Methyl-to-double bond migration in methylene arenium rhodium complexes

Article abstract of DOI:10.1021/om990966n

The novel methylene arenium Rh(I)-Me complexes 5 and 10 were prepared by selective protonation of the corresponding xylylene complexes with HBF₄ in cold pentane. These complexes are stable both in the solid state and in solution. Upon reaction with carbon monoxide, the corresponding CO adducts 11 are obtained, which undergo under ambient conditions selective methyl migration to the coordinated double bond of the methylene arenium moiety. This provides an uncommon example of a migratory insertion process in a saturated complex that is promoted by the incoming ligand. No evidence for a concurrent methyl migration to the CO ligand was observed, indicating that the methyl-to-olefin migration is thermodynamically preferred over the carbonylation process. The observed activation entropy (ΔS^? = -36.1 eu) is in good agreement with an organized transition state for the migration process.

Full text of DOI:10.1021/om990966n

Organometallics 2000, 19, 2341-2345
2341
Meth yl-to-Dou ble Bon d Migr a tion in Meth ylen e Ar en iu m
Rh od iu m Com p lexes
Arkadi Vigalok and David Milstein*
Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel
Received December 7, 1999
The novel methylene arenium Rh(I)-Me complexes 5 and 10 were prepared by selective
protonation of the corresponding xylylene complexes with HBF4 in cold pentane. These
complexes are stable both in the solid state and in solution. Upon reaction with carbon
monoxide, the corresponding CO adducts 11 are obtained, which undergo under ambient
conditions selective methyl migration to the coordinated double bond of the methylene
arenium moiety. This provides an uncommon example of a migratory insertion process in a
saturated complex that is promoted by the incoming ligand. No evidence for a concurrent
methyl migration to the CO ligand was observed, indicating that the methyl-to-olefin
migration is thermodynamically preferred over the carbonylation process. The observed
activation entropy (∆Sq ) -36.1 eu) is in good agreement with an organized transition state
for the migration process.
In tr od u ction
ing the highly electron deficient methylene arenium
olefin ligand. These complexes, stable otherwise, upon
addition of CO undergo irreversible methyl migration
to the olefin, yielding the corresponding ethyl arenium
Rh(I) complexes, representing an unexpected ligand-
promoted migratory insertion process. Moreover, al-
though instances of Rh-alkyl bond carbonylation are
well documented,9 in our case we found no evidence for
methyl migration to the coordinated CO, while a rare,
facile, thermodynamically driven methyl migration to
the coordinated double bond of the arenium moiety is
observed.
Methylene arenium metal complexes represent a new
family of organometallic compounds that contain a
nonlabile, highly electron deficient olefin ligand.1 The
strong back-bonding of a late transition metal center,
such as monovalent Rh or Ir, to the positively charged
arenium ligand results in an increase of the electrophi-
licity of the metal center. Electrophilic (normally cat-
ionic) metal complexes are highly active in various
catalytic transformations involving ligand migration to
a coordinated double bond, such as in the polymerization
of olefins.2 For the migratory insertion to take place,
the two interacting ligands have to occupy cis-positions.3
The created empty coordination site in the reaction
product makes the â-hydrogen elimination from the
newly formed alkyl group an easy process. Addition of
another ligand can drive the reaction toward the inser-
tion product.4 When carbon monoxide is used, CO
insertion to give the corresponding acyl complex is likely
to take place. This is also the case when both possibili-
ties, alkene polymerization and CO-olefin copolymer-
ization, are available.5,6 In contrast to the widespread
insertion of olefins into the Rh-H bond, examples of
olefin insertion into a Rh-Me bond are extremely
rare.7,8 Here we present the synthesis and ligand
insertion chemistry of Rh(I) methyl complexes contain-
Resu lts a n d Discu ssion
P r ot on a t ion of Xylylen e R h (I) Met h yl Com -
p lexes. We have recently reported the synthesis and
characterization of the o- and p-xylylene Rh(I) com-
plexes 1 and 2, respectively.1 Reaction of a 1:1 mixture
of these two complexes with an excess (2.5-3 equiv) of
trifluoromethane sulfonic acid (triflic acid, HOTf) in
methylene chloride resulted in the arenium complex 3,
which was fully spectroscopically characterized and was
shown to have most of the positive charge ring local-
ized.1 When the same mixture was reacted with another
strong acid, HBF4, in pentane at -30 °C, a brown
precipitate was immediately formed. NMR analysis in
CDCl3 showed two products in an approximate ratio of
1:3: the dicationic methylene arenium complex 4,
similar to 3, and the methylene arenium Rh methyl
complex 5 (Scheme 1).
(1) Vigalok, A.; Shimon, L. J . W.; Milstein, D. J . Am. Chem. Soc.
1998, 120, 477.
(2) For a recent review see: Britovsek, G. J . P.; Gibson, V. C.; Wass,
D. F. Angew. Chem., Int. Ed. 1999, 38, 428.
(3) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G. In
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987.
(4) Brown, C. K.; Wilkinson, G. J . Chem. Soc. A 1970, 2753.
(5) For a recent review see: Drent, E.; Budzelaar, P. H. M. Chem.
Rev. 1996, 96, 663.
(6) For comparative analysis of alkyl migration to olefin and carbon
monoxide see: Rix, F. C.; Brookhart, M. J . Am. Chem. Soc. 1995, 117,
1137.
The 31P NMR spectrum of complex 5 exhibits a
doublet at 43.41 ppm (J RhP ) 131.4 Hz). The protons of
(8) For an example of Rh(III)-catalyzed alkene polymerization see:
(a) Flood, T. C.; Wang, L. J . Am. Chem. Soc. 1992, 114, 3169. (b) Wang,
L.; Lu, R. S.; Bau, R.; Flood, T. C. J . Am. Chem. Soc. 1993, 115, 6999.
(9) (a) Forster, D. J . Am. Chem. Soc. 1976, 98, 846. (b) Moloy, K.
G.; Wegman, R. W. Adv. Chem. Ser. 1992, 230, 323. (c) Oswald, A. A.;
Hendriksen, D. E.; Kastrup, R. V.; Mozeleski, E. J . Adv. Chem. Ser.
1992, 230, 395. (d) Ojima, I.; Zhang, A.; Korda, A.; Ingallina, P.; Clos,
N. Adv. Chem. Ser. 1992, 230, 277.
(7) The kH/kAlkyl ratio in a Rh(III) olefin complex was found to be
more than 106: Brookhart, M.; Hauptman, E.; Lincoln, D. M. J . Am.
Chem. Soc. 1992, 114, 10394.
10.1021/om990966n CCC: $19.00 © 2000 American Chemical Society
Publication on Web 05/12/2000

2342 Organometallics, Vol. 19, No. 12, 2000
Vigalok and Milstein
Sch em e 1
F igu r e 1. ORTEP view of a molecule of 7a with the
thermal ellipsoids at 50% probability. The hydrogen atoms
are omitted for clarity.
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 7a
(a) Bond Lengths
Rh(1)-C(1)
Rh(1)-C(11)
C(1)-C(11)
2.071(3)
2.134(3)
1.407(6)
Rh(1)-O(5)
Rh(1)-P(2)
Rh(1)-P(3)
2.146(2)
1.3557(12)
1.3626(11)
(b) Angles
P(2)-Rh(1)-P(3)
P(3)-Rh(1)-C(1)
P(3)-Rh(1)-C(11)
C(1)-Rh(1)-C(11)
163.10(3)
90.04(11) C(11)-C(1)-Rh(1)
84.23(11) C(1)-C(11)-C(12) 118.6(3)
39.1(2) O(5)-Rh(1)-P(3) 90.53(8)
C(1)-C(11)-Rh(1)
68.1(8)
72.9(2)
the coordinated double bond appear in the 1H NMR
spectrum as a triplet at 3.89 ppm, whereas the metal-
bound methyl group gives rise to a triplet of doublets
centered at 1.70 ppm. When 5 was subjected to a large
(10-20 equiv) excess of HBF4 in methylene chloride, a
rather slow reaction took place, complex 5 being con-
verted to 4 within several hours. This was surprising,
as we expected protonation of an unsaturated Rh(I)
methyl complex with a strong acid, followed by methane
reductive elimination, to be facile processes.10 For
comparison, we reacted complex 6,11 which is analogous
to 5, with strong acids (eq 1). The reactions were
instantaneous, with the color changing from dark red
to bright yellow due to formation of the corresponding
Rh-A complexes 7.
(1)-C(1) and Rh(1)-C(11) being 2.071(3) and 2.134(3)
Å, respectively. This asymmetric bonding is similar to
the one observed with the corresponding more rigid
chelating methylene cyclohexadiene Rh(I) complex.13
As complex 6 is extremely reactive toward strong
acids, we attribute the relative inertness of 5 to the
electron-withdrawing effect of the arenium ring. The
two reaction centers in complexes 1 and 2sthe exocyclic
methylene groups and the rhodium centersare both
accessible to proton attack, with attack at the former
site being in general a least 3 times more preferable.
As the methylene groups in (the mixture of) 1 and 2
could differ in their proton affinity, we reacted the
o-xylylene complex 814 with an excess of HBF4 in
pentane at -30 °C in order to directly compare the
reactivity of the metal center and the exocyclic meth-
ylene in the protonation reaction. A 1:3 mixture of the
corresponding dicationic arenium complex 9 and the
methyl arenium Rh(I) complex 10, respectively, im-
mediately precipitated as a brown solid (Scheme 2).
Complexes 9 and 10 possess NMR features similar to
those of complexes 4 and 5, respectively. Since the
reaction of complex 10 with an excess of HBF4 is
relatively slow and occurs only within a few hours to
give 9, we can assume that in the xylylene Rh(I) methyl
complexes the total reactivity of the exocyclic methylene
groups is approximately 3 times higher toward proto-
nation than that of the metal center, as manifested in
the ratio between complexes 9 and 10. Presumably, the
unobserved cationic xylylene Rh(I) complex A reacts
immediately with H+ at the exocyclic methylene group
(1)
The tosylate complex 7a was crystallized from
benzene, giving orange plates. An X-ray structural
analysis of 7a shows weak coordination of the tosylate
anion to the rhodium center, Rh(1)-O(5) 2.146(2) Å
(Figure 1). Selected bond distances and bond angles are
given in Table 1. The coordinated double bond length
of 1.407(6) Å is similar to that observed for other Rh(I)
PCP type olefin complexes.11-13 Noteworthy, the rhod-
ium atom is shifted along the double bond toward the
least substituted carbon atom with the bond lengths Rh-
(10) For facile Rh protonation and C-H reductive elimination in a
similar chelating system see: Hahn, C.; Spiegler, M.; Herdtweck, E.;
Taube, R. Eur. J . Inorg. Chem. 1998, 1425.
(11) Vigalok, A.; Kraatz, H.-B.; Konstantinovsky, L.; Milstein, D.
Chem. Eur. J . 1997, 3, 253.
(12) Vigalok, A.; Shimon, L. J . W.; Milstein, D. J . Chem. Soc. Chem.
Commun. 1996, 1673.
(13) Vigalok, A.; Milstein, D. J . Am. Chem. Soc. 1997, 119, 7873.
(14) Vigalok, A.; Rybtchinski, B.; Shimon, L. J . W.; Ben-David, Y.;
Milstein, D. Organometallics 1999, 19, 895.

Methyl-to-Double Bond Migration
Organometallics, Vol. 19, No. 12, 2000 2343
Sch em e 2
F igu r e 2.
Sch em e 4
Sch em e 3
Sch em e 5
methyl arenium complexes 5 and 10 no methyl migra-
tion to the olefin takes place until carbon monoxide is
added. Interestingly, no subsequent â-hydrogen elimi-
nation occurs in 12, showing that the migration product
is the thermodynamically stable product in this system.
The methyl group migration to the double bond of the
arenium moiety is obviously promoted by the CO
coordination. To make the migratory insertion process
possible, the two reacting groups must be mutually cis
to each other. The incoming CO could provide this type
of geometry. However, NOE analysis of 11 at low
temperatures showed no mutual effect of the methylene
and methyl group protons. Although lack of an observed
effect cannot rule out the possibility of the mutual cis
orientation in 11, it might be indicative of an unfavor-
able configuration. Alternatively, the necessary cis
positioning might be achieved by rapid insertion of the
CO ligand into the Rh-Me bond followed by decarbo-
nylation with an exchange in positions between the CO
and Me (Scheme 5). Irreversible methyl migration to
the double bond would drive the whole process toward
complex 12. However, we did not detect any (CO
migration) reactivity upon cooling of 11 to -65 °C, as
monitored by multinuclear NMR techniques. 31P NMR
follow-up of the conversion of 11 into 12 showed a
smooth transformation that obeyed first-order kinetics.
Running the reaction under 5 atm of CO had no effect
on the reaction rate, indicating that there is no CO
to give the dicationic methylene arenium complexes
(Scheme 3).
Meth yl-to-Olefin Migr a tion In d u ced by Ca r bon
Mon oxid e. When CO was bubbled through a solution
of 5 or 10 in CDCl3 for several seconds, formation of
the CO adduct 11 was immediately observed15 with a
color change from brown to red. The 31P NMR spectrum
of 11a exhibits a doublet at 56.45 ppm with the J RhP
)
1
100.7 Hz. The H NMR spectrum of 11a shows signals
attributed to both the coordinated methylene group (t,
3.60 ppm) and the Rh-CH3 group (td, 1.77 ppm). Upon
standing overnight at room temperature, clean conver-
sion of the CO adduct to the corresponding product of
methyl migration to the double bond, complex 12, took
place (Scheme 4).
The p-methyl-substituted complex 12a has been
subjected to a single-crystal X-ray analysis. The crystal
structure of 12a has already been reported, and it was
shown that the real representation of 12a is a combina-
tion of two resonance forms: the ethyl arenium form I
and the C-C agostic Rh(+) form II (Figure 2).14
Although there are many examples of rhodium-
catalyzed carbonylation reactions, examples of olefin
polymerization or oligomerization assisted by rhodium
complexes are exceedingly rare.8 In the case of the
(15) The dicationic CO rhodium adducts from 4 or 9 were also
observed (ref 14).

2344 Organometallics, Vol. 19, No. 12, 2000
Vigalok and Milstein
5: 31P{1H} NMR (C6D6; δ, ppm) 43.41 (δ, J RhP ) 131.4 Hz);
1H NMR 3.89 (td, J PH ) 8.5 Hz, J RhH ) 1.5 Hz, 2H, CdCH2),
3.23 (AB m, 4H, CH2-P), 2.36 (s, 6H, Ar-CH3), 1.95 (s, 3H,
Ar-CH3), 1.70 (td, J PH ) 5.3 Hz, J RhH ) 1.7 Hz, 3H, Rh-CH3),
1.49 (vt, J PH ) 6.5 Hz, 18H, t-Bu), 1.23 (vt, J PH ) 6.6 Hz, 18
H, t-Bu); 13C{1H} NMR 161.26 (br s, Ar), 143.60 (s, Ar), 134.58
(t, J PC ) 4.3 Hz, Ar), 110.66 (td, J PC ) 4.0 Hz, J RhC ) <1 Hz,
CdCH2), 41.47 (m, CdCH2), 36.5 (m, two overlapped C(CH3)3),
30.45 (t, J PC ) 2.8 Hz, C(CH3)3), 29.26 (br s, C(CH3)3), 22.57
(t, J PC ) 9.5 Hz, CH2-P), 19.36 (s, Ar-CH3), 16.95 (s, 2 Ar-
CH3), 5.54 (dt, J RhC ) 32.0 Hz, J PC ) 3.7 Hz, Rh-CH3).
dissociation at the preequilibrium stage. Kinetic mea-
surements at different temperatures yielded the activa-
tion parameters ∆Hq ) 12.6 kcal/mol; ∆Sq ) -36.1 eu.
As expected for a highly organized transition state, the
entropy of activation is considerably negative.
Thus, the overall methyl migration to the double bond
in methylene arenium rhodium complexes is promoted
by coordination of an extra ligand (carbon monoxide).
The reaction is irreversible, complex 12 being stable in
both the solid state and in solution.
Ca r bon yla tion of 5. CO (1 mL) was briskly bubbled
through a CDCl3 solution of 5 (in a mixture with the dicationic
4), resulting in the immediate color change from brown to red.
The 31P and 1H NMR spectroscopy revealed clean conversion
of 5 to the new CO adduct 11a : 31P{1H} NMR (CDCl3; δ, ppm)-
56.45 (d, J RhP ) 100.7 Hz); 1H NMR 3.60 (br t, J PH ) 10.0 Hz,
2H, CdCH2), 3.35 (AB quart., J HH ) 15.7 Hz, 4H, CH2-P),
Su m m a r y
We have prepared 16e Rh(I) methyl complexes con-
taining the electron-deficient methylene arenium moi-
ety. These complexes do not undergo methyl migration
to the olefin. Addition of carbon monoxide results in
saturated CO adducts which slowly undergo migration
of the methyl ligand to the methylene group, giving
ethyl arenium Rh(I) carbonyl complexes. Thus, it is
demonstrated that the incoming ligand (CO) actually
promotes the migratory insertion process. Unexpectedly,
alkyl migration to the coordinated double bond is
thermodynamically more favorable over the alkyl group
migration to CO (to give the corresponding metal-acyl
complex), which was not observed under the reaction
conditions. The observed highly negative value for the
entropy of activation indicates an organized transition
state for the methyl migration process.
2.28 (s, 6H, 2 Ar-CH3), 2.10 (s, 3H, Ar-CH3), 1.77 (td, J PH
)
4.1 Hz, J RhH ) 3.1 Hz, 3H, Rh-CH3), 1.49 (vt, J PH ) 6.5 Hz,
18H, t-Bu), 1.28 (vt, J PH ) 6.7 Hz, 18 H, t-Bu).
Con ver sion of 11a in to 12a . Upon standing at room
temperature for 8 h, the CO adduct 11a was quantitatively
converted into the ethyl arenium complex 12a : 31P{1H} NMR
(CDCl3; δ, ppm) 20.78 (d, J RhP ) 102.5 Hz); 1H NMR 3.73 (AB
m, 4H, CH2-P), 3.51 (quart., J HH ) 7.1 Hz, 2H, CH3CH2), 2.39
(t, J PH ) 1.8 Hz, 6H, 2 Ar-CH3), 2.16 (s, 3H, Ar-CH3), 1.47
(vt, J PH ) 7.2 Hz, 18H, t-Bu), 1.23 (vt, J PH ) 7.1 Hz, 18 H,
t-Bu), 0.81 (t, J HH ) 7.1 Hz, 3H, CH2CH3). Complexes 11b and
12b were prepared analogously to complexes 11a and 12a ,
respectively, and had nearly identical NMR spectra.
P r oton a tion of Com p lex 6. To a red solution of 6 (20 mg,
0.041 mmol) in 1 mL of THF was added 8 mg (0.042 mmol) of
HOTos‚H2O in 2 mL of THF, resulting in an immediate color
change to yellow. The solvent was evaporated, and the result-
ing yellow solid was washed with pentane, giving 25 mg (95%)
of pure 7a : 31P{1H} NMR (C6D6; δ, ppm) 64.82 (d, J RhP ) 129.3
Hz); 1H NMR 8.07 (d, J HH ) 7.5 Hz, 2H), 6.87 (br s, 2H), 2.23
(br s, 2H, doublet in 1H{31P} spectrum, J RhH ) 2.2 Hz, Rh-
(CH2dC)), 1.96 (s, 3H), 1.26 (vt, J PH ) 6.5 Hz, 18H, t-Bu), 1.21
Exp er im en ta l Section
Gen er al P r ocedu r es. All operations with air- and moisture-
sensitive compounds were performed in
a nitrogen-filled
glovebox (Vacuum Atmospheres with an MO-40 purifier). All
solvents were reagent grade or better. Pentane, benzene, and
THF were distilled over sodium/benzophenone ketyl. All
solvents were degassed and stored under high-purity nitrogen
after distillation. All deuterated solvents (Aldrich) were stored
under high-purity nitrogen on molecular sieves (3 Å).
1H, 31P, and 13C NMR spectra were recorded at 400, 162,
and 100 MHz, respectively, using a Bruker AMX400 spec-
trometer. 1H and 13C chemical shifts are reported in ppm
downfield from TMS and referenced to the residual solvent h1
(7.24 ppm chloroform-d, 7.15 ppm benzene-d6) and all-d solvent
peaks (77.00 ppm chloroform, 128.00 ppm benzene), respec-
tively. 31P chemical shifts are in ppm downfield from H3PO4
and referenced to an external 85% phosphoric acid sample.
All measurements were performed at 20 °C unless otherwise
specified.
Syn th esis a n d Ch a r a cter iza tion of Com p ou n d s 5-12.
Syn th esis of th e Rh -Me Meth ylen e Ar en iu m Com p lex
5. To a pentane solution (2 mL) of a 1:1 mixture of the o- and
p-xylylene Rh-Me complexes 1 and 21 (20 mg, 0.036 mmol),
respectively, was added 1.5 equiv of HBF4 (in dioxane solution).
The clear red solution immediately turned turbid yellow, and
a dark brown oily precipitate was deposited on the walls of
the reaction vial. The pentane layer was decanted, and the
resulting solid was washed twice with pentane (2 × 1 mL) and
then dissolved in CH2Cl2. Filtration and evaporation of the
solvent gave a mixture of 5 and the dicationic Rh(I) complex
4 (this cationic complex possesses the same NMR data as 3)
in a ratio of 3:1, as was revealed by the 31P{1H} NMR
spectroscopy. Owing to the very similar solubility properties
in organic solvents (methylene chloride, THF) of complexes 4
and 5, it was impossible to separate them by solubility
differences in these solvents or by fractional crystallization
from mixtures of these solvents with pentane.
(vt, J PH ) 6.1 Hz, 18H, t-Bu); 13C{1H} NMR 77.07 (dt, J RhC
18.1 Hz, J PC ) 3.7 Hz, Rh(CH2dC)), 34.01 (d of br t, J RhC
)
)
17.9 Hz, Rh(CH2dC)), 35.60 (m, CH2(CH2-P) overlapped with
C(CH3)3)), 34.96 (td, J RhC ) 1.0 Hz, J PC ) 6.7 Hz, C(CH3)3),
30.56 (t, J PC ) 2.6 Hz, C(CH3)3)), 29.50 (t, J PC ) 2.9 Hz,
C(CH3)3)), 21.11 (s, CH3-Ar), 16.89 (br t, J PC ) 7.2 Hz,
CH2-P).
Complexes 7b and 7c were prepared analogously and
possess similar NMR data. Complex 7c: Anal. Found (Calc)
C 47.21 (46.99), H 8.40 (8.25).
P r oton a tion of Com p lex 8. The methylene arenium
complex 8 was prepared analogously to the closely related
complex 11 by reacting the corresponding chloride complex14
with MeLi. Consequent addition of 3-5 equiv of HBF4 in
pentane at -30 °C resulted in formation of complexes 9 and
10 in a 3:1 ratio. Complexes 9 and 10 show NMR signals nearly
identical to those of 4 and 5, respectively. As with complexes
4 and 5, attempts to separate between 9 and 10 were not
successful because of their very similar solubility properties.
X-r a y Str u ctu r a l An a lysis of 7a . Complex 7a was crystal-
lized from benzene at room temperature. Crystal data:
C
29H53O3P2RhS, orange plates, 0.3 × 0.3 × 0.1 mm3, mono-
clinic, P21, a ) 9.745(2) Å, b ) 14.841(3) Å, c ) 11.449(3) Å, â
) 109.04(3)°, from 25 reflections, T ) 110 K, V ) 1565.2(5)
Å3, Z ) 2, Fw ) 646.62, Dc ) 1.372 Mg/m3, µ ) 0.742 mm-1
.
Data collection and treatment: Rigaku AFC5R four-circle
diffractometer, Mo KR, graphite monochromator (λ ) 0.71073
Å), 7530 reflections collected, 1.88° e θ e 27.64°, -12 e h e
12, 0 e k e 19, -14 e l e 14, ω scan method, scan width )
1.2°, scan speed 12°/min, typical half-height peak width )

Methyl-to-Double Bond Migration
Organometallics, Vol. 19, No. 12, 2000 2345
0.25°, 3 standards were collected 39 times each, with a 3%
MINERVA Foundation, Munich, Germany. D.M. is the
holder of Israel Matz Professorial Chair of Organic
Chemistry. We thank Dr. L. J . W. Shimon for perform-
ing the X-ray structural analysis of complex 7a .
change in intensity, 3714 independent reflections (Rint
0.036).
)
Solution and refinement: The structure was solved by direct
methods (SHELXS-92). Full-matrix least-squares refinement
was based on F2 (SHELXL-93). Hydrogens were calculated
from difference Fourier map and refined in a riding mode with
the exception of H1A and H1B on C1, which were located and
refined independently. A total of 346 parameters with one
restraint, final R1 ) 0.0275 (based on F2) for data with I >
2σI and R1 ) 0.0304 for all data based on 3708 reflections,
goodness-of-fit on F2 ) 1.062, largest electron density ) 0.834
Su p p or tin g In for m a tion Ava ila ble: Text describing the
crystal structure determination of 7a , an ORTEP diagram of
7a , tables of crystal data and structure refinement details,
atomic coordinates, bond lengths and angles, anisotropic
displacement parameters, and hydrogen atom coordinates for
complex 7a ; copies of 1H NMR spectra of complexes 4, 5 (in a
mixture), 11a , and 12a . This material is available free of
charge via the Internet at http://pubs.acs.org.
e/Å-3
.
Ack n ow led gm en t. This work was supported by the
Israel Science foundation, J erusalem, Israel, and by the
OM990966N