B-H activation at a rhodium(I) center: Isolation of a bimetallic complex relevant to the transition-metal-catalyzed dehydrocoupling of amine-boranes

Article abstract of DOI:10.1002/anie.200905185

(Figure Presented) A bimetallic complex that is relevant to the mechanism of transition-metal-catalyzed amine-borane dehydrocoupling has been isolated. The structure (see picture) contains three different amine-borane activation modes within the same molecule.

Full text of DOI:10.1002/anie.200905185

Angewandte
Chemie
DOI: 10.1002/anie.200905185
Amine–Boranes
À
B H Activation at a Rhodium(I) Center: Isolation of a Bimetallic
Complex Relevant to the Transition-Metal-Catalyzed
Dehydrocoupling of Amine–Boranes**
Adrian B. Chaplin and Andrew S. Weller*
In memory of Daniela Rais
The transition-metal-catalyzed dehydrocoupling of amine–
boranes and phosphine–boranes (H3B·EHR2; E = N, P ; R =
H, alkyl, aryl) to form oligomeric and polymeric materials,
with the concomitant release of H2, is attracting considerable
attention owing to the control that the metal fragment
imposes on the kinetics and product distributions involved
in these processes. This has particular relevance to chemical
hydrogen storage as a future energy vector and the synthesis
of new Group 13/15 polymeric materials that show useful
physical and electronic properties.[1,2]
There are a growing number of transition metal systems
that have been reported to effect the dehydrocoupling of
amine–boranes (H3B·NR2H).[3–11] For homogeneous systems,
two different mechanistic scenarios have been proposed on
the basis of experimental and computational studies: inner-
sphere,[5,8,12] in which the metal center takes part in BH/NH
activation (stepwise or concerted), and outer-sphere,[9] which
is conceptually related to alcohol oxidation using transition
metal catalysts. For an inner-sphere mechanism that involves
Despite their central role, the chemistry of complexes
such as I is only now beginning to be explored, although
model species incorporating H3B·EMe3 (E = N, P) were
reported 10 years ago by Shimoi and co-workers.[13] We have
recently described the isolation, structural characterization,
and subsequent reactivity of a number of sigma complexes of
type I using the {Rh(PiBu3)2}+ and {Rh(P(C5H9)2(h2-C5H7)}+
12-electron fragments.[6–8] Amino–borane complexes of type
III, which contain the {Rh(PiBu3)2}+ metal fragment, have
also been spectroscopically characterized.[6,8] Closely related
ruthenium h2-sigma complexes of H2BR (R = H, alkyl, aryl)
have recently been reported.[14] As far as we are aware,
however, there are no reports of products that result directly
À
from B H activation (II). Although oxidative addition of
three-coordinate borane species is well-established,[15] oxida-
tive addition of four-coordinate, base-stabilized borane com-
pounds is less common.[16] Base-stabilized boryl compounds
are known,[17] but the key hydrido complexes (II) remain
elusive. The isolation of such complexes would thus be
significant. Herein, we report the synthesis of such species,
dinuclear metal–metal bonded complexes with base-stabi-
À
initial B H activation at the metal center, suggested inter-
mediates include metal amine–borane sigma complexes (I)
À
À
and the products of B H activation (oxidative addition),
namely base-stabilized boryl compounds (II).
lized boryl and hydride ligands (II), which result from B H
activation at monometallic rhodium(I) fragments. The for-
mation of these metal–metal bonded complexes also has
significance with regard to colloidal[18] or cluster[10,11] rho-
dium(0) catalysts, which form by reduction of monometallic
rhodium(I) precursors by amine–boranes, providing insight
into the bonding modes of amine–boranes at more than one
metal center.[2,11]
[*] Dr. A. B. Chaplin, Prof. A. S. Weller
Department of Inorganic Chemistry, University of Oxford
Oxford, OX1 3QR (UK)
We have previously reported on the rhodium(III) com-
plexes [Rh(PR3)(binor-S)][BArF ] (1-R[BArF ]), where R =
4
4
Cy, iPr; binor-S = norbornadiene dimer, C14H16; ArF = C6H3-
E-mail: andrew.weller@chem.ox.ac.uk
À
(CF3)2, that contain a ligand that adopts a C C sigma
[**] We thank the EPSRC for support.
interaction with the metal.[19] These complexes act as a
latent source of the {Rh(PR3)}+ fragment by reductive
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905185.
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elimination of binor-S on addition of Lewis bases.[19,20]
short one. Although this is a new motif in metal–boron
chemistry, corresponding silyl and hydridoborate complexes
have been reported that show closely related bonding motifs
to both the s/agostic interaction (A[25] and B[26]) and the
Addition of H3B·NMe3 to 1-R[BArF ] results in the slow (ca.
4
2 h) formation of the dimeric complexes [Rh2(PR3)2(H)2(m-
H2BNMe3)2(m-H3B·NMe3)][BArF ]2 (2-R[BArF ]2), which are
4
4
isolated as air-sensitive crystalline solids in greater than 70%
yield. These complexes were characterized by NMR spec-
troscopy, X-ray crystallography, and ESI-MS. The solid-state
structure of 2-Cy[BArF ]2 was of sufficient quality that the
4
hydrogen atoms associated with the metal and boron centers
were reliably located (Figure 1). The X-ray crystal structure
bridging s ligand (C[27] and D[23]). Preliminary calculations
afforded the same structural core as is observed experimen-
tally, and the calculated bond orders are consistent with the
proposed structure (see Supporting Information); a starting
structure that places one boryl ligand at each metal center,
such as the silyl ligands in B, optimizes to the experimentally
observed structure, with both on the same metal (similar to
complex A).
Figure 1. Solid-state structure of 2-Cy2+. Thermal ellipsoids set at 50%
probability; phosphine H-atoms omitted for clarity. Selected bond
lengths [ꢀ] and angles [8]: Rh1–Rh2 2.6292(4), Rh1–P1 2.4025(10),
Rh2–P2 2.3559(10), Rh1–B1 2.747(5), Rh1–B2 2.086(4), Rh1–B3
2.086(4), Rh2–B1 2.757(5), Rh2–B2 2.217(4), Rh2–B3 2.233(4), Rh1–
H0A 1.46(4), Rh2–H0B 1.50(4), Rh1–H1A 1.88(5), Rh2–H1B 1.86(5),
B1–H1A 1.21(5), B1–H1B 1.22(5), B1–H1C 1.15(5), B2–H2A 1.43(5),
B2-H2B 1.12(5), B3–H3A 1.34(5), B3–H3B 1.10(5), P1-Rh1-Rh2
176.55(3), Rh1-Rh2-P2 171.91(3).
In solution, the solid-state structures are retained. For
example, in the 31P{1H} NMR spectrum of 2-iPr[BArF ]2, two
4
different 31P environments are observed that show coupling to
À
both rhodium atoms, thus confirming the M M bond. At
298 K, the high-field region of the H NMR spectrum shows
1
four signals. Two integral 1H resonances at d = À17.75 ppm
and d = À21.16 ppm do not sharpen on 11B decoupling; this
observation, combined with their chemical shift, identifies
them as rhodium hydrides (e.g., H0A and H0B, Figure 1). The
equivalent a-B agostic hydrogen atoms (e.g., H3A), assigned
to a signal at d = À9.46 ppm (br, 2H), do not exchange with
the terminal B-H atoms (e.g., H3B), which are observed at
d = 5.45 ppm (vbr, 2H). Thus, these 3c-2e interactions appear
to be relatively strong. The intact s ligand hydrogen atoms
(e.g., H1A, H1B, and H1C) are observed as a broad 3 H signal
at d = À3.56 ppm, demonstrating site-exchange between
bridging and terminal B-H motifs. Cooling the NMR sample
down slows this site exchange process; at 200 K this resonance
resolves into two signals, which are observed at d = À1.90
(vbr, approx. 2H) and À8.31 ppm (br, 1H). At 190 K, the
former resonance splits into a very broad signal at d =
À3.90 ppm, and another that is presumably also very broad
and could not be definitively assigned. The latter peak
remains essentially unchanged. These observations support
a mechanism that involves two site-exchange processes; one
of 2-iPr[BArF ]2 was also obtained, but, owing to extensive
4
disorder, only the gross heavy-atom core could be located.
The solution-phase NMR data for both dimeric complexes are
essentially the same, indicating similar structures.
In the solid-state, 2-Cy2+ exists as a dicationic dimer
formed from two {Rh(PCy3)}+ fragments that are bridged by
three H3B·NMe3 ligands. Two of these ligands have under-
À
gone B H activation, which results in two ligands in which
amine–boranes (B2 and B3) form both a s-bond (base-
stabilized boryl ligand) to Rh1, and an a-B agostic[21] B H
À
interaction with Rh2 (Figure 1). The third H3B·NMe3 ligand
remains intact and bridges both metal atoms by two three-
À
center–two-electron (3c–2e) interactions. The Rh B distan-
À
ces reflect these different bonding modes, viz. Rh1 B3
À
2.086(4) ꢀ, Rh2 B3 2.233(4) and Rh1···B1 2.747(5) ꢀ. The
À
first two bonds are consistent with those that have been
reported previously,[22,23] whereas the amine–borane bridging
two metal centers is, as far as we are aware, unique. The
related ditantalum BH3 adduct,[24] and monomeric rhodium-
(III) sigma complex of H3B·NHMe2,[6] both have much
that makes all three B H groups equivalent, and a second,
À
with a lower energy barrier, that exchanges just two B H
groups. The chemical shifts for the other hydride signals
remain unchanged on cooling, although one of the rhodium
hydrides resolves into a sharp doublet of doublets at 273 K.
The 11B NMR spectrum shows two environments at d = + 37.3
and À9.6 ppm in an approximate 2:1 ratio. These resonances
À
shorter M···B distances (ca. 2.3 ꢀ). The Rh Rh distance
À
(2.6292(4) ꢀ) is consistent with a Rh Rh single bond, albeit a
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are assigned to the base-stabilized boryl (B2/B3) and borane
(B1) ligands, respectively.[28] ESI-MS shows a strong molec-
ular-ion peak for {2-iPr[BArF ]}+ (m/z = 1608.5, calcd 1608.5),
4
thus confirming the empirical formula.
Monitoring of the formation of 2-iPr[BArF ]2 in situ by
4
NMR spectroscopy indicates the formation of a rhodium(I)
intermediate upon addition of H3B·NMe3 to 1-iPr[BArF ],
4
with the concomitant elimination of binor-S. The reaction
then proceeds (1.5 h, see the Supporting Information) to
afford 2-iPr[BArF ]2 as the only observed product. NMR
4
spectroscopy and in situ ESI-MS studies identified this
intermediate as [Rh(PiPr3)(H3B·NMe3)2][BArF ] (3-iPr-
4
[BArF ]). Notably, broad signals are observed at d =
4
1
À3.25 ppm (6H) in the H NMR spectrum, at d = 8.3 ppm in
the 11B NMR spectrum, and a doublet is observed at d =
85.0 ppm (1JRhP 169 Hz) in the 31P{1H} NMR spectrum;
cooling the sample to 190 K does not change these spectra
significantly. ESI-MS shows the molecular ion for 3-iPr+, m/
z = 409.26 (calcd 409.26). 3-iPr[BArF ] presumably proceeds
H3B·NHMe2 to 2-iPr[BArF ]2 (20 mol% rhodium, 15 h, 100%
4
4
to 2-iPr[BArF ]2 by B H oxidative addition, followed by
conversion) does result in slow dehydrocoupling and the
À
4
dimerization and then isomerization of the rhodium(III)
formation of (H2BNMe2)2.[3] Free H3B·NMe3 is formed at the
hydrido boryl compound. Precedent for the suggested bis-
onset of catalysis, demonstrating exchange of the amine–
(amine–borane) binding mode in 3-iPr[BArF ] comes from
boranes. Complex 1-iPr[BArF ] is also a competent catalyst,
4
4
the recently reported rhodium(I) complex [Rh{P(C5H9)2(h2-
with quantitative conversion occurring in a shorter reaction
time (5 h). Mercury poisoning experiments showed no
inhibition in rate, thus suggesting a homogenous process.
Although a number of species are present in solution (as
observed by NMR spectroscopy) during these catalytic runs
we have not been able to definitively assign them to analogues
of either 2-iPr[BArF ]2 or 3-iPr[BArF ], and so cannot com-
C5H7)}(h2-H3B·NMe3)(h1-H3B·NMe3)][BArF ].[7]
4
4
4
ment on their role in the mechanism of H3B·NHMe2
dehydrocoupling.
À
In conclusion, we report complexes arising from B H
activation of an amine–borane at a low-valent, late transition
metal center, that show a unique dimeric motif with three
different amine–borane activation modes. The isolation of
these dimeric complexes and the observation of rhodium(I)
intermediates during their formation provide a link between
homogenous and colloidal heterogeneous rhodium systems
used in the dehydrocoupling reactions of amine–boranes and
is thus significant to both areas.
Crossover experiments between 2-iPr[BArF ]2 and 2-Cy-
4
F
À
[BAr ]2 show that the Rh Rh bond remains intact on the
4
laboratory timescale, with no mixed-phosphine species
observed. However, addition of relatively strong Lewis
bases results in fragmentation of the dimeric motif. Addition
of acetonitrile results in elimination of H3B·NMe3 and the
formation of a rhodium(I) complex, which was tentatively
assigned as [Rh(PiPr3)(NCMe)3][BArF ]. This result demon-
4
Experimental Section
À
strates that the B H activation of H3B·NMe3 is reversible.
A solution of 1-iPr[BArF ] (67 mg, 0.051 mmol) and H3B·NMe3
4
Addition of PiPr3 to 2-iPr[BArF ]2 gives a 1:1 mixture of
4
(7.5 mg, 0.103 mmol) in 1,2-difluorobenzene (2 mL) was stirred at
room temperature for 20 min. Layering with pentane and cooling to
rhodium(I) and rhodium(III) complexes, 4-iPr[BArF ] and 5-
4
iPr[BArF ] respectively, each with a coordinated H3B·NMe3
58C afforded 2-iPr[BArF ]2 as pale yellow crystals (1,2-difluoroben-
4
4
zene solvate). Yield: 0.058 g (88%). 1H NMR (CD2Cl2, 500 MHz):
group. Independent synthesis confirmed their assignment (for
their solid-state structures, see the Supporting Information),
and they are closely related to the recently reported PiBu3
analogues.[6,8] The mechanism for the formation of 5-iPr-
d = 7.68–7.79 (m, 16H, BArF ), 7.56 (br, 8H, BArF ), 5.45 (vbr,
4
4
fwhm = 360 Hz, 2H, terminal-H2BNMe3; fwhm = full width at half
maximum), 2.76 (s, 18H, H2BNMe3), 2.51 (s, 9H, H3B·NMe3), 2.26
(apparent octet, J = 7 Hz, 6H, PCHMe2), 1.32 (br, 36H, PCHMe2),
À3.56 (vbr, fwhm = 220 Hz, 3H, H3B·NMe3), À9.46 (vbr, fwhm =
110 Hz, 2H, B-agostic-H2BNMe3), À17.75 (m, 1H, RhH-
(BH2NMe3)2), À21.16 ppm (br, 1H, RhH); 11B NMR (CD2Cl2,
[BArF ] is yet to be determined.
4
Complexes 2-R[BArF ]2 do not undergo dehydrocoupling
4
owing to the lack of an NH proton. In contrast, addition of
Angew. Chem. Int. Ed. 2010, 49, 581 –584
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www.angewandte.org
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Communications
160 MHz): d = 37.3 (br, 2B, H2BNMe3), À6.6 (s, 2B, BArF ), À9.6 ppm
A. Friedrich, M. Drees, S. Schneider, Angew. Chem. 2009, 121,
922 – 924; Angew. Chem. Int. Ed. 2009, 48, 905 – 907.
[10] J. L. Fulton, J. C. Linehan, T. Autrey, M. Balasubramanian, Y.
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11949.
4
(br, 1B, H3B·NMe3); 31P{1H} NMR (CD2Cl2, 202 MHz): d = 69.0 (dd,
1
1
1P, JRhP = 104 Hz, 2JRhP = 68 Hz, RhPiPr3), 58.1 ppm (dd, 1P, JRhP
=
124 Hz, 2JRhP = 59 Hz, RhPiPr3(BH2NMe3)2); ESI-MS (1,2-F2C6H4,
608C, 4.5 kV) positive ion: m/z, 1608.4868 [M + (BArF )]+ (calcd
4
1608.4709); IR (KBr): n(BH) 2484 (w), 2383 (w); n(Rh-HB)
2026 cmÀ1 (br, s); Anal. calcd for C97H106B5F50N3P2Rh2·C6H4F2
(2585.66 gmolÀ1): C 45.06, H 4.13, N 1.63; found: C 45.27, H 4.36,
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N 1.82. 2-Cy[BArF ]2 was prepared in an analogous manner in 70%
4
yield. Full details of the synthesis, characterization, and reactivity of
2-R[BArF ]2 and 3-iPr[BArF ] are provided in the Supporting
4
4
Information.
Crystallographic data for 2-Cy[BArF ]2: C109H126B5F48N3P2Rh2,
4
3
¯
Mr = 2711.94, pale yellow block, 0.28 ꢁ 0.22 ꢁ 0.20 mm , triclinic, P1,
a = 17.21390(10),
b = 17.64870(10),
c = 22.98490(10) ꢀ,
a =
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Received: September 16, 2009
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Published online: November 24, 2009
À
Keywords: B H activation · boranes · oxidative addition ·
rhodium · X-ray diffraction
.
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