A R T I C L E S
Tennyson et al.
obtained with a VG analytical ZAB2-E instrument (ESI or CI).
Elemental analyses were performed by Midwest Microlabs (India-
napolis, IN). Gas chromatography (GC) was performed on an
Agilent 6850 gas chromatograph (HP-1 column, L ) 30 m, I.D. )
0.32 mm, linear gradient: 50–300 °C, 20 °C min–1). All syntheses
were performed under ambient conditions unless specified otherwise.
Electrochemistry. Electrochemical experiments were conducted
on CH Instruments Electrochemical Workstations (series 660D) using
a gastight, three-electrode cell under an atmosphere of dry nitrogen.
The cell was equipped with gold working and tungsten counter
electrodes, as well as a silver wire quasi-reference electrode. Unless
specified otherwise, measurements were performed using 1.0 mM
solutions of analyte in dry CH2Cl2 with 0.1 M [tetra-n-butylammoni-
um][PF6] as the electrolyte and decamethylferrocene (Fc*) as the
internal standard. Differential pulse voltammetry measurements were
performed using 50 mV pulse amplitudes and 2 mV data intervals.
Chronoamperometry experiments were performed using a 25 µm
diameter gold ultramicroelectrode as the working electrode, enabling
independent determination of D0 and n by plotting i(t)/iss vs t-1/2 and
using the Cottrell equation.73 Data deconvolution and fitting were
performed using the Origin 8.0 software package. All potentials listed
herein were determined by cyclic voltammetry at 100 mV s-1 scan
rates and referenced to a saturated calomel electrode (SCE) by shifting
(Fc*)0/+ to -0.057 V (CH2Cl2).74
General Spectroscopic Considerations. UV-visible absorption
spectra were recorded on a Perkin-Elmer Lambda 35 spectrometer.
All room-temperature measurements were made using matched 6Q
Spectrosil quartz cuvettes (Starna) with 1 cm path lengths and 3.0
mL of sample solution. Absorption spectra were acquired in CH2Cl2
under ambient conditions for all complexes. Extinction coefficients
(ε) were determined from Beer’s law measurements using 10, 20,
30, and 40 µM concentrations of the analyte.
1,3-Dimesitylnapthoquinimidazolium Chloride [1H][Cl]. 2,3-
Dichloronaphthoquinone (1.13 g, 4.98 mmol) and N,N′-dimesityl-
formamidine (2.80 g, 9.99 mmol) were dissolved with 25 mL of
CH3CN in a heavy-walled flask equipped with a stir bar. The flask
was sealed, and the reaction was then heated to 110 °C. After 48 h,
the reaction was allowed to cool to room temperature, whereupon
NaHCO3 (420 mg, 5.00 mmol) was added, and the reaction mixture
was then heated to 60 °C. After 16 h, the vessel was allowed to
cool to room temperature, and then the reaction mixture was filtered
through Celite to remove NaCl and the filtrate solvent was removed
under reduced pressure. The resulting residue was then taken up
in a minimal amount of CH2Cl2, filtered through a 0.2 µm PTFE
filter, and added to 20 volume equivalents of Et2O. The precipitated
solids were collected via filtration, washed successively with THF
and Et2O, and then dried to afford 2.32 g (4.93 mmol, 99% yield)
of the desired product as a yellow powder. Spectral data were
consistent with literature values.68
Figure 2. 1,3-Dimesitylnapthoquinimidazolylidene (1), an NHC comprising
a redox-active naphthoquinone moiety.
Given the utility of NHC-supported Group 10 catalysts for a
broad range of organic transformations,17,25,26,58-62 and the
relative ease of incorporating redox-active functionalities within
NHC frameworks,63-67 we envisioned that a complex compris-
ing both attributes could enable RSC. We thus sought to prepare
a family of Group 10 complexes bearing redox-active NHCs,
to study their spectroscopic, structural, and electrochemical
features, and to explore their potential for catalyzing organic
transformations. We previously communicated naphthoquin-
imidazolylidene 1 and showed that its electron-donating char-
acter could be modified via reduction of the quinone to a
semiquinone radical anion (Figure 2).68 Building on these
preliminary findings, we believed that Group 10 complexes
supported by this redox-active NHC could exhibit interesting
electronic properties in addition to catalyzing cross-coupling
reactions and enabling redox-switchable control over these
reactions. The results of our comprehensive studies are presented
herein.
Experimental Section
Materials and Methods. Nickel(II), palladium(II), and plati-
num(II) chloride were purchased from Pressure Chemicals. 1,3-
Dimesitylformamidine,69 [NiCl2(PPh3)2],70 trans-[PdCl2(PhCN)2],71
and trans-[PtCl2(PhCN)2]72 were prepared as previously described.
All other materials and solvents were of reagent quality and used
as received. 1H and 13C {1H} NMR spectra were recorded using a
Varian 300, 400, 500, or 600 MHz spectrometer. Chemical shifts
δ (in ppm) are referenced to tetramethylsilane using the residual
1
solvent as an internal standard. For H NMR: CDCl3, 7.24 ppm.
For 13C NMR: CDCl3, 77.0 ppm. Coupling constants (J) are
expressed in hertz. High-resolution mass spectra (HRMS) were
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¨
[AgCl(1)] (2). To a solution of [1H][Cl] (1.18 mg, 2.51 mmol)
in CH2Cl2 (15 mL) was added 3 Å molecular sieves, followed by
Ag2O (290 mg, 1.25 mmol) in CH2Cl2 (10 mL), and the resulting
reaction mixture was allowed to stir at room temperature. After
16 h, the reaction mixture was filtered through a 0.2 µm PTFE
filter, the filtrate solvent was removed under reduced pressure and
the resulting solid was washed successively with THF and Et2O,
and then dried to afford 1.42 g (2.46 mmol, 98% yield) of the
desired product as a bright yellow solid. Spectral data were
consistent with literature values.68
General Procedure for the Synthesis of trans-[MCl2(1)2]
Complexes. To a suspension of 2 (116 mg, 0.201 mmol) in toluene
(3 mL) was added [NiCl2(PPh3)2] (65 mg, 99 µmol for 3a), trans-
[PdCl2(PhCN)2] (40 mg, 0.10 mmol for 3b), or trans-[PtCl2(PhCN)2]
(49 mg, 0.10 mmol for 3c) in toluene (3 mL), and the resulting
mixture was then heated to 110 °C. After 16 h, the mixture was
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