Inorganic Chemistry
Article
Finnigan electrospray ionization mass spectrometer (ESI-MS).
Elemental analysis was conducted with a PerkinElmer 2400 instru-
ment. Cyclic voltammetry was carried out on a CH Instruments 620D
potentiostat. A standard three electrode cell was used under an
atmosphere of argon with 0.1 M Bu4NPF6 in spectrophotometric
grade acetonitrile as the supporting electrolyte. Glassy carbon (3 mm
diameter) and Pt wire were used as working and counter electrodes,
respectively. A nonaqueous reference electrode was used to minimize
IR drop at the solvent interface. This consisted of a Ag wire in the
same supporting electrolyte separated by a vycor frit. All experiments
were calibrated using ferrocene as an internal pseudoreference due to
the relative instability of the reference electrode employed. All redox
potentials are reported in reference to the saturated calomel electrode
(SCE) which has been reported at 0.40 V negative of the Fc+/0 couple
under identical conditions.67 The formal redox potential E° was
determined from cyclic voltammetry as (Epa + Epc)/2, where Epa and
Epc are the anodic and cathodic peak potentials, respectively. Where E°
could not be calculated due to irreversible behavior, potentials are
reported as either the Epa minimar or Epc maxima. UV−vis absorption
spectra were recorded on an Agilent 8456 diode array spectropho-
tometer in spectrophotometric grade acetonitrile. UV−vis−NIR
spectroelectrochemical experiments were conducted using a Varian
Cary 500 Scan spectrophotometer in tandem with a custom
spectroelectrochemical flow cell whose design is based upon a
literature description.68 This consisted of a Pt-foil/gauze working
electrode and Pt-gauze counter electrode (isolated via a fine porosity
glass frit). The same Ag-wire reference electrode was again used for
controlled potential electrolysis experiments. Electron paramagnetic
resonance (EPR) measurements were made in a two-electrode
capillary tube with an X-band (9.48 GHz) Bruker System EMX at a
low temperature of 110 K. All of the in situ electrochemically
generated one-electron oxidized species (12+−82+) in CH3CN/0.1 M
Bu4NPF6 are EPR silent at 295 K due to significant contribution of
ruthenium dπ orbitals to the singly occupied molecular orbital leading
to fast relaxation times, and EPR silence in fluid solutions.
according to a reported procedure by refluxing the benzyloxy
protected precursor (2 mmol) in ethanol overnight under an argon
atmosphere with 1,4-cyclohexadiene (20 mmol) and 0.5 g of 10%
palladium on carbon. After cooling the crude mixture was passed
through a Celite plug with excess dichloromethane. Analytically pure
ligand was recovered following removal of the dichloromethane ex
vacuo and recrystallization from hot methanol.
5,7-Bis(4-methoxyphenyl)-8-benzyloxyquinoline. This was
1
synthesized in 84% yield. H NMR δ[CDCl3]: 3.83−3.85 (m, 6H),
5.12 (s, 2H), 6.96 (d, 2H, J = 1.2 Hz), 7.02 (d, 2H, J = 1.2 Hz), 7.22−
7.24 (m, 5H), 7.35 (dd, 1H, J = 4.0, 9.0 Hz), 7.39 (d, 2H, J = 9.0 Hz),
7.62 (s, 1H), 7.64 (d, 2H, J = 9.0 Hz), 8.25 (m, 1H), 8.97 (m, 1H)
ppm. 13C NMR δ[CDCl3]: 55.42, 76.19, 113.69, 113.99, 114.69,
116.30, 120.89, 127.27, 127.66, 128.03, 128.60, 129.70, 130.55, 131.15,
131.22, 131.62, 133.60, 134.69, 135.69, 137.52, 143.66, 149.70, 150.69,
159.09, 159.21 ppm. ESI-MS [M + H+] m/z: calcd 448.1913; obsd
448.1277.
5,7-Bis(4-diphenylamino)phenyl-8-benzyloxyquinoline. This
was synthesized in 72% yield. 1H NMR δ[CDCl3]: 5.22 (s, 2H), 7.03−
7.09 (m, 5H), 7.13−7.20 (m, 12H), 7.25−7.31 (m, 9H), 7.36 (d, 4H, J
= 4.8 Hz), 7.40 (dd, 1H, J = 2.7, 5.1 Hz), 7.60 (d, 4H, J = 4.5 Hz), 8.36
(d, 1H, J = 5.4 Hz), 9.10 (d, 1H, J = 3.0 Hz) ppm. 13C NMR
δ[CDCl3]: 11.12, 14.20, 23.10, 23.89, 24.60, 29.04, 29.83, 30.52, 34.13,
38.83, 66.91, 76.49, 120.95, 123.00, 123.25, 123.37, 123.44, 124.47,
124.71, 127.24, 127.26, 128.14, 128.71, 129.38, 129.47, 129.59, 130.96,
132.25, 133.09, 133.79, 134.76, 135.75, 137.76, 143.76, 147.17, 147.46,
147.73, 147.81, 149.80, 150.88, 199.59, 215.98 ppm. ESI-MS [M +
H+] m/z: calcd 722.3171; obsd 722.2783.
Deprotection of R-OQN Ligands. The 5,7-substituted 8-
hydroxyquinoline ligand was isolated according to a reported
procedure by refluxing the benzyloxy protected precursor (2 mmol)
in ethanol overnight under an argon atmosphere with 1 g (12 mmol)
of 1,4-cyclohexadiene and 0.5 g of 10% palladium on carbon. After
cooling, the crude mixture was passed through a Celite plug with
excess dichloromethane. Analytically pure ligand was recovered
following removal of the dichloromethane ex vacuo and recrystalliza-
tion from hot methanol.
Computational Details. All calculations were carried out using
density functional theory (DFT) with the B3LYP functional as
implemented in the Gaussian 09 program package.69 The LANL08
basis set70 was used for Ru, and 6-31G(d,p) is used for other
elements.71,72 The optimization calculations were carried out using the
polarizable continuum model (PCM) with the dielectric constant of
acetonitrile.73 A vibrational frequency analysis coupling with PCM
model was carried out in order to confirm the minimum-energy
geometry in solution, followed by time-dependent density functional
theory (TD-DFT).74 Doublet species were calculated using an
unrestricted spin approach.
5,7-Bis(4-methoxyphenyl)-8-hydroxyquinoline. This was syn-
1
thesized in 91% yield. H NMR δ[d6-DMSO]: 3.82 (s, 3H), 3.84 (s,
3H), 6.67 (d, 2H, J = 5.4 Hz), 6.74 (d, 2H, J = 5.4 Hz), 7.46 (d, 2H, J
= 5.4 Hz), 7.49 (s, 1H), 7.57 (dd, 1H, J = 2.7, 5.1 Hz), 7.75 (d, 2H, J =
5.4 Hz), 7.62 (s, 1H), 8.25 (dd, 1H, J = 0.9, 5.1 Hz), 8.92 (dd, 1H, J =
0.9, 2.4 Hz), 9.85 (s, 1H) ppm. ESI-MS [M + H+] m/z: calcd
358.1443; obsd 358.1311.
5,7-Bis(4-diphenylamino)phenyl-8-hydroxyquinoline. This
1
was synthesized in 82% yield. H NMR δ[CDCl3]: 7.00−7.42 (m,
Materials. Tetrakis(triphenylphosphine)palladium, 10% palladium
on charcoal, 4-methoxyphenylboronic acid, 4-(diphenylamino)-
phenylboronic acid, 1,4-cyclohexadiene, potassium carbonate, potas-
sium hydroxide, potassium hexafluorophosphate, Celite, and acetoni-
trile (spectrophotometric grade, Aldrich) were purchased from Aldrich
and used as received. ACS reagent grade solvents methanol, acetone,
acetonitrile, and diethyl ether (Pharmco) were used as received.
Tetrabutylammonium hexafluorophsophate (Aldrich) was recrystal-
lized thrice from hot ethanol prior to use. Compounds 5,7-diiodo-8-
benzyloxyquinoline,75 5-phenyl-8-hydroxyquinoline,52 5,7-diphenyl-8-
25H), 7.63 (s, 2H), 7.55 (d, 4H, J = 9.0 Hz), 8.42 (d, 1H, J = 8.1 Hz),
8.81 (s, 1H) ppm. 13C NMR δ[CDCl3]: 11.12, 14.21, 23.11, 23.89,
24.60, 28.51, 29.04, 29.84, 30.52, 34.14, 38.83, 66.95, 115.43, 116.38,
117.69, 121.49, 121.88, 121.96, 122.77, 122.92, 123.02, 123.15, 123.56,
123.60, 124.26, 124.62, 125.75, 127.65, 129.17, 129.38, 129.46, 129.73,
130.20, 130.46, 130.91, 131.57, 133.29, 134.82, 138.85, 147.02, 147.21,
147.79, 147.93, 199.58, 216.00 ppm. ESI-MS [M + H+] m/z: calcd
632.2702; obsd 632.2617.
General Synthetic Method for [Ru(bpy)2(R-OQN)][PF6]
Complexes (4+−8+). A 50 mL flask was charged with 10 mL of
methanol and the solution purged with argon for 10 min. To the flask
was added 0.10 mmol of Ru(bpy)2Cl2·2H2O, 0.11 mmol of the
appropriately substituted hydroxyquinoline, and 1.1 mL of 0.1 M
aqueous potassium hydroxide. With an argon atmosphere maintained,
the purple suspension was allowed to reflux with stirring for 5 h
resulting in a deep reddish/brown solution. The methanol was then
removed on a rotary evaporator resulting in a crude aqueous solution
of the [Ru(bpy)2(R-OQN)]Cl salt. Additional water was added (5
mL), the pH was adjusted to 7 using 0.1 M aqueous HCl, and the trace
unreacted ligand was removed by gravity filtration. To the deep red
homogeneous filtrate was added dropwise 1 M aqueous KPF6 until a
dark reddish brown precipitate developed. The solid was isolated by
vacuum filtration on a medium porosity sintered funnel. Recrystalliza-
44
hydroxyquinoline,52 Ru(bpy)2Cl2,76 2+, and 3+ were prepared
according to literature methods.
General Method for Synthesis of Benzyl Protected R-OQN
Ligands. To an argon degassed 20 mL solution of toluene/ethanol/
water (2:1:1) was added 0.73 g (1.5 mmol) of 5,7-diiodo-8-
benzyloxyquinoline, 3.5 mmol of the appropriate boronic acid, 0.97
g (7 mmol) of potassium carbonate, and 0.74 g (0.6 mmol) of
tetrakis(triphenylphosphine)palladium. The reaction mixture was
refluxed under an argon atmosphere with stirring for 12 h. Upon
cooling 30 mL of dichloromethane was added, and the organic phase
was washed with water and brine and dried over MgSO4.
Dichloromethane was subsequently removed ex vacuo, and the
benzyloxy protected ligand precursor was precipitated from hot
methanol. The 5,7-substituted 8-hydroxyquinoline ligand was isolated
J
dx.doi.org/10.1021/ic5002623 | Inorg. Chem. XXXX, XXX, XXX−XXX