Unusual transmetallation-induced formation of a C2-symmetric tetrapallada-macrocycle

Article abstract of DOI:10.1039/c0cc03314f

We report the formation of a tetrapallada-macrocycle induced by an unusual transmetallation, in which an anionic bidentate chelate ligand is replaced by a phenyl ligand from phenylboronic acid, leaving the chloride ligands intact.

Full text of DOI:10.1039/c0cc03314f

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Unusual transmetallation-induced formation of a C2-symmetric
tetrapallada-macrocyclew
Vincent T. Annibale, Liisa M. Lund and Datong Song*
Received 17th August 2010, Accepted 4th September 2010
DOI: 10.1039/c0cc03314f
We report the formation of a tetrapallada-macrocycle induced
by an unusual transmetallation, in which an anionic bidentate
chelate ligand is replaced by a phenyl ligand from phenylboronic
acid, leaving the chloride ligands intact.
complicated mixtures, from which a small amount of complex
5 was isolated,7 a rather surprising example of transmetallation
has been observed when phenylboronic acid is used as the
transmetallation reagent. As shown in Scheme 2, when the
mixture of 1 and B1.3 equiv. of phenylboronic acid in a wet
benzene/acetone mixed solvent is heated at 70 1C, within hours
orange X-ray diffraction quality single crystals of analytically
pure 4 can be isolated in 50% yield.z Complex 4 is slightly
soluble in dichloromethane and 1,2-dichloroethane, but
insoluble in benzene, acetone, hexanes, diethyl ether, THF,
methanol, water, chloroform and DMSO.
The formation of macrocycles and cages induced by organo-
metallic transformations are emerging, where C–H activation
has proved particularly fruitful.1,2 Transmetallation-induced
formation of macrocycles has not been exploited. Note that
there are examples of metal-exchange processes to assemble
nanostructures which have also been termed ‘‘transmetallation’’
reactions.3,4 We use the term transmetallation in the
typical organometallic context.5 In this communication
we describe how transmetallation of aryl groups from
boronic acid to palladium(II) results in the formation of a
tetrapallada-macrocycle.
As shown in Fig. 2a, the molecular structure of 4 can be
viewed as two [PhPd(m-Cl)2Pd(Ph2nacnac)] units assembled
via two Pd–C (b-carbon of [Ph2nacnac]ꢀ) bonds. Each
[Ph2nacnac]ꢀ ligand in 4 uses two N-donors to chelate to a
Pd center in one [PhPd(m-Cl)2Pd(Ph2nacnac)] unit, while the
sp3 hybridized b-carbon coordinates to a Pd center in the other
unit. The six-membered chelate ring comprised of the
In our recent investigations into palladium b-diiminate
chemistry, we have explored the reactivity of [PdCl(Ph2nacnac)L]
(where L is a monodentate ligand) towards various trans-
metallation reagents, aiming to generate olefin polymerization
pre-catalysts, [PdR(Ph2nacnac)L], where R is an alkyl or aryl
group. The precursor [[PdCl(Ph2nacnac)L] can be synthesized
by treating [Pd(Ph2nacnac)Cl]2, 1 with a monodentate ligand
{(Ph2nacnac)Pd} moiety adopts
a boat conformation.
A similar coordination mode for [Ar2nacnac]ꢀ has been
reported by Feldman and co-workers in a dinuclear tricationic
Pd(II) complex,8 and our group in an unusual trinuclear
Pd(II)–Ph2nacnac complex with amido-chloro double-
bridges.9 The Pd2–C3A bond length in 4 is 2.099(3) A, similar
to the literature values.8,9 The ipso-carbon of the phenyl ligand
and the b-carbon donor of the [Ph2nacnac]ꢀ ligand are
mutually cis and the coordination environment around each
Pd(II) center is square planar. The two methyl groups of each
[Ph2nacnac]ꢀ ligand are no longer equivalent: the methyl
group containing C5 (Fig. 2a) is situated in the shielding
region of a phenyl ligand, while the methyl group containing
C1 has no phenyl ligand nearby. Accordingly, the two singlets
L.6 Alternatively, if
could replace the chloride ligands from 1 itself, the resulting
coordinatively unsaturated organopalladium species
a suitable transmetallation reagent
[PdR(Ph2nacnac)] or its oligomers might be used directly as
activator-free olefin polymerization catalysts. Herein we
report our initial discovery while exploring the two routes
described above.
As shown in Scheme 1, the mononuclear complex
[PdCl(Ph2nacnac)Py], 2 can be prepared in 97% yield by
treating 1 with excess pyridine. Complex 2 reacts with 1 equiv.
of MeLi to afford the conventional5 transmetallation product
3 in 40% yield. The structures of 2 and 3 have been confirmed
by X-ray crystallography (Fig. 1).z Each [Ph2nacnac]ꢀ ligand
in 2 and 3 binds to a Pd(II) center through the two N-donor
atoms in a chelating fashion. Complexes 2 and 3 feature the
complete delocalization of p-electron density over the nacnac
backbones and sp2 hybridized b-carbons.
1
at 0.87 and 2.06 ppm in the H NMR spectrum of 4 can be
assigned to the two methyl groups containing C5 and C1,
respectively. As shown in Fig. 2b, complex 4 is C2-symmetric
and chiral with both phenyl ligands pointing towards the front
side of the tetrapallada-macrocycle. The front opening of the
macrocycle is larger than that at the back, i.e., Cl2–Cl2A and
Cl1–Cl1A distances are B5.4 and B4.0 A, respectively. The
Pd1–Pd1A and Pd1–Pd2A distances are B4.6 and B3.5 A,
respectively, indicating small void space inside the macrocycle.
Although the treatment of the dimeric 1 with methyllithium
or Grignard reagents consistently produces Pd black and
Davenport Chemical Research Laboratories, Department of
Chemistry, University of Toronto, 80 St. George Street, Toronto,
Ontario, Canada. E-mail: dsong@chem.utoronto.ca;
Fax: +1-416-978-7013; Tel: +1-416-978-7014
w Electronic supplementary information (ESI) available: Experimental
and spectroscopic details of 2–5. CCDC 779764, 779765, 779766, and
779767. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0cc03314f
Scheme 1 Synthesis and reactivity of 2.
c
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Chem. Commun., 2010, 46, 8261–8263 8261

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Scheme 2 Synthesis of complex 4.
in the catalytic cycle of Suzuki-Miyaura cross-coupling
reactions. The direct observation of diorganopalladium(II)
intermediates resulting from transmetallation using boronic
acids is rare given that the subsequent reductive elimination is
usually quite facile, unless the Pd–C bond is strengthened by
fluorine substitution on the C-donor ligands.10 In contrast,
complex 4, which is formed from the direct transmetallation
using a boronic acid at an elevated temperature, has remarkable
thermal stability and is inert towards air and moisture.
Furthermore, 4 contains mutually cis aryl and alkyl ligands
on Pd(II) centers, which are neither supported by carbene or
phosphine ligands nor stabilized by fluorine substitution on
the aryl or alkyl ligands. The formation of 4 is presumably
driven by the poor solubility. The fates of the displaced
[Ph2nacnac]ꢀ ligand and the [B(OH)2]+ fragment are yet to
be determined, and the formation of certain highly stable
species from these fragments might also contribute to the
driving force for the formation of 4. The ring restraint in 4
may also retard the reductive elimination process.
Fig. 1 X-Ray crystal structure of complexes 2 (left) and 3 (right).
The thermal ellipsoids are plotted at 30% probability.
In summary, we have shown two types of reactivities
between Pd-nacnac complexes and transmetallation reagents:
the conventional transmetallation between 2 and MeLi and
the unusual transmetallation between 1 and phenylboronic
acid, where the anionic chelating [Ph2nacnac]ꢀ ligand instead
of a simple chloride is replaced from a Pd(II) center, leading to
the subsequent formation of a tetrapallada-macrocycle, 4.
Current efforts are focused on expanding this methodology
to form other discrete molecular architectures (including
larger macrocycles), investigating the fate of the displaced
[Ph2nacnac]ꢀ ligand and the [B(OH)2]+ fragment, and
inducing reductive elimination of 4 to form C–C bonds.
We thank the Natural Science and Engineering Research
Council of Canada (NSERC), the Canadian Foundation for
Innovation (CFI), the Ontario Research Fund and the
University of Toronto (Connaught Foundation) for funding.
V. T. A. also gratefully thanks the Government of Ontario for
an Ontario Graduate Scholarship (OGS), and NSERC for a
Canada Graduate Scholarship (CGS).
Fig. 2 X-Ray crystal structure of complex 4. (a) Side view; (b) Top
view (down the C2 axis): the phenyl and methyl groups of nacnac
ligands are omitted and the phenyl ligands are reduced to the
ipso-carbons for clarity. The thermal ellipsoids are plotted at 30%
probability. For clarity all H atoms are omitted. Selected bond lengths
(A) and angles (1): Pd1–N1, 2.009(3); Pd1–N2, 2.012(2); Pd1–Cl1,
2.3347(8); Pd1–Cl2, 2.3371(8); Pd2–C18, 1.988(3); Pd2–C3A, 2.099(3);
Pd2–Cl2, 2.3851(8); Pd2–Cl1, 2.5101(8); N1–Pd1–N2, 85.87(10);
N2–Pd1–Cl1, 93.75(8); N1–Pd1–Cl2, 93.79(7); Cl1–Pd1–Cl2,
86.63(3); C18–Pd2–C3A, 87.41(13); C18–Pd2–Cl2, 91.39(9);
C3A–Pd2–Cl1, 99.47(9); Cl2–Pd2–Cl1, 81.75(3); Pd1–Cl1–Pd2,
85.15(2); Pd1–Cl2–Pd2, 88.00(3).
Notes and references
z Selected crystallographic data, for 2:
a
=
6.4520(5) A,
b = 10.7360(10) A, c = 29.278(4) A, V = 2028.0(4) A3, Z = 4, space
group Pc21b, T = 150 K, total data 6717, unique data 2883,
parameters 246, R1 = 0.0370 (observed data), wR2 = 0.0769 (all data);
for 3: a = 11.1137(6) A, b = 23.4596(15) A, c = 11.9041(6) A,
b = 98.157(2)1, V = 3072.3(3) A3, Z = 8, space group C2/c, T = 150 K,
total data 12476, unique data 3530, parameters 688, R1 = 0.0371
(observed data), wR2 = 0.0967 (all data); for 4: a = 21.5383(8) A,
b = 13.6270(3) A, c = 18.8184(6) A, b = 124.5226(11)1, V = 4550.6(2) A3,
Z = 4, space group C2/c, T = 150 K, total data 17143, unique data
5211, parameters 264, R1 = 0.0334 (observed data), wR2 = 0.0881
(all data).
The macrocycle can be described as being ‘crown-shaped’,
similar to other cyclic hosts such as cyclodextrins and
calixarenes. Similarly designed platforms may find use in
chiral recognition or as receptors.
The direct transmetallation of an aryl group from an
arylboronic acid to a palladium(II) centre is an important step
c
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Chem. Commun., 2010, 46, 8261–8263 8263