Synthesis and structure of amido- and imido(pentafluorophenyl)borane zirconocene and hafnocene complexes: N-H and B-H activation

Article abstract of DOI:10.1002/chem.201200704

Treatment of Me₂S·B(C₆F₅) _nH_3-n (n=1 or 2) with ammonia yields the corresponding adducts. H₃N·B(C₆F₅)H₂ dimerises in the solid state through N-H...H-B dihyd

Full text of DOI:10.1002/chem.201200704

FULL PAPER
DOI: 10.1002/chem.201200704
Synthesis and Structure of Amido- and Imido(pentafluorophenyl)borane
À
À
Zirconocene and Hafnocene Complexes: N H and B H Activation
Elizabeth A. Jacobs,[a] Anna Fuller,[a] Simon J. Coles,[b] Garth A. Jones,[a]
Graham J. Tizzard,[b] Joseph A. Wright,[a] and Simon J. Lancaster*[a]
Abstract: Treatment of Me2S·B-
(C6F5)nH3Àn (n=1 or 2) with ammonia
yields the corresponding adducts.
H3N·B(C6F5)H2 dimerises in the solid
b-B-agostic chelate bonding of one of
the two amidoborane ligands in the
solid state. The agostic hydride is invar-
iably coordinated to the outside of the
metallocene wedge. Exceptionally,
nance structure. Treatment of the zwit-
terions [Cp2MMe(m-Me)B(C6F5)3] (M=
Zr, Hf) with Li[NH2B(C6F5)nH3Àn] (n=
2) results in [Cp2MMe{NH2B(C6F5)2H}]
complexes, for which the spectroscopic
data, particularly JACTHNGUETRNNU(G B,H), again suggest
b-B-agostic interactions. The reactions
proceed similarly for the structurally
G
E
ACHTUNGTRENNUNG
R
ACHTUNGTRENNUNG
G
E
ACHTUNGTRENNUNG
À
À
state through N H···H B dihydrogen
interactions. The adducts can be depro-
tonated to give lithium amidoboranes
1
[Cp2Hf
G
G
has
a structure in which the two amidobor-
ane ligands adopt an intermediate co-
ordination mode, in which neither is
Li
n=2 reagent with [Cp2ZrCl2] leads to
disubstitution, but [Cp2Zr{NH2B-
(C6F5)2H}2] is in equilibrium with the
product of b-hydride elimination
[Cp2Zr(H){NH2B(C6F5)2H}], which
proves to be the major isolated solid.
The analogous reaction with
A
N
encumbered
(C6F5)3] precursor (Cp’’=1,3-C5H3-
(SiMe3)2, n=1 or 2) to give
[Cp’’2ZrMe{NH2B(C6F5)nH3Àn}], both
[Cp’’2ZrMeACTHNGUTERNNU(G m-Me)B-
C
definitively
agostic.
[Cp2Hf
N
ACHTUNGTRENNUNG
N
N
ACHTUNGTRENNUNG
doborane ligand chelating through an
agostic interaction, but the bond-length
A
ACHTUNGTRENNUNG
A
U
of which have been structurally charac-
terised and show chelating, agostic ami-
doborane coordination. In contrast, the
analogous hafnium chemistry leads to
the recovery of [Cp’’2HfMe2] and the
distribution suggests
a contribution
from a zwitterionic amidoborane reso-
[Cp2HfCl2] gives a mixture of [Cp2Hf-
À
G
E
Keywords: boranes · density func-
tional calculations · hafnium · met-
allocenes · zirconium
formation of Li[HB
hydride abstraction.
ACHTNUGNERT(NGNU C6F5)3] through
A
R
and
A
R
ACHTUNGTRENNUNG
Introduction
a result, the tendency to form agostic interactions is similar
for the ethyl and amidoborane ligands, and amidoboranes
[3,4]
À
Ammoniaborane and ethane are isostructural and isoelec-
tronic, but have very different physical properties, primarily
because of the substantial differences in polarity.[1] However,
the contrast between amidoborane and ethyl ligands is
much less dramatic; metal coordination changes the distri-
bution of charge so that the boron-bound hydrogen becomes
can provide insight into C H activation processes.
Ammoniaborane currently has attracted considerable at-
tention as a chemical hydrogen carrier.[1,5–8] Thermal hydro-
gen liberation requires high temperatures and gives volatile
by-products.[8] Hydrogen can be released through solvolysis,
but the cost of regeneration from the resulting borate is pro-
hibitive.[9] As a consequence, interest was focused on a cata-
lytic approach and a number of late-transition-metal com-
plexes have proven effective.[1,3,5–8]
less hydridic than in ammoniaborane, while in the ethyl
group the C H bond is more polarised than in ethane. As
[2]
À
Interest specifically in Group 4 metallocene amidoboranes
has been given added impetus by the discovery that titano-
cene complexes are effective for the catalytic dehydrogena-
tion of amine–borane adducts.[10–14] Spectroscopic, kinetic
and theoretical investigations of the mechanism have pro-
duced a range of proposed intermediates (structures I–V,
[a] E. A. Jacobs, A. Fuller, G. A. Jones,+ J. A. Wright, S. J. Lancaster
Energy Materials Laboratory, School of Chemistry
University of East Anglia, Norwich Research Park
Norwich, NR4 7TJ (UK)
Fax : (+44)1603-592003
E-mail: S.Lancaster@uea.ac.uk
À
À
Scheme 1), resulting from N H and B H activation. How-
ever, to date structural characterisation of relevant species is
limited to Vb and VIa, which were prepared through halide
to amidoborane exchange by Roesler and co-workers.[15]
The amidoborane ligands are bound in a chelate fashion
[b] S. J. Coles, G. J. Tizzard
EPSRC National Crystallography Service
School of Chemistry, University of Southampton
University Road, Southampton, SO17 1BJ (UK)
[+] Computational queries should be addressed to Garth A. Jones.
À
through Zr N and Zr···H interactions, which the authors
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200704.
termed b-B-agostic.[4]
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lent yield (Scheme 3). Both 1 and 2 crystallise from solvent
mixtures of dichloromethane and light-petroleum without
incorporation of solvent. The coordinated ammonia gives
Scheme 1.
Scheme 3.
In earlier work, we demonstrated the synthesis of amido-
borane and ultimately nitridoborane complexes through se-
À
quential N H activation of the adduct H3N·B
(C6F5)3 follow-
rise to broad signals at d=1.73 and 1.46 ppm in the
1H NMR spectra of 1 and 2, respectively. While broad BH
multiplets are observed in the 1H NMR spectra, these
groups are much more easily identified in the 11B spectra,
where 1 gives a doublet at d=À15.8 ppm and 2 a triplet at
d=À19.2 ppm.
ing the reaction with metal bases (Scheme 2).[16–21] Metallo-
The solid-state structure of 1 (Figure 1) was determined
as the deuterated benzene solvate and has the expected es-
sential tetrahedral geometry about boron. The benzene mol-
ecule forms a heteroaryl-pairing interaction with one of the
pentafluorophenyl groups: the rings are almost co-parallel
and the centroid–centroid separation is 3.660 ꢁ.[24] The NH
hydrogens are engaged in weak intermolecular hydrogen-
bonding interactions, resembling those of the protic amine
adducts of tris(pentafluorophenyl)borane.[25]
Scheme 2.
cene complexes of the [NH2B
through treatment of the electrophilic zwitterions [Cp2MMe-
(m-Me)B(C6F5)3] with Li[NH2B
(C6F5)3].[16] This work has
played an important role in illustrating N H activation
pathways, but B(C6F5)3 derived ligands are of limited value
(C6F5)3]À ligand were prepared
ACHTUNGTRENNUNG
G
E
N
ACHTUNGTRENNUNG
À
ACHTUNGTRENNUNG
as models for intermediates in the catalytic reactions, be-
cause of the absence of b-hydrides. Therefore, our goal is to
À
examine the reactivity of amidoborane ligands with both B
À
H and N H groups.
We recently developed a convenient synthesis of the
mono- and bis(pentafluorophenyl)borane adducts of dime-
thylsulfide.[22] These precursors have facilitated the synthetic
and structural investigation of the chemistry of Li, Zr and
Hf complexes of the [NH2BACHTUNGTRENNUNG
(C6F5)H2]À ligands, with both B H and N H functions re-
À
À
Figure 1. ORTEP representation of the structure of 1·C6D6 showing 50%
À
M···H B interactions are a prominent feature. Preliminary
results, describing the products on treating [Cp2HfCl2] with
À
probability ellipsoids. Selected bond lengths [ꢁ] and angles [8]: B1 N2
À
À
À
À
1.6034(19), B1 C1 1.6309(18), B1 C11 1.6324(19); N2 B1 C1
À
À
À
À
LiACHTUNGTRENNUNG[NH2BACHTUNGTRENNUNG
(C6F5)2H], have been communicated.[23]
111.02(10), N2 B1 C11 107.97(10), C1 B1 C11 112.70(10).
Results and Discussion
Compound 2 was structurally characterised by single-crys-
tal X-ray diffraction (Figure 2). The molecular geometry is
unremarkable. The supramolecular architecture is dominat-
The labile dimethylsulfide ligand is exchanged by treating
solutions of Me2S·B(C6F5)2H and Me2S·B(C6F5)H2 with am-
monia, giving the pentafluorophenyl-substituted ammonia
boranes H3N·B(C6F5)2H (1) and H3N·B(C6F5)H2 (2) in excel-
À
À
A
U
ed by dimerisation through N H···H B dihydrogen bonding
interactions.[26] At 2.12 ꢁ, the H···H separation is well within
the 2.2 ꢁ cut-off adopted by Crabtree.[27] Hydrogen bonds
N
ACHTUNGTRENNUNG
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Synthesis and Structure of Zirconocene and Hafnocene Complexes
FULL PAPER
Figure 2. ORTEP representation of the structure of 2 showing 50% prob-
Figure 3. ORTEP representation of the structure of 3b showing 50%
probability ellipsoids; hydrogen atoms bound to carbon have been omit-
À
ability ellipsoids. Selected bond lengths [ꢁ] and angles [8]: B1 N2
À
À
À
1.6151(15), B1 C1 1.6136(15), H1a···H2a’ 2.121; N2 B1 C1 108.48(8),
À
ted for clarity. Selected bond lengths [ꢁ] and angles [8]: B1 N2 1.544(2),
À
À
À
B1 H1a H2a’ 103.57, N2 H2a···H1a’ 159.39.
À
À
À
À
À
À
B1 C1 1.661(2), B1 C11 1.648(3); N2 B1 C1 114.38(13), N2 B1 C11
À
À
À
À
111.78(14), C11 B1 C1 105.21(13), B1 N2 Li1 114.32(13).
to organofluorine acceptors link dimeric pairs of 2 into
chains.
served in 2, a similar reduction was observed on deprotona-
tion of 1 to yield 3a.
Monitoring the reaction of 1 with nBuLi by 1H NMR
spectroscopy indicates the formation of a new compound 3.
While the deprotonation proceeds in toluene, the cleanest
isolated product was obtained from reaction in tetrahydro-
Monitoring the reaction of zirconocene dichloride with
1
two equivalents of donor-free 3 in toluene by H NMR spec-
troscopy reveals a relatively slow reaction, leading to a com-
plex product mixture. The signal for [Cp2ZrCl2] is replaced
within two hours by three new cyclopentadienyl resonances
and no further changes were observed after stirring the reac-
tion overnight. On one occasion, fractional crystallisation re-
sulted in a low yield of a minor product, which was deter-
furan to give the solvate [(thf)2LiACHTUNTGRENNUG{NH2BACHUTGNTREN(NGUN C6F5)2H}] (3a).
Since 3a proved poorly crystalline, [12]crown-4 was added,
facilitating structural characterisation as [([12]crown-4)Li-
ACHTUNGTRENNUNG{NH2BACHTUNGTRENNUNG(C6F5)2H}] (3b; Scheme 4).
mined by X-ray crystallography to be [Cp2Zr
ACHTUNGTRENNUNG{NH2B-
AHCUTNERTGG(UNNN C6F5)2H}2] (5·PhMe; Scheme 5). In all other instances, the
1
major crystalline product 6 was isolated. The H NMR spec-
trum ([D2]dichloromethane) of 6 consists of a sharp singlet
at d=5.7 ppm for the cyclopentadienyl ligand. A single
broad resonance for the NH2 group is observed at d=
1.9 ppm. Integration indicates that there is only one amido-
borane group. We assigned a peak integrating to 1H at d=
3.8 ppm to a ZrH resulting from reversible b-hydride elimi-
nation, leading to formulation of this compound as
Scheme 4.
[Cp2Zr(H)
ACHTUNTGRENNG{U NH2BACHTUNTGREN(NUGN C6F5)2H}] (6). The ZrH is a J=5 Hz dou-
The structure of 3b (Figure 3) is related to that of
[([12]crown-4)Li{NH2B
(C6F5)3}].[17] The reduced steric con-
blet, which we tentatively propose is owing to coupling to
the agostic hydride. The by-product [H2NBACHTUNGTER(UNNG C6F5)2]n was iso-
N
N
À À
straints lead to a closing of the Li N B angle from 121.35
À
to 114.328 and a shortening of the Li N bond length from
2.048 to 2.015 ꢁ. There is no interaction between the BH
and lithium, which are separated by 3.279 ꢁ. In contrast to
[([12]crown-4)Li
ACHTUNGTRENNUNG{NH2BACHTUNGTERNN(UGN C6F5)3}], there are no intramolecular
hydrogen bonds in 3b, and the NH2 group is solely engaged
À
À
in a weak intermolecular N H···F C interaction.
Deprotonation of 2 to form [(thf)2LiACHTNUGTRENN{UG NH2BAHCTUNGTREN(NUGN C6F5)H2}]
(4a; Scheme 4) produces very minor changes to the 11B and
19F NMR chemical shifts, but the loss of the NH3 resonance
1
in the H NMR spectrum is apparent with concomitant ap-
1
pearance of a new signal at d=À0.11 ppm for NH2. The J-
(B,H) coupling constant, 82 Hz, is 20 Hz lower than that ob-
Scheme 5.
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S. J. Lancaster et al.
lated by treatment of the crude reaction mixture with di-
chloromethane, followed by extraction and preferential crys-
tallisation. Compound 6 proved poorly soluble in aromatic
solvents, in which it gives rise to a cyclopentadienyl reso-
nance, which is broadened by chemical exchange, presuma-
bly between isomers analogous to Va and Vb. It appears
that dichloromethane strongly favours one of the possible
isomers. Compounds 5 and 6 are believed to participate in
an equilibrium, which strongly favours 6 in the crude reac-
tion mixture (Scheme 5), such that spectroscopic data for 5
cannot be confidently assigned.
the tetrahydrofuran solvate, 8·4ACTHUNGETRNNU(G thf). Integration of the
1H NMR spectra confirmed that compound 8 was a bis(ami-
doborane), and we see no evidence for an amidoborane hy-
dride analogous to 6, which is consistent with the reduced
tendency of hafnium (relative to zirconium) to undergo b-
hydride elimination.
The crystalline solid normally isolated from the reaction
of [Cp2HfCl2] with the donor-free lithium salt, 3, was not the
bis(amidoborane) 8, but an unexpected second compound 9,
which was identified as [Cp2HfACHTUNGTRENNUG{NHBACHTUTGNERN(NUGN C6F5)2H}] by X-ray
crystallography (Scheme 6). Multinuclear NMR data for
compound 8 have been assigned from spectra of the crude
reaction mixture, following elimination of resonances deter-
mined to be owing to isolated 9 (see below). In the
1H NMR spectrum, singlets were assigned to the cyclopenta-
dienyl and amido groups; however, the BH resonance was
not observed, while the 11B signal was unusually broad.
These observations suggest dynamic behaviour in solution
and may be owing to solution phase exchange of the two
amidoborane ligands between agostic and non-agostic bond-
ing modes.
The distribution of products in crude reaction mixtures
containing both 8 and 9 is not perturbed by modest heating,
which indicates that 9 does not result from the decomposi-
tion of 8. [Cp2HfCl2] reacts with the first equivalent of 3 to
give the amidoborane chloride. The second equivalent of 3
reacts either (i) through metathesis to give 8 or (ii) as
a base, eliminating HCl and yielding 9 (Scheme 6). The by-
product of this process 1 was identified by multinuclear
NMR spectroscopy, but we note that it can also be formed
by hydrolysis. We have found no evidence for the formation
of a zirconium analogue of compound 9.
The proposed identity of 6 as a zirconium hydride is sup-
ported by the observation that it reacts slowly with dichloro-
methane to form
a new compound [Cp2Zr(Cl)ACHTUNTGERNUN{G NH2B-
ACHTUNGTRENNUNG
early stages of the reaction between Cp2ZrCl2 and 3. A
sample of 7 was prepared by treating 6 with dichlorome-
thane overnight. The 11B and 19F spectra of 6 and 7 are simi-
lar and exhibit only minor chemical shift differences from
the lithiated ligand. While Roeslerꢂs NH2BH3 systems ex-
1
change BH3 resonances in the H NMR spectra at tempera-
tures down to À808C,[15] there is no BH exchange here, and
we can observe the coupling constant reducing from J=
94 Hz in 3a to J=60 and 62 Hz in 6 and 7, respectively. This
À
is consistent with weakening of the B H bond associated
with a b-B-agostic interaction.
Treating [Cp2HfCl2] with two equivalents of 3 (Scheme 6)
leads to a crude product mixture with two major resonances
in the cyclopentadienyl region of the 1H NMR spectrum.
The solid state structure of 5·PhMe was determined by
diffraction methods (Figure 4). The geometry about zirconi-
um is essentially tetrahedral, and the key parameters are
presented in Table 1. The most striking feature of 5·PhMe is
the presence of two amidoborane ligands. Our previously
Scheme 6.
The ratio of these signals proved to be extremely sensitive
to the reaction conditions and, although the compounds re-
sponsible were reproducibly observed, their proportions
varied. Unfortunately the compounds proved difficult to
separate, frustrating many attempts at fractional crystallisa-
tion. A number of crystalline samples were eventually ob-
tained and screened by single-crystal X-ray crystallography.
In this fashion, we were able to structurally characterise
[Cp2HfACHTUNGTRENNUNG{NH2BACHTUNGTRENNUNG(C6F5)2H}2] (8·PhMe, see below), the hafnium
analogue of 5. However, we were unable to obtain sufficient
8·PhMe for elemental analysis and spectroscopic characteri-
sation of the isolated solid.
The selectivity is not improved in the presence of tetrahy-
drofuran, and here as well, the quantity of the crystalline
sample isolated was sufficed only to obtain the structure of
Figure 4. ORTEP representation of the structure of 5·PhMe showing
50% probability ellipsoids; a toluene molecule and hydrogen atoms
À
except for H1a H2c have been omitted for clarity. Intramolecular hydro-
gen bond lengths [ꢁ] and angles [8]: H1a···F36 2.187, H2b···F26 2.226;
À
À
N1 H1a···F36 126.09, N2 H2a···F26 130.74.
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Synthesis and Structure of Zirconocene and Hafnocene Complexes
FULL PAPER
Table 1. Selected distances [ꢁ] and angles [8] for 5·PhMe, 8·PhMe, 8·4-
ACHTUNGTRENNUNG
(thf) and 9.[a]
5·PhMe
8·PhMe
8·4
G
9
À
M N1
À
M N2
2.263(8)
2.335(8)
2.201
2.209
2.294
2.236(6)
2.226(5)
2.179
2.175
2.637
2.249(2)
2.283(2)
2.198
2.193
2.09(3)
3.363
1.538(4)
1.592(4)
77.30(8)
131.11
90.98(15)
122.05(17)
2.282(6)
–
2.182
2.180
2.10(6)
–
1.509(10)
–
[b]
[c]
À
M Cp1
À
M Cp2
À
M H1c
À
M H2c
3.259
2.840
À
B1 N1
1.520(13)
1.602(13)
79.3(3)
132.04
92.3(6)
120.0(6)
1.553(12)
1.563(13)
78.8(3)
133.03
102.7(5)
109.7(5)
À
B2 N2
À
À
N1 M N2
–
[b,c]
À
À
Cp1 M Cp2
133.64
86.7(4)
–
À
À
M N1 B1
À
À
M N2 B2
[a] Numbering refers to the Scheme used for 5·PhMe and 8·PhMe.
Figure 5. ORTEP representation of the structure of 8·4ACTHNUTRGENUG(N thf) showing
[b] Centroid of C1 to C5. [c] Centroid of C6 to C10.
50% probability ellipsoids; two tetrahydrofuran molecules and hydrogen
atoms bound to carbon atoms have been omitted for clarity. Intramolecu-
lar hydrogen bond lengths [ꢁ] and angles [8]: H11a···O71 2.300,
H11b···F26 2.188, H41a···O76 2.141, H41b···F62 2.345, H41b···O71 2.555;
characterised NH2BACHTUNGTRENNUNG(C6F5)3 complexes were restricted to one
À
À
À
N1 H11a···O71 158.75, N1 H11b···F26 138.49, N41 H41a···O76 173.30,
amidoborane ligand.[16–18] While examples are known for
main group NH2BH3 complexes,[28] compound 5 is a rare in-
stance of ligation by two substituted amidoborane ligands.[29]
The two amidoborane ligands in 5·PhMe exhibit markedly
À
À
N41 H41b···F62 128.06, N41 H41b···O71 148.73.
slightly shorter than the sum of the van der Waals radii, the
À À
À
À
different Zr N B angles 92.3(6) and 120.0(6), which cou-
pled with the relatively short Zr···H contact of 2.294 ꢁ does
indicate an agostic interaction to one of the amidoborane li-
resulting influence on the Hf1 N41 B42 angle marks this as
a highly significant interaction.
Given the agostic bonding in 8·4ACHTUNTRGNEUNG(thf), the structure
À À
gands. For comparison, Vb and VIa have Zr N B angles of
8·PhMe was expected to exhibit similar M···H interactions.
However, 8·PhMe provides a surprising contrast to that of
5·PhMe and 8·4ACTHUNTGRNEUNG(thf) (Figure 6). At 102.7(5) and 109.7(5)8,
86.0 and 87.98, respectively.[15] The simplicity of the spectra
suggest that this solid-state preference for the agostic inter-
action on the outside of the metallocene wedge is retained
À À
the Hf N B angles lie between the range expected for non-
in solution.[30] The Zr N bond lengths in 5·PhMe reflect this
difference in bonding pattern, with the shorter 2.263(8) ꢁ
bond of the agostic ligand being essentially identical in
length to that in VIa, while the and Zr N2 bond length is
2.335(8) ꢁ. A shorter N1 B1 bond at 1.520(13) ꢁ is ob-
agostic amidoborane ligands of ca. 1208 and the near 908
À
angles found for agostic interactions in 5·PhMe and 8·4ACHTUNGTRENNUNG(thf).
A similar pattern is found for the Hf···H distances of 2.637
and 2.840 ꢁ; they are much greater than the agostic contact
in 8·4ACHTUGNTERN(UNG thf), 2.09 ꢁ, but still significantly shorter than the
Hf···H distance for the non-agostic borane hydride 3.363 ꢁ
À
À
served for the agostically bonded ligand in comparison to
À
À
both the non-agostic ligand, where N2 B2 is 1.602(13) ꢁ,
in 8·4
(thf). At 2.236(6) and 2.226(5) ꢁ, the Hf N bond
and the free borane (1), in which the corresponding bond
length is 1.6034(19) ꢁ. The structural chemistry of the b-B-
agostic amidoboranes mirrors that of b-agostic alkyls: the
À
lengths are both shorter than that of the agostically bonded
group 8·4ACTHNUTRGNEUG(N thf) 2.249(2) ꢁ and the monoamidoborane com-
Ca Cb bond length in [(MeCp)2Zr(Et)ACHTUNGTRNEUNG
(PMe3)]+ is
1.47(2) ꢁ versus 1.532(2) ꢁ for the C C bond in
À
ethane.[31,32]
The structure of 8·4
diffraction methods (Figure 5). Unlike 8·PhMe, which is dis-
cussed below, the solvent molecules in 8·4(thf) show hydro-
ACHTUNGTRNE(NUNG thf) was also determined by X-ray
AHCTUNGTRENNUNG
gen bonding to the NH2 groups. The second hydrogen atom
on each of the NH2 groups is engaged in an intramolecular
hydrogen bond to an ortho-fluorine. Closer inspection of
8·4ACHTUNGTRENNUNG(thf) reveals that the structural parameters of the two
À
À
amidoborane ligands are very different. The Hf1 N11 B12
angle is 122.05(17)8, which, while more obtuse than the tet-
rahedral ideal, is smaller than the value of 135.14(17)8
found for [Cp2Hf(Me)
ACHTUNTGRENNGU{NH2BACHUTNGTREN(NUGN C6F5)3}], in which the steric
demands of the borane fragment is greater.[17] In contrast,
Figure 6. ORTEP representation of the structure of 8·PhMe showing
50% probability ellipsoids; a toluene molecule and hydrogen atoms
À
À
the Hf1 N41 B42 angle is only 90.98(15)8. This remarkably
acute angle results from a B H···Hf agostic bond. While the
À
except for H1a H2c have been omitted for clarity. Intramolecular hydro-
À
gen bond lengths [ꢁ] and angles [8]: H1a···F12 2.133, H2a···F42 2.240;
À
À
crystallographically determined H···Hf distance is only
N1 H1a···F12 131.47; N2 H2a···F42 126.02.
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S. J. Lancaster et al.
(C6F5)3}] 2.270(2) ꢁ.[17] Instead of
plex [Cp2Hf(Me)ACHTUNGTRENNUNG{NH2BAHCTUNGTRNENGUN
a definitive case of one of the two amidoborane ligands en-
gaging in an agostic interaction, the structure of 8·PhMe ap-
pears to exhibit two weak interactions.[33]
The structure of [Cp2HfACTHNUGTRENNU{G NH2BACHUTNGTREN(NUGN C6F5)2H}2] was optimised
in the gas phase by using the hybrid DFT approach B3LYP,
as outlined in the Computational Details. Two independent
optimisations were performed by taking the crystallographic
coordinates from both 8·4ACTHNUTRGENUGN(thf) and 8·PhMe as starting
points. Both optimisations converged to the same structures.
À À
The B N Hf angles are 90.43 and 122.068, which is in very
close agreement to those found experimentally for 8·4ACTHUNTGRNEUNG(thf).
Thus, the DFT calculations clearly point to a preference for
an arrangement, in which one of the amidoborane ligands is
bound in a chelate-fashion with a b-B-agostic interaction
and a Hf···H separation of 2.13 ꢁ versus 3.33 ꢁ for the non-
chelating ligand. The calculations also predict the observed
Figure 8. ORTEP representation of the structure of 9·PhMe showing
50% probability ellipsoids; a toluene molecule and hydrogen atoms
except for H1b and H1n have been omitted for clarity.
À
shortening of the B N bond from 1.62 ꢁ to 1.56 ꢁ for the
agostic ligand. Examination of the molecular orbitals re-
vealed that HOMO-13 has the greatest b-B-agostic charac-
ter (Figure 7).
ligand, thus complementing the amidoborane and nitrido-
borane examples we have recently reported.[16–21] The coor-
dination sphere of 9·PhMe is all the more remarkable, be-
cause imido zirconocene and hafnocene complexes have
previously required stabilisation with bulky nitrogen sub-
stituents. Even then, such compounds could only be isolated
with basic donor ligands occupying the coordination site
taken by an agostic interaction in 9·PhMe. The bonding pic-
ture, however, is more complex than simple designation as
À
an imidoborane implies. At 2.282(6) ꢁ, the Hf N bond
length in 9·PhMe is similar to those found in 8·PhMe and
8·4ACTHNUGRTENUNG(thf) and substantially longer than those previously re-
ported for hafnium imides.[34] In addition to the imidoborane
9A (Scheme 7), it is possible to draw a number of alterna-
tive resonance forms for compound 9. The pattern of rela-
Figure 7. Calculated structure for 8 depicting HOMO-13, which has both
À
À
B N and B H···Hf bonding character.
À
À
tively long Hf N, short B N and long Hf···H distances is
most consistent with the zwitterionic resonance structure
9B.
The structure of 9·PhMe was determined by diffraction
methods (Figure 8). Rather than two amidoborane groups,
there is a single ligand coordinated in the metallocene
wedge. The data were sufficient to locate a hydrogen atom
bound to the boron and engaged in a b-B-agostic interaction
with hafnium. At 2.10(6) ꢁ, the H···Hf distance is close to
1
that observed in 8·4
N
Scheme 7.
À
ACHTUNGTRENNUNG(B,H)=64 Hz is also consistent with weakening of the B H
bond commensurate with an agostic interaction. The diffrac-
tion data were not adequate to locate the hydrogen atom
bound to nitrogen, but the requirement to balance the
[Cp2Hf]2+ fragment means that the ligand must be dianionic,
We examined the structure of 9 by using the same DFT
approach that was employed for 8 to gain further insight
[NHB
(C6F5)2H]2À, so the structure was refined with a hydro-
into the nature of bonding of the [NHBACTHNGUTERNNUG
(C6F5)2H]2À ligand.
gen atom riding on the nitrogen. Spectroscopic support for
this formulation is provided by a NH signal integrating to
one proton at d=7.59 ppm.
However, despite predicting a b-B-agostic chelating amido-
borane ligand for 8, which is in close agreement with the ex-
perimental geometry of 8·4ACTHNURTGNEU(GN thf), DFT calculations fail to ac-
The [NHB
A
curately predict the structure of 9. The optimised geometry
a Lewis acid stabilised terminal imido or imidoborane
is one in which the ligand is bound in a chelate fashion, but
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Synthesis and Structure of Zirconocene and Hafnocene Complexes
FULL PAPER
À
both the Hf N bond length of 2.01 ꢁ and the Hf···H dis-
tance at 1.99 ꢁ are significantly smaller than those found ex-
À À
perimentally, while the calculated Hf N B angle of 92.68 is
much larger.
Treatment of [Cp2MCl2] (M=Zr or Hf) with two equiva-
lents of the LiACHTUNGTRENNUNG[NH2BACHTUNGTREN(NUNG C6F5)H2] reagents 4 or 4a lead to
crude product mixtures with complex multinuclear NMR
spectra. Despite repeated attempts at fractional crystallisa-
tion, we were unable to isolate a pure sample. However, the
lithium reagent [(thf)2Li
with zwitterions [Cp2M(Me)
Hf)[35] to give the methyl amidoborane complexes
[Cp2M(Me){NH2B(C6F5)2H}] (M=Zr, 10 and M=Hf, 11),
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
respectively (Scheme 8). Compounds 10 and 11 were puri-
fied by recrystallisation from toluene, but did not yield X-
ray-quality crystals. The spectroscopic data for 10 and 11 are
very similar. For 10, the NH2 group is observed as a broad
signal at d=1.5 ppm. The BH gives a characteristic broad
quartet at d=À0.32 ppm in the 1H NMR spectrum and
a doublet at d=À22.8 ppm in the 11B spectrum. At 64 Hz,
Scheme 9.
1
than those in 12. In the H NMR spectra of both 12 and 13,
the BH resonances are obscured by the SiMe3 signals, and
the group is best observed in the 11B spectra. The 11B signal
moves from d=À19 ppm in 4a to d=À25 ppm in 13. The
resonance was too broad in [D6]benzene to determine 1J-
the 1J
ACHTUNGTRENNUNG
1
The value of J
N
ACHTUNGTREN(NNUG B,H). In [D2]dichloromethane, the resonance was sufficient-
ly well resolved to measure a JACTHNGUETRNNU(G B,H) value of approximate-
1
tural similarity extends to an agostic interaction.
ly 76 Hz. Although similar to the coupling constant ob-
served for 12, we note that there are two boron hydrides
and that this value is an average.
The analogous hafnium reactions proceed very differently.
Treatment of [Cp’’2HfMeACTHNUTRGENN(UG m-Me)BAHCUTNGTNER(NUGN C6F5)3] with either 3a or
4a results in clean conversions, giving similar and very
1
simple H NMR spectra. Integration of the cyclopentadienyl
Scheme 8.
and methyl regions and comparison to an authentic sample
revealed recovery of [Cp’’2HfMe2]. The crude 19F NMR
spectra were more complex, however, fractional crystallisa-
Treatment of the same zwitterions with [(thf)2Li
ACHTUNGTRENNUNG(C6F5)H2}] (4a) leads to complex product mixtures, from
which we were unable to isolate individual compounds. To
better stabilise the resulting complexes, we examined the re-
{NH2B-
tion lead to the structural characterisation of Li[HB
We propose that the hydridoborate salt results from the ab-
straction of hydride from 3a or 4a by B(C6F5)3 (Scheme 9).
In support of this hypothesis, treatment of 3a with B(C6F5)3,
in the absence of metallocene, yields Li[HB
(C6F5)3].[36]
ACHTUNGTRENNUNG(C6F5)3].
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
activity of the lithium reagents towards [Cp’’2MMe
(C6F5)3] (Cp’’=1,3-(SiMe3)2C5H3; M=Zr or Hf).
[Cp’’2ZrMe(m-Me)B(C6F5)3] reacts cleanly with [(thf)2Li-
{NH2B(C6F5)2H}] (3a) to give [Cp’’2M(Me){NH2B(C6F5)2H}]
A
ACHTUNGTRENNUNG
G
Surprisingly, given the established similarity between zir-
conium and hafnium, rather different mechanisms appear to
be in operation (Scheme 8).[37] For zirconium, the nucleo-
philic lithium reagent is presumed to react with the electro-
philic metal centre, produced through reversible dissociation
A
ACHTUNGTRENNUNG
R
A
R
ACHTUNGTRENNUNG
(12; Scheme 9). The key spectroscopic data are similar to
those of 10. The rather bulky substituents lead to hindered
rotation of the Cp’’ ligands, and the SiMe3 environments are
of the [MeBACTHNGUTERNNUG
(C6F5)3]À anion. With hafnium instead, the lithi-
um salt is reacting with free BACTHNGUETRN(NUG C6F5)3, resulting from reversi-
1
inequivalent. At 74 Hz, the higher value of J
N
À
the 64 Hz found for 10 may indicate a weaker agostic inter-
action, which is to be expected given the extent of steric hin-
drance.
ble cleavage of the B C bond. For hafnium at least, the two
mechanisms must be finely balanced, because, in the forma-
tion of 11, anion dissociation is evidently the predominant
pathway.
Conducting the reaction between [Cp’’2ZrMe
ACHTUNGTRENN(UNG m-Me)B-
A
The solid state structure of 12 was determined by single-
crystal X-ray diffraction (Figure 9). The bonds of the
[Cp’’2ZrMe] fragment are slightly longer than those ob-
ties in attempting to synthesise amidopentafluorophenylbor-
ane complexes, and were able to isolate the crystalline solid
[Cp’’2Zr(Me)
{NH2B
G
(C6F5)3],[38] presumably be-
served in [Cp’’2ZrMeACTHNGUTRENN(UG m-Me)BACTHUNGTRENNUNG
of the Cp’’ ligands is again hindered on the NMR time scale,
cause of the more strongly bound and, as a consequence of
À
and the SiMe3 groups are inequivalent. Both the Zr NH2
the b-B-agostic interaction, more sterically demanding
[NH2BACHTUNGRETNNU(G C6F5)2H] ligand in comparison to the monodentate
À
and Zr Me resonances are found at slightly lower frequency
Chem. Eur. J. 2012, 00, 0 – 0
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S. J. Lancaster et al.
Figure 10. ORTEP representation of the structure of 13 showing 50%
À
probability ellipsoids; hydrogen atoms except for H1a H1d have been
omitted for clarity. Intramolecular hydrogen bond length [ꢁ] and angle
Figure 9. ORTEP representation of the structure of 12 showing 50%
À
[8]: H1a···F16 2.182; N1 H1a···F16 132.71.
À
probability ellipsoids; hydrogen atoms except for H1a H1d have been
omitted for clarity.
convenient precursors to the adducts H3N·BACHTUNTGRENUNG(C6F5)nH3Àn. De-
protonation gives nucleophilic amidoborane reagents, which
react with electrophiles to give the first complexes of the
À
and weakly bound [MeB
2.295(2) ꢁ lies between those of the agostic and non-agostic
(C6F5)3]. The Zr1 N1 distance of
amidoborane groups in 5·PhMe (Table 2). At 91.97(14)8, the
[NH2BACHTUNGTERNUN(G C6F5)nH3Àn] ligands (where n=1 or 2). The zircono-
À
À
Zr1 N1 B1 angle is remarkably similar to that found for
the agostic amidoborane group in 5·PhMe and clearly indi-
cates a similar interaction. The hydride was located in the
difference map and the Zr1···H1c separation of 2.18(2) ꢁ is
within the range of agostic bonding.
cene and hafnocene examples serve as models for intermedi-
ates in the catalytic dehydrogenation of ammoniaborane.
À
The [NH2B
N and b-B-agostic interactions.
G
We observe a remarkable degree of divergence between
the hafnocene and zirconocene chemistry. [Cp2Zr
(C6F5)2H}2] exhibits an agostic interaction and is in equilibri-
um with the [Cp2Zr(H){NH2B(C6F5)2H}] through a BH acti-
vation pathway. By contrast, [Cp2Hf{NH2B(C6F5)2H}2] only
manifests a definitive agostic interaction in the presence of
tetrahydrofuran. [Cp2Hf{NHB(C6F5)2H}] is formed alongside
the bis(amidoborane) through a parallel reaction pathway
and provides an example of multiple NH activation of possi-
ble relevance to catalytic systems. The failure of DFT meth-
ods to accurately model 9 will be the subject of future inves-
ACHTUNGTRENNUNG{NH2B-
AHCTUNGTRENNUNG
Table 2. Selected distances [ꢁ] and angles [8] for 12 and 13.
G
ACHTUNGTRENNUNG
12
13
A
ACHTUNGTRENNUNG
À
Zr1 N1
2.295(2)
2.243
2.235
2.320(2)
2.18(2)
79.37(8)
131.52
2.281(3)
2.230
2.228
2.319(3)
2.224
77.84(11)
132.48
[a]
[b]
À
Zr1 Cp1
G
ACHTUNGTRENNUNG
À
Zr1 Cp2
À
Zr1 C91
À
Zr1 H1c
À
À
N1 Zr1 C91
[a,b]
À
À
Cp1 Zr1 Cp2
À
À
Zr1 N1 B1
91.97(14)
88.63(18)
tigations. The sterically hindered [Cp’’2ZrMeACTHUNRGTENNG{U m-MeBACHTUNGTRENNUNG(C6F5)3}]
and its hafnium analogue react with the lithium amidobor-
ane reagents through different mechanisms, with the zirco-
[a] Centroid of C1 to C5. [b] Centroid of C6 to C10.
The molecular structure of 13 is shown in Figure 10. For
the common [Cp’’2ZrMe] fragment, the structural parame-
ters are similar to those of 12, but the bonds are slightly
shorter, commensurate with the reduced size of the mono(p-
nocene forming [Cp’’2ZrMeACTHNUGRTENU{NG NH2BACTHGNUERTN(NUNG C6F5)nH3Àn}] complexes.
Experimental Section
À
À
entafluorophenyl)borane group. The 88.63(18)8 Zr1 N1 B1
angle is the most acute of all those reported herein, an
All reactions were performed under a dry nitrogen atmosphere by using
standard Schlenk techniques in pre-dried glassware. Solvents were dried
by using an appropriate drying agent and distilled under nitrogen prior to
use: dichloromethane (CaH2), light petroleum (Na/dyglyme/benzophe-
none), tetrahydrofuran (sodium/benzophenone) and toluene (sodium).
[12]crown-4 was degassed and dried over activated 4 ꢁ molecular sieves
prior to use.
À
effect dictated by the b-B-agostic interaction, with a Zr1
H1c distance of 2.224 ꢁ (Table 2).
Conclusion
NMR samples were prepared by using degassed deuterated solvents
dried over activated 4 ꢁ molecular sieves. NMR spectra were obtained
by using a Bruker Avance DPX300 spectrometer at 238C, J values are
Facile exchange between Me2S·BH3 and B
ACHTNUGNERT(NGNU C6F5)3 provides
access to B(C6F5)nH3Àn (where n=1 or 2), which serve as
ACHTUNGTRENNUNG
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Synthesis and Structure of Zirconocene and Hafnocene Complexes
FULL PAPER
given in Hz. Chemical shifts are reported in ppm and referenced to resid-
ual solvent resonances (1H, 13C);[39] 19F is relative to CFCl3; 11B is relative
to Et2O·BF3. Elemental analyses were carried out at the Department of
Health and Human Sciences, London Metropolitan University.[40]
42.93, H 1.87, N 2.82; elemental analysis calcd (%) for C22H14BF10NZr
(6): C 45.22, H 2.41, N 2.40; found: C 45.19, H 2.36, N 2.59.
6: 1H NMR (300.1 MHz, CD2Cl2): d=5.7 (s, 10H; C5H5), 3.8 (d, 1H, 2J-
A
ACHTUNGTRENNUNG(H,B)=
60 Hz; BH); 13C NMR (75.5 MHz {1H}, CD2Cl2): d=104.6 ppm (C5H5);
H3N·B
ACHTUNGTRENNUNG
11B NMR (96.3 MHz, CD2Cl2): d=À25.6 ppm (d, 1J
Me2S·B
ACHTUNGTRENNUNG
19F NMR (282.4 MHz, CD2Cl2): d=À134.4 (brs, 4F; o-F), À158.1 (brs,
2F; p-F), À163.7 ppm (brs, 4F; m-F). IR (ATR): v˜ =3366 (NH), 1884,
(BH), 1559 cmÀ1 (ZrH).
the solvent was removed under reduced pressure and the crude product
was recrystallised from a mixture of dichloromethane and light petrole-
um at À258C (3.40 g, 83%). Crystals suitable for X-ray analysis were
grown at room temperature from the NMR sample. 1H NMR
(300.1 MHz, C6D6): d=3.18 (brs, 1H; BH), 1.73 ppm (brs, 3H; NH3);
ACHUTNGTREG[NNNU H2NBAHCTNUTREGNN(GNU C6F5)2]n: A sample of [H2NBACHTUNGTRENNGU(C6F5)2]n was isolated following
treatment of the crude reaction mixture from the preparation of 5 and 6
with dichloromethane and fractional crystallisation. 1H NMR
(300.1 MHz, CD2Cl2): d=4.9 ppm (s, 2H; NH2); 11B NMR (96.3 MHz,
11B NMR (96.3 MHz, C6D6): d=À15.8 ppm (d, 1J
ACHTUNGTRENNUNG
19F NMR (282.4 MHz, C6D6): d=À134.9 (d, 3J
ACHTUNGTRENNUNG
3
CD2Cl2): d=À5.2 ppm (s); 19F NMR (282.4 MHz, CD2Cl2): d=À137.5
À157.5 (t, J
N
3
analysis calcd (%) for C12H4BF10N: C 39.71, H 1.11, N 3.86; found: C
39.88, H 1.02, N 3.98.
(m, 2F; o-F), À152.5 (t, 1F, J
G
F); elemental analysis calcd (%) for C12H2BF10N: C 39.93, H 0.56, N
3.88; found: C 39.93, H 0.58, N 3.75.
H3N·B
ACHTUNGTRENNUNG
of Me2S·BACHTUNGTRENNUNG
AHCUTNGERTN[GNUN Cp2Zr(Cl)ACHTNUTRGEG{NNUN NH2BCAHTUNGTREN(NUNG C6F5)2H}] (7): Treatment of 6 with dichloromethane
over 16 h resulted in near quantitative conversion to 7. 1H NMR
(300.1 MHz, CD2Cl2): d=6.16 (s, 10H; C5H5), 2.9 (s, 2H; NH2),
treated with ammonia. The crude product was recrystallised from a mix-
ture of dichloromethane and light petroleum at À258C (4.38 g, 86%).
1H NMR (300.1 MHz, C6D6): d=2.44 (m, 2H; BH2), 1.46 ppm (brs, 3H;
À0.14 ppm (q, 1H, 1J
(H,B)=60 Hz; BH); 13C NMR (75.5 MHz {1H},
ACHTUNGTRENNUNG
NH3); 11B NMR (96.3 MHz, C6D6): d=À19.2 ppm (t, 1J
ACHTUNGTREN(NUNG B,H)=102 Hz;
ACHTUNGTREN(NUGN F,F)=23 Hz, 2F; o-
CD2Cl2): d=113.8 ppm (C5H5); 11B NMR (96.3 MHz, CD2Cl2): d=
BH2); 19F NMR (282.4 MHz, C6D6): d=À134.4 (d, J
3
1
À17.8 ppm (d, J
(B,H)=62 Hz; BH); 19F NMR (282.4 MHz, CD2Cl2): d=
ACHTUNGTRENNUNG
F), À159.8 (t, 3J
(F,F)=20 Hz, 1F; p-F), À164.4 ppm (m, 2F; m-F); ele-
G
À134.0 (brs, 4F; o-F), À156.9 (t, 3J
(F,F)=20 Hz, 2F; p-F), À162.7 ppm
N
mental analysis calcd (%) for C6H5BF5N: C 36.60, H 2.56, N 7.11; found:
C 36.69, H 2.65, N 7.01.
(brs, 4F; m-F); elemental analysis calcd (%) for C22H13BClF10NZr (7): C
42.70, H 2.12, N 2.26; found: C 42.60, H 1.98, N 2.35.
ACHTUNGTRENNUNG[(thf)2LiACHTUNGTRENNUNG{NH2BACHTUNGTRENNUNG(C6F5)2H}] (3a): A solution of 1 (0.30 g, 0.8 mmol) in THF
AHCNUTETGRGNN[UN Cp2HfACHTUGNTERNNUG{NH2BACHTUNGETRN(GNUN C6F5)2H}2] (8·4ACHTNUGTRENN(GUN thf)): Cp2HfCl2 (0.44 g, 1.1 mmol) was
(10 mL) was cooled to À788C and nBuLi (0.8 mmol) was added before
warming to room temperature. Removal of all volatiles yielded a colour-
less opaque solid in quantitative yield. 1H NMR (300.1 MHz, C6D6): d=
added to a freshly prepared solution of donor-free 3 (1.1 mmol) in tolu-
ene (15 mL) at À788C before warming to room temperature and stirring
for 12 h. The resulting pale brown solution was filtered and all volatiles
were removed to yield a sticky colourless solid, which was recrystallised
from a solvent mixture of THF and light petroleum to yield a few X-ray-
3.9 ppm (q, 1J
ACHTUNGTRENNUNG(H,B)=90 Hz; BH), NH2 peak obscured by THF reso-
nance; 11B NMR (96.3 MHz, C6D6): d=À15.0 ppm (d, 1J
ACHTUNGTRENUN(NG B,H)=94 Hz;
(F,F)=23 Hz, 4F; o-
BH); 19F NMR (282.4 MHz, C6D6): d=À136.2 (d, 3J
N
quality crystals of 8·4
ACHTUNGTRNE(NUNG thf).
F), À162.4 (t, 3J
(F,F)=20 Hz, 2F; p-F), À165.4 ppm (m, 4F; m-F). De-
E
A
E
U
spite simple NMR spectra (see Supporting Information), satisfactory ele-
mental analyses data could not be obtained.
solution of 3 (0.71 g, 1.92 mmol) in toluene (10 mL) was cooled to À788C
and treated with Cp2HfCl2 (0.36 g, 0.96 mmol) in toluene (10 mL). The
reaction was left to stir at À788C for two hours before warming to room
temperature. The solvent was removed under reduced pressure and the
crude product extracted with toluene (20 mL). Colourless crystals of
compound 9 were obtained by concentrating the toluene solution, layer-
ing with light petroleum and cooling to À258C for three days (9: 0.07 g,
11%).
[([12]crown-4)Li
{NH2B
E
A
solution of
1
(0.30 g,
0.8 mmol) in THF (10 mL) was cooled to À788C and nBuLi (0.8 mmol)
was added before warming to room temperature. [12]crown-4 (0.1 mL,
0.8 mmol) was added before removal of the volatiles under reduced pres-
sure to yield a colourless solid. Recrystallisation from a solution of di-
chloromethane and light petroleum yielded X-ray-quality crystals of the
title compound (0.31 g, 72%). 1H NMR (300.1 MHz, CDCl3): d=3.71 (s,
16H; CH2), 3.22 ppm (brs, 2H; NH2); 13C NMR (75.5 MHz {1H}, CDCl3):
d=66.9 ppm (CH2); 11B NMR (96.3 MHz, CDCl3): d=À15.5 ppm (d, 1J-
8: 1H NMR (300.1 MHz, C6D6): d=5.3 (s, 10H; C5H5), 2.2 ppm (s, 4H;
NH2); 11B NMR (96.3 MHz, C6D6): d=À16.8 ppm (brs; BH); 19F NMR
(282.4 MHz, C6D6): d=À134.9 (brs, 8F; o-F), À155.3 (t, 3J
ACHTUNGTRENUN(NG F,F)=21 Hz,
A
4F; p-F), À161.4 ppm (brs, 8F; m-F). BH resonance not observed in the
1H NMR spectrum.
(F,F)=23 Hz, 4F; o-F), À163.2 (brs, 2F; p-F), À166.2 ppm (m, 4F; m-F);
elemental analysis calcd (%) for C20H19BF10LiNO4: C 44.07, H 3.51, N
2.57; found: C 43.94, H 3.45, N 2.51.
9: 1H NMR (300.1 MHz, C7D8): d=7.59 (s, 1H; NH), 5.17 (s, 10H;
C5H5), À0.83 ppm (q, 1J
(H,B)=60 Hz, 1H; BH); 13C NMR (75.5 MHz
ACHTUNGTRENNUNG
{1H}, C7D8): d=103.6 ppm (C5H5); 11B NMR (96.3 MHz, C7D8): d=
ACHTUNGTRENNUNG[(thf)2LiACHTUNGTRENNUNG{NH2BACHTUNGTRENNUNG(C6F5)H2}] (4a): A solution of 2 (0.22 g, 1.1 mmol) in THF
À27.5 ppm (d, 1J
(B,H)=63 Hz; BH); 19F NMR (282.4 MHz, C7D8): d=
ACHTUNGTRENNUNG
(2 mL) was cooled to À788C and nBuLi (1.1 mmol) was added before
warming to room temperature. Removal of all volatiles yielded a colour-
less opaque solid in quantitative yield. 1H NMR (300.1 MHz, C6D6): d=
À133.7 (brs, 4F; o-F), À156.3 (t, 3J
(F,F)=21 Hz, 2F; p-F), À162.4 ppm
G
(brs, 4F; m-F); elemental analysis calcd (%) for: C22H12BF10NHf: C,
2.48 (q, 1J
G
39.46; H, 1.81; N, 2.09; found: C, 39.37; H, 1.69; N, 1.97.
11B NMR (96.3 MHz, C6D6): d=À19.0 ppm (t, 1J
ACHTUNGTRENNUNG
A
CATHGNURTNE{NUNG NH2BACHTUNTGRENN(GU C6F5)2H}] (10): A solution of BACHUTGTNRENNUG(C6F5)3 (0.24 g,
19F NMR (282.4 MHz, C6D6): d=À135.7 (d, 3J
À161.9 (t, 3J
(F,F)=20 Hz, 1F; p-F), À164.8 ppm (m, 2F; m-F). Despite
A
giving clean NMR spectra (see Supporting Information), satisfactory ele-
mental analyses could not be obtained.
0.5 mmol) in toluene (4 mL) was added at À788C. The reaction mixture
was allowed to stir for 20 min at À788C and then for 1 h at room temper-
ature. All volatiles were removed to yield a clear yellow oil. The zircono-
cene product was extracted with light petroleum (10 mL) and filtered
through Celite. The title compound was crystallised from toluene at
À258C overnight (0.036 g, 12%). 1H NMR (300.1 MHz, C6D6): d=5.2 (s,
10H; C5H5), 1.5 (brs, 2H; NH2), 0.03 (s, 3H; CH3), À0.32 ppm (q, 1J-
ACHTUNGTRENNUNG[Cp2ZrACHTUNGTRENNUNG{NH2BACHTUNGTRENNUNG(C6F5)2H}2] (5) and [Cp2Zr(H)ACTUHNTGRENNUN{G NH2BACHTUNGTRENUN(G C6F5)2H}] (6): A so-
lution of Cp2ZrCl2 (0.15 g, 0.52 mmol) in toluene (20 mL) was cooled to
À788C and to this, 3 (0.38 g, 1.05 mmol) in toluene (5 mL) was added.
The reaction mixture was allowed to stir for 20 min and was then stirred
at room temperature for a further hour. The resulting toluene solution
was filtered and cooled to À258C yielding 6 (0.05 g, 16%). (Note: on one
occasion 5 was found to be the crystalline product.) Elemental analysis
calcd (%) for C34H16B2F20N2Zr (5): C 43.20, H 1.71, N 2.96; found: C
AHCTUNGTRENNUNG
(H,B)=66 Hz, 1H; BH); 13C NMR (75.5 MHz {1H}, C6D6): d=110.1
(C5H5), 22.9 ppm (CH3); 11B NMR (96.3 MHz, C6D6): d=À22.8 ppm (d,
1J
ACHTUNGTRENNUNG
Chem. Eur. J. 2012, 00, 0 – 0
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S. J. Lancaster et al.
o-F), À156.1 (t, 3J
(F,F)=21 Hz, 2F; p-F), À162.1 ppm (m, 4F; m-F); ele-
light petroleum (10 mL) and filtered through Celite. X-ray-quality crys-
tals were obtained from a solution in toluene cooled to À258C overnight
(0.21 g, 29%). 1H NMR (300.1 MHz, C6D6): d=6.4 (m, 4H; C5H3(Si-
N 2.03; found: C 52.71, H 2.32, N 1.89.
A
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG{NH2BACHTUNGTRENNUNG(C6F5)2H}] (11): Following the procedure outlined for
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
1.4 mmol) and treated with 3 (0.52 g, 1.4 mmol). All volatiles were re-
moved to yield a clear yellow oil. The product was extracted with light
petroleum (10 mL) and filtered through Celite. The product was crystal-
lised from toluene at À258C overnight (0.085 g, 9%). 1H NMR
(300.1 MHz, C6D6): d=5.3 (s, 10H; C5H5), 1.7 (brs, 2H; NH2), À0.2 (s,
(CH3)3); 11B NMR (96.3 MHz, C6D6):
(CH3), 0.82 (SiACHTNUTRGENN(UG CH3)3), 0.68 ppm (SiACHTUTGNRENNUGN
1
d=À25.1 ppm (brs); 11B NMR (96.3 MHz, CD2Cl2): d=À25.6 ppm (t, J-
(B,H)=76 Hz; BH2); 19F NMR (282.4 MHz, C6D6): d=À133.7 (m, 2F; o-
F), À158.9 (t, 3J
(F,F)=21 Hz, 1F; p-F), À163.8 ppm (m, 2F; m-F); ele-
N
3H; CH3), À0.36 ppm (q, 1J
(H,B)=66 Hz, 1H; BH); 13C NMR
ACHTUNGTRENNUNG
mental analysis calcd (%) for C29H49BF5NSi4Zr: C 48.30, H 6.85, N 1.94;
found: C 48.14, H 6.94, N 1.88.
(75.5 MHz {1H}, C6D6): d=109.5 (C5H5), 26.4 ppm (CH3); 11B NMR
(96.3 MHz, C6D6): d=À23.6 ppm (d, 1J
N
X-ray crystallography: Crystals were suspended in oil, and one was
mounted on a glass fibre and fixed in the cold nitrogen stream of the dif-
fractometer. Data were collected by using MoKa (l=0.71073 ꢁ) radiation
using an Oxford Diffraction Xcalibur-3 CCD diffractometer equipped
with a graphite monochromator (2 and 12), a Bruker-Nonius Apex II dif-
fractometer equipped with confocal mirrors (5·PhMe and 8·PhMe) or
a Rigaku Saturn724+ diffractometer equipped with a confocal mono-
chromator (13). Data were processed using CrysAlisPro (2 and 12),[41]
DENZO and COLLECT (5·PhMe and 8·PhMe),[42] or CrystalClear SM
Expert (13).[43] Structures were determined by the direct methods rou-
tines in the SHELXL (2 and 12),[44] SUPERFLIP (5·PhMe and 13)[45] or
SIR-2004 (8·PhMe),[46] and refined by full-matrix least-squares methods
on F2 in SHELXL.[44] Non-hydrogen atoms were refined with anisotropic
thermal parameters. All hydrogen atoms were freely refined in 2 as was
H1c in 12, while all other hydrogen atoms were included in idealised po-
sitions and their Uiso values were set to ride on the Ueq values of the
parent carbon atoms for all other cases. At the end of the refinement, the
Flack parameter47 for 12 was À0.011(19). A summary of crystallographic
data for previously-unreported structures is given in Table 3.
2F; p-F), À162.0 ppm (m, 4F; m-F); elemental analysis calcd (%) for
11·0.5PhMe: C 43.50, H 2.75, N 1.91; found: C 43.35, H 2.07, N 1.67.
ACHTUNGTRENNUNG[Cp’’2Zr(Me)ACHTUNGTRENNUNG{NH2BACHTUNGTRENNUNG(C6F5)2H}] (12): A solution of BAHCUTNGTREN(GNNU C6F5)3 (0.50 g,
1.0 mmol) in toluene (5 mL) was added to Cp’’2ZrMe2 (0.53 g, 1.0 mmol)
in toluene (4 mL) at À788C. To this, 3 (0.36 g, 1.0 mmol) dissolved in tol-
uene (4 mL) was added at À788C. The reaction mixture was allowed to
stir for 20 min. at À788C and then for 1 h at room temperature. All vola-
tiles were removed to yield an opaque yellow solid. The product was ex-
tracted with light petroleum (10 mL) and filtered through Celite. X-ray-
quality crystals were obtained from a solution in toluene cooled to
À258C overnight (0.14 g, 16%). 1H NMR (300.1 MHz, C6D6): d=6.4 (m,
2H; C5H3(Si
C5H3(Si(CH3)3)2), 2.4 (brs, 2H; NH2), 0.4 (s, 3H; CH3), 0.102 (s, 18H; Si-
(CH3)3), 0.095 ppm (s, 18H; Si
(CH3)3); 13C NMR (75.5 MHz {1H}, C6D6):
d=134.9 (CH), 127.5 (C(Si(CH3)3)), 122.0 (CH), 120.2 (C(Si(CH3)3)),
(CH3)3); 11B NMR (96.3 MHz, C6D6):
ACHTUNGTRENNUNG(CH3)3)2), 6.3 (m, 2H; C5H3(SiCAHTUNTGRNE(NUGN CH3)3)2), 6.0 (m, 2H;
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
116.9 (CH), 26.9 (CH3), 0.7 ppm (Si
ACHTUNGTRENNUNG
d=À22.1 ppm (d, 1J
ACHTUNGTRENNUNG
(B,H)=74 Hz; BH); 19F NMR (282.4 MHz, C6D6):
d=À130.9 (brs, 4F; o-F), À157.0 (t, 3J
(F,F)=20 Hz, 2F; p-F),
CCDC-869673 (2), 869674 (5.PhMe), 869675 (8.PhMe), 869676 (12) and
869677 (13) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
À162.3 ppm (m, 4F; m-F); elemental analysis calcd (%) for
C35H48BF10NSi4Zr: C 47.39, H 5.45, N 1.58; found: C 47.25, H 5.37, N
1.63.
A
E
E
Computational Details: All calculations were performed by using the
Gaussian09 package,[48] employing the hybrid B3LYP functional in com-
bination with the 6–31G* basis set. Relativistic corrections for the Hf
centre were applied by using the effective core potentials from the Stutt-
G
ACHTUNGTRENNUNG
1.0 mmol) was combined with 4 (0.20 g, 1.0 mmol). All volatiles were re-
moved to yield an opaque yellow solid. The product was extracted with
Table 3. Summary of crystallographic data for 2, 5·PhMe, 8·PhMe, 12 and 13.
2
5·PhMe
8·PhMe
12
13
formula
Mr
crystal system
space group
C6H5BF5N
196.92
monoclinic
P21/c
C34H16B2F20N2Zr, C7H8
1037.46
monoclinic
P21/n
C34H16B2F20HfN2, C7H8
1124.73
triclinic
¯
P1
C35H48BF10NSi4Zr
887.13
monoclinic
P21
C29H49BF5NSi4Zr
721.08
triclinic
¯
P1
crystal description, colour
crystal dimensions [mm]
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
block, colourless
0.17ꢃ0.20ꢃ0.25
6.1113(3)
14.8010(7)
8.2999(4)
90
99.763(4)
90
739.88(6)
4
block, colourless
0.03ꢃ0.04ꢃ0.06
13.0729(7)
12.5806(10)
23.8159(16)
90
101.461(4)
90
3838.8(4)
4
block, colourless
0.04ꢃ0.06ꢃ0.10
12.1466(2)
13.2988(3)
13.9244(2)
72.0800(10)
88.9900(10)
67.3890(10)
1962.64(6)
2
block, colourless
0.20ꢃ0.20ꢃ0.30
10.6544(4)
18.2512(6)
10.9722(3)
90
100.600(3)
90
2097.20(12)
2
chunk, colourless
0.10ꢃ0.11ꢃ0.12
11.2025(4)
11.2815(4)
16.2901(11)
103.258(7)
96.341(7)
113.883(8)
1783.86(22)
2
b [8]
g [8]
V [ꢁ3]
Z [K]
1calcd [gcmÀ3
2qmax [8]
T [K]
]
1.768
60
140(1)
1.795
50
120(2)
1.903
55
120(2)
1.405
55
140(2)
1.342
50
100(2)
data collected
14386
23302
35081
32655
46059
unique data
2162
6706
8921
9396
8167
reflns obsd [I>2s(I)]
m [mm]
1733
0.192
4048
0.421
7427
2.789
8854
0.447
7463
0.489
R1 [I>2s(I)]
0.032
0.112
0.053
0.030
0.047
wR2 (all data)
0.094
0.213
0.113
0.065
0.108
largest diff. peak and hole
0.35, À0.28
1.19, À0.62
1.69, À3.23
0.62, À0.29
1.19, À0.62
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Synthesis and Structure of Zirconocene and Hafnocene Complexes
FULL PAPER
gart group.[49] Full optimisations were performed and these were charac-
terised by diagonalisation of the Hessian matrix, as implemented in
Gaussian. In both cases, no imaginary frequencies were seen, indicating
minimum energy structures. Reported energies are in electronvolts. Car-
tesian coordinates for the calculated structures of 8 and 9 have been de-
posited as Supporting Information.
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Acknowledgements
We acknowledge the support of the EPSRC and the University of East
Anglia. EJ is grateful for the award of a Katritzky scholarship. We are
grateful to Prof M. Bochmann for helpful discussions. The ab initio calcu-
lations presented in this paper were carried out on the High Performance
Computing Cluster supported by the Research and Specialist Computing
Support service at the University of East Anglia. We thank Dr J. Sass-
mannshausen for advice regarding the implementation of the relativistic
corrections.
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À
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Received: March 2, 2012
Published online: && &&, 0000
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
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These are not the final page numbers!

Synthesis and Structure of Zirconocene and Hafnocene Complexes
FULL PAPER
Chelate bonding: Lithium amidopenta-
fluorophenylboranes react with elec-
trophilic Group 4 metallocenes
Metallocenes
E. A. Jacobs, A. Fuller, S. J. Coles,
G. A. Jones, G. J. Tizzard, J. A. Wright,
S. J. Lancaster* ............... &&&& — &&&&
À
À
through N H and B H activation to
give novel amidoborane and imidobor-
ane complexes, which are bound in
chelate fashion with a b-B-agostic
interaction (see scheme).
Synthesis and Structure of Amido- and
Imido(pentafluorophenyl)borane
Zirconocene and Hafnocene
À
À
Complexes: N H and B H Activation
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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13
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ÞÞ
These are not the final page numbers!