrangement from benzyl cation to
tropylium is unlikely.33,34,41–43
These results could either mean that we have tropylium
structures, or that the reactivity of benzhydryl cations with
Table 3 DG for the rearrangement: benzhydryl cation 1 / tropylium
(Fig. 5)a,b
Free energy to
form tropylium
Free energy to form alternate
tropylium structure, if relevant
Cation
1a
1b
1c
1d
1e
+1.1
+0.5
+2.1
+1.9
+2.4
+0.6
+2.1
+4.2
+4.2
Second, in work done for an orthogonal project, we nd that
our measured gas phase acidity of peruorobenzhydryl cation
a
b
All values are in kcal molꢀ1
.
Calculations at B3LYP/6-31+G(d).
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1a-D is the same, whether we generate the cation in the gas nucleophilic silane hydride is Et3SiH (4), while the least nucle-
phase or from solution.44 That is, if we generate 1a-D as ophilic species is iPr3SiH (3). In the gas phase, iPr3SiH (3) is
described herein, or from solution (via electrospray), we nd more nucleophilic than Et3SiH (4). The higher nucleophilicity of
that both species have the same acidity. Our calculations indi- iPr3SiH in the gas phase could be due to the stability that the
cate that tropylium should be about 15 kcal molꢀ1 more acidic product iPr3Si+ cation gains from the polarizable isopropyl
than benzhydryl cation. Therefore, the similar acidity for the groups. In solution, those bulky isopropyl groups may inhibit
two differentially generated cations implies both have the same solvation of iPr3Si+ relative to Et3Si+, which results in the
structure.44 Electrospray ionization (ESI) is widely accepted as reversal of the hydricity of the parent neutral compounds. This
a gentle method which captures the ions that already exist in is reminiscent of the well-known reversal of acidity of methanol
solution.45 Since in solution, these ions are believed to be versus tert-butanol in solution versus the gas phase.46 In order to
benzhydryl cations, it would stand to reason that the ions lend insight into this possible explanation, we calculated the
brought into the mass spectrometer by ESI would also be free energy of solvation in methylene chloride for the silyl
benzhydryl in structure.10
cations. We nd that the triethylsilyl cation (generated from 4)
We should also note that ultimately, while the evidence has a DGsolvation of ꢀ52 kcal molꢀ1, while the triisopropyl cation
taken together points to a benzhydryl cationic structure, the (generated from 3) has a DGsolvation of ꢀ49 kcal molꢀ1. There-
identity of the structure does not strictly affect the results fore, the triethylsilyl cation is slightly better solvated, reecting
herein. The rate constants are used to ascertain the silane the observed trend. The solvation model does not include
hydride nucleophilicities, and if the cations are of varying specic solvation, which would be expected to further enhance
structure, it will not affect those nucleophilicity trends.
the differences between the two cations.47
Another possible reason for the differences between the gas
phase and solution silane nucleophilicity, suggested by
a referee, is that the dichloromethane solvent may form weak
Cl–Si interactions, such that a CH2Cl2–SiR3H species might be
the true hydride donor in solution.
Discussion
With rate constants (Table 2) in hand, we looked to Mayr eqn
(1), to ascertain the kinetic hydricity, or nucleophilicity, of the
silanes. At this early point in our research, rather than derive
any specic parameter values, we instead made a “Mayr-type”
graph, plotting the log of the rate constant for the reaction
between the series of nucleophiles and electrophiles (y axis)
versus the log of the rate constant of the electrophiles with 3 as
our reference nucleophile (x axis) (Fig. 7).
The results show that the trend for silane nucleophilicity in
the gas phase is 5 < 4 < 3 < 2. The trend in solution (methylene
chloride) is 3 < 5 < 2 < 4.10 This change in trend between the gas
phase and solution is of interest, since such changes usually
reveal the role of solvent. One possible reason for the differ-
ences between the gas phase and the solution phase could be
solvation of the silyl cation product. In solution, the most
Hydricity is of importance for understanding reactions in
which silane hydrides serve as hydride sources. One intriguing
example is the use of silane hydrides to reduce N-heterocyclic
carbene (NHC)-activated carbon dioxide.7,8 Utilization of
carbon dioxide has become of paramount importance, given the
rising carbon dioxide concentration in our atmosphere. Radian,
Zhang and Ying found that CO2 activated by NHCs (to form an
imidazolium carboxylate) will react with silane hydrides to form
methanol.7,8 In terms of substrates that overlap with ours, they
found that hydride transfer by Me2PhSiH (2) is more effective
than Et3SiH (4). We nd this of interest, as our gas phase
nucleophilicity trend is consistent with this result, while the
solvent nucleophilicity trend is not. That is, our gas phase
results on hydricity may be useful for predictive power in
applications utilizing silane hydrides.
Ultimately, the kinetic hydricity of the studied silanes are
different in the gas phase and in solution. Such results show the
importance of gas phase studies, to establish how solvent is
affecting our understanding of reactivity.
In terms of electrophilicity, the gas phase trend for the
benzhydryl cations is 1e < 1d < 1c < 1b < 1a, with 1a being most
electrophilic. Mayr and coworkers have examined 1b and 1c in
solution; the electrophilicity trend for these cations is the same
as that in the gas phase. This is consistent with the Mayr
interpretation that electrophile solvation energies change pro-
portionally with electrophilic reactivities; we see this pro-
portionality in the absence of solvent. Furthermore, a survey of
other uorinated benzhydryl cations studied by Mayr indicates
that uorine substitution on the 3-position increases electro-
philicity the most; F substitution on the 5 position also
enhances electrophilicity, but not as much as on the 3-position;
and F substitution on the 4-position tends to have a negative
Fig. 7 Plot of benzhydryl cation and silane data. The y-axis represents
the log of the rate constant for the reaction of each electrophile with
each nucleophile. The x-axis is the log of the rate constant of each
electrophile with 3.
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effect on electrophilicity.12 These trends are consistent with that previously.27,28,51,54,55 The experiments are run under pseudo-
which we see in the gas phase.11,12,25,48
rst-order conditions with the neutral silane hydride reactant
In terms of absolute rate constants, the reactions are faster in excess, relative to the reactant cations. Reading the pressure
in the gas phase than in solution. For the reaction of cation 1b of the neutral compounds from the ion gauges is not always
with Et3SiH 4, the solution phase rate constant is 6.04 ꢂ 107 accurate; therefore, we “back out” the neutral substrate pres-
Mꢀ1 sꢀ1. In the gas phase, this reaction rate constant is 1.11 ꢂ sure from fast control reactions (described previously).27,55–59
1011 Mꢀ1 sꢀ1. For the reaction of cation 1c with 4, the solution Under our conditions, the ionic species are generated, then
phase rate constant is 2.51 ꢂ 107 Mꢀ1 sꢀ1, while the gas phase k argon gas is pulsed in, so that the ions are cooled. Thus, only
is 0.70 ꢂ 1011 Mꢀ1 sꢀ1. Thus, for these reactions, the rates are thermoneutral and exergonic reactions are observable.13,60
roughly 2000 times faster in the gas phase.
All calculations were performed using density functional
This work indicates the importance of examining properties theory (B3LYP/6-31+G(d)) as implemented in Gaussian 09 and
in the gas phase; comparison of intrinsic reactivity with solu- 16.61–66 All ground state geometries were fully optimized and
tion phase behavior can ferret out the details of media effects. frequencies were calculated; no scaling factor was applied. The
optimized structures had no negative frequencies. The transi-
tion structures corresponding to the reaction of toluene with
Conclusions
benzyl cation, and toluene with benzhydryl cation, correspond
This work represents the rst attempt toward developing a gas to partially optimized structures in which the distance between
phase hydricity scale. Kinetic hydricity is of import both for carbocation (benzyl/benzhydryl) and carbon atom para to the
general understanding of nucleophilicity and electrophilicity, methyl carbon of toluene was constrained to 2.0 A. The partially
as well as for improved design of hydrides for energy applica- optimized transition structure resulted in the one imaginary
tions. Our studies indicate that the abstraction of hydride from frequency corresponding to the bond formation between the
a series of silane hydrides follows a trend that differs from that carbocation (benzyl/benzhydryl) and toluene. The temperature
in solution, revealing the effect of solvent. Our work also delves for the calculations was set to be 298 K. Calculations in meth-
into the reactivity and structure of our cations, using H/D ylene chloride were also conducted at B3LYP/6-31+G(d), using
studies, calculations, and further experimentation. Having a polarized continuum model; specically, we utilized the SMD
established that hydricity can be studied as described herein, variation of IEFPCM of Truhlar and co-workers.67
we are moving toward expanding the cations and silane
hydrides studied. Overall, the ability to independently investi-
gate electrophilicity and nucleophilicity in the gas phase would
Conflicts of interest
allow us to ascertain the solvent effects by direct comparison to There are no conicts of interest to declare.
solution phase electrophile and nucleophile data. Such studies
would be a breakthrough in the understanding of organic
reactivity, because many of the phenomena observed in
Acknowledgements
electrophile-nucleophile studies in solution cannot be We thank the NSF and the NSF Extreme Science and Engi-
adequately interpreted.49
neering Discovery Environment (XSEDE) for support. We are
also grateful to Professors Herbert Mayr and Steven Kass for
incredibly insightful discussions.
Experimental
All of the benzhydryl cation precursors (the benzhydryl alco-
hols) were commercially available, except for the deuterated
substrate described herein, which was synthesized following
literature procedure.50
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