The function of proteins involved in electron transfer is dependent on cofactors attaining the necessary reduction potentials. We establish a mode of cluster redox tuning through steric pressure on a synthetic model related to Photosystem II. Resembling the cuboidal [CaMn₃O₄] subsite of the biological oxygen evolving complex (OEC), [Mn4O4] and [YMn₃O₄] complexes featuring ligands of different basicity and chelating properties were characterized by cyclic voltammetry. In the absence of ligand-induced distortions, increasing the basicity of the ligands results in a decrease of cluster reduction potential. Contraction of Y-oxo/Y–Mn distances by 0.1/0.15 Å enforced by a chelating ligand results in an increase of cluster reduction potential even in the presence of strongly basic donors. Related protein-induced changes in Ca-oxo/Ca–Mn distances may have similar effects in tuning the redox potential of the OEC through entatic states and may explain the cation size dependence on the progression of the S-state cycle

Agapie, Theodor

Lee, Heui Beom

Caltech Authors - Main

S1 
 
Redox Tuning via Ligand-Induced Geometric Distortions at a YMn3O4 
Cubane Model of the Biological Oxygen Evolving Complex 
Heui Beom Lee† and Theodor Agapie*† 
† Department of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E 
California Blvd MC 127-72, Pasadena, CA 91125, USA 
 
 
 
Supporting information 
 
Experimental procedures  
     Synthesis of H3triam 2 
     Synthesis of 4-Mn and 4-Mn-ox 3 
     Synthesis of 2-Y 4 
     Synthesis of 3-Y-red and 3-Y 5 
NMR spectroscopy 6-8 
Electrospray mass spectrometry 8 
X-ray crystallography 9-12 
Electrochemistry 13-15 
Values of ligand pKa in DMSO, effective basicity, and cluster redox potential 15 
EPR spectroscopy 16 
References 17 
 
 
 
 
 
 
 
 
 
 
 
S2 
 
Experimental procedures 
 
General considerations 
All reactions were performed at room temperature in an N2-filled glovebox or by using standard Schlenk 
techniques unless otherwise specified. Glassware was oven dried at 150 oC for at least 2h prior to use, and 
allowed to cool under vacuum. All reagents were used as received unless otherwise stated. Anhydrous 
tetrahydrofuran (THF) was purchased from Aldrich in 18 L Pure-PacTM containers. Anhydrous CH2Cl2, 
CH3CN, diethyl ether, benzene and THF were purified by sparging with nitrogen for 15 minutes and then 
passing under nitrogen pressure through a column of activated A2 alumina. NMR solvents were purchased 
from Cambridge Isotope Laboratories, dried over calcium hydride, degassed by three freeze-pump-thaw 
cycles and vacuum-transferred prior to use. Paramagnetic 1H NMR spectra were recorded on a Varian 300 
MHz instrument, with shifts reported relative to the residual solvent peak. Elemental analyses were 
performed at the California Institute of Technology. 
 
Synthesis of H3triam 
Prepared according to literature1, the HCl salt of 
1,1,1-tris(aminomethyl)ethane (1.5 g, 6.62 mmol, 
1 equiv) was suspended in DMF (100 mL) and 
treated with Et3N (10 mL). The resulting mixture 
was cooled to 0 oC in an ice bath then treated with pivaloyl chloride (5 mL) dropwise. The mixture was 
warmed to room temperature and stirred for 2 days at room temperature. All volatiles were removed under 
reduced pressure, and the residue was treated with CH2Cl2 and saturated aqueous NaHCO3. The CH2Cl2 
layer was separated, washed with water, dried over anhydrous MgSO4, and filtered. All volatiles were 
removed from the filtrate, and the residue was washed with copious amounts of Et2O. H3triam was obtained 
was an off-white powder. Yield: 750 mg, 31 %. 
 
1H NMR (500 MHz, CDCl3): δ 7.09 (t, J = 5 Hz, 3H, NH), 2.90 (d, J = 5 Hz, 6H, CH2), 1.24 (s, 27H, 
−C(CH3)3), 0.77 (s, 3H, CH3) ppm. 
13C NMR (125.7 MHz, CDCl3): 180.12, 42.25, 41.48, 39.04, 27.85, 19.27 ppm. 
HRMS (FAB+): calcd. For C20H40N3O3: 370.3070; found: 370.3059 [M+H] 
 
 
S3 
 
Synthesis of 4-Mn 
A solution of H3triam (265 mg, 0.717 mmol, 
1.1 equiv) in THF (200 mL) was treated with 
KH (300 mg, ~10 equiv) and stirred for 18 
hours at room temperature. Excess KH was 
removed by filtration, and the filtrate was 
treated with 1-Mn (860 mg, 0.653 mmol, 1 
equiv). After stirring for 18 hours at room 
temperature, all volatiles were removed from 
the crude reaction mixture. The residue was washed with generous amounts of Et2O and benzene, then 
redissolved in CH2Cl2 and filtered through a pad of Celite. All volatiles were removed from the filtrate. The 
residue was washed three times with small amounts of MeCN, redissolved in CH2Cl2 and filtered through 
a pad of Celite. Volatiles were removed from the filtrate under reduced pressure, yielding 4-Mn as a brown 
powder. Yield: 820 mg, 83 %. Crystals suitable for X-ray crystallography were obtained from slow vapor 
diffusion of Et2O into a concentrated solution of 4-Mn in CH2Cl2. 1H NMR (300 MHz, CD2Cl2): δ 35.5, 
30.0, 17.8, 13.3, 9.7, −5.9 ppm. 
Synthesis of 4-Mn-ox 
A solution of 4-Mn (220 mg, 0.146 mmol, 1 
equiv) in THF (10 mL) was treated with a 
solution of Ag(OTf) (40 mg, 0.155 mmol, 
1.05 equiv) in MeCN (3 mL) and stirred for 
18 hrs at room temperature. All volatiles 
were removed from the crude reaction 
mixture. The residue was washed with 
generous amounts of Et2O and ben. The 
residue was dissolved in CH2Cl2 and filtered 
through a pad of Celite. All volatiles were removed from the filtrate. The residue was washed three times 
with small amounts of THF, redissolved in CH2Cl2 and filtered through a pad of Celite. Volatiles were 
removed from the filtrate under reduced pressure, yielding 4-Mn-ox as a brown powder. Yield: 133 mg, 
55 %. Crystals of 4-Mn-ox suitable for EPR spectroscopy were obtained from slow vapor diffusion of Et2O 
into a concentrated solution in CH2Cl2. 1H NMR (300 MHz, CD2Cl2): δ 114.5, 74.6, 13.1, 11.0, −18.5 ppm. 
Analysis calculated for [LMn4O4(triam)](OTf) [C78H75F3Mn4N9O13S]: C 56.60, H 4.57, N 7.62; found: C 
56.34, H 4.46, N 7.97. 
S4 
 
Synthesis of 2-Y 
A solid mixture of 1-Y (930 mg, 0.565 
mmol, 1 equiv) and H2N4O2 (143 mg, 
0.62 mmol, 1.1 equiv) was treated with 
THF (18 mL) and stirred at room 
temperature for 16 hours. The resulting 
red precipitate was collected, washed 
with THF until the filtrate was no longer 
red, dissolved in CH2Cl2, and filtered 
through Celite. Volatiles were removed 
from the filtrate under reduced pressure, yielding 2-Y as a red powder. Crystals suitable for X-ray 
crystallography were obtained from slow vapor diffusion of Et2O into a concentrated solution of 2-Y in py 
(250 mg, 26%). 1H NMR (300 MHz, CD2Cl2): δ 68.2, 61.2, 18.6, 17.1, 11.8, 11.0, −17.0, 19.2 ppm. Analysis 
calculated for [LYMn3O4(N4O2)(OAc)(DMF)](OTf) [C73H69F3Mn3N11O15SY]: C 52.09, H 4.13, N 9.15; 
found: C 52.07, H 4.23, N 9.37. 
 
 
 
 
 
 
 
 
 
 
 
 
S5 
 
Synthesis of 3-Y-red 
A solid mixture of 1-Y (420 mg, 0.255 
mmol, 1 equiv) and the triacetamide (85 
mg, 0.312 mmol, 1.2 equiv) was treated 
with MeCN (18 mL) and stirred briefly 
at room temperature. NaOtBu (81 mg, 
0.843 mmol, 3.3 equiv) was added as a 
solid and the mixture was stirred at room 
temperature for 16 hours. The resulting 
orange-brown precipitate was collected, 
washed with MeCN until the filtrate was 
no longer brown, dissolved in CH2Cl2, and filtered through Celite. Volatiles were removed from the filtrate 
under reduced pressure, yielding 3-Y-red as a red powder. (120 mg, 32%). 1H NMR (300 MHz, CD2Cl2): 
δ 24.9, 11.2, 10.2, 9.3, −22.43 ppm. 
Synthesis of 3-Y 
A solution of 3-Y-red (28 mg, 0.019 
mmol, 1 equiv) in CH2Cl2 (3 mL) was 
treated with a solution of Fc(OTf) (8 
mg, 0.024 mmol, 1.2 equiv) in 
CH2Cl2 (2 mL). The mixture was 
stirred at room temperature for 16 
hours. All volatiles were removed 
from the resulting red-brown solution, 
and the residue was washed with Et2O. 
The residue was dissolved in cold 
THF (−30 oC) and filtered through a pad of Celite. Volatiles were removed from the filtrate under reduced 
pressure, yielding 3-Y as a red powder. Yield: 28 mg, 90%. Crystals suitable for X-ray crystallography 
were obtained from slow vapor diffusion of Et2O into a concentrated solution of 3-Y in THF. 1H NMR (300 
MHz, CD2Cl2): δ 15.0, 11.5, 9.2, −15.4, −17.2 ppm. Analysis calculated for [LYMn3O4(Ntriam)](OTf)· 
CH2Cl2 [C71H62Cl2F3Mn3N10O13SY]: C 50.85, H 3.73, N 8.35; found: C 50.56, H 4.02, N 8.09. 
 
 
S6 
 
NMR spectroscopy 
 
Figure S1. 1H NMR of H3triam in CDCl3. 
 
Figure S2. 13C NMR of H3triam in CDCl3. 
 
Figure S3. 1H NMR of 4-Mn in CD2Cl2. 
S7 
 
 
Figure S4. 1H NMR of 4-Mn-ox in CD2Cl2. 
 
Figure S5. 1H NMR of 2-Y in CD2Cl2. 
 
Figure S6. 1H NMR of 3-Y-red in CD2Cl2. 
S8 
 
 
Figure S7. 1H NMR of 3-Y in CD2Cl2. 
 
Electrospray mass spectrometry 
 
Figure S8. ESI-MS of 4-Mn. m/z = 1505 consistent with [LMn4O4(triam)]+. 
 
Figure S9. ESI-MS of 2-Y. m/z = 1460 consistent with [LYMn3O4(N4O2)(OAc)]+. 
 
Figure S10. ESI-MS of 3-Y. m/z = 1443 consistent with [LYMn3O4(Ntriam)]+. 
S9 
 
X-ray crystallography 
Suitable crystals were mounted on a nylon loop using Paratone oil, then placed on a diffractometer 
under a nitrogen stream. X-ray intensity data were collected on a Bruker D8 VENTURE Kappa Duo 
PHOTON 100 CMOS detector employing Mo-Kα or Cu-Kα radiation (λ = 0.71073 Å or 1.54178 Å 
respectively) at a temperature of 100 K. All diffractometer manipulations, including data collection, 
integration and scaling were carried out using the Bruker APEX3 software. Frames were integrated using 
SAINT. The intensity data were corrected for Lorentz and polarization effects and for absorption using 
SADABS. Space groups were determined on the basis of systematic absences and intensity statistics using 
XPREP. Using Olex2, the structures were solved by direct methods using ShelXT and refined to 
convergence by full-matrix least squares minimization using ShelXL. All non-solvent non-hydrogen atoms 
were refined using anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions 
and refined using a riding model. Graphical representation of structures with 50% probability thermal 
ellipsoids was generated using Diamond visualization software. Two component twin refinement for 4-Mn. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
S10 
 
Table S1. Crystal and refinement data for complexes 4-Mn, 2-Y, and 3-Y. 
 
Compound 4-Mn 2-Y 3-Y 
CCDC 1897119 1897117 1897118 
Empirical formula C81H85Mn4N9O12 
C92.48H88.48F3Mn3N14.9O
15SY 
C82H60F3Mn3N10O16SY 
Formula weight 1596.33 1991.34 1784.19 
Temperature/K 100.0 100.0 99.99 
Crystal system triclinic triclinic monoclinic 
Space group P-1 P-1 C2/c 
a/Å 12.1663(5) 13.9021(6) 25.8213(17) 
b/Å 15.8988(7) 16.1176(7) 17.6106(12) 
c/Å 21.2417(10) 21.2112(8) 35.578(2) 
α/° 85.786(3) 87.635(3) 90 
β/° 85.114(3) 77.947(3) 103.105(2) 
γ/° 72.129(2) 83.333(3) 90 
Volume/Å3 3891.5(3) 4615.8(3) 15757.0(18) 
Z 2 2 8 
ρcalcg/cm3 1.362 1.433 1.504 
µ/mm-1 5.702 4.943 1.307 
F(000) 1660.0 2049.0 7256.0 
Crystal size/mm3 0.5 × 0.5 × 0.1 0.1 × 0.1 × 0.05 0.1 × 0.1 × 0.1 
Radiation CuKα (λ = 1.54178) CuKα (λ = 1.54178) MoKα (λ = 0.71073) 
2Θ range for data 
collection/° 5.848 to 150.194 7.034 to 149.336 4.412 to 54.95 
Index ranges -15 ≤ h ≤ 15, -19 ≤ k ≤ 19, 0 ≤ l ≤ 26 
-17 ≤ h ≤ 17, -20 ≤ k ≤ 
20, -26 ≤ l ≤ 26 
-33 ≤ h ≤ 33, -22 ≤ k ≤ 
22, -46 ≤ l ≤ 45 
Reflections collected 13231 101142 125697 
Independent reflections 13231 [Rint = merged, Rsigma = 0.0974] 
18768 [Rint = 0.0560, 
Rsigma = 0.0428] 
18015 [Rint = 0.0634, 
Rsigma = 0.0390] 
Data/restraints/paramet
ers 13231/2139/943 18768/0/1065 18015/49/942 
Goodness-of-fit on F2 1.072 1.055 1.061 
Final R indexes [I>=2σ 
(I)] 
R1 = 0.1746, wR2 = 
0.4017 
R1 = 0.0624, wR2 = 
0.1781 
R1 = 0.0905, wR2 = 
0.2546 
Final R indexes [all 
data] 
R1 = 0.2030, wR2 = 
0.4246 
R1 = 0.0695, wR2 = 
0.1842 
R1 = 0.1204, wR2 = 
0.2849 
Largest diff. peak/hole / 
e Å-3 3.46/-1.42 1.99/-2.23 3.20/-1.12 
 
 
 
S11 
 
Table S2. Mn-oxo and Mn-Mn distances (Å) in complexes 1-Mn, 2-Mn, and 4-Mn. Average distances 
underlined for emphasis. 
 1-Mn 2-Mn 4-Mn 
Mn(1)-O(1) 2.233(2) 1.883(2) 1.88(1) 
Mn(1)-O(2) 2.012(2) 1.889(2) 1.92(2) 
Mn(1)-O(3) 1.862(2) 2.007(3) 1.89(1) 
Mn(2)-O(1) 1.869(2) 1.877(2) 1.92(1) 
Mn(2)-O(3) 1.877(2) 1.868(2) 2.01(2) 
Mn(2)-O(4) 1.847(2) 1.860(2) 1.98(1) 
Mn(3)-O(2) 1.994(2) 1.908(2) 1.89(1) 
Mn(3)-O(3) 1.848(2) 2.087(2) 1.98(1) 
Mn(3)-O(4) 1.936(2) 1.879(2) 2.14(2) 
Mn(4)-O(1) 1.898(2) 2.033(2) 1.92(2) 
Mn(4)-O(2) 2.201(2) 1.977(2) 1.88(1) 
Mn(4)-O(3) 1.937(2) 2.037(2) 1.85(1) 
Mn-O average 1.960(2) 1.942(2) 1.94(1) 
    
Mn(1)-Mn(2) 3.0724(6) 2.8997(6) 2.889(5) 
Mn(1)-Mn(3) 2.9921(6) 2.9886(7) 3.008(4) 
Mn(2)-Mn(3) 2.9174(6) 2.9021(7) 2.992(5) 
Mn(1)-Mn(4) 2.9134(6) 2.8275(6) 2.783(4) 
Mn(2)-Mn(4) 2.7663(6) 2.8307(6) 2.823(5) 
Mn(3)-Mn(4) 2.8809(6) 2.8401(8) 2.797(4) 
Mn-Mn average 2.9238(6) 2.8814(7) 2.882(5) 
 
 
 
 
 
 
 
 
 
S12 
 
 
Figure S11. Overlay of the structures of 1-Y and 2-Y. The planes defined by the three Mn centers in each 
complex were fixed together. (Left) Full structure. (Right) Zoom-in of the [YMn3O4] core. The good 
overlap of the bridging acetate moieties in both complexes indicates a small change in Y-oxo/Y-Mn 
distances. 
 
 
Figure S12. Overlay of the structure of 1-Y and 3-Y. The planes defined by the three Mn centers in each 
complex were fixed together. (Left) Full structure. (Right) Zoom-in of the [YMn3O4] core. The wider Y-
O(acetate)-C(acetate) angles [126.8(2), 128.7(2), 136.1(2)] compared to the Y-N(amidate)-C(amidate) 
angles [124.4(5), 124.7(5), 125.1(4)] indicate a contraction in Y-oxo/Y-Mn distances in 3-Y compared to 
1-Y. 
 
 
Electrochemistry 
S13 
 
Measurements were performed under an inert N2 atmosphere in the glovebox using a Pine Instrument 
Company AFCBP1 bipotentiostat using the AfterMath software package. Cyclic voltammograms were 
recorded on 1.0 mM solutions of the relevant complex in the glovebox at 20 °C with an auxiliary Pt-coil 
counter electrode, Ag-wire reference electrode, and 3.0 mm glassy carbon working electrode (BASI). The 
electrolyte solution was 0.1 M [nBu4N][PF6] in dimethyl formamide. All reported values are referenced to 
an internal ferrocene/ferrocenium couple. Square-wave voltammograms were recorded using the following 
experimental parameters: amplitude 0.1 V, period 1 s, increment 10 mV, sampling width 1 ms. Peak 
cathodic and anodic currents from cyclic voltammograms at variable scan rates were plotted against the 
square root of the scan rate and fitted to a line. 
 
 
Figure S13. Cyclic voltammogram of 4-Mn at various scan rates and plot of peak current vs. square root 
of scan rate. 
 
Figure S14. Square-wave voltammogram of 4-Mn. 
S14 
 
 
 
Figure S15. Cyclic voltammogram of 2-Y at various scan rates and plot of peak current vs. square root of 
scan rate. 
 
Figure S16. Square-wave voltammogram of 2-Y. 
 
 
 
 
S15 
 
 
Figure S17. Cyclic voltammogram of 3-Y at various scan rates and plot of peak current vs. square root of 
scan rate. 
 
Figure S18. Square-wave voltammogram of 3-Y. 
 
 
 
Table S3. Values of ligand pKa in DMSO, effective basicity, and cluster redox potential. 
 
Complex Ligand 1 Ligand 2 pKa ligand 
1 
pKa ligand 
2 
Effective 
basicity 
Redox potential 
(V vs. Fc/Fc+) 
1-Mn Acetate  12.6 - 9.45 250 
2-Mn Acetamide CF3-
benzoate 
25.9  15.35 −15 
3-Mn Acetamide Acetate 25.9 12.6 16.1 −150 
4-Mn Acetamide  25.9 - 19.42 −465 
1-Y Acetate  12.6 - 9.45 −430 
2-Y Acetoxime Acetate 25.2 12.6 15.75 −860 
 
S16 
 
EPR spectroscopy 
Samples were prepared as solutions (c.a. 1 mM) in 1:1 CH2Cl2:2-MeTHF and rapidly cooled in liquid 
nitrogen to form a frozen glass. X-band CW-EPR experiments presented in this study were acquired at the 
Caltech EPR facility. X-band CW EPR spectra were acquired on a Bruker (Billerica, MA) EMX 
spectrometer using Bruker Win-EPR software (ver. 3.0). Temperature control was achieved using liquid 
helium and an Oxford Instruments (Oxford, UK) ESR-900 cryogen flow cryostat and an ITC-503 
temperature controller. 
 
Figure S19. X-band CW-EPR of 4-Mn-ox. Data acquisition parameters: frequency = 9.366 MHz, power = 
2 mW, conversion time = 20.48 ms, modulation amplitude = 8 G. The variable-temperature behavior of 4-
Mn-ox is reminiscent of a recently reported [LMnIIIMn3IVO4(diamidate)(OAc)]+ complex.2 
 
 
 
 
S17 
 
References 
 
  (1) Qin, C.-J.; James, L.; Chartres, J. D.; Alcock, L. J.; Davis, K. J.; Willis, A. C.; Sargeson, A. M.; 
Bernhardt, P. V.; Ralph, S. F., An Unusually Flexible Expanded Hexaamine Cage and Its CuII Complexes: 
Variable Coordination Modes and Incomplete Encapsulation. Inorg. Chem. 2011, 50, 9131. 
  (2) Lee, H. B.; Shiau, A. A.; Oyala, P. H.; Marchiori, D. A.; Gul, S.; Chatterjee, R.; Yano, J.; Britt, R. D.; 
Agapie, T., Tetranuclear [MnIIIMn3IVO4] Complexes as Spectroscopic Models of the S2 State of the Oxygen 
Evolving Complex in Photosystem II. J. Am. Chem. Soc. 2018, 140, 17175. 
 


Redox Tuning via Ligand-Induced Geometric Distortions at a YMn₃O₄ Cubane Model of the Biological Oxygen Evolving Complex

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Redox Tuning via Ligand-Induced Geometric Distortions at a YMn₃O₄ Cubane Model of the Biological Oxygen Evolving Complex

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