Dehydrogenative boron homocoupling of an amine-borane

Article abstract of DOI:10.1002/anie.201304382

A meeting of borons: The homocoupling of a B-B bond in an amine-borane is reported to give a diborane coordinated to a Rh^I metal center (see picture; C gray/black, O red, P blue, B green, N orange). The pathway for the occurrence of this reaction was also studied using density functional theory to calculate the free energies of each intermediate. Copyright

Full text of DOI:10.1002/anie.201304382

.
Angewandte
Communications
DOI: 10.1002/anie.201304382
B-B Coupling
Dehydrogenative Boron Homocoupling of an Amine-Borane**
Heather C. Johnson, Claire L. McMullin, Sebastian D. Pike, Stuart A. Macgregor,* and
Andrew S. Weller*
Dedicated to Professor Larry Sneddon
The dehydrogenative coupling of amine-boranes as catalyzed
by transition-metal fragments offers the potential for con-
trolled hydrogen release and the formation of oligomeric and
polymeric materials in which head-to-tail coupling yields
ꢀ
products with B N bonds that are isoelectronic with techno-
logically pervasive polyolefins.[1–3] Because of this, the area
has received considerable attention recently and there are
now a wide range of catalysts available, which operate using
inner-sphere- or outer-sphere-type mechanisms,[4] that dehy-
drogenatively couple amine-boranes of the general formula
H3B·NRR’H (R, R’ = H, alkyl) to give monomeric, cyclic, or
=
polymeric amino-borane materials based on H2B NRR’. By
contrast, the homocoupling[5] of amine-boranes to form well-
ꢀ
defined products with B B single bonds has not been
reported, although dehydrogenation of H3B·NH3 by [Pd-
(NCMe)4][BF4] has been reported to form insoluble poly-
[6]
ꢀ
meric materials with B B bonds. This preference for
heterocoupling likely stems from the fact that B-H/N-H
activation of amine-boranes gives amino-boranes that are
well set up for further oligomerization through the formation
ꢀ
ꢀ
of dative B N bonds, a process that is also driven thermo-
Scheme 1. Homocoupling to form B B bonds.
dynamically by the differences in relative s-bond strengths
between B B (70 kcalmol ) and B N (107 kcalmol ) single
bonds. Well-defined homocoupling of boranes, as mediated
undergoes B-H activation at {Rh(PR3)n}+ fragments to give
bimetallic hydrido-boryl products (n = 1, R3 = Cy3),[17] or in
the presence of the alkene tert-butylethene (TBE, n = 2, R3 =
iBu2tBu) catalytic hydroboration occurs to afford
Me3N·BH2CH2CH2tBu, I.[18] The suggested mechanism for
this process involves reversible B-H activation to give
a hydrido-boryl complex, alkene insertion, and subsequent
reductive elimination of I. Homocoupling of H3B·NMe3 was
not observed, possibly because the approach of the second
equivalent of H3B·NMe3 to the metal is hindered. However,
the Ir pincer system Ir(tBuPOCOPtBu)(H)2 [tBuPO-
COPtBu = k3P, C, P-1,3-(OPtBu2)2C6H3] catalyzes the dehydro-
polymerization of H3B·NMeH2, for which polymer growth
kinetics suggest a coordination insertion mechanism consis-
ꢀ1
ꢀ1
ꢀ
ꢀ
ꢀ
by transition metals, is essentially limited to B B bond
formation in polyhedral boranes, for example pentaborane(9)
(A),[7,8] guanidine bases (B),[9] and most recently the homo-
coupling of HBCat and related derivatives to give the
corresponding diboranes (C)[10–12] (Scheme 1). By contrast,
the homocoupling of phosphines or silanes is well estab-
lished.[13,14]
The homocoupling of boranes requires the B-H activation
of two boranes at a metal center; we, and others, have recently
reported on B-H activation at group 9 metal centers in both
amine- and amino-boranes.[15,16] In particular, H3B·NMe3
tent with the activation of two amine-boranes at the metal
[3]
[*] H. C. Johnson, S. D. Pike, Prof. A. S. Weller
Department of Chemistry, University of Oxford
Mansfield Road, Oxford, OX1 3TA (UK)
E-mail: andrew.weller@chem.ox.ac.uk
ꢀ
center before B N bond formation. Taking clues from this
and using a related pincer system based on the {Rh-
(Xantphos)}+ fragment,[19] we now report that H3B·NMe3
undergoes stoichiometric homo-dehydrocoupling to form
the diborane(4) H4B2·2NMe3 II (Scheme 1D), a compound
previously synthesized from the combination of NMe3 with
B3H7L (L = THF, SMe2).[20]
Dr. C. L. McMullin, Prof. S. A. Macgregor
School of Engineering & Physical Sciences
Institute of Chemical Sciences, Heriot-Watt University
Edinburgh, EH14 4AS (UK)
Addition of H3B·NMe3 to the precursor [Rh(k2
-
E-mail: s.a.macgregor@hw.ac.uk
P, P
Xantphos)(NBD)][BArF ][21] under a H2 atmosphere resulted
[**] We thank the EPSRC for funding.
4
in rapid hydrogenation of the diene and coordination of the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304382.
amine-borane to the resulting RhIII dihydride to give
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[Rh(k3P, O, P-Xantphos)(H)2(h1-H3B·NMe3)][BArF ],
2,
in
alkene TBE to 2 results in rapid (less than 15 minutes) anti-
4
essentially quantitative yield as measured by NMR spectros-
copy (Scheme 2). Complex 2 was also characterized by single-
crystal X-ray diffraction (Figure 1), which demonstrates
a pseudo octahedral RhIII center with an H3B·NMe3 ligand
Markovnikov hydroboration of the alkene to form [Rh(k2
-
P, P
Xantphos)(h2-Me3N·H2BCH2CH2tBu)][BArF ] 3, in which the
4
resulting alkyl borane I interacts with a RhI center through
two Rh-H-B interactions (Scheme 3). Figure 1 shows the
Scheme 3. Complex 3 and the catalytic hydroboration of TBE. [BArF ]ꢀ
anions not shown.
4
Scheme 2. Synthesis of complex 2. [BArF ]ꢀ anions not shown.
4
solid-state structure of 3, and solution spectroscopic
data are in full accord with this and are very similar to
those reported previously for coordination of this
borane with the {Rh(PiBu2tBu)2}+ fragment, a complex
that is also formed from hydroboration of TBE.[18] We
propose this process occurs by way of an initial
sacrificial hydrogenation of TBE to form a RhI species
that then undergoes rapid B-H activation, which in the
presence of further TBE follows through to hydro-
boration. This hydroboration is also catalytic (5 mol%,
0.013m 3, ToN 20, three hours) and at the end of
catalysis, 3 is recovered as the only organometallic
product.
Complex 3 provides a starting point to investigate
the reaction of H3B·NMe3 with the RhI {Rh-
(Xantphos)}+ fragment. Addition of excess (20 equiv-
alents) H3B·NMe3 to 3 resulted in the slow (one hour)
transformation to form a new complex 4, alongside 2,
in an approximately 50:50 ratio as measured by NMR
spectroscopy (Scheme 4). Free I was also released.
Figure 1. Displacement ellipsoid plots (30%) of the cationic portion of 2 (left)
and 3 (right). Only one of the two independent cations in the unit cell is shown
for 2 and a crystallographically imposed mirror plane bisects each cation. All
carbon-bound hydrogen atoms are omitted for clarity. Selected bond lengths
ꢀ
ꢀ
ꢀ
ꢀ
(ꢀ): 2: Rh1 P1, 2.2683(10); Rh1 O1, 2.199(3); Rh1 B1, 2.759(6); Rh1 H3,
ꢀ
ꢀ
ꢀ
ꢀ
1.759; B1 N1, 1.609(8). 3: Rh1 P1, 2.2678(12); Rh1 P2, 2.2494(12); Rh1 O1,
ꢀ
ꢀ
3.431(3); Rh1 B1, 2.162(5); B1 N1, 1.604(4).
ꢀ
coordinated through a single Rh-H-B interaction (Rh1 B1,
2.759(10) ꢀ), and a mer-k3 Xantphos ligand (Rh1 O1,
ꢀ
2.199(3) ꢀ).[22] Complex 2 is closely related to acetone and
NCMe adducts such as [Rh(k3P, O, P-Xantphos)(H)2(NCMe)]-
[BArF ].[21] The solution NMR data (Supporting Information)
4
are in full accord with the solid-state structure and are
consistent with the h1-coordination mode of the borane.[23]
The combined NMR and structural data also suggest that the
borane is only weakly bound, and consistent with this it can be
liberated by addition of NCMe to 2 to form the corresponding
adduct (see above).[21] Complex 2 is a rare example of an
amine-borane complex with a pincer-type ligand,[24] although
a simple borane adduct Ir(H)2(tBuPOCOPtBu)(BH3) has
been described,[25] and a related silane complex is also
Scheme 4. Synthesis of complex 4. [BArF ]ꢀ anions not shown.
4
known.[26] Complex
2
does not react with additional
Complex 4 could be separated from 2 by fractional recystal-
lization and was identified by NMR spectroscopy, ESI-MS,
and single-crystal X-ray diffraction as [Rh(k2P, P-Xantphos)(h2-
H3B·NMe3, remaining unchanged upon addition of 20 equiv-
alents under a H2 atmosphere.
Complex 2 is unstable when not under an atmosphere of
H2, slowly decomposing to give unidentified materials,
presumably through the loss of H2 to give a RhI amine-
borane species that undergoes B-H activation to a RhIII
hydrido-boryl, which is unstable in the absence of excess
H3B·NMe3. To probe this, addition of two equivalents of
H4B2·2NMe3)][BArF ]. Figure 2 shows the solid-state struc-
4
ture that demonstrates that the homocoupling of two
H3B·NMe3 molecules has occurred on the metal center to
afford the diborane H4B2·2NMe3 II. The hydrogen atoms of
the borane were located and show that coordination with the
metal center occurs through two vicinal Rh-H-B interactions,
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also consistent with significant B-H-Rh interactions. Dibor-
ane II is tightly bound to the metal center, and is not displaced
by excess NCMe, similar to M(CO)4(H4B2·2PMe3).[27] This
also means that the system is not catalytic, because H3B·NMe3
will not displace II.
Complex 4 is formed with 2 in approximately equal
amounts, alongside I, and a mechanism accounting for these
transformations, supported by DFT calculations[31] on a model
Me-Xantphos system, is shown in Scheme 5. Displacement of
Figure 2. Displacement ellipsoid plots (30%) of the cationic portion of
4. All carbon-bound hydrogen atoms are omitted for clarity. Selected
ꢀ
ꢀ
bond lengths (ꢀ) and angles (8): Rh1 P1, 2.2834(9); Rh1 P2,
ꢀ
ꢀ
ꢀ
2.2720(10); Rh1 B1, 2.405(4); Rh1 B2, 2.411(5); Rh1 O1, 3.304(3);
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
B1 B2, 1.678(7); B1 N1, 1.621(7); B2 N2, 1.619(6); P1 Rh1 P2,
101.56(4); angle between planes: P1Rh1P2/B1B2Rh1, 24.38.
leading to an eclipsed conformation of the diborane, and thus
Scheme 5. Proposed key steps in the formation of 4 and 2 from 3.
ꢀ
overall C1 symmetry. The B B distance, 1.678(7) ꢀ, is
[BArF ]ꢀ anions not shown. DFT-computed relative free energies are
4
consistent with a single bond, and is shorter than those
observed for Cr(CO)4(H4B2·2PMe3) (1.748(11) ꢀ)[27] and
[Cu(H4B2·2PMe3)2]I (1.80(2) ꢀ),[28] which both show similar
conformations for the bidentate diborane. In contrast to 4,
these are formed from the preformed diborane and the metal
fragment. The cation adopts a RhI pseudo-square-planar
structure, although the B-B axis is twisted somewhat from
being planar with the P2Rh plane, 24.38. We propose that this
distortion is electronic in origin, to allow for maximal overlap
of the bridging Rh-H-B interactions, as calculations on
a model Me-Xantphos system (i.e. where the Xantphos Ph
groups are replaced with Me substituents) show a similar
geometry (Supporting Information). The Rh-B distances
(2.405(4), 2.411(5) ꢀ) lie in between those measured in 2
and 3.
indicated in kcalmolꢀ1. [Rh]=[Rh(Xantphos)] (experimental) or
[Rh(Me-Xantphos)] (calculations).
I from 3 by H3B·NMe3 leads to adduct E (G =+ 0.3 kcal
molꢀ1). This can undergo rapid, but reversible B-H activation
to an initial RhIII hydrido-boryl intermediate that is trapped
with excess H3B·NMe3 to give F (G =+ 1.6 kcalmolꢀ1). A
rate-limiting process that involves B-B coupling then leads to
the RhIII intermediate G (G =+ 3.6 kcalmolꢀ1), which in the
presence of unreacted 3 and excess H3B·NMe3 undergoes
ligand redistribution to afford 2 and 4 along with displaced I.
Complex 2 does not promote B-B coupling and thus the
reaction stops. However, addition of excess cyclohexene
(which is not hydroborated in this system) to the mixture of 2,
4, and excess H3B·NMe3 leads to the generation of a putative
RhI species, alongside cyclohexane (GC-MS), that can then
mediate the coupling (Scheme 4). In this way, nearly quanti-
tative yields of 4 (greater than 99%) can be acheived. The
strong binding of II in 4 means that it is not displaced by
H3B·NMe3, (DG for this exchange was calculated to be
+ 20.6 kcalmolꢀ1) and thus the system is not catalytic. The
structures and bonding in diborane metal complexes of
guanidine bases (Scheme 1B) have recently been dis-
cussed,[32] in which the bonding was proposed to vary from
being dominated by B-H-M interactions to cases where
The solution NMR data of 4 are consistent with the gross
solid-state structure. However only one environment was
observed in the 31P{1H} NMR spectrum (d = 26.2 ppm,
J(RhP) 172 Hz); while only two BH (d = 1.51, ꢀ8.47 ppm),
one NMe3, and one Xantphos methyl environment are
1
observed in the H NMR spectrum. This suggests a fluxional
process is occurring at room temperature that makes both
phosphorus and {BH2NMe3} groups equivalent. Because two
different BH environments are observed (one terminal and
one bridging), we discount a mechanism for this that involves
dissociation of one Rh-H-B interaction, rotation around the
[29]
ꢀ
ꢀ
ꢀ
B B bond, and recoordination. Instead a simple inversion
B B···M bonding prevails (elongation of the B B bond and
only small upfield chemical shift change of the B-H unit). We
believe the first description is more accurate here because
of the Xantphos ligand would account for the observed C2
symmetry. Such behavior has been noted previously.[30] Cool-
ing a solution (CD2Cl2) of 4 to 218 K arrests this process so
that, for example, two different Rh-H-B (d = ꢀ8.02,
ꢀ8.55 ppm) and 31P (d = 29.0 ppm, J(RhP) 164 Hz; d =
28.2 ppm, J(RhP) 168 Hz) environments are observed, the
latter in an ABX pattern. These upfield chemical shifts are
ꢀ
1) the B B distance in II was calculated to shorten upon
complexation in 4 (from 1.76 ꢀ to 1.70 ꢀ) and 2) an atoms-in-
molecules (AIM) analysis of the topology of the electron
density in 4 highlighted the presence of bond critical points
(bcps) between Rh and the hydrogen atoms bridging the Rh-
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B1 and Rh-B2 connectivities. No bcp was located between Rh
and either B1 or B2, although a ring critical point was seen
The formation of the final observed products (2 + 4 + I)
from G (+ 3 + H3B·NMe3) was calculated to be exergonic by
10.2 kcalmolꢀ1 (see Scheme 5). A mechanism for this process
might involve displacement of II in G by H3B·NMe3 to give 2,
although this process is rather unfavorable (DG =+ 10.2 kcal
molꢀ1). Alternatively, H2 loss from G would form 4 (DG =
ꢀ2.6 kcalmolꢀ1) with H2 then reacting with E to give 2 (DG =
ꢀ7.9 kcalmolꢀ1). A series of associative ligand displacement
processes can also not be discounted. Calculated structures of
2, 3, and 4 agree well with the crystallographic data
(Supporting Information).
ꢀ
between Rh and the center of the B1 B2 bond. The
dominance of B-H-M interactions is also consistent with the
spectroscopic and structural markers highlighted above for 4.
Further calculations allow the details of the mechanism
outlined in Scheme 5 to be elucidated. Both transformations
E!F and F!G are multistep processes and the calculations
confirmed B-H activation in E is more accessible (DG° =
+ 8.9 kcalmolꢀ1) than the subsequent B-B coupling (DG° =
+ 27.5 kcalmolꢀ1). The calculated pathway for the formation
of G from F is shown in Scheme 6. The most stable form of F
In summary, we report the metal-mediated homocoupling
of an amine-borane (H3B·NMe3) that gives a diborane
coordinated to a RhI center. The mechanism for this process
is suggested to operate through sequential B-H activation, the
ꢀ
second of these combined with a B B bond forming step.
Aspects of this mechanism might also be generally applicable
to related B-N coupling events that lead to dehydropoly-
merization when using substrates such as H3B·NMeH2. Such
a process would require B-H and N-H activation coupled with
ꢀ
B N bond formation and no loss of amino-borane from the
metal center, as has been suggested[3,35,36] and shown for the
closely related phosphine-borane dehydrocoupling on a RhI
fragment.[37] Whether a cationic rhodium complex such as 3
catalyzes the dehydropolymerization of H3B·NMeH2 has yet
to be reported, but the close similarity to systems that do, such
as Ir(tBuPOCOPtBu)(H)2, encourages further investigations.
Scheme 6. Computed pathway for formation of G from F by way of
Received: May 21, 2013
Published online: July 19, 2013
B-B coupling and rearrangement. Calculated free energies (kcalmolꢀ1
)
are relative to 3 plus two H3B·NMe3 units. [Rh]=[Rh(Me-Xantphos)].
Keywords: amine-borane · boron homocoupling ·
.
(G =+ 1.6 kcalmolꢀ1) has the hydride ligand (H1) trans to one
Xantphos arm and isomerisation to move H1 into an axial
position is induced by H5 transfer between the boron centers.
This leads to Int1 (G =+ 14.8 kcalmolꢀ1) in which the two
{BH2NMe3} fragments are now in a cis arrangement. B-B
coupling can now occur and is triggered by activation of the
B1-H5 bond via TS2 (G =+ 28.5 kcalmolꢀ1) to give dihydride
Int2 (G =+ 15.0 kcalmolꢀ1). The {B2H4(NMe3)2} unit is now
established, albeit with bridging hydrides on the Rh-B1 and
B1-B2 connectivities. The required rearrangement involves
B1-H3 bond activation to give the dihydrogen hydride species
Int3 (G =+ 21.5 kcalmolꢀ1). Such reductive coupling of
hydride ligands has been noted upon B-H activation of
a neighboring amine-borane ligand.[33] H4 can then shift from
a bridging to a terminal position on B2 and this also induces H6
to bridge the Rh-B2 bond (Int4, G =+ 21.7 kcalmolꢀ1). B1-H3
bond coupling (with concomitant oxidative cleavage of the
dihydrogen ligand) completes the formation of G (G =
3.6 kcalmolꢀ1). B-B coupling therefore proceeds with an
overall computed barrier of 29.1 kcalmolꢀ1 with the highest-
lying transition state corresponding to a rearrangement of the
{RhB2H6(NMe3)2} unit (TS4) rather than the actual B-B
coupling event (TS2). Although this barrier is rather high for
a process occurring at room temperature, it does include
a significant entropic contribution that is likely to be over-
estimated in the calculations (for example, in TS2 the TDS
term is + 13.8 kcalmolꢀ1).[34]
dehydrogenative coupling · rhodium · X-ray diffraction
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