Highly Fluorinated Ir(III)-2,2′:6′,2″-Terpyridine-Phenylpyridine-X Complexes via Selective C-F Activation: Robust Photocatalysts for Solar Fuel Generation and Photoredox Catalysis

Article abstract of DOI:10.1021/jacs.6b03246

A series of fluorinated Ir(III)-terpyridine-phenylpyridine-X (X = anionic monodentate ligand) complexes were synthesized by selective C-F activation, whereby perfluorinated phenylpyridines were readily complexed. The combination of fluorinated phenylpyridine ligands with an electron-rich tri-tert-butyl terpyridine ligand generates a "push-pull" force on the electrons upon excitation, imparting significant enhancements to the stability, electrochemical, and photophysical properties of the complexes. Application of the complexes as photosensitizers for photocatalytic generation of hydrogen from water and as redox photocatalysts for decarboxylative fluorination of several carboxylic acids showcases the performance of the complexes in highly coordinating solvents, in some cases exceeding that of the leading photosensitizers. Changes in the photophysical properties and the nature of the excited states are observed as the compounds increase in fluorination as well as upon exchange of the ancillary chloride ligand to a cyanide. These changes in the excited states have been corroborated using density functional theory modeling.

Full text of DOI:10.1021/jacs.6b03246

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Article
Highly Fluorinated Ir(III)- 2,2#:6#,2#-Terpyridine -Phenylpyridine-
X Complexes via Selective C-F Activation: Robust Photocatalysts
for Solar Fuel Generation and Photoredox Catalysis
Jonathan A. Porras, Isaac N. Mills, Wesley J. Transue, and Stefan Bernhard
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b03246 • Publication Date (Web): 07 Jul 2016
Downloaded from http://pubs.acs.org on July 7, 2016
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Highly Fluorinated Ir(III)ꢀ 2,2′:6′,2″ꢀTerpyridine ꢀPhenylpyridineꢀX Complexes via
Selective CꢀF Activation: Robust Photocatalysts for Solar Fuel Generation and Photoredox
Catalysis
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Jonathan A. Porras†, Isaac N. Mills†, Wesley J. Transue†, and Stefan Bernhard†
†Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
Abstract. A series of fluorinated Ir(III)ꢀterpyridineꢀphenylpyridineꢀX (X = anionic monodentate
ligand) were synthesized by selective CꢀF activation, whereby perfluorinated phenylpyridines
were readily complexed. The combination of fluorinated phenylpyridine ligands with an
electron rich triꢀtertꢀbutyl terpyridine ligand generates a “pushꢀpull” force on the electrons upon
excitation, imparting significant enhancements of the stability, electrochemical, and
photophysical properties of the complexes. Application of the complexes as photosensitizers for
the photocatalytic generation of hydrogen from water and the redox photocatalysis of the
decarboxylative fluorination of several carboxylic acids showcase the performances of the
complexes in highly coordinating solvents, in some cases exceeding performances of the leading
photosensitizers. Changes in the photophysical properties and nature of the excited states are
observed as the compounds increase in fluorination, as well as exchange of the ancillary chloride
ligand to a cyanide. These changes in the excited state are corroborated using DFT modeling.
Introduction.
Activation of CꢀF bonds in aromatic and aliphatic compounds has become a prominent
route not only for the synthesis of fluorinated organic compounds1ꢀ2, but also for accessing
transition metal complexes that are themselves fluorinated.3–13 In order to use CꢀF activation
strategically, understanding the chemoselectivity of the process is necessary.14 It has been shown
that CꢀH bond activation was typically preferred over CꢀF with second and third row transition
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metals, likely due to a higher kinetic barrier despite CꢀF activation being energetically favorable.
15–22 However, there have been a few complexes of Rh and Ir that have shown preference of CꢀF
activation over CꢀH, likely due to oxidative addition/reductive elimination producing a more
stable product.23–29 It appears that factors such as oxidation state of the metal center and kinetic
versus thermodynamic preference of the products have significant impacts on controlling the
selectivity of such reactions. Ultimately, having control of CꢀF versus CꢀH activation is key
given that fluorination is a useful strategy for the stabilizing and tuning of several second and
third row transition metal complexes, including Ir (III).
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Ir(III) complexes have seen use in a diverse array of applications ranging from
biomedical applications including emissive labels,30–35 and antiꢀtumor drugs36 to photocatalytic
applications including photosensitizing water splitting,37–39 photosensitizing organic
transformations,40–48 and dyeꢀsensitized solar cells,49 to photonic materials applications including
the fabrication of organic lightꢀemitting diodes (OLEDs).50–53 Ir(III) complexes have such broad
applications and strong performances due to their high phosphorescence quantum efficiency,
long excited state lifetimes, ease of tunability, and ability to be synthesized with a diverse array
of ligand functionalities and architectures. All of these properties are consistent with the strong
spinꢀorbit coupling of Ir(III), which gives rise to efficient intersystem crossing from the singlet to
triplet excited states, thereby imparting high phosphorescent efficiencies. Given the ease of
tunability of Ir(III) complexes, controlling the energies of metal to ligand charge transfer
(MLCT) and intraligand charge transfer (ILCT) processes can be achieved through modulation
of the ligand structure and functionalities. The strategies used to most greatly tune the complexes
typically involve decreasing the energy of the highest occupied molecular orbital (HOMO) by
introducing electronꢀwithdrawing groups, increasing the energy of the lowest unoccupied
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molecular orbital (LUMO) by introducing electronꢀdonating groups, and limiting the overall
conjugation of the ligands to increase the energy of any πꢀ π* emissive states.54
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Several electronꢀwithdrawingꢀmoieties have been utilized to tune the energies of the
HOMOs in Ir(III) complexes including sulfonyl,55,56 trifluoromethyl,57–64 pentafluoroꢀsulfur,65
trifluromethoxy,66 nitriles,67,68 and perfluoroꢀcarbonyl groups.69 While such functional groups
have resulted in excellent modulation of the HOMO levels in Ir(III) complexes, their
implementation into ligand structures and ability to utilize more than one in a single ligand
complicates their use. In contrast, fluorine is significantly simpler to implement into ligand
structures, therefore can be used to more easily tune energy levels by control of the number of
fluorine atoms. Therefore, synthesizing complexes with varying degrees of fluorination is
paramount to understanding the effects on the energy levels, properties, as well as catalytic
activity within a series of complexes. Ideally, perfluorinated complexes should display the most
dramatic difference in terms of the orbital energy levels and should be investigated. While
several highly fluorinated Ir(III) complexes have been published,70–72 their syntheses relied on
the reaction with an asymmetrical, polyfluorinated phenylꢀpyridine ligand or depended on the
use of directing groups which would promote the Ir(III) cyclometallation,73 further complicating
their synthesis. Therefore, a simplified protocol to include highly fluorinated, symmetrical
ligands into Ir(III) complexes would be advantageous.
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The ligand architecture [Ir(tpy)(ppy)X]+ has only recently gained attention due to its
overall enhanced stability, improved photophysical properties, and diverse catalytic applications.
74–78
It is advantageous that the HOMO and LUMO are well partitioned within the structures.
Specifically, the HOMO is primarily located on the cyclometallating ligand and ancillary ligand,
and the LUMO is predominantly positioned on the tridentate ligand. Utilizing an electronꢀrich
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tridentate ligand with a strongly electronꢀwithdrawing cyclometallating ligand, efficiently creates
a “pushꢀpull” force on the electrons upon excitation. In this work, a series of such “pushꢀpull”
type complexes are shown to have photophysical and electrochemical properties that are directly
impacted by differing degrees of fluorination, as well as the strength of the ancillary ligand. To
achieve various degrees of fluorination, the complexes were synthesized utilizing CꢀF activation,
which has not been used as a deliberate synthetic tool for Ir(III) complexes. In implementing CꢀF
activation, this works shows high selectivity in the specific stereoisomer that is formed in the
cyclometallating reaction. As such, synthesis of complexes with perfluorophenyl ligands has
been drastically simplified. Details on the electronic structure of the complexes are collected
through computational modeling, photophysical spectroscopy and electrochemical experiments.
Finally, the complexes are evaluated for catalytic applications including photocatalytic hydrogen
evolution and redox photocatalysis of the decarboxylative fluorination of carboxylic acids.
Experimental
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General. The compounds 4,4′,4″ꢀtriꢀtertꢀbutylꢀ2,2′:6′,2″ꢀterpyridine (ttbutpy), IrCl3•4H2O, and
all solvents were used as received from commercial sources. The ligands 5ꢀmethylꢀ2ꢀ
phenylpyridine (mppy), 2ꢀ(4ꢀfluorophenyl)ꢀ5ꢀmethylpyridine (4ꢀFmppy), 2ꢀ(2ꢀfluorophenyl)ꢀ5ꢀ
methylpyridine (2ꢀFmppy), 2ꢀ(2,4ꢀdifluorphenyl)ꢀ5ꢀmethylpyridine (2,4ꢀdFmppy), 2ꢀ(2,6ꢀ
difluorophenyl)ꢀ5ꢀ5methylpyridine (2,6ꢀdFmppy), and 5ꢀmethylꢀ2ꢀ(2,4,6ꢀtrifluorphenyl)pyridine
(2,4,6ꢀtFmppy) were prepared from the corresponding acylꢀpyridinium salts as described by
Lowry et. al.38 The ligands 5ꢀmethylꢀ2ꢀ(perfluorophenyl)pyridine (PFmppy) and 5ꢀmethoxyꢀ2ꢀ
(perfluorophenyl)pyridine (PFMeOppy) were prepared using the procedure described by Do et.
al.79 1H, 13C, and 19F NMR spectra were obtained using Bruker Avance 300 MHz and 500 MHz
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19
spectrometers. H and C NMR spectra were referenced to residual solvent signals. F NMR
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spectra were referenced to CFCl3 by using the PF6 anion (ꢀ71.11 ppm, doublet) as an internal
standard. ESIꢀMS was performed with 50 ꢁM methanol solutions using a ThermoꢀFisher LCQ
instrument. Elemental analyses were conducted by Robertson Microlit Laboratories
(Ledgewood, NJ).
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Electrochemistry. Cyclic voltammetry experiments were performed using a CHꢀInstruments
Electrochemical Analyzer 600C potentiostat with a three electrode system consisting of a
platinum coil counter electrode, a silver wire pseudoꢀreference, and a 1 mm2 platinum disk
working electrode. Scans were performed with positive scan polarity, at 0.10 V/S, under an
atmosphere of argon, using argonꢀpurged acetonitrile solutions that contained 0.10 M tetraꢀnꢀ
butylammonium hexafluorophosphate as the supporting electrolyte and 0.5 mM of the analyte.
An internal standard of ferrocene was added to each solution, referencing potentials to SCE via
the oxidation of ferrocene at 0.40 V.80
Computational Methodology. DFT Calculations were performed using the Gaussian ‘09 suite.81
The triplet and singlet groundꢀstate and excitedꢀstate were evaluated for the complexes using the
B3LYP functional and the LANL2DZ basis set. No symmetry conditions were specified. For
TDꢀDFT calculations, the optimized singlet groundꢀstate geometry was used for the 150 lowest
excitations and solvent (acetonitrile) was specified. Orbitals were visualized using Avogadro: an
openꢀsource molecular builder and visualization tool Version 1.1.1.82 TDꢀDFT calculations of
UVꢀVis absorption spectra were visualized using Gaussum,83 where transitions were expanded
into Gaussian curves with a fullꢀwidth at halfꢀmaximum (fwhm) set to 4000 cmꢀ1.
Photophysical Characterization. Room temperature photophysical measurements were
conducted using argonꢀpurged 10 ꢁM acetonitrile solutions in screwꢀtop, quartz cuvettes. UVꢀvis
absorption spectra were collected with
a
Shimadzu UVꢀ1800 spectrophotometer.
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Photoluminescence characterization experiments were performed using a Fluorologꢀ3
spectrophotometer equipped with dual monochromators and a photomultiplier tube (PMT) at a
right angle geometry. All compounds were excited at 380 nm. Excited state lifetimes were
determined by pulsing samples at 266 nm using the fourth harmonic of a Nd:YAG (Continuum
Minilite II) laser. Emission decays were monitored with an oscilloscope (Tektronix TDS 3032B)
and converted into a linear regression using a Labview PC interface. Emission quantum yields
were determined by comparison against a 10 ꢁM [Ru(bpy)3](PF6)2 reference in acetonitrile, with
an established quantum yield (Φref = 0.062).84 Quantum yields were calculated using the equation
Φs = Φref (Is/Iref)(Aref/As)(ηs/ηref), where Φs is the quantum yield of the sample, Φref is the quantum
yield of the reference, Is and Iref are the maximum emission intensities for the sample and the
reference, As and Aref are the absorbances of the sample and reference at the excitation
wavelength, and ηref and ηs are the refractive indices of the solvents. Radiative decay constants
(kr) were calculated using the equation kr = (Φs/τs) where τs is the excited state lifetime of the
sample and subsequently nonradiative decay constants (knr) were calculated using the
relationship knr = (1/τs) – kr . Emission intensities were corrected for the detector’s response over
the spectral range.
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Photocatalytic Hydrogen Generation. Photocatalytic generation of H2 was carried out using
the fixed protocol described by Cline et. Al.85 Screw top EPA vials (40 mL) contained 10 mL of
solvent (8 mL acetonitrile, 1 mL triethylamine, 1 mL water), 0.075 mM photosensitizer, and 300
nmol K2PtCl4. Control vials were prepared without the photosensitizer, the catalyst, or
trimethylamine, or with all components present but without illumination. The vials were placed
in a 16 well, temperature controlled photoreactor mounted on an orbital shaker. Each vial was
equipped with a pressure transducer as well as a bottom LED for illumination (Luxeon V Dental
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blue LEDs, LXHLꢀLRD5 with collimating optics Fraen FHSꢀHNBIꢀLL01ꢀH). The vials were
sealed and subsequently degassed with 7 cycles of vacuum and argon, after which the vials were
equilibrated to atmospheric pressure at 22 oC. The orbital shaker was started (100 rpm) and the
samples were illuminated. Generation of H2 was monitored over time by conversion of the
pressure transducer readings into pressure traces via a Labview PC interface. Illumination ceased
when no further increase in the traces was observed. Following illumination, quantification of
H2 produced was accomplished by injecting 1 mL of the vial headspace in a GOWꢀMAC gas
chromatograph (thermal conductivity detector, Ar carrier gas), preꢀcalibrated using 10% H2/Ar
gas mixtures.
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Photoredox Catalysis. The photoredox catalysis studies were performed using modified
reaction conditions reported by Ventre et. al.86 Screw top EPA vials (20 mL) containing 5.1 mL
of solvent (3:1 acetonitrile/water), 510 ꢁmol of the corresponding carboxylic acid, 1.02 mmol of
Na2HPO4, 1.53 mmol of Selectfluor, and 0.255 ꢁmol (0.05 mol %) of photosensitizer were
equipped with pressure transducers in a 16 well, temperature controlled photoreactor on top of an
orbital shaker. Samples were degassed with 7 cycles of vacuum and argon, after which the vials
were equilibrated to atmospheric pressure. The orbital shaker was started (100 rpm) and the
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samples were illuminated from the bottom at 22 C (Luxeon V Dental blue LEDs, LXHLꢀLRD5
with collimating optics Fraen FHSꢀHNBIꢀLL01ꢀH). Generation of CO2 was monitored over time
by conversion of the pressure transducer readings into pressure traces via a Labview PC
interface. Illumination ceased after three hours. The crude reaction mixtures were directly
analyzed by 19F NMR using hexafluorobenzene as an internal standard.
Synthesis of [Ir(ttbutpy)Cl3] (1a). In a 40 mL EPA vial, IrCl3•4H2O (321 mg, 0.800
mmol),4,4′,4″ꢀtriꢀtertꢀbutylꢀ2,2′:6′,2″ꢀterpyridine (326 mg, 0.880 mmol, 1.10 eq.), and a stirbar
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were purged with argon for 15 minutes before injecting 13 mL of ethylene glycol. The vessel
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was purged with argon for an additional 10 minutes before stirring at 160 C , in a preꢀheated
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aluminum heating block, for 22 minutes, in the dark. Once cooled, the reaction was diluted with
25 mL of water and the red precipitate was collected on a Büchner funnel, after washing with
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1
water and diethyl ether. Yield: 86%. H NMR (500 MHz, DMSOꢀd6): δ. 9.06 (m, 2H), 8.86 (m,
13
2H), 8.79 (m, 2H), 7.96 (m, 2H), 1.79 (m, 9H), 1.47 (m, 18H). Concentrations necessary for C
NMR could not be obtained due to the poor solubility of 1a in DMSOꢀd6.
Synthesis of [Ir(ttbutpy)(mppy)Cl](PF6) Complexes. In a typical reaction, a 40 mL EPA vial
was charged with [Ir(ttbutpy)Cl3] (50 mg, 0.071 mmol), cyclometallating ligand (0.213 mmol,
3.00 eq.), and a stirbar. The vessel was purged with argon for 15 minutes before 13 mL of
ethylene glycol was injected, and the vessel was purged for an additional 10 minutes. The
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reaction was stirred at 183 C in a preꢀheated aluminum heating block, in the dark, for 18 hours.
Once cooled, the reaction was diluted with 20 mL of water and sonicated. The reaction was
filtered through a pad of Celite, and the filtrate was poured into a 125 mL separatory funnel.
The waterꢀethylene glycol mixture was extracted with 5 x 25 mL of diethyl ether. The aqueous
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phase was gently heated in a 40 C water bath to remove residual diethyl ether before adding
~300 mg of KPF6. The mixture was stirred for 1 hour at room temperature before isolating the
complex on a Büchner funnel, washing with water and diethyl ether. The complexes were then
purified by evaporation of methanol from a methanolꢀwater mixture.
1
[Ir(ttbutpy)(mppy)Cl](PF6) (2a). Yield: 44%. H NMR (500 MHz, acetoneꢀd6): δ. 9.96 (s, 1H),
8.97 (s, 2H), 8.79 (s, 2H), 8.35 (d, J = 8.36 Hz, 1H), 8.14 (d, J = 8.31 Hz, 1H), 7.88 (d, J = 7.77
Hz, 1H), 7.72 (d, J = 6.02 Hz, 2H), 7.55 (dd, J = 1.88, 6.02 Hz, 2H), 6.95 (t, J = 7.48 Hz, 1H),
6.74 (t, J = 7.42 Hz, 1H), 6.11 (d, J = 7.60 Hz, 1H), 2.63 (s, 3H), 1.65 (s, 9H), 1.40 (s, 18H). 19F
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ꢀ
NMR (470.54 MHz, acetoneꢀd6): δ. ꢀ71.11 (d, J = 707.62, [PF6 ]). 13C NMR (125.77 MHz,
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acetoneꢀd6): δ 166.0, 165.6, 164.9, 159.4, 156.6, 152.3, 151.7, 145.3, 142.8, 141.6, 135.3, 131.6,
130.9, 126.4, 125.7, 124.9, 124.1, 122.6, 120.9, 37.6, 36.6, 31.1, 30.5, 18.7. MS (m/z ESI,
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ꢀ +
ꢀ +
CH3OH) Calculated: 797.3 [MꢀPF6 ] Found: 797.4 [MꢀPF6 ] . Elem. Anal. Calcd. For
[C39H45ClF6IrN4P]·1/4 H2O: C, 49.47; H, 4.84; N, 5.92. Found: C, 49.45; H, 4.87; N, 5.72.
[Ir(ttbutpy)(4ꢀFmppy)Cl](PF6) (2b). Yield: 33%. 1H NMR (500 MHz, acetoneꢀd6): δ. 9.92 (s,
1H), 8.98 (s, 2H), 8.81 (s, 2H), 8.32 (d, J = 8.36 Hz, 1H), 8.15 (d, J = 8.32 Hz, 1H), 7.97 (dd, J =
5.59, 8.56 Hz, 1H), 7.72 (d, J = 6.00 Hz, 2H), 7.56 (dd, J = 1.96, 6.01 Hz, 2H), 6.73 (td, J = 2.42,
8.86 Hz, 1H), 5.79 (dd, J = 2.38, 8.99 Hz, 1H), 2.62 (s, 3H), 1.66 (s, 9H), 1.40 (s, 18H). 19F
NMR (470.54 MHz, acetoneꢀd6): δ. ꢀ71.11 (d, J = 707.35, [PF6 ]), ꢀ109.17 (s, 1F). 13C NMR
ꢀ
(125.77 MHz, acetoneꢀd6): δ.166.2, 165.7, 163.9, 163.4 (d, J = 252.9 Hz), 159.2, 156.4, 152.2,
151.5, 146.02 (d, J = 5.6 Hz), 141.7, 141.8 (d, J = 2.2 Hz), 135.1, 127.6 (d, J = 9.1 Hz), 126,4,
124.2, 122.8, 120.9, 117.8 (d, J = 18.3 Hz), 111.7 (d, J = 22.7 Hz), 37.6, 36.5, 31.0, 30.4, 18.6.
ꢀ +
ꢀ +
MS (m/z ESI, CH3OH) Calculated: 815.3 [MꢀPF6 ] Found: 815.5 [MꢀPF6 ] . Elem. Anal. Calcd.
For [C39H44ClF7IrN4P]: C, 48.77; H, 4.62; N, 5.83. Found: C, 49.53; H, 4.61; N, 5.80.
1
[Ir(ttbutpy)(2ꢀFmppy)Cl](PF6) (2c). Yield: 61%. H NMR (500 MHz, acetoneꢀd6): δ. 10.05 (s,
1H), 8.97 (s, 2H), 8.80 (m, 2H), 8.47 (d, J = 8.51 Hz, 1H), 8.18 (d, J = 8.37 Hz, 1H), 7.74 (d, J =
6.01 Hz, 2H), 7.55 (dd, J = 1.88, 6.00 Hz, 2H), 6.78 (m, 1H), 6.72 (dd, J = 8.18, 12.42 Hz, 1H),
5.95 (m, 1H), 2.64 (s, 3H), 1.64 (s, 9H), 1.39 (s, 18H). 19F NMR (470.54 MHz, acetoneꢀd6): δ. ꢀ
71.11(d, J = 707.35, [PF6 ]), ꢀ112.54 (s, 1F). 13C NMR (125.77 MHz, acetoneꢀd6): δ.166.2, 165.7,
ꢀ
162.0, 161.0 (d, J = 264.3 Hz), 159.2, 156.4, 152.3, 152.2, 145.1, 141.8, 135.6, 132.6 (d, J = 5.5
Hz), 132.1 (d, J = 8.9 Hz), 127.5 (d, J = 2.8 Hz), 126.4, 124.8 (d, J = 19.7 Hz), 124.2, 122.6,
112.1 (d, J = 22.7 Hz), 37.6, 36.5, 31.0, 30.4, 18.6. MS (m/z ESI, CH3OH) Calculated: 815.3 [Mꢀ
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ꢀ +
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PF6 ] Found 815.4 [MꢀPF6 ] . Elem. Anal. Calcd. For [C39H44ClF7IrN4P]·1 H2O: C, 47.87; H,
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4.74; N, 5.73. Found: C, 47.27; H, 4.34; N, 5.52.
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[Ir(ttbutpy)(2,4ꢀdFmppy)Cl](PF6) (2d). Yield: 39%. H NMR (500 MHz, acetoneꢀd6): δ. 10.02
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(d, J = 1.99 Hz, 1H), 8.99 (s, 2H), 8.82 (s, 2H), 8.43 (d, J = 9.17 Hz, 1H), 8.20 (d, J = 10.32 Hz,
1H), 7.74 (d, J = 6.01 Hz, 2H), 7.57 (dd, J = 2.12, 6.05 Hz, 2H), 6.65 (ddd, J = 2.32, 9.31, 11.81
Hz, 1H), 5.70 (dd, J = 2.30, 8.12 Hz, 1H), 2.64 (s, 3H), 1.65 (s, 9H), 1.40 (s, 18H). 19F NMR
ꢀ
(470.54 MHz, acetoneꢀd6): δ. ꢀ71.11 (d, J = 707.34, [PF6 ]), ꢀ106.61 (d, J = 9.98 Hz, 1F), ꢀ108.82
(d, J = 10.00 Hz, 1F). 13C NMR (125.77 MHz, acetoneꢀd6): δ.166.5, 165.9, 164.3 (d, J = 12.5
Hz), 161.8 (dd, J = 110.3, 9.7 Hz), 160.2 (d, J = 12.8 Hz), 159.2, 156.3, 152.3, 152.1, 147.5 (d, J
= 6.6 Hz), 142.0, 135.6, 129.3 (dd, J = 5.3, 3.0 Hz), 126.5, 124.4, 124.2, 122.9, 114.3 (dd, J =
18.1, 3.0 Hz), 100.7 (t, J = 27.0 Hz), 37.6, 36.6, 31.0, 30.5, 18.6. MS (m/z ESI, CH3OH)
ꢀ +
ꢀ +
Calculated: 833.3 [MꢀPF6 ] Found: 833.4 [MꢀPF6 ] . Elem. Anal. Calcd. For
[C39H43ClF8IrN4P]·1/2 H2O: C, 47.44; H, 4.49; N, 5.67. Found: C, 47.35; H, 4.13; N, 5.60.
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[Ir(ttbutpy)(PFmppy)Cl](PF6) (2e). Yield: 36%. H NMR (500 MHz, acetoneꢀd6): δ. 10.05 (s,
1H), 8.95 (s, 2H), 8.83 (s, 2H), 8.49 (d, J = 8.50 Hz, 1H), 8.27 (d, J = 8.47 Hz, 1H), 7.74 (d, J =
6.02 Hz, 2H), 7.59 (dd, J = 2.06, 6.02 Hz, 2H), 2.65 (s, 3H), 1.64 (s, 9H), 1.42 (s, 18H). 19F
ꢀ
NMR (470.54 MHz, acetoneꢀd6): δ. ꢀ71.11 (d, J = 707.39, [PF6 ]), ꢀ134.11 (dd, J = 14.95, 25.13
Hz, 1F), ꢀ139.46 (ddd, J = 5.11, 15.09, 24.51 Hz, 1F), ꢀ153.34 (ddd, J = 5.05, 18.90, 24.51 Hz,
1F), ꢀ162.29 (t, J = 19.16 Hz, 1F). 13C NMR (125.77 MHz, acetoneꢀd6): δ.166.5, 166.2, 161.1
(m), 159.7, 156.8, 152.3 (d, J = 17.3 Hz), 152.7, 152.4, 142.2, 136.9, 126.6, 126.5 (d, J = 3.0 Hz)
125.4 (d, J = 19.8 Hz), 124.2, 124.1, 124.0 (d, J = 8.0 Hz), 122.2, 122.1 (d, J = 5.48 Hz), 118.3
ꢀ +
(d, J = 79.5 Hz), 37.5, 36.6, 31.0, 30.5, 18.7. MS (m/z ESI, CH3OH) Calculated: 869.2 [MꢀPF6 ]
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Found: 869.5 [MꢀPF6 ] . Elem. Anal. Calcd. For [C39H41ClF10IrN4P]·1/2 H2O: C, 45.77; H, 4.14;
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N, 5.47. Found: C, 45.73; H, 3.93; N, 5.37.
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[Ir(ttbutpy)(PFMeOppy)Cl](PF6) (2f). Yield: 50%. H NMR (500 MHz, acetoneꢀd6): δ. 9.99 (d,
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J = 2.83 Hz, 1H), 8.95 (s, 2H), 8.84 (d, J = 1.85 Hz, 2H), 8.52 (dd, J = 1.21, 9.28 Hz, 1H), 8.05
(dd, J = 2.77, 9.12 Hz, 1H), 7.80 (d, J = 5.97 Hz, 2H), 7.59 (dd, J = 2.13, 6.05 Hz, 2H), 4.14 (s,
3H), 1.64 (s, 9H), 1.42 (s, 18H). 19F NMR (470.54 MHz, acetoneꢀd6): δ. ꢀ71.11 (d, J = 707.37,
ꢀ
[PF6 ]), ꢀ134.37 (dd, J = 14.84, 25.16 Hz, 1F), ꢀ141.04 (ddd, J = 4.34, 14.90, 19.33 Hz, 1F), ꢀ
154.43 (ddd, J = 4.30, 18.86, 23.85 Hz, 1F), ꢀ162.37 (t, J = 19.14 Hz, 1F). 13C NMR (125.77
MHz, acetoneꢀd6): δ.166.6, 166.3, 159.7, 157.5 (d, J = 0.4 Hz), 156.8, 156.2 (m), 152.5, 152.4 (d,
J = 6.4 Hz), 146.7 (m), 144.8 (m), 140.7, 130.0 (m), 124.2 (d, J = 29.4 Hz), 126.6 (d, J = 19.8
Hz), 126.5, 126.0, 122.3 (d, J = 5.0 Hz), 122.2, 117.4 (d, J = 34 Hz), 57.2, 37.6, 36.5, 31.0, 30.5.
ꢀ +
ꢀ +
MS (m/z ESI, CH3OH) Calculated: 885.3 [MꢀPF6 ] Found: 885.4 [MꢀPF6 ] . Elem. Anal. Calcd.
For [C39H41ClF10IrN4OP]·1/4 H2O: C, 45.26; H, 4.04; N, 5.41. Found: C, 45.61; H, 3.93; N,
5.51.
Synthesis of [Ir(ttbutpy)(mppy)CN](PF6) Complexes. In a typical reaction, a 40 mL EPA vial
was charged with [Ir(ttbutpy)Cl3] (60 mg, 0.086 mmol), cyclometallating ligand (0.255 mmol,
3.00 eq.), and a stirbar. The vessel was purged with argon for 15 minutes before 15 mL of
ethylene glycol was injected, and the vessel was purged for an additional 10 minutes. The
reaction was stirred at 183 oC in a preꢀheated aluminum heating block, in the dark, for 18 hours.
The vessel was cooled to room temperature, in the dark, and the heating block was cooled to 90
oC. The reaction was heated and an aqueous KCN solution (22 mg, 0.340 mmol, 4 eq. in 0.5 mL
of water) was injected and left to stir for 2 hours at 90 oC. Once cooled, the reaction was diluted
with 15 mL of water and 5 mL of ethanol, sonicated, and filtered through a pad of Celite. The
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filtrate was poured into a 125 mL separatory funnel and extracted 5 x 25 mL with diethyl ether.
The aqueous phase was gently heated in a 40 oC water bath to remove residual diethyl ether
before adding ~ 500 mg of KPF6. The mixture was stirred for 1 hour before isolating the
complex on a Büchner funnel, washing with water and diethyl ether. The complexes were then
purified by evaporation of methanol from a methanolꢀwater mixture.
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[Ir(ttbutpy)(4ꢀFmppy)CN](PF6) (3a). Yield: 33%. H NMR (500 MHz, acetoneꢀd6): δ. 9.82 (s,
1H), 9.07 (s, 2H), 8.89 (s, 2H), 8.34 (d, J = 8.41 Hz, 1H), 8.19 (d, J = 10.02 Hz, 1H), 7.99 (dd, J
= 5.26, 8.63 Hz, 1H), 7.89 (d, J = 6.05 Hz, 2H), 7.60 (d, J = 8.17 Hz, 2H), 6.74 (td, J = 2.58,
8.84 Hz, 1H), 5.64 (dd, J = 2.56, 8.61 Hz, 1H), 2.65 (s, 3H), 1.68 (s, 9H), 1.42 (s, 18H). 19F
NMR (470.54 MHz, acetoneꢀd6): δ. ꢀ71.11 (d, J = 707.29, [PF6 ]), ꢀ109.26 (s, 1F). 13C NMR
ꢀ
(125.77 MHz, acetoneꢀd6): δ.166.0, 165.7, 164.9, 164.0 (d, J = 253.3 Hz), 159.8 (d, J = 4.40 Hz),
158.8, 155.5, 154.3, 152.4, 142.4 (d, J = 2.13 Hz), 141.7, 136.1, 128.0 (d, J = 1.76 Hz), 127.8 (d,
J = 8.6 Hz), 126.6, 124.6, 123.3, 121.4, 117.1 (d, J = 17.3 Hz), 112.2 (d, J = 23.1 Hz), 37.6, 36.6,
ꢀ +
ꢀ +
31.0, 30.4, 18.5. MS (m/z ESI, CH3OH) Calculated 806.0 [MꢀPF6 ] Found 806.5 [MꢀPF6 ] .
Elem. Anal. Calcd. For [C40H44F7IrN5P]·1/3 KPF6: C, 47.47; H, 4.38; N, 6.92. Found: C, 47.56;
H, 4.19; N, 6.89.
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[Ir(ttbutpy)(2,4ꢀdFmppy)CN](PF6) (3b). Yield: 47%. H NMR (500 MHz, acetoneꢀd6): δ. 9.92
(s, 1H), 9.08 (s, 2H), 8.90 (s, 2H), 8.47 (dd, J = 1.91, 8.53 Hz, 1H), 8.25 (dd, J = 1.85, 8.50 Hz,
1H), 7.91 (d, J = 9.03 Hz, 2H), 7.62 (dd, J = 2.14, 6.06 Hz, 2H), 6.65 (ddd, J = 2.33, 9.23, 12.66
Hz, 1H), 5.54 (dd, J = 2.34, 7.61 Hz, 1H), 2.67 (s, 3H), 1.67 (s, 9H), 1.42 (s, 18H). 19F NMR
ꢀ
(470.54 MHz, acetoneꢀd6): δ. ꢀ71.11 (d, J = 707.23, [PF6 ]), ꢀ106.42 (d, J = 10.09 Hz, 1F), ꢀ
108.30 (d, J = 10.10 Hz, 1F). 13C NMR (125.77 MHz, acetoneꢀd6): δ.166.4, 165.9, 164.7 (d, J =
11.7 Hz), 162.7 (dd, J = 11.5, 3.5 Hz), 162.2 (d, J = 7.3 Hz), 161.6 (dd, J = 5.0, 1.3 Hz), 160.7
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(d, J = 11.5 Hz), 158.8, 155.5, 155.3, 152.6, 142.0, 136.6, 126.7, 124.9, 124.8, 124.7, 123.5,
113.3 (dd, J = 2.9, 7.0 Hz), 101.2 (t, J = 27.6 Hz), 37.7, 36.6, 31.0, 30.5, 18.6. MS (m/z ESI,
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ꢀ +
CH3OH) Calculated: 824.3 [MꢀPF6 ] Found: 824.5 [MꢀPF6 ] . Elem. Anal. Calcd. For
[C40H43F8IrN5P]·1/2 KPF6: C, 45.28; H, 4.09; N, 6.60. Found: C, 46.40; H, 3.57; N, 6.55.
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[Ir(ttbutpy)(PFMeOppy)CN](PF6) (3c). Yield: 22%. H NMR (500 MHz, acetoneꢀd6): δ. 9.81
(s, 1H), 9.00 (s, 2H), 8.90 (s, 2H), 8.55 (d, J = 9.15 Hz, 1H), 8.09 (d, J = 9.06 Hz, 1H), 7.96 (d, J
= 5.70 Hz, 2H), 7.64 (d, J = 5.46 Hz, 2H), 4.16 (s, 3H), 1.65 (s, 9H), 1.43 (s, 18H). 19F NMR
ꢀ
(470.54 MHz, acetoneꢀd6): δ. ꢀ71.11 (d, J = 707.25, [PF6 ]), ꢀ134.25 (dd, J = 16.29, 27.06 Hz,
1F), ꢀ140.23 (m,1F), ꢀ154.10 (ddd, J = 4.23, 18.80, 26.84 Hz, 1F), ꢀ161.63 (t, J = 18.98, 1F). 13C
NMR (125.77 MHz, acetoneꢀd6): δ.166.6, 166.3, 159.5, 159.5, 158.3 (d, J = 1.2 Hz), 156.1,
152.8, 152.7, 143.8, 143.4, 127.0 (d, J = 20.8 Hz), 126.8, 126.7 (d, J = 8.8 Hz), 126.0, 125.1 (d, J
= 11.3 Hz), 124.8, 124.4, 123.6 (m), 122.7, 122.6 (d, J = 17.1 Hz), 57.2, 37.6, 36.7, 31.0, 30.5.
ꢀ +
ꢀ +
MS (m/z ESI, CH3OH) Calculated: 876.3 [MꢀPF6 ] Found: 876.4 [MꢀPF6 ] . Elem. Anal. Calcd.
For [C40H41F10IrN5OP]: C, 47.06; H, 4.05; N, 6.86. Found: C, 46.72; H, 4.01; N, 6.67.
Results and Discussion
Synthesis & CꢀF Activation. To achieve the desired “pushꢀpull” design, an [Ir(tpy)Cl3]
precursor with an electron rich terpyridine, 1a was needed. Like previous triꢀchloride precursors,
reacting 4,4′,4″ꢀtriꢀtertꢀbutylꢀ2,2′:6′,2″ꢀterpyridine with IrCl3•4H2O in ethylene glycol gave 1a in
high yield. Exclusion of light and air, extension of reaction time, increase of reaction
temperature, and dilution of the reaction were necessary for optimal yields.
The chloroꢀcomplexes (2aꢀ2f) were synthesized by CꢀF or CꢀH activation with the
appropriate mppy derivative in ethylene glycol, as shown in Scheme 1. Optimal yields were
achieved with higher reaction temperature, extended reaction time, and significant dilution for
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overcoming the poor solubility of 1a in ethylene glycol. The corresponding cyano complexes
(3aꢀ3c) were synthesized via a oneꢀpot reaction of the corresponding chloro complex with
aqueous KCN after cyclometallation, as shown in Scheme 2. All complexes were isolated as the
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PF6 salts after anionꢀmetathesis with aqueous KPF6.
Scheme 1. Synthetic Pathways and Labels of Ir(III) Complexesa Isolated yields in parentheses.
a Reaction Conditions: mppy ligand, ethylene glycol, argon atmosphere, 182 oC, overnight.
Scheme 2. ChlorideꢀCyanide exchange of Ir(III) Complexesa
a Reaction Conditions: (a) mppy ligand, ethylene glycol, argon atmosphere, 182 oC, overnight. (b) KCN (aq),
ethylene glycol, argon atmosphere, 90 oC, 2 h.
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The mppy ligand derivatives were synthesized via Kröhnke condensation of the
corresponding phenylacylꢀpyridinium salt with methacrolein38 or via a CuI catalyzed cross
coupling of pentafluorobenzene with the corresponding 2ꢀbromopyridine precursor.79
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The addition of electronegative fluorine atoms to the mppy ligand was needed in order to
achieve the desired “pushꢀpull” design by altering the HOMO energy level of the complexes. It
has been thought that cyclometallations of Ir(III) with mppyꢀtype ligands have shown preference
of CꢀH over CꢀF activation, with oꢀfluorinated mppy ligands. However, in the reaction of 2,4ꢀ
dFmppy with 1a, the monoꢀfluorinated complex 2b was the primary product, with small
quantities of the diꢀfluorinated complex 2d detected by analysis of the crude reaction mixture
19
with F NMR and ESIꢀMS (Supporting Information). A similar behavior was observed in the
reaction of 1a with 2ꢀFmppy, whereby the CꢀF activation product 2a was the reaction product,
19
with small quantities of the monofluorinated 2c detected by F NMR and ESIꢀMS (Supporting
Information). Purification of these reactions by crystallization after precipitation of the PF6 salt
did produce only the pure CꢀF activation products. To synthesize 2d, 2,4,6ꢀtFmppy was instead
used as the cyclometallating ligand, whereas to synthesize 2c, 2,6ꢀdFmppy was used. CꢀF
activation was also observed by the formation of the perfluorinated complexes when
pentafluorophenyl ligands were used. Given the preference of CꢀF activation over CꢀH, it was
then possible to synthesize complexes 2a and 2b using CꢀF or CꢀH activation, as shown in
Scheme 3. It should be noted that yields of the reactions using CꢀF activation were comparable in
the synthesis of 2a (44% using 2ꢀFmppy, 40% using mppy), but were significantly higher in the
synthesis of 2b (48% using 2,4ꢀdFmppy versus 33% for 4ꢀFmppy). In all cases, the ancillary
ligand for the complexes synthesized via CꢀF activation remained a chloride and was not
exchanged to fluoride, as confirmed by 19F NMR and mass spectrometry. Over the course of the
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reaction, the Ir(III) precursor is transiently reduced to Ir(I), oxidizing the ethylene glycol solvent
and liberating protons. These liberated protons react with the fluoride produced by the CꢀF
insertion, generating hydrofluoric acid, which was then shown to react with borosilicate glass
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ꢀ
reaction vials by the appearance of BF4 and BF3OHꢀ detected via 19F NMR of the crude reaction
mixture (Supporting Information)87. Formation of SiF6 was anticipated,88 however it could not
2ꢀ
be detected by 19F NMR.
Scheme 3. Syntheses of 2a and 2b using CꢀF and CꢀH activation under identical reaction conditions. Isolated yields
in parentheses. Note: The CꢀF activation products are the major product of the reactions, with trace amounts of the
CꢀH products being detected by 19F NMR and ESIꢀMS analysis of the crude reaction mixture (Supporting
Information). Crystallization furnishes only the pure CꢀF activation products.
The preferential CꢀF activation of the complexes over CꢀH is likely due to the formation
Ir(I) species typically generated during cyclometallation reactions performed in oxidizable
solvents and the elevated reaction temperatures favoring CꢀF activation over CꢀH. In the
presence of oxidizable ethylene glycol, 1a is likely reduced to a transient Ir(I) species, which
upon oxidative addition to a fluorinated mppy ligand, is then reꢀoxidized to the final Ir(III)
species. Given the elevated reaction temperatures, thermodynamic control of the products would
produce more of the CꢀF activation product over the CꢀH product. To better understand the
reaction, attempts to identify the oxidized products were made by reaction with 2,4ꢀ
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dintrophenylhydrazine, followed by characterizing the corresponding hydrazone that preciptated
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from the solution by H NMR (Supporting Information). While the product corresponded to the
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hydrazone formed from the reaction with glyoxal, the origin of the glyoxal from the oxidation by
the Ir could not be distinguished from the oxidation of the solvent by residual oxygen in the
reaction, or from that which originated from the solvent oxidation over time at elevated
temperatures, making additional detailed characterizations and quantifications difficult. Control
reactions using solvents that are inert to oxidation including 2,3ꢀdimethylꢀ2,3ꢀbutanediol, 2,5ꢀ
dimethylꢀ2,5ꢀhexanediol, and tertꢀbutanol were inconclusive since the lower solubility of the
starting materials could have prevented any CꢀF activation from occurring. Attempts to run the
reactions at lower temperatures were made, however no reaction occurred indicating an elevated
energy barrier for the reaction.
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It should be noted that recent work by Huang et. al 31 had performed a similar reaction of
[Ir(pꢀtolylꢀtpy)Cl3] (where (pꢀtolylꢀtpy) is 4'ꢀ(4ꢀmethylphenyl)ꢀ 2,2':6',2''ꢀterpyridine) with 2,4ꢀ
dFppy, and mischaracterized the product as the difluorinated [Ir(pꢀtolylꢀtpy)(2,4ꢀdFppy)Cl]+.
Closer inspection of mass spectrometry experiments reveals a trace amount of the difluorinated
(CꢀH activation product) in the presence of the primarily monofluorinated (CꢀF activation
product). Similar ratios of the CꢀF and CꢀH products were observed in mass spectrometry
experiments conducted in this work (Supporting Information). While the 1H NMR and elemental
analysis of the product could correspond to the CꢀH activation product, the lack of 19F NMR and
13C NMR, whereby monoꢀ versus diꢀfluorination of the would be most obvious, makes
identification of the reaction product difficult.
Computational Modeling. The electronic structures and geometries of the complexes were
modeled using DFT calculations, specifically with the B3LYP functional and the LANL2DZ
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basis set for the singlet and triplet states. The frontier orbitals for the singlet and triplet states of
the monofluorinatedꢀchloro complex 2c and the perfluorinatedꢀcyano complex 3c are depicted in
Figure 1. The LUMO of each complex is located primarily on the terpyridine ligand with
contributions from the d orbitals of the metal center, owing to the strongly electronꢀdonating tertꢀ
butyl groups on the terpyridine. The HOMO of the complexes display contributions from the
phenyl ring of mppy ligand, the iridium d orbitals, and the ancillary ligand.
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Figure 1. Frontier orbitals of 2c (left) and 3c (right) from Gaussian ‘09 DFT calculations. Notable reduction in the
metal character of the LUMOs, HSOMOs, and LSOMOs is observed between the monoꢀfluorinated, chloroꢀcomplex
2c and the perfluorinated, cyanoꢀcomplex 3c. The HSOMO ➝ LSOMO transition in 2c is mixed MLCT/ILCT
whereas in 3c the loss of metal character results in the transition being primarily ILCT.
For the chloro complexes, as the mppy ligands increase in fluorination, the electronic
structure of the LUMO remains unchanged. However, in the cyano complexes, the LUMOs show
a decrease in metal character as the complexes increase in fluorination (Supporting Information).
The electronic structure of the HOMO of the chloro complexes shows increase in the electron
density of the phenyl ring in the mppy ligand as you increase fluorination. A similar trend is
observed in the cyano complexes. The addition of a methoxy group to the pyridine ring in chloro
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complex 2f and cyano complex 3c results in a significant increase in the contribution of the
pyridine ring to the HOMO.
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As the chloro complexes become more fluorinated, the HOMO energy levels of the
complexes are reduced compared to the unsubstituted 2a. Fluorination also results in a
broadening of the HOMOꢀLUMO gap within a 0.36 eV range for the chloro complexes. Both of
these trends are also seen in the corresponding cyano complexes. Exchange of the chloride to a
stronger field cyanide ligand results in a decrease in the HOMO levels by 0.26 eV, as well as
broadening of the HOMOꢀLUMO gaps.
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UVꢀVis Absorption Spectroscopy. The absorption spectra of complexes 2b, 2f, and 3c
collected in acetonitrile are shown in Figure 2. Complex 2b’s spectrum is representative of 2aꢀ
2d, whereas 2f is representative of the perfluorinated, chloro complexes. The spectrum of 3c is
representative of the cyano complexes 3aꢀ3c. All photophysical properties are presented in Table
1. The spectra of chromophores with a [Ir(tpy)(ppy)X]+ ligand structure, and the origins of the
observed transitions are similar to published spectra74 as indicated by TDꢀDFT calculations
(Figures S3ꢀS5 in Supporting Information). A weak feature near 475 nm (HOMO➝LUMO,
mixed MLCT/ILCT) is present in all of the chloro complexes 2aꢀ2f which is adjacent to stronger
transition at 390 nm (HOMOꢀ1➝LUMO and HOMO➝LUMO+1, mixed MLCT/ILCT). Upon
exchange of the chloride to a cyanide, these features subsequently disappear. TDꢀDFT
calculations of 3c indicate that these transitions, albeit weaker, are instead blueꢀshifted to 402 nm
(HOMO➝LUMO, mixed MLCT/ILCT) and 343 nm (HOMOꢀ2➝LUMO and
HOMO➝LUMO+2, mixed MLCT/ILCT) respectively. Subsequently, the region between 280
nm and 320 nm lacks any strong absorption bands for the chloro complexes with the exception of
the perfluorinated complexes 2e and 2f. Two bands near 280 nm (HOMOꢀ7➝LUMO and
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HOMOꢀ6➝LUMO+1, mixed MLCT/ILCT) and 320 nm (HOMOꢀ4➝LUMO and HOMOꢀ
5➝LUMO, mixed MLCT/ILCT) are prominent in these complexes, and is contrary to the
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spectra observed from previously published chloro analogues. Instead, this region bears
similarities to that of the cyano complexes 3a – 3c, which are characterized by mixed
MLCT/ILCT transitions at 280 nm (HOMOꢀ4➝LUMO+1 and HOMOꢀ3➝LUMO+1) and 320
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nm (HOMOꢀ4➝LUMO and HOMOꢀ3➝LUMO). The emergence of these features in 2e and 2f
is likely due to the strong electronꢀwithdrawing nature of the perfluorophenyl moiety. Finally, a
strong feature at 240 nm is present in all complexes originating from an ILCT (ancillary ligand p
orbitals, π orbitals of the central ring of the terpyridine, and π orbitals of the phenyl ring of the
mppy to π * orbitals of the terpyridine and π * orbitals of the pyridine ring of the mppy). This
feature is blue shifted as complexes increase in fluorination, as well as upon exchange of a
chloride to a cyanide due to increasing stabilization of the frontier orbitals.
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Figure 2. UVꢀVis absorption spectra for 2b (black), 2f (orange), and 3c (blue dashes). All spectra were collected in
acetonitrile (10 µM) at room temperature.
Table 1. Photophysical Properties
emission
Φ/
%
λmax/
τa/
ꢁs
knr (105) /
sꢀ1
kr (105)/
sꢀ1
complex
absorption λmax/ nm (intensity/104 Mꢀ1 cmꢀ1)
nm
537
525
525
515
498
498
488
480
470
2a
2b
2c
2d
2e
2f
3a
3b
3c
237 (5.37), 256sh (3.75), 270 (3.55), 280sh (3.34), 303 (2.74), 326sh (1.71), 361 (0.61), 382sh (0.54), 465 (0.14)
205 (5.50), 238 (5.93), 256sh (4.31), 269 (4.08), 277sh (3.74), 296sh (3.04), 323 (2.15), 381 (0.54), 450 (0.13)
204 (5.41), 236 (6.32), 256sh (4.26), 269 (3.87), 282 (3.65), 302 (3.00), 323sh (2.10), 358 (0.62), 382 (0.57), 455 (0.13)
206 (5.28), 237 (6.01), 257 (4.22), 268sh (3.81), 280 (3.40), 304sh (2.59), 323 (2.17), 375sh (0.57), 450 (0.14)
203 (6.46), 232 (6.35), 252sh (5.05), 282 (4.34), 320sh (2.61), 366 (0.74), 436 (0.14)
204 (6.91), 232 (7.01), 258 (5.81), 280 (5.27), 320 (3.27), 365 (0.81), 435 (0.14)
3.02
3.17
3.77
3.87
3.05
3.15
3.35
3.50
9.93
60.7 ± 5.2
64.6 ± 5.5
69.3 ± 5.9
58.6 ± 5.0
50.0 ± 4.3
47.5 ± 4.1
55.8 ± 5.1
61.8 ± 5.9
41.4 ± 4.2
1.30
1.12
0.81
1.07
1.64
1.67
1.32
1.09
0.59
2.01
2.04
1.84
1.51
1.64
1.51
1.67
1.77
0.42
216 (6.72), 238 (5.28), 271 (5.41), 280 (5.26), 316 (2.89), 430 (0.01)
216 (6.69), 270 (5.28), 280 (4.89), 317 (3.02), 428 (0.01)
216 (7.08), 255sh (5.49), 271 (6.21), 280 (6.11), 316 (3.40), 337sh (1.72),
a Excited state lifetimes were measured to within ± 50 ns.
Emission Spectroscopy. The complexes are strongly emissive in degassed, room temperature
acetonitrile, with emission colors ranging from yellow to blueꢀgreen (Figure 3). Complexes 2aꢀ
2d have broad, structureless emissions consistent with the mixed metal to ligand charge transfer
(MLCT) and intraligand charge transfer (ILCT) character of cyclometallated iridium complexes.
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In contrast, complexes 2eꢀ2f and 3aꢀ3c have greater vibrational substructure in their emission,
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indicating a greater degree of ILCT emission than MLCT.
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Figure 3. Room temperature emission spectra of degassed 10 µM solutions in acetonitrile. Complexes were excited
at 380 nm. Vibrational substructure is visible in the spectra of complexes with predominantly ILCT transitions (2eꢀ
2f, 3aꢀ3c).
Increasing fluorination of the mppy ligand results in a blueꢀshift of the emission maxima
of the complexes, owing to an increase in the stabilization of the HOMO. When comparing 2e
and 2f, the emission maxima of the complexes remains unchanged despite the addition of the
methoxy group to 2f. Exchange of the chloride ligand to a cyanide also blueꢀshifts the emission
maxima when compared to the corresponding chloro analogues. As the mppy ligand is
fluorinated, the HOMOꢀLUMO gap is broadened. A similar trend is seen in the cyano analogues
3aꢀ3c. It is worth noting that despite 2b and 2c being monofluorinated, they have higher
quantum yields than the unsubstituted 2a. When compared to other common iridium
complexes,38 the excited state lifetimes of the complexes are notably high, averaging 3.3 ꢀs, with
the exception of 3c which is three times as long. The knr rate constants of 2aꢀ3c are lower than
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other published Ir(III) complexes with the [Ir(tpy)(ppy)X]+ ligand structure, 74 indicating that the
complexes have more desirable photophysical properties (Figure 4). In addition, excluding 2e
and 2f, all complexes have lower knr rate constants compared to the most popular “pushꢀpull”
derivative of the [Ir(ppy)2(bpy)]+ family, [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 (knr = 1.39 x 105 sꢀ1).89
The complexes also have high kr rate constants, but are lower relative to
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6 (kr = 2.96 x 105 sꢀ1). The overall high quantum efficiency of these
complexes can be attributed to their “pushꢀpull” design.
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Figure 4. Energy gap law correlation of complexes 2aꢀ3c. Cyan circles indicate complexes with emissions from a
mixed MLCT/ILCT. Magenta squares indicate complexes with emissions having higher ILCT character. Differences
in excited state character are evident from the vibrational substructure of the luminescence spectra (Figure 3) as well
as the grouping of these data points.
Analyzing the electronic structure of the singly occupied molecular orbitals (SOMOs)
explains the change in the emission structure and photophysical properties as the complexes
increase in fluorination, as well as exchange the chloro ligand to a cyano. As shown in Figure 1,
2c has strong contribution of the iridium center in the electronic structures of its SOMOs. This
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high metal character and greater degree of spinꢀorbit coupling results in an emission from a
primarily MLCT, which is also seen in complexes 2bꢀ2d. In contrast, 3c has significantly less
metal character in the SOMOs, resulting in the emission of 3c from a predominantly ILCT. The
presence of the corresponding transitions in the UVꢀVis spectra of the perfluorinated chloro
complexes 2e and 2f, which are identical to that of the cyano complexes, also indicates a
similarity in their excited states. The significantly longer excited state lifetime and lower
quantum yields in comparison to other complexes from the series is symptomatic of this switch
in the nature of the excited state. It should also be noted that when the perfluorinated complexes
2e and 2f are analyzed in accordance to the Energy Gap Law, their excited states behave
similarly to that of the cyano complexes 3aꢀ3c, an observation that is highlighted in Figure 4.
DFT calculations have been previously utilized to predict the emission energy of Ir(III)
complexes.38,90,91One method involves calculating the energy difference of the singlet and triplet
excited states at the optimized triplet geometry.38,91 This method has been successful with
structurally diverse sets of iridium (III) complexes, but the dramatic differences in the character
of the excited states of 2aꢀ3c tremendously diminished the accuracy of these predictions (R2 =
0.60, Supporting Info). Instead, if the energy difference between the singlet ground states and
triplet excited states at their respective optimized geometries is used, a significant improvement
is made in modeling the emission energies, especially given the diversity of excited states within
this family of complexes (R2 = 0.96, Figure 5).90,91
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Figure 5. Relationship between experimental emission energy and the total energy difference between the singlet
ground and triplet excited states at their respective optimized geometries. Cyan circles indicate the chloro complexes
2aꢀ2f and magenta circles indicate cyano complexes 3aꢀ3c.
Electrochemical Characterization. Cyclic voltammetry experiments for the synthesized
iridium compounds were performed in acetonitrile (Table 2). All complexes show only one
oxidation that can be attributed to a metalꢀcentered process with some involvement of the mppy
ligand and the ancillary ligand. In the chloro complexes, the oxidation is quasiꢀreversible (Figure
6a), whereas in the cyano complexes the oxidation is irreversible (Figure 6b). Previous
complexes from the [Ir(tpy)(ppy)X]+ family have shown up to two oxidations,74–77 however none
had such a strong of a “pushꢀpull” design as the complexes in this work, which results in
increased electron density in the mppy and ancillary ligands. As fluorination increases in both
the chloro and cyano complexes, the oxidation potential shifts positively due to stabilization of
the HOMO on the mppy and ancillary ligands. In the cases of 2f and 3c, the addition of a
methoxy group to the pyridine ring of the cyclometallating ligand results in a decrease in the
oxidation potential, given its strongly electron donating nature and participation in the HOMO.
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The oxidation potential is also shifted positively when the chloride ligand is exchanged for a
cyanide, owing to its stronger ligandꢀfield character.
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Table 2. Electrochemical Properties of Chloro Complexes 2aꢀ2f and Cyano Complexes 3aꢀ3c
9
oxidationa
reductiona
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Epa/V
I: E1/2/V
(∆E/mV)
II:
Epc/V
III:
Ered/V
compound
1.61
1.70
1.69
1.87
1.88
1.83
ꢀ1.23 (45)
ꢀ1.17 (65)
ꢀ1.21 (69)
ꢀ1.18 (38)
ꢀ1.19 (51)
ꢀ1.14 (97)
ꢀ1.69
ꢀ1.63
ꢀ1.68
ꢀ1.62
ꢀ1.65
ꢀ1.63
ꢀ2.11
ꢀ2.13
ꢀ2.17
ꢀ1.80
ꢀ1.93
ꢀ1.91
2a
2b
2c
2d
2e
2f
oxidationa
reductiona
Epa/V
I: E1/2/V
(∆E/mV) (∆E/mV)
II: E1/2/V
compound
1.95
1.73 (sh), 2.01
2.00
ꢀ1.17 (68) ꢀ1.74 (83)
3a
3b
3c
ꢀ1.17 (66) ꢀ1.74 (86)
ꢀ1.17 (55) ꢀ1.75 (81)
a Potentials were measured in acetonitrile using 0.10 M tetraꢀnꢀbutylammonium hexafluorophosphate as the
supporting electrolyte with 0.5 mM of the analyte. Cyclic voltammetry experiments were performed using a three
electrode system, with positive scan polarity, at 0.10 V/S, under an atmosphere of argon, using argonꢀpurged
solutions. Potentials are given in V vs SCE, with peak separations given in mV.
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Figure 6. Cyclic voltammograms of a) 2b and b) 3a. Roman numerals indicate the unique reductions seen in the
complexes. Voltammograms were recorded using argonꢀdegassed 0.10
M
tetraꢀnꢀbutylammonium
hexafluorophosphate (acetonitrile) solutions containing 0.5 mM of analyte, at 0.10 V/s with a threeꢀelectrode
system. All potentials are referenced to SCE using ferrocene as an internal standard (fc/fc+ = 0.40 V).80
The first reduction observed for all complexes occurs on the terpyridine ligand of
complexes with the [Ir(tpy)(ppy)X]+ configuration. This reduction is completely reversible in all
complexes studied. While the complexes all have the same triꢀtertꢀbutylꢀterpyridine ligand,
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differences in the first reduction potentials are still observed based on changes to the mppy and
ancillary ligands. The absence of fluorine atoms in 2a results in a slightly lower reduction
potential compared to the fluorinated analogues. In addition, it appears that the position of the
fluorine atoms leads to differences in the reduction potential. Complex 2c, despite having the
same number of fluorine atoms as 2b, has a noticeable lower reduction potential. This difference
is likely due to the metaꢀ position of the fluorine atom to the cyclometallating carbon in 2c
compared to 2b. Meanwhile the methoxy group in 2f increases the first reduction compared to
the methyl analogue 2e, despite both being the most fluorinated. In contrast, the first reduction
remains unchanged within the structurally different cyano complexes 3aꢀc.
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The second reduction was assigned to the cyclometallating ligand of the Ir complexes. In
the chloro complexes 2aꢀ2f, this reduction is found to be irreversible, whereas it is fully
reversible in the cyano complexes 3aꢀ3c. Much like its first reduction, the second reduction of 2a
is lower than that of the other complexes within the chloro series. Subsequently 2c’s second
reduction, much like its first reduction, is also lower than its isomer 2b, despite the same degree
of fluorination of the phenyl ring. In addition, the methoxy group of 2f is responsible for the
higher second reduction compared to the methyl analogue 2e. To contrast, the second reduction
observed in the cyano complexes remains fairly unchanged by fluorination. Given that the
cyanide ligand has stronger back bonding to the Ir(III) center than a chloride, dissociation is
unlikely, resulting in not only the reversibility of this reduction in the cyano complexes, but also
its stability regardless of ligand fluorination.
A third reduction is only observed in the chloro complexes 2aꢀ2f, and is related to the
dissociation of the chloride ligand. This reduction is irreversible much like the second reduction
in the chloro complexes. The loss of the chloride is more difficult as the complexes are
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substituted by one fluorine, and a small change is again observable in the different monoꢀ
fluorinated complexes 2b and 2c. However, the complexes with 2 or more fluorine atoms show
the loss of the chloride to be significantly easier. Perfluorination seems to increase the third
redox potential regardless of the substitution of the pyridine ring. Overall, difluorinated analogue
2d shows it is the most susceptible to chloride loss.
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Using the redox potentials from cyclic voltammetry and the emission maxima, excited
state redox potentials were determined to better evaluate the photosensitization properties of the
complexes (Table 3).92 While the complexes’ excited states are not as strongly reducing as
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6,
a
known water reducing catalyst89 and “pushꢀpull”
photosensitizer, or the unsubstituted parent complex [Ir(ppy)2(bpy)]PF6,93 the complexes are
significantly stronger oxidants when excited.
Table 3. Calculated Excited State Redox Potentials92
compound
E ([M*]+/[M]2+)/Va,b E ([M*]+/[M]0)/Va,c
ꢀ0.70
ꢀ0.66
ꢀ0.67
ꢀ0.54
ꢀ0.61
ꢀ0.66
ꢀ0.59
ꢀ0.57
ꢀ0.64
1.07
1.19
1.15
1.22
1.30
1.35
1.37
1.42
1.47
2a
2b
2c
2d
2e
2f
3a
3b
3c
93
ꢀ0.85
ꢀ0.89
0.68
1.21
[Ir(ppy)2(bpy)]PF6
89
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6
aPotentials are given in V vs SCE. bE([M*]+/[M]2+) = Eox – Eλem. cE([M*]+/[M]0) = Ered + Eλem
.
Photocatalytic Hydrogen Evolution. Given their appealing photochemical and electrochemical
properties, the complexes were evaluated as photosensitizers for photocatalytic hydrogen
evolution from water. The reactions were carried out in 8:1 acetonitrile/water using K2PtCl4 as
the water reducing catalyst and triethylamine (TEA) as a sacrificial donor following previously
established protocols.85 Photocatalytic activity was determined against [Ir(dMeOphtpy)(4ꢀ
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