Convenient synthesis of mixed ferrocenes

Article abstract of DOI:10.1021/om960517c

The deprotonated form of [(fluorene)FeCp]PF₆ functions as an efficient cyclopentadienyliron (CpFe) transfer reagent to other cyclopentadienyls (Cp′) to give mixed ferrocenes (CpFeCp′). By using this method, many new mixed ferrocenes have been p

Full text of DOI:10.1021/om960517c

304
Organometallics 1997, 16, 304-306
Con ven ien t Syn th esis of Mixed F er r ocen es
Si-Geun Lee, Hee-Kyu Lee, Su Seong Lee, and Young Keun Chung*
Department of Chemistry and Center for Molecular Catalysis, College of Natural Sciences,
Seoul National University, Seoul 151-742, Korea
Received J une 27, 1996X
Summary: The deprotonated form of [(fluorene)FeCp]PF6
functions as an efficient cyclopentadienyliron (CpFe)
transfer reagent to other cyclopentadienyls (Cp′) to give
mixed ferrocenes (CpFeCp′). By using this method,
many new mixed ferrocenes have been prepared, and one
of the mixed ferrocenes has been used to make bi- and
trimetallic compounds.
methylamino)pentafulvenes. We have screened several
Fe(I) and Fe(II) complexes as CpFe transfer reagents.
Among the compounds which we have screened, CpFe-
(η5-oxocyclohexadienyl) and CpFe(naphthalene)+ have
some marginal utility. Only sometimes are they suc-
cessful in transferring the CpFe moiety. However, the
deprotonated form of [CpFe(fluorene)] PF6 can be ap-
plied very successfully to transfer of the CpFe moiety
in most cases. Several years ago, the thermal and
electron-transfer-induced isomerization of Fe(η6-fluor-
enyl)(η5-C5H5) were reported.7 In this communication,
we demonstrate the usefulness and the scope of the
CpFe transfer reaction using the deprotonated form of
[CpFe(fluorene)]PF6.
[CpFe(fluorene)]PF6 (1) was synthesized by a known
route.8 Treatment of 1 with t-BuOK in THF generated
the deprotonated fluorene iron complex 2, which has
been reported earlier by Treichel and J ohnson.9 When
2 was treated with hydrocarbon-substituted cyclopen-
tadienides, mixed ferrocenes were obtained in reason-
able to high yield (eq 1).10 The scope of the substituted
cyclopentadienides can be seen in Table 1.11
Due to its high stability and its versatility as starting
material in the synthesis of useful compounds, ferrocene
plays a key role in many areas of research, and
extensive chemistry associated with ferrocene has been
published.1 An important advantage of the ferrocene-
containing complexes is that their electron-donor ability
may be fine-tuned by the choice of number and nature
of the substituents.2 Thus, the synthesis of ferrocenes
with tailor-made properties has been a goal for many
synthetic chemists. Many useful synthetic methods
have been published.3 However, most of the published
methods are applicable only in special cases.4 In fact,
there seems to be a dearth of systematic investigations
into the complexation behavior of substituted cyclopen-
tadienyl ligands because there are no general routes to
synthesize unsymmetrically substituted ferrocenes.
Several years ago, Manriquez et al.5 reported the use
of Cp*Fe(acac) as a Cp*Fe transfer reagent. However,
attempted use of CpFe(acac) as a CpFe transfer reagent
was not successful. The use of [CpFe(p-xylene)]PF6 in
ligand exchange reactions to prepare substituted met-
allocenes has been reported,6 but in this reaction, the
cyclopentadiene derivatives are limited only to 6-(di-
(7) (a) Pomazanova, N. A.; Novikova, L. N.; Ustnyuk, N. A.;
Kravtsov, D. N. Metalloorg. Khim. 1989, 2, 422 (Chem. Abstr. 1990,
112, 139384c). (b) Kukharenko, S. V.; Strelets, V. V.; Ustynyuk, N. A.;
Novikova, L. N.; Denisovitch, L. I.; Peterleitner, M. G. Metalloorg.
Khim. 1991, 4, 299 (Chem. Abstr. 1991, 115, 29560m).
(8) (a) Nesmeyanov, A. N.; Vol’kenau, N. A.; Bolesova, I. N. Dokl.
Akad. Nauk SSSR 1966, 166, 607. (b) Helling, J . F.; Hendrickson, W.
A. J . Organomet. Chem. 1977, 141, 99.
(9) (a) Treichel, P. M.; J ohnson, J . W. J . Organomet. Chem. 1975,
88, 207. (b) Treichel, P. M.; J ohnson, J . W. Inorg. Chem. 1977, 16, 749.
(c) J ohnson, J . W.; Treichel, P. M. J . Chem. Soc., Chem. Commun. 1976,
688. J ohnson, J . W.; Treichel, P. M. J . Am. Chem. Soc. 1977, 99, 1427.
(10) The cyclopentadiene derivatives used in this study were
prepared by the reported methods. For the entries 1-3, see: Lee, B.
Y.; Moon, H.; Chung, Y. K.; J eong, N. J . Am. Chem. Soc. 1994, 116,
2163. For entry 4, see: Rausch, M. D.; wang, Y.-P. Organometallics
1991, 10, 1438. For entry 5, see: Threlkel, R. S.; Bercaw, J . E. J .
Organomet. Chem. 1977, 136, 1. For entries 7 and 8, see: Leblanc, J .
C.; Moise, C. J . Organomet. Chem. 1976, 120, 65. For entry 10, see:
Lee, M.-T.; Foxman, B. M.; Rosenblum, M. Organometallics 1985, 4,
547. Typical procedure for thepreparation of mixed ferrocenes: The
substituted cyclopentadiene (0.173 g, 0.87 mmol) (entry 1 in Table 1)
was dissolved in 10 mL of THF at -78 °C. To the resulting solution,
n-BuLi (1.3 mmol, 0.52 mL of a 2.5 M solution in n-hexane) was added
dropwise during 1 h. The resulting solution was transferred by cannula
to another flask containing 2 (generated in situ by the reaction of 1
(0.188 g, 0.44 mmol) with t-BuOK (0.08 g, 0.7 mmol) for 30 min) in 10
mL of THF at 0 °C. The reaction mixture was heated at 60 °C for 6 h.
After cooling to room temperature, water (30 mL) and hexane (30 mL)
were added to the reaction mixture and a few drops of 1 N HCl solution
were added. The organic layer was separated, dried over anhydrous
MgSO4, filtered, and concentrated. Purification was effected by column
chromatography on silica gel eluting with hexane. Compound 3 was
obtained in 75% yield (0.10 g, 0.33 mmol). 1H NMR (CDCl3) δ 7.55-
7.21 (m, 5 H, Ph), 4.36-4.13 (m, 3 H), 4.04 (s, 5 H, Cp), 2.59-2.48 (m,
2 H, CH2), 1.55-1.27 (m, 4 H, CH2), 0.88 (t, 3 H, CH3) ppm; HRMS
(m/ z) M+ calcd 318.1071, obsd 318.1053. Anal. Calcd for C20H22Fe: C,
75.48; H, 6.97. Found: C, 75.55; H, 7.38. 7: Mp 158-160 °C; 1H NMR-
(CDCl3) δ 5.42 (s, 2 H), 4.62 (t, 1.95 Hz, 4 H), 4.27 (t, 1.95 Hz, 4 H),
4.15 (s, 10 H) ppm; HRMS (m/ z) M+ calcd 396.0264, obsd 396.0586.
Anal. Calcd for C22H20Fe2: C, 66.71; H, 5.09. Found: C, 66.47; H, 5.04.
8: 1H NMR (CDCl3) δ 7.55-7.445 (m, 2 H, Ph), 7.38-7.21 (m, 3 H,
Ph), 3.81 (s, 5 H, Cp), 2.01 (s, 6 H, CH3), 1.95 (s, 6 H, CH3) ppm; HRMS
(m/ z) M+ calcd 318.1071, obsd 318.1273. Anal. Calcd for C20H22Fe: C,
75.48; H, 6.97. Found: C, 75.39; H, 7.48.
X Abstract published in Advance ACS Abstracts, J anuary 1, 1997.
(1) (a) Togni, A., Hayashi, T., Eds. Ferrocenes: homogeneous cataly-
sis, organic synthesis, and material science; VCH: Weinheim, Germany,
1995. (b) Oriol, L.; Serrano, J . L. Adv. Mater. 1995, 7, 348. (c) Coville,
N. J .; du Plooy, K. E.; Pickl, W. Coord.Chem. Rev. 1992, 116, 1.
(2) (a) Dong, T.-Y.; Chang, C.-K.; Huang, C.-H.; Wen, Y. S.;Lee, S.
L.; Chen, J . A.; Yeh, W. Y.; Yeh, A. J . Chem. Soc., Chem. Commun.
1992, 526. (b) Dong, T.-Y.; Chang, C.-K.; Huang, C.-H.; Wen, Y. S.;
Lee, S. L.; Chen, J . A.; Yeh, W. Y.; Yeh, A. J . Am. Chem. Soc. 1993,
115, 6357. (c) Geiger, W. E. J . Organomet. Chem. Libr. 1990, 22, 142.
(d) Zanello, P.; Opromolla, G.; Giorgi, G.; Sasso, G.; Togni, A. J .
Organomet. Chem. 1996, 506, 61.
(3) (a) Rockett, B. W.; Marr, G. J . Organomet. Chem. 1991, 416,
327-398. (b) Dong, T.-Y.; Lai, L.-L. J . Organomet. Chem. 1996, 509,
131.
(4) (a) Oberhoff, M.; Duda, L.; Karl, J .; Mohr, R.; Erker, G.; Fro¨hlich,
R.; Grehl, M. Organometallics 1996, 15, 4005. (b) Dong, T.-Y.; Lai, L.-
L. J . Organomet. Chem. 1996, 509, 131. (c) Guillaneux, D.; Kagan, H.
B. J . Org. Chem. 1995, 60, 2502. (d) Buchmeiser, M.; Schottenberger,
H. Organometallics 1993, 12, 2472. (e) Clark, T. J .; Killian, C. M.; Nile,
T. A. J . Organomet. Chem. 1993, 462, 247. (f) Gassman, P. G.;
Mickelson, J . W.; Sowa, J . R., J r. J . Am. Chem. Soc. 1992, 114, 6942.
(g) Plenio, H. Organometallics 1992, 11, 1856. (h) Bunel, E. E.; Valle,
L.; J ones, N. L.; Carroll, P. J .; Gonzalez, M.; Munoz, N.; Manriquez, J .
M. Organometallics 1988, 7, 789.
(5) (a) Bunel, E. E.; Valle, L.; Manriquez, J . M. Organometallics
1985, 4, 1680. (b) Bunel, E. E.; Valle, L.; J ones, N. L.; Carroll, P. J .;
Barra, C.; Gonzalez, M.; Munoz, N.; Visconti, G.; Aizman, A.; Man-
riquez, J . M. J . Am. Chem. Soc. 1988, 110, 6596.
(6) (a) Bickert, P.; Hildebrandt, B.; Hefner, K. Organometallics 1984,
3, 653. (b) Paquette, L. A.; Schirch, P. F. T.; Hathaway, S. J .; Hsu,
L.-Y.; Gallucci, J . C. Organometallics 1986, 5, 490.
S0276-7333(96)00517-1 CCC: $14.00 © 1997 American Chemical Society

Communications
Organometallics, Vol. 16, No. 3, 1997 305
Ta ble 1. Yield s of Mixed F er r ocen es
Several results presented in Table 1 are worthy of
comment. (1) This synthetic route is quite generally
applicable to the synthesis of mixed ferrocenes bearing
hydrocarbon substituent(s). (2) This route is quite
efficient and produces moderate to high yields. Moise
and Leblanc11 reported the formation of 11 by refluxing
a solution of FeCl2 with NaCp and the anion from
6-methyl-6-phenylfulvene in THF for 2 days. However,
they obtained 11 in only 11% yield. By using the CpFe
transfer reaction, 11 was obtained in 81% yield. (3) The
reaction is quite straightforward, and therefore, the
products are easily separated in pure form. In most
cases, byproducts were ferrocene and fluorene. (4) The
successful conversion of 6 to 7 demonstrates the utility
of this new synthetic route for the preparation of
bimetallic 1,1-diferrocenylethylene. The low yield of 7
was due to its partial decomposition during column
chromatography on silica gel. After isolation, 7 can be
stored in the freezer for several days. Compound 7
cannot be obtained by other routes. (5) Substituted
cyclopentadiene derivatives used in this study were
easily obtained from readily available organic com-
pounds. Most of the cyclopentadiene derivatives used
in this study were known or could be easily synthesized.
(6) Multigram quantities of mixed ferrocenes can be
obtained.12 Thus, the route described in this report is
quite useful from a synthetic point of view.
a
An isomeric mixture.
At present we have no conclusive mechanism to
explain the formation of mixed ferrocenes. However, it
is well-known that arene ligands in (arene)FeCp+
cations are readily displaced by other ligands6,13,14 due
to their stability in the free state and the longer bond
length15 of Fe to the arenes compared to the cyclopen-
tadienyl ligands. Thus, we expected that for 2 a ring
slippage and ring displacement by a 2e or 6e nucleophile
may be possible without difficulty. As we expected,
treatment of 2 with P(OMe)3 in THF led to a ligand
+ 13
exchange to yield CpFe{P(OMe)3}3
.
The reaction
may involve the nucleophilic substitution by Cp′ anion
with a concomitant ring slippage and, finally, ring
displacement and the liberation of fluorene and forma-
tion of CpFeCp′. A similar interpretation has been
presented to explain the formation of [(arene)M(CO)3]n+
(M ) Cr, n ) 0; M ) Mn, n ) 1) in the reaction of
[(naphthalene)M(CO)3]n+ or (N-methylpyrrole)Cr(CO)3
with arenes.16
(11) Compounds 4, 5, and 9-14 are known compounds. The forma-
tion of 4, 5, 9, and 12-14 were confirmed by checking 1H NMR spectra.
Compounds 10 and 11 were confirmed by 1H NMR spectra, elemental
analysis, and high-resolution mass. See the following papers. 4:
Rosenblum, M.; Howells, W. G.; Banerjee, A. K.; Bennet, C. J . Am.
Chem. Soc. 1962, 84, 2726. Kamezawa, N. J . Magn. Reson. 1973, 11,
88. 5: Hallam, B. F.; Pauson, P. L. J . Chem. Soc. 1956, 3030. 9: King,
R. B.; Bisnette, M. B. Inorg. Chem. 1964, 3, 796. Treichel, P. M.;
J ohnson, J . W. J . Organomet. Chem. 1975, 88, 207. 10: Horspool, W.
M.; Sutherland, R. G.; Thomson, B. J . J . Chem. Soc. C 1971, 1550.
Cardin, C. J .; Crawford, W.; Watts, W. E.; Hathaway, B. J . J . Chem.
Soc., Dalton Trans. 1979, 970. 11: Leblanc, J . C.; Moise, C. J .
Organomet. Chem. 1976, 120, 65. 12: Bunel, E. E.; Valle, L.; Man-
riquez, J . M. Organometallics 1985, 4, 1680. 13: Nesmeyanov, A. N.;
Sazonova, V. A.; Drozd, V. N. Dokl. Akad. Nauk SSSR 1965, 165, 575.
Lee, M.-T.; Foxman, B. M.; Rosenblum, M. Organometallics 1985, 4,
547. 14: Pouchert, C. J .; Behnke, J . The Aldrich Library of 13C and
1H FT-NMR Spectra; Aldrich Chemical: Milwaukee, WI, 1992; Vol.
1(3), 771A.
(12) The cyclopentadiene ligand (6.60 g, 39 mmol, 1.5 equiv) (entry
8 in Table 1) in 30 mL of THF was treated with n-BuLi (5 mL of 10 M
solution in hexanes, 50 mmol) at -78 °C. The resulting solution was
transferred by cannula to another flask containing 2 (generated in situ
by the reaction of 1 (11.24 g, 26 mmol) and t-BuOK (39 mmol, 4.38 g
in 120 mL of THF) at 0 °C). The reaction mixture was heated at 60 °C
for 6 h. After cooling of the mixture to room temperature, water (100
mL) and hexane (300 mL) were added to the reaction mixture and,
subsequently, 50 mL of saturated NH4Cl solution. The organic layer
was separated and purified. The yield based on 2 was 81% (6.10 g).
(13) (a) Gill, T. P.; Mann, K. R. J . Organomet. Chem. 1981, 216, 65.
(b) Lee, C. C.; Gill, U. S.; Iqbal, M.; Azogu, C. I.; Sutherland, R. G. J .
Organomet. Chem. 1982, 231, 151. Compounds 2 (generated in situ
by the reaction of 0.86 g (2 mmol) of 1 and t-BuOK (0.38 g, 3.4 mmol)
in 10 mL of THF at 0 °C) and P(OMe)3 (0.94 mL, 8 mmol) were
dissolved in 20 mL of THF. The solution was heated at reflux for 5 h
and cooled to room temperature. A solution of HBF4‚OEt2 (4 mmol)
was added to the reaction mixture to provide a suitable counteranion.
The resulting solution was concentrated and the product precipitated
by adding an excess of Et2O. The precipitates was filtered, recrystal-
lized from CH2Cl2/Et2O, and dried. The product was obtained in 71%
(0.82 g) yield. The 1H NMR spectrum of the product was the same as
that reported by Gill and Mann.13a FAB mass (M+): m/ z 493.
(14) (a) Roberts, R. M. G.; Wells, A. S. Inorg. Chim. Acta 1986, 112,
171. (b) Maigrot, N.; Ricard, L.; Charrier, C.; Mathey, F. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 534. (c) Darchen, A. J . Chem. Soc., Chem.
Commun. 1983, 768.
(15) Houlton, A.; Roberts, R. M. G.; Silver, J .; Wells, A. S.; Frampton,
C. S. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1990, C46,
1018.
(16) (a) Sun, S.; Yeung, L. K.; Sweigart, D. A.; Lee, T.-Y.; Lee, S. S.;
Chung, Y. K.; Switzer, S. R.; Pike, R. D. Organometallics 1995, 14,
2613. (b) Ku¨ndig, E. P.; Perret, C.; Spichiger, S.; Bernardinelli, G. J .
Organomet. Chem. 1985, 286, 183. Goti, A.; Semmelhack, M. F. J .
Organomet. Chem. 1994, 470, C4.

306 Organometallics, Vol. 16, No. 3, 1997
Communications
One of the ways to prepare bi- and polymetallic
compounds is through the utilization of a compound
having cyclopentadienyl or phenyl rings.17 Thus, we
expected that 5 could be used to make bi- or trimetallic
compounds. Reaction of 5 with Cr(CO)6 gave 15 and
16 in 18% and 62% yields, respectively (eq 2).18
The molecular structure of 16 was studied by X-ray
crystallography (Figure 1).19 At first, we expected that
the two Cr(CO)3 moieties would be in syn arrangement.
However, the two Cr(CO)3 moieties are in anti confor-
mation presumably due to steric congestion. To reduce
the steric congestion between the two Cr(CO)3 moieties,
the two phenyl groups are not in the same plane as the
cyclopentadienyl ring. Recently, we and our collabora-
tor20 reported the control of ligand substitution and
addition to (arene)Cr(CO)3 complexes by attachment of
a self-closing redox-switch ferrocenyl group. We expect
that 15 and 16 will show similar chemical behavior.
F igu r e 1. ORTEP drawing of 16 with the atomic number-
ing. Selected bond distances (Å): Fe-C(1), 2.049(4); Fe-
C(3), 2.042(4); Fe-C(4), 2.024(4); C(1)-C(2), 1.445(5); C(2)-
C(3), 1.419(6); C(3)-C(4), 1.414(6); C(1)-C(11), 1.476(5).
Selected bond angles (deg): C(1)-C(2)-C(3), 107.1(3);
C(1)-C(5)-C(4), 108.9(4); C(2)-C(1)-C(5), 106.9(3); C(1)-
C(2)-C(17), 125.7(3); Cr(1)-C(03)-O(03), 179.5(3).
The synthetic method described in this communica-
tion is generally applicable to the synthesis of mixed
ferrocenes. In particular, it will be very useful for the
construction of mixed ferrocenes suitable for the syn-
theses of charge-transfer complexes and thermotropic
liquid crystals.21 The physical studies of the biferrocene
compound 7 and the heterobimetallic compounds 15 and
16 are in progress.
(17) (a) Chung, T.-M.; Chung, Y. K. Organometallics 1992, 11, 2822.
(b) Lee, B. Y.; Moon, H.; Chung, Y. K.; J eong, N.; Carpenter, G.
Organometallics 1993, 12, 2973. (c) Kim, J .-A.; Chung, T.-M.; Chung,
Y. K.; J ung, J .-H.; Lee, S. W. J . Organomet. Chem. 1995, 486, 211. (d)
Kang, Y. K.; Chung, Y. K.; Lee, S. W. Organometallics 1995, 14, 4905.
(18) Preparation of compounds 15 and 16: Compound 4 (0.12 g, 0.35
mmol) and Cr(CO)6 (0.31 g, 1.4 mmol) were dissolved in a mixture of
n-Bu2O (50 mL) and THF (5 mL). The resulting solution was heated
at reflux for 3 d. The reaction mixture was filtered, concentrated, and
column chromatographed on silica gel eluting with a mixture of hexane
and ethyl acetate (v/v, 10:1) and then with CH2Cl2. The first eluted
compound was 15, and the next was 16. Compounds 15 and 16 were
obtained in 18% (0.03 g, 0.06 mmol) and 62% yield (0.13 g, 0.22 mmol),
Ack n ow led gm en t. The authors are grateful for the
financial support from the SNU DaeWoo Research Fund
and the Center for Molecular Catalysis.
respectively. 15: Mp 53-56 °C; IR ν(CO) 1950, 1864 cm-1 1H NMR
;
(CDCl3) δ 7.4-7.2 (m, 5 H, Ph), 5.8-5.2 (m, 5 H, Ph), 4.71 (dd, 1.46,
2.44 Hz, 1 H), 4.51 (dd, 1.46, 2.44 Hz, 1 H), 4.44 (t, 2.44 Hz, 1 H), 4.19
(s, 5 H, Cp) ppm; HRMS (m/ z) M+ calcd 474.0010, obsd 474.0014. Anal.
Calcd for C25H18FeCrO3: C, 63.31; H, 3.82. Found: C, 63.50; H, 3.99.
Su p p or tin g In for m a tion Ava ila ble: Tables giving crys-
tal data and refinement details, atomic coordinates, thermal
parameters, and bond distances and angles for 16 (7 pages).
Ordering information is given on any current masthead page.
16: Mp 194-196 °C; IR ν(CO) 1946, 1859 cm-1 1H NMR (CDCl3) δ
;
5.8-5.2 (m, 10 H, Ph), 4.86 (d, 2.68 Hz, 2 H), 4.52 (t, 2.68 Hz, 1 H),
4.33 (s, 5 H, Cp) ppm. Anal. Calcd for C28H18Cr2FeO6: C, 55.11; H,
2.97. Found: C, 55.06; H, 2.91.
OM960517C
(19) Single crystals suitable for X-ray analysis were obtained by CH2-
Cl2 and hexane: monoclinic; a ) 13.749(2), b ) 9.841(3), c ) 18.711(5)
Å; â )106.92(2)°; space group P21/a; Z ) 4; Dcalcd ) 1.673 g cm-3; 3017
independent reflections (2.28° < θ < 24.96°); solution of the structure
by the conventional heavy-atom method (SHELXS-86); hydrogen
positions calculated according to ideal geometry with a C-H bond
length 1.08 Å. Refinement by use of SHELXL-93 with anisotropic
temperature factors for all non-hydrogen atoms gave R ) 0.0387 and
wR2 ) 0.0925 for 3017 unique reflections with I > 2σ(I).
(21) (a) Deschenaux, R.; Izvolenski, V.; Turpin, F.; Guillon, D.;
Heinrich, B. J . Chem. Soc., Chem. Commun. 1996, 439. (b) Deschenaux,
R.; Kosztics, U.; Scholten, U.; Guillon, D.; Ibn-Elhaj, M. J . Mater. Chem.
1994, 4, 1351. (c) Constable, E. C.; Edwards, A. J .; Manez, R. M.;
Raithby, P. R.; Cargill Thompson, A. M. W. J . Chem. Soc., Dalton
Trans. 1994, 645. (d) Bosque, R.; Lopez, C.; Sales, J .; Solans, X.; Bardia,
M. F. J . Chem. Soc., Dalton Trans. 1994, 735. (e) Bosque, R.; Bardia,
M. F.; Lopez, C.; Sales, J .; Silver, J .; Solans, X. J . Chem. Soc., Dalton
Trans. 1994, 747. (f) Bhadhabe, M. M.; Das, A.; J effery, J . C.;
McCleverty, J . A.; Novas Badiola, J . A.; Ward, M. D. J . Chem. Soc.,
Dalton Trans. 1995, 2769.
(20) Yeung, L. K.; Kim, J . E.; Chung, Y. K.; Sweigart, D. A.
Organometallics 1996, 15, 3891.