Preparation of the palladium(II) dimethylamide Pd(2,6-(Ph2PCH2)2C6H 3)(NMe2) at low temperatures and its ligand exchange with the dicyclohexylamide

Article abstract of DOI:10.1016/S0022-328X(99)00511-2

The title complex Pd(2,6-(Ph₂PCH₂)₂C₆H ₃)(NMe₂) (2) has been prepared from deprotonation of [Pd(2,6-(Ph₂PCH₂)₂C₆H ₃)(NHMe₂)]OTf (1) by a stoichiometric amount of dicyclohexylamide anion at low temperatures. VT NMR experiments showed that 2 was stable below -10°C, and slowly underwent decomposition to give a couple of unidentified species at elevated temperatures. However, the reaction of 1 with an excess of lithium dicyclohexylamide (>ten equivalents) generated the monomeric palladium(II) hydride (2,6-(Ph₂PCH₂)₂C₆H₃)PdH (3). The reaction of (Ph₂PCH₂)₂C₆H₃)Pd(OTf) with an excess of lithium dicyclohexylamide also produced the hydride. A d₈-THF solution of 3 was stable for 16 h at ambient temperature. However, an attempt to isolate 3 resulted in the formation of a small amount of the hydride-bridged dipalladium [(Pd(2,6-(Ph₂PCH₂)₂C₆H ₃))₂(μ-H)]⁺ (4) and mostly decomposed species.

Full text of DOI:10.1016/S0022-328X(99)00511-2

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Journal of Organometallic Chemistry 592 (1999) 194–197
Preparation of the palladium(II) dimethylamide
Pd(2,6-(Ph2PCH2)2C6H3)(NMe2) at low temperatures and its ligand
exchange with the dicyclohexylamide
Sang Yeoul Ryu a, Honggon Kim b, Hoon Sik Kim b, Soonheum Park a,*
a Department of Chemistry, Dongguk Uni6ersity, Kyong-Ju 780-714, South Korea
b CFC Alternati6es Research Center, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul 130-650, South Korea
Received 31 July 1999; accepted 20 August 1999
Abstract
The title complex Pd(2,6-(Ph2PCH2)2C6H3)(NMe2) (2) has been prepared from deprotonation of [Pd(2,6-
(Ph2PCH2)2C6H3)(NHMe2)]OTf (1) by a stoichiometric amount of dicyclohexylamide anion at low temperatures. VT NMR
experiments showed that 2 was stable below −10°C, and slowly underwent decomposition to give a couple of unidentified species
at elevated temperatures. However, the reaction of 1 with an excess of lithium dicyclohexylamide (\ten equivalents) generated
the monomeric palladium(II) hydride (2,6-(Ph2PCH2)2C6H3)PdH (3). The reaction of (Ph2PCH2)2C6H3)Pd(OTf) with an excess of
lithium dicyclohexylamide also produced the hydride. A d8-THF solution of 3 was stable for 16 h at ambient temperature.
However, an attempt to isolate 3 resulted in the formation of a small amount of the hydride-bridged dipalladium [(Pd(2,6-
(Ph2PCH2)2C6H3))2(m-H)]+ (4) and mostly decomposed species. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Dimethylamido palladium complex; Catalytic amination; Palladium hydride; Beta-hydrogen transfer
[8], recent study revealed that substantial dialkylamido
species were involved in the catalytic amination of
arylhalides via CꢀN reductive elimination at ambient
temperature [2]. On the premise of previous examples
and mechanistic information for b-hydrogen elimina-
tion of alkylamido complexes, stability examination for
such complexes bearing a rigid terdentate ligand is of
interest in the context with the possible roles as inter-
mediates in many catalytic reactions.
We have previously reported that cationic dimethy-
lamine complexes of palladium(II) containing rigid ter-
dentate ligands, [Pd(2,6-(R2PCH2)2C6H3)(NHMe2)]+
underwent chlorine abstraction to produce chloropalla-
dium(II) complexes in the presence of a base [9] (Eq.
(1)). In the reaction, we suggested a palladium(II)
dimethylamide as transient species, which immediately
reacted with chlorinated solvents forming the chloro
complexes. In this paper, we describe the synthesis and
thermal stability of the dimethylamido palladium(II)
complex having a tridentate ligand with PCP donor sets
with a hope that rigidity of the ligand framework could
inhibit decomposition.
1. Introduction
Alkylamido complexes of late transition metals are
known as reactive intermediates in metal-catalyzed ami-
nation [1,2]. Such complexes, however, are rare because
a facile b-hydrogen transfer proceeds to generate metal
hydrides or reduced species [3]. It is also noteworthy
that alkylamido complexes with a flexible ligand frame-
work such as monodentating tertiary phosphines un-
dergo b-hydrogen elimination more easily than the
complexes with a rigid ligand framework [3–6]. Al-
though b-hydrogen elimination of such complexes is
common, a direct decomposition pathway has been
rarely observed. Recently it has been reported that
b-hydrogen transfer of the benzylamido complex
Ir(PPh3)2(CO)(N(CH2Ph)Ph) proceeds via a dissociative
pathway involving three coordinated 14 e intermediate
[7]. Even though many efforts to isolate dimethylamido
complexes of late transition metals were not successful
* Corresponding author. Fax: +82-561-7702518.
E-mail address: shpark@mail.dongguk.ac.kr (S. Park)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00511-2

S.Y. Ryu et al. / Journal of Organometallic Chemistry 592 (1999) 194–197
195
[Pd(2,6-(R2PCH2)2C6H3)(NHMe2)]++B−
this solution to 0°C, the complex started to decompose,
resulting in the signals broadening and the decreases of
ꢀꢀꢀ−ꢀꢀBꢀHꢀꢀꢁ Pd(2,6-(R2PCH2)2C6H3)Cl+NDMe2 (1)
1
peak intensities in the H- and 31P{1H}-NMR spectra.
CDCl /CD Cl
2
3
2
(R=Ph, Cy; B− =DABCO, NHMe2)
Finally this compound in d8-THF completely decom-
posed within 30 min at 10°C, giving a couple of uniden-
tified phosphorus-containing compounds as evidenced
by the signals observed at l 32.0 and l 33.6 in the
31P{1H}-NMR spectrum but no hydride species. This
observation is in contrast with the above result, where
we observed the dinuclear species from the reaction of
[Pd(2,6-(Ph2PCH2)2C6H3)(NHMe2)](OTf) with an ex-
cess of lithium dicyclohexylamide. The difference be-
tween the two independent reactions is the amount of
lithium dicyclohexylamide used, i.e. the former with
excess and the latter with a nearly equimolar amount.
Further VT NMR experiments by using more than a
tenfold excess of lithium dicyclohexylamide showed
that the species generated from the reaction at low
temperatures exhibited another signal, broad at l 34.7
(in d8-THF) in the 31P{1H}-NMR. Judging from the
NMR resonances, we suggest this species dicyclohexy-
lamide derivative presumably formed via exchange with
dicyclohexylamide anion used in excess. This species,
however, was unstable and slowly converted into the
monomeric hydrido complex (2,6-(Ph2PCH2)2C6H3)-
PdH (3). The complex can be verified by the upfield
triplet hydride resonance at l −2.18 (2J(PH)=12.6
2. Results and discussion
When the dimethylamine complex [Pd(2,6-
(Ph2PCH2)2C6H3)(NHMe2)](OTf) (1) reacted with an
excess of lithium dicyclohexylamide in tetrahydrofuran
at −78°C, slight color-change of the solution from
colorless to pale-orange was observed. Removal of the
solvent under high vacuum at low temperatures (B
0°C) gave a pale-orange residue, which was extracted
with dry d6-acetone. The 31P{1H}-NMR spectrum of
the extract displayed several resonances; one was sharp
at l 43.0 and the others were rather broad at l 35
(Dw1/2$42 Hz). The 1H-NMR spectrum of this solution
exhibited an upfield hydride resonance at l −8.60 as a
quintet (2J(PH)=14.2 Hz). This hydride was verified as
the cationic hydride-bridged dipalladium species
[(Pd(2,6-(Ph2PCH2)2C6H3))2(m-H)]+ (4) by evidences of
1
the H- and 31P{1H}-NMR spectra. The NMR spectral
data of this complex in d6-acetone are fully consistent
with those of the precedent that was previously pre-
pared by the reaction of [Pd(2,6-(Ph2PCH2)2-
C6H3)(acetone)]+ and NaO2CH [10].
1
Hz) in the H-NMR and single sharp resonance at l
In order to investigate the stability and decomposi-
tion features of the dimethylamido species, VT NMR
experiments have been performed. Addition of d8-THF
at −78°C to an approximately equimolar mixture of
[Pd(2,6-(Ph2PCH2)2C6H3)(NHMe2)](OTf) and lithium
dicyclohexylamide in a screw-capped NMR tube al-
lowed the spectroscopic detection of dimethylamido
species. The 31P{1H}-NMR spectrum of the d8-THF
solution at −50°C cleanly displayed a single resonance
at l 30.9. The 31P{1H}-NMR resonance corresponding
to the starting compound [Pd(2,6-(Ph2PCH2)2C6H3)-
(NHMe2)]+, which appears at l 37.7 in d8-THF, was
not observed at all, indicating that the coordinated
dimethylamine immediately deprotonated by the highly
45.3 in the 31P{1H}-NMR spectroscopy. This complex 3
with the phenyl substituted terdentate ligand has not
been reported. However, the analogous monomeric hy-
drido complex with sterically bulky t-butyl substituents,
(2,6-(tBu2PCH2)2C6H3)PdH, was previously prepared
[12]. The 1H-NMR spectral data observed for 3 is
closely related with those of (2,6-(tBu2PCH2)2C6H3)-
PdH; for comparison, the hydride signal of (2,6-
(tBu2PCH2)2C6H3)PdH resonates at
l
−3.86
(2J(PH)=13.5 Hz) in CDCl3. A d8-THF solution of 3
was stable at ambient temperature for at least 16 h.
However, an attempt to isolate this complex from the
solution was unsuccessful, resulting in the formation of
a small amount of the hydride-bridged dipalladium
complex, [(Pd(2,6-(Ph2PCH2)2C6H3))2(m-H)]+ (1H-
NMR (in d8-THF): −8.58 quintet (2J(PH)=14.2 Hz),
1
hindered dicyclohexylamide anion. The H-NMR spec-
trum of the solution, at −50°C, exhibited methyl reso-
nances at l 2.76 as a 1:2:1 triplet splitting with the very
small coupling constant (4J(PH)=2.1 Hz) assignable to
the dimethylamido moiety, and methylene resonances
at l 3.99 as a pseudo triplet due to the virtual trans-
31P{1H}-NMR:
species.
l 46.6), and mostly decomposed
In the reactions, it is believed that the hydride was
from the dicyclohexylamide but not the dimethylamide.
This was confirmed by the independent reaction of
Pd(2,6-(Ph2PCH2)2C6H3)(OTf) with an excess of lithium
dicyclohexylamide in d8-THF resulting in the formation
of the monomeric palladium(II) hydride, clearly indi-
cating that the dicyclohexylamide is responsible for the
hydride source. Thus, a plausible pathway for the ob-
served reactions involves the facile ligand exchange
2
couplings (ꢀ J(PH)+4J(PH)ꢀ=8.6 Hz) [11]. The rela-
tive intensity ratio observed for the methyl and
methylene resonances is 3:2. We assigned this species as
the dimethylamido complex, Pd(2,6-(Ph2PCH2)2C6H3)-
(NMe2) (2). The complex 2 was stable at low tempera-
1
tures up to −10°C; The H- and 31P{1H}-NMR reso-
nances observed for 2 remained intact for at least more
than 3 h without any signal broadening. Warming of

196
S.Y. Ryu et al. / Journal of Organometallic Chemistry 592 (1999) 194–197
with the dicyclohexylamide to form a highly reactive
species toward b-hydrogen transfer. The greater
propensity for b-hydrogen elimination of the dicyclo-
hexylamide against the dimethylamide is likely due to
the steric congestion of the bulkier substituents which
facilitates b-hydrogen abstraction resulting in the gener-
ation of the hydride. An alternative pathway involving
free radicals from the PdꢀN homolysis of the highly
hindered amide cannot be ruled out (see Scheme 1 for
an outline of the reaction mechanisms).
sieves. All other reagents were from various commercial
companies. The complex [Pd(2,6-(Ph2PCH2)2C6H3)-
(NHMe2)](OTf) was prepared according to the previous
1
method [9]. H- (199.975 MHz) and 31P{1H}- (80.950
MHz) NMR spectra were measured on a Varian Gem-
ini 200 spectrometer using the deuterium signal of the
1
solvent as an internal lock frequency. H and 31P{1H}
chemical shifts were reported relative to TMS and 85%
H3PO4, respectively.
3.1. NMR spectroscopic data for complexes 2–4
For Pd(2,6-(Ph2PCH2)2C6H3)(NMe2) (2): 1H-NMR
3. Experimental
4
(d8-THF): l 2.76 t (6H, PdꢀNMe2, J(PH)=2.1 Hz), l
2
All preparations of air sensitive compounds were
carried out on a standard Schlenk line or in a glove box
under an argon atmosphere. Anhydrous dimethy-
lamine, lithium dicyclohexylamide, and deuterated
NMR solvents were purchased from the Aldrich Chem-
ical Co. d8-THF was dried and deaerated by means of
solvent diffusion under high vacuum from sodium/ben-
zophenone ketyl, and stored over activated molecular
3.99 t (4H, CH2, ꢀ J(PH)+4J(PH)ꢀ=8.6 Hz). 31P{1H}-
NMR (d8-THF):
l 30.9 s. For (2,6-(Ph2PCH2)2-
C6H3)PdH (3): 1H-NMR (d8-THF): l −2.18 t (1H,
2
PdꢀH, J(PH)=12.6 Hz). 31P{1H}-NMR (d8-THF): l
45.3 s. For [(Pd(2,6-(Ph2PCH2)2C6H3))2(m-H)]OTf (4):
1H-NMR (d6-acetone):
l
−8.60 quintet (1H,
PdꢀHꢀPd, 2J(PH)=14.2 Hz), l 4.02 t (4H, CH2,
2
ꢀ J(PH)+4J(PH)ꢀ=9.2 Hz).
l 6.9–7.8 m (23H,
Scheme 1.

S.Y. Ryu et al. / Journal of Organometallic Chemistry 592 (1999) 194–197
197
Table 1
NMR data (ppm) in d8-THF for the palladium complexes bearing P,C,P pincer ligands
Complex
1H
31P{1H}
Ref.
[(2,6-(Ph2PCH2)2C6H3)Pd(NHMe2)]+ (1)
(2,6-(Ph2PCH2)2C6H3)Pd(NMe2) (2)
(2,6-(Ph2PCH2)2C6H3)Pd(NCy2)
(2,6-(Ph2PCH2)2C6H3)PdꢀH (3)
2.20 d (PdꢀHNMe2; 3J(HH)=5.8 Hz) a
2.76 t (PdꢀNMe2; 4J(PH)=2.1 Hz) b
41.1 a, 37.7
30.9
34.7
45.3
[9]
b
This work
This work
This work
[12]
−2.18 t (PdꢀH; 2J(PH)=12.6 Hz)
(2,6-(tBu2PCH2)2C6H3)PdꢀH
−3.86 t (PdꢀH; 2J(PH)=13.5 Hz) a
−8.60 quintet (PdꢀHꢀPd; 2J(PH)=14.2 Hz) c
[(Pd(2,6-(Ph2PCH2)2C6H3))2(m-H)]+ (4)
43.0 c, 46.6
[10]
a In CDCl3.
b At −50°C.
c In d6-acetone.
phenyl). 31P{1H}-NMR: l 43.0 s (in d6-acetone), l 46.6
s (in d8-THF) (see Table 1).
Acknowledgements
This work was supported by KOSEF (971-0306-043-
2) and the Korean Ministry of Education through the
Research Fund (BSRI-96-3443).
3.2. Reaction of [Pd(2,6-(Ph2PCH2)2C6H3)-
(NHMe2)](OTf) (1) with lithium dicyclohexylamide
To the mixture of [Pd(2,6-(Ph2PCH2)2C6H3)-
(NHMe2)]OTf (1) (50 mg, 0.065 mmol) and LiNCy2
(122 mg, 0.65 mmol) was added a cooled THF (10 ml)
at −78°C. The reaction mixture was stirred for 2 h.
Removal of the solvent under high vacuum at low
temperature (B0°C) resulted in a pale-orange solid,
which was extracted with d6-acetone. The NMR spec-
troscopic data showed that a small amount of [(Pd(2,6-
(Ph2PCH2)2C6H3))2(m-H)](OTf) (4) was present in the
solution as well as several unidentified species. Complex
4 can be isolated from acetone–n-hexane in ca. 15%
yield.
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3.3. VT NMR experiments
A 0.5 ml sample of d8-THF was injected, by means of
a gas tight syringe at −78°C, into a mixture of [Pd(2,6-
(Ph2PCH2)2C6H3)(NHMe2)](OTf) and lithium dicyclo-
hexylamide in a screw-capped NMR tube (Wilmad,
528-TR), which was prepared in a glove box under
argon. This sample was inserted into the pre-cooled
NMR probe at −50°C, and the 1H- and 31P{1H}-
NMR spectroscopic data at various temperatures were
obtained by switching the radio frequencies for both
nuclei. A series of experiments were similarly performed
by employing a different mole ratio of lithium dicyclo-
hexylamide to the palladium complex. The formations
of 2–4 were verified by 1H- and 31P{1H}-NMR
spectroscopy.
a bridging
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