From Nitrogen Fixation Catalysts to Photoreductants

Introduction

Opening Remarks

Transition metal complexes are routinely used as catalysts for the reductive cleavage of a diverse group of strong chemical bonds. However, the ability to easily modify the properties of transition metal complexes with molecular precision through judicious tuning of the supporting ligand platform or metal center makes structure-function studies a powerful tool to better understand such principles.

In fact, explorations of P3SiFe preceded P3BFe and were crucial to the development of the latter. P3XFe (X = B, Al, Ga) platforms used in the structure-function studies described in Chapter 2 to investigate the effect of the apical Lewis acid atom on structure, binding and N2RR activity.

Figure 1.1. Previous isostructural P 3 X Fe N 2 RR catalysts reported by Peters and co-workers

Group 6 Homoleptic Arylisocyanides: Powerful Photoreductants

Protection of the metal center in M(CNAr)6 by addition of orthosterics results in lower photosubstitution quantum yields for M(CNDipp)6 versus M(CNPh)6 (M = Mo, W). Comparison of the photophysical properties of Ru(bpy)32+, fac-Ir(ppy)3 and the group 6 tris(diisocyanides) M(CNtBuAr3NC)3 (M = Cr, Mo).

Figure 1.5. (A) Tungsten(0) hexakis(arylisocyanides) studied by our group, (B) a simplified molecular orbital diagram depicting their electronic structure, and (C) a simplified Jablonski diagram showing the dynamics that lead to pop

Overview of Individual Chapters

Through systematic variation of ligand electronics, we demonstrate that simple substitutions in the CNDippCCAr platform provide a straightforward method by which to rationally modulate the ground and excited state properties of W(CNDippCCAR)6 complexes. Alternatively, the study of the photophysical properties of W(CN-1-(2-iPr)-Naph)6 reveals the potential benefits of using fused-ring arizocyanide ligands in the design of this class of photosensitizers, such as increased steric shielding and hardness in the tungsten center resulting in relatively long life.

A series of compounds P3XFe–N2[0/1−] (X = B, Al, Ga) have similar electronic structures and N2 activation levels as determined by spectroscopic, structural, electrochemical, and computational (DFT) studies. That the tris(phosphine)silyl ligand proves so effective for osmium-mediated N2RR catalysis but is largely ineffective for isostructural [Fe]–N2−.

Introduction

Given the great importance of the boron center, we became interested in further investigating the effect of the substitution of the apical X(III) atom in the P3XFe framework on the binding, electronic structure and activity of N2RR. To address this, we report here the synthesis and characterization of a series of P3XFe–Br and P3XFe–N2[0/1−] compounds supported by the analogous tris(phosphino)alane group 13 (P3Al = (o-iPr2PC6H4)3Al)21 and tris(phosphino)gallan (P3Ga = (o-. iPr2PC6H4)3Ga)22 ligands.

Results and Discussion

• Synthesis and Characterization of P 3 X Fe–N 2 [0/1−] Complexes
• Electronic Structure and Bonding in P 3 X Fe Systems
• N 2 RR Activity

In contrast, 4b and 4c crystallize in the trigonal space group R3, which are threefold symmetric about the apical axis defined by X–Fe–N2. The 200 mV anodic shift in the reduction potential of Fe–N2[0/1−] for 3b and 3c relative to 3a suggests that exchange of the apical Lewis acidic element in the P3X framework results in only modest electronic perturbations at the iron center.

Figure 2.1. Solid-state structures of 2b, 3b, and 4b with thermal ellipsoids set at 50% probability

Conclusion

N2 is known to react cleanly with H2 to generate (P3B)(µ-H)Fe(L)(H) (L = H2, N2),28 species that can re-enter the N2RR catalytic cycle.10 Treatment of P3AlFe- N2 and P3GaFe-N2 with H2 results in a mixture of products, including ligand decomposition, whose profile matches that observed at the end of catalyst speciation reactions (see experimental section). The above results suggest that P3AlFe and P3GaFe platforms are relatively robust under both sets of N2RR catalytic conditions explored. Although little catalyst decomposition/deactivation occurs, this does not correlate well with the lower N2RR efficiency of P3AlFe and P3GaFe versus P3BFe, or the higher NH3 yields obtained with [H2NPh2][OTf]/Cp* conditions 2Co.

Instead, our observations are more consistent with P3AlFe and P3GaFe exhibiting greater selectivity for HER versus N2RR. Bimolecular coupling of Fe=NNH2[1+/0] N2RR intermediates with weak N-H bonds is predicted to be an operatively unproductive HER pathway on the P3BFe system.12 In this regard, it is worth noting that Fe-imido (Fe ≡ N-R; R = absolute,17 p-methoxyphenyl14) and Fe-disilylhydrazido (Fe=NNSi2)15,25 complexes accessible and stable on the P3BFe and (AltraPhos)Fe platforms could not be isolated with P3Al. Preliminary reactivity studies confirm that P3AlFe and P3GaFe are relatively robust under the catalytic conditions, suggesting that the lower observed turnover numbers are likely the result of greater HER versus N2RR selectivity rather than differences in core structure stability.

Experimental Section

• Experimental Methods
• Synthetic Details and Characterization Data
• Ammonia Production and Quantification Studies
• Cyclic Voltammetry
• Miscellaneous Experiments
• X-Ray Data Tables
• DFT Calculations

After removing the Et2O in vacuo, the remaining yellow-orange residue was dissolved in toluene (15 mL) and cooled to -78 oC in a glove box. Lyophilization of the solution gave a green residue, which was washed with HMDSO (3 x 15 mL) on a coarse frit filled with celite. The Et2O was then evaporated to give a solid precatalyst layer at the bottom of the Schlenk tube.

The temperature of the system was allowed to equilibrate for 5 minutes and then the tube was sealed with a Teflon screw cap. This solution was allowed to freeze, then the headspace of the tube was evacuated and the tube was sealed. The volatiles from the reaction mixture were vacuum transferred into this collection flask at room temperature.

After the vacuum transfer was complete, the collection flask was sealed and warmed to room temperature. For 5b and 5c, Et2O filtrate analysis revealed the presence of both [M(solv)x][P3XFe-N2] and free phosphine resulting from ligand decompn.

Figure 2.4. 1 H NMR spectrum (400 MHz, C 6 D 6 , 25 °C) of P 3 Al (1b).

The tripodal tris(phosphine)hydrazido complexes P3EFe=NNH2+ are spectroscopically (E = B)15a and structurally characterized (E = Si),15b, generated via double protonation of P3EFe-N2-. Using the pre-cooled pipette, the acid solution was layered dropwise over the frozen solution of [K(THF)2][P3SiOs-N2]. Variable temperature NMR spectral analysis of the reaction of [K(THF)2][P3SiOs–N2] with 1 equiv of HBArF4 at –78 °C.

IR spectrum for the reaction of [K(THF)2][P3SiOs–N2] with 1 equiv of HBArF4; deposited as a thin film of THF-d8. General procedure for the variable temperature NMR spectral analysis of the reaction of P3SiM–N2 (M = Os, Ru, Fe) with 5 equiv Cp*2Co. Both compounds were dissolved in THF-d8 and then the Cp*2Co solution was transferred to the vial containing P3SiM-N2.

Catalytic Nitrogen-to-Ammonia Conversion by Osmium and Ruthenium

Introduction

The heavier group 8 elements Ru and Os have played a significant role in the history and development of ammonia synthesis and M-N2 model chemistry. For example, an active heterogeneous Ru catalyst is used in the Kellog Advanced Ammonia Process (KAAP),5 and the first metal dinitrogen complex discovered contained ruthenium ((NH3)5Ru–N22+).6 Achievements in Ru- and Os- model chemistry has included (1) demonstration of terminal nitride (M≡N[3/2+]) coupling to form (isolable or transient) N2-bridged bimetals of the type LnM–N2–MLn[6/5/4+] ( M . = Ru or Os);7,8 (2) photochemically induced homolytic N2 cleavage to generate terminal Os≡N products;9 (3) selective protonation and hydrogenation of terminal osmium nitrides to generate NH3;10 and (4 ) stepwise NH3 oxidation to N2 via diimide (HN=NH) and hydrazine (N2H4) intermediates by cofacial Ru porphyrins.11. Despite these advances, there are no examples of synthetic Ru or Os complexes that demonstrate stepwise protonation of M-N2 to generate M-NxHy species or that catalyze N2RR.12 The study of a homologous isostructural series of complexes (Fe, Ru, Os) will help delineate important factors for N2RR catalyst design.

A limitation regarding achieving N2RR catalysis by Ru or Os is that low-grade redox states (< 2+) can be difficult to access for these metals.13 In systems where such states are accessible, it is common for the electron(s) be ligand-localized rather than metal; this is especially true for d7 systems.14 For M-N2 species, this should in turn lead to a less activated and therefore less easily functionalized N2 ligand. A notable exception is [M]–N2[0/1−] redox pairs (M = Ru, Os; Figure 3.1) reported by our group, where the use of a rigid, chelating tris(phosphine)silyligand allows access to low- valent M(I) and M(0) terminal dinitrogen compounds.14a These species exhibit v(NN) stretching frequencies indicative of strongly activated, terminally bound N2 ligands. Furthermore, the N2 ligand of [Os]–N2− can be protonated to yield a structurally characterized [Os]=NNH2+ hydrazido complex, a likely intermediate by analogy to related [Fe]=NNH2+ species.

Figure 3.1. Ruthenium and osmium N 2 RR catalysts studied herein and the pertinent physical data from refs 14a and 18

Results and Discussion

In contrast, exposure of [Os]–N2− to stoichiometric HBArF4 in THF-d8 soln. at −78 °C gives a mixture of hydride products [Os](N2)(H) (major) and. Os]H3 (minor), as determined by variable-temperature NMR spectroscopy (Scheme 3.1).22 On heating, [Os]–N2 is also observed as a byproduct. Os]H3 can be synthesized independently by reacting [Os]–Cl with LiEt3BH or by exposing [Os](N2)(H) or [Os]–N2 to an H2 atmosphere.

We conclude that at the catalytically relevant temperature of -78 °C, Cp*2Co is able to reduce [Os]–N2 to [Os]–N2−, a step that may be entropically driven via formation of {[ U.S]- . Similarly, reaction of [Os]-N2− with 3 equiv produces HOTf by thawing 2-MeTHF (−135 °C) an orange mixture from which light orange {[Os]=NNH2}{OTf} can be isolated by precipitation (Scheme 3.2). Os]=NNH2+ represents the first case of an Os-N2 species to be converted to a protonated Os-NxHy product.

Table 3.1. N 2 RR mediated by [M]–N 2 −

Conclusion

Combined with the observation that Cp*2Co can reduce [Os]–N2 to [Os]–N2− at low temperature, and that [Os]–N2− can be protonated to obtain [Os]=NNH2+, the observation is that [Os]–N2− ]=NNH2+ can facilitate NH3 formation and support an N2RR electron transfer-proton transfer pathway that, at least initially, is via a distal route.

Experimental Section

• Experimental Methods
• Synthetic Details and Characterization Data
• Ammonia Production and Quantification Studies
• Miscellaneous Experiments
• X-Ray Data Tables

The reaction was allowed to stir at room temperature for 1 h before the solution volume was concentrated to 2 mL, quenched with pentane, and cooled to −30 °C. The contents of the vial were then allowed to freeze and equilibrate at 77 K for 10 min. The reaction was stirred vigorously for two days at room temperature, after which the color of the reaction had changed from brown to yellow.

The reaction was stirred vigorously for two days at room temperature, after which the color of the reaction changed from red-brown to yellow. Precatalyst (2.0 μmol) was weighed into a vial in a nitrogen filled precatalyst.* The precatalyst was then quantitatively transferred to a Schlenk tube as a suspension in Et2O. This tube was transferred from the box to a bath of liquid nitrogen and transferred to a fume hood.

This was allowed to cool in the refrigerator until the contents of the vial were frozen. The reaction was stirred at room temperature for 15 min before being filtered through glass filter paper into a J-Young NMR tube.

Figure 3.3. 1 H NMR spectrum (500 MHz, THF-d 8 , 25 °C) of [P 3 Si Os=NNH 2 ][OTf].

Third Generation W(CNAr) 6 Photosensitizers Supported by Fused-Ring and

Introduction

Results and Discussion

• Synthesis and Characterization of Alkynyl-Bridged and Fused-Ring Arylisocyanide
• Synthesis and Characterization of W(CNDipp CC Ar) 6 and W(CN-1-(2- i Pr)-Naph) 6
• Absorption and Steady-State Emission of W(CNDipp CC Ar) 6 and W(CN-1-(2- i Pr)-
• Excited-State Dynamics and Reduction Potentials of W(CNDipp CC Ar) 6 and W(CN-1-

Conclusion