Synthesis and structure of [Zr{η5:η1-C 9H6B(NiPr2)NPh}2]: A new complex with a boron-bridged amido-indenyl ligand

Article abstract of DOI:10.1016/S0020-1693(02)01504-9

A boron-bridged amido-cyclopentadienyl ligand was recently shown to form constrained-geometry titanium-complexes that are capable of olefin polymerisation. Here, the synthesis of the corresponding amido-indenyl ligand η¹-C₉H₇

Full text of DOI:10.1016/S0020-1693(02)01504-9

Inorganica Chimica Acta 350 (2003) 467ꢁ474
/
www.elsevier.com/locate/ica
Synthesis and structure of [Zr{h5:h1-C9H6B(NiPr2)NPh}2]:
a new complex with a boron-bridged amido-indenyl ligand
Holger Braunschweig a,*, Carsten von Koblinski b, Frank Michael Breitling a,
Krzysztof Radacki a, Chunhua Hu b,1, Lars Wesemann c,1, Thiemo Marx c,1,
Ingo Pantenburg c,1
a Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, Exhibition Road, London SW7 2AY, UK
b Institut fur Anorganische Chemie, RWTH Aachen, Templergraben 55, D-52056 Aachen, Germany
¨
c Institut fur Anorganische Chemie, Universitat zu Koln, Greinstraße 6, D-50939 Koln, Germany
¨
¨
¨
¨
Received 15 July 2002; accepted 16 September 2002
Dedicated in honor of Professor Pierre Braunstein
Abstract
A boron-bridged amido-cyclopentadienyl ligand was recently shown to form constrained-geometry titanium-complexes that are
capable of olefin polymerisation. Here, the synthesis of the corresponding amido-indenyl ligand h1-C9H7B(NiPr2)N(H)Ph (C9H7ꢀ
vinyl isomerisation are discussed. Deprotonated 2a reacts with ZrCl4 to give the strongly distorted
/
indenyl) (2a/2v) and its allylꢁ
/
zirconocene-type complex [Zr{h5:h1-C9H6B(NiPr2)NPh}2] (3) in a 2:1 reaction. All compounds have been characterised by
multinuclear NMR methods and structures of both the ligand 2a and the complex 3 have been confirmed by X-ray diffraction.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Boron; Boranes; Amido-cyclopentadienyl ligand; Constitutional isomers; Zirconocene
1. Introduction
amido fragment, or the bridging moiety, with silicon and
carbon to be the most common elements in the bridge.
Recently, we started to investigate the chemistry of
both strained [3] and unstrained [4] [1]b borametalloce-
nophanes. This class of compounds exhibits an interest-
ing chemistry due to the special characteristics of the
incorporated boron atom. For example, the Lewis acidic
three coordinate boron centre in the bridge of Group IV
metallocenophanes enhances their catalytic activities [5]
Since 1990 [1] amido-cyclopentadienyl ligands (I) and
corresponding ‘constrained geometry’ complexes (CGC)
(IIa) have attracted considerable interest, because the
latter exhibit some distinct advantages as catalysts for
olefin polymerisation in comparison to metallocene-
based ZieglerꢁNatta type catalysts [2]. CGC show a
/
better copolymerisation performance with regard to
higher olefins and an increased stability towards MAO
even at elevated temperatures [1g]. A great number of
ligands of this type have been synthesised over the past
years by variation of either the cyclopentadienyl or
with respect to ZieglerꢁNatta type olefin polymerisation
/
[4a,4b,4c,6]. In due course we extended our synthetic
strategy to the synthesis of boron bridged amido-
cyclopentadienyl ligands and the first boron-bridged
CGC of titanium [7] in order to obtain catalysts that
combine the advantageous properties of both the CGC
and [1] borametallocenophanes (vide supra).
* Corresponding author. Tel./fax: ꢂ44-20-7594-5900.
E-mail address: h.braunschweig@ic.ac.uk (H. Braunschweig).
1
X-ray crystallography.
/
Synthesis of Group IV CGC is generally straightfor-
ward, applying the protonated ligand and a Group IV
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(02)01504-9

468
H. Braunschweig et al. / Inorganica Chimica Acta 350 (2003) 467ꢁ474
/
Scheme 1.
precursor such as [M(NR2)4] (Mꢀ
Et) [8] or the deprotonated ligand and a precursor such
as [MCl4(thf)x] (MꢀTi, xꢀ2 [1c]; MꢀZr, Hf, xꢀ0, 2
/
Ti, Zr, Hf, Rꢀ
/
Me,
prototropic rearrangement (1,3-shift) can generally be
achieved at elevated temperature or in the presence of
bases. In this paper, we also report on our investigations
on the isomerisation of the ligand precursor and a
related intermediate.
/
/
/
/
[9]), respectively (Scheme 1). However, formation of
metallocene-like complexes IIb with two ligands per
transition metal can often be observed under these
conditions, especially in the case of the larger metals
zirconium and hafnium, and when the transition metal
halides are used as precursors [9b,10]. Transition metal
amides give the metallocene derivatives only when an
excess of the ligand (more than a 1:1 ratio with respect
to the transition metal) is employed [8c,8d], but they are
only applicable as precursors when the ligands show a
sufficiently high acidity, e.g. for less substituted cyclo-
pentadienyl systems. Though formally being 20-electron
complexes [9b], the metallocenes incorporating two
linked amido-cyclopentadienyl ligands can catalyse the
polymerisation of ethylene and propylene when acti-
vated with MAO [8c,8d].
2. Experimental
2.1. Syntheses
In the present paper we report on the synthesis and
structural details of a new boron bridged amino-indenyl
ligand precursor and a corresponding zirconocene com-
plex incorporating two of these ligands.
Starting from lithium indenide and boron halides, the
boron can in principle end up being attached to either 1-
(allyl), 2- (vinyl) or 3-position (vinyl) of the indenyl
All manipulations were carried out under a dry
nitrogen atmosphere with common Schlenk techniques.
Solvents and reagents were dried by standard proce-
dures, distilled, and stored under nitrogen over mole-
cular sieves. iPr2NBCl2 [13] and lithium indenide
(Li[C9H7]) [14] were obtained according to literature
procedures, whereas lithium anilide (Li[NHC6H5]) was
obtained by stoichiometric addition of nBuLi to a
solution of aniline in toluene. NMR: Varian Unity 500
at 499.843 MHz (1H, external standard TMS), 150.364
system (IIIaꢁc). In most cases, only 1-indenyl and 3-
/
indenyl products can be observed [11], with the former
being the kinetically favoured and the latter being the
thermodynamically favoured product [4c,4d,11,12]. Iso-
merisation of the kinetically favoured product by
MHz (11B, BF3×
125.639 MHz (13C{1H}, APT, internal standard TMS);
/
OEt2 in C6D6 as external standard),

H. Braunschweig et al. / Inorganica Chimica Acta 350 (2003) 467ꢁ
/474
469
all NMR spectra were recorded in CD2Cl2 as solvent
unless otherwise stated. Mass spectra were recorded on a
Finnigan MAT 95 (70 eV) and elemental analyses (C, H,
N) were obtained from a Carlo-Erba elemental analyser,
model 1106.
solid. Recrystallisation from hexane at ꢃ
/
30 8C yielded
2a as colourless cubic crystals (8.54 g, 96%).
1H NMR (ꢃ
0.66 (d, 3H, 3Jꢀ
Me); 1.57 (m, 1H, CHiPr); 3.04 (m, 1H, CHiPr); 3.72 (m,
1H, BÃCHind); 5.25 (s, 1H, NH); 6.70ꢁ7.50 (m, 11H,
CHind, C6H5). 13C NMR (ꢃ
60 8C): dꢀ19.45, 23.32
(Me); 42.52 (br, BÃ
CHind); 42.78, 46.62 (CHiPr); 117.45,
/
80 8C): dꢀ
/
0.14 (d, 3H, Jꢀ
/6.71, Me);
3
/
6.71, Me); 1.25, 1.27 (2d, 3H, 3Jꢀ
/7.02,
/
/
2.1.1. h1-C9H7B(NiPr2)Cl (1a)
Li[C9H7] (2.11 g, 17.3 mmol) was suspended in 20 ml
i
hexane and a solution of Pr2NBCl2 (3.15 g, 17.3 mmol)
/
/
/
118.14, 119.11, 120.62, 121.33, 123.51, 124.42, 128.09,
128.64, 128.93, 139.70 (CHind, C6H5); 144.34, 148.60
in 15 ml hexane was added dropwise at 0 8C. The white
suspension was allowed to come to ambient temperature
and stirred for 16 h. The precipitated LiCl was filtered
off and washed with 10 ml hexane. The filtrate was
(Cind). 11B NMR: dꢀ
303(40) [Mꢂꢃ
[Mꢂꢃ
C9H7ÃC3H7], 115(25) [C9H7]. C21H27BN2
/
30.9. MS; m/z (%): 318(30) [Mꢂ],
/
Me], 203(80) [Mꢂꢃ
C9H7], 161(100)
/
/
/
(318.27) Calc. C, 79.25; H, 8.55; N, 8.80. Found: C,
78.66; H, 8.59; N, 8.78%.
concentrated and stored at ꢃ
colourless crystals (3.66 g, 98%).
1H NMR (C6D6): dꢀ
1.25, 1.27 (br 2d, 6H, Me); 3.14
(m, 2H, CHiPr); 3.80 (m, 1H, BÃ
CHind); 6.40ꢁ7.60 (m,
6H, CHind). 13C NMR (ꢃ
60 8C): dꢀ22.38, 23.23 (br,
Me); 40.43 (CHiPr); 48.38 (br, BÃ
CHind); 121.48,
122.33, 123.92, 126.11, 131.91, 136.74 (CHind); 145.68,
147.35 (Cind). 11B NMR: dꢀ
37.6. MS; m/z (%): 261(70)
[Mꢂ], 246(77) [MꢂꢃMe], 218(19) [Mꢂꢃ
C3H7],
146(64) [Mꢂꢃ
C9H7], 115(61) [C9H7], 43(100) [C3H7].
/
30 8C to yield 1a as
/
2.1.4. h1-C9H7B(NiPr2)N(H)Ph (2v)
/
/
Li[NHC6H5] (1.30 g, 13.1 mmol) was suspended in 40
ml toluene and a solution of 1v (2.84 g, 13.1 mmol) in 25
ml toluene was added dropwise at 0 8C. The reaction
mixture was allowed to come to ambient temperature
and stirred for 16 h. Precipitated LiCl was removed
/
/
/
/
/
/
from the yellowꢁgreen suspension by filtration, washed
/
/
with 10 ml hexane and the solvent of the filtrate was
remove in vacuo. Recrystallisation of the crude product
C15H21BClN (216.60) Calc. C, 68.87; H, 8.09; N, 5.35.
Found: C, 68.36; H, 8.12; N, 5.31%.
from hexane at ꢃ
crystalline solid (3.70 g, 89%).
1H NMR (ꢃ
20 8C): dꢀ1.04 (d, 3H, Jꢀ
1.06 (d, 3H, Jꢀ6.72, Me); 1.53 (d, 3H, Jꢀ
1.54 (d, 3H, Jꢀ7.02, Me); 3.45 (m, 2H, CH2ind); 3.53,
3.61 (2m, 1H, CHiPr); 5.33 (s, 1H, NH); 6.40ꢁ7.60 (m,
10H, CHind, C6H5). 13C NMR (ꢃ
60 8C): dꢀ21.65,
22.19, 23.98, 24.23 (Me); 40.45 (CH2ind); 43.01, 49.73
CHiPr); 119.33, 119.50, 121.66, 123.55, 124.07, 125.27,
126.00, 128.21, 129.01, 137.60 (CHind, C6H5); 144.34,
145.44 (Cind); 148.55 (BÃ 28.2. MS;
Cind). 11B NMR: dꢀ
m/z (%): 318(57) [Mꢂ], 303(100) [Mꢂꢃ
Me], 216(25)
[Mꢂꢃi
Pr2NBH], 115(19) [C9H7]. C21H27BN2 (318.27)
/
30 8C yielded 2v as a colourless
2.1.2. h1-C9H7B(NiPr2)Cl (1v)
3
/
/
/6.72, Me);
3
3
Compound 1a (3.92 g, 18.1 mmol) was dissolved in 50
ml hexane, a catalytic amount of triethylamine (0.2 ml,
1.4 mmol) was added and the solution was stirred for 16
h at ambient temperature. The solution was concen-
/
/6.71, Me);
3
/
/
/
/
trated and stored at ꢃ
crystalline solid (3.84 g, 98%).
1H NMR (C6D6): dꢀ
0.83, 1.40 (2d, 6H, Me); 3.11
(m, 2H, CH2ind); 3.44, 3.79 (2m, 1H, CHiPr); 6.38 (m,
/
30 8C to yield 1v as a colourless
(/
/
/
/
1H, 2-CHind); 7.00ꢁ
/
7.60 (m, 4H, CHind). 13C NMR: dꢀ
/
/
22.26, 23.22 (Me); 40.42 (CH2ind); 46.88, 51.36 (CHiPr);
122.31, 123.91, 124.88, 126.59, 137.20 (CHind); 144.21,
/
Calc. C, 79.25; H, 8.55; N, 8.80. Found: C, 78.55; H,
8.58; N, 8.78%.
147.54 (Cind), 149.43 (br, BÃ
MS; m/z (%): 261 (100) [Mꢂ], 246(88) [Mꢂꢃ
218(41) [MꢂꢃC3H7], 146(34) [Mꢂꢃ
C9H7], 115(32)
/
Cind). 11B NMR: dꢀ
/34.7.
/Me],
2.1.5. [Zr{h5:h1-C9H6B(NiPr2)NPh}2] (3)
/
/
[C9H7], 44(66) [C3H7]. C15H21BClN (216.60) Calc. C,
68.87; H, 8.09; N, 5.35. Found: C, 68.33; H, 8.14; N,
5.34%.
Compound 2a (1.27 g, 4.00 mmol) was dissolved in 40
ml toluene and 10 ml diethylether. LiBu (1.6 M) solution
in hexane (5.00 ml, 8.00 mmol) was added dropwise at
0 8C. The resulting white suspension was allowed to
come to ambient temperature and subsequently stirred
for 6 h. The mixture was then cooled to ꢃ
ZrCl4 (0.93 g, 4.00 mmol) was added. The redꢁ
2.1.3. h1-C9H7B(NiPr2)N(H)Ph (2a)
As described for 1a, Li[C9H7] (3.42 g, 28.0 mmol) was
i
reacted with Pr2NBCl2 (5.10 g, 28.0 mmol). After
/
40 8C and
/
brown
removing LiCl by filtration, the filtrate was added
dropwise at 0 8C to a suspension of Li[NHC6H5] (2.77
g, 28.0 mmol) in 50 ml toluene. The white suspension
was allowed to come to ambient temperature and stirred
for 16 h. The precipitated LiCl was filtered off, washed
with 20 ml hexane and the solvent of the filtrate was
removed in vacuo to give a slightly yellow amorphous
suspension was warmed to ambient temperature and
stirred for 16 h. Precipitated LiCl was filtered off and
washed with 10 ml toluene. The filtrate was concen-
trated and stored for 3 days at ambient temperature to
yield 3 as yellow crystals (1.37 g, 95%).
1H NMR: dꢀ
(m, 2H, CHiPr); 3.55 (m, 2H, CHiPr); 5.30 (m, 2H,
/0.85, 1.01, 1.16, 1.25 (4d, 6H, Me); 3.21

470
H. Braunschweig et al. / Inorganica Chimica Acta 350 (2003) 467ꢁ474
/
CHind); 6.80ꢁ
/
7.70 (m, 20H, CHind, C6H5). 13C NMR:
CHiPr);
0.039/0.081, GOF 0.969; max/min residual electron
ꢃ3
˚
0.47 e A
dꢀ21.47, 22.06, 26.32, 27.69 (Me); 44.40, 46.52 (
/
/
density 0.34/ꢃ
/
.
100.23, 121.54, 123.55, 125.33, 125.77, 126.13, 127.34,
128.68, 129.91 (CHind, C6H5); 159.37 (Ci). 11B NMR:
dꢀ
/
27.5. MS; m/z (%): 723(100) [Mꢂ], 707(40) [Mꢂꢃ
/
3. Results and discussion
Me], 115(50) [C9H7]. C42H50B2N4Zr (723.73) Calc. C,
69.70; H, 6.96; N, 7.74. Found: C, 68.93; H, 7.24; N,
7.13%.
3.1. Ligand synthesis
Synthesis of the diaminoindenyl borane 2a is straight-
i
forward by subsequent reaction of Pr2NBCl2 with 1
2.2. X-ray structure determination
equiv. each of lithium indenide and lithium anilide in a
hexane slurry at ambient temperature (Eq. (1)).
2.2.1. Crystal structure of compound 2a (C23H20BN2)
The intensity data were collected on a Enrafꢁ
/
Nonius
CAD4 diffractometer with Mo Ka radiation (lꢀ
/
˚
0.71073 A, graphite monochromator) at ꢃ
/
50 8C with
v ꢁ2u scans [15]. The data were corrected by LP factors,
/
and no absorption correction was done. The structure
was solved by the direct method [16] and refined by full-
matrix least-square method [17]. All non-hydrogen
atoms were refined with anisotropic displacement para-
meters. Hydrogen atoms are located from the difference
Fourier map and refined isotropically. Atom scattering
factors were taken from International Tables for X-Ray
Crystallography (1983) [18].
ð1Þ
Compound 2a, C23H20BN2, white plate of dimensions
No workup is required for the intermediate amino(in-
denyl)borane (1a), although this compound can be
easily isolated and purified by recrystallisation. Both
compounds 1a and 2a can be obtained via this route in
almost quantitative yield and multinuclear NMR spec-
tra proved them to be free of impurities. 1H NMR
spectra for both compounds reveal that the allylic boron
indenyl derivatives were formed exclusively by this
reaction sequence [4d], since only the allylic protons
0.44ꢄ
group P
13.018(4) A, aꢀ
103.99(4)8, Vꢀ954.2(9) A , Zꢀ
3591 reflections collected in the range 6.0ꢁ
3338 reflections independent, 1310 reflections observed
(I ꢀ2s(I)); 324 parameters, R1/wR2-values 0.088/0.217,
R1/wR2-values at I ꢀ
/
0.36ꢄ
/
0.16 mm, crystallises in the triclinic space
8.222(3), bꢀ10.106(8), cꢀ
111.61(4)8, bꢀ95.73(3)8, gꢀ
2, mꢀ
0.063 mmꢃ1
50.08 in 2u,
/
1 (No. 2); aꢀ
/
/
/
˚
/
/
/
3
˚
/
/
/
;
/
/
/
2s(I) 0.1848/0.2193, GOF 0.868;
ꢃ3
max/min residual electron density 0.246/ꢃ
/
0.284 e A
.
could be detected at 3.80 and 3.72 ppm, respectively. 13
C
˚
NMR spectra provide further proof since the boron
substituted carbon atoms show signals at 48.38 and
42.52 ppm, respectively, that are generally attributed to
allyl boranes, and no signals in the expected region for
2.2.2. Crystal structure of compound 3 (C42H50B2N4Zr)
The intensity data were collected on an Stoe IDPS
image plate diffractometer with Mo Ka radiation (lꢀ
/
˚
0.71073 A, graphite monochromator) at ꢃ
/
103 8C. The
vinyl boranes at 145ꢁ150 ppm could be observed [12e].
/
data was corrected by LP factors and no absorption
correction was done. The structure was solved by the
direct method and refined by full-matrix least-square
method [17,19,20]. All non-hydrogen atoms were refined
with anisotropic displacement parameters. Hydrogen
atoms are located from the difference Fourier map and
refined isotropically. Atom scattering factors were taken
from International Tables for X-Ray Crystallography
(1983) [18].
At ambient temperature, the nitrogen bonded isopro-
1
pyl groups of 2a show coalescence in both H and 13C
NMR spectra, whereas the isopropyl groups in 1a are
resolved at this temperature. This observation is due to a
localisation of the BÃ
in the former boron is bonded to two nitrogen atoms
which lowers the double-bond character of both BÃ
bonds and allows for easier rotation of the NiPr2 moiety.
At low temperature (ꢃ
80 8C), the 1H NMR spectrum of
/N bond in the latter case, whereas
/N
/
Compound 3, C42H50B2N4Zr, yellow cube of dimen-
2a shows a resolution of the signals for the isopropyl
groups, with four doublets at 0.14, 0.66, 1.25 and 1.27
ppm in an integral ratio of 3H:3H:3H:3H for the methyl
groups and two multiplets at 1.57 and 3.04 for the CH-
protons. Correlation of the dublets with the correspond-
ing multiplets was achieved by a low temperature 2D-
COSY NMR experiment. The first two doublets and the
first multiplet are attributed to the isopropyl group,
sions 0.1ꢄ
/
0.1ꢄ0.1 mm, crystallises in the monoclinic
/
space group C2/c (No. 15); aꢀ
/
19.797(3), bꢀ
/
15.400(7),
3
˚
˚
cꢀ
mꢀ
range 3.8ꢁ
3270 reflections observed (I ꢀ
R1/wR2-values 0.062/0.088, R1/wR2-values at I ꢀ
/
12.917(2) A, bꢀ
/
91.22(3)8, Vꢀ
/
3937.3(9) A , Zꢀ
/4,
/
0.260 mmꢃ1; 18 604 reflections collected in the
56.38 in 2u, 4456 reflections independent,
2s(I)); 322 parameters,
2s(I)
/
/
/

H. Braunschweig et al. / Inorganica Chimica Acta 350 (2003) 467ꢁ
/474
471
state are similar as it was already observed in the case of
the related Me2NB(C13H9)2 (C13H9ꢀfluorenyl) [4d].
The splitting of these methyl groups into two sets of
/
signals appears to be due to a hindered rotation with
i
N bond. Rotation of the Pr-group
respect to the CÃ
/
would reduce the distance between distinct methyl
protons and the indenyl plane to approximately 130
pm in comparison to about 230 pm in the solid state
conformation, therefore, preventing free rotation [21].
The signals that correspond to the isopropyl group in
trans position to the indenyl group are expectedly less
deshielded, and show likewise resolution of both methyl
groups into two sets of signals. This somehow unex-
pected observation may be due to a obstructed rotation
of this isopropyl group with respect to the CÃ
/N bond,
which may be caused by the hindered rotation of the
neighbouring isopropyl group which is attached to the
same nitrogen centre (vide supra).
A suitable single crystal of 2a was obtained from a
solution in hexane at ꢃ
lises in the space group P
symmetry (Fig. 1).
The boron and both nitrogen atoms are in a trigonal-
planar environment. The BÃN1 and BÃN2 distances of
143.0(6) and 141.3(6) pm, respectively, indicate presence
of some p-interaction in both cases. The BÃC1 distance
of 161.0(7) is similar to the corresponding distance in
iPr2NB(h1-C5H5)(h1-C9H7) [4d], but in contrast to the
crystal structure of the latter compound, the indenyl
plane is not pointed away from the nitrogen bonded
isopropyl group but towards it. This leads to a
/
30 8C. The compound crystal-
˚
Fig. 1. Molecular structure of 2a. Selected distances (A) and angles (8):
BÃN1 1.430(6), BÃN2 1.413(6), BÃC1 1.610(7), N1ÃC11 1.425(6),
N1ÃBÃN2 118.9(4), N1ÃBÃC1 117.1(4), C21ÃN2ÃC24 113.9(4),
C24ÃN2ÃBÃN1 ꢃ8.7, C21ÃN2ÃBÃN1 168.9.
/1 and the molecule adopts C1
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
which is cis to the indenyl group. Their significant high
field shift indicates a transfer of ring current from the p-
system of the indenyl group towards the neighbouring
isopropyl group. In the crystal (vide infra), the affected
isopropyl group points deep into the anisotropic cone of
the p-system and should be, therefore, fairly deshielded,
provided the molecular structures in solution and solid
/
Scheme 2.

472
H. Braunschweig et al. / Inorganica Chimica Acta 350 (2003) 467ꢁ474
/
comparably strong interaction of the isopropyl group
with the p-system as was previously discussed in
conjunction with the NMR spectroscopic data.
o(indenyl)borane as an intermediate, which underwent
rapid rearrangement.
Compound 1v and 2v were analysed by multinuclear
NMR techniques in solution. In comparison to the
allylic isomers 1a and 2a, in the 13C NMR spectra they
show signals for the boron bonded vinylic carbon atoms
at 149.43 and 148.55 ppm, respectively, and therefore, in
the expected region. The 11B NMR spectra of 1v and 2v
show signals at 34.7 and 28.2 ppm, respectively,
indicating a slightly deshielding effect on the boron
atom by the attached vinylic system in comparison to
their allylic counterparts 1a and 2a with signals at 37.6
and 30.9 ppm, respectively. In the 1H NMR spectrum of
2v, coalescence for the isopropyl groups is observed at
3.2. Indenyl borane isomerisation
Isomerisation of indenyl boranes is well-known and
occurs generally at elevated temperature or in the
presence of bases [4d,12]. We, therefore, investigated
the isomerisation of 1a and 2a that were obtained
exclusively as the allylic isomers following our reaction
sequence. Compound 1a readily undergoes isomerisa-
tion by 1,3-prototropic rearrangement to the thermo-
dynamically favoured product 1v in the presence of
catalytic amounts of NEt3, whereas 2a does not
isomerise neither in the presence of NEt3 nor in
refluxing benzene. The thermodynamically favoured
ligand precursor 2v, therefore, can be obtained only by
reaction of the intermediate 1v with lithium anilide, but
not directly from the allylic product 2a (Scheme 2).
These observations may be explained by the effect
various numbers of nitrogen atoms attached to the
boron centre have on its Lewis acidity. A higher number
of nitrogen atoms lowers the Lewis acidity of the boron
atom, and hence, reduces the p-contribution of an
attached vinylic system. At the same time, it is well-
known that in pentamethylcyclopentadienyl boranes the
activation energy for sigmatropic rearrangements is
drastically higher for less Lewis acidic amino substituted
derivatives in comparison to highly Lewis acidic alkyl or
halide substituted compounds [22]. Therefore, the in-
ability of 2a to rearrange 2v can be attributed to the low
Lewis acidity of the boron centre caused by two amino
groups, one of them even having a phenyl group
attached which can donate further p-electron density
[23]. These arguments explain as well, why previously
synthesised (iPr2N)BCl(C9H7) was exclusively obtained
as the thermodynamically favoured isomer 1v [24]. In
this case the compound was synthesised via the dichlor-
ambient temperature, whereas at ꢃ20 8C the signals are
/
resolved into four doublets for the methyl protons and
two multiplets for the CH-protons. Assignment of the
signals follows the same pattern as described for 2a (vide
supra).
3.3. Complex synthesis
Attempts to react the ligand precursor 2a and
[Zr(NEt2)4] with amine elimination to yield a zirconium
CGC failed probably due to insufficient acidity of the
ligand precursor. Deprotonation of ligand precursor 2a
with BuLi and subsequent reaction with 1 equiv. ZrCl4
in a 4:1 mixture of toluene and diethyl ether at ꢃ40 8C
/
gave the zirconocene complex 3 in high yield (Eq. 2) [25].
After removing LiCl and excess ZrCl4, recrystallisation
from toluene at ambient temperature afforded yellow
cubic crystals of 3 that were suitable for X-ray structure
determination. Compound 3 is moderately soluble in
common aliphatic and aromatic solvents, and fairly
stable towards air, especially in its crystalline from.
Although the general formation of both rac and meso
forms can be expected in the case of 3, the multinuclear
NMR data strongly suggest the formation of only one
diastereomer. As to be expected, NMR spectra of 3 are
rather complex, but reveal single sets of signals for the
(2)

H. Braunschweig et al. / Inorganica Chimica Acta 350 (2003) 467ꢁ
/474
473
Cipso atoms of the phenyl groups and the CH-moieties of
the isopropyl groups*the latter being doubled as to be
expected for a hindered rotation with respect to the BÄ
N double bond*and thus giving evidence for the
the analogue complexes [(h5-C5H5)2ZrCl2] [27],
[Me2NB(h5-C5H4)2ZrCl2] [4c] or even [{(h5-C5H4)Si-
Me2(NPh)}2Zr] [8c,8d]. The intramolecular distance of
zirconium and boron with 284.7(2) pm is about 30 pm
shorter than the analogue distance in the complex
[Me2NB(h5-C5H4)2ZrCl2] [4c], and only about 27 pm
longer than the sum of the respective van der Waals
/
/
/
presence of only one diastereomer. It should be noted,
though that the methyl groups show a double set of
1
signals in the H and 13C NMR spectra. This could be
radii. No data for classical ZrÃ
/
B bonds are available for
accounted for by a hindered rotation of the isopropyl
groups due to sterical hindrance as previously discussed
for the free ligand. The 11B NMR signal at 27.5 ppm is
in the expected region for aryl diamino boranes [26].
The compound crystallises in the space group C2/c
and the molecule adopts C2-symmetry in the crystal
(Fig. 2). Both boron and nitrogen atoms are in a
direct comparison. It should be noted though, that a
corresponding close contact between iron and boron
was observed in [1] boraferrocenophanes [iPr2NB(h5-
C5H4)2Fe], however, without indication of a bonding
interaction between the two atoms [3c]. The interaction
of zirconium and boron in 3 is subject of further
investigations.
trigonal-planar environment. The boronÃ
tance for BÃN2 of 140.8(1) pm indicates a boronÃ
nitrogen double bond, whereas the boronÃnitrogen
distance for BÃN1 of 144.6(7) pm is slightly elongated,
probably due to steric constraints in the compound.
The zirconiumÃnitrogen distances of 215.7(0) pm are
significantly longer than the zirconiumÃnitrogen dis-
/nitrogen dis-
/
/
/
4. Conclusions
/
With h1-C9H7B(NiPr2)N(H)Ph (2a/v) we synthesised
a new boron bridged amino-cyclopentadienyl precursor
that is potentially suitable for the synthesis of new CGC
with increased activity towards polymerisation of ole-
fines. Isomerisation studies on the boron bonded
indenyl moiety of the ligand precursor and a reaction
intermediate showed a inhibited isomerisation in the
presence of two attached amino groups on the boron
atom, whereas in the case of only one amino group rapid
isomerisation occurs. Finally, we established the synth-
esis of the zirconocene type complex [Zr{h5:h1-
C9H6B(NiPr2)NPh}2] (3) incorporating two of the
described ligands in high yield. Work on the application
of the ligand precursor to the synthesis of CGC, the
/
/
tance in the related Zr CGC [{(h5-C5Me4)SiMe2(Nt-
Bu)}ZrCl2] (205.2(2) pm) [8e] and slightly longer than in
the analogue zirconocene complex [{(h5-C5H4)Si-
Me2(NPh)}2Zr] (212.7(2) and 213.7(2) pm) [8c,8d].
This may be due to an electronic ‘oversaturation’ of
the zirconium centre with formally 20 electrons [9b] in
the case of 3 and the bridging boron atoms withdrawing
p-electron density from the nitrogen centres. The
deformation angle indcÃ
/
ZrÃ
/
ind?c of 138.1(8)8, the ZrÃ
ZrÃN1? angle
/
indc distance of 222.8(4) pm and the N1Ã
/
/
of 98.2(9)8 indicate strong distortion in comparison to
nature of the ZrÃ
/
B interaction in the described zirco-
nocene and the activity of this complex are in progress.
5. Supplementary material
Crystallographic data (excluding structure factors) for
the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre
CCDC Nos. 193253 (2a) and 193255 (3). Copies of the
data can be obtained free of charge on application to
The Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK [fax:
it@ccdc.cam.ac.uk or www: http://www.ccdc.cam.a-
ꢂ44-1223-336-033; e-mail: depos-
/
c.uk].
Acknowledgements
This work was supported by BASF AG Ludwigsha-
fen, BMBF, DFG, EPSRC, Fonds der Chemischen
Industrie, R. Soc. F.M.B. thanks the Fonds der
Chemischen Industrie for a doctoral scholarship.
˚
Fig. 2. Molecular structure of 3. Selected distances (A) and angles (8):
ZrÃ
N2 1.408(1), BÃ
138.1(8), N1ÃZrÃN1? 98.2(9), N1Ã
/
N1 2.157(0), ZrÃ
C2 1.604(3), N1Ã
BÃC2 104.0(2).
/
B 2.847(2), ZrÃ
/
indc 2.228(4), BÃ
/
N1 1.446(7), BÃ
/
/
/
C10 1.418(2), indcà ind?c
/
ZrÃ
/
/
/
/
/

474
H. Braunschweig et al. / Inorganica Chimica Acta 350 (2003) 467ꢁ474
/
(e) D.W. Carpenetti, L. Kloppenburg, J.T. Kupec, J.L. Petersen,
Organometallics 15 (1996) 1572.
References
[9] (a) H.V.R. Dias, Z. Wang, S.G. Bott, J. Organomet. Chem. 508
(1996) 91;
[1] (a) P.J. Shapiro, E.E. Bunel, W.P. Schaefer, J.E. Bercaw,
Organometallics 9 (1990) 867;
(b) J. Okuda, F. Amor, K.E. du Plooy, T. Eberle, K.C. Hultzsch,
T.P. Spaniol, Polyhedron 17 (1998) 1073.
[10] (a) U. Bohme, K.H. Thiele, J. Organomet. Chem. 472 (1994) 39;
¨
(b) W.E. Piers, P.J. Shapiro, E.E. Bunel, J.E. Bercaw, Synlett
(1990) 74.;
(c) J. Okuda, Chem. Ber. 123 (1990) 1649;
(d) J.C. Stevens, F.J. Timmers, D.R. Wilson, G.F. Schmidt, P.N.
Nickias, R.K. Rosen, G.W. Knight, S.Y. Lai (Dow), Eur. Pat.
Appl. EP-416-815-A2, 1991;
(b) K.E. du Plooy, U. Moll, S. Wocadlo, W. Massa, J. Okuda,
Organometallics 14 (1995) 3129;
(c) L. Duda, G. Erker, R. Frohlich, F. Zippel, Eur. J. Inorg.
¨
Chem. (1998) 1153.;
(d) H.G. Alt, K. Fottinger, W. Milius, J. Organomet. Chem. 572
¨
(e) J.A. Canich (Exxon), Pat. Appl. WO 91/04257, 1991;
(f) T. Eberle, T.P. Spaniol, J. Okuda, Eur. J. Inorg. Chem. (1998)
237;
(1999) 21;
(e) K. Kunz, G. Erker, S. Doring, S. Bredeau, G. Kehr, R.
¨
Frohlich, Organometallics 21 (2002) 1031.
¨
(g) A.L. McKnight, R.M. Waymouth, Chem. Rev. 98 (1998) 2587
(and references therein);
[11] For an exception see R. Duchateau, S.J. Lancaster, M. Thornton-
Pett, M. Bochmann, Organometallics 16 (1997) 4995.
[12] (a) B.M. Mikhailov, T.K. Baryshnikova, V.S. Bogdanov, Zh.
Obsch. Him. 43 (1973) 1949;
(h) J. Okuda, T. Eberle, in: A. Togni, R.A. Halterman (Eds.),
Metallocenes, vol. 2 (Chapter 7, and references therein), Wiley-
VCH, New York, 1998;
(i) G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem. 111
(1999) 448;
(b) G.E. Herberich, E. Barday, A. Fischer, J. Organomet. Chem.
567 (1998) 127;
(j) G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem., Int.
Ed. Engl. 38 (1999) 428 (and references therein).
[2] (a) H.H. Brintzinger, D. Fischer, R. Mulhaupt, B. Rieger, R.M.
¨
(c) I.D. Gridnev, A. Meller, Main Group Metal Chem. 21 (1998)
271;
(d) J. Knizek, I. Krossing, H. Noth, W. Ponikwar, Eur. J. Inorg.
¨
Waymouth, Angew. Chem. 107 (1995) 1255;
Chem. (1998) 505.;
(e) E. Barday, B. Frange, B. Hanquet, G.E. Herberich, J.
Organomet. Chem. 572 (1999) 225.
(b) H.H. Brintzinger, D. Fischer, R. Mulhaupt, B. Rieger, R.M.
¨
Waymouth, Angew. Chem., Int. Ed. Engl. 34 (1995) 1143 (and
references therein);
[13] W. Gerrard, H.R. Hudson, E.F. Mooney, J. Chem. Soc. (1960)
5168.
(c) W. Kaminski, J. Chem. Soc., Dalton Trans. (1998) 1413 and
references therein.;
[14] L. Brandsma, Preparative Organometallic Chemistry, vol. 2,
Springer, Berlin, 1990, p. 45.
(d) W. Kaminski, Chem. Rev. 4 (2000) 100 (and references
therein).
[3] (a) H. Braunschweig, R. Dirk, M. Muller, P. Nguyen, D.P. Gates,
¨
[15] Enraf-Nonius, CAD4 Operations Manual, Delft, 1977.
[16] G.M. Sheldrick, Acta Crystallogr., A 46 (1990) 467.
[17] SHELXL93, Program for the Refinement of Crystal Structures,
I. Manners, Angew. Chem. 109 (1997) 2433;
(b) H. Braunschweig, R. Dirk, M. Muller, P. Nguyen, D.P. Gates,
¨
G.M. Sheldrick, University of Gottingen, Gottingen, Germany,
¨ ¨
I. Manners, Angew. Chem., Int. Ed. Engl. 36 (1997) 2338;
(c) A. Berenbaum, H. Braunschweig, R. Dirk, U. Englert, J.C.
Green, A.J. Lough, I. Manners, J. Am. Chem. Soc. 122 (2000)
5765.
1993.
[18] T. Hahn (Ed.), International Tables for X-Ray Crystallography,
vol. A, D, Reidel, Dordrecht, 1983.
[19] WINGX 1.63: L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
[20] SIR 92-program for crystal structure solution, A. Altomare, G.
Cascarano, C. Giacovazzo, A. Guagliardi, J. Appl. Crystallogr.
26 (1993) 343.
[4] (a) H. Braunschweig, C. von Koblinski, R. Wang, Eur. J. Inorg.
Chem. (1999) 69;
(b) H. Braunschweig, C. von Koblinski, M.O. Kristen, Patent OZ
0050/49643, 1998, patent pending;
[21] As shown by computational modelling by a geometrical ap-
proach.
(c) H. Braunschweig, C. von Koblinski, M. Mamuti, E. Englert,
R. Wang, Eur. J. Inorg. Chem. (1999) 1899;
(d) H. Braunschweig, C. von Koblinski, M. Neugebauer, U.
Englert, X. Zheng, J. Organomet. Chem. 619 (2001) 305.
[5] J.A. Klang, D.B. Collum, Organometallics 7 (1988) 1532.
[6] A.J. Ashe, III, X. Fang, J.W. Kampf, Organometallics 18 (1999)
2288.
[22] (a) P. Jutzi, A. Seufert, Chem. Ber. 112 (1979) 2481;
(b) P. Jutzi, B. Krato, M. Hursthouse, A. Howes, Chem. Ber. 120
(1987) 565.
[23] For observation of a 1,3-sigmatropic rearrangement in an indenyl
bis(dialkylamino) borane see Ref. [12b].
[24] G. Zi, H.W. Li, Z. Xie, Organometallics 21 (2002) 1136.
[25] The fact that ZrCl4 reacts with 2 equiv. of the ligand although a
1:1 mixture was employed is not entirely unexpected and was
observed before in the case of [{(h5-C5Me4)SiMe2(NtBu)}2Zr]
[10a].
[7] H. Braunschweig, C. von Koblinski, U. Englert, Chem. Commun.
(2000) 1049.
[8] (a) A.K. Hughes, A. Meetsma, J.H. Teuben, Organometallics 12
(1993) 1936;
(b) W.A. Herrmann, M.J.A. Morawietz, J. Organomet. Chem.
482 (1994) 169;
[26] H. Noth, B. Wrackmeyer, in: P. Diehl, E. Fluck, R. Kosfeld
¨
(eds.), Nuclear Magnetic Resonance, Spectroscopy of Boron
Compounds, vol. 14, NMR, Basic Principles and Progress,
Springer, Berlin, 1978.
(c) W.A. Herrmann, M.J.A. Morawietz, T. Priermeier, Angew.
Chem. 106 (1994) 2025;
(d) W.A. Herrmann, M.J.A. Morawietz, T. Priermeier, Angew.
Chem., Int. Ed. Engl. 33 (1994) 1946;
[27] K. Prout, T.S. Cameron, R.A. Forder, S.R. Critchley, B. Denton,
G.V. Rees, Acta Crystallogr., B 30 (1974) 2290.