Article
Organometallics, Vol. 28, No. 19, 2009 5779
Scheme 1. Formation and Reactivity of [(η5-Me3SiC5H4)Fe-
(CO)(PPh3)]-Naþ
Figure 3. Proposed aggregation of 2a in hexanes solution
(M- = [(η5-Me3SiC5H4)Fe(CO)(PPh3)]).
coordinated to the face of a cationic metallic tetramer, is a
probable structure for the species in hexane. This type of
arrangement, which would increase solubility, maximize
dπ(Fe)f π*(CO- - -M), and produce the very low νCO bands,
i.e. 1693 cm-1 (Na) and 1681 cm-1 (Li), is known in alkali-
metal chemistry and the crystal structure of methyllithium
contains a lithium tetramer with a carbon atom coordinated
to each face of the tetrahedron.11 In addition many dimers
and aggregates of lithium and sodium are known.12
electron density at the transition-metal center. When this
tight ion pair is perturbed by addition of a crown ether, or
[PPN]þCl-, increasing electron density at Fe facilitates loss
of the PPh3 ligand and the resulting 16-electron system
scavenges CO to produce the dicarbonyl anion. Such loss
of the poorer π-acceptor is consistent with observations by
Chen and Ellis upon reduction of metal complexes contain-
ing both CO and PR3.9 The fate of the Fe atom that was the
source of the second CO is not clear at this time.
To our surprise, the silyl migration chemistry and forma-
tion of [(η5-Me3SiC5H4)Fe(CO)(PPh3)]-Naþ also proceed
smoothly in hexanes. The reaction is complete in 3 h upon
addition of only 3 equiv of NADA at room temperature to
produce a species with a single νCO band at 1693 cm-1 (and
shoulder at 1745 cm-1). The anion is readily quenched as
above, with higher yields and little or no formation of
dicarbonyl derivatives. Similarly, when LDA is added to 1
in hexane, a new broad νCO band at 1681 cm-1 is formed
(however, only after adding 20 equiv at room temperature
and stirring for 12 days). To our knowledge, no νCO bands as
low as 1681 and 1693 cm-1 have been seen for cyclopenta-
dienyliron carbonyl compounds.
Reactivity of [(η5-Me3SiC5H4)Fe(CO)(PPh3)]-Mþ with
ClCH2SiMe2R (R = H, SiMe3). An important potential of
the new metalate system of interest to us is the capacity to
synthesize oligosilyl (and related) complexes where the
phosphine-substituted Fp complex cannot be prepared via
the usual photochemical treatment due to photochemically
induced rearrangements.5d,13 It has been noted that photo-
chemical reactions of disilanes (e.g., Me3SiSiMe2H) lead to
silylene formation/expulsion when catalyzed by FpR com-
plexes, but non-photochemical thermal rearrangement
chemistry is observed when the phosphine-substituted ana-
logues are the catalysts or reagents.14a,b
The new ferrrate 2 reacts readily as noted in eq 1 to form the
simple alkyl and silyl complexes 5 and 6, and full spectroscopic
analysis of these materials is provided in the Experimental
Section. Additionally, we have reacted [(η5-Me3SiC5H4)Fe-
(CO)(PPh3)]-Naþ with the chloromethylsilane species
ClCH2SiMe2R (R = H, SiMe3). The reactions are rapid, and
the final products (η5-Me3SiC5H4)Fe(CO)(PPh3)SiMe2CH2R
(R = H (6), SiMe3 (7)) are obtained in good yield (eq 2).
When the hexane solution of [(η5-Me3SiC5H4)Fe(CO)-
(PPh3)]-Naþ is titrated with THF, the Fe-CO- - -M νCO band
at 1730 cm-1 reemerges until it completely replaces the νCO
band at 1693 cm-1. The νCO band at 1693 cm-1 reflects a
maximization of the dπ(Fe) f π*(CO- - -M) interaction sug-
gestive of a considerable CdO character. In addition, the
convenient solubility of the salt in hexanes suggests salt
aggregation between Naþ and the metalate. The well-estab-
lished solubility of alkyllithium reagents in hexanes (whether
because of Li-C covalency10a,b or, as some quantum mechan-
ical calculations suggest, the encapsulation of the lithium atom
core by the hydrophobic alkyl groups10c) is augmented in the
present case by the large hydrophobic Me3Si group on the
cyclopentadienyl ring and PPh3, thus distinguishing these new
metalates from the simple [Fp]- salts.
This chemistry is different from that using the parent Fp
anion, where in both cases the initially isolated product
was FpCH2SiMe2R.13a,d,e Similarly, the reaction of the
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