The hafnium-mediated NH activation of an amido-borane

Article abstract of DOI:10.1039/c1cc11320h

Treatment of Cp₂HfCl₂ with two equivalents of LiNH₂BH(C₆F₅)₂ in toluene solution yields Cp₂Hf{NHBH(C₆F₅)₂}, which has been crystallographically c

Full text of DOI:10.1039/c1cc11320h

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COMMUNICATION
The hafnium-mediated NH activation of an amido-boranew
Elizabeth A. Jacobs, Anna-Marie Fuller, Simon J. Lancaster* and Joseph A. Wright
Received 7th March 2011, Accepted 5th April 2011
DOI: 10.1039/c1cc11320h
Treatment of Cp2HfCl2 with two equivalents of LiNH2BH(C6F5)2
in toluene solution yields Cp2Hf{NHBH(C6F5)2}, which has been
crystallographically characterised. The otherwise base-free
[NHBH(C6F5)2] complex is stabilised by an agostic interaction.
[NH2BH(C6F5)2] (2). The identity of 2 was confirmed through
crystallisation of the 12-crown-4 chelate.w
Following similar reaction conditions to those described for
the synthesis of Cp2Zr(Cl)NH2BH3,2 the treatment of Cp2ZrCl2
with one equivalent of the thf solvate of 2 in toluene solution
between ꢁ78 1C and room temperature resulted in a product
mixture giving an extremely complex 1H NMR spectrum
with at least seven resonances in the cyclopentadienyl region
alone. Under these conditions, similarly complex spectra were
obtained in the analogous reaction with Cp2HfCl2. However,
on one occasion we were able to isolate a very small quantity
of a crystalline material from a thf/light petroleum solvent
mixture. Single crystal X-ray diffraction revealed the hafnocene
bis(amidoborane), Cp2Hf{NH2BH(C6F5)2}2 (3) as a tetrahydro-
furan (thf) solvate, the structure of which is described below. All
attempts to reproduce the synthesis and isolation of 3 have
proven unsuccessful.6
Group 4 metallocene complexes have recently been demon-
strated to be effective for the catalytic dehydrogenation of
amine–borane adducts. Manners and co-workers have proposed
a titanium amidoborane hydride, stabilised by agostic B–Hꢀ ꢀ ꢀTi
interactions, as an intermediate in the catalytic cycle.1 Roesler
et al. have isolated the zirconium amidoborane hydride,
Cp2Zr(H)NH2BH3 which also has an agostic b–H interaction.2
We have shown that amidoborane complexes of all the
group 4 metals are accessible through the reaction of the
Brønsted acidic adduct H3NꢀB(C6F5)3 with metal bases.3 In
particular circumstances, further NH activation resulted in the
formation of nitridoborane complexes.4
The elusive nature of compound 3 was addressed by the
screening of a series of alternative reaction conditions. In one
such experiment the treatment of Cp2HfCl2 with two equivalents
of the ether-free salt 2 in toluene between ꢁ78 1C and room
temperature resulted in a crude reaction mixture giving only
three resonances in the cyclopentadienyl region of the 1H
NMR spectrum, indicating three components in the crude
reaction mixture (Fig. S3, ESIw). At d 5.29, 5.28 and 5.16 all
three signals were found at lower frequency than Cp2HfCl2,
which was not evident in the spectrum. Fractional crystal-
lisation from mixtures of toluene and light petroleum yielded
colourless blocks. Characterisation by multinuclear NMR
spectroscopy indicated the formation of a new hafnocene
complex, with the signal at d 5.16 proving to belong to the
isolated crystals. Broad 1H NMR resonances at d 7.59 and
ꢁ0.83 were consistent with NH and BH groups, respectively,
We have recently developed a convenient synthesis of
the dimethylsulfide adduct of bis(pentafluorophenyl)borane.5
This adduct undergoes facile donor exchange with ammonia to
yield H3NꢀBH(C6F5)2 (1) (Scheme 1).w Herein we communicate
the first complexes of the [NH2BH(C6F5)2] ligand and the
subsequent NH activation to form a remarkable imidoborane
hafnium complex.
Treatment of a toluene solution of 1 with n-BuLi results
in deprotonation and formation of the lithium salt [Li]
but did not give the expected integration for thf-free 3. The 11
B
and 19F NMR spectra were consistent with a single BH(C6F5)2
environment. The structure was ultimately determined by
X-ray diffraction methods revealing 4 as the hafnocene complex
Cp2Hf{NHBH(C6F5)2}. In contrast to 3, the synthesis of 4 has
proven to be fully reproducible.z
Scheme 1 The synthesis of complexes 1–4.
Energy Materials Laboratory, School of Chemistry, University of
East Anglia, Norwich, NR4 7TJ, UK.
E-mail: S.Lancaster@uea.ac.uk; Fax: +44 (0)1603 592003;
Tel: +44 (0)1603 592009
w Electronic supplementary information (ESI) available: Compounds
1–4, ORTEP figures for 1 and 2, a table of hydrogen bonds for 3, a 1H
NMR spectrum of the crude reaction mixture for 4, synthetic details
for compounds 1, 2 and 3. CCDC 814619–814622. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc11320h
Fig. 1 shows the structure of the bis(amidoborane) complex
3.7 There are two thf molecules within the crystal lattice. There
are a further two thf molecules (one of which is disordered
over two positions), engaged in hydrogen bonding to the NH2
groups (Table S1, ESIw). The second hydrogen atom on each
NH2 group is involved in an intramolecular hydrogen bond to
an o-F atom.8
c
5870 Chem. Commun., 2011, 47, 5870–5872
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Chart 1 Resonance forms of compound 4.
wedge between the two cyclopentadienyl groups. While a
hydrogen atom was located on the boron atom, the data did
not suffice to crystallographically locate hydrogen bound to
nitrogen. The presence of a single hydrogen atom on nitrogen
was inferred by the requirement for charge balance with the
[Cp2Hf]2+ fragment and the structure modelled accordingly.
This treatment is supported by the single proton integration of
Fig. 1 ORTEP representation of the structure of 3ꢀ4(thf) showing
50% probability ellipsoids; two molecules of thf, and hydrogen atoms
on the Cp ring and thf molecules have been omitted for clarity.
Selected distances [A] and angles [1]: Hf(1)–N(11) 2.283(2),
Hf(1)–N(41) 2.249(2), Hf(1)–centroid[C(1)–C(5)] 2.501(3), Hf(1)–
centroid[C(6)–C(10)] 2.498(3), Hf(1)ꢀ ꢀ ꢀH(42) 2.09(3), N(11)–B(12)
1.592(4), N(41)–B(42) 1.538(4); N(11)–Hf(1)–N(41) 77.30(8),
centroid[C(1)–C(5)]–Hf(1)–centroid[C(6)–C(10)] 131.11, B(12)–N(11)–
Hf(1) 122.05(17), B(42)–N(41)–Hf(1) 90.98(15).
1
the resonance at d 7.59 ppm in the H NMR spectrum. The
coordination environment of the hafnium atom is completed
by the agostic interaction described below. Formulation of
the [NHBH(C6F5)2] ligand as an imidoborane is at odds with
the observation that at 2.282(6) A the Hf–N distance is similar
to that found for 3 and Cp2Hf(CH3){NH2B(C6F5)3} and
substantially longer than those previously reported for hafnium
imides.9 A number of resonance forms can be drawn for complex
4 (A–C, Chart 1).10 The pattern of relatively long Hf–N, short
B–N and long Hf–H distances suggest that, rather than A, the
greatest contribution is from resonance structure B.
The bonding of the Cp2Hf fragment is unremarkable.
The amidoborane ligands exhibit distorted tetrahedral
geometries about the nitrogen centres. For Hf(1)–N(11)–B(12)
the angle is 122.05(17)1, which is more obtuse than the
tetrahedral ideal but smaller than the 135.14(17)1 found for
Cp2Hf(CH3){NH2B(C6F5)3},3b reflecting the intermediate
steric demands of the borane. While the two Hf–N bond
lengths are similar, the second Hf(1)–N(41)–B(42) angle is
only 90.98(15)1. This dramatic distortion is the result of a
b-agostic Hf–H interaction.
Despite this distinction the [NHBH(C6F5)2] group can be
formally regarded as a Lewis acid stabilised terminal (NH)
imido ligand. We have now been able to demonstrate the
successful stabilisation of substituent-free terminal amido,
imido and nitrido groups through this strategy.3,4 Perhaps
even more remarkable is that in addition to the bulky nitrogen
substituent those previously structurally characterised examples
of terminal imido zirconocene and hafnocene complexes also
required basic donor ligands. In 4 this role is taken by the
agostic BH group.
Fig. 2 depicts the solid state structure of complex, 4ꢀPhMe.
There is a single [NHBH(C6F5)2] ligand coordinated in the
Compounds 3 and 4 are rare examples of structurally
characterised hafnium complexes with agostic interactions.
At 2.09(3) A and 2.10(6) A the H–Hf distances are very close
to the sum of the covalent radii.11,12 In 3 particularly there are
similarities to the bonding patterns seen in Cp2ZrX{NH2BH3}
(where X = H, Cl).2 A titanium complex in which there is an
agostic interaction with a BH(C6F5)2 fragment has been
reported.13 While the B–H distance for the agostic hydride
in 4 is not significantly different from that in 1, the reduction in
the BH coupling constant from 112 Hz in 1 to 63 Hz in 4 is
consistent with a weakening of the B–H bond.14
The isolation of 4 in respectable yield requires the treatment of
Cp2HfCl2 with two equivalents of 2. The first step is presumably
a metathesis reaction to yield Cp2Hf(Cl){NH2BH(C6F5)2} (I)
(Scheme 2). From I, three mechanisms can be envisaged for
the generation of 4 (i–iii). In (i) the second equivalent of 2
functions as a base, deprotonating I and eliminating LiCl to
generate 4. If instead the second equivalent also reacts through a
metathesis pathway with I the result will be the hafnocene
bis(amidoborane) (3). Roesler et al. proposed essentially the
same mechanism to form Cp2Zr(NH2BH3)2; however, these
complexes were believed to be unstable with respect to
b-hydride elimination and the formation of amidoborane
hydrides Cp2Zr(H)(NH2BH3).2 If the current system follows
Fig. 2 ORTEP representation of the structure of 4ꢀPhMe showing
50% probability ellipsoids; hydrogen atoms except for H(1b) and
H(1n) and a toluene molecule have been omitted for clarity. Selected
distances [A] and angles [1]: Hf(1)–N(1) 2.282(6), Hf(1)–
centroid[C(1)–C(5)] 2.486(8), Hf(1)–centroid[C(1)–C(5)] 2.449(10),
Hf(1)ꢀ ꢀ ꢀH(1b) 2.10(6), N(1)–B(1) 1.509(10), centroid[C(1)–C(5)]–
Hf(1)–centroid[C(6)–C(10)] 133.65, B(1)–N(1)–Hf(1) 86.7(4).
c
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temperature. The solvent was removed under reduced pressure and
the crude product extracted with toluene (20 cm3). Colourless crystals
were obtained by concentrating the toluene solution, layering with
light petroleum and cooling to ꢁ25 1C for 3 days (0.07 g, 0.10 mmol,
11%). Elemental analysis (%): Found: C, 39.37; H, 1.69; N, 1.97.
Calculated for C22H12BF10NHf: C, 39.46; H, 1.81; N, 2.09. dH/ppm
(300.1 MHz, 293 K, C7D8): 7.59 (1H, s NH), 5.17 (10H, s, C5H5),
ꢁ0.83 (1H, q, JB,H 60, BH). dC/ppm (75.5 MHz, C7D8):
d
103.6 (C5H5). dB/ppm (96.3 MHz, 293 K, C7D8): ꢁ27.5 (d, JB,H 63).
dF/ppm (282.4 MHz, 293 K, C7D8): ꢁ133.7 (4F, br, o-F), ꢁ156.3
3
(2F, t, JF,F 21, p-F), ꢁ162.4 (4F, br, m-F).
Crystals were suspended in oil, mounted on a glass fibre and fixed in
the cold nitrogen stream on an Oxford Diffraction Xcalibur-3 CCD
diffractometer equipped with Mo-Ka radiation and graphite mono-
chromator. Intensity data were measured by o-scans.
Crystal data for 3ꢀ4thf: C34H16 B2F20HfN2, 4(C4H8O), M =
1321.01, Monoclinic, space group P21/n, a = 13.4352(3) A, b =
16.4726(3) A, c = 23.8938(4) A, b = 100.440(2)1, V = 5200.46(17) A3,
Z = 4, T = 140(2) K, l(Mo-Ka) = 0.71073 A, R1 [I 4 2s(I)] =
0.034, wR2 (all data) = 0.078.
Crystal data for 4ꢀPhMe: C22H12BF10HfN, C7H8, M = 761.76,
Orthorhombic, space group P212121, a = 6.707(5) A, b = 15.957(5) A,
c = 25.116(5) A, V = 2688(2) A3, Z = 4, T = 140(2) K, l(Mo-Ka) =
0.71073 A, R1 [I 4 2s(I)] = 0.035, wR2 (all data) = 0.062.
Scheme 2 Three proposed mechanisms (i–iii) for formation of 4.
an analogous pathway (ii) the formation of 4 follows from the
resulting hafnocene amidoborane hydride (II) undergoing an
intramolecular a-NH activation eliminating H2. However, we
recognise that 3 may also be an intermediate for direct
a-NH activation and formation of 4 through elimination of
H3NꢀBH(C6F5)2 (mechanism (iii)). Distinguishing between the
mechanisms is not straightforward. The yield of 4 is not
quantitative and the crude reaction mixture invariably contains
H3NꢀBH(C6F5)2 because this is the ultimate product of
hydrolysis of all the hafnium complexes in Scheme 2 and
therefore its presence does not immediately discount mechanism
(ii). [H2NB(C6F5)2]n the putative b-hydride elimination product
from mechanism (ii) has not been unequivocally identified but
the 19F NMR spectrum of the crude reaction mixture includes
additional resonances which may be consistent with this material.
This point requires further investigation.
1 M. E. Sloan, A. Staubitz, T. J. Clark, C. A. Russell, G. C. Lloyd-
Jones and I. Manners, J. Am. Chem. Soc., 2010, 132, 3831.
2 T. D. Forster, H. M. Tuononen, M. Parvez and R. Roesler, J. Am.
Chem. Soc., 2009, 131, 6689.
3 (a) A. J. Mountford, W. Clegg, R. W. Harrington,
S. M. Humphrey and S. J. Lancaster, Chem. Commun., 2005,
2044; (b) A. J. Mountford, W. Clegg, S. J. Coles,
R. W. Harrington, P. N. Horton, S. M. Humphrey,
M. B. Hursthouse, J. A. Wright and S. J. Lancaster, Chem.–Eur.
J., 2007, 13, 4535.
4 (a) A. J. Mountford, S. J. Lancaster and S. J. Coles, Acta
Crystallogr., Sect. C: Cryst. Struct. Commun., 2007, 63, m401;
(b) A. Fuller, W. Clegg, R. W. Harrington, D. L. Hughes and
S. J. Lancaster, Chem. Commun., 2008, 5776.
In summary, two equivalents of [Li][NH2BH(C6F5)2]
react with Cp2HfCl2 to yield the remarkable imidoborane
complex Cp2Hf{NHBH(C6F5)2} (4). Compound 4 and the
crystallographically characterised bis(amidoborane) 3 are
stabilised by agostic interactions. Confirmation of the mechanism
of formation of 4 and determination of its reactivity are
the subjects of current investigation. We aim to establish
whether the reaction proceeds through a bis(amidoborane)
intermediate, and if so to determine the conditions under
which it can be stabilised by thf solvation or whether
Cp2Hf{NH2BH(C6F5)2}2ꢀ4thf (3) is simply a by-product
obtained when the reaction is conducted in the presence of
tetrahydrofuran.
5 A. Fuller, D. L. Hughes, S. J. Lancaster and C. M. White,
Organometallics, 2010, 29, 2194.
6 The irreproducibility of the isolation of 3 precludes the reporting of
a synthetic procedure or the provision of supporting spectroscopic
characterisation.
7 With the exception of BH3 based amidoborane ligands, compound
3, to the best of our knowledge, is only the second example of a
transition metal complex containing two amidoborane groups, the
first being a copper complex: J. L. Schneider, V. G. Young Jr. and
W. B. Tolman, Inorg. Chem., 2001, 40, 165.
8 The formation of hydrogen-bonded 6-membered rings in which
organofluorine acts as an acceptor is common for compounds of
this type: A. J. Mountford, S. J. Lancaster, S. J. Coles,
P. N. Horton, D. L. Hughes, M. B. Hursthouse and
M. E. Light, Inorg. Chem., 2005, 44, 5921.
9 Typical HfQN bond lengths are 1.80–1.86 A: H. Hamaki,
N. Takeda and N. Tokitoh, Organometallics, 2006, 25, 2457 and
references therein.
10 For examples of early transition metal hydrides related to
resonance C see (a) E. Bulak, G. E. Herberich, I. Manners,
H. Mayer and P. Paetzold, Angew. Chem., Int. Ed. Engl., 1988,
27, 958; (b) E. Bulak and P. Paetzold, Z. Anorg. Allg. Chem., 2000,
626, 1277.
We are grateful to the University of East Anglia for
financial support and to Prof Manfred Bochmann for helpful
discussions.
Notes and references
z Synthesis and characterisation of 4. All manipulations were
performed under dry and oxygen-free nitrogen using Schlenk-line
techniques. Toluene and light petroleum were dried over appropriate
drying agents (Na, Na/dyglyme respectively), and distilled under
nitrogen prior to use. 1H, 13C, 19F and 11B NMR spectra were
recorded on a Bruker DPX 300, J values are given in Hz. Chemical
shifts are reported in d units downfield from TMS (1H, 13C), CFCl3
11 B. Cordero, V. Gomez, A. E. Platero-Prats, M. Reves,
J. Echeverrıa, E. Cremades, F. Barragan and S. Alvarez, Dalton
Trans., 2008, 2832.
12 We are aware of one other example of such an interaction with
hafnium: A. Al-Humydi, J. C. Garrison, M. Mohammed,
W. J. Youngs and S. Collins, Polyhedron, 2005, 24, 1234, in which
the distances are 1.24(4) A and 2.05(4) A, respectively.
13 L. Cabrera, E. Hollink, J. C. Stewart, P. Wei and D. W. Stephan,
Organometallics, 2005, 24, 1091.
14 The nature of b-agostic interactions in titanium amidoborane
complexes of relevance to those in this study has recently been
discussed: D. J. Wolstenholme, K. T. Traboulsee, A. Decken and
G. S. McGrady, Organometallics, 2010, 29, 5769.
(
19F), Et2OꢀBF3
(
11B), with the solvent as the reference. Elemental
analyses were carried out at the London Metropolitan University.
A thf-free solution of [Li][NH2BH(C6F5)2] (0.71 g, 1.92 mmol) in
toluene (10 cm3) was cooled to ꢁ78 1C and treated with a solution of
Cp2HfCl2 (0.36 g, 0.96 mmol) in toluene (10 cm3). The reaction was
left to stir at ꢁ78 1C for two hours before warming to room
c
5872 Chem. Commun., 2011, 47, 5870–5872
This journal is The Royal Society of Chemistry 2011