Di-[1,10]-phenanthrolinyl Diazines: A New Family of Bis-tridentate Chelators

Article abstract of DOI:10.1021/ol0172572

(Formula Presented) A series of three bis-tridentate bridging ligands has been prepared in which two 1,10-phenanthroline units have been symmetrically appended to a central pyridazine, pyrimidine, or pyrazine ring. These ligands have been treated with [Ru

Full text of DOI:10.1021/ol0172572

ORGANIC
LETTERS
2002
Vol. 4, No. 8
1253-1256
Di-[1,10]-phenanthrolinyl Diazines: A
New Family of Bis-tridentate Chelators
Darren Brown, Seema Muranjan, Youngchan Jang, and Randolph Thummel*
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003
thummel@uh.edu
Received December 18, 2001 (Revised Manuscript Received February 25, 2002)
ABSTRACT
A series of three bis-tridentate bridging ligands has been prepared in which two 1,10-phenanthroline units have been symmetrically appended
to a central pyridazine, pyrimidine, or pyrazine ring. These ligands have been treated with [Ru(tpy-d11)Cl3] to afford both mono- and bimetallic
complexes that show very self-consistent NMR properties.
Recent interest in electron or energy transfer between two
or more metal centers confined to a supramolecular assembly
has prompted the preparation of a wide range of bridging
ligands.1 Many of these ligands have been bi- or tridentate
chelators in which a pyridine ring is the coordinating unit.
Thus, 2,2′-bipyridines (bpy) and 2,2′:6′,2′′-terpyridines (tpy)
have been connected by a variety of tethers and the influence
of intermetallic distance or π-conjugation has been evaluated.
More intimate bridging ligands will hold metals closer
together and presumably improve communication. Such
intimacy can be attained when a common aromatic ring is
shared between the two chelating moieties. Thus, the family
of di-2′-pyridyl diazines (1-4) has been extensively studied
as bridging ligands and scaffolds for the formation of
metallodendrimers.2 These species may be viewed as two
bpys that have been superimposed so as to create a common
central ring and also provide a symmetrical bis-bidentate
binding situation. Ligand 4 presents a steric problem upon
dinuclear coordination due to an unfavorable interaction
between the two pyridine H3′ atoms.
One could imagine an analogous situation for the super-
position of two tpy units to form a bis-tridentate binding
system. If the two tpys share a common central ring, one
has the well-known tetra-(2′-pyridyl)-pyrazine (5).3 If the tpys
share a distal ring, bridging diazines analogous to 1-4 can
be designed, and bis-2,2′-bipyrid-6-yl analogues of 2 and 3
have been reported.4 We have devised synthetic approaches
to three such bridging diazine ligands where the less flexible
R. R.; Johnson, J. O. Inorg. Chem. 1987, 26, 210. (g) Ernst, S.; Kasack,
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10.1021/ol0172572 CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/16/2002

Scheme 1
1,10-phenanthroline (phen) moiety has been incorporated in
place of the pendent bpy subunits.
an appropriate diacetyl diazine precursor and 2 equiv of
8-amino-7-quinolinecarbaldehyde (12)6 (Scheme 3). The 4,6-
The synthesis of 3,6-diphen-pyridazine (6) is analogous
to the route employed for the preparation of 15 (Scheme 2).
Scheme 3
Scheme 2
diacetylpyrimidine (13) was prepared by first coupling (1-
ethoxyethenyl)tri-n-butylstannane7 with 4,6-dichloropyrim-
idine followed by acid hydrolysis of the ethoxyvinyl groups.8
The condensation with 12 proceeded smoothly to provide a
50% yield of 7. The diacetylpyrazine 14 was prepared in
28% yield by the free radical acetylation of pyrazine using
acetaldehyde and t-butylhydroperoxide in the presence of
ferric sulfate.9 Condensation with quinoline aminoaldehyde
12 gave the expected 8 in 41% yield.
All three bridging ligands were readily characterized by
1H NMR. The 2-phen moieties gave very similar and
characteristic patterns in the aromatic region. The central ring
was distinguished by either one singlet (δ 9.36 for 6 and
10.25 for 8) or two singlets (δ 9.51 and 10.61 for 7). The
deshielding of these protons was consistent with their
proximities to nitrogen.
The reaction of 2-cyanophen with hydrazine in refluxing
ethanol provided a 73% yield of the dihydrotetrazine 10,
which was subsequently oxidized with nitric acid in acetic
acid. The bright red-colored tetrazine 11 was obtained in
40% yield. Cycloaddition of acetylene to 11 in refluxing
DMF followed by the extrusion of dinitrogen provided a 47%
yield of the desired pyridazine 6.
Both the diphen-pyrimidine 7 and the diphen-pyrazine 8
were synthesized by a double Friedla¨nder condensation using
Treating the ligands 6-8 with 1 equiv of [Ru(tpy-d11)-
Cl3] led to the incorporation of a single Ru(tpy-d11) subunit
at one of the two equivalent tridentate sites and consequent
formation of the red mononuclear complex (Schemes 4 and
5).
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1254
Org. Lett., Vol. 4, No. 8, 2002

Scheme 4
Use of the deuterated reagent allowed us to observe NMR
respectively. The complex of 7 is similar to ones prepared
by the Lehn4d and Siegal4b groups using analogous ligands
involving a bis-bpy pyrimidine-type bridge. The metals are
oriented in a syn fashion so that a pincer-type array results
(Scheme 4). The structure drawing may be somewhat
deceptive in that it depicts the molecule such that the two
tpy ligands appear to lie in parallel planes. However,
tridentate coordination by the phen-pyrimidine moiety will
cause a pinching that may force the tpys closer together than
they appear. A molecular mechanics calculation11 bears out
this hypothesis, and a more detailed structural analysis awaits
the growth of an appropriate crystal.
signals only from the bridging ligand, which greatly facili-
tated analysis, particularly for the mononuclear complexes
where all the protons on the bridging ligand were nonequiva-
lent.10 For the mononuclear complex of 6, the incorporated
Ru(tpy-d11) subunit effectively blocks the other tridentate
binding site so that a second Ru(tpy-d11) moiety cannot be
introduced in an N6 binding mode. Nevertheless, the pendant
uncomplexed phen can rotate away from the Ru(II) center
and a second metal might be introduced at this bidentate
site.
The bimetallic complex of 8 positions the Ru(tpy-d11)
subunits in an anti orientation, and one additional C-C bond
intervenes between the two metal centers. Even with the
pinching effect, the tpys are expected to lie in parallel planes.
Both complexes have high symmetry and are achiral. The
NMR spectrum of each complex exhibits the typical pattern
for a 2-substituted phen, while the central ring of 7 shows
two singlets for the pyrimidine protons. Since the bridging
ligand is approximately planar, H5 now lies in a more
deshielding region and is shifted downfield by 0.23 ppm,
appearing at δ 9.74. On the other hand, H2 is held between
the two tpy ligands where it is heavily shielded and, thus,
shifted upfield by 4.11 ppm to δ 6.50. The bimetallic
complex of 8 shows only a single two-proton resonance for
the pyrazine protons H2 and H5. This proton should feel
one-half of the shielding and deshielding effects felt by H2
and H5 of the corresponding complex of 7. Thus, one can
Scheme 5
When 7 and 8 were treated directly with 2.2 equiv of
[Ru(tpy-d11)Cl3], the bimetallic complexes were obtained as
their green hexafluorophosphate salts in 41 and 34% yields,
(10) Chirayil, S. C.; Hegde, V.; Jahng, Y.; Thummel, R. P. Inorg. Chem.
1991, 30, 2821.
(11) Molecular mechanics calculations performed with PC MODEL from
Serena Software, Bloomington, IN.
Org. Lett., Vol. 4, No. 8, 2002
1255

calculate an expected upfield shift of +1.94 ppm. The
observed singlet at δ 8.49 agrees fairly well with this
prediction, showing an upfield shift of 1.76 ppm.
(CHE-9714998) for financial support of this work. We also
thank John Hazelrigg for assistance and Professor Garry
Hanan for helpful discussions.
Further studies on this interesting family of ligands are
underway and will include a careful assessment of photo-
physical and electrochemical properties, variation of the
appended phen moiety, heterodinuclear complexes, and the
self-assembly of ligand-bridged supramolecular structures.
Supporting Information Available: Experimental pro-
cedures and characterization relating to the syntheses of 6-8,
10, 11, and the Ru(II) complexes of 6-8. This material is
available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the Robert A. Welch
Foundation (E-621) and the National Science Foundation
OL0172572
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Org. Lett., Vol. 4, No. 8, 2002