Synthesis, structures, and reactions of dirhodium complexes bearing a 1,2-Diphenyl-3,4-diphosphinidenecyclobutene ligand (DPCB)

Article abstract of DOI:10.1021/om8010799

A di-μ-hydroxo-dirhodium complex bearing a low-coordinate phosphorus ligand, [Rh₂(μ-OH)₂(DPCB)₂] (2; DPCB = 1,2-diphenyl-3,4-bis[(2,4,6-tri-rert-butylphenyl)phosphinidene]cyclobutene), is prepared from [Rh₂(μ-Cl

Full text of DOI:10.1021/om8010799

902
Organometallics 2009, 28, 902–908
Synthesis, Structures, and Reactions of Dirhodium Complexes
Bearing a 1,2-Diphenyl-3,4-diphosphinidenecyclobutene Ligand
(DPCB)
Masaaki Okazaki,*,† Akito Hayashi,† Ching-Feng Fu,‡ Shiuh-Tzung Liu,‡ and
Fumiyuki Ozawa*,†
International Research Center for Elements Science, Institute for Chemical Research, Kyoto UniVersity,
Uji, Kyoto 611-0011, Japan, and Department of Chemistry, National Taiwan UniVersity,
Taipei, Taiwan 106, Republic of China
ReceiVed NoVember 11, 2008
A di-µ-hydroxo-dirhodium complex bearing a low-coordinate phosphorus ligand, [Rh2(µ-OH)2(DPCB)2]
(2; DPCB ) 1,2-diphenyl-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene), is prepared from
[Rh2(µ-Cl)2(DPCB)2] (1) and KOH. Complex 2 serves as a good precursor for hydrido complexes, affording
[Rh2(µ-H)(µ-OH)(DPCB)2] (3) and [Rh2(µ-H)2(DPCB)2] (7) by the reactions with HSiMe2Ph and HCO2H,
respectively. Although complex 7 is too unstable to be isolated, its formation is evidenced by a trapping
experiment using 1,3-cyclohexadiene to give [Rh(η3-C6H9)(DPCB)] (8). Complex 2 reacts with CO to
afford [Rh2(µ-κP,P;κP,C-DPCB)(CO)4] (9) and [Rh2(µ-CO)2(DPCB)2] (10), which are interconvertible
in solution. Complex 9 is the stable form under a CO atmosphere even in the presence of free DPCB, but
readily converted to 10 by purging the CO gas from the solution. The X-ray structures of 2, 3, 9, 10, and
related [Rh2(µ-H)(µ-OMe)(DPCB)2] (4) are reported. Two Rh(DPCB) moieties in 10 are oriented
orthogonal to each other in the crystal; namely, one of the Rh centers has a square-planar configuration,
whereas the other is in a tetrahedral arrangement with the µ-CO and DPCB ligands, showing flexible
electronic properties of DPCB.
(H2CdPH), imine (H2CdNH), and phosphane (PH3) have
shown that the π* orbital level of the CdP bond is much lower
than those of Ct O and CdN bonds.3 On the other hand, the
lone-pair orbital of phosphaethene is situated at a comparable
level to that of phosphane. Accordingly, phosphaalkenes undergo
strong σ-donation and π-back-donation interactions with transi-
tion metals. We have demonstrated using 1,2-diphenyl-3,4-
bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutenes (DPCB-
Y; Y stands for the p-substituents of 1,2-diphenyl groups) that
this ligand property is useful for catalysis, leading to highly
efficient organic transformations in conjunction with group 8
and 10 metals.4,5
Introduction
There has been considerable recent interest in the coordination
chemistry of low-coordinate phosphorus compounds due to their
unique ligand properties, differing significantly from common
tertiary phosphanes.1 Phosphaalkenes having a PdC double
bond are among the central subjects in such chemistry.2 The
MO calculations on carbon monoxide (Ct O), phosphaethene
* To whom correspondence should be addressed. E-mail: (M.O.)
mokazaki@scl.kyoto-u.ac.jp. (F.O.) ozawa@scl.kyoto-u.ac.jp.
† Kyoto University.
‡ National Taiwan University.
(1) (a) Le Floch, P. Coord. Chem. ReV. 2006, 250, 627. (b) Yoshifuji,
M. Pure Appl. Chem. 2005, 77, 2011. (c) Mathey, F. Angew. Chem., Int.
Ed. 2003, 42, 1578. (d) Weber, L. Angew. Chem., Int. Ed. 2002, 41, 563.
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Chem. 2007, 18, 363. (b) Masuda, J. D.; Martin, D.; Lyon-Saunier, C.;
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26, 5050. (d) Nakamura, A.; Kawasaki, S.; Toyota, K.; Yoshifuji, M. J.
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In this study, we synthesized several organorhodium com-
plexes bearing DPCB ligand (Y ) H) and examined their
structures and reactivities. A particular interest has been focused
on hydridorhodium complexes, because hydrido species have
often been observed as key intermediates in DPCB-Y complex-
catalyzed reactions.4 For their preparation, we employed the
rhodium hydroxide [Rh2(µ-OH)2(DPCB)2] as a common precur-
sor. In the past several decades, a great deal of attention has
been focused on hydroxo complexes of late transition metals,
because of their interesting reactivities and potential relevance
to catalytic reactions.6 For example, M-OH species often react
with organic compounds with acidic nature, along with only
by-production of H2O. Thus, hydroxo complexes may serve as
clean and versatile precursors of a variety of complexes.7 The
synthesis and reactions of rhodium hydroxides have been
(3) Ozawa, F.; Kawagishi, S.; Ishiyama, T.; Yoshifuji, M. Organome-
tallics 2004, 23, 1325.
(4) (a) Ozawa, F.; Yoshifuji, M. Dalton Trans. 2006, 4987. (b) Ozawa,
F.; Yoshifuji, M. C. R. Chim. 2004, 7, 747.
10.1021/om8010799 CCC: $40.75
2009 American Chemical Society
Publication on Web 01/15/2009

Dirhodium Complexes Bearing a DPCB Ligand
Organometallics, Vol. 28, No. 3, 2009 903
examined with tertiary phosphanes as ancillary ligands.8-11
However, most of them are highly air-sensitive even in the solid
state, probably due to high electron density of the metal centers.
Accordingly, despite their utility, the study of hydroxorhodium
complexes has remained insufficiently explored. On the other
hand, we found that the DPCB ligand successfully stabilizes
rhodium hydroxide without notable loss of the reactivity.
Scheme 1
Results and Discussion
Synthesis of [Rh2(µ-OH)2(DPCB)2] (2). DPCB was intro-
duced to rhodium by ligand displacement with [Rh2(µ-
Cl)2(C2H4)4] in toluene at room temperature (Scheme 1).
Treatment of 1 with aqueous KOH in toluene at 80 °C formed
[Rh2(µ-OH)2(DPCB)2] (2), which was isolated in 98% yield as
a dark yellow solid, fairly stable in air. The 31P{1H} NMR signal
appeared at δ 164.1 as a doublet with the 1JRhP coupling of 244
Hz. The 1H NMR spectrum exhibited a singlet signal at δ -1.66,
assignable to µ-OH protons; the chemical shift is comparable
to those of tertiary phosphane analogues.8c,9i,10,11a The signal
instantly disappeared upon treatment with D2O.
(Rh-O ) 2.077(3), Rh-O* ) 2.082(3) Å) distances are
comparable to those of phosphane analogues (Rh-P )
2.185-2.241 Å, Rh-O ) 2.064-2.127 Å).8e,11a The interatomic
distance of Rh · · · Rh* is 3.273 Å, showing the absence of
bonding interaction.
Reaction of 2 with HSiMe2Ph. We have recently found that
the ruthenium chloride [Ru2Cl2(µ-Cl)2(CO)2(DPCB-OMe)2] is
cleanly reduced to hydride by the treatment with HSiMe2Ph and
water, where an Ru-OH species undergoes metathesis with
HSiMe2Ph to give an Ru-H species and HOSiMe2Ph.5a Thus,
the reaction of 2 with HSiMe2Ph was examined as the first
attempt to synthesize hydridorhodium complexes (eq 1). Com-
plex 2 was treated with excess HSiMe2Ph (3 equiv/Rh) in THF
at room temperature for 27 h. After removal of volatiles, the
residue was washed with pentane and dried under vacuum,
giving a dark red powder of [Rh2(µ-H)(µ-OH)(DPCB)2] (3) in
84% yield.
Figure 1 shows the crystal structure of 2, which has a center
of symmetry at the intersection of Rh-Rh* and O-O* vectors.
Each rhodium center takes a square-planar geometry. The Rh-P
(Rh-P1 ) 2.1879(12), Rh-P2 ) 2.1863(12) Å) and Rh-O
(5) (a) Hayashi, A.; Yoshitomi, T.; Umeda, K.; Okazaki, M.; Ozawa,
F. Organometallics 2008, 27, 2321. (b) Jensen, R. S.; Umeda, K.; Okazaki,
M.; Ozawa, F.; Yoshifuji, M. J. Organomet. Chem. 2007, 692, 286. (c)
Murakami, H.; Matsui, Y.; Ozawa, F.; Yoshifuji, M. J. Organomet. Chem.
2006, 691, 3151. (d) Katayama, H.; Nagao, M.; Nishimura, T.; Matsui, Y.;
Umeda, K.; Akamatsu, K.; Tsuruoka, T.; Nawafune, H.; Ozawa, F. J. Am.
Chem. Soc. 2005, 127, 4350. (e) Nagao, M.; Asano, K.; Umeda, K.;
Katayama, H.; Ozawa, F. J. Org. Chem. 2005, 70, 10511. (f) Gajare, A. S.;
Jensen, R. S.; Toyota, K.; Yoshifuji, M.; Ozawa, F. Synlett 2005, 144. (g)
Jensen, R. S.; Gajare, A. S.; Toyota, K.; Yoshifuji, M.; Ozawa, F.
Tetrahedron Lett. 2005, 46, 8645. (h) Ozawa, F.; Ishiyama, T.; Yamamoto,
S.; Kawagishi, S.; Murakami, H.; Yoshifuji, M. Organometallics 2004, 23,
1698. (i) Murakami, H.; Minami, T.; Ozawa, F. J. Org. Chem. 2004, 69,
4482. (j) Gajare, A. S.; Toyota, K.; Yoshifuji, M.; Ozawa, F. J. Org. Chem.
2004, 69, 6504. (k) Gajare, A. S.; Toyota, K.; Yoshifuji, M.; Ozawa, F.
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Yamamoto, S.; Minami, T.; Yoshifuji, M. J. Am. Chem. Soc. 2002, 124,
10968. (m) Minami, T.; Okamoto, H.; Ikeda, S.; Tanaka, R.; Ozawa, F.;
Yoshifuji, M. Angew. Chem., Int. Ed. 2001, 40, 4501. (n) Ozawa, F.;
Yamamoto, S.; Kawagishi, S.; Hiraoka, M.; Ikeda, S.; Minami, T.; Ito, S.;
Yoshifuji, M. Chem. Lett. 2001, 972. (o) Ikeda, S.; Ohhata, F.; Miyoshi,
M.; Tanaka, R.; Minami, T.; Ozawa, F.; Yoshifuji, M. Angew. Chem., Int.
Ed. 2000, 39, 4512.
X-ray diffraction analysis of 3 revealed the dirhodium
structure bearing µ-H and µ-OH ligands (Figure 2). The rhodium
atoms adopt slightly distorted square-planar configurations and
(6) (a) Giije, J. W.; Roesky, H. W. Chem. ReV. 1994, 94, 895. (b)
Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163.
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Organometallics 1999, 18, 2718, and references therein.
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Mizuno, T.; Alper, H. J. Mol. Catal. A 1997, 123, 21. (c) Grushin, V. V.;
Kuznetsov, V. F.; Bensimon, C.; Alper, H. Organometallics 1995, 14, 3927.
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Chem. 1989, 369, 335. (e) Brune, H.-A.; Hemmer, R.; Unsin, J.; Holl, K.;
Thewalt, U. Z. Naturforsh., B 1988, 43, 487.
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2007, 46, 4937. (b) Nishimura, T.; Hirabayashi, S.; Yasuhara, Y.; Hayashi,
T. J. Am. Chem. Soc. 2006, 128, 2556. (c) Kina, A.; Iwamura, H.; Hayashi,
T. J. Am. Chem. Soc. 2006, 128, 3904. (d) Kina, A.; Yasuhara, Y.;
Nishimura, T.; Iwamura, H.; Hayashi, T. Chem. Asian J. 2006, 1, 707. (e)
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(f) Yoshida, K.; Hayashi, T. J. Am. Chem. Soc. 2003, 125, 2872. (g) Hayashi,
T.; Takahashi, M.; Takaya, Y.; Ogasawara, M. J. Am. Chem. Soc. 2002,
124, 5052. (h) Yoshida, K.; Ogasawara, M.; Hayashi, T. J. Am. Chem. Soc.
2002, 124, 10984. (i) Hayashi, T.; Tokunaga, N.; Yoshida, K.; Han, J. W.
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4594.
(11) (a) Gevert, O.; Wolf, J.; Werner, H. Organometallics 1996, 15,
2806. (b) Herrmann, W. A.; Kulpe, J. A.; Konkol, W.; Bahrmann, H. J.
Organomet. Chem. 1990, 389, 85.
Figure 1. ORTEP drawing of 2 · 2(C5H12) with thermal ellipsoids
at the 50% probability level. Hydrogen atoms and pentane molecules
are omitted for clarity. Selected bond distances (Å) and angles (deg):
Rh-P1 ) 2.1879(12), Rh-P2 ) 2.1863(12), Rh-O ) 2.077(3),
Rh-O* ) 2.082(3), P1-Rh-P2 ) 82.59(4), O-Rh-O* )
76.19(12), P1-Rh-O ) 100.24(8), P2-Rh-O* ) 100.92(8).
Asterisks indicate atoms generated by the symmetric operation (-x,
-y, -z).

904 Organometallics, Vol. 28, No. 3, 2009
Okazaki et al.
Figure 3. ORTEP drawing of 4 with thermal ellipsoids at the 50%
probability level. Hydrogen atoms expect for the µ-H ligand are
omitted for clarity. Selected bond distances (Å) and angles (deg):
Rh1-Rh2 ) 2.7682(5), Rh1-P1 ) 2.1705(9), Rh1-P2 ) 2.2229(9),
Rh2-P3 ) 2.1769(9), Rh2-P4 ) 2.2177(10), Rh1-H ) 1.79(4),
Rh2-H ) 1.88(4), Rh1-O ) 2.022(3), Rh2-O ) 2.017(3), O-C1
) 1.397(5), P1-Rh1-P2 ) 83.18(4), P3-Rh2-P4 ) 83.08(4),
Rh1-O-Rh2 ) 86.52(10), Rh1-O-C1 ) 129.5(2), Rh2-O-C1
) 133.8(3), A-B ) 10.0(1).
Figure 2. ORTEP drawing of 3 · Et2O with thermal ellipsoids at
the 50% probability level. Hydrogen atoms except for the µ-H
ligand and a Et2O molecule are omitted for clarity. Selected bond
distances (Å) and angles (deg): Rh1-Rh2 ) 2.7486(12), Rh1-P1
) 2.1660(19), Rh1-P2 ) 2.2178(19), Rh2-P3 ) 2.174(2),
Rh2-P4 ) 2.202(2), Rh1-H ) 1.87(5), Rh2-H ) 1.90(5),
Rh1-O ) 2.087(4), Rh2-O ) 2.098(4), P1-Rh1-P2 ) 83.79(7),
P3-Rh2-P4 ) 83.36(7), Rh1-O-Rh2 ) 82.11(16). Dihedral
angel (deg): A-B ) 23.2(2).
Scheme 2
are connected with each other by a metal-metal bond (2.7486(12)
Å). Reflecting the higher trans influence of hydrido ligand than
hydroxo ligand, the Rh1-P2 (2.2178(19) Å) and Rh2-P4
(2.202(2) Å) bonds are clearly longer than the Rh1-P1
(2.1660(19) Å) and Rh2-P3 (2.174(2) Å) bonds, respectively.
1
The H NMR signal of the µ-hydrido ligand appeared at δ
-9.34 as a triplet of triplets of triplets, due to the couplings
with 31P nuclei at the trans and cis positions (2JPH ) 85, 28 Hz)
and with 103Rh nuclei (1JRhH ) 15 Hz), showing the occurrence
of a tight connection among the µ-hydrido ligand and rhodium
centers. On the other hand, the 31P{1H} NMR signal at δ 156.1
(2.7682(5) Å) is somewhat longer, and the Rh-O bonds
(2.022(3), 2.017(3) Å) are clearly shorter than 3.
Reaction of 2 with HCO2H. The reaction of 2 with
HSiMe2Ph formed mono-µ-hydrido complex 3 exclusively, and
no further reduction took place even in the presence of a large
excess of HSiMe2Ph (>20 equiv/Rh). Since stronger reducing
agents such as NaBH4 and EtLi provided complex mixtures,
we next examined the reaction with formic acid to synthesize
di-µ-hydrido complex 7 (Scheme 2).
1
(d, JRhP ) 244 Hz) assignable to the phosphorus trans to the
µ-OH ligand was significantly broadened, while the other at δ
1
1
175.6 (dd, JRhP ) 208 Hz, JPP ) 12 Hz) remained as a sharp
signal. Since the OH proton signal was broadened as well, it is
likely that complex 3 undergoes a rapid exchange of the µ-OH
ligand with residual water in solution, probably via the process
given in eq 2. Actually, the µ-OH ligand in 3 was readily
replaced by MeOH to give 4, having a µ-OMe ligand, in 67%
yield after isolation as dark brown crystals (eq 1).
When 2 was treated with HCO2H (1 equiv/Rh) at 0 °C in
1
toluene, the 31P{1H} NMR signal at δ 164.1 (d, JRhP ) 244
Hz) instantly disappeared, and a new broad signal, assignable
to di-µ-formato complex 5, appeared at δ 163.2 (d, 1JRhP ) 262
Hz). Although this complex was too unstable to be isolated,
the related di-µ-acetato complex 6 could be obtained in 98%
yield, using MeCO2H instead of HCO2H. The 1H NMR data of
both complexes were almost identical to each other, except for
the signals of bridging ligands. Complex 5 readily decomposed
with liberation of CO2 at room temperature. In the presence of
1,3-cyclohexadiene, η3-cyclohexenyl complex 8 was formed,
as confirmed by NMR spectroscopy. This observation is
As seen from the ORTEP drawing given in Figure 3, complex
4 has a structure very similar to 3, while the Rh-Rh bond

Dirhodium Complexes Bearing a DPCB Ligand
Organometallics, Vol. 28, No. 3, 2009 905
Figure 5. ORTEP drawing of 10 with thermal ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond distances (Å) and angles (deg): Rh1-Rh2 ) 2.6802(11),
Rh1-P1 ) 2.282(2), Rh1-P2 ) 2.279(2), Rh1-C1 ) 2.008(8),
Rh1-C2 ) 2.028(8), Rh2-P3 ) 2.329(2), Rh2-P4 ) 2.325(2),
Rh2-C1 ) 1.975(9), Rh2-C2 ) 1.986(10), C1-O1 ) 1.179(10),
C2-O2 ) 1.167(10), P1-Rh1-P2 ) 82.93(8), C1-Rh1-C2 )
94.6(4), P3-Rh2-P4 ) 85.78(8), C1-Rh2-C2 ) 97.0(3),
Rh1-C1-O1 ) 145.8(7), Rh2-C1-O1 ) 129.4(7), Rh1-C2-O2
) 145.7(7), Rh2-C2-O2 ) 130.2(7), A-B ) 80.6(4).
Figure 4. ORTEP drawing of 9 · C5H12 with thermal ellipsoids at
the 50% probability level. Hydrogen atoms and a pentane molecule
are omitted for clarity. Selected bond distances (Å) and angles (deg):
Rh1-Rh2 ) 2.7452(6), Rh1-P1 ) 2.2592(8), Rh1-P2 ) 2.4180(9),
Rh2-P1 ) 2.3909(8), Rh1-C1 ) 1.881(3), Rh1-C2 ) 1.905(3),
Rh2-C3 ) 1.877(3), Rh2-C4 ) 1.912(3), Rh2-C5 ) 2.203(2),
P1-C5 ) 1.743(2), P2-C8 ) 1.696(2), P1-Rh1-P2 ) 85.84(2),
C1-Rh1-C2 ) 95.00(11), P1-Rh1-C1 ) 101.84(7), P2-Rh1-C2
) 106.36(9), Rh1-Rh2-C5 ) 80.45(6), C3-Rh2-C4 ) 97.10(11),
Rh1-Rh2-C3)87.93(8),C4-Rh2-C5)96.53(10),Rh1-Rh2-P1
) 51.63(2), P1-Rh2-C5 ) 44.37(6).
π-allyl complex bearing two carbonyl ligands. The C3-Rh2-C4
angle (97.10(11)°) is within the range of π-allyl complexes
having a d8 metal center.
Unlike the crystal structure, two phosphorus nuclei of the
DPCB ligand were observed equivalently at δ 152.9 (dd, 1JRhP
) 151, 44 Hz) in toluene at room temperature, showing the
occurrence of a rapid migration of the Rh(CO)2 moiety between
the PdC bonds (eq 4). The fractional behavior did not freeze
even at -90 °C.
consistent with the formation of [Rh2(µ-H)2(DPCB)2] (7), which
undergoes the insertion of 1,3-cyclohexadiene to give 8.
Reaction of 2 with CO. Next, complex 2 was treated with
CO. It has been documented that hydroxycarbonyl complexes
formed by the insertion of CO into a Rh-OH bond undergo
decarboxylation to give hydrido complexes.12 However, the
reaction of 2 with CO in pentane afforded the novel dirhodium
complex [Rh2(µ-κP,P;κP,C-DPCB)(CO)4] (9) instead, along
with dissociation of one of the DPCB ligands (eq 3). Slow
evaporation of the solvent under the flow of CO gas provided
dark brown crystals with the composition 9 · C5H12 in 55% yield,
suitable for X-ray diffraction analysis.
Complex 9 is stable in solution under a CO atmosphere even
in the presence of free DPCB, but readily converted to [Rh2(µ-
CO)2(DPCB)2] (10) when the CO gas was purged from the
solution (eq 5). The IR and NMR data were consistent with the
dirhodium structure having two µ-CO ligands. A strong νCO
absorption band appeared in a typical region of bridging
carbonyls (1766 cm-1). The 13C{1H} NMR signal of µ-CO
ligands (δ 172.1) was observed as a triplet due to the coupling
with two 103Rh nuclei (1JRhC ) 28 Hz).
Figure 4 shows the X-ray crystal structure. There are two
nonequivalent rhodium centers, connected by a metal-metal
bond (2.7452(6) Å). One (Rh1) adopts a significantly distorted
five-coordinate structure having P1, P2, C1, C2, and Rh2 as
coordination atoms. On the other hand, the other rhodium atom
(Rh2) is combined with the Rh1-P1-C5 moiety in η3-fashion.13
Thus, the partial structure around Rh2 may be regarded as a
(12) (a) Johansson, R.; Wendt, O. F. Organometallics 2007, 26, 2426.
(b) Torresan, I.; Michelin, R. A.; Marsella, A.; Zanardo, A.; Pinna, F.;
Strukul, G. Organometallics 1991, 10, 623. (c) Bennett, M. A. J. Mol. Catal.
1987, 41, 1.
(13) A similar coordination mode of DPCB has been reported for a
tungsten complex: Yoshifuji, M.; Ichikawa, Y.; Yamada, N.; Toyota, K.
Chem. Commun. 1998, 27.
Figure 5 shows the X-ray structure of 10. Although the quality
of diffraction data was low (R1 ) 0.170), an interesting structural

906 Organometallics, Vol. 28, No. 3, 2009
Okazaki et al.
Scheme 3
highly flexible ligand property of DPCB compensates for the
imbalance of charge distribution between rhodium atoms,
leading to the unusual structural feature of 10.
Conclusion
It has been found that the di-µ-hydroxo-dirhodium complex
bearing DPCB ligands (2) serves as a versatile starting material
for organorhodium complexes. Complex 2 is stable in air, but
sufficiently reactive toward a variety of substrates including
HSiMe2Ph and CO, affording complexes with unique structural
features. The carbonyl complexes 9 and 10 are of particular
interest. DPCB undergoes the µ-κP,P;κP,C-coordination with
two rhodium atoms in 9 and forms a novel Rh2(µ-CO)2
framework with square-planar and tetrahedral rhodium centers
in 10. Both structures demonstrate highly flexible electronic
ligand properties of DPCB.
feature emerged.14 Complex 10 is a 30-electron dimer having
a rhodium-rhodium bond (2.6802(11) Å). The Rh1 center has
a square-planar geometry,15a whereas the Rh2 atom is in a
distorted tetrahedral configuration.15b Two Rh(DPCB) units are
oriented orthogonal to each other with the dihedral angle of
80.6(4)°. Reflecting the highly unsymmetrical dirhodium frame-
work, both µ-CO ligands are tilted toward the Rh2 atom by 8.2
(C1-O1) and 7.8° (C2-O2), respectively. Furthermore, the
Rh2-C1 and Rh2-C2 bonds (1.975(9), 1.986(10) Å) are shorter
than the Rh1-C1 and Rh1-C2 bonds (2.008(8), 2.028(8) Å),
respectively.
Experimental Section
The crystal structures of [L2M(µ-CO)2ML′2] type complexes
have been rationalized by Cotton (Scheme 3).16,17 When two
metal centers bear the same or very similar ancillary ligands (L
) L′), they form symmetrical structures of type I or II. On the
other hand, when the ancillary ligands with significantly different
electronic properties are bound to each metal (L * L′; typically
those in donor and acceptor relations), the complexes adopt
unsymmetrical structures of type III, involving square-planar
and tetrahedral geometries of metal centers and asymmetric
bridging of CO ligands. In this context, the X-ray structure of
10 is quite unique, being categorized into type III despite the
coordination of the same DPCB ligands to both rhodium atoms.
The highly distorted coordination structure of 10 should be
attributed principally to steric repulsion between DPCB ligands.
The orthogonal arrangement of Rh(DPCB) units most effectively
relieves the steric congestion. In this case, the nonequivalent
rhodium centers with square-planar and tetrahedral configura-
tions must bear electronic conditions quite distinct from each
other. Actually, DFT calculations suggested that the Rh1 atom
is much more negatively charged than the Rh2 atom,18 and this
is an important reason that the complexes of type III commonly
have ancillary ligands with significantly different properties at
each metal center. As described in the introductory part, DPCB
serves not only as a strong π-acceptor but also as an effective
σ-donor toward transition metals. Therefore, it is likely that this
General Considerations. All manipulations were carried out
under a dry nitrogen atmosphere using standard Schlenk techniques.
Toluene and pentane were distilled from sodium benzophenone
ketyl. Chloroform-d was purified by passing successively through
alumina and activated MS4A columns and degassed by freeze-
pump-thaw cycles prior to use. Benzene-d6 and dichloromethane-
d2 were dried over LiAlH4 and CaH2, respectively, and stored over
activated MS4A. Dehydrated diethyl ether, tetrahydrofuran, and
dichloromethane were obtained from commercial sources. The
compounds [Rh2(µ-Cl)2(C2H4)4]19 and DPCB3 were synthesized by
the literature methods. Other chemicals were purchased and used
as received.
NMR spectra were recorded on a Bruker Avance 400 spectrom-
eter at 20 °C unless otherwise noted. Chemical shifts are reported
1
in δ, referenced to H (of residual protons) and 13C signals of
deuterated solvents as internal standard or to the 31P signal of 85%
H3PO4 as an external standard. IR spectra were recorded on a
JASCO FT/IR-410 instrument. Elemental analysis was performed
by the ICR Analytical Laboratory, Kyoto University.
Preparation of [Rh2(µ-Cl)2(DPCB)2] (1). To a solution of
DPCB (827.0 mg, 1.095 mmol) in toluene (10 mL) was added
[Rh2(µ-Cl)2(C2H4)4] (216.4 mg, 0.556 mmol) at room temperature.
The color of the solution immediately turned from yellow to dark
brown. After 1 h, insoluble materials were removed by filtration
through a Celite pad, and volatiles were removed under reduced
pressure. The residue was washed with diethyl ether at 0 °C and
dried under vacuum, giving a dark brown microcrystalline powder
1
of 1 (878.7 mg, 90%). Mp: 225 °C (dec). H NMR (CDCl3): δ
(14) The poor R1 value is mainly due to insufficient disorder treatment
for tert-butyl groups. Based on the standard deviations, it has been concluded
that the structural data on the P2Rh(CO)2RhP2 core deserve discussion.
(15) (a) The sum of the four angles around Rh1: 360.2°. (b) The sum
of the four angles around Rh2: 405.2°.
(16) (a) Cotton, F. A. Prog. Inorg. Chem. 1976, 21, 1. (b) Colton, R.;
McCormick, M. J. Coord. Chem. ReV. 1980, 31, 1.
(17) For examples of di-µ-carbonyl-dirhodium complexes, see: (a) Singh,
P.; Dammann, C. B.; Hodgson, D. J. Inorg. Chem. 1973, 12, 1335. (b)
Burch, R. R.; Muetterties, E. L.; Schultz, A. J.; Gebert, E. G.; Williams,
J. M. J. Am. Chem. Soc. 1981, 103, 5517. (c) James, B. R.; Mahajan, D.;
Rettig, S. J.; Williams, G. M. Organometallics 1983, 2, 1452. (d) Freeman,
M. A.; Young, D. A. Inorg. Chgem. 1986, 25, 1556. (e) Shulman, P. M.;
Burkhardt, E. D.; Lundquist, E. G.; Pilato, R. S.; Geoffroy, G. L.
Organometallics 1987, 6, 101. (f) Cecconi, F.; Chilardi, C. A.; Midollini,
S.; Orlandini, A.; Zanello, P.; Heaton, B. T.; Huang, L.; Iggo, J. A.; Bordoni,
S. J. Organomet. Chem. 1988, 353, C5. (g) Douglas, S.; Lowe, J. P.; Mahon,
M. F.; Warren, J. E.; Whittlesey, M. K. J. Organomet. Chem. 2005, 690,
5027.
(18) DFT calculations for [Rh(µ-CO)(DPCB)]2 were carried out with
the Gaussian 98 program (Revision A.9, Gaussian, Inc., 1998) using B3LYP
in conjunction with the standard LanL2DZ basis set and effective core
potentials for Rh and 6-31G(d) basis set for other atoms. This examination
showed the Mulliken atomic charges of Rh1 (-0.14) and Rh2 (+0.38).
1.40 (s, 36H, p-tBu), 1.76 (s, 72H, o-tBu), 6.63 (d, J ) 7.7 Hz,
8H, o-Ph), 6.79 (t, J ) 7.8 Hz, 8H, m-Ph), 7.02 (t, J ) 7.4 Hz, 4H,
p-Ph), 7.46 (s, 8H, m-PAr). 13C{1H} NMR (CDCl3): δ 31.5, 34.6,
35.3, 39.1, 122.7, 126.5, 127.2, 128.0, 128.1 (d, J ) 7.5 Hz), 132.9,
142.1 (m, J ) 62, 33 Hz), 152.0, 156.4, 157.9 (dd, J ) 88, 10 Hz).
31P{1H} NMR (CDCl3): δ 160.6 (d, J ) 250 Hz). Anal. Calcd for
C104H136Cl2P4Rh2: C, 69.91; H, 7.67. Found: 69.97; H, 7.84.
Preparation of [Rh2(µ-OH)2(DPCB)2] (2). To a dark brown
solution of 1 (372.2 mg, 0.208 mmol) in toluene (15 mL) was added
an aqueous solution (9 mL) of KOH (2.5 g, 44.6 mmol) at room
temperature. The resulting mixture was stirred at 80 °C for 10 h.
The dark yellow organic layer was separated, washed with H2O,
and transferred to another Schlenk tube through a cotton-tipped
cannula. After volatiles were removed under reduced pressure, the
residue was washed with pentane at 0 °C and dried under vacuum
to give an analytically pure sample of 2 as a dark yellow
microcrystalline powder (266.8 mg, 73%). Evaporation of the
(19) Cramer, R. Inorg. Synth. 1974, 15, 14.

Dirhodium Complexes Bearing a DPCB Ligand
Organometallics, Vol. 28, No. 3, 2009 907
Table 1. Details of the X-ray Structure Determinations of 2 · 2(C5H12), 3 · Et2O, 4, 9 · C5H12, and 10
2 · 2(C5H12)
3 · Et2O
4
9 · C5H12
10
formula
fw
C114H162O2P4Rh2
1894.14
C108H148O2P4Rh2
1807.96
C105H140OP4Rh2
1747.87
C61H80O4P2Rh2
1145.01
C106H136O2P4Rh2
1771.85
cryst syst
space group
monoclinic
P21/c
triclinic
P1
triclinic
P1
monoclinic
P21/c
monoclinic
C2/c
j
j
a, Å
b, Å
c, Å
16.318(6)
16.990(6)
19.833(7)
90
104.493(6)
90
15.134(4)
19.251(6)
20.244(6)
71.59(2)
67.53(2)
68.04(2)
15.1401(15)
19.3117(19)
20.181(2)
71.572(11)
68.830(9)
68.024(10)
2
4989.7(9)
12.783(4)
23.929(7)
18.804(6)
90
97.609(5)
90
23.782(5)
28.125(6)
34.019(8)
90
110.534(3)
90
R, deg
ꢀ, deg
γ, deg
Z
2
2
4
8
V, Å3
5324(3)
2024
0.417
3.25 to 27.48
41 918
12 123 (0.0564)
12 123/0/602
1.242
R1 ) 0.0812
wR2 ) 0.1462
R1 ) 0.0979
wR2 ) 0.1544
0.812 and -0.545
4951(3)
1924
0.445
3.05 to 27.48
40 250
21 705 (0.0616)
21 705/0/1062
1.205
R1 ) 0.1221
wR2 ) 0.1961
R1 ) 0.1573
wR2 ) 0.2142
0.904 and -1.000
5701(3)
2392
0.679
3.00 to 27.48
45 351
13 005 (0.0350)
13 005/0/673
1.068
R1 ) 0.0347
wR2 ) 0.0811
R1 ) 0.0399
wR2 ) 0.0843
1.371 and -1.166
21309(8)
7504
0.413
3.09 to 27.48
80 384
24 235 (0.0903)
24 235/0/1063
1.338
R1 ) 0.1700
wR2 ) 0.3018
R1 ) 0.2073
wR2 ) 0.3222
1.013 and -0.528
F(000)
1856
0.439
µ, mm-1
θ range, deg
3.00 to 27.48
40 137
21 779 (0.03411)
21 779/ 0/1050
1.082
R1 ) 0.0586
wR2 ) 0.1667
R1 ) 0.0718
wR2 ) 0.1795
3.321 and -0.912
no. of reflns collected
no. of indep reflns (Rint)
no. of data/restraints/params
goodness of fit on F2
final R indicesa [I > 2σ(I)]
R indices (all data)
largest diff peak and hole, e Å-3
pentane washings gave additional product of 2 (89.3 mg, 25%;
totally 98%). Mp: 140 °C (dec). 1H NMR (C6D6): δ -1.66 (s, 2H,
µ-OH), 1.23 (s, 36H, p-tBu), 2.00 (s, 72H, o-tBu), 6.74 (t, J ) 7.4
Hz, 8H, m-Ph), 6.84 (t, J ) 7.4 Hz, 4H, p-Ph), 7.00 (d, J ) 7.1
Hz, 8H, o-Ph), 7.44 (s, 8H, m-PAr). 13C{1H} NMR (C6D6): δ 31.5,
34.6, 35.2, 39.2, 122.4, 125.6, 126.6, 127.2, 129.3 (d, J ) 6 Hz),
134.1, 142.2 (m), 151.9, 156.6, 157.6 (m). 31P{1H} NMR (C6D6):
δ 164.1 (d, J ) 244 Hz). Anal. Calcd for C104H138O2P4Rh2: C, 71.38;
H, 7.95. Found: C, 70.99; H, 8.22.
Preparation of [Rh2(µ-H)(µ-OH)(DPCB)2] (3). To a solution
of 2 (251.8 mg, 0.143 mmol) in THF (8.0 mL) was added
HSiMe2Ph (135 µL, 0.881 mmol) at room temperature. The solution
gradually changed from dark yellow to dark red. After 27 h, volatiles
were removed under reduced pressure, and the residue was washed
with pentane at 0 °C and dried under vacuum to give a dark red
powder of 3 (208.7 mg, 84%). Mp: 120 °C (dec). 1H NMR (C6D6):
δ -9.34 (ttt, J ) 85, 28, 15 Hz, 1H, µ-H), 1.42 (s, 36H, p-tBu),
1.83 (s, 36H, o-tBu), 1.90 (s, 36H, o-tBu), 6.77 (t, J ) 6.9 Hz, 4H,
m-Ph), 6.80 (t, J ) 6.6 Hz, 4H, m-Ph), 6.88 (m, 4H, p-Ph), 7.07
(d, J ) 7.2 Hz, 4H, o-Ph), 7.09 (d, J ) 7.5 Hz, 4H, o-Ph), 7.60 (s,
2H, m-PAr), 7.61 (s, 2H, m-PAr), 7.65 (s, 4H, m-PAr). 13C{1H}
NMR (C6D6): δ 31.6, 31.7, 34.0, 35.2, 35.4, 35.5, 39.2, 39.4, 122.4,
122.8, 122.9, 126.8, 127.4, 127.3, 128.5, 132.2, 134.0, 134.3, 134.4
(d, J ) 5 Hz), 134.5 (d, J ) 5 Hz), 141.7 (dd, J ) 56, 30 Hz),
144.9 (dd, J ) 57, 29 Hz), 151.8, 152.2, 155.7, 156.9, 159.0 (m),
162.5 (m). 31P{1H} NMR (C6D6): δ 175.6 (dd, J ) 208, 12 Hz),
156.1 (d, J ) 244 Hz). Anal. Calcd for C104H138OP4Rh2: C, 72.04;
H, 8.02. Found: C, 71.74; H, 8.18.
) 266 Hz). Anal. Calcd for C105H140OP4Rh2: C, 72.15; H, 8.07.
Found: C, 71.92; H, 8.09.
Preparation of [Rh2(µ-HCO2)2(DPCB)2] (5). To a solution of
2 (12.5 mg, 0.00741 mmol) in toluene-d8 (0.5 mL) was added
formic acid (0.6 µL, 0.0159 mmol) at 0 °C. After 10 min, complex
5 was exclusively formed, as confirmed by NMR spectroscopy.
1H NMR (toluene-d8): δ 1.36 (s, 36H, p-tBu), 1.93 (s, 72H, o-tBu),
6.74 (t, J ) 7.5 Hz, 8H, m-Ph), 6.87 (t, J ) 7.5 Hz, 4H, p-Ph),
6.91 (d, J ) 8.4 Hz, 8H, o-Ph), 7.61 (s, 8H, m-PAr). 31P{1H} NMR
(toluene-d8): δ 163.2 (d, J ) 262 Hz).
Preparation of [Rh2(µ-MeCO2)2(DPCB)2] (6). To a suspension
of 2 (100.0 mg, 0.0571 mmol) in pentane (1.0 mL) was added acetic
acid (6.9 µL, 0.120 mmol) at 0 °C. The color of the suspension
immediately turned from dark yellow to dark brown. After 1 h,
the precipitate was collected by filtration, washed with pentane at
0 °C, and dried under vacuum to give a dark brown power of 6
(102.2 mg, 98%). Mp: 197 °C (dec). IR (KBr): 1595, 1449 cm-1
.
1H NMR (C6D6): δ 1.39 (s, 36H, p-tBu), 1.71 (s, 6H, O2CMe),
2.01 (s, 72H, o-tBu), 6.78 (t, J ) 7.5 Hz, 8H, m-Ph), 6.88 (t, J )
7.5 Hz, 4H, p-Ph), 7.00 (d, J ) 7.5 Hz, 8H, o-Ph), 7.63 (s, 8H,
m-PAr). 13C{1H} NMR (C6D6): δ 24.4, 31.5, 34.5, 35.5, 39.1, 122.6,
126.9, 128.1, 128.6, 129.6 (m), 133.3, 143.4 (m), 153.1, 157.2, 158.9
(m), 191.6 (m). 31P{1H} NMR (C6D6): δ 165.9 (d, J ) 263 Hz).
Anal. Calcd for C108H142O4P4Rh2: C, 70.73; H, 7.80. Found: C,
70.54; H, 8.10.
Preparation of [Rh(η3-C6H9)(DPCB)] (8). To a suspension of
2 (49.9 mg, 0.0285 mmol) in toluene (2.5 mL) containing 1,3-
cyclohexadiene (5.5 µL, 0.0577 mmol) was added formic acid (2.2
µL, 0.0583 mmol) at room temperature. The solution immediately
turned dark reddish-brown. After 0.5 h, volatiles were removed
under reduced pressure, and the residue was washed with pentane
at 0 °C and dried under vacuum to give a dark reddish-brown
powder of 8 (27.3 mg, 51%). Slow diffusion of a concentrated THF
solution into MeOH gave analytically pure crystals with the
composition 8 · MeOH. Mp: 220 °C (dec). 1H NMR (C6D6): δ 1.41
(s, 18H, p-tBu), 1.71 (s, 18H, o-tBu), 1.91 (s, 18H, o-tBu), 2.26
(br, 4H, CHCH2CH2), 4.79 (m, 2H, allyl Hsyn), 5.69 (td, J ) 6.5,
2.6 Hz, 1H, allyl Hcentral), 6.81 (t, J ) 7.4 Hz, 4H, m-Ph), 6.90 (t,
J ) 7.4 Hz, 2H, p-Ph), 7.05 (d, J ) 7.2 Hz, 4H, o-Ph), 7.65 (s,
2H, m-PAr), 7.70 (s, 2H, m-PAr). 13C{1H} NMR (C6D6): δ 23.0,
29.8, 31.7, 33.6, 34.4, 35.4, 39.1, 39.1, 68.6 (dd, J ) 36, 9 Hz),
101.2 (m, J ) 3 Hz), 122.5, 122.8, 127.5, 128.5, 134.0, 135.4 (m),
144.7 (dd, J ) 55, 32 Hz), 151.5, 156.0, 156.1, 167.5 (ddd, J )
34, 30, 5 Hz). 31P{1H} NMR (C6D6): δ 165.4 (d, J ) 241 Hz).
Preparation of [Rh2(µ-H)(µ-OMe)(DPCB)2] (4). A solution of
3 (113.9 mg, 0.0657 mmol) in THF (1.5 mL) was prepared, layered
with MeOH (6.0 mL), and allowed to stand at room temperature
1
for 2 days to give dark brown crystals of 4 (96.1 mg, 67%). H
NMR (C6D6): δ -9.80 (ttt, J ) 84, 26, 15 Hz, 1H, µ-H), 1.37 (s,
18H, p-tBu), 1,41 (s, 18H, p-tBu), 1.81 (s, 36H, o-tBu), 1.92 (s,
36H, o-tBu), 3.88 (s, 3H, OMe), 6.77 (t, J ) 7.2 Hz, 4H, m-Ph),
6.79 (t, J ) 7.5 Hz, 4H, m-Ph), 6.87 (t, 2H, J ) 6.9 Hz, p-Ph),
6.89 (t, 2H, J ) 7.2 Hz, p-Ph), 7.04 (t, J ) 8.1 Hz, 4H, o-Ph), 7.12
(t, J ) 8.1 Hz, 4H, o-Ph), 7.59 (s, 2H, m-PAr), 7.60 (s, 2H, m-PAr),
7.67 (s, 4H, m-PAr). 13C{1H} NMR (C6D6): δ 32.0, 32.0, 32.1,
34.4, 35.5, 35.6, 35.7, 35.8, 35.8, 39.6, 39.7, 39.8, 39.8, 69.6, 122.8,
122.8, 123.2, 123.2, 123.3, 127.2, 127.5, 127.7, 127.8, 128.9, 128.9,
132.6, 134.4, 134.4, 134.6, 134.8, 134.9 (m), 142.1 (m), 145.3 (m),
152.1, 152.2, 152.5, 156.1, 157.0, 157.3, 158.2 (m), 162.8 (m).
31P{1H} NMR (C6D6): δ 174.6 (dd, J ) 220, 31 Hz), 156.9 (d, J

908 Organometallics, Vol. 28, No. 3, 2009
Okazaki et al.
Anal. Calcd for C59H81OP2Rh (8 · MeOH): C, 72.97; H, 8.41. Found:
C, 72.70; H, 8.24.
34.5, 35.3, 39.3, 122.8, 128.1, 128.5, 132.6, 148.6 (m), 151.7, 156.8,
172.1 (t, J ) 28 Hz). 31P{1H} NMR (C6D6): δ 173.9 (d, J ) 235
Hz).
Preparation of [Rh2(µ-KP,P;KP,C-DPCB)(CO)4] (9). A sus-
pension of 2 (71.2 mg, 0.0411 mmol) in pentane (8.0 mL) was
degassed by freeze-pump-thaw cycles, and carbon monoxide (1
atm) was introduced from a CO balloon. The dark red suspension
changed to a dark brown solution. Slow evaporation of the solvent
under the flow of CO gas at room temperature for 2 days formed
dark brown crystals with the composition 8 · C5H12 (24.0 mg, 55%).
X-ray Diffraction Studies. All measurements were performed
on a Rigaku Mercury CCD diffractometer with graphite-mono-
chromated Mo KR radiation (λ ) 0.71070 Å). The intensity data
were collected at 173 K and corrected for Lorentz and polarization
effects and absorption (numerical). The structures were solved by
DIRDIF99 and refined on F2 for all reflections (SHELXL-97).20
All non-hydrogen atoms were refined anisotropically. The µ-H
ligands in 3 and 4 were located from the differential Fourier maps
and refined isotropically. The other hydrogen atoms were placed
using AFIX instructions. Full crystallographic data have been
deposited with the Cambridge Crystallographic Data Centre: CCDC
No. 671945 (2 · (2C5H12)), 671947 (3 · Et2O), 671948 (4), 671949
(9 · C5H12), 671950 (10). A summary of the crystallographic data
is given in Table 1.
1
Mp: >300 °C. IR (KBr): 2033, 2008, 1977, 1962 cm-1. H NMR
(toluene-d8): δ 1.27 (s, 18H, p-tBu), 1.77 (s, 18H, o-tBu), 1.85 (s,
18H, o-tBu), 6.71-6.81 (m, 10H, Ph), 7.33 (s, 2H, m-PAr), 7.47
(s, 2H, m-PAr). 13C{1H} NMR (toluene-d8): δ 31.3, 35.2, 35.3, 39.9,
40.0, 123.1, 123.6, 127.2, 128.0, 128.3, 131.3 (td, J ) 13, 5 Hz),
132.3, 133.0 (dd, J ) 8, 7 Hz), 144.4 (t, J ) 8 Hz), 152.4, 157.6,
158.5, 194.7 (dt, J ) 74, 28 Hz). 31P{1H} NMR (toluene-d8): δ
152.9 (dd, J ) 151, 44 Hz). Anal. Calcd for C61H80O4P2Rh2
(8 · C5H12): C, 63.98; H, 7.04. Found: C, 63.60; H, 6.86.
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research on Priority Areas (No.
18064010, “Synergy of Elements”) from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
Preparation of [Rh2(µ-CO)2(DPCB)2] (10). A solution of 2
(150.8 mg, 0.086 mmol) in THF (4.0 mL) was degassed by
freeze-pump-thaw cycles, and carbon monoxide (1 atm) was
introduced from a CO balloon. The solution immediately turned
dark brown. The mixture was stirred for 30 min at room temperature
under a CO atmosphere. After purging the CO gas from the solution,
volatiles were removed under vacuum, and the residue was washed
with Et2O at 0 °C and dried under vacuum to give a dark brown
powder of 10 (111.8 mg, 73%). Recrystallization from THF and
MeOH formed single crystals, suitable for X-ray structural analysis.
Supporting Information Available: Crystallographic data of
2 · (2C5H12), 3 · Et2O, 4, 9 · C5H12, and 10 in cif format. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM8010799
1
IR (KBr): 1766 cm-1. H NMR (C6D6): δ 1.35 (s, 36H, p-tBu),
1.82 (s, 72H, o-tBu), 6.80 (m, 12H, m- and p-Ph), 6.96 (d, J ) 6.9
(20) Sheldrick, G. M. SHELX-97; University of Go¨ttingen: Germany,
1997.
Hz, 4H, o-Ph), 7.61 (s, 8H, m-PAr). 13C{1H} NMR (C6D6): δ 31.6,