Journal of the American Chemical Society
Communication
Scheme 3. Synthesis of 3
but its concentration decreased significantly over time. For
COE hydrogenation, an additional minor species that shows
Rh(olefin) resonances is observed to grow in and then
disappear. Addition of H to complex 1 in the absence of
2
substrate resulted in the formation of an unidentified
precipitate over 8 h, while recharging post-catalysis resulted
in no turnover. Both observations suggest a slow H -promoted
2
decomposition. To resolve this rather complex set of
observations, we modeled the processes occurring using
27
COPASI.
of 1 observed in solution cleanly convert to a single species.
This is also the case with the synthesis of 2.
Compound 1 was evaluated as a catalyst for the hydro-
genation of COD or COE to give cyclooctane (COA) (eq 1):
Scheme 4A shows the resulting fits to the kinetics data,
which arise from a model that operates for either COD or
COE starting points and was iterated on the observed
concentrations of substrates, intermediates, products, organo-
metallic speciation, and slow decomposition with a constant
1
(2 mol %)
COD or COE ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ ⎯→ COA
excess of H . The COE hydrogenation cycle was first modeled,
2
+
H
(1)
2
and specific rate constants were fixed, after which the model
was used for the COD analysis. The resulting simulation
recreates the multiple temporal profiles satisfactorily, giving
confidence that it captures the essential elements of the
catalytic manifold. Briefly described, slow hydrogenation of 1
results in an intermediate species, Rh(COE), which we
propose is the species observed in low concentrations during
COE hydrogenation. Rh(COE) then undergoes either further
While this is a well-known reaction for homogeneous Rh
2
2
precatalysts, as far as we are aware, this is the first example in
23,24
which a Zintl cluster acts as a homogeneous catalyst.
Initial in situ screening using NMR spectroscopy (2 mol %
1
, ∼4 atm H , [alkene] = 0.31 M, C D , 298 K) showed that
2
0
6
6
COD was hydrogenated considerably more slowly than COE
(
8.5 h vs 15 min). Aware of the problems with mass transport
25
effects in NMR-scale reactions, we collected kinetics data
(endergonic) reaction with H to eventually form INT, or
2
using a system open to flowing H (1 atm) and stirred, with
simple substitution by COD to return 1 and give free COE
(observed). Under COD hydrogenation conditions, reaction of
INT with COD again returns 1. With COE, Rh(COE) is
formed. Both cycles produce COA. While the observed
induction period in COE hydrogenation is explained by slow
Rh(COE) buildup, for both substrates compound 1 is the
principal resting state. The deceleration at longer reaction
times in both COE and COD hydrogenations and the
reduction in [1] are captured by the inclusion of a slow
decomposition process with H2 that reduces [Rh]total, as
observed experimentally in the absence of COD. Complex 2 is
not an active catalyst, consistent with the strongly bound dppe
ligand, whereas the reaction with 3 is significantly slower than
that with 1, taking 1 week to effect only 50% conversion of
2
individual time/concentration points coming from separate
1
quenched experiments as measured using H NMR spectros-
copy. Under these conditions, hydrogenation was much slower
(
days for COD hydrogenation), suggesting a positive order in
2
H . The temporal profiles for the two substrates were also very
different from one another (Scheme 4A). While monitoring of
Scheme 4. (A) Kinetic Data (○) and Simulated Data for
Hydrogenation of (top) COD and (bottom) COE; (B)
COD (NMR tube, 4 atm H ). Interestingly, free COE is the
2
major product at this point (COE:COA = 9:1). Binding of the
two Ni(COD) fragments to the core of 1 significantly alters
the electronics of the cluster (as evidenced by pronounced
changes to bond metric datasee Figure S21). We postulate
that this has an effect on the strength of the Rh−COD
interaction, which in turn alters the kinetics of the reaction.
In order to probe the viability of the proposed mechanism,
Because of the 18-electron configuration of 1, the most
4
2
accessible pathway begins with an η −η dissociation of the
COD ligand, with a barrier of +19.6 kcal/mol. The resulting
1
6-electron Rh(I) intermediate then reversibly adds H to
2
form a Rh(III) dihydride intermediate at +16.8 kcal/mol
relative to 1. Migratory insertion then produces an agostically
stabilized alkyl hydride that can rearrange via a non-agostic
alkyl hydride intermediate, allowing reductive coupling to form
Rh(COE) at −1.9 kcal/mol. The overall barrier for the
formation of Rh(COE) from 1 is +27.1 kcal/mol with a
turnover-limiting step corresponding to the reductive coupling
process. Displacement of COE by COD may occur at this
point to reform 1, or Rh(COE) may itself undergo
hydrogenation. This second hydrogenation occurs by a similar
the COD hydrogenation showed steadily decelerating
consumption of substrate and production of COA, for COE
a more complex sinusoidal profile was observed, with
induction, acceleration, and then deceleration phases. The
generation of a colloidal catalyst from 1 in COD hydro-
genation was discounted, as addition of Hg during productive
26
catalysis did not affect the observed rate. Complex 1 was
observed as the principal resting state using both substrates,
C
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX