Aminoborane σ complexes: Significance of hydride co-ligands in dynamic processes and dehydrogenative borylene formation

Article abstract of DOI:10.1021/om400040q

Systems of the type [(p-cym)Ru(PR₃)(H)(H₂BN ⁱPr₂)]⁺ (R = Cy, Ph) can be synthesized from (p-cym)Ru(PR₃)Cl₂ and H₂BNⁱPr ₂/Na[BAr^f₄] and are best formulated as (hydrido)ruthenium κ¹-aminoborane complexes. VT-NMR measurements have been used to probe the σ-bond metathesis process leading to Ru-H/H-B exchange, yielding an activation barrier of ΔG ^? = 7.5 kcal mol^-1 at 161 K. Moreover, in contrast to the case for related non-hydride-containing systems, reactivity toward alkenes constitutes a viable route to a metal borylene complex via sacrificial hydrogenation.

Full text of DOI:10.1021/om400040q

Communication
pubs.acs.org/Organometallics
Aminoborane σ Complexes: Significance of Hydride Co-ligands in
Dynamic Processes and Dehydrogenative Borylene Formation
David A. Addy, Joshua I. Bates, Michael J. Kelly, Ian M. Riddlestone, and Simon Aldridge*
Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K.
S
* Supporting Information
ABSTRACT: Systems of the type [(p-cym)Ru(PR3)(H)-
(H2BNiPr2)]+ (R = Cy, Ph) can be synthesized from (p-cym)Ru-
(PR3)Cl2 and H2BNiPr2/Na[BArf4] and are best formulated as
(hydrido)ruthenium κ1-aminoborane complexes. VT-NMR measure-
ments have been used to probe the σ-bond metathesis process leading
to Ru−H/H−B exchange, yielding an activation barrier of ΔG⧧ = 7.5
kcal mol−1 at 161 K. Moreover, in contrast to the case for related non-hydride-containing systems, reactivity toward alkenes
constitutes a viable route to a metal borylene complex via sacrificial hydrogenation.
he chemistry of metal borylene complexes, LnM(BX)m,
has developed rapidly over the past 10 years, to the extent
less common, and alternative structural descriptions as metal
hydroborate or boryl dihydride species have been investigated
(Chart 1).7b,12,13 In the current study, the reactions of (p-
T
that systems previously viewed as chemical novelties are now
well understood from the perspective of geometric and
electronic structure.1 Potentially useful reactivity patterns
have also begun to be mapped outfor stoichiometric systems
at least (e.g., in the controlled functionalization of organic/
organometallic substrates).1 Catalytic applications, on the other
hand, remain all but unknown,2 primarily due to the
incompatibility of existing synthetic routes (e.g., salt metathesis,
photolytic transfer, and halide abstraction) with catalytic
turnover.1
Direct borylene synthesis by the dehydrogenation of a
dihydroborane,3 in contrast, offers an exciting new approach
which, by analogy with related silylene systems, might be more
amenable to incorporation into a catalytic cycle.4 Moreover, the
viability of direct borane to borylene conversion has been
demonstrated by the dehydrogenation of Ru(PCy3)2(H)Cl(κ2-
H2BMes) to Ru(PCy3)2(H)Cl(BMes) (Mes = C6H2Me3-
2,4,6),3 and the intermediates and discrete mechanistic steps
implicit in such a transformation have recently been
elucidated.5 That said, the scope of this chemistry beyond
one groundbreaking example has yet to be realized, and with
this in mind, we hypothesized that the hydrogenation of a
sacrificial alkene might constitute an alternative and more
broadly applicable methodology. Central to the success of such
an approach is controlling the competition between alkene
hydrogenation and hydroboration.6 While the choice of alkene
is potentially important, the formation of the target borylene
systems appears in the current study to be crucially dependent
on using a borane complex precursor featuring an ancillary
hydride ligand.
Chart 1. Alternative Descriptions of a (Hydrido) σ-Borane
Complex: Metal Hydroborate or Boryl Dihydride
cym)Ru(PR3)Cl2 (1a, R = Cy; 1b, R = Ph; p-cym = p-
14
i
MeC6H4 Pr) with excess H2BNiPr2/Na[BArf4] led to the
formation of the cationic complexes [(p-cym)Ru(PR3)(H)-
(H2BNiPr2)][BArf4] (3a,b; Arf = C6H3(CF3)2-3,5) as yellow-
orange crystalline materials in 50−60% isolated yield (Scheme
1). For the PCy3 system, stepwise addition of the borane and
halide abstraction agent reveals that (p-cym)Ru(PCy3)H(Cl)
(2a)14 is accessible (together with H(Cl)BNiPr2)15a via initial
hydride/halogen exchange. Subsequent addition of Na[BArf4]
confirms that 2a is a viable intermediate in the formation of 3a.
3a,b have both been characterized by standard spectroscopic
and analytical techniques. Of interest are 11B chemical shifts
Scheme 1. Syntheses of [(p-cym)Ru(PR3)(H)(κ1-
a
H2BNiPr2)][BArf4] (3a, R = Cy; 3b, R = Ph)
In recent work we have been interested in the chemistry of
aminoborane complexes [LnM(H2BNR2)],7−10 systems which
have been implicated in processes such as the dehydrocou-
pling/polymerization of amineboranes11 but for which
relatively little fundamental chemistry has been mapped out.
σ-Borane complexes featuring additional hydride coligands are
a
Reagents and conditions: (a) H2BNiPr2 (2.0 equiv), C6H5F;14 (b)
Na[BArf4] (1.1 equiv), C6H5F, room temperature, 5 min, 50−60%.
Received: January 18, 2013
Published: March 11, 2013
© 2013 American Chemical Society
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Communication
(36 and 35 ppm, respectively), which are similar to those of
both “free” H2BNiPr2 (35.2 ppm)15b,c and [CpRu(PPh3)2(κ1-
H2BNiPr2)][BArf4] (4b; 38 ppm), the latter of which features
an unambiguously κ1-bound aminoborane fragment (vide
infra). By comparison, the κ2-aminoborane ligand in [Cp*Ru-
(PCy3)(κ2-H2BNiPr2)][BArf4] (5a; see the Supporting In-
formation), for example, gives rise to a much lower field signal
(δB 54 ppm) consistent with a tighter M···B contact.7g That
said, 11B chemical shifts similar to those for 3a,b have also been
reported for four-coordinate ruthenium hydroborate systems,16
and the additional facts (i) that only one Ru−H−B resonance
could be discerned in the room-temperature 1H NMR
spectrum of either 3a or 3b and (ii) that selective 11B{1H}
experiments yielded broad resonances giving no definitive
coupling constant data prompted us to examine in detail the
mode of attachment of the boron-containing ligand by
crystallographic and computational approaches.
BNC2 unit itself. Thus, the B−N distance (1.392(8) Å) is
strongly indicative of BN double-bond character, being (i)
similar to those found in Cl2BNPh2 (1.380(6) Å)17 and κ2
complexes containing the H2BNiPr2 ligand (e.g., 1.379(8) Å for
[Ir(IMes)2(H)2(κ2-H2BNiPr2)]+)9b and (ii) significantly shorter
than either the sum of the respective covalent radii (1.58 Å)18
or than distances in complexes of the H3B·NHiPr2 ligand (ca.
1.60 Å).9d Moreover, the geometry at N(32) is planar
(Σ(angles) = 359.8°).
Given the limitations in locating hydrogen atoms by X-ray
diffraction, additional information was sought from DFT
studies. While the potential energy hypersurface for 3b is
clearly a shallow one, a conformation lying 0.76 kcal mol−1
above the global minimum was found to model well the
alignment of the planar BNC2 heavy-atom framework19 and
additionally features B−μ-H (1.62, 1.38 Å) and Ru−H
distances (1.65, 1.72 Å) in good agreement with the
crystallographic study (Supporting Information).
The geometric data therefore imply that a hydroborate
structure of type I (Figure 1) involving significant covalent
bonding interactions with three hydrogen atoms is unlikely and
that an alternative motif based around a κ1-aminoborane
fragment (i.e., II) is plausible.20 With this in mind, we
synthesized for structural comparison the isoelectronic
[CpRuL2]+ system 4b, which features the same H2BNiPr2
fragment but no hydride coligand.7h While a number of
structural features (Figure 2), namely the short BN distance
Crystallographically, the solid-state structure of 3a features a
disordered borane fragment; that of 3b, however, suffers from
no such problems, and hydrogen atoms could also be located in
the difference Fourier map (Figure 1). Superficially, the solid-
Figure 1. Molecular structures and limiting structural descriptions for
3a,b. Here and elsewhere, the anion, most H atoms, and minor
disorder component(s) are omitted and phosphine substituents are
shown in wireframe format for clarity. H atoms in 3b were located in
the difference Fourier map and refined isotropically with no restraints.
Key bond lengths (Å) and angles (deg) for 3b: Ru(1)−P(2) =
2.324(1), Ru(1)−B(31) = 2.264(6), Ru(1)−H(11) = 1.63(6),
Ru(1)−H(17) = 1.69(6), B(31)−N(32) = 1.392(8), B(31)···H(11)
= 1.62(6), B(31)−H(17) = 1.33(6), B(31)−H(311) = 1.08(6);
Ru(1)−B(31)−N(32) = 128.3(4), B(31)−N(32)−C(33) = 119.7(5),
B(31)−N(32)−C(36) = 124.7(5), C(33)−N(32)−C(36) = 115.4(5).
Figure 2. Molecular structure of [CpRu(PPh3)2(κ1-H2BNiPr2)]-
[BArf4], 4b. Key bond lengths (Å) and angles (deg): Ru(1)−B(50)
= 2.353(4), Ru(1)−P(2) = 2.338(1), Ru(1)−P(4) = 2.334(1),
Ru(1)−H(28) = 1.70(5), B(50)−N(51) = 1.386(5), B(50)−H(28)
= 1.23(4), B(50)−H(501) = 1.08(5); Ru(1)−B(50)−N(51) =
131.4(3), B(50)−N(51)−C(52) = 120.2(3), B(50)−N(51)−C(55)
= 123.9(3), C(52)−N(51)−C(55) = 115.6(3).
(1.386(5) Å for 4b) and planar BNC2 skeleton, are common to
the two systems, the presence of a hydride coligand in 3b does
lead to some geometric perturbation of the aminoborane
fragment. Most notably, the geometry at the boron center in 3b
(as defined by N(32) and the two aminoborane hydrogens,
H(17) and H(311)) is less planar than in 4b (Σ(angles) =
343.5°; cf. 352.3°).
From a structural perspective, 3a,b therefore offer a platform
on which to study the degenerate exchange between a single
metal-bound hydride and a σ-coordinated BH bond (i.e., M−
H/H−B σ-bond metathesis).4,21 In the case of 3a, the dynamics
of exchange can be probed by VT-NMR in CDCl2F solution.
Thus, at room temperature, a single high-field resonance
integrating to 2H is measured at δH −12.53 ppm (for HA and
HB), and an AB multiplet is observed for the p-cymene
aromatic hydrogens. In effect, at this temperature, the cation
possesses a mirror plane on the NMR time scale. At very low
temperatures (T = 161 K), however, two distinct hydride
resonances are observed (at δH −12.73 and −12.48 ppm), and
four arene CH signals can be resolved, consistent with a static
state structure resembles those of isoelectronic charge-neutral
systems such as Cp*Ru(PiPr3)(κ2-H2BXMes) (X = Cl, H; Mes
=2,4,6- Me3C6H2), which feature a [BH2XMes]− ligand bound
to the ruthenium center via two equivalent bridging hydrogen
atoms.16b Closer inspection, however, reveals that this is
unlikely to be a valid structural description in the case of 3b.
Thus, in comparison to the Cp*Ru complexes, there is greater
asymmetry in the binding of the boron ligand, manifested by (i)
the B−H(Ru) distances (1.33(6) and 1.62(6) Å, cf. 1.25(2) and
1.27(2) Å for Cp*Ru(PiPr3)(κ2-H3BMes)),16b with the longer
B···H distance being slightly shorter than a similar contact
measured for {C6H3(OPtBu2)2}Ir(H)2(κ1-BH3) (1.74(5) Å),
which is described as being “outside the limits of a bonding
interaction”,12 and (ii) the BNC2 heavy-atom framework, which
is canted to one side of the metal center, such that Ru(1) lies
0.40 Å out of the least-squares plane defining B(31), N(32),
C(33), and C(36). More telling still is the geometry of the
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Scheme 3. Reaction of [Cp*Ru(PR3)(κ2-H2BNiPr2)][BArf4]
(5a) with TBE: Formation of
structure featuring nonequivalent Ru−H bonds and which is
chiral at ruthenium. Modeling of the coalescence behavior
allows the activation barrier ΔG⧧ = 7.5 kcal mol−1 to be
estimated at 161 K. While fluxional processes have previously
been identified in (hydrido)metal σ-borane complexes,7a,b,12,13c
3a offers, for the first time, unambiguous determination of the
barrier for simple M−H/H−B pairwise exchange.
[Cp*Ru(H){PCy2(C6H8)}][BArf4] and
a
tBuCH2CH2B(H)NiPr2
Given the low energy but nonproductive (i.e., degenerate) σ-
bond metathesis processes occurring for 3a,b, we hypothesized
that useful (facile) reactivity might stem from insertion of
unsaturated substrates into the Ru−H bond. In particular, with
borane dehydrogenation in mind, we examined the reactivity
toward the well-known dihydrogen acceptor 3,3-dimethylbu-
tene (TBE). Comparative studies utilizing the related (but non-
hydride-containing) complexes 4b and 5a were also under-
taken. In practice, the reaction of 3a with TBE leads to the
formation of two new boron-containing species. The major
product (65% isolated yield) gives rise to a broad 11B NMR
signal at δB 77 ppm and can subsequently be shown to be the
borylene complex [(p-cym)Ru(PCy3)(H)(BNiPr2)][BArf4]
(6a), formed by net dehydrogenation of 3a (Scheme 2). The
a
Reagents and conditions: (a) TBE (10.0 equiv), C6H5F, 60 °C, 8
days, 45%.
elimination,28 it should be noted that the hydrido (κ2-borane)
complex Ru(PCy3)2(H)Cl(κ2-H2BMes) undergoes spontane-
ous H−H bond formation without the need for a sacrificial
alkene.3 On the other hand, 3a does not undergo dehydrogen-
ation in the absence of TBE under any conditions we have
examined. While these experimental observations hint at the
mechanistic relevance of both the alkene and ancillary hydride
ligand, in-depth investigations by quantum chemical methods
of potential steps leading to the dehydrogenation of 3a (and
related systems) are clearly necessary and will be reported in
due course.
Scheme 2. Synthesis of Borylene Complex 6a by
a
Dehydrogenation of 3a and Molecular Structure of 6a
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, figures, and CIF files giving synthetic, spectro-
scopic, and crystallographic data for compounds 3a,b, 4b, 5a,
6a, [Cp*Ru(H){PCy2(C6H8)}][BArf4] and BuCH2CH2B(H)-
t
a
Reagents and conditions: (a) TBE (10.0 equiv), C6H5F, room
NiPr2 and details of the DFT calculations on 3b and 6a, and all
CIFs. This material is available free of charge via the Internet at
http://pubs.acs.org.
temperature, 30 min, 65%. Key bond lengths (Å) and angles (deg):
Ru(1)−P(2) = 2.336(1), Ru(1)−B(31) = 1.911(4), Ru(1)−H(8) =
1.54(6), B(31)−N(34) = 1.361(5), B(31)...H(8) = 1.71(6); Ru(1)−
B(31)−N(34) = 173.0(4).
AUTHOR INFORMATION
Corresponding Author
*S.A.: email, Simon.Aldridge@chem.ox.ac.uk; tel, +44 1865
285201; fax, +44 1865 272690; web, http://users.ox.ac.uk/
∼quee1989.
■
positions of both the metal-bound hydrogen atom and the
linear Ru−B−N unit determined crystallographically for this
species are well reproduced by DFT calculations (Supporting
Information). The second (minor) boron-containing species,
tBuCH2CH2B(H)NiPr2 (Supporting Information), arises from
alkene hydroboration, presumably via a metal-mediated
process, since the reaction of TBE with H2BNiPr2 does not
proceed under otherwise analogous conditions in the absence
of 3a.22,23 In contrast, the reaction of the simple (i.e. non-
hydride-containing) κ1-H2BNiPr2 complex 4b with TBE results
in simple borane displacement chemistry. Interestingly,
tBuCH2CH2B(H)NiPr2 is the sole boron-containing product
formed in the reaction of the κ2-H2BNiPr2 system 5a with TBE.
The metal-containing product in this case (Scheme 3 and the
Supporting Information) features a chelating phosphino-allyl
ligand resulting from multiple C−H activation processes in the
putative [Cp*Ru(PCy3)]+ cation.24
3a is therefore unique in the current study not only in
featuring both hydride and σ-borane ligands but also in its
ability to undergo dehydrogenative borylene formation. As
such, it represents, to our knowledge, the first unambiguous
report of the use of a sacrificial H2 acceptor in such
chemistry.1,25 While there is mechanistic precedent for both
(i) alkene insertion into a Ru−H bond26,27 and (ii) a
subsequent Ru−C/B−H exchange step leading to alkane
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the EPSRC PDRA (D.A.A.), studentships
(M.J.K., I.M.R.), the National Mass Spectrometry Service
Centre, and the NSERC PDF (J.I.B.).
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■
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dx.doi.org/10.1021/om400040q | Organometallics 2013, 32, 1583−1586