(R/S)A2- or (R,S/S,R)p,p′- [Pd(κ2-P,P-{P(OC6H3But 2-2,4)2N(Me)C(O)N(Me)PPh2}Cl2]: Chirality created by ring tilting

Article abstract of DOI:10.1021/ic049222e

The reaction of the unsymmetric bisphosphanyl urea ligand P(OC ₆H₃Bu^t₂-2,4)₂N(Me)C(O) N(Me)PPh₂ with [Pd(cod)Cl₂] (cod = 1,5-cyclooctadiene) results in the chiral palladacycle (R

Full text of DOI:10.1021/ic049222e

Inorg. Chem. 2004, 43, 6543−6545
(R/S)A2- or (R,S/S,R)p,p -[Pd(
K
2-P,P-
{
P(OC6H3But2-
′
2,4)2N(Me)C(O)N(Me)PPh2}Cl2]: Chirality Created by Ring Tilting
Olaf Ku1
hl*,†,‡ and Steffen Blaurock§
Institut fu¨r Chemie und Biochemie, EMAU Greifswald, Soldmannstrasse 16,
D-17487 Greifswald, Germany, and Institut fu¨r Anorganische Chemie,
UniVersita¨t Leipzig, Johannesallee 29, D-04103 Leipzig, Germany
Received June 15, 2004
The reaction of the unsymmetric bisphosphanyl urea ligand
P(OC6H3But2-2,4)2N(Me)C(O)N(Me)PPh2 with [Pd(cod)Cl2] (cod
1,5-cyclooctadiene) results in the chiral palladacycle (R,S)A2-[Pd-
2-P,P- P(OC6H3But2-2,4)2N(Me)C(O)N(Me)PPh2
Cl2]. The chirality
of the title compound is caused by the tilting of the central, six-
In the present contribution, we will describe a palladium
complex containing a bidentate phosphoramidate ligand
where the complex displays planar and axial chiral elements.
Title complex 1 can be synthesized by mixing THF solu-
tions of the ligand and [Pd(cod)Cl2] (cod ) 1,5-cycloocta-
diene) followed by precipitation with hexane.8 No attempt
was made to separate the pair of enantiomers.
)
(κ
{
}
membered PdP2N2C ring along one of the two P−N vectors and
comprises two chiral planes and one chiral axis.
Crystals of 1‚THF suitable for an X-ray diffraction study
were obtained from THF/hexane solution (Figure 1).9-12 The
palladium atom has the usual square planar coordination of
Pd(II) with a P1-Pd1-P2 angle of 93.20(3)°. The central
Pd1-P1-N1-C1-N2-P2 ring is tilted by 58° along the
N1-P2 vector effectively creating two chiral planes (Fig-
ure 2). The two Pd-Cl bond lengths are essentially equal
(233.1(1) and 233.60(9) pm) whereas the two Pd-P bond
lengths (217.77(9) and 221.78(9) pm) are not. A similar
Chirality is a very important concept in chemistry and has
major implications in catalysis, biochemistry, and pharma-
cology to name but a few.1 Chirality is described as central,
axial, and planar chirality depending on the dimension of
the chiral element.2 Prominent examples include asymmetric
carbon and metal atoms,3 a form of atropisomerism known
as “axial chirality”,4 and substituted arene transition metal
carbonyl complexes.5 Axial chirality is defined by any one-
dimensional chiral element known as an axis. This chirality
often, but not always, originates from a hindered rotation
along the phenyl-phenyl C-C bond and is therefore a form
of atropisomerism.4 Other examples of axially chiral com-
pounds include allenes, alkylidene cycloalkanes, and spi-
ranes.6 Chirality caused by a tilted ring system is compara-
tively rare7a whereas distorted chiral metalated macrocycles
play an important role in biochemistry.7b,c
(8) Complex 1 was prepared by adding 285 mg (1 mmol) of [Pd(COD)-
Cl2] to a solution of 713 mg (1 mmol) of P(OC6H3But2-2,4)2N(Me)-
C(O)N(Me)PPh2 in 20 mL of THF. After stirring for 2 h, the solution
was concentrated under reduced pressure and layered with 20 mL of
hexane. The pale yellow product was filtered off and dried in vacuo.
Crystals suitable for single crystal X-ray structure determination were
grown from the mother liquor at -20 °C. Yield 632 mg (73%); mp
178-80 °C (dec). NMR (CDCl3, 213 and 300 K): 1H δ 7.91 (m,
4H, o-Ph), 7.68 (m, 2H, p-Ph), 7.57 (m, 4H, m-Ph), 7.42 (s, 2H,
3
3
3-C6H3But2), 7.24 (d, JHH ) 8.4 Hz, 2H, 5-C6H3But2), 6.76 (d, JHH
) 8.4 Hz, 2H, 6-C6H3But2), 3.18 (d, JPH ) 5.2 Hz, 3H, Me), 3.01
3
(d, JPH ) 5.6 Hz, 3H, Me), 1.49 (s, 9H, But), 1.29 (s, 9H, But).
3
13C{1H}: δ 157.24 (s, CO), 148.04 (s, C1), 147.88 (s, C6), 139.51 (d,
† EMAU Greifswald.
JPC ) 7.55 Hz, i-Ph), 133.99 (s, C2), 133.87 (s, C4), 133.61 (d, JPC
)
‡ Current address: Institut fu¨r Angewandte Photophysik, TU Dresden,
George-Ba¨hr-Strasse 1, D-01069 Dresden, Germany. E-mail: kuhl@iapp.de.
§ Universita¨t Leipzig.
2.31 Hz, o-Ph), 130.17 (s, m-Ph), 130.04 (s, p-Ph), 117.90 (s, C5),
117.79 (s, C3), 38.27 (s, NMe), 35.89 (s, CMe), 35.18 (s,CMe), 32.46
(s, NMe), 32.05 (s, CMe), 31.01 (s, CMe). 31P{1H}: δ 86.99 (s, PO2N),
72.49 (s, PNPh2). IR (KBr, cm-1): 1680 (s, CO). Anal. Calcd for
C47H66Cl2N2O4P2Pd (962.44): C 58.65, H 6.91, N 2.92. Found: C
58.23, H 7.12, N 2.76.
(1) ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A.,
Yamamoto, H., Eds.; Springer: Berlin, 1999.
(2) (a) Cahn, R. S.; Ingold, C. K. J. Chem. Soc. 1951, 612. (b) Cahn, R.
S.; Ingold, C. K.; Prelog, V. Experienta 1956, 12, 81. (c) Cahn, R. S.;
Ingold, C. K.; Prelog, V. Angew. Chem. 1966, 78, 413. (d) Prelog,
V.; Helmchen, G. Angew. Chem. 1982, 94, 614.
(3) (a) Paiaro, G.; Panunzi, A. J. Am. Chem. Soc. 1964, 86, 5148. (b)
Flood, T. C.; Campbell, K. D.; Downs, H. H.; Nakanishi, S.
Organometallics 1983, 2, 1590.
(9) Crystallographic study (215 K): a ) 1100.4(2) pm, b ) 1232.3(2)
pm, c ) 1909.2(3) pm, R ) 104.548(3)°, â ) 99.170(3)°, γ )
91.801(3)°, with Z ) 2 in the triclinic space group P1h. R(int) ) 0.0214,
R1 ) 0.0595, wR2 ) 0.1253 for 14310 unique reflections. All
hydrogen atoms were found and refined. Absorption correction was
performed with SADABS.
(4) Bringmann, G.; Menche, D. Acc. Chem. Res. 2001, 34, 615.
(5) Salzer, A. Coord. Chem. ReV. 2003, 242, 59.
(10) Sheldrick, G. M. SADABS, A Program for Empirical Absorption
Correction; University of Go¨ttingen: Go¨ttingen, Germany, 1998.
(11) SHELXTL PLUS, XS, Program for Crystal Structure Solution, XL,
Program for Crystal Structure Determination, XP, InteractiVe Mo-
lecular Graphics; Siemens Analytical X-ray Institute Inc.: Madison,
WI, 1990.
(6) Hauptmann, S. Organische Chemie; Deutscher Verlag fu¨r Grund-
stoffindustrie: Leipzig, 1985.
(7) (a) Sumby, C. J.; Steel, P. J. Organometallics 2003, 22, 2358. (b)
Fekner, T.; Gallucci, J.; Chan, M. K. J. Am. Chem. Soc. 2004, 126,
223. (c) Fekner, T.; Gallucci, J.; Chan, M. K. Org. Lett. 2004, 6, 989.
(12) Farugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
10.1021/ic049222e CCC: $27.50
Published on Web 09/16/2004
© 2004 American Chemical Society
Inorganic Chemistry, Vol. 43, No. 21, 2004 6543

COMMUNICATION
two groups of three adjacent atoms that form two planes
without any atoms common to both. In contrast, the tilted
ring in 1 is best described as consisting of two groups of
four adjacent atoms that form two planes having two atoms
in common that form a chiral axis. In addition, both of the
two planes are chiral.
The tilt vector P1-N2 not only creates the chirality of
the molecule, but also is responsible for a general inequiva-
lence of the two phosphorus positions. As the ligand is
unsymmetrical with two different phosphorus atoms, the
plane (P1, Pd1, P2, N2) becomes chiral in its own right. Of
the two diastereomers possible, only the one where the
phosphino phosphorus is on the tilt vector (P1) is observed
for 1.
In a related molybdenum complex, both diastereomers are
obtained in a ratio of 85:15.15 As the present Pd(II) complex
is structurally closely related to the Mo(0) complex previ-
ously reported, the occurrence of a major and a minor
diastereomer was expected.
Figure 1. ORTEP plot of 1 showing 50% probability levels. Hydrogen
atoms have been omitted, and atoms of peripheric substituents were reduced
in size for clarity.
The absence of the minor diastereomer is confirmed by
multinuclear NMR studies (1H, 31P{1H}, and 13C{1H}) of a
bulk sample. The phosphorus NMR spectrum displays no
coupling between the two different phosphorus centers. As
dynamic behavior is known in tetrahedral nickel(0) com-
plexes of similar ligands,16 low-temperature NMR (1H and
31P{1H}) was performed to find out whether the absence of
P-P coupling in the NMR spectrum of 1 is due to the
palladium or dynamic behavior of the P-Pd bonds. As
expected, the spectra at 213 and 300 K were identical except
for a small temperature-dependent shift. Thus, the complex
can be considered to be temperature stable up to 300 K with
no racemization due to a fast dissociation-association
mechanism occurring. Racemization by ring inversion can
be ruled out by the findings of Suarez et al.17 on a related
rhodium complex. The authors show that four aryl substit-
uents on the two phosphorus atoms are quite sufficient to
prevent any racemization without bond fission.
The reason for the absence of the other pair of enantiomers
remains unclear. Since there is more space available on the
square planar palladium as compared to the octahedral
molybdenum complex, steric considerations in the resulting
complex do not seem to play a major role. However, steric
reasons cannot be ruled out for indermediates or transition
states during the displacement of the cod ligand.
The chirality of 1 can be determined applying the well-
known rules developed by Cahn, Ingold, and Prelog.2
Complex 1 posseses two chiral planes (P1, Pd1, P2, N2 and
P1, N1, C1, N2). The pilot atoms for the chiral planes are
Pd1 (P1, N1, C1, N2) denoted p and O2/O3 (P1, Pd1, P2,
N2) denoted p′ in Chart 1. As palladium has a higher priority
than oxygen, the chiral plane p takes priority over p′. Within
p, the order of priority as shown in Chart 1 is P1, N2, and
Figure 2. Central metallacycle in 1 and adjacent atoms.
inequality is observed for the two P-N bonds (168.9(3) and
171.4(3) pm). The two longer bonds both contain the P1
atom of the tilt vector P1-N2. No inequivalencies in M-P
and P-N bond lengths were reported in similar Pd, Pt, and
Mo complexes of CO(PPh2NR)2 (R ) Me, Et).13 Thus, the
electronic influence of the aryloxy substituents on P2 is the
likely cause for this observed bond shortening. Whereas most
Pd-phosphoramidate bond lengths are longer than 220 pm,13
bond lengths between 217.5(5) and 218.2(2) pm have been
reported for Pd(II) complexes of N,N′-bisphosphityl-N,N′-
dialkylhydrazines (alkyl ) methyl,14a ethyl14b). All other bond
lengths and angles are unremarkable. The unit cell has a cen-
ter of inversion converting the one enantiomer into the other.
The two chiral planes are linked by two common atoms,
the P1-N2 tilt vector. This vector effectively forms a chiral
axis that is not independent of the two chiral planes. The
chiral information provided by this chiral axis is sufficient
to fully describe the chirality of the molecule, but the
information is equally well provided by the two chiral planes.
It is worth mentioning that the tilted metallacycle in 1 is
not merely another puckered ring system. A puckered six-
membered metallacycle can be described as consisting of
(13) Slawin, A. M. Z.; Wainwright, M.; Woollins, J. D. Dalton Trans. 2001,
2724 and references therein.
(14) (a) Reddy, V. S.; Katti, K. V.; Barnes, C. L. Inorg. Chem. 1995, 34,
5483. (b) Slawin, A. M. Z.; Wainwright, M.; Woollins, J. D. Dalton
Trans. 2002, 513. (c) Korostylev, A. V.; Bondarev, O. G.; Lyubimov,
S. Y.; Kovalevski, A. Y.; Petrovskii, P. V.; Davankov, A. V.; Gavrilov,
K. N. Inorg. Chim. Acta 2000, 303, 1. (d) Wong, E. H.; Sun, X.;
Gabe, E. J.; Lee, F. L.; Charland, J.-P. Organometallics 1991, 10,
3010. (e) Arduengo, A. J., III; Dias, H. V. R.; Calabrese, J. C.
Phosphorus, Sulfur Silicon Relat. Elem. 1994, 87, 1.
(15) Ku¨hl, O. Dalton Trans. 2003, 949.
(16) (a) Ku¨hl, O.; Junk, P. C.; Hey-Hawkins, E. Z. Anorg. Allg. Chem.
2000, 626, 1591. (b) Ku¨hl, O.; Langel, W. Inorg. Chem. Commun.
2003, 6, 74.
(17) Suarez, A.; Mendez-Rojas, M. A.; Pizzano, A. Organometallics 2002,
21, 4611.
6544 Inorganic Chemistry, Vol. 43, No. 21, 2004

COMMUNICATION
Chart 1. Chirality of 1 as Described by the Chiral Planes (Tilt Angle
Emphasized)
reported crystal structures. Only one pair of enantiomers is
possible in such complexes, and the chirality can be described
using either one of the chiral planes or the chiral axis.
However, the two phosphorus atoms remain formally in-
equivalent, and it remains to be seen if dynamic NMR would
be able to resolve the small difference of the two chemically
equivalent P atoms seen in the solid state. The chirality of 1
can be accurately described in terms of axial as well as
double planar chirality. As the description by axial chirality
has the advantage of simplicity, it should be preferred.2
As geometrical constraints could be responsible for tilt
vectors in otherwise planar rings, this type of chirality is
likely to be more common than presently thought. Chirality
caused by tilting requires a potentially planar metallacycle
whose planarity is prevented by geometrical constraints.
Geometrical constraints include large bond angle deviations
from the mean value (e.g., 108° for a pentagon or 120° for
a hexagon) or a broad spread of bond lengths (e.g., 139 pm
for C-N, 170 pm for P-N, and 220 pm for Pd-P in 1).
For a potentially planar metallacycle, a degree of rigidity is
required that is not provided by pure single bonds. Some
double bond character will be needed, probably on all bonds.
In the metallacycle formed in 1, that is obviously the case
for the Pd-P bonds (σ-donor and π-acceptor properties), the
C-N bonds (amide character), and also the P-N bonds for
which a varying degree of double bond character is discussed
in the literature.19
Chart 2. Chirality of 1 as Described by A2 (Dotted Line)
thus, the plane is S for the left-hand molecule. In the other
molecule, created by the center of inversion within the
crystal, this plane becomes R. The other chiral plane p′ is
assigned the priorities P2, Pd1, P1 with the symbol R and is
changed to S by the center of inversion.18 Therefore, the
molecule is obtained as the racemate (S/R; R/S)p,p′
.
If the chiral axis is utilized to describe the chirality of the
compound, the situation simplifies. There are two chiral axis
(tilt vectors) possible, denoted P1-N2 and P2-N1, respec-
tively. Complex 1 contains the axis P1-N2 with the lower
priority P atom. It should thus be labeled A2 (A1 would be
the axis P2-N1), Chart 2. If one now views the molecule
along A2 from the near end (P1), the left-hand molecule in
Chart 1 becomes S and the other R. S and R name the sense
of direction along the shortest angle between the two chiral
planes going from the higher priority plane to the lower one.
The racemate would therefore be described as (R/S)A2.
Although chirality in closely related transition metal
complexes has only been reported for unsymmetrically
substituted bisphosphino urea ligands,15 symmetrically sub-
stituted ones such as (PPh2NR)2CX (R ) Me, Et; X ) O,
S)13 also lead to chiral complexes as evidenced by their
In summary, we have described a metallacycle with a
novel type of chirality displaying two chiral planes and one
chiral axis. We have shown that for simplicity and consis-
tency it is preferable to describe the chirality of these
complexes in terms of axial rather than double planar
chirality. It is likely that tilting is a general pathway to induce
chirality into an otherwise planar ring system.
Acknowledgment. O.K. thanks J. Heinicke for his
ongoing support and Dr. C. Drost and Ms. R. Za¨be for low
temperature NMR measurements.
Supporting Information Available: Crystallographic details
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
(18) Following the CIP rules strictly, Cl would take priority over P, and in
the present case p′ would be R. As both Cl atoms have identical
priority, p′ could not be determined unambiguously if one of the
P-Pd-Cl angles exceeds 180°. Therefore, endocyclic ordering was
preferred.
IC049222E
(19) Zubiri, M. R. I.; Woollins, J. D. Comments Inorg. Chem. 2003, 24,
189.
Inorganic Chemistry, Vol. 43, No. 21, 2004 6545