Rapid Polyolefin Hydrogenolysis by a Single-Site Organo-Tantalum Catalyst on a Super-Acidic Support: Structure and Mechanism.

The novel electrophilic organo-tantalum catalyst AlS/TaNpx (1) (Np=neopentyl) is prepared by chemisorption of the alkylidene Np3 Ta=CHt Bu onto highly Brønsted acidic sulfated alumina (AlS). The proposed catalyst structure is supported by EXAFS, XANES, ICP, DRIFTS, elemental analysis, and SSNMR measurements and is in good agreement with DFT analysis. Catalyst 1 is highly effective for the hydrogenolysis of diverse linear and branched hydrocarbons, ranging from C2 to polyolefins. To the best of our knowledge, 1 exhibits one of the highest polyolefin hydrogenolysis activities (9,800 (CH2 units) ⋅ mol(Ta)-1 ⋅ h-1 at 200 °C/17 atm H2 ) reported to date in the peer-reviewed literature. Unlike the AlS/ZrNp2 analog, the Ta catalyst is more thermally stable and offers multiple potential C-C bond activation pathways. For hydrogenolysis, AlS/TaNpx is effective for a wide variety of pre- and post-consumer polyolefin plastics and is not significantly deactivated by standard polyolefin additives at typical industrial concentrations.

Single-Site Carbon-Supported Metal-Oxo Complexes in Heterogeneous Catalysis: Structure, Reactivity, and Mechanism.

When early transition metal complexes are molecularly grafted onto catalyst supports, well-defined, surface-bound species are created, which are highly active and selective single-site heterogeneous catalysts (SSHCs) for diverse chemical transformations. In this minireview, we analyze and summarize a less conventional type of SSHC in which molybdenum dioxo species are grafted onto unusual carbon-unsaturated scaffolds, such as activated carbon, reduced graphene oxide, and carbon nanohorns. The choice of earth-abundant, low-toxicity, versatile metal constituents, and various carbon supports illustrates "catalyst by design" principles and yields insights into new catalytic systems of both academic and technological interest. Here, we summarize experimental and computational investigations of the bonding, electronic structure, reaction scope, and mechanistic pathways of these unusual catalysts.

Influence of Rare-Earth Ion Radius on Metal-Metal Charge Transfer in Trinuclear Mixed-Valent Complexes.

We report the synthesis and characterization of a highly conjugated bisferrocenyl pyrrolediimine ligand, Fc2PyrDIH (1), and its trinuclear complexes with rare earth ions─(Fc2PyrDI)M(N(TMS)2)2 (2-M, M = Sc, Y, Lu, La). Crystal structures, nuclear magnetic resonance (NMR) spectra, and ultraviolet/visible/near-infrared (UV/vis-NIR) data are presented. The latter are in good agreement with DFT calculations, illuminating the impact of the rare earth ionic radius on NIR charge transfer excitations. For [2-Sc]+, the charge transfer is at 11,500 cm-1, while for [2-Y]+, only a d-d transition at 8000 cm-1 is observed. Lu has an ionic radius in between Sc and Y, and the [2-Lu]+ complex exhibits both transitions. From time-dependent density functional theory (TDDFT) analysis, we assign the 11,500 cm-1 transition as a mixture of metal-to-ligand charge transfer (MLCT) and metal-to-metal charge transfer (MMCT), rather than pure metal-to-metal CT because it has significant ligand character. Typically, the ferrocenes moieties have high rotational freedom in bis-ferrocenyl mixed valent complexes. However, in the present (Fc2PyrDI)M(N(TMS)2)2 complexes, ligand-ligand repulsions lock the rotational freedom so that rare-earth ionic radius-dependent geometric differences increasingly influence orbital overlap as the ionic radius falls. The Marcus-Hush coupling constant HAB trends as [2-Sc]+ > [2-Lu]+ > [2-Y]+.

Mechanistic study of homoleptic trisamidolanthanide-catalyzed aldehyde and ketone hydroboration. Chemically non-innocent ligand participation.

Carbonyl bond hydroboration is a valuable synthetic route to functionalized alcohols but relies on sometimes unselective and sluggish reagents. While rapid and selective aldehyde and ketone hydroboration mediated by trisamidolanthanide catalysts is known, the origin of the selectivity is not well-understood and is the subject of this contribution. Here the aldehyde and ketone HBpin hydroboration reaction mechanisms catalyzed by La[N(SiMe3)2]3 are investigated both experimentally and theoretically. The results support initial carbonyl oxygen coordination to the acidic La center, followed by intramolecular ligand-assisted hydroboration of the carbonyl moiety by bound HBpin. Interestingly, ketone hydroboration has a higher energetic barrier than that of aldehydes due to the increased steric encumbrance and decreased electrophilicity. Utilizing NMR spectroscopy and X-ray diffraction, a bidentate acylamino lanthanide complex associated with the aldehyde hydroboration is isolated and characterized, consistent with the relative reaction rates. Furthermore, an aminomonoboronate-lanthanide complex produced when the La catalyst is exposed to excess HBpin is isolated and characterized by X-ray diffraction, illuminating unusual aminomonoboronate coordination. These results shed new light on the origin of the catalytic activity patterns, reveal a unique ligand-assisted hydroboration pathway, and uncover previously unknown catalyst deactivation pathways.

Selective Lanthanide-Organic Catalyzed Depolymerization of Nylon-6 to ϵ-Caprolactam.

Nylon-6 is selectively depolymerized to the parent monomer ϵ-caprolactam by the readily accessible and commercially available lanthanide trisamido catalysts Ln(N(TMS)2 )3 (Ln=lanthanide). The depolymerization process is solvent-free, near quantitative, highly selective, and operates at the lowest Nylon-6 to ϵ-caprolactam depolymerization temperature reported to date. The catalytic activity of the different lanthanide trisamides scales with the Ln3+ ionic radius, and this process is effective with post-consumer Nylon-6 as well as with Nylon-6+polyethylene, polypropylene or polyethylene terephthalate mixtures. Experimental kinetic data and theoretical (DFT) mechanistic analyses suggest initial deprotonation of a Nylon terminal amido N-H bond, which covalently binds the catalyst to the polymer, followed by a chain-end back-biting process in which ϵ-caprolactam units are sequentially extruded from the chain end.

Rapid atom-efficient polyolefin plastics hydrogenolysis mediated by a well-defined single-site electrophilic/cationic organo-zirconium catalyst.

Polyolefins comprise a major fraction of single-use plastics, yet their catalytic deconstruction/recycling has proven challenging due to their inert saturated hydrocarbon connectivities. Here a very electrophilic, formally cationic earth-abundant single-site organozirconium catalyst chemisorbed on a highly Brønsted acidic sulfated alumina support and characterized by a broad array of experimental and theoretical techniques, is shown to mediate the rapid hydrogenolytic cleavage of molecular and macromolecular saturated hydrocarbons under mild conditions, with catalytic onset as low as 90 °C/0.5 atm H2 with 0.02 mol% catalyst loading. For polyethylene, quantitative hydrogenolysis to light hydrocarbons proceeds within 48 min with an activity of > 4000 mol(CH2 units)·mol(Zr)-1·h-1 at 200 °C/2 atm H2 pressure. Under similar solventless conditions, polyethylene-co-1-octene, isotactic polypropylene, and a post-consumer food container cap are rapidly hydrogenolyzed to low molecular mass hydrocarbons. Regarding mechanism, theory and experiment identify a turnover-limiting C-C scission pathway involving ß-alkyl transfer rather than the more common σ-bond metathesis.

Chemodivergent Organolanthanide-Catalyzed C-H α-Mono-Borylation of Pyridines.

Chemodivergent synthetic methodologies enable the efficient introduction of structural diversity into high-value organic products via simple chemical alterations. In this regard, C-H activation and functionalization of pyridinoid azines are important transformations in the synthesis of many natural products, pharmaceuticals, and functional materials. Reflecting on azinyl nitrogen lone-pair steric repulsion, its tendency to irreversibly coordinate metal ion catalysts, and the electron deficiency of pyridine, C-H functionalization at the important α-position remains challenging. Thus, developing earth-abundant catalysts for α-selective azine mono-functionalization is an attractive target for chemical synthesis. Here, the selective organolanthanide-catalyzed α-mono-borylation of a diverse series of 18 pyridines is reported using Cp*2LuCH(TMS)2 (Cp* = η5-C5Me5) (TMS = SiMe3) and affording valuable precursors for subsequent functionalization. Experimental and theoretical mechanistic data reported here support the intermediacy of a C-H-activated η2-lanthanide-azine complex, followed by intermolecular α-mono-borylation via σ-bond metathesis. Notably, varying the lanthanide identity and substrate substituent electronic character promotes marked chemodivergence of the catalytic selectivity: smaller/more electrophilic lanthanide3+ ions and electron-rich substrates favor selective α-C-H functionalization, whereas larger/less electrophilic lanthanide3+ ions and electron-poor substrates favor selective B-N bond-forming 1,2-dearomatization. Such lanthanide series catalytic chemodivergence is, to our knowledge, unprecedented.

Efficient Polyester Hydrogenolytic Deconstruction via Tandem Catalysis.

Using a mechanism-based solvent-free tandem catalytic approach, commodity polyester plastics such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN) are rapidly and selectively deconstructed by combining the two air- and moisture-stable catalysts, Hf(OTf)4 and Pd/C, under 1 atm H2 , affording terephthalic acid (or naphthalene dicarboxylic acid for PEN) and ethane (or butane for PBT) in essentially quantitative yield. This process is effective for both laboratory grade and waste plastics, and comingled polypropylene remains unchanged. Combined experimental and DFT mechanistic analyses indicate that Hf(OTf)4 catalyzes a mildly exergonic retro-hydroalkoxylation reaction in which an alkoxy C-O bond is first cleaved, yielding a carboxylic acid and alkene, and this process is closely coupled to an exergonic olefin hydrogenation step, driving the overall reaction forward.

Synthesis and Structure-Activity Characterization of a Single-Site MoO2 Catalytic Center Anchored on Reduced Graphene Oxide.

Molecularly derived single-site heterogeneous catalysts can bridge the understanding and performance gaps between conventional homogeneous and heterogeneous catalysis, guiding the rational design of next-generation catalysts. While impressive advances have been made with well-defined oxide supports, the structural complexity of other supports and the nature of the grafted surface species present an intriguing challenge. In this study, single-site Mo(═O)2 species grafted onto reduced graphene oxide (rGO/MoO2) are characterized by XPS, DRIFTS, powder XRD, N2 physisorption, NH3-TPD, aqueous contact angle, active site poisoning assay, Mo EXAFS, model compound single-crystal XRD, DFT, and catalytic performance. NH3-TPD reveals that the anchored MoO2 moiety is not strongly acidic, while Mo 3d5/2 XPS assigns the oxidation state as Mo(VI), and XRD shows little rGO periodicity change on MoO2 grafting. Contact angle analysis shows that MoO2 grafting consumes rGO surface polar groups, yielding a more hydrophobic surface. The rGO/MoO2 DRIFTS assigns features at 959 and 927 cm-1 to the symmetric and antisymmetric Mo═O stretching modes, respectively, of an isolated cis-(O═Mo═O) moiety, in agreement with DFT computation. Moreover, the Mo EXAFS rGO/MoO2 structural data are consistent with isolated (C-O)2-Mo(═O)2 species having two Mo═O bonds and two Mo-O bonds at distances of 1.69(3) and 1.90(3) Å, respectively. rGO/MoO2 is also more active than the previously reported AC/MoO2 catalyst, with reductive carbonyl coupling TOFs approaching 1.81 × 103 h-1. rGO/MoO2 is environmentally robust and multiply recyclable with 69 ± 2% of the Mo sites catalytically significant. Overall, rGO/MoO2 is a structurally well-defined and versatile single-site Mo(VI) dioxo heterogeneous catalytic system.

A High Yield Synthesis of an Octastannacubane and a Bis(silyl) Stannylene via Reductive Elimination of a Silane.

Thermolysis of tris(silyl) tin hydride 2 at 70 °C for 3 hours results in elimination of tBu2 MeSiH and generation of bis(silyl) stannylene 3 which dimerizes instantaneously yielding distannene 4. Compound 3 can be trapped by NHCMe yielding stannylene-NHCMe complex 5. Upon heating (70 °C, 24 h) 4 yields stannyl radical 8 along with pentastannatricyclo[2.1.0.02, 5 ]pentane 10 (ca. 30 %) and traces (ca. 5 %) of the novel octastannacubane 9. Remarkably, octastannacubane 9 is produced in 70 % yield by mild heating (50 °C) of 1,1,2,2-tetrasilyldistannane 11, along with tBu2 MeSiH. Octastannacubane 9 was characterized by X-ray crystallography, NMR and UV/Vis spectroscopy. Based on DFT quantum-mechanical calculations the 11 → 9 transformation occurs via reductive elimination of two tBu2 MeSiH molecules from 11 yielding a distannyne, (or its bis-stannylene isomer), followed by its tetramerization.

Synthesis of Silyl Aluminum Reagents: Relevance Toward Atomic Layer Deposition of Metal Silicides and the Serendipitous Synthesis of a Novel Al-Hydride Cluster.

The novel mono-silyl [(R3Si)AlX2]2, di-silyl [(R3Si)2AlX]2, tri-silyl (R3Si)3Al·Et2O, and -ate-complex [(R3Si)4Al]-·Li+(Et2O)2 have been synthesized by reaction of AlX3 (X = Cl, Br) with silyl lithium reagents (tBuMe2SiLi, Et3SiLi) in Et2O. Treatment of these compounds with Me3N yields the corresponding amine-coordinated silyl aluminum complexes (R3Si)AlX2·NMe3, (R3Si)2AlX·NMe3, and (R3Si)3Al·NMe3. An intramolecular amine-coordinated mono-silyl aluminum complex Me2N(CH2)3(tBuMe2Si)2SiAlCl2 was prepared by the reaction of Me2N(CH2)3(tBuMe2Si)2SiLi with AlCl3 in Et2O. In addition, reaction of [(tBuMe2Si)2AlBr]2 with LiAlH4 yields the novel aluminum hydride cluster [(tBuMe2Si)2Al(µ-H)AlH3]6 which upon addition of TMEDA yields the ion pair [((tBuMe2Si)2AlH)2(µ-H)]-[AlH2(TMEDA)2]+. The amine-coordinated di- and tri-silyl aluminum complexes possess higher thermal stability than the analogous etherate complexes and are reasonably volatile (100-140 °C, 0.2 Torr). The materials presented herein were analyzed via thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) to assess their viability as potential ALD precursors.

Bis-Ferrocenyl-Pyridinediimine Trinuclear Mixed-Valent Complexes with Metal-Binding Dependent Electronic Coupling: Synthesis, Structures, and Redox-Spectroscopic Characterization.

A family of metal dichloride complexes having a bis-ferrocenyl-substituted pyridinediimine ligand was systematically synthesized ((Fc2PDI)MCl2, M = Mg, Zn, Fe, and Co) and characterized crystallographically, spectroscopically, electrochemically, and computationally. Electronic coupling between the ligand ferrocene units is switched on upon binding to a MCl2 fragment, as evidenced by both sequential oxidation of the ferrocenes in cyclic voltammetry (ΔEox ≈ 200 mV) and by Inter-Valence Charge Transfer electronic excitations in the near IR. Additionally, UV-vis spectra are used to directly observe orbital mixing between the ferrocenyl units and the imine π system since breaking of the orbital symmetry results in allowed transitions (ϵ = 2800 M-1cm-1 vs ϵ ≈ 200 M-1cm-1 in free ferrocene) as well as broadening and red-shifting of the ferrocenyl transitions-indicating organic character in formerly pure metal-centered transitions. DFT analysis reveals that interaction between the ferrocenes and the MCl2 fragment is small and suggests that communication is mediated by better energy matching between the ferrocene and organic π* orbitals upon coordination, allowing superexchange coupling through the LUMO. Furthermore, single crystal diffraction data obtained from oxidation of one and both ferrocenes show distortions, introducing the empty dxy/dx2-y2 orbitals into the secondary coordination sphere of the MCl2 fragment. Such structural rearrangements are infrequent in ferrocenyl mixed-valent compounds, and implications for catalysis as well as electronic communication are discussed.

Polyethylene Terephthalate Deconstruction Catalyzed by a Carbon-Supported Single-Site Molybdenum-Dioxo Complex.

Polyethylene terephthalate (PET) is selectively depolymerized by a carbon-supported single-site molybdenum-dioxo catalyst to terephthalic acid (PTA) and ethylene. The solventless reactions are most efficient under 1 atmosphere of H2 . The catalyst exhibits high stability and can be recycled multiple times without loss of activity. Waste beverage bottle PET or a PET + polypropylene (PP) mixture (simulating the bottle + cap) proceeds at 260 °C with complete PET deconstruction and quantitative PTA isolation. Mechanistic studies with a model diester, 1,2-ethanediol dibenzoate, suggest the reaction proceeds by initial retro-hydroalkoxylation/ß-C-O scission and subsequent hydrogenolysis of the vinyl benzoate intermediate.

The Reactions of Carbon Monoxide with Silyl and Silenyl Lithium - Synthesis and Isolation of the First Stable Tetra-Silyl Di-Ketyl Biradical and 1-Silaallenolate Lithium.

Reactions of carbon monoxide (CO) with tBu2 MeSiLi and (E)-(tBu2 MeSi)(tBuMe2 Si)C=Si(SiMetBu2 )Li⋅2 THF (4) were studied both experimentally and computationally. Reaction of tBu2 MeSiLi with CO in hexane yields the first stable tetra-silyl di-ketyl biradical [(tBu2 MeSi)2 COLi]. 2 (3). Reaction of 4 with CO yields selectively and quantitatively the first reported 1-silaallenolate, (tBu2 MeSi)(tBuMe2 Si)C=C=Si(SiMetBu2 )OLi⋅THF (5). Both 3 and 5 were characterized by X-ray crystallography and biradical 3 also by EPR spectroscopy. Silaallenolate 5 reacts with Me3 SiCl to produce siloxy substituted 1-silaallene (tBu2 MeSi)(tBuMe2 Si)C=C=Si(SiMetBu2 )OSiMe3 . The reaction of 4 with CO provides a new route to 1-silaallenes. The mechanisms of the reactions of tBuMe2 SiLi and of 4 with CO were studied by DFT calculations.

Generation and EPR Spectroscopy of the First Silenyl Radicals, R2 C=Si. -R: Experiment and Theory.

The first two persistent silenyl radicals (R2 C=Si. -R), with a half-life (t1/2 ) of about 30 min, were generated and characterized by electron paramagnetic resonance (EPR) spectroscopy. The large hyperfine coupling constants (hfccs) (a(29 Siα )=137.5-148.0 G) indicate that the unpaired electron has substantial s character. DFT calculations, which are in good agreement with the experimentally observed hfccs, predict a strongly bent structure (∡C=Si-R=134.7-140.7°). In contrast, the analogous vinyl radical, R2 C=C. -R (t1/2 ≈3 h), exhibits a small hfcc (a(13 Cα )=26.6 G) and has a nearly linear geometry (∡C=C-R=168.7°).

Generation and Characterization of the First Persistent Platinum(I)-Centered Radical.

The first persistent platinum(I)-centered radical was generated by homolytic cleavage of a Pt-HgSiR3 bond of a mercury-substituted platinum(II) complex. The PtI radical was characterized by EPR spectroscopy, chemical trapping experiments, and density functional theory (DFT) calculations.

Convenient Synthesis of Deuterosilanes by Direct H/D Exchange Mediated by Easily Accessible Pt(0) Complexes.

Easily accessible, simple phosphino-platinum(0) complexes catalyze (0.1-1 mol % equivalent) the deuteration of silanes in good yields under mild conditions (60 °C, 1 atm). The catalysis is mediated by platinum(II) deuteride/hydride complexes that are in equilibrium with the precursor Pt(0) complexes. The Pt(II) complexes can also be inserted into the Si-H bond of silanes to give intermediate Pt(IV) complexes. The proposed mechanism for catalysis is supported by density functional theory calculations.

Activation of Homolytic Si-Zn and Si-Hg Bond Cleavage, Mediated by a Pt(0) Complex, via Novel Pt-Zn and Pt-Hg Compounds.

The thermally stable [(tBuMe2 Si)2 M] (M=Zn, Hg) generate R3 Si(.) radicals in the presence of [(dmpe)Pt(PEt3 )2 ] at 60-80 °C. The reaction proceeds via hexacoordinate Pt complexes, (M=Zn (2 a and 2 b), M=Hg (3 a and 3 b)) which were isolated and characterized. Mild warming or photolysis of 2 or 3 lead to homolytic dissociation of the Pt-MSiR3 bond generating silyl radicals and novel unstable pentacoordinate platinum paramagnetic complexes (M=Zn (5), Hg (6)) whose structures were determined by EPR spectroscopy and DFT calculations.