Palladium-MOP Chemistry: Pseudo-cis-Allyl MOP Complexes and Flexible Olefin Bonding. Preliminary Communication

Article abstract of DOI:10.1002/hlca.200490014

We show that both palladium(0) and palladium(II) metal centers are capable of coordinating two monodentate MOP (=(R)-2-(diarylphosphino)-1,1′ -binaphthalene) ligands in a pseudo-cis orientation, despite published statements to the contrary. In addition to [Pd(η³-C ₃H₅)(MeO-MOP)₂]BF₄ (MeO-MOP=(R)-2-(diphenylphosphino)-2′-methoxy-1,1′-binaphthalene), the first examples of chiral bis κC¹-prop-2-enyl (η ¹-CH₂CH=CH₂) complexes [cis-Pd(κC ¹-C₃H₅)₂(MeO-MOP or MOP) ₂], are shown to be relatively stable. Further, coordinated MOP and MeO-MOP both show stronger propensity towards novel intramolecular π-olefin complexation than the CN-MOP analogue. The solid-state structure of [Pd(fumaronitrile)(MOP)₂] is reported.

Full text of DOI:10.1002/hlca.200490014

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Palladium-MOP Chemistry: Pseudo-cis-Allyl MOP Complexes and Flexible
Olefin Bonding
Preliminary Communication
by Pascal Dotta, P. G. Anil Kumar, and Paul S. Pregosin*
Laboratory of Inorganic Chemistry, ETHZ, CH-8093Z¸rich
and Alberto Albinati*
Department of Structural Chemistry (DCSSI), University of Milan, I-20133 Milan
We show that both palladium(0) and palladium(II) metal centers are capable of coordinating two
monodentate MOP (ˆ(R)-2-(diarylphosphino)-1,1'-binaphthalene) ligands in a pseudo-cis orientation, despite
published statements to the contrary. In addition to [Pd(h3-C3H5)(MeOÀMOP)2]BF4 (MeOÀMOP ˆ (R)-2-
(diphenylphosphino)-2'-methoxy-1,1'-binaphthalene), the first examples of chiral bis kC1-prop-2-enyl
(h1-CH2CHˆCH2) complexes [cis-Pd(kC1-C3H5)2(MeOÀMOP or MOP)2], are shown to be relatively stable.
Further, coordinated MOP and MeOÀMOP both show stronger propensity towards novel intramolecular p-olefin
complexation than the CNÀMOP analogue. The solid-state structure of [Pd(fumaronitrile)(MOP)2] is reported.
Applications of homogeneous catalysis involving chiral monodentate phosphine
ligands continue to increase. One of the more-successful classes, the so-called MOP
ligands, 1 3, introduced by Hayashi [1a], has been used in enantioselective allylic
alkylation [1b,c], hydrosilylation1) [1d g], and allylic reduction [1a,h].
Since the MOP ligand is relatively large, there is some uncertainty as to the number
of MOP molecules capable of coordinating to a transition metal. Specifically, for 3
(ˆ(R)-(2'-methoxy-[1,1']binaphthalen-2-yl)diphenylphosphine), which has a cone
angle of ca. 2008 [1b], it has been stated [1b] that for −MeOÀMOP... the p-allyl Pd
cannot accommodate two molecules of phosphine ligand because of the steric
bulkiness...×. In connection with our studies on enantioselective allylic alkylation [2],
we have prepared several new chiral Pd-MOP complexes and show here that a) indeed,
two MOP ligands can readily complex Pd in a cis-orientation, b) an unprecedented bis
1
)
trans-PdCl2((R)-MeO-MOP)2 is known [1g].

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kC1 allyl-bis(MOP) compound is relatively stable, and c) several MOP complexes of
PdII reveal a diene p-olefin bonding mode.
As the allylic alkylation and hydrosilylation reactions involve oxidative addition
reactions to Pd0, we prepared a stable fumaronitrile (trans-but-2-enedinitrile) Pd0
complex by adding two equivalents of the smallest conventional MOP, 1 (ˆ(S)-
([1,1']binaphthalen-2-yl)diphenylphosphine) to [Pd2(dba)3] (dba ˆ dibenzylidene-
acetone ˆ 1,5-diphenylpenta-1,4-dien-3-one) followed by addition of fumaronitrile.
The solid-state structure of the product, [Pd(fumaronitrile)(1)2], 4 (ˆ bis[(S)-
([1,1']binaphthalen-2-yl)diphenylphosphine-kP][(2,3-h)-but-2-enedinitrile]palladium),
is shown in Fig. 1. Interestingly, the two MOP ligands sit comfortably in a pseudo cis-
orientation around the Pd-atom with a modest PÀPdÀP angle of ca. 1148, i.e., there are
no untoward steric interactions that might have resulted in a PÀPdÀP angle of > 1208.
We note that this structure shows the two PPh2 groups placed relatively close to each
other, and the two naphthalenyl moieties in rather remote positions. The fumaronitrile
olefinic C-atoms are located approximately in the PÀPdÀP plane. The various bond
lengths within the coordination sphere are normal; however, the PdÀP distance is
somewhat long, Pd(1)ÀP(1) ˆ 2.358(1) ä2).
To expand on the zero-valent class of MOP complexes, Pt(diphenylacetylene)2 was
allowed to react with 2 equiv. of 1 to afford [Pt(PhCꢀCPh)(1)2], in solution as the only
product (Eqn. 1). This complex was not isolated; however, its 195Pt-NMR spectra
revealed the triplet multiplicity expected for a bis-phosphine complex (1J(Pt,P) ˆ
3590 Hz, d 195Pt ˆ À4968, CD2Cl2, 233 K), thereby confirming the presence of two
complexed MOP ligands within the coordination sphere.
Pt(PhCꢀCPh)2 ‡ 2 1 ! Pt(PhCꢀCPh)(1)2 À PhCꢀCPh
(1)
As a structural model for the PdII oxidation state, we allowed 1 equiv. of the
dinuclear allyl complex [Pd2(h3-C3H5)2(m-Cl)2], to react with 2 equiv. of AgBF4 and
4 equiv. of MeOÀMOP, 3. The yellow-orange product was isolated (53%); the 31P-
NMR spectrum, shown in Fig. 2, contains three components in the ratio 1.0 :0.8 :1.4 and
a small amount of phosphine oxide. Based on detailed NMR studies, the three MOP-
containing species are suggested to be 6 8.
Compound 6 (ˆ bis[(R)-(2'-methoxy-[1,1']binaphthalen-2-yl)diphenylphosphine-
kP](h3-prop-2-enyl)palladium(1‡)3) represents the now expected bis-phosphine
product. Its 31P-NMR reveals an AB spin system, plus the five 1H- and three
13C-NMR resonances of the allyl-ligand. The axial and equatorial nature of the two
PPh2 groups renders the two 31P signals (just barely) non-equivalent with respect to the
allyl group.
2
)
In keeping with this longish bond, we note that the 13C chemical shifts for the fumaronitrile olefinic C-
atoms in the two diastereomers of 4, d13C ˆ 32.5 and 36.7, are found at ca. 10 ppm higher frequency
relative to those of related Pd0 fumaronitrile complexes [2a], i.e., the p-bonding to the olefin is not quite so
marked.
NMR-Data of 6 (d in ppm, J in Hz): 1H-NMR (CD2Cl2, 400 MHz): 3.21 (m, 3J(P,H) ˆ 10.4, HantiÀC(1));
3.52 (m, 3J(P,H) ˆ 10.4, HantiÀC(3)); 4.06 (br. t, 3J(H,H) ˆ 5.4, HsynÀC(3)); 4.29 (br. t, 3J(H,H) ˆ 4.9,
HsynÀC(1)); 5.74 (m, HÀC(2)). 13C-NMR (CD2Cl2, 100 MHz): 78.7 (C(1)); 80.1 (C(3)); ca. 123(C(2)).
31P-NMR (CD2Cl2, 121 MHz): 29.7, 30.0 (2J(P,P) ˆ 36).
3
)

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Fig. 1. ORTEP View of the MOP-complex 4. Selected bond lengths [ä] and bond angles [8]: PdÀC(1L),
2.119(2); Pd(1)ÀP(1), 2.3588(8); P(1)ÀC(6), 1.855(2); P(1)ÀC(111), 1.828(2); P(1)ÀC(121), 1.847(2);
C(1L)ÀC(1L)*, 1.431(5); C(2L)ÀN(1), 1.130(4); P(1)ÀPd(1)ÀP(1)*, 114.11(3); P(1)ÀPd(1)ÀC(1L),
142.4(1); P(1)ÀPd(1)ÀC(1L)*, 103.4(1); C(1L)*ÀC(1L)ÀC(2L), 117.8(3); C(1L)ÀC(2L)ÀN(1), 177.5(4).
The molecule lies on a C2 axis passing through the Pd-atom and the midpoint of the coordinated double bond.
Symmetry related (starred) atoms are obtained by the symmetry operation À x; y; À z. Crystal data for 4:
C68H48N2P2Pd, M ˆ 1061.42, monoclinic, space group C2 (no. 5), a ˆ 22.318(7), b ˆ 8.930(3), c ˆ 16.679(5) ä,
b ˆ 117.199(5) deg, U ˆ 2956.8(15) ä3, Z ˆ 2, d ˆ 1.192 g cmÀ3 m ˆ 4.08 cmÀ1, Tˆ 295(2) K. All atoms were
refined anisotropically by full-matrix least-squares on F2. Final agreement factors are: R1 ˆ 0.0290 (for 6595
unique reflections with I > 2s(I)), 0.0311 (for all 6862 independent reflections).
The bis-allyl complex, 7 (ˆ bis[(R)-2'-methoxy-[1,1']binaphthalen-2-yl)diphenyl-
phosphine-kP]bis(prop-2-enyl-kC1)palladium), represents the first example of a stable
bis-kC1-allyl chiral compound. Generally, end-on allyl complexes are rarely observed.
For complexes of chiral ligands, we know of only one other example, 9, in which this
isomeric form is stable [3].
1
The kC1 allyl form in 7 is recognizable by its characteristic H- and 13C-NMR
resonances, i.e., diastereotopic aliphatic CH2 protons and an ABX spin system at
relatively high frequency, typical for a vinyl group (Fig. 3)4).
The presence of the two MOP ligands in 7 is confirmed by pulsed-gradient spin-
echo (PGSE) diffusion measurements [4 6]. The smaller diffusion constants, D, for 7,
1
in CH2Cl2 (D ˆ 8.71 and 8.63  10À10 m2 sÀ1 from H- and 31P-NMR measurements,
respectively), reflect the observed difference in volume between a single MOP ligand
NMR Data of 7 (d in ppm, J in Hz): 1H-NMR (CD2Cl2, 400 MHz): 3.24 (m, 3J(H,H) ˆ 7.3, 2J(H,H) ˆ
3J(P,H) ˆ ca. 15, PdCH2CHˆCH2); 3.35 (m, 3J(H,H) ˆ 7.2, 2J(H,H) ˆ 3J(P,H) ˆ ca. 15, 2 H,
PdCH2CHˆCH2); 5.06 (dd, 3J(H,H) ˆ 16.9, 2J(H,H) ˆ 3.7, 2 H, PdCH2CHˆCH2 , trans-H); 5.18
(dd, 3J(H,H) ˆ 10.2, 2J(H,H) ˆ 3.7, 2 H, PdCH2CHˆCH2 , cis-H); 5.32 (m, 2 H, PdCH2CHˆCH2).
13C-NMR (CD2Cl2, 100 MHz): 29.5 (PdCH2CHˆCH2); 124.4 (PdCH2CHˆCH2); 126.3(PdCH 2CHˆCH2).
31P-NMR (CD2Cl2, 121 MHz): 21.3.
4
)

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Fig. 2. 31P-NMR Spectra of 6 8 at ambient temperature. Complex 6 shows an AB spectrum, complex 7 a singlet,
and 8 and AX spin system. The 31P-NMR line widths in 8 indicate an exchange process and 2D exchange
spectroscopy at 253K confirms that the two MeO ÀMOP ligands exchange places.

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Fig. 3. The proton resonances of the vinyl group of 7. The 1D spectra show the normal 1H chemical shifts for a
vinyl group at d ca. 5.0 5.4. The cross-peaks arise from two sections of a 2D C,H one-bond correlation and show
the expected vinyl 13C chemical shifts at d ca. 124 126, and not those expected for a Pd-allyl ligand.
(D ˆ 10.59 and 10.89  10À10 m2 sÀ1 from 1H- and 31P-NMR data, resp.) and a complex
containing two such large molecules. The ratio of D-values, ca. 1.24, is exactly what is
expected for ca. double the volume5). Clearly, there is facile transfer of an allyl ligand
from one metal center to another, perhaps due to the ease of h3 to kC1 isomerisation.
Diene complex 8 (ˆ{(R)-[2'-methoxy-[1,1']binaphthalen-2-yl-(1',2',3',4'-h)]diphen-
ylphosphine-kP}[(R)-(2'-methoxy-[1,1']binaphthalen-2-yl)diphenylphosphine-kP]pal-
2
ladium(2‡)) shows two very different 31P resonances (d ˆ 19.4 and 33.7, J(P,P) ˆ
83Hz), with the former signal associated with the chelate ring. This difference arises
due to a novel PdÀMOP interaction, i.e., one MOP ligand serves as a six-electron
The value 1.24 is arrived at by taking the average of the two values (31P and 1H) for the ligand and dividing
it by the average of the two for the complex.
5
)

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donor. The h4-complexation in 8 was identified via a set of 13C,1H correlations with the
key 13C signals for the complexed arene assigned (at 253K) to d 103.6 (C(1')), 127.7
1
(C(2')), 80.4 (C(3')), and 86.3(C(4 ')). The corresponding H-signals for protons at
C(3') and C(4') appear at d 5.49 and 7.35. The use of 1, the unsubsubstituted MOP
analog, affords the three analogous products. Clearly this chemistry is not restricted to
MOP derivatives that possess electron-donating substituents.
Although there is now some literature for MOP naphthalenyl bonding [7], the
−normal× p-(or s)-complexation occurs via the fully substituted-C(1) and C(2) and not
via C(3) and C(4). Consequently, the p-bond complexation in 8 is unique for Pd.
Interestingly, the same reaction with 2 (R ˆ CN) instead of either 1 or 3 gave only
complexes analogous to 6 and 7, some phosphine oxide, but no p-bond complexation
analogous to 8. We believe that the CN group of 2 decreases the electron-donating
capacity of the naphthalene moiety, thus suppressing chelate formation in 8.
Consequently, the structural chemistry of the MOP class is not trivial, and postulated
reaction mechanisms involving this group of compounds may need to clearly
differentiate between 2 and 1 or 3.
Our results clearly show that both Pd0 and PdII are capable of coordinating two
MOP ligands in pseudo cis-orientation. Interestingly, and, perhaps, because these MOP
ligands are quite large, h3 to kC1 isomerisation is not only facile, but the end-on ligated
form is relatively stable. Further, the choice of MOP ligand is not trivial in that both
MOP complexes 1 and 3 show stronger propensity towards the novel intramolecular
p-olefin complexation than does the CN-analogue, 2.
P. S. P. thanks the Swiss National Science Foundation and the ETH Z¸rich for financial support. We also
thank Johnson Matthey for the loan of PdCl2 and K2PtCl4.
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Received June 28, 2003