Synthesis of 4,5-diheteroarylphenanthrenes and their dinuclear ruthenium(II) bis(2,2′:6′,2″-terpyridine) complexes possessing severe helical twists

Article abstract of DOI:10.1039/b921667g

Three 4,5-diheteroarylphenanthrenes 2a, 2b and 2c and two dinuclear Ru(II) bis(terpyridine) complexes 13 and 14 possessing severe helical twists were synthesized. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b921667g

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Synthesis of 4,5-diheteroarylphenanthrenes and their dinuclear
ruthenium(II) bis(2,20:60,200-terpyridine) complexes possessing
severe helical twistsw
Bo Wen, Jeffrey L. Petersen and Kung K. Wang*
Received (in College Park, MD, USA) 16th October 2009, Accepted 23rd December 2009
First published as an Advance Article on the web 20th January 2010
DOI: 10.1039/b921667g
Three 4,5-diheteroarylphenanthrenes 2a, 2b and 2c and two
dinuclear Ru(II) bis(terpyridine) complexes 13 and 14 possessing
severe helical twists were synthesized.
single operation. Presumably, the transformation from
8a to 2a proceeded through a cascade sequence of reac-
tions involving two prototropic rearrangements to form the
benzannulated enyne–allene units in 9.1c The subsequent
Schmittel cyclization reactions9 to generate the corresponding
biradicals followed by the intramolecular radical–radical
couplings and two prototropic rearrangements to regain
aromaticity then gave 2a as reported previously.1c Because
the relative reaction rates of the steps of the cascade sequence
have not been determined, it is also possible that the first
benzannulated enediyne unit in 8a could undergo the cascade
transformation before the second unit would begin its cyclization
sequence. Similarly, 2b and 2c were obtained from 8b and 8c,
respectively.
The 1H NMR spectrum of 2a shows a set of AB quartet
signals at d 4.42 and 4.20 (J = 21.0 Hz) from the diastereo-
topic methylene hydrogens on the five-membered rings,
manifesting the presence of a helical twist. Such an AB pattern
was also observed for 2b and 2c. Indeed, the X-ray crystal
structures of 2a and 2c (Fig. 2)z reveal severe structural
distortion due to nonbonded steric interactions between the
two heteroaromatic substituents. The two heteroaromatic
substituents are bent away from each other, causing a
pronounced 45.11 twist between the mean planes of rings A
and C of 2a and a 55.81 twist of that of 2c. As observed in the
case of 2 (Ar = phenyl), the two heteroaromatic substituents
are oriented in essentially twisted parallel positions, but are at
a 55.81 angle from the mean plane of either ring A or C of 2a
and at a 47.01 angle from those of 2c. The orientations of
the heteroaromatic substituents also place several aromatic
hydrogens in the magnetic shielding regions of the aromatic
ring currents, causing significant upfield shifts. In the crystal
structures, the perpendicular distance between the planes of
The 4,5-disubstituted phenanthrenes are nonplanar and possess
a helical twist.1 The X-ray crystallographic structure of 4,5-
dimethylphenanthrene (1) shows a 27.91 twist between the
mean planes of the two outer benzene rings (Fig. 1).1b Such a
structural distortion was caused by the nonbonded steric
interactions between the two substituents at the C4 and C5
positions. We recently reported a new synthetic pathway
leading to the diindeno-fused 4,5-diarylphenanthrenes
2
(Ar = phenyl, 3,5-dimethylphenyl or 4-biphenylyl) via cascade
cyclization reactions of the corresponding benzannulated
enyne–allenes.1c We now have used a modified pathway for
the synthesis of 4,5-diheteroarylphenanthrenes 2a, 2b and 2c
bearing pyridyl, 2,20-bipyridyl and 2,20:60,200-terpyridyl sub-
stituents, respectively. The presence of two units of pyridyl,
bipyridyl or terpyridyl substituents allows them to serve
as potential building blocks for complex formation with
transition metals to produce supramolecular systems.2 A wide
variety of ruthenium terpyridine complexes have been reported
because they display interesting photophysical, photochemical,
and electrochemical properties.3 Two dinuclear ruthenium(II)
bis(terpyridine) complexes of 2c possessing severe helical twists
were thus synthesized.
The synthetic sequence outlined in Scheme 1 for 2a, 2b and
2c involved condensations between 2 equiv. of 34 and diketone
41c to give diol 5 as an essentially 1 : 1 mixture of two
diastereomers. Reduction of 5 with triethylsilane in the presence
of trifluoroacetic acid furnished 6 also as an essentially
1 : 1 mixture of diastereomers. The subsequent Sonogashira
reactions5 with 4-ethynylpyridine (7a)6 produced 8a. The
structures of the two diastereomers of 8a were established by
X-ray structure analyses. Similarly, 8b and 8c were synthesized
by coupling with 5-ethynyl-2,20-bipyridine (7b)7 and 40-ethynyl-
2,20:60,200-terpyridine (7c),7a,8 respectively. Treatment of 8a
with potassium tert-butoxide in refluxing toluene produced
4,5-diheteroarylphenanthrene 2a bearing a helical twist in a
˚
the two heteroaromatic substituents is ca. 2.80 A for 2a
˚
and ca. 3.03 A for 2c, much shorter than the usual p system
C. Eugene Bennett Department of Chemistry,
West Virginia University Morgantown, WV 26506, USA.
E-mail: kung.wang@mail.wvu.edu
w Electronic supplementary information (ESI) available: Experimental
procedures, X-ray structures, 1H and 13C NMR spectra, UV-vis and
luminescence spectra for 2a, 2b and 2c. CCDC 750093 and 750096.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/b921667g
Fig. 1 Structures of 4,5-disubstituted phenanthrenes.
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of 2a–c can be expected to be slow as observed previously for 2
(Ar = phenyl).1c
The UV–vis absorption spectra of 2a, 2b and 2c, recorded in
CH2Cl2, show absorption bands in the near-UV region from
ca. 235 to 305 nm and less intense bands in the visible region
with maxima at ca. 405 nm. The CH2Cl2 solutions of 2a–c
show a bright yellow color. Upon excitation at 360 nm, they
exhibit blue emission with maxima at ca. 460 nm.
The presence of two terpyridyl substituents in 2c provides
opportunities for the formation of dinuclear ruthenium(II)
bis(terpyridine) complexes possessing helical twists.
A
reported procedure for the synthesis of Ru(II) bis(terpyridine)
complexes11 was adopted by first treating 2c with RuCl3
to form the Ru(III) trichloride complex 10 (Scheme 2). Treat-
ment of 10 with 40-ethoxy-2,20:60,200-terpyridine (11)8b or
40-chloro-2,20:60,200-terpyridine (12)8a,b followed by the addi-
tion of a large excess of NH4PF6 then formed the brown
precipitates of [(40-EtOtpy)Ru(2c)Ru(40-EtOtpy)](PF6)4 (13)
or [(40-Cltpy)Ru(2c)Ru(40-Cltpy)](PF6)4 (14), respectively.
1H NMR studies of CD3CN solutions of the heteroleptic
species 13 and 14 show the presence of minor amounts
of the homoleptic species [Ru(40-EtOtpy)2](PF6)2 and
[Ru(40-Cltpy)2](PF6)2, respectively, as observed previously in
the synthesis of other heteroleptic ruthenium complexes.11
Because of the presence of an unpaired electron in the t2g
orbital of the low-spin d5 Ru(III) ions, the 1H NMR signals of
10 are broadened and shifted dramatically.12 The signals at
d ꢁ32.35 and ꢁ35.56 can be attributed to the hydrogens at the
6 and 600 positions of the terpyridyl system, where they are
closest to the paramagnetic Ru(III) ions. The presence of these
two distinct signals also indicates a relatively slow rate of
rotation on the NMR time scale around the carbon–carbon
single bonds attaching the terpyridyl units to the phenanthryl
system. The AB quartet signals of 10 appear at d 7.75
Scheme 1 Synthesis of 4,5-diheteroarylphenanthrenes.
˚
van der Waals contact distance of ca. 3.4 A between parallel
aromatic hydrocarbons in crystals10 and the graphite layer
˚
distance of 3.35 A.
At 28 1C, the 1H NMR spectrum of 2c exhibits broad humps
for the aromatic hydrogens on the terpyridyl substituents. At
60 1C, these signals are less broad and a singlet at d 7.91
attributable to the four hydrogens at the 30 and 50 positions of
the two central pyridyl rings could be clearly discerned. At
ꢁ20 1C, multiple signals of the terpyridyl substituents start to
appear. These observations suggest restricted rotations around
the carbon–carbon single bonds connecting the terpyridyl
substituents to the C4 or the C5 position of the central
phenanthryl system. Similarly, at 28 1C broad signals in the
aromatic region attributable to the hydrogens on the bipyridyl
substituents of 2b were also observed. The rates of racemization
Fig. 2 ORTEP drawings of the crystal structures of 2a and 2c. The
thermal ellipsoids are scaled to enclose 30% probability.
Scheme 2 Synthesis of Ru(II) bis(terpyridine) complexes.
Chem. Commun., 2010, 46, 1938–1940 | 1939
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2c could allow them to be resolved, providing potentially new
sets of ligands for asymmetric homogeneous catalysis.14
We thank the National Science Foundation (Grant CHE-
0909613) for financial support.
Notes and references
z Crystal data for 2a: C46H40N2, M = 620.80, monoclinic, space
˚
group C2/c, colorless, a = 17.936(1), b = 10.398(1), c = 20.003(1) A,
3
b = 116.370(1)1, V = 3342.4(4) A , Z = 4, Dc = 1.234 g cmꢁ3
,
=
˚
m(Mo-Ka) = 0.71 cmꢁ1, T = 293 K, 3820 unique data with Rint
0.0439. Final R1 = 0.0554, wR2 = 0.1495 for 3052 reflections with
I 4 2s(I), GOF = 1.046. CCDC 750096. Crystal data for 2c:
C66H52N6ꢂ2CH2Cl2, M = 1098.99, monoclinic, space group C2/c,
˚
yellow,
a
=
27.005(2),
b
=
14.736(1),
c
=
17.778(2) A,
b = 128.084(2)1, V = 5568.6(8) A , Z = 4, Dc = 1.311 g cmꢁ3
,
=
3
˚
Fig.
3
UV–vis absorption spectra of 13 (—), 14 (ꢂ ꢂ ꢂ) and
m(Mo-Ka) = 2.62 cmꢁ1, T = 293 K, 6323 unique data with Rint
[Ru(40-EtOtpy)2](PF6)2 (---) in CH3CN at room temperature.
0.0494. Disordered CH2Cl2 molecules treated with SQUEEZE routine
in PLATON. Final R1 = 0.0665, wR2 = 0.1843 for 3872 reflections
with I 4 2s(I), GOF = 0.959. CCDC 750095.
(J = 21.0 Hz) and 4.83 (J = 21.0 Hz). For 13 without the
presence of paramagnetic ruthenium ions, the AB quartet
signals appear at d 4.46 (J = 21.6 Hz) and 4.30 (J = 21.6 Hz).
The 1H NMR signals of the ethoxyl groups and the hydrogens
on the diindeno-fused phenanthryl system are sharp with well-
defined splitting patterns. Similarly, for 14 the AB quartet
signals appear at d 4.46 (J = 21.6 Hz) and 4.30 (J = 21.6 Hz).
Again, due to slow rate of rotation around the carbon–carbon
single bonds attaching the terpyridyl units to the phenanthryl
system, the signals of the aromatic hydrogens on the terpyridyl
groups of 13 and 14 are broad.
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The UV–vis spectra of 13 and 14 show ligand-centered
bands of the aromatic terpyridyl structures in the near-UV
region (Fig. 3).11,13 The less intense bands in the visible region
can be attributed to the spin-allowed metal-to-ligand charge-
transfer (MLCT) transitions involving promotion of an elec-
tron from the metal t2g orbital to a p* antibonding orbital of
the ligand with absorption maxima at ca. 495 to 505 nm.
Compared to the parent [Ru(tpy)2](PF6)2 complex, which
exhibits an absorption maximum at 474 nm,11 these MLCT
bands undergo a red shift. Such a red shift is reminiscent of
what was observed in the cases of [Ru(40-EtOtpy)2](PF6)2
complex (l = 485 nm) bearing electron donating ethoxyl
groups, [Ru(40-Cltpy)2](PF6)2 complex (l = 480 nm) bearing
chloro substituents, and [Ru(40-Phtpy)2](PF6)2 complex
(l = 487 nm) bearing phenyl groups.11 Luminescence was
not observed for 13 and 14 at room temperature.
In conclusion, three 4,5-diheteroarylphenanthrenes 2a, 2b
and 2c possessing severe helical twists were synthesized from
the corresponding benzannulated enyne–allenes. The struc-
tures of 2a and 2c were established by X-ray crystal structure
analyses, permitting direct measurements of the extent of the
structural distortion. The presence of two terpyridyl units in 2c
allowed it to be used as a ligand for the synthesis of helical
dinuclear Ru(II) bis(terpyridine) complexes. The two Ru(II)
units are in close proximity to each other, making it possible
for electronic interactions between the two heteroaromatic p
systems for energy- and electron-transfer processes.3 The high
configurational stability of the helical structures of 2a, 2b and
13 A. Winter, J. Hummel and N. Risch, J. Org. Chem., 2006, 71,
4862–4871.
14 G. Chelucci and R. P. Thummel, Chem. Rev., 2002, 102,
3129–3170.
ꢀc
This journal is The Royal Society of Chemistry 2010
1940 | Chem. Commun., 2010, 46, 1938–1940