Characterization of acetylacetonato carbonyl diphenyl-2-pyridylphosphine rhodium(I): Comparison with other carbonyl complexes

Article abstract of DOI:10.1016/j.molstruc.2013.01.061

Different rhodium(I)/(III) diphenyl-2-pyridylphosphine complexes were isolated and successfully characterized. The [Rh(acac)(CO)(DPP)] (DPP = diphenyl-2-pyridylphosphine) complex crystallizes in the P1? space group with four molecules per unit cell. The results clearly show that the differences between the two independent molecules are mainly centered around the orientation of the pyridyl ring within the two square planer molecules. The results also indicate that the phosphine ligands act as monodentate ligands in both molecules, with Rh-P and Rh-CO bond distances of 2.243(1); 2.235(1) and 1.791(4); 1.776(4) ? respectively. A comparison of the ν(CO) stretching frequencies of a relatively large number of rhodium complexes indicated little overlap between the ν(CO) of different types of complexes (e.g. Rh(I) vs Rh(III)) and relatively small standard deviations within each type of complex. DFT calculations were used to determine the preferred pyridyl ring orientation. These calculations indicated that at least 12 areas of minimum energy, which exists as broad, low energy wells, are theoretically suitable for DPP group orientation within this kind of structure.

Full text of DOI:10.1016/j.molstruc.2013.01.061

Journal of Molecular Structure 1038 (2013) 220–229
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Characterization of acetylacetonato carbonyl diphenyl-2-pyridylphosphine
rhodium(I): Comparison with other carbonyl complexes
⇑
⇑
Walter Purcell , Jeanet Conradie , Trevor T. Chiweshe, Johan A. Venter, Hendrik G. Visser,
Michael P. Coetzee
Department of Chemistry, University of the Free State, 205 Nelson Mandela Drive, Bloemfontein 9300, South Africa
h i g h l i g h t s
"
"
"
"
Rh(I)/Rh(III) complexes exhibit n(CO) stretching frequencies in narrow IR range.
Two independent [Rh(acac)(CO)(DPP)] molecules exit due to the difference in pyridyl ring orientations.
DFT calculations indicate 12 areas of minimum energy for the DPP orientation in [Rh(acac)(CO)(DPP)].
DFT calculations corroborate the crystal structure observations.
a r t i c l e i n f o
a b s t r a c t
Article history:
Different rhodium(I)/(III) diphenyl-2-pyridylphosphine complexes were isolated and successfully charac-
terized. The [Rh(acac)(CO)(DPP)] (DPP = diphenyl-2-pyridylphosphine) complex crystallizes in the P1
space group with four molecules per unit cell. The results clearly show that the differences between
the two independent molecules are mainly centered around the orientation of the pyridyl ring within
the two square planer molecules. The results also indicate that the phosphine ligands act as monodentate
ligands in both molecules, with Rh–P and Rh–CO bond distances of 2.243(1); 2.235(1) and 1.791(4);
ꢀ
Received 22 October 2012
Received in revised form 22 January 2013
Accepted 24 January 2013
Available online 1 February 2013
Keywords:
Rhodium(I)
1.776(4) Å respectively. A comparison of the
m(CO) stretching frequencies of a relatively large number
of rhodium complexes indicated little overlap between the
m
(CO) of different types of complexes (e.g.
Diphenyl-2-pyridylphosphine
Carbonyl
Oxidative addition
Methyl iodide
Donor ligands
Rh(I) vs Rh(III)) and relatively small standard deviations within each type of complex. DFT calculations
were used to determine the preferred pyridyl ring orientation. These calculations indicated that at least
12 areas of minimum energy, which exists as broad, low energy wells, are theoretically suitable for DPP
group orientation within this kind of structure.
Ó 2013 Published by Elsevier B.V.
1. Introduction
on these kinds of catalytic processes and cycles is the greater
understanding of the relationships between activity and catalyst
Organometallic complexes of rhodium have attracted a lot of
attention in the last few decades, mainly due to the metal’s
scope and versatility as homogeneous and heterogeneous cata-
lysts in a large variety of industrial processes [1–4]. These pro-
cesses include the production of acetic acid via the Monsanto
process [5,6] and the commercial production of L-dopa [7] for
the treatment of Parkinson’s disease. Recent interest in the metal
was mainly driven by the discovery that complexes such as
structure, as well as ways to better predict, understand and con-
trol catalyst molecular architecture.
The conversion of methanol to acetic acid such as in the
Monsanto process using [Rh(CO)2I2]ꢀ as catalyst [10] involves six
different steps, including oxidative addition, 1,1-insertion or CO
insertion, CO association and reductive elimination. A group of
[Rh(LL)(CO)(BX3)] complexes (LL = mono ionic bidentate ligands
and BX3 = different phosphines, phosphites, arsines and stibines)
came to our attention as possible candidates for catalysis or model
complexes. The ability of these complexes to undergo oxidative
addition at relatively mild conditions as well as the possibility to
change the steric bulk and electronic properties of the complexes
with ease, made them ideal candidates to investigate the effect
of these changes on the rate of oxidative addition reactions.
Numerous structural and kinetic studies [11] were undertaken
to elucidate mechanistically the influences of factors such as
Rh2(l-O2CCF3)4 [8] and Rh2(l-HNCOCF3)4 [9] exhibit carcino-
static activity. Developing new, more effective catalysts depend
on the understanding of the mechanism of action of rhodium
complexes in existing catalytic processes. The key to research
⇑
Corresponding authors. Tel.: +27 (0)514012200; fax: +27 (0)865255382.
E-mail addresses: purcellw@ufs.ac.za (W. Purcell), conradj@ufs.ac.za (J. Con-
radie).
0022-2860/$ - see front matter Ó 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.molstruc.2013.01.061

W. Purcell et al. / Journal of Molecular Structure 1038 (2013) 220–229
221
Table 1
Carbonyl stretching frequencies for different rhodium complexes.
Table 1 (continued)
Complex
LL
PX3
m
(CO)
Refs.
(cmꢀ1
)
Complex
LL
PX3
m
(CO)
Refs.
(cmꢀ1
)
PCy3
P(p-
MeOC6H4)3
2049
2045
[26,32]
[26,32]
[Rh(LL)(CO)2]
acac
cupf
Ox
2063,
1998
2087,
2013
2065,
1992
2080,
2005
2074,
2002
2080,
1998
2088,
2024
2086,
2026
2082,
2008
2074,
2008
2068,
2014
2074,
1998
2074,
2008
2065,
2008
2084,
2014
[24,25]
[26]
neocupf PPh3
dmavk
fctfa
2048
2070
2056
[32]
[37]
[30]
PPh3
PPh3
[27]
[Rh(LL)(COCH3)(PX3)I]b
acac
cupf
dmavk
macsm
stsc
DPP
DPP
PPh3
PPh3
PCy3
1721
1692
1716
1712
1710
[15,33]
[33]
[37]
[38]
[38]
Sox
[27]
tmhd
dbm
btfac
tfac
[28]
[11,28]
[29]
cupf
P(OCH2)3CCH3 1704
[32]
[Rh(LL)(COCH3)(NSO2)(PPh3)]b macsm
PMe2Ph
1716
1720
1709
1704
1701
1713
1698
[39]
[40]
[41]
[42]
[42]
[41]
[43]
[Rh(LL)3(COCH3)(Cl)]+b
[Rh(LL)(COCH3)I2]b
[Rh(LLl)(COCH3)I2]b
[Rh(LL)(COCH3)I2]b
[Rh(LL)(COCH3)I2]b
[Rh(LL)(COCH3)I2]b
PMe2Ph
dppm
dppmo
dppms
dppe
–
–
–
–
–
–
[11,28]
[11,28]
[30]
hfac
fctfa
fca
dppp
Hacac = 2,4-pentanedione
(acetylacetone);
Hba = 1-phenyl-1,3-butanedione;
[30]
Hbfcm = 1-ferrocenyl-3-phenylpropane-1,3-dione; Hdbm = 1,3-diphenyl-1,3-propan-
edione; Hdfcm = 1,3-diferrocenylpropane-1,3-dione; Hfca = 1-ferrocenylbutane-1,3-
dione; Hfctca = 1-ferrocenyl-4,4,4-trichlorobutane-1,3-dione; Hfctfa = 1-ferrocenyl-
4,4,4-trifluorobutane-1,3-dione; Hhfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione;
Htfa = 1,1,1-trifluoro-2,4-pentanedione; Hbtfb = 1,1,1-trifluoro-4-phenyl-2,4-butane-
dione, Htmdh = tetramethylheptanedione, trifluorobenzoylacetone; Htta = thenoyl-
trifluoroacetone; Hbap = 1-phenylpentane-1,3-dione; Hbab = 1-phenylhexane-1,
3-dione; Hneocupf = N-nitroso-N-naphthylhydroxylamine; Hsacac = thioacetylace-
tone; Hdmavk = dimethylaminovinylketone; Hmacsm = methyl(2-methyl-amino-
1-cyclopentene-1-dithiocarboxylate); Hstsc = salicylaldehydethiosemicarbazose;
Htavk = 2-aminovinyl-5,5,5-trifluoro 4 pentanone; Hcupf = N-hydroxy-N-nitroso-
benzeneamine; HOx = 8-hydroxyquinoline; HSox = salicylaldoxide; Hdmh = dib-
enzoylmethane dppm = Ph2PCH2PPh2; dppmo = Ph2PCH2P(O)Ph2; dppms = Ph2-
bfcm
dfcm
bap
[30]
[30]
[31]
bab
[36]
[Rh(LL)(CO)(PX3)]
acac
PPh3
1986
1980
1980
1972
[24,25]
[32]
[11,28]
[32]
P(p-ClC6H4)3
P(p-Tol)3
P(p-
PCH2P(S)Ph2; dppe = Ph2PCH2CH2PPh2; and dppp = Ph2P(CH2)3PPh2.
b
MeOC6H4)3
DPP
PCy3
PPh3
DPP
P(Ph2C6H4)3
P(p-ClC6H4)3
P(p-
m(COCH3).
1975
1989
1982
1981
1978
1971
1965
[15,33]
[26,32]
[26,32]
[33]
[26,32]
[26,32]
[26,32]
cupf
nucleophilicity of the metal centers, solvent interactions and steric
bulk on the rate of oxidative addition and CO insertion.
Recently diphenyl-2-pyridylphosphine (DPP) [12] came to our
attention as ligand due to its ability to not only coordinate to the
metal ion with the phosphorus atom [13], but also with the pyridyl
nitrogen making the phosphine ligand a potential bidentate ligand
[14]. The increase in electron density due to the presence of a
nitrogen atom in the phosphine also has the potential to alter
the Lewis basicity of the phosphine and ultimately influence the
oxidative addition reactions for the metal complexes. With this
in mind we prepared and structurally characterized [Rh(acac)
(CO)(DPP)] [15] to investigate the phosphine’s mode of bonding
as well as possible oxidative addition products for the reaction be-
tween methyl iodide and the mono carbonyl complex.
MeOC6H4)3
P(p-Tol)3
P(p-Tol)3
P(p-Tol)3
P(p-Tol)3
P(p-Tol)3
P(p-Tol)3
PPh3
1965
1980
1983
1986
1983
1998
1984
1976
1978
1960
1960
1974
1960
[26,32]
[28]
[11,28]
[29]
[11,28]
[11,28]
[29]
[32]
[11]
[37]
[32]
tmhd
dbm
btfac
tfac
hfac
btfac
neocupf PPh3
sacac
dmavk
PPh3
PPh3
P(p-ClC6H4)3
PPh3
P(p-
tavk
[34]
[34]
2. Experimental
MeOC6H4)3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
fctfa
fca
bfcm
dfcm
tta
1986
1980
1977
1977
1981
1982
1981
[30]
[30]
[30]
[30]
[35]
[36]
[36]
2.1. General considerations
All chemicals used in the syntheses were of reagent grade and
were used without further purification. The RhCl3ꢁ3H2O and diphe-
nyl-2-pyridylphosphine were purchased from Sigma–Aldrich. Sol-
vents were purified and dried prior to use by standard
procedures. 1H and 31P NMR spectra were measured on a Bruker
AVANCE DPX 300 MHz spectrometer and the 13C NMR spectra on
a Bruker AVANCE II 600 MHz spectrometer. 1H NMR and proton
decoupled 13C NMR data are listed in the order: chemical shift
(d, reported in ppm and referenced to TMS), integral value, multiplic-
ity, coupling constant (J, in Hz) and assignment. Proton decoupled
31P NMR data are listed in the order: chemical shift (d), multiplicity
and coupling constant (J, in Hz). For the characterization of the
bap
bab
[Rh(LL)(CO)(PX3)(CH3)I]
acac
PPh3
P(p-ClC6H4)3
P(p-
2060
2064
2056
[11]
[32]
[32]
MeOC6H4)3
DPP
DPP
PPh3
P(p-ClC6H4)3
2048
2061
2052
2052
[15,33]
[15,33]
[26,32]
[26,32]
cupf

222
W. Purcell et al. / Journal of Molecular Structure 1038 (2013) 220–229
Rh(III) alkyl and acyl species [Rh(acac)(CO)(DPP)] (5 mg,
0.0101 mmol) was dissolved in 0.7 ml CDCl3 and transferred to a
chloroform and left to stand for 24 h. Yellow crystals suitable for
X-ray structure determination were isolated. Yield: 29%. IR data
5 mm diameter NMR tube. Methyl iodide (6.31
ll, 0.101 mmol)
(solid state): m
(CO) = 1975 cmꢀ1. Elemental Anal. RhC23H21NO3P,
was added and 1H and 31P NMR spectra recorded every 0.5 h. The
CHN analysis was performed on a LECO Truspec micro-analyzer.
A Shimadzu ICPS-7510 ICP-OES with a radial-sequential plasma
spectrometer was used for the wet chemical analysis of all the rho-
dium samples in the current study [16]. The torch of the ICP-OES
was mounted vertically with the plasma being viewed in the radial,
instead of the axial, mode due to the better detection limits
achieved with this configuration. The rhodium content in all of
the rhodium compounds was analyzed using a cobalt internal stan-
dard method at an atomic wavelength for rhodium of 343.489 nm
and ionic wavelength for cobalt of 228.616 nm [16]. The rhodium
standard (1000 ppm) was purchased from Aldrich Chemicals while
the analytical grade HCl (32%), HNO3 (65%) and Co(NO3)2ꢁ6H2O
were obtained from Merck Chemicals. Dilution of the solutions
was done using double distilled water and Schott Duran grade
(A) type glassware was used for all chemical analysis.
(calculated values in brackets): C, 54.74 (55.14), H, 4.30 (4.30), N,
2.45 (2.84), Rh, 20.54 (20.86). 1H NMR (300 MHz, CDCl3, 20 °C): d
3
4
8.70 (1H, dd, JH–H = 4.6 Hz, JH–H = 2.1 Hz, 3-H pyridyl-ring), 7.95
3
3
4
(1H, m, JH–H = 7.7 Hz, JH–H = 6.0 Hz, JH–H = 2.1 Hz, 5-H pyridyl-
3
ring), 7.74 (4H, m, 3- & 5-H phenyl), 7.62 (1H, m, JH–H = 7.7 Hz,
4JH–H = 1.7 Hz, 6-H pyridyl-ring), 7.45–7.15 (7H, 4-H pyridyl-ring,
2- & 6-H phenyl, 4-H phenyl), 5.35 (1H, s, CH-acac), 2.02 (3H, s,
CH3-acac), 1.45 (3H, s, CH3-acac), 13C{1H} NMR (151 MHz, CDCl3,
1
2
20 °C): d 187.79 (dd, JRh–C = 75.9 Hz, JP–C = 24.2 Hz, CO), 187.44
(s, C-acac), 185.37 (s, C-acac), 157.54 (d, JP–C = 71.0 Hz, 1-C pyri-
1
3
dyl-ring), 149.95 (d, JP–C = 14.3 Hz, 3-C pyridyl-ring), 135.43 (d,
3JP–C = 9.4 Hz, 5-C pyridyl-ring), 134.86 (d, JP–C = 11.6 Hz, 2- & 6-
2
2
C phenyl), 132.15 (d, JP–C = 51.6 Hz, 6-C pyridyl-ring), 131.16 (d,
1JP–C = 26.7 Hz, 1-C phenyl), 130.33 (d, JP–C = 2.2 Hz, 4-C phenyl),
4
3
127.93 (d, JP–C = 10.7 Hz, 3-
&
5-C phenyl), 123.78 (d,
4JP–C = 2.1 Hz, 4-C pyridyl-ring), 100.67 (s, CH-acac), 27.55
Intensity data was collected at ꢀ100 °C on a Bruker APEX(II)
(s, CH3-acac), 26.58 (s, CH3-acac), 31P{1H} NMR (121 MHz, CDCl3,
1
CCD area detector diffractometer with graphite monochromated
20 °C): d 50.41 (d, JRh–P = 176.0 Hz).
Mo Ka
radiation (50 kV, 30 mA). The data was collected with /
and
-
-scans at 0.5° width while data reduction was performed
2.2.3. [Rh(acac)(CO)(DPP)(CH3)I]
with SAINT+ [17]. The crystal structure was solved by direct meth-
ods using SHELXTL [18]. Non-hydrogen atoms were first refined by
full matrix least-squares calculations based on F2 using SHELXTL.
Hydrogen atoms were first located in the difference map, then
positioned geometrically and allowed to ride on their respective
parent atoms. Diagrams and publication material were generated
using SHELXTL and PLATON [19].
Methyl iodide (0.64 g, 4.55 mmol) was added to a well stirred
solution containing [Rh(acac)(CO)(DPP)] (0.25 g, 0.46 mmol) in
5 ml chloroform. The reaction was allowed to proceed for 20 min
at room temperature during which the color changed from orange
to red brown. The solvent was removed by evaporation to yield the
final powdered brown product. Yield: 60%. IR data (solid state):
m
(CO) = 2048 cmꢀ1. Elemental Anal. RhC24H24INO3P (calculated val-
ues in brackets): C, 45.68 (45.37), H, 3.47 (3.92), N, 1.90 (2.21), Rh,
2.2. Synthesis
16.95 (16.20). 1H NMR (300 MHz, CDCl3, 20 °C): d 8.79 (1H, dd,
4
3JH–H = 4.6 Hz, JH–H = 2.1 Hz, 3-H pyridyl-ring), 8.06 (2H, m, 3- or
2.2.1. [Rh(acac)(CO)2]
[Rh(acac)(CO)2] was synthesized as previously reported [20].
Yield: 69% IR data (solid state):
tal Anal. RhO4C7H7: (calculated values in brackets): C, 32.41
(32.58), H, 2.85 (2.74), Rh, 39.68 (39.88)%. 1H NMR (300 MHz,
CDCl3, 20 °C): d 5.63 (s, CH-acac), 2.09 (s, CH3-acac), 13C{1H} NMR
5-H phenyl), 7.66 (2H, m, 3- or 5-H phenyl), 7.55 (1H, m,
4
3JH–H = 7.7 Hz, JH–H = 3.5 Hz, 6-H pyridyl-ring), 7.50–7.15 (8H,
m
(CO) = 2063, 1998 cmꢀ1. Elemen-
4- & 5-H pyridyl-ring, 2- & 6-H phenyl, 4-H phenyl), 5.39 (1H, s,
CH-acac), 1.99 (3H, s, CH3-acac), 1.81 (3H, s, CH3-acac), 1.21 (3H,
dd, 2JRh–H = 2.0 Hz, 3JP–H = 2.0 Hz, Rh–CH3), 31P{1H} NMR
1
(121 MHz, CDCl3, 20 °C): d 39.09 (d, JRh–P = 128.4 Hz).
(151 MHz, CDCl3, 20 °C):
d
187.2 (s, C-acac), 183.6 (d,
1JRh–C = 72.9 Hz, 2 ꢂ CO), 101.6 (s, CH-acac), 27.0 (s, 2 ꢂ CH3-acac).
2.2.4. [Rh(acac)(COCH3)(DPP)I]
Methyl iodide (0.76 g, 4.55 mmol) was added to a well stirred
solution containing [Rh(acac)(CO)(DPP)] (0.19 g, 0.44 mmol) in
5 ml chloroform. The reaction was allowed to proceed for
200 min. at room temperature during which the color changed
from orange to red brown. The solution was removed by evapora-
tion to yield the red brown powdered product. Yield: 80%. IR data
2.2.2. [Rh(acac)(CO)(DPP)]
[Rh(acac)(CO)2] (0.2 g, 0.78 mmol) was dissolved in 10 ml
methanol and the solution was slightly heated to 30 °C for 5 min
to ensure homogeneity. Diphenyl-2-pyridylphosphine (0.2 g,
0.76 mmol) was added gently whilst stirring and the solution
changed color from yellow to red and a yellow product precipi-
tated immediately from the solution. The precipitate was removed
by filtration, washed with methanol and dried in a fume cupboard.
[Rh(acac)(CO)(DPP)] (0.109 g, 0.22 mmol) was dissolved in 5 ml
(solid state):
m
(COCH3) = 1721 cmꢀ1. Elemental Anal. RhC24H24-
INO3P (calculated values in brackets): C, 45.38 (45.37), H, 3.98
(3.82), N, 2.90 (2.21), Rh, 16.15 (16.20). 31P{1H} NMR (121 MHz,
1
CDCl3, 20 °C): d 27.2 (d, JRh–P = 150.9 Hz).
ν(CO2)
ν(CO1)
[Rh(LL)(CO)2]
[Rh(LL)(CO)(PX3)]
[Rh(LL)(CO)(PX3)(CH3)I]
[Rh(LL)(COCH3)(PX3)I]
ν(COCH3)
2000
1700
1900
1800
2100
ν(CO)/cm-1
Fig. 1. Carbonyl stretching frequencies in rhodium complexes.

W. Purcell et al. / Journal of Molecular Structure 1038 (2013) 220–229
223
2.3. Computational chemistry
of competition by the other carbonyl ligand. The addition of CH3I re-
sults in the oxidation of Rh(I) to Rh(III) with the simultaneous addi-
Density functional theory (DFT) calculations were carried out using
the ADF (Amsterdam Density Functional) programme [21] with the
GGA (Generalized Gradient Approximation) functional PW91
(Perdew-Wang, 1991) [22]. The TZP (Triple f polarized) basis set, with
a fine mesh for numerical integration, a spin-restricted formalism and
full geometry optimization with tight convergence criteria, was used.
tion of CH3 and I to the metal center as illustrated by the
to 2048 cmꢀ1. The spontaneous CO insertion reaction resulted in the
appearance of the (CO)
(COCH3) at 1721 cmꢀ1. The decrease in
indicated an increase in the metal–carbon -back-bonding and a de-
m(CO) shift
m
m
p
crease in carbon–oxygen triple bond character.
A comparison of the IR data for the monodentate carbonyl com-
plexes obtained in this study and those for previously synthesized
complexes revealed very interesting results. In the case of the
3. Results
Rh-acac complexes the
m(CO) for the DPP ligand appeared to be
on the lower end of the
m
(CO) scale for both the mono carbonyl
3.1. Synthesis
and alkyl product if it was compared to the different phosphines
studied to date (see Table 1). This could easily be explained by
the presence of the pyridine ring in DPP. The additional electron
density present in the phosphine as a result of the nitrogen
increased the electron density on the Rh center resulting in an in-
The elemental analyses and different NMR results clearly indi-
cate the successful preparation and characterization of the differ-
ent rhodium DPP carbonyl complexes in normal atmospheric
conditions. The synthesis and isolation of the target compounds
under these less demanding experimental conditions (Schlenk
techniques and inert atmosphere [12]) and in different solvents
illustrate the ease of synthesis of these type of complexes. Charac-
terization of the products using IR indicates that the following con-
secutive reactions took place:
crease in
synergistic
ing the (CO) (weaker carbon oxygen bond). This situation changed
however for the corresponding cupf complex. The results in Table 1
clearly showed that the (CO) for the cupferrate ligand was
p-back donation to the carbonyl carbon, thus increasing
r
-back-donation to the metal and subsequently lower-
m
m
approximately the same as for triphenylphosphine and in the same
order as all the other phosphine ligands listed.
½RhðacacÞðCOÞ2ꢃ þ!DPP½RhðacacÞðCOÞðDPPÞꢃ þ¡CH3I
Close inspection of the reported m(CO) for this relatively large
½RhðacacÞðCOÞꢀðCDOPPÞðCH3ÞðIÞꢃ
ð1Þ
ð2Þ
group of rhodium complexes, obtained from different solvents as
well as KBr, indicated that almost no carbonyl stretching frequency
overlapping occurred between any of the different types of com-
plexes indicated in Fig. 1. Also important from these results is that
neither the solvent nor the solid state plays a very important role
RhðIIIÞ alkyl
½RhðacacÞðCOÞðDPPÞðCH3ÞðIÞꢃ ¡ ½RhðacacÞðDPPÞðCOCH3ÞðIÞꢃ
RhðIIIÞ alkyl
RhðIIIÞ acyl
in the position of the
m
(CO) for the different type of complexes.
(CO2) of the dicarbonyl complexes ap-
The dicarbonyl complex [Rh(acac)(CO)2] clearly show two dis-
tinctive carbonyl peaks at 2063 and 1998 cmꢀ1 which changes to
The limits for (CO1) and
m
m
peared to be between 2063–2088 and 1992–2026 cmꢀ1 respec-
tively, while the carbonyl stretching frequency limits for the
rhodium(I) monocarbonyl complexes appeared to be between
1960 and 1998 cmꢀ1. The results in Table 1 also indicated that
one peak with m
(CO) at 1975 cmꢀ1 after the addition of DPP due to
the subsequent substitution of one of the carbonyl ligands and com-
pares favorably with the 1981 cmꢀ1 (KBr) that was obtained for the
same complexes that was reported by Wajda-Hermanowicz [12].
This shift in m(CO) is in accordance with previous studies which indi-
cate an increase in electron density on the metal center which can be
attributed to the electronegativity of the phosphine and the absence
Table 3
Selected bond lengths (Å) and angles (°) for [Rh(acac)(CO)(DPP)].
Molecule 1
Bond
Molecule 2
Bond
Table 2
Length (Å)
Length (Å)
Crystal data and refine parameters for [Rh(acac)(CO)(DPP)].
Rh1–C11
Rh1–O13
C13–O12
Rh1–O12
Rh1–P1
P1–C131
P1–C111
P1–C121
N1–C111
N1–C115
1.791(4)
2.035(2)
1.280(4)
2.073(2)
2.243(1)
1.827(3)
1.835(3)
1.832(3)
1.380(4)
1.397(5)
Rh2–C21
Rh2–O23
C23–O11
Rh2–O22
Rh2–P2
P2–C221
P2–C231
P2–C211
N2–C211
N2–C215
1.776(4)
2.034(2)
1.272(4)
2.068(2)
2.235(1)
1.825(3)
1.834(3)
1.843(3)
1.362(4)
1.378(4)
Identification code
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
sad
RhC23H23NO3P
495.805
293(2) K
0.71073 Å
Triclinic
ꢀ
P1
Unit cell dimensions
a = 10.085(5) Å; b = 12.793(3) Å
c = 18.629(5) Å;
b = 100.058(3)°;
2145.6(7) Å3
4
a
c
= 104.571(5)°
= 106.828(3)°
Bond
Angle (°)
Bond
Angle (°)
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta
Absorption correction
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
C11–Rh1–O13
C11–Rh1–O12
O13–Rh1–O12
C11–Rh1–P1
178.52(14)
91.75(12)
88.16(9)
89.62(11)
90.56(7)
176.40(6)
117.99(10)
112.85(9)
113.77(10)
125.6(2)
128.1(2)
178.0(3)
102.63(13)
103.71(13)
104.35(14)
117.7(3)
C21–Rh2–O23
C21–Rh2–O22
O23–Rh2–O22
C21–Rh2–P2
178.37(18)
90.16(17)
88.54(10)
93.23(15)
88.10(8)
176.14(7)
115.46(11)
108.65(10)
120.42(10)
126.0(3)
128.0(3)
174.8(4)
105.31(15)
100.92(14)
104.62(14)
117.4(3)
1.533 Mg/m3
0.894 mmꢀ1
1008
0.19 ꢂ 0.13 ꢂ 0.12 mm3
O13–Rh1–P1
O12–Rh1–P1
C131–P1–Rh1
C111–P1–Rh1
C121–P1–Rh1
C13–O12–Rh1
C15–O13–Rh1
O11–C11–Rh1
C131–P1–C111
C131–P1–C121
C111–P1–C121
C111–N1–C115
N1–C111–P1
O23–Rh2–P2
O22–Rh2–P2
C221–P2–Rh2
C231–P2–Rh2
C211–P2–Rh2
C23–O22–Rh2
C26–O23–Rh2
O21–C21–Rh2
C221–P2–C231
C221–P2–C211
C231–P2–C211
C211–N2–C215
N2–C211–P2
1.76–28.28°
ꢀ11 6 h 6 13, ꢀ17 6 k 6 17, ꢀ20 6 l 6 24
14,946
10,333 [R(int) = 0.0208]
97.0%
None
Full-matrix least-squares on F2
10333/0/526
1.013
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole
r(I)]
R1 = 0.0368, wR2 = 0.0902
R1 = 0.0571, wR2 = 0.1009
1.267 and ꢀ0.750 e Åꢀ3
119.8(2)
115.2(2)

224
W. Purcell et al. / Journal of Molecular Structure 1038 (2013) 220–229
Fig. 2. Numbering scheme of the two different [Rh(acac)(CO)(DPP)] molecules (30% probability displacement ellipsoids).
rhodium(I) monocarbonyl, rhodium(III) alkyl and acyl complexes
Table 4
would be between 1978 3, 2055 7 cmꢀ1 and 1710 6 cmꢀ1
respectively if normal distribution was assumed. These results also
allowed that the monocarbonyl stretching frequency for newly
synthesized rhodium complexes could be approximated according
to the following equations:
Selected Rh–CO bond distances of [Rh(LL0)(CO)(PX3)] complexes.
Complex
Rh–CO (Å)
Rh–P (Å)
Refs.
[Rh(cupf)(CO)(PPh3)]
[Rh(acac)(CO)(DPP)]
1.78(1)
2.232(3)
2.243(1)a
2.227(1)
2.156(2)
2.239(2)
2.244(4)
2.275(1)
2.232(1)
2.2376(9)
2.2387(9)
[26,32]
[15,33]
[32]
[26,32]
[11]
[24,25]
[37]
[44]
1.791(4)a
1.802(5)
1.772(9)
1.781(9)
1.801(8)
1.784(5)
1.801(5)
1.797(4)
1.788(5)
[Rh(neocupf)(CO)(PPh3)]
[Rh(cupf)(CO){P(OCH2)3CCH3}]
[Rh(tfdmaa)(CO)(PPh3)]
[Rh(acac)(CO)(PPh3)]
[Rh(dmavk)(CO)(PPh3)]
[Rh(fctfa)(CO)(PPh3)]
m
ðCOÞmono
ðCOÞmono
¼
¼
m
ðCO1Þ ꢀ 100 cmꢀ1 or
ðCO2Þ ꢀ 30 cmꢀ1
ð3Þ
ð4Þ
m
m
while the rhodium(III) alkyl and acyl stretching frequency could be
approximated according to the following equations:
[Rh(bap)(CO)(PPh3)] isomer 1
[Rh(bap)(CO)(PPh3)] isomer 2
[45]
[45]
a
This study.
m
m
m
ðCOÞalkyl
ðCOÞalkyl
ðCOÞalkyl
¼
¼
¼
m
m
m
ðCO1Þ ꢀ 20 cmꢀ1 or
ðCO2Þ þ 50 cmꢀ1 or
ðCOÞrhodiumðIÞ þ 75 cmꢀ1
ð5Þ
ð6Þ
ð7Þ
the Rh(III) alkyl and acyl complexes were limited between
2045–2070 and 1692–1721 cmꢀ1 respectively. Statistical calcula-
tions indicated that interesting predictions could be made from
the data in Table 1. These results predicted at a 95% probability
[23] that the
dicarbonyl complex would be between 2076
2008 5 cmꢀ1
(CO2) while the stretching frequencies for the
m
(CO) stretching frequencies for any rhodium(I)
5
m(CO1) and
and
m
Fig. 3. A representation of free DPP viewed along the
rnb–P axis, demonstrating the enantiomeric configurations P (clockwise) and M (anti-clockwise).

W. Purcell et al. / Journal of Molecular Structure 1038 (2013) 220–229
225
1. This was demonstrated by the intramolecular M–COꢁ ꢁ ꢁN(pyridyl
ring) bond distances of 6.072(2) and 3.290(1) Å as well as the dihe-
dral angles of 61.51(8)° and 47.4(1)° (OCRhP and pyridyl ring as
planes) for Molecules 1 and 2 respectively. Closer inspection of
the carbon–phosphur–carbon angles in the two molecules also
illustrated a smaller bond angle of 100.92(14)° for C221–P2–
C211 compared to the rest of the CPC bond angles and correlated
well with the identical complex which was isolated from diethyl
ether [15]. The pyridyl nitrogen of the phosphine ligand in the
monoclinic crystal was also directed more towards the Rh–CO
bond, as was the case with Molecule 2 in the current study. The dif-
ferent orientations of the pyridyl group in the three structures,
spurred us to undertake a computational chemistry study to deter-
mine the preferred conformation (if any) of the DPP group in
[Rh(acac)(CO)(DPP)].
(a)
(c)
(b)
3.3. Computational chemistry
The free DPP molecule [46], as well as coordinated DPP can exist
in two different helicities, that have the phenyl/pyridyl rings ori-
ented in a clockwise or counter clockwise fashion (referred to as
P and M helicity respectively) similar to PPh3 [47,48], see Fig. 3.
In solution, inversion of the helicity occured readily. In the solid
state DPP and PPh3-containing complexes generally exist as a race-
mic mixture consisting of units from each helicity in the same unit
cell.
Experimental crystal structures [49] and theoretical DFT calcu-
lations showed that the lowest energy conformation of PPh3 coor-
dinated to tetrahedral [47], trigonal-bipyramidal [47], octahedral
[50] or square planar metal centers [51] lay in a broad potential en-
ergy well having a broad range of energetically acceptable phenyl
ring orientations via independent P–Ci bond rotations (Ci = Cipso of
the phenyl ring Ph). In spite of the wide range of low energy orien-
tations, the orientation of coordinated PPh3 possessed some dis-
tinct features that could be derived from steric principles.
Costello [47] has termed the plane incorporating all points of min-
imum steric compression of a specific complex, the plane of nadir
energy. The plane of nadir energy is illustrated in Fig. 4 for square
planar metal centers [51]. The different R substituents on the P
atom of a tertiary phosphine PR3 should preferably be oriented
as near as possible to the plane of nadir energy to have a minimum
of steric interaction with the ligands bonded to the complex. In the
case of R = Ph the three phenyl rings each will be oriented as near
as possible to the plane of nadir energy, with a correlated tilting of
Fig. 4. (a) A projection demonstrating the plane of nadir energy (i.e. the plane
incorporating all points of minimum steric compression [47], shown by the dotted
blue lines) associated with the ligands L attached to a square planar metal center.
The predicted minimum energy conformation of PPh3 in ML3PPh3 is also shown. (b)
The preferred minimum energy conformation of PPh3 (as viewed along the P–M
bond axis) within square planar [51a] complexes relative to the plane of nadir
energy. (c) The six possible positions of N of pyridyl in DPP all correspond to the
same minimum energy conformation of DPP in square planar ML3DPP. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
m
m
ðCOÞacyl
ðCOÞacyl
¼
¼
m
m
ðCO1Þ ꢀ 366 cmꢀ1
ð8Þ
ð9Þ
ðCOÞrhodiumðIÞ ꢀ 268 cmꢀ1
3.2. Crystal structure
ꢀ
The title complex crystallized in the triclinic space group P1
with four molecules per unit cell (chloroform as solvent) which
is a different for the same complex that crystallized in the mono-
clinic space group P21/c using diethyl ether as solvent [15]. The
crystal data for [Rh(acac)(CO)(DPP)] is given in Table 2. The most
important bond lengths and angles of the two isomorphic
[Rh(acac)(CO)(DPP)] molecules are listed in Table 3 while the
numbering scheme is shown in the perspective drawing (Fig. 2).
The results clearly showed that the DPP acts a monodentate ligand
in both the molecules with the carbonyl ligand bonded cis to the
phosphine while the rest of the coordination sphere is completed
by the two acac oxygen atoms. A comparative study between the
two molecules indicated that both adopt a square planar geometry
with bond angles varying between 88.10(8)° and 93.23(15)°. The
bond distances and angles were of the same magnitude as would
be expected for isomorphic systems. The Rh–CO bond length of
1.791 Å for both the mono and triclinic systems correlated well
with those found for other [Rh(LL0)(CO)(PX3)] complexes as can
be seen in Table 4. The difference between the two molecules
was centered around the orientation of the DPP pyridine ring in
the crystal structure. The orientation of the pyridyl nitrogen atom
was directed towards the carbonyl ligand in Molecule 2 while the
same atom was directed away from the carbonyl group in Molecule
the phenyl rings (via M–P (h) and P–Ci bond rotations (x)) in order
to minimize both inter ring–ligand and inter ring–ring interactions
[51]. The favoured conformation of PPh3 coordinated to a square
planar metal center, illustrated in Fig. 4, can be derived using
the following principles [51]: (P helicity, view along P–M axis,
Co = Cortho of the phenyl ring Ph), (i) superimpose Co of the vertical
ring A onto the nadir plane perpendicular to the square plane and
allow ring A to tilt towards the smallest ligand, (ii) allow ring B to tilt
in the space below the smallest ligand in the quadrant between the
nadir plane below the complex and a horizontal plane through the
SQP of the complex, (iii) tilt ring C over the largest ligand and (iv) al-
low correlated tilting of rings A, B and C to minimize inter ring–ring
and inter ring–ligand interactions [51]. We will show that these
principles also hold for DPP coordinated to a square planar metal
center. PPh3 (with three Ph groups, each of D6h symmetry) coordi-
nated to a square planar metal center thus having only ONE pre-
ferred conformation for P helicity (as described above) and ONE
preferred conformation for M helicity. In contrast, the pyridyl ring
in DPP could adopt any of three positions and two orientations for
P helicity (similar for M helicity) that corresponded to the preferred
conformation of complex-bound PPh3. The predicted minimum en-
ergy conformation of DPP coordinated to a square planar metal

226
W. Purcell et al. / Journal of Molecular Structure 1038 (2013) 220–229
Fig. 5. Geometry of SQP crystals containing on DPP group, as viewed along the P–M axis. CSD reference code shown [49]. For comparative reasons P helicities are shown.
center could thus be any of the six illustrated in Fig. 4. We will see
that the position of N of the pyridyl group in DPP does not have an
influence on the minimum energy conformation of DPP.
The orientation of the DPP in selected SQP crystals [49] contain-
ing one DPP group is presented in Fig. 5. The orientation of DPP in
these experimental crystal structures was similar and is character-
ized by a near vertical ring A, tilted to the right (P helicity, except
for Molecule 1 of this study) and oriented as near as possible to the
nadir plane perpendicular to the square plane of the SQP complex, a
tilted ring B in the quadrant below the square plane and to the
right of the nadir plane and a near horizontally tilted ring C to
the left. The orientation of DPP in the structures in Fig. 5a is such
that the vertical ring A is tilted towards the smallest ligand, consis-
tant with the principles for deriving the favoured conformations of

W. Purcell et al. / Journal of Molecular Structure 1038 (2013) 220–229
227
Fig. 6. 2-D potential energy surface (PES) of [Rh(acac)(CO)(DPP)] representing the relative energy of different conformations of DPP in [Rh(acac)(CO)(DPP)] as a function of
x
(rotation of a phenyl ring about the P–Cpyridyl bond) and h (rotation of DPP about Rh–P bond). The dark blue areas present the lowest energy geometries (conformations)
(1 a.u. = 2625.5 kJ molꢀ1). The geometry of the complex relating to the minimum energy areas are presented in Fig. 7 according to the numbers indicated. (For interpretation
of the references to color in this figure legend, the reader is referred to the web version of this article.)
PPh3 coordinated to a square planar metal center [51]. We dis-
cussed the orientation of DPP here by looking at the steric orienta-
tion of the rings without referring to the specific position of N of
the Py group.
Fig. 6 gives the calculated PES of [Rh(acac)(CO)(DPP)] as a func-
tion of dihedral (torsion) angles (rotation of the pyridyl ring
about the P–CPh bond) and h (rotation of DPP about the Rh–P bond).
x
The PES is obtained by rotation of the DPP group through 360° and
Fig. 7. The experimental (I–VI) and the DFT calculated minimum energy conformations (i–xii) of [Rh(acac)(CO)(DPP)] as viewed along the P–Rh axis. In (a–c) the vertical
phenyl ring is tilted towards CO and in (d) the vertical phenyl ring is tilted towards Ob-diketonato. The numbering of the calculated structures corresponds to the numbering of
the minimum energy areas of the PES in Fig. 6.

228
W. Purcell et al. / Journal of Molecular Structure 1038 (2013) 220–229
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the simultaneous rotation of the pyridyl ring from 0° to 360° at
each point, resulting in 1296 restrained optimizations. Every point
on the graph thus presents a conformation of [Rh(acac)(CO)(DPP)]
with two dihedrals related to the pyridyl ring restricted and the
rest of the molecule fully optimized. The other two phenyl rings
were thus free to rotate in order to minimize steric interaction to-
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The orientation of the DPP group in the conformations in
Fig. 7(i and ii) were similar to the previously published crystal
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conformations Fig. 7(iii–vi) are similar to the Molecule 1 and
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Molecule
2 of the crystal structure of [Rh(acac)(CO)(DPP)]
presented in this study. The only difference was that the vertical
ring in the experimental structures is orientated slightly
differently, probably due to packing effects. The other six
minimum energy conformations have one (Fig. 7(vii–viii)) or two
(vii–xii) rings near perpendicular to the paper, orientations not
experimentally observed for [Rh(acac)(CO)(DPP)] complexes.
However, we observe that the orientation of DPP in Fig. 7(x–xii)
correspond to the orientation of DPP in the SQP -S,S0-Pt-DPP
complex with crystal code KUDJAV in Fig. 5b. The experimental
and DFT calculated results did not show any preference for the
position of N (illustrated in Fig. 4) of the pyridyl group in the min-
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The crystal structure of [Rh(acac)(CO)(DPP)] clearly showed that
the DPP acts a monodentate ligand in the structurally characterized
compounds. [Rh(acac)(CO)(DPP)] crystallized in the triclinic space
ꢀ
group P1 with four molecules per unit cell, contrary to previously
[14] (a) U. Abrama, R. Albertob, J.R. Dilworthc, Y. Zhengc, K. Ortne, Polyhedron 18
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monoclinic space group [15]. The conformation analysis of DPP
coordinated to [Rh(acac)(CO)(DPP)] resulted in 12 areas of
minimum energy, most lying in a broad energy well of low energy
orientations. The conformation of four of the 12 areas of minimum
energy agreed with the crystal structures of the P and M helicities of
Molecules 1 and 2 of [Rh(acac)(CO)(DPP)] presented in this study.
DFT calculations also confirmed the experimental observations that
more than one preferred N position of the pyridyl group exists for
the complex-bound DPP in these type of complexes.
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The authors thank the Research Fund of the University of the
Free State, the National Research Fund of the Republic of South
Africa and Dr. L. Twigge for her contribution to the NMR
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