O- and m-(bromotetrafluorophenyl)palladium(II) complexes: Atropisomerism studies by 19F NMR and measurement of through-space F-F coupling constants

Article abstract of DOI:10.1021/om970529d

Palladium(II) complexes with the asymmetric aryl groups o-C₆BrF₄ and m-C₆BrF₄ have been synthesized, namely [Pd₂(C₆BrF₄)₂(μ-Cl) ₂(tht)₂], trans-[Pd(C₆BrF₄)Cl(tht)₂] (tht = tetrahydrothiophene), trans-[Pd(C₆BrF₄)₂L₂], and cis-[Pd(C₆BrF₄)₂L₂] (L₂ = 2tht, COD, Me₂-bipy (4,4′-dimethyl-2,2′-bipyridine), OPPy₂Ph (Py = 2-pyridyl), 2PMe₃, 2CNMe). A mixture of atropisomers (syn-Br,Br and anti-Br,Br) is found for most bis(aryl) complexes, and the ratio of syn:anti atropisomers depends upon the R group, the cis or trans geometry of the complexes, and the ancillary ligand: A ratio close to 1:1 is observed for the cis- and trans-m-C₆BrF₄ derivatives and for the trans-o-C₆BrF₄ complexes, but the anti atropisomer is preferred in the cis-o-C₆BrF₄ derivatives, particularly if the ancillary ligands extend out of the coordination plane. Strong through-space coupling between syn F_ortho atoms of mutually cis R groups is observed in the ¹⁹F NMR spectra for cis-[Pd(C₆BrF₄)₂L₂] complexes. J-values were determined which prove useful to unequivocally identify the atropisomers formed and to detect distortions in the structure of the complexes. Atropisomerization by aryl rotation is observed in several cases at different rates: Faster for m-C₆BrF₄ than for o-C₆BrF₄ derivatives, and faster for small-in-plane than for bulk-in-plane ancillary ligands. The fastest rotation occurs in the complexes with CNMe, which do not show atropisomers by ¹⁹F NMR at room temperature.

Full text of DOI:10.1021/om970529d

5416
Organometallics 1997, 16, 5416-5423
o- a n d m -(Br om otetr a flu or op h en yl)p a lla d iu m (II)
Com p lexes: Atr op isom er ism Stu d ies by 19F NMR a n d
Mea su r em en t of Th r ou gh -Sp a ce F -F Cou p lin g Con sta n ts
Ana C. Albe´niz, Arturo L. Casado, and Pablo Espinet*
Departamento de Quı´mica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid,
E-47005 Valladolid, Spain
Received J une 23, 1997X
Palladium(II) complexes with the asymmetric aryl groups o-C6BrF4 and m-C6BrF4 have
been synthesized, namely [Pd2(C6BrF4)2(µ-Cl)2(tht)2], trans-[Pd(C6BrF4)Cl(tht)2] (tht )
tetrahydrothiophene), trans-[Pd(C6BrF4)2L2], and cis-[Pd(C6BrF4)2L2] (L2 ) 2tht, COD, Me2-
bipy (4,4′-dimethyl-2,2′-bipyridine), OPPy2Ph (Py ) 2-pyridyl), 2PMe3, 2CNMe). A mixture
of atropisomers (syn-Br,Br and anti-Br,Br) is found for most bis(aryl) complexes, and the
ratio of syn:anti atropisomers depends upon the R group, the cis or trans geometry of the
complexes, and the ancillary ligand: A ratio close to 1:1 is observed for the cis- and trans-
m-C6BrF4 derivatives and for the trans-o-C6BrF4 complexes, but the anti atropisomer is
preferred in the cis-o-C6BrF4 derivatives, particularly if the ancillary ligands extend out of
the coordination plane. Strong through-space coupling between syn Fortho atoms of mutually
cis R groups is observed in the 19F NMR spectra for cis-[Pd(C6BrF4)2L2] complexes. J -values
were determined which prove useful to unequivocally identify the atropisomers formed and
to detect distortions in the structure of the complexes. Atropisomerization by aryl rotation
is observed in several cases at different rates: Faster for m-C6BrF4 than for o-C6BrF4
derivatives, and faster for small-in-plane than for bulk-in-plane ancillary ligands. The fastest
rotation occurs in the complexes with CNMe, which do not show atropisomers by 19F NMR
at room temperature.
Ch a r t 1
In tr od u ction
The use of highly symmetric aryl groups is advanta-
geous for the identification of many organometallic
compounds but can be a drawback in the study of
certain dynamic processes. Thus, to study the aryl
rotation about the M-R bond (M ) Pd, Pt; R ) C6F5),
it is necessary to use ancillary ligands that render the
two sides of the coordination plane inequivalent and
make the rotation observable by the equivalence it
produces. Recently, we proved that in complexes
[M(C6F5)X(OPPynPh3-n)] (Chart 1; M ) Pd, Pt; X )
C6F5, halide; n ) 2, 3) there is a restricted rotation about
the Pd-C6F5 bond in the square planar compound that
makes the two halves of the C6F5 group equivalent.1,2
In order to prove this rotation, we had to discount that
the equivalence was produced by movements of the
ligand. In fact, for one of the complexes (M ) Pd; X )
Cl; n ) 3) the equivalence was observed at a faster rate
and was due to associative substitution of the Py groups
producing “inversion” of the chelating ligand (that is,
this inversion produced an apparent rotation at a faster
rate than the real rotation). Other ligand fluxional
processes can also produce a deceptive effect of aryl
rotation about the M-C bond.3
A way to overcome this uncertainty is to use nonsym-
metrical o-C6BrF4 or m-C6BrF4 derivatives. The pres-
ence of atropisomers is unequivocally proof of restricted
rotation, and the only way to produce atropisomeriza-
tion is rotation about the M-C bond, so there is no need
to disprove other mechanisms. Moreover, the rotation
can also be observed in complexes where the coordina-
tion plane is a symmetry plane.
The cis-bis(bromotetrafluorophenyl)palladium deriva-
tives described here display complex static 19F NMR
spectra. Their complete study shows that they contain
inter-aryl (including fairly large through-space) F-F
couplings between F atoms belonging to different groups
in the same molecule. Through-space F-F coupling
constants have been observed in many organic deriva-
tives whose structures place these atoms at short
X Abstract published in Advance ACS Abstracts, December 1, 1997.
(1) Casares, J . A.; Coco, S.; Lin, L.-S.; Espinet, P. Organometallics
1995, 14, 3058-3067.
(2) Some other examples of restricted aryl rotation about the M-C
bond are as follows: (a) Brown, J . M.; Pe´rez-Torrente, J .; Alcock, N.
Organometallics 1995, 14, 1195-1203. (b) Rieger, A. L.; Carpenter,
G. B.; Rieger, P. H. Organometallics 1993, 12, 842-847. (c) Selnau,
H. E.; Merola, J . S. Organometallics 1993, 12, 1583-1591. (d) Alsters,
P. L.; Boersma, J .; Smeets, W. J . J .; Spek, A. L.; Van Koten, G.
Organometallics 1993, 13, 1639-1647. (e) Anderson, G. K.; Cross, R.
J .; Manojlovic-Muir, L.; Muir, K. W.; Rocamora, M. Organometallics
1988, 7, 1520-1525. (f) Baumga¨rtner, R.; Brune, H.-A. J . Organomet.
Chem. 1988, 350, 115-127. (g) Probitts, E. J .; Saunders, D. R.; Stone,
M. H.; Mawby, R. J . J . Chem. Soc. Dalton Trans. 1986, 1167-1173.
(h) J ones, W. D.; Feher, F. J . Inorg. Chem. 1984, 23, 2376-2388. (i)
Wada, M.; Sameshima, K. J . Chem. Soc., Dalton Trans. 1981, 240-
244.
(3) Abel, E. W.; Orrell, K. G.; Osborne, A. G.; Pain, H. M.; Sik, V.;
Hursthouse, M. B.; Abdul Malik, K. M. J . Chem. Soc., Dalton Trans.
1994, 3441-3449.
S0276-7333(97)00529-3 CCC: $14.00 © 1997 American Chemical Society

(Bromotetrafluorophenyl)palladium(II) Complexes
Organometallics, Vol. 16, No. 25, 1997 5417
distances.4 However, although many cis-bis(fluorophe-
nyl)metal derivatives have been reported so far,5 this
type of coupling, responsible in part for the complicated
spin systems observed, had not been recognized before,
except in one case where it was attributed to scalar four-
or five-bond cross ring coupling.3
used previously for other R groups such as Me, C6F5,
and 2,5-C6Cl2F3.5,6,9
cis-[PdCl2(COD)] + 2LiR f
cis-[PdR2(COD)] + 2LiCl (3)
Complex 3m was obtained in higher yield (71%) than
3o (59%). Moreover, depending on the reaction condi-
tions used in the arylation reaction that produces 3o,
[Pd2(µ-Cl)2{6-(o-C6BrF4)-η3-cyclooct-1-en-5-yl}2] (12) could
be isolated in 10% yield (see Experimental Section). The
latter is formed by insertion of one double bond of COD
into the Pd-R bond in the mono-arylated complex cis-
[Pd(o-C6BrF4)Cl(COD)], as reported for similar C6F5
complexes.10 The insertion byproduct was not found in
the preparation of 3m . This suggests that, again for
steric reasons, the second arylation of cis-[Pd(o-C6-
BrF4)Cl(COD)] is much slower than that for the analo-
gous derivative with m-C6BrF4 so that the insertion
reaction can compete with it, decreasing the preparative
yield in 3o.
Resu lts a n d Discu ssion
Syn th esis. Complexes containing one or two bro-
motetrafluorophenyl groups bound to palladium were
synthesized. The arylation reactions were carried out
using convenient precursors for each stoichiometry and
geometry. The reaction of trans-[PdCl2(tht)2] (tht )
tetrahydrothiophene) with LiR leads to mixtures of
trans-[PdR2(tht)2] (1) and cis-[PdR2(tht)2] (2) (eq 1).
trans-[PdCl2(tht)2] + 2LiR f
[PdR2(tht)2] + 2LiCl (1)
The trans to cis ratio depends on the aryl group: 1o:
2o ) 9:1 (R ) o-C6BrF4); 1m :2m ) 1:1 (R ) m-C6BrF4).
The trans and cis isomers can be separated as described
in the Experimental Section. The trans complexes
isomerize slowly in CDCl3 solution to the corresponding
cis isomers (eq 2), as observed before for the correspond-
ing C6F5 or C6Cl2F3 derivatives.6,7 The process is first
Complexes 1, 2, or 3 react fast with stoichiometric
amounts of ligands, such as Me2bipy (4,4′-dimethyl-2,2′-
bipyridine), OPPy2Ph (Py ) 2-pyridyl), PMe3, or CNMe,
to give new complexes [Pd(C6BrF4)2L′2] (4-9, see Tables
1 and 2) which preserve the cis or trans geometry of
the starting products (eq 4).
[PdR2L2] + 2L′ f [PdR2L′2] + 2L
(4)
trans-[PdR2(tht)2] f cis-[PdR2(tht)2]
(2)
All of the bis(bromotetrafluorophenyl)palladium com-
plexes prepared (see Tables 1 and 2), except 5o, 5m ,
and 7m , appear as a mixture of atropisomers syn-Br,-
Br and anti-Br,Br at room temperature (Figure 1). For
9o and 9m , with an asymmetric ligand, the syn-Br,Br
isomer is further split in two, depending on whether the
Br and P substituents are in the same (syn-Br,P) or in
opposite (anti-Br,P) faces of the palladium coordination
plane. This gives a total of three observable atropiso-
mers: Two syn-Br,Br and one anti-Br,Br, which be-
comes chiral racemic. Other possible conformers with
the oxygen atom toward Pd have not been detected. The
conformation with the phenyl group close to Pd is
proposed according to the structure found for the
analogous C6F5 complex (Chart 1).1
order in Pd complex, and the observed rate constants
are kiso ) 1.3 × 10-6 s-1 for 1o and kiso ) 8.1 × 10-6 s-1
for 1m (10 mM solutions at 318.2 K). The ratios at
equilibrium are 1o:2o ) 35:65; 1m :2m ) 4:96.
It is known that trans-cis isomerization during the
arylation process is catalyzed by the aryllithium reagent
via the formation of [PdR3L]- intermediates.8 The
observed results suggest that the first product of the
arylation of trans-[PdCl2(tht)2] is trans-[PdR2(tht)2] and
that the catalysis of isomerization is less efficient for R
) o-C6BrF4, probably because the formation of [PdR3L]-
is less favorable for o-C6BrF4 than for the less hindered
m-C6BrF4 or C6F5.
Arylation of cis-[PdCl2(COD)] by organolithium re-
agents LiR leads to cis-[PdR2(COD)] (3o, R ) o-C6BrF4;
3m , R ) m-C6BrF4). This synthetic approach (eq 3) was
The atropisomer ratio depends both on the cis or trans
geometry and on the aryl group of the complexes. Ratios
of 1:1, or very close to this value, are found for trans-
[Pd(m-C6BrF4)2L2], trans-[Pd(o-C6BrF4)2L2], and cis-
[Pd(m-C6BrF4)2L2]. In contrast, for the complexes cis-
[Pd(o-C6BrF4)2L2], the anti-Br,Br diastereoisomer is
more abundant and the difference increases for ligands
that extend out of the coordination plane. These
abundances correspond to the equilibrium distribution
(atropisomerization is fast compared to the preparative
times) and reflect the influence of the Br substituent
on the stability of the complexes: Unnoticeable for the
m-C6BrF4 derivatives but important for the hindered
o-C6BrF4 derivatives, making the most crowded syn-Br,-
Br atropisomer less stable.
(4) (a) Ernst, L.; Ibrom, K. Angew. Chem., Int. Ed. Engl. 1995, 34,
1881-1882. (b) Ernst, L.; Ibrom, K.; Marat, K.; Mitchell, R. H.; Bodwell,
G. J .; Bushnell, G. W. Chem. Ber. 1994, 127, 1119-1124. (c) Lyga, J .
W.; Henrie, R. N.; Meier, G. A.; Creekmore, R. W.; Patera, R. M. Magn.
Reson. Chem. 1993, 31, 323-328. (d) Gribble, G. W.; Olson, E. R. J .
Org. Chem. 1993, 58, 1631-1634. (e) Mallory, F. B.; Mallory, C. W. J .
Am. Chem. Soc. 1985, 107, 4816-4819. (f) Mallory, F. B.; Mallory, C.
W.; Ricker, W. M. J . Am. Chem. Soc. 1975, 97, 4770-4771 and
references therein. (g) Mallory, F. B. J . Am. Chem. Soc. 1973, 95, 7747-
7752 and references therein.
(5) (a) Garc´ıa, M. P.; J ime´nez, M. V.; Lahoz, F. S.; Oro, L. A. Inorg.
Chem. 1995, 34, 2153-2159. (b) Uso´n, R.; Fornie´s, J .; Falvello, L. R.;
Toma´s, M.; Casas, J . M.; Mart´ın, A.; Cotton, F. A. J . Am. Chem. Soc.
1994, 116, 7160-7165. (c) Falvello, L. R.; Fornie´s, J .; Fortun˜o, C.;
Mart´ınez, F. Inorg. Chem. 1994, 33, 6242-6246. (d) Lo´pez, G.; Ruiz,
J .; Garc´ıa, G.; Vicente, C.; Casabo´, J .; Molins, E.; Miravitlles, C. Inorg.
Chem. 1991, 30, 2605-2610. (e) Uso´n, R.; Fornie´s, J . Adv. Organomet.
Chem. 1988, 28, 219-297. (f) Deacon, G. B.; Nelson-Reed, K. T. J .
Organomet. Chem. 1987, 322, 257-268.
In order to help with the assignment of the 19F NMR
spectral patterns for the bis-arylated complexes, deriva-
(6) Espinet, P.; Mart´ınez-Ilarduya, J . M.; Pe´rez-Briso, C.; Casado,
A. L.; Alonso, M. A. J . Organomet. Chem., in press.
(7) Minniti, D. Inorg Chem. 1994, 33, 2631-2634.
(8) Ozawa, F.; Kurihara, K.; Yamamoto, T.; Yamamoto, A. J .
Organomet. Chem. 1985, 279, 233-243.
(9) Rudler-Chaurin, M.; Rudler, H. J . Organomet. Chem. 1977, 134,
115-119.
(10) Albe´niz, A. C.; Espinet, P.; J eannin, Y.; Philoche-Levisalles, M.;
Mann, B. E. J . Am. Chem. Soc. 1990, 112, 6594-6600.

5418 Organometallics, Vol. 16, No. 25, 1997
Ta ble 1. 19F NMR Da ta for (o-Br om otetr a flu or op h en yl)p a lla d iu m (II) Com p lexesa
Albe´niz et al.
no.
1o
complexb
atropisomer
anti-Br,Br
%
δ3
δ4
δ5
δ6
3J 3-4
5J 3-6
3J 4-5
3J 5-6
trans-PdR2(tht)2
50
50
72
28
72
28
50
50
-127.8
-127.3
-127.1
-127.0
-126.5
-126.4
-128.1
-127.4
-129.3
-128.0
-127.5
-128.5
-128.3
-127.3
-127.3
-125.5
-126.7
-127.0
-125.6
-128.5
-129.1
-126.5
-158.0
-157.9
-159.1
-159.2
-158.4
-158.4
-159.1
-159.1
-160.0
-160.1
-160.1
-160.0
-160.0
-159.9
-159.9
-159.0
-159.3
-159.5
-159.6
-158.2
-162.5
-157.0
-156.8
-156.7
-157.2
-157.5
-156.8
-156.9
-157.6
-157.6
-158.7
-157.7
-158.0
-158.5
-158.5
-158.3
-158.1
-157.9
-157.2
-157.1
-158.2
-157.2
-160.9
-155.5
-117.6
-116.8
-114.2
-112.0
-110.1
-111.5
-116.0
-115.4
-114.2
-112.1
-111.9
-112.5
-112.5
-112.5
-111.9
-106.5
-108.2
-111.3
-109.3
-119.2
-118.1
-118.0
21.3
21.8
21.2
21.2
22.1
21.5
20.8
20.8
20.0
21.1
21.2
21.1
21.3
21.4
21.3
22.0
22.0
22.0
22.0
20.5
13.6
13.7
11.3
11.2
10.3
10.9
13.6
13.4
20.5
20.5
19.4
19.7
19.6
20.8
19.2
19.2
19.0
19.5
19.4
19.4
19.4
19.5
19.5
20.1
20.1
20.1
20.1
18.9
33.2
33.2
32.0
31.9
30.7
31.0
33.5
33.4
31.9
33.5
32.9
30.2
30.8
30.8
31.6
31.2
31.2
30.9
syn-Br,Br
anti-Br,Br
syn-Br,Br
anti-Br,Br
syn-Br,Br
anti-Br,Br
syn-Br,Br
2o
cis-PdR2(tht)2
3o
cis-PdR2(COD)
trans-PdR2(PMe3)2
4oc
5od
6o
trans-PdR2(CNMe)2
cis-PdR2(PMe3)2
anti-Br,Br
syn-Br,Br
anti-Br,Br
syn-Br,Br
anti-Br,Br
syn-Br,Br
anti-Br,Br
64
36
56
44
55
45
64
12.4
10.8
11.4
11.3
11.7
10.1
10.1
10.1
10.1
9.8
7o
8o
9o
cis-PdR2(CNMe)2
cis-PdR2(Me2bipy)
cis-PdR2(OPPhPy2)
syn-Br,Br-anti-Br,P
syn-Br,Br-syn-Br,P
trans
cis
28
8
81
19
10od
[PdR(µ-Cl)(tht)]2
trans-PdRCl(tht)2
29.2
30.3
11oe
21.0
10.9
19.3
a
b
At 282 MHz, CDCl3, 293 K: δ in ppm from CFCl3; J in Hertz. R ) o-C6F4Br. c anti-Br,Br, 4J 6-P ) 4.8 Hz, 6J 4-P ) 2.6 Hz; syn-Br,Br,
4J 6-P ) 5.7 Hz, J 4-P ) 2.6 Hz. In acetone-d6, only one atropisomer at room temperature.
J
) 1.7 Hz.
3-5
6
d
e 4
Ta ble 2. 19F NMR Da ta for (m -Br om otetr a flu or op h en yl)p a lla d iu m (II) Com p lexesa
no.
1m
complexb
atropisomer
anti-Br,Br
%
δ2
δ4
δ5
δ6
5J 2-5
3J 4-5
3J 5-6
trans-PdR2(tht)2
50
50
52
48
52
48
50
50
-89.5
-89.5
-87.2
-87.3
-88.9
-89.1
-88.0
-88.0
-85.7
-87.2
-87.3
-87.4
-85.0
-85.1
-85.2
-88.6
-89.3
-86.1
-91.9
-91.9
-91.7
-91.7
-90.2
-130.8
-130.8
-131.9
-131.9
-131.0
-131.0
-132.1
-132.1
-132.7
-133.0
-133.0
-132.7
-133.6
-133.6
-132.1
-132.1
-132.2
-132.1
-129.9
-129.9
-129.8
-129.8
-129.8
-161.6
-161.6
-162.3
-162.3
-161.7
-161.6
-162.4
-162.5
-163.4
-162.8
-162.9
-163.6
-163.9
-163.8
-161.8
-162.3
-162.6
-162.1
-161.7
-161.7
-161.5
-161.5
-161.1
-113.3
-113.3
-110.7
-110.7
-111.5
-111.6
-111.7
-111.7
-109.6
-110.6
-110.7
-110.6
-109.2
-109.1
-112.3
-109.9
-109.2
-111.5
-115.1
-115.1
-114.8
-114.8
-113.7
12.4
12.4
flux.
flux.
20.3
20.3
flux.
flux.
31.7
31.7
flux.
flux.
syn-Br,Br
anti-Br,Br
syn-Br,Br
anti-Br,Br
syn-Br,Br
anti-Br,Br
syn-Br,Br
2m
cis-PdR2(tht)2
3m c
4m
cis-PdR2(COD)
trans-PdR2(PMe3)2
10.2
12.4
12.5
11.4
21.6
20.5
20.5
21.0
30.7
32.3
32.4
31.5
5m d
6m
trans-PdR2(CNMe)2
cis-PdR2(PMe3)2
anti-Br,Br
syn-Br,Br
e
e
11.5
9.6
20.1
21.1
20.4
20.6
21.8
21.8
21.8
21.8
20.8
20.8
31.6
29.6
31.9
30.1
30.9
30.9
30.9
30.9
28.3
28.3
7m
cis-PdR2(CNMe)2
cis-PdR2(Me2bipy)
8m d
anti-Br,Br
syn-Br,Br
anti-Br,Br
50
50
54
10.7
11.7
10.3
10.3
9.7
9.7
9.0
9.0
9m c
cis-PdR2(OPPhPy2)
[PdR(µ-Cl)(tht)]2
trans-PdRCl(tht)2
syn-Br,Br-anti-Br,P
syn-Br,Br-syn-Br,P
trans-anti-Br,Br
trans-syn-Br,Br
cis-anti-Br,Brg
27
19
35
35
15
15
10m
cis-syn-Br,Brg
11m f
10.3
20.8
29.4
a
b
d
f 3
At 282 MHz, CDCl3, 293 K: δ in ppm from CFCl3; J in Hz. R ) m-C6BrF4. c At 233K. In acetone-d6. e Signals overlap.
J
) 2.9
2-4
5
4
g
Hz, J 2-6 ) 4.0 Hz, J 4-6 ) 3.4 Hz. Broad signals.
[Pd2R2(µ-Cl)2(tht)2] + 2tht f
2trans-[PdRCl(tht)2] (6)
tives with one aryl group per palladium were pre-
pared: [Pd2R2(µ-Cl)2(tht)2] (R ) o-C6BrF4 (10o), m-C6-
BrF4 (10m )) were made by transarylation between
[PdR2(tht)2] and PdCl2 (eq 5). Cleavage of the chloro-
bridges by tht afforded trans-[PdRCl(tht)2] (11o, 11m ,
eq 6).10,11
Ch a r a ct er iza t ion of t h e Com p lexes a n d NMR
An a lysis. The complexes were characterized by el-
(11) Uso´n, R.; Fornie´s, J .; Navarro, R.; Garc´ıa, M. P. Inorg. Chim.
Acta 1979, 33, 69-75.
[PdR2(tht)2] + PdCl2 f [Pd2R2(µ-Cl)2(tht)2] (5)

(Bromotetrafluorophenyl)palladium(II) Complexes
Organometallics, Vol. 16, No. 25, 1997 5419
F igu r e 2. 19F 282 MHz COSY (CDCl3, F2 and F6 regions,
233 K) of cis-[Pd(m-C6BrF4)2(OPPhPy2)] (9m ). The cross
peaks between the F2′-F6, F2′-F2, F2-F6′, and F6-F6′
signals of the (()-anti-Br,Br isomer reveal inter-ring
couplings (indicated with arrows). The other signals are
F2 and F2′ (b) and F6 and F6′ (g), corresponding to the syn-
Br,Br-syn-Br,P isomer; F2 and F2′ (d) and F6 and F6′ (e),
arising from the syn-Br,Br-anti-Br,P isomer.
F igu r e 1. Structural formulae for atropisomers of com-
plexes cis-[Pd(o-C6BrF4)2L2]: A, cis-syn-Br,Br-[Pd(o-C6-
BrF4)2L2]; B, cis-anti-Br,Br-[Pd(o-C6BrF4)2L2]; C, cis-syn-
Br,Br-anti-Br,P-[Pd(o-C6BrF4)2(OPPy2Ph)]; D, (()-cis-anti-
Br,Br-[Pd(o-C6BrF4)2(OPPy2Ph)]; E, cis-syn-Br,Br-syn-Br,P-
[Pd(o-C6BrF4)2(OPPy2Ph)].
ture. The only exceptions are 5o and 5m for which only
one isomer is observed at room temperature (Tables 1
and 2).
emental analysis and IR and 1H, 19F, and 31P NMR
techniques. Relevant data are collected in the Experi-
mental Section and in Tables 1 and 2.
The cis-[Pd(C6BrF4)2L2] complexes are not that simple.
Even in the cases where the two C6BrF4 group are
chemically equivalent, their fluorine atoms become
magnetically inequivalent due to the existence of strong
inter-ring 19F-19F coupling and an eight spin system
must be considered. Such inter-ring couplings are
clearly seen in the 19F COSY spectrum of the anti-Br,-
Br isomer of complex 9m (Figure 2), where the Fortho
nuclei on different inequivalent m-C6BrF4 rings are
coupled to each other giving the following cross peaks:
F2-F6′ and F2′-F6 (same side of the coordination plane,
close coupling); and F2-F2′ and F6-F6′ (opposite sides,
distant coupling). In the case of the anti-Br,Br isomer
of complex 9o, only the distant coupling F6-F6′ is
observed. Since the isomers anti-Br,Br of complexes 9
are asymmetric, both rings become inequivalent, pro-
viding a 19F first-order spin system. Consequentely,
simple homodecoupling experiments allow one to mea-
sure of the inter-ring 19F-19F coupling constants (Table
3).
Neither 1H nor 31P NMR spectra were useful for
identification of the atropisomers in solution because
of coincidence of resonances. The relative orientation
of the C6BrF4 groups exerts little influence on the
chemical shifts of the other ligands. However, 19F NMR
spectroscopy is extremely precise for the detection and
identification of the atropisomers formed.
The simplest 19F spectral pattern is found in the
mono-arylated derivatives, 11o and 11m . Assignments
of the different 19F signals for these complexes were
made using 19F-19F-COSY and homonuclear 19F-decou-
pling experiments. In this way, chemical shift values
and intra-ring 19F-19F scalar coupling constants could
be determined and then used in the analysis of the more
complex spectra of the bis-arylated derivatives. The
scalar coupling constants connecting mutually meta 19F
nuclei are very small, hence the signals appear as
doublets of doublets. The 19F chemical shifts follow the
6
3
5
4
order δF > δF > δF > δF in o-C6BrF4 complexes and
δF2 > δF6 > δF4 > δF5 in m-C6BrF4 complexes. For each
C6BrF4 group, only small changes in the chemical shifts
(less than 3-4 ppm) are observed upon change of the
ancillary ligand.
The assignment of the F2 signals for both the syn-
Br,Br atropisomers of complexes 9 is made on electronic
and steric arguments: The higher frequency F2 reso-
nance is assigned to the syn-Br,Br-syn-Br,P isomer
because of the anisotropic shielding of the close Ph group
and because it is the less abundant one, in agreement
with the higher sterically hindrance. The lower F2
signal is assigned to the syn-Br,Br-anti-Br,P isomer.
The rest of the cis complexes, including the syn-Br,-
Br atropisomers of complexes 9, have at least one
element of symmetry, and generate second order spins
systems F3F3′F4F4′F5F5′F6F6′ (for the ortho isomer) or
F2F2′F4F4′F5F5′F6F6′ (for the meta isomer), where Fn and
Fn′ now stand for chemically equivalent but magneti-
In the complexes trans-[Pd(C6BrF4)2L2], both C6BrF4
groups (whether ortho or meta) are chemically equiva-
lent. Two atropisomers are present, syn-Br,Br and anti-
Br,Br, and two independent four-nuclei first-order spin
systems are observed (one for each atropisomer), show-
ing that there is no noticeable inter-ring coupling.
Although the signals overlap, the analysis of the two
overlapped systems can be made from their fine struc-
(12) Uso´n, R.; Fornie´s, J .; Navarro, R; Gallo, A. Transition Met.
Chem. 1980, 5, 284-288.

5420 Organometallics, Vol. 16, No. 25, 1997
Albe´niz et al.
turned out to be smaller) could we estimate inter-ring
coupling constants (Table 3).
The analysis of the 19F spectra reveals the occurrence
in the cis complexes of inter-ring F-F couplings ranging
from very small to 52 Hz. Through-space coupling has
been interpreted as a result of direct overlap of two lone-
pair orbitals of close fluorine atoms giving rise to a
bonding and an antibonding molecular orbital delocal-
ized over both fluorine atoms; although there is no net
F-F bonding, nuclear spin information is transmitted
through this interaction.4g When two fluoroaryl groups
are bound to a metal in a cis arrangement, the syn Fortho
atoms of both groups are very close to each other and
the orbital interaction just described becomes very
significant. This coupling sometimes leads to compli-
cated spin systems, but on the other hand, it is very
useful to identify the atropisomers formed in a cis-bis-
(aryl) complex. Thus, syn-Br,Br and anti-Br,Br atropi-
somers of the cis-[Pd(o-C6BrF4)2L2] derivatives are easily
distinguished since the former shows a large inter-ring
F6-F6′ coupling while the latter does not. The same
applies to cis-[Pd(m-C6BrF4)2L2] derivatives: The anti-
Br,Br atropisomer shows strong inter-ring coupling
between the chemically inequivalent syn F2-F6′ atoms,
while the syn atropisomer, with chemically equivalent
syn F2-F2′ or F6-F6′ atoms, lacks this coupling.
F igu r e 3. 19F 282 MHz NMR spectra (CDCl3, F6 region)
for cis-syn-Br,Br-[Pd(o-C6BrF4)2(tht)2] (2o). (a) Initial sig-
nal; (b) signal under F3,F3′ irradiation; (c) signal under
F5,F5′ decoupling. Analysis of spectra b and c allows the
determination of through-space 19F-19F coupling constants
J 6-6′
.
Ta ble 3. 19F -19F In ter -Rin g Cou p lin gs in
The probable scalar contribution to the inter-ring
coupling between Fortho atoms can be estimated in 9o
(Table 3), where these anti atoms are too distant to
expect a significant through-space contribution. This
6J F-F is found to be small (<2 Hz), as also confirmed by
the value found for 9m (3.3 Hz). Thus, most of the
contribution found for syn Fortho atoms is probably to be
attributed to through-space coupling.
cis-Bis(br om otetr a flu or op h en yl)p a lla d iu m (II)
Com p lexesa
F‚‚‚F
F‚‚‚F
no.
atropisomer
syn-Br,Br
anti-Br,Br
syn-Br,Br
anti-Br,Br
syn-Br,Br
anti-Br,Br
syn-Br,Br
anti-Br,Br
anti-Br,Br
syn-Br,Br-anti-Br,P
anti-Br,Br
close
J /Hz
distant
J /Hz
2o
F6-F6′
F6-F6′
F6-F6′
F6-F6′
23.9
F6-F6′
F6-F6′
F6-F6′
3.0
<3
3o
7o
8o
9o
9m
52.0
22.5
25.6
The magnitude of the through-space coupling is
correlated with the nonbonding F‚‚‚F distance.4a In
these cis complexes, the values of close Fortho-Fortho
couplings range from 52 to 9.9 Hz (Table 3), which would
correspond to F‚‚‚F distances ranging from about 265
to about 320 pm. These distances are reasonable for
the geometry of the complexes but reveal the existence
of distortions of an ideally square-planar coordination
with the two aryl rings perpendicular to the coordina-
tion plane. The highest values are observed for the syn-
Br,Br atropisomers of 9o (L ) OPPy2Ph) and 3o (L )
COD), and this can be explained assuming that the
crowding in the z axis of two syn ortho-bromine atoms
and the nonplanar ligands forces a tilt of the aryl groups
in order to increase the distance between Br atoms,
placing the syn Fortho atoms closer to each other. Lower
J values ranging from 25.6 to 22.5 Hz are found for the
syn-Br,Br atropisomers of the other o-C6BrF4 derivatives
involving ligands with no steric demand in the z axis
(tht, CNMe, Me2bipy).
3.7
F6-F6′
F6-F6′
4.0
<2
F6-F6′
50.9
F2-F2′
F6-F6′
3.3
3.3
F2-F6′
F2′-F6
F2-F2′
F6-F6′
F2-F2′
F6-F6′
25.5
11.9
26.9
9.9
13.2
26.2
syn-Br,Br-anti-Br,P
syn-Br,Br-syn-Br,P
a
At 282 MHz, CDCl3.
cally inequivalent nuclei. The measurement of the
inter-ring 19F-19F coupling constants is not simple in
this case. Fortunately, scalar couplings between mutu-
ally meta 19F nuclei within the o-C6BrF4 rings are close
to zero (in fact, the selective decoupling of F4 resonance
does not alter the F6 signal), so the spin system
involving F6,F6′ nuclei can be rewritten as F3F3′F5F5′F6F6′.
This six-spin system can be further reduced to just four
by using selective irradiation of a chosen F3 or F5 signal,
giving AA′XX′ patterns that are easy to analyze (Figure
3). Some values of inter-ring coupling constants could
be measured in this way (Table 3).
Two different close Fortho-Fortho coupling constants are
found for each atropisomer of the m-C6BrF4 derivative
9m , one medium and one small (26.2 and 13.2 Hz for
the syn-Br,Br isomer; 26.9 and 9.9 Hz for the anti-Br,-
Br), supporting the existence of a tilt distortion of the
aryl groups. It seems that the meta-Br atoms do not
produce severe steric hindrance since, regardless their
position, the Fortho atoms close to the capping phenyl
group of the ligand are the ones tilted away.
The approach just described is difficult to apply to the
complexes cis-[Pd(m-C6BrF4)2L2]. The scalar coupling
between mutually meta 19F nuclei for these derivatives,
4J 2-6, although small (3-4 Hz), cannot be neglected, and
the second-order spin systems cannot be simplified.
Although the distortions just discussed can explain
the observations made, it must be kept in mind that
4
Only for the syn-Br,Br isomers of 9m (for which J 2-6

(Bromotetrafluorophenyl)palladium(II) Complexes
Organometallics, Vol. 16, No. 25, 1997 5421
similar to those values found for C6F5 rotation in
[Pd(C6F5)2(OPPy2Ph)] (∆Hq ) 52.7 kJ mol-1, ∆Sq
)
-18.2 J K-1 mol-1), supporting that m-C6BrF4 and C6F5
are sterically equivalent as far as rotation is concerned.
Atropisomerization also occurs for the o-C6BrF4 de-
rivatives, but at a much slower rate. Thus, for 9o at
323 K, only broadening, but still no coalescence of the
19F signals, is observed. In fact, all of the complexes
show two atropisomers at room temperature, except 5o
which shows only one with broad lines and can be
resolved in two at lower temperatures (∆Gq
) 55.8
283
kJ mol-1). The differences in the energy of activation
between the analogous derivatives of o-C6BrF4 and
m-C6BrF4 must be related to the bulkiness of the ortho-
Br compared to the F substituent. Moreover, the fact
that 5o and 7o, both having the aryl group flanked by
one C atom in the coordination plane, present very
different rotation barriers reveals that for the o-C6BrF4
derivatives this simplification is not valid and the bulk
of the Br substituent in the direction perpendicular to
the coordination plane is the main hindrance to rotation
in the cis isomer.
Exp er im en ta l Section
All reactions involving organolithium reagents or PMe3 were
carried out under dinitrogen atmosphere. Commercial re-
agents were used without further purification. Solvents were
freshly distilled from standard drying agents. Methyl isocya-
nide (CNMe, used as a dichloromethane solution previously
titrated by 1H NMR using 4-bromoanisol as an internal
standard),15 bis(2-pyridyl)phenylphosphine-P-oxide (OPPy2-
Ph),16 cis-[PdCl2(COD)],17 and trans-[PdCl2(tht)2]18 were pre-
pared as described in the literature. Infrared spectra (KBr
pellets or Nujol mull) were recorded on a Perkin-Elmer
FT1720X spectrophotometer. Elemental C, H, and N analyses
were carried out on a Perkin-Elmer 2400 CHN microanalyzer.
NMR spectra were recorded on a Bruker ARX 300 spectrom-
eter (300 MHz for 1H; 282 MHz for 19F; 121 MHz for 31P)
equipped with a VT-100 variable-temperature unit. Temper-
ature calibration was performed before measurements using
the temperature-dependent chemical shift differences of the
1H resonances of methanol (below room temperature) or
ethylene glycol (high temperatures). Chemical shifts are
reported as δ (ppm) from TMS, CCl3F, or H3PO4.
LiR solu tion s (R ) o-C6Br F 4, m -C6Br F 4). To a stirred
solution of 1,2-C6Br2F4 (1.92 g, 6.22 mmol) in dry diethyl ether
(40 mL) at -78 °C was added dropwise, over a period of 25
min, a solution of LinBu (6.22 mmol) in Et2O (20 mL), prepared
by dilution of a commercial solution (1.6 M in n-hexane). A
green solution of Li(o-C6BrF4) was obtained, and it was stirred
for an additional 20 min at -78 °C before being reacted with
the corresponding palladium complex (see below). Solutions
of Li(m-C6BrF4) (pale pink) were prepared in the same way
starting from 1,3-C6Br2F4.
F igu r e 4. Experimental and simulated (DNMR6 program)
19F 282 MHz NMR spectra (CDCl3, F2 region) for cis-[Pd(m-
C6BrF4)2(OPPy2Ph)] (9m ). First-order atropisomerization
constants (katrop) obtained by line shape analysis are given
at each temperature. See Figure 2 for signal assignment.
other probable distortions, such as variations in the
R-Pd-R angle, elongations of the Pd-R bonds, or out-
of-plane tilting of the R group, are probably occurring
to relieve inter-ring repulsions, particularly in the o-C6-
BrF4 derivatives.
Atr op isom er iza tion . In our previous study, we
showed that krot depends on the “in-plane” size of the
ligands flanking the C6F5 group in the coordination
plane.1 The results obtained here by 19F NMR spec-
troscopy in CDCl3 solution within the accessible tem-
perature range (230-330 K) support this observation.
The m-C6BrF4 group seems to be sterically equivalent
to C6F5 as far as rotation is concerned. For L ) CNMe,
the rotation is very fast and no atropisomers are
observed even at low temperature. This is the less-
hindered rotation. For the bulky (in the plane) L2
)
Me2bipy, 2PMe3, the most hindered situations are
observed, whereas slow rotations are observed for L2 )
COD, OPPy2Ph, 2tht. A low-temperature 19F NOESY
of complex 9m shows that chemical exchange affects all
three atropisomers. Aryl group rotation is the only
process that can bring about the simultaneous inter-
conversion of every atropisomer, so this experiment
unequivocally identifies the fluxional process operating.
Line shape analysis was applied to variable-tempera-
ture 19F spectra for complex 9m . Simulation of the F2
subspectra (Figure 4) was carried out using the DNMR6
program (see Experimental Section).13,14 Atropisomer-
ization rate constants, katrop, were found at different
temperatures, and an Eyring plot gave the following
activation parameters: ∆Hq ) 56.2 ( 1.2 kJ mol-1 and
∆Sq ) -12 ( 4 J K-1 mol-1. These values are very
(14) The equilibrium between atropisomers shows a small depen-
dence on temperature. Thus, as temperature raises, the ratio of
atropisomers for complex 9m changes slightly and approaches to the
statistical distribution: syn-Br,Br-syn-Br,P:syn-Br,Br-anti-Br,P:anti-
Br,Br (two enantiomers) ) 19:27:54 (243 K); syn-Br,Br-syn-Br,P:syn-
Br,Br-anti-Br,P:anti-Br,Br (two enantiomers) ) 25:25:50 (323 K). Since
the populations of four sites are very close (ca. 25%), we assumed
identical atropisomerization rates between every pair of syn-anti
conformations.
(15) Weber, W. P.; Gokel, G. W.; Ugi, I. K. Angew. Chem., Int. Ed.
Engl. 1972, 11, 530-531.
(16) (a) OPPy2Ph: Newcome, G.; Hagen, D. C. J . Org. Chem. 1978,
947-949. (b) PPy2Ph: Espinet, P.; Go´mez-Elipe, P.; Villafan˜e F. J .
Organomet. Chem. 1993, 450, 145-150.
(17) Drew, D.; Doyle, J . R. Inorg. Synth. 1972, 13, 52-53.
(18) Uso´n, R.; Fornie´s, J .; Martinez, F.; Toma´s, M. J . Chem. Soc.,
Dalton Trans. 1980, 888-894.
(13) DNMR6; Quantum Chemical Program Exchange (QCPE 633);
Indiana University, Bloomington, IN, 1995.

5422 Organometallics, Vol. 16, No. 25, 1997
Albe´niz et al.
[P d R2(th t)2] (tr a n s R ) o-C6Br F 4 (1o), m -C6Br F 4 (1m ),
cis R ) o-C6Br F 4 (2o), m -C6Br F 4 (2m )). To a Li(o-C6BrF4)
(6.22 mmol) solution in Et2O (60 mL) at -78 °C was added
finely ground trans-[PdCl2(tht)2] (1.000 g, 2.828 mmol). The
mixture was stirred for 15 min at -78°C, and then the
temperature was allowed to increase slowly for 40 min, 2 drops
of distilled water were added, and it was evaporated to
dryness. The residue was extracted with dichloromethane (80
mL) to give a yellow solution; the CH2Cl2 solution was
evaporated to ca. 5 mL. Ethanol (5 mL) was added, and the
mixture was cooled to -28 °C and let stand for 3 h to give
pale yellow crystals, which were filtered, washed with cold
ethanol (2 × 5 mL), and air-dried (1.810 g, 87% crude yield).
The crystals were a mixture of isomers 1o and 2o which were
separated in two steps: First, fractional crystallization in
dichloromethane at -28 °C leading to two batches of pure
isomers and an intermediate crop containing the isomeric
mixture. The latter was column chromatographed (silicagel
neutral 60 mesh, CH2Cl2:n-hexane ) 1:5). After separation,
pure trans-[Pd(o-C6BrF4)2(tht)2] (1o) (1.358 g, 63%) and cis-
[Pd(o-C6BrF4)2(tht)2] (2o) (0.181 g, 11%) were obtained. 1o:
IR (cm-1, KBr) 1484 (vs), 1421 (vs), 1080 (vs), 1003 (vs), 814
(m, dCH), 5.90 (m, dCH), 2.87 (m, CCH2), 2.85 (m, CCH2).
Anal. Calcd for C20H12Br2F8Pd: C, 35.83; H, 1.80. Found: C,
35.72; H, 1.70. 12: IR (cm-1, KBr) 1504 (vs), 1462 (vs), 1113
(s), 1064 (s), 946 (s), 817 (s), 277, 260 cm-1 (m, Pd-Cl); 1H
NMR (CDCl3, 293 K) δ 6.14 (bs, 2H, dCH), 5.59 (bs, 2H, dCH),
3.66 (d, J ) 13 Hz, 2H, PdCH), 3.39 (bs, 2H), 2.96 (m, 2H),
2.69 (m, 4H), 2.38 (m, 6H), 1.95 (d, 13 Hz, 2H, PdCCH), 1.27
(m, 2H); 19F NMR (CDCl3, 293 K) δ -125.6 (bd, J ) 14 Hz,
F3), -137.2 (bd, J ) 14 Hz, F6), -156.0 (dd, J 4-5 ) 21.2 Hz,
J 5-6 ) 20.2 Hz, F5), -156.2 (dd, J 3-4 ) 20.3 Hz, J 4-5 ) 21.2
Hz, F4). Anal. Calcd for C28H24Br2Cl2F8Pd2: C, 35.18; H, 2.53.
Found: C, 34.88; H, 2.40. Complex cis-[Pd(m-C6BrF4)2(COD)]
(3m ) (71%) was obtained following method A and using
[Li(m-C6BrF4)]: IR (cm-1, KBr) 1466 (vs), 1418 (vs), 1055 (vs),
1
874 (vs), 722 (w), 675 (m); H NMR (CDCl3, 293 K) δ 5.80 (s,
dCH), 2.75 (s, CCH2). Anal. Calcd for C20H12Br2F8Pd: C,
35.83; H, 1.80. Found: C, 35.71; H, 1.89.
tr a n s-[P d R2L2] (R ) o-C6Br F 4, L ) P Me3 (4o), CNMe
(5o); R ) m -C6Br F 4, L ) P Me3 (4m ), CNMe (5m )). To a
solution of 1o (0.100 g, 0.135 mmol) in dichloromethane (20
mL) was added a slight excess of PMe3 (0.30 mL, 0.300 mmol),
and the mixture was stirred for 30 min. The resulting solution
was evaporated to dryness, and the residue was treated with
ethanol (3 mL). This afforded white 4o, which was washed
with cold ethanol (2 × 1 mL) and air-dried (0.0781 g, 81%):
IR (cm-1, KBr) 1480 (vs), 1413 (vs), 1078 (vs), 1002 (vs), 814
1
(vs), 754 (vs); H NMR (CDCl3, 293 K) δ 2.69 (m, SCH2), 1.89
(m, CCH2). Anal. Calcd for C20H16Br2F8PdS2: C, 32.52; H,
2.18. Found: C, 32.43; H, 2.22. 2o: IR (cm-1, KBr) 1485 (vs),
1423 (vs), 1086 (vs), 1008 (vs), 817 (vs), 767 (vs); 1H NMR
(CDCl3, 293 K) δ 2.90 (m, SCH2), 1.95 (m, CCH2). Anal. Calcd
for C20H16Br2F8PdS2: C, 32.52; H, 2.18. Found: C, 32.63; H,
2.17. Complexes trans-[Pd(m-C6BrF4)2(tht)2] (1m ) (25%) and
cis-[Pd(m-C6BrF4)2(tht)2] (2m ) (20%) were obtained similarly
using Li(m-C6BrF4). 1m : IR (cm-1, KBr) 1466 (vs), 1413 (vs),
1052 (vs), 1023 (vs), 871 (vs), 723 (w), 658 (s); 1H NMR (CDCl3,
293 K) δ 2.68 (m, SCH2), 1.85 (m, CCH2). Anal. Calcd for
1
(vs), 754 (vs); H NMR (CDCl3, 293 K) δ 1.40 (m, J ) 3.5 Hz,
CH3), 1.11 (m, J ) 3.5 Hz, CH3); 31P{1H} NMR (CDCl3, 293 K)
δ -13.90 (s). Anal. Calcd for C18H18Br2F8P2Pd: C, 30.26; H,
2.54. Found: C, 30.36; H, 2.44. Complexes 4m (88%), 5o
(95%), and 5m (95%) were prepared similarly. 4m : IR (cm-1
,
KBr) 1461 (vs), 1408 (vs), 1046 (vs), 1024 (vs), 866 (vs), 723
1
(w), 658 (s); H NMR (CDCl3, 293 K) δ 1.09 (m, J ) 3.6 Hz,
CH3); 31P{1H} NMR (CDCl3, 293 K) δ -13.12 (s). Anal. Calcd
for C18H18Br2F8P2Pd: C, 30.26; H, 2.54. Found: C, 30.46; H,
2.45. 5o: IR (cm-1, KBr) 2256 (vs, st CN), 1484 (vs), 1419
(vs), 1080 (vs), 1005 (vs), 818 (vs), 756 (vs); 1H NMR (acetone-
d6, 293 K) δ 3.52 (s, CH3). Anal. Calcd for C16H6Br2F8N2Pd:
C, 29.82; H, 0.94; N, 4.35. Found: C, 29.98; H, 1.15; N, 4.25.
5m : IR (cm-1, KBr) 2256 (vs, st CN), 1466 (vs), 1418 (vs), 1053
(vs), 1031 (vs), 872 (vs), 725 (m), 666 (s); 1H NMR (acetone-d6,
293 K) δ 3.55 (t, CH3). Anal. Calcd for C16H6Br2F8N2Pd: C,
29.82; H, 0.94; N, 4.35. Found: C, 30.02; H, 1.16; N, 4.84.
C
20H16Br2F8PdS2: C, 32.52; H, 2.18. Found: C, 32.56; H, 2.06.
2m : IR (cm-1, KBr) 1466 (vs), 1420 (vs), 1051 (vs), 872 (vs),
1
723 (w), 669 (m); H NMR (CDCl3, 293 K) δ 2.88 (m, SCH2),
1.87 (m, CCH2). Anal. Calcd for C20H16Br2F8PdS2: C, 32.52;
H, 2.18. Found: C, 32.21; H, 2.09.
cis-[P d R2(COD)] (R ) o-C6Br F 4 (3o), m -C6Br F 4 (3m )).
Meth od A. To a Li(o-C6BrF4) solution (6.30 mmol) at -78
°C, prepared as described above, finely ground cis-[PdCl2-
(COD)] was added (1.000 g, 2.828 mmol). The cool bath was
removed, and the mixture was stirred for 2 h while the
temperature increased. The resulting suspension was treated
with 2 drops of distilled water and evaporated to dryness. The
residue was extracted with dichloromethane (100 mL) to give
a yellow solution that was then evaporated to ca. 2 mL.
Ethanol (5 mL) was added, and the mixture was cooled to -28
°C and let stand for 3 h. A crystalline pale yellow solid was
obtained, cis-[Pd(o-C6BrF4)2(COD)] (3o), which was then fil-
tered out, washed with cold ethanol (3 × 1 mL), and air-dried
(1.138 g, 60%). Meth od B. To a Li(o-C6BrF4) solution (6.30
mmol) at -78 °C, prepared as described above, finely ground
cis-[PdCl2(COD)] was added (1.000 g, 2.828 mmol). The
mixture was stirred overnight (ca. 17 h) while the bath
temperature raised slowly. The resulting suspension was
treated with 2 drops of distilled water and evaporated to
dryness. The residue was extracted with dichloromethane
(100 mL) to give a green-yellow solution that was evaporated
to ca. 5 mL. Crystallization at -28 °C for 2 h yielded a yellow
solid [Pd2(µ-Cl)2{6-(o-C6BrF4)-η3-cyclooct-1-en-5-yl}2] (12), which
was filtered, washed with cold dichloromethane (2 × 4 mL),
and air-dried (0.4926 g, 26%). The mother liquors were then
evaporated to dryness, ethanol (5 mL) was added, and the
mixture was cooled to -28 °C. After 3 h a pale greenish solid
crystallized, which was a mixture of 3o and 12. It was filtered,
washed with cold ethanol (3 × 2 mL), and air-dried (0.777 g,
41% crude yield). The mixture was then column chromato-
graphed (silicagel neutral 60 mesh, CH2Cl2:n-hexane ) 1:5),
giving pure pale yellow cis-[Pd(o-C6BrF4)2(COD)] (3o) (0.606
g, 32%). 3o: IR (cm-1, KBr) 1484 (vs), 1419 (vs), 1089 (vs),
cis-[P d R2L2] (R ) o-C6Br F 4, L ) P Me3 (6o), CNMe (7o),
1
1/2Me2bip y (8o), /2OP P y2P h (9o), th t (2o); R ) m -C6Br F 4,
L ) P Me3 (6m ), CNMe (7m ), 1/2Me2bip y (8m ), 1/2OP P y2P h
(9m ), th t (2m )). To a solution of 3o (0.100 g, 0.149 mmol) in
dichloromethane (50 mL) was added PMe3 (0.30 mL, 0.300
mmol), and the mixture was stirred for 30 min. The solution
was evaporated to dryness, and the residue was triturated with
n-hexane (3 mL). This afforded white cis-[Pd(o-C6BrF4)2-
(PMe3)2] (6o), which was washed with n-hexane (3 × 1 mL)
and air-dried (0.096 g, 90%): IR (cm-1, KBr) 1484 (vs), 1419
1
(vs), 1079 (vs), 1004 (vs), 812 (vs), 760 (vs); H NMR (CDCl3,
293 K) δ 1.33 (m, J H-P ) 8.54 Hz, CH3), 1.31 (m, J H-P ) 8.54
Hz, CH3); 31P{1H} NMR (CDCl3, 293 K) δ -19.80 (bs, 2P),
-20.3 (bs, 2P). Anal. Calcd for C18H18Br2F8P2Pd: C, 30.26;
H, 2.54. Found: C, 30.41; H, 2.77. Complexes 6m , 7o, 7m ,
8o, 8m , 9o, 9m , 2o, and 2m were prepared similarly in yields
higher than 90% but using the suitable complex cis-[PdR2-
(COD)] (3) and ligand L (200% excess of tht was used for
displacing COD). 6m : IR (cm-1, KBr) 1462 (vs), 1414 (vs),
1044 (vs), 867 (vs), 736 (m), 661 (m); 1H NMR (CDCl3, 293 K)
δ 1.30 (m, J H-P ) 8.79 Hz, CH3); 31P{1H} NMR (CDCl3, 293 K)
δ -18.77 (m). Anal. Calcd for C18H18Br2F8P2Pd: C, 30.26; H,
2.54. Found: C, 30.36; H, 2.48. 7o: IR (cm-1
, KBr) 2249 (vs,
st CN), 1484 (vs), 1417 (vs), 1080 (vs), 1006 (vs), 822 (s), 761
1
(s); H NMR (CDCl3, 293 K) δ 3.38 (s, CH3). Anal. Calcd for
C
16H6Br2F8N2Pd: C, 29.82; H, 0.94; N, 4.35. Found: C, 30.21;
H, 1.27; N, 4.22. 7m : IR (cm-1, KBr) 2241 (vs, st CN), 1462
1
(vs), 1420 (vs), 1053 (vs), 872 (vs), 724 (m), 668 (s); H NMR
1
1008 (vs), 820 (vs), 766 (vs); H NMR (CDCl3, 293 K) δ 5.95
(CDCl3, 293 K) δ 3.38 (t, CH3). Anal. Calcd for C16H6Br2F8N2-

(Bromotetrafluorophenyl)palladium(II) Complexes
Organometallics, Vol. 16, No. 25, 1997 5423
Pd: C, 29.82; H, 0.94; N, 4.35. Found: C, 30.16; H, 1.18; N,
4.72. 8o: IR (cm-1, KBr) 1484 (vs), 1418 (vs), 1083 (vs), 1006
(vs), 823 (s), 766 (s); 1H NMR (CDCl3, 293 K) δ 7.90 (s, H-Py),
7.80 (d, J ) 5.6 Hz, H-Py), 7.78 (d, J ) 5.6 Hz, H-Py), 7.25 (d,
J ) 5.6 Hz, H-Py), 7.22 (d, J ) 5.6 Hz, H-Py), 2.53 (s, CH3).
Anal. Calcd for C24H12Br2F8N2Pd: C, 38.61; H, 1.62; N, 3.75.
Found: C, 39.00; H, 1.63; N, 3.68. 8m : IR (cm-1, KBr) 1462
SCH2), 2.01 (m, CCH2). Anal. Calcd for C14H16BrClF4S2Pd:
C, 30.79; H, 2.95. Found: C, 30.34; H, 2.71. 11m was obtained
from 10m in similar yield: IR (cm-1, KBr) 1467 (vs), 1423 (vs),
1062 (vs), 1060 (vs), 876 (vs), 725 (m), 675 (m), 314 (s, st Pd-
Cl); 1H NMR (CDCl3, 293 K) δ 3.03 (bs, SCH2), 2.02 (bs, CCH2).
Anal. Calcd for
C14H16BrClF4S2Pd: C, 30.79; H, 2.95.
Found: C, 30.48; H, 2.61.
1
(vs), 1418 (vs), 1056 (vs), 872 (vs), 724 (m), 675 (s); H NMR
Kin etic Stu d ies on th e Isom er iza tion of tr a n s-[P d R2-
(th t)2] (1o, 1m ). NMR tubes (5 mm) were charged with CDCl3
solutions (10-2 mol L-1) of complexes 1o or 1m and placed into
a thermostated probe at 318.2 K (( 0.2 K). Conversion(x)-
time(t) data were then acquired from 1H NMR SCH2 signal
areas for the cis and trans isomers and fitted to equation ln-
(x) vs t, getting first-order constants kiso (standard errors are
also given).
(acetone-d6, 293 K) δ 8.50 (s, H-Py), 7.92 (d, J ) 5.6 Hz, H-Py),
7.47 (d, J ) 5.6 Hz, H-Py), 2.57 (s, CH3). Anal. Calcd for
C
24H12Br2F8N2Pd: C, 38.61; H, 1.62; N, 3.75. Found: C, 39.01;
H, 1.98; N, 3.59. 9o: IR (cm-1, KBr) 1485 (vs), 1419 (vs), 1086
(vs), 1010 (s), 821 (s), 766 (s); 31P{1H} NMR (CDCl3, 293 K) δ
20.72 (s). Anal. Calcd for C28H13Br2F8N2OPPd: C, 39.91; H,
1.56; N, 3.33. Found: C, 40.25; H, 1.91; N, 3.21. 9m : IR
(cm-1, KBr) 1467 (vs), 1418 (vs), 1058 (vs), 1022 (vs), 872 (vs),
733 (s), 673 (s); 31P{1H} NMR (CDCl3, 293 K) δ 20.26 (s). Anal.
Calcd for C28H13Br2F8N2OPPd: C, 39.91; H, 1.56; N, 3.33.
Found: C, 40.26; H, 1.95; N, 3.04.
Sim u la tion of NMR Sp ectr a . First-order rate constants
for atropisomerization katrop were obtained from line shape
analysis by matching the observed variable-temperature 19F
NMR spectra for complex 9m with those simulated using the
DNMR6 program.13 The temperature dependence of the
chemical shifts was analyzed in the slow exchange region, and
a linear dependence was found; δ values for the coalescence
region were calculated according to this linear correlation and
used in the simulation. The rate constant matrix was con-
structed assuming that the exchange rates which involve the
concerted rotation of two rings are negligible, whereas ex-
change rates involving single rotations are of the same value.
An Eyring plot of ln(katrop/T) vs 1/T was represented. Activa-
tion parameters, ∆Hq and ∆Sq, were calculated from the slope
and the intercept, respectively, of the best fit line drawn by a
least-squares analysis. Uncertainties in the activation pa-
rameters were calculated from the uncertainties of the slope
and the intercept of the best fit line.
[P d 2R 2(µ-Cl)2(t h t )2] (R ) o-C6Br F 4 (10o), m -C6Br F 4
(10m )). A mixture of 1o (0.200 g, 0.271 mmol) and PdCl2
(0.048 g, 0.271 mmol) was refluxed in acetone (30 mL) for 2 h.
The resulting warm yellow solution was filtered through Celite
and evaporated to dryness to give yellow 10o, which was
washed with diethyl ether, filtered, and air-dried (0.211 g, 85%
yield): IR (cm-1, KBr) 1489 (vs), 1434 (vs), 1092 (vs), 1012 (vs),
826 (vs), 776 (vs), 288 (s, st Pd-Cl); 1H NMR (acetone-d6, 293
K) δ 2.89 (bs, SCH2), 2.06 (bs, CCH2). Anal. Calcd for
C
20H16Br2Cl2F8S2Pd2: C, 26.23; H, 1.76. Found: C, 26.16; H,
1.76. 10m was prepared similarly from 1m (81% yield): IR
(cm-1, KBr) 1471 (vs), 1424 (vs), 1061 (vs), 1046 (vs), 872 (vs),
1
726 (m), 676 (s), 292 (s, st Pd-Cl); H NMR (acetone-d6, 293
K) δ 2.95 (bs, SCH2), 2.05 (bs, CCH2). Anal. Calcd for
C
20H16Br2Cl2F8S2Pd2: C, 26.23; H, 1.76. Found: C, 26.49; H,
1.84.
tr a n s-[P d R Cl(t h t )2] (R ) o-C6Br F 4 (11o), m -C6Br F 4
Ack n ow led gm en t. We are very grateful to Dr. J .
A. Casares and Dr. J . M. Mart´ınez for helpful discus-
sions. Financial support was granted by the Direccio´n
General de Investigacio´n Cient´ıfica y Te´cnica (Project
No. PB96-0363) and the J unta de Castilla y Leo´n
(Project No. VA 40-96). The Ministerio de Educacio´n y
Ciencia is gratefully acknowledged for a fellowship to
A.L.C.
(11m )). To a suspension of 10o (0.210 g, 0.230 mmol) in
acetone (60 mL) was added tht (0.06 mL, 0.680 mmol). The
resulting solution was stirred for 40 min and evaporated to
dryness. The residue was triturated with n-hexane (5 mL)
and stored at -28 °C for 2 h. Yellow trans-[Pd(o-C6BrF4)Cl-
(tht)2] (11o) was obtained, which was filtered, washed with
cold n-hexane (3 × 1 mL), and air-dried (0.153 g, 61%): IR
(cm-1, KBr) 1485 (vs), 1424 (vs), 1082 (s), 1010 (vs), 822 (vs),
768 (s), 303 (s, st Pd-Cl); 1H NMR (CDCl3, 293 K) δ 3.02 (bs,
OM970529D