Axial ligand tuning of a nonheme iron(IV) oxo unit for hydrogen atom abstraction -- Sastri et al. 104 (49): 19181 Data Supplement - HTML Page - index.htslp -- Proceedings of the National Academy of Sciences

Sastri et al. 10.1073/pnas.0709471104.

Supporting Information

Files in this Data Supplement:

SI Figure 7
SI Table 2
SI Figure 8
SI Figure 9
SI Figure 10
SI Figure 11
SI Table 3
SI Figure 12
SI Figure 13
SI Materials and Methods
SI Table 4
SI Figure 14
SI Figure 15

SI Figure 7

Fig. 7. UV-visible spectral changes of [Fe(IV)(O)(TMC)(OOCCF₃)]⁺ (1-OOCCF₃) upon addition of PPh₃. The reaction conditions were as follows. 1-OOCCF₃ was prepared by addition of PhIO (0.6 mM) to [Fe(II)(TMC)(OOCCF₃)]⁺ (0.5 mM) in CH₃CN at 25°C. PPh₃ (10 mM) was then added to the solution of 1-OOCCF₃ at 0°C, and the kinetics were monitored at 835 nm.

SI Figure 8

Fig. 8. Hammett plots of log k_rel of 1-X [1-N₃ (Top), 1-OOCCF₃ (Middle), and 1-NCCH₃ (Bottom)] against s_p⁺ of p-Y-2,6-t-Bu₂C₆H₂OH in CH₃CN at 25°C.

SI Figure 9

Fig. 9. Plots of log k_rel of 1-OOCCF₃ (Upper) and 1-NCCH3 (Lower) against O-H BDE of p-Y-2,6-t-Bu₂C₆H₂OH in CH₃CN at 25°C.

SI Figure 10

Fig. 10. Second-order rate constants determined in the reactions of [Fe^IV(O)(TMC)(X)]ⁿ⁺ (0.5 mM) with CHD (a), xanthene (b), and fluorene (c) in CH₃CN at 0°C [1-N3 (black squares); 1OOCCF₃ (green circles); 1-NCCH3 (red inverted triangles)].

SI Figure 11

Fig. 11. Plots of log k₂' of 1-OOCCF₃ (Upper) and 1-NCCH₃ (Lower) against C-H BDE of substrates.

SI Figure 12

Fig. 12. Cyclic and differential pulse voltammograms of 1-OOCCF₃ (Upper) and 1-N₃ (Lower) in CH₃CN. Glassy carbon and Pt gauge were used as working electrode and auxiliary counter electrode, respectively. The potentials were measured against Ag/Ag⁺ (0.01 M) reference electrode and reported against Fc⁺/Fc (Fc, ferrocence) couple. The cyclic voltammograms were run at a scan rate of 100 mV/s.

SI Figure 13

Fig. 13. Plots showing the correlation between the reactivity of CHD (Top), xanthene (Middle), and 2,4-t-Bu₂C₆H₃OH (Bottom) and Ep,c of [Fe^IV(O)(TMC)(X)]ⁿ⁺in CH₃CN at 25°C.

SI Figure 14

Fig. 14. Energy profiles for C-H hydroxylation of cyclohexane (CH) by 1-NCCH₃, indicated here as ^2S+1K_TMC(AN), where AN is CH₃CN. The triplet barrier on the highest level is 24.7 kcal/mol and on the quintet surface it is 13.8 kcal/mol. Adapted from figure 6 in ref. 1

1. Hirao H, Kumar D, Que L, Jr, Shaik S (2006) J Am Chem Soc 128:8590-8606.

SI Figure 15

Fig. 15. Energy profiles for C-H hydroxylation of cyclohexane (CH) by 1-OOCCF₃, indicated here as ^2S+1K_TMC(TF), where TF is CF₃CO₂^-. The triplet barrier on the highest level is 25.8 kcal/mol and on the quintet surface it is 13.1 kcal/mol. Adapted from figure 7 in ref. 1.

1. Hirao H, Kumar D, Que L, Jr, Shaik S (2006) J Am Chem Soc 128:8590-8606.

Table 2. Data for the reactions of [FeIV(O)(TMC)(X)]ⁿ⁺ with p-Y-2,6-t-Bu₂C₆H₂OH in CH₃CN at 25°C

				k _obs for 1-X, s^-1
Y	BDE,* kcal/mol	s_p⁺	pK_a^†	1 -NCCH₃	1 -OOCCF₃	1 -N₃
CN	84.24	0.66	---	n.d.	n.d.	0.8 ´10^-4
H	82.8	0	17.30	0.2 ´10^-4	2.3 ´10^-4	3.4 ´10^-4
tert -Bu	81.24	-0.26	17.80	1.1 ´10^-4	4.5 ´10^-4	5.1 ´10^-4
Me	81.02	-0.31	17.73	0.9 ´10^-4	9.2 ´10^-4	9.2 ´10^-4
OMe	78.31	-0.78	18.20	57 ´10^-4	132 ´10^-4	149 ´10^-4

The reaction conditions were as follows. 1-X were prepared by reacting [FeII(TMC)(X)]ⁿ⁺ (0.5 mM) with PhIO (0.6 mM) in CH₃CN at 25°C. The spectral changes were monitored at 820 nm for 1-NCCH₃, 835 nm for 1-OOCCF₃, and 850 nm for 1-N₃. Rate constants were averaged by three determinations, and standard deviation is less than 10% of the given values. n.d., not determined due to the low reactivity of the intermediates in the reaction of para-CN-2,6-t-Bu₂C₆H₂OH.

*Data were obtained from Brigati G, Lucarini M, Mugnaini V, Pedulli GF (2002) J Org Chem 67:4828-4832.

^†Data were obtained from Brodwell FG, Zhang XM (1995) J Phys Org Chem 8:529-535.

Table 3. Second-order rate constants to determine KIE values in the oxidation of DHA and xanthene by [FeIV(O)(TMC)(X)]ⁿ⁺ (1-X)

	k₂, M^-1·s^-1			k ₂, M^-1·s^-1
1 -X	DHA	DHA-d₄	k _H/k_D	xanthene	xanthene-d₂	k _H/k_D
1 -NCCH₃	0.50	0.05	10	1.80	0.14	16
1 -OOCCF₃*	1.30	0.07	19	7.60	0.37	20
1 -N₃	2.40	0.14	17	9.60	0.55	17

The reaction conditions were as follows. 1-X (where X is NCCH₃, OOCCF₃, and N₃) were prepared from [FeII(TMC)(X)]ⁿ⁺ (0.5 mM) by reacting with PhIO (0.6 mM) in CH₃CN at 25°C. Pseudofirst-order kinetics were monitored upon addition of appropriate substrates. Rate constants were averaged by three determinations, and standard deviation is less than 10% of the given values.

*Reactions were carried out at 0°C.

Table 4. Calculated BDE(O-H) values for [Fe^III(OH)(TMC)(X)]ⁿ⁺ complexes

X	BDE, kcal/mol
CH₃CN	84.6 (78.0)
NCS	83.5 (77.0)
TF	83.1 (76.4)
N₃	84.0 (77.4)
F	83.9 (77.5)
SR	85.0 (78.7)

Shown are BDE values with ZPE corrections in parentheses. Data are based on UB3LYP/LACVP.

SI Materials and Methods

All chemicals obtained from Aldrich Chemical Co. were the best available purity and were used without further purification unless otherwise indicated. Solvents were dried according to published procedures and distilled under Ar before use (1). Iodosylbenzene (PhIO) was prepared by a literature method (2). Preparation and handling of air-sensitive materials were done under an inert atmosphere either on a Schlenk line or in a glove box. The deuterated substrate 9,10-dihydroanthracene-d4, was prepared by taking 9,10-dihydroanthracene (0.5 g, 2.7 mmol) in DMSO-d6 (3 ml) along with NaH (0.2 g, 8.1 mmol) under an inert atmosphere (3). After the deep red solution was stirred at room temperature for 8 h, the reaction was quenched with ²H₂O (5 ml). The crude product was filtered and washed with copious amounts of H₂O. ¹H NMR confirmed >99% deuteration. Xanthene-d2 was prepared similarly. The dimeric product of 2,4di-tert-butylphenol, 2,2'-dihydroxy-3,3,5,5'-tetra-tert-butyl-1,1'-diphenyl, was prepared by published procedures (4).

Iron(II) complexes [Fe^II(TMC)(X)]ⁿ⁺ (X = NCCH₃, CF3COO^-, N₃^-) were prepared by adding 1.2 equiv of appropriate tetraalkylammonium salts to Fe^II(TMC)(CF₃SO₃)₂ (5-7). Iron(IV)-oxo complexes [Fe(O)(TMC)(X)]ⁿ⁺ (1-X) were prepared by reacting [Fe^II(TMC)(X)]ⁿ⁺ (0.5 mM) with 1.2 equiv of PhIO (0.6 mM) in CH₃CN at ambient temperature (5-7). The salt [Fe^II(TMCS)](PF₆) was prepared as previously described (8). The corresponding iron(IV)-oxo complex [Fe^IV(O)(TMCS)]⁺ (1'-SR) was generated by treating a methanol solution of [Fe^II(TMCS)](PF₆) (1 mM) with 1.1 equiv of m-chloroperbenzoic acid in the presence of 6 equiv of potassium tert-butoxide (9). For reactivity studies of 1'-SR,1 ml of 1'-SR (1 mM) was generated at 0°C and immediately diluted with chilled acetonitrile to bring the concentration of 1'-SR to 0.5 mM. An appropriate amount of substrate was then added to this solution, and the resulting reaction was monitored by UV-visible spectroscopy.

UV-visible spectra were recorded on a Hewlett Packard 8453 spectrophotometer equipped with Optostat^DN variable-temperature liquid-nitrogen cryostat (Oxford Instruments) or a circulating water bath. Product Analysis was performed on DIONEX Pump Series P580 equipped with a variable wavelength UV-200 detector (HPLC), Agilent Technologies 6890N gas chromatography equipped with a FID detector (GC), or Thermo Finnigan (Austin, Texas) Focus DSQ (dual state quadrapole) mass spectrometer interfaced with Finnigan Focus gas chromatography (GC-MS). Electrospray ionization mass spectra (ESI MS) of intermediates were collected on a Thermo Finnigan (San Jose, CA, USA) LCQ Advantage MAX quadrupole ion trap instrument, by infusing samples directly into the source at 25 ml/min using a syringe pump.

The spray voltage was set at 4.7 kV and the capillary temperature at 70°C. Frozen glass EPR measurements were done on JEOL JES-FA200 at 4K. All electrochemical experiments were performed under N₂ atmosphere using a CH Instruments (CHI 630B) electrochemical analyzer. ¹H NMR was measured with Bruker DPX-250 spectrometer.

Kinetic studies were performed by adding appropriate amounts of substrates to the solutions of 1-X and 1'-SR, and spectral changes of the intermediates were directly monitored by a UV-vis spectrophotometer. Rate constants, k_obs, were determined by pseudo-first-order fitting of the decrease of absorption bands at 820 nm for 1-NCCH₃, 835 nm for 1-OOCCF₃, and 850 nm for 1-N₃ and 1'-SR. All of the rate constants are averages of at least three determinations. Product analysis for the oxidation of PPh₃ was performed by injecting reaction solutions directly into HPLC. Product analysis for the oxidation of 2,4-di-tert-butylphenol and 9,10-dihydroanthracene by 1-X was performed by injecting reaction solutions directly into GC and GC-MS, by following procedures reported by Lansky and Goldberg (10). Product yields were determined by comparison with standard curves of known authentic samples from three determinations.

The cyclic and differential pulse voltammetric measurements of 1-X were carried out at 25°C in CH₃CN containing iron complexes (1 mM) and tetrabutylammonium hexafluorophosphate (TBAPF₆, 0.1 M) as supporting electrolyte in one chamber. Either glassy carbon or platinum disk (diameter = 3 mm) was used as a working electrode and counter electrode were platinum wire. The potential was measured by using a Ag/Ag⁺ (0.01 M) reference electrode and reported versus a Fc⁺/Fc (Fc, ferrocene) couple. Measurements for 1'-SR (2 mM) were carried out at -30°C in a 1:1 CH₃CN/CH₃OH solvent mixture containing 0.1 M TBAPF6, using a glassy carbon working electrode, a Ag/AgNO₃ (0.05 M) reference electrode, and a platinum auxiliary electrode. The 1'-SR complex was initially prepared in pure MeOH following the previously described procedure (vide supra) and then diluted at -30°C with an equal volume of CH₃CN. The potential is reported versus the Fc⁺/Fc couple. For comparison, cyclic voltammetric measurements of 1-NCCH₃, 1-OOCCF₃, and 1-N₃ were carried out under similar conditions. In these cases, the complexes were first generated in pure acetonitrile using PhIO as oxidant and then diluted at -30°C with an equal volume of CH₃OH.

All of the geometries were optimized with Jaguar 5.5 (11) at the UB3LYP/LACVP level [refs. 12-15; the LACVP series is derived from LANL2DZ (see ref. 16 and 17)] (UB3LYP/B1). Molecule structures were drawn with Molekel (18, 19). In ref. 44, the blended TSR equation (equation 2 there) is given as a weighted average of rate constants k_Q and k_T. It is not easy to define an effective barrier based on this formulation. If we assume that the weighted arithmetic average is roughly approximated as the weighted geometric average (xk_Q + (1 - x)k_T = k_Q^x·k_T^(1-x); x = w_Q, 1 - x = w_T), we can then use Eq. 3 as in the present text. With the great difference between the two calculated rate constants, the arithmetic average is affected only by the xk_Q term and behaves precisely as Eq. 3 in the text.

1. Armarego WLF, Perrin DD, eds (1997) Purification of Laboratory Chemicals (Pergamon, Oxford).

2. Saltzman H, Sharefkin JG, eds (1973) Organic Syntheses (Wiley, New York), Vol V, p 658.

3. Goldsmith CR, Jonas RT, Stack TDP (2001) J Am Chem Soc 124:83-96.

4. van der Vlugt JI, Hewat AC, Neto S, Sablong R, Mills AM, Lutz M, Spek AL, Müller C, Vogt D (2004) Adv Synth Catal 346:993-1003.

5. Sastri CV, Park MJ, Ohta T, Jackson TA, Stubna A, Seo MS, Lee J, Kim J, Kitagawa T, Münck E, Que L, Jr, Nam W (2005) J Am Chem Soc 127:1249412495.

6. Rohde J-U, Que L, Jr (2005) Angew Chem Int Ed 44:2255-2258.

7. Rohde J-U, In J-H, Lim MH, Brennessel WW, Bukowski MR, Stubna A, Münck E, Nam W, Que L, Jr (2003) Science 299:1037-1039.

8. Fiedler AT, Halfen HL, Halfen JA, Brunold TC (2005) J Am Chem Soc 127:1675-1689.

9. Bukowski MR, Koehntop KD, Stubna A, Bominaar EL, Halfen JA, Münck E, Nam W, Que L, Jr (2005) Science 310:1000-1002.

10. Lansky DE, Goldberg DP (2006) Inorg Chem 45:5119-5125.

11. Schrödinger, Inc. (2004) Jaguar 5.5 (Schrödinger, Portland).

12. Becke AD (1992) J Chem Phys 96:2155-2160.

13. Becke AD (1992) J Chem Phys 97:9173-9177.

14. Becke AD (1993) J Chem Phys 98:5648-5652.

15. Lee C, Yang W, Parr RG (1988) Phys Rev B 37:785-789.

16. Hay PJ, Wadt WR (1985) J Chem Phys 82:299-310.

17. Friesner RA, Murphy RB, Beachy MD, Ringnalda MN, Pollard WT, Dunietz BD, Cao Y (1999) J Phys Chem A 103:1913-1928.

18. Flükiger P, Lüthi HP, Portmann S, Weber, J (2002) MOLEKEL 4.3; (Swiss Center for Sci Comput, Manno, Switzerland).

19. Portmann S, Lüthi HP (2000) Chimia 54:766-770.