Dehydrocoupling of dimethylamine borane catalyzed by Rh(PCy 3)2H2Cl

Article abstract of DOI:10.1021/ic302804d

The Rh(III) species Rh(PCy₃)₂H₂Cl is an effective catalyst (2 mol %, 298 K) for the dehydrogenation of H ₃B·NMe₂H (0.072 M in 1,2-F₂C ₆H₄ solvent) to ultimately afford

Full text of DOI:10.1021/ic302804d

Article
pubs.acs.org/IC
Dehydrocoupling of Dimethylamine Borane Catalyzed by
Rh(PCy3)2H2Cl
Laura J. Sewell, Miguel A. Huertos, Molly E. Dickinson, and Andrew S. Weller*
Department of Chemistry, Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, United
Kingdom
Guy C. Lloyd-Jones
School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom
S
* Supporting Information
ABSTRACT: The Rh(III) species Rh(PCy3)2H2Cl is an
effective catalyst (2 mol %, 298 K) for the dehydrogenation
of H3B·NMe2H (0.072 M in 1,2-F2C6H4 solvent) to ultimately
afford the dimeric aminoborane [H2BNMe2]2. Mechanistic
studies on the early stages in the consumption of H3B·NMe2H,
using initial rate and H/D exchange experiments, indicate
possible dehydrogenation mechanisms that invoke turnover-
limiting N−H activation, which either precedes or follows B−
H activation, to form H2BNMe2, which then dimerizes to
give [H2BNMe2]2. An additional detail is that the active
catalyst Rh(PCy3)2H2Cl is in rapid equilibrium with an
inactive dimeric species, [Rh(PCy3)H2Cl]2. The reaction of
Rh(PCy3)2H2Cl with [Rh(PCy3)H2(H2)2][BArF ] forms the halide-bridged adduct [Rh(PCy3)2H2(μ-Cl)H2(PCy3)2Rh][BArF ]
4
4
(ArF = 3,5-(CF3)2C6H3), which has been crystallographically characterized. This dinuclear cation dissociates on addition of
H3B·NMe2H to re-form Rh(PCy3)2H2Cl and generate [Rh(PCy3)2H2(η2-H3B·NMe2H)][BArF ]. The fate of the catalyst at low
4
catalyst loadings (0.5 mol %) is also addressed, with the formation of an inactive borohydride species, Rh(PCy3)2H2(η2-H2BH2),
observed. On addition of H3B·NMe2H to Ir(PCy3)2H2Cl, the Ir congener Ir(PCy3)2H2(η2-H2BH2) is formed, with concomitant
generation of the salt [H2B(NMe2H)2]Cl.
borane H3B·NMe2H, A (Scheme 1a), there is nominally a
single final product, [H2BNMe2]2, C, this being formed via an
initial dehydrogenation of A and then dimerization of the
resulting aminoborane H2BNMe2, Z. This apparent
simplicity has allowed for deeper insight into both the boron
products formed during dehydrocoupling and the role of the
metal catalyst.27−32 In addition to H2BNMe2, the linear
dimer H3B·NMe2BH2·NMe2H, B, has also been observed as an
intermediate in some systems.4,27,28,33−35 Complex B can arise
from direct coupling of two A’s, as has been shown for Ti(II)-
based systems,10 although recent results suggest the active
catalyst is Ti(III),36 or from coupling of A and Z at a metal
center.28,29 For the transition-metal catalysts, inner-sphere
activation via σ-B−H−M interactions37 is implicit and N−H
activation is involved in the rate-limiting step in many cases.
Outer-sphere dehydrogenation mechanisms have been pro-
posed to operate in a manner related to alcohol oxidation using
bifunctional catalysts;14,15,38 while d0 metal catalyst systems
(groups 2 and 13) show complementary, but different,
INTRODUCTION
■
The dehydrocoupling of amine boranes, H3B·NR2H or
H3B·NRH2 (R = alkyl), as catalyzed by transition-, alkaline-
earth-, and main-group-metal−ligand complexes, has attracted
considerable recent interest.1−5 This is due to the potential for
control over H2 release kinetics necessary for chemical
hydrogen storage applications, for which the parent amine
borane, H3B·NH3, has a high concentration (wt %) of
hydrogen,6−9 or the formation via dehydropolymerization of
H3B·NRH2 of novel B−N polymeric materials that are
isoelectronic with polyolefins.10−12 Mechanistic studies probing
dehydrogenation and subsequent coupling for H3B·NH3
generally rely on the observation of non-metal-containing
boron intermediates or final products, although there are
reports that comment in detail on the specific role of the
metal.13−18 For the primary amine boranes H3B·NRH2 final
products can be polyaminoboranes, arising from dehydropoly-
merization, or borazines. Recent advances have demonstrated
the isolation of metal-bound aminoboranes18−22 and oligome-
rization products23 or the observation of hydrogen redistrib-
ution (transfer hydrogenation) reactions between amino-
boranes and amine boranes.24−26 For the secondary amine
Received: December 20, 2012
Published: April 1, 2013
© 2013 American Chemical Society
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Scheme 1. (a) Observed Intermediates and Final Product C
in the Dehydrocoupling of A Using Transition-Metal
Catalysts27−29 and (b) General Scheme for Dehydrocoupling
of A Based upon Studies Using the {Rh(PCy3)2}+ System33
Scheme 2. Neutral and Cationic Catalysts for
Dehydrocoupling of H3B·NMe2H Using {Rh(PCy3)2H2}+
Fragments
cationic {Rh(PCy3)2H2}+ fragment, i.e., under the conditions
found in cationic catalysis. We find the mode of consumption
of A is consistent with the constant oxidation state Rh(III)
portion of the overall scheme for dehydrocoupling (Scheme 1).
Aspects of this work have been briefly discussed in introducing
1 as a plausible catalyst in the cationic dehydrogenation
system.33
mechanisms.39−41 Systems that involve multimetallic activation
of amine boranes have also been reported.42,43
We have recently reported on the use of a variety of
{M(L2)}+ fragments [M = Rh, Ir; L2 = (PR3)2 or chelating
phosphine] to probe the dehydrocoupling of A or
H3B·NMeH2.23,29,34,35,44−48 Using [Rh(PCy3)2][BArF ] as a
4
RESULTS AND DISCUSSION
precatalyst [ArF = 3,5-C6H3(CF3)2], we were able to propose a
detailed mechanistic model, supported and informed by kinetic
simulations, that demonstrates a complex and nuanced
mechanistic landscape in which the cationic rhodium catalyst
shuttles between a fast Rh(I)/Rh(III) regime and a slower
constant oxidation state rhodium(III) dihydride regime,
Scheme 1b.33 In particular, this mechanism invokes C as a
modifier in catalysis, in which it acts in an autocatalytic role
moving the system between the slow rhodium(III) dihydride
regime and the fast Rh(I) regime by promoting reductive
elimination of H2. Simulations, verified by experiment, also
suggested the presence of an additional parallel catalyst in low,
but invariant, concentrations that promoted the pseudo-first-
order consumption of A. On the basis of preliminary
experiments, we suggested a plausible formulation for this
additional catalyst was neutral Rh(PCy3)2H2Cl,49 1 (Scheme
2). This would form from rapid hydrogenation of [Rh-
(PCy3)2Cl]2,50,51 which itself is likely formed via traces of
chloride that could be in low but saturated concentrations in
the solvent. Neutral complex 1 operates relatively rapidly to
dehydrogenate A to form aminoborane Z, which then can
either enter the “cationic” cycle or simply dimerize to form final
product C. Pertinently, Duckett and co-workers have suggested
on the basis of NMR diffusion measurements (DOSY) that 1 is
in fact a chloride-bridged dimer in solution, similar to
H2Rh(PPh3)2(μ-Cl)2Rh(PPh3)2H2.52 Interestingly, the struc-
ture of closely related Rh(PiPr3)2H2Cl shows it to be a
monomer in the solid state,53 although this does not rule
against a dimeric formulation in solution.
■
Reactivity Studies on Rh(PCy3)2H2Cl. Before describing
the role of 1 in catalysis directly, we first discuss its likely form
in the cationic system where {Rh(PCy3)2H2}+ is present in
excess (5 mol % total catalyst loading, 0.072 M substrate). In
this system the observed resting state is reported to be the
Rh(III) σ-amine borane complex [Rh(PCy3)2H2(η2-
H3B·NMe2H)]+, 2, as shown by 31P{1H} NMR spectroscopy.33
Under these conditions of catalyst concentration, a detection
limit of ca. 10% of the total catalyst concentration is not
unreasonable by 31P{1H} NMR spectroscopy, providing a
threshold for detection of ca. 0.5 mol % and above for any other
species present. Combination of equal amounts of 1 with
[Rh(PCy3)2H2(H2)2][BArF ],54 as a source of the {Rh-
4
(PCy3)2H2}+ fragment, immediately forms the new chloride-
bridged complex [Rh(PCy3)2H2(μ-Cl)H2(PCy3)2Rh][BArF ],
4
3, which was characterized by NMR spectroscopy, electrospray
ionization mass spectrometry (ESI-MS),55 and single-crystal X-
ray diffraction (Scheme 3). The solid-state structure of 3,
Figure 1, exhibits a Rh−Cl−Rh core, in which the chloride sits
on a special position in the unit cell, resulting in half of the
molecule being generated by crystallographically imposed
symmetry. In solution at room temperature (298 K, CD2Cl2)
broad signals are observed in the 1H and 31P{1H} NMR spectra
for 3, in particular a broad environment at δ −24.2 in the
1
hydride region of the H NMR spectrum and a single broad
environment at δ 49.6 in the 31P{1H} NMR spectrum. Cooling
to 200 K resolved these broad signals so that three separate, but
very similar, species were observed in both the 31P{1H} and the
1H NMR spectra, at individual chemical shifts that correspond
well with the weighted-average room temperature chemical
shifts, suggesting rapid interconversion among the three at 298
In this Article we explore the mechanism by which 1
dehydrogenates A and also comment on the likely species
present in catalysis when 1 is combined with an excess of a
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during catalysis), it rapidly (time of mixing) reacts to return 1
and 2 (Scheme 3). At the end of catalysis, at low
[H3B·NMe2H], 3 might also re-form (vide infra). Complex 3
is also broken up in the presence of H3B·NMe3 and
H3B·NMe2BH2·NMe2H (B) to form 1 and the corresponding
Scheme 3. Synthesis and Reactivity of 3
amine borane adducts [Rh(PCy3)2H2(η2-H3B·NMe3)][BArF ]
4
and [Rh(PCy3)2H2(η2-H3B·NMe2BH2·NMe2H)][BArF ], re-
4
spectively.33
Complex 1 undergoes H/D exchange with D2 to afford
Rh(PCy3)D2Cl, as previously reported by James.49 Presumably
this occurs via a monomeric (or monobridged dimer) σ-bound
intermediate, Rh(PCy3)2H2(D2)Cl, that then undergoes a σ-
CAM (CAM = complex-assisted metathesis) exchange
process.56,57 Related to this, Duckett has reported that the
monometallic pyridine adduct reversibly forms on addition of
pyridine to 1,52 demonstrating reversible coordination of a
Lewis base.
Catalysis. Using our standard open conditions under a slow
flow of Ar, i.e., not in a sealed NMR tube (298 K, 0.072 M A, 2
mol % 1, 1,2-F2C6H4 solvent), complex 1 efficiently promotes
the dehydrogenation of A to ultimately afford the cyclic
aminoborane [H2BNMe2]2, C (Scheme 4). The reaction
Scheme 4. Dehydrocoupling of H3B·NMe2H, A
essentially goes to completion (i.e., ToN = 48), taking 1.7 h
to reach 95% conversion. This can be compared to the
Rh(PHCy2)3Cl catalyst, reported by Manners and co-workers,
that in the presence of B(C6F5)3 (to remove one phosphine)
mediates complete conversion of A to C in 10 h at 1 mol %.58
A
time−concentration profile for 1 as the catalyst is shown in
Figure 2a. A significant concentration of the aminoborane
H2BNMe2, Z, is observed, which dimerizes to form C. The
second-order rate constant for this process has been
determined in various solvents, in which a metal fragment is
not implicated in the dimerization.26,29,33 Although the overall
kinetics for catalysis are complex, the consumption of A follows
Figure 1. Solid-state structure of complex 3. The [BArF ]− anion and
4
most hydrogen atoms are omitted for clarity. Displacement ellipsoids
are shown at the 30% probability level. The crystallographically
equivalent atoms are generated by the operation −x, y, −z + 1/2.
Selected bond distances (Å) and angles (deg): Rh1−P1, 2.3258(8);
Rh1−P2, 2.3295(7); Rh1−H1, 1.51(3); Rh1−H2, 1.51(3); Rh1−Cl1,
2.4813(3); Rh1−Rh1′, 4.831(8); P1−Rh1−P2, 156.75(3); Rh1−Cl1−
Rh1′, 153.61(5).
pseudo-first-order kinetics, k = (1.03
0.05) × 10−3 s−1
(Figure 2b). This behavior is consistent with the cationic
catalyst system at 5 mol % (0.072 M A) with a parallel catalyst
in low concentration (0.5 mol % or less), which also shows a
pseudo-first-order decay of A, and for which kinetic modeling
suggests a pseudo-first-order rate constant similar to that
0.01) × 10−3 s−1.33 Under open
K (Figure S-1, Supporting Information), viz., δ 49.7 [d, J(RhP)
= 115 Hz, ∼10%], 47.5 [d, J(RhP) = 114 Hz, ∼10%], 46.1 [d,
J(RhP) = 113 Hz, 80%]. We assign these three species to an
equilibrium mixture of 3 and the dissociated monomers, neutral
determined here: (0.58
conditions but at 1 mol % (i.e., an effective 0.5 mol %
concentration of 1), complex 3 also catalyzes the dehydrogen-
ation of A (Figure S-4, Supporting Information) and also
follows a pseudo-first-order profile with a rate constant k =
(0.37 0.01) × 10−3 s−1, again broadly consistent with that
measured in the cationic system.
Under sealed NMR tube conditions (298 K, 0.072 M A, 2
mol % 1, 1,2-F2C6H4 solvent), complex 1 is a competent
catalyst for the dehydrogenation of A to ultimately afford C. In
contrast to the open system, this reaction does not go to
completion, with only 70% conversion observed (i.e., ToN =
35). A time−concentration profile for this reaction is shown in
Figure S-5 (Supporting Information). Although complete
conversion to C is not observed, addition of more A to the
catalyst system (by opening the NMR tube to Ar, addition of
1 and cationic [Rh(PCy3)2H2L2][BArF ]54 (L = CH2Cl2 or
4
agostic interaction), in a ratio of 8:1:1, respectively, at 200 K.
1
The 31P{1H} and H NMR spectra of independently prepared
[Rh(PCy3)2H2L2][BArF ] at 200 K support this assignment,
4
i.e., δ 47.5 [d, J(RhP) = 110 Hz]. The observation of a single
1
31P and H (hydride) environment for 3 at 200 K suggests a
shallow potential energy profile for small changes in the Rh−
Cl−Rh angle that allows for the equivalence of the hydrides and
phosphorus environments.
Although complex 3 would likely form when 1 is in the
presence of an excess of a latent source of {Rh(PCy3)2H2}+,
under the additional constraint of excess H3B·NMe2H (i.e.,
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Figure 2. (a) 11B concentration over time for the dehydrocoupling of H3B·NMe2H (initial concentration 0.072 M) using Rh(PCy3)2H2Cl (0.1 mL
samples diluted with 0.25 mL of 1,2-C6H4F2 under argon). (b) First-order plot of consumption of H3B·NMe2H. Key: black circles, H3B·NMe2H, A;
black triangles, H2BNMe2, Z; gray triangles, H3B·NMe2BH2·NMe2H, B; black squares, [H2BNMe2]2, C; gray squares, HB(NMe2)2 (trace).
Table 1. Initial Rates for the Dehydrocoupling of H3B·NMe2H, A, Using Rh(PCy3)2H2Cl in a Sealed System (High-Pressure
a
NMR Tube, 298 K, 1,2-F2C6H4 Solvent) at Given Initial Concentrations of A
entry
[Rh] (M)
[H3B·NMe2H] (M)
[D3B·NMe2H] (M)
[H3B·NMe2D] (M)
[D3B·NMe2D] (M)
initial rate (10−5 M s−1
)
1
2
3
4
5
6
7
8
9
0.00144
0.00144
0.00144
0.00072
0.00288
0.00144
0.00144
0.00144
0.00114
0.072
0.036
0.144
0.072
0.072
13.8 0.4
5.5 0.4
26.6 0.4
9.9 0.4
19.5 0.4
11.5 0.4
2.6 0.4
2.7 0.4
14.0 0.4
0.072
0.072
0.072
b
0.072
a
b
Catalysis does not run to completion under these conditions; see the text. A 25-fold excess of C was added.
Figure 3. Initial rate versus concentration for (a) [DMAB] and (b) [Rh(PCy3)2H2Cl]1/2
.
more A, and then resealing) gives essentially the same
conversion (70%) and a very similar reaction profile (Figure
S-6, Supporting Information). This demonstrates the catalyst
remains active and does not decompose significantly. We assign
this incomplete conversion under sealed conditions to
inhibition by H2 formed from the dehydrocoupling, as no
inhibition is observed in the open system. Interestingly, under
these sealed tube conditions, we see very little of the linear
dimer intermediate B, unlike in the cationic system,35 although
Z is still observed in appreciable concentrations.
Given the relative complexity of the overall system and the
significant challenge in modeling the holistic temporal
evolution of the starting materials, intermediates, and final
products, we chose to study the mechanism for dehydrogen-
ation of A using the method of initial rates,59 combined with
isotopic labeling, to determine the order of the reaction and the
rate-limiting processes.
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Initial Rate and Labeling Experiments. Table 1 shows
the results of initial rate experiments conducted under sealed
NMR tube conditions (298 K, 1, 1,2-F2C6H4 solvent). These
data were fitted to the approximately linear region of A
consumption over the first 180 s of catalysis.
intermediate resulting from B−H activation (vide infra; see
D, Scheme 6), attenuating the rate-limiting N−H activation
that is proposed to occur via β-hydrogen transfer to a vacant
site on the metal. Under all these conditions of catalysis (open
and sealed) we did not observe an induction period, the
solutions retained their homogeneous appearance through the
course of catalysis, and the rate of catalysis was not significantly
affected by the addition of Hg(0). These observations point
toward a homogeneous rather than a heterogeneous
process,74,75 although caution should always be exercised in
definitively ruling out a heterogeneous process.76
Having established that a dimer−monomer equilibrium
operates in catalysis, and N−H activation is involved in the
turnover-limiting step, H/D exchange experiments allowed for
further insight into the mechanism, and in particular for
probing of the relative order of N−H and B−H activation.
Treatment of Rh(PCy3)2D2Cl with H3B·NMe3 (which has no
NH and therefore does not undergo dehydrogenation) resulted
in no H/D exchange at the Rh−H or B−D sites, in contrast to
other related, cationic systems.23,35 In these examples exchange
is suggested to occur via a B−H/Rh−H σ-CAM56,57 process
that generates a base-stabilized boryl (see Scheme 6) which
then can re-form the σ-complex with scrambling of H and D.
Such base-stabilized boryls are also invoked in the hydro-
boration of alkenes by H3B·NMe3 as catalyzed by a {Rh-
(PR3)2}+ fragment.77 In contrast, when D3B·NMe2D was
subjected to catalysis using 1 (2 mol %) but under 4 atm of
H2, this resulted in rapid H/D exchange at boron in the amine
borane starting material as measured by 11B NMR spectroscopy
(Figure S-8, Supporting Information) but not at N, to the
detection limits of 1H NMR spectroscopy, with the
concomitant formation of the final product [(D/H)2BNMe2]2
and HD(diss) (Scheme 5). This indicates that B−H coordination
Entries 1−3 demonstrate a first-order dependence on [A],
and Figure 3a shows this relationship graphically. Entries 1, 4,
and 5 (Figure 3b) show that the initial rate is linearly
dependent on [Rh(PCy3)2H2Cl]1/2, which is characteristic of a
fast monomer−dimer equilibrium being present during
catalysis, in which the dimer is dominant but sits off the
cycle and the monomer is the active species.60 This inference is
consistent with Duckett’s assignment of 1 as a dimer.52 Such an
order dependence and dimer−monomer equilibria have
previously been noted in Pd-catalyzed alkene arylations,61
hydrolysis of methylparathion,62 and Heck couplings.63 Like-
wise, transfer hydrogenation processes using Shvo’s cata-
lyst,64,65 including amine borane dehydrogenation,14 also
invoke such a kinetic regime. Monomer−dimer equilibria
have also been suggested for cyclohexene hydrogenation by
[Rh(PR3)2Cl]266 and Rh-catalyzed hydroboration.67 We see no
evidence for the formation of mixed-valence dimers, such as
[Rh(PR3)2H2(μ-Cl)2Rh(PR3)2] (R = Phephos, p-tolyl), which
are in equilibrium with the corresponding rhodium(I) chloride-
bridged dimers by loss of H2.66,68,69 Indeed, extended exposure
of 1 to a vacuum did not remove H2, consistent with previous
reports.70 By contrast to 1, a first-order dependence on the
catalyst has been measured in the dehydrogenation of
H3B·NMeH2 using Ir(tBuPOCOPtBu)H2 [tBuPOCOPtBu =
κ3-1,3-(OPtBu2)2C6H3], consistent with a monomeric cata-
lyst.12 There was no change in the initial rate when an excess
(25-fold) of C was added in addition to A [(14.0 0.4) × 10−5
M s−1, cf. entry 1]. This rules out an autocatalytic role for the
final product, in contrast to the cationic system.33
Scheme 5
Isotopic labeling experiments give further insight into the
likely mechanism of dehydrogenation. Entry 7 shows a
substantial primary kinetic isotope effect (KIE) (kH/kD = 5.3
1.2) for N−H/N−D activation when using H3B·NMe2D,
indicating that irreversible N−H transfer is likely to be involved
in the rate-limiting process. A similar KIE has been noted for
amine borane dehydrocoupling using TiCp2 systems.27 When
using D3B·NMe2H, a much smaller (presumably secondary
and/or equilibrium, vide infra) KIE is observed (kH/kD = 1.2
0.1), entry 6, suggesting that B−H cleavage is not involved in
the rate-limiting process. Consistent with this conclusion,
and activation at the metal center is fast and reversible
compared to irreversible dehydrogenation, consistent with the
small isotope effect for B−D cleavage, vide supra. A similar
scenario has been reported for the dehydrogenation of
H3B·NMe2H using {(η5-C5Me4H)2Ti}2N2 as the catalyst in
which B−H activation is proposed to precede rate-limiting N−
H activation.30 Base-stabilized boryls have also been suggested
to form in Ir systems on reaction with H3B·NH3.78 We suggest
that it is steric factors that suppress B−H activation of
H3B·NMe3 with 1, although we cannot discount the possibility
that the inability to form N−H···Cl−Rh secondary interactions
when using H3B·NMe3 might raise the barrier to B−H
activation by removing a lower energy pathway for approach
of the amine borane to the metal. Related interactions have
been proposed for the dehydrogenation of H3B·NH3 by
ruthenium bis(trimethylsilyl)amino catalysts.21 Treatment of
Rh(PCy3)2H2Cl with Et3B·NMe2H,79 which would probe N−H
activation only, due to the lack to B−H, also resulted in no
reaction. It is possible that this lack of reaction is also due to
increased steric demand compared to that of A.
double-labeled D3B·NMe2D afforded a kH/kD of 5.1
1.2,
entry 8, identical within experimental error to that observed
with H3B·NMe2D. Under a H2 atmosphere (ca. 4 atm) with
H3B·NMe2H as the substrate, the initial rate did not change
appreciably compared to standard sealed tube conditions, and
the reaction also ran to 70% conversion under these conditions
(ToN = 35). We suggest that this reflects the low solubility of
H2 in 1,2-F2C6H4,71 meaning that hydrogen only modifies the
catalytic cycle at low [H3B·NMe2H] near the end of catalysis,
possibly by competitively forming a σ-H2 complex with one of
the intermediates. H2 has been shown to reversibly bind to
i
Ir(PR3)2H2Cl (R = Cy, Pr) to give the corresponding
dihydrogen adducts.72,73 To our knowledge, the Rh congeners
have not been reported. We find no evidence of reaction
1
between 1 and H2 (4 atm, 1,2-F2C6H4 solution) by H NMR
spectroscopy: no chemical shift change or broadening of the
sharp hydride resonance at δ −22.9 is observed under these
conditions. We thus suggest that H2 coordinates to an
Suggested Catalytic Cycle. On the basis of these
observations, we suggest the mechanism for the initial
dehydrogenation of A by 1 is as outlined in Scheme 6. Dimeric
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Scheme 6. Proposed Mechanisms for the Dehydrogenation of H3B·NMe2H, A, Using Rh(PCy3)2H2Cl, 1, To Ultimately Form
a
the Cyclic Dimer [H2BNMe2]2, C, via Dimerization of Z, As Determined by Monitoring the Early Phase of Catalysis
a
See the text for a discussion of the role of the minor intermediate B.
1, sitting off cycle, is in rapid equilibrium with the
corresponding monomer. This can then undergo reversible
B−H activation (cycle I) or irreversible N−H activation (cycle
II). Cycle I presumably proceeds via an initial σ-CAM process,
similar to that postulated for H/D exchange in 1,56 as both B−
H oxidative addition to form a Rh(V) species and H2 loss from
170 to form a Rh(I) species are likely disfavored. The resulting
base-stabilized boryl (intermediate D) then proceeds in an
irreversible N−H β-H transfer to eliminate H2BNMe2, Z,
and regenerate the active catalyst. Cycle I captures the
significant KIE associated with NH/ND, while the small KIE
associated with BH/BD is presumably due to an equilibrium
isotope effect. Cycle II proceeds via irreversible N−H activation
(protonation) to give an aminoborane intermediate, E, possibly
with a supporting B-agostic interaction.36,80 The small KIE
associated with BH/BD might be due to a secondary isotope
effect or B−H activation becoming synchronous with N−H
activation. The collected data do not allow for the
discrimination between these two pathways, I or II. However,
the rapid H/D exchange observed in the substrate A must
invoke an intermediate (F) that is closely related to D on cycle
I.
Although our studies probe the very early stages of the
reaction, as Figure 2a shows, a small but significant amount of
H3B·NMe2BH2·NMe2H, B, is also formed as an intermediate
and then consumed. Formation of B might occur via a metal-
mediated combination of A and Z,28,29 or from two molecules
of A with concomitant release of H2.27 Consumption of B
likewise could re-form A and Z,28,33 or proceed by intra-
molecular dehydrocylization.27,33
Deactivation at Low Catalyst Loadings. The studies
reported above were conducted using 2 mol % catalyst loadings.
On moving to a lower catalyst loading of 0.5 mol % for 1 in an
open system (298 K, 0.072 M A, 1,2-F2C6H4 solvent), we
found that irreversible catalyst deactivation occurred, resulting
in only 60% consumption of A, with a profile that did not fit a
simple kinetic model. Addition of more A did not restart
catalysis to any significant level. This deactivation is in sharp
contrast to catalysis using 3 at the same effective loadings,
which has the counterpart {Rh(PCy3)2H2}+ fragment coordi-
nated with 1 and returns 100% conversion of A to C at 0.5 mol
% loadings at a rate similar to that of the cationic system at 5
mol % (vide supra). We thus suggest that this cationic fragment
acts to stabilize the more active Rh(PCy3)2H2Cl against
decomposition by some as yet undetermined mechanism.
Using the Ir congener to 1, Ir(PCy3)2H2Cl, 4, in catalysis gave
insight into likely decomposition products. In contrast to
complex 1, complex 4 does not turnover to dehydrogenate A.
Instead, under catalytic conditions (4, 20 mol %, 298 K, 1,2-
F2C6H4 solvent) a slow (50000 s) consumption of only 2 equiv
of amine borane is observed. The final product is not C, but
instead the salt [H2B(NMe2H)2]Cl is formed (as identified by
NMR spectroscopy85,86). The organometallic partner to this is
the borohydride complex Ir(PCy3)2H2(η2-H2BH2), 5, Scheme
7, giving mass balance to this process (see the Supporting
This proposed mechanism is similar to that suggested for
cationic catalyst systems of Rh and Ir, in which the oxidation
state of the metal does not change.29,35 It also has similarities to
those reported for Ti,27,30 Cr,81 and Mn82 catalysts and aspects
of the mechanism proposed for alkaline-earth metals.40 This
cycle differs from those that invoke concerted B−H/N−H
activation pathways, which have correspondingly more leveled
B−H and N−H kH/kD values than reported here.14,15 The
products of both B−H activation45 and N−H activation36,80 of
amine boranes have been isolated. Turculet and co-workers
have recently reported calculations that suggest a stepwise N−
H, followed by a higher energy B−H, activation in the
dehydrogenation of H3B·NH3 (Cy-PSiP)Ru(N(SiMe3)2) [Cy-
PSiP = κ3-(2-R2PC6H4)2SiMe], although here N−H activation
is calculated to occur via an intramolecular deprotonation
Scheme 7. Reaction of 4 with H3B·NMe2H
21
mechanism and subsequent elimination of HN(SiMe3)2 and
is somewhat related to that calculated for ammonia borane
dehydrogenation using Ni(NHC)2 systems (NHC = N-
heterocyclic carbene).16,83,84
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Notes
Information for full details). Complex 5 can be independently
synthesized by addition of Na[BH4] to 4 and analogous way to
Ir(PtBu3)2H2(η2-H2BH2)87 and is essentially inactive for the
dehydrogenation of A. The equivalent Rh complex which we
suggest might also form at low loadings of 1, Rh(PCy3)2H2(η2-
H2BH2), 6,51 is also inactive. Consistent with this, 6 is observed
to form as the significant species (95%, 31P{1H} NMR
spectroscopy) under open conditions (1, 0.5 mol %, 60%
conversion of A, 16 h). Heinekey and co-workers has reported
a catalytically dormant product in amine borane dehydrogen-
ation catalysis using the Ir(tBuPOCOPtBu)H2 catalyst that
invokes a borohydride-like structure, although it is better
formulated as a σ-BH3 complex of the parent dihydride.88
Interestingly, Manners and co-workers have reported that
Ir(PHCy2)2Cl does catalyze the dehydrogenation of A, while
no [H2B(NMe2H)2][Cl] is reported to be formed.58 Clearly,
these subtle changes in the phosphine compared to 4 influence
the course of this catalysis.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the EPSRC for funding (DTA for L.J.S.) and the
European Union (M.A.H., Marie Curie Action, FP7, “Dehy-
drocouple”). G.C.L.-J. is a Royal Society Wolfson Research
Merit Award holder. We also thank Johnson Matthey for the
generous loan of Rh salts.
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CONCLUSIONS
■
We have shown here that Rh(PCy3)2H2Cl, 1, is an effective
catalyst for the dehydrogenation of H3B·NMe2H, confirming
our initial suggestion that it is a cocatalyst present in low, but
constant, concentration in the cationic {Rh(PCy3)2H2}+
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89
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ASSOCIATED CONTENT
■
S
* Supporting Information
Full experimental details for complexes 3, 5, and 6, variable-
temperature NMR data for 3, X-ray characterization of 3, 5, and
6, details of mechanistic studies, and CIF data for 3.2C6H5F.
This material is available free of charge via the Internet at
http://pubs.acs.org. Crystallographic data have been deposited
with the Cambridge Crystallographic Data Centre (CCDC
915250, 3) and can be obtained via www. ccdc.cam.ac.uk/data_
request/cif.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: andrew.weller@chem.ox.ac.uk.
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