Supporting Information for: Catalytic N 2 Reduction to Silylamines and Thermodynamics of N 2 Binding at Square Planar Fe

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1 Supporting Information for: Catalytic N 2 Reduction to Silylamines and Thermodynamics of N 2 Binding at Square Planar Fe Demyan E. Prokopchuk, a Eric S. Wiedner, a Eric D. Walter, b Codrina V. Popescu, c Nicholas A. Piro, d W. Scott Kassel, d R. Morris Bullock, a Michael T. Mock a* a Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA michael.mock@pnnl.gov b Environmental Molecular Sciences Laboratory, Richland, WA, 99352, USA c Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, NY 13346, USA d Department of Chemistry, Villanova University, 800 E. Lancaster Ave., Villanova, PA 19085, USA Present Address: Department of Chemistry and Biochemistry, Albright College, 1621 N. 13 th Street, Reading, PA 19604, USA Modified Synthesis of Ph 2PCH 2CH 2P(CH 2OH) 2. 2 Modified Synthesis of P 4N 2. 2 NMR Spectra 3 IR Spectra 8 Additional EPR Data 9 Additional Catalytic Trials 9 Electrochemistry 10 Mössbauer Spectroscopy 14 Thermochemical Cycles 14 Pressure Dependence on N 2 Binding Free Energy, DG N2 14 Crystallographic details 15 DFT Calculations 18 S1

2 Modified Synthesis of Ph 2 PCH 2 CH 2 P(CH 2 OH) 2. The workup protocol described here yields the analytically pure product. 1 A 250 ml Schlenk flask was charged with a Teflon coated stir bar and PPh 2 CH 2 CH 2 PH 2 (3.04 g, 12.3 mmol) was dissolved in CH 3 CN (100 ml). Paraformaldehyde was added (3.63 g, 12.3 mmol, 10 equiv). The solution was heated at 65 C for 15 hours, cooled to room temperature, and the excess formaldehyde was removed by filtering through a medium pore glass frit. The solvent was removed under vacuum at 65 C, followed by heating at 100 C under high vacuum for 3 hours to remove oligomeric formaldehyde byproducts. The flask was refilled with N 2 and the colorless, viscous residue was dissolved in CH 2 Cl 2 (10 ml). Diethyl ether (100 ml) was added while stirring vigorously, which caused the solution to become cloudy white in appearance. After about 2 hours, the solution became colorless with an oily residue appearing at the bottom of the flask. The colorless solution was decanted and dried under vacuum to yield the analytically pure product as a white powder (2.85 g, 75%). The NMR spectra matched the reported literature values. 1 Anal. Calcd for C 16 H 20 O 2 P 2 : C, 62.74; H, Found: C, 63.08; H, Modified Synthesis of P 4 N 2. The procedure described here results in higher product yields and begins with analytically pure Ph 2 PCH 2 CH 2 P(CH 2 OH) 2. 1 A 100 ml Schlenk flask was charged with a Teflon coated stir bar, Ph 2 PCH 2 CH 2 P(CH 2 OH) 2 (400 mg, 1.31 mmol 1.0 equiv), and dissolved in ethanol (15 ml). The solution was cooled to 0 C and aniline was injected via syringe (5.2 mmol, 0.48 ml, 4.0 equiv; care must be taken to inject excess aniline to a solution of the dissolved phosphine to prevent formation of other phosphorus-containing products). The solution was then heated at 50 C for 30 min. and cooled to room temperature. In a separate Schlenk flask charged with a Teflon coated stir bar, Ph 2 PCH 2 CH 2 P(CH 2 OH) 2 (400 mg, 1.31 mmol 1.0 equiv) was dissolved in ethanol (15 ml) and injected to the cooled solution via syringe. The mixture was heated at 80 C for 16 h, during which a flocculent white precipitate gradually appeared. The solution was cooled to room temperature, isolated on a medium pore glass frit, washed with 10 ml diethyl ether, and dried under high vacuum to yield analytically pure P 4 N 2 as a flaky white powder (320 mg). The filtrate was recycled by drying under high vacuum at 35 C, redissolving in ethanol (20 ml), and heating again at 80 C for 16 h, after which more flocculent white precipitate appeared. The same workup procedure yielded a second crop of P 4 N 2 (284 mg). Recycling the filtrate again in the same manner yielded a third crop of P 4 N 2 (34 mg). Total yield: 638 mg, 67 %. The NMR spectra match the reported literature values. 1 S2

3 NMR Spectra Figure S1-1 H NMR spectrum of Fe II Br 2 (500 MHz, CD 2 Cl 2, -25 C). Figure S2-13 C{ 1 H} NMR spectrum of Fe II Br 2 (126 MHz, CD 2 Cl 2, -25 C). S3

4 Figure S3-31 P{ 1 H} NMR spectrum of Fe II Br 2 (202 MHz, CD 2 Cl 2, -25 C). Figure S4-1 H NMR spectrum of Fe I Br (500 MHz, C 4 H 8 O, 25 C). S4

5 Figure S5-1 H NMR spectrum of Fe 0 (N 2 ) (500 MHz, C 4 D 8 O, 25 C). Figure S6-13 C{ 1 H} NMR spectrum of Fe 0 (N 2 ) (126 MHz, C 4 D 8 O, 25 C). S5

6 Figure S7-31 P{ 1 H} NMR spectrum of Fe 0 (N 2 ) (202 MHz, C 4 D 8 O, 25 C), experimental (top) and simulated (bottom). See main text for simulation parameters. Figure S8-15 N{ 1 H} NMR spectrum of Fe 0 (N 2 ) (51 MHz, C 4 D 8 O, 25 C). Free 15 N 2 appears at ppm. S6

7 Figure S9-1 H NMR spectrum of [Fe I ][B(C 6 F 5 ) 4 ] (500 MHz, C 6 D 5 Cl, 25 C). Figure S10-1 H NMR spectrum of [Fe II ][B(C 6 F 5 ) 4 ] 2 (500 MHz, C 6 D 5 Cl, 25 C). Signals marked with an asterisk (*) are residual pentane. S7

8 IR Spectra Figure S11 - N 2 stretching region of Fe 0 (N 2 ) (THF) under 14 N 2 (black) and 15 N 2 (red). Figure S12 - N 2 stretching region of [Fe I ] + (toluene) under 14 N 2 (black) and 15 N 2 (red). S8

9 Additional EPR Data Figure S13 - EPR spectrum of Fe I Br (toluene glass, 105 K). Table S1 - Best fit values and coupling constants from EPR simulations of Fe I Br, [Fe I ] +, and [Fe I (N 2 )] +. Compound g x g y g z A 31P (MHz) A Br (MHz) Fe I Br (anisotropic) [Fe I ] [Fe I (N 2 )] Additional Catalytic Trials Table S2 Additional Silylation Catalysis Experiments (see main text for reaction scheme). Run Cat. M R 3 SiCl solvent N 2 Pressure (atm) mmol M/R 3 SiCl Time (h) N(SiMe 3 ) 3 equiv/fe 1 Fe 0 (N 2 ) KC 8 R=Me THF Fe 0 (N 2 ) Na R=Me toluene Fe 0 (N 2 ) Na R=Me THF Fe 0 (N 2 ) Na R=Me THF Fe 0 (N 2 ) KC 8 R=Et toluene S9

10 Electrochemistry Figure S14 - CVs of Fe 0 (N 2 ) at various scan rates under 1 atm N 2. * = trace impurity. Figure S15 - Linear dependence of peak current (I p ) vs. square root of scan rate (υ 1/2 ) for Fe 0 (N 2 ). Figure S16 - CVs of [Fe I ] + at various scan rates under 1 atm N 2. S10

11 Figure S17 - Simulated CVs of [Fe I ] + at various scan rates. See Table S3 for simulation parameters. Figure S18 Linear dependence of peak current (I p ) vs. square root of scan rate (υ 1/2 ) for [Fe I ] +. Figure S19 - CVs of [Fe II ] 2+ at various scan rates under 1 atm N 2. S11

12 Figure S20 - Linear dependence of peak current (I p ) vs. square root of scan rate (υ 1/2 ) for [Fe II ] 2+. Conditions: 100 mm n Bu 4 NB(C 6 F 5 ) 4 electrolyte in fluorobenzene. Figure S21 - CVs of [Fe II ] 2+ under N 2 and Ar (100 mv s -1 ). The solid black trace is the initial CV under 1 atm N 2 while the dashed black trace is the CV of a N 2 sparged solution after being sparged with Ar. Figure S22 - High pressure electrochemistry experiments of [Fe I ] + (1-100 atm N 2, 100 mv s -1 ). * = 1-AcFc internal reference. Loss in analyte peak current with increasing pressure is likely due to trace contaminant(s) in the gas stream. S12

13 Figure S23 Simulated CVs of [Fe I ] + at 1 atm and 100 atm N 2. See Table S3 for simulation parameters. Table S3 - Parameters used in the CV simulations. Electron Transfer Reactions E ' (V) α k S (cm/s) [Fe II ] 2+ + e [Fe I ] a 0.01 a [Fe I ] + + e [Fe 0 ] [Fe I (N 2 )] + + e Fe 0 (N 2 ) Chemical Reactions b K eq k fwd (s 1 ) [Fe I ] + [Fe I (N 2 )] + (1 atm N 2 ) c [Fe I ] + [Fe I (N 2 )] + (100 atm N 2 ) [Fe 0 ] Fe 0 (N 2 ) (1 atm N 2 ) d [Fe 0 ] Fe 0 (N 2 ) (100 atm N 2 ) Other Parameters Diffusion coefficient Fe e cm 2 /s Initial concentration [Fe I ] M Surface area cm 2 Resistance 10,000 Ω Double layer capacitance 0.2 µf Temperature K a Estimated by manual iteration to qualitatively reproduce the shape of the Fe II/I couple at 100 atm N 2. b N 2 binding was modeled as a pseudo-first order reaction. As a result, K eq and k fwd values at 100 atm N 2 are larger than the values at 1 atm N 2. c Estimated by manual iteration until the CV qualitatively matched the experimental CVs recorded at fast scan rate. d Arbitrarily set to a value faster than the CV timescale. e Measured for Fe 0 (N 2 ) using DOSY NMR. S13

14 Mössbauer Spectroscopy Table S4 - Experimental Mössbauer parameters of the main species for all new iron complexes. Complex Isomer shift (δ) mm/s Quadrupole splitting (ΔE Q ) mm/s Relative Area, % FWHM (mm/s) Fe II Br (1) 1.73(1) Fe I Br 0.38(1), 0.44(1) -0.09(1) 0.79(1) 1.02(1) 0.99(1) Fe 0 (N 2 ) 0.21(1) 0.14(4) 0.4(4) 0.43(2) [Fe II ] (1) 0.66(4) 0.70(4) 4.0(2)/3.5(2) / Thermochemical Cycles [Fe 0 ] [Fe I ] + + e - ΔG = 23.06E = kcal mol 1 [Fe I (N 2 )] + + e - Fe 0 (N 2 ) ΔG = 23.06E = 36.0 kcal mol -1 [Fe I ] + + N 2(g) [Fe I (N 2 )] + ΔG = log(k eq ) = 0.4 kcal mol -1 [Fe 0 ] + N 2(g) Fe 0 (N 2 ) ΔG N2 = -7.0 kcal mol -1 [Fe II ] 2+ + e - [Fe I ] + ΔG = 23.06E = 7.6 kcal mol 1 [Fe I (N 2 )] + [Fe II (N 2 )] 2+ + e - ΔG > V > 21.7 kcal mol -1 (E not observed) [Fe I ] + + N 2(g) [Fe I (N 2 )] + ΔG = log(k eq ) = 0.4 kcal mol -1 [Fe II ] 2+ + N 2(g) [Fe II (N 2 )] 2+ ΔG N2 > 29.7 kcal mol -1 Pressure Dependence on N 2 Binding Free Energy, DG N2 Since [Fe 0 ] + N 2(g) Fe 0 (N 2 ) and K $% = [)$* (, - )] [)$ * ] 1 2-, then S14

15 G,6 = RTln [Fe > (N 6 )] [Fe > ] P,6 where P,6 = 1 atm, i.e. standard state conditions. If P,6 P,6, then: G,6 = RTln [Fe > (N 6 )] [Fe > ] P,6 P,6 and: G,6 = RTln [Fe > (N 6 )] [Fe > ] P,6 RTln P,6 G,6 = G,6 RTln P,6 Crystallographic details Refinement of Fe II Br 2 : Data were collected on a yellow-green plate (0.40 x 0.25 x 0.10 mm 3 ) grown from a mixture of dichloromethane and pentane. The data set consisting of reflections (23213 unique, R int = 8.69%) was collected over Θ = to The metric symmetry and systematic absences were consistent with the centrosymmetric monoclinic space group P2 1 /c. The asymmetric unit contains two molecules of 1. The data were refined as a pseudomerohedral twin with the twin ratio freely refined and converging on ~74:26. Due to the twinning, rigid bond and similarity restraints on anisotropic displacement parameters were used on all atoms, and restraints to approximate isotropic behavior (ISOR) were used on two carbons. The goodness of fit on F 2 was with R 1 = 7.78% [I>2σ(I)], wr 2 = % (all data) and with a largest difference peak and hole of and e/å 3. S15

16 Figure S24 Molecular Structure of [Fe II Br 2 ] with 50% probability ellipsoids. Only one molecule from the asymmetric unit is shown and hydrogen atoms have been omitted for clarity. Selected bond distances ( ) and angles (Å): Fe1-Br1 = 2.470(1), Fe1-Br2 = 2.504(1), Fe1-P1 = 2.269(2), Fe1-P2 = 2.309(2), Fe1-P3 = 2.170(2), Fe1-P4 = 2.176(2); Br1-Fe1-Br2 = (5), P1-Fe1-P2 = (6), P3-Fe1-P4 = 78.30(6). Refinement of Fe I Br: Data were collected on an orange plate (0.40 x 0.18 x 0.06 mm 3 ) grown from a mixture of THF and pentane. The data set consisting of reflections (16292 unique, R int = 5.98%) was collected over Θ = to The symmetry was consistent with the triclinic space group P1 and the asymmetric unit contains two independent molecules of 3. The goodness of fit on F 2 was with R 1 = 3.90% [I>2σ(I)], wr 2 = 9.05% (all data) and with a largest difference peak and hole of and e/å 3. Figure S25 - Molecular Structure of Fe I Br with 50% probability ellipsoids. Only one molecule from the asymmetric unit is shown and hydrogen atoms have been omitted for clarity. Selected bond distances ( ) and angles (Å): Fe1-Br1 = (6), Fe1-P1 = 2.226(1), Fe1-P2 = (8), Fe1- P3 = (7), Fe1-P4 = (9), Fe1 N1 = 3.336(2); P1-Fe1-P2 = (3), P3-Fe1-P4 = 79.63(3). Fe 0 (N 2 ): Data were collected on a red block (0.5 x 0.5 x 0.2 mm 3 ) grown from a mixture of toluene and pentane. The data set consisting of reflections (6019 unique, R int = 7.54%) was collected over Θ = to The symmetry was consistent with the triclinic space group P1 and the asymmetric unit contains one molecules of 4. The goodness of fit on F 2 was with R 1 = 5.81% [I>2σ(I)], wr 2 = 16.51% (all data) and with a largest difference peak and hole of and e/å 3. S16

17 Figure S 26 - Molecular Structure of Fe 0 (N 2 ) with 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond distances ( ) and angles (Å): Fe1-N1 = 1.818(3), N1- N2 = 1.101(5), Fe1-P1 = (9), Fe1-P2 = 2.172(1), Fe1-P3 = 2.113(1), Fe1-P4 = (9), Fe1 N4 = 3.535(2); P1-Fe1-P2 = (4), P3-Fe1-P4 = 79.22(4). Refinement of [Fe I ][B(C 6 F 5 ) 4 ]: Data were collected on a red plate (0.20 x 0.15 x 0.05 mm 3 ) grown from a mixture of PhF and pentane. The data set consisting of reflections (11029 unique, R int = 7.39%) was collected over Θ = to The symmetry was consistent with the triclinic space group P1. The asymmetric unit contains one [Fe(P 4 N 2 )] + cation and one [B(C 6 F 5 ) 4 ] anion. The goodness of fit on F 2 was with R 1 = 4.45% [I>2σ(I)], wr 2 = 8.83% (all data) and with a largest difference peak and hole of and e/å 3. Figure S 27 - Molecular Structure of [Fe I ] + [B(C 6 F 5 ) 4 ] - with 50% probability ellipsoids. The B(C 6 F 5 ) 4 - anion and most hydrogen atoms have been omitted for clarity. Selected bond distances ( ) and angles (Å): Fe1-P1 = 2.236(1), Fe1-P2 = (8), Fe1-P3 = (7), Fe1-P4 = 2.158(1), Fe1 H20 = 2.786, Fe1 N1 = 3.188(2); C20-H20-Fe1 = 122.2, P1-Fe1-P2 = (3), P3-Fe1- P4 = 80.95(3). S17

18 DFT Calculations Figure S28 - Mulliken Spin density (isovalue = 0.005) of [Fe I ] + with spin population at Fe (1.24). Figure S29 Mulliken Spin density (isovalue = 0.005) of [Fe I (N 2 )] + with spin population at Fe (1.12) and N β (-0.08). Ph N P P N Ph Fe I Ph P Ph Ph P Ph + + N 2(g) Ph N P P N N N Fe I Ph Ph P Ph Ph P Ph N2 Scheme S1 N 2 binding equilibrium for [Fe I ] + and [Fe I (N 2 )] + benchmarked in Table S5. S18

19 Table S5 Benchmarking DFT functionals and basis sets for [Fe I ] +, N 2(g), and [Fe I (N 2 )] +. ΔG exp (kcal mol -1 ): 0.4 Basis Set Functional ΔG calc M06L TZVP/TZVPFit/SDD(Fe) 4.9 M06L 6-31G(d)/SDD(Fe) 1.2 B3P G(d)/SDD(Fe) 3.0 ωb97xd 6-31G(d)/SDD(Fe) 8.3 Cartesian Coordinates, Enthalpies (H, Hartree), and Free Energies (G, Hartree) M06L/TZVP/TZVPFit/SDD(Fe) N 2(g) H = G = N N [Fe I ] + H = G = Fe P P P P N N C C H C C C C H C C H C H C H C H C C H C H C H C H C H H C H C H C H C H C H C H C H S19

20 C H H C H C H C H C H H C H H C H C H H C H C H C H H C H H C H C H H C H C H C H C H C H C H [Fe I (N 2 )] + H = G = Fe P P P P N N N N C C C H C H H C H C H C H H C H C H C C H C C H H C C C H C H C H H C H C H C H H C H H C H C H H C H S20

21 C H H C H C H C H C H C H C H C H C H C H C H C H C H C H C H C H C H C H C H B3P86/6-31G(d)/SDD(Fe) N 2(g) H = G = N N [Fe I ] + H = G = Fe P P P P N N C C H C C C C H C C H C H C H C H C C H C H C H C H C H H C H C H C H C H C H C H C H C H H C H C H C H C H S21

22 H C H H C H C H H C H C H C H H C H H C H C H H C H C H C H C H C H C H [Fe I (N 2 )] + H = G = Fe P P P P N N N N C C C H C H H C H C H C H H C H C H C C H C C H H C C C H C H C H H C H C H C H H C H H C H C H H C H C H H C H C H C H C H S22

23 C H C H C H C H C H C H C H C H C H C H C H C H C H C H ωb97xd/6-31g(d)/sdd(fe) N 2(g) H = G = N N [Fe I ] + H = G = Fe P P P P N N C C H C C C C H C C H C H C H C H C C H C H C H C H C H H C H C H C H C H C H C H C H C H H C H C H C H C H H C H H C H C H H C H S23

24 C H C H H C H H C H C H H C H C H C H C H C H C H [Fe I (N 2 )] + H = G = Fe P P P P N N N N C C C H C H H C H C H C H H C H C H C C H C C H H C C C H C H C H H C H C H C H H C H H C H C H H C H C H H C H C H C H C H C H C H C H C H C H C S24

25 H C H C H C H C H C H C H C H C H M06L/6-31G(d)/SDD(Fe) N 2(g) H = G = N N [Fe I ] + H = G = Fe P P P P N N C C H C C C C H C C H C H C H C H C C H C H C H C H C H H C H C H C H C H C H C H C H C H H C H C H C H C H H C H H C H C H H C H C H C H H C H H C H C S25

26 H H C H C H C H C H C H C H [Fe I (N 2 )] + H = G = Fe P P P P N N N N C C C H C H H C H C H C H H C H C H C C H C C H H C C C H C H C H H C H C H C H H C H H C H C H H C H C H H C H C H C H C H C H C H C H C H C H C H C H C H C H C H C H S26

27 C H C H C H boat,chair-[fe II ] 2+ (S = 1) H = G = Fe P P P P N N C C H C C C C H C C H C H C H C H C C H C H C H C H C H H C H C H C H C H C H C H C H C H H C H C H C H C H H C H H C H C H H C H C H C H H C H H C H C H H C H C H C H C H C H C H chair,chair-[fe II ] 2+ (S = 1) H = S27

28 G = Fe P P P P N N C C C H C H C C H C H H C H H C H C H C H H C H C C H H C H C H C H C H C C H C H C H C H C H C H C H C H C C H C H C H C H C H C H C H C H C H C H H C H C H H C H H C H C H H C H [Fe II ] 2+ (S = 0) H = G = Fe P P P P N N C C C S28

29 H C H H C H C H C H H C H C H C C H C C H H C C C H C H C H H C H C H C H H C H H C H C H H C H C H H C H C H C H C H C H C H C H C H C H C H C H C H C H C H C H C H C H C H [Fe II ] 2+ (S = 2) H = G = Fe P P P P N N C C H C C C C H C C H C H S29

30 C H C H C C H C H C H C H C H H C H C H C H C H C H C H C H C H H C H C H C H C H H C H H C H C H H C H C H C H H C H H C H C H H C H C H C H C H C H C H (1) Wiedner, E. S.; Roberts, J. A. S.; Dougherty, W. G.; Kassel, W. S.; DuBois, D. L.; Bullock, R. M. Inorg. Chem. 2013, 52, S30