Synthesis of extended ethynylnaphthalene-based ruthenium(II) 2,2′:6′,2″-terpyridine complexes

Article abstract of DOI:10.1016/j.tetlet.2004.08.140

A 'synthesis-at-metal' approach is used for the preparation of a new class of luminescent metal complexes having closely-spaced triplet excited states. A 'synthesis-at-metal' approach is described for the preparation of extended ethynylnaphthalene-based ruthenium(II) 2,2′:6′,2″-terpyridine complexes.

Full text of DOI:10.1016/j.tetlet.2004.08.140

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 7883–7885
Synthesis of extended ethynylnaphthalene-based
ruthenium(II) 2,20:60,200-terpyridine complexes
Andrew C. Benniston,* Sarah Mitchell, Sarah A. Rostron and Songjie Yang
Molecular Photonics Laboratory, School of Natural Sciences (Chemistry), University of Newcastle,
Bedson Building, Newcastle upon Tyne NE1 7RU, UK
Received 25 July 2004; revised 16 August 2004; accepted 24 August 2004
Available online 11 September 2004
Abstract—A ꢀsynthesis-at-metalꢁ approach is described for the preparation of extended ethynylnaphthalene-based ruthenium(II)
2,20:60,200-terpyridine complexes.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Efficient and directional transport of information at the
nanoscale level is the basic concept behind research into
molecular-scale photonic devices.1 It is envisaged that in
such devices input and output photonic signals are from
well-separated terminals, with the organic connecting
framework facilitating intramolecular communication
(Fig. 1). Numerous input and output modules have been
identified but some of the most popular are associated
with ruthenium(II) and osmium(II) polypyridyl (e.g.,
2,20:60,200-terpyridine, 2,20-bipyridine, etc.) complexes.2
In examples to date, the ruthenium unit is the input
centre, whereas the output signal is generated at the
osmium-based terminal. An excitation energy transfer
gradient ensures that photonic input at the ruthenium
unit ends up at the osmium group. However, the proper
choice of connector is vital for promoting communica-
tion between the two metal terminals, and so identifica-
tion of the best unit is essential for the molecular device
to work efficiently.
In previous work on ruthenium(II) complexes we identi-
fied an alkyl- or alkoxy-bridged biphenylynyl unit
attached at the 40-position of 2,20:60,200-terpyridine as a
connector that could facilitate/retard energy transduc-
tion.3,4 However, for energy migration over unusually
long distances the naphthalene unit has been targeted
as a more promising conduit.5 Thus, in this communica-
tion we describe an approach to produce extended ethyn-
ylnaphthalene-based arrays using a ꢀsynthesis at metalꢁ
approach.6 One particular advantage of this method is
that the metal centre is introduced early in the synthesis
and avoids isolation of the ligand; which is often rather
insoluble and, therefore, difficult to purify.
The main synthetic approach to the targetted systems is
illustrated in Scheme 1. The method relies on Sonogash-
ira coupling protocols7 but also uses the fact that in aro-
matics the iodo replacement reaction proceeds at lower
temperatures when compared to the bromo derivative.8
Thus, commercially available 4-bromonaphthylamine 1
was converted to the mixed halogenated naphthalene
derivative 2 using simple Sandmeyer chemistry. Reac-
tion of 2 with trimethylsilylacetylene at room tempera-
ture, followed by deprotection and column
chromatography, (alumina, hexane) afforded 4 in 91%
yield. Coupling of 4 with 1equiv of 2, again at room
temperature, accordingly produced the dibrominated
derivative 5 in an average 41% yield. Although Sono-
gashira coupling of 5 with 7 was possible, we found that
conversion of 5 to the iodo derivative 6 prior to coupling
Figure 1. Simple illustration of the transport of information along a
molecular conduit.
Keywords: Ruthenium; Terpyridine; Naphthalene; Luminescence.
*
Corresponding author. Tel.: +44 0191 222 5706; fax: +44 0191 222
6929; e-mail: a.c.benniston@ncl.ac.uk
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.08.140

7884
A. C. Benniston et al. / Tetrahedron Letters 45 (2004) 7883–7885
TMS
H
NH2
Br
I
H
TMS
0.10
0.05
0.00
(iii)
(ii)
(v)
(i)
4
Br
Br
Br
2
3
1
(iv)
Br
Br
I
I
6
5
N
N
2+
N
N
H
Ru
(vi)
-
N
N
2PF6
7
300
400
500
600
700
800
N
N
N
N
N
Wavelength (nm)
Ru2+
N
N
Ru2+
N
N
Figure 2. Absorption spectra recorded for RuL1Ru (ꢀ), RuNAPRu
(-Æ-Æ-Æ) and RuL2Ru (ꢁ ꢁ ꢁ) in deoxygenated butyronitrile solution at
20ꢁC. Also shown is the emission spectral profile recorded for RuL1Ru
at 20ꢁC.
N
N
N
-
4PF6
n
n = 1 RuL1Ru; n = 3 RuL2Ru
1: (vii)
2: 6, (vi)
spectroscopy, electrospray/MALDI mass spectrometry
and elemental analysis.
N
N Ru2+
N
N
N
N
N
(vi)
Ru2+
N
TMS
N
N
H
3
-
N
-
N
2PF
The absorption and luminescence spectra of the three
complexes were recorded in butyronitrile (Fig. 2). Each
complex displays the characteristic MLCT absorption
band centred around 500nm; kMLCT = 50 5nm (RuL1-
Ru), 501nm (RuL2Ru) and 502nm (RuNAPRu). Addi-
tional bands are evident in the region 350–450nm for
RuL1Ru and RuNAPRu, which can be assigned to elec-
tronic transitions associated with the naphthalene units.
It is noted that the main naphthalene absorption band
for RuL2Ru is lower in energy by ca. 15nm, indicating
extended conjugation along the molecular bridge. Lumi-
nescence from the complexes is situated at the red-end of
the visible spectral region, centred around 715nm.
Similar low-energy luminescence has been observed in
binuclear ruthenium(II) terpy complexes connected
solely via acetylene bridges.14 In general, the insertion
of an aromatic residue between the alkynylene
groups raises the energy of the luminescence profile.
This is clearly not the case for naphthalene insertion.
Long-wavelength emitters are currently under intense
investigation15 and, therefore, complexes based on mul-
ti-linked di(ethynyl)naphthalene units could find impor-
tant applications in this area.
2PF
6
6
8
(iii)
N
N
N
N
Ru2+
Ru2+
N
N
N
N
-
N
N
N
N
4PF
6
RuNAPRu
Scheme 1. Synthetic methodology used in the preparation of extended
complexes. Reagents and conditions: (i) NaNO2, KI, HCl; (ii)
PdCl2(PPh3)2, CuI, Et3N, benzene, D, 18h; (iii) KF, MeOH, rt, 16h;
(iv) 2, PdCl2(PPh3)2, CuI, Et3N, benzene, rt, 21h; (v) n-BuLi, I2, THF,
ꢀ78ꢁC, 4h; (vi) PdCl2(PPh3)2, CuI, Et3N, THF/CH3CN, D, 28h; (vii)
K2CO3, MeOH.
gave the bis-ruthenium(II) 2,20:60,200-terpyridine deriva-
tive RuL1Ru9 in an unoptimised 37% yield. In order to
extend the molecular axis, 710 was coupled to 3 and once
again deprotected to afford derivative 8 in 51% yield.
Surprisingly, removal of the tetramethylsilyl group from
8 using KF in MeOH gave not only the expected depro-
tected product but also the homodinuclear cross-coupled
derivative RuNAPRu (17% yield).11 Similar cross-
coupling has been reported using KF on supported alu-
mina under microwave irradiation but still required the
presence of a copper catalyst.12 It is not obvious at this
stage what species acts as the catalyst in the formation of
the dimeric complex. On the other hand, deprotection of
8 using K2CO3 in MeOH did produce the desired acetyl-
ene derivative, which was then coupled to 6 to afford
RuL2Ru13 in an unoptimised yield of 7%. The rather
low yield of this product can be attributed to the pro-
duction of RuNAPRu as side product of the coupling
reaction. Alternatively, it was found that in situ depro-
tection of 8 in the presence of 6 under Sonogashira con-
ditions also afforded the desired bimetallic complex. The
authenticity of all new compounds was checked by
standard analytical techniques, including 1H NMR
Preliminary variable temperature luminescence studies
on the complexes are also very encouraging and suggest
that the naphthalene bridge might play an active role in
energy migration. At present, the synthesis of mixed
Ru(II)/Os(II) complexes based on L1 and L2 is under-
way. Such complexes should enable the detailed investi-
gation of long-range energy transfer processes and will
be reported at a later date. It should be noted that the
triplet manifold is likely to be crowded.
Acknowledgements
We thank the EPSRC (GR/S00088/01) for financial sup-
port and the EPSRC funded mass spectrometry service

A. C. Benniston et al. / Tetrahedron Letters 45 (2004) 7883–7885
7885
(1893.2) (MꢀPF6)+; 874.1 (874.1) (Mꢀ2PF6)2+. Elemental
analysis calcd (found) for C86H54N12Ru2P4F24ÆH2O: C,
50.25 (50.02); H, 2.75 (2.76); N, 8.18 (8.15).
at Swansea for running the electrospray and MALDI
mass spectra. Discussions with Prof. A. Harriman are
greatly appreciated.
10. Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni,
S. Chem. Rev. 1996, 96, 759–834.
11. Analytical data: 1H NMR ((CD3CN, 500MHz) d = 7.22
(m, 8H, CHterp), 7.43 (2 · d, 8H, J = 5.5Hz, CHterp), 7.99
(m, 12H, 8 · CHterp, 4 · CHnap), 8.11 (q, 4H, J = 6.1Hz,
CHnap), 8.47 (t, 2H, J = 8.2Hz, CHterp), 8.53 (d, 4H,
J = 8.2Hz, CHterp), 8.60(m, 6H, 4 · CHterp, 2 · CHnap),
8.80(d, 4H, J = 8.2Hz, CHterp), 8.85 (m, 2H, CHnap), 9.06
(s, 4H, CHterp). ES-MS m/z calcd (found) for
C88H54N12Ru2P3F18 1917.2 (1917.0) (MꢀPF6)+; 886.1
(886.0) (Mꢀ2PF6)2+. Elemental analysis calcd (found)
for C88H54N12Ru2P4F24: C, 51.27 (51.51); H, 2.64 (3.02);
N, 8.15 (8.01).
References and notes
1. Weiss, E. A.; Ahrens, M. J.; Sinks, L. E.; Gusev, A. V.;
Ratner, M. A.; Wasielewski, M. R. J. Am. Chem. Soc.
2004, 126, 5577–5584.
2. Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez,
S.; Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.;
Flamigni, L. Chem. Rev. 1994, 94, 993–1019.
3. Benniston, A. C.; Harriman, A.; Li, P.; Sams, C. Tetra-
hedron Lett. 2003, 44, 4167–4169.
4. Benniston, A. C.; Harriman, A.; Li, P.; Sams, C. Chem.
Phys. Phys. Chem. 2004, 875–877.
5. El-ghayoury, A.; Harriman, A.; Ziessel, R. J. Phys. Chem.
A 2000, 104, 7906–7915.
12. Kabalka, G. W.; Wang, L.; Namboodiri, V.; Pagni, R. M.
Tetrahedron Lett. 2000, 41, 5151–5154.
13. Analytical data: 1H NMR ((CD3CN, 500MHz) d = 7.18
(m, 8H, CHterp), 7.36 (d, 4H, J = 5.9Hz, CHterp), 7.41 (d,
6. Tzalis, D.; Tor, Y. Chem. Commun. 1996, 1043–1044.
7. Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron
Lett. 1975, 50, 4467–4470.
4H, J = 5.9Hz, CHterp), 7.95 (m, 16H, 8 · CHterp
,
4 · CHnap), 8.12 (d, 8H, J = 8.3Hz, CHnap), 8.43 (t, 2H,
J = 8.2Hz, CHterp), 8.49 (d, 4H, J = 8.1Hz, CHterp), 8.58
8. Pan, Y.; Peng, Z. Tetrahedron Lett. 2000, 41, 4537–4540.
9. Analytical data: 1H NMR (CD3CN, 500MHz) d = 7.19
(m, 8H, CHterp), 7.38 (d, 4H, J = 4.9Hz, CHterp), 7.42 (d,
(d, 4H, J = 8.5Hz, CHterp), 8.76 (m, 12H, 4 · CHterp
,
8 · CHnap), 9.04 (s, 4H, CHterp). MALDI-MS m/z calcd
(found) for
C110H66N12Ru2P3F18 2193.3 (2193.3)
4H, J = 4.9Hz, CHterp), 7.96 (m, 12H, 8 · CHterp
,
(MꢀPF6)+.
4 · CHnap), 8.16 (q, 4H, J = 6.7Hz, CHnap), 8.44 (t, 2H,
J = 8.2Hz, CHterp), 8.50(d, 4H, J = 8.3Hz, CHterp), 8.59
14. Benniston, A. C.; Grosshenny, V.; Harriman, A.; Ziessel,
R. Angew. Chem., Int. Ed. Engl. 1994, 33, 1884–
1885.
(d, 4H, J = 8.0Hz, CHterp), 8.77 (m, 6H, 4 · CHterp
,
2 · CHnap), 8.84 (m, 2H, CHnap), 9.04 (s, 4H, CHterp). ES-
MS m/z calcd (found) for C86H54N12Ru2P3F18 1893.2
15. Treadway, J. A.; Strouse, G. F.; Ruminski, R. R.; Meyer,
T. J. Inorg. Chem. 2001, 40, 4508–4509.