Catching the first oligomerization event in the catalytic formation of polyaminoboranes: H3B·NMeHBH2·NMeH 2 bound to iridium

Article abstract of DOI:10.1021/ja2040738

We report the first insertion step at a metal center for the catalytic dehydropolymerization of H₃B·NMeH₂ to form the simplest oligomeric species, H₃B·NMeHBH₂· NMeH₂, by the addition of 1 equiv of H₃B·NMeH ₂ to [Ir(PCy₃)₂(H)₂(η ²-H₃B·NMeH₂)][BAr^F ₄] to give [Ir(PCy₃)₂(H)₂(η ²-H₃B·NMeHBH₂·NMeH ₂)][BAr^F₄]. This reaction is also catalytic for the formation of the free linear diborazane, but this is best obtained by an alternative stoichiometric synthesis.

Full text of DOI:10.1021/ja2040738

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pubs.acs.org/JACS
Catching the First Oligomerization Event in the Catalytic Formation
of Polyaminoboranes: H3B NMeHBH2 NMeH2 Bound to Iridium
3
3
Heather C. Johnson,† Alasdair P. M. Robertson,‡ Adrian B. Chaplin,† Laura J. Sewell,† Amber L. Thompson,†
Mairi F. Haddow,‡ Ian Manners,‡ and Andrew S. Weller*,†
†Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford OX1 3QR, U.K.
‡School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, U.K.
S Supporting Information
b
Scheme 1. Dehydropolymerization of 1
ABSTRACT: We report the first insertion step at a metal
center for the catalytic dehydropolymerization of H3B
NMeH2 to form the simplest oligomeric species, H3B
3
3
NMeHBH2 NMeH2, by the addition of 1 equiv of H3B
3
3
NMeH2 to [Ir(PCy3)2(H)2(η2-H3B NMeH2)][BArF ] to
4
3
give[Ir(PCy3)2(H)2(η2-H3B NMeHBH2 NMeH2)][BArF ].
This reaction is also catalytic for the formation of the free
linear diborazane, but this is best obtained by an alternative
stoichiometric synthesis.
4
3
3
Scheme 2. Synthesis of Complexes 4 and 5a
olyaminoboranes, [BH2NHR]n, are boronÀnitrogen analo-
P
gues of polyolefins.1 However, in comparison with the vast
area that is encompassed by their ubiquitous carbon analogues,2
their synthesis and properties have been little studied. Catalytic
methods for the synthesis of high-molecular-weight examples of
these potentially interesting materials based upon dehydropo-
lymerization of primary amine borane substrates have recently
a Anions are not shown. Solvent = 1,2-C6H4F2.
We recently reported the synthesis of [Ir(PCy3)2(H)2(η2-
H3B NMe2H)][BArF ] (A) [ArF = 3,5-C6H3(CF3)2] via addi-
4
3
tion of H3B NMe2H to [Ir(PCy3)2H2(H2)2][BArF ] (3) along
been reported. For example, treatment of H3B NMeH2 (1) with
3
4
3
an appropriate catalyst allows for the preparation of high-
molecular-weight poly(methylaminoborane), [BH2NMeH]n
(Scheme 1).3
with the observation that this complex slowly (t1/2 = 24 h)
undergoes dehydrogenation to form the aminoborane complex
[Ir(PCy3)2(H)2(η2-H2BdNMe2)][BArF ].7 In the anticipation
4
of similar reactivity for 1, we prepared [Ir(PCy3)2(H)2(η2-H3B
A variety of catalyst systems have been investigated for the
dehydropolymerization of 1,3a,4 among the current best being those
based upon Ir(H)2(POCOP) (POCOP = [k3-1,3-(OPtBu2)2-
C6H3])4c and [Rh(Ph2P(CH2)xPPh2)]+ (x = 3À5),4b which pro-
duce polyaminoboranes with high molecular weights and reason-
able polydispersity indices. However, if fine control over both
molecular weight and polymer architecture is to be achieved,
knowledge of the mechanistic details of polymer growth is
essential, as has been so successfully demonstrated in the
synthesis of tailored polyolefins using transition-metal catalysts.2
In contrast, the key steps in the polymerization of primary amine
boranes (e.g., 1) are yet to be fully resolved.4c These would likely
be related to coordination/insertion events in olefin polymeri-
zation as catalyzed by a transition metal.5 Herein we report the
direct observation of this oligomerization event for 1 by the
(catalytic) synthesis of the simplest coupled unit arising from the
3
NMeH2)][BArF ] (4) in an equivalent way (Scheme 2) in quan-
4
titative NMR yield. The solid-state structure of 4 (Figure 1)
confirms an η2 binding mode for the amine borane,7,8 as do the
solution NMR spectroscopic data (see the Supporting In-
formation), which are similar to those for A. Surprisingly, pure
4 did not undergo dehydrogenation to give an aminoborane (i.e.,
H2BdNMeH)-ligated complex, but instead remained unchanged
after 48 h at 298 K; this is also in contrast to related rhodium9 and
neutral ruthenium10 systems that promote the dehydrogenation
of primary amine boranes H3B NRH2 (R = Me, tBu) to give the
3
corresponding metal-bound aminoboranes.
Although dehydrogenation was not observed for pure 4,
addition of a further equivalent of 1 to a 1,2-C6H4F2 solution
of 4 resulted in a relatively fast (4 h) reaction to afford H2 and a
new complex identified as [Ir(PCy3)2(H)2(η2-H3B NMeHBH2
3
3
NMeH2)][BArF ] (5). Crystalline material suitable for an X-ray
formal dehydrocoupling of 1 at a metal center to give H3B
3
3
4
diffraction experiment was obtained using the [BArCl4]À salt of
NMeHBH2 NMeH2 (2). The parent inorganic butane H3B
3
NH2BH2 NH3 has recently been prepared by a noncatalytic
3
route involving the addition of NH3 to H2B(μ-H)NH2BH2,6
and we also report that an analogous procedure using H2B(μ-H)-
NHMeBH2 produces 2 as a pure material.
Received: May 3, 2011
Published: June 23, 2011
r
2011 American Chemical Society
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|

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Scheme 3. Catalytic Production of 2
thus confers C1 symmetry to the molecule. A pair of tightly
coupled AB doublets [J(PP) = 287 Hz] were observed in the
31P{1H} NMR spectrum. The 11B NMR spectrum showed two
broad signals at δ +17.1 and À5.8, the former shifted to high
frequency relative to free 2 (see below), consistent with coordi-
nation to a metal center. Electrospray ionization mass spectro-
metry showed a clear parent ion for the cation (m/z 843.58
obsvd, 843.58 calcd) that also demonstrated the correct isotopic
pattern. The free oligomer 2 can be released from the metal
center by addition of excess MeCN to a C6H5F solution of 5
[δ(11B): À5.3 (t, 108 Hz), À17.8 (q, 92 Hz)]. However, under
these conditions, 2 is not stable and reacts to give further
products, including [H2BNMeH]3. We speculate that the adduct
Figure 1. Structure of the cationic portion of 4,11 with thermal
ellipsoids at the 30% probability level. The minor disordered component
is not shown. Selected bond lengths (Å): Ir1ÀB1, 2.210(7); Ir1ÀP1,
2.3281(8); Ir1ÀP2, 2.3347(8); B1ÀN1, 1.580(8). Selected bond angles
(deg): P1ÀIr1ÀP2, 160.76(3); B1ÀN1ÀC1, 111.0(6).
complex [Ir(PCy3)2H2(NCMe)2][BArF ] promotes this reaction.7
4
Under 10 mol % (0.025 M) loading at 298 K, 3 or 4 slowly pro-
moted catalytic dehydrogenation of 1 to give free 2 (Scheme 3).
However, this reaction was not clean, and other species were
observed (∼10% concentration relative to 2 over the course of
catalysis), with H2B(μ-H)NHMeBH2 as the principal compo-
nent (∼5%) alongside smaller amounts of [H2BNMeH]3 and
[HBNMe]3. During catalysis the resting state (31P NMR) is 5,
alongside a minor unidentified species. 5 is also a competent
precatalyst for the formation of 2 from 1 (10 mol %), while in the
absence of catalyst no reaction was observed. We did not observe
a growing oligomeric chain on the metal upon addition of excess
1 to 5, although [HBNMe]3 formed upon heating (40 °C, 3
days). It thus appears that 3 is selective for the single oligomer-
ization event of 1, in contrast to other catalyst systems that
promote rapid dehydropolymerization,3a,4aÀ4c although interest-
ingly these systems also yield [HBNMe]3 as the final thermo-
dynamic product.
Figure 2. (left) Structure of the cationic portion of À5,11 with thermal
ellipsoids at the 30% probability level. The [BArCl
] anion has been
4
omitted. Selected bond lengths (Å): Ir1ÀB1, 2.228(5); Ir1ÀP1,
2.3477(10); Ir1ÀP2, 2.3308(10); B1ÀN1, 1.576(5); N1ÀB2,
1.612(5); B2ÀN2, 1.607(6). Selected bond angles (deg): P1ÀIr1ÀP2,
159.60(3); B1ÀN1ÀB2, 114.5(3); N1ÀB2ÀN2, 109.6(3). (right)
Newman projection of bound 2.
Since the catalytic production of 2 produced compositionally
impure material, 2 was independently prepared via modification
of the method recently reported by Shore and Zhao for the
preparation of the linear diborazane H3B NH2BH2 NH3.6 A
5 (ArCl = 3,5-C6H3Cl2).11,12 The solid-state structure of the
cation (Figure 2) shows 2 η2-bound to the {Ir(PCy3)2(H)2}+
fragment. Although the preformed cyclic oligomer [H2BNMeH]3
has been shown to coordinate with Rh fragments,13 as have the
related species H3B NMe2BH2 NMe2H8b,13 and [H2BNMe2]2,8b,14
3
3
solution of H3B THF and H3B NH2Me heated to 60 °C over 70
3
3
h produced H2B(μ-H)NHMeBH2. Subsequent ring opening by
addition of a methylamine solution at À78 °C furnished 2, which
was isolated in 72% yield as a white solid upon removal of the
solvent. Recrystallization from CH2Cl2/hexane at À40 °C pro-
duced X-ray-quality crystals, and 2 was shown to crystallize with a
single molecule in the asymmetric unit. Interestingly, 2 crystal-
lizes in the antiperiplanar conformation (see Figure 3), in con-
trast to the gauche arrangement observed upon metal coordina-
tion in 5 (viz. dihedral B1ÀN1ÀB2ÀN2: 59.9° in 5, 179.9° in 2).
In comparison with 5, the BÀN distances are all slightly shorter
in free 2, with the greatest difference found for B1ÀN1, as might
be expected. Multinuclear NMR characterization in solution was
consistent with the structure revealed by X-ray analysis and the
observations made for 5 (and liberated 2). Three signals
attributed to BÀH groups in a 1:1:3 ratio (δ 2.05, 1.76, 1.31,
respectively) were observed in the 1H{11B} spectrum (CD2Cl2),
with the BH3 signals shifting considerably to lower frequency
upon coordination to the metal in 5, as expected. The 11B NMR
3
3
this is the first report of both the synthesis of 2 and its
coordination to a metal center. The bond lengths and angles
are unremarkable compared to those of related Ir(III) complexes
(e.g., 4 and [Ir(PCy3)2(H)2(η2-H3B NMe3)][BArF ]7). Inter-
4
3
estingly, bound 2 adopts a gauche conformation similar to that
found for H3B NH2BH2 NH36 but different from that expected
3
3
for 2-methylpentane. Two quadrupolar-broadened integral 1 H
IrÀHÀB signals were observed in the 1H NMR spectrum (500
MHz) at δ À6.15 and À6.32, thus showing that BH exchange at
the Ir center does not occur, similar to A.7 Three NH environ-
ments (δ 4.13, 4.02, 2.99) and a complex IrÀH signal (2 H)
suggestive of inequivalent hydride environments were also ob-
served. In the 1H{11B} NMR spectrum, as well as signals due to
the BH3 group, two environments were observed for the central
BH2 unit (δ 2.48, 2.37). The stereogenic nitrogen center, N1,
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COMMUNICATION
drawn from the study of the dehydrocoupling of H3B NMe2H to
3
form dimeric aminoboranes, which suggested that the metal is
involved in both dehydrogenation and oligomerization steps,7,20
with the corresponding linear diborazane H3B NMe2BH2
3
3
NMe2H (or a close relative21) observed as an intermediate during
dehydrocoupling.8b,19,20,22 Addition of H3B NMe3 (which con-
3
tains no NH functional group) to 4 did not give a mixed linear
diborazane but instead afforded [Ir(PCy3)2(H)2(η2-H3B NMe3)]-
3
[BArF ]7 and [HBNMe]3 after 24 h. We suggest that this might
4
occur because the bulky H3B NMe3 hinders the BÀN coupling
3
step in the relatively crowded {Ir(PCy3)2(H)2}+ fragment in C;
instead, aminoborane is liberated and trimerizes7,22 to give
[H2BNMeH]3, which then undergoes dehydrogenation to afford
[HBNMe]3.13,22 Support for this scenario comes from the addi-
tion of one equivalent of [H2BNMeH]313 to 3, which rapidly (<1 h)
gave [HBNMe]3 and 3. Addition of 1 to 4-N-D2 resulted in the
eventual formation of 5 with deuterium incorporated at both N
positions of bound 2, presumably as a result of initial exchange of
amine boranes bound to the metal center. H2, HD, and D2 were
also observed. However, as D was also incorporated into the
hydrides (possibly via exchange of bound H2 with D2/HD) as
well as into the PCy3 ligands, we cannot rule out alternative
mechanisms for the oligomerization step, potentially involving
intermolecular NÀH proton transfer from 1 to the metal21 or the
involvement of cyclometalated phosphines.
Figure 3. Solid-state structure of isolated 2, with thermal ellipsoids at
the 30% probability level.11 Selected bond lengths (Å): N1ÀB1,
1.4887(19); N1ÀB2, 1.5730(19); N1ÀC1, 1.5906(19); N2ÀC2,
1.4822(19); N2ÀB2, 1.5864(19). Selected bond angles (deg): B1À
N1ÀB2, 108.73(11); B1ÀN1ÀC1, 110.89(11); B2ÀN1ÀC1, 115.71(11);
C2ÀN2ÀB2, 112.62(11); N1ÀB2ÀN2, 109.14(10).
Scheme 4. Suggested Mechanism for the Formation of 5a
The outlined mechanism also accounts for the lack of further
oligomerization events, as the terminal NÀH group in 5 is
distant from the metal center and thus less likely to participate
in H2 elimination. This leads us to suggest that the isolation of 5
is a rather specific case and might not be directly applicable to
the mechanism of formation of polyaminoboranes at other
metal centers,4aÀc although is it likely that some mechanistic
steps are common. Importantly, it is this selectivity for the
synthesis of 2, the product of the first oligomerization event in
the formation of polyaminoboranes, that no doubt allows for its
isolation.
a [Ir] = [Ir(PCy3)2]+.
spectrum of 2 matched that liberated from 5.15 Addition of
preformed 2 (see below) to 3 generated 5 cleanly.
Limited insight into the mechanism of oligomerization can be
gleaned from labeling and crossover experiments. Addition of D2
to 4 resulted in H/D exchange at boron and iridium only (with
the concomitant observation of H2 and HD), while addition of 1
’ ASSOCIATED CONTENT
S
Supporting Information. Full experimental data for the
b
equiv of H3B NMeD2 to 3resultedinthe formationof[Ir(PCy3)2-
3
syntheses; crystallographic data for 2, 4, and 5, including atomic
positional and thermal parameters (CIF). This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(H)2(η2-H3B NMeD2)][BArF ] (4-N-D2), which showed no
4
3
evidence for incorporation of D into the Ir hydrides over 4 h. This
suggests that BÀH activation in 4 is a low-energy process relative
to NÀH activation, consistent with calculations on related
’ AUTHOR INFORMATION
H3B NMe2H systems.7,8b Spin-saturation-transfer experiments
3
Corresponding Author
andrew.weller@chem.ox.ac.uk
on 4 that showed chemical exchange between IrÀH and
IrÀHÀB only, consistent with this. These observations, along-
side the requirement of an excess of 1 to promote oligomerization
and the lack of dehydrogenation of 4 to form an aminoborane
species, point to the tentative mechanism suggested in Scheme 4.
BÀH activation7 in 4 and σ coordination of additional 1 gives
intermediate B, which then loses H2, possibly promoted by BH-
’ ACKNOWLEDGMENT
The authors thank the University of Oxford and the EPSRC
for support; the Diamond Light Source for the award of
beam time on I19 (MT1858) and Dr. Kirsten E. Christensen
for support; and Professor Stuart Macgregor (Heriot-Watt
University) for insightful discussions.
(δÀ) (+δ)HN dihydrogen interactions,8a,16 to form the ami-
3 3 3
noborane/base-stabilized boryl17 complex C.18 BÀN coupling
followed by BÀH bond formation leads to complex 5. The
coupling of two H3B NMe2H ligands at a TiCp2 center to form
3
the corresponding linear diborazane was recently proposed on the
basis of kinetic studies,19 while for the Ir(H)2(POCOP) system,
polymer-growth kinetics suggested a chain-growth mechanism
invoking both metal-mediated dehydrogenation of 1 (to form
an aminoborane) and BÀN coupling.4c Similar conclusions were
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