Reversible Coordination of H2 by a Distannyne

Article abstract of DOI:10.1021/jacs.7b11798

The terphenyl tin(II) hydride [AriPr₄Sn(μ-H)]₂ (1) (AriPr₄ = C₆H₃-2,6(C₆H₃-2,6-ⁱPr₂)₂) was shown to form an equilibrium with the distannyne AriPr₄SnSnAriPr₄ (2) and H₂ in toluene at 80 °C. The equilibrium constant and Gibbs free energy for the dissociation of H₂ are 2.23 × 10^-4 ± 4.9% and 5.89 kcal/mol ± 0.68%, respectively, by ¹H NMR spectroscopy and 2.33 × 10^-4 ± 6.2% and 5.86 kcal/mol ± 0.73%, respectively, by UV-vis spectroscopy, indicating that the hydride 1 is strongly favored. Further heating of 2 at ca. 100 °C afforded the known pentagonal-bipyramidal Sn₇ cluster Sn₅(SnAriPr₄)₂ (3). Mechanistic studies show that 3 is formed from distannyne 2, which is generated from 1. The order of the reaction for the conversion of 2 into 3 was found to be zero, and the rate constant is 1.77 × 10^-5 M s^-1 at 100 °C. Hydride 1 was further characterized by cyclic voltammetry, and its pK_a was found to be 18.8(2) via titration with 1,8-diazabicyclo[5.4.0]undec-7-ene. The bond dissociation free energy was estimated to be 51.1 kcal/mol ± 3.4% on the basis of its pK_a and reduction potential. Studies with deuterium indicate ready exchange of D₂ with the hydrides in 1.

Full text of DOI:10.1021/jacs.7b11798

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Communication
Reversible Coordination of H2 by a Distannyne
Shuai Wang, Tobias J Sherbow, Louise A Berben, and Philip P. Power
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11798 • Publication Date (Web): 22 Dec 2017
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Reversible Coordination of H2 by a Distannyne
Shuai Wang†, Tobias J. Sherbow†, Louise A. Berben†, and Philip P. Power†*
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† Department of Chemistry, University of California Davis, 1 Shield Avenue, Davis, CA 95616, United States.
Supporting Information Placeholder
relatively few instances where reversible absorption
and release of hydrogen has been effected under
mild conditions for main group compounds.26-30
Two examples involve use of metal free frustrated
Lewis pair systems,26-29 and anti-aromatic boron
containing organic rings.30 In addition, several main
group compounds have been shown to react with H2
under ambient conditions to yield hydrides.31-36 The-
se include low valent group 13 and 14 element com-
pounds, which feature frontier orbitals with small
energy separations and suitable symmetry to react
with H2,37 and whose reactivity can mimic that of
transition metal complexes.38 In 2008, it was shown
that low valent dimeric tin(II) hydrides, [ArSn(μ-
ABSTRACT:
The terphenyl tin(II) hydride,
[AriPr4Sn(μ-H)]
(1) (AriPr4 = C6H3-2,6(C6H3-2,6-
2
iPr2)2) was shown to form an equilibrium with the
distannyne, AriPr4SnSnAriPr4 (2) and H2 in toluene at
80 oC. The equilibrium constant (Keq) and Gibbs free
energy (ΔG) for the dissociation of H2 are 2.23 x 10-4 ±
1
4.9 % and 5.89 ± 0.68 % kcal/mol by H NMR spec-
troscopy, and 2.33 x 10-4 ± 6.2 % and 5.86 ± 0.73 %
kcal/mol by UV-Vis spectroscopy, indicating that the
hydride 1 is strongly favored. Further heating 2 at ca.
o
100 C afforded the known pentagonal bipyramidal
Sn7 cluster, Sn5(SnAriPr4)2 (3). Mechanistic studies
show that 3 is formed from the distannyne 2 which is
generated from 1. The order of the reaction for the
conversion of 2 into 3 was found to be zero and the
H)]2 (Ar
=
terphenyl), or their isomers,
rate constant is 1.77 x 10-5 M s-1 at 100 C. Hydride 1
o
ArSnSn(H)2Ar, which had been synthesized earlier
by various conventional reduction pathways,39-44 can
be generated via reacting H2 with the corresponding
was further characterized by cyclic voltammetry and
its pKa is 18.8 (2) via titration with 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU). The bond
dissociation free energy (BDFE ΔG) was estimated to
be 51.1 ± 3.4 % kcal/mol on the basis of its pKa and
reduction potential. Studies with deuterium indicate
ready exchange of D2 with the hydrides in 1.
distannynes, ArSnSnAr, under ambient conditions.31,
32
Herein, we establish reversibility for such reac-
tions and show that the tin(II) hydride, [AriPr4Sn(μ-
H)]2 (1), exists in equilibrium with the correspond-
ing distannyne, AriPr4SnSnAriPr4 (2), and hydrogen in
o
toluene solution at 80 C. The equilibrium constant
(Keq) and the Gibbs free energy (ΔG) were deter-
mined via H NMR and UV-Vis spectroscopy and
shown to strongly favor the tin(II) hydride. In addi-
tion, the pKa of 1 was determined to be 18.8 (1) by
The interaction of hydrogen with transition met-
als is a key process in organometallic chemistry.1-6
Furthermore, reversible oxidative addition and re-
ductive elimination reactions of H2 with transition
metal complexes constitute fundamental steps in
many catalytic cycles.7-8 Such processes have been
widely studied not only in organometallic chemis-
try,9 but also in surface chemistry,10-12 hydrogenase
enzymes.13-14 and in connection with their relevance
to chemical hydrogen storage.15-16 In effect the bind-
ing, storage and release of H2 under mild conditions
are of major importance,17-20 but despite the investi-
gations of several H2 carrier species, e.g. amine bo-
ranes,21 Mg,22-23 or Al24-25 systems, there have been
1
titration of
1
with the strong base, 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) in a THF so-
lution and using electrochemistry demonstrate that
the Sn-H bond strength is ca. 50 kcal mol-1. We also
demonstrate that the thermolysis of 1 to give the
cluster Sn5(SnAriPr4)2 (3) proceeds through the dis-
tannyne 2 which decomposes to give 3. The reaction
of D2 with 1 affords a mixture of isotopomers incor-
porating both deuterium and hydrogen in bridging
positions indicating ready H2/D2 exchange.
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Compound 1 was synthesized via the reduction of
In parallel experiments, detailed 1H NMR spectro-
scopic monitoring of the reaction at 80 oC in a
sealed tube indicated relatively rapid conversion of 1
to 2 in the first hour, after which time the rate of the
conversion decreased and eventually ceased reach-
ing equilibrium after ca. 2.5 h, where an integration
ratio of 2.35:1 was observed for 1 and 2. The equilib-
rium constant (Keq) of this reaction was calculated
(eq. 1) to be 2.23 x 10-4 ± 4.9 % at 80 oC, where the H2
concentration in toluene was calculated to be 0.549
mM (see supporting information),46 and the Gibbs
free energy (ΔG) of the equilibrium at 80 oC was also
determined to be 5.89 ± 0.68 % kcal/mol.
the corresponding tin(II) halide, [AriPr4Sn(μ-Cl)]2
with the reducing agent diisobutylaluminum hy-
dride (DIBAL) at ca. -78 oC in diethyl ether.41
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Scheme 1. Summary of the previous32, 41, 45 and
current work (panel) on [AriPr4Sn(μ-H)]2
(1)/AriPr4SnSnAriPr4 (2), and Sn5(SnAriPr4)2 (3).
iPr4
iPr4
ꢀAr SnSnAr ꢁ[H ]
2
Keq =
(1)
iPr4
ꢂ
ꢄ
2
[Ar Sn μꢃH ]
A further experiment which involved the heating
o
of a toluene solution of 1 at ca. 80 C with the head
space of the NMR tube filled with 1 atm of H2 gas
indicated no significant formation of 2 even after 16
h heating, and shows that the presence of H2 hin-
ders conversion of 1 into 2, further supporting the
reversible nature of the reaction.
1
K = 2.30 x 10-4 ± 4.9 % by 1H NMR
2.39 x 10-4 ± 6.2 % by UV-Vis
2
This work
Heating a d8-toluene solution of the hydride 1 at
ca. 80 oC for ca. 2.5 h in a sealed NMR tube afforded
two new doublet signals at 1.10 and 1.34 ppm and a
1
Figure 1. The H NMR spectrum of the d8-toluene
solution of 1 at 25 °C displaying the formation of 2
upon heating at 80 oC for 16 h.
1
new septet signal at 2.80 ppm in the H NMR spec-
The addition of D2 to the head space of an NMR
tube containing 0.6 mL of toluene solution of 1 (4.1
mM), and heating the solution for 16 h at 80 oC also
afforded no evidence of the formation of 2 (cf. corre-
sponding experiment with H2 above). However, the
119Sn NMR spectrum displayed a broad singlet (full
width at half maximum (FWHM) = 115 Hz) at 643.7
ppm, representing the formation of [AriPr4Sn(μ-D)]2
(1D).31 This is accompanied by the disappearance of
trum and the disappearance of the bridging hydride
signal at 9.06 ppm. These are indicative of the clean
formation of the distannyne, AriPr4SnSnAriPr4 (2)
(Figure 1). The integration ratio of 6:1 for 1 and 2 in-
dicates a 14.2 % of conversion of the hydride species
to the distannyne. Further heating the solution for
16 h led to no significant change in the extent of the
conversion. If however the H2 in the head space is
removed and replaced with N2, heating the solution
for a further 2.5 h resulted in a new integration ratio
for 1 and 2 of 2.34:1, indicating 30.0 % conversion of 1
to 2. Repetition of this process resulted in further
dehydrogenation until all 1 is converted to 2.
1
the bridging hydride signal at 9.06 ppm in the H
1
NMR spectrum. H NMR spectroscopy of a toluene
solution of 1 with a D2 filled head space heated at 80
oC for the shorter time of 2 h, resulted in a spectrum
that displayed a decrease in the intensity of the
bridging hydride signal at 9.06 ppm by 52 % along
2
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with the appearance of another broad signal at 9.00
Beyond the intermediate formation of 2, the
mechanistic details of the formation of the cluster,
Sn5(SnAriPr4)2 (3) from 1 are unknown. Upon raising
the temperature of a d8-toluene solution of 1 to ca.
90 oC, a new species, featuring a septet signal at 2.85
ppm and two doublets at 0.46 and 1.42 ppm was ob-
served along with 2 in the 1H NMR spectrum. These
correspond to the spectrum of 3,45 which was origi-
1
2
3
4
5
6
7
8
ppm. This new signal is due to the formation of the
mono-deuterated hydride, AriPr4Sn(μ-HD)SnAriPr4
(1HD). To further investigate this hypothesis, the
isotopmer 1D, was combined in a 1:1 ratio with com-
pound 1 in a toluene solution. The 119Sn NMR spec-
trum revealed a roughly 1:2:1 triplet signal at 657.9
ppm (JSn-H = 86 Hz), a broad singlet at 643.7 ppm,
representing 1 and 1D, respectively, as well as a dou-
blet at 650.6 ppm (JSn-H = 84 Hz), indicating cou-
pling to a single bridging hydrogen consistent with
o
nally obtained by refluxing (ca. 110 C) a toluene so-
9
lution of 1 for 16 h (or via the reduction of a 2:5 mix-
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ture of [AriPr4Sn(μ-Cl)]2, SnCl2 with KC8). An H
1
1
the formation of the 1HD isotopomer. The H NMR
NMR experiment involving the heating of a d8-
o
spectrum of the 1:1 mixture of 1 and 1D in d8-toluene
showed an integration ratio of 1:1 for the signals at
9.06 and 9.00 ppm, which represent the 1:2 ratio for
1 containing two hydrogen atoms and 1HD contain-
ing one hydrogen and one deuterium atom, further
supporting the ready stoichiometric scrambling of
the isotopologues.
toluene solution of 1 at ca. 100 C was carried out.
The formation rate of 3 was slow during the first 10 h
of heating, then increased rapidly during the follow-
ing 5 h, and slowed again in the last 4 h as the con-
version of 1 to 3 approaches completion (see Sup-
porting Information). To throw further light on the
formation pathway for 3, a similar experiment was
carried out by heating a toluene solution of 1 at 100
oC for 16 h with a H2 filled head space. No evidence
of the formation of either 2 or 3 was observed; in-
stead, only a new septet at 2.85 ppm and two dou-
blets at 1.09 and 1.11 ppm, which represent the for-
mation of AriPr4H, were observed along with the tin
metal deposition indicating the decomposition of 1.
In contrast, heating a toluene solution of 2 at ca. 100
oC in the absence of H2 afforded compound 3 in ca.
25 % yield and other unidentified products. The re-
action rate (k) and the rate of the formation of 3
were determined to be 6.11 x 10-5 ± 4.8 % M s-1 and
1.67 x 10-5 ± 7.2 % M s-1, respectively. These experi-
ments show that 3 is not formed directly from 1; in-
stead, it is formed from 2 which in this case is gen-
erated from 1.
In parallel with the NMR experiments, the equi-
librium of 1 with 2 and H2 was also studied via UV-
vis spectroscopy. A dilute toluene solution of 1 (0.16
mM) features a weak absorption band at 600 nm (ε
= 70 M-1 cm-1), in agreement with the previously re-
ported value for 1.41 However, two new bands at 410
nm and 597 nm, corresponding to those of 2,47 ap-
peared upon heating the sample at 80 oC (Figure 2).
The extent of conversion was calculated based on
the increasing intensity absorption at 410 nm. The
equilibrium constant Keq and the Gibbs free energy
ΔGo for the equilibrium were determined to be 2.33
x 10-4 ± 6.2 % and 5.86 ± 0.73 % kcal/mol by this
method, which are in good agreement with Keq and
1
ΔGo values determined by H NMR spectroscopy.
The small Keq and the positive ΔGo again illustrate
the strong preference for the hydride 1 over 2 in the
equilibrium.
The hydride species 1 was further characterized by
cyclic voltammetry (CV) performed on a 0.75 mM,
0.3 M Bu4NBF4 THF solution of 1 (Figure 3). 1 dis-
plays two reduction events at -1.63 and -2.00 V. The
first is quasi-reversible and the second irreversible,
and the second is barely observed at slow scan rates
which suggests that species generated by the second
reduction of 1 is fairly unstable (Figure S18).
1.2
1
0.8
0.6
0.4
0.2
0
2
1
To determine the number of electrons associated
with the reversible reduction at -1.63 V, we per-
formed differential pulse voltammetry (DPV) exper-
iments with a solution containing equimolar con-
centrations of 1 and FcCp2*. The area of each of the
reduction observed using DPV are 4.12 × 10-7 and
5.65 × 10-7 VA, respectively (Figure S19). This implies
that the reversible couple at -1.63 V is best assigned
as a one-electron reduction event.
350
450
550
650
λ (nm)
750
850
Figure 2. UV-Vis spectrum of 1 heated at 80 oC over 2.5
h (ca. 20 min intervals) in toluene (0.16 mM). The λmax
of 1 at 595 nm overlaps with the low energy λmax of 2 at
597 nm.
3
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the equilibrium (ΔG) indicated a strong preference
Page 4 of 6
for the hydride. The Sn7 cluster, Sn5(SnAriPr4)2 is the
thermolysis product of AriPr4SnSnAriPr4 rather than
of the Sn(II) hydride. Further characterization of 1
by cyclic voltammetry (Eꢅꢈꢆꢇꢇ/ꢇ = -1.63 V vs SCE), and
the pKa value of 18.8 (2) allowed the BDFE of the Sn-
H to be determined as ca. 50 kcal mol-1.
1
2
3
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Supporting Information
Experimental procedures for all compounds and their
spectral data, as well as details of the determination of
the equilibrium constants (Keq), ΔG values, the titra-
tion of 1 with DBU, electrochemical measurements and
the calculation of the Sn-H bond dissociation free en-
ergy. Supporting information is available free of charge
on the ACS Publications website.
Figure 3. Cyclic voltammogram of 0.35 mM solution
of 1 and 0.35 mM decamethylferrocene (FcCp2*), in
0.3 M Bu4NBF4 THF.
The irreversibility of the second reduction event
is more consistent with the reduction at a single
metal center, such as the divalent tin in the isomer
AriPr4SnSn(H)2AriPr4, which differs in energy from 1
by less than 3 kcal mol-1.35 Also, reduction of a small
equilibrium concentration of monomeric AriPr4SnH
species, which was calculated to be preferred ener-
getically over 1,35 cannot be completely ruled out
and its presence is consistent with previous work
involving the reaction of 1 and related species with
Lewis bases.49
AUTHOR INFORMATION
Corresponding Author
*P.P.P.: fax, +1-530-732-8995; tel, +1-530-752-6913; e-
mail, pppower@ucdavis.edu.
ORCID
Philip P Power: 0000-0002-6262-3209
Louise Berben: 0000-0001-6461-1829
ACKNOWLEDGMENT
After an unsuccessful attempt to obtain the pKa
of 1 with a mild base, Et3N (pKa of Et3NH+ = 14.9 in
THF),50 one equivalent of the strong base, 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU), was used to
titrate a THF solution of 1 (0. 75 mM) (eq. 2).
We wish to acknowledge the U.S. Department of Energy
(DE-FG02-07ER4675) Office of Basic Energy Sciences
for support of this work. LAB thanks the UCD academic
senate.
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