Supporting Information for:

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1 Supporting Information for: Catalytic Silylation of 2 and Synthesis of H 3 and 2 H 4 by et Hydrogen Atom Transfer Reactions using a Chromium 4 Macrocycle Alexander J. Kendall, Samantha I. Johnson, R. Morris Bullock, and Michael T. Mock* Center for Molecular Electrocatalysis, acific orthwest ational Laboratory,.O. Box 999, Richland, Washington 99352, United States Corresponding Author: michael.mock@pnnl.gov Table of Contents I. reparation of Cr complexes...s-1 II. Low oxidation state chromium-phosphine complexes...s-4 III. Quantification of itrogen...s-5 IV. Screening Tables...S-7 V. Additional Reactions...S-11 VI. Computational Details... S-19 VII. References...S-38 I. reparation of chromium complexes [Cr II Cl 2 (THF)] 1 and [Cr III Br 3 (THF) 3 ] 2 were synthesized using literature procedures. 4 Cr( 2 ) 2 synthesis. Modified preparation from the literature 1 : Cr II Cl 2 (THF) (400 mg, 2.05 mmol) was added as a solid to a solution of 2 2 (1.99 g, 4.1 mmol) in THF (240 ml) and allowed to stir for 7 days at room temperature under 2. The solvent was removed under reduced pressure, and the resulting solids were manually triturated twice with pentane (solute discarded) to yield a powdery solid. The resulting solids were mixed with approximately 5 grams diatomaceous earth. To the mixture was added 50 ml of Et 2 O, creating an orange-brownblue slurry. The mixture was filtered through diatomaceous earth and washed 2 x with 50 ml Et 2 O. The resulting orange solution (a mixture of ligand and trans- 4 Cr II Cl 2 ) was concentrated under reduced pressure to an orange solid, (component 1). The residual brown solids (stuck on the filter) were then washed with toluene (3 x 50 ml) yielding a brown solution and leaving blue solids on the filter. (ote that the toluene acts as a deliquescent promoter for the solids hence why they were pre-mixed with diatomaceous earth to promote clean filtrations). The brown S-1

2 toluene solution was concentrated under reduced pressure to a tacky brown solid composed of unidentified species (component 2). The remaining blue solids on the filter can be isolated using CH 2 Cl 2, resulting in a dark blue solution of fac-[crcl 3 ( 3 3)] (component 3). Component 1 was taken up into THF (10 ml) and 5 eq. of Rieke Mg was added (based on the assumption the entire mass was trans- 4 Cr II Cl 2 ). The mixture was allowed to stir overnight under 2. The mixture was then concentrated under reduced pressure to afford a tacky orange/brown solid. The solids were taken up into 1,4-dioxane (10 ml) and allowed to stir overnight. The resulting mixture was then filtered through diatomaceous earth and the resulting solution was concentrated to an orange/brown powder under reduced pressure. The solids were taken up into a minimal volume of THF (ca. 1 ml), filtered, and setup for a slow-diffusion crystallization with 20 ml of pentane at -30 C. Orange flakes or orange-red blocks were both isolated and determined to be spectroscopically pure 4 Cr( 2 ) 2 (110 mg, 5% isolated yield). Component 2 was taken up into THF (40 ml) and 5 eq. of Rieke Mg was added (based on the assumption the entire mass was trans- 4 Cr II Cl 2 ). The mixture was allowed to stir overnight under 2. The mixture was then concentrated under reduced pressure to afford a tacky brown solid. The mixture was manually triturated with pentane (3 x 50 ml) and the red solute discarded. (Caution! the red oil that is soluble in the pentane layer is pyrophoric and should be handled very carefully to avoid combustion in air). The remaining solids were triturated with Et 2 O (50 ml) and filtered through silica (100 mesh, 400 g, 35 mm column width, all under 2 ). An orange layer elutes (determined to be stable to silica for up to 40 minutes via 2D TLC) with Et 2 O which contains phosphine related organics and 4 Cr( 2 ) 2. A long brown streak (unwanted byproducts) should be avoided for collection. Several washes with Et 2 O over the triturated solids should help to dissolve and elute all of the 4 Cr( 2 ) 2 complex from the mixture. Et 2 O washes continued in 50 ml portions until no orange color was observed with the eluent (typically ending with ml of orange solution collected). 4 Cr( 2 ) 2 is air-sensitive and must be handled under 2. The resulting orange Et 2 O solution was concentrated under reduced pressure to yield an orange solid. The solids were taken up into a minimal volume of THF (ca. 2 ml), filtered, and setup for a slow-diffusion crystallization with 40 ml of pentane at -30 C. Orange flakes or orange-red blocks were both isolated and determined to be spectroscopically pure 4 Cr( 2 ) 2 (338 mg, 16% isolated yield). Tan solids (ligand related organics), colorless crystals, and red oil also can accompany crystallization efforts it is recommended to use a 20:1 pentane:thf ratio for slow diffusion. Typically several recrystallizations were required to properly isolate the 4 Cr( 2 ) 2 complex. Component 3 was used to isolate fac-[crcl 3 ( 3 3)] (see below). 4 Cr( 15 2 ) 2 synthesis: 4 Cr( 15 2 ) 2 was prepared by degassing a THF solution of 4 Cr( 2 ) 2 by three freeze-pump-thaw cycles and vigorously stirring this solution under an atmosphere of 15 2 gas for 12 h. S-2

3 trans-[(dmpe) 2 Cr 0 ( 2 ) 2 ] synthesis. Modified preparation from the literarure. 3 Synthesized as reported with the following change: when reducing trans-[(dmpe) 2 Cr II Cl 2 ], Reike magnesium was used (5 eq. per Cr) in THF at room temperature under 2 for 16 hours. The resulting mixture was concentrated under reduced pressure to an orange/brown paste and triturated with pentane, then filtered. The pentane solution contained both free dmpe ligand and trans- [(dmpe) 2 Cr 0 ( 2 ) 2 ]. Trans-[(dmpe) 2 Cr 0 ( 2 ) 2 ] was purified via slow evaporation to large red blocks that were washed with minimal amounts of cold (Me 3 Si) 2 O to yield spectroscopically to be pure trans-[(dmpe) 2 Cr 0 ( 2 ) 2 ] (yield range 74-88% isolated). Cr( 2 )(dmpe)( 3 3) synthesis. Followed literature procedure. 4 Recrystallized immediately before use (THF/pentane slow diffusion). cis-[cr( 2 ) 2 ( 2 2) 2 ] synthesis. Followed literature procedure. 5 Recrystallized immediately before use (THF/pentane slow diffusion). fac-[crcl 3 (k 3 -(,,) 2 2)] synthesis. Followed literature procedure. 5 S-3

4 fac-[crcl 3 ( 3 3)] synthesis. fac-[crcl 3 ( 3 3)] was purified from component 3 of the 4 Cr( 2 ) 2 synthesis (see above). The blue fac-[crcl 3 ( 3 3)] was found to be air-stable as a solid and in solution. In the crude mixture of component 3, another unidentified chromium species slowly oxidized in air over several days and precipitated from solution as a grey-green solid leaving behind a deep-blue solution after filtration. The solvent was removed under reduced pressure yielding a glassy blue solid that was readily crystallized by slow diffusion of pentane into a concentrated solution of CH 2 Cl 2 yielding dark blue needles of fac- [CrCl 3 ( 3 3)] (437 mg, 24%). II. Low Oxidation State Chromium-osphine Complexes In this section of the SI, we intend to point out some commonalities of the stable complexes to better understand the stability of chromium complexes. 1) Bond angles to form phosphine complexes. We have observed that there are very few phosphine complexes of chromium and even fewer phosphine complexes with chromium and 2. The reason this is important has to do with the difficulty in forming discrete low oxidation state chromium complexes with phosphine ligands. Our own attempts have only yielded a handful of stable complexes many of which form in very poor yields and are prone to thermal decomposition. Thermally stable Cr 0-2 complexes with phosphine ligands tend to have the following aspects: (1) close to 90 angle between ligands (see Table S1, (2) poly-dentate ligands produce more stable complexes (for example, Cr 0 (Me 3 ) 4 ( 2 ) 2 (bond angles of 90 by IR) is much less stable than Cr 0 (dmpe) 2 ( 2 ) 2 ) (very close to 90 angles), (3) thermodynamically preferred trans geometries tend to be thermally stable while cis geometries tend to decompose at room temperature (note that this is a general empirical observation and not theory based.) Table S1. Crystallographically determined -Cr- bond angles for isolable Cr 0 ( 2 )- phosphine complexes and selected unstable Cr complexes. Cr complex -Cr- Bond Reference Angle( ) cis-[cr( 2 ) 2 ( 2 2) 2 ] 77.2 M.T.Mock, Shentan Chen, R.Rousseau, M.J.O'Hagan, W.G.Dougherty, W.S.Kassel, D.L.DuBois, R.M.Bullock; Chem. Comm. 2011, 47, cis-[cr( 2 ) 2 ( Et (2,6-F) Et ) 2 ] 86.1, 86.4 J.D.Egbert, M.O'Hagan, E.S.Wiedner, R.Morris Bullock,.A.iro, W.Scott Kassel, M.T.Mock; Chem.Comm. 2016, 52, trans-[cr( 2 ) 2 ( Et (2,6-F) Et ) 2 ] 85.5 Ibid. S-4

5 [trans-cr(dmpe) 2 (CC)] 2 (μ 2-2 ) 82.9 W.A.Hoffert, A.K.Rappe, M..Shores; Inorg.Chem. 2010, 49, [trans-cr(dmpe) 2 (CCTMS)] 2 (μ 2-2 ) 83.1 Ibid. [trans-cr(dmpe) 2 (CCTIS)] 2 (μ 2-2 ) 82.7 Ibid. {[trans-cr(dmpe) 2 (CCTIS)] 2 (μ 2-2 )} Ibid. {[trans-cr(dmpe) 2 (CCTIS)] 2 (μ 2-2 )} Ibid. fac-[cr( 2 )(dmpe)( 3 3)] 89.5, 86.5 ( 3 3 ); 82.0 (dmpe) M.T.Mock, A.W.ierpont, J.D.Egbert, M.OHagan, Shentan Chen, R.Morris Bullock, W.G.Dougherty, W.Scott Kassel, R.Rousseau; Inorg. Chem. 2015, 54, Ibid. fac-[cr( 2 )(dmpm)( 3 3)] Could not be synthesized trans-[cr( 2 ) 2 ( 4 4)] 89.9 M.T.Mock, Shentan Chen, M.OHagan, R.Rousseau, W.G.Dougherty, W.Scott Kassel, R.Morris Bullock; J. Am. Chem. Soc. 2013, 135, trans-[cr( 2 ) 2 ( 4 2)] Stable at -30 C This work trans-[cr( 2 ) 2 (dmpe) 2 ] 83.4 J.E.Salt, G.S.Girolami, G.Wilkinson, M.Motevalli, M.Thornton-ett, M.B.Hursthouse; J. Chem. Soc., Dalton Trans. 1985, ) Hard Soft Acid Base (HSAB) Theory. HSAB theory 6 predicts that Cr will be a hard acid unless in the zero-valent oxidation state. Because the chemistry of Cr- 2 reduction involves redox swings at the Cr center, a hard-soft change is also occurring. Based on our observations that most homogenous Cr complexes with phosphines don t stay in solution during catalysis, it is likely that only ligands that remain tethered during a hard-soft chromium change will keep the species in solution. Otherwise, the HSAB mismatch likely leads to ligand dissociation and ultimately precipitation of chromium metal. We believe this is a major factor in the success of the 4 Cr( 2 ) 2 platform compared with other Cr species the specific ability to keep Cr in solution regardless of hard-soft chromium changes during catalysis. III. Quantification of itrogen 1 H MR was determined to be accurate for quantification (10:1 signal to noise) of H 4 Cl above 0.4 μm H 4 Cl in dmso-d 6. The lower detection reported in the manuscript (0.1 eq. / Cr) at least double that concentration for good measure. Experiments run multiple times were found to be reproducible within +/- 0.1 equivalents as reported. (SiMe 3 ) 3. itrogen yields were quantified using 1 H MR for its hydrolysis product, H 4 Cl (the accuracy of which was confirmed by a calibrated GC analysis of (SiMe 3 ) 3 against decane as an internal standard). The crude reaction mixture was filtered through diatomaceous earth and rinsed three times with THF. The filtrate was acidified with 100 equivalents of HCl (etherate) per Me 3 SiCl added. Solvent was then removed from the resulting mixture to yield solids. A ml of a stock solution of 8.5 mm 1,3,5-trimethoxybenzene (TMB) in DMSO-d 6 was added to the residual solids and thoroughly homogenized. The resulting solution was analyzed by 1 H MR ( 1 H relaxation delay set to 10 seconds based on H 4 Cl and TMB T 1 relaxation measurements) spectroscopy for the diagnostic H 4 + peak at 7.29 ppm (1:1:1 triplet, J = 50.9 Hz) and quantified against both TMB proton resonances. H 4 +. To determine the ammonia present after a reaction with protons and electrons, the Ashley method was followed exactly with only a minor modification to the glassware apparatus used to perform the H 3 vacuum transfer (Figure S1) having a pear receiving flask (25 ml flask) and a vacuum joint for attachment directly to a high-vacuum line maintained by an oil diffusion pump. S-5

6 o. 9 vacuum joint. Receiving flask Reaction flask Figure S1. Ammonia quantification apparatus. ote that the Teflon stopcocks for the threaded joints have been omitted for clarity each threaded tube corresponds with a screw-down valve for isolation. (For the custom glassware specifications, please contact the authors). H 3. For the TEMOH reactions with 4 Cr( 2 ) 2, free ammonia was quantified by a modification to the Ashley procedure as follows. Instead of basifying the reaction flask before the vacuum transfer (necessary when using protons and electrons), a direct vacuum transfer was employed. This was possible because of the absence of acid during the reaction. Thus, any H 3 detected came directly from the reaction with no additives. 2 H 5 +. The spectrophotometric hydrazine test (p-dimethylaminobenzaldehyde test) 7 was used to detect hydrazine produced during reactions with 2. Stock solutions of 2 H 6 Cl 2 were used to determine the limit of detection and linear range for quantification. A calibration curve (Figure S2) for hydrazine detection was determined. Samples were prepared and analyzed in exact accordance with Ashley s protocol (used for H 4 + detection as well). As with the Ashley protocol, when room temperature vacuum transfer conditions were employed, no 2 H 4 was observed to vacuum transfer to the receiving flask (Figure S1). Regardless, MR samples prepared from the receiving flask to quantify ammonium were also tested for hydrazinium. S-6

7 Figure S2. Hydrazine test calibration for linear region of spectrophotometric detection. IV. Screening Tables using trans-[cr 0 ( 2 ) 2 ( 4 4)]: trans-[cr 0 ( 2 ) 2 ( 4 4)] + "Si" + a Table S2. Silane reagent screen THF 22 C, 16h, 2 (SiMe 3 ) 3 HCl H 4 Cl Si TO () Me 3 SiCl 12.2 Et 3 SiCl 6.4 i r 3 SiCl 6.4 Me 3 SiBr 0.9 Me 3 SiI 0.1 ClMe 2 SiCH 2 CH 2 SiMe 2 Cl 5.8 MeSiCl 2 H 1.3 Single runs S-7

8 trans-[cr 0 ( 2 ) 2 ( 4 4)] + Me 3 SiCl + Red. Table S3. Reductant screen THF 22 C, 16h, 2 (SiMe 3 ) 3 HCl H 4 Cl Reductant TO () a 12.1 KC 8 (THF) 5.9 KC 8 (Toluene) <0.1 a/c 10 H 8 <0.1 a/k 7.5 Mg* <0.1 CoCp 2 <0.1 a/hg 4.2 Single runs trans-[cr 0 ( 2 ) 2 ( 4 4)] + Me 3 SiCl + a solvent 22 C, 16h, 2 (SiMe 3 ) 3 HCl H 4 Cl Table S4. Solvent screen Solvent TO () THF 12.2 Toluene 2.7 Et 2 O 5.6 Dioxane 5.0 MTBE 5.9 2,4,6-trimethylpyridine <0.1 2-MeTHF 0.5 DME 7.8 entane 4.8 Me 3 SiCl 4.2 (Me 3 Si) Single runs S-8

9 trans-[cr 0 ( 2 ) 2 ( 4 4)] + Me 3 SiCl + a 0 Table S5. Temperature screen THF (Temp) C, 16h, 2 (SiMe 3 ) 3 HCl H 4 Cl Temperature ( o C) TO () to Single runs trans-[cr 0 ( 2 ) 2 ( 4 4)] + Me 3 SiCl + a Table S6. Glassware passivation screen THF 22 C, 16h, 2 (SiMe 3 ) 3 HCl H 4 Cl Glassware TO () a Conc. HCl washed 9.3 Virgin glass 12.1 Silylated b 17.2 a Average of two runs each. b In exact accordance with urification of Laboratory Chemicals - (Sixth Edition). trans-[cr 0 ( 2 ) 2 ( 4 4)] + Me 3 SiCl + a Table S7. 2 pressure screen THF 22 C, 16h, 2 pressure (SiMe 3 ) 3 HCl H 4 Cl 2 pressure (atm) TO () Average of 3 runs S-9

10 Table S8. Reduction of two isomers and trans-cr 0 ( 2 ) 2 (dmpe) 2 Cr (0.1 mm) [Me 3 SiCl] a TO () trans-[cr 0 ( 2 ) 2 ( 4 4)] 100 mm 11.1 cis-[cr 0 ( 2 ) 2 ( 2 2) 2 ] 100 mm 4.8 trans-[cr 0 ( 2 ) 2 (dmpe) 2 ] 100 mm 5.2 trans-[cr 0 ( 2 ) 2 ( 4 4)] 1.0 M 14.5 cis-[cr 0 ( 2 ) 2 ( 2 2) 2 ] 1.0 M 3.0 trans-[cr 0 ( 2 ) 2 (dmpe) 2 ] 1.0 M 5.3 a equal stoichiometric equivalents of a added, virgin glassware, duplicate yields S-10

11 V. Additional Reactions Figure S3. H 4+ analysis for 4 Cr( 15 2 ) 2 in a catalytic run using ClSiMe 3 and the catalytic conditions listed in Table S2. 1 H MR spectrum shows the major product 15 H 4 Cl (doublet) and minor 14 H 4 Cl (1:1:1 triplet). Scheme S1. Independent confirmation of the chemical reversibility of 4 Cr II Cl 2 (right) and 4 Cr( 2 ) 2 (left) using the reagents from catalytic runs. 4 Cr( 2 ) 2 in 4 ml of THF was reacted with 1000 equiv. of TMSCl to yield a paramagnetic yellow solution within 16 h that was crystalized (68% crystalline isolated yield) and crystallographically identified as trans- 4 Cr II Cl 2. Conversely, crystalline trans- 4 Cr II Cl 2 was reacted with a (1000 equiv) overnight to yield an orange solution featuring one peak at 39 ppm in the 31 { 1 H} MR and a diagnostic 1 H MR spectrum for 4 Cr( 2 ) 2. When crystalized, this yielded orange blocks that were identifiable as the 4 Cr( 2 ) 2 complex (80% crystalline isolated yield). S-11

12 1 eq. 2 eq. 4 eq. Triflate absorbance IR: ν ( 14 ): 2072 (asym.), 1918 (sym.) cm -1 Figure S4. React IR plot showing 2 absorbance region for 4 Cr( 2 ) 2 as 2,4,6- trimethylpyridinium triflate (ColH[OTf]) is added in THF. ote that the change in height is caused by dilution of the solution no shift of the 2 bands is observed. S-12

13 4 Cr( 2 ) equivalents of acid AD 4 equivalents of CoCp 2 (5 min). 4 Cr( 2 ) equivalents of acid 24 hours post-addition 4 Cr( 2 ) equivalents of acid at 15 minutes post-addition 4 Cr( 2 ) 2 before acid Figure S5. 31 { 1 H} MR plot showing the singlet resonance for 4 Cr( 2 ) 2 as 2,4,6- trimethylpyridinium triflate (ColH[OTf]) is reacted in THF-d 8. ote that no change occurs until reducing agent (CoCp 2 ) is added. S-13

14 4 Cr( 2 ) equivalents of acid AD 4 equivalents of CoCp 2 (48 h). 4 Cr( 2 ) equivalents of acid AD 4 equivalents of CoCp2 (5 min.) 4 Cr( 2 ) equivalents of acid (15 min.) 4 Cr( 2 ) equivalents of acid (24 h) 4 Cr( 2 ) 2 CoCp 2 2,4,6-trimethyl pyridinium triflate 2,4,6-trimethyl pyridine Figure S6. 1 H MR spectra showing the reaction of 4 Cr( 2 ) 2 with 2,4,6-trimethylpyridinium triflate (ColH[OTf]) in THF-d 8. ote that no change occurs until reducing agent (CoCp 2 ) is added. Figure S7. 1 H MR spectrum of TEMOH (55 eq.) + 4 Cr( 2 ) 2 in THF-d 8 at room temperature. S-14

15 Figure S8. 1 H MR spectrum of vacuum transferred volatiles (ammonium test, see above) for TEMOH (97 eq.) + 4 Cr( 14 2 ) 2 in dmso-d 6 at room temperature. S-15

16 Figure S9. 1 H MR spectrum of vacuum transferred volatiles (ammonium test, see above) for TEMOH (97 eq.) + 4 Cr( 15 2 ) 2 in dmso-d 6 at room temperature. S-16

17 Figure S10. 1 H MR spectrum of vacuum transferred volatiles (ammonium test, see above) for TEMO (87 eq.) + 4 Cr( 14 2 ) 2 in dmso-d 6 at room temperature. ote that no ammonium is present in the spectrum. S-17

18 Figure S11. 2 H MR (46.1 MHz) spectrum of the reaction between 4 Cr( 15 2 ) 2 and 100 equiv TEMOD in THF-H 8 (not deuterated) at room temperature. The singlet at 0.65 ppm corresponds to 15 D 3 formed from the reduction of the terminally bound 15 2 ligands from 4 Cr( 15 2 ) 2, the resonances at 1.73 and 3.58 ppm correspond to the presence of partially deuterated THF, and the intense singlet at 6.5 ppm corresponds to the O-D of excess TEMOD. S-18

19 VI. Density Functional Theory Supporting Information Calculations were performed to understand the energetics of loss of 2 from the six-coordinate complex trans-[cr)( 2 ) 2 ( 4 4] as well as the silylation mechanism. These free energy calculations were completed using the B3LY hybrid functional 8 with Stuttgart/Dresden basis set with relativistic effective core potential (SDD) on Cr 9 and 6-31G** on all other atoms. 10 The Grimme D2 dispersion correction was used in all calculations. 11 Structures were optimized in the gas phase, followed by a harmonic vibrational frequency calculation at the same level of theory to obtain thermal and zero point energy (ZE) corrections. A subsequent single point solvation calculation in tetrahydrofuran (THF) was completed using charge density-based continuum solvent (SMD). 12 All free energies are referenced to the standard state of solutions at T= and p =1 atm 2. All calculations were completed using Gaussian Cr ΔG 0 out = ΔG 0 in = , + THF ΔG 0 THF = Cr 2 ΔG = 9.7 Cr Cr THF 4 Scheme S2. Loss of 2 from within (DG in ) and outside (DG out ) of the pocket created by phenyl substituents on the 4 4 ligand. The results of these calculations are shown in Scheme S2. The complex trans-[cr)( 2 ) 2 ( 4 4] has a pocket formed on one face of the ligand by the phenyl groups oriented upwards. This creates asymmetry with respect to the steric environment surrounding the 2 ligands, and two distinct possibilities for 2 loss loss from within this pocket and loss from outside of the pocket. Due to the sterically bulky and electron-rich nature of the phenyl groups surrounding 2, loss from within this pocket is 9.7 kcal/mol more favorable than from the open face of the complex, at 11.0 and 20.7 kcal/mol respectively. Molecule 2 has bond length of Å while molecule 3 has an bond length of Å, indicating minor 2 activation in both molecules. The possibility of solvent exchange with 2 was also investigated. The phenyl groups make it difficult to replace the pocket 2 with THF, but the open face can host a THF molecule. The resulting complex, 4, is also shown in Scheme S2. Despite the orientation of the THF oxygen towards Cr, the distance between Cr and O is calculated to be Å, indicating that THF is not covalently bound to the Cr complex. The bond length in 4 is equivalent to that S-19

20 in 2. Additionally, this orientation of solvent towards the open face of the macrocycle is higher in energy relative to 2. Silylation Mechanism 2 Cr 2 1 S=0 S=1/2 2 2 SiMe 3 Cr SiMe Cr 3 ΔG 0 = 6.8 ΔG 0 = 5.1 SiMe 3 Me 3 Si SiMe ΔG 0 = ΔG 0 in = 11.0 Cr 2 3 SiMe 3 ΔG 0 = S=1/2 Cr SiMe3 Scheme S3. Silylation was calculated to occur before and after loss of one 2 ligand. All reported free energies are in kcal/mol. Silylation by trimethylsilyl radical can occur from 1 to produce two equivalents of tris(trimethylsilyl)amine. The mechanism for the first portion of this reaction is shown in Schemes S2 and S3. As shown in Scheme S3, the reaction of the first trimethylsilyl radical can occur before or after loss of the in-pocket 2. As previously determined, loss of the in-pocket 2 to form 3 is endergonic by 11.0 kcal/mol. The subsequent silylation of the distal nitrogen atom is exergonic by 14.0 kcal/mol, forming 7. In comparison, the reaction of 1 with silyl radical is unfavorable by 5.1 kcal/mol to form complex 5. Loss of 2 is exergonic to form 7, which is poised to react with a second trimethylsilyl radical. Complex 5 features an bond length of 1.23 Å and a Cr bond length of 1.83 Å, but upon loss of the in-pocket 2, the bond lengthens slightly to 1.26 Å, while the Cr bond shortens to 1.69 Å. Despite the lengthened bond, the Cr angle is wider in 7, relative to 5, (168 vs 152, respectively). This could be a consequence of unfavorable steric interactions upon shortening of the Cr bond, as the Cr angle widens in order circumvent unfavorable steric interactions between the macrocycle and the trimethylsilyl group. The reaction of a second trimethylsilyl radical with 5, affording complex 6, was found to be unfavorable by 6.8 kcal/mol. The lowest energy conformer required dissociation of one Cr bond (Cr : 4.04 Å), similar to the dissociation step presented by Tanaka et al. 14 Thermodynamically, 6 is 15 kcal/mol less accessible than 7, suggesting that 2 loss from 5 would be the preferred pathway without initial dissociation of an 2 ligand. The experimental results showing loss of nearly all catalytic activity under increased 2 pressure would suggest the initial dissociation of an 2 ligand is a likely first step to enter the catalytic cycle. 7 S-20

21 Cr SiMe3 7 S=1/2 SiMe 3 ΔG 0 = 16.4 SiMe 3 ΔG 0 = Cr Me 3 Si Me 3 Si Me 3 Si 8 Cr SiMe3 9 SiMe 3 SiMe 3 Me 3 Si Me 3 Si Cr SiMe3 10 Cr Me 3 Si SiMe 3 Me 3 Si 11 Cr Cr SiMe 3 Cr + + (SiMe 3 ) 3 + (SiMe 3 ) 2 + (SiMe 3 ) 2 SiMe 3 Scheme S4. Second reaction with trimethylsilyl radical at the distal nitrogen to form 9 is calculated as the thermodynamically preferred product, with facile bond cleavage upon attack by a third silyl radical. Structures for 10 and 11, indicated by the dashed boxes, could not be optimized computationally. All reported free energies are in kcal/mol. The shortening of the Cr bond plays a critical step in subsequent silylation steps, as it limits reactivity at the proximal nitrogen atom. The thermodynamics of this reaction is shown in Scheme S4. Reaction with trimethylsilyl radical at the distal nitrogen atom is preferred by over 30 kcal/mol, due to the steric interactions of the trimethylsilyl group with the complex. Reaction at the proximal nitrogen bends the Cr angle, pushing the distal nitrogen down into the macrocycle. The complex 9 is thermodynamically preferred, with two silyl groups bound to the distal nitrogen and a nearly linear Cr angle, orienting the silylhydrazido intermediate away from the macrocycle. Structures for putative intermediates 10 and 11 could not be found computationally, which is indicated by the dashed boxes. Rather, both complexes spontaneously undergo bond cleavage. In the case of 10, tris(trimethylsilyl)amine is released, affording a Cr III nitride that can undergo further attack by silyl radicals. Alternatively, attack at the proximal nitrogen, as in complex 11, leads to the homolytic or heterolytic breaking at the bond, forming a mono-silylated nitride and releasing either a bis(trimethylsilyl)amide radical or anion. The radical amide can react with silyl radical in solution, and the anion can react with SiMe 3 Cl, as similar to mechanistic work done by Mézailles et al. on the silylation of nitrogen using a molybdenum catalyst. 15 Either path appears plausible, but work by Tanaka et al. on a related Mo system implies an intermediate such as 11 would be the kinetic product, while 10 would be the thermodynamic product. 14 Accordingly, a full mechanistic treatment of the reaction pathway requires both a thermodynamic and kinetic treatment and is in preparation for a future report. Upon - cleavage, we anticipate silylation to be facile as there is only one reactive site for generation of the second equivalent of (SiMe 3 ) 3. S-21

22 Table S9. Comparison of spin states Complex spin 1 spin 2 DG 0 (spin 1 spin 2 ) 1 Singlet triplet Singlet triplet Singlet triplet Singlet triplet Doublet quartet Doublet quartet 20.0 To ensure the correct spin states were reported, multiple spin states were calculated. The energy difference between these two spin states is reported in Table S9. For all complexes, the singlet conformations are the preferred low energy states and are used in all calculations. XYZ coordinates of relevant molecules 1- DG = kcal/mol Cr C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H S-22

23 C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C S-23

24 C H H DG = kcal/mol Cr C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H S-24

25 C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H DG = kcal/mol Cr C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H S-25

26 C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H S-26

27 H DG = kcal/mol Cr C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H S-27

28 H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H O C C C C H H H H H H H H DG = kcal/mol Cr C H C H C H C H C H C C H H C H H C H C H C H C S-28

29 H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H S-29

30 C H C H C H C H C C H H Si C H H H C H H H C H H H DG = kcal/mol Cr C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H S-30

31 C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H Si C H H H C H H H C H H H Si C H H H S-31

32 C H H H C H H H DG = kcal/mol Cr C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C H C C H H C H H C H C H C H C H C H C C H H C H C H C H C H C S-32