Supporting Information

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1 Supporting Information Non-Heme Diiron Model Complexes Can Mediate Direct NO Reduction: Mechanistic Insight Into Flavodiiron NO Reductases Hai T. Dong, a Corey J. White, a Bo Zhang, b Carsten Krebs, b Nicolai Lehnert a, * a Department of Chemistry, The University of Michigan, Ann Arbor, Michigan , United States b Department of Chemistry and Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S1

2 Table of Contents UV-Vis spectra: [Fe 2 ((Py 2 PhO 2 )MP)(OPr) 2 ](OTf) (1) and its reduced complex (2) (Figure S1)... S4 Titration of 1 with CoCp 2... S5-6 Temperature dependent absorption spectra of 2... S7 Product formed at -80 o C... S8 Absorbance versus time for the reaction of 2 with NO gas at -80 o C... S9 Product formed when warming up the solution from -80 o C to RT... S10 Product formed upon reaction of 2 with NO at RT... S11 Product formed when warming up the solution from -80 o C to RT vs. product formed directly at RT... S12 Repeat of the above characterizations with 1-OAc and 2-OAc... S13-15 Catalytic reactivity of 2-OAc observed in very quick succession cycles... S16 Comparison of 1-OAc before and after regeneration with acetic acid... S17 Cyclic voltammetry: [Fe 2 (BPMP)(OPr) 2 ]OTf vs. [CoCp 2 ][Fe2((Py2PhO2)MP)(OPr)2] (2)... S18 EPR spectra: Complex 1 vs. [Fe 2 (BPMP)(OPr)(O)] +... S19 Complex 2 vs. [Fe 2 (BPMP)(OPr)(O)] +... S20 Product at -80 o C at different temperatures... S21 Product at -80 o C vs. [Fe(TMG 3 tren)(no)](otf) 2... S22 Product warmed up from -80 o C vs. [Fe 2 (BPMP)(OPr)(O)] +... S23 Product formed directly at RT vs. [Fe 2 (BPMP)(OPr)(O)] +... S24 Solid state and gas phase IR spectra: Solid state IR of 1... S25 Solid state IR of 2... S26 Overlay of the IR spectra of 1 and of the reaction product of 2 with NO... S27 N 2 O yield at RT... S28 N 2 O formation from a RT reaction vs. a reaction at -80 o C, warmed up to RT... S29 N 2 O after 2 cycle of NO reduction using 2 eq. of acetic acid to regenerate 1-OAc... S30 Solution IR spectra: N 2 O and N 2 O formation in solution... S31 Complex 1 vs S32-80 o C product vs S33 15 N 18 O experiment to form product at -80 o C... S34 Changes in intensity of bands in the 1700 cm -1 and the N 2 O region... S H-NMR spectra: Complex 1... S37 S2

3 Complex 1-OAc after one turnover with NO and regeneration with acetic acid... S38 Complex 1-OAc from the first versus the second cycle... S39 Complex 2 generated in-situ by reacting 1 with CoCp 2... S40 2,6-Bis[((2-hydroxybenzyl)(2-pyridylmethyl)amino)methyl]-4-methylphenol (H 3 [(Py 2 PhO 2 )MP)... S41 TOF HPLC-MS Complex 1 with Fe... S42 Complex 1 with 57 Fe... S42 Mössbauer spectra: Complex 1 and 2... S43 Product at -80 o C... S44 Crystal Structure: Crystal structure of 2-OAc and essential structural parameters... S45-66 DFT Optimized structure of the diferrous dinitrosyl intermediate of 2... S67-69 References S3

4 Figure S1. Absorption spectra of complexes 1 and 2 in dichloromethane at 0.1 mm concentration at room temperature. S4

5 Figure S2. UV-Vis titration of complex 1 with CoCp 2 (up to 1 equivalent). Spectra were collected at 0.1 mm concentration of 1 in dichloromethane at room temperature. S5

6 Figure S3. Changes in the absorption spectrum of 1 after reaction with the second equivalent of CoCp 2. Spectra were obtained at a 0.1 mm concentration of 1 in dichloromethane at room temperature. S6

7 Comparison of compound 2 at different temperature and solid state 0.40 Absorbance Compound 2 at 20 o C Compound 2 at -80 o C Compound 2 crystalline material at RT in KBr Wavelength (nm) Figure S4. Top: spectral changes in the absorption spectrum of 2 upon cooling from room temperature to -80 o C. Note that these changes are fully reversible. In the process, the color of the solution changes from deep orange to light yellow. The spectra were collected at 0.1 mm concentration of 2 in dichloromethane. Bottom: Comparison of the UV-Vis spectrum obtained from crystalline material in KBr (black line) with the absorption spectra of complex 2 in solution at RT (red line) and -80 o C (blue line). S7

8 Absorbance NO reaction at -80 o C in CH 2 Cl 2 for 30 minutes 430 Complex 2 at -80 o C 15 minutes after adding NO gas 30 minutes after adding NO gas Wavelength (nm) Figure S5. UV-Vis spectral changes upon reaction of complex 2 with NO gas (syringed into the headspace of the reaction flask) at -80 o C. After ~30 minutes, no more changes in the absorption spectrum were observed, and the experiment was stopped. Spectra were collected at 0.1 mm concentration of 2 in dichloromethane. S8

9 Figure S6. A rate constant of s -1 is calculated from the absorbance increasing over the course of 30 minutes at 423 nm at -80 o C in the reaction of 2 with NO in dichloromethane (see Figure S5), using a single-exponential fit. The data show that the reaction has a significant rate, even at -80 o C. S9

10 0.8 Product at -80 o C warmed up to RT Absorbance Product at -80 o C 515 Product at RT Wavelength (nm) Figure S7. Changes in the absorption spectrum of the reaction product of 2 and NO gas (reaction run at -80 o C; see Figure S5) upon warming up of the solution to room temperature over the course of 45 minutes. Spectra were collected at a concentration of 0.1 mm 2 in dichloromethane. S10

11 Complex 2 Product after reaction with NO at RT Absorbance Wavelength (nm) Figure S8. Changes in the absorption spectrum of complex 2 upon reaction with NO gas at room temperature. These spectral changes occur in less than a minute. Spectra were collected at 0.1 mm concentration of 2 in dichloromethane. S11

12 Product comparison between RT and LT warmed up to RT Product warmed up from -80 o C to RT Product directly generated at RT Absorbance Wavelength (nm) Figure S9. Comparison of the absorption spectra of the product of the reaction of 2 with NO gas. Black: reaction performed at -80 o C, and then warmed up to room temperature. Red: reaction conducted at room temperature. Reactions were run at 0.1 mm 2 in dichloromethane. S12

13 Reaction of 2-OAc with NO gas at RT Absorbance Complex 2-OAc Product at RT Wavelength (nm) Figure S10. Changes in the absorption spectrum of 2-OAc upon reaction with NO gas at room temperature. These spectral changes occur in less than a minute. Spectra were collected at a concentration of 0.1 mm 2-OAc in dichloromethane. S13

14 Absorbance Reaction of 2-OAc with NO at -80 o C OAc at -80 o C Product after reaction with NO at -80 o C Wavelength (nm) Figure S11. Changes in the absorption spectrum of 2-OAc upon reaction with NO gas at -80 o C. Spectral changes stop after about 30 minutes of reaction time. Spectra were collected at a concentration of 0.1 mm 2-OAc in dichloromethane. S14

15 Product of 2-OAc at -80 o C warmed up to RT 0.8 Absorbance Product at -80 o C Product warmed up to RT Wavelength (nm) Figure S12. Changes in the absorption spectrum of the reaction product of 2-OAc and NO gas (reaction run at -80 o C; see Figure S11) upon warming up of the solution to room temperature over the course of 45 minutes. Spectra were collected at a concentration of 0.1 mm 2-OAc in dichloromethane. S15

16 Catalyst (2-OAc) regeneration in the presence of excess acetic acid and NO gas Product + acetic acid (regenerated 1-OAc) 2eq. CoCp 2 less than 30 seconds later 0.5 Absorbance Wavelength (nm) Figure S13. Changes in the absorption spectrum in the reaction between 2-OAc and excess NO gas in the presence of excess acetic acid. After NO addition, the product immediately starts to turn back into the starting material 1-OAc, evident from the shift in max. Spectra were collected at a concentration of 0.1 mm 2-OAc in dichloromethane. The shift in the yellow line, originating from the diferrous complex, is due to the immediate reaction between 2-OAc and NO as soon as CoCp 2 is added to the newly formed 1-OAc for the second reaction cycle. Similar experiments were repeated with 1 and 2, producing the same result. S16

17 Comparison of the absorption spectra after 3 cycles of NO reduction 0.9 Absorbance st cycle 3rd cycle Wavelength (nm) Figure S14. Changes in the absorption spectrum of the catalyst after 3 cycles of NO reduction. The max is still the same, but the intensity of the signal changes. This change does not seem to affect the yield of NO reduction, since quantitative amounts of N 2 O are detected from these cycles. S17

18 E 2 = mv 1/ E 1 = -282 mv 1/2 50 Current A mV/s 250mV/s 150mV/s Potential (mv) vs. Fc + /Fc Figure S15. Top: Cyclic voltammogram of a 5 mm solution of the diferrous complex [Fe 2 (BPMP)(OPr) 2 ]OTf in CH 2 Cl 2 at room temperature, obtained from previous work. 1 Tetrabutylammonium hexafluorophosphate was used as the electrolyte. Potentials are referenced to a Fc/Fc + standard. Bottom: Cyclic voltammogram of a 5 mm solution of the diferrous complex [CoCp 2 ][Fe 2 ((Py 2 PhO 2 )MP)(OPr) 2 ] in CH 2 Cl 2 at room temperature obtained at various scan rates. Tetrabutylammonium trifloromethansulfonate was used as the electrolyte. Potentials are referenced to a Fc/Fc + standard. S18

19 Figure S16. X-band EPR spectrum of a 2 mm solution of complex 1 in dichloromethane (blue) in comparison to that of the mixed-valent complex [Fe III Fe II (BPMP)(OPr)(O)] + (black) at the same concentration. 1 All data were collected at 2K. The data show that complex 1 is EPR silent. S19

20 Figure S17. X-band EPR spectrum of a 2 mm solution of complex 2 in dichloromethane (orange) in comparison to that of the mixed-valent complex [Fe III Fe II (BPMP)(OPr)(O)] + (black) at the same concentration. 1 All data were collected at 2K. The data show that complex 2 is EPR silent. S20

21 Figure S18. Temperature dependent X-band EPR spectra of the product formed from the reaction of 2 mm 2 with NO in dichloromethane at -80 o C. The data reveal two signals at g = 3.94 and g = 2.01 with a different temperature-dependence, indicating that they are associated with two different species. These signals, however, do only correspond to about ~5% of total iron (see Figure S19), so they originate from minor impurities. S21

22 Intensity Comparison between the -80 o C product and [Fe(TMG 3 tren)(no)](otf) 2 TMG3Tren NO complex Product after reaction with NO at -80 o C Magnetic Field (Gauss) Figure S19. X-band EPR spectrum of the product of the reaction of 2 mm 2 with NO at -80 o C in dichlorormethane (red), in comparison to that of a standard of the mononuclear {FeNO} 7 complex [Fe(TMG 3 tren)(no)](otf) 2 (black) 2 at the same concentration. All data were collected at 2K. See also Figure S18. S22

23 Figure S20. X-band EPR spectrum of the reaction product of 4 mm 2 with NO gas at -80 o C, which was subsequently warmed up to room temperature before the EPR samples were prepared (red). This is compared to the EPR spectrum of a 2 mm solution of the mixed-valent complex [Fe III Fe II (BPMP)(OPr)(O)] + (black). 1 All data were collected at 2K. The data show that the EPR signals observed at g = 3.94, 2.01 (red) at -80 o C (see Figure S18) disappear when the reaction mixture is allowed to warm up to room temperature. S23

24 Figure S21. X-band EPR spectrum of the product from the reaction of 4 mm 2 with excess NO gas at room temperature in dichloromethane in comparison to that of a 2 mm solution of the mixed-valent complex [Fe III Fe II (BPMP)(OPr)(O)] + (black). 1 All data were collected at 2 K. The data show that the room temperature product is EPR silent. S24

25 Figure S22. FT-IR spectrum of complex 1, collected from a pure solid at room temperature. S25

26 % Transmittance Reduction Product Wavenumber (cm -1 ) Figure S23. FT-IR spectrum of the reaction product of 2 with excess NO gas at room temperature, precipitated out of dichloromethane solution with diethylether, and measured in a KBr pellet. S26

27 % Transmittance Wavenumber (cm -1 ) Complex 1 Reduction Product Figure S24. Overlay of the IR spectra of the reaction product of 2 with NO (red) and complex 1 (black) in the cm -1 region, showing the possible formation of a bridging oxo complex. S27

28 10 N 2 O quantitation by gas headspace IR 8 mol of N 2 O y= x Chemically synthesized 2 + NO 2 generated by chemical reduction of 1 + NO Area under N 2 O curve Figure S25. Calibration curve for the quantitative detection of N 2 O by IR gas headspace analysis. Data for the reaction of 2 with NO gas at room temperature are indicated. See ref. 4 for the procedure to generate the calibration curve. The black line is the calibration curve generated from Piloty s acid, the black dots are the observed amounts of N 2 O formed in our experiments using chemically synthesized 2, and the blue dots are from complex 2 generated by chemical reduction of complex 1 by CoCp 2, quantified using the calibration curve. S28

29 Figure S26. Comparison of the IR gas headspace analysis of N 2 O formation between the reaction of 4 mm 2 with NO gas (a) at room temperature (red), and (b) at -80 o C, followed by warming up of the reaction mixture to room temperature (black). The data show that both reactions produce the same yield of N 2 O (= quantitative). S29

30 Expected Area: 4.11 Yield: 111% 0.10 Absorbance Area= FWHM= Wavenumber (cm -1 ) 0.10 Expected Area: 4.11 Yield: 99% Absorbance Area= FWHM= Wavenumber (cm -1 ) Figure S27. Comparison of the IR gas headspace analysis for N 2 O formation for the reaction of 8 mol of 2- OAc with NO gas in dichloromethane at room temperature. Top: first cycle, bottom: second cycle. Since CoCp 2 also reacts with NO gas directly to produce N 2 O, it is difficult to quantify the amount of N 2 O that is solely generated by 2-OAc when an excess amount of CoCp 2 is present in the reaction mixture. Therefore, we decided to run the reaction in cycles and only add quantitative amounts of CoCp 2 as needed to reduce 1-OAc back to 2- OAc. At the end of each cycle (after N 2 O detection), we purged out the solvent (CH 2 Cl 2 ), using N 2 gas from the Schlenk line, until only solid products remained. We then added the same amount of fresh CH 2 Cl 2 back in and subsequently added 2 equivalents of acetic acid. The red solution immediately turned blue, indicating the reformation of 1-OAc. Then, 2 equivalents of CoCp 2 were added to produce 2-OAc, which is then ready for the next cycle of NO reduction. Quantitative analysis of the product N 2 O was conducted by IR gas headspace analysis. S30

31 Figure S28. Detection of N 2 O and 15 N 2 18 O produced by the reaction of 2 with NO and 15 N 18 O gas, respectively, at -80 o C in dichloromethane, as observed by solution IR spectroscopy. S31

32 0.6 Solution IR of complex 1 and 2 in CH 2 Cl Absorbance Complex 2 Complex Wavenumber (cm -1 ) Figure S29. Solution IR spectra of complex 1 (blue) and complex 2 (black); the latter was generated by reducing 5 mm 1 with CoCp 2 in dichloromethane. S32

33 0.30 Solution IR of 2 and of the -80 o C product in CH 2 Cl Absorbance Product formed at -80 o C Complex Wavenumber (cm -1 ) Figure S30. Solution IR spectra of complex 2 (yellow) and of the reaction product of 5 mm 2 with NO gas in dichloromethane at -80 o C (black). The two signals at 2120 and 2150 cm -1 arise from possible impurities in the solvent. These signals are not consistent and have different intensities in different experiments. In addition, 15 N 18 O labelling experiments show that these signals are not isotope sensitive, and therefore, they do not arise from an NO-derived species. S33

34 Reaction with labeled NO at -80 o C Absorbance Product from reaction with 15 N 18 O at -80 o C Product from reaction with NO at -80 o C Wavenumber (cm -1 ) Figure S N 18 O labelling study using solution IR spectroscopy. The data show the product of the reaction of 5 mm 2 with NO/ 15 N 18 O gas (black/red spectra) in dichloromethane at -80 o C. The spectra demonstrate that the two signals at 1707 and 1726 cm -1 originate from N-O stretches in nitrosyl complexes that are formed in the low-temperature reaction (see text and Figure S18). S34

35 Figure S32. Solution IR spectra of the product of the reaction of 5 mm 2 with NO gas at -80 o C in dichloromethane. The data show the spectral changes when the sample is warmed up to room temperature in the sample holder. In this process (over time), the two N-O stretches at 1707 and 1726 cm -1 disappear, whereas at the same time the N 2 O signal increases in intensity. S35

36 NO decreases 0.05 Absorbance N 2 O increases Wavenumber (cm -1 ) Figure S33. Zoom into the solution IR data in Figure S31, focusing on the region of the N-O stretch of the nitrosyl complexes (left), and the N-N stretch of the product N 2 O. S36

37 Figure S34. 1 H-NMR spectrum of complex 1 in CD 2 Cl 2, referenced against solvent. S37

38 * * * * * * * Figure S35. 1 H-NMR spectrum of complex 1-OAc, regenerated after reaction of 2-OAc with NO (one cycle) using acetic acid in CD 2 Cl 2. The data are referenced against solvent. The finger print region is identical to 1- OAc (see overlay in Figure S36). The additional features are from CoCp 2, excess acid, and CH 2 Cl 2 in solution. S38

39 Figure S36. Overlay of the 1 H-NMR spectra of 1-OAc and regenerated 1-OAc (after reaction of 2-OAc with NO), with the finger print region zoomed in for clarity. S39

40 CoCp2 Figure S37. 1 H-NMR spectrum of complex 2 produced in situ by reduction of 1 with 2 equivalent of CoCp 2 in CD 2 Cl 2, referenced against solvent. S40

41 Figure S38. 1 H-NMR spectrum of the ligand H 3 [(Py 2 PhO 2 )MP] in CDCl 3, referenced against solvent. S41

42 Figure S39. TOF HPLC-MS of the natural abundant Fe complex 1. Figure S40. TOF HPLC-MS of the natural abundant 57 Fe complex 1. S42

43 0.0 ABSORPTION (%) A B VELOCITY (mm/s) Figure S K/53 mt Mossbauer characterization of complexes 1 (A, top) and 2 (B, bottom). The solid lines overlaying the experimental data are simulations using the parameters quoted in the main text. S43

44 A 10.0 ABSORPTION (%) B VELOCITY (mm/s) Figure S42. Mössbauer characterization of a sample of complex 2 reacted with NO gas (added by syringe transfer) at -80 o C for 60 min. The measurements were carried out on the same sample that was used for the spectra shown in Figures 9A and B in the main text. (A) Comparison of 4.2 K spectra recorded in a 53 mt magnetic field applied parallel (vertical bars) and perpendicular (solid line) to the γ radiation. The lack of field orientation dependence argues against the presence of a non-heme {FeNO} 7 complex with S=3/2 or a complex with S=1/2 ground state at any detectable amounts, in agreement with the EPR data, see Figure S19 The fieldorientation-independent features suggest a half-integer spin ground state with uniaxial spin expectation value; such systems produce only weak EPR features. (B) Spectrum recorded at 1.8 K in a 53 mt externally applied magnetic field. The magnetically split features become better defined at 1.8 K, indicating that the spectrum is not fully in the slow-relaxation limit at 4.2 K. The black line overlaying the raw spectrum is a Spin Hamiltonian simulation in the slow-relaxation limit of a ferromagnetically-coupled mixed-valent Fe(II)/Fe(III) species (30% total iron), using the following parameters: S tot =9/2, D = -1 cm -1, E/D =0.33, g = 2.0, δ 1 = 0.57 mm/s, ΔE Q1 = 0.92 mm/s, η 1 = 0, A z1 = 11.8 T (high-spin ferric site, 15% of total intensity) and δ 2 = 1.22 mm/s, ΔE Q2 = 2.89 mm/s, η 2 = 0.2, A z2 = 5.9 T (high-spin ferrous site, 15% of total intensity). Note that the spectra are only sensitive to the y-component of the A tensor. S44

45 Figure S43. Crystal structure of 2-OAc grown from diffusion of hexane into a CH 2 Cl 2 solution of the complex. Hydrogen atoms, solvent molecules and the cobaltocenium counter cation are omitted for clarity. S45

46 Table S1. Crystal data and structure refinement for 2-OAc. Identification code Empirical formula hd1152a_sq C51 H53 Cl4 Co Fe2 N4 O7 Formula weight Temperature Wavelength Crystal system, space group 85(2) K Å Monoclinic, C2/c Unit cell dimensions a = (7) Å alpha = 90 deg. b = (3) Å beta = (2) deg. c = (4) Å gamma = 90 deg. Volume (4) Å 3 Z, Calculated density 8, Mg/m 3 Absorption coefficient mm -1 F(000) 4720 Crystal size Theta range for data collection Limiting indices x x mm to deg. -38<=h<=38, -23<=k<=23, -21<=l<=22 Reflections collected / unique / [R(int) = ] Completeness to theta = % Absorption correction Semi-empirical from equivalents Max. and min. transmission and Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters / 37 / 664 Goodness-of-fit on F Final R indices [I>2sigma(I)] R1 = , wr2 = R indices (all data) R1 = , wr2 = Extinction coefficient n/a Largest diff. peak and hole and e. Å -3 S46

47 Table S2. Atomic coordinates ( x 10 4 ) and equivalent isotropic displacement parameters (Å 2 x 10 3 ) for 2-OAc. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. x y z U(eq) Co(1) (1) (1) Co(2) (1) Fe(1) 3430(1) 2712(1) 2924(1) 35(1) Fe(2) 3427(1) 4473(1) 3390(1) 32(1) O(1) 3076(1) 3574(1) 3062(1) 35(1) O(2) 3690(1) 1821(1) 2732(1) 41(1) O(3) 3736(1) 5364(1) 3728(2) 41(1) O(4) 3769(1) 3256(1) 2272(2) 44(1) O(5) 3821(1) 4333(1) 2688(1) 39(1) O(6) 3933(1) 2870(1) 3976(2) 48(1) O(7) 3846(1) 3920(1) 4389(1) 42(1) N(1) 2843(1) 2434(1) 1914(2) 35(1) N(2) 2998(1) 2162(1) 3462(2) 41(1) N(3) 2946(1) 4772(1) 4001(2) 34(1) N(4) 2860(1) 4928(1) 2450(2) 33(1) C(1) 2642(1) 3545(2) 2945(2) 35(1) C(2) 2358(1) 3276(2) 2260(2) 40(1) C(3) 1909(1) 3231(2) 2158(2) 50(1) C(4) 1726(1) 3449(2) 2709(3) 54(1) C(5) 2012(1) 3736(2) 3373(2) 46(1) C(6) 2463(1) 3788(2) 3503(2) 39(1) C(7) 1241(2) 3385(3) 2590(4) 82(2) C(8) 2552(1) 3033(2) 1662(2) 40(1) C(9) 2605(1) 1875(2) 2137(2) 39(1) C(10) 2626(1) 1914(2) 2968(2) 43(1) C(11) 2282(2) 1677(2) 3215(3) 53(1) C(12) 2330(2) 1691(2) 3980(3) 63(1) C(13) 2717(2) 1941(2) 4495(3) 60(1) C(14) 3039(2) 2182(2) 4211(2) 50(1) C(15) 3018(1) 2241(2) 1283(2) 41(1) C(16) 3277(1) 1595(2) 1409(2) 38(1) C(17) 3210(1) 1150(2) 785(2) 42(1) C(18) 3441(1) 544(2) 847(2) 45(1) C(19) 3737(1) 362(2) 1548(2) 44(1) C(20) 3812(1) 792(2) 2174(2) 41(1) C(21) 3596(1) 1425(2) 2118(2) 37(1) C(22) 2758(1) 4138(2) 4202(2) 37(1) C(23) 2590(1) 5210(2) 3506(2) 34(1) C(24) 2504(1) 5081(2) 2661(2) 33(1) C(25) 2086(1) 5148(2) 2135(2) 38(1) C(26) 2031(1) 5056(2) 1363(2) 44(1) C(27) 2396(1) 4903(2) 1145(2) 40(1) S47

48 C(28) 2802(1) 4840(2) 1707(2) 35(1) C(29) 3201(1) 5131(2) 4719(2) 38(1) C(30) 3380(1) 5812(2) 4591(2) 37(1) C(31) 3299(1) 6382(2) 4976(2) 42(1) C(32) 3478(1) 7016(2) 4914(2) 46(1) C(33) 3740(1) 7083(2) 4446(2) 44(1) C(34) 3813(1) 6530(2) 4038(2) 43(1) C(35) 3642(1) 5881(2) 4101(2) 38(1) C(36) 3902(1) 3855(2) 2291(2) 38(1) C(37) 4178(2) 4040(2) 1793(3) 56(1) C(38) 3997(1) 3321(2) 4483(2) 45(1) C(39) 4286(2) 3117(2) 5273(3) 73(2) C(40) 4937(2) 1528(3) 3409(3) 81(2) C(41) 5383(2) 1593(3) 3469(3) 77(2) C(42) 5487(2) 2286(3) 3472(5) 97(2) C(43) 5095(2) 2652(3) 3416(5) 111(3) C(44) 4756(2) 2186(3) 3385(4) 97(2) C(45) 4873(2) 4266(3) 4185(3) 74(2) C(46) 5334(1) 4317(3) 4578(3) 61(1) C(47) 5466(1) 4988(2) 4471(3) 59(1) C(48) 5085(2) 5345(3) 4022(3) 71(1) C(49) 4725(2) 4904(3) 3843(3) 76(2) Cl(1) 4271(1) 424(1) 4339(1) 86(1) Cl(1A) 4619(5) 928(8) 4757(11) 115(7) Cl(2) 3878(1) 1157(1) 5302(1) 70(1) C(50) 4043(2) 1210(2) 4479(3) 70(1) Cl(3) 4443(2) 6535(3) 2554(5) 53(2) Cl(4) 3725(3) 5816(6) 1462(7) 66(2) C(51) 4136(5) 5793(5) 2364(6) 45(3) Cl(3A) 4426(2) 6430(5) 2489(5) 121(3) Cl(4A) 3688(3) 5885(5) 1302(5) 76(2) C(51A) 3901(5) 6058(8) 2245(6) 85(4) S48

49 Table S3. Bond lengths [Å] and angles [deg] for 2-OAc. Co(1)-C(43) 2.010(7) Co(1)-C(43)# (7) Co(1)-C(42) 2.018(7) Co(1)-C(42)# (7) Co(1)-C(41) 2.018(6) Co(1)-C(41)# (6) Co(1)-C(40)# (6) Co(1)-C(40) 2.023(6) Co(1)-C(44)# (6) Co(1)-C(44) 2.037(6) Co(2)-C(46)# (4) Co(2)-C(46) 2.021(4) Co(2)-C(47) 2.023(4) Co(2)-C(47)# (4) Co(2)-C(48) 2.023(6) Co(2)-C(48)# (6) Co(2)-C(45)# (4) Co(2)-C(45) 2.029(4) Co(2)-C(49) 2.037(5) Co(2)-C(49)# (5) Fe(1)-O(2) 2.022(2) Fe(1)-O(1) 2.100(2) Fe(1)-O(6) 2.107(2) Fe(1)-O(4) 2.143(3) Fe(1)-N(2) 2.220(3) Fe(1)-N(1) 2.247(3) Fe(2)-O(3) 2.013(2) Fe(2)-O(1) 2.077(2) Fe(2)-O(5) 2.093(2) Fe(2)-O(7) 2.188(2) Fe(2)-N(3) 2.253(3) Fe(2)-N(4) 2.253(3) O(1)-C(1) 1.336(4) O(2)-C(21) 1.325(4) O(3)-C(35) 1.314(4) O(4)-C(36) 1.252(4) O(5)-C(36) 1.267(4) O(6)-C(38) 1.257(4) O(7)-C(38) 1.266(4) N(1)-C(9) 1.469(4) N(1)-C(8) 1.482(4) N(1)-C(15) 1.490(5) N(2)-C(10) 1.338(5) N(2)-C(14) 1.345(5) N(3)-C(22) 1.484(4) N(3)-C(23) 1.486(4) N(3)-C(29) 1.490(4) N(4)-C(28) 1.333(4) N(4)-C(24) 1.349(4) S49

50 C(1)-C(2) 1.401(5) C(1)-C(6) 1.411(5) C(2)-C(3) 1.390(5) C(2)-C(8) 1.504(5) C(3)-C(4) 1.389(6) C(3)-H(3) C(4)-C(5) 1.390(6) C(4)-C(7) 1.501(6) C(5)-C(6) 1.388(5) C(5)-H(5) C(6)-C(22) 1.498(5) C(7)-H(7A) C(7)-H(7B) C(7)-H(7C) C(8)-H(8A) C(8)-H(8B) C(9)-C(10) 1.511(5) C(9)-H(9A) C(9)-H(9B) C(10)-C(11) 1.398(6) C(11)-C(12) 1.367(6) C(11)-H(11) C(12)-C(13) 1.386(7) C(12)-H(12) C(13)-C(14) 1.378(7) C(13)-H(13) C(14)-H(14) C(15)-C(16) 1.496(5) C(15)-H(15A) C(15)-H(15B) C(16)-C(17) 1.406(5) C(16)-C(21) 1.416(5) C(17)-C(18) 1.389(5) C(17)-H(17) C(18)-C(19) 1.382(6) C(18)-H(18) C(19)-C(20) 1.388(5) C(19)-H(19) C(20)-C(21) 1.414(5) C(20)-H(20) C(22)-H(22A) C(22)-H(22B) C(23)-C(24) 1.511(5) C(23)-H(23A) C(23)-H(23B) C(24)-C(25) 1.383(5) C(25)-C(26) 1.388(5) C(25)-H(25) C(26)-C(27) 1.384(5) C(26)-H(26) C(27)-C(28) 1.382(5) C(27)-H(27) C(28)-H(28) S50

51 C(29)-C(30) 1.506(5) C(29)-H(29A) C(29)-H(29B) C(30)-C(31) 1.395(5) C(30)-C(35) 1.420(5) C(31)-C(32) 1.392(5) C(31)-H(31) C(32)-C(33) 1.388(6) C(32)-H(32) C(33)-C(34) 1.385(5) C(33)-H(33) C(34)-C(35) 1.410(5) C(34)-H(34) C(36)-C(37) 1.508(6) C(37)-H(37A) C(37)-H(37B) C(37)-H(37C) C(38)-C(39) 1.506(5) C(39)-H(39A) C(39)-H(39B) C(39)-H(39C) C(40)-C(41) 1.402(8) C(40)-C(44) 1.415(9) C(40)-H(40) C(41)-C(42) 1.405(8) C(41)-H(41) C(42)-C(43) 1.419(8) C(42)-H(42) C(43)-C(44) 1.407(9) C(43)-H(43) C(44)-H(44) C(45)-C(49) 1.418(9) C(45)-C(46) 1.424(6) C(45)-H(45) C(46)-C(47) 1.420(7) C(46)-H(46) C(47)-C(48) 1.418(7) C(47)-H(47) C(48)-C(49) 1.396(8) C(48)-H(48) C(49)-H(49) Cl(1)-C(50) 1.766(5) Cl(1A)-C(50) 1.833(13) Cl(2)-C(50) 1.758(5) C(50)-H(50A) C(50)-H(50B) C(50)-H(50C) C(50)-H(50D) Cl(3)-C(51) 1.731(9) Cl(4)-C(51) 1.759(12) C(51)-H(51A) C(51)-H(51B) Cl(3A)-C(51A) 1.753(11) S51

52 Cl(4A)-C(51A) 1.684(10) C(51A)-H(51C) C(51A)-H(51D) C(43)-Co(1)-C(43)# (5) C(43)-Co(1)-C(42) 41.2(2) C(43)#1-Co(1)-C(42) 118.0(3) C(43)-Co(1)-C(42)# (3) C(43)#1-Co(1)-C(42)#1 41.2(2) C(42)-Co(1)-C(42)# (4) C(43)-Co(1)-C(41) 68.5(3) C(43)#1-Co(1)-C(41) 152.8(2) C(42)-Co(1)-C(41) 40.7(2) C(42)#1-Co(1)-C(41) 165.0(2) C(43)-Co(1)-C(41)# (2) C(43)#1-Co(1)-C(41)#1 68.5(3) C(42)-Co(1)-C(41)# (2) C(42)#1-Co(1)-C(41)#1 40.7(2) C(41)-Co(1)-C(41)# (3) C(43)-Co(1)-C(40)# (2) C(43)#1-Co(1)-C(40)#1 68.5(3) C(42)-Co(1)-C(40)# (2) C(42)#1-Co(1)-C(40)#1 68.8(3) C(41)-Co(1)-C(40)# (2) C(41)#1-Co(1)-C(40)#1 40.6(2) C(43)-Co(1)-C(40) 68.5(3) C(43)#1-Co(1)-C(40) 164.8(2) C(42)-Co(1)-C(40) 68.8(3) C(42)#1-Co(1)-C(40) 126.9(2) C(41)-Co(1)-C(40) 40.6(2) C(41)#1-Co(1)-C(40) 108.5(2) C(40)#1-Co(1)-C(40) 119.7(3) C(43)-Co(1)-C(44)# (3) C(43)#1-Co(1)-C(44)#1 40.7(3) C(42)-Co(1)-C(44)# (3) C(42)#1-Co(1)-C(44)#1 69.1(3) C(41)-Co(1)-C(44)# (2) C(41)#1-Co(1)-C(44)#1 68.5(3) C(40)#1-Co(1)-C(44)#1 40.8(3) C(40)-Co(1)-C(44)# (3) C(43)-Co(1)-C(44) 40.7(3) C(43)#1-Co(1)-C(44) 126.5(3) C(42)-Co(1)-C(44) 69.1(3) C(42)#1-Co(1)-C(44) 107.0(3) C(41)-Co(1)-C(44) 68.5(3) C(41)#1-Co(1)-C(44) 119.2(2) C(40)#1-Co(1)-C(44) 153.6(3) C(40)-Co(1)-C(44) 40.8(3) C(44)#1-Co(1)-C(44) 164.2(4) C(46)#2-Co(2)-C(46) C(46)#2-Co(2)-C(47) 138.9(2) C(46)-Co(2)-C(47) 41.1(2) C(46)#2-Co(2)-C(47)#2 41.1(2) S52

53 C(46)-Co(2)-C(47)# (2) C(47)-Co(2)-C(47)# (11) C(46)#2-Co(2)-C(48) 110.9(2) C(46)-Co(2)-C(48) 69.1(2) C(47)-Co(2)-C(48) 41.03(19) C(47)#2-Co(2)-C(48) (19) C(46)#2-Co(2)-C(48)#2 69.1(2) C(46)-Co(2)-C(48)# (2) C(47)-Co(2)-C(48)# (19) C(47)#2-Co(2)-C(48)# (19) C(48)-Co(2)-C(48)# C(46)#2-Co(2)-C(45)# (18) C(46)-Co(2)-C(45)# (18) C(47)-Co(2)-C(45)# (19) C(47)#2-Co(2)-C(45)# (19) C(48)-Co(2)-C(45)# (2) C(48)#2-Co(2)-C(45)#2 68.5(2) C(46)#2-Co(2)-C(45) (18) C(46)-Co(2)-C(45) 41.17(18) C(47)-Co(2)-C(45) 68.92(19) C(47)#2-Co(2)-C(45) (19) C(48)-Co(2)-C(45) 68.5(2) C(48)#2-Co(2)-C(45) 111.5(2) C(45)#2-Co(2)-C(45) 180.0(3) C(46)#2-Co(2)-C(49) 111.0(2) C(46)-Co(2)-C(49) 69.0(2) C(47)-Co(2)-C(49) 68.48(18) C(47)#2-Co(2)-C(49) (18) C(48)-Co(2)-C(49) 40.2(2) C(48)#2-Co(2)-C(49) 139.8(2) C(45)#2-Co(2)-C(49) 139.2(2) C(45)-Co(2)-C(49) 40.8(2) C(46)#2-Co(2)-C(49)#2 69.0(2) C(46)-Co(2)-C(49)# (2) C(47)-Co(2)-C(49)# (18) C(47)#2-Co(2)-C(49)# (18) C(48)-Co(2)-C(49)# (2) C(48)#2-Co(2)-C(49)#2 40.2(2) C(45)#2-Co(2)-C(49)#2 40.8(2) C(45)-Co(2)-C(49)# (2) C(49)-Co(2)-C(49)# O(2)-Fe(1)-O(1) (10) O(2)-Fe(1)-O(6) 93.02(10) O(1)-Fe(1)-O(6) 92.96(9) O(2)-Fe(1)-O(4) 91.47(10) O(1)-Fe(1)-O(4) 92.99(9) O(6)-Fe(1)-O(4) 93.91(11) O(2)-Fe(1)-N(2) 90.16(10) O(1)-Fe(1)-N(2) 84.60(10) O(6)-Fe(1)-N(2) 93.56(12) O(4)-Fe(1)-N(2) (11) O(2)-Fe(1)-N(1) 85.73(10) O(1)-Fe(1)-N(1) 87.57(9) S53

54 O(6)-Fe(1)-N(1) (11) O(4)-Fe(1)-N(1) 95.47(10) N(2)-Fe(1)-N(1) 77.10(11) O(3)-Fe(2)-O(1) (9) O(3)-Fe(2)-O(5) 88.44(10) O(1)-Fe(2)-O(5) 94.83(9) O(3)-Fe(2)-O(7) 93.76(10) O(1)-Fe(2)-O(7) 87.07(9) O(5)-Fe(2)-O(7) 97.20(10) O(3)-Fe(2)-N(3) 87.83(10) O(1)-Fe(2)-N(3) 88.74(9) O(5)-Fe(2)-N(3) (10) O(7)-Fe(2)-N(3) 92.45(10) O(3)-Fe(2)-N(4) 94.88(10) O(1)-Fe(2)-N(4) 83.66(9) O(5)-Fe(2)-N(4) 94.23(10) O(7)-Fe(2)-N(4) (10) N(3)-Fe(2)-N(4) 76.70(10) C(1)-O(1)-Fe(2) (19) C(1)-O(1)-Fe(1) (18) Fe(2)-O(1)-Fe(1) (11) C(21)-O(2)-Fe(1) 131.6(2) C(35)-O(3)-Fe(2) 131.8(2) C(36)-O(4)-Fe(1) 133.0(2) C(36)-O(5)-Fe(2) 136.6(2) C(38)-O(6)-Fe(1) 133.0(2) C(38)-O(7)-Fe(2) 133.1(2) C(9)-N(1)-C(8) 110.8(3) C(9)-N(1)-C(15) 112.5(3) C(8)-N(1)-C(15) 108.3(3) C(9)-N(1)-Fe(1) 108.7(2) C(8)-N(1)-Fe(1) (19) C(15)-N(1)-Fe(1) 106.5(2) C(10)-N(2)-C(14) 118.7(3) C(10)-N(2)-Fe(1) 114.8(2) C(14)-N(2)-Fe(1) 125.0(3) C(22)-N(3)-C(23) 110.4(3) C(22)-N(3)-C(29) 109.2(3) C(23)-N(3)-C(29) 111.4(2) C(22)-N(3)-Fe(2) (19) C(23)-N(3)-Fe(2) 110.9(2) C(29)-N(3)-Fe(2) 107.5(2) C(28)-N(4)-C(24) 118.2(3) C(28)-N(4)-Fe(2) 123.4(2) C(24)-N(4)-Fe(2) 114.4(2) O(1)-C(1)-C(2) 120.3(3) O(1)-C(1)-C(6) 120.7(3) C(2)-C(1)-C(6) 119.1(3) C(3)-C(2)-C(1) 119.4(3) C(3)-C(2)-C(8) 122.0(3) C(1)-C(2)-C(8) 118.6(3) C(4)-C(3)-C(2) 122.5(4) C(4)-C(3)-H(3) S54

55 C(2)-C(3)-H(3) C(3)-C(4)-C(5) 117.2(4) C(3)-C(4)-C(7) 121.5(4) C(5)-C(4)-C(7) 121.3(4) C(6)-C(5)-C(4) 122.4(4) C(6)-C(5)-H(5) C(4)-C(5)-H(5) C(5)-C(6)-C(1) 119.4(3) C(5)-C(6)-C(22) 121.1(3) C(1)-C(6)-C(22) 119.4(3) C(4)-C(7)-H(7A) C(4)-C(7)-H(7B) H(7A)-C(7)-H(7B) C(4)-C(7)-H(7C) H(7A)-C(7)-H(7C) H(7B)-C(7)-H(7C) N(1)-C(8)-C(2) 112.9(3) N(1)-C(8)-H(8A) C(2)-C(8)-H(8A) N(1)-C(8)-H(8B) C(2)-C(8)-H(8B) H(8A)-C(8)-H(8B) N(1)-C(9)-C(10) 112.7(3) N(1)-C(9)-H(9A) C(10)-C(9)-H(9A) N(1)-C(9)-H(9B) C(10)-C(9)-H(9B) H(9A)-C(9)-H(9B) N(2)-C(10)-C(11) 121.4(4) N(2)-C(10)-C(9) 116.7(3) C(11)-C(10)-C(9) 121.9(4) C(12)-C(11)-C(10) 119.2(4) C(12)-C(11)-H(11) C(10)-C(11)-H(11) C(11)-C(12)-C(13) 119.6(4) C(11)-C(12)-H(12) C(13)-C(12)-H(12) C(14)-C(13)-C(12) 118.2(4) C(14)-C(13)-H(13) C(12)-C(13)-H(13) N(2)-C(14)-C(13) 122.8(4) N(2)-C(14)-H(14) C(13)-C(14)-H(14) N(1)-C(15)-C(16) 115.1(3) N(1)-C(15)-H(15A) C(16)-C(15)-H(15A) N(1)-C(15)-H(15B) C(16)-C(15)-H(15B) H(15A)-C(15)-H(15B) C(17)-C(16)-C(21) 118.9(3) C(17)-C(16)-C(15) 118.2(3) C(21)-C(16)-C(15) 122.9(3) C(18)-C(17)-C(16) 122.0(4) S55

56 C(18)-C(17)-H(17) C(16)-C(17)-H(17) C(19)-C(18)-C(17) 119.1(3) C(19)-C(18)-H(18) C(17)-C(18)-H(18) C(18)-C(19)-C(20) 120.2(3) C(18)-C(19)-H(19) C(20)-C(19)-H(19) C(19)-C(20)-C(21) 121.8(3) C(19)-C(20)-H(20) C(21)-C(20)-H(20) O(2)-C(21)-C(20) 118.9(3) O(2)-C(21)-C(16) 123.3(3) C(20)-C(21)-C(16) 117.8(3) N(3)-C(22)-C(6) 111.6(3) N(3)-C(22)-H(22A) C(6)-C(22)-H(22A) N(3)-C(22)-H(22B) C(6)-C(22)-H(22B) H(22A)-C(22)-H(22B) N(3)-C(23)-C(24) 112.7(3) N(3)-C(23)-H(23A) C(24)-C(23)-H(23A) N(3)-C(23)-H(23B) C(24)-C(23)-H(23B) H(23A)-C(23)-H(23B) N(4)-C(24)-C(25) 122.3(3) N(4)-C(24)-C(23) 116.1(3) C(25)-C(24)-C(23) 121.5(3) C(24)-C(25)-C(26) 118.9(3) C(24)-C(25)-H(25) C(26)-C(25)-H(25) C(27)-C(26)-C(25) 118.9(3) C(27)-C(26)-H(26) C(25)-C(26)-H(26) C(28)-C(27)-C(26) 118.7(3) C(28)-C(27)-H(27) C(26)-C(27)-H(27) N(4)-C(28)-C(27) 123.0(3) N(4)-C(28)-H(28) C(27)-C(28)-H(28) N(3)-C(29)-C(30) 114.3(3) N(3)-C(29)-H(29A) C(30)-C(29)-H(29A) N(3)-C(29)-H(29B) C(30)-C(29)-H(29B) H(29A)-C(29)-H(29B) C(31)-C(30)-C(35) 119.3(3) C(31)-C(30)-C(29) 119.8(3) C(35)-C(30)-C(29) 120.9(3) C(32)-C(31)-C(30) 122.2(4) C(32)-C(31)-H(31) C(30)-C(31)-H(31) S56

57 C(33)-C(32)-C(31) 118.7(3) C(33)-C(32)-H(32) C(31)-C(32)-H(32) C(34)-C(33)-C(32) 120.3(3) C(34)-C(33)-H(33) C(32)-C(33)-H(33) C(33)-C(34)-C(35) 122.0(4) C(33)-C(34)-H(34) C(35)-C(34)-H(34) O(3)-C(35)-C(34) 120.0(3) O(3)-C(35)-C(30) 122.5(3) C(34)-C(35)-C(30) 117.5(3) O(4)-C(36)-O(5) 126.0(3) O(4)-C(36)-C(37) 118.0(3) O(5)-C(36)-C(37) 115.9(3) C(36)-C(37)-H(37A) C(36)-C(37)-H(37B) H(37A)-C(37)-H(37B) C(36)-C(37)-H(37C) H(37A)-C(37)-H(37C) H(37B)-C(37)-H(37C) O(6)-C(38)-O(7) 126.3(3) O(6)-C(38)-C(39) 115.6(3) O(7)-C(38)-C(39) 118.1(3) C(38)-C(39)-H(39A) C(38)-C(39)-H(39B) H(39A)-C(39)-H(39B) C(38)-C(39)-H(39C) H(39A)-C(39)-H(39C) H(39B)-C(39)-H(39C) C(41)-C(40)-C(44) 108.3(5) C(41)-C(40)-Co(1) 69.5(4) C(44)-C(40)-Co(1) 70.1(4) C(41)-C(40)-H(40) C(44)-C(40)-H(40) Co(1)-C(40)-H(40) C(40)-C(41)-C(42) 108.8(5) C(40)-C(41)-Co(1) 69.9(4) C(42)-C(41)-Co(1) 69.6(4) C(40)-C(41)-H(41) C(42)-C(41)-H(41) Co(1)-C(41)-H(41) C(41)-C(42)-C(43) 106.9(5) C(41)-C(42)-Co(1) 69.6(3) C(43)-C(42)-Co(1) 69.1(4) C(41)-C(42)-H(42) C(43)-C(42)-H(42) Co(1)-C(42)-H(42) C(44)-C(43)-C(42) 108.9(5) C(44)-C(43)-Co(1) 70.7(4) C(42)-C(43)-Co(1) 69.7(4) C(44)-C(43)-H(43) C(42)-C(43)-H(43) S57

58 Co(1)-C(43)-H(43) C(43)-C(44)-C(40) 107.1(5) C(43)-C(44)-Co(1) 68.6(4) C(40)-C(44)-Co(1) 69.1(3) C(43)-C(44)-H(44) C(40)-C(44)-H(44) Co(1)-C(44)-H(44) C(49)-C(45)-C(46) 107.9(5) C(49)-C(45)-Co(2) 69.9(3) C(46)-C(45)-Co(2) 69.1(2) C(49)-C(45)-H(45) C(46)-C(45)-H(45) Co(2)-C(45)-H(45) C(47)-C(46)-C(45) 107.4(5) C(47)-C(46)-Co(2) 69.5(3) C(45)-C(46)-Co(2) 69.7(3) C(47)-C(46)-H(46) C(45)-C(46)-H(46) Co(2)-C(46)-H(46) C(48)-C(47)-C(46) 107.8(4) C(48)-C(47)-Co(2) 69.5(3) C(46)-C(47)-Co(2) 69.4(3) C(48)-C(47)-H(47) C(46)-C(47)-H(47) Co(2)-C(47)-H(47) C(49)-C(48)-C(47) 108.6(5) C(49)-C(48)-Co(2) 70.4(3) C(47)-C(48)-Co(2) 69.5(3) C(49)-C(48)-H(48) C(47)-C(48)-H(48) Co(2)-C(48)-H(48) C(48)-C(49)-C(45) 108.3(4) C(48)-C(49)-Co(2) 69.4(3) C(45)-C(49)-Co(2) 69.3(3) C(48)-C(49)-H(49) C(45)-C(49)-H(49) Co(2)-C(49)-H(49) Cl(2)-C(50)-Cl(1) 109.0(3) Cl(2)-C(50)-Cl(1A) 107.0(6) Cl(2)-C(50)-H(50A) Cl(1)-C(50)-H(50A) Cl(2)-C(50)-H(50B) Cl(1)-C(50)-H(50B) H(50A)-C(50)-H(50B) Cl(2)-C(50)-H(50C) Cl(1A)-C(50)-H(50C) Cl(2)-C(50)-H(50D) Cl(1A)-C(50)-H(50D) H(50C)-C(50)-H(50D) Cl(3)-C(51)-Cl(4) 111.9(7) Cl(3)-C(51)-H(51A) Cl(4)-C(51)-H(51A) Cl(3)-C(51)-H(51B) S58

59 Cl(4)-C(51)-H(51B) H(51A)-C(51)-H(51B) Cl(4A)-C(51A)-Cl(3A) 112.8(8) Cl(4A)-C(51A)-H(51C) Cl(3A)-C(51A)-H(51C) Cl(4A)-C(51A)-H(51D) Cl(3A)-C(51A)-H(51D) H(51C)-C(51A)-H(51D) Symmetry transformations used to generate equivalent atoms: #1 -x+1,y,-z+1/2 #2 -x+1,-y+1,-z+1 S59

60 Table S4. Anisotropic displacement parameters (Å 2 x 10 3 ) for 2-OAc. The anisotropic displacement factor exponent takes the form: -2 pi 2 [ h 2 a* 2 U h k a* b* U12 ] U11 U22 U33 U23 U13 U12 Co(1) 49(1) 41(1) 134(1) 0 49(1) 0 Co(2) 36(1) 85(1) 53(1) -34(1) 11(1) 0(1) Fe(1) 42(1) 25(1) 38(1) -2(1) 15(1) 2(1) Fe(2) 35(1) 24(1) 38(1) -2(1) 13(1) 1(1) O(1) 39(1) 24(1) 42(1) -2(1) 15(1) 1(1) O(2) 52(1) 30(1) 42(1) -3(1) 18(1) 4(1) O(3) 42(1) 33(1) 53(2) -9(1) 22(1) -3(1) O(4) 52(1) 34(1) 52(2) -7(1) 23(1) -4(1) O(5) 40(1) 35(1) 45(1) -4(1) 18(1) -2(1) O(6) 54(2) 40(1) 44(1) -8(1) 7(1) 9(1) O(7) 48(1) 33(1) 41(1) -3(1) 8(1) 6(1) N(1) 45(2) 24(1) 38(2) -3(1) 15(1) 0(1) N(2) 59(2) 27(1) 45(2) 2(1) 26(2) 4(1) N(3) 39(1) 25(1) 37(2) 0(1) 12(1) 2(1) N(4) 37(1) 23(1) 39(2) 0(1) 12(1) 1(1) C(1) 37(2) 24(2) 46(2) 2(1) 15(2) 1(1) C(2) 44(2) 26(2) 50(2) -4(1) 14(2) -1(1) C(3) 45(2) 34(2) 65(3) -17(2) 12(2) -5(2) C(4) 45(2) 41(2) 78(3) -15(2) 25(2) -5(2) C(5) 49(2) 32(2) 64(2) -6(2) 27(2) -3(2) C(6) 45(2) 23(2) 50(2) 1(1) 19(2) 0(1) C(7) 51(3) 83(4) 112(4) -41(3) 29(3) -18(2) C(8) 45(2) 26(2) 46(2) -3(1) 8(2) 0(1) C(9) 45(2) 25(2) 52(2) -4(1) 21(2) -5(1) C(10) 62(2) 20(2) 54(2) -1(1) 29(2) 2(1) C(11) 70(3) 33(2) 68(3) -2(2) 40(2) -5(2) C(12) 88(3) 41(2) 80(3) 7(2) 55(3) 0(2) C(13) 94(3) 42(2) 60(3) 7(2) 45(3) 13(2) C(14) 78(3) 32(2) 47(2) 4(2) 29(2) 12(2) C(15) 53(2) 32(2) 38(2) -2(1) 16(2) -1(1) C(16) 48(2) 28(2) 43(2) -4(1) 23(2) -6(1) C(17) 50(2) 36(2) 45(2) -8(2) 22(2) -7(2) C(18) 51(2) 38(2) 53(2) -16(2) 23(2) -4(2) C(19) 45(2) 32(2) 60(2) -10(2) 25(2) 0(1) C(20) 42(2) 34(2) 51(2) -3(2) 20(2) -1(1) C(21) 44(2) 26(2) 46(2) -5(1) 23(2) -4(1) C(22) 48(2) 26(2) 43(2) 2(1) 22(2) 1(1) C(23) 36(2) 25(2) 41(2) -1(1) 12(1) 4(1) C(24) 35(2) 20(1) 45(2) -2(1) 16(1) 0(1) C(25) 40(2) 30(2) 44(2) -4(1) 15(2) 0(1) C(26) 40(2) 38(2) 46(2) -3(2) 6(2) 6(1) C(27) 47(2) 33(2) 39(2) -2(1) 12(2) 1(1) C(28) 42(2) 26(2) 39(2) 1(1) 16(2) 3(1) S60

61 C(29) 44(2) 33(2) 37(2) -1(1) 15(2) 1(1) C(30) 43(2) 30(2) 37(2) -3(1) 10(2) 0(1) C(31) 57(2) 39(2) 34(2) -6(1) 19(2) 0(2) C(32) 61(2) 31(2) 42(2) -10(1) 13(2) 1(2) C(33) 52(2) 33(2) 44(2) -5(2) 12(2) -4(2) C(34) 48(2) 36(2) 47(2) -9(2) 19(2) -6(2) C(35) 42(2) 31(2) 40(2) -7(1) 12(2) 0(1) C(36) 40(2) 37(2) 38(2) -2(1) 15(2) 2(1) C(37) 71(3) 43(2) 68(3) -6(2) 41(2) -4(2) C(38) 53(2) 37(2) 42(2) 0(2) 10(2) 8(2) C(39) 102(4) 51(3) 48(2) -3(2) 0(2) 25(2) C(40) 74(3) 84(4) 95(4) -25(3) 41(3) -21(3) C(41) 73(3) 70(3) 97(4) -24(3) 39(3) 2(3) C(42) 62(3) 71(3) 172(7) -47(4) 58(4) -13(3) C(43) 80(4) 64(3) 204(8) -53(4) 68(5) -8(3) C(44) 64(3) 99(4) 147(6) -33(4) 61(4) -1(3) C(45) 51(2) 105(4) 72(3) -53(3) 28(2) -17(3) C(46) 51(2) 77(3) 58(3) -28(2) 21(2) 3(2) C(47) 45(2) 78(3) 54(2) -27(2) 16(2) -5(2) C(48) 59(3) 93(4) 55(3) -31(3) 10(2) 8(2) C(49) 48(2) 118(5) 57(3) -44(3) 9(2) 8(3) Cl(1) 96(1) 76(1) 87(1) 9(1) 28(1) 17(1) Cl(1A) 120(10) 101(10) 181(17) 104(12) 127(12) 67(10) Cl(2) 84(1) 63(1) 68(1) 18(1) 29(1) 6(1) C(50) 97(4) 50(2) 70(3) 14(2) 35(3) 2(2) Cl(3) 40(2) 59(3) 60(3) 4(2) 16(2) -12(1) Cl(4) 49(2) 62(3) 90(6) 8(3) 24(3) 1(2) C(51) 53(7) 31(5) 58(6) -5(4) 27(5) -1(4) Cl(3A) 105(4) 174(6) 82(3) 1(3) 29(3) -56(4) Cl(4A) 86(3) 81(4) 65(2) -28(3) 28(2) -24(3) C(51A) 79(7) 101(9) 67(5) -11(6) 13(5) -4(7) S61

62 Table S5. Hydrogen coordinates ( x 10 4 ) and isotropic displacement parameters (Å 2 x 10 3 ) for 2-OAc. x y z U(eq) H(3) H(5) H(7A) H(7B) H(7C) H(8A) H(8B) H(9A) H(9B) H(11) H(12) H(13) H(14) H(15A) H(15B) H(17) H(18) H(19) H(20) H(22A) H(22B) H(23A) H(23B) H(25) H(26) H(27) H(28) H(29A) H(29B) H(31) H(32) H(33) H(34) H(37A) H(37B) H(37C) H(39A) H(39B) H(39C) H(40) H(41) H(42) H(43) H(44) S62

63 H(45) H(46) H(47) H(48) H(49) H(50A) H(50B) H(50C) H(50D) H(51A) H(51B) H(51C) H(51D) S63

64 Table S6. Torsion angles [deg] for 2-OAc. Fe(2)-O(1)-C(1)-C(2) 128.2(3) Fe(1)-O(1)-C(1)-C(2) -51.9(4) Fe(2)-O(1)-C(1)-C(6) -51.4(4) Fe(1)-O(1)-C(1)-C(6) 128.5(3) O(1)-C(1)-C(2)-C(3) 178.2(3) C(6)-C(1)-C(2)-C(3) -2.3(5) O(1)-C(1)-C(2)-C(8) -0.9(5) C(6)-C(1)-C(2)-C(8) 178.6(3) C(1)-C(2)-C(3)-C(4) 0.5(6) C(8)-C(2)-C(3)-C(4) 179.5(4) C(2)-C(3)-C(4)-C(5) 1.7(6) C(2)-C(3)-C(4)-C(7) (4) C(3)-C(4)-C(5)-C(6) -2.0(6) C(7)-C(4)-C(5)-C(6) 178.6(4) C(4)-C(5)-C(6)-C(1) 0.2(5) C(4)-C(5)-C(6)-C(22) 176.2(3) O(1)-C(1)-C(6)-C(5) (3) C(2)-C(1)-C(6)-C(5) 2.0(5) O(1)-C(1)-C(6)-C(22) 5.5(5) C(2)-C(1)-C(6)-C(22) (3) C(9)-N(1)-C(8)-C(2) 56.5(4) C(15)-N(1)-C(8)-C(2) (3) Fe(1)-N(1)-C(8)-C(2) -63.6(3) C(3)-C(2)-C(8)-N(1) (4) C(1)-C(2)-C(8)-N(1) 66.1(4) C(8)-N(1)-C(9)-C(10) -87.3(3) C(15)-N(1)-C(9)-C(10) 151.2(3) Fe(1)-N(1)-C(9)-C(10) 33.5(3) C(14)-N(2)-C(10)-C(11) -0.1(5) Fe(1)-N(2)-C(10)-C(11) (3) C(14)-N(2)-C(10)-C(9) (3) Fe(1)-N(2)-C(10)-C(9) 15.6(4) N(1)-C(9)-C(10)-N(2) -34.1(4) N(1)-C(9)-C(10)-C(11) 148.5(3) N(2)-C(10)-C(11)-C(12) -0.9(5) C(9)-C(10)-C(11)-C(12) 176.4(3) C(10)-C(11)-C(12)-C(13) 0.2(6) C(11)-C(12)-C(13)-C(14) 1.5(6) C(10)-N(2)-C(14)-C(13) 1.9(5) Fe(1)-N(2)-C(14)-C(13) 167.3(3) C(12)-C(13)-C(14)-N(2) -2.6(6) C(9)-N(1)-C(15)-C(16) -50.4(4) C(8)-N(1)-C(15)-C(16) (3) Fe(1)-N(1)-C(15)-C(16) 68.6(3) N(1)-C(15)-C(16)-C(17) 138.4(3) N(1)-C(15)-C(16)-C(21) -43.6(5) C(21)-C(16)-C(17)-C(18) 1.0(5) C(15)-C(16)-C(17)-C(18) 179.1(3) C(16)-C(17)-C(18)-C(19) 2.0(6) S64

65 C(17)-C(18)-C(19)-C(20) -2.1(6) C(18)-C(19)-C(20)-C(21) -0.6(6) Fe(1)-O(2)-C(21)-C(20) (2) Fe(1)-O(2)-C(21)-C(16) 3.5(5) C(19)-C(20)-C(21)-O(2) (3) C(19)-C(20)-C(21)-C(16) 3.5(5) C(17)-C(16)-C(21)-O(2) 178.3(3) C(15)-C(16)-C(21)-O(2) 0.2(5) C(17)-C(16)-C(21)-C(20) -3.6(5) C(15)-C(16)-C(21)-C(20) 178.4(3) C(23)-N(3)-C(22)-C(6) 52.0(4) C(29)-N(3)-C(22)-C(6) 174.7(3) Fe(2)-N(3)-C(22)-C(6) -69.1(3) C(5)-C(6)-C(22)-N(3) (4) C(1)-C(6)-C(22)-N(3) 63.0(4) C(22)-N(3)-C(23)-C(24) -91.7(3) C(29)-N(3)-C(23)-C(24) 146.9(3) Fe(2)-N(3)-C(23)-C(24) 27.3(3) C(28)-N(4)-C(24)-C(25) 0.0(4) Fe(2)-N(4)-C(24)-C(25) (2) C(28)-N(4)-C(24)-C(23) (3) Fe(2)-N(4)-C(24)-C(23) 24.7(3) N(3)-C(23)-C(24)-N(4) -35.5(4) N(3)-C(23)-C(24)-C(25) 147.4(3) N(4)-C(24)-C(25)-C(26) -0.1(5) C(23)-C(24)-C(25)-C(26) 176.8(3) C(24)-C(25)-C(26)-C(27) -0.3(5) C(25)-C(26)-C(27)-C(28) 0.7(5) C(24)-N(4)-C(28)-C(27) 0.5(5) Fe(2)-N(4)-C(28)-C(27) 156.6(3) C(26)-C(27)-C(28)-N(4) -0.9(5) C(22)-N(3)-C(29)-C(30) (3) C(23)-N(3)-C(29)-C(30) -53.3(4) Fe(2)-N(3)-C(29)-C(30) 68.4(3) N(3)-C(29)-C(30)-C(31) 128.3(3) N(3)-C(29)-C(30)-C(35) -53.7(4) C(35)-C(30)-C(31)-C(32) -1.8(5) C(29)-C(30)-C(31)-C(32) 176.2(3) C(30)-C(31)-C(32)-C(33) 0.8(6) C(31)-C(32)-C(33)-C(34) 1.3(6) C(32)-C(33)-C(34)-C(35) -2.5(6) Fe(2)-O(3)-C(35)-C(34) (3) Fe(2)-O(3)-C(35)-C(30) 24.0(5) C(33)-C(34)-C(35)-O(3) (3) C(33)-C(34)-C(35)-C(30) 1.4(5) C(31)-C(30)-C(35)-O(3) 179.2(3) C(29)-C(30)-C(35)-O(3) 1.2(5) C(31)-C(30)-C(35)-C(34) 0.7(5) C(29)-C(30)-C(35)-C(34) (3) Fe(1)-O(4)-C(36)-O(5) -7.8(6) Fe(1)-O(4)-C(36)-C(37) 173.1(3) Fe(2)-O(5)-C(36)-O(4) -6.6(6) Fe(2)-O(5)-C(36)-C(37) 172.4(3) S65

66 Fe(1)-O(6)-C(38)-O(7) -23.9(7) Fe(1)-O(6)-C(38)-C(39) 156.7(4) Fe(2)-O(7)-C(38)-O(6) -0.2(7) Fe(2)-O(7)-C(38)-C(39) 179.3(3) C(44)-C(40)-C(41)-C(42) -0.7(8) Co(1)-C(40)-C(41)-C(42) 58.9(5) C(44)-C(40)-C(41)-Co(1) -59.7(5) C(40)-C(41)-C(42)-C(43) 0.2(8) Co(1)-C(41)-C(42)-C(43) 59.3(6) C(40)-C(41)-C(42)-Co(1) -59.1(4) C(41)-C(42)-C(43)-C(44) 0.4(10) Co(1)-C(42)-C(43)-C(44) 60.0(6) C(41)-C(42)-C(43)-Co(1) -59.6(5) C(42)-C(43)-C(44)-C(40) -0.9(9) Co(1)-C(43)-C(44)-C(40) 58.6(5) C(42)-C(43)-C(44)-Co(1) -59.4(6) C(41)-C(40)-C(44)-C(43) 1.0(8) Co(1)-C(40)-C(44)-C(43) -58.3(6) C(41)-C(40)-C(44)-Co(1) 59.3(4) C(49)-C(45)-C(46)-C(47) -0.1(5) Co(2)-C(45)-C(46)-C(47) -59.5(3) C(49)-C(45)-C(46)-Co(2) 59.4(3) C(45)-C(46)-C(47)-C(48) 0.6(5) Co(2)-C(46)-C(47)-C(48) -59.0(3) C(45)-C(46)-C(47)-Co(2) 59.6(3) C(46)-C(47)-C(48)-C(49) -0.8(5) Co(2)-C(47)-C(48)-C(49) -59.8(3) C(46)-C(47)-C(48)-Co(2) 59.0(3) C(47)-C(48)-C(49)-C(45) 0.8(5) Co(2)-C(48)-C(49)-C(45) -58.5(3) C(47)-C(48)-C(49)-Co(2) 59.2(3) C(46)-C(45)-C(49)-C(48) -0.4(5) Co(2)-C(45)-C(49)-C(48) 58.5(3) C(46)-C(45)-C(49)-Co(2) -58.9(3) Symmetry transformations used to generate equivalent atoms: #1 -x+1,y,-z+1/2 #2 -x+1,-y+1,-z+1 S66

67 Figure S44. Optimized structure of the proposed diferrous dinitrosyl intermediate of complex 2 with hydrogen atoms omitted for clarity. The structure was optimized using the BP86 functional and the TZVP basis set. S67

68 Table S7. XYZ coordinates for the DFT-optimized structure of the dinitrosyl intermediate. Fe Fe O O O O O C H C H C H C H C C H H C H C H C H C H C C H H C H C H C H C H C C H H C H C H C H C H S68