An unprecedented dinuclear alkylrhodium(III) complex built up by two 14-electron [RhCl2(alkyl)(PR3)] units

Article abstract of DOI:10.1021/ja020560t

Reaction of HCl with [RhCl(C₂H₄)(PR₃)]₂ affords the dinuclear alkylrhodium(III) complex [RhCl₂(C₂H₅)(PR₃)]₂, the structure of which has been determined crystallographically. PR₃ is the formerly unknown trialkyl phosphine ^tBu₂PCH₂CH₂C₆H₃-2,6-Me₂, prepared in three steps from ^tBuPCl₂. Treatment of the title compound with CO gives the mononuclear rhodium dicarbonyl cis-[RhCl(CO)₂(PR₃)], being the first fully characterized complex of this type. Copyright

Full text of DOI:10.1021/ja020560t

Published on Web 07/25/2002
An Unprecedented Dinuclear Alkylrhodium(III) Complex Built up by Two
14-Electron [RhCl2(alkyl)(PR3)] Units
Giuseppe Canepa, Carsten D. Brandt, and Helmut Werner*
Institut fu¨r Anorganische Chemie der UniVersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
Received April 19, 2002
Scheme 1
The recent discovery that the functionalized tertiary phosphine
tBu2PCH2CH2C6H5 (1), in the presence of olefinrhodium(I) com-
pounds, undergoes C-H activation of the six-membered ring under
unusually mild conditions1 prompted us to prepare some analogues
of 1 and study their reactivity toward d8 metal centers. Since
treatment of [RhCl(C8H14)2]2 or [RhCl(C2H4)2]2 with 1 (pentane,
25 °C) leads exclusively to the insertion of rhodium into the phenyl
C-H bond situated in ortho-position to the CH2CH2PtBu2 sub-
stituent, we were particularly interested to find out what the behavior
t
25 °C), it reacts under the same conditions with ethene to regen-
erate 4.
of a phosphine such as Bu2PCH2CH2C6H3-2,6-Me2 (3) is which
has the two ring carbon atoms next to the â-phosphinoethyl moiety
blocked by methyl groups. We were aware of the outstanding work
by Milstein et al.2 illustrating that the pincer-type ligand C6H-1,3-
(CH2PtBu2)2-2,4,6-Me3 already reacts at room temperature with
[MCl(C8H14)2]2 (M ) Rh, Ir) by C-C bond cleavage to give the
methylrhodium(III) and -iridium(III) derivatives [MCl(CH3)(κ3-
P,C,P-C6H-2,4-(CH2PtBu2)2-3,5-Me2)], respectively.
The reaction of 4 with HCl, undertaken to generate the five-
coordinate dichloro(hydrido)rhodium(III) complex [RhHCl2(C2H4)-
(3)] structurally related to 5, furnished a surprising result. Passing
a slow stream of dry HCl gas through a suspension of 4 (123 mg,
0.14 mmol) in CH2Cl2 (3 mL) for 10 s at room temperature pro-
duces, after removal of the solvent and recrystallization from di-
chloromethane, an orange solid (mp 102 °C dec, yield 83%), the
microanalysis of which is in agreement with the expected composi-
tion [RhHCl2(C2H4)(3)]. However, as shown by the X-ray crystal
structure analysis, the isolated product is not a hydrido but the di-
nuclear ethylrhodium(III) compound 6 which is built up by two
14-electron [RhCl2(C2H5)(3)] units (Figure 1).5 These units are
linked by two bridging chlorides which are unsymmetrically situated
between the two metal centers. The midpoint of the planar Rh2Cl2
ring with distances Rh(1)-Cl(2) ) 2.3561(6) Å and Rh(1)-Cl-
(2A) ) 2.5257(6) Å constitutes a center of inversion. Since the
terminal chlorides Cl(1) and Cl(1A) lie exactly and the phosphorus
atoms P(1) and P(1A) nearly in the plane of the Rh2Cl2 ring, the
coordination geometry around Rh(1) and Rh(1A) can be best
described as square-pyramidal. The bond angles Cl(1)-Rh(1)-
C(1) and Cl(2)-Rh(1)-C(1) are 87.77(9)° and 94.97(9)°, respec-
tively. We note that the bond length Rh(1)-C(1) of 2.059(3) Å is
significantly (ca. 0.11 Å) shorter than in the Milstein complex
[RhCl(CH3)(κ3-P,C,P-C6H-2,4-(CH2PtBu2)2-3,5-Me2)].2a
The reactivity of the ethylrhodium(III) compound 6 is rather
unusual. Treatment of 6 with phosphine 3, undertaken to prepare
the mononuclear complex [RhCl2(C2H5)(3)2] via chloride-bridge
cleavage, affords the dichloro(hydrido) derivative 7 (orange solid,
mp 103 °C). Therefore, we assume (see Scheme 2) that in solution
an equilibrium between a Rh(C2H5) and a RhH(C2H4) isomer exists
and that 7 is formed from the latter by displacement of the olefin.
The same ethene(hydrido)rhodium(III) species is possibly also
an intermediate in the reaction of 6 with CO. Stirring a solution of
6 in CH2Cl2 under a CO atmosphere for 1 h gives a mixture of
products which, according to the 31P NMR spectrum, consists of
dimer 8 and the phosphonium salt 3‚HCl. Alternatively, dimer 8
can be obtained cleanly and without any byproducts if a suspension
of 9 (prepared from [RhCl(C8H14)2]2 and 3 in the molar ratio 1:2)
in pentane is stirred under carbon monoxide. In ca. 10 s a yellow
solution is formed from which, after partial evaporation of the
The preparation of the new phosphine 3 occurred in three steps.
Dropwise addition of a solution of ClCH2CH2C6H3-2,6-Me2 (19.7
g, 0.12 mol) in THF (40 mL) to a suspension of finely divided Mg
(2.8 g, 0.12 mol) in THF (10 mL), followed by stirring under reflux
for 30 min, resulted in the formation of ClMgCH2CH2C6H3-2,6-
Me2. If the solution of this Grignard reagent was added at 0 °C to
a solution of tBuPCl2 (18.6 g, 0.12 mol) in THF (60 mL), a white
solid (MgCl2) precipitated. Removal of the solvent, repeated extrac-
tion of the residue with diethyl ether and fractional distillation (0.002
bar) gave the chlorophosphine PCl(tBu)(CH2CH2C6H3-2,6-Me2) (2)
as a colorless air- and moisture-sensitive solid. The reaction of 2
(13.0 g, 0.51 mol) in benzene (40 mL) with a 1.6 M solution of
tBuLi in pentane (44 mL, 0.74 mol) at 5 °C afforded after hydrolysis
with degassed water (20 mL), extraction with diethyl ether (40 mL)
and fractional distillation (0.002 bar) the wanted phosphine 3 as
an oily liquid (bp 105 °C at 0.002 bar), characterized by NMR and
mass spectra as well as by derivatization to the corresponding
phosphonium salt [HP(tBu)2(CH2CH2C6H3-2,6-Me2)]Cl (3‚HCl).
In contrast to other trialkylphosphines such as P(tBu)2CH3,3 3
does not react with [RhCl(olefin)2]2 (olefin ) C2H4, C8H14) to give
a mononuclear olefin-rhodium(I) complex trans-[RhCl(olefin)(3)2].
With [RhCl(C2H4)2]2 as the starting material and a 2-fold excess
of 3, the dinuclear compound 4 (yellow air-stable solid, mp 77 °C
dec, yield 93%) is formed instead.4 The attempted conversion of
4, under a hydrogen atmosphere, with 2 equiv of 3 to [RhCl(3)2]n
(n ) 1 or 2) affords the dihydrido complex 5 in almost quantitative
yield (Scheme 1). Typical features of 5 (light-yellow solid, mp 104
2
°C dec) are the high-field resonance at δ -23.0 [dt, J(Rh,H) )
26.2, 3J(P,H) ) 14.5 Hz] in the 1H NMR and the Rh-H stretching
vibration at 2122 cm-1 in the IR spectrum. While 5 is completely
inert toward cyclooctene and 3,3-dimethyl-1-butene (pentane,
* To whom correspondence should be addressed. E-mail: helmut.werner@
mail.uni-wuerzburg.de.
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J. AM. CHEM. SOC. 2002, 124, 9666-9667
10.1021/ja020560t CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Figure 1.
Scheme 2 a
up by two 14-electron fragments, but in all of these compounds
the metal is in the oxidation state +I and not +III. Second, the
mononuclear four-coordinate dicarbonyl compound 10 represents,
to the best of our knowledge, the first isolated and fully character-
ized rhodium(I) complex of the general composition cis-[RhCl-
(CO)2(PR3)]. It has been reported by various authors,8 that those
dicarbonylrhodium(I) derivatives are key intermediates in the
reactions of [RhCl(CO)2]2 with PR3 and of [RhCl(CO)(PR3)]2 with
CO, but due to the preferred dissociation of one Rh-CO bond the
attempted isolation of these intermediates failed. We assume that
the bulkiness of phosphine 3 somewhat inhibits the conversion of
10 to the more stable dinuclear complex 8, and experiments to prove
this hypothesis are underway.
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft. G.C. thanks the Fonds der Chemischen
Industrie for his PhD grant. Valuable advice by Dr. J. Wolf and
Dr. R. Bertermann is gratefully acknowledged.
Supporting Information Available: Complete list of NMR data
for 2, 3, 3‚HCl, 4, 5, 6, 7, 8, and 10 (PDF). An X-ray crystallographic
file (CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
References
a (L ) 3)
(1) Canepa, G.; Brandt, C. D.; Werner, H. Organometallics 2001, 20, 604-
606.
solvent in vacuo, a pale yellow solid (mp 195 °C dec, yield 87%)
precipitates. Since the IR spectrum of 8 (in KBr) displays only
one ν(CO) band at 1962 cm-1, and the 31P NMR spectrum (in CD2-
(2) (a) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D. J. Am. Chem.
Soc. 1996, 118, 12406-12415. (b) Gandelman, M.; Vigalok, A.; Shimon,
L. J. W.; Milstein, D. Organometallics 1997, 16, 3981-3986. (c)
Rybtchinski, B.; Milstein, D. Angew. Chem. 1999, 111, 918-932; Angew.
Chem., Int. Ed. 1999, 38, 870-883.
2
Cl2), a single resonance at δ 78.6 [d, J(P,Rh) ) 174.6 Hz], there
(3) Werner, H.; Kukla, F. Inorg. Chim. Acta 1995, 235, 253-261.
(4) For X-ray crystal structure analysis of 4 see: Brandt, C. D. PhD
Dissertation, Universita¨t Wu¨rzburg, to be submitted.
is no doubt that the two phosphine ligands are in trans disposition
with respect to the central Rh2Cl2 fragment. If the time for the
reaction of 9 and CO (1 bar) in pentane is extended from 10 s to
1 h and the solvent removed in the presence of CO, light-yellow
air-stable crystals are isolated which are analyzed as the dicarbonyl
complex 10 (mp 94 °C dec, yield 95%). In contrast to 8, the IR
spectrum of 10 (in KBr) shows two CO stretching modes at 2086
and 1999 cm-1. The 13C NMR spectrum of 10 (in CD2Cl2) exhibits
two doublet of doublet resonances for the CO carbon atoms of
which that at δ 181.1 with the larger 13C-31P coupling constant of
112.5 Hz is assigned to the CO group trans to the phosphine and
(5) Crystal parameters of 6: C40H72Cl4P2Rh2, T ) 193 K, P21/c, Z ) 2, a )
10.0093(6) Å, b ) 15.4273(9) Å, c ) 14.0090(8) Å, â ) 92.7580(10)°,
V ) 2160.7(2) Å3, dcalc ) 1.479 g/cm3, R1 ) 0.0311, wR2 ) 0.0709.
Data were collected on a Bruker Smart Apex diffractometer with D8-
Goniometer using graphite monochromated Mo KR (λ ) 0.71073 Å)
radiation, and were corrected for Lorentz, polarization, and absorption
effects. The structure was solved by direct methods (SHELXS-97) and
was refined using the full-matrix least-squares method (SHELXL-97). Full
details are described in the Supporting Information.
(6) The nearest relative of 6 we are aware of is the hydrido(silyl)rhodium-
(III) complex [Rh(µ-Cl)H{Si(CH2Ph)3}(PiPr3)]2, prepared from [RhCl-
(PiPr3)2]2 and HSi(CH2Ph)3: Osakada, K.; Koizumi, T.; Yamamoto, T.
Bull. Chem. Soc. Jpn. 1997, 70, 189-195.
(7) (a) Dickson, R. S. Organometallic Chemistry of Rhodium and Iridium;
Academic Press: London, New York, 1983; p. 58. (b) Jardine, F. H.;
Sheridan, P. S. In ComprehensiVe Coordination Chemistry; Wilkinson
G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon: New York, 1987;
p 908.
(8) Gallay, J.; De Montauzon, D.; Poilblanc, R. J. Organomet. Chem. 1972,
38, 179-197. (b) Rotondo, E.; Battaglia, G.; Giordano, G.; Cusmano, F.
P. J. Organomet. Chem. 1993, 450, 245-252. (c) Hughes, R. P. In
ComprehensiVe Organometallic Chemistry; Wilkinson, G., Stone, F. G.
A., Abel, E. W., Eds.; Pergamon: New York, 1982; p 300.
2
that at δ 184.8 with J(C,P) ) 16.2 Hz to the CO group trans to
the chloride. The mononuclear complex 10 is probably an inter-
mediate in the formation of 8 from 9 since in vacuo it is easily
converted either in solution or in the solid state to 8.
The results presented here highlight two different aspects. First,
the formation and the structure of the dinuclear alkylrhodium(III)
compound 6 is without precedent.6 There are numerous rhodium
complexes [Rh(µ-X)(L)(L′)]2 known,7 which like 6 are also built
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