The Synthesis, Characterization and Dehydrogenation of Sigma-Complexes of BN-Cyclohexanes

Article abstract of DOI:10.1002/chem.201502986

The coordination chemistry of the 1,2-BN-cyclohexanes 2,2-R₂-1,2-B,N-C₄H₁₀ (R₂=HH, MeH, Me₂) with Ir and Rh metal fragments has been studied. This led to the solution (NMR spectroscopy) and solid-state (X-ray diffraction) characterization of [Ir(PCy₃)₂(H)₂(η²η²-H₂BNR₂C₄H₈)][BAr^F₄] (NR₂=NH₂, NMeH) and [Rh(iPr₂PCH₂CH₂CH₂PiPr₂)(η²η²-H₂BNR₂C₄H₈)][BAr^F₄] (NR₂=NH₂, NMeH, NMe₂). For NR₂=NH₂ subsequent metal-promoted, dehydrocoupling shows the eventual formation of the cyclic tricyclic borazine [BNC₄H₈]₃, via amino-borane and, tentatively characterized using DFT/GIAO chemical shift calculations, cycloborazane intermediates. For NR₂=NMeH the final product is the cyclic amino-borane HBNMeC₄H₈. The mechanism of dehydrogenation of 2,2-H,Me-1,2-B,N-C₄H₁₀ using the {Rh(iPr₂PCH₂CH₂CH₂PiPr₂)}⁺ catalyst has been probed. Catalytic experiments indicate the rapid formation of a dimeric species, [Rh₂(iPr₂PCH₂CH₂CH₂PiPr₂)₂H₅][BAr^F₄]. Using the initial rate method starting from this dimer, a first-order relationship to [amine-borane], but half-order to [Rh] is established, which is suggested to be due to a rapid dimer-monomer equilibrium operating.

Full text of DOI:10.1002/chem.201502986

DOI: 10.1002/chem.201502986
Full Paper
&
Coordination Chemistry
The Synthesis, Characterization and Dehydrogenation of Sigma-
Complexes of BN-Cyclohexanes
Amit Kumar,[a] Jacob S. A. Ishibashi,[b] Thomas N. Hooper,[a] Tanya C. Mikulas,[c]
David A. Dixon,[c] Shih-Yuan Liu,*[b] and Andrew S. Weller*[a]
Dedicated to Professor Larry Sneddon in recognition of his outstanding contributions to the transition metal chemistry of boron
Abstract: The coordination chemistry of the 1,2-BN-cyclo-
hexanes 2,2-R2-1,2-B,N-C4H10 (R2 =HH, MeH, Me2) with Ir and
Rh metal fragments has been studied. This led to the solu-
tion (NMR spectroscopy) and solid-state (X-ray diffraction)
ates. For NR2 =NMeH the final product is the cyclic amino-
borane HBNMeC4H8. The mechanism of dehydrogenation of
2,2-H,Me-1,2-B,N-C4H10 using the {Rh(iPr2PCH2CH2CH2PiPr2)}+
catalyst has been probed. Catalytic experiments
characterization of [Ir(PCy3)2(H)2(h2h2-H2BNR2C4H8)][BArF ]
indicate the rapid formation of
a
dimeric species,
4
(NR2 =NH2, NMeH) and [Rh(iPr2PCH2CH2CH2PiPr2)(h2h2-
[Rh2(iPr2PCH2CH2CH2PiPr2)2H5][BArF ]. Using the initial rate
4
H2BNR2C4H8)][BArF ] (NR2 =NH2, NMeH, NMe2). For NR2 =NH2
method starting from this dimer, a first-order relationship to
[amine-borane], but half-order to [Rh] is established, which
4
subsequent metal-promoted, dehydrocoupling shows the
eventual formation of the cyclic tricyclic borazine [BNC4H8]3,
via amino-borane and, tentatively characterized using DFT/
GIAO chemical shift calculations, cycloborazane intermedi-
is suggested to be due to
equilibrium operating.
a rapid dimer–monomer
Introduction
The metal-catalysed dehydrocoupling of amine-boranes is an
important methodology for the production of polyaminobor-
anes that are isoelectronic analogues of polyolefins. The parent
compound H3B·NH3 is also of significant interest with regard to
its ability to act as a chemical hydrogen store, due to its high
weight %H (19.6%) and the ability to release H2 for subse-
quent utilization in a fuel cell.[1–6] Such catalytic methodologies
offer control of kinetics, product distributions and the temper-
atures of H2 loss when compared to simple thermal activation.
Cyclic amine-boranes[7] such as 1,2-BN cyclohexanes (e.g., 1–
3, Scheme 1A),[8,9] BN-methylcyclopentane (I, Scheme 1C)[10,11]
[a] A. Kumar, Dr. T. N. Hooper, Prof. A. S. Weller
Department of Chemistry, University of Oxford
Mansfield Road, Oxford, OX1 3TA (UK)
E-mail: andrew.weller@chem.ox.ac.uk
[b] J. S. A. Ishibashi, Prof. S.-Y. Liu
Department of Chemistry, Boston College
Chestnut Hill, Massachusetts, 02467-3860 (USA)
E-mail: shihyuan.liu@bc.edu
[c] T. C. Mikulas, Prof. D. A. Dixon
Department of Chemistry, The University of Alabama
Tuscaloosa, Alabama 35487-0336 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502986.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons At-
tribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
Scheme 1. Cyclic BN compounds and their subsequent dehydrogenation.
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and bis-BN cyclohexane (II, Scheme 1C)[12] are attractive candi-
dates for H2 storage applications as they release H2 on heating
to form well-defined molecular species (Scheme 1B and C) for
which viable regeneration routes can be developed.[8,10] H2 loss
that is promoted by a transition-metal-based catalyst offers
As far as we are aware, the coordination chemistry of cyclic
amine-boranes has not been explored, although rhodium
sigma-complexes of the cyclic amino-borane oligomers
[21]
[22]
[H2BNMe2]2
dehydrogenative cyclization of diaminemonoboranes has been
reported to form cyclic diaminoboranes using a
and [H2BNMeH]3
have been described. The
a
significant reduction in the temperature of release
(Scheme 1C), and heterogeneous (e.g., FeCl2 precatalyst)[10]
and homogeneous (Ru-based)[12] systems have been developed
for use with cyclic amine-boranes. Related B-substituted acyclic
systems also undergo dehydrogenation using CoCl2 as a preca-
talyst to form B-substituted borazines.[13] However, the nature
of likely intermediates in these dehydrocoupling processes
have not been determined, either because of the thermal con-
ditions required in the absence of catalyst (e.g., 1508C), the
generally heterogeneous nature of the catalyst system, or lack
of observable intermediates in homogeneous systems (Ru
catalysts).
Sigma-amine-borane complexes[14,15] are key intermediates in
inner-sphere transition-metal-catalysed amine-borane dehydro-
coupling;[1] and they are now well-established in terms of both
fundamental coordination chemistry, BÀH/NÀH activation pro-
cesses and, increasingly, BÀN coupling events.[1] Of particular
relevance to this work are those complexes that arise from
interaction of either an {Ir(PCy3)2(H)2}+[16–18] or a {Rh(chelating-
diphosphine)}+[19,20] fragment with amine-boranes. The former
promotes dehydrogenation of the coordinated amine-boranes
rather slowly, but leads to the isolation of metal bound inter-
mediates (Scheme 2A), while the latter promotes dehydrogen-
ation much more rapidly, leading to the use of low catalyst
loadings (e.g., Scheme 2B).
[Ru(PCy3)2(H2)2(H)2] catalyst;[23,24] the metal-catalysed dehydro-
coupling of base-stabilised diborane(6) [H2B(hpp)]2 to give
[HB(hpp)]2 (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyri-
midinate) has been reported, alongside subsequent coordina-
tion chemistry;[25,26] and there is an early report of sigma com-
plexes formed from cyclic diboranes.[27] Shore and co-workers
have developed the coordination chemistry and reactivity of
related cyclic anionic organohydroborates of the early
transition metals.[28]
In this contribution we report the coordination chemistry of
various N-substituted cyclic 1,2-BN-cyclohexanes, using
{Ir(PCy3)2(H)2}+ or {Rh(chelating-diphosphine)}+ fragments, to
afford the resulting sigma-complexes. We also comment on
their subsequent dehydrogenation/dehydrocoupling that leads
to insight into both: 1) the active species involved, and 2) the
metal-free cyclic amino-borane intermediates formed during
these transition-metal-catalysed routes, which operate at a
significantly lower temperature than non-catalysed or hetero-
geneously catalysed alternatives.
Results and Discussion
Synthesis of cyclic amine-boranes
To provide a comparison of the effect of increasing substitu-
tion at nitrogen with regard to both coordination chemistry
and subsequent dehydrogenation, three 1,2-BN-cyclohexanes
were prepared (Scheme 1A): 2,2-H2-1,2-B,N-C4H10 (1), 2,2-H,Me-
1,2-B,N-C4H10 (2) and 2,2-Me2-1,2-B,N-C4H10 (3). The synthesis of
compound 1 has recently been reported by hydroboration of
a bistrimethylsilyl-substituted homoallylamine by one of us.[9]
Compound 2 is, to our knowledge, unreported in the open lit-
erature as an isolated compound[29] while compound 3 was
originally reported by Wille and Goubeau in 1972.[30] We have
isolated compounds 2 and 3 (see Experimental Section) as an
analytically pure powder or liquid, respectively (Scheme 3). The
1
11B and H B-H NMR chemical shifts for these three compounds
are also given for later comparison with their coordination
complexes.
Reactivity with [Ir(PCy3)2(H2)2(H)2][BArF ]
4
Addition of one equivalent of cyclic amine-borane 1 to a C6H5F
solution of in situ generated [Ir(PCy3)2(H2)2(H)2][BArF ], III (a
4
source of the {Ir(PCy3)2(H)2}+ fragment),[16,17] resulted in the for-
mation of the complex [Ir(PCy3)2(H)2(h2h2-H2BNH2C4H8)][BArF ]
4
1
(4)[31] in quantitative yield as measured by H, 11B and 31P NMR
spectroscopies (ArF =3,5-(CF3)2C6H3, Scheme 4). Complex 4 can
4
be characterised as a Shimoi-type sigma-amine-borane com-
plex.[14,15,32] Analytically pure, crystalline, material was isolated
Scheme 2. Well-defined H3B·NR3 sigma complexes (R3 =MexH3–x; x=1–3) and
catalysed dehydrocoupling. [BArF ]À anions are not shown.
4
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Scheme 3. Synthesis of the 1,2-BN-cyclohexanes and key NMR spectroscopic
data [J(BH) in parenthesis in CD2Cl2].
Scheme 4. Synthesis of cyclic amine borane complexes of the {Ir(PCy3)2(H)2}+
fragment. [BArF ]À anions are not shown.
4
by recrystallisation at low temperature from C6H5F/pentane
solution.
The solid-state structure of complex 4 is shown in Figure 1,
and confirms the formulation. The IrÀH were located in the
final difference map, while the bridging BÀHÀIr were not locat-
ed with any reliability and placed in calculated positions. The
Ir···B [2.217(4) Š], B1ÀC1 [1.588(6) Š] and B1ÀN1 [1.605(6) Š]
distances are all consistent with an amine-borane taking part
in two Ir···HÀB 3 center-2 electron interactions, comparing
closely with sigma complexes of H3B·NMe3 [Ir···B, 2.207(7) Š],[17]
H3B·NMeH2 [2.210(7) Š][33] and H3B·NH3 [2.209(5) Š][18] with the
same metal fragment. Amine-boranes acting in a monodentate
bonding mode through a single BÀH bond, or as BÀH activat-
ed boryls, show significantly longer (ca. 2.6 Š or longer)[34,35]
and shorter MÀB distances (less than 2.1 Š), respectively.[36,37]
Figure 1. Solid-state structure of the cationic portion of complex 4, side (A)
and plan (B) view. Displacement ellipsoids are shown at the 30% probability
level. Selected bond lengths [Š] and angles [8]: Ir1-B1, 2.217(4); Ir1-P1,
2.3145(11); Ir1-P2, 2.3244(10); B1-C1, 1.588(6); B1-N1, 1.605(6); P1-Ir1-P2,
155.87(4).
through two BÀH···Ir interactions, in which the cyclic amine-
borane is undergoing a fluxional process that gives a time-
averaged mirror plane that makes the two sets of hydride li-
gands equivalent. A simple ring-flip is suggested, rather than
a rotation around the B1ÀIr1 vector that would also make the
phosphine ligands equivalent,[40] and for the free amine-borane
this ring-flip has been shown to proceed through a low barrier
(8.8(Æ0.2) kcalmolÀ1).[9]
The cyclic amine-borane adopts
a
chair conformation
(Figure 1B), meaning there is no plane of symmetry in the
molecule in the solid state.
The corresponding sigma complex of 2, [Ir(PCy3)2(H)2(h2h2-
In solution (CD2Cl2), the 11B NMR spectrum of 4 shows a diag-
nostic,[38,39] and significant, downfield shift on coordination
with the metal, d=19.8 ppm, when compared to free ligand,
d=À11.3 ppm. This signal is broad (fwhm=350 Hz) masking
H2BNHMeC4H8)][BArF ] (5), can be prepared in a manner similar
4
to 4. The synthesis of 5 by displacement of the dihydrogen
ligand in III takes longer than for 4: 90 min compared to on
time of mixing, respectively. The solid-state structure of com-
plex 5 is shown in Figure 2, which shows it to be very similar
to that of 4, with the amine-borane also adopting a chair con-
formation. Unlike 4, the BN-cyclohexane ligand is disordered,
occupying four chemically identical, but crystallographically
different sites (see the Supporting Information). The Ir···B
distance measured using this model, 2.230(4) Š, is within error
the same as for 4, as is the P1-Ir-P2 angle of 155.13(3)8.
1
the expected reduction in J(BH).[15] The H NMR spectrum dis-
plays a single IrÀH environment at d=À20.53 ppm [t, J(PH)=
16 Hz] and a single IrÀHÀB environment at d=À6.23 ppm (br)
that sharpens on decoupling of 11B and is shifted 8.04 ppm up-
1
field from 1. Finally, the H NMR spectrum shows a single NÀH
environment at d=4.07 ppm (confirmed by 1H/1H COSY and
HSQC experiments). The 31P{1H} NMR spectrum displays two
environments that show mutual 31P–31P coupling, consistent
with a trans orientation: J(PP)=268 Hz. These solution data are
consistent with a sigma-amine-borane complex, interacting
Despite repeated attempts, only a few crystals of complex 5
were produced, with the complex forming as an oil on at-
tempts to re-crystallise, meaning that analytically pure material
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20 mol%) results in the slow (72 h, TOF ꢀ0.07 hÀ1, sealed NMR
tube) dehydrocoupling to form the final amino-borane derived
tricyclic borazine product [BNC4H8]3 6 [d=34.5 ppm, s; lit. 34.8,
C6D6],[9,30] (Scheme 5). Inspection of the 11B NMR spectrum after
2 h shows compound 1 and a significant proportion of a new
signal assigned to the new monomeric amino-borane
HBNHC4H8, 7 [d=41.1 ppm, d, J(BH) 124 Hz]. This chemical
shift and coupling pattern is similar to other, transient, amino-
boranes,[17,42,43] as well as stable HBNMeC4H8 8 (vide infra).
After 24 h the signal due to 7 had essentially disappeared,
with 6 now observed as a significant product. Also apparent
after 24 h is a set of broad peaks centred around d=À5 ppm
that also show J(BH) coupling. Over a further 48 h (72 h in
total) these signals reduce in intensity at the overall gain of 6,
and the temporal behaviour of the system suggests they are
due to intermediates that follow 7 and precede 6. The final or-
ganometallic product observed is the pentahydride
[Ir(PCy3)2H5].[41] Reformation of compound 1 was not observed
during these later processes; the observation of which would
point to H-redistribution processes.[42,44]
Figure 2. Solid-state structure of the cationic portion of complex 5. Only one
disordered component of the cyclic amine-borane ligand is shown. Displace-
ment ellipsoids are shown at the 30% probability level. Selected bond
lengths [Š] and angles [8]: Ir1-B1, 2.230(4); Ir1-P1, 2.3269(7); Ir1-P2, 2.3266(7);
B1-C1, 1.588(5); B1-N1, 1.566(6); P1-Ir1-P2, 155.13(3).
Due to their transient nature, overlapping signals, and lack
of charge, we have not been able to use detailed NMR spectro-
scopic or ESI-MS techniques to determine the identity of these
intermediate species. The chemical shift/coupling constant
data suggest four-coordinate BH groups that are not metal-
bound, possibly due to a cycloborazane species (dimers and/or
trimers, e.g., n=0 or 1, Scheme 5) and isomers thereof.
for microanalysis was not available. Nevertheless, NMR spec-
troscopy demonstrates the formulation of 5 in the bulk. In the
1H NMR spectrum the IrÀH groups are now observed as two
relative 1-H signals, at d=À20.58 and À20.82 ppm, due to the
asymmetry now imposed by the NMe group on the cyclic
amine-borane that would not be removed by a low energy
ring-flip. Likewise, two IrÀHÀB signals are observed (d=À6.24,
À6.35 ppm), and a more complicated aliphatic region com-
pared with 4 is noted, as all the methylene CÀH groups are
now inequivalent. The 11B NMR spectrum shows a signal at d=
22.4 ppm, similar to that measured for complex 4 and shifted
downfield from free ligand (d=À6.5 ppm). As with 4, there are
two environments observed in the 31P{1H} NMR spectrum that
show trans J(PP)-coupling [J(PP)=276 Hz].
In order to put the structures of these intermediates on
a firmer footing we have used DFT geometry optimization cou-
pled with GIAO 11B chemical shift calculations to help in their
identification. Table 1 shows selected examples, with full details
given in the Supporting Information. The chemical shifts of
compounds 7 [exptl d=41.1 ppm, calcd d=38.4 ppm] and 6
[exptl d=34.5 ppm, calcd d=31.9 ppm] are reproduced well,
Addition of 3 to a C6H5F solution of [Ir(PCy3)2(H2)2(H)2][BArF ]
4
resulted in no reaction to the detection limit of 31P{1H} NMR
spectroscopy (ca. 5%) in a sealed NMR tube. However, the
1H NMR spectrum showed a very small peak at d=À4.06 ppm
(less than 5%) that might be assigned to a sigma complex by
comparison with the well-characterized examples 4 and 5. This
might suggest an initial equilibrium is established between the
starting bis-dihydrogen complex and a corresponding BN-cy-
clohexane sigma-complex that favours the starting material
(presumably due to the steric clash between the PCy3 and the
NMe2 groups). With the same metal fragment we have previ-
ously commented upon similar relative differences in the
strength of the IrÀHÀB sigma interaction when comparing
H3B·NR3 (R=H or Me).[17,18] Over time (8 h), decomposition to
[Ir(PCy3)2H5][41] and [Ir2{(PCy3)2}2H5][BArF ][17] is observed.
4
Catalytic dehydrocoupling using the Ir–BN complexes
When prepared pure, complexes 4 and 5 are stable for at least
24 h in 1,2-F2C6H4 solution with no significant change observed
by NMR spectroscopy. However, addition of five additional
equivalents of the cyclic amine-borane 1 to complex 4 (i.e.,
Scheme 5. Dehydrocoupling of 1 catalysed by 4. 11B NMR spectrum (ppm
scale) at 2, 24 and 72 h.
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with a consistent small, approximately 2.5 ppm, difference be-
tween experiment and calculation (Table 1). Based on these
calculations diborazane and triborazane species would be ex-
pected to show signals between d=0 to À7 ppm and d=À2
to À9 ppm, respectively, in the experimentally determined
spectrum as is observed (Scheme 5). Partially dehydrogenated
trimers would be expected to show an additional signal about
d= +37 ppm, very similar to 6, and thus might be obscured.
The oligomerisation of amino-boranes to form cyclic borazane
products is well-known, and these can, in certain cases, be fur-
ther dehydrogenated by a transition metal-catalyst to form
cyclic borazines.[22,45,46]
Scheme 6. Dehydrogenation of 2 catalysed by 5. 11B NMR spectrum after
24 h (ppm scale). *: Assigned to trace 1-OH-2-Me-1,2-B,N-C4H8[30] (confirmed
by EI-MS). The signal at d=À6.1 ppm is assigned to residual [BArF ]À.
4
Addition of 5 equivalents of 2 to a 1,2-F2C6H4 solution of III
(as a precursor to 5) in a sealed NMR tube results in dehydro-
genation and the formation of the monomeric cyclic amino-
borane HBNMeC4H8 8 (Scheme 6) after 24 h (TOF ꢀ0.2 hÀ1).
Compound 8 was initially reported by Wille and Goubeau,[30]
and has been independently prepared by intramolecular
hydroboration of the N-methylhomoallylamine-borane adduct
and subsequent one-pot thermal dehydrogenation (see Experi-
mental Section). Compound 8 does not cyclotrimerise or cyclo-
dimerise and remains monomeric in solution, as evidenced by
a characteristic down-field chemical shift in the 11B NMR spec-
trum d=40.8 ppm [J(BH) 125 Hz], very similar to 7. No other
significant boron-containing products were observed during
this process. The final organometallic species observed were 5
techniques to play an important role in lowering the barrier to
BÀH and NÀH activation steps.[18,49–51] We have not explored
the mechanism for the dehydrogenation process in 4 and 5 in
detail, but other studies using this iridium fragment have
shown that NÀH activation of amine-boranes is rate-determin-
ing and preceded by BÀH activation.[18,49]
Interestingly, in this context, addition of D2 to complex 4 in
C6H5F solution results in the loss of the IrÀH, IrÀHÀB and NÀH
resonances in the 1H NMR spectrum, and the appearance of
corresponding signals in the 2H NMR spectrum while the
31P{1H} NMR spectrum remains unchanged (Scheme 7). Dis-
solved HD and H2 were also observed [d=4.47 ppm, t, J(HD)=
43 Hz; d=4.50 ppm, respectively]. This points to both rapid
IrÀH/D2 exchange,[40] and that sequential NÀH or BÀH activa-
tion (in either order) are of approximately similar energies and
reversible. An alternative mechanism would be a concerted
and reversible NH and BH activation that leads to amino-
borane, 7 that then re-adds D2. That no free amino-borane 7,
or the final cyclic trimer 6, were observed argues against
a mechanism involving such reversible dehydrogenation.[52]
Addition of D2 followed by H2 re-establishes the NÀH (d=
4.07 ppm), Ir···HÀB (d=À6.23 ppm) and IrÀH (d=À20.53 ppm)
signals, showing that this process is reversible. NÀH activation
1
and Ir(PCy3)2H5 as identified by H and 31P{1H} NMR spectrosco-
py, in a 0.6:1 ratio, respectively.
The {Ir(PCy3)2(H)2}+ fragment thus acts as a slow catalyst (or
precatalyst) for the dehydrogenation of these cyclic amine-bor-
anes, as found for their acyclic counterparts.[17,18,42] That the
onward dehydrogenation does not occur in 4 or 5 in the ab-
sence of additional amine-borane is a further demonstration of
the promotional role that amine-borane plays in dehydrogena-
tion chemistry. This is likely through the formation of BÀH···HÀ
N dihydrogen bonds,[47,48] which are commonplace in amine-
borane chemistry, and have been shown by computational
Table 1. Calculated 11B NMR chemical shifts of representative dimers and trimers (s=standard (BF3·OEt2) - molecule).
Molecule
s (ppm)
Molecule
s (ppm)
+38.4 [exptl +41.1]
+31.9 [exptl 34.5]
À9.3
À2.2
À10.7 (B1)
+34.3 (B2)
À9.1 (B3)
À11.0 (B1)
À9.3 (B2)
À5.6 (B3)
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Scheme 7. Reversible H/D exchange in complex 4. [BArF ]À anions are not
4
shown.
is generally considered to have a larger barrier than BÀH acti-
vation in cationic systems.[1,17,53,54] For example, addition of D2
to [Ir(PCy3)2(H)2(h2h2-H3BNMeH2)][BArF ] results in H/D exchange
4
only at IrÀH and BÀH.[33] However, products that arise from
formal NÀH activation over BÀH activation have been isolat-
ed[55,56] or postulated[57,58] for neutral systems and are also pro-
posed as intermediates in amine-borane dehydropolymerisa-
tion.[20,59] It thus appears likely that the cyclic nature of the
amine-borane in 5 results in more levelled BÀH/NÀH activation
energies. NÀH activation may be additionally assisted by intra-
molecular hydrogen bonding.[57,60] Related acyclic phosphido-
borane complexes have been isolated and shown to undergo
rapid and reversible PÀH/BÀH bond activation as probed by
H/D scrambling experiments.[61] As significant D incorporation
into the PCy3 ligand is also observed, we cannot discount
more complicated mechanisms for H/D exchange that involve
cyclometallated phosphine ligands.
Scheme 8. 11B NMR spectra (ppm scale) showing the temporal evolution of
the dehydrogenation of 2 (2 equivalents) by V to form
[Rh(iPr2PCH2CH2CH2PiPr2)(h2h2-H2BNHMeC4H8)][BArF ] (10) and amino-borane
4
8. [BArF ]À anions are not shown.
4
NMR spectrum at room temperature [d=57.9 ppm, J(RhP)=
162 Hz]. Cooling to 190 K reveals two signals [d=57.8 ppm,
dd, J(PP)=56 Hz, J(RhP)=160 Hz; d=56.9 ppm, dd, J(PP)=
56 Hz, J(RhP)=160 Hz]. This suggests a fluxional process that
makes equivalent the two phosphine groups. A mechanism
that invokes a bidentate to monodentate change in amine-
borane binding and then a rotation around the remaining
RhÀH bond is suggested.[40] The 11B NMR spectrum shows
a broad signal at d=32.1 ppm, downfield shifted from free 2
by 38.6 ppm, consistent with a bidentate binding mode of the
amine-borane at a RhI centre.[21]
Reactivity of cyclic amine–boranes with
[Rh(iPr2PCH2CH2CH2PiPr2)(h6-C6H5F)][BArF ]
4
Addition of 2 equivalents of the cyclic amine-borane 2 to
[Rh(iPr2PCH2CH2CH2PiPr2)(h6-C6H5F)][BArF ], V,[62] in 1,2-F2C6H4
4
solution and monitoring in situ using 31P{1H} and 11B NMR spec-
troscopy showed that after 5 min a new complex was formed
[Rh(iPr2PCH2CH2CH2PiPr2)(h2h2-H2BNHMeC4H8)][BArF ]
(10;
4
Scheme 8), alongside unreacted V and 2. Also observed is the
amino-borane 8. Over time V and 2 are consumed, so that
after 8 h 10 and 8 remain. Over this time period a small
amount of dimer [Rh2(iPr2PCH2CH2CH2PiPr2)2H5][BArF4] (12) also
Figure 3 shows the solid-state structure of the cationic por-
tion of complex 10, demonstrating that the cyclic amine-
borane, 2, interacts with the Rh centre in a bidentate manner
through two RhÀHÀB interactions at a pseudo-square planar
RhI centre. Although the bridging hydrogen atoms were locat-
ed in the final difference map they were ultimately placed in
calculated positions. The amine-borane is equally disordered
over two positions, which can be modelled as either the NMe
pointing axial or equatorial with respect to the RhP2 plane. The
Rh···B distance measured from this model at 2.150(6) Š is
slightly shorter, but still similar, to that in closely related
1
forms (ꢀ5% by 31P{1H} and H NMR spectroscopy), vide infra.
Complex 10 can be produced as a red, analytically pure, crys-
talline material in moderate (40–45%) yield by recrystallization
of the reaction mixture from 1,2-F2C6H4/pentane at À358C.
When isolated pure, 10 is relatively stable, as is the case with
the iridium complexes. After 2 days, isolated samples of 10 in
C6H5F solution decompose to give only V and an unidentified
borane species [d(11B)=5.17 ppm (s)]. The observation of both
the starting material, V, and amino-borane, 8, under conditions
of excess 2 suggests that initial substitution of the fluoroben-
zene ligand is slower than subsequent amine-borane
promoted dehydrogenation in 10 (Scheme 8).
[Rh(Ph2PCH2CH2CH2PPh2)(h2h2-H3B·NMe3)][BArF ], 2.199(3) Š.[19]
4
The cyclic amine-borane adopts a chair conformation in 10.
Addition of 2 equivalents of 1 to V results, after only
30 min, in the formation of the new complex [Rh(iPr2P
1
The room temperature H NMR spectrum of 10 shows a set
of broadened environments for the chelating ligand and
amine-borane that give little further information. Two high-
field signals at d=À4.9 ppm and d=À5.4 ppm are assigned
to the RhÀHÀB groups. Although two signals are observed,
showing that the amine-borane lacks a plane of symmetry due
to the NMe group, only one signal is observed in the 31P{1H}
CH2CH2CH2PiPr2)(h2h2-H2BNH2C4H8)][BArF ] (9) and the complete
4
consumption of the amine-borane to form the amino-borane 7
(Scheme S2, Supporting Information). Over 8 h, this mixture
evolves by further dehydrocoupling to give cyclotriborazine 6
and complex 9 as the organometallic product. At the early
Chem. Eur. J. 2016, 22, 310 – 322
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Figure 3. Solid-state structure of the cationic portion of complex 10 (side
and top view). Only hydrogen atoms associated with the NH and Rh···HB
interaction are shown, and only one disordered component is shown. Dis-
placement ellipsoids are shown at the 30% probability level. Selected bond
lengths [Š] and angles [8]: Rh1-B1, 2.150(6); Rh1-P1, 2.2167(12); Rh1-P2,
2.2030(12); B1-N1, 1.518(8); P1-Rh1-P2, 92.95(5).
Figure 4. Solid-state structures of the cationic portion of complexes 9 and
11. Only hydrogen atoms associated with the Rh···HB interaction are shown.
Displacement ellipsoids are shown at the 30% probability level. Selected
bond lengths (Š) and angles (8) (9): Rh1-B1, 2.155(5); Rh1-P1, 2.2174(12);
Rh1-P2, 2.2170(12); B1-N1, 1.588(6); P1-Rh1-P2, 93.52(5). (11) Rh1-B1,
2.172(3);
stages of the reaction, NMR signals due to amine-borane and
1
Rh1-P1, 2.2204(8); Rh1-P2, 2.2182(8); B1-N, 1.599(4); P1-Rh1-P2, 94.14(3).
the sigma-complex 9 are too broad to be observed in the H
and 11B NMR spectra, but the 31P{1H} NMR spectrum is sharper,
thus suggesting a rapid exchange between bound and free
amine-borane. In support of this rapid exchange hypothesis,
when all of 1 is consumed, and the opportunity for exchange
is reduced, sigma-complex 9 is observed as a characteristic
broad signal at d=29.3 ppm in the 11B NMR spectrum while
the RhÀHÀB groups are observed at d=À4.83 ppm in the
1H NMR spectrum as a broad signal (relative integral of 2H).
The 31P{1H} NMR spectrum shows a single environment that
couples to 103Rh [d=57.1 ppm, J(RhP)=160 Hz]. As this rapid
exchange is not observed in 10 or 11, we suggest that the in-
creasing levels of substitution on nitrogen slow down this pro-
cess, which might indicate an associative mechanism for
ligand substitution. Recrystallisation of the reaction mixture
from 1,2-F2C6H4/pentane resulted in a small number of red
crystals of complex 9 that required mechanical separation
from co-crystallised orange V. The solid-state structure of com-
plex 9 is shown in Figure 4, and is closely related to 10. In par-
ticular, the Rh···B and BÀN distances are the same within error
or very similar, respectively: 2.155(5) and 1.588(6) Š. As com-
plex 9 cannot be isolated pure in bulk, we have not pursued
the H/D exchange experiments.
(11) which can be recrystallised by addition of pentane. Com-
plex 11 has been characterized by NMR spectroscopy and
single-crystal X-ray diffraction, and shows very similar data to
1
that of 9 and 10. The H NMR spectrum shows a single NMe2
environment at d=2.85 ppm (6H) and a single RhÀHÀB envi-
ronment at d=À5.17 ppm, while the 11B NMR spectrum shows
a down-field shifted signal at d=34.4 ppm. The solid-state
structure (Figure 4) also reflects these similarities with the
pseudo-square planar RhI centre coordinated with the cyclic
amine-borane in a bidentate manner. Comparing 9, 10, and 11,
there is no change in the Rh···B bond distances [2.155(5),
2.150(6), 2.172(3) Š, respectively] within the experimental error.
There is a slight change in chemical shift of the boron atom
when compared with free ligand [Dd= +40.6, +38.6,
+37.4 ppm], which suggests a trend in that increasing N-
substitution leads to a decreasing M···B interaction that is not
captured by an associated change in bond lengths.[38,63]
Catalytic dehydrogenation using the Rh–BN complexes
Addition of cyclic-amine-borane 3 to V in 1,2-F2C6H4 solution
results in the slow (24 h) substitution of the arene and the for-
mation of [Rh(iPr2PCH2CH2CH2PiPr2)(h2h2-H2BNMe2C4H8)][BArF4]
We have explored the catalytic dehydrogenation/dehydrocou-
pling of the cyclic amine-boranes
1 and 2 using the
{Rh(iPr2PCH2CH2CH2PiPr2)}+ fragment. Using 10 mol% V slow
Chem. Eur. J. 2016, 22, 310 – 322
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(4 days, sealed system, TOF ꢀ0.1 hÀ1) dehydrocoupling of
1 occurs to ultimately afford the tricyclicborazine 6. As with
catalyst 4, intermediate species centred around d=À5 ppm
are observed in the 11B NMR spectrum (Scheme 9, Table 1). At
(Scheme 11). A single crystal X-ray diffraction study demon-
strated the gross structure, but the hydrides were not located
(see Supporting Information). Complex 12 is similar to previ-
ously reported [Rh2(bis-phosphine)2H5]+ cations that are
formed by hydrogenation of [Rh(bis-phosphine)]+ precursors,
through dimerisation/deprotonation.[66–68] It is stable to H2 loss
under vacuum (10À3 Torr) in the solid-state and in solution. We
did not observe any evidence for the formation of boronium
cations (e.g., [L·BHNHMe(CH2)4]+) which might indicate a
hydride abstraction route to form intermediate monocationic
systems with odd numbers of hydrides.[69]
the end of catalysis complexes
9
and dimeric 12
[Rh2(iPr2PCH2CH2CH2PiPr2)2H5][BArF ] are observed as the
4
organometallic species.
Complex 12 is a competent catalyst itself for the dehydro-
genation of compound 2, taking approximately 2 h to effect
complete conversion ([Rh]=10 mol%, sealed NMR tube, TOF=
5 hÀ1). When comparing 10 and 12 as catalysts (10 mol% [Rh])
both follow an overall first-order profile (k=1.18(3)”10À5 sÀ1
;
2.16(7)”10À4 sÀ1, respectively) with dimeric 12 much faster
than monomeric 10. No induction period was observed.[19,20]
As these are sealed conditions, inhibition by H2 may well be
occurring, complicating a detailed kinetic analysis, as observed
previously for Rh systems.[20,70] Unfortunately, we have been
unable to reliably measure the rate in an open system (under
Ar) due to partial decomposition of 8 upon sampling.[30]
Addition of a hindered base (2,6-di-tertbutylpyridine, 5
equivalents relative to catalyst) to the mixture with 10/2 result-
ed in faster catalysis (k=1.4(1)”10À4 sÀ1) and the observation
of 12 as the only organometallic species at the end of catalysis.
Addition of [H(OEt2)2][BArF ]/10 equivalents of 2 to dimer 12 re-
4
sulted in the formation of 10 and catalysis at a slower rate,
similar to starting from 10 (k=2.3(1)”10À5 sÀ1). Addition of
[H(OEt2)2][BArF4]/CH3CN to dimer 12 resulted in the formation
Scheme 9. 11B NMR spectra (ppm scale) showing the temporal evolution of
the dehydrogenation of 1 by V (10 mol%) to ultimately form 6.
of [Rh(iPr2PCH2CH2CH2PiPr2)(NCMe)2][BArF ] (13; Scheme 11).
4
Complex 13 was also independently synthesized by the addi-
tion of CH3CN to V. These, and our previous observations, sug-
gest: 1) dimeric 12 is a more active (pre)catalyst than mono-
meric 10; 2) under catalytic conditions, 12 forms from 10 by
slow dehydrogenation of 2, subsequent oxidative addition of
H2 and dimerization to form intermediate B, which adds fur-
ther H2 and is reversibly deprotonated to form [H(solvent)x]
Dehydrogenation of methyl-substituted amine-borane
2
using V (10 mol%) results in the formation of amino-borane 8,
taking 17 h (TOF ꢀ0.6 hÀ1; Scheme 10) in a sealed NMR tube.
Following the catalyst speciation by 31P{1H} NMR spectroscopy
showed that V was replaced by 10 and then 12, the ratio of
which changed over time, to finally give a ratio of 10:12 of 1:9
after 17 h.
[BArF ] and 12 (Scheme 12); 3) H+ inhibits catalysis presumably
4
by channelling the resting state away from 12; 4) addition of
base promotes the formation of 12.
That dimeric 12 is an active catalyst or precatalyst for amine-
borane dehydrogenation has resonance with previous reports
of dimeric {Rh(L2)}2 being implicated in dehydrocoupling of
acyclic amine-boranes,[19,58,70,71] although in some systems
dimers have been discounted on the basis of computational
analysis.[49] The likely involvement of potential dimer/monomer
equilibria during catalysis using 12 as a precursor was probed
using the method of initial rates, monitoring over the first 5%
of turnover in the pseudo-zero-order regime of catalysis
(Figure 5).
Scheme 10. Dehydrocoupling of 2 using catalyst V to afford amino-borane
8.
The formation of the dimer [Rh2(iPr2PCH2CH2CH2PiPr2)2H5]
[BArF ] (12) is interesting, and to explore its role further it was
4
synthesized independently by addition of [H(OEt2)2][BArF ][64] to
These data show a first-order dependence on [2] and a half-
order dependence on [12]. This is consistent with a rapid
dimer/monomer equilibrium in which the dimer is dominant
but sits off the cycle and a monomer is the active species.
Such equilibria have been suggested before for arene
4
Fryzuk’s dinuclear [Rh(iPr2PCH2CH2CH2PiPr2)(m-H)]2 complex[65]
under an atmosphere of H2. Complex 12 can be isolated in
good yield as a microcrystalline material and has been charac-
terized by 1H, 31P NMR spectroscopy and microanalysis
Chem. Eur. J. 2016, 22, 310 – 322
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Scheme 11. Synthesis and reactivity of complex 12. [BArF ]À anions are not shown.
4
Scheme 12. Suggested relationships between 12 and 10. (S)=solvent.
[BArF ]À anions are not shown.
4
alkylations,[72] alkene hydrogenation[65] and hydroboration,[73]
amine-borane dehydrogenation,[58] CÀS bond activations,[74]
and arylation of BCl-1,2-azaborines,[75] amongst others. Perhaps
most closely related to the system under discussion here are
dimer/monomer equilibria operating for Shvo’s catalyst in both
amine-borane dehydrogenation and hydrogen transfer reac-
tions.[76,77] In these cases a dimer is suggested to be in
equilibrium with two, different, monomers (of equal relative
concentration); one of which is active in catalysis.
For the system here we speculate a rapid equilibrium is es-
tablished between 12 and cationic (C) and neutral (D) mono-
mers, for which one of the latter is the active catalyst
(Scheme 13). We discount the alternative reason for half-order
dependence that involves dissociation of either chelating
phosphine or H2 from 12[78] as this does not fit with other ex-
perimental observations. Reversible monomer/dimer equilibria
for [Rh(L2)H5]+ species involving reversible protonation have
previously been noted,[66,67] but we suggest that this is not oc-
curring due to the positive effect that exogenous base has on
the observed rate when starting from 10, conditions that
favour the formation of 12. Consistent with the rapid equilibri-
um proposed, addition of MeCN (10 equivalents) to 12 result-
ed in the immediate formation of the monomeric MeCN-
adduct 13 plus gas evolution (H2), which could arise from C.
Also formed are uncharacterised hydride products [1H NMR:
d=À11.79 and À17.45 ppm] that decompose after 1 h,
suggestive of a reactive neutral species such as D.
Figure 5. Initial rate versus concentration for the dehydrogenation of 2
mediated by 12 in a sealed NMR tube. A) [2]; B) [12]1/2, inset shows relation
to [12]1.
Scheme 13. Suggested dimer/monomer equilibrium for 12. [BArF ]À anions
4
are not shown. (S)=weakly bound solvent.
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metal, 0.061 mmol, 0.2 mol% palladium). The vessel was sealed
and flushed with hydrogen before pressurizing to 45 psi with hy-
drogen. The reaction was heated for 16 h with monitoring of the
internal pressure and refilling as often as required. The reaction
was then cooled to room temperature, and the solids were filtered
off giving 1.68 g crude product (41% crude yield, 12.8 mmol). This
crude product was dissolved in THF, and lithium aluminum hydride
(1.12 g, 29.5 mmol, 2.3 equiv) was added carefully at room temper-
ature. The reaction was stirred for 24 h at room temperature, and
the solids were filtered off in the glovebox. The filtrate was cooled
to À788C, and methanol (10 mL) was added dropwise to quench
excess LiAlH4 and to install the proton on the product. The reac-
tion was warmed to room temperature over 30 min, and the vola-
tiles were removed in vacuo. The residue was extracted with
hexane (4”80 mL), and the hexane was removed using a rotary
evaporator. This residue was rinsed with cold pentane to furnish
Conclusion
Presented here is the first study of the coordination chemistry,
and subsequent dehydrogenation, of BN-cyclohexanes. Per-
haps unsurprisingly the sigma-amine borane complexes
formed with the [Ir(PCy3)2(H)2]+ and [Rh(PiPr2(CH2)3PiPr2)]+ frag-
ments are broadly similar to those that result from coordina-
tion of acyclic amine-boranes, such as H3B·NMe3, although
there are interesting differences in reactivity (e.g., the more
levelled BÀH/NÀH activation energies when probed with exog-
enous D2). The real insight that comes from these systems is
their ability to mediate (albeit slowly) the dehydrogenation of
the coordinated cyclic-amine boranes at room temperature
that allows for intermediate species to be observed in the de-
hydrocoupling of the parent 2,2-H2-1,2-B,N-C4H10; as well as
(for the Rh system) the kinetics of dehydrogenation to be
probed, which show half-order behaviour for the catalyst, sug-
gesting a rapid dimer–monomer equilibrium is operating. Such
insight is valuable in both determining dehydrocoupling path-
ways of cyclic amine-boranes and determining the speciation
of the active catalyst species.
1
374 mg of desired product (12% yield overall). H NMR (500 MHz,
C6D6): d=2.93–2.07 (m, B-H signals), 1.93 (m, 3H), 1.72 (brs, 3H),
1.54 (brs, 2H), 1.35 (brs, 1H), 1.23 (brs, 1H), 0.87 (brs, 1H),
0.59 ppm (brs, 1H); 13C NMR (126 MHz, C6D6): d=55.1, 42.4, 28.8,
26.2, 16.6 ppm (br); 11B NMR (96 MHz, C6D6): d=À6.5 ppm (t,
J(BH)=96.4 Hz); HRMS (EI+): m/z calcd for C5H13NB [M]+
98.114105, found 98.114265.
N,N-Dimethyl-1,2-azaborinane 3: The preparation of this com-
pound was adapted from that of Wille and Goubeau.[30] In a glove-
box, a 300 mL pressure vessel was charged with tetrahydrofuran
(100 mL) and N,N-dimethylhomoallylamine (3.00 g, 30.3 mmol,
1 equiv). Borane-tetrahydrofuran solution (36 mL, 1m in THF,
36 mmol, 1.2 equiv) was added dropwise. The pressure vessel was
sealed, and the reaction was heated to 1008C for 18 h. The reac-
tion was cooled to room temperature then opened in the air. The
volatiles were removed in vacuo. The resulting residue, a viscous,
colourless liquid, was subjected to silica gel chromatography in the
air with 2:3 CH2Cl2/hexane as the eluent system to furnish a colour-
less, viscous liquid (102 mg, 3%). 1H NMR (300 MHz, C6D6): d=
2.96–2.13 (m, overlapping B-H signals), 2.06 (app t, 2H), 1.95 (s,
6H), 1.76 (brs 2H), 1.34–1.16 (m, 2H), 0.92 (brs, 2H). 13C NMR
(126 MHz, C6D6): d=62.4, 50.3, 26.4, 25.0, 14.8 (br). 11B NMR
(96 MHz, C6D6): d=À3.0 (t, J=96.8 Hz). HRMS (EI+) m/z calcd for
C6H15NB [M]+ 112.12976, found 112.12923.
Experimental Section
All manipulations were performed under an argon atmosphere
using standard Schlenk and glove-box techniques. Glassware was
oven dried at 1308C, overnight, and flamed under vacuum prior to
use. Pentane and MeCN were dried using a Grubbs-type solvent
purification system (MBraun SPS-800) and degassed by successive
freeze-pump-thaw cycles.[79] C6H5F and 1,2-F2C6H4 were dried over
CaH2, vacuum distilled and stored over 3 Š molecular sieves. [IrHP-
Cy2(h2-C6H9)PCy2(h3-C6H8)][BArF ],[16]
[Rh(PiPr2(CH2)3PiPr2)(C6H5F)]
4
[BArF ],[62] Na[BArF4],[80] Rh(PiPr2(CH2)3PiPr2)]2(m-H)2
and [H(OEt2)2]
[65]
4
[BArF ][64] were prepared by literature methods. BN-cyclohexane (1),
4
was prepared as described by Luo et al.[9] NMR spectra were re-
corded on a Bruker AVIII-500 spectrometer at room temperature,
or a Varian Unity/Inova 300 spectrometer 500 spectrometer. In
1
C6H5F and 1,2-C6H4F2 solvents H NMR spectra were referenced to
the centre of the downfield solvent multiplet, d=7.11 and
7.07 ppm, respectively. 31P and 11B NMR spectra were referenced
against 85% H3PO4 (external) and BF3·OEt2 (external), respectively.
The spectrometer was pre-locked and pre-shimmed to the solvent
mixture of 0.3 mL of 1,2-C6H4F2 and 0.1 mL of C6D6. Chemical shifts
(d) are quoted in ppm and coupling constants (J) in Hz. ESI-MS
were recorded on a Bruker micrOTOF instrument interfaced with
a glove-box.[81] Electron impact high-resolution mass spectrometry
(EI-HRMS) was performed at the Mass Spectrometry Facilities and
Services Core of the Environmental Health Sciences Center at
Oregon State University. Microanalysis was performed by Elemental
Microanalysis Ltd. For hydrogenation reactions a high pressure
NMR tube equipped with a J. Young’s valve and the dissolved com-
pound of interest was cooled to 77 K. The tube was evacuated and
H2 admitted (1 atm). The tube was sealed and warmed to 298 K, re-
sulting in a pressure of approximately 4 atm (298/77 ꢀ4).
[Ir(H)2(PCy3)2(h2h2-H2BNH2(CH2)4)][BArF ] (4): In a Young’s flask,
4
[IrHPCy2(h2-C6H9)PCy2(h3-C6H8)][BArF4] (44 mg, 2.7”10À2 mmol) in
C6H5F was hydrogenated at 4 atm as described in the general pro-
cedures. It was stirred for 20 min to produce a colourless solution
of [Ir(H)2(PCy3)2(H2)2][BArF ] which was rapidly transferred under
4
argon to a Schlenk flask containing 1 (2.3 mg, 2.7”10À2 mmol).
The resulting colourless solution was stirred for 2 min at 258C,
then layered with pentane and held at À308C for 72 h to afford
the product as colourless crystal. Yield: 20 mg, 43%. 1H NMR
(500 MHz, CD2Cl2): d=7.76 (s, 8H, [BArF ]À), 7.60 (s, 4H, [BArF ]À),
4
4
4.07 (br, 2H, NH2), 3.23 (br, 2H, CH2 next to NH2), 1.93-1.4 (m, 35H,
PCy3 (33H) and CH2 (2H)), 1.36-1.29 (m, 33H, PCy3), 0.92 (m, 4H,
CH2), À6.23 (br, 2H, BH2), À20.53 ppm (t, 2JHP =16 Hz, IrH2). The
NH2/CH2 resonances were identified on the basis of the number of
cross-peaks in the 1H/1H COSY spectrum. Furthermore the HSQC
spectrum shows no cross peak between the signal at d=
4.07 ppm, while the signal at d=3.23 ppm is coupled to a 13C NMR
Synthesis and characterization
2
signal at 48 ppm. 31P{1H} NMR (202 MHz, CD2Cl2): d=37.12 (d, JPP
=
N-Methyl-1,2-azaborinane 2: B-Cl-N-Me-1,2-BN-cyclohexene was
prepared as described by Chrostowska and Liu.[82] In a glovebox,
a Fischer–Porter tube was charged with the starting material
(4.00 g, 30.5 mmol) and palladium on carbon (64 mg, 10 wt% Pd
268 Hz, 1P), 35.26 ppm (d, 2JPP =268 Hz, 1P); 11B NMR (160 MHz,
CD2Cl2): d=19.8 (br, bound BH2), À6.63 ppm (s, [BArF ]À); ESI-MS
4
(1,2-C6H4F2, 608C) positive ion: m/z 840.5431 [M+] (calcd
840.5492); elemental microanalysis: calcd [C72H92B2F24IrNP2]
Chem. Eur. J. 2016, 22, 310 – 322
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(1703.28 gmolÀ1): C 50.77, H 5.44, N 0.82; found: C 50.32, H 5.33, N
1.03.
[Rh(iPr2PCH2CH2CH2PiPr2)(h2h2-H2BNHMeC4H8)][BArF ] (10): 1,2-
4
F2C6H4 (0.5 mL) was added to a Young’s flask charged with 2
(4.5 mg, 4.5 X 10À2, 2.5 equivalent) and V (25 mg, 1.8”10À2 mmol).
The resulting orange solution was stirred for 30 min to obtain
a red solution. Diffusion of pentane into this solution at À358C for
72 h afforded 10 as red crystals. Yield: 10 mg, 41%. 1H NMR
(500 MHz, CD2Cl2): d=7.76 (s, 8H, [BArF ]À), 7.60 (s, 4H, [BArF ]À),
[Ir(H)2(PCy3)2(h2h2-H2BNMeH(CH2)4)][BArF ] (5): In a high pressure
4
NMR tube equipped with
a
J. Young’s valve, [IrHPCy2(h2-
C6H9)PCy2(h3-C6H8)][BArF4] (16 mg, 1”10À2 mmol) in C6H5F was hy-
drogenated at 4 atm as described in the general procedures. It was
4
4
stirred for 20 min to produce
a
colourless solution of
3.94 (br, 1H, NH), 3.33 (br d, 1H, CH2), 2.91 (d, 3JHH =5 Hz, 3H,
NMe), 2.76 (m, 1H, CH2), 2.37 (br, 1H, CH2), 1.99–1.92 (br, 6H, CH
(4H) and CH2 (2H)), 1.65 (m, 1H, CH2), 1.5–1.0 (br m, 26H, CH3
(24H) and CH2 (2H)), 0.52 (br, CH2CH2CH2),-4.9 (br, 1H, BH2),
À5.38 ppm (br, 1H, BH2); 31P{1H} NMR (202 MHz, CD2Cl2): d=
57.87 ppm (d, JRhP =162 Hz); 31P{1H} NMR (202 MHz, CD2Cl2, 190 K):
d=57.75 (overlapping dd, 1P, JRhP =160 Hz, JPP =56 Hz), 56.85 ppm
(overlapping dd, 1P, JRhP =160 Hz, JPP =56 Hz); 11B NMR (160 MHz,
[Ir(H)2(PCy3)2(H2)2][BArF ] which was rapidly transferred under argon
4
to another high pressure NMR tube containing 2 (1 mg, 1”
10À2 mmol). Gentle inversion of NMR tube for 1.5 h resulted in the
colourless solution of 5 and Ir(H5)(PCy3)2 in 20:1 ratio. Compound 5
was characterized in situ by NMR spectroscopy and ESI-MS. A few
single crystals suitable for X-ray diffraction studies was obtained by
1
diffusion of pentane at À358C. H NMR (500 MHz, C6H5F): d=8.34
(s, 8H, [BArF ]À), 7.67 (s, 4H, [BArF ]À), 4.01 (br, 1H, NH), 3.13 (m, 1H,
4
4
CD2Cl2): d=32.1 (br, BH2), À6.60 ppm (s, [BArF ]À); ESI-MS (1,2-
4
CH2), 2.69 (m, 1H, CH2), 2.46 (s, 3H, NMe), 1.93-1.56 (m, 37H, PCy3
(33H) and CH2 (4H)), 1.39-1.21 (m, 33H, PCy3), 0.87 (m, 1H, CH2),
0.67 (br, 1H, CH2), À6.24 (br, 1H, BH2), À6.35 (br, 1H, BH2), À20.58
(br, 1H, IrH2), À20.82 ppm (br, 1H, IrH2); 31P{1H} NMR (202 MHz,
C6H4F2, 608C) positive ion: m/z 478.2417 [M+] (calcd 478.2408); el-
emental microanalysis: calcd [C52H60B2F24NP2Rh] (1341.49 gmolÀ1):
C 46.56, H 4.51, N 1.04; found: C 46.17, H 4.16, N 0.68.
2
2
[Rh(iPr2PCH2CH2CH2PiPr2)(h2h2-H2BNMe2C4H8)][BArF ] (11): To
C6H5F): d=36.47 (d, JPP =276 Hz, 1P), 33.22 ppm (d, JPP =276 Hz,
1P); 11B NMR (160 MHz, C6H5F): d=22.4 (br, bound BH2), À6.10 ppm
4
a high pressure NMR tube equipped with a J. Young’s valve and
charged with 3 (0.69m in 1,2-F2C6H4, 0.04 mL, 2.6”10À2 mmol, 2
equivalent) and V (18 mg, 1.3 X 10À2 mmol) was added 1,2-F2C6H4
(0.4 mL). The NMR tube was gently inverted at 258C and the reac-
tion was followed by periodic NMR spectroscopy. After 24 h the re-
action was complete and the resulting red solution was layered
with pentane and kept at 258C for 72 h to afford the product as
(s, [BArF ]À); ESI-MS (1,2-C6H4F2, 608C) positive ion: m/z 854.5533
4
[M+] (calcd 854.5649).
N-Methyl-1,2-azaborinene 8: This material has been reported by
Wille and Goubeau, who invoke the intermediacy of the amino-
borane 2 (which was not isolated in their study). Their procedure
has been adapted here. N-Methylhomoallylamine (6.63 g,
77.9 mmol, 1 equiv) was cooled to 08C in 30 mL ether. Borane-tet-
rahydrofuran solution (0.9m in THF, 103 mL, 93.4 mmol, 1.2 equiv)
was added slowly via cannula. After the addition was completed,
the ice bath was removed and the reaction was allowed to warm
for 20 min before the solvent was removed in vacuo. The clear, col-
ourless residue was rinsed with pentane (3” 50 mL) in a glovebox,
and the remaining insoluble residue was dissolved in benzene and
heated to reflux (908C) for 24 h. After cooling to room tempera-
ture, the benzene solvent was distilled off under N2, and the resi-
due was submitted to vacuum transfer to a liquid nitrogen-cooled
Schlenk flask. This procedure yielded 325 mg of a 1.25:1 molar
ratio mixture of product/benzene (2% yield assuming equal densi-
ty of benzene and product). 1H NMR (300 MHz, CD2Cl2): d=2.92–
2.80 (m, 5H, overlapping large singlet), 1.73–1.62 (m, 2H), 1.49–
1.37 (m, 2H), 0.84 (brs, 2H) B-H proton visible at 5.0–3.75 ppm (q,
1H); 11B NMR (96 MHz, CD2Cl2): d=40.62 ppm (d, J=125 Hz);
1H NMR (300 MHz, CD2Cl2): d=2.85 (m, overlapping with a singlet
5H), 1.66 (m, 2H), 1.44 (m, 2H), 0.84 ppm (m, 2H); 11B NMR
(97 MHz, CD2Cl2): d=40.0 ppm (d, J=125 Hz).
1
red crystals. Yield: 8 mg, 45%. H NMR (500 MHz, CD2Cl2): d=7.76
(s, 8H, [BArF ]À), 7.60 (s, 4H, [BArF ]À), 2.99 (br, 2H, CH2), 2.85 (s, 6H,
4
4
NMe2), 1.92 (m, 4H, CH), 1.79 (br, 2H, CH2), 1.65 (br, 2H, CH2), 1.50
(br, 2H, CH2), 1.23 (br, 24H, CH3), 0.5 (br, 6H, CH2CH2CH2),
À5.17 ppm (br, 2H, BH2); 31P{1H} NMR (202 MHz, CD2Cl2): d=
58.03 ppm (d, JRhP =163 Hz); 11B NMR (160 MHz, CD2Cl2): d=34.4
(br, BH2), À6.6 ppm (s, [BArF ]À); ESI-MS (1,2-C6H4F2, 608C) positive
4
ion: m/z 492.2548 [M+] (calcd 492.2565); elemental microanalysis:
calcd [C53H62B2F24NP2Rh] (1355.52 gmolÀ1): C 46.96, H 4.61, N 1.03;
found: C 46.65, H 4.10, N 0.62.
[{Rh(PiPr2(CH2)3PiPr2)}2(H)2(m-H)3][BArF ]
(12):
[Rh(PiPr2(CH2)3-
4
PiPr2)]2(m-H)2 (23 mg, 0.03 mmol) and [H(OEt2)2][BArF ] (30 mg,
4
0.03 mmol) were added to a Young’s flask. Addition of 1 mL of 1,2-
F2C6H4 led to the formation of a dark red solution which was im-
mediately hydrogenated at 4 atm as described in the general pro-
cedures. The resulting solution was stirred for 1 h to obtain a dark
orange solution. The solution was filtered into a crystallisation
tube, layered with pentane and kept at À188C for 24 h from which
dark red crystals were obtained in 70% yield (34 mg). This complex
decomposes in CH2Cl2 solution (12 h) to give unidentified products,
but shows greater stability in 1,2-F2C6H4 (no decomposition after
24 h). See the Supporting Information for a solid-state structure as
determined by single crystal X-ray diffraction. 1H NMR (400 MHz,
CD2Cl2): d=7.76 (s, 8H, [BArF ]À), 7.60 (s, 4H, [BArF ]À), 1.96 (m, 8H,
[Rh(PiPr2(CH2)3PiPr2)(h2h2-H2BNH2C4H8)][BArF ]
(9):
1,2-F2C6H4
4
(0.5 mL) was added to a high-pressure NMR tube equipped with
a J. Young’s valve and charged with compound 1 (1.2 mg, 1.4”
10À2 mmol, 1.2 equivalents) and V (16 mg, 1.2 X 10À2 mmol).
Gentle rotation of NMR tube for 30 min resulted in reddish orange
solution consisting of 9, amino-borane 7 and V as measured by
NMR spectroscopy. Compound 9 was characterized in situ by NMR
spectroscopy and ESI-MS. Diffusion of pentane at À358C gave mix-
ture of crystals corresponding to 9 (red) and V (orange). Red crys-
tals were mechanically separated from orange crystals for single
4
4
CH), 1.56–1.1 (m, 60H, CH3 (48H) and CH2 (12H)), À8.85 (br, 3H),
À18.87 ppm (br, 2H); 1H NMR (500 MHz, 1,2-C6H4F2): d=8.34 (s,
8H, [BArF ]À), 7.70 (s, 4H, [BArF ]À), 1.96 (m, 8H, CH), 1.55-1.11 (m,
4
4
60H, CH3 (48H) and CH2 (12H)), À8.25 (br, 3H), À18.90 ppm (br,
1
2H); H NMR (500 MHz, 1,2-C6H4F2, 250 K): d=8.34 (s, 8H, [BArF ]À),
4
1
crystal X-ray diffraction studies. H NMR (500 MHz, 1,2-C6H4F2): d=
1.93 (m, 8H, CH), 1.40–0.91 (m, 60H, CH3 (48H) and CH2 (12H)),
À7.91 (br, 2H), À9.52 (br, 1H), À18.71 ppm (br, 2H); 31P{1H} NMR
(162 MHz, CD2Cl2): d=65.71 (br), 62.68 ppm (br); 31P{1H} NMR
(202 MHz, 1,2-C6H4F2): d=66.49 (br), 62.22 (br).31P{1H} NMR
8.33 (s, 8H [BArF ]À), 7.69 (s, 4H, [BArF ]À), 4.54 (m, NH2), 3.28 (br,
4
4
CH2), 3.18 (br, CH2), 2.99 (br, CH2) 1.76 (m, CH), 1.23–1.16 (m, CH3),
1.06 (br, CH2CH2CH2), À4.83 ppm (br, BH2); 31P{1H} NMR (202 MHz,
1,2-C6H4F2): d=57.08 ppm (d, JRhP =160 Hz); 11B NMR (160 MHz, 1,2-
(202 MHz, 1,2-C6H4F2, 250 K): d=66.36 (br d,
JRhP =100 Hz),
C6H4F2): d=29.3 (br, BH2), À6.19 ppm (s, [BArF ]À); ESI-MS (1,2-
60.00 ppm (br d, JRhP =100 Hz); 11B NMR (128 MHz, CD2Cl2): d=
4
C6H4F2, 608C) positive ion: m/z 464.2284 [M+] (calcd 464.2248).
À6.63 ppm (s, [BArF ]À); 11B NMR (160 MHz, 1,2-C6H4F2): d=
4
Chem. Eur. J. 2016, 22, 310 – 322
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320
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
À6.19 ppm (s, [BArF ]À); elemental microanalysis: calcd
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133, 19326.
4
[C62H85BF24P4Rh2] (1626.34 gmolÀ1): C 45.77, H 5.27; found: C 45.33,
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H 4.84.
[Rh(iPr2P(CH2)3PiPr2)(NCMe)2][BArF ] (13): V (20 mg, 1.4 ” 10À2
4
mmol) was dissolved in 1,2-F2C6H4 (1 mL) in a Schlenk flask to
which CH3CN (16 mL, 0.28 mmol, 20 equivalents) was added. Addi-
tion of CH3CN immediately changed the colour of solution from
orange to pale yellow. Resulting solution was stirred for 10 min.
1,2-F2C6H4 and unreacted CH3CN were removed in vacuo to obtain
pale yellow solid of 13. Yield: 14 mg, 70%. 1H NMR (500 MHz,
CD2Cl2): d=7.76 (s, 8H, [BArF ]À), 7.61 (s, 4H, [BArF ]À), 2.25 (s, 6H,
NCCH3), 1.97 (br, 6H, CH2), 1.33 (m, 16H, CH3 (12H) and CH (4H)),
1.19 ppm (m, 12H, CH3); 31P{1H} NMR (202 MHz, CD2Cl2): d=
42.51 ppm (d, JRhP =167 Hz); ESI-MS (1,2-C6H4F2, 608C) positive ion:
m/z 461.1748 [M+] (calcd 461.1722); elemental microanalysis:
calcd [C51H52BF24N2P2Rh] (1324.62 gmolÀ1): C 46.24, H 3.96, N 2.11;
found: C 46.13, H 3.95, N 1.98.
4
4
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functional and the DFT-optimized DZVP2 basis set for all
atoms.[86] Vibrational frequencies were calculated to show that the
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[31] In this contribution we use the nomenclature h2h2 to describe the
bonding in two B–H···M 3c–2e interactions. However an alternative k2-
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Acknowledgements
The Rhodes Trust (A.K.) and the EPSRC (EP/M024210,
EP/J02127X) are thanked for support. Financial support was
also provided in part by the National Institute of Environmen-
tal Health Sciences, NIH (P30 ES00210); S.-Y.L. and D.A.D. thank
the US Department of Energy (DE-EE-0005658) for financial
support and the Camille Dreyfus Teacher–Scholar Awards Pro-
gram. D.A.D. thanks the Robert Ramsay Chair Fund of The Uni-
versity of Alabama for support. J.S.A.I. thanks the LaMattina
Family Graduate Fellowship in Chemical Synthesis for support
and Mr. Z. X. Giustra and Dr. P. G. Campbell for helpful discus-
sions regarding synthesis of cyclic amine-boranes.
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Keywords: amine-borane · catalysis · iridium · phosphine ·
rhodium
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Received: July 30, 2015
Published online on November 25, 2015
Chem. Eur. J. 2016, 22, 310 – 322
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