Transmetalation reactions with nitrogen-containing "pincer"-class ligands on platinum(II) centers

Article abstract of DOI:10.1139/v01-054

The transmetalation reaction of the aryllithium compound [Li(NCN)]₂ (NCN is the monoanionic "pincer" ligand [C₆H₃(CH₂NMe₂)₂-2,6]-) with the cyclometalated arylplatinum complex [PtCl(NCN)] a

Full text of DOI:10.1139/v01-054

709
Transmetalation reactions with nitrogen-
containing “pincer”-class ligands on platinum(II)
centers
Martin Albrecht, Stuart L. James, Nora Veldman, Anthony L. Spek, and
Gerard van Koten
Abstract: The transmetalation reaction of the aryllithium compound [Li(NCN)]2 (NCN is the monoanionic “pincer”
ligand [C6H3(CH2NMe2)2-2,6]–) with the cyclometalated arylplatinum complex [PtCl(NCN)] afforded the bisaryl plati-
num(II) complex [Pt(NCN)2] (3) containing one η3-N,C,N-terdentate and the other η1-C-monodentate-bonded pincer
ligand. Spectroscopic analyses on 3 suggest that η3 to η1 interconversion of the ligand binding mode (or vice versa) is
inhibited. Two independent X-ray structure determinations on crystals of 3 revealed the existence of a rare polymorph
containing one and three crystallographically independent molecules, respectively, in the unit cell. A similar
transmetalation reaction with lithium and platinum complexes containing heteroleptic NCNRR′ ligands (NCNRR′ is
[C6H3(CH2NRR′)-2,6]– with R = R′ = Me or R = Me, R′ = Et) pointed to the formation of a heterodinuclear cationic
bisaryl platinum lithium species as an intermediate of a preequilibrium to the final transmetalation products, involving,
rapid transcyclometalation (TCM) reactions. These TCM reactions comprise the exchange of the monoanionic NCNRR′
ligands between the platinum(II) and lithium centers. A consequence of the latter properties is that the strong Pt—N
bonds in [PtX(NCN)] complexes are considerably weakened by the presence of Li+ cations.
Key words: transmetalation, transcyclometalation (TCM), platinum, bisaryl complex, polymorphism.
Résumé : La réaction de transmétallation du composé aryllithium [Li(NCN)]2 (NCN = ligand « en pince » monoanio-
nique [C6H3(CH2NMe2)2-2,6]–) avec le complexe d’arylplatine cyclométallé [PtCl(NCN)] conduit à la formation du
complexe bisaryle de platine(II) [Pt(NCN)2] (3) qui contient un ligand η3-N,C,N-terdentate et l’autre ligand en forme de
pince lié de façon η1-C-monodentate. Les analyses spectroscopiques de 3 suggèrent qu’il y a une inhibition à
l’interconversion η3 à η1 (ou vice versa) du mode de fixation du ligand. Deux déterminations indépendantes de la struc-
ture des cristaux de 3 par diffraction des rayons X révèlent l’existence d’une polymorphie rare dans laquelle chacune
des formes comprend respectivement une et trois molécules cristallographiquement indépendantes par maille. Une réac-
tion de transmétallation semblable avec des complexes de lithium et de platine contenant les ligands hétéroleptiques
NCNRR′ (NCNRR′ = [C6H3(CH2NRR′)-2,6]– avec R = R′ = Me ou R = Me, R′ = Et) suggère qu’il y a formation d’une
espèce bisaryle de platine et de lithium hétérodinucléaire cationique comme intermédiaire d’un prééquilibre vers les
produits finaux de transmétallation et qu’il implique des réactions rapides de transcyclométallation (TCM). Ces réac-
tions de TCM incluent l’échange des ligands monoanioniques NCNRR′ entre les centres de platine(II) et de lithium. En
conséquence de ces propriétés, les fortes liaisons Pt—N présentes dans les complexes de [PtX(NCN)] sont de beaucoup
affaiblies par la présence des cations Li+.
Mots clés : transmétallation, transcyclométallation (TCM), platine, complexe bisaryle, polymorphie.
[Traduit par la Rédaction] Albrecht et al. 718
Introduction
particular, making and breaking of Caryl—M σ-bonds is of
great significance, e.g., in Caryl—CR cross coupling reactions
(Heck: R = alkene; Sonogashira: R = alkyne; Suzuki:
The reversible formation of an intermediate metal-to-
carbon bond is a crucial step in many catalytic cycles (1). In
Received October 18, 2000. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on July 9, 2001.
Dedicated to Brian James on the occasion of his 65th birthday in admiration for his great contribution to homogeneous catalysis
and our friendship.
M. Albrecht, S.L. James, and G. van Koten.1 Debye Institute, Department of Metal-Mediated Synthesis, Utrecht University,
Padualaan 8, 3584 CH Utrecht, The Netherlands.
N. Veldman2 and A.L. Spek.3 Bijvoet Center for Biomolecular Research, Department of Crystal and Structure Chemistry, Utrecht
University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
1Corresponding author (telephone: +31-30-253-3120; Fax: +31-30-252-3615; e-mail: g.vankoten@chem.uu.nl).
2Deceased.
3Corresponding author pertaining to crystallographic details (e-mail: a.l.spek@chem.uu.nl).
Can. J. Chem. 79: 709–718 (2001)
DOI: 10.1139/cjc-79-5/6-709
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Can. J. Chem. Vol. 79, 2001
Scheme 1. General representation of TCM reactions with terdentate coordinating ligands; interconversion of one metallacycle into an-
other.
Scheme 2. TCM reaction of [PtCl(NCN)] with PCHP to give
[PtCl(PCP)] and NCHN (12d).
to the analogous M—N bond is anticipated to be the driving
force. Indeed, the selective and quantitative replacement of
the amines by phosphorus donors has been demonstrated to
initiate the TCM process. In this report we present evidence
that also cyclometalated aryllithium and platinum com-
pounds can undergo TCM reactions. This leads even for η3-
coordinated NCN ligands to the formation of mixtures of
cyclometalated compounds.
Results and discussion
Initial attempts to transcyclometalate aryl ligands of the type
NC(H)NRR′ (NCNRR′ denotes the monoanionic bis(ortho)-sub-
stituted aryl ligand [C6H3(CH2NRR′)2-2,6]–) with either the
R = aryl) and in Caryl—E bond activation processes (E = car-
bon or heteroatom) (for examples of cross coupling see
ref. 2a; for examples of bond activation see ref. 2b).
Intramolecular stabilization of this C—M bond by
heteroatom coordination, i.e., formation of metallacycles,
has been demonstrated to be an efficient methodology for
stabilizing the metal-to-carbon bond. As a consequence, the
rate of (catalytic) processes is often considerably reduced,
which provides insight into mechanistic details and thus im-
portant information for efficient catalyst design (3, 4). Other
applications of transition metal complexes containing Caryl
σ–bonding chelates include the engineering and processing
of materials properties, e.g., for gas storage (5) and
sensoring purposes (6).
Up to now, cyclometalated complexes containing C—M σ-
bonds have been accessible predominantly via two different
methods: (i) transmetalation of (main group) organo-
metallics (C-Mg, C-Li, C-Sn) with a suitable metal precur-
sors (7); or (ii) direct cyclometalation via C—E bond
activation (E = e.g., C, Si, halide) (8). Recently, transcy-
clometalation (TCM) reactions have been introduced as a
new methodology for the generation of cyclometalated
transition-metal complexes. In close relation to (organic)
transesterification reactions (9), TCM reactions consist of
the substitution of one (monoanionic) cyclometalated ligand
by another one (Scheme 1). In some cases, this has the ad-
vantage of being a cleaner reaction than direct cyclo-
metalation. Previous examples, though at that time not
recognized as TCM reactions, include a variety of ligand
systems, which were predominantly activated with palladium
complexes (10). More recently, the TCM reaction of a
terdentate coordinated NCN-type pincer-ligand (11) by an-
other bis(ortho)-chelating PCP pincer-ligand has been de-
scribed to occur on a ruthenium(II) (i.e., M in Scheme 1 is
RuCl(PPh3)) and a platinum(II) (Scheme 2) metal center, re-
spectively, (12). As a primary conclusion of these studies
(12a), the higher bond strength of the M—P bond compared
neutral cyclometalated complex [PtCl(NCNMe )] (2), or its
2
cationic analogue [Pt(NCNMe ) (H2O)](BF4) (13) gave no de-
2
tectable reaction. Obviously, the Pt—N bond in [PtCl(NCN)]
cannot be displaced by an amino group of the NCHNRR′
ligand, and consequently, a TCM process is prevented since in
these reactions ligand substitution has been identified as an
initial step. Hence, half an equivalent of the dimeric aryl-
lithium species [Li(NCNMe )]2 (1-Li) (14) was reacted with
2
the arylplatinum(II) complex 2 in polar solvents and at ele-
vated temperatures (THF, reflux temperature). This afforded
1
the bisaryl platinum(II) complex [Pt(η3–NCNMe )(η –C–
2
NCNMe )] (3) in 60% yield (Scheme 3). This complex con-
2
tains two differently bonded pincer ligands, one monodentate
η1-C and the other terdentate η3-N,C,N coordinating. This
compound may be envisaged as an intermediate in the full
substitution of one NCN ligand by another in the TCM reac-
tion, cf. the isolation of [Pt(η3–P,C,P–PCP)(η1–C–NCN)] in
the TCM reaction of 1 with PCHP (Scheme 2). This is further
underlined by the reactivity of 3 towards aq HCl, which re-
sults in rapid Pt—C bond cleavage of the η1-coordinated
ligand and formation of the starting materials 2 and 1 as the
diprotonated chloride salt (Scheme 3). Remarkably, these con-
ditions affect neither the Pt—N nor the Pt—C bonds of the
η3-N,C,N chelating ligand, and are a good indication for the
excellent stability of the [Pt(NCN)] unit. This is also in concert
with the fact that 2 does react with the bis-phosphinoarene
PCHP but not with the bis-aminoarene NCHN.
Unambiguous confirmation of the connectivity pattern in
3 was accomplished by detailed X-ray structure analyses of
single crystals, which were grown by slow evaporation of a
saturated Et2O solution.
Solid-state structure of 3
The molecular structure of 3, measured at room tempera-
ture, is shown in Fig. 1 and reveals a platinum(II) center that
is ligated by a terdentate, η3-N,C,N-bound, and a mono-
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Albrecht et al.
711
Scheme 3. Synthesis of the homoleptic bisaryl platinum(II) complex 3 and its reactivity towards MeI.
Fig. 1. Perspective view of the molecular structure of 3 (50%
probability level, crystal I).
Two different structural motifs have been identified, for
the orientation of the CH2NMe2 groups with respect to the
arylplatinum unit. The first comprises short interatomic C-
H···N contacts between the aryl protons H(201) and H(203)
and the nitrogen atoms of the η1-coordinated NCN ligand
(Caryl-H···N 2.627 and 2.679 Å, respectively; see Fig. 2 and
Table 2). The second motif has the benzylic protons H(205)
and H(212) of the η1-coordinating pincer ligand positioned
near the virtual z axis of the platinum center (Pt···H 2.803
and 2.704 Å, respectively). The filled dz2 orbitals in such d8
metal centers, particularly in bisaryl platinum(II) complexes,
have been established to be important frontier orbitals. As a
consequence, the observed C-H···Pt arrangement provides
additional stabilization to this particular CH2NMe2 con-
former (12a, 16).
N(202)
N(101)
C(206)
C(102)
C(201)
Pt
C(101)
C(202)
C(106)
Surprisingly, a second crystal structure determination,
which was performed at 150 K, revealed a polymorph (17).
While the overall connectivity was the same as established
for the first X-ray analysis, distinctly different intra-
molecular orientations for the CH2NMe2 substituents were
identified for the crystal structures. Representative sections
of the two polymorphous crystal structures (I and II) are
shown in Fig. 3. Form I (data collected at room temperature,
Fig. 3a) contains one molecule per asymmetric unit, in
which the two free amine arms are both positioned on the
same side of the aryl ring to which they are connected, i.e.,
they have a cisoid arrangement. The asymmetric unit in the
polymorph II (150 K), however, contains three independent
molecules, two of which have the cisoid conformation as ob-
served in I, while the third (molecule 3, Fig. 3b) has the free
amine groups positioned on opposite sides of the aryl ring
(transoid). It is noteworthy that no phase transition was ob-
served on cooling of crystal I to 150 K.
N(102)
N(201)
dentate, η1-C-bound, NCN pincer ligand, resulting in a trans
positioning of the two aryl groups across the metal square
plane. The puckered five-membered chelate ring conforma-
tions, bond lengths, and bond angles of the [Pt(η3-NCNMe )]
2
moiety are normal for square planar platinum(II) complexes
containing this ligand (Table 1) (13, 15). As a consequence
of the different denticity of the ligands, the metal-to-carbon
σ-bond of platinum to the monodentate bonded NCN ligand
is considerably longer (Pt—C(201) 2.126(5) Å) than the one
to the terdentate coordinating NCN ligand (Pt—C(101)
1.963(6) Å). Moreover, the aromatic ring of the η1-C-
bonded ligand system is considerably distorted (the internal
angle at Cipso is 114.3(5)° for the η1-C-NCN, and 121.4(5)°
for the η3-N,C,N-bonded ligand), which is presumably a
consequence of the steric crowding around the platinum nu-
cleus. The noncoordinating tertiary amines are directed away
from the metal center.
Structural aspects of 3 in solution
As a result of the ligand arrangement at the platinum center
(viz. two metal-bound aryl units and two noncoordinating
tert-amine moieties), complex 3 displays good solubility in
apolar solvents (e.g., in Et2O and pentane) which is unusual
1
for related platinum-halide species such as 2 (13). In the H
NMR spectrum (C6D6 solution), two distinct signals for the
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Can. J. Chem. Vol. 79, 2001
Table 1. Interatomic bond lengths (Å) and angles (deg) of 3.
Crystal II
Crystal I
Molecule 1
Molecule 2
Molecule 3
Bond lengths
Pt—C(101)
Pt—C(201)
Pt—N(101)
Pt—N(102)
C(101)—C(102)
C(101)—C(106)
C(201)—C(202)
C(201)—C(206)
Bond angles
1.963(6)
2.126(5)
2.104(5)
2.107(5)
1.385(8)
1.388(9)
1.419(8)
1.424(8)
1.92(2)
1.95(2)
2.15(2)
2.117(14)
2.104(16)
1.40(3)
1.38(3)
1.41(3)
1.41(3)
2.00(2)
2.12(2)
2.075(18)
2.090(17)
1.34(3)
1.39(3)
1.40(2)
1.40(3)
2.131(19)
2.096(14)
2.102(16)
1.42(3)
1.41(3)
1.41(3)
1.39(3)
C(101)-Pt-C(201)
C(101)-Pt-N(101)
C(101)-Pt-N(102)
C(201)-Pt-N(101)
C(201)-Pt-N(102)
N(101)-Pt-N(102)
C(102)-C(101)-C(106)
C(202)-C(201)-C(206)
174.1(2)
80.9(2)
80.1(2)
177.7(8)
79.4(7)
80.8(7)
102.4(7)
97.3(6)
160.3(6)
123.5(19)
116.5(18)
177.6(7)
80.9(7)
80.2(7)
101.4(7)
97.5(7)
161.1(6)
119.3(19)
116.4(18)
176.8(8)
78.4(7)
81.1(8)
101.0(7)
99.4(7)
159.5(7)
125(2)
97.8(2)
101.3(2)
160.9(2)
121.4(5)
114.3(5)
113.8(17)
Fig. 2. Short intramolecular contacts of the noncoordinating
ortho-substituents from the η1-coordinating pincer ligand.
Table 2. Short intramolecular distances (Å) and angles (deg) in 3.
Donor–acceptor system
D-H···A
Distances (Å)
Angles (deg)
D-H···A
H···A
D···A
C(207)-H(205)···Pt
C(210)-H(212)···Pt
C(203)-H(201)···N(201)
C(205)-H(203)···N(202)
2.803
2.704
2.627
2.679
3.391(6)
3.324(6)
2.914(9)
2.960(9)
119.8
122.2
98.5
N(202)
C(210)
H(203)
98.3
H(212)
N(101)
were assigned to the ArCH2N protons of the η1-C and the η3-
N,C,N-coordinating pincer systems, respectively, (12a, 13).
The low field shift of the noncoordinating CH2N protons is
remarkable and points to additional deshielding of these nu-
clei. Such effects may originate from weak intramolecular
interactions (e.g., hydrogen bonding) with the nucleophilic
platinum(II) center (Fig. 2) (12a, 18). The properties of the
platinum(II) center as hydrogen bond acceptor have recently
been applied for the fabrication of organometallic proton
sponges (12a, 19). Further confirmation of the structure of 3
was obtained from 13C NMR spectroscopy. In particular, the
two signals at 172.6 and 182.2 ppm are characteristic for
Pt(NCN) type complexes containing an aryl anion in mutual
trans-position to the metal-bound pincer-carbon NCN (13).
As expected, two sets of benzylic carbons (at 68.4 and
80.8 ppm) and nitrogen-bound methyl carbons (at 45.9 and
54.7 ppm) were observed, which belong to the monodentate
coordinating and to the chelating ligand, respectively. The
symmetric structure in solution, indicated by 1H and 13C
NMR spectra, is consistent with a purely transoid arrange-
ment of the free arms, or fast exchange between transoid and
cisoid forms on the NMR time scale.
C(205)
Pt
C(201)
C(203)
N(102)
H(201)
H(205)
C(207)
N(201)
NMe2 groups are observed, which are located at 2.41 and
2.58 ppm, respectively. The resonance at 2.41 ppm is typical
for noncoordinating amine methyl groups, but is shifted to
higher frequency when compared to free ligand (δH (NMe2) =
2.21 ppm in 1). In contrast, the signal at 2.58 ppm displays a
coupling pattern (3JHPt = 42 Hz) which is diagnostic for nitro-
gen coordination to the metal center, even though these nuclei
seem to be considerably shielded when compared to related
groups, in 2 and analogues where the chloride in 2 is replaced
by another halide or a para-tolyl ligand, cf. δH (NMe2) =
2.95–3.18 (13a). Similar features are also observed for the
resonances at 4.38 and 3.57 ppm, which, on the basis of either
the absence or presence of 195Pt coupling (3JHPt = 47.0 Hz),
Variable temperature NMR experiments were carried out
to probe in particular whether dynamic processes could be
feasible involving a fluxional switch between η3- and η1-
bonding of each of the pincer ligands. At low temperature
(–80°C, CD2Cl2) the signals due to the free amine arms
broaden. Although this may be due to the onset of freezing
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Albrecht et al.
713
Scheme 4. Selective acid-mediated cleavage of the unsupported
Fig. 3. Polymorphism in crystalline 3: (a) crystal structure of
crystal I, which is characterized by one single independent mole-
cule of 3. Note that the noncoordinating nitrogens are in cisoid
configuration with respect to the bisecting aryl plane of the η1-
bound ligand; (b) the second polymorph, crystal II, contains
three independent molecules of 3 in the unit cell. Molecule 1
and 2 display a similar cisoid confirmation as identified for crys-
tal I, whereas molecule 3 contains the non-coordinating amines
in a transoid position.
Pt—C bond.
(a)
ammonium structure as 4. The accumulation of positive
charges and (or) steric crowding in close proximity to the
metal center apparently prevents the reaction of 4 with an-
other equivalent of MeI. The methyl groups of the terdentate
pincer ligand in 4 are chemically inequivalent (viz. two sin-
molecule 1
molecule 2
molecule 3
(b)
1
glet resonances with 195Pt couplings in the H NMR spec-
trum at δH = 2.69 and 2.74, respectively). Also, the benzylic
protons of the η3-bonded ligand in 4 became diastereotopic
as shown by the AB-type pattern with δA at 3.83 ppm and δB
at 5.03 ppm, respectively. These observations suggest a re-
duced symmetry in 4, which may be due to a frozen wag-
ging of the methylene CH2 groups of the five-membered
metallacycles. In solution, puckering effects are usually fast
and cannot be slowed down significantly by cooling (22).
However, the steric constraints of the bulky ammonium
groups on the η1-bonded ligand may reduce the fluxionality
of the metallacycles considerably. This is corroborated by
the anticipated relative position of the aryl planes of the η1-
bonded and η3-bonded pincer ligands, which are most likely
significantly tilted from an ideal perpendicular orientation,
cf. the solid-state structure of related bis-ammonium com-
plexes (12a).
out of a cisoid–transoid equilibrium, temperatures low
enough to resolve the interconverting species could not be
attained. Fluxional processes involving coordination of the
free amine groups with associated decoordination of the
bonded amine groups could also occur at higher tempera-
1
tures. However, neither H NMR spectra in toluene-d8 solu-
tion at 110°C nor spin saturation transfer experiments
revealed any evidence for a dynamic behavior of 3 on the
NMR time scale. These results show that even intramolecu-
lar substitution of coordinated amines by free amine sites is
not occurring (vide infra).
Bisaryl platinum(II) complexes containing heteroleptic
pincer ligands
The presence of noncoordinating tertiary amines in 3 was
further established by derivatization with MeI, which gave
It must be noted that during the formation of 3, a TCM
type reaction could be operative, which is undetectable as it
is degenerative in the products. To distinguish a trans-
metalation reaction, i.e., Li–Pt exchange on NCN, from an
eventually competitive TCM type process, a chemical modi-
fication in one ligand framework was introduced. Ideally,
such a modification should allow to distinguish between the
different amine donors but not cause significantly different
coordination properties. Therefore, the prochiral pincer
ligand precursor NCHNEtMe (5) containing one methyl and
one ethyl group bound to each nitrogen donor, was used as a
marker.4 Lithiation and subsequent transmetalation of this
ligand readily afforded the corresponding platinum complex
[PtCl(NCNEtMe)] (6) (23).
the bis-ammonium salt [Pt(η3-NCNMe )(η -NCN )](I)2 (4)
(Scheme 3). Interestingly, no arenium species was formed,
as might be expected on basis of previous results related to
the reaction of NCN platinum(II) aryl complexes with MeI
(20). Two different explanations may rationalize these obser-
vations: Either the bulk of the noncoordinating CH2NMe2
groups of the η1-monodentate bound pincer ligand efficiently
reduce the accessibility of the platinum center or alterna-
tively, oxidative addition of MeI may take place to form an
intermediate platinum(IV) species (4a, 21), followed by irre-
versible trapping of the platinum-bound methyl group by the
noncoordinated amine groups, and formation of a cationic
1
Me3
2
4 It has been shown previously that replacement of all methyl groups by ethyl substituents severely disturbes nitrogen coordination and re-
duces the accessibility of the metal center rigorously. As a consequence, the platinum center in complexes such as [PtX(NCNEt )] is not
2
available anymore for, e.g., SO2 (23) or I2 (4b) coordination. Moreover, X-ray structure analyses revealed that the puckering of the
Me 2
metallacycle in, e.g., [PtI(NCNEt )] is considerably smaller than in the corresponding NCN
complexes (23).
2
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Can. J. Chem. Vol. 79, 2001
Scheme 5. Heteroleptic bisaryl platinum(II) complex formation by using modified pincer ligands containing ethyl-labeled amino groups.
lithium species or in the cyclometalated starting material.
The high field region of the H NMR spectra is diagnostic:
Fig. 4. Snapshot of the transmetalation reaction between tanta-
lum and zinc on a pincer ligand. Note the structural analogy of
this complex with intermediate A as proposed for the
transcyclometalation between platinum and lithium (see
Scheme 6).
1
the signals at δH = 2.69 and 2.10 are indicative for platinum-
bound and noncoordinated NMe2 groups, respectively.
Furthermore, two sets of resonances were observed for the
nitrogen-bound ethyl substitutents. The set at lower field
(δH = 2.64 (q) and 1.16 (t)) is located at frequencies similar
to those observed for nonmetalated 5, whereas the set at
higher field (δH = 2.33 (m) and 0.99 (t)) showed the ex-
pected pattern for nitrogen coordination to platinum (24),
with characteristic coupling constants of the methylene
NCH2 protons to the platinum center (3JHPt = 78 Hz). A
product ratio between 7 and 8 of 3:5 was estimated based on
the signals due to the NCH2CH3 methyl groups. Interest-
ingly, this ratio is nearly constant and independent of the
chosen starting materials, i.e., the cycloplatinated starting
materials 2 or 6. This implies an equilibrium situation which
precedes the ultimate formation of the transmetalation prod-
ucts.
N2
Cl
Zn
C
Cl
Product analyses using variable temperature 1H NMR lead
to similar results as obtained for 3. In the measured tempera-
ture range (–80 to +100°C, toluene-d8 solution), no signifi-
cant line-broadening was observed. More importantly, the
product ratio between 7 and 8 remained constant, implying a
high kinetic barrier for the transition of 7 to 8 and vice versa
(Scheme 4). These results exclude a potential equilibrium
situation after the transmetalation sequence.
When the complexes 3, or 7 and 8 were treated with an
excess of aq HCl, rapid fission of the unsupported Pt—C
bond was observed, thus giving the cyclometalated platinum
chloride complexes 2 and 6, respectively, along with the
protonated pincer ligands 1·2HCl and 5·2HCl (Scheme 4).
Since the non-chelating Pt—C bond is cleaved selectively
(the Pt—C bond in complex 2 was stable towards HCl at
60°C for several hours), quantification of the product ratio
between 2 and 6 provides an a posteriori method for the de-
termination of the ratio between 7 and 8.
N1
Ta
Cl
Reaction of 6 with 1-Li is anticipated to give selectively
the bisaryl complex 8 containing the NEtMe groups coordi-
nated to the platinum center, provided the reaction is a
straightforward transmetalation reaction involving substitu-
tion of the Li+ cation in 1-Li by [Pt(NCNEtMe)]+ (Scheme 5).
Analogously, the product of a transmetalation of 5-Li with 2
would afford complex 7 as the exclusive product. On the
contrary, an eventually operative TCM reaction would be in-
dicated in these cross-experiments by the concomitant
presence of both complexes 7 and 8, since this process in-
volves (repetitive) formation and cleavage of metallacycles
with reformation of the corresponding [Li(NCNRR′)] com-
pounds.
Analysis of the product mixtures obtained from the cross-
experiments (performed under experimental conditions that
were analogous to those applied for the formation of 3)
pointed to the presence of both 7 and 8. The NMR spectra of
the product mixtures of both reactions were complicated
(24). However, they were remarkably consistent, irrespective
of whether the ethyl-labeled nitrogen donors were in the
Mechanistic consequences for TCM reactions with
[Li(NCNRR′)] complexes
Related transmetalation studies have emphasized the rele-
vance of bimetallic intermediates comprising an aryl
carbanion bridging to two (different) metal centers through
3c—2e or 3c—4e bonding (Fig. 4) (25). A similar
© 2001 NRC Canada

Albrecht et al.
715
Scheme 6. Rapid and reversible TCM process in a preequilibrium of the transmetalation reaction. Note that structure A is the common
intermediate of both the TCM and the transmetalation process.
Table 3. Crystallographic data for both polymorphs of complex 3.
Crystal I
Crystal II
Empirical formula
Formula weight
T (K)
C24H38N4Pt
577.67
298
C24H38N4Pt
577.67
150
Crystal system
Space group
Crystal size (mm)
Crystal color
Unit cell dimensions
a (Å)
b (Å)
c (Å)
β (deg)
V (Å3)
Monoclinic
P2l/c (no. 14)
0.50 × 0.50 × 0.50
Colorless
Monoclinic
P2l/c (no. 14)
0.38 × 0.38 × 0.15
Colorless
10.3234(7)
19.1763(12)
12.3632(8)
93.271(5)
2443.5(3)
4
16.954(2)
27.422(2)
18.023(2)
121.271(10)
7161.8(15)
12
Z
Dcalc (g cm–3)
µ (mm–1) (Mo Kα)
Abs. correction
Transmission range
(sin θ/λ) Max (Å–1)
Reflections collected, unique
Rint
1.5703(2)
5.76
PLATON (DELABS)
0.386–1.000
0.65
1.6073(3)
5.89
PLATON (DELABS)
0.585–1.000
0.65
8598, 5592
0.0301
14374, 13561
0.0581
Parameters, restraints
R1,a wR2,b S
w–1 c
Resid. density (e Å–3)
271, 0
538, 0
0.0372, 0.0811, 1.027
0.0680, 0.1363, 1.017
2
2
σ2(Fo ) + (0.0250P)2 + 5.13P
σ2(Fo ) + (0.0170P)2 + 100.31P
–0.79 < 1.18
–1.99 < 2.13
aR1 = Σ||Fo| – |Fc||/Σ|Fo|, for all I > 4σ(I).
2
2
bwR2 = [Σ[w(Fo – Fc2)2]/Σ[w(Fo )2]]1/2
.
cP = (max(Fo , 0) + 2Fc2)/3.
2
intermediate (A) is likely to be involved in the
transmetalation of 1-Li or 5-Li by the cyclometalated plati-
num(II) complexes 6 and 2, respectively, (Scheme 6). Due
to the low stability of such complexes, it is expected that in-
termediate A equilibriates rapidly with the starting materials.
Notably, it is not possible to deduce the precursors, i.e., 2
and 5-Li or 6 and 1-Li, from complex A and formation of
either of these two sets of starting materials occurs on
nucleophilic attack of Cl– on the platinum center. This
interconversion of 2 and 5-Li to 6 and 1-Li and vice versa
corresponds to a transcyclometalation (TCM) reaction be-
tween the platinum and lithium centers. Alternative attack of
the Cl– nucleophile on the lithium center in intermediate A
induces the formation of LiCl and the respective bisaryl
platinum(II) product, which is probably an irreversible reac-
tion sequence. Apparently, the equilibrium situation of the
TCM reaction with complex A as an intermediate must be
reached rapidly. This is an interesting further reaction trajec-
tory for a TCM process and contrasts to earlier established
reactions where one starting material was metal-free. The
relatively low selectivity of this TCM reaction (Scheme 6)
in terms of product distribution (nearly equal yields of 7 and
8) is most likely a direct consequence of the only small dif-
ferences in the bond strength of the platinum center to either
NMe or NEtMe type amines. Similar arguments account for
2
the Li—NMe and Li—NEtMe bond strengths. According to
2
© 2001 NRC Canada

716
Can. J. Chem. Vol. 79, 2001
1
spectroscopic analyses (NMR signal integration), there is a
slight preference for initial Li—NEtMe bond cleavage rather
H2O–acetone. H NMR (D2O, 200 MHz) δ: 7.41 (d, 4H,
3JHH = 7.3, ArH), 7.27 (t, 2H, 3JHH = 7.3, ArH), 5.04 (d, 2H,
3
than Li—NMe fission, since the major product of the trans-
3JHH = 13.2 Hz, JHPt not observed, CHHNMe2), 4.84 (d,
2H, JHH = 11.6, CHHNMe3), 4.52 (d, 2H, JHH = 11.6,
CHHNMe3), 3.83 (d, 2H, JHH = 13.4, JHPt not observed,
2
3
3
metalation is complex 8 having metallacycles containing
NEtMe type amines. The low product selectivity contrasts
with previously reported TCM processes, which involved
amine and phosphines donors on the pincer ligands
(Scheme 2). In those systems, exclusive formation of
[PtCl(PCP)] and NCHN resulted from a more pronounced
donor-metal bond strength difference, since the Pt—P bond
is significantly more stable than the corresponding M—N
bond.
3
3
3
CHHNMe2), 3.24 (s, 18H, NMe3), 2.75 (s, 6H, JHPt = ca.
40, NCH3Me), 2.73 (s, 6H, JHPt = 36.2, NMeCH3). 13C
3
NMR (D2O) δ: 175.8 (Cipso), 149.2, 132.8, 130.5, 123.6,
123.5 (all ArC), 78.9 (CH2NPt), 74.9 (CH2N), 56.1
(PtNCH3Me), 52.9 (NMe3), 51.7 (PtNMeCH3). Anal. calcd.
for C26H44I2N4Pt·H2O (879.57): C 35.50, H 5.27, N 6.37;
found: C 35.50, H 5.42, N 6.21.
[Pt(η3-NCNMe2)(η1-NCNEtMe)] (7) and [Pt(η3-
NCNEtMe)(η1-NCNMe2)] (8)
Experimental
These complexes have been prepared following a similar
protocol as for 3. The products were obtained as mixtures,
which precipitated as off-white powders from pentane–Et2O
systems. All attempts to separate the products were unsuc-
cessful.
Reactions involving organolithium derivatives were car-
ried out using standard Schlenk techniques under an inert at-
mosphere of dry, oxygen-free nitrogen. Hexane, Et2O, and
THF were distilled from Na–benzophenone, CH2Cl2 from
1
CaH2 prior to use. All H and 13C NMR spectra were re-
corded on a Bruker AC300 or a Varian Inova 300 spectrome-
ter, operating at 300 and 75 MHz, respectively. Spectra were
obtained at 25°C, unless stated otherwise, and are referenced
to external TMS (δ = 0.00 ppm, J in Hz). Elemental analyses
were obtained from Kolbe, Mikroanalytisches Laboratorium
(Mülheim, Germany).
Method A
From 1 (0.17 g, 0.89 mmol) in pentane (4 mL), n-BuLi
(1.5 M in hexane, 0.60 mL, 0.9 mmol), and 6 (0.37 g,
0.82 mmol). Yield: 0.22 g (44%).
Method B
The ligand precursors NC(H)NMe (1), NC(H)NEtMe (5),
2
From 5 (0.26 g, 1.2 mmol) in pentane (6 mL), n-BuLi
(1.5 M in hexane, 0.8 mL, 1.2 mmol), and 2 (0.46 g,
1.1 mmol). Yield: 0.35 mg (53%).
and the platinum complexes [PtCl(NCNMe )] (2) and
2
[PtCl(NCNEtMe)] (6), were prepared according to described
1H NMR (C6D6) δ: 7.83 (d, JHH = 7.4, ArH), 7.55 (s,
3
procedures (13, 15, 23).
ArH), 7.43 (t, 3JHH = 7.4, ArH), 7.31–7.19 (m, ArH), 7.04 (t,
3JHH = 7.5, ArH), 6.93 (d, JHH = 7.4, ArH), 6.64 (d, JHH
=
3
3
[Pt(η3-NCNMe2)(η1-NCNMe2)] (3)
3
7.5, ArH), 4.47 (s, CH2N (8)), 3.57 (s, JHPt = 43.7, CH2NPt
To a pentane solution of 1 (0.25 g, 1.3 mmol in 4 mL)
was added n-BuLi (1.5 M hexane solution, 0.87 mL,
1.3 mmol) at –80°C. The reaction mixture was allowed to
warm to room temperature over 14 h. All volatiles were then
removed in vacuo and the residue was redissolved in THF.
To this solution was added the platinum complex 2 (0.50 g,
1.2 mmol) and the mixture was heated to reflux temperature
for 4 h. After cooling to room temperature, the solvents were
removed and the residue was extracted with Et2O (3 ×
20 mL). The combined ether layers were concentrated to
5 mL and pentane was overlayered, which caused slow crys-
3
(7)), 3.40 (s, CH2N (7)), 3.29 (s, JHPt = 30.3, CH2NPt (8)),
3
3
2.69 (s, JHPt = 38.0, PtNCH3 (8)), 2.64 (q, JHH = 7.1,
3
NCH2CH3 (7)), 2.58 (s, JHPt = 43.6, PtNCH3 (7)), 2.41 (s,
NCH3 (8)), 2.33 (q, JHH = 7.1, JHPt = 78.0, PtNCH2CH3
(8)), 2.10 (s, NCH3 (7)), 1.16 (t, JHH = 7.1, 2.25H,
3
3
3
3
NCH2CH3 (7)), 0.99 (t, 3.75H, JHH = 7.1, PtNCH2CH3 (8)).
13C NMR (C6D6) δ: 181.6 (Cipso), 172.7 (Cipso), 147.5, 145.1,
143.8, 140.2, 130.0, 127.4, 126.9, 122.9, 122.6, 122.5,
119.2, 119.0 (all ArC), 80.9 (2JCPt = 28.0, CH2NPt (8)), 77.7
(2JCPt = 65.0, CH2NPt (7)), 66.8 (CH2N (7)), 62.6 (CH2N
(8)), 54.8 (2JCPt not resolved, PtNCH3 (8)), 54.0 (2JCPt
=
1
tallization of the product. Yield: 0.41 g (60%). H NMR
3
3
15.3, PtNCH3 (7)), 52.0 (NCH2CH3 (7)), 51.6 (PtNCH2CH3
(8)), 42.0 (NCH3 (7)), 41.6 (NCH3 (8)), 13.5 (NCH2CH3
(7)), 12.8 (PtNCH2CH3 (8)).
(C6D6) δ: 7.70 (d, 2H, JHH = 7.0, ArH), 7.40 (t, 1H, JHH
=
=
7.0, ArH), 7.17 (t, 1H, 3JHH = 8.0, ArH), 6.93 (d, 2H, 3JHH
8.0, ArH), 4.38 (s, 4H, CH2N), 3.57 (s, JHPt = 47, 4H,
3
3
CH2NPt), 2.58 (s, JHPt = 42, 12H, PtNCH3), 2.41 (s, 12H,
NCH3). 13C NMR (C6D6) δ: 182.2 (Cipso, JHPt not resolved),
1
Reaction with HCl
1
172.6 (Cipso, JHPt not resolved), 147.4, 145.1, 127.4, 122.7,
A solution of the bisarylplatinum complex 3, or 7 and 8 in
CH2Cl2 was saturated with freshly prepared anhyd HCl.
Stirring was maintained for 1 h and all volatiles removed in
vacuo. Identification of the products (1H NMR) was
performed by comparison with authentic samples of 2 and 6,
respectively.
122.3, 119.0 (all ArC), 80.8 (CH2NPt), 68.4 (CH2N), 54.7
(PtNCH3), 45.9 (NCH3). Anal. calcd. for C24H38N4Pt (587.66):
C 49.90, H 6.63, N 9.70; found: C 50.07, H 6.90, N 9.59.
[Pt(η3-NCNMe3)(η1-NCNMe3)](I)2 (4)
To a stirred solution of 3 (0.50 g, 0.87 mmol) in CH2Cl2
(5 mL) was added a solution of MeI (0.28 g, 2.0 mmol) in
CH2Cl2 (2 mL) and the mixture stirred for 3 h. The volatiles
were removed to leave 4 as an off-white solid (0.72 g, 97%).
Analytically pure 4 was obtained by recrystallization from
Structure determination and refinement of 3
Intensities were measured on an Enraf–Nonius CAD4-T
diffractometer with rotating anode (Mo Kα, λ = 0.71073 Å).
Crystal data and details on data collection and refinement
© 2001 NRC Canada

Albrecht et al.
717
are collected in Table 3.5 The structures were solved with
Patterson methods (DIRDIF-92 (26)) and refined against F2
of all reflections (SHELXL-93 (27)). Non-hydrogen atoms
were refined freely with anisotropic displacement parame-
ters, hydrogen atoms were introduced at calculated positions
and refined riding on their carrier atoms. Weights were opti-
mized in the final refinement cycles. Neutral atomic scatter-
ing factors and anomalous dispersion corrections were taken
from the International Tables of Crystallography. All calcu-
lations, graphical illustrations, and checking for higher sym-
metry were performed with the PLATON package (28).
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Conclusions
The use of labeled NCN pincer ligands suggested that the
transmetalation of [Li(NCN)] with cyclometalated
[PtCl(NCN)] precursors is preceded by a comparatively fast
and reversible TCM process between lithiated and platinated
metallacycles. Such a TCM reaction does not involve C—H
bond activation and therefore follows a distinctly different
reaction coordinate than previously analyzed TCM pro-
cesses. Importantly, in this case the product distribution is
most likely determined by steric preferences rather than
metal-to-heteroatom bond strength arguments as established
previously. A cationic heterodinuclear bisaryl platinum lith-
ium species has been suggested as a key intermediate. At-
tack of chloride on the platinum center of this intermediate
induces a TCM reaction, whereas attack on lithium com-
pletes the transmetalation trajectory with formation of the
corresponding bisaryl platinum(II) complex and LiCl. Inves-
tigations towards the stabilization and characterization of the
proposed intermediate of this novel TCM process are cur-
rently in progress.
11. G. van Koten. Pure Appl. Chem. 61, 1681 (1989).
12. (a) M. Albrecht, P. Dani, M. Lutz, A.L. Spek, and G. van
Koten. J. Am. Chem. Soc. 122, 11 822 (2000); (b) P. Dani, M.
Albrecht, G.P.M. van Klink, and G. van Koten. Organo-
metallics, 19, 4468 (2000).
Acknowledgements
This work was supported in part (N.V., A.L.S.) by the
Dutch Council for Chemical Sciences (CW-NWO).
13. (a) J. Terheijden, G. van Koten, F. Muller, D.M. Grove, K.
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(b) J.T.B.H. Jastrzebski, G. van Koten, M. Konijn, and C.H.
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tawa, ON K1A 0S2, Canada (http://www.nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electronically). Crystallographic data
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© 2001 NRC Canada