Variable coordination modes and catalytic dehydrogenation of B-phenyl amine-boranes

Article abstract of DOI:10.1039/c6dt00197a

The chemistry of N-substituted amine-boranes and their reactivity towards transition metal centres is well established but the chemistry of B-substituted amine-boranes is not. Here we present the coordination chemistry of H₂PhB·NMe₃ towards a range of Rh(i) fragments with different P-Rh-P ligand bite angles, {Rh(PⁱPr₃)₂}⁺, {Rh(PⁱBu₃)₂}⁺, {Rh(ⁱPr₂P(CH₂)₃PⁱPr₂)}⁺, {Rh(Ph₂P(CH₂)_nPPh₂)}⁺ (n = 3, 5), as characterised by NMR spectroscopy and single-crystal X-ray diffraction. This reveals a difference in the coordination mode of the amine-borane, with large bite angle fragments favouring η²-coordination through a sigma-interaction with BH₂, whereas fragments with small bite angles favour η⁶-coordination through the aryl group of the amine-borane. The catalytic dehydrocoupling of H₂PhB·NMe₂H is also explored, with the aminoborane HPhB=NMe₂ found to be the sole dehydrogenation product. Stoichiometric reactivity with H₂PhB·NMe₂H again showed small bite angle fragments to prefer η⁶-aryl coordination, while the larger bite angle {Rh(PⁱPr₃)₂}⁺ gave rapid dehydrogenation to form a mixture of the Rh(iii) dihydride [Rh(PⁱPr₃)₂(H)₂(η²-H₂PhB·NMe₂H)][BAr^F₄] and the low coordinate aminoboryl complex [Rh(PⁱPr₃)₂(H)(BPhNMe₂)][BAr^F₄]. These results suggest that precatalysts which η⁶-bind arenes strongly should be avoided for the dehydrocoupling of amine-boranes bearing aryl substituents.

Full text of DOI:10.1039/c6dt00197a

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DOI: 10.1039/C6DT00197A
Dalton Transactions
ARTICLE
Variable Coordination Modes and Catalytic Dehydrogenation of B-
Phenyl Amine-Boranes
,Received 00th January 20xx,
Accepted 00th January 20xx
Amit Kumar, Isobel K. Priest, Thomas N. Hooper,* and Andrew S. Weller*
The chemistry of N-substituted amine-boranes and their reactivity towards transition metal centres is well established but
the chemistry of B-substituted amine-boranes is not. Here we present the coordination chemistry of H2PhB·NMe3 towards
a range of Rh(I) fragments with different P-Rh-P ligand bite angles, {Rh(PiPr3)2}+, {Rh(PiBu3)2}+, {Rh(iPr2P(CH2)3PiPr2)}+,
{Rh(Ph2P(CH2)nPPh2)}+ (n = 3, 5), as characterised by NMR spectroscopy and single–crystal x–ray diffraction. This reveals a
difference in coordination mode of the amine-borane, with large bite angle fragments favouring η2-coordination through a
sigma-interaction with BH2 whereas fragments with small bite angles favour η6-coordination through the aryl group of the
amine-borane. The catalytic dehydrocoupling of H2PhB·NMe2H is also explored, with the aminoborane HPhB=NMe2 found
to be the sole dehydrogenation product. Stoichiometric reactivty with H2PhB·NMe2H again showed small bite angle
fragments to prefer η6-aryl coordination, while the larger bite angle {Rh(PiPr3)2}+ gave rapid dehydrogenation to form a
mixture of the Rh(III) dihydride [RhH2(PiPr3)2(η2-H2PhB·NMe2H)][BArF4] and the low coordinate aminoboryl complex
[RhH(PiPr3)2(BPhNMe2)][BArF4]. These results suggest that precatalysts which η6-bind arenes strongly should be avoided for
DOI: 10.1039/x0xx00000x
www.rsc.org/
the
dehydrocoupling
of
amine-boranes
bearing
aryl
substituents.
Introduction
Amine–boranes, defined by the simplest example H3B·NH3,
have been the subject of significant interest and research
effort in the past decade with regard to their potential as
molecular hydrogen storage materials (i.e. dehydrogenation) 1
and
as
precursors
to
polyaminoboranes
(i.e.
dehydrocoupling).2 Much of this research has focussed on
developing homo- and heterogeneous catalytic methodologies
for dehydrogenation/dehydrocoupling that allow for control of
kinetics and final product distributions.3 N-alkyl substituted
amine-boranes, particularly those bearing methyl groups4
(although aryl substituents are also known5) have received the
bulk of attention because of their thermal stability (N–alkyl
especially as N–aryl undergo spontaneous dehydrocoupling5b)
relative ease of synthesis, high weight % H, and as precursors
to polyaminoboranes.4 The coordination chemistry, and
subsequent reactivity, of such species is also well developed,
Scheme 1: Selected examples of B-substituted amine-boranes and the products of
dehydrogenation.
respect to dehydrocoupling, has only recently attracted
significant attention. Manners and co-workers reported the
synthesis of B-substituted amine-boranes containing relatively
exotic substituents [e.g. C6F5 or SR, Scheme 1(i)]9 that undergo
dehydrogenation to form the corresponding aminoboranes.
Liu and co-workers have developed a range of cyclic amine-
boranes [selected examples shown in Scheme 1(ii)],10 that can
be dehydrocoupled by transition metal catalysts to form
discrete, well-characterized products.11 In some cases
intermediate sigma–complexes can be isolated, e.g. A Scheme
2.11f Liu and Manners have independently described the
often operating through 3 centre–2 electron (3c-2e) sigma
6
M···H–B interactions.3-4,
Developments in B-substituted
analogues have, surprisingly, lagged behind; perhaps due to
their more challenging synthesis,7 and potential instability due
to weaker B–N bonds.7b, 8
The reactivity and coordination chemistry of B-alkyl (or
heteroalkyl) substituted amine-boranes, particularly with
Scheme 2: Coordination complexes of B-substituted amine-boranes, and B–aryl
precursors used in this study. ArF = 3,5–(CF3)2C6H3
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dehydrogenation
of
the
B-methyl
amine-boranes show that dehydrocoupling of H2PhB·NMe2H forms an unusual
View Article Online
DOI: 10.1039/C6DT00197A
MeH2B·NMexH3-x [x = 0, 1, 2; Scheme 1(iii)] by catalytic and example of a B–substituted acyclic aminoborane which
non-catalytic (thermal) routes.7
undergoes subsequent B–H activation to form a B–substituted
Reports of B-aryl amine-boranes are scarce. H2PhB·NMe3 amino–boryl complex.
(I)12 has been shown to form sigma-complexes with suitable
group 6 and 7 metal fragments (e.g. B, Scheme 2).13
H2PhB·NMe2H (II) is known14 but its coordination chemistry or
dehydrocoupling has not been reported. The B-phenyl amine-
borane H2PhB·NH3 can be dehydrocoupled using
[Pd(NCMe)4][BF4]2 to form a material tentatively identified as
Results and Discussion
Synthesis of Precursors
H2PhB·NMe3 (I)12 and H2PhB·NMe2H (II)14 have been
reported, and their original syntheses comes from the reaction
of diboranes (PhBH2)2 with NMe3 or NMe2H respectively. An
alternative, expedient, synthesis of (I) and (II) is based on the
methods of Hawthorne,22 Shimoi,13 and Liu.7a Li[PhBH3],
prepared by reaction of phenylboronic acid with lithium
aluminium hydride in diethyl ether,23 was combined with the
appropriate ammonium salt, [NMe3H]Cl or [NMe2H2]Cl, to give
H2PhB·NMe3 (I) and H2PhB·NMe2H (II) respectively, which
were isolated as white solids in good yield. NMR spectroscopic
data for (I) in CD2Cl2 are consistent those previously
described13 [e.g. BH2: δ(1H) 2.37; δ(11B) –0.8, t, J(BH) 97 Hz],
while as far as we are aware NMR data for (II) have not been
previously reported: BH2: δ(1H) 2.34; δ(11B) –4.7, t, J(BH) 95 Hz.
In contrast to N–aryl amine boranes, such as H3B·NPhH2,5b
compounds (I) and (II) were found be stable towards thermal
dehydrocoupling or B–N bond cleavage, remaining unchanged
on heating (C6H5F, 80ºC, 12 h).
[PhBNHx]n,
but
insolubility
prevented
further
characterisation.15
We report here a detailed study into the coordination
chemistry of B–aryl substituted amine-boranes (I) and (II) with
{Rh(L2)}+ fragments (L2 = (PR3)2 or chelating diphosphine) in
which the steric and electronic (bite angle, β 16) demands of
the phosphine ligands are varied. Unlike B–alkyl (or N–alkyl)
substituted amine-boranes, B–aryl analogues offer two
potential binding motifs: either through the aryl (e.g. η6) or 3c-
2e Rh···H–B interactions (e.g. η2), Scheme 3. The relative
strength of amine–borane sigma binding with increasing bite
angle has been commented upon before in [Rh(L2)(η2–
H3B·NMe3)][BArF ] complexes, with larger L–Rh–L bite angles
4
favouring tighter Rh···H2B interactions (as measured by NMR
spectroscopy).17 Conversely, larger bite angles in the simple
arene complexes [Rh(L2)(η6–C6H5F)][BArF ] result in weaker
4
Rh···arene interactions, as measured by collision–induced
dissociation in Electrospray Mass Spectrometry (ESI-MS) and
solution equilibrium measurements.18 These trends
presumably reflect the optimisation of bonding between the
d8–Rh(I)–{ML2} fragment and either the B–H sigma donating
orbitals (or other two electron Lewis bases)13 or the π–arene
orbitals,19 as modified by the L–Rh–L angle.20 This can be
interpreted by the energy of the C2v–{ML2}+ LUMO that is of π-
symmetry (b1) becoming lower in energy with increasing bite
angle,17a, 21 thus finding a worse match with the arene HOMO
and a better one with the relatively low lying B–H σ-orbitals. In
this contribution we demonstrate empirically that with B–aryl
amine–boranes the L–Rh–L bite–angle dictates which mode of
binding is observed (i.e. η6 or η2), present equilibrium
thermochemical data on the relative binding strengths of each
motif when the two binding modes are finely balanced, and
Coordination Chemistry of H2PhB·NMe3
Reaction of a stoichiometric amount of (I) with [Rh(L2)(η6-
C6H5F)][BArF ] [L2 = (PiPr3)2,24 (PiBu3)2,25 iPr2P(CH2)3PiPr2,18b
4
Ph2P(CH2)3PPh2 and Ph2P(CH2)5PPh217b] in 1,2-difluorobenzene
solvent resulted in displacement of the fluorobenzene ligand
and formation of new complexes in solution as determined by
NMR spectroscopy. These fragments were chosen to probe
changes in phosphine bite–angle, while keeping the electronic
contribution from the phosphine substituent as constant as
possible. For example PiBu3 and PiPr3 have different cone
angles of 143º & 160º respectively but similar electronic
properties;26 L–Rh–L bite angles can be varied in
Ph2P(CH2)nPPh2 (n = 3 or 5); and mono–dentate versus
chelating coordination modes can be probed with PiPr3 and
iPr2P(CH2)3PiPr2. These fragments have also been used to form
well–defined sigma amine–borane complexes with, for
example, H3B·NR3 type ligands,11f, 17, 27 whose structures and
solution NMR spectroscopic markers are well–established.
The reaction of [Rh(PiPr3)2(η6-C6H5F)][BArF ] with (I)
4
resulted in the immediate formation of a deep purple solution.
Recrystallisation by addition of pentane gave blue crystalline
material in 69% isolated yield, identified by NMR spectroscopy
and single crystal X-ray diffraction as [Rh(PiPr3)2(η2-
H2PhB·NMe3)][BArF ] (1) in which the amine–borane binds
4
through two Rh···H–B 3c-2e interactions (Scheme 4). In the
11B{1H} NMR spectrum a single broad peak is observed at δ
34.8, with a characteristic downfield shift (35.6 ppm) of the
borane resonance compared to free ligand (δ –0.8) that signals
Scheme 3: Potential coordination modes of B–aryl amine–boranes with {Rh(L2)}+
fragments, and previous observations regarding bite angle and strength of binding
of a generic arene and amine–borane fragments.
1
η2 binding.27-28 In the H NMR spectrum the BH2 resonance is
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observed at δ –6.36 (2 H relative integral), an upfield shift of the η2-coordination mode.
ARTICLE
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DOI: 10.1039/C6DT00197A
8.73 ppm compared to free ligand. The aryl protons [δ 7.37,
Forcing the P–Rh–P bite angle to be significantly smaller,
3H; δ 7.25, 2H] are not significantly shifted from free ligand.13 while keeping the electronic contribution of the P–substituents
In the 31P{1H} NMR spectrum a doublet is observed at δ 69.6 the same, is achieved by use of the chelating phosphine
[J(PRh) = 176 Hz], shifted 14.1 ppm downfield from the starting complex [Rh(iPr2P(CH2)3PiPr2)(η6-C6H5F)][BArF ]. Reaction of
4
material.24 In the solid state the complex crystallises with two this with a stoichiometric amount of amine-borane (I) resulted
cations (and two anions) in the asymmetric unit. An overlay of in the formation of an orange solution, rather than the purple
the independent cations (ESI) did not reveal any significant one observed for 1. X-ray diffraction quality crystals were
difference in amine-borane geometry but the PiPr3 ligands vary obtained
from
a
1,2-difluorobenzene/pentane
slightly in position and conformation [e.g. P(1)-Rh(1)-P(2) recrystallisation, from which a single crystal X–ray diffraction
106.19(3) Å, P(3)-Rh(2)-P(4) 101.86(3)º] which we attribute to study demonstrated η6-binding of the arene, rather than
crystal packing effects due to a rather flat potential energy Rh···H–B bonding: [Rh(iPr2P(CH2)3PiPr2)(η6-PhH2B·NMe3)][BArF ]
4
surface as only one set of resonances could be observed in the (2) (Scheme 4B). The P–Rh–P bite angle [94.04(4)°] is
1H, 11B and 31P{1H} NMR spectra. This observation of different significantly smaller than in η2-bound complex 1 [e.g.
ligand conformation/bite angles of two independent 101.86(3)º]. Consistent with this different binding mode, that
molecules in the asymmetric unit has been noted in amine- does not involve the borane fragment, in the 11B{1H} NMR
borane complexes of H3B·NMe2BH2·NMe2H and [Me2NBH2]2 spectrum a single resonance is observed at δ –2.1 that is now
with the {Rh(PiBu3)2}+ fragment.27 In the solid–state short only slightly shifted from free amine-borane (δ –0.8). In the
Rh···B distances [2.150(3) and 2.159(3) Å] are consistent with a 31P{1H} NMR spectrum a single species is observed [δ 45.0;
η2-binding mode, by comparison to previously reported J(RhP) = 195 Hz] a chemical shift that is barely changed when
structures,17b, 27, 29 including sigma amine–borane complexes of compared with [Rh(iPr2P(CH2)3PiPr2)(η6-C6H5F)][BArF ].18b No
4
t
closely related BuCH2CH2BH2·NMe3.30 This distance in the resonance was observed in the high-field region of the 1H NMR
structurally similar [Rh(PiPr3)2(η2-H3B·NMe3)][BArF ] is slightly spectrum that would signal Rh···H2B interactions, but peaks at
4
shorter [2.1376(3) Å],17a perhaps as a result of the extra steric δ 6.93 [relative integral 1 H] and δ 6.31 [4 H, a 2 + 2
demand caused by B-substitution in 1. High quality X-ray coincidence] demonstrate η6-binding through the phenyl
diffraction data allowed the hydrogen atoms of the BH2 unit to moiety of (I).31 Thus, changing the bite angle from 101.83(3)º
be located in the difference map and refined freely, confirming in (1) to 94.04(4)° in (2) also changes the coordination mode
from η2 to η6.
This preference comes into fine balance when the mono–
dentate phosphine PiBu3 is used, that has a cone angle of 143º
and thus might be expected to have a smaller P–Rh–P bite
angle than (1).17a Reaction of (I) with the precursor complex
[Rh(PiBu3)2(η6-C6H5F)][BArF ] again led to formation of a purple
4
solution. However, more complicated NMR data were
observed than for either (1) or (2) which suggested the
presence of two species in solution. In the 11B{1H} NMR
spectrum (CD2Cl2) two peaks are observed at δ 29.5 and –2.1
in a ratio of 10:11 respectively. The peak observed at δ 29.5
suggested the formation of a sigma complex with a η2-Rh···H2B
interaction, being shifted 30.3 ppm downfield compared to I,
cf. complex (1). The higher field signal at δ –2.1 is only shifted
1.3 ppm upfield compared with free ligand suggesting an
alternative coordination mode for the amine-borane, more like
(2). In the 31P{1H} NMR spectrum two resonances are observed
in the same ratio as measured in the 11B NMR spectrum, one
at δ 34.1 [d, J(RhP) = 177 Hz] with a similar downfield shift and
coupling constant to (1), consistent with sigma complex
formulation; while a signal at δ 25.2 [d, J(RhP) = 202 Hz]
Scheme 4: (A) Synthesis, selected NMR spectroscopic data and (B) molecular
suggests a binding mode as for (2). These data indicate both
structures of [Rh(PiPr3)2(η2-H2PhB·NMe3)][BArF4] (1) and [Rh(iPr2P(CH2)3PiPr2)(η6-
η2–Rh···H2B and η6-aryl bound complexes are present in
–
H2PhB·NMe3)][BArF4] (2). [BArF
]
4
anions and selected H atoms are omitted for
solution. The 1H NMR spectrum is consistent with this
description. In the high field region a broad resonance is
observed at δ –5.06 (Rh···H2B) which integrates to 1.1 H
clarity. Ellipsoids shown at 50% probability level. Selected bond lengths (Å) and
angles (°). (1) [values are also given for the second cation in the asymmetric unit
which is not shown in the figure]: Rh(1)-P(1) 2.2581(6), Rh(1)-P(2) 2.2748(7),
Rh(1)-B(1) 2.150(3), B(1)-N(1) 1.623(4), Rh(2)-P(3) 2.2628(7), Rh(2)-P(4) 2.2655(7),
Rh(2)-B(2) 2.159(3), B(2)-N(2) 1.629(4); P(1)-Rh(1)-P(2) 106.19(3), P(3)-Rh(2)-P(4)
101.86(3). (2) [Only major component of disorder shown]. Ellipsoids shown at 50%
probability level. Selected bond lengths (Å) and angles (°): Rh(1)-P(1) 2.2340(9),
Rh(1)-P(2) 2.2403(8), Rh(1)-Ph Centroid 1.848, B(1)-N(1) 1.635(4); P(1)-Rh(1)-P(2)
relative to the [BArF ]– signals, and 2 singlets are observed at δ
4
2.76 and 2.50 corresponding to NMe3 protons in the different
coordination modes of the amine-borane. In addition,
resonances can be observed upfield of the aryl region
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3.3 J K–1 mol–1) upon loss of phenyl group free VrioewtaAtrtiioclne Ownlianes
observed in the epimerisation of 2-phDeOnIy:l1-0c.-140,3c9-/6C-6dDimT0e0t1h9y7lA-
1,3-dioxane;32 while for the anion exchange equilibrium
between [1-closo-CB11H6Br6]– and [BArF ]– η6-binding through
4
an aryl group of [BArF ]– was also shown to be enthalpically
4
favoured but entropically disfavoured (ΔS° = –87.6 ± 0.8 J K–1
mol–1).33
Layering a 1,2-difluorobenzene solution of this mixture
with pentane at –30 °C led to formation of purple crystals and
an orange oil. Isolation of a crystal suitable for X-ray diffraction
by mechanical separation allowed the solid–state structure of
the purple material to be determined (Scheme 5C). As for
complex (1), two cations are present in the asymmetric unit;
an overlay of these independent structures (ESI) did not reveal
significant differences in amine-borane binding and
orientation, although some conformational differences and a
difference in ligand bite angle was observed for the PiBu3
ligands. The structure shows a close interaction between the
rhodium centre and the BH2 moiety in [Rh(PiBu3)2(η2-
H2PhB·NMe3)][BArF ] (η2–3) with Rh···B distances of 2.153(6)
4
and 2.172(6) Å consistent with η2-binding. Although the
hydrogen atoms could not be located in the difference map
and were placed in calculated positions, the metrical data are
consistent with this description as well as the NMR data. The
two P–Rh–P bite angles measured for each independent
molecule, 95.14(5) and 98.84(5)º, are smaller than for (1), but
larger than for (2), consistent with the equilibrium observed in
Scheme 5: (A) Equilibrium between η2–3 and η6–3 , (B) Van’t Hoff plot and (C)
molecular structure of [Rh(PiBu3)2(η2-H2PhB·NMe3)][BArF4] (η2–3). The second
cation in asymmetric unit, [BArF4]- anions and selected H atoms are omitted for
clarity. Only major component of disorder shown. Ellipsoids shown at 50%
probability level. Selected bond lengths (Å) and angles (°) (values are also given for
the second cation in the asymmetric unit which is not shown in the figure); Rh(1)-
P(1) 2.2254(14), Rh(1)-P(2) 2.2436(14), Rh(1)-B(1) 2.153(6), B(1)-N(1) 1.605(8),
Rh(2)-P(3) 2.2353(14), Rh(2)-P(4) 2.2289(14), Rh(2)-B(2) 2.172(6), B(2)-N(2)
1.617(9); P(1)-Rh(1)-P(2) 98.84(5), P(3)-Rh(2)-P(4) 95.14(5).
solution. [Rh(PiBu3)2(η6-C6H5F)][BArF ], a model for η6–binding
4
of (I), has a P–Rh–P angle of 94.14(4)º, placing the
approximate tipping point between the two structural motifs
as lying between 94 and 95º.
Bulk mechanical separation of the crystals from the oil for
further analysis was not possible, but we propose the orange
oil to be the η6-phenyl bound amine-borane complex
[Rh(PiPr3)2(η6-PhH2B·NMe3)][BArF ] (η6–3). Dissolving the
4
indicative of η6-aryl coordination. These two complexes are
mixture of blue and orange products isolated gave a solution
that showed the same NMR spectra as a freshly prepared
sample.
formulated as [Rh(PiBu3)2(η2-H2PhB·NMe3)][BArF ], (η2–3), and
4
[Rh(PiBu3)2(η6-PhH2B·NMe3)][BArF ], (η6–3), Scheme 5A.
4
A variable temperature NMR spectroscopy study was
carried out to determine if exchange between these isomers
was occurring in solution. At 298 K in 1,2-difluorobenzene, the
concentrations of η2–3 and η6–3 were found to be
approximately equal. Lowering the temperature to 240 K
resulted in a relative increase in η6–3 while at higher
temperature (330 K) (η2–3) was favoured, demonstrating the
two isomers to be in dynamic equilibrium. The equilibrium
constant at each temperature was calculated from integration
of the 31P{1H} NMR spectra; and the resulting Van’t Hoff plot
(Scheme 5B) allowed for determination of the thermodynamic
parameters for this exchange: ΔH° = –11.25 ± 0.48 kJ mol-1; ΔS°
= –39.1 ± 1.7 J K-1 mol-1; ΔG(298 K) = 0.4 ± 0.4 kJ mol-1. Thus
binding between the two modes is approximately
thermoneutral. The negative enthalpy indicates η6-binding of
the aryl group is stronger than the η2-binding through BH2 but
this is moderated by the associated negative entropy, which is
likely to be the result of loss of free rotation of the phenyl
group upon η6-binding. A similar entropy change (ΔS° = –16.3 ±
Figure 1: X-ray molecular structure of [Rh(Ph2P(CH2)5PPh2)(η6-PhH2B·NMe3)][BArF
]
4
(4). [BArF4]- anion and selected H atoms omitted for clarity. Ellipsoids shown at
50% probability level. Selected bond lengths (Å) and angles (°): Rh(1)-P(1)
2.2416(8), Rh(1)-P(2) 2.2469(7), Rh(1)-Ph Centroid 1.860, B(1)-N(1) 1.638(3); P(1)-
Rh(1)-P(2) 92.21(4).
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activation all as modified by the bite angle.3, 35 We therefore
View Article Online
DOI: 10.1039/C6DT001t9h7Ae
sought to probe the effect of the ligands on
dehydrocoupling of secondary amine-borane (II) by comparing
two electronically similar precatalysts but with very different
P–Rh–P
angles:
[Rh(PiPr3)2(η6-C6H5F)][BArF ]
and
4
[Rh(iPr2P(CH2)3PiPr2)(η6-C6H5F)][BArF ] which form η2 and η6–
4
complexes with (I), i.e. (1) and (2) respectively.
Reaction of 5 mol% of these two precatalysts with (II) in
1,2-difluorobenzene (25°C, closed system) resulted in very
slow3 consumption of (II) in both cases: less than 15%
conversion in 5 hours (TOF less than 0.5 hr–1). Although slow,
dehydrogenation is also not fast with other amine–boranes
17a
and these systems 11f,
.
The major 11B–containing product
Figure 2: Plot of Rh···B distance against bite angle β showing different binding
modes for complexes 1, 2, 3, 4 and 7. The values for independent cations in the
asymmetric units of 1 and 3 are given separately.
displayed a single resonance at δ 39.4 which split into a
doublet [J(BH) = 123 Hz] in the 11B NMR spectrum. This was
assigned as aminoborane HPhB=NMe2 (6) from its
characteristic 11B NMR chemical shift and the presence of a
single B-H bond (Scheme 6). For example aminoboranes
bearing two alkyl groups at nitrogen show similar 11B NMR
chemical shift values (e.g. H2B=NMe2, δ 37.5; H2B=NEt2, δ 36.6;
H2B=NiPr2, δ 35.1);36 while the recently reported B-substituted
HMeB=NMe2, displays a doublet at δ 41.2 [J(BH) = 123 Hz] in
the 11B NMR spectrum,7b and cyclo–HB=NMeC4H8 is observed
at δ 40.8 [J(BH) = 125 Hz].11f Heating to 77 °C in a sealed NMR
tube resulted in the complete consumption of (II) in less than 1
hour for both catalysts. The product (> 95% by 11B NMR
spectroscopy) of dehydrocoupling was again found to be free
aminoborane HPhB=NMe2 from the in situ NMR spectrum.
Unfortunately, due to its apparent instability and similar
volatility to the 1,2-difluorobenzene solvent, separation and
isolation of pure (6) was not possible due to decomposition
upon vacuum distillation. Nevertheless NMR data are
unambiguous, and as we show in situ generated (6) can also be
used for onward reactivity. The formation of (6) is in contrast
with the metal catalysed dehydrocoupling of H2BMe·NMe2H
which forms cyclic B–dimethyl–N–tetramethyldiborazane,7b
[Me2NBHMe]2, as well the aminoborane, HMeB=NMe2. The B–
phenyl group in (6) inhibits any appreciable dimerization to the
corresponding diborazane.
To extend this study into the effect of bite angle
[Rh(Ph2P(CH2)5PPh2)(η6-C6H5F)][BArF ] was used as a starting
4
material, which has a flexible chelating phosphine with a 5–
carbon backbone. This ligand has been shown to be able to
access to a wide range of bite angles, and values of 93.98(4) to
117.3(1)° have been determined crystallographically.16b,
34
Reaction of a stoichiometric amount of this starting material
with (I) resulted in an orange solution with NMR data
characteristic of an η6-aryl bound. Crystallisation from layering
a dichloromethane solution with pentane allowed a single
crystal X-ray diffraction study to be carried out and confirmed
the η6-coordination mode in [Rh(Ph2P(CH2)5PPh2)(η6-
PhH2B·NMe3)][BArF ] (4) (Figure 1). The phosphine ligand bite
4
angle was found to be only 92.21(4)°, the smallest observed
crystallographically for this ligand but consistent with the
observed binding mode. By contrast the corresponding
H3B·NMe3 complex is η2-bound, [Rh(Ph2P(CH2)5PPh2)(η2-
H3B·NMe3)][BArF ], and shows a P-Rh-P bite angle of 98.18(3)°.
4
This shows that the observed bite angle for a flexible ligand
such as Ph2P(CH2)5PPh2 is very dependent on the ancillary
ligands. The analogous complex formed with the smaller bite
angle
aryl
diphosphine,
[Rh(Ph2P(CH2)3PPh2)(η6-
PhH2B·NMe3)][BArF ] [5, 87.955(15)°, ESI] also shows a η6-
4
[Rh(Ph2P(CH2)3PPh2)(η6-C6H5F)][BArF ] has been shown to
be an excellent catalyst for dehydrocoupling of H3B·NMe2H
coordination mode. The data for the ligation of (I) is
summarised in Figure 2; in which a plot of Rh···B distance
against bite angle shows that larger bite angles give η2-
complexes, smaller bite angles result in η6-complexes, with a
crossover point at approximately 95°. The Rh···B distance in
the η2–binding mode appears to be rather insensitive to bite
angle.
4
Catalytic Dehydrogenation of H2PhB·NMe2H (II)
{Rh(P2)}+ fragments have been shown to catalyse the
dehydrocoupling of secondary and primary amine-boranes;17,
27 and the ligand bite angle has been shown to affect the rate
of dehydrocoupling of H3B·NMe2H in particular.17 Although
empirically it is found that the smallest bite angle was
reflected in larger turnover frequencies, the precise factors
behind these differences are not yet fully delineated and likely
involve a combination of relative accessibility of Rh(I)/Rh(III)
oxidation states/ease of H2 loss/relative barriers to BH and NH
Scheme 6: Catalytic dehydrogenation of amine-borane (II) to form aminoborane 6
at 77ºC [cat.]
=
[Rh(PiPr3)2(η6-C6H5F)][BArF
]
or [Rh(iPr2P(CH2)3PiPr2)(η6-
4
C6H5F)][BArF4]. Inset shows 11B NMR spectrum after 15 minutes, [cat.]
=
[Rh(iPr2P(CH2)3PiPr2)(η6-C6H5F)][BArF4] .
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and others, have previously commented upon the role of B–
View Article Online
37
H···H–N interactions in lowering barrier DtoOId: 1e0h.1y0d3r9o/Cco6DuTp0li0n1g9.7A
Given the η6 binding mode we cannot discount an outer-
sphere mechanism in which π–coordination38 of the metal
activates the amine–borane to alternative dehydrogenation
pathways. However, the N–H resonance does not change
significantly on coordination [δ 3.55 versus δ 3.52] suggesting
only a minimal perturbation to this bond.
By using a metal fragment which can adopt a large ligand
bite angle, {Rh(PiPr3)2}+ in which η2-coordination is favoured
(i.e. complex 1) this effect of competitive aryl binding can
,
–
Figure 3: X-ray molecular structure of (7). [BArF
]
anion and selected H atoms
omitted for clarity. Ellipsoids shown at 50% probability level.
Rh(1)-P(1) 2.2441(11), Rh(1)-P(2) 2.2422(10), Rh(1)-Ph
4
Centroid 1.851, B(1)-N(1) 1.618(6); P(1)-Rh(1)-P(2) 94.27(4).
(0.2 mol%, open system, TOF ~ 1250 hr–1).17b However this was
also a slow catalyst for dehydrocoupling of (II) at room
temperature, with full conversion to (6) only observed after 23
hours at 5 mol% catalyst loading (25°C, TOF ~ 1 hr–1). In order
to probe the causes of the slow dehydrogenation of (II) with
this catalyst, stoichiometric studies were performed.
Scheme 7: Complexes 10 and 11. [BArF4]– anions not shown.
potentially be avoided. Upon mixing equal amounts of
[Rh(PiPr3)2(η6-C6H5F)][BArF ] and (II) in 1,2-F2C6H4 solvent a
4
blue solution was immediately formed which rapidly
decolourised (less than 5 min) to yield a very pale yellow
solution. This blue colour likely results from the sigma-complex
Stoichiometric Reactivity of H2PhB·NMe2H (II)
[Rh(PiPr3)2(η2-H2PhB·NMe2H)][BArF ], (9), although its short
The presence of the phenyl group which provides a
competitive (η6) site for amine-borane binding at the metal
centre is a possible cause of the slow dehydrogenation of (II),
as B–H activation at the metal centre requires the formation of
4
lifetime meant full characterisation was not possible. 31P{1H}
NMR spectroscopy measured in situ after 2 minutes revealed
three doublets [δ 69.3, J(RhP) = 174 Hz; δ 64.4 J(RhP) = 109
Hz; and δ 48.5, J(RhP) = 116 Hz] in an approximate ratio of
3
a precursor sigma complex.2b, Preferential η6-coordination
through the aryl ring makes this less likely.
Addition of a slight excess of (II) (1.2 equiv.) to
[Rh(iPr2P(CH2)3PiPr2)(η6-C6H5F)][BArF ] results in the formation
4
of
an
η6–bound
complex
[Rh(iPr2P(CH2)3PiPr2)(η6-
PhH2B·NMe2H)][BArF ] (7) alongside
a
small amount of
4
dehydrogenation product (6). Complex (7) was characterized
by NMR spectroscopy and single crystal X–Ray diffraction
(Figure 3). The solid–state structure reveals a ligand bite angle
of 94.27(4)° which is very similar to that in (2) [94.04(4)°]
which also displayed a η6-coordination mode. In addition to
the expected resonances in the NMR spectra a N–H resonance
is observed in the 1H NMR spectrum at δ 3.55. A similar
complex is formed on reaction of [Rh(Ph2P(CH2)3PPh2)(η6-
C6H5F)][BArF ] with (II), as characterised by NMR spectroscopy:
Figure 4: X-ray molecular structures of [RhH2(PiPr3)2(η2-H2PhB·NMe2H)][BArF4] (10), left,
4
[Rh(Ph2P(CH2)3Ph2)(η6-PhH2B·NMe2H)][BArF ] (8). When three
4
one independent cation from the asymmetric unit is shown; and
equivalents of (II) were combined with [Rh(iPr2P(CH2)3PiPr2)(η6-
–
[RhH(PiPr3)2(BPhNMe2)][BArF4
]
(11), right. [BArF
]
anions, minor components of
4
C6H5F)][BArF ] slow dehydrocoupling (hours) to form
4
disorder and selected H atoms omitted for clarity. Ellipsoids shown at 50% probability
level. Selected bond lengths (Å) and angles (°). (10) [values are also given for the
second cation in the asymmetric unit which is not shown in the figure]: Rh(1)-P(1)
2.336(2), Rh(1)-P(2) 2.330(2), Rh(1)-B(1) 2.274(9), B(1)-N(1) 1.650(11), Rh(2)-P(3)
2.3227(19), Rh(2)-P(4) 2.327(2), Rh(2)-B(2) 2.333(8), B(2)-N(2) 1.608(11); P(1)-Rh(1)-
P(2) 158.35(8), P(3)-Rh(2)-P(4) 155.75(8). (11): Rh(1)-P(1) 2.3292(18), Rh(1)-P(2)
2.350(2), Rh(1)-B(1) 1.929(9), B(1)–C(1) 1.536(14), B(1)-N(1) 1.411(14); P(1)-Rh(1)-P(2)
157.55(7), P(1)-Rh(1)-B(1) 100.2(3), P(2)-Rh(1)-B(1) 100.6(3), Rh(1)-B(1)-N(1) 138.0(8),
Rh(1)-B(1)-C(1) 98.3(7), N(1)-B(1)-C(1) 123.7(8)
aminoborane (6) occurs, with η6–bound (7) observed at the
end of catalysis.
That η6-coordination of ligand (II) in (7) and (8) is preferred
to η2-binding suggests that this competitive binding mode
contributes to the slow dehydrogenation rate under catalytic
conditions for these chelating systems. However, that
dehydrogenation does occur catalytically, albeit slowly,
indicates that if an inner sphere mechanism is operating,
access to the η2-coordination mode through BH2 is possible,
but the equilibrium lies heavily in favour of η6-coordination.
Complexes (7) and (8) do not dehydrogenate to any significant
degree in the absence of exogenous amine–borane, and we,
10:45:45 respectively. After 10 minutes the resonance at δ
69.3 had disappeared leaving the remaining two in a 1:1 ratio.
We propose the doublet at δ 69.3 is therefore likely to result
6 | J. Name., 2012, 00, 1-3
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from (9). In the 11B{1H} NMR spectrum no signals for free (II) or retained, and the B–Caryl bond distance of 1.536(14) Å is
View Article Online
(6) were observed. There was no further change after 24 consistent with a single bond. The anglesDaOrIo: u10n.1d03b9o/rCo6nDTs0u0m197toA
hours. Recrystallisation at –26 °C resulted in the formation of 360º, demonstrating sp2 character, while the Rh–B(1)–C(1)
two distinct crystalline products: colourless block– and pale angle of 98.3(7)° is much smaller than the equivalent metric in
yellow plate–type crystals that could be separated (C) [132.7(9) Å]. Overall these metrics point to an amino–boryl
mechanically. Although relatively poor crystal quality species, rather than an alternative borylene structure.42 There
compounded with significant disorder of the phosphine alkyl appears to be a vacant site that sits cis to Rh–H and Rh–B.
i
groups in both complexes prevented collection of high-quality There are no close25, 43 Rh···C interactions from the Pr ligands
data in single crystal X-ray diffraction experiments, the data that would point to an agostic interaction [shortest Rh···C
was sufficient to identify the products as [Rh(PiPr3)2(H)2(η2- 3.227 Å]. There is, however, a relatively short Rh···C distance
H2PhB·NMe2H)][BArF ]
(10)
and to the ipso–phenyl carbon atom [2.634(8) Å] that, when
combined with the compressed Rh–B–C angle, suggests a
Rh···C(ipso) interaction. The next closest distance to the phenyl
4
[Rh(PiPr3)2(H)(BPhNMe2)][BArF ] (11), Scheme 7 and Figure 4.
4
The solid–state structure of (10) contains two independent group is 3.042(9) Å (ortho carbon), longer than would be
cations (and two anions) in the asymmetric unit with broadly expected for a η2–arene type interaction. The geometry is
i
similar metric parameters, but disorder is observed in the Pr reminiscent of the η2–benzyl interactions that interact via
groups of the phosphine ligand. The N–H and B–H hydrogen methylene and ipso carbon atoms.44 Including the Rh···C(ipso)
atoms were placed in calculated positions, the Rh–H hydride interaction complex (11) can be described as a 16-electron
ligands could not be reliably placed and so were omitted, Rh(III) species, and (11) is also related to the 16–electron
although their presence was confirmed by NMR spectroscopy Rh(PiPr3)2(Bcat)HCl that comes from oxidative addition of
and ESI–MS of pure, isolated material, vide infra. The HBcat to a Rh(I) precursor (cat = catechol).45 Interestingly, in
structure, and NMR data, of (10) are similar to the closely the system when a chelating phosphine is used an η6–complex
related complex [Rh(PiPr3)2(H2)(η2-H3B·NMe3)][BArF ],17a with is isolated, Rh(iPr2PCH2CH2PiPr2){(η6-cat)Bcat}, paralleling the
4
the PiPr3 ligands in trans orientation, cis Rh-H functionality and observations described herein.
Rh-B distances of 2.274(9) and 2.333(8) Å respectively]. The
In the 31P{1H} NMR spectrum of (11) a doublet is observed
31P{1H} NMR spectrum (CD2Cl2) of isolated (10) confirmed this [δ 48.8, J(RhP) = 120 Hz]. The chemical shift of δ 50.1 for the
species is one of the products observed in the mixture, δ 64.7 aminoborane observed in the 11B{1H} NMR spectrum places
[d, J(RhP) = 108 Hz]. Presumably the amine–borane in (10) is the complex as a boryl, rather than a borylene42 [cf (C) δ 50.1].
1
undergoing a fluxional process that makes the phosphine The H NMR spectrum displays the terminal Rh-H at δ –21.71
ligands equivalent at room temperatures, similar to that [doublet of triplets, J(RhH) = 64 Hz, J(PH) = 12 Hz], the
observed in related complexes39 in which an η2 to η1 change in unusually large Rh-H coupling constant31a indicates a hydride
coordination is accompanied by a rotation around the Rh···H–B bound to low coordinate Rh(III) centre; e.g. (C) J(RhH) = 43
bond. In the 1H NMR spectrum a broad resonance at δ –2.08 is Hz.40a or [Rh(PtBu3)2(H2)][BArF ] J(RhH) = 59 Hz.25 Two distinct
4
assigned to the Rh···H2B interaction, and a sharper one at δ – resonances are observed for the N-Me groups demonstrating a
19.05 (integral 2 H) assigned to Rh–H. The 11B{1H} NMR lack of rotation around the B-N bond on the NMR timescale,
spectrum shows a signal at δ 8.0 assigned to the amine– consistent with a B=N multiple bond character. The phenyl
borane. ESI–MS confirmed the formulation of the cation to be region shows 3 signals in the ratio 1:3:1 (in addition to the
[Rh(PiPr3)2(H2)(η2-H2PhB·NMe2H)] (m/z:
=
560.32 (found), [BArF ]– resonances), demonstrating that there is not free
4
560.32 (calculated)). Complex (10) forms by sequential B–H/N– rotation around B(1)–C(1).
H activation at a Rh(III) centre, to form a Rh(III) dihydride,
A plausible mechanism for the formation of complex (11)
which then coordinates another equivalent of (II) to liberate invokes dehydrogenation of (II) to form (10) and aminoborane
free (6). Such activation processes are well established.3, 27
(6), followed by much faster reaction of the latter with residual
The second product isolated from the reaction mixture, [Rh(PiPr3)2(η6-C6H5F)][BArF ] to form (11), overall in equal ratio
4
[Rh(PiPr3)2(H)(BPhNMe2)][BArF ] (11), is more unusual. The
4
solid–state structure shows a complex in which the phosphine
ligands are arranged in a trans orientation, and a molecule of
aminoborane (6) has undergone overall oxidative addition of
the B-H bond at the {Rh(PiPr3)2}+ fragment to form a terminal
hydride (located in the final difference map) and a direct Rh–B
bond, i.e. an amino-boryl species.
Group 9 amino-boryl species have been isolated previously,
coming from B–H activation, e.g. [Rh(IMes)2(H){B(H)NMe2}]+
[C, IMes = N,N–bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] 40
or [Rh(κ3-POP-Xantphos)(H)(BH=NiPr2)(NCMe)][BArF ].41 The Rh–
4
B distance in (11) [1.929(9) Å] is, within error, the same as that
found in (C) [1.960(9) Å]. A relatively short B-N bond [1.411(14)
Å, cf (C) 1.390(15) Å] suggests aminoborane character is
Scheme 8: Reactivity of (II) and (6)
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to (10). When three equivalents of (II) were combined with activation with a secondary B–substituted amine–borane to
View Article Online
[Rh(PiPr3)2(η6-C6H5F)][BArF ] the rapid (15 minutes) formation afford a Rh(III) dihydride complex and a B–substituted
DOI: 10.1039/C6DT00197A
4
of only (10) was observed followed by the slow aminoborane. With constrained, chelating, phosphines η6
dehydrocoupling (hours) to form aminoborane (6), during complexes can be isolated instead in which the amine–borane
which time (10) is observed as a resting state (Scheme 8). From moiety is intact and dehydrogenation is slow. This difference in
this reaction mixture (10) could be isolated pure in good yield stoichiometric reactivity balances out the reported large
(57%) by layering with pentane and storage at –25 °C. A differences in catalytic dehydrocoupling rate of {Rh(PR3)2(H2)}+
solution of pure (10) did not show any changes, suggesting fragment (slow) versus {Rh(chelating phosphine)}+ (fast) with
(11) does not form from (10). Using aminoborane (6) H3B·NMe2H, which does not bear aryl substituents. Thus,
generated catalytically (Scheme 6) reaction (overall oxidative although {Rh(PiPr3)2(H2)}+ dehydrocouples H2PhB·NMe2H
addition) with [Rh(PiPr3)2(η6-C6H5F)][BArF ] is very rapid (on slowly, the η6 coordination mode observed with
4
time of mixing) to form (11). In contrast there is no reaction of {Rh(iPr2P(CH2)3PiPr2)}+ means that B–H (and subsequent N–H)
(6) with the Rh(I) sigma-complex (1) over 24 hr, showing that activation by an inner sphere coordination/activation
aminoborane (6) will not displace amine–borane (I) under mechanism are also slowed so that now both fragments
these conditions. A proposed mechanism is summarised in operate at a similar rate. Such observations are potentially
Scheme 9.
important in the design of systems that dehydropolymerise
Complex (11) reacts with 2 equivalents of amine–borane arene–substituted amine-boranes (i.e. BN polystyrene
(II) to give (10) and (6) on time of mixing. The current data do analogues) as rapid dehydrogenation to form putative
not discriminate between two possible mechanisms for this aminoborane intermediates that then can undergo B–N bond
forming process are likely central to any successful catalyst
system. We thus suggest that systems that form strong
adducts with arene π-systems are less likely to be good
candidates for such transformations.
Experimental
General Experimental Details
All manipulations, unless otherwise stated, were
performed under an atmosphere of argon, using standard
Schlenk and glove-box techniques. Glassware was oven dried
at 130 °C overnight and flamed under vacuum prior to use.
Dichloromethane, diethyl ether and pentane were dried using
a Grubbs type solvent purification system (MBraun SPS-800)
and degassed by successive freeze-pump-thaw cycles.46 CD2Cl2
Scheme 9: Proposed mechanism for the formation of (10) and (11), and reactivity
and 1,2-F2C6H4 was distilled under vacuum from CaH2 and
stored over 3 Å molecular sieves, 1,2-F2C6H4 was stirred over
of (11) with amine–borane (II)
alumina for two hours prior to drying. NMR spectra were
transformation. A sigma–bond metathesis/β–elimination of
(11) with (II) to eliminate (6), or a reversible reductive
elimination of (6) to give a {Rh(PiPr3)2}+ fragment which
undergoes reaction with (II) as described (Scheme 8).
Whatever the mechanism, complex (10) and aminoborane (6)
are the ultimate products when excess amine–borane is
present, consistent with their observation during catalysis.
Pure complex (10) was found to be a slow catalyst for the
dehydrogenation of (II) to form (6). Slow amine–borane
dehydrogenation when catalysts sit in a Rh(III) dihydride
resting state has been noted previously.35b
recorded on a Bruker AVD 500 MHz spectrometer at room
temperature unless otherwise stated. For NMR spectra
measured in situ in 1,2-F2C6H4, the spectrometer was pre-
locked and pre-shimmed using a C6D6 (0.1 mL) and 1,2-F2C6H4
1
(0.3 mL) sample and H NMR spectra were referenced to the
centre of the downfield solvent multiplet (δ 7.07). 31P and 11B
NMR spectra were referenced against 85% H3PO4 (external)
and Et2O·BF3 (external) respectively. Chemical shifts are
quoted in ppm and coupling constants in Hz. ESI-MS were
recorded on a Bruker MicrOTOF instrument. In all ESI-MS
spectra there was a good fit to both the principal molecular ion
and the overall isotopic distribution. Microanalyses were
performed by Stephen Boyer at the London Metropolitan
University.
Conclusions
We have shown here that the bite angle in {Rh(P2)}+ type Metal precursor compounds [Rh(PiPr3)2(C6H5F)][BArF ],24
4
fragments can have a significant effect in determining whether [Rh(PiBu3)2(C6H5F)][BArF ],25
4
Rh···H2B η2–sigma amine–borane complexes or Rh···arene η6 [Rh(iPr2P(CH2)3PiPr2)(C6H5F)][BArF ],18b
4
complexes are formed with B–substituted amine-boranes. [Rh(Ph2P(CH2)3PPh2)(C6H5F)][BArF ]17b
and
4
Wider bite angles (i.e. monodentate phosphines) tend to [Rh(Ph2P(CH2)5PPh2)(C6H5F)][BArF ]17b were prepared by
4
favour η2 coordination modes, and relatively rapid B-H/N–H
8 | J. Name., 2012, 00, 1-3
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literature methods and all other starting materials were used Resulting orange solution was stirred for 30 minutes and
View Article Online
DOI: 10.1039/C6DT00197A
as received.
layered with pentane at −25˚C to afford the product as orange
1
crystals. Yield: 20.0 mg, 77%. H NMR (500 MHz, CD2Cl2): δ
PhH2B·NMe3 (I) and PhH2B·NMe2H (II)
7.76 (s, 8H, [BArF ]-), 7.60 (s, 4H, [BArF ]-), 6.93 (broad m, 1H,
4
4
Li[PhBH3] was prepared from PhB(OH)2 and Li[AlH4] as PhH), 6.31 (broad m, 4H, PhH), 2.55 (s, 9H, NMe3), 1.88 (br, 6H,
reported in the literature.23 In a typical synthesis, Li[PhBH3] CH2), 1.30 (br, 4H, CH), 1.19 (broad m, 12 H, CH3), 1.12 (broad
(350 mg, 3.57 mmol) and the appropriate ammonium chloride m, 12 H, CH3), BH2 signals not detected due to quadrupolar
salt, [NMe3H]Cl (291 mg, 3.57 mmol) or [NMe2H2]Cl (341 mg, broadening. On 11B decoupling BH2 resonance appears at δ
3.57 mmol) were added to a Schlenk flask and immediately 2.38 (s). 31P{1H} NMR (202 MHz, CD2Cl2): δ 45.0 (d, JRhP = 195
dissolved in diethyl ether (20 mL). The mixture (a suspension Hz). 11B{1H} NMR (160 MHz, CD2Cl2): δ −2.1 (br, BH2), -6.6 (s,
of white solid) was stirred vigorously for 2 hours and evolution [BArF ]-). 11B NMR (160 MHz, CD2Cl2): δ −2.1 (br, BH2), -6.6 (s,
4
of hydrogen was observed. The mixture was evaporated to [BArF ]-). ESI-MS (1,2-F2C6H4, 60 °C) positive ion: m/z 528.26
4
dryness in vacuo and pentane (100 ml) added and the mixture [M+] (calc. 528.26). Elemental Microanalysis: Calc.
stirred vigorously. The solution was transferred by filter [C56H62B2F24NP2Rh] (1391.32 gmol-1): C, 48.32; H, 4.49; N, 1.01.
cannula to another Schlenk and the remaining white solid Found: C, 48.19; H, 4.38; N, 0.92.
washed with pentane (2 x 10 mL). The combined fractions [Rh(PiBu3)2(η2–H2PhB·NMe3)][BArF ] (η2-3) and [Rh(PiBu3)2(η6–
4
were evaporated to dryness in vacuo to yield the amine- PhH2B·NMe3)][BArF ] (η6-3): To a Young’s NMR tube charged
4
borane PhH2B·NMe3 (309 mg, 58%) or PhH2B·NMe2H (340 mg, with [Rh(PiBu3)2(C6H5F)][BArF ] (10 mg, 6.82 X 10-3 mmol) and
4
64%) as white solids which were stored in the glove box.
PhH2B·NMe3 (1.0 mg, 6.82 X 10-3 mmol, 1.0 equivalents) was
Both compounds have been reported previously although added 1,2-F2C6H4 (0.4 mL). Mixing resulted in blue/purple
prepared by slightly different synthetic routes. The synthesis of solution and NMR spectra were taken of this sample in situ to
22, 47
PhH2B·NMe3 has been reported several times12-13,
and show products η2-3 and η6-3 in approximately equal ratio (see
NMR spectroscopy data have been reported in C6D6.13 Our main text). The solvent from this sample was removed in
data for PhH2B·NMe3 matched that previously reported. The vacuo and CD2Cl2 added and the NMR spectroscopy repeated
synthesis of PhH2B·NMe2H has been reported by a different to reveal virtually identical data in CD2Cl2 and this is reported
route14 but no NMR data was given and so is reported below below. Bulk isolation of either η2-3 or η6-3 was not possible
for the first time.
due to formation of a mixture of purple crystals (determined
PhH2B·NMe2H (II): 1H NMR (400 MHz, CD2Cl2): δ 7.38 by single crystal X-ray diffraction to be η2-3) and an orange oil
(apparent d, 3JHH = 6.9 Hz, 2H, PhH), 7.21 (apparent t, 3JHH = 7.4 therefore microanalysis of the sample was not possible. H
1
Hz, 2H, PhH), 7.12 (apparent t, 3JHH = 7.4 Hz, 1H, PhH), 3.52 (br, NMR (500 MHz, CD2Cl2): δ 7.72 (s, 8H, [BArF ]-), 7.56 (s, 4H,
4
1H, NH), 2.50 (s, 3H, NMe), 2.49 (s, 3H, NMe), 2.34 (br, 2H, [BArF ]-), 7.33 (broad m, ~1H, PhH, η2-3), 7.18 (broad m, ~0.5H,
4
BH2). 11B{1H} NMR (128.4 MHz, CD2Cl2): δ -4.7 (s). 11B NMR PhH, η2-3), 7.12 (broad m, ~1H, PhH, η2-3), 6.95 (broad m,
(128.4 MHz, CD2Cl2): δ -4.7 (t, JBH = 97 Hz). 13C{1H} NMR ~0.5H, PhH, η6-3), 6.10 (broad m, ~1H, PhH, η6-3), 5.94 (broad
(100.62 MHz, CD2Cl2): δ 149.0 (br, C-ipso), 135.6 (s, Ar), 127.3 m, ~1H, PhH, η6-3), 2.76 (s, ~4.5H, NMe3, η2-3), 2.50 (s, ~4.5H,
(s, Ar), 127.3 (s, Ar), 125.4 (s, Ar), 42.2 (s, NMe).
NMe3, η6-3), 1.98 (broad m, 6H, CH), 1.63 (broad m, ~6H, CH2),
1.55 (broad m, ~6H, CH2), 1.06 (apparent broad d, J ∼ 5 Hz,
36H, CH3), –5.06 (br, ~1H, BH2, η2-3). The signal for BH2 in η6-3
Synthesis of Metal Complexes
[Rh(PiPr3)2(η2–H2PhB·NMe3)][BArF ] (1): To a Schlenk flask was not observed due to quadrupolar broadening and overlap
4
charged with [Rh(PiPr3)2(C6H5F)][BArF ] (25.0 mg, 1.8 X 10-2 with other resonances. A H{11B} NMR spectrum showed this
1
4
mmol) and PhH2B·NMe3 (4 mg, 2.7 X 10-2 mmol, 1.5 peak at δ 2.41. 31P{1H} NMR (202 MHz, CD2Cl2): δ 34.1 (d, JPRh
=
equivalents) was added 1,2-F2C6H4 (0.5 mL). The resulting 177 Hz, η2-3), 25.2 (d, JPRh = 202 Hz, η6-3). 11B{1H } NMR (160
blue/purple solution was layered with pentane at −25˚C to MHz, CD2Cl2): δ 29.5 (br, BH2, η2-3), -2.1 (br, BH2, η6-3), -6.61
afford the product as blue crystals. Yield: 18 mg, 69%. 1H NMR (s, [BArF ]-). ESI-MS (1,2-F2C6H4, 60 °C) positive ion: m/z 656.41
4
(500 MHz, CD2Cl2): δ 7.76 (s, 8H, [BArF ]-), 7.60 (s, 4H, [BArF ]-), [M+] (calc. 656.42).
4
4
7.37 (br, 3H, PhH), 7.25 (br, 2H, PhH), 2.83 (s, 9H, NMe3), 2.07 [Rh(Ph2P(CH2)5PPh2)(η6–PhH2B·NMe3)][BArF ] (4): To
a
4
(broad m, 6H, CH), 1.32 (apparent dd, J ∼ 12, J ∼ 7, 36H, CH3), Schlenk flask charged with [Rh(Ph2P(CH2)5PPh2)(C6H5F)][BArF ]
4
–6.36 (br, 2H, BH2). 31P{1H} NMR (202 MHz, CD2Cl2): δ 69.6 (d, (15.0 mg, 9.98 X 10-3 mmol) and PhH2B·NMe3 (1.5 mg, 9.98 X
JRhP = 176 Hz). 11B{1H} NMR (160 MHz, CD2Cl2): δ 34.8 (br, 10-3 mmol, 1 equivalents) was added 1,2-F2C6H4 (1.0 mL). The
BH2), -6.6 (s, [BArF ]-). 11B NMR (160 MHz, CD2Cl2): δ 34.8 (br, resulting orange solution was stirred for 30 minutes. The
4
BH2), -6.6 (s, [BArF ]-). ESI-MS (1,2-F2C6H4, 60 °C) positive ion: solvent was removed in vacuo and the orange oily solid
4
m/z 572.32 [M+] (calc. 572.32). Elemental Microanalysis: Calc. redissolved in CH2Cl2 (1 mL). This solution was layered with
[C59H70B2F24NP2Rh] (1435.38 g mol-1): C, 49.35; H, 4.91; N, 0.98. pentane at −25˚C to afford the product as dark orange crystals.
1
Found: C, 49.32; H, 4.82; N, 1.01.
Yield: 5.0 mg, 32%. H NMR (500 MHz, CD2Cl2): δ 7.72 (s, 8H,
[Rh(iPr2P(CH2)3PiPr2)(η6–PhH2B·NMe3)][BArF ] (2): To a Schlenk [BArF ]-), 7.56 (s, 4H, [BArF ]-), 7.41-7.31 (broad overlapping m,
4
4
4
flask charged with [Rh(iPr2P(CH2)3PiPr2)(C6H5F)][BArF ] (25.0 20H, PhH), 7.12 (apparent broad t, JHH ~ 5.5 Hz, 1H, PhH), 5.21-
4
mg, 1.9 X 10-2 mmol) and PhH2B·NMe3 (3.1 mg, 2.1 X 10-2 5.17 (broad overlapping m, 4H, PhH), 2.47 (broad m, overlap
mmol, 1.1 equivalents) was added 1,2-F2C6H4 (0.5 mL). with 2.45 signal, 2H, CH2), 2.45 (s, 9H, NMe3), 2.30 (broad m,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
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1
4H, CH2), 1.91 (broad m, 4H, CH2). The signal for BH2 in 4 was spectroscopy. H NMR (500 MHz, 1,2-F2C6H4): δVi8ew.3A4rtic(les,On8liHne,
F
-
F
-
3
DOI: 10.1039/C6DT00197A
not observed due to quadrupolar broadening and overlap with [BAr ] ), 7.70 (s, 4H, [BAr ] ), 5.68 (apparent d, JHH = 6.1 Hz,
4
4
other resonances. A H{11B} NMR spectrum showed this peak 2H, PhH), 5.01 (apparent t, 3JHH = 6.1 Hz, 2H, PhH), 2.79 (br, 1H,
at δ 2.51. 31P{1H} NMR (202 MHz, CD2Cl2): δ 26.3 (d, JRhP = 204 NH), 3.04 – 2.67 (br, 2H, BH2), 2.48 (s, 6H, NMe2), 2.40 (br, 4H,
Hz). 11B{1H} NMR (160 MHz, CD2Cl2): δ −2.2 (br, BH2), -6.6 (s, CH2), 1.87 (broad m, 2H, CH2). The remaining phenyl signals
1
[BArF ]-). 11B NMR (160 MHz, CD2Cl2): δ −2.2 (br, BH2), -6.6 (s, were obscured by the 1,2-F2C6H4 signals. BH2 signal was
4
[BArF ]-). ESI-MS (1,2-F2C6H4, 60 °C) positive ion: m/z 692.22 observed at δ 2.81 (s, 2H) in the 1H{11B} NMR spectrum. 31P{1H}
4
[M+] (calc. 692.23). Elemental Microanalysis: Calc. NMR (202 MHz, 1,2-F2C6H4): δ 25.2 (d, JRhP = 193 Hz). 11B{1H}
[C70H58B2F24NP2Rh] (1556.04 gmol-1): C, 54.03; H, 3.76; N, 0.90. NMR (160 MHz, 1,2-F2C6H4): δ −5.0 (br, BH2), -6.6 (s, [BArF ]-).
4
Found: C, 53.92; H, 3.67; N, 1.00.
[Rh(PiPr3)2(H)2(η2–H2PhB·NMe2H)][BArF ] (10): To a sealed
4
[Rh(Ph2P(CH2)3PPh2)(η6–PhH2B·NMe3)][BArF ] (5): To
a
NMR tube charged with [Rh(PiPr3)2(C6H5F)][BArF ] (25.0 mg, 1.8
4
4
Schlenk flask charged with [Rh(Ph2P(CH2)3PPh2)(C6H5F)][BArF ] X 10-2 mmol) and PhH2B·NMe2H (7.2 mg, 5.4 X 10-2 mmol, 3
4
(30.0 mg, 2.04 X 10-2 mmol) and PhH2B·NMe3 (3.0 mg, 2.04 X equivalents)
was added 1,2-F2C6H4 (0.5 mL). Resulting
10-2 mmol, 1 equivalents) was added 1,2-F2C6H4 (1.0 mL). The colourless solution was mixed by inversion for 15 minutes and
resulting orange solution was stirred for 30 minutes and then transferred to a crystallization tube. The 1,2-F2C6H4
layered with pentane at −25˚C to afford the product as orange solution was layered with pentane and kept −25˚C for two days
1
crystals. Yield: 15.0 mg, 48%. H NMR (500 MHz, CD2Cl2): δ to afford the product as colourless crystals. Yield: 15 mg, 57%.
7.72 (s, 8H, [BArF ]-), 7.56 (s, 4H, [BArF ]-), 7.46 (broad m, 10H, 1H NMR (500 MHz, CD2Cl2): δ 7.76 (s, 8H, [BArF ]-), 7.60 (s, 4H,
4
4
4
3
PhH), 7.41-7.36 (broad m, 10H, PhH), 6.82 (apparent t, JHH
=
[BArF ]-), 7.44 (broad m, 2H, PhH), 7.39 (broad m, 3H, PhH),
4
6.1 Hz, 1H, PhH), 5.55 (apparent d, 3JHH = 6.1 Hz, 2H, PhH), 4.94 3.92 (s, 1H, NH), 2.64 (s, 6H, NMe2), 1.90 (br, 6H, CH), 1.22
3
(apparent t, JHH = 6.1 Hz, 2H, PhH), 2.48 (s, 9H, NMe3), 2.45 (apparent dd, J ∼ 13, J ∼ 6, 36H, CH3), –2.08 (br, 2H, BH2), –
(broad m, overlap with 2.48 signal, 4H, CH2), 1.85 (broad m, 19.05 (broad m, 2H, RhH2). H{31P} NMR (500 MHz, CD2Cl2): δ
1
2H, CH2). The signal for BH2 in 5 was not observed due to 7.76 (s, 8H, [BArF ]-), 7.60 (s, 4H, [BArF ]-), 7.44 (broad m, 2H,
4
4
quadrupolar broadening and overlap with other resonances. A PhH), 7.39 (broad m, 3H, PhH), 3.92 (s, 1H, NH), 2.64 (s, 6H,
1H{11B} NMR spectrum showed this peak at δ 2.59. 31P{1H} NMe2), 1.90 (br, 6H, CH), 1.22 (broad d, J ∼ 6, 36H, CH3), –2.08
NMR (202 MHz, CD2Cl2): δ 25.5 (d, JRhP = 192 Hz). 11B{1H} (br, 2H, BH2), –19.05 (broad d, JRhH = 17, 2H, RhH2). 31P{1H}
NMR (160 MHz, CD2Cl2): δ −2.0 (br, BH2), -6.6 (s, [BArF ]-). 11B NMR (202 MHz, CD2Cl2): δ 64.7 (d, JRhP = 108 Hz). 11B{1H} NMR
4
NMR (160 MHz, CD2Cl2): δ −2.0 (br, BH2), -6.6 (s, [BArF ]-). ESI- (160 MHz, CD2Cl2): δ 8.0 (br, BH2), -6.6 (s, [BArF ]-). 11B NMR
4
4
MS (1,2-F2C6H4, 60 °C) positive ion: m/z 664.19 [M+] (calc. (160 MHz, CD2Cl2): δ 8.0 (br, BH2), -6.6 (s, [BArF ]-). ESI-MS
4
664.19). Elemental Microanalysis: Calc. [C68H54B2F24NP2Rh] (1,2-F2C6H4, 60 °C) positive ion: m/z 560.32 [M+] (calc. 560.32).
(1527.99 gmol-1): C, 53.45; H, 3.56; N, 0.92. Found: C, 53.38; H, Elemental Microanalysis: Calc. [C58H70B2F24NP2Rh] (1435.38
3.43; N, 0.97.
gmol-1): C, 48.93; H, 4.96; N, 0.98. Found: C, 48.96; H, 4.62; N,
0.98.
[Rh(iPr2P(CH2)3PiPr2)(η6–PhH2B·NMe2H)][BArF ] (7): To
a
4
Schlenk flask charged with [Rh(iPr2P(CH2)3PiPr2)(C6H5F)][BArF ] [Rh(PiPr3)2(H)(BPhNMe2)][BArF ] (11): To a sealed NMR tube
4
4
(25.0 mg, 1.9 X 10-2 mmol) and PhH2B·NMe2H (2.8 mg, 2.1 X charged with [Rh(PiPr3)2(C6H5F)][BArF ] (25.0 mg, 1.8 X 10-2
4
10-2 mmol, 1.1 equivalents) was added 1,2-F2C6H4 (0.5 mL). mmol) and PhH2B·NMe2H (2.4 mg, 1.8 X 10-2 mmol) was added
Resulting orange solution was stirred for 24 h and layered with 1,2-F2C6H4 (0.5 mL). Addition of 1,2-F2C6H4 immediately
pentane at −25˚C to afford the product as orange crystals. resulted in a purple solution which turned to colourless in 5
1
Yield: 16 mg, 62 %. H NMR (500 MHz, CD2Cl2): δ 7.76 (s, 8H, minutes. Resulting colourless solution was mixed by inversion
[BArF ]-), 7.60 (s, 4H, [BArF ]-), 6.88 (broad m, 1H, PhH), 6.31 for 24 h which turned the colourless solution to yellow. The
4
4
(broad m, 2H, PhH), 6.26 (broad m, 2H, PhH), 3.55 (br, 1H, NH), yellow solution was transferred to a crystallization tube,
2.52 (s, 6H, NMe2), 1.86 (br, 6H, CH2), 1.30 (br, 4H, CH), 1.18 layered with pentane and kept −25˚C for two days which
(broad m, 12, CH3), 1.11 (broad m, 12, CH3). BH2 signals not resulted in the formation of majority of pale yellow crystals
detected due to quadrupolar broadening. 31P{1H} NMR (202 and some colourless crystals. Pale yellow crystals were
MHz, CD2Cl2): δ 45.3 (d, JRhP = 196 Hz). 11B{1H} NMR (160 mechanically separated from the mixture for characterization.
1
MHz, CD2Cl2): δ −6.0 (br, BH2), -6.6 (s, [BArF ]-). 11B NMR (160 Yield: 10 mg, 38%. H NMR (500 MHz, CD2Cl2): δ 7.76 (s, 8H,
4
MHz, CD2Cl2): δ −6.0 (br, BH2), -6.6 (s, [BArF ]-). ESI-MS (1,2- [BArF ]-), 7.67 (m, 1H, Ph), 7.60 (s, 4H, [BArF ]-), 7.55 (broad d,
4
4
4
F2C6H4, 60 °C) positive ion: m/z 514.24 [M+] (calc. 514.25). 2H, Ph), 7.54 (br, 2H, PhH), 3.07 (s, 3H, NMe), 2.95 (s, 3H,
Elemental Microanalysis: Calc. [C55H60B2F24NP2Rh] (1377.31 NMe), 2.23 (br, 6H, CH), 1.26 (apparent dd, J ∼ 13 Hz, J ∼ 6 Hz,
gmol-1): C, 47.96; H, 4.39; N, 1.02. Found: C, 47.60; H, 4.29; N, 18H, CH3), 1.18 (apparent dd, J ∼ 13 Hz, J ∼ 6, 18H, CH3), –
1.13.
21.71 (doublet of triplets, JRhH = 64 Hz, JPH = 12 Hz, 1H, RhH).
[Rh(Ph2P(CH2)3Ph2)(η6-PhH2B·NMe2H)][BArF ] (8): To a Young’s The pale yellow solution of (11) in CD2Cl2 was not stable and
4
NMR tube charged with [Rh(Ph2P(CH2)3PPh2)(C6H5F)][BArF ] decomposed in 6 h to form dark yellow solution of
4
(16.0 mg, 1.1 X 10-2 mmol) and PhH2B·NMe2H (1.5 mg, 1.1 X uncharacterized complexes. H{31P} NMR (500 MHz, CD2Cl2,
1
10-2 mmol) was added 1,2-F2C6H4 (0.5 mL). The resulting hydride region): δ –21.71 (d, JRhH = 64 Hz, 1H, RhH). 31P{1H}
orange solution was left at room temperature for 10 minutes NMR (202 MHz, CD2Cl2): δ 48.8 (d, JRhP = 120 Hz). 11B{1H} NMR
to form (8) which was in-situ characterized by the NMR (160 MHz, CD2Cl2): δ 50.1 (br, RhB), –6.6 (s, [BArF ]-). 11B NMR
4
10 | J. Name., 2012, 00, 1-3
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(160 MHz, CD2Cl2): δ 50.1 (br, RhB), –6.6 (s, [BArF ]-). 13C{1H}
4
Acknowledgements
View Article Online
DOI: 10.1039/C6DT00197A
The EPSRC (EP/J02127X) and the Rhodes Trust for funding.
NMR (100.62 MHz, CD2Cl2): Shifts due to [BArF ]- anion: δ
4
161.7 (q, JBC = 50 Hz), 134.8 (s), 128.8 (quartet of quartet, 2JFC
=
3
26 Hz, JBC = 3 Hz), 124.6 (q, JFC = 272 Hz), 117.4 (apparent
3
septet, JFC = 4 Hz); Shifts due to cation: δ 132.2 (s, Ph), 130.7
Notes and references
(s, Ph), 127.8 (s, Ph), 43.33 (s, NMe), 40.41 (s, NMe), 24.6
(overlapping doublets, JPC = 12 Hz, CHiPr), 19.9 (s, CH3iPr), 19.1
(s, CH3iPr). ESI-MS (1,2-F2C6H4, 60 °C) positive ion: m/z 556.29
[M+] (calc. 556.29). Elemental Microanalysis: Calc.
[C58H66B2F24NP2Rh] (1419.35 gmol-1): C, 49.07; H, 4.69; N, 0.99.
Found: C, 49.45; H, 4.20; N, 0.75.
1.
(a) C. W. Hamilton, R. T. Baker, A. Staubitz and I.
Manners, Chem. Soc. Rev., 2009, 38, 279-293; (b) A.
Staubitz, A. P. M. Robertson and I. Manners, Chem. Rev.,
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(a) E. M. Leitao, T. Jurca and I. Manners, Nat Chem, 2013,
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5629-5641.
H. C. Johnson, T. N. Hooper and A. S. Weller, in Synthesis
and Application of Organoboron Compounds, eds. E.
Fernández and A. Whiting, Springer International
Publishing, 2015, vol. 49, pp. 153-220.
Catalytic Generation of Aminoborane PhHB=NMe2 (6)
The aminoborane PhHB=NMe2 (II) was generated catalytically
by heating a mixture of amine-borane PhH2B·NMe2H (0.9 mg,
0.0070 mmol) and [Rh(PiPr3)2(C6H5F)][BArF ] (0.5 mg, 0.00035
4
mmol, 5 mol%) in 1,2-F2C6H4 (0.35 mL) in a high pressure
Young’s tap NMR tube for 1 hour (77 °C). Attempts to isolate
(6) from this mixture by vacuum distillation resulted in
decomposition of the aminoborane to uncharacterised
products however, we found this solution was sufficiently pure
for further reaction. The NMR data reported was measured in
situ in 1,2-F2C6H4 after complete catalytic conversion.
PhHB=NMe2 (6): 1H NMR (500 MHz, 1,2-F2C6H4): δ 5.05 (broad
q, JHB = 120 Hz, 1H, BH), 3.02 (s, 3H, NMe), 2.88 (s, 3H, NMe),
phenyl resonances were obscured due to the solvent (1,2-
F2C6H4) peaks. 11B{1H} NMR (160 MHz, 1,2-F2C6H4): δ 39.4 (s).
11B NMR (160 MHz, 1,2-F2C6H4): δ 39.4 (d, 1JBH = 123 Hz).
4.
5.
A. Staubitz, A. P. M. Robertson, M. E. Sloan and I.
Manners, Chem. Rev., 2010, 110, 4023-4078.
(a) C. A. Jaska, K. Temple, A. J. Lough and I. Manners, J.
Am. Chem. Soc., 2003, 125, 9424-9434; (b) H. Helten, A.
P. M. Robertson, A. Staubitz, J. R. Vance, M. F. Haddow
and I. Manners, Chem. Eur. J., 2012, 18, 4665-4680; (c) S.-
K. Kim, S.-A. Hong, H.-J. Son, W.-S. Han, A. Michalak, S.-J.
Hwang and S. O. Kang, Dalton Trans., 2015, 44, 7373-
7381.
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Katoh, M. Uruichi and H. Ogino, J. Am. Chem. Soc., 1999,
121, 11704-11712; (b) G. J. Kubas, Metal Dihydrogen and
σ-Bond Complexes, Springer, New York, 2001; (c) G.
Alcaraz and S. Sabo-Etienne, Angew. Chem. Int. Ed., 2010,
49, 7170-7179.
(a) P. G. Campbell, J. S. A. Ishibashi, L. N. Zakharov and S.-
Y. Liu, Aust. J. Chem., 2014, 67, 521-524; (b) N. E. Stubbs,
A. Schäfer, A. P. M. Robertson, E. M. Leitao, T. Jurca, H. A.
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Tomás, J. Mol. Struct.: THEOCHEM, 1998, 455, 51-57; (b)
D. J. Grant, M. H. Matus, K. D. Anderson, D. M. Camaioni,
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A. J. Lough and I. Manners, Eur. J. Inorg. Chem., 2011,
2011, 5279-5287; (b) A. P. M. Robertson, M. F. Haddow
and I. Manners, Inorg. Chem., 2012, 51, 8254-8264.
P. G. Campbell, A. J. V. Marwitz and S.-Y. Liu, Angew.
Chem. Int. Ed., 2012, 51, 6074-6092.
(a) P. G. Campbell, L. N. Zakharov, D. J. Grant, D. A. Dixon
and S.-Y. Liu, J. Am. Chem. Soc., 2010, 132, 3289-3291; (b)
W. Luo, L. N. Zakharov and S.-Y. Liu, J. Am. Chem. Soc.,
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Zakharov and S.-Y. Liu, J. Am. Chem. Soc., 2011, 133,
19326-19329; (d) W. Luo, D. Neiner, A. Karkamkar, K.
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137; (f) A. Kumar, J. S. A. Ishibashi, T. N. Hooper, T. C.
6.
Reaction of Aminoborane (6) with Sigma-Complex (1)
PhHB=NMe2 (6) was generated catalytically as above using
PhH2B·NMe2H
(0.9
mg,
0.0070
mmol)
and
7.
8.
9.
[Rh(PiPr3)2(C6H5F)][BArF ] (0.5 mg, 0.00035 mmol, 5 mol%).
4
Clean conversion to (6) was checked by 11B NMR spectroscopy
and this solution transferred via cannula to an NMR tube
containing [Rh(PiPr3)2(η2–H2PhB·NMe3)][BArF ] (1) (10.0 mg,
4
0.0070 mmol). The reaction was mixed and no colour change
was observed. Immediate NMR spectroscopy showed no
reaction between (6) and (1) and no change in these spectra
was observed after 24 h mixing of the solution by inversion at
room temperature.
Reaction of Aminoborane (6) with [Rh(PiPr3)2(C6H5F)][BArF ] –
An Alternative synthesis of (11)
4
10.
11.
PhHB=NMe2 (6) was generated catalytically as above using
PhH2B·NMe2H
(1.8
mg,
0.0133
mmol)
and
[Rh(PiPr3)2(C6H5F)][BArF ] (0.9 mg, 0.00067 mmol, 5 mol%).
4
Clean conversion to (6) was checked by 11B NMR spectroscopy
and this solution transferred via cannula to an NMR tube
containing [Rh(PiPr3)2(η6-C6H5F)][BArF ] (18.4 mg, 0.0133
4
mmol). The solution was mixed and immediate NMR
spectroscopy showed almost quantitative conversion (> 95%)
to (11) with NMR spectra (measured in situ in 1,2-F2C6H4)
matching those reported above. Crystallisation of this solution
by layering with pentane and storage at -18°C resulted in
formation of crystals of (11).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
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View Article Online
DOI: 10.1039/C6DT00197A
The binding mode of B–aryl substituted amine-boranes at {Rh(bisphoshine)}+ fragments
can manipulated by variation of the P-Rh-P bite-angle.