Synthesis and characterization of ruthenium(II)-pyridylamine complexes with catechol pendants as metal binding sites

Article abstract of DOI:10.1021/ic902070q

A novel tris(2-pyridylmethyl)amine (TPA) derivate having two catechol moieties linked by amide linkages at the 6-positions of two pyridyl groups was synthesized. The ligand, N,N-bis[6-{3,4-(dihydroxy)benzamide}-2-pyridyl-methyl]- N-(2-pyridylmethyl)amine (Cat₂-TPA; L2), and its precursor, N,N-bis[6-{3,4-bis(benzyloxy)-benzamide}-2-pyridyl-methyl]-N-(2-pyridylmethyl) -amine ((Bn₂Cat)₂-TPA; L1), formed stable ruthenium(II) complexes, [RuCl(L2)]PF₆ (2) and [RuCl(L1)]PF₆ (1), respectively. The crystal structure of [RuCl(L2)]Cl (2′) was determined by X-ray crystallography to show two isomers in terms of the orientation of one catechol moiety. In complex 2, the ligand bearing catechols acts as a pentadentate ligand involving coordination of one of the amide oxygen atoms in addition to that of the tetradentate TPA moiety and two metal-free catechol moieties as metal-binding sites. The coordination of L2 results in the preorganization of the two catechols to converge them to undergo intramolecular π-π interactions. The ¹H NMR spectrum of 2 in DMSO-d ₆ revealed that only one isomer was present in the solution. This selective formation could be ascribed to the formation of an intramolecular hydrogen-bonding network among the hydroxyl groups of the catechol moieties, as suggested by X-ray analysis. This intramolecular hydrogen bonding could differentiate the pK_a values of the hydroxy groups of the catechol moieties into three kinds, as indicated by spectroscopic titration with tetramethylammonium hydroxide (TMAOH) in DMF. The complexation of 2 with other metal ions was also examined. The reaction of 2 with [Cu(NO₃) ₂(TMEDA)] (TMEDA = N,N,N′,N′-tetramethylethylenediamine) in methanol allowed us to observe the selective formation of a binuclear complex, [RuCl(L2^2-){Cu(TMEDA)}]PF₆ (3), which was characterized by ESI-MS, UV-vis, and ESR spectroscopies. Its ESR spectrum in methanol suggested that the coordination of the Cu(II)-TMEDA unit to the converged catechol moieties would be different from conventional κ²-O,O′:η²-coordination: it exhibits a novel bridging coordination mode, bis-κ¹-O:η¹- coordination, to form the binuclear Ru(II)-Cu(II) complex.

Full text of DOI:10.1021/ic902070q

Inorg. Chem. 2010, 49, 3737–3745 3737
DOI: 10.1021/ic902070q
Synthesis and Characterization of Ruthenium(II)-Pyridylamine Complexes with
Catechol Pendants as Metal Binding Sites
Takahiko Kojima,*,† Norihisa Hirasa,‡ Daisuke Noguchi,§ Tomoya Ishizuka,† Soushi Miyazaki,‡ Yoshihito Shiota,
Kazunari Yoshizawa, and Shunichi Fukuzumi*,‡,^
†Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba,
Ibaraki 305-8571, ‡Department of Material and Life Science, Graduate School of Engineering, Osaka University
and SORST (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871, §Department of Chemistry, Graduate School of
Sciences, and Institute for Materials Chemistry and Engineering, Kyushu University, Hakozaki, Higashi-Ku,
Fukuoka 812-8581, Japan, and ^Department of Bioinspired Science, Ewha Womans University, Seoul,
120-750, Korea
Received October 20, 2009
A novel tris(2-pyridylmethyl)amine (TPA) derivate having two catechol moieties linked by amide linkages at the 6-
positions of two pyridyl groups was synthesized. The ligand, N,N-bis[6-{3,4-(dihydroxy)benzamide}-2-pyridyl-methyl]-
N-(2-pyridylmethyl)amine (Cat2-TPA; L2), and its precursor, N,N-bis[6-{3,4-bis(benzyloxy)-benzamide}-2-pyridyl-
methyl]-N-(2-pyridylmethyl)-amine ((Bn2Cat)2-TPA; L1), formed stable ruthenium(II) complexes, [RuCl(L2)]PF6 (2)
and [RuCl(L1)]PF6 (1), respectively. Thecrystal structure of [RuCl(L2)]Cl (20) wasdetermined by X-ray crystallography
to show two isomers in terms of the orientation of one catechol moiety. In complex 2, the ligand bearing catechols acts as
a pentadentate ligand involving coordination of one of theamide oxygen atoms in additionto that of the tetradentate TPA
moiety and two metal-free catechol moieties as metal-binding sites. The coordination of L2 results in the preorganization
of the two catechols to converge them to undergo intramolecular π-π interactions. The 1H NMR spectrum of 2 in
DMSO-d6 revealed that only one isomer was present in the solution. This selective formation could be ascribed to the
formation of an intramolecular hydrogen-bonding network among the hydroxyl groups of the catechol moieties, as
suggested by X-ray analysis. This intramolecular hydrogen bonding could differentiate the pKa values of the hydroxy
groups of the catechol moieties into three kinds, as indicated by spectroscopic titration with tetramethylammonium
hydroxide (TMAOH) in DMF. The complexation of 2 with other metal ions was also examined. The reaction of 2 with
[Cu(NO3)2(TMEDA)] (TMEDA = N,N,N0,N0-tetramethylethylenediamine) in methanol allowed us to observe the
selective formation of a binuclear complex, [RuCl(L22-){Cu(TMEDA)}]PF6 (3), which was characterized by ESI-MS,
UV-vis, and ESR spectroscopies. Its ESR spectrum in methanol suggested that the coordination of the Cu(II)-TMEDA
unit to the converged catechol moieties would be different from conventional κ2-O,O0:η2-coordination: it exhibits a novel
bridging coordination mode, bis-κ1-O:η1-coordination, to form the binuclear Ru(II)-Cu(II) complex.
Introduction
the two oxygen atoms via deprotonation as dianionic and
bidentate catecholato ligands to form a stable five-membered
chelate ring.1 Nature utilizes this characteristic of catechol
moieties in siderophores, which play indispensable roles in
the uptake and transport of metal ions.2,3 In the structure of
enterobactin for the uptake and transport of iron, as a
representative of natural siderophores, catechol moieties
are organized and converged by a cyclic triester backbone
to facilitate the strong binding of the Fe(III) ion.4
Catechol (1,2-dihydroxybenzene; H2Cat) and its deriva-
tives have been known to form stable complexes with metal
ions.1 Usually, catechols coordinate to metal ions through
*To whom correspondence should be addressed. E-mail: kojima@
chem.tsukuba.ac.jp (T.K.), fukuzumi@chem.eng.osaka-u.ac.jp (S. F.).
(1) (a) Pierpont, C. G.; Buchanan, R. M. Coord. Chem. Rev. 1981, 38,
45–87. (b) Pierpont, C. G. Coord. Chem. Rev. 2001, 219-221, 415–433.
(c) Sever, M. J.; Wilker, J. J. Dalton Trans. 2004, 1061–1072. (d) Albrecht,
€
M.; Janser, I.; Frohlich, R. Chem. Commun. 2005, 157–165. (e) Jang, H. G.; Cox,
D. D.; Que, L., Jr. J. Am. Chem. Soc. 1991, 113, 9200–9204. (f) Kurihara, M.;
Kato, N.; Kojima, T.; Ishii, Y.; Matsuda, Y. Inorg. Chem. 1995, 34, 4888–4895.
(g) Shavaleev, N. M.; Davles, E. S.; Adams, H.; Best, J.; Weinstein, J. A. Inorg.
Chem. 2008, 47, 1532–1547. (h) Bruijnincx, P. C. A.; Viciano-Chumillas, M.;
Lutz, M.; Spek, A. L.; Reedjik, J.; van Koten, G.; Gebbink, J. M. K. Chem.-Eur.
J. 2008, 14, 5567–5576.
(2) For Fe: Crosa, J. H.; Walsh, C. T. Microbiol. Mol. Biol. Rev. 2002, 66,
223–249.
(3) For Mo: Bellenger, J.-P.; Arnaud-Neu, F.; Asfari, Z.; Myneni, S. C.
B.; Stiefel, E. I.; Kraepiel, A. M. L. J. Biol. Inorg. Chem. 2007, 12, 367–376.
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r
2010 American Chemical Society
Published on Web 03/23/2010
pubs.acs.org/IC

3738 Inorganic Chemistry, Vol. 49, No. 8, 2010
Kojima et al.
Scheme 1. Preorganization of Catechol Moieties by Metal Complexa-
tion to Form a Metal-Complex Ligand
result in the convergence of the functional groups to allow
them to undergo intramolecular noncovalent interactions
such as π-π interactions.9 Thus, we applied this methodol-
ogy to the preorganization of catechol moieties linked to the
TPA ligand via an amide linkage to construct a novel metal-
complex ligand with converged catechol moieties.
In this paper, we describe the synthesis and characteriza-
tion of a novel TPA derivative having two catechol units via
amide linkages and its Ru(II) complex. In addition, the
complexation of the Ru(II) complex with a Cu(II)-diamine
unit was also studied to propose a novel coordination mode
of converged catechol moieties.
The combination of catechol moieties with other metal
binding sites having different characters such as pyridyl-
amines, that is “ditopic”, offers more than two different
coordination environments in one molecule. This type of
ditopic ligand is useful for selective coordination of metal
ions to favorable sites depending on the characteristics of the
metal ions, giving rise to well-defined heterometallic multi-
nuclear metal complexes to develop new integrated function-
ality.5 So far, it has been reported that catechol moieties have
been linked to, for example, 2,20-bipyridine,6 2,20;60,200-ter-
pyridine,6 and macrocyclic ligands7 to afford ditopic ligands.
In order to develop a new strategy to construct hetero-
metallic multinuclear complexes, we designed a novel ditopic
ligand using a flexible multidentate scaffold that links to
catechol moieties via an amide linkage. Complexation of
an appropriate metal ion to the flexible multidentate site fixes
the conformation and configuration of the ligand by virtue of
a template effect of the metal ions to preorganize the
geometry of the catechol sites which can enjoy free orienta-
tions in solution, as shown in Scheme 1, in place of the
organic backbones observed in natural siderophores. Such
preorganization of the catechol moieties allows us to employ
a metal-complex ligand with catechols as metal-binding sites.
On the basis of this strategy, we can create novel multinuclear
metal complexes with multiredox systems involving mutual
electronic interactions among those redox centers, including
redox-noninnocent catechol moieties.8
Experimental Section
Materials and Apparatus. Dichloromethane (CH2Cl2), car-
bon tetrachloride (CCl4), triethylamine (NEt3), and tetrahydro-
furan (THF) were distilled over CaH2 under N2 prior to use.
Chemicals such as dimethylformamide (DMF) were distilled
and stored under N2. N-Bromosuccinimide (NBS) was recrys-
tallized from hot water prior to use. Other chemicals were used
as received. N,N-Bis(6-amino-2-pyridylmethyl)-N-(2-pyridyl-
methyl)amine 3HCl (diamino-TPA 3HCl) was prepared as
3
3
previously reported.9b The dichloro(p-cymene)-ruthenium(II)
dimer was purchased from Aldrich and used without further
purification. [Cu(NO3)2(TMEDA)] (TMEDA = N,N,N0N0-tet-
ramethylethylenediamine) was synthesized as reported.11
UV-vis absorption spectra were recorded on a Jasco Ubest-
55 UV/vis spectrophotometer at room temperature. All NMR
measurements were carried out on JEOL-JNM-AL300 and
Bruker AVANCE 400 spectrometers at 25 ꢀC. IR spectra of
solid samples were recorded on a Thermo Nicolet NEXUS 870
FT-IR instrument using 2 cm-1 standard resolution at ambient
temperature in KBr pellets. ESI-MS spectra were measured on a
Perkin-Elmer SCIEX API 150 EX mass spectrometer in positive
detection mode, equipped with an ion spray interface. The
sprayer was held at a potential of þ5.0 kV, and compressed
N2 was employed to assist liquid nebulization. The orifice
potential was maintained at þ20 V.
Synthesis of Benzyl 3,4-Bis(benzyloxy)benzoate.12 A solution
of proto-catechuic acid (10.0 g, 0.065 mol), benzyl chloride
(30 mL, 0.26 mol), and Na2CO3 (45.0 g, 0.42 mol) in DMF
(150 mL) was refluxed for 48 h. The mixture was filtered, and the
filtrate was dried up under reduced pressure. The residue was
purified by column chromatography on a silica gel (CH2Cl2,
Rf = 0.8). A white liquid of the compound was obtained. The
yield was 85% (23.5 g). 1H NMR (CDCl3, δ (ppm)): 5.20 (s, 2H,
Bn-CH2), 5.23 (s, 2H, Bn-CH2), 5.32 (s, 2H, Bn-CH2), 6.93
(d, 1H, J = 7 Hz, H5), 7.31-7.47 (m, 15H, Bn-Ph), 7.66-7.68
(m, 2H, H2, H6). MALDI-TOF MS: m/z = 446.90 (M - 2Hþ).
Synthesis of 3,4-Bis(benzyloxy)benzoic Acid.12 A solution of
benzyl 3,4-bis(phenylmethoxy)benzoate (10 g, 30 mmol) and
NaOH (2.88 g, 72.0 mmol) in 11:5 v/v methanol/water (160 mL)
was refluxed for 2 h and then cooled. After filtration, the filtrate
was dried up under reduced pressure. Concentrated HCl was
added, and then a white solid of the compound was obtained.
The yield was 92% (7.2 g). 1H NMR ((CD3)2CO, δ (ppm)): 5.21
(s, 2H, Bn-CH2), 5.25 (s, 2H, Bn-CH2), 7.15 (d, 1H, J = 8 Hz,
H5), 7.30-7.40 (m, 6H, Bn-Ph), 7.50-7.53 (m, 4H, Bn-Ph), 7.63
(d, 1H, J = 2 Hz, H2), 7.67 (dd, 1H, J = 8, 2 Hz, H6). IR (KBr):
We have reported on the synthesis and characterization of
Ru(II) complexes having tris(2-pyridylmethyl)amine (TPA)
derivatives with two functional groups via amide linkages as
ligands.9,10 In those complexes, the TPA derivatives coordi-
nate to the Ru(II) centers as pentadentate ligands involving
coordination of the amide oxygen that is stabilized by an
intramolecular hydrogen bonding.9,10 The amide oxy-
gen coordination and the intramolecular hydrogen bonding
(5) Guerra, K. P.; Delgado, R. Dalton Trans. 2008, 539–550.
(6) Shukla, A. D.; Whittle, B.; Bajaj, H. C.; Das, A.; Ward, M. D. Inorg.
Chim. Acta 1999, 285, 89–96.
(7) Jonasdottir, S.; Kim, C.-G.; Kampf, J.; Coucouvanis, D. Inorg. Chim.
Acta 1996, 243, 255–270.
(8) (a) Piepont, C. G. Coord. Chem. Rev. 2001, 216-217, 99–125.
(b) Evangelio, E.; Ruiz-Molina, D. Eur. J. Inorg. Chem. 2005, 2957–2971.
(c) Shimazaki, Y.; Kabe, R.; Huth, S.; Tani, F.; Naruta, Y.; Yamauchi, O. Inorg.
Chem. 2007, 46, 6083–6090. (d) Ohtsu, H.; Tanaka, K. Chem.-Eur. J. 2005, 11,
3420–3426.
(9) (a) Kojima, T.; Hayashi, K.; Matsuda, Y. Chem. Lett. 2000, 1008–
1009. (b) Kojima, T.; Hayashi, K.; Matsuda, Y. Inorg. Chem. 2004, 43, 6793–
6804. (c) Kojima, T.; Miyazaki, S.; Hayashi, K.; Shimazaki, Y.; Tani, F.; Naruta,
Y.; Matsuda, Y. Chem.-Eur. J. 2004, 10, 6402–6410. (d) Kojima, T.; Hayashi,
K.; Tachi, Y.; Naruta, Y.; Suzuki, T.; Uezu, K.; Shiota, Y.; Yoshizawa, K. Bull.
Chem. Soc. Jpn. 2005, 78, 2152–2158. (e) Kojima, T.; Noguchi, D.; Nakayama,
T.; Inagaki, Y.; Shiota, Y.; Yoshizawa, K.; Ohkubo, K.; Fukuzumi, S. Inorg.
Chem. 2008, 47, 886–895.
ν = 1687 (CdO), 1677, 1601, 1306, 1277, 1225 cm-1
.
Synthesis of 3,4-Bis(benzyloxy)benzoyl Chloride.12 To a de-
gassed solution of 3,4-bis(phenylmethoxy)benzoic acid (1 g,
(11) Pavkovic, S. F.; Miller, D.; Brown, J. N. Acta Crystallogr. 1977, B33,
2894–2896.
(10) Jitsukawa, K.; Oka, Y.; Yamaguchi, S.; Masuda, H. Inorg. Chem.
2004, 43, 8119–8129.
(12) Wang, W.-L.; Chai, S. C.; Ye, Q.-Z. Bioorg. Med. Chem. Lett. 2009,
19, 1080–1083.

Article
Inorganic Chemistry, Vol. 49, No. 8, 2010 3739
Chart 1
brown solution was added NaPF6 (17.4 mg, 0.104 mmol), and
the solution was concentrated to a small volume to obtain a
yellow powder of 1, which was washed with a minimum volume
of ethanol and diethyl ether and then dried. The yield was 58%
(62.5 mg). Anal. Calcd for C60H52N6O6ClRuPF6 1.5H2O: C,
3
57.12; H, 4.39; N, 6.66. Found: C, 57.06; H, 4.18; N, 6.66. ESI-
MS: m/z 1089.33 ([M-PF6]þ), 549.21 ([M-PF6-Cl-2Hþ2Na]2þ),
527.16 ([M-PF6-Cl]2þ). 1H NMR (CD3CN, δ (ppm)): 4.34 and
4.47 (ABq, JAB= 18 Hz, CH2(a)), 4.64 and 4.95 (ABq, JAB= 15
Hz, CH2(b0)), 4.65 and 5.14 (ABq, JAB= 14 Hz, CH2(b)), 4.78
(s, 2H, Bn-CH2), 5.03 (s, 2H, Bn-CH2), 5.08 (s, 2H, Bn-CH2),
5.12 (s, 2H, Bn-CH2), 6.48 (d, 1H, J = 8.8 Hz, cat0-H5), 6.63 (d,
1H, J = 8.6 Hz, cat-H5), 6.94 (dd, 1H, J = 8.8, 1.5 Hz, cat0-H6),
7.07-7.57 (m, 29H, aromatic), 7.64 (t, 1H, J = 8.4 Hz, pyr0-H4),
7.75 (t, 1H, J = 8.1 Hz, pyr-H4), 8.43-8.46 (m, 2H, pyr-H6
and pyr0-H5), 10.06 (s, 1H, N-H of amide (coordinated)), 10.30
(s, 1H, N-H of amide (uncoordinated and hydrogen-bonded)).
Absorption maxima (λmax, nm): 305 (ε = 3.4 ꢀ 104 M-1 cm-1),
455 (ε = 8.6 ꢀ 103 M-1 cm-1).
Synthesis of [RuCl(L2)]PF6 (2). A solution containing
[RuCl2(p-cymene)]2 (49.2 mg, 0.0804 mmol) and (H2Cat)2-
TPA (L2) (100 mg, 0.169 mmol) in methanol (20 mL) was
refluxed for 24 h. After filtration, the filtrate was dried up under
reduced pressure. The residue was dissolved in a small volume
of methanol, and then NH4PF6 (16.0 mg, 0.098 mmol) was
added. The mixture was concentrated under reduced pressure to
obtain a brown precipitate. The precipitate was filtered and
washed with CH3CN and then dried. A reddish orange powder
of 2 was obtained. The yield was 56% (40.0 mg). Anal. Calcd for
C32H28N6O6ClRuPF6 CH3CN 4H2O: C, 41.42; H, 3.89; N,
2.84 mmol) in CH2Cl2 (10 mL) was added oxalyl dichloride
(10 mL, 0.3 mol) under N2. The mixture was stirred for 12 h
at room temperature. After filtration, the filtrate was dried
up under reduced pressure, and then a white solid of the
acid chloride was obtained. The yield was 81% (0.85 g). The
obtained compound was used without further purification. IR
(KBr, cm-1): ν = 1739 (CdO), 1677, 1590, 1515, 1277, 1225.
Synthesis of N,N-Bis[6-N,N-bis{3,4-bis(benzyloxy)-benzamide}-
2-pyridyl-methyl]-N-(2-pyridylmethyl)amine ((Bn2Cat)2-TPA, L1).
To a degassed solution of 3,4-bis(phenylmethoxy)benzoyl chlor-
ide (2.98 g, 8.47 mmol) and NEt3 (12 mL) in THF (36 mL) was
3
3
1
10.15. Found: C, 41.36; H, 3.98; N, 9.93. H NMR (CD3CN,
δ (ppm)): 4.34 and 4.53 (ABq, JAB= 18 Hz, CH2(a)), 4.48 and
4.58 (ABq, JAB= 18 Hz, CH2(b)), 4.75 and 4.90 (ABq, JAB= 15
Hz, CH2(b0)), 6.10 (s, 1H, cat-H2), 6.39 (d, 1H, J = 8.0 Hz, cat-
H5), 6.67 (d, 1H, J = 8.0 Hz, cat-H50), 6.94 (dd, 1H, J = 8.0, 2.0
Hz, cat-H6), 7.07 (dd, 1H, J = 8.0, 2.0 Hz, cat0-H6), 7.15-7.26
(m, 6H, aromatics), 7.68 (td, 1H, J = 8.0, 2.0 Hz, pyr-H4), 7.76
(t, 1H, J = 8.0 Hz, pyr0-H40), 7.85 (t, 1H, J = 8.0 Hz, pyr0-H4),
8.36 (d, 1H, J = 8.0 Hz, pyr0-H5), 8.41 (d, 1H, J = 8.0 Hz, pyr-
H6), 9.81 (s, 1H, N-H of amide (uncoordinated and hydro-
gen-bonded)), 10.05 (s, 1H, N-H of amide (coordinated)).
ESI-MS: m/z 729.2 ([M-PF6]þ), 347.1 ([M-PF6-Cl]2þ). Absorp-
tion maxima (λmax, nm): 311 (ε = 3.7 ꢀ 104 M-1 cm-1), 442 (ε =
added diamino-TPA 3HCl (1.01 g, 3.13 mmol) in THF (36 mL)
3
dropwise over a period of 30 min under N2 at 0 ꢀC, and then N,N-
dimethylaminopyridine (DMAP) (0.91 g) was added to the
solution. The mixture was refluxed for 14 h and then filtered.
The filtrate was dried up under reduced pressure and purified
by column chromatography on activated alumina (ethyl acetate,
Rf = 0.9), followed by that on silica gel (ethyl acetate f
methanol, Rf = 0.1). The ligand was obtained as a white liquid
in 50% yield (2.00 g). ESI-MS: m/z 975.46 ([M þ Na]þ), 953.42
([M þ H]þ). 1H NMR (CDCl3, δ (ppm)): 3.79 (s, 4H, CH2(b)),
3.91 (s, 2H, CH2(a)), 5.21 (s, 4H, Bn-CH2), 5.22 (s, 4H, Bn-CH2),
6.96 (d, 2H, J = 8.4 Hz, cat-H5), 7.15 (dd, 1H, J = 8, 2 Hz, pyr-
H5), 7.29-7.39 (m, 14H, cat-H6, pyr0-H3 and Bn-Ph), 7.42-7.48
(m, 10H, Bn-Ph), 7.55 (d, 1H, J = 8 Hz, pyr-H3), 7.61 (d, 2H,
J = 2 Hz, cat-H2), 7.66(td, 1H, J = 8, 2 Hz, pyr-H4), 7.71 (t, 2H,
J = 8 Hz, pyr0-H4), 8.19 (d, 2H, J = 8 Hz, pyr0-H5), 8.46 (s, 2H,
N-H), 8.53 (dd, 1H, J = 8, 2 Hz, pyr-H6). The numbering of
protons is shown in Chart 1.
9.6 ꢀ 103 M-1 cm-1
)
Synthesis of [RuCl((HCat)2-TPA){Cu(TMEDA)}]PF6 (3).
Complex 2 (30 mg, 0.034 mmol) and [Cu(N,N,N0,N0-tetra-
methylethylenediamine)(NO3)2] ([Cu(TMEDA)(NO3)2]) (23.6 mg,
0.0686 mmol) were dissolved into methanol (10 mL). To
the solution was added NEt3 (19 μL), and the mixture was
stirred for 30 min. The resultant chestnut-colored solution was
filtered. To the filtrate was added TBAPF6 (66.5 mg, 0.172
mmol), and the solution was concentrated to a small volume
to obtain a dark brown powder of 3, which was washed with
a minimum volume of CH3CN and then dried. The yield was
52% (16.2 mg). ESI-MS: m/z 906.1 ([RuCl((HCat)2-TPA){Cu-
(TMEDA)}]þ). Absorption maxima (λmax, nm): 276 (ε = 4.6 ꢀ
104 M-1 cm-1), 360 (ε = 3.0 ꢀ 104 M-1 cm-1). The sample for
the elemental analysis was prepared by precipitation with the
addition of NH4PF6 instead of TBAPF6 from the above filtrate
due to the crystallization problem. The addition of slightly
acidic NH4PF6 caused a protonation of the complex. Anal.
Calcd for C32H27N6O6RuClCuC6H16N2 2PF6 CH3OH H2O:
Synthesis of N,N-Bis{6-(3, 4-dihydroxybenzamide)-2-pyridyl-
methyl}-N-(2-pyridylmethyl)amine ((H2Cat)2-TPA, L2). A de-
gassed solution of L1 (137 mg, 0.148 mmol) and iodotrimethyl-
silane (0.5 mL, 4 mmol) in CH3CN (10 mL) was stirred for
24 h. The reaction was quenched by adding MeOH, and then the
reaction mixture was filtered. The filtrate was dried up under
reduced pressure, and the residue was washed with diethyl ether
and then washed with a small volume of MeOH. The yield was
85% (74 mg). ESI-MS: m/z 593.14 ([M þ H]þ). 1H NMR
(DMSO-d6, δ (ppm)): 4.48 (s, 4H, CH2(b)), 4.73 (s, 2H, CH2(a)),
6.81 (d, 2H, J = 8 Hz, cat-H5), 7.26 (d, 2H, J = 7 Hz, pyr0-H3),
7.36-7.40 (m, 4H, cat-H2 and -H6), 7.53 (dd, 1H, J = 8, 6 Hz,
pyr-H5), 7.70 (d, 1H, J = 8 Hz, pyr-H3), 7.86 (t, 2H, J = 8 Hz,
pyr0-H4), 7.99-8.05 (m, 3H, pyr-H5 and pyr0-H5), 8.67 (dd, 1H,
J = 4, 2 Hz, pyr-H6), 10.34 (s, 2H, N-H). The numbering of
protons is shown in Chart 1.
3
3
3
C, 37.00; H, 3.82; N, 8.85. Found: C, 37.29; H, 3.60; N, 8.55.
X-Ray Crystallography on Complex 20. A single crystal of the
chloride salt, [RuCl(L2)]Cl H2O (20), was obtained by recrys-
3
Synthesis of [RuCl(L1)]PF6 (1). To a refluxed deep green
solution of RuCl3 3H2O (22.5 mg, 0.0862 mmol) in ethanol
tallization from CH3OH/THF (see Table 1 for crystallographic
details).13 The crystal was mounted on a glass capillary with
silicon grease and immediately cooled down to 93 K. All
measurements were performed on a Rigaku Mercury CCD
3
(15 mL) was added (Bn2Cat)2-TPA (L1) (80 mg, 0.086 mmol)
as a solid, and the mixture was refluxed for 24 h. To the resultant

3740 Inorganic Chemistry, Vol. 49, No. 8, 2010
Kojima et al.
Table 1. X-Ray Crystallographic Data for [RuCl(L2)]Cl (20)
Scheme 2. Synthesis of L1 and L2
20
formula
fw
C32H30N6O7Cl2Ru
782.60
cryst syst
space group
T, K
monoclinic
P21/c (No. 14)
93
13.1592(2)
16.1463(3)
18.4551(7)
97.0165(8)
3891.8(2)
4
˚
a, A
˚
b, A
˚
c, A
β, deg
3
˚
V, A
Z
no. of reflns measured
7156
5351
441
no. of observations
no. of params refined
R1a
0.078 (I > 2.0σ(I))
0.216 (I > 2.0σ(I))c,d
1.03
Rwb
GOF
P
P
P
a R1 =
Fo| - |Fc /Σ |Fo|. b Rw = [ (w(Fo2 - Fc2)2)/ w(Fo2)2]1/2
.
c w = 1/(σ2(Fo)). d w = 1/[σ2(Fo2) þ (0.1126P)2 þ 9.5888P], where P =
(Max(Fo2, 0) þ 2Fc2)/3 .
diffractometer at 93 K with graphite-monochromated Mo KR
˚
radiation (λ = 0.71070 A) up to 2θmax = 55.0ꢀ.
The structure was solved by direct methods and expanded
using Fourier techniques. All non-hydrogen atoms were refined
anisotropically except two disordered Cl-’s having 0.5 popula-
tions for each. Refinement was carried out with full-matrix least-
squares on F with scattering factors from ref 14 and including
anomalous dispersion effects. All calculations were performed
using the Crystal Structure crystallographic software package,15
and structure refinements were made by using SHELX-97.16
Electrochemical Measurements. Cyclic voltammetry (CV)
and differential pulse voltammetry (DPV) measurements were
performed at 298 K on an ALS electrochemical analyzer in
deaerated MeCN (1) and DMF (2 and 3) containing 0.1 M [(n-
Bu)4N]ClO4 (TBAP) or [(n-Bu)4N]PF6 (TBAPF6) as a support-
ing electrolyte. A conventional three-electrode cell was used
with a platinum working electrode (surface area of 0.3 mm2) and
a platinum wire as the counter electrode. The Pt working
electrode (BAS) was routinely polished with an alumina suspen-
sion and rinsed with acetone before use. The measured poten-
tials were recorded with respect to the Ag/AgNO3 (0.01 M)
reference electrode. All potentials (vs Ag/Agþ) were converted
to values vs SCE by adding 0.29 V.17
observed spectrum. The g values were calibrated with a Mn2þ
marker.
DFT Calculations. Density functional theory (DFT) calcula-
tions on the Ru-Cu complexes were carried out at the
B3LYP18/LANL2DZ level of theory using the Gaussian 03
program.19 The Los Alamos ECP with double-ζ basis for the
Ru and Cu atoms20 and the D95 basis set with standard double-
ζ basis for other atoms21 were used. To look in detail at the weak
π-π interaction between the catechol moieties, second-order
Mφller-Plesset perturbational (MP2)22 calculations were per-
formed for structures optimized at the B3LYP/LANL2DZ level
of theory.
Results and Discussion
Synthesis of Ligands. The synthesis of the ligands was
made in accordance with Scheme 2. In order to introduce
catechol moieties to TPA, we first synthesized a benzyl-
protected catechol derivative, 3,4-bis(benzyloxy)benzoic
acid, that was converted quantitatively to the correspond-
ing acid chloride by using oxalyl chloride. The reaction of
N,N-bis(6-amino-2-pyridylmethyl)-N-(2-pyridylmethyl)-
amine (diamino-TPA) with the acid chloride in the pre-
sence of 4-N,N-dimethylaminopyridine (DMAP) as a
catalyst and triethylamine (NEt3) as a base in THF gave
N,N-bis[6-{3, 4-bis(benzyloxy)-benzamide}-2-pyridylmethyl]-
N-(2-pyridylmethyl)amine ((Bn2Cat)4-TPA, L1) as a
precursor in 50% yield. This compound was deprotected
mildly in the presence of trimethylsilyl iodide to give the
target compound, N,N-bis{6-(3,4-dihydroxybenzamide)-
2-pyridylmethyl}-N-(2-pyridylmethyl)amine ((H2Cat)2-
TPA; L2) in 85% yield.
ESR Measurements. The ESR spectrum of 3 in distilled
MeOH was recorded on a JEOL X-band spectrometer (JES-
RE1XE) with a quartz ESR tube (4.5 mm i.d.) at 77 K. The ESR
spectrum was measured under nonsaturating microwave power
conditions. The magnitude of modulation was chosen to opti-
mize the resolution and the signal-to-noise (S/N) ratio of the
(13) We could not obtain enough amount of the complex to attain its
elemental analysis data; however, the structure determined did not show any
-
discrepancy with the spectroscopic data of the corresponding PF6 salt.
Thus, we concluded that the structure of the PF6- salt should be the same as
the Cl- salt.
(14) Creagh, D. C.; McAuley, W. J. International Tables for Crystallo-
graphy; Wilson, A. J. C., Ed.; Kluwer Academic Pulishers: Boston, 1992; Vol. C,
Table 4.2.6.8, pp 219-222.
(15) CrystalStructure 3.7.0: Crystal Structure Analysis Package; Rigaku
and Rigaku/MSC: The Woodlands, TX, 2000-2005.
(16) Sheldrick, G. M. SIR 97 and SHELX 97, Programs for Crystal
€
€
Structure Refinement; University of Gottingen: Gottingen, Germany, 1997.
(17) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Nonaqueous
Systems; Marcel Dekker: New York, 1990.
(18) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A. D. J. Chem.
Phys. 1993, 98, 5648. (c) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37,
785. (d) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(e) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem.
1994, 98, 11623.
(19) Frisch, M. J. et al. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004. The full list of authors is given in the Supporting
Information.
(20) Dunning, T. H.; Hay, P. J. In Modern Theoretical Chemistry;
Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, p 1.
(21) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(22) Mfller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618.

Article
Inorganic Chemistry, Vol. 49, No. 8, 2010 3741
Figure 1. ORTEP drawings the cation moiety of 20: two isomers A (a) and B (b). (c) Intermolecular hydrogen bonding between two adjacent catechol
moieties for isomer A. Hydrogen atoms are omitted for clarity except the amide N-H hydrogen atoms in A and B.
Synthesis of Ru(II) Complexes of L1 (1) and L2 (2). We
have reported on the synthesis of bisamide-TPA with
various functional groups and their Ru(II) complexes by
“hard” base;23 therefore, the Ru(II) coordination occurs
at the TPA unit preferentially over that at the catechol
moieties. Thus, we could obtain complex 2 in moderate
yield.
using RuCl3 3H2O or [RuCl2(DMSO)4] as a metal
3
source.9,10 On the basis of that, the reaction of L1 with
RuCl3 3H2O in ethanol as reported previously gave a
Crystal Structure of Complex 20. Recrystallization of a
powder of 2 from CH3OH/THF gave a single crystal of an
3
Ru(II) complex of L1, [RuCl(L1)]PF6 (1), in 58% yield.
However, the preparation of a Ru(II) complex of L2 was
not successful with the use of those starting materials
previously used. Thus, we changed the starting material
to a bis-μ-chloro dinuclear precursor complex, [RuCl2(p-
cymene)]2, and the reaction of L2 with the precursor was
performed in methanol at reflux for 24 h. The reaction
proceeded smoothly to give an orange solution, and the
addition of NH4PF6 afforded [RuCl(L2)]PF6 (2) as a
reddish orange precipitate in 56% yield.
unexpected chloride salt, [RuCl(L2)]Cl H2O (20), which
3
was suitable for X-ray crystallography to establish its
molecular structure. ORTEP drawings of the cation
moiety of 20 are depicted in Figure 1 with partial number-
ing scheme. As can be seen in the figure, L2 coordinates to
the Ru(II) center as a pentadentate ligand including an
amide oxygen. We found disorder of the catechol moiety
including O2 and O3 to conclude that the crystal con-
tained two isomers A and B in terms of the direction of the
catechol moiety linked to the coordinated amide arm as
shown in Figure 1a and b, respectively.
In this synthetic reaction, the characteristics of the
Ru(II) ion play an indispensable role for its selective
binding to the TPA moiety rather than the catechol
moieties. The Ru(II) ion favors heteroaromatics due to
stabilization of their coordination by π-back-bonding
from the filled dπ orbitals (t2g) to the π* orbitals of
heteroaromatics in addition to their σ-bonding by σ-
donation of the lone pair(s) to the empty dσ orbital (eg)
of Ru(II). The catechol oxygen is categorized into the
The Ru(II) center possesses an octahedral geometry
and one of the amide oxygen atoms in L2 coordinates to
the Ru(II) ion at the position trans to the tertiary amino
group of L2. The bond length between the chloride anion
(23) (a) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorgic Chemistry, 4th
ed.; HarperCollins College Publisher: New York, 1993; pp 344-355. (b) Pearson,
R. G. J. Am. Chem. Soc. 1963, 85, 3533–3539.

3742 Inorganic Chemistry, Vol. 49, No. 8, 2010
Kojima et al.
0
˚
Table 2. Selected Bond Lengths (A) and Angles (deg) of 2
Scheme 3. Reversible Deprotonation-Protonation Reaction of 1
Ru1-Cl1 2.505(1)
Ru1-O1 2.088(4)
Ru1-N1 2.032(4)
Ru1-N2 2.045(5)
Ru1-N3 1.993(4)
Ru1-N5 2.103(5)
Cl1-Ru1-N1 175.0 (1)
O1-Ru1-N5 103.5(2)
N1-Ru1-N2 82.3(2)
N2-Ru1-N5 86.4(2)
O1-Ru1-N3 90.1(2)
O1-Ru1-N2 175.0(2)
N2-Ru1-N3 86.2(2)
N3-Ru1-N5 166.3(2)
˚
and the Ru(II) ion is 2.505(1) A (see Table 2), which is
longer than those found in other [RuCl(bisamide-TPA)]þ
reported so far. The amide oxygen binds to the Ru(II)
˚
center in the distance of 2.088(4) A, which is a little
shorter than those in other related Ru(II)-bisamide-
TPA complexes.9b,10 The dissymmetry for the bond lengths
LMCT band from Cl- to Ru(II) at 455 nm. Upon the
addition of 1 equiv of NEt3, the orange solution turned
reddish orange and the absorption spectrum changed to
show an increase in the intensity of the absorption of
the LMCT band (see Supporting Information). This
change is similar to that observed for [RuCl((1-Naph)2-
TPA)]PF6,9b but with a smaller magnitude. The original
spectrum was recovered by adding 1 equiv of HClO4.
Thus, this reversible spectral change by adding base and
acid is attributed to reversible deprotonation and pro-
tonation of the coordinated amide moiety as shown in
Scheme 3.
˚
˚
of Ru1-N3 (1.993(4) A) and Ru1-N5 (2.103(5) A), which
are trans to each other, is also typical for the Ru-
(bisamide-TPA) complexes, due to chelation of the amide
moiety to make the N3 atom strongly interact with Ru1 and
steric requirements for the two neighboring functional
groups to hold the appropriate distance.
In each isomer, the two catechol moieties exhibit
intramolecular π-π interaction with interatomic dis-
tances of 3.42 and 3.51 A. The catechol oxygen atoms
˚
of O3 in the isomer A and O2 and O3 in the isomer B form
hydrogen bonds with water molecules of crystallization.
In addition to the hydrogen bonding with water mole-
cules of crystallization, the O2 atom of the catechol
moiety in the isomer A showed intermolecular hydrogen
bonding with each other in the crystal, as shown in
Figure 1c.
Complex 2. The ESI-MS spectrum of 2 in MeOH was
measured to observe a peak cluster assigned to [RuCl(L2)]þ
at m/z = 729.2 with a specific isotopic pattern for a mono-
nuclear ruthenium complex. This mass number is consis-
tent with that calculated for the cation moiety of 2, and the
computer-simulated isotopic pattern of the mass spectrum
shows good agreement with the observed one. The result of
elemental analysis also exhibited good agreement with the
calculated values for 2 4H2O CH3CN.
In addition, the amide N-H of the uncoordinating
amide arm directs to the coordinated amide oxygen in the
coordinating amide arm, forming intramolecular hydro-
˚
gen bonding with the interatomic distance of 2.938(7) A
for O1 N6 and the angle of 148.9(4)ꢀ for O1 H-N6
to stabilize the structure as observed in other related
3
3
The 1H NMR spectrum of 2 was measured in CD3CN.
First of all, the peaks assigned to the methylene protons of
the TPA moiety were observed in the range of 4 to 5 ppm,
and they were split into three AB quartets: 4.34 and 4.53
ppm (JAB = 18 Hz), 4.48 and 4.58 ppm (JAB = 18 Hz),
and 4.75 and 4.90 ppm (JAB = 15 Hz). This is indicative of
the coordination of one of the amide oxygen atoms in L2
to the Ru(II) center in addition to the tetradentate TPA
moiety as well as complex 1 and other structurally
characterized Ru(II) complexes bearing pentadentate
bisamide-TPA ligands. A relatively sharp singlet at
9.81 ppm was assigned to the uncoordinated amide
N-H, which forms intramolecular hydrogen bonding
with the coordinated amide oxygen, and a broad singlet
at 10.05 ppm was ascribed to the N-H in the coordinated
amide linkage, as reported previously.9b,25 Other peaks
can be assigned by using 1H-1H COSY spectroscopy. It
should be noted that the product obtained is only one
species on the bases of NMR measurements, in contrast
to the fact that the two isomeric structures were found in
the crystal as depicted in Figure 1. Two doublet peaks
observed at 6.39 and 6.67 ppm were assigned to those of
the uncoordinated catecholamide moiety, and these sig-
nals exhibited upfield shifts relative to those observed
for L2 by more than 0.4 ppm. This clarifies the fact that
the 5 and 6 positions of the catechol moiety connected to
3 3 3
3 3 3
complexes.9a,b
Spectroscopic Characterization of Ru(II) Complexes.
Complex 1. Complex 1. The ESI-MS spectrum of 1 in
MeCN showed a peak cluster assigned to [RuCl((Bn2-
Cat)2-TPA)]þ (m/z=1089.3), and its isotopic pattern of
the spectrum was consistent with the computer simula-
tion.
1
The H NMR spectrum (300 MHz) of 1 in CD3CN
showed three AB quartets assigned to the methylene
protons at 4.34 and 4.47 ppm (JAB = 18 Hz), 4.64 and
4.95 ppm (JAB = 15 Hz), and 4.65 and 5.14 ppm (JAB =
14 Hz). These indicate that one of the amide oxygen
atoms coordinates to the ruthenium center to hold an
asymmetric structure in MeCN. A doublet at 8.45 ppm
(J = 5 Hz) was assigned to axial pyr-H6, because the
typical coupling constant of the axial pyr-H6 of other
Ru(II) complexes with TPA derivatives was about 5 Hz.24
Assignments for other aromatic protons were not possi-
ble due to severe overlap of signals. Singlet peaks assigned
to the CH2 groups of the benzyl arms were observed at
4.78, 5.03, 5.08, and 5.12 ppm, reflecting the asymmetric
coordination environment.
The UV-visible spectrum of 1 in MeCN showed an
MLCT band from Ru(II) to pyridine at 305 nm and an
(24) Kojima, T.; Amano, Y.; Ishii, M.; Ohba, Y.; Okaue, Y.; Matsuda, Y.
Inorg. Chem. 1998, 37, 4076–4085.
(25) In DMSO-d6, we could observe singlets due to OH groups of the
catechol moieties in the range of 9.36-9.96 ppm.

Article
Inorganic Chemistry, Vol. 49, No. 8, 2010 3743
the deprotonation of the coordinated N-H proton
as observed for other Ru(II)-(bisamide-TPA) complexes
mentioned above. It has been reported that the pKa values
of catechols having amide groups are pKa1 ∼ 8 and
pKa2 ∼ 13.26 The coordination of the amide oxygen
attached to the catechol results in lowering the pKa
values. Together with the fact that the pKa value of the
coordinated amide moiety has been estimated to be ∼6
for [RuCl((1-Naph)2-TPA)]þ ((1-Naph)2-TPA = N,N-
bis{6-(1-naphthoylamide)-2-pyridylmethyl}-N-(2-pyridy-
lmethyl)amine) in a CH3CN/Britton-Robinson buffer,9b
we assume that the coordinated amide N-H proton and
one of the catechol O-H protons are simultaneously
deprotonated at this stage. This suggests that one of the
four catechol O-H protons in 2 should be discriminated
in terms of acidity. The second spectral change was
observed in the course of the addition of 2-3 equiv of
TMAOH in total, showing an isosbestic point at 320 nm
as depicted in Figure 2b. In this step, only one proton was
removed. Another discriminated proton would exist
in 2, and its acidity should be lower than the former.
The final step required two more equivalents (3-5 equiv)
of TMAOH to exhibit no further spectral change with an
isosbestic point at 332 nm as shown in Figure 2c. At this
stage, the absorption at 369 nm gained further intensity.
This step can be attributed to the deprotonation of the
least acidic protons of catechols, that is, a chelated proton
between the two oxygen atoms. On the basis of these
observations, we propose an intramolecular hydrogen
bonding network between the two converged catechols
in 2 as depicted in Scheme 4.
Complexation of 2 with a Cu(II)-Diamine Unit. The
complexation capability of the preorganized catechols of
2 was examined by using a Cu(II)-diamine unit: In the
Irwing-Williams series, Cu(II) has been demonstrated to
form the most stable complexes with a bidentate chelating
ligand to afford a five-membered chelate ring.27 Thus,
we chose [CuII(NO3)2(TMEDA)] (TMEDA = N,N,N0
N0-tetramethylethylenediamine)11 as a precursor, having
two coordinatively labile sites to facilitate the catechol
binding. No reaction of 2 with the precursor in MeOH
occurred in the absence of a base. Upon the addition of
NEt3 into the solution, the reaction proceeded smoothly
with a color change to exhibit a chestnut color in the
solution. The addition of TBAPF6 gave a dark brown
powder of a new species, 3.
Figure 2. Absorption spectral change of 2 in the course of the addition
of TMAOH in DMF: (a) 0-2 equiv; (b) 2-3 equiv; (c) 3-5 equiv. Arrows
indicate the directions of absorbance changes.
the uncoordinated amide linkage experience the ring
current of the counterpart, indicating the existence of
the intramolecular π-π interaction between the two
catechol moieties in 2. This situation holds for both
isomer A and isomer B, as can be seen in Figure 1,
precluding a definitive standpoint to nail down the species
observed in the NMR spectrum in CD3CN.
MP2 calculations were performed to evaluate the en-
ergy difference between the two isomers A and B opti-
mized at the B3LYP/LANL2DZ level of theory (see the
Supporting Information). Isomer B was calculated to be
more stable than isomer A by 1.9 kcal/mol. This energy
difference corresponds to the population ratio of A/B
being 4:96, suggesting that the species observed in the
NMR spectrum could be isomer B. The interatomic
distances of the catechol moieties in the optimized struc-
ture of isomer B were slightly shorter than those of isomer
A. Such stronger intramolecular π-π interaction of the
catechol moieties can be a major factor of stabilization for
isomer B compared to isomer A.
In the absorption spectrum of 2 in MeOH, an LMCT
band, which was assigned to that from Cl- to Ru(II)
as a main factor, was observed at 442 nm. A similar
LMCT band was observed for [RuIICl(bisamide-TPA)]þ.
Upon the addition of tetramethylammonium hydroxide
(TMAOH) to the DMF solution of 2, we could observe
three-step spectral change. In the course of the addition of
0-2 equiv of TMAOH, the spectrum changed, exhibiting
an isosbestic point at 318 nm as depicted in Figure 2a. The
LMCT band exhibited a slight red shift to give the
absorption maximum at 458 nm, and a new absorption
emerged at 369 nm. This spectral change accompany-
ing the red shift of the LMCT band can be ascribed to
The new species showed the absorption maxima at 276
and 360 nm. This complex 3 was submitted to ESI-MS
measurements and was shown to exhibit a peak cluster
at m/z = 906.1, which was assigned to [RuCl((HCat)2-
TPA){Cu(TMEDA)}]þ.28 The computer simulation gave
rise to an excellent agreement with the isotopic pattern
and mass number, as shown in Figure 3. On the basis of
this result, this complex is a monocation, including only
one Cu(II)-TMEDA unit in spite of the fact that 2
contains two distinct catechol moieties.
The titration was performed by adding [Cu(NO3)2-
(TMEDA)] to the solution of 2 in MeOH to determine
(26) (a) Arnould, J. C.; Bertrandie, A.; Bird, T. G. C.; Boucherot, D.;
Jung, F.; Lohmann, J. J.; Olivier, A. J. Med. Chem. 1992, 35, 2631–2642.
(b) Paulini, R.; Lerner, C.; Diederich, F.; Jakob-Roetne, R.; Zu€rcher, G.; Borroni,
E. Helv. Chim. Acta 2006, 89, 1856–1887. (c) El Hage Chahine, J.-M.; Bauer,
A.-M.; Baraldo, K.; Lion, C.; Ramiandrasoa, F.; Kunesch, G. Eur. J. Inorg.
Chem. 2001, 2287–2296.
(27) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry,
4th ed.; HarperCollins College Publisher: New York, 1993; pp 348-349.
(28) When NH4PF6 was used in place of TBAPF6 in the purification
process, a protonated form of 3, [RuCl((HCat, H2Cat)-TPA){Cu(TMEDA)}]-
(PF6)2, was obtained. See the Experimental Section.

3744 Inorganic Chemistry, Vol. 49, No. 8, 2010
Scheme 4. Proposed Deprotonation Sequence for 2
Kojima et al.
Figure 3. ESI-MS spectrum of 3: (a) observed, (b) simulated isotopic
pattern.
Figure 5. ESR spectra of 3 (a) and [Cu(catecholato)(TMEDA)] (b) in
MeOH at 77 K. Conditions: Microwave power, 1 mW; 100 kHz modula-
tion amplitude, 2 mT; frequency, 9.203 GHz (a) and 9.208 GHz (b).
[Cu(catecholato)(TMEDA)]29 (CuII-catecholate) was
synthesized to measure its ESR spectrum (Figure 5b). In
the complex 3, the g|| and g^ values are 2.239 and 2.060, res-
pectively, with a hyperfine coupling constant for Cu(II) of
A||=184 G. In the CuII-catecholate, the g|| and g^ values
are 2.241 and 2.071, respectively, and the hyperfine cou-
pling constant A|| is 184 G as shown in Figure 5b. In both
cases, the ESR signals of Cu(II) centers indicate that the
Cu(II) centers in both complexes are with a monomeric
tetragonal geometry in the (dx2-y2)1 ground state.30 These
observations indicate that the Cu(II) center in 3 is in a
different environment from that of the Cu-catecholate and
support the unique coordination described in Scheme 5.
DFT calculations were performed for the structure
optimization of 3. The optimized structure is depicted in
Figure 6. This structure is more stable by -3.5 kcal/mol in
terms of the heat of formation than that including the O,
Figure 4. Absorption spectral change of 2 in MeOH in the course of the
addition of 1 equiv of [Cu(NO3)2(TMEDA)] in the presence of an excess
amount of NEt3. Red line, 2; black lines, 0.2-0.8 equiv addition of
[Cu(NO3)2(TMEDA)]; blue line, 1 equiv of [Cu(NO3)2(TMEDA)].
the number of Cu(II)-TMEDA units to be allowed to
bind to the catechol moieties in 2. We could observe
drastic spectral change in the course of the addition of
1 equiv of the Cu(II) complex, as shown in Figure 4;
however, no additional change was observed in its further
addition. Thus, we concluded that the convergent cate-
chol moieties could accept only one Cu(II) unit. This
result should be relevant to that in the titration of 2 with
TMAOH: After one proton is removed, another discri-
minated proton is deprotonated, as depicted in Scheme 4.
The ESR spectrum of 3 (1.0 ꢀ 10-5 M) in MeOH solu-
tion at 77 K is shown in Figure 5a. As a comparison,
(30) (a) Wojciechowski, K.; Bitner, A.; Bernardinelli, G.; Brynda, M.
Dalton Trans. 2009, 1114–1122. (b) Nielsen, P.; Toftlund, H.; Boas, J. F.;
Pilbrow, J. R.; Moubaraki, B.; Murray, K. S.; Neville, S. M. Inorg. Chim. Acta
2008, 361, 3453–3461.
(29) Kodera, M.; Kawata, T.; Kano, K.; Tachi, Y.; Itoh, S.; Kojo, S. Bull.
Chem. Soc. Jpn. 2003, 76, 1957–1964.

Article
Inorganic Chemistry, Vol. 49, No. 8, 2010 3745
Scheme 5. Complexation of the Cu(II)-TMEDA Unit with 2
moieties (2), there is intramolecular π-π interaction between
the two catechol moieties due to their convergence by virtue
of the preorganization of those functional groups. This
convergence could be achieved by a template effect of the
selective coordination of Ru(II) to the TPA moiety of
the ligand. In addition to the intramolecular π-π interaction,
the catechol moieties have been suggested to form a plausible
intramolecular hydrogen bonding network, which gives rise
to the selective formation of one isomer out of four possible
isomers in terms of orientation of the two catechol moieties.
The converged catechol moieties on the surface of the
Ru(II) complex can act as a metal-complex ligand. We could
confirm the coordination of a copper-diamine unit to the
catechol site. The number of Cu(II) allowed to bind is
restricted to only one, even though two chelatable catechols
should be ready to form stable five-membered chelate ring
structures. Thus, we propose that the coordination of the
Cu(II)-diamine unit to the converged moieties is in a novel
coordination mode, such as a bridging mode between the two
catechol moieties to form a unique binuclear Ru(II)-Cu(II)
complex with multiredox centers and proton-transfer sites.
This complex can provide a new frontier and a new basis of
development of novel electronic structures of multinuclear
transition metal complexes to afford their interesting proper-
ties and reactivities.
Figure 6. DFT-optimized structure of 3 at the B3LYP/LANL2DZ level
of theory. Hydrogen atoms are omitted for clarity.
O0-bidentate chelation of the catechol moiety with the
coordinated amide linkage to the Cu(II)-TMEDA unit
(see Supporting Information). In this structure, the cate-
chol moieties are tilted from the coordinated amide-
pyridine plane to bind the square-planar Cu(II)-TMEDA
fragment. The bond lengths of Cu(II)-O(cat) were
˚
calculated to be 1.933 and 1.968 A, and the bond angle
of O-Cu-O was 92.0ꢀ. Those bond lengths are fairly
longer than typical values for Cu(II)-O (terminal
aryloxy) bonds in a four-coordinate Cu(II) complex.31
The long Cu-O bond lengths are probably derived from
significant distortion of the catechol-amide-pyridine
skeletons, as mentioned above, to accommodate the
square planar coordination environment of the Cu(II)
center.
Acknowledgment. This work was supported by Grants-
in-Aid (No. 16550057, 19205019, 20108010) from Japan
Promotion of Science and Technology (JSPS), a Global
COE program, “The Global Education and Research
Center for Bio-Environmental Chemistry”, from MEXT,
Japan, and by KOSEF/MEST through the WCU project
(R31-2008-000-10010-0). S.M. appreciates the support
from a JSPS postdoctoral fellowship (no. 10727).
Summary
Supporting Information Available: Crystallographic data of 20
in the cif format, UV-vis spectral change of 1 with the addition
of NEt3 and HClO4, CV change of 1 upon the addition of NEt3,
DFT-optimized structures of the isomers A and B in Figure 1,
another possible Cu(II)-TMEDA adduct, and 1H NMR spectra
of 1 and 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
We have described the synthesis and characterization of
Ru-TPA complexes bearing catechol moieties connected by
amide linkages. In the ruthenium complex with catechol
(31) Guy Orpen, A.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,
D. G.; Taylor, R. J. Chem. Soc., Dalton Tans. 1989, S1–S83.