As-As bond formation via reductive elimination from a zirconocene bis(dimesitylarsenide) compound

Article abstract of DOI:10.1021/om300415q

A new zirconocene bis(arsenide) derivative, Cp₂Zr(AsMes ₂)₂ (1; Cp = cyclopentadienyl, Mes = 2,4,6- trimethylphenyl), has been prepared by the metathetical reaction of 2 equiv of LiAsMes₂ with Cp₂ZrCl₂ and structurally characterized. Efforts to prepare Cp₂ZrCl(AsMes₂) (2) by reaction of 1 equiv of LiAsMes₂ with Cp₂ZrCl₂ yielded a mixture of products that could not be separated, including 1 and 2, as identified by ¹H NMR spectroscopy. Compound 1 thermally decomposes with formation of As₂Mes₄, suggestive of reductive elimination to form an As-As bond. Further evidence for reductive elimination comes from effective interception of a putative zirconium(II) intermediate with diphenylacetylene to give Cp₂Zr(C₄Ph₄).

Full text of DOI:10.1021/om300415q

Note
pubs.acs.org/Organometallics
As−As Bond Formation via Reductive Elimination from a
Zirconocene Bis(dimesitylarsenide) Compound
L. Taylor Elrod, Hussain Boxwala, Hira Haq, Annie W. Zhao, and Rory Waterman*
Department of Chemistry, University of Vermont, Burlington, Vermont 05405, United States
S
* Supporting Information
ABSTRACT: A new zirconocene bis(arsenide) derivative, Cp2Zr-
(AsMes2)2 (1; Cp = cyclopentadienyl, Mes = 2,4,6-trimethylphen-
yl), has been prepared by the metathetical reaction of 2 equiv of
LiAsMes2 with Cp2ZrCl2 and structurally characterized. Efforts to
prepare Cp2ZrCl(AsMes2) (2) by reaction of 1 equiv of LiAsMes2
with Cp2ZrCl2 yielded a mixture of products that could not be
separated, including 1 and 2, as identified by 1H NMR spectroscopy.
Compound 1 thermally decomposes with formation of As2Mes4, suggestive of reductive elimination to form an As−As bond.
Further evidence for reductive elimination comes from effective interception of a putative zirconium(II) intermediate with
diphenylacetylene to give Cp2Zr(C4Ph4).
he organometallic chemistry of complexes featuring
metal−arsenic σ or π bonds is vastly less developed
In this report, the new complex Cp2Zr(AsMes2)2 (1) has
been prepared by the metathetical reaction of LiAsMes2 with
zirconocene dichloride. Interestingly, 1 is susceptible to a
ligand-induced reductive elimination event to form As2Mes4,
which appears to be the first example of such a transformation
for arsenic.
T
than that for phosphorus.1 This is becoming a somewhat odd
observation with the surge in interest regarding catalytic bond
formation between heavier main-group elements in recent
years.2 An increased understanding of these bonds is of
importance due to multiple mechanistic possibilities in such
reactions. For example, group 4 metal complexes are known to
engage in dehydrocoupling via σ-bond metathesis or α-
elimination steps.2c,d,j,k,3 Reductive elimination is a well-
known reaction for group 4 metal complexes,4 but this step
has not yet been implicated in catalytic dehydrocoupling
reactions, despite this being a reported stoichiometric route to
diphosphines and distibines.5
RESULTS AND DISCUSSION
■
Preparation and Characterization of Complexes.
Reaction of Cp2ZrCl2 with 2 equiv of LiAsMes2, generated in
situ by reaction of MeLi and Mes2AsH, afforded a dark blue
solution from which analytically pure purple crystals of
Cp2Zr(AsMes2)2 (1) formed in 49% yield (eq 1). The reaction
Much of what is known in zirconium and group 4 arsenido
chemistry comes from Hey-Hawkins and co-workers, who
prepared Cp2Zr[As(SiMe3)2]2 and Cp′2ZrCl[As(SiMe3)2] (Cp′
−
−
= C5H5 , C5MeH4 ), provided structural characterization, and
explored the reactivity of the Zr−As bond.6 The original
complex in this series, Cp2Zr(AsPh2)2, was spectroscopically
characterized as reported by Wade et al. in 1984.7 In the solid-
state structure of Cp2Zr[As(SiMe3)2]2, inequivalent arsenic
centers are observed, a feature attributed to ligand-to-metal π
donation from one rather than both centers. This behavior was
originally described by Baker and co-workers for related
bis(phosphide) derivatives.8
We have investigated zirconium−arsenido chemistry using
the (N3N)Zr (N3N = N(CH2CH2NSiMe3)33−) platform.9 In
those studies, we were acutely interested in the first observation
of dehydrocoupling of arsenic and the related mechanistic
differentiation based on primary versus secondary substitution
at arsenic. Additionally, we have also observed zirconium-
catalyzed hydroarsination of terminal alkynes. This prior work
in organoarsine catalysis has led to a reconsideration of the
family of metallocene complexes, as particularly related to As−
As bond formation.
reliably provides approximately 50% yield. However, in some
instances higher isolated yields are possible. The low isolated
yield appears to be a function of crystallization, as NMR spectra
on crude reaction products appear to be predominately
complex 1.
The NMR spectra of complex 1 are simple but highly
indicative. A singlet at δ 5.53 is assigned to the cyclopentadienyl
ligand, and singlet resonances at δ 2.62 and 2.16 are assigned as
the methyl substituents in the 1H NMR spectrum. Likewise, the
13C NMR spectrum clearly indicates cyclopentadienyl, mesityl,
and methyl resonances, on the basis of the chemical shift.
Infrared spectroscopic data were collected on complex 1, but
these were largely uninformative. Fingerprint data are reported
for confirmation.
Received: May 15, 2012
Published: July 3, 2012
© 2012 American Chemical Society
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Organometallics
Note
Crystals suitable for X-ray diffraction analysis were grown
from pentane solution, and the molecular structure of 1 is
shown in Figure 1. The crystal was plagued with a disordered
However, for reactions of group 4 metallocenes with phosphide
anions often give the double-substitution product, despite the
stoichiometry.1b
Decomposition of Cp2Zr(AsMes2)2 (1). Despite high air
and moisture sensitivity, compound 1 exhibits good stability as
a solid under nitrogen at −30 °C. However, upon extended
storage as a solid at ambient temperature, some decomposition
was observed. To better understand this behavior, the
decomposition process was monitored spectroscopically in
benzene-d6 solution. Solution samples of 1 were stable in
benzene-d6 at ambient temperature for extended periods under
nitrogen. Upon heating for hours at 90 °C, several new mesityl-
containing compounds were observed. Among these,
1
(Mes2As)2 (3) was detected by H NMR spectroscopy (eq
3).11 The formation of 3 suggested that reductive elimination
Figure 1. Molecular structure of Cp2Zr(AsMes2)2 (1) with thermal
ellipsoids drawn at the 50% level. Hydrogen atoms are omitted for
clarity.
may be occurring. The concomitant deposition of a precipitate
during the reaction indicated that the resultant zirconocene
products were unstable.
It was hypothesized that the zirconium byproduct of
reductive elimination may be hampering a clean reaction. An
effort to trap the putative Zr(II) complex was made to facilitate
the reaction. Thus, a benzene-d6 solution of complex 1 was
heated in the presence of 5 equiv of diphenylacetylene, which
resulted in complete conversion to 3 and quantitative formation
of Cp2Zr(C4Ph4) (eq 4). Larger excesses of diphenylacetylene
solvent molecule of crystallization. For convenience, this
disordered solvent was removed from the data set using the
SQUEEZE function of the PLATON software package.10 As
expected, there is differential bonding between the two arsenic
atoms and zirconium. Not only is Zr−As(2) approximately 0.07
Å shorter than Zr−As(1), the sum of angles at As(2) is
approaching planar at approximately 355° (Table 1). This
difference and the lengths of the Zr−As bonds are highly
similar to those seen by Hey-Hawkins.6a Interestingly, the As−
C bond lengths are shorter for As(2), which is involved in
greater ligand-to-metal π bonding than As(1). This is a small
difference (Δ ≈ 0.010−0.019 Å), but it is also consistent with
the trend for As−Si bonds as measured for Cp2Zr[As-
6a
(SiMe3)2]2 and may be a consequence of hybridization at
arsenic.
Attempts to prepare Cp2ZrCl(AsMes2) (2) by reaction of
Cp2ZrCl2 with 1 equiv of in situ generated LiAsMes2 afforded a
1
mixture of 1 and starting zirconocene (eq 2). The H NMR
prompted the reaction to proceed at a greater qualitative rate.
While this is not definitive evidence of a Zr(II) intermediate
during this reaction, oxidative coupling of alkynes at formally
Zr(II) compounds, is a well-described transformation.12
The increase in qualitative rate of diarsine formation upon
addition of diphenylacetylene is suggestive of a ligand-induced
reductive elimination. This is a somewhat unsurprising feature
of this chemistry, recalling that associative, ligand-induced
reductive elimination reactions have been known for
zirconocene complexes for more than 30 years.4b However,
As−As bond formation by reductive elimination appears to be
unique in the literature.
spectrum of crude reaction mixtures showed resonances for
starting Cp2ZrCl2 and 1. Additional resonances were seen,
which may have corresponded to the targeted product, but this
could not be isolated from 2 or more efficient conversion
solicited. This is an interesting observation in light of the
relative ease with which Hey-Hawkins and co-workers could
prepare Cp′2ZrCl[As(SiMe3)2] by the same synthetic strategy.6a
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Compound 1
Zr−As(1)
Zr−As(2)
Zr−Cp(1)
Zr−Cp(1)
2.7286(4)
2.6581(4)
2.183(3)
As(1)−C(11)
As(1)−C(17)
As(1)−C(23)
As(1)−C(29)
1.973(3)
1.974(3)
a
b
1.963(3)
2.183(3)
1.956(3)
C(11)−As(1)−C(17)
C(11)−As(1)−Zr
C(17)−As(1)−Zr
104.17(12)
115.24(9)
C(29)−As(1)−C(23)
C(29)−As(1)−Zr
C(23)−As(1)−Zr
104.76(12)
118.54(9)
131.64(10)
126.91(10)
a
b
Distance to a centroid formed by atoms C(1)−C(5). Distance to a centroid formed by atoms C(6)−C(10).
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Organometallics
Note
full-matrix least-squares procedures on F2 with SHELXTL (version
6.14).16 All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms on carbon were included in calculated positions
and were refined using a riding model. A disordered solvent molecule
(ether or pentane) was included in the unit cell but could not be
satisfactorily modeled. Therefore, that solvent was treated as a diffuse
contribution to the overall scatting without using specific atom
positions by the SQUEEZE function in the PLATON program.10
SQUEEZE output data (.lis file) are included as Supporting
Information. Crystal data and structure refinement parameters are
given in Table 2.
In summary, a new bis(arsenido) complex has been prepared,
and its decomposition via apparent reductive elimination to
form an As−As has been examined. While there have been
reports that diphosphines can be formed via reductive and
oxidative processes5a,13 and there is evidence for Sb−Sb
formation by reductive elimination from a bis(stibido) complex
of hafnium,5b this process is, to the best of our knowledge, a
unique route to As−As bonds and may represent an inroad to
novel synthesis involving arsenic.
EXPERIMENTAL DETAILS
■
Table 2. Crystal Data and Structure Refinement Parameters
for Complex 1
Manipulations were performed under a nitrogen with dry, oxygen-free
solvents using an M. Braun glovebox. Benzene-d6 was degassed and
then dried over NaK alloy. Celite-454 was heated to a temperature
greater than 180 °C under dynamic vacuum for at least 8 h. NMR
spectra were recorded with a Bruker AXR 500 MHz spectrometer in
benzene-d6 and are reported with reference to residual solvent
resonances (δ 7.15 and δ 128.0). Infrared spectra were collected on a
Bruker Alpha FT-IR spectrometer with an ATR head. Elemental
analysis was performed on an Elementar microCube instrument.
Reagents were obtained from commercial suppliers and dried by
conventional means. Zirconocene dichloride was purchased from
Strem Chemicals, and dimesitylarsane14 was prepared by reduction of
Mes3As15 with lithium metal in THF followed by quenching with
degassed water and crystallization.
a
formula
C56H66As2Zr
980.15
a
Mr
cryst syst
color
orthorhombic
purple
a/Å
20.9687(18)
22.0733(19)
9.3097(8)
90
b/Å
c/Å
α = β = γ/deg
unit cell V/Å3
space group
Z
4309.0(6)
Pnn2
4
Preparation of Cp2 Zr(AsMes2)2 (1). A 10 mL diethyl ether
solution of dimesitylarsane (304.5 mg, 0.9688 mmol) was cooled to
−30 °C, and methyllithium (0.61 mL, 0.98 mmol) was added slowly.
The resulting yellow solution was warmed to ambient temperature and
stirred for 1 h, and then it was cooled to −30 °C. Zirconocene
dichloride (141.6 mg, 0.484 mmol) was suspended in 10 mL of diethyl
ether and cooled to −30 °C. The cold Mes2AsLi solution was added
dropwise to the cold suspension of Cp2ZrCl2, resulting in a dark blue
solution. The solution was filtered through Celite after 1 h of stirring
at ambient temperature. Solvent was removed under reduced pressure
until a solid appeared, and then the solution was warmed to ambient
temperature to redissolve the solids. The solution was filtered then
cooled to −30 °C for several days, yielding 1 as purple crystals in two
crops (201 mg, 0.273 mmol, 49%). 1H NMR (500 MHz): δ 6.87 (s, 8
H, CH), 5.53 (s, 10 H, C5H5), 2.62 (s, 24 H, CH3), 2.16 (s, 12 H,
CH3). 13C{1H} NMR (125.8 MHz): δ 144.4 (s, C6H2Me3), 136.8 (s,
C6H2Me3), 129.2 (s, C6H2Me3), 128.3 (s, C6H2Me3), 105.8 (s, Cp),
25.7 (s, CH3), 20.9 (s, CH3). IR: 2953 w, 2910 w, 1597 w, 1430 m,
1365 m, 1291 w, 1011 m, 848 m, 803 s, 550 m cm−1. Anal. Calcd for
C46H54As2Zr: C, 65.15; H, 6.42. Found: C, 65.08; H, 6.49.
Attempted Preparation of Cp2ZrCl(AsMes2) (2). A 7 mL
diethyl ether solution of dimesitylarsane (152.2 mg, 0.4844 mmol) was
cooled to −30 °C, and methyllithium (0.30 mL, 0.49 mmol) was
added slowly. The resulting yellow solution was warmed to ambient
temperature and stirred for 1 h, and then it was cooled to −30 °C.
Zirconocene dichloride (141.6 mg, 0.484 mmol) was suspended in 5
mL of diethyl ether and cooled to −30 °C. The cold Mes2AsLi
solution was added dropwise to the cold suspension of Cp2ZrCl2,
resulting in a purple solution. The solution was reacted and worked up
as above to yield a light purple powder, which was found to be an
impure mixture of 1 and Cp2ZrCl2 by NMR spectroscopy.
θ range/deg
μ/mm−1
N
1.34−25.04
1.819
40 600
Nind
7616
Rint
0.0314
Flack x
0.014(6)
0.0279
a
R1 (I > 2σ(I))
b
wR2 (I > 2σ(I))
0.0639
Δρmax/e Å−3
Δρmin/e Å−3
GOF on R1
0.486
−0.356
1.042
a
Calculated with complete cell contents prior to SQUEEZE operation.
b
[c]
2
R1 = ∑||Fo| − |Fc||/∑|Fo|.
wR2 = {∑[w(Fo − Fc 2)2]/
2
∑[w(Fo )2]}1/2
.
ASSOCIATED CONTENT
■
S
* Supporting Information
A table, a figure, and a CIF file giving crystallographic data, a
SQUEEZE output file for 1, and an NMR spectrum of the
reaction of 1 with diphenylacetylene (pfd). This material is
available free of charge via the Internet at http://pubs.acs.org.
CCDC 873948 (1) has been deposited at the Cambridge
Crystallographic Data Centre.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rory.waterman@uvm.edu. Tel: 802-656-0278.
Reaction of 1 with Diphenylacetylene. A J. Young NMR tube
was charged with a 1 mL benzene-d6 solution of 1 (12.0 mg, 1.41 ×
10−2 mmol) and diphenylacetylene (12.6 mg, 7.05 × 10−2 mmol). The
Notes
1
The authors declare no competing financial interest.
tube was sealed and an initial H NMR spectrum collected. The dark
purple solution was then placed in a 100 °C oil bath for 2 h, resulting
in an orange solution. A final 1H NMR experiment was run,
confirming complete conversion to 3 and Cp2Zr(C4Ph4).
X-ray Crystallography. X-ray diffraction data were collected on a
Bruker APEX 2 CCD platform diffractometer (Mo Kα, λ = 0.710 73
Å) at 125 K. A suitable crystal of 1 was mounted in a nylon loop with
Paratone-N cryoprotectant oil. The structure was solved using direct
methods and standard difference map techniques and was refined by
ACKNOWLEDGMENTS
■
This work was supported by the U.S. National Science
Foundation (NSF, CHE-0747612). X-ray facilities were
provided by the NSF (CHE-1039436). H.H. and A.W.Z.
thank the Essex High School Women-in-Science Program for
the opportunity to conduct research. We thank Benjamin A.
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Organometallics
Note
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