Dimerization of phosphasilaferracycles: Formation and structures of isomeric Fe2Si2P2 six-membered metallacylcles

Article abstract of DOI:10.1021/om034264g

Irradiation of Cp(CO)₂FeSiMe₂PPh₂ led to formation of an isomeric mixture of Cp₂Fe₂(CO) ₂[μ₂-{μ²(Si,P)-Me₂SiPPh ₂}]₂ through generatio

Full text of DOI:10.1021/om034264g

Organometallics 2004, 23, 1971-1973
1971
Dim er iza tion of P h osp h a sila fer r a cycles: F or m a tion a n d
Str u ctu r es of Isom er ic F e2Si2P 2 Six-Mem ber ed
Meta lla cylcles
Masaaki Okazaki,* Kazuyuki Satoh, Kyeong A. J ung, Hiromi Tobita,* and
Hiroshi Ogino†
Department of Chemistry, Graduate School of Science, Tohoku University,
Sendai 980-8578, J apan
Received October 28, 2003
Summary: Irradiation of Cp(CO)2FeSiMe2PPh2 led to
formation of an isomeric mixture of Cp2Fe2(CO)2[µ2-
{κ2(Si,P)-Me2SiPPh2}]2 through generation of Cp(CO)-
Fe{κ2(Si,P)-Me2SiPPh2} and its successive dimerization.
The three-membered-ring complex reacted smoothly with
acetone to give the insertion product Cp(CO)Fe{κ2(Si,P)-
Me2SiOCMe2PPh2}.
Numerous examples of transition-metal complexes
containing a η2-CH2PR2 ligand have been reported, and
most have been synthesized through either cyclometa-
lation of methylphosphine1 or coordination of a phos-
phine part in a coordinatively unsaturated phosphi-
nomethyl complex.1 The present study is concerned with
the silicon analogue, which is expected to be highly
reactive due to the polarized Si-P bond. The first and
only example of such a phosphasilametallacycle was
reported by Paine et al., who utilized the reaction of a
tungsten phosphenium complex with silylenoid reagents
for the synthesis.2 However, no reactivity studies have
been explored as yet. We report here the generation and
dimerization of the phosphasilaferracycle Cp(CO)Fe-
{κ2(Si,P)-Me2SiPPh2} as well as the structures of two
isomeric dimers.
Cp(CO)2FeSiMe2PPh2 (1) was synthesized by the
reaction of Cp(CO)2FeSiMe2Cl with LiPPh2 in Et2O.
Molecular distillation of the mixture at 100 °C/3.6 ×
10-3 mmHg gave analytically pure 1 in 30% yield.
Successful introduction of the PPh2 moiety onto silicon
was confirmed by 29Si NMR spectroscopy, which re-
vealed a doublet signal at δ 43.9 coupled with the 31P
nuclei (1J PSi ) 61.1 Hz).
(2) was observed (vide infra). Upon prolonged photolysis,
2 was converted to an isomeric mixture of Cp2Fe2(CO)2-
[µ2-{κ2(Si,P)-Me2SiPPh2}]2 (3), in which trans-3 gradu-
ally diminished. After photolysis for 50 min, the NMR
yields of cis-3 and trans-3 were 38% and 5%, respec-
tively.
trans-3 and cis-3 were isolated as follows: a toluene
solution of 1 was irradiated in a sealed Pyrex tube for
15 min. After removal of the volatiles, recrystallization
of the residue from dichloromethane-hexane (1:3) at
-30 °C gave orange crystals of trans-3 in 19% yield.
Prolonged photolysis of 1 in toluene for 2 h led to the
formation of cis-3 as a major product, although there
were many other weak peaks in the 1H NMR spectrum.
The reaction mixture was concentrated in vacuo, and
recrystallization of the residue from dichloromethane
at -30 °C gave orange crystals of cis-3 in 37% yield.
Complexes 3 are the first metallacycles containing an
M2Si2P2 six-membered ring, whose structures were
determined by X-ray diffraction studies.
In an attempt to synthesize a phosphasilaferracycle,
a solution of 1 (3.0 mg) in benzene-d6 (0.5 mL) was
irradiated using a 450 W medium-pressure Hg lamp,
and the reaction was monitored by NMR spectroscopy
The molecular structure of trans-3 is depicted in
Figure 1. The asymmetric unit consists of two indepen-
dent half-molecules of the dimer, with no major differ-
ences between the two. The molecule contains an
Fe2Si2P2 six-membered ring, in which two iron centers
adopt a three-legged piano-stool geometry and the ring
is in a chair configuration.4 The two cyclopentadienyl
ligands are in a mutually trans relationship.
1
(eq 1).3 In the initial stage, the growth of H and 31P
NMR signals assigned to Cp(CO)Fe{κ2(Si,P)-Me2SiPPh2}
* To whom correspondence should be addressed. Tel: +81-22-217-
6539. Fax: +81-22-217-6543. E-mail: mokazaki@mail.tains.tohoku.ac.jp
(M.O.); tobita@mail.tains.tohoku.ac.jp (H.T.).
† Present address: Miyagi Study Center, The University of the Air,
Sendai 980-8577, J apan.
Compound cis-3 crystallized with one molecule of
dichloromethane per asymmetric unit. The molecular
(1) (a) AI-J ibori, S.; Crocker, C.; McDonald, W. S.; Shaw, B. L. J .
Chem. Soc., Dalton Trans. 1981, 1572. (b) Karsch, H. H.; Deubelly,
B.; Hofmann, J .; Pieper, U.; Mu¨ller, G. J . Am. Chem. Soc. 1988, 110,
3654 and references cited therein.
(2) Reisacher, H.-U.; Duesler, E. N.; Paine, R. T. J . Organomet.
Chem. 1997, 539, 37.
(4) Metallacycles with a chair form: (a) Amarasekera, J .; Rauchfuss,
T. B.; Rheingold, A. L. Inorg. Chem. 1987, 26, 2017. (b) Giolando, D.
M.; Papavassiliou, M.; Pickardt, J .; Rauchfuss, T. B.; Steudel, R. Inorg.
Chem. 1988, 27, 2596. (c) Michalczyk, M. J .; Recatto, C. A.; Calabrese,
J . C.; Fink, M. J . J . Am. Chem. Soc. 1992, 114, 7955. (d) Pleus, R. J .;
Saak, W.; Pohl, S. Z. Anorg. Allg. Chem. 2001, 627, 250.
(3) The time-dependent distribution of products is illustrated in the
Supporting Information.
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1972 Organometallics, Vol. 23, No. 9, 2004
Communications
31.2 (cis-3), coupled with two magnetically inequivalent
phosphorus atoms.
Irradiation of 1 in a dilute benzene-d6 solution for 1
min led to the formation of 2 as a major product (55%).
Under the present conditions, the subsequent dimer-
ization of 2 proceeded slowly. The structure of 2 was
uniquely determined by spectroscopic analysis. In the
1H NMR spectrum of 2, the methyl groups on the silicon
atom are inequivalent and are observed as two doublets,
at δ 0.58 (3J PH ) 8.4 Hz) and 0.74 (3J PH ) 7.2 Hz). The
1H NMR signal of the Cp ligand appears at δ 4.23 as a
doublet coupled with the 31P nuclei (3J PH ) 1.2 Hz),
indicating coordination of the phosphine part to the iron
center. The 31P NMR signal of 2, observed at δ -46.0,
exhibits an upfield shift characteristic of three-mem-
bered phosphametallacycles.6 The infrared spectrum
exhibits only one intense band in the terminal CO
region (1909 cm-1). The presence of a single νCO band
and its wavenumber indicate that dissociation of one
carbonyl ligand in 1 occurred and was followed by
coordination of the PPh2 moiety.
Complex 2 was light-sensitive, and prolonged pho-
tolysis of the dilute solution led to decomposition,
accompanied by the formation of 3 in low yields. When
a solution of 2 was allowed to stand at room tempera-
ture, 2 was slowly converted to trans-3 in 79% yield.3
This observation clearly shows that the dimerization of
2 to give trans-3 is a nonphotochemical process. Fur-
thermore, photolysis of the isolated trans-3 in benzene-
d6 for 5 min caused isomerization to give cis-3 in 75%
yield. Thus, the isomerization of 3 is expected to proceed
via photochemical dissociation of a carbonyl ligand
followed by recoordination from the opposite side of the
iron (Scheme 1).
F igu r e 1. Molecular structure of trans-3 (half-molecule
A). Selected bond distances (Å) and angles (deg): Fe(A)-
P(A) ) 2.234(3), Fe(A)-Si(A) ) 2.317(3), P(A)-Si(A)* )
2.380(4); P(A)-Fe(A)-Si(A) ) 98.4(1), Fe(A)-P(A)-Si(A)*
) 122.5(1), Fe(A)-Si(A)-P(A)* ) 124.6(1). Asterisks indi-
cate atoms generated by the symmetry operation -x + 2,
-y, -z + 1.
In the presence of CO or PMe3, 2 did not give the
adducts Cp(L)(CO)FeSiMe2PPh2 (L ) CO, PMe3) but
gave only the dimerization products. Interestingly,
introduction of PMe3 accelerated the dimerization of 2
to give trans-3 instantaneously at room temperature.
These experimental results seem to disfavor a mecha-
nism involving the dimerization of the coordinatively
unsaturated phosphinosilyliron(II) complex Cp(CO)-
FeSiMe2PPh2, formed via reversible dissociation of the
phosphine moiety in 2.
Instead, we propose two plausible mechanisms in-
volving the dimerization of phosphasilaferracycle 2 via
cleavage of the Si-P bond (Scheme 1). Hester and Yang
reported the photoreaction of Cp(CO)2FeCH2PEt2 to give
Cp(CO)Fe(η2-CH2PEt2), in which no dimer formation
was observed.7 The unique reactivity of the silicon
analogue 2 is attributable to a significant contribution
of a canonical structure corresponding to an internally
base-stabilized silylene complex A.8,9 In path A, the two
molecules of 2 interact with each other to give a head-
to-tail dimerization product.9 In an alternative explana-
tion (path B), complex 1 catalyzes the dimerization
F igu r e 2. Molecular structure of cis-3. Selected bond
distances (Å) and angles (deg): Fe(1)-P(1) ) 2.247(3),
Fe(2)-P(2) ) 2.250(3), Fe(1)-Si(1) ) 2.329(3), Fe(2)-Si-
(2) ) 2.346(3), P(1)-Si(2) ) 2.343(3), P(2)-Si(1) ) 2.348-
(3); P(1)-Fe(1)-Si(1) ) 100.46(9), Fe(1)-Si(1)-P(2) )
120.6(1), Si(1)-P(2)-Fe(2) ) 120.1(1), Fe(2)-Si(2)-P(1) )
119.5(1).
structure of cis-3 is illustrated in Figure 2. The confor-
mation of the Fe2Si2P2 six-membered ring is best
described as a twisted boat,5 on which the two cyclo-
pentadienyl ligands are located at mutually cis posi-
tions.
(5) Metallacycles with a twisted boat form: (a) Hoehne, S.; Lindner,
E.; Schilling, B. J . Organomet. Chem. 1977, 139, 315. (b) Schore, N.
E.; Young, S. J .; Olmstead, M.; Hofmann, P. Organometallics 1983, 2,
1769. (c) Fornie´s, J .; Mart´ınez, F.; Navarro, R.; Urriolabeitia, E. P.;
Welch, A. J . J . Chem. Soc., Dalton Trans. 1993, 2147.
(6) (a) Garrou, P. E. Chem. Rev. 1981, 81, 229. (b) Lindner, E.; Fawzi,
R.; Mayer, H. A.; Eichele, K.; Hiller, W. Organometallics 1992, 11, 1033.
(7) Hester, D. M.; Yang, G. K. Orgnometallics 1991, 10, 369.
(8) For a review concerning reactivity of silylene complexes: Oka-
zaki, M.; Tobita, H.; Ogino, H. Dalton Trans. 2003, 493.
The 31P NMR signals of 3 are observed at δ 19.9
(trans-3) and 10.9 (cis-3). The 29Si{1H} NMR spectrum
of 3 exhibits a virtual triplet at δ 54.5 (trans-3) and δ

Communications
Organometallics, Vol. 23, No. 9, 2004 1973
Sch em e 1. P la u sible F or m a tion Mech a n ism for 3
process: nucleophilic attack of 1 toward the silicon atom
in 2 cleaves the Si-P bond and generates the base-
stabilized phosphido(silylene) intermediate B. The phos-
phido ligand of B then attacks the silicon atom in
another 2 to cleave the Si-P bond and induces the
formation of six-membered ring. Both mechanisms can
explain the PMe3-accelerated dimerization of 2. In path
A, PMe3 facilitates the ring opening of 2, probably
through the transient formation of the PMe3-stabilized
phosphido(silylene) complex Cp(CO)(PPh2)Fe(dSiMe2‚
PMe3),9 and thus accelerates the formation of trans-3.
In path B, PMe3 can work as an efficient catalyst instead
of 1. At this time there is no direct evidence available
to distinguish between the two mechanisms.
is shifted considerably downfield compared to the values
for common iron phosphine complexes of the type Ln-
Fe-PPh2R (R ) alkyl). This downfield shift is charac-
teristic of the phosphorus-containing five-membered
metallacycles.6 As the precursor does not react with
acetone at ambient temperature, it is likely that the
highly enhanced reactivity of 2 is consistent with the
significant contribution of A in 2, on which acetone is
coordinated to the electron-deficient silicon atom to
generate the acetone-stabilized phosphido(silylene) in-
termediate. Intramolecular nucleophilic attack of the
PPh2 part to the carbonyl carbon of the coordinated
acetone then occurs to yield 4.
In conclusion, the highly reactive phosphasilametal-
lacycle 2 was successfully synthesized and was shown
to dimerize gradually to produce 3. Acetone insertion
into the Si-P bond of 2 proceeded instantaneously at
room temperature to give 4. These results are consistent
with the significant contribution of the canonical struc-
ture corresponding to phosphido(silylene) complex A in
2.
Facile cleavage of the Si-P bond in phosphasilafer-
racycle 2 was demonstrated by the reaction of 2 with
acetone, which occurred instantaneously at ambient
temperature to give the insertion product 4 in 68% yield
(eq 2). The 1H and 13C{1H} NMR spectra of 4 each
Ack n ow led gm en t. This work was supported by
Grants-in-Aid for Scientific Research (Nos. 13440193,
14204065, 14044010, 14078202, and 15655017) from the
Ministry of Education, Culture, Sports, Science and
Technology of J apan. M.O. wishes to thank the Nissan
Science Foundation and Tokuyama Science Foundation
for financial support.
exhibit four peaks assignable to chemically inequivalent
methyl groups on the silicon and carbon atoms, sup-
porting the formation of the five-membered metalla-
cycle. The chemical shift of the 31P NMR signal, δ 126.1,
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details for the synthesis and characterization of all
compounds and full details of the crystal structure analysis
as CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org.
(9) Silylene complexes are stabilized by coordination of phosphines
onto the silylene silicon atoms: (a) Handwerker, H.; Paul, M.; Blu¨mel,
J .; Zybill, C. Angew. Chem., Int. Ed. Engl. 1993, 32, 1313. (b)
Kawamura, K.; Nakazawa, H.; Miyoshi, K. Organometallics 1999, 18,
1517. (c) Gusev, D. G.; Fontaine, F.-G.; Lough, A. J .; Zargarian, D.
Angew. Chem., Int. Ed. 2003, 42, 216.
OM034264G