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Poly(divinyloligocyclobutanes) (pDVOCBs) are a distinct class of butadiene-derived, chemically recyclable polyolefins prepared from iron-catalyzed [2 + 2] cycloaddition followed by ruthenium-catalyzed ADMET polymerization. These polymers are highly crystalline, with high melting temperatures and unique rotator phases. The high degree of crystallinity in these materials not only leads to high stiffness over a broad temperature range but also results in brittle failure for polymers of modest molecular weights. Iron-catalyzed cross-[2 + 2] cycloaddition of butadiene and α,ω-dienes was accomplished, where the number of methylene units between the four-membered rings was systematically varied. Subsequent ADMET polymerization of the resulting telechelic oligomers providedmore » a new class of chemically recyclable polyolefins that exhibit distinct thermomechanical properties. Specifically, methylene units induce changes in crystallinity that, in combination with the increased molecular weight, favored the formation of stable necks upon tensile deformation, with ∼10× enhanced ductility and improved toughness compared to pDVOCB. Through combined experimental and computational studies, the effects of methylene spacers on the crystal-to-rotator transition of the cyclobutane polymers were established. Ultimately, these methylene-modified cyclobutane polymers expand the range of properties and potential applications of this class of recyclable polyolefins.« less

Addition of Ph₂SiH₂ to [(depe)₂Mo(N)][BAr^F₄] (depe = 1,2-bis(diethylphosphino)ethane, BAr^F₄ = B(3,5-(CF₃)₂C₆H₃)₄) at 60 °C generated the silyl imido molybdenum hydride complex, trans- [(depe)₂Mo(NSiHPh₂)H][BAr^F₄], a surrogate for a proposed intermediate complex in the photodriven hydrogenation to free ammonia. Irradiation of a THF solution of trans-[(depe)₂Mo(NSiHPh₂)H]- [BAr^F₄] with blue light under H₂ produced free amine along with [(depe)₂MoH₅][BAr^F₄] in 76% yield. This transformation occurred in the absence of a precious metal photocatalyst, suggesting that it was needed only for the initial addition of H₂ to the molybdenum nitride during the first N−H bond-forming step in the photodriven hydrogenation. Deuterium labeling and crossovermore » studies support concerted Si−H bond addition across the Mo≡N bond, enabled by the nucleophilicity of the nitride. Subsequent hydrogenation involves an intramolecular H migration from Mo to the imido ligand, as supported by electronic absorption spectroscopy, transient absorption spectroscopy, initial rate measurements, and deuterium kinetic isotope effect measurements. These findings provide insights into the photodriven hydrogenation of [(depe)₂Mo(N)][BAr^F₄] to ammonia and the role of the photocatalyst in this transformation.« less

Reduction of (depe)₂CrCl₂ (depe = 1,2-bis- (diethylphosphino)ethane) and (dep-benz)₂CrCl₂ (dep-benz = 1,2-bis(diethylphosphino)benzene) under 1 atm of N₂ furnished the dinitrogen complexes (depe)₂Cr(N₂)₂ and (dep-benz)₂Cr(N₂)₂, respectively. One-electron oxidation of these products with FcBAr^F ₄ (Fc = ferrocenium, BAr^F ₄ = B(3,5-(CF₃)₂C₆H₃)₄) yielded the unusual, high-spin tetrahedral complexes [(depe)₂Cr][BAr^F ₄] and [(dep-benz)₂Cr][BAr^F ₄] with concomitant loss of dinitrogen. Reaction of the chromium(I) derivatives with Ph₃CN₃ furnished rare examples of chromium(IV) nitrides as confirmed spectroscopically and by X-ray crystallography. While [(depe)₂Cr(≡N)][BAr^F ₄] underwent association of isocyanides accompanied by partial ligand dissociation, neither chromium nitride was reactive toward H₂ or diphenylsilane under thermal ormore » photochemical conditions. These results distinguish the unique properties of the chromium(IV) nitrides as compared to heavier group 6 congeners and demonstrate both the feasibility of nitride synthesis and the limitations of dinitrogen cleavage and subsequent N−H bond formation.« less

Polyurethanes (PUs) are the sixth most commonly utilized plastic class, yet ∼80% of commodity material is landfilled or incinerated at the end of life. Disposal of thermosets is particularly problematic as cross-linking prevents the repurposing of disposed material. Thus, there is considerable interest in the development of PUs derived from inexpensive feedstocks that can be inherently chemically deconstructed. Ring opening metathesis polymerization (ROMP) of the naturally occurring sesquiterpene β-caryophyllene in the presence of dihydroxy chain terminators afforded the polyol hydroxy-terminated polycaryophyllene (HTPCR). Incorporation of HTPCR into PUs through reaction with polyisocyanates produced polymers with thermal and rheological properties comparable tomore » commodity materials. The feasibility of chemical degradation of both thermoplastic and thermoset materials was also demonstrated through ruthenium-mediated metathesis, utilizing the metathesis-active olefins within the repeat caryophyllene monomer unit. Overall, this work highlights the value of biorenewable, chemically reprocessable polysesquiterpenes in the PU space.« less

Poly(1,n′-divinyl)oligocyclobutane (pDVOCB) has emerged as a class of poly(cycloolefin) that is amenable to chemical recycling and demonstrates promising thermomechanical properties. However, their high melting temperatures coupled with insolubility makes melt processing challenging due to thermo-oxidation of internal alkenes. To address these issues, we describe a series of polymers incorporating modifications to the pDVOCB backbone and analyze the effects on material stability and processability. Intentional migration of the internal 1,2-disubstituted alkenes to an exocyclic trisubstituted position yields isomerized pDVOCB (IpDVOCB) which exhibits a depression of thermal transitions by up to 50 °C. Conversely, elimination of stereoirregularity between enchained DVOCB oligomers throughmore » alkene saturation yields hydrogenated pDVOCB (HpDVOCB), resulting in elevated thermal transitions by up to 30 °C. Furthermore, these shifts are attributed to changes in crystal defect density which is strongly influenced by chain stereoregularity. Understanding these behaviors guides future polymer design and expands the control and use of this new class of recyclable poly(cycloolefin)s.« less

Oxidation of the Chatt-type tungsten dinitrogen compound, trans-(depe)₂W(N₂)₂ (depe = Et₂PCH₂CH₂PEt₂), with [(η⁵-C₅H₅)₂Fe][BAr^F₄] (BAr^F4 = B(3,5-(CF₃)₂C₆H₃)₄) resulted in isolation of [(depe)₂WN][BAr^F₄], a rare example of a tungsten(IV) nitride prepared from N₂ cleavage. A bimetallic μ-N₂ ditungsten intermediate supported by terminal N₂ ligands was identified, and irradiation with visible light promoted dinitrogen cleavage and formation of [(depe)₂WN][BAr^F₄]. Performing the analogous one-electron oxidation of the related tungsten dinitrogen compound, trans-(dppe)₂W(N₂)₂ (dppe = Ph₂PCH₂CH₂PPh₂), furnished the corresponding cationic, 17-electron tungsten dinitrogen complex, [(dppe)₂W(N₂)₂][BAr^F₄], that was characterized by X-ray diffraction and vibrational and EPR spectroscopies. The generation of [(dppe)₂W(N)][BAr^F₄] was observed in low yieldmore » from the in situ formed mixed N₂-bridged compound, [(N₂)(depe)₂W(μ-N₂)W(dppe)₂(N₂)][BAr^F₄]₂, and was confirmed by independent synthesis using 1-azidoadamantane. Addition of ammonia or water to [(depe)₂WN][BAr^F₄] resulted in formation of the cationic imide and hydroxide complexes, [(depe)₂W(NH)(X)][BAr^F₄] (X = NH₂, OH). Irradiation of [(depe)₂WN][BAr^F₄] with 440 nm visible light in the presence of Ir(ppy)₃ (ppy = 2-phenylpyridine) under 4 atm of dihydrogen resulted in hydrogenation of the tungsten nitride to the cationic tungsten pentahydride, [(depe)₂WH₅][BAr^F₄], with the release of free ammonia in 21% yield, a rare example of ammonia generation from dinitrogen and dihydrogen from a well-defined tungsten nitride.« less

Here, the synthesis of ammonia from its elements, N₂ and H₂, is the most atom-economical and thermodynamically preferred route but presents a high kinetic barrier and thus is rare using molecular compounds. Irradiation of a molecular molybdenum nitride prepared from N₂ cleavage with visible light in the presence of an iridium photocatalyst and 1–4 atm of H₂ produced high yields of ammonia along with the formation of a cationic, formally molybdenum(VI) pentahydride as the major molybdenum-containing product. Continued irradiation of the molybdenum hydride under an N₂ atmosphere resulted in regeneration of the molybdenum nitride that was recycled and used formore » additional hydrogenation to generate more ammonia, demonstrating superstoichiometric batch ammonia synthesis using only N₂ and H₂ with molecular compounds under ambient conditions.« less

Ammonia synthesis from N,N,O,O-supported manganese(V) nitrides and 9,10-dihydroacridine using proton-coupled electron transfer and visible light irradiation in the absence of precious metal photocatalysts is described. While the reactivity of the nitride correlated with increased absorption of blue light, excited-state lifetimes determined by transient absorption were on the order of picoseconds. Furthermore, this eliminated excited-state manganese nitrides as responsible for bimolecular N–H bond formation. Spectroscopic measurements on the hydrogen source, dihydroacridine, demonstrated that photooxidation of 9,10-dihydroacridine was necessary for productive ammonia synthesis. Transient absorption and pulse radiolysis data for dihydroacridine provided evidence for the presence of intermediates with weak E–H bonds,more » including the dihydroacridinium radical cation and both isomers of the monohydroacridine radical, but notably these intermediates were unreactive toward hydrogen atom transfer and net N–H bond formation. Additional optimization of the reaction conditions using higher photon flux resulted in higher rates of the ammonia production from the manganese(V) nitrides due to increased activation of the dihydroacridine.« less

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