Highly variable Zr-CH2-Ph bond angles in tetrabenzylzirconium: Analysis of benzyl ligand coordination modes

Article abstract of DOI:10.1021/om300820b

Analysis of a monoclinic modification of Zr(CH₂Ph)₄ by single-crystal X-ray diffraction reveals that the bond angles Zr-CH ₂-Ph in this compound span a range of 25.1° , which is much larger than previously observed for the orthorhombic form (12.1°). In accord with this large range, density functional theory calculations demonstrate that little energy is required to perturb the Zr-CH₂-Ph bond angles in this compound. Furthermore, density functional theory calculations on Me ₃ZrCH₂Ph indicate that bending of the Zr-CH₂-Ph moiety in the monobenzyl compound is also facile, thereby demonstrating that a benzyl ligand attached to zirconium is intrinsically flexible, such that its bending does not require a buffering effect involving another benzyl ligand.

Full text of DOI:10.1021/om300820b

Article
pubs.acs.org/Organometallics
Highly Variable Zr−CH2−Ph Bond Angles in Tetrabenzylzirconium:
Analysis of Benzyl Ligand Coordination Modes
Yi Rong, Ahmed Al-Harbi, and Gerard Parkin*
Department of Chemistry, Columbia University, New York, New York 10027, United States
S
* Supporting Information
ABSTRACT: Analysis of a monoclinic modification of Zr(CH2Ph)4 by single-crystal X-ray diffraction reveals that the bond
angles Zr−CH2−Ph in this compound span a range of 25.1°, which is much larger than previously observed for the orthorhombic
form (12.1°). In accord with this large range, density functional theory calculations demonstrate that little energy is required to
perturb the Zr−CH2−Ph bond angles in this compound. Furthermore, density functional theory calculations on Me3ZrCH2Ph
indicate that bending of the Zr−CH2−Ph moiety in the monobenzyl compound is also facile, thereby demonstrating that a
benzyl ligand attached to zirconium is intrinsically flexible, such that its bending does not require a buffering effect involving
another benzyl ligand.
only expected to influence the intrinsic reactivity of the bond
M−CH2Ph but could also provide a means to modulate the
reactivity of a metal center by stabilizing coordinatively
unsaturated centers. Tetrabenzylzirconium, first reported in
1969,3 has proven to be of much value in the development of
zirconium chemistry (especially with respect to the synthesis of
catalysts for olefin polymerization)4−7 and represents an
interesting example of a complex that features different benzyl
coordination modes.8 For example, the Zr−CH2−Ph bond
angles of Zr(CH2Ph)4 have been reported to range from
87.0(3)° to 99.1(3)°.8a Here, we report the structure of another
crystalline form of Zr(CH2Ph)4 that exhibits an even greater
range of Zr−CH2−Ph bond angles, namely, from 81.6(1)° to
106.7(2)°. Consistent with this large range, density functional
theory calculations demonstrate that little energy is required to
perturb the Zr−CH2−Ph bond angles in this compound. In
addition, we also analyze the distribution of the various benzyl
coordination modes by employing the Cambridge Structural
Database.9
INTRODUCTION
■
Benzyl ligands coordinate to transition metal centers in
manifold ways. Thus, in addition to η1-coordination, interaction
via the phenyl group is also possible, with η2-, η3-, η4-, η5-, and
η7-coordination modes having been discussed in the literature
(Figure 1).1,2 The coordination mode of a benzyl ligand is not
Received: August 24, 2012
Figure 1. Benzyl ligand coordination modes discussed in the literature.
© XXXX American Chemical Society
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Table 1. Crystallographic Data for Polymorphs of Zr(CH2Ph)4
ref 8a
refs 8b,c
this work
crystallization method
lattice
n-heptane at −25 °C
orthorhombic
Pbca
toluene at −25 °C
orthorhombic
toluene at room temp
monoclinic
P21
space group
a/Å
Pbca
16.387(1)
20.022(1)
13.758(6)
90
19.945(6)
13.716(7)
16.306(5)
90
10.2238(10)
9.6635(9)
11.2356(11)
90
b/Å
c/Å
α/deg
β/deg
90
90
101.295(1)
90
γ/deg
90
90
V/Å3
4514(2)
1.341
4461
1088.6(2)
1.390
d/g cm−3
1.36
temp/K
293(2)
233
150(2)
Table 2. Metrical Data for Polymorphs of Zr(CH2Ph)4
Zr−CH2−Ph/deg
Zr−CH2/Å
Zr···Cipso/Å
Zr···Cortho(short)/Å
Zr···Cortho(long)/Å
δipso/Å
δortho(short)/Å
δortho(long)/Å
a
Monoclinic
C11
81.63(14)
82.36(13)
98.67(15)
106.73(15)
2.270(2)
2.278(2)
2.262(3)
2.2929(19)
2.509(2)
2.533(2)
2.873(2)
3.063(2)
3.022(2)
2.969(3)
3.472(2)
3.774(2)
3.089(2)
3.174(2)
3.628(2)
3.820(2)
0.24
0.26
0.61
0.77
0.75
0.69
1.21
1.48
0.82
0.90
1.37
1.53
C21
C31
C41
b
Orthorhombic
C1
C2
C3
87.0(3)
90.2(3)
93.9(3)
99.1(3)
2.259(5)
2.248(5)
2.255(5)
2.258(4)
2.614(4)
2.684(4)
2.773(3)
2.879(4)
3.072(5)
3.249(4)
3.298(5)
3.519(5)
3.347(6)
3.361(5)
3.535(4)
3.589(6)
0.36
0.44
0.52
0.62
0.81
1.00
1.04
1.26
1.09
1.11
1.28
1.33
C4
a
b
This work. Reference 8a.
RESULTS AND DISCUSSION
Structural Characterization of Monoclinic Zr(CH2Ph)4.
■
Previous X-ray diffraction studies have revealed Zr(CH2Ph)4 to
exist as orthorhombic crystals, with space group Pbca, as
summarized in Table 1. It is, therefore, noteworthy that we
have obtained a monoclinic crystalline form of Zr(CH2Ph)4, as
illustrated in Figure 2, that differs from the previously reported
Figure 3. Comparison of the molecular structure of monoclinic (left)
and orthorhombic (right) forms of Zr(CH2Ph)4. Hydrogen atoms are
omitted for clarity.
monoclinic structure are (i) the dihedral angle between the
two [C−Zr−C] planes10 is reduced from 90° to 69° and (ii)
one of the benzyl ligands points in a direction that destroys the
C2 axis. Secondly, in addition to this variation in conformation,
the zirconium−benzyl interactions in the two polymorphs are
also different (Table 2). For example, the Zr−CH2−Ph bond
angles of the monoclinic form span a range of 25.1°, which is
substantially greater than for the orthorhombic form (12.1°).
Furthermore, monoclinic Zr(CH2Ph)4 exhibits Zr−CH2−Ph
angles that are both more acute (81.6°) and more obtuse
(106.7°) than observed for the orthorhombic form, for which
the range is 87.0−99.1°.
Classification of Benzyl Ligands in Zr(CH2Ph)4: Criteria
for Identifying the Benzyl Ligand Coordination Mode.
While the M−CH2−Ph bond angle distinguishes whether a
benzyl ligand coordinates in an η1-manner (with an idealized
value of 109.5°) or an ηx-manner (x > 1), differentiation
Figure 2. Molecular structure of monoclinic Zr(CH2Ph)4. Hydrogen
atoms are omitted for clarity.
orthorhombic structure in some significant ways. Firstly,
whereas the conformation of the benzyl ligands in ortho-
rhombic Zr(CH2Ph)4 are arranged in such a manner as to give
an approximate S4 molecular symmetry, the molecular structure
of the monoclinic form deviates considerably from this
idealized geometry, as illustrated in Figure 3. The two most
notable features that remove the S4 symmetry for the
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Table 3. Metrical Data for Selected Benzyl Ligand Coordination Modes
M−CH2−Ph/deg
δipso/Å
δortho(short)/Å
δortho(long)/Å
δmeta(short)/Å
δmeta(long)/Å
δpara/Å
notes
η1
η2
η2
η3
η4
η7
109.5
90.0
97.0
69.3
57.4
62.1
0.84
0.44
0.59
0
0.85−1.58
1.58−2.21
1.10
2.20−2.75
2.14
2.75−3.23
2.14
3.24
2.57
a
b
c
1.10
1.27
0
1.27
2.36
2.36
2.81
1.14
1.14
1.96
1.96
d
e
−0.42
−0.30
0
0
0.69
0.69
0.98
−0.12
−0.10
−0.03
−0.02
−0.10
f
a
Derived values for an idealized M−CH2−Ph = 109.5° obtained by using the CSD averages d(M−CH2Ph) = 2.195 Å and d(CH2−Cipso) = 1.483 Å
b
for η1-benzyl compounds. The ranges for δortho and δmeta correspond to rotation about the C−Ph bond. Derived values for an idealized M−CH2−Ph
= 90.0° obtained by using the CSD averages d(M−CH2Ph) = 2.300 Å for η2-benzyl compounds and d(CH2−Cipso) = 1.483 Å. Values for δortho and
c
δmeta are for a M−C−C−C torsion angle of 90°. Derived values for M−CH2−Ph = 97.0° obtained by using the CSD averages d(M−CH2Ph) =
d
2.300 Å for η2-benzyl compounds and d(CH2−Cipso) = 1.483 Å. Values for δortho and δmeta are for a M−C−C−C torsion angle of 90°. Derived values
obtained assuming that d(M−CH2Ph) = 2.090 Å, which is the average of known η3-compounds, and that d(M−CH2Ph) = d(M−Cortho(short)).
e
Derived values obtained assuming that d(M−CH2Ph) = 2.653 Å, corresponding to that in the only known η7-compound, and that d(M−CH2Ph) =
f
d(M−Cortho(short)) = d(M−Cortho(long)). Values listed are for the only structurally reported η7-compound.
between the various expanded hapticities requires analysis of
the M···C distances involving the phenyl group.1a,b,8a
Specifically, as the hapticity increases, the ipso, ortho, meta,
and para carbon atoms approach the metal center and the
Table 4. Criteria for Assigning Benzyl Ligand Coordination
Modes
M−CH2−Ph/deg
δipso/Å
δortho(short)/Å
δortho(long)/Å
η1
η2
η3
η4
>97
≤97
≤97
≪90
>0.5
≤0.5
≤0.5
<0.0
coordination mode can be classified by comparing the M···Cipso
,
M···Cortho, M···Cmeta, and M···Cpara distances to the M−CH2
bond length, i.e., δipso, δortho, δmeta, and δpara (Figure 4), an
approach that is based on Andersen’s analysis.1a
≤0.5
≤0.5
>0.5
≤0.5
that a M−CH2−Ph angle of ≤97° be used as a criterion for
η2‑coordination (Table 4), recognizing that such distinctions
have little meaning at the borderline. With respect to
distinguishing between η2- and η3-benzyl coordination, we
propose that the latter is identified by δortho(short) ≤ 0.5 Å, on the
basis that Zr−arene bond lengths15 may be up to ca. 0.5 Å
longer than the mean Zr−CH2Ph bond length for compounds
listed in the Cambridge Structural Database (2.298 Å).
A plot of M−CH2−Ph bond angle versus δortho(short)
(Figure 5) illustrates the regions that correspond to η1-, η2-,
and η3-benzyl coordination modes, and examples of com-
pounds that belong to these classes are provided in
Table 5.16−31 Of these coordination modes, η1 is the most
prevalent (92.9%), followed by η2 (6.1%) and η3 (0.9%).
η4‑Benzyl coordination requires small values for both δortho(short)
and δortho(long), and analysis of the compounds listed in the
Figure 4. Definition of δipso and δortho
.
With respect to interpreting these values, it is first pertinent
to consider some idealized situations (Table 3). For example,
an η1-benzyl ligand with an idealized M−CH2−Ph tetrahedral
angle of 109.5° is characterized by a δipso value of 0.84 Å, while
an η2-benzyl ligand with a M−CH2−Ph angle of 90.0° is
characterized by a δipso value of 0.44 Å. An idealized η3-benzyl
ligand is characterized by a situation in which the M−CH2−Ph
angle is <90.0° and one of the ortho carbon atoms approaches
the metal center within a distance that is comparable to that of
the methylene carbon; that is, δipso and δortho(short) have values of
0.0 Å. An idealized η4-benzyl ligand requires both δortho values to
be 0.0 Å, while an idealized η7-benzyl ligand also requires all
δmeta and δpara values to be 0.0 Å.
Despite the idealized data presented in Table 3, the
classification of the coordination mode of the benzyl ligand
in a compound is, nevertheless, a subjective issue. For example,
a compound with a M−CH2−Ph angle as small as 97.1° has
been classified as η1,11 while a compound with a M−CH2−Ph
angle as large as 97.5° has been classified as η2.12,13 However, all
other compounds listed in the Cambridge Structural Database9
that have been assigned η2-benzyl coordination have bond
angles M−CH2−Ph less than 97°.14 On this basis, we propose
Figure 5. Classification of benzyl ligands according to M−CH2−Ph
bond angle and δortho. η1-Coordination (92.9%) is the most prevalent,
followed by η2 (6.1%), η3 (0.9%), and η7 (0.1%).
C
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a
Table 5. Examples of Benzyl Compounds Classified According to Their Coordination Mode
compound
M−CH2−Ph/deg
δipso/Å
δortho(short)/Å δortho(long)/Å
ref
η1
(η2-3,5-Me2Pz)2Zr(η1-CH2Ph)(η2-CH2Ph)
97.1
98.6
0.59
0.62
0.64
0.73
0.80
0.77
0.94
0.95
1.12
1.31
1.11
1.19
1.31
1.19
1.26
1.37
1.67
1.61
1.82
2.02
1.39
1.41
1.35
1.69
1.78
1.61
1.74
1.81
2.03
2.31
11
13
19
17
18
14c
18
14d
34
16
[OONO]Zr(η1-CH2Ph)2
[η3-MeC(NC7H6)CHC(N-p-Tol)Me]Zr(η1-CH2Ph)(η2-CH2Ph)
{[2,6-CH2N(C6F5)]2NC5H3}2Zr(η1-CH2Ph)(η2-CH2Ph)
[η2-N(CHMePh)(PPh2)]Zr(η1-CH2Ph)2(η2-CH2Ph)
[NNO]Zr(η1-CH2Ph)(η2-CH2Ph)
99.6
104.2
108.3
106.7
115.8
116.6
130.4
144.4
[η2-N(CHMePh)(PPh2)]Zr(η1-CH2Ph)2(η2-CH2Ph)
(pyCMe2O)2Zr(η1-CH2Ph)(η2-CH2Ph)
t
(Cp1,2,4‑Bu )CeCH2Ph
3
Tp*Zr(η1-CH2Ph)3
η2
[η2-N(CHMePh)(PPh2)]Zr(η1-CH2Ph)2(η2-CH2Ph)
Cp*Mo(NO)(CH2SiMe3)(η2-CH2Ph)
[η3-MeC(NC7H6)CHC(N-p-Tol)Me]Zr(η1-CH2Ph)(η2-CH2Ph)
Cp*Th(η2-CH2Ph)3
82.5
83.0
83.6
84.1
84.4
84.5
85.4
85.8
87.1
87.3
90.3
93.1
95.8
96.1
0.25
0.29
0.28
0.25
0.31
0.31
0.29
0.31
0.32
0.34
0.40
0.44
0.55
0.55
0.59
0.82
0.67
0.72
0.91
0.69
0.88
0.80
0.89
0.90
0.75
0.67
0.94
1.13
0.99
0.98
0.98
0.97
0.93
1.04
0.95
0.92
0.97
0.94
1.25
1.41
1.47
1.31
18
1d
19
21
20
17
22
21
22
22
21
34
14c
14d
[Cp2Zr(CH3CN)(η2-CH2Ph)][(BPh4]
{[2,6-CH2N(C6F5)]2NC5H3}2Zr(η1-CH2Ph)(η2-CH2Ph)
Cp*U(η2-CH2Ph)3
Cp*Th(η2-CH2Ph)3
Cp*U(η2-CH2Ph)3
Cp*U(η2-CH2Ph)3
Cp*Th(η2-CH2Ph)3
t
(Cp1,2,4‑Bu )CeCH2Ph
3
[NNO]Zr(η1-CH2Ph)(η2-CH2Ph)
(pyCMe2O)2Zr(η1-CH2Ph)(η2-CH2Ph)
η3
Ni(PMe3)(η1-CH2Ph)(η3-CH2Ph)
69.7
70.0
71.8
72.6
72.9
72.9
73.6
73.6
75.1
76.4
76.4
87.7
90.6
90.9
0.02
0.03
0.07
0.10
0.08
0.06
0.12
0.13
0.17
0.14
0.20
0.36
0.38
0.41
0.24
0.33
0.21
0.34
0.37
0.22
0.24
0.27
0.32
0.34
0.41
0.41
0.51
0.50
0.82
0.76
0.98
0.94
1.01
0.95
1.09
1.00
1.00
1.01
1.13
1.44
1.39
1.49
28
28
30
31
25
26
31
31
29
24
27
1f
Ni(PMe3)(η1-CH2Ph)(η3-CH2Ph)
{κ2-C,N-(Ar)NC(Me)C(CH2)[OB(C6F5)3]}Ni(η3-CH2Ph) (Ar = 2,6-Pri2C6H3)
[κ2-P,O-2-P(Cy)2-4-Me-C6H3(SO3)]Ni(η3-CH2Ph)
[P(OCH3)3]3Co(η3-CH2Ph)
[Pri2P(CH2)3PPri2]Rh(η3-CH2Ph)
[κ2-P,C−2−P(2-OMePh)2-4-Me-C6H3(SO3)]Ni(η3-CH2Ph)
[κ2-P,O-2-P(Cy)2-4-Me-C6H3(SO3)]Ni(η3-CH2Ph)
[κ2-N,O-PhC(O)C6H4NC(Ph)OB(C6F5)3]Ni(η3-CH2Ph)
{[NH(Me)CH2CH2(η5-C5H4)] (CO)Re(η3-CH2Ph)}+ReO4
−
(NHC-2,6-Pri2-C6H3)(CF3SO3)Ni (η3-CH2Ph)
[Cp*Zr(η3-CH2Ph)(η7-CH2Ph)][B(CH2Ph)(C6F5)3]
[(Me3Si)2NC(NCy)2]2Er(η3-CH2Ph)
22
22
[(Me3Si)2NC(NCy)2]2Y(η3-CH2Ph)
η7
[Cp*Zr(η3-CH2Ph)(η7-CH2Ph)][B(CH2Ph)(C6F5)3]
62.1
−0.30
−0.12
−0.10
1f
a
Note that compounds with multiple benzyl ligands have an entry for each structurally different benzyl ligand.
Cambridge Structural Database (Figure 6) indicates that the
only compound with both δortho(short) and δortho(long) < 0.5 Å is
the η7-benzyl complex [Cp*Zr(CH2Ph)2]+.1f As such, despite
the fact that η4-benzyl complexes are frequently considered,1,8a
there are no benzyl compounds listed in the Cambridge
Structural Database that can be clearly assigned such a
coordination mode according to the criteria listed in Table 4.32
In this regard, of the η3-complexes that are listed in the
Cambridge Structural Database, the compound that most
closely approaches η4-coordination is Me3PNi(CH2Ph)2, for
which δortho(short) = 0.33 Å and δortho(long) = 0.76 Å.28
According to the criteria listed in Table 4, two of the benzyl
ligands in the monoclinic form of Zr(CH2Ph)4 are coordinated
in an η1-manner, while two are coordinated in an η2-manner
(Table 2), as illustrated by their location in Figure 5.
Specifically, the two η2-benzyl ligands have acute Zr−CH2−
Ph angles of 81.6(1)° and 82.4(1)°, while the two η1-benzyl
ligands have obtuse Zr−CH2−Ph angles of 98.7(2)° and
106.7(2)°. In addition, the two η2-benzyl ligands have δortho(short)
values of 0.75 and 0.69 Å, which indicate that there is little
η3‑character associated with the interaction. Similar analysis for
the orthorhombic form of Zr(CH2Ph)4 classifies three of the
benzyl ligands as η2 [with Zr−CH2−Ph angles of 87.0(3)°,
90.2(3)° and 93.9(3)°] and one as η1 [with a Zr−CH2−Ph
angle of 99.1(3)°], as illustrated in Figure 5.
Examination of structurally characterized zirconium com-
pounds indicates that the flexibility of benzyl ligands is by no
means restricted to Zr(CH2Ph)4, such that Zr−CH2−Ph angles
that range from 62.1° 1f to 144.4° 16 in different compounds
have been reported. The ability of crystal packing forces to
influence zirconium−benzyl interactions has been reported by
Arnold, who noted that {CyNC[N(SiMe3)2]NCy}Zr(CH2Ph)3
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fluxional, and exchange between η2- and η1-benzyl ligands is
facile.35a
Computational Evaluation of the Flexibility of Benzyl
Ligands Attached to Zirconium. In order to investigate the
nature of Zr(CH2Ph)4 in solution, the molecular structure was
investigated computationally by performing density functional
theory (DFT) geometry optimization calculations (B3LYP).
For this purpose, the geometry of Zr(CH2Ph)4 was optimized
using (i) constrained Zr−CH2−Ph bond angles that corre-
spond to the monoclinic and orthorhombic structures, (ii) S4
symmetry,37 and (iii) no constraints. Significantly, the energies
of each of these geometry-optimized structures (Figure 10)
differ by <2 kcal mol−1, despite the fact that the Zr−C−C
angles vary significantly between the structures (Table 6). This
result is in accord with the observation that two different
molecular structures of Zr(CH2Ph)4 could exist in the solid
state.
To obtain a further appreciation of the energetic penalty
associated with bending the Zr−C−C bonds, the S4 symmetric
structure was geometry optimized subject to constraining one
of the Zr−C−C bond angles to a series of values that range
from 70° to 150°. These data, as presented in Figure 11 and
Table 7, indicate that one of the Zr−C−C bond angles can be
varied over a large range without exerting a significant energetic
penalty. For example, the energy of the molecule fluctuates by
<1.5 kcal mol−1 over the range 85−120°. However,
constraining one of the Zr−C−C bond angles to a specific
value is accompanied by changes in the other benzyl ligands to
accommodate the induced perturbation, as illustrated by the
variation in the range of Zr−C−C bond angles for each
structure (Table 6). For this reason, the energy profile is not
characterized by a single minimum.
Figure 6. Classification of benzyl ligands according to δortho(short) and
δortho(long)
.
exists as two polymorphs with Zr−CH2−Ph angles that span
the ranges 88.7(4)−123.2(4)° and 104.6(2)−115.9(2)°.1g,33
Furthermore, a particularly interesting example of the flexibility
of the benzyl ligand is provided by the observation that
t
(Cp1,2,4‑Bu )CeCH2Ph exists with two distinctly different
3
geometries in the asymmetric unit, with Ce−CH2−Ph bond
angles of 93.1(4)° and 130.4(3)°.34
Consistent with the inequivalent nature of the benzyl ligands
in the solid-state structure of Zr(CH2Ph)4, the solid-state
13C{1H} NMR spectrum exhibits a 1:1:2 set of signals for the
four methylene carbon atoms at 76.4, 74.2 and 70.9 ppm,
respectively (Figure 7), rather than a single resonance. In
In order to eliminate the buffering effect provided by the
other benzyl ligands, geometry optimization calculations were
also performed on Me3ZrCH2Ph, which features only one
benzyl ligand. Significantly, while the most stable geometry-
optimized structure possesses a Zr−C−C bond angle of 92.8°,
the energy of the molecule changes by less than 2 kcal mol−1
over the range 80−125° (Figure 12). Thus, the flexibility of the
benzyl ligands in Zr(CH2Ph)4 is not merely attributable to a
buffering effect due to the presence of other benzyl ligands, but
is intrinsic to the Zr−CH2−Ph moiety. Specifically, the energy
required to decrease the Zr−CH2−Ph bond angle is
compensated by interaction of the phenyl group with the
electronically unsaturated zirconium center, while the energy
required to increase the Zr−CH2−Ph bond angle is
compensated by the formation of agostic interactions with
the methylene group,38 as illustrated in Figure 13. For example,
the large Ti−CH2−Ph angle (139.0°) and short Ti···H
interactions (2.32 and 2.37 Å) for one of the benzyl ligands
of Cp*Ti(CH2Ph)3 have been interpreted in terms of a double
agostic interaction.39 Furthermore, species with α-agostic
interactions have also been proposed as intermediates in olefin
polymerization catalyzed by [Cp2Zr(CH2Ph)]+.7k
Comparison with the silicon counterpart, Me3SiCH2Ph,
provides evidence that these secondary interactions with
zirconium are responsible for the flexibility of the benzyl ligand
of Me3ZrCH2Ph, because silicon does not form benzyl
compounds with a very large range of Si−CH2−Ph bond
angles.40,41 Thus, not only is the Si−CH2−Ph angle for the
most stable structure (114.4°) much larger than that for the
zirconium compound (92.8°), but the energy of the molecule
increases substantially as the Si−CH2−Ph angle deviates from
Figure 7. Solid-state 13C NMR spectrum of Zr(CH2Ph)4 (methylene
region).
solution, however, the benzyl ligands are chemically equivalent
on the NMR time-scale,35 as illustrated in Figures 8 and 9.
Spectroscopically, η1-coordination of benzyl ligands is asso-
ciated with δ Hortho > 6.5, δ Cipso ≈ 150, and 1JC−H for the CH2
group of ∼120 Hz, while η2-coordination is identified by
1
δ Hortho < 6.5, δ Cipso ≈ 140, and JC−H for the CH2 group of
∼135 Hz.36 In this regard, Zr(CH21Ph)4 is characterized by
δ Hortho = 6.38, δ Cipso = 139.5, and JC−H = 135 for the CH2
group, which support the presence of η2-benzyl ligands.
However, while these values are in accord with the presence
of some degree of η2-benzyl coordination in solution, they do
not distinguish between a situation in which the η2-benzyl
ligands are equivalent and one in which the molecule is
E
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1
Figure 8. H NMR spectrum of Zr(CH2Ph)4.
Figure 9. 13C NMR spectrum of Zr(CH2Ph)4 (methylene region).
Table 6. Geometry-Optimized Structures for Zr(CH2Ph)4
relative energy/
Zr−C−C/deg
kcal mol−1
no constraints
87.7
100.2
81.7
87.9
100.2
82.4
105.1
100.2
98.6
106.1
100.2
106.7
99.0
0.00
0.69
1.04
1.93
S4 symmetry
a
monoclinic
b
orthorhombic
87.1
90.2
93.9
a
Geometry-optimized structure with Zr−CH2−Ph angles constrained
b
to those of the monoclinic structure. Geometry-optimized structure
with Zr−CH2−Ph angles constrained to those of the orthorhombic
structure.
CONCLUSIONS
■
In summary, the Zr−CH2−Ph bond angles in the monoclinic
modification of Zr(CH2Ph)4 span a much larger range (25.1°)
than those reported for the orthorhombic form (12.1°).
Density functional theory calculations demonstrate that little
energy is required to perturb the Zr−CH2−Ph bond angles in
this compound, thereby providing support for the existence of
two different molecular structures in the solid state.
Furthermore, density functional theory calculations also
indicate that bending of the Zr−CH2−Ph moiety in the
monobenzyl compound Me3ZrCH2Ph is facile, thereby
demonstrating that a benzyl ligand attached to zirconium is
intrinsically flexible, such that its bending does not require a
buffering effect involving another benzyl ligand. This flexibility
of the benzyl ligand could provide a means to protect a metal
center during a catalytic transformation.5,7k Despite this
Figure 10. Geometry-optimized structures of Zr(CH2Ph)4 subject to
various constraints.
this value (Figure 12). For example, reducing the Si−CH2−Ph
bond angle to 85° increases the energy of the molecule by
19.1 kcal mol−1, while increasing the angle to 150° increases the
energy to 14.3 kcal mol−1, both of which are much greater than
the corresponding values of 0.4 and 6.8 kcal mol−1 for the
zirconium system.
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Figure 11. Variation in energy of Zr(CH2Ph)4 as a function of varying a single Zr−CH2−Ph bond angle after allowing the geometry to reoptimize.
The energies are relative to that of the S4 constrained geometry, as indicated with an asterisk.
Table 7. Energy Changes Associated with Bending the Zr−C−C Angle of One of the Benzyl Ligands (#1) in Zr(CH2Ph)4
Zr−C−C/deg
#1
constrained
Zr−C−C/deg
#2
Zr−C−C/deg
#3
Zr−C−C/deg
#4
bond angle
distribution/
deg
bond angle
range/
deg
relative
energy/
kcal mol−1
70.0
75.0
80.0
85.0
90.0
95.0
100.0
93.4
93.3
93.4
94.0
93.8
93.9
93.4
104.3
105.4
106.4
109.0
109.3
110.3
111.5
106.8
105.8
105.3
104.8
104.6
104.5
104.2
70.0−106.8
75.0−105.8
80.0−106.4
85.0−109.0
90.0−109.3
93.9−110.3
93.4−111.5
100.2
36.8
30.8
26.4
24.0
19.3
16.4
18.1
0.0
6.50
3.10
1.09
0.10
−0.30
−0.09
0.31
a
a
a
a
100.2
100.2
100.2
100.2
0.00
105.0
110.0
115.0
120.0
125.0
130.0
135.0
140.0
105.7
105.7
105.3
105.1
105.7
105.4
98.9
88.0
87.4
86.2
86.1
89.3
91.8
101.7
95.9
87.5
88.2
88.8
89.6
91.1
94.5
88.8
88.0
87.5−105.7
87.4−110.0
86.2−115.0
86.1−120.0
89.3−125.0
91.8−130.0
88.8−135.0
88.0−140.0
18.2
22.6
28.8
33.9
35.7
38.2
46.2
52.0
−1.22
−0.98
−0.56
0.12
0.96
2.09
3.07
102.7
4.07
a
Geometry-optimized value when constrained to S4 symmetry.
are reported in ppm relative to SiMe4 (δ = 0) and were referenced
externally to the methylene peak of adamantane (δ = 38.5).44
flexibility, however, the majority of structurally characterized
benzyl compounds feature η1-coordination modes.
Synthesis of Zr(CH2Ph)4. Zr(CH2Ph)4 was synthesized via the
reaction of PhCH2MgCl45 with ZrCl4 by a modification of the
literature method.3b A solution of benzylchloride (13.6 g, 0.11 mol) in
THF (200 mL) was slowly added to a stirred suspension of
magnesium turnings (11.0 g, 0.45 mol) in THF (50 mL) over a
period of ca. 1 h, such that the temperature of the reaction vessel was
maintained at ca. 25 °C. The mixture was stirred at room temperature
overnight and then filtered. The volatile components were removed
from the filtrate in vacuo to give PhCH2MgCl as an off-white powder,
which was treated sequentially with ZrCl4 (6.0 g, 0.026 mol) and Et2O
(150 mL) at −15 °C. The mixture was stirred at −15 °C overnight and
filtered at 0 °C. The precipitate was washed with Et2O (200 mL) at
0 °C and then extracted into toluene (200 and 100 mL). The volatile
components were removed from each extraction in vacuo, resulting in
the formation of Zr(CH2Ph)4 as orange crystalline blocks suitable for
X-ray diffraction (2.1 and 1.0 g, 26%). The synthesis and the
purification of tetrabenzylzirconium were conducted in the absence of
EXPERIMENTAL SECTION
General Considerations. All manipulations were performed using
a combination of glovebox, high-vacuum, and Schlenk techniques
under an argon atmosphere unless otherwise specified.42 Solvents were
■
1
purified and degassed by using standard procedures. H NMR spectra
were measured on Bruker 400 Cyber-enabled Avance III and Bruker
500 DMX spectrometers. 1H chemical shifts are reported in ppm
relative to SiMe4 (δ = 0) and were referenced internally with respect to
the protio solvent impurity (δ = 7.16 for C6D5H).43 13C NMR spectra
are reported in ppm relative to SiMe4 (δ = 0) and were referenced
internally with respect to the solvent (δ = 128.06 for C6D6).43
Coupling constants are given in hertz. Solid-state 13C{1H} NMR
experiments were performed on a Bruker 400 Cyber-enabled Avance
III at a field of 9.40 T (corresponding to a 13C resonance frequency of
100.62 MHz) using the CP-MAS pulse sequence, with an acquisition
time of 0.03 s and a spin rate of 104 Hz. Solid-state 13C NMR spectra
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Figure 12. Variation in energy of Me3ECH2Ph as a function of the E−CH2−Ph bond angle after allowing the geometry to reoptimize (E = Zr, Si).
Figure 13. Geometry-optimized structures of Me3ZrCH2Ph. Reducing the Zr−CH2−Ph bond angle from that in the fully optimized structure
(92.8°) is accompanied by an increased interaction with the phenyl group, while increasing the angle is accompanied by the formation of agostic
interactions with the CH2 group. The geometries have approximate Cs symmetry such that at acute angles the benzyl ligand approaches η4 rather
than η3 coordination.
quantum chemistry programs.47 Geometry optimizations were
performed with the B3LYP density functional48 using the 6-31G**
(C, H, and Si) and LACVP (Zr) basis sets.49
1
light to avoid any photochemical decomposition. H NMR (C6D6):
3
1.55 [s, 8H of Zr{(CH2)C6H5}4], 6.38 [d, JH−H = 7, 8Hortho of
3
Zr{(CH2)C6H5}4], 6.96 [t, JH−H = 7, 4Hpara of Zr{(CH2)C6H5}4],
3
7.06 [t, JH−H = 7, 8Hmeta of Zr{(CH2)C6H5}4]. 13C NMR (C6D6):
1
3
72.5 [tt, JC−H= 135, JC−H = 4, 4C of Zr{(CH2)C6H5}4], 124.5 [dt,
ASSOCIATED CONTENT
3
1JC−H = 162, JC−H = 8, 4Cpara of Zr{(CH2)C6H5}4], 128.7 [m, 8Cortho
■
1
3
S
* Supporting Information
of Zr{(CH2)C6H5}4], 131.0 [dd, JC−H= 159, JC−H = 8, 8Cmeta of
Zr{(CH2)C6H5}4], 139.5 [s, 4Cipso of Zr{(CH2)C6H5}4]. Solid-state
13C{1H} NMR (only CH2 group listed): 76.4 (1C), 74.0 (1C), 70.4
(2C) at −10 °C; 76.4 (1C), 74.2 (1C), 70.9 (2C) at room
temperature; 76.4 (1C), 74.4 (1C), 71.1 (2C) at 50 °C.
CIF files and Cartesian coordinates for geometry-optimized
structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
X-ray Structure Determinations. X-ray diffraction data were
collected on a Bruker Apex II diffractometer. The structures were
solved using direct methods and standard difference map techniques
and were refined by full-matrix least-squares procedures on F2 with
SHELXTL (version 6.14).46
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: parkin@columbia.edu.
Notes
Computational Details. Calculations were carried out using DFT
as implemented in the Jaguar 7.6 (release 110) suite of ab initio
The authors declare no competing financial interest.
H
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(10) The selected planes are those that contain the axis that
corresponds most closely to a molecular S4 axis. The angles between
other planes are 68° and 88°.
(11) Sanz, M.; Mosquera, M. E. G.; Cuenca, T.; de Arellano, C. R.;
Schormann, M.; Bochmann, M. Polyhedron 2007, 26, 5339−5348.
(12) Beckerle, K.; Manivannan, R.; Spaniol, T. P.; Okuda, J.
Organometallics 2006, 25, 3019−3026.
(13) Furthermore, a benzyl compound with a bond angle of 98.6°
was described as possessing partial η2-character. See: Groysman, S.;
Goldberg, I.; Kol, M.; Genizi, E.; Goldschmidt, Z. Organometallics
2003, 22, 3013−3015.
ACKNOWLEDGMENTS
■
We thank the U.S. Department of Energy, Office of Basic
Energy Sciences (DE-FG02-93ER14339), for support of this
research. A.A.-H. thanks the government of Saudi Arabia for a
scholarship. Dr. John Decatur, Michael Appel, and Wenbo Li
are thanked for assistance with the solid-state 13C NMR
experiments.
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dx.doi.org/10.1021/om300820b | Organometallics XXXX, XXX, XXX−XXX