The Simplest Amino-borane H2B=NH2 Trapped on a Rhodium Dimer: Pre-Catalysts for Amine-Borane Dehydropolymerization

Article abstract of DOI:10.1002/anie.201600898

The μ-amino-borane complexes [Rh₂(L^R)₂(μ-H)(μ-H₂B=NHR′)][BAr^F₄] (L^R=R₂P(CH₂)₃PR₂; R=Ph, ⁱPr; R′=H, Me) form by addition of H₃BNMeR′H₂ to [Rh(L^R)(η⁶-C₆H₅F)][BAr^F₄]. DFT calculations demonstrate that the amino-borane interacts with the Rh centers through strong Rh-H and Rh-B interactions. Mechanistic investigations show that these dimers can form by a boronium-mediated route, and are pre-catalysts for amine-borane dehydropolymerization, suggesting a possible role for bimetallic motifs in catalysis. Bridges of boron: Mechanistic investigations show that rhodium dimers bridged by amino-borane can form by a boronium-mediated route starting from amine-borane. These types of complexes are pre-catalysts for amine-borane dehydropolymerization, suggesting a possible role for bimetallic motifs in catalysis.

Full text of DOI:10.1002/anie.201600898

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Communications
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International Edition: DOI: 10.1002/anie.201600898
German Edition: DOI: 10.1002/ange.201600898
Amine–Borane Dehydropolymerization
=
The Simplest Amino-borane H2B NH2 Trapped on a Rhodium Dimer:
Pre-Catalysts for Amine–Borane Dehydropolymerization
Amit Kumar, Nicholas A. Beattie, Sebastian D. Pike, Stuart A. Macgregor,* and
Andrew S. Weller*
Abstract:
The
m-amino–borane
complexes
There is recent evidence that these processes occur at a metal
center in which the catalyst needs to perform two roles:
1) formal dehydrogenation of amine-borane to form a latent
R
F
R
=
=
[Rh2(L )2(m-H)(m-H2B NHR’)][BAr ] (L R2P(CH2)3PR2;
4
i
R = Ph, Pr; R’ = H, Me) form by addition of H3B·NMeR’H2
to [Rh(LR)(h6-C6H5F)][BArF ]. DFT calculations demonstrate
source of amino-borane (H2B NRH), and 2) subsequent
=
4
[2–6]
À
that the amino–borane interacts with the Rh centers through
strong Rh-H and Rh-B interactions. Mechanistic investigations
show that these dimers can form by a boronium-mediated
route, and are pre-catalysts for amine-borane dehydropolyme-
rization, suggesting a possible role for bimetallic motifs in
catalysis.
B N bond formation.
For some systems a coordination/
insertion mechanism is proposed, although the precise
structure of the propagating species is currently unresolved
(Scheme 1B).[3,5,6] This is in contrast to olefin polymerization,
in which the feedstock (for example, ethene or propene) is
already unsaturated, and the active species and propagating
mechanisms are well-defined.[7] A clearer understanding of
how the catalyst dehydrogenates amine-borane, traps inter-
P
olyamino-boranes ([H2BNRH]n) are potentially exciting
new materials that are isoelectronic with technologically
pervasive polyolefins, but are chemically distinct because of
À
mediate amino-boranes, and promotes B N bond-formation,
is central to harnessing the full potential of systems that
À
À
(dÀ)HB NH(d +) polarization. They are formed by the
ultimately deliver new well–defined B N polymeric materials
on a useful scale.
dehydropolymerization of amine-boranes (H3B·NRH2; R =
H or Me, for example; Scheme 1A),[1] and metal-catalyzed
routes to polyamino-boranes offer the potential for fine
control over molecular weight and polymer stereochemistry.
=
Unlike ethene (H2C CH2), which is stable under ambient
conditions, the isoelectronic amino-borane (H2B NH2) has
=
only been prepared in low temperature matrices and oligo-
merizes above À1508C.[2,8] Adding steric bulk to the nitrogen
[9]
=
atom increases stability, so that, for example, H2B NMeH
t
[10]
=
or H2B N BuH can be observed as transient species using
in situ NMR spectroscopy before they also oligomerize. There
=
are two examples where unstable H2B NH2 can be trapped
by coordination to a single metal center. These originate after
dehydrogenation of a putative s-ammonia borane[11] complex,
2
[12]
=
forming Ru(PCy3)2(H)2(h -H2B NH2) A
and (Cy-PSiP)-
2
3
[13]
=
Ru(H)(h -H2B NH2) B, Cy-PSiP = k -(Cy2PC6H4)2SiMe).
=
We now report that H2B NH2 can be trapped by
a bimetallic [Rh2(R2PCH2CH2CH2PR2)2]2+ fragment to give
a novel bridging amino-borane bonding motif. We provide
mechanistic evidence for formation of the complex from
a monometallic precursor, and show that such dimeric amino-
borane species may be important in dehydropolymerization
pathways. This report builds upon previous observations that
indirectly implicate bimetallic motifs during catalysis.[14–16]
Scheme 1. A) Amine-borane dehydropolymerization; B) a suggested
coordination/insertion mechanism, P=polymer chain; C) examples of
=
H2B NH2 coordinated to a metal center.
[*] A. Kumar, Dr. S. D. Pike, Prof. A. S. Weller
Addition of
a
slight excess of H3B·NH3 to
a
1
Department of Chemistry, Chemistry Research Laboratories,
University of Oxford, Mansfield Road, Oxford, OX1 3TA (UK)
E-mail: andrew.weller@chem.ox.ac.uk
[D8]THF solution of [Rh(LPh)(h6-C6H5F)][BArF ]
4
(LPh = Ph2P(CH2)3PPh2, ArF = 3,5-(CF3)2C6H3) resulted in
the rapid formation of a bimetallic monocation, which was
identified by NMR spectroscopy, electrospray ionization mass
spectrometry (ESI-MS), and single-crystal X-ray diffraction,
N. A. Beattie, Prof. S. A. Macgregor
Institute of Chemical Sciences, Heriot Watt University, Edinburgh,
EH14 4AS (UK)
E-mail: S.A.Macgregor@hw.ac.uk
Ph
F
4
=
as [Rh2(L )2(m-H)(m-H2B NH2)][BAr ] 3. One equivalent
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201600898.
of the boronium[9,17–20] cation [THF·BH2·NH3][BArF ] was
4
also formed (d(11B) 0.5 (t),
J
BH = 108 Hz; lit.[19]
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KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
[Et2O·BH2·NH3][BArF ] d(11B) 0.2, JBH = 125 Hz).
4
In situ solution NMR data for 3 show a signal at d(11B)
51.5, a single 31P environment (d(31P) 18.2, JRhP = 142 Hz), and
a broad peak at d(1H) À7.45 (integral ca. 3H relative to the
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phenyl groups). ESI-MS shows
a
mono-cation at
m/z = 1060.16 (calcd 1060.16) with the correct isotope pat-
tern. Crystallization (THF/pentane/-188C) gave a small
number of crystals, for which a single-crystal X-ray diffraction
Ph
=
study showed a H2B NH2 unit bridging a {(Rh2(L )2(m-H)}
unit (Supporting Information, Figure S21). However, insuffi-
cient material was obtained upon which to collect reliable
NMR data. Complex 3 is unstable in solution at room
temperature, decomposing after four hours to give a mixture
in which [Rh(LPh)(THF)2][BArF ] 6 was present in approx-
4
imately 30% yield.[21] To put the structure and spectroscopic
i
data on a firm footing, the equivalent reaction using the Pr-
substituted
chelating
phosphine
gave
complex 4,
F
[Rh2(LiPr)2(m-H)(m-H2B NH2)][BAr ], and 5 (Scheme 2).
=
4
Figure 1. Solid-state structure of the cationic portion of complex 4.
Displacement ellipsoids are shown at the 50% probability level.
Selected bond distances (Š) and angles (8): Rh1···Rh2, 2.7874(4);
À
À
À
À
Rh1 B1, 2.070(5); Rh2 B1, 2.055(5); B1 N1 1.377(6); P1 Rh1,
À
À
À
2.2550(10); P2 Rh1, 2.3063(10); Rh1 H1, 1.718; Rh2 H2, 1.723;
]plane (N1B1H1H2)/plane (N1B1Rh1Rh2), 54.1; ]plane (Rh1P1P2)/
plane (Rh2P3P4), 100.2; ](NH2)/(BH2) 24.38.
Scheme 2. Formation of amino-borane coordinated dimers 3 and 4.
[BArF ]À anions are not shown.
4
This reaction was slower than that observed for LPh.
Complex 4 can also be isolated in 78% yield as orange
crystalline material using an alternative route (see below,
Scheme 5). In the absence of H3B·NH3, complex 4 is stable for
at least two days in [D8]THF solution. However, when formed
in situ 4 decomposes over 24 hrs into a mixture of products,
one
of
which
can
be
characterized
as
[Rh2(LiPr)2(H)2(m-H)3][BArF ].[22] The room temperature so-
4
lution NMR data obtained for 4 are very similar to those for 3:
d(11B) 51.1; d(31P) 40.8, JRhP = 142 Hz; d(1H) À8.64 (3H,
broad). Progressive cooling to 180 K splits the high field
hydride resonance into two signals, in a 2:1 ratio; while two
31P environments were also observed, suggesting a fluxional
process at room temperature. An Eyring plot yields the
Scheme 3. Limiting valence bond descriptions for complex 4, and
examples of bridging hydridoborate and borylene complexes.
[Rh]={Rh(LiPr)}, charge not shown.
activation
data:
DH° = 31.1 Æ 1.3 kJmolÀ1,
DS° =
À27 Æ 1 JKÀ1 molÀ1,
DG(298 K)° = 39.2 Æ 1.6 kJmolÀ1
;
where the negative entropy of activation suggests an intra-
molecular process (Supporting Information, Figures S2–3).
The solid-state structure of complex 4 is shown in Fig-
distances suggest lengthened, but unbroken bonds (for
example, 1.360 Š). The NH2 group is slightly twisted with
respect to the BH2 group (24.38; Figure 1B). The whole
ure 1A. A dimeric Rh2 unit is accompanied by one [BArF ]À
4
anion, confirming that it is a mono-cation. Two {Rh(LiPr)}+
=
=
fragments are bridged by a hydride and a H2B NH2 unit. The
H2B NH2 fragment lies 54.18 from the Rh-Rh vector so as to
À
À
B N distance (1.377(6) Š) is consistent with a significant
accommodate appropriate overlap between the B H bonds
À
B N p-interaction, and is similar to that measured in A
and the two rhodium centers. These are best described as
being two distorted square planes (for example, P1/P2/H3/
H1) twisted with respect to one another by 1028 (Figure 1C).
This motif, which is similar to that observed for D, is fully
consistent with the low temperature NMR data, and are
recreated well in the DFT calculated structure (Supporting
Information, Figures S24–26). Each metal center in 4 is best
(1.396(3) Š) and B (1.359(8) Š), as well as the bridging
borylene complex C (1.399(3) Š; Scheme 3).[23] The Rh···B
distances (2.070(5) and 2.055(5) Š) are similar to those found
in
the
amino-borane
complexes A,
B,
and
F
[24]
=
[Ir(PCy3)2(H)2(H2B NMe2)][BAr ] (spanning 1.956(2) to
4
2.140(13) Š), but significantly shorter than those measured in
the bridging thexylborohydride complex D (2.330(3) Š).[25]
The hydrogen atoms were located but refined using a riding
I
[26]
À
described as Rh , with no M M bond.
The end-on
=
=
{Rh2(m-H2B NH2)} binding mode contrasts with H2C CH2
À
model. Within the limits of X-ray diffraction the B H
that bridges two metal centers symmetrically using both
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carbon atoms, in either m-h2:h2 or m-h1:h1 bonding modes,[27,28]
highlighting the differences between these isosteres.[29]
Surprisingly, the amino-borane in 4 is quite strongly
bound. It is only slowly displaced by excess acetonitrile (7%
in 50 min) to give a mixture of species, one of which is
[Rh(LiPr)(NCMe)2][BArF ].[22] No reaction occurs with tolu-
4
ene,
which
might
be
expected
to
form
a
[Rh(LiPr)(h6-C6H5Me)]+ complex if a monomeric {Rh(LiPr)}+
fragment were accessible.[30] Addition of cyclohexene, shown
to be a probe for free H2B NH2, gave no reaction. In
[2]
=
contrast,
H2
rapidly
reacts
with
4
to
form
[Rh2(LiPr)2(H)2(m-H)3][BArF ].[22]
4
There are two limiting forms for the structure of 4 (and
quasi-isostructural 3): 1) a bridging amino-borane at two RhI
centers, or 2) a bridging borylene dihydride (RhIII), Scheme 3.
The observed d(11B) chemical shift of 51 ppm is more
consistent with the former as amino-boranes bound to one
metal center show chemical shifts around 40–50 ppm,[12,13,24,31]
while bridging borylenes[32] are generally observed between
90 and 100 ppm.[23,33]
To probe the bonding of the amino-borane ligand in 4,
DFT calculations were used as the basis for a Quantum
Theory of Atoms in Molecules (QTAIM) analysis of the total
electron density. The results are presented in Figure 2A,
along with selected bond critical point (BCP) metrics. Fig-
ure 2B provides comparative BCP data for the bridging
borylene complex C, the hydridoborate complex D, and
Figure 2. A) Contour plot of the electron density of the central part of
4 presented in the {Rh1B1Rh2} plane with projected stationary points,
bond paths, bond critical points (BCP; green), and ring critical points
(RCP; red); the associated table shows selected BCP metrics (a.u.;
average data for indicated bonds) and computed d(11B) chemical
shifts. B) Calculated BCP metrics (a.u.; average data for indicated
[(PPh3)2Rh(H)(m-H)(m-Cl)2Rh(H)(PPh3)2]+, E,
a
well-
defined RhIII dimer with both terminal and bridging
hydrides.[34] Average data are presented for all complexes
where appropriate, although the discussion will focus on the
bonding around a single rhodium center (Rh1).
bonds) for comparator complexes C (including the computed 11
B
chemical shift), D and E (1(r)=electron density, 51(r)=Laplacian of
electron density, e=bond ellipticity, H(r)=local energy density). All
geometries are based on the crystallographically determined heavy
atom positions with hydrogen atoms optimized with the BP86 func-
tional. For a full summary of parameters see Figures S24–27 and
associated Tables in the Supporting Information.
In 4, the {Rh1/B1/H1} moiety displays bond paths
between all three centers, and these enclose a ring critical
À
À
point (RCP). Thus, 4 has direct Rh1 B1 and Rh1 H1
À
bonding interactions, while the B1 H1 bond is also intact.
À
Comparison with the Rh1 B1 interaction in C provides
similar 1(r) and H(r) values, but highlights a much reduced
bond ellipticity (e) of 0.08; this low value indicates dominant
s-bond character, whereas the value of 0.47 in 4 reflects the
asymmetry introduced by the B1-H1 unit. In D, the absence of
Rh-B BCPs confirms a lack of any direct Rh-B interaction,
ligand in 4 interacts with the rhodium centers through
À
stretched B H bonds that engage in strong Rh-H and Rh-B
interactions. Further support for this assertion comes from the
computed d(11B) chemical shifts (Figure 2) and the
Pipek–Mezey localized orbitals, where a strong bonding
interaction spanning all three Rh1, B1, and H1 centers was
identified (see Figure 3).
À
and this also reduces the average ellipticity of the Rh1 H1
À
and B1 H1 bonds. Also noticeable are the higher values of
p(r) and H(r) for the terminal B1-H4 bond in D compared to
the bridging B-H bonds in both that structure and, in
particular, 4, all of which is consistent with a weakening of
the latter. For E, the Rh1-H1 BCP has larger values for 1(r)
and H(r) than the Rh1-H1 BCP in 4, as well as a minimal
The mechanism of the room temperature fluxional
process observed for 4 was also probed with DFT calculations
and a single transition state was found to account for this
process (Scheme 4). This is accessed by cleavage of one (blue)
B-H bond to give a transition state structure featuring two
Rh-H-Rh bridging hydrides; movement of the original (red)
Rh-H-Rh hydride into a Rh-H-B bridging position then
completes the exchange (4’). Repeating this process from 4’
exchanges a second B-H hydrogen (black) into the Rh-H-Rh
bridging position (4’’). The computed free energy of activa-
tion is 55.2 kJmolÀ1, somewhat higher than the experimental
value (39.2 Æ 1.6 kJmolÀ1) but still consistent with facile room
temperature exchange.
À
e value. These data indicate a terminal Rh H s-bond and
stress the differences in bridging character of H1 and H2 in 4.
À
À
BCP data for the Rh1 H3 Rh2 bonds in 4, D, and E are very
similar, suggesting that this moiety varies little across these
three systems.
Taken together, the QTAIM analyses suggest that 4 is best
described as a m-amino-borane RhI species; a m-borylene
hydride RhIII formulism can certainly be ruled out in light of
À
À
À
the intact B1 H1/B1 H2 bonds and the lack of Rh1 H1/
Understanding how bimetallic species such as 3 and 4 are
formed, and subsequently react, is important for delineating
À
Rh2 H2 terminal hydride character. The m-amino-borane
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[Et2O·BH2·NH3][BArF ][19] to [Rh(LiPr)H]2 in Et2O solvent,
4
resulted in the immediate formation of 4 and gas evolution
(H2), which is consistent with the mechanism shown.
A dimeric species similar to 3 was also formed when one
equivalent of H3B·NMeH2 was added to 1 in THF solution.
This was characterized by in situ NMR spectroscopy and
Ph
F
=
ESI-MS as [Rh2(L )2(m-H)(m-H2B NMeH)][BAr ] 8:
4
d(1H) À6.84; d(31P{1H}) 22.2, 21.5; d(11B) 50.6.[21]
[THF·BH2·NMeH2][BArF ] was also formed (d(11B) 2.8 (t),
4
J
HB = 123 Hz; lit. Et2O adduct d(11B, CD2Cl2) 1.7 (t), JHB
=
Figure 3. Pipek–Mezey localized orbital, highlighting the bonding inter-
121 Hz[9]). A more complex mixture of species was formed
with H3B·NMe2H, suggesting steric factors may be important
in the formation of these aminoborane dimers, although
a signal observed at d(11B) 52.7 suggests dimer formation.
Complexes 3, 4, and 8 presumably form via a s-complex
À
action of the B1 H1 bond with center Rh1 (see Supporting Informa-
tion, Figure S28, for details and related orbitals spanning the
{Rh2B1H2} and {Rh1H3Rh2} moieties).
[Rh(LR)(H3B·NRH2)][BArF ], R = H (F Scheme 5) or Me. In
4
THF solution, using the LPh ligand, these s-complexes were
not observed as boronium formation and subsequent forma-
tion of 3 is fast. For LiPr, an intermediate s-complex could be
observed on the way to 4, [Rh(LiPr)(H3B·NH3)][BArF ],
Scheme 4. Proposed fluxional process occurring in 4 (and 3). Hydro-
gen atoms shown by filled circles. See Supporting Information for DFT
calculated geometries and energies.
4
presenting NMR data consistent with structure F.[21] Using
À
H3B·NMe3 (in which the N H bonds are absent)
[Rh(LiPr)(H3B·NMe3)][BArF ] (7) was isolated and structur-
4
their role in amine-borane dehydrocoupling. The single
ally characterized, confirming the in situ NMR studies
(Supporting Information, Figure S23). The rapid reaction of
equivalent of boronium [THF·BH2·NH3][BArF ] (5) formed
4
indicates that a hydride abstraction route may be operating,
as recently outlined by Conejero and co-workers for the
dehydrocoupling of H3B·NMe2H by cationic {Pt-NHC}+
catalysts,[17] as well as that occurring in cationic
Ru/Ir-systems[35] or with B(C6F5)3.[19] We reasoned that
a similar process would yield 5 by B-H activation[16] and
subsequent attack by THF (Scheme 5), alongside {Rh(LR)H}
[Et2O·BH2·NH3][BArF ] with [Rh(LiPr)H]2 to form 4 suggests
4
protonation is not slow for this system; currently we cannot
À
determine whether B H activation or boronium formation is
the rate limiting process, although it is likely that either could
be promoted by excess amine-borane via N-H···H-B inter-
actions.[37] Calculations on the {Pt-NHC}+/H3B·NMe2H
system suggest boronium formation is rate limiting.[17]
Complex 1 (0.5 mol%, THF, 3 hrs, open system) pro-
moted the dehydrocoupling of H3B·NH3 (1.2 equiv of H2
evolved by gas burette; Supporting Information, Figures
S4–S7) to form oligomeric species such as B-(cyclotriboraza-
nyl)amine-borane (BCTB),[3,38] and insoluble polyamino-
borane.[3] With more soluble H3B·NMeH2, polymethyla-
mino-borane was formed [H2BNMeH]n, which was isolated
by precipitation from hexanes (Mw = 30600 gmolÀ1, “ = 2.6),
alongside H2 (1.1 equiv, gas burette). Consistent with the
rapid formation of dimers such as 8 in THF, no induction
period was observed (as measured by H2 evolution) and
similar TOF values were recorded (ca. 200 hrÀ1 for 1 equiv
H2), starting from monomeric 1 or in situ formed dimeric 8
(Scheme 6).[39] Changing the solvent to non-nucleophilic
1,2-F2C6H4, and using 1 or in situ generated 8 as a catalyst,
did not present an induction period and also revealed a faster
TOF (for 8, ca. 1000 hrÀ1 with 1 equiv of H2 released).[40]
Sub-catalytic in situ experiments in this solvent[21] show that
Scheme 5. Mechanism of formation of 3 and 4 by boronium protona-
tion of neutral dimer H. (S)=THF or Et2O. [BArF ]À anions are not
4
shown.
dimer
8,
[(BH2)2NMeH(m-H)]
and
boronium
that would dimerize to give neutral [Rh(LR)H]2 (for example,
[(NH2Me)2BH2][BArF ] are present;[41] the latter is suggested
4
complex H). Subsequent protonation[17] by boronium 5 and
to arise from NMeH2 formed from B N bond cleavage in
À
=
elimination of H2 would give H2B NH2 trapped on a rhodium
H3B·NMeH2.[17] Thus, it is likely that similar active species are
present in THF or 1,2-F2C6H4. The lack of induction period is
in direct contrast to xantphos-based rhodium catalysts, which
show induction periods for H3B·NMeH2 dehydrocoupling in
C6H5F,[5,15] suggesting that a different kinetics regime or
mechanism is in operation.
dimer. To test this hypothesis, addition of 5 to the neutral
dimer is required. [Rh(LPh)H]2 is unknown, and our attempts
to prepare it have not been successful. [Rh(LiPr)H]2 is a known
complex, first prepared by Fryzuk in 1989,[36] and addition of
one
equivalent
of
the
known
boronium
salt
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the amino-borane-bridged dimers observed here.[15] Decon-
voluting these systems under conditions of high amine-borane
concentration is thus a significant challenge to address if
precise control over the resulting polyamino-borane is to be
achieved by metal/ligand design. Nevertheless, the observa-
tion of novel and unexpected bridging amino-borane com-
plexes as the first-formed species, offers tantalizing clues as to
the nature of the actual catalysts; and also suggests that
boronium cations may play a more general role in amine-
borane dehydrocoupling than generally appreciated.[17,19]
Acknowledgements
The EPSRC (A.S.W. and S.A.M., EP/M024210/1; N.A.B.,
DTP Studentship), the Rhodes Trust (A.K.), G. M. Adams (G.
P. C. analysis).
Scheme 6. H2 evolution experiments using 1 or 8, and H3B·NMeH2
(0.5 mol% [Rh], 0.41m amine-borane, THF, 298 K). [BArF ]À anions are
4
not shown.
Keywords: amino-borane · dehydrocoupling · DFT ·
Determination of the resting state in catalysis was
hampered by the addition of excess amine-borane
(H3B·NH3 or H3B·NMeH2) to the preformed dimeric species
3 or 4 in THF, resulting in a mixture of products that have
been resistant to characterization. Turning to the pure and
well-characterized dimer 4, initial rate measurements in
a closed system (4 mol% rhodium, THF) were more infor-
mative, and a first-order dependence for either H3B·NH3 or
H3B·NMeH2, as well as catalyst 4, were measured for the early
pseudo zero-order phase of catalysis (Supporting Informa-
tion, Figures S19 and 20). Such behavior is not consistent with
catalytic mechanisms · rhodium dimers
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6651–6656
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a rapid dimer–monomer equilibrium for which an order of
1
[4] = would be expected,[22,36,42] a view supported by the
2
stoichiometric reactions with acetonitrile or toluene (see
above). Under these conditions complexes 2 or 4 do not
promote full conversion of amine-borane (for 4, 70%
conversion of H3B·NH3 after 10 hrs). Informed by the sub-
catalytic experiments and H2 addition studies, we propose
that [Rh2(LiPr)2(H)2(m-H)3][BArF ][22] is formed during catal-
4
ysis.
Consistent
with
this
hypothesis,
poorer catalyst for
isolated
[Rh2(LiPr)2(H)2(m-H)3][BArF ] is
a
4
H3B·NH3 dehydrocoupling in a sealed system (4 mol%
[Rh], 30% conversion after 10 hrs) than both 2 and 4.
Interestingly, degassing the closed system restarted catalysis,
indicating that inhibition by the H2 formed during dehydro-
coupling is partially reversible (Supporting Information,
Figure S10). Co-promotion of dehydrocoupling by boronium
is discounted, as these studies show that isolated 4 is an active
pre-catalyst in its absence. Consistent with this statement,
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4
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Angew. Chem. Int. Ed. 2016, 55, 6651 –6656
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6655

Angewandte
Communications
Chemie
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Received: January 26, 2016
Revised: March 22, 2016
Published online: April 21, 2016
6656 www.angewandte.org
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 6651 –6656