Reactions of Half-Sandwich Ethene Complexes of Rhodium(I ...

Organometallics
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(in D -toluene), or IC F (in C D ) were dramatically affected
distances (1.898(3) and 1.130(4) Å) fall in the range of values
found for Rh(III) carbonyl complexes containing the η 5 -Cp*
ligand (1.800−2.041 and 1.040−1.149 Å, respectively). 56 The
Rh−C acyl distance (2.083(3) Å) is slightly longer than those
observed in other Rh(III) acyl complexes containing the η 5 -Cp*
ligand (2.010−2.071 Å).
Mechanistic Studies. It is interesting to trace a parallelism
between the reactions observed and the reaction of [Rh(η 5 -Cp*)-
(η 2 -C 2 H 4 )(PMe 3 )] with MeI, which led to [Rh(η 5 -Cp*)(η 2 -C 2 H 4 )-
(Me)(PMe 3 )]I through nucleophilic attack of the metal center
at the positively charged carbon atom. 37 Substitution of the
coordinated ethene by iodide finally gives [Rh(η 5 -Cp*)I(Me)-
(PMe 3 )]. The iodine-bound carbon of a perfluoroalkyl iodide
also bears a high positive charge, but it is sterically and electrostatically
shielded from nucleophilic attack by the negatively
charged fluorine substituents, especially in secondary and tertiary
perfluoroalkyl iodides. 57,58 In addition, the electron-withdrawing
character of the perfluoroalkyl group makes the iodine atom
electrophilic, allowing nucleophilic attack and subsequent release
of a perfluoroalkyl anion. 57−60 Such anions have been trapped by
Hughes and co-workers in the reactions of Rh(I), 29 Mo(II), 42,43
W(II), 42,43 or Pt(II) 61 complexes with perfluoroalkyl iodides. In a
benchmark study, 42 the same authors reported that the ethene
complexes [M(η 5 -Cp) 2 (η 2 -C 2 H 4 )] (M = Mo or W) react with
IC(CF 3 ) 3 to give [M(η 5 -Cp) 2 I{CH 2 CH 2 C(CF 3 ) 3 }] via nucleophilic
attack of the metal at the iodine, followed by addition of
the generated C(CF 3 ) − 3 anion to the coordinated ethene. 42,43
Alternatively, the possibility of a radical mechanism should
be considered, since perfluoroalkyl iodides are able to react with
electron donors by single electron transfer to give I − and an R F·
radical. 11,62 Considering these precedents, we attempted to find
experimental evidence for the intermediacy of R F· radicals or
R − F anions in the ethene perfluoroalkylation reactions.
As D 8 -toluene is expected to react with free R F· radicals to
give a D 7 -benzyl radical and DR F , 61 some representative
reactions were conducted in this solvent. However, no DR F
was detected by NMR spectroscopy in any experiment.
As perfluoroalkyl radicals easily add to olefins, 11,63,64 the
reactions between [Rh(η 5 -Cp*)(η 2 -C 2 H 4 )(PMe 3 )] and ICF-
(CF 3 ) 2 were performed in the presence of a 4-fold excess of
norbornene. However, the outcome of the reaction was not
appreciably altered, and no significant amounts of norbornene
perfluoroalkylation products were detected.
In addition, the reactions of [Rh(η 5 -Cp*)(η 2 -C 2 H 4 )(PMe 3 )]
or [Rh(η 5 -Cp*)(η 2 -C 2 H 4 ) 2 ] with ICF(CF 3 ) 2 were carried out
in the presence of the radical trap (2,2,6,6-tetramethylpiperidin-
1-yl)oxyl (TEMPO), using C 6 D 6 as solvent. While in the first
case the result of the reaction was essentially the same as in the
absence of the radical trap, in the second case most of the
starting materials remained unreacted after 16 h. As no radicaltrapping
products were detected, the reaction inhibition is
tentatively attributed to competitive formation of a halogenbonded
adduct between TEMPO and the perfluoroalkyl
iodide. 60 The formation of this adduct did not compete effectively
with the rapid reaction between complex [Rh(η 5 -Cp*)-
(η 2 -C 2 H 4 )(PMe 3 )] and ICF(CF 3 ) 2 .
We also attempted to trap radical or carbanionic
intermediates by carrying out the reactions in the presence of
CH 3 OD as previously reported. 29,42,61,65 In these experiments,
R F· radicals should preferentially cleave the weaker C−H
bond 66 to give HR F , whereas R − F anions should abstract a D +
cation to give DR F . Thus, the reactions of [Rh(η 5 -Cp*)-
(η 2 -C 2 H 4 )(PMe 3 )] with ICF(CF 3 ) 2 (in D 8 -toluene), IC(CF 3 ) 3
8 6 5 6 6
by the presence of CH 3 OD (1.5−2.5 equiv). Under these conditions,
the formation of complex 3a, 3c, or3d was inhibited or
severely reduced, DR F being the main fluoroorganic product.
Moreover, significant amounts of DC(CF 3 ) 3 or DC 6 F 5 were
detected when the reactions between [Rh(η 5 -Cp*)(η 2 -C 2 H 4 )-
(PMe 3 )] and IC(CF 3 ) 3 or IC 6 F 5 were run in C 6 D 6 saturated
with D 2 O. Minor amounts of HR F were also detected in all these
experiments, even in the absence of CH 3 OD, which might be
attributed to carbanion protonation by residual water or to the
contribution of a minor reaction pathway involving perfluoroalkyl
radicals. 67 The reactions of [Rh(η 5 -Cp*)(η 2 -C 2 H 4 ) 2 ]with
ICF(CF 3 ) 2 or ICF(CF 3 )CF 2 CF 3 in C 6 D 6 were not significantly
affected when they were carried out in the presence of CH 3 OD or
in D 2 O-saturated solvent. In contrast, the analogous reactions
involving IC(CF 3 ) 3 gave considerable amounts of DC(CF 3 ) 3 .
Evidence for both radical and anions was obtained in the
reaction of [Rh(η 5 -Cp*)(η 2 -C 2 H 4 )(PMe 3 )] with I n C 4 F 9 in C 6 D 6 .
Thus, the formation of the oxidative addition product 4g was
partially inhibited by carrying out the reaction in the presence of
norbornene or TEMPO (2 equiv), and significant amounts of
D n C 4 F 9 were observed by carrying it out in the presence of
CH 3 OD (2.5 equiv). This suggests that, in this case, both ionic
and radical pathways should contribute significantly to the overall
reaction mechanism.
Finally, the detection of cis- and trans-octafluoro-2-butene in
the reaction of [Rh(η 5 -Cp*)(η 2 -C 2 H 4 )(PMe 3 )] with ICF(CF 3 )-
CF 2 CF 3 is evidence for the generation of the CF 3 CF 2 (CF 3 )CF −
anion, which decomposes by elimination of F − to give the alkenes.
The released fluoride anion could trap a proton from residual
moisture or react with glass to form the H n F − n+1 and SiF 2− 6 anions
of the isolated salt. No perfluoroalkenes were detected in other
reaction mixtures examined by 19 F NMR spectroscopy.
On the basis of these results, we propose (Scheme 7) that
ethene perfluoro(alkylation or arylation) could occur by
Scheme 7
nucleophilic attack of the metal on the iodine atom to generate
the ionic intermediate A, which could undergo addition of the
R − F anion on the coordinated ethene to give complexes 3.
When L = C 2 H 4 , dinuclear species 1 could be formed by loss of
ethene and dimerization, although evidence for the formation
of R − F anions was found only in the case of 1c.
Compounds [Rh(η 5 -Cp*)I 2 (PR 3 )] and other unidentified byproducts
observed in the reactions of [Rh(η 5 -Cp*)(η 2 -C 2 H 4 )-
(PMe 3 )] with IR F could result from the evolution of intermediate A
after the dissociation of the ion pair and the destruction of the
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dx.doi.org/10.1021/om2009588 | Organometallics 2012, 31, 1287−1299