Catalytic Dehydrocoupling of Amine-Boranes using Cationic Zirconium(IV)-Phosphine Frustrated Lewis Pairs

Article abstract of DOI:10.1021/acscatal.6b02211

A series of novel, intramolecular Zr(IV)/P frustrated Lewis pairs (FLPs) based on cationic zirconocene fragments with a variety of ancillary cyclopentadienyl and 2-phosphinoaryloxide (-O(C₆H₄)PR₂, R = ^tBu and 3,5-CF₃-(C₆H₃)) ligands are reported and their activity as catalysts for the dehydrocoupling of dimethylamine-borane (Me₂NH·BH₃) assessed. The FLP system [(C₉H₇)₂ZrO(C₆H₄)P^tBu₂][B(C₆F₅)₄] is shown to give unprecedented turnover frequencies (TOF) for a catalyst based on a group 4 metal (TOF ≥ 600 h^-1), while also proving to be the most efficient FLP catalyst reported to date. The mechanism of this reaction has been probed using analogous intermolecular Zr(IV)/P FLPs, permitting deconvolution of the reactions taking place at both the Lewis acidic and basic sites. Elucidation of this mechanism revealed an interesting cooperative two-cycle process where one cycle is FLP mediated and the other, a redistribution of a linear diborazane intermediate, relies solely on the presence of a Zr(IV) Lewis acid.

Full text of DOI:10.1021/acscatal.6b02211

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Article
Catalytic Dehydrocoupling of Amine Boranes using
Cationic Zirconium(IV)-Phosphine Frustrated Lewis Pairs
Owen J. Metters, Stephanie R. Flynn, Christiana Dowds, Hazel A. Sparkes, Ian Manners, and Duncan F Wass
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02211 • Publication Date (Web): 09 Aug 2016
Downloaded from http://pubs.acs.org on August 15, 2016
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ACS Catalysis
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Catalytic Dehydrocoupling of Amine Boranes using Cationic Zirconi-
um(IV)-Phosphine Frustrated Lewis Pairs
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Owen J. Metters, Stephanie R. Flynn, Christiana K. Dowds, Hazel A. Sparkes, Ian Manners* and Dunꢀ
can F. Wass*
School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS
ABSTRACT: A series of novel, intramolecular Zr(IV)/P frustrated Lewis pairs (FLPs) based on cationic zirconocene fragments
t
with a variety of ancillary cyclopentadienyl and 2ꢀphosphinoaryloxide (–O(C H )PR , R = Bu and 3,5ꢀCF ꢀ(C H )) ligands are
6
4
2
3
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3
reported and their activity as catalysts for the dehydrocoupling of dimethylamineꢀborane (Me NH•BH ) assessed. FLP system
2
3
t
[
(C H ) ZrO(C H )P Bu ][B(C F ) ] is shown to give unprecedented turnover frequencies (TOF) for a catalyst based on a group 4
9 7 2 6 4 2 6 5 4
ꢀ1
metal (TOF = > 600 h ), whilst also proving to be the most efficient FLP catalyst reported to date. The mechanism of this reaction
has been probed using analogous intermolecular Zr(IV)/P FLPs, permitting deconvolution of the reactions taking place at both the
Lewis acidic and basic sites. Elucidation of this mechanism revealed an interesting cooperative twoꢀcycle process where one cycle
is FLP mediated, the other, a redistribution of a linear diborazane intermediate, relies solely on the presence of a Zr(IV) Lewis acid.
Keywords: Frustrated Lewis Pairs, FLP, Amine-borane, Dehydrocoupling, Zirconocenes
7
1
.
Introduction
FLPs have also been shown to mediate these dehydrogenation
reactions. Initially the focus was on metalꢀfree FLP systems
which were able to dehydrocouple dimethylamine borane
Catalytic dehydrogenation and dehydrocoupling of amine boꢀ
ranes is of broad current interest due to their potential applicaꢀ
8
1
(Me NH•BH ) stoichiometrically (Scheme 1). More recently
2
3
tions as hydrogen storage materials, reagents for hydrogen
2
however, there have been reports of FLPs based on main
group elements which are able to mediate this transformation
in a catalytic fashion. In 2013 Uhl, Slootweg et al. reported an
intramolecular Al/P FLP capable of dehydrogenating
Me NH•BH under melt conditions (45 °C, 9.3 mol %), comꢀ
transfer to organic or inorganic substrates and as precursors to
3
BNꢀbased ceramics and polymeric materials. There exists a
wide range of transition metalꢀbased catalysts which facilitate
4
these transformations, however work has also been carried
2
3
out exploring the use of catalysts based on main group eleꢀ
ments. As a consequence dehydrogenation methodologies
which employ catalysts based on elements from group 2 (Mg,
plete consumption of the monomer is achieved by heating to
0 °C for 45 mins, however, only a 71% yield of the desired
9
9
5
product was obtained. Aldridge et al. have developed a P/B
FLP, based on a dimethylxanthene backbone, which is able to
dehydrogenate a wider range of amineꢀborane substrates
Ca) and group 3 (Al, Ga, Sc, Y) are now known. Furthermore
simple Brønsted acid/base and Lewis acid catalysts can be
used to promote hydrogen release from ammoniaꢀborane
6
(RR’NH•BH , R = R’ = H, R = Me and R’ = H, R = R’ = Me)
3
(H N•BH ).
3
3
however still requires elevated temperatures and long reaction
10
times (1 mol%, CH Cl or THF, 55 °C, 24ꢀ48 h) (Scheme 2).
2
2
Scheme 1. Main-group FLPs capable of mediating the stoi-
chiometric dehydrogenation of Me NH•BH
Scheme 2. P/B FLP developed by Aldridge et al. capable of
mediating the catalytic dehydrocoupling of a range of
amine-boranes
2
3
In recent years solution phase combinations of sterically enꢀ
cumbered Lewis acids and Lewis bases frustrated Lewis pairs,
1
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Alongside these breakthroughs, we have developed a range of
Page 2 of 14
less to yellow) is then observed, which is complete within 1 h,
resulting in quantitative conversion to a new species as eviꢀ
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zirconium(IV) based FLPs with the aim of combining the fasꢀ
cinating small molecule activation chemistry of FLPs with the
well established catalytic chemistry of the transition metals.
3
1
denced by the P NMR spectra (2 δ = 55.9 ppm, 3 δ = 58.1
ppm, 4 δ = 57.6 ppm). The chemical shift of these resonancꢀ
P
P
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P
These transition metalꢀbased FLP systems (1) have been
shown to rapidly dehydrocouple several amineꢀborane subꢀ
strates under ambient conditions to yield the expected products
es, when compared to that of 1, suggest the presence of a Zr–P
interaction in all cases (for comparison the free ligand δ = ꢀ
5.7 ppm). Attempts to isolate 2ꢀ4 by layering PhCl solutions of
the species with hexane were unsuccessful and yielded intracꢀ
table oils. The characterisation of 2ꢀ4 was therefore carried out
P
1
1b
(Scheme 3).
1
13
31
in situ ( H, C and P NMR spectroscopy and ESIꢀMS).
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Scheme 4. Synthesis of Intramolecular Zr/P FLP systems
2-4 with a variety of ancillary ligands.
This methodology had to be adapted when using the more
electron deficient phosphine moiety, as protonolysis by [H–
DTBP][B(C F ) ] was found not to yield the desired product,
Scheme 3. Catalytic dehydrocoupling of amine-boranes
using Zr/P FLP 1.
6
5 4
Herein we report a series of novel intramolecular Zr–P FLP
systems featuring variations to the ancilliary ligands bound to
Zr and also incorporating a weakly Lewis basic phosphine
which was attributed to the less basic nature of the phosphine.
Upon addition of [H–DTBP][B(C ], protonation of the
pendant phosphine moiety did not occur. This is not unexꢀ
F )
6 5 4
+
(
RP(3,5ꢀCF (C H )) ). These were subsequently applied to the
pected as the pK
known to be 1.97 (in H
under the same conditions is 4.95 (in H
quence, [CPh ][B(C ] was used to mediate the methyl abꢀ
straction reaction and generate the cationic species 5 (Scheme
5).
of the related compound [(pꢀFPh) P–H] is
a 3
13 +
3
6
3
2
catalytic dehydrocoupling of Me NH•BH . Crucial insights
into the mechanism of this reaction was gained by using a
range of previously reported intermolecular Zr(IV)/P FLPs of
O) however the pKa of [DTBP–H]
2
14
2
3
O). As a conseꢀ
2
F )
6 5 4
3
R
2
the type [Cp ZrOMes][B(C F ) ] // PR’ (R = Me/H, R’ =
6
5 4
3
t
12
Bu, Cy, Et, Ph, Mes, C F ). Such systems permitted deconꢀ
6
5
volution of the mechanism owing to the ability to separate
Lewis acid and Lewis base mediated reactions.
2. Results and Discussion
2.1 Synthesis of Novel Intramolecular Zr-P FLP Sys-
tems
The synthetic approach employed is analogous to that previꢀ
11
ously used by us to access intramolecular FLP system 1. This
involved synthesis of the relevant dimethylzirconocene preꢀ
cursor (R ZrMe ) followed by protonolysis with the correꢀ
2
2
t
sponding alcohol (HO(C H )P Bu ). Subsequently the catalytiꢀ
6
4
2
cally active cationic species was generated by reaction with
H–DTBP][B(C F ) ] (DTBP = 2,6ꢀdiꢀtertbutylpyridine) with
[
6
5 4
Scheme 5. Synthesis of Intramolecular Zr/P FLP system 5
with an electron deficient phosphine.
concomitant release of 1 equiv. methane (Scheme 4).
31
The generation of cationic species 2ꢀ4 can be monitored by P
3
1
NMR spectroscopy. Upon addition of [H–DTBP][B(C F ) ] to
Interestingly, when this reaction was monitored by P NMR
6
5 4
31
the neutral precursors, protonation of the pendant phosphine
moiety is observed. This is manifested as a new resonance in
spectroscopy, no change in the P NMR spectrum was obꢀ
served upon formation of 5. This strongly suggests the absence
of a Zr–P interaction. Formation of 5 is however clearly eviꢀ
denced by the H NMR spectrum where loss of the resonance
corresponding to the Zr–Me (δ = ꢀ0.07 in PhCl) and the apꢀ
3
1
the P NMR spectra (δ = 20ꢀ25 ppm in all cases) displaying a
P
1
characteristic P–H splitting pattern (doublet, JPH ca. 400 Hz).
Effervescence (CH ) and a concomitant color change (colorꢀ
4
H
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t
pearance of a new resonance corresponding to triphenylethane
(MeCPh , δ = 2.09). Again, isolation of the cationic species
was attempted by precipitation into hexanes. However, this
only resulted in the generation of intractable solutions and 5
was therefore used in situ.
compound 2 down to the single Bu group, offered in comꢀ
pound 3, leads to a decrease in TOF. This decrease continues
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H
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with removal of the bulky Bu moiety in 1 and finally the lowꢀ
est TOF is observed with the least sterically hindered system 4
(TOF 2 > 3 > 1 > 4). However, this is also mirrored by the
electronic properties of these species. Indenyl ligands (present
in 2) are known to be significantly more electron donating
2
.2 Dehydrocoupling of Me NH•BH Using 2-5
16
2
3
than Cp ligands in 1, with Cp (1) and (Me Si)Cp (4) thought
2
2
Catalyst systems 2ꢀ5 (Figure 1) were trialled in the catalytic
to be similar. Due to this it is difficult to discern whether this
change in rate is electronic or steric in nature
dehydrocoupling of Me NH•BH . Initially a 5 mol% catalyst
2
3
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1
loading was employed and the reaction monitored by B{ H}
NMR spectroscopy. The results are shown in Table 1, where
the previously reported catalyst 1 is shown for comparison.
Further examination of the product distribution and intermediꢀ
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ates present during the reactions by B{ H} NMR spectroscoꢀ
py revealed not only the expected dehydrocoupling product,
cyclodiborazane [Me NꢀBH ] , but also the linear diborazane
2
2 2
Me NHꢀBH ꢀMe NꢀBH (I) and aminoborane Me N=BH (II)
2
2
2
3
2
2
intermediates (Figure 2). The amounts of each intermediate (I
and II) varies with the ancillary ligand employed. Increasing
the steric bulk in the order 4 < 1 < 3 < 2 leads to less I being
observed, indicating that the predominant mechanism in these
cases involves preferential formation of II. Due to the rapid
nature of these reactions, however, the exact ratios of these
intermediates could not be calculated.
Figure 1. FLP systems 2-5 trialled in the dehydrocoupling of
Me NH•BH
2
3
From these data it can be seen that catalyst 5, possessing the
electron withdrawing phosphine, shows no activity even after
heating to 80 °C for 7 days. This suggests that the phosphine
moiety is required to possess a certain degree of basicity in
order to mediate the dehydrocoupling. This strongly implies
that NH deprotonation is a key step in the catalytic cycle. This
is in good agreement with the behavior noted for the main
group systems and also corroborates the mechanism previousꢀ
1
1b
ly proposed by our group.
Table 1. Catalytic dehydrocoupling of Me NH•BH using
2
3
FLP systems 2-5 with the previously reported catalyst 1
included for comparison. All reactions were conducted in
chlorobenzene in sealed NMR tubes
11
1
Figure 2. B{ H} NMR spectra after ca. 10 minutes (1 mol%
Zr], 25 °C, PhCl), for the catalytic dehydrocoupling of
Me NH•BH3 with (from back to front) 4, 2, 3 and 1. * =
[
2
Me NH•BH , * = H BꢀNMe ꢀBH ꢀNHMe , ◊ = [Me NꢀBH ] .
[
Zr]
Temperature
(°C)
Time
(mins)
TOF
(h )
2
3
3
2
2
2
2
2 2
Catalyst
-1
Note that the terminal BH peak for * overlaps with *. Minor
(
mol%)
3
amounts of HB(NMe ) (~29 ppm), Me N=BH (36.6 ppm) and
2
2
2
2
1
2
3
4
5
2
5
5
5
5
25
25
25
25
25
14
1
210
>600
282
138
0
Me N(B H ) (~ꢀ17 ppm) are also observed.
2
2
5
The presence of both I and II in such dehydrocoupling reacꢀ
tions is unusual. There are thought to be three different mechꢀ
4
9
anisms by which Me NH•BH is converted to the cyclic diboꢀ
2 3
razane product. One possible mechanism is an ‘onꢀmetal’ proꢀ
>60
17
cess where linear diborazane I, the sole intermediate, is genꢀ
erated from a metalꢀmediated intermolecular dehydrocoupling
of two molecules of Me NH•BH . In a further metalꢀmediated
Further to this it is observed that 2 is a highly efficient catalyst
2
3
for the dehydrocoupling of Me NH•BH and, in fact, possesses
step, dehydrogenative cyclisation could occur to yield the cyꢀ
clic diborazane product. In an alternative ‘offꢀmetal’ mechaꢀ
nism only one of the steps is thought to be metal mediated. In
2
3
the highest TOF (Turnover Frequency) of any group 4 catalyst
ꢀ
1
(
(
> 600 h ), the previous highest being a Zrꢀamide species
Dipp
[NSiN] Zr(NMe ) (µꢀCl)Li(THF) ) reported by Rivard et
this step one molecule of Me NH•BH is dehydrogenated to
2 3
18
2 2
15
3
ꢀ
1
al. (TOF = 420 h ). Decreasing the steric bulk present in
produce the aminoborane II. The aminoborane then spontaꢀ
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Page 4 of 14
neously dimerises to form the cyclic diborazane, [Me Nꢀ
This leads us to propose that two different reaction mechaꢀ
2
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BH ] . The third possible mechanism, and one which may be
nisms occurring simultaneously. The first cycle involves simꢀ
2
2
2
useful in this discussion, has been proposed by Schneider et al,
calculations showing that rearrangement between
Me NH•BH , [Me NꢀBH ] and II is approximately therꢀ
ple deprotonation of Zrꢀκ amineꢀborane adduct by a suffiꢀ
ciently basic phosphine to form II, a phosphonium species
2
3
2
2 2
[R PꢀH] and a ZrꢀH. An intermediate zirconium amidoꢀborane
3
ꢀ
1
moneutral (2.0 kcal mol ) and may take place if “kinetically
feasible”.
species is not observed in our experiments, suggesting a conꢀ
certed pathway with simultaneous deprotonation and hydride
abstraction in line with other FLPꢀtype reactions. Subsequent
19
In the previous literature it is the offꢀmetal mechanism which
is the favoured model for FLP catalysed dehydrocoupling of
release of H from the phosphonium species and the ZrꢀH renꢀ
2
8,11b
ders the process catalytic. This is similar to the mechanism
Me NH•BH .
However, with the recent report by Aldridge
2
3
11b
proposed in our previous work. The second proposed proꢀ
cess involves insertion of a second equivalent of Me NH•BH
et al. strongly supporting the viability of a linear chain growth
on an FLP catalyst, it appears that the previously proposed
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to yield the linear diborazane (I) prior to a subsequent cyclisaꢀ
tion step. The exact nature of this alternative ‘onꢀmetal’ pathꢀ
way, however, remains unclear and the roles of the Lewis acid
and Lewis base in each step was judged to require further inꢀ
vestigation. As discussed above, the formation of I may also
be reversible and this could prove to be the origin of II in the
reaction. To this end we utilised a series of our recently reꢀ
ported analogous intermolecular Zr/P FLP systems to probe
mechanisms may require some modification. In their work
Aldridge et al. isolated several key intermediates in the P/B
FLP catalysed dehydrocoupling of amineꢀboranes suggesting
that insertion of 1 equiv. of amineꢀborane into an FLPꢀbound
amineꢀborane results in the formation of linear oligomeric
species akin to those observed with our systems (Scheme 6).
12
this reaction.
2
.3 Dehydrocoupling of Me NH•BH Using Intermolecu-
2
3
lar FLPs
Initially it was necessary to determine if the intermolecular
analogues 7ꢀ19 (Scheme 7) retained their catalytic activity
after removal of the aryl tether. It was found that treatment of
Me NH•BH with 10 mol% [Cp* ZrOMes][B(C F ) ] // PR
2
3
2
6
5 4
3
t
(
R = Bu (7), Cy (8), Et (9), Ph (10), Mes (11), C F (12)) in
Scheme 6. Growth of a linear dimeric species on a P/B FLP
synthesized by Aldridge et al.
6 5
PhCl (25 °C) led to a sluggish reaction resulting in < 5% conꢀ
version to [Me NꢀBH ] over 24 h in all cases, as calculated by
2
2 2
11
In order to confirm the identity of I as a catalytically relevant
intermediate, an authentic sample was treated with 5 mol% 1
B NMR spectroscopy, and when R = Ph, Mes and C F no
6 5
conversion was observed. Changing the ancillary ligands on
Zr from Cp* (pentamethylcyclopentadienyl) to Cp (14ꢀ19) led
to a marked improvement in the reactivity, as shown in Figure
4. This is consistent with previous observations, where intraꢀ
molecular Zr/P FLPs, bearing Cp ancillary ligands, give sigꢀ
nificantly more rapid reactivity when compared to their Cp*
1
1
in PhCl at 25 °C. Monitoring the reaction by B NMR specꢀ
troscopy revealed a product distribution similar to that obꢀ
served for the reaction of Me NH•BH with 1. Species I, II
2
3
1
1
1
and Me NH•BH were all identified in the B{ H} NMR
2
3
spectrum shown in Figure 3. Full conversion to the cyclic diꢀ
borazane was evident after 20 mins.
11
cousins. Ligands with intermediate steric bulk, specifically in
complexes 2 and 3, lead to more subtle effects, with complex
2
in particular giving a highly active catalyst despite having
more bulky indenyl ligands. In contrast to Cp*, the indenyl
ligands (and indeed the ligand in complex 3) may orientate in
such a way as to still not hinder substrate binding. Clearly the
different electronic characteristics of these ligand may then
come into play in yielding a more active catalyst. The possiꢀ
5
3
bility for more facile η to η ring slippage for indenyl during
the catalytic cycle mat also play a role.
11
1
Figure 3. B{ H} NMR spectrum (300 MHz, PhCl, 25 °C, 2
mins) of (I) + 5 mol% 1. ꢀ MeNH(B H ) (ꢀ18.8 ppm), ꢁ
2
5
ꢀ
[
B(C F ) ] (ꢀ17.5 ppm), ꢂ Me NH•BH (ꢀ14.4 ppm), ꢀ Me NHꢀ
6 5 4 2 3 2
BH ꢀMe NꢀBH (ꢀ14.4 and 0.84 ppm), ꢁ [Me NꢀBH ] (4.02
Scheme 7. Generation of previously reported intermolecu-
lar FLP systems.
2
2
3
2
2 2
ppm), ꢂ Me N=BH (36.6 ppm)
2
2
4
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FLP systems 15-19 (10 mol%, PhCl, 25 °C, 14 h) gave low
conversion (< 5%) to [Me NꢀBH ] even after 14 h, with 17,
In order to further probe the mechanism of the reaction,
Me NH•BH was treated with 10 mol%
[Cp ZrOMes][B(C F ) ] (6) and [Cp ZrOMes][B(C F ) ] (13)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
2 2
2
3
*
18 and 19 showing no conversion over the same period. The
lack of conversion using 19 bearing an electron withdrawing
phosphine is consistent with the result observed with 5. Deꢀ
spite this, 14 shows 97% conversion to [Me NꢀBH ] in 7.5 h.
2
6
5 4
2
6 5 4
in the absence of a Lewis base. In neither case was dehydroꢀ
11
coupling was observed by B NMR spectroscopy (24 h, 25
o
C, PhCl), however a new Zrꢀamine borane complex was idenꢀ
2
2 2
11
1
Interestingly, Figure 4 shows an induction period this is atꢀ
tributed to the formation of I prior to its consumption to genꢀ
erate [Me NꢀBH ] . In operando NMR spectroscopy (see
tified ( B{ H} δ = ꢀ11.5 (broad singlet, Me NH•BH ), ꢀ16.9
(s, [B(C F ) ] )). In the case of 6, this complex (20) was isolatꢀ
2
3
ꢀ
6
5 4
2
2 2
ed through
a stoichiometric reaction between 5 and
later) gives no evidence for any gross changes to the catalyst
structure during this initiation period; there is also no evidence
for the formation of heterogeneous or colloidal species.
Me NH•BH , the solid state structure of 20 is shown in Figure
2
3
6. From the solid state structure of 20 it is clear that
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
19
1
Me NH•BH is bound in a κ fashion, however, the H NMR
2
3
spectrum shows a broad resonance for three equivalent hyꢀ
drides, suggesting a dynamic structure in solution, where the
hydrides bound to Zr are exchanging on the NMR timescale.
Structurally characterised transition metal complexes of amine
boranes bearing NꢀH moieties are rare as the isolation of such
compounds is usually hindered by subsequent dehydrocouꢀ
19e,19g
pling reactivity.
Subsequent reaction of 20 with phosphine Lewis bases (PhCl,
5 °C, < 10 mins) led to dehydrogenation, release of
Me N=BH (II) and formation of the corresponding phosphoꢀ
2
2
2
nium salt, [HPR ][B(C F ) ] and a Zrꢀhydride. Aminoborane
3
6 5 4
II subsequently dimerised to form the cyclic diborazane. Proꢀ
tonation of the Zrꢀhydride by the phosphonium species to reꢀ
lease H is sluggish (PhCl, 25 °C, < 6 h) in the case of the Zr
2
species bearing the Cp* ligands. This is consistent with the
slow catalytic turnover achieved with catalyst systems 7-12.
o
Figure 4. Reaction of Me NH•BH with 10 mol% 14-19 (25 C,
2
3
PhCl, 14 h). ꢃ = 14, ꢁ = 15, ꢀ = 16, 17, 18 and 19 show no reacꢀ
tion with Me NH•BH
Varying the phosphine is seen to have a dramatic effect on this
2
3
t
transformation. More basic phosphines (PR R = Bu, Cy, Et)
3
2.4 Mechanistic Investigation
show the deprotonation/dehydrogenation reactivity described
above. Analogous treatment of 20 with PPh , PMes or
Monitoring the reaction between Me NH•BH and a catalytic
3
3
2
3
1
1
1
P(C F ) showed no reaction after 6 h. This is consistent with
amount of 14 (10 mol%, PhCl, 25 °C, 7.5 h) by B{ H} NMR
spectroscopy led to a similar distribution of reaction intermeꢀ
diates as that observed for the reactions with catalysts 1-4
6 5 3
the fact that 14-16 are catalysts for the dehydrocoupling reacꢀ
tion, whereas 17-19 show little to no conversion. This dehyꢀ
drogenation of a Zrꢀbound amine borane could be one mechaꢀ
(
Figure 1), with both Me NHꢀBH ꢀMe NꢀBH (I) and
2 2 2 3
nism for the dehydrocoupling of Me NH•BH , however, this
Me N=BH (II) generated simultaneously (Figure 5 and Figure
2
3
2
2
process does not account for the observation of the linear diboꢀ
razane (I). Nevertheless, these findings do indicate the necesꢀ
sity for both the Lewis acidic and Lewis basic fragments in the
initial dehydrogenation of Me NH•BH , yielding either
S1).
2
3
Me N=BH (II) or Me NHꢀBH ꢀMe NꢀBH (I). Crucially, in
2
2
2
2
2
3
support of this hypothesis, addition of further equivalents of
Me NH•BH to 20 leads to the formation of no new products
2
3
on a catalytically relevant timescale (8 h, 25 °C)
11
Figure 5. B{H} NMR spectrum (300 MHz, PhCl, 25 °C, 280
mins) of Me NH•BH + 10 mol% 14. ꢀ MeNH(B H ) (ꢀ18.8
2
3
2
5
ꢀ
ppm), ꢁ [B(C F ) ] (ꢀ17.5 ppm), ꢂ Me NH•BH (ꢀ14.4 ppm), ꢀ
6
5 4
2
3
Me NHꢀBH ꢀMe NꢀBH (I) (ꢀ14.4 and 0.84 ppm), ꢁ [Me Nꢀ
2
2
2
3
2
BH ] (4.02 ppm), ꢂ Me N=BH (II) (36.6 ppm)
2
2
2
2
5
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ACS Catalysis
Page 6 of 14
o
Figure 6. Solid state structure of the cation present in 20 as deterꢀ
Figure 8. (I) with 20 mol% 13 (25 C, PhCl, 12 h). ꢂ = Me NHꢀ
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
mined by Xꢀray crystallography. Second unique cation, nonessenꢀ
BH ꢀMe NꢀBH (I), ꢁ = [Me NꢀBH ] , ꢃ = Me NH•BH , ꢀ =
2
2
3
2
2 2
2
3
ꢀ
tial hydrogens and two [B(C F ) ] counterions are omitted for
Me N=BH (II), ꢂ = Me N(B H )
6
5 4
2 2 2 2 5
clarity. Selected bond lengths (Å) and angles (º): Zr1ꢀO1 1.968(3),
N1ꢀB1 1.587(6), Cp*ꢀZrꢀCp* 135.14(7). NB. ZrꢀB distance =
The proposed mechanism for the Zr(IV)/P FLP catalysed deꢀ
hydrocoupling of Me NH•BH is shown in Scheme 8. Cycle A
2
3
2
.709(5) Å
is analogous to the previously reported mechanism for this
11b
To probe the intermediacy of linear diborazane (I) a chloroꢀ
transformation, wherein the FLP mediates an intramolecular
benzene solution of was treated with 20 mol%
I
loss of H from Me NH•BH forming the aminoborane II. The
2
2
3
t
[Cp ZrOMes][B(C F ) ] // P Bu (14) (PhCl, 25 °C, 6 h). In
2
6
5 4
3
aminoborane spontaneously dimerises to form the cyclic diboꢀ
razane product, [Me NꢀBH ] . Alternatively, in the presence of
this case complete conversion to [Me NꢀBH ] was observed
2
2 2
2
2 2
in 6 h (Figure 7). It is evident from Figure 2 that, upon conꢀ
sumption of I, both II and the parent amineꢀborane,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a second equivalent of Me NH•BH , an intermolecular dehyꢀ
2 3
drocoupling event could occur to yield the linear diborazane I.
This then feeds into Cycle B. Cycle B is the phosphine indeꢀ
pendent redistribution of I, and involves initial complexation
of I to the Zr centre in an analogous fashion to that observed
Me NH•BH are formed. Redistribution of such linear diboraꢀ
2
3
zanes to form amineꢀborane dehydrocoupling products has
2
0
been previously reported by our group.
for 20. Loss of the terminal Me NH group, as previously reꢀ
2
20
ported in other redistribution reactions, then occurs, resulting
in a µꢀamido diborane species, Me NH(B H ). As shown in
2
2
5
Figures 7 and 8, Me NH(B H ) is observed in solution
2
2
5
throughout these reactions and is thus not thought to be bound
to the Zr and, in fact, addition of Me NH(B H ) to 6 or 13
2
2
5
results in no reaction (vide infra). The fate of Me NH(B H )
2
2
5
remains uncertain. We believe it is likely that the formation of
Me NH•BH and II could be via an alternative, concerted proꢀ
2
3
cess from I as indicated in Scheme 8 (Cycle B). When formed,
Me N=BH would spontaneously dimerise to form [Me Nꢀ
2
2
2
BH ] . The formation of this dimer is thought to be the driving
2
2
t
force for this step. In the presence of P Bu , Me NH•BH
3
2
3
could be dehydrocoupled to reform the linear diborazane I.
However, as mentioned above, it is thought that such a transꢀ
formation cannot occur in the absence of phosphine.
o
Figure 7. (I) with 20 mol% 14 (25 C, PhCl, 6h). ꢂ = Me NHꢀ
2
BH ꢀMe NꢀBH (I), ꢁ = [Me NꢀBH ] , ꢃ = Me NH•BH , ꢀ =
2
2
3
2
2 2
2
3
This mechanism also provides some insight into the cause of
the differing TOF depending on steric bulk of the ancillary
ligands in the intramolecular systems (1-4). The increase in
steric bulk is thought to preclude the formation of linear diboꢀ
Me N=BH (II), ꢂ = Me N(B H )
2
2
2
2
5
In order to determine if this second step in the cycle also reꢀ
quires both Lewis acidic and Lewis basic fragments, the Lewis
t
base (P Bu ) was removed. Reaction of (I) with 20 mol% 13
razane (II) as two equivalents of Me NH•BH are not able to
2 3
3
(PhCl, 25 °C, 14 h) resulted in a redistribution of linear diboꢀ
organise around the sterically congested catalytic site. It is
therefore thought that in the case of the most sterically bulky
system, 2, cycle A is far more dominant and decreasing steric
bulk allows cycle B to become more viable.
razane intermediate (I) to yield Me NH•BH and Me N=BH
2
3
2
2
(II), which subsequently cyclodimerized, as expected, to form
the cyclic dimer, [Me NꢀBH ] . We observe that Me NH•BH
2
2 2
2
3
is not consumed in the absence of exogenous phosphine (no
reaction is observed between complex 13 alone and
Me NH•BH )
2
3
6
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1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
i
o
Figure 9. Pr NH•BH with 10 mol% 14 (25 C, PhCl, 14h). ꢃ =
2
3
i
i
Pr NH•BH , ꢃ = Pr N=BH
2
3
2
2
As in the case of Me NH•BH , reaction of 6 and 13 with
Pr NH•BH resulted in no detectable conversion to dehydroꢀ
coupling products, but formation of an amineꢀborane complex
was again observed. In the case of the system bearing the Cp*
ligand set, this complex (21) has been isolated and crystalloꢀ
graphically characterised (Figure 10).
2
3
i
2
3
Scheme 8. Proposed reaction mechanism for the catalytic
dehydrocoupling of Me NH•BH using a Zr(IV)/P FLP
2
3
2.5 Model System for Proposed Cycle A
The intermolecular nature of FLP 14 also allowed us to further
probe both cycles A and B. Additional insight into the validity
of cycle A can be gained through a systematic study of the
i
dehydrogenation of Pr NH•BH by 14. Due to the increased
2
3
i
steric bulk around nitrogen, dehydrogenation of Pr NH•BH
2
3
only yields one product, the corresponding aminoborane
i
17f
( Pr N=BH ), through an intramolecular loss of H . This
2
2
2
substrate therefore provides an ideal model for cycle A where
we propose this intramolecular H elimination to be a catalytiꢀ
2
cally viable pathway. Intramolecular FLP 1 has been shown to
i
dehydrogenate Pr NH•BH (1 mol% [Zr], PhCl, 25 °C, 19 h)
2
3
Figure 10. Solid state structure of the cation present in 21 as deꢀ
termined by Xꢀray crystallography. Nonessential hydrogens, solꢀ
vent of crystallisation (PhCl) and [B(C F ) ] counterion are omitꢀ
ted for clarity. Selected bond lengths (Å) and angles (º): Zr1ꢀO1
1.974(1), N1ꢀB1 1.590(3), Cp*ꢀZrꢀCp* 135.07(3). NB ZrꢀB disꢀ
tance = 2.722(3) Å
i
to yield Pr N=BH , however revisiting this reaction seemed
pertinent in light of the current study. Upon treatment of a
PhCl solution of Pr NH•BH with 10 mol% 14 (25 °C, 14 h) a
3% conversion to Pr N=BH was observed with no other
2
2
1
1b
ꢀ
6
5 4
i
2
3
i
7
2 2
intermediates apparent (Figure 9).
The solid state structure of 21, as was to be expected, proves
to be nearly identical to that of 20, with the amineꢀborane
again bound in a κ fashion. In a similar manner to 20 treatꢀ
ment of 21 with a stoichiometric amount of P Bu results in
deprotonation of the bound amineꢀborane and formation of
Pr N=BH . These results, when combined with the data in
Figure 9, confirm the validity of a mechanistic pathway inꢀ
volving the intramolecular loss of H from an amineꢀborane
mediated by a Zr/P FLP.
2
19
t
3
i
2
2
2
7
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ACS Catalysis
Page 8 of 14
2
.6. Model System for Proposed Cycle B
MeNH=BH
to form [MeNHꢀBH ] , which is further dehydroꢀ
2 2 3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
genated through now wellꢀestablished hydrogen transfer reacꢀ
Gaining more detailed insight into cycle B has proven to be
more of a challenge, as our initial attempts to isolate the proꢀ
posed intermediates have been unsuccessful. Attempts to synꢀ
thesise a Zrꢀbound linear diborazane have been precluded by
the redistribution chemistry described above. Efforts to block
this reactivity were also made by capping the linear diboraꢀ
2d,20
tions.
3.
Summary
A range of intramolecular Zr(IV)/P FLP catalysts have been
prepared that are competent in the dehydrocoupling of
Me NH•BH Moreover, FLP system 2, exhibited the highest
zane with other Lewis bases (DMAP, PMe – See SI for furꢀ
3
2
3
ther discussion) but this strategy also proved unsuccessful. In
addition, attempts to isolate a Zrꢀꢁꢀamidodiborane complex
were made, but addition of Me N(B H ) to 6 or 13 resulted in
the formation of no new products, as observed by B NMR
spectroscopy. Two possible conclusions may be drawn from
this. Firstly, that such complexes are transient in solution and
therefore isolation is impossible, or secondly, that the converꢀ
sion of Me N(B H ) to the observed products is in fact not
TOF yet reported for a catalyst based on a group (IV) transiꢀ
tion metal. Studies of intermolecular FLP analogues allowed
elucidation of a novel reaction mechanism comprising two
cooperative cycles which provides a new concept for FLPꢀ
catalysed reactions. The first cycle involves a twoꢀstep process
involving amineꢀborane coordination and subsequent phosꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
5
1
1
phineꢀmediated H loss. The second cycle is based on Lewis
2
2
2
5
acidꢀmediated redistribution of a linear diborazane intermediꢀ
ate. The concept that the Lewis acidic and Lewis basic fragꢀ
ments can mediate transformations independently, in addition
to acting as an FLP, may have wideꢀreaching consequences for
other FLP catalysed reactions. Further studies are underway to
widen the substrate scope for these reactions with, in the case
of group 15ꢀ15 adducts, the formation of polymeric materials
as a particular target.
metalꢀmediated. The latter, however, appears unlikely as it is
known that, in the absence of a catalyst, but in the presence of
Me NH, Me N(B H ) readily undergoes a ringꢀopening reacꢀ
2
2
2
5
20
tion to yield the corresponding linear diborazane I.
Experiments using alternative linear diborazanes have providꢀ
ed insight into the decomposition pathway of I. Upon reaction
of the linear diborazane H BꢀNMeHꢀBH ꢀNMe H with differꢀ
ent substituents at nitrogen with 10 mol% 13 (PhCl, 25 °C,
4 h) formation of Me NH•BH was observed by B NMR
spectroscopy (Figure 11). This was accompanied by formation
of trace amounts of Nꢀmethylborazine, [HBꢀNMe] , and ꢁꢀNꢀ
methylamidodiborane, MeNH(B H ).
3
2
2
20
11
1
2 3
4. Experimental
3
4
.1 General Considerations
2
5
Unless otherwise stated, all manipulations were undertaken
under an atmosphere of argon or nitrogen using standard
glovebox (MꢀBraun O < 0.1 ppm, H O < 0.1 ppm) and
2
2
Schlenk line techniques and all glassware was oven and vacuꢀ
um dried prior to use. Cp ZrCl , Cp* ZrCl MeLi (1.6M in
2
2
2
2
t
Et O), P Bu , PCy PEt , PPh , PMes and P(C F ) were purꢀ
2
3
3,
3
3
3
6 5 3
chased from SigmaꢀAldrich and used as received.
CPh ][B(C F ) ] was purchased from Acros Organics and
[
3
6 5 4
used as received. Me NH•BH was purchased from sigma
2
3
o
ꢀ
Aldrich and purified by sublimation prior to use (25 C, 2 x 10
2
i
Torr).
Pr NH•BH ,
Me NHꢀBH ꢀMe NꢀBH ,
2
3
2
2
2
3
[Cp* ZrOMes][B(C F ) ] (5), [Cp ZrOMes][B(C F ) ] (6),
2
6
5 4
2
6 5 4
t
[Cp ZrO(C H )P Bu ][B(C F ) ] (1) and orthoꢀtBu P(C H )OH
2
6
4
2
6
5 4
2
6
4
1
1,20,21
were synthesized according to literature protocols.
other reagents were used as obtained unless otherwise stated.
Common laboratory solvents (Et O, DCM, Hexane, THF)
were purified using a Grubbs type purification system. Nonꢀ
standard solvents (Chlorobenzene, Pentane) were purchased
All
2
2
2
Figure 11. Reaction of Me NHꢀBH ꢀMeNHꢀBH with 10 mol%
13 (25 C, PhCl, 14h). ꢂ = Me NHꢀBH ꢀMeNHꢀBH , ꢃ =
2
2
3
o
2
2
3
from SigmaꢀAldrich and distilled from CaH prior to use.
2
Me NH•BH , ꢂ = MeNH(B H ), ꢁ [HBꢀNMe]
2
3
2
5
3
NMR spectra were recorded using JEOL ECPꢀ300 (300 MHz),
Varianꢀ400 (400 MHz), Varian NMRS500 (500 MHz) specꢀ
trometers. Deuterated solvents were obtained from Sigma Alꢀ
drich (d ꢀbenzene, d ꢀTHF and d ꢀDCM) or Apollo Scientific
The origin of these intermediates provides useful information
about Cycle B in Scheme 8. MeNH(B H ) appears to arise
from elimination of the terminal Me NH moiety from Me NHꢀ
2
5
2
2
6
8
2
BH ꢀNMeHꢀBH , with the formation of Me NH•BH providꢀ
2
3
2
3
(
d ꢀPhBr) and distilled from CaH prior to use. Spectra of air
5
2
ing further evidence for the presence of both free amine and
free BH3 in solution. The lack of detectable amounts of
Me N=BH or [Me NꢀBH ] indicates that any aminoborane
sensitive compounds were recorded using NMR tubes fitted
with JꢀYoung valves. NMR spectra of boron containing comꢀ
pounds were obtained in quartz NMR tubes fitted with Jꢀ
Young valves.
2
2
2
2 2
that is generated arises from the internal BN unit of the linear
diborazane. In this case formation of [HBꢀNMe] provides
further evidence for the presence of MeNH=BH as direct
observation of this aminoborane by B NMR spectroscopy
under ambient conditions would prove impossible. The boraꢀ
zine [HBꢀNMe] is thought to arise through trimerisation of
3
Xꢀray diffraction experiments were carried out on a Bruker
APEX II diffractometer using Mo Kα radiation (λ = 0.71073
Å). For further details see the supporting information.
2
1
1
2
1
3
8
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ACS Catalysis
Mass spectrometry experiments were carried out by the Uniꢀ
further gas evolution was observed. The solvent was removed
in vacuo yielding the desired complexes.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
versity of Bristol Mass Spectroscopy Service on a Bruker Dalꢀ
tronics micrO TOF II with a TOF analyser. All samples were
run in preꢀdried PhCl
t
t
(
BuC H ) Zr(Me)(OC H P( Bu) ) Viscous oil (702 mg, 95%)
5 4 2 6 4 2
1
4
.2 Synthesis of Intramolecular Zr/P FLPs (2-4)
H NMR (400 MHz, C D ): δ 7.61 (dt, 1H, J = 7.6, 1.8 Hz,
6
6
4
.2.1 Synthesis of R ZrMe precursors
H6), 7.12ꢀ7.16 (m, 1H, H3), 6.77 (t, 1H, J = 7.6 Hz, H4), 6.55
2
2
23
(dd, 1H, J = 5.1, 2.6 Hz, H5), 6.11ꢀ6.15 (m, 2H, Cp), 5.90ꢀ5.94
Ind ZrMe Following a modified literature procedure. Meꢀ
2
2
3
(
m, 2H, Cp), 5.87ꢀ5.89 (m, 4H, Cp), 1.25 (d, 18H, J = 11.3
HP
thyl lithium (1.6 M in Et O, 32.3 mL, 51.6 mmol) was added
2
Hz, PC(CH ) ), 1.19 (s,18H, CpC(CH ) ), 0.75 (s, 3H, ZrCH );
3
3
3 3
3
dropwise to a solution of indene (3.00 mL, 25.8 mmol) in Et O
2
13
1
2
C{ H} NMR (125 MHz, C D ): δ 169.8 (d, J = 23.7 Hz,
6
6
CP
(35 mL) at room temperature to give a yellow/orange solution.
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
C1), 138.8 (s, ipsoꢀCp(tBu)), 136.0 (d, J = 3.2 Hz, C6),
CP
1 4
This solution was stirred for 30 minutes, before addition of
130.1 (s, C3), 125.2 (d, J = 25.3 Hz, C2), 120.1 (d, J
=
CP
CP
ZrCl (3.00 g, 12.9 mmol) slurried in hexane (40 mL). The
4
3.4 Hz, C5), 118.1 (s, C4), 110.8, 110.7, 109.8, 107.2 (Cp),
resulting mixture was stirred for 2 hours, during which a white
precipitate (LiCl) formed. The solvent was removed in vacuo,
and the residue dissolved in hot hexane (50 mL) and filtered
through celite. Removal of the solvent in vacuo gave the deꢀ
sired product as a white powder (3.54 g, 78%) that was recrysꢀ
tallized from hexane at ꢀ40 °C. All recorded data is consistent
1
2
3
2.3 (d, J = 24.7 Hz, PC(CH ) ), 30.9 (d, J = 16.3 Hz,
CP 3 3 CP
PC(CH ) ), 29.3 (s, CpC(CH ) ), 26.3 (d, J = 6.6 Hz,
3 3
3 3
CP
3
1
1
ZrCH ), 22.9 (s, CpC(CH ) ) P{ H} NMR (161 MHz,
3
3 3
C D ): δ 10.17 (s).
6
6
2
4
t
with literature values.
Ind Zr(Me)(OC H P( Bu) ) White solid (567 mg, 95%)
2
6
4
2
1
1
H NMR (400 MHz, C D ): δ 7.36 (dd, 4H, J = 2.9, 6.7 Hz,
6
6
H NMR (400 MHz, toluene-d ): δ 7.57 (dt, 1H, J = 7.7, 1.8
8
H ), 7.06 (dd, 4H, J = 3.1, 6.7 Hz, H ), 5.95 (d, 4H, J = 2.9
4,7
6,5
Hz, H6), 7.28 (dq, 2H, J = 8.4, 1.0 Hz, H ), 7.21 (dq, 2H, J =
4,7
Hz, H ), 5.79 (t, 2H, J = 3.1 Hz, H ), ꢀ0.68 (s, 6H, Zr(CH ) ).
1
,3
2
3 2
8.4, 1.0 Hz, H ), 7.10 (ddd, 1H, J = 8.1, 7.1, 1.7 Hz, H3),
4,7
6.87 (ddd, 2H, J = 8.4, 6.6, 1.2 Hz, H ), 6.80 (ddd, 2H, J =
6,5
25
8.4, 6.6, 1.2 Hz, H ), 6.76 (dt, 1H, J = 7.4, 1.3 Hz, H4), 6.33
6,5
Me Si(C H ) ZrMe Adapted from a literature procedure.
2
5
4 2
2
(ddd, 1H, J = 8.1, 5.0, 1.3 Hz, H5), 6.06 (ddd, 2H, J = 3.2, 2.1,
0.9 Hz, H ), 5.96 (t, 2H, J = 3.3 Hz, H ), 5.73 (ddd, 2H, J =
Me Si(C H ) ZrCl (244 mg, 0.70 mmol) was suspended in
2
5
4 2
2
1
,3
2
hexane (20 mL) and cooled to ꢀ78 °C. Methyl lithium (1.6 M
3
3
ꢀ
.2, 2.1, 0.9 Hz, H ), 1.20 (d, 18H, J = 11.4 Hz, C(CH3)3),
1,3 H,P
13 1
in Et O, 0.83 mL, 1.33 mmol) was added dropwise and the
2
0.1 (s, 3H, ZrCH3) C{ H} NMR (125 MHz, toluene-d ): δ
8
reaction warmed to room temperature and stirred for 2 hours.
Solvent was removed in vacuo and the residue redissolved in
hexane. The resulting solution was filtered through celite, the
volume reduced to ~5 mL and cooled to ꢀ20 °C which resulted
in the precipitation of white crystals of the title compound
1
68.6 (d, J = 23.9 Hz, C1), 135.6 (d, J = 7.1, C6), 129.8 (s,
C3), 125.4 (d, J = 22.7 Hz, C2), 125.2 (s, C3a,7a), 125.0 (s,
C3a,7a), 124.4 (s, C ), 124.3 (m, C ), 124.0 (s, C ), 120.29
5,6
4,7
5,6
(d, J = 3.3 Hz, C5), 118.6 (s, C4), 117.6 (d, J = 1.9 Hz, C2),
1
01.5 (d, J = 1.2 Hz, C ), 98.9 (d, J = 1.3 Hz, C ), 32.8 (d,
1,3
1,3
(
172 mg, 80%). All recorded data consistent with literature
1
26
JCP = 7.5 Hz, ZrCH ), 32.4 (d, J = 24.9 Hz, C(CH ) ), 31.0
3 CP 3 3
2 31 1
values.
(d, J = 15.7 Hz, C(CH3)3). P{ H} NMR (161 MHz, tolu-
1
CP
H NMR (400 MHz, toluene-d ): δ 6.88 (t, 4H, J = 2.2 Hz,
8
ene-d ): δ 10.23 (s) E. A. – Calc (%): C 69.07, H 6.85. Found
8
Cp), 5.76 (t, 4H, J = 2.2 Hz, Cp), 0.52 (s, 6H, Si(CH ) ), ꢀ0.35
3 2
(%): C 68.92, H 6.93
(
s, 6H, Zr(CH3)2).
t
Me Si(C H ) Zr(Me)(OC H P( Bu) ) Viscous colourless oil
t
2
5
4 2
6
4
2
(
Bu-C H ) ZrMe Methyl lithium (1.6 M in hexanes, 0.54
1
5
4 2
2
(
227 mg, 98%) H NMR (400 MHz, PhCl-d ): δ 7.76 (dt, 1H,
5
mL, 0.86 mmol) was added dropwise to a stirred solution of
J = 7.5, 1.5 Hz, H6), 7.32 (dt, 1H, J = 7.1, 1.2 Hz, H3), 6.96
the zirconocene dichloride (165 mg, 0.41 mmol) in Et O (15
2
(dt, 1H, J = 7.7, 1.6 Hz, H4), 6.80ꢀ6.82 (m, 2H, Cp), 6.77ꢀ6.80
mL) at ꢀ78 °C. After addition, the reaction was warmed to
room temperature and stirred overnight. Solvent was removed
in vacuo and the resulting residue was extracted with hexanes
and filtered through a celite plug. Solvent was removed in
vacuo yielding a white solid (272 mg, 87%) of the title comꢀ
(
1
0
m, 1H, H5), 6.44ꢀ6.46 (m, 4H, Cp), 5.70ꢀ5.73 (m, 2H, Cp),
3
.36 (d, 18H, J = 10.2 Hz, C(CH ) ), 0.73 (s, 3H, SiCH ),
HP
3 3
3
13
1
.60 (s, 3H, SiCH ), 0.52 (s, 3H, ZrCH ) C{ H} NMR (100
3
3
MHz, PhCl-d ): δ 169.9 (d, J = 22.1 Hz, C1), 135.5 (d, J = 3.5
5
Hz, C6), 130.3 (d, J = 0.5 Hz, C3), 124.2 (d, J = 22.7 Hz, C2),
122.0 (s, ipsoꢀCpSi), 119.7 (d, J = 2.2 Hz, C5), 119.5 (d, J =
2
6
pound. All recorded data is consistent with literature values.
1
H NMR (400 MHz, CDCl ): δ 5.80ꢀ5.83 (m, 4H, Cp), 5.70ꢀ
2.7 Hz, C4), 118.4 (s, Cp), 113.6 (s, Cp), 110.6 (s, Cp), 110.5
3
1
5
.75 (m, 4H, Cp), 1.10 (s, 18H, CpC(CH ) ), 0.01 (s, 6H,
(s, Cp), 107.5 (s, Cp), 32.2 (d, J = 24.0 Hz, C(CH ) ), 31.8
3 3
CP
3 3
2
Zr(CH3)2).
(s, ZrCH ), 30.8 (d, J = 15.9 Hz, C(CH ) ), ꢀ4.96, ꢀ5.67 (s,
3 CP 3 3
31 1
Si(CH ) ). P{ H} NMR (161 MHz, PhCl-d ): δ 9.96 (s).
3
2
5
+
ESI-MS: 529.1621 [MꢀH]
4.2.2 Synthesis of neutral complexes [R Zr(Me)(O^P(tBu) )]
2
2
General method: A solution of the dimethyl zirconocene (1
equiv.) and phosphinoalcohol (1 equiv.) were individually
dissolved in the minimum amount of hexane prior to combinꢀ
ing. The resulting solutions were stirred overnight and until no
4.2.3
Synthesis
of
cationic
complexes
t
[R Zr(O^P( Bu) )][B(C F ) ] (2-5)
2
2
6 5 4
Data for the [B(C F ) ] anion reported separately:
6
5 4
9
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ACS Catalysis
Page 10 of 14
1
1
1
19
2
B{ H} NMR (96 MHz, CD Cl ): δ ꢀ17.60 (s). F NMR (376 Cp), 117.3 (d, J = 6.7 Hz, C3), 116.0 (br s, Cp), 115.4 (s,
2
2
CP
1
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
MHz, DCM-d ): δ ꢀ133.17 (s), ꢀ163.70 (s), ꢀ167.71 (s).
ipso-CpSi), 37.6 (d, J = 6.0 Hz, PC(CH ) ), 30.0 (d, J
=
CP
2
CP
3 3
1
3
1
C{ H} NMR (125 MHz, DCM-d ): δ 148.26 (d, J = 245.3
4.6 Hz, PC(CH ) ), ꢀ5.2 (br s, SiCH ), ꢀ7.3 (br s, SiCH ).
3 3 3 3
31 1
2
Hz, oꢀB(C F ) ), 136.68 (d, J = 242.4 Hz, pꢀ B(C F ) ), 134.76
(d, J = 254.1 Hz, mꢀ B(C F ) ), 124.30 (br, ipso-CB)
P{ H} NMR (161 MHz, PhCl-d ): δ 57.57 (s). ESI-MS:
5
+
6
5 4
6
5 4
513.1313 m/z [M]
6
5 4
Via protonolysis with [DTBP(H)][B(C F ) ]
4.3 Synthesis of Intramolecular FLP System 5
4.3.1 Synthesis of electron deficient phosphinoalcohol
Bis[3,5-bis(trifluoromethyl)phenyl]chlorophosphine
6
5 4
In a glovebox, stoichiometric amounts of the relevant neutral
t
complex [R Zr(Me)(O∧P( Bu) )] and [DTBP(H)][B(C F ) ]
2
2
6 5 4
were weighed into separate vials and dissolved in the miniꢀ
mum amount of PhF (note that PhCl and PhBr can be used
interchangeably). The solution of [DTBP(H)][B(C F ) ] was
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Magnesium turnings (700 mg) were covered with THF and a
solution of 1,3ꢀbis(trifluoromethyl)ꢀ5ꢀbromobenzene (4.91
mL, 28.5 mmol) in THF (20 mL) added dropwise with cooling
6
5 4
added dropwise to the vial containing the zirconium complex.
Gas evolution was evident and the resulting solution was
stirred for 1 hour, yielding bright yellow solutions. Due to
inherent instability, the complexes were used in situ to investiꢀ
gate the reactivity towards Me NH•BH .
(0 °C). The reaction was stirred at room temperature for 1
hour, leading to formation of a brown solution. The reaction
mixture was cooled to 0 °C and a solution of diethylphosphoꢀ
ramidous dichloride (2.00 mL, 13.7 mmol) in THF (10 mL)
added dropwise. The reaction mixture was warmed to room
temperature and stirred overnight. The solvent was removed in
vacuo and the resulting residue dissolved in hexane, filtered
through celite and concentrated to ~20 mL. Hydrogen chloride
solution (2.0 M in Et2O, 13.7 mL, 27.4 mmol) was added
dropwise at room temperature and the reaction stirred for 2
hours, yielding a white precipitate of the amine hydrochloride.
Subsequent filtration and removal of solvent in vacuo yielded
the desired chlorophosphine as a white solid (4.45 g, 66%).
2
3
t
Ind Zr(OC H P( Bu) )][B(C F ) ] (2) Near quantitative yield
2
6
4
2
6
5 4
1
1
by H NMR. H NMR (400 MHz, PhCl-d ): δ 7.59ꢀ7.70 (m,
5
5
7
H, H6 and H ), 7.53 (pseudo t, 1H, J = 7.3 Hz, H3), 7.30ꢀ
.40 (m, 5H, H6,5 and H4), 6.57ꢀ6.61 (m, 1H, H5), 6.27ꢀ6.31
4,7
(
m, 2H, H ), 5.80ꢀ5.85 (m, 2H, H ), 5.74ꢀ5.80 (m, 2H, H ),
1,3 2 1,3
3 13 1
1.36 (18H, d, J = 14.6 Hz, PC(CH ) ). C{ H} NMR (125
MHz, PhCl-d ): δ 166.2 (d, J = 15.6 Hz, C1), 133.6 (d, J
= 4.2 Hz, C5), 132.4 (d, J = 1.2 Hz, C4), 130.1 (s, C6,5),
HP
3 3
2
4
5
CP
CP
3
27
CP
All recorded data consistent with literature.
129.1 (s, C3a,5a), 127.9 (s, C ), 125.5 (s, C ), 125.3 (s, C ),
31
1
6,5
3a,5a
4,7
P{ H} NMR (121 MHz, CDCl ): δ 70.4 (s).
1
3
3
1
3
(
24.4 s, (s, C ), 123.0 (d, J = 27.0 Hz, C2), 122.8 (d, J
=
4,7
CP
CP
2
.4 Hz, C6), 122.3 (s, C ), 117.7 (d, J = 5.0 Hz, C3), 103.6
s, C ), 103.5 (s, C ), 37.0 (s, J = 7.4 Hz, PC(CH ) ), 29.7
2
CP
1
Bis[3,5-bis(trifluoromethyl)phenyl]phosphine
1
,3
1,3
CP
3 3
2
31
1
(
d, J = 4.5 Hz, PC(CH ) ) P{ H} NMR (161 MHz, PhCl-
CP
3 3
A solution of (3,5ꢀCF ꢀC F ) PCl (2.56 g, 5.21 mmol) in Et O
3
6
3 2
2
d5): δ 55.9 (s).ESI-MS: 589.1771 m/z [M + MeOH]
(12 mL) was added dropwise to a suspension of LiAlH (198
4
mg, 4.27 mmol) in Et O (40 mL) at room temperature. The
2
t
t
solution was heated at reflux for 2 hours, then quenched with
[( BuC H ) Zr(OC H P( Bu) )][B(C F ) ] (3) Near quantitaꢀ
5
4 2
6
4
2
6
5 4
1
1
degassed H O (0.15 mL). Filtration through celite and removal
2
tive yield by H NMR. H NMR (400 MHz, PhCl-d ): δ 7.13
5
of solvent in vacuo yielded the desired phosphine as a White
(
6
=
dt, 1H, J = 0.9, 7.7, H6), 7.03 (ddd, 1H, J = 1.6, 6.0 Hz, H4),
.80ꢀ6.85 (m, 1H, H5), 6.70ꢀ6.75 (m, 2H, Cp), 6.39 (dq, 1H, J
1.0, 4.5 Hz, H3), 6.25ꢀ6.30 (m, 2H, Cp), 6.00ꢀ6.08 (m, 4H,
solid (2.10 g, 88%). All recorded data consistent with literaꢀ
28
ture.
1
3
Cp), 1.11 (d, 18H, J = 14.8 Hz, PC(CH ) ), 0.90 (s, 18H,
H NMR (300 MHz, CDCl ): δ 7.55 (s, 1H, ArH), 7.44 (ps d,
3
1
HP
3 3
13
1
CpC(CH ) ). C{ H} NMR (125 MHz, PhCl-d ): δ 167.1 (d,
2H, J = 5.7 Hz, ArH), 4.56 (d, 1H, JPH = 223.9 Hz, PH).
3
3
5
2
4
3
1
1
JCP = 15.2 Hz, C1), 149.9 (s, ipsoꢀCp(tBu)), 133.8 (d, J
=
CP
P NMR (121 MHz, CDCl ): δ ꢀ41.1 (d, J = 216.8 Hz).
3 PH
3
1
2
.8 Hz, C5), 133.3 (d, J = 1.4 Hz, C4), 122.1 (d, J = 21.3
CP CP
2
Hz, C2), 121.9 (s, C6), 118.2 (d, J = 4.8 Hz, C3), 115.7,
CP
(Bis(3,5-bis(trifluoromethyl)phenyl)phosphanyl)phenol
2ꢀIodophenol (371 mg, 1.69 mmol), (3,5ꢀCF ꢀC PH (773
mg, 1.69 mmol), Cs CO (1.10 g, 3.37 mmol) and palladiꢀ
1
13.3, 113.2, 111.1 (Cp), 37.4 (d, J = 4.9 Hz, PC(CH ) ), 31.7
3 3
(
s, CpC(CH ) ), 30.3 (d, J = 4.7 Hz, PC(CH ) ), 30.1 (s,
3
F )
6 3 2
3
3
3 3
3
1
1
CpC(CH ) ) P{ H} NMR (161 MHz, PhCl-d ): δ 58.05 (s).
2
3
3
3
5
+
um(II)acetate (37 mg, 0.17 mmol) were combined in a
Schlenk tube and dissolved in toluene (15 mL). The reaction
mixture was heated at 100 °C for 16 hours. The solution was
filtered through a silica plug , eluted with DCM and the solꢀ
vent removed in vacuo to give the desired product that was
further purified by flash chromatography: silica, DCM:hexane
ESI-MS: 569.2483 m/z [M]
Me Si(C H ) Zr(OC H P(tBu) )][B(C F ) ] (4) Near quantiꢀ
[
2
5
4 2
6
4
2
6
5 4
1
1
tative yield by H NMR. H NMR (400 MHz, PhCl-d ): δ
5
7
.41 (t, 1H, J = 7.8 Hz, H5), 7.14ꢀ7.16 (m, 1H, H4), 7.06ꢀ7.08
(m, 1H, H6), 6.93 (br s, 2H, Cp), 6.63ꢀ6.65 (m, 1H, H3), 6.28
(
50:50). Brown solid (817 mg, 88%).
(
1
br s, 2H, Cp), 6.20 (br s, 2H, Cp), 5.37 (br s, 2H, Cp), 1.12 (d,
1
H NMR (300 MHz, C D ): δ 7.93 (s, 2H, PꢀArH), 7.82ꢀ7.83
3
6
6
8H, J = 13.3 Hz, C(CH ) ), 0.82 (br s, 3H, SiCH ), 0.56 (br
HP
3 3
3
(pseudo d, 4H, J = 6.9 Hz, PꢀArH), 7.44 (ddd, 1H, J = 1.7, 7.4,
13
1
2
s, 3H, SiCH3). C{ H} (100 MHz, PhCl-d ): δ 165.3 (d, J
=
1
1
5
CP
8
.1, H6), 7.02 (dt, 1H, J = 0.9, 7.5, H3), 6.91ꢀ6.97 (m, 2H, H4
4
3
15.4 Hz, C1), 134.4 (d, J = 1.1 Hz, C5), 132.8 (d, J
=
31
1
13
1
CP
CP
and H5). P{ H} NMR (121 MHz, C D ): δ ꢀ11.0 (s) C{ H}
3
6
6
.5 Hz, C4), 126.9 (br s, Cp), 122.7 (d, J = 4.4 Hz, C6),
CP
NMR (125 MHz, C D ): δ 158.3 (d, J = 15.7 Hz, C1), 138.7
1
6
6
21.9 (d, J = 26.7 Hz, C2), 119.0 (br s, Cp), 118.4 (br s,
1
CP
(
d, J = 15.7 Hz, ipso-PꢀAr), 134.6 (d, J = 8.5 Hz, C5), 133.4
CP
10
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Page 11 of 14
ACS Catalysis
2
(
pseudo d, J = 21.3 Hz, PꢀAr), 132.8 (s, C6), 131.7 (dq, J
=
[Cp* ZrOMes][B(C F ) ] (6). The resulting red solution was
2 6 5 4
CF
1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6.3, 34.0 Hz, ipsoꢀC(CF )), 123.3 (q, J = 273.0 Hz, C(CF )),
123.2 (qu, J = 3.9 Hz, PꢀAr), 121.9 (d, J = 3.7 Hz, C3),
3
CF
3
added to a glass vial containing Me NH•BH (9.5 mg, 0.16
2
3
3
CF
mmol), the solution was mixed to ensure full dissolution of the
amineꢀborane before transferring to a quartz JꢀYoungs NMR
tube. The tube was subsequently removed from the glovebox
and the relevant spectra obtained. In all cases the reactions
118.1 (d, J = 9.2 Hz, C2), 115.9 (d, J = 1.6 Hz, C4)
4
.3.2 Synthesis of cationic complex [(C H ) Zr(OC H P(mꢀ
5 5 2 6 4
show <5% conversion to [Me NꢀBH ] after 24 h
2
2 2
CF C H ) )][B(C F ) ] (5)
3
6
3 2
6 5 4
In a glovebox, a solution of the dimethyl zirconocene (1
equiv.) and phosphinoalcohol (1 equiv.) were individually
dissolved in the minimum amount of PhF prior to combining.
The resulting solutions were stirred for 30 minutes.
4.7 Dehydrocoupling of Me
14-19
NH•BH by complexes
3
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
In a glovebox [Cp ZrOMes][B(C F ) ] (19 mg, 0.018 mmol)
2
6 5 4
[
Ph C][B(C F ) ] (1 equiv.) was weighed into a separate vial
3 6 5 4
and the relevant phosphine (0.018 mmol, 14 PtBu 4 mg, 15
3
and dissolved in the minimum amount of PhF (note that PhCl
and PhBr can be used interchangeably). The solution of
[Ph C][B(C F ) ] was added dropwise to the vial containing
PCy 5 mg, 16 PEt 2 mg, 17 PPh 5 mg, 18 PMes 7 mg, 19
3
3
3
3
P(C F ) 10 mg) were weighed into glass vials. The phosphine
6
5 3
3
6 5 4
was dissolved in PhCl (0.5 mL) and mixed with
Cp ZrOMes][B(C F ) ]. In the cases of 15-17 a colour change
the zirconium complex. The resulting solution was stirred for
1 hour. Due to inherent instability, the complexes were used
immediately to investigate the reaction towards Me NH•BH .
[
2
6 5 4
from orange to yellow was observed indicative of a persistent
ZrꢀP bond. The resulting solutions were added to a glass vial
containing Me NH•BH (9.5 mg, 0.16 mmol), the solution was
2
3
Attempted isolation of both complexes resulted in decomposiꢀ
2
3
tion.
mixed to ensure full dissolution of the amineꢀborane before
transferring to a quartz JꢀYoungs NMR tube. The tube was
subsequently removed from the glovebox and the relevant
1
H NMR (300 MHz, PhCl:toluene-d ): δ Aromatic signals
8
are obscured by PhF signals and could not be unambiguously
identified. δ 6.68ꢀ6.72 (m, 2H, H4 and H5), 5.83 (s, 10H,
spectra obtained. 15-17 showed < 5% conversion to [Me Nꢀ
2
31
1
11
C H ) P{ H} NMR (121 MHz, PhCl:toluene-d ): δ ꢀ13.9
BH ] after 14 h. The reaction using 14 was followed by B
5
5
8
2 2
(s).
NMR spectroscopy and the stacked spectra are shown in Figꢀ
ure S1.
4
.4 Dehydrocoupling of Me NH•BH by complexes 2-
2 3
5
4.8 Synthesis of compound 20
General Method - In a glovebox, a PhCl stock solution of the
cationic zirconocene complex (0.025 M) was made as detailed
above. 0.5 mL of the solution (0.012 mmol) was added to a
glass vial of preweighed Me NH•BH (15 mg, 0.25 mmol). A
In
a
glovebox
a chlorobenzene (1 mL) solution of
Me
NH•BH (4 mg, 0.07 mmol) was added dropwise to a
2
3
chlorobenzene (1 mL) solution of 6 (78 mg, 0.07 mmol). An
immediate colour change from orange to yellow was observed.
The resulting solution was precipitated into a large volume (20
mL) of rapidly stirred pentane. The solvent was decanted off
before washing with pentane (3 x 5 mL). The resulting yellow
solid was dried in vacuo (65 mg, 79 %). Crystals of 20 suitable
for analysis by Xꢀray crystallography were obtained by layerꢀ
ing a PhCl solution of 20 with pentane (5 days).
2
3
colour change from yellow to colourless and evolution of gas
was evident. The solution was transferred to a JꢀYoungs NMR
tube, removed from the glovebox and the relevant spectra obꢀ
tained.
11
TOF figures were calculated by integration of the B NMR
spectra.
1
H NMR (500 MHz, d -PhBr): δ 0.54 (3H, br s,
5
Me NH•BH ), 1.49 (30H, s, Cp*), 1.77 (6H, s, orthoꢀCH ),
2
3
3
4
.5 Reaction of FLP system 1 (5 mol%) with Me NH-
2
2.02 (3H, s, paraꢀCH ), 2.12 (6H, s, Me NH•BH ), 3.60 (1H,
3
2
13
3
BH -Me N-BH
3
2
2
br s, Me NH•BH ), 6.56 (2H, s, ArꢀH) C NMR (125 MHz,
2
3
In a glovebox 1 (9 mg, 0.008 mmol) and Me NHꢀBH ꢀMe Nꢀ
2
2
2
d -PhBr): δ 15.3 (s, Cp*), 24.0 (s, orthoꢀCH ), 26.2 (s, paraꢀ
5
3
BH (18.6 mg, 0.16 mmol) were weighed out into glass vials
3
CH ), 47.7 (s, Me NH•BH ), 128.5 (s, Cp*), 159.1 (s, ipsoꢀC).
3
2
3
and combined in PhCl (0.5 mL). The solution was transferred
to a quartz JꢀYoungs NMR tube and then removed from the
Other aromatic peaks are obscured by the PhBr solvent. Sigꢀ
ꢀ
nals corresponding to [B(C F ) ] are also present as reported
6
5 4
1
1
glovebox. The reaction was monitored by B NMR spectrosꢀ
11
ꢀ
above. B NMR (96 MHz, d -PhBr): δ ꢀ16.9 (s, [B(C F ) ] ),
5
6 5 4
copy and found to give complete conversion to [Me NꢀBH2]2
2
ꢀ11.5 (br s, Me NH•BH )
2
3
in 20 mins.
4
.9 Deprotonation of 20 with PR3
4.6 Dehydrocoupling of Me NH•BH by complexes 7-
2
3
In a glovebox 20 (20 mg, 0.016 mmol) and the corresponding
12
phosphine (0.016 mmol, PtBu 3 mg, PCy 5 mg, PEt 2 mg,
3
3
3
In a glovebox [Cp* ZrOMes][B(C F ) ] (6) (21 mg, 0.018
2
6
5 4
PPh 5 mg, PMes 6 mg, P(C F ) 9 mg) were weighed into a
3 3 6 5 3
mmol) and the relevant phosphine (0.018 mmol, 7 PtBu 4 mg,
3
glass vial and dissolved in chlorobenzene (0.5 mL). The reꢀ
sulting solution was transferred to a quartz JꢀYoungs NMR
tube and removed from the glovebox. The reaction was moniꢀ
8 PCy 5 mg, 9 PEt 2 mg, 10 PPh 5 mg, 11 PMes 7 mg, 12
3
3
3
3
P(C F ) 10 mg) were weighed into glass vials. The phosphine
6
5 3
11
31
was dissolved in PhCl (0.5 mL) and mixed with
tored by B and P NMR spectroscopy.
11
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ACS Catalysis
Page 12 of 14
ꢀ
PhCl solvent. Signals corresponding to [B(C F ) ] are also
6
5 4
11
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
present as reported above. B NMR (96 MHz, d -PhCl): δ ꢀ
5
4
.10 Reaction between Me NH-BH -Me N-BH and 20
2 2 2 3
ꢀ
i
1
6.9 (s, [B(C F ) ] ), ꢀ9.5 (br s, Pr NH•BH )
6
5 4
2
3
mol% 14
In a glovebox [Cp ZrOMes][B(C F ) ] (37 mg, 0.036 mmol)
and the PtBu (8 mg, 0.036 mmol) were weighed into glass
vials. The phosphine was dissolved in PhCl (0.5 mL) and
mixed with [Cp ZrOMes][B(C F ) ]. The resulting solution
was added to a glass vial containing Me NHꢀBH ꢀMe NꢀBH
2
6 5 4
3
ASSOCIATED CONTENT
Additional spectra, further synthetic details and crystallographic
tables. This material is available free of charge via the Internet at
http://pubs.acs.org.
2
6 5 4
2
2
2
3
(
18.6 mg, 0.16 mmol), the solution was mixed to ensure full
dissolution of the amineꢀborane before transferring to a quartz
JꢀYoungs NMR tube. The tube was subsequently removed
from the glovebox and the relevant spectra obtained (Figure
S2).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
AUTHOR INFORMATION
Corresponding Author
*
duncan.wass@bristol.ac.uk
*
ian.manners@bristol.ac.uk
4
.11 Reaction between Me NH-BH -Me N-BH and 20
2 2 2 3
Author Contributions
mol% 6
All authors have given approval to the final version of the manuꢀ
script.
.
In a glovebox [Cp ZrOMes][B(C F ) ] (37 mg, 0.036 mmol)
2
6 5 4
was weighed into a glass vial and dissolved in PhCl (0.5 mL).
The resulting solution was added to a glass vial containing
Me NHꢀBH ꢀMe NꢀBH (18.6 mg, 0.16 mmol), the solution
2
2
2
3
ACKNOWLEDGMENT
was mixed to ensure full dissolution of the amineꢀborane beꢀ
fore transferring to a quartz JꢀYoungs NMR tube. The tube
was subsequently removed from the glovebox and the relevant
spectra obtained (Figure S3).
O.J.M and S.F. would like to thank the University of Bristol and
EPSRC for funding.
REFERENCES
i
(1) (a) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton Trans.,
4.12 Reaction between Pr NH•BH and 10 mol% 14
2
3
2
007, 2613ꢀ2626 (b) Staubitz, A.; Robertson, A. P. M.; Sloan,
In a glovebox [Cp ZrOMes][B(C F ) ] (18 mg, 0.018 mmol)
and the PtBu (4 mg, 0.018 mmol) were weighed into glass
vials. The phosphine was dissolved in PhCl (0.5 mL) and
mixed with [Cp ZrOMes][B(C F ) ]. The resulting solution
was added to a glass vial containing Pr NH•BH (20.5 mg,
M. E.; Manners, I. Chem. Rev., 2010, 110, 4023 (c) Smythe,
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3
5
21 (d) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners,
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3
0
.18 mmol), the solution was mixed to ensure full dissolution
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012, 134, 16805
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4.13 Synthesis of compound 21
An analogous methodology was used for the synthesis of 21 as
was employed for the synthesis of 20. In a glovebox a chloroꢀ
benzene (1 mL) solution of Pr NH•BH (4 mg, 0.03 mmol)
was added dropwise to a chlorobenzene (1 mL) solution of 6
(40 mg, 0.03 mmol). An immediate colour change from orꢀ
ange to yellow was observed. The resulting solution was preꢀ
cipitated into a large volume (20 mL) of rapidly stirred penꢀ
tane. The solvent was decanted off before washing with penꢀ
tane (3 x 5 mL). The resulting yellow solid was dried in vacuo
i
2
3
(
4) (a) Leitao, E. M.; Jurca, T. J.; Manners, I. Nat. Chem., 2013, 5,
8
17 (b) Johnson, H. C.; Hooper, T. N.; Weller A. S. Topics in
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(
35 mg, 79 %)
Crystals of 21 suitable for analysis by Xꢀray crystallography
were obtained by layering a PhCl solution of 21 with pentane
2 days).
(
1
5
598 (e) Marziale, A. N.; Friedrich, A.; Klopsch, I.; Drees, M.;
i
H NMR (500 MHz, d -PhCl): δ 1.04 (12H, d, Pr NH•BH ),
1.69 (30H, s, Cp*), 1.97 (6H, s, orthoꢀCH ), 2.18 (3H, s, paraꢀ
CH3), 3.09 (2H, m, Pr NH•BH ), 3.35 (1H, br s,
5
2
3
Celinski, V. R.; auf der Günne, J. S.; Schneider, S. J. Am.
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3
i
2
3
1
3
Me NH•BH ), 6.72 (2H, s, ArꢀH) C NMR (125 MHz, d -
2
3
5
i
PhCl): δ 11.6 (s, Cp*), 19.4 (s, orthoꢀCH ), 19.7 (s, Pr), 26.2
(s, paraꢀCH ), 54.3 (s, Pr), 123.3 (s, metaꢀC), 128.5 (s, Cp*),
55.7 (s, ipsoꢀC). Other aromatic peaks are obscured by the
3
i
3
1
12
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0
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Page 14 of 14
Authors are required to submit a graphic entry for the Table of Contents (TOC) that, in conjunction with the manuscript title,
should give the reader a representative idea of one of the following: A key structure, reaction, equation, concept, or theorem,
etc., that is discussed in the manuscript. Consult the journal’s Instructions for Authors for TOC graphic specifications.
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ACS Paragon Plus Environment