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Can. J. Chem. Vol. 76, 1998
[1]
[2]
[3]
[4]
(7). The crude product mixture contained the ether adduct of 1,
which was isolated base-free by distillation of
Zn(C6F5)2 (Et2O)2 under high vacuum at 140°C, resulting in a
15% yield of Zn(C6F5)2. However, significant amounts of an
unidentified colorless liquid by-product was also produced in
this reaction.
In our hands, this procedure produced very little Zn(C6F5)2
but large amounts of the liquid by-product, which we identi-
fied as an isomeric mixture of the Diels–Alder products
formed upon reaction with tetrafluorobenzyne and toluene.2 It
is not clear which species in the reaction mixture is the source
of tetrafluorobenzyne but, given the thermal stability of 1 it-
self, it is likely to be an intermediate in this process or perhaps
BrMgC6F5. In a second route to 1, loss of CO2 from the dicar-
boxylate compound Zn(O2CC6F5)2 (eq. [2]) provides the driv-
ing force for relatively clean formation of 1 directly (8). Third,
eq. [3] represents a relatively safe route to pentafluorophenyl
zinc derivatives, but the bases employed in the published pro-
cedure (9) are not easy to remove; for our purposes, we must
have base free 1.
monomeric with linear geometries (12), and most of those
whose structures have been determined bear this out (13). The
exception is diphenylzinc (13a), which is dimeric in the solid
state by virtue of an interaction between an ipso carbon of one
σ-bound phenyl group and the zinc center of another
Zn(C6H5)2 unit. This interaction was rationalized as being akin
to an electrophilic attack on the zinc phenyl group by an adja-
cent Lewis acidic zinc center. This feature is not replicated in
the structure of perflouro diphenylzinc, 1, presumably because
the C6F5 groups are much less susceptible to electrophilic at-
tack even though the zinc centers must also be more powerful
electrophiles.
Thus, in contrast to its perproteo cousin, 1 behaves like
most other ZnR2 derivatives and is monomeric in the solid state
with with C(1)-Zn-C(7) approaching ideal linearity at
172.6(2)°. The Zn—C bonds in 1 are typical of ZnR2 com-
pounds, which range from 1.93–1.95 Å in compounds where
R is not sterically bulky. These lengths are slightly shorter than
those found in four coordinate Zn(C6F5)2 L2 compounds (cf.
Zn—C lengths of 1.999(4) and 2.102(3)
Å
in
Despite the potential hazards associated with the more re-
active reagent pentafluorophenyllithium, we turned to the
fourth procedure, also developed by Weidenbruch (8), which
produces 1 much more cleanly (eq. [4]). A diethyl ether solu-
tion of ZnCl2 was added to in situ generated C6F5Li, and the
resulting mixture was allowed to warm slowly to room tem-
perature. Work-up led to base-free 1, which gave a 19F NMR
spectrum essentially identical to that reported, with no trace of
the Diels–Alder by-product in evidence. For larger scale
preparations (>2 g), multiple distillations were required to
fully remove the diethyl ether, which was judged complete
when the distillate was a free-flowing powder as opposed to
the oily white solid characteristic of 1 contaminated with its
ether adduct.
While a few Lewis base adducts 1 L2 have been structurally
characterized (L = THF (10); tetramethyltetrazene (11)), that
of base free 1 has not. We therefore obtained X-ray quality
crystals of 1 and evaluated its solid-state structure; an ORTEP
diagram is shown in Fig. 1, while metrical parameters are
given in Table 1, and atomic coordinates are given in Table 2.
Two coordinate organozinc compounds are typically
Zn(C6F5)2(THF)2) (10). The two C6F5 rings in 1 are almost
orthogonal to one another, with an angle between the planes
of the aromatic rings of 76.7(2)°. This compares to 67.1° for
the same parameter in Zn[2,4,6-(CF3)3C6H3]2 (13c).
In addition to alleviating intramolecular steric interactions
between the C6F5 rings, the orthogonality of the C6F5 rings in
1 allows for intramolecular stacking interactions between rings
on adjacent molecules in the crystal structure of the compound.
These stacking interactions engender a layered structure in the
crystal; Fig. 2 shows a chemical 3-D representation of the mo-
lecular arrangement in the crystal looking down on a portion
of one layer (A) and from the side (B). The distances between
the centroids of each C6F5 ring and its stacking partner are
3.503 and 3.563 Å, similar to the ring separations observed in
stacking interactions between phenyl and pentafluorophenyl
rings (14). Between the layers, a weak interaction between one
of the meta fluorines, F(2), and a Zn center of an adjacent
molecule may be present, as judged by the intermolecular
separation of 2.849(2) Å. Although this is at the long end of
the range (2.532(6)–2.733(6) Å) of previously observed intra-
molecular C-F Zn interactions (15), the intermolecular nature
of the contact here might be expected to be slightly longer.
Furthermore, the distance is within the sum of the van der
2
D.J. Parks and W.E. Piers. Unpublished results.
© 1998 NRC Canada