Incorporation of Cation Affects the Redox Reactivity of Fe-NNN Complexes on C-H Oxidation.

Cations such as Lewis acids have been shown to enhance the catalytic activity of high-valent Fe-oxygen intermediates. Herein, we present a pyridine diamine ethylene glycol macrocycle, which can form Zn(II)- or Fe(III)-complex with the NNN site, while allowing redox-inactive cations to bind to the ethylene glycol moiety. The addition of alkali, alkali earth, and lanthanum ions resulted in positive shifts to the Fe(III/II) redox potential. Calculation of dissociation constants showed the tightest binding with a Ba2+ ion. Density functional theory calculations were used to elucidate the effects of redox inactive cations toward the electronic structures of Fe complexes. Although the Fe-NNN complexes, both in the absence and presence of cations, can catalyze C-H oxidation of 9,10-dihydroanthracene, to give anthracene [hydrogen atom transfer (HAT) product], anthrone, and anthraquinone [oxygen atom transfer (OAT) products], highest overall activity and OAT/HAT product ratios were obtained in the presence of dications, that is, Ba2+ and Mg2+, respectively.

Influence of catalyst nuclearity on copper-catalyzed aerobic alcohol oxidation.

Reactions of CuX with the bis(triazolyl) ligand Hbtm [bis(1-benzyl-1H-1,2,3-triazol-4-yl)phenylmethanol] in CH2Cl2 afforded trinuclear copper(ii) complexes with a core structure (µ-X)Cu3(µ-κ3-N,O,N-btm)3(L)2+ [X = Cl, L = CH3OH (1); X = Br, L = H2O (2)], while a similar reaction of [Cu(CH3CN)4](PF6) with the mono(triazolyl) ligand HPhtm [(1-benzyl-1H-1,2,3-triazol-4-yl)diphenylmethanol] resulted in the mononuclear complex [Cu(κ2-N,O-Phtm)(κ2-N,O-HPhtm)(κ1-N-HPhtm)][PF6] (3). The structural characterization of these complexes was made by single-crystal X-ray crystallography in combination with elemental and ESI mass analyses. Catalytic studies toward aerobic oxidation of benzyl alcohol to benzaldehyde revealed that the trinuclear 1 and 2 exhibited higher activities than the mononuclear 3 in both CH3CN and EtOH/H2O solvent systems.

Tunable Porosity of Cross-Linked-Polyhedral Oligomeric Silsesquioxane Supports for Palladium-Catalyzed Aerobic Alcohol Oxidation in Water.

Polyhedral oligomeric silsesquioxane (POSS)-based materials, poly-POSS-Tn [n = 8 (1), 10 (2), 12 (3), and mix (4)], were prepared in high yields via free radical polymerization of corresponding pure forms of methacrylate-functionalized POSS monomers, MMA-POSS-Tn (n = 8, 10, 12), and the mixture form, MMA-POSS-Tmix. Powder X-ray diffraction (XRD) spectra and BET analysis indicate that 1-4 are amorphous materials with high surface areas (683-839 m2 g-1). The surface areas and total pore volumes follow the trend: poly-POSS-T12 > poly-POSS-T10 > poly-POSS-Tmix > poly-POSS-T8. In addition, on the basis of Barrett-Joyner-Halenda (BJH) analysis, poly-POSS-T12 contains the highest amount of mesopores. The Pd nanoparticles immobilized on poly-POSS-Tn [n = 8 (5), 10 (6), 12 (7), and mix (8)] are well dispersed with 4-6 wt % Pd content and similar average particle sizes of 6.2-6.5 nm, according to transmission electron microscopy-energy dispersive X-ray analysis (TEM-EDX) and microwave plasma-atomic emission spectroscopy (MP-AES). At 90 °C, the stabilized Pd nanoparticles in 5-8 catalyzed aerobic oxidation of benzyl alcohol to benzaldehyde in 72-100% yields at 6 h using a mixture of a H2O/Pluronic (P123) solution. The PdNp@poly-POSS-T8 catalyst (5) exhibited the lowest catalytic activity, as a result of its lowest surface areas, total pore volumes, and amounts of mesopores. With the catalyst 8, various benzyl alcohol derivatives were converted to the corresponding aldehydes in good to excellent yields. However, with alcoholic substrates featuring electron-withdrawing substituents, high conversions were achieved with 1 equiv of K2CO3 additive and longer reaction times.

Benzimidazole-triazole ligands with pendent triazole functionality: unexpected formation and effects on copper-catalyzed aerobic alcohol oxidation.

A series of benzimidazole-triazole ligands (NN') having a pendent triazole arm with different triazole substituents including CH2Ph (3a), cyclo-C6H11 (3b), and CH2SiMe3 (3c) were obtained in moderate yields from Cu-catalyzed oxidative C-N cyclization of the respective amine-triazole compounds N,N'-bis((1-R-1,2,3-triazol-4-yl)methyl)benzene-1,2-diamine (2a-2c). Treatment of CuCl2 with one equiv. of the benzimidazole-triazole ligands afforded the corresponding CuII complexes with the empirical formula of Cu(NN')Cl2 (4a-4c). Crystal structures of 4b and 4c reveal mononuclear and dinuclear CuII complexes, respectively. Despite the differences in triazole substituents and their solid state structures, ESR spectra indicate the same molecular structures in CH3CN solution whereas CV data suggest similar redox potentials for 4a-4c. Catalytic activities for aerobic oxidation of benzyl alcohol to benzaldehyde follow this trend: 4c > 4a > 4b. In addition, the catalytic system 4c/TEMPO/Cu0/NMI (TEMPO = 2,2,6,6-tetramethyl-1-piperidinyloxyl, NMI = N-methylimidazole) exhibited high activities for oxidation of activated alcohols (i.e., benzyl alcohol derivatives and allylic alcohol) in CH3CN at room temperature.

Synthesis of cyclic polyesters: effects of alkoxy side chains in salicylaldiminato tin(II) complexes.

A new class of salicylaldiminato tin(II) catalysts having different alkoxy side chains has been developed. The ligands were modified to have different lengths and flexibilities such as ­(CH2)2­ (2a), ­(CH2)3­ (2b), ­(ortho-C6H4)CH2­ (2c) and ­(CH2)2­O­(CH2)2­ (2d). Complexes 2a, b were characterized crystallographically revealing a more constrained environment around the metal in complex 2a. These catalysts are active for the solvent-free polymerization of L-lactide and ε-caprolactone. Complex 2a having a shorter side chain was shown to better promote intramolecular transesterification affording cyclic polylactides and cyclic poly(ε-caprolactone). Complexes 2b and 2d having longer side chains produced cyclic poly(ε-caprolactone) as a major product but failed to give cyclic polylactides.

Synthesis of aromatic functionalized cage-rearranged silsesquioxanes (T8, T10, and T12) via nucleophilic substitution reactions.

Organic-inorganic hybrid nano-building blocks of aromatic nitro-, aldehyde-, and bromo-functionalized polyhedral oligomeric silsesquioxanes were easily prepared through nucleophilic substitutions, starting from the reactions between octakis(3-chloropropyl)octasilsesquioxane and phenoxide derivatives. These phenoxide anions not only supply the substitution functions to a silsesquioxane cage, but can also induce a cage-rearrangement leading to the formation of octa-, deca-, and dodecahedral silsesquioxane cages.

Bis[2,6-bis-(2-meth-oxy-phen-yl)pyridinium] di-µ-bromido-bis-[dibromidocuprate(II)].

The title salt, (C(19)H(18)NO(2))(2)[Cu(2)Br(6)], was obtained from an attempt to synthesize the copper(II) complex of 2,6-bis-(2-meth-oxy-phen-yl)pyridine (L) from a reaction between CuBr(2) and one equivalent of L in CH(2)Cl(2) at room temperature. The resulting compound is the salt of the 2,6-bis-(2-meth-oxy-phen-yl)pyridinium cation and 0.5 equivalents of a hexa-bromido-dicuprate(II) dianion. Both meth-oxy groups of the cationic pyridinium moiety are directed towards the N atom of the pyridine ring as a result of intra-molecular N-H⋯O hydrogen bonds. The centrosymmetric hexabromidodicuprate dianion possesses a distorted tetra-hedral geometry at the copper ion. The Cu-Br bond lengths are 2.3385 (7) and 2.3304 (7) Šfor the terminal bromides, whereas the bond length between the Cu atom and two bridging bromides is slightly longer [2.4451 (6) Å].

Synthesis of high-molecular-weight poly(ε-caprolactone) catalyzed by highly active bis(amidinate) tin(II) complexes.

A series of bis(amidinate) tin(II) complexes is synthesized and shown to rapidly polymerize ε-caprolactone (ε-CL) in the presence and absence of benzyl alcohol giving high-molecular-weight poly(ε-CL) (M(n) up to 160,600 Da). Ligands having electron donating groups were found to accelerate the polymerization by making the complex more nucleophilic.

Reverse orders of reactivities in the polymerization of cyclic esters using N(2)O(2) aluminium alkoxide complexes.

Three aluminium alkoxide complexes containing N(2)O(2) bis(phenoxy)-amine ligands were synthesized from reactions of the corresponding N(2)O(2) ligands with Al(O(i)Pr)(3) in toluene. Different amine side chains of the ligands included pyridine (1), CH(2)NMe(2) (2), and CH(2)NEt(2) (3). The related chloro aluminium analog (4) was prepared from a reaction between AlCl(3) and the potassium salt of the N(2)O(2) ligand having CH(2)NMe(2) side chain. X-Ray crystallography reveals that complexes 3 and 4 have a monomeric five-coordinate aluminium center. Complexes 1-3 catalyzed the polymerization of epsilon-caprolactone (epsilon-CL) at 70 degrees C in toluene with the relative reactivities of 1<2< 3. In contrary, only complex 1 was active for the polymerization of lactide under the same polymerization conditions. (1)H NMR spectroscopy shows that treatment of 2 with 1 equivalent of lactide afforded the ring-opened product L(2)Al-OCH(Me)C(O)OCH(Me)C(O)O(i)Pr. Electronic effects are believed to be responsible for the observed trend in the epsilon-CL polymerization rates. On the other hand, steric hindrance at the amine side chain is the main contributor to the observed rates of lactide polymerization.