Easy and reversible C-H activation of a substituted benzene

Article abstract of DOI:10.1021/om0010159

The reaction of either [RhCl(C₈H₁₄)₂]₂ or [RhCl(C₂H₄)₂]₂ with tBu₂PCH₂CH₂C₆H₅ (3) affords at room temperature the five-coordinate arylhydridorhodium(III) complex 4, the molecular structure of which has been determined by X-ray crystallography. The C-H metalation is completely reversible, as is shown by the formation of trans-[RhCl(CO)(3)₂] from 4 and CO. Compound 4 also reacts with PhC triple bond CH, H₂, and AgPF₆ to give products 7-9 containing the intact phosphine ligand 3.

Full text of DOI:10.1021/om0010159

604
Organometallics 2001, 20, 604-606
Ea sy a n d Rever sible C-H Activa tion of a Su bstitu ted
Ben zen e
Giuseppe Canepa, Carsten D. Brandt, and Helmut Werner*
Institut fu¨r Anorganische Chemie, Universita¨t Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany
Received November 27, 2000
Sch em e 1a
Summary: The reaction of either [RhCl(C8H14)2]2 or
[RhCl(C2H4)2]2 with tBu2PCH2CH2C6H5 (3) affords at
room temperature the five-coordinate arylhydridorho-
dium(III) complex 4, the molecular structure of which
has been determined by X-ray crystallography. The C-H
metalation is completely reversible, as is shown by the
formation of trans-[RhCl(CO)(3)2] from 4 and CO.
Compound 4 also reacts with PhCtCH, H2, and AgPF6
to give products 7-9 containing the intact phosphine
ligand 3.
The bis(triisopropylphosphine)rhodium(I) complex
[RhCl(PiPr3)2]2 (1) is probably one of the most reactive
rhodium(I) compounds known to date.1 It reacts not only
with H2, O2, N2, CO, and C2H4 but also with terminal
alkynes to give stepwise π-alkyne, alkynyl hydrido, and
vinylidene rhodium derivatives.2 While 1 is easily
accessible from [RhCl(C8H14)2]2 (2) and 4 equiv of
PiPr3,1b the analogous complex [RhCl(PtBu3)2]2 cannot
be obtained by a similar route.3 Since we knew that even
small differences in the size of the phosphine ligand can
change the stability and reactivity of compounds of the
general composition [RhCl(PR3)2]n significantly, we
considered instead of PtBu3 the somewhat less bulky
derivative tBu2PCH2CH2C6H5 (3), which was recently
prepared in our laboratory,4 as a candidate to isolate
an analogue of 1.
a
L ) tBu2PCH2CH2C6H5 (3).
13C) couplings, each of these signals is split into a
doublet of doublets of doublets. Moreover, the two
doublets of doublets at δ 65.7 and 43.0 in the 31P NMR
spectrum of 4 confirm that the two phosphorus atoms
are chemically nonequivalent.
The result of the X-ray crystal structure analysis of
4 is shown in Figure 1.7 It reveals that during the
reaction of 2 and 3 a C-H metalation of one of the
phenyl groups has indeed taken place. The coordination
geometry around the rhodium center corresponds to a
distorted trigonal bipyramid with the two phosphorus
atoms in the apical positions. The Rh-P distances are
slightly longer than in the related, more symmetri-
cal chelate complex [RhHCl{tBu2PCH2C6H3CH2PtBu2-
κ3(P,C,P)}] (A) obtained from RhCl3‚3H2O and 1,3-bis-
[(tert-butylphosphino)methyl]benzene in iPrOH/H2O un-
der reflux.8 In contrast, the Rh-C31 bond length of 4
(1.967(5) Å) is slightly shorter than in A (1.999(7) Å)
and in the Milstein compound [Rh(CH3)Cl{tBu2PCH2-
C6H-3,5-(CH3)2CH2PtBu2-κ3(P,C,P)}] (2.02(2) Å).9 The
P-Rh-P axis of 4 is significantly bent (160.18(5)°),
which could be due both to steric hindrance between
the phosphine substituents and the strain of the six-
Under conditions similar to those used for the prepa-
ration of 1, the reaction of 2 with a 4-fold excess of 3 in
pentane at room temperature resulted in the formation
of the yellow solid 4, the analytical composition of which
1
corresponds to that of [RhCl(3)2].5 However, the H and
31P NMR spectra of 4 (see Scheme 1) reveal that the
product is not a rhodium(I) complex containing two
intact phosphine ligands but an arylhydridorhodium-
(III) species. The most typical features are the high-field
1
resonance in the H NMR spectrum at δ -18.11 for the
RhH proton and the 13C NMR signal at δ 146.9 for the
metalated carbon atom of the six-membered ring.6 Due
to 103Rh-1H (or 103Rh-13C) and 2-fold 31P-1H (or 31P-
(1) (a) Preparation in situ: Busetto, C.; D’Alfonso, A.; Maspero, F.;
Perego, G.; Zazzetta, A. J . Chem. Soc., Dalton Trans. 1977, 1828-
1834. (b) Isolation: Werner, H.; Wolf, J .; Ho¨hn, A. J . Organomet. Chem.
1985, 287, 395-407. (c) X-ray crystal structure analysis: Binger, P.;
Haas, J .; Glaser, G.; Goddard, R.; Kru¨ger, C. Chem. Ber. 1994, 127,
1927-1929.
(2) Reviews: (a) Werner, H. Nachr. Chem. Technol. Lab. 1992, 40,
435-444. (b) Werner, H. J . Organomet. Chem. 1994, 475, 45-55.
(3) (a) Wolf, J . Unpublished results. (b) The preparation of “crude”
[RhCl(PtBu3)2] has been reported,3c but the authors as well as we could
not obtain a pure sample of the given composition. (c) Yoshida, T.;
Otsuka, S.; Matsumoto, M.; Nakatsu, K. Inorg. Chim. Acta 1978, 29,
L257-L259.
(5) The preparation of 4 is as follows. A suspension of 2 (1.51 g,
2.11 mmol) in 10 mL of pentane was treated under continuous stirring
with 3 (2.11 g, 8.43 mmol). A yellow solution was formed, which was
evaporated to dryness in vacuo. After the oily residue was layered with
10 mL of pentane and stored for 8 h, a yellow solid was obtained. It
was separated from the mother liquor, washed five times with 4 mL
portions of pentane, and dried. The pentane washings were combined
and then evaporated to ca. 3 mL in vacuo. The concentrated solution
was stirred for 3 h at room temperature, which gave a second fraction
of the product: yield 2.29 g (85%); mp 97 °C dec. Alternatively,
compound 4 could also be prepared from 5 (303 mg, 0.78 mmol) and 3
(780 mg, 3.12 mmol): yield 808 mg (81%).
(4) Werner, H.; Canepa, G.; Ilg, K.; Wolf, J . Organometallics 2000,
19, 4756-4766.
10.1021/om0010159 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 01/26/2001

Communications
Organometallics, Vol. 20, No. 4, 2001 605
Sch em e 2a
F igu r e 1. ORTEP diagram of 4. The metal-bonded hy-
drogen is not exactly located; the other hydrogen atoms are
omitted for clarity. Selected bond distances (Å) and angles
(deg): Rh-P1, 2.3746(14); Rh-P2, 2.3344(13); Rh-Cl,
2.4687(13); Rh-C31, 1.967(5); P1-Rh-P2, 160.18(5); P1-
Rh-Cl, 98.01(5); P1-Rh-C31, 96.89(13); P2-Rh-Cl, 99.74-
(5); P2-Rh-C31, 87.54(13); Cl-Rh-C31, 103.40(14); Rh-
P2-C28, 110.62(15); P2-C28-C29, 113.8(3); C28-C29-
C30, 109.0(4); C29-C30-C31, 121.1(4); C30-C31-Rh,
126.8(4).
a
L ) tBu2PCH2CH2C6H5 (3).
as starting material for the preparation of 4. If 5 is
treated with 2 instead of 4 equiv of 3, the intermediate
[RhCl(C2H4)(3)]2 (6) is detected by 1H and 31P NMR
spectroscopy,6 which reacts with excess 3 to give 4.
Compound 6 is accessible in analytically pure form from
4 and 5 in the molar ratio of 2:1 in pentane and isolated
as a yellow solid in 86% yield.
membered chelate ring. We note that besides 2 also the
dimeric bis(ethene)rhodium(I) derivative 5 can be used
If a suspension of 4 in pentane is stirred at room
temperature under an atmosphere of carbon monoxide,
a gradual change of color occurs and after ca. 30 s a
light yellow solid precipitates. The spectroscopic data
of this compound indicate that instead of the anticipated
six-coordinate 1:1 adduct of 4 and CO the square-planar
carbonyl complex 7 is formed (Scheme 2). The practically
quantitative yield of 7 from the rhodium(III) precursor
4 illustrates that the insertion of the metal into one of
the ring C-H bonds of the phosphine 3 is completely
reversible. With regard to the structure of 7, the
noteworthy aspect is that in solution at low temperature
at least three rotamers can be observed, the existence
of which is probably due to the steric bulk of the tert-
butyl groups.10
(6) Selected spectroscopic data for 4 and 6-10 are as follows. 4: 1H
NMR (C6D6, 600 MHz) δ -18.11 (ddd, J (RhH) ) 22.9, J (PH) ) 15.9
and 9.5 Hz, 1H, RhH); 13C NMR (C6D6, 150.9 MHz) δ 146.9 (ddd,
J (RhC) ) 34.2, J (PC) ) 12.0 and 5.8 Hz, RhC), 144.3 (d, J (PC) ) 8.6
Hz, RhCCCH2), 42.2 (dd, J (RhC) ) 5.7, J (PC) ) 5.2 Hz, RhCCCH2),
19.1 (d, J (PC) ) 29.3 Hz, PCH2); 31P NMR (C6D6, 162.0 MHz) δ 65.7
(dd, J (RhP) ) 120.4, J (PP) ) 366.2 Hz, tBu2P of chelate ring), 43.0
(dd, J (RhP) ) 110.2, J (PP) ) 366.2 Hz, tBu2P of monodentate ligand).
6: 1H NMR (C6D6, 200 MHz) δ 3.56, 3.06 (both m, 4H each, C2H4),
2.84 (m, 4H, PCH2CH2), 1.56 (m, 4H, PCH2); 13C NMR (C6D6, 50.3 MHz)
δ 44.7 (d, J (RhC) ) 14.9 Hz, C2H4), 32.9 (s, PCH2CH2), 22.4 (d,
J (PC) ) 15.6 Hz, PCH2); 31P NMR (C6D6, 81.0 MHz) δ 65.8 (d,
J (RhP) ) 185.7 Hz). 7: IR (KBr) ν(CO) 1937 cm-1 31P NMR (toluene-
;
d8, 162.0 MHz, 223 K) δ 58.9 (dd, J (RhP) ) 118.7, J (PP) ) 312.0 Hz,
tBu2P of rotamer I), 58.1 (d, J (RhP) ) 120.4 Hz, tBu2P of rotamer II),
47.4 (dd, J (RhP) ) 123.8, J (PP) ) 312.0 Hz, tBu2P of rotamer I), 46.6
(d, J (RhP) ) 120.4 Hz, tBu2P of rotamer III). 8: 1H NMR (C6D6, 300
MHz, 313 K) δ 3.23 (m, 4H, PCH2CH2), 2.53 (m, 4H, PCH2), 1.36 (dt,
J (PH) ) 3.2, J (RhH) ) 1.1 Hz, RhdCdCH); 13C NMR (C6D6, 75.4 MHz,
313 K) δ 290.6 (m, RhdC), 116.2 (m, RhdCdC), 33.3 (s, PCH2CH2),
23.1 (vt, N ) 15.3 Hz, PCH2); 31P NMR (C6D6, 81.0 MHz, 308 K) δ
The reaction of 4 with phenylacetylene proceeds
similarly to that with CO. Treatment of a solution of 4
with PhCtCH in toluene at room temperature affords,
after chromatographic workup (Al2O3, neutral, activity
grade III, hexane) and recrystallization of the oily
residue from pentane, a blue-violet solid whose elemen-
tal analysis corresponds to 8 (Scheme 2). Typical
spectroscopic data of 8 are the signal for the RhdCd
52.5 (d, J (RhP) ) 137.3 Hz). 9: IR (KBr) ν(RhH) 2138 cm-1 1H NMR
;
(C6D6, 400 MHz) δ 3.25 (m, 4H, PCH2CH2), 2.31 (m, 4H, PCH2), -22.63
(dt, J (RhH) ) 26.3, J (PH) ) 14.7 Hz, 2H, RhH2); 13C NMR (C6D6, 100.6
MHz) δ 34.4 (s, PCH2CH2), 26.2 (vt, N ) 15.3 Hz, PCH2); 31P NMR
(C6D6, 162.0 MHz) δ 65.6 (d, J (RhP) ) 115.3 Hz). 10: 1H NMR (acetone-
d6, 200 MHz) δ 3.20, 2.71, 2.53, 2.33 (all m, 2H each, PCH2 and
PCH2CH2); 13C NMR (acetone-d6, 50.3 MHz) δ 142.5 (d, J (PC) ) 9.3
Hz, ipso-C of C6H5 uncoord), 111.5 (ddd, J (RhC) ) 3.7, J (PC) ) 9.2
and 4.7 Hz, ipso-C of C6H5 coord), 40.8 (dd, J (PC) ) 25.0 and 2.0 Hz,
PCH2CH2-η6-C6H5), 30.6 (s, PCH2CH2-η6-C6H5); 31P NMR (acetone-d6,
81.0 MHz) δ 81.5 (dd, J (RhP) ) 211.1, J (PP) ) 15.3 Hz, tBu2P of chelate
ligand), 68.6 (dd, J (RhP) ) 203.4, J (PP) ) 15.3 Hz, tBu2P of mono-
dentate ligand), -142.7 (sept, J (PF) ) 707.0 Hz, PF6-).
1
CH proton at δ 1.36 in the H NMR and two low-field
resonances for the vinylidene carbon atoms at δ 290.6
and 116.2 in the 13C NMR spectrum.6 Monitoring the
reaction of 4 with PhCtCH in toluene-d8 in an NMR
tube suggests that an alkynylhydridorhodium(III) spe-
cies is formed as an intermediate, which rearranges
smoothly to the final product.
(7) Crystal data for 4: crystals from acetone at room temperature;
crystal size 0.20 × 0.20 × 0.10 mm; monoclinic, space group P21/n (No.
14), Z ) 4; a ) 8.8783(18) Å, b ) 17.190(3) Å, c ) 21.126(4) Å, â )
98.92(3)°, V ) 3185.2(11) Å3, dcalcd ) 1.333 g cm-3; 2θ(max) ) 52.74°
(Mo KR, λ ) 0.710 73 Å, graphite monochromator, ψ scan, T ) 173(2)
K; 32 867 reflections scanned, 6512 unique, 4075 observed (I > 2σ(I)),
direct methods (SHELXS-97), 340 parameters, reflex/parameter ratio
19.15; R1 ) 0.0450, wR2 ) 0.1026; residual electron density +0.897/-
The C-H metalation of 3 leading to 4 is also reversed
upon stirring a suspension of 4 in pentane at room
(9) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D. J . Am.
Chem. Soc. 1996, 118, 12406-12415.
(10) Bushweller, C. H.; Rithner, C. D.; Butcher, D. J . Inorg. Chem.
1984, 23, 1967-1970.
1.259 e Å-3
.
(8) Nemeh, S.; J ensen, C.; Binamira-Soriaga, E.; Kaska, W. C.
Organometallics 1983, 2, 1442-1447.

606 Organometallics, Vol. 20, No. 4, 2001
Communications
Sch em e 3
rhodium(I) precursor [Rh(C8H14)2(acetone)2]PF6 and 2
equiv of C6H5CH2CH2PiPr2.4
In conclusion, we have shown that the reaction of 2,
frequently used as a starting material for the prepara-
tion of rhodium(I) complexes [RhCl(PR3)2]n (n ) 1, 2),1b,13
with the sterically demanding phosphine 3 leads to the
arylhydridorhodium(III) compound 4 by C-H activation
of one of the six-membered rings. The most noteworthy
features are (i) that the insertion of the metal into the
C-H bond proceeds under unusually mild conditions,
as in the reaction of the cationic species [Rh(C8H14)2-
(solv)n]+ with the benzylic phosphine tBu2PCH2C6H2-
Me3,14 and (ii) that in the presence of CO, phenylacet-
ylene, or H2 this insertion process is completely
reversible. The five-coordinate rhodium(III) complex 4
also provides access to the cationic half-sandwich-type
compound 10, which could not be obtained by estab-
lished routes.4,15
temperature in the presence of hydrogen. Under these
conditions, the dihydrido complex 9 (pale yellow solid)
1
is formed, the H NMR spectrum of which displays a
hydride signal at δ -22.63. It is somewhat shifted
upfield compared with that in 4. From the appearance
of a single resonance in the high-field region and the
splitting of this signal into a doublet of triplets, we
conclude that both the hydrido and the phosphine
ligands are stereochemically equivalent. Since it is
known that the related compound [RhH2Cl(PiPr3)2] has
a trigonal-bipyramidal structure,11 a similar coordina-
tion geometry of 9 is most likely.
An attempt to abstract the chloro ligand of 4 with
AgPF6 and generate the cationic 14-electron rhodium-
(III) species [RhH{C6H4-2-CH2CH2PtBu2-κ2(C,P)}(3)]+
led instead to the isolation of the half-sandwich-type
compound 10 (Scheme 3).12 The inequivalence of the two
PtBu2 units is confirmed by the appearance of two
signals in the 31P NMR spectrum at δ 81.5 and 68.6,
which due to 31P-103Rh and 31P-31P couplings are split
into doublets of doublets. A related complex with iPr
instead of tBu substituents at the phosphorus atoms has
recently been prepared from the labile bis(acetone)-
Ack n ow led gm en t. We gratefully acknowledge fi-
nancial support from the Deutsche Forschungsgemein-
schaft (Grant No. SFB 347) and the Fonds der Chemi-
schen Industrie, the latter in particular for a Ph.D.
fellowship (for G.C.). We also thank Dr. R. Bertermann
and Dr. M. Gru¨ne for detailed NMR measurements and
Dr. J . Wolf for valuable advice.
Su p p or tin g In for m a tion Ava ila ble: A table giving the
elemental analysis data for compounds 4 and 6-10 as well as
tables of crystallographic data, data collection, and solution
and refinement details, positional and thermal parameters,
and bond distances and angles for 4. This material is available
free of charge via the Internet at http://pubs.acs.org.
(11) Harlow, R. L.; Thorn, D. L.; Baker, R. T.; J ones, N. L. Inorg.
Chem. 1992, 31, 993-997.
OM0010159
(12) The preparation of 10 is as follows. A solution of 4 (136 mg,
0.21 mmol) in 6 mL of toluene was treated at -60 °C with a solution
of AgPF6 (54 mg, 0.21 mmol) in 2 mL of ether. When the solution was
warmed to room temperature, a change of color from yellow to brown
occurred and a white solid precipitated. The solution was filtered, the
filtrate was evaporated in vacuo, and the residue was extracted twice
with 4 mL of CH2Cl2. The combined extracts were brought to dryness
in vacuo, the residue was dissolved in 1 mL of acetone, and 6 mL of
ether was added with stirring. A pale brown solid was obtained, which
was separated from the mother liquor, washed twice with 5 mL each
of ether and pentane, and dried: yield 138 mg (88%); mp 107 °C dec.
(13) (a) van Gaal, H. L. M.; van den Bekerom, F. L. A. J . Organomet.
Chem. 1977, 134, 237-248. (b) Werner, H.; Feser, R. Z. Naturforsch.
1980, 35B, 689-693.
(14) Rybtchinsky, B.; Konstantinovsky, L.; Shimon, L. J . W.; Vigalok,
A.; Milstein, D. Chem. Eur. J . 2000, 6, 3287-3292.
(15) (a) Singewald, E. T.; Mirkin, C. A.; Levy, A. D.; Stern, C. L.
Angew. Chem. 1994, 106, 2524-2526; Angew. Chem., Int. Ed. Engl.
1994, 33, 2473-2475. (b) Singewald, E. T.; Shi, X.; Mirkin, C. A.;
Schofer, S. J .; Stern, C. L. Organometallics 1996, 15, 3062-3069.