Pd-catalyzed diarylation of aniline-a way to a non-linear bis(terpyridyl) ligand providing increased electronic communication

Article abstract of DOI:10.1039/b804461a

An amine-linked bis(terpyridyl) ligand, prepared via Pd-catalyzed diarylation of aniline, mediates unusually strong metal-metal interaction in its Ru₂ polypyridyl complex.

Full text of DOI:10.1039/b804461a

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Pd-catalyzed diarylation of aniline—a way to a non-linear bis(terpyridyl)
ligand providing increased electronic communication†
Olof Johansson,*a Lars Erikssonb and Reiner Lomoth*a
Received 19th March 2008, Accepted 1st May 2008
First published as an Advance Article on the web 23rd May 2008
DOI: 10.1039/b804461a
An amine-linked bis(terpyridyl) ligand, prepared via Pd-
catalyzed diarylation of aniline, mediates unusually strong
metal–metal interaction in its Ru2 polypyridyl complex.
Bridging bis-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridyl (tpy) ligands continue to be
of wide interest in many research fields. In particular the
ruthenium(II) complexes of “back-to-back” ligands have been
thoroughly investigated as light- and redox-active terminals in
molecular electronic devices and energy conversion schemes.1 In
view of such applications, the degree of electronic communication2
between two metal centers mediated by the ligands is of crucial
importance. Non-linear bis-tpy ligands on the other hand have
often been used in the synthesis of metallodendrimers3 and self-
assembly of metallomacrocycles.4 However, even the simplest non-
linear bis-tpy ligand, bis(4ꢀ-(2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridyl))ether, where an
oxygen atom bridges two tpy units provides only poor electronic
communication between two Ru polypyridyl units.5 Here we
present an alternative non-linear amine-linked bis-tpy ligand
prepared via Pd-catalyzed diarylation of aniline, and show that
this bridging motif provides strong metal–metal interaction in a
dinuclear Ru polypyridyl complex.
Scheme 1
In the 1H NMR of L (CDCl3), a characteristic singlet that
integrates to four protons was observed at 8.20 ppm (3ꢀ,5ꢀ-tpyH)
and mass peaks (ESI-MS) were detected at m/z 556.7 (M + H)+
and m/z 1133.2 (2M + Na)+ which confirmed the structure of L.
Single crystals for X-ray diffraction analysis were obtained by
recrystallization from CH3CN–toluene.‡ The structure (Fig. 1)
shows a 123.60◦ angle between the two terpyridyl units, and
exhibits a dihedral angle of 40.5◦ between the terpyridyl least
square planes. The pyridine rings of the terpyridyl moieties
adopt the all-trans conformation typical for such compounds with
NCCN torsional angles in the range 159.57(16)–177.21(16)◦.
The synthesis of bis-tpy derivatives6 usually relies on con-
densation methodologies,7 C–C coupling strategies of preformed
terpyridyl moieties,8 or 4ꢀ-position substitution reactions on com-
mercially available 4ꢀ-chloro-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine.9,10 We recently
reported a strategy for amine substitutions at the 4ꢀ-position
involving Pd-catalyzed C–N bond formation that avoid the harsh
conditions typically needed with amine nucleophiles.11 The use
of aniline derivatives as nucleophilic components in palladium
catalyzed amination of nitrogen heterocycles has been successfully
accomplished,12 and we anticipated that this strategy could be
successful also in the preparation of an amine-linked bis-tpy
ligand by diarylation of aniline. Using Pd(dba)2/SPhos (dba =
dibenzylideneacetone, SPhos = 2-dicyclohexylphosphino-2ꢀ,6ꢀ-
dimethoxybiphenyl), the bis-tpy ligand L was prepared in one
step from commercially available starting materials (Scheme 1).‡
The ligand was typically obtained in 60–65% yield using 1%
◦
Pd at 100 C for 18 h in toluene, and the remaining 4ꢀ-chloro-
Fig. 1 ORTEP view of L at 50% probability level.
2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine and the intermediate phenyl(4ꢀ-(2,2ꢀ:6ꢀ,2ꢀꢀ-
terpyridyl))amine were removed by washing the obtained solid
with CH3CN.13
To illustrate the potential of L as a bridging ligand in din-
uclear assemblies, the corresponding ruthenium(II) polypyridyl
complex was prepared. Heating the ligand with two equiva-
lents of Ru(ttpy)Cl3 (ttpy is 4ꢀ-tolyl-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine) and N-
ethylmorpholine in ethylene glycol at 196 ◦C using microwave
irradiation gave ‘Ru(L)Ru’ (Fig. 2) in 67% isolated yield after
column chromatography (silica, KNO3 (sat’d) in CH3CN–H2O
aDepartment of Photochemistry and Molecular Science, Uppsala
University, BOX 523, 751 20, Uppsala, Sweden. E-mail: olof.johansson@
fotomol.uu.se, reiner.lomoth@fotomol.uu.se; Fax: +46-18-4716844;
Tel: +46-18-4713632
bDepartment of Physical, Inorganic, and Structural Chemistry, Stockholm
University, 106 91, Stockholm, Sweden
1
(8 : 1)) and recrystallisation.§ The H NMR spectrum (CD3CN)
† CCDC reference number 677273. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b804461a
showed two singlets at 8.68 and 9.01 ppm each integrating to
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The electronic absorption spectrum of ‘Ru(L)Ru’ is charac-
terized by an intense metal-to-ligand charge transfer (MLCT)
band peaking at 513 nm (19 508 cm−1, 58 × 103 M−1 cm−1) that
is partly bleached upon oxidation to the RuIIRuIII state. In the
mixed-valence state the peak of the MLCT band shifts to 493 nm
(20 292 cm−1, 36.5 × 103 M−1 cm−1) and additional absorption
bands in the red and near infrared are observed. The band at
768 nm (13 024 cm−1, 5.4 × 103 M−1 cm−1) can be attributed to
a ligand-to-metal charge transfer (LMCT) transition from the
amine substituted bridging ligand to the RuIII center while the
1517 nm band (6592 cm−1, 5.8 × 103 M−1 cm−1) is assigned to an
intervalence charge transfer (IVCT) transition. These assignments
Fig. 2 Structure of ‘Ru(L)Ru’.
four protons (3ꢀ,5ꢀ-tpyH protons of L and the two ttpy ligands) in
agreement with the proposed structure. All molecular ions from
sequential loss of the four PF6− counterions (at m/z 1840.3, 847.5,
516.7, and 351.3) were detected by mass spectrometry (ESI-MS).
The electrochemical properties of ‘Ru(L)Ru’ were studied by
differential pulse voltammetry and cyclic voltammetry in CH3CN
solution. In the cyclic voltammogram (Fig. 3), two reversible
III
are corroborated by the spectrum of the isovalent Ru2 complex
that lacks the IVCT band and features instead a more intense
LMCT band at 858 nm (11 655 cm−1, 11.5 × 103 M−1cm).
The IVCT band has a Gaussian profile (inset Fig. 4) and, as
for most valence localized systems (class II), is somewhat broader
(Dm˜1/2 = 4.6 × 103 cm−1) than estimated with eqn (1)
(Dm˜1/2)2 = 16kBTkln 2 = 2.31 × 103(EIVCT − DG◦)
oxidation processes are observed at E 1 (1) = 0.71 V (vs. Fc)
2
and at E 1 (2) = 0.90 V that arise from the RuIII/II couples
(at 298 K in cm−1)
(1)
2
of the two metal centers. The magnitude of their separation
within the limits of Hush’s classical model (3.9 × 103 cm−1).17 Here
EIVCT is the energy of the IVCT transition (m˜max) that equals the
reorganization energy k for the intramolecular electron transfer in
a symmetric mixed-valence system (DG◦ = 0).
(DE 1 = 190 mV) indicates considerable stability of the mixed-
2
valence state with a comproportionation constant of Kc
=
exp(DE 1 F/RT) = 1.63 × 103 (at 298 K) and a free energy
of stabi2liztion due to metal–metal interaction of DG = 12 RT
ln(Kc/4) = 7.45 kJ mol−1.14 Interestingly, no separation between
the metal-centered redox processes was reported for the directly
linked “back-to-back” dinuclear Ru complex1c or for the O-
linked bis(terpyridyl)ether analogue.5 In contrast, strong metal–
metal interaction was reported for triruthenium complexes with
N-linked tri(4-ethynylphenyl)amine bridges.15 Also in a series of
diruthenium complexes with bis(4-pyridyl)-type bridging ligands
the maximum metal–metal interaction was observed with the
bis(4-pyridyl)amine ligand and it has been suggested that the
electrons in the lone pair of the amine N are responsible for the
efficient electronic coupling of the aromatic ring systems.16
Fig. 4 Absorption spectrum of ‘Ru(L)Ru’ (Ru2II, —) and spectra after
oxidation at 0.81 V (RuIIRuIII, ---) and 1.12 V (Ru2III, -·-) (CH3CN, 0.1 M
TBAPF6). Inset: Near infrared range of the spectrum of the mixed valence
complex ( ) and its representation (—) by Gaussian bands for the LMCT
(-·-) and IVCT (---) transition.
Within the same theoretical framework the magnitude of
electronic coupling Hab between the Ru centers is given by eqn (2)
Hab (cm−1) = [(4.2 × 10−4)eDm˜1/2EIVCT
]
1/2/d
(2)
Fig. 3 Cyclic voltammogram of ‘Ru(L)Ru’ at 0.1 V s−1 (CH3CN, 1 mM,
0.1 M TBAPF6). Inset: differential pulse voltammetry.
where e is the extinction coefficient at the band maximum and d is
˚
the electron transfer distance in A.
˚
On the reductive side differential pulse voltammograms (inset,
Fig. 3) resolve two close lying peaks at −1.58 V and −1.66 V. These
arise presumably from the one electron reductions of the two pe-
ripheral ttpy ligands and the small separation would be consistent
with a minor interaction between the ligand radicals as compared
to the metal centers.
From the structure of the ligand a Ru–Ru distance of 11 A can
be inferred that results in a value of Hab = 7.8 × 102 cm−1 for
the mixed-valence complex. Comparison to the electrochemical
data shows that resonance exchange with DGr = Hab2/k =
1.1 kJ mol−1 makes a significant, but not dominating, contribution
to the metal–metal interaction. Since electrostatic effects cannot
3650 | Dalton Trans., 2008, 3649–3651
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be expected to differ substantially between ‘Ru(L)Ru’ and e.g. its
ether analogue these results suggest that L is not only an efficient
mediator for resonance exchange but also for inductive effects.
In summary, the non-linear amine-linked bis-tpy ligand de-
scribed herein can be employed as a bridging ligand in dinuclear
or polynuclear complexes. Compared to similar bis-tpy bridging
ligands ‘Ru(L)Ru’ features substantially stronger metal–metal
interaction in the mixed-valence state. From the ∼120◦ angle ob-
served in the X-ray crystal structure, it is intriguing to consider its
potential use in the self-assembly of hexagonal metallomacrocycles
with pronounced metal–metal interactions.
3 (a) E. C. Constable, R. W. Handel, C. E. Housecroft, A. Fa`rran,
Morales, B. Ventura, L. Flamigni and F. Barigelletti, Chem.–Eur. J.,
2005, 11, 4024; (b) E. C. Constable, Chem. Commun., 1997, 1073.
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and G. R. Newkome, Inorg. Chim. Acta, 2007, 360, 1780; (b) S.-H.
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5 E. C. Constable, A. M. W. Cargill Thompson, P. Harveson, L. Macko
and M. Zehnder, Chem.–Eur. J., 1995, 1, 360.
This work was financially supported by the Swedish Energy
Agency, the Knut and Alice Wallenberg Foundation, the Carl
Trygger Foundation and NEST-STRP, SOLAR-H (EU Contract
516510).
6 (a) For recent reviews dealing with 2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine synthesis,
see: U. S. Schubert, H. Hofmeier and G. R. Newkome, in Modern
Terpyridine Chemistry, WILEY-VCH, Weinheim, 2006, ch. 2, pp. 7–
35; (b) M. Heller and U. S. Schubert, Eur. J. Org. Chem., 2003, 947;
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and U. S. Schubert, Synthesis, 2006, 2873; (b) S. Vaduvescu and P. G.
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Cargill Thompson, J. Chem. Soc., Dalton Trans., 1992, 3467; (d) F.
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5701; (c) A. Khatyr and R. Ziessel, J. Org. Chem., 2000, 65, 3126.
9 (a) P. R. Andres and U. S. Schubert, Synthesis, 2004, 1229; (b) U. S.
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Notes and references
‡ The reaction was typically performed in a sealed vial on a 0.4 mmol
scale (Cl-tpy) in 3 mL argon-degassed toluene. Analytical data for bis(4ꢀ-
(2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridyl))phenylamine (L): 1H NMR (CDCl3): d 7.22–7.28 (m,
5H), 7.33 (m, 2H), 7.42 (m, 2H), 7.81 (dt, J = 7.7, 1.5 Hz, 4H), 8.20 (s,
4H), 8.55 (m, 4H), 8.59 (d, J = 8.0 Hz, 4H). 13C (CDCl3): d 114.9, 121.4,
123.8, 126.2, 127.1, 130.3, 136.8, 144.9, 149.1, 155.4, 156.1, 157.1. ESI-
MS: m/z = 556.7 (M + H+), 1133.2 (2M + Na+). X-Ray crystallography
experimental data for L: C36H25N7, monoclinic, space group C2/c (no.
◦
˚
˚
˚
15), a = 30.670(3) A, b = 8.4665(12) A, c = 22.428(2) A, a = 90 , b =
◦
◦
94.063(12) , c = 90 , V = 5809.4(11) A , Dcalc = 1.271 g cm−3, T = 293(2)
3
˚
10 (a) E. C. Constable, C. E. Housecroft and Y. Tao, Synthesis, 2004, 869;
(b) 4ꢀ-Bromo-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine as an electrophile: B. Whittle, S. R.
Batten, J. C. Jeffery, L. H. Rees and M. D. Ward, J. Chem. Soc., Dalton
Trans., 1996, 4249.
K, Z = 8, Rint = 0.0623, R = 0.0430 for 2998 observed unique reflections.
1
§ Analytical data for Ru(L)Ru: H NMR (CD3CN): d 2.54 (s, 6H), 7.16
(m, 4H), 7.32 (m, 4H), 7.44 (m, 4H), 7.59 (d, J = 8.0 Hz, 4H), 7.63 (m,
1H), 7.69 (m, 4H), 7.79 (t, J = 7.7 Hz, 2H), 7.85 (dt, J = 7.7, 1.5 Hz, 4H),
7.91 (m, 2H), 7.99 (dt, J = 7.7, 1.5 Hz, 4H), 8.12 (d, J = 8.1 Hz, 4H),
8.39 (d, J = 7.7 Hz, 4H), 8.67 (d, J = 7.7 Hz, 4H), 8.68 (s, 4H), 9.01 (s,
4H). ESI-MS m/z = 1840.3 (M − PF6)+, 847.5 (M − 2PF6)2+, 516.7 (M −
3PF6)3+, 351.3 (M − 4PF6)4+. Anal. Calcd for C80H59N13Ru2P4F24: C 48.42,
H 3.00, N 9.18. Found: C 48.19, H 3.22, N 9.17%.
11 O. Johansson, Synthesis, 2006, 2585.
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Anderson, D. Zim, L. Jiang, A. Klapars and S. L. Buchwald, J. Am.
Chem. Soc., 2003, 125, 6653.
13 A similar amine-linked bis-tpy ligand, bis(4ꢀ-(2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridyl))-
amine was described by R.-A. Fallahpour, in a recent review (see
ref. 6(c)).
1 (a) A. C. Benniston, A. Harriman, P. Li, P. V. Patel and C. A. Sams,
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Sour, J. Chem. Soc., Chem. Commun., 1993, 434; (d) E. C. Constable,
A. M. W. Cargill Thompson and S. Greulich, J. Chem. Soc., Chem.
Commun., 1993, 1444.
14 The factor 4 accounts for a purely statistical contribution to the
comproportionation constant, i.e. Kc = 4 for a completely non-
interacting mixed-valence system.
15 K. Onitsuka, N. Ohara, F. Takei and S. Takahashi, Dalton Tans., 2006,
3693.
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17 N. S. Hush, Prog. Inorg. Chem., 1967, 8, 391.
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 3649–3651 | 3651
©