Radical Scission of Symmetrical 1,4-Dicarbonyl Compounds: C−C Bond Cleavage with Titanium(IV) Enolate Formation and Related Reactions

Visible Light-Driven Cascade Carbon-Carbon Bond Scission for Organic Transformations and Plastics Recycling

Significant efforts are devoted to developing artificial photosynthetic systems to produce fuels and chemicals in order to cope with the exacerbating energy and environmental crises in the world now. Nonetheless, the large-scale reactions that are the focus of the artificial photosynthesis community, such as water splitting, are thus far not economically viable, owing to the existing, cheaper alternatives to the gaseous hydrogen and oxygen products. As a potential substitute for water oxidation, here, a unique, visible light-driven oxygenation of carbon-carbon bonds for the selective transformation of 32 unactivated alcohols, mediated by a vanadium photocatalyst under ambient, atmospheric conditions is presented. Furthermore, since the initial alcohol products remain as substrates, an unprecedented photodriven cascade carbon-carbon bond cleavage of macromolecules can be performed. Accordingly, hydroxyl-terminated polymers such as polyethylene glycol, its block co-polymer with polycaprolactone, and even the non-biodegradable polyethylene can be repurposed into fuels and chemical feedstocks, such as formic acid and methyl formate. Thus, a distinctive approach is presented to integrate the benefits of photoredox catalysis into environmental remediation and artificial photosynthesis.

Synthesis and characterization of poly(ethene–ketone–arylene–ketone)s containing pendant methylthio groups via metal-free catalyzed copolymerization of aryldiynes with DMSO

A metal-free copolymerization of aryldiynes with DMSO as a monomer gave polyeneketones in good yields and high molecular weight.

The titanium tris-anilide cation [Ti(N[tBu]Ar)3]+ stabilized as its perfluoro-tetra-phenylborate salt: structural characterization and synthesis in connection with redox activity of 4,4′-bipyridine dititanium complexes

A rare cationic d⁰ metal tris-amide complex, containing an intriguing pyramidal TiN₃ core geometry, namely {Ti(N[^tBu]Ar)₃}⁺, is isolated as its [B(C₆F₅)₄]⁻ salt.

Toward the understanding of radical reactions: experimental and computational studies of titanium(III) diamine bis(phenolate) complexes

Radical reactions of titanium(III) [Ti((tBu2)O2NN')Cl(S)] (S = THF, 1; S = py, 2; (tBu2)O2NN' = Me2N(CH2)2N(CH2-2-O-3,5-(t)Bu2C6H2)2) are described. Reactions with neutral electron acceptors led to metal oxidation to Ti(IV), [Ti((tBu2)O2NN')Cl(TEMPO)] (4) being formed with the TEMPO radical and [Ti((tBu2)O2NN')Cl2] (9) with PhN═NPh. [Ti((tBu2)O2NN')Cl2] was also formed when [Ti((tBu2)O2NN')Cl(S)] was oxidized by [Cp2Fe][BPh4], but the [Cp2Fe][PF6] analogue yielded [Ti((tBu2)O2NN')ClF] (8). The reactions of [Ti((tBu2)O2NN')Cl(S)] with O2 gave [Ti((tBu2)O2NN')Cl]2(μ-O) (3). The DFT calculated Gibbs energy for the above reaction showed it to be exergonic (ΔG298 = -123.6 kcal·mol(-1)). [Ti((tBu2)O2NN')(CH2Ph)(S)] (S = THF, 5; py, 6) are not stable in solution for long periods and in diethyl ether gave 1:1 cocrystals of [Ti((tBu2)O2NN')(CH2Ph)2] (7) and [Ti((tBu2)O2NN')Cl]2(μ-O) (3), most probably resulting from a disproportionation process of titanium(III) followed by oxygen abstraction by the resulting Ti(II) species. The oxidation of [Ti((tBu2)O2NN')(κ(2)-{CH2-2-(NMe2)-C6H4})] (10), which is a Ti(III) benzyl stabilized by the intramolecular coordination of the NMe2 moiety, led to a complex mixture. Recrystallization of this mixture under air led to a 1:1 cocrystal of two coordination isomers of the titanium oxo dimer (3). In one of these isomers, one metal is pentacoordinate and the dimethylamine moiety of the diamine bis(phenolate) ligand is not bonded to the metal, displaying a coordination mode of the ligand never observed before. The other titanium center is distorted octahedral with two cis-phenolate moieties. In the second unit, the coordination of the two ancillary ligands to the titanium centers reveals mutually cis-phenolate groups in one-half of the molecule and trans-coordinated in the other titanium center, keeping a distorted octahedral environment around each titanium.

Synthesis, Characterization, and Reactions of Isolable (β-Diketiminato)Nb(III) Imido Complexes

We have investigated both the chemical reduction of (BDI)Nb(V) imido complexes (BDI = HC[C(Me)NAr](2); Ar = 2,6-(i)Pr(2)-C(6)H(3)) to the formal Nb(III) oxidation state and the ability of these Nb(III) complexes to behave as two-electron reductants. The reduction of the Nb(V) species was found to depend heavily on the nature of available supporting ligands, but the chemistry of the reduced compounds proceeded cleanly with a number of unsaturated organic reagents. Accordingly, the novel Nb(V) bis(imido) complexes supported by the monoazabutadiene (mad) ligand (mad)Nb(N(t)Bu)(NAr)(L') (L' = py, thf) were formed by either KC(8) reduction of (BDI)Nb(N(t)Bu)Cl(2)(py) in the absence of strong π-acids or by H(2) reduction of the Nb(V) dimethyl complex (BDI)Nb(N(t)Bu)Me(2) in THF. These products are likely formed though an intramolecular, 2 e(-) reductive C-N bond cleavage, as has been observed previously for related Group 4 systems, suggesting that transient Nb(III) intermediates were present in both cases. In the presence of 1,2-bis(dimethylphosphino)ethane (dmpe), KC(8) reduction of (BDI)Nb(N(t)Bu)Cl(2)(py) was arrested at the Nb(IV) oxidation state to give (BDI)Nb(N(t)Bu)Cl(dmpe), which was characterized by solution-state EPR spectroscopy as a Nb-centered paramagnet with strong coupling to the two equivalent phosphorus nuclei (A(iso){(93)Nb} = 120.5×10(-4) cm(-1), A(iso){(31)P} = 31.0×10(-4) cm(-1), g(iso) = 1.9815). When strong π-acids were used to intercept the thermally unstable Nb(III) complex (BDI)Nb(N(t)Bu)(py) prior to reductive cleavage of the ligand C-N bond, the thermally stable Nb(III) species (BDI)Nb(N(t)Bu)(CX)(2)(L″) (X = O, L″ = py; X = NXyl, L″ = CNXyl; Xyl = 2,6-Me(2)-C(6)H(3)) were obtained in good yields. The Nb(III) complexes (BDI)Nb(N(t)Bu)py, (BDI)Nb(N(t)Bu)(CO)(2)(py) and (BDI)Nb(N(t)Bu)(CO)(2) were subsequently investigated for their ability to serve as two-electron reducing reagents for both metal-ligand multiple bond formation and for the reduction of organic π-systems. The reduction of mesityl azide by (BDI)Nb(N(t)Bu)(py) and diphenylsulfoxide by (BDI)Nb(N(t)Bu)(CO)(2) led to the monomeric bis(imido) and dimeric oxo complexes (BDI)Nb(N(t)Bu)(NMes)(py) and [(BDI)Nb(N(t)Bu)](2)(μ2-O)(2), respectively. MeLi addition to (BDI)Nb(N(t)Bu)(CO)(2)(py) resulted in the formation of a Nb-acylate via methide addition to one of the carbonyl carbons. The acylate product was revealed to have a short Nb-C(acylate) bond distance (2.059(4) Å), consistent with multiple Nb-C bond character resulting from Nb(III) back-bonding into the acylate carbon. The interaction of (BDI)Nb(N(t)Bu)(CO)(2) with two equivalents of 4,4'-dichlorobenzophenone resulted in the clean, quantitative formation of the corresponding pinacol coupling product, but introduction of the ketone in 1: 1 molar ratios resulted in mixtures of the pinacol product and the starting material, suggesting that ketone coordination to the Nb(III) complex may be reversible. Relatedly, addition of 1-phenyl-1-propyne to (BDI)Nb(N(t)Bu)(CO)(2) formed a thermally unstable 1: 1 Nb/alkyne complex, as characterized by NMR and IR spectroscopies; reaction of this species with HCl/MeOH yielded a 2: 1 mixture of 1-phenyl-1-propene and the free alkyne, suggesting a high degree of covalency in the Nb-C bonds.

Fluorinated diarylamido complexes of lithium, zirconium, and hafnium

Deprotonation of N-(2-fluorophenyl)-2,6-diisopropylaniline (H[ (i) PrAr-NF]) with 1 equiv of n-BuLi in toluene at -35 degrees C produced cleanly [ (i) PrAr-NF]Li. Subsequent recrystallization of [ (i) PrAr-NF]Li in diethyl ether generated the bis(ether) adduct [ (i) PrAr-NF]Li(OEt 2) 2. An X-ray study of [ (i) PrAr-NF]Li(OEt 2) 2 showed it to be a four-coordinate species with the coordination of the fluorine atom to the lithium center. The reactions of [ (i) PrAr-NF]Li with MCl 4(THF) 2 (M = Zr, Hf), regardless of the stoichiometry employed, afforded the corresponding dichloride complexes [ (i) PrAr-NF] 2MCl 2 (M = Zr, Hf). Alkylation of [ (i) PrAr-NF] 2MCl 2 with a variety of Grignard reagents generated [ (i) PrAr-NF] 2MR 2 (M = Zr, Hf; R = Me, i-Bu, CH 2Ph). The X-ray structures of [ (i) PrAr-NF] 2ZrCl 2, [ (i) PrAr-NF] 2HfCl 2, [ (i) PrAr-NF] 2ZrMe 2, [ (i) PrAr-NF] 2Zr( i-Bu) 2, and [ (i) PrAr-NF] 2Hf(CH 2Ph) 2 are all indicative of the coordination of the fluorine atoms to these group 4 metals, leading to a C 2-symmetric, distorted octahedral geometry for these molecules.

Terminal, anionic carbide, nitride, and phosphide transition-metal complexes as synthetic entries to low-coordinate phosphorus derivatives

Anionic terminal one-atom nitride, phosphide, and carbide complexes are excellent starting materials for the synthesis of ligands containing low-coordinate phosphorus centers in the protecting coordination sphere of the metal complex. Salt-elimination reactions with chlorophosphanes lead to phosphaisocyanide, iminophosphinimide, and diorganophosphanylphosphinidene complexes in which the unusual phosphorus ligands are stabilized by coordination. X-ray structure analyses and density-functional calculations illuminate the bonding in these compounds.

A niobaziridine hydride system for white phosphorus or dinitrogen activation and N- or P-atom transfer

This short review describes a breakthrough embodied by the synthesis of a niobaziridine hydride complex. This reactive entity reacts directly with white phosphorus to provide a bridging diphosphorus diniobium complex that upon reduction splits to afford a terminal niobium phosphide anion, isolated as its sodium salt. Reactions of the latter with acid chlorides constitute a new synthesis of phosphaalkynes, while treatment with chlorodiorganophosphanes leads to complexed 1,1-diorganophosphanylphosphinidene systems. Additionally, reactions of the sodium salt of the niobium phosphide anion with divalent main group element salts (E = Ge, Sn, or Pb) provide complexed triatomic EP2 triangles. Dinitrogen cleavage was realized via reduction of a heterodinuclear niobium/molybdenum dinitrogen complex, and this provided an entry to a nitrogen-15 labeled terminal nitride anion of niobium as its sodium salt. In a fashion analogous to the aforementioned phosphaalkyne synthesis, acid chlorides are transformed upon reaction with the niobium nitride anion into corresponding nitrogen-15 labeled organic nitriles. Complete synthetic cycles are achieved in both the phosphaalkyne and the organic nitrile syntheses, as the oxoniobium(v) byproduct can be recycled in high yield to the title niobaziridine hydride complex.

Heterobimetallic Reductive Cross-Coupling of Benzonitrile with Carbon Dioxide, Pyridine, and Benzophenone

Described herein are heterobimetallic radical cross-coupling reactions between the benzonitrile adduct of the molybdenum(III) complex Mo(N[t-Bu]Ar)3 (Ar = 3,5-C6H3Me2) and titanium(III) complexes with carbon dioxide, pyridine, and benzophenone. The titanium(III) system employed was either Ti(N[t-Bu]Ar)3 (Ar = 3,5-C6H3Me2) or Ti(N[t-Bu]Ph)3. Crystal structure studies are described for the Mo/PhCN/CO2/Ti coupled system and for an analogue of the Mo/PhCN/Ph2CO/Ti coupled system in which PhCN is replaced with 2,6-Me2C6H3CN. In the case of the couplings involving pyridine and benzophenone, C-C bond formation takes place with dearomatization, with the new C-C bond being formed between the nitrile carbon of PhCN and the para carbon of pyridine or one of the benzophenone phenyl groups. Of the radical metal complex/substrate adducts invoked in this work, that between titanium(III) and CO2 is the only one not directly observable. In all cases, the selective cross-coupling reactions are interpreted as arising by heterodimerization of titanium(III) substrate complexes (substrate = CO2, py, or Ph2CO) with the persistent molybdenum-PhCN radical adduct. All of the heterobimetallic coupling products are diamagnetic, and the metal ions Ti and Mo in them both are assigned to the formal 4+ oxidation state.

Synthesis of a four-coordinate titanium(iv) oxoanion via deprotonation and decarbonylation of complexed formate

Deprotonation of the titanium formate complex [Ar(t-Bu)N]3TiOC(O)H with LiN(i-Pr)2 resulted in the release of free CO and the formation of a titanium(IV) oxoanion complex, isolated as its lithium salt.

Synthesis and X-ray crystal structure of Cr(NR2)3 (R = CH(Me)Ph)

The reaction of CrCl3 with three equivalents of the lithium salt of [R-(R*,R*)]-(+)-bis(α-methylbenzyl)amine produces the trigonal planar Cr(III) tris(amido) complex. The X-ray structure of Cr[N{CH(Me)Ph}2]3 is reported.Key words: chromium, X-ray crystal structure, amido ligands, chiral ligands.