2,9-Di-(2′-pyridyl)-1,10-phenanthroline: A tetradentate ligand for Ru(II)

Article abstract of DOI:10.1021/ja047410y

The tetradentate ligand 2,9-di-(2′-pyridyl)-1,10-phenanthroline is synthesized in 62% yield by the Stille coupling of 2,9-dichloro-1,10-phenanthroline and 2-(tri-n-butylstannyl)pyridine. Treatment of this ligand with RuCl₃·3H₂O and a 4-substituted pyridine results in the formation of a complex in which the tetradentate ligand occupies the equatorial plane and two pyridines are bound axially. The interior N-Ru-N angles vary from 76.1° to 125.6°, showing considerable distortion from the 90° ideal. The lowest energy electronic transition is sensitive to the electronegativity of the 4-substituent on the axial pyridines, varying from 516 nm for the CF₃ group to 580 nm for the NMe₂. The oxidation potentials mirror this trend, spanning a range of 1.36-1.03 V, while the reduction potentials show less variation (-0.97 to -1.08 V). The complexes are nonemissive, presumably due to competitive nonradiative processes caused by distortion of the system. Copyright

Full text of DOI:10.1021/ja047410y

Published on Web 08/17/2004
2,9-Di-(2′-pyridyl)-1,10-phenanthroline: A Tetradentate Ligand for Ru(II)
Ruifa Zong and Randolph P. Thummel*
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003
Received May 3, 2004; E-mail: thummel@uh.edu
Polypyridine derivatives of Ru(II) are the most widely studied
of all transition metal complexes. Interest in these systems stems
from the well-understood nature of their photophysical and
electrochemical properties as well as their ease of synthesis, leading
to the availability of a wide variety of substituted analogues.1 Of
the chelating polypyridines, 2,2′-bipyridine (bpy) is the bidentate
prototype and 2,2′:6′,2′′-terpyridine (tpy) is the tridentate. The next
higher homologue, quaterpyridine (1), is well known but commonly
acts as a bridging ligand with Ru(II), twisting about the central
bond to adopt a helical conformation which binds two Ru(II) centers
in a bidentate fashion.2,3 We reasoned that restricting the central
bond of this ligand by incorporating a phenanthroline subunit might
prohibit bridging and induce mononuclear coordination. This
Communication will describe the new tetradentate ligand 2,9-di-
(2′-pyridyl)-1,10-phenanthroline (2) that promises to provide a novel
family of well-behaved Ru(II) complexes.
Figure 1. ORTEP drawings of the Ru(2) planar fragment (left) and the
cation (right) for [Ru(2)(4-NMe2py)2](PF6)2‚(C3H6O).
Ligand 2 was prepared in 62% yield by Pd(PPh3)4-catalyzed Stille
coupling of 2,9-dichloro-1,10-phenanthroline4 and 2-(tri-n-butyl-
stannyl)pyridine.5 The NMR spectrum of 2 exhibits 7 proton peaks
and 11 13C signals,6 indicating a symmetric structure. Treatment
of 2 with RuCl3 in refluxing absolute ethanol, followed by the
introduction of 4-dimethylaminopyridine (4-NMe2py), 4-methylpy-
ridine (4-Mepy), or 4-trifluoromethylpyridine (4-CF3py) gave a trans
complex [Ru(2)(4-X-py)2]2+ in yields of 95, 50, and 5%, respec-
tively. The low yield for the 4-CF3py system is attributed to the
Figure 2. Absorption spectra of [Ru(2)(4-X-py)2](PF6)2 in 5 × 10-5
CH2Cl2, where X ) CF3 (red), Me (green), and NMe2 (blue).
M
much weaker coordinating ability of the pyridine such that
[Ru(2)(4-CF3py)(Cl)]+ is the major product (52%). All three
complexes gave NMR spectra with the expected seven peaks for 2
as well as two 4H doublets for the pyridine protons. Particularly
characteristic was the resonance of H6′ on the distal pyridine rings
of 2, which was downfield shifted by 1.13-1.17 ppm. These protons
lie in the molecular plane of 2 and are pushed close to one another,
explaining the strong deshielding effect. The complexes were further
characterized by MALDI-MS, IR, and elemental analyses. About
5% of [Ru(2)2]2+ was also isolated from all three reactions and
but well organized. The exterior Ru-N bonds are longer, averaging
2.17 Å, while the interior Ru-N bonds measure only 1.94 Å. The
central N23-Ru-N26 angle of 82.3° is not too distorted from the
90° ideal, while the adjacent N1-Ru-N26 and N22-Ru-N23 are
compressed to 76.1° and the outer N1-Ru-N22 angle is expanded
to 125.6°. The result is that N1 and N22 are separated by only
3.86 Å, while in a planar conformation of uncomplexed 2 a
separation of 5.19 Å is calculated.8 The complexes appear to be
stable to the reaction conditions of refluxing aqueous ethanol and,
in the presence of excess 4-X-pyridine, no other products were
detected.
Variation of the 4-substituent on the axial pyridines of the
complex exerts a strong effect on spectroscopic properties. Figure
2 shows the absorption spectra in the visible region, where four
major bands are readily discernible and summarized along with
the UV bands in Table 1. Typically these bands are assigned to a
metal-to-ligand charge-transfer (MLCT) state involving a metal
1
identified by its MALDI-MS and H NMR (Supporting Informa-
tion).
A single-crystal structure analysis of [Ru(2)(4-NMe2py)2](PF6)2‚
C3H6O revealed that ligand 2 coordinates in a tetradentate fashion,
forming a well-behaved Ru(2) plane (Figure 1).7 The torsion angle
of N1-N26-N23-N22 is only -0.2(2)°, and the ruthenium is
located just 0.0024(3) Å away from this least-squares plane. The
cation has C2V symmetry with the two auxiliary pyridines being
approximately coplanar, bonding to ruthenium at a normal distance
of about 2.10 Å. The tetradentate coordination is highly distorted
9
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J. AM. CHEM. SOC. 2004, 126, 10800-10801
10.1021/ja047410y CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
a
Table 1. Electronic Absorption Data for [Ru(2)(4-X-py)2](PF6)2
The complexes are not emissive at room temperature and 77 K.
This observation is most likely explained by a weak ligand field
associated with the highly distorted coordination geometry. These
distortions cause a lowering of the d-d-states, which provides an
alternate pathway for depopulation of the excited state. A similar
argument has been set forth to explain the lack of room-temperature
X
λmax(log ꢀ)
NMe2
225(4.46), 259(4.71), 281(4.87), 313(4.78), 445(3.61),
479(3.67), 514(3.71), 580(3.81)
Me
226(sh, 4.53), 241(4.64), 279(4.81), 315(4.66), 332(sh, 4.59),
411(3.70), 456(3.58), 487(3.74), 544(3.91)
CF3
224(4.47), 241(4.60), 277(4.77), 307(4.51), 338(4.56),
372(4.21), 437(3.56), 469(3.74), 516(3.89)
luminescence for the less distorted [Ru(tpy)2]2+ 10
.
In summary, unlike quaterpyridine 1, ligand 2 binds to a
ruthenium primarily as a tetradentate ligand, generating a planar
Ru(2) subunit which may be a useful building block for supramo-
lecular assembly, as demonstrated by the preparation of several
trans-bis-pyridine adducts. Further work will involve variations on
the central phenanthroline or the distal pyridine moieties to enlarge
this family of novel tetradentate ligands.
a Recorded in 5 × 10-5 M CH2Cl2; wavelength in nm and extinction
coefficient in M-1 cm-1
.
Table 2. Cyclic Voltammetric Data for 2 and
a
[Ru(2)(4-X-py)2](PF6)2
E
1/2
ox(∆E)
E
red (∆E)
1/2
2
-1.07ir
-1.55ir
-1.56ir
-1.53ir
-1.39ir
Acknowledgment. We thank the Robert A. Welch Founda-
tion (E-621) and the Division of Chemical Sciences, Office of
Basic Energy Sciences, U.S. Department of Energy (Contract No.
DE-FG03-02ER15334), for financial support of this work. We also
thank Dr. James Korp for assistance with the X-ray analysis.
X ) NMe2
X ) Me
X ) CF3
1.03(111)
1.27(160)
1.36(189)
-1.08(114)
-1.04(174)
-0.97(169)
a Recorded in CH2Cl2 containing 0.1 M NBu4PF6; E1/2 in V vs SCE and
∆E in mV; scan rate ) 100 mV/s; irreversible process estimated by
differential peaks.
Supporting Information Available: Synthetic details including the
reported byproducts, 1H NMR and IR spectra, expanded Figure 1, and
sample CV (PDF); X-ray crystallographic data (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
d-orbital and the π*-orbital of a ligand. The long-wavelength bands
are consistent with MLCT to the more electronegative ligand 2.
The energies of these bands are influenced by the axial ligands,
where electron-donating substituents (λmax ) 580 nm for NMe2)
destabilize the d-level relative to electron-withdrawing substituents
(λmax ) 516 nm for CF3). All four bands correlate well with
Hammett σp constants.9
References
(1) (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85-277. (b) Balzani, V.; Juris,
A. Coord. Chem. ReV. 2001, 211, 97-115. (c) Duerr, H.; Bossmann, S.
Acc. Chem. Res. 2001, 34, 905-917. (d) Kalyanasundaram, K. Photo-
chemistry of Polypyridine and Porphyrin Complexes; Academic Press:
San Diego, CA, 1992.
On the basis of AM-1 calculations, the LUMO and thereafter
higher energy orbitals for ligand 2 are estimated at -1.50, -1.02,
-0.80, and -0.07 eV, while the LUMO for 4-NMe2py, 4-Mepy,
and 4-CF3py are estimated at -0.00, -0.11, and -0.51 eV,
respectively. While we argue that low-energy MLCT is due mainly
to dπ(Ru)-to-π*(2), the higher energy bands at 445, 411, and 372
nm likely involve the participation of the auxiliary pyridines, which
becomes most important in the case of 4-CF3py, the best π-acceptor.
The Hammett correlation further supports these assignments.
Cyclic voltammetric measurements in dichloromethane displayed
two one-electron, ligand-centered reductions as well as one metal-
centered oxidation, and the data are summarized in Table 2.
Oxidation involves the removal of an electron from a metal
d-orbital, while reduction involves the addition of an electron to
the more electron-accepting ligand. Thus, we observe a decrease
in oxidation potential as the auxiliary 4-NMe2py donates electron
density to ruthenium, raising the HOMO (dπ) of the metal.
Reduction of the 4-NMe2py and 4-Mepy complexes clearly involves
the ligand 2, where only small differences are observed as compared
to reduction of the free ligand itself. The 4-CF3py system exerts a
strong electron-withdrawing effect and thus causes a small increase
in the reduction potential for this system.
(2) Constable, E. C.; Cathey, C. J.; Hannon, M. J.; Tocher, D. A.; Walker, J.
V.; Ward, M. D. Polyhedron 1999, 18, 159-173.
(3) A Ru(II) complex of a tetradentate derivative of 1 has been reported:
Renouard, T.; Fallahpour, R. A.; Nazeeruddin, M. K.; Humphry-Baker,
R.; Gorelsky, S. I.; Lever, A. B. P.; Graetzel, M. Inorg. Chem. 2002, 41,
367-378.
(4) (a) Lewis, J.; O’Donoghue, T. D. J. Chem. Soc., Dalton Trans. 1979,
736-742. (b) Yamada, M.; Nakamura, Y.; Kuroda, S.; Shimao, I. Bull.
Chem. Soc. Jpn. 1990, 63, 2710-2712.
(5) (a) Nierengarten, H.; Rojo, J.; Leize, E.; Lehn, J.-M.; Van Dorsselaer, A.
Eur. J. Inorg. Chem. 2002, 573-579. (b) Fargeas, V.; Favresse, F.;
Mathieu, D.; Beaudet, I.; Charrue, P.; Lebret, B.; Piteau, M.; Quintard,
J.-P. Eur. J. Org. Chem. 2003, 1711-1721.
(6) 1H NMR (300 MHz, CDCl3): δ 9.12 (d, J ) 8.1 Hz, 2H), 8.91 (d, J )
8.4 Hz, 2H), 8.79 (dd, J ) 4.5, 1.8 Hz, 2H), 8.43 (d, J ) 8.7 Hz, 2H),
8.03 (dt, J ) 8.1, 1.8 Hz, 2H), 7.88 (s, 2H), 7.44 (m, 2H). 13C NMR (75
MHz, CDCl3): δ 156.2, 155.9, 149.1, 145.7, 137.1, 137.0, 129.1, 126.7,
124.2, 122.2, 120.6.
(7) Crystal data for [Ru(2)(4-NMe2py)2](PF6)2‚(C3H6O): T ) 223(2) K,
triclinic, P1h, Z ) 2, a ) 8.6475(5) Å, b ) 11.3708(7) Å, c ) 22.7506(14)
Å, R ) 102.309(1)°, â ) 93.762(1)°, γ ) 96.210(1)°, V ) 2163.7(2) Å3,
R1 ) 0.0398, wR2 ) 0.0996, GOF on F2 ) 1.071.
(8) Molecular modeling and analysis were conducted with CS Chem3D Ultra,
CambridgeSoft, 2001.
(9) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165-195.
(10) Kirchhoff, J. R.; McMillin, D. R.; Marnot, P. A.; Sauvage, J.-P. J. Am.
Chem. Soc. 1985, 107, 1138-1141.
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