Synthesis and characterization of palladium complexes supported by an NPN-phosphido ancillary ligand

Article abstract of DOI:10.1021/om200701h

The Pd^II coordination chemistry of the phosphido ligand [(2-Me₂NC₆H₄)₂P]^- (NPN) is reported, along with the results of studies documenting the utility of such complexes as catalyst precursors for the Heck arylation of olefins. Examples of κ¹-, κ²-, and κ³-NPN coordination to Pd are described. The secondary phosphine (NPN)H (1) was reacted with 0.5 equiv of [(C₃H₅)PdCl]₂ to afford the dimeric species [(κ²-NPN)PdCl]₂ (2) in 96% yield. Similarly, the reaction of 1 with 0.33 equiv of [Pd(OAc)₂] ₃ led to the formation of dimeric [(κ²-NPN)PdOAc] ₂ (3) in 98% yield. Complexes 2 and 3 were both crystallographically characterized to reveal dipalladium species that feature bridging phosphido groups, as well as terminal chloride and acetate ligands, respectively. Treatment of 3 with Me₃SiOTf resulted in the formation of the analogous triflate complex [(κ²-NPN)PdOTf]₂ (4; 78% yield). Treatment of 2 with (C₃H₅)MgCl afforded the thermally sensitive, phosphido-bridged complex [(κ¹-NPN) Pd(η³-C₃H₅)]₂ (5) in 87% yield. The solid-state structure of 5 was confirmed by X-ray crystallography. Precoordination of BPh₃ to the NPN phosphido group was attempted in order to discourage the formation of phosphido-bridged (NPN)Pd species. Thus, the reaction of 1 with 1 equiv each of KCH₂Ph and BPh₃ led to the formation of [N(P?BPh₃)N]K (6), which was isolated in 88% yield. The related complex [N(P?BH₃)N]H (7) was prepared by treatment of 1 with BH₃?THF, and crystallographic characterization confirmed the preferential coordination of the NPN ligand phosphorus to BH₃. The reaction of 6 with 0.5 equiv of [Pd(C ₃H₅)Cl]₂ led to the formation of [κ³-N(P?BPh₃)N]Pd(η¹-C ₃H₅), which was isolated in 45% yield. Complexes 2-4 were established as precatalysts for the Heck coupling of ethyl acrylate and haloarenes.

Full text of DOI:10.1021/om200701h

Article
pubs.acs.org/Organometallics
Synthesis and Characterization of Palladium Complexes Supported
by an NPN-Phosphido Ancillary Ligand
Morgan C. MacInnis,† Robert McDonald,‡ and Laura Turculet*,†
†Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3
‡X-ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
S
* Supporting Information
ABSTRACT: The PdII coordination chemistry of the phosphido
ligand [(2-Me2NC6H4)2P]− (NPN) is reported, along with the
results of studies documenting the utility of such complexes as
catalyst precursors for the Heck arylation of olefins. Examples of
κ1-, κ2-, and κ3-NPN coordination to Pd are described. The
secondary phosphine (NPN)H (1) was reacted with 0.5 equiv of
[(C3H5)PdCl]2 to afford the dimeric species [(κ2-NPN)PdCl]2
(2) in 96% yield. Similarly, the reaction of 1 with 0.33 equiv of [Pd(OAc)2]3 led to the formation of dimeric [(κ2-NPN)PdOAc]2 (3) in
98% yield. Complexes 2 and 3 were both crystallographically characterized to reveal dipalladium species that feature bridging phosphido
groups, as well as terminal chloride and acetate ligands, respectively. Treatment of 3 with Me3SiOTf resulted in the formation of the
analogous triflate complex [(κ2-NPN)PdOTf]2 (4; 78% yield). Treatment of 2 with (C3H5)MgCl afforded the thermally sensitive,
phosphido-bridged complex [(κ1-NPN)Pd(η3-C3H5)]2 (5) in 87% yield. The solid-state structure of 5 was confirmed by X-ray
crystallography. Precoordination of BPh3 to the NPN phosphido group was attempted in order to discourage the formation of
phosphido-bridged (NPN)Pd species. Thus, the reaction of 1 with 1 equiv each of KCH2Ph and BPh3 led to the formation of
[N(P·BPh3)N]K (6), which was isolated in 88% yield. The related complex [N(P·BH3)N]H (7) was prepared by treatment of 1 with
BH3·THF, and crystallographic characterization confirmed the preferential coordination of the NPN ligand phosphorus to BH3. The
reaction of 6 with 0.5 equiv of [Pd(C3H5)Cl]2 led to the formation of [κ3-N(P·BPh3)N]Pd(η1-C3H5), which was isolated in 45% yield.
Complexes 2−4 were established as precatalysts for the Heck coupling of ethyl acrylate and haloarenes.
bis(phosphino)silyl pincer ligands of the type [κ 3-(2-
INTRODUCTION
■
R2PC6H4)2SiMe]− (R = Ph, Cy; κ3-PSiP),3a−e including examples
Monoanionic tridentate ligands comprised of a central anionic
of Ir species that can undergo facile intermolecular C−H and N−
X-type donor fragment flanked by two neutral L-type donors
H bond activation chemistry,3a as well as palladium and nickel
(i.e., κ3-LXL, where X is typically C or N) have proven to be
complexes that participate in unusual Si−C bond cleavage
particularly useful in supporting platinum-group-metal com-
reactions.3b To complement these investigations, and encouraged
plexes that exhibit unique reactivity and structural features.1,2
by the rich and diverse chemistry exhibited by platinum-group-
Given this progress, it is surprising that the exploration of such
metal complexes featuring bis(phosphino)amido ligation,1,2e−l we
ligand architectures featuring alternative central donor atoms
have initiated a synthetic and reactivity study of platinum-group-
including silicon3 and phosphorus4,5 has received relatively little
metal complexes featuring phosphido-based NPN ligands,
attention; indeed, despite the remarkable advancements that
including [(2-Me2NC6H4)2P]− (NPN). We report herein on the
have been achieved by use of tridentate bis(phosphino)amido
ability of [(2-Me2NC6H4)2P]− and the BPh3 adduct [(2-Me2-
(PNP) ligands in platinum-group-metal chemistry,1,2e−l the
NC6H4)2P·BPh3]− to support monodentate, bidentate, and tridentate
synthesis and characterization of related complexes supported
coordination complexes of PdII and the application of dimeric,
by phosphido-based (i.e., R2P−) pincer ancillary ligands is
phosphido-bridged [(κ2-NPN)PdX]2 (X = Cl, OAc, OTf) species of
limited to the work of Mazzeo, Peters, and co-workers,4b,j as
this type as precatalysts for the Heck arylation of ethyl acrylate.
well as van der Vlugt and co-workers.4k In these reports
complexes of the type (κ 3-PPP)MX (M = Pd, Pt) are reported,
RESULTS AND DISCUSSION
■
including, in the case of the Pd analogues, their use in the
allylation of aldehydes.4b Given that the incorporation of
Treatment of 2-lithio-N,N-dimethylaniline with 0.5 equiv of
PCl3 followed by reduction with LiAlH4 afforded the desired
phosphido fragments into pincer ligand frameworks is likely
to promote the formation of late-metal complexes that may
exhibit new and unusual reactivity, it is evident that the further
development of platinum-group-metal complexes supported by
phosphido-based multidentate ligands is warranted.
secondary phosphine [(2-Me2NC6H4)2P]H (1, (NPN)H) in
1
79% isolated yield. The H NMR spectrum of 1 exhibits a
characteristic doublet at 5.52 ppm (1JPH = 221 Hz)
We have recently reported on the synthesis and reactivity of
coordinatively unsaturated late-metal complexes supported by new
Received: July 28, 2011
Published: November 9, 2011
© 2011 American Chemical Society
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corresponding to the P−H resonance, while the 31P NMR
spectrum of 1 features a doublet at −59.3 ppm (1JPH = 220 Hz).
The reaction of 1 directly with PdII starting materials led to
the isolation of phosphido-bridged dimeric complexes of the
type [(κ2-NPN)PdX]2 (2, X = Cl; 3, X = η1-OAc). Treatment
of 1 with /2 equiv of [PdCl(C3H5)]2 in benzene at 65 °C led
to the precipitation of red crystalline 2, which was isolated in
96% yield (Scheme 1). The X-ray crystal structure of 2
defined by the coordination sphere of each Pd center in the
dimeric structure is 83.78(4)°. The Pd···Pd distance of 3.196 Å
in 2 is long,6 suggesting that Pd−Pd bonding does not play a
significant role in the observed structure.7 The Pd−P distances
in 2 (2.2166(5)−2.2495(5) Å) are comparable to the Pd−
phosphido distance of 2.2533(9) Å found in the monomeric
phosphido complex [κ3-(2-iPr2PC6H4)2P]PdCl.4b By compar-
ison with the latter monomeric κ3-PPP complex, the dinuclear
bridged structure adopted by 2 establishes that phosphido
bridging within a dinuclear species is preferred over a
monomeric κ3-NPN motif, owing to the comparatively poor
donor ability of the dimethylamino fragments within the NPN
ligand.
1
Scheme 1. Synthesis of the Palladium Complexes 2−5
1
The room-temperature H NMR spectrum of 2 (methylene
chloride-d2) features a single resonance corresponding to the
ligand dimethylamino protons at 2.76 ppm, as well as one set of
aromatic proton resonances. The observed spectrum suggests
two possible scenarios: (a) compound 2 is monomeric in
solution and features effective C2V symmetry or (b) compound
2 is dimeric in solution as well as in the solid state and is
fluxional, giving rise to an averaged 1H NMR spectrum at room
1
temperature. Variable-temperature H NMR spectroscopy of 2
(Figure 2, methylene chloride-d2) revealed substantial line-
shape changes. In particular, three decoalescence events were
observed at low temperature for the resonance corresponding
to the dimethylamino protons, consistent with rapid exchange
of free and bound NMe2 groups in a dimeric complex, as well as
slowed Caryl−N bond rotation and inversion at both free and
bound NMe2 nitrogen donors. When the solution was cooled
to −55 °C, the resonance corresponding to the dimethylamino
protons decoalesced into two broad resonances centered at
3.25 and 2.10 ppm (ca. 1:1 ratio), respectively, consistent with
the slowed exchange of free and bound NMe2 groups. The
ΔG⧧ value was estimated as ∼10 kcal mol−1 for this process.8
At −70 °C decoalescence of the resonance centered at 3.25
ppm was observed, giving rise to two singlets at 3.35 and 3.11
ppm (ca. 1:1 ratio, Figure 2), respectively. Finally, at −80 °C
decoalescence of the resonance centered at 2.10 ppm was
observed, giving rise to two broad peaks centered at 2.58 and
1.45 ppm (ca. 1:1 ratio, Figure 2), respectively. The latter
processes are consistent with slowed Caryl−N bond rotation and
inversion at both the free and bound NMe2 nitrogen donors.
confirms the formation of a phosphido-bridged dimeric Pd
complex in which the NPN ligand is coordinated to the metal
center in a κ2-NPN fashion (Figure 1); solution and refinement
1
Thus, at the low-temperature limit the H NMR spectrum of 2
is in agreement with the C2-symmetric structure observed in the
solid state, suggesting that 2 is likely dimeric and fluxional in
solution, resulting in an averaged spectrum at elevated
temperatures. Further support for this structural assignment
was provided by variable-temperature 31P NMR spectroscopy.
The 31P NMR spectrum of 2 (methylene chloride-d2) features a
single resonance at −54.0 ppm, and no line-shape changes were
observed when the spectrum was acquired at low temperature;
these data are consistent with an absence of monomer−dimer
equilibrium for 2, suggesting that the line-shape changes that
Figure 1. ORTEP diagram for 2 shown with 50% displacement
ellipsoids. Only one of the two crystallographically independent half-
molecules of 2 is shown (with the full structure generated via
application of crystallographically imposed 2-fold rotational symme-
try), and hydrogen atoms have been removed for clarity. The
crystallographically unique atoms are labeled. Selected interatomic
distances (Å) and angles (deg): Pd−P, 2.2213(5); Pd−P′, 2.2462(5);
Pd′−P, 2.2166(5); Pd′−P′, 2.2495(5); Cl−Pd−P, 175.65(2); Cl−Pd−
P′, 101.18(2); Cl−Pd−N, 95.15(4); P−Pd−P′, 76.00(2); P−Pd−N,
86.34(4); P′−Pd−N, 153.53(5).
1
are observed in the variable-temperature H NMR spectra of
this compound are indeed due to fluxional processes within the
dipalladium complex.
The reaction of 1 with /3 equiv of [Pd(OAc)2]3 in benzene
1
data for all crystallographically characterized compounds
featured herein are collected in Table 1. Each Pd center in 2
also features a terminal chloride ligand, resulting in approximate
square-planar coordination geometry at the metal center. The
dimer features a slightly unsymmetrical puckered Pd2P2 core,
such that the dihedral angle between the two square planes
at room temperature resulted in the formation of 3 in 98%
isolated yield (Scheme 1) with the apparent loss of acetic acid.
The X-ray crystal structure of 3·C6H6 (Figure 3) indicates the
formation of a phosphido-bridged dimeric Pd complex that is
analogous to 2 (Pd1···Pd2 = 3.1897(4) Å). As in the case of 2,
1
the H NMR spectrum of 3 (benzene-d6) is consistent with an
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Table 1. Crystallographic Data for 2, 3·C6H6, 5, and 7
2
3·C6H6
5
7
empirical formula
formula wt
cryst dimens
cryst syst
space group
a (Å)
C32H40Cl2N4P2Pd2
826.32
C42H52N4O4P2Pd2
951.62
C38H50N4P2Pd2
837.56
C16H24N2PB
286.15
0.31 × 0.26 × 0.22
monoclinic
P2/n
0.38 × 0.24 × 0.21
triclinic
0.32 × 0.16 × 0.08
monoclinic
P21/n
0.54 × 0.09 × 0.08
orthorhombic
Pbca
P1
̅
19.7889(14)
9.4338(7)
20.2742(15)
90
11.7857(15)
11.8320(15)
15.682(2)
91.9195(18)
96.8964(18)
95.3691(17)
2159.3(5)
2
9.8192(4)
18.1484(7)
20.4887(8)
90
13.8992(16)
14.8869(17)
16.4172(19)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
114.9868(10)
90
92.9071(4)
90
90
90
V (Å3)
3430.6(4)
4
3646.4(2)
4
3397.0 (7)
8
Z
ρcalcd (g cm−3
)
1.600
1.464
1.526
1.119
μ (mm−1
)
1.325
0.951
1.106
0.154
transmissn range
2θ limit (deg)
index ranges
0.7592−0.6841
54.96
0.8253−0.7140
52.84
0.9177−0.7198
54.90
0.9878−0.9214
51.00
−25 ≤ h ≤ 25
−12 ≤ k ≤ 12
−26 ≤ l ≤ 26
28 274
−14 ≤ h ≤ 14
−14 ≤ k ≤ 14
−19 ≤ l ≤ 19
14 929
−12 ≤ h ≤ 12
−23 ≤ k ≤ 23
−26 ≤ l ≤ 26
31 701
−16 ≤ h ≤ 16
−18 ≤ k ≤ 18
−19 ≤ l ≤ 19
23 216
total no. of data collected
no. of indep rflns
Rint
7861
8723
8325
3154
0.0257
0.0271
0.0304
0.0915
no. of obsd rflns
no. of data/restraints/params
goodness of fit
6906
6448
7184
2074
7861/0/379
1.055
8723/0/435
0.946
8325/3/443
1.034
3154/8/209
1.012
2
2
R1 (Fo ≥ 2σ(Fo ))
0.0220
0.0314
0.0227
0.0574
2
2
wR2 (Fo ≥ −3σ(Fo ))
0.0592
0.0759
0.0567
0.1550
largest peak, hole (e Å−3
)
0.558, −0.249
0.516, −0.280
0.738, −0.493
0.308, −0.211
Figure 3. ORTEP diagram for 3·C6H6 shown with 50% displacement
ellipsoids. Hydrogen atoms have been removed for clarity. Selected
interatomic distances (Å) and angles (deg): Pd1−P1, 2.2091(8);
Pd1−P2, 2.2670(8); Pd2−P1, 2.2488(8); Pd2−P2, 2.2163(8); P1−
Pd1−P2, 75.26(3); Pd1−P1−Pd2, 91.37(3); P1−Pd1−O1, 172.77(6);
P2−Pd1−N1, 155.71(6); P1−Pd2−P2, 75.48(3); Pd1−P2−Pd2,
90.70(3); P2−Pd2−O3, 167.16(7); P1−Pd2−N3, 158.96(7).
with Me3SiOTf in benzene solution resulted in precipitation of
the presumptive triflate analogue 4, which was isolated in 78%
yield.
Attempts to generate a Pd−H complex by the reaction of 2
with LiEt3BH were not successful and led to the formation of
intractable reaction mixtures. However, 2 reacted cleanly with
(C3H5)MgCl to form the new thermally sensitive allyl−Pd
complex 5 (Scheme 1). The X-ray crystal structure of 5
confirms the formation of a phosphido-bridged dimeric η3-allyl
Pd complex that features κ1-NPN coordination (Figure 4). As
Figure 2. Variable-temperature 1H NMR spectra of 2 (methylene
chloride-d2) showing line-shape changes in the dimethylamino proton
region of the spectrum (* and □ correspond to unique NMe
resonances observed at low temperature).
averaged structure at room temperature, as indicated by the
presence of only one NMe2 resonance at 2.60 ppm, as well as
only one set of aromatic proton resonances. The reaction of 3
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Table 2. Catalytic Performance of 2−4 in the Heck Arylation
of Ethyl Acrylate
a
b
entry
cat.
X
R
temp (°C)
time (h)
yield (%)
1
2
2
2
2
2
2
2
2
2
2
2
3
4
I
NO2
NO2
NO2
Me
110
140
140
110
140
110
110
110
140
140
140
140
10
5
>98
>98
62
2
I
Figure 4. ORTEP diagram for 5 shown with 50% displacement
ellipsoids. Hydrogen atoms have been removed for clarity, and only
the major component of the disordered allyl fragment is shown.
Selected interatomic distances (Å) and angles (deg): Pd1−P1,
2.3418(5); Pd2−P1, 2.3192(5); Pd1−P2, 2.3440(5); Pd2−P2,
2.3181(5); Pd1−C1, 2.205(2); Pd1−C2, 2.154(2); Pd1−C3,
2.177(2); Pd1−P1−Pd2, 96.197(19); P1−Pd2−P2, 78.798(18);
Pd1−P2−Pd2, 96.165(19); P1−Pd1−P2, 77.827(18); P1−Pd1−C3,
176.83(6); P2−Pd1−C1, 170.26(8).
3
I
2
4
I
20
10
20
10
20
10
10
20
5
>98
>98
>98
91
5
I
Me
6
Br
Br
Br
Br
Cl
Cl
I
NO2
NO2
Me
7
8
20
9
Me
>98
87
10
11
12
13
NO2
Me
0
NO2
NO2
>98
>98
in the case of 2 and 3, the dimer features a puckered Pd2P2
core (Pd1···Pd2 = 3.47 Å). However, unlike the former com-
plexes, η3-allyl coordination in 5 results in a κ1-NPN bonding
motif in which both N-donor fragments remain uncoordinated
to Pd.
I
140
5
a
b
0.5 mol % Pd (0.25 mol % catalyst). Determined on the basis of
1H NMR data; no conversion was observed in the absence of catalyst.
The room-temperature 1H NMR spectrum of 5 (toluene-d8)
features broad resonances that are consistent with fluxional
character. A single broad resonance corresponding to the ligand
dimethylamino protons is observed at 2.52 ppm, and a broad
resonance corresponding to the terminal allylic protons is
observed at 3.09 ppm. When the solution was cooled to −50 °C,
the resonance corresponding to the dimethylamino protons
decoalesced into two resonances at 2.71 and 2.30 ppm (ca. 1:1
ratio). Line-shape changes were also observed for the allylic
protons upon cooling; however, these resonances remained quite
broad down to −80 °C and exhibited substantial overlap with
each other as well as with the resonances corresponding to the
dimethylamino protons, and as such we are unable to comment
definitively on the nature of the dynamic properties of 5.
Complex 5 decomposed in room-temperature benzene solution
over the course of several hours to form a mixture of unidentified
products (1H, 31P NMR).
desired Heck coupling product was the only arene-containing
product formed on the basis of 1H NMR spectroscopic analysis
of the crude reaction mixtures. Similarly high conversions
were observed when using 4-iodotoluene after either 20 h at
110 °C or 10 h at 140 °C, using 2 as the precatalyst. With 4-
bromonitrobenzene, >90% conversions to the desired Heck
product were observed under similar reaction conditions. By
comparison, the coupling of 4-bromotoluene and ethyl acrylate
required heating to 140 °C in order to attain high conversions.
For the significantly less reactive 4-chloronitrobenzene, 87%
conversion was obtained after 10 h at 140 °C, while the
relatively deactivated substrate 4-chlorotoluene gave no
conversion even after heating for 20 h at 140 °C. Attempts to
couple ethyl acrylate and bromobenzene under these reaction
conditions using 2 as the catalyst resulted in nearly exclusive
formation of biphenyl (1H NMR), presumably arising from
homocoupling of bromobenzene. In addition, attempts to
reduce the catalyst loading to 0.1 and 0.05 mol % of Pd in the
coupling of ethyl acrylate and 4-iodonitrobenzene resulted in
the formation of nitrobenzene (1H NMR; > 98% conversion to
nitrobenzene after 20 h at 140 °C using 2 as the catalyst at a
loading of 0.05 mol % Pd). Overall, the performance of [(κ 2-
NPN)PdX]2 complexes as precatalysts for the Heck arylation of
olefins was comparable to that previously determined for
related monomeric (κ3-PNP)PdX complexes, as reported by
Ozerov and co-workers.10a While compounds 2−4 are clearly
dinuclear species in the solid state (as evidenced by the crystal
structures of 2 and 3), it is not currently known whether
mononuclear variants of these compounds play a role in the
generation of catalytically active Pd species in situ.
The utility of (PCP)PdII pincer complexes as catalysts in
C−C coupling reactions is well-established,2a,b,9 and amido-
centered monomeric (PNP)PdII complexes have also recently
been shown to act as effective precatalysts for the Heck
arylation of olefins.10,11 In a preliminary effort to assess the
catalytic efficacy of dimeric (κ2-NPN)PdII phosphido species
such as 2−4, the utility of these complexes as precatalysts in the
Heck coupling of aryl halides and ethyl acrylate was probed (eq 1).
Catalytic runs were carried out at Pd loadings of 0.5 mol %
(0.25 mol % of precatalyst) between 110 and 140 °C and for
reaction times ranging from 2 to 20 h. As a control, no
degradation was observed (31P NMR) after heating 2 at 140 °C
for over 14 h under the catalytic reaction conditions. Selected
catalytic results are given in Table 2.
In an effort to further explore the coordination chemistry of
NPN-type ligation, we considered that the precoordination of a
Lewis acid to the phosphido donor might encourage the
formation of monomeric (κ3-NPN)Pd complexes. In this
regard, treatment of 1 with KCH2Ph followed by BPh3 led to
The coupling of ethyl acrylate and substituted iodo-, bromo-,
and chloroarenes was investigated in the course of these studies.
With 4-iodonitrobenzene, conversions of >98% were achieved
after 5 h at 140 °C using 2, 3, or 4 as the precatalyst, and the
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the formation of the BPh3 adduct 6 (Scheme 2). The 1H NMR
spectrum of 6 (benzene-d6) is consistent with the formation of
formation of a [κ3-N(P·BPh3)N]Pd species, as alternative
formulations, including [κ2-N(P·BPh3)N]Pd(C3H5) and the
dinuclear, phosphido-bridged complex {[κ 1-(N·BPh3)PN]Pd-
(C3H5)}2, require η3-allyl coordination in order to achieve a
16-electron configuration. The coordination of the allyl
fragment to Pd rather than B is supported by the observation
of C−P coupling, which is consistent with coordination of both
the NPN ligand and the allyl ligand to Pd. The isolation of 8
establishes the coordination of a Lewis acid to the phosphido
donor of the NPN ligand as a viable strategy for encouraging
the formation of mononuclear complexes of such multidentate
phosphido ligands.
Scheme 2. Synthesis of Compounds 6−8
SUMMARY AND CONCLUSIONS
■
The facile synthesis of a phosphine precursor to an NPN
diaminophosphido ligand has been accomplished, and prelimi-
nary coordination chemistry studies with Pd have been
conducted. In the course of these studies [(2-Me2NC6H4)2P]−
ligation has been shown to support monodentate and bidentate
coordination complexes of PdII. Complexes of the type [(κ2-
NPN)PdX]2 (X = Cl, OAc, OTf) serve as precatalysts for the
Heck arylation of ethyl acrylate and exhibit catalytic perform-
ance that is comparable (on a per-Pd basis) to that of related
monomeric (κ3-PNP)PdX species. In solution these dinuclear
complexes exhibit dynamic behavior, likely resulting from
exchange of free and bound NMe2 ligand arms. The complex
[(κ2-NPN)PdCl]2 serves as a precursor for the η3-allyl species
[(κ1-NPN)Pd(η3-C3H5)]2, in which η3-allyl coordination has
displaced an NMe2 donor from the Pd coordination sphere.
This phenomenon is in contrast to the κ2-NPN binding mode
observed for the acetate complex 3, which features monodentate
acetate ligands, and likely reflects the more favorable interaction
between the electron-rich Pd center and the η3-allyl ligand
relative to the hard acetate ligands. In an effort to discourage the
formation of phosphido-bridged dinuclear complexes, precoor-
dination of BPh3 to NPN was pursued. Upon reaction of
[N(P·BPh3)N]K with [PdCl(C3H5)]2, the η1-allyl complex [κ3-
N(P·BPh3)N]Pd(η1-C3H5) (8) was isolated, which establishes
the coordination of a Lewis acid to the phosphido donor of the
NPN ligand as a viable strategy for encouraging the formation of
mononuclear κ3-NPN complexes. These studies establish the
versatility of NPN− ligation, which has proven capable of
adopting κ1-, κ2-, and pincer-like κ3-NPN binding motifs.
a phosphido species, as indicated by the disappearance of the
P−H resonance in 1, and is consistent with the formation of a
1
1:1 (NPN)·BPh3 adduct. The observation of a single H NMR
resonance corresponding to the NMe2 protons in 6 (2.25 ppm)
supports a formulation where BPh3 is coordinated to the
phosphido P atom, rather than to an NMe2 group. The 31P{1H}
NMR spectrum of 6 features a resonance at −36.4 ppm, while
the 11B{1H} NMR spectrum features a slightly broad resonance
at −6.0 ppm (cf. free BPh3 at 68.0 ppm).12 On the basis of
these NMR data we tentatively formulate 6 as an adduct of the
type [N(P·BPh3)N]K that features coordination of the
phosphido group to boron. Although we were unable to obtain
X-ray-quality crystals of 6, we were able to crystallographically
characterize the related complex [N(P·BH3)N]H (7),
which was prepared by treatment of 1 with BH3·THF
(Scheme 2). The solid-state structure of 7 (Figure 5) confirmed
EXPERIMENTAL SECTION
■
General Considerations. All experiments were conducted under
nitrogen in an MBraun glovebox or using standard Schlenk techniques.
Dry, oxygen-free solvents were used unless otherwise indicated. All
nondeuterated solvents were deoxygenated and dried by sparging with
nitrogen and subsequent passage through a double-column solvent
purification system provided by MBraun Inc. Tetrahydrofuran and
diethyl ether were purified over two activated alumina columns, while
benzene and pentane were purified over one activated alumina column
and one column packed with activated Q-5. All purified solvents were
stored over 4 Å molecular sieves. Benzene-d6, toluene-d8, and
methylene chloride-d2 were degassed via three freeze−pump−thaw
cycles and stored over 4 Å molecular sieves. The compounds
[(C3H5)PdCl]2 and [Pd(OAc)2]3 were purchased from Strem and
used as received. 2-Lithio-N,N-dimethylaniline14a and KCH2Ph14b
were prepared according to previously published procedures. LiAlH4
was purified by extraction into Et2O, followed by filtration to remove
insoluble components. Distilled water was deoxygenated by sparging
with nitrogen for ca. 40 min. All other reagents were purchased from
Aldrich and used without further purification. Unless otherwise stated,
Figure 5. ORTEP diagram for 7 shown with 50% displacement
ellipsoids. Selected hydrogen atoms have been removed for clarity.
the preferential coordination of the NPN ligand phosphorus
to BH3.
1
Treatment of 6 with /2 equiv of [PdCl(C3H5)]2 led to the
formation of the new Pd allyl complex 8 (Scheme 2). Complex
8 is formulated as [κ3-N(P·BPh3)N]Pd(η1-C3H5) on the basis
of solution NMR characterization. The 1H NMR spectrum of 8
(benzene-d6) is consistent with a 1:1 (NPN)·BPh3 adduct. The
11B{1H} NMR spectrum of 8 features a broadened resonance at
16.5 ppm, which is consistent with coordinated BPh3 (vide
supra). The η1 coordination of the allyl ligand is supported by
the observation of the allyl 13C NMR resonances (benzene-d6)
at 132.6, 118.3 (d, JCP = 12 Hz), and 32.8 ppm (d, JCP = 17
Hz).13 Such η1-allyl coordination is consistent with the
6412
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Article
1H, 13C, 31P, and 11B NMR characterization data were collected at
300 K on a Bruker AV-500 spectrometer operating at 500.1, 125.8,
202.5, and 160.5 MHz (respectively), with chemical shifts reported in
parts per million downfield of SiMe4 (for 1H and 13C), 85% H3PO4 in
D2O (for 31P), or BF3·OEt2 (for 11B). Variable-temperature NMR data
(CHarom), 126.9 (CHarom), 123.3 (CHarom), 48.8 (NMe2), 23.5
(CH3CO2). 31P{1H} NMR (202.5 MHz, benzene-d6): δ −52.0. Anal.
Calcd for C36H46N4O4P2Pd2: C, 49.50; H, 5.31; N, 6.41. Found: C,
50.03; H, 5.56; N, 5.66. A single crystal of 3·C6H6 suitable for X-ray
diffraction analysis was grown from benzene solution at room
temperature.
1
were collected on a Bruker AC-250 spectrometer. H and 13C NMR
[(κ2-NPN)PdOTf]2 (4). Neat Me3SiOTf (0.083 mL, 0.10 g, 0.46
mmol) was added via syringe to a room-temperature solution of 3
(0.20 g, 0.23 mmol) in ca. 5 mL of benzene. A yellow precipitate
formed immediately upon addition. The resulting reaction mixture was
stirred at room temperature for 30 min. The volatile components were
then removed in vacuo, and the remaining residue was washed with
benzene (5 mL), followed by pentane (3 × 7 mL), and dried in vacuo
chemical shift assignments are based on data obtained from 13C-
1
1
1
DEPT, H−1H COSY, H−13C HSQC, and H−13C HMBC NMR
experiments. In some cases, fewer than expected unique 13C NMR
resonances were observed, despite prolonged acquisition times.
Elemental analyses were performed by Desert Analytics, Inc. of
Tucson, AZ, and Canadian Microanalytical Service Ltd. of Delta,
British Columbia, Canada.
1
to afford 4 as a yellow solid (0.19 g, 78%). H NMR (250 MHz,
(NPN)H (1). A solution of 2-lithio-N,N-dimethylaniline (4.13 g,
32.5 mmol) in ca. 50 mL of Et2O was added dropwise via cannula to a
precooled (−78 °C), stirred solution of PCl3 (1.42 mL, 2.23 g, 16.3
mmol) in ca. 50 mL of Et2O. The resulting reaction mixture was
warmed to room temperature over the course of 3 h. An off-white
precipitate formed during this time. The reaction mixture was cooled
once again to −78 °C, and a solution of LiAlH4 (0.65 g, 17.0 mmol) in
ca. 30 mL of Et2O was added via cannula. The reaction mixture was
warmed to room temperature and stirred for an additional 4 h. The
mixture was then cooled to 0 °C, and the reaction was quenched by
dropwise addition of 40 mL of degassed water. The organic fraction
was cannula transferred away from the aqueous layer, which was
extracted with diethyl ether (2 × 100 mL). All organic fractions were
combined and dried over anhydrous MgSO4 under a nitrogen
atmosphere. Following filtration, the volatile components were
removed in vacuo, affording the target compound (3.50 g, 79%) as
a colorless oil that solidified upon standing at −35 °C. 1H NMR (500
MHz, benzene-d6): δ 7.34 (m, 2 H, Harom), 7.11 (m, 2 H, Harom), 6.94
methylene chloride-d2): δ 8.00 (m, 2 H, Harom), 7.45 − 7.35 (4 H,
H
arom), 7.01 (m, 2 H, Harom), 2.75 (s, 12 H, NMe2). 13C{1H} NMR
(125.8 MHz, methylene chloride-d2): δ 158.0 (Carom), 138.9 (CHarom),
133.9 (m, CHarom), 129.8 (Carom), 127.6 (m, CHarom), 123.3 (m,
CHarom), 49.0 (NMe2). 31P{1H} NMR (101.3 MHz, methylene
chloride-d2): δ −31.3. 19F{1H} NMR (235.4 MHz, methylene
chloride-d2): δ −77.2. Anal. Calcd for C34H40F6N4O6P2Pd2S2: C,
38.76; H, 3.83; N, 5.32. Found: C, 38.77; H, 3.73; N, 5.12.
[(κ1-NPN)Pd(η3-C3H5)]2 (5). A precooled (−30 °C) solution of 2
(0.14 g, 0.17 mmol) in ca. 5 mL of THF was treated with
(C3H5)MgCl (2.0 M in THF, 0.17 mL, 0.34 mmol). The resulting
reaction mixture was warmed to room temperature over the course of
25 min. The volatile components were removed in vacuo, and the
remaining residue was extracted into benzene (ca. 10 mL). The
benzene extract was filtered through Celite and evaporated to dryness
to afford a yellow solid that was recrystallized from ca. 10 mL of Et2O
at −30 °C to afford 5 (0.12 g, 87%) as a yellow microcrystalline solid.
1H NMR (500 MHz, toluene-d8): δ 7.68 (br s, 2 H, Harom), 6.95 (m, 2
H, Harom), 6.80 (m, 2 H, Harom), 6.63 (m, 2 H, Harom), 5.13 (quin, 1 H,
η3-C3H5, J = 10 Hz), 3.09 (br s, 4 H, η3-C3H5), 2.52 (br s, 12 H,
NMe2). 13C{1H} NMR (125.8 MHz, toluene-d8): δ 128.4 (CHarom),
122.8 (CHarom), 120.1 (CHarom), 117.0 (t, η3-C3H5, J = 5 Hz), 46.2
(NMe2). 31P{1H} NMR (202.5 MHz, methylene chloride-d2): δ
−103.1. Repeated attempts to obtain satisfactory elemental analysis for
5 were not successful, likely due to the thermally sensitive nature of
this compound. A single crystal of 5 suitable for X-ray diffraction
analysis was grown from Et2O solution at −30 °C.
(m, 2 H, Harom), 6.85 (t, 2 H, Harom, J = 7 Hz), 5.52 (d, 1 H, PH, 1JPH
=
221 Hz), 2.56 (s, 12 H, NMe2). 13C{1H} NMR (125.8 MHz, benzene-
d6): δ 158.1 (d, Carom, JCP = 14 Hz), 136.4 (d, CHarom, JCP = 4 Hz),
133.2 (d, Carom, JCP = 13 Hz), 129.9 (CHarom), 124.6 (CHarom), 120.4
(CHarom), 45.6 (NMe2). 31P{1H} NMR (202.5 MHz, benzene-d6): δ
−59.3. Anal. Calcd for C16H21N2P: C, 70.57; H, 7.77; N, 10.29.
Found: C, 70.85; H, 7.72; N, 10.51.
[(κ2-NPN)PdCl]2 (2). A room-temperature solution of 1 (0.20 g,
0.74 mmol) in ca. 5 mL of benzene was added to a room-temperature
solution of [(C3H5)PdCl]2 (0.13 g, 0.37 mmol) in ca. 2 mL of
benzene. The resulting orange solution was transferred to a 250 mL
thick-walled resealable Schlenk tube adapted with a Teflon stopcock,
and the reaction mixture was heated at 65 °C for 2.5 h. The formation
of a red crystalline precipitate was observed. The reaction mixture was
cooled to room temperature, and in the glovebox, the supernatant
solution was removed by pipet. The remaining crystalline residue was
washed with pentane (3 × 5 mL) and subsequently dried in vacuo to
[N(P·BPh3)N]K (6). A precooled (−30 °C) solution of 1 (0.20 g,
0.74 mmol) in ca. 7 mL of THF was treated with a precooled (−30 °C)
solution of KCH2Ph (0.096 g, 0.74 mmol) in ca. 3 mL of THF. The
resulting red-orange solution was warmed to room temperature over the
course of 20 min. The reaction mixture was cooled to −30 °C, and a
solution of BPh3 (0.18 g, 0.74 mmol) in ca. 3 mL of THF was added,
resulting in a color change to bright yellow. The reaction mixture was
warmed to room temperature over the course of 20 min. The volatile
components of the reaction mixture were removed in vacuo to afford a
yellow solid that was washed with pentane (5 × 5 mL) to give 6 (0.36 g,
1
give 2 as a microcrystalline red-orange solid (0.29 g, 96%). H NMR
(500 MHz, methylene chloride-d2): δ 8.02 (m, 2 H, Harom), 7.35−7.29
(4 H, Harom), 7.00 (t, 2 H, Harom, J = 7 Hz), 2.76 (s, 12 H, NMe2).
13C{1H} NMR (125.8 MHz, methylene chloride-d2): δ 158.3 (Carom),
139.7 (m, CHarom), 132.6 (CHarom), 132.4 (apparent t, Carom, JCP = 18
Hz), 126.7 (CHarom), 122.9 (CHarom), 49.2 (NMe2). 31P{1H} NMR
(202.5 MHz, methylene chloride-d2): δ −54.0. Anal. Calcd for
C32H40Cl2N4P2Pd2: C, 46.51; H, 4.88; N, 6.78. Found: C, 46.49; H,
4.75; N, 6.58. A single crystal of 2 suitable for X-ray diffraction analysis
was grown from benzene solution at room temperature.
1
88%) as a bright yellow powder. H NMR (500 MHz, benzene-d6):
δ 7.70 (d, 6 H, BPhortho), 7.32 (m, 2 H, Harom), 7.06 (t, 6 H, BPhmeta, J =
7 Hz), 6.91 − 6.80 (7 H, BPhpara + Harom), 6.59 (t, 2 H, Harom, J = 7 Hz),
2.25 (s, 12 H, NMe2). 13C{1H} NMR (125.8 MHz, benzene-d6): δ 158.1
(d, Carom, J = 16 Hz), 141.5 (d, Carom, J = 20 Hz), 139.0 (CHarom), 136.1
(d, BPhortho, J = 9 Hz), 127.9 (BPhipso), 127.5 (BPhmeta + CHarom), 123.9
(BPhpara), 123.7 (CHarom), 119.7 (CHarom), 46.0 (NMe2). 31P{1H} NMR
(202.5 MHz, benzene-d6): δ −36.4. 11B{1H} NMR (160.5 MHz,
benzene-d6): δ −6.0 (br s). Anal. Calcd for C34H35BKN2P: C, 73.91; H,
6.38; N 5.07. Found: C, 73.59; H, 6.58; N, 4.77.
[(κ2-NPN)PdOAc]2 (3). A room-temperature solution of 1 (0.15 g,
0.55 mmol) in ca. 5 mL of benzene was added to a room-temperature
solution of [Pd(OAc)2]3 (0.12 g, 0.18 mmol) in ca. 2 mL of benzene.
The resulting bright orange solution was allowed to stand at room
temperature for 20 min. The volatile components were removed in
vacuo, and the remaining residue was washed with pentane (3 ×
5 mL) and dried in vacuo to afford 3 as a bright yellow-orange solid
[N(P·BH3)N]H (7). A solution of 1 (0.10 g, 0.37 mmol) in ca. 5 mL
of THF was treated with BH3·THF (1.0 M in THF, 0.37 mL, 0.37
mmol). The reaction mixture was allowed to stand at room
temperature for 1 h. The volatile components of the reaction mixture
were subsequently removed in vacuo to afford 7 (0.10 g, 98%) as a
white solid. 1H NMR (500 MHz, benzene-d8): δ 7.79 (m, 2 H, Harom),
1
(0.23 g, 98%). H NMR (500 MHz, benzene-d6): δ 8.38 (br s, 2 H,
H
arom), 6.88 (t, 2 H, Harom, J = 7 Hz), 6.80 (t, 2 H, Harom, J = 7 Hz),
1
3
6.74 (d, 2 H, Harom, J = 8 Hz), 2.60 (s, 12 H, NMe2), 2.13 (s, 3 H,
CH3CO2). 13C{1H} NMR (125.8 MHz, benzene-d6): δ 175.8
(CH3CO2), 158.2 (Carom), 139.7 (CHarom), 132.8 (m, Carom), 131.5
7.06 (d of quart, 1 H, PH, JPH = 410 Hz, JHH = 7 Hz), 7.04 (t, 2 H,
H
arom, J = 8 Hz), 6.84 − 6.81 (4 H, Harom), 2.21 (s, 12 H, NMe2), 2.07
(br m, 3 H, BH3). 13C{1H} NMR (125.8 MHz, benzene-d6): δ 157.6
6413
dx.doi.org/10.1021/om200701h|Organometallics 2011, 30, 6408−6415

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Article
(Carom), 135.2 (d, CHarom, J = 13 Hz), 132,5 (CHarom). 127.6 (Carom),
N1−C18A, N1−C17B, N1−C18B, N2−C27A, N2−C28A, N2−C27B,
and N2−C28B bond lengths were constrained to be equal (within 0.05 Å)
during refinement.
125.5 (d, CHarom, J = 12 Hz), 122.2 (d, CHarom, J = 5 Hz), 45.7
1
(NMe2). 31P{1H} NMR (202.5 MHz, benzene-d6): δ −23.2 (d, JPB
=
56 Hz). 11B{1H} NMR (160.5 MHz, benzene-d6): δ −37.8. Anal.
Calcd for C16H24BN2P: C, 67.16; H, 8.45; N 9.79. Found: C, 67.35; H,
8.37; N, 9.70. A single crystal of 7 suitable for X-ray diffraction analysis
was grown from Et2O solution at −30 °C.
Representative Procedure for the Catalytic Heck Arylation
of Ethyl Acrylate. All catalytic runs were conducted under a
nitrogen atmosphere in 100 mL resealable Schlenk tubes adapted with
a Teflon stopcock. The Pd catalyst 2 (2.1 mg, 0.005 mmol), K2CO3
(0.21 g, 1.5 mmol), haloarene (1.00 mmol), and ethyl acrylate (0.14
mL, 1.3 mmol) were dissolved in 3 mL of N-methylpyrrolidone in
a reaction tube containing a magnetic stir bar. The reaction vessel
was placed in a temperature-controlled oil bath set at the desired
[κ3-N(P·BPh3)N]Pd(η1-C3H5) (8). A solution of 6 (0.10 g, 0.18
mmol) in ca. 5 mL of benzene was treated with a solution of
[Pd(C3H5)Cl]2 (0.033 g, 0.090 mmol) in ca. 2 mL of benzene. The
resulting dark red reaction mixture was allowed to stand at room
temperature for 5 min. The reaction mixture was filtered through
Celite, and the volatile components were removed in vacuo. The
remaining residue was extracted into Et2O (ca. 10 mL), and the Et2O
extracts were filtered through Celite to afford a dark red solution that
was concentrated in vacuo to ca. 5 mL and refrigerated at −30 °C to
1
temperature. Conversions were determined on the basis of H NMR
spectroscopic data (average of at least two runs).
ASSOCIATED CONTENT
* Supporting Information
■
S
1
give 8 (0.054 g, 45%) as a dark red microcrystalline solid. H NMR
CIF files giving single-crystal X-ray diffraction data for 2,
3·C6H6, 5, and 7. This material is available free of charge via the
Internet at http://pubs.acs.org.
(500 MHz, benzene-d8): δ 7.57 (m, 6 H, BPhortho), 7.26−7.19 (9 H,
BPhmeta + BPhpara), 6.99 (m, 2 H, Harom), 6.88 (m, 2 H, Harom), 6.75
(m, 4 H, Harom), 5.62 (m, 1 H, η1-C3H5), 4.55 (m, 1 H, η1-C3H5), 4.45
(m, 1 H, η1-C3H5), 2.58 (m, 2 H, η1-C3H5), 1.98 (s, 12 H, NMe2).
13C{1H} NMR (125.8 MHz, benzene-d6): δ 157.5 (d, Carom, J = 13
Hz), 142.2 (BPhipso), 137.0 (d, Carom, J = 38 Hz), 132.6 (η1-C3H5),
132.0 (BPhortho), 131.8 (CHarom), 130.6 (CHarom), 127.9 (BPhmeta),
126.7 (d, CHarom, J = 5 Hz), 126.1 (BPhpara), 123.4 (CHarom), 118.3 (d,
η1-C3H5, JCP = 12 Hz), 47.5 (NMe2), 32.8 (d, η1-C3H5, JCP = 17 Hz).
31P{1H} NMR (202.5 MHz, benzene-d6): δ −0.8. 11B{1H} NMR
(160.5 MHz, benzene-d6): δ 16.5 (br s). Anal. Calcd for
C37H40BN2PPd: C, 67.24; H, 6.10; N 4.24. Found: C, 66.95; H,
5.74; N, 4.38.
AUTHOR INFORMATION
Corresponding Author
*E-mail: laura.turculet@dal.ca. Fax: 1-902-494-1310. Tel: 1-902-
494-7190.
■
ACKNOWLEDGMENTS
■
Acknowledgment is made to Dalhousie University and the
Natural Sciences and Engineering Research Council of Canada
(including a Discovery Grant for L.T. and a Postgraduate
Scholarship for M.C.M.). We also thank Dr. Michael Lumsden
(NMR3, Dalhousie) for technical assistance in the acquisition
of NMR data.
Crystallographic Solution and Refinement Details. Crystallo-
graphic data for 2, 3·C6H6, 5, and 7 were obtained at 193(±2) K on
either a Bruker PLATFORM/SMART 1000 CCD diffractometer or a
Bruker D8/APEX II CCD diffractometer using graphite-monochro-
mated Mo Kα (λ = 0.710 73 Å) radiation, employing a sample that was
mounted in inert oil and transferred to a cold gas stream on the
diffractometer. Programs for diffractometer operation, data collection,
and data reduction (including SAINT) were supplied by Bruker.
Gaussian integration (face-indexed) was employed as the absorption
correction method for 3·C6H6 and 5, while for 2 and 7 SADABS
(Bruker) was employed as the absorption correction method. For both
2 and 3·C6H6, the structure was solved by use of the Patterson search/
structure expansion, whereas direct methods were employed for 5 and
7. All structures were refined by use of full-matrix least-squares
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procedures (on F2) with R1 based on Fo ≥ 2σ(Fo ) and wR2 based
2
2
2
2
on Fo ≥ −3σ(Fo ). Anisotropic displacement parameters were
employed throughout for the non-hydrogen atoms, and all hydrogen
atoms were added at calculated positions and refined by use of a riding
model employing isotropic displacement parameters based on the
isotropic displacement parameter of the attached atom. During the
structure solution process for 2, two crystallographically independent
half-molecules of the target complex were located in the asymmetric
unit and refined in a satisfactory manner; for simplicity, discussion of
metrical parameters is limited to one of the crystallographically
independent half-molecules. During the structure solution process for
3·C6H6, attempts to refine peaks of residual electron density as solvent
benzene carbon atoms were unsuccessful. The data were corrected for
disordered electron density through use of the SQUEEZE procedure15
as implemented in PLATON.16 A total solvent-accessible void volume
of 374.8 Å3 with a total electron count of 99 (consistent with two
molecules of solvent benzene or one molecule per formula unit of 3)
was found in the unit cell. During the structure solution process for 5,
one of the allyl fragments was found to exhibit a conformational
disorder that was modeled in a satisfactory manner by using a 55:45
split occupancy. Distances within the two conformers of the
disordered allyl group in 5 were constrained to be equal (within
0.01 Å) during refinement: d(C4A−C5A) = d(C4B−C5B); d(C5A−
C6A) = d(C5B−C6B); d(C4A···C6A) = d(C4B···C6B). Disorder
within the dimethylamino fragments in 7 were modeled in a
satisfactory manner by using a 65:35 split occupancy; the N1−C17A,
6414
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Article
5669. (h) Bushnell, G. W.; Casado, M. A.; Stobart, S. R. Organo-
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(8) By use of the Gutowsky−Holm approximation: ΔGc⧧ = (−RTc)
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