2,6-Dipyrazinylpyridines and their ruthenium(II) complexes: A new polynucleating ligand family

Article abstract of DOI:10.1021/ic015545o

We report a one-step synthesis of the first member of a new class of supramolecule-forming ligands, 4-p-tolyl-2,6-dipyrazinylpyridine, and the peripheral coordination of an Fe(II) moiety onto its homoleptic Ru(II) complex.

Full text of DOI:10.1021/ic015545o

Inorg. Chem. 2001, 40, 5485-5486
5485
2,6-Dipyrazinylpyridines and Their Ruthenium(II) Complexes: A New Polynucleating Ligand Family
Rocco Liegghio, Pierre G. Potvin,* and A. B. P. Lever*
Department of Chemistry, York University, 4700 Keele Street, Toronto, ON, Canada M3J 1P3
ReceiVed May 20, 2001
Ruthenium complexes have been explored as photosensitizers
and photocatalysts1 and, more recently, as electrocatalysts.2 The
ligands used include polypyridines,3,4 polypyrimidines,5,6 poly-
pyrazines,6 dipyridinylpyrimidine,7 dipyridinylpyrazine,4,6,8,9 dipy-
rimidinylpyridine,10 and tetrapyridinylpyrazine.3 Some of these
are polynucleating and have served in supramolecular constructs,11
but most reports concern monometallic species with a limited
number of electrons that can be mobilized.8 Multiple-electron
reductions are desirable, for instance in CO2 reduction, to avoid
the significant overpotentials associated with single-electron
reductions.12 2,6-Dipyrazinylpyridines (Figure 1), curiously miss-
ing from the literature, are attractive in that they can potentially
link three metals, allowing reactivity at the central core to be
tuned and also providing an additional source or sink of electrons
for multielectron catalysis. They also represent an important new
gateway into supramolecular chemistry via linkage of additional
metal-ligand units to the peripheral pyrazine nitrogen atoms. This
pathway can also clearly lead to dendrimer formation. We report
a one-step synthesis of such a ligand and the assembly of mono-
and polymetallic complexes therefrom.
Figure 1. Tuning a reaction center with a dipyrazinylpyridine ligand.
a
Scheme 1
All attempts to make a ligand of this type using conventional
routes to terpyridines failed, yielding instead only higher con-
densation products.13 However, modified Chichibabin conditions
(Scheme 1) provided, in one step, 4-p-tolyl-2,6-di(2-pyrazinyl)-
pyridine (L) in 84% isolated yield. We have successfully applied
this process to the syntheses of the 4-(4-tert-butylphenyl) and 4-(4-
hydroxyphenyl) analogues.14 NMR, EI-MS, elemental analysis,
and X-ray crystallographic analysis15 (Figure 2) confirmed the
structure of the L ligand.
a Conditions: (i) 0.5 equiv of p-tolualdehyde, excess NH4OH, 1:1
CH3OH-5% KOH/room temperature/9 h, (ii) 0.5 equiv of Ru(DMSO)4Cl2,
1,2-ethanediol/reflux/16 h, then excess aqueous NH4PF6.
(1) Hayashida, N.; Yamamoto, T. Bull. Chem. Soc. Jpn. 1999, 72, 1153.
Jing, B.; Zhang, H.; Zhang, M.; Lu, Z.; Shen, T. J. Mater. Chem. 1998,
8, 2055. Choi, C.; Mishra, L.; Mutai, T.; Araki, K. Bull. Chem. Soc.
Jpn. 2000, 73, 2051. Abdel-Shafi, A. A.; Beer, P. D.; Mortimer, R. J.;
Wilkinson, F. J. Phys. Chem. A 2000, 104, 192. Yanagida, M.; Singh,
L. P.; Sayama, K.; Hara, K.; Katoh, R.; Islam, A.; Sugihara, H.; Arakawa,
H.; Nazeeruddin, M. K.; Gra¨tzel, M. J. Chem. Soc., Dalton Trans. 2000,
2817.
(2) Nallas, G. N. A.; Brewer, K. J. Inorg. Chim. Acta 1996, 253, 7. Navarro,
M.; De Giovani, W. F.; Romero, J. R. J. Mol. Catal. A: Chem. 1998,
135, 249. Kloster, G. M.; Anson, F. C. Electrochim. Acta 1999, 44, 2271.
(3) Jones, S. W.; Vrana, L. M.; Brewer, K. J. J. Organomet. Chem. 1998,
554, 29.
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Chem. 1990, 29, 3926.
Figure 2. ORTEP drawing at 30% probability, with H atoms omitted,
of one of two crystallographically distinct molecules in the crystal structure
of L.
(6) Thanasekaran, P.; Rajagopal, S.; Srinivasan, C. J. Chem. Soc., Faraday
Trans. 1998, 94, 339.
(7) Phillips, I. G.; Steel, P. J. Aust. J. Chem. 1998, 51, 371.
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(9) Bridgewater, J. S.; Volger, L. M.; Molnar, S. M.; Brewer, K. J. Inorg.
Chim. Acta 1993, 208, 179.
[RuL2](PF6)2 was prepared by reaction of L with Ru-
(DMSO)4Cl2 (Scheme 1) in quantitative yield after recrystalli-
zation (hot CH3CN/Et2O). NMR, MALDI-MS, and X-ray crys-
16
(10) Greon, J. H.; van Leeuwen, P. W. N. M.; Vrieze, K. J. Chem. Soc.,
Dalton Trans. 1998, 113.
(15) C20H15N5: M 325.37, triclinic, a ) 9.2990(4) Å, b ) 10.7650(4) Å,
c ) 17.6110(9) Å, R ) 102.400(3)°, â ) 95.362(2)°, γ ) 111.857(3)°,
V ) 1568.52(12) Å3, T ) 150(2) K, space group P1h (No. 2), Z ) 4,
µ(Mo KR) ) 0.086 mm -1; 7009 unique reflections [Rint ) 0.032]; Rw-
(F2) ) 0.1443 (all data), R ) 0.0997 (all data), 0.0553 (I > 2σ(I)).
(16) Evans, I. P.; Spencer, A.; Wilkinson, G. J. Chem. Soc., Dalton Trans.
1973, 204.
(11) Lehn, J.-M. Supramolecular Chemistry. Concepts and PerspectiVes;
VCH: Weinheim, New York, 1995.
(12) Fujita, E. Coord. Chem. ReV. 1999, 373, 185.
(13) Chamchoumis, C.; Potvin, P. G. J. Chem. Res., Miniprint 1998, 0870.
(14) Liegghio, R.; Tharmalingam, T.; Potvin, P. G.; Lever, A. B. P. To be
published.
10.1021/ic015545o CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/22/2001

5486 Inorganic Chemistry, Vol. 40, No. 22, 2001
Communications
Figure 3. ORTEP drawing at 50% probability of the cation in the crystal
structure of [RuL2](PF6)2‚2CH3CN, with H atoms omitted for clarity.
Unlabeled atoms are symmetry related to labeled ones, and both phenyl
rings are disordered. Selected bond lengths (and esd’s): Ru(1)-N(1)
2.065(9) Å, Ru(1)-N(2) 2.066(7) Å, Ru(1)-N(4) 1.891(10) Å, Ru(1)-
N(5) 2.074(7) Å.
tallography17 (Figure 3) confirmed its structure. The Ru-N bond
lengths averaged 1.98 Å to the pyridine nitrogen atoms and 2.07
Å to the pyrazine nitrogen atoms, i.e., entirely within the ranges
found in terpyridine complexes.18 The MLCT band of [RuL2]-
(PF6)2 at 498 nm (ꢀ 1.66 × 105 M-1 cm-1) was typical of species
of this class. In the solid state, it emitted weakly at room
temperature near 667 nm (τ ) 18 ns), albeit more strongly than
did the terpyridine analogue [Ru(ttpy)2]2+ (τ ) 0.95 ns at 640
nm,19 ttpy is 4′-p-tolyl-2,2′:6′,2′′-terpyridine), intensifying to a
strong structured emission at 77 K with a peak at 685 nm and a
shoulder at 654 nm (τ ) 697 ns). This is evidently phosphores-
cence from the lowest lying MLCT state. The cyclic voltammo-
gram consisted of a reversible RuIII/II couple (1.62 V vs SCE),
and three reduction steps (-0.83, -1.04, and ca. -1.3 V vs SCE),
all at more positive potentials than with [Ru(ttpy)2](PF6)2,20 owing
to the electron-withdrawing effect of the pyrazine rings. We have
also prepared14 RuL(CO)Cl2, its 4-octyloxyphenyl analogue, and
[RuL(CO)2Cl]Cl as 2,6-dipyrazinylpyridine analogues of the
known terpyridine complexes.21
Figure 4. 1H NMR spectra (600 MHz, 1:1 CD3CN-D2O) of [RuL2]-
(PF6)2 titrated with (A) 0, (B) 0.9, and (C) 2.7 equiv of Na3[Fe(CN)5-
NH3]. Inset: UV-visible spectra (1:1 CH3CN-H2O) using (D) 0, (E) 1,
and (F) 3 equiv of Fe.
ligand is labile under these conditions (Figure 4). UV-visible
spectroscopy showed a new Fe-to-pyrazine charge-transfer band22
near 720 nm. In the 1H NMR spectrum, 1 equiv of Na3[Fe(CN)5-
NH3] caused a split of the aromatic signals into 1:2:1 (or, by
fortuitous overlap, 1:3) clusters, as the unsymmetrical [RuL-
(LFe(CN)5)]- formed exclusively and quantitatively. Further
addition of [Fe(CN)5(NH3)]3- led to mixtures of pentacyanofer-
rated adducts, which are currrently being examined. RuL(CO)-
Cl2 could likewise be titrated. The electrocatalytic and supramo-
lecular properties of this new versatile ligand are being explored.
Acknowledgment. The authors thank Prof. D. V. Stynes for
assistance with the crystallography, Re´mi Beaulac and Christian
Reber (Universite´ de Montre´al) for recording the emission spectra,
and the Natural Sciences and Engineering Research Council of
Canada for funding.
An important feature of this new ligand family is the ability to
bind other Lewis acids at the peripheral nitrogen atoms.22 [RuL2]-
(PF6)2 was titrated with aqueous Na3[Fe(CN)5NH3], whose NH3
(17) C40H30F12N10P2Ru‚2CH3CN, M 1123.86, tetragonal, a ) b ) 12.1640-
(2) Å, c ) 61.5970(12) Å, R ) â ) γ ) 90°, V ) 9114.1(3) Å3, T )
293(2) K, space group I41/a:2 (No. 88), Z ) 8, µ(Mo KR) ) 0.513 mm
-1; 5204 unique reflections [Rint ) 0.0457]; Rw(F2) ) 0.2224 (all data),
R ) 0.1699 (all data), 0.0916 (I > 2σ(I)).
Supporting Information Available: Crystallographic data in CIF
format for L and [RuL2](PF6)2. This material is available free of charge
via the Internet at http://pubs.acs.org.
(18) Zadykowicz, J.; Potvin, P. G. J. Coord. Chem. 1999, 47, 395.
(19) Barigelletti, F.; Flamigni, L.; Balzani, V.; Collin, J.-P.; Sauvage, J.-P.;
Sour, A.; Constable, E. C.; Cargill Thompson, A. M. W. J. Chem. Soc.,
Chem. Commun. 1993, 942.
(20) Sauvage, J. P.; Collin, J. P.; Chambron, J. C.; Guillerez, S.; Coudret, C.
Chem. ReV. 1994, 94, 993.
(21) Ben-Hadda, T.; Mountassir, C.; Le Bozec, H. Polyhedron 1995, 14, 953.
IC015545O
(22) Toma, H. E.; Lever, A. B. P. Inorg. Chem. 1986, 25, 176. Toma, H. E.;
Auburn, P. R.; Dodsworth, E. S.; Golovin M. N.; Lever, A. B. P. Inorg.
Chem. 1987, 26, 4257. Toma, H. E.; Santos P. S.; Lever, A. B. P. Inorg.
Chem. 1988, 27, 3850.