Structural trends in [Rh(PhCOCHCO(CH2)nCH 3)(CO)(PPh3)] (n = 0-3) and related complexes: Crystal structure of [Rh(PhCOCHCO(CH2)2CH3)(CO) (PPh3)]

Article abstract of DOI:10.1007/s11243-013-9708-6

The [Rh(PhCOCHCO(CH₂)₂CH₃)(CO)(PPh ₃)], 3, complex was characterized by means of solid-state X-ray crystallography. It was shown by means of statistical averages and histograms that the bond lengths and angles o

Full text of DOI:10.1007/s11243-013-9708-6

Transition Met Chem (2013) 38:429–440
DOI 10.1007/s11243-013-9708-6
Structural trends in [Rh(PhCOCHCO(CH2)nCH3)(CO)(PPh3)]
(n 5 0–3) and related complexes: crystal structure
of [Rh(PhCOCHCO(CH2)2CH3)(CO)(PPh3)]
•
Nomampondomise F. Stuurman Alfred Muller
•
Jeanet Conradie
Received: 14 December 2012 / Accepted: 18 February 2013 / Published online: 9 March 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract The [Rh(PhCOCHCO(CH2)2CH3)(CO)(PPh3)],
3, complex was characterized by means of solid-state X-ray
crystallography. It was shown by means of statistical
averages and histograms that the bond lengths and angles of
3 are in the typical range for [Rh(PhCOCHCO(CH2)nCH3)-
(CO)(PPh3)] (n = 0–3) and [Rh(b-diketonato)(CO)(PPh3)]
complexes. DFT calculations on the preferred orientation of
PPh3 in 3 show that the size of the Ph and (CH2)2CH3 side
groups on the b-diketonato ligand of 3 does not influence the
preferred orientation of PPh3 in this complex. Complexes 1
and 2 (with alkyl chain length of n = 0, 1, respectively)
show p–p stacking in the solid crystalline state, whereas
complexes 3 and 4 (with n = 2, 3, respectively) do not. This
results in different packing arrangements across the series,
ranging from both isomers present (50:50) at independent
positions for complexes 1 and 2 (with n = 0, 1), to one
isomer disordered for complex 3 (with n = 2), and an
89.7:10.3 isomer distribution in the same space in the unit
cell for complex 4 (with n = 3). It is postulated that in the
solid crystalline state, the increasing alkyl chain length
disrupts the supramolecular complementarity between pairs
of the same isomer and that the increasing length of the alkyl
chain starts to interact with the PPh3. The orientation of
PPh3 in complexes 1–4 is in agreement with the preferred
minimum energy orientation of PPh3 in experimental crystal
structures of [Rh(b-diketonato)(CO)(PPh3)] complexes.
Introduction
The catalytic importance of both tertiary phosphines [1, 2]
and rhodium [3] in transition metal complexes is well
documented. Classic examples of rhodium compounds
used as industrial homogeneous catalysts are as follows:
the methanol carbonylation to produce acetic acid in the
presence of cis [Rh(CO)2I2]- (Monsanto process) [4],
alkene hydroformylation by the [RhH(CO)2(PPh3)2] cata-
lyst [5], hydrogenation of olefins and acetylenes with the
help of [RhCl(PPh3)3] (Wilkinson’s catalyst) [6], and the
use of [Rh(CH3COCHCOCH3)(CO)2] in the hydroformyl-
ation of olefins [7]. The catalytic reactivity of the rhodium
complexes is caused to a large degree by the nature of the
ligand surroundings [8]. In our laboratory, numerous
structural [9–12] and kinetic studies [13–18] have been
undertaken on rhodium(I) square planar complexes of the
type Rh(b-diketonato)(CO)(PPh3), in order to investigate
the influence of factors such as nucleophilicity of the metal
centres, solvent interactions and steric bulk on the rate of
oxidative addition, CO insertion and mechanistic elucida-
tion. An analysis of the structure of this specific class of
rhodium complexes versus different ligand characteristics
might lead to a better understanding of the relationship
between activity and catalyst structure, as well as to the
ways of predicting catalytic activity. In this work, we
present the structure of [Rh(PhCOCHCO(CH2)2CH3)-
(CO)(PPh3)] by means of single-crystal X-ray structure
analysis and density functional theory (DFT) calculated
N. F. Stuurman ꢀ J. Conradie (&)
Department of Chemistry, University of the Free State,
P.O. Box 339, Bloemfontein 9300, Republic of South Africa
e-mail: conradj@ufs.ac.za
A. Muller
Department of Chemistry, University of Johannesburg (APK
Campus), P.O. Box 524, Auckland Park, Johannesburg 2006,
Republic of South Africa
J. Conradie
Department of Chemistry and Centre for Theoretical
and Computational Chemistry, University of Tromsø,
¨
9037 Tromso, Norway
123

430
Transition Met Chem (2013) 38:429–440
geometries. The geometrical parameters and the orientation
of PPh3 in this complex 3 and related [Rh(b-diketo-
nato)(CO)(PPh3)] complexes are compared and discussed.
was 1.136. The largest peak on the final difference electron
-3
densitysynthesiswas1.27 e.A at0.91AfromH6A2andthe
˚
˚
-3
deepest hole -0.80 e.A at 1.03 A from H14.
˚
˚
The aromatic, methylene and methyl H atoms were
placed in geometrically idealized positions (C–H =
˚
0.93–0.98 A) and were constrained to ride on their parent
Experimental
atoms with Uiso(H) = 1.2Ueq(C) for aromatic and methy-
lene and Uiso(H) = 1.5Ueq(C) for methyl hydrogen atoms.
The methyl H atoms were located from a Fourier difference
map and refined as a rigid rotor. Non-hydrogen atoms were
refined with anisotropic displacement parameters. The
carbons of the propyl, and part of the b-diketone backbone,
showed large thermal vibrations and were subsequently
treated with disordered refinement over two positions. To
keep the refinement stable, several geometrical restraints
(SAME, FLAT) and ellipsoid restraints (SIMU, DELU)
were employed. Default parameters of the ellipsoid
restraints were adjusted to keep displacements within rea-
sonable limits. Atomic scattering factors were taken from
the International Tables for Crystallography Volume C
[27]. The molecular plot was drawn using the DIAMOND
program [28] with a 50 % thermal envelope probability for
non-hydrogen atoms. Hydrogen atoms were drawn as
Synthesis
Complex 3 was synthesized as described previously [19].
Characterization data: M.p. 135–138 °C. IR (cm-1) =
1,981. 1H NMR (d/ppm, CDCl3) isomer A: 1.0 (3H, t,
CH3), 1.8 (2H, m, CH2), 2.5 (2H, t, CH2), 6.2 (1H, s, CH),
7.0–8.0 (20H, m, aromatic); isomer B: 0.6 (3H, t, CH3), 1.2
(2H, m, CH2), 2.0 (2H, t, CH2), 6.1 (1H, s, CH), 7.0–8.0
(20H, m, aromatic); ratio isomer A : isomer B (in CDCl3
solution at 25 °C) = 1.00:0.77. 31P NMR (d/ppm, CDCl3)
isomer A: 48.80 (1J(31P–103Rh) = 175 Hz), isomer B:
48.56 (1J(31P–103Rh) = 175 Hz). Elemental Anal. Calc. for
RhC31PH28O3: C, 63.9; H, 4.8. Found: C, 63.8; H, 4.6 %.
Crystallography
˚
arbitrary-sized spheres with radius of 0.135 A.
The crystal of 3 was mounted on a glass fibre and used for
the X-ray crystallographic analysis. The X-ray intensity
data were collected on a Bruker X8 Apex II 4 K CCD
diffractometer area detector system, equipped with a
graphite monochromator and Mo Ka fine-focus sealed tube
Computational details
DFT calculations were carried out using the Amsterdam
Density Functional 2012 programme [29] with the gen-
eralized gradient approximation functional PW91 (Per-
dew–Wang 1991) [30, 31]. The TZP (triple-f polarized)
basis set, with a fine mesh for numerical integration, a spin-
restricted formalism and full geometry optimization,
applying tight convergence criteria, was used. All calcu-
lations have consistently been performed without symme-
try constraint (C1), and all structures have been calculated
as singlet states. The computed results assume an ideal gas.
The standard deviation from the experimental average
value, where n values were measured of x, is calculated by
[32] using the following formula:
˚
(k = 0.71073 A), operated at 1.5 kW power (50 kV,
30 mA). The detector was placed at a distance of 3.8 cm
from the crystal. Crystal temperature during the data col-
lection was kept constant at 100(2) K, using an Oxford
700 series cryostream cooler.
The initial unit cell and data collection were achieved by
the Apex2 software [20], utilizing COSMO [21] for optimum
collection of more than a hemisphere of reciprocal space. A
total of 918 frames were collected with a scan width in u and
an exposure time of 60 s frame-1. The frames were integrated
using a narrow-frame integration algorithm and reduced with
the SAINT-Plus [22] and XPREP [22] software packages,
respectively. The integration of the data using a monoclinic
cell yielded a total of 23,159 reflections to a maximum h angle
of 28.43°, of which 6,528 reflections were independent with a
Rint = 0.1082. Analysis of the data showed no significant
decay during the data collection. Data were corrected for
absorption effects using the multi-scan technique SADABS
[23], with minimum and maximum transmission coefficients
of 0.8835 and 0.9852, respectively. The structure was solved
by the direct methods package SIR97 [24] and refined using
the WinGX software package [25] incorporating SHELXL
[26]. The final anisotropic full-matrix least-squares refine-
ment on F2 with 380 parameters converged at R1 = 0.0879
fortheobserveddataandwR2 = 0.1450foralldata. TheGOF
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
2
ðx ꢁ xÞ
n ꢁ 1
Results and discussion
Synthesis
[Rh(PhCOCHCO(CH2)nCH3)(CO)(PPh3)] complexes 1–4
(see Fig. 1 for complex numbering) result from nucleophilic
attack of PPh3 on the dicarbonyl complex [Rh(PhCOCH-
CO(CH2)nCH3)(CO)2]. The two possible isomers of the
123

Transition Met Chem (2013) 38:429–440
431
was performed in hot n-hexane as solvent. Crystals suitable
for single-crystal X-ray diffraction study of the products
were obtained, after recrystallization from acetone. A
molecular diagram of [Rh(PhCOCHCO(CH2)2CH3)
(CO)(PPh3)] showing atom labelling is presented in Fig. 2.
Crystal data and details of data collection and refinement
are given in Table 1. Selected bond lengths and angles for
the molecule are listed in Table 2. The structure crystal-
lizes in the monoclinic system, space group P 21/c, with
R
R
C1 O1
C2
C3 O2
C3 O2
PPh3
CO
K
c
2
Rh
C
Rh
PPh3
CO
C1 O1
Isomer B
SP-4-3
Isomer A
SP-4-2
1 R = CH3
2 R = CH2CH3
3 R = CH2CH2CH3
4 R = CH2CH2CH2CH3 Kc = 0.80
Kc = 0.47
Kc = 0.70
Kc = 0.77
˚
˚
˚
a = 10.0340(9) A, b = 26.771(3) A, c = 9.9180(6) A,
a = 90°, b = 101.746(3)°, c = 90° and Z = 4. Isomer A
of 3 (n = 2) was isolated in the solid state with a 79:21 %
disorder in the –(CH2)2CH3 chain and part of the chelated
b-diketonato backbone (see Fig. 1).
Fig. 1 Structural isomers of [Rh(PhCOCHCO(CH2)nCH3)(CO)(PPh3)]
complexes1–4(n = 0–3),includingthenumberingschemeofatoms.The
equilibrium constant Kc in CDCl3 at 25 °C is given for each complex [19]
The rhodium atom in the main component of 3 has a
square planar coordination sphere. Rh, P, CCO and the b-
diketonato backbone (O1C1C2C3O2) lie in the same plane
product [Rh(PhCOCHCO(CH2)nCH3)(CO)(PPh3)] are
labelled isomer A (IUPAC SP-4-2) [33] and isomer B (IU-
PAC SP-4-3 [33]). In solution, both isomers exist in a fast
equilibrium with each other [19]. The isomer expected to be
predominant due to the larger trans-influence is the isomer
where the PPh3 group is trans to the oxygen nearest to an
electron-donating b-diketonato side group [34, 35]. How-
ever, since the Gordy scale group electronegativities of the b-
diketonato side groups in the [Rh(PhCOCHCO(CH2)nCH3)
(CO)(PPh3)] complexes are very similar (vPh = 2.21 [36],
vCH3 = 2.34 [36], v(CH2)CH3 = 2.31, v(CH2)2CH3 = 2.41
and v(CH2)3CH3 = 2.22 [19]), it is in this case not possible to
predict the predominant isomer on grounds of electronega-
tivity. A similar difficulty also arises in the solid-state
structures for the above compounds: for complexes 1 [37]
and 2 [38], both geometrical isomers crystallized at inde-
pendent positions in the same crystal lattice, that is, at a 50:50
ratio. For complex 4 [39], both geometrical isomers crys-
tallized in the same space in the unit cell at an 89.7:10.3 ratio.
In the case of complex 3 presented in this study, only isomer
A (with partial disorder of the b-diketone) was isolated in the
1
˚
(RMS deviation of fitted atoms = 0.0569 A). The bond
angles in the rhodium polyhedron are within 3° of those
expected 90° for dsp2 hybridization. Deviations of up to 8°
were found in the sp2 hybridized C atoms of the b-diketonato
skeleton, possibly due to strain from chelation with the
metal. The b-diketonato phenyl ring of 3 forms a dihedral
angle of 4.8(6)° with the pseudo aromatic core of the b-
diketonato ligand. The phosphorus atom displays a distorted
tetrahedral geometry and is surrounded by the rhodium atom,
and three carbon atoms of the phenyl rings bound to it. The
bond angles around P differ up to 11° from 109°280, which is
the angle for a regular tetrahedron. The (CH2)2CH3 sub-
stituent on the b-diketonato forms a flat chain, with a rms
deviation of fitted atoms from the plane through C4, C5, C6
˚
˚
and C7, of 0.0205 A for the main component (0.0718 A for
the smaller disordered part).2 This plane is at a dihedral angle
of 61.6(6)° for the major component (54.6(8)° for the dis-
ordered part) to the plane formed by the b-diketonato
backbone (O1C1C2C3O2). The observed orientation of the
(CH2)2CH3 substituent in isomer A can primarily be ascribed
to p-packing interactions of CHꢀꢀꢀO and CHꢀꢀꢀPh, which are
illustrated in Fig. 4b and tabulated in Tables 3 and 4.
´
solid state. According to Trzeciak and Ziolkowski, the
presence of only a single product isomer in the solid state of
[Rh(RCOCHCOR’)(CO)(PPh3)] complexes could be a
consequence of the crystallization conditions [40, 41].
Rh-L bonds and L-Rh-L’ angles in [Rh(b-diketonato)-
(CO)(PR3)] complexes
´
Combined with this explanation by Trzeciak and Ziolkowski
could also be the fact that the size of the alkyl substituent (and
the intermolecular interactions thereof) versus the size of the
phenyl substituent contributes to the final packing arrange-
ment observed. The crystal structure of isomer A of complex
3 is presented in the next section of this study.
Table 5 gives comparative results of selected bond lengths
and angles for isomers A and B, respectively, for the four
1
The deviation of the atoms from the plane through O1C1C2AC3AO2A
˚
are O1 = 0.084(4) A, C1 = 0.040(5) A, C2A = -0.044(7) A,
˚
˚
˚
˚
O2A = 0.031(6) A,
˚
Rh = -0.013(3) A,
C3A = -0.060(7) A,
˚
CCO = -0.089(5) A and P = 0.053(4).
Structure of [Rh(PhCOCHCO(CH2)2CH3)(CO)(PPh3)], 3
2
The deviation of the atoms from the plane through C4C5C6 are
˚
˚
˚
C4A = -0.007(2) A, C5A = 0.007(2) A, C6A = 0.007(2) A; for
The substitution reaction [Rh(PhCOCHCO(CH2)3CH3)(CO)2]
˚
disordered part: C4B = -0.062(20) A, C5B = 0.062(21) A,
˚
˚
C6B = 0.060(20) A.
? PPh3 giving [Rh(PhCOCHCO(CH2)2CH3)(CO)(PPh3)] ? CO
123

432
Transition Met Chem (2013) 38:429–440
Fig. 2 Molecular diagrams of [Rh(PhCOCHCO(CH2)2CH3)(CO)(PPh3)], 3 (n = 2), showing atom labelling: a showing the 79:21 % disorder
with hydrogen atoms removed for clarity, and b the main component of 3
complexes [Rh(PhCOCHCOCH3)(CO)(PPh3)] 1 [37], [Rh-
(PhCOCHCO(CH2)CH3)(CO)(PPh3)] 2 [38], [Rh(PhCO-
CHCO(CH2)2CH3)(CO)(PPh3)] 3 and [Rh(PhCOCHCO-
(CH2)3CH3)(CO)(PPh3)] 4 [39].
Table 1 Crystal data and structure refinement for [Rh(PhCOCH-
CO(CH2)2CH3)(CO)(PPh3)], 3
Empirical formula
C31H28O3P Rh
Formula weight
Temperature
582.41
Using the Cambridge Structural Database (CSD) [42],
we established that structures of 48 [Rh(b-diketo-
nato)(CO)(PR3)] complexes containing a tertiary phos-
phine have been reported in the literature since the first
structure in 1978. So far, only the [Rh(PhCOCH-
CO(CH2)nCH3)(CO)(PPh3)] structures with n = 0, 1 or 3
contain both isomers A and B (Fig. 1). A statistical anal-
ysis of the important bond lengths and angles of these 48
[Rh(b-diketonato)(CO)(PR3)] complexes is presented in
Table 5 and visualized in Fig. 3.
293(2) K
˚
0.71069 A
Wavelength
Crystal system
Space group
Monoclinic
P 21/c
˚
a = 10.0340(9) A
Unit cell dimensions
˚
b = 26.771(3) A
˚
c = 9.9180(6) A
a = 90°
b = 101.746(3)°
c = 90°
All bond lengths and angles of complexes 1–4 of this
study are in the typical range for [Rh(b-diketonato)(CO)
(PR3)] compounds. Data of the 48 [Rh(b-diketonato)(CO)
(PR3)] crystal structures gave average M–L bonds Rh–
3
˚
Volume
2608.4(4) A
Z
4
Density (calculated)
Absorption coefficient
F(000)
1.483 Mg/m3
0.747 mm-1
1,192
˚
O1 = 2.072(13) A, Rh–O2 = 2.039(13) A, Rh–P = 2.220
˚
˚
˚
(38) A, and Rh–C = 1.798(37) A. Selected average angles
are C1–C2–C3 = 126.1(1.8)°, O1–Rh–O2 = 88.5(9)° and
P–Rh–C = 89.4(2.2)°. A visual presentation of the varia-
tion in the Rh–L bonds in [Rh(b-diketonato)(CO)(PR3)]
complexes is presented in Fig. 3.
Crystal size
0.17 9 0.10 9 0.02 mm3
Theta range for data collection
Index ranges
1.52–28.43°.
-9 B h B 13,
-35 B k B 35,
-13 B l B 13
˚
The difference in the average Rh–O1 = 2.072(13) A
Reflections collected
23,159
˚
(trans to PPh3) and Rh–O2 = 2.030(13) A (trans to CO,
Independent reflections
6,528 [R(int) = 0.1082]
99.2 %
atom numbering according to Fig. 1), Rh–O bond for these
48 [Rh(b-diketonato)(CO)(PR3)] crystal structures, indi-
cates that the tertiary phosphine group has a larger trans-
influence than the CO-group in [Rh(b-diketonato)(CO)
(PR3)] compounds, leading to the longer Rh–O bond trans
to the PPh3. These values are very similar to the average
values for the 5 [Rh(b-diketonato)(CO)(PPh3)] complexes,
containing a symmetrical b-diketonato ligand, namely Rh–
Completeness to theta = 28.43°
Maximum and minimum
Transmission
Semi-empirical from
equivalents
Refinement method
0.9852 and 0.8835
6528/201/380
Data/restraints/parameters
Goodness-of-fit on F2
1.136
Final R indices [I [ 2 sigma(I)]
R indices (all data)
R1 = 0.0879, wR2 = 0.1450
R1 = 0.1387, wR2 = 0.1631
˚
O1 = 2.072(15) A (trans to PPh3) and Rh–O2 = 2.031
˚
(9) A (trans to CO). This illustrates that the electron-
donating properties of the R and R’ substituents on the
-3
˚
1.362 and -1.067 e.A
Largest diff. peak and hole
123

Transition Met Chem (2013) 38:429–440
433
˚
Table 2 Selected bond lengths (A) and angles (°) for
[Rh(PhCOCHCO(CH2)2CH3)(CO)(PPh3)], 3
Table 4 Hydrogen bonds for [Rh(PhCOCHCO(CH2)2CH3)(CO)
(PPh3)], 3
˚
˚
˚
Atoms
Bond
Atoms
Angle
C–HꢀꢀꢀO
d(C–H)/A d(HꢀꢀꢀO)/A d(CꢀꢀꢀO)/A (CHO)/°
Rh–C
1.793(7)
1.94(3)
C–Rh–O2B
C–Rh–O1
163.2(11)
91.0(2)
C(13)–H(13)ꢀꢀꢀOi 0.95
2.56
2.54
3.415(9)
3.35(1)
150.6
144.3
Rh–O2B
Rh–O1
Rh–O2A
Rh–P
C(26)–
H(26)ꢀꢀꢀO(2A)
C(46)–
0.95
2.062(4)
2.064(7)
2.2414(18)
1.822(6)
1.826(7)
1.829(7)
1.276(8)
1.147(8)
1.398(17)
1.399(10)
1.501(9)
1.281(9)
O2B–Rh–O1
C–Rh–O2A
O1–Rh–O2A
C–Rh–P
91.3(8)
0.95
2.37
2.708(9)
100.6
178.4(3)
87.9(2)
H(46)ꢀꢀꢀO(1)
Symmetry transformations used to generate equivalent atoms:
i = 1 - x, -y, 2 - z
P–C31
92.5(2)
P–C11
O2B–Rh–P
O1–Rh–P
86.5(8)
P–C21
174.55(13)
88.7(2)
O1–C1
C–O
O2A–Rh–P
C3A–C2A–C1
C3B–C2B–C1
123.8(8)
128(2)
reveals p–p stacking of the conjugated chelated b-diketone
backbone for n = 1, 2 (see Table 6, with p–p stacking
parameters of complexes 1 and 2). It is also further noted
that in both of the above cases, the p–p stacking occurs
between pairs of similar isomers in a flipped head-to-head
arrangement (Fig. 4a). This arrangement can be regarded
as supramolecular interactional complementarity of each
isomer with itself, thus favouring the observed crystalli-
zation for these two structures.
C1–C2B
C1–C2A
C2A–C3A
C3A–O2A
b-diketonato ligand RCOCHCOR’ do not have an obser-
vable effect on the Rh–O bond length nearest to it.
For the structures of n = 2–3, the situation changes as
we no longer observe p–p stacking of the b-diketone
backbone, albeit the C–HꢀꢀꢀO and C–Hꢀꢀꢀp interactions,
which are observed in all structures from the series. In the
case of the (CH2)2CH3, it seems that the propyl substituent
is interfering with the previously observed p–p stacking,
resulting in the crystallization of only one of the isomers. It
also might be argued that interaction with the bulky PPh3
ligand becomes more pronounced as the (CH2)nCH3 chain
length increases; however, it appears that in the case of the
butyl substituent (complex 4), the cone angle value shows
no effect from the alkyl chain substituent of the b-diketone.
It should be noted, though, that the PPh3 ligand is free to
rotate, thus enabling it to optimally adjust its orientation,
accommodating its substituents to occupy as much space as
possible (while still maintaining the preferred orientation
of the Cipso–P bonds). This orientation adjustment can be
clearly seen from the torsion angles h of the phenyl ring B
nearest to CO that show a distinct region larger than 40° for
complexes 1 and 2, while complexes 3 and 4 have values
less than 20° for the phenyl ring C nearest to CO (see
values in Table 5). These small torsion angle values show
the steric demand from the increasing alkyl chain, which
thus deviates from the trend. It should also be noted that in
the case of complex 4, the space occupied by the phenyl
and butyl substituents on the b-diketone is virtually similar
and seemingly indistinguishable during the crystallization
process, hence the crystallization of the two isomers
occurring in the same space in the unit cell.
Packing of complexes 1–4
As mentioned previously, the complexes of [Rh(b-dike-
tone)(CO)(PPh3)] n = 0–3 crystallize with different ratios
of the two isomers in the unit cell. These differences in
crystallization can be ascribed to several parameters [41],
amongst these the packing arrangements induced by the
varying alkyl chain length of one of the b-diketone sub-
stituents. The following trends could be identified within
the series of complexes 1–4, with varying alkyl chain
length of n = 0–3. For n = 0 and 1, both isomers were
found independently inside the unit cell, at a 50:50 ratio,
whereas n = 2 showed only one isomer, while n = 3 dis-
played a 89.7:10.3 distribution of isomers crystallizing
together in the same position. Analysis of the interactions
found in the crystal lattice, using PLATON [44, 45],
Table 3 Analysis of X–HꢀꢀꢀCg (Ph ring) interactions for
[Rh(PhCOCHCO(CH2)2CH3)(CO)(PPh3)], 3
X–H
Cg(J)a HꢀꢀꢀCg(J) H-Perp HꢀꢀꢀCg(J) X–HꢀꢀꢀCg
C5A–H5A1 Cg3
C36–H36
Cg1i
2.91
2.79
2.64
3.00
2.83
2.73
2.37
2.92
3.894(16) 174
3.607(8)
3.60(4)
3.59(4)
144
163
120
C5B–H5B1 Cg3
C6B–H6B1 Cg2ii
a
Cg(J) refers to the centroid of the following Ph rings: Cg1 = C11–
C16; Cg2 = C21–C26; Cg3 = C31–C36. Symmetry codes: i = 1-x,
-y, 1-z, ii = x, y, -1 ? z
It can be postulated that the absence of isomer co-
crystallization in complex 3 is due to two linked factors:
123

434
Transition Met Chem (2013) 38:429–440
123

Transition Met Chem (2013) 38:429–440
435
12
10
8
12
10
8
6
6
4
4
2
2
0
0
d(Rh-O1)/Å
d(Rh-O2)/Å
16
14
12
10
8
18
16
14
12
10
8
6
6
4
4
2
2
0
0
d(Rh-P)/Å
d(Rh-C) / Å
Fig. 3 Histograms of the Rh–L bonds in [Rh(b-diketonato)(CO)(PR3)] complexes. Atom numbering corresponds to Fig. 1
(b) the propyl alkyl chain in complex 3 is also long
enough (compared to 1 and 2) to disrupt p stacking
between pairs of isomers. This disruption is also
observed for complex 4, but due to the steric
complementarity of the butyl and phenyl groups, the
co-crystallization is partially achieved.
Table 6 p–p stacking geometrical parameters for [Rh(PhCOCH-
CO(CH2)nCH3)(CO)(PPh3)], complexes 1 and 2, with alkyl chain
length of n = 0–1
Complex
Ct–Ct distance
Perpendicular distance
Slippage
1
Isomer B
Isomer A
2
3.6278
4.0740
3.5941
3.6533
0.493
1.803
The disruptive nature of the alkyl chain is indeed pres-
ent, as it is noted that the [Rh(PhCOCHCOPh)(CO)(PPh3)]
complex [46] also shows the p–p stacking as observed for
complexes 1 and 2.
Isomer B
Isomer A
3.6404
4.3219
3.4461
3.6481
1.174
2.317
PPh3 orientation
(a) due to the smaller volume occupied by the propyl
alkyl chain in 3, there is no steric complementarity
between butyl and Ph, as found in 4. Hence, complex
3 cannot instigate co-crystallization in the same space
as compared to 4; and
Background on PPh3 orientation
PPh3 (both in the free molecule, as well as coordinated
PPh3) exists in two different helicities, having the phenyl
substituents oriented in a clockwise or counterclockwise
Fig. 4 Partial packing diagrams
of complexes 1, 3, and 4,
showing a a representative
example of the p–p stacking
present in complexes in 1 and 2,
and b–c the disappearance
thereof observed in 3 and 4
123

436
Transition Met Chem (2013) 38:429–440
fashion (referred to as P and M helicity, respectively), as
illustrated in Fig. 5a [47–49]. In the solid state, PPh3-
containing complexes generally exist as a racemic mixture
consisting of units from each helicity in the same unit cell,
while in solution, inversion of the helicity occurs readily
[50]. The orientation of complex-bound PPh3 can be
described in terms of the dihedral (torsion) angle x = M–
P–Ci–Co (Ci=Cipso and Co=Cortho of the phenyl ring Ph) and
the dihedral angle h = L–M–P–Ci. The three phenyl rings
attached to P will be referred to as PhA, PhB and PhC, see
Fig. 5b.
phenyl ring C oriented towards the side of Ob (if viewed
along the P-M axis); and Group 2 with phenyl ring A tilted
towards Ob and with phenyl ring C oriented towards the
side of CO (if viewed along the P–M axis). xA and xC for
both groups are similar, while xB generally is slightly
larger for Group 2 than for Group 1: For Group 1, xA
(experimental -33 7°) B xB (experimental -38
5°) \ xC (experimental -62 5°), while for Group 2, xA
(experimental -33 10°) \ xB (experimental -48
4°) \ xC (experimental -63 5°).
It has been found that the lowest energy conformation of
PPh3 in M-PPh3 complexes lies in a broad potential energy
well, and an equally broad range of energetically acceptable
phenyl ring orientations is accessible via independent P–C
bond rotations. Relaxation (via M–P (h) and P–Ci bond
rotations (x)) in PPh3-containing complexes occurs in order
to minimize both inter-ring–ligand and inter-ring–ring
interactions. In spite of the wide range of low-energy ori-
entations, the orientation of complex-bound PPh3 possesses
some distinct features that can be related to the plane
incorporating all points of minimum steric compression,
called the plane of nadir energy [47, 51]. The favoured
degenerate conformations of complex-bound PPh3 in square
planar [(PPh3)(ML1L2L3)], [M(acac)(L0)(PPh3)], [M(acac)
(L0)(DPP)] (DPP = diphenyl-2-pyridyl phosphine) and
[Rh(b-diketonato)(L0)(PPh3)] complexes, relative to the
plane of nadir energy, are illustrated in Fig. 5c [39, 49, 52].
PPh3 orientation in different isomers of the same complex
(complexes 1, 2 and 4)
Another remarkable observation is that the PPh3 ligands of
the two isomers of [Rh(PhCOCHCOCH3)(CO)(PPh3)], 1,
adopt similar spatial orientations, despite different weak
interactions with the phenyl substituents, observed for these
two isomers in the solid state [37]. The Rh(CCOCH-
COC)(CO)(PPh3) moieties of these two isomers can be
superimposed [53–55] showing remarkable similarities (see
Fig. 7a for the graphical representation of this). The same
observation is true for the two isomers of [Rh(PhCOCH-
CO(CH2)CH3)(CO)(PPh3)], 2 [38] (see Fig. 7b), and also of
[Rh(PhCOCHCO(CH2)3CH3)(CO)(PPh3)], 4 (see Fig. 7c).
This observation illustrates that the size of the side groups R
and R’ on the b-diketonato ligand (RCOCHCOR’)- does not
influence the preferred conformation of coordinated PPh3 in
[Rh(b-diketonato)(CO)(PPh3)] complexes.
PPh3 orientation in experimental structures
DFT calculations on PPh3 orientation
A summary of the orientation of the Ph rings of PPh3 in the
14 available [Rh(b-diketonato)(CO)(PPh3)] complexes [42]
is given in Table 7, whereas the PPh3 orientation of
[Rh(PhCOCHCO(CH2)nCH3)(CO)(PPh3)] complexes 1–4
is shown in Fig. 6. The experimental [Rh(b-diketo-
nato)(CO)(PPh3)] crystal structures can be divided into two
groups: Group 1 with phenyl ring A tilted towards CO, and
Figure 8 gives the calculated PES of 3, as a function of
dihedral (torsion) angles x (rotation of phenyl ring A, PhA,
about the P–CPh bond) and h (rotation of PPh3 about the
M–P bond). The PES is obtained by rotation of the PPh3
group through 360° and the simultaneous rotation of one of
the phenyl groups (PhA) through 180°, at each point. Every
(a)
(b)
(c)
A
A
H
A
A
ω
P
C
P
PP
L'
C
L3M
L
P
C
θ
PhB
B
B
PhC
B
P
M
Fig. 5 a The enantiomeric P (clockwise) and M (anti-clockwise)
configurations of PPh3, illustrated by viewing along the rnb–P axis
[49]. b Definition of dihedral (torsion) angles x (rotation of a phenyl
ring about P–CPh bond) and h (rotation of PPh3 about M–P bond). The
rings are numbered A, B, and C. The C and H on the rings will be
referred to as ipso, ortho, meta, and para.
c
The preferred
conformation minimum energy orientation of PPh3 (as viewed along
the P–M bond axis), within square planar complexes relative to the
plane of nadir energy (plane perpendicular to paper indicated by the
dotted blue lines). L is more bulky than L0 [49]
123

Transition Met Chem (2013) 38:429–440
437
Table 7 Crystallographic data for type [Rh(b-diketonato)(CO)(PPh3)] complexes
R
R’
xA
xB
xC
GROUP 1 : PhA tilted towards CO
1 ZAWQAP 1a
1 ZAWQAP 2b
2 OJEWOQ 1a
2 OJEWOQ 2b
ACRHCP
Ph
CH3
Ph
-27.2
-24.1
-26.4
-31.3
-40.2
-28.9
-41.8
-43.1
-37.0
-33
-30.5
-36.8
-28.0
-36.1
-40.6
-44.4
-41.2
-41.5
-40.8
-38
-68.2
-59.9
-71.7
-59.0
-58.1
-59.0
-56.9
-59.8
-64.8
-62
CH3
CH2CH3
Ph
Ph
CH2CH3
CH3
C4H3S
CH3
CF3
CH3
TFAPRH
CF3
ACRHCP01
CH3
FODJEN
CH2CH3
CH(CH3)2
CASPER
CF3
Experimental average
7
5
5
GROUP 2 : PhA tilted towards Ob-diketonato
3
Ph
(CH2)2CH3
-38.9
-31.5
-23.4
-30.5
-19.0
-46.8
-43.5
-52.0
-46.9
-42.6
-47.5
-55.7
-44.9
-48.3
-48
-60.7
-60.1
-72.6
-64.0
-63.9
-63.0
-59.0
-63
4 isomer A
4 isomer B
DUWHIM
KUNBIE
ROGVEO 1c
ROGVEO 2c
Experimental average
Ph
(CH2)3CH3
(CH2)3CH3
Ph
C(CH3)3
Fc
CF3
CF3
Ph
Ph
Ph
Ph
-33 10
4
5
R and R’ are the side groups on the b-diketonato ligand RCOCHCOR’. CSD reference codes are used [42]
a
Isomer A
b
Isomer B
c
Two crystals in same unit cell
Fig. 6 Experimental geometry showing the PPh3 orientation of [Rh(PhCOCHCO(CH2)nCH3)(CO)(PPh3)] complexes 1–4 (P helicities), as
viewed along the P-Rh axis. For comparative reasons, P helicities are shown
123

438
Transition Met Chem (2013) 38:429–440
˚
˚
Fig. 7 An overlay of the Rh(CCOCHCOC)(CO)(PPh3) moieties of
the two isomers of a [Rh(PhCOCHCOCH3)(CO)(PPh3)], 1 (RMSD =
˚
0.0898 A, Max deviation = 0.1634 A), b [Rh(PhCOCHCOCH2CH3)
(CO)(PPh3)], 2 (RMSD = 0.0701 A, Max deviation = 0.2189 A),
and [Rh(PhCOCHCO(CH2)3CH3)(CO)(PPh3)], (RMSD =
0.2210 A, Max deviation = 0.4008 A)
c
4
˚
˚
˚
point on the graph thus presents a conformation of 3, with
two dihedrals related to one phenyl ring (PhA) restricted
and the rest of the molecule fully optimized. The other
two phenyl rings (PhB and PhC) are thus free to rotate, in
order to minimize steric interaction towards lower elec-
tronic energy geometry. The PES results in 10 areas of
minimum energy (marked i to x in Fig. 8). Careful
examination of the geometries of these ten minimum
energy structures results in only four different confor-
mations of complex 3, of which the P helicity confor-
mations are illustrated in Fig. 9. The DFT calculated
relative energies of the four conformations of P helicity
given in Fig. 9 are as follows:
The four P helicity conformations all have a vertical
phenyl ring (PhA), with Co as near as possible to the nadir
energy plane, perpendicular on the square plane through P,
CCO and the two b-diketonato oxygen atoms, but they
differ with respect to the direction in which the vertical
ring PhA is tilted, that is, either towards Ob-diketonato or
towards CO. Phenyl ring B (below right) is orientated with
Ho as near as possible to nadir plane below SQP, and
phenyl ring C with larger dihedral folds is orientated over
Ob-diketonato or towards CO, with its P–Ci bond near-parallel
to the square plane.
DFT calculations in this study showed no preference for
the conformation of complex 3 where the vertical phenyl
ring A is tilted towards Ob-diketonato (Group 1) over the
conformation of 3 where the vertical phenyl ring A is tilted
towards CO (Group 2). The steric influence of Ob-diketonato
ðaÞ ð0 kJ/molÞ\ðdÞ ð1:9 kJ/molÞ\ðcÞ ð2:1 kJ/molÞ
\ðbÞ ð3:1 kJ/molÞ:
Fig. 8 2-D and 3-D potential energy surface (PES) of [Rh(PhCOCH-
CO(CH2)2CH3)(CO)(PPh3)], 3, representing the relative energy of
different conformations of PPh3 in 3, as a function of x (rotation of a
phenyl ring about the P–CPh bond) and h (rotation of PPh3 about the
Rh–P bond). The dark blue areas present the lowest energy
geometries (conformations). (1 a.u. = 2625.5 kJ mol-1). The
geometry of the complexes relating to the minimum energy areas
marked i–iii is presented in Fig. 9a; the geometry for the minimum
energy areas viii and ix is presented in Fig. 9b; geometry for the
minimum energy areas vi and vii is presented in Fig. 9c; and the
geometry for the minimum energy areas iv, v, and x is presented in
Fig. 9d
123

Transition Met Chem (2013) 38:429–440
439
Fig. 9 The DFT calculated minimum energy conformations a–d of
[Rh(PhCOCHCO(CH2)2CH3)(CO)(PPh3)], 3, as viewed along the
P-Rh axis. In a and c, the vertical phenyl ring A is tilted towards CO,
a Presents the conformation of the minimum energy areas marked i,
ii, and iii; b the conformation of areas viii and ix; c conformation of
areas vi and vii; and d the conformation of areas iv, v and x, of the
PES in Fig. 8. (Only P (clockwise) configurations shown)
and in b and d, the vertical phenyl ring is tilted towards Ob-diketonato
.
and CO on the preferred orientation of PPh3 in 3 is thus
similar. The DFT calculations further show that the size of
the Ph and (CH2)2CH3 side groups on the b-diketonato
ligand PhCOCHCO(CH2)2CH3 does not influence the
preferred orientation of PPh3 in this complex. This result is
in agreement with the experimental [Rh(b-diketo-
nato)(CO)(PPh3)] structures (Table 7). The direction of
orientation of the vertical phenyl ring A of PPh3 in
experimental [Rh(b-diketonato)(CO)(PPh3)] structures is
not influenced by the bulkiness of the R and R’ side groups
on the b-diketonato ligand RCOCHCOR’ and showed no
preference to be orientated towards Ob-diketonato over CO.
both isomers A and B of [Rh(b-diketonato)(CO)(PR3)]
complexes generally exist in solution, it was only possible
in three cases to structurally characterize both isomers in
the solid state, viz for [Rh(PhCOCHCO(CH2)nCH3)-
(CO)(PPh3)], where n = 0, 1 or 3. For complex 3 (with
n = 2 in the latter series), only isomer A crystallized in the
solid state. It might be the case that the crystallized com-
ponent of the second isomer is too small to be detected
from the residual electron density map. It is postulated that
the size of the (CH2)2CH3-alkyl chain disrupts the observed
packing between pairs of similar isomers, which favoured
the observed crystallization of both isomers in a 50:50 ratio
in the unit cell. The role of crystallization conditions versus
packing in the solid state will be investigated by us in the
future.
Conclusions
The structural parameters of [Rh(PhCOCHCO(CH2)2CH3)-
(CO)(PPh3)], 3, and the orientation of PPh3 in 3 are typical
for [Rh(b-diketonato)(CO)(PR3)] complexes. Although
Acknowledgments Financial assistance by the South African
National Research Foundation and the Central Research Fund of the
University of the Free State is gratefully acknowledged.
123

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Transition Met Chem (2013) 38:429–440
27. International Tables for Crystallography, Kluwer Academic
Publishers, Dordrecht, The Netherlands, 1002, Vol C
28. Brandenburg K, Putz H (2005) DIAMOND, Release 31a Crystal
Impact GbR, Bonn, Germany
29. ADF2012.01 SCM Theoretical Chemistry, Vrije Universiteit,
Amsterdam, The Netherlands
Appendix
Cif file of crystal structure is deposited at the Cambridge
Crystallographic Data Centre: deposition number CCDC
915813.
30. Perdew JP, Chevary JA, Vosko SH, Jackson KA, Pederson MR,
Singh DJ, Fiolhais C (1992) Phys Rev B46:6671
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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
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