Pd-(MOP) Chemistry: Novel Bonding Modes and Interesting Charge Distribution

Article abstract of DOI:10.1021/om030555d

The new Pd-MOP acetyl acetonate complexes [Pd(acac)(10a or 10b)]BF ₄, 12a,b, were prepared starting from [PU(acac)CH₃CN) ₂]BF₄, whereas [Pd(acac)(11-H)], 13, was obtained directly from Pd(acac)₂ by ad

Full text of DOI:10.1021/om030555d

Organometallics 2003, 22, 5345-5349
5345
P d -(MOP ) Ch em istr y: Novel Bon d in g Mod es a n d
In ter estin g Ch a r ge Distr ibu tion
Pascal Dotta, P. G. Anil Kumar, and Paul S. Pregosin*
Laboratory of Inorganic Chemistry, ETHZ, Ho¨nggerberg, CH-8093 Zu¨rich
Alberto Albinati* and Silvia Rizzato
Department of Structural Chemistry (DCSSI), University of Milan, 20133 Milan, Italy
Received J uly 28, 2003
The new Pd-MOP acetyl acetonate complexes [Pd(acac)(10a or 10b)]BF4, 12a ,b, were
prepared starting from [Pd(acac)(CH3CN)2]BF4, whereas [Pd(acac)(11-H)], 13, was obtained
directly from Pd(acac)2 by adding 11. [Ligand 10a , MeO-MOP ) (R)-2-(diphenylphosphino)-
2′-methoxy-1,1′-binaphthyl; ligand 11, HO-MOP ) (R)-2-(diphenylphosphino)-2′-hydroxy-
1,1′-binaphthyl]. The solid-state structures for cationic 12a and neutral 13 were determined
via X-ray diffraction methods and reveal that both structures contain Pd-C σ-bonds arising
from the MOP naphthyl backbone. In structure 13, the hydroxyl function in 11 has lost a
proton to afford a keto-anionic chelating ligand. 13C NMR studies confirm that the solution
structures are the same as those found in the solid state and, for 12, describe how the organic
cation distributes the positive charge.
In tr od u ction
charge resides primarily on the nitrogen atom.3,4 The
MeO-MOP compound, 10, is thought to prefer a bonding
mode in which the C1-C6 double bond is π-complexed,
i.e., a phosphine-olefin chelate ligand as in 7.3b In some
ways, 7 is not surprising, as Binap and MeO-Biphep
both complex this double bond rather readily in Ru(II)
chemistry, e.g., 8.5-7
The interest in how the MOP donor binds a transition
metal is related to the question of chirality transfer. It
would be useful to know if the MOP class favors
chelation, rather than a monodentate mode. In the latter
situation, a relatively rigid chiral pocket might arise via
restricted rotation around M-P and/or P-C bonds.
Alternatively, perhaps two MOP ligands may complex
in order to form a sterically crowded pocket.
The chiral biaryl-based bidentate phosphine auxilia-
ries Binap, 1, and MeO-Biphep, 2 (see Scheme 1), are
now in common use in enantioselective homogeneous
catalysis.1 Hayashi and co-workers,2 in a series of fine
papers, have pointed out that the MOP auxiliaries, e.g.,
3 and 4, provide a useful monodentate alternative to
Binap.
One member of the MOP class, MAP, 5, introduced
by Kocovsky3 and Ding,4 has been shown to display an
unexpected bonding mode. Primarily, the MAP ligand
tends to form complexes of Pd(II) in which there is a
σ-bond from the ipso carbon, C1, to the metal. A
structural fragment demonstrating this type of interac-
tion is shown in Scheme 2, as 6, where the positive
We show here that, with Pd(II), the MeO-MOP
ligands 10 can indeed demonstrate structures similar
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10.1021/om030555d CCC: $25.00 © 2003 American Chemical Society
Publication on Web 11/07/2003

5346 Organometallics, Vol. 22, No. 25, 2003
Dotta et al.
Sch em e 1. Ch ir a l Bia r yl-Ba sed P h osp h in e
Liga n d s
F igu r e 1. ORTEP view of the cation of 12a .
Sch em e 2. Str u ctu r a l F r a gm en ts Sh ow in g th e
Va r iou s Bon d in g Mod es
F igu r e 2. ORTEP view of 13.
Sch em e 3
to 6 and that the HO-MOP ligand, 11, carries this
tendency much further and affords the keto-anion
structure, 9.
Resu lts a n d Discu ssion
Solid -Sta te Str u ctu r es. The new acetyl acetonate
complexes [Pd(acac)(10a or 10b )]BF4, 12a ,b , and
[Pd(acac)(11-H)], 13, were prepared as shown in Scheme
3. In the preparation of 13 one acac ligand functions as
a base. The solid-state structures for cationic 12a and
neutral 13 were determined via X-ray diffraction meth-
ods, and views of these are shown in Figures 1 and 2,
with a summary of the most salient bond distances and
bond angles in Table 1.
chelate with both structures revealing Pd-C σ-bonds.
The local geometries can be considered as distorted
square planar, with bond angles varying from the
classical values by as much as ca. 10°.
The immediate coordination sphere of both complexes
consists of the two acac Pd-O interactions and the P,C

Pd-(MOP) Chemistry
Organometallics, Vol. 22, No. 25, 2003 5347
Ta ble 1. Bon d Len gth s (Å) a n d An gles (d eg) for
P d (II) Com p ou n d s 12a a n d 13
Ta ble 2. δ 13C (in p p m ) for th e Com p lexes a n d
Liga n d s
12a
13
Bond Lengths
2.219(2)
2.040(6)
2.052(6)
2.226(8)
2.615(8)
1.36(1)
Pd1-P1
Pd1-O1L
Pd1-O2L
Pd1-C1
Pd1-C6
C6-O1
2.1938(6)
2.057(2)
2.072(2)
2.129(2)
2.766(2)
1.236(3)
1.495(3)
1.483(3)
C1-C6
1.41(1)
1.47(1)
C1-C2
position
10a
12a
10b
12b
11
13
Bond Angles
90.6(2)
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
122.1
134.4
129.1
130.2
113.0
155.5
125.4
126.8
123.7
128.3
86.7
140.2
129.5
147.1
113.3
166.3
125.9
131.4
127.4
130.4
122.2
134.2
128.9
130.0
113.5
155.1
a
86.9
139.8
129.3
146.9
113.4
166.4
126.3
a
118.3
133.7
128.8
130.2
117.7
151.4
125.0
126.8
123.4
128.2
70.5
146.1
128.8
142.8
127.6
193.7
126.5
127.9
123.9
128.7
O1L-Pd-O2L
O1L-Pd-C1
O1L-Pd-P
C1-Pd-P
89.54(8)
94.38(8)
177.83(6)
83.93(7)
175.10(8)
92.09(6)
99.1(3)
177.0(2)
83.0(2)
170.3(2)
87.2(2)
C1-Pd-O2L
P-Pd-O2L
a
a
The two Pd-C bond separations, 2.226(8) and
2.129(2) Å, for 12a and 13, respectively, are quite
different and only slightly long for Pd-C σ-bonds.8 In
the π-allyl Pd-MAP cation, 14, the analogous Pd-C
a
127.8
130.3
a
Not assigned.
F igu r e 3. 13C long-range correlation for 13. The chemical
shift of the ketone CdO is clearly revealed by the correla-
tion to H4, whereas H5 and H7 correlate to the pseudo-
sp3 carbon, C1, (CD2Cl2).
bond length is 2.338(11) Å, i.e., very much longer than
for either 12a or 13.4a The two Pd-P bond lengths at
2.219(2) and 2.194(1) Å, for 12a or 13, respectively, are
fairly short, as might be expected for P-donors trans to
an oxygen donor.8 The two different Pd-O separations
within each complex are reasonable, although they are
both significantly longer for 13 than for 12a , in keeping
with the observed difference in the Pd-C distances. The
two C6-O1 distances, 1.36(1) Å for 12a and 1.236(3) Å
for 13, clearly indicate that, in 13, we are dealing with
an organic ketone (whose expected CdO length is ca.
1.22 Å9). Further, for 13, we note that the C4-C5 bond
length, at 1.331(4) Å,9 is consistent with the localized
CdC bonding found in 9. The Pd-C1-C1′ angles of
113.5(5)° and 114.6(2)° in the two structures suggest
some tetrahedral character for C1. Taken together these
X-ray data suggest that, for complexes 12a and 13, C1
represents an alkyl-like donor. The bonding within the
hydrocarbon of 12a ,b is best discussed together with the
13C data.
F igu r e 4. 13C long-range correlation for 12b. The chemical
shift of the MeO-C ipso-carbon, C6, resonance is clear, at
δ ) 166.4, via the correlation to H4, whereas H5 and H7
correlate to the pseudo-sp3 carbon, C1, (CD2Cl2).
NMR Solu tion Resu lts. Selected 13C NMR data for
the ligands and their complexes, 12 and 13, are shown
in Table 2. In solution we observed only a single species
for both 12 and 13. As there is little to distinguish 12a
from 12b, only the former will be discussed. Figures 3
and 4 show the 13C,1H long-range correlations which
provide the key assignments for the fully substituted
carbon resonances. The proton AX spin system, from the
signals for H4 and H5, is readily assigned via COSY
spectroscopy. Further, the second AX pattern (in the
phosphine naphthyl moiety) is readily detected via a
31P,1H correlation and thus is easily differentiated from
H4 and H5. One can also use the 3J (C6,H(MeO))
interaction to confirm the assignment of C6.
The ketone carbon in 13, at δ ) 193.7, is readily
observed (see Figure 3) and found at a much higher
frequency than one would expect for a phenolic sp2
carbon. Moreover, the organic 13C NMR literature11,12
(8) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,
D. G.; Taylor, R. J . Chem. Soc., Dalton Trans. 1989, S1-S83.
(9) Allen, F. H.; Kennard, O.; Watson, D. G.; Orpen, A. G.; Brammer,
L.; Taylor, R. J . Chem. Soc., Perkin Trans. 2 1987, S1-S19.
(10) During the preparation of the manuscript Chan and co-workers
published the preparation of a related ketone-like Pd-complex, based
on a di-imine ligand: Xu, L.; Shi, Q.; Li, X.; J ia, X.; Huang, X.; Wang,
R.; Zhou, Z.; Lin, Z.; Chan, A. S. C. Chem. Commun. 2003, 1666-1667.

5348 Organometallics, Vol. 22, No. 25, 2003
Dotta et al.
Sch em e 4. ∆(δ 13C) Va lu es (in p p m ) for
Com p ou n d s 12a a n d 13, ∆(δ 13C) ) δ
to complex Pd(II) when given the opportunity. The
presence of either of the electron-donating substituents,
Me2N or MeO, can lead to molecular structures, in both
the solid and solution states, that favor a σ-bond
between C1 and the Pd atom. If the MOP oxygen atom
carries an H atom, this is readily lost to afford the keto-
anion. The keto-anion structure is easily recognized via
its characteristic 13C carbonyl chemical shift. Since the
parent MOP ligand, 3 in Scheme 1, may not readily form
structures such as 12 or 13, it seems likely that
individual ligands within the MOP class may well
behave quite differently under catalytic conditions. With
respect to how the MOP class functions as a chiral
auxiliary, it remains to be seen whether more than one
MOP ligand prefers to complex a transition metal, and
studies in this direction are in progress.
13C(coor d in a ted liga n d ) - δ 13C(fr ee liga n d )
suggests that this is exactly the correct δ value for the
carbonyl of an R,â-unsaturated ketone.
For both 12a and 13 the ipso carbons, C1, involved
in the Pd bonding, appear at low frequency, δ ) 86.7,
70.5, respectively, in the aliphatic region of the spec-
trum, thereby supporting their formulation as pseudo
sp3 carbons. As expected, based on the relatively short
Pd-C bond length in 13, this C1 resonance appears at
the lowest frequency.
Salt 12a contains a primarily organic cation, in that,
formally, the local coordination sphere about the Pd
atom is neutral. The 13C signal of the â-carbon of an
Exp er im en ta l Section
All manipulations were carried out under a nitrogen atmo-
sphere. Pentane and ether were distilled from NaK, and
CH2Cl2 from CaH2. The (R)-MOP ligands were prepared by
published procedures.13 Pd(acac)2 was purchased from Strem.
[Pd(acac)(CH3CN)2](BF4) was prepared by reaction of Pd(acac)2
with 1 equiv of HBF4 in CH3CN. NMR spectra were recorded
with Bruker DPX-400 and DPX-500 spectrometers at room
temperature. Chemical shifts are given in ppm and coupling
constants (J ) in hertz. Elemental analyses and mass spectro-
scopic studies were performed at the ETHZ.
R,â-unsaturated organic ketone or ester should be found
at ca. 140-150 ppm,11,12 so that the observed chemical
shift value of C4, 142.8 ppm, in 13 is as expected.
However, the observed chemical shift of 147.1 ppm for
Cr ystallogr aph y. Air-stable, red crystals of 12a[BF4]‚(CH3)2-
CO were obtained from CH2Cl2/ether. Dark orange crystals of
13‚CH2Cl2, suitable for X-ray diffraction, were obtained by
crystallization from CH2Cl2/pentane and are air stable.
For the data collection, prismatic single crystals of both com-
pounds were mounted on a glass fiber at a random orientation.
For compound 12a a Bruker SMART CCD diffractometer was
used for a room-temperature data collection. For 13, data were
acquired at 90(2) K using a Bruker APEX CCD diffractometer.
The space groups were determined from the systematic
absences, while the cell constants were refined with the data
reduction software SAINT14 at the end of the data collection.
The experimental conditions for the data collection, plus
crystallographic and other relevant data, are listed in Tables
3 and S1.
C4 in 12a suggests that this carbon carries some of the
cation positive charge (see Scheme 4 and Table 2).
Scheme 4 gives the 13C coordination chemical shifts for
12a and 13. These data reveal relatively large ∆δ values
for the resonances of C4 and C6 in 12a , 16.9 and 10.8
ppm, respectively.
Continuing for 12a , the methoxy-bearing carbon, C6
at 166.3 ppm (rather than at 155.5 as found in the
ligand itself), appears at relatively high frequency,
suggesting that this carbon position, along with C4 and
C6, also shares in the positive charge of the cation.
We note that the 31P chemical shifts for 12a and 13,
δ ) 48.5, 46.5, respectively, appear at considerably
higher frequency than for trans-PdCl2(10)2, δ ) 29.7.
Com m en ts. It seems clear that the MOP class will
find one of several possible chelating modes with which
The collected intensities were corrected for Lorentz and
polarization factors14 and empirically for absorption using the
SADABS program.15 The standard deviations on intensities
2
were calculated in term of statistics alone, while those on Fo
were calculated as shown in Table 3 and Table S1.
Str u ctu r a l Stu d y of 12a [BF 4]‚(CH3)2CO. The structure
was solved by direct and Fourier methods and refined by full
2
matrix least squares,16 minimizing the function [∑w(Fo
-
(1/k)Fc2)2] and using anisotropic displacement parameters for
(13) Uozumi, Y.; Tanahashi, A.; Lee, S.; Hayashi, T. J . Org. Chem.
1993, 1945-1948.
(14) SAINT: SAX Area Detector Integration; Siemens Analytical
Instrumentation, 1996.
(15) Sheldrick, G. M. SADABS; Universita¨t Go¨ttingen. To be
published.
(11) Kalinowski, H. O.; Berger, S.; Braun, S. 13C NMR Spektroskopie;
Georg Thieme Verlag: Stuttgart, 1984.
(12) Pretsch, E.; Seibl, J .; Simon, W. Strukturaufklarung organischer
Verbindungen; Springer-Verlag: Berlin, 1986.
(16) Sheldrick, G. M. SHELX-97. Structure Solution and Refinement
Package; Universita¨t Go¨ttingen, 1997.

Pd-(MOP) Chemistry
Ta ble 3. Exp er im en ta l Da ta for th e X-r a y
Organometallics, Vol. 22, No. 25, 2003 5349
Stirring was continued for 15 min, followed by removal of the
solvents under reduced pressure. The crude product was
recrystallized from CH2Cl2/ether and washed twice with ether
(10 mL portions). Yield: 70.7 mg (87%). Anal. Calcd for
Diffr a ction Stu d y of Com p ou n d s
12a [BF 4]‚(CH3)2CO a n d 13‚CH2Cl2
formula
mol wt
C
41H38 BF4O4PPd C38H32Cl2O3PPd
C
38H32O3BF4PPd (760.87): C, 59.99; H, 4.24. Found: C, 59.72;
818.89
744.91
H, 4.48. MS (MALDI): 673.1 (M+ - BF4, 100%), 574.1 (M+
-
data coll. T, K
diffractometer
cryst syst
space group (no.)
a, Å
293(2)
90(2)
1
3
Brucker SMART
orthorombic
P212121 (19)
9.082(2)
18.718(5)
22.526(6)
Brucker APEX
monoclinic
P21 (4)
8.7586(4)
17.9736(8)
11.2010(5)
112.061(1)
1634.2(1)
2
BF4 - acac, 32%). H NMR (CD2Cl2, 400 MHz): 8.84 (d, J HH
3
) 9.3, H-4), 7.98 (d, J HH ) 7.8, H-10), 7.48 (H-9), 7.42 (H-8),
3
3
7.39 (d, J HH ) 9.3, H-5), 7.23 (d, J HH ) 8.2, H-7), 5.04 (s,
L3-H), 3.60 (s, O-CH3), 1.73 (s, L1-CH3), 1.67 (s, L5-CH3). 13C
NMR (CD2Cl2, 100 MHz): 188.7 (L2-C), 184.9 (L4-C), 99.5
(L3-C), 57.7 (O-CH3), 28.4 (L1-C), 26.1 (L5-C). For additional
13C data, see Table 2. 31P NMR (CD2Cl2, 161 MHz): 48.5 (s).
Syn th esis of [P d (a ca c)(10b)](BF 4), 12b. To a stirred
solution of 17.3 mg (0.046 mmol) of [Pd(acac)(CH3CN)2](BF4)
in 1.5 mL of acetone was slowly added a solution of 32 mg
of 10b in 3 mL of CH2Cl2. The resulting solution was stirred
for 10 min at room temperature, followed by removal of
the solvents under reduced pressure. The red solid obtained
was washed twice with pentane. Yield: 44 mg (97%). Anal.
Calcd for C54H64O3BF4PPd‚H2O (1003.32): C, 64.64; H, 6.63.
Found: C, 64.64; H, 6.59. MS (HiResMALDI): found 897.3651
(M+ - BF4, 100%), calcd 897.3641. 1H NMR (CD2Cl2, 500
b, Å
c, Å
â, deg
V, Å3
3829(2)
4
1.420
5.86
Z
F
(calcd), g cm-3
1.514
8.18
µ, cm-1
radiation
Mo KR (graphite monochrom.,
λ ) 0.71073 Å)
θ range, deg
no. data collected
no. indep data
2.11 < θ < 26.27
20 744
6843
2.27 < θ < 27.54
17 143
7471
no. obsd reflns (no)
4632
7222
2
2
[|Fo| > 2.0σ(|F| )]
no. of params refined (nv) 431
415
3
3
Rint
0.0705
0.0272
0.0281
0.0649
1.046
MHz): 8.96 (d, J HH ) 9.4, H-4), 8.10 (d, J HH ) 8.0, H-10),
3 3
R(obsd reflns)a
Rw2(obsd reflns)b
cGOF
0.0587
0.1071
1.038
7.51 (d, J HH ) 9.4, H-5), 7.30 (d, J HH ) 8.3, H-7), 5.13 (s,
L3-H), 3.78 (s, O-CH3), 1.86 (s, L1-CH3), 1.75 (s, L5-CH3), 1.33
(s, 18 H, C(CH3)3), 1.31 (s, 18 H, C(CH3)3). 13C NMR (CD2Cl2,
125 MHz): 187.7 (L2-C), 184.4 (L4-C), 99.4 (L3-C), 58.5
(O-CH3), 34.9 (C(CH3)3), 30.9 (C(CH3)3), 28.5 (L1-C), 26.2
(L5-C). For additional 13C data, see Table 2. 31P NMR
(CD2Cl2, 202 MHz): 51.4 (s).
R ) ∑(|Fo - (1/k)Fc|)/∑|Fo|. Rw ) [∑w(Fo - (1/k)Fc2)2/
a
b
2
2
∑w|Fo | ]. c GOF ) [∑w(Fo - (1/k)Fc2)2/(no - nv)]1/2
.
2 2
2
all atoms. In the difference Fourier maps a clathrated acetone
molecule was located and refined using isotropic temperature
Syn th esis of [P d (a ca c)(11-H)], 13. To a stirred solution
of 26.8 mg (0.088 mmol) of Pd(acac)2 in 1 mL of CH2Cl2 was
slowly added a solution of 40 mg (0.088 mmol) of 7 in 3 mL of
CH2Cl2. The solution was stirred for 18 h at room temperature,
at which point an in situ 31P NMR showed one major product
(ca. 95%). The solvent was evaporated under reduced pressure
and the crude product recrystallized from CH2Cl2/pentane. The
product was washed twice with pentane. Yield: 32 mg (55%).
Anal. Calcd for C37H29O3PPd (659.03): C, 67.43; H, 4.44.
Found: C, 67.27; H, 4.60. MS (MALDI): 559 (M+ - acac, 35%),
-
factors. Both the acetone molecule and the BF4 anion are
highly disordered, as can be seen from the large atomic
displacements, and therefore their geometries are only ap-
proximate. It proved impossible to find a reasonable model for
the disorder of the counterion, so that it was constrained to
maintain a tetrahedral geometry. No extinction correction was
deemed necessary. Upon convergence (see Table S1), the final
Fourier difference map showed no significant peaks. The
contribution of the hydrogen atoms, in their calculated position
(C-H ) 0.95 (Å), B(H) ) 1.2/1.5B(Cbonded) (Å2)), was included
in the refinement using a riding model. Refining the Flack’s
parameter17 tested the handedness of the structure.
Str u ctu r a l Stu d y of 13‚CH2Cl2. The structure was solved
and refined as above. Anisotropic displacement parameters
were used for all atoms. A clathrated solvent molecule
(CH2Cl2) was found from the difference Fourier maps and
included in the refinement. The solvent molecule is disordered
over two positions. Both orientations were refined together
with their occupancy factors (0.81 and 0.19, respectively), using
anisotropic ADPs. The handedness of the structure was tested
by refining the Flack’s parameter.17 All calculations were car-
ried out by using the PC version of SHELX-97.16 The scattering
factors used, corrected for the real and imaginary parts of the
anomalous dispersion, were taken from the literature.18
Syn th esis of [(10a )P d (a ca c)](BF 4), 12a . To a stirred
solution of 40 mg (0.107 mmol) of [Pd(acac)(CH3CN)2](BF4) in
3 mL of acetone and 1 mL of CH2Cl2 was slowly added 50 mg
(0.107 mmol) of 10a in 3 mL of acetone at room temperature.
1
453 (HO-MOP, 100%). H NMR (CD2Cl2, 500 MHz): 7.81 (d,
3J HH ) 9.7, H-4), 7.49 (H-10), 7.13 (t, 3J HH ) ca. 7.3, H-9), 6.99
3
3
3
(t, J HH ) ca. 7.3, H-8), 6.81 (d, J HH ) 7.7, H-7), 6.51 (d, J HH
) 9.7, H-5), 5.16 (s, L3-H), 1.83 (s, L1-CH3), 1.79 (s, L5-CH3).
13C NMR (CD2Cl2, 125 MHz): 187.1 (L2-C), 186.0 (L4-C), 98.9
(L3-C), 28.2 (L1-C), 27.4 (L5-C). For additional 13C data, see
Table 2. 31P NMR (CD2Cl2, 202 MHz): 46.5 (s).
Ack n ow led gm en t. P.S.P. thanks the Swiss Na-
tional Science Foundation and the ETH Zurich for
financial support. A.A. thanks MIUR for a research
grant (PRIN 2002). We also thank J ohnson Matthey,
U.K., for the loan of precious metal salts.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details and a full listing of crystallographic data,
including tables of positional and isotropic equivalent displace-
ment parameters, calculated positions of the hydrogen atoms,
anisotropic displacement parameters, bond distances, bond
angles, and torsion angles. ORTEP figures showing the full
numbering schemes. This material is available free of charge
via the Internet at http://pubs.acs.org.
(17) Flack, H. D. Acta Crystallogr. 1983, A 39, 876.
(18) International Tables for X-ray Crystallography; Wilson, A. J .
C., Ed.; Kluwer Academic Publisher: Dordrecht, The Netherlands,
1992; Vol. C.
OM030555D