Synthesis, structures, and alkene hydrosilation activities of neutral tripodal amidozirconium alkyls

Article abstract of DOI:10.1039/b200789d

Aminolysis of Zr(CH₂C₆H₅)₄ with CH[(CH₃)₂SiNHAr]₃ (Ar = para-tolyl and para-fluorophenyl) affords CH[(CH₃)₂-SiNAr]₃ZrCH₂C ₆

Full text of DOI:10.1039/b200789d

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Synthesis, structures, and alkene hydrosilation activities of neutral
tripodal amidozirconium alkyls
Li Jia,*a Jiquan Zhao,a Errun Dinga and William W. Brennesselb
a Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USA.
E-mail: lij4@lehigh.edu
b Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA
Received 22nd January 2002, Accepted 2nd May 2002
First published as an Advance Article on the web 29th May 2002
Aminolysis of Zr(CH2C6H5)4 with CH[(CH3)2SiNHAr]3 (Ar = para-tolyl and para-fluorophenyl) affords CH[(CH3)2-
SiNAr]3ZrCH2C6H5 1a and 1b (a: Ar = para-tolyl; b: Ar = para-fluorophenyl). Solution and solid state X-ray
structural data suggest an η2-bonding mode for the benzyl group in 1a and 1b. Treating 1a and 1b with N,N-
dimethylanilinium chloride or triphenylmethyl chloride yields CH[(CH3)2SiNAr]3ZrCl (2a and 2b). Complex 2a
exhibits a fast dimer–monomer equilibrium in benzene. Single-crystal X-ray diffraction of 2a reveals a dimeric
structure bridged by two µ-Cl atoms. Metathesis of 2a and 2b with n-butyllithium in toluene and then in hexane
yields CH[(CH3)2SiNAr]3ZrnBu (3a and 3b). The crystal structures and the IR spectra of 3a and 3b in Nujol mulls
reveal weak β-agostic interactions, but solution NMR spectra of 3a and 3b give no evidence for such a motif. Further,
the larger Zr–Cα–Cβ angles and the longer Zr–Cβ distances suggest the β-agostic interactions in 3a and 3b are
weaker than those in the previously reported cases of Group 4 metals. Complexes 1a, 1b, 3a, and 3b are active alkene
hydrosilation catalysts. The catalytic activities exhibited by these complexes are in contrast to the inertness of the
previously reported neutral molecules of the type X3MR (X = amide, alkoxide, or siloxide; M = Group 4 metal;
R = alkyl or hydride). We attribute the improved activity to three factors: the small ancillary groups of the ligand,
the accessibility of the coordination sites cis to the Zr-alkyl group due to the small “bite angle” of the ligand, and
the weakened N(p)
Zr(d) π donation due to the coordination geometry imposed by the tripodal ligand.
M–N angles in the (TMS2N)3MCl series (M = Ti, Zr, and Hf )
Introduction
should follow the order N–Ti–N > N–Zr–N ≈ N–Hf–N,
because the ionic radius of Ti is the smallest and should cause
the strongest steric repulsion of the TMS2N-ligands. This is
actually not observed (N–Ti–N = 114.3(3)Њ, N–Zr–N =
114.1(1)Њ, and N–Hf–N = 115.4(2)Њ). Adding to the com-
parison, the average N–Th–N angle in (TMS2N)3Th(µ-BH4) is
115.9(2)Њ.7a It appears, at least in some cases, not completely
satisfactory to explain the large angles only on a steric basis. We
suspect that the large-angle coordination geometries were
High electrophilicity is a necessary condition for Group 4 d0
organometallic complexes to exhibit catalytic activities based
on alkene insertion and σ-bond metathesis. The vast majority
of recent research in this area has subsequently focused on
complexes with a net positive charge.1,2 However, Jordan,
Bercaw, and Bochmann have demonstrated that neutral zir-
conium molecules isoelectronic to the cationic metallocene
complexes are also active catalysts for ethylene polymerization
and alkyne hydrogenation although their activities are generally
one to two magnitudes lower than those of the cationic com-
plexes.3,4 It seems logical to expect that high electrophilicity can
also be achieved using electron-withdrawing ligands such as
alkoxides and amides and yet molecules with the general
formula X3MR (X = alkoxide, siloxide, or amide; M = Group 4
metal; R = alkyl or H) are very inert.5 Although most X3MR
molecules contain voluminous X ligands, the benzyltitanium
silsesquioxide reported by Duchateau et al. is sterically rather
open but still lacks the Lewis acidity to form adducts with
acetonitrile,5a suggesting that the low reactivity cannot be
adopted in part because it facilitated the X(p)
M(d) π inter-
action (see discussions on structure–reactivity relationships
below), which is believed to exist in the bonding between the
Group 4 metals and the X ligands.7 The π donation decreases
the electrophilicity and consequently the reactivity of the metal.
The X3MR complexes with small X–M–X angles are thus very
interesting for us.
In a series of elegant reports, Gade et al. has systematically
investigated the synthesis of Group 4 metal complexes with
tripodal amide ligands and their reactivities with organic
carbonyl compounds.8 The small N–M–N angle forced by the
tripodal ligand framework was a general structural feature of
Gade’s complexes. For the aforementioned reasons, we believe
that this feature warrants investigations on the alkene insertion
and σ-bond metathesis of these types of complexes. Among
the tripodal frameworks reported by Gade, type A has a small
binding site that appears slightly mismatched with the relatively
large zirconium ion.8g This property is particularly attractive
to us for generating Zr-alkyl species with the smallest possible
N–Zr–N angles.
ascribed solely to the steric bulkiness of the ligand X. On the
᎐
other hand, the tetra-coordinate species (᎐SiO) ZrH generated
᎐
3
by supporting ZrR4 complexes on a partially dehydrated
silica surface is believed to be the active component for hydro-
genolysis of polyethylene and polymerization of ethylene.6 The
striking reactivity differences between the soluble molecules
and the putative surface-immobilized species are thus puzzling.
A general structural feature of X3MR molecules, including
the halides (R = halide), is that the X–M–X angles exceed the
ideal tetrahedral angle of 109.5Њ (the average of the three X–
M–X angles reported in the literature range from 111 to
116Њ).5,7 The steric encumbrance of the X ligands is usually
cited as the reason for the large angles. If so, the average N–
We have communicated that the hydrido and benzylzirco-
nium complexes supported by the ligand A with Ar = para-tolyl
undergo ethylene multiple insertions and produce polyethylene
at very slow rates.9 This reactivity was not previously observed
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This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b200789d

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field chemical shifts of the ortho protons of the benzyl groups
(δ 6.04 and 5.95 ppm for 1a and 1b, respectively) and the large
1H–13C coupling constants of the methylene unit (1JH–C = 129
and 131 Hz for 1a and 1b, respectively).10 The coupling con-
stants suggest that the phenyl–Zr interaction is slightly stronger
in 1b than in 1a.
The solid state structure of 1b was determined by X-ray dif-
fraction (Fig. 1). Complex 1b is isostructural to 1a, the structure
for X3MR complexes. We report here a full account of the
synthesis, characterization, and catalytic alkene-hydrosilation
activities of the zirconium complexes supported by the ligand
type A having peripheral substituents with varied electronic
characters (Ar = para-tolyl and para-fluorophenyl). The β-
agostic structures observed in some of the alkyl complexes
indicate a relatively high level of electronic unsaturation of
the zirconium in these complexes. The catalytic hydrosilation
activity is unprecedented for the X3MR complexes. In addition
to the steric effects of the tripodal ligands on the reactivity, we
will also discuss the possible electronic effects caused by the
orbital interactions between zirconium and the tripodal ligands.
Results and discussion
Synthesis of benzyl complexes (1a and 1b)
The benzyl complexes 1a and 1b proved to be valuable starting
materials for the synthesis of zirconium complexes supported
by the specific tripodal amide ligands. They are obtained in a
straightforward manner by aminolysis of tetrabenzylzirconium
using the amine precursors of the corresponding ligands
(Scheme 1). The preparation of 1a reaches completion in 1 day
Fig. 1 ORTEP24 drawing of 1b. Thermal ellipsoids shown at 50%
probability.
of which was communicated earlier.9 Selected interatomic dis-
tances and angles for 1b are listed in Table 1. The benzyl group
in 1b is η2-bonded to zirconium in the solid state the same as in
1a. The Zr–CH2–Cipso angle in 1b (88.24(14)Њ) is smaller than
those in 1a (91.4(5) and 93.1(5)Њ in two crystallographically
independent molecules), and the Zr–Cipso contact in 1b (2.648(2)
Å) is shorter than those in 1a (2.753(7) and 2.795(8) Å in the
two crystallographically independent molecules).9 Thus, both
the solid state X-ray data and the solution data suggest a
stronger phenyl–Zr interaction in 1b than in 1a. This difference
can apparently be explained by the electron-withdrawing ability
of the fluorinated ligand resulting in an increase in zirconium
electrophilicity. The Zr–N distances are unexceptional, and the
nitrogen atoms of the arylamide are nearly planar as in all early
transition metal amide compounds. The planes that contain the
zirconium arylamides twist away slightly from the pseudo-C3
axis that passes C(1) and Zr(1) by 7 to 22Њ.
Synthesis of chloride complexes 2a and 2b
Gade recently reported the synthesis of the etherate and the
LiCl adduct of 2a by reaction of ZrCl4 with the lithium salt of
the corresponding ligand.8c Lewis base-free chloride complexes
with tripodal amide ligands bulkier and more electron-donating
than the ones used here were synthesized using the same
metathesis approach.8a We found that Lewis base-free 2a and 2b
can be obtained by reaction of 1a and 1b, respectively, with a
proton source or an alkyl abstractor (Scheme 1). The reactions
are quantitative, and the isolated yields are good. Attempts to
determine whether 2a and 2b exist as dimers or monomers in
benzene solution by freezing point depression were unsuccess-
ful due to the insufficient solubility of the two compounds.
Interestingly, the 1H NMR spectrum of 2a is dependent on the
concentration. The change is reversible and most noticeably
occurs in the aromatic region (Fig. 2). Our explanation of the
Scheme 1 Synthesis of tripodal amidozirconium complexes. a: Ar =
para-tolyl; b: Ar = para-fluorophenyl. Methyl groups attached to Si
atoms in the ligand framework are omitted for clarity.
at 60 ЊC and is essentially quantitative by NMR, while the
preparation of 1b requires 85 ЊC and takes 3 days. The tem-
perature is crucial for the preparation of 1b. Reactions at
temperatures above 90 ЊC result in mixtures that contain 1b but
are difficult to purify.
Both 1a and 1b are in time-average C3v symmetry on the
NMR time scale at room temperature in benzene-d6 solution.
The same symmetry property is observed for all complexes that
will be reported in the following text and will not be reiterated.
The benzyl groups in both 1a and 1b are bonded to zirconium
in the η2 mode in benzene-d6 solution, as evidenced by the high-
1
concentration dependence is that the H NMR spectra are the
average spectra of a fast equilibrium between the monomer
and the dimer of 2a. The spectrum at high concentrations dis-
playing two doublets of AX spins essentially belongs to the
dimer. The spectrum at low concentrations displaying a singlet
of A2 spins belongs to the monomer. At intermediate concen-
trations, the differences between the average chemical shifts of
the A2 and AX spins are comparable to the coupling constant
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Table 1 Selected interatomic distances (Å), angles (Њ), and torsion angles (Њ) in 1b
Zr(1)–C(26)
Zr(1)–C(27)
Zr(1)–N(1)
2.246(2)
2.648(2)
2.0773(18)
Zr(1)–N(2)
Zr(1)–N(3)
2.0680(17)
2.0644(17)
C(27)–C(26)–Zr(1)
N(1)–Zr(1)–N(2)
88.24(14)
108.86(7)
N(1)–Zr(1)–N(3)
N(2)–Zr(1)–N(3)
99.92(7)
101.08(7)
C(4)–N(1)–Zr(1)–C(1)
C(12)–N(2)–Zr(1)–C(1)
158.07(19)
163.59(17)
C(20)–N(3)–Zr(1)–C(1)
173.5(2)
Zr–N distances in the minor component of the disorder are in
parentheses (Table 2). The geometry of the tripodal ligands
around the two metal centers is very similar to that of 1b
described above. The X-ray diffraction data of 2b suffered from
loss of co-crystallized solvent molecules, but were sufficient to
show the dimeric nature of 2b with two Cl bridges in the solid
state.
Synthesis of n-butyl complexes 3a and 3b
Gade recently reported that Lewis base-free methyl complexes
with tripodal amide ligands bulkier and more electron-donating
than the ones used here could be synthesized by reaction of the
corresponding chloride complexes with LiMe or MgBrMe.8a
However, reactions of 2a with LiMe or MgBrMe inevitably
form the methylzirconium etherates. Our attempts to use Me3Al
and Et2Zn as the alkylating reagents for the synthesis of Lewis
base-free alkylzirconium complexes also proved unsuccessful.
On the other hand, synthesis of the Lewis base-free butyl com-
plexes 3a and 3b is successful using n-butyllithium (Scheme 1).
The reactions are carried out in toluene, but precipitation of
LiCl is not obvious presumably because LiCl remains coordin-
ated with zirconium. Stirring the mixtures in hexane extracts
the product and leaves LiCl in the solid. The 1H NMR spectra
of 3a and 3b in benzene-d6 solution provide no evidence for
agostic interactions between the butyl and zirconium, showing
normal chemical shifts and coupling constants (1JCα–H = 114
and 1JCβ–H = 120 Hz for 3a, 1JCα–H = 116 and 1JCβ–H = 120 Hz for
3b).11–14
1
Fig. 2 Concentration-dependent H NMR spectra of 2a (360 MHz,
25 ЊC, benzene-d6).
(J = 7.3 Hz), resulting in the second-order spectra of AB spins.
No change of the H NMR spectrum of 2b can be detected at
1
The crystal structures of 3a and 3b were determined by X-ray
diffraction (Figs. 4 and 5). Selected interatomic distances and
bond angles are listed in Tables 3 and 4. As generally observed
for the tripodal amide complexes,8,9 the N–Zr–N angles are
very small, and the Zr–N distances are unexceptional. Some
interesting features exist concerning the bonding between the
butyl and the zirconium. The structure of 3b contains a butyl
group that appears involved in a weak interaction with zir-
conium at the β-position. The Cβ–Cα–Zr angle of 95.7(10)Њ is
much larger than the Cβ–Cα–M angles previously observed for
β-agostic Group 4 metal alkyls (86.3(6)Њ in (DMPE)Cl3TiEt
and 84.7(5)Њ in Cp2ZrEt(PMe3)ϩ),12,14 but is much smaller
than the Cβ–Cα–M angle in non-agostic structures, where the
angle is typically in the range from 108 to 126Њ.11,12 The Cβ–Zr
various concentrations.
The crystal structure of 2a shows that it is a dimer bridged by
two µ-Cl atoms in the solid state (Fig. 3). The selected geometric
parameters are listed in Table 2. The Si and N atoms on one side
of the dimer are positionally disordered and refined at the 77 :
23 ratio of occupancy. The opposite side also shows a similar
disorder but to such a small extent as to be negligible. The
distance of 2.798(5)
Å is long compared to that in
Cp2ZrEt(PMe3)ϩ (2.629(9) Å). The short Cα–Zr distance of
2.19(2) Å is common for tetracoordinate zirconium alkyl com-
plexes,5d although the contribution from the β-agostic inter-
action can not be ruled out. The butyl group in 3a is disordered
and satisfactorily modeled over two positions at a 50 : 50
occupancy ratio. The two positions represent the coexistence
of a weakly β-agostic butyl and a non-agostic butyl. The Cβ–
Cα–Zr angle and the Cβ–Zr distance of the agostic Zr-butyl
group of 3a are similar to those in 3b (Table 4). IR spectra in
Nujol mulls of both complexes exhibit weak low-frequency
ν(C–H) bands (2571, 2500, and 2466 cmϪ1 for 3a, and 2574,
2504, and 2443 cmϪ1 for 3b), indicative of weakened C–H
bonds.11–13 Thus, in the continuum of β-agostic interaction, the
solid state structures of 3a and 3b serve as examples of very
weak interactions, which seems to recede in solution.
Fig. 3 Molecular structure of 2a. Hydrogen atoms are omitted for
clarity.
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Table 2 Selected interatomic distances (Å), angles (Њ), and torsion angles (Њ) in 2a
Zr(1)–Cl(1)
Zr(1)–Cl(2)
Zr(2)–Cl(1)
Zr(2)–Cl(2)
Zr(1)–N(1)
2.5761(8)
2.6385(8)
2.6428(8)
2.5660(8)
Zr(1)–N(2)
Zr(1)–N(3)
Zr(2)–N(4)
Zr(2)–N(5)
Zr(2)–N(6)
2.081(4) (1.951(14))
2.064(4) (2.082(14))
2.033(2)
2.048(2)
2.076(2)
2.020(5) (2.030(18))
N(1)–Zr(1)–N(2)
N(1)–Zr(1)–N(3)
N(2)–Zr(1)–N(3)
100.59(13) (100.5(5))
100.39(15) (96.0(6))
98.14(13) (96.9(5))
N(4)–Zr(2)–N(5)
N(4)–Zr(2)–N(6)
N(5)–Zr(2)–N(6)
98.20(9)
100.94(10)
99.07(9)
C(15)–N(1)–Zr(1)–C(1)
C(22)–N(2)–Zr(1)–C(1)
C(29)–N(3)–Zr(1)–C(1)
171.6(5)
162.4(4)
173.7(4)
C(36)–N(4)–Zr(2)–C(2)
C(43)–N(5)–Zr(2)–C(2)
C(50)–N(6)–Zr(2)–C(2)
161.2(3)
164.1(3)
171.2(3)
Table 3 Selected interatomic distances (Å), angles (Њ), and torsion angles (Њ) in 3a
Zr(1)–C(29)
Zr(1)–C(30)
Zr(1)–C(29Ј)
Zr(1)–C(30Ј)
Zr(1)–N(1)
2.19(2)
2.798(5)
2.27(2)
3.252(5)
2.0706(16)
Zr(1)–N(2)
Zr(1)–N(3)
C(29)–C(30)
C(29Ј)–C(30Ј)
2.0751(16)
2.0452(17)
1.542(9)
1.538(9)
C(30)–C(29)–Zr(1)
C(30Ј)–C(29Ј)–Zr(1)
N(1)–Zr(1)–C(29)
N(2)–Zr(1)–C(29)
N(3)–Zr(1)–C(29)
N(1)–Zr(1)–C(29Ј)
95.7(10)
115.9(11)
114.2(4)
122.5(3)
112.3(5)
120.8(4)
N(2)–Zr(1)–C(29Ј)
N(3)–Zr(1)–C(29Ј)
N(1)–Zr(1)–N(2)
N(1)–Zr(1)–N(3)
N(2)–Zr(1)–N(3)
117.7(3)
110.3(4)
102.09(6)
102.48(7)
100.75(7)
C(8)–N(1)–Zr(1)–C(1)
C(15)–N(2)–Zr(1)–C(1)
171.49(16)
158.89(17)
C(22)–N(3)–Zr(1)–C(1)
175.07(18)
Fig. 5 ORTEP drawings of 3b. Thermal ellipsoids shown at 50%
probability. (a) Top view. Hydrogen atoms are omitted for clarity. (b)
Side view with peripheral groups omitted.
Fig. 4 ORTEP drawings of 3a. Thermal ellipsoids shown at 50%
probability. (a) Top view. Hydrogen atoms are omitted for clarity. (b)
Side view with peripheral groups omitted.
catalysts are very stable for several days in toluene at a 90 ЊC
reaction temperature. The catalytic reactions are highly
selective and practically quantitative in all experiments.
Methylenecyclopentane is accumulated during the course of
hydrosilation of 1,5-hexadiene and is eventually hydrosilated
(entries 7 and 8, Table 5). For catalysts 1a and 1b, initial
Catalytic alkene hydrosilation
Compounds 1a, 1b, 3a, and 3b can be used as catalyst
precursors for 1-hexene hydrosilation (Table 5). All the pre-
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Table 4 Selected interatomic distances (Å), bond angles (Њ), and torsion angles (Њ) in 3b
Zr(1)–C(26)
Zr(1)–C(27)
Zr(1)–N(1)
2.209(2)
2.842(2)
2.0684(18)
Zr(1)–N(2)
Zr(1)–N(3)
C(26)–C(27)
2.0540(18)
2.0660(18)
1.535(3)
C(27)–C(26)–Zr(1)
N(1)–Zr(1)–C(26)
N(2)–Zr(1)–C(26)
N(3)–Zr(1)–C(26)
97.12(14)
116.12(8)
114.76(8)
115.36(7)
N(1)–Zr(1)–N(2)
N(1)–Zr(1)–N(3)
N(2)–Zr(1)–N(3)
104.05(7)
102.56(7)
102.14(7)
C(8)–N(1)–Zr(1)–C(1)
C(14)–N(2)–Zr(1)–C(1)
179.91(17)
170.9(2)
C(20)–N(3)–Zr(1)–C(1)
172.75(19)
Table 5 Catalytic olefin hydrosilationa
Entry
Alkene
Catalyst (2 mol%)
Reaction half-lifeb/h Reaction time/h
Product
Isolated yield (%)
1
2
1a
1b
—
—
187
134
87
94
3
4
3a
3b
6
4
69
50
90
92
5
6
3a
3b
20
11
340
297
90
90
7
8
3a
18
9
183
89
91
3b
110
a Reaction carried out at 90 ЊC in neat alkene. b Based on silane consumption estimated by 1H NMR integration.
cleavage of the benzyl group is very slow compared to the sub-
mixture was removed under high vacuum at 60 ЊC. 1H and 13
C
sequent turnovers of the catalytic process, for example, at the
completion of hydrosilation of 1-hexene, 37% of 1a and 66% of
1b remain unconverted to the active species (entries 1 and 2,
Table 5). Initiation of the catalytic process is fast for the butyl
complexes 3a and 3b, allowing direct comparison of their cata-
lytic activities. Catalyst 3b is always more efficient than 3a
(entries 3 vs. 4, 5 vs. 6, and 7 vs. 8, Table 5). The half-lives of
phenylsilane in reactions catalyzed by 3b are generally one half
to two thirds of those in the corresponding reactions catalyzed
by 3a. The activities of 3a and 3b, however, are several mag-
nitudes lower than those of the lanthanide and yttrium hydro-
silation catalysts.15
NMR clearly showed that the hexyl derivative, B, was the only
component in the pale yellow semi-solid residue. Note that B
can be independently synthesized through 1-hexene insertion
into the isolated zirconium hydride complex.9
With regard to the mechanism of the hydrosilation, it is
unlikely that any low-valent zirconium species are involved as in
the Cp2ZrCl2/BuLi systems.16 We propose that the mechanism
is similar to that of the lanthanide and yttrium-catalyzed hydro-
silation comprising of alkene insertion into the metal–H bond
and σ-bond metathesis between the H–Si bond and the metal–
alkyl bond (Scheme 2).15
Structure–reactivity relationship of the X3MR complexes
The catalytic activities of alkene hydrosilation and ethylene
polymerization that we have communicated,9 albeit low, have
never been observed for the X3MR complexes (X = amide,
alkoxide, or siloxide; R = alkyl or H).5a It is interesting to
question why the differences in reactivity are present.
Sterically, voluminous amide or alkoxide ligands are usually
employed to stabilize X3MR complexes when X is a monoden-
tate anionic ligand in order to suppress the ligand redistribution
tendency of the electropositive metals. Gade’s tripodal ligands
allow significant reduction of the overall steric hindrance
around the metal without compromising the stability with
respect to ligand redistribution. The complexes discussed here
show no signs of decomposition after several days at 90 ЊC in
toluene-d8 solution as assessed by 1H NMR. In addition to the
reduction of the size, the small binding site of the tripodal
ligands also makes the zirconium easily accessible by forcing it
out of the N–N–N plane (average N–Zr–N angle = 101Њ in 1a,
103Њ in 1b, 101.8Њ in 3a, and 102.9Њ in 3b). The small size and
Scheme 2 Proposed hydrosilation mechanism.
The structural integrity of the tripodal framework and thus
the mechanism are supported by the following experiment. In
a 3a-catalyzed 1-hexene hydrosilation experiment, 1-hexene in
two-fold excess to phenylsilane was used. After the silane was
completely consumed, the volatile component of the reaction
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small binding site of the tripodal ligands undoubtedly sterically
opens up the coordination sites cis to the Zr–alkyl bond and
favor the pre-coordination of reaction substrates such as
alkenes and silanes.
In addition to the steric effects, the small N–Zr–N angles are
possibly advantageous for enhancing the reactivity from the
catalytically active complexes, especially for complexes not in a
bent-metallocene framework.
Experimental
All experiments were conducted using standard Schlenk line
and high-vacuum (10Ϫ5 Torr) line techniques or in a glove box
filled with nitrogen. Hydrocarbon solvents were distilled from
Na/benzophenone under nitrogen and stored in vacuo over Na/
K alloy in flasks sealed by Teflon valves. Hexamethyldisiloxane
and benzene-d6 were dried over Na/K alloy, freeze–pump–thaw
degassed, and kept over Na/K alloy. CDCl3 was dried by reflux-
ing over CaH2, distilled, and kept over CaH2. Triphenylmethyl
chloride was purchased from Aldrich and purified by sub-
limation. N,N-Dimethylanilinium chloride was prepared by
mixing an equal molar amount of N,N-dimethylaniline and
HCl (1.0 M in diethyl ether) in diethyl ether followed by
repetitive washing with diethyl ether. Both N,N-dimethylaniline
and the ether solution of HCl were purchased from Aldrich
and were used directly. Tripodal ligands CH[(CH3)2SiNH(p-
viewpoint of the N(p)
Zr(d) π donation. In general, the X(p)
M(d) π interactions affect the electronic properties on two
fronts, the energy of the individual orbitals of d character and
the overall electron density at the metal center. We will first
discuss the energy of the individual d orbitals. The coordination
of a substrate cis to the M–R bond in C3v symmetry requires
2
2
low-lying metal-centered e orbitals (essentially dxy, dx y , dyz,
and dxz orbitals) (Scheme 3).
Ϫ
8f
18
tolyl)]3,8f CH[(CH3)2SiNH(4-FC6H4)]3 and Zr(CH2C6H5)4
were prepared using literature methods.
Synthesis
Synthesis of CH[(CH3)2SiN( p-CH3C6H4)]3ZrCH2C6H5 1a.
Zr(CH2C6H5)4 (114 mg, 0.25 mmol) and CH[(CH3)2SiNH(p-
tolyl)]3 (127 mg, 0.25 mmol) were loaded into a 50 mL reaction
flask in the glove box and dissolved in toluene (20 mL). The
toluene solution was stirred and heated at 60 ЊC for 24 hours
in the dark. Removal of the solvent in vacuo resulted in an off-
white powder. Extraction of the solid with hexane (≈15 mL)
and filtration yielded a pale yellow solution. Cooling the solu-
tion at Ϫ40 ЊC yielded pale yellow crystals of 2 (154 mg, 90%).
Anal. calc. for C35H47N3Si3Zr: C, 61.35; H, 6.91; N, 6.13.
Scheme 3 Change of π-interactions between metal-d and ligand-p
orbitals.
The e combinations of the ligand p orbitals would best over-
2
2
lap with the dxy, dx y , dyz, and dxz orbitals if the X–M bonds
Ϫ
were perpendicular to the z-axis (trigonal pyramidal geometry
with X–M–X angles being 120Њ). When the angles between the
M–X bonds and the z-axis increase (i.e., the X–M–X angles
decrease), the magnitude of the e overlaps decreases (Scheme
3). The e overlap in the axial direction involving dyz and dxz
orbitals would be completely non-bonding if the angle were
135Њ (Scheme 3). Consequently, the metal-centered e orbitals
will be stabilized when the X–M–X angles are small. The
1
Found: C, 61.20; H, 6.42; N, 6.02%. H NMR (20 ЊC, C6D6):
᎐
δ Ϫ0.50 (s, 1H, CH᎐), 0.36 (s, 18H, [(CH ) Si–] ), 2.13 (s, 2H,
᎐
3
2
3
ZrCH2C6H5), 2.16 (s, 9H, [p-C6H4CH3]3), 6.04 (d, J = 7.2 Hz,
2H, ortho ZrCH2C6H5), 6.59 (“t”, J = 7.4 Hz, 2H, meta
ZrCH2C6H5), 6.70 (t, J = 7.5 Hz, 1H, para ZrCH2C6H5), 6.96
(d, J = 7.2 Hz, 6H, [p-C6H4CH3]3), 7.01 (d, J = 7.2 Hz, 6H,
[p-C6H4CH3]3). 13C{1H} NMR (20 ЊC, C6D6): δ 4.1 ([(CH3)2-
2
opposite is true for the a overlap with the dz orbital (Scheme 3).
The e orbitals are required for substrate cis coordination, while
the a orbital is insignificant for substrate coordination since its
access is blocked by the collinear M–R bond.
᎐
Si–] ), 4.2 (HC᎐), 20.8 ([p-C H CH ] ), 73.3 (ZrCH C H ),
We now consider the overall electron density at the metal
center, which is affected by the total amount of π donation from
the ligand p orbitals to the metal d orbitals. Since the average
angles between the z-axis and the planes of the amides range
from 165–174Њ in the crystal structures of 1a, 1b, 3a, and 3b, the
horizontal e overlaps are fuller than the axial e and a overlaps in
these cases. When the diminution of the two sets of e overlaps
and the enhancement of the weak axial a interaction are both
considered, the geometry with small N–Zr–N angles should
cause a net decrease of overall electron density at the zirconium
center compared to the geometry with large N–Zr–N angles. To
summarize the above discussion, the small N–Zr–N angles
are beneficial for both the stabilization of the empty d orbitals
cis to the Zr-alkyl group (e orbitals) and the overall decrease
of electron density at the metal center. The geometry-induced
orbital effects are in fact reminiscent of the ansa-effects in the
bent metallocene systems discussed by Parkin et al.17
᎐
3
6
4
3 3
2
6
5
123.3 (para ZrCH2C6H5), 125.4 (ortho or meta [p-C6H4CH3]3),
126.5 (ortho or meta ZrCH2C6H5), 130.2 (ortho or meta
ZrCH2C6H5), 130.5 (ortho or meta [p-C6H4CH3]3), 131.9 (para
or ipso [p-C6H4CH3]3), 139.7 (ipso ZrCH2C6H5), 146.5 (para or
ipso [p-C6H4CH3]3).
Synthesis of CH[(CH3)2SiN( p-FC6H4)]3ZrCH2C6H5 (1b).
Compound 1b was synthesized following a procedure similar
to that for 1a using Zr(CH2C6H5)4 (228 mg, 0.500 mmol)
and CH[(CH3)2SiNH(p-FC6H4)]3 (259 mg, 0.50 mmol). The
reaction required 3 days at 85 ЊC. After removal of toluene,
the black residue was extracted with hexamethyldisiloxane
(≈15 mL) at 80 ЊC. Quick filtration resulted in a yellow solution.
Cooling the solution slowly to Ϫ40 ЊC and keeping it overnight
at such a temperature yielded pale yellow crystals of 2 (210 mg,
60%). Anal. calc. for C32H38F3N3Si3Zr: C, 55.13; H, 5.49;
1
N, 6.03. Found: C, 54.93; H, 5.42; N, 6.91%. H NMR (20 ЊC,
᎐
C D ): δ Ϫ0.58 (s, 1H, CH᎐), 0.24 (s, 18H, [(CH ) Si–] ), 1.95
᎐
6
6
3
2
3
Summary
(s, 2H, ZrCH2C6H5), 5.95 (d, J = 7.2 Hz, 2H, ortho ZrCH2-
C6H5), 6.49 (“t”, J = 7.4 Hz, 2H, meta ZrCH2C6H5), 6.62 (t,
J = 7.2 Hz, 1H, para ZrCH2C6H5), 6.68–6.86 (m, 12H, [p-C6-
We have described the synthesis, characterization, and reactivi-
ties of a group of Zr-alkyl complexes. They possess catalytic
activities unprecedented for the X3MR type complexes, but are
much less reactive than the more electropositive lanthanide
complexes and the Group 4 complexes with a formal cationic
charge. It appears that in addition to the inductive electronic
and steric effects, coordination geometry-caused orbital effects
are important factors to be considered for future design of
13
1
᎐
H4F]3). C{ H} NMR (20 ЊC, C D ): δ 3.2 (HC᎐), 4.2
᎐
6
6
([(CH3)2Si–]3), 73.9 (ZrCH2C6H5), 116.3 (d, 2JC–F = 21 Hz, meta
[p-C6H4F]3), 124.2 (para ZrCH2C6H5), 126.3 (overlap of ortho
[p-C6H4F]3 and ortho or meta ZrCH2C6H5), 130.8 (ortho or meta
ZrCH2C6H5), 138.4 (ipso ZrCH2C6H5), 145.6 (ipso [p-C6H4F]3),
159.4 (1JC–F = 239 Hz, para [p-C6H4F]3).
J. Chem. Soc., Dalton Trans., 2002, 2608–2615
2613

View Article Online
Table 6 Crystal data and structure refinements for 1b, 2a, 3a, and 3b
1b
2a
3a
3b
Empirical formula
Formula weight
Crystal system
Space group
a/Å
b/Å
C32H38F3N3Si3Zr
697.14
Orthorhombic
P212121
11.6166(6)
14.3844(8)
20.310(1)
90
3393.7(3)
4
C56H80Cl2N6Si6Zr2
1259.14
Monoclinic
P21/c
13.700(3)
25.525(5)
18.183(4)
94.187(4)
6341(2)
4
C32H49N3Si3Zr
651.23
Monoclinic
C2/c
42.402(3)
9.9224(7)
17.614(1)
108.717(1)
7018.8(8)
8
C29H40F3N3Si3Zr
663.13
Monoclinic
P21/n
9.576(1)
19.106(2)
18.087(2)
97.160(2)
3283.3(6)
4
c/Å
β/Њ
Volume/Å3
Z
Absorption coefficient/mmϪ1
Crystal color, habit
Crystal size/mm3
Reflections collected
Independent reflections
Refined parameters
Goodness-of-fit on F 2
Final R indices [I > 2σ(I )]
0.472
0.566
0.440
0.484
Colorless, block
0.29 × 0.26 × 0.26
25607
5994 [R(int) = 0.0223]
385
Pale yellow, block
0.29 × 0.22 × 0.19
37121
11187 [R(int) = 0.0324]
794
Pale yellow, block
0.28 × 0.22 × 0.19
20413
6196 [R(int) = 0.0233]
393
Colorless, block
0.26 × 0.26 × 0.25
21519
5808 [R(int) = 0.0249]
358
0.995
1.006
1.025
1.010
R1 = 0.0210
wR2 = 0.0508
R1 = 0.0231
wR2 = 0.0519
R1 = 0.0362
wR2 = 0.0766
R1 = 0.0530
wR2 = 0.0824
R1 = 0.0263
wR2 = 0.0677
R1 = 0.0374
wR2 = 0.0712
R1 = 0.0283
wR2 = 0.0766
R1 = 0.0377
wR2 = 0.0808
R indices (all data)
Synthesis of CH[(CH3)2SiN( p-CH3C6H4)]3ZrCl (2a). Com-
pound 1a (0.684 g, 1.0 mmol) and C6H5NMe2ؒHCl (0.142 mg,
1.00 mmol) were loaded into a 50 mL flask in the glove box.
The flask was attached to the vacuum line and evacuated,
and toluene (30 mL) was condensed into the flask at Ϫ78 ЊC.
The flask was backfilled with nitrogen and warmed to room
temperature slowly with stirring. After the reaction mixture was
stirred for 6 h at 25 ЊC, toluene was removed in vacuo. The
residue was extracted with boiling hexane (10 mL) and quickly
filtered. The filtrate was allowed to stand at room temperature
overnight. Pale yellow crystals were obtained (0.357 g, 56.8%).
Anal. calc. for C28H40ClN3Si3Zr: C, 53.50; H, 6.39; N, 6.69.
was stirred in hexane for 1 h at the room temperature and was
filtered to remove LiCl. Hexane was then removed in vacuo.
Next, hexamethyldisiloxane (10 mL) was condensed into the
flask. Heating the mixture at 80 ЊC resulted in a yellow homo-
geneous solution. Pale yellow crystals formed overnight at
room temperature (0.487 g, 58%). Anal. calc. for C32H49N3-
Si3Zr: C, 59.08; H, 7.54; N, 6.46. Found: C, 57.92; H, 7.60; N,
1
᎐
6.27%. H NMR (20 ЊC, C D ): δ Ϫ0.58 (s, 1H, HC᎐), 0.45 (s,
᎐
6
6
18H, [(CH3)2Si–]3), 0.48 (t,
J = 7.2 Hz, 2H, ZrCH2-
CH2CH2CH3), 0.67 (t, J = 7.2 Hz, 3H, ZrCH2CH2CH2CH3),
0.75 (m, 2H, ZrCH2CH2CH2CH3), 1.04 (m, 2H, ZrCH2-
CH2CH2CH3), 2.11 (s, 9H, [p-C6H4CH3]3), 7.02 and 7.07 (AB
1
Found: C, 53.20; H, 6.45; N, 6.74%. H NMR (20 ЊC, C6D6):
spin system, J = 7.2 Hz, 12H, [p-C6H4CH3]3). 13C{1H} NMR
᎐
᎐
δ Ϫ0.45 (s, 1H, HC᎐), 0.32 (s, 18H, [(CH ) Si–] ), 2.15 (s, 9H,
(20 ЊC, C D ): δ 2.9 (HC᎐), 4.2 ([(CH ) Si–] ), 13.8 (ZrCH -
᎐
᎐
3
2
3
6
6
3
2
3
2
[p-C6H4CH3]3), 6.90 (d, J = 7.3 Hz, 6H, [p-C6H4CH3]3), 7.00
CH2CH2CH3), 20.74 ([p-C6H4CH3]3), 27.2 (ZrCH2CH2CH2-
CH3), 28.52 (ZrCH2CH2CH2CH3), 70.0 (ZrCH2CH2CH2CH3),
125.0 (ortho or meta [p-C6H4CH3]3), 130.7 (ortho or meta
[p-C6H4CH3]3), 131.9 (ipso or para [p-C6H4CH3]3), 144.7 (ipso
or para [p-C6H4CH3]3).
(d, J = 7.3 Hz, 6H, [p-C6H4CH3]3). 13C{1H} NMR (20 ЊC,
᎐
C D ): δ 3.7 (s, HC᎐), 3.9 (s, [(CH ) Si–] ), 20.8 ([p-C H CH ] ),
᎐
6
6
3
2
3
6
4
3 3
124.6 (ortho or meta [p-C6H4CH3]3), 130.5 (ortho or meta
[p-C6H4CH3]3), 132.4 (para or ipso [p-C6H4CH3]3), 145.5 (para
or ipso [p-C6H4CH3]3).
Synthesis of CH[(CH3)2SiN( p-FC6H4)]3ZrnBu (3b). Com-
pound 3b was synthesized following a procedure similar to that
for 3a except that 2b (0.640 g, 1.00 mmol) and nBuLi (0.40 mL,
2.5 M in hexane) were used as the starting materials. The
product was isolated from its hexane solution at Ϫ78 ЊC as
colorless crystals (0.43 g, 64.9%). Anal. calc. for C29H40F3N3-
Si3Zr: C, 52.57; H, 6.04; N, 6.34. Found: C, 52.47; H, 6.10; N,
Synthesis of CH[(CH3)2SiN( p-FC6H4)]3ZrCl (2b). Com-
pound 2b was synthesized following a procedure similar to that
for 2a except that 1b (0.696 g, 1.00 mmol) and triphenylmethyl
chloride (0.278 g, 1.00 mmol) were used as the starting
materials. After the reaction was finished, the volume of tolu-
ene was reduced to 15 mL. The resulting slurry was heated at
100 ЊC and quickly filtered. The filtrate was allowed to stand
overnight at room temperature. White square crystals were
obtained (0.51 g, 79.7%). Anal. calc. for C25H31ClF3Si3Zr: C,
1
᎐
6.24%. H NMR (20 ЊC, C D ): δ Ϫ0.67 (s, 1H, HC᎐), 0.30 (s,
᎐
6
6
18H, [(CH3)2Si–]3), 0.48 (t, J = 7.2 Hz, 2H, ZrCH2CH2CH2-
CH3), 0.49 (t, J = 7.2 Hz, 3H, ZrCH2CH2CH2CH3), 0.68 (m,
2H, ZrCH2CH2CH2CH3), 0.89 (m, 2H, ZrCH2CH2CH2CH3),
46.87; H, 4.84; N, 6.56. Found: C, 46.75; H, 4.83; N, 6.59%. 1H
᎐
6.80–6.88 (m, 12H, [p-C6H4F]3. 13C{1H} NMR (20 ЊC, C6D6):
NMR (20 ЊC, C D ): δ Ϫ0.56 (s, 1H, HC᎐), 0.11 (s, 18H,
᎐
6
6
[(CH3)2Si–]3), 6.56–6.57 (m, 6H ortho [p-C6H4F]3), 6.87 (m,
6H, meta [p-C6H4F]3). 13C{1H} NMR (20 ЊC, CDCl3): δ 3.8
᎐
δ 2.9 (s, HC᎐), 3.9 ([(CH ) Si–] ), 13.8 (ZrCH CH CH CH ),
᎐
3
2
3
2
2
2
3
27.0 (ZrCH2CH2CH2CH3), 28.1 (ZrCH2CH2CH2CH3), 70.8
2
᎐
2
([(CH ) Si–] ), 5.2 (s, HC᎐), 115.5 (d, J = 22 Hz, meta [p-
᎐
3
2
3
FC
(s) (ZrCH2CH2CH2CH3), 116.6 (d, JFC = 21.6 Hz, meta [p-
3
3
C6H4F]3), 125.9 (d, JFC = 8 Hz, ortho [p-C6H4F]3), 145.4 (ipso
[p-C6H4F]3), 158.8 (d, 1JFC = 239 Hz, para [p-C6H4F]3).
C6H4F]3), 126.1 (d, JFC = 15.3 Hz, ortho [p-C6H4F]3), 143.3
(ipso [p-C6H4F]3), 158.8 (d, 1JFC = 239, para [p-C6H4F]3).
Synthesis of CH[(CH3)2SiN( p-CH3C6H4)]3ZrnBu (3a). Com-
pound 2a (0.810 g, 1.29 mmol) was loaded into a 50 mL flask in
the glove box. The flask was attached to the vacuum line and
evacuated. Toluene (20 mL) was condensed into the flask under
vacuum at Ϫ78 ЊC. The flask was backfilled with nitrogen, and
nBuLi (0.52 mL, 2.5 M in hexane) was injected into the flask at
Ϫ78 ЊC. The reaction mixture was warmed to room tempera-
ture slowly and was stirred for 3 h. Toluene was then removed,
and hexane (5 mL) was condensed into the flask. The mixture
General procedure for catalytic hydrosilations
A reaction flask with a Teflon valve was charged with catalyst
(0.08 mmol), PhSiH3 (4.00 mmol), and olefin (4.00 mmol) in
the glove box. No solvent was added. The flask was sealed
and placed in a 90 ЊC oil bath. Small aliquots of the reaction
mixture were withdrawn periodically, and their 1H NMR
spectra were taken to monitor the progress of the reaction.
When the reaction was completed, the product was separated
2614
J. Chem. Soc., Dalton Trans., 2002, 2608–2615

View Article Online
1998, 17, 5663 . Note the zirconium and hafnium complexes are
from the catalyst by distillation under reduced pressure. The
NMR spectra of the products are compared to the data
reported in the literature.15,16
active ethylene polymerization catalysts but are zwitterionic dimers,
not neutral complexes; (b) J. L. Bennett and P. T. Wolczanski,
J. Am. Chem. Soc., 1997, 119, 10696; (c) C. P. Schaller, C. Cummins
and P. T. Wolczanski, J. Am. Chem. Soc., 1996, 118, 591; (d ) C.
Cummins, G. D. Van Duyne, C. P. Schaller and P. T. Wolczanski,
Organometallics, 1991, 10, 164; (e) R. W. Chestnut, L. D. Durfee,
P. E. Fanwick, I. P. Rothwell, K. Folting and J. C. Huffman,
Polyhedron, 1987, 6, 2026; ( f ) T. V. Lubben, P. T. Wolczanski and
G. D. Van Duyne, Organometallics, 1984, 3, 977; (g) R. A. Andersen,
Inorg. Chem., 1979, 18, 1724.
X-Ray structure determination
Crystals of 1b, 2a, 3a, and 3b were placed on the tip of a
0.1 mm diameter glass capillary and mounted on a Siemens
or Bruker SMART Platform CCD diffractometer for data
collection at 173(2) K using graphite-monochromated Mo-Kα
radiation (0.71703 Å). The intensity data were corrected for
Lorentz polarization effects, absorption, and decay.19,20 Final
cell constants were calculated from the xyz centroids of 3377,
3883, 3495, and 4085 strong reflections for structures 1b, 2a, 3a,
and 3b, respectively. Data are summarised in Table 6.
6 (a) F. Lefebvre, A. De Mallmann and J.-M. Basset, Eur. J. Inorg.
Chem., 1999, 3, 361; (b) V. Dufaud and J.-M. Basset, Angew. Chem.,
Int. Ed., 1998, 37, 806; (c) J. Corker, F. Lefebvre, C. Lécuyer,
V. Dufaud, F. Quignard, A. Choplin, J. Evans and J.-M. Basset,
Science, 1996, 271, 966; (d ) F. Quignard, C. Lécuyer, A. Choplin and
J.-M. Basset, J. Chem. Soc., Dalton Trans., 1994, 1153.
The structures were solved by direct methods using SIR9221
for 1b and 2a and using SHELXS-9722 for 3a and 3b,
which provided most non-hydrogen atoms. The space groups
were determined based on systematic absences and intensity
statistics. Full-matrix least squares/difference Fourier cycles
were performed using SHELXL-97,23 which located the remain-
ing non-hydrogen atoms. All non-hydrogen atoms were refined
with anisotropic displacement parameters. All hydrogen atoms
were placed in ideal positions and refined as riding atoms with
isotropic displacement parameters related to the parent atom.
One side of the dimeric 2a is disordered over two positions
(77 : 23). Some variances in the Zr–N distances are present
in the minor component of the disorder (Zr1–N1Ј, Zr1–N2Ј,
Zr1–N3Ј), due to the particular modeling of the disorder. The
butyl group of 3a is modeled as disordered over two positions
(50 : 50), and methyl group C21 is rotationally disordered. All
the methyl groups attached to silicon atoms in 3b are rotation-
ally disordered.
7 (a) H. W. Turner, R. A. Andersen, A. Zalkin and D. H. Templeton,
Inorg. Chem., 1979, 18, 1221; (b) C. Airoldi, D. Bradley,
H. Chudzynska, M. B. Hursthouse, A. Mali and P. R. Raithby,
J. Chem. Soc., Dalton Trans., 1980, 2010; (c) S. L. Latesky,
J. Keddington, A. K. McMullen and I. P. Rothwell, Inorg. Chem.,
1985, 24, 995.
8 (a) L. H. Gade, P. Renner, H. Memmler, F. Fecher, C. H. Galka,
M. Laubender, S. Rdojevic, M. McPartlin and J. W. Lauher, Chem.
Eur. J., 2001, 7, 2563; (b) M. Lutz, M. Haukka, T. A. Pakkanen
and L. H. Gade, Organometallics, 2001, 20, 2631; (c) P. Renner,
C. Galka, H. Memmler, U. Kauper and L. H. Gade, J. Organomet.
Chem., 1999, 591, 71; (d ) A. Schneider, L. H. Gade, M. Breuning,
G. Bringmann, I. J. Scowen and M. McPartlin, Organometallics,
1998, 17, 1643; (e) H. Memmler, U. Kauper, L. H. Gade, I. J. Scowen
and M. McPartlin, Chem. Commun., 1996, 1751; ( f ) B. Findeis,
M. Schubart, C. Platzek, L. H. Gade, I. J. Scowen and M.
McPartlin, Chem. Commun., 1996, 219; (g) H. Memmler, W. Kevin,
L. H. Gade and J. W. Lauher, Inorg. Chem., 1995, 34, 4062;
(h) M. Schubart, B. Findeis, L. H. Gade, W.-S. Li and M. McPartlin,
Chem. Ber., 1995, 128, 329; (i) S. Friedrich, H. Memmler,
L. H. Gade, W.-S. Li and M. McPartlin, Angew. Chem., Int. Ed.
Engl., 1994, 33, 676; (j) L. H. Gade, C. Becker and J. W. Lauher,
Inorg. Chem., 1993, 32, 2308.
CCDC reference numbers 183853–183856.
See http://www.rsc.org/suppdata/dt/b2/b200789d/ for crystal-
lographic data in CIF or other electronic format.
9 L. Jia, E. Ding, A. L. Rheingold and B. Rhatigan, Organometallics,
2000, 19, 963.
10 S. L. Latesky, A. K. McMullen, G. P. Niccolai and I. P. Rothwell,
Organometallics, 1985, 4, 902.
11 M. Brookhart, M. L. H. Green and L. Wong, Prog. Inorg. Chem.,
1988, 36, 1.
12 Z. Dawoodi, M. L. H. Green, V. S. B. Mtetwa, K. Prout,
A. J. Schultz, J. M. Williams and T. F. Koetzle, J. Chem. Soc., Dalton
Trans., 1986, 1629.
Acknowledgements
This work is supported by ACS-Petroleum Research Fund
(Grant number 35538–G3) and Lehigh University.
13 M. E. Thompson, S. M. Baxter, A. R. Bulls, B. J. Burger,
M. C. Nolan, B. D. Santarsiero, W. P. Schaefer and J. E. Bercaw,
J. Am. Chem. Soc., 1987, 109, 203–219.
14 R. F. Jordan, P. K. Bradley, N. C. Baenziger and R. E. LaPointe,
J. Am. Chem. Soc., 1990, 112, 1289.
References and notes
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