Facile meso to rac isomerization of the bis-planar chiral ferrocenyldiphosphine bis (1-(diphenylphosphino)-η5-indenyl)iron (II)

Article abstract of DOI:10.1021/om020200e

The facile meso to rac isomerization of the bis-planar chiral ferrocenyldiphosphine was reported. The isomerization took place overnight in tetrahydrofuran at room temperature. The isomerization was accelerated by salts but did not occurred on chloroform

Full text of DOI:10.1021/om020200e

Organometallics 2002, 21, 2827-2829
2827
F a cile m eso to r a c Isom er iza tion of th e Bis-P la n a r Ch ir a l
F er r ocen yld ip h osp h in e
Bis(1-(d ip h en ylp h osp h in o)-η5-in d en yl)ir on (II)
Owen J . Curnow* and Glen M. Fern
Department of Chemistry, University of Canterbury, Private Bag 4800,
Christchurch, New Zealand
Received March 12, 2002
Summary: The meso isomer of the ferrocenyldiphosphine
bis(1-(diphenylphosphino)-η5-indenyl)iron(II) isomerizes
to the rac isomer overnight in tetrahydrofuran at room
temperature. The isomerization is accelerated by salts
but does not occur in chloroform solvent.
Ferrocenylphosphines continue to be intensively in-
vestigated for their utility in homogeneous catalysis;
chiral derivatives are of particular interest for asym-
metric catalysis.1 The introduction of a chiral substitu-
ent or the generation of planar chirality is usually used
to create the chirality. Despite the large number of
planar chiral ferrocenylphosphines that have been
reported and used in asymmetric catalysis, no racem-
ization has been observed in these systems. Compounds
containing two planar chiral units may exhibit rac and
meso isomers. We recently reported the preparation of
the diindenyl analogue of 1,1′-bis(diphenylphosphino)-
ferrocene (dppf), bis(1-(diphenylphosphino)-η5-indenyl)-
iron(II) (1), and the characterization of its rac and meso
isomers by X-ray crystallographic studies of their tet-
racarbonylmolybdenum complexes.2 Unfortunately, we
had been unable to isolate the two isomers. We report
here the isolation of the rac isomer and the remarkably
facile isomerization of the meso isomer to the rac isomer.
F igu r e 1. ORTEP drawing of rac-1 indicating the num-
bering of the atoms. The thermal ellipsoids have been
drawn at 50% probability. Selected bond lengths (Å): Fe-
CNT ) 1.672, Fe-C(1) ) 2.062(3), Fe-C(2) ) 2.045(3), Fe-
C(3) ) 2.067(3), Fe-C(8) ) 2.090(3), Fe-C(9) ) 2.104(3),
C(1)-C(2) ) 1.446(4), C(1)-C(8) ) 1.448(4), C(2)-C(3) )
1.425(4), C(3)-C(9) ) 1.432(4), C(4)-C(9) ) 1.427(4), C(4)-
C(5) ) 1.359(5), C(5)-C(6) ) 1.441(5), C(6)-C(7) ) 1.349(4),
C(7)-C(8) ) 1.436(4), C(8)-C(9) ) 1.447(4), C(1)-P )
1.829(3), P-C(10) ) 1.850(3), P-C(20) ) 1.833(3). Bond
angles (deg): C(1)-P-C(10) ) 101.51(15), C(1)-P-C(20)
) 100.85(15), C(10)-P-C(20) ) 102.37(14), CNT-C(1)-P
) 176.07. Dihedral angle (deg): C(1)-CNT-CNT′-C(1′)
) 123.07.
Addition of ferrous chloride to 2 equiv of lithi-
um 1-(diphenylphosphino)indenide in tetrahydrofuran
(Scheme 1) initially produces a mixture of two phos-
phorus-containing compounds, as shown by peaks in the
31P NMR spectrum at -22.26 and -26.53 ppm. After
the mixture was stirred overnight, only the peak at
-22.26 ppm was observed.3 Crystals of this compound
were obtained and shown by X-ray crystallography4 to
be the rac isomer of 1 (Figure 1). If the reaction is
stopped after 2 h, the two compounds can be isolated
as a mixture.5 1H and 13C NMR spectroscopy of the
mixture shows the two compounds to have similar
spectra, with only minor differences in the chemical
shifts. Since the rac and meso isomers are expected to
Sch em e 1. Syn th esis a n d Isom er iza tion of 1
* To whom correspondence should be addressed. Fax: +64 3 364
2110. E-mail: o.curnow@chem.canterbury.ac.nz.
(1) (a) Wagner, G.; Herrmann, R. In Ferrocenes: Homogeneous
Catalysis, Organic Synthesis, and Materials Science; Togni, A., Ha-
yashi, T., Eds.; VCH: Weinheim, Germany, 1995; pp 173-218. (b) Togni,
A. In Metallocenes: Synthesis, Reactivity, and Applications; Togni, A.,
Halterman, R. L., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Vol.
2, pp 685-722. (c) Hayashi, T. In Ferrocenes: Homogeneous Catalysis,
Organic Synthesis, and Materials Science; Togni, A., Hayashi, T., Eds.;
VCH: Weinheim, Germany, 1995; pp 105-142. (d) Togni, A. Angew.
Chem., Int. Ed. Engl. 1996, 35, 1475.
have similar NMR patterns, it is reasonable to say that
the other compound is the meso isomer of 1.
The conversion of the meso isomer to the rac isomer
requires one of the indenyl rings to flip over and
coordinate to the iron atom by the other face. To the
best of our knowledge, this has not been observed in
any ferrocenylphosphine, although racemization of acyl-
(2) Adams, J . J .; Berry, D. E.; Browning, J .; Burth, D.; Curnow, O.
J . J . Organomet. Chem. 1999, 580, 245.
10.1021/om020200e CCC: $22.00 © 2002 American Chemical Society
Publication on Web 06/11/2002

2828 Organometallics, Vol. 21, No. 14, 2002
Communications
ferrocenes in nitroalkane solvents in the presence of
strong acids, such as HClO4 or AlCl3, has been reported.6
Other processes resulting in the flipping over of a
cyclopentadienyl ligand have been observed in metal-
locene systems of the lanthanides7 and group 38 and
group 49 metals. A number of the group 4 ring-flipping
processes are photochemically initiated.9a-g Also of
particular note, Hollis and Fu have independently
described a diphosphazirconocene in which a phosphole
flips over via a proposed intermediate in which the
phosphole is coordinated by only the P atom.10
Herberich and co-workers have described systems in
which cyclopentadienyl rings are exchanged between
(3) To a solution of 1-(diphenylphosphino)indene (1.8 g, 6.0 mmol)
in tetrahydrofuran (50 mL) at -80 °C was added a solution of n-BuLi
(3.75 mL, 1.6 M, 6.0 mmol). After 2 h, FeCl2 (0.38 g, 3 mmol) was added
and the reaction mixture was stirred for 12 h at ambient temperature
to give a dark green solution. The solvent was removed in vacuo, and
the residue was loaded onto a Celite column and washed with diethyl
ether (to remove unreacted 1-(diphenylphosphino)indene). Subsequent
elution with dichloromethane yielded 1.26 g (64%) of rac-1 as a dark
blue powder. Dark blue crystallographic-quality crystals were obtained
by recrystallization from CH2Cl2/diethyl ether. Data for rac-1: 1H NMR
F igu r e 2. Conversion of meso-1 to rac-1 in tetrahydrofu-
ran at various temperatures: 23 °C, k1 ) [1.59(3)] × 10-5
s-1; 30 °C, k1 ) [3.01(9)] × 10-5 s-1; 40 °C, k1 ) [5.89(16)]
× 10-5 s-1; 50 °C, k1 ) [1.20(4)] × 10-4 s-1
.
3
3
(CDCl3) δ 3.07 (d, J HH ) 2 Hz, 2H, H-2), 4.92 (d, J HH ) 2 Hz, 2H,
H-3), 6.4-7.4 (m, 28H, H4-7 and Ph); 13C{1H} NMR (CDCl3) δ 66.1
3
1
2
metal centers via triple-decker sandwich intermediates
and which result in coordination of the rings by the
other face. This involves an intermolecular exchange
process.11 That such a process may occur in some
ferrocene systems is evidenced by the cyclopentadienyl-
exchange reactions between ferrocenes, using the Lewis
acid AlCl3,12 and the synthesis of [Cp3Fe2]+ by Kudi-
nov.13 Other examples of cyclopentadienyl-transfer reac-
tions from ferrocenes include the following: transfer of
acylferrocenes to Re,14 a redistribution reaction between
ferrocene and 1,1′-dimethylferrocene at 250 °C after 3
days,15 and the synthesis of ruthenocene from ferrocene
and RuCl3 at 250 °C.16
(d, J PC ) 4 Hz, C-3), 68.1 (d, J PC ) 9 Hz, C-1), 72.0 (d, J PC ) 4 Hz,
2
3
C-2), 90.3 (d, J PC ) 4 Hz, C-9), 91.0 (d, J PC ) 25 Hz, C-8), 122.5 (s,
C-5), 122.9 (s, C-6), 123.6 (s, C-4), 124.1 (d, 3J PC ) 9 Hz, C-7), 127.6 (s,
p-Ph), 128.0 (d, 3J PC ) 5 Hz, m-Ph), 128.3 (d, 3J PC ) 8 Hz, m-Ph), 129.3
2
2
(s, p-Ph), 131.7 (d, J PC ) 18 Hz, o-Ph), 135.2 (d, J PC ) 22 Hz, o-Ph),
1
1
136.7 (d, J PC ) 7 Hz, ipso-Ph), 139.8 (d, J PC ) 10 Hz, ipso-Ph);
31P{1H} NMR (CDCl3) δ -22.26 (s).
(4) Crystal data for rac-1: C42H32FeP2; Mr ) 654.47; orthorhombic,
Pbcn; a ) 12.959(6) Å, b ) 12.396(5) Å, c ) 19.296(8) Å, V ) 3100(2)
Å3, Z ) 4, F(000) ) 1360, Dcalcd ) 1.402 g cm-3, Mo KR radiation (λ )
0.710 73 Å), µ ) 0.6221 mm-1, T ) 168(2) K; θ range 2.11-26.44°;
3142 unique reflections, 204 parameters; R1 ) 0.0351, wR2 ) 0.0783
(I > 2σ(I)), largest difference peak and hole 0.308 and -0.427 e Å-3
.
Data were collected on a Siemens P4 Smart CCD area detector; the
structure was solved by direct methods and refined by full-matrix least-
squares methods on F2.
(5) The same procedure as for the preparation of rac-1 was used,
except that the reaction solution is stirred for only 2 h before the
solvent is removed in vacuo. Yields obtained for mixtures do not vary
significantly from that obtained for the rac isomer. Data for meso-1:
The isomerization of meso-1 to rac-1 was followed by
31P NMR spectroscopy at a variety of temperatures
(Figure 2). The isomerization is first order in ferrocene
concentration, and the activation parameters were
found to be ∆Hq ) 57 ( 4 kJ mol-1 and ∆Sq ) -145 (
15 J mol-1 K-1. In neat tetrahydrofuran, the rate is
[1.59(3)] × 10-5 s-1 at 23 °C. During the synthesis of 1,
however, the rate was found to be [3.60(9)] × 10-5 s-1
([1] ) 0.0605 M, T ) 23 °C). Extrapolation back to t )
0 gives an initial rac to meso ratio of 55:45. Possible
causes of the rate increase include excess indenyldiphen-
ylphosphine and dissolved salts. It was found that
additional indenyldiphenylphosphine did not increase
the rate, whereas addition of LiCl increased the rate
significantly (at [LiCl] ) 0.081 M and [1] ) 0.020 M, k1
3
3
1H NMR (CDCl3) δ 3.48 (d, J HH ) 2 Hz, 2H, H-2), 3.81 (d, J HH ) 2
Hz, 2H, H-3), 6.88-7.53 (m, 28H, H4-7 and Ph); 13C{1H} NMR (CDCl3)
1
δ 64.4 (s, C-3), 66.9 (d, J PC ) 13 Hz, C-1), 74.5 (s, C-2), 90.3 (s, C-9),
91.6 (d, 2J PC ) 22 Hz, C-8), 124.3 (s, C-5), 124.9 (s, C-6), 127.7 (s, p-Ph),
3
3
127.9 (s, C-4), 128.0 (d, J PC ) 3 Hz, m-Ph), 128.1 (d, J PC ) 10 Hz,
3
2
C-7), 128.2 (d, J PC ) 8 Hz, m-Ph), 129.0 (s, p-Ph), 132.4 (d, J PC ) 20
2
1
Hz, o-Ph), 135.4 (d, J PC ) 21 Hz, o-Ph), 137.4 (d, J PC ) 11 Hz, ipso-
Ph), 139.1 (d, J PC ) 14 Hz, ipso-Ph); 31P{1H} NMR (CDCl3) δ -26.53
1
(s).
(6) (a) Falk, H.; Lehner, H.; Paul, J .; Wagner, U. J . Organomet.
Chem. 1971, 28, 115. (b) Slocum, D. W.; Tucker, S. P.; Engelmann, T.
R. Tetrahedron Lett. 1970, 621.
(7) (a) Giardello, M. A.; Conticello, V. P.; Brard, L.; Sabat, M.;
Rheingold, A. L.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994,
116, 10212. (b) Haar, C. M.; Stern, C. L.; Marks, T. J . Organometallics
1996, 15, 1765. (c) Hultzsch, K. C.; Spaniol, T. P.; Okuda, J . Organo-
metallics 1997, 16, 4845.
(8) Yoder, J . C.; Day, M. W.; Bercaw, J . E. Organometallics 1998,
17, 4946.
(9) (a) Wild, F. R. W. P.; Zsolnai, L.; Huttner, G.; Brintzinger, H.-
H. J . Organomet. Chem. 1982, 232, 233. (b) Wiesenfeldt, H.; Reinmuth,
A.; Barsties, E.; Evertz, K.; Brintzinger, H.-H. J . Organomet. Chem.
1989, 369, 359. (c) Schmidt, K.; Reinmuth, A.; Rief, U.; Diebold, J .;
Brintzinger, H.-H. Organometallics 1997, 16, 1724. (d) Kaminsky, W.;
Schauwienold, A.-M.; Freidanck, F. J . Mol. Catal. A 1996, 112, 37. (e)
Rheingold, A. L.; Robinson, N. P.; Whelan, J .; Bosnich, B. Organome-
tallics 1992, 11, 1869. (f) Collins, S.; Hong, Y.; Taylor, N. J . Organo-
metallics 1990, 9, 2695. (g) Vitz, E. D.; Wagner, P. J .; Brubaker, C.
H., J r. J . Organomet. Chem. 1976, 107, 301. (h) Ringwald, M.;
Stuermer, R.; Brintzinger, H.-H. J . Am. Chem. Soc. 1999, 121, 1524.
(i) Diamond, G. M.; Rodewald, S.; J ordan, R. F. Organometallics 1995,
14, 5. (j) Christopher, J . N.; Diamond, G. M.; J ordan, R. F.; Petersen,
J . L. Organometallics 1996, 15, 4038. (k) Diamond, G. M.; J ordan, R.
F.; Petersen, J . L. Organometallics 1996, 15, 4045. (l) Diamond, G.
M.; J ordan, R. F.; Petersen, J . L. Organometallics 1996, 15, 4030.
(10) (a) Hollis, T. K.; Wang, L.-S.; Tham, F. J . Am. Chem. Soc. 2000,
122, 11737. (b) Bellemin-Laponnaz, S.; Lo, M. M. C.; Peterson, T. H.;
Allen, J . M.; Fu, G. C. Organometallics 2001, 20, 3453.
(11) (a) Herberich, G. E.; J ansen, U. J . Organometallics 1995, 14,
834. (b) Herberich, G. E.; Dunne, B. J .; Hessner, B. Angew. Chem. 1989,
101, 798. (c) Herberich, G. E.; Bueschges, U.; Dunne, B. J .; Hessner,
B.; Klaff, N.; Koeffer, D. P. J .; Peters, K. J . Organomet. Chem. 1989,
372, 53. (d) Herberich, G. E.; Englert, U.; Marken, F.; Hofmann, P.
Organometallics 1993, 12, 4039.
(12) (a) Bublitz, D. Can. J . Chem. 1964, 42, 2381. (b) Bublitz, D. E.
J . Organomet. Chem. 1969, 16, 149. (c) Hayashi, T.; Okada, Y.;
Shimizu, S. Transition Met. Chem. 1996, 21, 418. (d) Astruc, D.;
Dabard, R. J . Organomet. Chem. 1975, 96, 283. (e) Astruc, D.; Dabard,
R. J . Organomet. Chem. 1976, 111, 339.
(13) Kudinov, A. R.; Fil’chikov, A. A.; Petrovskii, P. V.; Rybinskaya,
M. I. Russ. Chem. Bull. (Engl. Transl.) 1999, 48, 1352.
(14) Spradau, T. W.; Katzenellenbogen, J . A. Organometallics 1998,
17, 2009.
(15) Allcock, H. R.; McDonnell, G. S.; Riding, G. H.; Manners, I.
Chem. Mater. 1990, 2, 425.
(16) Gauthier, G. J . Chem. Soc., Chem. Commun. 1969, 690.

Communications
Organometallics, Vol. 21, No. 14, 2002 2829
Sch em e 2. Som e P ossible Mech a n ism s for th e Isom er iza tion of 1
) [3.59(14)] × 10-4 s-1 at 30 °C). LiCl is the major salt
present during the synthesis. In the noncoordinating
solvent chloroform, no isomerization was observed after
1 week, indicating that the tetrahydrofuran solvent is
intimately involved in the mechanism. On the basis of
these observations, our proposed mechanism involves
coordination of THF (one or two molecules) and slippage
of the indenylphosphine ligand until the indenide dis-
sociates and the ligand coordinates by only the phos-
phorus atom (although we cannot yet rule out complete
dissociation of Ph2P(C9H6)-), thus generating the zwit-
terionic intermediate 7 (Scheme 2), which could be
stabilized by the presence of ionic species. Thus, solvent-
assisted dissociation of indenide from 6 to 7 is the rate-
determining step. Recoordination of the five-membered
indenide ring could then occur by either face. The large
negative entropy of activation supports an associative
rate-determining step and solvent-stabilized intermedi-
ate. In further support of this mechanism, dmpe has
been observed to completely displace indenide from
(C9H7)Rh(C2H4)2 to form [Rh(dmpe)2][C9H7].17
rings (illustrated by the offset π stacking conformation
in the solid-state crystal structure),18 since this cannot
occur in the meso isomer without creating significant
steric interactions between the diphenylphosphino
groups. The question of why this isomerization has not
been observed in other ferrocenylphosphines also needs
to be addressed: indenyl ligands generally undergo
more facile ring-slippage reactions than cyclopentadi-
enyl ligands, and the usual explanation for this is
generation of aromaticity in the six-membered ring upon
ring slippage of the indenyl ligand.19 However, for
complete dissociation of an indenyl ligand, to give our
proposed intermediate 7, the six-membered ring does
not become aromatic. This suggests that the ring-slipped
intermediates 5 and 6, in which the six-membered ring
does become aromatic, are readily formed.
To summarize, we have reported an unprecedented
and facile meso to rac isomerization of the ferrocenyl-
diphosphine bis(1-(diphenylphosphino)-η5-indenyl)iron-
(II) of relevance to the chemistry of other planar chiral
ferrocenes with phosphine substituents. In particular,
one cannot assume the stability of other planar chiral
ferrocenylphosphine systems. Further studies are being
carried out to define the mechanism and test its
generality.
Although a 1,3-proton shift could not result in isomer-
ization, a 1,5-proton shift, after ring slippage via 3 or 5
and 8, to give an intermediate such as 4 or 9 could.
However, these intermediates are not likely to be as
energetically favorable as 7, since their formation
involves a loss of aromaticity at some stage, and we
would not expect the rate to be significantly affected by
salts.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, atomic positional parameters, and bond distances and
angles for rac-1. This material is available free of charge via
the Internet at http://pubs.acs.org.
The higher stability of the rac isomer versus the meso
isomer may be a result of π stacking between the benzo
OM020200E
(17) (a) Kakkar, A. K.; Taylor, N. J .; Marder, T. B. Organometallics
1989, 8, 1765. (b) Marder, T. B.; Williams, I. D. J . Chem. Soc., Chem.
Commun. 1987, 1478.
(18) Hunter, C. A.; Lawson, K. R.; Perkins, J .; Urch, C. J . J . Chem.
Soc., Perkin Trans. 2 2001, 651.
(19) O’Connor, J . M.; Casey, C. P. Chem. Rev. 1987, 87, 307.