Inorganic Chemistry
Article
trans‐Me3SnO‐NN‐OSnMe3 + CO2(g)
→ Me3SnO‐C(O)‐OSnMe3 + N2O(g)
(9)
The calculated Gibbs energy change for addition of N2O to
Me3Sn-O-SnMe3 (2Me) is ΔGo298K = +40.3 kcal·mol−1 as it can
be seen in Figure 7. The computed ΔGo
value for CO2
298K
insertion into the Sn−O bond of 2Me to make the monomeric
carbonate complex, 3Me, as shown in Figure 9, is ΔGo
=
298K
−1.8 kcal·mol−1. Confidence in that value is gained by the
derived experimental value from the graph in SI Figure S6 of
ΔGo
= −3.6 1.2 kcal·mol−1 for the analogous reaction
298K
with 3Cy. Utilizing these data yields an estimate of ΔGo
=
298K
−44 kcal·mol−1 for displacement of bound N2O by CO2 in
reaction 9. This is surprisingly close to the estimate made for
reaction 8 of ΔGo
= −40 kcal·mol−1. The relatively close
298K
agreement assigned to conversion of a hyponitrite to a
carbonate for Na2O and Me3SnOSnMe3 appears to imply
that the dominant factor determining the relative stability may
lie in the hypothetical O2− transfer reaction shown in reaction
10 and relative Lux acidity27 of N2O and CO2, and that this is
closely related to resonance delocalization and charge
stabilization in the carbonate ion being greater than that of
the hyponitrite ion.
Figure 10. Reaction profile (kcal·mol−1) for the unfavorable addition
of N2O to H2O and Me3SnOSnMe3. Data for reaction of H2O and
N2O leading to trans-H2N2O2 are from Thynell29 [carbon (gray),
nitrogen (blue), oxygen (red), tin (green), hydrogen (white)].
size and vacant 5d orbitals. In contrast, H migration requires
substantial weakening of the O−H bond, contributing to its
higher activation energy.
N2O22−(g) + CO2(g) → N2O(g) + CO32−(g)
(10)
The Gibbs energy at 298 K for the acid base reaction shown in
The rapid reaction of protic acids to eliminate H2N2O2
which subsequently decomposes to H2O and N2O in an acid
catalyzed reaction is a characteristic reaction of hyponitrites.1,2
Reaction of 1Ph or 1Cy with the transition metal carbonyl
hydride H-Cr(CO)3C5Me5 is rapid and produces Ph3Sn-
Cr(CO)3C5Me5 and H2N2O2 in high yields in organic solvents.
Since tin hyponitrites are stable, anhydrous, and store well
under dry argon, combined with the fact that the chromium
hydride may be prepared in high purity and that R3Sn-
Cr(CO)3C5Me5, CrSn, products are nonvolatile, the reaction
of tin hyponitrites with H-Cr(CO)3C5Me5 presents a
convenient method to prepare and vacuum transfer anhydrous
hyponitrous acid which may be of synthetic utility.
It is well-known that organic hyponitrites are decomposed
by radicals and either alkyl or alkoxy radicals may be produced
during this process. The tin hyponitrites, since they are large
and also contain vacant 5d orbitals, are a ready radical receptor,
and radical addition at the Sn center to form a hypervalent tin
radical intermediate appears to be more facile for stannyl as
opposed to alkyl hyponitrites. Investigation of reactions of the
17 electron organometallic radical •Cr(CO)3C5Me5 and 1Ph
shows concentration dependent behavior, and a plethora of
products are formed. These complex reactions are under
additional mechanistic investigation, but several observations
may be made at this time. At low radical concentrations, where
concerted attack of two •Cr(CO)3C5Me5 radicals is
disfavored, elimination of N2O from the tin hyponitrites is
moderately accelerated as proposed via generation of the
Ph3SnO• radical which serves as a radical carrier for nitrous
oxide elimination as shown in Scheme 2.
reaction 10 is computed to be −48.1 and −46.0 kcal·mol−1 for
2−
the cis- and trans-N2O2 anions, respectively, at the B3LYP-
D3(BJ)/Def2-TZVP level of theory.20−22 These values are
only slightly higher than those computed for reactions 8 and 9,
supporting the hypothesis that the most relevant energetic
contribution to these reactions is related to the difference in
resonance delocalization and charge stabilization in the ions
and simple binding reactions of CO2 and N2O to Na2O or
trans-Me3SnOSnMe3, as discussed earlier, may be similar in
reaction energy. Additional experimental work to further
support that hypothesis is planned.
In benzene solution in sealed NMR tube studies, the first
order decay of the hyponitrite complexes and their impervious
response to added PPh3, O2, CO2, and N2O were surprising.
This is in keeping with the computed mechanism that involves
migration of the Sn group from O to N as shown in Figure 7. It
is of interest to compare the mechanism computed in this work
for N2O elimination from the hyponitrite 1Me to the well
studied mechanism for dissociation of trans-H2N2O2 hyponi-
trous acid computed previously by Morokuma28 and more
recently by Thynell.29 Figure 10 shows a comparison of the
mechanism for the reverse reaction: N2O addition to H2O
(previously computed)30 or to Me3SnOSnMe3 (computed in
this work, see Figure 7) to yield trans-H2N2O2 or hyponitrite
1Me, respectively.
N2O capture processes by either water or Me3SnOSnMe3 to
form trans-hyponitrous acid or the trans-hyponitrite 1Me
complex are both highly endergonic processes (ΔGo
≈
298K
+53 and +40 kcal·mol−1, respectively). The transition states
and intermediates for both tin and hyponitrous acid are very
similar with no readily discernible difference. The Gibbs
energy barrier however is substantially higher for water than it
is for tin (+120 vs +65 kcal·mol−1). The ability of tin to
become hypervalent is the most probable reason for this. In
either TS, tin is able to obtain substantial donation from both
the N and the O lone pairs while migrating due to its larger
At higher Cr• concentrations, a more complex behavior was
observed. Carbon dioxide was evolved, and in addition, an
unstable intermediate complex as Cr(Cp*)(CO)2(NNO-
SnPh3) is formed together with detectable CO2 evolution.
When reactions of 1Ph are done with H-Cr(CO)3C5Me5
(which like H-Cr(CO)3C5H5 is both a weak acid and a facile
H atom transfer reagent), there is a more rapid evolution of
H
Inorg. Chem. XXXX, XXX, XXX−XXX