A tert-butyl-substituted amino-borane bound to an iridium fragment: A latent source of free H2BNtBuH

Article abstract of DOI:10.1016/j.jorganchem.2012.07.018

Release of the primary amino-borane H₂BN^tBuH from [Ir(PCy₃)₂(H)₂(η²-H ₂BN^tBuH)][BAr4F], formed from dehydrogenation of [Ir(PCy₃)₂(H)₂(η²-H ₃B·N^tBuH₂)][BAr4F], results in the eventual formation of the corresponding cyclic borazine [HBN^tBu] ₃. Observations point to the fact that off-metal hydrogen redistribution reactions play a major role in the final product formation, with H₃B·N^tBuH₂ being formed alongside [HBN^tBu]₃.

Full text of DOI:10.1016/j.jorganchem.2012.07.018

Journal of Organometallic Chemistry 721-722 (2012) 17e22
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
A tert-butyl-substituted amino-borane bound to an iridium fragment: A latent
source of free H2B]NtBuH
*
Heather C. Johnson, Andrew S. Weller
Department of Chemistry, Chemical Research Laboratories, Mansfield Road, Oxford OX1 3TA, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Release of the primary amino-borane H2B]NtBuH from [Ir(PCy3)2(H)2(h2-H2B]NtBuH)]½BArF ꢀ, formed
Article history:
4
from dehydrogenation of [Ir(PCy3)2(H)2(h2-H3B$NtBuH2)][BArF ꢀ, results in the eventual formation of the
corresponding cyclic borazine [HBNtBu]3. Observations point4to the fact that off-metal hydrogen redis-
tribution reactions play a major role in the final product formation, with H3B$NtBuH2 being formed
alongside [HBNtBu]3.
Received 24 May 2012
Received in revised form
3 July 2012
Accepted 6 July 2012
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Amine-borane
Catalysis
Mechanism
Amino-borane
Iridium
1. Introduction
stable products (such as those arising from the hydroboration of
cyclohexene [12,16]), polymer growth kinetics [7], or catalytic
The metal-mediated dehydrocoupling of amine-boranes poten-
tially provides kinetic control over both H2 loss and BeN bond
forming processes that allows for the development of catalysts for
chemical hydrogen storage applications [1e6] and the formation of
BeN polymeric materials that are isoelectronic with ubiquitous, and
technologically important, polyolefins [7,8]. The mechanism of
dehydrocoupling has attracted significant interest [6,9e12], as
manipulation of the various steps on the cycle potentially allows for
control of rates of hydrogen evolution or the BeN products formed
[1]. Much of this work has centered around the dehydrocoupling
reactions of H3B$NH3 and H3B$NMe2H which either give generally
ill-defined oligomeric/polymeric materials or dimeric [H2BNMe2]2
respectively. The primary amine-borane H3B$NMeH2 can also
undergo dehydrocoupling forming oligomeric and polymeric
materials [13e15], although trimethylborazine is the eventual
thermodynamic product of such reactions (Scheme 1) [7].
redistribution reactions [16]. We have recently reported a related
methodology for their study, in which the amino-borane H2B]
NMe2 is bound to a metal fragment, [Ir(PCy3)2(H)2(h2eH2B]
NMe2)]½BArF4ꢀ, which then acts as a latent source of free H2B]NMe2
when released by addition of MeCN to the metal centre
(Scheme 2) [17].
Central to the control of the dehydrocoupling of primary amine-
boranes such as H3B$NMeH2, with the goal of the delivery of
tailored polyaminoboranes, is the understanding of the underlying
chemistry of free H2B]NRH. In particular, questions still remain
over the subsequent reactivity of the amino-borane, once formed.
One scenario is the initial trimerisation to form a trimeric borazane,
as has been suggested [14], which then undergoes further dehy-
drogenation to form a borazine (Scheme 1). The study of such
processes would rely on a reliable latent source of H2B]NRH. Sabo-
Etienne and Alcaraz have reported H2B]NMeH bound to a neutral
Ru-fragment, accessed via dehydrogenation of the precursor
amine-borane [18], but no further chemistry was reported. For our
[Ir(PCy3)2(H)2]þ systems we have been unable access the equivalent
complex and dehydrogenation reactions result instead in metal-
mediated coupling to form a linear dimer, H3B$NMeHBH2$NMeH2,
although H2B]NMeH is implicated in the reaction. [19].
Central to the understanding of the mechanism of dehy-
drocoupling is the formation by initial dehydrogenation, and
subsequent reactivity, of amino-borane intermediates H2B]NRR0
(R ¼ e.g. H, Me; R0 ¼ H, Me). As such species are highly reactive,
their study often relies on their transient observation in reactions
by 11B NMR spectroscopy, or indirect observation by trapping of
We thus chose instead to study the primary amino-borane
H2B]NtBuH bound to the [Ir(PCy3)2(H)2]þ fragment. Free H2B]
NtBuH has not, to our knowledge, been isolated, and its spectro-
scopic data has only been briefly mentioned as an intermediate in
* Corresponding author. Tel.: þ44 1865 285151.
E-mail address: andrew.weller@chem.ox.ac.uk (A.S. Weller).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.07.018

18
H.C. Johnson, A.S. Weller / Journal of Organometallic Chemistry 721-722 (2012) 17e22
Scheme 1. General overview for the dehydrocoupling of primary amine-boranes.
Scheme 2. ½BArF4ꢀꢁ anions not shown.
the dehydrogenation of H3B$NtBuH2 [12,20]. The cyclic dimer
[H2BNHtBu]2 [21] can be heated to generate reactive H2B]NtBuH in
situ [22]; while the metal-mediated dehydrocoupling of
H3B$NtBuH2 has been described [23e25], and in some cases the
solid-state structures of the resulting amino-borane coordination
(ArCl ¼ 3,5-C6H3Cl2) [28]. The NMR data for 1 are similar to that
reported for [Ir(PCy3)2(H)2(h2-H3B$NMe2H)]½BArFꢀ [17] and show
4
that the amine-borane is relatively tightly bound to the Ir centre
through two 3 centree2 electron bonds. The 298 K 31P{1H} NMR
spectrum shows two broad environments at
d 39.7 and d 32.8.
complexes have been reported: [Rh(IMes)2(H)2(
h
2-H2B]
Cooling to 250 K resolves these into a mutually coupled pair of
sharp doublets, J(PP) 298 Hz. This large coupling constant is
consistent with a trans geometry, and thus chemical inequivalence
forced by a static amine-borane ligand that is not undergoing site
exchange between terminal and bridging BeH groups [29]. In the
1H NMR spectrum a diagnostic quadrupolar broadened integral 2H
NHtBu)]½BArF4ꢀ (IMes ¼ N,N0-bis(2,4,6-trimethylphenyl)imidazol-2-
ylidene)
and
(Cy-PSiP)RuH(
h
2-H2B]NtBuH)
(Cy-
PSiP ¼ (Cy2PC6H4)2SiMe) [26,27].
In this contribution, we report the synthesis of a latent source of
H2B]NtBuH bound to a metal centre, and its subsequent reaction
chemistry on release. In this we find that, when released by addi-
tion of excess MeCN, simple trimerisation of H2B]NtBuH to form
[H2BNtBuH]3 [25] is not occurring, and the ultimate product of the
reactions, the borazine [HBNtBu]3, potentially forms via a different
route that involves a hydrogen redistribution reaction [16].
signal is observed at
doublets [2H, J(PH) 16, 17 Hz] is observed at
The remaining unbound BeH group is too broad to be observed at
298 K, but at 250 K is clearly seen at 6.15. The NH2 group is
observed at
4.21 as a broad integral 2H signal. The 11B NMR
spectrum shows a broad peak assigned to the borane at 11.2.
d
ꢁ5.83 assigned to IreHeB, while a doublet of
d
ꢁ20.25 for the IreH.
d
d
d
2. Results and discussion
Restricted rotation of amine-boranes bound to Ir-centres has been
reported previously [17], and contrasts those of rhodium that, due
to a weaker RheHeB interaction, undergo site exchange between
bound and free BeH more readily [15,30].
Dehydrogenation of the amine-borane in 1 takes place slowly at
room temperature (72 h at 298 K) to give the amino-borane bound
complex [Ir(PCy3)2(H)2(h2-H2B]NtBuH)]½BArF4ꢀ, 2 (Scheme 3). This
process occurs with quantitative yield (by NMR spectroscopy). The
Addition of H3B$NtBuH2 to [Ir(PCy3)2(H)2(H2)2]½BArF4ꢀ [17]
generates the sigma amine-borane complex [Ir(PCy3)2(H)2(h2
-
H3B$NtBuH2)]½BArFꢀ 1 (Scheme 3). Complex 1 has been character-
ized by NMR spec4troscopy anꢁd electrospray ionization mass spec-
trometry (ESI-MS). The ½BArF4ꢀ salt is formed as an intractabꢁle oily
solid, but solid material can be isolated from the ½BArc4lꢀ salt
Scheme 3. ½BArF4ꢀꢁ anions not shown.

H.C. Johnson, A.S. Weller / Journal of Organometallic Chemistry 721-722 (2012) 17e22
19
small amount of H2 (dissolved) that would be formed was not
observed, but as the reaction is slow and its solubility in 1,2-C6H4F2
would be expected to be low, [31], this is not unreasonable. We
speculate this process operates via a constant oxidation-state
sigma-CAM type mechanism [32], as calculated for the dehydro-
genation of closely related H3B$NMe2H to give H2B]NMe2 when
bound to the same {Ir} fragment [17]. This process can be acceler-
ated by addition of the hydrogen acceptor tert-butylethene (tbe)
which acts as a sacrificial hydrogen acceptor. The corresponding
alkane, tert-butylethane, is formed. The NMR data for 2 are
consistent with its formulation and are similar to those reported for
other amino-borane complexes [17,18,26,30,33e35]. In particular
the 31P{1H} NMR spectrum now shows a single resonance at
while the 1H NMR spectrum shows the NH group at
4.44 as
a relative integral 1H signal. A broad signal at
ꢁ6.31 sharpens and
splits on 11B decoupling to reveal two integral 1H signals for the
now inequivalent IreHeB groups, while two broadened integral
1H hydride resonances are observed at
ꢁ13.94 and ꢁ14.97. The
11B NMR spectrum shows a very broad signal at
46.6, which is
d 33.1,
d
d
d
d
d
similar to that reported for related amino-borane complexes of Ir
[17,26,33].
With complex 2 in hand we next investigated the release of
H2B]NtBuH from the metal by addition of excess MeCN to form
[Ir(PCy3)2(H)2(NCMe)2][BArF4], 3, Scheme 4. Immediately after
addition, liberated H2B]NtBuH is observed in the 11B NMR spec-
trum as a triplet at
d 35.0 [J(HB) 126 Hz] [12,20]. NMR data for
Fig. 1. 11B NMR spectra (1,2-C6H4F2) showing the temporal evolution of species on
addition of excess MeCN to 2.
complex 3 are as reported previously [17]. This solution changes in
composition over time, as is shown in Fig. 1. Only three 11B-con-
taining species are observed: H2B]NtBuH, borazine [HBNtBu]3 and
the amine-borane H3B$NtBuH2. Fig. 2 shows how these evolve with
time. To our surprise H2B]NtBuH did not simply trimerise to give
the cyclotriborazane [H2BNtBuH]3 [21] [
d
ꢁ5.1 J(HB) 106 Hz, [25],]
as has been suggested to occur for H2B]NMeH [14], but instead the
corresponding borazine [HBNtBu]3 was formed [
25.7, J(HB)
127 Hz; lit. 27.2 J(HB) 125 Hz, d6-benzene [25]], alongside the
formation of an approximately equal proportion of H3B$NtBuH2
d
d
[
d
ꢁ21.9, J(HB) 95 Hz, identical to a pure sample in the same
solvent] as measured by 11B concentration, at a similar rate to that
of [HBNtBu]3 (Fig. 2) [36]. After ca. 14 h the concentration of
H3B$NtBuH2 starts to drop while that of [HBNtBu]3 increases. The
overall mass balance remains the same throughout. The cyclo-
triborazane [H2BNtBuH]3 was not observed to the detection limit of
11B NMR spectroscopy (w1%). We also do not observe the formation
of the diaminoborane, HB(NtBuH)2, or the aminodiborane,
B2H5(NHtBu) [20] which have been shown to arise from metal-
mediated BeN bond cleavage in H3B$NtBuH2 [12,23,24]. After
36 h [HBNtBu]3 is the only boron-containing product in solution
(Fig. 2).
Fig. 2. Time-concentration graph for the evolution of H2B]NtBuH, generated by
addition of excess MeCN to 2. -, H2B]NtBuH; :, H3B$NtBuH2; C, [HBNtBu]3. After
36 h [HBNtBu]3 is the only boron-containing product in solution.
In the control experiment complex 3 slowly dehydrogenates
H3B$NtBuH2 to give H2B]NtBuH and [HBNtBu]3 as observed by 11
B
increase the overall concentration of [HBNtBu]3. Due to this parallel
process we cannot definitively rule out the possibility of an equi-
librium being established between H3B$NtBuH2 and [HBNtBu]3 to
form H2B]NtBuH. However, given the thermodynamic stability of
NMR spectroscopy (25% conversion to the borazine at 14 h),
consistent with the overall time/concentration profile described
above. This underlying process thus consumes the formed
H3B$NtBuH2 to produce background levels of H2B]NtBuH and
Scheme 4. ½BArF4ꢀꢁ anions not shown.

20
H.C. Johnson, A.S. Weller / Journal of Organometallic Chemistry 721-722 (2012) 17e22
borazines [7] such a process is unlikely, in contrast to the hydrogen
redistribution reactions observed between amine-boranes and
amino-boranes (vide infra) [16].
Extrapolation of these results might suggest that in our system
the bulkier cyclotriborazane [H2BNtBuH]3 is unlikely to be an
intermediate in the formation of [HBNtBu]3, albeit this inference
obtained using the proxy system of [H2BNMeH]3. Alternatively if
[H2BNtBuH]3 is formed then it must undergo rapid hydrogen
redistribution (with the concomitant formation of H3B$NtBuH2) to
form borazine. We also do not observe linear dimer species such as
(unreported) H3B$NBuHBH2$NBuH2. Such oligomeric species have
been suggested to form in the dehydrocoupling chemistry of the
parent amine-borane H3B$NH3, to ultimately give borazine
[12,40e42], where autocatalysis by H2B]NH2 is also suggested to
occur [43]. They have also been shown to undergo redistribution
reactions with amino-boranes [16]. In our system, if they are being
formed then they must also have a short lifetime. A detailed study
of the possible involvement of [H2BNtBuH]3 and H3BNtBuHBH2Nt-
BuH2 will have to wait until practicable synthetic routes are re-
ported for these materials. Notwithstanding this the formation of
significant amounts of amine-borane indicates hydrogen redistri-
bution reactions (from whatever intermediate) are occurring,
further demonstrating this pathway has a role in the overall
mechanism of dehydrocoupling [16,43].
The formation of significant amounts of H3B$NtBuH2 suggests
that a hydrogen redistribution reaction has occurred. Similar
chemistry has recently been reported by Manners et al. in as much
that amino-boranes such as H2B]NiPr2 can act as hydrogen
acceptors with amine-boranes H3B$NRR0H (R ¼ Me, R0 ¼ Me, H)
[16], resulting in hydrogen redistribution products (Scheme 5).
Intrigued that similar chemistry was happening in our system, as
evidenced by the formation of H3B$NtBuH2, we investigated the
likely route of formation of H3B$NtBuH2. At a first approximation it
might be expected that H2B]NtBuH would trimerise to give the
cyclic cyclotriborazane [H2BNtBuH]3 [25]. This could then poten-
tially react with more H2B]NtBuH by a hydrogen redistribution
reaction (exemplified in Scheme 5) to form [HBNtBu]3 and
H3B$NtBuH2. If initial trimerisation was slow compared with
hydrogen redistribution this might account for the observed
behaviour. To probe this, ideally the cyclotriborazane [H2BNtBuH]3
would be reacted with H2B]NtBuH. Although the crystal structure
of this material has recently been reported [25], it is only produced
in low yields from the Al(NMe2)3 catalyzed dehydrogenation of
H3B$NtBuH2 or by pyrolysis of H3B$NtBuH2 [21]. As an alternative
[37] we instead used the sterically less encumbered methyl-
substituted cyclotriborazane [H2BNMeH]3, which is readily
prepared [38]. Addition of MeCN to 2 to release H2B]NtBuH in the
presence of one equivalent (per boron) of [H2BNMeH]3 resulted in
the formation of 3, [HBNtBu]3 and H3B$NtBuH2, as before. However
the [H2BNMeH]3 remains untouched (Scheme 6, Eq. (1)). Experi-
3. Conclusions
We have demonstrated for the first time that release of
a primary amino-borane from a metal centre results in the eventual
formation of the corresponding cyclic borazine. Intriguingly for the
H2B]NtBuH systems described here observations indicate that off-
metal hydrogen redistribution reactions play a major role in the
final product formation. We can observe this behaviour in our
particular system as the generation of H3B$NtBuH2 from such
a process is not masked by the fact that it is not also a starting
substrate and that the metal centre (as the MeCN adduct) only
dehydrogenates it slowly once formed. However it is likely that
such processes are not system specific, and could occur whatever
the metal fragment and starting amine- or amino-boranes. The
dehydrocoupling of amine-boranes sits in a remarkably complex
and nuanced landscape. Initial dehydrogenation to form amino-
boranes is just the first step in a complex set of interconnecting
reactions that involve both on-metal and off-metal processes. Off-
metal hydrogen redistribution reactions add yet further
complexity. These results further support previous observations
[12,16,17,41] that whether the amino-borane is bound or unbound
to the metal can dictate the course of the reaction, and that the
observation of borazine suggests free amino-borane is generated.
Such insight will be important in delivering systems that can
ments using [Ir(PCy3)2(H)2(h2-H2B]NMe2)]½BArF ꢀ as
a latent
source of H2B]NMe2 also resulted in no c4onsumption of
[H2BNMeH]3 and the formation only of the cyclic dimer
[H2BNMe2]2 (via free H2B]NMe2) as reported previously (Scheme
6, Eq. (2)) [17]. The control reaction of 3 with [H2BNMeH]3 resulted
in no reaction, and this lack of dehydrogenation is similar to that
reported for other systems [39] but contrasts observations for
colloidal Rh-catalysts [14] or [Ir(PCy3)(H2)2(H)2]½BArF4ꢀ [19] that do
dehydrogenate [H2BNMeH]3 to give [HBNMe]3.
Scheme 5. Manners’ hydrogen redistribution reaction.
Scheme 6. Complex 3 is generated by addition of MeCN to the appropriate amino-borane precursor.

H.C. Johnson, A.S. Weller / Journal of Organometallic Chemistry 721-722 (2012) 17e22
21
dehydrocouple amine-boranes to give desirable products (e.g.
polyaminoboranes) while avoiding the production of unwanted
borazines.
4.2. [Ir(H)2(PCy3)2(h2-H2B]NtBuH)]½BArF ꢀ (2)
4
[Ir(H)2(PCy3)2(H3B$NtBuH2)]½BAr4Fꢀ (50 mg [Ir(H)PCy2(
h
2-C6H9)
PCy2(h3-C6H8)]½BArF ꢀ, 0.03 mmol) was formed in situ in 1,2-C6H4F2
in a Young’s flask.4The solution was degassed by freeze-pump-
thawing twice to remove any residual H2. 3,3-dimethylbut-1-ene
4. Experimental
All manipulations, unless otherwise stated, were performed
under an argon atmosphere using standard Schlenk and glove-box
techniques. Glassware was oven dried at 130 ꢂC overnight and
flamed under vacuum prior to use. Pentane, toluene and MeCN
were dried using a Grubbs type solvent purification system
(MBraun SPS-800) and degassed by successive freeze-pump-thaw
cycles [44]. 1,2-C6H4F2 and C6H5F were dried over CaH2, vacuum
distilled and stored over 3 Å molecular sieves. 3,3-dimethylbut-1-
ene was dried over Na, vacuum distilled and stored over 3 Å
molecular sieves. H3B$NtBuH2 was purchased from Aldrich and
(15 mL, 0.115 mmol) was added to the solution using a gastight
syringe. The solution was stirred for an hour, during which the
colour changed from colourless to yellow. The solvent was removed
in vacuo to yield a ‘sticky’ yellow solid. Pentane (5 mL) was added
with sonication for 10 minutes and then decanted. The resulting
solid was a cream powder. The solid was washed once with pentane
(5 mL) and dried in vacuo. Yield: 35 mg (66%). Despite repeated
attempts, material suitable for single crystal X-ray diffraction could
not be isolated.
1H NMR (500 MHz, 1,2-C6H4F2):
d
8.33 (br, 8H, ½BArF4ꢀꢁ), 7.69 (br,
sublimed prior to use (5 ꢃ 10ꢁ2 mbar, 298 K). [Ir(H)PCy2(
h
2-C6H9)
4H, ½BArF4ꢀꢁ), 4.44 (br, 1H, NH), 1.50 (s, 9H, tBu), 2.20e1.10 (m, 66H,
Cy), ꢁ6.31 (br, 2H, BH2), ꢁ13.94 (br, 1H, IrH), ꢁ14.97 (br, 1H, IrH).
PCy2(h3-C6H8)]½BArF ꢀ
[17],
[Ir(H)PCy2(h -
2-C6H9)PCy2(h3
4
C6H8)]½BArc4lꢀ [19], H3B$NMeHBH2$NMeH2 [19], [H2BNMeH]3 [38]
and [Ir(H)2(PCy3)2(H2B]NMe2)]½BArF4ꢀ [17] were prepared by
literature methods. NMR spectra were recorded on a Unity Plus
500 MHz spectrometer at room temperature, unless otherwise
stated. In 1,2-C6H4F2, 1H NMR spectra were referenced to the centre
1H {11B} NMR (500 MHz, 1,2-C6H4F2):
d
8.33 (br, 8H, ½BArF4ꢀꢁ), 7.69
(br, 4H, ½BArF4ꢀꢁ), 4.44 (br, 1H, NH), 1.50 (s, 9H, Bu), 2.20e1.10 (m,
66H, Cy), ꢁ6.26 (br, 1H, BH2), ꢁ6.36 (br, 1H, BH2), ꢁ13.94 (br, 1H,
IrH), ꢁ14.97 (br, 1H, IrH).
t
31P{1H} NMR (202 MHz, 1,2-C6H4F2):
d 33.1 (s).
of the downfield solvent multiplet,
d
¼ 7.07. 31P and 11B NMR
11B NMR (160 MHz, 1,2-C6H4F2):
d
46.6 (v. br), ꢁ6.1 (s, ½BArF4ꢀꢁ).
spectra were referenced against 85% H3PO4 (external) and BF3$OEt2
ESI-MS (1,2-C6H4F2, 60 ꢂC, 4.5 kV): m/z 840.5401 [M]þ (calc.
(external) respectively. The spectrometer was pre-locked to CD2Cl2.
840.5492).
Elemental
Chemical shifts (d) are quoted in ppm and coupling constants (J) in
microanalysis:
Calc.
C72H92B2F24IrNP2
Hz. ESI-MS were recorded on a Bruker MicrOTOF instrument
interfaced with a glove-box [45]. Microanalyses were performed at
London Metropolitan University.
(1703.26 gmolꢁ1): C, 50.77; H, 5.44; N, 0.82. Found: C, 50.72; H,
5.42; N, 0.76.
4.3. General procedure for H2B]NtBuH release
4.1. [Ir(H)2(PCy3)2(h2-H3B$NtBuH2)]½BArclꢀ (1)
4
2 (8 mg, 0.005 mmol) was added to a high pressure NMR tube
and dissolved in 0.4 mL 1,2-C6H4F2. MeCN (8 mL, 0.153 mmol) was
added via a gastight syringe and the sample immediately frozen in
liquid N2 before monitoring with 11B NMR spectroscopy. Immedi-
ately, liberated H2B]NtBuH was observed with no 2 present. 1H
and 31P{1H} NMR spectroscopy indicated full conversion to 3.
[Ir(H)2(H2)2(PCy3)2½BArc4lꢀ was formed in situ by the hydroge-
nation of [Ir(H)PCy2(
h
2-C6H9)PCy2(h3-C6H8)½BArc4lꢀ (40 mg,
0.03 mmol) at 4 atm in 1,2-C6H4F2. After 30 min, the colourless
solution was opened under argon and rapidly transferred to
a Schlenk containing solid H3B$NtBuH2 (2.6 mg, 0.03 mmol) and
stirred for 15 min. Pentane (30 mL) was added, and the mixture
turned cloudy and was cooled to ꢁ18 ꢂC for 6 days, after which
a cream-coloured solid had formed. The solid was washed with
pentane (2 ꢃ 5 mL) and dried in vacuo. Yield: 13 mg (30%). The
preparation of 1 with the ½BArF4ꢀꢁ anion waꢁs conducted in situ and
gave similar NMR spectra to the ½BArc4lꢀ salt, although solid
material could not be isolated. Despite repeated attempts, material
suitable for single crystal X-ray diffraction could not beꢁisolated.
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1H NMR (500 MHz, 1,2-C6H4F2):
d
7.57 (br, 8H, ½BArc4lꢀ ), 4.17 (br,
2H, NH2), 1.60 (s, 9H, NtBu), 2.24e1.19 (m, 66H, Cy), ꢁ5.83 (br, 2H,
2
2
s
-bound BH2), ꢁ20.25 (overlapping dd, JHP w16, JHP w17, 2H,
IrH2). The remaining BH signal is not observed, presumably as too
broad. Other ½BArc4lꢀꢁ peak is obscured by the solvent.
1H NMR (500 MHz, 1,2-C6H4F2, 250 K):
d
7.61 (br, 8H, ½BArc4lꢀꢁ),
6.15 (br, 1H, BH not s
-bound), 4.22 (br, 2H, NH2), 1.59 (s, 9H, NtBu),
2.24e1.19 (m, 66H, Cy), ꢁ5.94 (br, 2H,
s
-bound BH2), ꢁ20.07 (m,
2H, IrH2). Other ½BArc4lꢀꢁ peak is obscured by the solvent.
31P{1H} NMR (202 MHz, 1,2-C6H4F2):
d 39.7 (br d, 1P), 32.8 (br d,
1P).
31P{1H} NMR (202 MHz, 1,2-C6H4F2, 250 K):
1P), 32.7 (d, 2JPP ¼ 283, 1P).
d
39.4 (d, 2JPP ¼ 283,
11B NMR (160 MHz, 1,2-C6H4F2):
d
11.2 (br, BH3), ꢁ6.5 (s, ½BArF ꢀꢁ).
ESI-MS (C6H5F, 60 ꢂC, 4.5 kV): m/z 842.5512 [M]þ (4calc.
842.5648).
Elemental
microanalysis:
Calc.
C64H94B2Cl8IrNP2
(1436.85 gmolꢁ1): C, 53.50; H, 6.59; N, 0.97. Found: C, 53.28; H,
6.50; N, 1.02.
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19322e19325.

22
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