Hafnium imido complexes containing silyl ligands

Article abstract of DOI:10.1016/S0022-328X(01)01386-9

The hafnium imido dichloride disilyl complex (2,6-i-Pr₂C₆H₃)N=Hf(THF) ₂(SiPh₃)₂ (2) in low yield. Compound 2 was crystallographically characterized. The hafnium monosilyl complex (2,6-iPr

Full text of DOI:10.1016/S0022-328X(01)01386-9

Journal of Organometallic Chemistry 643–644 (2002) 431–440
www.elsevier.com/locate/jorganchem
Hafnium imido complexes containing silyl ligands
Ivan Castillo, T. Don Tilley *
Department of Chemistry, Uni6ersity of California at Berkeley, Berkeley, CA 94720-1460, USA
Received 7 August 2001; accepted 9 October 2001
Dedicated to Professor Franc¸ois Mathey on the occasion of his 60th birthday
Abstract
The hafnium imido dichloride (2,6-i-Pr2C6H3)NꢀHf(THF)2Cl2 (1) reacts with two equivalents of Ph3SiLi(THF)3 to yield the
corresponding disilyl complex (2,6-i-Pr2C6H3)NꢀHf(THF)2(SiPh3)2 (2) in low yield. Compound 2 was crystallographically
characterized. The hafnium monosilyl complex (2,6-i-Pr2C6H3)NꢀHf(4-t-BuC5H4N)2Cl[Si(SiMe3)3] (4) was synthesized in a similar
manner from (2,6-i-Pr2C6H3)NꢀHf(4-t-BuC5H4N)2Cl2 (3) and one equivalent of (Me3Si)3SiK in good yield. The Cp*-containing
hafnium imido complex {Cp*HfCl[m-N(2,6-i-Pr2C6H3)]}2 (5) was prepared by thermolysis of Cp*Hf[Si(SiMe3)3][NH(2,6-i-
Pr2C6H3)]Cl via loss of (Me3Si)3SiH. Complex 5 was crystallographically characterized. Attempts to prepared derivatives of 5 with
reactive s-bonds resulted in the formation of Cp*-cyclometallated species. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Hafnium imido complexes; Silyl ligands; Thermolysis
Therefore, we expect a new generation of more active
silane dehydropolymerization catalysts to feature ancil-
lary ligands other than cyclopentadienyl [8].
1. Introduction
The chemistry of early transition and f-metal com-
plexes in catalytic processes has developed dramatically
in recent years. Olefin polymerization [1], hydrogena-
tion [2], hydrosilylation [3], silane dehydropolymeriza-
tion [4], and alkane activation [5] are among the
transformations that have been accomplished with such
species. Most of the research to date has focused on the
use of metallocene-derived systems. Although several
researchers have undertaken the investigation of non-
metallocene-based systems [6], much remains to be
learned about the role played by non-cyclopentadienyl
ancillary ligands in early transition and f-metal complex
reactivity.
Investigations in our group have focused on silane
dehydropolymerizations with Group 4 metallocene cat-
alysts, among which CpCp*Zr[Si(SiMe3)]Me is the
most active to date (Cp=h5-C5H5, Cp*=h5-C5Me5)
[7d]. Altering the silyl, alkyl, or hydride ligands in the
‘mixed ring’ systems CpCp*M(SiR3)X (M=Zr, Hf;
R=H, alkyl, aryl; X=H, alkyl, aryl, Cl) [7] has not
led to significant improvements in catalytic activity.
Ancillary ligand set modifications might create a
more electrophilic environment at the metal center,
which may lead to more active catalysts for s-bond
metathesis reactions. For example, Basset and co-work-
ers have recently described catalytic CꢁH and CꢁC
bond activations under mild conditions [9]. The re-
markable reactivity of the silica- and alumina-sup-
ported zirconium and tantalum centers used in the
Basset systems is believed to be derived from the en-
hanced electrophilicity conferred by the silica and alu-
mina surfaces, which act as hard ligands. In addition,
the coordinatively unsaturated single-site metal hydride
species generated on the solid supports are immobilized
to prevent dimerization, which could inhibit catalytic
activity.
We believe that replacement of soft cyclopentadienyl
ligands with hard, bulky imido ligands [10] will enhance
the electrophilicity of transition metal centers, while
sterically protecting them from dimerization. Within
this context, recent efforts in our laboratories have
focused on the synthesis of Group 5 and 6 metal
complexes with imido ancillary ligands [11]. Thus far,
* Corresponding author.
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01386-9

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the activity of such complexes towards s-bonds has
remained limited to hydrogenations [11c] and phenylsi-
lane activations [11a]. It is expected that more elec-
trophilic Group 4 metals may display enhanced s-bond
metathesis reactivity in d0 complexes that incorporate
imido ancillary ligands. Herein, we report our syntheses
of hafnium imido complexes that feature reactive s-
bonds for elementꢁelement and elementꢁhydrogen bond
activations.
(1)
The most prominent 1H-NMR features of 2 include a
doublet at l 1.12, which corresponds to the i-Pr groups
of the imido ligand, and the associated methine septet at
1
l 4.48. The aromatic resonances in the H-NMR spec-
trum integrate to 33 H, in agreement with the presence
of six phenyl groups and one (2,6-i-Pr2C6H3)N ligand.
The 29Si-NMR spectrum contains a single resonance at
l 35.1, confirming the presence of equivalent silyl lig-
ands.
2. Results and discussion
2.1. Synthesis and characterization of
(2,6-i-Pr2C6H3)NꢀHf(THF)2(SiPh3)2 (2)
The solid-state structure of 2 was determined by
X-ray crystallography. Yellow cubic crystals of 2 were
obtained from a cold, concentrated pentane solution.
The coordination environment around the Hf atom is
that of a pseudo-trigonal bipyramid, with the two THF
ligands in the axial positions. The OꢁHfꢁO angle of
169.9° is close to linear as expected for the axial ligands
of a trigonal bipyramid, with HfꢁO bond lengths of 2.17
Our initial approach to the synthesis of Group 4
imido complexes featuring reactive s-bonds focused on
the known hafnium compound (2,6-i-Pr2C6H3)Nꢀ
Hf(THF)2Cl2 (1) [10e] (THF=tetrahydrofuran). How-
ever, addition of lithium silyl reagents R3SiLi(THF)3
[R3=(SiMe3)3, t-BuPh2, Mes2H] [12] to solutions of 1 at
low temperatures in a variety of solvents resulted in the
formation of intractable mixtures of products, which
contained large amounts of the corresponding silane
R3SiH, and (2,6-i-Pr2C6H3)NH2. Identical results were
obtained whether one or two equivalents of the lithium
silyl reagents were used. The analogous reaction be-
tween two equivalents of Ph3SiLi(THF)3 [12d] and 1 in
diethyl ether at −80 °C led to the isolation of low
yields (510%) of the hafnium disilyl derivative (2,6-i-
Pr2C6H3)NꢀHf(THF)2(SiPh3)2 (2, Eq. (1)). To our
knowledge, complex 2 represents the first example of a
Group 4 metal complex featuring imido and silyl lig-
ands. Although 2 was fully characterized by spectro-
scopic methods and single crystal X-ray crystallography,
attempts to optimize its yield, even in the dark, were
unsuccessful. This prevented a detailed study of the
reactivity of the HfꢁSi bond of 2.
,
and 2.18 A to O(1) and O(2), respectively. The imido
and triphenylsilyl ligands occupy the equatorial posi-
tions, likely due to steric constraints. The short HfꢁN
,
bond length of 1.82 A is characteristic of hafnium imido
complexes [10f], as is the CipsoꢁNꢁHf angle of 162.4°,
which is indicative of multiple bonding between
hafnium and nitrogen [10f]. Complex 2 possesses nearly
,
identical HfꢁSi bond lengths of 2.87 and 2.85 A to Si(1)
and Si(2), respectively. The HfꢁSi distances are almost
,
identical to that of CpCp*Hf[Si(SiMe3)3]Cl (2.88 A) [7c].
The sum of the angles between the ligands in the
equatorial plane is very close to 360°, as expected for a
planar trigonal geometry. Notably, the angle between
the two silyl ligands of 129.7° is the largest of all three,
indicative of the large steric demand of such groups. An
ORTEP diagram of 2 is presented in Fig. 1.
Unfortunately, the reactivity of 2 and analogous
hafnium disilyl complexes could not be explored due to
its low yield, and the lack of success in preparing related
compounds with other silyl ligands. Since all reactions
of 1 with lithium silyl reagents led to the formation of
large amounts of the corresponding silane and diiso-
propyl aniline, regardless of the solvent used in such
reactions, we believe that the THF ligands might be
acting as a source of hydrogen atoms. Thus, we devised
the synthesis of hafnium imido complexes of the type
(2,6-i-Pr2C6H3)NꢀHfL2Cl2 (L=two-electron donor),
featuring the potentially more inert pyridine ligands.
2.2. Preparation of
(2,6-i-Pr2C6H3)NꢀHf(4-t-BuC5H4N)2Cl2 (3)
Rothwell and co-workers have previously reported
the preparation of hafnium imido compounds of the
Fig. 1. ORTEP diagram of (2,6-i-Pr2C6H3)NꢀHf(THF)2(SiPh3)2 (2).

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433
2.3. Synthesis and reacti6ity of
(2,6-i-Pr2C6H3)NꢀHf(4-t-BuC5H4N)2Cl[Si(SiMe3)3] (4)
type
(2,6-i-Pr2C6H3)NꢀHf(py%)2[HN(2,6-i-Pr2C6H3)]2
(py%=4-pyrrolidinopyridine) [10f], which are potential
precursors to the corresponding hafnium imido dichlo-
rides. Initial efforts focused on the treatment of (2,6-i-
As in the case of 1, no hafnium silyl complexes were
isolated from reactions with either one or two equiva-
lents of lithium silyls. Although many solvents and
reaction temperatures were investigated, complex mix-
tures of products were obtained in all cases. As men-
tioned above, we reasoned that the formation of silanes
and aniline in these reactions was related to the pres-
ence of THF in the reaction mixtures. Since lithium
silyl reagents typically have THF associated with them,
we decided to explore the reactivity of THF-free silyl
anions. The potassium silyl compound (Me3Si)3SiK [13]
Pr2C6H3)NꢀHf(py)2[HN(2,6-i-Pr2C6H3)]2
(py=pyri-
dine, which was prepared by addition of two equiva-
lents of pyridine to pentane solutions of Hf[HN(2,6-i-
Pr2C6H3)]4 [10f]) with two equivalents of Me3SiCl, to
replace the amido ligands with chlorides. Heating or-
ange solutions of (2,6-i-Pr2C6H3)NꢀHf(py)2[HN(2,6-i-
Pr2C6H3)]2 in the presence of two equivalents of
Me3SiCl led to the precipitation of a pale yellow pow-
der, which was assigned as impure (2,6-i-Pr2C6H3)-
NꢀHf(py)2Cl2 (by comparing its IR spectrum with that
of its fully characterized analogue 3, vide infra). Unfor-
tunately, this compound could not be obtained in ana-
lytically pure form, as its solubility in polar (pyridine,
acetonitrile, methylene chloride, chloroform) and non-
polar (pentane, toluene, benzene, diethyl ether) solvents
is negligible. Washing the solid with several portions of
pentane and diethyl ether did not provide material that
could be identified as (2,6-i-Pr2C6H3)NꢀHf(py)2Cl2 by
combustion analysis. Formation of insoluble species is
likely responsible for incomplete reaction of (2,6-i-
Pr2C6H3)NꢀHf(py)2[HN(2,6-i-Pr2C6H3)]2 with Me3SiCl,
which results in the isolation of mixtures of products as
the pale yellow powder described above.
represents
a source of the base-free silyl anion
(Me3Si)3Si−. Thus, treatment of 3 with one equivalent
of (Me3Si)3SiK resulted in quantitative formation (by
1H-NMR spectroscopy in benzene-d6) of (2,6-i-
Pr2C6H3)NꢀHf(4-t-BuC5H4N)2Cl[Si(SiMe3)3] (4, Eq.
(3)), which represents the second example of a hafnium
complex featuring imido and silyl ligands.
Replacing pyridine with 4-t-Bu-pyridine as a ligand
in the preparation of (2,6-i-Pr2C6H3)NꢀHf(4-t-
BuC5H4N)2[HN(2,6-i-Pr2C6H3)]2 afforded a product
that is considerably more soluble in aliphatic solvents
than its pyridine counterpart. Treatment of toluene
solutions of (2,6-i-Pr2C6H3)NꢀHf(4-t-BuC5H4N)2[HN-
(2,6-i-Pr2C6H3)]2 with two equivalents of Me3SiCl at
elevated temperatures resulted in the formation of the
desired hafnium imido complex (2,6-i-Pr2C6H3)NꢀHf(4-
t-BuC5H4N)2Cl2 (3, Eq. (2)), which is soluble in both
benzene and toluene. Analytically pure samples of 3
were obtained by recrystallization from cold, concen-
trated toluene solutions. The spectroscopic data for 3 is
consistent with a trigonal bipyramidal structure similar
to that of 1, with the two t-Bu-pyridine ligands in
equivalent axial positions, giving rise to one set of
resonances for the t-Bu and aromatic protons in the
1H-NMR spectrum at l 0.79, 6.67, and 8.80, respec-
tively, in a 9:2:2 ratio.
(3)
Compound 4 was identified by its 1H-NMR spec-
trum, which includes a resonance at l 0.29, correspond-
ing to the silyl ligand. Likewise, the t-Bu resonance of
the pyridine ligands appears at l 0.81, and integrates in
a ratio of 2:3 relative to that of the silyl group. The i-Pr
groups of the imido ligand give rise to broad reso-
nances at l 1.49 for the methyl groups, and l 4.72 for
the methine protons. This broadening is likely due to
hindered rotation about the CipsoꢁN bond of the imido
ligand, which is slow on the NMR time scale at ambi-
ent temperature. This hindered rotation can be at-
tributed to the steric bulk of the ꢁSi(SiMe3)3 group.
1
Variable temperature H-NMR spectroscopy studies of
toluene-d8 solutions of 4 revealed the presence of two
well resolved sets of isopropyl resonances at −80 °C,
which correspond to a rate constant k=1410 s−1 at
the coalescence temperature of 22 °C. This yields a
value for the free energy of activation DG‡=13 kcal
mol−1 [14]. Unfortunately, we have not been able to
analyze the solid state structure of 4, since attempts to
obtain X-ray quality crystals have so far resulted in
formation of small, stacked flakes.
Preliminary reactivity studies revealed that the Hf
center of 4 is sterically encumbered due to the presence
of the bulky imido and silyl ligands. Thus, exposure of
benzene-d6 solutions of 4 to an atmosphere of dihydro-
gen at ambient temperature and up to 60 °C for 4 h
(2)

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I. Castillo, T. Don Tilley / Journal of Organometallic Chemistry 643–644 (2002) 431–440
Hf. This involves the thermolytic a-H abstraction from
the metal-bound amide by a hydrocarbyl or silyl group
(Eq. (4)).
(4)
Chart 1.
1
The hafnium silyl complex Cp*Hf[Si(SiMe3)3]Cl2 [16]
is a convenient starting material for the preparation of
compounds with amide and silyl ligands. Addition of
solid LiNHAr (Ar=2,6-i-Pr2C6H3) to benzene-d6 or
toluene solutions of Cp*Hf[Si(SiMe3)3]Cl2 likely re-
sulted in generation of the desired complex
Cp*Hf[Si(SiMe3)3](NHAr)Cl. Although this compound
was not isolated, heating the solutions to 65 °C for 12
h resulted in quantitative formation of (Me3Si)3SiH (by
1H-NMR spectroscopy in benzene-d6) and a new
hafnium-based product. While the new hafnium
product {Cp*HfCl[m-N(2,6-i-Pr2C6H3)]}2 (5) is associ-
ated with a single Cp* resonance at l 1.83 in the
1H-NMR spectrum, no resonance that could be at-
tributed to a NH group was detected. Likewise, no wNH
stretching band was observed in the IR spectrum of 5.
The structure of compound 5 appears to be that of a
dimeric hafnium m-imido complex (Eq. (5)), based on
the presence of two sets of doublets at l 1.29 and 1.51
resulted in no changes in the H-NMR spectrum. Like-
wise, addition of one equivalent of PhSiH3 to benzene-
d6 solutions of 4 led to no reaction after 4 h at room
temperature, or upon heating to 80 °C for 2 h, as
1
evidenced by H-NMR spectroscopy. Upon heating to
100 °C for 2 h, an intractable mixture of hafnium-con-
taining products was obtained, but PhSiH3 remained
intact.
2.4. Synthesis of Cp*Hf(ꢀNAr) deri6ati6es
(Cp*=C5Me5, Ar=2,6-i-Pr2C6H3)
In addition to the cyclopentadienyl-free imido com-
plexes, we are interested in mixed cyclopentadienyl
imido derivatives of the Group 4 metals. Such cy-
clopentadienyl-imido complexes are interesting in that
they are isolobal with the Group 3 and lanthanide
metallocenes (Chart 1). Given the rich s-bond metathe-
sis chemistry of the metallocene complexes of the
Group 3 and lanthanide metals, preparation of hafnium
1
in its H-NMR spectrum, corresponding to diastereo-
topic ꢁCHMe2 groups on each of the imido ligands.
(5)
cyclopentadienyl imido compounds presents an attrac-
tive alternative that would be amenable to comparisons
of s-bond metathesis reactivity.
Examples of Group 4 cyclopentadienyl imido com-
plexes exist in the literature [10e,12,17], but all of them
possess additional donors that render them less elec-
trophilic than their lanthanide metallocene analogues.
In addition, there are no examples of such complexes
with reactive s-bonds.
In the examples of known Cp*M(ꢀNAr) complexes,
generation of the Group 4 metalꢁnitrogen multiple
bond was achieved by base-induced a-H abstraction
from a metal amide [10e,15]. This method led to the
formation of base adducts of the metal imido com-
plexes. We devised a synthesis of coordinatively unsatu-
rated Cp*Hf(ꢀNAr) complexes that circumvents the use
of bases or ethereal solvents that could coordinate to
The solid state structure of 5 confirms the bridging
nature of the imido ligands, which likely arises from the
electron deficient nature of the hafnium center. Appar-
ently, the coordination of two s-type donors better
satisfies the electrophilicity of Hf than one non-bridg-
ing, s+p imido donor [17]. The low intensity of the
reflections in the X-ray data set collected did not allow
anisotropic refinement of the carbon and nitrogen
atoms, and only the hafnium and chlorine atoms were
refined anisotropically. Nonetheless, the connectivity of
5 was firmly established, and the metric parameters
obtained are consistent with those of known hafnium
compounds [17]. The asymmetric unit consists of one
half of the molecule, which is related to the other half
by an inversion center. Complex 5 is characterized by
,
unequal HfꢁN distances of 2.05 and 2.11 A, as well as
,
bond lengths of 2.41 (HfꢁCl), and 2.22 A (Hf-centroid).

I. Castillo, T. Don Tilley / Journal of Organometallic Chemistry 643–644 (2002) 431–440
435
The N(1)ꢁHf(1)ꢁN(1)* angle of 84.9° is complemented
by the Hf(1)ꢁN(1)ꢁHf(1)* angle of 95.1°, giving rise to
a nearly square planar Hf2N2 core. An ORTEP diagram
of 5 is presented in Fig. 2.
respectively, by protonation of one of the hydrocarbyl
ligands with one equivalent of diisopropyl aniline at
−80 °C. We hypothesized that the presence of two
reactive HfꢁC bonds would lead to imido complexes
with a pre-formed HfꢁMe or HfꢁCH2Ph bond upon
thermolysis (Eq. (6)).
Complex 5 can also be prepared by thermolysis of
Cp*Hf(NHAr)MeCl (synthesized by addition of one
equivalent of ArNH2 to cold solutions of Cp*HfMe2Cl
1
[18] and identified by H-NMR spectroscopy), which
1
proceeds via the clean elimination of methane (by H-
NMR spectroscopy in benzene-d6). This method avoids
the use of (THF)3LiSi(SiMe3)3, which is more difficult
to obtain than the MeLi or MeMgCl employed in the
preparation of Cp*HfMe2Cl.
(6)
Attempts to prepare derivatives of 5 that incorporate
reactive s-bonds (HfꢁC, HfꢁSi, or HfꢁH) have proven
unsuccessful, pressumably due to the hindered nature of
the Hf center of 5. No reaction was observed upon
addition of the afforementioned lithium and potassium
Heating benzene-d6 solutions of 6 and 7 to 75 °C
resulted in the elimination of methane and toluene,
1
respectively (by H-NMR spectroscopy). Both reactions
required long reaction times of 48 h to proceed to
completion, and led to the formation of several
hafnium-containing products (by 1H-NMR spec-
troscopy). The main products obtained in both cases
were assigned as hafnium amide species, based on the
observed 1H-NMR resonances corresponding to NH
hydrogens (at l 5.46 and 5.39, respectively). In addi-
tion, the appearance of four major resonances in a
1:1:1:1 ratio in the Cp* region of the spectra indicates
the formation of cyclometallated complexes of hafnium
with h1,h5-CH2C5Me4 ligands. Thermolytic hydrogen
atom abstraction from a Cp* ligand has previously
been observed in Group 4 metallocene chemistry [21].
Although preparative scale themolyses of 6 and 7 in
benzene did not yield crystalline products that could be
completely characterized, the tentative structure of the
themolysis products is shown in Eq. (7).
1
silyl reagents to benzene-d6 solutions of 5 (by H-NMR
spectroscopy). While the use of THF has been pre-
cluded by its potential to coordinate to the hafnium
center, addition of the silyl reagents to solutions of 5 in
diethyl ether or hydrocarbon solvents has resulted in
isolation of 5. Complex mixtures of products were
obtained upon heating, likely due to the thermal de-
composition of the silyl anions. Similar results were
observed when Grignard (MeMgCl, PhCH2MgCl, Np-
MgCl; Np=neopentyl) and lithium alkyl [NpLi,
Me3SiCH2Li, (Me3Si)2CHLi] reagents were used in at-
tempts to prepare compounds with HfꢁC bonds.
Due to the lack of success in preparing derivatives of
5 that contain silyl, hydrocarbyl, or hydride ligands, a
different route to the synthesis of such compounds was
(7)
undertaken. Thus, Cp*HfMe2[NH(2,6-i-Pr2C6H3)] (6)
and Cp*Hf(CH2Ph)2[NH(2,6-i-Pr2C6H3)] (7) were pre-
pared from Cp*HfMe3 [19] and Cp*Hf(CH2Ph)3 [20],
Formation of the Cp* metallated complexes could
proceed through initial a-abstraction to generate a tran-
sient hafnium imido species. Such imido species would
then rearrange to the proposed metallated cyclopenta-
dienyl derivatives by intramolecular addition of a Cp*
CꢁH bond across the HfꢀN bond [21,22]. Precedent for
this type of reactivity exists in the conversion of
¸¹¹¹¹º
Cp*Hf(CH2Ph)2 to toluene and Cp*HfCH2-o-C6H4,
2
2
which is proposed to proceed by initial a-H abstraction
[21b]. The transiently generated hafnium benzylidene
complex undergoes Cp* ring hydrogen abstraction to
produce the isolated cyclometallated intermediate
Cp*(h1,h5-CH2C5Me4)HfCH2Ph.
Fig. 2. ORTEP diagram of {Cp*HfCl[m-N(2,6-i-Pr2C6H3)]}2 (5).

436
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3. Conclusions
(CH2Ph)3 [20], (THF)3LiSi(SiMe3)3 [12a], (THF)3LiSi-t-
BuPh2 [12b], (THF)3LiSiHMes2 [12c], (THF)3LiSiPh3
[12d], and KSi(SiMe3)3 [13] were prepared by literature
methods. NMR spectra were recorded on Bruker
AMX-300, AMX-400, or DRX-500 spectrometers at
ambient temperature unless otherwise noted. Elemental
analyses were performed by the Microanalytical Labo-
ratory in the College of Chemistry at the University of
California, Berkeley. Infrared spectra were recorded on
a Mattson Infinity 60 FTIR instrument. Samples were
prepared as KBr pellets unless otherwise noted, and
The first hafnium complexes that incorporate imido
and silyl ligands have been prepared. Although the
poor synthetic yields of the disilyl complex (2,6-i-
Pr2C6H3)NꢀHf(THF)2(SiPh3)2 (2) did not allow us to
examine its reactivity, its solid state structure confirmed
the presence of HfꢁSi bonds. The s-bond metathesis
reactivity of the silyl complex (2,6-i-Pr2C6H3)NꢀHf(4-t-
BuC5H4N)2Cl[Si(SiMe3)3] (4) appears to be low, since it
does not react with PhSiH3 even at elevated tempera-
tures. The low reactivity is likely due to the steric
congestion around the Hf center.
data are reported in units of cm−1
.
During our attempts to synthesize coordinatively un-
saturated Cp*Hf(ꢀNAr) derivatives, we discovered that
the aromatic group (2,6-i-Pr2C6H3) is not large enough
4.2. Synthesis of (2,6-i-Pr2C6H3)NꢀHf(THF)2(SiPh3)2
(2)
to prevent dimerization to
a
m-imido species
A Schlenk tube equipped with a stirbar was charged
with (2,6-i-Pr2C6H3)NꢀHf(THF)2Cl2 (1) (0.50 g, 0.88
mmol) and Ph3SiLi(THF)3 (0.94 g, 1.80 mmol). Tolu-
ene (ca. 50 ml) was added at −80 °C via cannula while
stirring. The mixture was allowed to warm to room
temperature (r.t.) overnight. Volatile materials were
evaporated under vacuum. The products were extracted
with 50 ml of C5H12 and cannula filtered into another
Schlenk flask. The yellow solution was then concen-
trated to a volume of ca. 10 ml and cooled to −35 °C.
Yellow crystals of 2 were obtained in a low yield of
10% (80 mg, 0.08 mmol). m.p. 137–142 °C. IR. 3050
(m), 2957 (s), 2868 (s), 1619 (m), 1459 (m, sh), 1427 (w),
1336 (m), 1260 (w, sh), 1188 (s), 1113 (m), 833 (w, br),
{Cp*HfCl[m-N(2,6-i-Pr2C6H3)]}2 (5). The dimeric na-
ture of 5 prevented its derivatization to complexes
containing reactive HfꢁSi, HfꢁC, or HfꢁH bonds.
The thermolyses of Cp*HfMe2(NHAr) (6) and
Cp*Hf(CH2Ph)2(NHAr) (7, Ar=2,6-i-Pr2C6H3) [which
we expected to lead to the formation of complexes of
the type Cp*Hf(ꢀNAr)Me and Cp*Hf(ꢀNAr)(CH2Ph),
respectively] led to the formation of hafnium amide
specie. It appears that the Cp* ligand is deleterious in
theses systems, since it is prone to intramolecular CꢁH
activations, which yield cyclometallated complexes of
the type Cp*(h1,h5-CH2C5Me4)HfR (R=Me, CH2Ph).
1
803 (w), 733 (m), 699 (m, sh), 484 (w). H-NMR (500
4. Experimental
MHz, benzene-d6): l 0.89 (m, 8H, b-THF), 1.12 (d,
12H, CHMe2), 4.26 (m, 8H, a-THF), 4.48 (sept, 2H,
CHMe2), 7.18 (m, 9H, Ar), 7.24 (m, 12H, Ar), 7.64 (m,
12H, Ar). 13C{1H}-NMR (126 MHz): l 24.64 (b-THF),
24.97 (CHMe2), 27.56 (CHMe2), 76.31 (a-THF),
122.22, 127.21, 127.42, 129.65, 135.81, 136.56, 141.64,
146.48 (aromatics). 29Si-NMR (99 MHz): l 35.13. Anal.
Calc. for C56H63HfNO2Si2: C, 66.15; H, 6.24; N, 1.38.
Found: C, 65.55; H, 6.19; N, 1.10%.
4.1. General
Unless otherwise specified, all manipulations were
performed under a N2 or Ar atmosphere using standard
Schlenk techniques or an inert atmosphere dry box.
Dry, oxygen-free solvents were employed throughout.
Olefin free C5H12 was obtained by treatment with con-
centrated H2SO4, then 0.5 N KMnO4 in 3 M H2SO4,
followed by NaHCO3, and finally MgSO4. Thiophene-
free benzene and C6H5CH3 were obtained by pretreat-
ing the solvents with concentrated H2SO4, followed by
Na2CO3, and CaCl2. C5H12, C6H6, C6H5CH3, THF, and
Et2O were distilled from sodium/benzophenone and
stored under nitrogen prior to use, whereas benzene-d6
and toluene-d8 were vacuum distilled from Na–K alloy.
Pyridine was distilled from Na and stored under nitro-
gen. 4-t-BuC5H4N was dried over CaH2 and distilled
under nitrogen. Me3SiCl was distilled under nitrogen
prior to use. Reagents were purchased from commercial
suppliers and used without further purification unless
otherwise specified. (2,6-i-Pr2C6H3)NꢀHf(THF)2Cl2
[10e], Hf[HN(2,6-i-Pr2C6H3)]4 [10f], Cp*Hf[Si(SiMe3)3]-
Cl2 [16], Cp*HfMe2Cl [18], Cp*HfMe3 [19], Cp*Hf-
4.3. Synthesis of
(2,6-i-Pr2C6H3)NꢀHf(4-t-BuC5H4N)2Cl2 (3)
To
a stirred orange solution of Hf[HN(2,6-i-
Pr2C6H3)]4 (6.60 g, 7.47 mmol) in 200 ml of C5H12 was
added 4-t-Bu-pyridine (2.30 ml, 15.7 mmol) via syringe.
A yellow solid crashed out of solution upon addition.
After stirring the mixture for 1 h, the volume was
reduced to ca. 50 ml under vacuum, and then the
supernatant solution was discarded. Traces of C5H12
were removed from the yellow solid under vacuum. The
yellow product was then dissolved in a minimum
amount of C6H5CH3, and chilled to −35 °C. Yellow
crystals of (2,6-i-Pr2C6H3)NꢀHf(4-t-BuC5H4N)2[NH-
(2,6-i-Pr2C6H3)]2 were obtained in 88% yield (6.42 g,

I. Castillo, T. Don Tilley / Journal of Organometallic Chemistry 643–644 (2002) 431–440
437
6.57 mmol). Part of this material (3.00 g, 3.07 mmol)
was placed in a Schlenk tube and dissolved in ca. 30 ml
of C6H5CH3. To this solution was added Me3SiCl (0.80
ml, 6.14 mmol) via syringe. The orange mixture was
heated to 65 °C under an atmosphere of
nitrogen for ca. 18 h. A yellow solution was obtained,
which was cannula filtered into another Schlenk flask.
This solution was concentrated under vacuum until
small yellow crystals could be seen on the sides of the
flask. Cooling to −80 °C resulted in formation of
yellow crystals, which were isolated and washed twice
with ca. 10 ml of C5H12. Removal of residual solvents
in vacuo afforded 3 in 87% yield (1.88 g, 2.44 mmol).
m.p. 240–242 °C. IR 3047 (m), 2965 (s), 2925 (s), 2867
(m), 1937 (w), 1842 (w), 1793 (w), 1614 (s), 1542 (w,
sh), 1501 (m, sh), 1460 (m), 1421 (s), 1366 (m), 1318
(m), 1274 (m), 1232 (m, sh), 1181 (s), 1102 (m), 1069
(m), 1017 (m, sh), 926 (w, br), 889 (w, sh), 828 (s), 800
(m, br), 753 (s), 724 (w, sh), 569 (s), 541 (w), 508 (m),
(CHMe2), 30.12 (CHMe2), 35.42 (CMe3), 120.60,
122.48, 123.01, 126.03, 129.66, 152.72, 165.45 (aromat-
ics). 29Si-NMR (99 MHz): l −98.76 (Si(SiMe3)3), −
4.35 (Si(SiMe3)3). Anal. Calc. for C39H70ClHfN3Si4: C,
51.63; H, 7.78; N, 4.63. Found: C, 52.11; H, 7.60; N,
4.37%.
4.5. Synthesis of {Cp*HfCl[v-N(2,6-i-Pr2C6H3)]}2 (5)
To stirred C6H5CH3 (ca. 15 ml) solution of
Cp*HfCl2[Si(SiMe3)3] (0.50 g, 0.80 mmol) in a Schlenk
tube was added solid LiNH(2,6-i-Pr2C6H3) (0.15 g, 0.80
mmol). The yellow solution obtained changed color to
orange upon heating to 65 °C overnight. After cooling
the reaction mixture to r.t., volatile materials were
evaporated in vacuo. The orange solid obtained was
washed with ca. 5 ml of C5H12 and redissolved in ca. 10
ml of C6H5CH3. Cooling the concentrated C6H5CH3
solution to −35 °C afforded yellow–orange crystals of
5 in 72% yield (0.30 g, 0.29 mmol). m.p. 137–142 °C.
IR: 3033 (m), 2958 (s), 2866 (s), 1619 (m), 1425 (w),
1336 (m), 1296 (m), 1265 (w, sh), 1190 (s), 1114 (m),
833 (w, br), 821 (w), 733 (m), 700 (m, br), 612 (w), 484
1
413 (w, sh). H-NMR (300 MHz, benzene-d6) l 0.79 (s,
18H, t-Bu), 1.35 (d, 12H, CHMe2), 5.62 (sept, 2H,
CHMe2), 6.67 (d, 4H, 3-C5H4N), 6.95 (t, 1H, Ar), 7.20
(d, 2H, Ar), 8.80 (d, 4H, 2-C5H4N). 13C{1H}-NMR
(126 MHz) l 27.53 (CMe3), 27.63 (CMe3), 30.18
(CHMe2), 34.97 (CHMe2), 119.26, 120.86, 122.75,
123.46, 125.11, 142.82, 152.20 (Aromatics). Anal. Calc.
for C36H43Cl2HfN3: C, 51.84; H, 6.24; N, 6.05. Found:
C, 51.90; H, 6.24; N, 5.94%.
1
(w). H-NMR (500 MHz, benzene-d6) l 1.29 (d, 6H,
CHMe2), 1.51 (d, 6H, CHMe2), 1.83 (s, 15H, Cp*), 4.36
(sept, 2H, CHMe2), 6.92 (t, 1H, Ar), 7.13 (d, 2H, Ar).
13C{1H}-NMR (126 MHz): l 19.98 (CHMe2), 23.55
(CHMe2), 26.73 (C5Me5), 37.27 (CHMe2), 118.96
(C5Me5), 123.33, 125.85, 132.12, 145.07 (aromatics).
Anal. Calc. for C22H32ClHfN: C, 50.39; H, 6.15; N,
2.67. Found: C, 49.95; H, 6.12; N, 2.54%.
4.4. Synthesis of
(2,6-i-Pr2C6H3)NꢀHf(4-t-BuC5H4N)2Cl[Si(SiMe3)3] (4)
In a Schlenk tube equipped with a stirbar were mixed
3 (0.50 g, 0.72 mmol) and (Me3Si)3SiK (0.21 g, 0.72
mmol). The solid mixture was stirred and cooled to
−40 °C, and 50 ml of cold toluene were added slowly
via cannula. Upon warming to ambient temperature, a
yellow–orange solution formed. After further stirring
for 1 h, volatile materials were removed under vacuum.
The remaining yellow–orange solid was extracted with
ca. 60 ml of C5H12, and the orange solution obtained
was cannula filtered into another Schlenk tube. Con-
centrating the solution to a volume of ca. 15 ml and
cooling to −35 °C afforded orange microcrystals of 4
in 66% yield (0.43 g, 0.48 mmol). m.p. 149–153 °C
(dec.). IR: 2961 (s), 2892 (s), 2869 (s), 1953 (w, br), 1854
(w), 1795 (w, br), 1616 (s), 1586 (m), 1544 (w, sh), 1498
(m, sh), 1461 (m), 1423 (s), 1343 (s), 1275 (m), 1258 (s),
1240 (s), 1202 (w), 1156 (w, br), 1100 (m), 1068 (m),
1022 (s), 961 (m), 834 (s, br), 752 (m), 728 (m), 679 (m),
622 (m, sh), 570 (m, sh), 543 (w, sh), 452 (w, br).
1H-NMR (500 MHz, benzene-d6): l 0.29 (s, 27H,
SiMe3), 0.81 (s, 18H, t-Bu), 1.49 (d, 12H, CHMe2), 4.72
(br, 2H, CHMe2), 6.69 (d, 4H, 3-C5H4N), 7.00 (t, 1H,
Ar), 7.39 (d, 2H, Ar), 9.28 (d, 4H, 2-C5H4N). 13C{1H}-
NMR (126 MHz): l 5.54 (SiMe3), 25.60 (CMe3), 28.47
4.6. Synthesis of Cp*HfMe2[NH(2,6-i-Pr2C6H3)] (6)
To a stirred, cold (−80 °C) solution of Cp*HfMe3
(0.59 g, 1.64 mmol) in ca. 20 ml of C5H12 was added
(2,6-i-Pr2C6H3)NH2 (0.31 ml, 1.64 mmol) dropwise via
syringe. The mixture was slowly allowed to reach ambi-
ent temperature, and at that point the volatile materials
were removed under vacuum until the solution reached
a volume of ca. 5 ml. Cooling to −35 °C afforded tan
crystalline 6 in 72% yield (0.61 g, 1.17 mmol). m.p.
43–45 °C. IR: 3063 (m, wNH), 3018 (m), 2959 (s), 2913
(s), 2867 (s), 2760 (m), 1898 (w), 1848 (w, br), 1793 (w),
1618 (m, sh), 1590 (m, sh), 1459 (s), 1434 (s), 1380 (m),
1360 (m), 1337 (m), 1302 (m, sh), 1259 (s), 1227 (m),
1203 (m), 1140 (m), 1116 (m), 1042 (m, sh), 1026 (m,
sh), 955 (w), 890 (m, sh), 865 (m), 793 (m, br), 745 (s,
sh), 696 (s, sh), 624 (w, br), 577 (w, sh), 554 (w, br), 474
1
(s, br). H-NMR (300 MHz, benzene-d6): l 0.08 (s, 6H,
HfMe2), 1.26 (d, 12H, CHMe2), 1.82 (s, 15H, Cp*),
2.94 (sept, 2H, CHMe2), 5.14 (s, 1H, NH), 6.97 (t, 1H,
Ar), 7.13 (d, 2H, Ar). 13C{1H}-NMR (126 MHz): l
11.18 (HfMe2), 23.85 (C5Me5), 30.40 (CHMe2), 48.17
(CHMe2), 119.18 (C5Me5), 121.89, 123.33, 138.88,
147.88 (aromatics). Anal. Calc. for C24H39HfN: C,

438
I. Castillo, T. Don Tilley / Journal of Organometallic Chemistry 643–644 (2002) 431–440
,
19.1425(3), b=12.7530(2), c=20.1801(4) A, i=
55.43; H, 7.56; N, 2.69. Found: C, 55.56; H, 7.38; N,
2.82%.
3
,
95.572(1)°. The cell volume is 4903.2(1) A with Z=4,
D
calc=1.377 g cm−3, and an absorption coefficient of
2.214 mm−1. The max and min transmission were in
the range of 0.977–0.619. A absorption correction from
equivalents was applied. F(000)=2088; 23 296 reflec-
tions were collected of which 9078 (Rint=0.078) were
independent. The data were refined by a full-matrix
least-squares on F2. The ratio of data/parameters was
4589/559, while the goodness-of-fit on F2 is 1.17. R[I\
4.7. Synthesis of Cp*Hf(CH2Ph)2[NH(2,6-i-Pr2C6H3)]
(7)
To
a
stirred, cold (−80 °C) solution of
Cp*Hf(CH2Ph)3 (1.07 g, 1.83 mmol) in ca. 25 ml of
C5H12 was added (2,6-i-Pr2C6H3)NH2 (0.35 ml, 1.83
mmol) dropwise via syringe. The solution was allowed
to warm to r.t., and it was further stirred for 1 h. The
volume of the solution was then reduced to ca. 10 ml in
vacuo. Cooling to −35 °C afforded off-white crystals
of 7 in 78% yield (0.96 g, 1.43 mmol). m.p. 144–147 °C
(dec.). IR: 3056 (m, wNH), 3014 (m), 2957 (s), 2921 (s),
2868 (s), 1928 (w, br), 1845 (w), 1791 (w, br), 1719 (w),
1593 (s), 1485 (s), 1435 (s), 1381 (m), 1357 (w), 1324
(m), 1254 (s), 1232 (m), 1201 (s), 1178 (w), 1148 (w, br),
1111 (m, br), 1029 (s), 993 (m, sh), 931 (w, sh), 890 (w),
864 (m, sh), 794 (m, sh), 745 (s, sh), 694 (s, sh), 584 (w,
3|(I)]: R 0.047, Rw 0.051, Rall 0.106. The largest differ-
−3
,
ence peak and hole: 2.78 and −2.19 e A
.
4.8.3. X-ray crystal structure determination of 5
A yellow, cubic crystal was mounted on a glass
capillary using Paratone N hydrocarbon oil and placed
under a stream of cold nitrogen on a Siemens SMART
diffractometer with a CCD area detector (Mo–Ka radi-
,
ation, u=0.71073 A). Preliminary orientation matrix
1
br), 554 (w, sh), 518 (w, sh), 414 (w). H-NMR (300
and unit cell parameters were determined by collecting
60 10-s frames. A hemisphere of data was collected at a
temperature of 127 K using ꢀ scans of 0.30° and a
collection time of 20 s per frame. Frame data were
integrated using SAINT. An absorption correction was
applied using SADABS. No correction for decay was
necessary. The structure was solved using direct meth-
ods (SIR92) and the hafnium and chlorine atoms were
refined anisotropically. The carbon and nitrogen atoms
were refined isotropically. Hydrogen atoms were in-
cluded in idealized positions but were not refined.
MHz, benzene-d6): l 1.28 (d, 12H, CHMe2), 1.80 (d,
2H, CH2Ph), 1.86 (s, 15H, Cp*), 2.20 (d, 2H, CH2Ph),
2.84 (sept, 2H, CHMe2), 5.64 (s, 1H, NH), 6.10–7.36
(13H, aromatics). 13C{1H}-NMR (126 MHz): l 11.24
(CHMe2), 24.59 (C5Me5), 29.24 (CH2Ph), 75.46
(CHMe2), 120.27 (C5Me5), 122.86, 122.97, 123.17,
127.75, 129.31, 140.40, 147.35, 149.60 (aromatics).
Anal. Calc. for C36H47HfN: C, 64.32; H, 7.05; N, 2.08.
Found: C, 63.95; H, 6.98; N, 1.94%.
4.8. X-ray single crystal structure determination of 2
and 5
4.8.4. Crystal data for 5
(C22H32Cl2Hf2N2; 1048.89 g mol−1; yellow cube of
0.03×0.04×0.07 mm dimension): the hafnium imido
chloride complex crystallizes in the monoclinc system
(C2/c) with the following unit cell: a=21.3765(7), b=
4.8.1. X-ray crystal structure determination of 2
A yellow, block-like crystal was mounted on a glass
capillary using Paratone N hydrocarbon oil and placed
under a stream of cold nitrogen on a Siemens SMART
diffractometer with a CCD area detector (Mo–Ka radi-
,
17.8103(5), c=10.9785(4) A, i=94.762(1)°. The cell
3
,
volume is 4165.3(2) A with Z=4, Dcalc=1.680 g
cm−3, and an absorption coefficient of 5.156 mm−1
.
,
ation, u=0.71073 A). Preliminary orientation matrix
The max and min transmission were in the range of
0.894–0.743. A absorption correction from equivalents
was applied. F(000)=2080; 9755 reflections were col-
lected of which 3602 (Rint=0.101) were independent.
The data were refined by a full-matrix least-squares on
F2. The ratio of data/parameters was 1636/111, while
the goodness-of-fit on F2 0.99. R[I\3|(I)]: R 0.041,
and unit cell parameters were determined by collecting
60 10-s frames. A hemisphere of data was collected at a
temperature of 160 K using v scans of 0.30° and a
collection time of 20 s per frame. Frame data were
integrated using SAINT. An absorption correction was
applied using SADABS. No correction for decay was
necessary. The structure was solved using direct meth-
ods (SIR92) and all non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included in ideal-
ized positions but were not refined.
Rw 0.042, Rall 0.127. The largest difference peak and
−3
,
hole: 1.03 and −0.93 e A
.
4.8.2. Crystal data for 2
5. Supplementary material
(C56H63HfNO2Si2; 1016.78 g mol−1; yellow block of
0.28×0.16×0.16 mm dimension; air sensitive): the
hafnium disilyl complex crystallizes in the monoclinc
system (P21/c) with the following unit cell: a=
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Center, CCDC nos. 168426 and 168427 for com-

I. Castillo, T. Don Tilley / Journal of Organometallic Chemistry 643–644 (2002) 431–440
439
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pounds 2 and 5, respectively. Copies of this information
may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
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ac.uk or www: http://www.ccdc.cam.ac.uk).
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Acknowledgements
The authors wish to thank the National Science
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