TMQ is an important precursor in industrial vitamin E synthesis. We report a "green chemistry approach" with respect to the highly selective and environmentally friendly oxidation of 2,3,6-trimethylphenol (TMP) to trimethyl-1,4-benzoquinone (TMQ) with molecular oxygen as oxidant and a copper catalyst immobilized in a molten salt. n-Butanol as co-solvent has a positive effect on the activity and selectivity. The structurally characterized catalyst, a 1-n-butyl-3-methylimidazolium oxotetracuprat, is formed in situ via hydrolysis of CuCl2 in the presence of imidazolium chloride. We propose a mechanism of oxidative phenolate activation at a [Cu4(mu4-O)]6+ core as electronically coupled electron acceptor, formation of a copper-bound phenolate radical anion, spin delocalization into the aromatic ring, and attack by triplet oxygen at the para position. Attack of Cu(I) as reduction equivalent at the peroxy radical, proton-mediated elimination of a copper(II)-hydroxo species, will either substitute a copper(I) site in the reduced oxo cluster or take up an electron from the reduced mixed valent cluster [Cu4(mu4-O)]6+ to regenerate the oxidized cluster as the active electron acceptor.

The synthesis and characterization of a series of Ni, Co, and Fe complexes bearing a tridentate bis(phosphino)silyl ligand (κ 3 -(2-Ph 2 PC 6 H 4 ) 2 SiMeH, [PSiP]-H, 1) are reported. 1 reacted with Ni(PMe 3 ) 4 to afford the mononuclear nickel(0) complex [η 2 (Si−H)-PSiP]Ni(PMe 3 ) (2). The halogeno nickel complexes [PSiP]Ni(X)(PMe 3 ) (X = Cl (3), Br (4), I (5)) were synthesized in the reactions of 2 with Me 3 SiCl or MeHSiCl 2 , EtBr, and MeI. Complex 2 underwent ligand substitution of PMe 3 by CO to give [η 2 (Si−H)-PSiP]Ni(CO) (6). Complex 3 reacted with NaOMe to deliver [PSiP]Ni-(OMe)(PMe 3 ) (7) through anionic ligand substitution, while the neutral ligand replacement of PMe 3 by CO in 3 afforded the rare hexacoordinate 20-electron nickel(II) complex [PSiP]Ni(Cl)(CO) 2 (8). Unexpectedly, reaction of 1 with NiMe 2 (PMe 3 ) 3 produced the tetracoordinate nickel(0) complex [Me 2 PSiP] 2 Ni (9). The complex [Me 2 PSiP]Ni(CO) 2 (10) was acquired from 9 after the substitution of one [PSiP] ligand by two carbonyl ligands. 1 reacted with Co(PMe 3 ) 4 or CoCl(PMe 3 ) 3 to afford the hydrido cobalt(II) complex [PSiP]CoH(PMe 3 ) (11) or hydrido cobalt(III) complex [PSiP]Co(H)(Cl)(PMe 3 ) (13). Complex 12, [PSiP]Co(H)(I)(PMe 3 ), could be obtained from the reaction of MeI with 11 or 13. Treatment of 13 with 1 equiv of MeLi or n-BuMgBr in THF resulted in the clean formation of cobalt(I) complex [PSiP]Co(PMe 3 ) 2 ( 14) via reductive elimination. The simple anhydrous inorganic salt NiCl 2 or CoCl 2 could also react with 1 in the presence of PMe 3 to form the corresponding silyl complexes 3 and [PSiP]Co(Cl)(PMe 3 ) (15) via Si−H bond cleavage. 1 reacted with Fe(PMe 3 ) 4 to form the hexacoordinate octahedral hydrido iron(II) complex [PSiP]Fe(H)(PMe 3 ) 2 ( 16). The molecular structures of complexes 2−5, 10, 12, 13, 15, and 16 were determined by X-ray single crystal diffraction. 16 has excellent catalytic reactivity for the reduction of aldehydes and ketones.

While selective C-H and C-F activations of fluoroaromatic imines and ketones with transition metal complexes supported by PMe3 have been successfully achieved in recent publications, insight into the molecular mechanism and energetics of those reactions is still lacking. Focusing on three typical substrates, 2,6-difluorobenzophenone imine (A) and 2,6-difluorobenzophenone (B), and 2,4'-difluorobenzophenone (C), the present work theoretically studied their C-H and C-F cyclometalation reactions promoted by the activator Co(PMe3)4 or CoMe(PMe3)4. It is found that reaction A + Co(PMe3)4 favors the C-F activation, reaction A + CoMe(PMe3)4 prefers the C-H activation, whereas both the C-H and C-F activation pathways may be viable for reactions B + CoMe(PMe3)4 and C + CoMe(PMe3)4. The experimentally observed C-H and C-F cyclometalation products have been rationalized by analyzing the thermodynamic and kinetic properties of two activation pathways. From calculated results combined with the experimental observations, we believe that three factors, i.e. the oxidation state of the metal center in the activators, the anchoring group of substrates, and substituted fluoroatom counts of the aromatic ring in substrates, affect the selectivity of C-H and C-F activations of fluoroaromatic ketones and imines. Calculated results are enlightening about the rational design of activators and substrates of fluoroaromatic imines and ketones to obtain the exclusive C-H or C-F bond activation product.

We report the synthesis of a novel bidentate Nheterocyclic silylene (NHSi) ligand, N-(LSi:)-N-methyl-2-pyridinamine (1) (L = PhC(NtBu) 2 ), and the first bischelate disilylene iron hydride, [(Si,N)(Si,C)Fe(H)(PMe 3 )] (2), and monosilylene iron hydride, [(Si,C)Fe(H)(PMe 3 ) 3 ] (2′), through C sp 2 −H activation of the NHSi ligand. Compounds 1 and 2 were fully characterized by spectroscopic methods and single-crystal X-ray diffraction analysis. Density functional theory calculations indicated the multiple-bond character of the Fe−Si bonds and the π back-donation from Fe(II) to the Si(II) center. Moreover, the strong donor character of ligand 1 enables 2 to act as an efficient catalyst for the hydroboration reaction of carbonyl compounds at room temperature. Chemoselective hydroboration is attained under these conditions. This might be the first example of hydroboration of ketones and aldehydes catalyzed by a silylene hydrido iron complex. A catalytic mechanism was suggested and partially experimentally verified.

A novel potentially tridentate N-heterocyclic carbene (NHC) precursor, anionic salicylaldimine-functionalized imidazolium bromide, [3,5-t Bu 2 -2-(HO)C 6 H 2 CHdNCH 2 CH 2 (CH-{NCHCHN i Pr})Br] (HL‚HBr, 2), was designed. The reaction of in situ-generated monoanionic tridentate salicylaldiminato-functionalized NHC LNa with Ni(PPh 3 ) 2 Br 2 affords a novel monoligand Ni(II) bromide, [3,5-t Bu 2 -2-(O)C 6 H 2 CHdNCH 2 CH 2 (C{NCHCHN i Pr})]NiBr (LNiBr , 3), in good yield. Complex 3 can also be synthesized by the direct reaction of nickelocene (Cp) 2 Ni or bis-indenyl Ni(II) complex (Ind) 2 Ni with 2 in high yield via the cyclopentadiene or indene elimination reaction, respectively. Complex 3 has been fully characterized including X-ray structural determination. Preliminary study indicated that 3 shows good catalytic activity for the polymerization of styrene in the presence of NaBPh 4 at 80 °C.

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The reaction of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) with one equivalent of a novel imidazolium salt of iron(II), [FeBr 3 (C 4 H 8 O)](HIPr) 3 C 4 H 8 O (1), afforded the anionic iron(II) complex bearing an N-heterocyclic carbene ligand [Fe(IPr)Br 3 ](HIPr) 3 C 7 H 8 (2), which shows extremely high activity in comparison with the other iron(II)-based precatalysts in the cross-coupling reaction of 4-tolylmagnesium bromide with cyclohexyl bromide.