Role of Zr,Al Hydride Intermediate Structure and Dynamics in Alkene Hydroalumination with XAlBui2 (X = H, Cl, Bui), Catalyzed by Zr η5 Complexes

Article abstract of DOI:10.1021/acs.organomet.5b00370

The zirconocene complexes L₂ZrCl₂ (L = C₅H₅, C₅H₄Me, Ind, C₅Me₅; L₂ = rac-Me₂C(2-Me-4-Bu^t-C₅H₂)₂, meso-Me₂C(2-Me-4-Bu^t-C₅H₂)₂, rac-Me₂C(3-Bu^t-C₅H₃)₂, rac-Me₂C(Ind)₂, rac-Me₂Si(Ind)₂, and rac-C₂H₄(Ind)₂) were tested as catalysts in alkene hydroalumination by the isobutylalanes XAlBuⁱ₂ (X = H, Cl, Buⁱ). A low-temperature NMR spectroscopy study on the structure and dynamics of Zr,Al intermediates formed in the L₂ZrCl₂-XAlBuⁱ₂ systems showed that the intra- and intermolecular exchange in the bimetallic clusters, controlled by the steric factor of the η⁵ ligand and organoaluminum compound nature, determines the activity of the whole catalytic system. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00370

Article
pubs.acs.org/Organometallics
Role of Zr,Al Hydride Intermediate Structure and Dynamics in Alkene
Hydroalumination with XAlBui2 (X = H, Cl, Bui), Catalyzed by Zr η5
Complexes
Lyudmila V. Parfenova,*,† Pavel V. Kovyazin,† Ilya E. Nifant’ev,‡ Leonard M. Khalilov,†
and Usein M. Dzhemilev†
†Institute of Petrochemistry and Catalysis, Russian Academy of Sciences, prospect Oktyabrya, 141, 450075 Ufa, Russian Federation
‡Department of Chemistry, Lomonosov Moscow State University, 1-3 Leninskiye Gory, 119991 Moscow, Russian Federation
S
* Supporting Information
ABSTRACT: The zirconocene complexes L2ZrCl2 (L = C5H5,
C5H4Me, Ind, C5Me5; L2 = rac-Me2C(2-Me-4-But-C5H2)2, meso-
Me2C(2-Me-4-But-C5H2)2, rac-Me2C(3-But-C5H3)2, rac-Me2C(Ind)2,
rac-Me2Si(Ind)2, and rac-C2H4(Ind)2) were tested as catalysts in
alkene hydroalumination by the isobutylalanes XAlBui2 (X = H, Cl,
Bui). A low-temperature NMR spectroscopy study on the structure and
dynamics of Zr,Al intermediates formed in the L2ZrCl2−XAlBui2
systems showed that the intra- and intermolecular exchange in the
bimetallic clusters, controlled by the steric factor of the η5 ligand and
organoaluminum compound nature, determines the activity of the
whole catalytic system.
appear to be convenient for monitoring, as well as the absence
of paramagnetic species, which, for example, in the case of
titanium complexes preclude observation of the genesis of
intermediate complexes of catalytic systems due to pronounced
NMR signal broadening.
INTRODUCTION
■
Catalytic alkene hydroalumination has found wide use in
organic and organometallic synthesis as an efficient method for
the synthesis of practically important organoaluminum
compounds (OACs) of a specified structure and as a method
for regioselective reduction and functionalization of unsaturated
compounds.1 This type of reaction can be catalyzed by
complexes of various transition metals (Ti, Zr, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, etc.). It is noted that the activity of the catalytic
system is strongly affected by both the nature of substituents in
the initial OAC and the electronic structure of the transition
metal.2 Various bimetallic hydride intermediates are assumed as
active centers of the reactions.3
The catalytic action of zirconocene complexes in alkene
hydroalumination is of considerable interest, since these
compounds not only perform the reaction under mild
conditions with high regio- and stereoselectivity but also
allow establishment of the reaction mechanism, particularly, the
intermediate structure and interconversions using dynamic
NMR spectroscopy, which provides the most complete
information and allows direct monitoring of the reaction.
Extensive application of this method to the studies of catalytic
systems based on zirconium η5 complexes and OACs became
possible due to several reasons. First, a broad range of catalytic
reactions can be implemented in these systems, from hydro-,
carbo-, and cyclometalation to polymerization of unsaturated
compounds. Second, these systems are convenient for
fundamental investigations, since η5 ligands bound to zirconium
atoms act like magnetic probes reflecting the electronic state of
the transition-metal atom. Furthermore, the reaction times
Experimental studies of the mechanism of alkene hydro-
alumination with XAlBui2 (where X = H, Cl, Bui) in the
presence of Cp2ZrCl2 by dynamic NMR demonstrated that
bimetallic Al,Zr-hydride complexes may act as the key reaction
intermediates.4,5 A quantum chemical study of this reaction6
specified the initial steps of the ligand exchange between
HAlBui2 and Cp2ZrCl2 and proposed the structure of bimetallic
intermediates that can exist in the system (Scheme 1). Among
the intermediates, complexes 6a, 7a, and 11a were found
experimentally.4,5,7 The possibility of complex 10a formation
has been considered in the literature8 as well. Further
theoretical studies demonstrated9 that intermediates 3a, 4a,
and 5a containing a free Zr−H bond can act as catalytically
active sites of the hydroalumination, and the reactivity of these
complexes is dependent on the number of bridging Zr−H−Al
bonds: the greater their number, the less active the complex
becomes.
An introduction of activating agents (MAO or [Ph3C][B-
(C6F5)4)]) into the catalytic systems L2ZrCl2−XAlBui2 makes
possible the polymerization of the unsaturated compounds.10 It
is assumed that neutral or cationic bimetallic hydride complexes
containing [L2ZrH3] or [L2ZrH2] moieties, depending on the
Received: April 30, 2015
© XXXX American Chemical Society
A
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Scheme 1. Quantum Chemical Model of Catalytic Alkene Hydroalumination by HAlBui2
nature of the η5 ligand, are formed as intermediates in the
Scheme 2
reaction.8,11,12
However, the question of how the OAC and zirconocene
structures affect the activity of these catalytic systems remains
open. To solve the problem, we studied the structure,
dynamics, and reactivity of the bimetallic hydride complexes
formed in the reaction of L2ZrCl2 with XAlBui2 and considered
these results in the context of the catalytic system efficiency in
the alkene hydroaluminations.
RESULTS AND DISCUSSION
■
Catalytic Activity of Zr η5 Complexes in the Hydro-
alumination of Terminal Alkenes with XAlBui2 (X = H, Cl,
Bui). In order to elucidate the factors determining the activity
of the hydroaluminating catalytic systems, we studied the
catalytic action of zirconocene complexes L2ZrCl2 (1a−j: L =
C5H5 (a), C5H4Me (b), C5Me5 (c); L2 = rac-Me2C(2-Me-4-
But-C5H2)2 (d), meso-Me2C(2-Me-4-But-C5H2)2 (e), rac-Me2C-
(3-But-C5H3)2 (f), Ind (g); rac-Me2C(Ind)2 (h), rac-Me2Si-
(Ind)2 (i), rac-C2H4(Ind)2 (j)) in alkene hydroalumination
in the yield of the hydroalumination products irrespective of
the OAC structure (Figure 1d).
with the isobutylalanes XAlBui2 (X = H, Cl, Bui) (Scheme 2).
The reaction yields the hydroalumination products 2.
The use of rac isomers of complexes 1d,f with bulky ligands
as catalysts in the reaction of alkene with HAlBui2 provides a
high yield of hydrometalation product as well as in the case of
1c (Figure 2). The replacement of HAlBui2 with AlBui3 in these
systems dramatically decreases the alkene conversion. rac-
Zirconocenes with ansa-indenyl ligands 1h−j and the meso
isomer of the sterically hindered bis-cyclopentadienyl complex
1e (the starting sample of 1e contained ∼17% of 1d impurity)
showed a catalytic activity lower than that of complex 1a in the
hydroalumination with HAlBui2.
As follows from Figure 3, conducting the reaction of alkene
with HAlBui2 in the presence of a stoichiometric amount of 1a
(1a:HAlBui2:alkene ratio of 1:3:1) was accompanied by a sharp
increase in the reaction rate, so that 90% conversion of the
alkene was achieved in 2 h. In the catalytic version, when the
initial OAC concentration was increased or the amount of the
catalyst was reduced to the molar ratio 1a:HAlBui2:alkene 1:6:5
Previously, we demonstrated4 that, in the presence of
complex 1a, at a reactant molar ratio 1a:OAC:alkene of
1:60:50, the highest hydroalumination rate among the chosen
OACs was observed for AlBui3. In the case of ClAlBui2, a long-
term induction period (up to 3 h) was found.13 Diisobutyla-
luminum hydride showed the lowest activity among the studied
OACs (Figure 1a).
The catalysis by complex 1b leads almost to the same results;
however, in the case of ClAlBui2, the induction period
disappears and the yield of hydroalumination products
decreases (Figure 1b). In the presence of the most sterically
hindered catalyst 1c, the rate of alkene hydroalumination
substantially increases when HAlBui2 is used, whereas the
reaction with ClAlBui2 or AlBui3 is slowed down (Figure 1c). A
replacement of the cyclopentadienyl ligands in the zirconium
complex with indenyl ligands provides a considerable decrease
B
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i
i
Figure 1. Effect of η5 ligand and OAC structure (HAlBui2
■
●
), ClAlBu 2 ( ), AlBu 3 ( )) on octane yield in the reaction of 1-octene
▲
(
hydroalumination (molar ratio L2ZrCl2:AOC:alkene 1:60:50, C6H6, t = 20 °C): (a) (C5H5)2ZrCl2 (1a); (b) (C5H4Me)2ZrCl2 (1b); (c)
(C5Me5)2ZrCl2 (1c); (d) Ind2ZrCl2 (1g).
HAlBui2 exists mainly as a trimer and ClAlBui2 is a dimer, while
AlBui3 is monomeric.14 Quantum chemical modeling of self-
association of HAlBui2 and ClAlBui2 shows that, in nonpolar
solvents at room temperature, the dimeric form predominates
for ClAlBui2 and a mixture of dimeric and trimeric forms
prevails for HAlBui2.15 Although these compounds had been
thoroughly studied, our experimental results made a certain
contribution to the knowledge of the structure of the initial
1
OACs. The H NMR spectra of HAlBui2 recorded at room
temperature in benzene were found to exhibit signals for four
hydride atoms at δH 3.94, 3.24, 2.97, and 2.77 ppm (Figure S1a
in the Supporting Information). These signals were correlated
in the EXSY spectra (no cross-peaks between them were
present in the COSY spectra) (Figure 4a). A similar
observation was reported previously.12 The signal at δH 3.94
ppm, which was assigned to the hydride atom of the HAlBui2
monomer, had the lowest intensity. Dimerization should be
accompanied by an upfield shift of the hydride resonance line
due to the formation of bridging Al−H−Al bonds. Therefore,
the next signal at δH 3.24 ppm was assigned to the dimeric
form. The most intense resonance line at δH 2.97 ppm
corresponds to the trimer, while the highest-field signal out of
the four signals (δH 2.77 ppm) apparently belongs to the
tetramer.
Figure 2. Activity of ansa-zirconocenes in the reaction of 1-octene
hydroalumination by XAlBui2 (molar ratio L2ZrCl2:AOC:alkene
1:60:50, C6H6, t = 20 °C): rac-Me2C(2-Me-4-But-C5H2)2ZrCl2
+
t
HAlBui2 ( ), rac-Me2C(3-Bu -C5H3)2ZrCl2 + HAlBui2 ( ), rac-
■
●
C2H4(Ind)2ZrCl2 + HAlBui2
(
), rac-Me2C(Ind)2ZrCl2 + HAlBui2
▼
(▶), rac-Me2Si(Ind)2ZrCl2 + HAlBui2 ( ), rac-Me2C(2-Me-4-Bu -
t
⧫
i
C5H2)2ZrCl2 + AlBui3 ( ), rac-C2H4(Ind)2ZrCl2 + AlBu 3 (*), meso-
□
Me2C(2-Me-4-But-C5H2)2ZrCl2 + HAlBui2
(
▲
).
When the HAlBui2 concentration in benzene is 3.3 mol/L (in
relation to the monomer), the monomer:dimer:trimer:tetramer
molar ratio is 1:29:226:23 and the monomer concentration is
∼4 × 10−3 mol/L. The addition of 0.6 equiv of ClAlBui2 to 1
equiv of HAlBui2 in toluene changes the [HAlBui2]:
[(HAlBui2)2]:[(HAlBui2)3]:[(HAlBui2)4] molar ratio to
1:6:5:0.4 (Figure S1b in the Supporting Information). As
follows from the ratio, the percentage of the diisobutylalumi-
num hydride monomer (δH 3.94 ppm) increases with the
appearance of ClAlBui2. Probably, ClAlBui2 has an influence on
the equilibrium shown in Scheme 3 by producing the mixed
oligomers [(HAlBui2)n·(ClAlBui2)m],16 which provide new
Figure 3. Hexane yield in the reaction of 1-hexene with HAlBui2 vs
1
signals in the H NMR spectrum at δH equal to 3.47, 3.18,
amount of catalyst 1a (C6H6, t = 24 °C). Molar ratio
i
and 2.88 ppm (Figure S1b; T = 260 K, toluene). This fact is
confirmed by the presence of slow intermolecular exchange
between the mixed oligomers with [HAlBui2]n in the EXSY
spectrum at 260 K (Figure 4b). Thus, the involvement of the
OAC monomer in the reaction with Zr complexes during the
hydroalumination is quite probable. The percentage of the
HAlBui2 monomer in the system can probably increase, owing
to the additional amount of ClAlBui2 arising in the early steps
upon Cl−H exchange in the L2ZrCl2 + HAlBui2 reaction.
Zr,Al-Hydride Intermediates in the Reaction of L2ZrCl2
with XAlBui2 (X = H, Cl, Bui). A. Systems Based on the C5H5
Ligand. In order to elucidate the structure and dynamics of the
■
●
▲
1a:HAlBu 2:alkene: 1:3:1 ( ), 1:6:5 ( ), 1:60:50 ( ).
or 1:60:50, a considerable decrease in the hydroalumination
product yield was observed.
Since the rate of the catalytic hydroalumination is largely
determined by the structure of the OAC, we carried out a study
of solutions of HAlBui2 and ClAlBui2 in hydrocarbon solvents.
Exchange Spectroscopy Studies on the Structure and
Dynamics of the Initial Isobutyl-Substituted OACs. The
isobutyl-substituted OACs tend to self-associate to various
extents. For example, it is known that in nonpolar solvents
C
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Figure 5. Variable-temperature 1H NMR study on the system
(C5H5)2ZrCl2 (1a)−HAlBui2 (1:3) in C7D8 (the intensity of upfield
signals is increased for clarity).
and 5.59 ppm and two groups of signals correlated in the
COSY spectra in the region of hydride atoms bonded to Zr and
Al: (a) a triplet at δH −1.06 ppm and a doublet at δH −2.30
ppm (1:2 ratio) and (b) broadened signals at δH 3.56, −1.22,
−2.30, and −3.05 ppm (1:1:1:1 ratio) (Table 1). The more
intense resonance line of the Cp ligand at δH 5.59 ppm was
related to the triplet at δH −1.06 ppm in the ratio 10:1. The
ratio of the areas of the cyclopentadienyl signal at δH 5.66 ppm
and a signal at δH 3.56 ppm was 10:1 as well.
Thus, both complexes contain [(C5H5)2ZrH3] moieties and
in the latter complex the symmetry is reduced, giving rise to
three nonequivalent signals in the upfield region. If one
assumes that 6a is a more symmetric complex, the symmetry
may be reduced as a result of one more HAlBui2 molecule
entering, providing structure 8a, the hydride signal of this
moiety being observed in the middle part of the spectrum
(Table 1). A rise in the concentration of HAlBui2 in the mixture
entails an increase in the percentage of nonsymmetric complex
8a. In low-temperature EXSY spectra (230−240 K) slow
exchange is observed between the hydride atoms of complexes
6a and 8a (Figure S4 in the Supporting Information). In
addition, the signal at δH 3.56 ppm for the Al−H−Al bond of
complex 8a was correlated with the signals of oligomers of
HAlBui2. As the temperature of the reaction mixture 1a +
HAlBui2 was increased to 300 K, gradual coalescence of signals
for both the Cp ligands and hydride hydrogens took place,
resulting in a single Cp resonance line at δH 5.69 ppm (δC 104.5
ppm) and broadened hydride signals at δH −1.00 and −2.11
ppm with a 10:1:2 ratio, respectively (Figure 5). The rate
constant and the activation parameters for the dynamic process
6a ↔ 8a were k220 = 10.9 s−1, ΔG⧧220 = 11.98 0.05 kcal/mol,
ΔH⧧ = 6.11 0.54 kcal/mol, and ΔS⧧ = −26.80 9.97 cal/
(mol K). The temperature increase provided the exchange
between the hydride atoms of Zr−H−Al bonds of the
complexes and the oligomers of HAlBui2 (Figure 6).
Apparently, the exchange occurs via dissociation of the Zr,Al-
hydride complexes with elimination of the monomer that serves
as a connecting unit between the complexes and the [HAlBui2]n
(Scheme 4).
Figure 4. EXSY spectra of (a) HAlBui2 in C6D6 (3.3 mol/L, 300 K, τ =
0.3 s) and (b) the mixture HAlBui2 + ClAlBui2 in C7D8 (C(HAlBui2)
1.2 mol/L, C(ClAlBui2) 0.7 mol/L, 260 K, τ = 0.3 s).
Scheme 3
intermediates involved in catalytic hydroalumination, we
studied the reaction of (C5H5)2ZrCl2 (1a) and XAlBui2 (X =
H, Bui). In the reaction with HAlBui2, a complete dissolution of
complex 1a in d6-benzene was observed at 20 °C and a
1a:HAlBui2 molar ratio of 1:3. Previously, formation of the
individual compound (C5H5)2Zr(μ-H)3(AlBui2)2(μ-Cl) (6a)7
or (C5H5)2Zr(μ-H)3(AlBui2)3(μ-Cl)2 (10a)6,8 was assumed for
this system (Scheme 1).
We attempted to prepare complex 6a by an alternative
synthesis, namely, by the reaction of 0.5 equiv of
[(C5H5)2ZrH2]2 (12a) with 1 equiv of ClAlBui2 and 1 equiv
of HAlBui2 (Scheme 5). The NMR chemical shifts of the
bridging hydride atoms and cyclopentadienyl ligands in the
According to our NMR investigations, the reaction of 1a with
HAlBui2 (1:3) yields a mixture of at least two complexes in a
1
2.7:1 ratio. The H NMR spectra of this mixture recorded at
220 K (Figure 5) exhibit two signals for the Cp rings at δH 5.66
D
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a
1
Table 1. H NMR Spectra of Zr,Al-Hydride Complexes
complex
T, K
Zr−H−Al
L
(C5H5)2Zr(μ-H)3(AlBui2)2(μ-Cl) (6a)
220
−2.30 (d, 4.8 Hz, 2H)
−1.06 (d, 6.8 Hz, 1H)
−1.75 (d, 4.8 Hz, 2H)
−0.67 (t, 7.2 Hz, 1H)
5.59 (s, 10H)
(C5H4Me)2Zr(μ-H)3(AlBui2)2(μ-Cl) (6b)
220
220
5.87 (m, 8H, CH)
5.45 (m, 8H, CH)
1.72 (s, 6H, CH3)
7.37 (m, 4H, CH)
6.87 (m, 4H, CH)
6.32 (m, 2H, CH)
5.19 (m, 4H, CH)
7.85 (d, 7.6 Hz, 2H, CH)
7.39 (m, CH, COSY)
6.83 (m, CH, COSY)
6.92 (m, 4H, CH)
5.81 (m, 2H, CH)
0.52 (NOESY, SiMe3)
7.43 (d, 8.8 Hz, 2H, CH)
7.15 (m, CH, COSY)
6.89 (m, 2H, CH)
6.81 (m, 2H, CH)
6.40 (m, 2H, CH)
5.75 (m, 2H, CH)
Ind2Zr(μ-H)3(AlBui2)2(μ-Cl) (6f)
−1.11 (d, 5.6 Hz, 2H)
0.07 (t, 7.6 Hz, 1H)
Me2Si(Ind)2Zr(μ-H)3(AlBui2)2(μ-Cl) (6h)
220
220
−1.55 (m, 2H)
1.06 (m, COSY)
C2H4(Ind)2Zr(μ-H)3(AlBui2)2(μ-Cl) (6i)
−1.00 (m, 2H)
0.62 (m, COSY)
2.65, 3.05 (m, NOESY, −CH2CH2−)
1.99 (s, 30H, CH3)
(C5Me5)2ZrH(μ-H)2(AlBui2)2(μ-Cl) (5c)
220
195
−1.27 (m, 1H)
−0.66 (d, 9.6 Hz, 1H)
4.38 (dd, 9.6 Hz, 4.0 Hz, 1H) (Zr−H)
−0.91 (m, 1H)
−0.33 (m, 1H)
2.28 (m, COSY) (Zr−H)
rac-Me2C(2-Me-4-But-C5H2)2ZrH(μ-H)2(AlBui2)2(μ-Cl) (5d)
6.69 (m, 1H, CH)
6.35 (m, 1H, CH)
4.88 (m, 1H, CH)
4.80 (m, 1H, CH)
5.67 (d, 2.4 Hz, 2H, CH)
4.94 (m, 2H, CH)
2.22 (s, 6H, CH3)
1.58 (m, 3H, CH3)
1.30 (m, 3H, CH3)
1.33 (m, 18H, CH3)
6.66 (m, 1H, CH)
6.45 (m, 1H, CH)
5.77 (m, 1H, CH)
5.54 (m, 1H, CH)
4.86 (m, 1H, CH)
4.53 (m, 1H, CH)
5.66 (s, 10H)
meso- Me2C(2-Me-4-But-C5H2)2ZrH(μ-H)2(AlBui2)2(μ-Cl) (5e)
300
194
−1.07 (m, 1H)
1.19 (m, COSY)
3.51 (m, COSY) (Zr−H)
rac-Me2C(3-But-C5H2)2ZrH(μ-H)2(AlBui2)2(μ-Cl) (5f)
−0.94 (m, 1H)
−0.78 (m, 1H)
2.05 (m, COSY) (Zr−H)
(C5H5)2Zr(μ-H)3(AlBui2)3(μ-Cl)(μ-H) (8a)
(C5H4Me)2Zr(μ-H)3(AlBui2)3(μ-Cl)(μ-H) (8b)
220
220
−3.05 (m, 1H)
−2.30 (m, 1H)
−1.22 (m, 1H)
3.56 (m, 1H) (Al−H−Al)
−2.54 (m, 1H)
−1.75 (m, 1H)
−0.85 (m, 1H)
3.71 (m, 1H) (Al−H−Al)
6.09 (m, 2H, CH)
5.87 (m, 2H, CH)
5.52 (m, 2H, CH)
5.37 (m, 2H, CH)
1.76 (s, 6H, CH3)
5.68 (s, 10H)
(C5H5)2Zr(μ-H)3(AlBui2)3(μ-Cl)2 (10a)
(C5H5)2Zr(μ-H)3(AlBui2)(AlBui3) (16a)
220
230
−2.27 (m, 2H)
−1.04 (m, 1H)
−2.48 (m, 1H)
5.56 (s, 10H)
−1.83 (d, 7.6 Hz, 1H)
−1.15 (d, 6.4 Hz, 1H)
−0.71 (br s, 1H)
−1.68 (br s, 2H)
(C5H5)2Zr(μ-H)3(AlEt2)2(μ-Cl) (19b)
(C5H5)2Zr(μ-H)3(AlBui2)3(μ-H)2 (20a)
293
210
5.68 (m, 8H, CH)
5.44 (m, 8H, CH)
1.73 (s, 6H, CH3)
5.45 (s, 10H)
−2.20 (d, 15.2 Hz, 2H)
−1.34 (t, 15.2 Hz, 1H)
3.28 (br s, 2H) (Al−H−Al)
E
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Table 1. continued
a
δ values given in ppm at 400.13 MHz in d8-toluene.
1
Figure 7. H NMR of complex 6a (a), 6a + ClAlBui2 (b), and 6a +
HAlBui2 (c) in C7D8 at 220 K (the intensity of upfield signals is
increased for clarity).
Figure 6. EXSY spectrum of the system (C5H5)2ZrCl2 (1a)−HAlBui2
(1:3) in d8-toluene at 290 K (τ = 0.3 s).
Scheme 5
Scheme 4
atoms of 6a and 10a, indicating the existence of fast
intermolecular exchange between them. Apparently, the lower
molecular masses found for complexes 6a, 8a, and 10a by the
cryoscopic method in comparison to the theoretical values are
due to the fact that these compounds tend to dissociate and
exist in equilibrium with other Zr,Al-hydride complexes and
OAC oligomers.
Additionally, we investigated the complex (C5H5)2Zr(μ-
H)3(AlBui2)3(μ-H)2 (9a), which was postulated to form in the
1a−HAlBui2 system in our previous studies6,9 and was
detected8 in the reaction of 1/2 equiv of [(C5H5)2ZrH2]2
with 3 equiv of HAlBui2. We found that complex 9a is actually
observable in this reaction at temperatures below 240 K
(Scheme 5 and Figures S5 and S6 in the Supporting
Information). The EXSY spectra at 240 K exhibit the hydride
exchange both within the complex molecules and with
[HAlBui2]n oligomers (Figure S8 in the Supporting Informa-
tion). Molecular mass determination (cryoscopy in benzene)
for the presumed cluster 9a does not confirm the existence of
temperature range from 209 to 282 K fully corresponded to
structure 6a, resulting from the reaction of 1a with HAlBui2
(Scheme 4).
However, the molecular mass of the compound determined
by cryoscopy in benzene proved to be lower than the
theoretical value (541 g/mol) and was equal to 487 11 g/
mol. The addition of HAlBui2 to complex 6a obtained by the
alternative synthesis gives rise to 1H NMR signals of compound
8a (Figure 7 and Scheme 5). However, in this case no
agreement between the theoretical and experimental molecular
masses of the complex was observed as well (Mexptl = 529 10
g/mol, Mtheor.= 684 g/mol). Meanwhile, the addition of
ClAlBui2 to 6a gives rise to downfield signals in the low-
1
temperature H NMR spectra corresponding to the cyclo-
pentadienyl ligands and the hydride atoms of a new complex,
which contains the [(C5H5)2ZrH3] moiety as well and is very
similar to 6a (Figure 7); hence, structure 10a was ascribed to
the new complex. In this case, the molecular mass did not
this species, at least above 0 °C (Mexptl.= 378
27 g/mol,
increase significantly either, being 533 15 g/mol (Mtheor
=
M
theor.= 649 g/mol). Thus, at room temperature, complex 9a,
718 g/mol). A temperature rise to 300 K induces coalescence
of the signals for cyclopentadienyl groups and bridging hydride
like 6a, 8a, and 10a, exists in a fast exchange with smaller
clusters, for example, 13a and 14a, and oligomers [HAlBui2]n.
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The obtained NMR data indicate unambiguously that complex
9a is not formed in the 1a−HAlBui2 system.
complexes 16a and 6a and broadening of signals for the
bridging hydride atoms. The addition of ClAlBui2 to a mixture
of the complexes increased the content of 6a. In the EXSY
spectra recorded in the temperature range 260−280 K we
detected slow intermolecular exchange of hydrides between
particular types of the Zr−H−Al bonds in 16a and 6a: namely,
the signals for internal atoms and the signals for external atoms
exchanged in pairs (Figure 9b). The observed fact may be a
Further, the reactivity of complexes 6a and 8a−10a with
respect to alkenes was studied. Earlier, we considered complex
6a to be nonreactive, because the addition of alkene to this
complex in the NMR tube did not produce any visible changes
1
in the H NMR spectrum over a 1 h period. Studies in a
magnetically stirred solution demonstrated (Figure 8) that 6a
Figure 8. Yield of hydrometalation product in the reaction of hexene-1
i
i
■
▲
with complexes 6a ( ), 8a (6a + HAlBu 2) ( ), 10a (6a+ ClAlBu 2)
⧫
●
( ), and 11a ( ) (C6H6, t = 22 °C).
reacts with a terminal alkene quite slowly, so that 92%
conversion of the alkene is achieved only in 19 h. The addition
of 1 equiv of ClAlBui2 or HAlBui2 to 6a induces a slight
decrease in the reaction rate due to an equilibrium shift toward
complex 8a or 10a, which should be less reactive in
hydroalumination according to theoretical data.6,9 However,
in this case 77−80% conversion of the alkene was noted after
15−20 h of the reaction. Complex 9a proved to be somewhat
more reactive than 6a during the first 3 h of the reaction, but
finally, after 19 h, the yield of the hydroalumination product
was in the same range as the yields of the reactions involving
complexes 6a, 8a, and 10a.
The low-temperature NMR study of the reaction of 1a with
AlBui3 at a ratio of 1:4.5 demonstrated the initial formation of
alkyl chloride complex 15a,4 which was then converted to
compounds 16a and 6a upon isobutylene elimination (Scheme
6 and Figure S9 in the Supporting Information). The structure
of complex 16a was identified on the basis of three doublets for
magnetically nonequivalent hydride atoms of the Zr−H−Al
bridging bonds in a 1:1:1 ratio present in the high-field region
(δH −1.15, −1.83, and −2.48) of the low-temperature (230 K)
1H NMR spectrum (Table 1). A temperature increase induced
coalescence of the signals for cyclopentadienyl ligands of
Figure 9. Upfield region of COSY (a) and EXSY (b) spectra of the
system (C5H5)2ZrCl2 (1a)−AlBui3 (1:4.5) in d8-toluene at 270 K.
consequence of the exchange of AlBui3 and ClAlBui2 molecules
between 16a and 6a via partial dissociation of the bimetallic
clusters to give complex 13a (Scheme 6). In this system the
dynamic processes have a lower rate in comparison with the
1a−HAlBui2 system; this is indicated by the higher minimum
temperature at which the slow exchange can be detected.
Apparently, the absence of fast exchange between the hydride
clusters, resulting in longer lifetimes of intermediates with a free
Zr−H bond and the lack of possibility of formation of larger
clusters, is the reason for the high reactivity of the 1a−AlBui3
catalytic system toward alkenes.
Scheme 6
B. System Based on the C5H4Me Ligand. The NMR spectra
recorded during the reaction of (C5H4Me)2ZrCl2 (1b) with
HAlBui2 (1:5) showed, apart from the initial OAC, the presence
of complexes 6b and 8b, similar to the case for the
cyclopentadienyl analogues, in a 3.4:1 ratio (Table 1 and
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Figure S10 in the Supporting Information). Here, as in the case
of the bimetallic cyclopentadienyl complexes, the hydride
exchange occurs within the complexes, between the complexes,
and between the complexes and HAlBui2 oligomers (Figure S13
in the Supporting Information). The constants and the
activation parameters of 6b ↔ 8b exchange correspond to
those for 6a ↔ 8a: namely, k220 = 12.0 s−1, ΔG⧧ = 11.94
220
0.04 kcal/mol, ΔH⧧ = 6.19 0.40 kcal/mol, and ΔS⧧ = −25.94
7.52 cal/(mol K).
It is noteworthy that in an attempt to synthesize complexes
7b or 17b, analogues of the cyclopentadienyl complex prepared
in our earlier works,4,5 by the reaction of [(C5H4Me)2ZrH2]2
(12b) with ClAlR2 (R = Et, Bui) at 20 °C in benzene or toluene
was not successful. Instead of the expected 7b (17b), the
reaction provided trihydride complexes 6b (19b) (Scheme 7).
Scheme 7
Figure 10. EXSY spectrum of the system Ind2ZrCl2 (1g)−HAlBui2
(1:12) in d8-toluene at 275 K (τ = 0.3 s).
In the case of rac complexes containing ansa-bonded indenyl
ligands (1i,j), we observed almost the same pattern as for 1g
(Figures S35−S42 in the Supporting Information). The
intermediates detected at 220 K were also identified as being
trihydride clusters 6i,j with bridging Zr−H−Al bonds (Table
1). This assignment is supported by the absence of additional
splitting of the proton signals of the ansa ligands, which would
inevitably appear upon generation of the stereogenic center on
Zr if nonsymmetric complexes 5i,j or 8i,j were formed.
The system showed low activity toward terminal alkenes.
The formation of hydroalumination products was observed no
earlier than 2 h after placement of the alkene directly in the
NMR tube.
D. Systems Based on Sterically Hindered Cp Ligands. The
reaction of (C5Me5)2ZrCl2 (1c) with HAlBui2 gives a complex
containing a [L2ZrH3] moiety. Thus, the 1H NMR spectrum of
a 1c + HAlBui2 mixture (1:26) at temperatures below 280 K
exhibited, apart from the lines for the remaining initial complex
1c, a signal for the hydride atom of the terminal Zr−H bond at
δH 4.38 ppm, which was correlated in the COSY spectra with
upfield signals at δH −0.66 and −1.27 ppm (Figure S16 in the
Supporting Information). The integral intensity ratio for these
signals was 1:1:1 (Table 1). A temperature increase to 300 K
induced broadening of the hydride signals, which finally ceased
to be detected in the spectra (Figure S15 in the Supporting
Information).
Complexes 5c, 13c, and 20c could be proposed as the
possible structures of the observed intermediate (Scheme 8). In
view of the fact that these complexes are prone to form tri- and
tetrametallic clusters, the existence of 13c is unlikely. Structure
20c was not detected, because the spectra did not exhibit
proton signals for the bridging Al−H−Al bond. Therefore, the
observed intermediate can most likely be described as 5c.
The EXSY spectra recorded at 265 K prove the existence of
exchange between the hydride atoms within complex 5c and
the exchange of these hydrides with [HAlBui2]n oligomers.
Moreover, in the spectra we found cross-peaks between the
signals of [HAlBui2]n hydrides and the downfield broadened
signal at 6.85 ppm (Figure 11), which can be attributed to the
hydride atoms of free (CMe5)2ZrHCl18 (3c). Apparently, the
existence of uncomplexed zirconocene hydride in the system is
According to monitoring of this reaction directly in the NMR
spectrometer cell, along with the complexes 6b (19b) formed,
the mixture contained both the starting 12b and 1b. Probably,
complex 7b (17b) formed in the first step rapidly react swith
ClAlR2 via monomer 4b (18b), giving rise to 1b and to
elimination of HAlR2. The latter reacts with intermediate 4b
(18b), providing the complex 6b (19b). The structures of
complexes 6b,c proved to be similar to the structure of complex
6a described above.
Upon the addition of alkene (1-hexene) to the 1b−HAlBui2
reaction mixture in an NMR tube at room temperature, signals
for the complex (C5H4Me)2Zr(n-C6H13)Cl and RAlBui24 (2)
appear in the 1H and 13C NMR spectra (Figures S17 and S18 in
the Supporting Information). Alkylalane RAlBui2 forms the
17
associates RAlBui2·HAlBui2 in the presence of excess HAlBui2,
1
according to H NMR data (the signal of the monomeric
associated HAlBui2 occurs at δH 3.97 ppm).
C. Systems Based on Indenyl Ligands. The NMR spectra of
the reaction mixture Ind2ZrCl2 (1g)−HAlBui2 (1:12) at 300 K
contained one broadened signal for hydride atoms at −0.94
ppm, the ratio of which with the resonance lines of the indenyl
ligand attested to the formation of a complex containing a
[L2ZrH3] moiety. Lowering the temperature to 280 K and
below gave rise to two upfield signals: a broadened triplet at δH
0.05 ppm and a doublet at δH −1.04 ppm with an intensity ratio
of 1:2 (Table 1). The downfield part of the spectrum displayed
two signals for cyclopentadienyl protons of the indenyl ligand: a
triplet at δH 6.35 and a doublet at δH 5.31 ppm. The intensity
ratio of the four aformentioned signals was 1 (Zr−H−Al):2
(Zr−H−Al):2 (Ind):4 (Ind). A decrease in the temperature to
220 K did not result in considerable changes in the shape of the
signals for hydride and indenyl hydrogens. Hence, the complex
was identified as structure 6g. The EXSY spectra at 280 K
exhibited cross-peaks corresponding to the exchange of the
hydride atoms between the bridging Zr−H−Al bonds and the
trimer HAlBui2 (Figure 10).
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Scheme 8
1
Figure 12. Variable-temperature H NMR study on the system rac-
Me2C(2-Me-4-But-C5H2)2ZrCl2 (1d)−HAlBui2 (1:17) in C7D8
(asterisks indicate an admixture of meso isomer).
temperature decrease from 230 to 195 K caused additional
splitting into halves of the downfield signals for the ligand
protons and the upfield signal for the bridging hydride atoms.
These results can be interpreted in the following way. The
NMR spectral pattern observed at 230−280 K contributes to
the existence of a structure with the [L2ZrH3] moiety
containing a terminal Zr−H bond. Thus, in the case of
formation of open structures such as 51d−54d (51d−53d and
52d−54d are enantiomer pairs), the spectra should exhibit four
signals for H1−H4 protons of the cyclopentadienyl groups of
the ansa ligand, which are expected to be magnetically
nonequivalent upon reduction of the symmetry of the complex
(Scheme 9). This corresponds to the NMR spectral pattern at
Scheme 9
Figure 11. EXSY spectrum of the system (CMe5)2ZrCl2 (1c)−
HAlBui2 (1:26) in d8-toluene at 265 K (τ = 0.3 s).
caused by the tendency of the bimetallic clusters to dissociate
due to the steric hindrance in the η5 ligand.
A more complex pattern is observed in the reaction of
sterically hindered ansa-bonded cyclopentadienyl rac complexes
1d,f with HAlBui2. The 1H NMR spectrum of the 1d + HAlBui2
system (1:17) recorded at 230 K showed two broadened signals
in the downfield region for the protons of bound bis-
cyclopentadienyl ligand (δH 6.52 and 4.90 ppm) and a
broadened signal for the bridging hydride atoms in a upfield
region (δH −0.58 ppm). The signal ratio was 1:1:1 (Table 1).
This situation occurred in the temperature range from 230 to
280 K. In the COSY spectra, the signal at δH −0.58 ppm was
correlated with the signal at δH 2.28 ppm, which can be
assigned to the terminal hydride atom at Zr. In the EXSY
spectra recorded at 260−270 K, we detected cross-peaks
corresponding to the slow exchange of two hydride atoms at δH
−0.58 and 2.28 ppm and the exchange of these hydride
hydrogens with the [HAlBui2]n oligomers (Figure S21 in the
Supporting Information). A further temperature increase
induced pronounced line broadening for both hydride bridge
and ligand proton signals (Figure 12). Finally, at 300 K, some
of these lines were no longer detected in the spectra. The
temperatures below 210 K; this is apparently manifested due to
slowing of the hydride exchange in the complex. The H2−H4
and H1−H3 proton exchange observed as the temperature is
raised can be attributed to intramolecular rearrangement of the
terminal and bridging bonds in 51d ↔ 52d and in their
enantiomers 53d ↔ 54d. At temperatures above 230 K, a
substantial contribution is made by the intermolecular exchange
with dissociation of the bridging Zr−H−Al bonds and
elimination of OAC molecules, as indicated by the data of
the EXSY spectra.
A similar situation was observed for the 1f + HAlBui2 system
(1:13). In this case, the low-temperature spectra exhibited an
additional pair of signals for the third proton of the
cyclopentadienyl moieties of the ansa ligands in the
intermediate 5f (Figures S28 and S29 in the Supporting
Information).
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Upon the addition of alkene to the reaction mixture in the
NMR tube, the reaction occurred almost instantaneously: the
signals for the intermediate hydride atoms and η5 ligand
related to the structural and dynamic features of the bimetallic
hydride intermediates formed in these systems. It was shown
that the use of catalysts with less electron donating and
sterically crowded ligands leads to the formation of the stable
Zr,Al-hydride clusters L2Zr(μ-H)3(AlBui2)2(μ-Cl) (L = C5H5,
C5H4Me, Ind; L2 = rac-Me2C(Ind)2, rac-Me2Si(Ind)2, rac-
C2H4(Ind)2), L2Zr(μ-H)3(AlBui2)3(μ-Cl)(μ-H), and L2Zr(μ-
H)3(AlBui2)3(μ-Cl)2 (L = C5H5, C5H4Me), which tend to form
bridging Zr−H−Al bonds; hence, these complexes have low
activity in the reaction with alkene. We also found intra- and
intermolecular exchange between the hydride atoms in these
clusters and [HAlBui2]n oligomers. As a result, the complexes
exist in equilibrium with each other and HAlBui2 self-associates,
while the intermolecular exchange involves the OAC monomer
and occurs via dissociation of bimetallic complexes. As the
[HAlBui2]n concentration increases, i.e., the reaction ap-
proaches the catalytic version, the equilibrium shifts toward
large clusters of low activity in which the alkene insertion is
hampered due to the competing intermolecular exchange with
OAC oligomers.
1
disappeared, and simultaneously the H and 13C NMR spectra
started to exhibit resonance lines for RAlBui2·HAlBui2.
A somewhat different situation was observed in the reaction
of HAlBui2 with meso isomer 1e. As in the case of 1c,d,f, the 1H
NMR spectra of the reaction mixture contained signals for the
terminal hydride atom at δH 3.51 ppm and signals for bridging
hydrides at δH 1.19 and −1.07 ppm, which can be attributed to
structure 5e (Scheme 10). The temperature variation did not
Scheme 10
In the reaction of AlBui3 with (C5H5)2ZrCl2, alkyl chloride
exchange and isobutylene elimination give the intermediates
(C5H5)2Zr(μ-H)3(AlBui2)(AlBui3) and (C5H5)2Zr(μ-
H)3(AlBui2)2(μ-Cl), which exist with intermolecular exchange
of AlBui3 and ClAlBui2 groups. The absence of fast exchange
between these hydride clusters increases the lifetime of the
active sites with free Zr−H bonds, and this is responsible for
the high activity of the (C5H5)2ZrCl2−AlBui3 catalytic system
toward alkene.
induce a change in the signal shape, and the EXSY spectra did
not show cross-peaks characteristic of hydride exchange; this
implies the absence of both intra- and intermolecular dynamic
processes. According to NOESY spectra, the terminal hydride
Hα atom occurs in the close vicinity of the But groups and the
H2 protons of the ansa ligand. This shielding precludes
association of this hydride with the organoaluminum moiety,
which probably terminates both the intramolecular exchange
between the possible structures 5e ↔ 5e′ and the
intermolecular exchange 5e ↔ [HAlBui2]n.
The observed complex 5e did not react with the alkene:
according to NMR data, no signs of reaction were observed
even after 24 h. This may also be due to the ligand steric
influence, which prevents the substrate coordination to the
terminal Zr−H bond in complex 5e.
In the reactions catalyzed by Zr complexes with bulky ligands
(L = C5Me5, rac-Me2C(2-Me-4-But-C5H2)2, rac-Me2C(3-But-
C5H3)2), Zr,Al-bimetallic active sites containing a [L2ZrH3]
moiety with a free Zr−H bond are formed, and the steric
hindrance in the ligand prevents the formation of low-activity
intermediates; therefore, high yields of hydroalumination
products are achieved. The meso isomer of the sterically
hindered cyclopentadienyl complex Me2C(2-Me-4-But-
C5H2)2ZrCl2 gives an intermediate with a shielded free Zr−H
bond, which makes the catalytic system inactive.
In the reaction of 1c−f with ClAlBui2 or AlBui3, the alkyl
chloride ligand exchange was not manifested in the NMR
spectra. Moreover, if one assumes that this exchange occurs in a
catalytic version, the rate of formation of the key hydride
intermediates via C−H activation in the isobutyl group and
isobutene elimination from the alkylzirconocene complex
should be very low19 due to the presence of sterically hindered
η5 ligands in the molecule. Apparently, the low rate of these two
steps is responsible for the low catalytic activity of complexes
1c−f in the alkene hydroalumination with ClAlBui2 (AlBui3)
(Figure 1d).
Finally, the L2ZrCl2−XAlBui2 systems can be classified as
labile systems in which intra- and intermolecular hydride
exchange between Zr and Al, controlled by the steric factor of
the η5 ligand, OAC nature, and the reaction conditions
(reactant ratio), plays the key role in the catalytic process.
EXPERIMENTAL SECTION
■
General Procedures. All operations were carried out under argon
using Schlenk techniques. Solvents were dried by refluxing over
HAlBui2 and freshly distilled prior to use. The commercial organo-
aluminum compounds HAlBui2 (99%, Aldrich), ClAlBui2 (97%,
Strem), ClAlEt2 (97%, Strem), and AlBui3 (95%, Strem) were used
in the reactions. Caution! The pyrophoric nature of aluminum alkyl and
hydride compounds requires special safety precautions in their handling.
The complexes (C5H5)2ZrCl2 (1a),20 (C5H4Me)2ZrCl2 (1b),21
Ind2ZrCl2 (1g),22 rac-Me2C(Ind)2ZrCl2 (1h),23 rac-Me2C(2-Me-4-
But-C5H2)2ZrCl2 (1d) (contains ∼7% of 1e),10g meso-Me2C(2-Me-4-
But-C5H2)2ZrCl2 (1e) (contains ∼17% of 1d),10g and rac-Me2C(3-tBu-
C5H3)2ZrCl2 (1f)10g were synthesized from ZrCl4 (Aldrich, 99.5%).
Zirconocenes (C5Me5)2ZrCl2 (1c; 97%, Acros), rac-Me2Si(Ind)2ZrCl2
(97%, Strem), and rac-C2H4(Ind)2ZrCl2 (Strem) were purchased and
used as received. The synthesis of [(C5H5)2ZrH2]2 and
[(C5H4Me)2ZrH2]2 from the corresponding zirconocene dichlorides
was performed as described previously.7,4
CONCLUSION
■
Studies on the catalytic activity of the systems L2ZrCl2−XAlBui2
(L = C5H5, C5H4Me, Ind, C5Me5; L2 = rac-Me2C(2-Me-4-But-
C5H2)2, meso-Me2C(2-Me-4-But-C5H2)2, rac-Me2C(3-But-
C5H3)2, rac-Me2C(Ind)2, rac-Me2Si(Ind)2, rac-C2H4(Ind)2); X
= H, Cl, Bui) in the alkene hydroalumination showed that the
maximum effect was attained by using complexes with more
sterically crowded cyclopentadienyl ligands in combination with
HAlBui2. Catalysts with less bulky ligands are the most active in
the reaction of alkenes with AlBui3 or ClAlBui2. The indenyl
zirconium complexes provide a considerable decrease in the
yield of the hydroalumination products irrespective of the OAC
structure.
The 1H and 13C NMR spectra were recorded on a Bruker
AVANCE-400 spectrometer (400.13 MHz (1H), 100.62 MHz (13C)).
The observed dependence of the catalytic system activity on
the OAC nature and the ligand structure in the zirconocene is
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Toluene-d8 and benzene-d6 were employed as the solvents and internal
standards. 1D and 2D NMR spectra (COSY HH, HSQC, HMBC,
EXSY (NOESY)) were recorded using standard Bruker pulse
sequences. The constants and activation parameters of intermediate
exchange were found using line shape analysis of NMR spectra with
the program Bruker TopSpin 3.0.
The hydrolysis products of reaction mixtures were analyzed on a
Chrom-5 chromatograph (flame ionization detector, 2 m × 3 mm
column, 15% Peg-6000, 50−190 °C).
The cryoscopic measurements were carried out in benzene by a
standard procedure24 in a glass cell equipped with a Beckmann
thermometer (the accuracy of melting point determinations was 0.005
°C).
Study of the Catalytic Activity of L2ZrCl2 in the Reaction of
XAlBui2 with Alkenes. A flask with a magnetic stirrer was filled under
argon with 1a−j (0.20 mmol), 1-alkene (10 mmol), 3 mL of benzene,
and OAC (12 mmol). The reaction was carried out with stirring. At
specific time intervals, samples (0.2 mL) of the reaction mixture were
syringed into tubes filled with argon, and then the samples were
treated with 10% HCl at 0 °C. The products were extracted with
benzene, and the organic layer was dried above Na2SO4. The yield of
alkylalanes was determined via the amount of hydrolysis products of
the reaction mixture (alkane relative to initial terminal alkene) by
GLC.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Russian Foundation of Basic Research
(Grant No. 12-03-33089mol_a_ved), Russian Ministry of
Education and Science (Grant No. 8426) and Russian Science
Foundation (Grant No. 15-13-00053) for financial support.
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mL of toluene-d8 in an argon-filled glovebox. Then the mixture was
cooled to 240 K and 0.3 mmol of HAlBui2 was added dropwise until
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ASSOCIATED CONTENT
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* Supporting Information
Figures giving 1D and 2D NMR spectra. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.organomet.5b00370.
AUTHOR INFORMATION
■
Corresponding Author
*L.V.P.: e-mail, luda_parfenova@mail.ru, ink@anrb.ru; tel, +7
347 2843527; fax, +7 347 2842750.
K
DOI: 10.1021/acs.organomet.5b00370
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DOI: 10.1021/acs.organomet.5b00370
Organometallics XXXX, XXX, XXX−XXX