Nucleophilic de-coordination and electrophilic regeneration of "hemilabile" pincer-type complexes: Formation of anionic dialkyl, diaryl, and dihydride PtII complexes bearing no stabilizing π-acceptors

Article abstract of DOI:10.1002/chem.200400514

Novel anionic dialkyl, diaryl, and dihydride platinum(II) complexes based on the new πlong-armπ hemilabile PCN-type ligand C₆H ₄[CH₂P(tBu)₂](CH₂) ₂N(CH₃)₂ with the general formula Li ⁺ [Pt(PCN)(R)₂]^- (R = Me (4), Ph (6) and H (9)) were prepared by reaction of [Pt(PCN)(R)] complexes (obtained from the corresponding chlorides) with an equivalent of RLi, as a result of the opening of the chelate ring. Alkylating agents based on other metals produce less stable products. These anionic d⁸ complexes are thermally stable although they bear no stabilizing π acceptors. They were characterized by ¹H, ³¹P{¹H}, ¹³C, and ⁷Li NMR spectroscopy; complex 9 was also characterized by single crystal X-ray crystallography, showing that the Li ⁺ ion is coordinated to the nitrogen atom of the open amine arm and to the hydride ligand (trans to the P atom) of a neighboring molecule (H-Li = 2.15 A), resulting in a dimeric structure. Complexes 4 and 9 exhibit high nucleophilic reactivity, upon which the pincer complex is regenerated. Reaction of 4 with water, methyl iodide, and iodobenzene resulted in the neutral complex [Pt(PCN)(CH₃)] (3) and methane, ethane, or toluene, respectively. Labeling studies indicate that the reaction proceeds by direct electrophilic attack on the metal center, rather than attack on the alkyl ligand. The anionic dihydride complex 9 reacted with water and methyl iodide to yield [Pt(PCN)(H)] (8) and H₂ or methane, respectively.

Full text of DOI:10.1002/chem.200400514

D. Milstein, et al.
Chem. Eur. J. 2004, 10, 4673 – 4684
DOI: 10.1002/chem.200400514
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4673

FULL PAPER
Nucleophilic De-coordination and Electrophilic Regeneration of
“Hemilabile” Pincer-Type Complexes: Formation of Anionic Dialkyl, Diaryl,
and Dihydride PtII Complexes Bearing No Stabilizing p-Acceptors
Elena Poverenov,[a] Mark Gandelman,[a] Linda J. W. Shimon,[b] Haim Rozenberg,[b]
Yehoshoa Ben-David,[a] and David Milstein*[a]
Abstract: Novel anionic dialkyl, diaryl,
and dihydride platinum(ii) complexes
based on the new “long-arm” hemila-
ceptors. They were characterized by
upon which the pincer complex is re-
generated. Reaction of 4 withwater,
methyl iodide, and iodobenzene result-
7
1H, 31P{1H}, 13C, and Li NMR spectros-
copy; complex 9 was also characterized
bile
C6H4[CH2P(tBu)2](CH2)2N(CH3)2 with
the general formula
Li+
PCN-type
ligand
by single crystal X-ray crystallography,
showing that the Li ion is coordinat-
ed to the nitrogen atom of the open
amine arm and to the hydride ligand
(trans to the P atom) of a neighboring
ed
in
the
neutral
complex
+
[Pt(PCN)(CH3)] (3) and methane,
ethane, or toluene, respectively. Label-
ing studies indicate that the reaction
proceeds by direct electrophilic attack
on the metal center, rather than attack
on the alkyl ligand. The anionic dihy-
dride complex 9 reacted withwater
[Pt(PCN)(R)2]À (R=Me (4), Ph( 6)
and H (9)) were prepared by reaction
of [Pt(PCN)(R)] complexes (obtained
from the corresponding chlorides) with
an equivalent of RLi, as a result of the
opening of the chelate ring. Alkylating
agents based on other metals produce
less stable products. These anionic d8
complexes are thermally stable al-
though they bear no stabilizing p ac-
À
molecule (H Li=2.15 Š), resulting in
a dimeric structure. Complexes 4 and 9
exhibit high nucleophilic reactivity,
and
methyl
iodide
to
yield
[Pt(PCN)(H)] (8) and H2 or methane,
respectively.
Keywords: hemilabile complexes ·
hydrides
· N,P ligands · pincer
ligands · platinum
Introduction
of suchspecies. Examples of anionic transition metal com-
plexes that do not contain such p-acceptor ligands are
rare[2,4,5,7] and their synthesis is a considerable challenge.
Anionic transition-metal complexes can exhibit remarkable
nucleophilic behavior toward organometalic complexes and
organic molecules. Their reactivity as alkylating reagents
Complexes of PCP and NCN pincer-type ligands (A) have
[12]
attracted muchrecent attention.
They have been very
withvarious electrophiles [1,2] and as reducing hydride donors
useful in various areas, including bond activation, catalysis,
and the stabilization of unstable, elusive compounds.[12–15]
The hardness of the chelating N donor in NCN-type ligands
versus softness of P donor in PCP systems results in very
different behavior of the corresponding NCN- and PCP-
based complexes. While each of these systems has its unique
[3–6]
withpolar molecules
is a subject of current interest. Usu-
[6,8]
ally, p-acceptor ligands, suchas carbonyls
and olefins,[9]
and electron-withdrawing phosphines[10] or halide ligands[11]
are required in order to stabilize the high electron density
on the metal center; this reduces the nucleophilic character
chemistry, it is conceivable that a mixed PCN-type system
[16]
could benefit from advantages of bothligands.
The differ-
ent electron-donor properties of the phosphine and amine li-
gands and the more labile coordination of the amine arm in
[a] E. Poverenov, M. Gandelman, Y. Ben-David, Prof. Dr. D. Milstein
Department of Organic Chemistry
The Weizmann Institute of Science
Rehovot, 76100 (Israel)
Fax : (+972)893-44-142
E-mail: david.milstein@weizmann.ac.il
[b] Dr. L. J. W. Shimon, Dr. H. Rozenberg
Unit of Chemical Research Support
The Weizmann Institute of Science
Rehovot, 76100 (Israel)
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DOI: 10.1002/chem.200400514
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4673 – 4684
comparison to the phosphine[17] open up additional reactivi-
ty patterns. The hemilabile[18] coordination of a PCN ligand
could make the system adaptable to electronic, coordinative,
and steric requirements in different steps of a catalytic cycle
or of a stoichiometric reaction, enabling a desirable balance
between stability and reactivity. Here we report on a new
“long-arm” PCN system, which enables the synthesis of
novel, anionic dialkyl and diphenyl complexes of PtII that do
not bear stabilizing p-accepting ligands. In addition, an un-
precedented anionic d8 dihydride complex was synthesized
and was spectroscopically and crystallographically character-
ized. Key features of the formation of these complexes are
the chelate ring-opening of the PCN system and a significant
counterion effect. These anionic complexes show high nucle-
ophilic reactivity towards various electrophiles, upon which
the pincer complex framework is regenerated.
Scheme 2.
(cod=cyclooctadiene), quantitative formation of 2 was ob-
served after 30 min of heating at 1008C.
The 31P{1H} NMR of 2 shows a singlet at 64.20 ppm with
1
Pt satellites (J(Pt,P)=4001 Hz). In the H NMR spectrum,
the NMe2 group gives rise to singlets at 2.83 ppm (J(Pt,H)=
9 Hz) and 2.82 ppm (J(Pt,H)=8 Hz) withPt satellites. In
the 13C{1H} NMR the ipso carbon atom exhibits a doublet at
149.80 ppm (J(P,C)=11 Hz), and the methyl groups of
NMe2 appear as a singlets at 49.29 and 49.27 ppm. The fact
that the two methyl groups of the dimethylamine unit have
different chemical shifts indicates coordination of the amine
to the metal center; this results in the lack of a plane of
symmetry between the methyl groups. Colorless plates of 2
suitable for a single-crystal X-ray analysis were obtained by
recrystallization of 2 from a benzene/pentane two-phase
mixture. The platinum atom is located in the center of a
square-planar structure in which the chloride is located trans
to the aromatic ring, and the amine arm is coordinated to
the metal center (Figure 1). The majority of known pincer-
Results and Discussion
Synthesis of a “long-arm” PCN-type ligand: The new PCN
ligand 1 bearing two methylene groups in the amine arm
was prepared in order to enhance the labile coordination
effect. It was synthesized from 3-(bromomethyl)benzyl alco-
hol, which was obtained by esterification, bromination, and
reduction of 3-(methyl)benzoic acid according to a literature
procedure.[19] The resulting 3-(bromomethyl)benzyl alcohol
was treated witha slight excess of sodium or potassium cya-
nide to give 3-(cyanomethyl)benzyl alcohol, which was re-
duced withLiAlH to yield 3-(aminoethyl)benzyl alcohol.
4
This alcohol was methylated to give 3-(dimethylamino-
ethyl)benzyl alcohol, which upon reaction with an aqueous
solution of HBr at room temperature yielded the HBr salt
of 3-(dimethylaminoethyl)benzyl bromide. Treatment of this
salt with2.4 equivalents of tBu2PH yielded the pure ligand 1
(Scheme 1).
Synthesis and characterization of a “long-arm” PCN-based
platinum chloride complex: The PCN-based platinum chlo-
À
ride complex 2 was obtained as a result of C H activation
and methane liberation reactions (Scheme 2). When a solu-
tion of ligand 1 in THF was added to [Pt(cod)(CH3)(Cl)]
Figure 1. An ORTEP view of a molecule of complex 2. Hydrogen atoms
are omitted for clarity.
type complexes have two symmetric (five-membered) rings,
providing the system with relative rigidity and stability, since
five-membered organometalic rings are generally the most
stable.[1a] The PCN-based complex 2 has a less stable, six-
membered ring, formed by the amine arm, and hence labile
coordination of the amine ligand is expected. Compatible
À
with this expectation, the Pt N bond length(2.188 Š) of
complex 2 is relatively long in comparison withthe usual
À
Pt N bond length(2.08 Š) in symmetric NCN sys-
Scheme 1.
tems.[20,21,22] On the other hand, the Pt P bond lengthof
À
Chem. Eur. J. 2004, 10, 4673 – 4684
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D. Milstein, et al.
FULL PAPER
complex 2 (2.226 Š), which constitutes part of a five-mem-
À
bered ring, is similar to other reported Pt P bond lengths of
PCP complexes (2.25 Š).[23,24] Additional bond lengths and
angles of 2 are given in Table 1.
Table 1. Selected bond lengths [Š] and angles [8] for complex 2.
À
Pt1 C1
2.021(10)
2.188(7)
2.226(2)
2.419(2)
1.481(12)
1.486(11)
1.809(10)
1.864(9)
C1-Pt1-N1
C1-Pt1-P2
C1-Pt1-Cl1
N1-Pt1-P2
N1-Pt1-Cl1
P2-Pt1-Cl1
C11-N1-C12
C11-N1-Pt1
94.4(3)
84.9(3)
174.9(2)
175.2(2)
87.3(2)
93.89(8)
108.1(7)
109.6(6)
À
Pt1 N1
À
Pt1 P2
À
Pt1 Cl1
À
N1 C11
À
N1 C12
À
P2 C21
À
P2 C23
Figure 2. An ORTEP view of a molecule of complex 3. Hydrogen atoms
are omitted for clarity.
Formation of anionic dialkyl and diaryl complexes: As men-
tioned in the introduction, formation of stable anionic com-
plexes usually requires the presence of strong p-acceptor li-
gands in order to stabilize the electron density on the metal
center. However, such ligands decrease the nucleophilic fea-
tures of the anionic complexes. The hemilabile coordination
of the s-donating amine arm of the new PCN ligand turned
out to be useful in addressing this stability versus reactivity
problem. Upon reaction of complex 2 withone equivalent
of methyllithium, the neutral monomethyl PtII complex 3
was formed and was fully characterized by NMR spectrosco-
py (Scheme 3). The 31P{1H} NMR spectrum of 3 exhibits a
Table 2. Selected bond lengths [Š] and angles [8] for complex 3.
À
Pt1 C1
1.971(5)
2.192(4)
2.217(13)
2.408(13)
1.500(7)
1.500(7)
1.849(5)
1.870(5)
C1-Pt1-N1
C1-Pt1-P2
C1-Pt1-Cl3
N1-Pt1-P2
N1-Pt1-Cl3
P2-Pt1-Cl3
C14-N1-C12
C14-N1-Pt1
82.84(19)
83.58(15)
174.36(14)
166.25(12)
93.22(12)
100.48(5)
108.3(4)
À
Pt1 N1
À
Pt1 P2
À
Pt1 Cl3
À
N1 C14
À
N1 C12
À
P2 C21
À
P2 C23
109.9(3)
À
nance at 73.75 ppm withPt P satellites (J(Pt,P)=2200 Hz).
À
The relatively small Pt P coupling (as compared withthe
typical coupling of this system of about 4000 Hz) points to
coordination of a strong sigma donor trans to phospho-
rous.[1a] Two sets of coordinated methyl groups were ob-
served at 0.37 (d, J(Pt,H)=67 Hz, J(P,H)=7 Hz) and
Scheme 3.
1
0.06 ppm (d, J(Pt,H)=51 Hz, J(P,H)=4 Hz) in the H NMR
spectrum, and at 5.11 (d, J(P,C)=20 Hz, J(Pt,C)=669 Hz)
singlet withPt satellites at 61.19 ppm ( J(Pt,P)=4228 Hz). In
1H NMR spectrum the coordinated methyl group gives rise
to a doublet withsatellites at 0.57 ppm ( J(P,H)=1 Hz,
J(Pt,H)=41 Hz) and the two methyl groups on the amine
arm give rise to two signals withPt satellites at 2.37 (s,
J(Pt,H)=20.4 Hz) and 2.27 ppm (s, J(Pt,H)=22 Hz). In
and À7.82 ppm (d, J(P,C)=5 Hz, J(Pt,C)=477 Hz) in 13C
1
{1H} NMR spectrum. According to the H and 13C{1H} NMR
data, the amine arm of the PCN ligand is not coordinated to
the metal center, since the methyl groups of NMe2 exhibit a
single resonance without Pt satellites. It is likely that Li+ is
coordinated to the nitrogen atom of the opened amine arm,
13
1
7
À
C{ H} NMR spectrum, the Pt CH3 carbon appears at
contributing to the stability of the anionic complex. The Li
À1.65 ppm (d, J(P,C)=8 Hz, J(Pt,C)=508 Hz) and the
carbon atoms of NMe2 appear at 48.74 and 48.71 ppm.
Complex 3 was recrystallized from a benzene/pentane
two-phase mixture at room temperature to give colorless
crystals appropriate for a single-crystal X-ray analysis. The
platinum atom is located in the center of a square-planar
structure, in which the methyl group is trans to the aromatic
ring, and the amine arm is coordinated to the metal center
(Figure 2). Selected bond lengths and angles are given in
Table 2.
NMR spectrum of complex 4 exhibits a broad singlet at
0.35 ppm, while the free Li+ ion from LiCl gives a narrow
peak at 0 ppm. Crystallographic evidence for Li coordina-
tion were obtained for the analogous dihydride complex
(vide infra).
The same mode of reactivity was observed with other or-
ganolithium compounds. Thus, reaction of 2 witha stoichio-
metric amount of phenyllithium led to formation of the
monophenyl PtII complex 5. This complex exhibits a single
resonance at 59.86 ppm (J(Pt,P)=4145 Hz) in the 31P{1H}
NMR spectrum, and in 13C {1H} NMR spectrum the ipso
signal due to the coordinated phenyl ring appears at
169.45 ppm, while the ipso signal of the PCN system of 5 ap-
Remarkably, when a second equivalent of MeLi was
added, the anionic dimethyl complex
4 was formed
(Scheme 3). The 31P{1H} NMR spectrum shows a single reso-
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Chem. Eur. J. 2004, 10, 4673 – 4684

Pincer-Type Complexes
4673 – 4684
pears at 164.31 ppm. Addition of two or more equivalents of
PhLi to
2
gave the anionic diphenyl PtII complex 6
(Scheme 4). Similarly to 4, th e31P{1H} NMR spectrum of 6
exhibits a downfield-shifted single resonance at 72.94 ppm
Scheme 5.
complex 3 and CH3D, which was detected by GC/MS. Signif-
icantly, complex 4 is capable of reaction even withweak
electrophiles. For example, upon reaction with an excess of
iodobenzene or bromobenzene, complex 3 and toluene were
formed, the latter being detected by GC. This reaction was
slower than the reactions with MeI or water and quantita-
tive formation of 3 was observed only after 24 h.
Mechanistically, these reactions may proceed through
direct attack of the electrophile on the alkyl group or
through oxidative addition of the electrophile to the Pt
center, followed by reductive elimination of the organic
product (Scheme 6). Addressing this interesting issue, com-
Scheme 4.
À
witha small Pt P coupling (J(Pt,P)=1764 Hz) and the two
methyl groups of NMe2 give rise to one peak in the 13C{1H}
1
and H NMR spectra withno coupling to Pt. In the 13C{1H}
spectrum two ipso signals at 174.44 (d, J(P,C)=120 Hz) and
168.69 ppm (d, J(P,C)=21 Hz) were found, confirming the
coordination of two phenyl groups to the Pt center. The ipso
signal of the PCN system of 6 appears at 157.03 ppm
(J(Pt,C)=620 Hz).
Thus, the “long-arm” PCN-based neutral alkyl and aryl
complexes show remarkable reactivity towards nucleophilic
attack, resulting in chelate-ring opening and formation of
novel, stable anionic complexes. This reactivity pattern was
not observed before withPCP,
plex 4 was treated withCD I, and the product ethane was
3
checked by GC/MS, revealing that CH3CH3/CD3CH3/
CD3CD3 were formed in a ratio of 59:14:8. Formation of
NCN, or other pincer-type sys-
tems. In fact, withthe electron-
rich[Pt(NCN)(X)] (X =halide)
complexes, even nucleophilic
substitution of halide by RLi to
give
stable
neutral
[Pt(NCN)(R)] complexes takes
place only withan electron-
withdrawing R group (R=aryl,
acetylide), while with alkyllithi-
um reagents [Pt(NCN)(alkyl)]
products were unstable and
could not be isolated.[22] In this
context, the labile features of
the amine arm of the PCN
ligand not only allow coordina-
tion of a second nucleophile
due to facile generation of a
Scheme 6.
vacant site, but also de-coordination of the amine ligand de-
creases the electron density on the metal center, encourag-
ing the nucleophilic attack.
CH3CH3 and CD3CD3 suggests that the oxidative addition
scenario is probably involved, since direct nucleophilic
attack is expected to lead only to the mixed CH3CD3
(Scheme 6).
Reactivity of the anionic dialkyl complexes: The nucleophil-
ic behavior of the anionic dimethyl complex 4 was explored,
revealing that it is highly reactive with different electro-
philes, regenerating the pincer complex framework
(Scheme 5). For example, reaction of 4 withMeI led to for-
mation of the monomethyl neutral complex 3, which was de-
tected by 31P{1H} NMR spectroscopy immediately after ad-
dition of the electrophile. Formation of ethane was detected
by GC/MS and a yellow precipitate of LiI was formed. Like-
Interestingly, the neutral monomethyl complex is also ca-
pable of acting as a methyl donor. Reaction of complex 3
withmethyl iodide and iodobenzene yielded complex 7 and
ethane or toluene, respectively. However, these reactions
were slower than the corresponding reactions of the dimeth-
yl complex 4 and formation of complex 7 was complete only
after 24 h(MeI) or 48 h(PhI) at room temperature. The
mechanism of these reactions may involve oxidative addi-
tion to form a PtIV intermediate, followed by reductive elim-
ination (Scheme 7).
wise, reaction of 4 withD O led to immediate formation of
2
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D. Milstein, et al.
FULL PAPER
27 Hz and J(Pt,H)=136 Hz. As observed withthe mono-
methyl complex 3, complex 8 reacted withmethyl iodide to
yield complex 7 and an equivalent amount of methane
(Scheme 8). However, in this case, conversion of the mono-
hydride to the iodide complex was relatively rapid, probably
due to bothelectronic and steric reasons, whichmakes
8
more reactive than the methyl complex 3 in oxidative addi-
tion reactions.
Remarkably, when excess of Et3BHLi was added to 8, th e
anionic dihydride complex 9 was formed (Scheme 8). Com-
plex 9 exhibits a singlet at 91.69 ppm with a relatively small
Pt P coupling (J(Pt,P)=2295 Hz) in the 31P{1H} NMR spec-
trum. In H NMR spectrum, the hydride trans to the phos-
À
1
Scheme 7.
phorous atom appears as a double doublet at À3.00 ppm
with J(Pt,H)=1035 Hz, J(P,H)=152 Hz, and J(H,H)=8 Hz,
while the second hydride (cis to the phosphorous atom) ex-
hibits a triplet at À7.07 ppm with J(Pt,H)=700 Hz, J(P,H)=
8 Hz, and J(H,H)=8 Hz. Similarly to the anionic complexes
described above, the methyl groups of the NMe2 moiety
Formation and reactivity of an anionic dihydride complex:
Anionic late-transition-metal hydrides that have no strong
p-acceptor ligands are a very interesting rare class of com-
plexes. Mono- and dihydride d6 anionic complexes, suchas
[{Mo(h-C5H5)2(H)Li}4],[4] and K+[(Ph3P)2Ph2PC6H4RuH2]À·
C10H8·(C2H5)2O[5] have been prepared and demonstrated re-
markable hydridic chemistry. Anionic d8 hydride complexes
in the absence of p-accepting ligands are expected to have a
very high electron density on the metal center. This is, per-
haps, one of the reasons why, to the best of our knowledge,
no synthesis and characterization of d8 anionic hydride com-
plexes have been reported. Using the hemilabile aptitude of
the amine arm, we succeed in the preparation of an anionic
d8 platinum dihydride complex through direct nucleophilic
attack on the metal center. Upon reaction of complex 2 with
one equivalent of LiEt3BH the expected monohydride com-
plex 8 was formed (Scheme 8). This complex was fully char-
acterized by multinuclear NMR spectroscopy. The 31P{1H}
1
show one peak in H and 13C{1H} NMR spectra at 2.12 and
46.31 ppm, respectively; this indicates de-coordination of
the amine arm from the platinum center.
Crystals of the dihydride complex, suitable for a single-
crystal X-ray analysis, were obtained by evaporation of the
THF solvent from a highly concentrated solution of 9
(Figure 3, top). The Pt atom is located in the center of a dis-
torted square-planar structure, and the two hydride ligands
occupy trans and cis positions to the phosphorous atom. As
indicated by the spectroscopic data in solution, the amine
arm has opened, allowing coordination of a second hydride
to the metal center. Significantly, the Li+ ion is coordinated
to the nitrogen atom and to a THF molecule. In addition,
the high electron density on the metal center and on the hy-
dride ligands is stabilized by a Li+ ion from a neighboring
molecule (Figure 3, middle and bottom). A short distance
between the hydride trans to the phosphorous atom and the
NMR spectrum of
8 exhibits a single resonance at
1
82.06 ppm with J(Pt,P)=4014 Hz; in H NMR spectrum the
hydride gives rise to a doublet at À2.55 ppm with J(P,H)=
À
neighboring Li ion (H Li=
2.15 Š) is observed. Taking into
consideration the ionic radii of
Li+ (0.68 Š) and HÀ (1.54 Š),
sucha distance is short enough
to represent a bond and it is
likely that these interactions
lead to formation of the dimeric
structure. In addition, a weak
stabilizing interaction between
the Pt atom and the neighbor-
À
ing Li ion (Pt Li=2.68 Š) was
found (the anionic radius of
Pt2+ is 0.80 Š, therefore this
distance is too long to be con-
sidered a real bond). The Li–Li’
distance found in 9 of 3.13 Š is
relatively long compared with
known organolithium dimers.[25]
The fact that 9 crystallized as a
dimer is in agreement withthe
general tendency of organolithi-
um compounds to form oligom-
Scheme 8.
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Pincer-Type Complexes
4673 – 4684
Table 3. Selected bond lengths and angles of complex 9.
À
Pt1 C10
2.10(5)
2.68(8)
2.25(13)
1.33(15)
1.66(4)
2.10(9)
1.97(10)
3.13(16)
2.153
C10-Pt1-P2
C10-Pt1-Li1
84.1(14)
131.1(2)
131.9(18)
78.0(6)
105.0(6)
113.0(6)
173.1(13)
53.4(14
À
Pt1 Li1
À
Pt1 P2
P2-Pt1-Li1A
C10-Pt1-H1A
P2-Pt1-H1A
Li1A-Pt1-H1A
C10-Pt1-H1B
Li1A-Pt1-H1B
H1A-Pt1-H1B
À
Pt1 H1A
À
Pt1 H1B
À
N1 Li1
À
0100 Li1
À
Li1 Li1A
À
Li1 H1B
106.0(6)
Interestingly, complex 9 can be alternatively prepared by
reaction of 8 withone equivalent of litihum diisopropyl
amide (LDA) at 858C for 5 h (Scheme 8). We believe that
complex 8 undergoes nucleophilic attack (surprisingly LDA
reacts as a nucleophile and not as a base), which leads to
opening of the amine arm and coordination of diisopropyl
amide to get an unobserved anionic intermediate; this in
turn undergoes b-H elimination to form the dihydride com-
plex 9 (Scheme 8).
Similar to the anionic dialkyl and diaryl complexes, com-
plex 9 is highly reactive with electrophiles (Scheme 9). For
example, it reacted readily withwater and methyl iodide at
room temperature, resulting in immediate formation of the
monohydride complex 8 and H2 or methane, respectively.
Scheme 9.
It is important to note the considerable effect of the coun-
terion on these anionic systems. As shown by the X-ray
structure of complex 9, the Li cation coordinates to the ni-
trogen atom of the open amine arm and interacts with the
hydride ligands. In addition, the strong coordination of Li+
is reflected in the high stability of the anionic complexes re-
ported here toward TMEDA or [12]crown-4, which possess
high affinity for lithium ions. All our attempts to extract Li+
from the anionic complexes were unsuccessful. Also, we
found that the nature of counterion is very important. For
example, use of Na[Et3BH] (instead of Li[Et3BH]) in reac-
tion withcomplex 2 resulted in a more difficult, slow forma-
tion of an unstable complex 9’, whereas when MeLi was re-
placed by MeMgCl only formation of the monomethyl com-
plex 3 and no formation of an anionic dimethyl complex was
observed (Scheme 10). This significant cation effect is very
likely a result of its ability to coordinate to the nitrogen
atom, thus facilitating the nucleophilic attack and stabilizing
the product. Li+ coordination to the amine arm is more fa-
vorable than that of the other cations studied here.
Figure 3. Top: An ORTEP view of a molecule of complex 9. Middle: An
ORTEP view of a dimeric molecule of complex 9. Hydrogen atoms,
except for hydrides, are omitted for clarity. Bottom: A view of the Li-Pt-
H dimeric core of complex 9.
ers. It is usually attributed to the high ionicity of lithium
bonds and hence large energies of association.[25,26] Addi-
tional bond lengths and angles are given in Table 3.
Chem. Eur. J. 2004, 10, 4673 – 4684
www.chemeurj.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4679

D. Milstein, et al.
FULL PAPER
1H, 13C, 31P, and Li NMR spectra were
recorded at 400, 100, 162, and
7
155 MHz, respectively, by using
a
Bruker AMX-400 NMR spectrometer.
All spectra were recorded at 238C. 1H
NMR and 13C{1H} NMR chemical
shifts are reported in ppm downfield
from tetramethylsilane. 1H NMR
chemical shifts were referenced to the
residual hydrogen signal of the deuter-
ated solvents (7.15 ppm, benzene;
7.24 ppm, chloroform; 3.58 ppm, tetra-
hydrofuran). In 13C{1H} NMR meas-
urements the signals of C6D6
(128.0 ppm), CDCl3 (77.0 ppm), and
[D8]THF (67.4 ppm) were used as a
reference. 31P NMR chemical shifts
are reported in ppm downfield from
Scheme 10.
H3PO4 and referenced to an external
85% solution of phosphoric acid in
Conclusion
D2O. In 7Li NMR measurements LiCl was used as a standard (LiCl in
THF=0 ppm). Screw-cap 5 mm NMR tubes were used in the NMR
follow-up experiments. Abbreviations used in the description of NMR
data are as follows: br, broad; s, singlet; d, doublet; t, triplet; m, multip-
let. GC analysis was performed witha HP6890 chromatographequipped
witha FID detector using capillary columns Supelcowax 10 (30 m”
Novel anionic dialkyl, diaryl, and dihydride platinum(ii)
complexes that do not bear p acid ligands were obtained by
reaction of the new “long-arm” hemilabile [Pt(PCN)(R)]
complexes withan equivalent of RLi. Formation of these
0.2 mm”0.2 mm) and HP5 (30 m”0.32 mm”0.25 mm) and
a stainless
8
very electron-richd complexes is based on the opening of
steel packed column 10% Carbowax 20M +80/100 Supersupport 6’”1/
8’’. The retention time of the products was referenced to mesitylene.
the six-membered chelate ring of the amine arm; this pro-
vides the required coordination site and reduces the electron
density at the metal prior to nucleophilic attack. This is an
unprecedented demonstration of pincer complex hemilabili-
ty upon nucleophilic attack. The thermal stablity of these
complexes is surprising, since they do not contain strong p
acceptors that can stabilize the anionic metal center. The
stability of the complexes is counterion dependent, the Li+
ion providing the highest stabilization. An X-ray crystallo-
graphic study of the anionic dihydride 9 shows that the Li+
ion is coordinated to the nitrogen atom of the open amine
arm and to the hydride ligand (trans to P) of a neighboring
molecule, resulting in a dimeric structure. The anionic com-
plexes exhibit high nucleophilic reactivity, upon which the
pincer ligand framework is regenerated. Labelling studies
indicate that the reaction proceeds through direct electro-
philic attack on the metal center, rather than attack on the
alkyl ligand. Current studies are exploring synthetic and
mechanistic aspects of the reactivity of these complexes and
the possible extension of the pincer hemilability approach as
a synthetic tool in the preparation of highly nucleophilic,
anionic complexes of other late transition metals without p-
accepting ligands
Synthesis of the PCN ligand 1: The starting material for preparation of
ligand 1, was 3-(bromomethyl)benzyl alcohol, which was obtained by es-
terification, bromination, and reduction of 3-(methyl)benzoic acid ac-
cording to a literature procedure.[19]
1) Preparation of 3-(hydroxymethyl)benzyl cyanide: 3-(Bromomethyl)-
benzyl alcohol (46.0 g, 0.23 mol) was dissolved in absolute MeOH
(310 mL) and a solution of KCN (18.3 g, 0.28 mol) in H2O (100 mL) was
added. The mixture was refluxed for 5 h and then concentrated to a low
volume under vacuum. The mixture was transferred to an extraction
funnel and extracted with ethyl acetate (3”250 mL). The organic layer
was separated, dried over Na2SO4, and filtered, and the solvent was
evaporated under vacuum. The resulting crude oil was distilled under
vacuum (b.p. 1408–1508C at 0.8 mmHg), yielding 3-(hydroxymethyl)ben-
zyl cyanide (21.20 g; 63% yield). 1H NMR (CDCl3): d=7.15–7.35 (4H;
À
À
Ar), 4.61 (s, 2H; CH2 OH), 3.69 (s, 2H; CH2 CN), 2.85 ppm (brs, 1H;
CH2 OH); 13C{1H} NMR (CDCl3): d=142.08(s, Ar), 130.07 (s, Ar),
À
129.22 (s, Ar), 126.93 (s, Ar), 126.27 (s, Ar), 126.43 (s, Ar), 117.97 (s,
À
À
À
CH2 CN), 64.39 (s, CH2 CN), 23.43 ppm (s, CH2 0H) (assignment of
13C{1H} NMR was confirmed by 13C DEPT); IR (film): n˜ =2252 cmÀ1
(CN).
2) Preparation of 3-(aminoethyl)benzyl alcohol: AlCl3 (19.7 g) dissolved
in dry THF (250 mL) was added to a solution of LiAlH4 in THF (1m,
148 mL). After 5 min of stirring at room temperature, a solution of of 3-
(hydroxymethyl)benzyl cyanide (20.8 g, 0.14 mol) in of dry diethyl ether
(300 mL) was added. The resulting mixture was stirred at room tempera-
ture for 20 h, followed by addition of H2O (100 mL) and H2SO4 (6n,
200 mL). The acidic mixture was extracted with diethyl ether (3”
300 mL) and the aqueous layer was treated with KOH to pH 11. H2O
(850 mL) was added, and the solution was saturated with NaCl and ex-
tracted with ethyl acetate (4”200 mL). The combined organic layers
were dried over Na2SO4, filtered, and concentrated in vacuo to yield 3-
(aminoethyl)benzyl alcohol (16.2 g, 76% yield), which was clean enough
Experimental Section
1
to be used in the next step without further purification. H NMR (C6D6):
General procedures: All experiments withmetal complexes and phos-
phine ligands were carried out under an atmosphere of purified nitrogen
in a Vacuum Atmospheres glovebox equipped with an MO 40-2 inert gas
purifier or by using standard Schlenk techniques. All solvents were re-
agent grade or better. All nondeuterated solvents were refluxed over
sodium/benzophenone ketyl and distilled under an argon atmosphere.
Deuterated solvents were used as received. All the solvents were de-
gassed withargon and kept in the glovebox over 4 Š molecular sieves.
Commercially available reagents were used as received. The precursor
[Pt(cod)(CH3)(Cl)] was prepared according to a literature procedure.[27]
À
À
d=6.98–7.24 (4H; Ar), 4.65 (s, 2H; CH2OH), 2.72 (t, 2H; CH2 CH2
À
À
NH2), 2.55 ppm (t, 2H; CH2 CH2 NH2).
3) Preparation of 3-(dimethylaminoethyl)benzyl alcohol: Formic acid
(7.5 g ) was slowly added to 3-(aminoethyl)benzyl alcohol (7.5 g, 0.5 mol)
cooled in an ice-water bath, avoiding a violent reaction; this was followed
by the addition of formaldehyde (37% solution, 9.5 mL). The flask was
equipped witha reflux condenser, and the mixture was stirred at 80 8C
for 24 h, while heavy bubbling was observed in the beginning of heating.
The reaction was cooled and HCl (6n, 50 mL) was added, followed by
4680
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 4673 – 4684

Pincer-Type Complexes
4673 – 4684
extraction with diethyl ether (3”75 mL). NaOH (50% aqueous solution)
was added to the aqueous phase to give pH 11; then the mixture was sa-
turated withNaCl, followed by extraction withetyhl acetate (3”
300 mL). The combined organic layers were dried over Na2SO4, filtered,
and concentrated in vacuo. The resulting crude mixture was distilled
under vacuum (b.p. 1238C at 0.5 mmHg) yielding pure 3-(dimethyl-
149.80 (d, J(P,C)=11 Hz, ipso), 142.23 (d, J(P,C)=1 Hz, Ar), 137.69 (d,
J(P,C)=1 Hz, Ar), 124.71 (s, Ar), 123.71 (s, Ar), 121.41 (d, J(P,C)=
À
À
17 Hz, Ar), 63.77 (d, J(P,C)=1 Hz, CH2 CH2 N), 63.75 (d, J(P,C)=1 Hz,
À
À
À
À
CH2 CH2 N), 49.29 (s, N CH3), 49.27 (s, N CH3), 34.65 (d, J(P,C)=
À
À
À À
27 Hz, Ar CH2 P), 34.44 (d, J(P,C)=34 Hz, P C (CH3)3), 29.21 ppm (d,
J(P,C)=3 Hz, J(Pt,C)=19 Hz, P C (CH3)3) (assignment of 13C{1H}
À À
aminoethyl)benzyl alcohol as
a
viscous oil in 80% yield. 1H NMR
NMR signals was confirmed by 13C DEPT); elemental analysis calcd (%)
for C19H33NPClPt: C 43.12, H 7.42; found: C 42.98, H 7.41.
À
(CDCl3): d=7.02–7.28 (4H; Ar), 4.58 (s, 2H; CH2 OH), 3.45 (brs, 1H;
À
À
À
À
À
CH2 OH), 2.71(t, 2H; CH2 CH2 N), 2.45 (t, CH2 CH2 N), 2.19 ppm (s,
Reaction
of
[Pt(PCN)(Cl)]
(2)
with
MeLi—formation
of
6H; N(CH3)2); 13C{1H} NMR (CDCl3): d=141.61 (s, Ar), 140.24 (s, Ar),
[Pt(PCN)(CH3)] (3): A solution of [Pt(PCN)(Cl)] (20 mg, 0.037 mmol) in
THF (1 mL) was cooled to À408C and a solution of MeLi in diethyl
ether (1.57m, 23 mL, 0.037 mmol), also cooled to À408C was added.
31P{1H} NMR spectroscopy revealed the formation of complex 3. The sol-
vent was evaporated, and the resulting white solid was washed with pen-
tane and dissolved in benzene. After benzene evaporation, complex 3
(18 mg 0.035 mmol) was obtained as a white solid (95% yield). Colorless
crystals of 3 suitable for single-crystal X-ray diffraction were obtained
from a benzene/pentane two-phase mixture at room temperature. 31P{1H}
NMR (C6D6): d=61.19 ppm (s, J(Pt,P)=4228 Hz); 1H NMR (C6D6): d=
7.32 (d, J(H,H)=7 Hz, 1H; Ar), 7.22 (dt, J(H,H)=7 Hz, J(P,H)=2 Hz,
1H; Ar), 7.00 (d, J(H,H)=7 Hz, 1H; Ar), 2.01 (d, J(P,H)=10 Hz,
À
128.44 (s, Ar), 127.59 (s, Ar), 127.14 (s, Ar), 124.65 (s, Ar), 64.77 (s, CH2
À
À
À
OH), 61.22 (s, CH2 CH2 NH2), 45.11 (s, N(CH3)2), 33.82 ppm (s, CH2
CH2 NH2) (assignment of 13C{1H} NMR was confirmed by 13C DEPT).
À
4) Preparation of HBr salt of 3-(dimethylaminoethyl)benzyl bromide: In a
round-bottomed 500 mL flask 3-(dimethylaminoethyl)benzyl alcohol
(10.1 g, 0.056 mol) was dissolved in absolute MeOH (90 mL), followed by
addition of an aqueous solution of HBr (48%, 90 mL). The mixture was
stirred at room temperature for 24 h, followed by solvent removal under
high vacuum. The obtained crude solid was re-crystalized from an
MeOH/diethyl ether mixture, and the collected snow white solid was
dried under high vacuum, yielding the HBr salt of 3-(dimethylamino-
ethyl)benzyl bromide (16.4 g, 90% yield). 1H NMR (CD3OD): d=7.27–
À
À
À
À
J(Pt,H)=36 Hz, 2H; Ar CH2 P), 2.57 (m, 2H; CH2 CH2 N), 2.38 (s,
À
À
À
À
À
7.43 (4H; Ar), 4.58 (s, 2H; CH2 Br), 3.43 (m, 2H; CH2 CH2 N), 3.10
J(Pt,H)=20.4 Hz, 3H; N CH3), 2.37 (s, J(Pt,H)=22 Hz, 3H; N CH3),
(m, CH2 CH2 N), 2.97 ppm (s, 6H; N(CH3)2); 13C{1H} NMR (CD3OD):
d=140.47 (s, Ar), 137.94 (s, Ar), 130.64 (s, Ar), 130.41 (s, Ar), 129.86 (s,
2.27 (m, 2H; CH2 CH2 N), 1.22 (d, J(P,H)=12.8 Hz, 18H; P tBu),
À
À
À
À
À
0.57 ppm (d, J(P,H)=1 Hz, J(Pt,H)=41 Hz, 3H; Pt CH3); 13C{1H} NMR
À
À
À
Ar), 129.16 (s, Ar), 59.57 (s, CH2 CH2 NH2), 43.64 (s, N(CH3)2), 33.72
(C6D6): d=149.78 (d, J(P,C)=5 Hz, ipso), 143.54 (s, Ar), 124.41 (s, Ar),
(s, CH2 CH2 NH2), 31.55 ppm (s, CH2 Br), (assignment of 13C{1H} NMR
122.86 (s, Ar), 121.09 (s, Ar), 120.93 (s, Ar), 65.73 (s, Ar CH2 CH2 N),
À
À
À
À
À
À
was confirmed by 13C DEPT).
65.71 (s, Ar CH2 CH2 N), 48.74 (s, N CH3), 48.71 (s, N CH3), 38.59 (d,
À À
À
À
À
À
À
À
À
J(P,C)=16 Hz, Ar CH2 P), 34.41 (d, J(P,C)=27 Hz, P C (CH3)3), 29.58
5) Preparation of ligand 1: The HBr salt of 3-(dimethylaminoethyl)benzyl
bromide (8.3 g, 0.026 mol) was dissolved in dry MeOH (45 mL) in a glass
pressure vessel in a dry box. A solution of di-tert-butyl phosphine (9.0 g,
0.062 mol) in dry MeOH (15 mL) was added, causing the slightly orange
solution to become colorless. The mixture was stirred at 458C for 48 h
outside the box in the sealed tube, cooled to room temperature and then
re-introduced into the dry box. Triethylamine (18 mL) was added and the
solution was stirred for 30 min. The solvents were removed under
vacuum, resulting in a mixture of a white solid and viscous oil. The mix-
ture was treated with diethyl ether and filtered, followed by washing with
a copious amount of diethyl ether. The combined diethyl ether fractions
were dried under vacuum to give a viscous oil. 31P{1H} and 1H NMR
spectra of the crude product showed very little impurities. The oil was
purified by chromatography in the dry box through a short silica column
withhexane/THF (1:1) as an eluent, yielding a pure product (7.40 g, 94%
yield). 31P{1H NMR (C6D6): d=32.12 ppm (s); 1H NMR (C6D6): d=7.38
(s,1H; Ar), 7.31 (d, J(H,H)=7 Hz, 1H; Ar), 7.16 (t, J(H,H)=7 Hz, 1H;
À À
(d, J(P,C)=3 Hz, J(Pt,C)=26 Hz, P C (CH3)3), À1.65 ppm (d, J(P,C)=
8 Hz, J(Pt,C)=508 Hz, Pt CH3) (assignment of 13C{1H} NMR signals was
À
confirmed by 13C DEPT); elemental analysis calcd (%) for C20H36NPPt:
C 46.51, H 6.98; found: C 46.60, H 7.05.
Reaction of [Pt(PCN)(Cl)] (2) with MeLi—formation of Li+
[Pt(PCN)(CH3)2]À (4): A solution of MeLi in diethyl ether (1.57m,
47 mL, 0.074 mmol) was added to
a solution of complex 2 (20 mg,
0.037 mmol) in THF (1 mL). A yellow solution formed and 31P{1H} NMR
spectroscopy revealed formation of a new complex. The solvent was
evaporated and the resulting yellow solid was dissolved in pentane. The
pentane was evaporated in vacuo, yielding a quantitative (by NMR spec-
troscopy) amount of complex 4. In addition, 4 can be readily prepared by
reaction of the neutral monomethyl complex 3 withone equivalent of
MeLi. Complex 4 is stable for about a monthat À378C and for a couple
of days at room temperature, upon which it is converted to the mono-
methyl complex 3, probably by reaction withtrace water. 31P{1H} NMR
([D8]THF): d=73.75 ppm (J(Pt,P)=2220 Hz); 1H NMR ([D8]THF): d=
6.79 (d, J(H,H)=6 Hz, 1H; Ar), 6.77(d, J(H,H)=6 Hz, 1H; Ar), 6.53 (d,
À
Ar), 6.97 (d, J(H,H)=7 Hz, 1H; Ar), 2.77 (d, J(P,H)=6 Hz, 2H; Ar
À
À
À
À
À
À
À
CH2 P), 2.73 (m, 2H; Ar CH2 CH2 N), 2.48 (m, 2H; Ar CH2 CH2
N), 2.11 (s, 6H; N(CH3)2), 1.08 ppm (d, J(P,H)=21 Hz, 18H;
P(C(CH3)3)2); 13C{1H} NMR (C6D6): d=142.08 (d, J(P,C)=10 Hz, Ar),
141.20 (s, Ar), 130.73 (s, Ar), 130.65 (s, Ar), 126.37 (s, Ar), 126.35 (s, Ar),
À
À
J(H,H)=6 Hz, 1H; Ar), 3.06 (m, 2H;CH2 CH2 N), 2.99 (d, J(P,H)=
À
À
À
À
9 Hz, J(Pt,H)=27 Hz, 2H; Ar CH2 P), 2.55 (m, 2H;CH2 CH2 N), 2.17
(s, 6H; N(CH3)2), 1.17 (d, J(P,H)=10 Hz,18H; P(tBu)2), 0.37 (d,
À
À
À
À
À
À
À
À
62.12 (s, Ar CH2 CH2 N), 45.67 (s, N (CH3)2), 35.05 (s, Ar CH2 CH2
J(Pt,H)=67 Hz, J(P,H)=7 Hz, 3H; Pt CH3), 0.06 ppm (d, J(Pt,H)=
N), 31.97 (d, J(P,C)=24 Hz, P C (CH3)3), 30.07 (d, J(P,C)=14 Hz, P
51 Hz, J(P,H)=4 Hz, 3H; Pt CH3); 13C NMR ([D8]THF): d=177.75 (s,
À À
À
À
À
À
À
C (CH3)3, 29.23 ppm (d, J(P,C)=25 Hz, Ar CH2 P) (assignment of
J(Pt,C)=692 Hz, ipso), 152.26 (d, J(P,C)=15 Hz, Ar), 149.49 (d, J(P,C)=
3 Hz, Ar), 125.01 (s, J(Pt,C)=32 Hz, Ar), 120.55 (s, Ar), 119.86 (d,
13C{1H} NMR was confirmed by 13C DEPT).
À
À
À
À
J(P,C)=13 Hz, Ar), 66.29 (s, CH2 CH2 N), 64.16 (s, CH2 CH2 N), 46.14
Reaction of ligand 1 with [Pt(cod)(CH3)(Cl)]—formation of
[Pt(PCN)(Cl)] (2): A solution of the PCN ligand 1 (30 mg, 0.097 mmol)
in THF (2.5 mL) was added to a solution of [Pt(cod)(CH3)(Cl)] (cod=cy-
clooctadiene; 34.6 mg, 0.097 mmol) in THF (2.5 mL). The mixture was
À
(s, N (CH3)2), 40.90 (d, J(P,C)=21 Hz, P(C(CH3)3)2), 34.32 (d, J(P,C)=
À
À
7 Hz, J(Pt,C)=28 Hz, Ar CH2 P), 30.24 (d, J(P,C)=5 Hz, P-
À
(C(CH3)3)2), 5.11 (d, J(P,C)=20 Hz, J(Pt,C)=669, Pt CH3), À7.82 ppm
(d, J(P,C)=5 Hz, J(Pt,C)=477 Hz); 7Li NMR ([D8]THF): d=0.35 ppm
stirred at 1008C for 30 min, resulting in
a colorless solution.
31P{1H} NMR spectroscopy revealed the formation of 2. The solvent was
evaporated and the resulting white solid was washed with pentane and
dissolved in benzene. The benzene was evaporated, yielding 2 (45.3 mg,
0.084 mmol) as white crystals (87% yield). Colorless crystals of 2 suitable
for single-crystal X-ray diffraction were obtained from a benzene/pentane
two-phase mixture at room temperature. 31P{1H NMR (CDCl3): d=
(brs).
Reaction
of
[Pt(PCN)(Cl)]
(2)
with
PhLi—formation
of
[Pt(PCN)(C6H5)] (5):
A
solution of PhLi in pentane (1.4m, 26 mL,
0.037 mmol) was added to a solution of complex 2 (20 mg, 0.037 mmol)
in THF (1 mL). The solution turned brown-yellow and according to
31P{1H} NMR spectrum, complex 5 was immediately formed at room tem-
perature. The solvent was evaporated and the resulting solid was washed
withpentane and dissolved in benzene. After removing of the solvent
under vacuum, the phenyl complex 5 (24 mg, 0.037 mmol) formed as a
white-yellow solid (97% yield). 31P {1H} NMR (C6D6): d=59.86 ppm (s,
1
64.20 ppm (s, J(P,Pt)=4001 Hz); H NMR (CDCl3): d=6.92 (d, J(H,H)=
7 Hz, 1H; Ar), 6.77 (dt, J(H,H)=7 Hz, J(P,H)=2 Hz, 1H; Ar), 6.59 (d,
J(H,H)=7 Hz, 1H; Ar), 2.84 (d, J(P,H)=11 Hz, J(Pt,H)=21 Hz, 2H;
À
À
À
Ar CH2 P), 2.83 (s, J(Pt,H)=9 Hz, 3H; N CH3), 2.82 (s, J(Pt,H)=8 Hz,
1
À
À
À
À
À
3H; N CH3), 2.65 (m, 2H; CH2 CH2 N), 2.56 (m, 2H; CH2 CH2 N),
J(Pt,P)=4145 Hz); H NMR (C6D6): d=3.55 (d, J(P,H)=9 Hz, J(Pt,H)=
39 Hz, 2H; Ar CH2 P), 3.04 (m, 2H; CH2 CH2 N), 2.67 (m, 2H; CH2
1.25 ppm (d, J(P,H)=16 Hz, 18H; P tBu); 13C{1H} NMR (CDCl3): d=
À
À
À
À
À
À
Chem. Eur. J. 2004, 10, 4673 – 4684
www.chemeurj.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4681

D. Milstein, et al.
FULL PAPER
À
À
CH2 N), 2.62 (s, J(Pt,H)=28 Hz, 3H; N CH3), 2.61 (s, J(Pt,H)=27 Hz,
turned yellow, and after 24 h 31P {1H} NMR spectroscopy revealed forma-
À
3H; N CH3), 1.51 ppm (d, J(P,H)=13 Hz, 18H; P-tBu); the aromatic
tion of complex 8 (reaction withone equivalent of
a solution of
region was crowded due to the coordinated phenyl group and the aro-
matic ring of the PCN ligand; 13C{1H} NMR (C6D6): d=169.45 (d,
J(P,C)=2 Hz, ipso of coordinated phenyl), 164.31 (d, J(P,C)=1.5 Hz, ipso
of ligand), 150.20 (d, J(P,C)=3 Hz, Ar), 144.59 (s, Ar), 143.43 (s,
Ar),143.44 (s, Ar), 127.08 (s, Ar), 124.53 (s, J(Pt,C)=60 Hz, Ar), 123.76
(s, Ar), 122.51 (s, Ar), 121.41 (s, Ar), 121.26 (s, Ar), 65.58 (s, CH2 CH2
N), 65.56 (s, CH2 CH2 N), 50.03 (s, N CH3), 50.00 (s, N CH3), 38.44 (d,
J(P,C)=24 Hz, Ar CH2 P), 35.10 (d, J(P,C)=27 Hz, P C (CH3)3),
30.04 ppm (d, J(P,C)=4 Hz, J(Pt,C)=19 Hz, P C (CH3)3). elemental
Li[Et3BH] in THF (1m, 38 mL 0.037 mmol) led to immediate formation
of this complex). The solvent was evaporated and the resulting yellow-
white solid was dissolved in pentane. After pentane evaporation pure 8
(17 mg, 0.034 mmol) was obtained (92% yield). 31P{1H} NMR (C6D6): d=
82.06 ppm (J(Pt,P)=4014 Hz); 1H NMR (C6D6): d=7.36 (d, J(H,H)=
10 Hz 1H; Ar), 7. 24 (dt, J(H,H)=10 Hz, J(P,H)=1 Hz,1H; Ar), 7.04 (d,
J(H,H)=10 Hz 1H; Ar), 3.00 (d, J(P,H)=15.2 Hz, J(Pt,H)=50 Hz, 2H;
À
À
À
À
À
À
À
À
À À
À
À
Ar CH2 P), 2.78 (s, J(Pt,H)=47 Hz, 3H; N(CH3)), 2.77 (s, J(Pt,H)=
À À
À
À
À
49 Hz, 3H; N(CH3)), 2.65 (m, 2H;CH2 CH2 N), 2.27 (m, 2H;CH2
analysis calcd (%) for C25H38PtPN: C 51.90, H 6.57; found: C 51.78, H
6.48.
À
CH2 N), 1.27 (d, J(P,H)=21 Hz, 18H; P(tBu)2), À2.55 ppm (d, J(P,H)=
27 Hz, J(Pt,H)=136.5 Hz, 1H; Pt H); 13C NMR (C6D6): d=173.01 (d,
À
Reaction of [Pt(PCN)(Cl)] (2) with PhLi—formation of Li+
[Pt(PCN)(C6H5)2]À (6): A solution of PhLi in pentane (1.4m, 79 mL,
0.104 mmol) was added to a solution of complex 2 (20 mg, 0.037 mmol)
in THF (1 mL). After 3 hat room temperature a new complex was
formed according to 31P{1H} NMR spectroscopy. The solvent was evapo-
rated and the orange solid was washed with pentane and dissolved in
benzene. After evaporation of the benzene solvent under vacuum, quan-
titative formation (by NMR spectroscopy) of 6 was observed. Complex 6
can be alternatively prepared by reaction of the neutral mono-phenyl
complex 5 withone equivalent of PhLi. Complex 6 is stable for a couple
of months at À378C and for a week at room temperature. 31P {1H} NMR
([D8]THF): d=72.94 ppm (J(Pt,P)=1764 Hz); 1H NMR ([D8]THF): d=
J(P,C)=2 Hz, J(Pt,C)=653 Hz, ipso), 151.63 (d, J(P,C)=11 Hz, J(Pt,C)=
94 Hz, Ar), 142.68 (s, Ar), 124.34 (s, J(Pt,C)=20 Hz, Ar), 123.51 (d,
J(P,C)=1 Hz Ar), 121.54 (d, J(P,C)=16 Hz, J(Pt,C)=48 Hz, Ar), 63.83
À
À
À
À
À
(s, CH2 CH2 N), 63.82 (s, CH2 CH2 N), 54.95 (s, N (CH3)2), 54.93 (s,
À
N (CH3)2), 36.51 (d, J(P,C)=33 Hz, J(Pt,C)=105 Hz, P(C(CH3)3)2),
À
À
33.65 (d, J(P,C)=30 Hz, J(Pt,C)=87 Hz, Ar CH2 P), 29.61 ppm (d,
J(P,C)=3 Hz, J(Pt,C)=26 Hz, P (C(CH3)3)2); IR (film): n˜ =1863 cm
(Pt H). elemental analysis calcd (%) for C19H44PtPN: C 45.42, H 6.77;
found: C 45.29, H 6.72.
À1
À
À
Reaction of [Pt(PCN)(Cl)] (2) with Li+[Et3BH]À or reaction of
[Pt(PCN)(H)] (8) with LDA—formation of Li+[Pt(PCN)(H)2]À THF (9):
À
À
3.75 (d, J(P,H)=7 Hz, J(Pt,H)=19 Hz, 2H; Ar CH2 P), 2.39 (m, 2H;
1) A solution of super hydride Li[Et3BH] in THF (1m, 190 mL,
0.200 mmol) was added to a solution of complex 2 (20 mg, 0.037 mmol)
in THF (1 mL). The solution became brown, and after 3 h 31P{1H} NMR
spectroscopy revealed formation of complex 9. The solvent was evaporat-
ed, and the brown solid was washed with pentane and extracted with
benzene. Evaporation of the solvent gave complex 9 in quantitative yield
(by NMR spectroscopy). It was very sensitive to adventitious moisture,
converting to the neutral complex 8.
À
À
À
À
CH2 CH2 N), 2.02 (m, 2H; CH2 CH2 N), 1.95 (s, 6H; N(CH3)2),
1.08 ppm (d, J(P,H)=11 Hz, 18H; P(tBu)2); the aromatic region of the
spectrum was very crowed due to two coordinated phenyls and the aro-
matic ring of the PCN ligand; 13C {1H} NMR ([D8]THF): d=174.44 (d,
J(P,C)=120 Hz, ipso of phenyl, coordinated trans to P), 168.69 (d,
J(P,C)=21 Hz, ipso of phenyl, cis to P), 168.68 (d, J(P,C)=20 Hz, ipso of
coordinated ligand), 157.03 (s), 149.59 (s), 143.73 (s), 142.04 (s, J(Pt,C)=
25 Hz), 140.42 (s, J(Pt,C)=48 Hz), 129.63 (s), 128.99 (s), 128.02 (s),
127.66 (s), 125.61 (s, J(Pt,C)=23 Hz), 125.02 (d, J(P,C)=6 Hz), 124.93
(s), 123.95 (s),122.77 (s), 118.55 (d, J(P,C)=101 Hz) (these last 15 signals
arise from the coordinated phenyl groups and to the aromatic ring of
2)
A solution of LDA in THF/heptane/ethylbenzene (2m, 80 mL
0.162 mmol) was added to a solution of the monohydride complex 8
(80 mg, 0.162 mmol) in THF (1 mL). The resulting solution was heated
for 5 hat 85 8C and 31P {1H} NMR spectroscopy revealed formation of
complex 9 (quantitative yield by NMR spectroscopy). The concentrated
THF solution of 9 was gently evaporated, causing precipitation of color-
less crystals, suitable for a single-crystal X-ray analysis. 31P{1H} NMR
([D8]THF): d=91.69 ppm (J(Pt,P)=2295 Hz); 1H NMR ([D8]THF): d=
6.91 (d, J(H,H)=7 Hz, 1H; Ar), 6.70 (d, J(H,H)=7 Hz, 1H; Ar), 6.60
À
À
À
À
À
ligand), 64.46 (s, CH2 CH2 N), 59.64 (s,CH2 CH2 N), 46.24 (s, N
À
À
(CH3)2), 45.61 (d, J(P,C)=24 Hz, Ar CH2 P), 38.20 (d, J(P,C)=10 Hz,
P(C(CH3)3)2), 30.84 ppm (d, J(P,C)=4 Hz, P-(C(CH3)3)2); 7Li NMR
([D8]THF): d=À0.21 ppm (brs).
Reaction of [Pt(PCN)(CH3)] (3) with MeI and iodobenzene—formation
of [Pt(PCN)(I)] (7): A slight excess of iodomethane (4 mL, 0.062 mmol)
was added to a solution of 3 (15 mg, 0.029 mmol) in THF (1 mL). After
two days at room temperature 31P{1H} NMR spectroscopy revealed the
quantitative formation of a new complex. GC analysis revealed formation
of ethane. After evaporation of the THF, the resulting white solid was
washed with pentane and dissolved in benzene, which was evaporated to
give the clean complex 7. The same mode of reaction was observed when
3 was reacted witha slight excess of iodobenzene (0.091 mmol, 19 mL).
À
À
(dt, J(H,H)=7 Hz, J(P,H)=1 Hz ,1H; Ar), 3.21 (m, 2H; CH2 CH2 N),
À
À
3.04 (d, J(P,H)=2 Hz, J(Pt,H)=31 Hz, 2H; Ar CH2 P), 2.47 (m, 2H;
À
À
CH2 CH2 N), 2.12 (s, 6H; N(CH3)2), 1.20 (d, J(P,H)=12 Hz, 18H;
P(tBu)2), À3.00 (dd, J(Pt,H)=1035 Hz, J(P,H)=152 Hz, J(H,H)=8 Hz,
À
1H; Pt H), À7.07 ppm (t, J(Pt,H)=700 Hz, J(P,H)=8 Hz, J(H,H)=
8 Hz, 1H; Pt H); 13C {1H} NMR ([D8]THF): d=154.71 (d, J(P,C)=
À
14 Hz, ipso), 149.00 (s, J(Pt,C)=37 Hz, Ar), 126.32 (s, Ar), 123.18 (s,
Reaction of [Pt(PCN)(H)] (8) with MeI—formation of [Pt(PCN)(I)] (7):
A slight excess of MeI (4 mL, 0.062) was added to a solution of 8 (15 mg,
0.030 mmol) in THF (1 mL). After 30 min formation of complex 7 was
revealed by 31P{1H} NMR spectroscopy. The THF was evaporated, and 7
was washed by pentane and dissolved in benzene. After evaporation of
benzene solvent, clean complex 7 was obtained. 31P {1H} NMR (C6D6):
d=62.76 ppm (J(Pt,P)=3928 Hz); 1H NMR (C6D6): d=7.08 (dd,
J(H,H)=7 Hz, J(P,H)=2 Hz, 1H; Ar), 7.04 (d, J(H,H)=7 Hz, 1H; Ar),
6.77 (d, J(H,H)=7 Hz 1H; Ar), 2.83 (s, J(Pt,H)=34 Hz, 3H; N(CH3)),
2.82 (s, J(Pt,H)=34 Hz, 3H; N(CH3)), 2.67 (d, J(P,H)=2 Hz, J(Pt,H)=
J(Pt,C)=26 Hz, Ar), 122.29 (s, Ar), 120.22 (d, J(P,C)=16 Hz, Ar), 64.61
À
À
À
À
À
(s, CH2 CH2 N), 64.57 (s, CH2 CH2 N), 46.31 (s, N (CH3)2), 33.90 (d,
À
À
J(P,C)=22 Hz, Ar CH2 P), 30.50 (d, J(P,C)=16 Hz, P(C(CH3)3)2),
30.05 ppm (d, J(P,C)=5 Hz, P (C(CH3)3)2); 7Li NMR ([D8]THF): d=
0.53 ppm (brs); IR (film): n˜ =1610 (Pt H, cis to P), 1456 cm (Pt H,
À
À1
À
À
+
trans to P; H interacts withLi from the a neighboring molecule).
X-ray structure determination and refinement of complexes 2, 3, and 9:
The crystals were mounted in a nylon loop and flash frozen in a cold ni-
trogen stream (120 K) on a Nonius Kappa CCD diffractometer with
MoKa radiation (l=0.71071 Š). Accurate unit cell dimensions were ob-
tained from 208 of data. The data were processed with the Denzo-Scale-
pack package. The structure was solved by direct methods (SHELXS-97)
and refined by full-matrix least-squares (SHELXL-97). Idealized hydro-
gen atoms were placed and refined in the riding mode. Crystal data are
given in Table 4. CCDC-239558 (2), CCDC-239557 (3), and CCDC-
239559 (9) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via www.ccdc.cam.
ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+
44)1223-336-033; or deposit@ccdc.cam.uk).
À
À
À
À
À
30 Hz, 2H; Ar CH2 P), 2.39 (m, 2H;CH2 CH2 N), 2.01 (m, 2H;CH2
CH2 N), 1.31 ppm (d, J(P,H)=14 Hz, 18H; P(tBu)2); 13C NMR (C6D6):
À
d=149.20 (d, J(P,C)=10 Hz, ipso), 146.05 (s, Ar),143.71 (s, Ar), 142.57
(d, J(P,C)=1 Hz, Ar), 124.42 (s, Ar), 121.33 (d, J(P,C)=16 Hz, Ar), 64.53
À
À
À
À
À
(s, CH2 CH2 N), 64.50 (s, CH2 CH2 N), 52.31 (s, N (CH3)2), 52.29
À
(s,N (CH3)2), 36.74 (d, J(P,C)=32 Hz, P(C(CH3)3)2), 35.35 (d, J(P,C)=
À
À
À
26 Hz, Ar CH2 P), 30.01 ppm (d, J(P,C)=3 Hz, P (C(CH3)3)2).
Reaction of [Pt(PCN)(Cl)] (1) with Na+[Et3BH]À—formation of
[Pt(PCN)(H)] (8):
A solution of Na[Et3BH] in THF (1m, 38 mL,
0.037 mmol) was added to a solution of complex 2 (20 mg, 0.037 mmol)
in THF (1 mL). Upon addition of the hydride reagent the solution
4682
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 4673 – 4684

Pincer-Type Complexes
4673 – 4684
Table 4. Experimental crystallographic data for complexes 2, 3, and 9.
0.034 mmol) in THF (0.7 mL) in a 5 mm screw cap NMR tube witha
septum. Formation of the neutral monohydride complex 8 was immedi-
ately revealed by 31P{1H} NMR spectroscopy, which was completely con-
verted to 7 after 30 min. GC/MS analysis on 10 mL of the gas phase from
the screw cap NMR tube revealed formation of CHD3 (Mw =19 gmolÀ1).
CD3I was used instead of CH3I in order to get unambiguous evidence of
methane formation using GC/MS.
2
3
9
color, shape
formula
Fw
colorless, plates colorless, prism colorless, plate
C19H33ClPNPt
536.97
C20H36NPPt
516.56
C23H43NPPt
580.57
crystal system
space group
a [Š]
b [Š]
c [Š]
monoclinic
P21/c (no. 14)
16.431(3)
7.487(2)
17.060(3)
90
rhombohedral monoclinic
¯
R3 (no. 148)
P21/n (no. 14)
9.680(2)
12.483(3)
20.667(4)
90
24.224(3)
24.224(3)
17.900(4)
90
a [8]
b [8]
108.35(1)
90
1.791
7.259
0.1”0.05”0.0
16431
90
120
1.697
7.021
0.1”0.1”0.1
24580
5649
97.05(3)
90
1.562
5.759
0.1”0.05”0.1
24580
Acknowledgements
g [8]
1calcd [g cmÀ3
]
This work was supported by the Israel Science Foundation, by the Miner-
va Foundation, Munich(Germany) and by teh Helen and Martin
Kimmel Center for Molecular Design. D.M. is the holder of the Israel
Matz Professorial Chair of Organic Chemistry.
m (MoKa) [mmÀ1
crystal size [mm]
total reflections
]
unique reflections
2676
5890
Rint
0.136
0.067
0.057
parameters/restraints 216/0
217/0
0.038/0.050
1.611
264/0
0.0336/0.0530
2.164
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R1/wR2
0.0461/0.0541
residual density [eŠ3] 2.056
Reactions with electrophiles
Reaction of Li+[Pt(PCN)(CH3)2]À (4) with water: Upon injection of
excess D2O (6 mL, 0.333 mmol) into a solution of 4 (20 mg, 0.033 mmol)
in THF (0.7 mL) in a 5 mm screw cap NMR tube equipped witha
septum, formation of the neutral monomethyl complex 3 was immediate-
ly revealed by 31P{1H} NMR spectroscopy. GC/MS analysis of a 10 mL
sample of the gas phase from the screw cap tube revealed formation of
CH3D (Mw =17 gmolÀ1). D2O was used instead of H2O, since detection
of CH3D is easier than that of CH4 (CH4 is ambiguous in GC/MS detec-
tion, since the MS peak at 16 could also stem from O2).
Reaction of Li+[Pt(PCN)(CH3)2]À (4) with iodomethane: A slight excess
of MeI (5 mL, 0.074 mmol) was injected into a solution of 4 (20 mg,
0.033 mmol) in THF (0.7 mL) in a 5 mm screw cap NMR tube equipped
with a septum. Formation of the neutral monomethyl complex 3 was im-
mediately revealed by 31P{1H} NMR spectroscopy, while after 48 h com-
plex 3 was completely converted to 7. A sample of 10 mL of the gas phase
from the screw cap NMR tube was analyzed by GC/MS revealing forma-
tion of C2H6 (Mw =30 gmolÀ1). In order to learn about the mechanism of
ethane formation CD3I (4.5 mL, 0.074 mmol) was used under the same
conditions; this resulted in a mixture of CH3CH3, CH3CD3, and CD3CD3
in the ratio of 59:14:8 as observed by GC/MS.
Reaction of Li+[Pt(PCN)(CH3)2]À (4) with iodobenzene: Excess iodo-
benzene (41 mL, 0.372 mmol) was added to a solution of 4 (20 mg,
0.033 mmol) in THF (0.8 mL). Conversion of 4 to 3 was followed by
31P{1H} NMR spectroscopy and after 24 hno more 4 was observed. The
main product was the monomethyl complex 3, which started conversion
to the iodide complex 7. The organic product of the reaction, toluene,
was detected by GC (0.018 mmol, yield 55%), using mesitylene as an in-
ternal standard.
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3866; Angew. Chem. Int. Ed. 2001, 40, 3750; b) M. E. van der Boom,
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Reaction of Li+[Pt(PCN)(H)2]À (9) with water: Excess H2O (12 mL,
0.67 mmol) was injected into a solution of 9 (40 mg, 0.068 mmol) in THF
(0.7 mL) in a 5 mm screw cap NMR tube equipped witha septum; this
resulted in immediate formation of the neutral monohydride complex 8
as revealed by 31P{1H} NMR spectroscopy. Since, it is difficult to detect
unambiguously formation of H2 by using NMR or GC methods, trapping
of hydrogen gas was performed by using the complex [Ir(PNP)(coe)]+
À
PF6 (PNP=C5NH3(CH2P(tBu)2)2, coe=cyclooctene). This complex
reacts with hydrogen gas to yield an iridium dihydride cationic com-
plex.[28] 1 mL of the gas phase from the screw cap NMR tube was injected
into a screw cap NMR tube containing a the complex [Ir(PNP)(COE)]+
À
PF6 and after 12 h formation of the dihydride complex was revealed by
31P{1H} (singlet at 65.03 ppm) and 1H NMR spectroscopy (hydride signal
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at À23.75 ppm).
Reaction of Li+[Pt(PCN)(H)2]À (9) with iodomethane: A slight excess of
CD3I (4.5 mL, 0.074 mmol) was injected into a solution of 9 (20 mg,
Chem. Eur. J. 2004, 10, 4673 – 4684
www.chemeurj.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4683

D. Milstein, et al.
FULL PAPER
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Received: May 24, 2004
Published online: August 20, 2004
4684
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 4673 – 4684