Optically active bimetallic complexes with a tantalum atom as a chiral center. synthesis, characterization, and x-ray structures of two diastereoisomers: Cp′CpTa*(CO)(μ-PMe2)W(CO)4L* (Cp′ = 1-tBu-3-4-Me2-C5H2; L* = (R)-(+)-Phenyl(o-anisyl)methylphosphine (PAMP))

Article abstract of DOI:10.1021/om9600497

Reaction of the racemic metallophosphine Cp′CpTa(CO)(PMe₂) (1; Cp′ = 1-^tBu-3,4-Me₂-C₅H₂) with the tungsten fragment [W(CO)₅] affords the corresponding μ-bimetallic phosphido complex 2, which in a photochemical transformation gives the dibridged derivative Cp′CpTa-(μ-CO)(μ-PMe₂)W(CO)₄ (3). Addition of an optically active phosphine (PAMP) to 3 leads to a pair of optically active diastereoisomers (4a and 4b) which can be separated by fractionnai crystallization. Both diastereoisomers 4a and 4b crystallize in the orthorhombic noncentrosymmetric space group P2₁2₁2₁. X-ray structure determinations of 4a and 4b allowed the assignment of absolute configurations of the tantalum chiral centers (S for 4a and R for 4b). The absolute configuration of the phosphorus atom of the PAMP ligand is the same (S) in both structures.

Full text of DOI:10.1021/om9600497

Organometallics 1996, 15, 2399-2403
2399
Op tica lly Active Bim eta llic Com p lexes w ith a Ta n ta lu m
Atom a s a Ch ir a l Cen ter . Syn th esis, Ch a r a cter iza tion ,
a n d X-r a y Str u ctu r es of Tw o Dia ster eoisom er s:
Cp ′Cp Ta *(CO)(µ-P Me2)W(CO)4L* (Cp ′ )
1-tBu -3,4-Me2-C5H2; L* )
(R)-(+)-P h en yl(o-a n isyl)m eth ylp h osp h in e (P AMP ))
Philippe Sauvageot,† Olivier Blacque,† Marek M. Kubicki,*,† Sylvain J uge´,‡ and
Claude Mo¨ıse*,†
Laboratoire de Synthe`se et d’Electrosynthe`se Organome´talliques (URA 1685),
Universite´ de Bourgogne, Faculte´ des Sciences, 6 boulevard Gabriel, 21000 Dijon, France, and
Equipe “Re´activite´s specifiques”, Universite´ de Cergy Pontoise, BP 8428,
47 avenue des Ge´nottes, 95806 Cergy Pontoise Cedex, France
Received J anuary 25, 1996X
Reaction of the racemic metallophosphine Cp′CpTa(CO)(PMe2) (1; Cp′ ) 1-tBu-3,4-Me2-
C5H2) with the tungsten fragment [W(CO)5] affords the corresponding µ-bimetallic phosphido
complex 2, which in a photochemical transformation gives the dibridged derivative Cp′CpTa-
(µ-CO)(µ-PMe2)W(CO)4 (3). Addition of an optically active phosphine (PAMP) to 3 leads to
a pair of optically active diastereoisomers (4a and 4b) which can be separated by fractionnal
crystallization. Both diastereoisomers 4a and 4b crystallize in the orthorhombic noncen-
trosymmetric space group P212121. X-ray structure determinations of 4a and 4b allowed
the assignment of absolute configurations of the tantalum chiral centers (S for 4a and R for
4b). The absolute configuration of the phosphorus atom of the PAMP ligand is the same
(S) in both structures.
In tr od u ction
sibility is frequently encountered in square-pyramidal
and pseudotetrahedral complexes. The molecular struc-
tures of group 4, 5, and 6 metal bis(cyclopentadienyl)
derivatives contain the metal atom in a pseudotetrahe-
dral arrangement; the asymmetry of the metallic center
thereby requires the presence of two different cyclopen-
tadienyl rings. This feature can be readily achieved by
introduction of alkyl substituents on one of the two Cp
rings. We have already used such a strategy to syn-
thesize and to resolve chiral bis(cyclopentadienyl)-
titanium derivatives.5 For some time we have been
interested in the building of bi- and triheterometallic
systems based on metallophosphines as starting materi-
als. These are the group 5 bis(cyclopentadienyl) deriva-
tives.6 To the best of our knowledge, no example of an
optically active group 5 organometallic complex has
been reported to date. We have therefore undertaken
the synthesis of chiral metalloligands with the aim of
their resolution and of the preparation of optically active
polymetallic structures. We wish to report here our
results in this stereochemical area.
Organometallic stereochemistry has undergone in-
tense development over the last few decades for two
main reasons: chiral organometallic complexes may be
employed from a mechanistic point of view for studying
the effects of ligand substitutions or of reactions on the
metal-ligand bond,1 but catalytic or stoichiometric
enantioselective organic syntheses are by far the major
applications.2 Chirality may arise from the presence of
an asymmetric ligand or derive from the metal center,
which lies in an asymmetric environment. As reviewed
by Brunner3 and Sokolov,4 this latter structural pos-
† Universite´ de Bourgogne.
‡ Universite´ de Cergy Pontoise.
X Abstract published in Advance ACS Abstracts, April 15, 1996.
(1) (a) Chou, C.-K.; Miles, D. L.; Bau, R.; Flood, T. C. J . Am. Chem.
Soc. 1978, 100, 7271. (b) Davison, A.; Martinez, N. J . Organomet.
Chem. 1974, 74, C17. (c) Brunner, H.; Wallner, G. Chem. Ber. 1976,
109, 1053. (d) Merrifield, J . H.; Ferna`ndez, J . M.; Buhro, W. E.;
Gladysz, J . A. Inorg. Chem. 1984, 23, 4022. (e) Cagle, P. C.; Arif, A.
M.; Gladysz, J . A. J . Am. Chem. Soc. 1994, 116, 3655. (f) Peng, T.-S.;
Arif, A. M.; Gladysz, J . A. J . Chem. Soc., Dalton Trans. 1995, 1857
and references cited therein.
(2) (a) Enders, D., Gais, H.-J ., Keim, W., Ed. Organic Synthesis Via
Organometallics; Vieweg and Sohn: Wiesbaden, Germany, 1993. (b)
Kagan, H. B.; Riant, O. Chem. Rev. 1992, 92, 1007. (c) Noyori, R. Chem.
Soc. Rev. 1989, 18, 187. (d) Brunner, H. Top. Stereochem. 1988, 18,
129. (e) Bosnich, B., Ed. Asymmetric Catalysis; Martinus Nijhoff:
Dordrecht, The Netherlands, 1986. (f) Hegedus, L. S. Transition Metals
in the Synthesis of Complex Organic Molecules; University Science
Books: Mill Valley, CA, 1994. (g) Paquette, L. A.; Sivik, M. R.; Bzowej,
E. I.; Stanton, K. J . Organometallics 1995, 14, 4865. (h) Noyori, R.
Asymmetric Catalysis in Organic Synthesis; Wiley: New York, 1994.
(i) Halterman, R. L. Chem. Rev. 1992, 965.
(5) (a) Mo¨ıse, C.; Leblanc, J . C.; Tirouflet, J . J . Am. Chem. Soc. 1975,
97, 6272. (b) Mo¨ıse, C.; Leblanc, J . C.; Tirouflet, J . Nouv. J . de Chim.
1977, 1, 211.
(6) (a) Lavastre, O.; Bonnet, G.; Leblanc, J . C.; Moı¨se, C. Polyhedron
1995, 2, 307. (b) Boni, G.; Kubicki, M. M.; Mo¨ıse, C. Bull. Soc. Chim.
Fr. 1994, 131, 895. (c) Challet, S.; Lavastre, O.; Mo¨ıse, C.; Leblanc, J .
C.; Nuber, B. New J . Chem. 1994, 18, 1155. (d) Bonnet, G.; Kubicki,
M. M.; Mo¨ıse, C.; Lazzaroni, R.; Salvadori, P.; Vitulli, G. Organome-
tallics 1992, 11, 964. (e) Bonnet, G.; Lavastre, O.; Leblanc, J . C.; Moı¨se,
C. New J . Chem. 1988, 12, 551. (f) Oudet, P.; Kubicki, M. M.; Mo¨ıse,
C. Organometallics 1994, 13, 4278. (g) Bonnet, G.; Lavastre, O.; Boni,
G.; Leblanc, J . C.; Mo¨ıse, C. C. R. Acad. Sci. Paris, Ser. 2 1994, 319,
1293. (h) Boni, G.; Sauvageot, P.; Marpeaux, E.; Moı¨se, C. Organome-
tallics 1995, 14, 5652.
(3) Brunner, H. Adv. Organomet. Chem. 1980, 18, 151.
(4) Sokolov, V. I. Chirality and Optical Activity in Organometallic
Compounds; Gordon and Breach: New York, 1990.
S0276-7333(96)00049-0 CCC: $12.00 © 1996 American Chemical Society

2400 Organometallics, Vol. 15, No. 9, 1996
Sauvageot et al.
Ta ble 1. Cr ysta llogr a p h ic Da ta for
Cp ′Cp Ta (CO)(µ-P Me2)W(CO)4P AMP (4a ,b)
Exp er im en ta l Section
Gen er a l Ma ter ia ls a n d Mea su r em en ts. All reactions
were carried out under an argon atmosphere with use of
standard Schlenk techniques. The solvents and eluants were
dried and distilled under argon from sodium and benzophe-
none immediately before use. Column chromatography was
performed under argon and with silica gel (70-230 mesh).
Cp′CpTa(CO)(PMe2) (1) and the optically active phosphine
“(R)-PAMP” were prepared by previously reported proce-
dures.7,8
1H and 31P NMR spectra were recorded on a Bruker AC 200
spectrometer; chemical shifts are given in ppm relative to Me4-
Si (1H) or (external) H3PO4 (31P). Infrared spectra were
recorded on a Nicolet 205 IR-FT. CD spectra were recorded
(at 25 °C) on a J obin Yvon CD6 dichrograph, with THF
solutions (1.0 × 10-3 M for 4a , 6.6 × 10-4 M for 4b, and 1.6 ×
10-3 M for Cp2Ta(CO)(µ-PMe2)W(CO)4PAMP6h) in a 1 cm cell.
Elemental analyses (C, H) were performed by the “Service
Central d’Analyse du CNRS” (Gif-sur-Yvette, France).
Syn th esis of Cp ′Cp Ta (CO)(µ-P Me2)W(CO)5 (2). To a
solution of 1 (250 mg, 0.52 mmol) in 15 mL of THF was added
an excess of W(CO)5(THF) produced by irradiation of W(CO)6
(210 mg, 0.60 mmol) in THF. Then, the mixture was stirred
for 45 min at room temperature. The solvent was removed in
vacuo and the crude reaction product chromatographed on
silica gel with toluene as eluant. Recrystallization from THF/
pentane (1:6) afforded 290 mg of green crystals of 2 (69%). IR
(ν(CO), THF): 2051 (m), 1975 (s), 1913 (vs) cm-1. Anal. Calcd
for C24H28O6PTaW: C, 35.67; H, 3.49. Found: C, 35.55; H,
3.47.
4a
4b
mol formula
fw
WTaP2O6C37H43
1010.5
WTaP2O6C37H43
1010.5
cryst size, mm
cryst syst
space group
cell dimens
a, Å
0.3 × 0.15 × 0.15
orthorhombic
P212121 (No. 19)
0.4 × 0.3 × 0.3
orthorhombic
P212121 (No. 19)
13.965(5)
18.203(4)
14.604(6)
3712(2)
4
10.122(2)
11.395(3)
32.734(6)
3775(1)
4
b, Å
c, Å
V, Å3
Z
d
calcd, g cm-3
1.808
1.778
F(000)
1960
62.015
1960
60.975
linear abs, µ, cm-1
radiation, Å
λ(Mo KR) )
0.710 73
2-25
λ(Mo KR) )
0.710 73
2-25
θ range, deg
scan type
ω-2θ
ω
scan speed, deg min-1
scan width, deg
1.7-8.3
∆ω ) 0.47 +
0.347 tan θ
8212
1.5-8.3
∆ω ) 0.90 +
0.347 tan θ
8024
no. of rflns measd
temp, K
296(1)
296(1)
decay, %
-4.7, corrected
I g 3σ(I)
5218
-21, corrected
I g 3σ(I)
5513
cutoff for obsd data
no. of unique obsd
data (NO)
no. of variables (NV)
R(F)
535
364
0.038
0.044
0.040
0.064
0.078
0.060
Rw(F)
wt, w-1
)
Syn th esis of Cp ′Cp Ta (µ-CO)(µ-P Me2)W(CO)4 (3).
A
[σ2(I) + (pFo2)2]1/2, p
solution of 2 (260 mg, 0.32 mmol) in 100 mL of THF was
irradiated in Pyrex filtering vessels with a high-pressure
Hanau TQ 150 mercury lamp for 6 h. After evaporation of
the solvent, the crude product was chromatographed on silica
gel. Elution with toluene/THF (4:1) afforded two fractions. The
first band was unreacted 2 (50 mg). The second band was red.
After the solvent was removed, 3 was obtained as a red solid.
Yield: 130 mg (52%). IR (ν(CO), THF): 1999 (m), 1930 (s),
GOF
F
0.405
1.349
max/Fmin, e Å-3
1.12a/-0.34
1.80a/-0.52
a
Close to the tantalum atom.
with neutral-atom scattering factors.10 Intensities were cor-
rected for Lorentz and polarization effects as well as for linear
decay.
1908 (m), 1880 (s), 1705 (w) cm-1
.
Systematic extinctions indicated the same noncentrosym-
metric orthorhombic space group, P212121, for both crystals.
The structures were solved and refined by conventional three-
dimensional Patterson, difference Fourier, and full-matrix
least-squares methods. All nonhydrogen atoms in 4a were
refined with anisotropic thermal parameters. Because of the
poor quality of the crystals of 4b, some non-hydrogen atoms
were treated in an isotropic model. The correct initial choice
of enantiomorph for 4a and 4b was checked by inversion of
all positional parameters. In both cases, an increase of R
factors (from 0.040 to 0.051 for 4a and from 0.068 to 0.075 for
4b) was observed without improvement of estimated standard
deviations. The hydrogen atoms were placed in calculated
positions and included in the final cycles of refinements in a
riding model with Biso fixed at 1.3Beq for the carbon atoms
bearing them. The atomic coordinates for the structures of
4a and 4b are given in Tables 2 and 3, respectively (Supporting
Information).
Syn th esis of Cp ′Cp Ta (CO)(µ-P Me2)W(CO)4P MeP h (o-
An ) (4). To a solution of 3 (130 mg, 0.17 mmol) in 40 mL of
THF was added PAMP (40 mg, 0.17 mmol). After 30 min at
room temperature, the solution changed from red to green. The
solvent was then removed under reduced pressure, and the
crude reaction product was chromatographed on silica gel with
ether/pentane (6:14) as eluant. After the solvent was removed,
a green solid was obtained. Yield: 160 mg (92%). IR (ν(CO),
THF): 1993 (m), 1904 (m), 1885 (vs), 1867 (vs), 1839 (s) cm-1
Anal. Calcd for C37H43O6P2TaW: C, 43.98; H, 4.29. Found:
C, 43.31; H, 4.27.
.
Sep a r a tion of th e Dia ster eoisom er s 4a a n d 4b. 4 was
dissolved in diethyl ether. The solution was concentrated close
to saturation. Pentane was added, and the green solution was
cooled to 5 °C for 2 days. Two types of crystals differing in
shape and color (green needles for 4a and brown pyramids for
4b) were concurrently grown. Separation of crystals was
carried out manually in order to perform crystallographic,
NMR, and chiroptical studies. 4a : optical rotation, [R]20
Resu lts a n d Discu ssion
578
+66°, [R]20
+117°, [R]20
+301° (c 1, THF). 4b: optical
546
436
The racemic metallophosphine [Cp′CpTa(CO)PMe2]
(1; Cp′ ) 1-tBu-3,4-Me2-C5H2) is obtained as a green
solid according to the procedure outlined in Scheme 1
by reaction of the chiral carbonyl hydride Cp′CpTa(CO)-
(H)7 with PMe2Cl followed by treatment with KOH.
Stereostability at the asymmetric tantalum center in
rotation, [R]20
-82°, [R]20
-128°, [R]20
-434° (c 0.66,
578
546
436
THF).
X-r a y Str u ctu r e Deter m in a tion . The sizes of the crystals
used for data collections carried out on an Enraf-Nonius CAD4
diffractometer and the pertinent crystallographic data are
given in Table 1. The unit cells were determinated from 25
reflections selected by the CAD4 automatic routines. All
calculations were carried out by use of the MOLEN package9
1
1 is clearly evidenced by the H NMR spectrum, which
(9) Fair, C. K. MOLEN, Structure Determination System; Enraf-
Nonius: Delft, The Netherlands, 1990.
(10) International Tables for X-Ray Crystallography; Kynoch
Press: Birmingham, U.K., 1974; Vol. 4, p 99.
(7) Sauvageot, P.; Mo¨ıse, C. Bull. Soc. Chim. Fr., in press.
(8) J uge´, S.; Stephan, M.; Laffitte, J . A.; Genet, J . P. Tetrahedron
Lett. 1990, 31, 6357.

Optically Active Bimetallic Complexes
Organometallics, Vol. 15, No. 9, 1996 2401
Ta ble 4. 1H a n d 31P NMR Da ta for 1-3
Sch em e 2
1H NMR (ppm, C6D6)
com-
plex
31P NMR
(ppm, C6D6)
Cp and Cp′
PMe2
1
2
3
4.46 (s, 5H)
1.50 (s, br, 6H)
-91.5
4.39 (dd, 7a and
2.5 Hz, 1H)
3.82 (d, 2.5 Hz, 1H)
2.03 (s, 3H, Me)
1.94 (s, 3H, Me)
t
0.97 (s, 9H, Bu)
4.52 (d, 1.9 Hz,a
5H)
2.00 (d, 5.1 Hz, 3H) -118.8 (s, J PW
171 Hz)
1.66 (d, 5.6 Hz, 3H)
)
3.84 (s, br, 1H)
3.69 (s, br, 1H)
1.85 (s, 3H, Me)
1.45 (s, 3H, Me)
t
1.00 (s, 9H, Bu)
the complexation of this metallophosphine to the W(CO)5
fragment. Such a behavior of the phosphorus nuclei is
easily explained by the participation of the nonlocal
diamagnetic contribution in the overall shielding.13 In
this context, the 31P resonance in 3 (+34.4 ppm) is
strongly deshielded and indicates the presence of a
direct Ta-W bond.14
4.49 (d, 2.2 Hz,a
5H)
1.92 (d, 8.5 Hz, 3H) 34.4 (s, J PW )
163 Hz)
3.92 (d, 2.7 Hz, 1H) 1.46 (d, 8.5 Hz, 3H)
3.83 (dd, 5.6a and
2.7 Hz, 1H)
1.66 (s, 3H, Me)
1.52 (s, 3H, Me)
0.89 (s, 9H, Bu)
t
We have recently shown that Lewis bases L (isoni-
triles or phosphines) are able to cleave regiospecifically
the metal-metal bond in dibridged structures of type
3, Cp2Ta(µ-CO)(µ-PMe2)M(CO)4. We have already shown
that such a cleavage leads to the formation of an
M(CO)4L fragment bound to the metallocene through a
phosphido bridge.6h The stereo- or electrochemical
opening of the metal-metal bond or of the carbonyl
bridge by the nucleophiles depends on the size of the
incoming ligand L as well as on that of the metal M.
Trans geometries are favored with a small chromium
atom, but the cis ones may be easily formed in the
presence of larger metals such as tungsten. The size of
the L ligand also influences the nature of geometrical
isomers: small phosphines give cis geometries, while
the bulkier ones favor the trans isomers. In the case of
tungsten, a cis arrangement is exclusively formed,
except for the bulky PPh3 phosphine, which gives a
small amount of trans isomer (5%). Our strategy of
resolution uses these observations.
a
31P coupling.
Sch em e 1
displays two sets of two different signals for the two
methyl groups and the two ring protons of the substi-
tuted cyclopentadienyl ligand. Furthermore, a temper-
ature dependence is observed for the resonance signal
of the PMe2 group. As mentioned in Table 4 this signal
appears as a broad singlet at 295 K and as a doublet
(2J PH ) 4 Hz) at 330 K, whereas two well-resolved
doublets (∆ν ) 56 Hz, 2J PH ) 4 Hz) are displayed when
the temperature is decreased to 260 K. This behavior
is indicative of a low stereostability at the phosphorus
center, as has been previously reported for related niobo
derivatives.6d According to our previous results dealing
with the reactivity of the unsubstituted metallophos-
phine [Cp2Ta(CO)(PMe2)] toward group 6 pentacarbonyl
fragments,6h complex 1 is allowed to react with the
tungsten fragment [W(CO)5] in order to obtain the
corresponding bimetallic derivative 2. A photochemical
treatment of 2 leads to the µ-PMe2, µ-CO dibridged
complex 3 in a moderate yield (52%).
NMR data (Table 4) support the structures ascribed
to 2 and 3, and the corresponding patterns are similar
to those observed for the complexes with nonsubstituted
Cp rings described earlier.6h In particular, for complex
3 the µ-CO IR absorption centered at 1705 cm-1
indicates the presence of the bridging carbonyl group.
The 31P chemical shifts of the dimethylphosphido ligand
merit some comments. The corresponding resonance in
metallophosphine 1 is only slightly deshielded with
respect to that reported for PMe2H (-99 ppm).11 The
negative coordination chemical shift12 is observed upon
The room-temperature reaction of 3 with the very
bulky, optically active NMDPP (neomenthyldiphen-
ylphosphine) is unsuccessful; heating causes significant
degradation of the starting dibridged complex with
regeneration of some amount of 2. In contrast, reaction
of 3 with PAMP ((R)-(+)-phenyl(o-anisyl)methylphos-
phine; ee ) 98%) proceeds quickly, affording after
workup the pair of diastereoisomers 4a and 4b.
1
4a and 4b show well-separated signals in H and 31P
NMR spectra (Table 5); no diastereoselectivity can be
deduced from the ratio of their integrated intensities.
Attempts to perform a separation of diasteroisomers by
chromatography lead only to a slight enrichment in one
form; however, crystallization of the mixture 4a + 4b
from ether/pentane affords two kinds of distinctly
colored crystals which can be easily manually separated.
(12) Mann, B. E.; Masters, C.; Shaw, B. L.; Stainbank, R. E. Inorg.
Nucl. Chem. Lett. 1971, 7, 881.
(13) Kubicki, M. M.; Le Gall, J .-Y.; Kergoat, R.; Gomes de Lima, L.
C. Can. J . Chem. 1987, 65, 1292.
(14) (a) Carty, A. J .; MacLaughlin, S. A.; Nucciarone, D. In Methods
in Stereochemical Analysis; Marchand, A. P., Ed.; Phosphorus-31 NMR
Spectroscopy in Stereochemical Analysis 8; VCH: Deerfield Beatch,
FL, 1987; Chapter 16. (b) Casey, C. P.; Bullock, R. M. Organometallics
1984, 3, 1102.
(11) Manatt, S. L.; J uvinall, G. L.; Elleman, D. D. J . Am. Chem.
Soc. 1963, 85, 2664.

2402 Organometallics, Vol. 15, No. 9, 1996
Ta ble 5. 1H a n d 31P NMR Da ta for 4a a n d 4b
Sauvageot et al.
1H NMR (ppm, C6D6)
31P NMR (ppm, C6D6)
complex
Cp and Cp′
PMe2
PMePh(o-An)
PMe2
PMePh(o-An)
4a
4.69 (d, 2 Hz,a 5H)
3.85 (s, br, 1H)
1.73 (d, 4.4 Hz, 3H)
1.47 (d, 5.1 Hz, 3H)
6.4-8 (m, Ph)
3.30 (s, OMe)
-117.7 (d, 30 Hz)
-13.9 (d, 30 Hz)
3.80 (s, br, 1H)
2.11 (d, 5.4 Hz, PMe)
2.14 (d, 1.7 Hz,a 3H, Me)
1.61 (s, 3H, Me)
t
1.04 (s, 9H, Bu)
4b
4.72 (d, 2 Hz,a 5H)
3.83 (s, br, 1H)
1.69 (d, 4.4 Hz, 3H)
1.37 (d, 5.3 Hz, 3H)
6.4-8 (m, Ph)
3.25 (s, OMe)
-118.1 (d, 30 Hz)
-15.4 (d, 30 Hz)
3.76 (s, br, 1H)
2.15 (d, 5.3 Hz, PMe)
2.12 (d, 1.7 Hz,a 3H, Me)
1.60 (s, 3H, Me)
t
1.02 (s, 9H, Bu)
a
31P coupling.
F igu r e 2. Molecular structure of 4a (50% probability
level).
F igu r e 1. Circular dichroism curves of 4a and 4b (solid
line) and Cp2Ta(CO)(µ-PMe2)W(CO)4PAMP (dashed line).
Sch em e 3
F igu r e 3. Molecular structure of 4b (30% probability
level).
The purity of each sample has been checked by NMR
spectroscopy.
(dotted line) is reported in Figure 1. The very weak
intensity of this curve clearly demonstrates the minor
contribution of the W(CO)4PAMP moiety to the chirop-
tical properties of 4a and 4b.
It may be assumed that the stereochemistry of the
chiral phosphine remains unchanged upon its coordina-
tion to the tungsten atom; therefore, the optical isomers
4a and 4b, which differ only in the absolute configura-
tion at their tantalum centers, are expected to be
optically active in solution. As mentioned in the litera-
ture,3,5,15 it can be expected that these two diasteroiso-
mers should display mirror-image CD spectra.
Figure 1 shows the CD curves obtained on THF
solutions of 4a and 4b (solid line) and recorded in the
350-650 nm region. These CD spectra are defined by
two intense Cotton effects, at about 360 and 420 nm,
and by another weak signal of opposite sign at about
580 nm. As expected, they show an almost enantiomeric
(mirror-image) shape, which must arise from the enan-
tiomeric relationship between the tantalum centers in
these two diastereoisomers. To confirm the domination
of the tantalum chromophore, the CD spectrum obtained
for a THF solution of Cp2Ta(CO)(µ-PMe2)W(CO)4PAMP
The absolute configurations of the tantalum and of
the phosphorus atoms have been clearly defined from
the structure determination of 4a and 4b.
The crystal structures of 4a and 4b are built from
discrete dinuclear molecules. These molecules contain
the tantalocene (pseudotetrahedral geometry) and tet-
racarbonylphosphinotungsten (distorted octahedron) moi-
eties singly bridged by the dimethylphosphide ligand.
The overall geometries of both molecules are closely
related and exhibit similar metric parameters (Table 6).
They form a pair of diastereoisomers which differ from
one another in the optical isomerism of the metallic (Ta)
center. The chiralities of the asymmetric phosphorus
atoms (P2) are the same and correspond to that of the
free PAMP ligand.8 Thus, the diastereoisomer 4a is
S(Ta),S(P), while 4b is R(Ta),S(P) if we assume the
following priority numbers over the tantalum atom:16
(Cp′ ligand), 2 (Cp ligand), 3 (P atom), and 4 (carbonyl
1
(15) Attar, S.; Nelson, J . H.; Fischer, J .; de Cian, A.; Sutter, J . P.;
Pfeffer, M. Organometallics 1995, 14, 4559.

Optically Active Bimetallic Complexes
Organometallics, Vol. 15, No. 9, 1996 2403
Ta ble 6. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Cp ′Cp Ta (CO)(µ-P Me2)W(CO)4P AMP (4a ,b)
0.26 Å in 4b. Thus, W(CO)4L fragments are pushed
toward the non-substituted and less hindered C5H5 ring.
It is evident that steric intramolecular effects operate
in the determination of the overall molecular geometry.
It is worth noting that the conformations of the chiral
phosphine atoms (P2) are different in the crystals of 4a
and 4b. This is shown by the values of the dihedral
angles P1-W-P2/W-P2-C24, which are equal to 34.3
and 65.4° for 4a and 4b, respectively. The Ta-P1
distances of 2.68 Å are long but correspond well with
the values found for other group 5 metallocene-bridging
phosphido ligands. The W-P1 distances of 2.628(3) Å
(4a ) and 2.641(5) Å (4b) are also long and together with
the largely opened Ta-P1-W angles (127.85(1) and
127.1(2)°) indicate the presence of steric repulsions
between both organometallic fragments. Thus, as pre-
viously stated, the metallophosphines Cp2M(CO)PR2 are
very bulky ligands.6,21 The W-P2 distances of 2.549-
(3) Å (4a ) and 2.511(6) Å (4b) are short, as expected,
and fall in the range observed for other cis-M(CO)4 (M
) Mo,W; 2.52-2.56 Å) complexes.22 They are, however,
shorter than those found in Fe(C5Me4PPh2)2W(CO)4
(2.60 Å)23 and in [Cp2Mo(H)(µ-PMe2)]2W(CO)4 (2.62 Å),24
where the phosphorus atoms are of the “metallophos-
phine” type. The P1-W-P2 angles (96.3(1) and 99.8-
(2)°) fall in the range usually observed in cis-M(CO)4
diphosphines (87-97°)22 but are smaller than in Cp2-
Zr(H)(µ-PPh2)2W(CO)4 (103.3(1)°)25 and in [Cp2Mo(H)-
(µ-PMe2)]2W(CO)4 (105.1(1)°).24
4a
4b
Ta-W
Ta-CP1
Ta-CP2
Ta-P1
Ta-C1
W-P1
W-P2
W-C2
W-C3
W-C4
W-C5
4.7723(7)
2.076
2.079
4.7598(9)
1.98
2.06
2.685(3)
2.04(1)
2.628(3)
2.549(3)
2.02(2)
1.98(1)
2.02(1)
1.95(1)
2.675(5)
1.99(2)
2.641(5)
2.511(6)
2.00(2)
1.95(3)
1.93(3)
2.04(3)
CP1-Ta-CP2
P1-Ta-C1
P1-W-P2
136.0
136.4
83.4(4)
96.3(1)
127.8(1)
82.4(7)
99.8(2)
127.1(2)
Ta-P1-W
C atom). The orientation of methyl substituents on the
bridging phosphorus atom with respect to the metal-
locene Cp′CpTa-CO is of the endo type. The same endo
structure has been already observed in the complexes
Cp2Nb(CO)(µ-PMe2)W(CO)4(PMe2Ph)17 and Cp2Ta(CO)-
(µ-PMe2S)W(CO)5.18 However, exo-type geometries are
formed in the presence of bulkier substituents on the
i
phosphorus atom such as Ph19 and Pr.6d Our previous
extended Hu¨ckel calculations6f,20 explain the influence
of substituent size on the overall geometry of bimetallic
complexes. In order to minimize the repulsions with
the CO ligand, the methyl groups of the PMe2 bridge in
an endo conformation should be symmetrically displayed
over the C1-Ta-P1 plane (as observed in both struc-
tures). Consequently, the tungsten atom should lie in
this plane. However, the positions of the W atom
deviate significantly from this plane: 0.46 Å in 4a and
Su p p or tin g In for m a tion Ava ila ble: For 4a and 4b,
tables of X-ray crystallographic data, including atomic coor-
dinates, anisotropic thermal parameters, and interatomic
distances and angles and figures giving stereoviews (15 pages).
Ordering information is given on any current masthead page.
OM9600497
(16) (a) Stanley, K.; Baird, M. C. J . Am. Chem. Soc. 1975, 97, 6598.
(b) Sloan, T. E. Top. Stereochem. 1981, 12, 1. (c) Ferna`ndez, J . M.;
Gladysz, J . A. Inorg. Chem. 1986, 25, 2674.
(17) Oudet, P.; Kubicki, M. M.; Mo¨ıse, C. Acta Crystallogr. 1995,
C51, 811.
(18) Challet, S.; Kubicki, M. M.; Leblanc, J . C.; Mo¨ıse, C.; Nuber,
B. J . Organomet. Chem. 1994, 483, 47.
(19) Bonnet, G.; Lavastre, O.; Kubicki, M. M.; Leblanc, J . C.; Mo¨ıse,
C. Proceedings from the XVth International Union of Crystallography;
Bordeaux, France, 1990; p C227.
(21) Barre´, C.; Kubicki, M. M.; Leblanc, J . C.; Mo¨ıse, C. Inorg. Chem.
1990, 29, 5244.
(22) (a) Churchill, M. R.; Rheingold, A. L.; Keiter, R. L. Inorg. Chem.
1981, 20, 2730. (b) Cotton, F. A.; Darensbourg, D. J .; Klein, S.;
Kolthammer, B. W. S. Inorg. Chem. 1982, 21, 1651. (c) Varshney, A.;
Gray, G. M. Inorg. Chem. 1991, 30, 1748.
(23) Ninoreille, S.; Broussier, R.; Amardeil, R.; Kubicki, M. M.;
Gautheron, B. Bull. Soc. Chim. Fr. 1995, 132, 128.
(24) Barre´, C. Ph.D. Thesis, Universite´ de Dijon, 1992.
(25) Targos, T. S.; Rosen, R. P.; Whittle, R. R.; Geoffroy, G. Inorg.
Chem. 1985, 24, 1375.
(20) (a) Barre´, C.; Boudot, P.; Kubicki, M. M.; Moı¨se, C. Inorg.Chem.
1995, 34, 284. (b) Kubicki, M. M.; Oudet, P.; Martin, C.; Barre´, C. J .
Chem. Soc., Dalton Trans. 1995, 3699.