Dihydrogen loss from a 14-electron rhodium(III) bis-phosphine dihydride to give a rhodium(I) complex that undergoes oxidative addition with aryl chlorides

Article abstract of DOI:10.1021/om800373d

Dihydrogen loss from the 14-electron Rh(III) bis-phosphine dihydride [Rh(PⁱBu₃)₂H₂][BAr^F ₄] forms a complex tentatively identified as [Rh(PⁱBu ₃)₂(L₂)][BAr^F₄] (L ₂ = solvent or agostic interactions), which reacts with dichloro ethane or fluorobenzene to form [Rh(PⁱBu₃) ₂(ClCH₂CH₂Cl)][BAr^F₄] and [Rh(PⁱBu₃)₂(η⁶-C ₆H₅F)][BAr^F₄], respectively. [Rh(PⁱBu₃)₂(L)₂[-[BAr ^F₄] or its adducts undergo oxidative addition of aryl halides (C₆H₅X; X = Cl, Br) at room temperature to give the dimeric species [Rh(PⁱBu₃)₂(C ₆H₅)(μ-X)]₂.

Full text of DOI:10.1021/om800373d

2918
Organometallics 2008, 27, 2918–2921
Dihydrogen Loss from a 14-Electron Rhodium(III) Bis-Phosphine
Dihydride To Give a Rhodium(I) Complex That Undergoes
Oxidative Addition with Aryl Chlorides
Thomas M. Douglas, Adrian B. Chaplin, and Andrew S. Weller*
Department of Chemistry, Inorganic Chemistry Laboratories, UniVersity of Oxford, Oxford OX1 3QR, U.K.
ReceiVed April 29, 2008
Summary: Dihydrogen loss from the 14-electron Rh(III) bis-
phosphine dihydride [Rh(PiBu3)2H2][BArF ] forms a complex
4
tentatiVely identified as [Rh(PiBu3)2(L2)][BArF ] (L2 ) solVent
4
or agostic interactions), which reacts with dichloroethane or
fluorobenzene to form [Rh(PiBu3)2(ClCH2CH2Cl)][BArF ] and
4
[Rh(PiBu3)2(η6-C6H5F)][BArF ], respectiVely. [Rh(PiBu3)2(L)2]-
4
[BArF ] or its adducts undergo oxidatiVe addition of aryl halides
4
(C6H5X; X ) Cl, Br) at room temperature to giVe the dimeric
species [Rh(PiBu3)2(C6H5)(µ-X)]2.
The use of haloarene substrates is ubiquitous in organic
chemistry, and the oxidative addition of C-halogen bonds to
low-coordinate transition metals, in particular Pd(0),1 is a key
step in cross-coupling reactions.2 Recently a number of examples
of oxidative addition of aryl halides to rhodium and iridium
complexes have been reported,3 reactions which are suggested
to proceed through low-coordinate electronically unsaturated
intermediates. Examples of well-characterized group 9 species
capable of oxidative addition with aryl halides are particularly
useful in the study of the emerging fields of C-C and C-N
coupling4 and polychloroarene dehydrohalogenation5 reactions
mediated by group 9 metals.
Figure 1. [Rh(PiBu3)2H2][BArF ] (1; 40% probability ellipsoids).
4
The anion, minor disordered components, and most hydrogen atoms
have been omitted for clarity. Key bond distances (Å) and angles
(deg): Rh1-P1, 2.3067(12); Rh1-P2, 2.2990(11); Rh1-C3, 2.90(3);
Rh1-C15, 2.891(5); P1-Rh1-P2, 175.09(4).
We have previously reported that bis-dihydrogen dihydride
i
complexes [Rh(PR3)2H2(η2-H2)2][BArF ] (R ) Pr, Cy; ArF )
the H2 atmosphere results in loss of the dihydrogen ligands and
formation of dihydride species, suggested to be stabilized by
solvent molecule coordination or by agostic C-H · · · Rh interac-
tions. We now report that, on changing the phosphine ligand to
triisobutylphosphine (PiBu3), removing the H2 atmosphere
results instead in the generation of a complex with no hydride
ligands that can be considered as the kinetic equivalent of “12-
4
C6H4(CF3)2) are formed from addition of H2 to CD2Cl2 solutions
of [Rh(PR3)2(nbd)][BArF ] (nbd ) norbornadiene).6 Removing
4
* To whom correspondence should be addressed. E-mail:
andrew.weller@chem.ox.ac.uk.
(1) (a) Stambuli, J. P.; Incarvito, C. D.; Buhl, M.; Hartwig, J. F. J. Am.
Chem. Soc. 2004, 126, 1184–1194. (b) Barrios-Landeros, F.; Hartwig, J. F.
J. Am. Chem. Soc. 2005, 127, 6944–6945. (c) Barder, T. E.; Walker, S. D.;
Martinelli, J. R.; Buchwald, S. L. J. Am. Chem. Soc. 2005, 127, 4685–
4696.
electron” [Rh(PiBu3)2][BArF ]. This complex undergoes clean
4
oxidative addition with unactivated aryl bromides and chlorides
at room temperature to give coordinatively unsaturated Rh(III)
aryl halides.
(2) Corbet, J. P.; Mignani, G. Chem. ReV. 2006, 106, 2651–2710.
(3) (a) Douvris, C.; Reed, C. A. Organometallics 2008, 27, 807–810.
(b) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc. 2004, 126, 3068–
3069. (c) Willems, S T. H.; Budzelaar, P. H. M.; Moonen, N. N. P.; Gelder,
R.; Smits, J. M. M.; Gal, A. W. Chem. Eur. J. 2002, 8, 1310–1320. (d)
Gatard, S.; Celenligil-Cetin, R.; Guo, C.; Foxman, B. M.; Ozerov, O. V.
J. Am. Chem. Soc. 2006, 128, 2808–2809. (e) Fan, L.; Parkin, S.; Ozerov,
O. V. J. Am. Chem. Soc. 2005, 127, 16772–16773, and references therein.
Oxidative addition of CH2Cl2 or benzyl chloride has been reported for
cationic Rh(I) complexes: Dorta, R.; Shimon, L. J. W.; Rozenberg, H.;
Milstein, D. Eur. J. Inorg. Chem. 2002, 1827–1834.
Addition of H2 (4 atm) to [Rh(PiBu3)2(nbd)][BArF ] in CH2Cl2
4
solution results in the immediate formation of [Rh(PiBu3)2H2]-
[BArF ] 1 (eq 1), which has been characterized by NMR and
4
IR spectroscopy, ESI-MS, and X-ray crystallography. Complex
1 can be characterized in CD2Cl2 solution under an argon
atmosphere. The molecular structure of 1 is shown in Figure 1
and reveals two Rh · · · H-C agostic interactions trans to the
(located) hydride ligands (Rh · · · C(3) ) 2.90(3) Å, Rh · · · C(15)
) 2.891(5) Å) and is closely related to “14-electron” iridium
bis-phosphine and N-heterocyclic carbene hydrides reported by
Caulton7 and Nolan,8 respectively. 1H and 31P{1H} NMR spectra
are consistent with the solid-state structure. In the 31P{1H} NMR
(4) See for example: (a) Grushin, V. V.; Alper, H. Chem. ReV. 1994,
94, 1047–1062. (b) Ishiyama, T.; Hartwig, J. J. Am. Chem. Soc. 2000, 122,
12043–12044. (c) Wang, X.; Lane, B. S.; Sames, D. J. Am. Chem. Soc.
2005, 127, 4996–4997. (d) Lewis, J. C.; Berman, A. M.; Bergman, R. G.;
Ellman, J. A. J. Am. Chem. Soc. 2008, 130, 2493–2500.
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Herrero, J.; Oliva´n, M. New J. Chem. 2001, 25, 775–776. (c) Esteruelas,
M. A.; Herrero, J.; Lo´pez, F. M.; Mart´ın, M.; Oro, L. A. Organometallics
1999, 18, 1110–1112.
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Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1999, 121, 97–106.
(8) Scott, N. M.; Pons, V.; Stevens, E. D.; Heinekey, D. M.; Nolan,
S. P. Angew. Chem., Int. Ed. 2005, 44, 2512–2515.
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Weller, A. S. Inorg. Chem. 2005, 44, 3162–3171.
10.1021/om800373d CCC: $40.75
2008 American Chemical Society
Publication on Web 05/29/2008

Communications
Organometallics, Vol. 27, No. 13, 2008 2919
Figure 2. [Rh(PiBu3)2(ClCH2CH2Cl)][BArF ] (3b) and [Rh(PiBu3)2(η6-C6H5F)][BArF ] (3c) (40% probability ellipsoids). The anions, minor
4
4
disordered components, and most hydrogen atoms have been omitted for clarity.
spectrum a single environment is observed that shows coupling
to rhodium (δ 28.3, J(RhP) ) 107 Hz). The 1H NMR spectrum
displays a high-field doublet of triplets for hydrides, while a
spectrum (CD2Cl2). The 31P{1H} NMR spectrum of 3a remains
relatively sharp and unchanged in chemical shift between 298
and 190 K, suggesting that reversible C-H activation of one
of the alkylphosphines, which would result in inequivalent
phosphorus environments as recently reported by Caulton for
i
single set of resonances is observed for the Bu protons,
indicating rapid exchange between agostic and free C-H groups.
This exchange is not frozen out at 190 K. The IR spectrum
(CD2Cl2) shows a very broad, medium-intensity band charac-
teristic of agostic interactions at 2668 cm-1.7 Using a slightly
t
(PNP)Rh (PNP ) Bu2PCH2SiMe2)2N), is probably not occur-
ring.11 Crystalline material obtained from CH2Cl2/pentane
solutions showed a grossly disordered solid-state structure from
which, unfortunately, no useful structural information could be
obtained. Microanalysis and ESI-MS data were consistent with
a formulation that does not contain CH2Cl2, a 1H NMR spectrum
(C6H5F) of dissolved crystals showed no liberated CH2Cl2, and
different protocol (starting from Rh(PtBu3)2H2Cl and Na[BArF ])
4
the analogous complex [Rh(PtBu3)2H2][BArF ] (2) was also
4
synthesized (Supporting Information).
2
a solid-state H NMR experiment of a sample of 3a prepared
in CD2Cl2 showed no deuterium signal, consistent with the
absence of coordinated CD2Cl2. In the solid state, in the absence
of coordinated solvent, such a complex would undoubtedly have
supporting Rh · · · HC agostic interactions12 or interactions with
the [BArF ]- anion,9 while in solution a bound CH2Cl2 solvent
4
molecule13 or agostic interactions are likely present, given that
we have no evidence for anion coordination. Infrared spectro-
scopic measurements on microcrystalline 3a (KBr) or dissolved
3a (CD2Cl2) show no characteristic stretches for agostic C-H
interactions, thus suggesting anion and solvent coordination,
respectively. However, given that those signals observed in 1
are particularly broad, we cannot discount that they are present
but are too broad to be observed. A d8 ML2 fragment would be
expected to be bent rather than linear and thus be well set up
for interactions with solvent, arene, or agostic C-H bonds.14
Whatever the precise structure of 3a, solvent-trapping experi-
ments confirm that the {Rh(PiPr3)2}+ fragment has been formed.
Evaporating a pale yellow solution of 1 to dryness and placing
the residue under vacuum (16 h, 5 × 10-2 Torr) resulted in the
slow loss of H2 to give an air-sensitive orange solid that we
tentatively formulate as [Rh(PiBu3)2(L2)][BArF ] (3a) (eq 2, L2
4
) solvent, agostic or anion coordination). Alternatively, rapid
H2 removal from 1 in CH2Cl2 solution can be achieved by
addition of tert-butylethylene (tbe, 5 equiv, 5 min) as a hydrogen
acceptor, evaporating the solution to dryness, and washing the
resulting orange powder with pentane to remove excess tbe.
1H and 31P{1H} NMR spectra are identical for both preparation
1
methods, indicating that tbe is not bound to the metal. The H
NMR spectrum (CD2Cl2, 298-190 K) of 3a shows peaks due
(10) (a) Korbe, S.; Schreiber, P. J.; Michl, J. Chem. ReV. 2005, 106,
5208–5249. (b) Clarke, A. J.; Ingleson, M. J.; Kociok-Kohn, G.; Mahon,
M. F.; Patmore, N. J.; Rourke, J. P.; Ruggiero, G. D.; Weller, A. S. J. Am.
Chem. Soc. 2004, 126, 1503–1517.
to a single PiBu3 environment, a featureless hydride region, and
no evidence for coordination of the [BArF ]- anion through a
4
η6-arene interaction.9 The 31P{1H} and 19F NMR spectra
(CD2Cl2) both show single environments at δ 44.9 (J(RhP) )
207 Hz) and δ -61.1, respectively. The latter observation further
suggests that the anion is not bound through a η6-arene
interaction in solution.9a Additional evidence comes from
(11) Verat, A. Y.; Pink, M.; Fan, H.; Tomaszewski, J.; Caulton, K. G.
Organometallics 2008, 27, 166–168.
(12) (a) Urtel, H.; Meier, C.; Eisentrager, F.; Rominger, F.; Joschek,
J. P.; Hofmann, P. Angew. Chem., Int. Ed. 2001, 40, 781. (b) Baratta, W.;
Mealli, C.; Herdtweck, E.; Ienco, A.; Mason, S. A.; Rigo, P. A. J. Am.
Chem. Soc. 2004, 126, 5549–55629. (c) Ingleson, M. J.; Mahon, M. F.;
Weller, A. S. Chem. Commun. 2004, 21, 2398–2399.
changing the [BArF ]- anion in 3a for the carborane anion
4
[closo-HCB11(CH3)11]-,10 which resulted in no change in the
(13) For examples of coordinated CH2Cl2 see: (a) Butts, M. D.; Scott,
B. L.; Kubas, G. J. J. Am. Chem. Soc. 1996, 118, 11831. (b) Huang, D. J.;
Huffman, J. C.; Bollinger, J. C.; Eisenstein, O.; Caulton, K. G. J. Am. Chem.
Soc. 1997, 119, 7398. (c) Taw, F. L.; Mellows, H.; White, P. S.; Hollander,
F. J.; Bergman, R. G.; Brookhart, M.; Heinekey, D. M. J. Am. Chem. Soc.
2002, 124, 5100.
chemical shift or coupling constant in the 31P{1H} NMR
(9) (a) Douglas, T. M.; Molinos, E.; Brayshaw, S. K.; Weller, A. S.
Organometallics 2007, 26, 463–465. (b) Powell, J.; Lough, A.; Saeed, T.
J. Chem. Soc., Dalton Trans. 1997, 4137–4138.
(14) Otsuka, S. J. Organomet. Chem. 1980, 200, 191–205.

2920 Organometallics, Vol. 27, No. 13, 2008
Communications
Figure 3. Synthesis of 4a,b (left) and a plot illustrating first-order oxidative addition at different C6H5Cl concentrations (right).
formation (NMR) of new species that have been characterized
as the products of aryl halide oxidative addition at the cationic
Rh(I) center: [Rh(PiBu3)2(C6H5)(µ-X)]2[BArF ] (4a, X ) Cl;
4 2
4b, X ) Br). The reaction with bromobenzene is instant at
room temperature (<5 min), but that with chlorobenzene
takes longer (8 h). In this latter case the immediate formation
of an intermediate species is observed. The crystal structure
of [Rh(PiBu3)2(C6H5)(µ-Cl)]2 (4a) is presented in Figure 4
and shows a dimeric complex with bridging chloride ligands.
Each Rh(III) is formally a 16-electron species with a vacant
site in the coordination sphere, lying opposite the aryl group
(Rh · · · C > 3.4 Å). The intermediate species in this oxidative
addition reaction has been identified spectroscopically as
[Rh(PiBu3)2(η6-C6H5Cl)][BArF ] (5) by comparison of spec-
4
troscopic data with those of 3c. When 5 is formed in situ
from addition of chlorobenzene to 3a in CD2Cl2, its
transformation to 4b follows first-order kinetics with a rate
Figure 4. [Rh(PiBu3)2(C6H5)(µ-Cl)]2[BArF ] (4a) (40% probability
that is independent of chlorobenzene concentration (Figure
4 2
3; k ) (1.78 ( 0.05) × 10-4 s-1), indicating that the rate-
ellipsoids). The anions, minor disordered components, and hydrogen
atoms have been omitted for clarity.
limiting step is oxidative cleavage of the bound aryl halide
to the metal center, possibly via a η6 to η2 ring slippage
mechanism.16 The formation of 4a,b via a η6-haloarene
intermediate parallels that observed by Budzelaar and co-
workers on the neutral Rh(nacnac)(coe) complex (nacnac )
ArNC(Me)CHC(Me)NAr, Ar ) 2,6-Me2C6H3, coe ) cis-
cyclooctene) to give dimeric Rh(III) aryl halides.3c The
structure of 4a is also closely related to the dimeric alkyl-
Thus, repeating the synthesis in ClCH2CH2Cl rather than CD2Cl2
solvent afforded an air-sensitive complex with NMR spectro-
scopic characteristics very similar to those of 3a in CD2Cl2
solution (δ(31P) 32.1, J(RhP) ) 204 Hz) that could also be
characterized by X-ray crystallography as the dichloroethane
rhodium(III) complex [RhCl2(CH2CH3){PtBu2(CH2CH2C6H3-
2,6-Me2)}]2.17 Addition of an excess (10 equiv) of PiBu3 to
a CH2Cl2 solution of 3a inhibited oxidative addition of
haloarenes, consistent with the mechanism proposed. Com-
plexes 3b,c also underwent oxidative addition of aryl halides
to give the same products, but in the case of 3c the rate is
slower, presumably due to competitive exchange between the
arenes.
adduct15 [Rh(PiBu3)2(ClCH2CH2Cl)][BArF ] (3b) (Figure 2).
4
Use of fluorobenzene as the solvent gave the arene adduct
[Rh(PiBu3)2(η6-C6H5F)][BArF ] (3c) (Figure 2). 3b,c can also
4
be cleanly generated by adding the appropriate ligand to CH2Cl2
solutions of 3a (eq 2).
Interestingly 2 does not lose H2 readily and addition of tbe
to CH2Cl2 solutions results in slow decomposition. We speculate
that this is because the bulky tert-butyl groups in 2 cannot fold
back sufficiently to allow access to a trigonal transition-state
with a dihydrogen-like ligand, most likely required for the loss
In conclusion, we report the H2 loss from a “14-electron”
Rh(III) complex to give a series of solvent-stabilized low-
coordinate cationic Rh(I) species (3a-c) in solution, which
are capable of oxidative addition of unactivated aryl halides
at room temperature. These products are themselves unsatur-
ated, and it is tempting to speculate that this feature will
allow them to take part in further reactivity that will result
in useful coupling reactions. Studies are currently underway
to probe this, and preliminary results indicate that 3a will
promote C-C coupling between 4-bromotoluene and (4-tert-
butylphenyl)boronic acid in the presence of base. Finally,
we note that species such as 3b are inferred, but not isolated,
of H2 from 1. Similar attempts to remove all the hydrides from
i
[Rh(PR3)2H2(η2-H2)2][BArF ] (R ) Pr or Cy) gave mixtures
4
of products, showing that the PiBu3 ligands provide just the right
balance of steric and electronic support to facilitate H2 loss and
stabilize the resulting complex.
Addition of bromo- or chlorobenzene to 3a in CH2Cl2
solution at room temperature resulted in the quantitative
(16) Ahlquist, M.; Norrby, P. O. Organometallics 2007, 26, 550–553.
(17) Canepa, G.; Brandt, C. D.; Werner, H. J. Am. Chem. Soc. 2002,
124, 9666–9667.
(15) Moxham, G. L.; Brayshaw, S. K.; Weller, A. S. Dalton Trans. 2007,
1759–1761.

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Organometallics, Vol. 27, No. 13, 2008 2921
intermediates in cycloaddition reactions mediated by Rh(I)
bis-phosphines.18
James Osbourne for experimental assistance in the prelimi-
nary C-C coupling study.
Acknowledgment. We thank the EPSRC and the Uni-
versity of Oxford for support, Amber Thompson for as-
sistance with X-ray crystallography, and Michael Willis and
Supporting Information Available: Text, figures, and tables
giving full experimental details, kinetic data, and characterization
data and CIF files giving crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(18) Gilbertson, S. R.; Hoge, G. S. Tetrahedron Lett. 1998, 39, 2075–
2078.
OM800373D