The Fluoro Analogue of Wilkinson's Catalyst and Unexpected Ph-Cl Activation

Article abstract of DOI:10.1021/ja049844z

The fluoro analogue of Wilkinson's catalyst, [(Ph₃P)₃RhF] (1), was synthesized and fully characterized. Both solution behavior and solid-state geometry parameters of 1 were found to be surprisingly similar to those of Wilkinson's catalyst. Unlike Wilkinson's catalyst however, 1 exhibited most unusual reactivity toward the notoriously inert C?Cl bond of nonactivated chloroarenes. The novel Ph?Cl activation with 1 includes fluorine transfer from the metal to a phosphine ligand and evidently phenyl transfer from the phosphine to Rh to produce an electron-rich σ-phenylrhodium intermediate. Copyright

Full text of DOI:10.1021/ja049844z

Published on Web 02/25/2004
The Fluoro Analogue of Wilkinson’s Catalyst and Unexpected Ph-Cl
Activation†
Vladimir V. Grushin* and William J. Marshall
Central Research & DeVelopment, E. I. DuPont de Nemours and Co., Inc., Experimental Station,
Wilmington, Delaware 19880-0328
Received January 9, 2004; E-mail: vlad.grushin-1@usa.dupont.com
Scheme 1. Synthesis and an ORTEP Drawing of 1
Chlorotris(triphenylphosphine)rhodium, [(Ph3P)3RhCl], often
referred to as Wilkinson’s catalyst, is one of the most versatile
catalysts and a key compound in modern organometallic chemistry.1
The bromo and iodo analogues, [(Ph3P)3RhX] (X ) Br, I), are also
well-characterized complexes that have been known for almost 40
years.1 In contrast, the fluoride [(Ph3P)3RhF] (1) has been mentioned
in the literature only twice, both times without characterization or
systematic studies.2 Herein we report a simple and efficient synthesis
of 1, its full characterization, and most unusual reactivity which
might be of considerable interest to the new, promising, and rapidly
growing area of late transition metal fluoride complexes.3
We found that [(Ph3P)4Rh2(µ-OH)2] reacted with Et3N‚3HF
(TREAT HF; 2/3 equiv) to produce a new dinuclear fluoride
[(Ph3P)4Rh2(µ-F)2] (2; for X-ray and NMR data, see Supporting
Information). Treatment of 2 with PPh3 led to 14 as an orange-
yellow solid, which can also be prepared in up to >90% yield, and
more conveniently, by reacting [(COD)2Rh2(µ-OH)2] with TREAT
HF in the presence of excess PPh3 (Scheme 1).5
The formation and stability of 1 is somewhat peculiar. The i-Pr3P
analogue of 2 has been shown6 to react only with strongly π-acidic
ligands L (CO, RNC, Ph2C2, and CH2dCH2) to give stable trans-
[(i-Pr3P)2Rh(F)L], but not with weak π-acids such as Py or MeCN.
This trend has been rationalized6 in terms of the favorable trans-
arrangement of the π-acid and fluoride which may be viewed as a
powerful π-base3a,d,7 or as a poorly polarizable, “hard” base bearing
a localized negative charge.3c In either case, a π-acid trans to the
F ligand is expected to alleviate dπ-pπ filled/filled repulsion.7 Since
PPh3 is very weakly π-acidic, 1 was expected to be less stable than
Wilkinson’s catalyst.
To our surprise however, the Rh-P bond distances and coor-
dination angles that we found in the X-ray structure of 1 (Scheme
1) were remarkably similar to those of [(Ph3P)3RhCl], both the
orange and red forms (Table 1).1 Furthermore, the degree of
phosphine dissociation from 1 (eq 1) in benzene under rigorously
anhydrous conditions was measured by 31P NMR and calculated
at 30% for [1] ) 5 × 10-4 mol dm-3. Under identical conditions,
the same degree of dissociation (30%) was determined for Wilkin-
son’s catalyst (eq 1). Thus, both the solid state and solution data
for 1 and [(Ph3P)3RhCl] are remarkably similar. This result is
unusual, as most often late transition metal fluorides are distinct,
differing considerably from their iodo, bromo, and chloro coun-
terparts.3
Table 1. Geometry Parameters for 1 and [(Ph3P)3RhCl]
[(Ph3P)3RhF], 1
[(Ph3P)3RhCl] (ref 1)
parameter,
Å or deg
(this work)
orange
red
Rh-X
2.070 (2)
2.193 (1)
2.325 (1)
2.325 (1)
166.9 (1)
159.7 (1)
2.404 (4)
2.225 (4)
2.304 (4)
2.338 (4)
166.7 (2)
159.1 (2)
2.376 (4)
2.214 (4)
2.322 (4)
2.334 (4)
156.2 (2)
152.8 (1)
Rh-P trans to X
Rh-P trans to P
Rh-P trans to P
X-Rh-P
P-Rh-P
In sharp contrast with all of the above similarities was the unusual
reactivity that 1 exhibited toward haloarenes. Oxidative addition
of most reactive iodoarenes to [(Ph3P)3RhX] (X ) Cl, I) is known8,9
to occur only at elevated temperatures (>100 °C), in the presence
of a large excess of ArI. Incomparably more inert10 PhCl did not
react with [(Ph3P)3RhCl] under such conditions.11
To our surprise, the fluoro complex 1 was found to easily cleave
the most unreactive C-Cl bond of chlorobenzene (eq 2). Both the
mild conditions required for reaction 2 (1-3 h at 80-100 °C) and
the formation of 3 were totally unexpected.
The new fluorophosphine complex 312 (Figure 1) was formed
in ca. 70% 31P NMR yield (isolated yield: 50%), along with
[(Ph3P)3RhCl] (ca. 30%) which possibly formed from 3 via
phosphine ligand exchange. Several experiments were carried out
in order to gain insight into the mechanism of reaction 2.
First, it was shown that the reaction of 1 with p-chlorotoluene
led to 4-MeC6H4Ph and only traces of Ph2 (<1%, GC-MS).
Second, it was found that the thermal decomposition of 1 in
benzene at 80 °C occurred readily and selectively to produce two
Rh complexes 413 and 514 in a 1:1 ratio (eq 3).
2[(Ph P) RhF] [(Ph P) Rh (µ-F) ]
h
+ 2PPh3
(1)
3
3
3
4
2
2
1
2
Also, like [(Ph3P)3RhCl],1 1 readily reacted with CO and
decarbonylated DMF (90 °C) to produce the well-known complex
trans-[(Ph3P)2Rh(F)(CO)] which was identified by its 31P NMR and
IR (νCO ) 1957 cm-1 in Nujol) data.3a
† Contribution No. 8501.
9
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10.1021/ja049844z CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
I- produced and 1 would lead to the highly nucleophilic F- that
can react19 (SN2) with MeI to give MeF. Indeed, 2 that cannot
undergo phosphine dissociation, did not produce MeF upon
treatment with MeI.
In conclusion, the fluoro analogue of Wilkinson’s catalyst has
been synthesized, fully characterized, and shown to possess
exceptional reactivity toward nonactivated chloroarenes due to the
novel, facile Rh-F/P-Ph s Rh-Ph/P-F exchange.
Acknowledgment. We thank Dr. Stuart A. Macgregor for a
fruitful discussion and for sharing results prior to publication, and
Drs. Cathy Radzewich, Ken Moloy, and Viacheslav Petrov for
reading the manuscript and for valuable comments.
Supporting Information Available: Experimental procedures and
NMR (PDF) and X-ray analysis data for 1, 2, 3, and [(Ph3P)2Rh(Ph)-
X2] (X ) I, Br) (CIF and PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 1. ORTEP drawing of 3 with all H atoms omitted for clarity.
Scheme 2. Mechanism for Reactions 2 and 3
References
(1) Jardine, F. H. Prog. Inorg. Chem. 1981, 28, 63.
(2) (a) 1 has been characterized by elemental analysis2b and once used, without
characterization, in synthesis.2c Very recently, [(Et3P)3RhF] was reported
and characterized in solution.2d (b) Van Gaal, H. L. M.; Van den Bekerom,
F. L. A.; Verlaan, J. P. J. J. Organomet. Chem. 1976, 114, C35. (c) Jones,
C. M.; Doherty, N. M. Polyhedron 1995, 14, 81. (d) Braun, T.; Noveski,
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(3) For reviews, see: (a) Doherty, N. M.; Hoffmann, N. W. Chem. ReV. 1991,
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(4) NMR data for 1 (C6D6, 20 °C), δ. 1H: 7.0 (m, 3H, m, p-Ph); 7.9 (m, 2H,
o-Ph). 19F: -286.3 (ddt, coupling observable in the presence of excess
PPh3, JF-Rh ) 77.6 Hz, JF-P(trans) ) 172.4 Hz, JF-P(cis) ) 28.5 Hz). 31P:
32.4 (ddd, 2P, JP-Rh ) 154.3 Hz, JP-P ) 39.0 Hz, JP-F ) 28.5 Hz); 57.4
(ddt, 1P, JP-Rh ) 181.4 Hz, JP-P ) 39.0 Hz, JP-F ) 172.4 Hz).
(5) The formation of [(COD)Rh(PPh3)F] upon treatment of [(COD)RhF]n with
1 equiv of PPh3 per Rh has been reported: Vicente, J.; Gil-Rubio, J.;
Bautista, D. Inorg. Chem. 2001, 40, 2636.
Stoichiometry considerations suggested that the formation of one
molecule of 4 from two molecules of 1 should be accompanied by
the production of 1 molecule of [(Ph3P)3RhPh]. The latter (like its
σ-Me analogue)15 was found to be unstable under the reaction
conditions (80 °C), rapidly decomposing to 5.
Finally, it was observed that added free PPh3 (4 equiv per Rh)
did not decelerate reactions 2 and 3, indicating that the rate-
determining step of both reactions does not require phosphine pre-
dissociation.
Scheme 2 accounts for the above observations. It is believed
that the rate-limiting step is the rearrangement of 1 to the electron-
rich σ-Ph intermediate A, which is fully expected16 to undergo
oxidative addition to give B, followed by reductive elimination
leading to 3. In the absence of ArCl, A and as yet unreacted 1
undergo ligand exchange to produce more stable 4 and C that
undergoes cyclometalation to 5. The enhanced stability of 4, as
compared to that of 1, is manifested by the lack of phosphine
dissociation (eq 1) and is likely3 due to the fluoride ligand being
trans to more π-acidic Ph2PF. The mechanism of the Rh-F/P-Ph
to Rh-Ph/P-F rearrangement remains unknown and may involve
a metallophosphorane forming upon F-transfer to the P atom,17
followed by Ph-transfer to the metal center.
When 1 was treated with much more reactive PhI and PhBr
(excess in benzene; 80 °C, 1-3 h), new [(Ph3P)2Rh(Ph)X2] (X )
I or Br) were produced in high yield, isolated, and fully character-
ized by NMR and X-ray data. These reactions likely involve the
formation of [(Ph3P)2(Ph2PF)RhX] (X ) I or Br; see eq 2 and
Scheme 2) which then undergo oxidative addition of the more
reactive PhX (X ) I, Br) and dissociation of Ph2PF.18
(6) Gil-Rubio, J.; Weberndorfer, B.; Werner, H. J. Chem. Soc., Dalton Trans.
1999, 1437.
(7) Caulton, K. G. New J. Chem. 1994, 18, 25.
(8) (a) Mori, K.; Mizoroki, T.; Ozaki, T. Bull. Chem. Soc. Jpn. 1976, 49,
758. (b) We found that oxidative addition of PhI to [(Ph3P)3RhCl] produced
[(Ph3P)2Rh(Ph)Cl(I)]8a and also [(Ph3P)2Rh(Ph)Cl2] and [(Ph3P)2Rh(Ph)-
I2] in a 2:1:1 ratio (31P NMR), due to halide exchange.
(9) Ishiyama, T.; Hartwig, J. J. Am. Chem. Soc. 2002, 122, 12043.
(10) Grushin, V. V.; Alper, H. Chem. ReV. 1994, 94, 1047.
(11) Heating [(Ph3P)3RhCl] in neat PhCl at 100 °C for 2.5 h led almost
exclusively to precipitation of [(Ph3P)4Rh2(µ-Cl)2] due to PPh3 dissociation,
as if the reaction were run in a neutral solvent such as toluene.1 Only
trace amounts (<2%) of the oxidative addition product [(Ph3P)2Rh(Ph)-
Cl2] were detected by 31P NMR (21.9 ppm, d, JP-Rh ) 103.6 Hz, see:
Fawcett, J.; Holloway, J. H.; Saunders: G. C. Inorg. Chim. Acta 1992,
202, 111).
(12) NMR data for 3 (CD2Cl2, 20 °C), δ. 1H: 7.0-7.7 (m, Ph). 19F: -110.7
(ddt, JF-P ) 856 Hz, JF-P ) 20.0 Hz, JF-Rh ) 14.1 Hz). 31P: 35.5 (ddd,
2P, JP-Rh ) 139.0 Hz, JP-P ) 40.7 Hz, JP-F ) 20.0 Hz); 181.4 (ddt, 1P,
JP-F ) 856 Hz, JP-Rh ) 222.1 Hz, JP-P ) 40.7 Hz).
(13) NMR data for 4 (C6D6, 20 °C), δ. 19F: -109.7 (ddt, 1F, JF-P ) 848 Hz,
JF-P ) 30 Hz, JF-Rh ) 15 Hz, P-F); -267.9 (ddt, JF-Rh ) 58.5 Hz,
JF-P(trans) ) 217 Hz, JF-P(cis) ) 27 Hz, Rh-F). 31P: 32.3 (dddd, 2P, JP-Rh
) 150 Hz, JP-P ) 41.5 Hz, JP-F ) 27 Hz; JP-F ) 15 Hz); 188.8 (dddt,
1P, JP-F ) 848 Hz, JP-F ) 217 Hz, JP-Rh ) 207.4 Hz, JP-P ) 41.5 Hz).
(14) Identified by its 31P NMR spectrum: Kuznetsov, V. F.; Yap, G. P. A.;
Bensimon, C.; Alper, H. Inorg. Chim. Acta 1998, 280, 172.
(15) Keim, W. J. Organomet. Chem. 1968, 14, 179.
(16) (a) Semmelhack, M. F.; Ryono, L. Tetrahedron Lett. 1973, 31, 2967. (b)
We found that [(Ph3P)3RhPh] (C) readily reacted with PhCl (neat; 80 °C;
<1 h) to give [(Ph3P)3RhCl] and Ph2 via C-Cl oxidative addition and
C-C reductive elimination (Scheme 2).
(17) (a) Macgregor, S. A.; Neave, G. W. Organometallics 2004, 23, 891. (b)
A somewhat similar mechanism may govern the formation of [(C6F5)Ir-
(PEt3)2(PEt2F)] from [MeIr(PEt3)3] and C6F6: Blum, O.; Frolow, F.;
Milstein, D. J. Chem. Soc., Chem. Commun. 1991, 258.
(18) (a) Ph2PF could not be detected due to facile disproportionation to Ph2-
PPPh2 and Ph3PF2,18b both of which were indeed produced by the reaction
of 1 with PhBr. (b) Riesel, L.; Haenel, J.; Ohms, G. J. Fluorine Chem.
1988, 38, 335.
While the reactions of 1 with PhX (X ) Cl, Br, I) did not result
in C-F bond formation, treatment of 1 with excess MeI gave rise
to MeF (ca. 15% yield; 19F NMR: -267.1, q, JF-H ) 45.9 Hz),
along with other products that were not identified. This C-F bond
formation was not due to an oxidative addition-reductive elimina-
tion sequence but rather due to the formation of [Ph3PMe]+ I- from
the MeI and dissociated Ph3P (eq 1). Halide exchange between the
(19) Grushin, V. V. Angew. Chem., Int. Ed. 1998, 37, 994.
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