Regioselective C-H activation of cyclometalated bis-tridentate ruthenium complexes

Article abstract of DOI:10.1021/om200784j

A series of bis-tridentate Ru(II) complexes consisting of trimethyl-4,4′,4″-tricarboxylate-2,2′:6′, 2″-terpyridine (Me₃tctpy) and derivatized 6-phenyl-2,2′- bipyridine (pbpy) ligands are reported. Each complex is attached to a terminal tripheny

Full text of DOI:10.1021/om200784j

Article
pubs.acs.org/Organometallics
Regioselective C−H Activation of Cyclometalated Bis-Tridentate
Ruthenium Complexes
Stacy S. R. Muise,† Holly A. Severin,† Bryan D. Koivisto,† Kiyoshi C. D. Robson,† Eduardo Schott,†,‡
and Curtis P. Berlinguette*,†
†Department of Chemistry, The Institute for Sustainable Energy, Environment and Economy, and Centre for Advanced Solar
Materials, University of Calgary, 2500 University Drive N.W., Calgary, Canada T2N 1N4
‡
́
Departamento de Ciencias Quimicas, Universidad Andres Bello, Republica 275, Santiago, Chile
́
S
* Supporting Information
ABSTRACT: A series of bis-tridentate Ru(II) complexes consisting of trimethyl-4,4′,4″-
tricarboxylate-2,2′:6′,2″-terpyridine (Me3tctpy) and derivatized 6-phenyl-2,2′-bipyridine
(pbpy) ligands are reported. Each complex is attached to a terminal triphenylamine (TPA)
substituent at the central ring of pbpy through a thiophene bridge to benefit light absorption,
while the anionic ring of pbpy is functionalized with substituents to modulate the metal-based redox potential. The
cyclometalation step was found to favor the isomer where the electron-donating groups (EDGs; i.e., −OEt, −SEt) are situated
ortho to the organometallic bond rather than the sterically favored para position, while the para isomer is formed in exclusivity
when electron-withdrawing groups (e.g., −CF3) are installed on the anionic ring. Moreover, the distribution of the isomeric
products is affected by the identity of the chalcogen: ortho:para = 1:0 and 3:1 where EDG = −OEt and −SEt, respectively.
Because our molecular scaffold rules out certain cyclometalation pathways (e.g., oxidative addition, agostic interactions, σ-bond
metathesis), we are able to experimentally establish that the observed regioselectivity is in accordance with an electrophilic
metalation where the relative stabilities of the products and carbanionic intermediates govern the ratio of the isomers formed.
not without precedent,16,17 we set out to better understand the
mechanistic details of this regioselectivity by modifying our
bichromic scaffold with bulky chalcogen-containing substituents
about the anionic ring (Chart 1; derivatives where −R1 is
INTRODUCTION
■
Ruthenium(II) coordination complexes bearing a single
cyclometalating ligand [e.g., 6-phenyl-2,2′-bipyridine (pbpy);
2-phenylpyridine (ppy)] have received a lot of recent attention
owing to their ability to generate high power conversion
efficiencies in the dye-sensitized solar cell (DSSC).1−13 Indeed,
a
Chart 1. Designation of Compounds
Gratzel et al.2 and our program11 have demonstrated that cell
̈
efficiencies in excess of 8% can be reached by Ru(II) complexes
bearing bidentate and tridentate cyclometalating ligands,
respectively. An important aspect of this class of dyestuffs is
that the character of the highest occupied molecular orbital
(HOMO) is often localized to the anionic ring of the
cyclometalated ligand and the metal.5 Chemical modification
of this anionic ring therefore enables acute control of the metal-
based oxidation potential, thus offering an additional handle for
optimizing DSSC chromophores.14
When designing bis-tridentate Ru(II) complexes for light-
harvesting applications, additional chromophoric units can
compensate for the narrow absorption spectra and modest
extinction coefficients characteristic of these compounds.4,15
On this basis, we recently developed a bichromic manifold that
couples a triphenylamine (TPA) group to the cyclometalated
Ru core where the oxidation potential of each chromophore
can be modified by the judicious placement of electron-
donating groups (EDGs) and electron-withdrawing groups
(EWGs).12 Our studies involving the modification of the
anionic ring of the pbpy chelate revealed a curious trend:
EWGs (e.g., −CF3) were positioned para to the organometallic
bond of the cyclometalated product, whereas EDGs (e.g.,
−OMe) favored the ortho position.12 While this phenomenon is
a
Counterion = NO3− for 1 and 2 and HCO3− for 3 and 4. Complexes
5−7 were previously reported and are included for reference
purposes.12
substituted with methoxy groups and −R3 with EWGs are also
reported to aid in the characterization of the different isomers.)
These alkoxy and thiolate substituents were selected to examine
how electronic and steric interactions affect the frontier orbitals
Received: August 21, 2011
Published: December 2, 2011
© 2011 American Chemical Society
6628
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Organometallics
Article
132.2, 129.4, 128.9, 125.7; HRMS (EI) m/z = 152.0291 [(M)+] (calcd
for C8H8OS+ m/z = 152.0296).
and the cyclometalation pathway, and also because chalcogens
are common constituents in DSSC dyes18 (it has been
postulated that highly polarizable chalcogens can enhance the
interaction with the electrolyte used in high-performance
DSSCs19,20). This article unravels the underlying factors that
dictate the ortho-assisted C−H activation step to aid in our
ability to chemically control the frontier molecular orbitals of
these complexes.
1-(3-(Ethylthio)phenyl)ethanone (P2). To a solution of P1 (645
mg, 4.24 mmol) in EtOH (50 mL) was added potassium carbonate
(594 mg, 4.24 mmol) and excess ethyl bromide (2 mL). The mixture
was heated slightly below reflux overnight. The solvent was removed
in vacuo, and the residue was purified by column chromatography
[SiO2: CH2Cl2; Rf = 0.53] to yield 0.77 g of the product as a pale
yellow oil in quantitative yield. 1H NMR (CDCl3): δ 7.82 (s, 1H, Hr),
3
3
7.66 (d, 1H, J = 7.7 Hz, Hn), 7.42 (d, 1H, J = 7.8 Hz, Hp), 7.30 (t,
3
3
EXPERIMENTAL SECTION
1H, J = 7.7 Hz, Ho), 2.92 (q, 2H, J = 7.4 Hz, −SCH2CH3), 2.52 (s,
3H, Hm), 1.26 (t, 3H, 3J = 7.4 Hz, −SCH2CH3). 13C NMR (CDCl3): δ
197.7, 138.0, 137.7, 133.0, 129.0, 128.0, 125.7, 27.4, 26.7, 14.3. HRMS
(EI): m/z 180.0615 [(M)+] (calcd for C10H12OS+ m/z = 180.0609).
1-(2-(3-Ethoxyphenyl)-2-oxoethyl)pyridinium Iodide (P4). To a
flask containing 1-(3-ethoxyphenyl)ethanone (3.44 g, 21.0 mmol) in
pyridine (20 mL) was added iodine (6.38 g, 25.1 mmol). The reaction
mixture was then stirred at 100 °C for 2.5 h. After cooling the reaction
mixture to room temperature, the brown precipitate was isolated by
vacuum filtration, washed with Et2O, and air-dried. The solid was
recrystallized from hot EtOH to yield 4.83 g (62.4%) of the product as
a tan solid. 1H NMR (MeOH): δ 8.93 (d, 2H, 3J = 6.7 Hz, Hs), 8.73 (t,
1H, 3J = 7.9 Hz, Hu), 8.22 (t, 2H, 3J = 6.8 Hz, Ht), 7.70 (d, 1H, 3J = 7.7
■
Preparation of Compounds. All reagents were purchased from
Aldrich and used without further purification except for RuCl3·3H2O
(Pressure Chemical Company) and trimethyl-4,4′,4″-tricarboxylate-
2,2′:6′,2″-terpyridine (L5; Helio Chemical Company, Switzerland).
Purification by column chromatography was carried out using silica
(Silicycle: Ultrapure Flash Silica). Analytical thin-layer chromatog-
raphy (TLC) was performed on aluminum-backed sheets precoated
with silica 60 F254 adsorbent (0.25 mm thick; Merck, Germany) and
1
visualized under UV light. Routine H and 13C NMR spectra were
recorded at 400 and 100 MHz, respectively, on a Bruker AV 400
instrument at ambient temperature. Chemical shifts (δ) are reported in
parts per million (ppm) from low to high field and referenced to
residual nondeuterated solvent. Standard abbreviations indicating
multiplicity are used as follows: s = singlet; d = doublet; t = triplet;
m = multiplet. All proton assignments correspond to the generic
molecular schemes that are provided (Figure 1). Organic precursors
3
Hz, Hn), 7.59 (s, 1H, Hr), 7.53 (t, 1H, J = 8.0 Hz, Ho), 7.31 (d, 1H,
3J = 8.1 Hz, Hp), 6.43 (s, 2H, Hm), 4.14 (q, 2H, 3J = 7.0 Hz,
−OCH2CH3), 1.43 (t, 3H, 3J = 7.0 Hz, −OCH2CH3). 13C NMR
(MeOH): δ 191.2, 161.1, 147.9, 147.8, 136.3, 131.6, 129.3, 129.2,
122.7, 121.9, 114.7, 65.2, 15.2. HRMS (ESI): m/z 242.1175 [(M)+]
+
(calcd for C15H16NO2 m/z = 242.1176).
1-(2-(3-(Ethylthio)phenyl)-2-oxoethyl)pyridinium Iodide (P5). To
a solution of P2 (2.12 g, 11.7 mmol) in pyridine (15 mL) was added
iodine (3.59 g, 14.1 mmol). The mixture was heated to 100 °C under
N2 for 2.5 h. After the reaction mixture was cooled to room
temperature, a brown-tan precipitate was collected by vacuum
filtration, washed with Et2O, and air-dried. The resulting golden
brown solid was triturated with EtOH to yield 3.70 g (81.8%) of the
1
3
product as an off-white solid. H NMR (CDCl3): δ 8.99 (d, 2H, J =
6.4 Hz, Hs), 8.75 (t, 1H, 3J = 7.9 Hz, Hu), 8.29 (t, 2H, 3J = 6.9 Hz, Ht),
3
3
7.91 (s, 1H, Hr), 7.85 (d, 1H, J = 7.3 Hz, Hn), 7.73 (d, 1H, J = 7.8
3
Hz, Hp), 7.61 (t, 1H, J = 7.7 Hz, Ho), 6.50 (s, 2H, Hm), 3.10 (q, 2H,
3J = 7.4 Hz, −SCH2CH3), 1.27 (t, 3H, J = 7.3 Hz, −SCH2CH3). 13C
3
NMR (CDCl3): δ 190.4, 146.4, 146.2, 138.0, 134.2, 133.4, 129.8,
127.9, 126.9, 125.2, 66.3, 26.0, 14.0. HRMS (ESI): m/z 258.0941
[(M)+] (calcd for C15H16NOS+ m/z 258.0947).
4-(5-Bromothiophen-2-yl)-6-(3-ethoxyphenyl)-2,2′-bipyridine
(P6). P4 (2.79 g, 7.10 mmol), P3 (2.09 g, 7.10 mmol), ammonium
acetate (14.2 g, 184 mmol), and formamide (25 mL) were stirred at
120 °C under N2 for 18 h. After the solution was cooled to room
temperature, the precipitate was isolated by vacuum filtration and
further purified by column chromatography [SiO2: CH2Cl2/EtOAc,
9:1; Rf = 0.76]. Recrystallization from hot absolute EtOH yielded 2.06
g (62.9%) of the product as a light brown powder. 1H NMR (CDCl3):
δ 8.69 (d, 1H, 3J = 4.7 Hz, Ha), 8.61 (d, 1H, 3J = 8.0 Hz, Hd), 8.50 (d,
1
Figure 1. Generic labeling scheme for H NMR signal assignments.
1-(3-mercaptophenyl)ethanone21 (P1), (E)-3-(5-bromothiophen-2-yl)-1-pre-
cursors (pyridin-2-yl)prop-2-en-1-one22 (P3), N,N-diphenyl-4-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)aniline23 (P8), and 4-methoxy-N-(4-methoxy-
phenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline24
(P9), N,N-diphenyl-4-(5-(6-(3-(trifluoromethyl)phenyl)-2,2'-bipyridin-4-yl)-
thiophen-2-yl)aniline (L6),12 4-methoxy-N-(4-methoxyphenyl)-N-(4-(5-(6-
(3-(trifluoromethyl)phenyl)-2,2'-bipyridin-4-yl)thiophen-2-yl)phenyl)aniline
(L7),12 N,N-diphenyl-4-(5-(6-phenyl-2,2'-bipyridin-4-yl)thiophen-2-yl)aniline
(L8),11 and complexes 5,11 6,11 and 711 were prepared as previously reported.
1-(3-Mercaptophenyl)ethanone (P1). The following is a mod-
ification of a previously reported procedure (no characterization data
were presented).21 To a solution of 3-acetylbenzenesulfonyl chloride
(2.50 g, 11.4 mmol) in anhydrous toluene (30 mL) was slowly added
Ph3P (9.00 g, 34.3 mmol) under a N2 atmosphere. Water (10 mL) was
added to the mixture, which was then stirred for 10 min before an
extraction step with 10% NaOH (2 × 25 mL). The alkaline aqueous
extract was successively washed with toluene (2 × 20 mL), acidified
with 1 M HCl (60 mL), and extracted with CH2Cl2 (3 × 30 mL). The
organic extract was dried with MgSO4, and the solvent was removed
in vacuo to yield 1.09 g (62.5%) of the product as a pale yellow oil.
3
1H, 4J = 1.5 Hz, He), 7.84 (dt, 1H, J = 7.7, 4J = 1.8 Hz, Hc), 7.78 (d,
4
4
3
1H, J = 1.6 Hz, Hm), 7.72 (t, 1H, J = 1.9 Hz, Hq), 7.68 (d, 1H, J =
3
3
7.9 Hz, Hn), 7.42 (d, 1H, J = 4.0 Hz, Hf), 7.40 (t, 1H, J = 7.9 Hz,
Ho), 7.33 (dt, 1H, 3J = 4.8, 4J = 1.0 Hz, Hb), 7.10 (d, 1H, 3J = 3.9 Hz,
Hg), 6.98 (dd, 1H, 3J = 8.1, 4J = 2.4 Hz, Hp), 4.14 (q, 2H, 3J = 7.0 Hz,
−OCH2CH3), 1.47 (t, 3H, 3J = 7.0 Hz, −OCH2CH3). 13C NMR
(CDCl3): δ 159.7, 157.5, 156.7, 156.1, 149.3, 143.4, 142.5, 140.7,
137.1, 131.4, 130.0, 126.0, 124.2, 121.7, 119.5, 116.5, 115.5, 115.4,
114.3, 113.7, 63.8, 15.1. HRMS (EI): m/z 438.0236 [(M)+] (calcd for
C22H17N2OS81Br+ m/z 438.0225).
4-(5-Bromothiophen-2-yl)-6-(3-(ethylthio)phenyl)-2,2′-bipyridine
(P7). A mixture of P3 (2.50 g, 8.50 mmol), P5 (3.27 g, 8.50 mmol),
and ammonium acetate (17.11 g, 221 mmol) in formamide (25 mL)
was stirred and heated at 120 °C under N2 overnight. After cooling the
reaction mixture to room temperature the solid was filtered to produce
a waxy solid. The solid was dissolved in CH2Cl2 (150 mL), and
the resulting solution was washed with water (2 × 50 mL) then brine
1H NMR (CDCl3) δ 7.80 (s, 1H, Hr), 7.66 (d, 1H, J = 7.8 Hz, Hn),
3
3
3
7.39 (d, 1H, J = 7.8 Hz, Hp), 7.27 (t, 1H, J = 7.8, Ho), 3.55 (s, 1H,
H
SH), 2.52 (s, 3H, Hm); 13C NMR (CDCl3) δ 197.5, 137.9, 133.6,
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dx.doi.org/10.1021/om200784j|Organometallics 2011, 30, 6628−6635

Organometallics
Article
(2 × 50 mL), dried with MgSO4, and dried in vacuo to yield a tan
powder. Purification by column chromatography using a CH2Cl2/
EtOAc gradient [SiO2: CH2Cl2/EtOAc, 9:1; Rf = 0.93] and
recrystallization in EtOH yielded 2.19 g (56.8%) of the product as
an off-white solid. 1H NMR (CDCl3): δ 8.66 (d, 1H, 3J = 4.1 Hz, Ha),
mmol) and P8 (500 mg, 1.35 mmol) in THF/water (9:1; 125 mL)
was sparged with N2 for 10 min, K2CO3 (933 mg, 6.73 mmol) and
Pd(PPh3)4 (109 mg, 0.09 mmol) were added, and the reaction was left
to reflux for 14 h under N2. The reaction mixture was then cooled and
poured into H2O. The product was extracted with Et2O (3 × 75 mL)
and washed with brine (3 × 100 mL). The organic layer was dried with
MgSO4 prior to the removal of the solvent in vacuo. The residual
brown oil was purified by column chromatography [SiO2: CH2Cl2/
EtOAc (9:1); Rf = 0.94] to yield 702 mg (93.8%) of the product as a
bright yellow solid. 1H NMR (CDCl3): δ 8.72 (d, 1H, 3J = 4.8 Hz, Ha),
3
4
8.55 (d, 1H, J = 8.0 Hz, Hd), 8.45 (d, 1H, J = 1.2 Hz, He), 8.09 (s,
3
4
1H, Hr), 7.87 (m, 1H, Hn), 7.81 (dt, 1H, J = 7.6, J = 1.5 Hz, Hc),
4
7.69 (d, 1H, J = 1.2 Hz, Hm), 7.40−7.36 (m, 2H, Hp, Ho), 7.33 (d,
1H, 3J = 3.9 Hz, Hf), 7.29 (t, 1H, 3J = 5.9 Hz, Hb), 7.05 (d, 1H, 3J = 3.8
3
3
Hz, Hg), 3.02 (q, 2H, J = 7.3 Hz, −SCH2CH3), 1.36 (t, 3H, J = 7.3
Hz, −SCH2CH3). 13C NMR (CDCl3): δ 156.8, 156.6, 155.8, 149.1,
143.2, 142.3, 139.7, 137.4, 137.0, 131.3, 129.7, 129.3, 127.8, 125.9,
124.6, 124.1, 121.5, 116.2, 115.4, 114.3, 27.9, 14.5. HRMS (EI): m/z
454.0005 [(M)+] (calcd for C22H17BrN2S2 m/z 453.9996).
3
4
8.63 (d, 1H, J = 8.0 Hz, Hd), 8.61 (d, 1H, J = 1.5 Hz, He), 8.17 (s,
1H, Hn), 7.96 (dt, 1H, 3J = 6.6, 4J = 2.2 Hz, Hr), 7.89 (d, 1H, 4J = 1.5
Hz, Hm), 7.85 (dt, 1H, 3J = 7.7, 4J = 1.8 Hz, Hc), 7.64 (d, 1H, 3J = 3.8
3
Hz, Hf), 7.52 (d, 2H, J = 8.7 Hz, Hh), 7.49−7.41 (m, 2H, Hp, Hq),
7.33 (ddd, 1H, 3J = 7.5, 3J = 4.8, 4J = 1.1 Hz, Hb), 7.28 (t, 4H, 3J = 7.9
4-(5-(6-(3-Ethoxyphenyl)-2,2′-bipyridin-4-yl)thiophen-2-yl)-N,N-
diphenylaniline (L1H). P6 (498 mg, 1.14 mmol) and N,N-diphenyl-4-
(4,4,5,5,-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (P8) (470 mg,
1.27 mmol) were solubilized in a THF/H2O (9:1, 125 mL) solution
and sparged with N2 for 10 min. To this solution were then added
K2CO3 (875 mg, 6.33 mmol) and Pd(PPh3)4 (102 mg, 0.09 mmol),
and the reaction mixture was set to reflux for 14 h under N2. The
reaction mixture was cooled to room temperature and then poured into
water. The product was extracted with Et2O and washed with brine.
Organic fractions were combined and dried with MgSO4, filtered, and
concentrated by removing the solvent in vacuo. The product was
purified by column chromatography [SiO2: CH2Cl2/EtOAc, 19:1; Rf =
3
3
Hz, Hk), 7.26 (d, 1H, J = 4.3 Hz, Hg), 7.14 (d, 4H, J = 7.5 Hz, Hj),
7.09 (d, 2H, 3J = 8.6 Hz, Hi), 7.06 (t, 2H, 3J = 7.4 Hz, Hl), 3.05 (q, 2H,
3J = 7.3 Hz, −SCH2CH3), 1.38 (t, 3H, J = 7.3 Hz, −SCH2CH3); 13C
3
NMR (CDCl3) δ 156.9, 156.5, 156.2, 149.2, 148.0, 147.5, 146.2, 143.4,
140.2, 139.8, 137.4, 137.1, 129.8, 129.6, 129.4, 128.0, 127.8, 126.9,
126.8, 125.0, 124.9, 124.8, 124.1, 123.5, 123.4, 121.7, 116.4, 115.5,
28.1, 14.6. HRMS (EI): m/z 617.1951 [(M)+] (calcd for C40H31N3S2
+
m/z 617.1959).
4-(5-(6-(3-(Ethylthio)phenyl)-2,2′-bipyridin-4-yl)thiophen-2-yl)-
N,N-bis(4-methoxyphenyl)aniline (L4H). P7 (475 mg, 1.05 mmol)
and P9 (502 mg, 1.16 mmol) were solubilized in a THF/water
solution (125 mL, 9:1 v/v) and sparged with N2 for 10 min. Following
the addition of K2CO3 (801 mg, 5.80 mmol) and Pd(PPh3)4 (102 mg,
0.09 mmol), the reaction mixture was left to reflux overnight under an
inert atmosphere. After the reaction mixture was cooled to room
temperature it was poured into H2O. The product was extracted with
Et2O (2 × 100 mL) and washed with brine (2 × 100 mL). The organic
fraction was dried with MgSO4 prior to the removal of the solvent
in vacuo. The resulting oil was purified by column chromatography
[SiO2: CH2Cl2/EtOAc (9:1); Rf = 0.91] to yield 335 mg (47.4%) of
the product as a bright yellow solid. 1H NMR (CDCl3): δ 8.70 (d, 1H,
1
0.49] to yield 468 mg (68.3%) of the product as a yellow solid. H
NMR (CDCl3): δ 8.72 (dd, 1H, 3J = 4.7, 4J = 0.8 Hz, Ha), 8.64 (d, 1H,
3J = 8.0 Hz, Hd), 8.60 (d, 1H, 4J = 1.6 Hz, He), 7.89 (d, 1H, 4J = 1.6 Hz,
3
4
4
Hm), 7.84 (td, 1H, J = 7.6, J = 1.8 Hz, Hc), 7.76 (t, 1H, J = 1.4 Hz,
3
3
Hr), 7.73 (d, 1H, J = 7.8 Hz, Hn), 7.63 (d, 1H, J = 3.9 Hz, Hf), 7.51
(d, 2H, 3J = 8.7 Hz, Hh), 7.42 (t, 1H, 3J = 7.9 Hz, Ho), 7.32 (ddd, 1H,
3J = 7.6 Hz, 3J = 4.8 Hz, 4J = 1.8 Hz, Hb), 7.30−7.25 (m, 5H, Hk, Hg),
7.13 (d, 4H, 3J = 8.3 Hz, Hj), 7.10−7.03 (m, 4H, Hi, Hl), 6.99 (dd, 1H,
3J = 7.5, 4J = 1.8 Hz, Hp), 4.16 (q, 2H, 3J = 7.0 Hz, −OCH2CH3), 1.48
(t, 3H, 3J = 7.0 Hz, −OCH2CH3). 13C NMR (CDCl3): δ 159.6, 157.2,
156.4, 156.3 149.2, 148.0, 147.5, 146.1, 143.3, 141.0, 139.9. 137.0,
129.9, 129.6, 127.9, 126.8, 126.7, 124.9, 124.1, 123.5, 123.5, 123.4,
121.7, 119.6, 116.5, 115.4, 115.2, 113.7, 63.8, 15.1. HRMS (EI): m/z
601.2162 [(M)+] (calcd for C40H31N3OS+ m/z 601.2188).
3
3J = 4.3 Hz, Ha), 8.62 (d, 1H, J = 8.0 Hz, Hd), 8.59 (s, 1H, He), 8.16
(s, 1H, Hr), 7.94 (m, 1H, Hn), 7.86 (s, 1H, Hm), 7.82 (dt, 1H, 3J = 7.8
Hz, 4J = 1.6 Hz, Hc), 7.59 (d, 1H, 3J = 3.8 Hz, Hf), 7.46−7.38 (m, 4H,
3
3
Hh, Hp, Ho), 7.31 (t, 1H, J = 6.7 Hz, Hb), 7.20 (d, 1H, J = 3.8 Hz,
3
3
Hg), 7.08 (d, 4H, J = 8.9 Hz, Hj), 6.92 (d, 2H, J = 8.0 Hz, Hi), 6.85
4-(5-(6-(3-Ethoxyphenyl)-2,2′-bipyridin-4-yl)thiophen-2-yl)-N,N-
bis(4-methoyphenyl)aniline (L2H). 4-Methoxy-N-(4-methoxyphen-
yl)-N-(4-(4,4,5,5,-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline
(P9) (510 mg, 1.18 mmol) and P6 (465 mg, 1.06 mmol) were
solubilized in a 9:1 THF/H2O (125 mL) solution and degassed for 10
min by sparging with N2. K2CO3 (820 mg, 5.91 mmol) and Pd(PPh3)4
(100 mg, 0.09 mmol) were then added, and the reaction mixture was
refluxed under N2 overnight. The reaction was then cooled to room
temperature and poured into H2O, and the product extracted with
Et2O. After the Et2O layer was washed with brine, the organic fractions
were collected and dried with MgSO4. The solvent was removed
in vacuo after filtration to yield an oil, which was solubilized in CH2Cl2
and preabsorbed on silica. The sample was purified by column
chromatography [SiO2: CH2Cl2/EtOAc, 9:1; Rf = 0.66] to yield 480
mg (68.2%) of the product as a bright yellow solid. 1H NMR (CDCl3):
δ 8.71 (d, 1H, 3J = 4.6 Hz, Ha), 8.63 (d, 1H, 3J = 7.9 Hz, Hd), 8.58 (d,
1H, 4J = 1.5 Hz, He), 7.89 (d, 1H, 4J = 1.5, Hm), 7.83 (td, 1H, 3J = 7.7,
4J = 1.7 Hz, Hc), 7.75 (s, 1H, Hr), 7.72 (d, 1H, 3J = 7.8 Hz, Hn), 7.62 (d,
3
3
(d, 4H, J = 8.9 Hz, Hk), 3.79 (s, 6H, −OCH3), 3.04 (q, 2H, J = 7.3
Hz, −SCH2CH3), 1.38 (t, 3H, J = 7.3 Hz, −SCH2CH3). 13C NMR
3
(CDCl3): δ 156.4, 156.2, 156.1, 156.0, 149.0, 148.7, 146.4, 143.1,
140.4, 139.9, 138.9, 137.3, 136.8, 129.4, 129.2, 127.6, 126.8, 126.7,
126.4, 125.6, 124.6, 123.9, 122.7, 121.4, 120.1, 116.0, 115.2, 114.8,
55.5, 27.8, 14.5. HRMS (EI): m/z 677.2187 [(M)+] (calcd for
+
C42H35N3O2S2 m/z 677.2171).
[Ru(P7)(L5)]NO3 (P10). To a suspension of P7 (553 mg, 1.22
mmol) in MeOH/H2O/THF (5:1:1, 210 mL) were added Ru(L5)Cl3
(750 mg, 1.22 mmol) and N-ethylmorpholine (0.5 mL). After the
reaction mixture was left to reflux overnight under an N2 atmosphere,
AgNO3 (622 mg, 3.66 mmol) was added followed by an additional 2 h
reflux. The hot solution was filtered, and then the solvent was removed
in vacuo. Purification of the solid by column chromatography [SiO2:
CH2Cl2/MeOH (9:1); Rf = 0.48] yielded 635 mg (50.9%) of the
product as a black powder. Low yields were attributed to the difficulty
of separating the ortho and para isomeric products; e.g., ortho/para 3:1
1
1
1H, 3J = 3.8 Hz, Hf), 7.45 (d, 2H, 3J = 8.8 Hz, Hh), 7.41 (t, 1H, 3J = 7.9
Hz, Ho), 7.32 (dd, 1H, 3J = 4.9, 4J = 1.0 Hz, Hb), 7.21 (d, 1H, 3J = 3.8
as determined by H NMR spectroscopy. H NMR (CDCl3): δ 9.10
3
(s, 2H, HE), 8.94 (d, 1H, J = 7.2 Hz, Hd), 8.92 (s, 1H, He), 8.86 (s,
2H, HD), 8.22 (s, 1H, Hm), 8.05 (d, 1H, 3J = 3.9 Hz, Hf), 7.84 (td, 1H,
3
3
Hz, Hg), 7.08 (d, 4H, J = 8.3 Hz, Hj), 6.98 (dd, 1H, J = 8.1, 4J = 1.9
3J = 7.8, J = 1.2 Hz, Hc), 7.70 (d, 1H, J = 7.9 Hz, Hn), 7.68 (d, 2H,
3J = 5.9 Hz, HA), 7.65 (dd, 2H, 3J = 5.9, 4J = 1.4 Hz, HB), 7.25 (d, 1H,
3J = 3.5 Hz, Hg), 6.99 (t, 1H, 3J = 6.4 Hz, Hb), 6.88 (t, 1H, 3J = 7.8 Hz,
Ho), 6.56 (d, 1H, 3J = 5.3 Hz, Ha), 6.37 (d, 1H, 3J = 7.7 Hz, Hp), 4.20
4
3
3
3
Hz, Hp), 6.92 (d, 2H, J = 8.7 Hz, Hi), 6.83 (d, 4H, J = 8.3 Hz, Hk),
4.16 (q, 2H, 3J = 7.2 Hz, −OCH2CH3), 3.79 (s, 6H, −OCH3), 1.47 (t,
3H, J = 7.2 Hz, −OCH2CH3). 13C NMR (CDCl3): δ 159.6, 157.3,
3
156.4, 156.3, 156.2, 149.2, 148.9, 146.6, 143.4, 141.0, 140.7, 139.4,
137.1, 129.9, 127.0, 126.7, 125.9, 124.0, 122.9, 121.7, 120.4, 119.6,
116.5, 115.4, 115.2, 115.0, 113.7, 63.8, 55.7, 15.1. HRMS (EI): m/z
661.2383 [(M)+] (calcd for C42H35N3O3S+ m/z 661.2399).
3
(s, 3H, HF), 3.94 (s, 6H, HC), 2.14 (q, 2H, J = 7.3 Hz, −SCH2CH3),
3
0.68 (t, 3H, J = 7.3 Hz, −SCH2CH3); HRMS (ESI): m/z 954.0112
[(M)+] (calcd for C43H33BrN5O6RuS2 m/z 954.0126). Anal. Calcd
+
for C43H33BrN6O9RuS2·2H2O: C, 48.77; H, 3.52; N, 7.94. Found: C,
48.87; H, 3.34; N, 7.75.
4-(5-(6-(3-(Ethylthio)phenyl)-2,2′-bipyridin-4-yl)thiophen-2-yl)-
N,N-diphenylaniline (L3H). After a solution of P7 (549 mg, 1.21
6630
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Organometallics
Article
[Ru(L1)(L5)]NO3 (1). To a quantity of L1H (242 mg, 0.40 mmol)
suspended in MeOH/H2O/THF (5:1:1, 210 mL) were added
Ru(L5)Cl3 (248 mg, 0.40 mmol) and N-ethylmorpholine (0.5 mL).
The solution was left to reflux for 14 h and was then cooled to room
temperature. To this solution was added AgNO3 (204 mg, 1.20 mmol)
prior to an additional 2 h reflux. The solution was filtered while warm
prior to the removal of the solvent in vacuo. The residue was purified
by column chromatography [SiO2: CH2Cl2/MeOH (9:1); Rf = 0.50],
[Ru(L4)(L5)]HCO3 (4). A mixture of P9 (130 mg, 0.30 mmol) and
P10 (262 mg, 0.26 mmol) in anhydrous DMF (25 mL) was sparged
with N2 for 20 min. Following the addition of K2CO3 (355 mg, 2.56
mmol) and Pd(PPh3)4 (36 mg, 0.03 mmol), the reaction mixture
was left overnight under N2 at 70 °C. The reaction mixture was then
dried in vacuo, preabsorbed onto silica, and purified by column
chromatography [SiO2: CH2Cl2/MeOH, (9:1); Rf = 0.39] to yield
120 mg (37.6%) of the product as a fine black powder. A gradient
elution (6:4) was employed to remove the second and third fractions
corresponding to the mono- and disaponified products, respectively.
1
yielding 328 mg (78.8%) of the product as a black crystalline solid. H
NMR (CDCl3): δ 9.10 (s, 2H, HE), 8.94 (d, 1H, 3J = 8.3 Hz, Hd), 8.91
3
1H NMR (CDCl3): δ 9.31 (d, 1H, J = 8.1 Hz, Hd), 9.12 (s, 1H, He),
4
4
(d, 1H, J = 1.2 Hz, He), 8.87 (d, 2H, J = 1.1 Hz, HD), 8.31 (d, 1H,
4J = 1.2 Hz, Hm), 8.19 (d, 1H, 3J = 3.9 Hz, Hf), 7.90 (dt, 1H, 3J = 7.8,
4J = 1.4 Hz, Hc), 7.68 (d, 2H, 3J = 5.9 Hz, HA), 7.63 (dd, 2H, 3J = 5.9,
4J = 1.6 Hz, HB), 7.60−7.56 (m, 3H, Hh, Hn), 7.43 (d, 1H, 4J = 3.9 Hz,
9.08 (s, 2H, HE), 8.85 (s, 2H, HD), 8.52 (d, 1H, 3J = 3.7 Hz, Hf), 8.32
3
3
(s, 1H, Hm), 7.94 (t, 1H, J = 7.8 Hz, Hc), 7.74 (d, 1H, J = 7.8 Hz,
Hn), 7.68 (d, 2H, 3J = 5.9 Hz, HA), 7.63 (d, 2H, 3J = 5.9 Hz, HB), 7.48
(d, 1H, J = 8.5 Hz, Hh), 7.36 (d, 1H, J = 3.9 Hz, Hg), 7.09 (d, 4H,
3J = 8.9 Hz, Hk), 7.07 (t, 1H, 3J = 7.8 Hz, Hb), 6.93 (d, 2H, 3J = 8.4 Hz,
3
3
3
Hg), 7.29 (t, 4H, J = 8.3 Hz, Hk), 7.18−6.98 (m, 9H, Hj, Hi, Hl, Hb),
3
3
6.83 (t, 1H, J = 7.8 Hz, Ho), 6.74 (d, 1H, J = 5.4 Hz, Ha), 5.98 (d,
3
3
1H, 3J = 7.9 Hz, Hp), 4.20 (s, 3H, HF), 3.95 (s, 6H, HC), 3.06 (q, 2H,
Hi), 6.87 (t, 1H, J = 7.7 Hz, Ho), 6.84 (d, 4H, J = 8.9 Hz, Hj), 6.53
3
3
3J = 6.9 Hz, −OCH2CH3), 0.49 (t, 3H, J = 6.9 Hz, −OCH2CH3).
3
(d, 1H, J = 5.3 Hz, Ha), 6.35 (d, 1H, J = 7.7 Hz, Hp), 4.19 (s, 3H,
HF), 3.94 (s, 6H, HC), 3.79 (s, 6H, −OCH3), 2.13 (q, 2H, 3J = 7.3 Hz,
HRMS (ESI): m/z 1103.2278 [(M)+] (calcd for C61H47N6O7RuS+
m/z 1103.2297). Anal. Calcd for C61H47N7O10RuS·2H2O: C, 60.69;
H, 4.26; N, 8.12. Found: C, 60.38; H, 4.09; N, 7.90.
3
−SCH2CH3), 0.68 (t, 3H, J = 7.3 Hz, −SCH2CH3). HRMS (ESI):
m/z 1179.2277 [(M)+] (calcd for C63H51N6O8RuS2 m/z =
+
1179.2280). Anal. Calcd for C64H52N6O11RuS2·5H2O: C, 57.52; H,
4.68; N, 6.29. Found: C, 57.88; H, 4.31; N, 6.17.
[Ru(L2)(L5)]NO3 (2). To a suspension of L2H (246 mg, 0.40
mmol) in MeOH/H2O/THF (5:1:1, 210 mL) were added Ru(L5)Cl3
(265 mg, 0.40 mmol) and N-ethylmorpholine (0.5 mL). After the
reaction mixture was left to reflux for 14 h, it was cooled to room
temperature, and AgNO3 (0.204 g, 1.20 mmol) was added before the
reaction was left to reflux for an additional 2 h. The solution was
gravity filtered while warm prior to removal of the solvent in vacuo to
yield a dark red solid. This solid was preabsorbed on silica and purified
by column chromatography [SiO2: CH2Cl2/MeOH, 9:1; Rf = 0.89] to
Physical Methods. Elemental analysis, electrospray ionization
mass spectrometry (ESI-MS), and electron impact (EI) mass
spectrometry data were collected at the Chemistry Instrumentation
Facility of the University of Calgary. Electrochemical measurements
were performed under anaerobic conditions with a Princeton Applied
Research VersaStat 3 potentiostat using a dry MeCN solvent, a glassy
carbon working electrode, a platinum counter electrode, a silver
pseudoreference electrode, and a 0.1 M NBu4BF4 supporting
electrolyte. Electronic spectroscopic data were collected on MeCN
solutions using a Cary 5000 UV−vis spectrophotometer (Varian).
Steady-state emission spectra were obtained at room temperature
using an Edinburgh Instruments FLS920 spectrometer equipped with
a Xe900 450-W steady-state xenon arc lamp, a TMS300-X excitation
monochromator, a TMS300-M emission monochromator, and a
Hamamatsu R2658P PMT detector and corrected for detector
response. Lifetime measurements were obtained at room temperature
using an Edinburgh Instruments FLS920 spectrometer equipped with
a Fianium SC400 super continuum white light source and a
Hamamatsu R3809U-50 multichannel plate detector, and data were
analyzed with Edinburgh Instruments F900 software. Curve fitting of
the data was performed using a nonlinear least-squares procedure in
the F900 software.
1
yield 367 mg (75.4%) of the product as a fine black powder. H NMR
3
(CDCl3): δ 9.10 (s, 2H, HE), 8.87 (s, 2H, HD), 8.85 (d, 1H, J = 7.1
Hz, Hd), 8.83 (s, 1H, He), 8.30 (s, 1H, Hm), 8.13 (d, 1H, 3J = 3.9 Hz,
3
3
Hf), 7.87 (t, 1H, J = 7.9 Hz, Hc), 7.66 (d, 2H, J = 5.9 Hz, HA), 7.61
(dd, 2H, 3J = 5.9, 4J = 1.3 Hz, HB), 7.57 (d, 1H, 3J = 7.8 Hz, Hn), 7.47
3
3
(d, 2H, J = 8.7 Hz, Hh), 7.31 (d, 1H, J = 3.8 Hz, Hg), 7.08 (d, 4H,
3J = 8.9 Hz, Hj), 7.01 (t, 1H, 3J = 6.5 Hz, Hb), 6.92 (d, 2H, 3J = 8.7 Hz,
3
3
Hi), 6.84 (d, 4H, J = 8.9 Hz, Hk), 6.81 (t, 1H, J = 7.7 Hz, Ho), 6.75
3
3
(d, 1H, J = 5.4 Hz, Ha), 5.97 (d, 1H, J = 8.0 Hz, Hp), 4.18 (s, 3H,
HF), 3.93 (s, 6H, HC), 3.79 (s, 6H, −OCH3), 3.05 (q, 2H, 3J = 6.9 Hz,
3
−OCH2CH3), 0.48 (t, 3H, J = 6.9 Hz, −OCH2CH3). HRMS (ESI):
m/z 1163.2495 [(M)+] (calcd for C63H51N6O9RuS+: m/z =
1163.2509). Anal. Calcd for C63H51N7O12RuS·2H2O: C, 59.71; H,
4.37; N, 7.74. Found: C, 59.36; H, 4.37; N, 7.52.
[Ru(L3)(L5)]HCO3 (3). A combination of P10 (250 mg, 0.25 mmol)
and P8 (115 mg, 0.31 mmol) in 25 mL of anhydrous DMF was
sparged with N2 for 20 min. K2CO3 (345 mg, 2.50 mmol) and
Pd(PPh3)4 (40 mg, 0.03 mmol) were then added to the reaction
mixture, which was left to stir at 70 °C overnight. The filtered solution
was then concentrated by rotary evaporation, followed by the addition
of H2O and CH2Cl2. The organic fraction was isolated and dried with
MgSO4 prior to the removal of solvent in vacuo. The solid was
preabsorbed onto silica and purified by column chromatography
[SiO2: CH2Cl2/MeOH (9:1); Rf = 0.50] to yield 0.25 g (85.0%) of the
product as a dark solid. A gradient elution (6:4) was employed to
remove the second and third fractions, which correspond to saponified
products. 1H NMR (CDCl3): δ 9.14 (d, 1H, 3J = 8.3 Hz, Hd), 9.10 (s,
2H, HE), 9.05 (d, 1H, 4J = 1.2 Hz, He), 8.86 (d, 2H, 4J = 1.1 Hz, HD),
Computational Methods. The Gaussian 03 computational
package25 was used to perform ground-state and transition-state
geometry optimization calculations employing Becke’s three-parameter
hybrid exchange functional and the Lee−Yang−Parr nonlocal
correlation functional B3LYP26−28 and the LANL2DZ basis set29,30
with an effective core potential for Ru, and a 6-31G* basis set was used
for S, C, N, O, and H atoms.31 Time-dependent density functional
theory calculations were also performed using this methodology, and
the first 60 singlet excited states were calculated. Calculations by the
first-principles method were used to obtain accurate excitation
energies and oscillator strengths. We modeled the solvent with the
polarizable continuum model using MeCN as the solvent.32
3
4
RESULTS
8.37 (d, 1H, J = 3.9 Hz, Hf), 8.34 (d, 1H, J = 1.2 Hz, Hm), 7.92 (t,
■
3
3
3
1H, J = 7.8 Hz, Hc), 7.75 (d, 1H, J = 7.3 Hz, Hn), 7.70 (d, 2H, J =
Synthesis and Structural Characterization. We pre-
viously developed a modular synthetic approach to isolate the
TPA-functionalized tridentate ligands relevant to this study
(Scheme 1).11,12 Each of these ligands can be accessed using
well-established synthetic methods and produced on reasonably
5.9 Hz, HA), 7.65 (dd, 2H, 3J = 5.9, 4J = 1.6 Hz, HB), 7.59 (d, 2H, 3J =
8.8 Hz, Hh), 7.46 (d, 1H, J = 3.9 Hz, Hg), 7.29 (t, 4H, J = 8.3 Hz,
Hk), 7.18−7.03 (m, 9H, Hj, Hi, Hl, Hb), 6.90 (t, 1H, J = 7.8 Hz, Ho),
4
3
3
3
3
6.54 (d, 1H, J = 5.4 Hz, Ha), 6.37 (d, 1H, J = 7.9 Hz, Hp), 4.21 (s,
3H, HF), 3.96 (s, 6H, HC), 2.14 (q, 2H, 3J = 6.9 Hz, −SCH2CH3), 0.69
(t, 3H, 3J = 6.9 Hz, −SCH2CH3). HRMS (ESI): m/z 1119.2060
large scales. The Krohnke reagents P4 and P5 are prepared
̈
[(M)+] (calcd for C61H47N6O6RuS2 m/z = 1119.2069). Anal. Calcd
+
from the respective substituted acetylphenones, commercially
available 3′-ethoxyacetophenone, and P2. Krohnke condensa-
̈
for C62H48N6O9RuS2·3H2O: C, 60.04; H, 4.39; N, 6.78. Found: C,
59.93; H, 4.54; N, 7.05.
tions with enone P3 yielded the bromothiophene-substituted
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a
Scheme 1. Syntheses of Ligands L1H−L4H.
a
Reaction conditions: (a) BrCH2CH3, K2CO3, EtOH, 78 °C, 14 h; (b) I2, pyridine, 100 °C, 2.5 h; (c) ammonium acetate, formamide, 120 °C, 14 h;
(d) Pd(PPh3)4, K2CO3, THF/H2O (9:1 v/v); 65 °C, 14 h.
pro-ligands P6 and P7, which were poised for further reactions
with Suzuki reagents P8 and P9 to furnish ligands L1H−L4H
in high yields.
Cyclometalated Ru(II) complexes 1 and 2 could be isolated
in high yields (e.g., >75%) through the reaction of the
Ru(L5)Cl3 synthon with L1H and L2H, respectively (Scheme 2).
not be separated by column chromatography. We therefore had
to rely on an alternative method to access the thioether
derivatives. The combination of Ru(L5)Cl3 with pro-ligand P7
(which is devoid of a TPA group) also yields an ortho/para 3:1
product distribution for P10; however, the absence of the
apolar TPA moiety enables the ortho product to be isolated
from the reaction mixture by column chromatography33
(Scheme 2). The ortho isomer of P10 could then be
functionalized with either P8 or P9 to afford 3 and 4,
respectively, using standard Suzuki conditions in DMF.
a
Scheme 2. Assembly of Complexes 1−4
The 1H NMR spectra reflect the mode of chalcogen
substitution of the N∧N∧C chelate. The spectra for 2 and 4,
for example, indicate that the −OEt group donates more
electron density into the anionic ring system than the −SEt
group; that is, resonances corresponding to Hn, Ho, Hp, Hd, and
He are upfield for 2 relative to 4 (Figure 2). The lower degree
a
Reaction conditions: (a) Step 1: MeOH/H2O/THF (5:1:1 v/v/v),
N-ethylmorpholine, 65 °C, N2, 14 h; step 2: AgNO3, 65 °C, N2, 2 h;
(b) ortho product (dashed enclosure) was reacted using K2CO3,
Pd(PPh3)4, DMF, 70 °C, N2, 14 h.
1
Figure 2. H NMR spectra for CDCl3 solutions of 2 (top) and 4
(bottom) at ambient temperature. Signals are assigned according to
the labeling scheme provided in Figure 1.
Note that the ortho-cyclometalated isomers were obtained in
exclusivity in both cases. Following this same synthetic protocol
for 3 and 4 (from L3H and L4H, respectively) yielded a
mixture of ortho- and para-substituted products (i.e., ortho/para
of shielding of these protons is rationalized by the relative
donation of the lone pairs of the respective chalcogens into the
aromatic system affecting the electron density at the metal
center. Molecular modeling shows that there is significant steric
1
3:1 for 3 as determined by H NMR spectroscopy) that could
6632
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Table 1. Summary of Spectroscopic and Electrochemical Properties for Cyclometalated Complexes and Ligands Recorded in
MeCN
a
d
ox
UV−vis absorbance data (nm)
λmax (ε × 103 M−1.cm−1
emission data
E1/2 (V vs NHE)
Ru(III)/
Ru(II)
TPA/
b
c
compound
)
λem (nm)
τ (ns); ϕ
TPA•+
L1H
1
390 (38.1)
534 (390)
3.18 (0.97); 0.89
1.12
1.16
0.92
e
e
690sh (4.3), 531 (31.2), 431 (49.9)
0.97
0.96
1.03
L2H
2
401 (36.3)
614 (400)
e
0.63 (1.02); 0.11
e
f
688sh (3.8), 532 (30.4), 437 (43.9)
391 (36.1)
0.96
L3H
3
526 (391)
e
2.94 (1.05); 0.80
e
1.12
1.18
0.92
0.92
686sh (3.5), 574sh (19.3), 530 (26.4), 430 (44.7)
L4H
4
402 (34.2)
611 (402)
e
3.26 (0.97); 0.17
e
686sh (3.3), 574sh (19.9), 530 (28.0), 435 (41.7)
686sh (3.4), 574sh (14.3), 530 (16.6), 430 (24.6)
686sh (3.1), 574sh (22.6), 516 (36.8), 425 (48.7)
392 (39.8)
1.04
1.04
1.15
e
e
e
e
P10
5
f
1.15
L5H
6
545 (391)
e
3.3 (0.94); 0.81
e
1.13
0.93
0.92
1.16
1.14
686sh (2.7), 574sh (20.6), 518 (34.5), 430 (38.6)
1.14
L6H
7
404 (33.0)
632 (403)
549 (429)
552 (389)
0.5 (0.97); 0.34
3.2 (1.02); 0.27
3.5 (0.99); 0.66
680sh (3.5), 583sh, 525 (26.3), 430 (39.8), 329 (42.7)
1.04
e
L7H
391 (36.0)
-
a
b
c
Recorded at ambient temperature. λex indicated in parenthesis in units of nm. Data recorded in deaerated solutions. χ2 indicated in parenthesis;
d
absolute quantum yield measured with an integrating sphere. Data collected using 0.1 M NBu4BF4 MeCN solutions at 100 mV/s
e
f
and referenced to a [Fc]/[Fc]+ internal standard followed by conversion to NHE; [Fc]/[Fc+] = +640 mV vs NHE in MeCN. Not observed. Peaks
could not be resolved by differential pulse voltammetry (DPV) experiments recorded in MeCN. shShoulder
gearing in ortho-substituted products that inhibits rotation
sufficiently high when R1 = H (i.e., 1 and 3), thus indicating that
the TPA unit needs to be furnished with EDGs (e.g., −OMe).
Optical Properties. UV−vis and fluorescence data of the
ligands and title complexes are listed in Table 1. Emission was
not observed for the cyclometalated complexes, a feature that is
commonly observed for complexes of this type owing to the
distorted octahedral metal environment and the energy gap
law.15,36,37 We invoke the latter argument to rationalize the
lower quantum yields for the −OMe-substituted TPA ligands
L2H and L4H relative to L1H and L3H.
The UV−vis data for each of the complexes are similar
except the bands centered at ca. 530 nm for the thiolate
derivatives 3 and 4, which exhibit slightly lower intensities
relative to the alkoxy derivatives. TD-DFT calculations were
performed on the geometry-optimized structures to aid in the
assignment of the bands (e.g., Figure S1). In the case of 4, for
example, the lowest energy band at ca. 700 nm is predicted to
correspond to the promotion of an electron from the HOMO−
1 orbital (localized primarily at the Ru and anionic phenyl ring)
to a LUMO distributed over the π*-system of the Me3tctpy
ligand. (We do not, however, rule out direct population of a
3MLCT state facilitated by the heavy Ru metal.15) The
prominent band centered at ca. 570 nm arises from transitions
between the HOMO that is predominantly TPA in character
and the LUMO+1, and a higher energy transition from a metal-
based HOMO−2 level to the LUMO+1. A major contribution
to the high-energy band at ca. 430 nm is provided by a
transition between the HOMO and LUMO+3, which is
predominantly distributed over the π-system of the bipyridyl
fragment of the N∧N∧C chelate. Note that all of these
transitions are appropriate for sensitizing n-type semiconduc-
tors in that they involve the movement of electron density
toward Me3tctpy following light absorption.
about the bond between the sp3-hydridized chalcogen and the
C
phenyl. We also contend that the relatively smaller sp3 orbitals
of the O atoms afford a greater degree of freedom for rotation
than those of the S atoms (although the ethyl groups prohibit
full rotation in both cases); thus, the larger and more diffuse sp3
orbitals of the S atom of 4 restrict bond rotation, diminishing
electron donation into the π-system of the anionic ring. Note
that the diffuse nature of the sulfur orbitals increases the
shielding of Ha because it resides in the cone of shielding of the
central ring of the adjacent tridendate ligand (qualitatively
described in the insets of Figure 2).
Electrochemical Behavior. The electrochemical proper-
ties of the free ligands and the corresponding metal complexes
were examined in MeCN by cyclic voltammetry (Table 1).
Each of the TPA-substituted ligands exhibits a single reversible
one-electron oxidation (E1/2,ox = +0.92−1.14 V) that can be
attenuated by substitution of the TPA (e.g., −OMe groups
lower the oxidation potential by ∼200 mV).11 The position of
this oxidation wave shifts to modestly higher potentials upon
coordination to the cationic Ru metal center; however, the TPA
and metal-based oxidation potentials of the metal complexes
can be independently modulated.
Consistent with our analysis of the NMR spectra that inferred
−SEt is acting as a weaker donor than −OEt, the Ru(II)-based
oxidation potential of 4 is anodically shifted by ∼80 mV relative
to 2. This observation has important implications in the context
of sensitizing n-type semiconductors, where the HOMO should
be localized to the TPA rather than the metal to induce
favorable charge separation.11,34,35 The weaker donor character
of the −SEt group, for example, results in a higher metal-based
oxidation potential to increase the potential for hole transfer to
the TPA unit (i.e., the fragment of the molecule which the
HOMO is localized to) following a light-induced metal-to-
ligand charge-transfer (MLCT) event. Notwithstanding, the
difference in energy between the HOMO and HOMO−1 is not
DISCUSSION
■
Our program has a longstanding interest in developing cyclo-
metalated Ru(II) chromophores for DSSC applications.3,14,38,39
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Scheme 3. Factors That Govern the C−H Activation Step for 1−4a
It is therefore critical that we have a clear understanding of the
requisite C−H activation process during the formation of the
target dyes, particularly in cases where different isomers can
be generated. An examination of the title complexes and their
byproducts offers some important clues into how steric
interactions and electronic parameters affect the cyclo-
metalation pathway (as it pertains to octahedral Ru(II) centers
within a polypyridyl ligand environment).
The various mechanisms for C−H activation relevant to
cyclometalation have been detailed in numerous reviews.40−43
In general, there are three established mechanisms for
cyclometalation: oxidative addition, electrophilic metalation,
and σ-bond metathesis (summarized in Figure S3). There is
also a less elaborated agostic pathway that possesses features
characteristic of both oxidative addition and electrophilic
metalation. Each of these pathways is classified according to
the orientation of the C−H bond as it approaches the metal,
how the C−H bond is cleaved, and the relative thermodynamic
stabilities of the organometallic products.
One particular study that offers some insight into the favored
ortho product distribution of 1−4 compares the C−H
activation process for a series of haloarenes and anisole. In
this study by Milstein et al.,44 it was established that the ortho-
metalated Ir pincer products were produced via an oxidative
addition pathway that involved the stabilization of the transition
state and the thermodynamic product by the coordination of
the halide/oxygen to an empty metal orbital. This same analysis
does not apply to our systems, however, as the metal will
inherently be bound to five pyridine rings prior to C−H
activation, and thus a second coordination site is not available
for oxidative addition. (The same argument excludes σ-bond
metathesis.) Moreover, an oxidative addition pathway demands
an ∠MHC angle of 130° in the transition state for an idealized
arrangement of the C−H bond and the metal d orbitals (i.e.,
this angle provides favorable overlap between the dRu and σC−H
orbitals while accommodating back-donation from the dπ
orbital to the σ*C−H to destabilize the C−H bond).42 This
bond angle in the transition state of our systems is predicted to
be far from this idealized value [i.e., ∠MHC = 100.1° (−OEt)
and 103.9° (−SEt); Figure S2]. An agostic pathway can also be
eliminated because the Lewis bases present under our reaction
conditions (e.g., MeOH, H2O) are stronger donors than the
C−H fragment of the phenyl ring and would therefore preclude
a dominant interaction from occurring. Furthermore, agostic
interactions typically require a geometric confinement of the
molecule; yet in our case the pbpy ligand is able to undergo free
rotation prior to cyclometalation because the O and S atoms at
R2 are unable to interact (due to geometric constraints) with
the metal to mediate the agostic interaction.
electron-withdrawing methyl-ester groups afford an electro-
philic metal center primed for electrophilic metalation, which
are often observed for electron-poor late transition metals.40
We also note that there exists a close contact between the Ru
and Cphenyl atoms that would enable an electrophilic mechanism
(Figure S2), while the large metal−chalcogen distances rule out
a directing effect induced by a Lewis base coordination.
Electrophilic metalations involving substituted aromatic mole-
cules typically show little selectivity between different C−H
bonds, and the observed selectivity is usually a consequence of
steric factors. This trend, however, is not observed in the cases
of 1−4. A nonlinear extrapolation of the electrochemical data
for the title compounds (and related complexes4,11,12,45
)
indicates that −OMe groups para and ortho to the organo-
metallic bond lower the metal-based oxidation potential by
∼130 and ∼70 mV, respectively. Consequently, the ortho
isomer is thermodynamically favored and prevails over the
higher steric congestion provided by the ethyl groups. There
also exists evidence in the literature that the preference for
C−H activation of aromatic rings ortho to fluorine substituents
arises because the carbanion is stabilized by the adjacent σ*C−F
orbital.16,17 We surmise that this same effect is operative for the
σ*C−O orbital to favor the ortho isomer. This line of reasoning
also translates to the thiolate derivatives; however, the poorer
orbital overlap leads to a lower thermodynamic preference for
substitution at the ortho position. Note that the difference in
the metal-based oxidation potentials of the para and ortho
isomers where X = SEt is nominal, which lends credence to the
σ*C−S orbital playing a role in stabilizing the relatively sterically
encumbered ortho product.
The relative isomeric ratios that are observed can therefore
be attributed to a combination of the chalcogen stabilizing the
carbanion of the activated phenyl ring through inductive effects
and the relative thermodynamic stabilities of the products due
to π-donation into the phenyl ring. Scheme 3 summarizes the
proposed chalcogen-assisted ortho-metalation step commencing
from the intermediate complex with five Ru−Npyridine bonds
and a vacant coordination site due to dissociation of the Cl−
ligand (i.e., A). It is the bidendate coordination mode of the
pbpy ligand, which enables free rotation of the benzene ring
bearing the chalcogen substituent, that is responsible for the
observation of the different isomers. Intermediate A can then
undergo an electrophilic metalation step para to the substituent
to form intermediate B prior to the formation of the para
product C.
In cases where the phenyl ring is substituted by π-donating
substituents, E is thermodynamically favored over C, thus
shifting the equilibria in favor of the ortho isomer. However,
steric congestion arising from the ethyl groups can inhibit
appropriate orbital overlap with the aromatic π-system, leading
to a diminution of the effective electron-donating character of
the substituents. When X = −SEt, this steric gearing, in tandem
Elimination of these other pathways led us to infer that the
formation of 1−4 proceeds via an electrophilic metalation
reaction (Figure S3). The high-valent Ru(III) precursor and
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Organometallics
Article
(11) Robson, K. C. D.; Koivisto, B. D.; Yella, A.; Sporinova, B.;
with the poor orbital overlap between the thiolate and the
aromatic π-system, suppresses electron donation into the aryl
ring to render C and E nearly degenerate. When X = −OEt,
this steric gearing stabilizes E relative to C (because of the more
prominent differences in lone pair donation from the O atom
into the phenyl ring) to govern the exclusive formation of the
ortho isomer. This effect is compounded by stabilization of the
carbanion by the σ*C−O orbital.
Nazeeruddin, M. K.; Baumgartner, T.; Gratzel, M.; Berlinguette, C. P.
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Summary. We have presented a series of bis-tridentate
Ru(II) complexes bearing a cyclometalating ligand functional-
ized with EDGs and EWGs. This study lays out experimental
evidence that sheds light on why the activation of the C−H
bond ortho to the EDGs is favored, even in cases where there is
significant steric congestion. We show that there is subtle
interplay between steric interactions and the relative stabilities
of both the products and the preceding intermediates that
govern the formation of each of the isomers. These results
offer important experimental insight into the C−H activation
process.
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ASSOCIATED CONTENT
■
S
* Supporting Information
TD-DFT data and transition-state modeling are provided. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: cberling@ucalgary.ca.
ACKNOWLEDGMENTS
■
This work was financially supported by the Canadian Natural
Science and Engineering Research Council (NSERC), Canada
Research Chairs, Canadian Foundation for Innovation (CFI),
Alberta Ingenuity, and Canada School of Energy and
Environment (CSEE). We are also grateful to NSERC
(K.C.D.R., S.M, H.A.S., and E.S.) and Alberta Innovates
(K.C.D.R.) for scholarships and P07-006-F, DI-26-10/I,
AT24100024, and CONICYT (E.S.) for student fellowships.
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N.; Zakeeruddin, S. M.; Walder, L.; Gratzel, M. J. Am. Chem. Soc. 1999,
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