Structures and reactivity of Zr(IV) chlorobenzene complexes

Article abstract of DOI:10.1021/ja044303v

The synthesis, structures, and unusual reactivity of (C₅R₅)₂ZrR′(ClPh)⁺ chlorobenzene complexes are described. The reaction of (C₅R₅)₂ZrR′₂ with [Ph₃C][B(C₆F₅)₄] in C₆D₅Cl affords [(C₅R₅)₂ZrR′(ClC₆D₅)][B(C₆F₅)₄] chlorobenzene complexes (1-d₅, R′ = CH₂Ph and (C₅R₅)₂ = (C₅H₅)₅; 2a-d-d5, R′ = Me and (C₅R₅)₂ = rac-(1,2-ethylene(bis)indenyl) (2a), (C₅H₅)₅ (2b), (C₅H₄Me)₂ (2c), (C₅Me₅)₂ (2d, C₅Me₅ = Cp*)). Complexes 1 and 2b,c are thermally robust but are converted to [{(C₅R₅)₅Zr(μ-Cl)}₂][B(C₆F₅)4]2 (4b,c) by a photochemical process in ClPh solution. In contrast, 2d undergoes facile thermal ortho-C-H activation to yield [Cp*₂Zr(η²-C,Cl-2-Cl-C₆H₅)][B(C₆F₅)₄] (5), which slowly rearranges to [(η⁴,η¹-C₅Me₅C₆H₄)Cp*ZrCl][B(C₆F₅)₄] (6) via β-Cl elimination and benzyne insertion into a Zr-C_Cp* bond. The higher thermal reactivity of 2d versus that of 1 and 2b,c is attributed to steric crowding associated with the Cp* ligands of 2d, which forces a ClPh ortho-hydrogen close to the Zr-Me group. Copyright

Full text of DOI:10.1021/ja044303v

Published on Web 11/06/2004
Structures and Reactivity of Zr(IV) Chlorobenzene Complexes
Fan Wu, Aswini K. Dash, and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
Received September 18, 2004; E-mail: rfjordan@uchicago.edu
The structures and reactivity of LnM(η1-XR) halocarbon com-
plexes of group 6-11 metals have been studied extensively.1,2
Several general reactions have been established for these systems,
including substitution of the halocarbon by stronger ligands, C-X
oxidative addition, nucleophilic displacement of the activated halide,
and X-directed C-H activation.1,2 It has been appreciated for some
time that d0-metal LnMR′+ cations, which are active species in olefin
polymerization and other reactions, also can be stabilized by
halocarbon coordination.3 However, little is known about the
properties of d0 LnMR′(XR)+ species.4 Here, we describe the
synthesis, structures, and unusual reactivity of (C5R5)2ZrR′(ClPh)+
chlorobenzene complexes.
consumed and 6-d4 is present in 90% yield. A small amount (<10%)
of the chloride complex [Cp*2ZrCl(ClC6D5)][B(C6F5)4] (7-d5) is
also formed. Control experiments show that 7-d5 is formed by a
photochemical process. Exposure of 2d-d5 to a 1000 W Hg-Xe
lamp for 1 h yields 7-d5 in 80% yield, whereas 7-d5 is not formed
when 2d-d5 is protected from light.
Scheme 1
The reaction of (C5R5)2ZrR′2 with [Ph3C][B(C6F5)4] in C6D5Cl
affords [(C5R5)2ZrR′(ClC6D5)][B(C6F5)4] complexes, as shown in
eq 1 (1-d5, R′ ) CH2Ph and (C5R5)2 ) Cp2; 2a-d-d5, R′ ) Me
and (C5R5)2 ) rac-(EBI) (2a), Cp2 (2b), Cp′2 (2c), Cp*2 (2d)).5,6
1-d5 was characterized by X-ray diffraction, while 2a-d-d5 were
characterized by NMR. The cation of 1-d5 (Figure 1) adopts a bent
metallocene geometry, and the chlorobenzene ligand is η1-
coordinated via the chlorine. The Zr-ClPh distance (Zr(1)-Cl(1)
) 2.746(1) Å) is intermediate between the sums of Zr and Cl
covalent radii (2.47 Å) and van der Waal radii (3.23 Å).7 The C-Cl
distance of the coordinated chlorobenzene (1.773(3) Å) is not
significantly changed from that in gas-phase chlorobenzene (1.737-
(5) Å).8 The Zr(1)-Cl(1)-C(18) angle is 115.0(1)°, and the ClPh
ring points away from the benzyl group (C(18)-Cl(1)-Zr(1)-
C(12) dihedral angle ) 139.1(2)°). The benzyl ligand is strongly
η2-distorted. The Zr(1)-C(11)-C(12) angle is smaller (82.3(2)°),
and the Zr(1)-C(12) distance is shorter (2.588(3) Å), compared to
the corresponding values in [Cp2Zr(CH2Ph)(CH3CN)][BPh4] (84.9-
(4)°, 2.648(6) Å).9 Chlorobenzene is a weaker donor than CH3CN,
making the Zr(IV) center in 1-d5 more electrophilic than that in
the CH3CN complex and resulting in a stronger Zr‚‚‚Ph interaction.
5-d4 was prepared independently (100%) from Cp*2ZrH+ via
the reaction of 2d-d5 with 1 atm H2 in C6D5Cl at 23 °C for 10 min
(Scheme 1). X-ray structural analysis of 5 (from 2d and H2 in ClPh)
confirmed the dative coordination of the ortho-Cl but was com-
plicated by rotational disorder of the chlorophenyl group. To
confirm the identity of 5 and the coordination of ortho-Cl, the CH3-
CN adduct, [Cp*2Zr(η2-C,Cl-2-Cl-C6H4)(CH3CN)][B(C6F5)4] (5-
CH3CN), was generated by addition of CH3CN to 5. The structure
of 5-CH3CN, free of disorder, is shown in Figure 1. The ortho-Cl
1-d5 and 2a-c-d5 are stable for several days at 23 °C in C6D5Cl
solution, but only if protected from light. Exposure of C6D5Cl
solutions of 1-d5 and 2b,c-d5 to room light for 8 days results in
∼20% conversion to dinuclear dicationic complexes [{(C5R5)2Zr-
(µ-Cl)}2][B(C6F5)4]2 (4b,c), which were characterized by X-ray
diffraction.10 The mechanism of this photochemical reaction is under
investigation.
Reaction of 2d-d5 at 23 °C yielded unexpected results (Scheme
1). After 1 day, 70% of 2d-d5 is converted to a 4/1 mixture of
[Cp*2Zr(η2-C,Cl-2-Cl-C6D4)][B(C6F5)4] (5-d4) and [(η4,η1-C5-
Me5C6D4)Cp*ZrCl][B(C6F5)4] (6-d4), and CH3D is formed. A small
amount of CH4 is also observed, likely due to a minor Cp* ring
methyl C-H activation process.11 Complex 6-d4 grows in with time
at the expense of 5-d4, and after 8 days, 2d-d5 is completely
Figure 1. ORTEP views of the cations of 1-d5, 5-CH3CN, 6, and 7-d5.
H/D atoms are omitted.
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C O M M U N I C A T I O N S
1
is datively bonded to Zr and occupies the central coordination site.
The Zr-Cl distance (2.831(1) Å) is longer than the Zr-ClPh
distance in 1-d5. The C-Cl distance (1.775(4) Å) is similar to that
in free chlorobenzene.
[B(C6F5)4] (10) quantitatively (2 days, 23 °C). A H-1H NOESY
correlation between the resonance of the Me group bound to the
cyclopentadiene sp3 carbon and a singlet aromatic hydrogen
resonance establishes that the aryl-Me group is located at C4, para
to Zr. This result is consistent with path i and the exclusive attack
of Cp* at the lateral benzyne CtC carbon without benzyne rotation,
but rules out path ii, which would generate the 5-Me isomer of 10
(i.e., 11). Attempts to trap benzyne from the proposed Cp*2ZrCl-
(C6H4)+ intermediate in the reaction of 5 were unsuccessful,
implying that the benzyne is more strongly bound than that in Cp*2-
ZrHF(C6F4).12a Stronger benzyne coordination is expected for
cationic versus neutral species, and for nonfluorinated versus
fluorinated benzynes, since d-π* back-bonding is not possible in
these d0-metal systems.
These results show that (C5R5)2ZrR′+ species can be stabilized
by intermolecular (1, 2, and 7) and intramolecular (5, 5-CH3CN,
and 9) Zr‚‚‚ClPh coordination. Noncrowded (C5R5)2ZrR′(ClPh)+
species are thermally robust but are converted to [{(C5R5)2Zr-
(µ-Cl)}2]2+ species by a photochemical process in ClPh solution.
In contrast, Cp*2ZrR′(ClPh)+ (R′ ) Me or H) undergoes facile
thermal ortho-C-H activation to yield 5, which rearranges to 6
via â-Cl elimination and benzyne insertion into a Zr-CCp* bond.
The higher thermal reactivity of 2d versus that of 1 and 2b,c is
attributed to steric crowding involving the Cp* ligands, which forces
a ClPh ortho-hydrogen close to the Zr-Me group in 2d.13 Efforts
to exploit the Cl-directed C-H activation chemistry in synthetic
applications are in progress.
6 was generated quantitatively by reaction of 5 in C6H5Cl (6
days, 23 °C). X-ray analysis (Figure 1) shows that 6 contains a
cyclopentadiene-phenyl ligand that is η4-coordinated through
C(1)-C(4) and σ-coordinated through C(21), and formally is
derived by insertion of benzyne into a Zr-CCp* bond. The Zr-
C(5) distance (2.803(3) Å) is longer than the distances between Zr
and C(1)-C(4) (2.638(3)-2.699(3) Å). Bond length alternation in
the C(1)-C(5) ring, displacement of C(5) by 0.113(3) Å from the
C(1)-C(4) plane, and sp3 hybridization of C(5) are all indicative
of an η4-cyclopentadiene structure. The NMR data for 6 are fully
consistent with the solid-state structure.
7-d5 was prepared independently (100%) by the reaction of 2d-
d5 with Me3SiCl in C6D5Cl (Scheme 1). X-ray analysis (Figure 1)
shows that 7-d5 contains a terminal Zr-Cl ligand and an η1-ClPh
ligand. The Zr-ClPh distance (2.698(1) Å) is similar to that in
1-d5, and the Cl-Ph distance (1.784(5) Å) is slightly elongated.
The Zr(1)-Cl(1)-C(21) angle is 118.5(1)°, and the ClPh ring points
toward the terminal Zr-Cl (C(21)-Cl(1)-Zr(1)-Cl(2) dihedral
angle ) 43.7(2)°). Reaction of 7-d5 with [NBu3CH2Ph]Cl yields
Cp*2ZrCl2 quantitatively.
The observation of CH3D as the major organic product in the
formation of 5-d4 from 2d-d5, and the faster formation of 5 from
in situ generated Cp*2ZrH+ (10 min) than from 2d itself (>1 day),
is consistent with Cl-directed ortho-C-H activation via a σ-bond
metathesis process.
Acknowledgment. We thank the National Science Foundation
for support (CHE-0212210), and Dr. Ian Steele for X-ray analyses.
Two plausible mechanisms for the conversion of 5 to 6 are shown
in Scheme 2. Path i involves â-Cl elimination of 5 to form a Zr(IV)
benzyne intermediate, Cp*2ZrCl(C6H4)+ (8), followed by benzyne
insertion into a Zr-CCp* bond. Jones showed that thermolysis of
Cp*2Zr(C6F5)H to form Cp*2Zr(o-C6F4H)F proceeds via a similar
benzyne intermediate, Cp*2ZrHF(C6F4), and was able to trap the
C6F4 group as the durene adduct.12a Path ii involves direct
nucleophilic displacement of the activated chloride of 5 by attack
of a Zr-CCp* bond at C2. A related SNAr2 mechanism was invoked
to explain the formation of Cp*2ZrHF and arene in the reactions
of Cp*2ZrH2 with fluoroarenes.12b
Supporting Information Available: Experimental procedures and
characterization data (PDF). Crystallographic data for 1-d5, 5-CH3CN,
6, and 7-d5 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
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Scheme 2
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(5) EBI ) 1,2-ethylene(bis)indenyl, Cp ) C5H5, Cp′ ) C5H4Me, Cp* ) C5-
Me5.
(6) 2a-c-d5 are generated within 4 h at 23 °C. In the first few minutes, [{-
(C5R5)2ZrMe}2Me][B(C6F5)4] (3a-c) are observed by NMR. However,
2d-d5 is formed quantitatively within 10 min at 23 °C, and no intermediate
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(13) The distance between C(12) and the closest C6D5Cl ortho-D in 1-d5 is
4.9 Å, and that between Cl(2) and the closest C6D5Cl ortho-D in 7-d5 is
2.7 Å.
To probe the mechanism of conversion of 5 to 6, the p-Me-
substituted complex [Cp*2Zr(η2-C,Cl-2-Cl-5-Me-C6H3)][B(C6F5)4]
(9) was generated by the reaction of [Cp*2ZrMe(p-Cl-MeC6H4)]-
[B(C6F5)4] with H2, and its reactivity was studied (Scheme 2).
Complex 9 rearranges to [{η4,η1-C5Me5-(4-Me-C6H3)}Cp*ZrCl]-
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