Oxidative addition of C-H bonds to RhCl(PMe3)3: Relevant to the catalytic transformation of hydrocarbons

Article abstract of DOI:10.1021/ja021375i

RhCl(PMe₃)₃ (1) reacts with benzene under irradiation to give the oxidative addition product, Rh(C₆H₅)(H)Cl(PMe₃)₃ (2). The reaction is promoted under CO₂ atmosphere. The structure of 2 was fully characterized by X-ray crystallography as well as NMR, IR, and elemental analysis. The adduct (2) is unstable in solution even at room temperature to regenerate benzene and 1. The thermolysis of 2 under a CO atmosphere produces benzaldehyde along with the reductive elimination product, benzene. On the other hand, the prolonged photoreaction of 1 with benzene under CO₂ resulted in the activation of the C-H bond and CO₂ to yield Rh(C₆H₅)(η²-CO₃)(PMe₃)₃ (3). Copyright

Full text of DOI:10.1021/ja021375i

Published on Web 06/10/2003
Oxidative Addition of C-H Bonds to RhCl(PMe3)3: Relevant to the Catalytic
Transformation of Hydrocarbons
Jun-Chul Choi and Toshiyasu Sakakura*
National Institute of AdVanced Industrial Science and Technology (AIST),
Higashi, AIST Central 5, Tsukuba 305-8565, Japan
Received November 22, 2002; E-mail: t-sakakura@aist.go.jp
Synthetic chemists have long envisioned the direct and selective
conversion of hydrocarbons under mild conditions. Transition metal
complexes are a powerful tool for achieving this goal because they
can activate unreactive C-H bonds of hydrocarbons.1 Previously,
we developed a RhCl(L)(PMe3)2-hν system (L ) CO, CH2dCH2,
PMe3) that catalyzes a photopromoted C-H bond activation.2-5
When the reaction medium is dense CO2, hydrocarbons including
methane and ethane are effectively functionalized.4,5 The generally
accepted C-H bond activation pathway for the RhCl(L)(PMe3)2-
hν system is the oxidative addition of hydrocarbons to a low valent
metal center.6-8 To date, however, the oxidative addition products
have not been isolated. In this paper, we report the isolation, full
structural characterization, and reactivity of Rh(C6H5)(H)Cl(PMe3)3
Figure 1. CO2 concentration effect on the yield of 2 from 1. The yield is
the sum of the amount of 2 and its isomer (see ref 9).
synthesized through oxidative addition of benzene to a Wilkinson-
type complex, RhCl(PMe3)3 (1), which is an efficient catalytic
precursor for carbonylation and dehydrogenation of hydrocarbons.4
Sixty equivalents of CO2 (about 10 atm) was added to a benzene
solution (0.4 mL) of 1 (0.014 mmol). The reaction mixture was
then irradiated with a 100 W high-pressure mercury lamp at room
temperature. After 24 h, 1 was selectively converted to a new
complex that kept the three phosphine ligands of the starting
complex as determined by 31P NMR.9 In addition, 1H and 13C NMR
spectra reveal the presence of a hydrido and a phenyl ligand on
the rhodium center. The combination of various spectral data
indicated the formation of mer-Rh(C6H5)(H)Cl(PMe3)3 (2) through
1
the oxidative addition of benzene to 1. The complicated H and
13C NMR patterns in the phenyl region indicated a restricted rotation
around the phenyl-rhodium bond even at room temperature. The
1
small signals in H and 31P NMR are presumably attributable to
the presence of the stereoisomer of 2 that is produced by exchanging
the phenyl and the hydrido ligands. The large JP-H value (215 Hz)
is consistent with the configuration having the hydrido ligand trans
to phosphorus.9
Figure 2. X-ray crystal structure of 2. Selected bond distances (Å) and
angles (deg): Rh1-H1 1.33(7), Rh1-C1 2.102(5), Rh1-Cl1 2.500(1),
Rh1-P1 2.312(1), Rh1-P2 2.320(1), Rh1-P3 2.363(1), C1-Rh1-H1
92(2), Cl1-Rh1-C1 94.2(1), P1-Rh1-P2 165.71(6), C1-Rh1-P1
85.7(1), C1-Rh1-P2 85.6(1).
Interestingly, the presence of CO2 promoted the formation of 2
from 1 (Figure 1). Although the precise mechanistic discussion is
premature,10 possible explanations for this effect are (i) the
formation of a small amount of CO and OdPMe3 from CO2 and
PMe3,5 followed by the preferential photoreaction of RhCl(CO)-
(PMe3)2, or (ii) the weak coordination of CO2 to the Rh center
resulting in the stabilization of productive intermediates such as
RhCl(C6H5)(H)(CO2)(PMe3)2. The following phenomena also give
some clues to the role of CO2: (i) Reductive elimination of benzene
from 2 was not retarded by CO2 (vide infra). (ii) Aresta reported
the formation of a rhodium(I) CO2 complex, RhCl(CO2)(PBu3)2,
from RhCl(PBu3)3 under atmospheric CO2. However, we have not
yet detected this species by NMR or IR.11
Figure 2.12 The structure is consistent with the solution structure
speculated by NMR and IR. The coordination sphere around the
rhodium center corresponds to an octahedron with H1 and C1 in
the cis positions. Although various molecules such as dihydrogen,
hydrogen halides, halogens, alkyl halides, acyl halides, silyl
hydrides, and aldehydes are known to oxidatively add to RhCl-
(PR3)3,13,14 examples of intermolecular oxidative addition of
hydrocarbons to RhCl(PR3)3 have not been reported.
The reactivity of the phenyl(hydrido) complex 2 is summarized
in Scheme 1. Although 2 was produced in the presence of excessive
CO2, insertion of CO2 into the Rh-H or Rh-Ph bond was not
observed. Complex 2 was unstable in solution at room temperature
under nitrogen. Benzene and 1 were gradually regenerated via
reductive elimination. The rate of the reaction was measured using
1H and 31P NMR spectroscopy to follow the disappearance of 2 at
Although complex 2 is rather unstable at room temperature,
single crystals of 2 were successfully obtained by recrystallization
in a toluene-hexane mixture at a low temperature. The molecular
structure of 2 determined by X-ray crystallography is shown in
9
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J. AM. CHEM. SOC. 2003, 125, 7762-7763
10.1021/ja021375i CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
(5) Bitterwolf, T. E.; Kline, D. L.; Linehan, J. C.; Yonker, C. R.; Addleman,
R. S. Angew. Chem., Int. Ed. 2001, 40, 2692-2694.
(6) (a) Margl, P.; Ziegler, T.; Blo¨chl, P. E. J. Am. Chem. Soc. 1996, 118,
5412-5419. (b) Maguire, J. A.; Boese, W. T.; Goldman, M. E.; Goldman,
A. S. Coord. Chem. ReV. 1990, 97, 179-192.
(7) (a) Rosini, G. P.; Soubra, S.; Vixamar, M.; Wang, S.; Goldman, A. S. J.
Organomet. Chem. 1998, 554, 41-47. (b) Rosini, G. P.; Wang, K.; Patel,
B.; Goldman, A. S. Inorg. Chim. Acta 1998, 270, 537-542. (c) Rosini,
G. P.; Zhu, K.; Goldman, A. S. J. Organomet. Chem. 1995, 504, 115-
121. (d) Boyd, S. E.; Field, L. D.; Partridge, M. G. J. Am. Chem. Soc.
1994, 116, 9492-9497. (e) Rosini, G. P.; Boese, W. T.; Goldman, A. S.
J. Am. Chem. Soc. 1994, 116, 9498-9505.
(8) (a) Bitterwolf, T. E.; Scallorn, W. B.; Bays, J. T.; Weiss, C. A.; Linehan,
J. C.; Franz, J.; Poli, R. J. Organomet. Chem. 2002, 652, 95-104. (b)
Bridgewater, J. S.; Netzel, T. L.; Schoonover, J. R.; Massick, S. M.; Ford,
P. C. Inorg. Chem. 2001, 40, 1466-1476. (c) Bridgewater, J. S.; Lee, B.;
Bernhard, S.; Schoonover, J. R.; Ford, P. C. Organometallics 1997, 16,
5592-5594. (d) Ford, P. C.; Netzel, T. L.; Spillett, C. T.; Pourreau, D.
B. Pure Appl. Chem. 1990, 62, 1091-1094. (e) Spillett, C. T.; Ford, P.
C. J. Am. Chem. Soc. 1989, 111, 1932-1933.
25 °C in THF-d8. The reaction is first-order in [2] with a rate
constant of 3.8 × 10-5 s-1. The presence of CO2 (60 equiv) did
not significantly alter the reaction. The treatment of 2 with
atmospheric CO at 25 °C in THF-d8 resulted in 80% conversion in
1 h as judged by 1H NMR. GC-MS analysis after 5 h revealed the
formation of benzaldehyde (6%) along with the reductive elimina-
tion product, benzene (74%). For the catalytic C-H bond carbo-
nylation by the RhCl(CO)(PMe3)2-hν system, Rh(C6H5)(H)Cl-
(PMe3)2 and Rh(C6H5)(H)Cl(CO)(PMe3)2 were postulated as catalytic
intermediates.7 The benzaldehyde formation from 2 shown in
Scheme 1 possibly proceeds through these intermediates.
(9) Analytical data of 2: 1H NMR (400 MHz in C6D6 at 25 °C) δ -18.26
(ddt, 1H, RhH, J(RhH) ) 19 Hz, J(PH) ) 28, J(PH) ) 15 Hz), 0.98
(apparent triplet due to virtual coupling, 18H, P(CH3)3), 1.09 (d, 9H,
P(CH3)3, J(PH) ) 6 Hz), 7.10 (m, 2H, Ph), 7.27 (m, 1H, Ph), 7.49 (t, 1H,
Ph, J(HH) ) 6 Hz), 8.92 (t, 1H, Ph, J(HH) ) 7 Hz). 31P{1H} NMR (160
MHz in C6D6 at 25 °C) δ -20.81 (dt, J(RhP) ) 79 Hz, J (PP) ) 29 Hz),
-7.37 (dd, J(RhP) ) 103 Hz, J(PP) ) 29 Hz). 13C{1H} NMR (100 MHz
in CD2Cl2 at -50 °C) δ 17.43 (apparent triplet due to virtual coupling,
P(CH3)3), 19.58 (d, P(CH3)3, J(PC) ) 22 Hz), 120.98, 125.07, 125.71,
128.82, 139.05, 145.64 (Ph). NMR data of the isomer: 1H NMR (400
MHz in C6D6 at 25 °C) δ -9.70 (ddt, 1H, RhH, J(RhH) ) 17 Hz, J(PH)
) 215 Hz, J(PH) ) 15 Hz), 8.61 (t, 1H, Ph, J(HH) ) 8 Hz). 31P{1H}
NMR (160 MHz in CD2Cl2 at -50 °C) δ -22.93 (dt, J(RhP) ) 74 Hz,
J(PP) ) 26 Hz), -9.10 (dd, J(RhP) ) 95 Hz, J(PP) ) 26 Hz). IR (KBr)
ν 2036 cm-1 (Rh-H). Anal. Calcd for C15H33ClP3Rh: C, 40.51; H, 7.48;
Cl, 7.97. Found: C, 40.72; H, 7.49; Cl, 8.09.
On the other hand, the prolonged photoreaction of 1 with benzene
under CO2 produced a small amount of colorless crystals of a
phenyl(carbonato) complex, mer-Rh(C6H5)(CO3)(PMe3)3 (3), re-
vealing the activation of the C-H bond and CO2. The structure
was determined by X-ray crystallography.15 The fluxional behavior
(10) The authors appreciate the helpful mechanistic discussion with one of
the reviewers. The PMe3 addition experiment suggested by the reviewer
resulted in the formation of [Rh(PMe3)4]Cl.
1
of the ortho-protons in H NMR revealed that in 3 the phenyl-
rhodium bond rotates more easily due to wider phenyl-Rh-P
angles than in 2. Complex 3 was presumably formed via 2 because
both possess a phenyl group and three trimethylphosphine ligands.
The addition of 1 equiv of water to the system did not promote the
formation of 3. A possible explanation for the origin of the
(11) (a) Aresta, M.; Nobile, F. Inorg. Chim. Acta 1977, 24, L49-L50. (b)
Gibson, D. H. Chem. ReV. 1996, 96, 2063-2096.
(12) Recrystalized from toluene-hexane at -40 °C. Crystal data for 2:
A
single crystal (0.20 × 0.30 × 0.40 mm) was sealed in a capillary glass
tube. The diffraction data were collected at 193 K on a Rigaku AFC-7R
automated four-cycle diffractometer by using graphite-monochromated
Mo KR radiation (λ ) 0.71069 Å) and the ω-2 method. A full matrix
least-squares refinement was used for non-hydrogen atoms with anisotropic
thermal parameters. Hydrogen atoms except for the Rh-H hydrogen were
located by assuming ideal positions (d ) 0.95) and were included in the
structure calculation without further refinement of the parameters. Crystal
data: C15H33ClP3Rh, Mr ) 444.71, monoclinic, space group P21/n (No.
14), a ) 9.787(3), b ) 11.944(5), c ) 17.892(4), â ) 94.14(2), V )
2085(1), Z ) 4, Dcalc ) 1.416 g cm-3, 2θ max ) 55, GOF ) 1.97. The
2-
carbonato ligand is the disproportionation of CO2 to CO3 and
CO, although the participation of adventitious water is not denied.16
The small amount of benzophenone detected in the reaction mixture
supports the CO2 disproportionation mechanism.
In conclusion, we have revealed that RhCl(PMe3)3 can break
the C-H bond of benzene under irradiation to give Rh(C6H5)(H)-
Cl(PMe3)3 in a high yield. The structure and the reactivities were
elucidated. It is noteworthy that RhCl(PMe3)3 is an active catalyst
for the photopromoted C-H bond activations.
final R factor was 0.048 (Rw ) 0.077) for 3929 reflections with Io
2σ(Io).
>
(13) Jardine, F. H. Prog. Inorg. Chem. 1981, 28, 63-202.
(14) Milstein, D. Organometallics 1982, 1, 1549-1551.
(15) Recrystalized from CH2Cl2-Et2O at room temperature. Crystal data for
3: A single crystal (0.20 × 0.30 × 0.40 mm) was sealed in a capillary
glass tube for the data collection, and diffraction data were collected at
296 K. Crystal data: C16H32O3P3Rh, Mr ) 468.25, tetragonal, space group
Acknowledgment. This research was supported in part by a
grant from the New Energy and Industrial Technology Development
Organization (NEDO) of Japan.
P4/n (No. 85), a ) 22.597(3), c ) 8.705(4), V ) 4445(2), Z ) 8, Dcalc
)
1.399 g cm-3, 2θ max ) 55, GOF ) 1.71. The final R factor was 0.049
(Rw ) 0.078) for 3929 reflections with Io > 2σ(Io). Analytical data of 3:
1H NMR (400 MHz in CD2Cl2 at 25 °C) δ 1.18 (apparent triplet due to
virtual coupling, 18H, P(CH3)3), 1.61 (d, 9H, P(CH3)3, J(PH) ) 10 Hz),
6.94 (m, 3H, Ph), 7.13 (br, 2H, Ph). 13C{1H} NMR (CD2Cl2) δ 12.31
(apparent triplet due to virtual coupling, P(CH3)3), 17.46 (d, P(CH3)3, J(PC)
) 31 Hz), 122.13, 126.89, 135.51, 145.07 (Ph), 166.24 (CO3). 31P{1H}
NMR (CD2Cl2) δ -3.64 (dd, J(RhP) ) 99 Hz, J(PP) ) 36 Hz), 1.51 (dt,
J(RhP) ) 126 Hz, J(PP) ) 36 Hz). 1H NMR (400 MHz in CD2Cl2 at
-80 °C) δ 1.08 (apparent triplet due to virtual coupling, 18H, P(CH3)3),
1.54 (d, 9H, P(CH3)3, J(PH) ) 10 Hz), 6.72 (d, 1H, Ph, J(HH) ) 7 Hz),
6.87 (m, 3H, Ph), 7.37 (d, 1H, Ph, J(HH) ) 7 Hz). IR (KBr) ν 1552
cm-1 (CO3). Anal. Calcd for C16H32O3P3Rh: C, 41.04; H, 6.89; O, 10.25.
Found: C, 40.95; H, 7.21; O, 10.35. HRMS(EI): calcd for C16H31O3P3-
Rh, 467.0540; found, 467.0545 (M+ - H).
Supporting Information Available: NMR and crystallographic
data of 2 and 3, 1H and 31P NMR data and a kinetic data for the
decomposition of 2 to 1, and the result of the PMe3 addition experiment
(PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
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