.
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pseudo-zero-order decay of [H B·NMe H] was measured,
3
2
while approximate pseudo-first-order consumption of
H B·NMe H was found at lower concentrations (Figure 3B).
3
2
[4]
Similar saturation kinetics were previously shown with 1.
During the early stages of catalysis, post-induction, the
measured rate shows a first order dependence on [5], rather
than half order that would suggest a rapid dimer–monomer
[27]
equilibrium in which the dimer lies off-cycle.
Kinetic isotope effects of 1.1 Æ 0.2 and 2.0 Æ 0.3 were
Scheme 6. Proposed pathways for the formation of the active catalyst
measured by using D B·NMe H and H B·NMe D respectively
(
3
2
3
2
under conditions of high relative [H B·NMe H] (500 equiv) starting
3
2
zero-order regions), suggesting that NÀH activation may be
from a) monomeric and b) dimeric precatalysts.
involved in, or in an equilibrium prior to, the turnover-
limiting step. Moreover, the induction period approximately
doubled from about 300 s with H B·NMe H and D B·NMe H
undergoes a slower NÀH activation to form a dimeric active
species; or b) the monomeric species are the active catalysts,
which could be formed when starting from 5 by opening up of
the phosphido bridges, perhaps by protonation by the amine–
borane. To explore this latter possibility, complex 5 was
3
2
3
2
to circa 600 s with H B·NMe D, indicating that NÀH activa-
3
2
tion is also involved in the rate-limiting process during the
formation of the catalytically active species. Consistent with
labelling experiments, BÀH activation in 5 is fast and
reversible as addition of D to 5 results in incorporation of
reacted with HCl (Et O solution) and MeI. In both cases
2
2
D into the BH groups within the time of mixing, possibly via
intractable mixtures resulted. Unfortunately ESI-MS, or
crossover experiments using different xantphos-containing
precursors, have not been definitive in discounting either the
dimer or the monomer as the active species. Catalysis in
a sealed NMR tube with 20 equivalents of H B·NMe H
3
a s-CAM-type process (s-CAM = s-complex-assisted meta-
[
28]
thesis).
These observations combined suggest that the
active catalytic species contains an NÀH-activated amine–
borane, possibly an amidoborane complex, that quasi-rever-
sibly coordinates a second equivalent of H B·NMe H. The
3
2
(5 mol% [Rh]) was employed to probe likely resting states.
3
2
À5
1
31
1
dehydrocoupling of H B·NMe H (0.072m, [5] = 7.2 10 m)
During catalysis, H and P{ H} NMR spectra showed broad
unresolved signals, suggestive of several species, while after
12 h, complex 4 was again the major organometallic product
formed. Under these conditions of higher catalyst loading,
3
2
in a sealed system, which enables build-up of H , shows a TOF
2
À1
of circa 450 h (relative to [Rh]), slower than in the open
system, indicating inhibition by H2. The decay of
À1
[
H B·NMe H] follows a first-order profile (post-induction
dimer 5 is strikingly faster than monomer 1 (TOF 240 h and
3
2
À1 [4]
period), again very similar to that seen with 1 under the same
4 h , respectively), whereas the dehydrocoupling at much
[
4]
conditions. Complex 4 is also active in catalysis, but shows
higher relative ratios of amine–borane operate at similar rates
[16]
À1
a longer induction period and a slower overall rate.
(see above; 500 equivalents, 0.2 mol% [Rh], TOF 1150 h
À1
Complex 5 (0.1 mol%) also catalyzes the dehydropolymeri-
zation of H B·NMeH2 to form [H BNMeH]n (M =
and 1000 h , respectively). This difference in rate may
suggest that active-species formation from 1 is dependent
3
2
n
À1
2
8700 gmol , PDI = 1.7, Scheme 5). This is a similar molec-
on the concentration of amine–borane, possibly aided by
[
29]
ular weight to the [H BNMeH] produced under analogous
outer-sphere BÀH···HÀN interactions. Overall, the current
2
n
À1
[4]
conditions with 1 (M = 22700 gmol , PDI = 2.1).
data suggest that if not the actual catalyst, dimeric species
such as 5 likely sit close to the real catalyst.
n
In summary, PÀC-activated dimeric complexes based
2+
upon the {Rh (xantphos’) } motif are very active catalysts
2
2
for the dehydrocoupling of H B·NMe H and the dehydropo-
3
2
Scheme 5. Dehydropolymerization of H B·NMeH using 5 as a cata-
lyst.
3
2
lymerization of H B·NMeH . Kinetic data suggest that the
3
2
mechanisms of dehydrocoupling by dimeric and previously
reported monomeric precatalysts may be closely related. The
implication that dimeric species are active suggests that
bimetallic cooperativity might be important for dehydropo-
lymerization and offers opportunities to further tune catalyst
properties as has successfully been demonstrated for olefin
Overall, these studies suggest that the mechanism of the
dehydrocoupling of amine–boranes by 5 and 1 at low catalyst
loadings (0.2 mol% Rh) are likely closely related: both 1 and
5
show induction periods as well as very similar kinetic
[
8]
profiles and isotope effects. We postulate that the active
polymerization processes. More generally given the wide
[
30]
species, whether a monomer or dimer, is accessed through NÀ use of xantphos as a ligand for many catalytic applications,
H activation. Complex 4 that was isolated at the end of
it will be interesting to see whether PÀC-activated dimers
catalysis could form from the addition of BH to a [Rh] -
prove to be a common motif in organometallic chemistry.
3
2
NMe BH unit. As shown in Scheme 6, we propose that the
2
3
active species forms by one of two pathways: a) PÀC
Keywords: amine–boranes · homogeneous catalysis · P ligands ·
rhodium · X-ray diffraction
III
activation in a Rh complex related to 3 (which would
[
4]
result from addition of H B·NMe H to 1 ) that is very fast
3
2
under conditions of a high relative amine–borane concen-
tration and forms a dimeric species related to 5 that then
How to cite: Angew. Chem. Int. Ed. 2015, 54, 10173–10177
Angew. Chem. 2015, 127, 10311–10315
1
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 10173 –10177