Amino-borane oligomers bound to a Rh(I) metal fragment

Article abstract of DOI:10.1039/c003055d

Coordination complexes of previously observed intermediates, H ₃B·NMe₂BH₂·NMe₂H and [H₂BNMeH]₃, in the transition metal catalysed dehydrocoupling of H₃B·NMe₂H and H ₃B·NMeH₂ have been isolated and structurally characterised using the [Rh{PR′₂(η²-C ₅H₇)}]⁺ fragment. Their onward reactivity shows that further dehydrogenation is not a simple intramolecular process. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c003055d

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Amino-borane oligomers bound to a Rh(I) metal fragmentw
Romaeo Dallanegra, Adrian B. Chaplin, Jennifer Tsim and Andrew S. Weller*
Received (in Cambridge, UK) 12th February 2010, Accepted 4th March 2010
First published as an Advance Article on the web 23rd March 2010
DOI: 10.1039/c003055d
Coordination complexes of previously observed intermediates,
H3BꢀNMe2BH2ꢀNMe2H and [H2BNMeH]3, in the transition
metal catalysed dehydrocoupling of H3BꢀNMe2H and
H3BꢀNMeH2 have been isolated and structurally characterised
using the [Rh{PR0 (g2-C5H7)}]+ fragment. Their onward
2
reactivity shows that further dehydrogenation is not a simple
intramolecular process.
Scheme 2 Synthesis of bis-s-amine-borane complexes.
with [Rh{PR02(Z2-C5H7)}]+ (R0 = cyclo-C5H9), 1[BArF4] and
2[BArF4], respectively (Scheme 2), which show an unique
bis-amine-borane bonding mode.12 These complexes proceed
to afford the products of dehydrocoupling, II and V, respec-
tively, both stoichiometrically and catalytically, although
intermediate species were not observed under the conditions
used. In the solid-state both show an intramolecular BHꢀ ꢀ ꢀHN
hydrogen bond. One possible mechanism for the formation of
II could be simple intramolecular H2 elimination to form III
directly on the metal centre, which then proceeds in a further
intramolecular dehydrocoupling to give II. Trimer V could
form in a similar fashion via involvement of VII. In order
to probe these events we have targeted the synthesis
of III and VII with the [Rh{PR02(Z2-C5H7)}]+ fragments
The transition metal catalysed dehydrocoupling of amine- and
phosphine-boranes is attracting considerable current attention
because of its central role in the generation of new group 13/15
materials and possible future energy transport vectors for the
delivery of H2.1 Secondary and primary amine-borane adducts
H3BꢀNMe2H (I) and H3BꢀNMeH2 (IV) undergo dehydro-
coupling to ultimately give [H2BNMe2]2 (II), [HBNMe]3 (V)
or oligomeric/polymeric2 materials alongside the release of H2
(Scheme 1). Experimental and computational studies have
established that the first step is dehydrogenation to form an amino-
borane.3–6 However, subsequent coupling/dehydrogenation
steps are less well resolved. Oligomeric H3BꢀNMe2BH2ꢀ
NMe2H III3,5,7,8 and [H2BNMeH]3 VII5,9 have been observed
as intermediates during the dehydrocoupling of I and IV,
respectively, using a number of catalyst systems; although
details of how these intermediates proceed to give the final
products are still scarce,10 and multiple pathways may be
operating.4,7,11 Such mechanistic information is important if
fine control over these processes is to be achieved.
i
(R0 = cyclo-C5H9, Pr).
Addition of III5,13 or the eee-isomer of VII14 to
the
complexes
[Rh{PR02(Z2-C5H7)}(Z6-C6H5F)][BArF
]
4
(ArF = C6H3(CF3)2; R0 = cyclo-C5H9,15 iPr see ESIw) in
1,2-F2C6H4 solution resulted in the clean and quantitative
(by NMR spectroscopy) formation of the new com-
plexes [Rh{PR02(Z2-C5H7)}(Z2 : Z1-III)][BArF4], 3[BArF4],
We have recently reported the synthesis, solution and
solid-state structures of amine-boranes I and IV complexed
and [Rh{PR02(Z2-C5H7)}(Z1 : Z1 : Z1-VII)][BArF
]
4
4[BArF
]
4
i
(R0 = cyclo-C5H9 a; Pr b), Scheme 3.z These new complexes
have been characterised by NMR spectroscopy, ESI-MS and
X-ray crystallography (Fig. 1), which show them to be sigma
amine-borane adducts that bond with the metal centre via
three 3 centre–2 electron bonds.y We discuss in detail the
i
analytical data for the R0 = Pr complexes—those for the
R0 = cyclo-C5H9 ligand are essentially the same (see ESIw).
The solid-state structure of 3b[BArF4] shows that the
diborazane ligand binds to the metal centre in a Z2 : Z1
Rhꢀ ꢀ ꢀHB coordination mode. The Rhꢀ ꢀ ꢀB distances, viz.
2.242(5) and 2.626(4) A, respectively, are consistent with this
Scheme 1 Products and intermediates of the dehydrocoupling of I
and IV.
Department of Chemistry, Inorganic Chemistry Laboratory,
University of Oxford, Oxford, UK OX1 3QR.
E-mail: andrew.weller@chem.ox.ac.uk; Tel: +44 (0)1865 285151
w Electronic supplementary information (ESI) available: Full experimental
and analytical data. X-Ray structure of [Rh{PiPr2(Z2-C5H7)}(Z6-C6H5F)]-
[BArF4]. CCDC 767949–767951. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c003055d
Scheme 3 Synthesis of the new complexes. Anions not shown.
ꢁc
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In CD2Cl2 solution at room temperature 3b[BArF4] shows
two BHꢀ ꢀ ꢀRh signals at d ꢂ0.02 (3H) and d ꢂ2.26 (2H) that
sharpen on decoupling 11B, the former also resolves into a
doublet, J(RhH) = 21 Hz. These are assigned to BH3 and BH2
groups, respectively, that undergo rapid site exchange between
coordinated and terminal BH groups. These signals are also
shifted upfield from the free ligand,5 consistent with coordina-
tion to the metal. Cooling to 200 K results in the collapse of
the 3H signal into a very broad 2H signal at d ꢂ1.34, assigned
to Rh–H–B, indicating that BH3 site exchange has been
slowed.3,18 The corresponding terminal BH signal is not
resolved. The BH2 signal does not split or change significantly
in chemical shift at 200 K, while the signal due to the alkene
(C7/C8) remains sharp, indicating that the BH2 group is still
undergoing site exchange at 200 K. In the 11B NMR spectrum
a broad peak centred at d ꢂ4.29 is observed. For 4b[BArF4] the
room temperature NMR data show that the cyclic borane
ligand is undergoing site exchange, with one Rhꢀ ꢀ ꢀHB
(3H, d ꢂ3.44) and one NMe3 (9H, d 2.62) signal observed.
The 11B NMR spectrum is a doublet of doublets at d ꢂ5.45,
J(HB) = 118, 87 Hz; the latter reduced coupling constant is
consistent with a Rhꢀ ꢀ ꢀHB interaction.19 Cooling to 200 K
halts the fluxional process and two very broad signals are
1
observed at d ꢂ8.80 (1H) and d ꢂ1.11 (2H) in the H NMR
spectrum. For both complexes the 31P{1H} NMR spectrum
shows a single environment coupling to 103Rh, and ESI-MS
shows strong molecular ions. These data show that the
solid-state structures are retained in solution.
When pure samples of 3a[BArF
] ] are
4
and 4a[BArF
4
dissolved in 1,2-F2C6H4 there is no reaction over 48 hours,
with unchanged material recovered. This is in contrast to
1[BArF4] and 2[BArF4] that proceed in six hours to give the
products of dehydrocoupling (Scheme 2). These observations
indicate that simple intramolecular dehydrogenation of III
and VII is not operating. This is the same as found for
[Rh(PiBu3)2(Z2-III)][BArF
]
that remains unchanged in
4
solution in the absence of excess amine-borane.3
The stability of pure 3a[BArF4] and 4a[BArF4] towards
intramolecular dehydrogenation prompted us to study their
reactions under catalytic conditions (Scheme 4). Addition of I
to a solution of 3a[BArF4] (20 mol%) immediately formed
complex 1[BArF4], with III displaced. After 26 hours all I and
Fig. 1 Solid-state structure of 3b[BArF4] (a), and 4b[BArF4] (b);
ellipsoids are depicted at the 50% probability level. Anions and minor
disordered components omitted for clarity. Selected bond lengths (A)
and angles (1). 3b[BArF4]: Rh1–P1, 2.2058(9); Rh1–B1, 2.242(5);
Rh1–B2, 2.626(4); Rh1–H1A, 1.85(5); Rh1–H1B, 1.84(5); Rh1–H2A,
1.96(4); Rh1–C7, 2.107(4); Rh1–C8, 2.121(4); B1–N1–B2, 103.3(3);
N1–B2–N2, 112.7(3); Rh1–H2A–B2, 113(3). 4b[BArF4]: Rh1–P1,
2.2354(8); Rh1–B1, 2.693(4); Rh1–B2, 2.808(4); Rh1–B3, 2.761(4);
Rh1–H1A, 1.84(4); Rh1–H2A, 1.94(4); Rh1–H3A, 1.86(4); Rh1–C6,
2.096(3); Rh1–C7, 2.094(3); B1–N2–B2, 112.9(3); B2–N1–B3, 111.7(3);
B3–N3–B1, 112.6(3); Rh1–H1A–B1, 123(3); Rh1–H2A–B2, 128(3);
Rh1–H3A–B3, 128(3).
III were consumed to give II as the major B–N product by 11
B
description.16 By contrast, III interacts with [Rh(PiBu3)2]+
through only the BH3 group.3 For 4b[BArF4] cycloborazane
VII binds to the metal through three long Z1 Rhꢀ ꢀ ꢀHB
interactions [2.693(4), 2.761(4), 2.808(4) A]. This tridentate
coordination mode for VII is reminiscent of that observed for
complexes of the [B3H8]ꢂ anion.17 For both complexes the
interaction of three BH bonds with the [Rh{PR02(Z2-C5H7)}]+
fragment is similar to that observed for 1[BArF ] and 2[BArF ].12
Scheme 4 Reactivity of complexes 3a[BArF4] and 4a[BArF4].
4
4
ꢁc
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NMR spectroscopy. When only III was added to 3a[BArF
]
4
y Crystallographic data. 3b[BArF4]: C47H51B3F24N2PRh, M = 1266.21,
triclinic, P1 (Z = 2), a = 13.11340(10) A, b = 15.09320(10) A, c =
ꢀ
(20 mol%), conversion to II was slower but still went to
completion after 96 hours. For 4a[BArF4] (20 mol%) addition
of IV results in the initial formation of 2[BArF4] with VII
displaced, and the complete conversion of IV to V in 14 hours;
but by contrast with the reaction of 3a[BArF4] with III, VII
was still persistent in solution after a further 127 hours.
Addition of VII to 4a[BArF4] (20 mol%) gave very slow
formation of V although VII was still present after 141 hours
(see ESIw), Scheme 4. When a 1 : 2 mixture of the eee and eea
isomers of VII was used, there is no significant change in the
ratio of isomers over this time period showing that an isomer
effect was not operating in the dehydrocoupling process here.
15.6507(2) A, a = 84.6142(4)1, b = 78.4318(4)1, g = 86.9282(4)1. V =
3019.40(5) A3, T = 150(2) K, 12 244 unique reflections [Rint = 0.0154].
Final R1 = 0.0466 [I > 2s(I)]. 4b[BArF4]: C46H51B4F24N3PRhꢀ
12(C5H12),
M
=
1315.09, monoclinic, P21/c (Z
=
4),
a
=
=
13.22060(10) A,
b
=
18.33470(10),
c
=
24.0722(2) A,
b
98.7365(4)1. V = 5767.30(7) A3, T = 150(2) K, 11 699 unique
reflections [Rint = 0.0231]. Final R1 = 0.0415 [I > 2s(I)].
1 (a) T. J. Clark, K. Lee and I. Manners, Chem.–Eur. J., 2006, 12,
8634–8648; (b) C. W. Hamilton, R. T. Baker, A. Staubitz and
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a simple metal-based
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intramolecular dehydrogenation involving the oligomeric
intermediates III and VII is not occurring with these systems,
and
a more complex regime, possibly involving B–N
bond cleavage7 or a bi-molecular outersphere mechanism,6a
is probably operating. Moreover the data also suggest that in
these systems VII is not a viable intermediate in the overall
dehydrocoupling, while III is. The former observation is in
contrast to results previously reported by Manners and
co-workers that show using colloidal Rh(0) catalysts VII is
an observed intermediate,5,9,20 demonstrating that even
though the final products are the same, the pathways to reach
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Notes and references
z Selected NMR data: 3b[BArF4] 1H NMR (500 MHz, CD2Cl2, 200 K):
d 7.74 (s, 8H, BArF4), 7.56 (s, 4H, BArF4), 4.25 (br s, 1H, NH), 3.70
(s, 2H, HCQCH), 2.58–2.53 (m, 12H, NH–CH3, N–CH3), 1.99
(m, 2H, PiPr–CH), 1.88 (br m, 1H, PCyp CH), 1.82 (dd, 2H, J(HH)
13, J(PH) 12, PCyp0–CH2), 1.37 (br dd, 2H, J(PH) 46, J(HH) 13
PCyp0–CH2), 1.14 (dd, 6H, J(PH) 14, J(HH) 7, PiPr–CH3), 1.09
(dd, 6H, J(PH) 14, J(HH) 7, PiPr–CH3), ꢂ1.34 (br, 2H, BH3), ꢂ2.48
(br, 2H, BH2). The remaining B–H signal was not observed, presumably
it was broad and/or obscured by the aliphatic signals. H{11B} NMR
1
(500 MHz, CD2Cl2, 200 K): ꢂ1.35 (v br, 2H, RhH2BH), ꢂ2.49
(br, 2H, BH2). 31P{1H} NMR (202 MHz, CD2Cl2, 200 K): d 121.59
[d, J(RhP) 172]. 11B NMR (160 MHz, CD2Cl2, 200 K): d very broad,
no clear signals observed. ESI-MS (C6H4F2, 60 1C, 4.5 kV): positive
ion: m/z 403.2066, [M]+ (23% calcd 403.2092). 4b[BArF4]: 1H NMR
(500 MHz, CD2Cl2, 200 K): d 7.73 (s, 8H, BArF4), 7.55 (s, 4H, BArF4),
3.73 (s, 2H, HCQCH), 3.20 (br s, 3H, NH), 2.56 (v br, 9H, N–CH3),
2.00 (m, 2H, PiPr–CH), 1.90 (br m, 1H, PCyp0–CH), 1.70 (virtual
triplet, 2H, J(HH) E J(PH) 13, PCyp0–CH2), 1.16 (dd, 12H, J(HH) 7,
J(PH) 14, PiPr–CH3) 1.23–0.97 (partially obscured multiplet, 2H,
PCyp0–CH2), ꢂ1.11 (br, 2H, RhHBH), ꢂ8.80 (v br, 1H, RhHBH).
The remaining RhHBH signals were not unequivocally identified,
presumably they are broad and/or obscured by the aliphatic signals.
1H{11B} NMR (500 MHz, CD2Cl2, 200 K): d ꢂ1.11 (br, 2H, RhHBH),
ꢂ8.85 (br, 1H, RhHBH). 31P{1H} NMR (202 MHz, CD2Cl2, 200 K): d
119.33 [d, J(RhP) 162]. 11B NMR (160 MHz, CD2Cl2, 200 K): d very
broad, no clear signal observed. ESI-MS (C6H4F2, 60 1C, 4.5 kV):
positive ion: m/z 416.2230, [M]+ (calcd 416.2218).
20 Shimoi and co-workers have recently suggested that VII may
undergo metal promoted dehydrogenation to give
V using
Cr–carbonyl catalysts in the dehydrocoupling of H3BꢀNH2Me.
See ref. 6a.
ꢁc
This journal is The Royal Society of Chemistry 2010
3094 | Chem. Commun., 2010, 46, 3092–3094