Substitution and isomerization of asymmetric β-diketonato rhodium(I) complexes: A crystallographic and computational study

Article abstract of DOI:10.1021/om1000138

Ligand substitution of PPh₃ for CO in the asymmetric square-planar β-diketonato complex [Rh(PhCOCHCOCH₂CH ₃)(CO)₂] leads to formation of the monosubstituted [Rh(PhCOCHCOCH₂CH₃)(CO)(PPh₃)] product. Two geometrical product isomers are possible, which unexpectedly crystallize in the same crystal lattice. DFT calculations of the substitution reaction support a two-step mechanism, involving temporary dissociation of a β-diketonato ligand and formation of a square-planar intermediate. Analysis of the mechanisms involved in product isomerization suggests several noncatalyzed and solvent-assisted interconversion pathways.

Full text of DOI:10.1021/om1000138

2446 Organometallics 2010, 29, 2446–2458
DOI: 10.1021/om1000138
Substitution and Isomerization of Asymmetric β-Diketonato Rhodium(I)
Complexes: A Crystallographic and Computational Study
Kathrin H. Hopmann,† Nomampondomise F. Stuurman,‡ Alfred Muller,§ and
Jeanet Conradie*,†,‡
†Center for Theoretical and Computational Chemistry and Department of Chemistry, University of Tromsø,
N-9037 Tromsø, Norway, ‡Department of Chemistry, University of the Free State, 9300 Bloemfontein,
Republic of South Africa, and Department of Chemistry, University of Johannesburg (APK Campus),
P.O. Box 524, Aucklandpark, Johannesburg 2006, Republic of South Africa
§
Received January 5, 2010
Ligand substitution of PPh3 for CO in the asymmetric square-planar β-diketonato complex [Rh-
(PhCOCHCOCH2CH3)(CO)2] leads to formation of the monosubstituted [Rh(PhCOCHCOCH2-
CH3)(CO)(PPh3)] product. Two geometrical product isomers are possible, which unexpectedly
crystallize in the same crystal lattice. DFT calculations of the substitution reaction support a two-
step mechanism, involving temporary dissociation of a β-diketonato ligand and formation of a
square-planar intermediate. Analysis of the mechanisms involved in product isomerization suggests
several noncatalyzed and solvent-assisted interconversion pathways.
Introduction
trans to the donor atom with the largest trans influence.11-14
X-ray crystal structures of the monophosphine product com-
plexes [Rh(RCOCHCOR0)(CO)(PPh3)] typically display only
one of two possible geometric isomers,12,14-16 which was taken
as an indication that only one isomer was formed in the
substitution reaction. As both donor atoms of β-diketonate
are oxygens, their relative trans influence should be governed by
the adjacent substituents, R and R0, on the ligand backbone.
The crystal structures of the complexes [Rh(C4H3SCOCHCO-
CF3)(CO)(PPh3)]14 and [Rh(CH3COCHCOCF3)(CO)(P(p-Cl-
Ph)3)]16 exhibit CO substitution trans to the oxygen adjacent to
the more electron-donating group (C4H3S and CH3, respect-
ively), indicating that the oxygen atom nearest a strongly
electron-attracting group, such as CF3, has the smallest trans
influence.17 This is in agreement with the Grinberg polarization
theory18 and the σ-trans effect,19 as the oxygen nearest the CF3
group will be a weaker σ-donor as a result of the electron
attraction by CF3. The apparent group electronegativities, χR, in
Gordy scale (in parentheses) of the substituents are R = C4H3S
(2.10)20 ∼ CH3 (2.34)21 < CF3 (3.01).22 The CO substitution
Square-planar rhodium(I) complexes of the type [Rh(β-
diketonato)(CO)2] and their substituted derivatives [Rh(β-
diketonato)(L)(L0)] (where L, L0 = CO, olefin, phosphine- or
phosphite-based ligands) are used as catalyst precursors in
variousreactions, inparticularhydroformylationof olefins1-7
but also CO2 hydrogenation8 and asymmetric allylic alkyl-
ation.9 The first [Rh(β-diketonato)(CO)2] complexes were re-
ported by Bonati and Wilkinson.10 Substitution reactions of
these complexes with olefins results in replacement of both CO
ligands, while reactions with triphenylphosphine (PPh3, Ph =
C6H5) or triphenylarsine (AsPh3) generally result in replacement
of only one of the CO units.10 The monosubstitution reaction
with PPh3 has been employed to study the relative thermo-
dynamic trans influence of the bonding atoms in β-diketonate
and similar bidentate ligands, because it was assumed that the
reaction with PPh3 would result in substitution of the CO group
*To whom correspondence should be addressed. Tel: þ27-51-
4012194. Fax: þ27-51-4446384. E-mail: conradj@ufs.ac.za.
ꢀ
(1) Mieczynska, E.; Trzeciak, A. M.; Ziolkowski, J. J. J. Mol. Catal.
1993, 80, 189.
(2) Trzeciak, A. M.; Mieczynska, M.; Ziolkowski, J. J. Top. Catal.
2000, 11/12, 461.
(3) Jin, H.; Subramaniam, B.; Ghosh, A.; Tunge, J. AIChE J. 2006,
52, 2575.
(4) van Rooy, A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.;
Goubitz, K.; Fraanje, J.; Veldman, N.; Spek, A. L. Organometallics
1996, 15, 835.
(11) Graham, D. E.; Lamprecht, G. J.; Potgieter, I. M.; Roodt, A.;
Leipoldt, J. G. Transition Met. Chem. 1991, 16, 193.
(12) Leipoldt, J. G.; Basson, S. S.; Nel, J. T. Inorg. Chim. Acta 1983,
74, 85.
(13) Leipoldt, J. G.; Grobler, E. C. Inorg. Chim. Acta 1982, 60, 141.
(14) Leipoldt, J. G.; Bok, L. D. C.; van Vollenhoven, J. S.; Pieterse,
A. I. J. Inorg. Nucl. Chem. 1978, 40, 61.
ꢀ
(15) Leipoldt, J. G.; Basson, S. S.; Potgieter, J. H. Inorg. Chim. Acta
(5) Nozaki, K.; Sakai, N.; Nanno, T.; Higashijima, T.; Mano, S.;
Horiuchi, T.; Takaya, H. J. Am. Chem. Soc. 1997, 119, 4413.
(6) Kainz, S.; Leitner, W. Catal. Lett. 1998, 55, 223.
1986, 117, L3.
(16) Steynberg, E. C.; Lamprecht, G. J.; Leipoldt, J. G. Inorg. Chim.
Acta 1987, 133, 33.
(17) Leipoldt, J. G.; Bok, L. D. C.; Basson, S. S.; Gerber, T. I. A.
ꢁ
(7) Francio, F.; Leitner, W. Chem. Commun. 1999, 1663.
(8) Angermund, K.; Baumann, W.; Dinjus, E.; Fornika, R.; Gorls,
H.; Kessler, M.; Kruger, C.; Leitner, W.; Lutz, F. Chem.—Eur. J. 1997,
3, 755.
Inorg. Chim. Acta 1979, 34, L293.
(18) Grinberg, A. A. Acta Physiochim., USSR 1935, 3, 573.
(19) Langford, C. H.; Gray, H. B. Ligand Substitution Processes;
W.A. Benjamin Inc.: New York, 1965; pp 31 ff.
(9) Hayashi, T.; Okada, A.; Suzuka, T.; Kawatsura, M. Org. Lett.
2003, 5, 1713.
(20) Conradie, M. M.; Muller, A. J.; Conradie, J. S. Afr. J. Chem.
2008, 61, 13.
(10) Bonati, F.; Wilkinson, G. J. Chem. Soc. 1964, 3156.
r
pubs.acs.org/Organometallics
Published on Web 05/12/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 11, 2010 2447
Scheme 1. Proposed Effects of trans Influence and Steric Factors on CO Substitution in [Rh(β-diketonato)(CO)2] Complexes (Based
on Suggestions by Leipoldt and Co-workers)12,15,16a
a Red dashed lines indicate bonds that are broken/formed. The product isomers can interconvert and are in equilibrium.23,26,27
.
selectivity might also be affected by steric interactions, which
could explain why the solid-state structures of related [Rh(RCO-
16
CHCOCF3)(CO)(PPh3)] complexes (where R = CH2CH3
that depends inter alia on solvent polarity and temperature
effects.23,26,27,29,30 Crystallization conditions appear to favor
one isomer over the other, but which isomer crystallizes does
not necessarily reflect the isomer ratio in solution.27 Only in a
single instance has it been possible to crystallize both geome-
trical isomers of an asymmetric β-diketonato Rh(I) complex,
[Rh(PhCOCHCOCH3)(CO)(PPh3)], in the same crystal
lattice.28 The β-diketonato substituents of [Rh(PhCOCHCO-
CH3)(CO)(PPh3)] have similar Gordy group electronegativ-
ities, χPh = 2.2122 and χCH3 = 2.34,21 which might have con-
tributed to the equimolar isomer ratio observed in the solid
state.
Recent density functional theory (DFT) studies of the
PPh3 substitution for CO in the symmetric [Rh(acac)(CO)2]
(acac = (CH3COCHCOCH3)-) complex have shown that
CO substitution does not proceed through a single step via an
associative interchange mechanism as proposed by Leipoldt
and co-workers (Scheme 1),16 but that the reaction instead
occurs through two interchange steps, involving temporary
loss of an Oacac ligand in the square-planar (SQP) inter-
mediate.31 We were interested to determine if a similar two-
step substitution mechanism operates in asymmetric β-dike-
tonato complexes and if both possible product isomers are
primary kinetic products, or if one isomer is formed from the
other. Here we present a combined experimental and com-
putational study of the substitution reaction of one CO
group in the asymmetric β-diketonato complex [Rh(PhCO-
CHCOCH2CH3)(CO)2] with PPh3. The β-diketonato sub-
stituents Ph and CH2CH3 were chosen on the basis of similar
12
15
(χR = 2.31),23 CH(CH3)2 (χR = 2.29),24 C(CH3)3 (χR
=
2.27),24 or Fc25 (χR = 1.87)22) exhibit substitution trans to the
oxygen adjacent to CF3, although this is expected to have the
smaller trans influence. Leipoldt and co-workers have attempted
to explain these findings on the basis of a one-step substitution
reaction, involving a trigonal-bipyramidal (TBP) transition state
(TS, Scheme 1).12,15,16 At the TS, the equatorial plane is occu-
pied by the entering group (PX3), the leaving ligand (CO0), and
the β-diketonato oxygen (O0) with the stronger trans influence
(Scheme 1, TStrans). If the substituent (R0) close to the oxygen
with stronger trans influence is bulky, it might preferably be
positioned in an axial position (TSsteric, Scheme 1),16 which leads
to replacement of the opposite CO ligand and formation of a
different product isomer.
ꢀ
Trzeciak and Ziolkowski have pointed out that the pre-
sence of a single [Rh(RCOCHCOR0)(CO)(PPh3)] product
isomer in the solid state is a consequence of the crystal-
lization conditions.26,27 They showed that both product
isomers of [Rh(RCOCHCOCF3)(CO)(PPh3)] (where R =
CH3, Ph, C4H3S) exist in solution.26 NMR studies on
[Rh(RCOCHCOPh)(CO)(PPh3)] (where R = CH3, CH2CH3,
CH2CH2CH3, CH2CH2CH2CH3)23,28 and [Rh(RCOCHCO-
Fc)(CO)(PPh3)] (where R = CH3, Ph, CF3)27 also confirmed
the presence of both isomers in solution. The isomers are
able to interconvert, with a resulting equilibrium constant (Kc)
steric and electronic properties (χPh = 2.2122 ≈ χCH2CH3
=
2.3123). We present crystallographic characterizations of the
dicarbonyl reactant and the carbonyl triphenyl-phosphine
products, followed by a theoretical analysis of the substitu-
tion reaction and possible isomerization pathways leading to
isomer interconversion during and after product formation.
(21) Kagarise, R. E. J. Am. Chem. Soc. 1955, 77, 1377.
(22) Du Plessis, W. C.; Vosloo, T. G.; Swarts, J. C. J. Chem. Soc.,
Dalton Trans. 1998, 2507.
(23) Stuurman, N. F.; Conradie, J. J. Organomet. Chem. 2009, 694,
259.
(24) Klaas, P. Synthesis, Electrochemical, Kinetic and Thermodynamic
Properties of New Ferrocene-containing Betadiketonato Rhodium(I) Com-
plexes with Biomedical Applications. M.Sc.Thesis, University of the Free
State, R.S.A., 2002.
Results and Discussion
(25) Lamprecht, G. J.; Swarts, J. C.; Conradie, J.; Leipoldt, J. G. Acta
Crystallogr. 1993, 49, 82.
(26) Trzeciak, A. M.; Zio1kowski, J. J. Inorg. Chim. Acta 1985, 96, 15.
(27) Conradie, J.; Lamprecht, G. J.; Otto, S.; Swarts, J. C. Inorg.
Chim. Acta 2002, 328, 191.
(28) Purcell, W.; Basson, S. S.; Leipoldt, J. G.; Roodt, A.; Preston, H.
Inorg. Chim. Acta 1995, 234, 153.
Synthesis and Crystallography. Nucleophilic attack of PPh3
on the dicarbonyl complex [Rh(PhCOCHCOCH2CH3)(CO)2]
leads to substitution of one CO group. Two different isomers
of the [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] product are
ꢀ
(29) Conradie, M. M.; Conradie, J. Inorg. Chim. Acta 2008, 361, 208.
(30) Conradie, M. M.; Conradie, J. Inorg. Chim. Acta 2008, 361, 2285.
(31) Hopmann, K. H.; Conradie, J. Organometallics 2009, 28, 3710.

2448 Organometallics, Vol. 29, No. 11, 2010
Hopmann et al.
Scheme 2. Substitution of PPh3 for CO in [Rh(PhCOCHCOCH2CH3)(CO)2] Gives an Isomeric Mixture
of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] Complexes, Isomer A (SP-4-2), and Isomer B (SP-4-3)
˚
Table 1. Selected Bond Lengths (A) and Angles (deg) for [Rh(PhCOCHCOCH2CH3)(CO)2] and the Two Structural Isomers A (SP-4-2)
and B (SP-4-3) of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)]
[Rh(PhCOCHCOCH2CH3)(CO)(PPh3)]
[Rh(PhCOCHCOCH2CH3)(CO)2
isomer A
isomer B
Rh-C1
1.849(2)
1.852(2)
2.0288(13)
2.0285(14)
1.135(2)
1.137(2)
89.14(9)
89.87(7)
90.53(7)
90.36(6)
126.34(19)
179.4(2)
Rh2-C42
1.788(5)
2.2387(9)
2.032(3)
2.067(3)
1.285(4)
1.282(5)
1.376(6)
1.380(6)
87.84(11)
179.1(5)
178.59(17)
91.68(14)
92.99(8)
87.47(12)
127.2(4)
Rh1-C12
Rh1-P1
1.797(4)
2.2376(9)
2.032(2)
2.060(3)
1.289(4)
1.263(5)
1.382(6)
1.390(6)
89.39(10)
177.7(5)
179.08(17)
90.84(15)
91.62(7)
88.15(14)
127.0(4)
Rh-C2
Rh2-P2
Rh-O3
Rh2-O5
Rh1-O2
Rh-O4
Rh2-O4
Rh1-O1
O1-C1
C35-O4
C5-O2
O2-C2
C33-O5
C3-O1
C1-Rh-C2
C2-Rh-O4
C1-Rh-O3
O3-Rh-O4
C5-C6-C7
O1-C1-Rh
C35-C34
C5-C4
C33-C34
C3-C4
O5-Rh2-O4
O6-C42-Rh2
C42-Rh2-O5
C42-Rh2-O4
O5-Rh2-P2
P2-Rh2-C42
O1-Rh1-O2
O3-C12-Rh1
C12-Rh1-O2
C12-Rh1-O1
O2-Rh1-P1
P1-Rh1-C12
C5-C4-C3
C35-C34-C33
Figure 1. Molecular diagrams of (I) [Rh(PhCOCHCOCH2CH3)(CO)2] with 30% probability displacement ellipsoids (data collection at
100 K) and the two geometrical isomers of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)], (II) isomer A (SP-4-2), and (III) isomer B (SP-4-3),
with 30% probability displacement ellipsoids (data collection at 293 K). Hydrogen atoms were omitted for clarity. The ethyl carbons are
disordered, with 0.74:0.26 for C1A and C1B and 0.66:0.34 for C31A and C31B.
possible, which here are labeled isomer A (IUPAC SP-4-232)
and isomer B (IUPAC SP-4-332 Scheme 2). Both isomers might
be formed directly from the substitution reaction or through a
subsequent isomerization reaction. A priori it is not clear if the
substitution reaction favors any of the two isomers, nor is it
possible to predict which isomer is more likely to crystallize.
The substitution reaction as shown in Scheme 2 was
performed in hot n-hexane as solvent. Crystals suitable for
single-crystal X-ray studies of the parent compound [Rh(Ph-
COCHCOCH2CH3)(CO)2] and the product [Rh(PhCOCH-
COCH2CH3)(CO)(PPh3)] were obtained after recrystallization
from acetone. The crystal data and details of data collection
and refinement are given in the Supporting Information
(Table S1), whereas selected bond lengths and angles are listed
in Table 1.
Solid-State Structure of [Rh(PhCOCHCOCH2CH3)(CO)2].
The molecular diagram of [Rh(PhCOCHCOCH2CH3)(CO)2]
is presented in Figure 1I. The dicarbonyl complex packs in the
P21/n space group with Z = 4, resulting in molecules lying on
general positions in the unit cell. The rhodium atom has a
nearly perfect square-planar coordination sphere (Rh atom
˚
displaced 0.0395(9) A from the plane formed by four coordi-
nating atoms, rms deviation of fitted atoms = 0.0047). The
bond angles in the rhodium polyhedron are also within experi-
mental error of the expected 90° for dsp2 hybridization; how-
ever, deviations of up to 6° were found in the sp2-hybridized
(32) http://www.iupac.org/publications/books/rbook/Red_Book_2005.
pdf, p 180. SQP complexes are designated SP-4, followed by a single digit
referring to the priority number of the ligating atom trans to the ligating
atom of priority number 1.

Article
Organometallics, Vol. 29, No. 11, 2010 2449
C atoms of the β-diketonato skeleton, possibly due to strain
from chelation with the metal.
The Ph and CH2CH3 substituents on the β-diketonato
ligand are twisted out of the plane formed by the acetylace-
tone backbone (dihedral of 50.92(13)° and 23.62(11)°, res-
pectively). Weak intermolecular CH O/Rh interactions
3 3 3
(between C11-H11 O4i and C13-H13 Rhii)33 are ob-
3 3 3
3 3 3
served with the Ph substituent on the β-diketonato ligand,
possibly resulting in the preferred orientation. The confor-
mation of CH2CH3 is mainly due to the packing arrange-
ments of neighboring molecules. The Ph and CH2CH3
substituents in [Rh(PhCOCHCOCH2CH3)(CO)2] have simi-
lar electron-donating properties, which is confirmed by
similar Rh-O bond lengths (both Rh-O3 and Rh-O4 are
Figure 2. Overlay of the Rh(CCOCHCOC)(CO)(PPh3) moieties
of the two isomers of (A) [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)]
and (B) [Rh(PhCOCHCOCH3)(CO)(PPh3)].28
37
˚
expected value for an aromatic C-C bond (1.394 A). In
˚
2.029 A). Also the Rh-C1 and Rh-C2 bonds, 1.849 and
both isomers, the Rh-O bond trans to PPh3 (2.067(3) and
˚
1.852 A, respectively, did not show a clear difference due to
˚
2.060(3) A) is longer than the Rh-O bond trans to CO
the trans influence of O4 and O3, respectively. The thermo-
dynamic trans influence is a ground-state phenomenon
leading to a weakening of the metal-ligand bond in the
trans position.34 On grounds of the similar Rh-CO bond
lengths, it thus is not possible to forecast which CO group
preferably will be replaced in the substitution reaction
˚
(2.032(3) and 2.032(2) A), indicating that the trans influence
of PPh3 is larger than that of CO in this type of complexes.38
All the bond angles of the Ph rings are equal to 120° within
the experimental error, e.g., C16-C15-C14 = 120.5(5)° and
C44-C45-C46 = 120.8(4)°. The phosphorus atom displays
a distorted tetrahedral geometry and is surrounded by the
rhodium atom and three carbon atoms of the Ph rings
bonded to it. The bond angles around P differ 11° from 109°,
which is the angle for a regular tetrahedron. The O-C-Rh
angle is close to linear (179.1(5)°), with a C-O bond length
˚
(Scheme 2). In general, the Rh-C (mean distance 1.834 A,
˚
˚
with a spread of 0.125 A) and Rh-O (mean distance 2.044 A,
with a spread of 0.146 A) bond distances are fairly sensitive
˚
to changes on the β-diketonato backbone.35
Solid-State Structure of [Rh(PhCOCHCOCH2CH3)(CO)-
(PPh3)]. NMR spectra in CDCl3 of [Rh(PhCOCHCOCH2-
CH3)(CO)(PPh3)] show the existence of both geometrical
product isomers in solution in a ratio of 1.00 (isomer A):0.70
(isomer B).23 Solution mixtures of product isomers have been
observed for other [Rh(RCOCHCOR0)(CO)(PPh3)] com-
plexes also,26,27,29,30 but typically only a single product iso-
mer is observed in the solid state.12,15,16,26,36 Factors such as
temperature and solvent polarities are likely to play a role in
determining which isomer in the mixture crystallizes. The
substitution product [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)]
crystallized in the triclinic system in the space group P1,
requiring two independent molecules (Z = 4) in the unit cell.
The two molecules turned out to be both isomers of
[Rh(PhCOCHCOCH2CH3)(CO)(PPh3)], namely, isomer A,
with PPh3 trans to the oxygen next to the Ph group (SP-4-2,
Figure 1II), and isomer B, with PPh3 trans to the oxygen next to
the CH2CH3 group (SP-4-3, Figure 1III). Simultaneous crys-
tallization of both product isomers of an asymmetric β-dike-
tonato Rh(I) complex in the same crystal lattice has been
reported only once previously.28
˚
˚
of 1.164(6) A for isomer A (177.7(5)° and 1.133(5) A for
˚
isomer B). The Rh-P bond length, 2.2387(9) A (isomer A)
and 2.2376(9) A (isomer B), is comparable to related [Rh(β-
˚
diketonato)(CO)(PPh3)] complexes with β-diketonato =
38
(CH3COCHCOCH3)- (2.244(2) A), (PhCOCHCOCH3)-
˚
(2.248(3) and 2.249(3) A for the two isomers),28 and (PhCO-
˚
39
CHCOPh)- (2.237(7) A). The plane of the Ph ring on the
˚
β-diketonato ligand makes an angle of 22.44(12)° with the
O-C-C-C-O plane of the β-diketonato skeleton of [Rh-
(PhCOCHCOCH2CH3)(CO)2] and 20.38(26)° and 28.33(18)°
for the two isomers of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)],
respectively. These angles are similar to the angle between the
plane ofthe Phringandtheplaneofthe β-diketonato ligandin
[Rh(PhCOCHCOCH3)(CO)(PPh3)] (21.23° and 27.80° for
the two isomers)38 and in [Rh(PhCOCHCOPh)(CO)(PPh3)]
(10.32° and 6.61° for the two Ph groups of the one molecule
and 11.39° and 7.82° for the two Ph groups of the other
molecule in the same unit cell).39
Steric Parameters. It is interesting that the PPh3 ligands of
the two isomers of [Rh(β-diketonato)(CO)(PPh3)] adopt simi-
lar spatial orientations, despite different weak interactions to
the Ph substituents (C16-H16 Rh1i, C16-H16 O2i, and
The differences in Rh-O, Rh-P, and Rh-C bond lengths
(Table 1) for the two isomers are within experimental error.
The ligand-Rh-ligand bond angles of [Rh(PhCOCHCO-
CH2CH3)(CO)(PPh3)] deviate by (2° from the expected 90°
for a dsp2 hybridization, while deviations of up to 7° were
found in the sp2-hybridized C atoms of the β-diketonato
skeleton. The average C-C bond lengths of the Ph rings of
3 3 3
3 3 3
C58-H58 O3ii).40 The Rh(CCOCHCOC)(CO)(PPh3) moi-
3 3 3
eties of the two isomers can be superimposed, showing remark-
able similarities (see Figure 2A for graphical representation).
As a result, the steric parameters of these two ligands are similar
(cone angles = 156.7° and 158.5°;41 solid angles = 26.41% and
26.36%42 for P1 and P2, respectively).43 A possible explanation
˚
˚
PPh3 (1.389 A) and the β-diketonato substituent (1.385 A)
in [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] are close to the
(37) Sutin, L. E. Tables of Interatomic Distances and Configuration in
Molecules and Ions, Supplement 1956-1959,; The Chemical Society:
London, p S16s.
(33) Symmetry codes are i = 1/2þx, 3/2-y, -1/2-z and ii = 1/2-x,
1/2þy, 1/2-z.
(38) Leipoldt, J. G.; Basson, S. S.; Bok, L. D. C.; Gerber, T. I. A.
Inorg. Chim. Acta 1978, 26, L35.
(34) Jordan, R. B. Reaction Mechanisms of Inorganic and Organo-
metallic Systems, 3rd ed.; Oxford University Press: New York, 2007.
(35) Averages taken from 31 observations from square-planar Rh-
(O-O0)(CO)2 complexes in the Cambridge Structural Database (CSD),
Version 5.30, August 2008 update.
(39) Lamprecht, D.; Lamprecht, G. J.; Botha, J. M.; Umakoshi, K.;
Sasaki, Y. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1997, 53,
1403.
(40) Symmetry codes are i = -1þx, y, z and ii = 1-x, 1-y, 1-z.
(41) For calculation of cone angles: Taverner, B. C. Steric v1.12B,
available at http://www.gh.wits.ac.za.
(36) Roodt, A.; Steyn, G. J. J. Recent Res. Dev. Inorg. Chem. 2000, 2, 1.

2450 Organometallics, Vol. 29, No. 11, 2010
Hopmann et al.
Figure 3. Solid angle projections on isomer B of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)]. (A) Presentation of the 8.5% overlap of the
solid angle projection of CO (blue) and PPh3 (green). (B) Solid angle projection of the (PhCOCHCOCH2CH3)- ligand. (C) Solid angle
projection of the Ph and CH2CH3 substituents on the (PhCOCHCOCH2CH3)- ligand.
is that due to the 90° angle between the CO and PPh3 ligands, the
carbonyl ligand is wedging itself between the phenyl substituents
of the PPh3 ligand, thereby forcing the PPh3 into a specific
conformation. The overlap from the solid angle calculations42
for the CO and PPh3 ligands displayed in Figure 3A is ca. 8.5%.
A different conformation of the PPh3 ligand would generally
result in more overlap and is more unlikely. Incidentally this
conformational effect is also observed in the previously reported
case, where both isomers crystallized in the same lattice
([Rh(PhCOCHCOCH3)(CO)(PPh3)], Figure 2B).28
Calculation of the solid angle42 of the (PhCOCHCO-
CH2CH3)- ligand also yields information regarding the
steric size of the Ph and CH2CH3 substituents. A solid angle
projection42 of the ligand shows two comparable lobes for
the substituents of the β-diketonato ligand (Figure 3B).
Separation of these (Figure 3C) shows that they shield
and [Rh(PhCOCHCOCH2CH3)(CO)2]. Distances are in ang-
12.48% and 12.43% of the sphere, indicating similar steric
properties. This observation may also contribute to the
rhodium complex.
stability of both isomers that are found in the solid state.
DFT Studies. The presence of both isomers A and B of
[Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] in solution and in
the solid-state X-ray crystal structure can be due to forma-
tion of both isomers through the substitution reaction or,
instead, through formation of only one isomer, followed by
conversion into the other isomer (Scheme 2). We have used
density functional theory to analyze the substitution path-
ways leading to formation of the two isomers as well as
possible noncatalyzed and solvent-assisted mechanisms in-
volved in product isomerization.
Figure 4. DFT-optimized reactant geometry comprising PPh3
stroms. The inset shows angles and overall conformation of the
˚
within 0.01 A of the experimental structure. These results
show that the employed computational method provides
adequate geometric structures.
Two aspects of the PPh3 geometry and mode of attack
should be considered here. First, PPh3 can exist in two
different helicities, which have the Ph substituents oriented
in a clockwise (P) or anticlockwise (M) fashion.45,46 In
solution, inversion of the helicity occurs readily, and the
possible chirality introduced by the PPh3 moiety is normally
disregarded.45 All calculations here were performed with
PPh3 in M helicity. Second, attack of PPh3 on [Rh(PhCO-
CHCOCH2CH3)(CO)2] can occur from two different faces,
leading to TBP transition states of chirality A and C, respec-
tively (Scheme 3).47 Each pathway can give rise to both
product isomers A and B (depending on which CO ligand
dissociates); that is, there are four possible reaction pathways.
The TS structures of chirality C are energetically slightly
preferred and the A_(C) pathway is presented here (figures
and energies for the B_(C), A_(A), and B_(A) pathways and
all coordinates are given in the Supporting Information).
Substitution Reaction. The optimized reactant complex
comprising PPh3 and [Rh(PhCOCHCOCH2CH3)(CO)2] is
shown in Figure 4. The SQP rhodium complex exhibits
˚
symmetric Rh-ligand bonds (1.88 A for both Rh-CO bonds
˚
and 2.07 A for both Rh-Oβ-diketonato bonds), indicating that
the β-diketonato oxygens have similar trans influence. This
supports the conclusions made based on the experimental
structure. Comparison of [Rh(PhCOCHCOCH2CH3)(CO)2]
to the X-ray crystal structure (Table 1) reveals that the
rhodium-ligand bond distances are overestimated by 0.03
44
˚
to 0.04 A, which is not considered to be critical. The
ꢁ
(45) Pinter, A; Haberhauer, G.; Hyla-Kryspin, I.; Grimme, S. Chem.
ꢁ
optimized carbon-carbon and carbon-oxygen bonds are
Commun. 2007, 3711.
(46) Costello, J. F.; Davies, S. G. J. Chem. Soc., Perkin Trans. 1998, 2,
1683.
(42) Solidanglecalculations: Guzei, I. A., Wendt, M. ProgramSolid-G;
UW-Madison, WI, 2004.
(47) http://www.iupac.org/publications/books/rbook/Red_Book_2005.
pdf, p 187. The structure is oriented so that the viewer looks down the TBP
axis, with the axial atom with higher priority closer to the viewer. Using this
orientation, if the sequence of the atoms in the trigonal plane is in a
clockwise fashion, the chirality symbol C is assigned, and if it is anti-
clockwise, the symbol A is assigned.
(43) Calculations were preformed with the Rh-P distance fixed to
2.28 A to eliminate deviations incorporated by differences in bond
˚
distances.
(44) Hehre, W. J. A. Guide to Molecular Mechanisms and Quantum
Chemical Calculations; Wavefunction Inc.: Irvine, CA, 2003; pp 153, 181.

Article
Organometallics, Vol. 29, No. 11, 2010 2451
Scheme 3. Attack of PPh3 Can Occur from Two Different Faces of [Rh(PhCOCHCOCH2CH3)(CO)2], Leading to TBP Complexes with
Chirality (A) or (C) (IUPAC nomenclature47)a
a Each path can give rise to either isomer A or B, depending on the CO that is substituted (dashed lines).
Scheme 4. CO Substitution in β-Diketonato Rh(I) Complexes Having a Bulky β-Diketonato Substituent R0: (A) Leipoldt and Co-workers
Proposed a One-Step Substitution Process with the Bulky Substituent Placed Axially;12,15,16 (B) Optimized Angles of the Two-Step
Substitution Process Computed in This Study Indicate Preferred Equatorial Placement of a Bulky Group R0 at TS2
Kinetic Pathway for the Formation of [Rh(PhCOCHCO-
CH2CH3)(CO)(PPh3)]. Isomer A (SP-4-2) of [Rh(PhCOCH-
COCH2CH3)(CO)(PPh3)] is formed if the CO group trans to
the Oβ-diketonato adjacent to the Ph substituent is displaced.
The reaction (A_(C) path, Scheme 3) proceeds through two
pseudo-trigonal-bipyramidal transition states and involves
formation of a SQP intermediate (Scheme 4B). The first TS
comprises (i) the nucleophilic attack of PPh3 on Rh, (ii) clea-
vage of a Oβ-diketonato bond, and (iii) rotation of one CO
ligand (Figure 5A). At the TS, the phosphorus atom is
influence series is PPh3 > CO > Oacac.
38,48 The series is likely
the same for the asymmetric β-diketonato studied here, given
the similar apparent group electronegativities of the substitu-
ents (χCH3 = 2.34,21 χCH2CH3 = 2.31,23 χPh = 2.2122). Thus,
nucleophilic attack by PPh3 will lead to cleavage of the
Rh-Oβ-diketonato bond instead of the Rh-CO bond.
The optimized intermediate (Figure 5B) exhibits a four-
coordinated SQP geometry, with the displaced β-diketonato
˚
oxygen positioned 2.54 A from the rhodium center. The
˚
Rh-P bond has shortened to 2.44 A, and the PPh3 group is
˚
positioned 2.64 A from rhodium, whereas the scissile Rh-
Oβ-diketonato bond is 2.29 A. The displacement vector of the
now in an optimal position to weaken the Rh-CO bond
trans to it. The second TS (Figure 5C) involves (i) the re-
formation of the Rh-Oβ-diketonato bond, (ii) cleavage of the
Rh-CO bond trans to PPh3, and (iii) formation of a new
SQP geometry. At the TBP transition state, the attacking
˚
imaginary frequency shows that this TS also comprises a
rotation of the equatorial CO group around the TBP axis
(arrow in Figure 5A). The movement converts the TBP TS
geometry into a SQP intermediate. A TBP intermediate could
not be located. Dissociation of a Rh-Oβ-diketonato bond
insteadof a Rh-CObond canberationalized with the relative
trans influence of the ligands. For [Rh(acac)(CO)2], the trans
˚
oxygen atom is positioned 2.32 A from rhodium, whereas the
˚
scissile Rh-CO bond is elongated to 2.09 A. Following CO
dissociation, the TBP geometry collapses to the SQP product
isomer A (Figure 5D). Comparison of the optimized product
to the X-ray structure (Table 1) shows a slight overestimation
˚
of Rh-ligand bonds of 0.03 to 0.07 A, similar to the results
observed for the dicarbonyl reactant structure.
(48) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev.
1973, 10, 335.

2452 Organometallics, Vol. 29, No. 11, 2010
Hopmann et al.
Figure 5. DFT-optimized geometries for formation of isomer A (SP-4-2) of [Rh(PhCOCHCOCH2CH3)(CO) (PPh3)] (A_(C) path,
Scheme 3). (A) TS1_A, (B) Inter_A, (C) TS2_A, (D) Prod_A (isomer A, Scheme 2).
Table 2. Computed Relative Energies for the Formation of Isomer A (Isomer B in Parentheses)
of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] from [Rh(PhCOCHCOCH2CH3)(CO)2] and PPh3
a
electronic energy/kJ mol-1 ΔH298K/kJ mol-1 ΔS298K/J K-1 mol-1 ΔG298K/kJ mol-1 ΔG342K/kJ mol-1
reactant
TS1_A (TS1_B)
Inter_A (Inter_B)
TS2_A (TS2_B)
Prod_A (Prod_B)
Prod_A, separated (Prod_B, separated)
0.0 (0.0)
0.0 (0.0)
0.0 (0.0)
0.0 (0.0)
53.9 (59.3)
42.4 (48.0)
65.5 (68.9)
17.1 (19.2)
-1.8 (-5.1)
0.0 (0.0)
57.8 (63.5)
45.2 (50.7)
68.3 (72.4)
17.5 (19.6)
-4.2 (-7.6)
30.3 (33.9)
24.1 (29.7)
50.8 (50.3)
15.0 (17.6)
19.7 (16.8)
27.1 (30.8)
23.4 (29.3)
46.5 (45.7)
14.0 (16.5)
14.9 (12.1)
-89.7 (-95.7)
-63.9 (-62.5)
-63.7 (-77.9)
-10.3 (-9.1)
55.9 (57.8)
a Results for the substitution reaction via path (C) in Scheme 3 are presented here. See Table S2 for results via path (A).
Formation of isomer B (SP-4-3) of [Rh(PhCOCHCO-
CH2CH3)(CO)(PPh3)] occurs analogously to formation of
isomer A, but results in displacement of the CO group trans
to the Oβ-diketonato adjacent to CH2CH3 (see Supporting
Information, Figure S1 for the (B_(C) path and Figure S3
for the (B_(A) path, Scheme 3). The optimized distances for
the forming and breaking bonds are very similar to the
pathways leading to formation of isomer A (Figure 5 for
the A_(C) path and Figure S2 for the A_(A) path).
Thermochemical Analysis of Substitution Reaction. Thermo-
chemical quantities (ΔS, ΔH, ΔG, 298 K) for the A_(C) and
B_(C) routes (Scheme 3) to formation of isomers A and B
are given in Table 2. The computed free energies of activa-
tion at 298 K are 53.9 and 59.3 kJ mol-1 for TS1_A and
TS1_B, respectively. The large negative entropies show that
both TSs are very ordered.49 The intermediates have relative
energies of 42.4 and 48.0 kJ mol-1 (ΔG298K), respectively,
indicating that they are unstable despite the SQP coordina-
tion around the rhodium atom. The second reaction step,
dissociation of CO, is rate limiting. The overall activation
barrier is 65.5 and 68.9 kJ mol-1 (ΔGq,298K) for formation
of isomer A and B, respectively. The difference in barrier is
so small that it cannot be considered significant. The reac-
tion free energies for formation of both product isomers are
small positive (17.1 and 19.2 kJ mol-1, respectively). How-
ever, the reaction is driven to completion by evaporation
of CO. Calculation of the separated [Rh(PhCOCHCO-
CH2CH3)(CO)(PPh3)] and CO units indicates a free energy
of -1.8 and -5.1 kJ mol-1 for formation of product isomer A
and B, respectively (298 K, Table 2). The reaction energies at
the experimental23 conditions (boiling point of n-hexane ∼69
°C or 342 K) are -4.2 and -7.6 kJ mol-1 for isomer A and B,
respectively (342 K, Table 3).50 The ΔG298K values of the free
products indicate that isomer B is 3.3 kJ mol-1 more stable
(50) The lower energy of the separated fragments is due to an increase
in entropy, as indicated above; these values are considered estimations
only. Also note that the reported energies correspond to gas phase values
and do not include solvent effects. This is considered a reasonable
approximation here, because the substitution reaction is performed in
very low polarity media (the dielectric constant of n-hexane is 1.89 at
20 °C) and polarizing effects of the surroundings should thus be small.
(49) Note that entropy calculations (and hence Gibbs free energies)
are sensitive to low-frequency modes; that is, the computed ΔS and ΔG
values should be considered estimations only.

Article
Organometallics, Vol. 29, No. 11, 2010 2453
Table 3. Computed Activation Energies at 298 K for Formation
of Isomer A (Isomer B in Parentheses)
of [Rh(PhCOCHCOCH2CH3)(CO)(PH3)] from
[Rh(PhCOCHCOCH2CH3)(CO)2] with PH3 as Nucleophile
at TS2 are similar to TS1 (87.7° to 89.2° or ∼88°, Scheme 4).
Thus, at TS2, the steric interactions between PPh3 and the
axial β-diketonato ligand (also referred to as cis ligand) are
larger than the interactions between PPh3 and the equatorial
β-diketonato substituents. Steric effects at TS2 (which is
rate-limiting) should therefore favor placement of bulky
groups in an equatorial position (see Supporting Informa-
tion, Figure S5). This is in line with Gray and Langford’s
observation that sterically demanding substituents on trans
ligands have less effect on reaction rates of SQP substitution
reactions than bulky substituents on cis ligands.19,54 Hence,
for [Rh(RCOCHCOR0)(CO)2] complexes, where R and R0
differ substantially in size, CO substitution should be promoted
trans to the bulkier β-diketonato substituent (Scheme 4), op-
posite the substitution model proposed by the Leipoldt
model.12,15,16
ΔHq/kJ mol-1
ΔSq/J K-1 mol-1
ΔGq/kJ mol-1
TS1
TS2
40.3 (40.6)
66.9 (66.9)
-76.9 (-76.3)
-65.1 (-60.6)
63.3 (63.3)
86.3 (84.9)
than isomer A (Table 2). NMR studies in CDCl3 instead
report an excess of isomer A (1.00 A:0.70 B).23 Single-point
calculations on the optimized products in CHCl3 revert the
isomer order, with isomer A being 2.4 kJ mol-1 more stable.
Thus, the equilibrium between isomers could be affected by
the polarity of the surroundings. It should be noted though
that the energy differences are small and fall within the
expected error margin of DFT calculations.
Electronic Effects. In order to eliminate steric effects due
to the Ph groups of PPh3, the substitution reactions employ-
ing phosphine (PH3) as nucleophile are also computed
(optimized coordinates and energies are given in the Sup-
porting Information, Table S3). This should make it possible
to isolate electronic effects originating from the β-diketonate
substituents. The same substitution pathway involving two
transition states and a square-planar intermediate(Scheme 4B)
is obtained. Activation parameters obtained for the formation
of isomer A or B were very similar (Table 3). For example, at
the first TS, involving nucleophilic attack of PH3, the free
energy of activation is the same for both product isomers
(63.3 kJ mol-1). The second TS, involving dissociation of
CO, has a free energy barrier of 86.3 and 84.9 kJ mol-1 for
isomer A and B, respectively. These results illustrate that
the electronic effects of the two β-diketonate substituents
on the reaction barriers are virtually identical, as could be
expected from the almost identical group electronegati-
vities of Ph22 and CH2CH3.23
Isomerization Pathways. The presented DFT study of the
substitution of PPh3 for CO in the asymmetric square-planar
β-diketonato complex [Rh(PhCOCHCOCH2CH3)(CO)2],
with β-diketonato substituents Ph and CH2CH3, reveals that
formation of both product isomers of [Rh(PhCOCHCO-
CH2CH3)(CO)(PPh3)] is energetically feasible. The presence
of both product isomers in the reaction mixture is thus likely
to be due to formation of both isomers in the substitution
reaction. However, as pointed out above, the product iso-
mers also have the possibility to isomerize; that is, formation
of one product complex might be followed by isomerization
to the other isomer. NMR studies on [Rh(CF3COCHCO-
Fc)(CO)(PPh3)] (Fc = ferrocenyl) have shown that if pure
crystals of one product isomer are dissolved, the second
isomer is rapidly formed.27 Product isomerization occurs in
solvents with very different polarity, and coordination pro-
pensity (e.g., C6D6, CDCl3, CD3CN), resulting in different
isomer ratios.27,29 Consequently, a central part of our theo-
retical investigation of β-diketonate Rh(I) complexes is to
establish possible isomerization pathways of the SQP com-
pounds. Different noncatalyzed and solvent-assisted path-
ways can be envisioned, depending on if they occur in the
low-polarity environment of the substitution reaction (i.e.,
n-hexane27) or in the presence of polar/coordinating solvent
(such as CH3CN or CHCl3, which typically are employed in
NMR studies27). Here we have considered three noncatalyzed
The computed values of this study can also be compared to
available experimental data of similar substitution reactions.
The measured reaction enthalpy for formation of [Rh(acac)-
(CO)(PPh3)] from [Rh(acac)(CO)2] of 20.1 kJ mol-1 in
CH2Cl2 at 30.0 °C51 compares well with the computed values
of 12.0 to 20.3 kJ mol-1 (Table 2 and S2). The entropy and
enthalpy of activation have been measured for the reaction of
[Rh(acac)(CO)2] with the nucleophiles P(OPh)3 andP(NC4H4)3
in toluene at 25 °C.52 Activation entropies of respectively
-113 and -125 J K-1 mol-1 were obtained, which are of
similar order as the activation entropies observed here (-89.7
to -102.0 J K-1 mol-1, Table 2 and S2). The enthalpy of
activation was reported to be 8 kJ mol-1 for both reactions,
which is smaller than the activation enthalpies computed here,
but might be due to differences in the nucleophiles.
Steric Effects. Leipoldt and co-workers have proposed
that steric effects at the TBP transition state affect the
selectivity of CO substitution, leading to formation of a
different product isomer than expected on electronic
grounds (Scheme 1).12,15,16 Several points can be made on
the basis of the optimized geometries.
The transition states for the first reaction step (TS1)
exhibit relatively small equatorial angles between incoming
PPh3 and leaving Oβ-diketonato ligand (88.8° to 91.2° or ∼90°,
Scheme 4), which might lead to steric interactions if entering
or leaving groups are bulky.53 Leipoldt and co-workers have
suggested that the bulkier β-diketonato substituent prefer-
ably will be placed axially to reduce steric interactions with
the incoming ligand.12,15,16 However, the optimized geome-
tries of TS1 show that the angle between PPh3 and the axially
placed ligands is of similar magnitude (90.8° to 91.5° or
∼91°, Scheme 4), leading to comparable axial steric interac-
tions. The optimized geometries for TS1_A and TS1_B show
steric interactions between the Ph groups of the attacking
PPh3 and both the equatorially and the axially placed
β-diketonato ligand (Supporting Information, Figure S4).
Thus, at TS1 there is no obvious preference for an axial or
equatorial placement of a bulky β-diketonato substituent.
At TS2, the PPh3 group is positioned trans to the leaving
ligand, which results in an equatorial angle between PPh3
and the attacking β-diketonato oxygen of ∼135° (Scheme 4).
The angles between PPh3 and the axial β-diketonato ligands
(51) Serron, S.; Huang, J.; Nolan, S. P. Organometallics 1998, 17,
534–539.
(52) Simanko, W.; Mereiter, K.; Schmid, R.; Kirchner, K.; Trzeciak,
A. M.; Ziolkowski, J. J. Organomet. Chem. 2000, 602, 59–64.
(53) Lin, Z.; Hall, M. B. Inorg. Chem. 1991, 30, 646.
(54) Basolo, F.; Chatt, J.; Gray, H. B.; Pearson, R. G.; Shaw, B. L.
J. Chem. Soc. 1961, 2207.

2454 Organometallics, Vol. 29, No. 11, 2010
Hopmann et al.
Scheme 5. Possible Isomerization Pathways of SQP β-Diketonate Complexes: (I) Isomerization of SQP Intermediate Formed during
the Substitution Reaction, (II) Pseudorotation of Product Complex through Quasi-tetrahedral (THD) TS, (III) Temporary Ligand
Dissociation and Rearrangement of the Three-Coordinated Intermediate, (IV) Coordination of L (PPh3/solvent molecule) and Consecutive
Berry Pseudorotations of TBP Intermediate, (V) Coordination of L and Isomerization of SQP Intermediate through an Interchange
Mechanisma
a R0 = Ph and R = CH2CH3 in this study.

Article
Organometallics, Vol. 29, No. 11, 2010 2455
Figure 6. Isomerization of SQP intermediates (Scheme 5I): (A) Inter_A_(C), (B) TSi_int, (C) Inter_B_(A).
isomerization mechanisms: (I) isomerization of the SQP inter-
mediate formed during the substitution reaction (Scheme 5I),
(II) isomerization of the product through a pseudorotation
mechanism (Scheme 5II),55-58 and (III) product isomerization
through a dissociative pathway (Scheme 5III).57 Additionally,
two solvent-assisted (or PPh3-assisted) pathways can occur in
the presence of coordinating solvent (or excess PPh3), (IV) a
Berry pseudorotation mechanism involving a TBP complex
(Scheme 5IV)59-61 or (V) an interchange mechanism involving
a SQP intermediate (Scheme 5V).
Isomerization in a Low-Polarity Environment. Mechanism
I (Scheme 5I) operates prior to product formation and
involves isomerization of the SQP intermediate formed
during the substitution reaction (Scheme 4B, Figure 5B).
We propose that this intermediate is able to isomerize by
interchanging the two β-diketonato oxygens (Scheme 5I).
The TS for conversion of Inter_A (where the Oβ-diketonato
next to the Ph group is dissociated, Figure 6A) to the
opposite SQP intermediate (where the Oβ-diketonato next to
the ethyl group is dissociated, Figure 6C) has a TBP structure
(Figure 6B). The equatorial plane of the TS is occupied by the
CO and the Oβ-diketonato groups. The computed interconversion
barrier is ΔGq,298K =11.6 kJ mol-1 relative to Inter_A
Figure 7. Pseudotetrahedral TS (TSi_prod) for isomerization
of product isomers (Scheme 5II).
isomerization of the Pt complex.58 We have optimized the
pseudorotation TS leading to interconversion between the
product isomers of [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)]
(Figure7). The TS canbe viewedasa rotation of theCOgroup
around the P-Rh bond to yield the opposite isomer. The
barrier relative to product isomer A is ΔGq,298K = 111.7 kJ
mol-1 (ΔHq,298K = 110.8 kJ mol-1, ΔSq,298K = -2.8 J K-1
mol-1), which is similar to the barrier for isomerization of
[Pt(Cl)(SnCl3)(PH3)2].58
(ΔHq,298K = 0.8 kJ mol-1, ΔSq,298K = -36.2 J K-1 mol-1
and ΔGq,298K = 54.0 kJ mol-1 (ΔHq,298K = 24.2 kJ mol-1
)
,
Mechanism III results in product isomerization through
dissociation of a ligand (Scheme 5III). Three different
ligands might be lost from a product complex: CO, PPh3,
or a β-diketonato oxygen. The computed cost for dissociat-
ing PPh3 is ΔGr298K = 110.1 kJ mol-1 (ΔHr298K = 164.2 kJ
mol-1, ΔSr298K = 172.9 J K-1 mol-1), making formation of
an [Rh(RCOCHCOR0)(CO)] intermediate unlikely but pos-
sible. Attempts to optimize a TS for rearrangement of the
three-coordinated intermediate failed; however, it is clear
ΔSq,298K = -100.1 J K-1 mol-1) relative to the reactant
(Figure 4). Given that the barrier for intermediate isomerization
is 10 to 15 kJ mol-1 lower than TS2 (Table 2), it is likely that a
given SQP intermediate isomerizes before it proceeds through
TS2 to liberate CO, thus giving rise to both product isomers.
Mechanism II involves isomerization of the product complex
through a pseudotetrahedral TS (Scheme 5II). Such a mechan-
ism has been analyzed for SQP palladium ([PhPd(PMe3)2-
OAc], [PhPd(PMe3)2I], [(CH2dCH)Pd(PH3)2Br])55,56 and
platinum ([Pt(Cl)(SnCl3)(PH3)2])58 complexes. Barriers of
ΔGq,298K = 75 to 85 kJ mol-1 (BP86, ΔHq,298K = 77 to 85 kJ
that the barrier would be higher than 110.1 kJ mol-1
Dissociation of CO from the product complex has a signifi-
cantly higher cost of ΔGr298K=147.8 kJ mol-1 (ΔHr298K
.
=
mol-1 55 and 85 kJ mol-1 (B3LYP)56 were obtained for the
)
195.4 kJ mol-1, ΔSr298K = 159.8 J K-1 mol-1) and is
therefore ruled out here. Temporary loss of a β-diketonato
ligand should be more feasible than loss of either CO or
PPh3, in particular as the dissociated ligand stays in close
vicinity, whereas the dissociated CO or PPh3 are likely to
diffuse away from the complex. However, attempts to opti-
mize a three-coordinated intermediate with one unbound
β-diketonato oxygen failed.
Pd complexes and ΔGq,298K = 109 kJ mol-1 (MP2, ΔHq,298K
=
112 kJ mol-1, ΔSq,298K = 8 J K-1 mol-1) for cis-trans
(55) Goossen, L.; Koley, D.; Hermann, H. L.; Thiel, W. Organo-
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(59) Berry, R. S. J. Chem. Phys. 1960, 32, 933.
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(61) Sakaki, S., Mizoe, N., Musashi, Y., Sugimoto, M. J. Mol.
Struct. (THEOCHEM) 1999, 461-462, 533.
Mechanisms IV and V involve coordination of a fifth
ligand to the product complex. If the resulting compound
is trigonal bipyramidal, it could isomerize through two
consecutive Berry pseudorotations (Scheme 5IV). During
the substitution reaction in n-hexane, a possible fifth ligand is

2456 Organometallics, Vol. 29, No. 11, 2010
Hopmann et al.
Figure 8. PPh3-assisted isomerization of product isomer A into isomer B (Scheme 5V): (A) Nucleophilic attack of PPh3 on isomer A
(TS1_PPh3_A), (B) SQP diphosphine intermediate, (C) TS for isomerization (TSi_PPh3), (D) isomerized SQP diphosphine
intermediate, (E) TS for PPh3 release (TS2_PPh3_B).
PPh3. Studies on PH3-catalyzed cis-trans isomerization of
[PtH(SiH3)(PH3)2] through a Berry pseudorotation report a
barrier of 113 kJ mol-1 (MP2, ΔEq).61 We attempted to test
such a mechanism, but optimization of a diphosphine TBP
intermediate failed. Instead, a SQP diphosphine compound
is formed, in which the Oβ-diketonato trans to PPh3 has
dissociated. The result is not surprising, as the substitution
reaction also proceeds through a SQP intermediate rather
than a TBP geometry. The SQP intermediate cannot isomer-
ize through a Berry pseudorotation, but we propose that
isomerization can occur through an interchange mechanism
analogous to mechanism I; see mechanism V, Scheme 5.
The optimized geometries for the PPh3-assisted isomeriza-
tion according to mechanism V are shown in Figure 8, with
the relative energies and thermodynamic parameters given in
Table S4 (Supporting Information). Attack of PPh3 on
[Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] leads to a TBP TS,
with one β-diketonato oxygen and the two PPh3 ligands in
the equatorial plane (Figure 8A). The computed barrier is
ΔGq,298K = 61.3 kJ mol-1. In the formed SQP intermediate,
the β-diketonato oxygen has dissociated (Figure 8B). Nu-
cleophilic attack of the free oxygen leads to a new TBP TS,
where the two β-diketonato oxygens and CO occupy the
equatorial plane (Figure 8C). The barrier for interconversion
is ΔGq,298K = 50.0 kJ mol-1. Dissociation of PPh3 from the
isomerized intermediate has a barrier of ΔGq,298K = 62.2 kJ
mol-1 (Figure 8E). The results indicate that PPh3-mediated
isomerization could be a feasible route to product isomeriza-
tion. Our results are also in agreement with the experimental
III, and V (Scheme 5). The barrier for pseudorotation
(mechanism II) is almost unaffected by inclusion of solvent
effects, resulting in a slight reduction to ΔGq,298K = 109.7 kJ
mol-1. The dissociative mechanism (mechanism III) shows
significantsolventeffects, resulting in a reductionin the cost of
PPh3 dissociation to 62.4 kJ mol-1 (ΔGr298K). In coordinating
solvent, however, the empty coordination site might be occu-
pied, preventing isomerization and recoordination of PPh3.
The PPh3-assisted isomerization (Scheme 5, mechanism V) is
likely to occur during the substitution reaction in n-hexane,
but could also take place in a polar environment when the
formed product complex is redissolved. If the solvent replaces
a PPh3 ligand from one complex, the liberated PPh3 could
catalyze isomerization of other complexes. Such a mechanism
has been suggested for isomerization of [PdX2L2] com-
plexes.57 Calculations of the solvent effect on the PPh3-
assisted isomerization according to mechanism V increase
the rate-limiting step to 80.4 kJ mol-1, which still can be
considered feasible.
In coordinating solvent, a solvent molecule could also
catalyze isomerization (mechanism V, Scheme 5). We have
tested this with CH3CN as coordinating solvent. The reaction
occurs analogously to the PPh3-assisted pathway through
formation of a SQP intermediate (optimized geometries and
energies are given in the Supporting Information, Figure S6
and Table S5). The overall Gibbs free energy barrier for
CH3CN-mediated product isomerization is 80.6 kJ mol-1
(including solvent correction), which is similar to the PPh3-
mediated pathways. However, the energies for mechanism
V might vary depending on size and nucleophilicity of the
coordinating solvent as well as the polarity of the environment.
Summary of Overall Substitution and Isomerization Me-
chanism. The DFT results show that formation of both
isomers A and B of [Rh(RCOCHCOR0)(CO)(PPh3)] occurs
through an interchange mechanism (Scheme 6), analogous to
ꢀ
observation of Trzeciak and Ziolkowski that the presence of
free phosphine molecules causes fast exchange between free
and rhodium-coordinated phosphine.26
Isomerization in a Polar Environment. We have performed
single-point calculations in acetonitrile (ε = 36.64) to estimate
the effect of polar solvents on the energetics of mechanisms II,

Article
Organometallics, Vol. 29, No. 11, 2010 2457
Scheme 6. Overall Mechanism for Formation and Isomerization of [Rh(RCOCHCOR0)(CO)(PPh3)] Complexesa
a L = free PPh3 or a solvent molecule. R0 = Ph and R = CH2CH3 in this study.
the mechanism computed for CO substitution in the sym-
metric [Rh(acac)(CO)2] complex.31 Reversible attack of PPh3
leads to dissociation of a β-diketonato oxygen (TS1_A or
TS1_B), resulting in formation of a SQP intermediate. The
two possible SQP intermediates are able to isomerize through
a trigonal-bipyramidal TS (TSi_int), which has a barrier
comparable to TS1 when R0 = Ph and R = CH2CH3.
Dissociation of CO through either TS2_A or TS2_B results
in formation of the SQP product isomers, isomer A and
isomer B. This step is rate-limiting and irreversible, as CO is
removed from the reaction mixture. The product isomers are
able to interconvert, either through a slow noncatalyzed
pseudorotation pathway (TSi_prod) or through a fast PPh3-
assisted mechanism (TSi_sol). In a coordinating solvent (such
as acetonitrile or chloroform), isomerization might also occur
through a solvent-assisted pathway (TSi_sol).
SQP diphosphine intermediate appears as a feasible route.
The theoretical study further shows that the β-diketonato
ligand does not act as a trans directing ligand, but functions
as a leaving group at the first TS and as an entering group
at the second TS. The substitution selectivity thus cannot
be related directly to the relative trans influence of the
β-diketonato oxygens. Steric interactions in the axial posi-
tion of the TBP transition states appear to be more pro-
nounced than for the equatorial position; that is, placement
of bulky groups should be favored equatorially, not axially
as was proposed by Leipoldt.12,15,16
Experimental Details
Synthesis. [Rh(PhCOCHCOCH2CH3)(CO)2] and [Rh(Ph-
COCHCOCH2CH3)(CO)(PPh3)] were synthesized as described
earlier.23 Crystals suitable for X-ray diffraction were obtained
after recrystallization from acetone.
Conclusions
X-ray Structure Determination. Crystals were mounted on
glass fibers. The X-ray intensity data were measured on a Bruker
X8 Apex II 4K CCD diffractometer area detector system
equipped with a graphite monochromator and Mo KR fine-
Substitution of PPh3 for CO in the SQP β-diketonato
complex [Rh(PhCOCHCOCH2CH3)(CO)2] leads to the
crystallization of two structural isomers of [Rh(PhCOCH-
COCH2CH3)(CO)(PPh3)] in the same crystal lattice. The two
β-diketonato side groups Ph and CH2CH3 exhibit similar
electronic properties, as reflected by their near equal group
electronegativities as well as the similar Rh-CCO bond
lengths (thermodynamic trans influence) in the reactant
[Rh(PhCOCHCOCH2CH3)(CO)2]. The steric size of the
two β-diketonato side groups Ph and CH2CH3 is also
similar, as determined by the contribution of each group to
the solid angle of the (PhCOCHCOCH2CH3)- ligand.
A DFT analysis of the substitution reaction shows that
nucleophilic attack of PPh3 on [Rh(PhCOCHCOCH2CH3)-
(CO)2] results in formation of a SQP intermediate, where one
Rh-Oβ-diketonato bond is broken. Different intermediates are
formed, depending on which Oβ-diketonato dissociates. The
intermediates have the possibility to isomerize, followed by
irreversible CO release to form the product isomers A and B.
Isomerization of the product can occur through different
noncatalyzed and solvent-assisted pathways. Especially a
novel PPh3-mediated isomerization mechanism involving a
˚
focus sealed tube (λ = 0.71073 A) operated at 1.5 kW power
(50 kV, 30 mA). The detector was placed at a distance of 3.75 cm
from the crystal. Crystal temperature during the data collection
for [Rh(PhCOCHCOCH2CH3)(CO)2] was kept constant at
100(2) K using an Oxford 700 series cryostream cooler. Collec-
tion for [Rh(PhCOCHCOCH2CH3)(CO)(PPh3)] was done at
room temperature (293 K). The initial unit cell and data collec-
tion were achieved with Apex2 software62 utilizing COSMO63
for optimum collection of more than a hemisphere of reciprocal
space. A total of 1127 (2473) ([Rh(PhCOCHCOCH2CH3)-
(CO)(PPh3)] parameters given in parentheses) frames were
collected with a scan width ω in j and an exposure time of 10
(20) s frame-1. The frames were integrated using a narrow-
frame integration algorithm and reduced with the Bruker
SAINT-Plus64 and XPREP64 software packages, respectively.
The integration of the data yielded a total of 14 494 (64 974)
reflections to a maximum θ angle of 28.30° (28.37°), of which
(62) Apex2 (Version 1.0-27); Bruker AXS Inc.: Madison, WI, 2005.
(63) COSMO, Version 1.48; Bruker AXS Inc.: Madison, WI, 2003.
(64) SAINT-Plus, Version 7.12 (including XPREP); Bruker AXS Inc.:
Madison, WI, 2004.

2458 Organometallics, Vol. 29, No. 11, 2010
Hopmann et al.
3154 (13 157) were independent with a Rint = 0.0342 (0.0408).
Analysis of the data showed no significant decay during the data
collection. Data were corrected for absorption effects using the
multiscan technique SADABS65 with minimum and maximum
transmission coefficients of 0.7293 and 0.8550 (0.8488 and
0.9641), respectively.
rhodium, where we used LANL2DZ (corresponding to the Los
Alamos Effective Core Potential plus DZ74). Additional calculations
employing an added f-polarization function on rhodium (coefficient
= 0.4 (LANL2DZþf040) or 1.35 (LANL2DZþf135))75,76 yielded
virtually identical geometries and similar energies (see Tables S6 and
S7, Supporting Information).
Structures were solved by the direct methods package
SIR9766 and refined using the WinGX software package67
incorporating SHELXL.68 The final anisotropic full-matrix
least-squares refinement on F2 with 164 (651) variables con-
verged at R1 = 0.0219 (0.0436) for the observed data and wR2
= 0.0543 (0.1055) for all data. The GOF was 1.105 (1.060). The
Frequency calculations were performed on all optimized
structures at the same level of theory as geometry optimizations.
Minima exhibited only positive frequencies, while all transition
states exhibited one negative frequency. Thermochemical data
and temperature corrections were computed at 298.15 K for all
optimized geometries. Additional thermochemistry calculations
at 342 K were performed with the freqchk functionality imple-
mented in Gaussian 03. Single-point calculations in CH3CN
solvent (dielectric constant = 36.64) and CHCl3 (dielectric
constant = 4.9) were performed with the solvent model
IEFPCM.
largest peak on the final difference electron density synthesis was
-3
0.53 e A at 0.88 A from Rh and the deepest hole -0.60 e A-3 at
˚
˚
˚
-3
˚
0.91 A from Rh (1.88 e A at 0.86 A from Rh2 and the deepest
hole -0.79 e A at 0.60 A from Rh2). The aromatic, methylene,
and methyl H atoms were placed in geometrically idealized
˚
˚
-3
˚
˚
˚
positions (C-H = 0.93-0.98 A) and constrained to ride on
their parent atoms with Uiso(H) = 1.2Ueq(C) for aromatic and
methylene and Uiso(H) = 1.5Ueq(C) for methyl. The methyl H’s
were located from a Fourier difference map and refined as a rigid
rotor. Non-hydrogen atoms were refined with anisotropic dis-
placement parameters. Atomic scattering factors were taken
from the International Tables for Crystallography Volume C.69
The molecular plot was drawn using the DIAMOND program70
with a 30% thermal envelope probability for non-hydrogen
atoms. Hydrogen atoms were drawn as arbitrarily sized spheres
Acknowledgment. This work was supported by the
Research Council of Norway and the National Research
Foundation of the Republic of South Africa (Grant
Unique Number 65507).
Supporting Information Available: Crystallographic data in
CIF format, optimized Cartesian coordinates, computed thermo-
chemical data for selected systems, and additional figures. This
material is available free of charge via the Internet at http://pubs.
acs.org.
˚
with a radius of 0.135 A.
The refinement of the two [Rh(PhCOCHCOCH2CH3)(CO)-
(PPh3)] isomers showed large thermal vibrations in the CH2CH3
chain. This was treated by disordered refinement techniques to
obtain more satisfactory refinement parameters. The refinement
was kept stable with additional geometric and anisotropic
restraints. Site occupancies for the terminal carbons in the ethyl
groups of both isomers were left to refine freely but restricted to
add up to one for each methyl, resulting in 0.737:0.263 for C1A
and C1B and 0.664:0.336 for C31A and C31B (Figure 1).
Computational Details. All calculations were performed with
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Gaussian 03 program package.73 Geometries were optimized in the
gas phase with the triple-ζbasis set 6-311G(d,p) on all atoms except
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