Synthesis and structural characterization of sterically crowded hydridotris(pyrazolyl)borato complexes: Unusual double 1,2-borotropic shift at a titanium centre

Article abstract of DOI:10.1016/j.jorganchem.2006.06.026

The potassium and thallium(I) derivatives of the hydrido-tris(3-Me₂Bz,5-Me-pyrazol-1-yl)borato ligand, K[Tp^Me2^Bz,Me] (1) and Tl[Tp^Me2^Bz,Me] (2), are reported. Their reactions with TiCl₄ und

Full text of DOI:10.1016/j.jorganchem.2006.06.026

Journal of Organometallic Chemistry 691 (2006) 4172–4180
www.elsevier.com/locate/jorganchem
Synthesis and structural characterization of sterically crowded
hydridotris(pyrazolyl)borato complexes: Unusual double
1,2-borotropic shift at a titanium centre
a,
b
b
Paolo Biagini *, Fausto Calderazzo , Fabio Marchetti ,
a
a
Anna Maria Romano , Silvia Spera
a
EniTecnologie S.p.A – Centro Ricerche di Novara ‘‘Istituto Guido Donegani’’, via G. Fauser 4, I-28100 Novara, Italy
b
`
Universita di Pisa, Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35, I-56126 Pisa, Italy
Received 10 May 2006; received in revised form 8 June 2006; accepted 8 June 2006
Available online 30 June 2006
This paper is dedicated to the memory of Roberto Santi, a highly respected scientist and a friend, deceased on December the 26th 2003.
Abstract
The potassium and thallium(I) derivatives of the hydrido-tris(3-Me2Bz,5-Me-pyrazol-1-yl)borato ligand, K[TpMe2Bz,Me] (1) and
Tl[TpMe2Bz,Me] (2), are reported. Their reactions with TiCl4 under mild conditions afforded, after rearrangement of the ligand, recognized
as a double 1,2-borotropic shift, the related Ti(IV) complex {hydrido-[bis(3-Me,5-Me2Bz)(3-Me2Bz,5-Me)-pyrazol-1-yl]borato}trichlo-
1
rotitanium(IV), [TpMe2Bz,Me**] TiCl3 (3). The H- and 13C NMR spectra of the new compounds are discussed, the molecular structure
of 3 has been determined by X-ray diffraction methods, the titanium centre has a pseudo-octahedral coordination, with the nitrogen and
chloride ligands in a fac geometry. A possible mechanism of formation of 3 is proposed.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Tris(pyrazolyl)borate ligands; Titanium; 1,2-borotropic shift
1. Introduction
fin polymerization catalysts, in the presence of several tran-
sition elements such as Group 4 metals [4], vanadium [5]
and nickel [6]. In some cases the presence of three bulky
groups in position 3 of the pyrazolyl ring produces high
steric crowding around the metal centre, so that the forma-
tion of isomeric complexes, through a 1,2-borotropic shift
which exchanges the positions 3 and 5 of the pyrazolyl ring,
can occur under particular conditions [4d,4f,6a]. These iso-
merizations are well documented in the literature for
several transition metals [7] and for aluminium [8]. The
1,2-borotropic shift represents a simple way to obtain
new complexes of Cs symmetry, of interest for catalysis
[4] and for bio-inorganic chemistry [2c,9].
Since the first report by Trofimenko [1], hydrido-
tris(pyrazol-1-yl)borato (Tp0) ligands have been extensively
used in coordination- and organometallic chemistry [2].
Tp0 ligands offer the possibility of an efficient three-dimen-
sional control around the metal centre and the modulation
of electronic effects, because as many as three positions are
available for substitution in each pyrazole ring. These fea-
tures have been applied in several fields of chemistry, from
modelling in metallo-enzymes, to catalysis and materials
science, through analytical chemistry and organic synthesis
[3]. It has recently been observed that Tp0 ligands, bearing
bulky substituents in position 3 or 5, generate efficient ole-
These observations prompted us to investigate the
reactions of sterically hindered Tp0 ligands and here we
report the potassium and thallium(I) derivatives of the
new hydrido-tris(3-Me2Bz,5-Me-pyrazol-1-yl)borato ligand,
*
Corresponding author. Tel.: +39 0 321 447817; fax: +39 0 321 447241.
E-mail address: paolo.biagini@PolimeriEuropa.com (P. Biagini).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.06.026

P. Biagini et al. / Journal of Organometallic Chemistry 691 (2006) 4172–4180
4173
K[TpMe2Bz,Me] (1) and Tl[TpMe2Bz,Me] (2), which are com-
pared in their chemical and spectroscopic features. The
reaction of 1 or 2 with TiCl4 affords, after a double
isomerization of the ligand, the related Ti(IV) complex {hydr-
ido-[bis(3-Me,5-Me2Bz)(3-Me2Bz,5Me)-pyrazol-1-yl]borato}-
trichlorotitanium(IV), [TpMe2Bz,Me**]TiCl3 (3) [10]. All
compounds were spectroscopically characterized and the
molecular structure of 3 is described.
205Tl–13C coupling constants are solvent – and temperature
dependent: in this case they were measured in CD2Cl2 at
room temperature. In the 13C NMR spectrum, 205Tl–13C
couplings were observed for the pyrazolyl CH resonance
(JTl–C4 = 25.6 Hz), for the aromatic carbons (JTl–C ortho
81.7 Hz; JTl–C meta = 37.6 Hz; JTl–C para = 16.7 Hz; JTl–Cq
=
=
55.6 Hz), for the quaternary carbons on the pyrazolyl ring
(JTl–C5 = 42.6 Hz) and for the methyl of the dimethylbenzyl
groups (JTl–C = 96.8 Hz). These data are in agreement with
those already reported for similar compounds [12]. The
quite large 13C–205Tl couplings for almost all the carbons
of the dimethylbenzyl group suggest that this substituent
is close to the thallium centre, therefore in position 3, for
the three pyrazole groups, while the methyl groups are in
position 5.
2. Results and discussion
The preparation of 5-phenyl-5-methyl-2,4-hexanedione
has been carried out by condensation of acetone with the
corresponding aromatic a-bromoketone, according to the
general method [11] for 1-aryl-2,4-diones. The related 3-
(10,10-dimethylbenzyl)-5-methyl-pyrazole has been pre-
pared by a conventional method [2c], namely the reaction
of the b-diketone with hydrazine, and converted to the cor-
responding K[TpMe2Bz,Me] (1) with the stoichiometric
amount of KBH4 at 240 ꢁC, without solvent. The potas-
sium derivative 1 has been further converted to the corre-
sponding thallium derivative 2, according to the literature
method [2], see Scheme 1.
The B-H stretch is quite similar: 2505 cmꢀ1 for 1 and
2511 cmꢀ1 for 2. In some cases, depending on the recrystal-
lization conditions, 1 shows two distinct bands (2519,
2461 cmꢀ1) in the B-H stretching region. This behaviour
could be ascribed to a solid-state effect, as these materials
1
have the same H NMR spectrum in CD2Cl2 solution.
Compounds 1 and 2 form with high regioselectivity, no
other isomers in solution being detected spectroscopically.
These observations could be interpreted by taking into
account the steric bulk of the dimethylbenzyl groups, which
probably prevents their location in positions 5 of the pyra-
zole ring, at variance with reports [13] on the synthesis of
similar dialkyl-hindered TpR3,R5 ligands (R3 ¼ R5).
Both 1 and 2 show an unusual solubility in aliphatic
hydrocarbons, probably due to multiple interaction of the
metal centre with the nitrogen atoms of the anion. Accord-
ingly, 1 and 2 were recrystallized from hexane.
The NMR spectra of 1 and 2 are in agreement with an
idealized C3v symmetry, showing all pyrazolyl groups to
be equivalent. The NMR characterization of 1 is straight-
forward, see Section 3. The thallium complex 2 and its
NMR characterization show some interesting aspects. In
The reaction of K[TpMe2Bz,Me] (1) or Tl[TpMe2Bz,Me] (2)
with one equivalent of TiCl4 in CH2Cl2 or toluene at 0 ꢁC,
see Scheme 2, affords the Ti(IV) trichloride complex
[TpMe Bz,Me
**]TiCl3 (3) which was obtained as a red–orange
2
1
the H NMR spectrum of 2, only the proton at position
solid in yields as high as 79%, after precipitation with hex-
ane at ꢀ18 ꢁC. Compound 3 is well soluble in polar and
aromatic solvents, has a poor solubility in aliphatic hydro-
carbons, is moderately stable in air in the solid state, while
its solutions rapidly decompose on exposure to air.
4 of the pyrazolyl ring clearly shows 205Tl–H coupling
(JTl–H = 6.4 Hz), while the methyl groups and the aromatic
Hortho protons of the dimethylbenzyl groups show just a
broadening. As already reported [12], 205Tl–H and
The structure of 3 has been assigned on the basis of
NMR analysis and confirmed in the solid state by an X-
ray diffraction experiment. The molecular structure of 3
is shown in Fig. 1, while Table 1 lists the geometrical
parameters. The metal has a pseudo-octahedral coordina-
tion. Moreover, as it is normally found in titanium–
trichloro–pyrazolylborato complexes [4h,7c,14], the three
chloride ligands are in a fac geometry. The pyrazolylborato
ligand shows a configuration of the N(1)–N(2) and N(5)–
N(6) rings resulting from a 1,2-borotropic shift and the
N(3)–N(4) ring only shows the bulky dimethylbenzyl group
in position 3. As a result of this encumbering presence, the
O
O
(i)
+
Br
(ii)
O
O
(iii)
˚
Ti–N(3) bond is 0.074 A longer than the average of the
(iv)
(v)
other two Ti–N bonds.
N
N
N
N
The molecules are held together by van der Waals inter-
actions. The unit cell contains, in addition to four
[TpMe2Bz,Me**]TiCl3 molecules, two toluene molecules,
which, being placed on inversion centres, are disordered
and convey some disorder also to the nearby portions of
the titanium-containing complex.
B
H
H
3
Scheme 1. Synthesis of [TpMe2Bz,Me ꢀ
as its potassium (1) or thallium
derivatives (2). (i): EtMgCl, t-BuOH, 0 ꢁC; (ii): refluxing toluene, 4 h; (iii):
NH2–NH2 Æ H2O in refluxing EtOH for 1.5 h; (iv): KBH4, 240 ꢁC, 4 h; (v):
(iv) + TlNO3 in H2O/organic solvents.
]

4174
P. Biagini et al. / Journal of Organometallic Chemistry 691 (2006) 4172–4180
[a]
[a]
N
N
N
N
Toluene, or CH Cl
2
2
TiCl
TiCl
3
+
4
H
B
N
N
0 ˚C
B
H
M
2
3
M= K (1); Tl (2)
(3)
[b]
Scheme 2. Preparation of 3, with the labeling scheme of the pyrazole groups adopted in the NMR characterization.
The latter shows: (a) two signals of relative intensity 2:1
assigned to pyrazolyl protons at 6.12 and 5.75 ppm, (b)
three signals for the methyl benzyl groups at 1.86, 1.42
and 1.25 ppm with relative intensities 6:6:6 and (c) two sig-
nals for the methyl group on the pyrazolyl ring, one at 2.69
and the other at very low-frequency, 0.92 ppm, with rela-
tive intensity 2:1.
The signals can be grouped as in Table 2, where are also
reported (for comparison) the chemical shifts of 1 and 2.
Besides, the H-B chemical shifts have also been reported,
obtained from boron-decoupled broad-band 1H NMR
spectra.
On the basis of the data in Table 2, it can be inferred
that there are two sets of pyrazolyl resonances for 3, one
having intensities corresponding to only one pyrazolyl ring
[a], and the other having intensities pertaining to two pyr-
azolyl rings [b] (see Scheme 2). The benzylic methyl groups
in set [b], being diastereotopic, give rise to two different sig-
nals at 1.42 and 1.25 ppm.
The assignment of the unusual low-frequency chemical
shift at 0.92 ppm to the methyl group on the pyrazole, set
[a], was confirmed by the COSY 1H–1H spectrum reported
in Fig. 3, where are clearly evident the cross-peak connect-
ing the H-4 protons at 6.12 ppm (set [b]) with the pyrazole
methyl group at 2.69 ppm and the cross-peak connecting
the H-4 proton at 5.75 ppm (set [a]) with the methyl group
at 0.92 ppm.
Fig. 1. View of the molecular structure of compound [TpMe2Bz,Me**]TiCl3
in {[TpMe2Bz,Me**]TiCl3 Æ 0.5 toluene} (3 Æ 0.5 toluene). Thermal ellipsoids
are at 30% probability.
Table 1
˚
Bond lengths (A) and angles (ꢁ) around the Ti atom in
{[TpMe2Bz,Me**]TiCl3 Æ 0.5toluene} (3 Æ 0.5 toluene)
Ti–Cl(1)
Ti–Cl(2)
Ti–Cl(3)
N(1)–N(2)
N(3)–N(4)
N(5)–N(6)
Cl(1)–Ti–Cl(2)
Cl(2)–Ti–Cl(3)
Cl(1)–Ti–N(3)
Cl(2)–Ti–N(1)
Cl(2)–Ti–N(5)
Cl(3)–Ti–N(3)
N(1)–Ti–N(3)
N(3)–Ti–N(5)
2.242(1)
2.255(1)
2.254(1)
1.376(4)
1.387(4)
1.371(4)
93.79(6)
93.94(6)
89.22(9)
89.2(1)
Ti–N(1)
Ti–N(3)
Ti–N(5)
N(2)–B
N(4)–B
N(6)–B
Cl(1)–Ti–Cl(3)
Cl(1)–Ti–N(1)
Cl(1)–Ti–N(5)
Cl(2)–Ti–N(3)
Cl(3)–Ti–N(1)
Cl(3)–Ti–N(5)
N(1)–Ti–N(5)
2.177(3)
2.241(3)
2.158(3)
1.548(5)
1.544(5)
1.561(5)
99.96(5)
171.2(1)
89.99(9)
174.3(1)
88.1(1)
However, the intensities are reversed with respect to the
assignments in the literature [4d,4f], i.e. 6.12 ppm (two pro-
tons) and 5.75 ppm (one proton). Moreover, in our case the
unique pyrazolyl ring is characterized by resonances shifted
towards lower frequencies with respect to the literature
data. Thus, in our case two pyrazolyl rings undergo a 1,2
borotropic shift to high-frequency while the low-frequency
signal maintains the original position. This hypothesis is
confirmed by the X-ray crystal structure (see above). A
peculiar feature of 3, disclosed by the X-ray structure is
91.0(1)
90.30(9)
87.2(1)
168.6(1)
81.6(1)
84.2(1)
˚
the distance of 3.44 A between C(14) and the centre of
the aromatic ring [C(8)–C(13)], see Fig. 4. Thus, the consis-
tent low-frequency shift (0.92 ppm) of the methyl in posi-
tion 5 (set [a]) (for comparison see the chemical shift of
methyl groups in the same position in other Tp0 titanium
complexes [14,15]) can reasonably be attributed to the
1
The H- and 13C NMR spectra of 3 are consistent with
the Cs-symmetry of the anion, as for comparison between
the 1H NMR spectrum of 1 (Fig. 2a) and that of 3
(Fig. 2b).

P. Biagini et al. / Journal of Organometallic Chemistry 691 (2006) 4172–4180
4175
CH3-(Bz)
a
b
CH3-(Pyr)
CH-(ar)
H-4
*
*
b
a
b
b
a
b
a
*
7.0
6.0
5.0
4.0
3.0
2.0
1.0
ppm
Fig. 2. 1H NMR spectra of K[TpMe2Bz,Me] (1) (a) and [TpMe2Bz,Me**]TiCl3 (3) (b) in CD2Cl2 at room temperature. For the labelling scheme, see Table 2
and Scheme 2. The signals marked with an asterisk are due to impurities. The signal at 5.30 ppm is due to CH2Cl2.
Table 2
Selected 1H NMR data for compounds 1, 2 and 3; in CD2Cl2 at room temperature
H-4
CH3-(Pyr)
CH3-(Bz)
H-B
K[TpMe2Bz,Me] (1)
(Relative intensity)
Tl[TpMe2Bz,Me] (2)
(Relative intensity)
[TpMe2Bz,Me**]TiCl3 (3)
(Relative intensity)
5.83
3
2.31
9
1.44
18
4.80
1
5.93 d
3
2.38
9
1.42 br
18
4.70
1
4.05a
1
6.12
2
5.75
1
2.69
6
0.92
3
1.86
6
1.42
6
1.25
6
[b]
[a]
[b]
[a]
[a]
[b]
[b]
a
In C7D8 solution.
Fig. 3. COSY 1H–1H of [TpMe2Bz,Me**]TiCl3 (3), in CD2Cl2 at room temperature.

4176
P. Biagini et al. / Journal of Organometallic Chemistry 691 (2006) 4172–4180
in solution, i.e. that the intramolecular interactions are
stronger than solid-state effects. In the 1H{B11} NMR spec-
trum the H-B resonance at 4.05 ppm (see Table 2) is quite
different from the chemical shift observed for complexes 1
and 2, thus suggesting, also for this proton, a different
chemical environment.
The observed B-H stretch at 2564 cmꢀ1 for 3 is in agree-
ment with the reported values for other similar dialkyl-
TpR3,R5 complexes of titanium(IV) [14,15].
The synthesis of 3 has some unusual features within the
chemistry of Tp0 complexes. To the best of our knowledge,
3 is the first compound which is directly obtained through a
double 1,2-borotropic shift, starting from the correspond-
ing symmetrical ligand. In fact in the other cases already
reported [4f,6a,17] such an isomerization always occurs
starting from a TpR* [TpR* = HB(3-alkylpyrazolyl)2(5-
alkylpyrazolyl)ꢀ] transition metal compound and generally
it is determined by the formation of the more sterically
demanding dinuclear pentacoordinated nickel(II) [6a] or
hexacoordinated zirconium(IV) [4f] complexes. Generally
a 1,2-borotropic shift is supposed to occur through inter-
mediate species in which the coordination mode of the
Tp0 ligand changes from the initial k3N,N0,N00 to k2 N,N0,
the transformation being favoured by increasing the tem-
perature and/or in the presence of polar coordinating sol-
vents, such as THF [4d,4f,6a–8]. In the present case, the
isomerization of the two pyrazole rings occurs at low tem-
perature and in the absence of oxygenated solvents [18]. A
plausible mechanism is shown in Scheme 3. The first inter-
action with the titanium centre may be realized through the
coordination of only one pyrazole ring, while the other
Fig. 4. Projection of the molecular structure of compound [TpMe2Bz,Me
]
**
TiCl3 in {[TpMe2Bz,Me**]TiCl3 Æ 0.5 toluene} (3 Æ 0.5 toluene), displaying the
short distance between the methyl C(14) and the centre of the aromatic
ring [C(8)–C(13)].
shielding effect of the aromatic ring current [16] which can
influence the magnetic field around the methyl group.
Obviously, this interpretation stands on the assumption
that the solid-state geometrical arrangement is maintained
H
B
Ph
N
N
N
N
TiCl
4
H
N
N
N
N
N
N
B
N
K
toluene, 0 °C
N
Ph
Ph
Ph
TiCl
4
K
Ph
Ph
H
- KCl
Ph
B
1,2-B shift
Ph
N
N
N
N
N
N
Ti
Ph
Cl
Cl
Cl
Scheme 3. Possible mechanism of formation of 3.

P. Biagini et al. / Journal of Organometallic Chemistry 691 (2006) 4172–4180
4177
two, which eventually still interact with the alkali metal,
3.2. 5-Phenyl-5-methyl-2,4-hexanedione
undergo the 1,2-borotropic shift, which enables the bulky
dimethylbenzyl groups to shift to position 5. Finally, also
the isomerized pyrazole rings bind to titanium and pro-
mote the elimination of the chloride anion.
To a 3 M THF solution (200 ml) of EtMgBr (0.6 mol),
300 ml of toluene were added, followed by the dropwise
addition, in 3 h at 0 ꢁC, of t-BuOH (57.5 ml, 0.6 mol) dis-
solved in 50 ml of toluene. At the end of the addition,
the reaction mixture was allowed to warm up to room tem-
perature and stirred overnight. Acetone (22.3 ml, 0.3 mol),
diluted with 30 ml of toluene, was slowly added in about
30 min. The resulting solution was treated with 36 ml
(0.21 mol) of 2-bromo-iso-butyrophenone and refluxed
for 4 h, the completion of the reaction being verified by
thin layer chromatography (disappearance of the bromo-
derivative). The reaction mixture was then hydrolysed, at
0 ꢁC, with 400 ml of H2SO4 (5%) and stirred for 3 h. The
organic layer was separated and the aqueous phase was
extracted with Et2O (4 · 100 ml). The resulting organic
solution was washed with NaHCO3 (5%) and water to neu-
trality and finally dried over Na2SO4. After evaporation of
the volatiles under reduced pressure, a yellow–orange
liquid was obtained (43 g), which was distilled affording
32 g [fraction bp (1 mmHg) 69–73 ꢁC] of the yellow 5-phe-
nyl-5-methyl-2,4-hexanedione (75% yield, based on 2-
bromo-iso-butyrophenone). 1H NMR (CDCl3): 15.4 (s,
1H, OH en.); 7.4–7.0 (m, 5H en. + 5H ket., Harom.); 5.31
(s, 1H, CH en.); 3.31 (s, 2H, CH2 ket.); 2.00 (s, 3H, CH3
ket.); 1.88 (s, 3H, CH3 en.); 1.50 (s, 6H, CH3-(Bz) en.);
1.47 (s, 6H, CH3-(Bz) ket.) (en. = 85 %; ket. = 15 %, at
room temperature). 13C NMR (CDCl3): 205.5 (CO, ket.);
201.8 (CO, en.); 201.5 (CO, ket.); 187.0 (C–OH en.);
145.1 (Cq arom. en.); 142.4 (Cq arom. ket.); 128.8, 127.1,
125.9 (CH arom. ket.); 128.2; 126.4, 126.0 (CH arom.
en.); 97.6 (CH en.); 52.7; 47.8; 30.0; 26.0; 24.3; 23.5. IR
Support to this hypothesis comes from an experiment
in which the reaction between K[TpMe2Bz,Me] and TiCl4
has been performed directly in a NMR tube, in d8-toluene
at 0 ꢁC at different intervals of time in an attempt to
follow the transformation of the system. However, the res-
onances of 3 were immediately detected, thus suggesting
that 3 is already present as such and does not arise from
an isomerization of a more symmetric titanium complex.
A substantial contribution to the stabilization of the
coordinatively unsaturated intermediate species of tita-
nium, described in Scheme 3, could derive from an inter-
action with the aromatic ring of the dimethylbenzyl
group, or from the formation of chloride-bridged dimers,
as already reported for Group 4 metals in their reactions
with Lewis bases [19]. The observations that some steri-
cally hindered Tp0 ligands can rearrange under mild con-
ditions and that the isomerization could be mediated by
electropositive metals (Scheme 3) greatly support the
hypothesis that similar processes may generate active spe-
cies under polymerization conditions, when an excess of
aluminium compounds is normally present, which might
compete with the transition metal for the coordination
of the Tp0 ligand.
3. Experimental
3.1. General procedures
All operations were carried out using standard Schlenk-
tube techniques, under an atmosphere of prepurified nitro-
gen. The reaction vessels were oven-dried prior to use.
Solvents were dried by conventional methods.
(neat) m=cmꢀ1: 3409br,w (mOH), 3088vw, 3060w, 3025w,
~
2976m, 2931m, 2876vw, 1724w, 1703w, 1626vs (mC@O),
1599vs (mC@O), 1496m, 1462w, 1447m, 1418m, 1385w,
1364m, 1303w, 1252s, 1188m, 1156vw, 1118m, 1097s,
1076w, 1054vw, 1031m, 937m, 864s, 793m, 766s, 741w,
700vs, 646w, 555w.
IR spectra were recorded at 2 cmꢀ1 resolution on a Nic-
olet Nexus FT-IR spectrophotometer; the complexes were
analysed in transmission as nujol mulls prepared under
exclusion of moisture and air. NMR spectra were recorded
with a Varian VXR 300 NMR spectrometer in CD2Cl2 [ref-
erence peaks: d(1H) = 5.30 ppm and d(13C) = 53.7 ppm] or
in CDCl3 solutions [reference peaks: d(1H) = 7.26 ppm and
d(13C) = 77.0 ppm] with a pulse width of 7 ls (40ꢁ) and a
3.3. 3-(10,10-Dimethylbenzyl)-5-methyl-pyrazole
To a solution of 19.97 g of 5-phenyl-5-methyl-2,4-
hexanedione (97.9 mmol) in 80 ml of EtOH, 25 ml of
NH2NH2 Æ H2O (0.51 mol) dissolved in 50 ml of EtOH,
were added dropwise and the reaction mixture was refluxed
for 2 h. After addition of 100 ml of Et2O, the resulting solu-
tion was washed with water (3 · 50 ml) to remove the
excess hydrazine, and dried over Na2SO4. After evapora-
tion of the solvent under reduced pressure, a pale yellow
solid was obtained (16.92 g), which was recrystallized from
hexane at ꢀ18 ꢁC affording 13.04 g (67% yield) of colour-
less large crystals of 3-(10,10-dimethylbenzyl)-5-methyl-pyr-
azole. Found: C, 77.5; H, 7.9; N, 14.0%. C13H16N2
requires: C, 77.6; H, 8.5; N, 13.9%. Mass spectrum (DCI)
1
relaxation delay of 1.5 s. H{11B} broad-band decoupled
NMR spectra were obtained with a Bruker Avance
1
400 MHz instrument. The H–1H COSY experiment was
performed by using the conventional Bruker pulse
sequence. Mass spectra were recorded on a Finningan
MAT 8400 scan instrument using the DCI technique and
iso-butane as matrix.
Acetone and t-BuOH were distilled over CaH2. EtMgBr
(3 M solution in THF, Acros), 2-bromo-iso-butyrophenone
(Aldrich), NH2NH2 Æ H2O (Fluka), KBH4 (Aldrich),
Tl(NO3) (Aldrich) and TiCl4 (Strem) were used without
further purification.
1
m/z: [M + H+] 201 (100%). H NMR (CD2Cl2): 10.12 (s,
broad, 1H, N-H); 7.27–7.17 (m, 5H, Harom.); 5.86 (s, 1H,

4178
P. Biagini et al. / Journal of Organometallic Chemistry 691 (2006) 4172–4180
H-4); 2.21 (s, 3H, CH3-5); 1.65 (s, 6H, CH3-(Bz)). 13C
NMR (CD2Cl2): 157.5 (1C, Cq); 149.4 (1C, Cq); 144.1
(1C, Cq); 128.5 (2C, CH arom.); 126.5 (2C, CH arom.);
126.3 (1C, CH arom.); 102.6 (1C, CH-4); 39.4 (1C, Cq-
(Bz)); 29.9 (2C, CH3-(Bz)); 12.3 (1C, CH3-5). IR (nujol
and the resulting solution was dried over Na2SO4. A pale
yellow sticky solid was obtained on evaporation of the sol-
vent in vacuo: hexane (50 ml) was added and the resulting
slurry was stirred for 1 h. The colourless solid was recov-
ered by filtration and dried in vacuo giving 4.9 g of 2. An
additional crop of product (1.8 g) was recovered by cooling
the mother liquor at ꢀ18 ꢁC (57% total yield). Found: C,
56.9; H, 5.8; N, 10.2%. C39H46BN6Tl requires: C, 57.5;
H, 5.7; N, 10.3%. Mass spectrum (DCI) m/z: [M + H+]
mull) m=cmꢀ1: 3161w, 3126vw, 3085m, 2727vw, 1599vw,
~
1570m, 1365w, 1298s, 1238w, 1202w, 1189vw, 1157m,
1115w, 1075m, 1031w, 1017s, 1000w, 944vw, 921vw,
903vw, 860br,m, 788m, 777w, 764m, 759s, 722m, 697vs,
566w, 555w, 507vw.
1
815 (100%). H NMR (CD2Cl2): 7.15 (m, 9H, Hmeta+para);
7.01 (m broad, 6H, Hortho); 5.93 (d, 3H, H-4, JTl–H
= 6.4 Hz); 4.70 (s, broad, 1H, H-B); 2.38 (s broad, 9H,
CH3-5); 1.42 (s, 18H, CH3-(Bz)). 13C NMR (CD2Cl2):
163.3 (d, 1C, C-3, JTl–C = 42.6 Hz); 152.1 (d, 1C, Cq arom.,
JTl–C = 55.6 Hz); 145.4 (s, 1C, C-5); 130.2 (d, 2C, CH-meta,
JTl–C = 37.6 Hz); 128.6 (d, 2C, CH-ortho, JTl–C = 81.7 Hz);
127.6 (d, 1C, CH-para, JTl–C = 16.7 Hz); 105.0 (d, 1C, CH-
4, JTl–C = 25.6 Hz); 41.3 (s, 1C, Cq-(Bz)); 33.1 (d, 2C, CH3-
(Bz), JTl–C = 96.8 Hz); 14.9 (s, 1C, CH3-5). IR (nujol mull)
3.4. Potassium hydrido-tris[3-(10,10-dimethylbenzyl)-5-
methyl-pyrazol-1-yl]borato, K½TpMe2Bz;Me2ꢁ (1)
A mixture of 0.98 g of KBH4 (18.1 mmol) and 10.97 g of
3-(10,10-dimethylbenzyl)-5-methyl-pyrazole
(54.6 mmol),
maintained under a dinitrogen atmosphere, was heated
gradually with stirring up to 240 ꢁC in about 2 h and kept
at this temperature until the di-hydrogen evolution was
over (3–5 h). The melt was cooled down to room tempera-
ture in a nitrogen atmosphere and 70 ml of hexane were
added. The resulting slurry was stirred for 1 h and the col-
ourless solid was then recovered by filtration and dried in
vacuo giving 5.2 g of 1. An additional crop of product
(1.9 g) was recovered by cooling the mother liquor at
ꢀ18 ꢁC (61% total yield). Found: C, 71.0; H, 7.2; N,
12.8%. C39H46BKN6 requires: C, 72.2; H, 7.1; N, 12.9%.
Mass spectrum (DCI) m/z: [M + H+] 650 (100%). 1H
NMR (CD2Cl2): 7.30–7.10 (m, 15H, Harom.); 5.83 (s, 3H,
H-4); 4.80 (s, broad, 1H, H-B); 2.31 (s, 9H, CH3-5); 1.44
(s, 18H, CH3-(Bz)). 13C NMR (CD2Cl2): 161.3 (1C, C-3);
152.0 (1C, Cq arom.); 144.2 (1C, C-5); 128.8 (2C, CH
arom.); 127.5 (2C, CH arom.);126.5 (1C, CH arom.);
102.0 (1C, CH-4); 40.3 (1C, Cq-(Bz)); 31.3 (2C, CH3-
m=cmꢀ1: 3080vw, 3055w, 2723vw, 2511w (mBH), 1531m,
~
1494s, 1422m, 1360s, 1340m, 1235w, 1184s, 1157vw,
1067s, 1029w, 1011m, 984w, 831w, 806w, 791m, 764vs,
752m, 722w, 703s, 698s, 653m, 623w, 562m.
3.6. {Hydrido-[(3-Me,5-Me2Bz)2(3-Me2Bz,5Me)-pyrazol-
Bz;Me2ꢂꢂꢁTiCl3 ð3Þ
1-yl]borato}trichloro-titanium(IV), ½TpMe
2
3.6.1. From 1, in toluene as solvent
In a 250 ml round-bottomed flask were introduced
1.56 g (2.4 mmol) of K½TpMe2Bz;Me2ꢁ (1), dissolved in 50 ml
of toluene. The colourless solution was cooled with an ice
bath and 0.3 ml of TiCl4 (2.7 mmol) in 20 ml of toluene
were added dropwise in about 30 min. The resulting
orange–brown solution was stirred at 0 ꢁC for 24 h. After
removal of KCl by filtration, the volume of the solution
was reduced to 30 ml in vacuo and 30 ml of hexane were
finally added. After cooling at ꢀ18 ꢁC, the red–orange
crystals which separated out were recovered by filtration
and dried in vacuo affording 1.09 g of 3 (59% yield). Found:
C, 62.0; H, 6.0; N, 9.8; Cl, 12.9%. C39H46BCl3N6Ti
requires: C, 61.3; H, 6.1; N, 11.0; Cl, 13.9%. Mass spectrum
(DCI, negative ion) m/z: [Mꢀ] 764 (80%); [Mꢀ ꢀ Cl] 727
(20%); [2 pz + Cl] 435 (40%); [pz + Cl] 235 (100%). 1H
NMR (CD2Cl2): 7.3–6.9 (m, 13H, Harom. [a + b]); 6.7–6.8
(m, 2H, Hortho[a]); 6.12 (s, 2H, H-4 [b]); 5.75 (s, 1H, H-4
[a]); 2.69 (s, 6H, CH3-(Pyr) [b]); 1.86 (s, 6H, CH3-(Bz)
[a]); 1.42 (s, 6H, CH3-(Bz) [b]); 1.25 (s, 6H, CH3–(Bz)
[b]); 0.92 (s, 3H, CH3-(Pyr) [a]). 13C NMR (CD2Cl2):
166.9 (1C, C-3 [a]), 158.3 (2C, C-3 [b]), 156.5 (2C, Cq arom.
[b]), 152.0 (1C, Cq arom. [a]), 150.0 (2C, C-5 [b]), 147.9 (1C,
C-5 [a]), 130.6–127.1 (CH arom.); 110.2 (1C, C-4 [a]); 109.6
(2C, C-4 [b]); 44.0 (1C, Cq-(Bz) [a]); 41.3 (2C, Cq-(Bz) [b]);
34.6 (2C, CH3-(Bz) [b]); 33.9 (2C, CH3-(Bz) [b]); 32.2 (2C,
CH3-(Bz) [a]); 19.5 (2C, CH3-(Pyr) [b]); 12.8 (1C, CH3-
(Pyr) [a]). IR (nujol mull) ~m=cmꢀ1: 3141vw, 3086vw,
3051w, 3021w, 2726vw, 2564m (mBH), 1600m, 1581w,
1541s, 1530s, 1495s, 1413s, 1365m, 1338m, 1275vw,
(Bz)) 14.0 (1C, CH3-5). IR (nujol mull) m=cmꢀ1: 3085vw,
~
3057w, 2726vw, 2519w (mBH), 2461w (mBH), 1598m,
1578w, 1533s, 1494s, 1446m, 1422w, 1365m, 1360m,
1346s, 1239w, 1202m, 1180m, 1096w, 1072s, 1030w,
1008m, 980w, 926vw, 905vw, 821w, 805m, 779m, 769s,
751w, 728vw, 701vs, 679vw, 654m, 649m, 624w, 571m
515w.
3.5. Thallium hydrido-tris[3-(10,10-dimethylbenzyl)-5-
methyl- pyrazol-1-yl]borato, Tl½TpMe2Bz;Me2ꢁ ð2Þ
In a round-bottomed flask were introduced 0.78 g of
KBH4 (14.4 mmol) and 8.89 g of 3-(10,10-dimethylbenzyl)-
5-methyl-pyrazole (44.2 mmol). By operating under a
nitrogen atmosphere, the solid mixture was gradually
heated up to 240 ꢁC and kept at this temperature for about
5 h, up to the end of di-hydrogen evolution. The melt was
finally cooled down to room temperature in a di-nitrogen
atmosphere and the resulting solid was dissolved in 40 ml
of THF. The tetrahydrofuran solution was then added to
a mixture of 5 g (18.8 mmol) of TlNO3, in 80 ml of H2O
and 80 ml of CH2Cl2. After stirring at room temperature
for 1 h, the product was extracted with Et2O (6 · 20 ml)

P. Biagini et al. / Journal of Organometallic Chemistry 691 (2006) 4172–4180
4179
1238w, 1208m, 1177s, 1135m, 1100vw, 1085w, 1074w,
1064w, 1052s, 1030w, 1007m, 949vw, 925vw, 905w, 852w,
836m, 815w, 806m, 793w, 769s, 746m, 729m, 717s, 700vs,
672w, 651w, 640m, 626w, 561w.
(No. 14) space group. After completion of the main mole-
cule, a sequence of broad maxima was found in the differ-
ence Fourier map around the inversion centre at 0, 0, 1/2,
Wyckoff position b, due to the presence of a disordered tol-
uene molecule. An idealized toluene molecule was then
found to fit the sequence of maxima and introduced in
the calculations with an occupancy factor of 1/2. The
3.6.2. From 1, in dichloromethane
In a 100 ml round-bottomed flask were introduced 0.97 g
(1.5 mmol) of K½TpMe2Bz;Me2ꢁ (1) dissolved in 25 ml of
CH2Cl2; a solution of 0.2 ml (1.8 mmol) of TiCl4 in 25 ml
of CH2Cl2 was then added dropwise in about 30 min at
room temperature. By operating as above described,
0.59 g (52% yield) of 3 were recovered. Analytical and spec-
troscopic data were in accord with those reported above.
ꢀ
action of the 1 operator brought the occupancy factor of
the site up to 1 and gave account of the disorder. The mul-
tiplicity of the site b gave a 3/toluene ratio of 0.5. The fol-
lowing refinement cycles brought some thermal parameters
of the phenyl groups in 3 to abnormally high values, possi-
bly due to some degree of disorder induced by the toluene
molecule. The refinement was, however, continued without
introducing the disorder in those groups.
3.6.3. From 2, in dichloromethane
In a 250 ml round-bottomed flask were introduced
0.97 g (1.2 mmol) of Tl½TpMe2Bz;Me2ꢁ (2), dissolved in 50 ml
of CH2Cl2. The colourless solution was cooled with an
ice bath and 0.16 ml of TiCl4 (1.4 mmol) in 20 ml of
CH2Cl2 were added dropwise in about 30 min. The solu-
tion, which turned to orange–brown within a few minutes,
was stirred at 0 ꢁC for 2 h. Then the solvent was removed in
vacuo and the residue was treated with 50 ml of toluene, the
pale yellow residue of TlCl was separated by decantation,
and the resulting red–orange solution was treated with
80 ml of hexane and cooled at ꢀ18 ꢁC. The red–orange
crystals which separated out were recovered by filtration
The calculations were done by using the SHELXTL pro-
gram [21] and some routines contained in the WINGX
suite [22].The final refinement cycle including 511 parame-
ters gave for the 7655 intensity data with I > 2r (I) a R1 fac-
tor of 0.0584, a wR2 factor of 0.1427 and a S factor of 1.010
calculated on F2.
Acknowledgements
The authors of the University of Pisa wish to thank the
`
`
Ministero dell’Universita e dell’Istruzione, dell’Universita e
della Ricerca, Programmi di Ricerca Scientifica di Rilev-
ante Interesse Nazionale 2004-5, for financial support.
1
and dried in vacuo affording 0.72 g of 3 (79% yield). H
NMR (C7D8): 7.20–6.80 (m, 15H, Harom. [a + b]); 5.77
(s, 2H, H-4 [b]); 5.51 (s, 1H, H-4 [a]); 4.05 (s, broad, 1H,
H-B); 2.82 (s, 6H, CH3-(Pyr) [b]); 1.99 (s, 6H, CH3–(Bz)
[a]); 1.23 (s, 6H, CH3-(Bz) [b]); 1.06(s, 6H, CH3-(Bz) [b]);
0.80 (s, 3H, CH3-(Pyr) [a]).
Appendix A. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 298918 for compound 3. Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (Fax: (int code) +44 1223 336 033, or e-mail: depos-
it@ccdc.cam.ac.uk or www:http://www.ccdc.cam.ac.uk.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2006.06.026).
3.7. Crystal structure determination of compound 3
Single crystals of (3 Æ 0.5 toluene), empirical composition
C
42.5H50BCl3N6Ti, FW 809.95 a.m.u., suitable for the X-
ray diffraction analysis, were obtained at room temperature
by slow diffusion of hexane into a toluene solution. The dif-
fractometric experiment was carried out by using a Bruker
AXS P4 instrument equipped with graphite-monochro-
˚
mated Mo-Ka radiation (k = 0.71073 A). All data were col-
References
lected in the x/2h scan mode, and three standard
reflections were monitored every 97 measurements for
checking crystal decay and equipment stability. For data
reduction the XSCANS program was used [20]. A suitable
crystal was glued at the end of a glass fibre and the unit cell
[1] S. Trofimenko, J. Am. Chem. Soc. 88 (1966) 1842.
[2] (a) S. Trofimenko, Scorpionates: The Coordination Chemistry of
Polypyrazolylborate Ligands, Imperial College Press, London, 1999;
(b) G. Parkin, Adv. Inorg. Chem. 42 (1995) 291;
´
(c) N. Kitajima, W.B. Tolman, Progr. Inorg. Chem. 43 (1995) 419;
(d) S. Trofimenko, Chem. Rev. 93 (1993) 943;
˚
parameters, a = 13.880(2), b = 16.260(2), c = 18.542(2) A,
a = c = 90, b = 93.97(1)ꢁ, were calculated from the setting
angles of 35 strong reflections. A set of 9287 intensity data
were collected between 1.7ꢁ 6 h 6 25.5ꢁ and corrected for
Lorentz polarization and absorption effects (w-scan
method, l = 0.435 mmꢀ1). After merging of equivalent
(e) S. Trofimenko, Chem. Rev. 72 (1972) 497.
[3] C. Pettinari, C. Santini, Polypyrazolylborate and scorpionate ligands,
in: J.A. McCleverty, T.J. Meyer (Eds.), Comprehensive Coordination
Chemistry II, in: A.B. P. Lever (Ed.) Elsevier Pergamon, Oxford, 1,
2004, 159 and ff.
[4] (a) F.A. Kunrath, R.F. de Souza, O.L. Casagrande, Macromol.
Rapid Commun. 21 (2000) 277;
P
P
reflections fRint ¼ ½ jF 2o ꢀ F o2ðmeanÞj= ðF 2oÞꢁ ¼ 0:0371g,
7655 independent reflections were obtained. The structure
was solved by the standard direct methods in the P21/n
(b) L.G. Furlan, M.P. Gil, O.L. Casagrande, Macromol. Rapid
Commun. 21 (2000) 1054;

4180
P. Biagini et al. / Journal of Organometallic Chemistry 691 (2006) 4172–4180
(c) M.P. Gil, J.H.Z. dos Santos, O.L. Casagrande, Macromol. Chem.
Phys. 202 (2001) 319;
(d) S. Murtuza, O.L. Casagrande, R.F. Jordan, Organometallics 21
(2002) 1882;
[11] (a) A.V. Kel’in, Y. Yu. Kozyrkov, Synthesis (1998) 729;
(b) E.P. Kohler, M. Tishler, J. Am. Chem. Soc. 57 (1935) 217.
[12] C. Lopez, D. Sanz, R.M. Claramunt, S. Trofimenko, J. Elguero, J.
Organometal. Chem. 503 (1995) 265.
(e) K. Michiue, R.F. Jordan, Macromolecules 36 (2003) 9707;
(f) K. Michiue, R.F. Jordan, Organometallics 23 (2004) 460;
(g) M.P. Gil, J.H.Z. dos Santos, O.L. Casagrande, J. Mol. Catal. A:
Chem. 209 (2004) 163;
[13] (a) M. Cano, J.V. Eras, C.J. Jones, J.A. McCleverty, S. Trofimenko,
Polyhedron 9 (1990) 619;
(b) O.M. Reinaud, A.L. Rheingold, K.H. Theopold, Inorg. Chem. 33
(1994) 2306.
(h) M.P. Gil, O.L. Casagrande, J. Organomet. Chem. 689 (2004) 286;
(i) G. Parkin, Adv. Inorg. Chem. 42 (1995) 291;
(j) A.H. Cowley, L. Geerts, C.M. Nunn, S. Trofimenko, J. Organo-
met. Chem. 365 (1989) 19;
(k) K. Yoon, G. Parkin, Polyhedron 14 (1995) 811;
(l) D.D. LeCloux, C.J. Tokar, M. Osawa, R.P. Houser, M.C. Keyes,
W.B. Tolman, Organometallics 13 (1994) 2855;
(m) G. Ferguson, M.C. Jennings, F.J. Lalor, C. Shanahan, Acta
Crystallogr., Sect. C: Cryst. Struct. Commun. 47 (1991) 2079;
(n) A. Rheingold, R.L. Ostrander, B.S. Haggerty, S. Trofimenko,
Inorg. Chem. 33 (1994) 3666;
[14] (a) D.L. Hughes, G.J. Leigh, D.G. Walker, J. Chem. Soc., Dalton
Trans. (1988) 1153;
(b) A. Antinolo, F. Carrillo-Hermosilla, A.E. Corrochano, J.
Fernandez-Baeza, M. Lanfranchi, A. Otero, M.A. Pellinghelli, J.
Organomet. Chem. 577 (1999) 174.
[15] (a) H. Cai, W.H. Lam, X. Yu, X. Liu, Z.Z. Wu, T. Chen, Z. Lin,
X.T. Chen, X.Z. You, Z. Xue, Inorg. Chem. 42 (2003) 3008;
(b) J.K. Kouba, S.S. Wreford, Inorg. Chem. 15 (1976) 2313;
(c) J. Ipaktschi, W. Sulzbach, J. Organomet. Chem. 426 (1992) 59.
[16] F.A. Bovey, Nuclear Magnetic Resonance Spectroscopy, Academic
Press, New York, 1969.
(o) E. Libertini, K. Yoon, G. Parkin, Polyhedron 12 (1993) 2539;
(p) R. Han, G. Parkin, S. Trofimenko, Polyhedron 14 (1995) 387.
[5] O.L. Casagrande, A.C.A. Casagrande, M.P. Gil, J.J. Zacca, R. F.
Jordan, Br. 9904045-0 (1999) to OPP Petrochemicals.
[6] (a) F.A. Kunrath, R.F. de Souza, O.L. Casagrande, N.R. Brooks,
V.G. Young, Organometallics 22 (2003) 4739;
[17] A. Albinati, M. Bovens, H. Ruegger, L.M. Venanzi, Inorg. Chem. 36
(1997) 5991.
[18] The reaction between TlTpMes and ZrCl4 or HfCl4, in toluene
as solvent has been recently reported to produce mixtures of
TpMesZrCl3/TpMes*ZrCl3 and TpMesHfCl3/TpMes*HfCl3, respec-
tively [4f].
´
(b) L.G. Furlan, F.A. Kunrath, R.S. Mauler, R.F. de Souza, O.L.
Casagrande, J. Mol. Catal. A: Chem. 214 (2004) 207;
(c) R. Santi, A.M. Romano, A. Sommazzi, M. Grande, C. Bianchini,
G. Mantovani, J. Mol. Catal. A: Chem. 229 (2005) 191.
[7] (a) S. Trofimenko, J.C. Calabrese, P.J. Domaille, J.S. Thompson,
Inorg. Chem. 28 (1989) 1091;
[19] (a) C.I. Bra¨nden, I. Lindqvist, Acta Chem. Scand. 14 (1960) 726;
(b) L. Burn, Acta Crystallogr. 20 (1966) 739;
(c) I.W. Bassi, M. Calcaterra, R. Intrito, J. Organometal. Chem. 127
(1977) 305;
(d) D.A. Young, J. Mol. Catal. 53 (1989) 433;
(e) E. Solari, C. Floriani, K. Schenk, J. Chem. Soc., Chem. Commun.
(1990) 963;
(b) J.C. Calabrese, S. Trofimenko, Inorg. Chem. 31 (1992) 4810;
(c) D.D. LeCloux, M.C. Keyes, M. Osawa, V. Reynolds, W.B.
Tolman, Inorg. Chem. 33 (1994) 6361.
(f) T. Lis, J. Utko, P. Sobota, Acta Crystallogr., Sect. C 49 (1993)
2089;
[8] (a) A. Looney, G. Parkin, Polyhedron 9 (1990) 265;
(b) M.H. Chisholm, N.W. Eilerts, J.C. Huffman, Inorg. Chem. 35
(1996) 445.
[9] M. Lukasiewicz, Z. Ciunik, S. Wolowiec, Polyhedron 19 (2000) 2119.
[10] The abbreviation TpMe2Bz;Me2ꢂꢂ is based on Trofimenko’s rules [2] and
denotes the presence of two dimethylbenzyl groups in position 5 – of
the pyrazolyl ring.
(g) F. Calderazzo, M. D’Attoma, F. Marchetti, G. Pampaloni, S.I.
Troyanov, J. Chem. Soc., Dalton Trans. (1999) 2275.
[20] XSCANS, X-ray Single Crystal Analysis System, Rel. 2.1 Bruker AXS
Inc., Madison, Wisconsin, USA, 1994.
[21] G.M. Sheldrick, SHELXTL-Plus, Rel. 5.03, Bruker AXS Inc., Madison,
Wisconsin, USA, 1995.
[22] L.J. Farrugia, J. Appl. Cryst. 32 (1999) 837.