Proton transfer from the triplet excited state of brominated naphthol to a difluoroboryl bridged Co^I-diglyoxime complex, forming Co^(III)H, was monitored via transient absorption. The second-order rate constant for Co^(III)H formation is in the range (3.5−4.7) × 10^9 M^(−1) s^(−1), with proton transfer coupled to excited-state deactivation of the photoacid. Co^(III)H is subsequently reduced by excess Co^I-diglyoxime in solution to produce Co^(II)H (k_(red) = 9.2 × 10^6 M^(−1) s^(−1)), which is then protonated to yield Co^(II)-diglyoxime and H_2

Dempsey, Jillian L.

Gray, Harry B.

Winkler, Jay R.

English

Caltech Authors - Main

S1 
 
 
 
 
Supporting Information 
 
 
 
 
Mechanism of H2 Evolution from a Phototriggered Hydridocobaloxime 
Jillian L. Dempsey, Jay R. Winkler, Harry B. Gray 
 
 
 
1. Synthetic Details ............................................................................................................ S2 
2. Photochemical Measurements ....................................................................................... S3 
3. Calculations of Ground State and Excited State pKa Values ......................................... S8 
4. Hydrogen Yields from Bulk Photolysis ....................................................................... S10 
5. Calculation of barriers.................................................................................................. S11 
 
 
  
S2 
 
1. Synthetic Details 
General methods  
Syntheses of air and moisture sensitive compounds were carried out in a nitrogen 
atmosphere glovebox.  Solvents for these syntheses were dried by a standard method1 or over 
activated sieves followed by passage over activated alumina.  CD3CN was obtained from 
Cambridge Isotope Laboratories, Inc.  All materials, unless noted, were used as received. 
Tetrabutylammonium hexafluorophosphate and 6-bromo-2-naphthol were purchased from 
Aldrich and recrystalized from ethanol. Co(dmgBF2)(CH3CN)2 was prepared according to the 
method of Bakac and Espenson.2 MeCo(dmgBF2)2(CH3CN) was prepared by the method of Ram 
et al.3 NMR spectra were recorded using a Varian Mercury 300 spectrometer.  1H NMR chemical 
shifts were referenced to residual solvents as determined relative to Me4Si (δ=0 ppm).  
[Na][Co(dmgBF2)2(CH3CN)] 
[Na][Co(dmgBF2)2(CH3CN)] was prepared in an analogous method to the literature 
preparation for [Na][Co(dpgBF2)2(CH3CN)].4 0.251 g (0.559 mmol, 1 eq.) of 
Co(dmgBF2)(CH3CN)2 was suspended in 4 mL of THF and transferred to a 20 mL scintillation 
vial containing 2.673 g (0.579 mmol, 1.04 eq.) of sodium mercury amalgam (0.5% Na). The 
reaction was stirred for 3 hours to yield a deep blue-black solution.  Solution was removed from 
the mercury and remaining blue solid was dissolved in CH3CN and removed. THF and CH3CN 
solutions were mixed and the solvent was reduced to <1 mL. 15 mL of Et2O was added to 
precipitate a blue solid, which was collected via filtration on medium frit. Product was dried 
under vacuum. Yield =71%. 1HNMR (CD3CN, 300 MHz): δ 2.09 (s, 12H), 1.96 (s, 3H). 
Sodium 6-Bromo-2-Naphtholate  
In an inert atmosphere, a 20 mL scintillation vial was charged with 21 mg (0.883 mmol) sodium 
hydride and a stirbar. A solution of 197 mg (0.883 mmol) of 6-bromo-2-naphthol in 4 mL dry, 
degassed acetonitrile, was transferred to the reaction vessel. A white suspension was 
immediately formed and vigorous bubbling was observed. Reaction was allowed to stir for 1 
hour, until solution became clear pale yellow.  Solution was filtered and solvent was removed. 
Yield = 90%. 1H NMR (CD3CN, 300 MHz): δ 7.67 (d, J = 1.2 Hz, 1H), 7.39 (d, J = 8.7 Hz, 1 H), 
7.18 (m, 2H), 6.92 (dd, J1 = 8.9 Hz, J2 = 2.6 H, 1H), 6.66 (d, J = 2.4 Hz, 1H)  
S3 
 
2. Photochemical Measurements 
 
Photochemical Methods 
UV-visible absorption measurements were carried out using a Hewlett Packard 8452 or 
Cary 50 UV-Vis spectrophotometer in 1 cm pathlength quartz cuvettes. 
Samples for transient absorption and room temperature luminescence measurements were 
prepared in dry, degassed CH3CN and placed into the cell of a high-vacuum 1-cm pathlength 
fused quartz cuvette (Starna Cells) connected to a 10 mL bulb and isolated from atmosphere and 
the bulb by a high-vacuum Teflon valve (Kontes). Samples for low temperature luminescence 
measurements were prepared in dry, degassed butyronitrile and placed in 1 cm diameter glass 
tubes sealed with a high-vacuum Teflon valve. 
Steady-state and time-resolved spectroscopic measurements were carried out at the 
Beckman Institute Laser Resource Center (California Institute of Technology).  Emission spectra 
were recorded on a Jobin Yvon Spec Fluorolog-3-11. Sample excitation was achieved via a 
xenon arc lamp with wavelength selection provided by a monochromator. Right angle emission 
was sorted using a monochromator and detected with a Hamamatsu R928P photomultiplier tube 
with photon counting. Appropriate long pass filters were utilized to minimize scattered excitation 
wavelength. Low temperature measurements were made in a liquid nitrogen filled glass finger 
dewar aligned in the fluorometer sample cavity. 
For time-resolved measurements, 266 nm laser excitation was provided by 8 ns pulses 
from the fourth harmonic of a 10 Hz Q-switched Nd:YAG laser (Spectra-Physics Quanta-Ray 
PRO-Series). Probe light for transient absorption kinetics measurements was provided by a 75-W 
arc lamp (PTI Model A 1010) that could be operated in continuous wave or pulsed modes.  
Timing between the laser and the probe light was controlled by a digital delay generator.  After 
passing through the sample collinearly with the laser beam, scattered excitation light was 
rejected by suitable long pass and short pass filters, and probe wavelengths were selected for 
detection by a double monochromator (Instruments SA DH-10) with 1 mm slits.  Transmitted 
light was detected with a photomultiplier tube (PMT, Hamamatsu R928).  The PMT current was 
amplified and recorded with a GageScope transient digitizer.  The data were converted to units of 
ΔOD (ΔOD = −log10(I/I0), where I is the time-resolved probe-light intensity with laser excitation, 
and I0 is the intensity without excitation). Samples measured on the microsecond timescale or 
faster were stirred continuously and measured using a laser repetition rate of 10 Hz, while 
S4 
 
samples measured on a millisecond timescale were excited with a single shutter-released laser 
pulse, stirred for 1 s after collecting data, then allowed to sit until the solution settled (2 s) before 
the next laser pulse. Data were averaged over approximately 50 shots. All instruments and 
electronics in these systems were controlled by software written in LabVIEW (National 
Instruments).  Data were log-time compressed prior to fitting. Data manipulation was performed 
with MATLAB R2008a (Mathworks, Inc.) and graphed with Igor Pro 5.01 (Wavemetrics). 
Absorption Spectra 
 
 
Figure S1 Absorption Spectra of 6-bromo-2-naphthol (red) and sodium 6-bromo-2-naphtholate (blue) in acetonitrile.  
 
Figure S2 Absorption Spectra of Co(II)(dmgBF2)2(CH3CN)2 (yellow), [Na][Co(I)(dmgBF2)2(CH3CN)] (blue), 
MeCo(III)(dmgBF2)2(CH3CN)] (red) in acetonitrile.  
S5 
 
 
Table 1 Absorption Characterization 
Sample UV-Vis Absorption/nm (Epsilon/(M-1 s-1)) 
6-Bromo-2-Naphthol 232 (59,450), 276 (5,111), 339 (1,851) 
Sodium 6-Bromo-2-Naphtholate 257 (62,785), 316 (9,484), 402 (2,489) 
Co(II)(dmgBF2)2(CH3CN)2 260 (4,645), 322 (1,810), 424 (2,660) 
[Na][Co(I)(dmgBF2)2(CH3CN)] 264 (7,905), 553 (4,712), 628 (4,546) 
Me-Co(III)(dmgBF2)2(CH3CN) 258 (6,042), 370 (2,090) 
 
Absorption spectra at time intervals of Photolysis 
A 4 mL sample in acetonitrile containing 672 μM 6-bromo-2-naphthol, 167 μM [Na] 
[CoI(dmgBF2)2(CH3CN)], and 100 mM [NBu4][PF6] in a 1 cm pathlength quartz cuvette  was 
photolyzed at 266 nm. Absorption spectra were measured at various time intervals during 
photolysis (Figure S3). Global analysis was performed to determine the concentrations of 
[CoI(dmgBF2)2(CH3CN)]–, CoII(dmgBF2)2(CH3CN)2, and [Na][6-bromo-2-naphtholate] at each 
time interval (6-bromo-2-naphtholate does not absorb in the region of interest). Interestingly, the 
absorption of [Na][6-bromo-2-naphtholate] made up a negligible component of the spectra, while 
the sum of [Co2+] and [Co+] was consistently ~ 170 μM (Table 1), suggesting a reactive pathway 
for the naphtholate decomposition, as observed by Pretali et al.5 
 
 
Figure S3 Absorption spectra (solid lines) and corresponding global fits (starred lines) monitored at arbitrary time points during 
photolysis. Sample initially contained 672 μM 6-bromo-2-naphthol, 167 μM [Na] [Co(dmgBF2)2(CH3CN)], and 100 mM 
[NBu4][PF6] in acetonitrile in a 1 cm pathlength cuvette. Sample 1-pink, sample 2-red; sample 3-yellow; sample 3-green; sample 
4-cyan; sample 5-blue; sample 4-black. 
  
Wavelength (nm)
Ab
so
rp
tio
n
S6 
 
Table 2 Sample components for each absorption measurements in Figure S3, obtained from global analysis in units of M. 
Sample [CoI] [CoII] [6-bromo-2-naphtholate] 
1 1.66E-04 3.30E-06 1.88E-05 
2 1.61E-04 8.80E-06 1.98E-05 
3 1.45E-04 2.68E-05 1.99E-05 
4 1.32E-04 3.71E-05 2.09E-05 
5 1.16E-04 5.20E-05 2.22E-05 
6 4.22E-05 1.19E-04 2.52E-05 
7 8.30E-06 1.49E-04 2.61E-05 
 
Luminescence Spectra of 6-Br-2-Naphthol and Sodium 6-Bromo-2-Naphtholate 
 
Figure S4 Normalized luminescence spectra of 6-bromo-2-naphthol (red) and sodium 6-bromo-2-naphtholate (blue), room 
temperature, in acetonitrile solution. λex (6-bromo-2-naphthol) 290 nm, λex (sodium 6-bromo-2-naphtholate) 380 nm. Weak 
phosphorescence at room temperature is observed for 6-bromo-2-naphthol but not sodium 6-bromo-2-naphtholate. 
 
Figure S5 Normalized luminescence spectra of 6-bromo-2-naphthol (red) and sodium 6-bromo-2-naphtholate (blue), 77K, in 
butyronitrile glass. λex (6-bromo-2-naphthol) 340 nm, λex (sodium 6-bromo-2-naphtholate) 340 nm. Both fluorescence and 
phosphorescence were measured for sodium 6-bromo-2-naphtholate, but the fluorescence of 6-bromo-2-naphthol was not 
measured at 77K due to experimental limitation. 
S7 
 
 
Figure S6 Normalized phosphorescence spectra of 6-bromo-2-naphthol at 77K (red, in butyronitrile glass) and at room 
temperature (orange, in acetonitrile solution). λex (77K) 340 nm, λex (room temperature) 290 nm. 
 
Figure S7 Normalized luminescence spectra of sodium 6-bromo-2-naphtholate at 77K (blue, in butyronitrile glass) and at room 
temperature (green, in acetonitrile solution). λex (77K) 340 nm, λex (room temperature) 380 nm. 
  
S8 
 
 
Figure S8 The slow kinetics process of kinetics traces measured at 630 nm was fit to a biexponential decay. The larger rate 
constant exhibits a linear relationship with the concentration of [Co(dmgBF2)2(CH3CN)]
–. All samples contained 600 μM 6-
bromo-2-naphthol, 100 mM [NBu4][PF6] and a varying concentration of [Na][Co(dmgBF2)2(CH3CN)]. A linear fit to the data 
series, kobs = kred*[Co(dmgBF2)2(CH3CN)]
–, gives kred = 9.2 x 10
6
 M
-1
 s
-1. 
3. Calculations of Ground State and Excited State pKa Values 
pKa of 6-Bromo-2-Naphthol in CH3CN  
The pKa of 6-bromo-2-naphthol in DMSO has been reported as 16.2.6 The pKa of 6-
bromo-2-naphthol in CH3CN can be estimated from the DMSO value by the method of Koppel 
and coworkers, shown in equation 1.7 
pKa of 6-bromo-2-naphthol in CH3CN = 11.80 + 0.884(pKa(DMSO)) = 26.1 (Eq. 1) 
pKa* of 6-Bromo-2-Naphthol in CH3CN  
The Förster cycle8-9  was used to estimate the pKa* of 6-bromo-2-naphthol, pKa – pKa* = 
(EHA - EA-)/(2.3kBT). For *1pKa, EHA and EA- are the difference between S0 and S1 (E0,0) in 
wavenumbers. For *1pKa, E0,0 is estimated from the intersection of normalized absorption and 
fluorescence spectra of the naphthol and naphtholate, and converted to wavenumbers. For *3pKa, 
EHA and EA- are the difference between S0 and T1 (E0,0) in wavenumbers. E0,0 is estimated from 
the highest energy of the structured emission observed in the low temperature phosphorescence 
spectra. 
 
Table 3 Absorption and Fluorescence Maxima in CH3CN at Room temperature 
 Abs Max (cm-1) Fluorescence Max (cm-1) E0,0 (cm-1) 
BrNaphOH 29,586 28,090 29,070 
[Na+][BrNaphO–] 24,876 21,834 23,148 
EHA-EA- 4,710 6,256 5,922 
 
Table 4 Phosphorescence data in butyronitrile at 77K 
 E0,0 (cm-1) 
BrNaphOH 20,790 
[Na+][BrNaphO–] 19,379 
EHA-EA- 1,411 
 
  
S9 
 
Table 5 Excited State pKa values in CH3CN estimated from the Förster Cycle 
Excited State EHA-EA- calculated from Temperature (K) pKa* 
Singlet Fluorescence Maxima 298 13.0 
Singlet Fluorescence E0,0 298 13.7 
Triplet Phosphorescence E0,0 77 14.6 
 
Estimates of pKa* of 6-Bromo-2-Naphthol in CH3CN coupled to excited state relaxation  
The Förster cycle8-9 estimates the pKa of *1,3[6-bromo-2-naphthol] upon deprotonation to 
*1,3[6-bromo-2-naphtholate]. However, the pKa is expected to be significantly lower if the 
deprotonation is coupled with relaxation of the excited state. The predicted pKa of the triplet state 
is estimated from the energy cycle pictured below (Figure S9) to be approximately -26.3. 
 
Figure S9 Energy cycle based on the Förster cycle predicted excited-state pKa and E0,0(T1←S0) of 6-bromo-2-naphtholate. pKa 
values were converted to free energy via ΔG° = -kBTln(Ka). 
Quenching of 3*[6-bromo-2-Naphthol] by [NBu4][OTs]  
To probe the excited-state pKa of 6-bromo-2-naphthol, quenching by the conjugate base 
of p-toluenesulfonic acid (pKa = 8.0 in CH3CN), tetrabutylammonium tosylate ([NBu4][OTs]) 
was monitored by transient absorption measurements of 3*[6-bromo-2-Naphthol] (Figure 13). As 
described by Pretali et al., the decay of *3[6-bromo-2-Naphthol] is biexponential, with rate 
constants of 6.4 x 104 s-1 and 4.0 x 105 s-1.5 In the presence of [NBu4][OTs], biexponential decays 
are still observed (Figure S11), with the coefficient for the larger rate constant roughly 3.3 times 
larger than the smaller one. Based on the larger rate constant, a second-order rate constant for 
proton transfer is approximately 5.1 x 106 M-1 s-1. 
 
S10 
 
 
Figure S10 Kinetics traces monitoring the absorption of 3*[6-bromo-2-naphthol] at 460 nm (λex = 266 nm) with varying 
concentrations of [NBu4][OTs]. Kinetics traces are best fit to biexponential decay kinetics. 
 
Figure S11 Rate constants for the biexponential decay of 3*[6-bromo-2-naphthol] in the presence of [NBu4][OTs]. 
 
4. Hydrogen Evolution Yields from Bulk Photolysis 
Experimental Details  
Bulk photolysis experiments were utilized to characterize the evolution of hydrogen from 
excitation of 6-bromo-2-naphthol in the presence of [Na][Co(dmgBF2)2(CH3CN)]. Reactions 
were run in a custom built 61 mL round bottom flask fitted with a 1 inch diameter quartz 
window, 24/40 ground glass joint, Kontes valve leading to a 14/20 joint sealed with a septum. In 
a glovebox, the reaction vessel was charged with 5 mM 6-bromo-2-naphthol, 50 mM NBu4PF6, 
and the noted amount of [Na][Co(dmgBF2)2(CH3CN)] in 40 mL acetonitrile with a stirbar and 
the vessel was sealed with a 24/40 glass stopper and Kontes valve.  The sample was placed in 
front of a ~100 mW 266 nm Nd:YAG laser and photolyzed while stirring for approximately 45 
minutes until the blue [Na][Co(dmgBF2)2(CH3CN)] had reacted to form yellow 
Co(dmgBF2)2(CH3CN)2.  
The amount of H2 produced evolved was quanitified by analyzing the gas mixture in the 
headspace using an Agilent 7890A GC-TCD. The total amount of H2 produced was calculated as 
S11 
 
a sum of the H2 in the headspace plus the H2 dissolved in the solvent (calculated using Henry’s 
Law with a constant of 290 L•atm/mol). 
 
Table 6 Hydrogen evolution yields from bulk photolysis experiments 
Run [Co(dmgBF2)2(CH3CN)–] (μM) % H2 in Headspace % Yield 
1 510 0.65 64 
2 446 0.39 38 
 
5. Calculations of Barriers 
Barriers were estimated as: 
∆𝐺‡ = −RTln �𝑘Z� 
k  = rate constant, M-1 s-1 
Z = collision factor, 1011 M-1 s-1 
R = molar gas constant, 0.001986 kcal K-1 mol-1 
T = temperature, 298 K 
 
 
 
References Cited 
 
(1) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 
1996, 15, 1518. 
(2) Bakac, A.; Espenson, J. H. J. Am. Chem. Soc. 1984, 106, 5197. 
(3) Ram, M. S.; Riordan, C. G.; Yap, G. P. A.; LiableSands, L.; Rheingold, A. L.; Marchaj, A.; 
Norton, J. R. J. Am. Chem. Soc. 1997, 119, 1648. 
(4) Hu, X.; Brunschwig, B. S.; Peters, J. C. J. Am. Chem. Soc. 2007, 129, 8988. 
(5) Pretali, L.; Doria, F.; Verga, D.; Profumo, A.; Freccero, M. J. Org. Chem. 2009, 74, 1034. 
(6) Bordwell, F. G.; Cheng, J. J. Am. Chem. Soc. 1991, 113, 1736. 
(7) Kütt, A.; Leito, I.; Kaljurand, I.; Sooväli, L.; Vlasov, V. M.; Yagupolskii, L. M.; Koppel, I. A. J. 
Org. Chem. 2006, 71, 2829. 
(8) Grabowski, Z. R.; Rubaszewska, W. J. Chem. Soc., Faraday Trans. 2 1977, 73, 11. 
(9) Forster, T. Z. Elektrochem. 1950, 54, 531. 
 
 


Mechanism of H_2 Evolution from a Photogenerated Hydridocobaloxime

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Mechanism of H_2 Evolution from a Photogenerated Hydridocobaloxime

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